Synthesis and secretion of GnRH

Animal Reproduction Science 88 (2005) 29–55

Synthesis and secretion of GnRH夽 Iain J. Clarke ∗ , Sueli Pompolo Prince Henry’s Institute of Medical Research, P.O. Box 5152, Clayton 3168, Australia

Abstract Comprehensive studies have provided a clear understanding of the effects of gonadal steroids on the secretion of gonadotropin releasing hormone (GnRH), but some inconsistent results exist with regard to effects on synthesis. It is clear that regulation of both synthesis and the secretion of GnRH are effected by neurotransmitter systems in the brain. Thus, steroid regulation of GnRH synthesis and secretion can be direct, but the predominant effects are transmitted through steroid-responsive neuronal systems in various parts of the brain. There is also emerging evidence of direct effects on GnRH cells. Overriding effects on synthesis and secretion of GnRH can be observed during aging, in undernutrition and under stressful situations; these involve various neuronal systems, which may have serial or parallel effects on GnRH cells. The effect of aging is accompanied by changes in GnRH synthesis, but comprehensive studies of synthesis during undernutrition and stress are less well documented. Altered GnRH and gonadotropin secretion that occurs in seasonal breeding animals and during the pubertal transition is not generally accompanied by changes in GnRH synthesis. Secretion of GnRH from the brain is a reflection of the inherent function of GnRH cells and the inputs that integrate all of the central regulatory elements. Ultimately, the pattern of secretion dictates the reproductive status of the organism. In order to fully understand the central mechanisms that control reproduction, more extensive studies are required on the neuronal circuitry that provides input to GnRH cells. © 2005 Elsevier B.V. All rights reserved. Keywords: GnRH; Gonadotropins; Reproduction; Neuroendocrinology; Estrogen

夽 This paper is part of the special issue entitled: GnRH in Domestic Animal Reproduction, Guest Edited by K.L. Macmillan and Jin-Gui Gong. ∗ Corresponding author. Present address: Department of Physiology, PO Box 13F, Monash University, Clayton, Vic. 3800, Australia. Tel.: +61 39594 4387; fax: +61 39594 6125. E-mail address: [email protected] (I.J. Clarke).

0378-4320/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.anireprosci.2005.05.003

30

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

1. Introduction Gonadotropin releasing hormone (GnRH) is the brain factor that provides primary drive for the reproductive axis. Without GnRH, the gonadotropes and the gonads do not function. This is well demonstrated in the human condition of Kallman’s syndrome (SchwanzelFukuda et al., 1989) and in the hypogonadal mouse (Charlton et al., 1983; Mason et al., 1986). Given that GnRH secretion, and action on the pituitary gonadotropes, is essential to the reproductive process, some knowledge of the factors that control the GnRH cells of the brain is of fundamental importance. In spite of this, there is a relatively poor understanding of the how various factors provide integrated information to the GnRH cells and regulate secretion. The function of GnRH cells can be influenced by a range of factors, such as gonadal feedback, stress, nutrition and season. This review focuses on how these intrinsic and extrinsic factors regulate synthesis and secretion of GnRH. Regarding the control of secretion, information is most abundant for studies done in sheep, since there is a robust means for accessing the hypophyseal portal blood vessels in this species (Clarke and Cummins, 1982). This allows minute-to-minute analysis of patterns of secretion in detail and the changes that occur during the estrous cycle, and in response to gonadal steroids, has been reviewed in detail (Clarke, 1996, 2002). Since the majority of information on secretion of GnRH has been obtained from sheep, this review is focussed on information from this species. In ovariectomised (OVX) females, the long-term effect of both progesterone (P) and estrogen (E) is to inhibit GnRH secretion, whereas the transient effect of unopposed elevation in E levels causes a positive feedback effect. In castrated males, testosterone has a powerful negative effect on GnRH secretion. These steroid effects can explain the patterns of luteinizing hormone (LH) secretion that are seen in gonadintact animals and explain the cyclic effects of LH secretion seen during the estrous cycle. Information on the regulation of the synthesis of GnRH has been derived from various species.

2. GnRH gene and amino acid sequence GnRH is a 10 amino acid peptide that was first characterized in mammals (Amoss et al., 1971; Matsuo et al., 1971). The mammalian gene, and the identification of a translated precursor peptide was reported by Seeburg and Adelman (1984) and Adelman et al. (1986). GnRH pre-prohormone mRNA was found in the diagonal band of Broca (dbB) and the preoptic (POA) of the rat brain (Shivers et al., 1983). Using techniques such as immunohistochemstry, in situ hybridisation, polymerase chain reaction and HPLC, expression of the GnRH gene and the peptide product was detected in the brains of a wide variety of species across the phylogenetic scale, from tunicates to humans (reviews in Fernald and White 1999; Lin et al., 1998b; Carolsfeld et al., 2000; Grove-Strawser et al., 2002; Gore, 2002a). Sixteen different forms of GnRH have been isolated and in the majority of species, two or more forms occur in anatomically and distinct neuronal populations (Gestrin et al., 1999; Urbanski et al., 1999; Latimer et al., 2000). The amino acid sequence of the hypohysiotropic GnRH is identical across mammals, with the exception of the guinea pig in which there are substitutions of amino acids 2 and 7 (Jimenez-Linan et al., 1997). The gene sequence of

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

31

the coding region and its molecular signal peptide sequence is highly conserved across a wide range of species, differing only in a few amino acids (see Millar, 2003, this volume). This chapter will focus on the cells of the mammalian brain that produce Type I GnRH and provide the hypophysiotropic drive to the gonadotropes of the pituitary gland. GnRH-I is found in the POA and hypothalamus, whereas cells that produce GnRH-II are found in the midbrain and non-hypophyiotropic centres of the hypothalamus and hippocampus in mice, rats, humans and monkeys (Miyamoto et al., 1982; White et al., 1995, 1998; Urbanski et al., 1999; Gestrin et al., 1999). GnRH-III was first sequenced in salmon and is produced in the olfactory and forebrain regions of several species of fish, reptiles and at least one mammal, the capybara (Hydrochaeris hydrochaeris) (Sherwood et al., 1983; Montaner et al., 1999; Carolsfeld et al., 2000). Using immunohistochemistry to study the sheep brain, and using all of the currently available antibodies (Millam et al., 1989; Okuzawa et al., 1990; Urbanski et al., 1999; Latimer et al., 2000), we have been unable to demonstrate the presence of GnRH-II or GnRH-III in the cell bodies (Pompolo and Clarke, unpublished data). Immunoreactive varicosities were seen in median eminence with these antibodies, but this always co-localised with immunoreactivity for GnRH-I and could be blocked by pre-absorption of the antibodies with GnRH-I (Pompolo and Clarke, unpublished data). Some recent studies suggest that GnRH-III acts as an FSH-releasing factor (Yu et al., 1997, 2000; Dees et al., 2001; Densmore and Urbanski, 2003; Temple et al., 2003), but there is no evidence that this peptide is secreted from the brain in mammals. We have been unable to obtain evidence of secretion of GnRH-II into hypophyseal portal blood in sheep (G.A. Lincoln, R.P. Millar and I.J. Clarke, unpublished data). The gene encoding preproGnRH-1 gene has been cloned in number of species (reviewed by Wierman et al., 1995; Lin et al., 1998; Fernald and White, 1999) and is approximately 4300-bp with four relatively short exons (1–4) separated by three large introns (A–C) (Fig. 1). Exon 1 encodes the 5 -untranslated region, exon 2 the signal peptide, the GnRH-I decapeptide and the first 11 amino acids of the GnRH-associated peptide (GAP). Exon 3 encodes amino acids 12–43 of GAP and exon 4 encodes the remainder of GAP and the 3 -untranslated region. In POA-AH of rat and human all three pro-GnRH-I introns are spliced out of the primary gene transcript resulting in a mature mRNA of about 560 bases, excluding the poly(A) tail (Adelman et al., 1986). Jakubowski and Roberts (1994) found that the first processing step of prepro-GnRH-I in the POA-anterior hypothalamus of rats

Fig. 1. Structure of the GnRH-I gene that has been conserved throughout evolution (Lin et al., 1998). As a general structure, the GnRH-I gene consists of four relatively short exons 1, 2, 3 and 4 (the rat gene has 138, 142, 96 and 186 bp, respectively). The exons are separated by 3 large introns A, B, C (the rate gene has 800, 1590 and 1350 bp respectively). Black bars indicate 5 - and 3 -UTR in the exons 1 and 4 respectively. The hatched bar indicates the signal sequence, the cross-hatched bar indicates the GnRH-I-coding region and the open bar indicates the GnRH-associated peptide (GAP) coding region (Modified version of the sequence in H. burtoni from White and Fernald 1998).

32

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

was the splicing out of intron B. Subsequently introns A and C are spliced out in no apparent order. Other studies in the mouse and in GT1 cell lines suggested that there is no apparent preferential order for the splicing out of introns (Yeo et al., 1996). In vitro studies, using HeLa cells, show that the splicing of introns B and C occurs with greater efficiency than that of intron A (Seong et al., 1999). More recent studies by Son et al. (2003) suggested that the precise and efficient excision of intron A and the joining of adjacent exons is probably the most critical regulatory step for the post-transcriptional regulation of GnRH regulation. During the GnRH processing, several transcripts can be detected in the nucleus of the cell including the unprocessed GnRH mRNA primary transcript, several processing intermediates containing one or more introns, and the mature GnRH mRNA (Jakubowski and Roberts, 1994; Yeo et al., 1996). There are relatively few GnRH neurons scattered through the POA and adjacent areas, with an estimated total population of 2400 GnRH cells in adult rhesus monkey brain (Marshall and Goldsmith, 1980). Studies on transcriptional control of GnRH has been done largely in GnRH-secreting cell lines such as GT1-7, GN10 and GN11 (Mellon et al., 1990; Radovick et al., 1991). The 5 flanking region of GnRH gene is highly homologous between species (Nelson et al., 1998). It comprises two major regions for transcription, an enhancer and a promoter. A conserved 173-bp promoter is found just upstream of the transcriptional start site and deoxyribonuclease I footprint analysis of the rat GnRH promoter revealed at least seven regulatory regions located between −173 and 112 (Eraly and Mellon, 1995). The promoter contains binding sites for transcription factors, such as POU-homeodomain Oct-1 and the neuron-restricted homodomain protein, Otx2 (Kelley et al., 2000, 2002). The proximal promoter also includes cis-acting elements involved in hormonal regulation by glucocorticoids (Chandran and DeFranco, 1999), E and P (Kepa et al., 1996; Roy et al., 1999) and 12-O-tetradecanoylphorbol 13-acetate (Bruder et al., 1996). A second important region for transcription of the rat GnRH promoter is a 300-bp enhancer that is −1863 to −1571 relative to the start site; this confers a 50–100-fold transcriptional activation over the promoter alone, at least in GT1-7 cells (Nelson et al., 1998; Whyte et al., 1995). Within the enhancer, specific binding sites for a number of transcription factors have been identified, such as the zinc finger protein, GATA-4 and pbx-related protein (Lawson et al., 1996; Kelley et al., 2002).

3. Anatomical localisation of GnRH in the brain The GnRH neurons originate from the olfactory placode (Schwanzel-Fukuda and Pfaff, 1989; Wray et al., 1989; Wray, 2001). From this embryonic origin, the cells migrate to and colonize the basal forebrain in and around the POA and the mediobasal hypothalamus (Wray, 2001, 2002). Studies in a number of species have shown a broadly similar arrangement of GnRH localization in the brain, although there is some interspecific variation. Immunohistochemical (Baker et al., 1975; King et al., 1974; Polkowska et al., 1980; Lehman et al., 1986) and radioimmunoassay (Wheaton et al., 1975) data show that the median eminence of mammalian species contains the greatest amount of GnRH, this being the area in which the peptide is stored in neuronal terminals prior to release into hypophyseal portal blood. Significant amounts of GnRH are also found in the neuronal terminals of the

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

33

organum vasculosum of the lamina terminalis, although the function of these projections is not known. The GnRH neuronal cell bodies that project to the median eminence are mostly found in the dbB, the POA and septal regions of the mammalian brain, although a discrete population is found in the mediobasal hypothalamus; the importance of the latter is species dependent (see Clarke, 1987b for detailed review). In the sheep, a small number of GnRH cells in the arcuate nucleus may be relevant to basal secretion of peptide, with evidence of regulation by ovarian steroids in the ewe (Boukhliq et al., 1999). The anatomical arrangement between the hypothalamus, median eminence and the pituitary gland is a masterpiece of design that allows for exquisite control of the gonadotropes of the pituitary gland. The GnRH cells project to the external (secretory) zone of the median eminence, where terminals are found in close proximity to the primary capillary bed of the hypophyseal portal system (Page and Dovey-Hartman, 1984). Neural mechanisms regulate the GnRH cells, although some intrinsic phasic mechanism probably exists within the cells (Andrew and Dudek, 1983).

