Modulation of spinal excitability: co-operation between neurokinin and excitatory amino acid neurotransmitters

Race/Ethnicity Does Not Moderate the Relationship Between Adverse Life Experiences and Temporal Summation of the Nociceptive Flexion Reflex and Pain: Results From the Oklahoma Study of Native American Pain Risk

Adverse life experiences (ALEs) are associated with hyperalgesia and chronic pain, but the underlying mechanisms are poorly understood. One potential mechanism is hyperexcitability of spinal neurons (ie, central sensitization). Given that Native Americans (NAs) are more likely to have ALEs and to have a higher prevalence of chronic pain, the relationship between ALEs and spinal hyperexcitability might contribute to their pain risk. The present study assessed temporal summation of the nociceptive flexion reflex (TS-NFR; a correlate of spinal hyperexcitability) and pain (TS-Pain) in 246 healthy, pain-free non-Hispanic whites and NAs. The Life Events Checklist was used to assess the number of ALEs. Multilevel growth models were used to predict TS-NFR and TS-Pain, after controlling for age, perceived stress, psychological problems, negative and positive affect, and painful stimulus intensity. ALEs and negative affect were significantly associated with greater pain, but not enhanced TS-Pain. By contrast, ALEs were associated with enhanced TS-NFR. Race did not moderate these relationships. This finding implies that ALEs promote hyperalgesia as a result of increased spinal neuron excitability. Although relationships between ALEs and the nociceptive flexion reflex/pain were not stronger in NAs, given prior evidence that NAs experience more ALEs, this factor might contribute to the higher prevalence of chronic pain in NAs. PERSPECTIVE: This study found a dose-dependent relationship between ALEs and spinal neuron excitability. Although the relationship was not stronger in NAs than non-Hispanic whites, given prior evidence that NAs experience more ALEs, this could contribute to the higher prevalence of chronic pain in NAs.

Calcium signalling through L-type calcium channels: role in pathophysiology of spinal nociceptive transmission

L-type voltage-gated calcium channels are ubiquitous channels in the CNS. L-type calcium channels (LTCs) are mostly post-synaptic channels regulating neuronal firing and gene expression. They play a role in important physio-pathological processes such as learning and memory, Parkinson's disease, autism and, as recognized more recently, in the pathophysiology of pain processes. Classically, the fundamental role of these channels in cardiovascular functions has limited the use of classical molecules to treat LTC-dependent disorders. However, when applied locally in the dorsal horn of the spinal cord, the three families of LTC pharmacological blockers - dihydropyridines (nifedipine), phenylalkylamines (verapamil) and benzothiazepines (diltiazem) - proved effective in altering short-term sensitization to pain, inflammation-induced hyperexcitability and neuropathy-induced allodynia. Two subtypes of LTCs, Ca_v 1.2 and Ca_v 1.3, are expressed in the dorsal horn of the spinal cord, where Ca_v 1.2 channels are localized mostly in the soma and proximal dendritic shafts, and Ca_v 1.3 channels are more distally located in the somato-dendritic compartment. Together with their different kinetics and pharmacological properties, this spatial distribution contributes to their separate roles in shaping short- and long-term sensitization to pain. Ca_v 1.3 channels sustain the expression of plateau potentials, an input/output amplification phenomenon that contributes to short-term sensitization to pain such as prolonged after-discharges, dynamic receptive fields and windup. The Ca_v 1.2 channels support calcium influx that is crucial for the excitation-transcription coupling underlying nerve injury-induced dorsal horn hyperexcitability. These subtype-specific cellular mechanisms may have different consequences in the development and/or the maintenance of pathological pain. Recent progress in developing more specific compounds for each subunit will offer new opportunities to modulate LTCs for the treatment of pathological pain with reduced side-effects.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Botulinum toxin: The Midas touch

Botulinum Toxin (BT) is a natural molecule produced during growth and autolysis of bacterium called Clostridium botulinum. Use of BT for cosmetic purposes has gained popularity over past two decades, and recently, other therapeutic uses of BT has been extensively studied. BT is considered as a minimally invasive agent that can be used in the treatment of various orofacial disorders and improving the quality of life in such patients. The objective of this article is to review the nature, mechanism of action of BT, and its application in various head and neck diseases.

Neuronal intrinsic properties shape naturally evoked sensory inputs in the dorsal horn of the spinal cord

Intrinsic electrophysiological properties arising from specific combinations of voltage-gated channels are fundamental for the performance of small neural networks in invertebrates, but their role in large-scale vertebrate circuits remains controversial. Although spinal neurons have complex intrinsic properties, some tasks produce high-conductance states that override intrinsic conductances, minimizing their contribution to network function. Because the detection and coding of somato-sensory information at early stages probably involves a relatively small number of neurons, we speculated that intrinsic electrophysiological properties are likely involved in the processing of sensory inputs by dorsal horn neurons (DHN). To test this idea, we took advantage of an integrated spinal cord–hindlimbs preparation from turtles allowing the combination of patch-clamp recordings of DHN embedded in an intact network, with accurate control of the extracellular milieu. We found that plateau potentials and low threshold spikes (LTS) -mediated by L- and T-type Ca²⁺channels, respectively- generated complex dynamics by interacting with naturally evoked synaptic potentials. Inhibitory receptive fields could be changed in sign by activation of the LTS. On the other hand, the plateau potential transformed sensory signals in the time domain by generating persistent activity triggered on and off by brief sensory inputs and windup of the response to repetitive sensory stimulation. Our findings suggest that intrinsic properties dynamically shape sensory inputs and thus represent a major building block for sensory processing by DHN. Intrinsic conductances in DHN appear to provide a mechanism for plastic phenomena such as dynamic receptive fields and sensitization to pain.

