Current Topics in Medicinal Chemistry - Volume 6, Issue 10, 2006

The inhibitory γ-aminobutyrate (GABA) and the excitatory glutamate (Glu) are physiologically and therapeutically important major signaling amino acids within the brain. A strong interest in the drug discovery community is directed to interplaying GABAergic and Gluergic processes, as reflected in the large number of publications devoted to this subject from the 1990's. In this issue of Current Topics in Medicinal Chemistry, a selection of reviews on the recent developments in this area is presented by researchers who have been active in the field over many years. The topic of this issue is "GABA and glutamate as targets in medicinal chemistry", with emphasis on glial-neuronal metabolic coupling, synaptic vs. nonsynaptic and direct vs. indirect co-signaling performed by GABA and/or Glu and the elucidation of its physiological and pathological significance. Several lines of functional and analytical evidence are reviewed which cover co-existence of GABA and Glu within the synapse and nonsynaptically. In this way, medicinal chemists have an updated survey of the fundamentals of the complex problem of therapeutic intervention associated with signaling by GABA and/or Glu. The first contribution in this issue describes the function of astroglia in the modulation of availability, release and clearance of Glu and GABA within the central nervous system (CNS). Arne Schousboe and Helle Waagepetersen give an overview of the metabolic coupling between neurons and astroglia. The GABA-Glu-Gln cycle ramifies astroglia and neurons at the expense of the tricarboxilic acid cycle involving the activation of astroglial Glu transporters. Specific strategy aimed at inhibiting terminal GABA transporter subtype GAT1 has been successfully applied in the development of the antiepileptic tiagabine. The second review by Gabriella Nyitrai et al discusses and evaluates methodological details and problems associated with brain tissue microdyalisis. These in vivo measurements are very important for monitoring changes in the extracellular level of GABA and Glu under control and pathological conditions such as ischemia and epilepsy. Reflecting extracellular concentrations, the alterations of the amount of GABA and Glu in dyalisate samples independent of neuronal activity precipitate a role for glia in nonsynaptic communication. The third review by Sylvester Vizi and Árpád Mike investigate whether the archetypical synaptic neurotransmitters GABA and Glu can operate via nonsynaptic transmission. The authors collect evidence for different forms of nonsynaptic transmission performed by the extrasynaptic GABA and Glu receptors (GluRs). In the light of recent progress in the field theoretical predictions of the concept of nonsynaptic transmission have also been investigated. Extrasynaptic receptors and the ambient neurotransmitters or drugs are expected to interact with higher affinity. This feature may enable extrasynaptic receptors to serve useful pharmacological targets. Highlighted by Katalin Schlett, proliferative effects should be distinguished from those which affect cell fate commitment and/or survival. Glu actions on neurogenesis show distinct differences in the developing and adult brain. Glu acts nonsynaptically on dividing progenitors and can influence proliferation and neuronal commitment. Pathological conditions including ischemia, epilepsy and stress induce cell loss in the brain providing a clue for therapeutic interventions aimed at enhancing neuronal replacement. Rewived by Robert Balazs, GluR activation may directly initiates a cascade of events primarily involving Ca2+ ion-mediated signaling and consecutive gene expression changes. Indirect effects of GluR stimulation are due to the production and release of neurotrophic factors, such as brain-derived neurotrophic factor and also involve glia-neuronal interaction. Neuronal loss can occur during development as well as in the adult brain. Contrasting physiological and excessive stimulations, usually associated with either trophic effects promoting neuronal plasticity or neurotoxicity, respectively, may compromise the therapeutic manipulation of GluRs. Salient features of the co-transmission by GABA and Glu in neural signaling are reviewed by József Somogyi. Several brain areas, including granule cells of the Dentate Gyrus, hippocampal mossy fibre terminals and their termination zone in the CA3 subfield, retina, brain stem and spinal cord are highlighted. The vesicular Glu (VGLUT3) and GABA (VIAAT) transporters are functionally compatible making possible package and release of Glu and GABA from the same terminals. His new hypothesis on combinatorial neural code gives a possible reason for the functional significance of co-transmission by GABA and Glu in nerve terminals...........

