Bombesin inhibits thyrotrophin secretion in rats

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Abstract
The effects of peripheral administration of bombesin on TRH and TSH secretion in rats were studied. Bombesin (200 .mu.g/kg) was injected i.v. and the rats were serially decapitated. TRH, TSH and thyroid hormone were measured in radioimmunoasay. The hypothalamic immunoreactive TRH (ir-TRH) content increased significantly after bombesin injection, whereas plasma concentrations tended to decrease, but not significantly. Plasma TSH levels decreased significantly in a dose-related manner with a nadir at 40 min after the injection. Plasma thyroid hormone levels did not change significantly. Plasma ir-TRH and TSH responses to cold were inhibited by bombesin, but the plasma TSH response to TRH was not affected. In the pimozide- or para-chlorophenylalanine pre-treated group, the inhibitory effect of bombesin on TSH levels was prevented, but not in the L-Dopa-or 5-hydroxytryptophan pre-treated group. These drugs alone had no effect on plasma TSH levels in terms of the dose used. The inactivation of TRH immunoreactivity in plasma or hypothalamus in vitro after bombesin injection did not differ from that of the controls. Bombesin may act on the hypothalamus to inhibit TRH release. Its effects may at least be partially modified by amines of the CNS.