The cannabinoid CB1 receptor antagonists rimonabant (SR141716) and AM251 directly potentiate GABA(A) receptors.

BACKGROUND AND PURPOSE: Rimonabant (SR141716) and the structurally related AM251 are widely used in pharmacological experiments as selective cannabinoid receptor CB(1) antagonists / inverse agonists. Concentrations of 0.5-10 µM are usually applied in in vitro experiments. We intended to show that these drugs did not act at GABA(A) receptors but found a significant positive allosteric modulation instead. EXPERIMENTAL APPROACH: Recombinant GABA(A) receptors were expressed in Xenopus oocytes. Receptors were exposed to AM251 or rimonabant in the absence and presence of GABA. Standard electrophysiological techniques were used to monitor the elicited ionic currents. KEY RESULTS: AM251 dose-dependently potentiated responses to 0.5 µM GABA at the recombinant α(1) ß(2) γ(2) GABA(A) receptor with an EC(50) below 1 µM and a maximal potentiation of about eightfold. The Hill coefficient indicated that more than one binding site for AM251 was located in this receptor. Rimonabant had a lower affinity, but a fourfold higher efficacy. AM251 potentiated also currents mediated by α(1) ß(2) , α(x) ß(2) γ(2) (x = 2,3,5,6), α(1) ß(3) γ(2) and α(4) ß(2) δ GABA(A) receptors, but not those mediated by α(1) ß(1) γ(2) . Interestingly, the CB(1) receptor antagonists LY320135 and O-2050 did not significantly affect α(1) ß(2) γ(2) GABA(A) receptor-mediated currents at concentrations of 1 µM. CONCLUSIONS AND IMPLICATIONS: This study identified rimonabant and AM251 as positive allosteric modulators of GABA(A) receptors. Thus, potential GABAergic effects of commonly used concentrations of these compounds should be considered in in vitro experiments, especially at extrasynaptic sites where GABA concentrations are low. LINKED ARTICLES: This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

Impact of subunit positioning on GABAA receptor function.

The major isoforms of the GABAA (gamma-aminobutyric acid type A) receptor are composed of two alpha, two beta and one gamma subunit. Thus alpha and beta subunits occur twice in the receptor pentamer. As it is well documented that different isoforms of alpha and beta subunits can co-exist in the same pentamer, the question is raised whether the relative position of a subunit isoform affects the functional properties of the receptor. We have used subunit concatenation to engineer receptors of well-defined subunit arrangement to study this question. Although all five subunits may be concatenated, we have focused on the combination of triple and dual subunit constructs. We review here what is known so far on receptors containing simultaneously alpha1 and alpha6 subunits and receptors containing beta1 and beta2 subunits. Subunit concatenation may not only be used to study receptors containing two different subunit isoforms, but also to introduce a point mutation into a defined position in receptors containing either two alpha or beta subunits, or to study the receptor architecture of receptors containing unconventional GABAA receptor subunits. Similar approaches may be used to characterize other members of the pentameric ligand-gated ion channel family, including nicotinic acetylcholine receptors, glycine receptors and 5-HT3 (5-hydroxytryptamine) receptors.

Abolition of zolpidem sensitivity in mice with a point mutation in the GABAA receptor gamma2 subunit.

Agonists of the allosteric benzodiazepine site of GABAA receptors bind at the interface of the alpha and gamma subunits. Here, we tested the in vivo contribution of the gamma2 subunit to the actions of zolpidem, an alpha1 subunit selective benzodiazepine agonist, by generating mice with a phenylalanine (F) to isoleucine (I) substitution at position 77 in the gamma2 subunit. The gamma2F77I mutation has no major effect on the expression of GABAA receptor subunits in the cerebellum. The potency of zolpidem, but not that of flurazepam, for the inhibition of [3H]flunitrazepam binding to cerebellar membranes is greatly reduced in gamma2I77/I77 mice. Zolpidem (1 microM) increased both the amplitude and decay of miniature inhibitory postsynaptic currents (mIPSCs) in Purkinje cells of control C57BL/6 (34% and 92%, respectively) and gamma2F77/F77 (20% and 84%) mice, but not in those of gamma2F77I mice. Zolpidem tartrate had no effect on exploratory activity (staircase test) or motor performance (rotarod test) in gamma2I77/I77 mice at doses up to 30 mg/kg (i.p.) that strongly sedated or impaired the control mice. Flurazepam was equally effective in enhancing mIPSCs and disrupting performance in the rotarod test in control and gamma2I77/I77 mice. These results show that the effect of zolpidem, but not flurazepam, is selectively eliminated in the brain by the gamma2F77I point mutation.

