Catecholamine-induced mechanical sensitization of cutaneous nociceptors in the rat

A mouse model of chronic primary pain that integrates clinically relevant genetic vulnerability, stress, and minor injury

Chronic primary pain conditions (CPPCs) affect over 100 million Americans, predominantly women. They remain ineffectively treated, in large part because of a lack of valid animal models with translational relevance. Here, we characterized a CPPC mouse model that integrated clinically relevant genetic (catechol-O-methyltransferase; COMT knockdown) and environmental (stress and injury) factors. Compared with wild-type mice, Comt+/- mice undergoing repeated swim stress and molar extraction surgery intervention exhibited pronounced multisite body pain and depressive-like behavior lasting >3 months. Comt+/- mice undergoing the intervention also exhibited enhanced activity of primary afferent nociceptors innervating hindpaw and low back sites and increased plasma concentrations of norepinephrine and pro-inflammatory cytokines interleukin-6 (IL-6) and IL-17A. The pain and depressive-like behavior were of greater magnitude and longer duration (≥12 months) in females versus males. Furthermore, increases in anxiety-like behavior and IL-6 were female-specific. The effect of COMT genotype × stress interactions on pain, IL-6, and IL-17A was validated in a cohort of 549 patients with CPPCs, demonstrating clinical relevance. Last, we assessed the predictive validity of the model for analgesic screening and found that it successfully predicted the lack of efficacy of minocycline and the CB2 agonist GW842166X, which were effective in spared nerve injury and complete Freund's adjuvant models, respectively, but failed in clinical trials. Yet, pain in the CPPC model was alleviated by the beta-3 adrenergic antagonist SR59230A. Thus, the CPPC mouse model reliably recapitulates clinically and biologically relevant features of CPPCs and may be implemented to test underlying mechanisms and find new therapeutics.

Subcutaneous ω-Conotoxins Alleviate Mechanical Pain in Rodent Models of Acute Peripheral Neuropathy

The peripheral effects of ω-conotoxins, selective blockers of N-type voltage-gated calcium channels (CaV2.2), have not been characterised across different clinically relevant pain models. This study examines the effects of locally administered ω-conotoxin MVIIA, GVIA, and CVIF on mechanical and thermal paw withdrawal threshold (PWT) in postsurgical pain (PSP), cisplatin-induced neuropathy (CisIPN), and oxaliplatin-induced neuropathy (OIPN) rodent models. Intraplantar injection of 300, 100 and 30 nM MVIIA significantly (p < 0.0001, p < 0.0001, and p < 0.05, respectively) alleviated mechanical allodynia of mice in PSP model compared to vehicle control group. Similarly, intraplantar injection of 300, 100, and 30 nM MVIIA (p < 0.0001, p < 0.01, and p < 0.05, respectively), and 300 nM and 100 nM GVIA (p < 0.0001 and p < 0.05, respectively) significantly increased mechanical thresholds of mice in OIPN model. The ED50 of GVIA and MVIIA in OIPN was found to be 1.8 pmol/paw and 0.8 pmol/paw, respectively. However, none of the ω-conotoxins were effective in a mouse model of CisIPN. The intraplantar administration of 300 nM GVIA, MVIIA, and CVIF did not cause any locomotor side effects. The intraplantar administration of MVIIA can alleviate incision-induced mechanical allodynia, and GVIA and MVIIA effectively reduce OIPN associated mechanical pain, without locomotor side effects, in rodent models. In contrast, CVIF was inactive in these pain models, suggesting it is unable to block a subset of N-type voltage-gated calcium channels associated with nociceptors in the skin.

A novel player in the field: Merkel disc in touch, itch and pain

The mechanosensitive Merkel cell-neurite complex comprising two distinct cell types in both hairy and glabrous skin has been widely recognized as touch receptor for more than 100 years. In 2014, three elegant studies further demonstrated that the Merkel cell-neurite complex mediates touch transduction via the mechanosensitive Piezo2 channel. However, whether it is involved in genesis of itch and pain sensations, has been unclear. Recently, we reported that Merkel cells modulate the development of mechanical itch under the conditions of dry skin and aging, whereas two other studies demonstrated that Piezo2 channel mediates mechanical pain. In this assay, we summarized the current knowledge of Merkel disk under both normal and pathological conditions, with a focus on its role in touch, itch, and pain.

