GABA sub(B) receptors within the ventral tegmental area are involved in the expression and acquisition of morphine-induced place preference in morphine-sensitized rats

The influence of intra-ventral tegmental area administration of gamma-amino-butyric-acid-B (GABA sub(B)) receptor agonist and antagonist on the expression and acquisition of morphine-induced conditioned place preference (CPP) in morphine-sensitized female rats was examined. Our pilot experiment showed that subcutaneous administration of morphine-(2.5, 5 and 7.5 mg/kg) induced CPP. Administration of one dose daily morphine (5 mg/kg) for 3 days followed by 5 days rest, enhanced the conditioning induced by ineffective doses of morphine (0.25, 0.5 and 1 mg/kg). Injections of GABA sub(B) receptor agonist, baclofen, (1.5 and 12 ?g/rat) reduced the expression of morphine CPP whereas the dose of 6 ?g/rat of the drug increased it. Baclofen also significantly reduced the acquisition of morphine CPP in morphine-sensitized animals. Administration of GABA sub(B) receptor antagonist, CGP 35348, significantly reduced the expression (12 ?g/rat) and acquisition (1.5, 6 and 12 ?g/rat) of morphine CPP in morphine-sensitized animals. In conclusion, results confirmed the importance of GABA sub(B) receptors within the ventral tegmental area of morphine CPP in morphine-sensitized female rats.