The relative amount of cRNA coding for gamma2 subunits affects stimulation by benzodiazepines in GABA(A) receptors expressed in Xenopus oocytes.

Benzodiazepine (BZD) potentiation of GABA-activated Cl(-)-current (I(GABA)) in recombinant GABA(A) receptors requires the presence of the gamma subunit. When alpha1, beta2 and gamma2S cRNA are expressed in a 1:1:1 ratio in Xenopus oocytes, BZD potentiation of I(GABA) is submaximal, variable and diminishes over time. Potentiation by BZDs is increased, more reproducible and is stabilized over time by increasing the relative amount of cRNA coding for the gamma2S subunit. In addition, GABA EC(50) values for alpha1beta2gamma2 (1:1:1) receptors are intermediate to values measured for alpha1beta2 (1:1) and alpha1beta2gamma2 (1:1:10) receptors. We conclude that co-expression of equal ratios of alpha1, beta2 and gamma2 subunits in Xenopus oocytes produces a mixed population of alpha1beta2 and alpha1beta2gamma2 receptors. Therefore, for accurate measurements of BZD potentiation it is necessary to inject a higher ratio of gamma2 subunit cRNA relative to alpha1 and beta2 cRNA. This results in a purer population of alpha1beta2gamma2 receptors.

Subunit arrangement of gamma-aminobutyric acid type A receptors.

The GABA(A) receptors are ligand-gated chloride channels. The subunit stoichiometry of the receptors is controversial; four, five, or six subunits per receptor molecule have been proposed for alphabeta receptors, whereas alphabetagamma receptors are assumed to be pentamers. In this study, alpha-beta and beta-alpha tandem cDNAs from the alpha1 and beta2 subunits of the GABA(A) receptor were constructed. We determined the minimal length of the linker that is required between the two subunits for functional channel expression for each of the tandem constructs. 10- and 23-amino acid residues are required for alpha-beta and beta-alpha, respectively. The tandem constructs either alone or in combination with each other failed to express functional channels in Xenopus oocytes. Therefore, we can exclude tetrameric or hexameric alphabeta GABA(A) receptors. We can also exclude proteolysis of the tandem constructs. In addition, the tandem constructs were combined with single alpha, beta, or gamma subunits to allow formation of pentameric arrangements. In contrast to the combination with alpha subunits, the combination with either beta or gamma subunits led to expression of functional channels. Therefore, a pentameric arrangement containing two alpha1 and three beta2 subunits is proposed for the receptor composed of alpha and beta subunits. Our findings also favor an arrangement betaalphagammabetaalpha for the receptor composed of alpha, beta, and gamma subunits.

Differential cross talk of ROD compounds with the benzodiazepine binding site.

We have recently identified a novel class of allosteric modulators of GABA(A) receptors, the ROD compounds that are structurally related to bicuculline. Here, the relationship of their site of action relative to other known modulatory sites of this receptor was investigated. Two types of ROD compounds, R1 (ROD164A, ROD185) and R2 (ROD222 and ROD259) could be differentiated. R1 compounds competitively inhibited binding of benzodiazepines in alpha1beta2gamma2 receptors, and their functional effects were partially inhibited by the benzodiazepine antagonist Ro15-1788 in a noncompetitive manner. The enhancement by an R1 compound was not additive with that by diazepam. R2 compounds in contrast failed to inhibit binding of benzodiazepines; the R2 compounds' functional effects were not inhibited by the benzodiazepine antagonist. The enhancement by an R2 compound was additive with that by diazepam. In contrast to benzodiazepines, both R1 and R2 type compounds were still able to enhance alpha1beta2 receptors. ROD164A in alpha1beta2gamma2 receptors was found to be partially antagonized by Ro15-1788 in a noncompetitive way. ROD178B did not affect gamma-aminobutyric acid induced currents, but was able to inhibit both enhancement by R1 and R2 type compounds as well as enhancement by diazepam. R1 and R2 type compounds as well as diazepam enhanced pentobarbital-induced currents in a Ro15-1788-sensitive way. We conclude that R1 type compounds act at the benzodiazepine binding site and additionally at a different R1 site, and that the R1, but not the R2 site is allosterically coupled to the benzodiazepine binding site. ROD178B is a competitive antagonist at the R1 site in that it shows allosteric interaction with the benzodiazepine binding site and displacement of benzodiazepines, and a negative allosteric modulator at the R2 site.

