Therapeutic potential of capsaicin-like molecules: studies in animals and humans

Resiniferatoxin: Nature's Precision Medicine to Silence TRPV1-Positive Afferents

Resiniferatoxin (RTX) is an ultrapotent capsaicin analog with a unique spectrum of pharmacological actions. The therapeutic window of RTX is broad, allowing for the full desensitization of pain perception and neurogenic inflammation without causing unacceptable side effects. Intravesical RTX was shown to restore continence in a subset of patients with idiopathic and neurogenic detrusor overactivity. RTX can also ablate sensory neurons as a "molecular scalpel" to achieve permanent analgesia. This targeted (intrathecal or epidural) RTX therapy holds great promise in cancer pain management. Intra-articular RTX is undergoing clinical trials to treat moderate-to-severe knee pain in patients with osteoarthritis. Similar targeted approaches may be useful in the management of post-operative pain or pain associated with severe burn injuries. The current state of this field is reviewed, from preclinical studies through veterinary medicine to clinical trials.

The effects of vanilloid analogues structurally related to capsaicin on the transient receptor potential vanilloid 1 channel

Transient receptor potential vanilloid 1 (TRPV1) is known as a receptor of capsaicin, a spicy ingredient of chili peppers. It is also sensitive to a variety of pungent compounds and is involved in nociception. Here, we focused on the structural characteristics of capsaicin, and investigated whether vanillylmanderic acid (VMA), vanillic acid (VAcid), vanillyl alcohol (VAlc), vanillyl butyl ether (VBE), and vanillin, containing a vanillyl skeleton similar to capsaicin, affected the TRPV1 activities. For detection of TRPV1 activity, intracellular Ca²⁺ concentration ([Ca²⁺]_i) was measured in HEK 293 cells heterologously expressing mouse TRPV1 (mTRPV1-HEK) and in mouse sensory neurons. Except for vanillin, four vanilloid analogues dose-dependently increased [Ca²⁺]_i in mTRPV1-HEK. The solutions that dissolved VMA, VAcid and vanillin at high concentrations were acidic, whereas those of VAlc and VBE were neutral. Neutralized VAcid evoked [Ca²⁺]_i increases but neutralized VMA did not. Mutation of capsaicin-sensing sites diminished [Ca²⁺]_i responses to VAcid, VAlc and VBE. VAcid, VMA, and vanillin suppressed the activation of TRPV1 induced by capsaicin. VAcid and VMA also inhibited the acid-induced TRPV1 activation. In sensory neurons, VMA diminished TRPV1 activation by capsaicin or acids. The present data indicate that these structural characteristics of chemical compounds on TRPV1 may provide strategies for the development of novel analgesic drugs targeting nociceptive TRPV1.

TRPV1 modulators: Synthesis and in vitro evaluation of 1-heteroaryl piperidinecarboxamide and piperazinylurea derivatives

A series of new 1-heteroaryl piperidinecarboxamide and piperazinylurea derivatives was synthesized and evaluated as TRPV1 modulators in a Ca(2+) channel assay in HEK-293 cells overexpressing the human recombinant TRPV1 channel. Structural variations in the putative key portions of the molecules afforded several compounds endowed with agonist and/or antagonist/desensitizing activity at low micromolar concentration. As promising examples from this series, the piperidine-3-carboxamide derivative 31 exerts agonist/desensitizing activity at low micromolar concentration, while piperazinylurea derivatives 39 and 41 act as antagonists with sub-micromolar potency.

Ameliorating Effect of Transcutaneous Electroacupuncture on Impaired Gastric Accommodation in Patients with Postprandial Distress Syndrome-Predominant Functional Dyspepsia: A Pilot Study

Patients with functional dyspepsia (FD) have both reduced gastric accommodation and impaired gastric motility that are difficult to treat. The aim of this study was to investigate the therapeutic potential of transcutaneous electroacupuncture (TEA) for both of these disorders in FD patients. Acute experiments were performed in FD patients to study the effect of TEA and sham-TEA on gastric accommodation assessed by a nutrient drink test and gastric motility assessed by the measurement of the electrogastrogram (EGG). TEA or sham-TEA was performed via cutaneous electrodes at acupoints ST36 and PC6 or sham-points nonacupoints. It was found that (1) gastric accommodation (maximum tolerable volume) was reduced in FD patients compared with the controls (P < 0.03). TEA improved gastric accommodation in FD patients (P < 0.02). (2) Acute TEA significantly increased the percentage and power of normal gastric slow waves in the fed state assessed in the FD patients by the EGG in comparison with sham-TEA. (3) TEA increased vagal activity assessed by the spectral analysis of the heart rate variability in the fed state in FD patients. It was concluded that needleless method of transcutaneous electroacupuncture may have a therapeutic potential for treating both impaired gastric accommodation and impaired gastric motility in patients with FD.

