NEUROTRANSMITTER CONTROL OF THYROTROPHIN SECRETION IN HYPOTHYROID RATS

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Abstract
The effect of drugs modifying dopaminergic, noradrenergic and serotonergic systems on the serum TSH [thyrotropin] levels was studied in the male rats made hypothyroid by giving 10 mg/l of propylthiouracil in tap water for 4 days. Apomorphine (2.5 mg/kg, given once-30 min before sacrifice; or 4 times-120, 90, 60 and 30 min before sacrifice), bromocryptine (10 and 20 mg/kg, 2 h before sacrifice) and piribedil (50 and 100 mg/kg, 4 h) decreased the serum TSH concentrations. The effect of a single dose of apomorphine (2.5 mg/kg, 30 min before sacrifice) was partially reversed by a pimozide pre-treatment (2.5 mg/kg, 2 h). Clonidine (1 mg/kg but not 0.01 or 0.1 mg/kg, 2 h before sacrifice) further elevated the high TSH levels whereas .alpha.-methyl-p-tyrosine (300 mg/kg, 2 h), phenoxybenzamine (50 mg/kg, 2 h) and diethyldithiocarbamate (300 mg/kg, 2 h) significantly decreased TSH secretion. The effect of clonidine (1 mg/kg, 2 h) was partially antagonized by phenoxybenzamine (20 mg/kg, 2 h). A high dose of 5-HTP [5-hydroxytryptophan] (300 mg/kg, 2 h) increased serum TSH concentrations whereas p-chlorophenylalanine (100 mg/kg, 2 h) decreased it. When both drugs were given together, the serum TSH levels did not change. L-tryptophan (100-300 mg/kg, 2 h) uniformly decreased the serum TSH concentrations in all experiments. In hypothyroid rats, the secretion of TSH may be inhibited by the dopaminergic system, and stimulated by the noradrenergic system. The effect of 5-HT [serotonin] pathways remained an open question.