Peripheral and spinal mechanisms of nociception in a rat reserpine-induced pain model

Chronic widespread pain is a serious medical problem, yet the mechanisms of nociception and pain are poorly understood. Using a reserpine-induced pain model originally reported as a putative animal model for fibromyalgia, this study was undertaken to examine the following: (1) expression of several ion channels responsible for pain, mechanotransduction, and generation/propagation of action potentials in the dorsal root ganglion (DRG), (2) activities of peripheral nociceptive afferents, and (3) alterations in spinal microglial cells. A significant increase in mRNA expression of the acid-sensing ion channel (ASIC)-3 was detected in the DRG, and the behavioral mechanical hyperalgesia was significantly reversed by subcutaneous injection of APETx2, a selective blocker of ASIC3. Single-fiber recordings in vitro revealed facilitated mechanical responses of mechanoresponsive C-fibers both in the skin and muscle although the proportion of mechanoresponsive C-nociceptors was paradoxically decreased. In the spinal dorsal horn, microglial cells labeled with Iba1 immunoreactivity was activated, especially in laminae I-II where the nociceptive input is mainly processed compared with the other laminae. The activated microglia and behavioral hyperalgesia were significantly tranquilized by intraperitoneal injection of minocycline. These results suggest that the increase in ASIC3 in the DRG facilitated mechanical response of the remaining C-nociceptors and that activated spinal microglia may direct to intensify pain in this model. Pain may be further amplified by reserpine-induced dysfunction of the descending pain inhibitory system and by the decrease in peripheral drive to this system resulting from a reduced proportion of mechanoresponsive C-nociceptors.

Noradrenergic pain modulation

Norepinephrine is involved in intrinsic control of pain. Main sources of norepinephrine are sympathetic nerves peripherally and noradrenergic brainstem nuclei A1-A7 centrally. Peripheral norepinephrine has little influence on pain in healthy tissues, whereas in injured tissues it has variable effects, including aggravation of pain. Its peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ionic channel properties of primary afferent nociceptors, while an interaction with the immune system may contribute in part to peripheral antinociception induced by norepinephrine. In the spinal cord, norepinephrine released from descending pathways suppresses pain by inhibitory action on alpha-2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct alpha-2-adrenergic action on pain-relay neurons (postsynaptic inhibition), and by alpha-1-adrenoceptor-mediated activation of inhibitory interneurons. Additionally, alpha-2C-adrenoceptors on axon terminals of excitatory interneurons of the spinal dorsal horn possibly contribute to spinal control of pain. At supraspinal levels, the pain modulatory effect by norepinephrine and noradrenergic receptors has varied depending on many factors such as the supraspinal site, the type of the adrenoceptor, the duration of the pain and pathophysiological condition. While in baseline conditions the noradrenergic system may have little effect, sustained pain induces noradrenergic feedback inhibition of pain. Noradrenergic systems may also contribute to top-down control of pain, such as induced by a change in the behavioral state. Following injury or inflammation, the central as well as peripheral noradrenergic system is subject to various plastic changes that influence its antinociceptive efficacy.

Sympathetic sprouting and changes in nociceptive sensory innervation in the glabrous skin of the rat hind paw following partial peripheral nerve injury

Previous studies have suggested that sympathetic sprouting in the periphery may contribute to the development and persistence of sympathetically maintained pain in animal models of neuropathic pain. In the present study, we examined changes in the cutaneous innervation in rats with a chronic constriction injury to the sciatic nerve. At several periods postinjury, hind paw skin was harvested and processed by using a monoclonal antibody against dopamine-beta-hydroxylase to detect sympathetic fibers and a polyclonal antibody against calcitonin gene-related peptide to identify peptidergic sensory fibers. We observed migration and branching of sympathetic fibers into the upper dermis of the hind paw skin, where they were normally absent. This migration was first detected at 2 weeks, peaked at 4-6 weeks, and lasted for at least 20 weeks postlesion. At 8 weeks postlesion, there was a dramatic increase in the density of peptidergic fibers in the upper dermis. Quantification revealed that densities of peptidergic fibers 8 weeks postlesion were significantly above levels in sham animals. The ectopic sympathetic fibers did not innervate blood vessels but formed a novel association and wrapped around sprouted peptidergic nociceptive fibers. Our data show a long-term sympathetic and sensory innervation change in the rat hind paw skin after the chronic constriction injury. This novel fiber arrangement after nerve lesion may play an important role in the development and persistence of sympathetically maintained neuropathic pain after partial nerve lesions.

