Inflammation-induced up-regulation of protein kinase Cgamma immunoreactivity in rat spinal cord correlates with enhanced nociceptive processing

Ethanol withdrawal-associated allodynia and hyperalgesia: age-dependent regulation by protein kinase C epsilon and gamma isoenzymes

Ethanol (EtOH) withdrawal increases sensitivity to painful stimuli in adult rats. In this study, withdrawal from a single, acute administration of EtOH dose-dependently produced mechanical allodynia and thermal hyperalgesia in postnatal day 7 (P7) rats. In contrast, P21 rats exhibited earlier and more prolonged mechanical allodynia but not thermal hyperalgesia. For both P7 and P21 rats, blood and spinal cord EtOH levels peaked at 30 minutes after administration, with P7 rats achieving overall higher spinal cord concentrations. Protein kinase C (PKC) has been implicated in mediating pain responses. Inhibitory PKC- and gamma-specific peptides attenuated mechanical allodynia and thermal hyperalgesia in P7 rats, whereas only the PKCgamma inhibitor prevented mechanical allodynia in P21 rats. Immunoreactive PKC in dorsal root ganglion and PKCgamma in lumbar spinal cord increased at 6 hours after EtOH administration in P7 rats. In P21 rats, the density of PKC immunoreactivity remained unchanged, whereas the density of PKCgamma immunoreactivity increased and translocation occurred. These studies demonstrate developmental differences in neonatal nociceptive responses after withdrawal from acute EtOH and implicate a role for specific PKC isozymes in EtOH withdrawal-associated allodynia and hyperalgesia.

PERSPECTIVE

This study examines age-specific nociceptive responses after ethanol exposure by using 2 different ages of rats. The results suggest that ethanol age-dependently alters sensitivity to mechanical and thermal stimuli via specific protein kinase C isozymes. These results begin to ascertain the mechanisms that produce abnormal pain after alcohol exposure.

Activated PKA and PKC, but not CaMKIIα, are required for AMPA/Kainate-mediated pain behavior in the thermal stimulus model

Secondary mechanical allodynia resulting from a thermal stimulus (52.5 degrees C for 45s) is blocked by intrathecal (i.t.) pretreatment with calcium-permeable AMPA/KA receptor antagonists, but not NMDA receptor antagonists. Spinal sensitization is presumed to underlie thermal stimulus-evoked secondary mechanical allodynia. We investigated whether this spinal sensitization involves activation and phosphorylation of calcium-dependent protein kinases (PKA, PKC and CaMKIIalpha), and examined if the noxious stimulus increases phosphorylated AMPA GLUR1 (pGLUR1 Ser-845 and pGLUR1 Ser-831). Secondary mechanical allodynia after thermal stimulation was not altered by i.t. pretreatment with control vehicles (saline or 5% DMSO). Comparable allodynia was observed after pretreatment with a selective CaMKIIalpha inhibitor (17 and 34nmol KN-93). In marked contrast, pretreatment with either a PKA (10nmol H89) or PKC (30nmol chelerythrine) inhibitor blocked allodynia. Western immunoblot analyses supported behavioral findings and revealed a thermal stimulus-evoked increase in spinal phosphorylated PKA and PKC, but not CaMKIIalpha. There was no increase in any of the total protein kinases. Although thermal stimulation did not change either pGLUR1 Ser-845 or pGLUR1 Ser-831, it was associated with an increase in cytosolic total GLUR1. Pretreatment with a selective calcium-permeable AMPA/KA receptor antagonist (5nmol joro spider toxin), but not an NMDA receptor antagonist (25nmol d-2-amino-5-phosphonovalerate, AP-5), blocked thermal stimulus-evoked increases in phosphorylated PKA and PKC, in addition to increased cytosolic GLUR1. These findings indicate that spinal sensitization in the thermal stimulus model does not involve CaMKIIalpha activation or AMPA GLUR1 receptor phosphorylation, and differs from that occurring in NMDAr-dependent pain states.

Regulation of Na v channels in sensory neurons

Voltage-gated Na(+) channels have an essential role in the biophysical properties of nociceptive neurons. Factors that regulate Na(+) channel function are of interest from both pathophysiological and therapeutic perspectives. Increasing evidence indicates that changes in expression or inappropriate modulation of these channels leads to electrical instability of the cell membrane and the inappropriate spontaneous activity that is observed following nerve injury, and that this might contribute to neuropathic pain. The role of Na(v) channels in nociception depends on modulation by factors such as auxiliary beta-subunits, cytoskeletal proteins and the phosphorylation state of neurons. In this review we describe the modulation of Na(v) channels on sensory neurons by auxiliary beta-subunits, protein kinases and cytoskeletal proteins.

L1 CAM expression is increased surrounding the lesion site in rats with complete spinal cord transection as neonates

L1 is a cell adhesion molecule associated with axonal outgrowth, fasciculation, and guidance during development and injury. In this study, we examined the long-term effects of spinal cord injury with and without exercise on the re-expression of L1 throughout the rat spinal cord. Spinal cords from control rats were compared to those from rats receiving complete mid-thoracic spinal cord transections at postnatal day 5, daily treadmill step training for up to 8 weeks, or both transection and step training. Three months after spinal cord transection, we observed substantially higher levels of L1 expression by both Western blot analysis and immunocytochemistry in rats with and without step training. Higher expression levels of L1 were seen in the dorsal gray matter and in the dorsal lateral funiculus both above and below the lesion site. In addition, L1 was re-expressed on the descending fibers of the corticospinal tract above the lesion. L1-labeled axons also expressed GAP-43, a protein associated with axon outgrowth and regeneration. Treadmill step training had no effect on L1 expression in either control or transected rats despite the fact that spinal transected rats displayed improved stepping patterns indicative of spinal learning. Thus, spinal cord transection at an early age induced substantial L1 expression on axons near the lesion site, but was not additionally augmented by exercise.

The behavioral and neuroanatomical effects of IB4-saporin treatment in rat models of nociceptive and neuropathic pain

One distinguishing feature of primary afferent neurons is their ability to bind the lectin IB(4). Previous work suggested that neurons in the inner part of lamina II (IIi), onto which IB(4)-positive sensory neurons project, facilitate nociceptive transmission following tissue or nerve injury. Using an IB(4)-saporin conjugate (IB(4)-SAP), we examined the contribution of IB(4)-positive neurons to nociceptive processing in rats with and without nerve injury. Intrasciatic injection of IB(4)-SAP (5 mug/5 mul) significantly decreased IB(4)-labeling and immunoreactive P(2)X(3) in the spinal cord and delayed the behavioral and neuroanatomical consequences of L5 spinal nerve ligation (SNL) injury. In the absence of injury, thermal and mechanical nociceptive thresholds increased 2 weeks post-treatment only in IB(4)-SAP-treated, but not control (saline or saporin only), rats. Acute NGF-induced hyperalgesia was also attenuated following IB(4)-SAP treatment. In the SNL model, mechanical allodynia failed to develop 1 and 2 weeks post-injury, but was fully established by 4 weeks. Moreover, neuropeptide Y immunoreactivity (NPY-ir), which increases in the spinal cord after nerve injury, was unchanged in IB(4)-SAP-treated animals whereas immunoreactive PKCgamma decreased 2, but not 4, weeks post-injury. Quantitative RT-PCR revealed a reduction in P(2)X(3) mRNA in L4 DRG of IB(4)-SAP-treated animals, but no change in TrkA expression. Our results suggest that IB(4)-positive neurons in L4 are required for the full expression of NGF-induced hyperalgesia and participate in the behavioral and anatomical consequences that follow injury to the L5 spinal nerve.

