Litorin (bombesin family) inhibits thyrotropin secretion in rats

The effects of the peripheral administration of litorin on thyrotropin-releasing hormone (TRH) and thyrotropin (TSH) secretion were studied in rats. Litorin (400 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) content increased significantly after litorin 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 20 min. after the injection. The plasma thyroid hormone levels showed no changes. The plasma ir-TRH and TSH responses to cold were inhibited by litorin, but the plasma TSH response to TRH was not affected. In the pimozide- or para-chlorophenylalanine-pretreated group, the inhibitory effect of litorin on TSH levels was prevented, but not in the L-DOPA- or 5-hydroxytryptophan-pretreated group. 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 litorin injection did not differ from that of the saline-treated group. These findings suggest that litorin acts on the hypothalamus to inhibit TRH release, and that its effects are modified by amines of the central nervous system.