Role of aspartate 298 in mouse 5-HT3A receptor gating and modulation by extracellular Ca2+

Asymmetric opening of the homopentameric 5-HT3A serotonin receptor in lipid bilayers

Pentameric ligand-gated ion channels (pLGICs) of the Cys-loop receptor family are key players in fast signal transduction throughout the nervous system. They have been shown to be modulated by the lipid environment, however the underlying mechanism is not well understood. We report three structures of the Cys-loop 5-HT_3A serotonin receptor (5HT₃R) reconstituted into saposin-based lipid bilayer discs: a symmetric and an asymmetric apo state, and an asymmetric agonist-bound state. In comparison to previously published 5HT₃R conformations in detergent, the lipid bilayer stabilises the receptor in a more tightly packed, ‘coupled’ state, involving a cluster of highly conserved residues. In consequence, the agonist-bound receptor conformation adopts a wide-open pore capable of conducting sodium ions in unbiased molecular dynamics (MD) simulations. Taken together, we provide a structural basis for the modulation of 5HT₃R by the membrane environment, and a model for asymmetric activation of the receptor.

Pentameric ligand-gated ion channels (pLGICs) are key players in neurotransmission and have been shown to be modulated by the lipid environment, however the underlying mechanism is not well understood. Here, the authors report structures of the pLGIC 5-HT_3A serotonin receptor reconstituted into lipid bilayer discs and reveal lipid–protein interactions as well as asymmetric activation of the homopentameric receptor.

Structure, Function and Physiology of 5-Hydroxytryptamine Receptors Subtype 3

5-hydroxytryptamine receptor subtype 3 (5-HT₃R) is a pentameric ligand-gated ion channel (pLGIC) involved in neuronal signaling. It is best known for its prominent role in gut-CNS signaling though there is growing interest in its other functions, particularly in modulating non-serotonergic synaptic activity. Recent advances in structural biology have provided mechanistic understanding of 5-HT₃R function and present new opportunities for the field. This chapter gives a broad overview of 5-HT₃R from a physiological and structural perspective and then discusses the specific details of ion permeation, ligand binding and allosteric coupling between these two events. Biochemical evidence is summarized and placed within a physiological context. This perspective underscores the progress that has been made as well as outstanding challenges and opportunities for future 5-HT₃R research.

Conformational transitions of the serotonin 5-HT3 receptor

The serotonin 5-HT₃ receptor is a pentameric ligand-gated ion channel (pLGIC). It belongs to a large family of receptors that function as allosteric signal transducers across the plasma membrane^1,2; upon binding of neurotransmitter molecules to extracellular sites, the receptors undergo complex conformational transitions that result in transient opening of a pore permeable to ions. 5-HT₃ receptors are therapeutic targets for emesis and nausea, irritable bowel syndrome and depression³. In spite of several reported pLGIC structures^4-8, no clear unifying view has emerged on the conformational transitions involved in channel gating. Here we report four cryo-electron microscopy structures of the full-length mouse 5-HT₃ receptor in complex with the anti-emetic drug tropisetron, with serotonin, and with serotonin and a positive allosteric modulator, at resolutions ranging from 3.2 Å to 4.5 Å. The tropisetron-bound structure resembles those obtained with an inhibitory nanobody⁵ or without ligand⁹. The other structures include an 'open' state and two ligand-bound states. We present computational insights into the dynamics of the structures, their pore hydration and free-energy profiles, and characterize movements at the gate level and cation accessibility in the pore. Together, these data deepen our understanding of the gating mechanism of pLGICs and capture ligand binding in unprecedented detail.

Auto-inhibition at a ligand-gated ion channel: a cross-talk between orthosteric and allosteric sites

BACKGROUND AND PURPOSE

A ligand is believed to produce either positive or negative responses, or to block both of them. However, an indole compound was found to promote both positive and negative effects at the 5-HT3 AB receptor, which displays a low level of spontaneous activity. The present study attempted to delineate the mechanisms underlying this phenomenon.

EXPERIMENTAL APPROACH

The spontaneously active V291S 5-HT3 A receptor was used to explore the properties of 5-hydroxyindole (5-HoI) and 5-methoxyindole (5-MoI), structural analogues of 5-HT, either alone or in combination with orthosteric probes.

