[3H]resiniferatoxin binding to pig dorsal horn membranes displays positive cooperativity

An external sodium ion binding site controls allosteric gating in TRPV1 channels

TRPV1 channels in sensory neurons are integrators of painful stimuli and heat, yet how they integrate diverse stimuli and sense temperature remains elusive. Here, we show that external sodium ions stabilize the TRPV1 channel in a closed state, such that removing the external ion leads to channel activation. In studying the underlying mechanism, we find that the temperature sensors in TRPV1 activate in two steps to favor opening, and that the binding of sodium to an extracellular site exerts allosteric control over temperature-sensor activation and opening of the pore. The binding of a tarantula toxin to the external pore also exerts control over temperature-sensor activation, whereas binding of vanilloids influences temperature-sensitivity by largely affecting the open/closed equilibrium. Our results reveal a fundamental role of the external pore in the allosteric control of TRPV1 channel gating and provide essential constraints for understanding how these channels can be tuned by diverse stimuli.

DOI:http://dx.doi.org/10.7554/eLife.13356.001

Humans and other mammals sense elevated heat and other painful stimuli via a sensory ion channel protein called TRPV1. Ion channels create pores in the outer membrane of cells and act as gates that open and close to regulate the flow of ions into and out of cells. This flow of ions generates electrical signals that sensory neurons use to communicate information about the environment to the brain. The TRPV1 channel is opened by heat, but also by venom toxins, acid and the active ingredient in hot chilli peppers.

The fluid that surrounds animal cells often contains high levels of sodium ions, and these ions flow through TRPV1 channels when they are open. Also, when sodium ions are removed from the fluid surrounding a cell, TRPV1 channels will spontaneously open. It is not known why this happens, but it suggests that sodium ions help to regulate the activity of the TRPV1 channel, and despite having been extensively studied, it remains unclear how this channel senses temperature and responds to other harmful stimuli.

Jara-Oseguera et al. have now investigated the role of sodium ions in activating TRPV1 in cells grown in the laboratory. The experiments involved a technique that records the movement of ions through TRPV1 channels in the cell surface membrane while exposing the cell to various stimuli that open or close the channels. The results showed that TRPV1 contains an important site on its outer surface that binds sodium ions and fine-tunes the channel’s properties. This enables the channel to be activated by potentially damaging temperatures. Further analysis revealed that TRPV1 undergoes at least two distinct physical changes in response to heat and that the binding of sodium ions to the channel’s outer structure regulates one of these changes.

Jara-Oseguera et al. went on to discover that the binding of a tarantula toxin to the outer surface of TRPV1 renders the channel insensitive to changes in temperature. These findings suggest that structural changes to the external part of this channel protein are closely associated with its ability to sense temperature, possibly because this region of the protein is directly responsible for detecting changes in temperature.

These findings shed light on how this channel detects and responds to dangerously high temperatures, as well to other harmful stimuli. An aim for future studies is to pinpoint the specific regions of the protein that form the sodium-binding sites and to test the hypothesis that temperature sensors are located on the outer surface of the protein.

DOI:http://dx.doi.org/10.7554/eLife.13356.002

Pharmacokinetics of eugenol and its effects on thermal hypersensitivity in rats

Neuropathic pain is a type of chronic pain following central or peripheral nervous system lesions that cause allodynia (pain initiated by a non-painful stimulus) and hyperalgesia (increased pain sensation following a painful stimulus). The first objective of the study was to evaluate the pharmacokinetics of eugenol, the principle chemical constituent of clove oil, following a gavage administration (40 mg/kg) in male Sprague-Dawley rats. The second objective was to evaluate the effect of repeated oral administrations of eugenol on hyperalgesia and allodynia using an experimental model of neuropathic pain in rats. Thermal and mechanical sensitivity (Hargreave's test and von Frey filaments) were determined in sciatic nerve cuff-implanted rats. Sensitivities were assessed following repeated oral administrations of 40 mg/kg of eugenol or saline for 5 days (n=6 per group). Pharmacokinetic parameters were calculated using noncompartmental methods. Serial blood samples were collected over 24 h. Concentrations of eugenol in blood and plasma peaked rapidly following oral administration. Mean T(1/2) values of eugenol in plasma and blood were long (14.0 and 18.3 h, respectively), suggesting a potential accumulation of the drug following repeated administrations. Reaction time to thermal stimuli appeared to increase constantly following repeated administrations of eugenol. On the last day of treatment, eugenol treatments resulted in a statistically significant prolongation of the reaction time to thermal stimuli in rats compared to the saline group (Mean+/-S.E.M.: 11.4+/-1.23 vs. 6.1+/-0.53 s, P<0.01). These results support the hypothesis that eugenol may alleviate neuropathic pain and that the cumulative effect of the drug may be in part responsible for this effect following repeated daily administrations.

