The Hypothalamus and Reproduction in the Female

The Hypothalamus and Reproduction in the Female H. OPEL

US Department of Agriculture, SEA-AR, Animal Physiology and Genetics Institute, Avian Physiology Laboratory, Beltsville, Maryland 20705 (Received for publication January 22, 1979) 1979 Poultry Science 58:1607-1618 White-crowned sparrows, chickens, and Japanese quail. Two general methods have been used to analyze the hypothalamic structures regulating gonadotrophin secretion: 1) neuroanatomical methods have been used to identify the neurosecretory elements and their neuronal circuitry, and 2) surgical techniques have been used to delimit participating hypothalamic regions and examine their physiology. Details of the neuroanatomy of the hypothalamic systems are described by Oksche and Farner (1974) and Oksche (1976; 1978). Excellent reviews of the neuroendocrine regulation of gonadotrophin secretion have been published by Assenmacher (1973), Kobayashi andWada (1973) and Follett and Davies (1975). Because most of the available data is obtained from male birds, no previous review has placed emphasis on the female. General Hypothalamic Structure and Nomenclature. The hypothalamus is a relatively small structure which forms the walls and floor of the lower part of the third ventricle of the brain (Fig. 1). It includes the optic chiasma, the tuber cinereum, the infundibulum, and the mammillary bodies. Dorsally, it is separated from the thalamus by the hypothalamic sulcus, a usually horizontal groove. Salient structural features of the avian hypothalamus are shown in Figure 2. The anatomical nomenclature used in the figure and in the text of this paper is that of Follett and Davies (1975). It was chosen because of its use in important work on female Coturnix (Stetson, 1972b; Follett and Davies, 1979) and in my work on turkeys (Opel, (1979a,b). The important difference in this nomenclature from earlier nomenclature used in birds is that the main neuroendocrine effector of gonadotrophin secretion in the tuberal (posterior) hypothalamus is called the infundibular nuclear complex (INC). This extensive complex, which occupies the medial and ventral portions of the tuberal region (Fig. 2), contains several morphologically different kinds of

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The hypothalamus controls reproduction by regulating the secretory activities of the pituitary gland. In response to stimuli received from the internal and external environments, it produces several peptide neurohormones that regulate anterior pituitary function. These neurohormones are released from the median eminence into the capillaries of the hypophyseal portal veins where they are carried to the adenohypophyseal cells to stimulate or inhibit release of particular trophic hormones. In female birds, growth of the ovarian follicles, synthesis of sex steroids, and ovulation are effected, at least in part, by neurohormonally mediated changes in rates of release of pituitary gonadotrophins. Also in response to environmental stimuli, the hypothalamus forms the neurohypophyseal hormones, which are transported down the long axons of the hypothalamo-hypophyseal tract for storage in the neural lobe of the pituitary and subsequent release into the systemic circulation. Although an obligatory role for neurohypophyseal hormones in avian reproduction is not clearly established, their timed release into the systemic circulation may evoke oviposition (Follett, 1969; Sturkie and Mueller, 1976). Any consideration of recent research on hypothalamic control of reproduction in female birds should place emphasis on two main areas of investigation, the localization of hypothalamic structures regulating gonadotrophin secretion and the role of the feedback of ovarian steroids in hypothalamic control of gonadotrophin release. Both areas are discussed in the following account. Some earlier papers on the female, as well as recent papers on male birds and on mammals, are quoted as key references. Localization of Hypothalamic Structures Regulating Gonadotrophin Secretion. Most of the available information on hypothalamic control of gonadotrophin secretion in birds comes from photosensitive species, especially ducks,

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neurosecretory neurons whose axons form part of the tubero-infundibular tract projecting to the median eminence (ME). The basally located infundibular nucleus corresponds to the tuberal (arcuate) nucleus described in much of the avian literature. Localization of Neurons Producing Luteinizing Hormone-Releasing Hormone (LHRH). A major problem in avian neuroendocrinology is the identification of the neuronal elements that produce and transport gonadotrophin-releasing hormones. Antisera to synthetic mammalian luteinizing hormone-releasing hormone (LHRH) are now being used for the immunocytochemical identification of LHRH-producing neurons in the avian brain. Because mammalian LHRH is similar but not identical in structure to avian LHRH (Jackson, 1971a,b; Bicknell and Follett, 1975; Bonney and Cunningham, 1977) questions arise concerning specificity of the mammalian antisera. However, in the most detailed investigations reported thus far (Bons et al., 1978a,b), procedures used to check specificity make it reasonably certain that the fluorescent substance demonstrated by the LHRH-immune sera is, in fact, avian LHRH. Clusters of LHRHimmunoreactive perikerya have been found in the preoptic region (POR) and adjacent structures in the rostral hypothalamus of the Mallard duck (Bons et al, 1977, 1978a,b; Oksche, 1978), Japanese quail (Oksche, 1978), and in the INC of the tuberal hypothalamus of the duck (McNeill et al, 1976; Bons et al., 1978b). The separate LHRH-producing systems in the rostral and tuberal hypothalamus of the duck,

