Extracellular amino acid concentrations in the dorsal spinal cord of freely moving rats following veratridine and nociceptive stimulation

Pluripotential GluN1 (NMDA NR1): Functional Significance in Cellular Nuclei in Pain/Nociception

The N-methyl-D-aspartate (NMDA) glutamate receptors function as plasma membrane ionic channels and take part in very tightly controlled cellular processes activating neurogenic and inflammatory pathways. In particular, the NR1 subunit (new terminology: GluN1) is required for many neuronal and non-neuronal cell functions, including plasticity, survival, and differentiation. Physiologic levels of glutamate agonists and NMDA receptor activation are required for normal neuronal functions such as neuronal development, learning, and memory. When glutamate receptor agonists are present in excess, binding to NMDA receptors produces neuronal/CNS/PNS long-term potentiation, conditions of acute pain, ongoing severe intractable pain, and potential excitotoxicity and pathology. The GluNR1 subunit (116 kD) is necessary as the anchor component directing ion channel heterodimer formation, cellular trafficking, and the nuclear localization that directs functionally specific heterodimer formation, cellular trafficking, and nuclear functions. Emerging studies report the relevance of GluN1 subunit composition and specifically that nuclear GluN1 has major physiologic potential in tissue and/or subnuclear functioning assignments. The shift of the GluN1 subunit from a surface cell membrane to nuclear localization assigns the GluN1 promoter immediate early gene behavior with access to nuclear and potentially nucleolar functions. The present narrative review addresses the nuclear translocation of GluN1, focusing particularly on examples of the role of GluN1 in nociceptive processes.

The multiple faces of ketamine in anaesthesia and analgesia

Objective

Ketamine is an anaesthetic agent with a unique dissociative profile and pharmacological effects ranging from the induction and maintenance of anaesthesia to analgesia and sedation, depending on the dose. This article provides information for the clinical use of ketamine in anaesthesia, in both conventional and special circumstances.

Methods

This is a non-systematic review of the literature, through a PubMed search up to February 2021.

Results

With a favourable pharmacokinetic profile, ketamine is used in hospital and prehospital settings for emergency situations. It is suitable for patients with many heart conditions and, unlike other anaesthetics, its potential for cardiorespiratory depression is low. Furthermore, it may be used when venous access is difficult as it may be administered through various routes. Ketamine is the anaesthetic of choice for patients with bronchospasm thanks to its bronchodilatory and anti-inflammatory properties.

Conclusion

With a favourable pharmacokinetic profile, ketamine is used in hospital and prehospital settings for emergency situations and is suitable for patients with many cardiac and respiratory conditions.

Tyrosine Kinase Inhibitors Reduce NMDA NR1 Subunit Expression, Nuclear Translocation, and Behavioral Pain Measures in Experimental Arthritis

In the lumbar spinal cord dorsal horn, release of afferent nerve glutamate activates the neurons that relay information about injury pain. Here, we examined the effects of protein tyrosine kinase (PTK) inhibition on NMDA receptor NR1 subunit protein expression and subcellular localization in an acute experimental arthritis model. PTK inhibitors genistein and lavendustin A reduced cellular histological translocation of NMDA NR1 in the spinal cord occurring after the inflammatory insult and the nociceptive behavioral responses to heat. The PTK inhibitors were administered into lumbar spinal cord by microdialysis, and secondary heat hyperalgesia was determined using the Hargreaves test. NMDA NR1 cellular protein expression and nuclear translocation were determined by immunocytochemical localization with light and electron microscopy, as well as with Western blot analysis utilizing both C- and N-terminal antibodies. Genistein and lavendustin A (but not inactive lavendustin B or diadzein) effectively reduced (i) pain related behavior, (ii) NMDA NR1 subunit expression increases in spinal cord, and (iii) the shift of NR1 from a cell membrane to a nuclear localization. Genistein pre-treatment reduced these events that occur in vivo within 4 h after inflammatory insult to the knee joint with kaolin and carrageenan (k/c). Cycloheximide reduced glutamate activated upregulation of NR1 content confirming synthesis of new protein in response to the inflammatory insult. In addition to this in vivo data, genistein or staurosporin inhibited upregulation of NMDA NR1 protein and nuclear translocation in vitro after treatment of human neuroblastoma clonal cell cultures (SH-SY5Y) with glutamate or NMDA (4 h). These studies provide evidence that inflammatory activation of peripheral nerves initiates increase in NMDA NR1 in the spinal cord coincident with development of pain related behaviors through glutamate non-receptor, PTK dependent cascades.

Conserved contributions of NMDA receptor subtypes to synaptic responses in lamina II spinal neurons across early postnatal development

NMDA receptors are heteromeric complexes that contribute to excitatory synaptic transmission and plasticity. The presence of specific variants of GluN2 subunits in these complexes enables diversity in NMDA receptor function and regulation. At brain synapses, there is a switch from slow GluN2B-mediated NMDA receptors to faster GluN2A-dominated NMDA receptors as well as an increase in the ratio of AMPA to NMDA receptors during early postnatal development. This glutamate receptor switch is observed across brain regions and is critical for synaptic maturation, circuit development, and associative learning. However, whether a similar receptor subunit switch occurs within pain processing neurons in the developing spinal cord remains untested. To investigate this, we performed whole-cell patch clamp recordings of excitatory synaptic responses from lamina II dorsal horn neurons of one to three week-old rats. We found that GluN2B and GluN2A both prominently contribute to NMDA receptor responses at neonatal lamina II synapses, with a small contribution from GluN2D as well. Surprisingly, we found that this molecular identity of NMDA receptor responses as well as the relative contribution of AMPA receptors versus NMDA receptors did not change at lamina II synapses across early postnatal development (P7 to P21). The lack of a developmental switch and persistence of slow-decaying GluN2B- and GluN2D-mediated synaptic responses throughout neuronal maturation in the dorsal horn has implications for understanding both the regulation of synaptic glutamatergic receptors as well as spinal mechanisms of pain processing.

Tumour-Derived Glutamate: Linking Aberrant Cancer Cell Metabolism to Peripheral Sensory Pain Pathways

Background

Chronic pain is a major symptom that develops in cancer patients, most commonly emerging during advanced stages of the disease. The nature of cancer-induced pain is complex, and the efficacy of current therapeutic interventions is restricted by the dose-limiting side-effects that accompany common centrally targeted analgesics.