KEY RESULTS

Two types of efficacy switching were initiated by altering ligand structure and concentration. At lower concentrations, a subtle structural change at position 5 of the indole molecule resulted in opposite effects. 5-HoI apparently elicited partial allosteric inverse agonism, whereas 5-MoI induced allosteric agonism. Interestingly, at a higher concentration, these indoles produced distinct auto-inhibition, manifested as a switch from positive to negative effects. 5-HoI induced a transition from orthosteric agonism to allosteric inverse agonism, whereas 5-MoI produced a shift from allosteric agonism to orthosteric inverse agonism. The auto-inhibition appears to involve communication between orthosteric and allosteric sites of the active receptor conformation and/or between inactive and active conformations. An additive effect of orthosteric and allosteric inverse agonism and insensitivity of allosteric agonism to orthosteric antagonism were also demonstrated.

CONCLUSIONS AND IMPLICATIONS

Together, the results suggest that the moiety at position 5 of the indole structure is a critical determinant of a ligand's properties at the 5-HT3 A receptor, providing new insights into understanding ligand-receptor interactions.

An outline of desensitization in pentameric ligand-gated ion channel receptors

Pentameric ligand-gated ion channel (pLGIC) receptors exhibit desensitization, the progressive reduction in ionic flux in the prolonged presence of agonist. Despite its pathophysiological importance and the fact that it was first described over half a century ago, surprisingly little is known about the structural basis of desensitization in this receptor family. Here, we explain how desensitization is defined using functional criteria. We then review recent progress into reconciling the structural and functional basis of this phenomenon. The extracellular-transmembrane domain interface is a key locus. Activation is well known to involve conformational changes at this interface, and several lines of evidence suggest that desensitization involves a distinct conformational change here that is incompatible with activation. However, major questions remain unresolved, including the structural basis of the desensitization-induced agonist affinity increase and the mechanism of pore closure during desensitization.

Inhibition of the prokaryotic pentameric ligand-gated ion channel ELIC by divalent cations

The modulation of pentameric ligand-gated ion channels (pLGICs) by divalent cations is believed to play an important role in their regulation in a physiological context. Ions such as calcium or zinc influence the activity of pLGIC neurotransmitter receptors by binding to their extracellular domain and either potentiate or inhibit channel activation. Here we have investigated by electrophysiology and X-ray crystallography the effect of divalent ions on ELIC, a close prokaryotic pLGIC homologue of known structure. We found that divalent cations inhibit the activation of ELIC by the agonist cysteamine, reducing both its potency and, at higher concentrations, its maximum response. Crystal structures of the channel in complex with barium reveal the presence of several distinct binding sites. By mutagenesis we confirmed that the site responsible for divalent inhibition is located at the outer rim of the extracellular domain, at the interface between adjacent subunits but at some distance from the agonist binding region. Here, divalent cations interact with the protein via carboxylate side-chains, and the site is similar in structure to calcium binding sites described in other proteins. There is evidence that other pLGICs may be regulated by divalent ions binding to a similar region, even though the interacting residues are not conserved within the family. Our study provides structural and functional insight into the allosteric regulation of ELIC and is of potential relevance for the entire family.

Length and amino acid sequence of peptides substituted for the 5-HT3A receptor M3M4 loop may affect channel expression and desensitization

5-HT3A receptors are pentameric neurotransmitter-gated ion channels in the Cys-loop receptor family. Each subunit contains an extracellular domain, four transmembrane segments (M1, M2, M3, M4) and a 115 residue intracellular loop between M3 and M4. In contrast, the M3M4 loop in prokaryotic homologues is <15 residues. To investigate the limits of M3M4 loop length and composition on channel function we replaced the 5-HT3A M3M4 loop with two to seven alanine residues (5-HT3A-A_{n = 2–7}). Mutants were expressed in Xenopus laevis oocytes and characterized using two electrode voltage clamp recording. All mutants were functional. The 5-HT EC₅₀'s were at most 5-fold greater than wild-type (WT). The desensitization rate differed significantly among the mutants. Desensitization rates for 5-HT3A-A₂, 5-HT3A-A₄, 5-HT3A-A₆, and 5-HT3A-A₇ were similar to WT. In contrast, 5-HT3A-A₃ and 5-HT3A-A₅ had desensitization rates at least an order of magnitude faster than WT. The one Ala loop construct, 5-HT3A-A₁, entered a non-functional state from which it did not recover after the first 5-HT application. These results suggest that the large M3M4 loop of eukaryotic Cys-loop channels is not required for receptor assembly or function. However, loop length and amino acid composition can effect channel expression and desensitization. We infer that the cytoplasmic ends of the M3 and M4 segments may undergo conformational changes during channel gating and desensitization and/or the loop may influence the position and mobility of these segments as they undergo gating-induced conformational changes. Altering structure or conformational mobility of the cytoplasmic ends of M3 and M4 may be the basis by which phosphorylation or protein binding to the cytoplasmic loop alters channel function.

