The β Subunit Determines the Ligand Binding Properties of Synaptic Glycine Receptors

Glycine receptor subunit composition alters the action of GABA antagonists

GABA receptor antagonists produce an unexpectedly significant inhibition of native glycine receptors in retina and in alpha1 or alpha2 homomeric glycine receptors (GlyRs) expressed in HEK 293 cells. In this study we evaluate this phenomenon in heteromeric glycine receptors, formed by mixing alpha1, alpha2, and beta subunits. Picrotoxinin, picrotin, SR95531, and bicuculline are all more effective antagonists at GlyRs containing alpha2 subunits than alpha1 subunits. Inclusion of beta subunits reduces the inhibitory potency of picrotoxinin and picrotin but increases the potency of SR95531 and bicuculline. As a result of these two factors, bicuculline is particularly poor at discriminating GABA and glycine receptors. Picrotin, which has been reported to be inactive at GABA receptors, blocks glycine currents in retina and in HEK293 cells, suggesting its utility as a selective glycine antagonist. However, picrotin is a more potent inhibitor of GABA than glycine in retinal neurons. We also tested if GABA and glycine receptor subunits can combine to form functional receptors. If GABAAR gamma2S subunits are co-expressed with GlyR alpha subunits, the mixed receptor is glycine-sensitive and GABA-insensitive. But the mixed receptor exhibits a non-competitive picrotoxinin inhibition that is not observed in the homomeric GlyRs. This suggests that glycine and GABA subunits can co-assemble to form functional glycine receptors.

Colocalization of synaptic GABA(C)-receptors with GABA (A)-receptors and glycine-receptors in the rodent central nervous system

Fast inhibition in the nervous system is preferentially mediated by GABA- and glycine-receptors. Two types of ionotropic GABA-receptor, the GABA(A)-receptor and GABA(C)-receptor, have been identified; they have specific molecular compositions, different sensitivities to GABA, different kinetics, and distinct pharmacological profiles. We have studied, by immunocytochemistry, the synaptic localization of glycine-, GABA(A)-, and GABA(C)-receptors in rodent retina, spinal cord, midbrain, and brain-stem. Antibodies specific for the alpha1 subunit of the glycine-receptor, the gamma2 subunit of the GABA(A)-receptor, and the rho subunits of the GABA(C)-receptor have been applied. Using double-immunolabeling, we have determined whether these receptors are expressed at the same postsynaptic sites. In the retina, no such colocalization was observed. However, in the spinal cord, we found the colocalization of glycine-receptors with GABA(A)- or GABA(C)-receptors and the colocalization of GABA(A)- and GABA(C)-receptors in approximately 25% of the synapses. In the midbrain and brain-stem, GABA(A)- and GABA(C)-receptors were colocalized in 10%-15% of the postsynaptic sites. We discuss the possible expression of heteromeric (hybrid) receptors assembled from GABA(A)- and GABA(C)-receptor subunits. Our results suggest that GABA(A)- and GABA(C)-receptors are colocalized in a minority of synapses of the central nervous system.

Hyperekplexia mutation R271L of alpha1 glycine receptors potentiates allosteric interactions of nortropeines, propofol and glycine with [3H]strychnine binding

Human alpha1 and hyperekplexia mutant alpha1(R271L) glycine receptors (GlyRs) were transiently expressed in human embryonic kidney 293 cells for [3H]strychnine binding. Binding parameters were determined using a ternary allosteric model. The hyperekplexia mutation increased the positive cooperativity of 0.3 mM propofol and glycine binding by about six times: the cooperativity factor beta was 0.26 for alpha1 GlyRs and 0.04 for alpha1(R271L) GlyRs. Thus, propofol restored the potency of glycine impaired by the mutation. Five nortropeines, i.e. substituted benzoates of nortropine and a new compound, nortropisetron were prepared and also examined on [3H]strychnine binding. They showed nanomolar displacing potencies amplified by the hyperekplexia mutation. The affinity of nor-O-zatosetron (2.6 nM) is one of the highest reported for GlyRs. This binding test offers an in vitro method to evaluate agents against neurological disorders associated with inherited mutations of GlyRs.

Glycine receptors in the striatum, globus pallidus, and substantia nigra of the human brain: an immunohistochemical study

Glycine receptors (GlyRs) are heteropentameric chloride ion channels that facilitate fast-response, inhibitory neurotransmission in the mammalian spinal cord and brain. GlyRs have four functional subunits, alpha1-3 and beta, which likely exist in heteromeric alphabeta combinations. Mutations in GlyR alpha1 and beta subunits are well known for their involvement in hyperekplexia, a paroxysmal motor disorder. In this study we present the first detailed immunohistochemical investigation at the regional, cellular, and subcellular levels of GlyRs in the human basal ganglia. The results show that GlyRs are present at the regional level in low concentrations in the striatum and globus pallidus and are present in the highest concentrations in the substantia nigra. At the cellular level, GlyRs are present only in discrete populations of neurons immunoreactive for choline acetyltransferase (ChAT), parvalbumin, and calretinin in the human striatum, on a subpopulation of parvalbumin- and calretinin-positive neurons in the globus pallidus, and in the substantia nigra GlyRs are present on approximately three-fourths of all pars compacta and one-third of all pars reticulata neurons. They also form a distinct band of immunoreactive neurons in the intermedullary layers of the globus pallidus. At the subcellular level in the substantia nigra pars reticulata (SNr), GlyRs show a localized distribution on the soma and dendrites that partially complements but does not overlap with the distribution of gamma-aminobutyric acid (GABA)A receptors. Our results demonstrate the precise cellular and subcellular localization of GlyRs in the human basal ganglia and suggest that glycinergic receptors may play an important complementary role to other inhibitory receptors in modulating cholinergic, dopaminergic, and GABAergic neuronal pathways in the basal ganglia.

Homology modeling and molecular dynamics simulations of the glycine receptor ligand binding domain

We present a homology based model of the ligand binding domain (LBD) of the homopentameric alpha1 glycine receptor (GlyR). The model is based on multiple sequence alignment with other members of the nicotinicoid ligand gated ion channel superfamily and two homologous acetylcholine binding proteins (AChBP) from the freshwater (Lymnaea stagnalis) and saltwater (Aplysia californica) snails with known high resolution structure. Using two template proteins with known structure to model three dimensional structure of a target protein is especially advantageous for sequences with low homology as in the case presented in this paper. The final model was cross-validated by critical evaluation of experimental and published mutagenesis, functional and other biochemical studies. In addition, a complex structure with strychnine antagonist in the putative binding site is proposed based on docking simulation using Autodock program. Molecular dynamics (MD) simulations with simulated annealing protocol are reported on the proposed LBD of GlyR, which is stable in 5 ns simulation in water, as well as for a deformed LBD structure modeled on the corresponding domain determined in low-resolution cryomicroscopy structure of the alpha subunit of the full-length acetylcholine receptor (AChR). Our simulations demonstrate that the beta-sandwich central core of the protein monomer is fairly rigid in the simulations and resistant to deformations in water.

