Amphetamine administered to the ventral tegmental area but not to the nucleus accumbens sensitizes rats to systemic morphine: lack of conditioned effects.

Different groups of rats received conditioning training with bilateral injections of amphetamine (2.5 micrograms/0.5 microliters/side) either into the ventral tegmental area (VTA) or the nucleus accumbens (NAC) and were tested on separate occasions for conditioned locomotor activity with saline and cross-sensitization to the locomotor activating effects of morphine (1.0 mg/kg, i.p.). Intra-VTA injections of amphetamine had no effect acutely but subsequently produced a sensitized locomotor response to morphine. On the other hand, intra-NAC amphetamine produced increased locomotion but with no evidence of sensitization either during training or on the test with morphine. No evidence for conditioning of the locomotor activating effects of amphetamine or for stimulus control of the expression of the sensitized response to morphine was obtained with either the intra-VTA or the intra-NAC injections. These results confirm earlier findings indicating that the site of action of amphetamine critical for the development of behavioral sensitization is the VTA and not the NAC and extend them to the case of cross-sensitization to morphine. They also demonstrate that behavioral sensitization can be obtained in the absence of conditioned effects and, therefore, that the two phenomena are distinct. Implications of the present findings for an understanding of the mode and site of action of amphetamine in the development of conditioned locomotor activity are discussed.