SB-205384: a GABA(A) receptor modulator with novel mechanism of action that shows subunit selectivity

GABA Regulates Electrical Activity and Tumor Initiation in Melanoma

Oncogenes can initiate tumors only in certain cellular contexts, which is referred to as oncogenic competence. In melanoma, whether cells in the microenvironment can endow such competence remains unclear. Using a combination of zebrafish transgenesis coupled with human tissues, we demonstrate that GABAergic signaling between keratinocytes and melanocytes promotes melanoma initiation by BRAFV600E. GABA is synthesized in melanoma cells, which then acts on GABA-A receptors in keratinocytes. Electron microscopy demonstrates specialized cell-cell junctions between keratinocytes and melanoma cells, and multielectrode array analysis shows that GABA acts to inhibit electrical activity in melanoma/keratinocyte cocultures. Genetic and pharmacologic perturbation of GABA synthesis abrogates melanoma initiation in vivo. These data suggest that GABAergic signaling across the skin microenvironment regulates the ability of oncogenes to initiate melanoma.

SIGNIFICANCE

This study shows evidence of GABA-mediated regulation of electrical activity between melanoma cells and keratinocytes, providing a new mechanism by which the microenvironment promotes tumor initiation. This provides insights into the role of the skin microenvironment in early melanomas while identifying GABA as a potential therapeutic target in melanoma. See related commentary by Ceol, p. 2128. This article is featured in Selected Articles from This Issue, p. 2109.

SB-205384 is a positive allosteric modulator of recombinant GABAA receptors containing rat α3, α5, or α6 subunit subtypes coexpressed with β3 and γ2 subunits

Many drugs used to treat anxiety are positive modulators of GABAA receptors, which mediate fast inhibitory neurotransmission. The GABAA receptors can be assembled from a combination of at least 16 different subunits. The receptor's subunit composition determines its pharmacologic and functional properties, and subunit expression varies throughout the brain. A primary goal for new treatments targeting GABAA receptors is the production of subunit-selective modulators acting upon a discrete population of receptors. The anxiolytic 4-amino-7-hydroxy-2-methyl-5,6,7,8,-tetrahydrobenzo[b]thieno[2,3-b]pyridine-3-carboxylic acid, but-2-ynyl ester (SB-205384) is widely considered to be selective for α3-containing GABAA receptors. However, it has been tested only on α1-, α2-, and α3-containing receptors. We examined the activity of SB-205384 at recombinant receptors containing the six different α subunits and found that receptors containing the α3, α5, and α6 subunits were potentiated by SB-205384, with the α6 subunit conferring the greatest responsiveness. Properties associated with chimeric α1/α6 subunits suggested that multiple structural domains influence sensitivity to SB-205384. Point mutations of residues within the extracellular N-terminal domain identified a leucine residue located in loop E of the agonist binding site as an important determinant of high sensitivity to modulation. In the α6 subunit the identity of this residue is species-dependent, with the leucine found in rat subunits but not in human. Our results indicate that SB-205384 is not an α3-selective modulator, and instead acts at several GABAA receptor isoforms. These findings have implications for the side-effect profile of this anxiolytic as well as for its use in neuronal and animal studies as a marker for contribution from α3-containing receptors.

