CPEB3 Deficiency Elevates TRPV1 Expression in Dorsal Root Ganglia Neurons to Potentiate Thermosensation

Cytoplasmic polyadenylation element binding protein 3 (CPEB3) is a sequence-specific RNA-binding protein that downregulates translation of multiple plasticity-related proteins (PRPs) at the glutamatergic synapses. Activity-induced synthesis of PRPs maintains long-lasting synaptic changes that are critical for memory consolidation and chronic pain manifestation. CPEB3-knockout (KO) mice show aberrant hippocampus-related plasticity and memory, so we investigated whether CPEB3 might have a role in nociception-associated plasticity. CPEB3 is widely expressed in the brain and peripheral afferent sensory neurons. CPEB3-KO mice with normal mechanosensation showed hypersensitivity to noxious heat. In the complete Freund''s adjuvant (CFA)-induced inflammatory pain model, CPEB3-KO animals showed normal thermal hyperalgesia and transiently enhanced mechanical hyperalgesia. Translation of transient receptor potential vanilloid 1 (TRPV1) RNA was suppressed by CPEB3 in dorsal root ganglia (DRG), whereas CFA-induced inflammation reversed this inhibition. Moreover, CPEB3/TRPV1 double-KO mice behaved like TRPV1-KO mice, with severely impaired thermosensation and thermal hyperalgesia. An enhanced thermal response was recapitulated in non-inflamed but not inflamed conditional-KO mice, with cpeb3 gene ablated mostly but not completely, in small-diameter nociceptive DRG neurons. CPEB3-regulated translation of TRPV1 RNA may play a role in fine-tuning thermal sensitivity of nociceptors.     

Breaking down the barriers: fMRI applications in pain, analgesia and analgesics

Background

The vanilloid receptor 1 (TRPV1) is critical in the development of inflammatory hyperalgesia. Several receptors including G-protein coupled prostaglandin receptors have been reported to functionally interact with the TRPV1 through a cAMP-dependent protein kinase A (PKA) pathway to potentiate TRPV1-mediated capsaicin responses. Such regulation may have significance in inflammatory pain. However, few functional receptor interactions that inhibit PKA-mediated potentiation of TRPV1 responses have been described.

Results

In the present studies we investigated the hypothesis that the μ opioid receptor (MOP) agonist morphine can modulate forskolin-potentiated capsaicin responses through a cAMP-dependent PKA pathway. HEK293 cells were stably transfected with TRPV1 and MOP, and calcium (Ca²⁺) responses to injection of the TRPV1 agonist capsaicin were monitored in Fluo-3-loaded cells. Pre-treatment with morphine did not inhibit unpotentiated capsaicin-induced Ca²⁺ responses but significantly altered capsaicin responses potentiated by forskolin. TRPV1-mediated Ca²⁺ responses potentiated by the direct PKA activator 8-Br-cAMP and the PKC activator Phorbol-12-myristate-13-acetatewere not modulated by morphine. Immunohistochemical studies confirmed that the TRPV1 and MOP are co-expressed on cultured Dorsal Root Ganglion neurones, pointing towards the existence of a functional relationship between the G-protein coupled MOP and nociceptive TRPV1.

Conclusion

The results presented here indicate that the opioid receptor agonist morphine acts via inhibition of adenylate cyclase to inhibit PKA-potentiated TRPV1 responses. Targeting of peripheral opioid receptors may therefore have therapeutic potential as an intervention to prevent potentiation of TRPV1 responses through the PKA pathway in inflammation.     

