IONIC MECHANISM OF SUBSTANCE-P ACTIONS ON NEURONS IN TRIGEMINAL ROOT-GANGLIA

1. Responses of primary sensory neurons to substance P applications by perfusion were studied with intracellular recording techniques in in vitro slice preparations of trigeminal root ganglia (guinea pigs). Application of substance P in micromolar doses produced reversible depolarizations of 2-7 mV in 48 out of 64 neurons. The depolarizing influence facilitated repetitive spike discharge evoked by current-pulse injection. Evidence of desensitization was observed during prolonged or repeated applications of the peptide. 2. The responses to substance P were associated with decreased input resistance, although increased input resistance was observed in neurons where the resting membrane potential was compensated with DC injection. In single-electrode voltage-clamp (SEVC) recordings, substance P evoked an inward shift in the holding current and reduced an outwardly rectifying component in the I-V relationships. The reversal potential for the substance P response could not be determined. These results suggested that the perikaryal response to substance P has a complex ionic mechanism involving activation and deactivation of membrane conductances. 3. Substance P-induced depolarizations were greatly attenuated during perfusion with solutions that were deficient in [Na+] or [Mg2+] and were not significantly affected during perfusion with low[Ca2+]-, Co2+-containing solutions. 4. In the voltage-clamp investigations, an inward current contributed to the substance P responses during combined application with the K+-channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). This current was not abolished by the inclusion of CsCl in the perfusing solution or by internal Cs+ application from the recording electrode, suggesting that an anomalous inward rectifier was not involved in the responses to substance P. The extrapolated reversal potential for substance P-evoked current under these conditions was near -20 mV. Removal of Mg2+ from the perfusing solutions containing K+-channel blockers resulted in a moderate reduction in the inward currents evoked by substance P application. 5. The most likely explanation for the depolarizing effects is that activation of membrane receptors for substance P opens nonselective ionic channels resulting in a net inward current; this leads to a K+-current blockade that is dependent on the external [Mg2+]. The combination of the receptor-activated inward current and a deactivated outward current produces the membrane depolarization that thereby facilitates the repetitive discharge ability of trigeminal neurons.