iodoresiniferatoxin and Pain

Negative affective state has a significant impact on pain, and genetic background is an important moderating influence on this interaction. The Wistar-Kyoto (WKY) inbred rat strain exhibits a stress-hyperresponsive, anxiety/depressive-like phenotype and also displays a hyperalgesic response to noxious stimuli. Transient receptor potential subfamily V member 1 (TRPV1) within the midbrain periaqueductal grey (PAG) plays a key role in regulating both aversive and nociceptive behaviour. In the present study, we investigated the role of TRPV1 in the sub-columns of the PAG in formalin-evoked nociceptive behaviour in WKY versus Sprague-Dawley (SD) rats. TRPV1 mRNA expression was significantly lower in the dorsolateral (DL) PAG and higher in the lateral (L) PAG of WKY rats, compared with SD counterparts. There were no significant differences in TRPV1 mRNA expression in the ventrolateral (VL) PAG between the two strains. TRPV1 mRNA expression significantly decreased in the DLPAG and increased in the VLPAG of SD, but not WKY rats upon intra-plantar formalin administration. Intra-DLPAG administration of either the TRPV1 agonist capsaicin, or the TRPV1 antagonist 5'-Iodoresiniferatoxin (5'-IRTX), significantly increased formalin-evoked nociceptive behaviour in SD rats, but not in WKY rats. The effects of capsaicin were likely due to TRPV1 desensitisation, given their similarity to the effects of 5'-IRTX. Intra-VLPAG administration of capsaicin or 5'-IRTX reduced nociceptive behaviour in a moderate and transient manner in SD rats, and similar effects were seen with 5'-IRTX in WKY rats. Intra-LPAG administration of 5'-IRTX reduced nociceptive behaviour in a moderate and transient manner in SD rats, but not in WKY rats. These results indicate that modulation of inflammatory pain by TRPV1 in the PAG occurs in a sub-column-specific manner. The data also provide evidence for differences in the expression of TRPV1, and differences in the effects of pharmacological modulation of TRPV1 in specific PAG sub-columns, between WKY and SD rats, suggesting that TRPV1 expression and/or functionality in the PAG plays a role in hyper-responsivity to noxious stimuli in a genetic background prone to negative affect.

Topics: Animals; Anxiety; Behavior, Animal; Capsaicin; Depression; Diterpenes; Genotype; Inflammation; Male; Pain; Periaqueductal Gray; Rats; Rats, Inbred WKY; Rats, Sprague-Dawley; RNA, Messenger; TRPV Cation Channels

The neural substrates and mechanisms mediating the antinociceptive effects of the endogenous bioactive lipid, N-palmitoylethanolamide (PEA), require further investigation. We investigated the effects of exogenous PEA administration into the anterior cingulate cortex (ACC), an important brain region linked with cognitive and affective modulation of pain, on formalin-evoked nociceptive behaviour in rats. Potential involvement of peroxisome proliferator-activated receptor isoforms (PPAR) α and γ or endocannabinoid-mediated entourage effects at cannabinoid1 (CB1) receptors or transient receptor potential subfamily V member 1 (TRPV1) in mediating the effects of PEA was also investigated. Intra-ACC administration of PEA significantly attenuated the first and early second phases of formalin-evoked nociceptive behaviour. This effect was attenuated by the CB1 receptor antagonist AM251, but not by the PPARα antagonist GW6471, the PPARγ antagonist GW9662, or the TRPV1 antagonist 5'-iodo resiniferatoxin. All antagonists, administered alone, significantly reduced formalin-evoked nociceptive behaviour, suggesting facilitatory/permissive roles for these receptors in the ACC in inflammatory pain. Post-mortem tissue analysis revealed a strong trend for increased levels of the endocannabinoid anandamide in the ACC of rats that received intra-ACC PEA. Expression of c-Fos, a marker of neuronal activity, was significantly reduced in the basolateral nucleus of the amygdala, but not in the central nucleus of the amygdala, the rostral ventromedial medulla or the dorsal horn of the spinal cord. In conclusion, these data indicate that PEA in the ACC can reduce inflammatory pain-related behaviour, possibly via AEA-induced activation of CB1 receptors and associated modulation of neuronal activity in the basolateral amygdala.

