Median eminence nitric oxide signaling

Brain Research Reviews 34 (2000) 27–41 www.elsevier.com / locate / bres

Full-length review

Median eminence nitric oxide signaling Vincent Prevot a , *, Sebastien Bouret a , George B. Stefano b , Jean-Claude Beauvillain a a

INSERM U 422, IFR 22, Neuroendocrinologie et physiopathologie neuronale, Place de Verdun, 59045 Lille, Cedex, France b Neuroscience Research Institute, State University of New York at Old Westbury, New York, NY 11568, USA Accepted 5 July 2000

Abstract It is becoming increasingly clear that nitric oxide (NO), an active free radical formed during the conversion of arginine to citrulline by the enzyme NO synthase (NOS), is a critical neurotransmitter and biological mediator of the neuroendocrine axis. Current evidence suggests that NO modulates the activity of both the hypothalamic-pituitary-gonadal axis and the hypothalamic-pituitary-adrenal axis. Supporting this hypothesis is the finding that the highest expression of neuronal NOS in the brain is found within the hypothalamus in areas where the cell bodies of the neurons from the different neuroendocrine systems are located. In this regard, the influence of neuronal NO on the regulation of the neuroendocrine neural cell body activity has been well-documented whereas little is known about NO signaling that directly modulates neurohormonal release into the pituitary portal vessels from the neuroendocrine terminals within the median eminence, the common termination field of the adenohypophysiotropic systems. Studies in rat suggest that NO is an important factor controlling both gonadotropin-releasing hormone (GnRH) and corticotropin-releasing hormone (CRH) release at the median eminence. The recent use of amperometric NO detection from median eminence fragments coupled to the use of selective NOS inhibitors demonstrated that a major source of NO at the median eminence might be endothelial in origin rather than neuronal. The present article reviews the recent progress in identifying the origin and the role of the NO produced at the median eminence in the control of neurohormonal release. We also discuss the potential implications of the putative involvement of the median eminence endothelial cells in a neurovascular regulatory process for hypothalamic neurohormonal signaling.  2000 Elsevier Science B.V. All rights reserved. Theme: Endocrine and autonomic regulation Topic: Neuroendocrine regulation: other Keywords: GnRH; LHRH; CRH; Nitric oxide; Median eminence; Hypothalamus; Endothelial cell

Contents 1. Introduction ............................................................................................................................................................................................ 2. NOS isoforms expressed at the median eminence....................................................................................................................................... 3. NO-donors and / or L-arginine affect neurosecretion at the median eminence ................................................................................................ 3.1. GnRH secretion .............................................................................................................................................................................. 3.2. CRH secretion ................................................................................................................................................................................ 3.3. Dopamine secretion ......................................................................................................................................................................... 4. NOS inhibitors reveal NO signaling pathway in the mediobasal hypothalamus affecting neuroendocrine secretion in the median eminence ...... 5. Direct measurement of NO release from median eminence fragments in vitro .............................................................................................. 5.1. Stimulated median eminence NO release affects GnRH and CRH secretions ....................................................................................... 5.2. NO produced at the median eminence affecting GnRH release is endothelial ....................................................................................... 5.3. Acute exposure of median eminence fragments to estradiol stimulates rapid endothelial NOS-derived NO release.................................. 6. NO target at the neuroendocrine nerve terminal ......................................................................................................................................... 7. NO and pulsatile GnRH secretion at the median eminence..........................................................................................................................

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*Corresponding author. Present address: ORPRC / OHSU, Division of Neuroscience, 505 NW 185th Avenue, Beaverton, OR 97006, USA. Tel.: 11-503-690-5305; fax: 11-503-690-5384. E-mail address: [email protected] (V. Prevot). 0165-0173 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0165-0173( 00 )00035-7

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8. NO and morphological plasticity .............................................................................................................................................................. 9. Physiological implication ......................................................................................................................................................................... Acknowledgements ...................................................................................................................................................................................... References...................................................................................................................................................................................................

1. Introduction The median eminence, both anatomically and physiologically, represents an interface between the hypothalamus and the anterior pituitary. This hypothalamic structure, arising from the differentiation of the floor of the third ventricle, is the common termination field for adenohypophysiotrophic systems. The median eminence is composed of the following: (1) an ependymal layer containing the cell bodies of tanycytes, median eminence specific ependymal cells that are stretched between the ventricular surface and the perivascular space of fenestrated capillaries that edge the external zone of the median eminence; (2) an internal zone or fiber layer, containing, among other structures, the axons of neurons projecting to the neural part of the pituitary; and (3) an external zone mainly composed of neuroendocrine nerve terminals, tanycytic processes and some astroglial cell bodies and processes. The adenohypophysiotrophic factors, or neurohormones, are released from the neuroendocrine nerve terminals located in the external zone of the median eminence into the fenestrated pituitary portal blood vessels. The median eminence, therefore, is in a central position to modulate hypothalamic neurosecretion. In this regard, only recently there have been any studies concerning the modulation of hypothalamic neurosecretion from a vascular perspective. Namely, can vascular signaling modulate hypothalamic neurosecretion? The present review will focus on this new intercellular communication process, expanding our original concepts concerned with hypothalamic regulation. Critical to our vascular-endothelial hypothesis is an understanding of nitric oxide (NO) signaling. Several lines of evidence suggested that NO, a gaseous intercellular messenger, may participate in the regulation of prehypophysiotrophic secretions at the median eminence. NO signaling appears to play a crucial role in the regulation of both hypothalamic-pituitary-gonadal axis and hypothalamic-pituitary-adrenal axis (for review see Ref. [13]). At the median eminence, NO affects both gonadotropin releasing hormone (GnRH) and corticotropin releasing hormone (CRH) release [24,45,56,67,75,77,78,85] that control luteinizing hormone (LH) / follicle stimulating hormone (FSH) and adrenocorticotropin (ACTH) release at the pituitary, respectively.

2. NOS isoforms expressed at the median eminence In the brain, under physiological conditions, NO is synthesized from L-arginine by two major nitric oxide

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synthase (NOS) isoforms: the neuronal NOS (nNOS), expressed in discrete neuronal populations [89], and the endothelial NOS (eNOS), expressed in the brain microvessel endothelium and the choroid plexus epithelial cells [63,91,93] and in discrete neuronal populations [27,68]. These two NOS isoforms are constitutively expressed and are calcium and calmodulin dependent enzymes [49]. One of the most highly expressing nNOS cell populations in the rat brain have been identified within the hypothalamus [14,89], suggesting that it exerts signaling activities. At the median eminence, strong nNOS immunoreactivity is detected in cell processes in the internal zone [22,39,107]. Double labeling studies showed that there was no overlapping distribution of nNOS and glial fibrillary acidic protein (GFAP) immunoreactivities within the median eminence, suggesting that nNOS is not present in glial and ependymal cell processes, but only in neuronal processes [39]. The eNOS immunoreactivity has not been extensively investigated at the level of the median eminence. Ceccatelli et al. [22] and Yamada et al. [107], the only reports, using an antibody recognizing both NOS isoforms [15], showed NOS immunolabeling at the pituitary portal blood vessel level. The use of a specific anti-eNOS monoclonal antibody revealed numerous eNOS immunoreactive cells in the vascular tissue (Fig. 1) and at the immediate proximity of the neuroendocrine nerve terminals of the external zone of the median eminence (Fig. 2). No immunoreactivity for eNOS is detected either in the subependymal layer or in the nervous parenchyma of the median eminence. It does occur in the portal blood capillaries loops that penetrate the median eminence parenchyma (Fig. 1).

3. NO-donors and / or L-arginine affect neurosecretion at the median eminence

3.1. GnRH secretion In vivo and in vitro studies using NOS inhibitors showed that the synthesis and secretion of NO was necessary for the basal secretion of GnRH / LH in the male rat [87] and crucial for the occurrence of the GnRH / LH surge in ovariectomized rats treated consecutively with estradiol and progesterone [9,10]. Intracerebroventricular injection of L-arginine (Arg), the NO precursor, stimulated LH secretion with peak values attaining the range normally seen on proestrus, in ovariectomized female rats treated with estradiol [8]. This last observation suggests that NO secretion by the hypothalamus has a progesterone-like effect on GnRH / LH secretion in the female rat. The key

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role of hypothalamic NO secretion influencing the preovulatory GnRH / LH surge is strengthened by the fact that intracerebroventricular injection of NOS antisense oligonucleotides suppresses the occurrence of the LH surge in ovariectomized female rats treated both with estradiol and progesterone [1]. At the median eminence, sensus stricto, different in vitro studies using either L-Arg or NO donors or NOS inhibitors showed that NO stimulates GnRH release from GnRH neuroendocrine terminals [10,67,85]. Recently Kohsaka et al. [51], infusing L-Arg in the median eminence arcuate nucleus complex and measuring GnRH release by push pull perfusion, showed that NO was able to stimulate GnRH release from rat median eminence in vivo, thus supporting the results of all the previous in vitro studies noted with median eminence-arcuate nucleus explants.

