Current Drug Targets-CNS & Neurological Disorders - Volume 2, Issue 4, 2003

This hot topic issue of Current Drug Targets - CNS and Neurological Disorders focuses on the GABAergic system for the development of novel and improved therapeutic approaches. GABA (γ-aminobutyric acid) is the major inhibitory neurotransmitter system in the mammalian brain and has been implicated in a large variety of neurological and psychiatric disorders, including anxiety, epilepsy, sleep and cognitive impairment. GABA is formed by the α-decarboxylation of Lglutamic acid, a reaction catalysed by glutamic acid decarboxylase (GAD), an enzyme encoded by two distinct genes to produce GAD65 and GAD67. GABA is packaged into presynaptic vesicles using the vesicular GABA transporter and when a depolarising stimulus arrives at the presynaptic nerve terminal, the vesicle fuses with the presynaptic membrane and GABA is released into the synaptic cleft. The released GABA may interact with the ionotropic GABAA receptors or the metabotropic GABAB receptors [1] or the ionotropic GABAC receptor [2]. It should be noted that whilst the GABAC receptor has been argued to be a subtype of the GABAA receptor family [3], in this Special Issue we have retained the GABAC nomenclature to emphasize the distinct neuroanatomical distribution and pharmacology of this receptor population [4]. The action of GABA within the synapse is terminated by uptake into neurons and glia by the plasma membrane GABA transporters, of which four genes have been described. Finally, GABA is metabolised to succinic semialdehyde in a reaction catalysed by GABA-α- oxoglutarate transaminase (GABA-T). Given the key role of the GABA system in mediating a wide variety of CNS functions it is therefore not surprising that the various enzymes, transporters and receptors involved in GABA function and metabolism have attracted considerable attention as potential therapeutic targets which is reflected in this Special Issue. Of the various therapeutic sites of intervention in GABAergic neurotransmission, the GABAA receptor has received most attention. Indeed, based upon the number of recognition sites that the GABAA receptor possesses (such as the GABA, benzodiazepine, neurosteroid, barbiturate, ethanol, anaesthetic, loreclezole and convulsant binding sites), this receptor alone could merit a Special Issue. Nevertheless, based upon over 40 years of clinical experience with the benzodiazepines (BZs) and the corresponding elucidation of the molecular mechanisms by which they exert their effects, it is the BZ binding site which forms the basis of the discussions of the GABAA receptor, with the acceptance that this is a blinkered view of the therapeutic potential of this receptor based more upon limitations of space rather than scientific rationale. Thus, the clinical safety and efficacy of BZs coupled to the realization that their diverse pharmacological effects (including anxiolysis, sedation, myorelaxation, cognitive impairment and anticonvulsant activity) are exerted via interaction with GABAA receptors containing an α1, α2, α3 or α5 subunit has resulted in the search for compounds which only express certain aspects of their pharmacology. Fundamental to this is an understanding of which GABAA subtypes mediate which particular aspect of the BZ pharmacology; a topic discussed in the review of “Validation of GABAA receptor subtypes as potential drug targets by using genetically modified mice” by T.W. Rosahl. It summarizes transgenic approaches to validate novel and existing drug targets by dissecting out the various effects of BZs and other GABAA receptor modulators, such as anaesthetics, on individual GABAA receptor subtypes. For example, in the search for compounds which are anxiolytic but have reduced sedation, two approaches have been employed; the development of non-selective compounds which interact with the four subtypes of GABAA receptor but with reduced efficacy relative to the existing full agonists used in the clinic (non-selective partial agonists) and, more recently, compounds which selectively enhance the actions of GABA at those GABAA receptor subtypes (α2 and / or α3- containing receptors) involved in anxiety (subtype-selective compounds). J.R. Atack reviews the status of preclinical and clinical studies on such non-sedating anxiolytics in “Anxioselective compounds acting at the GABAA receptor BZ binding site”. The major challenge for such compounds remains the difficulty in translating a novel pharmacology in preclinical species, whether it is achieved via non-selective partial agonism or subtype selectivity, into a meaningful advantage in the clinic. In contrast to developing subtype selective agonists as non-sedating anxiolytics, the approach of developing inverse agonists which specifically target α5 subunit containing GABAA receptors