Topics: Amides; Animals; Anti-Inflammatory Agents, Non-Steroidal; Cannabinoid Receptor Antagonists; Cohort Studies; Disease Models, Animal; Diterpenes; Ethanolamines; Fixatives; Formaldehyde; Gyrus Cinguli; Locomotion; Male; Microdissection; Microinjections; Pain; Pain Measurement; Palmitic Acids; PPAR gamma; Proto-Oncogene Proteins c-fos; Rats; Rats, Sprague-Dawley; Receptor, Cannabinoid, CB1

Topics: Animals; Bone Neoplasms; Disease Models, Animal; Diterpenes; Humans; Mice; Pain; Pain Management; Receptor, Cannabinoid, CB1; TRPV Cation Channels

N-arachidonoylserotonin (AA-5-HT, 1a) is an inhibitor of fatty acid amide hydrolase (FAAH) that acts also as an antagonist of transient receptor potential vanilloid-type 1 (TRPV1) channels and is analgesic in rodents. We modified the chemical structure of 1a with the aim of developing "hybrid" FAAH/TRPV1 blockers more potent than the parent compound or obtaining analogues with single activity at either of the two targets to study the mechanism of the analgesic action of 1a. Thirty-eight AA-5-HT analogues, containing a serotonin "head" bound to a variety of lipophilic moieties via amide, urea, or carbamate functionalities, were synthesized. Unlike 1a, most of the new compounds possessed activity at only one of the two considered targets. The amides 1b and 1c of alpha- and gamma-linolenic acid, however, showed "hybrid" activity similar to 1a. The carbamate 3f (OMDM106), although unable to antagonize TRPV1 receptors, was the most potent FAAH inhibitor in this study (IC50=0.5 microM). Compounds 3f and 1m (OMDM129), which exhibited activity at only FAAH or TRPV1, respectively, were 10-fold less potent than 1a at preventing formalin-induced hyperalgesia in mice.

Topics: Amidohydrolases; Analgesics; Animals; Arachidonic Acids; Biphenyl Compounds; Brain; Calcium; Carbamates; Cell Line; Endocannabinoids; Humans; Hydrolysis; Hyperalgesia; Indoles; Mice; Pain; Pain Measurement; Polyunsaturated Alkamides; Rats; Serotonin; Structure-Activity Relationship; TRPV Cation Channels

The purpose of this study was to determine the role of transient receptor potential vanilloid type 1 (TRPV1) receptor in modulating neuronal activity of the dorsolateral periaqueductal gray (dl-PAG) through excitatory and inhibitory synaptic inputs. First, whole cell voltage-clamp recording was performed to obtain the spontaneous miniature excitatory postsynaptic currents (mEPSCs) and inhibitory postsynaptic currents (mIPSCs) of the dl-PAG neurons. As 1 microM of capsaicin was applied into the perfusion chamber, the frequency of mEPSCs was increased from 3.21 +/- 0.49 to 5.64 +/- 0.64 Hz (P < 0.05, n = 12) without altering the amplitude and the decay time constant of mEPSCs. In contrast, capsaicin had no distinct effect on mIPSCs. A specific TRPV1 receptor antagonist, iodo-resiniferatoxin (i-RTX, 300 nM), decreased the frequency of mEPSCs from 3.51 +/- 0.29 to 2.01 +/- 0.2 Hz (P < 0.05, n = 8) but did not alter the amplitude and decay time. In addition, i-RTX applied into the chamber abolished the effect of capsaicin on mEPSC of the dl-PAG. In another experiment, spontaneous action potential of the dl-PAG neurons was recorded using whole cell current-clamp methods. Capsaicin significantly elevated the discharge rate of the dl-PAG neurons from 3.03 +/- 0.38 to 5.96 +/- 0.87 Hz (n = 8). The increased firing activity was abolished in the presence of glutamate N-methy-D-aspartate (NMDA) and non-NMDA antagonists, 2-amino-5-phosphonopentanoic acid, and 6-cyano-7-nitroquinoxaline-2,3-dione. The results from this study provide the first evidence indicating that activation of TRPV1 receptors increases the neuronal activity of the dl-PAG through selective potentiation of glutamatergic synaptic inputs.

