Proconvulsant-induced seizures in alpha(4) nicotinic acetylcholine receptor subunit knockout mice

The α4 Nicotinic Acetylcholine Receptor Is Necessary for the Initiation of Organophosphate-Induced Neuronal Hyperexcitability

Acute intoxication with organophosphorus (OP) cholinesterase inhibitors can produce seizures that rapidly progress to life-threatening status epilepticus. Significant research effort has been focused on investigating the involvement of muscarinic acetylcholine receptors (mAChRs) in OP-induced seizure activity. In contrast, there has been far less attention on nicotinic AChRs (nAChRs) in this context. Here, we address this data gap using a combination of in vitro and in vivo models. Pharmacological antagonism and genetic deletion of α4, but not α7, nAChR subunits prevented or significantly attenuated OP-induced electrical spike activity in acute hippocampal slices and seizure activity in mice, indicating that α4 nAChR activation is necessary for neuronal hyperexcitability triggered by acute OP exposures. These findings not only suggest that therapeutic strategies for inhibiting the α4 nAChR subunit warrant further investigation as prophylactic and immediate treatments for acute OP-induced seizures, but also provide mechanistic insight into the role of the nicotinic cholinergic system in seizure generation.

Intracerebroventricular administration of cigarette smoke condensate induced generalized seizures reduced by muscarinic receptor antagonist in rats

Tobacco smoking is considered the greatest risk factor for early death caused by noncommunicable diseases. Currently, there are more than one billion tobacco smokers in the world predisposed to many diseases including heart attack, stroke, cancer, and premature birth or birth defects related to the consumption of cigarettes. However, studies on the association between tobacco smoking and seizures or epilepsy are insufficient and not well documented. In the present study, the authors examined the convulsive effects of the intracerebroventricular administration of cigarette smoke condensate (CSC, 2μl/Rat) in rats and compared it with the intensity of seizures in the kainic acid (KA)-induced seizure model of epilepsy. The role of the cholinergic system was also investigated by testing the effect of the muscarinic acetylcholine receptors (mAChRs) antagonist atropine (2ml/kg) on CSC-induced seizures. The results indicate that a central injection of CSC produces an epileptic behavior similar to that induced by KA, the similarities include the following parameters: time latency of seizures, latency and duration of tonic-clonic seizures, duration of seizures, survival, and tonic-clonic rate. However, a pretreatment with atropine reduced seizures and all their parameters.

Neuronal nicotinic receptors in sleep-related epilepsy: studies in integrative biology

Although Mendelian diseases are rare, when considered one by one, overall they constitute a significant social burden. Besides the medical aspects, they propose us one of the most general biological problems. Given the simplest physiological perturbation of an organism, that is, a single gene mutation, how do its effects percolate through the hierarchical biological levels to determine the pathogenesis? And how robust is the physiological system to this perturbation? To solve these problems, the study of genetic epilepsies caused by mutant ion channels presents special advantages, as it can exploit the full range of modern experimental methods. These allow to extend the functional analysis from single channels to whole brains. An instructive example is autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), which can be caused by mutations in neuronal nicotinic acetylcholine receptors. In vitro, such mutations often produce hyperfunctional receptors, at least in heterozygous condition. However, understanding how this leads to sleep-related frontal epilepsy is all but straightforward. Several available animal models are helping us to determine the effects of ADNFLE mutations on the mammalian brain. Because of the complexity of the cholinergic regulation in both developing and mature brains, several pathogenic mechanisms are possible, which also present different therapeutic implications.

Key factors in the discovery and development of new antiepileptic drugs

Since the early 1990s, many new antiepileptic drugs (AEDs) that offer appreciable advantages in terms of their favourable pharmacokinetics, improved tolerability and lower potential for drug-drug interactions have entered the market. However, despite the therapeutic arsenal of old and new AEDs, approximately 30% of patients with epilepsy still suffer from seizures. Thus, there remains a substantial need for the development of more efficacious AEDs for patients with refractory seizures. Here, we briefly review the emerging knowledge on the pathological basis of epilepsy and how it might best be used in the design of new therapeutics. We also discuss the current approach to AED discovery and highlight some of the unique features of newer models of pharmacoresistance and epileptogenesis that have emerged in recent years.

