Noxious Chemical Stimuli Research Articles

Functional differences between neurochemically defined populations of inhibitory interneurons in the rat spinal dorsal horn

In order to understand how nociceptive information is processed in the spinal dorsal horn we need to unravel the complex synaptic circuits involving interneurons, which constitute the vast majority of the neurons in laminae I–III. The main limitation has been the difficulty in defining functional populations among these cells. We have recently identified 4 non-overlapping classes of inhibitory interneuron, defined by expression of galanin, neuropeptide Y (NPY), neuronal nitric oxide synthase (nNOS) and parvalbumin, in the rat spinal cord. In this study we demonstrate that these form distinct functional populations that differ in terms of sst2A receptor expression and in their responses to painful stimulation. The sst2A receptor was expressed by nearly all of the nNOS- and galanin-containing inhibitory interneurons but by few of those with NPY and none of the parvalbumin cells. Many galanin- and NPY-containing cells exhibited phosphorylated extracellular signal-regulated kinases (pERK) after mechanical, thermal or chemical noxious stimuli, but very few nNOS-containing cells expressed pERK after any of these stimuli. However, many nNOS-positive inhibitory interneurons up-regulated Fos after noxious thermal stimulation or injection of formalin, but not after capsaicin injection. Parvalbumin cells did not express either activity-dependent marker following any of these stimuli. These results suggest that interneurons belonging to the NPY, nNOS and galanin populations are involved in attenuating pain, and for NPY and nNOS cells this is likely to result from direct inhibition of nociceptive projection neurons. They also suggest that the nociceptive inputs to the nNOS cells differ from those to the galanin and NPY populations.

Pain‐related depression of the mesolimbic dopamine system in rats

Some stressors produce prodepressant behavioral effects by activating endogenous kappa opioid systems that in turn depress mesolimbic dopamine release. This study evaluated the role of this mechanism in pain‐related behavioral depression in rats. Adult male Sprague‐Dawley rats were prepared either with intracranial electrodes targeting the medial forebrain bundle (for behavior studies of intracranial self‐stimulation; ICSS) or cannulae for microdialysis measures of nucleus accumbens dopamine (NAcc DA). Changes in ICSS and NAcc DA were examined after treatment with a chemical noxious stimulus (intraperitoneal injection of dilute acid) or an exogenous kappa agonist (U69593). Additional studies examined sensitivity of acid and U69593 effects to blockade by two analgesics (the nonsteroidal antiinflammatory drug ketoprofen and the mu opioid receptor agonist morphine) or by the kappa antagonist norbinaltorphimine (norBNI). Both acid and U69593 depressed ICSS and NAcc DA release. However, the pain‐related effects of acid were blocked by pretreatment with ketoprofen and morphine, but not by norBNI. Conversely, effects of U69593 were blocked by norBNI but not by ketoprofen, and were only attenuated by morphine. These results support a role for the mesolimbic dopamine system, but not endogenous kappa opioid systems, in mediating pain‐related depression of behavior in rats.

Opioid withdrawal increases transient receptor potential vanilloid 1 activity in a protein kinase A-dependent manner

Hyperalgesia is a cardinal symptom of opioid withdrawal. The transient receptor potential vanilloid 1 (TRPV1) is a ligand-gated ion channel expressed on sensory neurons responding to noxious heat, protons, and chemical stimuli such as capsaicin. TRPV1 can be inhibited via μ-opioid receptor (MOR)-mediated reduced activity of adenylyl cyclases (ACs) and decreased cyclic adenosine monophosphate (cAMP) levels. In contrast, opioid withdrawal following chronic activation of MOR uncovers AC superactivation and subsequent increases in cAMP and protein kinase A (PKA) activity. Here we investigated (1) whether an increase in cAMP during opioid withdrawal increases the activity of TRPV1 and (2) how opioid withdrawal modulates capsaicin-induced nocifensive behavior in rats. We applied whole-cell patch clamp, microfluorimetry, cAMP assays, radioligand binding, site-directed mutagenesis, and behavioral experiments. Opioid withdrawal significantly increased cAMP levels and capsaicin-induced TRPV1 activity in both transfected human embryonic kidney 293 cells and dissociated dorsal root ganglion (DRG) neurons. Inhibition of AC and PKA, as well as mutations of the PKA phosphorylation sites threonine 144 and serine 774, prevented the enhanced TRPV1 activity. Finally, capsaicin-induced nocifensive behavior was increased during opioid withdrawal in vivo. In summary, our results demonstrate an increased activity of TRPV1 in DRG neurons as a new mechanism contributing to opioid withdrawal-induced hyperalgesia.