Characterization of bladder sensory neurons in the context of myelination, receptors for pain modulators, and acute responses to bladder inflammation

Bladder sensation is mediated by lumbosacral dorsal root ganglion neurons and is essential for normal voiding and nociception. Numerous electrophysiological, structural, and molecular changes occur in these neurons following inflammation. Defining which neurons undergo these changes is critical for understanding the mechanism underlying bladder pain and dysfunction. Our first aim was to define the chemical classes of bladder sensory neurons that express receptors for the endogenous modulators of nociceptor sensitivity, glial cell line-derived neurotrophic factor (GDNF), the related neurotrophic factor, artemin, and estrogens. Bladder sensory neurons of adult female Sprague-Dawley rats were identified with retrograde tracer. Diverse groups of neurons express these receptors, and some neurons express receptors for both neurotrophic factors and estrogens. Lumbar and sacral sensory neurons showed some distinct differences in their expression profile. We also distinguished the chemical profile of myelinated and unmyelinated bladder sensory neurons. Our second aim was to identify bladder sensory neurons likely to be undergoing structural remodeling during inflammation. Following systemic administration of cyclophosphamide (CYP), its renal metabolite acrolein causes transient urothelial loss, exposing local afferent terminals to a toxic environment. CYP induced expression of the injury-related immediate-early gene product, activating transcription factor-3 (ATF-3), in a small population of sacral nitrergic bladder sensory neurons. In conclusion, we have defined the bladder sensory neurons that express receptors for GDNF, artemin and estrogens. Our study has also identified a sub-population of sacral sensory neurons that are likely to be undergoing structural remodeling during acute inflammation of the bladder. Together these results contribute to increased understanding of the neurons that are known to be involved in pain modulation and hyperreflexia during inflammation.

Inhibition by capsaicin and its related vanilloids of compound action potentials in frog sciatic nerves

AIMS

Although capsaicin not only activates transient receptor potential vanilloid-1 (TRPV1) channels but also inhibits nerve conduction, the latter action has not yet been fully examined. The purpose of the present study was to know whether various vanilloids have an inhibitory action similar to that of capsaicin and further to compare their actions with that of local anesthetic procaine.

MAIN METHODS

Fast-conducting compound action potentials (CAPs) were recorded from frog sciatic nerve fibers by using the air-gap method.

KEY FINDINGS

Capsaicin reversibly and concentration-dependently reduced the peak amplitude of the CAP. TRPV1 antagonist capsazepine did not affect the capsaicin activity, and powerful TRPV1 agonist resiniferatoxin had no effect on CAPs, indicating no involvement of TRPV1 channels. Capsaicin analogs and other various vanilloids also inhibited CAPs in a concentration-dependent manner. An efficacy sequence of these inhibitions was capsaicin=dihydrocapsaicin>capsiate>eugenol>guaiacol≥zingerone≥vanillin>vanillylamine. Vanillic acid had almost no effect on CAPs; olvanil and curcumin appeared to be effective less than capsaicin. Capsaicin and eugenol were, respectively, ten- and two-fold effective more than procaine in CAP inhibition, while each of guaiacol, zingerone and vanillin was five-fold effective less than procaine.

SIGNIFICANCE

Various vanilloids exhibit CAP inhibition, the extent of which is determined by the property of the side chain bound to the vanillyl group, and some of them are more effective than procaine. These results may serve to unveil molecular mechanisms for capsaicin-induced conduction block and to develop antinociceptive drugs related to capsaicin.

How does the urothelium affect bladder function in health and disease? ICI-RS 2011

The urothelium is a multifunctional tissue that not only acts as a barrier between the vesical contents of the lower urinary tract and the underlying tissues but also acts as a sensory organ by transducing physical and chemical stresses to the attendant afferent nervous system and underlying smooth muscle. This review will consider the nature of the stresses that the urothelium can transduce; the transmitters that mediate the transduction process; and how lower urinary pathologies, including overactive bladder syndrome, painful bladder syndrome and bacterial infections, are associated with alterations to this sensory system. In particular, the role of muscarinic receptors and the TRPV channels system will be discussed in this context. The urothelium also influences the contractile state of detrusor smooth muscle, both through modifying its contractility and the extent of spontaneous activity; potential pathways are discussed. The potential role that the urothelium may play in bladder underactivity is introduced, as well as potential biomarkers for the condition that may cross the urothelium to the urine. Finally, consideration is given to vesical administration of therapeutic agents that influence urinary tract function and how the properties of the urothelium may determine the effectiveness of this mode of delivery.

