TRPV1 Activation Modulates Acetylcholine Release at Myoneural Junctions

Abstract

Transient receptor potential (TRP) proteins are non-selective cation channel proteins that are expressed throughout the body. Previous studies show that TRP Vanilloid 1 (TRPV1), capsaicin (CAP) receptor, is expressed in sensory neurons. Here we report that TRPV1 protein is expressed at the motor nerve terminals and that activation of TRPV1 by CAP suppresses acetylcholine release, in vitro. We analyzed twitch tension, spontaneous and nerve stimulus evoked acetylcholine release from phrenic nerve diaphragm nerve muscle preparations (NMP) isolated from isoflurane anesthetized adult wild type mice and that genetically lacked TRPV1 (TRPV1-/-). When acutely applied to isolated NMP, 1 µM CAP produced a concentration-dependent decline of twitch tension and a significant decline in the amplitude of stimulus evoked endplate currents (EPCs) and quantal content without any effect on the miniature endplate currents (mEPCs). The suppression of nerve stimulus evoked acetylcholine release by CAP was antagonized by capsazepine (CPZ; 10 µM), a TRPV1 antagonist. However, CAP did not suppress phrenic nerve stimulus evoked acetylcholine release in TRPV1−/− mice. Also, CAP treatment, in vitro, interfered with the localization of adapter protein 2 in cholinergic Neuro 2a cells. Wortmannin, (WMN; 10 µM; non-selective phosphoinositol kinase inhibitor), mimicked the effects of CAP by inhibiting the acetylcholine exocytosis. Our data suggest that TRPV1 proteins expressed at the motor nerve terminal are coupled to the exo-endocytic mechanisms to regulate neuromuscular functions.