Spinal modulation of the induction of central sensitization

Wind-up in lamina I spinoparabrachial neurons: a role for reverberatory circuits

Wind-up is a frequency-dependent increase in the response of spinal cord neurons, which is believed to underlie temporal summation of nociceptive input. However, whether spinoparabrachial neurons, which likely contribute to the affective component of pain, undergo wind-up was unknown. Here, we addressed this question and investigated the underlying neural circuit. We show that one-fifth of lamina I spinoparabrachial neurons undergo wind-up, and provide evidence that wind-up in these cells is mediated in part by a network of spinal excitatory interneurons that show reverberating activity. These findings provide insight into a polysynaptic circuit of sensory augmentation that may contribute to the wind-up of pain's unpleasantness.

Inhibition of electroacupuncture on nociceptive responses of dorsal horn neurons evoked by noxious colorectal distention in an intensity-dependent manner

Background

The transmission of visceral nociception can be inhibited by electroacupuncture (EA) at the spinal level. However, relationships between current intensity and EA-induced analgesia are still lacking. This study compares the effects of different intensities of EA at local acupoints and heterotopic acupoints on nociceptive responses of spinal wide dynamic range (WDR) neurons induced by noxious colorectal distension (CRD).

Materials and methods

Experiments were conducted on 40 Sprague Dawley rats anesthetized with 10% urethane. Discharges of WDR neurons in the L1–L3 segments of the dorsal horn of the spinal cord were recorded extracellularly by glass micropipettes. Different intensities of EA (0.5, 1, 2, 4, 6, and 8 mA, 0.5 ms, 2 Hz) were applied to contralateral “Zusanli” (ST 36) or “Neiguan” (PC 6), with either the same or different segmental innervation of the colon.

Results

In local acupoints, the increased discharges of WDR neurons evoked by CRD were significantly inhibited by EA at 0.5–8 mA. A positive relationship between current intensity and the inhibiting rate was observed within 0.5–4 mA, but the inhibiting rate reached a plateau when EA exceeded 4 mA. In heterotopic acupoints, the increased discharges of WDR neurons evoked by CRD were significantly inhibited by EA at 2–8 mA. A positive relationship between current intensity and the inhibiting rate was observed within 2–6 mA. Further increase in the current beyond 6 mA also resulted in a plateau effect.

Conclusion

Within a certain range, the nociceptive responses of dorsal horn neurons induced by CRD could be inhibited by EA in an intensity-dependent manner.

Neuritis and vinblastine-induced axonal transport disruption lead to signs of altered dorsal horn excitability

BACKGROUND

Many patients with neuropathic pain present without signs of nerve injury on routine clinical examination. Some of these patients may have inflamed peripheral nerves (neuritis). In this study, we have examined whether neuritis causes changes within the dorsal horn that may contribute to a central pain mechanism. Comparisons have been made to a model of axonal transport disruption induced using vinblastine, since neuritis disrupts such processes.

RESULTS

At the peak of cutaneous hypersensitivities, recordings from wide dynamic range neurons revealed increases in wind-up following neuritis but not vinblastine treatment. Ongoing activity from these neurons was unchanged. Vinblastine treatment caused a reduction in the responses of wide dynamic range neurons to noxious mechanical stimulation of the receptive field. The response of neurons to innocuous mechanical stimulation was also reduced in wide dynamic range neurons that were at a depth ≥550 µm following vinblastine treatment. An examination of the superficial dorsal horn revealed an increase in c-Fos-positive neurons in both groups following electrical stimulation of the sciatic nerve. The area of dorsal horn expressing substance P was also decreased following vinblastine treatment.

CONCLUSION

These findings indicate that a minor nerve insult, such as neuritis, can lead to changes within the dorsal horn that are consistent with a central neuropathic pain mechanism.

Pain in Urology: Pathophysiological Aspects of Pain and Chronicity

Chronic pain has been traditionally defined by pain duration, but this approach has limited empirical support and does not account for chronic pain multidimensionality. Defining chronic pain solely by duration is based on the view that acute pain signals potential tissue damage, whereas chronic pain results from central sensitization in which pain is sustained after nociceptive inputs have diminished. Chronic urological pain is a prevalent condition, which can represent a major challenge to health care providers due to its complex aetiology and poor response to therapy. In most cases, clear signs of on-going tissue trauma, inflammation or infection are not present. Despite this, more underhanded pathophysiological mechanisms, affecting the urinary system or other pelvic organ systems (musculoskeletal, neurologic, urologic, gynaecologic) and some psychological aspects may be present. In this article, some pathophysiological aspects of visceral pain are discussed; the definition of ‘chronic pain’, the mechanism of action of drugs used in the treatment of pain and the rationale for association therapy are also reviewed.

Evidence for Increased Magnetic Resonance Imaging Signal Intensity and Morphological Changes in the Brachial Plexus and Median Nerves of Patients With Chronic Arm and Neck Pain Following Whiplash Injury

Background Whiplash following a motor vehicle accident can result in chronic neck and arm pain. Patients frequently present with cutaneous hypersensitivities and hypoesthesia, but without obvious clinical signs of nerve injury. T2-weighted magnetic resonance imaging (MRI) has previously been used to identify nerve pathology. Objectives To determine whether there are signs of peripheral nerve pathology on MRI in patients with chronic arm and neck pain following whiplash injury. Methods This cross-sectional study used T2-weighted MRI to examine the brachial plexus and median nerve in patients and age-matched, healthy control subjects. Clinical examination included tests of plexus and nerve trunk mechanical sensitivity. Results The T2 signal intensity was greater in the brachial plexus and median nerve at the wrist in the patient group (mean intensity ratio = 0.52 ± 0.13 and 2.09 ± 0.33, respectively) compared to the control group (mean intensity ratio = 0.45 ± 0.07 and 1.38 ± 0.31, respectively; P<.05). Changes in median nerve morphology were also observed, which included an enlargement (mean area: patient group, 8.05 ± 1.29 mm²; control group, 6.52 ± 1.08 mm²; P<.05) and flattening (mean aspect ratio: patient group, 2.46 ± 0.53; control group, 1.62 ± 0.30; P<.05) at the proximal carpal row. All patients demonstrated signs of nerve trunk mechanical sensitivity. Conclusion These findings suggest that patients with chronic whiplash may have inflammatory changes and/or mild neuropathy, which may contribute to symptoms. J Orthop Sports Phys Ther 2018;48(7):523-532. Epub 24 Apr 2018. doi:10.2519/jospt.2018.7875.