4. GnRH secretion 4.1. The relationship between GnRH and gonadotropin secretion Original measurements of GnRH in plasma were made in anaesthetised rats using an anaesthetic (Althesin® ) that does not inhibit secretion (Sarkar et al., 1976). With this methodology, it was unequivocally shown that the signal for ovulation originated from the brain. A pro-estrous surge in GnRH was demonstrated, inferring that this was the cause of the preovulatory surge in LH secretion. Since original methods of collection of hypophyseal portal blood compromised pituitary function (Worthington, 1966; Porter and Smith, 1967), it was not possible to make simultaneous measurements of GnRH secretion and pituitary gonadotrophin secretion. With the development of the ovine model (vide supra) to sample hypophyseal portal blood in conscious animals and to simultaneously sample peripheral blood, for the measurement of LH secretion (Clarke and Cummins, 1982), it became clear that pulsatile pattern of LH secretion in OVX ewes is a direct reflection of hypothalamic secretion of GnRH (although direct steroid feedback effects on the gonadotrope can influence the amplitude of LH pulses). This was the final proof of the ‘neurohumoral theory’ of Harris (1955), which stated that secretion of each type of hormone from the anterior pituitary gland would reflect secretion of the relevant neuropeptide from the brain. The system allows for substantial amplification of the signal, since picogram quantities of GnRH in portal blood elicit pulses of LH in peripheral blood that are in the nanogram range. Given the dilution in the periphery, this is an amplification of many 1000-fold. The phasic pattern of GnRH and LH secretion allows for modulation of both frequency and amplitude, and this is clearly manifest across the estrous cycle of the ewe (vide infra). The high specificity of GnRH for the GnRH receptor on the pituitary gland confers specificity to the whole reproductive process (see Nett, this volume). The low GnRH concentrations in the hypophyseal portal blood and the presence of enzymes that degrade the peptide (McDermott et al., 1981) necessitate that the sampling of portal blood is required in order

34

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

to monitor GnRH secretion. Interestingly, the pattern of secretion of follicle stimulating hormone (FSH) is not tightly coupled to the secretion of GnRH, although GnRH action is required for the maintenance of secretion of this gonadotropin (see review by Clarke, 1996). GnRH input to the gonadotrope appears to maintain FSH synthesis, but control of the secretion of this gonadotropin seems to involve pituitary action of gonadal steroids and inhibin (Clarke et al., 1984, 1986; Lincoln and Fraser, 1987; Li et al., 1989). 4.2. GnRH secretion during the estrous cycle GnRH cells are regulated by the feedback effects of gonadal steroids, involving complex processes that are not clearly understood. Fig. 2 shows the pattern of steroid secretion across the estrous cycle of the ewe and indicates that GnRH cells are regulated by gonadal steroids via intermediary neurons that possess the relevant steroid receptors. This defines the pattern of GnRH secretion. GnRH neurons do not express P or androgen receptor (Herbison et al., 1996; Skinner et al., 2001), nor do they express E receptor ␣ (ER␣) (Shivers et al., 1983; Lehman and Karsch, 1993; Butler et al., 1999; Herbison and Pape, 2001). Although the GnRH cells of rat express ER␤ (Herbison and Pape, 2001; Hrabovszky et al., 2000), the role of this sub-type of ER appears to be of minor significance with respect to feedback control of GnRH cells (Krege et al., 1998; Lubahn et al., 1993). Lagrange et al. (1995) showed that E hyperpolarised (and thus down-regulated) GnRH cells in the guinea

Fig. 2. Diagramatic version of the ovine estrous cycle showing the progesterone-dominant luteal phase, the estrogen-dominant follicular phase and the surge-phase. Steroid hormones secreted by the ovary regulate the GnRH cells via neuronal intermediates within the brain.

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

35

pig hypothalamus, by opening K+ channels. More recently, it has become increasingly clear that steroids rapidly act on cells, in second or minutes, to exert effects that has been classified as “nongenomic” (Falkenstein and Wehling, 2000; Razandi et al., 2003). Recent work (Nunemaker et al., 2002) has utilised transgenic mice with green fluorescent protein incorporated into the GnRH gene to allow identification of individual cells within the brain. Using electrophysiological techniques these workers have reported direct effects of E on GnRH cells. It was further suggested (Nunemaker et al., 2002) that such E effects are modulated by afferents such as ionotropic GABA-ergic and glutamatergic neurotransmitter systems. In spite of this, the overwhelming evidence is that the majority of control of GnRH secretion is by steroids is via E-responsive systems within the brain, that relay peripheral signals by conventional synaptic input to the cells. This indirect effect on controlling GnRH secretion appears to involve both excitatory and inhibitory neurotransmitters/neuropeptides (Herbison, 1998; Tilbrook et al., 2002). The secretion of GnRH during the luteal phase of the cycle is characterised by high amplitude, low frequency pulses (Moenter et al., 1991; Clarke, 1987a, 1995a, 1995b). At the end of the luteal phase of the estrous cycle, with a fall in plasma P levels, the frequency of GnRH and LH pulses increases (Moenter et al., 1991; Clarke, 1995b) (Fig. 3). As the follicular phase ensues and plasma E levels rise (Fig. 2), GnRH pulse frequency increases and amplitude decreases (Fig. 3). The preovulatory LH surge is associated with an unambiguous rise in the secretion of GnRH (Clarke et al., 1987; Moenter et al., 1991) (Fig. 3). This is due to the un-opposed action of E on the brain to activate systems that are involved in the positive feedback mechanism (Clarke, 1995b). Thus, whilst under a negative feedback clamp, high levels of E un-opposed by P causes activation of central mechanisms that lead to positive feedback (Clarke, 1995b). The estrous cycle may be modelled by appropriate treatment with E and P in OVX ewes (Goodman et al., 1981; Evans et al., 1994a). This model appears to produce patterns of GnRH/LH secretion similar to those seen during the normal cycle, indicating that these two steroids play a predominant role in the regulation of GnRH cells in the female. In this model, withdrawal of P and the application of higher levels of E lead to an artificial follicular phase, followed by a preovulatory-like GnRH/LH surge. During the artificial follicular phase of this model, GnRH secretion progressively declines, as in the normal cycle (Clarke, 1995a) until the time that a GnRH/LH surge ensues (Evans et al., 1994b). This is a negative feedback effect of E in the period prior to the surge and this is consistent with data obtained in cyclic ewes (Moenter et al., 1991; Clarke, 1995b). With regard to the preovulatory LH surge, this can be replicated by a single injection of estradiol benzoate to OVX ewes (Clarke, 1993) or anestrous ewes (Clarke, 1988). Whilst this does not account for P priming of the brain, that occurs in normal cyclic animals, the mere fact that a surge is produced indicates the predominant role of E in the induction of the preovulatory event (Clarke, 1993). P acts in two ways on the secretion of GnRH in the sheep. During the luteal phase, this steroid limits GnRH pulse frequency (vide supra), whilst there is minimal effect at the level of the pituitary gland (Clarke and Cummins, 1984). Continuously elevated plasma P levels prevent the preovulatory LH surge (Scaramuzzi et al., 1971), which is due to blockade of the GnRH surge (Kasa-Vubu et al., 1992). P also acts during the luteal phase of the cycle to ‘prime’ the brain to the actions of E and this may affect the timing of the GnRH/LH surge (Caraty and Skinner, 1999). Harris et al. (1998) treated P-primed OVX ewes with E

36

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

Fig. 3. GnRH and LH levels at various stages of the ovine estrous cycle. During the progesterone-dominant luteal phase (A), GnRH and LH pulses are of low frequency and high amplitude. During the follicular phase, the frequency of GnRH/LH pulses increases and the amplitude decreases; there is a progressive decline in amplitude from the early follicular phase (24 h after an injection of Cloprostenol® to late-luteal phase ewes) (B) until the late follicular phase (48 h after an injection of Cloprostenol® to late-luteal phase ewes) (C). There is an unambiguous surge in GnRH secretion that initiates the preovulatory LH surge and ovulation (D), in concert with synonymous upregulation of the sensitivity of the pituitary gonadotropes to GnRH. Arrowheads in panels A–C indicate defined pulses. In the upper part of panel D, the larger data points represent estradiol levels in peripheral plasma and the smaller data points represent GnRH levels in hypophysial portal plasma; the data in the lower part represent peripheral plasma levels of LH. Panels A–C are adapted from Clarke (1995a) and panel D is adapted from Moenter et al. (1991).

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

37

for 8 h and then gave P for 5 h afterwards or between 5 and 10 h afterwards. Only when P was given in the period shortly after E treatment was the LH surge blocked. E appears to activate cells within the mediobasal hypothalamus to cause the GnRH/LH surge (Caraty et al., 1998). Cells of the arcuate nucleus and the ventromedial nucleus, and in the A1 field of the brainstem of the OVX ewe are activated by E, as indicated by the appearance of Fos within 1 h of E injection (Clarke, 1995b; Clarke et al., 2001; Rawson et al., 2001). The E-responsive cells of the brainstem are retrogradely labelled from the preoptic area (Rawson et al., 2001) are noradrenergic and contain ER␣ (Scott et al., 1999). In the arcuate/ventromedial region of the hypothalamus the types of cells that express ER␣ and show Fos responses to E were not well characterised (Clarke et al., 2001), but recent data (Pompolo et al., 2003b) show that many of these are glutamatergic. It appears that there is an indirect pathway from this region of the hypothalamus to GnRH cells (Pompolo et al., 2001), but this is not fully defined. The glutamatergic cells that are activated by E may form part of this pathway (Pompolo et al., 2003b). Although induction of cells by E can be detected in the relevant regions, and it is possible that these are related to control of GnRH synthesis/secretion, it is also possible that these elements are involved in other functions such as sexual behaviour. 4.3. GnRH secretion in rams Tilbrook et al. (2002) have reviewed studies carried out in rams. The relationship between GnRH and LH secretion is demonstrated in this sex, as in the female. Testosterone negatively regulates the secretion of GnRH in castrated rams, but gonadal steroids do not appear to regulate the gonadotropes directly in rams (Tilbrook et al., 1991). Inhibin clearly acts to regulate FSH secretion at the level of the pituitary gland in rams (Tilbrook et al., 1993, 1999, 2001).

5. GnRH synthesis 5.1. Effects of gonadectomy and gonadal steroids on GnRH synthesis in females The rate of production of mature GnRH peptide can potentially be influenced by transcription rate, mRNA stability, and post-translational processing (Gore and Roberts, 1997), but most studies have involved measurement of steady-state levels of mRNA, using techniques such as in situ hybridisation. There have, however, been some studies of regulation of transcription, as indicated below. Results obtained in animals of various species, examining levels of mRNA expression across the normal estrous cycle, and following castration, have not always produced consistent results, so the exact role of gonadal steroids in the regulation GnRH synthesis remains controversial (see reviews by Sagrillo et al., 1996; Gore and Roberts, 1997; Gore, 2002a). Expression during the normal cycle was found to be inversely proportional to plasma levels of oestradiol in rats, suggesting negative feedback regulation (Zoeller and Young, 1988; Park et al., 1990), but at least one study reported no cyclic changes in the levels of GnRH mRNA in rats (Malik et al., 1991). Using in situ hybridisation, one group (Porkka-Heiskanen et al., 1994)

