Actions of agonists, fipronil and ivermectin on the predominant in vivo splice and edit variant (RDLbd, I/V) of the Drosophila GABA receptor expressed in Xenopus laevis oocytes

Synaptic architecture of leg and wing premotor control networks in Drosophila

Animal movement is controlled by motor neurons (MNs), which project out of the central nervous system to activate muscles1. MN activity is coordinated by complex premotor networks that facilitate the contribution of individual muscles to many different behaviours2-6. Here we use connectomics7 to analyse the wiring logic of premotor circuits controlling the Drosophila leg and wing. We find that both premotor networks cluster into modules that link MNs innervating muscles with related functions. Within most leg motor modules, the synaptic weights of each premotor neuron are proportional to the size of their target MNs, establishing a circuit basis for hierarchical MN recruitment. By contrast, wing premotor networks lack proportional synaptic connectivity, which may enable more flexible recruitment of wing steering muscles. Through comparison of the architecture of distinct motor control systems within the same animal, we identify common principles of premotor network organization and specializations that reflect the unique biomechanical constraints and evolutionary origins of leg and wing motor control.

Functional validation of A2'N mutation of the RDL GABA receptor against fipronil via molecular modeling and genome engineering in drosophila

BACKGROUND

Insect RDL (resistant to dieldrin) receptors are essential pentameric ligand-gated chloride channels that mediate the neuroinhibitory effect of GABA, the chief inhibitory neurotransmitter in the central nervous system. These receptors serve as primary targets for various insecticides, including noncompetitive antagonists (NCAs) such as cyclodiene organochlorines and phenylpyrazoles, as well as allosteric modulators like meta-diamides and isoxazolines. This study focuses on a newly discovered A2'N mutation within the RDL receptors, identified in fipronil-resistant planthoppers. Despite in vitro electrophysiological studies have proposed its role in conferring target-site resistance, in vivo genetic functional validation of this mutation remains unexplored.

RESULTS

Our research employed toxicity bioassays, assessing various Rdl genotypes against a spectrum of insecticides, including fipronil, α-endosulfan, broflanilide, and isocycloseram. Results revealed distinct resistance profiles for A2'N and A2'S mutants, indicating different binding interactions of RDL receptors with NCAs. Significantly, the A2'N heterozygote showed substantial resistance to fipronil, despite its homozygous lethality. Molecular modeling and docking simulations further supported these findings, highlighting unique binding poses for fipronil and α-endosulfan.

CONCLUSION

This study confirmed that A2'N mutation of the RDL GABA receptor confer high resistance to fipronil in vivo. The observed resistance in A2'N mutants is likely attributable to a steric hindrance mechanism, wherein the introduction of larger side chains hampers fipronil binding, even in a heterozygous state. © 2023 Society of Chemical Industry.

Activity, selection response and molecular mode of action of the isoxazoline afoxolaner in Tetranychus urticae

BACKGROUND

Afoxolaner is a novel representative of the isoxazolines, a class of ectoparasiticides which has been commercialized for the control of tick and flea infestations in dogs. In this study, the biological efficacy of afoxolaner against the two-spotted spider mite Tetranychus urticae was evaluated. Furthermore, as isoxazolines are known inhibitors of γ-aminobutyric acid-gated chloride channels (GABACls), the molecular mode of action of afoxolaner on T. urticae GABACls (TuRdls) was studied using functional expression in Xenopus oocytes followed by two-electrode voltage-clamp (TEVC) electrophysiology, and results were compared with inhibition by fluralaner, fipronil and endosulfan. To examine the influence of known GABACl resistance mutations, H301A, I305T and A350T substitutions in TuRdl1 and a S301A substitution in TuRdl2 were introduced.

RESULTS

Bioasassays revealed excellent efficacy of afoxolaner against all developmental stages and no cross-resistance was found in a panel of strains resistant to most currently used acaricides. Laboratory selection over a period of 3 years did not result in resistance. TEVC revealed clear antagonistic activity of afoxolaner and fluralaner for all homomeric TuRdl1/2/3 channels. The introduction of single, double or triple mutations to TuRdl1 and TuRdl2 did not lower channel sensitivity. By contrast, both endosulfan and fipronil had minimal antagonistic activities against TuRdl1/2/3, and channels carrying single mutations, whereas the sensitivity of double and triple TuRdl1 mutants was significantly increased.