Glycine receptors of A-type ganglion cells of the mouse retina

A-type ganglion cells of the mouse retina represent the visual channel that transfers temporal changes of the outside world very fast and with high fidelity. In this study we combined anatomical and physiological methods in order to study the glycinergic, inhibitory input of A-type ganglion cells. Immunocytochemical studies were performed in a transgenic mouse line whose ganglion cells express green fluorescent protein (GFP). The cells were double labeled for GFP and the four alpha subunits of the glycine receptor (GlyR). It was found that most of the glycinergic input of A-type cells is through fast, alpha1-expressing synapses. Whole-cell currents were recorded from A-type ganglion cells in retinal whole mounts. The response to exogenous application of glycine and spontaneous inhibitory postsynaptic currents (sIPSCs) were measured. By comparing glycinergic currents recorded in wildtype mice and in mice with specific deletions of GlyRalpha subunits (Glra1spd-ot, Glra2-/-, Glra3-/-), the subunit composition of GlyRs of A-type ganglion cells could be further defined. Glycinergic sIPSCs of A-type ganglion cells have fast kinetics (decay time constant tau = 3.9 +/- 2.5 ms, mean +/- SD). Glycinergic sIPSCs recorded in Glra2-/- and Glra3-/- mice did not differ from those of wildtype mice. However, the number of glycinergic sIPSCs was significantly reduced in Glra1spd-ot mice and the remaining sIPSCs had slower kinetics than in wildtype mice. The results show that A-type ganglion cells receive preferentially kinetically fast glycinergic inputs, mediated by GlyRs composed of alpha1 and beta subunits.

Single-channel study of the spasmodic mutation alpha1A52S in recombinant rat glycine receptors

Inherited defects in glycine receptors lead to hyperekplexia, or startle disease. A mutant mouse, spasmodic, that has a startle phenotype, has a point mutation (A52S) in the glycine receptor alpha1 subunit. This mutation reduces the sensitivity of the receptor to glycine, but the mechanism by which this occurs is not known. We investigated the properties of A52S recombinant receptors by cell-attached patch-clamp recording of single-channel currents elicited by 30-10000 microM glycine. We used heteromeric receptors, which resemble those found at adult inhibitory synapses. Activation mechanisms were fitted directly to single channel data using the HJCFIT method, which includes an exact correction for missed events. In common with wild-type receptors, only mechanisms with three binding sites and extra shut states could describe the observations. The most physically plausible of these, the 'flip' mechanism, suggests that preopening isomerization to the flipped conformation that follows binding is less favoured in mutant than in wild-type receptors, and, especially, that the flipped conformation has a 100-fold lower affinity for glycine than in wild-type receptors. In contrast, the efficacy of the gating reaction was similar to that of wild-type heteromeric receptors. The reduction in affinity for the flipped conformation accounts for the reduction in apparent cooperativity seen in the mutant receptor (without having to postulate interaction between the binding sites) and it accounts for the increased EC50 for responses to glycine that is seen in mutant receptors. This mechanism also predicts accurately the faster decay of synaptic currents that is observed in spasmodic mice.

Mechanisms for picrotoxinin and picrotin blocks of alpha2 homomeric glycine receptors

Contrary to its effect on the gamma-aminobutyric acid type A and C receptors, picrotoxin antagonism of the alpha1 homomeric glycine receptors (GlyRs) has been shown to be non-use-dependent and nonselective between the picrotoxin components picrotoxinin and picrotin. Picrotoxin antagonism of the embryonic alpha2 homomeric GlyR is known to be use-dependent and reflects a channel-blocking mechanism, but the selectivity of picrotoxin antagonism of the embryonic alpha2 homomeric GlyRs between picrotoxinin and picrotin is unknown. Hence, we used the patch clamp recording technique in the outside-out configuration to investigate, at the single channel level, the mechanism of picrotin- and picrotoxinin-induced inhibition of currents, which were evoked by the activation of alpha2 homomeric GlyRs stably transfected into Chinese hamster ovary cells. Although both picrotoxinin and picrotin inhibited glycine-evoked outside-out currents, picrotin had a 30 times higher IC50 than picrotoxinin. Picrotin-evoked inhibition displayed voltage dependence, whereas picrotoxinin did not. Picrotoxinin and picrotin decreased the mean open time of the channel in a concentration-dependent manner, indicating that these picrotoxin components can bind to the receptor in its open state. When picrotin and glycine were co-applied, a large rebound current was observed at the end of the application. This rebound current was considerably smaller when picrotoxinin and glycine were co-applied. Both picrotin and picrotoxinin were unable to bind to the unbound conformation of the receptor, but both could be trapped at their binding site when the channel closed during glycine dissociation. Our data indicate that picrotoxinin and picrotin are not equivalent in blocking alpha2 homomeric GlyR.

Alterations in the brain-gut axis underlying visceral chemosensitivity in Nippostrongylus brasiliensis-infected mice

BACKGROUND & AIMS

Visceral hypersensitivity, a hallmark of irritable bowel syndrome, is generally considered to be mechanosensitive in nature and mediated via spinal afferents. Both stress and inflammation are implicated in visceral hypersensitivity, but the underlying molecular mechanisms of visceral hypersensitivity are unknown.

METHODS

Mice were infected with Nippostrongylus brasiliensis (Nb) larvae, exposed to environmental stress and the following separate studies performed 3-4 weeks later. Mesenteric afferent nerve activity was recorded in response to either ramp balloon distention (60 mm Hg), or to an intraluminal perfusion of hydrochloric acid (50 mmol/L), or to octreotide administration (2 micromol/L). Intraperitoneal injection of cholera toxin B-488 identified neurons projecting to the abdominal viscera. Fluorescent neurons in dorsal root and nodose ganglia were isolated using laser-capture microdissection. RNA was hybridized to Affymetrix Mouse whole genome arrays for analysis to evaluate the effects of stress and infection.