Vanilloid-mediated heterosynaptic facilitation of inhibitory synaptic input to neurons of the rat dorsal motor nucleus of the vagus

Vanilloid type-1 receptors (VR1) are abundant in the dorsal vagal complex, where their function is mostly unknown. We examined the role of VR1 in regulating synaptic inputs to neurons of the dorsal motor nucleus of the vagus (DMV). Using patch-clamp recordings from DMV neurons in brainstem slices, capsaicin was found to increase action potential-independent inhibitory input onto DMV neurons. This rapid effect was mimicked by application of the endogenous cannabinoid, anandamide and blocked by VR1 antagonists. The VR1-mediated facilitation of synaptic inhibition was reduced by ionotropic and metabotropic glutamate receptor antagonists, suggesting an indirect, heterosynaptic enhancement of GABA release caused by a VR1-mediated increase in glutamate release from presynaptic terminals of excitatory neurons. Application of L-glutamate also increased GABA release. The paired-pulse ratio was increased for IPSCs evoked after electrical stimulation of the nucleus tractus solitarius, but the effect was slower than for the enhancement of spontaneous and miniature IPSCs. Capsaicin also increased the frequency of glutamatergic postsynaptic currents in a VR1-mediated manner. Results of these studies suggest that VR1-containing glutamatergic terminals contact DMV neurons. Activation of VR1 potently enhances glutamate release onto GABAergic terminals, facilitating GABA release. Endogenous cannabinoids can thereby rapidly enhance inhibitory input to DMV neurons via VR1-mediated presynaptic mechanisms.

Effects of capsaicin on the sensorimotor function of the proximal stomach in humans

BACKGROUND

Capsaicin-sensitive afferents have been implicated in the control of visceral sensorimotor functions.

AIM

To study the effects of capsaicin on the sensorimotor function of the proximal stomach in humans.

METHODS

In 10 healthy subjects, a gastric barostat was used to study the effect of saline or capsaicin-containing red pepper sauce on the sensitivity to gastric distension and on gastric accommodation to a meal.

RESULTS

The administration of capsaicin was accompanied by a relaxation of the proximal stomach. After capsaicin, the compliance of the proximal stomach was increased and higher perception scores were reached for the same distending pressures. Dosing with capsaicin significantly decreased the pressures and corresponding wall tensions at the discomfort threshold. Gastric accommodation to a meal was not altered by capsaicin.

CONCLUSION

The acute administration of capsaicin decreases proximal gastric tone, inhibits phasic contractility of the proximal stomach and increases sensitivity to proximal gastric distension. These data suggest the involvement of vanilloid receptor 1 in the modulation of gastric sensorimotor function in humans.

Stimulatory effect of centrally injected capsaicin, an agonist of vanilloid receptors, on gastric acid secretion in rats

Capsaicin, the main pungent ingredient in chilli peppers, acts through specific vanilloid receptors on sensory neurons. The vanilloid receptors have been localized in the brain. We describe here a stimulatory effect of centrally injected capsaicin on gastric acid secretion in urethane-anesthetized rats. Injection of capsaicin (10-30 nmol per rat) into the lateral cerebroventricle markedly stimulated the secretion. Injection of capsazepine (30 nmol) or ruthenium red (30 nmol), antagonists for vanilloid receptors, into the lateral cerebroventricle inhibited the secretion induced by capsaicin, although these antagonists alone significantly stimulated the secretion. Injection of capsaicin into the fourth cerebroventricle also stimulated gastric acid secretion. The effects of centrally injected capsaicin into the lateral and fourth cerebroventricle were mediated via the vagus cholinergic nerve, because the effects were abolished by bilateral vagotomy at the cervical level. The present findings showed that central injection of capsaicin stimulated gastric acid secretion, via vanilloid receptors in the central nervous system (CNS), and through vagus nerve mechanisms in the perfused stomach of urethane-anesthetized rats.

Opiates: biphasic dose responses

It was shown that biphasic responses are commonly reported for opiates with respect to a broad range of animal models and endpoints. These endpoints include such diverse functions as blood pressure, muscle tension, breathing rates, hCG production, HIV production, neutrophil migration, ACTH production, protein binding, and neuronal functioning. Quantitative features of the dose-response relationships indicated that the maximum stimulatory responses were < or = 3-fold greater than the controls with most being between 10 to 70% greater than the controls. In contrast to the striking similarity in the maximum stimulatory response, there was marked variation with respect to the dose range of the stimulatory responses that varied from 10(1) to 10(10). Mechanistic assessments were conducted for most biphasic dose-response relationships and are addressed in detail.