Methods

This review focuses on how up-regulated glutamate production and export by the tumour converge at peripheral afferent nerve terminals to transmit nociceptive signals through the transient receptor cation channel, TRPV1, thereby initiating central sensitization in response to peripheral disease-mediated stimuli.

Results

Cancer cells undergo numerous metabolic changes that include increased glutamine catabolism and over-expression of enzymes involved in glutaminolysis, including glutaminase. This mitochondrial enzyme mediates glutaminolysis, producing large pools of intracellular glutamate. Up-regulation of the plasma membrane cystine/glutamate antiporter, system xc-, promotes aberrant glutamate release from cancer cells. Increased levels of extracellular glutamate have been associated with the progression of cancer-induced pain and we discuss how this can be mediated by activation of TRPV1.

Conclusion

With a growing population of patients receiving inadequate treatment for intractable pain, new targets need to be considered to better address this largely unmet clinical need for improving their quality of life. A better understanding of the mechanisms that underlie the unique qualities of cancer pain will help to identify novel targets that are able to limit the initiation of pain from a peripheral source–the tumour.

Glutaminase Increases in Rat Dorsal Root Ganglion Neurons after Unilateral Adjuvant-Induced Hind Paw Inflammation

Glutamate is a neurotransmitter used at both the peripheral and central terminals of nociceptive primary sensory neurons, yet little is known concerning regulation of glutamate metabolism during peripheral inflammation. Glutaminase (GLS) is an enzyme of the glutamate-glutamine cycle that converts glutamine into glutamate for neurotransmission and is implicated in producing elevated levels of glutamate in central and peripheral terminals. A potential mechanism for increased levels of glutamate is an elevation in GLS expression. We assessed GLS expression after unilateral hind paw inflammation by measuring GLS immunoreactivity (ir) with quantitative image analysis of L4 dorsal root ganglion (DRG) neurons after one, two, four, and eight days of adjuvant-induced arthritis (AIA) compared to saline injected controls. No significant elevation in GLS-ir occurred in the DRG ipsilateral to the inflamed hind paw after one or two days of AIA. After four days AIA, GLS-ir was elevated significantly in all sizes of DRG neurons. After eight days AIA, GLS-ir remained elevated in small (<400 µm²), presumably nociceptive neurons. Western blot analysis of the L4 DRG at day four AIA confirmed the elevated GLS-ir. The present study indicates that GLS expression is increased in the chronic stage of inflammation and may be a target for chronic pain therapy.

Involvement of multiple µ-opioid receptor subtypes on the presynaptic or postsynaptic inhibition of spinal pain transmission

The involvement of the μ-opioid receptor subtypes on the presynaptic or postsynaptic inhibition of spinal pain transmission was characterized in ddY mice using endomorphins. Intrathecal treatment with capsaicin, N-methyl-d-aspartate (NMDA) or substance P elicited characteristic nociceptive behaviors that consisted primarily of vigorous biting and/or licking with some scratching. Intrathecal co-administration of endogenous μ-opioid peptide endomorphin-1 or endomorphin-2 resulted in a potent antinociceptive effect against the nociceptive behaviors induced by capsaicin, NMDA or substance P, which was eliminated by i.t. co-administration of the μ-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP). The antinociceptive effect of endomorphin-1 was significantly suppressed by i.t.-co-administration of the μ2-opioid receptor antagonist Tyr-D-Pro-Trp-Phe-NH2 (D-Pro2-endomorphin-1) but not the μ1-opioid receptor antagonist Tyr-D-Pro-Phe-Phe-NH2 (D-Pro2-endomorphin-2) on capsaicin- or NMDA-elicited nociceptive behaviors. In contrast, the antinociceptive effect of endomorphin-2 was significantly suppressed by i.t.-co-administration of D-Pro2-endomorphin-2 but not D-Pro2-endomorphin-1 on capsaicin-, NMDA- or substance P-elicited nociceptive behaviors. Interestingly, regarding substance P-elicited nociceptive behaviors, the antinociceptive effect of endomorphin-1 was significantly suppressed by i.t.-co-administration of another μ2-opioid receptor antagonist, Tyr-D-Pro-Trp-Gly-NH2 (D-Pro2-Tyr-W-MIF-1), but not D-Pro2-endomorphin-1 or D-Pro2-endomorphin-2. The present results suggest that the multiple μ-opioid receptor subtypes are involved in the presynaptic or postsynaptic inhibition of spinal pain transmission.

Distribution of Spinal Sensitization Evoked by Inflammatory Pain Using Local Spinal Cord Glucose Utilization Combined with 3H-Phorbol 12,13-Dibutyrate Binding in Rats

Aims. Hyperalgesia following tissue injury is induced by plasticity in neurotransmission. Few investigators have considered the ascending input which activates the superficial of spinal cord. The aim was to examine neurotransmission and nociceptive processing in the spinal cord after mustard-oil (MO) injection. Both in vitro and in vivo autoradiographs were employed for neuronal activity and transmission in discrete spinal cord regions using the ¹⁴C-2-deoxyglucose method and ³H-phorbol 12,13-dibutyrate (³H-PDBu) binding sites. Methods. To quantify the hyperalgesia evoked by MO, the flinching was counted for 60 min after MO (20%, 50 μL) injection in Wistar rats. Simultaneous determination of ¹⁴C-2-deoxyglucose and ³H-PDBu binding was used for a direct observation of neuronal/metabolic changes and intracellular signaling in the spinal cord. Results. MO injection evoked an increase in flinching for 60 min. LSCGU significantly increased in the Rexed I-II with ³H-PDBu binding in the ipsilateral side of spinal cord. Discussion. We clearly demonstrated that the hyperalgesia is primarily relevant to increased neuronal activation with PKC activation in the Rexed I-II of the spinal cord. In addition, functional changes such as “neuronal plasticity” may result in increased neuronal excitability and a central sensitization.

Intrathecal glutamate release during hindlimb tourniquet inflation and femoral artery occlusion in rats

BACKGROUND/PURPOSE

A tourniquet is commonly used in limb surgery. Tourniquet inflation after a period of time may produce painful sensation. While the mechanisms of tourniquet-induced pain are still unknown, two components, pressure and ischemia, have been proposed. In this study, in vivo microdialysis was used to detect changes in intrathecal glutamate, an excitatory amino acid highly relevant to pain transmission, following hindlimb tourniquet application and femoral artery occlusion in the rat.