RESULTS

In mice previously infected with Nb, there was no change in intestinal afferent mechanosensitivity, but there was an increase in chemosensitive responses to intraluminal hydrochloric acid when compared with control animals. Gene expression profiles in vagal but not spinal visceral sensory neurons were significantly altered in stressed Nb-infected mice. Decreased afferent responses to somatostatin receptor 2 stimulation correlated with lower expression of vagal somatostatin receptor 2 in stressed Nb-infected mice, confirming a link between molecular data and functional sequelae.

CONCLUSIONS

Alterations in the intestinal brain-gut axis, in chemosensitivity but not mechanosensitivity, and through vagal rather than spinal pathways, are implicated in stress-induced postinflammatory visceral hypersensitivity.

The kinetics of inhibition of rat recombinant heteromeric alpha1beta glycine receptors by the low-affinity antagonist SR-95531

The GABA(A) antagonist SR-95531 (gabazine) is known to block glycine receptors, albeit with low affinity. We have studied the effect of SR-95531 on rat recombinant alpha1beta glycine receptors expressed in human embryonic kidney (HEK293) cells by recording macroscopic currents elicited by rapid glycine application to outside-out patches. SR-95531 has a fast unbinding rate (k(offSR), about 3000 s(-1)) and this means that the time course of its unbinding is comparable to the expected time course of the transmitter in the cleft. We also found that equilibrium applications of SR-95531 reduced the response to brief glycine applications by an amount inversely proportional to the duration of glycine application. The fast unbinding rate of SR-95531 from the glycine receptor will make it useful for establishing the time course of glycine concentration at glycinergic synapses.

Identification of residues mediating inhibition of glycine receptors by protons

We previously identified H109 of the glycine alpha1 subunit as a putative proton binding site. In the present studies, we explored additional proton binding site(s) as well as the mechanism underlying modulation of glycine receptors by protons. Whole-cell glycine currents were recorded from HEK 293 cells transiently expressing wild type or mutant glycine receptors. Individual mutation of 3 of 4 remaining extracellular histidine residue into alanine (i.e., alpha1 H107A, H215A or H419A), reduced the receptor sensitivity to protons to a varying extent. In contrast, mutation of alpha1 H201A did not affect proton sensitivity. Double, triple or quadruple histidine mutation of these residues caused a further reduction of proton sensitivity, suggesting multiple binding sites for proton action on glycine receptors. Furthermore, the substitution T133A, which mediates Zn(2+) inhibition, virtually abolished the proton effect on peak amplitude and current kinetics of glycine response. Replacement of T with S on position 133 partially restored receptor sensitivity to protons, suggesting the hydroxyl group of residue T133 is essential for proton-mediated modulation. In heteromeric alpha1beta receptors, mutations beta H132A and S156A, which correspond to H109 and T133 of the alpha1 subunit, respectively, also affected proton inhibition. In conclusion, multiple extracellular histidine residues (H107, H109, H215 and H419) and threonine residues of the alpha1 and beta Zn(2+) coordination sites are critical for modulation of the glycine receptor by protons.

Protein kinases regulate glycine receptor binding in brain stem auditory nuclei after unilateral cochlear ablation

Glycinergic synaptic inhibition is part of acoustic information processing in brain stem auditory pathways and contributes to the regulation of neuronal excitation. We found previously that unilateral cochlear ablation (UCA) in young adult guinea pigs decreased [3H]strychnine binding activity in several brain stem auditory nuclei. This study determined if the UCA-induced deficit could be regulated by protein kinase C (PKC), protein kinase A (PKA) or Ca2+/calmodulin-dependent protein kinase II (CaMKII). The specific binding of [3H]strychnine was measured in slices of the dorsal (DCN), posteroventral (PVCN) and anteroventral (AVCN) cochlear nucleus (CN), the lateral (LSO) and medial (MSO) superior olive, and the inferior colliculus (IC) 145 days after UCA. Tissues from age-matched unlesioned animals served as controls. UCA induced deficits in specific binding in the AVCN, PVCN, and LSO on the ablated side and in the MSO bilaterally. These deficits were reversed by 3 microM phorbol 1,2-dibutyrate, a PKC activator, or 0.2 mM dibutyryl-cAMP, a PKA activator. However, 50 nM Ro31-8220, a PKC inhibitor, and 2 microM H-89, a PKA inhibitor, had no effect in unlesioned controls and after UCA. In contrast, 4 microM KN-93, a CaMKII inhibitor, relieved or reversed the UCA-induced binding deficits and elevated binding in the IC. These findings suggest that a UCA-induced down-regulation of glycine receptor synthesis may have occurred via reduced phosphorylation of proteins that control receptor synthesis; this effect was reversed by diminishing CaMKII activity or increasing PKC and PKA activity.

Multiple association states between glycine receptors and gephyrin identified by SPT analysis

The scaffolding protein gephyrin is known to anchor glycine receptors (GlyR) at synapses and to participate in the dynamic equilibrium between synaptic and extrasynaptic GlyR in the neuronal membrane. Here we investigated the properties of this interaction in cells cotransfected with YFP-tagged gephyrin and GlyR subunits possessing an extracellular myc-tag. In HeLa cells and young neurons, single particle tracking was used to follow in real time individual GlyR, labeled with quantum dots, traveling into and out of gephyrin clusters. Analysis of the diffusion properties of two GlyR subunit types--able or unable to bind gephyrin--gave access to the association states of GlyR with its scaffolding protein. Our results indicated that an important portion of GlyR could be linked to a few molecules of gephyrin outside gephyrin clusters. This emphasizes the role of scaffolding proteins in the extrasynaptic membrane and supports the implication of gephyrin-gephyrin interactions in the stabilization of GlyR at synapses. The kinetic parameters controlling the equilibrium between GlyR inside and outside clusters were also characterized. Within clusters, we identified two subpopulations of GlyR with distinct degrees of stabilization between receptors and scaffolding proteins.

