Ca2+ influx through voltage-gated Ca2+ channels regulates 5-HT3 receptor channel desensitization in rat glioma x mouse neuroblastoma hybrid NG108-15 cells

Serotonin 5-HT3 receptor-mediated vomiting occurs via the activation of Ca2+/CaMKII-dependent ERK1/2 signaling in the least shrew (Cryptotis parva)

Stimulation of 5-HT₃ receptors (5-HT₃Rs) by 2-methylserotonin (2-Me-5-HT), a selective 5-HT₃ receptor agonist, can induce vomiting. However, downstream signaling pathways for the induced emesis remain unknown. The 5-HT₃R channel has high permeability to extracellular calcium (Ca²⁺) and upon stimulation allows increased Ca²⁺ influx. We examined the contribution of Ca²⁺/calmodulin-dependent protein kinase IIα (Ca²⁺/CaMKIIα), interaction of 5-HT₃R with calmodulin, and extracellular signal-regulated kinase 1/2 (ERK1/2) signaling to 2-Me-5-HT-induced emesis in the least shrew. Using fluo-4 AM dye, we found that 2-Me-5-HT augments intracellular Ca²⁺ levels in brainstem slices and that the selective 5-HT₃R antagonist palonosetron, can abolish the induced Ca²⁺ signaling. Pre-treatment of shrews with either: i) amlodipine, an antagonist of L-type Ca²⁺ channels present on the cell membrane; ii) dantrolene, an inhibitor of ryanodine receptors (RyRs) Ca²⁺-release channels located on the endoplasmic reticulum (ER); iii) a combination of their less-effective doses; or iv) inhibitors of CaMKII (KN93) and ERK1/2 (PD98059); dose-dependently suppressed emesis caused by 2-Me-5-HT. Administration of 2-Me-5-HT also significantly: i) enhanced the interaction of 5-HT₃R with calmodulin in the brainstem as revealed by immunoprecipitation, as well as their colocalization in the area postrema (brainstem) and small intestine by immunohistochemistry; and ii) activated CaMKIIα in brainstem and in isolated enterochromaffin cells of the small intestine as shown by Western blot and immunocytochemistry. These effects were suppressed by palonosetron. 2-Me-5-HT also activated ERK1/2 in brainstem, which was abrogated by palonosetron, KN93, PD98059, amlodipine, dantrolene, or a combination of amlodipine plus dantrolene. However, blockade of ER inositol-1, 4, 5-triphosphate receptors by 2-APB, had no significant effect on the discussed behavioral and biochemical parameters. This study demonstrates that Ca²⁺ mobilization via extracellular Ca²⁺ influx through 5-HT₃Rs/L-type Ca²⁺ channels, and intracellular Ca²⁺ release via RyRs on ER, initiate Ca²⁺-dependent sequential activation of CaMKIIα and ERK1/2, which contribute to the 5-HT₃R-mediated, 2-Me-5-HT-evoked emesis.

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.

Extrinsic afferent nerve sensitivity and enteric neurotransmission in murine jejunum in vitro

Enteric and extrinsic sensory neurons respond to similar stimuli. Thus they may be activated in series or in parallel. Because signal transmission via synapses or mediator release would depend on calcium, we investigated its role for extrinsic afferent sensitivity to chemical and mechanical stimulation. Extracellular multiunit afferent recordings were made in vitro from paravascular nerve bundles supplying the mouse jejunum. Intraluminal pressure and afferent nerve responses were recorded under control conditions and under four conditions designed to interfere with enteric neurotransmission. We found that phasic intestinal contractions ceased after switching perfusion to Ca(2+)-free buffer with or without a purinergic P2 receptor antagonist, pyridoxal phosphate-6-azo(benzene-2,4-disulfonic acid) (PPADS) or cadmium (blocking all Ca(2+)-channels) but not following omega-conotoxin GVIA (N-type Ca(2+)-channel blocker). Luminal HCl (pH 2) and 5-HT (500 microM) evoked peak firing of 17 +/- 4 impulses per second (imp/s) (n = 10) and 21 +/- 4 imp/s (n = 13) under control conditions. These responses were reduced to 4 +/- 2 imp/s and 5 +/- 2 imp/s by cadmium (n = 7, P < 0.05), to 7 +/- 2 imp/s and 6 +/- 1 imp/s by Ca(2+)-free perfusion (n = 6, P < 0.05), and to 3 +/- 1 imp/s and 4 +/- 1 imp/s by Ca(2+)-free perfusion with PPADS (n = 6, P < 0.05). Responses were unchanged by omega-conotoxin GVIA. Mechanical ramp distension of the intestinal segment to 60 cmH(2)O was not altered by any of the experimental conditions. We concluded that HCl and 5-HT activate extrinsic afferents via a calcium-dependent mechanism, which is unlikely to involve enteric neurons carrying N-type calcium channels. Extrinsic mechanosensitivity is independent of enteric neurotransmission. It appears that cross talk from the enteric to the extrinsic nervous system does not mediate extrinsic afferent sensitivity.