38

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

observed an increase in the number of cells expressing GnRH mRNA at 1200 h on the day of pro-estrus, but the level of mRNA expression/cell was not altered. Similar results were observed in rats (Park et al., 1990), but the increase was seen at 1800 h. Gore and Roberts (1995) used a ribonuclease protection assay and observed an increase in primary transcripts of GnRH and mature message at 1500 h. Others (Suzuki et al., 1995) have used quantitative reverse transcription-polymerase chain reaction (PCR) methodology and observed a peak of expression at 1100 h on the day of pro-estrus. Thus, an increase in GnRH mRNA expression and/or the number of cells in which GnRH expression appears to occur on the day of proestrus, but the timing of this increase can vary. Perhaps such differences relate to subtle differences in the timing of the preovulatory GnRH/LH surge in different colonies of rats. Early studies using radioimmunoassay showed that ovariectomy reduced levels of GnRH in the mediobasal hypothalamus in rats (Kalra, 1976) and sheep (Wheaton, 1979), primarily due to the reduction in content of GnRH in the median eminence (Kalra, 1976; Kobayashi et al., 1978) and this effect could be reversed in female rats by administration of E (Kalra, 1976). There appeared to be very little, if any, change in the GnRH content of hypothalamic nuclei of rats following ovariectomy (Kobayashi et al., 1978). Using different techniques, some studies showed that GnRH mRNA levels were increased in OVX rats compared to randomly cycling female rats (Toranzo et al., 1989), whereas others reported no change compared with intact pro-estrous rats (Kelly et al., 1989), or a reduction of expression compared to prepubertal rats (Kim et al., 1989) or diestrous rats (Roberts et al., 1989). These differing results are probably due to several factors, such as different models, the length of time after the ovariectomy, or time of the day that animals were killed, or in which stage of estrous cycle these animals were killed. Roberts et al. (1989) showed that GnRH expression was lower 9 days after ovariectomy than at shorter (2 days) or longer (16 and 24 days) time-points when compared with randomly cycling rats and Petersen et al. (1993) observed that GnRH mRNA decreased between 2 and 28 days after ovariectomy. Others (Kim et al., 1989) observed reduced levels of GnRH mRNA within 2 days after ovariectomy of immature, age-matched rats. Toranzo et al. (1989) observed an increase of 25% in the levels of GnRH mRNA when compared with randomly intact animals. On the other hand, Kelly et al. (1989) did not find any effect 14 days after OVX when compared with proestrous rats. E replacement in OVX rats induces both negative and positive feedback effect on LH secretion. On one hand, E treatment reverses the rise in basal plasma LH levels following ovariectomy but also stimulates a daily afternoon LH surges (Legan et al., 1975). Levels of GnRH expression are lower on the morning of the surge but increase in the afternoon when the surge occurs (Zoeller et al., 1988; Petersen and Barraclough, 1989; Toranzo et al., 1989; Rosie et al., 1990; Weesner et al., 1993a; Petersen et al., 1993, 1995, 1996). In early studies in sheep, Wheaton (1979) using radioimmunoassay, was unable to reverse the effect of ovariectomy by administering E, which is somewhat puzzling given the consistent responses achieved in rats. More recent work, using in situ hybridisation, showed a decrease in GnRH mRNA levels prior to an E-induced LH surge in OVX ewes (Dhillon et al., 1997; Harris et al., 1998). There have, however, been no comprehensive studies of the changes in GnRH mRNA levels across the estrous cycle in the ewe. Thus, studies in the rat show that mRNA levels increase prior to the LH surge, whereas studies in the sheep show

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

39

the opposite. In OVX ewes given a low continuous dose of E, P treatment does not affect the level of GnRH mRNA/cell, as indicated by in situ hybridisation studies (Robinson et al., 2000). In the OVX female monkey treated with either E or E + P, both treatments reduced GnRH mRNA levels, but there was no difference between E or E + P treatments compared to levels in untreated OVX animals (Krajewski et al., 2003). These data suggest that the action of P to reduce GnRH pulse frequency during the luteal phase of the cycle is not accompanied by an effect on the synthesis of GnRH. In the rat, the situation is somewhat more complicated, because P can cause a surge in GnRH secretion in the E-primed, OVX female (this does not occur in species such as the sheep). Such treatment advances the LH surge, when compared to treatment with E alone (DePaolo and Barraclough, 1979). In OVX-E treated prepubertal female rats, a single injection of P (to induce an LH surge) causes an increase in GnRH mRNA expression prior to the induced LH surge (Kim et al., 1989). On the other hand, administration of P markedly potentiated the negative feedback effect of E on pro-GnRH gene expression in OVX animals treated for 14 days (Toranzo et al., 1989). This demonstrates the duality of action of P on the GnRH/LH system in rats. 5.2. Effects of gonadectomy and gonadal steroids on GnRH synthesis in males Early studies on the effects of gonadectomy and gonadal steroid replacement on GnRH secretion in vivo and in vitro in the rat, has been extensively reviewed by Kalra and Kalra (1989). Wiemann et al. (1990) observed that the number of cells that expressed GnRH mRNA was lower in intact male rats compared to those castrated for 21 days, but did see a difference in the level of expression/cell. Selmanoff et al. (1991) also used in situ hybridisation and found that castration of male rats caused a significant increase in GnRH mRNA in the rostral hypothalamus of animals killed 9 days after castration. Another series of studies show that gonadectomy caused an increase of 54% in the level of expression of GnRH mRNA/cell in male rats14 days after gonadectomy, but this effect could be reversed by the 5␣-reduced derivative of testosterone, dihydrotestosterone (Li et al., 1995). In contrast to the above, one group (Spratt and Herbison, 1997) found that GnRH mRNA expression in long-term castrated rats was increased by either testosterone or E treatment. Thus, effects of steroids on GnRH expression (at least in rats) are not consistent. In castrated male sheep, testosterone treatment had no effect on the level of GnRH mRNA expression (Hileman et al., 1996), suggesting that the effect of testosterone to reduce secretion (vide supra) is not accompanied by an effect on synthesis. 5.3. Neurotransmitter effects on GnRH synthesis Whilst gonadal steroids such as E and P are the cyclic regulators of GnRH secretion, these are not considered to have direct effects on GnRH neurons (vide supra). It follows, therefore, that effects of steroids on GnRH synthesis must involve neuronal intermediaries (Fig. 2). GnRH cells receive input from various neuronal systems (Tilbrook et al., 2002). Studies in rats have shown that for some neurotransmitters, inhibitory effects can be seen in OVX animals, but stimulatory effects are seen in steroid treated-OVX animals. Such a dual effect can be seen with adrenergic agonists and neuropeptide Y (Kalra and Kalra, 1983). For

40

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

this reason, the experimental model in which effects of drugs are being examined should be carefully noted. The following is a review of our current understanding of which systems may regulate GnRH synthesis. GABA is thought to be an inhibitor of GnRH secretion (Herbison, 1998) and treatment of male rats with a GABAA agonist or benzodiazepine receptor ligand causes a decrease of GnRH mRNA, as measured by in situ hybridization (Li and Pelletier, 1993a, 1995a). Similar results are obtained in OVX female rats treated with a GABAA agonist (Leonhardt et al., 1995). On the other hand, Bergen et al. (1991) found that treatment of OVX rats with bicuculline (GABAA antagonist) decreased GnRH mRNA expression and Kang et al. (1995) showed an increase in GnRH mRNA in OVX-E + P-treated rats treated with the GABAA agonist muscimol. These latter data suggest a stimulatory role for the GABAergic system in the control of GnRH synthesis, so the effect of GABA on GnRH mRNA levels is not clear. Grattan et al. (1996) have presented evidence to suggest that the feedback effect of testosterone on GnRH neurons is mediated by androgen-sensitive GABA neurons in the preoptic region of the male rat brain. Serotonin stimulates LH release in presence of high levels of E (such as occur around the preovulatory surge), but inhibits LH release in OVX rats (Vitale and Chiocchio, 1993). Work in intact male rats (Li and Pelletier, 1995b) showed that serotonin reduces GnRH mRNA levels. Although GnRH mRNA levels were increased by the 5-HT2 receptor antagonist, this was not counter-acted by concomitant treatment with a 5-HT3 antagonist (Ondansetron). The latter authors suggested the serotoninergic system exerts a negative feedback effect on the biosynthesis of GnRH via activation of 5-HT2 receptor. To our knowledge, the effect of serotonin on GnRH mRNA expression in females is not known. Dopamine appears to be an important regulator of GnRH cells in rodent species (Kalra and Kalra, 1983; Li and Pelletier, 1992a, 1992b; Kalra, 1993). In ewes, pharmacological studies have implicated dopamine as an important neurotransmitter mediating the negative feed back effect of E on GnRH/LH secretion during the anestrous season (Le Corre and Chemineau, 1993; Meyer and Goodman, 1985). Whereas the feedback effects of E in ewes clearly involve D2 receptors (Anderson et al., 2001), this does not appear to be the case with testosterone feedback in males (Tilbrook and Clarke, 1992). There are no studies on dopaminergic regulation of GnRH expression in this species. In rodents, dopamine can exert a stimulatory (Choudhury et al., 1974; Clemens et al., 1977; Vijayan and McCann, 1978a, 1978b) or inhibitory effect when on LH secretion (Drouva and Gallo, 1977; Beck et al., 1978; Judd et al., 1978; Sarkar and Fink, 1981). Treatment for 14 days with a D2 receptor agonist increased GnRH mRNA expression in female and male rats by 32 and 67%, respectively, while the dopamine receptor antagonist haloperidol had the opposite effect. Nitric oxide (NO) is a diffusible gas that is formed in vivo by conversion of the amino acid l-arginine to l-citrulline, the reaction being catalysed by nitric oxide synthase (NOS). NO acts as neurotransmitter in the brain and stimulates LH release (Bonavera et al., 1993; Ceccatelli, 1997). Consistent with this, the infusion of NOS inhibitors into the lateral ventricles reduces GnRH mRNA levels in adult intact and castrated male rats (Wang et al., 1998). Noradrenaline stimulates LH secretion in steroid-primed female rats, but reduces LH secretion in OVX females (Kalra, 1993). The stimulatory effect appears to be via the ␣1 adrenergic receptor (Herbison, 1997). An increase in GnRH mRNA expression

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

41

in OVLT/POA region has been observed, using in situ hybridisation, within 1 h of intraventricular infusion of noradrenaline to OVX-E treated rats (He et al., 1993). Infusion of Prazosin (␣1 antagonist) into the POA of OVX-E treated rats reduces GnRH expression at the time of the E-induced LH surge (Weesner et al., 1993b), consistent with a stimulatory effect of noradrenaline at this time. Kim et al. (1994) treated OVX + E rats with an inhibitor of dopamine ␤ hydroxylase (diethyldithiocarbamic acid) 1 h before treatment with P and a decrease in GnRH mRNA expression was observed using Northern blot analysis. In the same animal model, these workers (Kang et al., 1998) showed that a noradrenergic neurotoxin (N-(2-chorethyl)-N-ethyl-2-bromobenzylamine) also reduced GnRH mRNA levels in the POA. The authors suggested that suppression of the noradrenergic transmission resulted in a marked decrease in P-induced GnRH levels. Thus, the literature consistently shows that noradrenergic input provides a stimulus to GnRH synthesis. Like noradrenaline, NPY may exert dual effects on GnRH/LH secretion in rats depending upon the steroid status (Kalra, 1993). In a singular report (Li et al., 1994), intracerebroventricular infusion of NPY or a Y1 receptor agonist to castrated adult male rats was found to stimulate the level of GnRH expression, as measured by in situ hybridisation. Opioids have an inhibitory effect on GnRH release and are thought to be one means by which negative tone is imposed on GnRH secretion (Gore and Terasawa, 2001; He et al., 1993; Kalra, 1993). In male rats, chronic morphine treatment reduced GnRH mRNA expression whereas naloxone (a broad acting antagonist), increased expression (Li and Pelletier, 1993b). Short-term (15 min) morphine treatment of female rats (of unspecified reproductive state) did not alter steady-state levels of mRNA (He et al., 1993), but when this treatment was followed by intraventricular infusion of noradrenaline, this blocked the NE-induced increase in GnRH mRNA. This inhibitory effect of opioids on the expression of GnRH mRNA may mediate the negative feedback effects of steroids on GnRH release through an inhibition of biosynthesis; this may involve counter-action of the stimulatory actions of adrenergic input. The amino acid glutamate is implicated in direct and indirect regulation of GnRH/LH secretion (Dhandapani and Brann, 2000). N-Methyl-d-aspartate (NMDA) receptor subtypes and kainate receptors are found on GnRH neurons and glutamatergic inputs to GnRH neurons have been demonstrated in several species (Goldsmith et al., 1994; Eyigor and Jennes, 2000; Gore, 2001, 2002b; Lin et al., 2003; Ottem et al., 2002; Pompolo et al., 2003a,b). Using a quantitative RNase protection assay, and in randomly cycling female rats, it was shown that N-methyl-d,l-aspartate and kainic acid effected a rapid (within 1 h) increase in GnRH mRNA expression (Gore and Roberts, 1994). A similarly rapid effect is seen with NMDA infusion to intact male rats (Petersen et al., 1991) and in androgen-sterilised female rats. Intravenous infusion of NMDA into OVX E + P-treated rats doubled the levels of GnRH mRNA levels in medially located neurons (determined by in situ hybridisation), but this was not blocked by MK801 (NMDA receptor antagonist) (Ottem and Petersen, 2002). Others have shown a decrease in GnRH mRNA after treatment with MK801 using Northern blot analysis (Seong et al., 1993), PCR (Suzuki et al., 1995) and RNase protection assays (Gore and Roberts, 1997). Using in situ hybridization, and in mice of both sexes, NMDA increased GnRH mRNA expression and the antagonist CGP40116 reduced expression (Ford and Ebling, 2000). On the other hand, a study in male mice with use of a ribonuclease protection assay (Wu et al., 2000) showed that NMDA caused a rapid

42

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

(within 15 min) decrease in cytoplasmic GnRH mRNA with a reduction in nuclear GnRH primary transcript RNA after 2 h. In neonatal mice (Ford and Ebling, 2000) the glutamate antagonist CGP 40116 did not alter GnRH mRNA expression at on Day 1 of postnatal life, but a reduction was observed at Day 5. Others (Liaw and Barraclough, 1993) found that treatment with NMDA increased GnRH mRNA expression on the day of birth, but not on Day 5. In summary, glutamatergic input appears stimulatory to GnRH synthesis.

