Multi-gene epigenetic silencing of tumor suppressor genes in T-cell lymphoma cells; delayed expression of the p16 protein upon reversal of the silencing

To understand better T-cell lymphomagenesis, we examined promoter CpG methylation and mRNA expression of closely related genes encoding p16, p15, and p14 tumor suppressor genes in cultured malignant T-cells that were derived from cutaneous, adult type, and anaplastic lymphoma kinase (ALK)-expressing T-cell lymphomas. p16 gene was epigenetically silenced in all but one of the 10 malignant T-cell lines examined, p15 gene silenced in roughly half of the lines, and p14 was the least frequently affected. Extensive methylation of the p16 promoter was seen in six out of 10 cutaneous T-cell lymphoma patient samples and corresponded with lack of p16 protein expression in the cases examined. Treatment of cultured T-cells with the DNA methyltransferase inhibitor, 5-aza-2-deoxy-cytidine, resulted in reversal of the p16 gene silencing. However, expression of p16 protein was delayed in relationship to p16 promoter demethylation and required up to 3 weeks to occur, seemingly reflecting late activation of the p16 gene. These findings indicate that epigenetic silencing affects in T-cell malignancies, often simultaneously, several tumor suppressor genes that impact on key cell functions. The observed differential silencing of p16 and p14, and to a lesser degree p15 gene, indicates that the silencing is governed by precise, promoter region-specific mechanisms. The study provides also further rationale for treatment of at least some types of T-cell lymphomas with DNA methyltransferase inhibitors to target the epigenetically silenced tumor suppressor genes.

Vanilloid receptor ligands: hopes and realities for the future

Neurons possessing C-fibers transmit nociceptive information into the central nervous system and participate in various reflex responses. These neurons carry receptors that bind capsaicin, recently identified as the vanilloid VR1 receptor. Excitation of these cells by capsaicin is followed by a lasting refractory state, termed desensitisation, in which the neurons fail to respond to a variety of noxious stimuli. Desensitisation to capsaicin has a clear therapeutic potential in relieving neuropathic pain and ameliorating urinary bladder overactivity, just to cite 2 important examples. Vanilloids may also be beneficial in the treatment of benign prostate hyperplasia (BPH). Since the majority of elderly patients have neuropathic pain co-existent with urinary incontinence and/or BPH, a drug that ameliorates pain and improves urinary symptoms at the same time promises to be of great clinical value in geriatric medicine. In fact, capsaicin has already been shown to have a role in the treatment of conditions that can arise in the elderly, including herpes zoster-related neuropathic pain, diabetic neuropathy, postmastectomy pain, uraemic itching associated with renal failure, and urinary incontinence. The potent VR1 agonist resiniferatoxin, now in phase II clinical trials, appears to be superior to capsaicin in terms of its tolerability profile. Recent discoveries enhance the therapeutic potential of vanilloids. The recognition that VR1 also functions as a principal receptor for protons and eicosanoids implies that VR1 antagonists may be of value in the treatment of inflammatory hyperalgesia and pain. Animal experimentation has already lent support to this assumption. The discovery of VR1-expressing cells in the brain as well as in non-neural tissues such as the kidney and urothelium places VR1 in a much broader perspective than peripheral pain perception, and is hoped to identify further, yet unsuspected, indications for vanilloid therapy. The realisation that VR1 and cannabinoid CB1 receptors have overlapping ligand recognition properties may also have far-reaching implications for vanilloid therapy. In fact, arvanil, a combined agonist of VR1 and CB1 receptors, has already proved to be a powerful analgesic drug in the mouse. From academic molecular biology laboratories to industrial drug discovery centres to the clinics, there is a steady flow of new data, forcing us to constantly revise the ways we are thinking about vanilloid receptor ligands and their hopes and realities for the future. This review covers the most promising current trends in vanilloid research with special emphasis on geriatric medicine.

Neurobehavioral activity in mice of N-vanillyl-arachidonyl-amide

We studied the cannabimimetic properties of N-vanillyl-arachidonoyl-amide (arvanil), a potential agonist of cannabinoid CB(1) and capsaicin VR(1) receptors, and an inhibitor of the facilitated transport of the endocannabinoid anandamide. Arvanil and anandamide exhibited similar affinities for the cannabinoid CB(1) receptor, but arvanil was less efficacious in inducing cannabinoid CB(1) receptor-mediated GTPgammaS binding. The K(i) of arvanil for the vanilloid VR(1) receptor was 0.28 microM. Administered i.v. to mice, arvanil was 100 times more potent than anandamide in producing hypothermia, analgesia, catalepsy and inhibiting spontaneous activity. These effects were not attenuated by the cannabinoid CB(1) receptor antagonist N-(piperidin-1-yl)-5-(4-chloro-phenyl)-1-(2, 4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide.HCl (SR141716A). Arvanil (i.t. administration) induced analgesia in the tail-flick test that was not blocked by either SR141716A or the vanilloid VR(1) antagonist capsazepine. Conversely, capsaicin was less potent as an analgesic (ED(50) 180 ng/mouse, i.t.) and its effects attenuated by capsazepine. The analgesic effect of anandamide (i.t.) was also unaffected by SR141716A but was 750-fold less potent (ED(50) 20.5 microg/mouse) than capsaicin. These data indicate that the neurobehavioral effects exerted by arvanil are not due to activation of cannabinoid CB(1) or vanilloid VR(1) receptors.

New perspectives on enigmatic vanilloid receptors

In spite of the rapid advances in our understanding of vanilloid-receptor pharmacology in the PNS, the function of vanilloid receptors in the brain has remained elusive. Recently, the endocannabinoid anandamide has been proposed to function as an endogenous agonist at the vanilloid receptor VR1. This is an exciting hypothesis because the localization of VR1 overlaps with that of anandamide and its preferred cannabinoid receptor CB(1) in various brain areas. The interaction of anandamide and/or related lipid metabolites with these two completely separate receptor systems in the brain clearly places VR1 in a much broader role than pain perception. At a practical level, the overlapping ligand recognition properties of VR1 and CB(1) might be exploited by medicinal chemistry. For example, arvanil, a 'chimeric' ligand that combines structural features of capsaicin and anandamide, promises to be an interesting lead for new drugs that interact at both vanilloid and cannabinoid receptors.

Distribution of mRNA for vanilloid receptor subtype 1 (VR1), and VR1-like immunoreactivity, in the central nervous system of the rat and human

The cloned vanilloid receptor VR1 has attracted recent attention as a molecular integrator of painful stimuli on primary sensory neurons. The existence of vanilloid-sensitive neurons in the brain is, however, controversial. In this study, we have used an antibody and a complementary RNA probe to explore the distribution of neurons that express VR1 in rat and in certain areas of human brain. In the rat, we observed VR1-expressing neurons throughout the whole neuroaxis, including all cortical areas (in layers 3 and 5), several members of the limbic system (e.g., hippocampus, central amygdala, and both medial and lateral habenula), striatum, hypothalamus, centromedian and paraventricular thalamic nuclei, substantia nigra, reticular formation, locus coeruleus, cerebellum, and inferior olive. VR1-immunopositive cells also were found in the third and fifth layers of human parietal cortex. Reverse transcription-PCR performed with rat VR1-specific primers verified the expression of VR1 mRNA in cortex, hippocampus, and hypothalamus. In the central nervous system, neonatal capsaicin treatment depleted VR1 mRNA from the spinal nucleus of the trigeminal nerve, but not from other areas such as the inferior olive. The finding that VR1 is expressed not only in primary sensory neurons but also in several brain nuclei is of great importance in that it places VRs in a much broader perspective than pain perception. VRs in the brain (and putative endogenous vanilloids) may be involved in the control of emotions, learning, and satiety, just to name a few exciting possibilities.

Novel natural vanilloid receptor agonists: new therapeutic targets for drug development

The discovery that compounds lacking a recognizable vanillyl-like motif might act as vanilloids has given new impetus to a search for novel vanilloid receptor agonists and antagonists in compound libraries. The availability of cell lines transfected with a cloned human vanilloid receptor will further expedite this search. In this article, the pharmacological properties of unsaturated dialdehydes and triprenyl phenols that represent two newly discovered chemical classes of vanilloids will be discussed. The existence of vanilloid receptors in several brain nuclei as well as in non-neuronal tissues predicts novel, innovative therapeutic indications for vanilloids. However, these findings also suggest that vanilloids might cause side-effects. An exploration of the uses of unsaturated dialdehydes in indigenous medicine might help identify new therapeutic targets for vanilloids and avoid unwanted actions.

