Effects of amitriptyline and gabapentin on bilateral hyperalgesia observed in an animal model of unilateral axotomy

Nociceptive responses in an animal model of peripheral nerve injury were studied. The left common sciatic nerve was exposed, tightly ligated at two locations and transected between the ligatures. A bilateral decrease in the nociceptive threshold to mechanical stimulation was observed within 3 h after the operation. The skin of the lateral dorsal part of the injured paw was hypoalgesic, while the medial dorsal paw innervated by the intact saphenous nerve and the contralateral dorsal paw exhibited hyperalgesia. Amitriptyline, an antidepressant, at 25, 50 and 100 mg/kg per day, p.o., and gabapentin, an anticonvulsant, at 30, 100 and 300 mg/kg per day, p.o., significantly inhibited the decrease in the mechanical nociceptive threshold in the injured and uninjured paws. The effects of amitriptyline at 25 and 50 mg/kg were evident at doses that did not cause neurologic deficits as assessed by the inclined screen test. Indomethacin, a cyclooxygenase inhibitor, and morphine (except at the highest dose of 30 mg/kg, s.c.) showed no analgesic effects in this model. The tail-flick latency was also significantly decreased compared with intact rats. Similar bilateral hyperalgesia was observed when axotomy was performed using silk thread instead of chromic gut. When this axotomy model was applied to mice, the nociceptive thresholds in both paws immediately showed a significant decrease in the same manner as in rats. The bilateral and systemic hyperalgesia observed in this axotomy model, which resembles the clinical features of chronic neuropathic pain, suggests the involvement of the central nervous system in the maintenance of the chronic pain state.

Medullary N-type and P/Q-type calcium channels contribute to neuropathy-induced allodynia

The present study was designed to determine the contribution of N-type, P/Q-type and L-type calcium channels in the rostral ventromedial medulla to tactile allodynia following peripheral nerve injury. L5/L6 spinal nerve ligation in rats produced tactile allodynia, which was dose-dependently inhibited by intrarostral ventromedial medulla microinjection of the N-type calcium channel antagonist omega-conotoxin MVIIA. Similarly, intrarostral ventromedial medulla microinjection of the P/Q-type calcium channel antagonist omega-agatoxin IVA inhibited spinal nerve ligation-induced tactile allodynia, whereas intrarostral ventromedial medulla microinjection of the L-type calcium channel antagonist nimodipine had no effect. These results demonstrate that N-type and P/Q-type calcium channels in the rostral ventromedial medulla contribute to tactile allodynia following peripheral neuropathy, likely via neurotransmitter-mediated activation of descending facilitatory systems from the rostral ventromedial medulla.

Protease-activated receptors and inflammatory hyperalgesia

Recent advances in basic science pointed to a role for proteinases, through the activation of proteinase-activated receptors (PARs) in nociceptive mechanisms. Activation of PAR1, PAR2 and PAR4 either by proteinases or by selective agonists causes inflammation inducing most of the cardinal signs of inflammation: swelling, redness, and pain. Sub-inflammatory doses of PAR2 agonist still induced hyperalgesia and allodynia while PAR2 has been shown to be implicated in the generation of hyperalgesia in different inflammatory models. In contrast, sub-inflammatory doses of PAR1 increases nociceptive threshold, inhibiting inflammatory hyperalgesia, thereby acting as an analgesic agent. PARs are present and functional on sensory neurons, where they participate either directly or indirectly to the transmission and/or inhibition of nociceptive messages. Taken together, the results discussed in this review highlight proteinases as signaling molecules to sensory nerves. We need to consider proteinases and the receptors that are activated by proteinases as important potential targets for the development of analgesic drugs in the treatment of inflammatory pain.

