D-AMPHETAMINE-LIKE STIMULUS PROPERTIES ARE PRODUCED BY MORPHINE INJECTIONS INTO THE VENTRAL TEGMENTAL AREA BUT NOT INTO THE NUCLEUS-ACCUMBENS

We investigated whether injections of morphine into the ventral tegmen tal area (VTA) or the nucleus accumbens (NAS) could produce amphetamin e-like stimulus properties and locomotor stimulant effects. Rats were trained to discriminate 1.0 mg/kg D-amphetamine from saline using both VI-30 and FR-16 reinforcement schedules and they were then tested fol lowing bilateral injections of morphine sulfate (2.5, 5.0 and 10.0 mu g/side) either into the VTA or the NAS. Intra-VTA injections of morphi ne produced significant increases in amphetamine-lever responding that were comparable to increases observed following intra-NAS D-amphetami ne (2.5, 5.0 and 10.0 mu g/side). Such increases were not observed, ho wever, following intra-NAS injections of morphine. Tests for the effec ts of intracerebral morphine and D-amphetamine on locomotor activity p roduced a similar pattern of results. Locomotor activity was increased following intra-VTA morphine and intra-NAS D-amphetamine injections, but not after intra-NAS morphine injections. Together, these findings indicate that activation of opioid receptors within the VTA, but not t he NAS, can produce a behavioral state which mimics to some degree the state produced by systemic and intra-NAS injections of D-amphetamine.