Enzymatic synthesis of capsaicin analogs and their effect on the T-type Ca2+ channels

Capsaicin (Cap) and its analogs (CAPanalogs) have diverse effects in sensory neurons including analgesia, implying they modulate other cellular targets besides the TRPV1 Cap receptor. Since Cap and CAPanalogs are not largely available and their chemical synthesis is cumbersome, they have been obtained through a direct lipase-catalyzed reaction. Capsiate, the ester CAPanalog, was synthesized using a novel enzymatic transacylation one-pot strategy. Five different CAPanalogs were synthesized by amidation in 2-methyl-2-butanol with higher yields than previously reported. Voltage-dependent Ca(2+) channels (Ca(v)s) are among the main Ca(2+) entry paths into cells. They are classified as high-voltage-activated Ca(2+) channels (HVA) and low-voltage-activated Ca(2+) channels (LVA) constituted only by T-type channels. Though HVA Ca(v)s are Cap sensitive, it is not known if capsaicinoids inhibit LVA Ca(v)s which participate in the primary sensory neuron pain pathway. Here we first report that Cap, dihydrocapsaicin, N-VAMC(8), N-VAMC(9), and N-VAMC(10) can directly and partially reversibly inhibit T-type Ca(v)s, whereas olvanil, capsiate, and vanillylamine cannot. The Cap inhibition of T-type Ca(v)s was independent of TRPV1 activation.

Mechanisms of disease: involvement of the urothelium in bladder dysfunction

Although the urinary bladder urothelium has classically been thought of as a passive barrier to ions and solutes, a number of novel properties have been recently attributed to urothelial cells. Studies have revealed that the urothelium is involved in sensory mechanisms (i.e. the ability to express a number of sensor molecules or respond to thermal, mechanical and chemical stimuli) and can release chemical mediators. Localization of afferent nerves next to the urothelium suggests that urothelial cells could be targets for neurotransmitters released from bladder nerves or that chemicals released by urothelial cells could alter afferent nerve excitability. Taken together, these and other findings highlighted in this article suggest a sensory function for the urothelium. Elucidation of mechanisms that influence urothelial function might provide insights into the pathology of bladder dysfunction.

Capsaicin receptor: TRPV1 a promiscuous TRP channel

TRPV1, the archetypal member of the vanilloid TRP family, was initially identified as the receptor for capsaicin, the pungent ingredient in hot chili peppers. The receptor has a diverse tissue distribution, with high expression in sensory neurons. TRPV1 is a nonselective cation channel with significant permeability to calcium, protons, and large polyvalent cations. It is the most polymodal TRP channel, being activated by numerous stimuli, including heat, voltage, vanilloids, lipids, and protons/cations. TRPV1 acts as a molecular integrator of physical and chemical stimuli in peripheral nociceptor terminals and plays a critical role in thermal inflammatory hyperalgesia. In addition, TRPV1 may regulate a variety of physiological functions in different organ systems. Various second messenger systems regulate TRPV1 activity, predominantly by serine-threonine phosphorylation. In this review, we provide a concise summary of the information currently available about this channel.

Pathophysiology and treatment of functional dyspepsia

Functional dyspepsia is a highly prevalent symptom complex and a heterogenous disorder. Recent studies showed potential associations between specific pathophysiologic disturbances and dyspeptic symptoms. Delayed gastric emptying reported in about 30% of patients with functional dyspepsia is associated with the symptoms of postprandial fullness, nausea, and vomiting. Impaired gastric accommodation present in 40% of functional dyspepsia patients is found to be associated with early satiety. Hypersensitivity to gastric distension is observed in 37% of functional dyspepsia patients and associated with the symptoms of postprandial pain, belching, and weight loss. Psychosocial factors and altered response to duodenal lipids or acid have also been identified as pathophysiologic mechanisms. Therapeutic options are still limited but targeted therapy directed at the underlying pathophysiology seems desirable. Thus, efforts to further elucidate underlying pathophysiologic mechanisms and identify the appropriate patient population using some type of pathophysiologic testing will be required.

Spinal neurophysiologic correlates of the analgesic actions of intravesical dimethyl sulfoxide and capsaicin in the rat

Peripheral analgesia produced by the intravesical instillation of dimethyl sulphoxide (DMSO) and capsaicin has been used to treat visceral pain originating in the urinary bladder. The present study sought to determine the neurophysiologic consequences of the intravesical instillation of these compounds by measuring spinal neuronal responses evoked by urinary bladder distension (UBD) in the rat. Subjects were spinally transected, decerebrate female Sprague-Dawley rats. The effect of 0.5 mL of solution of 10% or 50% DMSO, 100 micromol/L capsaicin, or the same volume of saline instilled into the bladder on excitatory neuronal responses to UBD was studied by using single-unit extracellular recordings of L6-S2 dorsal horn spinal cord neurons. Fifty-six dorsal horn neurons that were excited by UBD in a graded fashion were identified. All neurons were also excited by noxious or non-noxious cutaneous stimuli. Two hours after intravesical instillation, solutions of 50% DMSO or 100 micromol/L of capsaicin produced a reduction of the slope of stimulus-response functions for neuronal activity evoked by graded UBD. These data support a local effect of intravesical 50% DMSO or capsaicin and suggest the use of this model to study novel peripheral treatment strategies for bladder pain.