Unravelling the mystery of capsaicin: a tool to understand and treat pain

A large number of pharmacological studies have used capsaicin as a tool to activate many physiological systems, with an emphasis on pain research but also including functions such as the cardiovascular system, the respiratory system, and the urinary tract. Understanding the actions of capsaicin led to the discovery its receptor, transient receptor potential (TRP) vanilloid subfamily member 1 (TRPV1), part of the superfamily of TRP receptors, sensing external events. This receptor is found on key fine sensory afferents, and so the use of capsaicin to selectively activate pain afferents has been exploited in animal studies, human psychophysics, and imaging studies. Its effects depend on the dose and route of administration and may include sensitization, desensitization, withdrawal of afferent nerve terminals, or even overt death of afferent fibers. The ability of capsaicin to generate central hypersensitivity has been valuable in understanding the consequences and mechanisms behind enhanced central processing of pain. In addition, capsaicin has been used as a therapeutic agent when applied topically, and antagonists of the TRPV1 receptor have been developed. Overall, the numerous uses for capsaicin are clear; hence, the rationale of this review is to bring together and discuss the different types of studies that exploit these actions to shed light upon capsaicin working both as a tool to understand pain but also as a treatment for chronic pain. This review will discuss the various actions of capsaicin and how it lends itself to these different purposes.

Protein kinase C modulation of thermo-sensitive transient receptor potential channels: Implications for pain signaling

A variety of molecules are reported to be involved in chronic pain. This review outlines the specifics of protein kinase C (PKC), its isoforms and their role in modulating thermo-sensitive transient receptor potential (TRP) channels TRPV1-4, TRPM8, and TRPA1. Anatomically, PKC and thermo-sensitive TRPs are co-expressed in cell bodies of nociceptive dorsal root ganglion (DRG) neurons, which are used as physiological correlates of peripheral and central projections involved in pain transmission. In the past decade, modulation of painful heat-sensitive TRPV1 by PKC has received the most attention. Recently, PKC modulation of other newly discovered thermo-sensitive pain-mediating TRPs has come into focus. Such modulation may occur under conditions of chronic pain resulting from nerve damage or inflammation. Since thermo-TRPs are primary detectors of acute pain stimuli, their modulation by PKC can severely alter their function, resulting in chronic pain. Comprehensive knowledge of pain signaling involving interaction of specific isoforms of PKC with specific thermo-sensitive TRP channels is incomplete. Such information is necessary to dissect out modality specific mechanisms to better manage the complex polymodal nature of chronic pain. This review is an attempt to update the readers on current knowledge of PKC modulation of thermo-sensitive TRPs and highlight implications of such modulation for pain signaling

Peripheral nerve injury increases glutamate-evoked calcium mobilization in adult spinal cord neurons

BACKGROUND

Central sensitization in the spinal cord requires glutamate receptor activation and intracellular Ca2+ mobilization. We used Fura-2 AM bulk loading of mouse slices together with wide-field Ca2+ imaging to measure glutamate-evoked increases in extracellular Ca2+ to test the hypotheses that: 1. Exogenous application of glutamate causes Ca2+ mobilization in a preponderance of dorsal horn neurons within spinal cord slices taken from adult mice; 2. Glutamate-evoked Ca2+ mobilization is associated with spontaneous and/or evoked action potentials; 3. Glutamate acts at glutamate receptor subtypes to evoked Ca2+ transients; and 4. The magnitude of glutamate-evoked Ca2+ responses increases in the setting of peripheral neuropathic pain.

RESULTS

Bath-applied glutamate robustly increased [Ca2+]i in 14.4 ± 2.6 cells per dorsal horn within a 440 x 330 um field-of-view, with an average time-to-peak of 27 s and decay of 112 s. Repeated application produced sequential responses of similar magnitude, indicating the absence of sensitization, desensitization or tachyphylaxis. Ca2+ transients were glutamate concentration-dependent with a Kd = 0.64 mM. Ca2+ responses predominantly occurred on neurons since: 1) Over 95% of glutamate-responsive cells did not label with the astrocyte marker, SR-101; 2) 62% of fura-2 AM loaded cells exhibited spontaneous action potentials; 3) 75% of cells that responded to locally-applied glutamate with a rise in [Ca2+]i also showed a significant increase in AP frequency upon a subsequent glutamate exposure; 4) In experiments using simultaneous on-cell recordings and Ca2+ imaging, glutamate elicited a Ca2+ response and an increase in AP frequency. AMPA/kainate (CNQX)- and AMPA (GYKI 52466)-selective receptor antagonists significantly attenuated glutamate-evoked increases in [Ca2+]i, while NMDA (AP-5), kainate (UBP-301) and class I mGluRs (AIDA) did not. Compared to sham controls, peripheral nerve injury significantly decreased mechanical paw withdrawal threshold and increased glutamate-evoked Ca2+ signals.

CONCLUSIONS

Bulk-loading fura-2 AM into spinal cord slices is a successful means for determining glutamate-evoked Ca2+ mobilization in naïve adult dorsal horn neurons. AMPA receptors mediate the majority of these responses. Peripheral neuropathic injury potentiates Ca2+ signaling in dorsal horn.

Timing in the absence of supraspinal input III: regularly spaced cutaneous stimulation prevents and reverses the spinal learning deficit produced by peripheral inflammation

In the absence of brain input, spinal systems can adapt to new environmental relations. For example, spinally transected rats given a legshock each time the leg is extended exhibit a progressive increase in flexion duration that minimizes net shock exposure, a simple form of instrumental learning. This capacity for learning is modulated by prior stimulation; both variable shock and inflammation produce a lasting inhibition of learning. An extended exposure to fixed spaced shock has no adverse effect on learning and opposes the consequences of variable shock. The present studies expand on these findings and demonstrate that fixed stimulation ameliorates the impact of peripheral inflammation. Spinally transected rats were administered 900 fixed spaced legshocks before (Experiment 1) or 1,800 legshocks after (Experiment 2) a subcutaneous hindpaw injection of capsaicin. Learning was assessed 24 hr later. Treatment with fixed shock attenuated the capsaicin-induced inhibition of learning. These findings suggest that fixed stimulation promotes adaptive plasticity and may foster recovery after injury.

