Dermorphin inhibits basal and cold induced thyrotropin secretion in rats

The effects of dermorphin on thyrotropin-releasing hormone (TRH) and thyrotropin (TSH) secretion in rats were studied. Dermorphin (400 micrograms kg-1) was injected i. v. and the rats were serially decapitated. TRH, TSH and thyroid hormones were measured by radioimmunoassay. The hypothalamic immunoreactive TRH (ir-TRH) content decreased significantly after dermorphin injection, whereas its plasma concentration 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 plasma thyroid hormone levels were not changed significantly. The plasma ir-TRH and TSH responses to cold were inhibited by dermorphin, but the plasma TSH response to TRH was not. Naloxone partially blocked the inhibitory effect of dermorphin on TSH levels. In the para-chlorophenylalanine or pimozide pretreated groups the inhibitory effect of dermorphin on TSH levels was prevented, but not in the groups pretreated with 5-hydroxytryptophan or L-DOPA. These drugs alone did not affect plasma TSH levels in terms of the dose used. The inactivation of TRH immunoreactivity by plasma of hypothalamus in vitro after dermorphin injection did not differ from that of the control. These findings suggest that dermorphin acts on the hypothalamus to inhibit TRH release, and its effects are mediated via opioid receptor and modified by amines of the central nervous system.