Ovarian hormones can bias female rats to use one memory system over another when navigating a novel or familiar environment, resulting in a memory bias. High levels of estrogen (E) promotes places memory while low level of E promotes response memory. However, little is known about the effects of progesterone (P) on memory bias. Experiment 1 determined whether P affects memory bias. Ovariectomized (OVX) female rats were trained in a plus shaped maze, which assesses memory system bias, and received one of three hormonal treatments: Low 17β estradiol (E2), high E2 or high E2 + P. P did affect memory bias by reversing the effects of high E2 when rats receive P one hour prior to testing. To understand the mechanisms by which P affect memory bias in the hippocampus (HPC), antibodies directed at nPR, mPRβ and mPRδ were examined. The effects of low E2, high E2, and high E2+P were examined on immunoreactivity to these receptors in the HPC. All three receptor-types were found in the female rat HPC and were found to be insensitive to hormone administration. The presence of these receptors suggests that P can exert both genomic and non-genomic effects in the HPC. Other brain areas involved in memory bias remain to be examined further