METHODS

Male Wistar rats were used. For the tourniquet study, 6 rats of the study group received 30 minutes right hindlimb tourniquet inflation and another 6 rats as the control group received only tourniquet application without inflation. In the femoral artery occlusion study, 6 rats of the study group received 30 minutes right femoral artery occlusion and another 6 rats as the control group received only sham operation without femoral artery occlusion. Cerebrospinal fluid dialysates were collected prior to, during, and after tourniquet application or femoral artery occlusion. Glutamate was measured by HPLC.

RESULTS

A significant increase in intrathecal glutamate release was found during the tourniquet inflation period, and it returned to baseline after tourniquet deflation. No change of glutamate release was noted during femoral artery occlusion or after femoral artery reperfusion.

CONCLUSION

The intrathecal glutamate release was increased by the hindlimb tourniquet inflation, but not influenced by femoral artery occlusion in the rat.

Low formalin concentrations induce fine-tuned responses that are sex and age-dependent: a developmental study

The formalin test is increasingly applied as a model of inflammatory pain using high formalin concentrations (5–15%). However, little is known about the effects of low formalin concentrations on related behavioural responses. To examine this, rat pups were subjected to various concentrations of formalin at four developmental stages: 7, 13, 22, and 82 days of age. At postnatal day (PND) 7, sex differences in flinching but not licking responses were observed with 0.5% formalin evoking higher flinching in males than in females. A dose response was evident in that 0.5% formalin also produced higher licking responses compared to 0.3% or 0.4% formalin. At PND 13, a concentration of 0.8% formalin evoked a biphasic response. At PND 22, a concentration of 1.1% evoked higher flinching and licking responses during the late phase (10–30 min) in both males and females. During the early phase (0–5 min), 1.1% evoked higher licking responses compared to 0.9% or 1% formalin. 1.1% formalin produced a biphasic response that was not evident with 0.9 or 1%. At PND 82, rats displayed a biphasic pattern in response to three formalin concentrations (1.25%, 1.75% and 2.25%) with the presence of an interphase for both 1.75% and 2.25% but not for 1.25%. These data suggest that low formalin concentrations induce fine-tuned responses that are not apparent with the high formalin concentration commonly used in the formalin test. These data also show that the developing nociceptive system is very sensitive to subtle changes in formalin concentrations.

Differential release of neurotransmitters from superficial and deep layers of the dorsal horn in response to acute noxious stimulation and inflammation of the rat paw

Experimental evidence suggests that release of neurotransmitters in response to acute noxious stimulation and inflammation can differ in superficial and deeper dorsal horn (DH) laminae. Using two different microdialysis probes, we studied changes in levels of glutamate, aspartate, arginine and GABA in dialysates collected from the surface of the spinal cord and within the DH induced by pinching the paw or paw inflammation. In penthotal anaesthetized rats, a flexible microdialysis probe was placed on the dorsal surface of the L4-L5 or L6-S2 spinal segments. In other rats, a rigid microdialysis probe was implanted within the DH of the same segments. Samples were collected every minute before, during and after pinching the hind paw (acute pain), and every half an hour after injecting either carrageenan or saline into the same paw (inflammation-induced pain). Amino acids were measured by capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIFD). Pinching the paw induced a significant but short lasting increase in extracellular glutamate and aspartate in dialysates from the surface of the DH. Carrageenan, but not saline, injected into the paw significantly increased concentrations of glutamate, aspartate and arginine both on the surface and within the DH of L4-L5 and also within the DH of the L6-S2 segments. The GABA level was significantly increased following carrageenan only within the DH. The maximum increase on the surface was detected 60-120 min after the onset of inflammation whereas the response within the DH reached a maximum between 150 and 180 min after carrageenan. These results indicate that unlike acute mechanical noxious stimulation which enhances amino acid neurotransmitters in surface dialysate, inflammation induced neurotransmitter release in all layers of the DH suggesting sensitization of the DH.

Glutaminase immunoreactivity and enzyme activity is increased in the rat dorsal root ganglion following peripheral inflammation

Following inflammation, primary sensory neurons in the dorsal root ganglion (DRG) alter the production of several proteins. Most DRG neurons are glutamatergic, using glutaminase as the enzyme for glutamate production, but little is known about glutaminase following inflammation. In the present study, adjuvant-induced arthritis (AIA) was produced in rats with complete Freund's adjuvant into the hindpaw. At 7 days of AIA, DRG were examined with glutaminase immunohistochemistry, Western blot immunoreactivity, and enzyme activity. Image analysis revealed that glutaminase was elevated most in small-sized neurons (21%) (P < 0.05). Western blot analysis revealed a 19% increase (P < 0.05) in total glutaminase and 21% in mitochondrial glutaminase (P < 0.05). Glutaminase enzyme activity was elevated 29% (P < 0.001) from 2.20 to 2.83 moles/kg/hr. Elevated glutaminase in primary sensory neurons could lead to increased glutamate production in spinal primary afferent terminals contributing to central sensitization or in the peripheral process contributing to peripheral sensitization.

Pharmacological enhancement of δ-subunit-containing GABA(A) receptors that generate a tonic inhibitory conductance in spinal neurons attenuates acute nociception in mice

The development of new strategies for the treatment of acute pain requires the identification of novel nonopioid receptor targets. This study explored whether δ-subunit-containing GABA(A)Rs (δGABA(A)Rs) in neurons of the spinal cord dorsal horn generate a tonic inhibitory conductance in vitro and whether δGABA(A)R activity regulates acute nociception. Whole-cell recordings revealed that δGABA(A)Rs generate a tonic inhibitory conductance in cultured spinal neurons and lamina II neurons in spinal cord slices. Increasing δGABA(A)R function by applying the δGABA(A)R-preferring agonist 4,5,6,7-tetrahydroisoxazolo [5,4-c]pyridine-3-ol (THIP) increased the tonic current and inhibited neuronal excitability in spinal neurons from wild-type (WT) but not δ subunit null-mutant (Gabrd(-/-)) mice. In behavioral studies, baseline δGABA(A)R activity did not regulate acute nociception; however, THIP administered intraperitoneally or intrathecally attenuated acute nociception in WT but not Gabrd(-/-) mice. In the formalin nociception assay, the phase 1 response was similar for WT and Gabrd(-/-) mice. In contrast, the phase 2 response, which models central sensitization, was greater in Gabrd(-/-) mice than WT. THIP administered intraperitoneally or intrathecally inhibited phase 1 responses of WT but not Gabrd(-/-) mice and had no effect on phase 2 responses of WT mice. Surprisingly, THIP reduced the enhanced phase 2 response in Gabrd(-/-) mice. Together, these results suggest that δGABA(A)Rs in spinal neurons play a major physiological and pharmacological role in the regulation of acute nociception and central sensitization. Spinal δ-subunit-containing GABA(A) receptors were identified with electrophysiological methods and behavioral models as novel targets for the treatment of acute pain.

