Evidence for dopaminergic control of thyrotrophin secretion in the rat

The effects of two dopamine agonists (apomorphine and bromocriptine) and a dopamine antagonist (pimozide) on cold- or thyrotrophin releasing hormone (TRH)-induced TSH secretion were studied in normal male rats. Apomorphine given in various doses (0-5-10 mg/kg body wt) 10 min before exposure to cold significantly depressed TSH secretion. Large doses of bromocriptine (5-10 mg/kg body wt) given 1 h before exposure to cold, also blocked this response whereas a smaller dose (2-5 mg/kg body wt) given 30 min, 1, 3 or 6 h before cold exposure or repeated doses (0-1-2-5 mg/kg body wt) for 3 days did not modify cold-induced TSH secretion. Pimozide given in various doses (-25-2-5 mg/kg body wt) 1 h before exposure to cold did not alter the cold response, but 2-5 mg/kg reversed the inhibition caused by apomorphine or bromocriptine. None of these drugs affected TRH-induced TSH secretion. These results suggest that there are no dopaminergic receptors on the pituitary thyrotrophs, but that dopamine might be an inhibitory transmitter in the brain involved in the regulation of TSH secretion in the rat.