Gonadotropin-releasing Hormone Nerve Terminals Research Articles

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

Despite intense investigation, the demonstration of morphological plasticity in the external zone of the median eminence concerning the gonadotropin-releasing hormone system has never been reported. In this study, we investigate whether dynamic transformations of the gonadotropin-releasing hormone nerve terminals and/or tanycytes in the external zone of the median eminence of the hypothalamus occurred during the rat estrous cycle, by following individual gonadotropin-releasing hormone-immunoreactive nerve terminals on serial ultrathin sections observed by electron microscopy. Female rats were killed at 16.00 diestrus II ( n=3), i.e. when estrogen levels are basal and gonadotropin-releasing hormone release is low, and at 16.00 proestrus ( n=4), i.e. when estrogen levels peak and the preovulatory gonadotropin-releasing hormone surge occurs. Our results show that, in the median eminence obtained from proestrus rats, 12±2% of the gonadotropin-releasing hormone nerve terminals were observed to make physical contact with the parenchymatous basal lamina, i.e. the pericapillary space. In the median eminence obtained from diestrus II rats, no contacts were observed. On proestrus, numerous physical contacts between gonadotropin-releasing hormone nerve terminals and the basal lamina occurred by evagination of the basal lamina and/or by emerging processes from gonadotropin-releasing hormone nerve terminals. The quantification of the evagination of the basal lamina revealed that the basal lamina was at least twofold more tortuous in appearance during proestrus. These results demonstrate for the first time the existence of dynamic plastic changes in the external zone of the median eminence, allowing gonadotropin-releasing hormone nerve terminals to contact the pericapillary space on the day of proestrus, thus facilitating the release of the neurohormone into the pituitary portal blood.

Morphological characterization of relationship between gap junctions and gonadotropin releasing hormone nerve terminals in the rat median eminence

The present study aimed to reveal possible morphological relationships between gonadotropin-releasing hormone (GnRH) nerve terminals and gap junctions in the median eminence. Coronal brain sections from castrated male rats were dual immunostained with GnRH and either connexin 26, 32, or 43, and examined by confocal laser microscopy. Connexin 43-immunoreactive puncta were distributed between GnRH-immunoreactive fibers, and some of them were colocalized with GnRH-immunoreactivities. Dual immunostaining with connexin 43 and glial fibrillary acidic protein revealed that most of the puncta were located in astrocytes. At the immunoelectron microscopic level, connexin 43-immunoreactivities were mainly located on the plasma membranes of glial-like processes. Few connexin 26- or connexin 32-immunoreactivities were found in the median eminence. The present results indicate the possibility that gap junctions play a role in the GnRH release at the median eminence.

Evidence for terminal regulation of GnRH release by excitatory amino acids in the median eminence in female rats: a dual immunoelectron microscopic study.

The present study was designed to determine whether excitatory amino acids directly act on the gonadotropin-releasing hormone (GnRH) nerve terminals to release the peptide. The median eminence taken from ovariectomized rats was dual immunostained with GnRH and ionotropic glutamate receptor subtypes (NR1, GIuR1, GluR2/3, GluR6/7 and KA2), and their colocalization was examined under electron microscopy. The connection of fibers immunopositive for GnRH and glutamate was also examined. Of the glutamate receptor subtypes, NR1- and KA2-immunoreactivities were colocalized with GnRH-immunoreactivity in nerve terminals of the median eminence. In addition, some glutamate-immunopositive nerve terminals were shown to abut the many GnRH-immunopositive nerve terminals. No synaptic contacts were observed on these immunopositive nerve terminals. These results suggest that GnRH release is regulated at the GnRH nerve terminals by excitatory amino acids in a non-synaptic manner in the median eminence.

Semi-quantitative 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

The ultrastructural appearance of gonadotropin releasing hormone-immunoreactive elements was studied in the external zone of the median eminence of adult female Wistar rats. On the one hand, the purpose of the study was to determine the distribution of gonadotropin releasing hormone terminals towards the parenchymatous basal lamina at the level of hypothalamo-hypophyseal portal vessels, throughout the estrous cycle. On the other hand, we have semi-quantified the gonadotropin releasing hormone content in nerve terminals or preterminals during this physiological condition. A morphometric study was coupled to a colloidal 15 nm gold postembedding immunocytochemistry procedure. Animals were killed at 09.00 on diestrus II, 09.00, 10.00, 13.00, 17.00 and 18.00 on proestrus and 09.00 on estrus ( n=4–8 rats/group). A preliminary light microscopic study was carried out to identify an antero-posterior part of median eminence strongly immunostained by anti-gonadotropin releasing hormone antibodies but which was, in addition, easily spotted. This last condition was necessary to make a good comparison between each animal. Contacts between gonadotropin releasing hormone nerve terminals and the basal lamina were observed only the day of proestrus. Such contacts, however, were rare and in the great majority of cases, gonadotropin releasing hormone terminals are separated from basal lamina by tanycytic end feet. The morphometric analysis showed no significant variation in average distance between gonadotropin releasing hormone terminals and capillaries throughout the estrous cycle. Consequently, it did not appear that a large neuroglial plasticity exists during the estrous cycle. However, the observation of contacts only on proestrus together with some ultrastructural images evoke the possibility of a slight plasticity. The semi-quantitative results show that the content of gonadotropin releasing hormone in the nerve endings presented two peaks on proestrus: one at 09.00 (23±5 particles/ μm 2, P<0.03) before the onset of luteinizing hormone surge, and the second at 18.00 (16±2 particles/ μm 2, P<0.01) concomitantly with the luteinizing hormone surge, when compared to baseline values on proestrus 10.00 (8±1 particles/ μm 2).

Pre- and postsynaptic actions of GABA on the release of hypothalamic gonadotropin-releasing hormone (GnRH)

The role of γ-aminobutyric acid (GABA) in the regulation of gonadotropin hormone-releasing hormone (GnRH) release was studied using rat hypothalamic slices in vitro. Perifusion with GABA (10 −8−10 −4 M) caused a dose-dependent increase in GnRH release. The GABA A receptor agonist isoguvacine (10 −5M) also stimulated GnRH release, whereas the GABA B agonist baclofen (10 −6M) had no effect. The specific GABA A antagonist SR95103 (10 −6M) caused a reduction of basal GnRH release and completely blocked that induced by GABA (10 −4M). When nerve transmission was blocked with tetrodotoxin (TTX, 10 −6M), GnRH release was slightly reduced but the stimulatory effects of both GABA and isoguvacine were abolished. The GABA-induced stimulation of GnRH release was also prevented when the hypothalamic slices were treated with a corticotropin releasing hormone (CRH) antagonist (α-helical CRF 9−41, 10 −6M) or the opioid antagonist naloxone (10 −6M). Treatment with CRH (10 −8M) resulted in a decrease in GnRH release and this effect was not reversed in the presence of GABA. Finally, GABA was found to stimulate the release of the opioid peptides β-endorphin, dynorphin and met-enkephalin. These results lead us to conclude that GABA exerts two opposing effects upon GnRH neuronal activity: it acts in an inhibitory fashion at GnRH nerve terminals and in a stimulatory fashion at GnRH perikarya; the latter might occur through GABAergic inhibition of CRH release and, therefore, of opioid peptide release. Lastly, all the effects of GABA upon GnRH release appear to be mediated through GABA A receptors.