Dopamine directly modulates GABAA receptors.

Dopamine is a critical neuromodulator that activates GPCRs in mammals or ligand-gated ion channels in invertebrates. The present study demonstrates that dopamine (0.1-10 mm) exerts novel, opposing effects on different populations of mammalian (rat) GABAA receptors. Using whole-cell patch-clamp electrophysiology, we observed direct dopamine-mediated inhibition of tonic-level (1 µm) GABA-evoked currents in untransfected striatal neurons that could be recapitulated in HEK293 cells containing α1ß3 or α1ß2γ2 subunits. Surprisingly, direct activation by dopamine was seen in the absence of GABA with α1ß2γ2, α5ß3γ2, or α1ß3γ2 transfections. This activity was also present in α1ß3γ2 receptors containing a mutant ß3 subunit (H267A [(Z)ß3]) insensitive to trace levels of inhibitory Zn(2+). Dopamine activation required ß and γ subunits but not α subunits ((Z)ß3γ2 EC50 value, 660 µm). Dopamine activity was fully blocked by picrotoxin but not GABAA competitive antagonists, and was strongly correlated with spontaneous receptor activity. We also report opposing effects of bicuculline and gabazine, such that bicuculline surprisingly activated non-α-containing (ß3γ2) GABAA receptors, whereas gabazine suppressed spontaneous activity in these receptors. Our results suggest that dopamine may directly inhibit GABAA receptors that are both immediately adjacent to dopamine release sites in the striatum and activated by tonic GABA. Furthermore, synaptic/phasic release of dopamine may directly enhance signaling at some spontaneously active noncanonical GABAA receptors that lack α subunits.

Anterograde trafficking signals in GABAA subunits are required for functional expression.

Pentameric GABAA receptors are composed from 19 possible subunits. The GABAA ß subunit is unique because the ß1 and ß3 subunits can assemble and traffic to the cell surface as homomers, whereas most of the other subunits, including ß2, are heteromers. The intracellular domain (ICD) of the GABAA subunits has been implicated in targeting and clustering GABAA receptors at the plasma membrane. Here, we sought to test whether and how the ICD is involved in functional expression of the ß3 subunit. Since θ is the most homologous to ß but does not form homomers, we created two reciprocal chimeric subunits, swapping the ICD between the ß3 and θ subunits, and expressed them in HEK293 cells. Surface expression was detected with immunofluorescence and functional expression was quantified using whole-cell patch-clamp recording with fast perfusion. Results indicate that, unlike ß3, neither the ß3/θIC nor the θ/ß3IC chimera can traffic to the plasma membrane when expressed alone; however, when expressed in combination with either wild-type α3 or ß3, the ß3/θIC chimera was functionally expressed. This suggests that the ICD of α3 and ß3 each contain essential anterograde trafficking signals that are required to overcome ER retention of assembled GABAA homo- or heteropentamers.

Glutamate receptor trafficking: endoplasmic reticulum quality control involves ligand binding and receptor function.

The glutamate receptor (GluR) agonist-binding site consists of amino acid residues in the extracellular S1 and S2 domains in the N-terminal and M3-M4 loop regions, respectively. In the present study, we sought to confirm that the conserved ligand-binding residues identified in the AMPA receptor S1S2 domains also participate in ligand binding of GluR6 kainate receptors. Amino acid substitutions were made in the GluR6 parent at R523, T690, and E738 to alter their potential interactions with ligand. Mutant receptors were expressed in human embryonic kidney 293 cells, confirmed by Western blot analysis, and tested by [3H]kainate binding and patch-clamp recording. Each of the binding site mutations was sufficient to reduce [3H]kainate binding to undetectable levels and eliminate functional responses to glutamate or kainate. As with our studies of other nonfunctional mutants (Fleck et al., 2003), immunocytochemical staining and cell-surface biotinylation studies showed that the mutant receptors were retained intracellularly and did not traffic to the cell surface. Endoglycosidase-H digests and colocalization with endoplasmic reticulum (ER) markers demonstrated that the mutant receptors are immaturely glycosylated and retained in the ER. Immunoprecipitation, native PAGE, and functional studies confirmed that the GluR6-binding site mutants are capable of multimeric assembly, indicating their retention in the ER does not result from a gross protein folding error. Together, these results confirm the role of R523, T690, and E738 directly in ligand binding to GluR6 and further support our previous report that nonfunctional GluRs are retained intracellularly by a functional checkpoint in ER quality control.