Spinal dorsal horn neurone targets for nociceptive primary afferents: do single neurone morphological characteristics suggest how nociceptive information is processed at the spinal level

It has become increasingly clear that nociceptive information is signalled by several anatomically distinct populations of primary afferents that target different populations of neurones in the spinal cord. It is probable that these different systems all give rise to the sensation pain and hence, an understanding of their separate roles and the processes that they employ, may offer ways of selectively targeting pain arising from different causes. The review focuses on what is known of the anatomy of neurones in LI-III of the spinal dorsal horn that are implicated in nociception. The dendritic geometry and synaptic input of the large LI neurones that receive input from primary afferents containing substance P that express neurokinin 1 (NK(1)) receptors suggests that these neurones may monitor the extent of injury rather than the specific localisation of a discrete noxious stimulus. This population of neurones is also critically involved in hyperalgesia. In contrast neurones in LII with the morphology of stalked cells that receive primary afferent input from glomerular synapses may be more suitable for fine discrimination of the exact location of a noxious event such as a sting or parasite attack. The review focuses as far as possible on precisely defined anatomy in the belief that only by understanding these anatomical relationships will we eventually be able to interpret the complex processes occurring in the dorsal horn. The review attempts to be an accessible guide to a sometimes complex and highly specialised literature in this field.

Activation of protein kinase C antagonizes the opioid inhibition of calcium current in rat spinal dorsal horn neurons

Spinal dorsal horn (SDH) is one of important regions in both nociceptive transmission and antinociception. Opioid peptides produce analgesia via regulation of neurotransmitter release through modulation of voltage-dependent Ca(2+) channel (VDCC) in neuronal tissues. The modulatory effect of micro-opioid receptor (MOR) activation on VDCC was investigated in acutely isolated rat SDH neurons under the conventional whole-cell patch-clamp recording mode. The Ba(2+) current passing through VDCC was reversibly inhibited by a MOR agonist, [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin (DAMGO, 1 microM). Among 108 SDH neurons tested, VDCC of 39 neurons (36%) were inhibited by MOR activation, while other 69 neurons (64%) were not affected. The L-, N-, P/Q-, and R-type VDCC components shared 58.4+/-18.9%, 29.3+/-12.1%, 8.7+/-7.2%, and 3.4+/-4.8% of the total VDCC, respectively. Among VDCC subtypes inhibited by MOR activation, L- and N-types were 61.4+/-12.8% and 30.7+/-14.4%, respectively, while both P/Q- and R-types were 7.9+/-11.8%. A depolarizing pre-pulse increased the amplitude of VDCC and suppressed most of the inhibitory effect of MOR activation. Application of 1 microM phorbol-12-myristate-13-acetate completely antagonized the inhibitory effect of MOR activation without any alteration of basal VDCC amplitude. In contrast, the response of MOR activation was not altered by application of 4-alpha-phorbol (1 microM), 2-[3-Dimethylaminopropyl]indol-3-yl]-3-(indol-3-yl) maleimide (GF109203X, 1 microM), forskolin (1 microM), N-(2-[p-Bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide hydrochloride (H-89, 1 microM). These results indicate that activation of MOR coupled to G-proteins inhibits VDCC, and that this G-protein-mediated inhibition is antagonized by PKC-dependent phosphorylation.

Persistent Pain as a Disease Entity: Implications for Clinical Management

Pain has often been regarded merely as a symptom that serves as a passive warning signal of an underlying disease process. Using this model, the goal of treatment has been to identify and address the pathology causing pain in the expectation that this would lead to its resolution. However, there is accumulating evidence to indicate that persistent pain cannot be regarded as a passive symptom. Continuing nociceptive inputs result in a multitude of consequences that impact on the individual, ranging from changes in receptor function to mood dysfunction, inappropriate cognitions, and social disruption. These changes that occur as a consequence of continuing nociceptive inputs argue for the consideration of persistent pain as a disease entity in its own right. As with any disease, the extent of these changes is largely determined by the internal and external environments in which they occur. Thus genetic, psychological and social factors may all contribute to the perception and expression of persistent pain. Optimal outcomes in the management of persistent pain may be achieved not simply by attempting to remove the cause of the pain, but by addressing both the consequences and contributors that together comprise the disease of persistent pain.

[Opioid-induced hyperalgesia. Pathophysiology and clinical relevance]

Opioids are the drugs of choice for the treatment of moderate to severe acute and chronic pain. However, clinical evidence suggests that opioids can elicit increased sensitivity to noxious stimuli suggesting that administration of opioids can activate both pain inhibitory and pain facilitatory systems. Acute receptor desensitization via uncoupling of the receptor from G-proteins, up-regulation of the cAMP pathway, activation of the N-methyl-D-aspartate (NMDA) receptor system, as well as descending facilitation, have been proposed as potential mechanisms underlying opioid-induced hyperalgesia. Numerous reports exist demonstrating that opioid-induced hyperalgesia is observed both in animal and human experimental models. Brief exposures to micro-receptor agonists induce long-lasting hyperalgesic effects for days, which might by reflected by clinical observations that large doses of intraoperative micro-receptor agonists increased postoperative pain and morphine consumption. Furthermore, the prolonged use of opioids in patients often requires increasing doses and may be accompanied by the development of abnormal pain. Successful strategies that may decrease or prevent opioid-induced hyperalgesia include the concomitant administration of drugs like NMDA-antagonists, alpha(2)-agonists, or non-steroidal anti-inflammatory drugs (NSAIDs), opioid rotation or combinations of opioids with different receptor selectivity.

Exaggerated nociceptive responses on morphine withdrawal: roles of protein kinase C ε and γ

On withdrawal from opioids many patients experience a heightened sensitivity to stimuli and an exaggerated pain response. The phenomenon has been little studied in infants. We present evidence that in postnatal day 7 rats an exaggerated nociceptive ventral root response of spinal cords in vitro and withdrawal-associated thermal hyperalgesia in vivo are dependent on protein kinase C (PKC), and we document the roles of PKC and gamma isozymes. In vitro, the slow ventral root potential (sVRP) is a nociceptive-related response in spinal cord that is depressed by morphine and recovers to levels significantly above control on administration of naloxone. A broad-spectrum PKC antagonist, GF109213X, blocked withdrawal hyperresponsiveness of the sVRP whereas an antagonist specific to Ca(++)-dependent isozymes, Go69076, did not. Consistent with this finding, a specific peptide inhibitor of calcium-independent PKC, but not an inhibitor of calcium-dependent PKC gamma, blocked withdrawal hyperresponsiveness of the sVRP. Similarly, in vivo in 7-day-old rat pups, inhibition of PKC, but not PKC gamma, prevented thermal hyperalgesia precipitated by naloxone at 30 min post-morphine. In contrast, thermal hyperalgesia during spontaneous withdrawal was inhibited by both PKC and gamma inhibitors. The consistency between the in vivo and in vitro findings with respect to naloxone-precipitated withdrawal provides further evidence that the sVRP reflects nociceptive neurotransmission. In addition the difference between naloxone-precipitated and spontaneous withdrawal in vivo suggests that in postnatal day 7 rats, morphine exposure produces an early phase of primary afferent sensitization dependent upon PKC translocation, followed by a later phase involving spinal sensitization mediated by PKC gamma.