Evidence that pregabalin reduces neuropathic pain by inhibiting the spinal release of glutamate

Pregabalin is an anti-convulsant that successfully treats many neuropathic pain syndromes, although the mechanism of its anti-hyperalgesic action remains elusive. This study aims to help delineate pregabalin's anti-hyperalgesic mechanisms. We assessed the effectiveness of pregabalin at decreasing mechanical and cold hypersensitivity induced in a rat model of neuropathic pain. Thus, we compared the effectiveness of pre- or post-treatment with systemic or intrathecal (i.t.) pregabalin at reducing the development and maintenance of the neuropathic pain symptoms. Pregabalin successfully decreased mechanical and cold hypersensitivity, as a pre-treatment, but was less effective at suppressing cold hypersensitivity when administered as a post-treatment. Furthermore, both i.t. and systemic administration of pregabalin were effective in reducing the behavioral hypersensitivity, with the exception of systemic post-treatment on cold hypersensitivity. We also examined pregabalin's effects at inhibiting hind paw formalin-induced nociception in naïve rats and formalin-induced release of excitatory amino acids in the spinal cord dorsal horn (SCDH) both in naïve rats and in rats with neuropathic pain. Pregabalin dose-dependently reduced nociceptive scores in the formalin test. We also present the first evidence that pregabalin reduces the formalin-induced release of glutamate in SCDH. Furthermore, i.t. pregabalin reduces the enhanced noxious stimulus-induced spinal release of glutamate seen in neuropathic rats. These data suggest that pregabalin reduces neuropathic pain symptoms by inhibiting the release of glutamate in the SCDH.

Fixative composition alters distributions of immunoreactivity for glutaminase and two markers of nociceptive neurons, Nav1.8 and TRPV1, in the rat dorsal root ganglion

Most, if not all, dorsal root ganglion (DRG) neurons use the neurotransmitter glutamate. There are, however, conflicting reports of the percentages of DRG neurons that express glutaminase (GLS), the enzyme that synthesizes glutamate, ranging from 30% to 100% of DRG neurons. Defining DRG neuron populations by the expression of proteins like GLS, which indicates function, is routinely accomplished with immunolabeling techniques. Proper characterization of DRG neuron populations relies on accurate detection of such antigens. It is known intuitively that fixation can alter immunoreactivity (IR). In this study, we compared the effects of five formaldehyde concentrations between 0.25% and 4.0% (w/v) and five picric acid concentrations between 0.0% and 0.8% (w/v) on the IR of GLS, the voltage-gated sodium channel 1.8 (Na(v)1.8), and the capsaicin receptor TRPV1. We also compared the effects of five incubation time lengths from 2 to 192 hr, in primary antiserum on IR. Lowering formaldehyde concentration elevated IR for all three antigens, while raising picric acid concentration increased Na(v)1.8 and TRPV1 IR. Increasing IR improved detection sensitivity, which led to higher percentages of labeled DRG neurons. By selecting fixation conditions that optimized IR, we found that all DRG neurons express GLS, 69% of neurons express Na(v)1.8, and 77% of neurons express TRPV1, indicating that some previous studies may have underestimated the percentages of DRG neurons expressing these proteins. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

The roles of nerve growth factor and cholecystokinin in the enhancement of morphine analgesia in a rodent model of central nervous system inflammation

Animal models of inflammatory pain are characterized by the release of inflammatory mediators such as cytokines and neurotrophic factors, and enhanced analgesic sensitivity to opioids. In this study, we examine the mechanisms underlying this effect, in particular the roles of cholecystokinin (CCK) and nerve growth factor (NGF), in an animal model of central nervous system (CNS) inflammation induced by spinal administration of lipopolysaccharide (LPS). Although spinal administration of LY-225910 (25 ng), a CCK-B antagonist, enhanced morphine analgesia in naïve rats, it was unable to do so in LPS-treated animals. Conversely, spinal CCK-8S administration (1 ng) decreased morphine analgesia in LPS-treated rats, but not in naïve animals. Further, spinal anti-NGF (3 microg) was able to reduce morphine analgesia in LPS-treated rats, but not in naïve animals, an effect that was reversed by spinal administration of LY-225910. While CCK-8S concentration was increased in spinal cord extracts of LPS animals as compared to controls, morphine-induced spinal CCK release in the extracellular space, as measured by in-vivo spinal cord microdialysis was inhibited in LPS animals as compared to controls, and this was reversed by anti-NGF pretreatment. Finally, chronic spinal administration of beta-NGF (7 microg/day) for 7 days enhanced spinal morphine analgesia, possibly by mimicking a CNS inflammatory state. We suggest that in intrathecally LPS-treated rats, spinal CCK release is altered resulting in enhanced morphine analgesia, and that this mechanism may be regulated to an important extent by NGF.

Activation of micro, delta or kappa opioid receptors by DAMGO, DPDPE, U-50488 or U-69593 respectively causes antinociception in the formalin test in the naked mole-rat (Heterocephalus glaber)

Data available on the role of the opioid systems of the naked mole-rat in nociception is scanty and unique compared to that of other rodents. In the current study, the effect of DAMGO, DPDPE and U-50488 and U-69593 on formalin-induced (20 microl, 10%) nociception were investigated. Nociceptive-like behaviors were quantified by scoring in blocks of 5 min the total amount of time (s) the animal spent scratching/biting the injected paw in the early (0-5 min) and in the late (25-60 min) phase of the test. In both the early and late phases, administration of 1 or 5 mg/kg of DAMGO or DPDPE caused a naloxone-attenuated decrease in the mean scratching/biting time. U-50488 and U-69593 at all the doses tested did not significantly change the mean scratching/biting time in the early phase. However, in the late phase U-50488 or U-69593 at the highest doses tested (1 or 5 mg/kg or 0.025 or 0.05 mg/kg, respectively) caused a statistically significant and naloxone-attenuated decrease in the mean scratching/biting time. The data showed that mu, delta or kappa-selective opioids causes antinociception in the formalin test in this rodent, adding novel information on the role of opioid systems of the animal on pain regulation.

