Sex differences in open-field behavior in the rat: The inductive and activational role of gonadal hormones

Abstract

Male rats are generally found to be significantly less active and to defecate more than female rats in the open-field test. In developmental studies, these differences were shown to emerge in the peripuberal period at 50–60 days of age. After gonadectomy in adult life, the sex difference in open-field activity was reduced in magnitude but did persist indicating that it was not solely dependent on the presence of gonadal secretions in either sex. In contrast, the sex difference in open-field defecation did not reach statistical significance after gonadectomy with ovariectomized females exhibiting increased defecation frequency. Estrogen-progesterone replacement stimulated activity and decreased defecation scores to pre-gonadectomy levels in females but lacked effect in males. Administration of testosterone propionate (TP) to female rats in neonatal life produced developmental changes in open-field behavior similar to those seen in the male. In addition, the response of TP treated females to gonadectomy and hormone replacement was similar to that found for the male. These results suggest that the presence of testosterone in neonatal life plays a role in the masculinisation of open-field behavior and that its effect is partly mediated by the presence of cyclic ovarian secretions in the adult female rat and their absence in the male and TP treated female.