FIG. 2. Schematic sagittal drawing of the hypothalamus of birds showing regions and nuclei regulating pituitary secretions and the neural pathways between them. The aldehyde-fuchsin (AF) positive neurosecretory tracts are shown by dashed lines and monoamine fiber tracts are shown by solid lines. Shading shows areas heavily innervated with aminergic nerve terminals. The rostral (anterior) hypothalamus includes the preoptic region (POR), the paraventricular nucleus (PVN), and the supraoptic region (SOR). A , distribution of AF positive neurosecretory cells producing oxytocin and arginine vasotocin. The tuberal (anterior) hypothalamus includes the infundibular nuclear complex (INC) and the nucleus hypothalamicus posterior medialis (NHPM). •, a, •, • , o, different types of neurosecretory cells of INC. AME, anterior median eminence; OC, optic chiasma; PD, pars distalis; PME, posterior median eminence; PN, pars nervosa (modified from Follett and Davies, 1975).

as visualized by Bons et al. (1978a), are depicted in Figure 3. In their preparations, the concentration of LHRH-producing neurons was greatest in the rostral hypothalamus. The main clusters of LHRH-immunoreactive perikerya were found in the periventricular POR. All investigators agree that axons from the LHRHproducing neurons project to the external layers of the anterior and posterior divisions of the ME and terminate near the portal capillary beds. This supports other evidence showing that both of the well-defined divisions of the ME in birds are involved in control of gonadotrophin secretion (Assenmacher, 1973; Oksche and Farner, 1974). Oksche (1978) reported that significant numbers of axons from the LHRH-producing neurons in the POR of the duck and quail terminate in other intra- and extrahypothalamic sites, including the lamina terminalis. As expected, the available immunocyto-

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FIG. 1. Sagittal section of the chicken brain showing the location of the hypothalamus and the pituitary gland. H, hypothalamus; ME, median eminence; OC, optic chiasma; PD, pars distalis; PN, pars nervosa; III, third ventricle.

HYPOTHALAMUS AND FEMALE REPRODUCTION

chemical data show that the neurosecretory systems that form LHRH are separate from the neurosecretory system producing neurohypophyseal hormones (Bons et al, 1978b, Oksche, 1978). An immunoreactive-arginine vasotocin system originates in the paraventricular (PVN) and supraoptic (SON) nuclei and terminates in the external layer of the anterior ME and in the neural lobe of the pituitary. The initial findings on the localization of LHRH-producing neurons in birds are consistent with a larger body of evidence in mammals (Zimmerman, 1977) showing the LHRH-producing neurons probably exist in fields or zones in the rostral and tuberal hypothalamus and do not follow the boundaries of recognized hypothalamic nuclei. As with the data in mammals (Zimmerman, 1977), the data in birds are probably incomplete due primarily to false negative results. In comparing the immunocytochemical data, it is worth noting that in birds, as in mammals (Schally et al, 1971), one neurohormone may regulate the release of LH and FSH. Synthetic mammalian LHRH evokes release of both LH and FSH from turkey pituitary cells in vivo (Opel, 1976; Scanes et al, 1977) and in vitro (Godden et al, 1977). An explanation of how one neurohormone might independently regulate LH and FSH release in birds via regional differentiation of the ME and anterior pituitary is provided by Ishii et al. (1975). The observations in the duck and quail

