Mechamsms of potassium- and capsaicin-induced axonal calcitonin gene-related peptide release: Involvement of L- and T-type calcium channels and TRPV1 but not sodium channels

We have previously shown that capsaicin, noxious heat, protons and potassium ions (K+) induce a graded, calclum- and receptor-dependent increase of immunoreactive calcitonin gene-related peptide (iCGRP) release from isolated rat sciatic axons. Morphological evidence for axonal vesicular exocytosis has also been presented. Here we determine the differential contribution of voltage-gated calcium and sodium channels to high extracellular potassium and capsaicin-induced iCGRP secretion. Blockade of L-type calcium channels significantly decreased the K+ -induced axonal response (nimodipine (10 mu M) by 66% and methoxyverapamil, D600 (50 mu M), by 77%). Interestingly, however, D600 was unable to reduce the capsaicin-induced iCGRP release. Omega-Conotoxin GVIA (1 mu M), a N-type blocker, and omega-agatoxin TK (0.1 mu M), a P/Q-type blocker, had no significant effect. Also the anticonvulsant gabapentin (50 mu M and 100 mu M), reported to impede calcium channels, was ineffective. Inhibition of low threshold T-type calcium channels by mibefradil (10 mu M) significantly reduced potassium (by 47%) but not capsaicin-stimulated iCGRP release. Reduction of total sodium channel conductance by tetrodotoxin (1 mu M), lidocalne (10 mu M, 50 mu M or 500 mu M) or by replacement of extracellular sodium with choline-chloride did not result in a reduction of either potassium- or capsaicin-induced axonal iCGRP release. These results suggest that slow depolarization by high extracellular potassium activates axonal low threshold (T-type) as well as high threshold-activated (L-type) voltage-gated calcium channels to mediate iCGRP release, and that capsaicin-induced release is largely dependent on calcium influx through TRPV1. Action potential generation and propagation are not required for axonal release mechanisms. (c) 2007 IBRO. Published by Elsevier Ltd. All rights reserved.