Evolutionary history of the ligand-gated ion-channel superfamily of receptors

Actions of fluorinated alkanols on GABA(A) receptors: relevance to theories of narcosis

Previous work demonstrates that various anesthetics enhance the effect of gamma-aminobutyric acid (GABA), and this enhancement has been proposed as an explanation for how anesthetics cause anesthesia. This explanation extends to both fluorinated and unfluorinated alkanols. In the present study, we tested the capacity of fluorinated alkanols to enhance the function of the GABA(A) receptors expressed in Xenopus oocytes. CF3CH2OH, CF3(CF2)2CH2OH and CF3(CF2)4CH2OH potentiated GABA(A) receptor function, but CF3(CF2)5CH2OH did not. The degree of potentiation decreased in proportion to the chain length of the alkanols. These findings were not specific for receptors expressed in oocytes, as similar results were obtained with muscimol-stimulated 36Cl- uptake using mouse brain membrane vesicles. Although CF3(CF2)5CH2OH has been reported to enhance the capacity of desflurane to produce immobility in vivo, in our in vitro studies, this compound reduced potentiation of GABA-gated response by anesthetics such as isoflurane, enflurane, and pentobarbital. CHF2(CF2)5CH2OH, which has in vivo anesthetic effects, also failed to potentiate GABA(A) receptor function. These results indicate that the GABA(A) receptor is not the only receptor affected by fluorinated alkanols and that other receptors contribute to the capacity of alkanols to produce immobility. In particular, CF3(CF2)5CH2OH and CF3CH2OH inhibited N-methyl-D-aspartate receptor-mediated responses, which raises the possibility that this receptor is important for actions of fluorinated alkanols.

IMPLICATIONS

We find a consistent parallel between the immobilization produced by fluorinated alkanols and their actions on N-methyl-D-aspartate receptors but do not find a consistent parallel between immobilization and effects on gamma-aminobutyric acid type A receptors. Thus, we suggest that N-methyl-D-aspartate, but not gamma-aminobutyric acid type A, receptors may mediate the capacity of anesthetics to produce immobilization.

Cloning, expression and electrophysiological characterization of glycine receptor alpha subunit from zebrafish

The glycine receptor is a ligand-gated anion channel protein, providing inhibitory drive within the nervous system. We report here the isolation and functional characterization of a novel alpha subunit (alphaZ1) of the glycine receptor from adult zebrafish (Danio rerio) brain. The predicted amino acid sequence is 86%, 81% and 77% identical to mammalian isoforms alpha1, alpha3 and alpha2, respectively. AlphaZ1 exhibits many of the molecular features of mammalian alpha1, but the sequence patterns in the M4 and C-terminal domains are more similar to alpha2/alpha3. Phylogenetic analysis indicates that alphaZ1 is more closely related to the mammalian alpha1 subunits, being positioned, however, on a distinct branch. The alphaZ1 messenger RNA is 9.5 kb, similar to that described previously for alpha1 messenger RNAs. When expressed in Xenopus oocytes or a human cell line (BOSC 23), alphaZ1 forms a homomeric receptor which is activated by glycine and antagonized by strychnine. This receptor demonstrates unexpectedly high sensitivity to taurine and can also be activated by GABA. These results are consistent with physiological findings in lamprey and goldfish, and they suggest that this teleost fish glycine receptor displays a lower selectivity to neurotransmitters than that reported for glycine mammalian receptors.

Enhancement of GABA-activated current by muscarine in rat dorsal root ganglion neurons

The modulation of GABA-gated ion channel responses to GABA, pentobarbital and diazepam by muscarine was studied in freshly isolated rat dorsal root ganglion neurons using a whole-cell patch-clamp technique. Muscarine enhanced current activated by 5 microM GABA dose-dependently with an EC50 of 40 +/- 2 microM. This potentiation was not blocked by pirenzepine, gallamine and atropine, the specific and non-specific muscarinic receptor antagonists. Muscarine shifted the GABA dose-response curve to the left, with the GABA EC50 decreased from 45 +/- 2 to 13 +/- 2 microM. The maximal response to GABA was suppressed to 89.3 +/- 4.6% as compared with the control (100%) by 80 microM muscarine. Muscarine potentiated GABA (1-100 microM)-activated current in a voltage-independent manner. Muscarine shifted the dose-response curve for pentobarbital enhancement of GABA-activated current to the left, and the enhancement of GABA-activated current by muscarine was additive to that of pentobarbital over all pentobarbital concentrations. Muscarine shifted the dose-response curve for diazepam (1-100 nM) enhancement of GABA-activated current to the left. However, muscarine attenuated the facilitatory effect of saturating concentrations of diazepam (> 100 nM). The potentiating effect of muscarine was blocked by 1 nM ethyl-beta-carboline-3-carboxylate, the inverse agonist of benzodiazepine receptors. These results suggest that GABA-gated ion channel responses to GABA and pentobarbital were potentiated by muscarine and the binding site(s) for muscarine might be related to those for diazepam.

Structural features of the ligand-binding domain of the serotonin 5HT3 receptor

The nicotinic acetylcholine receptor (AChR) and the serotonin type 3 receptor (5HT3R) are members of the ligand-gated ion channel gene family. Both receptors are inhibited by nanomolar concentrations of d-tubocurarine (curare) in a competitive fashion. Chemical labeling studies on the AChR have identified tryptophan residues on the gamma (gammaTrp-55) and delta (deltaTrp-57) subunits that interact with curare. Comparison of the sequences of these two subunits with the 5HT3R shows that a tryptophan residue is found in the homologous position in the 5HT3R (Trp-89), suggesting that this residue may be involved in curare-5HT3R interactions. Site-directed mutagenesis at position Trp-89 markedly reduces the affinity of the 5HT3R for the antagonists curare and granisetron but has little effect on the affinity for the agonist serotonin. To further examine the role of this region of the receptor in ligand-receptor interactions, alanine-scanning mutagenesis analysis of the region centered on Trp-89 (Thr-85 to Trp-94) was carried out, and the ligand binding properties of the mutant receptors were determined. Within this region of the receptor, curare affinity is reduced by substitution only at Trp-89, whereas serotonin affinity is reduced only by substitution at Arg-91. On the other hand, granisetron affinity is reduced by substitutions at Trp-89, Arg-91, and Tyr-93. This differential effect of substitutions on ligand affinity suggests that different ligands may have different points of interaction within the ligand-binding pocket. In addition, the every-other-residue periodicity of the effects on granisetron affinity strongly suggests that this region of the ligand-binding site of the 5HT3R (and by inference, other members of the ligand-gated ion channel family) is in a beta-strand conformation.

Evolution of GABA(A) receptor diversity in the human genome

Nowhere is the record of receptor evolution more accessible than in the organization of the 19 vertebrate genes coding for subunits of the major inhibitory neurotransmitter receptor in the central nervous system, the gamma-aminobutyric acid receptor (GABAAR). Co-expression of alpha, beta, and gamma subunit genes is necessary for the formation of a GABAAR that is potentiated by widely used anxiolytics, anticonvulsants, and hypnotics. The identification of alpha, beta, and gamma genes on chromosomes 4, 5, and 15 suggests that co-localization of a gamma gene with an alpha and beta may be important for brain function. We have now directly examined the organization of GABAAR subunit genes on human chromosomes. Estimates of physical distance using in situ hybridization to cells in interphase, and gene localization using hybridization to cells in metaphase demonstrate the existence of beta-alpha-alpha-gamma gene clusters in cytogenetic bands on chromosomes 4(p12) and 5(q34). Sequencing of PAC clones establishes intercluster conservation of a unique head-to-head configuration for alpha and beta genes on chromosomes 4, 5, and 15. Remarkably, phylogenetic tree analysis predicts the existence of a beta-alpha-gamma ancestral gene cluster in which internal duplication of an ancestral alpha was followed by cluster duplication, resulting in the relative chromosomal positions of modern GABAAR subunit genes in the human genome.