(Oksche, 1978) that axons of LHRH-producing cells in the rostral hypothalamus project to the lamina terminalis lend support to a widely-discussed hypothesis (Knigge et al, 1973) that LHRH, as well as other neurohormones, are secreted into the cerebral spinal fluid and delivered to the hypophyseal portal vessels via specialized ependymal cells (tanycytes) lining the ME. In Coturnix, tanycytes lining the ME can absorb protein size molecules injected into the third ventricle and transport them through the cytoplasm to processes terminating on portal capillaries (Kobayashi et al, 1972). Other evidence, however, argues against a role of tanycytes in the transport of LHRH in birds. McNeill et al. (1976) found no immunoreactive LHRH in tanycytes of the duck ME, but did find immunoreactive neurophysin, the carrier peptide for neurohypophyseal hormones. Uemura and Kobayashi (1977) reported that in the quail destruction of the ependymal lining of the ME did not prevent photostimulated growth of the testes. Localization of the Deep Hypothalamic Photoreceptor. Forty years ago, Benoit (1936, 1937) found that light applied directly to the hypothalamus of the Mallard duck through a glass tube or quartz rod stimulated testicular growth. In considering the physiological significance of this discovery, Benoit and Ott (1944) pointed out that the lens of the drake's eye focuses light on the hypothalamus. Since then, several studies have shown that light can penetrate through the skull to the hypothalamus of birds and mammals (van Tienhoven and Planck, 1974). The nature and location of the so-called "deep hypothalamic photoreceptor" has remained an enigma. Photo-pigments or specialized photoreceptors have not been found in hypothalamic tissues. Recent experiments in Coturnix suggest that the hypothalamic photoreceptor resides in the INC. Oliver and Bayle (1976) showed that implantation of a small pellet of radioluminous paint (RLP) into the INC of immature quail held under non-stimulatory light (6L:18D) induced testicular growth comparable to that seen in unoperated controls subjected to stimulatory light (18L:6D). Because of the weak luminance of the RLP pellet (.029 candela/m 2 ) the authors consider it likely that the photic stimulus was limited to the vicinity of the implant. Implantation of an RLP pellet into the POR or other areas of the rostral hypothalamus did not stimulate the testes. In a subsequent

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FIG. 3. Schematic drawing of the hypothalamus of the duck showing the location of LHRH-producing neurons (0) and fibers (0—0). INC, infundibular nuclear complex; ME, median eminence; OC, optic chiasma; POR, preoptic region; PD, pars distalis; PN, pars nervosa (modified from Bons et al., 1978b).

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The POR appears to be the critical area of the hypothalamus regulating the cyclic ovulatory surge of LH in the chicken (Ralph, 1959; Ralph and Fraps, 1959b) and turkey (Opel, 1979a). Lesions placed in the POR of these species (Fig. 4A,B) shortly before the LH surge for an impending ovulation consistently blocked ovulation. Ralph and Fraps (1959a) found that lesions of the POR of the chicken also prevented progesterone-induced ovulations. They suggested that the POR is either the site where feedback of progesterone triggers an ovulation or is a link in a neural complex medi-

FIG. 4. Summary diagram of possible hypothalamic sites regulating gonadotrophin secretion as delineated by lesions. Horizontal lines show sites regulating cyclic release of LH; verticle lines show sites regulating tonic secretion of gonadotrophin for growth and maintenance of the ovary; cross hatching shows sites influencing both cyclic and tonic secretions. (A) Regulatory sites in chickens (Ralph, 1959; Ralph and Fraps, 1959b; Kanematsu et al. (1966). Solid circles show where Ralph and Fraps found single lesions to block cyclic and tonic secretions. (B) Sites in turkey (Opel, 1979a,b); (C) sites in Coturnix according to (Stetson, 1972b); dashed line indicates division of tuberal hypothalamus into anterior portions regulating cyclic LH release and posterior portions regulating tonic secretions for ovarian maintenance. (D) Sites in Coturnix according to Follett and Davies (1979). AC, anterior commissure; INC, infundibular nuclear complex; OC, optic chiasma; POR, preoptic region; SOR, supraoptic region.

ating progesterone feedback. Other structures may regulate the cyclic LH surge in Japanese quail. Stetson (1972b) found that lesions in the preoptic-supraoptic region (POR-SOR) in the ventral, medial, or posterior divisions of the INC or in the posterior ME (Fig. 4C) chronically stopped ovulation, but caused only a slight decrease in weights of the ovary and oviduct. The large ovarian follicles were somewhat reduced in size, but the normal gradation in follicle sizes was maintained. This condition, reminiscent of the persistent estrous syndrome in the rat (Everett, 1969), indicates that the ovulatory surge of LH was blocked, while enough FSH and LH were available for stimulation of the follicle and some secretion of ovarian steroids. Because lesions of the posterior INC also inhibited LH secretion in male

