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Molecular Mechanisms of Antiseizure Drug Activity at GABAA Receptors

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Molecular Mechanisms of Antiseizure Drug Activity at GABAA Receptors View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Seizure 22 (2013) 589–600 Contents lists available at SciVerse ScienceDirect Seizure jou rnal homepage: www.elsevier.com/locate/yseiz Review Molecular mechanisms of antiseizure drug activity at GABAA receptors L. John Greenfield Jr.* Dept. of Neurology, University of Arkansas for Medical Sciences, 4301W. Markham St., Slot 500, Little Rock, AR 72205, United States A R T I C L E I N F O A B S T R A C T Article history: The GABAA receptor (GABAAR) is a major target of antiseizure drugs (ASDs). A variety of agents that act at Received 6 February 2013 GABAARs s are used to terminate or prevent seizures. Many act at distinct receptor sites determined by Received in revised form 16 April 2013 the subunit composition of the holoreceptor. For the benzodiazepines, barbiturates, and loreclezole, Accepted 17 April 2013 actions at the GABAAR are the primary or only known mechanism of antiseizure action. For topiramate, felbamate, retigabine, losigamone and stiripentol, GABAAR modulation is one of several possible Keywords: antiseizure mechanisms. Allopregnanolone, a progesterone metabolite that enhances GABAAR function, Inhibition led to the development of ganaxolone. Other agents modulate GABAergic ‘‘tone’’ by regulating the Epilepsy synthesis, transport or breakdown of GABA. GABAAR efficacy is also affected by the transmembrane Antiepileptic drugs chloride gradient, which changes during development and in chronic epilepsy. This may provide an GABA receptor Seizures additional target for ‘‘GABAergic’’ ASDs. GABAAR subunit changes occur both acutely during status Chloride channel epilepticus and in chronic epilepsy, which alter both intrinsic GABAAR function and the response to GABAAR-acting ASDs. Manipulation of subunit expression patterns or novel ASDs targeting the altered receptors may provide a novel approach for seizure prevention. ß 2013 British Epilepsy Association. Published by Elsevier Ltd. All rights reserved. À Seizures frequently result from an imbalance of excitation and central water-filled pore, which gates to conduct Cl ions when inhibition due to a failure of inhibitory neurotransmission. Most GABA is bound (Fig. 1). Individual subunit subtypes confer 12 agents that enhance GABAA receptor (GABAAR) function have different sensitivities to GABAAR modulators including BZs , 13 14 antiseizure properties due to their ability to increase inhibitory loreclezole and zinc ions (Table 1). neurotransmitter tone. The evidence linking epilepsy with GABAARs are the target not only of the BZs, but other ASDs 1 dysfunction of GABAergic inhibition is substantial, and has been including barbiturates and agents like tiagabine and vigabatrin 1–4 extensively reviewed . that increase GABA concentration at the synapse. Bicuculline and picrotoxin, which are GABAAR antagonists, induce seizures in 1. GABAARs and epilepsy animals and epileptiform activity in brain slice preparations. Several animal models of epilepsy have altered GABAAR number or GABAARs are pharmacologically complex, with binding sites for 1,2,15 function . GABAAR subunit expression is altered in the benzodiazepines (BZs), barbiturates, neurosteroids, general anes- 16 hippocampi of experimental animals with recurrent seizures thetics, loreclezole, and the convulsant toxins, picrotoxin and 17,18 and in patients with temporal lobe epilepsy . Angelman’s bicuculline. Protein subunits from seven different subunit fami- 5 6 syndrome is a human neurodevelopmental disorder associated lies assemble to form pentameric transmembrane chloride with severe mental retardation and epilepsy, which is linked to a channels (Fig. 1). In mammals, 16 subunit subtypes have been 20 7 8 deletion mutation on chromosome 15q11-13 in a region cloned, including 6 a, 3 b and 3 g subtypes, as well as d, p , e and 21 9 encoding the GABAAR b3 subunit . Two mutations in the g2 u and alternatively spliced variants of the b2 and g2 subtypes. 22 23 24 10 subunit that impair GABAAR function , K289 M and R43Q , Subunit expression is regulated by region, cell type and 11 have been linked to a human syndrome of childhood absence developmental stage , reducing the number of isoforms epilepsy and febrile seizures, and a loss-of-function mutation in expressed in specific brain regions and individual neurons. The the a1 subunit causes autosomal dominant Juvenile Myoclonic most common composition includes two a1, two b2 and a single 25 Epilepsy . The R43Q mutation in the g2 subunit reduces BZ g2 subunit; the d subunit is found instead of g in receptors 26 27–29 sensitivity by altering GABAAR assembly and trapping the expressed extrasynaptically. The subunits are arranged around a 30 receptor in the endoplasmic reticulum . Other point mutations in GABAAR subunits have also been associated with generalized epilepsies (see Fig. 2). Hence, GABAAR modulation by GABAergic * Tel.: +1 501 686 7236; fax: +1 501 686 7850. E-mail address: ljgreenfi[email protected] ASDs is likely critical to their antiseizure activity. 