SEMIQUANTITATIVE ULTRASTRUCTURAL ANALYSIS OF THE LOCALIZATION AND NEUROPEPTIDE CONTENT OF GONADOTROPIN-RELEASING-HORMONE NERVE-TERMINALS INTHE MEDIAN-EMINENCE THROUGHOUT THE ESTROUS-CYCLE OF THE RAT

The ultrastructural appearance of gonadotropin releasing hormone-immun oreactive elements was studied in the external zone of the median emin ence of adult female Wistar rats. On the one hand, the purpose of the study was to determine the distribution of gonadotropin releasing horm one 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 physiol ogical condition. A morphometric study was coupled to a colloidal 15 n m gold postembedding immunocytochemistry procedure. Animals were kille d at 09.00 on diestrus II, 09.00, 10.00, 13.00, 17.00 and 18.00 on pro estrus and 09.00 on estrus (n=4-8 rats/group). A preliminary light mic roscopic 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 l ast condition was necessary to make a good comparison between each ani mal. Contacts between gonadotropin releasing hormone nerve terminals a nd the basal lamina were observed only the day of proestrus. Such cont acts, however, were rare and in the great majority of cases, gonadotro pin releasing hormone terminals are separated from basal lamina by tan ycytic end feet. The morphometric analysis showed no significant varia tion in average distance between gonadotropin releasing hormone termin als and capillaries throughout the estrous cycle. Consequently, it did not appear that a large neuroglial plasticity exists during the estro us cycle. However, the observation of contacts only on proestrus toget her with some ultrastructural images evoke the possibility of a slight plasticity. The semi-quantitative results show that the content of go nadotropin releasing hormone in the nerve endings presented two peaks on proestrus: one at 09.00 (23 +/- 5 particles/mu m(2), P<0.03) before the onset of luteinizing hormone surge, and the second at 18.00 (16+/ -2 particles/mu m(2), P<0.01) concomitantly with the luteinizing hormo ne surge, when compared to baseline values on proestrus 10.00 (8+/-1 p articles/mu m(2)). (C) 1998 IBRO. Published by Elsevier Science Ltd.