Mechanisms 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

Abstract We have previously shown that capsaicin, noxious heat, protons and potassium ions (K+ ) induce a graded, calcium- 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 μM) by 66% and methoxyverapamil, D600 (50 μM), by 77%). Interestingly, however, D600 was unable to reduce the capsaicin-induced iCGRP release. Ω-Conotoxin GVIA (1 μM), a N-type blocker, and ω-agatoxin TK (0.1 μM), a P/Q-type blocker, had no significant effect. Also the anticonvulsant gabapentin (50 μM and 100 μM), reported to impede calcium channels, was ineffective. Inhibition of low threshold T-type calcium channels by mibefradil (10 μM) significantly reduced potassium (by 47%) but not capsaicin-stimulated iCGRP release. Reduction of total sodium channel conductance by tetrodotoxin (1 μM), lidocaine (10 μM, 50 μM or 500 μM) 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.