Transcription of the pain‐related TRPV1 gene requires Runx1 and C/EBPβ factors

Transient Receptor Potential Vanilloid type 1 channel (TRPV1) is an important endogenous transducer of noxious heat and chemical stimuli and is required during development of inflammatory hypersensitivity. The transcription factor Runx1 is known to play a relevant role in sensory neuron differentiation as it controls the expression of several sensory nociceptive receptors, including TRPV1. Here, we show that Runx1 up-regulates TRPV1 transcription activity by interacting directly with the proximal TRPV1 gene promoter sequence. Importantly, C/EBPβ a well-established heterodimer partner of Runx1 also binds to the TRPV1 promoter and cooperates with Runx1 to further stimulate TRPV1 transcription. Our results support a mechanism where Runx1-C/EBPβ-containing transcription regulatory complexes are recruited to the TRPV1 gene promoter to modulate TRPV1 expression in dorsal root ganglia neurons.

Analysis of Transient Receptor Potential Ankyrin 1 (TRPA1) in Frogs and Lizards Illuminates Both Nociceptive Heat and Chemical Sensitivities and Coexpression with TRP Vanilloid 1 (TRPV1) in Ancestral Vertebrates

Transient receptor potential ankyrin 1 (TRPA1) and TRP vanilloid 1 (V1) perceive noxious temperatures and chemical stimuli and are involved in pain sensation in mammals. Thus, these two channels provide a model for understanding how different genes with similar biological roles may influence the function of one another during the course of evolution. However, the temperature sensitivity of TRPA1 in ancestral vertebrates and its evolutionary path are unknown as its temperature sensitivities vary among different vertebrate species. To elucidate the functional evolution of TRPA1, TRPA1s of the western clawed (WC) frogs and green anole lizards were characterized. WC frog TRPA1 was activated by heat and noxious chemicals that activate mammalian TRPA1. These stimuli also activated native sensory neurons and elicited nocifensive behaviors in WC frogs. Similar to mammals, TRPA1 was functionally co-expressed with TRPV1, another heat- and chemical-sensitive nociceptive receptor, in native sensory neurons of the WC frog. Green anole TRPA1 was also activated by heat and noxious chemical stimulation. These results suggest that TRPA1 was likely a noxious heat and chemical receptor and co-expressed with TRPV1 in the nociceptive sensory neurons of ancestral vertebrates. Conservation of TRPV1 heat sensitivity throughout vertebrate evolution could have changed functional constraints on TRPA1 and influenced the functional evolution of TRPA1 regarding temperature sensitivity, whereas conserving its noxious chemical sensitivity. In addition, our results also demonstrated that two mammalian TRPA1 inhibitors elicited different effect on the TRPA1s of WC frogs and green anoles, which can be utilized to clarify the structural bases for inhibition of TRPA1.

The calcium-activated chloride channel anoctamin 1 acts as a heat sensor in nociceptive neurons

Nociceptors are a subset of small primary afferent neurons that respond to noxious chemical, thermal and mechanical stimuli. Ion channels in nociceptors respond differently to noxious stimuli and generate electrical signals in different ways. Anoctamin 1 (ANO1 also known as TMEM16A) is a Ca(2+)-activated chloride channel that is essential for numerous physiological functions. We found that ANO1 was activated by temperatures over 44 °C with steep heat sensitivity. ANO1 was expressed in small sensory neurons and was highly colocalized with nociceptor markers, which suggests that it may be involved in nociception. Application of heat ramps to dorsal root ganglion (DRG) neurons elicited robust ANO1-dependent depolarization. Furthermore, knockdown or deletion of ANO1 in DRG neurons substantially reduced nociceptive behavior in thermal pain models. These results indicate that ANO1 is a heat sensor that detects nociceptive thermal stimuli in sensory neurons and possibly mediates nociception.