Functional and desensitizing effects of the novel synthetic vanilloid-like agent 12-phenylacetate 13-acetate 20-homovanillate (PPAHV) in the perfused rat hindlimb

1. In the present study, the effects of the novel vanilloid agonist, 12-phenylacetate 13-acetate 20-homovanillate (PPAHV), on oxygen consumption (VO(2)) and vascular resistance (perfusion pressure, PP) were investigated in the constant flow, perfused rat hindlimb. The acute desensitizing properties of this novel synthetic agent were also examined. 2. Maximum stimulation of VO(2) was produced by 0.2 microM PPAHV (delta VO(2), 0.83+/-0.06 micromol g(-1) h(-1)) and was accompanied by mild vasoconstriction (increase in PP; 8.0+/-1.1 mmHg). The highest concentration of PPAHV tested (2 microM) caused inhibition of VO(2) (delta VO(2), -2.73+/-0.51 micromol g(-1) h(-1)) and strong vasoconstriction (delta PP, 42.0+/-1.2 mmHg). 3. Capsazepine (10 microM) caused a parallel shift to the right of both VO(2) and PP concentration-response curves for PPAHV (pK(b)=5.00), indicative of competitive binding to vanilloid receptors. 4. The stimulation of VO(2) produced by 0.2 microM PPAHV decreased, but was not completely abolished, after repeated infusion of PPAHV (change in VO(2), first infusion, 0.66+/-0.18 micromol g(-1) h(-1); sixth infusion, 0.29+/-0. 08 micromol g(-1) h(-1), P<0.05), an acute tachyphylactic response not previously seen with the repeated infusion of other vanilloid analogues. Conversely, the PP response to repeated PPAHV infusion increased (delta PP, first infusion, 5.8+/-0.7 mmHg; sixth infusion, 9.0+/-0.6 mmHg, P<0.05). 5. In conclusion, PPAHV produces vasoconstriction and a biphasic effect on VO(2) in the perfused rat hindlimb very similar to that induced by naturally occurring vanilloids. Both effects are blocked by the competitive antagonist capsazepine. Since, the metabolic response to low concentrations of PPAHV (stimulation of VO(2)) undergoes tachyphylaxis, the present data suggest that PPAHV desensitizes putative vanilloid receptors in the hindlimb.

Capsazepine, a vanilloid receptor antagonist, inhibits nicotinic acetylcholine receptors in rat trigeminal ganglia

Vanilloid receptors are activated by capsaicin, the pungent ingredient in hot pepper. They are also specifically and competitively inhibited by capsazepine (CPZ). To determine whether CPZ is specific to vanilloid receptors, its effects were tested on the currents evoked by nicotine in rat trigeminal ganglia. We found that 10 microM CPZ, a concentration frequently used to inhibit capsaicin's physiological responses attributed to capsaicin, reversibly inhibits (40%) the magnitude of the currents activated by 100 microM nicotine. We conclude that 10 microM capsazepine can alter the effects of channels other than those activated by capsaicin, and thus caution must be used in attributing all the CPZ-sensitive physiological effects to those only produced by blocking of vanilloid receptors.

Specific binding of [3H]resiniferatoxin by human and rat preoptic area, locus ceruleus, medial hypothalamus, reticular formation and ventral thalamus membrane preparations

Specific [3H]resiniferatoxin (RTX) binding detects the vanilloid (capsaicin) receptors and provides a biochemical means for exploring their pharmacology. In the present study we demonstrate specific vanilloid (RTX) binding sites in various brain areas not known to be innervated by primary afferent neurons. Specific high-affinity binding of [3H]RTX could be detected in membrane preparations of the posterior ("hypothalamic") and anterior ("septal") parts of the preoptic area, locus ceruleus, medial hypothalamus, brainstem reticular formation and ventral thalamic nuclei from naive rats. The determined levels of binding at 4 nM [3H]RTX were 23.0 +/- 4.5, 7.1 +/- 1.6, 29.9 +/- 2.3, 23.5 +/- 2.4, 9.9 +/- 2.2 and 8.1 +/- 1.9 fmol/mg, respectively; unfortunately, the high levels of non-specific binding (higher than 80%) in the present experiments made it impossible for us to characterize fully the binding properties of the receptors. However, no detectable specific [3H]RTX binding was present in membranes of brain nuclei from rats pretreated with 300 mg/kg capsaicin, a treatment which causes loss of response to capsaicin. Significant specific [3H]RTX binding was also absent in membrane preparations of the midbrain central gray matter, somatosensory cortex and cerebellum either from naive or capsaicin treated rats. In human brain specific [3H]RTX binding measured at 4 nM [3H]RTX showed a pattern of distribution similar to that in the rat brain. The corresponding levels of specific [3H]RTX binding in the preoptic area, locus ceruleus, medial hypothalamus, reticular formation and ventral thalamus were 44.9 +/- 2.4, 50.6 +/- 3.0, 36.1 +/- 2.9, 9.4 +/- 2.8 and 8.4 +/- 2.4 fmol/mg, respectively. Our findings corroborate previous biological evidence that vanilloid receptors are present in brain as well as in sensory afferent neurons.

