Response of splanchnic-driven neurons to substance P and eledoisin-related peptide

The effects of iontophoretically applied substance P (SP) and eledoisin-related peptide (ERP) on the extracellularly recorded activity of spontaneous or D,L-homocysteic acid (DLH)-driven thoracic (T7-T8) spinal neurons, identified by orthodromic activation by the greater splanchnic nerve, were examined in chloralose/urethane anesthetized cats. Of 24 neurons tested, SP and/or ERP (18-120 nA) produced a weak (ca 30%) excitatory effect on 5 neurons, a weak inhibitory effect on 9 neurons, and no effect on the remaining neurons. When compared to the action of either DLH or GABA, we found both the excitatory and inhibitory effects of SP and/or ERP were delayed and prolonged. These data do not support a role for SP as a major neurotransmitter in spinal circuits in which visceral (splanchnic) afferents participate.