Interactions of bradykinin and norepinephrine on rat cutaneous nociceptors in both normal and inflamed conditions in vitro

Many inflammatory chemical mediators excite or sensitize nociceptors, which had led some researchers to believe that they may interact with each other to maintain a persistent painful state. We examined how the excitatory mediators norepinephrine (NE) and bradykinin (BK) interact, using single fiber recordings from cutaneous nociceptors. We observed that NE augmented the BK-induced response in both control and adjuvant-inflamed rats in a way different from NE-induced excitation in inflamed animals only. BK also tended to augment the NE-induced response (examined only in inflamed rats). Our results provide the first evidence that BK and NE synergistically interact on nociceptors.

Decreasing sympathetic sprouting in pathologic sensory ganglia: a new mechanism for treating neuropathic pain using lidocaine

Lidocaine brings relief to those suffering from certain neuropathic pain syndromes in humans and in animal models. Evidence suggests that some neuropathic pain behaviors are closely associated with extensive sprouting of noradrenergic sympathetic fibers in the dorsal root ganglia (DRG). Using immunohistochemistry, we examined lidocaine's effects on abnormal sprouting of sympathetic fibers in two animal models: rats with unilateral spinal nerve ligation (SNL) and rats with complete sciatic nerve transection (CSNT). For the first time, we have demonstrated that systemic lidocaine beginning at the time of surgery via an implanted osmotic pump remarkably reduces sympathetic sprouting (2-3 fold) (e.g. the density of sympathetic fibers and the number of DRG neurons surrounded by sympathetic fibers) in axotomized DRGs in SNL rats. The effects of systemic lidocaine lasted more than 7 days after the termination of lidocaine administration. Similar results were obtained after topical application of lidocaine to the nerve trunk to block abnormal discharges originating in the neuroma in CSNT rats. Results strongly suggest that sympathetic sprouting in pathologic DRG may be associated with abnormal spontaneous activity originating in the DRG or the injured axons (e.g. neuroma). This finding provides new insight into the mechanisms underlying sympathetic sprouting and increases our current understanding of the prolonged therapeutic effects of lidocaine on neuropathic pain syndromes.

Reversal of experimental neuropathic pain by T-type calcium channel blockers

Experimental nerve injury results in exaggerated responses to tactile and thermal stimuli that resemble some aspects of human neuropathic pain. Neuronal hyperexcitability and neurotransmitter release have been suggested to promote such increased responses to sensory stimuli. Enhanced activity of Ca(2+) current is associated with increased neuronal activity and blockade of N- and P-types, but not L-type, calcium channels have been found to block experimental neuropathic pain. While T-type currents are believed to promote neuronal excitability and transmitter release, it is unclear whether these channels may also contribute to the neuropathic state. Rats were prepared with L(5)/L(6) spinal nerve ligation, and tactile and thermal hypersensitivities were established. Mibefradil or ethosuximide was administered either intraperitoneally, intrathecally (i.th.), or locally into the plantar aspect of the injured hindpaw. Systemic mibefradil or ethosuximide produced a dose-dependent blockade of both tactile and thermal hypersensitivities in nerve-injured rats; responses of sham-operated rats were unchanged. Local injection of mibefradil also blocked both end points. Ethosuximide, however, was inactive after local administration, perhaps reflecting its low potency when compared with mibefradil. Neither mibefradil nor ethosuximide given i.th. produced any blockade of neuropathic behaviors. The results presented here suggest that T-type calcium channels may play a role in the expression of the neuropathic state. The data support the view that selective T-type calcium channel blockers may have significant potential in the treatment of neuropathic pain states.