Protein kinase C-α upregulates sodium channel Nav1.9 in nociceptive dorsal root ganglion neurons in an inflammatory arthritis pain model of rat

Previous studies have found that increased expression of Nav1.9 and protein kinase C (PKC) contributes to pain hypersensitivity in a couple of inflammatory pain models. Here we want to observe if PKC can regulate the expression of Nav1.9 in dorsal root ganglion (DRG) in rheumatoid arthritis (RA) pain model. A chronic knee joint inflammation model was produced by intra-articular injection of the complete Freund's adjuvant (CFA) in rats. Nociceptive behaviors including mechanical, cold, and heat hyperalgesia were examined. The expression of Nav1.9 and PKCα in DRG was detected by a quantitative polymerase chain reaction, Western blot, and immunofluorescence. The in vitro and in vivo effects of a PKC activator (phorbol 12-myristate 13-acetate [PMA]) and a PKC inhibitor (GF-109203X) on the expression of Nav1.9 were examined. Moreover, the effects of PKC modulators on nociceptive behaviors were studied. Increased mechanical, heat, and cold sensitivity was observed 3 to 14 days after CFA injection. Parallel increases in messenger RNA and protein expression of Nav1.9 and PKCα were found. Immunofluorescence experiments found that Nav1.9 was preferentially colocalized with IB4+DRG neurons in RA rats. In cultured DRG neurons, PMA increased Nav1.9 expression while GF-109203X prevented the effect of PMA. PMA increased Nav1.9 expression in naïve rats while GF-109203X decreased Nav1.9 expression in RA rats. In naïve rats, PMA caused mechanical and cold hyperalgesia. On the other hand, GF-109203X attenuated mechanical and cold hyperalgesia in RA-pain model. Nav1.9 might be upregulated by PKCα in DRG, which contributes to pain hypersensitivity in CFA-induced chronic knee joint inflammation model of RA pain.     

ERK MAP Kinase Activation in Spinal Cord Regulates Phosphorylation of Cdk5 at Serine 159 and Contributes to Peripheral Inflammation Induced Pain/Hypersensitivity

Cyclin-dependent kinase 5 is a proline-directed serine/threonine kinase and its activity participates in the regulation of nociceptive signaling. Like binding with the activators (P35 or P25), the phosphorylation of Cdk5 plays a critical role in Cdk5 activation. However, it is still unclear whether Cdk5 phosphorylation (p-Cdk5) contributes to pain hyperalgesia. The aim of our current study was to identify the roles of p-Cdk5 and its upstream regulator in response to peripheral inflammation. Complete Freund''s adjuvant (CFA) injection induced acute peripheral inflammation and heat hyperalgesia, which was accompanied by sustained increases in phospho-ERK1/2 (p-ERK1/2) and phospho-Cdk5^S159 (p-Cdk5^S159) in the spinal cord dorsal horn (SCDH). CFA-induced p-ERK primarily colocalized with p-Cdk5^S159 in superficial dorsal horn neurons. Levels in p-ERK and p-Cdk5 were also increased in the 2^nd phase of hyperalgesia induced by formalin injection, which can produce acute and tonic inflammatory pain. MAP kinase kinase inhibitor U0126 intrathecal delivery significantly suppressed the elevation of p-Cdk5^S159, Cdk5 activity and pain response behavior (Heat hyperalgesia, Spontaneous flinches) induced by CFA or formalin injection. Cdk5 inhibitor roscovitine intrathecal administration also suppressed CFA-induced heat hyperalgesia and Cdk5 phosphorylation, but did not attenuate ERK activation. All these findings suggested that p-Cdk5^S159 regulated by ERK pathway activity may be a critical mechanism involved in the activation of Cdk5 in nociceptive spinal neurons contributes to peripheral inflammatory pain hypersensitivity.     

