Biomedical subjects

Alain Caraty

Publications and source records attributed to Alain Caraty.

8 recordsLinked to original sources

PubMed2006

KiSS-1 messenger ribonucleic acid expression in the hypothalamus of the ewe is regulated by sex steroids and season.

The KiSS-1 gene encodes a family of peptides called kisspeptins, which are endogenous ligands for the G protein-coupled receptor GPR54. Kisspeptin function appears to be critical for GnRH secretion and the initiation of puberty. To test the hypothesis that steroid hormones regulate KiSS-1 mRNA expression in the ewe, we examined the brains of ovary-intact (luteal phase) and ovariectomized (OVX) ewes, as well as OVX ewes that received estradiol (E) or progesterone (P) replacement. KiSS-1 mRNA-expressing cells were predominantly located in the arcuate nucleus (ARC). Here, expression was increased after OVX but returned to the level of gonad-intact animals with E treatment. Treatment with P partially restored KiSS-1 expression toward gonad-intact levels. Double-label immunohistochemistry revealed that approximately 86% of kisspeptin-immunoreactive cells in the ARC are also P-receptor positive. Finally, we tested the hypothesis that KiSS-1 mRNA is lower during anestrus, due to a non-steroid-dependent seasonal effect. In OVX ewes, expression in the ARC was lower at the time of year corresponding to anestrus. We conclude that KiSS-1 expression in the ARC of the ewe brain is negatively regulated by chronic levels of E and P, suggesting that both steroids may exert negative feedback control on GnRH secretion through altered kisspeptin signaling. Furthermore, a seasonal alteration in KiSS-1 expression in the ARC of OVX ewes strongly suggests that kisspeptin is fundamentally involved in the control of seasonal changes in reproductive function.

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PubMed2005

Progesterone-receptive dopaminergic and neuropeptide Y neurons project from the arcuate nucleus to gonadotropin-releasing hormone-rich regions of the ovine preoptic area.

Progesterone inhibits gonadotropin-releasing hormone (GnRH) secretion in sheep through an interneuronal system located in the mediobasal hypothalamus. This study focused on known inhibitors of GnRH secretion in sheep, dopamine and neuropeptide Y (NPY). As the distributions of tyrosine hydroxylase (TH)- and NPY-immunoreactive neurons overlap with progesterone receptors (PR) in the arcuate nucleus, we hypothesized that, if these neurons mediate, at least partially, the inhibitory feedback signal of progesterone, then they should co-express PRs. Fluorogold (FG), a retrograde tracer, was injected into the preoptic area of ovariectomized ewes pretreated with estrogen and progesterone. When the FG injection site encompassed at least 80 GnRH neurons, sections from the arcuate nucleus were processed using dual immunocytochemistry for PR and either TH or NPY. We found that 30% of PR-immunoreactive, 12% of TH-containing and 21% of NPY-synthesizing neurons project toward this GnRH-rich region. Of the PR/TH dual-labeled cells, which represent 21% of PR and 31% of TH cells, respectively, 22% displayed FG labeling. Of the PR/NPY neurons, which account for 19% of PR and 67% of NPY neurons, respectively, 26% were FG fluorescent. This study suggests that subsets of arcuate nucleus dopaminergic and NPY neurons may transduce, at least in part, the progesterone-mediated inhibition of GnRH secretion.

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PubMed2005

Progesterone-receptive beta-endorphin and dynorphin B neurons in the arcuate nucleus project to regions of high gonadotropin-releasing hormone neuron density in the ovine preoptic area.

Progesterone inhibits gonadotropin-releasing hormone (GnRH) secretion through interneuronal systems located in the mediobasal hypothalamus in ewes. Endogenous opioid peptides are implicated in this inhibition of GnRH secretion. The distributions of endogenous opioid peptides are known to overlap with progesterone receptors (PR) in the arcuate nucleus. We investigated whether PR is expressed by beta-endorphin and dynorphin B neurons in the arcuate nucleus and if a subset of double-labeled cells projects to the preoptic area where most GnRH neurons are detected. Injection of a retrograde tracer, Fluorogold, into the rostral preoptic area was performed in ovariectomized ewes pretreated with estrogen and progesterone. Brain sections were processed using double immunocytochemistry. Only brains of ewes with an injection site encompassing at least 80 GnRH neurons were processed for PR and then either beta-endorphin or dynorphin B immunocytochemistry. Antigen retrieval is essential for PR detection but causes Fluorogold to fade. Thus, quantitative analysis was performed on photographs taken before and after antigen retrieval. We found that 25-30% of PR-containing neurons, 20% of beta-endorphin cells and 22% of dynorphin B neurons in the arcuate nucleus project toward the preoptic area. From the PR/beta-endorphin double-labeled cells that represent 25 and 36% of PR and beta-endorphin cells, respectively, 35% were labeled with Fluorogold. From the PR/dynorphin B double-labeled cells that account for 39 and 62% of PR and dynorphin B neurons, respectively, 26% contained Fluorogold. These data strongly support the hypothesis that progesterone acts in the arcuate nucleus through beta-endorphin and dynorphin B neurons to affect preoptic area GnRH neurons.

