Differential effects of neonatal castration on the development of sexually dimorphic brain areas in the gerbil.

We compared the effects of neonatal castration within 6 h of birth in the Mongolian gerbil on the development of the sexually dimorphic area pars compacta (SDApc) and supraschiasmatic nucleus (SCN). Development of these brain areas was also related to a masculine courtship ultrasonic vocalization, the frequency-modulated upsweep. Castration immediately after birth resulted in differential effects with complete or partial reduction of SCN and SDApc volumes, respectively, as compared to male values. The rates of ultrasonic calling of males castrated as neonates were also decreased to female levels. In sham-operated males, calling rates were positively correlated with the volume of the left SDApc, but not the right. Both the left and the right SDApc volumes were correlated with calling rates in males castrated as neonates. The asymmetric relationship between vocal behavior and area volume was specific to the SDApc. We suggest that in the neonate (a) the sensitivity of the SDApc to the differentiating effects of androgens differs from the SCN and (b) the asymmetric link between brain structure and vocal behavior depends on the effects of androgen within 6 h of birth.