Differential Inhibition of Human Nav1.2 Resurgent and Persistent Sodium Currents by Cannabidiol and GS967

Many epilepsy patients are refractory to conventional antiepileptic drugs. Resurgent and persistent currents can be enhanced by epilepsy mutations in the Nav1.2 channel, but conventional antiepileptic drugs inhibit normal transient currents through these channels, along with aberrant resurgent and persistent currents that are enhanced by Nav1.2 epilepsy mutations. Pharmacotherapies that specifically target aberrant resurgent and/or persistent currents would likely have fewer unwanted side effects and be effective in many patients with refractory epilepsy. This study investigated the effects of cannbidiol (CBD) and GS967 (each at 1 μM) on transient, resurgent, and persistent currents in human embryonic kidney (HEK) cells stably expressing wild-type hNav1.2 channels. We found that CBD preferentially inhibits resurgent currents over transient currents in this paradigm; and that GS967 preferentially inhibits persistent currents over transient currents. Therefore, CBD and GS967 may represent a new class of more targeted and effective antiepileptic drugs.

Ketogenic diet as a successful early treatment modality for SCN2A mutation

SCN2A mutations have been described in a very broad spectrum of clinical phenotypes including benign (familial) neonatal/infantile seizures and early infantile epileptic encephalopathies (EIEE) as Ohtahara syndrome (OS), Dravet syndrome (DS), epilepsy of infancy with migrating focal seizures and West syndrome (WS). Treatment modalities for epilepsy caused by SCN2A mutations mainly consist of sodium channel blockers but ketogenic diet (KD) is also considered as an option of treatment for intractible seizures caused by SCN2A mutations. Because of the wide nature of the heterogeneity of mutations related to SCN2A gene, the clinical phenotypes vary in severity and treatment response to KD has been reported to be controversial. We present a patient diagnosed with OS associated with a novel SCN2A mutation (c.408G > A, p.Met136lle; OMIM®: 182390) who had a complete resolution of seizures and EEG abnormalities with KD commenced at 39 days of age. As far as we are aware our case is the youngest patient with SCN2A mutation treated with KD with complete resolution of epilepsy at an early age and has been seizure free of antiepileptic medications for a long duration.

Resibufogenin and cinobufagin activate central neurons through an ouabain-like action

Cinobufagin and resibufogenin are two major effective bufadienolides of Chan su (toad venom), which is a Chinese medicine obtained from the skin venom gland of toads and is used as a cardiotonic and central nervous system (CNS) respiratory agent, an analgesic and anesthetic, and as a remedy for ulcers. Many clinical cases showed that Chan su has severe side-effects on the CNS, causing shortness of breath, breathlessness, seizure, coma and cardiac arrhythmia. We used whole-cell recordings from brain slices to determine the effects of bufadienolides on excitability of a principal neuron in main olfactory bulb (MOB), mitral cells (MCs), and the cellular mechanism underlying the excitation. At higher concentrations, cinobufagin and resibufogenin induced irreversible over-excitation of MCs indicating a toxic effect. At lower concentrations, they concentration-dependently increased spontaneous firing rate, depolarized the membrane potential of MCs, and elicited inward currents. The excitatory effects were due to a direct action on MCs rather than an indirect phasic action. Bufadienolides and ouabain had similar effects on firing of MCs which suggested that bufadienolides activated neuron through a ouabain-like effect, most likely by inhibiting Na⁺/K⁺-ATPase. The direct action of bufadienolide on brain Na⁺ channels was tested by recordings from stably Na_v1.2-transfected cells. Bufadienolides failed to make significant changes of the main properties of Na_v1.2 channels in current amplitude, current-voltage (I-V) relationships, activation and inactivation. Our results suggest that inhibition of Na⁺/K⁺-ATPase may be involved in both the pharmacological and toxic effects of bufadienolide-evoked CNS excitation.

Neuronal signaling in central nervous system

A new method of axon recording through axon bleb has boosted the studies on the functional role of central nervous system (CNS) axons. Using this method, we have revealed the mechanisms underlying the initiation and propagation of the digital-mode signal, all-or-none action potentials (APs), in neocortical pyramidal neurons. Accumulation of the low-threshold Na(+) channel subtype Na(v)1.6 at the distal end of the axon initial segment (AIS) determines the lowest threshold for AP initiation, whereas accumulation of the high-threshold subtype Na(v)1.2 at the proximal region of the AIS promotes AP backpropagation to the soma and dendrites. Through dual recording from the soma and the axon, we have showed that subthreshold membrane potential (V(m)) fluctuations in the soma propagate along the axon to a long distance and probably reach the axon terminals. Paired recording from cortical neurons has revealed that these V(m) changes in the soma modulate AP-triggered synaptic transmission. This new V(m)-dependent mode of synaptic transmission is called analog communication. Unique properties of axonal K(+) channels (K(v)1 channels) may contribute to shaping the AP waveform, particularly its duration, and thus controlling synaptic strength at different levels of presynaptic V(m). The level of background Ca(2+) may also participate in mediating the analog signaling. Together, these findings enrich our knowledge on the principles of neuronal signaling in the CNS and help understand how the brain works.