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study, Oliver et al. (1977) found that implants of RLP into the tuberal or dorsal part of the INC caused significant testicular growth in quail held under 6L:18D or in complete darkness. Neural isolation of the hypothalamus from the rest of the brain did not suppress the photosexual reflex evoked by direct application of RLP to the INC. Two tentative conclusions can be drawn from these results: 1) photostimulation of the testes was obtained by stimulation of the tuberal hypothalamus without participation of retinal photoreceptors and 2) the tuberal hypothalamus has an antonomous capability to stimulate gonadotrophin secretion in response to light. Although there is mounting evidence that the pineal complex functions as an extraretinal photoreceptor in birds (Binkley et al, 1977, 1978), there is no clear proof that this gland is involved in the photoperiodic aspects of reproduction (Menaker and Oksche, 1974; Binkley et al, 1977). At the moment, the best argument against the role of the pineal in avian reproduction is a report by Harrison and Becker (1969) showing that pinealectomy does not alter the time of sequential ovipositions in chickens. Localization of Structures Controlling the Ovulatory Surge of LH. In photoperiodic birds such as the chicken, turkey, and Japanese quail, the neural apparatus controlling ovulation consists of two main components; a lightentrained biological clock (circadian rhythm system) that sets the period of the day when the cyclic ovulatory surge of LH is normally initiated, and a triggering mechanism that initiates the LH surge in response to feedback of one or more steroid hormones. In investigating the hypothalamic integration of these components, surgical analyses can be used to localize the structures most directly involved and to place them in functional sequence.

HYPOTHALAMUS AND FEMALE REPRODUCTION

The recent findings in the rat are consistent with the more limited data in birds, which admit to the possibilities that the rostral hypothalamus produces LHRH, houses the site critical for steroid feedback, and houses all or part of the circadian rhythm system setting the daily period of ovulation. We know little about the neuronal circuitry involved in regulating the ovulatory surge of LH. In the chicken, Sonoda et al. (1973) found that damage to the lateral forebrain bundle, an avenue of entry for afferents from higher brain areas, blocks ovulation. In mature turkeys about to enter lay (Opel, 1979b), a microknife manipulated under stereotaxic control was used to deafferentate or cut neural connections to and from the POR. Superior deafferentation of the POR, which severed most inputs from higher brain areas, produced a polyfollicular syndrome very like that observed by Follett and Davies (1979) in quail. All but an occasional ovulation was prevented, while the number and size of large ovarian follicles was increased. These results, similar to those produced by frontal or superior deafferentation of the POR in rats (HaMsz, 1969), suggest that at least a part of the timing mechanism regulating the ovulatory surge of LH lies outside the hypothalamus. Transection of the supraopticohypophyseal tract, a main pathway for afferents projecting from the POR-SOR directly to the ME, did not interfere with ovulation. Cuts that severed afferents projecting from the PORSOR to the INC, or cuts that completely isolated the INC from the rest of the brain, blocked ovulation and resulted in ovarian atrophy. Conversely, cuts that isolated the INC from all but antero-dorsal connections allowed normal follicular development and ovulation. These results admit to the possibility that a POR-INC pathway is critical for cyclic LH release. Localization of Structures Regulating "Tonic" Secretion of Gonadotrophins. Knowledge of hypothalamic structures controlling the "tonic" secretion of gonadotrophins (FSH and LH) comes from five domesticated species: the chicken, turkey, Japanese quail, Mallard duck, and pigeon. One generalization that can be made from the limited data available is that areas in both the rostral and tuberal hypothalamus are critical for development and maintenance of the functional ovary. In immature chickens (Kanematsu, 1968), ducks (Novikov and Roudneva, 1964, 1969), and quail (Follett and Davies, 1979) damage to either division of

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quail (Stetson, 1972a), Stetson suggested that the INC was not part of the neuroendocrine system triggering the ovulatory LH surge. Follett and Davies (1979), who lesioned mature quail about to enter lay, obtained somewhat different results. Lesions of the POR or INC resulted in regression of the ovary and oviduct. Lesions of the SOR, defined as located immediately anterior to the rostral edge of the optic chiasma (Fig. 4D), selectively blocked the ovulatory LH surge. Provided that both the SON and suprachiasmatic nucleus (SCN) were destroyed, all but an occasional ovulation was prevented. Oviduct weights did not differ from those of unoperated controls, but ovarian weights were greatly increased due to a pile-up of large ovarian follicles. The inconsistencies in the few data available on structures regulating the LH surge can be resolved only by additional and more varied research. The etiologies of the "persistent estrous-like" syndrome of Stetson (1972b) and the "polyfollicular syndrome" of Follett and Davies (1979) are probably the same. As pointed out by Stetson (1972b), a similar syndrome could have been overlooked in a longterm study in the chicken (Ralph and Fraps, 1959b). Considering the large lesions used, it is possible that the POR-SOR lesions of Stetson (1972b) selectively blocked the LH surge because they destroyed both the SON and SCN. However, the rostral hypothalamic area critical for ovulation in the quail may not be the same as that found in the chicken (Ralph, 1959) and turkey (Opel, 1979a). In the latter species, the SCN and the main body of the SON are too far from the midline to have been destroyed by the effective lesions placed in the medial POR. According to Ralph (1960), lesions of the SON do not prevent ovulation in the chicken. In further work on this problem, the progress made in similar investigations in the rat may be instructive. Early studies (Everett, 1964) indicated that a diffuse septo-preoptic-rostral hypothalamic area regulated the ovulatory LH surge: later the region was narrowed to the medial POR (Barraclough, 1973). More recently the SCN was claimed to be the effective locus (Gray et al, 1978). At the same time, Wiegand et al. (1978) obtained evidence indicating that both the medial POR and the SCN are involved. Their results suggest that the POR is the site regulating LH release in response to steroid feedback, while the SCN houses all or part of the biological clock regulating cyclic ovulation.