Resiniferatoxin-type phorboid vanilloids display capsaicin-like selectivity at native vanilloid receptors on rat DRG neurons and at the cloned vanilloid receptor VR1

1 Although the cloned rat vanilloid receptor VR1 appears to account for both receptor binding and calcium uptake, the identification of vanilloids selective for one or the other response is of importance because these ligands may induce distinct patterns of biological activities. 2 Phorbol 12,13-didecanoate 20-homovanillate (PDDHV) evoked 45Ca(2+)-uptake by rat dorsal root ganglion neurons (expressing native vanilloid receptors) in culture with an EC50 of 70 nM but inhibited [3H]-resiniferatoxin (RTX) binding to rat dorsal root ganglion membranes with a much lower potency (Ki>10,000 nM). This difference in potencies represents a more than 100 fold selectivity for capsaicin-type pharmacology. 3 45Ca2+ influx by PDDHV was fully inhibited by the competitive vanilloid receptor antagonist capsazepine, consistent with the calcium uptake occurring via vanilloid receptors. 4 PDDHV induced calcium mobilization in CHO cells transfected with the cloned rat vanilloid receptor VR1 with an EC50 of 125 nM and inhibited [3H]-RTX binding to these cells with an estimated Ki of 10,000 nM. By contrast, PDDHV failed to evoke a measurable calcium response in non-transfected CHO cells, confirming its action through VR1. 5 We conclude that PDDHV is two orders of magnitude more potent for inducing calcium uptake than for inhibiting RTX binding at vanilloid receptors, making this novel vanilloid a ligand selective for capsaicin-type pharmacology. These results emphasize the importance of monitoring multiple endpoints for evaluation of vanilloid receptor structure-activity relations. Furthermore, PDDHV now provides a tool to explore the biological correlates of capsaicin-type vanilloid pharmacology.

The cloned rat vanilloid receptor VR1 mediates both R-type binding and C-type calcium response in dorsal root ganglion neurons

[(3)H]Resiniferatoxin (RTX) binding and calcium uptake by rat dorsal root ganglion (DRG) neurons show distinct structure-activity relations, suggestive of independent vanilloid receptor (VR) subtypes. We have now characterized ligand binding to rat VR1 expressed in human embryonic kidney (HEK293) and Chinese hamster ovary (CHO) cells and compared the structure-activity relations with those for calcium mobilization. Human embryonic kidney cells (HEK293/VR1 cells) and Chinese hamster ovary cells transfected with VR1 (CHO/VR1 cells) bound [(3)H]RTX with affinities of 84 and 103 pM, respectively, and positive cooperativity (Hill numbers were 2.1 and 1.8). These parameters are similar to those determined with rat DRG membranes expressing native VRs (a K(d) of 70 pM and a Hill number of 1.7). The typical vanilloid agonists olvanil and capsaicin inhibited [(3)H]RTX binding to HEK293/VR1 cells with K(i) values of 0.4 and 4.0 microM, respectively. The corresponding values in DRG membranes were 0.3 and 2.5 microM. HEK293/VR1 cells and DRG membranes also recognized the novel vanilloids isovelleral and scutigeral with similar K(i) values (18 and 20 microM in HEK293/VR1 cells; 24 and 21 microM in DRGs). The competitive vanilloid receptor antagonist capsazepine inhibited [(3)H]RTX binding to HEK293/VR1 cells with a K(i) value of 6.2 microM and binding to DRG membranes with a K(i) value of 8.6 microM. RTX and capsaicin induced calcium mobilization in HEK293/VR1 cells with EC(50) values of 4.1 and 82 nM, respectively. Thus, the relative potencies of RTX (more potent for binding) and capsaicin (more potent for calcium mobilization) are similar in DRG neurons and cells transfected with VR1. We conclude that VR1 can account for both the ligand binding and calcium uptake observed in rat DRG neurons.

A non-pungent triprenyl phenol of fungal origin, scutigeral, stimulates rat dorsal root ganglion neurons via interaction at vanilloid receptors

1. A [3H]-resiniferatoxin (RTX) binding assay utilizing rat spinal cord membranes was employed to identify novel vanilloids in a collection of natural products of fungal origin. Of the five active compounds found (scutigeral, acetyl-scutigeral, ovinal, neogrifolin, and methyl-neogrifolin), scutigeral (Ki=19 microM), isolated from the edible mushroom Albatrellus ovinus, was selected for further characterization. 2. Scutigeral induced a dose-dependent 45Ca uptake by rat dorsal root ganglion neurons with an EC50 of 1.6 microM, which was fully inhibited by the competitive vanilloid receptor antagonist capsazepine (IC50=5.2 microM). 3. [3H]-RTX binding isotherms were shifted by scutigeral (10-80 microM) in a competitive manner. The Schild plot of the data had a slope of 0.8 and gave an apparent Kd estimate for scutigeral of 32 microM. 4. Although in the above assays scutigeral mimicked capsaicin, it was not pungent on the human tongue up to a dose of 100 nmol per tongue, nor did it provoke protective wiping movements in the rat (up to 100 microM) upon intraocular instillation. 5. In accord with being non-pungent, scutigeral (5 microM) did not elicit a measurable inward current in isolated rat dorsal root ganglion neurons under voltage-clamp conditions. It did, however, reduce the proportion of neurons (from 61 to 15%) that responded to a subsequent capsaicin (1 microM) challenge. In these neurons, scutigeral both delayed (from 27 to 72 s) and diminished (from 5.0 to 1.9 nA) the maximal current evoked by capsaicin. 6. In conclusion, scutigeral and its congeners form a new chemical class of vanilloids, the triprenyl phenols. Scutigeral promises to be a novel chemical lead for the development of orally active, non-pungent vanilloids.

Effects of systemic resiniferatoxin treatment on substance P mRNA in rat dorsal root ganglia and substance P receptor mRNA in the spinal dorsal horn

Capsaicin depletes the sensory neuropeptide substance P (SP) in the rat due to a combination of neuron loss and decreased synthesis in the surviving cells. Resiniferatoxin (RTX) mimics most, but not all, capsaicin actions. In the present study, the effects of RTX (300 microg/kg, s.c.) were examined on mRNA levels for SP and its receptor in the adult rat. The percentage of dorsal root ganglia (DRG) neuronal profiles showing an in situ hybridization signal for preprotachykinin mRNAs encoding SP was not altered following RTX treatment (up to 8 weeks), though the signal became perceptibly weaker. In accord, 2 weeks after RTX administration a 60% decrease was observed in the steady-state levels of SP-encoding mRNAs using Northern blot analysis, leaving the ratio of beta- and gamma-preprotachykinin mRNAs unchanged. No change was, however, observed in mRNA levels encoding tachykinins NK-1 receptors in the dorsal horn, the spinal targets for SP. The present findings suggest that RTX does not kill SP-positive DRG neurons, though it suppresses the synthesis of SP. Since RTX treatment does not alter NK-1 receptor expression, this reduced SP synthesis is likely to play a central role in the analgesic actions of RTX.

Dialdehyde sesquiterpenes and other terpenoids as vanilloids

Selected naturally occurring unsaturated dialdehyde sesquiterpenes and related bioactive terpenoids were assayed for vanilloid-like activity. Out of the 25 compounds tested, eight inhibited completely the specific binding of [3H]resiniferatoxin by rat spinal cord membranes: binding affinities ranged from 0.6 microM for cinnamodial to 19.0 microM for hebelomic acid F. These values were comparable to the binding affinity of capsaicin (2.7 microM). With the exception of four ligands, compounds that inhibited resiniferatoxin binding to rat spinal cord membranes were also pungent on the human tongue where they showed cross-tachyphylaxis with capsaicin. As expected from their reactive nature, these compounds possess additional sites of action, as reflected in the complex behavior of the stimulation of calcium influx by cinnamodial and cinnamosmolide at high concentrations. This observation might explain the unexpectedly weak membrane depolarization by cinnamodial compared to capsaicin. We conclude that a range of sesquiterpene dialdehydes and related terpenoids, both pungent and non-pungent, may function as vanilloids. These compounds may represent a new chemical lead for the development of vanilloid drugs, structurally unrelated to either capsaicin or resiniferatoxin.

