The role of neuron death in the development of the gender difference in the number of neurons in the rat superior cervical ganglion.

Adult male rats have more neurons in their superior cervical ganglia than do adult females. This difference arises over the first two postnatal weeks, and is apparently related to perinatal levels of circulating testosterone. Exposure of neonatal rats to testosterone or estradiol during the first postnatal weeks results in an increase in the number of neurons in the superior cervical ganglion seen at 15, 30 or 60 days postnatally. The present studies were undertaken to determine whether this observed increase in neurons is due to an increase in neuronal proliferation or to a decrease in neuronal death. Results of autoradiographic studies show no evidence of enhanced neuronal proliferation following postnatal exposure to estradiol, but do show an increased survival of a prenatally labeled population of cells. Counts of degenerating cells in the superior cervical ganglion show that during the peak period of normal neuronal degeneration, on postnatal day 5, 17-beta-estradiol or testosterone propionate treated animals have significantly fewer degenerating superior cervical ganglion cells than do vehicle-injected littermate controls. In addition, vehicle-injected females have more degenerating cells on day 5 than do vehicle-injected males. Taken together these results provide strong evidence that the increase in superior cervical ganglion neurons seen after neonatal exposure to estradiol results from a reduction in developmental neuron death.