CONCLUSIONS

Our results demonstrate that afoxolaner is a potent antagonist of GABACls of T. urticae and has a powerful mode of action to control spider mites. © 2022 Society of Chemical Industry.

Novel insecticidal 1,6-dihydro-6-iminopyridazine derivatives as competitive antagonists of insect RDL GABA receptors

BACKGROUND

The ionotropic γ-aminobutyric acid (GABA) receptor (iGABAR) is an important target for insecticides and parasiticides. Our previous studies showed that competitive antagonists (CAs) of insect iGABARs have the potential to be used for developing novel insecticides and that the structural modification of gabazine (a representative CA of mammalian iGABARs) could lead to the identification of novel CAs of insect iGABARs.

RESULTS

In the present study, a novel series of 1,3-di- and 1,3,5-trisubstituted 1,6-dihydro-6-iminopyridazines (DIPs) was designed using a versatile strategy and synthesized using facile methods. Electrophysiological studies showed that several target DIPs (30 μM) exhibited excellent antagonistic activities against common cutworm and housefly iGABARs consisting of RDL subunits. The IC₅₀ values of 3-(4-methoxyphenyl), 3-(4-trifluoromethoxyphenyl), 3-(4-biphenylylphenyl), 3-(2-naphthyl), 3-(3,4-methylenedioxyphenyl), and 3,5-(4-methoxyphenyl) analogs ranged from 2.2 to 24.8 μM. Additionally, several 1,3-disubstituted DIPs, especially 3-(4-trifluoromethoxyphenyl) and 3-(3,4-methylenedioxyphenyl) analogs, exhibited moderate insecticidal activity against common cutworm larvae, with >60% mortality at a concentration of 100 mg kg^-1 . Molecular docking studies showed that the oxygen atom on the three-substituted aromatic ring could form a hydrogen bond with Arg254, which may enhance the activity of these DIPs against housefly iGABARs.

CONCLUSION

This systematic study indicated that the presence of a carboxyl side chain shorter by one methylene than that of gabazine at the 1-position of the pyridazine ring is effective for maintaining the stable binding of these DIPs in insect iGABARs. Our study provides important information for the design of novel insect iGABAR CAs. © 2022 Society of Chemical Industry.

Xenopus Oocytes: A Tool to Decipher Molecular Specificity of Insecticides towards Mammalian and Insect GABA—A Receptors

The number of insect GABA receptors (GABAr) available for expression studies has been recently increased by the cloning of the Acyrthosiphon pisum (pea aphid) RDL subunits. This large number of cloned RDL subunits from pest and beneficial insects opens the door to parallel pharmacological studies on the sensitivity of these different insect GABAr to various agonists or antagonists. The resulting analysis of the molecular basis of the species-specific GABAr responses to insecticides is necessary not only to depict and understand species toxicity, but also to help at the early identification of unacceptable toxicity of insecticides toward beneficial insects such as Apis mellifera (honeybees). Using heterologous expression in Xenopus laevis oocytes, and two-electrode voltage-clamp recording to assess the properties of the GABAr, we performed a comparative analysis of the pharmacological sensitivity of RDL subunits from A. pisum, A. mellifera and Varroa destructor GABAr to three pesticides (fipronil, picrotoxin and dieldrin). These data were compared to similar characterizations performed on two Homo sapiens GABA-A receptors (α₂β₂γ₂ and α₂β₂γ₂). Our results underline a global conservation of the pharmacological profiles of these receptors, with some interesting species specificities, nonetheless, and suggest that this approach can be useful for the early identification of poorly specific molecules.

Insect RDL Receptor Models for Virtual Screening: Impact of the Template Conformational State in Pentameric Ligand-Gated Ion Channels

The RDL receptor is one of the most relevant protein targets for insecticide molecules. It belongs to the pentameric ligand-gated ion channel (pLGIC) family. Given that the experimental structures of pLGICs are difficult to obtain, homology modeling has been extensively used for these proteins, particularly for the RDL receptor. However, no detailed assessments of the usefulness of homology models for virtual screening (VS) have been carried out for pLGICs. The aim of this study was to evaluate which are the determinant factors for a good VS performance using RDL homology models, specially analyzing the impact of the template conformational state. Fifteen RDL homology models were obtained based on different pLGIC templates representing the closed, open, and desensitized states. A retrospective VS process was performed on each model, and their performance in the prioritization of active ligands was assessed. In addition, the three best-performing models among each of the conformations were subjected to molecular dynamics simulations (MDS) in complex with a representative active ligand. The models showed variations in their VS performance parameters that were related to the structural properties of the binding site. VS performance tended to improve in more constricted binding cavities. The best performance was obtained with a model based on a template in the closed conformation. MDS confirmed that the closed model was the one that best represented the interactions with an active ligand. These results imply that different templates should be evaluated and the structural variations between their channel conformational states should be specially examined, providing guidelines for the application of homology modeling for VS in other proteins of the pLGIC family.