Function of astroglia in the modulation of availability, release and clearance (inactivation) of Glu and GABA within the central nervous system is reviewed. Net synthesis of Glu through Gln synthethase exclusively localized in astrocytes can only occur by a metabolic coupling between neurons and astrocytes. Two (GLAST and GLT-1) of the five Glu transporters cloned preferentially expressed in astrocytes perform the astroglial Glu uptake of very high capacity. Moreover, astrocytes have been shown to mediate Glu release by a mechanism mimicking vesicular release. Biosynthesis of GABA in neurons is brought about by decarboxylation of Glu catalyzed by a pyridoxal phosphate requiring enzyme (GAD) that exists in two isoforms (GAD65 and GAD67) exhibiting different subcellular localization and regulatory properties. Detailed studies of GABA synthesis in GABAergic neurons using 13C NMR spectroscopy have provided evidence for direct involvement of the tricarboxylic acid cycle. Synaptically released GABA taken up into surrounding astrocytes is converted to either CO2 or Gln. Two reports on the release of GABA in rat dorsal root ganglia indicated that glial cells may perform GABA release as well. Gln formed from GABA in astrocytes can be transferred to GABAergic neurons and subsequently converted to GABA. Inhibition of either degradation or clearance of GABA has been successfully applied in the development of antiepileptics such as vigabatrin or tiagabine. So far, no specific strategy has been developed aimed at stimulating Glu transport.

In spite of several studies showing specific physiological functions of changes in the extracellular level of the major excitatory and inhibitory transmitters, Glu and GABA within the brain ([Glu]EXT, [GABA]EXT) the exact origin (neuronal vs. astroglial, synaptic vs. extrasynaptic) of Glu and GABA present in dialysate samples is still a matter of debate. For better understanding the significance of in vivo microdialysis data, here we discuss methodological details and problems in addition to regulation of [Glu]EXT and [GABA]EXT. Changes in [Glu]EXT and [GABA]EXT under pathological conditions such as ischemia and epilepsy are also reviewed. Based on recent in vivo microdialysis data we argue that ambient [Glu] EXT and [GABA]EXT may have a functional role. It is suggested that specific changes in concentrations of Glu and GABA in dialysate samples together with their alterations independent of neuronal activity indicate the involvement of Glu and GABA in the information processing of the brain as essential signaling molecules of nonsynaptic transmission as well. Since various drugs are able to interfere with extrasynaptic signals in vivo, studying the extracellular cell-to-cell communication of brain cells represents a new aspect to improve drugs modulating Gluergic as well as GABAergic neurotransmission.

The concept of nonsynaptic communication between neurons, once a heretic idea, has become a self-evident fact during the almost forty years since its original discovery [1]. In this review we investigate whether the archetypical synaptic transmitters of the central nervous system, Glu and GABA, can operate via nonsynaptic transmission. While experimental data supporting the general concept of nonsynaptic transmission has been progressively accumulating during these years, most of the evidence regarding nonsynaptic transmission by Glu and GABA are results of the last decade. In this paper we collect evidence for different forms of nonsynaptic transmission by the Gluergic and GABAergic system. We investigate two theoretical predictions of the concept of nonsynaptic transmission in the light of recent progress in the field: i) since extrasynaptic receptors experience a lower concentration of agonist, they are likely to have higher affinity for the agonist ii) extrasynaptic receptors are expected to be more important pharmacological targets.

It has been widely accepted that neurogenesis continues throughout life. Neural stem cells can be found in the ventricular zone of the embryonic and in restricted regions of the adult central nervous system, including subventricular and subgranular zones of the hippocampal dentate gyrus. The network of signaling mechanisms determining whether neural stem cells remain in a proliferative state or differentiate is only partly discovered. Recent advances indicate that glutamate (Glu), the predominant excitatory neurotransmitter in mature neurons, can influence immature neural cell proliferation and differentiation, as well. Despite many similarities, Glu actions on neurogenesis in the developing and adult brain show distinct differences and are far from being clear. Due to alterations of Glu transport mechanisms, extracellular Glu level is high in the embryonic CNS. Glu acts non-synaptically on dividing progenitors either by directly activating ionotropic and/or metabotropic Glu receptors or can influence other cells which are located in the vicinity of proliferating cells and produce molecules regulating neural precursor cell proliferation by other mechanisms. Due to the complexity of signaling pathways and to regional differences in neural precursors, Glu can influence proliferation and neuronal commitment as well, and acts as a positive regulator of neurogenesis. Brain injuries like ischemia, epilepsy or stress lead to severe neuronal death and additionally, influence neurogenesis, as well. Glu homeostasis is altered under these pathological circumstances, implying that therapeutic treatments mediating Glu signaling might be useful to increase neuronal replacement after cell loss in the brain.

