INFLUENCE OF MATERNAL GROOMING, SEX AND AGE ON FOS IMMUNOREACTIVITY IN THE PREOPTIC AREA OF NEONATAL RATS - IMPLICATIONS FOR SEXUAL-DIFFERENTIATION

The medial preoptic area (mPOA) of the hypothalamus contains a sexuall y dimorphic nucleus (SDN-POA) that is 5-7 times larger in males than f emales and which contributes to the development and expression of male -specific sex behaviors in adulthood. Aside from a critical role for e strogen, the mechanisms that establish and maintain this sex differenc e are largely unknown. Differences in the size of the SDN-POA are thou ght to be related to estrogen-associated effects on programmed cell de ath (apoptosis) during early neonatal development. The expression of m ale sex behavior is also influenced by maternal behavior during develo pment. During the postnatal period, the dam grooms the anogenital regi on of the pups to stimulate urination and defecation; however, male pu ps are groomed significantly more often than females and this maternal attention influences the expression of normal male sexual behavior in adulthood. Based on these observations, we hypothesized that differen t amounts of anogenital sensory stimulation might contribute to the se xually dimorphic development of the SDN-POA, specifically by providing for different levels of neuronal activation in the SDN-POA resulting in different degrees of cell death. Two experiments were conducted to test this hypothesis. In the first experiment, male and female rat pup s on postnatal day 3 (PN 3) received simulated anogenital grooming wit h a stiff bristle paint brush. One hour later, the brains were removed and sections through the POA were cut and processed for the immunocyt ochemical detection of Fos-like immunoreactivity (IR) as an indicator of neuronal activation. In the second experiment, male and female litt ermates were killed on PN 3, 5, 7 and 12 and the number of Fos-immunor eactive cells and pyknotic cells detected in the SDN-POA were counted and compared. Our data demonstrate that anogenital stimulation on PN 3 results in a rapid induction of Fos-immunoreactive in the POA of both males and females. However, the majority of Fos-immunoreactive cells were located in the ventral POA and were distinctly lacking in the SDN -POA. In experiment 2, again no Fos-immunoreactive cells were detected in the SDN-POA of animals examined on PN 5-12. However, there was an increase in the number of pyknotic cells in the area surrounding and i ncluding the SDN-POA of females relative to males at PN 5, 7 and 12. C ollectively, the data suggest that (1) anogenital grooming during earl y postnatal development induces a rapid activation of cells in the ven tral mPOA, but not in the SDN-POA of rats, (2) there is a greater inci dence of cell death in and around the SDN-POA of females vs. males dur ing neonatal development, particularly toward the end of the hormone-s ensitive critical period, and (3) Fos expression does not appear to be correlated with the sexually dimorphic development of, and/or program med cell death within, the developing SDN-POA.