Figure 2

Effects of Kv7 channel activation and blockade on the recovery cycle of electrical excitability. Stimulus–response curves were obtained using unconditioned test stimuli of 1 ms duration. These established the maximal CMAP to supramaximal nerve stimulation and the size of the submaximal target CMAP (~ 40% of maximum). The stimulus current necessary to produce the target potential using a 1 ms test stimulus is referred to as the “threshold” for that potential. Electrical excitability, i.e. the current required to maintain a 40% compound A-fibre action potential response, was determined at discrete interstimulus intervals following a supra-maximal stimulus (A). The representative example in panel B illustrates the concentration-dependent changes in the recovery cycle observed with bath application of flupirtine (3-30 μM) and their reversal following selective Kv7 channel blockade with XE991 (10 μM). Changes in the recovery cycle of A-fibres were quantified with the empirically determined values of refractoriness at 2 ms (D) and 2.5 ms (E) and excitability at 5 ms (F) and 7 ms (G). The relative refractory period (C) was determined respectively by linear interpolation and first-order exponential fits (see Methods). The RRP (p < 0.01) was reduced in a concentration-dependent manner (C). Flupirtine (30 μM) also reduced refractoriness at 2.5 ms (E; p < 0.01) but not at 2 ms (D). Similarly, flupirtine produced a concentration-dependent increase in the magnitude of post-spike superexcitability at 5 ms (F; p < 0.01 for flupirtine 10 μM and 30 μM) and at 7 ms (G; p < 0.01; for flupirtine 30 μM)