A machine learning methodology for the selection and classification of spontaneous spinal cord dorsum potentials allows disclosure of structured (non-random) changes in neuronal connectivity induced by nociceptive stimulation

Previous studies aimed to disclose the functional organization of the neuronal networks involved in the generation of the spontaneous cord dorsum potentials (CDPs) generated in the lumbosacral spinal segments used predetermined templates to select specific classes of spontaneous CDPs. Since this procedure was time consuming and required continuous supervision, it was limited to the analysis of two specific types of CDPs (negative CDPs and negative positive CDPs), thus excluding potentials that may reflect activation of other neuronal networks of presumed functional relevance. We now present a novel procedure based in machine learning that allows the efficient and unbiased selection of a variety of spontaneous CDPs with different shapes and amplitudes. The reliability and performance of the present method is evaluated by analyzing the effects on the probabilities of generation of different classes of spontaneous CDPs induced by the intradermic injection of small amounts of capsaicin in the anesthetized cat, a procedure known to induce a state of central sensitization leading to allodynia and hyperalgesia. The results obtained with the selection method presently described allowed detection of spontaneous CDPs with specific shapes and amplitudes that are assumed to represent the activation of functionally coupled sets of dorsal horn neurones that acquire different, structured configurations in response to nociceptive stimuli. These changes are considered as responses tending to adequate transmission of sensory information to specific functional requirements as part of homeostatic adjustments.

The selectivity of rostroventral medulla descending control of spinal sensory inputs shifts postnatally from A fibre to C fibre evoked activity

Brainstem descending control is crucial in maintaining the balance of excitation and inhibition in spinal sensory networks. In the adult, descending inhibition of spinal dorsal horn circuits arising from the brainstem rostroventral medial medulla (RVM) is targeted to neurons with a strong nociceptive C fibre input. Before the fourth postnatal week, the RVM exerts a net facilitation of spinal networks but it is not known if this is targeted to specific dorsal horn neuronal inputs. As the maturation from descending facilitation to inhibition occurs only after C fibre central synaptic maturation is complete, we hypothesized that RVM facilitation in young animals is targeted to A fibre afferent inputs. To test this, the RVM was stimulated while recording dorsal horn neuronal activity in vivo under isoflurane anaesthesia at postnatal day (P) 21 and P40 (adult). Electrical thresholds for A and C fibre evoked activity, spike counts and wind-up characteristics at baseline and during RVM stimulation (10–100 µA, 10 Hz) were compared. In adults, RVM stimulation selectively increased the threshold for C fibre evoked activity while at P21, it selectively decreased the threshold for A fibre evoked activity and these effects were correlated to the wind-up characteristics of the neuron. Thus, the postnatal shift in RVM control of dorsal horn circuits is not only directional but also modality specific, from facilitation of A fibre input in the young animal to inhibition of nociceptive C input in the adult, with additional contextual factors. The descending control of spinal sensory networks serves very different functions in young and adult animals.

Long-term potentiation of excitatory synaptic strength in spinothalamic tract neurons of the rat spinal cord

Spinal dorsal horn nociceptive neurons have been shown to undergo long-term synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD). Here, we focused on the spinothalamic tract (STT) neurons that are the main nociceptive neurons projecting from the spinal cord to the thalamus. Optical technique using fluorescent dye has made it possible to identify the STT neurons in the spinal cord. Evoked fast mono-synaptic, excitatory postsynaptic currents (eEPSCs) were measured in the STT neurons. Time-based tetanic stimulation (TBS) was employed to induce long-term potentiation (LTP) in the STT neurons. Coincident stimulation of both pre- and postsynaptic neurons using TBS showed immediate and persistent increase in AMPA receptor-mediated EPSCs. LTP can also be induced by postsynaptic spiking together with pharmacological stimulation using chemical NMDA. TBS-induced LTP observed in STT neurons was blocked by internal BAPTA, or Ni(2+), a T-type VOCC blocker. However, LTP was intact in the presence of L-type VOCC blocker. These results suggest that long-term plastic change of STT neurons requires NMDA receptor activation and postsynaptic calcium but is differentially sensitive to T-type VOCCs.

Differential presynaptic control of the synaptic effectiveness of cutaneous afferents evidenced by effects produced by acute nerve section

In the anaesthetized cat, the acute section of the saphenous (Saph) and/or the superficial peroneal (SP) nerves was found to produce a long-lasting increase of the field potentials generated in the dorsal horn by stimulation of the medial branch of the sural (mSU) nerve. This facilitation was associated with changes in the level of the tonic primary afferent depolarization (PAD) of the mSU intraspinal terminals. The mSU afferent fibres projecting into Rexed's laminae III-IV were subjected to a tonic PAD that was reduced by the acute section of the SP and/or the Saph nerves. The mSU afferents projecting deeper into the dorsal horn (Rexed's laminae V-VI) were instead subjected to a tonic PAD that was increased after Saph and SP acute nerve section. A differential control of the synaptic effectiveness of the low-threshold cutaneous afferents according to their sites of termination within the dorsal horn is envisaged as a mechanism that allows selective processing of sensory information in response to tactile and nociceptive stimulation or during the execution of different motor tasks.

Temporal regularity determines the impact of electrical stimulation on tactile reactivity and response to capsaicin in spinally transected rats

Nociceptive plasticity and central sensitization within the spinal cord depend on neurobiological mechanisms implicated in learning and memory in higher neural systems, suggesting that the factors that impact brain-mediated learning and memory could modulate how stimulation affects spinal systems. One such factor is temporal regularity (predictability). The present paper shows that intermittent hindleg shock has opposing effects in spinally transected rats depending upon whether shock is presented in a regular or irregular (variable) manner. Variable intermittent legshock (900 shocks) enhanced mechanical reactivity to von Frey stimuli (hyperreactivity), whereas 900 fixed-spaced legshocks produced hyporeactivity. The impact of fixed-spaced shock depended upon the duration of exposure; a brief exposure (36 shocks) induced hyperreactivity whereas an extended exposure (900 shocks) produced hyporeactivity. The enhanced reactivity observed after variable shock was most evident 60-180 min after treatment. Fixed and variable intermittent stimulation applied to the sciatic nerve, or the tail, yielded a similar pattern of results. Stimulation had no effect on thermal reactivity. Exposure to fixed-spaced shock, but not variable shock, attenuated the enhanced mechanical reactivity (EMR) produced by treatment with hindpaw capsaicin. The effect of fixed-spaced stimulation lasted 24h. Treatment with fixed-spaced shock also attenuated the maintenance of capsaicin-induced EMR. The results show that variable intermittent shock enhances mechanical reactivity, while an extended exposure to fixed-spaced shock has the opposite effect on mechanical reactivity and attenuates capsaicin-induced EMR.

