The glutamate synapse: a target in the pharmacological management of hyperalgesic pain states

Nerve growth factor and chronic daily headache: a potential implication for therapy

The pivotal role of nerve growth factor in inducing hyperalgesia and central sensitization has been emphasized in experimental pain models. Higher nerve growth factor levels have recently been found in the cerebrospinal fluid of patients with chronic daily headache. These levels were significantly correlated with the cerebrospinal fluid levels of substance P and calcitonin gene-related peptide, supporting the involvement of this neurotrophin in enhancing the production of the two sensory neuropeptides of the trigemino-vascular system in chronic daily headache. This may, in part, account for the long-lasting sensitization and activation of this system, which could contribute to headache chronicity. More recent research has shown a significant correlation between the higher cerebrospinal fluid levels of nerve growth factor and those of another neurotrophin, the brain-derived neurotrophic factor, as well as glutamate in chronic daily headache patients. These findings suggest the potential involvement of nerve growth factor-mediated upregulation of brain-derived neurotrophic factor in persistent head pain. Therefore, nerve growth factor appears to indirectly exert its effect through enhancing glutamatergic transmission involved in the processing of head pain via brain-derived neurotrophic factor. Based on these data, a potential application can be hypothesized for novel strategies targeting neurotrophins (nerve growth factor and brain-derived neurotrophic factor) and their receptors to chronic daily headache. To date, the majority of the molecules discovered in this regard have been scarcely or never proved in animal pain models and are far from clinical use in chronic pain, including chronic daily headache. If this approach is to be developed in the near future, research should be focused on identifying strategies with few central side effects and specific selective action on central sites involved in chronic head pain and more generally in chronic pain conditions. This will represent a very difficult challenge, taking into account the pleiotropic effect of nerve growth factor and the wide range of intracellular signalling pathways activated by this neurotrophin which are not limited to the nociceptive system.

Role of neuroinflammation in morphine tolerance: effect of tumor necrosis factor-α

Opioids have been used as potent analgesics in clinics for decades; however, their long-term administration leads to tolerance. Two possible mechanisms for drug tolerance are postulated as within-system and between-systems adaptation. The within-system tolerance is involved in the signal transduction of opioid receptors, including downregulation of opioid receptors, uncoupling of G-protein from opioid receptors, and β-arrestin recruitment to opioid receptors, which causes receptor desensitization and internalization/endocytosis. The between-systems tolerance comprehends the glutamatergic receptor system and glial activation with the release of proinflammatory cytokines, and thus the analgesic effect of morphine is reduced. Tumor necrosis factor-α (TNF-α) is a vital proinflammatory cytokine and exerts either a neurotoxic or neuroprotective effect on different diseases of the central nervous system. TNF-α has also been demonstrated to correlate with neuronal plasticity via activation of spinal glial cells and enhancement of glutamatergic transmission. Previous studies had revealed an increased expression of TNF-α in morphine tolerance. This review article focuses on the role of TNF-α in neuroinflammation and the glutamatergic receptor system in morphine tolerance. It may provide another adjuvant therapy for morphine tolerance, which extends the effectiveness of opioids in clinical pain management.

Spinal cord mechanisms mediating behavioral hyperalgesia induced by neurokinin-1 tachykinin receptor activation in the rostral ventromedial medulla

Hyperalgesia in animal injury models is linked to activation of descending raphespinal modulatory circuits originating in the rostral ventromedial medulla (RVM). A neurokinin-1 (NK-1) receptor antagonist microinjected into the RVM before or after inflammation produced by complete Freund's adjuvant (CFA) resulted in an attenuation of thermal hyperalgesia. A transient (acute) or a continuous infusion of Substance P (SP) microinjected into the RVM of non-inflamed animals led to similar pain hypersensitivity. Intrathecal pretreatment or post-treatment of a 5-HT3 receptor antagonist (Y-25130 or ondansetron) blocked the SP-induced hyperalgesia. The SP-induced hyperalgesia was both GABA(A) and NMDA receptor-dependent after pre- and post-treatment with selective antagonists at the spinal level. A microinjection of SP into the RVM also led to increased NMDA NR1 receptor subunit phosphorylation in spinal cord tissue. The GABA(A) receptor-mediated hyperalgesia involved a shift in the anionic gradient in dorsal horn nociceptive neurons and an increase in phosphorylated NKCC1 protein (isoform of the Na-K-Cl cotransporter). Following a low dose of SP infused into the RVM, intrathecal muscimol (GABA(A) agonist) increased SP-induced thermal hyperalgesia, phosphorylated NKCC1 protein expression, and NMDA NR1 subunit phosphorylation in the spinal cord. The thermal hyperalgesia was blocked by intrathecal gabazine, the GABA(A) receptor antagonist, and MK-801, the NMDA receptor channel blocker. These findings indicate that NK-1 receptors in the RVM are involved in SP-induced thermal hyperalgesia, this hyperalgesia is 5-HT3-receptor dependent at the spinal level, and involves the functional interaction of spinal GABA(A) and NMDA receptors.