Binding of neuroleptic drugs (trifluoperazine and rimcazole) to vanilloid receptors in porcine dorsal horn

Neuroleptic drugs were reported to modulate [3H]resiniferatoxin binding to vanilloid receptors in the spinal cord, with marked differences between rat and man. In the present study, we have used a [3H]resiniferatoxin binding assay using porcine dorsal horn membranes to explore further species differences in the interaction of neuroleptic drugs at spinal vanilloid receptors. Specific binding of 13 pM [3H]resiniferatoxin to porcine dorsal horn membranes (corresponding to a 7% fractional receptor occupancy) was affected by trifluoperazine in a bi-phasic fashion, with an initial 90% enhancement of binding preceding inhibition: a fit to the modified Hill equation yielded a cooperativity index of 1.8 and a Ki of 5 microM. Under similar conditions, rimcazole, by contrast, had a monophasic effect: it enhanced but, up to 100 microM, did not inhibit [3H]resiniferatoxin binding. These results are in accord with previous findings in human spinal cord but contrast with those in the rat. In experiments in which the concentration of [3H]resiniferatoxin was varied, 20 microM trifluoperazine reduced the Bmax by 33% (from 181 +/- 9 fmol/mg protein to 121 +/- 5 fmol/mg protein) without a measurable change in affinity or cooperativity. In parallel experiments, by contrast, neither capsaicin nor capsazepine (both at a concentration of 10 microM) affected the Bmax or cooperativity but, as expected, reduced the affinity from 61 +/- 8 pM to 120 +/- 11 pM or to 101 +/- 7 pM, respectively. Whereas vanilloid receptor agonists (resiniferatoxin and capsaicin) affected [3H]resiniferatoxin binding at low (approximately 7%) fractional receptor occupancies by the radioligand in a bi-phasic fashion, the competitive vanilloid receptor antagonist capsazepine failed to induce the initial binding enhancement. Thus, capsazepine appears to bind to vanilloid receptors in a non-cooperative fashion, or at least with much reduced positive cooperativity in this system. The mechanism by which neuroleptic drugs modulate resiniferatoxin binding is yet to be clarified and is clearly complicated as well as species-dependent; nonetheless, the reduced Bmax at higher concentrations suggests that it may at least in part be non-competitive.

Distinct structure-activity relations for stimulation of 45Ca uptake and for high affinity binding in cultured rat dorsal root ganglion neurons and dorsal root ganglion membranes