Experimental neurogenic cystitis

Recent advances in basic and clinical research indicate that interstitial cystitis (IC) is a form of neurogenic inflammation, thereby opening new avenues for research into this painful disease. With this in mind, we have recently developed a rat model of neurogenic inflammation of the bladder produced by a central nervous system viral disease. As in IC, the inflammation in this model develops without direct injury or trauma to the bladder, is non-infectious, and is limited to the bladder. Our most recent studies aimed at further testing the similarity of this animal model to IC by assessing the urine content in histamine with the occurrence of mast cell degranulation in the bladder wall. We further verified for a sex difference in the occurrence of the disease. Our results showed increased levels of urine histamine and mast cell activation during the early stages of the disease. We additionally observed that females had a greater degree of plasma extravasation, while males had a greater cellular infiltration together with worse behavioral signs. Gonadectomy prevented the bladder inflammation altogether in both males and females. These findings further validate our model of neurogenic cystitis to study the neurogenic component of IC.

Tachykinin-independent activity of capsaicin on in-vitro lamb detrusor

The capsicum alkaloid capsaicin is an afferent fibre exciter. In the vesical bladder, capsaicin acts by releasing peptides stored in afferent fibres. The aim of this work was to verify the activity of capsaicin on in-vitro lamb urinary bladder and to ascertain whether this alkaloid evokes peptide release. Capsaicin relaxed about 80% of the lamb detrusor muscle preparations tested and contracted about 20%. Whereas neurokinin A and substance P antagonists, administered alone or together, left the contractile responses to capsaicin unchanged, atropine and tetrodotoxin totally inhibited contraction. Ruthenium red and indometacin abolished contractions and relaxation. The substance P and neurokinin A antagonists and the NO-synthesis inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) left relaxation unchanged; conversely, the calcitonin gene-related peptide antagonist alpha h-CGRP (8-37) abolished this response. These results suggest that capsaicin relaxes lamb detrusor muscle not through tachykinins but by releasing CGRP from afferent fibres. Our observation that indometacin blocks the capsaicin response in in-vitro lamb urinary bladder also suggests a role of prostanoids.

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.

Botulinum-A toxin for treating detrusor hyperreflexia in spinal cord injured patients: a new alternative to anticholinergic drugs? Preliminary results

PURPOSE

We evaluated the efficacy of botulinum-A toxin injections into the detrusor muscle in patients with spinal cord injury, detrusor hyperreflexia and urge incontinence resistant to anticholinergic drugs. The purpose of treatment was to suppress incontinence episodes and increase functional bladder capacity.

MATERIALS AND METHODS

Included in our prospective nonrandomized study done at 2 clinics were 31 patients with traumatic spinal cord injury who emptied the bladder by intermittent self-catheterization. These patients had severe detrusor hyperreflexia and incontinence despite a high dose of anticholinergic medication. Pretreatment evaluation included a clinical examination and complete urodynamic investigation. Under cystoscopic control a total of 200 to 300 units of botulinum-A toxin were injected into the detrusor muscle at 20 to 30 sites (10 units per ml. per site), sparing the trigone. Clinical and urodynamic followup was planned for 6, 16 and 36 weeks after treatment. Patients were asked to decrease their intake of anticholinergic drugs during week 1 after treatment.

RESULTS

Of the 21 patients 19 underwent a complete examination 6 weeks after the botulinum-A toxin injections, and 11 at 16 and 36 weeks. At the 6-week followup complete continence was restored in 17 of 19 cases in which anticholinergic medication was markedly decreased or withdrawn. Less satisfactory results in 2 cases were associated with an insufficient dose of 200 units botulinum-A toxin. After the injections overall mean reflex volume and mean maximum cystometric bladder capacity plus or minus standard deviation significantly increased from 215.8 +/- 90.4 ml. to 415.7 +/- 211.1 (p <0.016) and 296.3 +/- 145.2 to 480.5 +/- 134.1 (p <0.016), respectively. There was also a significant decrease after treatment in mean maximum detrusor voiding pressure from 65.6 +/- 29.2 cm. water to 35 +/- 32. 1 (p <0.016). Mean post-void residual urine volume catheterized at the end of the urodynamic examination increased significantly from a mean of 261.8 +/- 241.3 ml. to 490.5 +/- 204.8 (p <0.016). Moreover, autonomic dysreflexia associated with bladder emptying that manifested as a hypertensive crisis during voiding disappeared after treatment in the 3 patients with tetraplegia. Satisfaction was high in all successfully treated patients and no side effects were observed. Ongoing improvement in urodynamic parameters and incontinence was already present in all patients reevaluated at 16 and 36 weeks.

