Effects of Various Drugs on Thyrotropin Secretion in Rats

Summary

The effects of α-neoendorphin, kyotorphin, melatonin or diphenylhydantoin (DPH) on thyrotropin-releasing hormone (TRH) and thyrotropin (TSH) release in rats were studied, α-neoendorphin (1.0 mg/kg), kyotorphin (1.0 mg/kg), melatonin (2.5 mg/kg) or DPH (75 mg/kg) was injected iv or ip, and the rats were serially decapitated. TRH, TSH and thyroid hormone were determined by radioimmunoassay.

The hypothalamic immunoreactive (ir-TRH) contents decreased significantly after melatonin injection, but not after α-neoendorphin, kyotorphin or DPH. The plasma ir-TRH concentrations decreased significantly after DPH injection, but not after α-neoendorphin, kyotorphin or melatonin. The plasma TSH levels decreased significantly in a doserelated manner with a nadir at 10 min. after melatonin, at 30 min. after DPH and at 40 min. after α-neoendorphin or kyotorphin injection. The plasma thyroid hormone levels did not change significantly after these drugs injection. The plasma ir-TRH and TSH responses to cold were inhibited by these drugs, but the plasma TSH response to TRH was not influenced. In the L-DOPA- or 5-hydroxy-tryptophan (5-HTP)-pretreated group, the inhibitory effect of α-neoendorphin or kyotorphin on TSH levels was prevented, but not in the haloperidol- or para-chloprophenylalanine (PCPA)-pretreated group. In the haloperidol- or PCPA-pretreated group, the inhibitory effect of melatonin on TSH levels was prevented, but not in the L-DOPA- or 5-HTP-pretreated group. These drugs alone did not affect plasma TSH levels in terms of the dose used. The inactivation of TRH immuno-reactivity by hypothalamus or plasma in vitro after these drugs injection did not differ from that of the control.

These findings suggest that α-neoendorphin, kyotorphin, melatonin or DPH acts at the hypothalamus level by inhibiting TRH release, and α-neoendorphin, kyotorphin or melatonin effects are at least partially modified by amines of the central nervous system.

Key-Words:

α-Neoendorphin - Kyotorphin, - Melatonin - Diphenylhydantoin - Thyrotropin-Releasing Hormone - Thyrotropin - Amines