Implications of pain in generalized anxiety disorder: efficacy of duloxetine

OBJECTIVE

To conduct a post hoc evaluation of the prevalence of clinically significant pain and the efficacy of duloxetine in patients with generalized anxiety disorder (GAD) and concurrent pain.

METHOD

Data from two 9- to 10-week double-blind, placebo-controlled, randomized clinical trials of duloxetine (60 to 120 mg) in DSM-IV-defined GAD were analyzed (study 1 was conducted from July 2004 to September 2005; study 2 was conducted from August 2004 to June 2005). Efficacy was assessed with the Hamilton Rating Scale for Anxiety (HAM-A), visual analog scales (VAS) for pain, the Hospital Anxiety Depression Scale (HADS), the Clinical Global Impressions-Improvement of Illness (CGI-I) scale, the Patient Global Impressions-Improvement (PGI-I) scale, and the Sheehan Disability Scale (SDS) global functional impairment scale.

RESULTS

Of 840 patients randomly assigned to treatment, 61.3% (302 duloxetine, 213 placebo) had VAS scores ≥ 30 mm on at least 1 of the pain scales, indicating clinically significant pain. Among those patients with concurrent pain at baseline, change from baseline to endpoint in the HAM-A total score (42.9% change in mean scores for duloxetine, 31.4% for placebo), HADS anxiety scale (40.3% vs. 22.8%), HADS depression scale (36.1% vs. 20.5%), HAM-A psychic factor (45.9% vs. 29.9%), and SDS global functional improvement score (45.5% vs. 22.1%) was significantly (all p's < .001) greater for duloxetine compared with placebo. Improvement on the CGI-I (p = .003) and PGI-I (p < .001) was also significantly greater for duloxetine. Response (HAM-A total score decrease ≥ 50%) (49% vs. 29%) and remission (HAM-A total score ≤ 7 at endpoint) (29% vs. 18%) rates were significantly greater for duloxetine compared with placebo (p < .001 and p = .041, respectively). Duloxetine demonstrated statistically significantly greater reduction in pain on all 6 VAS pain scales (all p's < .001 except headaches with p < .002) (for duloxetine, percent change in means from baseline to endpoint ranged from 40.1% to 45.2% across the 6 VAS scales; for placebo, 22.0% to 26.3%).

CONCLUSION

Duloxetine, relative to placebo, improves anxiety symptoms, pain, and functional impairment among patients with GAD with concurrent clinically significant pain.

TRIAL REGISTRATION

clinicaltrials.gov Identifiers: NCT00122824 (study 1) and NCT00475969 (study 2).

Timing in the absence of supraspinal input III: regularly spaced cutaneous stimulation prevents and reverses the spinal learning deficit produced by peripheral inflammation

In the absence of brain input, spinal systems can adapt to new environmental relations. For example, spinally transected rats given a legshock each time the leg is extended exhibit a progressive increase in flexion duration that minimizes net shock exposure, a simple form of instrumental learning. This capacity for learning is modulated by prior stimulation; both variable shock and inflammation produce a lasting inhibition of learning. An extended exposure to fixed spaced shock has no adverse effect on learning and opposes the consequences of variable shock. The present studies expand on these findings and demonstrate that fixed stimulation ameliorates the impact of peripheral inflammation. Spinally transected rats were administered 900 fixed spaced legshocks before (Experiment 1) or 1,800 legshocks after (Experiment 2) a subcutaneous hindpaw injection of capsaicin. Learning was assessed 24 hr later. Treatment with fixed shock attenuated the capsaicin-induced inhibition of learning. These findings suggest that fixed stimulation promotes adaptive plasticity and may foster recovery after injury.

Activation of the galanin receptor 2 in the periphery reverses nerve injury-induced allodynia

Background

Galanin is expressed at low levels in the intact sensory neurons of the dorsal root ganglia with a dramatic increase after peripheral nerve injury. The neuropeptide is also expressed in primary afferent terminals in the dorsal horn, spinal inter-neurons and in a number of brain regions known to modulate nociception. Intrathecal administration of galanin modulates sensory responses in a dose-dependent manner with inhibition at high doses. To date it is unclear which of the galanin receptors mediates the anti-nociceptive effects of the neuropeptide and whether their actions are peripherally and/or centrally mediated. In the present study we investigated the effects of direct administration into the receptive field of galanin and the galanin receptor-2/3-agonist Gal2-11 on nociceptive primary afferent mechanical responses in intact rats and mice and in the partial saphenous nerve injury (PSNI) model of neuropathic pain.

Results

Exogenous galanin altered the responses of mechano-nociceptive C-fibre afferents in a dose-dependent manner in both naive and nerve injured animals, with low concentrations facilitating and high concentrations markedly inhibiting mechano-nociceptor activity. Further, use of the galanin fragment Gal2-11 confirmed that the effects of galanin were mediated by activation of galanin receptor-2 (GalR2). The inhibitory effects of peripheral GalR2 activation were further supported by our demonstration that after PSNI, mechano-sensitive nociceptors in galanin over-expressing transgenic mice had significantly higher thresholds than in wild type animals, associated with a marked reduction in spontaneous neuronal firing and C-fibre barrage into the spinal cord.

Conclusions

These findings are consistent with the hypothesis that the high level of endogenous galanin in injured primary afferents activates peripheral GalR2, which leads to an increase in C-fibre mechanical activation thresholds and a marked reduction in evoked and ongoing nociceptive responses.

Activation of ERK1/2 in the primary injury site is required to maintain melittin-enhanced wind-up of rat spinal wide-dynamic-range neurons

Peripheral modulation of wind-up enhancement induced by peripheral tissue injury is investigated in rat spinal wide-dynamic-range (WDR) neurons. After subcutaneous (s.c.) injection of melittin, a pain-related peptidergic component separated from bee venom, the responsiveness of spinal cord WDR neuron to repeated suprathreshold (1.5T, the intensity threshold) electrical stimuli is enhanced. Comparing with the less effects on early response (0-100 ms), melittin significantly increases late response (100 ms to the next stimulus artifact) and after-discharge (starting from 2s after the last stimulus artifact) with 189% and 546%, respectively. Peripheral administration of a specific MEK inhibitor, 1,4-diamino-2,3-dicyano-1,4-bis-[o-aminophenylmercapto] butadiene (U0126, 1 microg) gradually suppresses, but not completely blocks melittin-enhanced wind-up to the similar level of baseline. The inhibitions of U0126 are mainly on late response and after-discharge with 49% and 65%, respectively. Peripheral administration of three doses of U0126 (0.1, 1, 10 microg) has no effects on melittin-induced local paw edema regardless of either pre- or post-treatment of the drug. We conclude that peripheral ERKs pathway in the primary injury site is required to maintain melittin-enhanced wind-up of rat spinal cord wide-dynamic-range neurons.