A non-pungent resiniferatoxin analogue, phorbol 12-phenylacetate 13 acetate 20-homovanillate, reveals vanilloid receptor subtypes on rat trigeminal ganglion neurons

Capsaicin, the vanilloid responsible for the pungent taste of hot peppers, binds to receptors found primarily in polymodal nociceptors. Capsaicin initially stimulates polymodal nociceptors and subsequently inhibits them from responding to a variety of stimuli. This property makes it useful clinically as an analgesic and anti-inflammatory compound. There is mounting, albeit indirect, evidence for the existence of several subtypes of vanilloid receptors. One such piece of evidence comes from studying analogues of capsaicin, such as phorbol 12-phenylacetate 13 acetate 20-homovanillate. This compound binds to (capsaicin) vanilloid receptors on sensory neurons, but unlike capsaicin it is non-pungent and does not produce hypothermia. To determine how sensory neurons respond to phorbol 12-phenylacetate 13 acetate 20-homovanillate, and to compare these responses with those evoked by capsaicin, whole-cell patch-clamp measurements were performed on cultured rat trigeminal ganglion neurons. It was found that 63% of the neurons held at -60 mV were activated by 3 microM, phorbol 12-phenylacetate 13 acetate 20-homovanillate, and 87% of these were also activated by 1 microM capsaicin. In a given neuron, phorbol 12-phenylacetate 13 acetate 20-homovanillate, like capsaicin, could activate kinetically distinct inward currents. The current-voltage curves characterizing phorbol 12-phenylacetate 13 acetate 20-homovanillate responses were asymmetric and had reversal potentials at -5.8 +/- 6.0 mV and 10.4 +/- 4 mV. The averaged dose-response curves for phorbol 12-phenylacetate 13 acetate 20-homovanillate were fit to the Hill equation and had binding constants (K(1/2)s) of 2.73 microM and 0.96 microM and Hill coefficients (ns) of approximately 1 for a rapidly- and slowly-activating current, respectively. These parameters are consistent with those obtained from binding experiments and calcium-influx experiments on sensory nerves. Repeated applications of phorbol 12-phenylacetate 13 acetate 20-homovanillate every 3 min caused a complete reduction in the rapidly-activating currents leaving only a reduced slowly-activating current. This provides strong evidence for the independence of these currents and the existence of subtypes of vanilloid receptors. Additional evidence for the existence of receptor subtypes is that 10 microM capsazepine, a specific and competitive inhibitor of capsaicin-evoked responses, did not inhibit the phorbol 12-phenylacetate 13 acetate 20-homovanillate-induced currents in some neurons and partially inhibited them in other neurons. Thus, there are capsazepine-sensitive and capsazepine-insensitive subtypes of vanilloid receptors. In summary, we have obtained electrophysiological and pharmacological evidence for distinct subtypes of vanilloid receptors.

The preparation and bioactivities of (-)-isovelleral

The resolution of synthetic (+/-)-isovelleral (1), via chromatographic separation of the two diastereomers of the (-)-menthoxyacetic acid diester of the corresponding (+/-)-diol (3), yielded both enantiomers of the bioactive fungal metabolite (+)-isovelleral (1). While the antimicrobial and cytotoxic activities of the two enantiomers are comparable, natural (+)-1 is approximately 10 times more mutagenic towards Ames' tester strain TA98 than (-)-1. The two enantiomers of the cyclopropane ring isomer 2 also possess negligible mutagenicity compared to (+)-1. Both (+)-1 and (-)-1 have the same affinity for the vanilloid receptor, but significant different affinity for the dopamine D1 receptor.

Effects of the capsaicin analogue resiniferatoxin on spinal nociceptive mechanisms in the rat: behavioral, electrophysiological and in situ hybridization studies

The effect of a single subcutaneous (s.c.) injection of the ultrapotent capsaicin analogue resiniferatoxin (RTX) on responses of adult rats to noxious thermal and mechanical stimulation was examined. The effects of RTX treatment on the nociceptive flexor reflex and activity-dependent increase in spinal excitability after conditioning C-fiber stimulation (CS) were also assessed. Finally, the expression of galanin message associated peptide (GMAP) mRNA in dorsal root ganglion (DRG) cells and the effects of the high affinity galanin receptor antagonist M35 on the flexor reflex in RTX-treated rats were evaluated. RTX, but not vehicle, produced marked thermal hypoalgesia on the hot plate test with partial recovery in about 50% of animals after about 2 weeks and no recovery in the remaining rats after 4 weeks. In all animals there was only a transient and moderate increase in paw withdrawal threshold to mechanical pressure. The flexor reflex in response to a C-fiber CS train was recorded 15-35 days after RTX or vehicle treatment. There was no difference between RTX and vehicle treated rats on baseline response, but RTX treatment lead to less wind-up during the CS and reduced hyperexcitability. This was particularly the case for rats which did not recover from RTX-induced hypoalgesia. The C-fiber mediated hyperexcitability was potentiated by the galanin receptor antagonist M35, more so in the non-recovered rats than in the partially recovered rats. The number of DRG cells expressing GMAP mRNA was significantly higher in non-recovered than in partially recovered rats. Thus, RTX produced marked and prolonged impairment of capsaicin-sensitive afferents and upregulation of the inhibitory neuropeptides GMAP and galanin in DRG neurons, which may underlie the prolonged effect of RTX.

Euphorbium: modern research on its active principle, resiniferatoxin, revives an ancient medicine

Resiniferatoxin, an ultrapotent capsaicin analog present in the latex of Euphorbia resinifera, interacts at a specific membrane recognition site (referred to as the vanilloid receptor), expressed by primary sensory neurons mediating pain perception as well as neurogenic inflammation. Desensitization to resiniferatoxin is a promising approach to mitigate neuropathic pain and other pathological conditions in which sensory neuropeptides released from capsaicin-sensitive neurons play a crucial role. Clinical trials to evaluate the potential of topical resiniferatoxin treatment to relieve pain associated with diabetic polyneuropathy and postherpetic neuralgia are in progress. Though resiniferatoxin was isolated only two decades ago, the dried latex of Euphorbia resinifera, called Euphorbium, has been in medicinal use since the time of recorded history. This review highlights the most important events in the history of this ancient medicine, from the first written record of the therapeutic potential of Euphorbium (at the time of the reign of the Roman Emperor Augustus) to the identification of its active principle as resiniferatoxin in 1975. A brief overview of the enormous contribution of resiniferatoxin to our current understanding of the anatomical localization, function, and pharmacology of vanilloid receptors is provided. Lastly, the mechanisms are summarized by which capsaicin and resiniferatoxin, despite sharing receptors, may have dissimilar biological actions.

Vanilloid receptors: new insights enhance potential as a therapeutic target

Compounds related to capsaicin and its ultrapotent analog, resiniferatoxin (RTX), collectively referred to as vanilloids, interact at a specific membrane recognition site (vanilloid receptor), expressed almost exclusively by primary sensory neurons involved in nociception and neurogenic inflammation. Desensitization to vanilloids is a promising therapeutic approach to mitigate neuropathic pain and pathological conditions (e.g. vasomotor rhinitis) in which neuropeptides released from primary sensory neurons play a major role. Capsaicin-containing preparations are already commercially available for these purposes. The use of capsaicin, however, is severely limited by its irritancy, and the synthesis of novel vanilloids with an improved pungency/desensitization ratio is an on-going objective. This review highlights the emerging evidence that the vanilloid receptor is not a single receptor but a family of receptors, and that these receptors recognize not simply RTX and capsaicin structural analogs but are broader in their ligand-binding selectivity. We further focus on ligand-induced messenger plasticity, a recently discovered mechanism underlying the analgesic actions of vanilloids. Lastly, we give a brief overview of the current clinical uses of vanilloids and their future therapeutic potential. The possibility is raised that vanilloid receptor subtype-specific drugs may be synthesized, devoid of the undesirable side-effects of capsaicin.