Amitriptyline preserves morphine’s antinociceptive effect by regulating the glutamate transporter GLAST and GLT-1 trafficking and excitatory amino acids concentration in morphine-tolerant rats

The present study was undertaken to examine the effect of amitriptyline on the antinociceptive effect of morphine and its underlying mechanisms in regulating glutamate transporters trafficking in morphine-tolerant rats. Long-term morphine infusion induced antinociceptive tolerance and down-regulation of glutamate transporters (GTs), GLAST, GLT-1, and EAAC1, expression in the rat spinal cord dorsal horn. Acute amitriptyline treatment potentiated morphine's antinociceptive effect, with a 5.3-fold leftward shift of morphine's dose-response curve in morphine-tolerant rats, and this was associated with GLAST and GLT-1 trafficking onto the cell surface. Similar to our previous studies, morphine challenge (10 microg/10 microl, i.t.) significant by increased the excitatory amino acids (EAAs) aspartate and glutamate level in the CSF dialysates of morphine-tolerant rats. Acute amitriptyline treatment not only suppressed this morphine-evoked EAA release, but further reduced the EAA concentration than baseline level. Furthermore, long-term morphine infusion up-regulated PKA and PKC protein expression in the spinal cord dorsal horn, while amitriptyline inhibited the increase in expression of phospho-PKA, PKCalpha, PKCbetaII, and PKCgamma. In morphine-tolerant rats, acute treatment with PKA inhibitor H89 and PKC inhibitor Gö6805 attenuated morphine tolerance and the morphine-induced CSF glutamate and aspartate elevation, and induced trafficking of GLAST and GLT-1 from cytosol onto the cell surface. These results show that acute amitriptyline treatment preserved morphine's antinociceptive effect in morphine-tolerant rats; the mechanisms may be involved in inhibition of phospho-PKA and PKC expression, and thus inducing the GLAST and GLT-1 trafficking onto glial cell surface which enhances the EAA uptake from the synaptic cleft and reduces EAA concentration in the spinal CSF.

Amitriptyline suppresses neuroinflammation and up-regulates glutamate transporters in morphine-tolerant rats

The present study was performed to evaluate the effects of the tricyclic antidepressant amitriptyline on morphine tolerance in rats. Male Wistar rats were implanted with two intrathecal (i.t.) catheters with or without a microdialysis probe, then received a continuous i.t. infusion of saline (control) or morphine (15 microg/h) and/or amitriptyline (15 microg/h) for 5 days. The results showed that amitriptyline alone did not produce an antinociceptive effect, while morphine alone induced antinociceptive tolerance and down-regulation of spinal glutamate transporters (GLAST, GLT-1, and EAAC1) in the rat spinal cord dorsal horn. Co-administration of amitriptyline with morphine attenuated morphine tolerance and up-regulated GLAST and GLT-1 expression. On day 5, morphine challenge (10 microg/10 microl) resulted in a significant increase in levels of the excitatory amino acids (EAAs), aspartate and glutamate, in CSF dialysates in morphine-tolerant rats. Amitriptyline co-infusion not only markedly suppressed this morphine-evoked EAA release, but also preserved the antinociceptive effect of acute morphine challenge at the end of infusion. Glial cells activation and increased cytokine expression (TNFalpha, IL-1beta, and IL-6) in the rat spinal cord were induced by the 5-day morphine infusion and these neuroimmune responses were also prevented by amitriptyline co-infusion. These results show that amitriptyline not only attenuates morphine tolerance, but also preserves its antinociceptive effect. The mechanisms involved may include: (a) inhibition of pro-inflammatory cytokine expression, (b) prevention of glutamate transporter down-regulation, and even up-regulation of glial GTs GLAST and GLT-1 expression, with (c) attenuation of morphine-evoked EAA release following continuous long-term morphine infusion.

