Sustained acidosis suppresses persistent sodium currents to regulate the excitability of medullary dorsal horn neurons in rats.

Abstract

Objectives: Acid-sensing ion channels (ASICs) are rapidly inactivated following activation by acidic extracellular pH. Consequently, mechanisms beyond ASICs are likely involved in modulating neuronal excitability under sustained acidic conditions. Therefore, this study investigated the impact of sustained acidic pH on neuronal excitability.

Methods: Membrane current and voltage changes induced by acidic pH were recorded from acutely isolated rat medullary dorsal horn neurons using the whole-cell patch-clamp technique.

Results: The steady-state inactivation relationship for extracellular pH revealed that most ASICs were completely inactivated at pH ≤ 6.5. Acidic pH depolarized medullary dorsal horn neurons with high affinity (EC50 of pH 6.9), a process mediated by ASIC activation. Acidic pH (≤6.9) also generated instantaneous action potentials; however, they immediately disappeared owing to the inactivation of voltage-gated Na⁺ channels. Action potentials reemerged depending on pH level, even under sustained acidic conditions. This reappearance of action potentials correlated with the extent to which acidic pH inhibited the persistent Na⁺ current mediated by voltage-gated Na⁺ channels.

Conclusion: These findings suggest that under pathological conditions characterized by sustained extracellular pH reduction, such as inflammation, a persistent Na⁺ current may serve as a sensor for modulating neuronal excitability in response to prolonged acidic pH levels.