Suppressive effects of glycyrrhetinic acid derivatives on tachykinin receptor activation and hyperalgesia

Glycyrrhetinic acid (GA), an aglycone of glycyrrhizin, isolated from the licorice root (Glycyrrhizia), and its semi-synthetic derivatives have a wide range of pharmacological effects. To investigate whether GA derivatives may be used as a new class of analgesics, we examined the effects of these compounds on human tachykinin receptors expressed in CHO-K1 cells. Among the GA derivatives examined, the disodium salt of olean-11,13(18)-dien-3β,30-O-dihemiphthalate inhibited the mobilization of [Ca(2+)](i) induced by substance P, neurokinin A, and neurokinin B in CHO-K1 cells expressing the human NK(1), NK(2), and NK(3) tachykinin receptors, respectively. In an inflammatory pain model, Compound 5 suppressed the capsaicin-induced flinching behavior in a dose-dependent manner. Compound 5 was also effective in suppressing pain-related behaviors in the late phase of the formalin test and reducing thermal hyperalgesia in the neuropathic pain state caused by sciatic nerve injury. Collectively, Compound 5 may be an analgesic candidate via tachykinin receptor antagonism.

N-antipyrine-3, 4-dichloromaleimide, an effective cyclic imide for the treatment of chronic pain: the role of the glutamatergic system

BACKGROUND

In recent years, cyclic imides have attracted the attention of the scientific community because of their promising therapeutic potential. Studies with the compound N-antipyrine-3,4-dichloromaleimide (NA-3,4-DCM) also demonstrated an antinociceptive effect in formalin or capsaicin models of nociception, and that it reduced acetic acid-induced abdominal writhing in mice.

METHODS

In this study, we examined the effects of NA-3,4-DCM on mechanical hypernociception in persistent pain-like behavioral models in mice. We also investigated the peripheral, topical, spinal, and supraspinal antinociceptive properties of NA-3,4-DCM and evaluated the involvement of the glutamatergic system on the antinociceptive effects of NA-3,4-DCM in mice.

RESULTS

NA-3,4-DCM, dosed systemically (intraperitoneally or per os), was capable of interfering with the development of mechanical hypernociception induced by intraplantar injection of carrageenan and complete Freund adjuvant in mice. Interestingly, repeated intraperitoneal or per os treatment with NA-3,4-DCM, administered after the induction of hypernociception, also reversed the mechanical sensitization induced by complete Freund adjuvant injection or partial ligation of the sciatic nerve in mice, with lower doses than gabapentin, a drug used clinically to treat chronic pain. When administered systemically, locally, spinally, or supraspinally, NA-3,4-DCM was able to inhibit the overt nociception of both phases of the formalin test. The systemic administration of NA-3,4-DCM also reduced the nociception induced by intraplantar or intrathecal injection of glutamate in mice. Furthermore, NA-3,4-DCM caused marked inhibition of the nociceptive response induced by intrathecal injection of a group I metabotropic glutamate receptors agonist (1S,3R)-aminocyclopentane-trans-1,3-dicardboxylic acid (ACPD) or N-methyl-d-aspartate (NMDA), without interfering with nociception induced by other non-NMDA receptor agonists (alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionic acid and kainate) or by substance P. Notably, in the same range of doses, the antinociception caused by the compound NA-3,4-DCM was not associated with nonspecific effects such as changes in locomotor activity or motor coordination.

CONCLUSION

These results provide strong evidence that NA-3,4-DCM produces antihypernociception in mice at peripheral, spinal, and supraspinal sites, and that interaction with the group I metabotropic glutamate receptors and NMDA receptors contributes to the mechanisms underlying its effect.

Effects of capsaicin on gastric acid secretion and mechanisms involved

Although capsaicin has multiple pharmacological actions, its effects on gastric acid secretion attract the most attention. Most studies show that low-dose capsaicin can inhibit gastric acid secretion while high-dose may accelerate gastric acid secretion. However, some other studies show that capsaicin does not affect gastric acid secretion at all. The difference in the effects of capsaicin on gastric acid secretion may be related with the location of vanilloid receptor subtype 1, dose of capsaicin, route of administration, and the release of some substances such as calcitonin gene-related peptide, neurokinin A, vasoactive intestinal peptide and substance P. Capsaicin is a potentially promising drug used for modulation of gastric acid secretion.

The role of peptides in central sensitization

Peptides released in the spinal cord from the central terminals of nociceptors contribute to the persistent hyperalgesia that defines the clinical experience of chronic pain. Using substance P (SP) and calcitonin gene-related peptide (CGRP) as examples, this review addresses the multiple mechanisms through which peptidergic neurotransmission contributes to the development and maintenance of chronic pain. Activation of CGRP receptors on terminals of primary afferent neurons facilitates transmitter release and receptors on spinal neurons increases glutamate activation of AMPA receptors. Both effects are mediated by cAMP-dependent mechanisms. Substance P activates neurokinin receptors (3 subtypes) which couple to phospholipase C and the generation of the intracellular messengers whose downstream effects include depolarizing the membrane and facilitating the function of AMPA and NMDA receptors. Activation of neurokinin-1 receptors also increases the synthesis of prostaglandins whereas activation of neurokinin-3 receptors increases the synthesis of nitric oxide. Both products act as retrograde messengers across synapses and facilitate nociceptive signaling in the spinal cord. Whereas these cellular effects of CGRP and SP at the level of the spinal cord contribute to the development of increased synaptic strength between nociceptors and spinal neurons in the pathway for pain, the different intracellular signaling pathways also activate different transcription factors. The activated transcription factors initiate changes in the expression of genes that contribute to long-term changes in the excitability of spinal and maintain hyperalgesia.

Morphine inhibits herpetic allodynia through mu-opioid receptors induced in Abeta-fiber neurons

Peripheral action of mu-opioid receptor agonist inhibits mechanical allodynia in mice with herpetic pain. Mechanical allodynia is mainly mediated by Abeta fibers, whereas mu-opioid receptors are present in C and Adelta fibers. This study was conducted to address this discrepancy. Neonatal capsaicin treatment, which almost abolished aversive response to capsaicin, did not affect herpetic allodynia and antiallodynic effect of local injection of morphine. Although mu-opioid receptor was chiefly expressed in small-sized and medium-sized sensory neurons in naive mice, it was induced in large sensory neurons in mice with herpetic pain. Viral propagation in the sensory ganglion may induce mu-opioid receptor expression in Abeta fibers, which may be responsible for the inhibitory action of local opioids on mechanical allodynia in mice with herpetic pain.