L-type calcium channels and NMDA receptors: a determinant duo for short-term nociceptive plasticity

In the dorsal horn of the spinal cord, pain-transmitting neurons exhibit action potential windup, a form of short-term plasticity, which consists of a progressive increase in neuronal response during repetitive stimulation of nociceptive input fibers. Windup depends on N-methyl-D-aspartate (NMDA) receptor activation, but previous in vitro studies indicated that windup also relies on intrinsic plateau properties of spinal neurons. In the present study, we considered the possible involvement of these properties in windup in vivo. For this purpose, we first studied a nociceptive flexion reflex in the rat. We showed that windup of the reflex is actually suppressed by blockers of L-type calcium current and Ca(2+)-activated non-specific cationic current (Ican), the two main depolarizing conductances of plateau potentials. We further showed that, during windup, NMDA receptors provide a critical excitatory component in a dynamic balance of excitatory and inhibitory inputs which ultimately activates L-type calcium channels. The nociceptive reflex involves at least two neuronal groups, which may express intrinsic amplification properties, motor neurons and dorsal horn neurons. By means of extracellular recordings in the dorsal horn, we showed that windup of dorsal horn neuron discharge was sensitive to the modulators of L-type calcium current. Altogether, our results suggest that, in vivo, windup also depends on the amplification properties of spinal neurons, the triggering of which requires previous activation of NMDA receptors.

Analgesic effects of serotonergic, noradrenergic or dual reuptake inhibitors in the carrageenan test in rats: Evidence for synergism between serotonergic and noradrenergic reuptake inhibition

The efficacy of antidepressant drugs with serotonergic, noradrenergic, or dual reuptake inhibition was evaluated in reversing carrageenan-induced thermal hyperalgesia and mechanical allodynia in rats. Duloxetine (1-30mg/kg, i.p.), a balanced serotonergic-noradrenergic reuptake inhibitor (SNRI), was equiefficacious and more potent than the SNRI venlafaxine (3-100mg/kg, i.p.) in reversing both thermal hyperalgesia and mechanical allodynia induced by carrageenan. In addition, the selective noradrenergic reuptake inhibitors (NRIs) thionisoxetine (0.03-10mg/kg, i.p.) and desipramine (1-30mg/kg, i.p.) also produced complete reversals of carrageenan-induced thermal hyperalgesia. However, only thionisoxetine exhibited a greater than 80% reversal of the carrageenan-induced mechanical allodynia. In contrast, the selective serotonergic reuptake inhibitors (SSRIs) paroxetine, sertraline, and fluoxetine (0.3-10mg/kg i.p.) had little or no effect in the carrageenan model. In order to understand whether the observed enhanced effectiveness of the dual SNRIs was due to a possible synergism between serotonergic and noradrenergic reuptake inhibition, the effects of the NRI thionisoxetine alone and in combination with an inactive dose of the SSRI fluoxetine were determined. In the presence of fluoxetine, the potency of thionisoxetine in reversing carrageenan-induced hyperalgesia and allodynia was significantly increased by approximately 100-fold and brain concentrations of thionisoxetine were increased by 1.1- to 5-fold. The present data indicate fluoxetine pharmacodynamically potentiated the analgesic effects of thionisoxetine over and above a metabolic interaction between these two drugs. The present findings thus indicate that, in the carrageenan model, dual serotonergic-noradrenergic reuptake inhibition by dual SNRIs, or SSRI-NRI combinations, produces synergistic analgesic efficacy.

Synergistic interactions between the dual serotonergic, noradrenergic reuptake inhibitor duloxetine and the non-steroidal anti-inflammatory drug ibuprofen in inflammatory pain in rodents

OBJECTIVES

The present study was undertaken to characterize whether the pharmacologic interaction between duloxetine, a balanced serotonergic and noradrenergic reuptake inhibitor, and the non-steroidal anti-inflammatory drug ibuprofen was simply additive, less than additive, or greater than additive (i.e., synergistic) in preclinical models of visceral and inflammatory pain, specifically acetic acid-induced writhing in mice and carrageenan-induced thermal hyperalgesia and mechanical allodynia in rats.

METHODS

In the writhing test, male CF-1 mice were injected intraperitoneally with 0.55% acetic acid and 5 min later the number of writhes was counted over a 5-min period. In the carrageenan models, male Sprague-Dawley rats were injected with a 1.5% carrageenan solution into the ventral surface of the hind paw; hypersensitivity to thermal and mechanical stimuli was subsequently evaluated 2h post-carrageenan.

RESULTS

Vehicle or a dose of duloxetine alone (1-100 mg/kg), ibuprofen alone (10-300 mg/kg), or duloxetine and ibuprofen in combination in a dose-ratio of 1:10 duloxetine:ibuprofen were orally administered 30 or 60 min before testing. Isobolographic analysis of the effects of duloxetine in combination with ibuprofen revealed a significant synergistic (greater than additive) interaction between duloxetine and ibuprofen both for reducing acetic acid-induced writhing and carrageenan-induced thermal hyperalgesia, but were additive for reversing mechanical allodynia.

CONCLUSIONS

Our data indicate that duloxetine and ibuprofen have synergistic efficacy in a visceral and an inflammatory pain model in rodents, and suggest that duloxetine and ibuprofen in combination may provide a useful approach to the clinical treatment of persistent pain, particularly inflammation-related pain.