Heartworm disease - Overview, intervention, and industry perspective

Dirofilaria immitis, also known as heartworm, is a major parasitic threat for dogs and cats around the world. Because of its impact on the health and welfare of companion animals, heartworm disease is of huge veterinary and economic importance especially in North America, Europe, Asia and Australia. Within the animal health market many different heartworm preventive products are available, all of which contain active components of the same drug class, the macrocyclic lactones. In addition to compliance issues, such as under-dosing or irregular treatment intervals, the occurrence of drug-resistant heartworms within the populations in the Mississippi River areas adds to the failure of preventive treatments. The objective of this review is to provide an overview of the disease, summarize the current disease control measures and highlight potential new avenues and best practices for treatment and prevention.

Untangling a Gordian knot: the role of a GluCl3 I321T mutation in abamectin resistance in Tetranychus urticae

BACKGROUND

The cys-loop ligand-gated ion channels, including the glutamate-gated chloride channel (GluCl) and GABA-gated chloride channel (Rdl) are important targets for drugs and pesticides. The macrocyclic lactone abamectin primarily targets GluCl and is commonly used to control the spider mite Tetranychus urticae, an economically important crop pest. However, abamectin resistance has been reported for multiple T. urticae populations worldwide, and in several cases was associated with the mutations G314D in GluCl1 and G326E in GluCl3. Recently, an additional I321T mutation in GluCl3 was identified in several abamectin resistant T. urticae field populations. Here, we aim to functionally validate this mutation and determine its phenotypic strength.

RESULTS

The GluCl3 I321T mutation was introgressed into a T. urticae susceptible background by marker-assisted backcrossing, revealing contrasting results in phenotypic strength, ranging from almost none to 50-fold. Next, we used CRISPR-Cas9 to introduce I321T, G314D and G326E in the orthologous Drosophila GluCl. Genome modified flies expressing GluCl I321T were threefold less susceptible to abamectin, while CRISPRed GluCl G314D and G326E flies were lethal. Last, functional analysis in Xenopus oocytes revealed that the I321T mutation might reduce GluCl3 sensitivity to abamectin, but also suggested that all three T. urticae Rdls are affected by abamectin.

CONCLUSION

Three different techniques were used to characterize the role of I321T in GluCl3 in abamectin resistance and, combining all results, our analysis suggests that the I321T mutation has a complex role in abamectin resistance. Given the reported subtle effect, additional synergistic factors in resistance warrant more investigation. © 2020 Society of Chemical Industry.

Potential of Competitive Antagonists of Insect Ionotropic γ-Aminobutyric Acid Receptors as Insecticides

Ionotropic γ-aminobutyric acid (GABA) receptors (GABARs) represent an important insecticide target. Currently used GABAR-targeting insecticides are non-competitive antagonists (NCAs) of these receptors. Recent studies have demonstrated that competitive antagonists (CAs) of GABARs have functions of inhibiting insect GABARs similar to NCAs and that they also exhibit insecticidal activity. CAs have different binding sites and different mechanisms of action compared to those of NCAs. Therefore, GABAR CAs should have the potential to be developed into novel insecticides, which could be used to overcome the developed resistance of insect pests to conventional NCA insecticides. Although research on insect GABAR CAs has lagged behind that on mammalian GABAR CAs, research on the CAs of insect ionotropic GABARs has made great progress in recent years, and several series of heterocyclic compounds, such as 3-isoxazolols and 6-iminopyridazines, have been identified as insect GABAR CAs. In this review, we briefly summarize the design strategies, structures, and biological activities of the novel GABAR CAs that have been found in the past decade. Updated information about GABAR CAs may benefit the design and development of novel GABAR-targeting insecticides.