Psychotropic and nonpsychotropic cannabis derivatives inhibit human 5-HT(3A) receptors through a receptor desensitization-dependent mechanism

Δ⁹ tetrahydrocannabinol (THC) and cannabidiol (CBD) are the principal psychoactive and nonpsychoactive components of cannabis. While most THC-induced behavioral effects are thought to depend on endogenous cannabinoid 1 (CB1) receptors, the molecular targets for CBD remain unclear. Here, we report that CBD and THC inhibited the function of human 5-HT(3A) receptors (h5-HT(3A)Rs) expressed in HEK 293 cells. The magnitude of THC and CBD inhibition was maximal 5 min after a continuous incubation with cannabinoids. The EC₅₀ values for CBD and THC-induced inhibition were 110 nM and 322 nM, respectively in HEK 293 cells expressing h5-HT(3A)Rs. In these cells, CBD and THC did not stimulate specific [³⁵S]-GTP-γs binding in membranes, suggesting that the inhibition by cannabinoids is unlikely mediated by a G-protein dependent mechanism. On the other hand, both CBD and THC accelerated receptor desensitization kinetics without significantly changing activation time. The extent of cannabinoid inhibition appeared to depend on receptor desensitization. Reducing receptor desensitization by nocodazole, 5-hydroxyindole and a point-mutation in the large cytoplasmic domain of the receptor significantly decreased CBD-induced inhibition. Similarly, the magnitude of THC and CBD-induced inhibition varied with the apparent desensitization rate of h5-HT(3A)Rs expressed in Xenopus oocytes. For instance, with increasing amount of h5-HT(3A)R cRNA injected into the oocytes, the receptor desensitization rate at steady state decreased. THC and CBD-induced inhibition was correlated with the change in the receptor desensitization rate. Thus, CBD and THC inhibit h5-HT(3A) receptors through a mechanism that is dependent on receptor desensitization.

The structural basis of function in Cys-loop receptors

Cys-loop receptors are membrane-spanning neurotransmitter-gated ion channels that are responsible for fast excitatory and inhibitory transmission in the peripheral and central nervous systems. The best studied members of the Cys-loop family are nACh, 5-HT3, GABAA and glycine receptors. All these receptors share a common structure of five subunits, pseudo-symmetrically arranged to form a rosette with a central ion-conducting pore. Some are cation selective (e.g. nACh and 5-HT3) and some are anion selective (e.g. GABAA and glycine). Each receptor has an extracellular domain (ECD) that contains the ligand-binding sites, a transmembrane domain (TMD) that allows ions to pass across the membrane, and an intracellular domain (ICD) that plays a role in channel conductance and receptor modulation. Cys-loop receptors are the targets for many currently used clinically relevant drugs (e.g. benzodiazepines and anaesthetics). Understanding the molecular mechanisms of these receptors could therefore provide the catalyst for further development in this field, as well as promoting the development of experimental techniques for other areas of neuroscience.In this review, we present our current understanding of Cys-loop receptor structure and function. The ECD has been extensively studied. Research in this area has been stimulated in recent years by the publication of high-resolution structures of nACh receptors and related proteins, which have permitted the creation of many Cys loop receptor homology models of this region. Here, using the 5-HT3 receptor as a typical member of the family, we describe how homology modelling and ligand docking can provide useful but not definitive information about ligand interactions. We briefly consider some of the many Cys-loop receptors modulators. We discuss the current understanding of the structure of the TMD, and how this links to the ECD to allow channel gating, and consider the roles of the ICD, whose structure is poorly understood. We also describe some of the current methods that are beginning to reveal the differences between different receptor states, and may ultimately show structural details of transitions between them.

