SEXUAL DIMORPHISM IN COPACKAGING OF LUTEINIZING-HORMONE-RELEASING HORMONE AND GALANIN INTO NEUROSECRETORY VESICLES OF HYPOPHYSIOTROPHIC NEURONS - ESTROGEN DEPENDENCY

Hypophysiotrophic neurons projecting to hypophyseal portal vessels in the median eminence of the hypothalamus maintain the operation of the master gland, the pituitary, by secreting releasing and release-inhibi ting hormones into the bloodstream. LHRH, synthesized in neurons of th e rat prosencephalon, is one of the key substances that governs the an terior pituitary-gonadal axis. Recently, it has been shown that the pe ptide galanin (GAL) is coproduced in a subpopulation of LHRH neurons a nd is a potent modulator of central processes regulating reproduction. A better understanding of the secretory mechanisms involved in pulsat ile hormone release from LHRH axons of the median eminence requires ex ploration of the organelle domain that displays the cosynthesized pept ides in terminal boutons. This study shows that LHRH- and GAL-immunore active axons overlap heavily in the lateral part of the median eminenc e. Double fluorescent labeling revealed colocalization of the peptides at the level of single axon terminals. By means of dual colloidal gol d immunolabeling, LHRH and GAL were detected in the same secretory ves icles at the ultrastructural level. The incidence of colocalizing vesi cles was high in the female (45%) and low in the male (3%) rat. Ovarie ctomy resulted in a dramatic decline in the number of LHRH/GAL-coexpre ssing vesicles (23%), which was reversed (55%) by the administration o f estradiol. The observations indicate a sex-related difference in the packaging of LHRH and GAL and suggest that the events are estrogen de pendent. Furthermore, the simultaneous release of GAL and LHRH from th e colocalizing vesicles provides a mechanism that might ensure the pot entiating effect of GAL on LHRH by synchronizing events at the recepto r sites in the anterior pituitary.