T-type calcium channels contribute to neuronal excitability and can be regulated by signals that produce reactive oxygen species (ROSs). Nelson et al. found that ascorbic acid (vitamin C), which is abundant in neurons, inhibited T-type calcium channels in dissociated dorsal root ganglion (DRG) cells and in brain slices containing thalamic nuclei. Only brain slices that contained the Cav3.2 subtype of channel exhibited inhibition, and this subtype selectivity was confirmed with channels expressed in HEK293 cells, a nonneuronal cell line. Domain swapping between Cav3.2 and Cav3.1 indicated that domain 1 of Cav3.2 was required for ascorbate inhibition. Furthermore, mutation of His191 to Gln in Cav3.2 abolished ascorbate-mediated inhibition, and introduction of a His at position 172 in Cav3.1 conferred ascorbate-mediated inhibition. Histidines bind divalent transition metals, which can contribute to metal-catalyzed oxidation. Metal chelation enhanced basal T-currents and prevented ascorbate-mediated inhibition of DRG neurons. Catalase, which degrades peroxide, also prevented ascorbate-mediated inhibition, as did a ROS scavenger. Cu2+, but not Zn2+, participates in metal-catalyzed oxidation, and increasing the Cu2+/Zn2+ ratio in the extracellular solution increased ascorbate-mediated inhibition, which is consistent with a metal-catalyzed oxidation mechanism for inhibition. Ascorbate applied at physiological concentrations inhibited firing of reticular thalamic neurons in response to low-threshold Ca2+ spikes and burst firing. Thus, ascorbate may be an endogenous modulator of Cav3.2-mediated neuronal activity.
M. T. Nelson, P. M. Joksovic, P. Su, H.-W. Kang, A. Van Deusen, J. P. Baumgart, L. S. David, T. P. Snutch, P. Q. Barrett, J.-H. Lee, C. F. Zorumski, E. Perez-Reyes, S. M. Todorovic, Molecular mechanisms of subtype-specific inhibition of neuronal T-type calcium channels by ascorbate. J. Neurosci. 27, 12577-12583 (2007). [Abstract] [Full Text]