During development, Glu receptors and N-methyl-D-aspartate receptors in particular initiate a cascade of signal transduction events and gene expression changes primarily involving Ca2+ ion-mediated signaling induced by activation of either Ca2+ ion-permeable receptor channels or voltage-sensitive Ca2+ ion channels. The consecutive activation of major protein kinase signaling pathways, such as Ras-MAPK/ERK and PI3-K-Akt, contributes to regulation of gene expression through the activation of key transcription factors, such as CREB, SRF, MEF-2, NF-kB. Metabotropic Glu receptors can also engage these signaling pathways and this may be mediated, in part, by transactivating receptor tyrosine kinases. Indirect effects of Glu receptor stimulation are due to the production and release of neurotrophic factors, such as brain derived neurotrophic factor and also involve glia-neuronal interaction through Glu-induced release of trophic factors from glia. The trophic effect of Glu receptor activation is developmental stage-dependent and may play an important role in determining the selective survival of neurons that made proper connections. During this sensitive developmental period interference with Glu receptor function may lead to widespread neuronal loss. However, NMDA receptor blockadeinduced neurodegeneration can also occur in the adult brain. Depending on the stimulus strength, Glu receptors mediate biphasic effects. In addition to synaptic transmission, physiological stimulation of Glu receptors can mediate trophic effects and promote neuronal plasticity. Excessive stimulation is neurotoxic. Attention must, therefore, be paid to these features, when therapeutic manipulation of excitatory amino acid receptors is considered in the clinical setting.

Salient features of the co-transmission by GABA and Glu in neural signaling are summarized. Experimental data have been accumulating which demonstrate; i) GABA-immunoreactivity in and GABA-release from constitutively Gluergic hippocampal mossy fibre terminals, ii) plasticity of the GABAergic phenotype of constitutively Gluergic granule cells of the Dentate Gyrus, iii) expression of GABAA receptor γ3 subunit in the mossy fibre termination zone in the CA3 subfield, iv) co-labeling of terminals for GABA and Glu in the retina, brain stem and spinal cord, and v) functional compatibility of vesicular Glu (VGLUT3) and GABA (VIAAT) transporters. It is not clear, however, whether or not Glu and GABA are released from the same terminals, and packaged in the same vesicles. Using multiple transmitters neurons may serve to reduce the metabolic cost and errors of signaling.

The granule cells of the Dentate Gyrus are one of the most exciting and intriguing cells in the central nervous system. Besides containing and releasing Glu, they have been shown to contain and release peptides (somatostatin, neuropeptide Y, neurokinin B, cholecystokinin, dynorphin, enkephalin), Zn++ ion, and brain-derived neurotrophic factor (BDNF). The recent addition of GABA to this list suggests that these cells can also function as inhibitory cells. Indeed, evidence has been presented of co-localization of all markers of the GABAergic phenotype in granule cells: GABA, the enzyme for its synthesis (Glu decarboxylase) and the membrane and vesicular transporters of GABA. These markers of the GABAergic phenotype are up-regulated after epileptic seizures. When this occurs, monosynaptic GABA receptormediated transmission emerges in the mossy fiber synapse thus restraining excitation and mediating antiepileptic and neuroprotective actions.

Co-localization of transporters able to recapture the released or endogenously synthesized transmitter (homotransporters) and of transporters that can selectively take up transmitters/modulators originating from neighbouring structures (heterotransporters) has been demonstrated to occur within the same axon terminal of several neuronal phenotypes. Activation of terminal heterotransporters invariably leads to the release of the transmitter specific to the terminal. Heterotransporters are also increasingly reported to exist on neuronal soma/dendrites and nerve terminals, on the basis of morphological experiments. The functions of somatodendritic heterotransporters has been investigated only in a very limited number of cases. Release-regulating GABA heterotransporters of the GAT-1 type exist on Glu nerve terminals in different rodent brain regions including spinal cord. Activation of GABA heterotransporters provokes release of Glu, which takes place by reversal of the Glu homotransporter and by anion channel opening. Interestingly, the release of Glu induced by GABA in spinal cord is dramatically enhanced in a transgenic mouse model of amyotrophic lateral sclerosis and this effect seems to represent the most precocious mechanism that increases extracellular Glu concentration, reported to occur in the pathomechanism.