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Data from the duck (Novikov and Roudneva, 1964; 1969) indicate that the SON and IN regulate tonic gonadotrophin secretion. The duck stands as the only species in which surgical analyses consistently implicate the classical aldehyde-fuchsin (AF) positive neurosecretory system, the known source of neurohypophyseal hormones, in control of gonadotrophin secretion. Recent immunocytochemical evidence on the location of LHRH-producing systems in the duck (Bons et al, 1977'; 1978a,b), described in an earlier section of this paper, would seem to explain this enigma. Axons of the LHRH-producing neurons in the rostral hypothalamus appear to run down along the caudal surface of the optic chiasma in parallel with the AF-positive supraoptico-hypophyseal tract. The large mechanical or electrolytic lesions used to transect the AF-positive tract (Assenmacher, 1973; Novikov and Roudneva, 1964, 1969) probably cut the axons of the LHRH-producing neurons in the rostral hypothalamus, thus blocking go-

nadotrophin release. Bouille and Bayle (1973) reported that in the pigeon, lesions of the "preoptic anterior" hypothalamus, and a far lateral region of the tuberal hypothalamus produced ovarian atrophy. In the other species studied, critical sites in the tuberal hypothalamus are centrally located. A different organization in neuronal systems regulating gonadotrophin secretion in the pigeon is not surprising. In contrast to the other birds studied, it is not very photosensitive, normally depends on visual stimuli from the male to initiate functional development of the ovary, and is a determinate rather than an indeterminate layer. Tactile stimuli from the brood patch rigidly sets clutch length at two eggs. For a comprehensive understanding of neuroendocrine mechanisms regulating gonadotrophin secretion in birds, additional research on birds like the pigeon and on other birds with other types of reproductive strategies are urgently needed. Since the early demonstration in mammals that the medial basal hypothalamus (the socalled hypophysiotrophic area) is capable of autonomous function in maintaining gonadotrophin secretions of the pituitary (see Halasz, 1969 for review), there has been considerable interest in the existence of a homologous area in the avian hypothalamus. Recent deafferentation experiments in the Japanese quail (Nozaki, 1975; Nozaki et al, 1975; Follett and Davies, 1979) and turkey (Opel, 1979b) suggest that the avian hypothalamus is not capable of autonomous control of gonadotrophin secretion. Complete isolation of the tuberal hypothalamus from all neural imputs, without interfering with its connections to the ME and anterior pituitary, prevented entrance of the ovarian follicles into the phase of rapid growth. At present, we can only guess how the rostral and tuberal parts of the hypothalamus are functionally correlated to regulate gonadotrophin secretion. Both appear to be a source of gonadotrophin releasing hormone. The tuberal hypothalamus may be directly photosensitive to environmental light. Much additional research on the comparative characteristics of the two divisions of the hypothalamus and of their interconnecting neuronal circuitry might lead to advances in our understanding of control of tonic as well as cyclic secretion of pituitary gonadotrophins. Steroid Feedback and Control of Gonadotrophin Secretion. Under natural conditions, the synthesis and release of FSH and LH in the

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the hypothalamus prevents photostimulated growth of the ovary. In mature chickens (Ralph and Fraps, 1959b; Egge and Chiasson, 1963; Kanematsu et al, 1966; Sonoda et al, 1973), ducks (Novikov and Roudneva, 1964), turkeys (Opel, 1979a), quail (Stetson, 1972b), and pigeons (Bouille and Bayle, 1973), lesioning of either division results in regression of the ovary and oviduct. Evidence on localization of hypothalamic structures regulating tonic secretion of gonadot r o p i n s in the chicken, turkey, and quail are summarized in Figure 4. In the chicken (Ralph, 1959; Ralph and Fraps, 1959b) and quail (Stetson, 1972b; Follett and Davies, 1979) the POR appears to be the effective locus in the rostral hypothalamus. Evidence on this point is not available in the turkey. Some disagreement seems to exist over involvement of specific parts of the tuberal hypothalamus in control and maintenance of the ovary, but this may be a consequence of technical problems. Not all investigators adequately explored the tuberal division; some did not present their data clearly in terms of stereotaxic coordinates or a topographical diagram of effective loci. Exploration of the basal tuberal region is beset by the problem of injury to the portal vessels of the ME which can mechanically stop the flow of LHRH to the pituitary. There seems to be general agreement that the INC, and the infundibular nucleus (IN) in particular, are sites regulating growth and functional maintenance of the ovary.

