beta-Neoendorphin inhibits thyrotrophin secretion in rats

The effects of beta-neoendorphin on thyrotrophin-releasing hormone (TRH) and thyrotrophin (TSH) secretion in rats were studied. beta-neoendorphin (500 micrograms/kg) was injected iv, and the rats were decapitated serially. TRH, TSH, thyroxine (T4) and 3,3',5-triiodothyronine (T3) were measured by means of a specific radioimmunoassay for each. Hypothalamic immunoreactive TRH (ir-TRH) content increased significantly after beta-neoendorphin injection, and plasma concentrations tended to decrease, but not significantly so. Plasma TSH levels decreased significantly in a dose-related manner with a nadir at 40 min. Plasma T4 and T3 levels did not change after the injection. Plasma ir-TRH and TSH responses to cold were significantly inhibited by beta-neoendorphin, but the plasma TSH response to TRH was not. Naloxone partially prevented the inhibitory effect of beta-neoendorphin on TSH release. In the haloperidol- or 5-hydroxytryptophan-pretreated group, the inhibitory effect of beta-neoendorphin on TSH release was prevented, but not in the L-dopa- or para-chlorophenylalanine-pretreated group. These drugs alone did not affect plasma TSH levels at the dose used. These findings suggest that beta-neoendorphin acts on the hypothalamus by inhibiting TRH release, which may be modified by amines of the central nervous system.