Current Topics in Medicinal Chemistry - Volume 2, Issue 8, 2002

GABA-A receptors are the major inhibitory neurotransmitter receptors in the brain and are the site of action of many clinically important drugs. These receptors are composed of five subunits that can belong to eight different subunit classes. Depending on their subunit composition, these receptors exhibit distinct pharmacological and electrophysiological properties. Recent studies on recombinant and native GABA-A receptors suggest the existence of far more receptor subtypes than previously assumed. Thus, receptors composed of one, two, three, four, or five different subunits might exist in the brain. Studies on the regional, cellular and subcellular distribution of GABA-A receptor subunits, and on the co-localization of these subunits at the light and electron microscopic level for the first time provide information on the distribution of GABA-A receptor subtypes in the brain. These studies will have to be complemented by electrophysiological and pharmacological studies on the respective recombinant and native receptors to finally identify the receptor subtypes present in the brain. The distinct cellular and subcellular location of individual receptor subtypes suggests that they exhibit specific functions in the brain that can be selectively modulated by subtype specific drugs. This conclusion is supported by the recent demonstration that different GABA-A receptor subtypes mediate different effects of benzodiazepines. Together, these results should cause a revival of GABA-A receptor research and strongly stimulate the development of drugs with a higher selectivity for α2-, α3-, or α5-subunit-containing receptor subtypes. Such drugs might exhibit quite selective clinical effects.

The GABA-A receptor system is implicated in a number of neurological diseases, making GABA-A receptor ligands interesting as potential therapeutic agents. Only a few different classes of structures are currently known as ligands for the GABA recognition site on the GABA-A receptor complex, reflecting the very strict structural requirements for GABA-A receptor recognition and activation. Within the series of compounds showing agonist activity at the GABA-A receptor site that have been developed, most of the ligands are structurally derived from the GABA-A agonists muscimol, THIP or isoguvacine. Using recombinant GABA-A receptors, functional selectivity has been shown for a number of compounds such as the GABA-A agonists imidazole-4-acetic acid and THIP, showing highly subunit-dependent potency and maximal response. In the light of the interest in partial GABA-A receptor agonists as potential therapeutics, structure-activity studies of a number of analogues of 4-PIOL, a low-efficacy partial GABA-A agonist, have been performed. In this connection, a series of GABA-A ligands has been developed showing pharmacological profiles from moderately potent low-efficacy partial GABA-A agonist activity to potent and selective antagonist effect. Only little information about direct acting GABA-A receptor agonists in clinical studies is available. Results from clinical studies on the effect of the GABA-A agonist THIP on human sleep pattern shows that the functional consequences of a direct acting agonist are different from those seen after administration of GABA-A receptor modulators.

Ligands of the benzodiazepine binding site of the GABA-A receptor come in three flavors: positive allosteric modulators, negative allosteric modulators and antagonists, all of which can bind with high affinity. The GABA-A receptor is a pentameric protein which forms a chloride selective ion channel and ligands of the benzodiazepine binding site stabilize three different conformations of this receptor channel. Classical benzodiazepines exert a positive allosteric effect by increasing the affinity of channel opening by the agonist γ-aminobutyric acid (GABA). We concentrate here on the major adult isoform, the α1β2γ2 GABA-A receptor. The binding pocket for benzodiazepines is located in a subunit cleft between γ2 and α1 subunits in a position homologous to the agonist binding site for GABA that is located between α1 and β2 subunits. It is reviewed here how we arrived at this picture. In particular, point mutations were performed in combination with subsequent analysis of the expressed mutant proteins using either electrophysiological techniques or radioactive ligand binding assays. The predictive power of these methods is assessed by comparing the results with the predictions that can be made on the basis of the recently published crystal structure of the acetylcholine binding protein that shows homology to the N-terminal, extracellular domain of the GABA-A receptor.

Considerable evidence has been provided these last years for the involvement of the GABA-A receptor complex in memory processes. Compounds that enhance the action of GABA, such as benzodiazepines, impair memory processing. On the contrary, compounds that reduce the action of GABA, such as ß-CCM, pentylenetetrazol or picrotoxin, have the opposite action, that is : enhance memory processing. All these actions seem to focus mainly on the acquisition (learning) processes. Depending on the dose, the same compounds also have effects on anxiety and on seizuring. Benzodiazepines are well-known anxiolytic and anticonvulsant agents whereas compounds that reduce the action of GABA have been found to produce anxiogenic and convulsant actions. The GABA-A receptor complex might thus be the location of a possible link between a pathological state (epilepsy) and two normal functions (anxiety and learning). This link is likely to involve common genetic pathways. In the normal subject, these data also emphasize the idea that normal memory processing involves a moderate level of anxiety.