Role of mitogen-activated protein kinase activation in injured and intact primary afferent neurons for mechanical and heat hypersensitivity after spinal nerve ligation

To investigate whether activation of mitogen-activated protein kinase (MAPK) in damaged and/or undamaged primary afferents participates in neuropathic pain after partial nerve injury, we examined the phosphorylation of extracellular signal-regulated protein kinase (ERK), p38 MAPK, and c-Jun N-terminal kinase (JNK) in the L4 and L5 dorsal root ganglion (DRG) in the L5 spinal nerve ligation (SNL) model. We first confirmed, using activating transcription factor 3 and neuropeptide Y immunoreactivity, that virtually all L4 DRG neurons are spared from axotomy in this model. In the injured L5 DRG, the L5 SNL induced the activation of ERK, p38, and JNK in different populations of DRG neurons. In contrast, in the uninjured L4 DRG, the L5 SNL induced only p38 activation in tyrosine kinase A-expressing small- to medium-diameter neurons. Intrathecal ERK, p38, and JNK inhibitor infusions reversed SNL-induced mechanical allodynia, whereas only p38 inhibitor application attenuated SNL-induced thermal hyperalgesia. Furthermore, the L5 dorsal rhizotomy did not prevent SNL-induced thermal hyperalgesia. We therefore hypothesized that p38 activation in the uninjured L4 DRG might be involved in the development of heat hypersensitivity in the L5 SNL model. In fact, the treatment of the p38 inhibitor and also anti-nerve growth factor reduced SNL-induced upregulation of brain-derived neurotrophic factor and transient receptor potential vanilloid type 1 expression in the L4 DRG. Together, our results demonstrate that the L5 SNL induces differential activation of MAPK in injured and uninjured DRG neurons and, furthermore, that MAPK activation in the primary afferents may participate in generating pain hypersensitivity after partial nerve injury.

Temperature-dependent double spikes in C-nociceptors of neuropathic pain patients

Five patients with small-fibre neuropathy characterized by temperature-dependent spontaneous pain, hyperalgesia/allodynia and signs of neurogenic inflammation were studied clinically and thermographically, and by microneurography. Thermography revealed hyperthermia confined to painful and hyperalgesic skin of distal extremities, in absence of sympathetic vasomotor denervation. Quantitative sensory testing documented either reduced thresholds or increased suprathreshold magnitude for heat pain. Microneurography identified 13 primary cutaneous C-nociceptors generating abnormal impulses in response to electrical stimuli and, in one patient, nociceptors firing spontaneously. All five patients showed examples of double spikes, in which a single brief electrical stimulus occasionally or regularly evoked two impulses. In one case, a second impulse occurred at one of three different delays. In all five patients, warming of the skin increased the probability of a second impulse occurring. Impulse doubling has previously been reported as occurring rarely in normal subjects and is attributable to unfiltering of multiple orthodromic impulses due to unidirectional conduction failure at branch points. A higher incidence of double firing in neuropathic pain patients is probably due to a reduced safety factor for conduction in the terminal arborizations of their C-nociceptors. These observations show that unidirectional conduction block provides a peripheral mechanism of temperature-dependent nociceptor hyperactivity in small-fibre neuropathy that may contribute to hyperalgesia.

The role of peripheral N-methyl-D-aspartate receptors in muscle hyperalgesia

The present study demonstrates that intramuscular administration of N-methyl-D-aspartate receptor antagonist AP5 dose-dependently attenuates complete Freund's adjuvant (CFA) induced muscular hyperalgesia. CFA significantly reduced mean bite force and success rate in the rats trained to produce a specific bite force. Pretreatment with AP5 reversed the overall magnitude of reduction in mean bite force and success rate in CFA inflamed rats, and significantly facilitated the recovery of these measures to pre-injection level. AP5 treatment 1 day after the CFA injection had little effect on CFA-mediated changes in bite force measurements. These data suggest that peripheral N-methyl-D-aspartate receptors play a critical role in the development of persistent muscle hyperalgesia, and provide important new insights for therapeutic alternatives that can be directed at the periphery.

