Co‐expression of Na V β subunits alters the kinetics of inhibition of voltage‐gated sodium channels by pore‐blocking μ‐conotoxins

Background and Purpose Voltage‐gated sodium channels ( VGSCs ) are assembled from two classes of subunits, a pore‐bearing α‐subunit ( Na V 1 ) and one or two accessory β‐subunits ( Na V β s ). Neurons in mammals can express one or more of seven isoforms of Na V 1 and one or more of four isoforms of Na V β . The peptide μ‐conotoxins, like the guanidinium alkaloids tetrodotoxin ( TTX ) and saxitoxin ( STX ), inhibit VGSCs by blocking the pore in Na V 1 . Hitherto, the effects of Na V β‐subunit co‐expression on the activity of these toxins have not been comprehensively assessed. Experimental Approach Four μ‐conotoxins (μ‐ TIIIA , μ‐ PIIIA , μ‐ SmIIIA and μ‐ KIIIA ), TTX and STX were tested against Na V 1 .1, 1.2, 1.6 or 1.7, each co‐expressed in X enopus laevis oocytes with one of Na V β1 , β2, β3 or β4 and, for Na V 1 .7, binary combinations of thereof. Key Results Co‐expression of Na V β‐subunits modifies the block by μ‐conotoxins: in general, Na V β1 or β3 co‐expression tended to increase k on (in the most extreme instance by ninefold), whereas Na V β2 or β4 co‐expression decreased k on (in the most extreme instance by 240‐fold). In contrast, the block by TTX and STX was only minimally, if at all, affected by Na V β‐subunit co‐expression. Tests of Na V β1  : β2 chimeras co‐expressed with Na V 1 .7 suggest that the extracellular portion of the Na V β subunit is largely responsible for altering μ‐conotoxin kinetics. Conclusions and Implications These results are the first indication that Na V β subunit co‐expression can markedly influence μ‐conotoxin binding and, by extension, the outer vestibule of the pore of VGSCs . μ‐Conotoxins could, in principle, be used to pharmacologically probe the Na V β subunit composition of endogenously expressed VGSCs .