The role of neurokinin 1 receptors in the maintenance of visceral hyperalgesia induced by repeated stress in rats

BACKGROUND & AIMS

The neurokinin 1 receptors (NK(1)Rs) and substance P (SP) have been implicated in the stress and/or pain pathways involved in chronic pain conditions. Here we examined the participation of NK(1)Rs in sustained visceral hyperalgesia observed in rats exposed to chronic psychological stress.

METHODS

Male Wistar rats were exposed to daily 1-hour water avoidance stress (WA) or sham WA for 10 consecutive days. We tested intraperitoneal or intrathecal injection of the NK(1)R antagonist SR140333 on the visceromotor reflex to colorectal distention in both groups at day 11. Real-time reverse-transcription polymerase chain reaction, Western blot, and immunohistochemistry were used to assess the expression of NK(1)Rs and/or SP in samples of colon, spinal cord, and dorsal root ganglia.

RESULTS

Both intraperitoneal and intrathecal SR140333 injection diminished the enhanced visceromotor reflex to colorectal distention at day 11 in stressed rats but did not affect the response in control animals. Real-time polymerase chain reaction and Western blotting demonstrated stress-induced up-regulation of spinal NK(1)Rs. Immunohistochemistry showed an increased number of NK(1)R-expressing neurons in the laminae I of the dorsal horn in stressed rats. The expression of NK(1)Rs was decreased in colon from stressed rats compared with control. The expression of SP gene precursor in dorsal root ganglia was unchanged in stressed rats compared with controls.

CONCLUSIONS

Stress-induced increased NK(1)R expression on spinal neurons and the inhibitory effect of intrathecal NK(1)R antagonist on visceral hyperalgesia support the key contribution of spinal NK(1)Rs in the molecular pathways involved in the maintenance of visceral hyperalgesia observed after chronic WA.

Mustard oil has differential effects on the response of trigeminal caudalis neurons to heat and acidity

Topical application of mustard oil (allyl isothiocyanate) to the skin or injection into joints induces hyperalgesia, allodynia, and neuroinflammation. However, when applied to the oral or nasal mucosa, mustard oil evokes a desensitizing pattern of irritation. Presently we investigated the responses of neurons in superficial laminae of trigeminal subnucleus caudalis (Vc) to noxious thermal (53 degrees C) and chemical (pentanoic acid; 200 mM) stimuli prior to and following lingual mustard oil application. A low concentration of mustard oil (0.125%) applied by constant flow (0.5 ml/min; 15 min), initially excited Vc neurons followed by partial desensitization. Responses to noxious heat were unchanged following mustard oil. A high concentration of mustard oil (1.25%) initially excited Vc neurons followed quickly (within 20 s) by nearly complete desensitization. The desensitization was transient since reapplication of mustard oil approximately 20 min later elicited a comparable response that also rapidly desensitized. Mustard oil also transiently cross-desensitized Vc responses to pentanoic acid (to 52%), in striking contrast to noxious heat-evoked responses which were significantly sensitized to approximately 160% of pre-mustard oil levels. The data suggest that the effect of mustard oil on subsequent lingual nociceptive responses is concentration dependent, transient, and modality specific.

Protein kinases regulate the phosphorylation of the GluR1 subunit of AMPA receptors of spinal cord in rats following noxious stimulation

The present project was designed to investigate the role of protein kinase A (PKA) and protein kinase C (PKC) in the regulation of phosphorylation of the GluR1 subunits of AMPA receptors in the spinal cord of rats after capsaicin injection. We found that after capsaicin injection, a significant upregulation of phosphorylated GluR1 both at Ser(831) and Ser(845) was detected on the side ipsilateral to the injection. Intrathecal treatment with a PKA inhibitor, H89 ([N-[2-((3-bromophenyl)-2-propenyl)amino)ethyl]-5-isoquinoline sulfonamide, HCl), or a PKC inhibitor, NPC15473 (2,6-diamino-N-([1-oxotridecyl)-2-piperidinyl]methyl)hexanamide), significantly blocked the increased phosphorylation at different serine sites without affecting the GluR1 protein itself. Our results suggest that increased phosphorylation of the GluR1 subunit of AMPA receptors contributes to central sensitization following acute peripheral inflammation, and the effect may occur at different phosphorylation sites through the activation of the PKA or PKC protein kinase cascades.