The differential contribution of tumour necrosis factor to thermal and mechanical hyperalgesia during chronic inflammation

Therapies directed against tumour necrosis factor (TNF) are effective for the treatment of rheumatoid arthritis and reduce pain scores in this condition. In this study, we sought to explore mechanisms by which TNF contributes to inflammatory pain in an experimental model of arthritis. The effects of an anti-TNF agent, etanercept, on behavioural pain responses arising from rat monoarthritis induced by complete Freund's adjuvant were assessed and compared with expression of TNF receptors (TNFRs) by dorsal root ganglion (DRG) cells at corresponding time points. Etanercept had no effect on evoked pain responses in normal animals but exerted a differential effect on the thermal and mechanical hyperalgesia associated with rat arthritis induced by complete Freund's adjuvant (CFA). Joint inflammation was associated with increased TNFR1 and TNFR2 expression on DRG cells, which was maintained throughout the time course of the model. TNFR1 expression was increased in neuronal cells of the DRG bilaterally after arthritis induction. In contrast, TNFR2 expression occurred exclusively on non-neuronal cells of the macrophage–monocyte lineage, with cell numbers increasing in a TNF-dependent fashion during CFA-induced arthritis. A strong correlation was observed between numbers of macrophages and the development of mechanical hyperalgesia in CFA-induced arthritis. These results highlight the potential for TNF to play a vital role in inflammatory hyperalgesia, both by a direct action on neurons via TNFR1 and by facilitating the accumulation of macrophages in the DRG via a TNFR2-mediated pathway.     

Behavioral and Electrophysiological Evidence for the Differential Functions of TRPV1 at Early and Late Stages of Chronic Inflammatory Nociception in Rats

We previously reported that vanilloid receptor type 1 (VR1, or TRPV1) was up-regulated in dorsal root ganglion (DRG) and the spinal dorsal horn after chronic inflammatory pain produced by complete Freund’s adjuvant (CFA) injection into the plantar of rat hind paw. In the present study, we found that subcutaneous or intrathecal application of capsazepine (CPZ), a TRPV1 competitive antagonist, could inhibit thermal hyperalgesia on day 1 and on day 14 but not on day 28 after CFA injection. With extracellular electrophysiological recording, the effect of CPZ on noxious electrical or heat stimulation evoked responses of wide dynamic range (WDR) neurons in the deep layers of the spinal dorsal horn was evaluated. Under noxious electrical stimulation to sciatic nerve, CPZ applied to the spinal cord produced an inhibition on Aδ- and C-fiber evoked responses of WDR neurons on day 1 and 14, but not on day 28. Under radiant heat stimulation to the receptive field skin, subcutaneous application of CPZ significantly inhibited the background activity and extended the response latency of WDR neurons on day 14. These results provide new evidence for the functional significance of TRPV1 at the early stage, but not the late stage, in the rat model of CFA-induced inflammatory pain. Special issue article in honor of Dr. Ji-Sheng Han. Hao Luo, Isabella Shi Xu, Yi Chen are Co-first authors.     

Expression and Regulation of Cav3.2 T-Type Calcium Channels during Inflammatory Hyperalgesia in Mouse Dorsal Root Ganglion Neurons

The Cav3.2 isoform of the T-type calcium channel is expressed in primary sensory neurons of the dorsal root ganglion (DRG), and these channels contribute to nociceptive and neuropathic pain in rats. However, there are conflicting reports on the roles of these channels in pain processing in rats and mice. In addition, the function of T-type channels in persistent inflammatory hyperalgesia is poorly understood. We performed behavioral and comprehensive histochemical analyses to characterize Cav3.2-expressing DRG neurons and examined the regulation of T-type channels in DRGs from C57BL/6 mice with carrageenan-induced inflammatory hyperalgesia. We show that approximately 20% of mouse DRG neurons express Cav3.2 mRNA and protein. The size of the majority of Cav3.2-positive DRG neurons (69 ± 8%) ranged from 300 to 700 μm2 in cross-sectional area and 20 to 30 μm in estimated diameter. These channels co-localized with either neurofilament-H (NF-H) or peripherin. The peripherin-positive cells also overlapped with neurons that were positive for isolectin B4 (IB4) and calcitonin gene-related peptide (CGRP) but were distinct from transient receptor potential vanilloid 1 (TRPV1)-positive neurons during normal mouse states. In mice with carrageenan-induced inflammatory hyperalgesia, Cav3.2 channels, but not Cav3.1 or Cav3.3 channels, were upregulated in ipsilateral DRG neurons during the sub-acute phase. The increased Cav3.2 expression partially resulted from an increased number of Cav3.2-immunoreactive neurons; this increase in number was particularly significant for TRPV1-positive neurons. Finally, preceding and periodic intraplantar treatment with the T-type calcium channel blockers mibefradil and NNC 55-0396 markedly reduced and reversed mechanical hyperalgesia during the acute and sub-acute phases, respectively, in mice. These data suggest that Cav3.2 T-type channels participate in the development of inflammatory hyperalgesia, and this channel might play an even greater role in the sub-acute phase of inflammatory pain due to increased co-localization with TRPV1 receptors compared with that in the normal state.     