Stress and the inflammatory response: a review of neurogenic inflammation

The subject of neuroinflammation is reviewed. In response to psychological stress or certain physical stressors, an inflammatory process may occur by release of neuropeptides, especially Substance P (SP), or other inflammatory mediators, from sensory nerves and the activation of mast cells or other inflammatory cells. Central neuropeptides, particularly corticosteroid releasing factor (CRF), and perhaps SP as well, initiate a systemic stress response by activation of neuroendocrinological pathways such as the sympathetic nervous system, hypothalamic pituitary axis, and the renin angiotensin system, with the release of the stress hormones (i.e., catecholamines, corticosteroids, growth hormone, glucagons, and renin). These, together with cytokines induced by stress, initiate the acute phase response (APR) and the induction of acute phase proteins, essential mediators of inflammation. Central nervous system norepinephrine may also induce the APR perhaps by macrophage activation and cytokine release. The increase in lipids with stress may also be a factor in macrophage activation, as may lipopolysaccharide which, I postulate, induces cytokines from hepatic Kupffer cells, subsequent to an enhanced absorption from the gastrointestinal tract during psychologic stress. The brain may initiate or inhibit the inflammatory process. The inflammatory response is contained within the psychological stress response which evolved later. Moreover, the same neuropeptides (i.e., CRF and possibly SP as well) mediate both stress and inflammation. Cytokines evoked by either a stress or inflammatory response may utilize similar somatosensory pathways to signal the brain. Other instances whereby stress may induce inflammatory changes are reviewed. I postulate that repeated episodes of acute or chronic psychogenic stress may produce chronic inflammatory changes which may result in atherosclerosis in the arteries or chronic inflammatory changes in other organs as well.

Measurement of nerve dysfunction in neuropathic pain

Neuropathic pain is difficult to diagnose and treat. Both can be made easier if the nerve dysfunction can be quantified. The two basic modes of evaluation are subjective and objective. This article presents currently used methods in both arenas. Discussion is also given to some basic neurophysiology necessary to understanding the use of these testing methods.

Distribution and regulation of alpha(2)-adrenoceptors in rat dorsal root ganglia

Using in situ hybridization with riboprobes the distribution of alpha(2A)-, alpha(2B)- and alpha(2C)-adrenoceptor mRNAs were studied in normal rat dorsal root ganglia and after unilateral peripheral nerve injury (total nerve transection) or inflammation. The most common adrenoceptor mRNA was of the alpha(2C) subtype (almost 80% of all neuron profiles) followed by the alpha(2A) subtype (almost 20%), whereas alpha(2B)-adrenoceptor mRNA was only found in small numbers of neuron profiles. The most dramatic effect of peripheral nerve injury was observed for the alpha(2A)-adrenoceptor mRNA, which increased to 45% of all neuron profiles. In contrast, alpha(2C) adrenoceptor mRNA showed a small decrease in this situation. Carrageenan-induced peripheral inflammation did not affect the percentage of alpha(2A)- or alpha(2C)-adrenoceptor mRNA-positive profiles. These findings suggest that, if any of the alpha(2) adrenoceptor, the alpha(2A) subtype represents the most likely candidate in DRG neurons to be involved in sympathetically maintained pain.

NSAID-induced cyclooxygenase inhibition differentially depresses long-lasting versus brief synaptically-elicited responses of rat spinal dorsal horn neurons in vivo

