The mechanism of non-blocking inhibition of sodium channels revealed by
conformation-selective photolabeling
Abstract
Hyperexcitability-related diseases include epilepsies, pain syndromes,
neuromuscular disorders, and cardiac arrhythmias. Sodium channel
inhibitors can be used to treat these conditions, however, their
applicability is limited by their nonspecific effect on physiological
function. They act by channel block (obstructing ion conduction, since
the binding site is within the channel pore), and by modulation
(delaying recovery from the non-conducting inactivated state). Channel
block inhibits healthy and pathological tissue equally, while modulation
can preferentially inhibit pathological activity. Therefore, an ideal
sodium channel inhibitor drug would act by modulation alone.
Unfortunately, thus far no such drug has been known to exist. Here we
present evidence that riluzole acts by this “ideal” mechanism,
“non-blocking modulation” (NBM). We propose that, being a relatively
small molecule, riluzole is able to stay bound to the binding site, but
nonetheless stay off the conduction pathway, by residing in one of the
“fenestrations” (cavities connecting the central cavity to the
membrane phase). Using precisely timed UV pulses to photolabel specific
conformations of the channel, we show that association to the local
anesthetic binding site requires prior inactivation. We discuss why
kinetics of binding is crucial for selective inhibition of pathological
activity, and how the NBM mechanism can be recognized using a special
voltage- and drug application-protocol. Our results identify riluzole as
the prototype of this new class of sodium channel inhibitors. Drugs of
this class are expected to selectively prevent hyperexcitability, while
having minimal effect on cells firing at a normal rate from a normal
resting potential.