The interaction between abamectin and RDL in the carmine spider mite: a target site and resistant mechanism study

The γ-aminobutyric acid receptor (GABAR) has been identified as a target site of some commonly used insecticides. Our pervious study documented an interesting phenomenon, i.e. GABA accumulation was involved in abamectin resistance in the carmine spider mite, Tetranychus cinnabarinus. However, the mechanism of this phenomenon remains to be clarified. In this study, we investigated the interaction between abamectin and GABAR. Firstly, an artificial increase of GABA content was conducted in T. cinnabarinus and toxicity assays showed that GABA accumulation could indeed increase the tolerance of T. cinnabarinus to abamectin in vivo. Subsequently a GABAR of T. cinnabarinus, RDL2, was expressed in Xenopus oocytes and its sensitiveness to abamectin was detected. The results revealed that RDL2 showed significant responses to a series of GABA concentrations and GABA inhibited the effect of abamectin in vitro, providing direct evidence of the abamectin resistance mediated by GABA content. Our data confirmed that GABAR is the action target of abamectin and the GABA accumulation is one of the mechanisms of abamectin resistance in spider mites.

The effects of insecticides on two splice variants of the glutamate-gated chloride channel receptor of the major malaria vector, Anopheles gambiae

BACKGROUND AND PURPOSE

Between half to 1 million people die annually from malaria. Anopheles gambiae mosquitoes are major malaria vectors. Unfortunately, resistance has emerged to the agents currently used to control A. gambiae, creating a demand for novel control measures. The pentameric glutamate-gated chloride channel (GluCl) expressed in the muscle and nerve cells of these organisms are a potentially important biological target for malaria control. The pharmacological properties of Anophiline GluCl receptors are, however, largely unknown. Accordingly, we compared the efficacy of four insecticides (lindane, fipronil, picrotoxin, and ivermectin) on two A. gambiae GluCl receptor splice variants with the aim of providing a molecular basis for designing novel anti-malaria treatments.

EXPERIMENTAL APPROACH

The A. gambiae GluCl receptor b1 and c splice variants were expressed homomerically in Xenopus laevis oocytes and studied with electrophysiological techniques, using two-electrode voltage-clamp.

KEY RESULTS

The b1 and c GluCl receptors were activated with similar potencies by glutamate and ivermectin. Fipronil was more potent than picrotoxin and lindane at inhibiting glutamate- and ivermectin-gated currents. Importantly, b1 GluCl receptors exhibited reduced sensitivity to picrotoxin and lindane. They also recovered from these effects to a greater extent than c GluCl receptors CONCLUSIONS AND IMPLICATIONS: The two splice variant subunits exhibited differential sensitivities to multiple, structurally divergent insecticides, without accompanying changes in the sensitivity to the endogenous neurotransmitter, glutamate, implying that drug resistance may be caused by alterations in relative subunit expression levels, without affecting physiological function. Our results strongly suggest that it should be feasible to develop novel subunit-specific pharmacological agents.

Species specific RNA A-to-I editing of mosquito RDL modulates GABA potency and influences agonistic, potentiating and antagonistic actions of ivermectin

The insect GABA receptor, RDL, is the target of several classes of pesticides. The peptide sequences of RDL are generally highly conserved between diverse insects. However, RNA A-to-I editing can effectively alter amino acid residues of RDL in a species specific manner, which can affect the potency of GABA and possibly insecticides. We report here that RNA A-to-I editing alters the gene products of Rdl in three mosquito disease vectors, recoding five amino acid residues in RDL of Aedes aegypti and six residues in RDLs of Anopheles gambiae and Culex pipiens, which is the highest extent of editing in RDL observed to date. Analysis of An. gambiae Rdl cDNA sequences identified 24 editing isoforms demonstrating a considerable increase in gene product diversity. RNA editing influenced the potency of the neurotransmitter, GABA, on An. gambiae RDL editing isoforms expressed in Xenopus laevis oocytes, as demonstrated by EC₅₀s ranging from 5 ± 1 to 246 ± 41 μM. Fipronil showed similar potency on different editing isoforms, with IC₅₀s ranging from 0.18 ± 0.08 to 0.43 ± 0.09 μM. In contrast, editing of An. gambiae RDL affected the activating, potentiating and inhibiting actions of ivermectin. For example, ivermectin potentiated currents induced by GABA at the EC₂₀ concentration in the unedited isoform but not in the fully edited variant. Editing of a residue in the first transmembrane domain or the cys-loop influenced this potentiation, highlighting residues involved in the allosteric mechanisms of cys-loop ligand-gated ion channels. Understanding the interactions of ivermectin with molecular targets may have relevance to mosquito control in areas where people are administered with ivermectin to treat parasitic diseases.