Isoflurane depresses windup of C fiber-evoked limb withdrawal with variable effects on nociceptive lumbar spinal neurons in rats

Windup is a progressive increase in responses of nociceptive spinal cord neurons to repeated electrical C fiber stimulation. We hypothesized that isoflurane would depress windup at approximately the minimum alveolar anesthetic concentration (MAC) required to suppress purposeful movement in response to noxious stimulation. We recorded windup responses in single lumbar spinal neurons (n = 17) to a series of 15 repetitive electrical stimuli delivered at 1 Hz to the hindpaw at C fiber strength; hindpaw withdrawal force was simultaneously recorded. The total number of action potentials per 15 stimuli (mean +/- sem as a percentage of each neuron's maximal response) was 83% +/- 5%, 84% +/- 5%, 67% +/- 7%, and 57% +/- 8% at 0.7, 0.9, 1.1, and 1.4 MAC, respectively. The 0.9 and 1.1 MAC values differed significantly from each other, whereas the 0.7 and 0.9 MAC values differed from the 1.4 MAC value (P < 0.05). The reduced firing was attributed to a depression of the initial C fiber-evoked responses in most units, and a reduction in windup slope over the initial 5 stimuli in 6 units. Muscle force was 67%, 11%, and 4% of the 0.7 MAC value at 0.9, 1.1, and 1.4 MAC, respectively. Isoflurane depressed excitability and variably affected windup of lumbar spinal cord neurons, while uniformly depressing windup of limb withdrawals in a concentration-dependent manner.

Efficacy of Duloxetine, a Potent and Balanced Serotonergic and Noradrenergic Reuptake Inhibitor, in Inflammatory and Acute Pain Models in Rodents

Duloxetine, a selective but balanced serotonergic and noradrenergic reuptake inhibitor, was evaluated in the acute nociceptive pain models of tail flick and hot plate in mice and in the persistent and/or inflammatory pain models of acetic acid-induced writhing in mice, carrageenan-induced thermal hyperalgesia and mechanical allodynia in rats, and capsaicin-induced mechanical allodynia in rats. In acute pain models, duloxetine had no significant effect on response latency in the mouse tail-flick test but produced modest increases in response latencies in the mouse hot plate test. Morphine produced dose-related analgesic effects in both the mouse tail-flick and hot plate tests. In models of inflammatory and/or persistent pain, duloxetine, morphine, and ibuprofen produced dose-related decreases in acetic acid-induced writhing in mice. Duloxetine, ibuprofen, and gabapentin also produced dose-dependent reversals of both thermal hyperalgesia and mechanical allodynia produced by carrageenan in rats. In addition, both duloxetine and morphine produced a significant reduction of capsaicin-induced mechanical allodynia in rats. Duloxetine and gabapentin were without substantial effect on the Rotorod test in mice, whereas morphine and ibuprofen produced a significant impairment. Our data indicate that duloxetine may be efficacious in the treatment of persistent and/or inflammatory pain states at doses that have modest or no effect on acute nociception or motor performance.

Electrophysiological characterization of facilitated spinal withdrawal reflex to repetitive electrical stimuli and its modulation by central glutamate receptor in spinal anesthetized rats

The present study is aimed to systematically investigate wind-up and after-discharge of the spinal withdrawal reflex assessed by recording single motor unit (SMU) electromyographic (EMG) response to different intensities [0.5-1.5xreflex threshold (T)] of repetitive [frequencies (0.5-200 Hz)] transcutaneous electrical stimuli for 5 s. The role of central glutamate receptors in modulation of the withdrawal reflex facilitation was observed and evaluated in order to explore the potential central mechanism. Stimulus intensities below reflex threshold, such as 0.8xT, but not 0.5xT, could by repetition elicit and facilitate withdrawal reflex. The facilitation (wind-up and after-discharge) of the withdrawal reflex is a result of central integration and is increased significantly for increasing stimulus intensity and frequency. Electrical stimuli at 3-5 Hz for 5 s are appropriate to elicit wind-up. In contrast, 10-20 Hz frequencies of electrical stimuli are adequate to evoke the after-discharge. For pharmacological intervention, suprathreshold (1.5xT) repeated (5 Hz) electrically evoked facilitated reflex (wind-up) were apparently depressed by intrathecal (i.t.) administration of MK-801 as well as CNQX (40 nmol/10 microl, respectively). However, wind-up of spinal reflexes evoked by subthreshold (0.8xT) electrical stimuli could only be depressed by the treatment with CNQX, not MK-801. The after-discharge of the withdrawal reflex elicited by 20 Hz electrical stimulation with either 0.8xT or 1.5xT intensity was depressed by i.t. treatment with CNQX. I.t. application of MK-801 only depressed 0.8xT the intensity of electrically evoked after-discharge. In conclusion, for the first time, the present study clearly demonstrates that, following the wind-up phase, the spinal withdrawal reflex pathways continue to fire spontaneously in a stimulus frequency- and intensity-dependent way (temporal and/or spatial summation). This inherited memory and the central non-N-methyl-d-aspartate (non-NMDA) receptor, but not the NMDA receptor, mainly involving pharmacological mechanisms, may play an important role in pathological conditions with spontaneous nociceptive firing. Furthermore, the after-discharge of the spinal reflex may be an important indicator for studies on central sensitization in many pathological pain conditions.

Calcium-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptors mediate development, but not maintenance, of secondary allodynia evoked by first-degree burn in the rat

Intrathecal pretreatment with N-methyl-D-aspartate (NMDA) receptor antagonists blocks development of spinal sensitization in a number of pain models. In contrast, secondary mechanical allodynia evoked by thermal injury (52.5 degrees C for 45 s) applied to the hind paw of the rat is not blocked by intrathecal pretreatment with NMDA receptor antagonists. It is, however, blocked by antagonists to the non-NMDA, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate (AMPA/KA) and calcium-permeable AMPA/KA receptors. These findings suggest a role for these receptors in the development of spinal sensitization. The present study used the same thermal injury model to assess the effects of the AMPA/KA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and specific calcium-permeable AMPA/KA receptor antagonists philanthotoxin (PHTx) and joro spider toxin (JST) when given as postinjury treatments. Intrathecal saline injection at 5 and 30 min postinjury had no effect on thermal injury-evoked allodynia as measured by calibrated von Frey filaments. In contrast, 36 nmol of CNQX given at either time point reversed allodynia. Intrathecal 13 nmol of PHTx or 9 nmol of JST (higher doses than that required for pretreatment) reversed allodynia at the 5-min time point, but neither drug was antiallodynic at the 30-min time point. Thus, secondary mechanical allodynia in this model is not maintained by calcium-permeable AMPA/KA receptors, but instead requires activation of calcium-impermeable AMPA/KA receptors. This finding supports a role for AMPA/KA receptor function in responses occurring during spinal sensitization.

