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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.

Cannabinoids, which are the active compounds of marijuana, produce some pharmacological effects similar to the opioids. In addition, there are functional interactions between the cannabinoid and opioid systems. In this study, we investigated the effects of intraperitoneal (i.p.) injection of opioid drugs on responses induced by intracentral amygdala (intra-CeA) microinjection of cannabinoid CB1 receptor agents in rats, using the elevated plus maze test of anxiety. I.p. injection of morphine (6 and 9 mg/kg) 30 min before testing, increased the percentage open arm time (%OAT) and the percentage open arm entries (%OAE), but not locomotor activity, showing an anxiolytic-like response. I.p. administration of the opioid receptor antagonist, naloxone (1 mg/kg) 15 min before testing, significantly reduced %OAT, but not %OAE and locomotor activity. The drug, however, tended to decrease locomotor activity. Intra-CeA administration of arachidonylcyclopropylamide (ACPA, an agonist shown to selectively activate CB1 receptors; 1.25 and 5 ng/rat) increased %OAT and %OAE but not locomotor activity, indicating an anxiolytic-like response. Coadministration of morphine (6 mg/kg, i.p.) plus ACPA (0.125 ng/rat, intra-CeA) increased the anxiolytic-like response. Administration of naloxone reversed the effects of intra-CeA injection of ACPA. Intra-CeA administration of the cannabinoid CB1 receptor antagonist, AM251 (2.5, 25, and 100 ng/rat) did not alter %OAT and %OAE, but the higher doses of the drug (25 and 100 ng/rat) reduced locomotor activity. Coadministration of morphine (6 mg/kg) or naloxone (0.1 mg/kg) with AM251 showed an anxiolytic-like response. In conclusion, the results may indicate an anxiolytic-like effect for cannabinoid CB1 receptors of the CeA and the existence of an interaction between the cannabinoid and the opioid systems in the modulation of anxiety.

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