A revised critical period for the sexual differentiation of the sexually dimorphic nucleus of the preoptic area in the rat.

The volume of the sexually dimorphic nucleus of the preoptic area (SDN-POA) of the rat is several times larger in males than in females. Several studies have established the importance of gonadal steroids perinatally in the sexual differentiation of the SDN-POA as well as a critical period for the permanent influences of exogenous androgen on the volume of the nucleus. Recent preliminary evidence from this laboratory had suggested, however, that the critical period for the effects of the removal of endogenous gonadal steroids on SDN-POA volume may not match that for the administration of exogenous gonadal steroids. A series of experiments was designed to examine further the effects of the removal of endogenous gonadal steroids on adult SDN-POA volume by castrating male rats at various ages. In spite of a rather clear definition of a postnatal critical period for the effects of exogenous steroid administration, the results of this study indicate that the volume of the SDN-POA is sensitive to the removal of endogenous gonadal steroids for a prolonged period of time, extending through at least day 29 postnatally. The data suggest that there may be multiple critical periods for the sexual differentiation of the SDN-POA and reinforce the concept that these critical periods are distinct from those for other sexually differentiated parameters.