Influence of pre-exposure to morphine on cannabinoid-induced impairment of spatial memory in male rats.

In the present study, we investigated the effects of repeated morphine pre-treatment on impairment of spatial memory acquisition induced by intra dorsal hippocampus (intra-CA1) administration of the non-selective cannabinoid CB1/CB2 receptor agonist, WIN55,212-2 in adult male rats. 2-day version of Morris water maze task has been used for the assessment of spatial memory. On the training day, rats were trained by a single training session of eight trials and 24 h later a probe trial test consist of 60s free swim period without a platform and the visible test was administered. Animals received pre-treatment subcutaneous (s.c.) injections of morphine, once daily for three days followed by five days drug-free treatment before training trials. The results indicated that bilateral pre-training intra-CA1 infusions of WIN55,212-2 (0.25 and 0.5 μg/rat) impaired acquisition of spatial memory on the training and test day. The amnesic effect of WIN55, 212-2 (0.5 μg/rat) was prevented in rats previously injected with morphine (20 mg/kg/day × 3 days, s.c.). Improvement in spatial memory acquisition in morphine-pretreated rats was inhibited by once daily administration of naloxone (1 and 2 mg/kg, s.c.) 15 min prior to injection of morphine for three days. The results suggest that sub-chronic morphine treatment may produced sensitization to cannabinoids, which in turn reversed the impairment of spatial memory acquisition induced by WIN55,212-2 and mu- opioid receptors may play an important role in this effect.