Positive N-methyl-d-aspartate receptor modulation by selective glycine transporter-1 inhibition in the rat dorsal spinal cord in vivo

In this study we have employed the selective glycine transporter-1 (GlyT-1) and GlyT-2 transporter inhibitors R-(-)-N-methyl-N-[3-[(4-trifluoromethyl)phenoxy]-3-phenyl-propyl]glycine (1:1) lithium salt (Org 24598) and 4-benzyloxy-3,5-dimethoxy-N-[1-(dimethylaminocyclopently)methyl]benzamide (Org 25543), respectively, and microdialysis perfusion to determine the effect of GlyT transporter inhibition on extracellular amino acid concentrations in the lumbar dorsal spinal cord of the halothane-anaesthetised rat. Reverse dialysis of Org 24598 (0.1-10 microM) induced a concentration-related increase in extracellular glycine accompanied by a progressive increase in citrulline, but not aspartate, glutamate or GABA, efflux. Org 25543 (10 microM) by the same route induced a similar increase in glycine levels without affecting the efflux of other amino acids quantified. To test the hypothesis that the increase in citrulline efflux resulted from activation of the N-methyl-D-aspartate receptor (NMDA-R)/nitric oxide synthase (NOS) signalling cascade, the sensitivity was determined of GlyT-1 inhibition-induced effects to NMDA-R antagonism or NOS inhibition. Co-administration by reverse dialysis of the selective NMDA-R channel blocker MK-801 (0.5 mM) or the selective antagonist of the strychnine-insensitive glycine site, 7-chlorokynurenic acid (1 mM), with Org 24598 (10 microM) did not affect the uptake inhibition-induced increase in glycine efflux, but did significantly attenuate the increase in extracellular citrulline. Similarly, co-administration with Org 24598 of the isoform non-selective and selective neuronal NOS inhibitors Nomega-nitro-L-arginine methyl ester (1 mM) or 1-(2-trifluoromethylphenyl)imidazole (0.2 mM), respectively, prevented Org 24598-induced citrulline efflux with no effect on increased glycine efflux. These data provide evidence that the observed increased in extracellular citrulline is a consequence of positive modulation of NMDA-R, secondary to increased extracellular glycine and support a protective role for GlyT-1 against fluctuations in extracellular glycine uptake at glutamatergic synapses in the dorsal spinal cord. Such a mechanism could be important to NMDA-R-mediated synaptic plasticity in the spinal cord and be of relevance to the clinical usage of GlyT-1 inhibitors.

Presynaptic mechanism for anti-analgesic and anti-hyperalgesic actions of kappa-opioid receptors

Glutamate neurotransmission plays an important role in the processing of pain and in chronic opioid-induced neural and behavioral plasticity, such as opioid withdrawal and opioid dependence. Kappa-opioid receptors also have been implicated in acute opioid modulation of pain and chronic opioid-induced plasticity, both of which are primarily mediated by mu-opioid receptors. Using whole-cell patch clamp recordings in brain slices in vitro and system analysis of pain behaviors in rats in vivo, this study investigated the functional role of glutamate synaptic transmission and kappa-opioid receptors in two behavioral pain conditions: m-opioid-induced analgesia (decreased pain) and mu-opioid withdrawal-induced hyperalgesia (increased pain). In the nucleus raphe magnus (NRM), a brainstem structure that controls spinal pain transmission, we found that kappa-receptor agonists presynaptically inhibited glutamate synaptic currents in both of the two cell types that are thought to respectively inhibit or facilitate spinal pain transmission. In rats, both glutamate receptor antagonists and the kappa agonist microinjected into the NRM attenuated mu-opioid-induced analgesia, which is most likely mediated through activation of such pain-inhibiting neurons. However, during opioid abstinence-induced withdrawal, the same doses of glutamate receptor antagonists and the kappa agonist administered in the NRM suppressed the withdrawal-induced hyperalgesia, which is thought to be mediated by activation of those pain-facilitating neurons during opioid withdrawal. These results demonstrate that kappa-opioid receptors antagonize mu-receptor-induced effects in both analgesic and hyperalgesic states, and suggest inhibition of glutamate synaptic transmission as a presynaptic mechanism for the kappa antagonism of these two mu receptor-mediated actions.