Prevention of fentanyl-induced delayed pronociceptive effects in mice lacking the protein kinase Cγ gene

It has recently been reported in several nociceptive models of rats that the antinociceptive effect of fentanyl, an opioid analgesic widely used in the management of per-operative pain, was followed by paradoxical delayed hyperalgesia dependent on N-methyl-D-aspartate (NMDA) mechanisms. Events upstream of the NMDA receptor, especially the activation of the protein kinase Cgamma (PKCgamma), have been involved in the persistence of pain states associated with central sensitisation. In order to evaluate the contribution of the PKCgamma in early and delayed fentanyl nociceptive responses, we studied these effects in knock-out mice deficient in such a protein. We found that fentanyl antinociception was followed by the spontaneous appearance of prolonged hyperalgesia in the paw pressure and formalin tests, and allodynia in the Von Frey paradigm. In PKCgamma deficient mice, an enhancement of the early fentanyl antinociceptive effects was observed, as well as a complete prevention of the fentanyl delayed hyperalgesic/allodynic effects. Finally, naloxone administration in mice that had recovered their pre-fentanyl nociceptive threshold, precipitated hyperalgesia/allodynia in wild-type but not in mutant mice. This study identifies the PKCgamma as a key element that links opioid receptor activation with the recruitment of opposite systems to opioid analgesia involved in a physiological compensatory pain enhancement.

Modulation of Nav1.7 and Nav1.8 peripheral nerve sodium channels by protein kinase A and protein kinase C

Voltage-gated Na+ channels (VGSC) are transmembrane proteins that are essential for the initiation and propagation of action potentials in neuronal excitability. Because neurons express a mixture of Na+ channel isoforms and protein kinase C (PKC) isozymes, the nature of which channel is being regulated by which PKC isozyme is not known. We showed that DRG VGSC Nav1.7 (TTX-sensitive) and Nav1.8 (TTX-resistant), expressed in Xenopus oocytes were differentially regulated by protein kinase A (PKA) and PKC isozymes using the two-electrode voltage-clamp method. PKA activation resulted in a dose-dependent potentiation of Nav1.8 currents and an attenuation of Nav1.7 currents. PKA-induced increases (Nav1.8) and decreases (Nav1.7) in peak currents were not associated with shifts in voltage-dependent activation or inactivation. The PKA-mediated increase in Nav1.8 current amplitude was prevented by chloroquine, suggesting that cell trafficking may contribute to the changes in Nav1.8 current amplitudes. A dose-dependent decrease in Nav1.7 and Nav1.8 currents was observed with the PKC activators phorbol 12-myristate, 13-acetate (PMA) and phorbol 12,13-dibutyrate. PMA induced shifts in the steady-state activation of Nav1.7 and Nav1.8 channels by 6.5 and 14 mV, respectively, in the depolarizing direction. The role of individual PKC isozymes in the regulation of Nav1.7 and Nav1.8 was determined using PKC-isozyme-specific peptide activators and inhibitors. The decrease in the Nav1.8 peak current induced by PMA was prevented by a specific epsilonPKC isozyme peptide antagonist, whereas the PMA effect on Nav1.7 was prevented by epsilonPKC and betaIIPKC peptide inhibitors. The data showed that Nav1.7 and Nav1.8 were differentially modulated by PKA and PKC. This is the first report demonstrating a functional role for epsilonPKC and betaIIPKC in the regulation of Nav1.7 and Nav1.8 Na+ channels. Identification of the particular PKC isozymes(s) that mediate the regulation of Na+ channels is essential for understanding the molecular mechanism involved in neuronal ion channel regulation in normal and pathological conditions.

Anandamide and vanilloid TRPV1 receptors

A large body of evidence now exists to substantiate that the endocannabinoid, anandamide, activates TRPV1 receptors. It is a low intrinsic efficacy TRPV1 agonist that behaves as a partial agonist in tissues with a low receptor reserve, while in tissues with high receptor reserve and in circumstances associated with certain disease states, it behaves as a full agonist. The efficacy of anandamide as a TRPV1 agonist is influenced by a succession of factors including receptor reserve, phosphorylation, metabolism and uptake, CB1 receptor activation, voltage, temperature, pH and bovine serum albumin. There are indications that the endocannabinoid system may play a role in the modulation of TRPV1 receptor activation. The activation of TRPV1 receptors by anandamide has potential implications in the treatment of inflammatory, respiratory and cardiovascular disorders. The relative importance of anandamide as a physiological and/or pathophysiological TRPV1 receptor agonist in comparison to other potential candidates has yet to be revealed.

Pregabalin and gabapentin reduce release of substance P and CGRP from rat spinal tissues only after inflammation or activation of protein kinase C

Gabapentin and pregabalin are amino acid derivatives of gamma-amino butyric acid that have anticonvulsant, analgesic, and anxiolytic-like properties in animal models. The mechanisms of these effects, however, are not well understood. To ascertain whether these drugs have effects on sensory neurons, we studied their actions on capsaicin-evoked release of the sensory neuropeptides, substance P and calcitonin gene-related peptide from rat spinal cord slices in vitro. Although release of immunoreactive peptides from non-inflamed animals was not altered by either drug, prior in vivo treatment by intraplantar injection of complete Freund's adjuvant enhanced release from spinal tissues in vitro, which was attenuated by gabapentin and pregabalin. These drugs also reduced release of immunoreactive neuropeptides in spinal tissues pretreated in vitro with the protein kinase C activator, phorbol 12,13-dibutyrate. Our results suggest that gabapentin and pregabalin modulate the release of sensory neuropeptides, but only under conditions corresponding to significant inflammation-induced sensitization of the spinal cord.

Phosphorylation of CREB and mechanical hyperalgesia is reversed by blockade of the cAMP pathway in a time-dependent manner after repeated intramuscular acid injections

Spinal activation of the cAMP pathway produces mechanical hyperalgesia, sensitizes nociceptive spinal neurons, and phosphorylates the transcription factor cAMP-responsive element binding protein (CREB), which initiates gene transcription. This study examined the role of the cAMP pathway in a model of chronic muscle pain by assessing associated behavioral changes and phosphorylation of CREB. Bilateral mechanical hyperalgesia of the paw was induced by administering two injections of acidic saline, 5 d apart, into the gastrocnemius muscle of male Sprague Dawley rats. Interestingly, the increases in immunoreactivity for CREB and phosphorylated CREB (p-CREB) in the spinal dorsal horn occur 24 hr, but not 1 week, after the second injection of acidic saline compared with pH 7.2 intramuscular injections. Spinal blockade of adenylate cyclase prevents the expected increase in p-CREB that occurs after intramuscular acid injection. The reversal of mechanical hyperalgesia by adenylate cyclase or protein kinase A inhibitors spinally follows a similar pattern with reversal at 24 hr, but not 1 week, compared with the vehicle controls. The p-CREB immunoreactivity in the superficial dorsal horn correlates with the mechanical withdrawal threshold such that increases in p-CREB are associated with decreases in threshold. Therefore, activation of the cAMP pathway in the spinal cord phosphorylates CREB and produces mechanical hyperalgesia associated with intramuscular acid injections. The mechanical hyperalgesia and phosphorylation of CREB depend on early activation of the cAMP pathway during the first 24 hr but are independent of the cAMP pathway by 1 week after intramuscular injection of acid.

