In the present study, the effects of bilateral injections of D2 dopamine receptor antagonist into nucleus accumbens on nicotine-induced improvement of memory impairment by morphine were investigated. A step-through passive avoidance task was used for the assessment of memory retention in male wistar rats. Post-training subcutaneous (s.c.) administration of morphine (5, 7.5 mg/kg) induced impairment of memory retrieval when tested 24 hour later .The response induced by morphine (5, 7.5 mg/kg) was restored by pre-test injection of the same dose of drug, indicating state-dependent memory. Pre-test intraperitoneal (i.p.) administration of nicotine (0.4mg/kg) improved post-training morphine elicited memory impairment. Pre-test administration of the dopamine D2 receptor antagonist, sulpiride (0.25, 0.5μg/rat), intra -NAc which had no effect alone prevented the nicotine reversal of morphine effect on memory. Our data indicate that D2dopamine receptors in the nucleus accumbens are involved in the reversing effect of nicotine on morphine induced state-dependency.
