Glutamate-dopamine interactions in the ventral striatum: role in locomotor activity and responding with conditioned reinforcement.

Previous evidence suggests that glutamatergic limbic afferents participate in the potentiation of responding with conditioned reinforcement produced by intra-accumbens d-amphetamine. The present experiments were designed to investigate glutamate-dopamine interactions in the ventral striatum in both conditioned reinforcement and locomotor activity. Glutamate receptor agonists and antagonists were infused into the nucleus accumbens both alone and in combination with 3 micrograms d-amphetamine, and the effects of these interactions on responding with conditioned reinforcement and locomotor activity were measured. The glutamate receptor agonists NMDA, AMPA and quisqualate (agonists at the NMDA, AMPA and metabotropic glutamate receptor subtypes, respectively) and the antagonists AP5 and CNQX, (antagonists at the NMDA and AMPA receptor subtypes, respectively) were used in these investigations. These compounds were used in a dose range of 0.3 to 3 nmol, except CNQX, which was used in 0.2 to 2 nmol doses. While all agonists and antagonists increased locomotor activity when administered alone, the antagonists attenuated the locomotor response to d-amphetamine. In contrast, the agonists AMPA and quisqualate enhanced d-amphetamine-induced locomotor activity, although NMDA interfered with the effects of d-amphetamine. In the conditioned reinforcement paradigm, both the agonists and the antagonists abolished amphetamine's potentiation of responding with conditioned reinforcement, suggesting that the glutamatergic transmission of information about the conditioned reinforcer could be blocked by glutamate receptor antagonists and disrupted by administration of the agonists. The dissociation between the effects of these excitatory amino acids on amphetamine-induced locomotor activity versus their effects on amphetamine's potentiation of responding with conditioned reinforcement provides insight into the nature of the reward enhancement by accumbens dopamine versus its locomotor stimulant effects.