Novel structural determinants of single channel conductance and ion selectivity in 5-hydroxytryptamine type 3 and nicotinic acetylcholine receptors

Nicotinic acetylcholine (nACh) and 5-hydroxytryptamine type 3 (5-HT(3)) receptors are cation-selective ion channels of the pentameric ligand-gated ion channel (pLGIC) superfamily. Multiple lines of evidence adduced over the last 30 years indicate that the lining of the channel of such receptors is formed by the alpha-helical second transmembrane (TM2) domain and flanking sequences contributed by each of the five subunits present within the receptor complex. Specific amino acid residues within, and adjacent to, the TM2 domain influence single channel conductance, ion selectivity, and other aspects of receptor function that include gating and desensitization. However, more recent work has revealed important structural determinants of single channel conductance and ion selectivity that are not associated with the TM2 domain. Direct experimental evidence indicates that the intracellular domain of eukaryotic pLGICs, in particular a region of the loop linking TM3 and TM4 termed the membrane-associated (MA) stretch, exerts a strong influence upon ion channel biophysics. Moreover, recent computational approaches, complemented by experimentation, implicate the extracellular domain as an additional important determinant of ion conduction. This brief review describes how our knowledge of ion conduction and selectivity in cation-selective pLGICs has evolved beyond TM2.

Isomerization of the proline in the M2-M3 linker is not required for activation of the human 5-HT3A receptor

Each subunit of the cation-selective members of the Cys-loop family of ligand-gated ion channels contains a conserved proline residue in the extracellular loop between the second and third transmembrane domains. In the mouse homomeric 5-hydroxytryptamine type 3A (5-HT(3)A) receptor, the effects of substitution of this proline by unnatural amino acids led to the suggestion that trans-cis isomerization of the protein backbone at this position is integral to agonist-induced channel opening [Nature (2005) vol. 438, pp. 248-252]. We explored the generality of this conclusion using natural amino acid mutagenesis of the homologous human 5-HT(3)A receptor. The conserved proline (P303) was substituted by either a histidine or tryprophan and the mutant receptors were expressed in Xenopus oocytes. These mutations did not significantly affect the magnitude of agonist-mediated currents, compromise channel gating by 5-HT or inhibition of 5-HT-induced currents by either picrotoxin or d-tubocurarine. The mutations did, however, result in altered dependence on extracellular Ca(2+) concentration and a 10-fold increase in the rate of receptor desensitization. These results demonstrate an important role for P303 in 5-HT(3)A receptor function but indicate that trans-cis isomerization at this proline is unlikely to be a general mechanism underlying the gating process.

The 5-HT3 receptor--the relationship between structure and function

The 5-hydroxytryptamine type-3 (5-HT3) receptor is a cation-selective ion channel of the Cys-loop superfamily. 5-HT3 receptor activation in the central and peripheral nervous systems evokes neuronal excitation and neurotransmitter release. Here, we review the relationship between the structure and the function of the 5-HT3 receptor. 5-HT3A and 5-HT3B subunits are well established components of 5-HT3 receptors but additional HTR3C, HTR3D and HTR3E genes expand the potential for molecular diversity within the family. Studies upon the relationship between subunit structure and the ionic selectivity and single channel conductances of 5-HT3 receptors have identified a novel domain (the intracellular MA-stretch) that contributes to ion permeation and selectivity. Conventional and unnatural amino acid mutagenesis of the extracellular domain of the receptor has revealed residues, within the principle (A-C) and complementary (D-F) loops, which are crucial to ligand binding. An area requiring much further investigation is the subunit composition of 5-HT3 receptors that are endogenous to neurones, and their regional expression within the central nervous system. We conclude by describing recent studies that have identified numerous HTR3A and HTR3B gene polymorphisms that impact upon 5-HT3 receptor function, or expression, and consider their relevance to (patho)physiology.

Calcium modulation of 5-HT3 receptor binding and function

Calcium modulates the 5-HT3 receptor response by reducing peak current amplitude and increasing rates of activation, deactivation and desensitisation, but the binding site(s) and mechanism(s) of this modulation are unknown. Here we study residues that may be involved in calcium binding in two partially overlapping regions of the extracellular domain (E213-E215-E218 and D204-E218-V219). The modulatory effects of calcium were assessed by radioligand binding and whole-cell patch-clamp. Comparisons of [3H]granisetron binding showed an increase in Kd in 10mM calcium that was abolished by the substitutions E213Q, E215Q, D204N and V219L. E218Q mutant receptors displayed no specific binding or function, and immunofluorescence showed that they did not reach the cell surface. E213Q increased inherent rates of desensitisation, but the relative effects of calcium on these rates, and on the reduction in current amplitude, were similar to wild type receptors. Current responses and calcium-mediated effects at E215Q mutant receptors were indistinguishable from wild type. D204N and V219L mutants were non-functional. A calcium impermeable mutant (E277A/S297R) revealed no changes in peak amplitude or kinetics with increased calcium. Our results are consistent with residues D204, E218 and V219 participating in receptor assembly, structure and/or trafficking to the plasma membrane, and we speculate that this might rely upon the stabilising effect of bound calcium. E213, E215, D204 and V219 may contribute to a calcium binding site that is responsible for the calcium-mediated effects on ligand binding. However, the major site for calcium-dependent modulation of the 5-HT3 current is located within the ion channel or cell interior.