Altered fast- and slow-twitch muscle fibre characteristics in female mice with a (S248F) knock-in mutation of the brain neuronal nicotinic acetylcholine receptor

We generated a mouse line with a missense mutation (S248F) in the gene (CHRNA4) encoding the alpha4 subunit of neuronal nicotinic acetylcholine receptor (nAChR). Mutant mice demonstrate brief nicotine induced dystonia that resembles the clinical events seen in patients with the same mutation. Drug-induced dystonia is more pronounced in female mice, thus our aim was to determine if the S248F mutation changed the properties of fast- and slow-twitch muscle fibres from female mutant mice. Reverse transcriptase-PCR confirmed CHRNA4 gene expression in the brain but not skeletal muscles in normal and mutant mice. Ca(2+) and Sr(2+) force activation curves were obtained using skinned muscle fibres prepared from slow-twitch (soleus) and fast-twitch (EDL) muscles. Two significant results were found: (1) the (pCa(50) - pSr(50)) value from EDL fibres was smaller in mutant mice than in wild type (1.01 vs. 1.30), (2) the percentage force produced at pSr 5.5 was larger in mutants than in wild type (5.76 vs. 0.24%). Both results indicate a shift to slow-twitch characteristics in the mutant. This conclusion is supported by the identification of the myosin heavy chain (MHC) isoforms. Mutant EDL fibres expressed MHC I (usually only found in slow-twitch fibres) as well as MHC IIa. Despite the lack of spontaneous dystonic events, our findings suggest that mutant mice may be having subclinical events or the mutation results in a chronic alteration to muscle neural input.

Nicotine normalizes intracellular subunit stoichiometry of nicotinic receptors carrying mutations linked to autosomal dominant nocturnal frontal lobe epilepsy

Autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is linked with high penetrance to several distinct nicotinic receptor (nAChR) mutations. We studied (alpha4)(3)(beta2)(2) versus (alpha4)(2)(beta2)(3) subunit stoichiometry for five channel-lining M2 domain mutations: S247F, S252L, 776ins3 in alpha4, V287L, and V287M in beta2. alpha4 and beta2 subunits were constructed with all possible combinations of mutant and wild-type (WT) M2 regions, of cyan and yellow fluorescent protein, and of fluorescent and nonfluorescent M3-M4 loops. Sixteen fluorescent subunit combinations were expressed in N2a cells. Förster resonance energy transfer (FRET) was analyzed by donor recovery after acceptor photobleaching and by pixel-by-pixel sensitized emission, with confirmation by fluorescence intensity ratios. Because FRET efficiency is much greater for adjacent than for nonadjacent subunits and the alpha4 and beta2 subunits occupy specific positions in nAChR pentamers, observed FRET efficiencies from (alpha4)(3)(beta2)(2) carrying fluorescent alpha4 subunits were significantly higher than for (alpha4)(2)(beta2)(3); the converse was found for fluorescent beta2 subunits. All tested ADNFLE mutants produced 10 to 20% increments in the percentage of intracellular (alpha4)(3)(beta2)(2) receptors compared with WT subunits. In contrast, 24- to 48-h nicotine (1 muM) exposure increased the proportion of (alpha4)(2)(beta2)(3) in WT receptors and also returned subunit stoichiometry to WT levels for alpha4S248F and beta2V287L nAChRs. These observations may be relevant to the decreased seizure frequency in patients with ADNFLE who use tobacco products or nicotine patches. Fluorescence-based investigations of nAChR subunit stoichiometry may provide efficient drug discovery methods for nicotine addiction or for other disorders that result from dysregulated nAChRs.

Epileptogenic Channelopathies: Experimental Models of Human Pathologies

The discovery of genetically determined epileptic syndromes associated with specific mutations of genes codifying for subunits of voltage or ligand-activated ion channels highlights the role of ion channels in epileptogenesis. In vitro and in vivo models of channel pathology have been used to define the functional consequence of the mutations identified in human epilepsies. The evaluation of gene-channel mutations based on molecular and physiological techniques have provided significant knowledge on the cellular mechanisms leading to inherited human epilepsies, and possibly to nongenetic human epilepsies due to "acquired" channel pathologies. We review some of the studies that have explored human epileptic disorders through experimental manipulations of these channels, highlighting some of the difficulties that have arisen using "in vitro" preparations or rodent models. These findings underscore the need for further studies to address the mechanisms involved in mutated-channel dysfunctions.