Glutamate receptors and endoplasmic reticulum quality control: looking beneath the surface.

Glutamate is the principal excitatory neurotransmitter in the mammalian central nervous system. The cellular regulation of glutamate receptor (GluR) ion channel function and expression is important for maintaining or adjusting target cell excitability to meet ever-changing demands, for example, in relation to developmental or use-dependent synaptic plasticity. Dysregulation of GluR function or expression may be a contributing factor in certain forms of epilepsy, stroke/ischemia, head trauma, cognitive impairments, and neurodegenerative disease. Recent years have seen substantial progress in understanding how GluRs operate in terms of their structural and functional properties, their synaptic targeting and membrane anchoring by PDZ-domain proteins, and their activity-dependent cycling at the plasma membrane. Yet precious little is known about the earliest events in GluR biogenesis or the mechanisms in place to ensure the GluRs that reach the cell surface are processed, folded, and oligomerized in an appropriate manner. Indeed, only a minor fraction of the GluR content of cells is expressed at any given time on the cell surface, whereas most of the remaining receptors exist in the endoplasmic reticulum (ER). The functional competence and significance of the ER fraction of receptors are presently unknown, but they are generally thought to represent immature, unassembled, or improperly assembled subunits. Some are ultimately destined for insertion in the plasma membrane. Others may be targeted for proteosomal degradation. Still others might provide a latent pool of fully functional receptors that can be recruited to enhance cell excitability in response to specific signals or under pathological conditions. This review will explore the structural and functional elements that regulate GluR assembly and export from the ER.

Mutagenesis and computational docking studies support the existence of a histamine binding site at the extracellular ß3+ß3- interface of homooligomeric ß3 GABAA receptors.

Histamine is an important neurotransmitter that exerts its physiological actions through H1-4 metabotropic receptors in mammals. It also directly activates ionotropic GABAA receptor (GABAAR) ß3 homooligomers and potentiates GABA responses in αß heterooligomers in vitro, but the respective histamine binding sites in GABAARs are unknown. We hypothesized that histamine binds at the extracellular ß+ß- interface at a position homologous to the GABA binding site of heterooligomeric GABAARs. To test this, we individually mutated several residues at the putative ligand binding minus side of a rat GABAAR ß3 wild type subunit and of a ß3 subunit that was made insensitive to trace Zn(2+) inhibition [ß3(H267A); called (Z)ß3]. (Z)ß3, (Z)ß3(Y62L), (Z)ß3(Q64A), (Z)ß3(Q64E), α1(Z)ß3, or α1(Z)ß3(Y62L) receptors were studied in HEK293T cells using whole cell voltage clamp recording. ß3, ß3(Y62C), ß3(Q64C), ß3(N41C), ß3(D43C), ß3(A45C) or ß3(M115C) receptors were examined in Xenopus oocytes using two-electrode voltage clamp. Histamine directly activated (Z)ß3 and ß3 homooligomers and potentiated GABA actions in α1(Z)ß3 heterooligomers. Receptors containing (Z)ß3(Y62L), ß3(Y62C) and ß3(D43C) showed markedly reduced histamine potency, but homo- and heterooligomers with (Z)ß3(Q64E) exhibited increased potency. The GABAAR αß(γ) competitive antagonist bicuculline elicited sub-maximal agonist currents through (Z)ß3 homooligomers, the potency of which was strongly decreased by (Z)ß3(Y62L). Mutations ß3(N41C), ß3(A45C) and ß3(M115C) disturbed receptor expression or assembly. Computational docking into the crystal structure of homooligomeric ß3 receptors resulted in a histamine pose highly consistent with the experimental findings, suggesting that histamine activates ß3 receptors via a site homologous to the GABA site in αßγ receptors.