Ethnic differences in capsaicin pain response in healthy subjects

Ethnic differences in pain perception have been documented in a variety of clinical pain conditions, generally indicating that African Americans report greater pain and suffering when compared to whites. A growing body of experimental pain studies has also shown that African Americans report greater sensitivity (i.e., lower pain threshold) and reduced pain tolerance to a variety of quantitative sensory testing methods when compared to non-Hispanic whites. Forty-six healthy African American (n = 21) and non-Hispanic white (n = 25) subjects participated in an experimental screening session involving pain induction by topical high concentration (10%, .35g) capsaicin cream applied evenly to a 6.25cm2 area of skin on the back of the non-dominant hand. An occlusive dressing (TegadermTM) was placed over the site to maintain the capsaicin within the area of application. A warm thermode maintained a specific (40° C) skin temperature at the capsaicin site. Pain was induced for approximately 30 minutes, and verbal pain ratings were obtained at one minute intervals. Repeated Measures Analysis of Variance (ANOVA) revealed a significant difference in pain ratings over time (p = 0.001) and a rating by ethnicity interaction (p = 0.003). The average capsaicin pain intensity rating over the 30 minute period was 33.2 (SE = 4.0) for African Americans and 27.3 (SE = 3.7) for non-Hispanic whites; the final ratings (at 30 minutes) were 56.0 (SE = 5.7) for African Americans and 38.9 (SE = 5.2) for non-Hispanic whites. These findings indicate ethnic differences in capsaicin induced pain responses. These findings suggest ethnic group differences in the central and/or peripheral nervous system processing of a noxious chemical stimulus, which may have implications for understanding ethnic group differences in the experience of a variety of painful conditions.

Mechanisms-based classifications of musculoskeletal pain: Part 3 of 3: Symptoms and signs of nociceptive pain in patients with low back (±leg) pain

As a mechanisms-based classification of pain ‘nociceptive pain’ (NP) refers to pain attributable to the activation of the peripheral receptive terminals of primary afferent neurones in response to noxious chemical, mechanical or thermal stimuli. The symptoms and signs associated with clinical classifications of NP have not been extensively studied. The purpose of this study was to identify symptoms and signs associated with a clinical classification of NP in patients with low back (±leg) pain.Using a cross-sectional, between-subjects design; four hundred and sixty-four patients with low back (±leg) pain were assessed using a standardised assessment protocol after which their pain was assigned a mechanisms-based classification based on experienced clinical judgement. Clinicians then completed a clinical criteria checklist indicating the presence/absence of various symptoms and signs.A regression analysis identified a cluster of seven clinical criteria predictive of NP, including: ‘Pain localised to the area of injury/dysfunction’, ‘Clear, proportionate mechanical/anatomical nature to aggravating and easing factors’, ‘Usually intermittent and sharp with movement/mechanical provocation; may be a more constant dull ache or throb at rest’, and the absence of ‘Pain in association with other dysesthesias’, ‘Night pain/disturbed sleep’, ‘Antalgic postures/movement patterns’ and ‘Pain variously described as burning, shooting, sharp or electric-shock-like’. This cluster was found to have high levels of classification accuracy (sensitivity 90.9%, 95% CI: 86.6–94.1; specificity 91.0%, 95% CI: 86.1–94.6).Pattern recognition of this empirically-derived cluster of symptoms and signs may help clinicians identify an assumed dominance of NP mechanisms in patients with low back pain disorders.