CONCLUSIONS

Botulinum-A toxin injections into the detrusor seem to be a safe and valuable therapeutic option in spinal cord injured patients with incontinence resistant to anticholinergic medication who perform clean intermittent self-catheterization. Successfully treated patients become continent again and may withdraw from or markedly decrease anticholinergic drug intake. A dose of 300 units botulinum-A toxin seems to be needed to counteract an overactive detrusor. The duration of bladder paresis induced by the toxin is at least 9 months, when repeat injections are required.

Botulinum-A toxin for treating detrusor hyperreflexia in spinal cord injured patients: a new alternative to anticholinergic drugs? Preliminary results

PURPOSE

We evaluated the efficacy of botulinum-A toxin injections into the detrusor muscle in patients with spinal cord injury, detrusor hyperreflexia and urge incontinence resistant to anticholinergic drugs. The purpose of treatment was to suppress incontinence episodes and increase functional bladder capacity.

MATERIALS AND METHODS

Included in our prospective nonrandomized study done at 2 clinics were 31 patients with traumatic spinal cord injury who emptied the bladder by intermittent self-catheterization. These patients had severe detrusor hyperreflexia and incontinence despite a high dose of anticholinergic medication. Pretreatment evaluation included a clinical examination and complete urodynamic investigation. Under cystoscopic control a total of 200 to 300 units of botulinum-A toxin were injected into the detrusor muscle at 20 to 30 sites (10 units per ml. per site), sparing the trigone. Clinical and urodynamic followup was planned for 6, 16 and 36 weeks after treatment. Patients were asked to decrease their intake of anticholinergic drugs during week 1 after treatment.

RESULTS

Of the 21 patients 19 underwent a complete examination 6 weeks after the botulinum-A toxin injections, and 11 at 16 and 36 weeks. At the 6-week followup complete continence was restored in 17 of 19 cases in which anticholinergic medication was markedly decreased or withdrawn. Less satisfactory results in 2 cases were associated with an insufficient dose of 200 units botulinum-A toxin. After the injections overall mean reflex volume and mean maximum cystometric bladder capacity plus or minus standard deviation significantly increased from 215.8 +/- 90.4 ml. to 415.7 +/- 211.1 (p <0.016) and 296.3 +/- 145.2 to 480.5 +/- 134.1 (p <0.016), respectively. There was also a significant decrease after treatment in mean maximum detrusor voiding pressure from 65.6 +/- 29.2 cm. water to 35 +/- 32. 1 (p <0.016). Mean post-void residual urine volume catheterized at the end of the urodynamic examination increased significantly from a mean of 261.8 +/- 241.3 ml. to 490.5 +/- 204.8 (p <0.016). Moreover, autonomic dysreflexia associated with bladder emptying that manifested as a hypertensive crisis during voiding disappeared after treatment in the 3 patients with tetraplegia. Satisfaction was high in all successfully treated patients and no side effects were observed. Ongoing improvement in urodynamic parameters and incontinence was already present in all patients reevaluated at 16 and 36 weeks.

CONCLUSIONS

Botulinum-A toxin injections into the detrusor seem to be a safe and valuable therapeutic option in spinal cord injured patients with incontinence resistant to anticholinergic medication who perform clean intermittent self-catheterization. Successfully treated patients become continent again and may withdraw from or markedly decrease anticholinergic drug intake. A dose of 300 units botulinum-A toxin seems to be needed to counteract an overactive detrusor. The duration of bladder paresis induced by the toxin is at least 9 months, when repeat injections are required.

Menthol desensitization of capsaicin irritation. Evidence of a short-term anti-nociceptive effect

Evidence is presented of a short-term antinociceptive effect of menthol that was discovered in the course of investigating menthol's potential to sensitize the mouth to capsaicin. Previous research had shown that treating the tongue with menthol 15 min before exposure to capsaicin could enhance the irritancy of capsaicin, and we wished to learn if this effect would increase as the time between exposure to menthol and capsaicin decreased. We found instead that when capsaicin followed menthol by only 3.5 min, or when it was presented in mixture with menthol for 2-3 min, sensory irritation was reduced rather than enhanced. We examined the duration of this apparent crossdesensitization in a second experiment by varying the delay between exposure to menthol and a block of three consecutive capsaicin stimuli. Cross-desensitization tended to decline as the interstimulus interval (ISI) increased to 5 min, and even when desensitization was maximal, it was significant only for the first of the three capsaicin stimuli. In the final experiment we investigated how menthol self- and cross-desensitization can influence the perception of menthol-capsaicin mixtures. During a series of five, 90-s stimulations, self- and cross-desensitization became evident at the beginning of the second exposure, but the effect on mixture intensity again diminished rapidly as stimulation continued. We infer from these results that method can transiently desensitize capsaicin-sensitive fibers, but that exposure to capsaicin rapidly overrides the effect. The implications these findings have for menthol's potential as a topical analgesic are discussed.