3.2. CRH secretion The effect of NO-donors, like sodium nitroprusside, on CRH secretion from hypothalamus explants, i.e., fragments containing both CRH cell bodies and nerve terminals, is not clear. Costa et al. [24] showed that the incubation of hypothalami with NO-donors or L-Arg does not alter basal CRH release, while Karanth et al. [45] and Raber et al. [78] showed that sodium nitroprusside stimulates CRH release within 20–30 min after its addition on hypothalamic explants. In part, this variation may be due to the instability of sodium nitroprusside, in that it releases NO rapidly before it can contact the tissue. We recently showed that in median eminence fragments, containing the CRH neuroendocrine terminals but not the CRH cell bodies, NO-donors, i.e., SNAP which releases NO slowly, stimulate CRH release within minutes after their addition to the incubation medium [77]. In this regard, the apparent contradiction also may be related to the sensitivity of the techniques as well as the time points in the observation. For example, we monitored NO-coupling to CRH release in real-time, and demonstrated that this is a rapid process, occurring within 10 min [77]. Thus, NO seems to have a direct and rapid stimulatory effect on CRH release at the nerve terminals. The stimulatory effect of NO on CRF release is supported by the recent work of Lee et al. [55] demonstrating in intact rats that intracerebroventricular administration of SN-1, a NO donor, significantly increases plasma ACTH, and that antibodies against CRF abolished that effect.

3.3. Dopamine secretion Intracerebroventricular injection of sodium nitroprusside decreases in a dose-dependent manner the tyrosine hydroxylase activity of the median eminence and thus inhibits the tuberoinfundibular dopaminergic neurons [35]. This decreased tyrosine hydroxylase activity is associated with a significant increase in serum prolactin levels [35].

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This finding is consistent with the fact that subcutaneous injection of NOS inhibitors in the female rat suppresses the prolactin surge on proestrus [9], as with the fact that systemic administration of L-Arg stimulates prolactin secretion in humans [80].

4. NOS inhibitors reveal NO signaling pathway in the mediobasal hypothalamus affecting neuroendocrine secretion in the median eminence In 1992, Rettori et al. [87] using N G -monomethyl-Larginine (NNMA), a NOS inhibitor, demonstrated that the prostaglandin E2-induced GnRH release by norepinephrine at the mediobasal hypothalamus occurred via NO release. They also showed that NO signaling at the mediobasal hypothalamus is involved in the control of GnRH release by GABA [92], Leptin [108,109] and oxytocin [86] and in the control of CRH release by interleukin (IL)-2 [45]. The stimulatory effect of IL-2 on CRH release mediated via NO secretion at the hypothalamus was confirmed later by Raber et al. [78]. By the same approach, Bonavera et al. [10] showed that the stimulatory effect of NMDA on GnRH release in vitro from median eminence-arcuate nucleus fragments was also mediated by NO release. Even though these experiments demonstrate that at the mediobasal hypothalamus NO secretion is involved in the control of neurohormonal secretion from median eminence nerve terminals, there is no direct evidence that NO signaling occurs locally at the median eminence or that these substances affect NO release directly.

5. Direct measurement of NO release from median eminence fragments in vitro There are many methods for measuring direct NO release from tissues. Until recently, only two studies attempted to measure NO release directly from incubated mediobasal hypothalamus explants (see Ref. [16]). One group [16] measured the conversion of [ 14 C] arginine incubated with tissue into [ 14 C] citrulline. The conversion of [ 14 C] arginine to [ 14 C] citrulline is stoichiometric with the formation of NO [16], providing an index of NOS activity. Using this method, Seilicovich et al. [92] and Canteros et al. [21] failed to detect a direct effect of GABA and norepinephrine on NO release, respectively, whereas when they determined NOS activity by incubating the treated hypothalami after homogenization with [ 14 C] arginine and cofactors [17], they succeeded in measuring a 3-fold increase in [ 14 C] citrulline / NO production when compared to the norepinephrine untreated hypothalami [21]. The authors concluded that norepinephrine does not directly stimulate NO release but induces an increase of the NOS content in the tissue. Using a similar approach

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Fig. 1.

Fig. 2.

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Fig. 3. Direct measurement of NO release from median eminence fragments using a NO-specific amperometric probe. Physiological NO release was evoked from median eminence fragment via the stimulation of the endogenous m 3 opiate receptor and the cannabinoid type 1 receptor by micromolar concentrations of morphine (A), (B) or anandamide (C) respectively [77]. (A) Micromolar morphine is able to stimulate NO release from median eminence fragments within minutes of its application, and this effect is antagonized by micromolar naloxone, a m opioid receptor antagonist. (B) The antagonism of morphine-stimulated NO production by median eminence fragments by L-NAME, a NOS inhibitor, and naloxone over time. (C) The antagonism of anandamide-stimulated NO production by median eminence fragments by L-NAME and SR 141716A, a selective cannabinoid type I receptor inhibitor, over time. Note that morphine (A,B) stimulates greater peak NO levels than anandamide (C). (From Ref. [77], with permission).

Fig. 1. Photomicrograph of a frontal section of the median eminence (ME) showing a dense eNOS-immunoreactivity (Texas Red fluorescence) within the pituitary portal vessel bed (arrows) in the external zone of the median eminence. Note that some eNOS-immunoreactivity is found in the parenchyma of the median eminence (arrowheads). This latter immunoreactivity might be located within the internal plexus capillary loops that arise from the external plexus and arborize in the subependymal zone. Some eNOS-immunoreactivity is also observed in capillaries of the arcuate nucleus (AN; little arrowheads). 3V, third ventricle. Magnification3200. Wistar rats (body weight 250–300 g) were decapitated and the brain was rapidly removed and frozen on dry ice and 12 mm-thick frontal sections were cut on a cryostat, mounted on chromealum / gelatine-coated slides and briefly air-dried. The tissue sections were fixed by immersion of the slides into 2208C 100% ethanol for 20 min. The sections were then rinsed and processed for indirect immunofluorescence as described earlier [73,76], i.e., incubated with a monoclonal anti-eNOS antibody (1:100, clone [38620, Transduction Laboratories, France) overnight at room temperature, rinsed, incubated 1 h with biotinylated goat anti-mouse antibody (1:200, Vector, France), rinsed, incubated 1 h with a streptavidin Texas Red (1:50, Amersham, France), rinsed, mounted and examined on a Leica Microscope DMRB. Control sections were treated identically except that the primary antibody was omitted. The specificity of the eNOS antibody was demonstrated commercially. Fig. 2. Photomicrograph showing GnRH fibers containing FITC immunofluorescence (arrows) in close apposition to the eNOS-immunoreactive external plexus capillaries (Texas Red staining, arrowheads) in the external zone of the median eminence. Magnification3400. The same experimental protocol was followed as described in Fig. 1 for the double-immunofluorescent labeling except that the sections were incubated with both mouse anti-eNOS (1:100) and rabbit anti-GnRH (1:1000, provided by Dr. G. Tramu, Bordeaux, France). The GnRH immunoreactivity was revealed with an FITC-labeled goat anti-rabbit IgG (1:200 Amersham, France). The specificity of the GnRH antibody has been described [76]. Note that due to the fixation of the tissues with ethanol the GnRH immunoreactivity is very weak when compared to the one obtained with the same antibody on paraformaldehyde-fixed sections [73,76]. 3V, third ventricle.

