Ovarian hormones modulate social recognition in female rats

The effects of estradiol and progesterone on social recognition were evaluated in a habituation-dishabituation procedure. Ovariectomized female rats were either given 18 µg/kg of estradiol benzoate followed by progesterone, 1 mg/rat, 48 h later or oil. The test consisted of 4 consecutive exposures to a juvenile rat and a final exposure to a novel juvenile. Although the time spent investigating the juvenile was reduced after repeated exposure in both groups, the reduction was larger in the group treated with ovarian hormones. When exposed to a novel juvenile, both groups displayed enhanced investigation. When the within-exposure investigation time was analyzed, a gradual reduction from the first to the last minute of the 5 min exposure was found in both groups. The reduction was faster in the group treated with estradiol + progesterone. There was no hormone effect on any other behavior pattern in the social recognition test. The hormone treatment produced a high level of receptive and proceptive behaviors according to a test for copulatory behavior performed immediately after the test for social recognition. These data show that ovarian hormones are not necessary for social recognition to occur, but they have a facilitatory effect. It is suggested that the contradictory findings reported in the literature can be explained by the fact that these hormones have rather subtle effects, perhaps detectable with the habituation-dishabituation procedure but not with the social discrimination procedure employed in most earlier studies.