Persistent antinociception through repeated self-injury in patients with borderline personality disorder

Patients with borderline personality disorder, mostly female, exhibit severe autoaggressive behavior, namely an intentionally performed, nonsuicidal self-injury and severe blunting of pain perception, the mechanism of which is hitherto not understood. Because the nociceptive system displays a high degree of plasticity, the aim of this study was to analyze the relationship of pain perception to self-injurious behavior. Pain perception of mechanical and chemical noxious stimuli was studied by quantitative sensory testing in 22 patients (15 female, 7 male) with borderline personality disorder (BPD) according to DSM-IV and 22 age- and gender-matched controls. BPD patients exhibited a significantly higher pain threshold to pinprick stimuli (2.7 times higher than healthy control subjects), and significantly lower pain ratings to mechanical (pinprick, −28%) and chemical (capsaicin, −38%) stimulation. Capsaicin-induced pain decayed significantly faster in BPD patients (τ=49seconds) than in controls (τ=76seconds). These alterations of pain perception were generally present in the female, but not in the male subgroup of BPD patients. Analysis of pain intensity vs unpleasantness suggested that primarily the unpleasantness aspect of the pain experience was reduced. Blunting of pain sensation was significantly predicted by the recency of self-injurious behavior (multiple r=0.58). In line with recent data, we suggest an excess of endogenous antinociception in BPD patients resulting from self-inflicted multiple injuries. This exaggerated pain control is conceived to operate via an uncoupling of the evaluative or emotional–affective from the sensory-discriminative dimension of pain.

Abstract 17098: Blood Pressure Response to Controlled Diesel Exhaust Inhalation in Human Subjects is Modified by Functional Variation in TRPV1

Background: Exposure to traffic-related air pollution is associated with increased risk of cardiovascular disease and mortality. Diesel exhaust (DE) inhalation is a model exposure, and we hypothesize that pollutants trigger acute cardiovascular events via neurogenic processes. We evaluated blood pressure (BP) response to DE and whether the effect was modified by genotype for the transient receptor potential receptor, vanilloid type 1 (TRPV1), a non-selective cation channel expressed in the lung and activated by noxious chemical stimuli. The t allele for the TRPV1 single nucleotide polymorphism rs8065080 at position 585 results in Ile substituting for Val, and has been associated with reduced nociception. Methods: Forty-seven non-smokers, age 18-49 with no prior cardiovascular or pulmonary disease, underwent controlled exposures to each condition [DE at 200 μ g/m 3 PM 2.5 and filtered air (FA)] for 120 minutes in a crossover experiment. Exposures were double-blind, randomized to order, and separated by at least 2 weeks. We measured BP pre-exposure, at 5 time points during exposure, and 3, 5, 7 and 24 hours from exposure start. We analyzed pressure change (from pre-exposure values) for each exposure to determine the difference between DE and FA. Results: Compared with FA, systolic blood pressure (SBP) increased at all time points measured during and after DE exposure (p=0.004); mean effect peaked 30 to 90 minutes after exposure began (3.9 mmHg, p=0.01). No effects were observed for heart rate or diastolic BP. SBP response varied by TRPV1 Ile585Val genotype, with t/t subjects showing the lowest SBP response to DE (see Figure). Conclusion: DE exposure was associated with a rapid increase in SBP, with a reduced response in subjects with the Ile-Ile variation of TRPV1. This suggests that TRPV1 activation by a component of DE may be a key step explaining observed vascular effects of air pollution, perhaps by initiating autonomic signaling or neurogenic inflammation.

Coexpression and activation of TRPV1 suppress the activity of the KCNQ2/3 channel

Transient receptor potential vanilloid 1 (TRPV1) is a ligand-gated nonselective cation channel expressed predominantly in peripheral nociceptors. By detecting and integrating diverse noxious thermal and chemical stimuli, and as a result of its sensitization by inflammatory mediators, the TRPV1 receptor plays a key role in inflammation-induced pain. Activation of TRPV1 leads to a cascade of pro-nociceptive mechanisms, many of which still remain to be identified. Here, we report a novel effect of TRPV1 on the activity of the potassium channel KCNQ2/3, a negative regulator of neuronal excitability. Using ion influx assays, we revealed that TRPV1 activation can abolish KCNQ2/3 activity, but not vice versa, in human embryonic kidney (HEK)293 cells. Electrophysiological studies showed that coexpression of TRPV1 caused a 7.5-mV depolarizing shift in the voltage dependence of KCNQ2/3 activation compared with control expressing KCNQ2/3 alone. Furthermore, activation of TRPV1 by capsaicin led to a 54% reduction of KCNQ2/3-mediated current amplitude and attenuation of KCNQ2/3 activation. The inhibitory effect of TRPV1 appears to depend on Ca2+ influx through the activated channel followed by Ca2+-sensitive depletion of phosphatidylinositol 4,5-bisphosphate and activation of protein phosphatase calcineurin. We also identified physical interactions between TRPV1 and KCNQ2/3 coexpressed in HEK293 cells and in rat dorsal root ganglia neurons. Mutation studies established that this interaction is mediated predominantly by the membrane-spanning regions of the respective proteins and correlates with the shift of KCNQ2/3 activation. Collectively, these data reveal that TRPV1 activation may deprive neurons from inhibitory control mediated by KCNQ2/3. Such neurons may thus have a lower threshold for activation, which may indirectly facilitate TRPV1 in integrating multiple noxious signals and/or in the establishment or maintenance of chronic pain.