This review describes the new research developments that have established the CNS-activity of some natural flavonoids. The properties of flavone, chrysin, apigenin and cirsiliol are described and a survey of the occurrence of ligands for the benzodiazepine binding site in the flavonoid field is attempted. Natural compounds, structurally related to flavonoids and with similar CNS-activities, are also included.A medicinal chemistry approach to improve the biochemical and pharmacological properties of the flavone nucleus is described alongside with the enumeration of the principal achievements obtained to date.Quantitative structure-activity relationships studies leading to the formulation of pharmacophore models presumably describing the characteristics of the flavone-binding site in the GABA-A -receptor are summarized.

Despite the fact that ethanol is one of the most widely used psychoactive agents, the mechanisms and sites of action by which it modifies brain functions are only now being elucidated. Studies over the last decade have shown that ethanol can specifically alter the function of several ligand-activated ion channels including Nmethyl- D-aspartate (NMDA), serotonin (5-HT3), glycine and GABA-A receptors. After several years of extensive research in this field, the resolution of what, where and how ethanol modifies GABA-A receptors continues to be controversial. For example, after demonstrating that ethanol was able to alter Cl- flux in synaptoneurosomes and cultured neurons, several electrophysiological studies were unable to show enhancement of the GABA-A receptor current in single neurons. The lack of positive results with low ethanol concentrations was interpreted as being due to receptor heterogeneity and differences in intracellular modulation by protein kinases and calcium.The existence of high receptor heterogeneity with respect to ethanol sensitivity has been supported by studies done in a variety of cell types which showed that ethanol potentiated some, but not other neurons. Adding to this complexity, it was shown that while some hippocampal GABA-A receptors can be affected by ethanol concentrations between 1 and 100 mM, others are only sensitive to concentrations above 200 mM. The curve of the relationship between low ethanol concentrations and current enhancement suggests a high degree of complexity in the molecular interaction because of its steepness and “inverted” U shape. Similarly, the effects of ethanol on GABA-A receptors seems much more complex than those of benzodiazepines, barbiturates and neurosteroids. The major problem encountered in advancing understanding of the mechanism of ethanol action in native neuronal receptors has been the large variability detected in ethanol sensitivity. For example, several studies have shown that only some groups of neurons are sensitive to pharmacologically relevant concentrations of ethanol (1-100 mM). This receptor sensitivity variability has not been resolved using recombinant expression systems. For example, studies performed in recombinant receptors, although important for elucidating molecular requirements, have shown that they are less sensitive to ethanol suggesting that neuronal substrates are important for ethanol actions.In this review, we discuss the possibility that ethanol's action on the GABA-A receptor may not be due solely to a direct interaction with the receptor protein, but that its effects could also be modulated by intracellular regulation, and that this latter effect is the more physiologically relevant one. Data in cortical and hippocampal neurons suggest that ethanol action on the receptor is labile, and that it also depends on repetitive stimulation and neuron integrity. In addition, the action of ethanol can be modified by activation of protein kinases and neuronal development. Finally, we discuss that the best approach for studying the interaction between the receptor and ethanol is through the combined use of recombinant receptors and overexpression in neurons.

Over the last two decades there has been a resurgence of interest in steroids as potential therapeutics for central nervous system disorders. This interest followed the discovery that neurosteroids and neuroactive steroids are potent modulators of GABA-A receptor function. This article traces those developments focussing particularly on the structure-activity relationships that have been identified through synthetic modification of established ligands, but also examines the influence of GABA-A receptor subunit composition for steroid modulation. The review then covers some of the physiological effects such steroids are liable to exert and their therapeutic potential for treating central nervous system disorders including epilepsy, anxiety and insomnia.

GABA-C receptors belong to the nicotinicoid superfamily of ionotropic receptors that include nicotinic acetylcholine receptors, bicuculline-sensitive GABA-A receptors, strychnine-sensitive glycine receptors and 5HT3 serotonin receptors. The GABA-C receptor concept arose from medicinal chemical studies of a conformationally restricted analog of GABA. Receptors matching the predicted properties of GABA-C receptors were cloned from the retina. Post cloning studies revealed the unique physiology and pharmacology of these relatively simple homomeric receptors. Three subtypes of GABA-C receptors have been cloned from mammalian sources and pharmacological differences between the ρ1, ρ2 and ρ3 GABA-C receptors have been described. There is evidence for functional GABA-C receptors in the retina, spinal cord, superior colliculus, pituitary and the gut and their involvement in vision, aspects of memory and sleep-waking behaviour. This review concentrates on the medicinal chemistry and molecular pharmacology of GABA-C receptor subtypes emphasising possible new investigational tools with which to investigate further GABA-C receptor function. The most useful currently available ligands that show some GABA-C receptor subtype selectivity are TPMPA, P4PMA, imidazole-4-acetic acid, 2-methyl-TACA and (±)-TAMP.