Ranolazine Reduces Central Neuron Excitability by Slowly Interacting with NaV Channels

Ranolazine inhibits the increased persistent Na+ current (persistent INa) conducted by NaV1.1 channels encoding epilepsy and migraine associated mutations. We therefore determined the effects of ranolazine on the electrical activity of cultured rat hippocampal neurons using empirical and computational modeling approaches. Ranolazine (3μM) produced a 24% reduction in the number of action potentials (APs) evoked in response to repetitive (1sec, 0.67Hz) depolarizing current injections (21±4 for control and 16±3 for ranolazine, pulse 9, p<0.05). With a single current injection of 4sec, spike cessation occurred at 2403±220 msec in the presence of 10μM ranolazine (4000±0 msec for control). Similar results were observed for the anticonvulsants phenytoin (3μM, 1387±184 msec) and lacosamide (30μM, 2441±53 msec), which bind to fast and slow-inactivated states of Na+ channels, respectively. Ranolazine enhanced the development of Na+ channel fast and slow inactivation evaluated with conditioning pre-pulses of either 100, 1000 or 10000 msec, consistent with progressive binding to inactivated states. Recovery of Na+ channel activity assessed using fast and slow inactivating voltage protocols was also delayed in the presence of ranolazine. Interestingly, the use-dependent inhibition (25Hz) of Na+ channel activity by ranolazine (10μM) was dependent on the duration of the voltage step (3.0±2.0% for 2ms and 33.8±13.5% for 20ms, p<0.05) suggesting the drug bound to inactivated state(s). Similar to phenytoin, ranolazine exhibited slow binding kinetics to HEK293 cells stably expressing hNav1.2 (KON= 1M−1msec−1 and KOFF= 5e−5msec−1). Computational simulations predicted equal inhibition of neuronal APs regardless of whether ranolazine binding was constrained to fast-inactivated or slow-inactivated states of the Na+ channel. Ranolazine had no or minimal effects on neuronal KV channels, GABA or NMDA neurotransmission. In summary, ranolazine inhibits the excitability of hippocampal neurons by slowly stabilizing the inactivated states of Na+ channels.