Roles of Ca2+, Mg2+, and Ba2+ Cations in the Regulation of TRPV1 Channels in Rat DRG Neurons

In experiments performed on a primary culture of neurons isolated from the dorsal root ganglia (DRGs) of rats, we estimated the level of intracellular calcium using fluorescence microscopy and a fluorescent probe for divalent cations, Fura 2. It was found that capsaicin (Caps) affects the Ca2+ permeability of the plasmalemma and does not cause Ca2+ release from the intracellular stores. As [Ca2+](in) in the neurons increases, desensitization of TRPV1 channels develops. The dependence of the amplitude of Caps application-caused calcium responses on the concentration of external Ca2+ is nonlinear and demonstrates saturation. The CaCl2 concentration in the solution, at which the amplitude of calcium flow through TRPV1 channels is half-maximum (K-M), was estimated as 0.498 mM. As was found, Ca2+, Mg2+, and Ba2+ cations are able to penetrate the membrane through TRPV1 channels. The simultaneous presence of Mg2+ and Ba2+ or Ca2+ and Ba2+ ions in the medium leads to the entry of these cations through TRPV1 channels with the subsequent development of desensitization to the action of Caps. At the same time, Ba2+, via replacing other divalent cations, eliminated inactivation of TRPV1 channels. In the presence of 2.5 mM BaCl2 and 1.0 mM MgCl2 in the external solution, the effect of Caps on TRPV1 channels also showed obvious desensitization. The obtained data indicate that desensitization of TRPV1 channels requires the presence of not only CaCl2, but also MgCl2 in the external solution. Moreover, Mg2+ can replace Ca2+ in the development of this effect.