Disturbances of motility and visceral hypersensitivity in irritable bowel syndrome: biological markers or epiphenomenon

Motility and visceral hypersentitivity are regarded as the primary mechanisms of symptom development in irritable bowel syndrome(IBS). While a variety of motor abnormalities have been described throughout the gastrointestinal tract in IBS, their specificity and relationship to symptoms remain unclear. Visceral hypersensitivity is ubiquitous in functional gastrointestinal disease and is especially common in IBS. Again, however, its specificity for IBS has been questioned. Many factors, including stress and psychopathology,complicate the interpretation of these phenomena and new re-search suggests that mucosal inflammation and luminal factors may be more fundamental to the etiology of this common disorder.

Increased hyperalgesia after tissue injury and faster recovery of allodynia after nerve injury in the GalR1 knockout mice

Evidence suggests that galanin and its receptors including GalR1 are involved in the modulation of nociception. To understand the contributions of this galanin receptor subtype to the analgesic effect of galanin, we systematically examined the nociception phenotype of the GalR1 knockout (KO) mice. (1) Baseline thresholds: Thermal escape latencies and tactile thresholds of the hind paws were not different between the GalR1 KO and wild type (WT) mice. (2) Thermal injury evoked hyperalgesia: Thermal injury (52 degrees C, 45 s) to one hind paw resulted in a reduction in the thermal escape latency as compared to the uninjured paw. The right/left difference score was significantly greater in the KO (5.9 +/- 0.8 s) than for the WT (2.8 +/- 0.7 s) indicating a greater hyperalgesia. (3) Formalin-induced flinching: Formalin paw injection (2.5%/20 microl) produced a two-phase flinching in both GalR1 KO and WT groups, that was detected by an automated flinching sensor device. Phase II flinching of KO (1510 +/- 90) was slightly greater than that observed for WT (1290 +/- 126), but the difference is not statistically significant. (4) Nerve injury evoked allodynia: Tactile thresholds were assessed prior to and at intervals up to 21 days after left L5 spinal nerve ligation and transection. In both GalR1 KO and WT mice, nerve injury caused thresholds to fall to 0.2-0.3g though 11 days. On days 14-21, GalR1 KO animals showed a significant recovery as compared to WT. In summary, GalR1 KO mice showed no difference from WT with respect to acute nociception, but showed a modest tendency towards increased hyperalgesia after tissue injury and inflammation. These results are consistent with a regulatory effect of galanin at GalR1 receptors on nociceptive processing.

Potential future therapies for irritable bowel syndrome: will disease modifying therapy as opposed to symptomatic control become a reality?

Irritable bowel syndrome can remit spontaneously, implying cure is possible. Predictors of good prognosis include a short history, acute onset(possibly postinfective origin), absence of psychological disorders, and resolution of chronic life stressors. Possible-disease modifying treatments with long-lasting effects include diet and anti-inflammatory and psychological treatments. Dietary modifications, which often involve excluding dairy and wheat products, are successful in some patients. Anti-inflammatory treatments have been subjected to one RCT in postinfective IBS without benefit. Probiotics may have benefit in altering bacterial flora and as anti-inflammatory agents, but further trials are needed before they can be recommended. Psychological treatments may produce long-lasting responses. Relaxation therapy appears to have a nonspecific benefit. Psychotherapy has been shown to have long-term benefit and is particularly acceptable to, and effective for, those with overt psychological distress. Hypnotherapy has been shown to be effective in randomized placebo controlled trials and has a sustained effect.

