Proton-induced currents in substantia gelatinosa neurons of the rat trigeminal subnucleus caudalis

Acid-sensing ion channels (ASICs) are widely expressed in both the peripheral and central nervous system, and contribute to the modulation of central nociceptive transmission under both physiological and pathophysiological conditions. In this study, we characterized the proton-induced membrane currents in acutely isolated rat substantia gelatinosa (SG) neurons of the trigeminal subnucleus caudalis using the whole cell patch-clamp technique. Exposure to acidic conditions (pH<6.5) induced the inward currents in a pH-dependent manner. Amiloride, a general ASIC antagonist, significantly blocked the proton-induced currents in a non-competitive manner. The pH 6.0-induced membrane current (I_pH6.0) was greatly attenuated in the Na⁺-free external solution, and the reversal potential of the proton-induced currents was similar to the theoretical Na⁺ equilibrium potential. The I_pH6.0 was reciprocally potentiated by a lower extracellular Ca²⁺ concentration. The modulation of I_pH6.0 by divalent cations and other modulators suggests that the proton-induced currents are mediated by multiple types of ASIC subunits, including ASIC1a and ASIC2a. Multi-cell RT-PCR analysis revealed that SG neurons express these subunits. Exposure to a pH 6.0 solution directly depolarized the membrane potential, and generated a burst of action potentials in a current-clamp mode. This acidic pH-induced depolarization was significantly blocked by amiloride. The present results suggest that ASICs expressed on SG neurons play important roles in the regulation of nociceptive transmission from the orofacial tissues.