Loss of leptin receptors on hypothalamic POMC neurons alters synaptic inhibition

Adaptive changes in hypothalamic neural circuitry occur in response to alterations in nutritional status. This plasticity at hypothalamic synapses contributes to the control of food intake and body weight. Here we show that genetic ablation of leptin receptor gene expression in pro-opiomelanocortin (POMC) neurons (POMC: Lepr(-/-) GFP) induces alterations at synapses on POMC neurons in the arcuate nucleus of the hypothalamus. Our studies reveal that POMC: Lepr(-/-) GFP mice have decreased frequency of spontaneous GABAergic, but not glutamatergic, postsynaptic currents at synapses on POMC neurons. The decay time course of GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs) onto POMC neurons in POMC: Lepr(-/-) GFP mice is significantly slower than that of sIPSCs in control animals. While analysis of individual miniature IPSCs shows lowered baseline activity, this tonic decrease is associated with an increased amplitude and slow decay of mini-IPSCs onto POMC neurons in POMC: Lepr(-/-) GFP mice. Moreover, POMC neurons receive greater total ionic flux per GABAergic event in the absence of leptin receptor signaling. In addition, treatment with the alpha 3 subunit-containing GABA(A) receptor modulator SB-205384 enhances GABAergic transmission only onto POMC neurons in POMC: Lepr(-/-) GFP mice. Single-cell RT-PCR analysis further supports the expression of the alpha 3 subunit of the GABA(A) receptor on POMC neurons in POMC: Lepr(-/-) GFP mice. Finally, the responses to the GABA(A) receptor agonist isoguvacine of POMC neurons are significantly smaller in POMC: Lepr(-/-) GFP than in control animals. Therefore our present work demonstrates that loss of leptin signaling in POMC neurons induces synaptic alterations at POMC synapses that may play an essential role in energy homeostasis.

Quantitative trait locus analysis identifies Gabra3 as a regulator of behavioral despair in mice

The Tail Suspension Test (TST), which measures behavioral despair, is widely used as an animal model of human depressive disorders and antidepressant efficacy. In order to identify novel genes involved in the regulation of TST performance, we crossed an inbred strain exhibiting low immobility in the TST (RIIIS/J) with two high-immobility strains (C57BL/6J and NZB/BlNJ) to create two distinct F2 hybrid populations. All F2 offspring (n = 655) were genotyped at high density with a panel of SNP markers. Whole-genome interval mapping of the F2 populations identified statistically significant quantitative trait loci (QTLs) on mouse chromosomes (MMU) 4, 6, and X. Microarray analysis of hippocampal gene expression in the three parental strains was used to identify potential candidate genes within the MMUX QTLs identified in the NZB/BlNJ × RIIIS/J cross. Expression of Gabra3, which encodes the GABA_A receptor α3 subunit, was robust in the hippocampus of B6 and RIIIS mice but absent from NZB hippocampal tissue. To verify the role of Gabra3 in regulating TST behavior in vivo, mice were treated with SB-205384, a positive modulator of the α3 subunit. SB-205384 significantly reduced TST immobility in B6 mice without affecting general activity, but it had no effect on behavior in NZB mice. This work suggests that GABRA3 regulates a behavioral endophenotype of depression and establishes this gene as a viable new target for the study and treatment of human depression.

Inhibitory transmission in locus coeruleus neurons expressing GABAA receptor epsilon subunit has a number of unique properties

Fast inhibitory synaptic transmission in the brain relies on ionotropic GABA(A) receptors (GABA(A)R). Eighteen genes code for GABA(A)R subunits, but little is known about the epsilon subunit. Our aim was to identify the synaptic transmission properties displayed by native receptors incorporating epsilon. Immunogold localization detected epsilon at synaptic sites on locus coeruleus (LC) neurons. In situ hybridization revealed prominent signals from epsilon, and mRNAs, some low beta1 and beta3 signals, and no gamma signal. Using in vivo extracellular and in vitro patch-clamp recordings in LC, we established that neuron firing rates, GABA-activated currents, and mIPSC charge were insensitive to the benzodiazepine flunitrazepam (FLU), in agreement with the characteristics of recombinant receptors including an epsilon subunit. Surprisingly, LC provided binding sites for benzodiazepines, and GABA-induced currents were potentiated by diazepam (DZP) in the micromolar range. A number of GABA(A)R ligands significantly potentiated GABA-induced currents, and zinc ions were only active at concentrations above 1 muM, further indicating that receptors were not composed of only alpha and beta subunits, but included an epsilon subunit. In contrast to recombinant receptors including an epsilon subunit, GABA(A)R in LC showed no agonist-independent opening. Finally, we determined that mIPSCs, as well as ensemble currents induced by ultra-fast GABA application, exhibited surprisingly slow rise times. Our work thus defines the signature of native GABA(A)R with a subunit composition including epsilon: differential sensitivity to FLU and DZP and slow rise time of currents. We further propose that alpha(3,) beta(1/3,) and epsilon subunits compose GABA(A)R in LC.