The ontogeny of neuropathic pain: Postnatal onset of mechanical allodynia in rat spared nerve injury (SNI) and chronic constriction injury (CCI) models

Neuropathic pain is known to occur in children but remains poorly understood and treated. The aim here was to establish a model of neuropathic pain in neonatal and young rodents. In the adult the spared nerve injury (SNI) model produced robust mechanical allodynia measured as a fall in cutaneous sensory threshold to 16% of controls, within one postoperative day and lasting at least 28 days. In contrast, animals aged 3, 10 and 21 days at the time of surgery did not display equivalent allodynia at any time up to 28 days later. A small, transient bilateral increased cutaneous sensitivity was observed at day 7 in P10 and P21 animals but this had gone by 14 days. SNI performed at 33 days led to a significant and persistent allodynia with the threshold falling to 55% of control values. A similar lack of neuropathic pain behaviour in younger animals was observed using the chronic constriction injury (CCI) model, which produced a clear allodynia in adult rats but no change in hindpaw sensitivity when performed at 10 days of age. Mechanical allodynia can be evoked in very young animals with inflammatory pain, so this developmental profile is selective for peripheral neuropathic pain and suggests a remarkable ability in young animals to compensate for the sensory consequences of nerve injury. The results are consistent with human neonatal responses to nerve injury; further study of underlying mechanisms are likely to yield important information about the pathogenesis and treatment of neuropathic pain.

A model of incisional pain: the effects of dermal tail incision on pain behaviours of Sprague Dawley rats

Hyperalgesia, a component of post-operative pain, is an enhanced responsiveness to painful challenges after the tissue damage caused by an incision. It is important to understand the mechanisms involved in the development of incisional pain, in order to treat the condition appropriately. The aim of this study was to develop a model of post-operative pain using the rat's tail. Under halothane-induced anaesthesia, female Sprague Dawley rats underwent 10mm longitudinal incisions through skin and fascia (n = 10) or 20 mm incisions through skin, fascia and muscle (n = 10) of the mid-portion of the tail. Control rats were only anaesthetised (n = 14). Withdrawal latencies to noxious mechanical and thermal challenges were recorded daily. A bar algometer was placed onto and 15 mm proximal to the incision with a force of 4N and the tail was immersed in 49 degrees C water. Daily withdrawal latencies were compared to pre-incision values using one way analysis of variance (ANOVA) with Dunnett's post-hoc test. Primary mechanical hyperalgesia lasted for 6 days after the 10 mm incisions (P < 0.0001) and for 7 days after the 20 mm incisions (P < 0.0001). Secondary mechanical hyperalgesia persisted for 1 day after the 10 mm incisions (P = 0.0013) and for 2 days after the 20 mm incisions (P = 0.0028). Thermal hyperalgesia was not elicited. This model is suitable to examine the mechanisms involved in post-operative pain.

Transcutaneous electrical nerve stimulation activates peripherally located alpha-2A adrenergic receptors

The alpha2A and alpha2C adrenergic receptor (AR) subtypes mediate antinociception when activated by the endogenous ligand norepinephrine. These receptors also produce antinociceptive synergy when activated concurrently with opioid receptor activation. The involvement of the opioid receptors in the mechanisms governing transcutaneous electrical nerve stimulation (TENS) has been well described. While spinal alpha-2 ARs do not appear to be involved in TENS antihyperalgesia in rats, the noradrenergic analgesic system also involves supraspinal and peripheral sites. Thus, a broader evaluation of the potential contribution of alpha-2 AR to TENS is warranted. The current study compared the antihyperalgesic efficacy of high (100 Hz) and low (4 Hz) frequency TENS in mutant mice lacking a functional alpha2A AR against their respective wildtype counterparts. The degree of secondary heat hyperalgesia induced by intra-articular injection of carrageenan/kaolin (3%) mixture did not differ among the experimental groups. However, the antihyperalgesia induced by both low and high frequency TENS was significantly diminished in alpha2A mutant mice compared to controls. The alpha2 adrenergic receptor selective antagonist, SK&F 86466, reversed TENS-mediated antihyperalgesia when delivered intra-articularly, but not when delivered intrathecally or intracerebroventricularly. These data suggest that peripheral alpha2 ARs contribute, in part, to TENS antihyperalgesia. This pharmacodynamic response is consistent with previous anatomical observations that alpha2A ARs are expressed on primary afferent neurons and macrophages near injured tissue.

