Distinct impacts of sodium channel blockers on the strength-duration properties of human motor cortex neurones

Abstract

Voltage-gated sodium channels (VGSCs) are essential for regulating axonal excitability in the human brain. While sodium channel blockers are known to modulate neuronal excitability, their in vivo effects on the human cortex remain poorly understood. Here, we employed novel transcranial magnetic stimulation (TMS) measures to investigate the effects of sodium channel blockers, carbamazepine and lacosamide, on the strength-duration behaviour of human cortical neurones, serving as an index of sodium channel function. Our data showed that single doses of both medications elevated resting motor thresholds compared to placebo, indicating reduced excitability; however, their impacts varied according to TMS pulse width. Carbamazepine raised thresholds proportionally across all pulse widths, whereas lacosamide disproportionately influenced thresholds for long-duration pulses. Crucially, lacosamide reduced the strength-duration time constant and increased rheobase, while carbamazepine had minimal effects on both. These results reveal subtle and unexpected differences in cortical neurone behaviour following VGSC-blocking medication administration. Lacosamides response aligns with the proposed mechanism of sodium conductance blockade, while carbamazepines effects suggest distinct VGSC interactions or potential off-target effects. Our findings advance the understanding of VGSC-blocking medication interactions in the human cortex and underscore the importance of employing specific TMS measures to gain deeper insights into medication mechanisms of action in vivo. Such measures could serve as valuable adjuncts in medication development and patient monitoring, enhancing understanding of medication action in clinical settings.