Tachykinin receptor modulators: novel therapeutics for rheumatoid arthritis

The activation of a cellular immune response in a genetically susceptible individual is widely recognised as a main step in triggering rheumatoid arthritis (RA). The tachykinins, substance P (SP) and neurokinin A (NKA), can play a major role in different immune diseases. In patients with inflammatory joint disease, elevated levels of SP have been demonstrated in the synovial fluid of affected joints. It is well known that SP and, to a lesser extent, NKA are deeply involved in the processing of nociceptive signals and exert many pro-inflammatory actions, which may be elicited by an increased neuronal neurokinin release in arthritis; the mechanism behind this increase remains to be fully elucidated. Different observations suggest that one approach to the treatment of RA might be to inhibit the local effects of neurokinins in the affected joints. This review will summarise the more relevant aspects of this topic.

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.

Intravesical capsaicin and resiniferatoxin therapy: spicing up the ways to treat the overactive bladder

PURPOSE

Pharmacological treatment of the overactive bladder relies on partially blocking the efferent parasympathetic innervation to the detrusor with anticholinergic drugs. However, often these drugs have troublesome side effects and doses are insufficient to restore continence in patients with detrusor instability. We present the background, basic and clinical research with intravesical instillation of capsaicin and resiniferatoxin as treatments for the overactive bladder.

MATERIALS AND METHODS

Capsaicin, the main pungent ingredient in hot peppers of the genus Capsicum, is a specific neurotoxin that desensitizes C fiber afferent neurons which may be responsible for signals that trigger detrusor overactivity.

RESULTS

In the last 6 years studies have demonstrated encouraging improvement in lower urinary tract symptoms with minimal long-term complications. Most of these studies have also demonstrated that the acute pain and irritation associated with capsaicin are major deterrents to widespread use. Therefore, resiniferatoxin, an ultra-potent analogue of capsaicin which appears to have similar efficacy but less acute side effects, may be more useful.

CONCLUSIONS

Intravesical capsaicin and resiniferatoxin are novel and promising treatments for the overactive bladder, with profound basic and clinical implications.

Capsaicin-induced desensitization is prevented by capsazepine but not by ruthenium red in guinea pig bronchi

In isolated guinea pig bronchi, the influence of ruthenium red, capsazepine and extracellular Ca2+ on capsaicin-induced desensitization was examined to investigate whether this desensitization was mediated via a specific receptor coupled with an ion channel. Pre-exposure of tissues to capsaicin (1, 3 or 10 microM) caused a dose-dependent desensitization to the second application of capsaicin. However, the contractile responses to exogenous tachykinins were not changed after pre-exposure of tissues to capsaicin. This capsaicin-induced desensitization was prevented by capsazepine (30 microM), but not by ruthenium red added to tissues 20 min before pretreatment with capsaicin (3 microM). While the excitatory contractile response to capsaicin was markedly reduced in the absence of extracellular Ca2+, the desensitization induced by capsaicin was not changed by the removal of extracellular Ca2+. In summary, the results from the present study suggest that in vitro functional desensitization induced by capsaicin in guinea pig bronchi may involve changes in the vanilloid receptor and occur through a ruthenium red-insensitive pathway.

Effects of ZD6169, a KATP channel opener, on bladder hyperactivity and spinal c-fos expression evoked by bladder irritation in rats

Cystometrographic recording and immunocytochemical techniques were used to examine the effects of ZD6169, an ATP-sensitive K+-channel opener, and capsaicin, an afferent neurotoxin, on urinary bladder hyperactivity and immediate early gene expression in the spinal cord induced by acetic acid (0.25%) irritation of the bladder. Chemical irritation of the bladder of the rat increased the frequency of voiding reflexes by 8 fold and increased c-fos expression in neurons in the dorsal commissure (DCM), sacral parasympathetic nucleus (SPN) as well as the medial and lateral dorsal horn (MDH, LDH) of L6 and S1 segments of the spinal cord. Pretreatment with ZD6169 (30 nM) for 1 h reduced the effect of acetic acid on voiding frequency as reflected by an increase in the intercontraction interval (ICI, 137+/-48% increase, P<0.05). ZD6169 also decreased the number of Fos positive neurons in the L6 spinal cord, in the DCM (62.1+/-7.1% decrease), SPN (48.8+/-7%), MDH (50+/-7.3%) and LDH regions (38. 8+/-10.5%). Similar reductions were noted in the S1 spinal cord: 65. 1+/-10.8% in DCM, 53.8+/-11% in SPN, 56+/-10.4% in MDH and 25.3+/-18. 1% in LDH. Capsaicin pretreatment (125 mg/kg, s.c., 4 days prior to the experiments) also reduced bladder hyperactivity (550% increase in ICI) and decreased the numbers of acetic acid-induced Fos positive neurons 78.8+/-6.3% in DCM, 73+/-7.8% in MDH, 59.2+/-16% in LDH and 45.2+/-17% in SPN of L6 segment of the spinal cord. These results suggest that ZD6169 can influence bladder hyperactivity by suppressing the firing of capsaicin-sensitive C-fiber bladder afferents which are known to modulate the micturition reflex.