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Bhat et al. [6] showed that NOS activity in mediobasal hypothalamic fragments was increased after 30 min incubation with a glutamate NMDA receptor agonist. Direct and real-time measurement of NO release from median eminence fragments was only performed recently by our group using a NO-specific amperometric probe [75,77,99]. This method, previously described by others [57,95,101], allows for the measurement of the concentration of NO gas in solution in real time with computer data acquisition. With this approach we demonstrated, for the first time, that activation of specific receptors expressed at the median eminence was able to stimulate a rapid and short burst of NO release, occurring within 10 min and in the 10–30 nM range, from median eminence fragments. This stimulated constitutive NO release exerts specific physiological alterations, demonstrating that NO signaling actually occurs in this hypothalamic area (Fig. 3). Indeed, activation of the m3 opiate receptor and cannabinoid type 1 receptor (CB1-R) by micromolar concentrations of morphine and anandamide, respectively, stimulated NO release from median eminence fragments within seconds after addition of these drugs to the medium [77]. The NO release lasted for 10 min and then returned to basal values (Fig. 3B and C). Interestingly, morphine stimulates greater peak NO levels than anandamide, suggesting that the amount of NO release could be different among the transduction pathways activated [96].

5.1. Stimulated median eminence NO release affects GnRH and CRH secretions By evaluating GnRH and CRH levels in the medium after each experiment, it appeared that the activation of the m3 opiate receptor, i.e., lack of opioid peptide effect, by morphine stimulated both GnRH and CRH release from median eminence fragments within 10 min, while the activation of the CB1-R by anandamide resulted only in GnRH release, and was unable to affect CRH release [77]. The morphine stimulated GnRH and CRH release and the anandamide stimulated GnRH release occurred via NO secretion since the effect of these two drugs on neurohormone release was abolished by addition in the medium of N√ -nitro-L-arginine methyl ester ( L-NAME), another NOS inhibitor. The differential effect of m3 opiate receptor and CB1-R pathway activation on acute CRH release could be linked to the different amount of NO released. In a second study, we showed that IL-10, cytokine that has the potential for affecting neuroendocrine and immune interactions and that was known to stimulate corticotrophin (ACTH) release [43,79], stimulates median eminence NO release in such amounts that it induces CRH secretion from median eminence explants [99]. Together, our studies not only show that NO signaling actually occurs in the median eminence but also that this NO signaling affects neuroendocrine secretions. Given this, it was still uncertain at that time where the NO release was originating from in the

median eminence since both the n- and e-NOS isoforms are constitutively expressed in this hypothalamic structure.

5.2. NO produced at the median eminence affecting GnRH release is endothelial When the anatomical distribution of nNOS immunoreactivity is compared to the distribution of GnRH immunolabeling in the median eminence, it strikingly appears that nNOS fibers and GnRH fibers are distributed separately in the internal and external zones, respectively [39]. This demonstrates that nNOS producing fibers are distant from the GnRH nerve terminals, making it improbable that these NO containing structures affect GnRH secretion. Conversely, constitutive eNOS is only expressed at the median eminence in the endothelial cells of the pituitary portal blood vessels, as shown in Fig. 1 and suggested by the anatomical study of Yamada et al. [107]. The portal blood vessel endothelial cells are located at the immediate proximity of the GnRH neuroendocrine terminals [47,50,76], and could thus easily modulate GnRH release via NO release. The hypothesis of the implication of median eminence endothelial cells in the regulation of neurosecretions is strengthened by the findings that median eminence NO release is stimulated by the activation of m3 opiate receptor and CB1-R present on this cell type [77]. Indeed, cannabinoids appear to have no binding site on the neuronal material of the ME [40,41] whereas CB-1R has been found in rat and human endothelial cells [26]. Furthermore, m3 opiate receptors have also been found on endothelial cells where they are coupled to NO release [18,94,95]. The endothelial origin of NO secreted from median eminence fragment is also consistent with the work of Aguan et al. [1], showing that central administration of eNOS antisense is more efficacious than nNOS antisense in suppressing the estradiol / progesterone-induced GnRH / LH surge in ovariectomized female rats.

5.3. Acute exposure of median eminence fragments to estradiol stimulates rapid endothelial NOS-derived NO release The endothelial origin of the NO generated at the median eminence and affecting neuroendocrine secretions is also supported by the recent discovery that acute exposure of median eminence fragments to 17b-estradiol stimulates GnRH release via endothelial NO secretion [75]. The 17b-estradiol-stimulated NO release was found to occur through the activation of a membrane estrogen receptor that appears to be located on portal blood endothelial cells. Indeed, physiological concentrations of 17bestradiol stimulates NO release within seconds after its addition to the median eminence fragments, and 17bestradiol conjugated to bovine serum albumin (E 2 -BSA), an impermeable cell membrane estradiol analog, also stimulates NO release. These results indicate that the

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Fig. 4. Schematic representation of estradiol coupling to endothelial NOS via a membrane receptor located on the vascular endothelial cells of the median eminence, resulting in a stimulation of GnRH release into the pituitary portal blood vessels. Once secreted, the endothelial NO may stimulate GnRH release either directly by activating the soluble guanylyl cyclase (SGC) [56,67] and / or the cyclooxygenase (COX) [87] within the GnRH nerve terminals, or indirectly by targeting the COX of the median eminence glial cells (Tan) [69]. While the SGC converts GTP into cGMP, COX converts arachidonic acid (AA) into prostaglandin E 2 (PG). Both cGMP and PG stimulate GnRH release [56,67,69,87]. E, endothelium; E2, 17b-estradiol; E2-BSA, 17b-estradiol conjugated to bovine serum albumin; ecNOS, endothelial constitutive nitric oxide synthase; ER, estrogen receptor; nNOS, neuronal nitric oxide synthase; Tan, tanycytes.

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estrogen receptor (ER) is located on the plasma membrane of the NOS-containing cells. We further demonstrated that both 17b-estradiol and E 2 -BSA succeeded in stimulating GnRH secretion via NO release, since estradiol-stimulated GnRH release was antagonized by L-NAME and inhibited by hemoglobin, a NO scavenger. Interestingly, L-N 5 -(1iminoethyl)ornithine (L-NIO), a more potent inhibitor of eNOS than nNOS [66,84], succeeded in inhibiting estradiol-stimulated GnRH release at a concentration known to inhibit eNOS selectively, strongly suggesting that the estrogen membrane receptor-NO coupling occurs in the median eminence endothelial cells (Fig. 4). This conclusion is supported by another recent study that demonstrates that estradiol stimulates eNOS derived NO release from endothelial cells via intracellular calcium transients coupling and that the estrogen receptor is found on the cell membrane of endothelial cells in culture [97]. In both studies [75,97], intriguingly, tamoxifen, a nuclear estrogen receptor (ER) antagonist, succeeded in inhibiting the estradiol-stimulated NO release as the signal transduction occurred through an estrogen membrane receptor. This result is in accordance with the emergent evidence that tamoxifen, or ICI-182,780, another ER antagonist, could cause both nuclear and membranous ERs antagonism [23,34,46,83,98] via interaction with the ER hormone binding domain [65]. In fact, in our study the antagonism of the estradiol-stimulated NO release by tamoxifen shows that this effect is actually mediated via a ER on the surface of the cell [75]. Median eminence endothelial cells express ER [54]. Razandi et al. [83] showed that ERa and ERb cDNAexpressing cells present both membrane and nuclear receptors. In another of our studies on cultured human endothelial cells, we demonstrated that the estradiol-stimulated NO release was preceded by 40 s by a rapid intracellular calcium transient within 6 s of nanomolar 17b-estradiol exposure to these cells, and this intracellular calcium transient was crucial for the activation of the eNOS [97] that is a calcium-dependent enzyme [17,25,31]. These results are consistent with those of Goetz et al. [34] and Kim et al. [46]. As shown by Razandi et al. [83] membrane ER is coupled to protein G activation, which stimulates inositol phosphate hydrolysis through the activation of phospholipase. We surmise the increase in IP3 generation might induce intracellular calcium mobilization

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and thus lead to the activation of the constitutive eNOS. Goetz et al. [34] showed that after estradiol addition on endothelial cells, the transient rise in intracellular calcium concentration induces within minutes the eNOS translocation from the membrane to intracellular sites close to the nucleus, a process that functionally activates eNOS. Interestingly, on more prolonged exposure to estradiol, most of the eNOS returns to the membrane. Altogether, these data strongly suggest that even if estradiol can have a long-term stimulatory effect on eNOS expression and activity via protein synthesis [38,63,102], it appears likely that at the median eminence estradiol might affect GnRH release via an acute effect on endothelial NO release. This leads to a new concept in neuroendocrinology, i.e., the implication of the vascular endothelium in the modulation of neural processes [94], e.g., as in median eminence neuroendocrine secretions [77].

