Thyrotropin-releasing hormone: hyperactivity and mesolimbic dopamine system in rats

The mechanism of stimulatory action of thyrotropin-releasing hormone (TRH) on spontaneous motor activity was investigated in rats. TRH produced a significant hyperactivity with intraperitoneal administration of 20 mg/kg or bilateral injection of 10 micrograms into the nucleus accumbens septi (NAS). Following bilateral injection of 6-hydroxydopamine into the mesolimbic dopamine (DA) pathway, the hyperactivity induced by TRH was not altered, whereas the response to apomorphine given intraperitoneally or DA injected into the NAS was clearly enhanced. The TRH-induced hyperactivity was remarkably suppressed by alpha-methyltyrosine and in contrast, augmented by pargyline. Systemic injection of aminooxyacetic acid in a dose producing behavioral depression reduced markedly the TRH-induced hyperactivity. Bilateral injection of ethanolamine O-sulphate (100 micrograms) into the NAS produced no behavioral depression per se, but remarkably attenuated the hyperactivity response to TRH or DA (20 micrograms) given intraperitoneally or into the NAS. Both TRH (10(-5) and 10(-4) M) and methamphetamine (10(-6)--10(-4) M increased the spontaneous release of 14C-DA from rat NAS slices. These findings suggest that TRH induces hyperactivity by enhancing DA release from nerve terminals in the NAS without a direct stimulation of the post-synaptic DA recptors. TRH and GABA, independently or via interaction between them, may play a reciprocal regulatory role in the activity of the mesolimbic DA system.