TRiPping down the oesophagus

TRiPping down the oesophagus

Simultaneous multisite recordings of neural ensemble responses in the motor cortex of behaving rats to peripheral noxious heat and chemical stimuli

Chronic motor cortex (MCx) stimulation (MCS) is an effective approach for patients with chronic, intractable neuropathic pain. However, the underlying neural mechanisms are less known. Combining an in vivo simultaneous multisite recording technique with a video-based behavioral tracker, simultaneous neuronal ensemble activities of the MCx and behavioral responses to noxious heat stimuli applied to bilateral hindpaw pads under naïve and inflammatory pain state were studied in freely behaving rats receiving prior implantation of microwire multielectrode array (2 × 4). Totally, 81 active units were sorted and separated from 40 microwire electrodes pre-implanted in the MCx of 5 rats. Under naïve state, 41% (33/81) units were responsive to contralateral, while 27% (22/81) were responsive to ipsilateral heat stimuli. However, the proportion of heat-responsive units under inflammatory pain state induced by subcutaneous bee venom (BV) injection was significantly increased when compared with saline control (BV vs. saline: 60% vs. 48% for contralateral and 51% vs. 37% for ipsilateral, P < 0.05, n = 81 units) as a consequence of recruitment of some previously heat non-responsive to heat sensitive units. Moreover, under the BV-inflamed condition, the discharge rate of the MCx neurons was significantly increased. The time course of increased spontaneous neuronal ensemble activities ( n = 81) was in parallel with that of pain-related behaviors following BV injection. It is concluded that there are pain-related neurons in the MCx that can be functionally changed by peripheral inflammatory pain condition.