Null mutation of the alpha4 nicotinic receptor subunit increases the propensity of muscarinic-mediated neuronal bursting in mouse hippocampal slices

Alpha4 subunit nicotinic cholinergic receptor (nAChR) knock out mice (KO) have a greater susceptibility to proconvulsant-induced seizures than do wild type (WT). The underlying mechanisms remain obscure. We tested whether such seizure-like activity was reflected in bursting activity of hippocampal neurons by recording with intracellular microelectrodes from CA1 pyramidal neurons in slices from WT and KO mice. Intriguingly, while carbachol-induced bursting activity occurred in only 21% of WT slices, qualitatively identical patterns of bursting occurred in 72% of KO slices. Extracellular recordings from CA1 and CA3 regions suggest that carbachol-mediated population activity was regionalized in our preparations. The relative weighting of excitatory to inhibitory synaptic potentials was similar between WT and alpha4 KO mice. However, burst-firing cells had a smaller input time constant than non-bursters. Low-concentration DHbetaE (selective alpha4beta2 nAChR antagonist) did not increase the propensity of WT slices to burst-fire, indicating that absence of alpha4 subunits per se, cannot explain the differences in activity between slices from WT and KO mice. These observations suggest that alpha4 nAChRs are unlikely to be involved in modulating the pattern of bursting neural activity, but their absence could induce subtle developmental changes in the sensitivity of hippocampal circuits to develop this behaviour.

A spontaneous mutation involving Kcnq2 (Kv7.2) reduces M-current density and spike frequency adaptation in mouse CA1 neurons

The M-type K+ current [IK(M)] activates in response to membrane depolarization and regulates neuronal excitability. Mutations in two subunits (KCNQ2 and KCNQ3; Kv7.2 and Kv7.3) that underlie the M-channel cause the human seizure disorder benign familial neonatal convulsions (BFNC), presumably by reducing IK(M) function. In mice, the Szt1 mutation, which deletes the genomic DNA encoding the KCNQ2 C terminus and all of CHRNA4 (nicotinic acetylcholine receptor alpha4 subunit) and ARFGAP-1 (GTPase-activating protein that inactivates ADP-ribosylation factor 1), reduces seizure threshold, and alters M-channel pharmacosensitivity. Genomic deletions affecting the C terminus of KCNQ2 have been identified in human families with BFNC, and truncation of the C terminus prevents proper KCNQ2/KCNQ3 channel assembly in Xenopus oocytes. We showed previously that Szt1 mice have a reduced baseline seizure threshold and altered sensitivity to drugs that act at the M-channel. Specifically, the proconvulsant M-channel blocker linopirdine and anticonvulsant enhancer retigabine display increased and decreased potency, respectively, in Szt1 mice. To investigate the effects of the Szt1 mutation on IK(M) function explicitly, perforated-patch electrophysiology was performed in CA1 pyramidal neurons of the hippocampus in brain slices prepared from C57BL/6J-Szt1/+ and control C57BL/6J+/+ mice. Our results show that Szt1 reduces both IK(M) amplitude and current density, inhibits spike frequency adaptation, and alters many aspects of M-channel pharmacology. This is the first evidence that a naturally occurring Kcnq2 mutation diminishes the amplitude and function of the native neuronal IK(M), resulting in significantly increased neuronal excitability. Finally, the changes in single-cell biophysical properties likely underlie the altered seizure threshold and pharmacosensitivity reported previously in Szt1 mice.