Differential expression of phosphorylated Ca2+/calmodulin-dependent protein kinase II and phosphorylated extracellular signal-regulated protein in the mouse hippocampus induced by various nociceptive stimuli

In the present study, we characterized differential expressions of phosphorylated Ca(2+)/calmodulin-dependent protein kinase IIalpha (pCaMKIIalpha) and phosphorylated extracellular signal-regulated protein (pERK) in the mouse hippocampus induced by various nociceptive stimuli. In an immunoblot study, s.c. injection of formalin and intrathecal (i.t.) injections of glutamate, tumor necrosis factor-alpha (TNF-alpha), and interleukin-1beta (IL-1 beta) significantly increased pCaMKIIalpha expression in the hippocampus, but i.p. injections of acetic acid did not. pERK1/2 expression was also increased by i.t. injection of glutamate, TNF-alpha, and IL-1beta but not by s.c. injections of formalin or i.p. injections of acetic acid. In an immunohistochemical study, we found that increased pCaMKIIalpha and pERK expressions were mainly located at CA3 or the dentate gyrus of the hippocampus. In a behavioral study, we assessed the effects of PD98059 (a MEK 1/2 inhibitor) and KN-93 (a CaMKII inhibitor) following i.c.v. administration on the nociceptive behaviors induced by i.t. injections of glutamate, pro-inflammatory cytokines (TNF-alpha or IL-1beta), and i.p. injections of acetic acid. PD98059 as well as KN-93 significantly attenuated the nociceptive behavior induced by glutamate, pro-inflammatory cytokines, and acetic acid. Our results suggest that (1) pERKalpha and pCaMK-II located in the hippocampus are important regulators during the nociceptive processes induced by s.c. formalin, i.t. glutamate, i.t. pro-inflammatory cytokines, and i.p. acetic acid injection, respectively, and (2) the alteration of pERK and pCaMKIIalpha in nociceptive processing induced by formalin, glutamate, pro-inflammatory cytokines and acetic acid was modulated in a different manner.

Microdialysis in central nervous system disorders and their treatment

Central nervous system (CNS) insults elevate endogenous toxins and alter levels of indicators of metabolic disorder. These contribute to neurotrauma, neurodegenerative diseases and chronic pain and are possible targets for pharmaceutical treatment. Microdialysis samples substances in the extracellular space for chemical analysis. It has demonstrated that toxic levels of glutamate are released and that toxic levels of the reactive species O(2)(-), H(2)O(2), HO. NO and HOONO are generated upon CNS injury. Agent administration by microdialysis can also help elucidate mechanisms of damage and protection, and to identify targets for clinical application. Microdialysis sampling indicates that circuits descending from the brain to the spinal cord transmit and modulate pain signals by releasing neurotransmitter amines and amino acids. Efforts are under way to develop microdialysis into a technique for intensive care monitoring and predicting outcomes of brain insults. Finally, microdialysis sampling has demonstrated in vivo elevation of glial cell line-derived neurotrophic factor following grafting of primed fetal human neural stem cells into brain-injured rats, the first in vivo demonstration of the release of a neurotrophic factor by grafted stem cells. This increased release correlated with significantly improved spatial learning and memory.

Modulation of NMDA receptors by intrathecal administration of the sensory neuron-specific receptor agonist BAM8-22

The sensory neuron-specific receptor (SNSR) is exclusively distributed in dorsal root ganglion (DRG) cells. We have demonstrated that intrathecal (i.t.) administration of SNSR agonists inhibits formalin-evoked responses and the development of morphine tolerance [Chen, T., Cai, Q., Hong, Y., 2006. Intrathecal sensory neuron-specific receptor agonists bovine adrenal medulla 8-22 and (tyr(6))-gamma2-msh-6-12 inhibit formalin-evoked nociception and neuronal fos-like immunoreactivity in the spinal cord of the rat. Neuroscience 141, 965-975]. The present study was undertaken to examine the possible impact of the activation of SNSR on NMDA receptors. I.t. administration of NMDA (6.8 nmol) induced nociceptive behaviors, including scratching, biting and lifting, followed by thermal hypoalgesia and hyperalgesia. These responses were associated with the expression of Fos-like immunoreactivity (FLI) throughout the spinal dorsal horn with highest effect seen in laminae I-II. I.t. NMDA also induced an increase in nitric oxide synthase (NOS) activity in superficial layers of the dorsal horn, but not around the central canal, as revealed by NADPH diaphorase histochemistry. Pretreatment with the SNSR agonist bovine adrenal medulla 8-22 (3, 10 and 30 nmol) dose-dependently diminished NMDA-evoked nocifensive behaviors and hyperalgesia. This agonist also reduced NMDA-evoked expression of FLI and NADPH reactivity in the spinal dorsal horn. Taken together, these data suggest that the activation of SNSR induces spinal analgesia by suppressing NMDA receptor-mediated activation of spinal dorsal horn neurons and an increase in NOS activity.

Intrathecal injection of glutamate receptor antagonists/agonist selectively attenuated rat pain-related behaviors induced by the venom of scorpion Buthus martensi Karsch

The present study investigated the involvement of spinal glutamate receptors in the induction and maintenance of the pain-related behaviors induced by the venom of scorpion Buthus martensi Karsch (BmK). (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5-10-imine hydrogen maleate (MK-801; 40nmol; a non-competitive NMDA receptor antagonist), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 40nmol; a non-NMDA receptor antagonist), dl-amino-3-phosphonopropionic acid (dl-AP3; 100nmol; a group I metabotropic glutamate receptor antagonist) and 4-aminopyrrolidine-2,4-dicarboxylate (APDC; 100nmol; a group II metabotropic glutamate receptor agonist) were employed. On intrathecal injection of glutamate receptor antagonists/agonist before BmK venom administration by 10min, BmK venom-induced spontaneous nociceptive responses could be suppressed by all tested agents. Primary thermal hyperalgesia could be inhibited by MK-801 and dl-AP3, while bilateral mechanical hyperalgesia could be inhibited by CNQX and dl-AP3 and contralateral mechanical hyperalgesia could be inhibited by APDC. On intrathecal injection of glutamate receptor antagonists/agonist after BmK venom injection by 4.5h, primary thermal hyperalgesia could be partially reversed by all tested agents, while bilateral mechanical hyperalgesia could only be inhibited by APDC. The results suggest that the role of spinal glutamate receptors may be different on the various manifestations of BmK venom-induced pain-related behaviors.