Heat hyperalgesia after incision requires TRPV1 and is distinct from pure inflammatory pain

Postoperative pain significantly impacts patient recovery. However, postoperative pain management remains suboptimal, perhaps because treatment strategies are based mainly on studies using inflammatory pain models. We used a recently developed mouse model of incisional pain to investigate peripheral and spinal mechanisms contributing to heat hyperalgesia after incision. Behavioral experiments involving TRPV1 KO mice demonstrate that, as previously observed in inflammatory models, TRPV1 is necessary for heat (but not mechanical) hyperalgesia after incision. However, in WT mice, neither the proportion of TRPV1 immunoreactive neurons in the DRG nor the intensity of TRPV1 staining in the sciatic nerve was different from that in controls up to 4 days after incision. This result was corroborated by immunoblot analysis of sciatic nerve in rats subjected to an incision, and is distinct from that following inflammation of the rat hind paw, a situation in which TRPV1 expression levels in sciatic nerve increases. In the absence of heat exposure, spinal c-Fos staining was similar between incised TRPV1 KO and WT mice. However, differences in c-Fos staining between heat exposed TRPV1 KO and WT mice after incision suggest that the incision-mediated enhancement of heat-evoked signaling to the spinal cord involves a TRPV1-dependent mechanism. Finally, heat hyperalgesia after incision was reversed by antagonism of spinal non-NMDA receptors, unlike inflammatory hyperalgesia, which is mediated via NMDA receptors . Thus, TRPV1 is important for the generation of thermal hyperalgesia after incision. Our observations suggest that all experimental pain models may not be equally appropriate to guide the development of postoperative pain therapies.

Mechanisms of antinociception of spinal galanin: how does galanin inhibit spinal sensitization?

Galanin by a spinal action has been shown to have an antihyperalgesic action. Thus, in rats with lumbar intrathecal (IT) catheters, the thermal hyperalgesia evoked by carrageenan paw injection was blocked by IT delivery of galanin(1-29) (Gal(1-29)) and galanin(2-11) (Gal(2-11)) with the rank order of activity being Gal(1-29)>Gal(2-11). We sought to determine whether this spinal action reflects an effect upon afferent transmitter release, e.g., substance P (SP), and/or on secondary neurons, e.g., signaling postsynaptic to neurokinin 1 (NK1) receptor activation. To address the question on afferent release, we investigated the effect of IT administration of galanin on tissue injury-induced spinal NK1 internalization (an indicator of SP release). Noxious stimulation (paw compression) produced an increase in NK1 internalization in dorsal horn lamina I. IT pretreatment of rats with Gal(1-29) and Gal(2-11) significantly attenuated the evoked NK1 internalization, with the rank order of activity being Gal(1-29)>Gal(2-11)>saline. To address the question of postsynaptic action, we examined the effects of IT galanin upon IT SP-induced thermal hyperalgesia and spinal PGE2 release. Application of SP (30 nmol) directly to spinal cord led to a decrease in thermal thresholds and a profound increase in PGE(2) concentration in spinal dialysates. Both phenomena were reversed by Gal(1-29) and Gal(2-11) (10nmol, IT). These findings suggest that the antihyperalgesic effect of spinal galanin is due to its action on sites both presynaptic (inhibition of SP release) and postsynaptic (blockade of SP-evoked hyperalgesia and PGE2 production) to the primary afferents.