Activation of extracellular signal-regulated protein kinases 5 in primary afferent neurons contributes to heat and cold hyperalgesia after inflammation

Heat and cold hyperalgesia is a common feature of inflammatory pain. To investigate whether activation of extracellular signal-regulated protein kinase 5 (ERK5), also known as big mitogen-activated protein kinase 1, in primary sensory neurons participates in inflammatory pain, we examined the phosphorylation of ERK5 in the dorsal root ganglion (DRG) after peripheral inflammation. Inflammation induced by complete Freund's adjuvant produced heat and cold hyperalgesia on the ipsilateral hind paw and induced an increase in the phosphorylation of ERK5, mainly in tyrosine kinase A-expressing small- and medium-size neurons. In contrast, there was no change in ERK5 phosphorylation in the spinal dorsal horn. ERK5 antisense, but not mismatch, oligodeoxynucleotide decreased the activation of ERK5 and suppressed inflammation-induced heat and cold hyperalgesia. Furthermore, the inhibition of ERK5 blocked the induction of transient receptor potential channel TRPV1 and TRPA1 expression in DRG neurons after peripheral inflammation. Our results show that ERK5 activated in DRG neurons contribute to the development of inflammatory pain. Thus, blocking ERK5 signaling in sensory neurons that has the potential for preventing pain after inflammation.     

Differential expression of microRNAs in mouse pain models

Background

MicroRNAs (miRNAs) are short non-coding RNAs that inhibit translation of target genes by binding to their mRNAs. The expression of numerous brain-specific miRNAs with a high degree of temporal and spatial specificity suggests that miRNAs play an important role in gene regulation in health and disease. Here we investigate the time course gene expression profile of miR-1, -16, and -206 in mouse dorsal root ganglion (DRG), and spinal cord dorsal horn under inflammatory and neuropathic pain conditions as well as following acute noxious stimulation.

Results

Quantitative real-time polymerase chain reaction analyses showed that the mature form of miR-1, -16 and -206, is expressed in DRG and the dorsal horn of the spinal cord. Moreover, CFA-induced inflammation significantly reduced miRs-1 and -16 expression in DRG whereas miR-206 was downregulated in a time dependent manner. Conversely, in the spinal dorsal horn all three miRNAs monitored were upregulated. After sciatic nerve partial ligation, miR-1 and -206 were downregulated in DRG with no change in the spinal dorsal horn. On the other hand, axotomy increases the relative expression of miR-1, -16, and 206 in a time-dependent fashion while in the dorsal horn there was a significant downregulation of miR-1. Acute noxious stimulation with capsaicin also increased the expression of miR-1 and -16 in DRG cells but, on the other hand, in the spinal dorsal horn only a high dose of capsaicin was able to downregulate miR-206 expression.

Conclusions

Our results indicate that miRNAs may participate in the regulatory mechanisms of genes associated with the pathophysiology of chronic pain as well as the nociceptive processing following acute noxious stimulation. We found substantial evidence that miRNAs are differentially regulated in DRG and the dorsal horn of the spinal cord under different pain states. Therefore, miRNA expression in the nociceptive system shows not only temporal and spatial specificity but is also stimulus-dependent.