The alpha3 and beta4 nicotinic acetylcholine receptor subunits are necessary for nicotine-induced seizures and hypolocomotion in mice

Binding of nicotine to nicotinic acetylcholine receptors (nAChRs) elicits a series of dose-dependent behaviors that go from altered exploration, sedation, and tremors, to seizures and death. nAChRs are pentameric ion channels usually composed of alpha and beta subunits. A gene cluster comprises the alpha3, alpha5 and beta4 subunits, which coassemble to form functional receptors. We examined the role of the beta4 subunits in nicotine-induced seizures and hypolocomotion in beta4 homozygous null (beta4 -/-) and alpha3 heterozygous (+/-) mice. beta4 -/- mice were less sensitive to the effects of nicotine both at low doses, measured as decreased exploration in an open field, and at high doses, measured as sensitivity to nicotine-induced seizures. Using in situ hybridization probes for the alpha3 and alpha5 subunits, we showed that alpha5 mRNA levels are unchanged, whereas alpha3 mRNA levels are selectively decreased in the mitral cell layer of the olfactory bulb, and the inferior and the superior colliculus of beta4 -/- brains. alpha3 +/- mice were partially resistant to nicotine-induced seizures when compared to wild-type littermates. mRNA levels for the alpha5 and the beta4 subunits were unchanged in alpha3 +/- brains. Together, these results suggest that the beta4 and the alpha3 subunits are mediators of nicotine-induced seizures and hypolocomotion.

GABAergic systems modulate nicotinic receptor-mediated seizures in mice

The pharmacology of nicotinic receptor-mediated seizures was investigated in C3H mice. Eleven nicotinic agonists and six antagonists were administered centrally (i.c.v.). Epibatidine and epiboxidine were the most potent agonists tested, whereas acetylcholine and the alpha7*-selective compounds 3-(2,4-dimethoxybenzylidene)-anabaseine (GTS-21) and anabasine, were the least potent. Nicotine-induced seizures were blocked by cotreatment with either the nonselective antagonist mecamylamine or the alpha7*-selective antagonist methyllycaconitine. The alpha4beta2*-selective antagonist dihydro-beta-erythroidine was ineffective at blocking seizures. However, high doses of all six antagonists tested were fully efficacious in producing seizures, with d-tubocurarine being the most potent and mecamylamine the least potent. Potential relationships between nicotinic receptor-mediated seizures and drug effects on GABA function were also investigated. No correlation was seen between potencies of the agonists in producing seizures and stimulating [3H]GABA release or between potencies of the antagonists in producing seizures and antagonist inhibition of nicotine-stimulated [3H]GABA release. However, a robust correlation was detected between potencies of the agonists in producing seizures and the IC50 values for inhibition of nicotine-stimulated [3H]GABA release produced by agonist-induced receptor desensitization. We also compared inbred mouse strain sensitivity to nicotine, picrotoxin, bicuculline, and kainate-induced seizures. Robust positive correlations were revealed for nicotine-induced seizures and seizures induced by either picrotoxin or bicuculline, both GABAA receptor antagonists. No correlation was found between nicotine-induced seizures and those induced by the excitatory amino acid receptor agonist kainate. Based on these findings, we present a model for nicotinic receptor-mediated seizures mediated through GABAergic systems.

Increased sensitivity to agonist-induced seizures, straub tail, and hippocampal theta rhythm in knock-in mice carrying hypersensitive alpha 4 nicotinic receptors

We studied a strain of exon replacement mice ("L9'S knock-in") whose alpha4 nicotinic receptor subunits have a leucine to serine mutation in the M2 region, 9' position (Labarca et al., 2001); this mutation renders alpha4-containing receptors hypersensitive to agonists. Nicotine induced seizures at concentrations (1 mg/kg) approximately eight times lower in L9'S than in wild-type (WT) littermates. At these concentrations, L9'S but not WT showed increases in EEG amplitude and theta rhythm. L9'S mice also showed higher seizure sensitivity to the nicotinic agonist epibatidine, but not to the GABA(A) receptor blocker and proconvulsant bicuculline. Dorsiflexion of the tail (Straub tail) was the most sensitive nicotine effect found in L9'S mice (0.1 mg/kg). The L9'S mice were hypersensitive to galanthamine- and tacrine-induced seizures and Straub tails. There were no apparent neuroanatomical differences between L9'S and WT mice in several brain regions. [(125)I]Epibatidine binding to brain membranes showed that the mutant allele was expressed at approximately 25% of WT levels, presumably because of the presence of a neomycin selection cassette in a nearby intron. (86)Rb efflux experiments on brain synaptosomes showed an increased fraction of function at low agonist concentrations in L9'S mice. These data support the possible involvement of gain-of-function alpha4 receptors in autosomal dominant nocturnal frontal-lobe epilepsy.