6. NO target at the neuroendocrine nerve terminal Soluble guanylyl cyclase, a heterodimeric protein, is a major target for NO in the central and peripheral nervous system [2,48]. The NO recognition site in guanylyl cyclase is a heme moiety to which NO binds with high affinity. Soluble guanylyl cyclase activation may be due to conformational change in the enzyme protein induced by NO [100]. At the mediobasal hypothalamus, Canteros et al. [20] and Bhat et al. [5] showed that NO-donors, dosedependently, elevate cGMP levels. On immortalized neuronal cell lines secreting GnRH (GT1 cells), Moretto et al. [67] and Lopez et al. [56] showed that the NO-induced GnRH release was blocked by the addition in the culture medium of Rp-8-Br-cGMPS, a cGMP analog that blocks cGMP-dependent protein kinase, and thus demonstrates that NO stimulates GnRH secretion by activating guanylyl cyclase. At the median eminence, we showed that 1H[1,2,4]oxadiazolo[3,4a ]quinoxalin-1-one (ODQ), a potent and selective inhibitor of NO-sensitive guanylyl cyclase [33], suppresses the acute stimulatory effect of 17b-estradiol-on GnRH release (75). The 17b-estradiol-stimulated NO release may initiate activation of the soluble guanylyl cyclase in GnRH nerve endings that results in cGMP production (Fig. 4), and thus, in a depolarization involving a cationic conductance [30] that leads to GnRH

Fig. 5. NO is an attractive signaling molecule that could induce and / or participate in the morphological changes of the external zone of the median eminence during the rat estrous cycle. These morphological plasticity leads to neurovascular contact for GnRH nerve terminals on the day of proestrus, and may result from tanycytic processes withdrawal and / or GnRH nerve terminals sprouting and / or endothelium outgrowth (see Fig. 6). (A), (B) Electron micrograph of the median eminence external zone showing an example of neurovascular physical contact (arrows) for a GnRH nerve terminal observed on two serial sections ((A) and (B)). Contacts between GnRH nerve terminals (arrowheads) and the pericapillary space (p.s.) are only observed on the day of proestrus in female rats and are surmised to facilitate the GnRH release into the pituitary portal vessels, and thus the occurrence of the preovulatory GnRH / LH surge [74,76]. (C) Diagram illustrating the relationship between the parenchymatous basal lamina (p.l.m.), delimiting the median eminence parenchyma from the pericapillary space, the tanycytes (tan) and the GnRH neuroendocrine terminals (GnRH) on diestrus (a) or on proestrus (b). e, endothelium; e.b.l., endothelial basal lamina; thin arrow, tanycytic and / or nerve terminals and / or endothelium plastic changes that may occur between each morphological state observed on proestrous (From Ref. [76], with permission).

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release [29]. At the median eminence, NO may also stimulate cyclooxygenase activity in neuroendocrine terminals [87] and / or in glia [69] resulting in the production of prostaglandin E2 (PGE2) (Fig. 4). Like soluble guanylyl cyclase, cyclooxygenases are heme-containing enzymes that can directly bind NO (see Ref. [32]), possibly resulting in an increase in enzyme activity. Increased PGE2 release would induce, in neuroendocrine nerve terminals, the depletion of intracellular calcium stores [72] and cAMP formation [70], which finally stimulates exocytosis of GnRH secretory granules [87].

7. NO and pulsatile GnRH secretion at the median eminence The immortalized GT1-1 cell line, secreting GnRH [64] in vitro, revealed that the pulsatile release of GnRH required to maximally sensitize gonadotrophs to GnRH stimulation, was an intrinsic characteristic of the GnRH neurons [53,61,103]. Indeed, basal secretion of GnRH from GT1 is pulsatile with a pulse frequency similar to that seen in castrated rat and mice [61,62]. Westel et al. [103] showed that in culture both gap junctions and synaptic contacts maintained the interaction within the neural network. This network organization allows clusters of cells to undergo secretion at the same time, and thus, synchronous GnRH release. Such physical contacts between GnRH neurons occur in vivo [104,105] but are unlikely to drive the pulsatile secretion of GnRH into the portal blood at the median eminence since they are rarely observed. Interestingly, it has been shown that GT1 cells express both NOS mRNA and protein [56,60] and that NO synthesis activity in these cells was involved in the genesis of pulsatile GnRH secretion in vitro [56]. In this regard, immortalized GnRH neurons express NOS whereas hypothalamic neurons do not [37,39]. In vivo, the observation that deafferentation of the mediobasal hypothalamus does not inhibit pulsatile release of LH [7,52], and that release of GnRH from hypothalamic explants is pulsatile [11,12] led to the concept that the mechanism synchronizing GnRH secretion resides within the mediobasal hypothalamus. These synchronizing events could even occur directly at the median eminence since Rasmussen [82] showed that median eminence explants were also able to secrete GnRH in a pulsatile mode. As hypothesized by Sakakibara et al. [88] for GT1 cells in vitro, the oscillations in cAMP levels within GnRH nerve terminals at the median eminence could constitute a biological clock for timing the pulsatile release of GnRH by individual fibers. NO gas, easily diffusible, could then be one of the key factors secreted locally leading to GnRH release synchronization from all or subpopulations of GnRH nerve terminals that are spread out the external zone of the median eminence that is 2.5 mm long in the rat [77]. According to our findings [75,77], and in agreement with

the results of others [1], the major source of median eminence NO that controls GnRH release may be endothelial. As shown in Fig. 2 and in previous studies [22,107], vascular endothelium expressing eNOS is located at the immediate proximity of GnRH nerve terminals in the external zone of the median eminence. The release of endothelial NO and its subsequent diffusion may allow large groups of GnRH nerve terminals to undergo release at the same time and thus would establish a synchronizing link to anatomically scattered GnRH nerve terminals. Indeed, according to dispersion studies, even with a halflife of few seconds, NO generated at a single point source should be able to influence function within a sphere with a diameter of approximately 0.3 mm (for review see Ref. [32]). Therefore, we surmise that endothelial NO is involved in the regulation of pulsatile and / or cyclic GnRH release and we further speculate that this vascular endothelial release is pulsatile and / or cyclic. Amperometric studies of endothelial NO secretion from median eminence fragments throughout the rat estrous cycle does demonstrate a cyclic NO secretory presence (Knauf C, Prevot V, Stefano GB, Beauvillain JC and Croix D, unpublished results). Interestingly in this study, NO secretion from median eminence fragments is significantly increased and maximal on proestrous, the day of the preovulatory GnRH / LH surge.

8. NO and morphological plasticity At the median eminence, we have recently demonstrated that dynamic morphological changes occurring in the external zone might facilitate GnRH release in the portal blood capillaries on the day of proestrus [74]. These morphological changes lead to actual physical contact between GnRH nerve terminals and the pericapillary space on proestrus, as hypothesized by Koslovski and Coates in 1985 [50]. This phenomenon implies both GnRH axon growth and simultaneous tanycytic process withdrawal as well as micro-endothelium outgrowth (Fig. 5). An attractive signaling candidate that could induce or participate in these plastic changes throughout the estrous cycle is NO. NO is able to induce rapid conformational changes in cultured neurohypophyseal astrocytes [81] as well as in vivo [3] and in endothelial cells [58]. Wu and Scott [106] and Scott et al. [90] showed that NO may serve as a second messenger molecule that may act in some fashion to govern the process of central regeneration and regrowth of magnocellular axons into the median eminence after hypophysectomy. After lesion, NOS expression was markedly increased in these neurons while nitroarginine, a competitive antagonist of NO synthesis successfully inhibited axonal regeneration. In line with the possible role of NO in axonal growth, recent studies have shown in the peripheral nervous system that after sciatic nerve ligature, upregulation of NOS expression in elongating fibers was

