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The corticotropin-releasing hormone (CRH) neurons of the hypothalamic parvocellular paraventricular nucleus (PVN) have a high potential for phenotypical plasticity, allowing them to rapidly modify their neuroendocrine output, depending upon the type of stressors. Indeed, these neurons coexpress other neuropeptides, such as cholecystokinin (CCK), vasopressin (VP), and neurotensin, subserving an eventual complementary function to CRH in the regulation of the pituitary. Unlike in rats, our previous data showed that in jerboas, CCK is not coexpressed within CRH neurons in control as well as stressed animals. The present study explored an eventual VP participation in the phenotypic plasticity of CRH neurons in the jerboa. We analyzed the VP expression within the PVN by immunocytochemistry in male jerboas submitted to acute stress. Our results showed that, contrary to CRH and CCK, no significant change concerned the number of VP-immunoreactive neurons following a 30-min immobilization. The VP/CRH coexpression within PVN and median eminence was investigated by double immunocytochemistry. In control as well as stressed animals, the CRH-immunopositive neurons coexpressed VP within cell bodies and terminals. No significant difference in the number of VP/CRH double-labeled cells was found between both groups. However, such coexpression was quantitatively more important into the posterior PVN as compared with the anterior PVN. This suggests an eventual autocrine/paracrine or endocrine role for jerboa parvocellular VP which is not correlated with acute immobilization stress. VP-immunoreactive neurons also coexpressed CCK within PVN and median eminence of control and stressed jerboas. Such coexpression was more important into the anterior PVN as compared with the posterior PVN. These results showed the occurrence of at least two VP neuronal populations within the jerboa PVN. In addition, the VP expression did not depend upon acute immobilization stress. These data highlight differences in the neuroendocrine regulatory mechanisms of the stress response involving CRH/CCK or VP. They also underline that adaptative physiological mechanisms to stress might vary from one mammal species to another. Copyright © 2007 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

The present investigation has examined which subunits of the GABA(A) receptor are expressed by gonadotropin-releasing hormone (GnRH) neurons in the juvenile and adult male mouse. Cells of defined morphology, located in the medial septum (MS) and rostral preoptic area (POA), were patch-clamped in the acute brain slice preparation and their cell contents extracted. A reverse transcriptase polymerase chain reaction (RT-PCR) procedure using nested primers was used to establish individual GnRH mRNA-expressing cells which were then evaluated for eleven GABA(A) receptor (alpha1-5, beta1-3, gamma1-3) subunit transcripts. Single and multiple GABA(A) receptor subunit mRNAs were detected in approximately 70% of all GnRH neurons. A range of different subunit mRNAs (alpha1, alpha2, alpha5, beta1, beta2, beta3, gamma2) were found in juvenile GnRH neurons, with the alpha1gamma2 and alpha5gamma2 combinations encountered most frequently within individual cells. The expression profile in adult GnRH neurons was more extensive than that detected in juveniles with alpha1, alpha2, alpha3, alpha5, beta1, beta2, beta3, gamma1 and gamma2 subunits all being detected. The major difference in subunit profile between GnRH neurons located in the MS and POA involved the beta subunits. The principal postnatal developmental change was one of increasing overall subunit heterogeneity in maturing POA GnRH neurons. The profile of GABA(A) receptor subunit mRNAs detected in male GnRH neurons was quite different to that reported by us for female GnRH neurons in the mouse using the same RT-PCR approach. Together, these findings indicate that postnatal GnRH neurons are likely to express a range of GABA(A) receptor subunit mRNAs in a sexually dimorphic and developmentally-regulated manner., (Copyright 2001 S. Karger AG, Basel)

Estrogen exerts a critical regulatory influence upon the biosynthetic and secretory activity of the gonadotropin-releasing hormone (GnRH) neurons. It seems likely that estrogen regulates the behavior of the GnRH neuron through multiple transsynaptic, neuronal-glial, and direct membrane modes of action. Advances in our understanding of these mechanisms over the last 3 years are highlighted. In addition, very recent studies have begun to provide evidence for the expression of estrogen receptors (ERs) in GnRH neurons in the rodent. Although not yet firmly established, the current consensus supports the hypothesis that GnRH neurons express ERbeta. Evidence exists for ERbeta mRNA expression by GnRH neurons throughout development and ERbeta immunoreactivity has now also been detected in these cells. Murine GnRH neurons have further been shown to express estrogen receptor-related receptor-alpha, an orphan receptor thought to constitutively activate estrogen response elements. Together, these findings provide a cornerstone for the reassessment of the role of ERs and related receptors in the direct genomic and potential nontranscriptional actions of estrogen upon the GnRH neuron., (Copyright 2001 Academic Press.)

