Trigeminal Ganglion Neurons of Mice Show Intracellular Chloride Accumulation and Chloride-Dependent Amplification of Capsaicin-Induced Responses

Intracellular Cl(−) concentrations ([Cl(−)](i)) of sensory neurons regulate signal transmission and signal amplification. In dorsal root ganglion (DRG) and olfactory sensory neurons (OSNs), Cl(−) is accumulated by the Na(+)-K(+)-2Cl(−) cotransporter 1 (NKCC1), resulting in a [Cl(−)](i) above electrochemical equilibrium and a depolarizing Cl(−) efflux upon Cl(−) channel opening. Here, we investigate the [Cl(−)](i) and function of Cl(−) in primary sensory neurons of trigeminal ganglia (TG) of wild type (WT) and NKCC1(−/−) mice using pharmacological and imaging approaches, patch-clamping, as well as behavioral testing. The [Cl(−)](i) of WT TG neurons indicated active NKCC1-dependent Cl(−) accumulation. Gamma-aminobutyric acid (GABA)(A) receptor activation induced a reduction of [Cl(−)](i) as well as Ca(2+) transients in a corresponding fraction of TG neurons. Ca(2+) transients were sensitive to inhibition of NKCC1 and voltage-gated Ca(2+) channels (VGCCs). Ca(2+) responses induced by capsaicin, a prototypical stimulus of transient receptor potential vanilloid subfamily member-1 (TRPV1) were diminished in NKCC1(−/−) TG neurons, but elevated under conditions of a lowered [Cl(−)](o) suggesting a Cl(−)-dependent amplification of capsaicin-induced responses. Using next generation sequencing (NGS), we found expression of different Ca(2+)-activated Cl(−) channels (CaCCs) in TGs of mice. Pharmacological inhibition of CaCCs reduced the amplitude of capsaicin-induced responses of TG neurons in Ca(2+) imaging and electrophysiological recordings. In a behavioral paradigm, NKCC1(−/−) mice showed less avoidance of the aversive stimulus capsaicin. In summary, our results strongly argue for a Ca(2+)-activated Cl(−)-dependent signal amplification mechanism in TG neurons that requires intracellular Cl(−) accumulation by NKCC1 and the activation of CaCCs.