Inhibition of spinal cytosolic phospholipase A(2) expression by an antisense oligonucleotide attenuates tissue injury-induced hyperalgesia

Activation of the spinal phospholipase A(2) (PLA(2)) -cyclooxygenase (COX) -prostaglandin signaling pathway is widely implicated in nociceptive processing. Although the role of spinal COX isoforms in pain signal transmission has been extensively characterized, our knowledge of PLA(2) enzymes in this cascade is limited. Among all PLA(2) groups, cytosolic calcium-dependent PLA(2) group IVA (cPLA(2)IVA) appears to be the predominant PLA(2) enzyme in the spinal cord. In the present study we sought to (i) characterize anatomical and cellular distribution and localization of cPLA(2)IVA in dorsal horn of rat spinal cord, (ii) verify efficacy and selectivity of intrathecal (IT) delivery of an antisense oligonucleotide (AS) targeting rat cPLA(2)IVA mRNA on spinal expression of this enzyme, and (iii) examine the effect of down-regulation of spinal cPLA(2)IVA on peripheral tissue injury-induced pain behavior. Here we demonstrate that cPLA(2)IVA is constitutively expressed in rat spinal cord, predominantly in dorsal horn neurons and oligodendrocytes but not in astrocytes or microglia. Intrathecal injection of AS significantly down-regulated both protein and gene expression of cPLA(2)IVA in rat spinal cord, while control missense oligonucleotide (MS) had no effect. Immunocytochemistry confirmed that the reduction occurred in neurons and oligodendrocytes. cPLA(2)IVA AS did not alter expression of several other PLA(2) isoforms, such as secretory PLA(2) (groups IIA and V) and calcium-independent PLA(2) (group VI), indicating that the AS was specific for cPLA(2)IVA. This selective knockdown of spinal cPLA(2)IVA did not change acute nociception (i.e. paw withdrawal thresholds to acute thermal stimuli and intradermal formalin-induced first phase flinching), however, it significantly attenuated formalin-induced hyperalgesia (i.e. second phase flinching behavior), which reflects spinal sensitization. Thus the present findings suggest that cPLA(2)IVA may specifically participate in spinal nociceptive processing.

Activin acutely sensitizes dorsal root ganglion neurons and induces hyperalgesia via PKC-mediated potentiation of transient receptor potential vanilloid I

Pain hypersensitivity is a cardinal sign of tissue damage, but how molecules from peripheral tissues affect sensory neuron physiology is incompletely understood. Previous studies have shown that activin A increases after peripheral injury and is sufficient to induce acute nociceptive behavior and increase pain peptides in sensory ganglia. This study was designed to test the possibility that the enhanced nociceptive responsiveness associated with activin involved sensitization of transient receptor potential vanilloid I (TRPV1) in primary sensory neurons. Activin receptors were found widely distributed among adult sensory neurons, including those that also express the capsaicin receptor. Whole-cell patch-clamp recording from sensory neurons showed that activin acutely sensitized capsaicin responses and depended on activin receptor kinase activity. Pharmacological studies revealed that the activin sensitization of capsaicin responses required PKCepsilon signaling, but not PI3K (phosphoinositide 3-kinase), ERK (extracellular signal-regulated protein kinase), PKA, PKCalpha/beta, or Src. Furthermore, activin administration caused acute thermal hyperalgesia in wild-type mice, but not in TRPV1-null mice. These data suggest that activin signals through its own receptor, involves PKCepsilon signaling to sensitize the TRPV1 channel, and contributes to acute thermal hyperalgesia.

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.

Peripheral and central mechanisms of pain generation

Pain research has uncovered important neuronal mechanisms that underlie clinically relevant pain states such as inflammatory and neuropathic pain. Importantly, both the peripheral and the central nociceptive system contribute significantly to the generation of pain upon inflammation and nerve injury. Peripheral nociceptors are sensitized during inflammation, and peripheral nerve fibres develop ectopic discharges upon nerve injury or disease. As a consequence a complex neuronal response is evoked in the spinal cord where neurons become hyperexcitable, and a new balance is set between excitation and inhibition. The spinal processes are significantly influenced by brain stem circuits that inhibit or facilitate spinal nociceptive processing. Numerous mechanisms are involved in peripheral and central nociceptive processes including rapid functional changes of signalling and long-term regulatory changes such as up-regulation of mediator/receptor systems. Conscious pain is generated by thalamocortical networks that produce both sensory discriminative and affective components of the pain response.

L-type calcium channels and NMDA receptors: a determinant duo for short-term nociceptive plasticity

In the dorsal horn of the spinal cord, pain-transmitting neurons exhibit action potential windup, a form of short-term plasticity, which consists of a progressive increase in neuronal response during repetitive stimulation of nociceptive input fibers. Windup depends on N-methyl-D-aspartate (NMDA) receptor activation, but previous in vitro studies indicated that windup also relies on intrinsic plateau properties of spinal neurons. In the present study, we considered the possible involvement of these properties in windup in vivo. For this purpose, we first studied a nociceptive flexion reflex in the rat. We showed that windup of the reflex is actually suppressed by blockers of L-type calcium current and Ca(2+)-activated non-specific cationic current (Ican), the two main depolarizing conductances of plateau potentials. We further showed that, during windup, NMDA receptors provide a critical excitatory component in a dynamic balance of excitatory and inhibitory inputs which ultimately activates L-type calcium channels. The nociceptive reflex involves at least two neuronal groups, which may express intrinsic amplification properties, motor neurons and dorsal horn neurons. By means of extracellular recordings in the dorsal horn, we showed that windup of dorsal horn neuron discharge was sensitive to the modulators of L-type calcium current. Altogether, our results suggest that, in vivo, windup also depends on the amplification properties of spinal neurons, the triggering of which requires previous activation of NMDA receptors.

Iteration of high-frequency stimulation enhances long-lasting excitatory responses in the spinal dorsal horn of rats: characterization by optical imaging of signal propagation

To investigate plastic changes in nociceptive sensitivity of the dorsal horn, slow excitatory responses elicited by iteration of high-frequency stimulation were spatiotemporally observed in spinal cord slices of young-adult rats using membrane excitation imaging techniques. Single-pulse stimulation to the dorsal root elicited membrane excitation in lamina II, and high-frequency pulse-train stimulation evoked long-lasting excitation that expanded widely in the dorsal horn. Iteration of high-frequency stimulation enhanced the strength and extent of the excitatory responses, but such augmentation of the excitatory responses disappeared in the presence of an NMDA receptor antagonist (CPP) and was hindered by an NK1 receptor antagonist (L-703.606). The results suggest that activation of both NMDA and NK1 receptors is involved in the enhancement of slow excitatory responses evoked by iteration of high-frequency stimulation.

