Substance P in the lumbar spinal cord of the rat affects the motor response to 5-HTP and TRH.

Substance P, administered intrathecally in the lumbar area of paraplegic rats activated the electromyogram (EMG) of the hindlimbs. This effect was markedly enhanced by previous denervation by 5,7-dihydroxytryptamine. Stimulation of the motoneurons by substance P was followed by a blunted response to the excitatory substances thyrotropin releasing hormone (TRH) and 5-hydroxytryptophan (5-HTP), given intraperitoneally 1 hr later. The decreased sensitivity to 5-HTP persisted for more than 24 hr. In several animals, however, a flaccid paralysis was observed after administration of substance P. In such cases the response to 5-HTP was enhanced 24 hr later. These observations suggest an interaction between 5-HT, TRH and substance P in the control of the excitability of motoneurons.