is described in K.A. Maubach's review “GABAA receptor subtype selective cognition enhancers”. It is well accepted that cognitive performance declines with age and as the demographics of the world population shifts towards the elderly, there is an increasing largely unmet medical need for cognition enhancers not only for the symptomatic treatment of Alzheimer's disease but also the mild cognitive impairment associated with the normal aging process and which may be a prelude to Alzheimer's disease. Preclinical animal studies with α5 selective inverse agonists are intriguing in that they enhance cognitive performance in normal animals but as with the subtype selective non-sedating anxiolytics, the crucial issue remains that of translating efficacy in animals models into meaningful improvements in the clinic. A problem with extrapolating from the pharmacology of a compound in vitro to that observed at the level of the whole animal (behaviour) is the lack of an understanding of the neuronal circuitry involved. The effect of GABAA receptor modulation on specific neuronal populations, for example hippocampal interneurons and pyramidal cells, is discussed by A. Semyanov in his review entitled “Cell type specificity of GABAA receptor mediated signaling in the hippocampus”. The problems associated with the development of novel anticonvulsant drug strategies in the absence of a detailed understanding of their effects on neuronal networks are highlighted. Moreover, the emergence of extrasynaptic GABAA receptors as a neuronal population exerting tonic inhibitory effects within the CNS highlights their potential as novel anticonvulsant targets. Although drugs which are selective either for anatomically discrete neuronal populations or for extra-synaptic rather than synaptic receptors are currently not available, Dr. Semyanov presents a persuasive case that such compounds would represent novel and effective anticonvulants. For many years, the GABAB receptor lived in the shadow of the GABAA receptor. Thus, whilst molecular biologists described a large variety of GABAA receptor subunits which offered a bewildering array of potential subtypes, and despite being implicated in a wide variety of neurological and psychiatric disorders, GABAB receptors remained decidedly dull and boring in comparison with their more glamourous cousins. However, all that has changed with the relatively recent cloning of GABAB receptors [5] which has reinvigorated interest in these receptors as potential drug targets. The review “GABAB receptors as potential therapeutic targets” by C.-M. Vacher and B. Bettler provides a thorough update on the molecular composition, the physiology and pharmacology of GABAB receptors and ongoing and future drug discovery efforts in the field. However, initial reports of GABAB subtype-selective compounds have proved difficult to reproduce and currently the best approach for achieving subtype selectivity appears to rest with the differential anatomical localization of the GABAB(1a,2) and GABAB(1b,2) subtypes. GABAC receptors are the least studied of the three major classes of GABA receptors and G.A.R. Johnston and colleagues summarize their therapeutic potential in “GABAC receptor as drug targets”. Major indications for drugs acting on GABAC receptors are in the treatment of visual, sleep and cognitive disorders. It is suggested that due to their relatively low abundance and more restricted neuroanatomical distribution in the CNS, GABAC receptor-selective drugs may provide an alternative to drugs acting through the more widespread GABAA or GABAB receptors. The last review of this issue by A. Sarup and colleagues focus on “GABA transporters and GABA-transaminase as drug targets”. The use of tiagabin and vigabatrin (γ-vinyl GABA) as anticonvulsants demonstrate the clinical utility of GABA transporters and GABA-transaminase, respectively. Whilst GABA transaminase exists as a single gene product and would therefore appear to offer little opportunity for further therapeutic specificity, the differential localization of the four members of the GABA transporter family in either neuronal or glial tissue may offer the opportunity to selectively modulate distinct aspects of GABAergic neurotransmission. Overall, this issue provides an overview of the current scientific knowledge of the physiology and pharmacology of the GABAergic system and highlights the potential for therapeutic intervention at a variety of points in the GABAergic neurotransmission pathway. Moreover, whilst drugs acting at these various targets currently exist and emphasize their therapeutic utility in a variety of neuropsychiatric and neurological disorders, refinements based upon subtype selectivity and / or anatomical localization offer the potential of greater clinical specificity. Within the next 5 or so years it is hoped that at least some of these novel strategies will face the ultimate challenge; that of demonstrating efficacy in the clinic. [1] Hill, D.R.; Bowery, N.G. Nature, 1981, 290, 149-152. [2] Johnson, G.A.R. Trends Pharmacol. Sci., 1996, 17, 319-323. [3] Barnard, E.A.; Skolnick, P.; Olsen, R.W.; Möhler, H.; Sieghart, W.; Biggio, G.; Braestrup, C.; Bateson, A.N.; Langer, S.Z. Pharmacol. Rev., 1992, 50, 291-313. [4] Bormann, J. Trends Pharmacol. Sci., 2000, 21, 16-19. [5] Kaupmann, K.; Huggel, K.; Heid, J.; Flor, P.J.; Bischoff, S.; Mickel, S.J.; McMaster, G.; Angst, C.; Bittiger, H.; Froestl, W.; Bettler, B. Nature, 1997, 386, 239-46.