This electrophysiological study examined the effects of NSAID administration on synaptically-elicited responses of rat single spinal dorsal horn neurons to natural stimulation of peripheral receptive fields. Nociceptive responses consisted of a fast initial discharge during the stimulus followed by a slowly-decaying afterdischarge. The cyclooxygenase inhibitor, indomethacin (2.0-8.0 mg/kg, i.v.), was without effect on the on-going rate of discharge but dose-dependently inhibited synaptically-elicited responses to noxious cutaneous mechanical stimulation (fast initial discharge: n = 3/3 with 2 mg/kg, 5/8 with 4 mg/kg, 5/6 with 8 mg/kg; slowly-decaying afterdischarge: n = 3/3 with 2 mg/kg, 6/8 with 4 mg/kg, 6/6 with 8 mg/kg) and thermal (fast initial discharge: n = 7/9 with 8 mg/kg; slowly-decaying afterdischarge: n = 3/4 with 4 mg/kg, n = 7/9 with 8 mg/kg). The inhibitory effect of indomethacin started within 2-4 min and lasted up to 120 min. To eliminate any effect of indomethacin via cutaneous sensory receptors it was tested on the responses of some neurons to high intensity electrical stimulation of the sciatic nerve; indomethacin depressed these evoked responses (fast initial discharge: n = 5/6 with 2 mg/kg, n = 7/7 with 4 mg/kg; slowly-decaying afterdischarge: n = 6/6 with 2 mg/kg, n = 7/7 with 4 mg/kg). The brief excitatory responses to innocuous pressure (fast initial discharge: n = 2/3 with 2 mg/kg, n = 6/8 with 4 mg/kg, n = 4/6 with 8 mg/kg) and hair (n = 2/7 with 2 and 4 mg/kg, respectively) stimulation in both non-nociceptive and wide dynamic range neurons were also depressed but to a lesser extent. However, the prolonged excitation of three wide dynamic range neurons to continuous hair stimulation was almost entirely inhibited by indomethacin. Overall, inhibition of the afterdischarge and the excitatory effect of long-lasting synaptic input were greater than inhibition of the fast synaptic input-evoked initial discharge. The evidence supports the suggestion that systemically-administered indomethacin has an effect in the spinal cord and demonstrates an action specifically in the dorsal horn. The data are interpreted to suggest that sensory inputs are more involved than input-independent excitation of dorsal horn neurons in leading to de novo synthesis of eicosanoids and that the time course of this synthesis brings the levels to a point where COX inhibition can have an observable effect during prolonged excitation. Although the data suggest that COX inhibition differentially inhibits nociceptive versus non-nociceptive mechanisms at the cellular level, irrespective of the modality of the stimulus, this is the first direct demonstration that prolonged activation of synaptic mechanisms are preferentially inhibited. According to this it would be predictable that NSAIDs would be more effective on nociceptive types of pain characterized by time or prolonged inputs of primary afferents.

Effect of subcutaneous administration of calcium channel blockers on nerve injury-induced hyperalgesia

Recent studies suggest that calcium contributes to peripheral neural mechanisms of hyperalgesia associated with nerve damage. In this animal behavioural study, we examined further the contribution of calcium in neuropathic pain by testing whether subcutaneous administration of either a calcium chelating agent or voltage-dependent calcium channel blockers attenuate nerve injury-induced hyperalgesia to mechanical stimulation. Studies were carried out in animals with partially ligated sciatic nerves, an established animal model of neuropathic pain. The nociceptive flexion reflex was quantified using an Ugo Basile Analgesymeter. Partial nerve injury induced a significant decrease in mechanical threshold compared to the sham operated controls. Daily subcutaneous injections of the calcium chelating agent, Quin 2 (20 microgram/2.5 microliter), significantly attenuated the nerve injury-induced hyperalgesia. Similarly, SNX-111, a N-type channel blocker, also significantly attenuated the nerve injury-induced hyperalgesia. SNX-230, a P and/or Q-type channel blocker, and nifedipine, a L-type channel blocker, had no effect on the hyperalgesia to mechanical stimulation. In control experiments, SNX-111 had no effect on mechanical thresholds when administered subcutaneously in either the hindpaw of normal animals or the back of the neck in nerve injury animals. This study shows that neuropathic pain involves a local calcium-dependent mechanism in the receptive field of intact neurons of an injured nerve, since it can be alleviated by subcutaneous injections of either a calcium chelating agent or SNX-111, a N-type calcium channel blocker. These agents may be effective, peripherally acting therapeutic agents for neuropathic pain.

Multiple receptors involved in peripheral alpha 2, mu, and A1 antinociception, tolerance, and withdrawal

