Previous Exposure to VTA Amphetamine Enhances Cocaine Self-Administration under a Progressive Ratio Schedule in an NMDA, AMPA/Kainate, and Metabotropic Glutamate Receptor-Dependent Manner

Previous exposure to amphetamine (AMPH) in the ventral tegmental area (VTA) enhances cocaine self-administration in a D1 dopamine receptor-dependent manner. The present study examined the contribution of VTA NMDA, AMPA/kainate, and metabotropic glutamate (mGlu) receptors to this effect. Rats in different groups received three intra-VTA injections, one every third day, of either saline (0.5 μl/side), AMPH (2.5 μg/0.5 μl/side), AMPH+CPP (NMDA receptor antagonist; 10 μM or 100 μM/0.5 μl/side), AMPH+CNQX (AMPA/kainate receptor antagonist; 0.3 mM or 1 mM/0.5 μl/side), AMPH+MCPG (mGlu receptor antagonist; 0.5 mM or 50 mM/0.5 μl/side), or the glutamate receptor antagonists alone. Starting 7–10 days after the last pre-exposure injection, rats were trained to self-administer cocaine (0.3 mg/kg/infusion) and then tested under a progressive ratio (PR) schedule of reinforcement for 6 consecutive days. As reported previously, VTA AMPH pre-exposed rats worked more and obtained more infusions of cocaine than saline pre-exposed animals. Coadministration of CPP, CNQX, or MCPG with AMPH during pre-exposure dose-dependently blocked this enhancement of cocaine self-administration. Rats pre-exposed to the glutamate receptor antagonists alone did not differ on the test days from the saline pre-exposed controls. These results indicate that, in a manner paralleling the induction of sensitization of the locomotor stimulating effects of AMPH, activation of NMDA, AMPA/kainate, and mGlu receptors during pre-exposure to AMPH in the VTA is necessary for the enhancement of cocaine self-administration to develop.