Median nerve trauma in a rat model of work-related musculoskeletal disorder

Anatomical and physiological changes were evaluated in the median nerves of rats trained to perform repetitive reaching. Motor degradation was evident after 4 weeks. ED1-immunoreactive macrophages were seen in the transcarpal region of the median nerve of both forelimbs by 5-6 weeks. Fibrosis, characterized by increased immunoexpression of collagen type I by 8 weeks and connective tissue growth factor by 12 weeks, was evident. The conduction velocity (NCV) within the carpal tunnel showed a modest but significant decline after 9-12 weeks. The lowest NCV values were found in animals that refused to participate in the task for the full time available. Thus, both anatomical and physiological signs of progressive tissue damage were present in this model. These results, together with other recent findings indicate that work-related carpal tunnel syndrome develops through mechanisms that include injury, inflammation, fibrosis and subsequent nerve compression.

SYM 2081, an agonist that desensitizes kainate receptors, attenuates capsaicin and inflammatory hyperalgesia

Excitatory amino acids acting at non-NMDA receptors contribute to transmission of nociceptive information. SYM 2081 ((2S,4R)-4-methyl glutamic acid) desensitizes kainate receptors, one subtype of non-NMDA receptors, to subsequent release of excitatory amino acids and thus may attenuate transmission of nociceptive information. To determine if SYM 2081 can prevent development of hyperalgesia, SYM 2081 (10, 50 or 100 mg/kg, i.p.) was administered prior to injection of capsaicin into the hindpaw of rats, which produces mechanical and heat hyperalgesia. To determine if SYM 2081 can reduce ongoing inflammatory hyperalgesia, SYM 2081 (10 or 100 mg/kg, i.p.) was administered after development of carrageenan-evoked hyperalgesia. Intraplantar injection of capsaicin produced an increase in hindpaw withdrawal frequency to mechanical stimuli (from 4+/-2 to 41+/-7%; mean+/-S.E.M.) and a decrease in withdrawal latency to heat (from 12.3+/-0.3 to 5.9+/-0.4 s) in rats that received vehicle. In contrast, rats that received SYM 2081 (100 mg/kg) prior to injection of capsaicin exhibited a lower hindpaw withdrawal frequency (18+/-4%) and a longer withdrawal latency (7.7+/-0.5 s). Intrathecal (1-100 microg/5 microl), but not intraplantar (10 or 100 microg/50 microl), injection of SYM 2081 attenuated the development of capsaicin-evoked heat hyperalgesia suggesting that SYM 2081's antihyperalgesic effects were due to its central effects. Furthermore, SYM 2081 completely reversed ongoing carrageenan-evoked mechanical hyperalgesia and partially (approximately 50%) reversed ongoing heat hyperalgesia. The present study demonstrates that administration of a high-potency ligand that selectively desensitizes kainate receptors attenuates the development of mechanical and heat hyperalgesia and attenuates ongoing inflammatory hyperalgesia.

Corticosterone acts directly at the amygdala to alter spinal neuronal activity in response to colorectal distension

Administration of glucocorticoids to the amygdaloid nucleus facilitates visceromotor responses to colorectal distension in rats. The aim of this study was to determine if colorectal hypersensitivity develops through central modulation of spinal neuronal activity. Stereotaxic delivery of corticosterone (n = 10) or cholesterol (control, n = 10) onto the dorsal margin of the amygdala was performed on male Fischer-344 rats. Seven days later, extracellular potentials of single L(6)-S(1) spinal neurons were examined for responses to colorectal distension (CRD, 20-80 mmHg, 20 s) in sodium pentobarbital anesthetized and paralyzed animals. The proportions of neurons that responded to noxious CRD in corticosterone-implanted (62/186, 33%) and cholesterol-implanted (55/163, 34%) animals were virtually identical. However, the mean excitatory response of spinal neurons to CRD in corticosterone-treated rats was significantly greater (26.7 +/- 2.2 vs. 16.4 +/- 1.8 imp/s, P < 0.01) and the duration was longer (37.0 +/- 3.9 vs. 25.8 +/- 1.5 s, P < 0.05) than in the control group. No significant differences were found in neural responses to nonnoxious and noxious mechanical stimulation of somatic fields between corticosterone-implanted and control groups. In conclusion, our data support the hypothesis that central stimulation of the amygdala by corticosterone sensitizes the lumbosacral spinal neurons that mediate visceromotor reflexes to CRD.