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.

Heterotopic inputs facilitate poststimulus afterdischarges of spinal WDR neurons in rats with chronic nerve constriction

Heterotopic inputs activate spinal wide dynamic range (WDR) neurons in rats with chronic constriction of one sciatic nerve (CCI rats). A possible contribution from these inputs, to long-lasting afterdischarges (ADs) of noxious evoked responses, was investigated during reversible input blockade from adjacent saphenous nerve and contralateral peripheral nerve territories. The results show significant AD reduction or suppression, indicating that heterotopic afferences contribute to mechanisms underlying prolonged ADs.

Potent analgesic effects of a putative sodium channel blocker M58373 on formalin-induced and neuropathic pain in rats

M58373, 4-[2-(4-hydroxy-4-{[N-(4-isopropoxyphenyl)-N-methylamino]methyl}piperidin-1-yl)ethyl]benzonitrile monohydrochloride, is a novel compound, which has an inhibitory activity on neurotoxin binding to the site 2 of voltage-gated sodium channels. In this study, we investigated the effects of M58373 on substance P release from sensory neurons in vitro and pain behaviors/responses in rats, compared with mexiletine. M58373 (1-10 microM) inhibited veratridine-induced release of substance P from dorsal root ganglion cells. In the formalin test, oral M58373 (0.3-10 mg/kg) reduced the time spent in nociceptive behaviors only in the late phase. In the neuropathic pain model, oral M58373 (1-10 mg/kg) attenuated mechanical allodynia and heat hyperalgesia in the nerve-injured paw without affecting normal responses in the uninjured paw. In contrast, oral mexiletine (10-100 mg/kg) had a narrow therapeutic dose range in both models because of the adverse effects on the central nervous system. These results suggest that M58373 is a favorable prototype for novel anti-neuropathic pain agents.

Protein kinase C inhibitor chelerythrine attenuates the morphine-induced excitatory amino acid release and reduction of the antinociceptive effect of morphine in rats injected intrathecally with pertussis toxin

Neuropathic pain syndromes respond poorly to opioid treatment. In our previous studies, we found that intrathecal (i.t.) injection of pertussis toxin (PTX) produces thermal hyperalgesia, which is poorly responsive to morphine and is accompanied by an increase in cerebrospinal fluid (CSF) levels of excitatory amino acids (EAAs) and protein kinase C (PKC) activation. In the present study, rats were implanted with an i.t. catheter for drug injection and a microdialysis probe for CSF dialysate collection. On the fourth day after injection of PTX (2 microg, i.t.), there was a significant reduction in the antinociceptive effect of morphine (10 microg, i.t.) which was accompanied by an increase in levels of EAAs. Pretreatment with the PKC inhibitor, chelerythrine (25 microg, i.t.) one hour before morphine injection markedly inhibited both effects. These results suggest that, in PTX-treated rats, PKC plays an important role in inhibiting the morphine-induced spinal EAA release, which might be related to the reduced antinociceptive effect of morphine.

Switching-on and -off of bistable spontaneous discharges in rat spinal deep dorsal horn neurons

Somatosensory deep dorsal horn spinal neurons were previously shown to present in vitro a bistable state of activity in which a fixed firing rate is maintained over prolonged periods in the absence of stimulation. Those periods of enhanced spinal spontaneous discharge may play a role in the genesis or maintenance of hyperalgesic states, where episodes of durable spontaneous pain are commonly reported. Here we show in vivo that a small percentage of deep spinal neurons (4% of the recorded population) are capable of rapidly shifting between low-frequency and high-frequency levels of spontaneous activity. At least one of the transitions between the two states was induced by stimulation of the receptive field, making this an interesting and unique case in which stable firing rates are switched-on or -off by somatosensory stimuli.

Secondary hyperalgesia in the monoarthritic rat is mediated by GABAB and NK1 receptors of spinal dorsal horn neurons: A behavior and c-fos study

Secondary hyperalgesia in the monoarthritic rat is accompanied by a decrease in nociceptive activation of spinal neurons expressing GABA(B) receptors and by the opposite effect in the cells expressing neurokinin 1 (NK1)-receptors. In order to ascertain the relative role of each receptor, the effects of intrathecal administration of SP-saporin (SP-SAP), baclofen or both were evaluated, using a model of secondary hyperalgesia that consists of mechanical stimulation of the hindlimb skin close to an inflamed joint. Four days after the induction of monoarthritis by intraarticular injection of Complete Freund's Adjuvant (CFA), a cannula was implanted at T(13)-L(1) level and 10 microl of saline or SP-SAP (10(-6) M) were intrathecally (i.t.) injected. Fourteen days after CFA-injection, half of the animals from each group received i.t. injections of 10 microl saline and the remainder were injected with the same volume of baclofen (1 microg). Ten minutes later, the animals were behaviorally evaluated by the von Frey test or submitted to noxious mechanical stimulation to analyze c-fos expression. The von Frey thresholds increased after the treatments, but more pronouncedly after baclofen or SP-SAP plus baclofen. In segments L(2)-L(3), the spinal area that receives input from the stimulated skin close to the inflamed joint, the numbers of Fos-immunoreactive neurons were reduced after the three treatments both in the superficial and deep dorsal horn. In segments T(13)-L(1), the numbers of Fos-immunoreactive neurons were significantly reduced after treatment with SP-SAP plus baclofen in both dorsal horn regions, and in the deep dorsal horn after baclofen treatment. We conclude that both GABA(B) and NK1 receptors of spinal dorsal horn neurons participate in secondary hyperalgesia in the monoarthritic rat, although the decrease in GABA inhibition appears to play a more important role than the increase in SP-mediated effects.

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.

