Involvement of GABA B receptors of the dorsal hippocampus on the acquisition and expression of morphine-induced place preference in rats

Abstract

In the present study, effects of intra-hippocampal CA1 (intra-CA1) injections of GABA B 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 (0.5–6 mg/kg) produced a dose-dependent conditioned place preference (CPP). Using a 3-day schedule of conditioning, it was found that the GABA B receptor agonist, baclofen (0.5–2 μg/rat; intra-CA1), or the GABA B receptor antagonist, phaclofen (1–3 μg/rat; intra-CA1), did not produce a significant place preference or place aversion. Intra-CA1 administration of baclofen (1 and 2 μg/rat; intra-CA1) decreased the acquisition of CPP induced by morphine (3 mg/kg; s.c.). On the other hand, intra-CA1 injection of phaclofen (1 and 2 μg/rat; intra-CA1) in combination with a lower dose of morphine (1 mg/kg) elicited a significant CPP. The response of baclofen (2 μg/rat; intra-CA1) was reversed by phaclofen (4 and 6 μg/rat; intra-CA1). Furthermore, intra-CA1 administration of baclofen but not phaclofen before testing significantly decreased the expression of morphine (3 mg/kg; s.c.)-induced place preference. Baclofen or phaclofen injections had no effects on locomotor activity on the testing sessions. It is concluded that the GABA B receptors in dorsal hippocampus may play an active role in morphine reward.