Srinivasa N. Raja

Injury-induced mechanical hypersensitivity requires C-low threshold mechanoreceptors

Mechanical pain contributes to the morbidity associated with inflammation and trauma, but primary sensory neurons that convey the sensation of acute and persistent mechanical pain have not been identified. Dorsal root ganglion (DRG) neurons transmit sensory information to the spinal cord using the excitatory transmitter glutamate, a process that depends on glutamate transport into synaptic vesicles for regulated exocytotic release. Here we report that a small subset of cells in the DRG expresses the low abundance vesicular glutamate transporter VGLUT3 (also known as SLC17A8). In the dorsal horn of the spinal cord, these afferents project to lamina I and the innermost layer of lamina II, which has previously been implicated in persistent pain caused by injury. Because the different VGLUT isoforms generally have a non-redundant pattern of expression, we used Vglut3 knockout mice to assess the role of VGLUT3+ primary afferents in the behavioural response to somatosensory input. The loss of VGLUT3 specifically impairs mechanical pain sensation, and in particular the mechanical hypersensitivity to normally innocuous stimuli that accompanies inflammation, nerve injury and trauma. Direct recording from VGLUT3+ neurons in the DRG further identifies them as a poorly understood population of unmyelinated, low threshold mechanoreceptors (C-LTMRs). The analysis of Vglut3-/- mice now indicates a critical role for C-LTMRs in the mechanical hypersensitivity caused by injury.Mechanical pain contributes to the morbidity associated with inflammation and trauma, but primary sensory neurons that convey the sensation of acute and persistent mechanical pain have not been identified. Dorsal root ganglion (DRG) neurons transmit sensory information to the spinal cord using the excitatory transmitter glutamate1, a process that depends on glutamate transport into synaptic vesicles for regulated exocytotic release. Here we report that a small subset of cells in the DRG expresses the low abundance vesicular glutamate transporter VGLUT3. In the dorsal horn of the spinal cord, these afferents project to lamina I and the innermost layer of lamina II, which has previously been implicated in persistent pain caused by injury2. Since the different VGLUT isoforms generally exhibit a nonredundant pattern of expression3, we used VGLUT3 knock-out (KO) mice to assess the role of VGLUT3+ primary afferents in the behavioral response to somatosensory input. The loss of VGLUT3 specifically impairs mechanical pain sensation and in particular the mechanical hypersensitivity to normally innocuous stimuli that accompanies inflammation, nerve injury and trauma. Direct recording from VGLUT3+ neurons in the DRG further identifies them as a poorly understood population of unmyelinated, low threshold mechanoreceptors (C-LTMRs)4,5. The analysis of VGLUT3 KO mice now indicates a critical role for C-LTMRs in the mechanical hypersensitivity caused by injury.

Intradermal injection of norepinephrine evokes pain in patients with sympathetically maintained pain

&NA; Tissue injuries, with or without involvement of nerves, may lead to ongoing pain and hyperalgesia to external stimuli. In a subset of patients, the pain is maintained by sympathetic efferent activity (SMP). We investigated if the peripheral administration of the &agr;‐adrenergic agonist, norepinephrine (NE), in physiologically relevant doses resulted in pain in patients with SMP. To establish the dose of intradermal NE required to induce cutaneous vasoconstriction, NE (1 nM–10 &mgr;M, 30 &mgr;l) was injected under a laser Doppler probe on the volar forearm of seven normal subjects. A decrease in blood flow was evident at a dose of 10 &mgr;M. Twelve patients (five male, seven female) diagnosed to have SMP based on the decrease in pain by a local anesthetic sympathetic blockade (70±6%) were enrolled in the study. Pain ratings were obtained continuously for 5 min after intradermal injections of saline and NE (0.1–10 &mgr;M) into their hyperalgesic zone and the mirror‐image contralateral side. Injections were done during the period of pain relief following a local anesthetic sympathetic blockade. Similar injections were made in eight control subjects. On the affected side of the patients, the two highest concentrations of NE (1 and 10 &mgr;M) caused significantly more pain than saline (P<0.05, ANOVA). In contrast, there was no significant pain induced by the NE injections in the unaffected side and in control subjects. Six of nine patients tested reported a marked decrease in pain and hyperalgesia following infusion of phentolamine (1 mg/kg over 10 min). Two of the three patients who did not receive pain relief following phentolamine infusion also did not report pain to the NE injections. We conclude that NE injections produce pain in SMP patients at doses that are at the threshold for producing vasoconstriction. These studies support a role for cutaneous adrenoceptors in the mechanisms of sympathetically maintained pain.