A role for the brainstem in central sensitisation in humans. Evidence from functional magnetic resonance imaging

Animal studies have established a role for the brainstem reticular formation, in particular the rostral ventromedial medulla (RVM), in the development and maintenance of central sensitisation and its clinical manifestation, secondary hyperalgesia. Similar evidence in humans is lacking, as neuroimaging studies have mainly focused on cortical changes. To fully characterise the supraspinal contributions to central sensitisation in humans, we used whole-brain functional magnetic resonance imaging at 3T, to record brain responses to punctate mechanical stimulation in an area of secondary hyperalgesia. We used the heat/capsaicin sensitisation model to induce secondary hyperalgesia on the right lower leg in 12 healthy volunteers. A paired t-test was used to compare activation maps obtained during punctate stimulation of the secondary hyperalgesia area and those recorded during control punctate stimulation (same body site, untreated skin, separate session). The following areas showed significantly increased activation (Z>2.3, corrected P<0.01) during hyperalgesia: contralateral brainstem, cerebellum, bilateral thalamus, contralateral primary and secondary somatosensory cortices, bilateral posterior insula, anterior and posterior cingulate cortices, right middle frontal gyrus and right parietal association cortex. Brainstem activation was localised to two distinct areas of the midbrain reticular formation, in regions consistent with the location of nucleus cuneiformis (NCF) and rostral superior colliculi/periaqueductal gray (SC/PAG). The PAG and the NCF are the major sources of input to the RVM, and therefore in an ideal position to modulate its output. These results suggest that structures in the mesencephalic reticular formation, possibly the NCF and PAG, are involved in central sensitisation in humans.

Disruption of the P2X7 purinoceptor gene abolishes chronic inflammatory and neuropathic pain

The P2X(7) purinoceptor is a ligand-gated cation channel, expressed predominantly by cells of immune origin, with a unique phenotype which includes release of biologically active inflammatory cytokine, interleukin (IL)-1beta following activation, and unique ion channel biophysics observed only in this receptor family. Here we demonstrate that in mice lacking this receptor, inflammatory (in an adjuvant-induced model) and neuropathic (in a partial nerve ligation model) hypersensitivity is completely absent to both mechanical and thermal stimuli, whilst normal nociceptive processing is preserved. The knockout animals were unimpaired in their ability to produce mRNA for pro-IL-1beta, and cytometric analysis of paw and systemic cytokines from knockout and wild-type animals following adjuvant insult suggests a selective effect of the gene deletion on release of IL-1beta and IL-10, with systemic reductions in adjuvant-induced increases in IL-6 and MCP-1. In addition, we show that this receptor is upregulated in human dorsal root ganglia and injured nerves obtained from chronic neuropathic pain patients. We hypothesise that the P2X(7) receptor, via regulation of mature IL-1beta production, plays a common upstream transductional role in the development of pain of neuropathic and inflammatory origin. Drugs which block this target may have the potential to deliver broad-spectrum analgesia.

Functional aspects and mechanisms of TRPV1 involvement in neurogenic inflammation that leads to thermal hyperalgesia

Neurogenic inflammation is produced by overstimulation of peripheral nociceptor terminals by injury or inflammation of tissues. Excessive activity of sensory neurons produces vasodilation, plasma extravasation and hypersensitivity. Mechanistically, neurogenic inflammation is due to the release of substances from primary sensory nerve terminals that act directly or indirectly at the peripheral terminals, either activating or sensitizing nociceptors, endothelial cells and immunocytes. Notably, small-diameter sensory neurons that are sensitive to capsaicin play a key role in the generation of neurogenic inflammation. The cloning of the vanilloid receptor 1 (TRPV1) has been a breakthrough that has propelled our understanding of the molecular mechanisms involved in neurogenic inflammation. TRPV1 pivotally contributes to the integration of various stimuli and modulates nociceptor excitability, thus making it a true gateway for pain transduction. In addition, TRPV1 is the endpoint target of intracellular signalling pathways triggered by inflammatory mediators. Phosphorylation-induced potentiation of TRPV1 channel activity, along with an incremented TRPV1 surface expression are major events underlying the nociceptor activation and sensitization that leads to thermal hyperalgesia. The important contribution of TRPV1 receptor to the onset and maintenance of neurogenic inflammation has validated it as a therapeutic target for inflammatory pain management. As a result, the development of specific TRPV1 antagonists is a central focus of current drug discovery programs.