The alpha9 nicotinic acetylcholine receptor shares pharmacological properties with type A gamma-aminobutyric acid, glycine, and type 3 serotonin receptors

In the present study, we provide evidence that the alpha9 nicotinic acetylcholine receptor (nAChR) shares pharmacological properties with members of the Cys-loop family of receptors. Thus, the type A gamma-aminobutyric acid receptor antagonist bicuculline, the glycinergic antagonist strychnine, and the type 3 serotonin receptor antagonist ICS-205,930 block ACh-evoked currents in alpha9-injected Xenopus laevis oocytes with the following rank order of potency: strychnine > ICS-205,930 > bicuculline. Block by antagonists was reflected in an increase in the acetylcholine (ACh) EC50 value, with no changes in agonist maximal response or Hill coefficient, which suggests a competitive type of block. Moreover, whereas neither gamma-aminobutyric acid nor glycine modified ACh-evoked currents, serotonin blocked responses to ACh in a concentration-dependent manner. The present results suggest that the alpha9 nAChR must conserve in its primary structure some residues responsible for ligand binding common to other Cys-loop receptors. In addition, it adds further evidence that the alpha9 nAChR and the cholinergic receptor present at the base of cochlear outer hair cells have similar pharmacological properties.

Modulation of the glycine response by Ca2+-permeable AMPA receptors in rat spinal neurones

1. In acutely isolated rat sacral dorsal commisural nucleus (SDCN) neurones, application of kainate (KA) reversibly potentiated glycine-evoked Cl- currents (IGly) in a concentration-dependent manner. 2. The cellular events underlying the interaction between non-NMDA receptors and glycine receptors were studied by using nystatin-perforated patch and cell-attached single-channel recording modes. 3. The action of KA was not accompanied by a shift in the reversal potential for IGly. In dose-response curves, KA potentiated IGly without significantly changing glycine binding affinity. 4. GYKI 52466 blocked while NS-102 had no effect on the KA-induced potentiation of IGly. 5. The potentiation was reduced when KA was applied in a Ca2+-free extracellular solution or in the presence of BAPTA AM, and was independent of the activation of voltage-dependent Ca2+ channels. 6. Pretreatment with KN-62, a selective Ca2+-calmodulin-dependent protein kinase II (CaMKII) inhibitor, abolished the action of KA. Inhibition of calcineurin converted the KA-induced potentiation to a sustained one. 7. Single-channel recordings revealed that KA decreased the mean closing time of glycine-gated single-channel activity, resulting in an increase in the probability of channel opening. 8. It is proposed that Ca2+ entry through AMPA receptors modulates the glycine receptor function via coactivation of CaMKII and calcineurin in SDCN neurones. This interaction may provide a new postsynaptic mechanism for control of inhibitory synaptic signalling and represent one of the important regulatory mechanisms of spinal nociception.

The Ligand Gated Ion Channel Database

The ligand gated ion channels (LGICs) are ionotropic receptors to neurotransmitters. Their physiological effect is carried out by the opening of an ionic channel upon binding of a particular neurotransmitter. These LGICs constitute superfamilies of receptors formed by homologous subunits. A database has been developed to handle the growing wealth of cloned subunits. This database contains nucleic acid sequences, protein sequences, as well as multiple sequence alignments and phylogenetic studies. This database is accessible via the worldwide web (http://www.pasteur.fr/units/neubiomol/LGIC.h tml), where it is continuously updated. A downloadable version is also available [currently v0.1 (98.06)].

Blue native PAGE as a useful method for the analysis of the assembly of distinct combinations of nicotinic acetylcholine receptor subunits

Oligomerization of complete and incomplete combinations of rat muscle-type nicotinic acetylcholine receptor (nAChR) subunits in Xenopus oocytes was studied by blue native PAGE and compared with acetylcholine-activated current in these cells. The rank order of expression level judged by current was alpha 1 beta 1 gamma delta >> alpha 1 beta 1 gamma > alpha 1 beta 1 delta > alpha 1 gamma delta >> alpha 1 delta >> alpha 1 gamma. alpha 1 and alpha 1 beta 1 were not functional. Protein complexes incorporating a heptahistidyl-tagged alpha 1 subunit were chromatographically purified from digitonin extracts of oocytes and resolved by blue native PAGE. In the absence of any co-expressed nAChR subunit, the majority of alpha 1 formed aggregates. Co-expression of beta 1 had no effect on alpha 1 aggregation, whereas both gamma and delta diminished alpha 1 aggregation in favor of discrete oligomers: alpha 1 formed tetramers together with gamma and dimers, trimers, and tetramers together with delta. When alpha 1 gamma was complemented with beta 1 to form a functional alpha 1 beta 1 gamma receptor, a small amount of a pentamer was found besides a prominent alpha 1-His7 beta 1 gamma trimer. Expression of the functional alpha 1 beta 1 delta receptor yielded marked amounts of a pentamer besides dimers and trimers. These results are discussed in terms of the assembly model of Green and Claudio (Cell 74, 57-69, 1994), substantiating that blue native PAGE is suited for the investigation of ion channel assembly.

Non-additive interaction between nicotinic cholinergic and P2X purine receptors in guinea-pig enteric neurons in culture

1. Acetylcholine (ACh)-activated currents and their interaction with ATP-activated currents were studied in primary cultures of myenteric neurons from guinea-pig small intestine using patch clamp techniques. Peak currents caused by co-application of ACh (1 mM) and ATP (300 microM) were 78 +/- 2 % of the sum of currents activated by each agonist alone (P < 0.05, n = 29). Reversal potentials measured during co-application of ACh and ATP did not differ from those measured during application of ACh or ATP alone. Addition of BAPTA (10 mM) to the pipette solution or replacement of extracellular Ca2+ with Na+ did not prevent occlusion. 2. Responses caused by co-application of 5-HT (300 microM), acting at 5-HT3 receptors, and ACh (3 mM) or ATP (1 mM) were additive (94 +/- 3 or 96 +/- 4 %, respectively, of the sum of currents activated by 5-HT and ACh or ATP alone; P > 0.05). Currents caused by GABA (1 mM), acting at GABAA receptors, and ACh (3 mM) or ATP (1 mM) were also additive (105 +/- 4 or 100 +/- 3 %, respectively, of the sum of currents activated by GABA and ACh or GABA and ATP applied separately; P > 0. 05). 3. Single channel currents caused by ACh and ATP in the same outside-out patches were less than additive (85 +/- 10 % of the predicted sum, P < 0.05). 4. P2X receptors and nicotinic cholinergic receptors (nAChRs) are linked in a mutually inhibitory manner in guinea-pig myenteric neurons. The functional interaction does not involve ligand binding sites, Ca2+-dependent mechanisms, a change in the driving force for Na+ or cytoplasmic signalling mechanisms.