     

Sickle cell mice exhibit mechanical allodynia and enhanced responsiveness in light touch cutaneous mechanoreceptors

Background

Interleukin-8 (IL-8) is known for its roles in inflammation and plays critical roles in the development of pain. Its expression increases in the brain after peripheral inflammation. Prefrontal cortex, including the anterior cingulate cortex (ACC), is a forebrain structure known for its roles in pain transmission and modulation. Painful stimuli potentiate the prefrontal synaptic transmission, however, little is known about the expression of IL-8 and its role in the enhanced ACC synaptic transmission in animals with persistent inflammatory pain.

Findings

In the present study, we examined IL-8 expression in the ACC, somatosensory cortex (SSC), and the dorsal horn of lumbar spinal cord following hind-paw administration of complete Freund's adjuvant (CFA) in mice and its effects on the ACC synaptic transmission. Quantification of IL-8 at protein level (by ELISA) revealed enhanced expression in the ACC and spinal cord during the chronic phases of CFA-induced peripheral inflammation. In vitro whole-cell patch-clamp recordings revealed that IL-8 significantly enhanced synaptic transmission through increased probability of neurotransmitter release in the ACC slice. ACC local infusion of repertaxin, a non-competitive allosteric blocker of IL-8 receptors, notably prolonged the paw withdrawal latency to thermal radian heat stimuli bilaterally in mice.

Conclusions

Our findings suggest that up-regulation of IL-8 in the ACC partly attributable to the enhanced prefrontal synaptic transmission in the mice with persistent inflammatory pain.     

Increased Synaptophysin Is Involved in Inflammation-Induced Heat Hyperalgesia Mediated by Cyclin-Dependent Kinase 5 in Rats

Mechanisms associated with cyclin-dependent kinase 5 (Cdk5)-mediated heat hyperalgesia induced by inflammation remain undefined. This study was designed to examine whether Cdk5 mediates heat hyperalgesia resulting from peripheral injection of complete Freund''s adjuvant (CFA) in the spinal dorsal horns of rats by interacting with synaptophysin, a well known membrane protein mediating the endocytosis-exocytosis cycle of synaptic vesicles as a molecular marker associated with presynaptic vesicle membranes. The role of Cdk5 in mediating synaptophysin was examined through the combined use of behavioral approaches, imaging studies, and immunoprecipitation following CFA-induced inflammatory pain. Results showed that Cdk5 colocalized with both synaptophysin and soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptors (SNAREs) consisting of VAMP-2, SNAP-25, and syntaxin 1A in spinal dorsal horn of rats. Increased synaptophysin expression of spinal cord horn neurons post intraplantar injection of CFA coincided with increased duration of heat hyperalgesia lasting from 6 h to 3 d. Intrathecal administration of roscovitine, a Cdk5 specific inhibitor, significantly depressed synaptophysin expression during peak heat hyperalgesia and heat hyperalgesia induced by peripheral injection of CFA. Data presented in this report indicated that calpain activity was transiently upregulated 6 h post CFA-treatment despite previous reports suggesting that calpain was capable of cleaving p35 into p25. Results from previous studies obtained by other laboratories demonstrated that significant changes in p35 expression levels within spinal cord horn neurons were not observed in the CFA-treated inflammatory pain model although significant upregulation of Cdk5 kinase was observed between 2 h to 7 d. Therefore, generation of p25 occurred in a calpain-independent fashion in a CFA-treated inflammatory pain model. Our results demonstrated that increased synaptophysin levels were involved in heat hyperalgesia mediated by Cdk5 in spinal cord dorsal horns of CFA-treated rats, suggesting that inhibiting abnormal activation of Cdk5-synaptophysin may present a novel target for diminishing inflammatory pain.     