Cannabinoid CB1 receptor inhibition of mechanically evoked responses of spinal neurones in control rats, but not in rats with hindpaw inflammation

Spinally administered cannabinoid receptor agonists are anti-nociceptive in a variety of models of acute and persistent pain. The present study investigated the effects of activation of spinal cannabinoid CB(1) receptors on mechanically evoked responses of spinal neurones in acute and inflammatory pain states. In vivo electrophysiology studies were carried out in anaesthetised rats. Effects of spinal administration of a selective cannabinoid CB(1) receptor agonist, arachidonyl-2-chloroethylamide (ACEA), on mechanically evoked responses of dorsal horn neurones in control rats and rats with peripheral hindpaw carrageenan-induced inflammation were compared. ACEA (0.27 nM-27 microM) significantly inhibited innocuous and noxious mechanically evoked responses of dorsal horn neurones in control rats. Pre-administration of the CB(1) receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1(2,4-dichlorophenyl)-4-methyl-1-H-pyrazole-3-carboxyamide, SR141716A, (0.43 microM) attenuated the inhibitory effects of ACEA (27 microM). ACEA did not alter mechanically evoked responses of dorsal horn neurones in rats with hindpaw carrageenan-induced inflammation. Following peripheral inflammation, there is a loss of spinal CB(1) receptor-mediated inhibition of mechanically evoked responses, which is suggestive of a functional down-regulation of CB(1) receptors under these conditions.

Morphology and axonal arborization of rat spinal inner lamina II neurons hyperpolarized by mu-opioid-selective agonists

The ventral or inner region of spinal substantia gelatinosa (SG; lamina II(i)) is a heterogeneous sublamina important for the generation and maintenance of hyperalgesia and neuropathic pain. To test whether II(i) neurons can be hyperpolarized by the mu-opioid agonist [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO; 500 nM) and to address possible downstream consequences of mu-opioid-evoked inhibition of II(i) neurons, we combined in vitro whole-cell, tight-seal recording methods with fluorescent labeling of the intracellular tracer biocytin and confocal microscopy. Twenty-one of 23 neurons studied had identifiable axons. Nine possessed axons that projected ventrally into laminae III-V; six of these were hyperpolarized by DAMGO. Three of four neurons with identifiable axons that projected to lamina I were hyperpolarized by DAMGO. Most neurons could be classified as either islet cells or stalked cells. Five of nine labeled islet cells and only two of seven stalked cells were hyperpolarized by DAMGO. Three were stellate cells: one resembled a spiny cell and three could not be classified. DAMGO hyperpolarized each of the stellate cells, the spiny cell, and 1 of the unclassified cells. Our data support the hypothesis that part of the action of mu-opioid agonists involves the inhibition of interneurons that are part of a polysynaptic excitatory pathway from primary afferents to neurons in the deep and/or superficial dorsal horn.

D-2-amino-5-phosphonopentanoic acid inhibits intrathecal pertussis toxin-induced thermal hyperalgesia and protein kinase Cgamma up-regulation

The aim of the present study was to examine the effect of intrathecal (i.t.) injection of pertussis toxin (PTX) on the nociceptive threshold and protein kinase C (PKC) expression in the rat spinal cord. The role of N-methyl-D-aspartic acid (NMDA) receptors in these changes was also examined. Male Wistar rats were implanted with two i.t. catheters, one of which was connected to a mini-osmotic pump and used to infuse saline or D-2-amino-5-phosphonopentanoic acid (D-AP5) (2 microg/h) starting on day 3 after i.t. catheter insertion. Two days later, a single injection of saline or PTX (2 microg) was given via the other catheter, followed by a flush with 10 microl of saline. On day 4 after PTX or saline injection, the thermal paw withdrawal latency was measured, then the rats were sacrificed by decapitation, and the dorsal part of the lumbosacral spinal segments was removed for PKC Western blotting assays. In PTX-treated rats, thermal hyperalgesia was observed, and the PKCgamma content of both the synaptosomal membrane and cytosolic fractions was significantly increased. The levels of alpha-, betaI-, or betaII-PKC isozymes in these fractions were unaffected by PTX treatment. Infusion of the NMDA antagonist, D-AP5, prevented both the thermal hyperalgesia and the increase in PKCgamma isoform expression in PTX-treated rats, and had no effect on these values in nai;ve rats. Intrathecal injection of the PKC inhibitor, chelerythrine (10 microg), significantly inhibited the thermal hyperalgesia observed in PTX-treated rats. These results show that i.t. injection of PTX induced thermal hyperalgesia accompanied by a selective increase in PKCgamma expression in both the synaptosomal membrane and cytosolic fractions of the dorsal horn of the rat lumbar spinal cord, and both effects were inhibited by the NMDA receptor antagonist, D-AP5.

Expression of calcitonin gene-related peptide, substance P and protein kinase C in cultured dorsal root ganglion neurons following chronic exposure to mu, delta and kappa opiates

The mechanisms involved in morphine tolerance are poorly understood. It was reported by our group that calcitonin gene-related peptide (CGRP)-like immunoreactivity (IR) was increased in the spinal dorsal horn during morphine tolerance [Ménard et al. (1996) J. Neurosci. 16, 2342-2351]. More recently, we observed that it was possible to mimic these results in cultured dorsal root ganglion (DRG) neurons allowing for more detailed mechanistic studies [Ma et al. (2000) Neuroscience 99, 529-539]. The aim of the present series of experiments was to further validate the DRG cell culture model by establishing which subtypes of opioid receptors are involved in the induction of CGRP in cultured rat DRG neurons, and to examine the signaling pathway possibly involved in the induction of CGRP-like IR following repeated opiate treatments. Other neuropeptides known to be expressed in DRG neurons, such as substance P (SP), neuropeptide Y (NPY) and galanin, were investigated to assess specificity. Following treatment with any of the three opioid agonists (mu, DAMGO; delta, DPDPE; kappa, U50488H), the number of CGRP- and SP-IR cultured DRG neurons increased significantly, and in a concentration-dependent manner, with the effects of kappa agonist being less pronounced. NPY and galanin were not affected.Double-immunofluorescence staining showed that the three opioid receptors were co-localized with both CGRP- and SP-like IR.Protein kinase C (PKC)-like IR was found to be significantly increased following a repetitive treatment with DAMGO. Double-immunofluorescence staining showed the co-localization of PKCalpha with CGRP- and SP-IR in cultured DRG neurons. Moreover, a combined treatment with DAMGO and a PKC inhibitor (chelerythrine chloride or Gö 6976) was able to block the effects of the opioid on increased CGRP-like IR. These data suggest that the three opioid receptors may be involved in the induction of CGRP and SP observed following chronic exposure to opiates, and that PKC probably plays a role in the signaling pathway leading to the up-regulation of these neuropeptides. These findings further validate the DRG cell culture as a suitable model to study intracellular pathways that govern changes seen following repeated opioid treatments possibly leading to opioid tolerance.

