Repeated administration of triiodothyronine (T 3) to rats enhances nigrostriatal and mesolimbic dopaminergic behavioural responses

Abstract

A once daily injection to rats of triiodothyronine (T 3; 100 μg/kg s.c.) for 10 days enhanced the locomotor responses of rats pretreated with tranylcypromine (5 mg/kg) to an injection into the nucleus accumbens of dopamine (5 μg bilaterally), when tested 24 hr after the last T 3 administration. This treatment did not, however, alter the control responses of tranylcypromine-pretrealed rats to injection of saline (1 μl bilaterally). Injection of T 3 for 10 days enhanced the circling responses of unilateral nigrostriatal-lesioned rats to apomorphine (0.5 mg/kg) but not to methamphetamine (0.5 mg/kg). The probable explanation for the latter result was the finding that repeated T 3 administration lowered basal striatal dopamine concentrations. The treatment also produced a trend towards reduced dopamine concentrations in the n. accumbens, although the difference did not reach statistical significance. These data suggest that repeated T 3 injection causes an increased post-synaptic function of both the nigrostriatal and mesolimbic dopamine systems in rat brain.