慢性炎症痛引起DRG感觉神经元P2X3受体表达的改变

目的探讨脊髓背根神经节（dorsal root ganglia,DRG）P2X3受体参与大鼠足底慢性炎症痛相关的热痛觉过敏机制。方法 1）用行为学的研究方法,以大鼠右侧后脚掌注射松节油加石蜡（各占50％）0．1ml建立后脚掌慢性痛模型,用热测痛的方法测量后脚掌皮下注射松节油后的痛阈,每天1次,连续测15d。2）用免疫组织化学技术观察大鼠后脚掌慢性炎症后第2天和第7天,炎症侧脊髓背根神经节（L4—6）神经元中P2X,受体阳性细胞类型的分布变化;以及正常脊髓背根神经节（L4—6）神经元中P2X,受体阳性细胞类型的分布作为对照。结果1）炎症后大鼠后脚掌侧痛阈出现降低,在第2天痛阈达到最低,后逐渐恢复,14d后恢复正常痛阈值。2）正常大鼠P2X,主要表达于DRG的中小神经元上,炎症后DRG（L4—6）,中小型P2X,受体阳性细胞数比对照组明显增加。细胞平均面积增大。结论后脚掌慢性炎症痛可以引起大鼠对伤害性热刺激的痛觉过敏,并导致脊髓背根神经节（L4—6）神经元qbP2X3受体阳性细胞数目增加,表明P2X3在DRG的中小神经元的改变可能对松节油引起脚掌炎症痛时热痛觉过敏的形成与维持起重要作用。     

Anesthesiologists' practice patterns for treatment of postoperative nausea and vomiting in the ambulatory Post Anesthesia Care Unit

Background

Induction of the COX-2 isoenzyme appears to play a major role in the genesis of central sensitization after nociceptive stimulation. This study aimed to investigate the efficacy of a single, oral dose of the specific COX-2 inhibitor-valdecoxib in attenuating the central sensitization – induced secondary hyperalgesia in a heat/capsaicin pain model in healthy volunteers.

Methods

The study was a randomized, double blind, placebo controlled, crossover, single dose efficacy trial using 20 healthy volunteers. Two hours following placebo or 40 mg, PO valdecoxib, participants underwent skin sensitization with heat/capsaicin, as well as supra-threshold pain and re-kindling measurements according to an established, validated pain model. Subjects rated pain intensity and unpleasantness on a visual analog scale and the area of secondary hyperalgesia was serially mapped.

Results

The area of secondary hyperalgesia produced after 40 mg of valdecoxib was no different than that after placebo. Furthermore, there were no significantly relevant differences when volunteers were treated with valdecoxib or placebo in relation to either cold- or hot pain threshold or the intensity of pain after supra-threshold, thermal pain stimulation.

Conclusion

We demonstrated that a single, oral dose of valdecoxib when does not attenuate secondary hyperalgesia induced by heat/capsaicin in a cutaneous sensitization pain model in healthy volunteers.     

The oxidized linoleic acid metabolite 12,13-DiHOME mediates thermal hyperalgesia during inflammatory pain

Eicosanoids play a crucial role in inflammatory pain. However, there is very little knowledge about the contribution of oxidized linoleic acid metabolites in inflammatory pain and peripheral sensitization. Here, we identify 12,13-dihydroxy-9Z-octadecenoic acid (12,13-DiHOME), a cytochrome P450-derived linoleic acid metabolite, as crucial mediator of thermal hyperalgesia during inflammatory pain. We found 12,13-DiHOME in increased concentrations in peripheral nervous tissue during acute zymosan- and complete Freund's Adjuvant-induced inflammatory pain. 12,13-DiHOME causes calcium transients in sensory neurons and sensitizes the transient receptor potential vanilloid 1 (TRPV1)-mediated intracellular calcium increases via protein kinase C, subsequently leading to enhanced TRPV1-dependent CGRP-release from sensory neurons. Peripheral injection of 12,13-DiHOME in vivo causes TRPV1-dependent thermal pain hypersensitivity. Finally, application of the soluble epoxide hydrolase (sEH)-inhibitor TPPU reduces 12,13-DiHOME concentrations in nervous tissue and reduces zymosan- and CFA-induced thermal hyperalgesia in vivo. In conclusion, we identify a novel role for the lipid mediator 12,13-DiHOME in mediating thermal hyperalgesia during inflammatory pain and propose a novel mechanism that may explain the antihyperalgesic effects of sEH inhibitors in vivo.     

