Dynamic changes in LHRH neurovascular terminals with various endocrine conditions in adults.

Luteinizing hormone-releasing hormone (LHRH) release is required for ovulation in mammals. Although evidence for the direct action of gonadal steroids on LHRH neurons has been minimal, their importance in inducing the preovulatory surge of LHRH is unequivocal. We have identified a subgroup of LHRH neurons with progestin receptors in guinea pigs. Given their central position, these neurons may constitute foci of initial activity, which are amplified throughout the population of LHRH neurons, resulting in increased LHRH neurosecretion on the afternoon of proestrus. Additionally, gonadal steroids may regulate LHRH secretion at the level of the terminals. Using immunoelectron microscopy and image analysis, we have illustrated the dramatic influence of gonadal steroids on individual LHRH terminals in the median eminence of rats. Indirectly, gonadal steroids may modulate LHRH release by modulating glial elements. Using double-label fluorescence confocal microscopy, we illustrate that LHRH terminals in the median eminence are encased by end-feet of tanycytes. Acting on glial elements, gonadal steroids may regulate access of LHRH terminals to the basal lamina and influence the amount of the neuropeptide reaching the portal vessels. We propose that during the preovulatory surge, LHRH release is coordinated by synergistic mechanisms operating at the level of particular subgroups of neuronal perikarya and/or discrete regions of the median eminence. These synergistic actions may ensure that LHRH is released in a precipitous fashion, to induce the surge of LH from the pituitary, required for ovulation.