Nitrous oxide increases serotonin release in the rat spinal cord

The mechanism of the antinociceptive action of nitrous oxide (N(2)O) is not fully understood. It was reported that N(2)O induces opioid peptide release in the rat midbrain, which can activate the descending inhibitory system in the spinal cord. Although effects of N(2)O on the noradrenergic descending inhibitory system have been established, effects of N(2)O on the serotonergic descending inhibitory system have not been extensively investigated. We measured the extracellular level of serotonin by using in vivo microdialysis in the dorsal horn of the spinal cord in rats. The serotonin release increased to 213.01 +/- 24.87% (mean +/- SEM) of the baseline level from 20 to 40 min after applying N(2)O, which was followed by a gradual decrease. It is suggested that the serotonergic descending pathway is activated by N(2)O.

Low dose ketamine: a therapeutic and research tool to explore N-methyl-D-aspartate (NMDA) receptor-mediated plasticity in pain pathways

Ketamine is a dissociative anaesthetic that has been used in the clinic for many years. At low, sub-anaesthetic doses, it is a relatively selective and potent antagonist of the N-methyl-D-aspartate (NMDA) receptor. It belongs to the class of uncompetitive antagonists and blocks the receptor by binding to a specific site within the NMDA receptor channel when it is open. Like other compounds of this class, ketamine can cause hallucinations or other untoward central effects which limit its use in the clinic. Nevertheless, because of the evidence on the importance of NMDA receptor-mediated pLasticity in chronic pain, low doses of ketamine have been explored in a wide range of pain conditions. The majority of studies with ketamine have shown efficacy; however, it has not been possible to separate safely the pain relief from the side effects of the drug. Hence, clinical use of ketamine as a pain treatment is very limited. Nevertheless, ketamine has served as a useful tool to provide a compelling rationale for developing other NMDA antagonists. Some of the new compounds of this class, particularly those acting at the NR2B subtype of the NMDA receptor, have shown promise in preclinical and clinical studies.

Colocalization of metabotropic glutamate receptors in rat dorsal root ganglion cells

Glutamate is the main excitatory transmitter in both central and peripheral nervous systems. Discovery of metabotropic glutamate receptors (mGluRs) made it clear that glutamate can have excitatory or inhibitory effects on neuronal function, with group I mGluRs enhancing cell excitability but group II and III mGluRs decreasing excitability. The present study investigated the colocalization of mGluR subtypes representing groups I, II, or III in rat L5 dorsal root ganglion (DRG) cells. The analyses show that group III has the highest expression, with 75.0% of DRG cells expressing mGluR8, followed by group II, with 51.6% expressing mGluR2/3, followed by group I, with only 6.8% expressing mGluR1alpha. mGluR8 is expressed by small, medium, and large diameter cells. In contrast, mGluR1alpha and mGluR2/3 are expressed by mainly small and medium cells. Approximately half of cells expressing group I mGluR1alpha also express either group II mGluR2/3 or group III mGluR8. These mGluR1alpha double-labeled populations are not likely to overlap since >1.0% of mGluR1alpha are triple-labeled. As expected from the high percentage of single-labeled mGluR2/3 and mGluR8 cells, there is a considerable population of double-labeled cells with approximately 30% of each population expressing both receptors. Due to the fact that the number of mGluR1alpha-expressing cells in the DRG is low, the percentage of triple-labeled cells is also low ( approximately 1-2%). The prevalence of groups II and III indicate that glutamate could have a substantial inhibitory effect of primary afferent function, reducing and/or fine-tuning sensory input before transmission to the spinal cord. These anatomical data highlight the potential inhibitory role glutamate may play in peripheral sensory transmission.

AAPS-FDA workshop white paper: microdialysis principles, application and regulatory perspectives

Many decisions in drug development and medical practice are based on measuring blood concentrations of endogenous and exogenous molecules. Yet most biochemical and pharmacological events take place in the tissues. Also, most drugs with few notable exceptions exert their effects not within the bloodstream, but in defined target tissues into which drugs have to distribute from the central compartment. Assessing tissue drug chemistry has, thus, for long been viewed as a more rational way to provide clinically meaningful data rather than gaining information from blood samples. More specifically, it is often the extracellular (interstitial) tissue space that is most closely related to the site of action (biophase) of the drug. Currently microdialysis (microD) is the only tool available that explicitly provides data on the extracellular space. Although microD as a preclinical and clinical tool has been available for two decades, there is still uncertainty about the use of microD in drug research and development, both from a methodological and a regulatory point of view. In an attempt to reduce this uncertainty and to provide an overview of the principles and applications of microD in preclinical and clinical settings, an AAPS-FDA workshop took place in November 2005 in Nashville, TN, USA. Stakeholders from academia, industry and regulatory agencies presented their views on microD as a tool in drug research and development.

The intracerebroventricular kainic acid-induced damage affects animal nociceptive behavior

In the present study, we examined nociceptive behaviors on various pain models after the pretreatment of kainic acid intracerebroventricularly. We found that intracerebroventricular administration of kainic acid shows significant neuronal damage on the hippocampal CA3 region in the brain slices stained with cresyl violet. Compared to the control group, intracerebroventricular pretreatment of kainic acid significantly attenuated nocifensive behaviors induced by intraplantar formalin (only in the 2nd phase), intrathecal glutamate, TNF-alpha or IL-1beta. However, nocifensive behaviors induced by intraperitoneal acetic acid (writhing test), intrathecal substance P or IFN-gamma were not affected by the pretreatment of kainic acid. These results suggest that (1) KA-induced alterations of nocifensive behaviors are related to the neuronal death of the hippocampal formation, especially CA3 pyramidal neurons and (2) nocifensive behaviors induced by formalin, acetic acid, SP, glutamate, and pro-inflammatory cytokines were modulated in a different manner.