V. Prevot et al. / Brain Research Reviews 34 (2000) 27 – 41 Fig. 6. Schematic representation of the hypothetical implication of endothelial NO in the modulation of the morphological changes in the external zone of the median eminence depicted on Fig. 5. NO may be one of the factors produced locally in the median eminence on proestrous that promotes sprouting of the GnRH nerve terminals (blue arrow) by interacting for example, as suggested by the studies on nerve regeneration in the peripheral nerve system [4,36], with the growth associated protein-43 (GAP-43) expressed in GnRH nerve terminals [73]. NO may further participate in the neuro-glial remodeling of the external zone of the median eminence by inducing rapid conformational changes of tanycytes (Tan) [3,81] resulting in tanycytic endfeets withdrawal (orange arrow) and thus facilitating the access of GnRH nerve terminals to the portal vasculature. NO could also participate to endothelium outgrowth by causing conformational changes in the endothelial cells [58] of the portal blood capillaries (orange double arrow). E, endothelium. 37

38

V. Prevot et al. / Brain Research Reviews 34 (2000) 27 – 41

associated with an upregulation of the growth associated protein-43 in 80% of neurons in regeneration [4,36]. This supports the hypothesis that NO might contribute to the growth and / or guidance of injured axons in elongation [36]. By analogy, with the models of synaptic plasticity, NO is proposed to be the elusive retrograde messenger for long-term depression and long-term potentiation [42]. We surmise that at the external zone of the median eminence the amount of NO released by the endothelium may vary with the physiological state. This in turn would constitute a signal to promote GnRH nerve terminal sprouting toward the pericapillary space. In line with this speculation is the expression of growth associated protein-43 in adult GnRH nerve terminals and the variation of growth associated protein-43 mRNA in GnRH neurons throughout the rat estrous cycle [73]. In this context, it is tempting to speculate that NO may induce cytoskeletal reorganization in the median eminence external zone leading to tanycytic endfeet removal and / or to GnRH nerve terminal outgrowth, allowing the later to contact the pericapillary space on the day of proestrus, facilitating GnRH release into the portal blood capillaries (Fig. 6).

9. Physiological implication Taken together, our findings suggest that at the median eminence endothelial activity is modulated by circulating signal molecules, e.g., estradiol, morphine, cannabinoids etc., acting through endothelial membrane receptors that may be coupled to intracellular signaling. The activation of these receptors might lead to an increase in intracellular calcium concentrations that rapidly, i.e., seconds, stimulate eNOS, leading to a rapid synthesis and release of NO that in turn modulates the release of neurohormones in the pituitary portal vessels via diffusion within its sphere of influence. The involvement of the vascular endothelium in the regulation of neurohormone release, i.e., at the median eminence, represents a new concept in neuroendocrinology. Neurohormone release has been mainly considered to occur by way of neuron-to-neuron interactions and / or as a result of glial influences [71]. We further surmise that endothelial activity at the median eminence is required to synchronize the activity of neuroendocrine systems that need to modify their pattern of secretion during critical physiological stages, e.g., preovulatory GnRH secretion leading to ovulation on the afternoon of the day of proestrus, for the GnRH neuroendocrine axis; or during an external aggression, e.g., massive CRF release. In this last case, stimulation of endothelium by non-proinflammatory cytokines, i.e., IL-10, may induce NO production further enhancing CRF release [99]. During the estrous cycle, the increased levels of plasma estrogen on proestrus may stimulate endothelial NO release at the median eminence

and thus may facilitate a rapid and synchronized GnRH release from nerve terminals leading to the preovulatory GnRH / LH surge. This particular hypothesis is supported by the recent finding that the estrous cycle in endothelial NOS knockout female mice is significantly longer when compared to that of wild type mice [28,44]. Indeed, in addition to deficiencies occurring at the ovary level, this dysfunction in estrous cyclicity, may be due to, at least in part, the absence of eNOS in the portal blood vessel endothelium. Otherwise, one may speculate that such an endothelio-neuro-glial communication process is not restricted to the median eminence area, but is also occurring in other brain areas where eNOS is expressed in vessel endothelium. This last conjecture may indeed be the case. The modulation of neuronal secretory activity at the median eminence by endothelial NO raises the question of the specificity of the signaling event, i.e., how a signaling molecule conveyed in the portal blood to the median eminence can selectively stimulate the targeted neurohormone release via endothelial NO release? Our results appear to show that the selectivity of NO action could depend on the amount of NO released or the level of NO hitting the target tissue as shown for human monocytes and invertebrate microglia [96]. Thus, the threshold of sensitivity to NO appears to differ among the neuroendocrine system considered as well, e.g., the GnRH nerve terminals seem to be more sensitive to NO than are the CRF nerve terminals [77]. Conversely, a given amount of NO may affect, at the same time, different neuroendocrine systems, and may as a consequence facilitate the communication between them, e.g., between the GnRH neuroendocrine axis and the CRF neuroendocrine axis, i.e., the reproductive and the stress / immune functions respectively [19,59]. In conclusion, the biomedical implications of vascularneuroendocrine communication are currently a matter for speculation. However, given this promising beginning, the possibility to acutely target the endothelium at the median eminence to inhibit or stimulate prehypophysiotrophic secretions will be important for future research exploration.

Acknowledgements This work was supported by INSERM (U422), the University of Lille II and the FEDER (LARC network), NIDA 09010, NIMH 47392 and NIH Fogarty INT 00045.

References [1] K. Aguan, V.B. Mahesh, L. Ping, G. Bhat, D.W. Brann, Evidence for a physiological role for nitric oxide in the regulation of the LH surge: effect of central administration of antisense oligonucleotides to nitric oxide synthase, Neuroendocrinology 64 (1996) 449–455. [2] W.P. Arnold, C.K. Mittal, S. Katsuki, F. Murad, Nitric oxide

V. Prevot et al. / Brain Research Reviews 34 (2000) 27 – 41

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

activates guanylate cyclase and increases guanosine 39:59-cyclic monophosphate levels in various tissue preparations, Proc. Natl. Acad. Sci. USA 74 (1977) 3203–3207. G.H. Beagley, P. Cobbett, Inhibition of nitric oxide synthase induces ultrastructural changes in the neurohypophysis of dehydrated rats, Neurosci. Lett. 222 (1997) 143–146. E. Bergman, K. Carlsson, A. Liljeborg, E. Manders, T. Hokfelt, B. Ulfhake, Neuropeptides, nitric oxide synthase and GAP-43 in B4binding and RT97 immunoreactive primary sensory neurons: normal distribution pattern and changes after peripheral nerve transection and aging, Brain Res. 832 (1999) 63–83. G. Bhat, V.B. Mahesh, K. Aguan, D.W. Brann, Evidence that brain nitric oxide synthase is the major nitric oxide synthase isoform in the hypothalamus of the adult female rat and that nitric oxide potently regulates hypothalamic cGMP levels, Neuroendocrinology 64 (1996) 93–102. G.K. Bhat, V.B. Mahesh, L. Ping, L. Chorich, V.T. Wiedmeier, D.W. Brann, Opioid-glutamate-nitric oxide connection in the regulation of luteinizing hormone secretion in the rat, Endocrinology 139 (1998) 955–960. C.A. Blake, C.H. Sawyer, Effects of hypothalamic deafferentation on the pulsatile rhythm in plasma concentrations of luteinizing hormone in ovariectomized rats, Endocrinology 94 (1974) 730–736. J.J. Bonavera, P.S. Kalra, S.P. Kalra, L-arginine / nitric oxide amplifies the magnitude and duration of the luteinizing hormone surge induced by estrogen: involvement of neuropeptide Y, Endocrinology 137 (1996) 1956–1962. J.J. Bonavera, A. Sahu, P.S. Kalra, S.P. Kalra, Evidence in support of nitric oxide (NO) involvement in the cyclic release of prolactin and LH surges, Brain Res. 660 (1994) 175–179. J.J. Bonavera, A. Sahu, P.S. Kalra, S.P. Kalra, Evidence that nitric oxide may mediate the ovarian steroid-induced luteinizing hormone surge: involvement of excitatory amino acids, Endocrinology 133 (1993) 2481–2487. ´ J.P. Bourguignon, P. Franchimont, Liberation intermittente de GnRH par l’hypothalamus de rat in vitro, C.R. Seances Soc. Biol. 175 (1981) 389–396. J.P. Bourguignon, P. Franchimont, Puberty-related increase in episodic LHRH release from rat hypothalamus in vitro, Endocrinology 114 (1984) 1941–1943. D.W. Brann, G.K. Bhat, C.A. Lamar, V.B. Mahesh, Gaseous transmitters and neuroendocrine regulation, Neuroendocrinology 65 (1997) 385–395. D.S. Bredt, C.E. Glatt, P.M. Hwang, M. Fotuhi, T.M. Dawson, S.H. Snyder, Nitric oxide synthase protein and mRNA are discretely localized in neuronal populations of the mammalian CNS together with NADPH diaphorase, Neuron 7 (1991) 615–624. D.S. Bredt, P.M. Hwang, S.H. Snyder, Localization of nitric oxide synthase indicating a neural role for nitric oxide, Nature 347 (1990) 768–770. D.S. Bredt, S.H. Snyder, Nitric oxide mediates glutamate-linked enhancement of cGMP levels in the cerebellum, Proc. Natl. Acad. Sci. USA 86 (1989) 9030–9033. D.S. Bredt, S.H. Snyder, Isolation of nitric oxide synthetase, a calmodulin-requiring enzyme, Proc. Natl. Acad. Sci. USA 87 (1990) 682–685. P. Cadet, T.V. Bilfinger, C. Fimiani, D. Peter, G.B. Stefano, Human vascular and cardiac endothelia express mu opiate receptor transcripts. Endothelium: J. Endothelial Cell Res. (2000) in press. A.E. Calogero, G. Bagdy, R. D’Agata, Mechanisms of stress on reproduction, Evidence for a complex intra-hypothalamic circuit, Ann. NY Acad. Sci. 851 (1998) 364–370. G. Canteros, V. Rettori, A. Franchi, A. Genaro, E. Cebral, A. Faletti, M. Gimeno, S.M. McCann, Ethanol inhibits luteinizing hormonereleasing hormone (LHRH) secretion by blocking the response of LHRH neuronal terminals to nitric oxide, Proc. Natl. Acad. Sci. USA 92 (1995) 3416–3420.