Immunolabelling of the rat intestinal tract with antibodies specific to the long form of the 5-hydroxytryptamine3 receptor

The mouse 5-hydroxytryptamine3 (5-HT3) type of serotonin receptors is expressed as two forms, 5-HT3R-A(L) and 5-HT3R-A(S), generated by alternative splicing of its primary transcript, that differ by a stretch of six amino acids in the second intracellular loop domain. Because this six-amino acid region contains a putative phosphorylation site that may be important for the function and/or regulation of 5HT3R-A(L) receptor, specifically, we developed polyclonal antibodies as appropriate tools for studies relevant to this question. Antibodies against a 20-amino acid peptide corresponding to the sequence of 5-HT3R-A(L) at the level of this six-amino acid region were obtained as soon as one month after injection of this synthetic peptide to rabbits. Immunocytochemistry with these antibodies led to a strong positive labelling of plasma membrane, reticulum and Golgi apparatus of COS-7 cells expressing cloned murine 5-HT3R-A(L), whereas COS-7 cells expressing similar levels of 5-HT3R-A(S) exhibited only a very weak labelling. Immunoblots of fusion proteins combining glutathion-S-transferase and the second cytoplasmic loop of 5-HT3R-A(L) or 5-HT3R-A(S) revealed a c. 20-fold selectivity of the antibodies for the first, long form, as evaluated by densitometric analysis of enhanced chemiluminescence detection. Similarly, immunoblots of COS-7 cells transfected with cloned 5-HT3 receptors showed that the anti-peptide antibodies detected a band at 53,000 mol. wt only in cells transfected with 5-HT3R-A(L). Under optimal conditions, antibodies immunoprecipitated 52% of 5-HT3R-A(L), but only 11% of 5-HT3R-A(S), solubilized from COS-7 cells transfected with the respective encoding plasmids. In the rat, no immunoautoradiographic labelling by the anti-peptide antibodies could be detected in brain structures which had previously been described to express preferentially a short form of the 5-HT3 receptor. In contrast, a strong immunolabelling was found in the intestinal mucosa, especially in the rat fetus (at the 17th embryonic day), suggesting the possible participation of the 5-HT3R-A(L) isoform in the development of this tissue. These results show that specific antibodies are useful tools for the visualization of the least abundant 5-HT3 receptor isoform in rat tissue.

Molecular and functional characterization of a calcium-sensitive chloride channel from mouse lung

A protein (mCLCA1) has been cloned from a mouse lung cDNA library that bears strong sequence homology with the recently described bovine tracheal, Ca2+-sensitive chloride channel protein (bCLCA1), bovine lung endothelial cell adhesion molecule-1 (Lu-ECAM-1), and the human intestinal Ca2+-sensitive chloride channel protein (hCLCA1). In vitro, its 3.1-kilobase message translates into a 100-kDa protein that can be glycosylated to an approximately 125-kDa product. SDS-polyacrylamide gel electrophoresis from lysates of mCLCA1 cDNA-transfected transformed human embryonic kidney cells (HEK293) reveals proteins of 130, 125, and 90 kDa as well as a protein triplet in the 32-38 kDa size range. Western analyses with antisera raised against Lu-ECAM-1 peptides show that the N-terminal region of the predicted open reading frame is present only in the larger size proteins (i.e. 130, 125, and 90 kDa), whereas the C-terminal region of the open reading frame is observed in the 32-38 kDa size proteins, suggesting a posttranslational, proteolytic processing of a precursor protein (125/130 kDa) into 90 kDa and 32-38 kDa components similar to that reported for Lu-ECAM-1. Hydrophobicity analyses predict four transmembrane domains for the 90-kDa protein. The mCLCA1 mRNA is readily detected by Northern analysis and by in situ hybridization in the respiratory epithelia of trachea and bronchi. Transient expression of mCLCA1 in HEK293 cells was associated with an increase in whole cell Cl- current that could be activated by Ca2+ and ionomycin and inhibited by 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid, dithiothreitol, and niflumic acid. The discovery of mCLCA1 opens the door for further investigating the possible contribution of a Ca2+-sensitive chloride conductance to the pathogenesis of cystic fibrosis.

Which molecular targets are most relevant to general anaesthesia?

1. In view of the large number of possible molecular targets of general anaesthetics, it is necessary to have some criteria for judging which targets are important for producing general anaesthesia and which are probably not. 2. We consider in detail two criteria: sensitivity to clinically relevant concentrations of anaesthetics and stereoselectivity to anaesthetic optical isomers. 3. The targets which currently emerge as most important belong to an anaesthetic-sensitive superfamily of genetically related fast neurotransmitter-gated receptor channels present at central synapses.

Glycine

Glycine consists of a single carbon molecule attached to an amino and a carboxyl group. Its small size helps it to function as a flexible link in proteins and allows for the formation of helices, an extracellular signaling molecule, recognition sites on cell membranes and enzymes, a modifier of molecular activity via conjugation and glycine extension of hormone precursors, and an osmoprotectant. There is substantial experimental evidence that free glycine may have a role in protecting tissues against insults such as ischemia, hypoxia, and reperfusion. This impressive catalogue of functions makes an interesting contrast with glycine's perceived metabolic role as a nonessential amino acid. Glycine interconverts with serine to provide a mechanism for the transfer of activated one-carbon groups. Glycine has just been viewed as a convenient source of nitrogen to add to solutions of nutrients. Although this may have unexpected benefits when such solutions are used in clinical practice, it does raise the specter of a possible confounding effect in experiments when glycine is added to control solutions to make them isonitrogenous.

A GABAA receptor alpha1 subunit tagged with green fluorescent protein requires a beta subunit for functional surface expression

gamma-Aminobutyric acid, type A (GABAA) receptors, the major inhibitory neurotransmitter receptors in the central nervous system, are heteropentameric proteins assembled from distinct subunit classes with multiple subtypes, alpha(1-6), beta(1-4), gamma(1-3), delta(1), and epsilon(1). To examine the process of receptor assembly and targeting, we tagged the carboxyl terminus of the GABAA receptor alpha1 subunit with red-shifted enhanced green fluorescent protein (EGFP). Xenopus oocytes were injected with cRNA of this fusion protein, alpha1-EGFP, alone or in combination with cRNA of GABAA receptor beta2, gamma2, or beta2+gamma2 subunits. Within 72 h after injection, EGFP fluorescence was visible in all fusion protein-injected cells. The fluorescence was associated with the plasmalemma only when the beta2 subunit was co-injected with alpha1-EGFP. Texas Red-conjugated immunolabeling of EGFP on nonpermeabilized cells demonstrated that EGFP was localized extracellularly. Hence, the COOH terminus of the alpha1 subunit is extracellular. Two-electrode voltage clamp of alpha1-EGFPbeta2- and alpha1-EGFPbeta2 gamma2-injected oocytes demonstrates that these cells express functional receptors, with EC50 values for GABA and diazepam similar to wild-type receptors. Thus, a COOH-terminal tag of the alpha1 subunit appears to be functionally silent, providing a useful marker for studies of GABAA receptor expression, assembly, transport, targeting, and clustering. Moreover, the beta2 subunit is required for receptor assembly and surface expression.

Modulation of agonist and antagonist interactions in serotonin 1A receptors by alcohols

The serotonin type IA (5-HT1A) receptors are members of a superfamily of seven transmembrane domain receptors that couple to GTP binding regulatory proteins (G-proteins). Serotonergic signalling has been shown to play an important role in alcohol tolerance and dependence. We have studied the effects of alcohols on ligand (agonist and antagonist) binding to bovine hippocampal 5-HT1A receptor in native as well as solubilized membranes. Our results show that alcohols inhibit the specific binding of the agonist OH-DPAT and the antagonist p-MPPF to 5-HT1A receptors in a concentration-dependent manner.

Using knockout and transgenic mice to study neurophysiology and behavior

Reverse genetics, in which detailed knowledge of a gene of interest permits in vivo modification of its expression or function, provides a powerful method for examining the physiological relevance of any protein. Transgenic and knockout mouse models are particularly useful for studies of complex neurobiological problems. The primary aims of this review are to familiarize the nonspecialist with the techniques and limitations of mouse mutagenesis, to describe new technologies that may overcome these limitations, and to illustrate, using representative examples from the literature, some of the ways in which genetically altered mice have been used to analyze central nervous system function. The goal is to provide the information necessary to evaluate critically studies in which mutant mice have been used to study neurobiological problems.

