ß-Casomorphin Inhibits Thyrotropin Secretion in Rats

Summary

The effects of β-casomorphin on thyrotropin-releasing hormone (TRH) and thyrotropin (TSH) secretion in rats were studied, β-casomorphin (300 μg/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 β-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 β-casomorphin, but the plasma TSH response to TRH was not. Naloxone partially blocked the inhibitory effect of β-casomorphin on plasma TSH levels. In the haloperidol-pretreated group, the inhibitory effect of β-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 β-casomorphin acts at the hypothalamus to inhibit TRH release, and its effect are modified by a dopaminergic system.

Key words

β-casomorphin - thyrotropin-releasing hormone - thyrotropin - haloperidol