A key issue for drug discovery in the post genomic era is target validation. This is particularly important when considering the CNS, where currently the majority of drug targets are neurotransmitter receptors that are known to exist as multi-gene families. The GABAergic system, which is the major inhibitory neurotransmitter system in the CNS, is no exception in that respect. The GABAA receptors, which are the site of action of a number of clinically used drugs such as benzodiazepines and barbiturates, exist in a large gene family. Existing drugs mediating their effects through the GABAA receptor are generally non-selective, i.e. will act at several subtypes of that receptor family. Thus, if we are both to refine existing therapeutic approaches, and develop novel approaches, a key question is to define which subtype(s) of the GABAA receptor family we should target; which will mediate the beneficial effects of a drug, and which could be responsible for unwanted side effects? One of the tools, which has been developed over the last decade to elucidate the function of a given gene, is the generation and analysis of gene-targeted mice. This review will summarize progress on identifying individual GABAA receptor subtypes as potential drug targets by using genetically modified mice.

With the exception of obsessive compulsive disorder, benzodiazepines (BZs) remain a major first line treatment for anxiety disorders. However, as well as being anxiolytic, BZs also cause sedation acutely, related to the fact that BZs are also used as hypnotics, and chronically may have abuse potential as well as cause physical dependence which manifests itself as the demonstration of a number of adverse events upon discontinuation. The molecular mechanisms of BZs are now well defined in that they enhance the actions of the inhibitory neurotransmitter GABA by binding to a specific recognition site on GABAA receptors containing α1, α2, α3 and α5 subunits. Compounds that bind at this modulatory site and enhance the inhibitory actions of GABA are classified as agonists, those that decrease the actions of GABA are termed inverse agonists whereas compounds which bind but have no effect on GABA inhibition are termed antagonists. The clinically used BZs are full agonists and between the opposite ends of the spectrum, i.e. full agonist and full inverse agonist, are a range of compounds with differing degrees of efficacy, such as partial agonists and partial inverse agonists. Attempts have been made to develop compounds which are anxioselective in that they retain the anxiolytic properties of the full agonist BZs but have reduced sedation and dependence (withdrawal) liabilities. Such compounds may interact with all four (i.e. α1-, α2-, α3- and α5-containing) GABAA receptor subtypes and have partial rather than full agonist efficacies. Examples of nonselective partial agonists include bretazenil, imidazenil, FG 8205, abecarnil, NS 2710, pagoclone, RWJ-51204 and (S)-desmethylzopiclone. Alternatively, a compound might have comparable binding affinity but different efficacies at the various subtypes, thereby preferentially exerting its effects at subtypes thought to be associated with anxiety (α2- and / or α3-containing receptors) rather than the subtype associated with sedation (α1-containing receptors). Examples of efficacy selective compounds include L-838417, NGD 91-3 and SL651498. For each compound, preclinical and where available clinical data will be reviewed. Emerging themes include the lack of definitive intrinsic efficacy data for certain compounds (e.g. abecarnil, ocinaplon, pagoclone) and the difficulty in translating robust anxiolysis and a separation between anxiolytic and sedative doses of non-selective partial agonists in preclinical species into consistent clinical benefit in man (e.g. bretazenil, abecarnil, pagoclone). With respect to efficacy selective compounds, NGD 91-3 was not anxiolytic in man but in the absence of efficacy data, these results are difficult to interpret. Nevertheless, efficacy selective compounds represent a novel approach to targeting specific subtypes of the GABAA receptor, the ultimate test of which will be evaluation in the clinic.

