Chlorpromazine and QX222 block 5-HT3 receptors in N1E-115 neuroblastoma cells

Allosteric binding site in a Cys-loop receptor ligand-binding domain unveiled in the crystal structure of ELIC in complex with chlorpromazine

Pentameric ligand-gated ion channels or Cys-loop receptors are responsible for fast inhibitory or excitatory synaptic transmission. The antipsychotic compound chlorpromazine is a widely used tool to probe the ion channel pore of the nicotinic acetylcholine receptor, which is a prototypical Cys-loop receptor. In this study, we determine the molecular determinants of chlorpromazine binding in the Erwinia ligand-gated ion channel (ELIC). We report the X-ray crystal structures of ELIC in complex with chlorpromazine or its brominated derivative bromopromazine. Unexpectedly, we do not find a chlorpromazine molecule in the channel pore of ELIC, but behind the β8-β9 loop in the extracellular ligand-binding domain. The β8-β9 loop is localized downstream from the neurotransmitter binding site and plays an important role in coupling of ligand binding to channel opening. In combination with electrophysiological recordings from ELIC cysteine mutants and a thiol-reactive derivative of chlorpromazine, we demonstrate that chlorpromazine binding at the β8-β9 loop is responsible for receptor inhibition. We further use molecular-dynamics simulations to support the X-ray data and mutagenesis experiments. Together, these data unveil an allosteric binding site in the extracellular ligand-binding domain of ELIC. Our results extend on previous observations and further substantiate our understanding of a multisite model for allosteric modulation of Cys-loop receptors.

Structural basis of open channel block in a prokaryotic pentameric ligand-gated ion channel

The flow of ions through cation-selective members of the pentameric ligand-gated ion channel family is inhibited by a structurally diverse class of molecules that bind to the transmembrane pore in the open state of the protein. To obtain insight into the mechanism of channel block, we have investigated the binding of positively charged inhibitors to the open channel of the bacterial homolog GLIC by using X-ray crystallography and electrophysiology. Our studies reveal the location of two regions for interactions, with larger blockers binding in the center of the membrane and divalent transition metal ions binding to the narrow intracellular pore entry. The results provide a structural foundation for understanding the interactions of the channel with inhibitors that is relevant for the entire family.

Physiological and immunocytochemical evidence that glutamatergic neurotransmission is involved in the activation of arm autotomy in the featherstar Antedon mediterranea (Echinodermata: Crinoidea)

The crinoid echinoderm Antedon mediterranea autotomises its arms at specialised skeletal joints known as syzygies that occur at regular intervals along the length of each arm. Detachment is achieved through the nervously mediated destabilisation of ligament fibres at a particular syzygy. The aim of this investigation was to identify neurotransmitters that are involved in the autotomy response. Physiological experiments were conducted on isolated preparations of syzygial joints, which can be induced to undergo autotomy-like fracture by applying stimulatory agents such as elevated [K(+)](o). Initial experiments with elevated [K(+)](o) showed that the autotomy threshold (the minimum amount of stimulation required to provoke autotomy) is lowest in syzygies at the arm base and rises distally. Of a range of neurotransmitter agonists tested, only l-glutamate invoked syzygial destabilisation, as did its analogues l-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate, but not l-(+)-2-amino-4-phosphonobutyrate (l-AP4) or N-methyl-d-aspartate (NMDA). The implication that l-glutamate stimulates syzygial fracture through AMPA/kainate-like receptors was supported by the finding that the action of l-glutamate was inhibited by the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Acetylcholine depressed the response of syzygial preparations to l-glutamate, suggesting a possible mechanism by which the autotomy threshold could be varied constitutively and facultatively. An immunocytochemical method employing a polyclonal antibody against l-glutamate conjugated to glutaraldehyde revealed l-glutamate-like immunoreactivity in all components of the putative neural pathway controlling the autotomy reflex, including the epidermis, brachial nerve, syzygial nerves and cellular elements close to the syzygial ligaments. We conclude that it is highly probable that l-glutamate acts as an excitatory neurotransmitter in the activation of arm autotomy in A. mediterranea.