Inhibition of spinal protein kinase Calpha expression by an antisense oligonucleotide attenuates morphine infusion-induced tolerance

Protein kinase C isoforms including the alpha isozyme have been implicated in morphine tolerance. In the present study, we examined the effect of intrathecal delivery of an antisense oligonucleotide targeting rat protein kinase Calpha mRNA on the expression of spinal protein kinase Calpha isozyme and spinal morphine tolerance. Continuous intrathecal infusion of rats with morphine produced an increase in paw withdrawal threshold to thermal stimulation on day 1, which disappeared by day 5. On day 6, a bolus intrathecal injection of morphine (a probe dose) produced significantly less analgesia in morphine-infused rats than in saline-infused rats, suggesting tolerance. Intrathecal treatment with the protein kinase Calpha antisense concurrent with spinal morphine infusion not only maintained the analgesic effect of morphine during the 5-day infusion, it also significantly increased responsiveness to the probe morphine dose on day 6. In comparison, the missense used in the same treatment paradigm had no effect. The inhibitory effect of protein kinase Calpha antisense on spinal morphine tolerance was dose-dependent, and reversible. Intrathecal treatment with the antisense, but not the missense, in rats decreased expression of spinal protein kinase Calpha mRNA and protein, as revealed by real-time quantitative reverse transcription-polymerase chain reaction and western blots. Expression of the gamma isozyme was not affected by the oligonucleotides. The antisense also attenuated protein kinase C-mediated phosphorylation in spinal cord. These results demonstrate that selective reduction in the expression of the spinal protein kinase Calpha isozyme followed by a decrease of local protein kinase C-mediated phosphorylation will reverse spinal morphine infusion-induced tolerance. This finding is consistent with the view that tolerance produced by morphine infusion is dependent upon an increase in phosphorylation by protein kinase C, and also it emphasizes that the protein kinase Calpha isozyme and its activation in spinal cord may specifically participate in the phenomenon of opiate tolerance.

Gabapentin potentiates N-methyl-D-aspartate receptor mediated currents in rat GABAergic dorsal horn neurons

We previously reported that gabapentin (GBP), a widely prescribed analgesic, enhances N-methyl-aspartate (NMDA) receptor mediated currents only when the intracellular level of protein kinase C is elevated. However, it is unclear how the potentiation of NMDA responses by GBP can lead to pain relief. To resolve this issue, we combined immunocytochemical and patch recording techniques to study the actions of GBP on NMDA receptors in dorsal horn cells isolated from rats with inflammation and to determine the gamma-aminobutyric acid (GABA) content in the recorded cells. We found that all GBP-responsive cells are GABA-immunoreactive and none of the GABA-negative neurons respond to GBP. Thus, GBP appears to enhance NMDA currents in GABAergic neurons. These observations suggest that GBP exerts its antinociceptive action by increasing the activity of these inhibitory neurons.

[Nociceptors and mediators in acute inflammatory pain]

OBJECTIVE

To bring together the most recent data concerning the physiology of nociceptors at a time when there has been significant progress in the understanding of these.

DATA SOURCES

References were obtained from computerised bibliographic data banks (Medline and others) and the authors' personal documents.

DATA SYNTHESIS

Nociceptive impulses are generated at the periphery in unmyelinated fibres called nociceptors, the responses of which depend on the tissue environment. Numerous mediators can activate, sensitise or "wake up" nociceptor: kinins (bradykinin, kallidin and their metabolites), pro-inflammatory cytokines (TNF alpha, IL-6, IL-1 beta, IL-8), anti-inflammatory cytokines (IL-4, IL-6, IL-10, IL-12, IL-13), prostanoids (PGE2, PGI2), lipo-oxygenases (leucotrienes such LTB4 or 15-HETE), the "central mediators of the immune response" (NF-kappa B), growth factors such as neurotrophins (NGF, BDNF, NT-3 and NT-4/5), peptides (substance P, CGRP, Neurokinin A), nitric oxide, histamine, serotonin, proteases, excitatory amino acids, adrenergic amines and opioids. The release of neuromediators by primary afferent fibres in the spinal cord may be summarised by successively considering calcium channels, presynaptic receptors, excitatory amino acids and peptides.

CONCLUSION

Sensitisation phenomena are not exclusively peripheral but also central in origin and these are interlinked.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Alteration in the voltage dependence of NMDA receptor channels in rat dorsal horn neurones following peripheral inflammation

1. It has been proposed that the activation of NMDA receptors and upregulation of protein kinase C (PKC) underlie the exaggerated and persistent pain experienced in the inflammatory state. However, there is no direct evidence to show that inflammation alters the function of NMDA receptors. 2. We examined the voltage-dependent properties of NMDA receptor channels in rat dorsal horn neurones that receive sensory inputs from an inflamed hindpaw. 3. Peripheral inflammation was induced by injections of complete Freund's adjuvant (CFA). Membrane currents were measured using the perforated patch-clamp technique. 4. After CFA treatment, the current-voltage relationship of NMDA receptor channels was shifted in the hyperpolarized direction. This resulted in enhanced NMDA responses at negative potentials. 5. The change was mediated by PKC because the voltage shift was blocked by the selective PKC inhibitors chelerythrine and bisindolylmaleimide I. 6. Furthermore, the Mg(2+) blockade of NMDA receptors was reduced. This reduction could account for the shift in the voltage dependence of NMDA receptor channels. 7. These results indicate that NMDA receptor channel characteristics in the dorsal horn are altered by inflammation, and that the changes observed could contribute to the hyperalgesia and allodynia associated with tissue injury.

Reduced hyperalgesia induced by nerve injury, but not by inflammation in mice lacking protein kinase C gamma isoform

Protein kinase C is one of protein kinases which might be involved in the nerve injury- or inflammation-induced hyperalgesia. The present study was designed to investigate the hyperalgesia with thermal paw-withdrawal test induced by sciatic nerve ligation or by intraplantar injection of a complete Freund's adjuvant solution in protein kinase C gamma knockout and its wild-type mice. Either sciatic nerve ligation or intraplantar injection of a complete Freund's adjuvant caused a marked decrease of the paw-withdrawal latency only on the ipsilateral, but not on the contralateral side of the paw in wild-type mice. This ipsilateral hyperalgesia induced by sciatic nerve ligation was significantly attenuated in protein kinase C gamma knockout mice. On the other hand, the ipsilateral hyperalgesia induced by complete Freund's adjuvant remained about the same in protein kinase C gamma knockout mice as in wild-type mice. The results indicate that protein kinase C gamma is involved in the development of the thermal hyperalgesia induced by nerve ligation, but not by complete Freund's adjuvant-induced inflammation.

Protein kinase C gamma-like immunoreactivity in the substantia gelatinosa of the medullary dorsal horn of the rat

We examined protein kinase C gamma-immunoreactivity (PKCgamma-IR) in the substantia gelatinosa (SG) of the rat medullary dorsal horn (MDH). The density of PKCgamma-IR in the MDH was most intense in the SG. The number of neurons with PKCgamma-IR were also much larger in the SG than in the other layers of the MDH. Double-immunohistochemical studies indicated light and electron microscopically that substance P-containing fibers and I-B4 (isolectin from Bandeiraea simplicifolia)-labeled fibers made synapses on SG neurons with PKCgamma-IR, indicating that SG neurons with PKCgamma might receive nociceptive primary afferent fibers. The results support the notion that PKCgamma in the MDH may contribute to the regulation of the nociception.

Protein kinase C gamma-like immunoreactivity of trigeminothalamic neurons in the medullary dorsal horn of the rat

We examined protein kinase C gamma-like immunoreactivity (PKCgamma-LI) of trigeminothalamic neurons in the rat medullary dorsal horn (MDH) after injecting a retrograde tracer, Fluoro-Gold (FG), into the thalamus. Over 90% of FG-labeled neurons in the marginal layer (lamina I) and a few FG-labeled neurons in the superficial part of the magnocellular layer (lamina III) showed PKCgamma-LI. No PKCgamma-neurons in the substantia gelatinosa (lamina II) were labeled with FG. PKCgamma-mediated regulation of trigeminothalamic neurons may contribute to the changes in MDH activity during persistent pain.