In vivo recruitment by painful stimuli of AMPA receptor subunits to the plasma membrane of spinal cord neurons

The persistent increase in pain sensitivity observed after injury, known as hyperalgesia, depends on synaptic plasticity in the pain pathway, particularly in the spinal cord. Several potential mechanisms have been proposed, including post-synaptic exocytosis of the AMPA subclass of glutamate receptors (AMPA-R), which is known to play a critical role in synaptic plasticity in the hippocampus. AMPA-R trafficking has been described in spinal neurons in culture but it is unknown if it can also occur in spinal neurons in vivo, or if it can be induced by natural painful stimulation. Here we have induced referred mechanical hyperalgesia in vivo by intracolonic instillation of capsaicin in mice and have observed a recruitment of GluR1 AMPA-R subunits to neuronal plasma membranes in the lumbar spinal cord. Intracolonic capsaicin induced a rapid (10 min) increase in GluR1, but not GluR2/3 in the synaptosomal membrane fraction which lasted at least 3 h and a decrease in GluR1 subunit in the cytosolic fraction. Capsaicin treatment also provoked CaMKII activation and pre-treatment with a specific CaMKII inhibitor prevented the GluR1 trafficking. Brefeldin-A, an antibiotic that inhibits exocytosis of proteins, not only prevented GluR1 trafficking to the membrane but also inhibited referred hyperalgesia in capsaicin-treated mice. Our results show that delivery of GluR1 AMPA receptor subunits to the cell membrane through a CaMKII activity-dependent exocytotic regulated pathway contributes to the development of hyperalgesia after a painful stimulus. We conclude that AMPA-R trafficking contributes to the synaptic strengthening induced in the pain pathway by natural stimulation.

Characterization of morphine-induced hyperalgesia in male and female rats

The pain enhancing (hyperalgesic) effect of morphine was characterized in relation to pain stimulus (thermal, mechanical), dose, mode of administration (acute, chronic), sex and mechanism. We found that a low (subanalgesic) dose of morphine enhanced the sensitivity to thermal and mechanical noxious stimuli in a dose- and sex-related manner. Morphine hyperalgesia was inversely related to dose (0.002-0.2mg/kg) and was more pronounced in female than male rats. The N-methyl-d-aspartate receptor antagonist, ketamine, antagonized morphine hyperalgesia. Tolerance developed to hyperalgesia following repeated (chronic) dosing with low dose morphine. Several additional findings were noted in rats tolerant to morphine-induced hyperalgesia. The efficacy of an analgesic dose of morphine was increased (female rats). Sex-related differences in morphine's analgesic action (male>female) were attenuated. Development of tolerance to the analgesic effect of morphine was delayed. The present findings may have an implication for the use of mu opioids in the clinical setting.

Potentiation of spinal N-methyl-D-aspartate-mediated nociceptive transmission by cocaine-regulated and amphetamine-regulated transcript peptide in rats

The present study examined the effects of cocaine-regulated and amphetamine-regulated transcript peptide (CARTp) fragment 55-102, on N-methyl-D-aspartate (NMDA)-mediated nociceptive transmission in vivo and in vitro. In-vivo experiments were conducted in Sprague-Dawley rats to evaluate the effects of CARTp on thermal hyperalgesia induced by NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Intrathecal NMDA (1, 2, 4 nmol) or AMPA (0.5, 1, 2 nmol) dose-dependently decreased the tail-flick latency. Intrathecal CARTp was without effect on the tail-flick latency. Interestingly, it significantly enhanced NMDA-induced, but not AMPA-induced, nociceptive effects. The in-vitro effects of CARTp on NMDA-induced or AMPA-induced depolarizations in substantia gelatinosa neurons were studied in rat spinal cord slices. CARTp (100, 300 nM), which caused no significant change of membrane potentials, increased the amplitude of NMDA-induced depolarizations in substantia gelatinosa neurons with little effect on AMPA-induced depolarizations. The present study demonstrates that exogenously applied CARTp selectively facilitates NMDA receptor-mediated nociceptive transmission.

