Stimulatory effect of PMRPamide (Phe-Met-Arg-Phe-NH2) on thyrotropin secretion in rats

The effect of PMRPamide (Phe-Met-Arg-Phe-NH2) on thyrotropin (TSH) and thyrotropin-releasing hormone (TRH) secretion in rats was studied. PMRPamide (600 micrograms/kg) was injected i.v. and the rats were serially decapitated. TRH, TSH and thyroid hormone were measured by radioimmuassay. The content of immunoreactive TRH (ir-TRH) in hypothalamus significantly decreased after PMRPamide injection, whereas its plasma concentration tended to increase, but not significantly. The level of TSH in plasma increased significantly in a dose-related manner with a zenith at 40 min after the injection, while the level of thyroid hormone showed no changes. The plasma response of ir-TRH to cold was inhibited by PMRPamide. The response of plasma TSH level to TRH was not affected by PMRPamide. The stimulatory effect of PMRPamide on TSH levels was prevented in groups pretreated with L-DOPA or 5-hydroxytryptophan, but not in those pretreated with para-chlorophenylalanine or pimozide. These drugs alone did not affect plasma TSH levels in terms of the dose used. The inactivation of TRH immunoreactivity by hypothalamus or plasma in vitro after PMRPamide injection did not differ from that of the control. These findings suggest that PMRPamide may inhibit the release of TSH from the hypothalamus and that this effect may be modified by amines of the central nervous system.