Near-Infrared Irradiation Photobiomodulation: The Need for Basic Science

Photomedicine and Laser SurgeryVol. 29, No. 3 Guest EditorialNear-Infrared Irradiation Photobiomodulation: The Need for Basic ScienceBrendan J. Quirk and Harry T. WhelanBrendan J. QuirkSearch for more papers by this author and Harry T. WhelanSearch for more papers by this authorPublished Online:6 Mar 2011https://doi.org/10.1089/pho.2011.3014AboutSectionsView articleView Full TextPDF/EPUB ToolsPermissionsDownload CitationsTrack CitationsAdd to favorites Back To Publication ShareShare onFacebookTwitterLinked InRedditEmail View article"Near-Infrared Irradiation Photobiomodulation: The Need for Basic Science." , 29(3), pp. 143–144FiguresReferencesRelatedDetailsCited byEffect of Red-to-Near Infrared Light and a Nitric Oxide Donor on the Oxygen Consumption of Isolated Cytochrome c Oxidase Brendan Quirk and Harry T. Whelan15 July 2021 | Photobiomodulation, Photomedicine, and Laser Surgery, Vol. 39, No. 7Digital Transformation of Photobiomodulation Therapy: A Step Forward to Become Mainstream? Marcelo Victor Pires de Sousa and Michael R. Hamblin7 May 2021 | Photobiomodulation, Photomedicine, and Laser Surgery, Vol. 39, No. 5Blue light exacerbates and red light counteracts negative insults to retinal ganglion cells in situ and R28 cells in vitroNeurochemistry International, Vol. 125Blue light negatively affects the survival of ARPE19 cells through an action on their mitochondria and blunted by red light9 September 2018 | Acta Ophthalmologica, Vol. 97, No. 1Is Photodynamic Therapy Resistance a Special Case of Photobiomodulation? Brendan J. Quirk, Albert W. Girotti, and Harry T. Whelan1 August 2018 | Photomedicine and Laser Surgery, Vol. 36, No. 8H2S and polysulfide metabolism: Conventional and unconventional pathwaysBiochemical Pharmacology, Vol. 149Effects of Low-Level Laser Therapy in Autism Spectrum Disorder29 June 2018Visual light effects on mitochondria: The potential implications in relation to glaucomaMitochondrion, Vol. 36Light-Emitting Diode Phototherapy Reduces Nocifensive Behavior Induced by Thermal and Chemical Noxious Stimuli in Mice: Evidence for the Involvement of Capsaicin-Sensitive Central Afferent Fibers7 April 2016 | Molecular Neurobiology, Vol. 54, No. 5Effect of Red-to-Near Infrared Light on the Reaction of Isolated Cytochrome c Oxidase with Cytochrome c1 December 2016 | Photomedicine and Laser Surgery, Vol. 34, No. 12Effect of near-infrared light on in vitro cellular ATP production of osteoblasts and fibroblasts and on fracture healing with intramedullary fixationJournal of Clinical Orthopaedics and Trauma, Vol. 7, No. 4Red light of the visual spectrum attenuates cell death in culture and retinal ganglion cell death in situ29 February 2016 | Acta Ophthalmologica, Vol. 94, No. 6Near infrared light induces post-translational modifications of human red blood cell proteins1 November 2015 | Photochemical & Photobiological Sciences, Vol. 14, No. 11Near infrared light induces post-translational modifications of human red blood cell proteins1 January 2015 | Photochemical & Photobiological Sciences, Vol. 14, No. 11Absorption of monochromatic and narrow band radiation in the visible and near IR by both mitochondrial and non-mitochondrial photoacceptors results in photobiomodulationJournal of Photochemistry and Photobiology B: Biology, Vol. 140Cardioprotection from Ischemia-Reperfusion Injury by Near-Infrared Light in Rats20 August 2014 | Photomedicine and Laser Surgery, Vol. 32, No. 9Photobiomodulation inside the brain: a novel method of applying near-infrared light intracranially and its impact on dopaminergic cell survival in MPTP-treated miceJournal of Neurosurgery, Vol. 120, No. 3Laser Therapy in Canine RehabilitationNon-invasive infra-red therapy (1072nm) reduces β-amyloid protein levels in the brain of an Alzheimer’s disease mouse model, TASTPMJournal of Photochemistry and Photobiology B: Biology, Vol. 123670 nm Laser Light and EGCG Complementarily Reduce Amyloid-β Aggregates in Human Neuroblastoma Cells: Basis for Treatment of Alzheimer's Disease?4 January 2012 | Photomedicine and Laser Surgery, Vol. 30, No. 1 Volume 29Issue 3Mar 2011 InformationCopyright 2011, Mary Ann Liebert, Inc.To cite this article:Brendan J. Quirk and Harry T. Whelan.Near-Infrared Irradiation Photobiomodulation: The Need for Basic Science.Photomedicine and Laser Surgery.Mar 2011.143-144.http://doi.org/10.1089/pho.2011.3014Published in Volume: 29 Issue 3: March 6, 2011PDF download

The Capsaicin Paradox: Pain Relief by an Algesic Agent

Chemosensitive primary sensory neurones expressing the TRPV1 receptor, a molecular integrator of diverse noxious stimuli, play a fundamental role in the sensation of pain. Capsaicin, the archetypical ligand of the TRPV1 receptor, is one of the most painful chemical irritants, and its acute administration onto the skin and mucous membranes elicits severe pain. However, repeated or high-dose applications of capsaicin, and/or its administration through specific routes dramatically decreases the sensitivity of the innervated tissues to noxious chemical and heat stimuli. This review surveys the mechanisms of the antinociceptive, anti-inflammatory and anti-hyperalgesic effects of vanilloid agonists applied topically, or perineurally, or injected into the subarachnoid space in animal experiments and to put these data into a clinical perspective. The great body of available experimental evidence indicates that vanilloid agonists exert their antinociceptive actions through TRPV1 receptor-mediated selective neurotoxic/neurodegenerative effects directed against somatic and visceral C-fibre nociceptive primary afferent fibres. It is expected that vanilloid agonists will broaden the palette of analgesic drugs which do not cause addiction and tachyphylaxis. Keywords: Analgesia, capsaicin, neurogenic inflammation, pain management, primary sensory neurone, spinal cord, TRPV1, Sensory efferent, Vasodilation

