DYNAMIC CHANGES IN LHRH NEUROVASCULAR TERMINALS WITH VARIOUS ENDOCRINE CONDITIONS IN ADULTS

Luteinizing hormone-releasing hormone (LHRH) release is required for o vulation in mammals. Although evidence for the direct action of gonada l steroids on LHRH neurons has been minimal, their importance in induc ing the preovulatory surge of LHRH is unequivocal. We have identified a subgroup of LHRH neurons with progestin receptors in guinea pigs. Gi ven their central position, these neurons may constitute foci of initi al activity, which are amplified throughout the population of LHRH neu rons, resulting in increased LHRH neurosecretion on the afternoon of p roestrus. Additionally, gonadal steroids may regulate LHRH secretion a t the level of the terminals. Using immunoelectron microscopy and imag e analysis, we have illustrated the dramatic influence of gonadal ster oids on individual LHRH terminals in the median eminence of rats. Indi rectly, gonadal steroids may modulate LHRH release by modulating glial elements. Using double-label fluorescence confocal microscopy, we ill ustrate that LHRH terminals in the median eminence are encased by end- feet of tanycytes. Acting on glial elements, gonadal steroids may regu late access of LHRH terminals to the basal lamina and influence the am ount of the neuropeptide reaching the portal vessels. We propose that during the preovulatory surge, LHRH release is coordinated by synergis tic mechanisms operating at the level of particular subgroups of neuro nal perikarya and/or discrete regions of the median eminence. These sy nergistic actions may ensure that LHRH is released in a precipitous fa shion, to induce the surge of LH from the pituitary, required for ovul ation. (C) 1994 Academic Press, Inc.