Gabapentin (1-(Gabapentin (1-(aminomethyl)cyclohexane acetic acid; Neurontin) is a novel antiepileptic drug that is orally active in various animal models of epilepsy, including maximal electroshock in rats and pentylenetetrazol-or audiogenically induced seizures in mice (1-3). Gabapentin has also been shown to be effective in decreasing the frequency of seizures in medically refractory patients with partial or generalized epilepsy (3, 4). Although originally synthesized as a lipophilic ␥-aminobutyric acid (GABA) 1 analogue, capable of penetrating the blood-brain barrier, gabapentin does not possess a high affinity for either GABA A or GABA B receptors, does not influence neural uptake of GABA and does not inhibit the GABA-metabolizing enzyme, GABA transaminase (EC 2.6.1.19) (3, 5). Moreover, gabapentin does not affect voltage-dependent sodium channels (the site of action of several antiepileptic drugs, including phenytoin, carbamazepine, and valproate) and is inactive in assays for a wide range of other neurotransmitter receptors, enzymes, and ion channels (5, 6).A single high affinity (K D ϭ 38 Ϯ 2.8 nM) binding site for [ 3 H]gabapentin in rat brain has been described (7). Radioligand binding to brain membranes was potently inhibited by a range of gabapentin analogues and by several 3-alkyl-substituted analogues of GABA, although GABA itself was only weakly active. Other antiepileptic drugs including phenytoin, diazepam, carbamazepine, valproate, and phenobarbitone were inactive. Gabapentin (IC 50 ϭ 80 nM) and (RS)-3-isobutyl-GABA (IC 50 ϭ 80 nM) were the most active compounds identified (7). The (Sϩ)-enantiomer of 3-isobutyl-GABA was significantly more active than the (RϪ)-enantiomer both in displacing Despite extensive research the mechanism of action of gabapentin remains unclear. In vivo behavioral studies have suggested the possible involvement of the glycine co-agonist site of the NMDA receptor complex in the anticonvulsant action of gabapentin; intracerebroventricular administration of D-serine (a glycine site agonist) reversed the protection afforded by gabapentin against chemically induced seizures in mice (9). However, radioligand binding assays have not shown gabapentin to inhibit strychnine-insensitive [ 3

Voltage-sensitive sodium channels are responsible for the initiation and propagation of the action potential and therefore are important for neuronal excitability. Complementary DNA clones encoding the beta 1 subunit of the rat brain sodium channel were isolated by a combination of polymerase chain reaction and library screening techniques. The deduced primary structure indicates that the beta 1 subunit is a 22,851-dalton protein that contains a single putative transmembrane domain and four potential extracellular N-linked glycosylation sites, consistent with biochemical data. Northern blot analysis reveals a 1,400-nucleotide messenger RNA in rat brain, heart, skeletal muscle, and spinal cord. Coexpression of beta 1 subunits with alpha subunits increases the size of the peak sodium current, accelerates its inactivation, and shifts the voltage dependence of inactivation to more negative membrane potentials. These results indicate that the beta 1 subunit is crucial in the assembly, expression, and functional modulation of the heterotrimeric complex of the rat brain sodium channel.

Neuropathic pain is a debilitating condition affecting millions of people around the world and is defined as pain that follows a lesion or dysfunction of the nervous system. This type of pain is difficult to treat, but the novel compounds pregabalin (Lyrica) and gabapentin (Neurontin) have proven clinical efficacy. Unlike traditional analgesics such as nonsteroidal antiinflammatory drugs or narcotics, these agents have no frank antiinflammatory actions and no effect on physiological pain. Although extensive preclinical studies have led to a number of suggestions, until recently their mechanism of action has not been clearly defined. Here, we describe studies on the analgesic effects of pregabalin in a mutant mouse containing a single-point mutation within the gene encoding a specific auxiliary subunit protein (␣2-␦-1) of voltage-dependent calcium channels. The mice demonstrate normal pain phenotypes and typical responses to other analgesic drugs. We show that the mutation leads to a significant reduction in the binding affinity of pregabalin in the brain and spinal cord and the loss of its analgesic efficacy. These studies show conclusively that the analgesic actions of pregabalin are mediated through the ␣2-␦-1 subunit of voltage-gated calcium channels and establish this subunit as a therapeutic target for pain control.

ObjectiveDravet syndrome is a severe form of intractable pediatric epilepsy with a high incidence of SUDEP: Sudden Unexpected Death in epilepsy. Cardiac arrhythmias are a proposed cause for some cases of SUDEP, yet the susceptibility and potential mechanism of arrhythmogenesis in Dravet syndrome remain unknown. The majority of Dravet syndrome patients have de novo mutations in SCN1A, resulting in haploinsufficiency. We propose that, in addition to neuronal hyperexcitability, SCN1A haploinsufficiency alters cardiac electrical function and produces arrhythmias, providing a potential mechanism for SUDEP.MethodsPostnatal day 15-21 heterozygous SCN1A-R1407X knock-in mice, expressing a human Dravet syndrome mutation, were used to investigate a possible cardiac phenotype. A combination of single cell electrophysiology and in vivo electrocardiogram (ECG) recordings were performed.ResultsWe observed a 2-fold increase in both transient and persistent Na+ current density in isolated Dravet syndrome ventricular myocytes that resulted from increased activity of a tetrodotoxin-resistant Na+ current, likely Nav1.5. Dravet syndrome myocytes exhibited increased excitability, action potential duration prolongation, and triggered activity. Continuous radiotelemetric ECG recordings showed QT prolongation, ventricular ectopic foci, idioventricular rhythms, beat-to-beat variability, ventricular fibrillation, and focal bradycardia. Spontaneous deaths were recorded in 2 DS mice, and a third became moribund and required euthanasia.InterpretationThese data from single cell and whole animal experiments suggest that altered cardiac electrical function in Dravet syndrome may contribute to the susceptibility for arrhythmogenesis and SUDEP. These mechanistic insights may lead to critical risk assessment and intervention in human patients.