Pulsed Radiofrequency Reduced Complete Freund’s Adjuvant-induced Mechanical Hyperalgesia via the Spinal c-Jun N-terminal Kinase Pathway

Pulsed radiofrequency (PRF) treatment involves the pulsed application of a radiofrequency electric field to a nerve. The technology offers pain relief for patients suffering from chronic pain who do not respond well to conventional treatments. We tested whether PRF treatment attenuated complete Freund’s adjuvant (CFA) induced inflammatory pain. The profile of spinal c-Jun N-terminal kinases (JNKs) phosphorylation was evaluated to elucidate the potential mechanism. Injection of CFA into the unilateral hind paw of rats induced mechanical hyperalgesia in both the ipsilateral and contralateral hind paws. We administered 500-kHz PRF treatment in 20-ms pulses, at a rate of 2 Hz (2 pulses per second) either to the sciatic nerve in the mid-thigh, or to the L4 anterior primary ramus just distal to the intervertebral foramen in both the CFA group and no-PRF group rats. Tissue samples were examined at 1, 3, 7, and 14 days following PRF treatments. Behavioral studies showed that PRF applied close to the dorsal root ganglion (DRG) significantly attenuated CFA-induced mechanical hyperalgesia compared to no-PRF group (P < .05). And western blotting revealed significant attenuation of the activation of JNK in the spinal dorsal horn compared to no-PRF group animals (P < .05). Application of PRF close to DRG provides an effective treatment for CFA-induced persistent mechanical hyperalgesia by attenuating JNK activation in the spinal dorsal horn.     

Involvement of spinal NR2B-containing NMDA receptors in oxaliplatin-induced mechanical allodynia in rats

Background

Two main classes of peripheral sensory neurons contribute to thermal pain sensitivity: the unmyelinated C fibers and thinly myelinated Aδ fibers. These two fiber types may differentially underlie different clinical pain states and distinctions in the efficacy of analgesic treatments. Methods of differentially testing C and Aδ thermal pain are widely used in animal experimentation, but these methods are not optimal for human volunteer and patient use. Thus, this project aimed to provide psychophysical and electrophysiological evidence that whether different protocols of infrared diode laser stimulation, which allows for direct activation of nociceptive terminals deep in the skin, could differentially activate Aδ or C fiber thermonociceptors in volunteers.

Results

Short (60 ms), high intensity laser pulses (SP) evoked monomodal "pricking" pain which was not enhanced by topical capsaicin, whereas longer, lower power pulses (LP) evoked monomodal "burning" pain which was enhanced by topical capsaicin. SP also produced cortical evoked EEG potentials consistent with Aδ mediation, the amplitude of which was directly correlated with pain intensity but was not affected by topical capsaicin. LP also produced a distinct evoked potential pattern the amplitude of which was also correlated with pain intensity, which was enhanced by topical capsaicin, and the latency of which could be used to estimate the conduction velocity of the mediating nociceptive fibers.

Conclusions

Psychophysical and electrophysiological data were consistent with the ability of short high intensity infrared laser pulses to selectively produce Aδ mediated pain and of longer pulses to selectively produce C fiber mediated thermal pain. Thus, the use of these or similar protocols may be useful in developing and testing novel therapeutics based on the differential molecular mechanisms underlying activation of the two fiber types (e.g., TRPV1, TRPV2, etc). In addition, these protocol may be useful in determining the fiber mediation of different clinical pain types which may, in turn be useful in treatment choice.