Neurotransmitter plasma membrane transporters do have much more to perform than simply terminating synaptic transmission and replenishing neurotransmitter pools. Findings in the past decade have evidenced their function in maintaining physiological synaptic excitability, and their actions in critical or pathological conditions, also. Conclusively these findings indicated a previously unrecognized role for neurotransmitter plasma membrane transporters in both, synaptic and nonsynaptic signaling. Major inhibitory and excitatory neurotransmitters within the brain, GABA and Glu, have long been considered to operate through independent systems (GABAergic or Gluergic), each of them characterized by its own localization, function and dedicated GABAergic or Gluergic cell phenotypes. Recent advances, however, have challenged this long-standing paradigm. Localization of GABA in Gluergic terminals and Glu in GABAergic cells were reported. Specific plasma membrane transporters for GABA and Glu are also co-localized in different brain areas. Although, their role in regulating each other's signal is still far from being understood, emerging lines of evidence on interplaying GABAergic and Gluergic processes through plasma membrane transporters opens up a new avenue in the field of more specific therapeutic intervention.

Editorial Potassium channels are membrane proteins which selectively allow potassium ions to flow across the cell membrane, following the electrochemical gradient. Since the extracellular potassium concentration is greatly lower than the intracellular one, the opening of potassium channels typically determines an outward current of these ions, causing a shift of the resting membrane potential towards the potassium equilibrium potential (hyperpolarisation) or the recovery of the resting potential in a depolarised membrane (repolarisation). Both these mechanisms can counteract the excitatory (depolarising) stimuli, generally due to the inward flows of other important cationic species (calcium and sodium). This fundamental role of potassium channels is inmost involved in almost all the main cell activities, such as the excitability of neurons and muscle cells, the shaping of action potentials, the coupling of many chemical and or mechanical stimuli with given intracellular events, the function of secretive cells, etc., and thus drugs activating potassium channel seem to represent interesting tools for the potential treatment of several pathological conditions. Because of their relaxing effects on smooth muscle cells, they have been investigated as vasodilators and anti-asthmatic agents. Their neuroprotective activity furnished a strong rational basis for the use in neurodegenerative disorders and/or in stroke. Other experimental studies indicated potassium channel activators as useful agents for treatment of epilepsy and pain. More recent evidence, indicating a relevant role of potassium channels expressed on the myocardial mitochondria in the "ischaemic pre-conditioning", suggested an intriguing scenario for potassium channel activators as innovative cardio-protective anti-ischaemic drugs. Although all these therapeutic perspectives have been well supported by a plethora of convincing experimental studies, the availability of potassium channel activators in the clinical practice is still quite limited. There are, at least, two main reasons for this apparent antinomia between the hypothesised potentialities of these drugs and their real application. Potassium channels are sub-divided into a very large number of types and subtypes, but only few of them have been selected for the development of selective drugs. Many drugs presently available show an appreciable selectivity for given potassium channel types, but, generally, these targets are largely expressed in many districts and, hence, this determines the presence of several side-effects accompanying a wished pharmacological activity. Therefore, the individuation of a given potassium channel subtype closely associated with a particular role (and, ideally, in a particular district), as well as the development of appropriate pharmacophore models able to confer to a potassium channel activator a satisfactory selectivity for a given sub-type (or, at least, to confer an adequate tissue-selectivity due to pharmacokinetic properties), seems to represent the most challenging issue for the pharmacologists and medicinal chemists working in the field of potassium channel drugs. In this special issue, William Dalby-Brown and colleagues present an intriguing and detailed review focused on the voltage-operated potassium channel Kv7 type as a recent target of the pharmacological/pharmaceutical investigation. Indeed, some subtypes of this channel play clear and distinct roles in human diseases, offering some promising perspectives for the development of selective modulators (openers as well as blockers). The large-conductance calcium-activated potassium (BK) channel, has been the topic of intense research, in the recent years. A relatively large amount of heterogeneous compounds acts as BK-openers and many of them have been obtained from the structural development of pioneer benzimidazolone derivatives. In his interesting paper, Sheng-Nan Wu and co-workers give an overview on the pharmacological roles of BK channels and on the BK-activating properties of several compounds, which are not structurally related with the benzimidazolone BK-activators and, therefore, can represent a useful and original template for the development of new chemical classes of BK-openers. The ATP-sensitive (KATP) one is surely the most studied, among the different types of potassium channels and about two decades of investigations furnished a wide collection of KATP-activators. Indeed, these channel modulators belong to extremely diversified chemical families and the large number of compounds of each family allowed to trace clear structure-activity relationships..........