Interferon-gamma induces characteristics of central sensitization in spinal dorsal horn neurons in vitro

Hyperexcitability of spinal dorsal horn neurons, also known as 'central sensitization', is a component of pain associated with pathological conditions in the nervous system. The aim of the present study was to analyze if the pro-inflammatory cytokine, interferon-gamma (IFN-gamma), which can be released for extended periods of time in the nervous system during inflammatory and infectious events, can alter synaptic activity in dorsal horn neurons and thereby contribute to such hyperexcitability. Treatment of cultured dorsal horn neurons with IFN-gamma for 2 weeks resulted in a significantly reduced clustering of alpha-amino-3-hydroxy-5-methylisoxazole (AMPA) receptor subunit 1 (GluR1) that was dependent on nitric oxide. The neurons displayed an increased frequency and amplitude of excitatory postsynaptic currents (EPSCs) upon IFN-gamma treatment. Treated dorsal horn neurons also exhibited increased responsiveness to stimulation of dorsal root ganglia (DRG) axons in a two-compartment model. Furthermore, disinhibition by the GABA(A) receptor antagonist picrotoxin (PTX) significantly increased EPSC frequency and induced bursting in untreated cultures but did not significantly increase the frequency in treated neurons, which displayed bursting even without PTX. GABA(A) agonists reduced activity more strongly in treated cultures and immunochemical staining for GABA(A) receptors showed no difference from controls. Since GluR1-containing AMPA receptors (AMPARs) occur predominantly on inhibitory neurons in the dorsal horn, we suggest that the IFN-gamma-mediated increase in spontaneous activity and responsiveness to DRG axon stimulation, decrease in sensitivity to PTX and tendency for EPSC bursting result from a reduced expression of GluR1 on these neurons and not from a reduction in active GABA(A) receptors in the network. IFN-gamma thereby likely causes disinhibition of synaptic activity and primary afferent input in the dorsal horn, which consequently results in central sensitization.

The medullary dorsal reticular nucleus enhances the responsiveness of spinal nociceptive neurons to peripheral stimulation in the rat

Single-unit spinal recordings combined with application of glutamate into the medullary dorsal reticular nucleus were used to assess the action of this nucleus upon deep dorsal horn neurons in rats. Injection of high glutamate concentrations (10 and 100 mm) induced a dramatic and long-lasting increase of the responses of wide-dynamic range neurons to electrical stimulation of the sciatic nerve in the noxious range, without affecting ongoing discharges. Post-stimulus time histograms revealed that this increase concerned the post-discharge, but not A- or C-fibre-mediated responses, which remained unchanged independently of the stimulation frequency applied. The onset of the glutamate-induced response enhancement occurred with a concentration-dependent time delay and developed slowly until its maximum. These data indicate that the medullary dorsal reticular nucleus exerts a facilitating action upon deep dorsal horn wide-dynamic range neurons by enhancing their capacity to respond to peripheral stimulation through prolongation of their discharge. This action is accompanied by the strengthening of wind-up of deep dorsal horn wide-dynamic range neurons, hence providing a plausible substrate for chronic pain states. These results are in agreement with previous behavioural studies suggesting a pronociceptive role for the dorsal reticular nucleus [Almeida et al. (1996) Brain Res. Bull., 39, 7-15; Almeida et al. (1999) Eur. J. Neurosci., 11, 110-122], and support the involvement of a reverberating circuit, previously described in morphological studies [Almeida et al. (1993) Neuroscience, 55, 1093-1106; Almeida et al. (2000) Eur. J. Pain, 4, 373-387], which probably operates only at a certain threshold of activation.

Aftersensations in experimental and clinical hypersensitivity

This study examined aftersensations after repetitive pinprick stimulation in nerve injury patients with allodynia and in capsaicin sensitized skin in control subjects. Forty-one patients and 12 male control subjects were stimulated repetitively with a computer-driven von Frey hair in sensitized skin and in contralateral normal skin at 0.2 and 2.0Hz. Pain during and after stimulation was scored on an electronic visual analogue scale and sampled on a computer. Maximum evoked pain, time to pain onset, duration of pain and duration of aftersensations were calculated from stored data. Aftersensations were significantly more frequent in sensitized skin than in contralateral normal skin in both control subjects and patients. Pain was evoked more frequently and maximum evoked pain score was significantly higher in sensitized skin compared to contralateral normal skin in both groups. Furthermore, evoked pain score was frequency dependent with a significantly higher pain score at 2.0 than at 0.2Hz. There was a significant positive correlation between maximum evoked pain scores and duration of aftersensations both in patients and control subjects. No difference was observed in maximum pain score, time to pain onset and duration of aftersensations between the groups. Findings suggest that aftersensations and evoked pain are mediated by a common mechanism, which most likely is central. The clinical manifestation of aftersensations can be mimicked in an experimental pain model so aftersensations may be another useful parameter in assessment of central sensitization.

Simultaneous recordings of wind-up of paired spinal dorsal horn nociceptive neuron and nociceptive flexion reflex in rats

To study the sensory-motor interaction of spinal processing underlying the neuronal mechanisms of the nociceptive flexion reflex (NFR) and its temporal facilitation, 16 spinal dorsal horn (DH) wide-dynamic-range (WDR) neurons and paired 16 single motor units (SMU) from the gastrocnemius soleus muscle (GS) were simultaneously recorded using extracellular single unit and electromyographic techniques in spinal, halothane-anesthetized rats. The paired DH WDR neuron and GS SMU showed a parallel increase in the firing rate and duration of spike responses to noxious pinch stimuli applied to their common cutaneous receptive field (cRF) on the ipsilateral hind paw skin. Innocuous brush or pressure evoked no, or less, firing in the SMU but evoked a graded increase in spike responses in the simultaneously-recorded WDR neuron. Moreover, both pressure and noxious pinch stimuli evoked a short-lasting after-discharge (for several min) in the WDR neuron but without any after-discharge in the simultaneously-recorded SMU. The paired WDR neuron and SMU also showed a parallel basal response (termed as early and late components according to latency), after-discharge and wind-up of the late response to repetitively applied supra-threshold electrical stimulation (intensity: >1.5 T, duration: 1 ms and frequency: 1 Hz for 15 s). Linear regression and cross-correlation histogram analyses showed that the DH WDR neuron had a significant correlation with the simultaneously-recorded SMU and they were functionally located in the spinally-organized NFR circuitry via polysynaptic connections. Systemic administration of fentanyl, an opioid receptor agonist, resulted in a parallel, naloxone-reversible suppression of both basal late response component and wind-up response in both WDR neuron and SMU paired; however, fentanyl suppressed only the early response of the SMU without any effect on that of the DH WDR neuron. The present results provide new direct evidence showing an essential role of spinal DH WDR neurons in the mediation of spinally-organized NFR as well as its temporal facilitation (wind-up). Based on these data, the spinal DH WDR neuron seems to function as a signal discriminator or frequency encoder of multireceptive primary afferent impulses that may determine excitable level of motor output and the occurrence of a behavioral NFR via polysynaptic connections. Consequently, the spinal WDR neuron-mediated NFR and its temporal facilitation are likely to be modulated by spinal endogenous opioid peptides via opioid receptors on the nociceptive sensory components of the spinally-organized NFR circuitry.