Developing patient-reported outcome measures for pain clinical trials : IMMPACT recommendations

a University of Washington, Seattle, WA 98195, USA b University of Rochester School of Medicine and Dentistry, Rochester, NY, USA c United States Food and Drug Administration, Rockville, MD, USA d Northwestern University, Chicago, IL, USA e Johnson and Johnson, Raritan, NY, USA f AstraZeneca, Wilmington, DE, USA g University of California San Diego, La Jolla, CA, USA h Merck and Company, Blue Bell, PA, USA i University of Texas, M.D. Anderson Cancer Center, USA j American Chronic Pain Association, Rocklin, CA, USA k Allergan, Inc, Irvine, CA, USA l National Institute of Dental and Craniofacial Research, Bethesda, MD, USA m University of Pennsylvania, Philadelphia, PA, USA n Johns Hopkins University, Baltimore, MD, USA o University Health Network and University of Toronto, Toronto, Canada p Novartis Pharmaceuticals, East Hanover, NJ, USA q VA Connecticut Healthcare System, West Haven, CT, USA r Yale University, New Haven, CT, USA s Celgene Corporation, Warren, NJ, USA t Pfizer Global Research and Development, Ann Arbor, MI, USA u Dalhousie University, Halifax, Nova Scotia, Canada v London Regional Cancer Centre, London, Ont., Canada

Diagnosis and Assessment of Pain Associated With Herpes Zoster and Postherpetic Neuralgia

UNLABELLED Accurate evaluation of pain plays a critical role in identifying new interventions for the treatment and prevention of herpes zoster and postherpetic neuralgia (PHN). Different types of pain and other sensory symptoms are found in patients with herpes zoster, and these vary greatly with respect to their presence, location, duration, intensity, and quality. The results of recent studies of herpes zoster and PHN and the development of new methods for assessing neuropathic pain provide a foundation for diagnosing and assessing the pain associated with herpes zoster. We review the results of recent research to identify the essential components that must be considered in developing an evidence-based description of pain associated with herpes zoster and PHN. PERSPECTIVE Comprehensive assessments of pain are necessary for clinical research on the epidemiology, natural history, pathophysiologic mechanisms, treatment, and prevention of pain in herpes zoster and PHN.

Pain coping strategies play a role in the persistence of pain in post-herpetic neuralgia.

&NA; Post‐herpetic neuralgia (PHN) is a neuropathic pain state that is often difficult to treat. Although frequently discussed in the clinical literature, little is known about the impact of pain on daily function and the extent to which psychosocial factors, in particular pain coping strategies, influence adaptation to this chronic illness. In the context of a crossover pharmacological trial, 68 patients with PHN completed a battery of psychological measures during a first drug‐free baseline period. Following discontinuation of approximately 8 weeks of treatment, 49 of these patients completed data collection during a second drug‐free assessment prior to beginning a second drug phase. Twice‐weekly telephone pain ratings were combined with questionnaire measures of perceived interference due to pain, overall activity level, depressive symptoms, and pain coping strategies. Cross‐sectional hierarchical regression analyses indicated that catastrophizing correlated with depressive symptoms but not pain, and coping self‐statements were correlated with higher levels of overall activity. Prospective hierarchical regression analyses indicated that catastrophizing at baseline predicted level of pain 8 weeks later, an effect that was independent of baseline pain and depressive symptoms. Patients who reported increasing their activity in response to pain also reported more perceived interference due to pain 8 weeks later. Higher levels of ignoring pain sensations at baseline were prospectively correlated with more depressive symptoms 8 weeks later. These findings support a role for the continued investigation of cognitive‐behavioral factors affecting the adaptation of elderly individuals experiencing PHN.