6. Changes in GnRH secretion and synthesis 6.1. Effects of puberty and aging Hypothalamic levels of GnRH content and release increase steadily during postnatal development (Sisk et al., 2001; Plant and Shahab, 2002) in rats and monkeys. A study of mice (Ford and Ebling, 2000) examined levels of GnRH mRNA across prenatal and postnatal life and during the sexual maturation of males and females using in situ hybridization. Expression increased between embryonic Day 17 and postnatal Day 10, with no sex difference. Using their RNase protection assay to study mice, Gore et al. (1999) also found that cytoplasmic GnRH mRNA levels increased gradually during post-natal development in both sexes. Levels of GnRH primary transcript (an index of GnRH gene transcription) showed a four-fold increase between postnatal Days 5 and 7 and reached adult levels at post-natal Day15 in males. In females the levels of GnRH primary transcript were high at embryonic Day 16, decreased to nadir at postnatal Day 5, and then increased at postnatal Day 7; levels were then stabilised and did not increase further in adulthood (Gore et al., 1999). In one study, using in situ hybridization , no difference in GnRH expression was found between pre-pubertal and adult male rats (Wiemann et al., 1989). In contrast, using the RNase protection assay, GnRH mRNA expression in the brains of rats was found to increase at postnatal Day 30 in females, followed by a further increase to adult levels at postnatal Day 45 (Jakubowski et al., 1991). Similar trends were observed in male rats, with an increase in expression between postnatal Days 22 and 26, followed a further increase at postnatal Day 40 (Dutlow et al., 1992). Levels of GnRH mRNA expression are greater in post-pubertal (60-day-old) hamsters than in pre-pubertal (21-day-old) hamsters, when measured by in situ hybridization (Parfitt et al., 1999). The apparent increase in GnRH secretion at puberty could reflect a change in the activity of GnRH neurons or central mechanisms controlling secretion. A study in the male rhesus monkey examined animals castrated at the age of 1 week and killed at the 4–7 weeks (neonates) or 12–15 months (juvenile) (El Majdoubi et al., 2000). This showed that GnRH mRNA levels were similar in both instances, but the study did not include post-pubertal animals. Thus, the weight of evidence from various species is that GnRH synthesis rises during development to reach adult rates prior to puberty; puberty is not due to a sudden increase in the rate of production of GnRH. Aging is characterized by attenuation of pulsatile secretion of LH release and reduced size of the preovulatory GnRH/LH surge (Wise et al., 2002). Rodents have been well studied during the aging process, which is characterized by irregularity in estrous cyclicity and persistent estrus in females. This transition to acyclicity occurs independent of ovarian follicular atresia, since resumption of ovulation may occur in response to external stimuli

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

43

(Lu et al., 1977; Huang et al., 1978). In rats, attenuation or delay in the preovulatory LH surge occurs in middle-aged females. In intact female rats and humans, GnRH mRNA levels increase with the aging process (Rance and Uswandi, 1996; Gore et al., 2000) presumably because of upregulation of expression due to lack of steroid feedback. On the other hand, GnRH gene expression is reduced in OVX female rats, indicating an overall reduction in the capacity of these cells to synthesise GnRH (Rubin et al., 1997; Miller and Gore, 2002). In contrast, GnRH mRNA levels are reduced with aging in intact male rats (Gruenewald et al., 2000). In summary, the increase in GnRH mRNA expression with aging in females is consistent with the loss of ovarian cyclicity and the removal of negative feedback of ovarian steroids on the GnRH cells of the brain; this is not seen on OVX animals because there is no negative tone. The anomalous result in males is not readily explicable, although sexual dimorphism of the steroid regulation of GnRH synthesis has been recently reported (Thanky et al., 2003). 6.2. Effects of season/photoperiod Some species, such as sheep and hamsters, are seasonal breeders, utilising photoperiod and other cues to regulate their circannual pattern of reproduction (Thiery et al., 2002). An extensive literature exists on the seasonal patterns of LH secretion (reflecting GnRH secretion) (Karsch et al., 1993; Lehman et al., 2002; Lincoln, 2002), but this is beyond the scope of the present review. The following is confined to seasonal effects on synthesis, which is less extensively reviewed in existing literature. Siberian hamsters breed under long-day photoperiod. In males of this species (intact or castrated with testosterone replacement), the expression of GnRH mRNA in the dbB/septal area was higher in short-day photoperiod than in long days (Ronchi et al., 1992). Thus, synthesis appeared to be increased during the nonbreeding period. Others (Brown et al., 2001) have observed no difference in mRNA levels using in situ hybridisation in intact male hamsters on either short or long-day photoperiods. Neither was any difference observed in gray hamsters (Kawamoto et al., 2000) on shortor long-day photoperiod. In another experimental series, hamsters were transferred from short (6L:18D for 4 weeks) to long-day photoperiod (16L:8D for 1 day) (Porkka-Heiskanen et al., 1997) and a transient increase was observed in the number of cells expressing GnRH mRNA. In a study of male prairie were housed under short or long days and at two different temperatures (mild-20 ◦ C or low-8 ◦ C), no difference in the level of GnRH mRNA expression was observed when measured by in situ hybridisation (Kriegsfeld et al., 2000). In castrated male sheep with or without testosterone replacement and kept for 3 weeks in short or long-day photoperiod, no difference was observed in GnRH mRNA levels although plasma concentrations of LH were lower under long days (Hileman et al., 1998). Thus, the weight of evidence suggests that seasonal breeding status is not the result of altered synthesis of GnRH. 6.3. Nutritional effects Food deprivation of sheep reduces plasma LH levels (Campbell et al., 1977; Henry et al., 2001) as result of inhibitory influences on secretion of GnRH (Thomas et al., 1990; I’Anson et al., 2000; Henry et al., 2001; Henry, 2003). In situ hybridisation studies by McShane et al. (1993) showed that the levels of GnRH mRNA were not altered in OVX lambs following

44

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

30% food-restriction for 7 weeks. Similar results have been obtained with food restriction of intact male rats for 5 (Bergendahl et al., 1992) or 17 days (Leonhardt et al., 1999). Others (Gruenewald and Matsumoto, 1993) have observed a decrease in the number of cells expressing GnRH mRNA in the dbB/mPOA of male rats subjected to food restriction for 3 months, while the level of expression/cell was unchanged. In male prairie voles, food restriction led to an increase in the number of GnRH cells detected by immunohistochemistry with an increase in cell size and concomitant increase in GnRH-immunoreactive fibre intensity the median eminence (Kriegsfeld et al., 2001); these authors suggested the this was probably a reflection of reduced secretion of GnRH. Interestingly, treatment of undernourished animals with leptin can correct the hypogonadotropic stated (Henry et al., 2001), suggesting that this fat-derived hormone relays important information to the brain regarding metabolic status and this is ‘sensed’ by pathways regulating reproduction. 6.4. Effects of stress It is well recognised that stress has deleterious effects on the reproductive axis (Tilbrook et al., 2002). Whilst many studies show that this is manifest in a reduction in gonadotropin secretion, few show direct effects on the GnRH system. Recent studies, Debus et al. (2002) showed that administration of a bacterial endotoxin, to induce immune/inflammatory stress, reduced pulsatile GnRH/LH secretion; the effect on GnRH synthesis was not measured.

References Adelman, J.P., Mason, A.J., Hayflick, J.S., Seeburg, P.H., 1986. Isolation of the gene and hypothalamic cDNA for the common precursor of gonadotropin-releasing hormone and prolactin release-inhibiting factor in human and rat. Proc. Natl. Acad. Sci. U.S.A. 83, 179–183. Amoss, M., Burgus, R., Blackwell, R., Vale, W., Fellows, R., Guillemin, R., 1971. Purification, amino acid composition and N-terminus of the hypothalamic luteinizing hormone releasing factor (LRF) of ovine origin. Biochem. Biophys. Res. Commun. 44, 205–210. Anderson, G.M., Connors, J.M., Hardy, S.L., Valent, M., Goodman, R.L., 2001. Oestradiol microimplants in the ventromedial preoptic area inhibit secretion of luteinizing hormone via dopamine neurones in anoestrous ewes. J. Neuroendocrinol. 13, 1051–1058. Andrew, R.D., Dudek, F.E., 1983. Burst discharge in mammalian neuroendocrine cells involves an intrinsic regenerative mechanism. Science 221, 1050–1052. Baker, B.L., Dermody, W.C., Reel, J.R., 1975. Distribution of gonadotropin-releasing hormone in the rat brain as observed with immunocytochemistry. Endocrinology 97, 125–135. Beck, W., Hancke, J.L., Wuttke, W., 1978. Increased sensitivity of dopaminergic inhibition of luteinizing hormone release in immature and castrated female rats. Endocrinology 102, 837–843. Bergen, H.T., Hejtmancik, J.F., Pfaff, D.W., 1991. Effects of gamma-aminobutyric acid receptor agonists and antagonist on LHRH-synthesizing neurons as detected by immunocytochemistry and in situ hybridization. Exp. Brain Res. 87, 46–56. Bergendahl, M., Wiemann, J.N., Clifton, D.K., Huhtaniemi, I., Steiner, R.A., 1992. Short-term starvation decreases POMC mRNA but does not alter GnRH mRNA in the brain of adult male rats. Neuroendocrinology 56, 913–920. Bonavera, J.J., Sahu, A., Kalra, P.S., Kalra, S.P., 1993. Evidence that nitric oxide may mediate the ovarian steroidinduced luteinizing hormone surge: involvement of excitatory amino acids. Endocrinology 133, 2481–2487. Boukhliq, R., Goodman, R.L., Berriman, S.J., Adrian, B., Lehman, M.N., 1999. A subset of gonadotropin-releasing hormone neurons in the ovine medial basal hypothalamus is activated during increased pulsatile luteinizing hormone secretion. Endocrinology 140, 5929–5936.

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

45

Brown, D.I., Garyfallou, V.T., Urbanski, H.F., 2001. Photoperiodic modulation of GnRH mRNA in the male Syrian hamster. Brain Res. Mol. Brain Res. 89, 119–125. Bruder, J.M., Spaulding, A.J., Wierman, M.E., 1996. Phorbol ester inhibition of rat gonadotropin-releasing hormone promoter activity: role of Fos and Jun in the repression of transcription. Mol. Endocrinol. 10, 35–44. Butler, J.A., Sjoberg, M., Coen, C.W., 1999. Evidence for oestrogen receptor alpha-immunoreactivity in gonadotrophin-releasing hormone-expressing neurones. J. Neuroendocrinol. 11, 331–335. Campbell, G.A., Kurcz, M., Marshall, S., Meites, J., 1977. Effects of starvation in rats on serum levels of follicle stimulating hormone, luteinizing hormone, thyrotropin, growth hormone and prolactin; response to LH-releasing hormone and thyrotropin-releasing hormone. Endocrinology 100, 580–587. Caraty, A., Fabre-Nys, C., Delaleu, B., Locatelli, A., Bruneau, G., Karsch, F.J., Herbison, A., 1998. Evidence that the mediobasal hypothalamus is the primary site of action of estradiol in inducing the preovulatory gonadotropin releasing hormone surge in the ewe. Endocrinology 139, 1752–1760. Caraty, A., Skinner, D.C., 1999. Dynamics of steroid regulation of GnRH secretion during the oestrus cycle of the ewe. Ann. Endocrinol. 60, 68–78. Carolsfeld, J., Powell, J.F., Park, M., Fischer, W.H., Craig, A.G., Chang, J.P., Rivier, J.E., Sherwood, N.M., 2000. Primary structure and function of three gonadotropin-releasing hormones, including a novel form, from an ancient teleost, herring. Endocrinology 141, 505–512. Ceccatelli, S., 1997. Expression and plasticity of NO synthase in the neuroendocrine system. Brain Res. Bull. 44, 533–538. Chandran, U.R., DeFranco, D.B., 1999. Regulation of gonadotropin-releasing hormone gene transcription. Behav. Brain Res. 105, 29–36. Charlton, H.M., Halpin, D.M.G., Iddon, C., Rosie, R., Levy, G., McDowell, I.F.W., Megson, A., Morris, J.F., Bramwell, A., Speight, A., Ward, B.J., Broadhead, J., Davey-Smith, G., Fink, G., 1983. The effects of daily administration of single and multiple injections of gonadotropin-releasing hormone on pituitary and gonadal function in the hypogonadal (hpg) mouse. Endocrinology 113, 535–544. Choudhury, S.A., Sharpe, R.M., Brown, P.S., 1974. The effect of pimozide, a dopamine antagonist, on pituitary gonadotrophic function in the rat. J. Reprod. Fertil. 39, 275–283. Clarke, I.J., 1987a. Control of GnRH secretion. J. Reprod. Fertil. Suppl. 34, 1–8. Clarke, I.J., 1987b. New concepts in gonadotrophin-releasing hormone action on the pituitary gland. Semin. Reprod. Endocrinol. 5, 345–352. Clarke, I.J., 1988. Gonadotrophin-releasing hormone secretion (GnRH) in anoestrous ewes and the induction of GnRH surges by oestrogen. J. Endocrinol. 117, 355–360. Clarke, I.J., 1993. Variable patterns of gonadotropin-releasing hormone secretion during the estrogen-induced luteinizing hormone surge in ovariectomized ewes. Endocrinology 133, 1624–1632. Clarke, I.J., 1995a. Evidence that the switch from negative to positive feedback at the level of the pituitary gland is an important timing event for the onset of the preovulatory surge in LH in the ewe. J. Endocrinol. 145, 271–282. Clarke, I.J., 1995b. The preovulatory LH surge. A case of a neuroendocrine switch. Trends Endocrinol. Metab. 6, 241–247. Clarke, I.J., 1996. The hypothalamo–pituitary axis. In: Hillier, S.G., Kitchener, H.C., Neilsen, J.P. (Eds.), Scientific Essentials of Reproductive Medicine. W.B. Saunders Co. Ltd., UK, pp. 120–132. Clarke, I.J., 2002. Two decades of measuring GnRH secretion. Reprod. Suppl. 59, 1–13. Clarke, I.J., Cummins, J.T., 1982. The temporal relationship between gonadotropin releasing hormone (GnRH) and luteinizing hormone (LH) secretion in ovariectomized ewes. Endocrinology 111, 1737–1739. Clarke, I.J., Cummins, J.T., 1984. Direct pituitary effects of estrogen and progesterone on gonadotropin secretion in the ovariectomized ewe. Neuroendocrinology 39, 267–274. Clarke, I.J., Cummins, J.T., Findlay, J.K., Burman, K.J., Doughton, B.W., 1984. Effects on plasma luteinizing hormone and follicle-stimulating hormone of varying the frequency and amplitude of gonadotropin-releasing hormone pulses in ovariectomized ewes with hypothalamo–pituitary disconnection. Neuroendocrinology 39, 214–221. Clarke, I.J., Findlay, J.K., Cummins, J.T., Ewens, W.J., 1986. Effects of ovine follicular fluid on plasma LH and FSH secretion in ovariectomized ewes to indicate the site of action of inhibin. J. Reprod. Fertil. 77, 575– 585.