Diversity of glycine receptors in the mouse retina: localization of the alpha4 subunit

Glycine and gamma-aminobutyric acid (GABA) are the major inhibitory neurotransmitters in the retina. Approximately half of the amacrine cells release glycine at their synapses with bipolar, other amacrine, and ganglion cells. Whereas the retinal distributions of glycine receptor (GlyR) subunits alpha1, alpha2, and alpha3 have been mapped, the role of the alpha4 subunit in retinal circuitry remains unclear. A rabbit polyclonal antiserum was raised against a peptide that comprises the C-terminal 14 amino acids of the mouse GlyR alpha4 subunit. Using immunocytochemistry, we localized the alpha4 subunit in the inner plexiform layer (IPL) in brightly fluorescent puncta, which represent postsynaptically clustered GlyRs. This was shown by double-labeling sections for GlyR alpha4 and synaptic markers (bassoon, gephyrin). Double-labeling sections for GlyR alpha4 and the other GlyR alpha subunits shows that they are mostly clustered at different synapses; however, approximately 30% of the alpha4-containing synapses also express the alpha2 subunit. We also studied the pre- and postsynaptic partners at GlyR alpha4-containing synapses and found that displaced (ON-) cholinergic amacrine cells prominently expressed the alpha4 subunit. The density of GlyR alpha4-expressing synapses in wildtype, Glra1(ot/ot), and Glra3(-/-) mouse retinas did not differ significantly. Thus, there is no apparent compensation of the loss of alpha1 or alpha3 subunits by an upregulation of alpha4 subunit gene expression; however, the alpha2 subunit is moderately upregulated.

A novel glycine receptor beta subunit splice variant predicts an unorthodox transmembrane topology. Assembly into heteromeric receptor complexes

The inhibitory glycine receptor is a ligand-gated ion channel with a pentameric assembly from ligand binding alpha and structural beta subunits. In addition to alpha subunit gene variants (alpha1-alpha4) and developmental alterations in subunit composition of the receptor protein complex, alternative splicing of alpha subunits has been found to contribute to glycine receptor heterogeneity. Here, we describe a novel splice variant of the glycine receptor beta subunit from mouse central nervous system, prevailing in macroglial cells, predominantly in astrocytes and extraneural tissues. As predicted by its cDNA sequence, the novel subunit betaDelta7 lacks amino acid positions 251-302 encoded by exon 7 of the Glrb gene. Transcripts and antigen of betaDelta7 were detected in cerebral cortex, liver, and heart. Lack of exon 7 results in a profoundly altered prediction of transmembrane topology as betaDelta7 lacks TM1 and TM2 present in the full-length variant. Despite these topological alterations, in vitro studies showed that the betaDelta7 polypeptide integrates into the plasma membrane, forming receptor complexes with the alpha1 subunit and gephyrin. Our data demonstrate that a topology deviating from the classical four transmembrane-fold is compatible with formation of glycine receptor protein complexes. However, co-expression of alpha1 with betaDelta7 subunits did not change glycine receptor channel properties. Rather, the high level of expression in non-neuronal cells having intimate contact with synaptic regions may account for a yet unknown function of this splice variant betaDelta7 in glycinergic neurotransmission.

Tropisetron modulation of the glycine receptor: femtomolar potentiation and a molecular determinant of inhibition

The 5-hydroxytryptamine type-3 receptor antagonist tropisetron is in clinical use as an anti-emetic drug. This compound also exerts both potentiating and inhibitory effects on the glycine receptor chloride channel. The inhibitory effects occur at micromolar concentrations, whereas the potentiating effects are shown here to occur at femtomolar concentrations at the homomeric alpha1 receptor. Potentiation occurred only when tropisetron was applied in the presence of glycine. We also sought to identify molecular determinants of tropisetron inhibition at the alpha1 glycine receptor by serially mutating residues located in or near known ligand-binding sites. We discovered that conservative mutations to N102 ablated tropisetron inhibition without affecting the magnitude or sensitivity of tropisetron potentiation. Several lines of evidence, including a structure-activity analysis of tropisetron, atropine and SB203186, suggest that N102 may bind to the tropisetron tropane nitrogen via H-bonding. Mutation of the N125 residue in the beta subunit, which corresponds to N102 in the alpha1 subunit, had little effect on tropisetron inhibitory potency. These results show that N102 is required for tropisetron inhibition but not potentiation and that inhibitory tropisetron binds in different orientations at different subunit interfaces. To our knowledge, tropisetron is the most exquisitely sensitive modulator yet identified for a cys-loop receptor.

Characterization of mice with targeted deletion of glycine receptor alpha 2

Glycine receptors are ligand-gated chloride channels that mediate inhibitory neurotransmission in the adult nervous system. During development, glycine receptor alpha 2 (GlyRalpha2) is expressed in the retina, in the spinal cord, and throughout the brain. Within the cortex, GlyRalpha2 is expressed in immature cells and these receptors have been shown to be active and excitatory. In the developing retina, inhibition of glycine receptor activity prevents proper rod photoreceptor development. These data suggest that GlyRalpha2, the developmentally expressed glycine receptor, may play an important role in neuronal development. We have generated mice with a targeted deletion of glycine receptor alpha 2 (Glra2). Although these mice lack expression of GlyRalpha2, no gross morphological or molecular alterations were observed in the nervous system. In addition, the cerebral cortex does not appear to require glycine receptor activity for proper development, as Glra2 knockout mice did not show any electrophysiological responses to glycine.

Receptor and transmitter release properties set the time course of retinal inhibition

Synaptic inhibition is determined by the properties of postsynaptic receptors, neurotransmitter release, and clearance, but little is known about how these factors shape sensation-evoked inhibition. The retina is an ideal system to investigate inhibition because it can be activated physiologically with light, and separate inhibitory pathways can be assayed by recording from rod bipolar cells that possess distinct glycine, GABA(A), and GABA(C) receptors (R). We show that receptor properties differentially shape spontaneous IPSCs, whereas both transmitter release and receptor properties shape light-evoked (L) IPSCs. GABA(C)R-mediated IPSCs decayed the slowest, whereas glycineR- and GABA(A)R-mediated IPSCs decayed more rapidly. Slow GABA(C)Rs determined the L-IPSC decay, whereas GABA(A)Rs and glycineRs, which mediated rapid onset responses, determined the start of the L-IPSC. Both fast and slow inhibitory inputs distinctly shaped the output of rod bipolar cells. The slow GABA(C)Rs truncated glutamate release, making the A17 amacrine cell L-EPSCs more transient, whereas the fast GABA(A)R and glycineRs reduced the initial phase of glutamate release, limiting the peak amplitude of the L-EPSC. Estimates of transmitter release time courses suggested that glycine release was more prolonged than GABA release. The time course of GABA release activating GABA(C)Rs was slower than that activating GABA(A)Rs, consistent with spillover activation of GABA(C)Rs. Thus, both postsynaptic receptor and transmitter release properties shape light-evoked inhibition in retina.