The stimulation of capsaicin-sensitive neurones in a vanilloid receptor-mediated fashion by pungent terpenoids possessing an unsaturated 1,4-dialdehyde moiety

1. The irritant fungal terpenoid isovelleral caused protective eye-wiping movements in the rat upon intraocular instillation and showed cross-tachyphylaxis with capsaicin, the pungent principle in hot pepper. 2. Isovelleral induced a dose-dependent calcium uptake by rat dorsal root ganglion neurones cultured in vitro with an EC50 of 95 nM, which was fully inhibited by the competitive vanilloid receptor antagonist capsazepine. 3. Isovelleral inhibited specific binding of [3H]-resiniferatoxin (RTX), an ultrapotent capsaicin analogue, to rat trigeminal ganglion or spinal cord preparations with an IC50 of 5.2 microM; in experiments in which the concentration of [3H]-RTX was varied, isovelleral changed both the apparent affinity (from 16 pM to 37 pM) and the co-operativity index (from 2.1 to 1.5), but not the Bmax. 4. The affinity of isovelleral for inducing calcium uptake or inhibiting RTX binding was in very good agreement with the threshold dose (2.2. nmol) at which it provoked pungency on the human tongue. 5. For a series of 14 terpenoids with an unsaturated 1,4-dialdehyde, a good correlation was found between pungency on the human tongue and affinity for vanilloid receptors on the rat spinal cord. 6. The results suggest that isovelleral-like compounds produce their irritant effect by interacting with vanilloid receptors on capsaicin-sensitive sensory neurones. Since these pungent diterpenes are structurally distinct from the known classes of vanilloids, these data provide new insights into structure-activity relations and may afford new opportunities for the development of drugs targeting capsaicin-sensitive pathways.

Synthesis and evaluation of phorboid 20-homovanillates: discovery of a class of ligands binding to the vanilloid (capsaicin) receptor with different degrees of cooperativity

A number of phorboid 20-homovanillates were prepared by condensation of phorbol 12,13-diesters and 12-dehydrophorbol 13-esters with Mem-homovanillic acid followed by removal of the protecting group with SnCl4 in THF. These compounds were evaluated for their ability to inhibit [3H]resiniferatoxin (RTX) binding to rat spinal cord membranes. Compounds bearing a lipophilic ester group on ring C were considerably active, but a surprising tolerance of the vanilloid receptor toward the location and the orientation of this ester group was disclosed. Unexpectedly, these ligands could also diminish, to a variable degree, the positive cooperativity which characterizes RTX binding to the vanilloid receptor. Phorbol 12-phenylacetate 13-acetate 20-homovanillate (PPAHV, 6a), a compound which abolished binding cooperativity, was further tested in a variety of in vivo assay used to characterize vanilloid-like activity. PPAHV showed only a marginal pungency and failed to induce a measurable hypothermia response at doses (up to 200 mg/kg) at which it effectively desensitized against neurogenic inflammation. These data suggest that the peculiar binding behavior of these ligands might be associated with a distinct spectrum of biological activity.

Similarities and differences in the structure-activity relationships of capsaicin and resiniferatoxin analogues

Structure-activity relationships in analogues of the irritant natural product capsaicin have previously been rationalized by subdivision of the molecule into three structural regions (A,B, and C). The hypothesis that resiniferatoxin (RTX), which is a high-potency ligand for the same receptor and which has superficial structural similarities with capsaicin, could be analogously subdivided has been investigated. The effects of making parallel changes in the two structural series have been studied in a cellular functional assay which is predictive of analgesic activity. Parallel structural changes in the two series lead to markedly different consequences on biological activity; the 3- and 4-position aryl substituents (corresponding to the capsaicin 'A-region') which are strictly required for activity in capsaicin analogues are not important in RTX analogues. The homovanillyl C-20 ester group in RTX (corresponding to the capsaicin 'B-region') is more potent than the corresponding amide, in contrast to the capsaicin analogues. Structural variations to the diterpene moiety suggest that the functionalized 5-membered diterpene ring of RTX is an important structural determinant for high potency. Modeling studies indicate that the 3D position of the alpha-hydroxy ketone moiety in the 5-membered ring is markedly different in the phorbol (inactive) analogues and RTX (active) series. This difference appears to be due to the influence of the strained ortho ester group in RTX, which acts as a local conformational constraint. The reduced activity of an analogue substituted in this region and the inactivity of a simplified analogue in which this unit is entirely removed support this conclusion.

Vanilloid receptor loss is independent of the messenger plasticity that follows systemic resiniferatoxin administration

Resiniferatoxin (RTX) depletes vanilloid (capsaicin) receptors from lumbar dorsal root ganglia (DRG) of the rat. In addition, RTX causes changes in neuropeptide and nitric oxide synthase expression in lumbar DRG neurons, similar to those described following axotomy; this latter phenomenon is referred to as messenger plasticity. These findings suggested that vanilloid receptor loss may be part of the plasticity that follows RTX treatment. Here we show that vanilloid receptor expression, as detected by [3H]RTX autoradiography, is not changed in lumbar DRGs of axotomized rats, nor is it altered in a rat model (chronic constriction injury) of neuropathic pain. Thus, the in vivo expression of vanilloid receptors detected by specific [3H]RTX binding does not require the presence of intraaxonally transported trophic factors such as nerve growth factor. We conclude that messenger plasticity and vanilloid receptor loss are mediated by distinct mechanisms.

Capsaicin-, resiniferatoxin-, and lactic acid-evoked vascular effects in the pig nasal mucosa in vivo with reference to characterization of the vanilloid receptor

Nasal cavity volume, mucosal and superficial skin blood flow as well as renal splenic vascular effects of capsaicin, resiniferatoxin and lactic acid were investigated, using a novel in vivo pig model. The present results show that locally intraarterially injected capsaicin, resiniferatoxin and lactic acid evoke similar vasodilatory responses, although with different duration, in the nasal mucosa and superficial skin as well as an increase in heart rate and mean arterial blood pressure. Nasal vascular responses evoked by capsaicin, resiniferatoxin and lactic acid were unaffected by the cyclooxygenase inhibitor diclofenac. Moreover, chlorisondamine did not alter the nasal vasodilatory responses evoked by capsaicin and lactic acid. However, chlorisondamine abolished sympathetic reflex-mediated vasoconstrictor effects of capsaicin in the spleen and kidney. Lactic acid-evoked vasodilation in the nasal mucosa and skin was inhibited by the 8-37 fragment of calcitonin gene-related peptide, a calcitonin gene-related peptide-receptor antagonist. Lactic acid-evoked vasoconstriction in the spleen and kidney was reduced but not abolished by chlorisondamine, suggesting that the effects of lactic acid are not exclusively reflex-mediated. Capsazepine did not inhibit the vasodilatation in the nasal mucosa evoked by capsaicin and lactic acid. [3H]Resiniferatoxin bound to pig nasal mucosa membranes with an affinity of 134 pM in a non-cooperative fashion; this binding behaviour contrasted to the apparent positive cooperativity (a Hill coefficient of 2.2) of specific resiniferatoxin binding to pig spinal cord preparations. Specific [3H]resiniferatoxin binding to nasal mucosa membranes was fully inhibited by capsaicin (Ki = 5 microM) and lactic acid (IC50 at pH 5.0) but not by capsazepine (up to 10 microM), in accord with the physiological findings. Capsazepine, by contrast, displaced [3H]resiniferatoxin from spinal vanilloid receptors with an affinity of 3 microM. These findings show the presence of vanilloid receptors in the pig nasal mucosa and suggest heterogeneity in the properties of vanilloid receptors in the pig. Furthermore, lactic acid evokes vascular effects similar to those of capsaicin and resiniferatoxin, possibly via interaction of protons and/or proton-generated substances at vanilloid receptors with a subsequent release of calcitonin gene-related peptide.