5-HT(3) receptors: role in disease and target of drugs

Serotonin type 3 (5-HT(3)) receptors are pentameric ion channels belonging to the superfamily of Cys-loop receptors. Receptor activation either leads to fast excitatory responses or modulation of neurotransmitter release depending on their neuronal localisation. 5-HT(3) receptors are known to be expressed in the central nervous system in regions involved in the vomiting reflex, processing of pain, the reward system, cognition and anxiety control. In the periphery they are present on a variety of neurons and immune cells. 5-HT(3) receptors are known to be involved in emesis, pain disorders, drug addiction, psychiatric and GI disorders. Progress in molecular genetics gives direction to personalised medical strategies for treating complex diseases such as psychiatric and functional GI disorders and unravelling individual drug responses in pharmacogenetic approaches. Here we discuss the molecular basis of 5-HT(3) receptor diversity at the DNA and protein level, of which our knowledge has greatly extended in the last decade. We also evaluate their role in health and disease and describe specific case-control studies addressing the involvement of polymorphisms of 5-HT3 subunit genes in complex disorders and responses to drugs. Furthermore, we focus on the actual state of the pharmacological knowledge concerning not only classical 5-HT(3) antagonists--the setrons--but also compounds of various substance classes targeting 5-HT(3) receptors such as anaesthetics, opioids, cannabinoids, steroids, antidepressants and antipsychotics as well as natural compounds derived from plants. This shall point to alternative treatment options modulating the 5-HT(3) receptor system and open new possibilities for drug development in the future.

Toxic effects of chlorpromazine on Carassius auratus and its oxidative stress

Under laboratory conditions, ecotoxicological effects of chlorpromazine (CPZ) on freshwater goldfish (Carassius auratus) were examined using the toxic culture experiment. The results showed that the median lethal concentration (LC(50)) of CPZ toxic to Carassius auratus in 24, 48 and 96 h was 1.11, 0.43 and 0.32 mg/L, respectively. Thus, CPZ is an extreme toxicant to goldfish. Furthermore, there were significantly positive correlations between the ecotoxicological effects of CPZ and its concentrations, and the toxicity became higher as the exposure time increased. The activity of superoxide dismutase (SOD) and catalase (CAT) in goldfish livers was significantly influenced by CPZ. At the same exposure time, the activity of SOD reduced first, and increased then, whereas the activity of CAT enhanced first and decreased then. At the same exposure levels of CPZ, the activity of SOD and CAT changed similarly, decreased first, then increased and decreased at last. Within the range of exposure concentrations, the changes in the activity of CAT can more easily reflect the oxidation stress in Carassius auratus by CPZ than those of SOD.

High affinity binding of epibatidine to serotonin type 3 receptors

Epibatidine and mecamylamine are ligands used widely in the study of nicotinic acetylcholine receptors (nAChRs) in the central and peripheral nervous systems. In the present study, we find that nicotine blocks only 75% of (125)I-epibatidine binding to rat brain membranes, whereas ligands specific for serotonin type 3 receptors (5-HT(3)Rs) block the remaining 25%. (125)I-Epibatidine binds with a high affinity to native 5-HT(3)Rs of N1E-115 cells and to receptors composed of only 5-HT(3A) subunits expressed in HEK cells. In these cells, serotonin, the 5-HT(3)R-specific antagonist MDL72222, and the 5-HT(3)R agonist chlorophenylbiguanide readily competed with (125)I-epibatidine binding to 5-HT(3)Rs. Nicotine was a poor competitor for (125)I-epibatidine binding to 5-HT(3)Rs. However, the noncompetitive nAChR antagonist mecamylamine acted as a potent competitive inhibitor of (125)I-epibatidine binding to 5-HT(3)Rs. Epibatidine inhibited serotonin-induced currents mediated by endogenous 5-HT(3)Rs in neuroblastoma cell lines and 5-HT(3A)Rs expressed in HEK cells in a competitive manner. Our results demonstrate that 5-HT(3)Rs are previously uncharacterized high affinity epibatidine binding sites in the brain and indicate that epibatidine and mecamylamine act as 5-HT(3)R antagonists. Previous studies that depended on epibatidine and mecamylamine as nAChR-specific ligands, in particular studies of analgesic properties of epibatidine, may need to be reinterpreted with respect to the potential role of 5-HT(3)Rs.

