Sex differences in object-in-place memory of adult rats.

Male rodents typically display an advantage over female conspecifics in tasks that assess memory for the identity or location of objects. However, the performance of female rodents on object recognition and object location tasks often is enhanced by elevated levels of ovarian steroids. The novel object-in-place task, combines elements of object recognition and object location tasks used to assess rodent object memory. Until now, only male rats have been tested on the object-in-place task, therefore, a study was conducted to compare the performance of males to females under different hormone conditions. Rats were given a sample phase to study the locations of four objects in an open field. After various delays, a test phase was administered with the locations of two objects reversed. Increased investigation of moved objects indicates memory for these objects and their original locations. Rats were able to discriminate moved from unmoved objects after a brief 5-min delay, regardless of biological sex or hormone status. However, gonadally intact males, but not diestrous females, were able to discriminate object locations after a 30-min delay. In contrast, ovariectomized females treated with estradiol and progesterone discriminated moved from unmoved objects after a 60-min delay while ovariectomized vehicle-treated females and gonadally intact males did not. Results indicate that female rats outperform males on a memory task that combines object recognition and location but only when circulating levels of the ovarian steroids, estradiol and progesterone, are elevated and only when memory is challenged by an extended retention interval.