␣-Scorpion toxins and sea anemone toxins bind to a common extracellular site on the Na ؉ channel and inhibit fast inactivation. Basic amino acids of the toxins and domains I and IV of the Na ؉ channel ␣ subunit have been previously implicated in toxin binding. To identify acidic residues required for toxin binding, extracellular acidic amino acids in domains I and IV of the type IIa Na ؉ channel ␣ subunit were converted to neutral or basic amino acids using site-directed mutagenesis, and altered channels were transiently expressed in tsA-201 cells and tested for 125 I-␣-scorpion toxin binding. Conversion of Glu 1613 at the extracellular end of transmembrane segment IVS3 to Arg or His blocked measurable ␣-scorpion toxin binding, but did not affect the level of expression or saxitoxin binding affinity. Conversion of individual residues in the IVS3-S4 extracellular loop to differently charged residues or to Ala identified seven additional residues whose mutation caused significant effects on binding of ␣-scorpion toxin or sea anemone toxin. Moreover, chimeric Na ؉ channels in which amino acid residues at the extracellular end of segment IVS3 of the ␣ subunit of cardiac Na ؉ channels were substituted into the type IIa channel sequence had reduced affinity for ␣-scorpion toxin characteristic of cardiac Na ؉ channels. Electrophysiological analysis showed that E1613R has 62-and 82-fold lower affinities for ␣-scorpion and sea anemone toxins, respectively. Dissociation of ␣-scorpion toxin is substantially accelerated at all potentials compared to wild-type channels. ␣-Scorpion toxin binding to wild type and E1613R had similar voltage dependence, which was slightly more positive and steeper than the voltage dependence of steady-state inactivation. These results indicate that nonidentical amino acids of the IVS3-S4 loop participate in ␣-scorpion toxin and sea anemone toxin binding to overlapping sites and that neighboring amino acid residues in the IVS3 segment contribute to the difference in ␣-scorpion toxin binding affinity between cardiac and neuronal Na ؉ channels. The results also support the hypothesis that this region of the Na ؉ channel is important for coupling channel activation to fast inactivation.Voltage-gated Na ϩ channels are responsible for the conduction of electrical impulses in most excitable tissues (1). The importance of their function is demonstrated by the effects of Na ϩ channel-specific neurotoxins that bind to at least six different receptor sites on the Na ϩ channel molecule and disrupt its normal behavior (reviewed in Refs. 2 and 3). These natural toxins are powerful tools for understanding and correlating ion channel structure and function, as exemplified by identification of molecular determinants for binding of the pore blocker tetrodotoxin, which has provided important information about the structure of the ion selectivity filter and pore (3, 4). Similarly, the identification of molecular determinants for binding of toxins that modify activation or inactivation will likely provide important ...

Vanilloid receptor 1 (TRPV1), a membrane-associated cation channel, is activated by the pungent vanilloid from chili peppers, capsaicin, and the ultra potent vanilloid from Euphorbia resinifera, resiniferatoxin (RTX), as well as by physical stimuli (heat and protons) and proposed endogenous ligands (anandamide, Narachidonyldopamine, N-oleoyldopamine, and products of lipoxygenase). Only limited information is available in TRPV1 on the residues that contribute to vanilloid activation. Interestingly, rabbits have been suggested to be insensitive to capsaicin and have been shown to lack detectable [ 3 H]RTX binding in membranes prepared from their dorsal root ganglia. We have cloned rabbit TRPV1 (oTRPV1) and report that it exhibits high homology to rat and human TRPV1. Like its mammalian orthologs, oTRPV1 is selectively expressed in sensory neurons and is sensitive to protons and heat activation but is 100-fold less sensitive to vanilloid activation than either rat or human. Here we identify key residues (Met 547 and Thr 550 ) in transmembrane regions 3 and 4 (TM3/4) of rat and human TRPV1 that confer vanilloid sensitivity, [ 3 H]RTX binding and competitive antagonist binding to rabbit TRPV1. We also show that these residues differentially affect ligand recognition as well as the assays of functional response versus ligand binding. Furthermore, these residues account for the reported pharmacological differences of RTX, PPAHV (phorbol 12-phenyl-acetate 13-acetate 20-homovanillate) and capsazepine between human and rat TRPV1. Based on our data we propose a model of the TM3/4 region of TRPV1 bound to capsaicin or RTX that may aid in the development of potent TRPV1 antagonists with utility in the treatment of sensory disorders.The receptor for capsaicin (a small vanilloid molecule extracted from "hot" chili peppers), designated vanilloid receptor 1 (also known as VR1 and TRPV1 1 (1)) has been cloned and shown to be a nonselective cation channel with high permeability to calcium. TRPV1 belongs to a superfamily of ion channels known as transient receptor potential channels (TRPs) several of which appear to be sensors of temperature (2, 3). TRPV1 can be activated by exogenous agonists (capsaicin and RTX) and by physical stimuli such as heat (Ͼ42°C) and protons (pH 5). Possible endogenous ligands released during tissue injury have also been suggested, including anandamide (arachidonylethanolamine or AEA) and products of lipoxygenases such as 12-hydroperoxyeicosatetraenoic acid, N-arachidonyldopamine (NADA), and N-oleoyldopamine (OLDA) (4 -7). Ji et al. (8) reported that TRPV1 is detectable at increased levels after inflammatory injury in rodents and speculated that the increased level of TRPV1 protein combined with the confluence of stimuli present in inflammatory injury states leads to a reduced threshold of activation of nociceptors that express TRPV1, i.e. hyperalgesia. Indeed the converse is true that TRPV1-deficient mice display reduced thermal hypersensitivity following inflammatory tissue injury (9). Structure-func...

