Substance P selectively blocks excitation of Renshaw cell by acetylcholine.

In cats, under Dial anaesthesia, Renshaw cells were excited by microiontophoretic applications of acetylcholine (ACh), aspartate, and glutamate. Substance P, in small doses (10-30 nA), selectively abolished the responses to ACh, leaving the discharges evoked by the amino acids unchanged or enhanced. Higher doses (greater than 50 nA) depressed all responses, but those evoked by amino acids went down last and recovered sooner. By contrast, neither synaptic responses to ventral root stimulation nor spontaneous discharges were affected by substance P, presumably owing to the high efficacy of synaptic transmission and the presence of diffusion barriers around junctional sites.