Proton-induced sodium current in freshly dissociated hypothalamic neurones of the rat

1. The proton-gated current was investigated in freshly dissociated ventromedial hypothalamic (VMH) neurones from 4-week-old Wistar rats, under whole-cell configuration by the use of the 'concentration-clamp' technique which combines intracellular perfusion with the rapid exchange of external solution within 1-2 ms under a single-electrode voltage-clamp condition. 2. The proton-gated current increased in a sigmoidal fashion as extracellular pH (pHo) decreased. In external solution containing 2 mM-Ca2+, the threshold of current activation was at pHo 6.5, and the maximum response appeared at pHo 4.1-3.9. The dissociation constant (Kd) and Hill coefficient were 10(-4.9) M (pHo = 4.9) and 1.5 respectively. 3. Decreasing extracellular Na+ concentration reduced the proton-gated current. The current reversed direction at the Na+ equilibrium potential (ENa), indicating that it was carried by Na+. 4. The activation phase kinetics of proton-induced current was single exponential. The time constant of activation (tau a) did not have a potential dependence but decreased slightly by decreasing pHo. The inactivation phase kinetics was two-exponential. The time constant of inactivation (tau i) consisted of fast and slow components (tau if and tau is, respectively). Like tau a, both tau if and tau is did not have any potential dependence, but they slightly increased with decreasing pHo. 5. The steady-state inactivation curve, constructed by decreasing pHo from various conditioning pHos to 4.1, revealed that the proton-induced current had a half-maximum inactivation at pHo 6.2. 6. The proton-induced current was suppressed as the extracellular Ca2+ concentration ([Ca2+]o) increased from almost free (0.01 mM) to 80 mM. Increasing [Ca2+]o increased tau a, but slightly decreased both tau if and tau is. 7. Recovery of proton-induced current from complete inactivation of proton-induced current depended on the degree of pHo change. A bigger change in pHo induced faster recovery than a smaller change. 8. External divalent cations inhibited the proton-induced current, and the inhibitory potency was in the order of Mn2+ greater than Co2+ greater than Ca2+ greater than Sr2+ greater than Ba2+ greater than Mg2+. 9. Tetrodotoxin (TTX) at relatively low concentration (less than 10(-7) M) did not inhibit the peak amplitude of the proton-induced current, but at a higher concentration (10(-6) M) it slightly inhibited the peak amplitude of the current and accelerated the inactivation process. Scorpion toxin markedly increased the peak amplitude of the proton-induced current and prolonged the inactivation phase. The tau is was also increased by scorpion toxin in a concentration-dependent manner. Veratridine had no effect on the proton-induced current. 10. The membrane properties of the proton-operated channel were similar to those of the voltage-gated Na+ channel rather than the Ca2+ channel.