Peripheral metabotropic glutamate receptors as drug targets for pain relief

The relatively new family of G-protein-coupled metabotropic glutamate receptors (mGluRs) is comprised of eight cloned subtypes, which are classified into three groups based on their sequence homology, signal transduction mechanisms and receptor pharmacology. It is now well-established that mGluRs in the central nervous system are essential for neuroplasticity associated with normal brain functions but are also critically involved in various neurological and psychiatric disorders. Recent anatomical and behavioural evidence suggests an important role of mGluRs in peripheral tissues in animal models of inflammatory and neuropathic pain. Once the cellular effects of peripheral mGluR activation and inhibition are better understood, certain peripheral mGluR subtypes may become important novel therapeutic targets for the relief of pain associated with peripheral tissue injury. Peripherally acting drugs that modulate nociceptive processing through mGluRs should have the advantage of lacking the central side effects commonly observed with drugs interfering with glutamatergic transmission in the central nervous system.

The neuropharmacology of centrally-acting analgesic medications in fibromyalgia

As demonstrated above, the anatomy and neuropharmacology of the pain pathways within the CNS, even to the level of the midbrain, are extraordinarily complex. Indeed, discussions of the effects of these agents on the neuropharmacology of the thalamus, hypothalamus, and cortex were excluded from this review owing to their adding further to this complexity. Also, the dearth of data regarding FMS pain pathophysiology necessitated a relatively generic analysis of the pain pathways. As mentioned in the introduction, the current thought is that central sensitization plays an important role in FMS. However, we see in this chapter that the behavioral state of central sensitization may be a result of alterations in either the ascending systems or in one or more descending systems. Studies to assess the presence or relative importance of such changes in FMS are difficult to perform in humans, and to date there are no animal models of FMS. Accepting these limitations, it is apparent that many drugs considered to date for the treatment of FMS do target a number of appropriate sites within both the ascending and descending pain pathways. The data regarding clinical efficacy on some good candidate agents, however, is extremely preliminary. For example, it is evident from the present analysis that SNRIs, alpha 2 agonists, and NK1 antagonists may be particularly well suited to FMS, although current data supporting their use is either anecdotal or from open-label trials [114,149]. Other sites within the pain pathways have not yet been targeted. Examples of these include the use of CCKB antagonists to block on-cell activation or of nitric oxide synthetase antagonists to block the downstream mediators of NMDA activation. Efficacy of such agents may give considerable insight into the pathophysiology of FMS. Finally, as indicated previously, FMS consists of more than just chronic pain, and the question of how sleep abnormalities, depression, fatigues, and so forth tie into disordered pain processing is being researched actively. Future research focusing on how the various manifestations of FMS relate to one another undoubtedly will lead to a more rational targeting of drugs in this complex disorder.

Altered glutamate receptor function during recovery of bladder detrusor-external urethral sphincter coordination in a rat model of spinal cord injury

Coordination of the bladder detrusor and the external urethral sphincter is a supraspinally controlled reflex that is essential for efficient micturition. This coordination is permanently lost after spinal cord transection but can recover chronically after incomplete spinal cord injury (SCI). As glutamatergic transmission plays a key role in all levels of detrusor-external urethral sphincter coordination, we examined the role of potential alterations in glutamatergic control in its recovery after SCI. Rats were subjected to standardized incomplete contusion injury. Detrusor-external urethral sphincter coordination was evaluated urodynamically at 5 days (subacute) and 8 weeks (chronic) after SCI. Sensitivity of coordinated activation of the external urethral sphincter in response to bladder distension to the alpha -amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid/kainate antagonist 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo(f)quinoxaline-7-sulfonamide disodium (NBQX) and to the N-methyl-D-aspartate (NMDA) antagonist R(--3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP) was determined by intrathecal application at the L6 spinal cord level during urodynamic recordings. We found that while detrusor contractions recovered at 5 days after SCI, coordinated activation of the external urethral sphincter was significantly impaired at 5 days and recovered only by 8 weeks. There was no difference in sensitivity of detrusor-external urethral sphincter coordination to NBQX at the subacute or chronic time points. However, external urethral sphincter response to bladder distension was sensitive to a 50% lower dose of CPP at 5 days compared with uninjured rats or chronic recovered SCI rats. Thus, alterations in NMDA receptor function appeared to be involved in recovery of detrusor-external urethral sphincter coordination after incomplete SCI.

