Peripheral and intracerebroventricular administration of human chorionic gonadotropin alters several hippocampus-associated behaviors in cycling female rats.

Our recent demonstration of receptors for luteinizing hormone (LH)/human chorionic gonadotropin (hCG) in rat brain with the highest density within the hippocampus and dentate gyrus suggests novel functional roles for gonadotropic hormones within the brain. The present study investigated whether 125I-hCG can cross the blood-brain barrier and reach hippocampus and the possible role of hCG in the regulation of several behavioral activities associated with the hippocampal formation in the rat. About 1/100th of peripherally injected 125I-hCG crossed the blood-brain barrier in an intact form and was found in cerebrospinal fluid and in hippocampus. Intraperitoneal (IP) or intracerebroventricular (ICV) injections of highly purified hCG on the morning of proestrus of cycling female rats resulted in changes in several hippocampus-associated behaviors. hCG-treated animals were generally less active and showed less exploratory behavior as compared to saline-injected control animals. There was no difference, however, in latency to enter the open field between hCG-treated and control animals. Taste neophobia was dramatically decreased following IP as well as ICV injection of hCG. No differences were found in the memory component of T-maze performance; however, the hCG-treated rats exhibited decreased stereotypic behavior. In summary, hCG can cross the blood-brain barrier, and peripheral or central administration of hCG affects several hippocampus-associated behaviors suggesting that hippocampal LH/hCG receptors are most likely involved in mediating these effects. Some of the observed behavioral changes have parallels in pregnant women.