Amino-acid residues involved in glutamate receptor 6 kainate receptor gating and desensitization.

The glutamate receptor (GluR) agonist-binding site consists of amino acid residues in the extracellular S1 and S2 segments in the N-terminal and M3-M4 loop regions, respectively. Molecular and atomic level structural analyses have identified specific S1 and S2 residues that interact directly with ligands, interact with one another in a dimeric configuration, and influence channel gating and desensitization properties of AMPA receptors. Other studies suggest that KA receptor gating and desensitization may differ mechanistically. In particular, a leucine (L) to tyrosine (Y) mutation in the S1 segment of AMPA receptors is sufficient to block desensitization, whereas KA receptors naturally contain a tyrosine residue at the equivalent position (Y751 in GluR6) but retain the fast-desensitizing phenotype. We hypothesized that KA receptor desensitization is preserved by a compensatory substitution in the S2 segment. We generated a series of GluR6 mutants that converted individual S2 domain residues to their AMPA receptor equivalents. Various S2 mutations had effects on the kinetics of desensitization and recovery from desensitization, but no single amino acid substitution was found to block desensitization, as in the L/Y mutant AMPA receptors, or to prevent desensitization to KA. Other mutations designed to neutralize residues thought to interact across the dimer interface had dramatic effects on channel gating and desensitization. These results are consistent with a close but imperfect structural homology between AMPA and KA receptors and support the role of conserved S1S2 domain interactions at the dimer interface in GluR channel function.

Antagonism of alpha 3 beta 4 nicotinic receptors as a strategy to reduce opioid and stimulant self-administration.

The iboga alkaloid ibogaine and the novel iboga alkaloid congener 18-methoxycoronaridine are putative anti-addictive agents. Using patch-clamp methodology, the actions of ibogaine and 18-methoxycoronaridine at various neurotransmitter receptor ion-channel subtypes were determined. Both ibogaine and 18-methoxycoronaridine were antagonists at alpha 3 beta 4 nicotinic receptors and both agents were more potent at this site than at alpha 4 beta 2 nicotinic receptors or at NMDA or 5-HT(3) receptors; 18-methoxycoronaridine was more selective in this regard than ibogaine. In studies of morphine and methamphetamine self-administration, the effects of low dose combinations of 18-methoxycoronaridine with mecamylamine or dextromethorphan and of mecamylamine with dextromethorphan were assessed. Mecamylamine and dextromethorphan have also been shown to be antagonists at alpha 3 beta 4 nicotinic receptors. All three drug combinations decreased both morphine and methamphetamine self-administration at doses that were ineffective if administered alone. The data are consistent with the hypothesis that antagonism at alpha 3 beta 4 receptors is a potential mechanism to modulate drug seeking behavior. 18-Methoxycoronaridine apparently has greater selectivity for this site than other agents and may be the first of a new class of synthetic agents acting via this novel mechanism to produce a broad spectrum of anti-addictive activity.

Novel iboga alkaloid congeners block nicotinic receptors and reduce drug self-administration.

