Robust sensitization to amphetamine following intra-VTA cholera toxin administration

Studies were conducted regarding the hypothesis that enhanced cAMP formation in the ventral tegmental area (VTA) affects the magnitude of the behavioral responses elicited by psychostimulant drugs. In the first paradigm, spontaneous and amphetamine-elicited locomotor activity was measured at various times following injection of cholera toxin (CTX), a known activator of adenylate cyclase, into the VTA. Adult male rats showed enhanced amphetamine-stimulated locomotor activity when tested 1 or 3 days after treatment with 0.5 microgram CTX into the VTA. Spontaneous activity was markedly increased 1 and 3 days following treatment with the higher dose of 1.0 microgram CTX into the VTA, and amphetamine was still capable of eliciting an increased level of locomotor activity above this high baseline. Using a paradigm in which repeated amphetamine injections were given on an intermittent schedule following injection of CTX into the VTA, it was observed that a single low dose of amphetamine (0.5 mg/kg) given 1 day after CTX (0.5 microgram) injection into the VTA led to a markedly potentiated locomotor activity response to subsequent treatment with amphetamine. Evaluation of this protocol (initial amphetamine dose 24 h after CTX injection, and challenge treatment of amphetamine at various times thereafter) showed that the sensitization was long-lasting and could be observed after an initial dose of amphetamine as low as 0.1 mg/kg. A sensitized response was also expressed when the challenge dose was given directly into the nucleus accumbens. These data suggest that injection of CTX into the VTA enhances the induction of locomotor sensitization to amphetamine.