Pathophysiology of pain

The processing and interpretation of pain signals is a complex process that entails excitation of peripheral nerves, local interactions within the spinal dorsal horn, and the activation of ascending and descending circuits that comprise a loop from the spinal cord to supraspinal structures and finally exciting nociceptive inputs at the spinal level. Although the "circuits" described here appear to be part of normal pain processing, the system demonstrates a remarkable ability to undergo neuroplastic transformations when nociceptive inputs are extended over time, and such adaptations function as a pronociceptive positive feedback loop. Manipulations directed to disrupt any of the nodes of this pain facilitatory loop may effectively disrupt the maintenance of the sensitized pain state and diminish or abolish neuropathic pain. Understanding the ascending and descending pain facilitatory circuits may provide for the design of rational therapies that do not interfere with normal sensory processing.

Amino acid release at the spinomedullary junction after inflammation of the TMJ region in male and female rats

Temporomandibular joint (TMJ) disorders are painful conditions that are more prevalent in women than men. This study tested the hypothesis that acute inflammation of the TMJ region evoked sex-related changes in amino acid transmitter concentrations at the trigeminal subnucleus/upper cervical cord (Vc/C2) junction, the major terminal zone for TMJ sensory afferents. Microdialysis samples were collected in male, intact and ovariectomized (OvX) female rats after injection of mustard oil into the TMJ region (TMJ-MO) under barbiturate anesthesia. Males displayed increases in glutamate, aspartate and serine at 5 min and secondary increases 40-45 min after TMJ-MO. Intact and OvX females given low dose estrogen (LE2) displayed increases in glutamate, aspartate and serine at 5 min but no secondary increase at 40 min, while OvX females given high dose estrogen (HE2) revealed no increases after TMJ-MO. Glycine increased 20 min after TMJ-MO in males and cycling females, but not in OvX rats. Perfusion of high potassium through the probe evoked similar increases in glutamate, aspartate and glycine in all groups. In separate experiments, perfusion of the glutamate-aspartate reuptake inhibitor, L-trans-2,4-pyrrolidine dicarboxylate (PDC), through the probe caused a prompt elevation in glutamate that was significantly greater in HE2 than LE2 females or males. These results suggested sex hormone status affects glutamatergic neurotransmission at the Vc/C2 junction by acting, in part, through modulation of glutamate reuptake. Altered amino acid transmitter release and/or availability at the Vc/C2 junction may contribute to differential processing of sensory input from the TMJ region in males and females.

Changes in pain behavior induced by formalin, substance P, glutamate and pro-inflammatory cytokines in immobilization-induced stress mouse model

In the present study, we examined the change of pain behaviors induced by formalin injected subcutaneously (s.c.) into the hind paw, or substance P (SP), glutamate, and pro-inflammatory cytokines (TNF-alpha, IL-1beta, and IFN-gamma) injected intrathecally (i.t.) in the mouse immobilization stress model. The mouse was restrained either once for 1h or five times for 5 days (once/day). In the formalin test, a single immobilization stress attenuated pain behaviors (licking, biting or scratching) in the second phase, while it had no effect on the pain behaviors revealed during the first phase. In addition, repeated immobilization stress attenuated pain behaviors revealed during the second phase but not in the first phase. A single as well as repeated immobilization stress decreased pain behaviors induced by substance P i.t. injection, but there were no significant changes in the glutamate test. In the pro-inflammatory cytokine pain model, a single immobilization stress decreased the pain behaviors induced by TNF-alpha, IL-1beta administered i.t. but not by IFN-gamma administered i.t. Moreover, a mouse applied with repeated immobilization stress did not show any changes in pain behaviors elicited by pro-inflammatory cytokines (TNF-alpha, IL-1beta and IFN-gamma) compared to the control group. These results suggest that a single and repeated immobilization stress differentially affects such nociceptive processing induced by formalin, SP, glutamate and pro-inflammatory cytokines in different manners.

A comparison of the glutamate release inhibition and anti-allodynic effects of gabapentin, lamotrigine, and riluzole in a model of neuropathic pain

The effects of treatment with the anti-convulsant agents, lamotrigine and riluzole were compared with gabapentin in a rat experimental model of neuropathic pain. Rats were treated intraperitoneally, with gabapentin (30, 100 and 300 mg/kg), lamotrigine (2, 10 and 50 mg/kg) or riluzole (6 and 12 mg/kg) prior to, and every 12 h for 4 days following chronic constriction injury (CCI) of the sciatic nerve. Mechanical and cold sensitivity were assessed prior to surgery (baseline) and then at 4, 8 and 12 days following CCI. The four-day treatment with each of the agents was effective at producing reductions in the development of mechanical and cold hypersensitivity for periods ranging from the fourth to 12th day. The highest doses of each of the agents were also assessed on formalin-induced nociceptive behaviors and on formalin-induced increases in extracellular glutamate (Glu) and aspartate (Asp) in the spinal cord dorsal horn (SCDH) of awake behaving rats using in vivo microdialysis. Nociceptive scores in formalin test were significantly decreased by gabapentin (300 mg/kg i.p.) and riluzole (12 mg/kg i.p.), but not by lamotrigine (50 mg/kg i.p.). Formalin-induced increases in glutamate levels in SCDH were lowered significantly, as compared with the controls, with all drugs both in the first phase and second phases, with the greatest effects for riluzole and gabapentin. Similar suppressive effects of the drugs were observed on formalin-induced increases in spinal aspartate, except that gabapentin and lamotrigine produced effects only during the second phase. Riluzole produced profound and prolonged reductions in the spinal levels of glutamate and aspartate both for basal and formalin-stimulated release. In conclusion, the results suggest that the anti-convulsant agents gabapentin, lamotrigine and riluzole may reduce the development of hyperalgesia in a rat model of neuropathic pain by reducing the spinal release of glutamate. Riluzole's pronounced suppressive effects on spinal EAA levels is attributed to its established role as a glutamate release inhibitor and an enhancer of glutamate transporter activity.