Structural determinants of Ca2+ permeability and conduction in the human 5-hydroxytryptamine type 3A receptor

Cation-selective cysteine (Cys)-loop transmitter-gated ion channels provide an important pathway for Ca2+ entry into neurones. We examined the influence on Ca2+ permeation of amino acids located at intra- and extracellular ends of the conduction pathway of the human 5-hydroxytryptamine type 3A (5-HT3A) receptor. Mutation of cytoplasmic arginine residues 432, 436, and 440 to glutamine, aspartate, and alanine (the aligned residues of the human 5-HT3B subunit (yielding 5-HT3A(QDA)) increased PCa/PCs from 1.4 to 3.7. The effect was attributable to the removal of an electrostatic influence of the Arg-436 residue. Despite its relatively high permeability to Ca2+, the single channel conductance of the 5-HT3A(QDA) receptor was depressed in a concentration-dependent and voltage-independent manner by extracellular Ca2+. A conserved aspartate, located toward the extracellular end of the conduction pathway and known to influence ionic selectivity, contributed to the inhibitory effect of Ca2+ on macroscopic currents mediated by 5-HT3A receptors. We introduced a D293A mutation into the 5-HT3A(QDA) receptor (yielding the 5-HT3A(QDA D293A) construct) to determine whether the aspartate is required for the suppression of single channel conductance by Ca2+. The D293A mutation decreased the PCa/PCs ratio to 0.25 and reduced inwardly directed single channel conductance from 41 to 30 pS but did not prevent suppression of single channel conductance by Ca2+. The D293A mutation also reduced PCa/PCs when engineered into the wild-type 5-HT3A receptor. The data helped to identify key residues in the cytoplasmic domain (Arg-436) and extracellular vestibule (Asp-293) that markedly influence PCa/PCs and additionally directly demonstrated a depression of single channel conductance by Ca2+.

On the voltage-dependent Ca2+ block of serotonin 5-HT3 receptors: a critical role of intracellular phosphates

Natively expressed serotonin 5-HT(3) receptors typically possess a negative-slope conductance region in their I-V curve, due to a voltage-dependent block by external Ca(2+) ions. However, in almost all studies performed with heterologously expressed 5-HT(3) receptors, this feature was not observed. Here we show that mere addition of ATP to the pipette solution is sufficient to reliably observe a voltage-dependent block in homomeric (h5-HT(3A)) and heteromeric (h5-HT(3AB)) receptors expressed in HEK293 cells. A similar block was observed with a plethora of molecules containing a phosphate moiety, thus excluding a role of phosphorylation. A substitution of three arginines in the intracellular vestibule of 5-HT(3A) with their counterpart residues from the 5-HT(3B) subunit (RRR-QDA) was previously shown to dramatically increase single channel conductance. We find this mutant to have a linear I-V curve that is unaffected by the presence of ATP, with a fractional Ca(2+) current (Pf%) that is reduced (1.8 +/- 0.2%) compared to that of the homomeric receptor (4.1 +/- 0.2%), and similar to that of the heteromeric form (2.0 +/- 0.3%). Moreover, whereas ATP decreased the Pf% of the homomeric receptor, this was not observed with the RRR-QDA mutant. Finally, ATP was found to be critical for voltage-dependent channel block also in hippocampal interneurons that natively express 5-HT(3) receptors. Taken together, our results indicate a novel mechanism by which ATP, and similar molecules, modulate 5-HT(3) receptors via interactions with the intracellular vestibule of the receptor.