Influence of the RDL A301S mutation in the brown planthopper Nilaparvata lugens on the activity of phenylpyrazole insecticides

We discovered the A301S mutation in the RDL GABA-gated chloride channel of fiprole resistant rice brown planthopper, Nilaparvata lugens populations by DNA sequencing and SNP calling via RNASeq. Ethiprole selection of two field N. lugens populations resulted in strong resistance to both ethiprole and fipronil and resulted in fixation of the A301S mutation, as well as the emergence of another mutation, Q359E in one of the selected strains. To analyse the roles of these mutations in resistance to phenylpyrazoles, three Rdl constructs: wild type, A301S and A301S+Q359E were expressed in Xenopus laevis oocytes and assessed for their sensitivity to ethiprole and fipronil using two-electrode voltage-clamp electrophysiology. Neither of the mutant Rdl subtypes significantly reduced the antagonistic action of fipronil, however there was a significant reduction in response to ethiprole in the two mutated subtypes compared with the wild type. Bioassays with a Drosophila melanogaster strain carrying the A301S mutation showed strong resistance to ethiprole but not fipronil compared to a strain without this mutation, thus further supporting a causal role for the A301S mutation in resistance to ethiprole. Homology modelling of the N. lugens RDL channel did not suggest implications of Q359E for fiprole binding in contrast to A301S located in transmembrane domain M2 forming the channel pore. Synergist bioassays provided no evidence of a role for cytochrome P450s in N. lugens resistance to fipronil and the molecular basis of resistance to this compound remains unknown. In summary this study provides strong evidence that target-site resistance underlies widespread ethiprole resistance in N. lugens populations.

Electrophysiological characterization of ivermectin triple actions on Musca chloride channels gated by l-glutamic acid and γ-aminobutyric acid

Ivermectin (IVM) is a macrocyclic lactone that exerts antifilarial, antiparasitic, and insecticidal effects on nematodes and insects by acting on l-glutamic acid-gated chloride channels (GluCls). IVM also acts as an allosteric modulator of various other ion channels. Although the IVM binding site in the Caenorhabditis elegans GluCl was identified by X-ray crystallographic analysis, the mechanism of action of IVM in insects is not well defined. We therefore examined the action of IVM on the housefly (Musca domestica) GluCl and γ-aminobutyric acid (GABA)-gated ion channel (GABACl). For both channels, IVM induced currents by itself, potentiated currents induced by low concentrations of agonists, and inhibited currents induced by high concentrations of agonists. Despite exerting common actions on both types of channels, GluCls were more susceptible to IVM actions than GABACls, indicating that GluCls are the primary target of IVM. Substitution of an amino acid residue in the third transmembrane segment (G312M in GluCls, and G333A and G333M in GABACls) resulted in significantly reduced levels or loss of activation, potentiation, and antagonism of the channels, indicating that these three actions result from the interaction of IVM with amino acid residues in the transmembrane intersubunit crevice.

Novel GABA receptor pesticide targets

The γ-aminobutyric acid (GABA) receptor has four distinct but overlapping and coupled targets of pesticide action importantly associated with little or no cross-resistance. The target sites are differentiated by binding assays with specific radioligands, resistant strains, site-directed mutagenesis and molecular modeling. Three of the targets are for non-competitive antagonists (NCAs) or channel blockers of widely varied chemotypes. The target of the first generation (20th century) NCAs differs between the larger or elongated compounds (NCA-IA) including many important insecticides of the past (cyclodienes and polychlorocycloalkanes) or present (fiproles) and the smaller or compact compounds (NCA-IB) highly toxic to mammals and known as cage convulsants, rodenticides or chemical threat agents. The target of greatest current interest is designated NCA-II for the second generation (21st century) of NCAs consisting for now of isoxazolines and meta-diamides. This new and uniquely different NCA-II site apparently differs enough between insects and mammals to confer selective toxicity. The fourth target is the avermectin site (AVE) for allosteric modulators of the chloride channel. NCA pesticides vary in molecular surface area and solvent accessible volume relative to avermectin with NCA-IBs at 20-22%, NCA-IAs at 40-45% and NCA-IIs at 57-60%. The same type of relationship relative to ligand-docked length is 27-43% for NCA-IBs, 63-71% for NCA-IAs and 85-105% for NCA-IIs. The four targets are compared by molecular modeling for the Drosophila melanogaster GABA-R. The principal sites of interaction are proposed to be: pore V1' and A2' for NCA-IB compounds; pore A2', L6' and T9' for NCA-IA compounds; pore T9' to S15' in proximity to M1/M3 subunit interface (or alternatively an interstitial site) for NCA-II compounds; and M1/M3, M2 interfaces for AVE. Understanding the relationships of these four binding sites is important in resistance management and in the discovery and use of safe and effective pest control agents.