Pathophysiological tissue changes associated with repetitive movement: a review of the evidence

Work-related musculoskeletal disorders (WMSDs) represent approximately one third of workers' compensation costs in US private industry, yet estimates of acceptable exposure levels for forceful and repetitive tasks are imprecise, in part, due to lack of measures of tissue injury in humans. In this review, the authors discuss the scope of upper-extremity WMSDs, the relationship between repetition rate and forcefulness of reaching tasks and WMSDs, cellular responses to injury in vivo and in vitro, and animal injury models of repetitive, forceful tasks. The authors describe a model using albino rats and present evidence related to tissue injury and inflammation due to a highly repetitive reaching task. A conceptual schematic for WMSD development and suggestions for further research are presented. Animal models can enhance our ability to predict risk and to manage WMSDs in humans because such models permit the direct observation of exposed tissues as well as motor behavior.

Glutamate, NMDA, and AMPA induced changes in extracellular space volume and tortuosity in the rat spinal cord

Glutamate release, particularly in pathologic conditions, may result in cellular swelling. The authors studied the effects of glutamate, N-methyl-D-aspartate (NMDA), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) on extracellular pH (pH(e)), extracellular potassium concentration ([K(+)](e)), and changes in extracellular space (ECS) diffusion parameters (volume fraction alpha, tortuosity lambda) resulting from cellular swelling. In the isolated spinal cord of 4-to 12-day-old rats, the application of glutamate receptor agonists induced an increase in [K(+)](e), alkaline-acid shifts, a substantial decrease in alpha, and an increase in lambda. After washout of the glutamate receptor agonists, alpha either returned to or overshot normal values, whereas lambda remained elevated. Pretreatment with 20 mmol/L Mg(++), MK801, or CNQX blocked the changes in diffusion parameters, [K(+)](e) and pH(e) evoked by NMDA or AMPA. However, the changes in diffusion parameters also were blocked in Ca(2+)-free solution, which had no effect on the [K(+)](e) increase or acid shift. The authors conclude that increased glutamate release may produce a large, sustained and [Ca(2+)](e)-dependent decrease in alpha and increase in lambda. Repetitive stimulation and pathologic states resulting in glutamate release therefore may lead to changes in ECS volume and tortuosity, affecting volume transmission and enhancing glutamate neurotoxicity and neuronal damage.

Spinal NK1 receptor is upregulated after chronic bladder irritation

It has been suggested that there is a significant upregulation of the NK1 receptor (NK1R) on neurons in the dorsal spinal cord after long-term somatic inflammation. This upregulation appears to play a significant role in central sensitization in chronic pain states. However, it is not clear whether such a change is also observed after chronic visceral (bladder) inflammation. Changes in NK1R immunoreactivity after chronic bladder irritation were investigated in order to evaluate the existence of hypersensitive states in the spinal cord after chronic bladder irritation. Experiments were performed on a total of 12 adult female Sprague-Dawley rats. In six animals, cyclophosphamide (CPA) was administered intraperitoneally for 2 weeks. Another six animals were given intraperitoneal saline injections and served as the control group. After these treatments, immunohistochemical staining for NK1Rs and substance P in rat lumbosacral spinal cord was performed. In CPA-treated animals, NK1R-positive areas and staining intensity within the dorsal spinal cord were significantly increased in the L5 to S2 spinal cord areas, especially in the L6 and S1 segments. In the L6 spinal segment, CPA-treatment enhanced NK1R immunostaining in the medial and the lateral dorsal horn, as well as in the lateral laminae including the sacral parasympathetic nucleus to a lesser extent. In CPA-treated animals, substance P staining intensity increased in the same regions in which NK1R immunoreactivity was increased. This finding probably implies the upregulation of spinal NK1R and the occurrence of central sensitization within the spinal cord after chronic visceral inflammation.

Role of peripheral N-methyl-D-aspartate (NMDA) receptors in visceral nociception in rats

BACKGROUND & AIMS

N-methyl-D-aspartate (NMDA) receptors are ligand-gated ion channels that have an important role in long-term potentiation and memory processing in the central nervous system. The aims in this study were to determine whether NMDA receptors are expressed in the peripheral nervous system and identify their role in mediating behavioral pain responses to colonic distention in the normal gut.