18-Methoxycoronaridine, a novel iboga alkaloid congener, reduces drug self-administration in animal models of addiction. Previously, we proposed that these effects are mediated by the ability of 18-methoxycoronaridine to inhibit nicotinic alpha3beta4 acetylcholine receptors. In an attempt to identify more potent 18-methoxycoronaridine analogs, we have tested a series of 18-methoxycoronaridine congeners by whole-cell patch clamp recording of HEK 293 cells expressing recombinant nicotinic alpha3beta4 receptors or glutamate NR1/NR2B N-methyl-d-aspartate (NMDA) receptors. The congeners exhibited a range of inhibitory potencies at alpha3beta4 receptors. Five congeners had IC(50) values similar to 18-methoxycoronaridine, and all of these were ineffective at NMDA receptors. The congeners also retained their ability to reduce morphine and methamphetamine self-administration. These data are consistent with the importance of nicotinic alpha3beta4 receptors as a therapeutic target to modulate drug seeking. These compounds may constitute a new class of synthetic agents that act via the nicotinic alpha3beta4 mechanism to combat addiction.

Effects of cimetidine-like drugs on recombinant GABAA receptors.

Even though conventional systemic doses of cimetidine and other histamine H(2) antagonists display minimal brain penetration, central nervous system (CNS) effects (including seizures and analgesia) have been reported after administration of these drugs in animals and man. To test the hypothesis that cimetidine-like drugs produce these CNS effects via inhibition of GABA(A) receptors, the actions of these drugs were studied on seven different, precisely-defined rat recombinant GABA(A) receptors using whole-cell patch clamp recordings. The H(2) antagonists famotidine and tiotidine produced competitive and reversible inhibition of GABA-evoked currents in HEK293 cells transfected with various GABA(A) receptor subunits (IC(50) values were between 10-50 microM). In contrast, the H(2) antagonist ranitidine and the cimetidine congener improgan had very weak (if any) effects (IC(50) > 50 microM). Since the concentrations of cimetidine-like drugs required to inhibit GABA(A) receptors in vitro (greater than 50 microM) are considerably higher than those found during analgesia and/or seizures (1-2 microM), the present results suggest that cimetidine-like drugs do not appear to produce seizures or analgesia by directly inhibiting GABA(A) receptors.

Histamine-gated ion channels in mammals?

There is ample pharmacological and physiological evidence for yet unidentified histamine receptors in mammalian brain that are linked to a Cl(-) conductance. In invertebrates, two histamine-gated chloride channels (HisCl α1 and α2) are already well known. HisCl channels are members of the Cys-loop receptor superfamily of ligand-gated ion channels and are closely related to the mammalian GABA(A) and glycine receptors (GlyR). Indeed, they share particularly strong homology within the ligand binding and ion channel domains. Here we discuss the possibility that mammalian HisCl channels might exist among the known GABA(A) or GlyR subunits. Studies published to date support this hypothesis, including evidence for direct histamine gating of GABA(A) ß homomers, histamine potentiation of GABA(A) αß and αßγ heteromeric receptors, and GABA(A) receptor blockade by some antihistamines. We explore what is known about the binding-site structure, function and pharmacology of invertebrate HisCl channels and other histamine binding sites to support and inform a broader search for HisCl channels among the mammalian GABA(A) and GlyR subunits. The discovery and identification of HisCl-like channels in mammals would not only enhance understanding of inhibitory signaling and histamine function in the mammalian brain, but also provide new avenues for development of therapeutic compounds targeting this novel histamine site. This commentary is therefore intended to foster consideration of a novel and potentially important target of histamine and histaminergic drugs in the CNS.

Functional consequences of natural substitutions in the GluR6 kainate receptor subunit ligand-binding site.

