Properties of the depolarization induced by TRH in the isolated frog spinal cord

We observed the excitatory effects of thyrotropin-releasing hormone (TRH) on isolated, intraarterially perfused spinal cords of frogs. TRH (10(-7)-10(-3) M) produced a transient depolarization and potentiated the spontaneous activities in both the ventral and dorsal roots. The TRH effects were related to both direct and indirect actions, and Na+ deficiency attenuated the direct actions. Serotonin was the only neurotransmitter candidate with which TRH exhibited an interaction. Iproniazid potentiated the TRH effects, in normal Ringer's solution. These results indicate that the TRH depolarization due to direct actions is Na+ dependent, and that indirect actions involve an interaction with a monoaminergic pathway.