A novel agonist, phorbol 12-phenylacetate 13-acetate 20-homovanillate, abolishes positive cooperativity of binding by the vanilloid receptor

Capsaicin binds to a specific recognition site, referred to as the vanilloid receptor, which it shares with the natural, ultrapotent agonist resiniferatoxin and with the competitive antagonist capsazepine. Upon binding to its receptor, capsaicin opens a cation channel leading to Ca2+ influx. The binding of capsaicin or resiniferatoxin by the vanilloid receptor follows a sigmoidal saturation curve, indicative of positive cooperativity. The biological significance of this positive cooperative behaviour is unknown, as is the mechanism responsible for it. We have developed a novel ligand, phorbol 12-phenylacetate 13-acetate 20-homovanillate (PPAHV), which binds to cultured rat sensory neurons (with a Ki of 3.1 +/- 0.4 microM), and induces Ca2+ uptake by them (with an ED50 of 1.8 +/- 0.3 microM) with similar affinities and in a non-cooperative manner (Hill coefficients are 0.99 and 1.06 for binding and Ca2+ uptake, respectively). The behaviour of PPAHV thus contrasts with resiniferatoxin or capsaicin not only in the lack of cooperativity but also in the relative potencies for resiniferatoxin binding versus Ca2+ uptake (resiniferatoxin is less potent and capsaicin is more potent for induction of Ca2+ uptake than for binding). In further experiments in which the concentration of [3H]resiniferatoxin was varied, 1 microM PPAHV likewise reduced the cooperativity index that characterizes resiniferatoxin binding to rat spinal cord membranes from 2.3 +/- 0.1 to 1.1 +/- 0.2; in parallel experiments, neither capsaicin nor capsazepine (both at a concentration of 2 microM) affected binding cooperativity. Moreover, PPAHV (1 microM) turned the bi-phasic dissociation curve of resiniferatoxin into a monophasic curve, eliminating the second, slow-dissociation phase. The present results suggest that positive cooperativity is a ligand-induced feature rather than an inherent property of vanilloid receptors. A comparison of the spectrum of biological activity of ligands which bind to vanilloid receptors with different degrees of cooperativity may provide an approach to explore the functional significance of this binding behaviour.

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.

Vanilloid (capsaicin) receptors in the rat: distribution in the brain, regional differences in the spinal cord, axonal transport to the periphery, and depletion by systemic vanilloid treatment

Vanilloid (capsaicin) receptors were visualized by [3H]resiniferatoxin (RTX) autoradiography in the brain of newborn as well as adult (both control and colchicine-treated) rats. Specific labelling was seen in the brain stem only, in the nucleus of the solitary tract extending into the area postrema and the spinal sensory nucleus of the trigeminal nerve. Also, a strong signal was seen in the dorsal horn, dorsal root, trigeminal and nodose ganglia. Membranes obtained from the cervical, thoracic, and lumbar segments of the spinal cord showed similar affinities for RTX and likewise for capsaicin and capsazepine; maximal receptor density was similar in the cervical and thoracic segments (approximately 70 fmol/mg protein) but was twice as high in the lumbar segment. 24 h after ligation of the vagal or the sciatic nerves, a strong accumulation of specific RTX binding sites was observed mainly proximal to the ligature, implying intraaxonal receptor transport from the nodose and dorsal root ganglia, respectively, to the periphery. Systemic (s.c.) vanilloid treatment depleted specific [3H]RTX binding sites from the brain stem, the sensory (dorsal root as well as trigeminal) ganglia, and the spinal cord. RTX was approximately 200-fold more potent than capsaicin for eliminating vanilloid receptors from the spinal cord. The present results suggest a discrete expression of vanilloid receptors in the brain stem (sensory nuclei); although intrinsic vanilloid receptor-expressing neurons are though to exist in the rat brain, they remain undetected by the present [3H]RTX autoradiography methodology.

Increased levels of GMAP, VIP and nitric oxide synthase, and their mRNAs, in lumbar dorsal root ganglia of the rat following systemic resiniferatoxin treatment

Using in situ hybridization, the expression of mRNA encoding galanin, vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY), and nitric oxide synthase (NOS), respectively, was studied in lumbar dorsal root ganglia of rats given a single s.c. dose of 300 micrograms kg-1 resiniferatoxin (RTX), an ultrapotent capsaicin analogue. In control animals, 10% of the DRG neurones were positive for galanin mRNA, whereas no message for VIP, NPY or NOS could be detected. One week after RTX treatment, a markedly increased number (approximately 30%) of the neurones expressed galanin mRNA. Simultaneously, VIP and NOS mRNA became detectable in 6-8% of the neurones. The number of galanin-positive neurones declined after 2 weeks and returned to control levels by 8 weeks. The increase in number of VIP-, or NOS-positive neurones persisted up to 4 weeks after RTX treatment and declined thereafter. Also, there was a small increase in NPY mRNA-positive neurones. In parallel immunohistochemical experiments, similar increases were observed for galanin message-associated protein (GMAP)-, VIP- and NOS-like immunoreactivities. Our findings suggest that RTX can cause changes (messenger plasticity) in galanin, VIP and NOS expression in capsaicin-sensitive sensory neurones of the rat, similar to those described following axotomy.

Resiniferatoxin binding to vanilloid receptors in guinea pig and human airways

We have used the [3H]resiniferatoxin (RTX) binding assay to characterize for the first time a vanilloid (capsaicin) receptor in tracheobronchial tissues of the guinea pig. Membranes obtained from the trachea and the main bronchi bound RTX with an affinity of 1 nM; the cooperativity index was close to unity, indicating noncooperative binding. Specific [3H]RTX binding was fully inhibited by capsaicin (Ki = 500 nM) and capsazepine (Ki = 100 nM), but it was not inhibited at all by the inactive RTX structural analog resiniferonol 9, 13, 14-orthophenylacetate (10 microM), confirming the specificity of the binding. Neither was RTX binding inhibited by the functional vanilloid antagonist ruthenium red (100 microM). The density of specific RTX binding sites was similar in the trachea (Bmax = 150 fmol/mg protein) and the bronchi (Bmax = 170 fmol/mg protein). In keeping with the marked resistance of hamsters to capsaicin actions, no specific RTX binding could be detected in the airways of this species. By contrast, we have been able to demonstrate specific RTX binding sites in human bronchi: the estimated affinity for RTX, 2 nM, was similar to that (7 nM) determined in guinea pig bronchi. We conclude that (1) the [3H]RTX binding assay affords a novel biochemical marker for vanilloid-sensitive nerves in the airways, and (2) this binding assay may be a useful tool to explore species-related differences in the expression and pharmacologic profile of vanilloid receptors in the airways.

Proton inhibition of [3H]resiniferatoxin binding to vanilloid (capsaicin) receptors in rat spinal cord

Protons and capsaicin activate overlapping subsets of sensory nerves by opening ion conductances of similar properties. We have used the [3H]resiniferatoxin binding assay utilizing rat spinal cord membranes to elucidate the possible interaction of protons at the vanilloid (capsaicin) receptor. Using low pH (pH 6.0 and pH 5.0) buffers, a time-dependent gradual decrease was observed in specific resiniferatoxin binding. Protons inhibited resiniferatoxin binding with an IC50 of pH 5.3 +/- 0.1. In experiments in which the concentration of [3H]resiniferatoxin was varied, protons reduced the Bmax value by approximately 40% with a corresponding 2-fold decrease in affinity. No change however, was observed in binding cooperativity (the Hill coefficients were 1.7 +/- 0.1 and 1.6 +/- 0.2 in the presence of pH 7.4 and pH 5.0 buffers, respectively). These changes in binding parameters are consistent with a non-competitive or, alternatively, mixed inhibitory mechanism. The remaining resiniferatoxin binding sites bound capsaicin with an affinity (Ki = 5.0 +/- 1.0 microM) very similar to that determined in the presence of a pH 7.4 buffer (Ki = 3.0 +/- 1.5 microM). A cyclooxygenase inhibitor, indomethacin (up to 10 microM), did not prevent the action of protons on resiniferatoxin binding; neither was it mimicked by prostanoids (prostaglandin I2 and E1, both at 100 microM). We conclude that protons interact at vanilloid receptors in the rat spinal cord; this interaction is either non-competitive or mixed in nature, and probably is not related to prostanoid generation. Protons and/or putative proton-generated mediators might represent endogenous modulators of the vanilloid receptor.