The vanilloid receptor 1 (VR1 or TRPV1) is a membrane-bound, nonselective cation channel expressed by peripheral sensory neurons. TRPV1 antagonists produce antihyperalgesic effects in animal models of inflammatory and neuropathic pain. Here, we describe the in vitro and in vivo pharmacology of a novel TRPV1 antagonist, AMG 9810,AMG 9810 is a competitive antagonist of capsaicin activation (IC 50 value for human TRPV1, 24.5 Ϯ 15.7 nM; rat TRPV1, 85.6 Ϯ 39.4 nM) and blocks all known modes of TRPV1 activation, including protons (IC 50 value for rat TRPV1, 294 Ϯ 192 nM; human TRPV1, 92.7 Ϯ 72.8 nM), heat (IC 50 value for rat TRPV1, 21 Ϯ 17 nM; human TRPV1, 15.8 Ϯ 10.8 nM), and endogenous ligands, such as anandamide, N-arachidonyl dopamine, and oleoyldopamine. AMG 9810 blocks capsaicin-evoked depolarization and calcitonin gene-related peptide release in cultures of rat dorsal root ganglion primary neurons. Screening of AMG 9810 against a panel of G protein-coupled receptors and ion channels indicated selectivity toward TRPV1. In vivo, AMG 9810 is effective at preventing capsaicin-induced eye wiping in a dose-dependent manner, and it reverses thermal and mechanical hyperalgesia in a model of inflammatory pain induced by intraplantar injection of complete Freund's adjuvant. At effective doses, AMG 9810 did not show any significant effects on motor function, as measured by open field locomotor activity and motor coordination tests. AMG 9810 is the first cinnamide TRPV1 antagonist reported to block capsaicin-induced eye wiping behavior and reverse hyperalgesia in an animal model of inflammatory pain.Activation of peripheral nociceptors in humans by capsaicin results in burning pain (Park et al., 1995). Capsaicin, and its ultrapotent analog resiniferatoxin, aided the identification and characterization of the vanilloid receptor 1 (aka VR1 and TRPV1). TRPV1 is a nonselective cation channel with high permeability to calcium (Caterina et al., 1997) and belongs to a superfamily of ion channels known as the transient receptor potential channels or TRPs (Clapham et al., 2001). In addition to activation by exogenous agonists such as capsaicin and resiniferatoxin, TRPV1 can be activated by physical stimuli, such as heat (Ͼ42°C) and protons (pH 5). Based on their structural similarity to capsaicin, several endogenous ligands have been proposed that include anandamide (AEA), 12-hydroperoxy-5,8,10,14-eicosatetraenoic acid,N-arachidonyl dopamine (NADA), N-oleoyldopamine (OLDA), and products of lipoxygenases (Hwang et al., 2000;Olah et al., 2001;Huang et al., 2002;Chu et al., 2003). TRPV1 is up-regulated during inflammation (Ji et al., 2002), and channel activity is modulated by the action of inflammaArticle, publication date, and citation information can be found at http://jpet.aspetjournals.org.doi :

Polypeptide neurotoxins alter ion channel gating by binding to extracellular receptor sites, even though the voltage sensors are in their S4 transmembrane segments. By analysis of sodium channel chimeras, a beta-scorpion toxin is shown here to negatively shift voltage dependence of activation and enhance closed state inactivation by binding to a receptor site that requires glycine 845 (Gly-845) in the S3-S4 loop at the extracellular end of the S4 segment in domain II of the alpha subunit. Toxin action requires prior depolarization to drive the S4 voltage sensors outward, but these effects are lost in the mutant G845N. The results reveal a voltage sensor-trapping model of toxin action in which the IIS4 voltage sensor is trapped in its outward, activated position by toxin binding.