The cloned capsaicin receptor integrates multiple pain-producing stimuli

Capsaicin, the main pungent ingredient in "hot" chili peppers, elicits buming pain by activating specific (vanilloid) receptors on sensory nerve endings. The cloned vanilloid receptor (VR1) is a cation channel that is also activated by noxious heat. Here, analysis of heat-evoked single channel currents in excised membrane patches suggests that heat gates VR1 directly. We also show that protons decrease the temperature threshold for VR1 activation such that even moderately acidic conditions (pH < or = 5.9) activate VR1 at room temperature. VR1 can therefore be viewed as a molecular integrator of chemical and physical stimuli that elicit pain. Immunocytochemical analysis indicates that the receptor is located in a neurochemically heterogeneous population of small diameter primary afferent fibers. A role for VR1 in injury-induced hypersensitivity at the level of the sensory neuron is presented.

Temporal characteristics of capsaicin desensitization and stimulus-induced recovery in the oral cavity

It was recently discovered that capsaicin desensitization of the tongue can be temporarily reversed during recurrent stimulation ("stimulus-induced recovery," SIR). The effects of concentration and the temporal pattern of stimulation on desensitization and SIR were studied in four experiments. In Experiment 1, three different concentrations of capsaicin (3.3, 33, or 330 microM) were delivered to the tongue in blocks of twelve stimuli at the rate of one per minute (treatment block) followed by a block of 20 exposures to 33 microM capsaicin (test block). The 33- and 330-microM treatments both caused significant desensitization, but by the end of the test block recovery was substantial only for the 33-microM condition. Experiment 2 provided a second test of the effect of concentration while also exploring whether intermittent stimulation was a requirement for SIR, i.e., stimuli were refreshed on the tongue every six minutes rather than every 60 s. SIR occurred but was pronounced only when the concentration of the test stimulus equaled or exceeded that of the treatment stimulus. In the third experiment we delivered capsaicin in a candy formulation to examine SIR under conditions in which stimulation was not interrupted at all. Although desensitization was evident during the first few minutes of succeeding exposures to the candy, peak irritation was unchanged throughout nine exposures over 3 days. In the final experiment, we returned to intermittent stimulation to examine the effect of lengthening ISI on SIR. The results showed that SIR remained virtually complete until ISI was lengthened to 60 s. We conclude that SIR is a robust phenomenon that is maximized by rapid exposure to capsaicin in a concentration at least as high as that of the desensitizing stimulus. The implications of these findings for hypotheses about the mechanism of SIR, its function in endogenous chemonociception, and for clinical use of capsaicin as a topical analgesic are discussed.

Review of the effectiveness of capsaicin for painful cutaneous disorders and neural dysfunction

BACKGROUND

Topical capsaicin is known to be a safe and effective pain management adjunct for rheumatoid arthritis, osteoarthritis, neuralgias, and diabetic neuropathy. However, studies and case reports in the literature have indicated that other conditions may also benefit from capsaicin: painful or itching cutaneous disorders from operations, injuries, or tumors; neural dysfunction; or inflammation of the airways and urinary tract.

METHODS

To determine the effectiveness of capsaicin for painful cutaneous disorders and neural dysfunction, the authors analyzed data from 33 reports (MEDLINE search of 1966-96) on the efficacy of capsaicin. Outcome measures consisted of the response rate and degree of pain relief. Results from placebo-controlled trials were pooled when possible; effect of treatment was estimated by the method of DerSimonian and Laird.

RESULTS

Pain relief for postmastectomy syndrome and cluster headache was greater with capsaicin than with placebo; also, psoriasis and pruritus responded better to capsaicin. Uncontrolled studies and case reports have indicated that pain or dysfunction was less at the end of capsaicin therapy for neck pain, loin pain/hematuria syndrome, oral mucositis, rhinopathy, reflex sympathetic dystrophy syndrome, detrusor hyperreflexia, and cutaneous pain due to tumor of the skin.

CONCLUSIONS

Capsaicin is effective for psoriasis, pruritus, and cluster headache; it is often helpful for the itching and pain of postmastectomy pain syndrome, oral mucositis, cutaneous allergy, loin pain/hematuria syndrome, neck pain, amputation stump pain, and skin tumor; and it may be beneficial for neural dysfunction (detrusor hyperreflexia, reflex sympathetic dystrophy, and rhinopathy). A universal problem for many of the studies analyzed was the absence of a "burning placebo" such as camphor.

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.

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.