Nmf11 is a novel ENU-induced mutation in the mouse glycine receptor alpha 1 subunit

Nmf11 is an N-ethyl-N-nitrosourea-induced recessive mouse mutation. In this article we show that the mutation is in the gene that encodes the glycine receptor alpha 1 subunit (Glra1). The new Glra1 mutation appears to affect glycine's inhibitory neurotransmission in the central nervous system (CNS) of the nmf11 homozygotes, which suffer from a severe startle disease-related phenotype and die by postnatal day 21. The nmf11 mutation involves a C-to-A transition of nucleotide 518, which results in the N46K substitution in the long extracellular NH(2) terminal or ligand-binding domain of the GLRA1 mature protein. The mutation does not result in reduced expression of GLRA1 at the mRNA or protein levels and the mutant glycine receptor localizes properly in synaptic sites of nmf11 homozygotes.

Molecular determinants of ginkgolide binding in the glycine receptor pore

Ginkgolides are potent blockers of the glycine receptor Cl- channel (GlyR) pore. We sought to identify their binding sites by comparing the effects of ginkgolides A, B and C and bilobalide on alpha1, alpha2, alpha1beta and alpha2beta GlyRs. Bilobalide sensitivity was drastically reduced by incorporation of the beta subunit. In contrast, the sensitivities to ginkgolides B and C were enhanced by beta subunit expression. However, ginkgolide A sensitivity was increased in the alpha2beta GlyR relative to the alpha2 GlyR but not in the alpha1beta GlyR relative to the alpha1 GlyR. We hypothesised that the subunit-specific differences were mediated by residue differences at the second transmembrane domain 2' and 6' pore-lining positions. The increased ginkgolide A sensitivity of the alpha2beta GlyR was transferred to the alpha1beta GlyR by the G2'A (alpha1 to alpha2 subunit) substitution. In addition, the alpha1 subunit T6'F mutation abolished inhibition by all ginkgolides. As the ginkgolides share closely related structures, their molecular interactions with pore-lining residues were amenable to mutant cycle analysis. This identified an interaction between the variable R2 position of the ginkgolides and the 2' residues of both alpha1 and beta subunits. These findings provide strong evidence for ginkgolides binding at the 2' pore-lining position.

Glycine receptors: recent insights into their structural organization and functional diversity

Strychnine-sensitive glycine receptors (GlyRs) are known to mediate synaptic inhibition in spinal cord, brainstem and other regions of the CNS. During the past 5 years, considerable progress has been made in delineating structural determinants of ligand binding and channel activation in recombinant GlyRs. Furthermore, immunohistochemical and gene inactivation studies have disclosed distinct distributions and functions of differentially expressed GlyR subtypes in retina, hippocampus and the dorsal horn of the spinal cord. Accordingly, GlyRs regulate not only the excitability of motor and sensory neurones, but are also essential for the processing of photoreceptor signals, neuronal development and inflammatory pain sensitization. Hence, these receptors constitute promising targets for the development of clinically useful compounds.

Glycinergic transmission

Inhibition in the mature central nervous system is mediated by activation of gamma-aminobutyric acid (GABA(A)) and glycine receptors. Both receptors belong to the same superfamily of ligand-gated ion channels and share common transmembrane topology and structural and functional features. Glycine receptors are pentameric ligand-gated anion channels composed of two different subunits, named alpha und beta, that assemble with a fixed stoichiometric ratio of two alpha to three beta subunits. Four genes encoding the alpha subunits exist, whereas only one gene encoding the beta subunit has been detected. Ligand binding occurs at the interface of alpha and beta subunits. The beta subunit, which is unable to form homo-oligomeric receptors, is responsible for assembly and channel properties. Moreover, this subunit carries a binding motif for the cytoplasmic protein gephyrin, which is believed to mediate synaptic clustering and anchoring at inhibitory synapses by interacting with the subsynaptic cytoskeleton. Synaptic gephyrin appears to restrict the mobility of glycine receptors diffusing in the plane of the plasma membrane, thereby generating dynamic plasma membrane domains contributing to the plasticity of inhibitory synapses. Glycine receptors are well established as playing important roles in controlling motor functions and sensory signaling in vision and audition and those in the dorsal horn of the spinal cord are now considered to be new targets for pain therapies. Like GABA(A) receptors, glycine receptors have been shown to be depolarizing during development. The functional meaning of the developmental switch from excitatory to inhibitory glycine receptor action remains to be elucidated.

Modulation of inhibitory and excitatory synaptic transmission in rat inferior colliculus after unilateral cochleectomy: an in situ and immunofluorescence study

We investigated whether inhibitory synaptic transmission mediated through glycinergic receptor, GABAA receptors, glutamic acid decarboxylase, the enzyme synthesizing GABA, and excitatory synaptic transmission through alpha-amino-3-hydroxi-5-methylisoxazole-4-propionic acid receptors and N-methyl-D-aspartate receptors are affected in the inferior colliculus by unilateral surgical cochleectomy. In situ hybridization and immunohistofluorescence studies were performed in normal and lesioned adult rats at various times following the lesion (1-150 days). Unilateral auditory deprivation decreased glycine receptor alpha1 and glutamic acid decarboxylase 67 expression in the contralateral central nucleus of the inferior colliculus. This decrease began one day after cochleectomy, and continued until day 8; thereafter expression was consistently low until day 150. The glycine receptor alpha1 subunit decrease did not occur if a second contralateral cochleectomy was performed either on day 8 or 150 after the first cochleectomy. Bilateral cochleectomy caused also a bilateral inferior colliculus diminution of glutamic acid decarboxylase 67 mRNA at post-lesion day 8 but there were no changes in glycine receptor alpha1 compared with controls. In contrast, the abundance of other alpha2-3, and beta glycine receptor, gephyrin, the anchoring protein of glycine receptor, the alpha1, beta2 and gamma2 subunits of GABAA receptors, GluR2, R3 subunits of alpha-amino-3-hydroxi-5-methylisoxazole-4-propionic acid receptors, and NR1 and NR2A transcripts of N-methyl-D-aspartate receptors was unaffected during the first week following the lesion. Thus, unilateral cochlear removal resulted in a selective and long-term decrease in the amount of the glycine receptor alpha1 subunit and of glutamic acid decarboxylase 67 in the contralateral central nucleus of the inferior colliculus. These changes most probably result from the induced asymmetry of excitatory auditory inputs into the central nucleus of the inferior colliculus and may be one of the mechanisms involved in the tinnitus frequently encountered in patients suffering from a sudden hearing loss.