Cross-regulation between colocalized nicotinic acetylcholine and 5-HT3 serotonin receptors on presynaptic nerve terminals

AIM

Substantial colocalization of functionally independent alpha4 nicotinic acetylcholine receptors and 5-HT(3) serotonin receptors on presynaptic terminals has been observed in brain. The present study was aimed at addressing whether nicotinic acetylcholine receptors and 5-HT(3) serotonin receptors interact on the same presynaptic terminal, suggesting a convergence of cholinergic and serotonergic regulation.

METHODS

Ca(2+) responses in individual, isolated nerve endings purified from rat striatum were measured using confocal imaging.

RESULTS

Application of 500 nmol/L nicotine following sustained stimulation with the highly selective 5-HT(3) receptor agonist m-chlorophenylbiguanide at 100 nmol/L resulted in markedly reduced Ca(2+) responses (28% of control) in only those striatal nerve endings that originally responded to m-chlorophenylbiguanide. The cross-regulation developed over several minutes. Presynaptic nerve endings that had not responded to m-chlorophenylbiguanide, indicating that 5-HT(3) receptors were not present, displayed typical responses to nicotine. Application of m-chlorophenylbiguanide following sustained stimulation with nicotine resulted in partially attenuated Ca(2+) responses (49% of control). Application of m-chlorophenylbiguanide following sustained stimulation with m-chlorophenylbiguanide also resulted in a strong attenuation of Ca(2+) responses (12% of control), whereas nicotine-induced Ca(2+) responses following sustained stimulation with nicotine were not significantly different from control.

CONCLUSION

These results indicate that the presynaptic Ca(2+) increases evoked by either 5-HT(3) receptor or nicotinic acetylcholine receptor activation regulate subsequent responses to 5-HT(3) receptor activation, but that only 5-HT(3) receptors cross-regulate subsequent nicotinic acetylcholine receptor-mediated responses. The findings suggest a specific interaction between the two receptor systems in the same striatal nerve terminal, likely involving Ca(2+)-dependent intracellular pathways that regulate these signaling systems at one or more levels.

Characteristics for enhanced response of serotonin-evoked ion dynamics in differentiated NG108-15 cells

Characteristics for the up-regulated response in the concentration of intracellular calcium ion ([Ca(2+)]( i )) and in the sodium ion (Na(+)) current by serotonin (5-HT) were investigated in differentiated neuroblastoma x glioma hybrid NG108-15 (NG) cells. The results for the changes in [Ca(2+)]( i ) by 5-HT were as follows, (1) The 5-HT-induced Ca(2+) response was inhibited by 3 x 10(-9) M tropisetron (a 5-HT(3) receptor blocker), but not by other types of 5-HT receptor blockers; (2) The 5-HT-induced Ca(2+) response was mainly inhibited by calciseptine (a L-type Ca(2+) blocker), but not by other types of Ca(2+) channel blockers or 10(-7) M TTX (a voltage-sensitive Na(+) channel blocker); (3) When the extracellular Na(+) was removed by exchange with choline chloride or N-methyl-D-glucamine, the 5-HT-induced Ca(2+) response was extremely inhibited. The results for the 5-HT-induced Na(+) current by the whole cell patch-clamp technique were as follows, (1) The 5-HT-induced Na(+) current in differentiated cells was significantly larger than that in undifferentiated cells; (2) The ED(50) value for 5-HT-induced Na(+) current in undifferentiated and differentiated cells was almost the same, about 4 x 10(-6) M each other; (3) The 5-HT-induced Na(+) current was completely blocked by 3 x 10(-9) M tropisetron, but not by other 5-HT receptor antagonists and 10(-7) M TTX. These results suggested that 5-HT-induced Ca(2+) response in differentiated NG cells was mainly due to L-type voltage-gated Ca(2+) channels allowing extracellular Na(+) to enter via 5-HT(3) receptors, but not through voltage-gated Na(+) channels.