Modular design of Cys-loop ligand-gated ion channels: functional 5-HT3 and GABA rho1 receptors lacking the large cytoplasmic M3M4 loop

Cys-loop receptor neurotransmitter-gated ion channels are pentameric assemblies of subunits that contain three domains: extracellular, transmembrane, and intracellular. The extracellular domain forms the agonist binding site. The transmembrane domain forms the ion channel. The cytoplasmic domain is involved in trafficking, localization, and modulation by cytoplasmic second messenger systems but its role in channel assembly and function is poorly understood and little is known about its structure. The intracellular domain is formed by the large (>100 residues) loop between the alpha-helical M3 and M4 transmembrane segments. Putative prokaryotic Cys-loop homologues lack a large M3M4 loop. We replaced the complete M3M4 loop (115 amino acids) in the 5-hydroxytryptamine type 3A (5-HT(3A)) subunit with a heptapeptide from the prokaryotic homologue from Gloeobacter violaceus. The macroscopic electrophysiological and pharmacological characteristics of the homomeric 5-HT(3A)-glvM3M4 receptors were comparable to 5-HT(3A) wild type. The channels remained cation-selective but the 5-HT(3A)-glvM3M4 single channel conductance was 43.5 pS as compared with the subpicosiemens wild-type conductance. Coexpression of hRIC-3, a protein that modulates expression of 5-HT(3) and acetylcholine receptors, significantly attenuated 5-HT-induced currents with wild-type 5-HT(3A) but not 5-HT(3A)-glvM3M4 receptors. A similar deletion of the M3M4 loop in the anion-selective GABA-rho1 receptor yielded functional, GABA-activated, anion-selective channels. These results imply that the M3M4 loop is not essential for receptor assembly and function and suggest that the cytoplasmic domain may fold as an independent module from the transmembrane and extracellular domains.

Chlorpromazine binding to Na+, K+-ATPase and photolabeling: involvement of the ouabain site monitored by fluorescence

This work reports the results of ultraviolet irradiation on the interaction of the phototoxic antipsychotic drug chlorpromazine (CPZ) with the sodium pump Na+, K+-ATPase. The study was performed by monitoring the fluorescence modifications of CPZ itself and of the specific probe anthroylouabain (AO). CPZ association with Na+, K+-ATPase was found to modify the kinetics of CPZ-photodegradation. It was demonstrated that UV irradiation produces a stable fluorescent photoproduct of CPZ covalently bound to Na+, K+-ATPase. The fluorescent probe AO, which specifically binds to the extracellular ouabain site of the pump, was used to localize the CPZ binding site. UV-irradiation of AO-labeled Na+, K+-ATPase treated with CPZ at concentration about 20 microM produced dose-dependent modifications of the AO fluorescence, e.g. increased quantum yield and blue shift. The results demonstrated that CPZ binds near the ouabain site. The photo-induced reaction of CPZ with AO-labeled Na+, K+-ATPase protected the ouabain site from the aqueous environment. It was also found that UV irradiation of CPZ-treated enzyme obstructs the binding of AO, which suggested occlusion of the ouabain site. This effect can be evaluated for a potential use of CPZ in photochemotherapy.