METHODS AND RESULTS

Immunohistochemical localization of the NR1 subunit showed that NMDA receptors are expressed on the cell bodies and peripheral terminals of primary afferent nerves innervating the colon. Dorsal root ganglia neurons retrogradely labeled from the colon in short-term culture responded to addition of NMDA with increased intracellular [Ca2+]. Activation of peripheral NMDA receptors in colonic tissue sections caused Ca2+-dependent release of the proinflammatory neuropeptides, calcitonin gene-related peptide and substance P. Behavioral pain responses to noxious mechanical stimulation were inhibited in a reversible, dose-dependent manner by intravenous administration of memantine, a noncompetitive antagonist of the NMDA receptor. Single fiber recordings of decentralized pelvic nerves showed that colorectal distention responsive afferent nerve activity was inhibited by memantine.

CONCLUSIONS

Peripheral NMDA receptors are important in normal visceral pain transmission, and may provide a novel mechanism for development of peripheral sensitization and visceral hyperalgesia.

Spinal substance P receptor expression and internalization in acute, short-term, and long-term inflammatory pain states

Inflammatory pain involves the sensitization of both primary afferent and spinal cord neurons. To explore the neurochemical changes that contribute to inflammatory pain, we have examined the expression and ligand-induced internalization of the substance P receptor (SPR) in the spinal cord in acute, short-term, and long-term inflammatory pain states. These inflammatory models included unilateral injection of formalin (8-60 min), carrageenan (3 hr), and complete Freund's adjuvant (CFA; 3 d) into the rat hindpaw as well as adjuvant-induced polyarthritis (21 d). In acute inflammatory pain there is ongoing release of substance P (SP) as measured by SPR internalization in lamina I neurons at both 8 and 60 min after formalin injection. Although there is no tonic release of SP in short-term inflammatory pain, at 3 hr after carrageenan injection, SP is released in response to both noxious and non-noxious somatosensory stimulation with SPR internalization being observed in neurons located in both laminae I and III-IV. In long-term inflammatory pain models (CFA and polyarthritis) the same pattern of SP release and SPR activation occurs as is observed in short-term inflammation with the addition that there is a significant upregulation of the SPR in lamina I neurons. These results suggest that SPR internalization might serve as a marker of the contribution of ongoing primary afferent input in acute and persistent pain states. These stereotypical neurochemical changes suggest that there are unique neurochemical signatures for acute, short-term, and long-term inflammatory pain.

Supraspinal contributions to hyperalgesia

Tissue injury is associated with sensitization of nociceptors and subsequent changes in the excitability of central (spinal) neurons, termed central sensitization. Nociceptor sensitization and central sensitization are considered to underlie, respectively, development of primary hyperalgesia and secondary hyperalgesia. Because central sensitization is considered to reflect plasticity at spinal synapses, the spinal cord has been the principal focus of studies of mechanisms of hyperalgesia. Not surprisingly, glutamate, acting at a spinal N-methyl-D-aspartate (NMDA) receptor, has been implicated in development of secondary hyperalgesia associated with somatic, neural, and visceral structures. Downstream of NMDA receptor activation, spinal nitric oxide (NO.), protein kinase C, and other mediators have been implicated in maintaining such hyperalgesia. Accumulating evidence, however, reveals a significant contribution of supraspinal influences to development and maintenance of hyperalgesia. Spinal cord transection prevents development of secondary, but not primary, mechanical and/or thermal hyperalgesia after topical mustard oil application, carrageenan inflammation, or nerve-root ligation. Similarly, inactivation of the rostral ventromedial medulla (RVM) attenuates hyperalgesia and central sensitization in several models of persistent pain. Inhibition of medullary NMDA receptors or NO. generation attenuates somatic and visceral hyperalgesia. In support, topical mustard oil application or colonic inflammation increases expression of NO. synthase in the RVM. These data suggest a prominent role for the RVM in mediating the sensitization of spinal neurons and development of secondary hyperalgesia. Results to date suggest that peripheral injury and persistent input engage spinobulbospinal mechanisms that may be the prepotent contributors to central sensitization and development of secondary hyperalgesia.