Spinal and Supraspinal Contributions to Central Sensitization in Peripheral Neuropathy

We will focus on spinal cord dorsal horn lamina I projection neurones, their supraspinal targets and involvement in pain processing. These spinal cord neurons respond to tonic peripheral inputs by wind-up and other intrinsic mechanisms that cause central hyper-excitability, which in turn can further enhance afferent inputs. We describe here another hierarchy of excitation - as inputs arrive in lamina I, neurones rapidly inform the parabrachial area (PBA) and periaqueductal grey (PAG), areas associated with the affective and autonomic responses to pain. In addition, PBA can connect to areas of the brainstem that send descending projections down to the spinal cord - establishing a loop. The serotonin receptor, 5HT3, in the spinal cord mediates excitatory descending inputs from the brainstem. These descending excitatory inputs are needed for the full coding of polymodal peripheral inputs from spinal neurons and are enhanced after nerve injury. Furthermore, activity in this serotonergic system can determine the actions of gabapentin (GBP) that is widely used in the treatment of neuropathic pain. Thus, a hierarchy of separate, but interacting excitatory systems exist at peripheral, spinal and supraspinal sites that all converge on spinal neurones. The reciprocal relations between pain, fear, anxiety and autonomic responses are likely to be subserved by these spinal-brainstem-spinal pathways we describe here. Understanding these pain pathways is a first step toward elucidating the complex links between pain and emotions.

Calcium flux in neuroblastoma cells is a coupling mechanism between non-genomic and genomic modes of estrogens

Estrogens have been demonstrated to rapidly modulate calcium levels in a variety of cell types. However, the significance of estrogen-mediated calcium flux in neuronal cells is largely unknown. The relative importance of intra- and extracellular sources of calcium in estrogenic effects on neurons is also not well understood. Previously, we have demonstrated that membrane-limited estrogens, such as E-BSA given before an administration of a 2-hour pulse of 17beta-estradiol (E2), can potentiate the transcription mediated by E2 from a consensus estrogen response element (ERE)-driven reporter gene. Inhibitors to signal transduction cascades given along with E-BSA or E2 demonstrated that calcium flux is important for E-BSA-mediated potentiation of transcription in a transiently transfected neuroblastoma cell line. In this report, we have used inhibitors to different voltage-gated calcium channels (VGCCs) and to intracellular store receptors along with E-BSA in the first pulse or with E2 in the second pulse to investigate the relative importance of these channels to estrogen-mediated transcription. Neither L- nor P-type VGCCs seem to play a role in estrogen action in these cells; while N-type VGCCs are important in both the non-genomic and genomic modes of estrogen action. Specific inhibitors also showed that the ryanodine receptor and the inositol trisphosphate receptor are important to E-BSA-mediated transcriptional potentiation. This report provides evidence that while intracellular stores of calcium are required to couple non-genomic actions of estrogen initiated at the membrane to transcription in the nucleus, extracellular sources of calcium are also important in both non-genomic and genomic actions of estrogens.