NMDA or non-NMDA receptor antagonists attenuate increased Fos expression in spinal dorsal horn GABAergic neurons after intradermal injection of capsaicin in rats

GABAergic neurons play an important role in the generation of primary afferent depolarization, which results in presynaptic inhibition and, if large enough, triggers dorsal root reflexes. Recent electrophysiological studies by our group have suggested that increased excitation of spinal GABAergic neurons by activation of N-methyl-D-aspartate (NMDA) and non-NMDA receptors following intradermal injection of capsaicin results in the generation of DRRs that contribute to neurogenic inflammation. The present study was to determine if changes in the expression of Fos protein occur in GABAergic neurons in the lumbosacral spinal cord following injection of capsaicin into the glabrous skin of one hind paw of anesthetized rats and if pretreatment with an NMDA receptor antagonist, D-(-)-2-amino-7-phosphonoheptanoic acid (AP7) or a non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocks Fos expression in these neurons. The experiments used western blots and immunofluorescence double labeling staining following capsaicin or vehicle injection. Western blots showed that Fos protein was increased on the ipsilateral side in spinal cord tissue 0.5 h after capsaicin injection. Pretreatment with AP7 or CNQX caused a decrease in capsaicin-induced Fos expression. Immunofluorescence double labeling showed that the proportion of Fos-positive GABAergic neuronal profiles was significantly increased following capsaicin injection (48.8+/-4.8%) compared to the vehicle injection (23.8+/-5.1%) in superficial laminae on the ipsilateral side in lumbosacral spinal cord (P<0.05). However, when the spinal cord was pretreated with AP7 (5 microg) or CNQX (0.2 microg), only 9.1+/-0.6% or 7.1+/-0.8% of GABA-immunoreactive neuronal profiles were stained for Fos following capsaicin injection. The blockade of the capsaicin-evoked Fos staining was dose-dependent. These findings suggest that GABAergic neurons take part in dorsal horn circuits that modulate nociceptive information and that the function of GABAergic neurons following capsaicin injection is partially mediated by NMDA and non-NMDA receptors.

Antihyperalgesic effects of the muscarinic receptor ligand vedaclidine in models involving central sensitization in rats

It is well established that muscarinic cholinergic agonists produce antinociceptive effects in a number of acute pain models. However, relatively little is known about the effects of muscarinic receptor agonists in models which involve central sensitization in pain pathways. The purpose of the present studies was to evaluate the effects of vedaclidine, a muscarinic receptor mixed agonist/antagonist across receptor subtypes, in models involving central sensitization. Vedaclidine (0.3-10 mg/kg s.c.) produced dose-related antihyperalgesic effects in the formalin test as well as a dose-related reversal of capsaicin-induced mechanical hyperalgesia in rats. In the carrageenan test, vedaclidine (0.1-30 mg/kg) produced a dose-related reversal of both mechanical and thermal hyperalgesia that were antagonized by the muscarinic receptor antagonist scopolamine. In addition, the antihyperalgesic effects of vedaclidine in the carrageenan test were synergistic with the antihyperalgesic effects of the non-steroidal antiinflammatory drug ketoprofen, as demonstrated by isobolographic analysis. The present studies demonstrate that vedaclidine produces antihyperalgesic effects in models involving central sensitization, suggesting that vedaclidine, and potentially other muscarinic receptor agonists, may have clinical utility in the management of pain states involving central sensitization, such as neuropathic and inflammatory pain states.

Chronic pain/dysfunction in whiplash-associated disorders

OBJECTIVE

The purposes of this article are (1) to review current knowledge of and recent concepts pertaining to the causes of chronic pain and/or dysfunction following whiplash-type injuries and (2) to acquaint those who treat these types of injuries with possible mechanisms of continued pain and or dysfunction following whiplash.

DATA COLLECTION

A review of the literature on mechanisms of injury and neurologic considerations was undertaken. A hand search of relevant medical, neuroscience, chiropractic, and online Index Medicus sources and other sources involving mechanisms of nociception, neurotransmitters, and receptors that might evolve from whiplash-type soft tissue injuries was conducted.

RESULTS

Pain is a complex phenomenon that has great variability. Chronic pain appears to involve a deficient descending inhibitory process and/or ongoing excitatory input.

CONCLUSIONS

There is a wide variety of reactions by individuals to any given type of stimulus. Injury may lead to increases in neuronal activity and prolonged changes in the nervous system. Chronic pain may be seen as part of a central disturbance accompanied by disinhibition or sensitization of central pain modulation, mirrored in the immune and endocrine systems. Patients with chronic whiplash syndrome may have a generalized central hyperexcitability from a loss of tonic inhibitory input (disinhibition) and/or ongoing excitatory input contributing to dorsal horn hyperexcitability. Dysfunction of the motor system may also occur, with or without pain. The purpose of treatment should be not only to relieve pain but also to allow for proper proprioception.

Changes in tactile stimuli-induced behavior and c-Fos expression in the superficial dorsal horn and in parabrachial nuclei after sciatic nerve crush

Neurons in the superficial laminae of the dorsal horn are dominated by input from peripheral nociceptors. Following peripheral nerve injury, low threshold mechanoreceptive Abeta-fibers sprout from their normal termination site in laminae III/IV into laminae I-II and this structural reorganization may contribute to neuropathic tactile pain hypersensitivity. We have now investigated whether a sciatic nerve crush injury alters the behavioral response in rats to tactile stimuli and whether this is associated with a change in the pattern of c-Fos expression in the dorsal horn and the parabrachial area of the brainstem. Sciatic nerve crush resulted in a patchy but marked tactile allodynia manifesting first at 3 weeks and persisting for up to 52 weeks. C-Fos expression in the dorsal horn and parabrachial region was never observed on brushing the skin of the sciatic nerve territory in animals with intact nerves, but was found after sciatic nerve crush with peripheral regeneration. We conclude that after nerve injury, low threshold mechanoreceptor fibers may play a major role in producing pain-related behavior by activating normally nociceptive-specific regions of the central nervous system such as the superficial laminae of the dorsal horn and the parabrachial area.