Therapeutic potential of PKC inhibitors in painful diabetic neuropathy

Diabetic neuropathy accompanied by anomalies in pain perception is one of the most frequent complications in insulin-dependent diabetes in humans. Many clinical and experimental studies have suggested that diabetes or hyperglycaemia alters pain sensitivity. In humans, diabetic neuropathy can be associated with burning, tactile hypersensitivity. Behavioural reactions of hyperalgesia in animal models of diabetes have been described. However, the aetiology of these disturbances is still unknown, although metabolic factors such as hyperglycaemia or neurotransmitter alteration may be involved. Activation of protein kinase C (PKC) has been implicated in changes in pain perception. Phorbol esters, which activate PKC, enhance the thermal hyperalgesia in diabetic mice and enhance nociceptive responses after tissue injury induced by formalin. Electrophysiological experiments have shown that activation of PKC leads to long-lasting enhancement of excitatory amino acid-mediated currents in dorsal horn neurones and trigeminal neurones. Thus, activation of PKC may underlie the neuronal sensitisation that produces hyperalgesia in diabetic neuropathy.

Hyperalgesia induced by peripheral inflammation is mediated by protein kinase C betaII isozyme in the rat spinal cord

We have addressed the molecular mechanism(s) of hyperalgesia, which depends on increased excitability of dorsal horn neurons and on sensitization of primary afferent nociceptors, during peripheral inflammation. Following unilateral adjuvant-induced inflammation in the rat hind paw, time-course changes in behavioral hyperalgesia and functional activities of Ca2+/phospholipid-dependent protein kinase C isozymes were examined. Inflammation was characterized by increase in paw diameter, and behavioral hyperalgesia was quantified as paw withdrawal latency from a radiant heat source. Behavioral hyperalgesia on the injected paw was significantly increased. This was accompanied by a significant increase in total functional membrane-associated protein kinase C activity, whereas total cytosolic protein kinase C activity was unchanged on the sides of the lumbar spinal cord both contralateral and ipsilateral to the inflammation. Importantly, on the side of lumbar cord ipsilateral to the inflamed paw, the activity of membrane-associated protein kinase CbetaII was increased following the same time-course as the paw withdrawal latency decrease, suggesting an increased translocation of protein kinase Cbetall to the membrane related to behavioral hyperalgesia. A defined mixture of purified gangliosides, which inhibits intracellular protein kinase C translocation and activation, decreased inflammation-induced paw withdrawal latency, and specifically decreased the activity of membrane-associated protein kinase Cbetall on the side of the spinal cord ipsilateral to the inflammation. Quantitative immunohistochemical analyses demonstrated intensified protein kinase CbetaII-like immunoreactivity on the side of the spinal cord ipsilateral to the inflammation. Time-course for increases in the activity of membrane-associated protein kinase CbetaII, and in intensity of protein kinase CbetaII-immunoreactivity, paralleled inflammation-mediated changes in paw withdrawal latency and paw diameter. Our findings indicate an apparent involvement of protein kinase CbetaII isozyme specifically in the molecular mechanism(s) of thermal hyperalgesia.

PKCgamma contributes to a subset of the NMDA-dependent spinal circuits that underlie injury-induced persistent pain

In previous studies we provided evidence that the gamma isoform of protein kinase C (PKCgamma) is an important contributor to the increased pain sensitivity that occurs after injury. Here we combined electrophysiological and behavioral approaches in wild-type and PKCgamma-null mice to compare the hyperexcitability of wide dynamic range neurons in lamina V of the spinal cord dorsal horn with the behavioral hyperexcitability produced by the same injury [application of a C-fiber irritant, mustard oil (MO), to the hindpaw]. Wild-type and null mice did not differ in their response to mechanical or thermal stimuli before tissue injury, and the magnitude of the response to the MO stimuli was comparable. In wild-type mice, MO produced a dramatic and progressive enhancement of the response of lamina V neurons to innocuous mechanical and thermal stimuli. The time course of the neuronal hyperexcitability paralleled the time course of the MO-induced behavioral allodynia (nocifensive behavior in response to a previously innocuous mechanical stimulus). Neuronal hyperexcitability was also manifest in the PKCgamma-null mice, but it lasted <30 min. By contrast, the behavioral allodynia produced by MO in the PKCgamma-null mice, although reduced to approximately half that of the wild-type mice, persisted long after the lamina V hyperexcitability had subsided. Because the MO-induced behavioral allodynia was completely blocked by an NMDA receptor antagonist, we conclude that PKCgamma mediates the transition from short- to long-term hyperexcitability of lamina V nociresponsive neurons but that the persistence of injury-induced pain must involve activity within multiple NMDA-dependent spinal cord circuits.

Gabapentin actions on N-methyl-D-aspartate receptor channels are protein kinase C-dependent

Gabapentin (Neurontin) (GBP) is a widely prescribed analgesic used in treating pain patients with peripheral nerve injuries, diabetic neuropathy and cancer. To understand the mechanism of its action, we used the whole-cell patch recording technique to study the effects of GBP on N-methyl-D-aspartate (NMDA)-evoked currents in single dorsal horn neurons isolated from normal rats and from rats with inflammation induced by the injection of complete Freund adjuvant (CFA) to the hindpaw. We found that GBP enhanced NMDA currents in normal neurons only when protein kinase C (PKC) was added to these cells. The enhancement resulted from an increase in the affinity of glycine for NMDA receptors by GBP. In contrast, in neurons isolated from CFA-treated rats, GBP enhanced NMDA responses without any PKC treatment. Since endogenous PKC in inflamed tissue is elevated, these results suggest that GBP exerts its effects only on those cells affected by inflammatory injuries. Thus, the effects of GBP on NMDA receptors are plastic; they depend on the phosphorylation states of cells or receptors. These observations point to a new strategy for drug design. A chemical whose action depends on the state of cells would maximize its effectiveness while keeping its side-effects to a minimum.

Involvement of spinal protein kinase Cgamma in the attenuation of opioid mu-receptor-mediated G-protein activation after chronic intrathecal administration of [D-Ala2,N-MePhe4,Gly-Ol(5)]enkephalin

The present study was designed to investigate the role of a protein kinase C (PKC) isoform in the uncoupling of the mu-opioid receptor from G-proteins after repeated intrathecal injection of a selective mu-receptor agonist, [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO), in the spinal cord of mice. The activation of G-proteins by opioids was measured by monitoring the guanosine-5'-o-(3-[(35)S]thio)triphosphate ([(35)S]GTPgammaS) binding. Mice were injected intrathecally with saline or DAMGO once a day for 1-7 d. At 24 hr after every injection the spinal cord membranes were prepared for the assay. The enhanced [(35)S]GTPgammaS binding by mu-agonists DAMGO, endomorphin-1, or endomorphin-2 was attenuated clearly in spinal cord membranes obtained from mice that were treated intrathecally with DAMGO for 5 and 7 d, but not for 1 or 3 d. By contrast, no change in levels of [(35)S]GTPgammaS binding induced by the delta-receptor agonist SNC-80 or kappa-receptor agonist U-50,488H was noted in membranes obtained from mice that were treated with DAMGO. Concomitant intrathecal administration of a specific PKC inhibitor Ro-32-0432 with DAMGO blocked the attenuation of DAMGO-induced G-protein activation that was caused by chronic DAMGO treatment. Western blotting analysis showed that chronic DAMGO treatment increased the levels of PKCgamma, but not PKCalpha, PKCbetaI, and PKCbetaII isoforms, in spinal cord membranes. Furthermore, mice lacking PKCgamma failed to exhibit the desensitization of the DAMGO-stimulated [(35)S]GTPgammaS binding after repeated DAMGO injection. These findings indicate that repeated intrathecal administration of DAMGO may activate the PKCgamma isoform and in turn cause a desensitization of mu-receptor-mediated G-protein activation in the mouse spinal cord.