Interaction between GABA(A) receptor beta subunits and the multifunctional protein gC1q-R.

gamma-Aminobutyric acid type A (GABA(A)) receptors were immunopurified from bovine brain using a monoclonal antibody directed against the alpha1 subunit. Of the several proteins that copurified, a 34-kDa protein was analyzed further. After enrichment and tryptic proteolysis, the resulting fragments were sequenced, and the protein was identified as gC1q-R. Using anti-gC1q-R and anti-GABA(A) receptor antibodies, mutual coimmunoprecipitation could be demonstrated from solubilized rat brain membranes. The stability of this interaction was estimated to be very high. Using the yeast two-hybrid system, various GABA(A) receptor subunit intracellular loop constructs were tested for an interaction with gC1q-R. All beta subunits, but not alpha 1 and gamma 2 subunits, were found to bind to gC1q-R. NH(2)- and COOH-terminally truncated beta 2 subunit loops were used to find the region responsible for the interaction with gC1q-R. A stretch of 15 amino acids containing 7 positively charged residues was identified (amino acids 399--413). This region contains residue Ser-410, which is a protein kinase substrate, and it is known that phosphorylation of this residue leads to an alteration in receptor activity. Localization studies suggested a predominantly intracellular localization. Our observations therefore suggest a tight interaction between gC1q-R and the GABA(A) receptor which might be involved in receptor biosynthesis or modulation of the mature function.

Two novel residues in M2 of the gamma-aminobutyric acid type A receptor affecting gating by GABA and picrotoxin affinity.

An amino acid residue was found in M2 of gamma-aminobutyric acid (GABA) type A receptors that has profound effects on the binding of picrotoxin to the receptor and therefore may form part of its binding pocket. In addition, it strongly affects channel gating. The residue is located N-terminally to residues suggested so far to be important for channel gating. Point mutated alpha1beta(3) receptors were expressed in Xenopus oocytes and analyzed using the electrophysiological techniques. Coexpression of the alpha(1) subunit with the mutated beta(3) subunit beta(3)L253F led to spontaneous picrotoxin-sensitive currents in the absence of GABA. Nanomolar concentrations of GABA further promoted channel opening. Upon washout of picrotoxin, a huge transient inward current was observed. The reversal potential of the inward current was indicative of a chloride ion selectivity. The amplitude of the inward current was strongly dependent on the picrotoxin concentration and on the duration of its application. There was more than a 100-fold decrease in picrotoxin affinity. A kinetic model is presented that mimics the gating behavior of the mutant receptor. The point mutation in the neighboring residue beta(3)A252V resulted in receptors that displayed an about 6-fold increased apparent affinity to GABA and an about 10-fold reduced sensitivity to picrotoxin.

Use of bicuculline, a GABA antagonist, as a template for the development of a new class of ligands showing positive allosteric modulation of the GABA(A) receptor.

Analogues of bicuculline devoid of the benzo ring fused to the lactone moiety were prepared by reacting 2-(tert-butyl-dimethylsiloxy)furans with 3,4-dihydroisoquinolinium salts. Some of these compounds (e.g., ROD185, 8) acted as modulators of the GABAA receptor, displacing ligands of the benzodiazepine binding site. They also strongly stimulated GABA currents mediated by recombinant GABA(A) receptors expressed in Xenopus oocytes.

Loreclezole as a simple functional marker for homomeric rho type GABA(C) receptors.

GABA(C) receptors are expressed in the whole brain, but predominantly in the retina. They can be identified by their unique pharmacology. The establishment of the entire pharmacology is, however, quite tedious. We show here that loreclezole dose dependently inhibits ionic currents elicited by GABA (gamma-aminobutyric acid) with an IC(50) of about 0.5 microM in homomeric rho1 GABA(C) receptors expressed in Xenopus oocytes. Thus, loreclezole may constitute a functional marker for these receptors.

