Interaction between thyrotropin-releasing hormone and the mesolimbic dopamine system.

The involvement of the mesolimbic dopamine (DA) system in the excitatory behavioral effects of thyrotropin-releasing hormone (TRH) has been a controversial topic. In this study TRH was injected into the nucleus accumbens, lateral ventricles or ventral tegmental area and changes in spontaneous motor activity and metabolism of DA in the nucleus accumbens and striatum measured. Injection of TRH into all three areas of the brain produced an increase in photocell counts of locomotor activity and, in the nucleus accumbens, a significant decrease in photocell counts of rearing was measured. Injection of TRH into the nucleus accumbens caused a marked increase in metabolism of DA in both the nucleus accumbens and striatum. A smaller increase in metabolism of DA was also observed after injection of TRH into the lateral ventricles, but no significant change was found after intra-ventral tegmental administration of TRH. These data indicate that while TRH probably acts in the nucleus accumbens to enhance the metabolism of DA, and presumably release of DA, the excitatory behavioral effect of TRH is only partially mediated by this dopaminergic mechanism.