Electrostatics and the ion selectivity of ligand-gated channels

The nicotinic acetylcholine receptor (nAChR) is a cation-selective ion channel that opens in response to acetylcholine binding. The related glycine receptor (GlyR) is anion selective. The pore-lining domain of each protein may be modeled as a bundle of five parallel M2 helices. Models of the pore-lining domains of homopentameric nAChR and GlyR have been used in continuum electrostatics calculations to probe the origins of ion selectivity. Calculated pKA values suggest that "rings" of acidic or basic side chains at the mouths of the nAChR or GlyR M2 helix bundles, respectively, may not be fully ionized. In particular, for the nAChR the ring of glutamate side chains at the extracellular mouth of the pore is predicted to be largely protonated at neutral pH, whereas those glutamate side chains in the intracellular and intermediate rings (at the opposite mouth of the pore) are predicted to be fully ionized. Inclusion of the other domains of each protein represented as an irregular cylindrical tube in which the M2 bundles are embedded suggests that both the M2 helices and the extramembrane domains play significant roles in determining ion selectivity.

Binding sites for exogenous and endogenous non-competitive inhibitors of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) is the paradigm of the neurotransmitter-gated ion channel superfamily. The pharmacological behavior of the AChR can be described as three basic processes that progress sequentially. First, the neurotransmitter acetylcholine (ACh) binds the receptor. Next, the intrinsically coupled ion channel opens upon ACh binding with subsequent ion flux activity. Finally, the AChR becomes desensitized, a process where the ion channel becomes closed in the prolonged presence of ACh. The existing equilibrium among these physiologically relevant processes can be perturbed by the pharmacological action of different drugs. In particular, non-competitive inhibitors (NCIs) inhibit the ion flux and enhance the desensitization rate of the AChR. The action of NCIs was studied using several drugs of exogenous origin. These include compounds such as chlorpromazine (CPZ), triphenylmethylphosphonium (TPMP+), the local anesthetics QX-222 and meproadifen, trifluoromethyl-iodophenyldiazirine (TID), phencyclidine (PCP), histrionicotoxin (HTX), quinacrine, and ethidium. In order to understand the mechanism by which NCIs exert their pharmacological properties several laboratories have studied the structural characteristics of their binding sites, including their respective locations on the receptor. One of the main objectives of this review is to discuss all available experimental evidence regarding the specific localization of the binding sites for exogenous NCIs. For example, it is known that the so-called luminal NCIs bind to a series of ring-forming amino acids in the ion channel. Particularly CPZ, TPMP+, QX-222, cembranoids, and PCP bind to the serine, the threonine, and the leucine ring, whereas TID and meproadifen bind to the valine and extracellular rings, respectively. On the other hand, quinacrine and ethidium, termed non-luminal NCIs, bind to sites outside the channel lumen. Specifically, quinacrine binds to a non-annular lipid domain located approximately 7 A from the lipid-water interface and ethidium binds to the vestibule of the AChR in a site located approximately 46 A away from the membrane surface and equidistant from both ACh binding sites. The non-annular lipid domain has been suggested to be located at the intermolecular interfaces of the five AChR subunits and/or at the interstices of the four (M1-M4) transmembrane domains. One of the most important concepts in neurochemistry is that receptor proteins can be modulated by endogenous substances other than their specific agonists. Among membrane-embedded receptors, the AChR is one of the best examples of this behavior. In this regard, the AChR is non-competitively modulated by diverse molecules such as lipids (fatty acids and steroids), the neuropeptide substance P, and the neurotransmitter 5-hydroxytryptamine (5-HT). It is important to take into account that the above mentioned modulation is produced through a direct binding of these endogenous molecules to the AChR. Since this is a physiologically relevant issue, it is useful to elucidate the structural components of the binding site for each endogenous NCI. In this regard, another important aim of this work is to review all available information related to the specific localization of the binding sites for endogenous NCIs. For example, it is known that both neurotransmitters substance P and 5-HT bind to the lumen of the ion channel. Particularly, the locus for substance P is found in the deltaM2 domain, whereas the binding site for 5-HT and related compounds is putatively located on both the serine and the threonine ring. Instead, fatty acid and steroid molecules bind to non-luminal sites. More specifically, fatty acids may bind to the belt surrounding the intramembranous perimeter of the AChR, namely the annular lipid domain, and/or to the high-affinity quinacrine site which is located at a non-annular lipid domain. Additionally, steroids may bind to a site located on the extracellular hydrophi

Pharmacological characterization of a rat 5-hydroxytryptamine type3 receptor subunit (r5-HT3A(b)) expressed in Xenopus laevis oocytes

The present study has utilized the two electrode voltage-clamp technique to examine the pharmacological profile of a splice variant of the rat orthologue of the 5-hydroxytryptamine type 3A subunit (5-HT3A(b)) heterologously expressed in Xenopus laevis oocytes. At negative holding potentials, bath applied 5-HT (300 nM - 10 microM) evoked a transient, concentration-dependent (EC50 = 1.1+/-0.1 microM), inward current. The response reversed in sign at a holding potential of -2.1+/-1.6 mV. The response to 5-HT was mimicked by the 5-HT3 receptor selective agonists 2-methyl-5-HT (EC50= 4.1+/-0.2 microM), 1-phenylbiguanide (EC50=3.0+/-0.1 microM), 3-chlorophenylbiguanide (EC50 = 140+/-10 nM), 3,5-dichlorophenylbiguanide (EC50 = 14.5+/-0.4 nM) and 2,5-dichlorophenylbiguanide (EC50 = 10.2+/-0.6 nM). With the exception of 2-methyl-5-HT, all of the agonists tested elicited maximal current responses comparable to those produced by a saturating concentration (10 microM) of 5-HT. Responses evoked by 5-HT at EC50 were blocked by the 5-HT3 receptor selective antagonist ondansetron (IC50=231+/-22 pM) and by the less selective agents (+)-tubocurarine (IC50=31.9+/-0.01 nM) and cocaine (IC50 = 2.1+/-0.2 microM). The data are discussed in the context of results previously obtained with the human and mouse orthologues of the 5-HT3A subunit. Overall, the study reinforces the conclusion that species differences detected for native 5-HT3 receptors extend to, and appear largely explained by, differences in the properties of homo-oligomeric receptors formed from 5-HT3A subunit orthologues.

The diversity of GABAA receptors. Pharmacological and electrophysiological properties of GABAA channel subtypes

The amino acid gamma-aminobutyric-acid (GABA) prevails in the CNS as an inhibitory neurotransmitter that mediates most of its effects through fast GABA-gated Cl(-)-channels (GABAAR). Molecular biology uncovered the complex subunit architecture of this receptor channel, in which a pentameric assembly derived from five of at least 17 mammalian subunits, grouped in the six classes alpha, beta, gamma, delta, sigma and epsilon, permits a vast number of putative receptor isoforms. The subunit composition of a particular receptor determines the specific effects of allosterical modulators of the GABAARs like benzodiazepines (BZs), barbiturates, steroids, some convulsants, polyvalent cations, and ethanol. To understand the physiology and diversity of GABAARs, the native isoforms have to be identified by their localization in the brain and by their pharmacology. In heterologous expression systems, channels require the presence of alpha, beta, and gamma subunits in order to mimic the full repertoire of native receptor responses to drugs, with the BZ pharmacology being determined by the particular alpha and gamma subunit variants. Little is known about the functional properties of the beta, delta, and epsilon subunit classes and only a few receptor subtype-specific substances like loreclezole and furosemide are known that enable the identification of defined receptor subtypes. We will summarize the pharmacology of putative receptor isoforms and emphasize the characteristics of functional channels. Knowledge of the complex pharmacology of GABAARs might eventually enable site-directed drug design to further our understanding of GABA-related disorders and of the complex interaction of excitatory and inhibitory mechanisms in neuronal processing.

