Role of central dopaminergic and 5-hydroxytryptaminergic projections in the behavioral responses elicited by thyrotropin-releasing hormone in rats.

Abstract

The systemic administration of thyrotropin-releasing hormone (TRH) to rats elicits locomotor activation, wet dog shakes, jaw movements, paw licking and tail rattle. Central dopamine (DA) and 5-hydroxytryptamine (5-HT) systems and peripheral vagal afferents have been implicated in these responses. To define this circuitry further, the effects of lesions of these pathways on the behavioral responses elicited by intraperitoneal (IP) injections of TRH were assessed in rats. Lesions of the DAergic innervation of the nucleus accumbens did not affect the locomotor activation, wet dog shakes, paw licking, jaw movements or tail rattle elicited by TRH. This is consistent with our in vivo microdialysis finding that TRH did not affect the release of DA in the nucleus accumbens at a dose that strongly increased locomotor activity. Depletion of spinal 5-HT significantly decreased the wet dog shakes induced by TRH, while depletion of forebrain 5-HT had no effect on any behavior. Bilateral vagotomy did not affect the locomotor response to TRH or any of the other behaviors measured. Taken together these results suggest that the DAergic mesolimbic, the 5-HTergic projections to the forebrain and vagal afferent systems are not mediators of the behavioral responses to systemic TRH. In contrast, the raphe-spinal 5-HTergic projection system may serve to modulate the wet dog shakes elicited by this peptide.