Currently the treatment of Alzheimer's disease (AD) and Mild Cognitive Impairment (MCI) is largely unrealised, with no preventive or curative therapies. The marketed acetylcholinesterase inhibitors (eg. donepezil, Aricept) are directed toward temporary symptomatic relief from impaired cognition, but have prominent adverse effects with minimal efficacy. In pursuit of novel cognition enhancers, the observation that classical benzodiazepines (BZ, eg. diazepam) are amnesic, coupled with the preservation of GABAA receptors in brain areas most affected by AD, highlighted the GABAA receptor as a potential therapeutic target. In contrast to the amnesic BZ agonists, the BZ inverse agonists (eg. DMCM) which attenuate GABAA receptor function, have been shown to improve performance in animal models of learning and memory. Unfortunately, such nonselective ligands also induce anxiety and convulsions. More recently, novel ligands have been developed (eg. 6,6-dimethyl-3-(2-hydroxyethyl)thio-1-(thiazol-2-yl)-6,7-dihydro-2-benzothiophen-4(5H)-one) that demonstrate binding selectivity and high inverse agonism for the α5 GABAA receptor subtype, which is preferentially located in the hippocampus, a region of the brain associated with learning and memory. Preclinical results are encouraging, since these α5 selective inverse agonists enhance memory in animal models, such as spatial learning in the Morris water-maze, but are devoid of the adverse effects associated with activity at other GABAA receptor subtypes in other brain regions. If the efficacy and safety profiles of α5 inverse agonists in humans prove to be similar to those seen in pre-clinical studies, these compounds would offer significant benefit to AD and MCI patients.

Inhibitory signaling mediated by ionotropic GABAA receptors generally acts as a major brake against excessive excitability in the brain. This is especially relevant in epilepsy-prone structures such as the hippocampus, in which GABAA receptor mediated inhibition is critical in suppressing epileptiform activity. Indeed, potentiating GABAA receptor mediated signaling is an important target for antiepileptic drug therapy. GABAA receptor mediated inhibition has different roles in the network dependent on the target neuron. Inhibiting principal cells will thus reduce network excitability, whilst inhibiting interneurons will increase network excitability; GABAergic therapeutic agents do not distinguish between these two alternatives, which may explain why, on occasion, GABAergic antiepileptic drugs can be proconvulsant. The importance of the target-cell for the effect of neuroactive drugs has emerged from a number of recent studies. Immunocytochemical data have suggested non-uniform distribution of GABAA receptor subunits among hippocampal interneurons and pyramidal cells. This has been confirmed by subsequent electropharmacological data. These have demonstrated that compounds which act on GABAA receptors or the extracellular GABA concentration can have distinct effects in different neuronal populations. Recently, it has also been discovered that presynaptic glutamate heteroreceptors can modulate GABA release in the hippocampus in a postsynaptic cell-specific manner. Since systemically administrated drugs may act on different neuronal subtypes, they can exhibit paradoxical effects. Distinguishing compounds that have target specific effects on GABAergic signaling may lead to novel and more effective treatments against epilepsy.