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PubMed2005

Molecular mechanisms involved in LH release by the ovine pituitary cells.

The luteinizing hormone-releasing hormone (LHRH) is a hypothalamic decapeptide and main positive regulator of luteinizing hormone (LH) secretion from pituitary cells. Insulin-like growth factor-I (IGF-1) also stimulates LH release and enhances the effect of LHRH. However, the molecular mechanisms involved in the interactions between LHRH and IGF-1 are unclear. Here, we first determined the effect of various types of LHRH [I (mammalian), II (chicken), III (lamprey), hyp9 and salmon] on both LH secretion and activation of MAPK (ERK1/2 and p38) in ovine pituitary cells. After 3h of treatment, LH secretion was significantly higher for LHRH-I than for the other LHRH tested. Interestingly, LHRH-III had no effect at any concentration used on the LH release by ovine pituitary cells. The phosphorylation of both MAPK ERK1/2 and p38 was also significantly higher after treatment with LHRH-I than LHRH-II, salmon LHRH or hyp9. These MAPKs were not activated or only very weakly activated by LHRH-III. We then used pharmacological inhibitors to show that MAPK ERK1/2 and PKCdelta participate in the LH release by ovine pituitary cells in response to LHRH-I. We identified the main substrates and signaling pathways [PI3K/Akt and MAPK (ERK1/2, p38 and JNK1/2] of IGF-1R and investigated the effect of IGF-1 on the stimulation of ovine pituitary cell LH secretion by the various LHRH. IGF-1 increases LH secretion in response to LHRH-I, LHRH-II, hyp9 and salmon LHRH but not the secretion after treatment with LHRH-III. Using specific inhibitors, we found that the MAPK ERK1/2 but not the PI3K/Akt signaling pathway is involved in the LH secretion in response to IGF-1. This is the first description of a common molecular mechanism, involving the MAPK ERK1/2, by which LHRH-R and IGF-1-R induce LH secretion in ovine pituitary cells.

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PubMed2005

Kisspeptin directly stimulates gonadotropin-releasing hormone release via G protein-coupled receptor 54.

We have recently described a molecular gatekeeper of the hypothalamic-pituitary-gonadal axis with the observation that G protein-coupled receptor 54 (GPR54) is required in mice and men for the pubertal onset of pulsatile luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion to occur. In the present study, we investigate the possible central mode of action of GPR54 and kisspeptin ligand. First, we show that GPR54 transcripts are colocalized with gonadotropin-releasing hormone (GnRH) neurons in the mouse hypothalamus, suggesting that kisspeptin, the GPR54 ligand, may act directly on these neurons. Next, we show that GnRH neurons seem anatomically normal in gpr54-/- mice, and that they show projections to the median eminence, which demonstrates that the hypogonadism in gpr54-/- mice is not due to an abnormal migration of GnRH neurons (as occurs with KAL1 mutations), but that it is more likely due to a lack of GnRH release or absence of GnRH neuron stimulation. We also show that levels of kisspeptin injected i.p., which stimulate robust LH and FSH release in wild-type mice, have no effect in gpr54-/- mice, and therefore that kisspeptin acts directly and uniquely by means of GPR54 signaling for this function. Finally, we demonstrate by direct measurement, that the central administration of kisspeptin intracerebroventricularly in sheep produces a dramatic release of GnRH into the cerebrospinal fluid, with a parallel rise in serum LH, demonstrating that a key action of kisspeptin on the hypothalamo-pituitary-gonadal axis occurs directly at the level of GnRH release. The localization and GnRH release effects of kisspeptin thus define GPR54 as a major control point in the reproductive axis and suggest kisspeptin to be a neurohormonal effector.

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PubMed2004

Neuroanatomical organization of gonadotropin-releasing hormone neurons during the oestrus cycle in the ewe.