46

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

Clarke, I.J., Pompolo, S., Scott, C.J., Rawson, J.A., Caddy, D., Jakubowska, A.E., Pereira, A.M., 2001. Cells of the arcuate nucleus and ventromedial nucleus of the ovariectomized ewe that respond to oestrogen: a study using Fos immunohistochemistry. J. Neuroendocrinol. 13, 934–941. Clarke, I.J., Thomas, G.B., Yao, B., Cummins, J.T., 1987. GnRH secretion throughout the ovine estrous cycle. Neuroendocrinology 46, 82–88. Clemens, J.A., Tinsley, F.C., Fuller, R.W., 1977. Evidence for a dopaminergic component in the series of neural events that lead to the pro-oestrous surge of LH. Acta Endocrinol. (Copenh.) 85, 18–24. Debus, N., Breen, K.M., Barrell, G.K., Billings, H.J., Brown, M., Young, E.A., Karsch, F.J., 2002. Does cortisol mediate endotoxin-induced inhibition of pulsatile luteinizing hormone and gonadotropin-releasing hormone secretion? Endocrinology 143, 3748–3758. Dees, W.L., Dearth, R.K., Hooper, R.N., Brinsko, S.P., Romano, J.E., Rahe, H., Yu, W.H., McCann, S.M., 2001. Lamprey gonadotropin-releasing hormone-III selectively releases follicle stimulating hormone in the bovine. Domest. Anim. Endocrinol. 20, 279–288. Densmore, V.S., Urbanski, H.F., 2003. Relative effect of gonadotropin-releasing hormone (GnRH)-I and GnRH-II on gonadotropin release. J. Clin. Endocrinol. Metab. 88, 2126–2134. DePaolo, L.V., Barraclough, C.A., 1979. Dose dependent effects of progesterone on the facilitation and inhibition of spontaneous gonadotropin surges in estrogen treated ovariectomized rats. Biol. Reprod. 21, 1015– 1023. Dhandapani, K.M., Brann, D.W., 2000. The role of glutamate and nitric oxide in the reproductive neuroendocrine system. Biochem. Cell Biol. 78, 165–179. Dhillon, H., Dunn, A.M., Esquivel, E., Hamernik, D.L., Wise, M.E., 1997. The estradiol-induced luteinizing hormone surge in the ewe is not associated with increased gonadotropin-releasing hormone messenger ribonucleic acid levels. Biol. Reprod. 57, 107–111. Drouva, S.V., Gallo, R.V., 1977. Further evidence for inhibition of episodic luteinizing hormone release in ovariectomized rats by stimulation of dopamine receptors. Endocrinology 100, 792–798. Dutlow, C.M., Rachman, J., Jacobs, T.W., Millar, R.P., 1992. Prepubertal increases in gonadotropin-releasing hormone mRNA, gonadotropin-releasing hormone precursor, and subsequent maturation of precursor processing in male rats. J. Clin. Invest. 90, 2496–2501. El Majdoubi, M., Sahu, A., Plant, T.M., 2000. Changes in hypothalamic gene expression associated with the arrest of pulsatile gonadotropin-releasing hormone release during infancy in the agonadal male rhesus monkey (Macaca mulatta). Endocrinology 141, 3273–3277. Eraly, S.A., Mellon, P.L., 1995. Regulation of gonadotropin-releasing hormone transcription by protein kinase C is mediated by evolutionarily conserved promoter-proximal elements. Mol. Endocrinol. 9, 848–859. Evans, N.P., Dahl, G.E., Glover, B.H., Karsch, F.J., 1994a. Central regulation of pulsatile gonadotropin-releasing hormone (GnRH) secretion by estrodiol during the period leading up to the preovulatory GnRH surge in the ewe. Endocrinology 134, 1806–1811. Evans, N.P., McNeilly, J.R., Webb, R., 1994b. Alterations in endogenous gonadotropin secretion and pituitary responsiveness to gonadotropin-releasing hormone in adult ewes, following indirect selection in prepubertal male lambs. Biol. Reprod. 51, 913–919. Eyigor, O., Jennes, L., 2000. Kainate receptor subunit-positive gonadotropin-releasing hormone neurons express c-Fos during the steroid-induced luteinizing hormone surge in the female rat. Endocrinology 141, 779– 786. Falkenstein, E., Wehling, M., 2000. Nongenomically initiated steroid actions. Eur. J. Clin. Invest. 3 (30 Suppl.), 51–54. Fernald, R.D., White, R.B., 1999. Gonadotropin-releasing hormone genes: phylogeny, structure, and functions. Front. Neuroendocrinol. 20, 224–240. Ford, H., Ebling, F.J., 2000. Glutamatergic regulation of gonadotropin releasing hormone mRNA levels during development in the mouse. J. Neuroendocrinol. 12, 1027–1033. Gestrin, E.D., White, R.B., Fernald, R.D., 1999. Second form of gonadotropin-releasing hormone in mouse: immunocytochemistry reveals hippocampal and periventricular distribution. FEBS Lett. 448, 289–291. Goldsmith, P.C., Thind, K.K., Perera, A.D., Plant, T.M., 1994. Glutamate-immunoreactive neurons and their gonadotropin-releasing hormone neuronal interactions in the monkey hypothalamus. Endocrinology 134, 858–868.

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

47

Goodman, R.L., Legan, S.J., Ryan, K.D., Foster, D.L., Karsch, F.J., 1981. Importance of variations in behavioural and feedback actions of oestradiol to the control of seasonal breeding in the ewe. J. Endocrinol. 89, 229–240. Gore, A.C., 2001. Gonadotropin-releasing hormone neurons, NMDA receptors, and their regulation by steroid hormones across the reproductive life cycle. Brain Res. Brain Res. Rev. 37, 235–248. Gore, A.C., 2002a. GnRH: The Master Molecule of Reproduction. Kuewer Academic Publishers, Boston, USA. Gore, A.C., 2002b. Gonadotropin-releasing hormone (GnRH) neurons: gene expression and neuroanatomical studies. Prog. Brain Res. 141, 193–208. Gore, A.C., Oung, T., Yung, S., Flagg, R.A., Woller, M.J., 2000. Neuroendocrine mechanisms for reproductive senescence in the female rat: gonadotropin-releasing hormone neurons. Endocrine 13, 315–323. Gore, A.C., Roberts, J.L., 1994. Regulation of gonadotropin-releasing hormone gene expression by the excitatory amino acids kainic acid and N-methyl-d,l-aspartate in the male rat. Endocrinology 134, 2026–2031. Gore, A.C., Roberts, J.L., 1995. Regulation of gonadotropin-releasing hormone gene expression in the rat during the luteinizing hormone surge. Endocrinology 136, 889–896. Gore, A.C., Roberts, J.L., 1997. Regulation of gonadotropin-releasing hormone gene expression in vivo and in vitro. Front. Neuroendocrinol. 18, 209–245. Gore, A.C., Roberts, J.L., Gibson, M.J., 1999. Mechanisms for the regulation of gonadotropin-releasing hormone gene expression in the developing mouse. Endocrinology 140, 2280–2287. Gore, A.C., Terasawa, E., 2001. Neural circuits regulating pulsatile luteinizing hormone release in the female guinea-pig: opioid, adrenergic and serotonergic interactions. J. Neuroendocrinol. 13, 239–248. Grattan, D.R., Rocca, M.S., Sagrillo, C.A., McCarthy, M.M., Selmanoff, M., 1996. Antiandrogen microimplants into the rostral medial preoptic area decrease gamma-aminobutyric acidergic neuronal activity and increase luteinizing hormone secretion in the intact male rat. Endocrinology 137, 4167–4173. Grove-Strawser, D., Sower, S.A., Ronsheim, P.M., Connolly, J.B., Bourn, C.G., Rubin, B.S., 2002. Guinea pig GnRH: localization and physiological activity reveal that it, not mammalian GnRH, is the major neuroendocrine form in guinea pigs. Endocrinology 143, 1602–1612. Gruenewald, D.A., Matsumoto, A.M., 1993. Reduced gonadotropin-releasing hormone gene expression with fasting in the male rat brain. Endocrinology 132, 480–482. Gruenewald, D.A., Naai, M.A., Marck, B.T., Matsumoto, A.M., 2000. Age-related decrease in hypothalamic gonadotropin-releasing hormone (GnRH) gene expression, but not pituitary responsiveness to GnRH, in the male Brown Norway rat. J. Androl. 21, 72–84. Harris, G.W., 1955. Neural Control of the Pituitary Gland. Arnold, London, UK. Harris, T.G., Robinson, J.E., Evans, N.P., Skinner, D.C., Herbison, A.E., 1998. Gonadotropin-releasing hormone messenger ribonucleic acid expression changes before the onset of the estradiol-induced luteinizing hormone surge in the ewe. Endocrinology 139, 57–64. He, J.R., Molnar, J., Barraclough, C.A., 1993. Morphine amplifies norepinephrine (NE)-induced LH release but blocks NE-stimulated increases in LHRH mRNA levels: comparison of responses obtained in ovariectomized, estrogen-treated normal and androgen-sterilized rats. Brain Res. 20, 71–78. Henry, B.A., 2003. Links between the appetite regulating systems and the neuroendocrine hypothalamus: lessons from the sheep. J. Neuroendocrinol. 15, 697–709. Henry, B.A., Goding, J.W., Tilbrook, A.J., Dunshea, F.R., Clarke, I.J., 2001. Intracerebroventricular infusion of leptin elevates the secretion of luteinising hormone without affecting food intake in long-term food-restricted sheep, but increases growth hormone irrespective of bodyweight. J. Endocrinol. 168, 67–77. Herbison, A.E., 1997. Noradrenergic regulation of cyclic GnRH secretion. Rev. Reprod. 2, 1–6. Herbison, A.E., 1998. Multimodal influence of estrogen upon gonadotropin-releasing hormone neurons. Endocr. Rev. 19, 302–330. Herbison, A.E., Pape, J.R., 2001. New evidence for estrogen receptors in gonadotropin-releasing hormone neurons. Front Neuroendocrinol. 22, 292–308. Herbison, A.E., Skinner, D.C., Robinson, J.E., King, I.S., 1996. Androgen receptor-immunoreactive cells in the ram hypothalamus: distribution and co-localization patterns with gonadotropin-releasing hormone, somatostatin and tyrosine hydroxylase. Neuroendocrinology 63, 120–131. Hileman, S.M., Kuehl, D.E., Jackson, G.L., 1998. Photoperiod affects the ability of testosterone to alter proopiomelanocortin mRNA, but not luteinizing hormone-releasing hormone mRNA, levels in male sheep. J. Neuroendocrinol. 10, 587–592.