Analgesic effects of the selective group II (mGlu2/3) metabotropic glutamate receptor agonists LY379268 and LY389795 in persistent and inflammatory pain models after acute and repeated dosing

Group II (mGluR2/3) metabotropic glutamate receptors have been implicated in the mechanisms of persistent pain states. In the present study, the effects of the selective group II metabotropic glutamate receptor agonists LY379268 and LY389795 were evaluated in the formalin test, carrageenan-induced thermal hyperalgesia and mechanical allodynia, and capsaicin-induced mechanical allodynia in rats. The agonists LY379268 and LY389795 produced dose-dependent decreases in formalin-induced behaviors that were antagonized by the mGlu2/3 receptor antagonist LY341495. The group II antagonist LY341495 produced parallel shifts in the LY379268 dose-response curve, consistent with a competitive antagonism. LY379268 decreased formalin-induced behaviors after intracisternal but not intrathecal administration, suggesting primarily a supraspinal site of action. Both LY379268 and LY389795 produced a dose-related reversal of carrageenan-induced thermal hyperalgesia and capsaicin-induced mechanical allodynia, but had no effect on carrageenan-induced mechanical allodynia. Both agonists also increased response latencies in the hot plate test, but were without effect in the tail-flick test. However, both agonists produced motor impairment on the inverted screen at doses that were analgesic. Moreover, tolerance to the analgesic effects of LY379268 developed after 4 days of once-daily repeated administration in the formalin, carrageenan, capsaicin and hot plate tests. The present findings indicate that group II (mGluR2/3) metabotropic glutamate receptors may be involved in the mechanisms of hyperalgesia and allodynia, however tolerance rapidly develops to these effects.

Changes of the neuropeptides content and gene expression in spinal cord and dorsal root ganglion after noxious colorectal distension

Visceral pain/hypersensitivity is a cardinal symptom of functional gastrointestinal disorders. With their peripheral and central (spinal) projections, sensory neurons in the dorsal root ganglia (DRG) are the "gateway" for painful signals emanating from both somatic and visceral structures. In contrast to somatic pain, the neurochemical pathways involved in visceral pain/hypersensitivity have not been well studied. We hypothesized the neuropeptide changes in spinal cord and DRG during visceral pain would mirror similar changes in somatic nociception. Noxious (painful) colorectal distension (CRD) was done by distending a rectal balloon up to 60 mm Hg phasically for 1 h in Sprague-Dawley rats. The spinal content of calcitonin gene-related peptide (CGRP), substance P (SP), galanin and vasoactive intestinal peptide (VIP) as well as their mRNAs in DRG were measured at 0, 4 and 24 h after the CRD. Visceromotor reflex (VMR) was measured by recording the electromyogram at the abdominal muscle in response to CRD. Distal colorectum was removed for evaluating the presence of inflammation. No significant evidence of histological inflammation was seen in the colonic mucosa/submucosa after repeated CRD, which is confirmed by myeloperoxidase assay. The spinal content of CGRP and SP decreased significantly 4 h after CRD, while galanin and VIP levels increased gradually and reached highest level at 24 h (p<0.05). The mRNAs in DRG of the neuropeptides were significantly upregulated after CRD (p<0.05). VMR recording showed the rat's colon became hypersensitive 4 h after CRD, a sequence parallel to the spinal changes of CGRP and SP in timeframe. Noxious mechanical distension of the colorectum causes an acute change in the spinal levels of excitatory neurotransmitters (CGRP and SP), probably reflecting central release of these peptides from sensory neurons and contributing to the hypersensitivity following the noxious CRD. This is followed by a slower change in the levels of the inhibitory neurotransmitter galanin and VIP. Such stimulation results in significant alternation of the gene expression in DRG, reflecting the plasticity of the neuronal response. In the absence of visceral inflammation, the aforementioned neuropeptides are important mediators in the processing of visceral pain/hypersensitivity.

Effect of nor-trimebutine on neuronal activation induced by a noxious stimulus or an acute colonic inflammation in the rat

Nor-trimebutine is the main metabolite of trimebutine that is used in the treatment of patients with irritable bowel syndrome. Nor-trimebutine has a blocking activity on sodium channels and a potent local anesthetic effect. These properties were used to investigate the effect of nor-trimebutine on spinal neuronal activation induced by models of noxious somato-visceral stimulus and acute colonic inflammation. Nor-trimebutine was administered in rats either subcutaneously 30 min before intraperitoneal administration of acetic acid or intracolonically 30 min before intracolonic infusion of trinitrobenzenesulfonic acid. Abdominal contractions were counted for 1 h as a marker of abdominal pain. c-fos expression was used as a marker of neuronal activation and revealed by immunohistochemistry 1h after intraperitoneal acetic acid injection and 2 h after colonic inflammation. Nor-trimebutine decreased Fos expression in the thoraco-lumbar (peritoneal irritation) and lumbo-sacral (colonic inflammation) spinal cord in laminae I, IIo V, VII and X. This effect was also observed in the sacral parasympathetic nucleus after colonic inflammation. Nor-trimebutine induced a significant decrease of abdominal contractions following intraperitoneal acetic acid injection. These data may explain the effectiveness of trimebutine in the therapy of abdominal pain in the irritable bowel syndrome.

Distribution and regulation of L-type calcium channels in deep dorsal horn neurons after sciatic nerve injury in rats

Deep dorsal horn neurons are involved in the processing of nociceptive information in the spinal cord. Previous studies revealed a role of the intrinsic bioelectrical properties (plateau potentials) of deep dorsal horn neuron in neuronal hyperexcitability, indicating their function in pain sensitization. These properties were considered to rely on L-type calcium currents. Two different isotypes of L-type calcium channel alpha 1 subunit have been cloned (Ca(V)1.2 and Ca(V)1.3). Both are known to be expressed in the spinal cord. However, no data were available on their subcellular localization. Moreover, possible changes in Ca(V)1.2 and Ca(V)1.3 expression had never been investigated in nerve injury models. Our study provides evidence for a differential expression of Ca(V)1.2 and Ca(V)1.3 subunits in the somato-dendritic compartment of deep dorsal horn neurons. Ca(V)1.2 immunoreactivity is restricted to the soma and proximal dendrites whereas Ca(V)1.3 immunoreactivity is found in the whole somato-dendritic compartment, up to distal dendritic segments. Moreover, these specific immunoreactive patterns are also found in electrophysiologically identified deep dorsal horn neurons expressing plateau potentials. After nerve injury, namely total axotomy or partial nerve ligation, Ca(V)1.2 and Ca(V)1.3 expression undergo differential changes, showing up- and down-regulation, respectively, both at the protein and at the mRNA levels. Taken together, our data support the role of L-type calcium channels in the control of intrinsic biolectrical regenerative properties. Furthermore, Ca(V)1.2 and Ca(V)1.3 subunits may have distinct and specific roles in sensory processing in the dorsal horn of the spinal cord, the former being most likely involved in long-term changes after nerve injury.