Visualization by [3H]resiniferatoxin autoradiography of capsaicin-sensitive neurons in the rat, pig and man

[3H]Resiniferatoxin autoradiography revealed high densities of binding sites in rat dorsal root ganglia as well as in the superficial dorsal horn of the spinal cord, known to contain the cell bodies and central terminals, respectively, of capsaicin-sensitive, sensory neurons. This binding was fully displaced by non-radioactive resiniferatoxin and was absent following administration of high, neurotoxic doses of capsaicin. The binding thus has the characteristics expected for the vanilloid (capsaicin) receptor. High density, specific resiniferatoxin binding was also observed in pig spinal cord and dorsal root ganglia. Finally, similar high density binding was detected in the dorsal horn of human spinal cord obtained post-mortem. We conclude that [3H]resiniferatoxin autoradiography may afford a novel neurochemical tool to identify capsaicin-sensitive neurons in the central as well as in the peripheral nervous system, to explore the ontogeny of these neurons, and to detect changes in vanilloid (capsaicin) receptor expression under pathophysiological conditions.

Visualizing vanilloid (capsaicin) receptors in pig spinal cord by [3H]resiniferatoxin autoradiography

Autoradiographic mapping using [3H]resiniferatoxin (RTX) revealed high densities of vanilloid binding sites over areas (Rexed laminae I and II) in pig spinal cord known to be rich in central terminals of capsaicin-sensitive neurons. Also, high affinity [3H]RTX binding was detected in membranes obtained from the corresponding areas: apparent binding affinity and cooperativity but not the maximal receptor density was influenced by the assay conditions (temperature, buffer composition). No specific binding could be detected in other areas of the spinal cord by either methodology suggesting that the vanilloid receptors are present exclusively on central terminals of the capsaicin-sensitive neurons. We conclude that [3H]RTX autoradiography may afford a novel neurochemical approach to detect localized changes in vanilloid receptor expression.

The calcitonin gene-related peptide (CGRP) phenotype is expressed early and up-regulated by resiniferatoxin (RTX) in mouse sensory neurons

Calcitonin gene-related peptide (CGRP) immunoreactivity was detected at day 2 in vitro (embryonic day 15) in developing mouse dorsal root ganglion (DRG) neurons in primary culture. During 2 weeks of culture the proportion of CGRP-immunoreactive (CGRP-IR) neurons remained around 65-70%, much higher than usually found in adult animals (45-50%). Treatment of cultures with the capsaicin analog resiniferatoxin (RTX; 0.3-30 nM) significantly augmented CGRP immunoreactivity per neuron at all ages investigated without increasing the number of CGRP-immunoreactive cells. The increased CGRP immunoreactivity was observed both in the axonal varicosities and in the perinuclear region of cell bodies. This RTX-induced increase in CGRP immunoreactivity was completely blocked by Ruthenium red (RR). Treatment with the non-esterified form of RTX (resiniferol 9, 13, 14 orthophenylacetate, ROPA) produced no increase. These results suggest that: (1) early expression of the CGRP phenotype is regulated in a cell-autonomous way in developing mouse DRG neurons in vitro; and (2) the RTX-induced increase in CGRP biosynthesis is most likely the result of activating the capsaicin/RTX receptor rather than directly activating the protein kinase C (PKC) pathway in vitro. The results may also reflect qualitative and quantitative differences in capsaicin/RTX sensitivity of sensory neurons between embryonal and adult ages.

Comparison of tachykinin NK1 receptors in human IM9 and U373 MG cells, using antagonist (FK888, (+/-)-CP-96,345, and RP 67580) binding

We have used one peptide (FK888) and two non-peptide ((+/-)-CP-96,345 and RP 67580) antagonists, along with the preferred endogenous agonist, substance P, to compare the pharmacological (binding) profile of NK1 receptors expressed by human B lymphoblastoma (IM9) and astrocytoma (U373 MG) cells. Of the ligands tested, substance P was the most potent in both cell lines: binding affinities were 0.1 nM for IM9 cells, and 0.3 nM for U373 MG cells, respectively. The high-affinity dipeptide antagonist, FK888, bound to NK1 receptors in both cell lines with similar potencies: Ki values were 1.2 nM and 3.6 nM for IM9 cells and U373 MG cells, respectively. Of the non-peptide antagonists, as expected, (+/-)-CP-96,345 displayed higher affinity (0.4 nM in IM9 cells, and 1.2 nM in U373 MG cells) than did RP 67580 (33 nM and 223 nM in IM9 cells and U373 MG cells, respectively) in both cell lines. We conclude that the pharmacological profile of NK1 receptors is similar in the human lymphoblastoma and astrocytoma cells, i.e. if NK1 receptor subtypes exist in humans, these cell lines are likely to express a similar subtype. Because IM9 cells grow faster and are easier to maintain, this cell line may be preferable to the astrocytoma cells as a primary screen to identify NK1 receptor antagonists.

The vanilloid (capsaicin) receptor: receptor types and species differences

1. Capsaicin was postulated to exert its pharmacological actions by interacting at a specific recognition site (receptor) expressed predominantly by primary afferent neurons. 2. The actual existence of this long-sought "capsaicin-receptor" has recently been demonstrated by the specific binding of [3H]resiniferatoxin (RTX), an ultrapotent capsaicin analog with a unique spectra of actions. 3. Since homovanillic acid is the key structural motif shared by capsaicin and RTX, their recognition site appears to be best termed the vanilloid receptor. 4. Central (sensory ganglia and spinal cord) vanilloid receptors of the rat bind RTX with high affinity in a cooperative fashion; moreover, they recognize capsaicin with higher affinity than the competp6ive antagonist, capsazepine. Peripheral (urinary bladder, urethra, airways, colon) vanilloid receptors, by contrast, bind RTX with lower affinity in a noncooperative manner. An opposite affinity for capsazepine relative to capsaicin appears to distinguish vanilloid receptors in the urinary bladder from those present in the airways or colon. These findings imply heterogeneity in the properties of vanilloid receptors. 5. The affinity of [3H]RTX binding in vitro is influenced by reducing agents, suggesting an in vivo modulatory role for endogenous reducing agents in vanilloid receptor functions. 6. The size of central vanilloid receptors (270 kDa) as measured by radiation inactivation and the cooperative binding both suggest a receptor cluster with cooperating subunits. 7. RTX binds to vanilloid receptors with orders of magnitude higher affinity than capsaicin; its ability to induce cooperative binding is also more pronounced. These differences in receptor binding along with the pharmacokinetical differences in tissue equilibration and in plasma binding may form a rational basis to explain the peculiar spectrum of actions of RTX. 8. Guinea pig spinal cord and airway membranes bind RTX with lower affinity than rat tissues. The receptor density is, however, higher in the guinea pig in keeping with the marked sensitivity of this species to vanilloid actions. 9. The apparently low level of specific [3H]RTX binding sites in the hamster and rabbit is in accord with the resistance of these species to vanilloid actions. 10. In post-mortem human spinal cord specific [3H]RTX binding sites can be detected; their binding parameters are similar to those determined in guinea pig spinal cord. 11. The vanilloid receptor appears to display both intraspecies heterogeneity and marked interspecies differences. 12. As yet, it is known whether the vanilloid receptor is operated by endogenous ligands. It is not known either which receptor superfamily (if any) it belongs to. The [3H]RTX binding assay has, however, the potential of answering these questions.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization by [3H]resiniferatoxin binding of a human vanilloid (capsaicin) receptor in post-mortem spinal cord

Membranes obtained from post-mortem human spinal cord specimens bound [3H]resiniferatoxin (RTX) with an affinity of 11 nM in a non-cooperative fashion. This binding behaviour contrasted with the high affinity [3H]RTX binding (Kd = 24 pM) to rat spinal cord membranes which displayed apparent positive cooperativity (cooperativity index = 1.8) but was in accord with the low affinity (Kd = 5 nM) non-cooperative RTX binding to guinea pig spinal cord preparations. We conclude that the [3H]RTX binding assay utilizing post-mortem human spinal cord membranes affords a novel biochemical approach to explore structure-activity relations at human vanilloid receptors.

