Possible involvement of nigrostriatal dopamine system in the inhibition of thyrotropin secretion in the rat.

The dopaminergic inhibition of cold-stimulated thyrotropin (TSH) secretion was studied in male rats. Serum TSH levels were decreased by apomorphine (1 mg/kg i.p.) but not by dopamine (DA, 0.2--5 mg/kg s.c.). This effect of apomorphine was abolished by haloperidol (1 mg/kg i.p.), metoclopramide and sulpiride (10 mg/kg i.p.) but not by domperidone (0.1--5 mg/kg i.p.). Domperidone does not cross the blood-brain barrier while the other DA receptor antagonists do so. High doses of domperidone itself inhibited the cold-induced TSH secretion whereas the other DA antagonists did not. DA (1--10 micrograms/rat) into the medial basal hypothalamus (MBH) had no effect but 10--50 micrograms/rat into the 3rd ventricle inhibited the cold-stimulated TSH secretion. 6-Hydroxydopamine infusion after desipramine pretreatment (25 mg/kg i.p.) did not affect TSH secretion when given into the MBH (2 micrograms/rat), the 3rd ventricle (100 micrograms/rat) or unilaterally into the substantia nigra (SN, 6 micrograms/nucleus), but bilateral nigral infusions abolished the TSH cold response. The inhibitory effect of apomorphine (0.1 and 0.5 mg/kg i.p.) was amplified only in the rats whose SN was unilaterally destroyed. These results show that tuberoinfundibular DA neurons do not affect TSH secretion. Instead, the inhibition is mediated through the hypothalamic projections of the nigrostriatal DA system.