Elimination of neurokinin-1 receptor neurons in caudal nucleus reverses the effects of systemic bicuculline on c-Fos expression in rat trigeminal sensory nucleus: I. High intensity electrical stimulation of the trigeminal ganglion

Although neurokinin-1 receptor (NK-1)-bearing neurons are distributed in lamina I of the trigeminal caudal nucleus (Vc) and constitute major projection neurons, little is known about their fundamental role(s) in nociceptive processing. This study examines the effect of intra cisterna magna injection of substance P (SP) conjugated to saporin (SP-Sap; 5 microM, 5 microl) [with/without systemic administration of bicuculline] on c-Fos expression in the trigeminal sensory nucleus (TSN) induced 2 h after 10 min repetitive electrical stimulation of the trigeminal ganglion (TG) at high intensity (1.0 mA, 5 Hz, 5 ms) in the urethane-anesthetized rat. In the SP-Sap-treated rats, the numbers of NK-1-immunopositive neurons in laminae I and III of the Vc decreased compared with rats similarly pretreated with saline (Sal; 5 microl) or blank-saporin (Bl-Sap; 5 microM, 5 microl). In Sal- or Bl-Sap-treated controls, high intensity stimulation induced c-Fos expression in neurons throughout the full extent of ipsilateral superficial layers of the Vc (VcI/II), magnocellular zone of the Vc (VcIII/IV) and the dorsal or dorsomedial subdivisions of the rostral TSN above the obex (trigeminal principal, oral (Vo) and interpolar nuclei). Preadministration of bicuculline (2 mg/kg, i.p.) decreased the numbers of c-Fos-immunopositive neurons in the VcI/II, VcIII/IV and Vo in Sal- or Bl-Sap-treated controls. In contrast, high intensity stimulation induced less c-Fos-immunopositive neurons in the VcI/II and Vo of rats treated with SP-Sap compared with those in Sal- or Bl-Sap-treated controls. In SP-Sap-treated rats preadministered with bicuculline, the numbers of c-Fos-immunopositive neurons in the VcI/II and Vo were increased compared with the SP-Sap-treated rats preadministered with Sal. These results suggest that NK-1-immunopositive neurons in laminae I and III of Vc play a pivotal role in the nociceptive specific processing in the TSN through GABA(A) receptors.

Capsaicin-evoked substance P release in rat dorsal horn increases after peripheral inflammation: a microdialysis study

Numerous in vitro studies suggest that inflammation is associated with enhanced release of substance P (SP) in the dorsal horn. To test the hypothesis that inflammation increases the evoked concentration of SP in the intact animal, we used in vivo microdialysis with a highly sensitive radioimmunoassay to monitor SP-like immunoreactivity (SP-LI) in the dorsal horn. Seven days after the induction of persistent unilateral inflammation with hindpaw injection of complete Freund's adjuvant, perfusion of the microdialysis probe with 10 microM capsaicin (a concentration which failed to induce SP-LI release in rats without inflammation) induced a significant increase of microdialysate SP-LI. Inclusion of an NMDA antagonist in the perfusion fluid completely blocked this capsaicin-evoked SP release. Administration of a five-fold higher dose of capsaicin did not further increase SP release. These results in a rat model of chronic arthritis suggest that persistent inflammatory signaling facilitates capsaicin-evoked SP release in the dorsal horn in vivo.

Involvement of spinal neurokinins, excitatory amino acids, proinflammatory cytokines, nitric oxide and prostanoids in pain facilitation induced by Phoneutria nigriventer spider venom

The major local symptom of Phoneutria nigriventer envenomation is an intense pain, which can be controlled by infiltration with local anesthetics or by systemic treatment with opioid analgesics. Previous work showed that intraplantar (i.pl) injection of Phoneutria nigriventer venom in rats induces hyperalgesia, mediated peripherally by tachykinin and glutamate receptors. The present study examined the spinal mechanisms involved in pain-enhancing effect of this venom. Intraplantar injection of venom into rat hind paw induced hyperalgesia. This phenomenon was inhibited by intrathecal (i.t.) injection of tachykinin NK1 (GR 82334) or NK2 (GR 94800) receptor antagonists, a calcitonin gene-related peptide (CGRP) receptor antagonist (CGRP8-37) and N-methyl-D-aspartate (NMDA; MK 801 and AP-5), non-NMDA ionotropic (CNQX), or metabotropic (AIDA and MPEP) glutamate receptor antagonists, suggesting the involvement of spinal neurokinins and excitatory amino acids. The role of proinflammatory cytokines, nitric oxide (NO), and prostanoids in spinally mediated pain facilitation was also investigated. Pharmacological blockade of tumour necrosis factor-alpha (TNFalpha) or interleukin-1beta (IL-1beta) reduced the hyperalgesic response to venom. Intrathecal injection of L-N6-(1-iminoethyl)lysine (L-NIL), but not of 7-nitroindazole (7-NI), inhibited hyperalgesia induced by the venom, indicating that NO, generated by the activity of the inducible form of nitric oxide synthase, also mediates this phenomenon. Furthermore, indomethacin, an inhibitor of cyclooxigenases (COX), or celecoxib, a selective inhibitor of COX-2, abolished venom-induced hyperalgesia, suggesting the involvement of spinal prostanoids in this effect. These data indicate that the spinal mechanisms of pain facilitation induced by Phoneutria nigriventer venom involves a plethora of mediators that may cooperate in the genesis of venom-induced central sensitization.