RNA editing of the GABA(A) receptor alpha3 subunit alters the functional properties of recombinant receptors

RNA editing provides a post-transcriptional mechanism to increase structural heterogeneity of gene products. Recently, the alpha3 subunit of the GABAA receptors has been shown to undergo RNA editing. As a result, a highly conserved isoleucine residue in the third transmembrane domain is replaced with a methionine. To determine the effect of this structural change on receptor function, we compared the GABA sensitivity, pharmacological properties and macroscopic kinetics of recombinant receptors containing either the edited or unedited forms of the alpha3 subunit along with beta3 and gamma2L. Editing substantially altered the GABA sensitivity and deactivation rate of the receptors, with the unedited form showing a lower GABA EC50 and slower decay. Comparable effects were observed with a mutation at the homologous location in the alpha1 subunit, suggesting a common role for this site in regulation of channel gating. Except for the response to GABA, the pharmacological properties of the receptor were unaffected by editing, with similar enhancement by a variety of modulators. Since RNA editing of the alpha3 subunit increases through development, our findings suggest that GABAergic neurotransmission may be more effective early in development, with greater GABA sensitivity and slower decay rates conferred by the unedited alpha3 subunit.

GABAergic synaptic communication in the GABAergic and non-GABAergic cells in the deep cerebellar nuclei

The deep cerebellar nuclei (DCN) are the final integrative units of the cerebellar network. The strongest single afferent to the DCN is formed by GABAergic Purkinje neuron axons whose synapses constitute the majority of all synapses in the DCN, with their action strongly regulating the intrinsic activity of their target neurons. Although this is well established, it remains unclear whether all DCN cell groups receive a functionally similar inhibitory input. We previously characterized three types of mouse DCN neurons based on the expression of glutamic acid decarboxylase isoform 67 (GAD67), their active membrane properties and morphological features. Here we describe the GABAergic synapses in these cell groups and show that spontaneous GABAergic synaptic activity can be seen in all three cell types. Since the majority of DCN neurons fire action potentials spontaneously at high frequencies both in vivo and in vitro, we expected that spontaneous GABAergic synaptic activities mediated by intra-DCN synaptic connections could be uncovered by their sensitivity to TTX. However, TTX had little effect on spontaneous synaptic activity. It seems, therefore that functional GABAergic connectivity within the DCN is sparse and/or weak at least under our experimental conditions. Even though present in all cell types, the spontaneous GABAergic events showed significant differences between the cell types. The synaptic currents in GABAergic cells had lower amplitude, lower frequency and slower kinetics than those of non-GABAergic cells. These differences could not be sufficiently explained by considering only cell size differences or a differential GABA(A)-receptor alpha-subunit composition. Rather, the main differentiating factor appears to be the dendritic localization of GABAergic synapses in the GABAergic cells.

The anti-convulsant stiripentol acts directly on the GABA(A) receptor as a positive allosteric modulator

Stiripentol (STP) has been used as co-therapy for treatment of epilepsy for many years. Its mechanism of action has long been considered to be indirect, as it inhibits the enzymes responsible for metabolism of other anti-convulsant agents. However, a recent report suggested that STP might also act at the neuronal level, increasing inhibitory GABAergic neurotransmission. We examined the effect of STP on the functional properties of recombinant GABA(A) receptors (GABARs) and found that it was a positive allosteric modulator of these ion channels. Its activity showed some dependence on subunit composition, with greater potentiation of alpha3-containing receptors and reduced potentiation when the beta1 or epsilon subunits were present. STP caused a leftward shift in the GABA concentration-response relationship, but did not increase the peak response of the receptors to a maximal GABA concentration. Although STP shares some functional characteristics with the neurosteroids, its activity was not inhibited by a neurosteroid site antagonist and was unaffected by a mutation in the alpha3 subunit that reduced positive modulation by neurosteroids. The differential effect of STP on beta1- and beta2/beta3-containing receptors was not altered by mutations within the second transmembrane domain that affect modulation by loreclezole. These findings suggest that STP acts as a direct allosteric modulator of the GABAR at a site distinct from many commonly used anti-convulsant, sedative and anxiolytic drugs. Its higher activity at alpha3-containing receptors as well as its activity at delta-containing receptors may provide a unique opportunity to target selected populations of GABARs.

