Involvement of dopamine and excitatory amino acid transmission in novelty-induced motor activity.

The increase in locomotor activity expressed by rats in a novel environment demonstrates individual variability, and the present study evaluated an hypothesis that the variability resides, in part, in differences in neurotransmission in the nucleus accumbens, ventral tegmental area or ventral pallidum. Rats were divided into equal groups expressing either a high or low response in a novel open field. Subsequently, dopamine, the excitatory amino acid agonist alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) or the mu opioid agonist [D-Ala2, MePhe4, Gly-ol5] enkephalin (DAMGO) was microinjected into one of the three brain nuclei, and motor activity was monitored. All three drugs produced a dose-dependent elevation in motor activity in all three brain nuclei. However, the motor response elicited by dopamine in the ventral pallidum and nucleus accumbens was significantly greater in the rats demonstrating a high locomotor response to novelty. Similarly, the motor response elicited by AMPA in the ventral pallidum, nucleus accumbens or ventral tegmental area was enhanced in the high versus low responding rats. In contrast, at no dose and in no brain nucleus was the motor response to DAMGO different between high and low responding rats. These data indicate that alterations in dopamine and excitatory amino acid but not enkephalin neurotransmission in the ventral pallidum, nucleus accumbens and ventral tegmental area are associated with differences in motor activity expressed by animals in a novel environment.