Opioid-NMDA receptor interactions may clarify conditioned (associative) components of opioid analgesic tolerance

Recent evidence suggests that acute administration of opioid analgesic drugs (such as morphine or heroin) produces delayed hyperalgesia. This hyperalgesic response is likely to result from hyperactivation of NMDA receptors triggered by stimulation of opioid receptors and may mediate acute tolerance. In support of this hypothesis, blockade of NMDA receptors attenuates opioid-induced delayed hyperalgesia and prolongs the duration of antinociceptive activity of morphine. Furthermore, the NMDA receptor-induced hyperalgesia is likely an unconditioned response to opioid receptor stimulation that becomes spatiotemporally associated with environmental cues accompanying repeated opioid exposure. This hypothesis conforms to the traditional Pavlovian requirement for conditioned and unconditioned responses to be qualitatively similar. In support of the role of NMDA receptor hyperactivation in morphine tolerance, NMDA receptor antagonists have been shown to block development of analgesic tolerance induced by repeated exposures to morphine. The view of the conditioned nature of opioid tolerance may be significantly extended by assuming that upon repeated drug administration an early-onset effect of a drug may become a predictive stimulus for a later-onset effect and, consequentially, it may become empowered to elicit the later-onset effect itself. Such 'intra-drug' conditioning hypothesis is well in line with the current experimental evidence but further studies will be needed to verify it directly.

Enhanced mu-opioid responses in the spinal cord of mice lacking protein kinase Cgamma isoform

The protein kinase C (PKC)gamma isoform is a major pool of the PKC family in the mammalian spinal cord. PKCgamma is distributed strategically in the superficial layers of the dorsal horn and, thus, may serve as an important biochemical substrate in sensory signal processing including pain. Here we report that mu-opioid receptor-mediated analgesia/antinociception and activation of G-proteins in the spinal cord are enhanced in PKCgamma knockout mice. In contrast, delta- and kappa-opioidergic and ORL-1 receptor-mediated activation of G-proteins in PKCgamma knockout mice was not altered significantly relative to the wild-type mice. Deletion of PKCgamma had no significant effect on the mRNA product of spinal mu-opioid receptors but caused an increase of maximal binding of the mu-opioid receptor agonist [3H][d-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin in spinal cord membranes obtained from PKCgamma knockout mice. These findings suggest that deletion of PKCgamma genes protects the functional mu-opioid receptors from degradation by phosphorylation. More importantly the present data provide direct evidence that PKCgamma constitutes an essential pathway through which phosphorylation of mu-opioid receptors occurs.

Effects of amyloid peptides on cell viability and expression of neuropeptides in cultured rat dorsal root ganglion neurons: a role for free radicals and protein kinase C

Chronic pain caused by nerve injury and inflammation is more common in the elderly. However, mechanisms underlying this phenomenon are unclear. Higher sensitivity of sensory neurons to free radicals has been suggested as one possibility. The production of free radicals can be induced by various agents, including the highly toxic protein beta-amyloid (A beta), which is found in higher amounts in the brains of Alzheimer's Disease patients. In dorsal root ganglion (DRG) cultures exposed to A beta, we examined cellular toxicity and peptide expression, in particular calcitonin gene-related peptide (CGRP), a peptide which is abundantly expressed by nociceptive afferents and is known to be involved in pain processes. Exposure of cultured rat DRG neurons to A beta(25--35) or A beta(1--40) (10 or 20 microM for 24--96 h) increased trypan blue-stained cells in a concentration- and time-dependent manner, thus, indicating cellular toxicity. These treatments also increased the number of CGRP immunoreactive (IR) neurons while decreasing the number of neuropeptide Y- and galanin-IR neurons. The free radical scavenger, superoxide dismutase, attenuated both the toxicity and neuropeptide changes induced by A beta, thus, suggesting that oxidative stress probably contributes to these effects. Exposure of cultured DRG neurons to A beta also increased the number of protein kinase C alpha (PKC alpha)-IR neurons. The PKC inhibitors, chelerythrine chloride and Gö6976, significantly augmented A beta-induced cellular toxicity while attenuating the increases in CGRP-and PKC alpha-IR cells, supporting the notion of a protective role for PKC in A beta insults. These in vitro data suggest that A beta peptides may, in addition to causing neurotoxicity, regulate neuropeptide expression in primary afferents. This finding could be relevant to the higher incidence of neuropathic pain that occurs with ageing.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Inhibition of spinal protein kinase C blocks substance P-mediated hyperalgesia

Substance P (SP) is an important neuromediator in the spinal processing of nociceptive afferent information. Our previous study has shown that spinal (intrathecal, IT) application of SP produces thermal hyperalgesia that is mediated by activation of the G-protein coupled NK1 receptor. The activation of some classes of the G-protein coupled receptors is known to produce diacylglycerol with consequent activation of protein kinase C (PKC). In the present study, we have demonstrated that intrathecal administration of a selective PKC inhibitor GF109203X (GF, 0.73 nmol) in rats chronically implanted with intrathecal catheters 15 min prior to IT-SP (48 nmol) completely blocked the SP-induced thermal hyperalgesia. The effect of GF was dose-dependent (0.073-0.73 nmol). Bisindolymaleimide V, the inactive homolog of GF, had no effect. Pretreatment with GF 3 h, but not 24 h, prior to SP still produced antinociception. Moreover, intrathecal treatment with GF (0.73 nmol) attenuated the formalin paw injection-induced flinching, preferentially at the 2nd phase, that is known to be associated with the release of endogenous SP at the spinal cord. These data suggest that activation of spinal PKC is involved in the SP-mediated hyperalgesia. Thus, SP, which is released in the spinal cord subsequent to persistent stimulation of small sensory afferents after tissue injury, may contribute to spinal hyperexcitability and persistent pain by enhancement of PKC-mediated phosphorylation of target molecules such as NMDA receptors.

Pain genes?: natural variation and transgenic mutants

Like many other complex biological phenomena, pain is starting to be studied at the level of the gene. Advances in molecular biological technology have allowed the cloning, mapping, and sequencing of genes, and also the ability to disrupt their function entirely (i.e. via transgenic knockouts). With these new tools at hand, pain researchers have begun in earnest the task of defining (a) which of the 70,000-150,000 mammalian genes are involved in the mediation of pain, and (b) which of the pain-relevant genes are polymorphic, contributing to both natural variation in responses and pathology. Although there are only a few known examples in which single gene mutations in humans are associated with pain conditions (e.g. an inherited form of migraine and congenital insensitivity to pain), it is likely that others will be identified. Concurrently, a variety of genes have been implicated in both the transmission and control of "pain" messages in animals. The present review summarizes current progress to these ends, focusing on both transgenic (gene-->behavior) and classical genetic (behavior-->gene) approaches in both humans and laboratory mice.

