Effects of substance P on neurones in the dorsal horn of the spinal cord of the cat.

The activity of single, physiologically identified neurones has been recorded both extra- and intracellularly in the 6th and 7th lumbar segments of the pentobarbitone-anaesthetized cat. In the majority of dorsal horn neurones studies (laminae 4 and 5 of Rexed) microiontophoretically applied synthetic substance P and the active fragment, substance P (4--11), were found to cause a slow and prolonged increase of the spontaneous firing rate and/or an enhancement of L-glutamate induced activity. Intracellular studies revealed that substance P caused a reversible depolarization of both dorsal horn neurones and motoneurones without a detectable alteration of the membrane resistance, antidromic action potential or postsynaptic potentials. These results are compatible with a possible role of substance P in sensory transmission in the spinal cord of the cat.