39

[21] G. Canteros, V. Rettori, A. Genaro, A. Suburo, M. Gimeno, S.M. McCann, Nitric oxide synthase content of hypothalamic explants: increase by norepinephrine and inactivated by NO and cGMP, Proc. Natl. Acad. Sci. USA 93 (1996) 4246–4250. [22] S. Ceccatelli, J.M. Lundberg, J. Fahrenkrug, D.S. Bredt, S.H. Snyder, T. Hokfelt, Evidence for involvement of nitric oxide in the regulation of hypothalamic portal blood flow, Neuroscience 51 (1992) 769–772. [23] Z. Chen, I.S. Yuhanna, Z. Galcheva-Gargova, R.H. Karas, M.E. Mendelsohn, P.W. Shaul, Estrogen receptor alpha mediates the nongenomic activation of endothelial nitric oxide synthase by estrogen, J. Clin. Invest. 103 (1999) 401–406. [24] A. Costa, P. Trainer, M. Besser, A. Grossman, Nitric oxide modulates the release of corticotropin-releasing hormone from the rat hypothalamus in vitro, Brain Res. 605 (1993) 187–192. [25] T.M. Dawson, S.H. Snyder, Gases as biological messengers: nitric oxide and carbon monoxide in the brain, J. Neurosci. 14 (1994) 5147–5159. [26] D.G. Deutsch, M.S. Goligorsky, P.C. Schmid, R.J. Krebsbach, H.H. Schmid, S.K. Das, S.K. Dey, G. Arreaza, C. Thorup, G. Stefano, L.C. Moore, Production and physiological actions of anandamide in the vasculature of the rat kidney, J. Clin. Invest. 100 (1997) 1538–1546. [27] J.L. Dinerman, T.M. Dawson, M.J. Schell, A. Snowman, S.H. Snyder, Endothelial nitric oxide synthase localized to hippocampal pyramidal cells: implications for synaptic plasticity, Proc. Natl. Acad. Sci. USA 91 (1994) 4214–4218. [28] D.L. Drazen, S.L. Klein, A.L. Burnett, E.E. Wallach, J.K. Crone, P.L. Huang, R.J. Nelson, Reproductive function in female mice lacking the gene for endothelial nitric oxide synthase, Nitric Oxide 3 (1999) 366–374. [29] S.V. Drouva, E. Laplante, J.P. Gautron, C. Kordon, Effects of 17 beta-estradiol on LH–RH release from rat mediobasal hypothalamic slices, Neuroendocrinology (1984) 38152–38157. [30] E.E. Fesenko, S.S. Kolesnikov, A.L. Lyubarsky, Induction by cyclic GMP of cationic conductance in plasma membrane of retinal rod outer segment, Nature 313 (1985) 310–313. [31] U. Forstermann, H. Kleinert, Nitric oxide synthase: expression and expressional control of the three isoforms, Naunyn Schmiedebergs Arch. Pharmacol. 352 (1995) 351–364. [32] J. Garthwaite, C.L. Boulton, Nitric oxide signaling in the central nervous system, Annu. Rev. Physiol. 57 (1995) 683–706. [33] J. Garthwaite, E. Southam, C.L. Boulton, E.B. Nielsen, K. Schmidt, B. Mayer Potent and selective inhibition of nitric oxide-sensitive guanylyl cyclase by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, Mol. Pharmacol. 48 (1995) 184–188. [34] R.M. Goetz, H.S. Thatte, P. Prabhakar, M.R. Cho, T. Michel, D.E. Golan, Estradiol induces the calcium-dependent translocation of endothelial nitric oxide synthase, Proc. Natl. Acad. Sci. USA 96 (1999) 2788–2793. [35] M.C. Gonzalez, E. Llorente, P. Abreu, Sodium nitroprusside inhibits the tyrosine hydroxylase activity of the median eminence in the rat, Neurosci. Lett. 254 (1998) 133–136. [36] T. Gonzalez-Hernandez, A. Rustioni, Nitric oxide synthase and growth-associated protein are coexpressed in primary sensory neurons after peripheral injury, J. Comp. Neurol. 404 (1999) 64–74. [37] A. Grossman, NO news is good news, Endocrinology 134 (1994) 1003–1005. [38] T. Hayashi, K. Yamada, T. Esaki, M. Kuzuya, S. Satake, T. Ishikawa, H. Hidaka, A. Iguchi, Estrogen increases endothelial nitric oxide by a receptor-mediated system, Biochem. Biophys. Res. Commun. 214 (1995) 847–855. [39] A.E. Herbison, S.X. Simonian, P.J. Norris, P.C. Emson, Relationship of neuronal nitric oxide synthase immunoreactivity to GnRH neurons in the ovariectomized and intact female rat, J. Neuroendocrinol. 8 (1996) 73–82. [40] M. Herkenham, A.B. Lynn, B.R. de Costa, E.K. Richfield, Neuronal

40

[41]

[42] [43]

[44]

[45]

[46]

[47]

[48]

[49] [50]

[51]

[52]

[53]

[54]

[55]

[56]

[57]

[58]

