Sexual differentiation of the neuronal plasma membrane: neonatal levels of sex steroids modulate the number of exo-endocytotic images in the developing rat arcuate neurons.

Exo-endocytotic images and intramembrane protein particles (IMP) were quantitatively assessed in freeze-fracture replicas from the plasma membrane of arcuate neurons of rats aged 0 (newborns), 10, 20 and 100 days postpartum. Membranes contained significantly (P less than 0.02) more IMPs in females than in males. Exo-endocytotic images were increased in newborn and 10-day-old males when compared to adult males or to developing females (48 +/- 6 vs 6 +/- 1 images/100 micron 2 in 10-day-old male and female rats, respectively). Androgenization of females with a single injection of testosterone propionate on the day of birth resulted in an increased number of exo-endocytotic images in developing animals (75 +/- 9 images/100 micron 2, 10-day-old rats) and in the abolishment of the sex differences in the number of IMPs.