Interaction of D-tubocurarine analogs with the 5HT3 receptor

D-Tubocurarine is a potent competitive antagonist of two members of the ligand-gated ion channel family, the muscle-type nicotinic acetylcholine receptor (AChR) and serotonin type-3 receptor (5HT3R). We have used a series of analogs of D-tubocurarine to determine the effects of methylation, stereoisomerization and halogenation on the interaction of D-tubocurarine with the 5HT3R. The affinities of the analogs for the 5HT3R span a 200-fold concentration range and fall into three broad groups. The first group, with affinity constants (Ki) < 150 nM, consists of D-tubocurarine and analogs modified at the nitrogens or 7' hydroxyl. The fact that these compounds all have high affinity for the 5HT3R suggests that these portions of the ligand do not make interactions with the receptor that are critical for high-affinity binding. The second group, with Ki's in the 1-5 microM range, consists of analogs modified at the 12'-hydroxyl or the adjacent 13'-carbon, which suggests that this portion of the ligand makes interactions that are important for high-affinity binding. The third, very low affinity, group is a compound with altered stereoconfiguration at the 1 carbon, demonstrating the importance of proper configuration of the antagonist in ligand-receptor interactions. For the most part, this pattern of selectivity is similar to that for the AChR, suggesting that the structures of the ligand-binding sites of these two receptors share common structural features.

Mutations at lipid-exposed residues of the acetylcholine receptor affect its gating kinetics

The firmest candidate among the transmembrane portions of the nicotinic acetylcholine receptor (AChR) to be in contact with the lipid bilayer is the fourth segment, M4. To explore the contribution of alphaM4 amino acid residues of mouse AChR to channel gating, we combined site-directed mutagenesis with single-channel recordings. Two residues in alphaM4, Cys418 and Thr422, were found to significantly affect gating kinetics when replaced by alanine. AChRs containing alphaC418A and alphaT422A subunits form channels characterized by a 3- and 5-fold reduction in the mean open time, respectively, suggesting an increase in the closing rate due to the mutations. The calculated changes in the energy barrier for the channel closing process show unequal and coupled contributions of both positions to channel gating. Single-channel recordings of hybrid wild-type alpha/alphaT422A AChR show that the closing rate depends on the number of alpha subunits mutated. Each substitution of threonine to alanine changes the energy barrier of the closing process by approximately 0.5 kcal/mol. Recordings of channels activated by high agonist concentration suggest that these mutations also impair channel opening. Both Cys418 and Thr422 have been postulated to be in contact with the lipid milieu and are highly conserved among species and subunits. Our results support the involvement of lipid-exposed residues in alphaM4 in AChR channel gating mechanism.

On the transcriptional regulation of neuronal nAChR genes

The promoters driving transcription of the neuronal nicotinic genes alpha 7 and beta 3 have been characterized in the chicken. Although their regulatory modalities are thoroughly different, they nevertheless lead to co-expression in the same neurons.

P2X1 and P2X3 receptors form stable trimers: a novel structural motif of ligand-gated ion channels

P2X receptors are cation channels gated by extracellular ATP. The seven known P2X isoforms possess no sequence homology with other proteins. Here we studied the quaternary structure of P2X receptors by chemical cross-linking and blue native PAGE. P2X1 and P2X3 were N-terminally tagged with six histidine residues to allow for non-denaturing receptor isolation from cRNA-injected, [35S]methionine-labeled oocytes. The His-tag did not change the electrophysiological properties of the P2X1 receptor. His-P2X1 was found to carry four N-glycans per polypeptide chain, only one of which acquired Endo H resistance en route to the plasma membrane. 3, 3'-Dithiobis(sulfosuccinimidylpropionate) (DTSSP) and two of three bifunctional analogues of the P2X receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) cross-linked digitonin-solubilized His-P2X1 and His-P2X3 quantitatively to homo-trimers. Likewise, when analyzed by blue native PAGE, P2X receptors purified in digitonin or dodecyl-beta-D-maltoside migrated entirely as non-covalently linked homo-trimers, whereas the alpha2 beta gamma delta nicotinic acetylcholine receptor (used as a positive control) migrated as the expected pentamer. P2X monomers remained undetected soon after synthesis, indicating that trimerization occurred in the endoplasmic reticulum. The plasma membrane form of His-P2X1 was also identified as a homo-trimer. If n-octylglucoside was used for P2X receptor solubilization, homo-hexamers were observed, suggesting that trimers can aggregate to form larger complexes. We conclude that trimers represent an essential element of P2X receptor structure.

KEYWORDS

blue native PAGE/cross-linking/P2X receptor/quaternary structure.

Brain nicotinic receptors: structure and regulation, role in learning and reinforcement

The introduction, in the late sixties, of the concepts and methods of molecular biology to the study of the nervous system had a profound impact on the field, primarily through the identification of its basic molecular components. These structures include, for example, the elementary units of the synapse: neurotransmitters, neuropeptides and their receptors, but also ionic channels, intracellular second messengers and the relevant enzymes, cell surface adhesion molecules, or growth and trophic factors [21,78,81, 52,79]. Attempts to establish appropriate causal relationships between these molecular components, the actual organisation of neural networks, and a defined behavior, nevertheless, still must overcome many difficulties. A first problem is the recognition of the minimum levels of organisation, from the molecular, cellular, or multicellular (circuit) to the higher cognitive levels, that determine the given physiological and/or behavioral performance under investigation. A common difficulty (and potential source of errors of interpretation) is to relate a cognitive function to a network organization which does not possess the required structural complexity and vice-versa. Another problem is to distinguish, among the components of the system, those which are actually necessary and those which, taken together, suffice for a given behavior to take place. Identification of such a minimal set of building blocks may receive decisive insights from the elaboration of neurally plausible formal models that bring together, within a single and coherent 'artificial organism', the neuronal network, the circulating activity, and the behavior they determine (see [42,43,45,72,30]). In this communication, we shall attempt, still in a preliminary fashion, to bring together: (1) our recent knowledge on the molecular biology of brain nicotinic receptors (nAChRs) and their allosteric properties and (2) integrated behaviors, such as cognitive learning, investigated for instance with delayed-response or passive avoidance tasks that are likely to involve nAChRs in particular at the level of reinforcement (or reward) mechanisms (see [18,29,135]).

A domain contributing to the ion channel of ATP-gated P2X2 receptors identified by the substituted cysteine accessibility method

P2X receptors are a family of ATP-gated ion channels thought to have intracellular N and C termini and two transmembrane segments separating a large extracellular domain. We examined the involvement of the second putative transmembrane domain (TM2) of the P2X2 subunit in ion conduction, using the substituted cysteine accessibility method (SCAM). This method tests the ability of hydrophilic reagents such as Ag+ or the methanethiosulfonates to modify covalently the sulfhydryl side chains exposed to aqueous environments. ATP-gated current was measured in HEK293 cells transiently expressing either wild-type or functional mutant P2X2 receptors containing a cysteine substitution in or around TM2. Application of Ag+ to gating channels had no sustained effect on wild-type P2X2 (WT) but irreversibly altered whole-cell currents in 15 mutants. By contrast, bath application of (2-aminoethyl)methanethiosulfonate (MTSEA) to closed channels inhibited 8 of the 15 residues affected by Ag+ when the channel was gating. Inhibition of the closed channel was prevented in seven of eight mutants when membrane-permeant MTSEA was scavenged by 20 mM intracellular cysteine, indicating that these seven mutants lie on the intracellular side of the channel gate. Further, MTSEA inhibited current through G342C in the absence of intracellular cysteine but augmented the current when cysteine was present, suggesting that this residue may be part of the gate. Taken together, the data help to the identify a functional domain of the channel pore by mapping residues on either side of the channel gate.