V. Prevot et al. / Brain Research Reviews 34 (2000) 27 – 41 localization of cannabinoid receptors in the basal ganglia of the rat, Brain Res. 547 (1991) 267–274. M. Herkenham, A.B. Lynn, M.R. Johnson, L.S. Melvin, B.R. de Costa, K.C. Rice, Characterization and localization of cannabinoid receptors in rat brain: a quantitative in vitro autoradiographic study, J. Neurosci. 11 (1991) 563–583. C. Holscher, Nitric oxide, the enigmatic neuronal messenger: its role in synaptic plasticity, Trends Neurosci. 20 (1997) 298–303. T.K. Hughes, P. Cadet, P.L. Rady, S.K. Tyring, R. Chin, E.M. Smith, Evidence for the production and action of interleukin-10 in pituitary cells, Cell. Mol. Neurobiol. 14 (1994) 59–69. A. Jablonka-Shariff, S. Ravi, A.N. Beltsos, L.L. Murphy, L.M. Olson, Abnormal estrous cyclicity after disruption of endothelial and inducible nitric oxide synthase in mice, Biol. Reprod. 61 (1999) 171–177. S. Karanth, K. Lyson, S.M. McCann, Role of nitric oxide in interleukin 2-induced corticotropin-releasing factor release from incubated hypothalami, Proc. Natl. Acad. Sci. USA 90 (1993) 3383–3387. H.P. Kim, J.Y. Lee, J.K. Jeong, S.W. Bae, H.K. Lee, I. Jo, Nongenomic stimulation of nitric oxide release by estrogen is mediated by estrogen receptor alpha localized in caveolae, Biochem. Biophys. Res. Commun. 263 (1999) 257–262. J.C. King, R.J. Letourneau, Luteinizing hormone-releasing hormone terminals in the median eminence of rats undergo dramatic changes after gonadectomy, as revealed by electron microscopic image analysis, Endocrinology 134 (1994) 1340–1351. R.G. Knowles, M. Palacios, R.M. Palmer, S. Moncada, Formation of nitric oxide from L-arginine in the central nervous system: a transduction mechanism for stimulation of the soluble guanylate cyclase, Proc. Natl. Acad. Sci. USA 86 (1989) 5159–5162. R.G. Knowles, S. Moncada, Nitric oxide synthases in mammals, Biochem. J. 298 (1994) 249–258. G.P. Koslowski, P.W. Coates, Ependymoneural specialisation between LHRH fibers and cells of the cerebroventricular system, Cell Tissue Res. 242 (1985) 301–311. A. Kohsaka, H. Watanobe, Y. Kakizaki, T. Suda, A comparative study of the effects of nitric oxide and carbon monoxide on the in vivo release of gonadotropin-releasing Hormone and neuropeptide Y from rat hypothalamus during the estradiol-induced luteinizing hormone surge: estimation by push–pull perfusion, Neuroendocrinology 69 (1999) 245–253. L.C. Krey, W.R. Butler, E. Knobil, Surgical disconnection of the medial basal hypothalamus and pituitary function in the rhesus monkey. I. Gonadotropin secretion, Endocrinology 96 (1975) 1073– 1087. L.Z. Krsmanovic, S.S. Stojilkovic, F. Merelli, S.M. Dufour, M.A. Virmani, K.J. Catt, Calcium signaling and episodic secretion of gonadotropin-releasing hormone in hypothalamic neurons, Proc. Natl. Acad. Sci. USA 89 (1992) 8462–8466. M.C. Langub Jr., R.E. Watson Jr., Estrogen receptor-immunoreactive glia, endothelia, and ependyma in guinea pig preoptic area and median eminence: electron microscopy, Endocrinology 130 (1992) 364–372. S. Lee, C.K. Kim, C. Rivier, Nitric oxide stimulates ACTH secretion and the transcription of the genes encoding for NGFI-B, corticotropin-releasing factor, corticotropin-releasing factor receptor type 1, and vasopressin in the hypothalamus of the intact rat, J. Neurosci. 19 (1999) 7640–7647. F.J. Lopez, M. Moretto, I. Merchenthaler, A. Negro-Vilar, Nitric oxide is involved in the genesis of pulsatile LHRH secretion from immortalized LHRH neurons, J. Neuroendocrinol. 9 (1997) 647– 654. H.I. Magazine, Detection of endothelial cell-derived nitric oxide: current trends and future directions, Adv. Neuroimmunol. 5 (1995) 479–490. H.I. Magazine, Y. Liu, T.V. Bilfinger, G.L. Fricchione, G.B. Stefano,

[59]

[60]

[61]

[62]

[63]

[64]

[65] [66]

[67]

[68]

[69]

[70]

[71]

[72]

[73]

[74]

[75]

Morphine-induced conformational changes in human monocytes, granulocytes, and endothelial cells and in invertebrate immunocytes and microglia are mediated by nitric oxide, J. Immunol. 156 (1996) 4845–4850. M.A. Magiakou, G. Mastorakos, E. Webster, G.P. Chrousos, The hypothalamic-pituitary-adrenal axis and the female reproductive system, Ann. NY Acad. Sci. 816 (1997) 42–56. P. Mahachoklertwattana, S.M. Black, S.L. Kaplan, J.D. Bristow, M.M. Grumbach, Nitric oxide synthesized by gonadotropin-releasing hormone neurons is a mediator of N-methyl-D-aspartate (NMDA)-induced GnRH secretion, Endocrinology 135 (1994) 1709–1712. G. Martinez de la Escalera, A.L. Choi, R.I. Weiner, Generation and synchronization of gonadotropin-releasing hormone (GnRH) pulses: intrinsic properties of the GT1-1 GnRH neuronal cell line, Proc. Natl. Acad. Sci. USA 89 (1992) 1852–1855. C. Masotto, A. Negro-Vilar, Gonadectomy influences the inhibitory effect of the endogenous opiate system on pulsatile gonadotropin secretion, Endocrinology 123 (1988) 747–752. A.M. McNeill, N. Kim, S.P. Duckles, D.N. Krause, H.A. Kontos, Chronic estrogen treatment increases levels of endothelial nitric oxide synthase protein in rat cerebral microvessels, Stroke 30 (1999) 2186–2190. P.L. Mellon, J.J. Windle, P.C. Goldsmith, C.A. Padula, J.L. Roberts, R. Weiner, Immortalization of hypothalamic GnRH neurons by genetically targeted tumorigenesis, Neuron 5 (1990) 1–10. M.E. Mendelsohn, R.H. Karas, Estrogen and the blood vessel wall, Curr. Opin. Cardiol. 9 (1994) 619–626. W.M. Moore, R.K. Webber, G.M. Jerome, F.S. Tjoeng, T.P. Misko, M.G. Currie, L-N 6 -(1-iminoethyl)lysine: a selective inhibitor of inducible nitric oxide synthase, J. Med. Chem. 37 (1994) 3886– 3888. M. Moretto, F.J. Lopez, A. Negro-Vilar, Nitric oxide regulates luteinizing hormone-releasing hormone secretion, Endocrinology 133 (1993) 2399–2402. T.J. O’Dell, P.L. Huang, T.M. Dawson, J.L. Dinerman, S.H. Snyder, E.R. Kandel, M.C. Fishman, Endothelial NOS and the blockade of LTP by NOS inhibitors in mice lacking neuronal NOS, Science 265 (1994) 542–546. S.R. Ojeda, H.F. Urbanski, M.E. Costa, D.F. Hill, M. MoholtSiebert, Involvement of transforming growth factor alpha in the release of luteinizing hormone-releasing hormone from the developing female hypothalamus, Proc. Natl. Acad. Sci. USA 87 (1990) 9698–9702. S.R. Ojeda, H.F. Urbanski, K.H. Katz, M.E. Costa, Stimulation of cyclic adenosine 39,59-monophosphate production enhances hypothalamic luteinizing hormone-releasing hormone release without increasing prostaglandin E2 synthesis: studies in prepubertal female rats, Endocrinology 117 (1985) 1175–1178. S.R. Ojeda, Y.I. Ma, B.J. Lee, V. Prevot, Glial-to-neuron signaling and neuroendocrine control of female puberty, Recent Progress in Hormone Research 55 (2000) 197–224. S.R. Ojeda, A. Negro-Vilar, Prostaglandin E2-induced luteinizing hormone-releasing hormone release involves mobilization of intracellular Ca 12 , Endocrinology 116 (1985) 1763–1770. V. Prevot, S. Bouret, D. Croix, G. Alonso, L. Jennes, V. Mitchell, A. Routtenberg, J.C. Beauvillain, GAP-43 mRNA expression in GnRH neurons during the rat estrous cycle, Endocrinology 141 (2000) 1648–1657. V. Prevot, D. Croix, S. Bouret, S. Dutoit, G. Tramu, G.B. Stefano, J.C. Beauvillain, Definitive evidence for the existence of morphological plasticity in the external zone of the median eminence during the rat estrous cycle: implication of neuro-glio-endothelial interactions in gonadotropin-releasing hormone release, Neuroscience 94 (1999) 809–819. V. Prevot, D. Croix, C.M. Rialas, P. Poulain, G.L. Fricchione, G.B. Stefano, J.C. Beauvillain, Estradiol coupling to endothelial nitric

V. Prevot et al. / Brain Research Reviews 34 (2000) 27 – 41

[76]

[77]

[78]

[79]

[80]

[81]

[82]

[83]

[84]

[85]

[86]

[87]

[88]

[89] [90]

[91]