1059-1311/$ – see front matter ß 2013 British Epilepsy Association. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.seizure.2013.04.015 590 L.J. Greenfield Jr. / Seizure 22 (2013) 589–600 Fig. 1. Model of a GABAA receptor in the plasma membrane. A. A space-filling model of the pentomer in side view (A) and top view (B) based on the high sequence homology with the nicotinic acetylcholine receptor. There are two binding sites for GABA, between a and b subunits (bent arrows), and one for BZs between the alpha and gamma subunits (arrow). C. A schematic view shows the topology of each subunit with a large extracellular loop containing a cysteine loop and 4 transmembrane domains (M1-M4), À the second of which forms the chloride ion channel. Binding of GABA allows the channel to open and conduct Cl ions, resulting in the fast inhibitory post-synaptic potential (IPSP). D. Putative arrangement of 5 subunits to form a pentamer with central chloride channel lined by the M2 subunit. [Derived from the published structure: RCSB PDB 198 Database ÁPDB ID: 2BG9 from Unwin, N. (2005) . Images modified from en.wikipedia.org/wiki/GABA_A_receptor, used with permission (public domain)]. 32 2. Benzodiazepines and nitrazepam. In 1965, diazepam was first used to treat status 33 epilepticus in humans. Clonazepam was introduced in the 1970’s 34 BZs were initially developed as anxiolytic agents in the 1950’s. primarily as an ASD, and clobazam, a 1,5 benzodiazepine, was 31 35 Chlordiazepoxide was introduced in 1960, followed by diazepam later developed as an ASD with less sedative effect. However, the Table 1 Antiseizure Drugs and their GABAAR Effects. Anti-seizure drug Subunit specificity Site of action GABAAR action Other mechanisms Benzodiazepines a1–3, a5; g a/g interface, Left shift of GABA Possible increased a1(H101) C/R curve, " GABAAR channel conductance open frequency Zolpidem a1 > a2,3; g a/g interface ‘‘ ’’ + Barbiturates b subunits? M3 residues? " channel open time & Use-dependent Na channel block burst duration Loreclezole b2, b3 B2(N289) Barbiturate-like? inhibits GABAAR at " concentration Ganaxolone a, b, d, # with e a1(Q241) Barbiturate-like, also " Open channel block at high conc. open frequency + 2+ Topiramate a6 > a4 > a1-a2? unknown Enhanced GABA currents Na /Ca channel block, AMPA/kainate receptor block Felbamate unknown unknown Barbiturate-like Blocks NMDA receptor currents Retigabine unknown unknown Increased GABA IPSCs Opens KCNQ2/3 potassium channels + Losigamone unknown unknown Enhanced GABA Use-dependent Na channel block receptor current Stiripentol a3, # with b1, e unknown Barbiturate-like CYP450 inhibition Vigabatrin GABA transaminase increases [GABA] – 2+ Gabapentin, Glutamic acid decarboxylase " GABA synthesis? a2-d subunit of voltage-gated Ca channel pregabalin + 2+ Valproic Acid Glutamic acid decarboxylase " GABA synthesis? block of Na , T-type Ca channels Tiagabine GAT-1 (GABA transporter) Blocks GABA reuptake L.J. Greenfield Jr. / Seizure 22 (2013) 589–600 591 199 Fig. 2. Model of a prototype GABAAR subunit (based on a1 subunit diagram from Olsen & Tobin, 1990) showing approximate locations of point mutations associated with 200 generalized epilepsies (see also Macdonald et al., 2012 ) in black, and locations of point mutations associated with ASD sites of action. See text for details. induction of tolerance limits their use as ASDs. The 1,5 BZs may receptor with high affinity for the hypnotic, zolpidem, defining the 50 produce less tolerance than the 1,4 BZs, but tolerance to 1,5 BZs ‘‘BZ-1’’ (or V-1) receptor type. The a2 and a3 subunits result in 36 does occur. receptors with moderate zolpidem affinity, termed BZ-2 receptors. BZ activity at GABAARs is a function of the drug’s affinity for the GABAARs containing the a5 subunit and/or the g3 subunit are BZ binding site and its intrinsic allosteric effect on the GABAAR. The sensitive to diazepam but not to zolpidem and are termed BZ-3 efficacy of individual compounds varies widely. Most BZs in receptors. GABAARs with the a4 or a6 subunits are insensitive to clinical use are full agonists that maximally enhance GABAAR most BZs. activity. Flumazenil, a competitive antagonist used to reverse BZ- The subunit composition not only determines the affinity for 37 induced sedation, binds to the BZ site without affecting GABAAR particular BZs, but also the clinical/behavioral effect of the BZ at 39 40 41 function. Abecarnil, imidazenil, and bretazenil are ‘‘partial that receptor. The role of the a subunits in BZ pharmacology was agonists’’ at the BZ site which have antiseizure efficacy in animal revealed by the discovery of a single histidine (H) residue found in 42 models and appear less prone to tolerance. Several b-carbolines all BZ-sensitive a subunits (H101 in the rat a1 subunit), but not in 43 are ‘‘inverse BZ agonists’’ that inhibit GABA binding and can the BZ-insensitive a4 or a6 subunits, which instead have a 44 induce seizures or anxiety.
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