HYPOTHALAMUS AND FEMALE REPRODUCTION

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Steroid Sensitive Areas in the Hypothalamus. Few attempts have been made to find steroid sensitive areas of the brain associated with gonadotrophin secretion in the female. Ralph and Fraps (1960) reported that the microinjection of progesterone into the rostral and tuberal hypothalamus and in the caudal neostriatum (Fig. 5a) induced premature ovulation in the hen. The more anterior effective sites were within the POR — the region where lesions had been previously shown to block spontaneous or progesterone-induced ovulation (Ralph, 1959; Ralph and Fraps, 1959a). Injection of progesterone into the pituitary did not force an ovulation. Stetson (1972c) found that implants of crystalline estradiol into the POR-SOR, in the ventral INC, or in the adenohypophysis of Japanese quail (Fig. 5b) produced a "persistent estrous"-like syndrome comparable to that previously produced (Stetson, 1972b) by lesioning the POR—SOR or INC. Because gonadotrophin potency of the pituitary was not altered by the implants, Stetson suggested that estradiol feedback was not inhibitory (see Stetson, 1972c for explanation). Brain areas sensitive to steroid feedback can be localized in terms of affinity for and retention of steroid hormones. Martinez-Vargas et al. (1975, 1976) have demonstrated that neurons in the brain of the ring dove take up and retain 3 H-estradiol-17j3. Heavy concentrations of estradiol were found in nuclei of cells of the medial POR and the INC. Lesser concentrations were found in several other hypothalamic and extrahypothalamic sites. From this it is tempting to assume that the estradiol target sites in the hypothalamus are those regulating gonadotrophin secretion. As a matter of fact, Stumpf and Sar (1977) have amassed considerable autoradiographic and immunocytochemical evi-

FIG. 5. Diagrams of the brain of the chicken (a) and the hypothalamus of the quail (b) showing where direct application of an ovarian steroid affects ovulation. (a) •, microinjection of progesterone evoked premature ovulation; o, injection of progesterone had no effect. Modified from Ralph and Fraps (1960). (b) •, implant of estradiol resulted in persistent estrous-like syndrome; o, implant of estradiol had no effect. Modified from Stetson (1972c); H, hyperstriatum; INC, infundibular nuclear complex; N, neostriatum; OC, optic chiasma; P, paleostriatum; PIT, anterior pituitary; POR, preoptic region.

dence in mammals to suggest that steroid hormone target cells and LHRH-producing neurons occupy corresponding sites in the brain. However, the possibility cannot be excluded that in the dove the hypothalamic loci concentrating estradiol are target sites regulating reproductive behavior (Hutchison, 1976). Other recent evidence argues for a role of the hypothalamus in steroid feedback regulation of gonadotrophin secretion. Using an in vitro incubation system, Tanaka et al. (1976) reported that in the chicken the uptake of 3 Hprogesterone by tissues of the hypothalamus and adenohypophysis fluctuate during the

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female is regulated by the feedback action of ovarian steroids. Two types of feedback systems exist, one stimulating the release of gonadotropins and the other inhibiting their release. A variety of evidence shows that the positive feedback of ovarian steroids, primarily to the hypothalamus, evokes the ovulatory surge of LH (Fraps, 1961, 1970; Follett and Davies, 1979). The negative feedback of ovarian steroids, probably acting partly at the level of the hypothalamus and partly on the pituitary, regulates gonadotrophin secretion required for growth, maturation, and functional maintenance of the ovary (Follett, 1973; Hinde et al., 1974; Davies, 1976).