[92]

oxide stimulates gonadotropin-releasing hormone release from rat median eminence via a membrane receptor, Endocrinology 140 (1999) 652–659. V. Prevot, S. Dutoit, D. Croix, G. Tramu, J.C. Beauvillain, Semiquantitative ultrastructural analysis of the localization and neuropeptide content of gonadotropin releasing hormone nerve terminals in the median eminence throughout the estrous cycle of the rat, Neuroscience 84 (1998) 177–191. V. Prevot, C.M. Rialas, D. Croix, M. Salzet, J.P. Dupouy, P. Poulain, J.C. Beauvillain, G.B. Stefano, Morphine and anandamide coupling to nitric oxide stimulates GnRH and CRF release from rat median eminence: neurovascular regulation, Brain Res. 790 (1998) 236– 244. J. Raber, G.F. Koob, F.E. Bloom, Interleukin-2 (IL-2) induces corticotropin-releasing factor (CRF) release from the amygdala and involves a nitric oxide-mediated signaling; comparison with the hypothalamic response, J. Pharmacol. Exp. Ther. 272 (1995) 815– 824. E.M. Rady Smith, P. Cadet, M.R. Opp, S.K. Tyring, T.K. Hughes Jr., Presence of interleukin-10 transcripts in human pituitary and hypothalamus, Cell Mol. Neurobiol. 15 (1995) 289–296. J.S. Rakoff, T.M. Siler, Y.N. Sinha, S.S. Yen, Prolactin and growth hormone release in response to sequential stimulation by arginine and synthetic TRF, J. Clin. Endocrinol. Metab. 37 (1973) 641–644. K.D. Ramsell, P. Cobbett, Nitric oxide induces morphological changes in cultured neurohypophysial astrocytes, J. Neuroendocrinol. 8 (1996) 235–240. D.D. Rasmussen, Episodic gonadotropin-releasing hormone release from the rat isolated median eminence in vitro, Neuroendocrinology 58 (1993) 511–518. M. Razandi, A. Pedram, G.L. Greene, E.R. Levin, Cell membrane and nuclear estrogen receptors (ERs) originate from a single transcript: studies of ERalpha and ERbeta expressed in chinese hamster ovary cells, Mol. Endocrinol. 13 (1999) 307–319. D.D. Rees, R.M. Palmer, R. Schulz, H.F. Hodson, S. Moncada, Characterization of three inhibitors of endothelial nitric oxide synthase in vitro and in vivo, Br. J. Pharmacol. 101 (1990) 746– 752. V. Rettori, N. Belova, W.L. Dees, C.L. Nyberg, M. Gimeno, S.M. McCann, Role of nitric oxide in the control of luteinizing hormonereleasing hormone release in vivo and in vitro, Proc. Natl. Acad. Sci. USA 90 (1993) 10130–10134. V. Rettori, G. Canteros, R. Renoso, M. Gimeno, S.M. McCann, Oxytocin stimulates the release of luteinizing hormone-releasing hormone from medial basal hypothalamic explants by releasing nitric oxide, Proc. Natl. Acad. Sci. USA 94 (1997) 2741–2744. V. Rettori, M. Gimeno, K. Lyson, S.M. McCann, Nitric oxide mediates norepinephrine-induced prostaglandin E2 release from the hypothalamus, Proc. Natl. Acad. Sci. USA 89 (1992) 11543–11546. H. Sakakibara, M. Conti, R.I. Weiner, Role of phosphodiesterases in the regulation of gonadotropin-releasing hormone secretion in GT1 cells, Neuroendocrinology 68 (1998) 365–373. E.M. Schuman, D.V. Madison, Nitric oxide and synaptic function, Annu. Rev. Neurosci. 17 (1994) 153–183. D.E. Scott, W. Wu, J. Slusser, A. Depto, S. Hansen, Neural regeneration and neuronal migration following injury. I. The endocrine hypothalamus and neurohypophyseal system, Exp. Neurol. 131 (1995) 23–38. B. Seidel, A. Stanarius, G. Wolf, Differential expression of neuronal and endothelial nitric.oxide synthase in blood vessels of the rat brain, Neurosci. Lett. 239 (1997) 109–112. A. Seilicovich, B.H. Duvilanski, D. Pisera, S. Theas, M. Gimeno, V. Rettori, S.M. McCann, Nitric oxide inhibits hypothalamic luteinizing hormone-releasing hormone release by releasing gamma-aminobutyric acid, Proc. Natl. Acad. Sci. USA 92 (1995) 3421–3424.

41

[93] A. Stanarius, I. Topel, S. Schulz, H. Noack, G. Wolf, Immunocytochemistry of endothelial nitric oxide synthase in the rat brain: a light and electron microscopical study using the tyramide signal amplification technique, Acta Histochem. 99 (1997) 411–429. [94] G.B. Stefano, Autoimmunovascular regulation: morphine and anandamide stimulated nitric oxide release, J. Neuroimmunol. 83 (1998) 70–76. [95] G.B. Stefano, A. Hartman, T.V. Bilfinger, H.I. Magazine, Y. Liu, F. Casares, M.S. Goligorsky, Presence of the mu3 opiate receptor in endothelial cells. Coupling to nitric oxide production and vasodilation, J. Biol. Chem. 270 (1995) 30290–30293. [96] G.B. Stefano, Y. Liu, M.S. Goligorsky, Cannabinoid receptors are coupled to nitric oxide release in invertebrate immunocytes, microglia and human monocytes, J. Biol. Chem. 271 (1996) 19238– 19242. [97] G.B. Stefano, V. Prevot, J.C. Beauvillain, T.V. Bilfinger, P. Cadet, C. Fimiani, I. Welters, G.L. Fricchione, Acute exposure of estrogen to human endothelia results in nitric oxide release mediated by an estrogen surface receptor coupled to intracellular calcium transients, Circulation 101 (2000) 1594–1597. [98] G.B. Stefano, V. Prevot, J.C. Beauvillain, C. Fimiani, I. Welters, P. Cadet, C. Breton, J. Pestel, M. Salzet, T.V. Bilfinger, Estradiol coupling to human monocyte nitric oxide release is dependent on intracellular calcium transients: evidence for an estrogen surface receptor, J. Immunol. 163 (1999) 3758–3763. [99] G.B. Stefano, V. Prevot, J.C. Beauvillain, T.K. Hughes, Interleukin10 stimulation of corticotrophin releasing factor median eminence in rats: evidence for dependence upon nitric oxide production, Neurosci. Lett. 256 (1998) 167–170. [100] T.G. Traylor, V.S. Sharma, Why NO?, Biochemistry 31 (1992) 2847–2849. [101] H. Tsukahara, D.V. Gordienko, M.S. Goligorsky, Continuous monitoring of nitric oxide release from human umbilical vein endothelial cells, Biochem. Biophys. Res. Commun. 193 (1993) 722–729. [102] C.P. Weiner, I. Lizasoain, S.A. Baylis, R.G. Knowles, I.G. Charles, S. Moncada, Induction of calcium-dependent nitric oxide synthases by sex hormones, Proc. Natl. Acad. Sci. USA 91 (1994) 5212– 5216. [103] W.C. Wetsel, M.M. Valenca, I. Merchenthaler, Z. Liposits, F.J. Lopez, R.I. Weiner, P.L. Mellon, A. Negro-Vilar, Intrinsic pulsatile secretory activity of immortalized luteinizing hormone-releasing hormone-secreting neurons, Proc. Natl. Acad. Sci. USA 89 (1992) 4149–4153. [104] J.W. Witkin, Synchronized neuronal networks: the GnRH system, Microsci. Res. Tech. 44 (1999) 11–18. [105] J.W. Witkin, H. O’Sullivan, A.J. Silverman, Novel associations among gonadotropin-releasing hormone neurons, Endocrinology 136 (1995) 4323–4330. [106] W. Wu, D.E. Scott, Increased expression of nitric oxide synthase in hypothalamic neuronal regeneration, Exp. Neurol. 121 (1993) 279– 283. [107] K. Yamada, P. Emson, T. Hokfelt, Immunohistochemical mapping of nitric oxide synthase in the rat hypothalamus and colocalization with neuropeptides, J. Chem. Neuroanat. 10 (1996) 295–316. [108] W.H. Yu, M. Kimura, A. Walczewska, S. Karanth, S.M. McCann, Role of leptin in hypothalamic-pituitary function, Proc. Natl. Acad. Sci. USA 94 (1997) 1023–1028. [109] W.H. Yu, A. Walczewska, S. Karanth, S.M. McCann, Nitric oxide mediates leptin-induced luteinizing hormone-releasing hormone (LHRH) and LHRH and leptin-induced LH release from the pituitary gland, Endocrinology 138 (1997) 5055–5058.