Modulating inhibitory ligand-gated ion channels

The glycine and gamma-aminobutyric acid receptors (GlyR and GABA(A)R, respectively) are the major inhibitory neurotransmitter-gated receptors in the central nervous system of animals. Given the important role of these receptors in neuronal inhibition, they are prime targets of many therapeutic agents and are the object of intense studies aimed at correlating their structure and function. In this review, the structure and dynamics of these and other homologous members of the nicotinicoid superfamily are described. The modulatory actions of the major biological macromolecules that bind and allosterically affect these receptors are also discussed.

Distinct physiological mechanisms underlie altered glycinergic synaptic transmission in the murine mutants spastic, spasmodic, and oscillator

Spastic (spa), spasmodic (spd), and oscillator (ot) mice have naturally occurring glycine receptor (GlyR) mutations, which manifest as motor deficits and an exaggerated "startle response." Using whole-cell recording in hypoglossal motoneurons, we compared the physiological mechanisms by which each mutation alters GlyR function. Mean glycinergic miniature IPSC (mIPSC) amplitude and frequency were dramatically reduced (>50%) compared with controls for each mutant. mIPSC decay times were unchanged in spa/spa (4.5 +/- 0.3 vs 4.7 +/- 0.2 ms), reduced in spd/spd (2.7 +/- 0.2 vs 4.7 +/- 0.2 ms), and increased in ot/ot (12.3 +/- 1.2 vs 4.8 +/- 0.2 ms). Thus, in spastic, GlyRs are functionally normal but reduced in number, whereas in spasmodic, GlyR kinetics is faster. The oscillator mutation results in complete absence of alpha1-containing GlyRs; however, some non-alpha1-containing GlyRs persist at synapses. Fluctuation analysis of membrane current, induced by glycine application to outside-out patches, showed that mean single-channel conductance was increased in spa/spa (64.2 +/- 4.9 vs 36.1 +/- 1.4 pS), but unchanged in spd/spd (32.4 +/- 2.1 vs 35.3 +/- 2.1 pS). GlyR-mediated whole-cell currents in spa/spa exhibited increased picrotoxin sensitivity (27 vs 71% block for 100 microM), indicating alpha1 homomeric GlyR expression. The picrotoxin sensitivity of evoked glycinergic IPSCs and conductance of synaptic GlyRs, as determined by nonstationary variance analysis, were identical for spa/spa and controls. Together, these findings show the three mutations disrupt GlyR-mediated inhibition via different physiological mechanisms, and the spastic mutation results in "compensatory" alpha1 homomeric GlyRs at extrasynaptic loci.

Characterization of the glycinergic input to bipolar cells of the mouse retina

Glycine and gamma-aminobutyric acid (GABA) are the major inhibitory transmitters of the mammalian retina, and bipolar cells receive GABAergic and glycinergic inhibition from multiple amacrine cell types. Here we evaluated the functional properties and subunit composition of glycine receptors (GlyRs) in bipolar cells. Patch-clamp recordings were performed from retinal slices of wild-type, GlyRalpha1-deficient (Glra1(spd-ot)) and GlyRalpha3-deficient (Glra3(-/-)) mice. Whole-cell currents following glycine application and spontaneous inhibitory postsynaptic currents (IPSCs) were analysed. During the recordings the cells were filled with Alexa 488 and, thus, unequivocally identified. Glycine-induced currents of bipolar cells were picrotoxinin-insensitive and thus represent heteromeric channels composed of alpha and beta subunits. Glycine-induced currents and IPSCs were absent from all bipolar cells of Glra1(spd-ot) mice, indicating that GlyRalpha1 is an essential subunit of bipolar cell GlyRs. By comparing IPSCs of bipolar cells in wild-type and Glra3(-/-) mice, no statistically significant differences were found. OFF-cone bipolar (CB) cells receive a strong glycinergic input from AII amacrine cells, that is preferentially based on the fast alpha1beta-containing channels (mean decay time constant tau = 5.9 +/- 1.4 ms). We did not observe glycinergic IPSCs in ON-CB cells and could elicit only small, if any, glycinergic currents. Rod bipolar cells receive a prominent glycinergic input that is mainly mediated by alpha1beta-containing channels (tau = 5.5 +/- 1.6 ms). Slow IPSCs, the characteristic of GlyRs containing the alpha2 subunit, were not observed in bipolar cells. Thus, different bipolar cell types receive kinetically fast glycinergic inputs, preferentially mediated by GlyRs composed of alpha1 and beta subunits.

Frequency-dependent modulation of glycine receptor activation recorded from the zebrafish larvae hindbrain

In vertebrates, most glycinergic inhibitory neurons discharge phasically at a relatively low frequency. Such a pattern of glycine liberation from presynaptic terminals may affect the kinetics of post-synaptic glycine receptors. To examine this influence, we have analyzed the behavior of glycine receptors in response to repetitive stimulation at frequencies at which consecutive outside-out currents did not superimpose (0.5-4 Hz). Neurotransmitter release was mimicked on outside-out patches from zebrafish hindbrain Mauthner cells using fast flow application techniques. The amplitude of outside-out currents evoked by short (1 ms) repetitive applications of a saturating concentration (3 mM) of glycine remained unchanged for application frequencies<or=1 Hz. When the application frequency was increased from 1 to 4 Hz, the amplitude of the outside-out currents decreased with time to reach a steady state level. This decrease in current amplitude was larger and occurred faster with increasing application frequencies. Recovery occurred when the stimulation frequency was decreased back to 1 Hz. The recovery time constant was independent on the application frequency. This frequency-dependent inhibition was also observed for non-saturating glycine concentrations. Our results indicate that glycine receptor activity is down-regulated when the stimulation frequency increases to values>1 Hz. Glycine-evoked current simulations using a simple Markov model describing zebrafish glycine receptor kinetic behavior, indicates that this down-regulation of glycine receptor efficacy is due to a progressive accumulation of the receptors in a long lasting desensitization state. Our simulations suggest that this down-regulation can occur even when spontaneous inhibitory currents were generated randomly at a frequency>1 Hz.