A novel positive allosteric modulator of the GABA(A) receptor: the action of (+)-ROD188.

(+)-ROD188 was synthesized in the search for novel ligands of the GABA binding site. It shares some structural similarity with bicuculline. (+)-ROD188 failed to displace [(3)H]-muscimol in binding studies and failed to induce channel opening in recombinant rat alpha1beta2gamma2 GABA(A) receptors functionally expressed in Xenopus oocytes. (+)-ROD188 allosterically stimulated GABA induced currents. Displacement of [(3)H]-Ro15-1788 indicated a low affinity action at the benzodiazepine binding site. In functional studies, stimulation by (+)-ROD188 was little sensitive to the presence of 1 microM of the benzodiazepine antagonist Ro 15-1788, and (+)-ROD188 also stimulated currents mediated by alpha1beta2, indicating a major mechanism of action different from that of benzodiazepines. Allosteric stimulation by (+)-ROD188 was similar in alpha1beta2N265S as in unmutated alpha1beta2, while that by loreclezole was strongly reduced. (+)-ROD188 also strongly stimulated currents elicited by either pentobarbital or 5alpha-pregnan-3alpha-ol-20-one (3alpha-OH-DHP), in line with a mode of action different from that of barbiturates or neurosteroids as channel agonists. Stimulation by (+)-ROD188 was largest in alpha6beta2gamma2 (alpha6beta2gamma2>>alpha1beta2gamma2=alpha5beta2gamma2++ +>alpha2beta2ga mma2= alpha3beta2gamma2), indicating a unique subunit isoform specificity. Miniature inhibitory postsynaptic currents (mIPSC) in cultures of rat hippocampal neurons, caused by spontaneous release of GABA showed a prolonged decay time in the presence of 30 microM (+)-ROD188, indicating an enhanced synaptic inhibitory transmission.

Electrophysiological evidence for the coexistence of alpha1 and alpha6 subunits in a single functional GABA(A) receptor.

The subunit combinations alpha1beta2gamma2, alpha6beta2gamma2, and alpha1alpha6beta2gamma2 of the GABA(A) receptor were functionally expressed in Xenopus oocytes. The properties of the resulting ion currents were characterized by using electrophysiological techniques. The concentration-response curve of the channel agonist GABA for alpha1alpha6beta2gamma2 showed a single apparent component characterized by an EC(50) of 107 +/- 26 microM (n = 4). It was different from the one for alpha1beta2gamma2, which had an EC(50) of 41 +/- 9 microM (n = 4), that for alpha6beta2gamma2, with an EC(50) of 6.7 +/- 1.9 microM (n = 5), and those for alpha1beta2 and alpha1alpha6beta2. There was no appreciable functional expression of alpha6beta2. Allosteric responses of alpha1alpha6beta2gamma2 to diazepam were intermediate to those of alpha1beta2gamma2 and alpha6beta2gamma2, and allosteric responses to flumazenil were comparable to the ones for alpha1beta2gamma2. The inhibition by furosemide of the currents elicited by GABA in alpha1alpha6beta2gamma2 [IC(50) = 298 +/- 116 microM (n = 7), assuming only one component] was not identical with inhibition of alpha6beta2gamma2 (IC(50) = 38 +/- 2 microM, n = 4), alpha1beta2gamma2 (IC(50) = 5,610 +/- 910 microM, n = 5), or a mixture of these components (assuming two components). These findings indicate unambiguously the formation of functional GABA(A) receptors containing two different alpha subunits, alpha1 and alpha6, with properties different from those of alpha1beta2gamma2 and alpha6beta2gamma2. Furthermore, we provide evidence for the facts that in the Xenopus oocyte (a) the formation of the different receptor types depends on the relative abundance of cRNAs coding for the different receptor subunits and (b) that functional dual subunit combinations alphabeta do not form in the presence of cRNA coding for the gamma subunit.

Identification of amino acid residues of GABA(A) receptor subunits contributing to the formation and affinity of the tert-butylbicyclophosphorothionate binding site.