Casein kinase ii (protein kinase ck2) regulates serotonin 5-ht(3) receptor channel function in ng108-15 cells

We have used whole-cell recording techniques in the mouse neuroblastoma X rat glioma hybrid NG108-15 cell line to test whether protein kinase CK2 (CK2; also known as casein kinase II) modulates the function of the serotonin 5-HT(3) receptor (5-HT(3)R) channel. The rapid application of 5-HT (50 microM) to NG108-15 cells elicits a 5-HT(3)R-mediated inward current response that rapidly reaches peak amplitude and then desensitizes in the continued presence of agonist. Internal dialysis with CK2 (20 micrograms/ml) via the patch pipette significantly increases the amplitude and decreases the rate of desensitization of the 5-HT(3)R-mediated responses. CK2 that had been heat-inactivated has no effect on either the amplitude or the kinetics of desensitization of the 5-HT(3)R responses. These data suggest that dialysis with protein kinase CK2 significantly enhanced current through the 5-HT(3)R channel, and that CK2 may be an important regulator of 5-HT(3)R channel function in the nervous system, possibly serving to facilitate the 5-HT-induced excitation of the cells.

Chick RGS2L demonstrates concentration-dependent selectivity for pertussis toxin-sensitive and -insensitive pathways that inhibit L-type Ca2+ channels

In neuronal cells, the influx of Ca2+ ions through voltage-dependent L-type calcium (L) channels couples excitation to multiple cellular functions. In addition to voltage, several neurotransmitters, hormones and cytokines regulate L channel gating via binding to G-protein-coupled receptors. Intracellular molecules that modify G-protein activity - such as regulator of G-protein-signalling (RGS) proteins - are therefore potential candidates for regulating Ca2+ influx through L channels. Here we show that a novel RGS2 splice variant from chick dorsal root ganglion (DRG) neurons, RGS2L, reduces bradykinin (BK)-mediated inhibition of neuronal L channels and accelerates recovery from inhibition. Chick RGS2 reduces the inhibition mediated by both the pertussis toxin (PTX)-sensitive (Gi/o-coupled) and the PTX-insensitive (presumably Gq/11-coupled) pathways. However, we demonstrate for the first time in a living cell that the extent of coupling to each pathway varies with RGS2L concentration. A low concentration of recombinant chick RGS2L (10 nM) preferentially reduces the inhibition mediated by the PTX-insensitive pathway, whereas a 100-fold higher concentration attenuates both PTX-sensitive- and PTX-insensitive-mediated components equally. Our data suggest that factors promoting RGS2L gene induction may regulate Ca2+ influx through L channels by recruiting low-affinity interactions with Gi/o that are absent at basal RGS2L levels.

Expression and function of 5-HT3 receptors in the enteric neurons of mice lacking the serotonin transporter

The actions of enteric 5-HT are terminated by 5-HT transporter (SERT)-mediated uptake, and gastrointestinal motility is abnormal in SERT -/- mice. We tested the hypothesis that adaptive changes in enteric 5-HT(3) receptors help SERT -/- mice survive despite inefficient 5-HT inactivation. Expression of mRNA encoding enteric 5-HT(3A) subunits was similar in SERT +/+ and -/- mice, but that of 5-HT(3B) subunits was fourfold less in SERT -/- mice. 5-HT(3B) mRNA was found, by in situ hybridization, in epithelial cells and enteric neurons. 5-HT evoked a fast inward current in myenteric neurons that was pharmacologically identified as 5-HT(3) mediated. The EC(50) of the 5-HT response was lower in SERT +/+ (18 microM) than in SERT -/- (36 microM) mice and desensitized rapidly in a greater proportion of SERT -/- neurons; however, peak amplitudes, steady-state current, and decay time constants were not different. Adaptive changes thus occur in the subunit composition of enteric 5-HT(3) receptors of SERT -/- mice that are reflected in 5-HT(3) receptor affinity and desensitization.

