Apomorpine inhibits the prolactin but not the TSH response to thyrotropin releasing hormone

Pretreatment of normal subjects with apomorphine, a dopamine receptor agonist, resulted in significant impairment of the subsequent prolactin (PRL) response to thyrotropin releasing hormone (TRH). The mean maximal increment of PRL was 27.9 ±2.4 ng/ml after TRH alone, and 11.9 ± 3.0 ng/ml (P < 0.001) after apomorphine plus TRH. In contrast, the thyrotropin (TSH) response to TRH was unaffected by apomorphine (10.5 ± 2.9 νs. 9.5 ± 1.8 μU/ml, P > 0.5). These results demonstrate that dopaminergic effects are capable of inhibiting PRL responses to TRH, probably via a direct effect on the lactotrope cell. They also suggest that dopaminergic influences are not important in the regulation of TSH secretion.