Neuropathic pain in rats is associated with altered nitric oxide synthase activity in neural tissue.

Peripheral nerve injury may lead to a chronic neuropathic pain state that results from an increase in excitability of central neurons. This central sensitization is mediated via an N-methyl-D-aspartic acid (NMDA) receptor and may involve the production of nitric oxide (NO). As NO is suggested to play a role in nociceptive transmission following nerve injury, we examined for altered NO synthase activity at multiple levels of peripheral and spinal neural tissue in a rat model of neuropathic pain. Peripheral neuropathy was induced in rats (N = 12) by ligation of the left L5 and L6 nerve roots. Six other rats had sham surgery. An ipsilateral decrease in paw withdrawal threshold to mechanical stimuli confirmed the presence of a neuropathic pain state. Samples of the lumbar and thoracic spinal cords, L4, L5, and L6 dorsal root ganglia (DRGs), and the sciatic nerves were obtained from the lesioned and contralateral sides at 2 and 4 weeks after neuropathic surgery (N = 6 per group). In the lumbar spinal cord, a bilateral decrease in nitric oxide synthase (NOS) activity was observed 2 and 4 weeks after neuropathic surgery. NOS activity was increased in the ipsilateral L5 and 6 DRGs 2 weeks following neuropathic surgery. An increase in NOS activity in the DRG may be an early mechanism for inducing more central changes. The bilaterally decreased NOS activity in the lumbar spinal cord may be secondary to a negative feedback mechanism resulting from increased NO production in the spinal dorsal root ganglia. Multiple alterations in expression of NOS activity that occur in both peripheral and central processing may play a role in the pain behavior resulting from peripheral nerve injury. (Preliminary results of these studies have been presented in abstract form at the annual meetings of the Society for Neuroscience, 1994, and the American Society of Anesthesiologists, 1994).

Lumbar sympathectomy failed to reverse mechanical allodynia- and hyperalgesia-like behavior in rats with L5 spinal nerve injury

The L5 spinal nerve ligation model of neuropathic pain in rats has been proposed as a model for sympathetically maintained pain (SMP) based on the effects of surgical or chemical sympathectomy on nerve injury induced behavior. In an attempt to confirm that the lesion produces an animal model of SMP, surgical sympathectomies were independently conducted in two different laboratories (Johns Hopkins and University Kiel) using male Sprague-Dawley (n = 30) or Wistar rats (n = 14). The L5 spinal nerve was ligated or cut and ligated. Using von Frey hairs, paw withdrawal threshold and incidence of paw withdrawal were tested concurrently before and after the sympathectomy. The sympathectomy was either verified by (a) glyoxylic acid staining of peripheral blood vessels of the hindpaw or (b) skin temperature measurements of the hindpaws. To blind the experimenter, surgeries and behavioral tests were performed by two different investigators and a sham sympathectomy was performed at Johns Hopkins. Decreased paw withdrawal thresholds and increased frequencies of paw withdrawal on the lesioned side were observed after the L5 lesion. Thus, the L5 spinal nerve ligation resulted in behavioral signs of allodynia and hyperalgesia to mechanical stimuli. Lumbar surgical sympathectomy 1-3 weeks after the lesion or prior to lesion with bilateral removal of the sympathetic ganglia L2-L4, however, did not reverse or prevent the behavioral changes induced by the nerve injury. The lack of effect of the sympathectomies was independent of the testing paradigm used. Experiments in Wistar and Sprague-Dawley rats yielded the same results. Potential reasons for the discrepancies between the present study and earlier reports are discussed. These results indicate that an L5 spinal nerve injury rat model is not a reliable model for SMP.

Genetic knockout and pharmacologic inhibition of neuronal nitric oxide synthase attenuate nerve injury-induced mechanical hypersensitivity in mice.

