Composition of TTX-S Nav current in rat muscle afferent neurons.

Abstract

Group III and IV muscle afferents carry pain and exercise pressor reflex (EPR) signals. However, aberrant signaling of the EPR in patients with cardiovascular disease can increase their risk of a myocardial infarction. Thus, blocking action potentials that signal this reflex could benefit these patients. Immunohistochemistry studies have shown that group III/IV neurons isolated from Sprague Dawley rats express tetrodotoxin-sensitive (TTX-S) voltage-dependent sodium (NaV) channels rNaV1.6 and rNaV1.7, but the contribution of these channels to the TTX-S current is unknown. We initially tested several blockers for selectivity against rNaV1.6 and rNaV1.7 channels expressed in HEK cells and found that µ-conotoxin PIIIA (PIIIA, 500 nM) selectively blocked rNaV1.6 current, while Protoxin II (PTxII, 10 nM) was more selective for rNaV1.7 channels. 4,9-Anhydrotetrodotoxin was found to block non-selectively. To evaluate the TTX-S component currents in muscle afferent neurons, we sequentially added the NaV1.6-selective PIIIA, followed by PTxII to block NaV1.7 current, and then 300 nM TTX to block the remaining TTX-S current. On average PIIIA blocked 14 ± 13% (mean ± SD) and PTxII blocked 70 ± 17% of the TTX-S NaV current in rat muscle afferent neurons. However, the NaV1.6 current fraction significantly varied with cell diameter with mean inhibitions of 0% for neurons < 27 µm, 17% between 27-32 µm (p < 0.05), 5% between 32-27 µm and 21% for neurons > 37 µm (p < 0.05), suggesting differential expression among group III/IV neurons. The TTX-S NaV current in group III/IV neurons is primarily comprised of NaV1.7 >> NaV1.6 = unidentified TTX-S channel activity. Blocking group III/IV action potentials with a selective NaV1.7 channel blocker should inhibit the EPR to reduce the myocardial risk in patients with cardiovascular disease and provide analgesia in patient with severe muscle pain.