Head turning induced by unilateral intracaudate thyrotropin-releasing hormone (TRH) injection in the cat

Unilateral intracaudate injection of TRH in cats (10 microgram) induced head turning responses similar to the responses produced by dopamine (DA) (10 microgram). Contralateral head turning responses were observed after injections in the central part of the caudate body, whereas, injections placed more laterally induced ipsilateral responses. While haloperidol (3 mg/kg i.m.) suppressed the DA-induced response, it appeared to potentiate the effects of TRH. Cyproheptadine (10 mg/kg i.p.) did not alter the TRH-induced responses. These results suggest that, although TRH induces DA-like effects on postural symmetry, the mediation of TRH effects bypasses striatal dopaminergic and serotonergic receptors. While intracaudate TRH and DA induced head turning, they did not alter the level of head motility as measured by the number of head movements. Haloperidol, but not cyproheptadine pretreatment, decreased the level of head motility providing evidence that postural symmetry is modulated within the striatum, whereas head motility is regulated by extrastriatal dopaminergic mechanisms.