Characterization of a method for measuring primary hyperalgesia of deep somatic tissue

Measuring primary hyperalgesia from deep somatic tissue (ie, muscle and joint) is difficult in laboratory animals but clinically important. In this study, we modified a newly developed method to measure primary hyperalgesia of muscle in rats and compared this with primary hyperalgesia from the knee. Compression withdrawal thresholds of the gastrocnemius muscle or the knee joint were measured with a device consisting of strain gauges attached to forceps. Compression of the muscle or joint with the forceps results in hind limb withdrawal, and thresholds were measured before and 4 hours after induction of inflammation by 3% carrageenan injected into the gastrocnemius muscle or 3% kaolin-carrageenan injected into the knee joint. Compression thresholds were significantly decreased 4 hours after induction of inflammation in the muscle or knee joint compared with thresholds before inflammation. Surprisingly, in animals with muscle inflammation, compression thresholds were also significantly decreased on the contralateral hind limb. Systemic morphine (5 mg/kg, intraperitoneal) or lidocaine (2%) injected into the inflamed tissue reversed the decreased compression threshold induced by deep tissue inflammation. However, local anesthetic applied to the skin overlying the muscle or knee joint did not affect the decreased threshold. Thus, we report a consistent and convenient method to measure primary hyperalgesia in deep tissues of rats. The measured hyperalgesia originates in the inflamed tissues and has no measurable contribution from skin.

PERSPECTIVE

The current method measures primary hyperalgesia directly from injured deep somatic tissues. Thus it is relevant to painful clinical conditions that are aggravated by mechanical pressure at the site of injury. As such, it might serve as a model for basic mechanistic studies as well as drug discovery for musculoskeletal pain syndromes.

Increased nociceptive response in mice lacking the adenosine A1 receptor

The role of the adenosine A1 receptor in nociception was assessed using mice lacking the A1 receptor (A1R-/-) and in rats. Under normal conditions, the A1R-/- mice exhibited moderate heat hyperalgesia in comparison to the wild-type mice (A1R+/+). The mechanical and cold sensitivity were unchanged. The antinociceptive effect of morphine given intrathecally (i.t.), but not systemically, was reduced in A1R-/- mice and this reduction in the spinal effect of morphine was not associated with a decrease in binding of the mu-opioid ligand DAMGO in the spinal cord. A1R-/- mice also exhibited hypersensitivity to heat, but not mechanical stimuli, after localized inflammation induced by carrageenan. In mice with photochemically induced partial sciatic nerve injury, the neuropathic pain-like behavioral response to heat or cold stimulation were significantly increased in the A1R-/-mice. Peripheral nerve injury did not change the level of adenosine A1 receptor in the dorsal spinal cord in rats and i.t. administration of R-PIA effectively alleviated pain-like behaviors after partial nerve injury in rats and in C57/BL/6 mice. Taken together, these data suggest that the adenosine A1 receptor plays a physiological role in inhibiting nociceptive input at the spinal level in mice. The C-fiber input mediating noxious heat is inhibited more than other inputs. A1 receptors also contribute to the antinociceptive effect of spinal morphine. Selective A1 receptor agonists may be tested clinically as analgesics, particularly under conditions of neuropathic pain.

Mirror-like spread of chronic pain

The spread of chronic pain from its initial site of presentation is common, but the mechanisms of the spread are unknown. Here the authors present neurophysiologic evidence of altered interhemispheric conduction in a patient with a mirror-like spread of complex regional pain syndrome symptoms.