Role of naloxone as an exogenous opioid receptor antagonist in spatial learning and memory of female rats during the estrous cycle.

Opioid neurotransmitters play a modulatory role in learning and memory processing. Their levels fluctuate throughout the reproductive cycle. The purpose of this research described herein was to study the effects of an exogenous opioid receptor antagonist, naloxone, on spatial learning and memory during the estrous cycle in female rats. Proestrus and estrus female rats were trained in a Y-maze. After a 4-hour delay, spatial recognition memory was assessed. The rats were administered naloxone (2 mg/kg) or saline before training, after training or before the retention test. The administration of naloxone to the estrus and proestrus rats before and after training had no significant effects on their preference for the novel arm of the Y-maze. Injection of naloxone to estrus rats before the retention test enhanced their preference for the novel arm of the Y-maze, whereas, in the case of the proestrus rats, naloxone decreased their preference for the novel arm. Therefore, it can be concluded that naloxone enhances the retrieval of spatial recognition memory in the estrus phase rat, but that it impairs retrieval in the proestrus phase rat. This finding indicates that there is an interaction between ovarian hormone levels and opioids in cognitive function, so that naloxone prevents the facilitatory role of ovarian hormones in the retrieval of spatial memory.