Deciphering the structural framework of glycine receptor anchoring by gephyrin

Glycine is the major inhibitory neurotransmitter in the spinal cord and brain stem. Gephyrin is required to achieve a high concentration of glycine receptors (GlyRs) in the postsynaptic membrane, which is crucial for efficient glycinergic signal transduction. The interaction between gephyrin and the GlyR involves the E-domain of gephyrin and a cytoplasmic loop located between transmembrane segments three and four of the GlyR beta subunit. Here, we present crystal structures of the gephyrin E-domain with and without the GlyR beta-loop at 2.4 and 2.7 A resolutions, respectively. The GlyR beta-loop is bound in a symmetric 'key and lock' fashion to each E-domain monomer in a pocket adjacent to the dimer interface. Structure-guided mutagenesis followed by in vitro binding and in vivo colocalization assays demonstrate that a hydrophobic interaction formed by Phe 330 of gephyrin and Phe 398 and Ile 400 of the GlyR beta-loop is crucial for binding.

Constraining the expression of nicotinic acetylcholine receptors by using pentameric constructs

Much of our understanding of ligand-gated ion channels comes from heterologous expression studies. However, this technique cannot produce receptors with a predetermined subunit composition for channels formed by several different subunits and cannot insert a single mutation copy if the subunit of interest is present in several copies in the channel. Here, we describe a novel approach that overcomes these problems by expressing pentameric constructs, in which the code of the five subunits is linked (i.e., beta4_beta4_alpha3_beta4_alpha3). This is the first time that a concatemer of the complete pentameric receptor has been expressed for channels in the cysteine-loop superfamily. The presence of the linker did not change the agonist or antagonist sensitivity of alpha3beta4 nicotinic receptors. We show evidence that the expressed receptors were made up of alpha3 and beta4 subunits in one pentameric fusion protein as designed in the construct. This approach can be applied to any nicotinic superfamily receptor to produce channels with a defined subunit arrangement and to introduce specific mutations at any desired location of the pentameric fusion protein.

Pretty subunits all in a row: using concatenated subunit constructs to force the expression of receptors with defined subunit stoichiometry and spatial arrangement

The members of the Cys-loop ligand-gated ion channel (LGIC) gene family play a major role in fast synaptic transmission, and these receptors represent an important class of targets for therapeutic agents. Each member of this gene family is a pentameric complex containing one or more different subunits, and a large number of subunits for each member have been identified. This large number of subunits could give rise to a bewildering array of possible subunit compositions and spatial arrangements within a single complex, not all of which may occur in vivo. Heterologous expression systems have been used to create specific combinations of individual subunits to mimic naturally occurring receptors. However, this approach is not without its problems. In this issue of Molecular Pharmacology, Groot-Kormelink et al. (page 559) describe a method for constructing "concatameric" receptors, in which five individual subunits are arranged in a predetermined order connected by a flexible linker. Expression of this construct results in the formation of receptors with a unique, predefined subunit stoichiometry and subunit arrangement within the receptor complex. Receptors formed from this construct are fully functional and have properties essentially identical to those formed from individual subunits. The application of this very general approach to other members of the LGIC family should markedly enhance our ability to understand how subunit composition influences receptor function, as well as provide a means for the expression of receptors of predefined subunit composition and arrangement as tools for the development of novel selective pharmacological and therapeutic agents.

GABA and glycine as neurotransmitters: a brief history

gamma-Aminobutyric acid (GABA) emerged as a potentially important brain chemical just over 50 years ago, but its significance as a neurotransmitter was not fully realized until over 16 years later. We now know that at least 40% of inhibitory synaptic processing in the mammalian brain uses GABA. Establishing its role as a transmitter was a lengthy process and it seems hard to believe with our current knowledge that there was ever any dispute about its role in the mammalian brain. The detailed information that we now have about the receptors for GABA together with the wealth of agents which facilitate or reduce GABA receptor mechanisms make the prospects for further research very exciting. The emergence of glycine as a transmitter seems relatively painless by comparison to GABA. Perhaps this is appropriate for the simplest of transmitter structures! Its discovery within the spinal cord and brainstem approximately 40 years ago was followed only 2 years later by the proposal that it be conferred with 'neurotransmitter' status. It was another 16 years before the receptor was biochemically isolated. Now it is readily accepted as a vital spinal and supraspinal inhibitory transmitter and we know many details regarding its molecular structure and trafficking around neurones. The pharmacology of these receptors has lagged behind that of GABA. There is not the rich variety of allosteric modulators that we have come to readily associate with GABA receptors and which has provided us with a virtual treasure trove of important drugs used in anxiety, insomnia, epilepsy, anaesthesia, and spasticity, all stemming from the actions of the simple neutral amino acid GABA. Nevertheless, the realization that glycine receptors are involved in motor reflexes and nociceptive pathways together with the more recent advent of drugs that exhibit some subtype selectivity make the goal of designing selective therapeutic ligands for the glycine receptor that much closer.

Phylogenetic shadowing of a histamine-gated chloride channel involved in insect vision

A recently identified gene, hclA (synonym: ort), codes for an ionotrophic histamine receptor subunit in Drosophila melanogaster, and known hclA mutations lead to defects in the visual system, neurologic disorders and changed responsiveness to neurotoxins. To investigate whether this novel class of receptors is common across the Insecta, we analysed the genomes of 15 other insect species (Diptera, Hymenoptera, Coleoptera, Lepidoptera) and revealed orthologs of hclA in all of them. The predicted receptor domain of HCLA is extensively conserved (86-100% of identity) among the 16 proteins. Minor changes in the amino acid sequence that includes the putative transmembrane domains (TMs) 1-3 were found in non-drosophilid species only. Substantial amino acid variability was observed in the signal polypeptides, the intracellular loop domains and in TM4, in good accordance with known data on sequence variations in ligand-gated ion channels. Pairwise comparisons revealed three consensus sequences for N-glycosylation, conserved in HCLAs of all species studied, as well as a drosophilid-specific putative phosphorylation site. Real-time PCR analysis demonstrated that hclA-mRNA is abundant in heads of adult Drosophila. However, species- and sex-specific variations of the hclA expression levels were also observed.

Study of the interaction of antiplasmodial strychnine derivatives with the glycine receptor

Strychnos icaja Baill. (Loganiaceae) is a liana found in Central Africa known to be an arrow and ordeal poison but also used by traditional medicine to treat malaria. Recently, many dimeric or trimeric indolomonoterpenic alkaloids with antiplasmodial properties have been isolated from its rootbark. Since these alkaloids are derivatives of strychnine, it was important, in view of their potential use as antimalarial drugs, to assess their possible convulsant strychnine-like properties. In that regard, their interaction with the strychnine-sensitive glycine receptor was investigated by whole-cell patch-clamp recordings on glycine-gated currents in mouse spinal cord neurons in culture and by [(3)H]strychnine competition assays on membranes from adult rat spinal cord. These experiments were carried out on sungucine (leading compound of the chemical class) and on the antiplasmodial strychnogucine B (dimeric) and strychnohexamine (trimeric). In comparison with strychnine, all compounds interact with a very poor efficacy and only at concentrations >1 microM with both [(3)H]strychnine binding and glycine-gated currents. Furthermore, the effects of strychnine and protostrychnine, a monomeric alkaloid (without antiplasmodial activity) also isolated from S. icaja and differing from strychnine only by a cycle opening, were compared in the same way. The weak interaction of protostrychnine confirms the importance of the G cycle ring structure in strychnine for its binding to the glycine receptor and its antagonist properties.

