Beta-casomorphin inhibits thyrotropin secretion in rats.

The effects of beta-casomorphin on thyrotropin-releasing hormone (TRH) and thyrotropin (TSH) secretion in rats were studied. beta-casomorphin (300 micrograms/kg) was injected iv, and the rats were serially decapitated. TRH, TSH and thyroid hormone were measured by radioimmunoassay. The hypothalamic immunoreactive TRH (ir-TRH) contents increased significantly after beta-casomorphin injection, whereas its plasma concentrations tended to decrease, but not significantly. The plasma TSH levels decreased significantly in a dose-related manner with a nadir at 40 min. after the injection. The thyroid hormone levels showed no change. The plasma ir-TRH and TSH responses to cold were inhibited by beta-casomorphin, but the plasma TSH response to TRH was not. Naloxone partially blocked the inhibitory effect of beta-casomorphin on plasma TSH levels. In the haloperidol-pretreated group, the inhibitory effect of beta-casomorphin on plasma TSH levels was prevented, but not in the L-DOPA, para-chlorophenylalanine- or 5-hydroxytryptophan-pretreated group. These drugs alone did not affect plasma TSH levels in terms of the dose used. The findings suggest that beta-casomorphin acts at the hypothalamus to inhibit TRH release, and its effect are modified by a dopaminergic system.