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A careful study by Williams and Sharp (1978) shows that, in chickens held under 14 hr of light per day, minor surges in plasma LH, progesterone, and testosterone are initiated at about the beginning of the dark period (Fig. 7). They proposed that an increase in blood levels of LH at the onset of darkness stimulates the

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maturing ovarian follicles to secrete progesterone and androgens. If the largest ovarian follicle is mature, the increase in LH at the onset of darkness stimulates the secretion of enough progesterone (and possibly androgen) to trigger the ovulatory surge of LH. Although estrogen feedback does not appear to be involved in initiating the ovulatory LH surge, experiments in the ovariectomized chicken (Wilson and Sharp, 1975) suggest that the positive feedback of steroids in evoking the LH surge is a two-phase process. The first, or priming phase, depends on priming of the hypothalamic LH release mechanism with both estrogen and progesterone. The second, or inductive phase, involves only an incremental change in progesterone. Steroid Feedback and Ovarian Growth. In sexually immature birds, long daylength directly stimulates gonadotrophin secretion (Davies et al., 1976). In the female, this is held in check until shortly before sexual maturity by the feedback of ovarian steroids. Ovariectomy, particularly under long days, leads to greatly increased gonadotrophin levels in the chicken (Sharp, 1973), quail (Gibson et al, 1975), and canary (Hinde et al, 1974). Estrogen treatment reduces LH secretion in ovariectomized birds

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ovulatory cycle. Kawashima et al. (1978) found that the cytosol fraction of the hypothalamus and anterior pituitary of the chicken contains a progesterone-binding component, indicating the presence of cytoplasmic receptors. In the chicken, the sensitivity of the pituitary gland does not increase at the time of the preovulatory LH surge (Bonney et al, 1974) and the effect of progesterone in inducing the LH surge can be blocked by an antiserum to LHRH (Fraser and Sharp, 1978). Other Aspects of Steroid Feedback and the Ovulatory LH Surge. With the development of radioimmunoassays for avian steroids and gonadotrophins, it is possible to quantitate changes in blood levels of these hormones in relation to the ovulatory cycle. This has provided a wealth of information concerning both the particular ovarian steroids involved in regulation of the ovulatory LH surge and the temporal relationships in the neuroendocrine control of ovulation. This information was recently reviewed for the first time by Follett and Davies (1979). Those data especially relevant to the present paper will be briefly outlined here. The positive feedback component regulating the ovulatory surge of LH matures at about the time the hen enters lay (Wilson and Sharp, 1975). The LH surge is restricted to a period of about 7 hr each day which is set by a lightentrained biological clock (Follett and Davies, 1979; Morris, 1979). Numerous reports in the chicken (Follett and Davies, 1979) and single reports in the turkey (Wentworth et al, 1976) and quail (Follett and Davies, 1979) show that a single surge of LH in peripheral blood rises above baseline levels for about 6 hr, reaches a peak 4 to 5 hr before ovulation, and is absent on days of no ovulation. In chickens (Follett and Davies, 1979), the LH surge is accompanied by a surge of progesterone, testosterone, and corticosterone, but not by a surge of estradiol or estriol. Figure 6 shows typical preovulatory surges of LH and progesterone in a regularly laying hen. Evidence in the turkey (Opel and Arcos, 1978) suggests that a preovulatory surge of progesterone takes place at about the time of the preovulatory LH surge.

HYPOTHALAMUS AND FEMALE REPRODUCTION

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clear. In the chicken (Sharp, 1975; Williams and Sharp, 1977) and turkey (Godden and Scanes, 1977) sexual maturation, as expressed by development of secondary sexual characteristics, is associated with a steep rise in gonadotrophin secretion. This rise may be due to increased sensitivity of the pituitary to LHRH when concentrations of circulating steroids are still quite low (Godden and Scanes, 1977; Bonney et al., 1974; Peterson and Webster, 1974; Senior, 1974). In the maturing chicken and turkey, plasma LH levels decline from the prepuberal maximum over a 2 to 3 week period until the first egg is laid. Since the rapidly growing follicles, which are known to be steroidogenic (Furr, 1969), develop during this time, the decline may be due to increased steroid secretion. At the final stage of follicular growth, when follicles begin to acquire the ability to ovulate in response to LH, the sensitivity of the pituitary to synthetic LHRH falls and the positive feedback system controlling the ovulatory surge of LH matures (Wilson and Sharp, 1975). The decreased sensitivity of the pituitary to LHRH is not due to an increased feedback action of estrogen, because estrogen levels are falling during the 2 to 3 weeks before the onset of lay. Wilson and Sharp suggested that the final priming of the positive feedback mechanism may result from a changing ratio of circulating levels of estrogens and progesterone.

o Q.

FIG. 7. Changes in blood levels of LH (broken line), progesterone (solid line) and androgen (dotted line) in a hen at about the time when the preovulatory surge of LH for the first ovulation of a sequence is initiated. The solid bar indicates the dark period. Modified from Williams and Sharp (1978).