The voltage-gated ion channels and their structural relatives are a large superfamily of membrane proteins specialized for electrical signaling and ionic homeostasis (1). Voltage-gated sodium channels are responsible for the increase in sodium permeability that initiates action potentials in electrically excitable cells (2) and are the molecular target for several groups of neurotoxins, which bind to different receptor sites and alter voltage-dependent activation, conductance, and inactivation (3, 4). Sodium channels are composed of one pore-forming ␣ subunit of ϳ2000 amino acid residues associated with one or two smaller auxiliary subunits, ␤1-␤4 (5-7). The ␣ subunit consists of four homologous domains (I-IV), each containing six transmembrane segments (S1-S6), and a re-entrant pore loop (P) between S5 and S6 (5). The S4 transmembrane segments are positively charged and serve as voltage sensors to initiate channel activation (8 -14). However, the molecular mechanism of voltage sensing by sodium channels and the other members of the voltage-gated ion channel family is unknown.The initial "sliding helix" (9) or "helical screw" (15) models for voltage sensing proposed that the S4 segments, which have positively charged amino acids at intervals of three residues, transport gating charges outward to activate sodium channels in response to depolarization by moving along a spiral pathway through the protein structure. This movement would preserve interactions with surrounding hydrophilic and negatively charged amino acid residues during gating and thereby stabilize the gating charges in the intramembrane environment. Many structure-function studies have supported this general model (see "Discussion"). In contrast, x-ray crystallographic studies of a bacterial voltage-gated K ϩ channel in complex with detergent and a site-directed antibody yielded a structure in which the S3 and S4 segments lay along the position of the intracellular surface of the membrane, dissociated from the remainder of the protein (16 -18). These results led to the concept that the voltage sensors function as loosely linked "paddles," pivoting through the phospholipid surrounding the core of the ion channel as a semi-rigid body rather than moving gating charge outward through the protein structure. This paddle model makes strikingly different predictions for polypeptide toxins that modify gating by interaction with the voltage sensors. Whereas polypeptide toxins might be able to bind the extracellular end of the voltage sensors in the resting state in a sliding helix or helical screw gating model, the S4 segments would not be expected to be available for toxin binding in the resting state in the paddle model.Scorpion venoms contain two groups of polypeptides toxins that alter sodium channel gating. The ␣-scorpion toxins, as well as sea anemone toxins and some spider toxins, bind to neurotoxin receptor site 3 and slow or block inactivation (19 -22). Amino acid residues that contribute to neurotoxin receptor site

The clinical efficacy of local anesthetic and antiarrhythmic drugs is due to their voltage-and frequencydependent block of Na+ channels. Quaternary local anesthetic analogs such as QX-314, which are permanently charged and membrane-impermeant, effectively block cardiac Na+ channels when applied from either side of the membrane but block neuronal Na+ channels only from the intracellular side. This difference in extracellular access to QX-314 is retained when rat brain rIlA Na+ channel a subunits and rat heart rHl Na+ channel a subunits are expressed transiently in tsA-201 cells. Amino acid residues in transmembrane segment S6 of homologous domain IV (IVS6) of Na+ channel a subunits have important effects on block by local anesthetic drugs. Although five amino acid residues in IVS6 differ between brain rIIA and cardiac rHl, exchange of these amino acid residues by sitedirected mutagenesis showed that only conversion of Thr-1755 in rHi to Val as in rIIA was sufficient to reduce the rate and extent of block by extracellular QX-314 and slow the escape of drug from closed channels after use-dependent bloclk Tetrodotoxin also reduced the rate of block by extracellular QX-314 and slowed escape of bound QX-314 via the extracellular pathway in rHl, indicating that QX-314 must move through the pore to escape.

Enzymatically isolated myocytes from ferret right ventricles (12-16 wk, male) were studied using the whole cell patch clamp technique. The macroscopic properties of a transient outward K + current /to were quantified. /to is selective for K +, with a PNa/P~ of 0.082. Activation of/to is a voltage-dependent process, with both activation and inactivation being independent of Na + or Ca ~+ influx. Steady-state inactivation is well described by a single Boltzmann relationship (V1/z = -13.5 mV; k = 5.6 mV). Substantial inactivation can occur during a subthreshold depolarization without any measurable macroscopic current. Both development of and recovery from inactivation are well described by single exponential processes. Ensemble averages of single /to channel currents recorded in cellattached patches reproduce macroscopic/to and indicate that inactivation is complete at depolarized potentials. The overall inactivation/recovery time constant curve has a bell-shaped potential dependence that peaks between -10 and -20 mV, with time constants (22°C) ranging from 23 ms (-90 mV) to 304 ms (-1O mV). Steady-state activation displays a sigmoidal dependence on membrane potential, with a net aggregate half-activation potential of +22.5 mV. Activation kinetics (0 to +70 mV, 22°C) are rapid, with Ito peaking in ~5-15 ms at +50 inV. Experiments conducted at reduced temperatures (12°C) demonstrate that activation occurs with a time delay. A nonlinear least-squares analysis indicates that three closed kinetic states are necessary and sufficient to model activation. Derived time constants of activation (22°C) ranged from 10 ms (+10 mV) to 2 ms (+70 mV). Within the framework of Hodgkin-Huxley formalism,/to gating can be described using an a 3i formulation.Address reprint requests to Dr.

Voltage-gated sodium channels in brain neurons were found to associate with receptor protein tyrosine phosphatase beta (RPTPbeta) and its catalytically inactive, secreted isoform phosphacan, and this interaction was regulated during development. Both the extracellular domain and the intracellular catalytic domain of RPTPbeta interacted with sodium channels. Sodium channels were tyrosine phosphorylated and were modulated by the associated catalytic domains of RPTPbeta. Dephosphorylation slowed sodium channel inactivation, positively shifted its voltage dependence, and increased whole-cell sodium current. Our results define a sodium channel signaling complex containing RPTPbeta, which acts to regulate sodium channel modulation by tyrosine phosphorylation.