Effects of acute and long-term treatments with thyrotropin-releasing hormone on locomotor activity and jumping behavior in mice

Abstract The acute and chronic effects of thyrotropin-releasing hormone (TRH) on ambulation and, in combination with apomorphine, on jumping behavior were investigated in mice. A single administration of TRH (1–10 mg/kg SC) produced an initial hyperactivity in a dose-dependent manner. Following administration of TRH (1–10 mg/kg SC) for 21 successive days, the stimulatory effect on locomotion progressively increased. Haloperidol exerted a biphasic action on hyperlocomotion induced by acute and repeated TRH, i.e., stimulation at lower doses (0.01–0.02 mg/kg SC) and inhibition at higher doses (0.05–1 mg/kg SC). When TRH was administered in combination with low doses of apomorphine, locomotor activity was inhibited but jumping behavior occurred. The inhibitory effect of low doses of apomorphine on locomotion was shifted from doses of 0.1–0.25 mg/kg SC of apomorphine for acute TRH (10 mg/kg) to 0.25–0.35 mg/kg for repeated TRH (10 mg/kg), whereas the stimulatory effect of higher doses of apomorphine (0.5–1 mg/kg SC) on locomotion tended to decrease with repeated TRH. Jumping behavior induced by the combined treatment of TRH and apomorphine was proportional to the dose of TRH but exhibited an inverted-U response to the dose of apomorphine. Chronic TRH (10 mg/kg) in combination with apomorphine (0.1–1 mg/kg SC) also produced jumping behavior, but the dose-response curve for apomorphine was shifted to the right. The present results suggest that repeated treatment with TRH in mice produces hyperlocomotion, despite attenuation of both pre- and postsynaptic receptor activity, and that the inhibitory effect of repeated TRH on presynaptic receptors may be more potent than that on postsynaptic receptors.