Morphine-induced impairment of spatial memory acquisition reversed by morphine sensitization in rats.

In the present study, the effects of systemic administration of morphine on spatial memory acquisition of a Morris water maze (MWM) task have been investigated in morphine-sensitized rats. Sensitization was obtained by subcutaneous (s.c.) injection of morphine, once daily for 3 days followed by 5 days free of the opioid before training. Then the rats were trained in the water maze task which was consisted of single training session of 8 trials and 24 h later a probe trial consist of 60 s free swim period without a platform and then non-spatial visual discrimination task was done. Our data indicated that pre-training single administration of morphine (2.5, 5 and 7.5 mg/kg) decreased the spatial memory acquisition in Morris water maze task with a maximum effect of 5 mg/kg of morphine. Amnesia induced by pre-training morphine was significantly reversed in morphine-sensitized (15 and 20 mg/kg) rats. The inhibition of morphine-induced amnesia in morphine-sensitized rats was decreased by once daily administration of naloxone (1 and 2 mg/kg) 30 min prior to injection of morphine during sensitization. The results suggest that morphine sensitization reverses the impairment of spatial memory acquisition induced by morphine and it is postulated that opioid receptors may play an important role in this effect.