Serotonin 5-HT(3) receptors in rat CA1 hippocampal interneurons: functional and molecular characterization

The molecular makeup of the serotonin 5-HT(3) receptor (5-HT(3)R) channel was investigated in rat hippocampal CA1 interneurons in slices using single-cell RT-PCR and patch-clamp recording techniques. We tested for the expression of the 5-HT(3A) (both short and long splice variants) and 5-HT(3B) subunits, as well as the expression of the alpha4 subunit of the neuronal nicotinic ACh receptors (nAChRs), the latter of which has been shown to co-assemble with the 5-HT(3A) subunit in heterologous expression systems. Both the 5-HT(3A)-short and alpha4-nAChR subunits were expressed in these interneurons, but we could not detect any expression of either the 5-HT(3B) or the 5-HT(3A)-long subunits. Furthermore, there was a strong tendency for the 5-HT(3A)-short and alpha4-nAChR subunits to be co-expressed in individual interneurons. To assess whether there was any functional evidence for co-assembly between the 5-HT(3A)-short and alpha4-nAChR subunits, we used the sulphydryl agent 2-aminoethyl methanethiosulphonate (MTSEA), which has previously been shown to modulate expressed 5-HT(3)Rs that contain the alpha4-nAChR subunit. In half of the interneurons examined, MTSEA significantly enhanced the amplitude of the 5-HT(3)R-mediated responses, which is consistent with the notion that the alpha4-nAChR subunit co-assembles with the 5-HT(3A) subunit to form a native heteromeric 5-HT(3)R channel in rat CA1 hippocampal interneurons in vivo. In addition, the single-channel properties of the 5-HT(3)R were investigated in outside-out patches. No resolvable single-channel currents were observed. Using non-stationary fluctuation analysis, we obtained an estimate of the single-channel conductance of 4 pS, which is well below that expected for channels containing both the 5-HT(3A) and 5-HT(3B) subunits.

Open probability of homomeric murine 5-HT3A serotonin receptors depends on subunit occupancy

1. The time course of macroscopic current responses of homomeric murine serotonin 5-HT3A receptors was studied in whole cells and excised membrane patches under voltage clamp in response to rapid application of serotonin. 2. Serotonin activated whole cell currents with an EC(50) value for the peak response of 2 microM and a Hill slope of 3.0 (n = 12), suggesting that the binding of at least three agonist molecules is required to open the channel. 3. Homomeric 5-HT3A receptors in excised membrane patches had a slow activation time course (mean +/- S.E.M. 10-90 % rise time 12.5 +/- 1.6 ms; n = 9 patches) for 100 microM serotonin. The apparent activation rate was estimated by fitting an exponential function to the rising phase of responses to supramaximal serotonin to be 136 s(-1). 4. The 5-HT3A receptor response to 100 microM serotonin in outside-out patches (n = 19) and whole cells (n = 41) desensitized with a variable rate that accelerated throughout the experiment. The time course for desensitization was described by two exponential components (for patches tau(slow) 1006 +/- 139 ms, amplitude 31 %; tau(fast) 176 +/- 25 ms, amplitude 69 %). 5. Deactivation of the response following serotonin removal from excised membrane patches (n = 8) and whole cells (n = 29) was described by a dual exponential time course with time constants similar to those for desensitization (for patches tau(slow) 838 +/- 217 ms, 55 % amplitude; tau(fast) 213 +/- 44 ms, 45 % amplitude). 6. In most patches (6 of 8), the deactivation time course in response to a brief 1-5 ms pulse of serotonin was similar to or slower than desensitization. This suggests that the continued presence of agonist can induce desensitization with a similar or more rapid time course than agonist unbinding. The difference between the time course for deactivation and desensitization was voltage independent over the range -100 to -40 mV in patches (n = 4) and -100 to +50 mV in whole cells (n = 4), suggesting desensitization of these receptors in the presence of serotonin does not reflect a voltage-dependent block of the channel by agonist. 7. Simultaneously fitting the macroscopic 5-HT3A receptor responses in patches to submaximal (2 microM) and maximal (100 microM) concentrations of serotonin to a variety of state models suggests that homomeric 5-HT3A receptors require the binding of three agonists to open and possess a peak open probability greater than 0.8. Our modelling also suggests that channel open probability varies with the number of serotonin molecules bound to the receptor, with a reduced open probability for fully liganded receptors. Increasing the desensitization rate constants in this model can generate desensitization that is more rapid than deactivation, as observed in a subpopulation of our patches.