Topics: Action Potentials; Afferent Pathways; Animals; Autonomic Pathways; Capsaicin; Cardiovascular Physiological Phenomena; Diterpenes; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; Female; Glutamic Acid; Inhibitory Postsynaptic Potentials; Male; Neurons; Organ Culture Techniques; Pain; Patch-Clamp Techniques; Periaqueductal Gray; Presynaptic Terminals; Rats; Rats, Sprague-Dawley; Synaptic Transmission; TRPV Cation Channels

Robust chemical or mechanical irritation of the colon of neonatal rats leads to chronic visceral hypersensitivity. The clinical and physiologic relevance of such noxious stimulation in the context of human irritable bowel syndrome is questionable. The aims of this study were to determine whether mild chemical irritation of the colon of neonatal rats produced persistent changes in visceral sensitivity and to evaluate the role of transient receptor potential vanilloid 1 (TRPV1) in the initiation and maintenance of visceral hypersensitivity.. Ten-day-old rat pups received an intracolonic infusion of 0.5% acetic acid in saline. TRPV1 inhibitors were administered 30 minutes before acetic acid sensitization. Sensitivity of the colon to balloon distention (CRD) in adults was measured by grading their abdominal withdrawal reflex and electromyographic responses. In adult rats, TRPV1 antagonist was injected intraperitoneally 30 minutes before CRD.. Neonatal acetic acid treatment resulted in higher sensitivity to CRD in adult rats compared with controls in the absence of histopathologic signs of inflammation. Treatment of colons of adult rats with acetic acid did not produce persistent sensitization. Antagonism of the TRPV1 before neonatal administration of acetic acid and after established visceral hypersensitivity attenuated sensitivity to CRD. TRPV1 expression was increased in dorsal root ganglia-containing colon afferent neurons.. We have described a new model for persistent colonic sensory dysfunction following a transient noxious stimulus in the neonatal period and a potentially important role for TRPV1 in initiation and maintenance of persistent visceral hypersensitivity.

Topics: Acetic Acid; Age Factors; Anilides; Animals; Animals, Newborn; Capsaicin; Catheterization; Cinnamates; Colon; Disease Models, Animal; Diterpenes; Electromyography; Ganglia, Spinal; Hyperalgesia; Irritable Bowel Syndrome; Male; Pain; Rats; Rats, Sprague-Dawley; Reflex, Abdominal; TRPV Cation Channels; Visceral Afferents

Bone cancer pain has a strong impact on the quality of life of patients but is difficult to treat. Therefore, the mechanisms of bone cancer pain require elucidation for the purpose of development of new therapeutics. A recent study showed that activation of transient receptor potential vanilloid subfamily 1 (TRPV1) was involved in bone cancer pain. In this study, we re-evaluated the analgesic effects of pharmacological blockade of TRPV1 using the potent TRPV1 antagonist 5-iodoresiniferatoxin (I-RTX) and examined whether bone cancer can change TRPV1 expression and distribution in the primary sensory neurons in a mouse model of bone cancer pain. Implantation of osteosarcoma into the femur induced ongoing and movement-evoked bone cancer-related pain behaviors. These behaviors were significantly reduced by i.p. administration of I-RTX, compared with vehicle. Western blot and reverse transcription-polymerase chain reaction (RT-PCR) analyses revealed that TRPV1 level was significantly increased in dorsal root ganglions (DRGs) ipsilateral to sarcoma implantation. Immunohistochemical analysis showed that implantation of osteosarcoma induced not only an increase in the percentage of TRPV1-positive neurons among DRG neurons (24.3+/-1.3% in sham mice and 31.2+/-1.3% in mice with osteosarcoma implantation, P<0.05) but also an overall shift in the distribution of area of profiles to the right. Colocalization study showed that the percentages of colocalization of TRPV1 with neurofilament 200 kD (NF200) and calcitonin gene-related peptide (CGRP) but not isolectin B4 (IB4) among DRG neurons in mice with osteosarcoma implantation were increased compared with those in sham mice (from 0.8+/-0.1% to 2.1+/-0.3% for TRPV1 and NF200 and from 21.1+/-1.3% to 26.5+/-0.2% for TRPV1 and CGRP). In conclusion, TRPV1 activation plays a critical role in the generation of bone cancer pain, and bone cancer increases TRPV1 expression within distinct subpopulation of DRG neurons. These findings may lead to novel strategies for the treatment of bone cancer pain.

