Effect of substance P on colonic mechanoreceptors, motility, and sympathetic neurons

Intracellular recording techniques were used in vitro to analyze the effects of substance P (SP) on synaptic transmission and electrical properties of sympathetic neurons in the inferior mesenteric ganglion (IMG) of the guinea pig. Intraluminal pressure-recording techniques were used to study the effects of SP on colonic motility. Superfusion of the ganglia with SP (10^-7 to 10^-6 M) depolarized the cell soma (2-12 mV) and increased cell input resistance (8-11 MΩ). These effects converted synchronous excitatory postsynaptic potentials, in response to electrical stimulation of preganglionic nerves, and asynchronous excitatory postsynaptic potentials, in response to activation of colonic mechanoreceptors, to action potentials. Administration of SP to only the colon increased basal intraluminal pressure and the frequency and amplitude of phasic changes in intraluminal pressure. These changes increased mechanoreceptor synaptic input to neurons in the IMG. We conclude that SP facilitates synaptic transmission along noradrenergic pathways and increases colonic motility.