The L293 residue in transmembrane domain 2 of the 5-HT3A receptor is a molecular determinant of allosteric modulation by 5-hydroxyindole

Allosteric modulation of ligand-gated ion channels can play important roles in shaping synaptic transmission. The function of the 5-hydroxytryptamine (serotonin) type 3 (5-HT(3)) receptor, a member of the Cys-loop ligand-gated ion channel superfamily, is modulated by a variety of compounds such as alcohols, anesthetics and 5-hydroxyindole (5-HI). In this study, the molecular determinants of allosteric modulation by 5-HI were explored in N1E-115 neuroblastoma cells expressing the native 5-HT(3) receptor and HEK 293 cells transfected with the recombinant 5-HT(3A) receptor using molecular biology and whole-cell patch-clamp techniques. 5-HI potentiated 5-HT-activated currents in both N1E-115 cells and HEK 293 cells, and significantly decreased current desensitization and deactivation. Substitution of Leu293 (L293, L15') in the second transmembrane domain (TM2) with cysteine (L293C) or serine (L293S) abolished 5-HI modulation. Other mutations in the TM2 domain, such as D298A and T284F, failed to alter 5-HI modulation. The L293S mutation enhanced dopamine efficacy and converted 5-HI into a partial agonist at the mutant receptor. These data suggest that 5-HI stabilizes the 5-HT(3A) receptor in the open state by decreasing both desensitization and 5-HT unbinding/channel closing; and L293 is a common site for both channel gating and allosteric modulation by 5-HI. Our observations also indicate existence of a second 5-HI recognition site on the 5-HT(3A) receptor, which may overlap with the 5-HT binding site and is not involved in the positive modulation by 5-HI. These findings support the idea that there are two discrete sites for 5-HI allosteric modulation and direct activation in the 5-HT(3A) receptor.

The 5-HT3B subunit confers spontaneous channel opening and altered ligand properties of the 5-HT3 receptor

Current receptor theory suggests that there is an equilibrium between the inactive (R) and active (R*) conformations of ligand-gated ion channels and G protein-coupled receptors. The actions of ligands in both receptor types could be appropriately explained by this two-state model. Ligands such as agonists and antagonists affect receptor function by stabilizing one or both conformations. The 5-HT3 receptor is a member of the Cys-loop ligand-gated ion channel superfamily participating in synaptic transmission. Here we show that co-expression of the 5-HT3A and 5-HT3B receptor subunits in the human embryonic kidney (HEK) 293 cells results in a receptor that displays a low level of constitutive (or agonist-independent) activity. Furthermore, we also demonstrate that the properties of ligands can be modified by receptor composition. Whereas the 5-hydroxytryptamine (5-HT) analog 5-methoxyindole is a partial agonist at the 5-HT3A receptor, it becomes a "protean agonist" (functioning as an agonist and an inverse agonist at the same receptor) at the 5-HT3AB receptor (after the Greek god Proteus, who was able to change his shape and appearance at will). In addition, the 5-HT analog 5-hydroxyindole is a positive allosteric modulator for the liganded active (AR*) conformation of the 5-HT3A and 5-HT3AB receptors and a negative allosteric modulator for the spontaneously active (R*) conformation of the 5-HT3AB receptor, suggesting that the spontaneously active (R*) and liganded active (AR*) conformations are differentially modulated by 5-hydroxyindole. Thus, the incorporation of the 5-HT3B subunit leads to spontaneous channel opening and altered ligand properties.

Differential effects of serotonin and dopamine on human 5-HT3A receptor kinetics: interpretation within an allosteric kinetic model

Serotonin type 3 (5-HT3) receptors are members of the pentameric Cys-loop superfamily of receptors that modulate synaptic neurotransmission. In response to agonist binding and unbinding, members of this superfamily undergo a series of conformational transitions that define their functional properties. In this study, we report the results of electrophysiological studies using rapid solution exchange designed to characterize and compare the actions of the high-efficacy agonist serotonin and the low-efficacy agonist dopamine on human 5-HT3A receptors expressed in human embryonic kidney HEK293 cells. In the case of serotonin, receptor activation rates varied with agonist concentration, and deactivation occurred as a single-exponential process with a rate that was similar to the maximal rate of desensitization. Receptors recovered slowly from long desensitizing pulses of serotonin with a sigmoidal time course. In the case of dopamine, receptor activation rates were independent of agonist concentration, receptor deactivation occurred as a complex process that was significantly faster than the maximal rate of desensitization, and recovery from desensitization occurred more quickly than with 5-HT and its time course was not sigmoidal. We developed an allosteric kinetic model for 5-HT3A receptor activation, deactivation, desensitization, and resensitization. Interpretation of our results within the context of this model indicated that the distinct modulatory actions of serotonin versus dopamine are largely attributable to the vastly different rates with which these two agonists induce channel opening and dissociate from open and desensitized states.