Differences in binding-site residues of GluR2 (AMPAR) and GluR6 (KAR) subunits have been identified that might account for their functional and pharmacological differences. Specifically, residues A518, A689 and N721 in GluR6 replace highly conserved threonine and serine residues found in other ionotropic glutamate receptor (iGluR) subunits. To define how these natural substitutions impact GluR6 function, we used patch clamp recording with ultrafast perfusion to characterize the effects of A518T, A689S and N721T on agonist potency, efficacy and response kinetics. We find these natural substitutions impact GluR6 function less than would be expected from reverse mutations in other iGluRs. There was little effect of individual or combined mutations on glutamate potency, deactivation or desensitization kinetics. Altered recovery kinetics were seen that were greatest after combined mutations. Kainate potency and response kinetics were also unchanged in the mutants, whereas kainate efficacy was reduced in A518T and increased the T/S/T mutant relative glutamate. Notably, A518T and A689S mutation permitted AMPA to bind as a weak competitive antagonist and the effects of these mutations were additive. N721T mutation further enhanced AMPA binding, allowing AMPA to activate and fully desensitize the receptors. Alternative mutations altering side chain length at position 518 produced far greater changes in glutamate affinity and response kinetics than did the natural mutations. We conclude that these nonconserved residues in GluR6 define the size of the agonist-binding pocket, exerting a steric influence on the bound agonist and the extent of binding-domain closure that can influence agonist potency, deactivation, desensitization and recovery kinetics.

Targeting AMPA receptor gating processes with allosteric modulators and mutations.

Allosteric modulators and mutations that slow AMPAR desensitization have additional effects on deactivation and agonist potency. We investigated whether these are independent actions or the natural consequence of slowing desensitization. Effects of cyclothiazide (CTZ), trichlormethiazide (TCM), and CX614 were compared at wild-type GluR1 and "nondesensitizing" GluR1-L497Y mutant receptors by patch-clamp recording with ultrafast perfusion. CTZ, TCM, or L/Y mutation all essentially blocked GluR1 desensitization; however, the effects of L/Y mutation on deactivation and glutamate EC50 were three to five times greater than for modulators. CTZ and TCM further slowed desensitization of L/Y mutant receptors but paradoxically accelerated deactivation and increased agonist EC50. Results indicate that CTZ and TCM target deactivation and agonist potency independently of desensitization, most likely by modifying agonist dissociation (koff). Conversely, CX614 slowed desensitization and deactivation without affecting EC50 in both wild-type and L/Y receptors. The S750Q or combined L497Y-S750Q mutations abolished all CTZ and TCM actions without disrupting CX614 activity. Notably, the S/Q mutation also restored L/Y deactivation and EC50 to wild-type levels without restoring desensitization, further demonstrating that desensitization can be modulated independently of deactivation and EC50 by mutagenesis and possibly by allosteric modulators.

Molecular actions of (S)-desmethylzopiclone (SEP-174559), an anxiolytic metabolite of zopiclone.

This study set out to profile the activity of (S)-desmethylzopiclone (SEP-174559) at subtypes of the gamma-aminobutyric acid type-A (GABA(A)) receptor and other neurotransmitter receptor ion channels. Recombinant receptors were expressed in human embryonic kidney 293 cells and examined functionally by patch-clamp recording with fast perfusion of agonist and drug solutions. Micromolar concentrations of SEP-174559 potentiated GABA(A) receptor currents evoked by subsaturating concentrations of GABA. The potentiation was related to a leftward shift in the GABA dose-response curves, suggesting the drug acts to increase GABA binding affinity. The potentiation strictly required the presence of the gamma2 subunit; no enhancement was seen for receptors containing instead the gamma1 subunit or lacking a gamma subunit altogether. SEP-174559 and its parent compound, racemic zopiclone, were not selective between alpha1-, alpha2-, or alpha3-bearing GABA(A) receptors. Within the nicotinic receptor superfamily, SEP-174559 did not affect serotonin type-3 receptor function but was found to inhibit nicotinic acetylcholine (nACh) receptors. The inhibition of nACh receptors was noncompetitive and was mimicked by zopiclone, alprazolam, and diazepam. In the glutamate receptor superfamily, SEP-174559 inhibited N-methyl-D-aspartate (NMDA) receptor currents but did not affect non-NMDA receptors. These data confirm that SEP-174559 has benzodiazepine-like actions at gamma2-bearing subtypes of the GABA(A) receptor and suggest additional actions of benzodiazepine-site ligands at nACh and NMDA receptors.