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 hormonerelease

Despite intense investigation, the demonstration of morphological plasticit y in the external zone of the median eminence concerning the gonadotropin-r eleasing hormone system has never been reported. In this study, we investig ate whether dynamic transformations of the gonadotropin-releasing hormone n erve terminals and/or tanycytes in the external zone of the median eminence of the hypothalamus occurred during the rat estrous cycle, by following in dividual gonadotropin-releasing hormone-immunoreactive nerve terminals on s erial ultrathin sections observed by electron microscopy. Female rats were killed at 16.00 diestrus II (n = 3), i.e. when estrogen levels are basal an d gonadotropin-releasing hormone release is low, and at 16.00 proestrus (n = 4), i.e. when estrogen levels peak and the preovulatory gonadotropin-rele asing hormone surge occurs. Our results show that, in the median eminence o btained from proestrus rats, 12 +/- 2% of the gonadotropin-releasing hormon e nerve terminals were observed to make physical contact with the parenchym atous basal lamina, i.e. the pericapillary space. In the median eminence ob tained from diestrus II rats, no contacts were observed. On proestrus, nume rous physical contacts between gonadotropin-releasing hormone nerve termina ls 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 proes trus. These results demonstrate for the first time the existence of dynamic plast ic changes in the external zone of the median eminence, allowing gonadotrop in-releasing hormone nerve terminals to contact the pericapillary space on the day of proestrus, thus facilitating the release of the neurohormone int o the pituitary portal blood. (C) 1999 IBRO. Published by Elsevier Science Ltd.