γ-Aminobutyric acid-B (GABAB) receptors are broadly expressed in the nervous system and have been implicated in a wide variety of neurological and psychiatric disorders. To date the only GABAB drug on the market is the agonist baclofen (Lioresal®) that is used to treat severe spasticity of cerebral and spinal origin. In addition baclofen is effective in animal models for many central and peripheral disorders, but sideeffects and the development of tolerance prohibited a more widespread use of this drug in man. Similarly GABAB antagonists show great therapeutic promise but their shortcomings, e.g. the lack of brain penetration or some proconvulsive potential, prevented clinical development. The cloning of GABAB receptors in 1997 revived interest in these receptors as drug targets. The long-awaited availability of the tools that were necessary to develop more selective and safer drugs stimulated an impressive activity in the field. The demonstration that GABAB receptors needed to heteromerize for function provided new insights into the structure of G-protein coupled receptors in general and enabled to identify allosteric GABAB drugs. Gene knockout mice revealed neuronal systems that are under tonic GABAB control and therefore best suited for therapeutic intervention. Significant advances were made in clarifying the relationship between GABAB receptors and the receptors for γ-hydroxybutyrate (GHB), a drug of abuse. Here we provide and update on the molecular composition, the physiology and the pharmacology of GABAB receptors and discuss to what extent our current knowledge influences ongoing and future drug discovery efforts.

GABAC receptors are the least studied of the three major classes of GABA receptors. The physiological roles of GABAC receptors are still being unravelled and the pharmacology of these receptors is being developed. A range of agents has been described that act on GABAC receptors with varying degrees of specificity as agonists, partial agonists, antagonists and allosteric modulators. Pharmacological differences are known to exist between subtypes of cloned GABAC receptors that have been cloned from mammalian sources. There is evidence for functional GABAC receptors in the retina, spinal cord, superior colliculus, pituitary and gastrointestinal tract. Given the lower abundance and less widespread distribution of GABAC receptors in the CNS compared to GABAA receptors, GABAC receptors may be a more selective drug target than GABAA receptors. The major indications for drugs acting on GABAC receptors are in the treatment of visual, sleep and cognitive disorders. The most promising leads are THIP, a GABAC receptor antagonist in addition to its well known activity as a GABAA receptor partial agonist, which is being evaluated for sleep therapy, and CGP36742, an orally active GABAB and GABAC receptor antagonist, which enhances cognition. Analogues of THIP and CGP36742, such as aza-THIP, that are selective for GABAC receptors are being developed. TPMPA and related compounds such as P4MPA, PPA and SEPI are also important leads for the development of systemically active selective GABAC receptor antagonists.

The fine-tuning and homeostatic balance of the GABAergic inhibitory tone in the central nervous system (CNS) is a prerequisite for controlling the excitatory neurotransmission. This principal mechanism for controlling excitation is inhibition which has been the topic of intensive research covering all known functional entities of the GABAergic synapse. The therapeutical scope for targeting the GABA system covers a large number of neurological and psychiatric disorders. This review focuses on the major inactivation systems for GABAergic neurotransmission, the GABA transporters (GATs) and the GABA catabolic enzyme GABA -transaminase (GABA-T) as drug targets. Tiagabin and Vigabatrin, two anti-epileptic drugs on the market today, specifically inhibit GABA transport and metabolism, respectively. However, previous and recent evidence has clearly demonstrated the importance and differential functional roles of glial and neuronal GABA uptake and the metabolic fate of the sequestered neurotransmitter GABA in these cells. Moreover, the diverse expression patterns of the GABA transporters, in combination with development of GAT inhibitors with novel pharmacological profiles may initiate a renaissance for these inactivation systems as drugs targets. In particular, further research to elucidate the specialized physiological function of the GATs combined with their differential spatial expression could be of fundamental importance for the understanding of concerted action with regard to the fine-tuning of the GABAergic inhibitory tone. As such, selective targeting and modulation of GABA transporter subtypes and cell-specific GABA uptake and metabolism is of therapeutical interest in GABA-related CNS disorders, including epilepsy.