Involvement of dopamine receptors of the dorsal hippocampus on the acquisition and expression of morphine-induced place preference in rats.
In the present study, the effects of bilateral intrahippocampal CA1 injections of dopamine receptor agonists and antagonists on the acquisition and expression of morphine-induced place preference were examined in male Wistar rats. Subcutaneous administration of different doses of morphine sulphate (0.5-10 mg/kg) produced a conditioned place preference (CPP) dose-dependently. Using a 3-day schedule of conditioning, it was found that dopamine D1 receptor agonist, SKF 38393 (0.01-1 microg/rat), dopamine D1 receptor antagonist, SCH 23390 (0.25-1 microg/rat), dopamine D(2/3) receptor agonist, quinpirole (0.3-3 microg/rat) or dopamine D2 receptor antagonist, sulpiride (0.04-5 microg/rat) did not produce significant place preference. The administration of SKF 38393 (1 microg/rat) significantly potentiated the acquisition of morphine (0.5 and 2.5 mg/kg)-induced place preference. This potentiation was reversed by SCH 23390 (1 microg/rat) pretreatment. Quinpirole injection (0.3 microg/rat) induced CPP in combination with the lower doses of morphine but decreased the response of the higher doses of morphine. These responses of quinpirole were reversed by sulpiride (5 microg/rat) pretreatment. SCH 23390 or sulpiride reduced the acquisition of morphine (7.5 mg/kg)-induced place preference. The administration of sulpiride, but not other drugs, during acquisition showed an increase in the locomotor activity on the testing days. SKF 38393, SCH 23390 or sulpiride, but not quinpirole when used before testing, reduced the expression of morphine-induced place preference. Sulpiride, but not other drugs, increased locomotion when used before testing. It is concluded that dorsal hippocampal dopamine receptors may play an active role in morphine reward.