Effects of intrathecal isoflurane administration on nociception and Fos expression in the rat spinal cord

Isoflurane has been used as an inhaled anaesthetic for nearly 30 years. Isoflurane inhalation during anaesthesia also produces an anti-nociceptive effect. Whether this occurs at the spinal or supraspinal level remains unknown. With a novel type of liquid isoflurane, the present study examined the effects of intrathecal isoflurane on the nociceptive response and Fos expression in the rat spinal cord. Thirty-six rats were randomly assigned to three groups as follows: group A (n = 6), intrathecal physiological saline 50 μl kg⁻¹; and group B and C (n = 6 each), intrathecal isoflurane at doses of 25 μl kg⁻¹ or 50 μl kg⁻¹, respectively. Noxious thermal (Hargreaves test), mechanical (von Frey test) and chemical (formalin 5%, 50 μl) stimuli were applied to a hind paw after intrathecal isoflurane injection to study its anti-nociceptive effect. In addition, the expression of Fos protein and c-fos mRNA in the spinal dorsal horns was detected by immunohistochemistry and real-time reverse transcriptase PCR, respectively. Compared with the physiological saline control, intrathecal isoflurane significantly suppressed spontaneous paw flinches in rats induced by formalin injection and paw withdrawal induced by thermal and mechanical stimuli in a dose-dependent manner. Immunohistochemistry and real-time reverse transcriptase PCR revealed that isoflurane administration inhibited formalin injection-induced c-fos expression in the spinal cord. These data suggest that isoflurane can exert anti-nociceptive effects at the spinal level by preventing neuronal activation.

Modulation of nociceptive-like behavior in zebrafish (Danio rerio) by environmental stressors.

Zebrafish have been demonstrated to react consistently to noxious chemical stimuli and present reliable phenotypes of stress, fear, and anxiety. In this article, we describe the modulation of nociceptive-like responses of zebrafish to fear-, stress-, and anxiety-eliciting situations. Animals were exposed to an alarm substance, confinement stress, or a novel environment before being injected with 1% acetic acid in the tail. The alarm substance and confinement stress reduced the display of erratic movements and tail-beating behavior elicited by acetic acid. The novelty of the environment, in contrast, increased the frequency of tail-beating behavior. The results suggest that descending modulatory control of nociception exists in zebrafish, with apparent fear- and stress-induced analgesia and anxiety-induced hyperalgesia.

VGLUT2 expression in primary afferent neurons is essential for normal acute pain and injury-induced heat hypersensitivity

Dorsal root ganglia (DRG) neurons, including the nociceptors that detect painful thermal, mechanical, and chemical stimuli, transmit information to spinal cord neurons via glutamatergic and peptidergic neurotransmitters. However, the specific contribution of glutamate to pain generated by distinct sensory modalities or injuries is not known. Here we generated mice in which the vesicular glutamate transporter 2 (VGLUT2) is ablated selectively from DRG neurons. We report that conditional knockout (cKO) of the Slc17a6 gene encoding VGLUT2 from the great majority of nociceptors profoundly decreased VGLUT2 mRNA and protein in these neurons, and reduced firing of lamina I spinal cord neurons in response to noxious heat and mechanical stimulation. In behavioral assays, cKO mice showed decreased responsiveness to acute noxious heat, mechanical, and chemical (capsaicin) stimuli, but responded normally to cold stimulation and in the formalin test. Strikingly, although tissue injury-induced heat hyperalgesia was lost in the cKO mice, mechanical hypersensitivity developed normally. In a model of nerve injury-induced neuropathic pain, the magnitude of heat hypersensitivity was diminished in cKO mice, but both the mechanical allodynia and the microgliosis generated by nerve injury were intact. These findings suggest that VGLUT2 expression in nociceptors is essential for normal perception of acute pain and heat hyperalgesia, and that heat and mechanical hypersensitivity induced by peripheral injury rely on distinct (VGLUT2 dependent and VGLUT2 independent, respectively) primary afferent mechanisms and pathways.