Kv7 channels are unique among K+ channels, since four out of the five channel subtypes have well-documented roles in the development of human diseases. They have distinct physiological functions in the heart and in the nervous system, which can be ascribed to their voltage-gating properties. The Kv7 channels also lend themselves to pharmacological modulation, and synthetic openers as well as blockers of the channels, regulating neuronal excitability, have existed even before the Kv7 channels were identified by cloning. In the present review we give an account on the focused efforts to develop selective modulators, openers as well as blockers, of the Kv7 channel subtypes, which have been undertaken during recent years, along with a discussion of the Kv7 ion channel physiology and therapeutic indications for modulators of the neuronal Kv7 channels.

The gating of large-conductance Ca2+-activated K+ (BKCa) channel is primarily controlled by intracellular Ca2+ and/or membrane depolarization. These channels play a role in the coupling of excitation-contraction and stimulussecretion. A variety of structurally distinct compounds may influence the activity of these channels. Squamocin, an Annonaceous acetogenin, could interact with the BKCa channel to increase the amplitude of Ca2+-activated K+ current in coronary smooth muscle cells. Its stimulatory effect is related to intracellular Ca2+ concentrations. In inside-out patches, application of ceramide to the bath suppressed the activity of BKCa channels recorded from pituitary GH3 cells and from retinal pigment epithelial cells. ICI-182,780, an estrogen receptor antagonist, was found to modulate BKCa-channel activity in cultured endothelial cells and smooth muscle cells in a mechanism unlinked to the inhibition of estrogen receptors. Caffeic acid phenethyl ester (CAPE) and its analogy, cinnamyl-3,4-dihydroxy-α-cyanocinnamate, could directly increase the activity of BKCa channels in GH3 cells. CAPE also reduced the frequency and amplitude of intracellular Ca2+ oscillations in these cells. The CAPE-stimulated activity in BKCa channels is thought to be unassociated with its inhibition of NF-kB activation. Cilostazol, an inhibitor of cyclic nucleotide phosphodiesterase, could stimulate BKCa channel-activity and reduce the firing of action currents simultaneously in GH3 cells. Therefore, the regulation by these compounds of BKCa channels may in part be responsible for their regulatory actions on cell functions.

Given their many physiological functions, KATP channels represent promising drug targets. Sulfonylureas like glibenclamide block KATP channels; they are used in the therapy of type 2 diabetes. Openers of KATP channels (KCOs) e.g. relax smooth muscle and induce hypotension. KCOs are chemically heterogeneous and include as different classes as the benzopyrans, cyanoguanidines, thioformamides, thiadiazines and pyridyl nitrates. Examples for new chemical entities more recently developed as KCOs include cyclobutenediones, dihydropyridine related structures, and tertiary carbinols. Structure-activity relationships of the main chemical classes of KCOs are discussed.

ATP-Sensitive potassium channel openers (KATPCOs) are a group of compounds with a broad spectrum of potential therapeutic applications, as they constitute efficient tools for dampening cell excitability. Interest in the KATPCOs was triggered in the early 1980s by the discovery of the benzopyran-based structure cromakalim (CRK), which is a powerful smooth muscle relaxant. CRK can be considered the archetype of KATPCOs and is by far the most mimicked structure. In many structure-activity studies various substitutions have been made at the different positions of the benzopyran ring permitting the optimal activity to be correlated with a specific set of structural characteristics and stereochemical features of the molecule. Thus, many potent benzopyran derivatives have been identified. The benzopyran nucleus itself has also been modified in both the aromatic ring and in the pyran moiety. The intention of this review is to bring together all the different structural classes of KATPCOs arising from the replacement of CRK benzopyran-based structure with various ring systems; design, structure-activity relationship, and synthesis will be given.