48

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

Hileman, S.M., Lubbers, L.S., Petersen, S.L., Kuehl, D.E., Scott, C.J., Jackson, G.L., 1996. Influence of testosterone on LHRH release, LHRH mRNA and proopiomelanocortin mRNA in male sheep. J. Neuroendocrinol. 8, 113–121. Hrabovszky, E., Shughrue, P.J., Merchenthaler, I., Hajszan, T., Carpenter, C.D., Liposits, Z., Petersen, S.L., 2000. Detection of estrogen receptor-beta messenger ribonucleic acid and 125I-estrogen binding sites in luteinizing hormone-releasing hormone neurons of the rat brain. Endocrinology 141, 3506–3509. Huang, H.H., Steger, R.W., Bruni, J.F., Meites, J., 1978. Patterns of sex steroid and gonadotropin secretion in aging female rats. Endocrinology 103, 1855–1859. I’Anson, H., Manning, J.M., Herbosa, C.G., Pelt, J., Friedman, C.R., Wood, R.I., Bucholtz, D.C., Foster, D.L., 2000. Central inhibition of gonadotropin-releasing hormone secretion in the growth-restricted hypogonadotropic female sheep. Endocrinology 141, 520–527. Jakubowski, M., Blum, M., Roberts, J.L., 1991. Postnatal development of gonadotropin-releasing hormone and cyclophilin gene expression in the female and male rat brain. Endocrinology 128, 2702–2708. Jakubowski, M., Roberts, J.L., 1994. Processing of gonadotropin-releasing hormone gene transcripts in the rat brain. J. Biol. Chem. 269, 4078–4083. Jimenez-Linan, M., Rubin, B.S., King, J.C., 1997. Examination of guinea pig luteinizing hormone-releasing hormone gene reveals a unique decapeptide and existence of two transcripts in the brain. Endocrinology 138, 4123–4130. Judd, S.J., Rakoff, J.S., Yen, S.S., 1978. Inhibition of gonadotropin and prolactin release by dopamine: effect of endogenous estradiol levels. J. Clin. Endocrinol. Metab. 47, 494–498. Kalra, S.P., 1976. Tissue levels of luteinizing hormone-releasing hormone in the preoptic area and hypothalamus, and serum concentrations of gonadotropins following anterior hypothalamic deafferentation and estrogen treatment of the female rat. Endocrinology 99, 101–107. Kalra, S.P., 1993. Mandatory neuropeptide-steroid signaling for the preovulatory luteinizing hormone-releasing hormone discharge. Endocr. Rev. 14, 507–538. Kalra, S.P., Kalra, P.S., 1983. Neural regulation of luteinizing hormone secretion in the rat. Endocr. Rev. 4, 311– 350. Kalra, S.P., Kalra, P.S., 1989. Do testosterone and estradiol-17B enforce inhibition or stimulation of luteinizing hormone-releasing hormone secretion. Biol. Reprod. 41, 559–570. Kang, S.H., Seong, J.Y., Cho, S., Cho, H., Kim, K., 1995. Acute increase of GABAergic neurotransmission exerts a stimulatory effect on GnRH gene expression in the preoptic/anterior hypothalamic area of ovariectomized, estrogen- and progesterone-treated adult female rats. Neuroendocrinology 61, 486–492. Kang, S.S., Son, G.H., Seong, J.Y., Choi, D., Kwon, H.B., Lee, C.C., Kim, K., 1998. Noradrenergic neurotoxin suppresses gonadotropin-releasing hormone (GnRH) and GnRH receptor gene expression in ovariectomized and steroid-treated rats. J. Neuroendocrinol. 10, 911–918. Karsch, F.J., Dahl, G.E., Evans, N.P., Manning, J.M., Mayfield, K.P., Moenter, S.M., Foster, D.L., 1993. Seasonal changes in gonadotropin-releasing hormone secretion in the ewe: alteration in response to the negative feedback action of estradiol. Biol. Reprod. 49, 1377–1383. Kasa-Vubu, J.Z., Dahl, G.E., Evans, N.P., Thrun, L.A., Moenter, S.M., Padmanabhan, V., Karsch, F.J., 1992. Progesterone blocks the estradiol-induced gonadotropin discharge in the ewe by inhibiting the surge of gonadotropin-releasing hormone. Endocrinology 131, 208–212. Kawamoto, K., Tanaka, S., Kawano, M., Hayashi, T., Tsuchiya, K., 2000. Effects of photoperiod and ambient temperature on the gonadotropin-releasing hormone neuronal system in the gray hamster, Tscherskia triton. Neuroendocrinology 72, 284–292. Kelley, C.G., Givens, M.L., Rave-Harel, N., Nelson, S.B., Anderson, S., Mellon, P.L., 2002. Neuron-restricted expression of the rat gonadotropin-releasing hormone gene is conferred by a cell-specific protein complex that binds repeated CAATT elements. Mol. Endocrinol. 16, 2413–2425. Kelley, C.G., Lavorgna, G., Clark, M.E., Boncinelli, E., Mellon, P.L., 2000. The Otx2 homeoprotein regulates expression from the gonadotropin-releasing hormone proximal promoter. Mol. Endocrinol. 14, 1246– 1256. Kelly, M.J., Garrett, J., Bosch, M.A., Roselli, C.E., Douglass, J., Adelman, J.P., Ronnekleiv, O.K., 1989. Effects of ovariectomy on GnRH mRNA, proGnRH and GnRH levels in the preoptic hypothalamus of the female rat. Neuroendocrinology 49, 88–97.

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

49

Kepa, J.K., Jacobsen, B.M., Boen, E.A., Prendergast, P., Edwards, D.P., Takimoto, G., Wierman, M.E., 1996. Direct binding of progesterone receptor to nonconsensus DNA sequences represses rat GnRH. Mol Cell Endocrinol. 117, 27–39. Kim, K., Lee, B.J., Cho, B.N., Kang, S.S., Choi, W.S., Park, S.D., Lee, C.C., Cho, W.K., Wuttke, W., 1994. Blockade of noradrenergic neurotransmission with diethyldithiocarbamic acid decreases the mRNA level of gonadotropin-releasing hormone in the hypothalamus of ovariectomized, steroid-treated prepubertal rats. Neuroendocrinology 59, 539–544. Kim, K., Lee, B.J., Park, Y., Cho, W.K., 1989. Progesterone increases messenger ribonucleic acid (mRNA) encoding luteinizing hormone releasing hormone (LHRH) level in the hypothalamus of ovariectomized estradiol-primed prepubertal rats. Brain Res. 6, 151–158. King, J.C., Parsons, J.A., Erlandsen, S.L., Williams, T.H., 1974. Luteinizing hormone-releasing hormone (LH-RH) pathway of the rat hypothalamus revealed by the unlabeled antibody peroxidase-antiperoxidase method. Cell Tissue Res. 153, 211–217. Kobayashi, R.M., Lu, K.H., Moore, R.Y., Yen, S.S., 1978. Regional distribution of hypothalamic luteinizing hormone-releasing hormone in proestrous rats: effects of ovariectomy and estrogen replacement. Endocrinology 102, 98–105. Krajewski, S.J., Abel, T.W., Voytko, M.L., Rance, N.E., 2003. Ovarian steroids differentially modulate the gene expression of gonadotropin-releasing hormone neuronal subtypes in the ovariectomized cynomolgus monkey. J. Clin. Endocrinol. Metab. 88, 655–662. Krege, J.H., Hodgin, J.B., Couse, J.F., Enmark, E., Warner, M., Mahler, J.F., Sar, M., Korach, K.S., Gustafsson, J.A., Smithies, O., 1998. Generation and reproductive phenotypes of mice lacking estrogen receptor beta. Proc. Natl. Acad. Sci. U.S.A. 95, 15677–15682. Kriegsfeld, L.J., Ranalli, N.J., Bober, M.A., Nelson, R.J., 2000. Photoperiod and temperature interact to affect the GnRH neuronal system of male prairie voles (Microtus ochrogaster). J. Biol. Rhythms 15, 306–316. Kriegsfeld, L.J., Ranalli, N.J., Trasy, A.G., Nelson, R.J., 2001. Food restriction affects the gonadotropin releasing hormone neuronal system of male prairie voles (Microtus ochrogaster). J. Neuroendocrin. 13, 791–798. Lagrange, A.H., Ronnekleiv, O.K., Kelly, M.J., 1995. Estradiol-17 beta and mu-opioid peptides rapidly hyperpolarize GnRH neurons: a cellular mechanism of negative feedback? Endocrinology 136, 2341–2344. Latimer, V.S., Rodrigues, S.M., Garyfallou, V.T., Kohama, S.G., White, R.B., Fernald, R.D., Urbanski, H.F., 2000. Two molecular forms of gonadotropin-releasing hormone (GnRH-I and GnRH-II) are expressed by two separate populations of cells in the rhesus macaque hypothalamus. Brain Res. Mol. Brain Res. 75, 287– 292. Lawson, M.A., Whyte, D.B., Mellon, P.L., 1996. GATA factors are essential for activity of the neuron-specific enhancer of the gonadotropin-releasing hormone gene. Mol. Cell Biol. 16, 3596–3605. Le Corre, S., Chemineau, P., 1993. Control of photoperiodic inhibition of luteinizing hormone secretion by dopaminergic and serotonergic systems in ovariectomized Ile-de-France ewes supplemented with oestradiol. J. Reprod. Fertil. 97, 367–373. Legan, S.J., Coon, G.A., Karsch, F.J., 1975. Role of estrogen as initiator of daily LH surges in the ovariectomized rat. Endocrinology 96, 50–56. Lehman, M.N., Coolen, L.M., Goodman, R.L., Viguie, C., Billings, H.J., Karsch, F.J., 2002. Seasonal plasticity in the brain: the use of large animal models for neuranatomical research. Reproduction S59, 149–165. Lehman, M.N., Karsch, F.J., 1993. Do gonadotropin-releasing hormone, tyrosine hydroxylase-, and b-endorphinimmunoreactive neurons contain oestrogen receptors? A double-label immunocytochemical study in the Suffolk ewe. Endocrinology 133, 887–895. Lehman, M.N., Robinson, J.E., Karsch, F.J., Silverman, A.J., 1986. Immunocytochemical localization of luteinizing hormone-releasing hormone (LHRH) pathways in the sheep brain during anestrus and the mid-luteal phase of the estrous cycle. J. Comp. Neurol. 244, 19–35. Leonhardt, S., Seong, J.Y., Kim, K.J., Thorun, Y., Wuttke, W., Jarry, H., 1995. Activation of central GABAA – but not of GABAB – receptors rapidly reduces pituitary LH release and GnRH gene expression in the preoptic anterior hypothalamic area of ovariectomized rats. Neuroendocrinology 61, 655–662. Leonhardt, S., Shahab, M., Luft, H., Wuttke, W., Jarry, H., 1999. Reduction of luteinzing hormone secretion induced by long-term feed restriction in male rats is associated with increased expression of GABA-synthesizing enzymes without alterations of GnRH gene expression. J. Neuroendocrin. 11, 613–619.