Injury threshold: whiplash-associated disorders

OBJECTIVES

To review current knowledge and recent concepts of the causes of injuries after minor impact automobile collisions and to acquaint those who treat these types of injuries with possible injury thresholds and mechanisms that may contribute to symptoms.

DATA SOURCES

A review of literature involving mechanisms of injury, tissue tensile threshold, and neurologic considerations was undertaken. A hand-search of relevant engineering, medical/chiropractic, and computer Index Medicus sources in disciplines that cover the variety of symptoms was gathered.

RESULTS

Soft-tissue injuries are difficult to diagnose or quantify. There is not one specific injury mechanism or threshold of injury. With physical variations of tissue tensile strength, anatomic differences, and neurophysiologic considerations, such threshold designation is not possible.

CONCLUSIONS

To make a competent assessment of injury, it is important to evaluate each patient individually. The same collision may cause injury to some individuals and leave others unaffected. With the variability of human postures, tensile strength of the ligaments between individuals, body positions in the vehicle, collagen fibers in the same specimen segment, the amount of muscle activation and inhibition of muscles, the size of the spinal canals, and the excitability of the nervous system, one specific threshold is not possible. How individuals react to a stimulus varies widely, and it is evident peripheral stimulation has effects on the central nervous system. It is also clear that the somatosensory system of the neck, in addition to signaling nociception, may influence the control of neck, eyes, limbs, respiratory muscles, and some preganglionic sympathetic nerves.

Wind-up of spinal cord neurones and pain sensation: much ado about something?

Wind-up is a frequency-dependent increase in the excitability of spinal cord neurones, evoked by electrical stimulation of afferent C-fibres. Although it has been studied over the past thirty years, there are still uncertainties about its physiological meaning. Glutamate (NMDA) and tachykinin NK1 receptors are required to generate wind-up and therefore a positive modulation between these two receptor types has been suggested by some authors. However, most drugs capable of reducing the excitability of spinal cord neurones, including opioids and NSAIDs, can also reduce or even abolish wind-up. Thus, other theories involving synaptic efficacy, potassium channels, calcium channels, etc. have also been proposed for the generation of this phenomenon. Whatever the mechanisms involved in its generation, wind-up has been interpreted as a system for the amplification in the spinal cord of the nociceptive message that arrives from peripheral nociceptors connected to C-fibres. This probably reflects the physiological system activated in the spinal cord after an intense or persistent barrage of afferent nociceptive impulses. On the other hand, wind-up, central sensitisation and hyperalgesia are not the same phenomena, although they may share common properties. Wind-up can be an important tool to study the processing of nociceptive information in the spinal cord, and the central effects of drugs that modulate the nociceptive system. This paper reviews the physiological and pharmacological data on wind-up of spinal cord neurones, and the perceptual correlates of wind-up in human subjects, in the context of its possible relation to the triggering of hyperalgesic states, and also the multiple factors which contribute to the generation of wind-up.

The NMDA receptor antagonist MK-801 attenuates c-Fos expression in the lumbosacral spinal cord following repetitive noxious and non-noxious colorectal distention

The effects of pretreatment with an NMDA receptor antagonist, MK-801, on c-Fos (Fos) expression in the lumbosacral spinal cord following repetitive, noxious (80 mmHg) or non-noxious (20 mmHg) colorectal distention (CRD) was examined immunocytochemically in awake and urethane anesthetized rats. In awake rats, noxious CRD induced Fos expression in the lumbosacral spinal cord. Pretreatment with MK-801 (0.1-1.0 mg/kg, i.p.) produced no change or an increase in noxious CRD induced-Fos expression and caused aversive side effects. In order to examine greater doses of MK-801, further experiments were performed in rats anesthetized with urethane. Both noxious and non-noxious CRD induced Fos in the lumbosacral spinal cord. Pretreatment with MK-801 (0.5, 1.0, 5.0 mg/kg, i.p.) dose-dependently attenuated noxious CRD-induced Fos by 20-40%. Five mg/kg MK-801 attenuated non-noxious CRD-induced Fos by 20%. Lesser doses did not significantly attenuate Fos expression. The laminar distribution of Fos following MK-801 pretreatment revealed a tendency towards the deeper laminae showing the greatest attenuation at the highest dose of MK-801. Protein plasma extravasation in the colon measured with Evan's blue dye showed no difference between rats without balloons, rats with balloons that were not distended and non-noxious CRD. There was significantly more extravasation following noxious CRD. Pretreatment with systemic MK-801 had no effect on plasma extravasation produced by noxious CRD. These data suggest that the induction of Fos in the lumbosacral spinal cord by noxious and non-noxious CRD is partially NMDA receptor mediated. However, NMDA receptor activation contributes significantly more to noxious than non-noxious CRD-induced Fos. Inflammation of the colon following noxious CRD likely contributes to sensitization of colonic afferents which may contribute to the increased NMDA receptor-mediated Fos following the noxious stimulus.

Evidence for subnucleus interpolaris in craniofacial muscle pain mechanisms demonstrated by intramuscular injections with hypertonic saline

The subnucleus interpolaris (Vi) has been identified as a major recipient for trigeminal ganglionic input from jaw muscles, and contains neurons with nociceptive properties similar to those in the subnucleus caudalis (Vc). Therefore, Vi may be another important site for processing craniofacial muscle nociception. The aims of present study were to define functional properties of Vi neurons that receive input from masseter muscle afferents by characterizing their responses to electrical, mechanical, and to chemical stimulation of the muscle. Ninety cells were identified as masseter muscle units in 11 adult cats. Most of these units (79%) received additional inputs from orofacial skin. Following the intramuscular injection of 5% hypertonic saline, 49% of the cells showed a significant modulation of either the resting discharge and/or responses to innocuous mechanical stimulation on their cutaneous receptive fields (RFs). The most common response to saline injection was an induction or facilitation of resting discharge which declined as an exponential decay function, returning to pre-injection level within 3-4 min. Forty-five percent of the muscle units that were tested with mechanical stimulation (13/29) showed a prolonged inhibition of mechanically-evoked responses. In most cases (8/13), the inhibitory response was accompanied by initial facilitation. The observations that Vi contained a population of neurons that receive small diameter muscle afferent inputs, responded to noxious mechanical stimulation on the muscle and to a chemical irritant that is known to produce pain in humans provide compelling evidence for the involvement of Vi in craniofacial muscle pain mechanisms.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.