A chimeric GABA(A) receptor subunit was constructed that contained the beta3 sequence from the N-terminus to the first two amino acids of the second transmembrane (TM2) domain. The remaining part of this chimera had the sequence of the alpha1 subunit. On co-expression with alpha1 subunits, this chimera was able to form heterooligomeric channels that were open in the absence of GABA. Picrotoxin and tert-butylbicyclophosphorothionate (TBPS) were able to block these channels with low potency. These channels exhibited high-affinity [3H]muscimol but no high-affinity [35S]TBPS binding sites. Introduction of V251, A252, and L253 of the beta3 subunit into the chimera resulted in the formation of closed channels that could be opened by GABA. The introduction of A252 and L253 of the beta3 subunit into this chimera was sufficient to reconstitute the specific high-affinity [35S]TBPS binding site in receptors composed of the chimera and alpha1 subunits. Replacement of other amino acids of the TM2 region of the chimera with corresponding amino acids of the beta3 subunit modulated the affinity of this [35S]TBPS binding site. Results obtained provide important information on the structure-function relationship of GABA(A) receptors.

Role of the conserved lysine residue in the middle of the predicted extracellular loop between M2 and M3 in the GABA(A) receptor.

In alpha1, beta2, and gamma2 subunits of the gamma-aminobutyric acid A (GABA(A)) receptor, a conserved lysine residue occupies the position in the middle of the predicted extracellular loop between the transmembrane M2 and M3 regions. In all three subunits, this residue was mutated to alanine. Whereas the mutation in alpha1 and beta2 subunits resulted each in about a sixfold shift of the concentration-response curve for GABA to higher concentrations, no significant effect by mutation in the gamma subunit was detected. The affinity for the competitive inhibitor bicuculline methiodide was not affected by the mutations in either the alpha1 subunit or the beta2 subunit. Concentration-response curves for channel activation by pentobarbital were also shifted to higher concentrations by the mutation in the alpha and beta subunits. Binding of [3H]Ro 15-1788 was unaffected by the mutation in the alpha subunit, whereas the binding of [3H]muscimol was shifted to lower affinity. Mutation of the residue in the alpha1 subunit to E, Q, or R resulted in an about eight-, 10-, or fivefold shift, respectively, to higher concentrations of the concentration-response curve for GABA. From these observations, it is concluded that the corresponding residues on the alpha1 and beta2 subunits are involved more likely in the gating of the channel by GABA than in the binding of GABA or benzodiazepines.

Dual mode of stimulation by the beta-carboline ZK 91085 of recombinant GABA(A) receptor currents: molecular determinants affecting its action.

In electrophysiological measurements the beta-carboline ethyl 6-benzyloxy-beta-carboline-3-carboxylate (ZK 91085) acts as a positive allosteric modulator on rat recombinant alpha1beta2gamma2 GABA(A) receptors and binds with high affinity (IC50-1.5 nM) to the [3H]-flunitrazepam site. Flumazenil was able to partially counteract the current modulation. These observations indicate an action of ZK 91085 at the benzodiazepine binding site. At the dual subunit combination alpha1beta2, which lacks the gamma subunit required for benzodiazepine modulation, we still observed a potentiation of GABA currents. Thus ZK 91085 acts via an additional site on the channel. At the subunit combination alpha1beta1, ZK 91085 potentiation is strongly reduced as compared to alpha1beta2. In binding studies, ZK 91085 was able to decrease [35S]-TBPS binding in alpha1beta2gamma2 and alpha1beta2 but not in alpha1beta1. This selectivity of ZK 91085 for receptors containing the beta2 isoform over those containing the beta1 isoform is reminiscent of the action of loreclezole. To identify amino acid residues important for the second type of modulation, we functionally compared wild type alpha1beta2 and mutant receptors for stimulation by ZK 91085. The mutation beta2N265S, that abolishes loreclezole effects, also abolishes ZK 91085 stimulation. The mutation beta2Y62L increased stimulation by ZK 91085 3-4 fold, locating an influencing entity of the second type of action of ZK 91085 at an alpha/beta subunit interface. Structural intermediates of ZK 91085 and the beta-carboline abecarnil, the latter of which only slightly potentiated GABA currents in alpha1/beta2, were analysed to determine structural requirements for modulation. ZK 91085 thus allosterically stimulates the GABA(A) receptor through two sites of action: the benzodiazepine site and the loreclezole site in contrast to classical beta-carbolines, that confer negative allosteric modulation through the benzodiazepine site.

EDPC: a novel high affinity ligand for the benzodiazepine site on rat GABA(A) receptors.