50

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

Li, J., Francis, H., Clarke, I.J., 1989. Effects of bovine follicular fluid on pulsatile secretion of gonadotrophinreleasing hormone and gonadotrophins in ovariectomized ewes. J. Neuroendocrin. 1, 61–64. Li, S., Garcia de Yebenes, E., Pelletier, G., 1995. Effects of dehydroepiandrosterone (DHEA) on GnRH gene expression in the rat brain as studied by in situ hybridization. Peptides 16, 425–430. Li, S., Hong, M., Fournier, A., St Pierre, S., Pelletier, G., 1994. Role of neuropeptide Y in the regulation of gonadotropin-releasing hormone gene expression in the rat preoptic area. Brain Res. Mol. Brain Res. 26, 69–73. Li, S., Pelletier, G., 1992a. Dopamine regulation of gonadotropin-releasing hormone (GnRH) gene expression in the female rat brain. Neurosci. Lett. 146, 207–210. Li, S., Pelletier, G., 1992b. Role of dopamine in the regulation of gonadotropin-releasing hormone in the male rat brain as studied by in situ hybridization. Endocrinology 131, 395–399. Li, S., Pelletier, G., 1993a. Chronic administration of muscimol and pentobarbital decreases gonadotropin-releasing hormone mRNA levels in the male rat hypothalamus determined by quantitative in situ hybridization. Neuroendocrinology 58, 136–139. Li, S., Pelletier, G., 1993b. Opioid regulation of gonadotropin-releasing hormone gene expression in the male rat brain as studied by in situ hybridization. Neuroreport 4, 331–333. Li, S., Pelletier, G., 1995a. Inhibitory effect of the potential endogenous benzodiazepine receptor ligand, octadecaneuropeptide (ODN), on gonadotropin-releasing hormone gene expression in the male rat brain. Neuroreport 6, 1354–1356. Li, S., Pelletier, G., 1995b. Involvement of serotonin in the regulation of GnRH gene expression in the male rat brain. Neuropeptides 29, 21–25. Liaw, J.J., Barraclough, C.A., 1993. N-methyl-d,l-aspartic acid differentially affects LH release and LHRH mRNA levels in estrogen-treated ovariectomized control and androgen-sterilized rats. Brain Res. 17, 112–118. Lin, W., McKinney, K., Liu, L., Lakhlani, S., Jennes, L., 2003. Distribution of vesicular glutamate transporter-2 messenger ribonucleic Acid and protein in the septum-hypothalamus of the rat. Endocrinology 144, 662–670. Lin, X.W., Otto, C.J., Peter, R.E., 1998. Evolution of neuroendocrine peptide systems: gonadotropin-releasing hormone and somatostatin. Comp. Biochem. Physiol. C. Pharmacol. Toxicol. Endocrinol. 119, 375–388. Lincoln, G.A., 2002. Neuroendocrine regulation of seasonal gonadotrophin and prolactin rhythms: lessons from the Soay ram model. Reprod. Suppl. 59, 131–147. Lincoln, G.A., Fraser, H.M., 1987. Compensatory response of the luteinizing-hormone(LH)-releasing hormone (LHRH)/LH pulse generator after administration of a potent LHRH antagonist in the ram. Endocrinology 120, 2245–2250. Lu, K.H., Huang, H.H., Chen, H.T., Kurcz, M., Mioduskewski, R., Meites, J., 1977. Positive feedback by estrogen and progesterone on LH release in old and young rats. Proc. Soc. Exp. Biol. Med. 154, 82–85. Lubahn, D.B., Moyer, J.S., Golding, T.S., Couse, J.F., Korach, K.S., Smithies, O., 1993. Alteration of reproductive function but not prenatal sexual development after insertional disruption of the mouse estrogen receptor gene. Proc. Natl. Acad. Sci. U.S.A. 90, 11162–11166. Malik, K.F., Silverman, A.J., Morrell, J.I., 1991. Gonadotropin-releasing hormone mRNA in the rat: distribution and neuronal content over the estrous cycle and after castration of males. Anat. Rec. 231, 457–466. Marshall, P.E., Goldsmith, P.C., 1980. Neuroregulatory and neuroendocrine GnRH pathways in the hypothalamus and forebrain of the baboon. Brain Res. 193, 353–372. Mason, A.J., Hayflick, J.S., Zoeller, R.T., Young III, W.S., Phillips, H.S., Nikolics, K., Seeburg, P.H., 1986. A deletion truncating the gonadotropin-releasing hormone gene is responsible for hypogonadism in the hpg mouse. Science 234, 1366–1371. Matsuo, H., Baba, Y., Nair, R.M., Arimura, A., Schally, A.V., 1971. Structure of the porcine LH- and FSH-releasing hormone. I. The proposed amino acid sequence. Biochem. Biophys. Res Commun. 43, 1334–1339. McDermott, J.R., Smith, A.I., Biggins, J.A., Hardy, J.A., Dodd, P.R., Edwardson, J.A., 1981. Degradation of luteinizing hormone-releasing hormone by serum and plasma in vitro. Regul. Pept. 2, 69–79. McShane, T.M., Petersen, S.L., McCrone, S., Keisler, D.H., 1993. Influence of food restriction on neuropeptideY, proopiomelanocortin, and luteinizing hormone-releasing hormone gene expression in sheep hypothalami. Biol. Reprod. 49, 831–839. Mellon, P.L., Windle, J.J., Goldsmith, P.C., Padula, C.A., Roberts, J.L., Weiner, R.I., 1990. Immortalization of hypothalamic GnRH neurons by genetically targeted tumorigenesis. Neuron 5, 1–10.

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

51

Meyer, S.L., Goodman, R.L., 1985. Neurotransmitters involved in mediating the steroid-dependent suppression of pulsatile luteinizing hormone secretion in anestrous ewes: effects of receptor antagonists. Endocrinology 116, 2054–2061. Millam, J.R., Craig-Veit, C.B., Adams, T.E., Adams, B.M., 1989. Avian gonadotropin-releasing hormones I and II in brain and other tissues in turkey hens. Comp. Biochem. Physiol. A 94, 771–776. Miller, B.H., Gore, A.C., 2002. N-methyl-d-aspartate receptor subunit expression in GnRH neurons changes during reproductive senescence in the female rat. Endocrinology 143, 3568–3574. Miyamoto, K., Hasegawa, Y., Minegishi, T., Nomura, M., Takahashi, Y., Igarashi, M., Kangawa, K., Matsuo, H., 1982. Isolation and characterization of chicken hypothalamic luteinizing hormone-releasing hormone. Biochem. Biophys. Res. Commun. 107, 820–827. Moenter, S.M., Caraty, A., Locatelli, A., Karsch, F.J., 1991. Pattern of gonadotropin-releasing hormone (GnRH) secretion leading up to ovulation in the ewe: existence of a preovulatory GnRH surge. Endocrinology 129, 1175–1182. Montaner, A.D., Affanni, J.M., King, J.A., Bianchini, J.J., Tonarelli, G., Somoza, G.M., 1999. Differential distribution of gonadotropin-releasing hormone variants in the brain of Hydrochaeris hydrochaeris (Mammalia, Rodentia). Cell Mol. Neurobiol. 19, 635–651. Nelson, S.B., Eraly, S.A., Mellon, P.L., 1998. The GnRH promoter: target of transcription factors, hormones, and signaling pathways. Mol. Cell. Endocrinol. 140, 151–155. Nunemaker, C.S., DeFazio, R.A., Moenter, S.M., 2002. Estradiol-sensitive afferents modulate long-term episodic firing patterns of GnRH neurons. Endocrinology 143, 2284–2292. Okuzawa, K., Amano, M., Kobayashi, M., Aida, K., Hanyu, I., Hasegawa, Y., Miyamoto, K., 1990. Differences in salmon GnRH and chicken GnRH-II contents in discrete brain areas of male and female rainbow trout according to age and stage of maturity. Gen. Comp. Endocrinol. 80, 116–126. Ottem, E.N., Godwin, J.G., Petersen, S.L., 2002. Glutamatergic signaling through the N-methyl-d-aspartate receptor directly activates medial subpopulations of luteinizing hormone-releasing hormone (LHRH) neurons, but does not appear to mediate the effects of estradiol on LHRH gene expression. Endocrinology 143, 4837– 4845. Page, R.B., Dovey-Hartman, B.J., 1984. Neurohemal contact in the internal zone of the rabbit median eminence. J. Comp. Neurol. 226, 274–288. Parfitt, D.B., Thompson, R.C., Richardson, H.N., Romeo, R.D., Sisk, C.L., 1999. GnRH mRNA increases with puberty in the male Syrian hamster brain. J. Neuroendocrinol. 11, 621–627. Park, O.-K., Gugneja, S., Mayo, K.E., 1990. Gonadotropin-releasing hormone gene expression during the rat estrous cycle: effects of pentobarbital and ovarian steroids. Endocrinology 127, 365–372. Petersen, S.L., Barraclough, C.A., 1989. Suppression of spontaneous LH surges in estrogen-treated ovariectomized rats by microimplants of antiestrogens into the preoptic brain. Brain Res. 484, 279–289. Petersen, S.L., Gardner, E., Adelman, J., McCrone, S., 1996. Examination of steroid-induced changes in LHRH gene transcription using 33P-and 35S-labeled probes specific for intron 2. Endocrinology 137, 234– 239. Petersen, S.L., McCrone, S., Keller, M., Gardner, E., 1991. Rapid increase in LHRH mRNA levels following NMDA. Endocrinology 129, 1679–1681. Petersen, S.L., McCrone, S., Keller, M., Shores, S., 1995. Effects of estrogen and progesterone on luteinizing hormone-releasing hormone messenger ribonucleic acid levels: consideration of temporal and neuroanatomical variables. Endocrinology 136, 3604–3610. Petersen, S.L., McCrone, S., Shores, S., 1993. Localized changes in LHRH mRNA levels as cellular correlates of the positive feedback effects of estrogen on LHRH neurons. Am. Zool. 33, 255–265. Plant, T.M., Shahab, M., 2002. Neuroendocrine mechanisms that delay and initiate puberty in higher primates. Physiol. Behav. 77, 717–722. Polkowska, J., Dubois, M.P., Domanski, E., 1980. Immunocytochemistry of luteinizing hormone releasing hormone (LHRH) in the sheep hypothalamus during variuos reproductive stages: correlation with the gonadotropic hormones of the pituitary. Cell Tissue Res. 208, 327–341. Pompolo, S., Pereira, A., Kaneko, T., Clarke, I.J., 2003a. Seasonal changes in the inputs to gonadotropin-releasing hormone neurons in the ewe brain; an assessment by conventional and confocal microscopy. J. Neuroendocrinol. 15, 538–545.

52

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

Pompolo, S., Pereira, A., Scott, C.J., Fujiyama, F., Clarke, I.J., 2003b. Evidence for estrogenic regulation of gonadotropin-releasing hormone neurons by glutamatergic neurons in the ewe brain: an immunohistochemical study using an antibody against vesicular glutamate transporter-2. J. Comp. Neurol. 465, 136–144. Pompolo, S., Rawson, J.A., Clarke, I.J., 2001. Projections from the arcuate/ventromedial region of the hypothalamus to the preoptic area and bed nucleus of stria terminalis in the brain of the ewe; lack of direct input to gonadotropin-releasing hormone neurons. Brain Res. 904, 1–12. Porkka-Heiskanen, T., Khoshaba, N., Scarbrough, K., Urban, J.H., Vitaterna, M.H., Levine, J.E., Turek, F.W., Horton, T.H., 1997. Rapid photoperiod-induced increase in detectable GnRH mRNA-containing cells in Siberian hamster. Am. J. Physiol. 273, R2032–R2039. Porkka-Heiskanen, T., Urban, J.H., Turek, F.W., Levine, J.E., 1994. Gene expression in a subpopulation of luteinizing hormone-releasing hormone (LHRH) neurons prior to the preovulatory gonadotropin surge. J. Neurosci. 14, 5548–5558. Porter, J.C., Smith, K.R., 1967. Collection of hypophysial stalk blood in rats. Endocrinology 81, 1182– 1185. Radovick, S., Ticknor, C.M., Nakayama, Y., Notides, A.C., Rahman, A., Weintraub, B.D., Cutler Jr., G.B., Wondisford, F.E., 1991. Evidence for direct estrogen regulation of the human gonadotropin-releasing hormone gene. J. Clin. Invest. 88, 1649–1655. Rance, N.E., Uswandi, S.V., 1996. Gonadotropin-releasing hormone gene expression is increased in the medial basal hypothalamus of postmenopausal women. J. Clin. Endocrinol. Metab. 81, 3540–3546. Rawson, J.A., Scott, C.J., Pereira, A.M., Jakubowiak, A., Clarke, I.J., 2001. Noradrenergic projections from the A1 field to the preoptic area in the brain of ewe and fos responses to oestrogen in th A1 cells. J. Neuroendocrinol. 13, 129–138. Razandi, M., Pedram, A., Park, S.T., Levin, E.R., 2003. Proximal events in signaling by plasma membrane estrogen receptors. J. Biol. Chem. 278, 2701–2712. Roberts, J.L., Dutlow, C.M., Jakubowski, M., Blum, M., Millar, R.P., 1989. Estradiol stimulates preoptic areaanterior hypothalamic proGnRH-GAP gene expression in ovariectomized rats. Brain Res. 6, 127–134. Robinson, J.E., Healey, A.E., Harris, T.G., Messent, E.A., Skinner, D.C., Taylor, J.A., Evans, N.P., 2000. The negative feedback action of progesterone on luteinizing hormone release is not associated with changes in GnRH mRNA expression in the ewe. J. Neuroendocrinol. 12, 121–129. Ronchi, E., Krey, L.C., Pfaff, D.W., 1992. Steady-state analysis of hypothalamic GnRH mRNA levels in male Syrian hamsters: influences of photoperiod and androgen. Neuroendocrinology 55, 146–155. Rosie, R., Thomson, E., Fink, G., 1990. Oestrogen positive feedback stimulates the synthesis of LHRH mRNA in neurones of the rostral diencephalon of the rat. J. Endocrinol. 124, 285–289. Roy, D., Angelini, N.L., Belsham, D.D., 1999. Estrogen directly respresses gonadotropin-releasing hormone (GnRH) gene expression in estrogen receptor-alpha (ERalpha)- and ERbeta-expressing GT1-7 GnRH neurons. Endocrinology 140, 5045–5053. Rubin, B.S., Lee, C.E., Ohtomo, M., King, J.C., 1997. Luteinizing hormone-releasing hormone gene expression differs in young and middle-aged females on the day of a steroid-induced LH surge. Brain. Res. 770, 267– 276. Sagrillo, C.A., Grattan, D.R., McCarthy, M.M., Selmanoff, M., 1996. Hormonal and neurotransmitter regulation of GnRH gene expression and related reproductive behaviors. Behav. Genet. 26, 241–277. Sarkar, D.K., Chiappa, S.A., Fink, G., Sherwood, N.M., 1976. Gonadotropin-releasing hormone surge in prooestrous rats. Nature 264, 461–463. Sarkar, D.K., Fink, G., 1981. Gonadotropin-releasing hormone surge: possible modulation through postsynaptic alpha-adrenoreceptors and two pharmacologically distinct dopamine receptors. Endocrinology 108, 862–867. Scaramuzzi, R.J., Tillson, S.A., Thorneycroft, I.H., Caldwell, B.V., 1971. Action of exogenous progesterone and estrogen on behavioral estrus and luteinizing hormone levels in the ovariectomized ewe. Endocrinology 88, 1184–1189. Schwanzel-Fukuda, M., Bick, D., Pfaff, D.W., 1989. Luteinizing hormone-releasing hormone (LHRH)-expressing cells do not migrate normally in an inherited hypogonadal (Kallmann) syndrome. Brain Res. Mol. Brain. Res. 6, 311–326. Schwanzel-Fukuda, M., Pfaff, D.W., 1989. Origin of luteinizing hormone-releasing hormone neurons. Nature 338, 161–164.