BACKGROUND: During the preovulatory surge of gonadotropin-releasing hormone (GnRH), a very large amount of the peptide is released in the hypothalamo-hypophyseal portal blood for 24-36H00. To study whether this release is linked to a modification of the morphological organization of the GnRH-containing neurons, i.e. morphological plasticity, we conducted experiments in intact ewes at 4 different times of the oestrous cycle (before the expected LH surge, during the LH surge, and on day 8 and day 15 of the subsequent luteal phase). The cycle stage was verified by determination of progesterone and LH concentrations in the peripheral blood samples collected prior to euthanasia. RESULTS: The distribution of GnRH-containing neurons throughout the preoptic area around the vascular organ of the lamina terminalis was studied following visualisation using immunohistochemistry. No difference was observed in the staining intensity for GnRH between the different groups. Clusters of GnRH-containing neurons (defined as 2 or more neurons being observed in close contact) were more numerous during the late follicular phase (43 +/- 7) than during the luteal phase (25 +/- 6), and the percentage of clusters was higher during the beginning of the follicular phase than during the luteal phase. There was no difference in the number of labelled neurons in each group. CONCLUSIONS: These results indicate that the morphological organization of the GnRH-containing neurons in ewes is modified during the follicular phase. This transitory re-organization may contribute to the putative synchronization of these neurons during the surge. The molecular signal inducing this plasticity has not yet been identified, but oestradiol might play an important role, since in sheep it is the only signal which initiates the GnRH preovulatory surge.

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PubMed2004

Regulation by estradiol of hypothalamic somatostatin gene expression: possible involvement of somatostatin in the control of luteinizing hormone secretion in the ewe.

In the ewe, the mediobasal hypothalamus (MBH) is the primary central site for estradiol to generate the preovulatory GnRH/LH surges and sexual behavior. This area contains numerous neurons expressing the estradiol receptor alpha, distributed in the ventromedial nucleus (VMN) and the infundibular nucleus (IN). A large proportion of these neurons express somatostatin, making this neuropeptide a potential candidate for transmission of the estradiol signal to the GnRH neurons located in the preoptic area. We tested this hypothesis using ovariectomized ewes that had been subjected to an artificial estrous cycle. In the first experiment, 22 h after progesterone removal, ewes received estradiol (treated ewes) or empty implants (control ewes) for 4 h and then were killed. Using in situ hybridization, we showed that this short estradiol treatment increased the somatostatin mRNA amount by about 50% in the VMN and 42% in the IN. In the second experiment, preovulatory estradiol signal was replaced by somatostatin intracerebroventricular (ICV) administration. This treatment abolished LH pulsatility and dramatically decreased the mean basal level of LH secretion while it did not affect the mean plasma GH concentration. We demonstrated that an increase in somatostatin mRNA occurs at the time of the negative feedback effect of estradiol on LH secretion during the early stage of the GnRH surge induction. As ICV somatostatin administration inhibits the pulsatile LH secretion by acting on the central nervous system, we suggest that somatostatin synthesized in the MBH could be involved in the estradiol negative feedback before the onset of the preovulatory surge.

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PubMed2003

Early decrease of proopiomelanocortin but not neuropeptide Y mRNA expression in the mediobasal hypothalamus of the ewe, during the estradiol-induced preovulatory LH surge.

In sheep, the mediobasal hypothalamus (MBH) has been shown to be the primary central site of estradiol (E2) action that induces both the preovulatory surge and sexual behaviour. However, the nature of the neurotransmitters or neuromodulators synthesized in the MBH during E2 stimulation remains to be clearly defined. After the cloning of the ovine cDNA sequences and using in situ hybridization, hypothalamic proopiomelanocortin (POMC), and preproneuropeptide Y (preproNPY) mRNA expression was studied in ovariectomized ewes that received a sequential treatment of progesterone and E2. As we showed that an exposition to E2 only for 4h well in advance on the LH surge onset is sufficient to induce the preovulatory surge and estrous behaviour, mRNA expression was evaluated in ewes treated with 6x30-mm E2 implants (experimental group) or with empty implants (control group) and slaughtered 4h after the start of the E2 treatment. Our results demonstrate that this short E2 treatment significantly decreased both the mean number of silver grains per POMC-containing cell (35%) and the mean number of POMC-cells (38%) in the ovine infundibular nucleus, whereas the treatment had no effect on preproNPY mRNA expression. These observations suggest that a reduction of POMC gene transcription could participate to the early neural mechanism of E2 feedback.

Amino Acid Sequence↗