The molecular and cellular characteristics of the gonadotropin-releasing hormone (GnRH) neurons have been difficult to ascertain due to their scattered distribution within the basal forebrain. Using morphological criteria coupled with single cell RT-PCR postidentification, we have developed a method for investigating native GnRH neurons in the mouse brain and used it to examine the development of GABA(A) receptor signalling in this phenotype. Following the harvesting of the cytoplasmic contents of individual GnRH neurons, single cell multiplex RT-PCR experiments demonstrated that GABAA receptor alpha1-5, beta1-3 and gamma2 & 3 subunit transcripts were expressed by both neonatal (postnatal day 5) and juvenile (day 15-20) GnRH neurons in a heterogeneous manner. Following puberty, this profile was reduced to a predominant alpha1, alpha5, beta1, gamma2 subunit complement in rostral preoptic area GnRH neurons of the adult female. Whole-cell patch-clamp recordings revealed little difference between juvenile and adult GnRH neurons in their resting membrane potential and spontaneous firing rates. All GnRH neurons were found to be subjected to a tetrodotoxin-insensitive, tonic GABAergic barrage signalling through the GABA(A) receptor. However, marked heterogeneity in the sensitivity of individual juvenile GnRH neurons to GABA was revealed and, in parallel with the change in subunit mRNA profile, this was dramatically reduced in the reproductively competent adult GnRH neurons. These findings provide the first electrical and molecular characterization of the GnRH phenotype and demonstrate a novel pattern of late postnatal reorganization of native GABA(A) receptor gene expression and signalling in the GnRH neuronal population.

Hypothalamic neuropeptide Y (NPY) and proopiomelanocortin (POMC)-derived peptides located in the arcuate nucleus (ARC) have been postulated to be good candidates to play a modulatory role during lactation. In the present study, we first quantified, by in situ hybridization, lactation-induced changes in NPY and POMC gene expression throughout the ARC. In a second phase of the study, we attempted to determine whether any relationship exists between neuropeptide gene expression and the suckling stimulus itself. For this, we used experimental groups of animals submitted to suppression of the suckling stimulus by removal of pups and the subsequent restoration of the suckling stimulus by the return of the litter. Since lactation is characterized by an estrogen-deficient status [15], we attempted using ovariectomized (2 or 21 days) diestrous females to describe the changes in NPY gene expression observed during lactation. Since the suckling stimulus induces a strong prolactin (PRL) release, we completed this study by using an intravenous injection of PRL antiserum in order to discriminate the effects of PRL per se on the observed suckling-induced changes in neuropeptide gene expression. Freely nursing lactating females exhibited a large increase in NPY mRNA expression as compared to diestrous females (10.10 +/- 0.50 vs. 4.51 +/- 0.35). After suppression of the suckling stimulus by removal of pups, this increase intensified during short-term suppression of 16 h (15.37 +/- 0.67) and was reversed following long-term suppression of 36 h (12.35 +/- 0.61). Ovariectomized diestrous animals showed significant changes in NPY mRNA expression as compared to lactating females (5.25 +/- 0.42 vs. 10.10 +/- 0.50). Lactating females submitted to PRL immunoneutralization by PRL antiserum showed a slight increase in NPY mRNA expression as compared to non-injected lactating females (13.75 +/- 0.51 vs. 12.95 +/- 0.59). Freely nursing lactating females showed a decrease in POMC mRNA expression (8.27 +/- 0.33) whereas suppression of suckling by removal of the pups (9.52 +/- 0.45) resulted in a return to diestrous POMC mRNA levels (10.77 +/- 0.36). We showed that restoration of suckling by the return of the litter induced an increase in POMC gene expression (12.55 +/- 0.66). By lowering circulating levels of PRL with PRL antiserum after restoration of suckling, we observed a decrease in POMC mRNA expression (9.81 +/- 0.46). Results of this study showed that the increase in NPY mRNA in the medial ARC during lactation did not appear to be due either to gonadal steroid-deficient status or to the suckling-induced hyperprolactinemia. If freely nursing lactating females showed a moderate decrease in POMC gene expression, restoration of the suckling stimulus by return of the pups provoked an increase in POMC gene expression which seemed to depend on high endogenous levels of PRL.

Attention has recently been focused on lactation-induced modifications of activity of neuronal populations in the arcuate nucleus (ARC) of the mediobasal hypothalamus. The ARC hosts the tubero-infundibular dopaminergic (TIDA system) responsible for the neuroendocrine control of prolactin (PRL), and other non-neuroendocrine neuronal populations, such as neuropeptide Y (NPY)- and proopiomelanocortin (POMC)-containing systems that are important modulators of hypothalamic gonadoliberin (GnRH) secretion. Our longstanding interest in the functional anatomy of the ARC led us to investigate whether the suckling stimulus would trigger an expression of Fos-ir in specific arcuate neuronal populations and to possibly characterize responsive neurons by using double-labeling immunohistochemistry. Freely nursing lactating females expressed strong Fos-ir in neurons of the ARC compared to diestrous females. Fos-ir was encountered in neurons not belonging to the TIDA system and that was for a large proportion identical to the POMCergic neurons. We showed that, in lactating females submitted to suppression of the suckling stimulus by removal of the pups, the pattern of expression of Fos-ir is similar to that seen in diestrous females and that, a pattern of expression of Fos-ir indistinguishable from that observed during free lactation is reinstated a short time after the return of the pups and restoration of the suckling stimulus, suggesting that this expression of Fos-ir strictly depends upon the presence of the newborns and the suckling stimulus. By lowering circulating levels of the PRL with bromocryptine-or PRL antiserum-treatment, we noticed a decrease in the number of (beta-endorphin + Fos)-ir neurons compared to non-injected freely nursing lactating females. By maintaining high levels of circulating PRL with haloperidol-treatment, we observed a number of colocalizations close to that observed in freely nursing lactating females. Our results suggest that during lactation a rostral subgroup of the arcuate POMCergic neuronal population is activated at least partially in response to the suckling-induced secretion of PRL and that this activation participates in maintaining the endocrine and/or metabolic demands of the lactational status.