Involvement of the ventral tegmental area (VTA) in morphine-induced memory retention in morphine-sensitized rats

In the present study, the effects of intra-ventral tegmental area (VTA) injections of morphine on memory retention of a one-trial passive avoidance task have been investigated in morphine-sensitized rats. Retrieval was examined 24 h after training and used as memory retention. Sensitization was obtained by subcutaneous (s.c.) injections of morphine, once daily for 3 and 5 days free of the opioid before training. Post-training administration of the both systemic (2.5, 5 and 7.5 mg/kg, s.c.) and intra-VTA (5 and 7.5 μg/rat) of morphine, dose-dependently decreased memory retention. The response induced by post-training administration of intra-VTA morphine (7.5 μg/rat) was significantly reversed in morphine-sensitized rats. The inhibition of morphine-induced amnesia in morphine-sensitized rats was decreased by once daily injections of naloxone (0.5, 1 and 2 mg/kg, s.c.), SCH 23390 (0.025, 0.05 and 0.1 mg/kg, s.c.) or sulpiride (25, 50 and 100 mg/kg, s.c.), during the sensitization. The results suggest that VTA has an important role in morphine-induced amnesia and morphine sensitization affects this process through opioid and dopamine receptors.