Resiniferatoxin-induced loss of vanilloid receptors is reversible in the urinary bladder but not in the spinal cord of the rat

Resiniferatoxin (RTX) induced a dose-dependent loss of vanilloid receptors (specific [3H]RTX-binding sites) in tissues containing peripheral (urinary bladder) and central (spinal cord) endings of capsaicin-sensitive neurons. This receptor loss in the spinal cord was entirely due to a reduction in the Bmax. When examined 24 h after s.c. RTX treatment, receptor loss required somewhat less RTX in the urinary bladder (ED50 = 10 micrograms/kg) than in the spinal cord (ED50 = 50 micrograms/kg), whereas the loss of the xylene-induced neurogenic inflammatory response in the bladder displayed an approximate ED50 of 5 micrograms/kg. In the bladder of rats pretreated with 30 micrograms/kg RTX, both receptor binding and neurogenic inflammatory response recovered almost completely within 2 month after treatment. In the bladder of rats that received a 10-fold higher RTX dose, a 50% recovery of binding and a 70% recovery of the Evans' blue extravasation response were found. By contrast, no recovery of specific [3H]RTX binding to spinal cord membranes was observed at either dose. These findings suggest that vanilloid receptor loss after RTX treatment can be either reversible (desensitization) or irreversible (most likely reflecting neurotoxicity), and that peripheral and central terminals of capsaicin-sensitive neurons have a differential sensitivity to these long-term vanilloid actions.

Topical capsaicin administration protects against trinitrobenzene sulfonic acid-induced colitis in the rat

We used the [3H]resiniferatoxin binding assay to demonstrate for the first time the existence of vanilloid receptors in the rat colon and to explore their expression during trinitrobenzene sulfonic acid-induced colitis. Membranes obtained from control colon bound [3H]resiniferatoxin with an affinity of 3 nM; the receptor density was 450 fmol/mg protein or 9 fmol/mg wet weight. Capsaicin and capsazepine, a competitive antagonist of capsaicin, inhibited specific resiniferatoxin binding with Ki values of 3 microM and 0.1 microM, respectively. Trinitrobenzene sulfonic acid induced a very rapid ulceration in the colon: 1 h after treatment 90% of the colon showed ulcerative damage. Coadministration of 640 microM capsaicin diminished the ulcerative effect of trinitrobenzene sulfonic acid to 64% when examined 1 h after trinitrobenzene sulfonic acid challenge; however, this protective action was lost 23 h later. Colon samples obtained 4 h, 24 h, and 1 week after trinitrobenzene sulfonic acid challenge bound resiniferatoxin, capsaicin, and capsazepine with affinities similar to those of control samples. The receptor density remained at an essentially constant level when expressed in fmol/mg protein but, in keeping with the increased wet weights, showed a reduction when expressed in fmol/mg wet weight. We conclude that acute capsaicin administration protects against the ulcerative action of trinitrobenzene sulfonic acid, most likely via the release of protective neuropeptides from capsaicin-sensitive nerve endings. The loss of this protective action is presumably due to a depletion of the protective neuropeptides rather than to a loss of vanilloid (capsaicin) receptors.

Capsaicin-induced relaxation in the rat isolated external urethral sphincter: characterization of the vanilloid receptor and mediation by CGRP

1. The potential role of capsaicin-sensitive nerves in the relaxation of the rat external urethral sphincter (REUS) was evaluated by demonstrating the existence of specific vanilloid (capsaicin) receptors and by investigating the sensory neurotransmitter(s) putatively involved in this relaxation. 2. Capsaicin (1 microM) relaxed REUS strips precontracted with noradrenaline (NA) (0.1 mM). This effect underwent desensitization and it was absent in preparations taken from adult capsaicin-pretreated rats. 3. Capsaicin-induced relaxation of NA-precontracted REUS was mimicked by calcitonin gene-related peptide (CGRP, 0.3-10 microM), but not by substance P (1 microM), vasoactive intestinal polypeptide (VIP, 1 microM), alpha-beta methylene ATP (10 microM), gamma-aminobutyric acid (GABA, 3 mM) or galanin (1 microM). A cross-tachyphylaxis between capsaicin (1 microM) and CGRP (1 microM) was observed. Both capsaicin and CGRP-induced relaxation were partially antagonized by the proposed CGRP antagonist, CGRP (8-37) (10 microM). 4. Electrical field stimulation (EFS, 2.5 Hz, 60 V, 1 ms, trains of 5 s every 5 min) of REUS evoked a contraction characterized by a largely adrenergic slowly developing tonic contraction with superimposed fast twitches due to the striated component of the strips. Both capsaicin (1 microM) and CGRP (0.01-1 microM) produced an almost complete inhibition of EFS-induced tonic contraction. A cross-tachyphylaxis between capsaicin and CGRP was observed. Furthermore, these inhibitory actions were unaffected by CGRP (8-37) (10 microM). 5. [3H]-resiniferatoxin displayed specific, saturable binding to rat urethral membranes. Data were consistent with a single site with a Kd of 105 pM and a Bmax of 40 fmol mg-1 protein. This binding was inhibited by capsaicin with a Ki of 0.6 microM and it was reduced by approximately 80% in preparations taken from rats that had undergone surgical ablation of the major pelvic ganglion 4 days earlier.6. In conclusion we have demonstrated the existence of vanilloid receptors on capsaicin-sensitive nerves innervating the rat urethra mainly through the major pelvic ganglion. The activation of this set of nerves could lead to a local release of CGRP that in turn elicits a remarkable urethral relaxation. Such a mechanism could be of relevance in physiological conditions to facilitate urine expulsion during micturition and in pathological conditions to help removal of noxious stimuli following mechanical/chemical irritation of the lower urinary tract.

Competitive inhibition by capsazepine of [3H]resiniferatoxin binding to central (spinal cord and dorsal root ganglia) and peripheral (urinary bladder and airways) vanilloid (capsaicin) receptors in the rat

Capsazepine was reported to block capsaicin- and resiniferatoxin (RTX)-induced responses both in vivo and in vitro with Schild plots suggesting a competitive mechanism of action. We have used the [3H]RTX binding assay, thought to represent the vanilloid (capsaicin) receptor, to explore the inhibitory mechanism of capsazepine at the receptor level in the rat. In competition assays, capsazepine inhibited [3H]RTX binding by spinal cord, dorsal root ganglion (DRG) and urinary bladder membranes with similar Ki values of 4.0 +/- 0.3, 3.5 +/- 0.5 and 5.0 +/- 1.0 microM (mean +/- S.E.M.; three determinations), respectively. By contrast, capsazepine was 35- to 50-fold more potent for inhibiting specific [3H]RTX binding in the airways (Ki = 0.12 +/- 0.02 microM; mean +/- S.E.M.; four determinations). In experiments in which the concentration of [3H]RTX was varied, 10 microM capsazepine reduced the affinity of the vanilloid receptor expressed by DRG and spinal cord membranes for [3H]RTX from 15 +/- 3 to 43 +/- 5 pM, and from 20 +/- 3 to 80 +/- 5 pM (mean +/- S.E.M.; three determinations), respectively, without a measurable change in Bmax or in cooperativity index; these shifts in affinity yield Ki values of 5.2 and 3.3 microM for DRG and spinal cord membranes, respectively. Capsaicin inhibited [3H]RTX binding by spinal cord, DRG and urinary bladder membranes with a 6- to 13-fold higher potency than did capsazepine; the Ki values were 0.3 +/- 0.1, 0.6 +/- 0.4 and 0.5 +/- 0.2 microM (mean +/- S.E.M.; three determinations), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Vanilloid (capsaicin) receptor in the rat: positive cooperativity of resiniferatoxin binding and its modulation by reduction and oxidation

Specific [3H]resiniferatoxin (RTX) binding is thought to represent the postulated vanilloid (capsaicin) receptor. In the present report, this binding has been reevaluated using a modified [3H] RTX binding assay in which the high nonspecific binding, which limited the previous characterization, was reduced by adding alpha 1-acid glycoprotein, a plasma protein that binds RTX, to the usual binding assay after RTX binding by the vanilloid receptor had been terminated. Specific [3H]RTX binding by both dorsal root ganglion (DRG) and spinal cord membranes of the rat followed sigmoidal saturation kinetics indicating apparent positive cooperativity. The cooperativity index determined by fitting the data to the Hill equation was 1.7 in DRG and 1.9 in spinal cord. Apparent dissociation constants were estimated as 24 pM for DRG and 11 pM for spinal cord preparations. As predicted by the modified Hill equation, at low receptor occupancy nonradioactive agonists (RTX, tinyatoxin, capsaicin) produced biphasic competition curves. The initial (enhancement) phase of these curves correlated with the biological potency of the agonist. Dithiothreitol reduced both positive cooperativity and apparent binding affinity; the oxidizing agent 5,5'-dithiobis-(2-nitrobenzoic acid) reduced the cooperativity index without a major effect on binding affinity. These findings suggest that the vanilloid receptor is a receptor cluster in which the subunits cooperate; cooperation is, at least in part, subject to redox modulation.