(Hinde et al, 1974). According to Davies (1976) estrogen feedback acts mainly on the pituitary, possibly by altering pituitary sensitivity to LHRH. Subcutaneous implantation of estrogen reduces the release of LH induced by electrical stimulation of the INC (Davies, 1976), indicating that part of the negative feedback of estrogen is exerted on the hypothalamus. The role of positive and negative steroid feedback in maturation of the ovary is not

SUMMARY Recent evidence confirms and expands earlier reports showing that the rostral hypothalamus controls ovulation and that the ovulating surge of LH is regulated by a feedback of ovarian hormones. Contradictory experimental results do not permit any definite conclusions regarding the exact hypothalmic loci involved. Knowledge of hypothalamic mechanisms regulating growth and maintenance of ovarian follicles is still sparse and largely inferential. The controversy regarding the location of neurons producing gonadotrophin releasing hormones appears to be resolved. LHRH-producing systems have been found in both the rostral and tuberal divisions of the hypothalamus. Evidence in Japanese quail indicates that the long-sought deep hypothalamic photoreceptor resides in the infundibular nuclear complex. REFERENCES Assenmacher, I., 1973. Reproductive endocrinology:

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OPEL Egge, A. S., and R. B. Chiasson, 1963. Endocrine effects of diencephalic lesions in the White Leghorn hen. Gen. Comp. Endocrinol. 3:346—361. Everett, J. W., 1964. Preoptic stimulative lesions and ovulation in the rat: "Thresholds" and LH-release time in late diestrous and proestrus. Page 346— 366 in Major problems in neuroendocrinology. A. Bajusz and M. Jasmin, ed. William and Wilkins, Baltimore. Everett, J. W., 1969. Neuroendocrine aspects of mammalian reproduction. Ann. Rev. Physiol. 31:383— 416. Follett, B. K., 1969. Effects of neurohypophyseal hormones and their synthetic analogues on lower vertebrates. Page 321—350 in International encyclopaedic of pharmacology and therapeutics. H. Heller and B. T. Pickering, ed. Pergamon Press, Oxford. Follett, B. K., 1973. The neuroendocrine regulation of gonadotrophin secretion in avian reproduction. Page 209—243 in Breeding biology of birds. D. S. Farner, ed. Nat. Acad. Sci., Washington, DC. Follett, B. K., and D. T. Davies, 1975. Photoperiodicity and the control of reproduction in birds. Page 199—224 in Avian physiology. M. Peaker, ed. Academic Press, New York. Follett, B. K., and D. T. Davies, 1979. The endocrine control of ovulation in birds. In Beltsville symposia in agricultural research, III. Animal Reproduction. H. W. Hawk, ed., Allanheld, Osmun and Co., Montclair, NJ (In press). Fraps, R. M., 1961. Ovulation in domestic fowl. Page 133-162 in Control of ovulation. C. A. Ville, ed. Pergamon Press, New York. Fraps, R. M., 1970. Photoregulation in the ovulation cycle of the domestic fowl. Page 281 — 306 in La photoregulation de la reproduction chez les oiseaux et les mamiferes. CNRS, Paris. Fraser, H. M., and P. J. Sharp, 1978. Prevention of positive feedback in the hen (Gallus domesticus) by antibodies to luteinizing hormone releasing hormone. J. Endocrinol. 76:181—182. Furr, B. J. A., 1969. Identification of steroids in the ovaries and plasma of laying hens and the site of production of progesterone in the ovary. Gen. Comp. Endocrinol. 13:506. Gibson, W. R., B. K. Follett, and B. Gledhill, 1975. Plasma levels of luteinizing hormone in gonadectomized Japanese quail exposed to short or long day lengths. J. Endocrinol. 6 4 : 8 7 - 1 0 1 . Godden, P. M. M., M. R. Luck, and C. G. Scanes, 1977. Effect of LH-RH and steroids on the release of LH and FSH from incubated turkey pituitary cells. Acta Endocrinol. 85:713-717. Godden, P. M. M., and C. G. Scanes, 1977. Gonadotrophin concentrations during growth and maturation in domestic turkey. Brit. Poultry Sci. 18:675— 685. Gray, G. D., P. Sodersten, D. Tallentire, and J. M. Davidson, 1978. Effects of lesions in various structures of the suprachiasmatic-preoptic region on LH regulation and sexual behavior in female rats. Neuroendocrinology 25:174—191. Hala'sz, B., 1969. The endocrine effects of isolation of the hypothalamus from the rest of the brain. In Frontiers in neuroendocrinology. W. F. Ganong

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