Topics: Animals; Behavior, Animal; Bone Neoplasms; Calcitonin Gene-Related Peptide; Cell Line, Tumor; Diterpenes; Ganglia, Spinal; Gene Expression Regulation, Neoplastic; Lectins; Male; Mice; Mice, Inbred C3H; Mice, Inbred C57BL; Mice, Knockout; Neurofilament Proteins; Neurons; Pain; Pain Measurement; Sarcoma; TRPV Cation Channels

Activation of transient receptor potential vanilloid-1 (TRPV1) channels in the periaqueductal gray (PAG) activates OFF antinociceptive neurons of the rostral ventromedial medulla (RVM). We examined in rats the effect of intra-ventrolateral (VL)-PAG injections of TRPV1 agonists and antagonists on the nocifensive response to heat in the plantar test, neurotransmitter (glutamate and GABA) release in the RVM, and spontaneous and tail flick-related activities of RVM neurons. The localization of TRPV1 in VL-PAG and RVM neurons was examined using various markers of glutamatergic and GABAergic neurons. Intra-VL-PAG injection of capsaicin increased the threshold of thermal pain sensitivity, whereas the selective TRPV1 antagonist 5'-iodo-resiniferatoxin (I-RTX) facilitated nociceptive responses, and blocked capsaicin analgesic effect at a dose inactive per se. Intra-VL PAG capsaicin evoked a robust release of glutamate in RVM microdialysates. I-RTX, at a dose inactive per se, blocked the effect of capsaicin, and inhibited glutamate release at a higher dose. Antinociception and hyperalgesia induced by capsaicin and I-RTX, respectively, correlated with enhanced or reduced activity of RVM OFF cells. Immunohistochemical analyses suggested that several TRPV1-immunoreactive (ir) neurons in both the VL-PAG and RVM are glutamatergic and surrounded by glutamatergic and GABAergic terminals. Our data suggest that VL-PAG neurons respond to TRPV1 stimulation by releasing glutamate into the RVM, thereby activating OFF cells and producing analgesia. The results obtained with the TRPV1 antagonist alone suggest that this pathway is tonically activated by endovanilloids.

Topics: Action Potentials; Animals; Brain Stem; Capsaicin; Diterpenes; Dose-Response Relationship, Drug; gamma-Aminobutyric Acid; Glutamic Acid; Hot Temperature; Male; Microdialysis; Microinjections; Neural Pathways; Neurons, Efferent; Pain; Pain Measurement; Pain Threshold; Periaqueductal Gray; Rats; Rats, Wistar; Reaction Time; Sensory System Agents; TRPV Cation Channels