Vanilloid receptors in the urinary bladder: regional distribution, localization on sensory nerves, and species-related differences

Using selective surgical ablations we have investigated the localization of vanilloid receptors (specific [3H] resiniferatoxin binding sites) on terminals of the pelvic, hypogastric, and pudendal nerves in the rat urinary bladder. Pelvic and hypogastric nerve resections resulted in 90% and 25% loss of specific [3H] resiniferatoxin (RTX) binding sites, respectively, whilst pudendic nerve resection had no measurable effect on the binding. In control animals, the density of vanilloid receptors was 1.7-fold higher in the neck than in the dome of the urinary bladder; the Bmax values were 57 +/- 8 and 34 +/- 7 fmol/mg protein, respectively. The binding characteristics of the vanilloid receptor were similar in the urinary bladder of the rat and mouse: Kd values were 87 +/- 15 and 61 +/- 11 pM, Bmax values were 37 +/- 2 and 60 +/- 10 fmol/mg protein, respectively. In contrast to the findings for the rat and mouse, in the urinary bladder of the guinea pig and the hamster the low level of specific [3H]RTX binding prevented the detailed characterization of vanilloid receptors. Nonetheless, at a fixed (60pM) concentration of [3H]RTX, specific binding both in the guinea pig and hamster urinary bladder was approximately 20% of that in the rat urinary bladder. In the urinary bladder of newborn rats, as in adults, a single class of specific [3H]RTX binding sites was found which bound RTX with an affinity of 110 +/- 20 pM and with a maximal binding capacity of 30 +/- 5 fmol/mg protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a peripheral vanilloid (capsaicin) receptor in the urinary bladder of the rat

Specific binding of [3H]resiniferatoxin (RTX) is thought to represent the vanilloid (capsaicin) receptor. In the present study, we have used this binding assay to identify for the first time a vanilloid receptor in the periphery and to compare it to central vanilloid receptors present in dorsal root ganglia (DRG) as well as in spinal cord of the rat. Rat urinary bladder membranes bound [3H]RTX with a Kd of 30 +/- 4 pM and a Bmax of 65 +/- 14 fmol/mg protein; the corresponding values were 19 +/- 3 pM and 104 +/- 14 fmol/mg protein in DRG, and 16 +/- 3 pM and 50 +/- 9 fmol/mg protein in spinal cord. Capsaicin inhibited [3H]RTX binding to membranes from urinary bladder, spinal cord, and DRG with similar potency (Ki values were 0.5 +/- 0.1 microM, 0.3 +/- 0.1, and 0.6 +/- 0.1 microM, respectively). Interestingly, [3H]RTX bound to urinary bladder in a non-cooperative fashion in contrast with the apparent positive cooperativity of [3H]RTX binding in both DRG and spinal cord (cooperativity index = 1.8 and 1.7, respectively). This finding suggests heterogeneity in the properties of the vanilloid receptors in the rat.

[3H]resiniferatoxin binding by the vanilloid receptor: species-related differences, effects of temperature and sulfhydryl reagents

Specific binding of [3H]resiniferatoxin (RTX) is thought to represent the vanilloid (capsaicin) receptor. In the present study, we have used this binding assay to elucidate the contribution of differential receptor expression to the capsaicin-resistance of hamsters and rabbits; binding parameters were compared to those of species (rats, mice) regarded as capsaicin-sensitive. Whereas the 5-fold lower affinity for [3H]RTX binding in the hamster (100 pM) as compared to the rat (20 pM) is unlikely to account for the 100-fold difference in the in vivo responses of RTX-induced inflammation and hypothermia, the lack of detectable specific [3H]RTX binding sites in the rabbit might represent the predominant mechanism of capsaicin-resistance in this species. Regulation of the vanilloid receptor was further characterized in the rat. In accord with the temperature dependence of both in vivo and in vitro capsaicin actions, we found a marked temperature dependence for association rates. Dissociation turned out to have complex kinetics dependent on time and receptor occupancy. Low pH (5.5-7.0) did not affect receptor binding. Preincubation with heavy metal cations and other sulfhydryl-reactive agents inhibited specific [3H]RTX binding indicating that the vanilloid receptor is a thiol-protein, and that free sulfhydryl groups play an essential role in agonist binding activity. Preliminary characterization suggested noncompetitive inhibition.

Pharmacologic and neurochemical evidence for the activation of capsaicin-sensitive sensory nerves by lipoxin A4 in guinea pig bronchus

Exogenous administration of lipoxin A4 (LXA4) to guinea pig isolated bronchus produced contractile effects in a concentration-dependent manner (1, 3, and 6 microM). These responses were potentiated when preparations were previously incubated with thiorphan (10 microM), an inhibitor of tachykinin breakdown, but were significantly depressed when sensory nerves were previously desensitized in vitro by capsaicin (10 microM for 15 min) challenge. Ruthenium red (10 microM for 20 min), a blocker of the cationic channel coupled to the capsaicin receptor, also produced, although in a weaker manner, a reduction in bronchomotor responses elicited by LXA4. On the other hand, preexposure to omega-conotoxin (0.1 microM for 45 min), a blocker of neuronal voltage-dependent Ca2+ channels, did not modify the LXA4 contractile effects. Furthermore, LXA4 (6 microM) superfusion of guinea pig bronchial tissue elicited a significant calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) release that was reduced by capsaicin (10 microM, 30 min) desensitization. Finally, LXA4 (10 microM) was unable to displace [3H]resiniferatoxin binding in dorsal root ganglion of rat and guinea pig. These findings support (1) a role for LXA4 in activating motor sensory function of capsaicin-sensitive nerves; (2) this activation mechanism is marginally ruthenium red-sensitive and omega-conotoxin-resistant; and (3) the interaction does not involve the recognized binding site on the vanilloid receptor. As a whole this study presents LXA4 as an endogenous mediator activating sensory nerves potentially involved in basic mechanisms of airway diseases.

Identification of alpha-1-acid glycoprotein (orosomucoid) as a major vanilloid binding protein in serum

We have used the vanilloid (capsaicin) receptor assay to search for modulators of binding activity. We report here that both capsaicin and its ultrapotent analog, resiniferatoxin (RTX), bind to the plasma protein alpha-1-acid glycoprotein (AGP) with high affinity (10.5 and 0.3 microM, respectively). AGP seems to be the dominant vanilloid (capsaicin/RTX) binding protein in serum. [3H] RTX binding to AGP is inhibited by chlorpromazine and by Trisbutoxyethylphosphate, indicating that vanilloids compete for a well-characterized drug binding domain on the AGP molecule. The 35-fold difference in the affinity of AGP for RTX and capsaicin may result in differences in the pharmacodynamics and pharmacokinetics of these two compounds; the contribution of AGP binding to the unique spectra of action of RTX or to the marked species differences in vanilloid actions, however, remains to be determined. An important practical application of AGP is its inclusion in the [3H]RTX binding assay utilizing sensory ganglion membranes to reduce nonspecific binding by up to 5-fold.

Vanilloid receptor loss in rat sensory ganglia associated with long term desensitization to resiniferatoxin

A dose-dependent loss of vanilloid receptors (specific [3H]resiniferatoxin binding sites) was found in sensory ganglia of rats 24 h after s.c. administration of resiniferatoxin (RTX), an ultrapotent capsaicin analog. This receptor loss displayed an ED50 of 30 micrograms/kg both in dorsal root and trigeminal ganglia; the ED50 was 6-fold higher than the ED50 for loss of the neurogenic inflammatory response and 30-60-fold higher than the ED50 for desensitization in the standard eye-wiping (chemogenic pain) response. The receptor loss appeared later (24 h) than the loss of the physiological responses (6 h) and showed modest recovery (to 20-30% of control levels) over the following 4 weeks. This vanilloid receptor loss may represent a novel, specific mechanism for vanilloid-induced chronic desensitization.