Abstract

In the present study, the effects of intra-central amygdala (CeA) injections of dopamine (DA) D2-like receptor agonist and antagonist on the acquisition and expression of morphine-induced place preference in male Wistar rats have been investigated. Subcutaneous administration of different doses of morphine sulphate (0.5-10 mg/kg) produced a dose-dependent conditioned place preference (CPP). Using a 3-day schedule of conditioning, it was found that the DA D2/D3 receptor agonist, quinpirole (0.3-3 mug/rat), or the DA D2 receptor antagonist, sulpiride (0.04-5 mug/rat), did not produce a significant place preference or place aversion. Intra-CeA administration of quinpirole (0.3 and 1 mug/rat) with an ineffective dose of morphine (0.5 mg/kg) elicited a significant CPP. On the other hand, quinpirole (0.3 mug/rat injection into the CeA induced CPP in combination with the lower doses of morphine (0.5 and 2.5 mg/kg), but decreased the response of higher dose (7.5 mg/kg) of morphine. This response of quinpirole was attenuated by sulpiride (0.2 mug/rat). Sulpiride by itself (0.04-5 mug/rat) reduced the acquisition of morphine (7.5 mg/kg)-induced place preference. The administration of the higher dose of sulpiride (1 and 5 mug/rat) or the higher dose of quinpirole (3 mug/rat) during acquisition decreased the locomotor activity of the animals on the testing days. The injection of the low dose of quinpirole (0.3 mug/rat) on the test day reduced the expression of morphine-induced CPP, but the high dose of quinpirole (3 mug/rat) potentiated this expression. The administration of sulpiride (5 mug/rat) attenuated the quinpirole response. The injection of sulpiride (1 and 5 mug/rat) abolished the expression of morphine-induced CPP. It is concluded that the CeA DA D2-like receptors may play an active role in morphine reward. (C) 2002 Elsevier Science Inc. All rights reserved.