Subcutaneous endothelin-1 (ET-1; 200 microM, 2 nmoles/paw) injected into the rat hind paw, has been shown to cause robust hind paw flinching (HPF) and paw licking, and to induce impulses selectively in primary nociceptors. Here we report that a much lower [ET-1] sensitizes the paw to a nocifensive withdrawal response to tactile stimulation (by von Frey hairs, VFH), a sensitization that involves local TRPV1 receptors. Injection of 10 microM ET-1 (0.1 nmole/paw) causes only marginal HPF but rapidly (20 mins after injection) lowers the force threshold for paw withdrawal (PWT) to VFH, to approximately 30% of pre-injection baseline. Such tactile allodynia persists for 3 hrs. In rats pre-injected with the TRPV1-antagonists capsazepine (CPZ; 1.33 mM) or 5'-iodoresiniferatoxin (I-RTX; 0.13 microM), 15 min before ET-1, a fast initial drop in PWT, as with ET-1 alone, occurs (to 40% or to 19% of baseline, respectively), but this earliest reduction then regresses back to the pre-injection PWT value more rapidly than with ET-1 alone. The recovery of allodynia from the maximum value is about two times faster for ET-1+CPZ and about 4 times faster for ET-1+ I-RTX, compared with that from ET-1 +vehicle (t(1/2) = 130, 60, and 250 mins, respectively). In contrast, spontaneous pain indicated by overt HPF from ET-1 is not attenuated by TRPV1 antagonists. Tactile allodynia is similarly abbreviated by antagonists of both ET(A) (BQ-123, 32 nmoles/paw) and ET(B) (BQ-788, 30 nmoles/paw) receptors, whereas HPF is abolished by this ET(A) antagonist but enhanced by the ET(B) antagonist. We conclude that low ET-1 causes tactile allodynia, which is characterized by a different time-course and pharmacology than ET-1-induced nociception, and that local TRPV1 receptors are involved in the maintenance of this ET-1-induced allodynia but not in the overt algesic action of ET-1.

Topics: Animals; Behavior, Animal; Capsaicin; Diterpenes; Dose-Response Relationship, Drug; Endothelin-1; Injections, Subcutaneous; Male; Nociceptors; Pain; Rats; Rats, Sprague-Dawley; Reaction Time; Receptor, Endothelin A; Touch; TRPV Cation Channels

Capsaicin antagonists including ruthenium red, capsazepine and iodo-resiniferatoxin (I-RTX) have recently been shown to inhibit the activation by noxious heat of the capsaicin receptor (TRPV1) expressed in non-neuronal host cells, and natively, in cultured dorsal root ganglion cells. Noxious heat has been shown to release immunoreactive calcitonin gene-related peptide (iCGRP) from the isolated rat skin. In this model, ruthenium red, I-RTX as well as capsazepine 10 microM caused no alteration in iCGRP release at 32 degrees C by themselves whereas capsazepine 100 microM doubled it reversibly. In wild-type mice 100 microM capsazepine also stimulated iCGRP release while it was without effect in TRPV1 knockout littermates. In the rat skin, both ruthenium red and capsazepine (10/100 microM) reduced and abolished, respectively, capsaicin-induced iCGRP release while I-RTX (1/10 microM) was ineffective. Only ruthenium red 100 microM showed an unspecific effect inhibiting iCGRP release induced by KCl. Ruthenium red and capsazepine (10/100 microM) caused no significant alteration of iCGRP release induced by heat stimulation at 47 degrees C. Employing 45 degrees C stimulation intensity, capsazepine and I-RTX (in the higher concentrations) showed a significant facilitatory effect on the heat response suggesting a partial agonistic action of the compounds. It is concluded that noxious heat-induced iCGRP release in the isolated rat skin occurs through a mechanism that is not inhibited by TRPV1 antagonism reflecting a different pharmacological profile of noxious heat transduction in terminals of sensory neurons compared to that in cultured cell bodies and TRPV1-transfected host cells.

Topics: Animals; Calcitonin Gene-Related Peptide; Capsaicin; Diterpenes; Dose-Response Relationship, Drug; Drug Interactions; Hot Temperature; In Vitro Techniques; Male; Mice; Mice, Knockout; Nociceptors; Pain; Rats; Rats, Wistar; Receptors, Drug; Ruthenium Red; Sensory Receptor Cells; Signal Transduction; Skin