Progesterone inhibition of sexual behavior is accompanied by an activation of aggression in female Syrian hamsters

Previous studies have shown that progesterone can have biphasic activational and inhibitory roles in the regulation of sexual behavior in female Syrian hamsters. The purpose of this study was to determine whether these biphasic effects on sexual behavior are mirrored in a reciprocal regulation of aggression. Ovariectomized female Syrian hamsters were treated with estradiol followed 2 and 3 days later by either 1) two oil injections (O-O), 2) an oil and a progesterone injection (O-P), 3) a progesterone and oil injection (P-O), or 4) two progesterone injections (P-P). Females were tested for aggression and sexual behavior in conjunction with these hormone treatments. On the final test session, P-P females had significantly lower lordosis durations than did O-P females, confirming the progesterone inhibition of sexual behavior previously reported. Without hormone treatment all females showed high baseline levels of aggression. On the last behavioral test, aggression in O-O females and P-O females was comparable to their baseline levels of aggression. In contrast, aggression in P-P females was significantly higher than the baseline levels of aggression. These results suggest that like its effects on sexual behavior, progesterone can have both inhibitory and activational effects on aggression in female Syrian hamsters.