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

53

Scott, C.J., Rawson, J.A., Pereira, A.M., Clarke, I.J., 1999. Oestrogen receptors in the brainstem of the female sheep: relationship to noradrenergic cells and cells projecting to the medial preoptic area. J. Neuroendocrinol. 11, 745–755. Seeburg, P.H., Adelman, J.P., 1984. Characterization of cDNA for precursor of human luteinizing hormone releasing hormone. Nature 311, 666–668. Selmanoff, M., Shu, C., Petersen, S.L., Barraclough, C.A., Zoeller, R.T., 1991. Single cell levels of hypothalamic messenger ribonucleic acid encoding luteinizing hormone-releasing hormone in intact, castrated, and hyperprolactinemic male rats. Endocrinology 128, 459–466. Seong, J.Y., Lee, Y.K., Lee, C.C., Kim, K., 1993. NMDA receptor antagonist decreases the progesterone-induced increase in GnRH gene expression in the rat hypothalamus. Neuroendocrinology 58, 234–239. Seong, J.Y., Park, S., Kim, K., 1999. Enhanced splicing of the first intron from the gonadotropin-releasing hormone (GnRH) primary transcript is a prerequisite for mature GnRH messenger RNA: presence of GnRH neuronspecific splicing factors. Mol. Endocrinol. 13, 1882–1895. Sherwood, N., Eiden, L., Brownstein, M., Spiess, J., Rivier, J., Vale, W., 1983. Characterization of a teleost gonadotropin-releasing hormone. Proc. Natl. Acad. Sci. U.S.A. 80, 2794–2798. Shivers, B.D., Harlan, R.E., Morrell, J.I., Pfaff, D.W., 1983. Absence of oestradiol concentration in cell nuclei of LHRH-immunoreactive neurones. Nature 304, 345–347. Sisk, C.L., Richardson, H.N., Chappell, P.E., Levine, J.E., 2001. In vivo gonadotropin-releasing hormone secretion in female rats during peripubertal development and on proestrus. Endocrinology 142, 2929– 2936. Skinner, D.C., Caraty, A., Allingham, R., 2001. Unmasking the progesterone receptor in the preoptic area and hypothalamus of the ewe: no colocalization with gonadotropin-releasing neurons. Endocrinology 142, 573–579. Son, G.H., Jung, H., Seong, J.Y., Choe, Y., Geum, D., Kim, K., 2003. Excision of the first intron from the gonadotropin-releasing hormone (GnRH) transcript serves as a key regulatory step for GnRH biosynthesis. J. Biol. Chem. 278, 18037–18044. Spratt, D.P., Herbison, A.E., 1997. Regulation of preoptic area gonadotrophin-releasing hormone (GnRH) mRNA expression by gonadal steroids in the long-term gonadectomized male rat. Brain Res. Mol. Brain Res. 47, 125–133. Suzuki, M., Nishihara, M., Takahashi, M., 1995. Hypothalamic gonadotropin-releasing hormone gene expression during rat estrous cycle. Endocrin. J. 42, 789–796. Temple, J.L., Millar, R.P., Rissman, E.F., 2003. An evolutionarily conserved form of gonadotropin-releasing hormone coordinates energy and reproductive behavior. Endocrinology 144, 13–19. Thanky, N.R., Slater, R., Herbison, A.E., 2003. Sex differences in estrogen-dependent transcription of gonadotropin-releasing hormone (GnRH) gene revealed in GnRH transgenic mice. Endocrinology 144, 3351–3358. Thiery, J.C., Chemineau, P., Hernandez, X., Migaud, M., Malpaux, B., 2002. Neuroendocrine interactions and seasonality. Domest. Anim. Endocrinol. 23, 87–100. Thomas, G.B., Mercer, J.E., Karalis, T., Rao, A., Cummins, J.T., Clarke, I.J., 1990. Effect of restricted feeding on the concentrations of growth hormone (GH), gonadotropins, and prolactin (PRL) in plasma, and on the amounts of messenger ribonucleic acid for GH, gonadotropin subunits, and PRL in the pituitary glands of adult ovariectomized ewes. Endocrinology 126, 1361–1367. Tilbrook, A.J., Clarke, I.J., 1992. Evidence that dopaminergic neurons are not involved in the negative feedback effect of testosterone on luteinizing hormone in rams in the non-breeding season. J. Neuroendocrinol. 4, 365–374. Tilbrook, A.J., de Kretser, D.M., Clarke, I.J., 1993. Human recombinant inhibin A suppresses plasma follicle stimulating hormone to intact levels but has no effect on luteinizing hormone in castrated rams. Biol. Reprod. 49, 779–788. Tilbrook, A.J., de Kretser, D.M., Clarke, I.J., 1999. Seasonal changes in the negative feedback regulation of the secretion of the gonadotrophins by testosterone and inhibin in rams. J. Endocrinol. 160, 155–166. Tilbrook, A.J., de Kretser, D.M., Clarke, I.J., 2001. Influence of the degree of stimulation of the pituitary by gonadotropin-releasing hormone on the action of inhibin and testosterone to suppress the secretion of the gonadotropins in rams. Biol. Reprod. 64, 473–481.

54

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

Tilbrook, A.J., de Kretser, D.M., Cummins, J.T., Clarke, I.J., 1991. The negative feedback effects of testicular steroids are predominantly at the hypothalamus in the ram. Endocrinology 129, 3080–3092. Tilbrook, A.J., Turner, A.I., Clarke, I.J., 2002. Stress and reproduction: central mechanisms and sex differences in non-rodent species. Stress 5, 83–100. Toranzo, D., Dupont, E., Simard, J., Labrie, C., Couet, J., Labrie, F., Pelletier, G., 1989. Regulation of progonadotropin-releasing hormone gene expression by sex steroids in the brain of male and female rats. Mol. Endocrinol. 3, 1748–1756. Urbanski, H.F., White, R.B., Fernald, R.D., Kohama, S.G., Garyfallou, V.T., Densmore, V.S., 1999. Regional expression of mRNA encoding a second form of gonadotropin-releasing hormone in the macaque brain. Endocrinology 140, 1945–1948. Vijayan, E., McCann, S.M., 1978a. Re-evaluation of the role of catecholamines in control of gonadotropin and prolactin release. Neuroendocrinology 25, 150–165. Vijayan, E., McCann, S.M., 1978b. The effect of systemic administration of dopamine and apomorphine on plasma LH and prolactin concentrations in conscious rats. Neuroendocrinology 25, 221–235. Vitale, M.L., Chiocchio, S.R., 1993. Serotonin, a neurotransmitter involved in the regulation of luteinizing hormone release. Endocr. Rev. 14, 480–493. Wang, H., Li, S., Pelletier, G., 1998. Role of nitric oxide in the regulation of gonadotropin-releasing hormone and tyrosine hydroxylase gene expression in the male rat brain. Brain Res. 792, 66–71. Weesner, G.D., Harms, P.G., McArthur, N.H., Wilson, J.M., Forrest, D.W., Wu, T.J., Pfaff, D.W., 1993a. Luteinizing hormone-releasing hormone gene expression in the bovine brain: anatomical localisation and regulation by ovarian state. Biol. Rep. 49, 431–436. Weesner, G.D., Krey, L.C., Pfaff, D.W., 1993b. Alpha 1 adrenergic regulation of estrogen-induced increases in luteinizing hormone-releasing hormone mRNA levels and release. Brain Res. Mol. Brain Res. 17, 77–82. Wheaton, J.E., 1979. Regional brain content of luteinizing hormone-releasing hormone in sheep during the estrous cycle, seasonal anestrus, and after ovariectomy. Endocrinology 104, 839–844. Wheaton, J.E., Krulich, L., McCann, S.M., 1975. Localization of luteinizing hormone-releasing hormone in the preoptic area and hypothalamus of the rat using radioimmunoassay. Endocrinology 97, 30–38. White, R.B., Eisen, J.A., Kasten, T.L., Fernald, R.D., 1998. Second gene for gonadotropin-releasing hormone in humans. Proc. Natl. Acad. Sci. U.S.A. 95, 305–309. White, S.A., Kasten, T.L., Bond, C.T., Adelman, J.P., Fernald, R.D., 1995. Three gonadotropin-releasing hormone genes in one organism suggest novel roles for an ancient peptide. Proc. Natl. Acad. Sci. U.S.A. 92, 8363–8367. Whyte, D.B., Lawson, M.A., Belsham, D.D., Eraly, S.A., Bond, C.T., Adelman, J.P., Mellon, P.L., 1995. A neuron-specific enhancer targets expression of the gonadotropin-releasing hormone gene to hypothalamic neurosecretory neurons. Mol. Endocrinol. 9, 467–477. Wiemann, J.N., Clifton, D.K., Steiner, R.A., 1989. Pubertal changes in gonadotropin-releasing hormone and proopiomelanocortin gene expression in the brain of the male rat. Endocrinology 124, 1760–1767. Wiemann, J.N., Clifton, D.K., Steiner, R.A., 1990. Gonadotropin-releasing hormone messenger ribonucleic acid levels are unaltered with changes in the gonadal hormone milieu of the adult male rat. Endocrinology 127, 523–532. Wierman, M.E., Bruder, J.M., Kepa, J.K., 1995. Regulation of gonadotropin-releasing hormone (GnRH) gene expression in hypothalamic neuronal cells. Cell Mol. Neurobiol. 15, 79–88. Wise, P.M., Smith, M.J., Dubal, D.B., Wilson, M.E., Rau, S.W., Cashion, A.B., Bottner, M., Rosewell, K.L., 2002. Neuroendocrine modulation and repercussions of female reproductive aging. Recent Prog. Horm. Res. 57, 235–256. Worthington, W.C., 1966. Blood samples from the pituitary stalk of the rat: method of collection and factors determining volume. Nature 210, 710–712. Wray, S., 2001. Development of luteinizing hormone releasing hormone neurones. J. Neuroendocrinol. 13, 3–11. Wray, S., 2002. Molecular mechanisms for migration of placodally derived GnRH neurons. Chem. Senses 27, 569–572. Wray, S., Grant, P., Gainer, H., 1989. Evidence that cells expressing luteinizing hormone-releasing hormone mRNA in the mouse are derived from progenitor cells in the olfactory placode. Proc. Natl. Acad. Sci. U.S.A. 86, 8132–8136.

I.J. Clarke, S. Pompolo / Animal Reproduction Science 88 (2005) 29–55

55

Wu, T.J., Gibson, M.J., Roberts, J.L., 2000. Effect of N-methyl-d,l-aspartate (NMA) on gonadotropin-releasing hormone (GnRH) gene expression in male mice. Brain Res. 862, 238–241. Yeo, T.T., Gore, A.C., Jakubowski, M., Dong, K.W., Blum, M., Roberts, J.L., 1996. Characterization of gonadotropin-releasing hormone gene transcripts in a mouse hypothalamic neuronal GT1 cell line. Brain Res. Mol. Brain Res. 42, 255–262. Yu, W.H., Karanth, S., Sower, S.A., Parlow, A.F., McCann, S.M., 2000. The similarity of FSH-releasing factor to lamprey gonadotropin-releasing hormone III (l-GnRH-III). Proc. Soc. Exp. Biol. Med. 224, 87–92. Yu, W.H., Karanth, S., Walczewska, A., Sower, S.A., McCann, S.M., 1997. A hypothalamic follicle-stimulating hormone-releasing decapeptide in the rat. Proc. Natl. Acad. Sci. U.S.A. 94, 9499–9503. Zoeller, R.T., Seeburg, P.H., Young, W.S., 1988. In situ hybridization histochemistry for messenger ribonucleic acid (mRNA) encoding gonadotropin-releasing hormone (GnRH): effect of estrogen on cellular levels of GnRH mRNA in female rat brain. Endocrinology 122, 2570–2577. Zoeller, R.T., Young, W.S., 1988. Changes in cellular levels of messenger ribonucleic acid encoding gonadotropinreleasing hormone in the anterior hypothalamus of female rats during the estrous cycle. Endocrinology 123, 1688–1689.