Neuronal nitric oxide synthase (nNOS) is a key enzyme for nitric oxide production in neuronal tissues and contributes to the spinal central sensitization in inflammatory pain. However, the role of nNOS in neuropathic pain remains unclear. The present study combined a genetic strategy with a pharmacologic approach to examine the effects of genetic knockout and pharmacologic inhibition of nNOS on neuropathic pain induced by unilateral fifth lumbar spinal nerve injury in mice. In contrast to wildtype mice, nNOS knockout mice failed to display nerve injury-induced mechanical hypersensitivity. Furthermore, either intraperitoneal (100 mg/kg) or intrathecal (30 μg/5 μl) administration of L-NG-nitro-arginine methyl ester, a nonspecific NOS inhibitor, significantly reversed nerve injury-induced mechanical hypersensitivity on day 7 post-nerve injury in wildtype mice. Intrathecal injection of 7-nitroindazole (8.15 μg/5 μl), a selective nNOS inhibitor, also dramatically attenuated nerve injury-induced mechanical hypersensitivity. Western blot analysis showed that the expression of nNOS protein was significantly increased in ipsilateral L5 dorsal root ganglion but not in ipsilateral L5 lumbar spinal cord on day 7 post-nerve injury. The expression of inducible NOS and endothelial NOS proteins was not markedly altered after nerve injury in either the dorsal root ganglion or spinal cord. Our findings suggest that nNOS, especially in the dorsal root ganglion, may participate in the development and/or maintenance of mechanical hypersensitivity after nerve injury.

Effect of genetic knockout or pharmacologic inhibition of neuronal nitric oxide synthase on complete Freund's adjuvant-induced persistent pain.

Abstract Nitric oxide (NO) acts as a neurotransmitter or neuromodulator involving in the modulation of thermal and/or inflammatory hyperalgesia. The neuronal nitric oxide synthase (nNOS) is a key enzyme for NO production in normal neuronal tissues, but its functional role in chronic pain remains unclear. The present study combined a genetic strategy with a pharmacologic approach to address the role of nNOS in the central mechanism of complete Freunds adjuvant (CFA)‐induced chronic inflammatory pain. Targeted disruption of the nNOS gene significantly reduced CFA‐induced mechanical pain hypersensitivity during the maintenance (but not the development) of inflammatory pain, while it failed to attenuate either development or maintenance of CFA‐induced thermal pain hypersensitivity. Intraperitoneal administration of L‐NG‐nitro‐arginine methyl ester (L‐NAME), a non‐specific NOS inhibitor, blocked CFA‐evoked thermal and mechanical pain hypersensitivity at both development (2 h) and maintenance (24 h) phase in wild type mice, but had no effect in the knockout mice. Furthermore, intrathecal injection of either L‐NAME or 7‐nitroindazole, a selective nNOS inhibitor, markedly attenuated mechanical pain hypersensitivity at both 2 and 24 h after CFA injection. Finally, spinal cord nNOS (but not endothelial NOS or inducible NOS) expression was up‐regulated at 24 h after CFA injection, occurring mainly in the ipsilateral superficial dorsal horn. Together, these data indicate that spinal cord nNOS may be essential for the maintenance of mechanical pain hypersensitivity and that it may also be sufficient for the development of mechanical pain hypersensitivity and for the development and maintenance of thermal pain hypersensitivity after chronic inflammation. Our findings suggest that spinal cord nNOS might play a critical role in central mechanisms of the development and/or maintenance of chronic inflammatory pain.

Neural activity originating from a neuroma in the baboon.

Single nerve fiber recordings were obtained from traumatically induced neuromas of the superficial radial nerve in baboons 1-7 months after injury. Eight to 18% of the fibers had spontaneous activity, and 67% of these were unmyelinated. Myelinated as well as unmyelinated fibers responded to mechanical stimulation of the neuroma whereas no fibers responded to similar stimulation of the normal nerve. Apparent crosstalk of action potential activity between fibers at the neuroma was observed which could be due to electrical coupling, though retrograde sprouting is another possible explanation. These abnormalities in neural activity originating from a neuroma in the primate are qualitatively similar to those noted in other species and may provide an explanation for certain abnormal sensory phenomena associated with peripheral nerve injury.