Mechanisms for picrotoxin block of alpha2 homomeric glycine receptors

It is well known that the convulsant alkaloid picrotoxin (PTX) can inhibit neuronal gamma-aminobutyric acid (GABA) and homomeric glycine receptors (GlyR). However, the mechanism for PTX block of alpha(2) homomeric GlyR is still unclear compared with that of alpha(1) homomeric GlyR, GABA(A), and GABA(C) receptors. Furthermore, PTX effects on GlyR kinetics have been poorly explored at the single-channel level. Hence, we used the patch-clamp technique in the outside-out configuration to investigate the mechanism of PTX suppression of currents carried by alpha(2) homomeric GlyRs stably transfected into Chinese hamster ovary cells. PTX inhibited the alpha(2) homomeric GlyR current elicited by glycine in a concentration-dependent and voltage-independent manner. Both competitive and noncompetitive mechanisms were observed. PTX decreased the mean open time of the GlyR channel in a concentration-dependent manner, suggesting that PTX can block channel openings and bind to the receptor in the open channel conformation. When PTX and glycine were co-applied, a small rebound current was observed during drug washout. Application of PTX during the deactivation phase of glycine-induced currents eliminated the rebound current and accelerated the deactivation time course in a concentration-dependent manner. PTX could not bind to the unbound conformation of GlyR, but could be trapped at its binding site when the channel closed during glycine dissociation. Based on these observations, we propose a kinetic Markov model in which PTX binds to the alpha(2) homomeric GlyR in both the open channel state and the fully liganded closed state. Our data suggest a new allosteric mechanism for PTX inhibition of wild-type homomeric alpha(2) GlyR.

Extracellular Domain Nicotinic Acetylcholine Receptors Formed by α4 and β2 Subunits

Models of the extracellular ligand-binding domain of nicotinic acetylcholine receptors (nAChRs), which are pentameric integral membrane proteins, are attractive for structural studies because they potentially are water-soluble and better candidates for x-ray crystallography and because their smaller size is more amenable for NMR spectroscopy. The complete N-terminal extracellular domain is a promising foundation for such models, based on previous studies of alpha7 and muscle-type subunits. Specific design requirements leading to high structural fidelity between extracellular domain nAChRs and full-length nAChRs, however, are not well understood. To study these requirements in heteromeric nAChRs, the extracellular domains of alpha4 and beta2 subunits with or without the first transmembrane domain (M1) were expressed in Xenopus oocytes and compared with alpha4beta2 nAChRs based on ligand binding and subunit assembly properties. Ligand affinities of detergent-solubilized, extracellular domain alpha4beta2 nAChRs formed from subunits with M1 were nearly identical to affinities of alpha4beta2 nAChRs when measured with [3H]epibatidine, cytisine, nicotine, and acetylcholine. Velocity sedimentation suggested that these extracellular domain nAChRs predominantly formed pentamers. The yield of these extracellular domain nAChRs was about half the yield of alpha4beta2 nAChRs. In contrast, [3H]epibatidine binding was not detected from the extracellular domain alpha4 and beta2 subunits without M1, implying no detectable expression of extracellular domain nAChRs from these subunits. These results suggest that M1 domains on both alpha4 and beta2 play an important role for efficient expression of extracellular domain alpha4beta2 nAChRs that are high fidelity structural models of full-length alpha4beta2 nAChRs.

Natural products as tools for studies of ligand-gated ion channels

Ligand-gated ion channels, or ionotropic receptors, constitute a group of membrane-bound proteins that regulate the flux of ions across the cell membrane. In the brain, ligand-gated ion channels mediate fast neurotransmission. They are crucial for normal brain function and involved in many diseases in the brain. Historically, natural products have been used extensively in biomedical studies and ultimately as drugs or leads for drug design. In studies of ligand-gated ion channels, natural products have been essential for the understanding of their structure and function. In the following a short survey of natural products and their use in studies of ligand-gated ion channels is given.

Functional characterisation of human glycine receptors in a fluorescence-based high throughput screening assay

The human glycine receptor subtypes alpha1beta and alpha2 have been expressed stably in HEK293 cells, and the functional characteristics of the receptors have been characterised in the FLIPR Membrane Potential Assay. The pharmacological properties obtained for nine standard ligands at the two receptors in this assay were found to be in good agreement with those from electrophysiology studies of the receptors expressed in Xenopus oocytes or mammalian cell lines. Hence, this high throughput screening assay will be of great use in future pharmacological studies of glycine receptors, particular in the search for novel compound structures acting at them.

The beta subunit increases the ginkgolide B sensitivity of inhibitory glycine receptors

We investigated the effect of ginkgolide B (GB), a component of the extract from the leaves of the Ginkgo biloba tree, on recombinant glycine receptors (GlyRs) expressed in Xenopus oocytes by using voltage-clamp recording. GB (0.01-10 microM) inhibited glycine-induced currents of homo-oligomeric alpha1, alpha2 and alpha 3 GlyRs, with the highest potency being found at the alpha1 GlyR (IC(50) value=0.61+/-0.1 microM). Coexpression of the alpha subunits with the beta subunit resulted in a shift of the IC(50) value of GB to nanomolar values, indicating selectivity of GB for beta subunit containing GlyRs. We also analyzed the mechanism of GB inhibition and the effect of point mutations introduced into the alpha1 subunit. Our results are consistent with a channel blocking effect, since (i) GB inhibited glycine currents non-competitively, and (ii) a point mutation in the pore forming M2 domain reduced GB potency. In conclusion, GB is a potent blocker of beta subunit containing GlyR channels and hence can be used to discriminate homo- from hetero-oligomeric GlyRs. As hetero-oligomeric GlyRs are known to be synaptically localized, GB represents a channel blocker that may be employed to separate extrasynaptic from synaptic glycine currents.