Role of local anesthetics on both cholinergic and serotonergic ionotropic receptors

A great body of experimental evidence indicates that the main target for the pharmacological action of local anesthetics (LAs) is the voltage-gated Na+ channel. However, the epidural and spinal anesthesia as well as the behavioral effects of LAs cannot be explained exclusively by its inhibitory effect on the voltage-gated Na+ channel. Thus, the involvement of other ion channel receptors has been suggested. Particularly, two members of the neurotransmitter-gated ion channel receptor superfamily, the nicotinic acetylcholine receptor (AChR) and the 5-hydroxytryptamine receptor (5-HT3R type). In this regard, the aim of this review is to explain and delineate the mechanism by which LAs inhibit both ionotropic receptors from peripheral and central nervous systems. Local anesthetics inhibit the ion channel activity of both muscle- and neuronal-type AChRs in a noncompetitive fashion. Additionally, LAs inhibit the 5-HT3R by competing with the serotonergic agonist binding sites. The noncompetitive inhibitory action of LAs on the AChR is ascribed to two possible blocking mechanisms. An open-channel-blocking mechanism where the drug binds to the open channel and/or an allosteric mechanism where LAs bind to closed channels. The open-channel-blocking mechanism is in accord with the existence of high-affinity LA binding sites located in the ion channel. The allosteric mechanism seems to be physiologically more relevant than the open-channel-blocking mechanism. The inhibitory property of LAs is also elicited by binding to several low-affinity sites positioned at the lipid-AChR interface. However, there is no clearcut evidence indicating whether these sites are located at either the annular or the nonannular lipid domain. Both tertiary (protonated) and quaternary LAs gain the interior of the channel through the hydrophilic pathway formed by the extracellular ion channel's mouth with the concomitant ion flux blockade. Nevertheless, an alternative mode of action is proposed for both deprotonated tertiary and permanently-uncharged LAs: they may pass from the lipid membrane core to the lumen of the ion channel through a hydrophobic pathway. Perhaps this hydrophobic pathway is structurally related to the nonannular lipid domain. Regarding the LA binding site location on the 5-HT3R, at least two amino acids have been involved. Glutamic acid at position 106 which is located in a residue sequence homologous to loop A from the principal component of the binding site for cholinergic agonists and competitive antagonists, and Trp67 which is positioned in a stretch of amino acids homologous to loop F from the complementary component of the cholinergic ligand binding site.

The antagonistic effect of chlorpromazine on cadmium toxicity

Adult male rats were injected sc with cadmium chloride (CdCl(2)) in a single dose of 7 mg/kg body wt. Twenty-four hours postinjection, exposure to CdCl(2) increased the hemoglobin absorbance of the testes from 0.36 +/- 0.01 to 2.46 +/- 0.02. Pretreatment of rats with chlorpromazine (CPZ) 3 mg/kg ip either for 1 or 2 days before exposure to CdCl(2) significantly (p < 0.05) reduced the testicular damage and the hemoglobin absorbance decreased to 1.03 +/- 0.02 and 0.92 +/- 0.04, respectively. After CdCl(2) injection there was a progressive increase in serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities. CdCl(2) injection induced hemorrhage and a diffuse area of coagulative necrosis in liver. Pretreatment with CPZ partially protected liver from the effect of CdCl(2). Two months postinjection, exposure to CdCl(2) significantly decreased the weights of testes, epididymis, and accessory sex organs. Furthermore, CdCl(2) induced a highly significant (p < 0.01) decrease in sperm cell concentration and the percentage of mobile cells. Moreover CdCl(2) induced degenerative changes in testes, epididymis, and seminal vesicles. Pretreatment with CPZ partially protected these organs from the toxic effects of CdCl(2). It could be concluded that chlorpromazine partially antagonized the toxic effects of cadmium on liver, testes, and other male reproductive organs of rats.