Comparison between the modes of action of novel meta-diamide and macrocyclic lactone insecticides on the RDL GABA receptor

Macrocyclic lactones, avermectins, and milbemycins are widely used to control arthropods, nematodes, and endo- and ectoparasites in livestock and pets. Their main targets are glutamate-gated chloride channels. Furthermore, macrocyclic lactones reportedly interact with insect RDL γ-aminobutyric acid (GABA) receptors, but their modes of action on insect RDL GABA receptors remain unknown. In this study, we attempted to better understand the modes of action of macrocyclic lactones on RDL GABA receptors. We observed that ivermectin and milbemectin behaved as allosteric agonists of the Drosophila RDL GABA receptor. G336A, G336S, and G336T mutations had profound effects on the activities of ivermectin and milbemectin, and a G336M mutation abolished the allosteric agonist and antagonist activities of these macrocyclic lactones. These results suggest that G336 in TM3 of the Drosophila RDL GABA receptor is important for the binding of macrocyclic lactones. Recently, it has been suggested that a novel RDL GABA receptor antagonist, 3-benzamido-N-(2-bromo-4-perfluoroisopropyl-6-(trifluoromethyl)phenyl)-2-fluorobenzamide (meta-diamide 7), binds to the transmembrane intersubunit pocket near G336 in the Drosophila RDL GABA receptor. Thus, we compared the effects of mutations around G336 and A302 mutations in TM2 on the activities of macrocyclic lactone and meta-diamide 7. The effects of L281C, V340Q, V340N, A302S, and A302N mutations on the activity of meta-diamide 7 differed from those on ivermectin and milbemectin. Molecular modeling studies showed that macrocyclic lactones docked in the intersubunit pocket near G336 in the Drosophila RDL GABA receptor in the open state. In contrast, meta-diamide 7 docked into the Drosophila RDL GABA receptor in the closed state. This suggests that the modes of action of macrocyclic lactone binding to the wild-type Drosophila RDL GABA receptor differ from those of meta-diamide binding.

Exon 3 splicing and mutagenesis identify residues influencing cell surface density of heterologously expressed silkworm (Bombyx mori) glutamate-gated chloride channels

Glutamate-gated chloride channels (GluCls) mediate fast inhibitory neurotransmission in invertebrate nervous systems. Insect GluCls show alternative splicing, and, to determine its impact on channel function and pharmacology, we isolated GluCl cDNAs from larvae of the silkworm (Bombyx mori). We show that six B. mori glutamate-gated chloride channel variants are generated by splicing in exons 3 and 9 and that exons 3b and 3c are common in the brain and third thoracic ganglion. When expressed in Xenopus laevis oocytes, the three functional exon 3 variants (3a, b, c) all had similar EC50 values for l-glutamate and ivermectin (IVM); however, Imax (the maximum l-glutamate- and IVM-induced response of the channels at saturating concentrations) differed strikingly between variants, with the 3c variant showing the largest l-glutamate- and IVM-induced responses. By contrast, a partial deletion detected in exon 9 had a much smaller impact on l-glutamate and IVM actions. Binding assays using [(3)H]IVM indicate that diversity in IVM responses among the GluCl variants is mainly due to the impact on channel assembly, altering receptor cell surface numbers. GluCl variants expressed in HEK293 cells show that structural differences influenced Bmax but not Kd values of [(3)H]IVM. Domain swapping and site-directed mutagenesis identified four amino acids in exon 3c as hot spots determining the highest amplitude of the l-glutamate and IVM responses. Modeling the GluCl 3a and 3c variants suggested that three of the four amino acids contribute to intersubunit contacts, whereas the other interacts with the TM2-TM3 linker, influencing the receptor response.