We examined the interactions among three classes of peripherally-acting antinociceptive agents (mu-opioid, alpha 2-adrenergic, and A1-adenosine) in the development of tolerance and dependence to their antinociceptive effects. Antinociception was determined by assessing the degree of inhibition of prostaglandin E2 (PGE2)-induced mechanical hyperalgesia, using the Randall-Selitto paw-withdrawal test. Tolerance developed within 4 hr to the antinociceptive effect of the alpha 2-adrenergic agonist clonidine; dependence also occurred at that time, demonstrated as a withdrawal hyperalgesia that was precipitated by the alpha 2-receptor antagonist yohimbine. These findings are similar to those reported previously for tolerance and dependence to mu and A1 peripheral antinociception (Aley et al., 1995). Furthermore, cross-tolerance and cross-withdrawal between mu, A1, and alpha 2 agonists occurred. The observations of cross-tolerance and cross-withdrawal suggest that all three receptors are located on the same primary afferent nociceptors. In addition, the observations suggest that the mechanisms of tolerance and dependence to the antinociceptive effects of mu, A1, and alpha 2 are mediated by a common mechanism. Although any of the agonists administered alone produce antinociception, we found that mu, A1, and alpha 2 receptors may not act independently to produce antinociception, but rather may require the physical presence of the other receptors to produce antinociception by any one agonist. This was suggested by the finding that clonidine (alpha 2-agonist) antinociception was blocked not only by yohimbine (alpha 2-antagonist) but also by PACPX (A1-antagonist) and by naloxone (mu-antagonist), and that DAMGO (mu-agonist) antinociception and CPA (A1-agonist) antinociception were blocked not only by naloxone (mu-antagonist) and PACPX (A1-antagonist), respectively, but also by yohimbine (alpha 2-antagonist). This cross-antagonism of antinociception occurred at the ID50 dose for each antagonist at its homologous receptor. To test the hypothesis that the physical presence of mu-opioid receptor is required not only for mu antinociception but also for alpha 2 antinociception, antisense oligodeoxynucleotides (ODNs) for the mu-opioid and alpha 2C-adrenergic receptors were administered intrathecally to reduce the expression of these receptors on primary afferent neurons. These studies demonstrated that mu-opioid ODN administration decreased not only mu-opioid but also alpha 2-adrenergic antinociception; A1 antinociception was unaffected. In contrast, alpha 2C-adrenergic ODN decreased antinociception induced by all three classes of antinociceptive agents. In conclusion, these data suggest that peripheral antinociception induced by mu, alpha 2, and A1 agonists requires the physical presence of multiple receptors. We propose that there is a mu, A1, alpha 2 receptor complex mediating antinociception in the periphery. In addition, there is cross-tolerance and cross-dependence between mu, A1, and alpha 2 antinociception, suggesting that their underlying mechanisms are related.

Complex regional pain syndromes: including "reflex sympathetic dystrophy" and "causalgia"

"Reflex sympathetic dystrophy" and "causalgia" are now classified by the International Association for the Study of Pain as Complex Regional Pain Syndromes I and II. Sympathetically maintained pain is a frequent but variable component of these syndromes, as the sympathetic and somatosensory pathways are no longer functionally distinct. Pain is the cardinal feature of CRPS, but the constellation of symptoms and signs may also include sensory changes, autonomic dysfunction, trophic changes, motor impairment and psychological changes. Diagnosis is based on the clinical picture, with additional information regarding the presence of sympathetically maintained pain or autonomic dysfunction being provided by carefully performed and interpreted supplemental tests. Clinical experience supports early intervention with sympatholytic procedures (pharmacological or nerve block techniques), but further scientific data is required to confirm the appropriate timing and relative efficacy of different procedures. Patients with recurrent or refractory symptoms are best managed in a multi-disciplinary pain clinic as more invasive and intensive treatment will be required to minimize ongoing pain and disability.

Clinical neurophysiology laboratory tests to assess the nociceptive system in humans

This paper presents some currently available neurophysiological tools that are helpful in the clinical setting to evaluate and document neuropathic disturbances that may be associated with pain. The specific tests described in this discussion are quantitative sensory tests (QSTs), autonomic tests (ATs), microneurography (MCNG), and laser evoked potentials (LEPs). Quantitative sensory testing of the nociceptive system includes the thermal stimulation (TST) and current perception threshold (CPT) tests. The ATs applicable to some patients with pain are sudomotor and vasomotor tests. The quantitative sudomotor axon reflex test (QSART), resting sweat output (RSO), and sympathetic skin response (SSR) are the tests for sudomotor involvement. The vasomotor system is tested by measuring skin temperature (surface thermistor or thermography) at rest and, in some cases, after provocative maneuvers. In addition, MCNG (intraneural recording of single nerve fibers or fascicles of nerves) allows examiners to look directly at muscle and skin sympathetic efferent output in normal subjects without pain or with experimental pain and in patients with neuropathic pain. This technique also provides a means of studying the physiology of primary afferent fibers in persons with neurogenic pain. Recent development of LEPs that incorporate the use of painful infrared laser-induced stimuli allow selective study of the nociceptive system, both the central and peripheral portions.

