Effect of vagatomy on hyperactivity and increased dopamine turnover induced by intraperitoneal administration of thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH) injected intraperitoneally at doses of 10 mg/kg and 20 mg/kg induced marked hyperactivity in rats. Although regional levels of brain dopamine and its metabolites (DOPAC and HVA) in vagotomized rats did not differ from those in sham-operated controls, the (DOPAC + HVA)/dopamine ratio, an indicator of dopamine turnover, was significantly higher in the nucleus accumbens of TRH-treated sham-operated rats than that in untreated sham-operated controls. TRH injection induced hyperactivity only in sham-operated rats and not in subdiapragmatic bileaterally vagotomized rats. Similarly, bilateral vagotomy completely abolished the TRH-induced increase in dopaminergic turnover in the nucleus accumbens. These results suggest that the dopaminergic system in the nucleus accumbens is involved in hyperactivity induced by TRH, and that TRH mainly affects peripheral receptors. The vagal nerve may be the major pathway from the visceral organs to the brain involved in the etiology of hyperactivity.