Amnesia induced by morphine in spatial memory retrieval inhibited in morphine-sensitized rats

The present study investigated the effect of morphine sensitization on the impairment of spatial memory retrieval induced by acute morphine in adult male rats. Spatial memory was assessed by 2-day Morris water maze task which included training and test day. On the training day, rats were trained by a single training session of 8 trials. On the test day, a probe trial consisting of 60 s free swim period without a platform and the visible test were administered. Morphine sensitization was induced by subcutaneous (s.c.) injection of morphine, once daily for 3 days followed by 5 days without drug treatment before training. The results indicated that acute administration of morphine (7.5 mg/kg, s.c.) before testing impaired spatial memory on the test day. Pre-test morphine-induced amnesia decreased in morphine-sensitized (15 and 20 mg/kg, s.c.) rats. Improvement in spatial memory retrieval in morphine-sensitized rats was inhibited by once daily administration of naloxone (1 and 2 mg/kg, s.c.) 30 min prior to the injection of morphine for three days. The results suggest that morphine sensitization reverses the impairment of spatial memory retrieval induced by acute morphine and it is implied that mu-opioid receptors may play an important role in this effect.