Cloning and functional expression of alternative spliced variants of the rho1 gamma-aminobutyrate receptor

The rho1 gamma-aminobutyrate receptor (GABArho1) is expressed predominantly in the retina and forms homomeric GABA-gated Cl- channels that are clearly different from the multisubunit GABAA receptors. In contrast to these, GABArho1 receptors desensitize very little and are not blocked by bicuculline. In addition to GABArho1, two new variants were identified in human retina cDNA libraries. Cloning and sequence analysis showed that both variants contain large deletions in the putative extracellular domain of the receptor. These deletions extend from a common 5' site to different 3' sites. The cDNA with the largest deletion, named GABArho1Delta450, contains a complete ORF identical to that of GABArho1 but missing 450 nt. This cDNA encodes a protein of 323 aa, identical to the GABArho1, but has a deletion of 150 aa in the amino-terminal extracellular domain. GABArho1Delta450 mRNA injected into Xenopus oocytes did not produce functional GABA receptors. The second GABArho1 variant (GABArho1Delta51) contains a 51-nt deletion. In Xenopus oocytes, GABArho1Delta51 led to the expression of GABA receptors that had the essential GABArho1 characteristics of low desensitization and bicuculline resistance. Therefore, alternative splicing increases the coding potential of this gene family expressed in the human retina, but the functional diversity created by the alternative spliced forms is still not understood.

Calcium-dependent desensitizing function of the postsynaptic neuronal-type nicotinic acetylcholine receptors at the neuromuscular junction

Several subunits that commonly have been regarded as neuronal-type nicotinic acetylcholine receptor (nAChR) subtypes, have been found in the postjunctional endplate membrane of adult skeletal muscle fibres. The postsynaptic function of these neuronal-type nAChR subtypes at the neuromuscular junction has been investigated by using aequorin luminescence and fluorescence confocal imaging. A biphasic elevation of intracellular Ca2+ is elicited by prolonged nicotinic action at the mouse muscle endplates. The fast and slow Ca2+ components are operated by a postsynaptic muscle- and colocalized neuronal-type nAChR, respectively. Neuromuscular functions may be regulated by a dual nAChR system to maintain the normal postsynaptic excitability. Certain neuronal-type nAChR may be endowed with the same functional role in the central nervous system also.

Propofol and other intravenous anesthetics have sites of action on the gamma-aminobutyric acid type A receptor distinct from that for isoflurane

Both volatile and intravenous general anesthetics allosterically enhance gamma-aminobutyric acid (GABA)-evoked chloride currents at the GABA type A (GABAA) receptor. Recent work has revealed that two specific amino acid residues within transmembrane domain (TM)2 and TM3 are necessary for positive modulation of GABAA and glycine receptors by the volatile anesthetic enflurane. We now report that mutation of these residues within either GABAA alpha2 (S270 or A291) or beta1 (S265 or M286) subunits resulted in receptors that retain normal or near-normal gating by GABA but are insensitive to clinically relevant concentrations of another inhaled anesthetic, isoflurane. To determine whether receptor modulation by intravenous general anesthetics also was affected by these point mutations, we examined the effects of propofol, etomidate, the barbiturate methohexital, and the steroid alphaxalone on wild-type and mutant GABAA receptors expressed in human embryonic kidney 293 cells. In most cases, these mutations had little or no effect on the actions of these intravenous anesthetics. However, a point mutation in the beta1 subunit (M286W) abolished potentiation of GABA by propofol but did not alter direct activation of the receptor by high concentrations of propofol. These data indicate that the receptor structural requirements for positive modulation by volatile and intravenous general anesthetics may be quite distinct.

Enhancement of glycine receptor function by ethanol is inversely correlated with molecular volume at position alpha267

Glycine and gamma-aminobutyric acid (GABA)A receptors are members of the "superfamily" of ion channels, and are sensitive to allosteric modulation by n-alcohols such as ethanol and butanol. We recently demonstrated that the mutation of Ser-267 to Ile in the alpha1 subunit abolished ethanol regulation of glycine receptors (Gly-R). In the present study, a pair of chimeric receptors was studied, in which a 45-amino acid domain comprising transmembrane domains 2 and 3 was exchanged between the Gly-Ralpha1 and gamma-aminobutyric acid rho1 subunits. Detailed pharmacologic analysis of these chimeras confirmed that this domain of the Gly-R confers enhancement of receptor function by ethanol and butanol. An extensive series of mutations at Ser-267 in the Gly-Ralpha1 subunit was also prepared, and the resulting homomeric receptors were expressed and tested for sensitivity to glycine, and allosteric modulation by alcohols. All of the mutant receptors expressed successfully in Xenopus oocytes. Mutation of Ser-267 to small amino acid residues such as Gly or Ala produced receptors in which glycine responses were potentiated by ethanol. As we have reported previously, the mutant Gly-Ralpha1 (Ser-267 --> Ile) was completely insensitive to ethanol; mutation of Ser-267 to Val had a similar effect. Mutation of Ser-267 to large residues such as His, Cys, or Tyr resulted in inhibition of Gly-R function by ethanol. These results demonstrate that the size of the amino acid residue at position alpha267 plays a crucial role in determining the functional consequences of allosteric modulation of the Gly-R by alcohols.

Cloning of the rat brain cDNA encoding for the SLC-1 G protein-coupled receptor reveals the presence of an intron in the gene

In order to isolate new G protein-coupled receptors expressed in the cerebral cortex, a set of degenerate oligonucleotides corresponding to the third and seventh transmembrane segment were synthetized. Their use in PCR on rat brain cortex mRNA amplified several cDNA fragments. One of them, a 526 bp sequence, encoded for what was at that time an unknown G protein-coupled receptor. An oligonucleotide derived from the sequence was then used as a probe to isolate the receptor cDNA from a rat brain cDNA library. It encodes for a 353aa protein with seven transmembrane segments, three consensus N-glycosylation sites at the amino terminus and several potential phosphorylation sites in the intracellular loops. This protein shares 91% overall identity with a recently cloned human somatostatin-like receptor of 402aa named SLC-1. This suggests that we have cloned the rat orthologue of the human SLC-1. However, the extracellular N-terminus of the human receptor is 49 amino acids longer and shows 50% identity with the rat one. Because the human sequence was deduced from genomic DNA, we suspected the presence of an intron in the gene. This was confirmed by PCR using primers spanning the intron. On the basis of the sequence of a 128 kb fragment of chromosome 22 encompassing the SLC-1 gene, we were able to deduce a corrected amino acids sequence for the human receptor. So both rat and human SLC-1 receptors are 353aa long, with three consensus N-glycosylation sites. They share 96% identity at the amino acid level and are encoded by a gene containing one intron in the coding sequence.

Critical elements determining diversity in agonist binding and desensitization of neuronal nicotinic acetylcholine receptors

To identify the molecular determinants underlying the pharmacological diversity of neuronal nicotinic acetylcholine receptors, we compared the alpha7 homo-oligomeric and alpha4beta2 hetero-oligomeric receptors. Sets of residues from the regions initially identified within the agonist binding site of the alpha4 subunit were introduced into the alpha7 agonist binding site, carried by the homo-oligomeric alpha7-V201-5HT3 chimera. Introduction of the alpha4 residues 183-191 into alpha7 subunit sequence (chimera C2) selectively increased the apparent affinities for equilibrium binding and for ion channel activation by acetylcholine, resulting in a receptor that no longer displays differences in the responses to acetylcholine and nicotine. Introduction of the alpha4 residues 151-155 (chimera B) produced a approximately 100-fold increase in the apparent affinity for both acetylcholine and nicotine in equilibrium binding measurements. In both cases electrophysiological recordings revealed a much smaller increase (three- to sevenfold) in the apparent affinity for activation, but the concentrations required to desensitize the mutant chimeras parallel the shifts in apparent binding affinity. The data were fitted by a two-state concerted model, and an alteration of the conformational isomerization constant leading to the desensitized state accounts for the chimera B phenotype, whereas alteration of the ligand binding site accounts for the chimera C2 phenotype. Point mutation analysis revealed that several residues in both fragments contribute to the phenotypes, with a critical effect of the G152K and T183N mutations. Transfer of alpha4 amino acids 151-155 and 183-191 into the alpha7-V201-5HT3 chimera thus confers physiological and pharmacological properties typical of the alpha4beta2 receptor.

