Abstract

The mammalian central nervous system undergoes a process of sexual structural differentiation, which is primarily determined by early interaction of gonadal sex steroids with the neuronal genome. The present study confirmed that the neuropil of the ventrolateral pan of the ventromedial hypothalamic nucleus (VL-VMHN) of the rat is sexually dimorphic in the density of axospinous and axódendritic types of synapses, which are numerous in the intact male. Pcnnatal exposure of the female to exogenous testosterone or castration of the newborn male "inverted" the difference, demonstrating the hormonal dependence of this sexual dimorphism. Another finding was, that in adult rats with transection of the fornix, the density of degenerating fibers terminating in the VL-VMHN is also sexually dimorphic. Suppression of endogenous testosterone by castration of the newborn male decreased the density of degenerating fornical fibers terminating in the VL-VMHN to a number comparable to that found in the females' nucleus. It is concluded that: 1. The fornix gives a neural input to the VL-VMHN as proven by orthograde degeneration. 2. The number of fornical termináis in the VL-VMHN is greater in the male than m the female. 3. This dimorphism depends on the organizational effect of gonadal sex steroids.