Pharmacological characterization of the vanilloid receptor in the rat isolated vas deferens

The present study set out to further characterize the vanilloid receptor in the rat isolated vas deferens. In this preparation, both capsaicin and resiniferatoxin (RTX) evoked a concentration-dependent inhibition in the amplitude of electrically-evoked contractions with pEC50 values of 7.62 +/- 0.03 and 12.2 +/- 0.21 respectively. Responses to capsaicin were fast in onset and faded rapidly over a 30-min exposure period, whereas those to RTX were slow in onset and well maintained, an observation believed to reflect pharmacokinetic differences in the rate of penetration to the vanilloid receptor. Responses to both agonists showed mutual cross-desensitization and were antagonized by both the vanilloid-receptor antagonist capsazepine and the ion-channel blocker ruthenium red. The capsaicin analogue, olvanil failed to either mimic or antagonize capsaicin-evoked responses in the rat isolated vas deferens, an effect at variance with previous observations in other tissues. The reason for these differences is unclear, but the possibility of multiple classes of receptor cannot at this stage be ruled out.

Oral capsaicin provides temporary relief for oral mucositis pain secondary to chemotherapy/radiation therapy

Pain from oral mucositis afflicts from 40% to 70% of patients receiving chemotherapy or radiation therapy. Current methods of clinical pain management (for example, topical anesthetics, systemic analgesics) have limited success. In a pilot study, we examined the ability of oral capsaicin to provide temporary relief of oral mucositis pain. Capsaicin, the active ingredient in chili peppers, desensitizes some neurons and has provided moderate pain relief when applied to the skin surface. Oral capsaicin in a candy (taffy) vehicle produced substantial pain reduction in 11 patients with oral mucositis pain from cancer therapy. However, this pain relief was not complete for most patients and was only temporary. Additional research is needed to fully utilize the properties of capsaicin desensitization and thus optimize analgesia.

[3H]resiniferatoxin binding by the human vanilloid (capsaicin) receptor

We report here that we were able to detect the human vanilloid receptor in all three major central endings of primary afferent neurons--in the dorsal horn of the spinal cord, in the cuneate and gracile nuclei and in the spinal nucleus of the trigeminal nerve--and to characterize the binding properties of the receptor in the dorsal horn. Specific [3H]resiniferatoxin (RTX) binding is thought to represent the vanilloid (capsaicin) receptor. [3H]RTX binding to membranes obtained from total human spinal cord and dorsal horn followed sigmoidal saturation kinetics indicating apparent positive cooperativity. The cooperativity index determined by fitting the data to the Hill equation was 1.37 +/- 0.02 in the total spinal cord and 1.77 +/- 0.16 in the dorsal horn. The apparent dissociation constants in whole spinal cord and dorsal horn membranes were 915 +/- 12 and 532 +/- 27 pM; the receptor densities were 140 +/- 6 and 227 +/- 15 fmol/mg protein, respectively. Membrane preparations from the spinal nucleus of the trigeminal nerve and the cuneate and gracile nuclei also bound [3H]RTX in a similar fashion. In parallel experiments, rat spinal cord membranes bound [3H]RTX with 20- to 40-fold higher affinity, somewhat greater positive cooperativity, but at a 3-fold lower receptor density. As predicted by the modified Hill equation, non-radioactive RTX at low receptor occupancy produced biphasic competition curves.(ABSTRACT TRUNCATED AT 250 WORDS)

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 and piperine: capsaicin-like stimulators of oxygen uptake in the perfused rat hindlimb

The naturally occurring capsaicin-like molecules, resiniferatoxin (RTX, Euphorbia spp.) and piperine (Piper nigrum), each stimulated oxygen uptake (VO2) in association with increased vascular resistance in a concentration-dependent manner when infused into the perfused rat hindlimb. 5 microM glyceryl trinitrate (GTN, a nitrovasodilator) significantly blocked the oxygen and pressure responses to both RTX and piperine, indicating a close relationship between changes in VO2 and the vasoconstriction. Concentrations greater than those required for maximal VO2 resulted in an inhibition of VO2, although perfusion pressure continued to increase. Time course studies showed that both RTX and piperine at high doses resulted in a tri-phasic response. An initial phase of transient VO2 stimulation was followed by a second phase of inhibition. A third phase involving an often larger but transient stimulation of VO2 followed removal of the agents and continued after the pressure returned to basal. The actions of RTX and piperine were similar to those of other active capsaicin-like molecules tested previously in this system, including capsaicinoids (Capsicum spp.), gingerols (Zingiber officinale), and shogoals (Zingiber officinale). RTX was the most potent, and piperine the least potent of this series. Although receptor involvement has yet to be unequivocally established, the data are consistent with the presence of a functional capsaicin-like (vanilloid) receptor in the vasculature of the rat hindlimb that mediates vasoconstriction and oxygen uptake. These findings may have implications for the future development of thermogenic agents.

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.