The [3H]resiniferatoxin (RTX) binding assay using membrane preparations has been used to identify and characterize the vanilloid receptors in the central and peripheral nervous system of different species. In the present study, using cultured adult rat dorsal root ganglion neurons either in suspension or attached to the tissue culture plates, we developed an assay to measure specific [3H]RTX binding by the intact cells. We were able to characterize the vanilloid binding characteristics of the neurons and compared those to the properties of vanilloid binding sites present in rat dorsal root ganglia membrane preparations. We found that [3H]RTX bound with similar affinity and positive cooperativity to attached neurons (cultured for 5 days before being assayed), neurons in suspension (using a filtration assay) and dorsal root ganglion membrane preparations. Dissociation constants obtained in the three assays were 47.6 +/- 3.5 pM, 38.4 +/- 3.1 pM and 42.6 +/- 3.1 pM, respectively. The cooperativity indexes determined by fitting the data to the Hill equation were 1.73 +/- 0.11, 1.78 +/- 0.12 and 1.78 +/- 0.09, respectively. The maximal binding capacity was 0.218 +/- 0.026 fmol/10(3) cells and 0.196 +/- 0.021 fmol/10(3) cells in the case of the attached cells and cells in suspension, respectively. Nonradioactive RTX, capsaicin, capsazepine and resiniferonol 20-homovanillylamide fully displaced specifically bound [3H]RTX from cells in suspension with Ki and Hill coefficient values of 42.5 +/- 5.3 pM, 2.06 +/- 0.16 microM, 3.16 +/- 0.21 microM and 32.4 +/- 4.1 nM and 1.79 +/- 0.17, 1.68 +/- 0.06, 1.72 +/- 0.11 and 1.81 +/- 0.12, respectively. Structure-activity analysis of different vanilloid derivatives revealed that the various compounds have distinct potencies for receptor binding and inducing 45Ca uptake in rat dorsal root ganglion neurons. Affinities for receptor binding and stimulation of 45Ca uptake of RTX, resiniferonol 20-homovanillylamide, RTX-thiourea, tinyatoxin, phorbol 12,13-dibenzoate 20-homovanillylamide and capsaicin were 38.5 +/- 2.9 pM, 25.7 +/- 3.0 nM, 68.5 +/- 3.8 nM, 173 +/- 25 pM, 7.98 +/- 0.83 microM and 4.93 +/- 0.35 microM as compared to 0.94 +/- 0.12 nM, 26.5 +/- 3.5 nM, 149 +/- 30 nM, 1.46 +/- 0.25 nM, 1.41 +/- 0.48 microM and 340 +/- 57 nM. Computer fitting of the data yielded Hill coefficient values indicating positive cooperativity of receptor binding; however, stimulation of 45Ca uptake appeared to follow a non-cooperative mechanism of action. The competitive capsaicin antagonist capsazepine inhibited specific binding of [3H]RTX by rat dorsal root ganglion membrane preparations with Ki and Hill coefficient values of 3.89 +/- 0.38 microM and 1.74 +/- 0.11. On the other hand it inhibited the induction of 45Ca uptake into the cells induced by capsaicin and RTX in a non-cooperative fashion with Ki values of 271 +/- 29 nM and 325 +/- 47 nM. Our results show that the membrane binding assay relates to the reality of receptor function in the intact, cultured neurons, both in terms of affinity and positive cooperativity. However the different vanilloid derivatives displayed markedly distinct structure-activity relations for high affinity receptor binding and stimulation of 45Ca uptake into rat dorsal root ganglion neurons. Among various explanations for this discrepancy, we favor the possibility that the two assays detect distinct classes of the vanilloid (capsaicin) receptor present in primary sensory neurons.

Comparison of [3H]resiniferatoxin binding by the vanilloid (capsaicin) receptor in dorsal root ganglia, spinal cord, dorsal vagal complex, sciatic and vagal nerve and urinary bladder of the rat

In the present report we compared the properties of [3H]resiniferatoxin (RTX) binding by the vanilloid receptors present at different parts of the primary afferent neurons of the rat. We found no major differences in either the affinity or the cooperativity of [3H]RTX binding by vanilloid receptors on the cell body, central terminals, peripheral terminals or axons. Specific binding of [3H]RTX to dorsal root ganglia, whole spinal cord, dorsal vagal complex, urinary bladder, and sciatic and vagal nerves all followed sigmoidal saturation kinetics indicating positive cooperativity among the binding sites. The cooperativity indexes determined by fitting the data to the Hill equation were 1.82 +/- 0.11, 2.21 +/- 0.04, 2.55 +/- 0.01, 1.91 +/- 0.11, 2.03 +/- 0.09 and 2.27 +/- 0.04, respectively. The dissociation constants in dorsal root ganglia, spinal cord, dorsal vagal complex, urinary bladder, and sciatic and vagal nerve membranes were 46.5 +/- 2.7, 29.3 +/- 5.1, 28.2 +/- 1.2, 60.8 +/- 4.4, 59.9 +/- 1.9 and 45.2 +/- 0.7 pM; the receptor densities were 219 +/- 14, 48 +/- 5, 67 +/- 1, 32 +/- 7, 61 +/- 9, and 100 +/- 20 fmol/mg protein, respectively. We could not show any major differences in the affinities of capsaicin and capsazepine in inhibition of [3H]RTX binding by the different membrane preparations either. In all cases the initial enhancement of [3H]RTX binding by nonradioactive RTX, capsaicin, and capsazepine confirmed the existence of positive cooperativity among the binding sites. We were unable to detect specific [3H]RTX binding sites in membrane preparations of the preoptic area, locus ceruleus, substantia nigra, striatum and paraventricular nuclei of the rat brain under our present conditions. Our results suggest the uniformity of the vanilloid receptors present at different parts of the primary afferent neuron.