Interactions of sympathetic and primary afferent neurons following nerve injury and tissue trauma

Sympathetic post-ganglionic neurons may be involved in the generation of pain, hyperalgesia and inflammation under pathophysiological conditions. Two categories of influence of the sympathetic neuron on afferent neurons can be distinguished and this distinction seems to be related to whether the coupling between afferent and sympathetic neuron develops after nerve lesion or after tissue trauma with inflammation (Fig. 15): A. Peripheral nerve lesion generates plastic changes of the afferent and sympathetic postganglionic neurons, depending on the type of nerve lesion (e.g. complete, partial). Both afferent and post-ganglionic neurons exhibit degenerative and regenerative changes and unlesioned neurons may show collateral sprouting in the periphery as well as in the dorsal root ganglion. This reorganization of the peripheral neurons may lead to chemical coupling between sympathetic and afferent neurons. The coupling is responsible for sensitization and/or activation of primary afferent neurons by the sympathetic neurons. The mediator probably is norepinephrine, but other substances cannot be excluded. The afferent neuron expresses or upregulates functional adrenoceptors. The type of adrenoceptor involved is probably alpha 2. The coupling may occur at different sites of the primary afferent neuron, e.g. at the lesion site, remote from the lesion site in the dorsal root ganglion or between nonlesioned sympathetic and afferent neurons which show collateral sprouting. The biochemical signals which trigger these changes probably are neurotrophic substances, their receptors which are synthesized by the peripheral neurons, Schwann cells and other cells in response to the peripheral lesions. B. Sympathetic nerve terminals in peripheral tissues may serve as mediator elements in hyperalgesia and inflammation following tissue trauma without nerve lesion. Experiments show that these functions are largely independent of activity in the sympathetic neurons and independent of vesicular release of transmitter substances (such as norepinephrine). Sensitization of nociceptive afferents for mechanical stimuli and venular plasma extravasation in the synovium which are induced by the inflammatory mediator bradykinin are, at least in part, dependent on the sympathetic terminal. The signal to venules and afferent receptors is synthesized and released from the sympathetic terminal or in association with it. It is a prostaglandin (probably PGE2). Sympathetically mediated (neurogenic) inflammation and neurogenic inflammation mediated by afferents may interact reciprocally and enhance the inflammatory process as well as the sensitization of nociceptive afferents. Norepinephrine may also lead to sensitization of nociceptive afferents under inflammatory conditions. This sensitization is presumably mediated by alpha 2-adrenoceptors in the sympathetic varicosities and by a prostaglandin (probably PGI2) which is synthesized and released by or in association with the sympathetic varicosities.

Alpha 1-adrenoceptor-mediated sympathetically dependent mechanical hyperalgesia in the rat

The model of rolipram (a type IV phosphodiesterase inhibitor) induced prolongation (> 3 days) of the mechanical hyperalgesia produced by the intradermal injection of prostaglandin E2 in the hairy skin of the hindpaw of the rat, measured by the Randall-Selitto paw-withdrawal test, was employed to study mechanisms involved in the contribution of the sympathetic postganglionic neuron to mechanical hyperalgesia. Lumbar surgical sympathectomy prevented rolipram-induced prolongation of prostaglandin E2 hyperalgesia. Decentralization of sympathetic postganglionic neurons innervating the hindpaw did not, however, effect rolipram-induced prolongation of prostaglandin E2 hyperalgesia. Phentolamine, an alpha-adrenoceptor antagonist, and prazosin, an alpha 1-selective adrenoceptor antagonist, when given systemically or intradermally at the site of injection of prostaglandin E2 and rolipram, blocked rolipram-induced prolongation of prostaglandin E2 hyperalgesia. Intrathecal administration of phentolamine and prazosin were, however, without effect on rolipram-induced prolongation of prostaglandin E2 hyperalgesia. Yohimbine, an alpha 2-adrenoceptor antagonist given systemically, intradermally or intrathecally also did not produce any alteration in rolipram-induced prolongation of prostaglandin E2 hyperalgesia. We propose that sympathetic postganglionic neurons are involved in rolipram-induced prolongation of prostaglandin E2 hyperalgesia and that this form of sympathetically dependent hyperalgesia, which is independent of activity in preganglionic sympathetic neurons, is mediated by a peripheral alpha 1-adrenergic mechanism.