Molecular dissection of benzodiazepine binding and allosteric coupling using chimeric gamma-aminobutyric acidA receptor subunits

Although gamma-aminobutyric acid (GABA)A receptor alpha subunits are important for benzodiazepine (BZD) binding and GABA-current potentiation by BZDs, the presence of a gamma subunit is required for high affinity BZD effects. To determine which regions unique to the gamma2S subunit confer BZD binding and potentiation, we generated chimeric protein combinations of rat gamma2S and alpha1 subunits using a modified protocol to target crossover events to the amino-terminal extracellular region of the subunits. Several chimeras with full open reading frames were constructed and placed into vectors for either voltage-clamp experiments in Xenopus laevis oocytes or radioligand binding experiments in human embryonic kidney 293 cells. Chimeras (chi) containing at least the amino-terminal 161 amino acids of gamma2S bound BZDs with wild-type affinity when coexpressed with alpha1 and beta2 subunits. Further analysis of the gamma2S binding site region uncovered two areas, gamma2S K41-W82 and gamma2S R114-D161, that together are necessary and sufficient for high affinity BZD binding. Surprisingly, although the 161-amino acid residue amino terminus of the gamma2S subunit is sufficient for high affinity BZD binding, it is not sufficient for efficient allosteric coupling of the GABA and BZD binding sites, as demonstrated by reduced diazepam potentiation of the GABA-gated current and GABA potentiation of [3H]flunitrazepam binding. Thus, by using gamma/alpha chimeras, we identified two gamma2 subunit regions required for BZD binding that are distinct from domain or domains responsible for allosteric coupling of the BZD and GABA binding sites.

Identification of the subunits of the nicotinic cholinergic receptors in the rat cochlea using RT-PCR and in situ hybridization

There are two tissues in the adult mammalian cochlea that are post-synaptic to cholinergic efferent fibers: The outer hair cells (OHCs) and the dendrites of the afferent fibers of the type I spiral ganglion cells. The unusual nicotinic-like pharmacology of cochlear cholinergic responses and the unique embryonic development of cochlear tissues suggest that the inner-ear nicotinic cholinergic receptor (nAChR) may be different from nAChRs described previously at synapses in the mammalian brain, autonomic ganglia, or skeletal muscle. In this study, we determined the mRNA expression of the alpha2-7, alpha9, and beta2-4 subunits of the nicotinic acetylcholine receptor (nAChR) family in the rat cochlea. In micro-dissected tissue from the organ of Corti, spiral ganglion, and the membranous lateral wall, we found mRNA expression of the alpha7 and alpha9 subunits in the organ of Corti and alpha5-7, and beta2 and beta3 in the spiral ganglion using RT-PCR. Employing in situ hybridization with 35S-riboprobes, we localized alpha9 in hair cells regions and alpha6, alpha7 and beta2 in the type I cells of the spiral ganglion. No evidence of nAChR subunit mRNA expression was found in supporting cells, but beta2 was expressed in type II spiral ganglion cells, which are neither cholinergic nor cholinoceptive.

Age-related glycine receptor subunit changes in the cochlear nucleus of Fischer-344 rats

Previous studies have shown that levels of binding for the strychnine-sensitive glycine receptor in the cochlear nucleus (CN) of Fischer (F344) rats decrease with age. Given the major role glycine plays in normal CN function, changes in glycine-receptor activity may contribute to central presbycusis. To further evaluate the impact of age on glycine receptors, in situ hybridization was used to assess, in three age groups of F344 rats, changes in levels of gene expression for four of its subunits. When compared with the 3-month-old rats, expression of mRNAs for alpha1 and beta subunits in the anteroventral CN decreased significantly in the 18- and 27-month-old age groups, while mRNA expression for the alpha2 subunit increased. If protein expressions are similar, these subunit changes may alter the function of glycine receptors, thereby affecting binding to its ligands.

Deficient development and maintenance of postsynaptic specializations in mutant mice lacking an ‘adult’ acetylcholine receptor subunit

At many synapses, ‘fetal’ neurotransmitter receptor subunits are replaced by ‘adult’ subunits as development proceeds. To assess the significance of such transitions, we deleted the gene encoding the adult acetylcholine receptor (AChR) epsilon subunit, which replaces its fetal counterpart, the gamma subunit, at the skeletal neuromuscular junction during early postnatal life. Several aspects of postnatal maturation, including synapse elimination, proceeded normally in the absence of the adult AChR, but structural development of the endplate was compromised. Later, inadequate compensation by the gamma subunit led to severely reduced AChR density in mutant endplates relative to controls. This decreased density led to a profound reorganization of AChR-associated components of the postsynaptic membrane and cytoskeleton. Together, these results suggest novel roles for AChRs in assembly of the postsynaptic apparatus.

Biology of the postsynaptic glycine receptor

Glycine is one of the major inhibitory neurotransmitters, and upon binding to its receptor it activates chloride conductances. Receptors are accumulated immediately opposite release sites, at the postsynaptic differentiations, where they form functional microdomains. This review describes recent advances in our understanding of the structure-function relationships of the glycine receptor, a member of the ligand-gated ion channel superfamily. Following purification of the receptor complex and identification of its integral and peripheral membrane protein components, molecular cloning has revealed the existence of several subtypes of the ligand-binding subunit. This heterogeneity is responsible for the distinct pharmacological and functional properties displayed by the various receptor configurations that are differentially expressed and assembled during development. This review also focuses on the molecular aspects of glycinergic synaptogenesis, highlighting gephyrin, the peripheral component of the receptor. The role of this cytoplasmic protein in anchoring and maintaining the channel complex in postsynaptic clusters is discussed. The glycine receptor recently moved into the spotlight as a paradigm in the approach to cell biology of the formation of the postsynaptic membrane.

The interaction of general anaesthetics with recombinant GABAA and glycine receptors expressed in Xenopus laevis oocytes: a comparative study