Rat recombinant alpha1beta2gamma2 gamma-aminobutyric acid type A (GABAA) receptors were functionally expressed in Xenopus laevis oocytes and analyzed for the action of EDPC (Ethyl 3-(1,3-dithian-2-yl)-1H-pyrrolo[2,3-c]pyridine-5-carboxylate) using electrophysiological techniques. EDPC inhibited GABA currents at low concentrations (IC50 approximately/= 2 nM). The inhibition by 100 nM EDPC could be reversed by 1 microM of the benzodiazepine antagonistflumazenil (Ro 15-1788), indicating a negative allosteric modulation via the benzodiazepine binding site. In line with this conclusion are radioactive ligand binding studies. EDPC inhibited the binding of 2 nM [3H]flunitrazepam to membranes from the cerebellum or the cortex with IC50 values of about 8 and 25 nM, respectively.

A novel serine kinase with specificity for beta3-subunits is tightly associated with GABA(A) receptors.

Tuning of gamma-aminobutyric acid type A (GABA(A)) receptor function via phosphorylation of the receptor potentially allows neurons to modulate their inhibitory input. Several kinases, both of the serine-threonine kinase and the tyrosine kinase families, have been proposed as candidates for such a modulatory role in vivo. However, no GABA(A) receptor-phosphorylating kinase physically associated with the receptor has been identified so far on a molecular level. In this study, we demonstrate a GABA(A) receptor-associated protein serine kinase phosphorylating specifically beta3-subunits of native GABA(A) receptors. The characteristics of this novel kinase clearly distinguish it from enzymatic activities that have been shown so far to phosphorylate the GABA(A) receptor. We putatively identify this protein kinase as the previously described GTAP34 (GABA(A) receptor-tubulin complex-associated protein of molecular mass 34 kDa). Using expressed recombinant fusion proteins, we identify serine 408 as a major target of the phosphorylation reaction, whereas serine 407 is not phosphorylated. This demonstrates the high specificity of the kinase. Phosphorylation of serine 408 is known to result in a decreased receptor function. The direct association of this kinase with the receptor indicates an important physiological role.

The benzodiazepine binding pocket of recombinant alpha1beta2gamma2 gamma-aminobutyric acidA receptors: relative orientation of ligands and amino acid side chains.

Wild-type alpha1beta2gamma2 gamma-aminobutyric acid (GABA)A receptors and receptors containing a point-mutated subunit gamma2F77Y were expressed by transient transfection in human embryonic kidney 293 cells. Mutant receptors bound the benzodiazepine binding site ligand [3H]flumazenil with similar, subnanomolar affinity as wild-type receptor. Displacement studies with diazepam showed that the affinity for this compound was reduced 250-fold on mutation, indicating that the tyrosine hydroxyl group interferes with diazepam binding. This differential behavior then was used to find the chemical entity presumably interacting with the phenyalanine residue in position 77 of the gamma2 subunit of wild-type receptors. Thirty-four substances were analyzed in this respect. Our results suggest that the phenyl substituent of diazepam is located close to gammaF77. Similarly, we investigated the possible location of alpha1T206 and gamma2M130. Electrophysiological data obtained with the wild-type receptor furthermore suggest a simple overlap between positive allosteric modulators acting at the benzodiazepine binding site with its antagonists.

Amino acid residue 200 on the alpha1 subunit of GABA(A) receptors affects the interaction with selected benzodiazepine binding site ligands.

Mutant alph1 subunits of the GABA(A) receptor were coexpressed in combination with the wild-type beta2 and gamma2 subunits in human embryonic kidney (HEK) 293 cells. The binding properties of various benzodiazepine site ligands were determined by displacement of ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5a]-[1,4]benzodia zepine-3-carboxylate ([3H]Ro 15-1788). The mutation G200E led to a decrease in zolpidem and 3-methyl-6-[3-(trifluoromethyl)phenyl]-1,2,4-triazolo[4,3-b]pyridazine (CL 218872) affinity amounting to 16- and 8-fold. Receptors containing a conservative T206V substitution showed a 41- and 38-fold increase in methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) and CL 218872 affinity combined with a decrease in diazepam and zolpidem affinity, amounting to 7- and 10-fold. Two mutations, Q203A and Q203S showed almost no effects on the binding of benzodiazepine site ligands, indicating that this residue is not involved in the binding of benzodiazepines and related compounds.