Sexual dimorphism in the synaptic input to gonadotropin releasing hormone neurons

GnRH neurons form the final common pathway regulating the secretion of gonadotropins from the anterior pituitary. Since the patterns of gonadotropin release display profound sexual dimorphism among mammals including the rodent, we undertook an ultrastructural analysis to determine whether these neurosecretory cells were differentially innervated between the sexes. As a further exploration of the organization of the neurocircuitry integrating GnRH neurons with the central nervous system, we also determined the degree to which GnRH cells and their processes were innervated by terminals containing either the endogenous opiate, beta-endorphin (BE) or GnRH itself. Sections from the diagonal band of Broca and the preoptic area of adult male and diestrus II female rats were immunocytochemically processed for dual localization of GnRH and BE. GnRH neurons cut through the plane of the nucleus were identified in 1 micron sections. Serial ultrathin sections were made and analyzed for 1) total synaptic input to both cell bodies and dendrites; 2) BE input; and 3) input arising from GnRH itself. We report that GnRH neuronal cell bodies in females received approximately twice the number of synapses as did those of males. The input to the GnRH dendrites, when measured as percent of plasma membrane in synaptic contact, also showed a profound sexual dimorphism with the female having a larger proportion of the dendrite in synaptic apposition. BE innervation contributed to this dimorphism at the level of both the cell body and dendrite. In contrast, the distribution and number of GnRH terminals did not differ between the sexes. In both they were confined to the dendritic arbor. We hypothesize that the capacity of the female rodent GnRH system to show neurogenic derived alterations in GnRH output not seen in the male may be due in part to these anatomical differences.