1. The effects of five structurally dissimilar general anaesthetics were examined in voltage-clamp recordings of agonist-evoked currents mediated by recombinant gamma-aminobutyric acid (GABA)A receptors composed of human alpha 1 beta 1 and gamma 2L subunits expressed in Xenopus laevis oocytes. A quantitative comparison of the effects of these agents was made upon recombinant glycine receptors expressed as a homo-oligomer of human alpha 1 subunits, or as a hetero-oligomer of human alpha 1 and rat beta subunits. 2. Complementary RNA-injected oocytes expressing GABAA receptors responded to bath applied GABA with an EC50 of 158 +/- 34 microM. Oocytes expressing alpha 1 and alpha 1 beta glycine receptors subsequent to cDNA injection displayed EC50 values of 76 +/- 2 microM and 66 +/- 2 microM, respectively, in response to bath applied glycine. 3. Picrotoxin antagonized responses mediated by homo-oligomeric alpha 1 glycine receptors with an IC50 of 4.2 +/- 0.8 microM. Hetero-oligomeric alpha 1 beta glycine receptors were at least 100-fold less sensitive to blockade by picrotoxin. 4. With the appropriate agonist EC10, propofol enhanced GABA and glycine-evoked currents to approximately the maximal response produced by a saturating concentration of either agonist (i.e. Imax). The calculated EC50 values were 2.3 +/- 0.2 microM, 16 +/- 3 microM and 27 +/- 2 microM, for GABAA alpha 1 beta 1 gamma 2L, glycine alpha 1 and alpha 1 beta receptors, respectively. At relatively high concentrations, propofol was observed to activate directly both GABAA and glycine receptors. 5. Pentobarbitone potentiated GABA-evoked currents to 117 +/- 8.5% of Imax with an EC50 of 65 +/- 3 microM. The barbiturate also produced a substantial enhancement of the glycine-evoked currents, Imax and EC50 values being 71 +/- 2% and 845 +/- 66 microM and 51 +/- 10% and 757 +/- 30 microM for homomeric alpha 1 and heteromeric alpha 1 beta glycine receptors respectively. At high concentrations, pentobarbitone directly activated GABAA, but not glycine, receptors. 6. The potentiation by propofol or pentobarbitone of currents mediated by alpha 1 homo-oligomeric glycine receptors was in both cases associated with a parallel sinistral shift of the glycine concentration-effect curve. The effects of binary combinations of pentobarbitone and propofol at maximally effective concentrations were mutually occlusive suggesting a common site, or mechanism, of action. 7. GABA-evoked currents were maximally potentiated by etomidate to 79 +/- 2% of Imax (EC50 of 8.1 +/- 0.9 microM). By contrast, glycine-induced currents mediated by alpha 1 and alpha 1 beta glycine receptor isoforms were enhanced only to 29 +/- 4% and 28 +/- 3% of Imax. Limited solubility precluded the calculation of EC50 values for the effect of etomidate at glycine receptors. None of the receptor isoforms examined were directly activated by etomidate. 8. The neurosteroid 5 alpha-pregnan-3 alpha-ol-20-one potentiated GABA-evoked currents to 69 +/- 4% of Imax, with an EC50 value of 89 +/- 6 nM. In contrast, both alpha 1 homo-oligomeric and alpha 1 beta hetero-oligomeric glycine receptors were insensitive to the action of this steroid. A direct agonist action of the steroid was discernible at GABAA, but not glycine, receptors. 9. Trichloroethanol, the active metabolite of the general anaesthetic chloral hydrate, enhanced glycine-evoked currents to 77 +/- 10% and 94 +/- 4% of Imax on alpha 1 and alpha 1 beta glycine receptors, with EC50 values of 3.5 +/- 0.1 mM and 5.9 +/- 0.3 mM respectively. On GABAA receptors, trichloroethanol had a lower maximum enhancement (52 +/- 5% of Imax), but a slightly higher potency (EC50 1.0 +/- 0.1 mM). Trichloroethanol activated neither GABAA, nor glycine, receptors. 10. The data demonstrate a variety of intravenous general anaesthetic agents, at clinically relevant concentrations, to augment preferentially GABA-evoked currents mediated by the alpha1beta1upsilon2L receptor subunit combination as compared to their effects on both alpha1 and alpha1beta glycine receptors. However, the presence on glycine receptors of lower affinity modulatory binding sites for pentobarbitone, propofol and trichloroethanol may aid in the identification of the molecular determinants of the CNS actions of these anaesthetics.

Modulation of nicotinic ACh-, GABAA- and 5-HT3-receptor functions by external H-7, a protein kinase inhibitor, in rat sensory neurones

1. The effects of external H-7, a potent protein kinase inhibitor, on the responses mediated by gamma-aminobutyric acid A type (GABAA)-, nicotinic acetylcholine (nicotinic ACh)-, ionotropic 5-hydroxytryptamine (5-HT3)-, adenosine 5'-triphosphate (ATP)-, N-methyl-D-aspartate (NMDA)- and kainate (KA)-receptors were studied in freshly dissociated rat dorsal root ganglion neurone by use of whole cell patch-clamp technique. 2. External H-7 (1-1000 microM) produced a reversible, dose-dependent inhibition of whole cell currents activated by GABA, ACh and 5-HT. 3. Whole-cell currents evoked by ATP, 2-methylthio-ATP, NMDA and KA were insensitive to external H-7. 4. External H-7 shifted the dose-response curve of GABA-activated currents downward without changing the EC50 significantly (from 15.0 +/- 4.0 microM to 18.0 +/- 5.0 microM). The maximum response to GABA was depressed by 34.0 +/- 5.3%. This inhibitory action of H-7 was voltage-independent. 5. Intracellular application of H-7 (20 microM), cyclic AMP (1 mM) and BAPTA (10 mM) could not reverse the H-7 inhibition of GABA-activated currents. 6. The results suggest that external H-7 selectively and allosterically modulates the functions of GABAA-, nicotine ACh- and 5-HT3 receptors via a common conserved site in the external domain of these receptors.

Caenorhabditis elegans levamisole resistance genes lev-1, unc-29, and unc-38 encode functional nicotinic acetylcholine receptor subunits

We show that three of the eleven genes of the nematode Caenorhabditis elegans that mediate resistance to the nematocide levamisole and to other cholinergic agonists encode nicotinic acetylcholine receptor (nAChR) subunits. unc-38 encodes an alpha subunit while lev-1 and unc-29 encode non-alpha subunits. The nematode nAChR subunits show conservation of many mammalian nAChR sequence features, implying an ancient evolutionary origin of nAChR proteins. Expression in Xenopus oocytes of combinations of these subunits that include the unc-38 alpha subunit results in levamisole-induced currents that are suppressed by the nAChR antagonists mecamylamine, neosurugatoxin, and d-tubocurarine but not alpha-bungarotoxin. The mutant phenotypes reveal that unc-38 and unc-29 subunits are necessary for nAChR function, whereas the lev-1 subunit is not. An UNC-29-GFP fusion shows that UNC-29 is expressed in body and head muscles. Two dominant mutations of lev-1 result in a single amino acid substitution or addition in or near transmembrane domain 2, a region important to ion channel conductance and desensitization. The identification of viable nAChR mutants in C. elegans provides an advantageous system in which receptor expression and synaptic targeting can be manipulated and studied in vivo.

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Assigning functions to residues in the acetylcholine receptor channel region (review)

This review is concerned with the functional domains of the nicotinic acetylcholine receptor (AChR) involved in ion permeation. These comprise the ion pore and its gate. The latter allows the channel to be almost exclusively closed in the absence of agonist and favours ion flux in its presence. Early photoaffinity labelling experiments using open-channel blockers and site-directed mutagenesis studies identified M2 of each AChR subunit as the transmembrane domain lining the walls of the ion pore. Several biochemical, electrophysiological, and mutagenesis studies as well as molecular modelling and in vitro studies of ion channel formation with synthetic peptides corroborate these findings. Point mutations combined with electrophysiological techniques have contributed to dissecting the AChR channel region assigning functions to individual amino acid residues, thus revealing structural and functional stratification of the M2 channel domain. Specific residues have been found to be structural determinants of conductance, ion selectivity, gating, and desensitization. The three-dimensional structure of the AChR protein at 9A resolution suggests a possible arrangement of the M2 alpha-helices in the open and closed states, respectively. In spite of the current wealth of knowledge on the AChR ion channel stemming from the combination of experimental approaches discussed in this review, the mechanistic structure by which the interaction with the agonist favours the opening of the cationic channel remains unknown.

Sites of alcohol and volatile anaesthetic action on GABA(A) and glycine receptors

Volatile anaesthetics have historically been considered to act in a nonspecific manner on the central nervous system. More recent studies, however, have revealed that the receptors for inhibitory neurotransmitters such as gamma-aminobutyric acid (GABA) and glycine are sensitive to clinically relevant concentrations of inhaled anaesthetics. The function of GABA(A) and glycine receptors is enhanced by a number of anaesthetics and alcohols, whereas activity of the related GABA rho1 receptor is reduced. We have used this difference in pharmacology to investigate the molecular basis for modulation of these receptors by anaesthetics and alcohols. By using chimaeric receptor constructs, we have identified a region of 45 amino-acid residues that is both necessary and sufficient for the enhancement of receptor function. Within this region, two specific amino-acid residues in transmembrane domains 2 and 3 are critical for allosteric modulation of both GABA(A) and glycine receptors by alcohols and two volatile anaesthetics. These observations support the idea that anaesthetics exert a specific effect on these ion-channel proteins, and allow for the future testing of specific hypotheses of the action of anaesthetics.