Upper thoracic postsynaptic dorsal column neurons conduct cardiac mechanoreceptive information, but not cardiac chemical nociception in rats

Postsynaptic dorsal column (PSDC) neurons transmit noxious visceral information from the lower thoracic and lumbosacral spinal cord. Cuneothalamic neurons in the PSDC pathway and upper thoracic (T 3–T 4) spinal neurons ascending through the ventrolateral funiculus (VLF) have been shown to transmit nociceptive cardiac information. Therefore, we hypothesized that upper thoracic PSDC neurons transmit noxious cardiac information. Neuronal responses to intrapericardially injected mechanical (1.0 ml saline) and noxious chemical (0.2 ml algogenic chemicals) stimuli were recorded from antidromically activated PSDC and VLF neurons in the T 3–T 4 spinal cord of anesthetized Sprague–Dawley rats. Of the PSDC neurons, 43% responded to mechanical stimulation, but only one responded to noxious chemical stimuli. Fifty-eight percent of VLF neurons responded to mechanical stimulation and all responded to noxious chemical stimulation. Fluoro-Ruby (FR)-labeled PSDC neurons in the T 3–T 4 spinal cord of Sprague–Dawley rats were processed for c-fos immunohistochemistry following intrapericardial stimulation with mechanical, chemical, or control stimuli. Sections were viewed under epifluorescence and light microscopy to detect FR-labeled neurons containing a c-fos immunoreactive (IR) nucleus. An average of 6 PSDC neurons per rat was found in the T 3 and T 4 spinal segments. The average number of c-fos-IR neurons per segment varied by type of stimulus: 12 (control), 67 (chemical) and 85 (mechanical) for T 3 and 8 (control), 37 (chemical) and 62 (mechanical) for T 4. None of the 200 PSDC neurons examined expressed c-fos-IR regardless of stimulus. Together, these results suggest that thoracic PSDC neurons transmit mechanical cardiac information, but they play a minimal role in cardiac nociception.

Central neuron-glia interactions and neuropathic pain

Neuropathic pain results from injury or disease causing dysfunction at any level of the somatosensory (primarily spinothalamic) system, including peripheral nociceptive axons, dorsal root ganglion (DRG), dorsal horn, spinothalamic pathway, and thalamus. The manifestations of neuropathic pain, including spontaneous pain, hyperalgesia, and thermal and mechanical allodynia, reflect excessive excitability of peripheral nociceptors (peripheral sensitization), central nociceptive neurons (central sensitization), or both. Sensitization reflects maladaptive plastic changes in the nociceptive system that result directly from axonal injury and from the effects of products of inflammation. There is abundant evidence that activation of microglia and astrocytes in the dorsal horn is common and constitutes an important amplification mechanism leading to neuropathic pain in the setting of peripheral nerve or spinal cord injury. Activated glial cells are also involved in sensitization of brainstem and thalamic neurons at a distance from the site of injury. The complex roles of the central glia and its mediators in the mechanisms of neuropathic pain have been extensively reviewed.1–9 The components of transmission of nociceptive information (“pain system”) include the nociceptive neurons of the DRG with unmyelinated C- and small myelinated Aδ axons that activate different types of neurons in the dorsal horn, particularly in lamina I and lamina V, which project via parallel pathways to the thalamus and to the brainstem. The primary DRG afferents also activate excitatory and inhibitory interneurons in lamina II that exert local control on nociceptive transmission.10–12 Similar organization occurs in the nociceptive trigeminal system. The nociceptive DRG neurons express different types of voltage-gated sodium (Na+) channels (particularly Nav1.7, Nav1.8, and Nav1.9), transient receptor potential (TRP) channels (including TRPV1 and TRPA), acid-sensitive channels, serotonin 5-HT3 receptors, and purinergic P2X receptors that are activated by noxious mechanical or chemical stimuli.11 In normal conditions, activation of Aδ and C-fiber nociceptors leads …