Antiallodynic and Antihyperalgesic Effects of Selective Competitive GLUK5 (GluR5) Ionotropic Glutamate Receptor Antagonists in the Capsaicin and Carrageenan Models in Rats

GLU(K5) kainate receptor subunits are abundant in pain pathways, including dorsal root ganglia and spinothalamic neurons, as well as in the thalamus and brain stem. A growing body of evidence indicates that the GLU(K5) kainate receptor subtype plays a prominent role in pain transmission, particularly in persistent pain. In the present studies, compounds from a novel series of amino acid GLU(K5) receptor antagonists were evaluated for their effectiveness in reversing capsaicin-induced mechanical allodynia as well as carrageenan-induced thermal hyperalgesia. In vitro, the amino acid compounds were efficacious in blocking glutamate-evoked calcium flux in cells expressing GLU(K5) but not GLU(K6) or GLU(A2), homomeric receptors. Electrophysiologically, the compounds exhibited selectivity for kainate receptors in dorsal root ganglion cells relative to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid hydrobromide and N-methyl-d-aspartate receptors in hippocampal pyramidal neurons. The amino acid compounds were poorly efficacious in the pain tests after s.c. or p.o. administration. However, compounds were highly efficacious after central intracisternal administration, and the rank order of potencies correlated with their rank order of affinities at GLU(K5) receptors determined in vitro, indicating that the lack of activity after systemic administration was due to poor oral bioavailability. To increase oral bioavailability, isobutyl or 2-ethyl-butyl ester prodrugs of the parent amino acids were prepared. The prodrugs, which produced robust plasma levels of parent amino acids, were highly efficacious in the capsaicin and carrageenan tests. The present studies provide further evidence that selective Glu(K5) kainate receptor subtype antagonists can reverse allodynia and hyperalgesia, particularly in persistent pain states.

Involvement of spinal μ1-opioid receptors on the Tyr-d-Arg-Phe-sarcosine-induced antinociception

The involvement of spinal mu-opioid receptor subtypes on the antinociception induced by i.t.-administered Tyr-D-Arg-Phe-sarcosine (TAPS), a N-terminal tetrapeptide analog of dermorphin, was determined in mice tail-flick test. Intrathecal administration of TAPS produced the marked inhibition of the tail-flick response in a dose-dependent manner. The antinociception induced by TAPS was completely eliminated by i.t.-co-administration of Tyr-D-Pro-Phe-Phe-NH2 (D-Pro2-endomorphin-2), the mu1-opioid receptor antagonist, whereas i.t. co-treatment with Tyr-D-Pro-Trp-Phe-NH2 (D-Pro2-endomorphin-1) or Tyr-D-Pro-Trp-Gly-NH2 (D-Pro2-Tyr-W-MIF-1), the mu2-opioid receptor antagonists, did not affect the TAPS-induced antinociception. In contrast, the antinociception induced by i.t.-administered [D-Ala2,N-MePhe4,Gly-ol5]enkephalin was significantly attenuated by i.t.-co-administration of D-Pro2-endomorphin-1 or D-Pro2-Tyr-W-MIF-1, but not D-Pro2-endomorphin-2. These results suggest that TAPS may stimulate spinal mu1-opioid receptors to produce the antinociception.

GD3 synthase gene knockout mice exhibit thermal hyperalgesia and mechanical allodynia but decreased response to formalin-induced prolonged noxious stimulation

Gangliosides are a family of sialic acid-containing glycosphingolipids that are highly enriched in the mammalian nervous system. In particular, b- and c-series gangliosides, all of which contain alpha-2,8 sialic acids, have been considered to play important roles in adhesion, toxin-binding, neurite extension, cell growth and apoptosis. However, the neurobiological functions of these series of gangliosides remain largely unknown. To clarify the function of b- and c-series gangliosides in pain sensation in vivo, we generated mice in whom the gene for the alpha-2,8-sialyltransferase (GD3 synthase), which is responsible for the generation of all b-series gangliosides as well as c-series gangliosides, was disrupted. Compared to the wild-type mice, the mutant mice exhibited increased sensory responses to thermal and mechanical stimuli as measured by a hot plate test and von Frey test. In contrast, the mutant mice showed decreased responses during the late phase of the formalin test. Paw edema and Fos expression in the spinal cord after formalin injection were significantly decreased in the mutant mice compared to the wild-type mice. No significant differences in the conduction velocity of the sciatic nerve, and no apparent morphologic differences in the spinal cord and the sciatic nerve were detected between the wild-type and the mutant mice. These results suggested that b- and c-series gangliosides are critical in the development and/or maintenance of the sensory nervous system responsible for the transmission of acute pain sensation and pain modulation. Moreover, they play an important role in the development of hyperalgesia induced by inflammation.

Agents that act by different mechanisms modulate the activity of protein kinase CβII isozyme in the rat spinal cord during peripheral inflammation

Hyperalgesia following unilateral complete Freund's adjuvant-induced inflammation was characterized by paw withdrawal latency to thermal stimulus. Paw withdrawal latencies were significantly shorter on the complete Freund's adjuvant-treated paw than on the contralateral paw of the complete Freund's adjuvant- and the sham-treated rats. Total cytosolic protein kinase C activity in the lumbar enlargement was unchanged on the sides of the spinal cord ipsi- and contra-lateral to the inflamed paw. Membrane-associated activities of protein kinase Calpha, protein kinase CbetaI and protein kinase Cgamma did not change significantly on the sides of the cord ipsi- and contra-lateral to the inflammation. However, membrane-associated activity of protein kinase CbetaII was increased in the cord section ipsilateral to the inflammation, suggesting that increased translocation/activation of protein kinase CbetaII is related to thermal hyperalgesia. Dextrorphan (an N-methyl-D-aspartate receptor antagonist), L-703,606 (an NK-1 receptor antagonist) and an antisense oligodeoxynucleotide for a selective knockdown of protein kinase Cbeta, reduced complete Freund's adjuvant-induced hyperalgesia, and reversed significant changes in the membrane activity of protein kinase CbetaII on the spinal cord section ipsilateral to the inflamed paw. Dextrorphan and protein kinase Cbeta antisense oligodeoxynucleotide were effective in reversing complete Freund's adjuvant-induced increase in the activity of protein kinase CbetaII ipsilateral to the inflammation at all the doses tested, but L-703,606 was effective only at the highest dose. Furthermore, in the presence of inflammatory stimulus, dextrorphan and L-703,606 did not alter the activities of membrane-associated protein kinase Calpha, protein kinase CbetaI, and protein kinase Cgamma in the section of the spinal cord ipsi- and contra-lateral to the inflammation. Protein kinase Cbeta antisense oligodeoxynucleotide had no significant effect on the membrane-associated activities of protein kinase Calpha and protein kinase Cgamma, but decreased the activities of both protein kinase CbetaI and protein kinase CbetaII and the expression of protein kinase Cbeta isozyme in the spinal cord. The data provide evidence that a common molecular event that converges to initiate and maintain hyperalgesia may include the translocation and activation of protein kinase CbetaII in the spinal dorsal horn.