Diversity of GABA(A) receptor synaptic currents on individual pyramidal cortical neurons

Miniature GABA(A) receptor-mediated inhibitory postsynaptic currents (mIPSCs) in cortical pyramidal neurons have previously been categorized into two types: small amplitude mIPSCs with a mono-exponential deactivation (mono-mIPSCs) and relatively larger mIPSCs with bi-exponential deactivation (bi-mIPSCs). The aim of this study was to determine if the GABA(A) channels that underlie these mIPSCSs are molecularly distinct. We found, using non-stationary noise analysis, that the difference in their amplitude could be not accounted for by their single channel conductance (both were 40 pS). Next, using alpha subunit selective GABA(A) receptor modulators, we examined the identity of the alpha subunits that may be expressed in the synapses that give rise to these mIPSCs. Zolpidem (100 and 500 nM, alpha1 selective) affected the deactivation of a subset of the mono-mIPSCs, indicating that alpha1 subunits are not highly expressed in these synapses. However, zolpidem (100 nM) prolonged the deactivation of all bi-mIPSCs, indicating a high abundance of alpha1 subunits in these synapses. SB-205384 (alpha3 selective) had no effect on the mono-mIPSCs but the bi-mIPSCs were prolonged. Furosemide (alpha4 selective) reduced the amplitude of only the mono-mIPSCs. L655,708 (alpha5 selective) reduced the amplitude of both populations and shortened the duration of the mono-mIPSCs. Finally, we found that the neuroactive steroid pregesterone sulphate reduced the amplitude of both mIPSC types. These results provide pharmacological evidence that synapses on cortical pyramidal neurons are molecularly distinct. The purpose of these different types of synapses may be to provide different inhibitory timing patterns on these cells.

Do preclinical seizure models preselect certain adverse effects of antiepileptic drugs

Classical screening tests (maximal electroshock, MES, and threshold pentylenetetrazol, PTZ) employ non-epileptic rodents and identify antiepileptic drugs (AEDs) with mechanisms of action associated with significant CNS side effects. Thus MES identifies drugs acting on Na+ channels that produce cerebellar toxicity. It may be possible to produce novel AEDs more selectively targeted at voltage-sensitive (VS) ion channels. There is little specific evidence for the likely success of this strategy with subunit selective agents targeted at the different VS Na+ channels. Drugs targeted at specific VS Ca++ channels (T, N, P/Q types) may be useful in generalised seizures. There are many as yet unexplored possibilities relating to K+ channels. GABA related drugs acting on PTZ clonic seizures tend to induce sedation and muscle hypotonia. Studies in mice, particularly with knock-in mutations, but also with subunit selective agents acting via the GABA(A) benzodiazepine site, suggest that it is possible to produce agents which do or do not induce particular side effects (sedative, hypnotic, anxiolytic, muscle relaxant, amnesia, anaesthesia). Whether these findings transfer to man has yet to be established. Acquired epilepsy in rodents (e.g. kindling or spontaneous seizures following chemically- or electrically-induced status epilepticus) or acquired epilepsy in man (following prolonged febrile seizures or traumatic brain injury) is associated with multiple changes in the function and subunit composition of ion channels and receptor molecules. Optimal screening of novel AEDs, both for efficacy and side effects, requires models with receptor and ion channel changes similar to those in the target human syndrome.