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

Inhibition of ligand-gated cation-selective channels by tamoxifen

The nonsteroidal antioestrogen tamoxifen has been shown to block a number of voltage-gated cation-selective channels but its effect on ligand-gated cation-selective channels has not been studied. We have investigated the action of tamoxifen and the related derivative toremifene on ligand-gated cationic nicotinic acetylcholine and 5-HT3 receptor channels. Tamoxifen and toremifene both inhibited cationic currents of adult-type human muscle nicotinic acetylcholine receptors expressed in Xenopus oocytes with similar IC50 values of 1.2 +/- 0.03 microM (nH = 0.84 +/- 0.02) and 1.2 +/- 0.1 microM (nH = 1.1 +/- 0.1), respectively. Tamoxifen could also block native 5-HT3 receptors in NG108-15 neuroblastoma/glioma hybrid cells with IC50 = 0.81 +/- 0.15 microM and nH of 1.3 +/- 0.3. The characteristics of block by tamoxifen at the 5-HT3 receptor were voltage- and use-independent. The inhibition of the 5-HT-evoked currents were not overcome by increasing concentrations of 5-HT consistent with a noncompetitive mechanism of block.

Nicotinic agonists competitively antagonize serotonin at mouse 5-HT3 receptors expressed in Xenopus oocytes

The 5-HT3 receptor (5-HT3R) is part of a superfamily of ligand-gated ion channels which includes nicotinic acetylcholine receptors (nAChR). cRNA derived from the long isoform cloned mouse 5-HT3R was used to drive expression of 5-HT3Rs in Xenopus oocytes. 5-HT-induced currents were monitored using two-electrode voltage-clamp. Eight nicotinic agonists, including ACh and nicotine, but not alpha-anatoxin, were found to antagonize 5-HT-induced currents. With the exception of 3-(2,4)-dimethoxybenzylidene-anabaseine (DMXB-anabaseine; GTS-21) this antagonism appeared to be competitive since it could be overcome by increasing concentrations of 5-HT. Potency of 5-HT3 antagonism was comparable to reported values for nAChR alpha7 activation. These results confirm the notion of families of receptors and further indicate that strong similarities can exist in some critical binding domains.

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)

Activation of sodium transport and intracellular sodium lowering by the neuroleptic drug chlorpromazine

Chlorpromazine (CPZ), a commonly used antipsychotic drug, at high concentration was found to reduce significantly the sodium content of both rat (Rattus norvegicus) and toad (Bufo marinus) liver cells. This reduction in intracellular sodium was demonstrated using 22Na+ flux and measurement of cell sodium content. The results suggest that the sodium-lowering effect of CPZ stemmed from a stimulation of sodium transport rather than from an inhibition of sodium influx (i.e., sodium channels), cell damage, or Na+:Na+ exchange. CPZ was found to interfere with the binding of ouabain to the sodium pump, although a simple reduction in sodium pump inhibition did not account for the sodium-lowering effect. CPZ was able to negate the effects of monensin, a sodium ionophore, suggesting a substantial capacity to activate sodium transport. The intracellular sodium-lowering action of CPZ through the activation of sodium transport represents a new property previously undescribed for this drug.

Radioligand binding and photoaffinity labelling studies show a direct interaction of phenothiazines at 5-HT3 receptors

The effects of a range of phenothiazines were examined on 5-hydroxytryptamine3 (5-HT3) receptors in membranes from NIE-115 neuroblastoma cells using radioligand binding. Chlorpromazine, fluphenazine, perphenazine, trifluoperazine and prochlorperazine inhibited specific binding of both the 5-HT3 receptor antagonist [3H]GR65630 and agonist [3H]meta-chlorophenylbiguanide (mCPBG), with Ki values ranging from 0.4 to 3.9 microM. The mode of action of chlorpromazine was further examined using photoaffinity labelling in the presence and absence of 5-HT. Saturation radioligand binding data with both [3H]GR65630 and [3H]mCPBG showed that photoaffinity labelling with chlorpromazine (1 microM) caused a decrease in the maximum number of binding sites observed (35% and 28% for agonist and antagonist, respectively). This decrease was not observed when the membranes were incubated in the presence of 5-HT. The results demonstrate a direct interaction of a range of phenothiazines at the 5-HT3 receptor binding site.