Postsynaptic target regulates functional responses induced by 5-HT3 serotonin receptors on axonal varicosities of NG108-15 hybrid neuroblastoma cells

Rat brain presynaptic 5-HT3 serotonin receptors, members of the ligand-gated ion channel superfamily, induce changes in nerve terminal [Ca2+]i in a manner distinct from that found for somatic 5-HT3 receptors. Here, we assessed the role of postsynaptic target in regulating the nature of presynaptic receptor-induced responses, using the hybrid neuroblastoma cell line NG108-15 as a model neuronal system that expresses 5-HT3 receptors. Using immunocytochemistry, 5-HT3 receptors were found to be present on the presynaptic-like varicosities of differentiated NG108-15 cells, indicating that these receptors possess an inherent ability to localize to potential presynaptic sites. In the absence of postsynaptic target, 5-HT3 receptors localized to the varicosities induce rapid but transient changes in [Ca2+]i that were initiated by voltage-gated Ca2+ channels, as assessed using Ca2+ channel blockers, these properties being typical of those found for somatic 5-HT3 receptors. In co-cultures containing rat myotubes, with which NG108-15 cells form functional cholinergic synapses, the 5-HT3 receptor-induced changes in [Ca2+]i in the axonal varicosities shifted over time (three to 10 days) to that found for brain nerve endings: sustained responses that were insensitive to blockade by antagonists of voltage-gated Ca2+ channels. The effect of co-culturing myotubes with the NG108-15 cells was mimicked by conditioned media from myotube cultures. These results indicate that regulatory molecules from the target postsynaptic cell dictate the functional responses elicited by presynaptic 5-HT3 receptors. Because the target-induced changes required several days before they were evident, we hypothesize that changes in protein expression, perhaps the consequence of altered gene regulation, underlie the changes in the responses to 5-HT3 receptor activation in the axonal varicosities of this neuronal cell line.

Functional differences between splice variants of the murine 5-HT(3A) receptor: possible role for phosphorylation

The murine 5-HT(3A) receptor subunit is expressed as either of two splice variants which are differentially regulated in vivo. The difference resides in a six-amino acid sequence within the cytoplasmic loop between transmembrane regions 3 and 4, which is present in the long form but not the short form. No physiological roles have yet been ascribed to the two splice variants. Whole cell patch clamp recording from transfected HEK 293 cells stably expressing either long or short form receptors showed very similar responses under control conditions. However, inclusion of 1 mM cAMP (activator of protein kinase A) in the patch pipette caused an initial increase in the desensitization rate of the long form, but a decrease in the short form. With the addition of 100 nM phorbol 12-myristate 13-acetate (PMA; activator of protein kinase C) to the pipette solution, responses elicited with 1 microM 5-HT revealed an increase in the current amplitude in the long but not the short form of the receptor. Over a longer time period, inclusion of PMA in the patch-pipette caused a faster run down of peak current amplitude in response to 30 microM 5-HT in the long form but did not affect the short form; there was no observed long-term effects of cAMP. We conclude that the long and short forms of the 5-HT(3) receptor are differentially modulated by agents that activate PKA and PKC. These different patterns of modulation could have markedly divergent consequences on receptor function.

Zn(2+) enhancement of the recombinant 5-HT(3) receptor is modulated by divalent cations

The modulation by Zn(2+) of recombinant murine 5-hydroxytryptamine(3A) (5-HT(3A)) receptor responses and its modification by Ca(2+) or Mg(2+) were studied using whole-cell voltage clamp and radioligand binding techniques. In the absence of other added divalent cations Zn(2+) enhanced the response to 5-HT by increasing maximum peak current (I(max)) to a maximum of 122.5%, decreasing the rate of desensitization (maximum t(1/2)=210%), and decreasing the EC(50) by approximately two fold. In the presence of Ca(2+) or Mg(2+), the effects of Zn(2+) on I(max) and t(1/2) were still manifest, although higher Zn(2+) concentrations were required; however, the effect on EC(50) was abolished. Zn(2+) also enhanced [3H]agonist but not [3H]antagonist binding. We propose there is more than one Zn(2+) binding site on the 5-HT(3) receptor molecule, and that one or more of these sites may also bind Ca(2+) and Mg(2+).