Synaptic and extrasynaptic GABA transporters as targets for anti‐epileptic drugs

Abstract

Epilepsy is a heterogeneous disorder that is characterized by the presence of both convulsive and non-convulsive seizures. Moreover, epilepsy is one of the most prevalent neurological disorders that does not discriminate according to age, gender, race, or socioeconomic status. Worldwide, there are over 50 million people afflicted by this disorder and approximately 50% of patients with epilepsy can be effectively treated with the currently available anti-epileptic drugs. For another 20%, their seizures are inadequately controlled or controlled at the expense of substantial adverse effects. Since 1993, 11 new anti-epileptic drugs have been introduced into the market for the treatment of epilepsy. Each of these new drugs has brought about an improved standard of care for a large number of patients with epilepsy in the form of reduced adverse events, a lower propensity for drug-drug interactions, and improved efficacy. Unfortunately, there still remains a significant unmet need for approximately 20-30% of the patients with refractory epilepsy. Until this population is seizure-free, there will always be a need for more effective and better tolerated drugs.A greater understanding of the mechanisms of existing anti-epileptic drugs (AEDs), and the molecular genetics and pathophysiology of various seizure types has resulted in an ever increasing number of molecular targets for drug Received November 3, 2008; revised manuscript received December 12, 2008; accepted January 11, 2009. Address correspondence and reprint requests to H. Steve White, Ph.D., Professor and Director, Anticonvulsant Drug Development Program, University of Utah, 417 Wakara Way, Suite 3211, Salt Lake City, Utah 84108. E-mail: swhite@hsc.utah.eduAbbreviations used: AED, anti-epileptic drug; AGS, audiogenic seizure; BGT-1, Betaine-GABA transporter-1; GAT, GABA transporters. AbstractInhibition of the GABA transporter subtype GAT1 by the clinically available anti-epileptic drug tiagabine has proven to be an effective strategy for the treatment of some patients with partial seizures. In 2005, the investigational drug EF1502 was described as possessing activity at both GAT1 and BGT-1. When combined with the GAT1 selective inhibitor tiagabine, EF1502 was found to possess a synergistic anti-convulsant action in the Frings audiogenic seizure-susceptible mouse model of reflex epilepsy. This effect was subsequently attributed to inhibition of BGT-1. In this study, the anti-convulsant effect of the GAT2/3 inhibitor SNAP-5114 was assessed in the Frings audiogenic seizure-susceptible mouse alone, and in combination with tiagabine and EF1502. The results showed that SNAP-5114 produced a synergistic anti-convulsant effect in combination with EF1502 but not when used in combination with tiagabine. These findings support anatomical evidence that GAT2/3 are most likely located at the synapse in close proximity to GAT1; whereas BGT-1 is located some distance away from the synapse and GAT1 and GAT2/3. Lastly, EF1502 and tiagabine were evaluated alone, and in combinatio...

A Subconvulsive Dose of Kainate Selectively Compromises Astrocytic Metabolism in the Mouse Brain In Vivo

“…Seizure severity was graded according to a modified Racine scale. 4,14 Fifteen minutes after kainate injection, the mice were injected intraperitoneally with a combination of [1-13 C]glucose (543 mg/kg) and [1,[2][3][4][5][6][7][8][9][10][11][12][13] C]acetate (504 mg/kg) and after another 15 minutes, the brains of the mice were subjected to microwave fixation at 4 kW for 1.7 seconds (model GA5013, Gerling Applied Engineering, Modesto, CA, USA). This is the most humane euthanization method instantly inactivating proteins in the brain.…”

“…If [4,[5][6][7][8][9][10][11][12][13] C]a-ketoglutarate remains in the astrocytic TCA cycle for further metabolism via this pathway, it will, after condensation with unlabeled acetyl CoA, give rise to [3-13 C]-and [1,2-13 C]a-ketoglutarate in the subsequent turn, which may give rise to glutamate and glutamine displaying the same labeling patterns. Based on the distinct 13 C labeling in glutamine after the first and second turn of TCA cycle metabolism, a number indicative of astrocytic TCA cycle activity can be calculated by relating the amount of isotopomers generated in the second turn of TCA cycle metabolism to the amount of that generated in the first turn, i.e., ( [1,[2][3][4][5][6][7][8][9][10][11][12][13] …”

“…In glutamatergic neurons, this [4,[5][6][7][8][9][10][11][12][13] C]glutamate may be released upon depolarization. Otherwise, [4,[5][6][7][8][9][10][11][12][13] C]glutamate may be converted to [4,[5][6][7][8][9][10][11][12][13] C]a-ketoglutarate for further TCA cycle metabolism or may in GABAergic neurons be converted to [1,[2][3][4][5][6][7][8][9][10][11][12][13] C]GABA. It should be noted that the largest pool of glutamate is situated in the glutamatergic neurons and the [4,[5][6][7][8][9][10]…”

“…Approximately 50 million people worldwide are diagnosed with epilepsy and only B50% of these are adequately treated. 1 Temporal lobe epilepsy is one of the most common forms of epilepsy and pharmacoresistance within these patients is particularly high accounting for up to 80% of patients. 2 Hence, there is a persistent need for development of anticonvulsant drugs displaying improved efficacy and reduced adverse effects and to this end, a thorough understanding of the etiology of temporal lobe epilepsy and seizure activity is required.…”

Selective GABA Transporter Inhibitors Tiagabine and EF1502 Exhibit Mechanistic Differences in Their Ability to Modulate the Ataxia and Anticonvulsant Action of the Extrasynaptic GABAAReceptor Agonist Gaboxadol

“…Proof of concept that GAT inhibitors mediate anticonvulsant activity in rodents has been firmly established (Nielsen et al, 1991;White et al, 1993White et al, , 2002White et al, , 2005Madsen et al, 2009Madsen et al, , 2010. Furthermore, it has been demonstrated that an inhibitor of BGT1 having no affinity for GABA i.e., 4,5,6,7-tetrahydrobenzo[d]isoxazol-3-ol (EF1502) when administered in combination with inhibitors of GAT1 (i.e., tiagabine) and N- [4,5,6,, results in a synergistic anticonvulsant effect in audiogenic seizure (AGS)-susceptible Frings mice (White et al, 2005).…”