title: 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 microM) by 66% and methoxyverapamil, D600 (50 microM), by 77%). Interestingly, however, D600 was unable to reduce the capsaicin-induced iCGRP release. Omega-Conotoxin GVIA (1 microM), a N-type blocker, and omega-agatoxin TK (0.1 microM), a P/Q-type blocker, had no significant effect. Also the anticonvulsant gabapentin (50 microM and 100 microM), reported to impede calcium channels, was ineffective. Inhibition of low threshold T-type calcium channels by mibefradil (10 microM) significantly reduced potassium (by 47%) but not capsaicin-stimulated iCGRP release. Reduction of total sodium channel conductance by tetrodotoxin (1 microM), lidocaine (10 microM, 50 microM or 500 microM) 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.
authors.affiliation: Institut für Physiologie und Pathophysiologie, Universität Erlangen-Nürnberg, Universitätsstrasse 17, D-91054 Erlangen, Germany.
authors.forename: M J S
authors.initials: MJ
authors.lastname: Spitzer
authors.forename: P W
authors.initials: PW
authors.lastname: Reeh
authors.forename: S K
authors.initials: SK
authors.lastname: Sauer
mesh_terms: D000588:Amines / Q000494:pharmacology; D000818:Animals; D015740:Calcitonin Gene-Related Peptide / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000502:physiology*; D020747:Calcium Channels, T-Type / Q000502:physiology*; D002211:Capsaicin / Q000494:pharmacology*; D003509:Cyclohexanecarboxylic Acids / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D045604:Extracellular Fluid / Q000187:drug effects*; D000077206:Gabapentin; D007124:Immunoenzyme Techniques / Q000379:methods; D066298:In Vitro Techniques; D008297:Male; D011188:Potassium / Q000494:pharmacology*; D051381:Rats; D012584:Sciatic Nerve / Q000187:drug effects / Q000378:metabolism; D026941:Sodium Channel Blockers / Q000494:pharmacology; D015222:Sodium Channels / Q000502:physiology; D018709:Statistics, Nonparametric; D050916:TRPV Cation Channels / Q000502:physiology*; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
chemical_list: D000588:Amines; D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D020747:Calcium Channels, T-Type; D003509:Cyclohexanecarboxylic Acids; D018691:Excitatory Amino Acid Antagonists; D026941:Sodium Channel Blockers; D015222:Sodium Channels; D050916:TRPV Cation Channels; C493922:Trpv1 protein, rat; D005680:gamma-Aminobutyric Acid; D000077206:Gabapentin; D015740:Calcitonin Gene-Related Peptide; D011188:Potassium; D002211:Capsaicin