Protein kinase C isozymes and the regulation of diverse cell responses

Individual protein kinase C (PKC) isozymes have been implicated in many cellular responses important in lung health and disease, including permeability, contraction, migration, hypertrophy, proliferation, apoptosis, and secretion. New ideas on mechanisms that regulate PKC activity, including the identification of a novel PKC kinase, 3-phosphoinositide-dependent kinase-1 (PDK-1), that regulates phosphorylation of PKC, have been advanced. The importance of targeted translocation of PKC and isozyme-specific binding proteins (like receptors for activated C-kinase and caveolins) is well established. Phosphorylation state and localization are now thought to be key determinants of isozyme activity and specificity. New concepts on the role of individual PKC isozymes in proliferation and apoptosis are emerging. Opposing roles for selected isozymes in the same cell system have been defined. Coupling to the Wnt signaling pathway has been described. Phenotypes for PKC knockout mice have recently been reported. More specific approaches for studying PKC isozymes and their role in cell responses have been developed. Strengths and weaknesses of different experimental strategies are reviewed. Future directions for investigation are identified.

Involvement of spinal protein kinase C in thermal hyperalgesia evoked by partial sciatic nerve ligation, but not by inflammation in the mouse

Activation of several protein kinases contributes to the development of hyperalgesia evoked by injuries. The present study was designed to investigate the role of protein kinase C in the spinal cord in thermal hyperalgesia evoked by sciatic nerve ligation or by intraplantar injection of complete Freund's adjuvant. The paw withdrawal latency on the ipsilateral side, but not on the contralateral side, was markedly decreased after sciatic nerve ligation. Intraplantar injection of complete Freund's adjuvant also caused markedly decreases of the paw withdrawal latency. Intrathecal pretreatment with protein kinase C inhibitor calphostin C (100 and 250 ng) attenuated the decrease of the paw withdrawal latency evoked by sciatic nerve ligation. In contrast, the decrease of the paw withdrawal latency evoked by inflammation was only slightly attenuated by intrathecal pretreatment with calphostin C. The results indicate that protein kinase C in the spinal cord is involved in the development of the thermal hyperalgesia evoked by nerve ligation and is much less involved in the thermal hyperalgesia by complete Freund's adjuvant's-induced inflammation.

Murine models of inflammatory, neuropathic and cancer pain each generates a unique set of neurochemical changes in the spinal cord and sensory neurons

The aim of this investigation was to determine whether murine models of inflammatory, neuropathic and cancer pain are each characterized by a unique set of neurochemical changes in the spinal cord and sensory neurons. All models were generated in C3H/HeJ mice and hyperalgesia and allodynia behaviorally characterized. A variety of neurochemical markers that have been implicated in the generation and maintenance of chronic pain were then examined in spinal cord and primary afferent neurons.Three days after injection of complete Freund's adjuvant into the hindpaw (a model of persistent inflammatory pain) increases in substance P, calcitonin gene-related peptide, protein kinase C gamma, and substance P receptor were observed in the spinal cord. Following sciatic nerve transection or L5 spinal nerve ligation (a model of persistent neuropathic pain) significant decreases in substance P and calcitonin gene-related peptide and increases in galanin and neuropeptide Y were observed in both primary afferent neurons and the spinal cord. In contrast, in a model of cancer pain induced by injection of osteolytic sarcoma cells into the femur, there were no detectable changes in any of these markers in either primary afferent neurons or the spinal cord. However, in this cancer-pain model, changes including massive astrocyte hypertrophy without neuronal loss, increase in the neuronal expression of c-Fos, and increase in the number of dynorphin-immunoreactive neurons were observed in the spinal cord, ipsilateral to the limb with cancer. These results indicate that a unique set of neurochemical changes occur with inflammatory, neuropathic and cancer pain in C3H/HeJ mice and further suggest that cancer induces a unique persistent pain state. Determining whether these neurochemical changes are involved in the generation and maintenance of each type of persistent pain may provide insight into the mechanisms that underlie each of these pain states.

Loose ligation of the rat sciatic nerve is accompanied by changes in the subcellular content of protein kinase C beta II and gamma in the spinal dorsal horn

This study examined whether loose ligation of the sciatic nerve was accompanied by specific changes in protein kinase C (PKC) betaII and gamma isozymes in the spinal dorsal horn. The isozyme staining pattern was visualized with immunocytochemistry. Their content in subcellular fractions was estimated from Western immunoblots. In control animals, PKC betaII immunoreactivity extended from lamina I into lamina III, while PKC gamma immunoreactivity was concentrated within laminae II and III. In ligated animals exhibiting thermal hyperalgesia, the content of both PKC betaII and gamma in the synaptosomal membrane fraction, but not crude cytosolic fraction, was significantly greater by an average of 40% from their respective controls. These data support suggestions that peripheral nerve injury engenders plastic changes in the dorsal horn to contribute to the development of persistent pain.

Cannabinoid CB(1) receptor expression in rat spinal cord

While evidence implicates the endogenous cannabinoid system as a novel analgesic target at a spinal level, detailed analysis of the distribution of the cannabinoid receptor CB(1) in spinal cord has not been reported. Here, immunocytochemical studies were used to characterize the CB(1) receptor expression in rat spinal cord. Staining was found in the dorsolateral funiculus, the superficial dorsal horn (a double band of CB(1) immunoreactivity (ir) in laminae I and II inner/III transition), and lamina X. Although CB(1)-ir was present in the same laminae as primary afferent nociceptor markers, there was limited colocalization at an axonal level. Interruption of both primary afferent input by dorsal root rhizotomy and descending input by rostral spinal cord hemisection produced minor changes in CB(1)-ir. This and colocalization of CB(1)-ir with interneurons expressing protein kinase C subunit gamma-ir suggest that the majority of CB(1) expression is on spinal interneurons. These data provide a framework and implicate novel analgesic mechanisms for spinal actions of cannabinoids at the CB(1) receptor.

Involvement of spinal protein kinase C in induction and maintenance of both persistent spontaneous flinching reflex and contralateral heat hyperalgesia induced by subcutaneous bee venom in the conscious rat

To further study the roles of spinal protein kinase C (PKC) in induction and maintenance of both the persistent spontaneous nociception and the contralateral heat hyperalgesia induced by subcutaneous (s.c.) bee venom injection, the effects of intrathecal (i.t.) treatment with a PKC inhibitor, chelerythrine chloride (CH), were evaluated in conscious rats. Pre-treatment i.t. with CH at three doses of 0.01, 0.1 and 1 nmol produced a dose-dependent suppressive effect on the flinching reflex with the inhibitory rates of 39, 48 and 59%, respectively, when compared with the pre-saline control group. Post-treatment i.t. with the drug at the highest dose used (1 nmol) also resulted in a 42% suppression of the flinching reflex compared with the control. Moreover, pre-treatment i.t. with CH at three doses of 0.01, 0.1 and 1 nmol also produced 12, 22 and 48% inhibition of the contralateral heat hyperalgesia in the pre-saline control group. Post-treatment i.t. with the drug at the highest dose used (1 nmol) also resulted in a 35% reversal effect on the established contralateral heat hyperalgesia. The present result suggests that activation of PKC in the spinal cord contributes to the induction and maintenance of both peripherally-dependent persistent spontaneous pain and contralateral heat hyperalgesia which is dependent upon central sensitization.