Morphological and electrophysiological evidence for an ionotropic GABA receptor of novel pharmacology

Evidence from toxicological studies suggested that an ionotropic GABA receptor of novel pharmacology (picrotoxin-insensitive, bicuculline-sensitive) exists in the chick embryo retina. In this report, we provide direct morphological and electrophysiological evidence for the existence of such an iGABA receptor. Chick embryo retinas (14-16 days old) incubated in the presence of kainic acid showed pronounced histopathology in all retinal layers. Maximal protection from this toxicity required a combination of bicuculline and picrotoxin. Individual application of the antagonists indicated that a picrotoxin-insensitive, bicuculline-sensitive GABA receptor is likely to be present on ganglion and amacrine, but not bipolar, cells. GABA currents in embryonic and mature chicken retinal neurons were measured by whole cell patch clamp. GABA was puffed at the dendritic processes in the IPL. Picrotoxin (500 microM, in the bath) eliminated all (>95%) the GABA current in the majority of ganglion and amacrine cells tested, but many cells possessed a substantial picrotoxin-insensitive component. This current was eliminated by bicuculline (200 microM). This current was not a transporter-associated current, since it was not altered by GABA transport blockers or sodium removal. The current-voltage relation was linear and reversed near E(Cl), as expected for a ligand-gated chloride current. Both pentobarbital and lorazepam enhanced the picrotoxin-insensitive current. We conclude that chicken retinal ganglion and amacrine cells express a GABA receptor that is GABA-A-like, in that it can be blocked by bicuculline, and positively modulated by barbiturates and benzodiazepines, but is insensitive to the noncompetitive blocker picrotoxin. Understanding the molecular properties of this receptor will be important for understanding both physiological GABA neurotransmission and the pathology of GABA receptor overactivation.

Cloning and functional expression of a human orthologue of rat vanilloid receptor-1

Capsaicin, resiniferatoxin, protons or heat have been shown to activate an ion channel, termed the rat vanilloid receptor-1 (rVR1), originally isolated by expression cloning for a capsaicin sensitive phenotype. Here we describe the cloning of a human vanilloid receptor-1 (hVR1) cDNA containing a 2517 bp open reading frame that encodes a protein with 92% homology to the rat vanilloid receptor-1. Oocytes or mammalian cells expressing this cDNA respond to capsaicin, pH and temperature by generating inward membrane currents. Mammalian cells transfected with human VR1 respond to capsaicin with an increase in intracellular calcium. The human VR1 has a chromosomal location of 17p13 and is expressed in human dorsal root ganglia and also at low levels throughout a wide range of CNS and peripheral tissues. Together the sequence homology, similar expression profile and functional properties confirm that the cloned cDNA represents the human orthologue of rat VR1.

Developmental change in GABAA receptor desensitization kinetics and its role in synapse function in rat cortical neurons

We examined the maturation of GABAA receptor synapses in cortical pyramidal neurons cultured from embryonic rats. The decay kinetics of GABAA receptor-mediated miniature postsynaptic currents (mPSCs) were compared with those of responses evoked by GABA in excised membrane patches. Fast perfusion of 1 or 10 mM GABA on membrane patches evoked currents with different desensitizing time courses in young and old neurons. For neurons older than 4 days in vitro (DIV), GABAA currents had a fast component of desensitization (median approximately 3 ms) seldom seen in patches from younger neurons. In contrast, mPSCs exhibited a substantial fast component of decay at 2-4 DIV that became more prominent with further development although the median value of its time constant remained unchanged. The selective alpha3 subunit positive modulator SB-205384 had no effect on mPSCs at any time in vitro but potentiated extrasynaptic activity. This suggests that synapse maturation does not proceed by a gradual exchange of early embryonic GABAA receptor subforms for adult forms. At all ages, the kinetic properties of mPSCs were heterogeneous. This heterogeneity extended to the level of mPSCs from single neurons and may be a normal aspect of synaptic functioning. These results suggest that inhibitory synapses in developing neurons are capable of selectively capturing GABAA receptors having fast desensitization kinetics. This functional preference probably reflects the developmental turning point from an inwardly looking trophic capacity of embryonic GABAA receptors to a role concerned with information processing.