Effects of clozapine and sulpiride on morphine state-dependent memory in the step-down passive avoidance test

The effects of antipsychotic drugs sulpiride and clozapine on morphine state-dependent memory of passive avoidance task were examined in mice. Post-training administration of morphine (5 mg/kg) led to state-dependent learning with impaired memory retrieval on the test day which was reversed by pre-test administration of the same dose of the opioid (5 mg/kg). In animals where memory was impaired by post-training morphine, the administration of either sulpiride or clozapine before pre-test morphine reduced the improvement of memory produced by the opioid. Co-administration of sulpiride with clozapine did not potentiate their antagonistic response. In conclusion, the inhibition of improvement of memory retrieval by morphine treatment on the test day by the two dopamine receptor antagonists seems to be induced through two different receptor mechanisms.