Dynorphin (1-13) effects on thyrotrophin-releasing hormone and thyrotrophin secretion in rats

The effects of dynorphin (1-13) on thyrotrophin-releasing hormone (TRH) and thyrotrophin (TSH) secretion in rats were studied. Dynorphin (500 micrograms/kg) was injected iv, and the rats were serially decapitated. TRH and TSH, thyroxine (T4) and 3,3',5-triiodothyronine (T3) were measured by radioimmunoassay. The hypothalamic immunoreactive TRH did not change significantly after dynorphin injection. Basal plasma TSH levels significantly decreased in a dose-related manner with a nadir at 40 min after dynorphin injection. The effect of dynorphin on TSH release was partially prevented by naloxone. The plasma TSH response to cold was significantly inhibited by dynorphin. The plasma TSH response to TRH did not differ from that of the control. In the L-DOPA or 5-hydrotryptophan-pretreated group, the inhibitory effect of dynorphin on TSH release was prevented, but not in the haloperidol-or para-chlorophenylalanine-pretreated group. These drugs alone did not affect plasma TSH levels. The plasma T4 and T3 levels did not change significantly after dynorphin injection. The findings suggest that dynorphin acts on the hypothalamus by inhibiting TRH release, which may be modified by amines of the central nervous system.