NR2B receptors are involved in the mediation of spinal segmental reflex potentials but not in the cumulative motoneuronal depolarization in vitro

Windup, the frequency dependent build-up of spinal neuronal responses is an electrophysiological model of the development of the central sensitization in the chronic pain states. NR2B subunit containing NMDA-type glutamate receptors are implicated in the windup of dorsal horn neurons, while their role at the motoneuronal level is controversial. The cumulative motoneuronal depolarization in hemisected rat spinal cord preparation is an in vitro model of windup. The role of NR2B receptors in this process, and in the mediation of dorsal root stimulation evoked ventral root reflex potentials was elucidated. Three selective NR2B antagonists; CP-101,606; CI-1041 and Co-101244 (1 microM) were used. They had only weak, but statistically significant inhibitory effect on the early part of ventral root response, and did not influence the cumulative depolarization. On the contrary, non-selective NMDA antagonist APV (40 microM) decreased both responses markedly. We conclude that the pharmacological sensitivities of windup at the sensory and motor levels are different. NR2B containing NMDA receptors have major role in the mediation of the windup of dorsal horn neurons, but their contribution to this phenomenon at the motor level is negligible.

Bidirectional modulation of windup by NMDA receptors in the rat spinal trigeminal nucleus

Activation of afferent nociceptive pathways is subject to activity-dependent plasticity, which may manifest as windup, a progressive increase in the response of dorsal horn nociceptive neurons to repeated stimuli. At the cellular level, N-methyl-d-aspartate (NMDA) receptor activation by glutamate released from nociceptive C-afferent terminals is currently thought to generate windup. Most of the wide dynamic range nociceptive neurons that display windup, however, do not receive direct C-fibre input. It is thus unknown where the NMDA mechanisms for windup operate. Here, using the Sprague-Dawley rat trigeminal system as a model, we anatomically identify a subpopulation of interneurons that relay nociceptive information from the superficial dorsal horn where C-fibres terminate, to downstream wide dynamic range nociceptive neurons. Using in vivo electrophysiological recordings, we show that at the end of this pathway, windup was reduced (24 +/- 6%, n = 7) by the NMDA receptor antagonist AP-5 (2.0 fmol) and enhanced (62 +/- 19%, n = 12) by NMDA (1 nmol). In contrast, microinjections of AP-5 (1.0 fmol) within the superficial laminae increased windup (83 +/- 44%, n = 9), whereas NMDA dose dependently decreased windup (n = 19). These results indicate that NMDA receptor function at the segmental level depends on their precise location in nociceptive neural networks. While some NMDA receptors actually amplify pain information, the new evidence for NMDA dependent inhibition of windup we show here indicates that, simultaneously, others act in the opposite direction. Working together, the two mechanisms may provide a fine tuning of gain in pain.

Substance P and enkephalinergic synapses onto neurokinin-1 receptor-immunoreactive neurons in the pre-Bötzinger complex of rats

Our previous studies have demonstrated that neurokinin-1 receptor (NK1R)-immunoreactive (ir) neurons in the pre-Bötzinger Complex (pre-BötC), the hypothesized kernel of respiratory rhythmogenesis, receive both glutamatergic excitatory and GABAergic or glycinergic inhibitory inputs. Neuromodulators, such as substance P (SP) and opioids, play important roles in normal respiratory activity and respiratory disorders. The identification of the relationship between neurotransmitters and NK1R-ir neurons at the cellular level is essential for understanding the synaptic interaction within the pre-BötC network. Using immunofluorescence and immunogold-silver staining, we wished to exploit SP and enkephalin (ENK) immunoreactivity and their relationships with glutamate, GABA, glycine, or NK1R in the pre-BötC in adult Sprague-Dawley rats. The pre-BötC contained a substantial amount of SP-ir and ENK-ir boutons. They were largely colocalized with glutamate and much less so with GABA. Glycine immunoreactivity was rarely found in either SP-ir or ENK-ir boutons. A number of SP-ir boutons were ENK-ir as well. Synapses were commonly found between SP-ir or ENK-ir terminals and NK1R-ir neurons in the pre-BötC. Most of them were asymmetric. Symmetric synapses made up 10% of all synapses examined between SP-ir boutons and NK1R-ir neurons, and 19% of ENK/NK1R synapses. Colocalization of SP and/or ENK with glutamate in boutons in the pre-BötC implies the combined synaptic release of excitatory amino acid and neuropeptides, which may exert combined post-synaptic effects onto NK1R-ir neurons and contribute to respiratory activity.

Pathophysiology of pain

Pain is a major symptom of many different diseases. Modern pain research has uncovered important neuronal mechanisms that are underlying clinically relevant pain states, and research goes on to define different types of pains on the basis of their neuronal and molecular mechanisms. This review will briefly outline neuronal mechanisms of pathophysiological nociceptive pain resulting from inflammation and injury, and neuropathic pain resulting from nerve damage. Pain is the sensation that is specifically evoked by potential or actual noxious (i.e. tissue damaging) stimuli or by tissue injury. Pain research has not only explored the neuronal and molecular basis of the "pain system" of the healthy subject but has also provided insights into the function and plasticity of the "pain system" during clinically relevant pains such as post-injury pain, inflammatory pain, postoperative pain, cancer pain and neuropathic pain. This review will briefly describe the "pain system" and then address neuronal mechanisms that are involved in clinical pain states.

Reduction by gabapentin of K+-evoked release of [3H]-glutamate from the caudal trigeminal nucleus of the streptozotocin-treated rat

Recently, we showed that gabapentin can inhibit a facilitatory effect of substance P (SP) on K(+)-evoked glutamate release in rat trigeminal slices (Maneuf et al., 2001), and we have now examined the effect of gabapentin on glutamate release in the trigeminal slice from the streptozotocin (STZ)-treated rat. 1. At 4 weeks following STZ treatment (50 mg kg(-1) i.p.), blood glucose was increased in the majority of cases, compared to the control level. All the treated animals showed a significant degree (P<0.001) of tactile allodynia (assessed using von Frey filaments) that did not appear to correlate with blood glucose levels. 2. In this study, we demonstrated that, after STZ treatment, 30 microM gabapentin reduced K(+)-evoked release of [(3)H]-glutamate in either normal (11 mM) or high (30 mM) glucose conditions by 24 and 22%, respectively. In the normal rat, gabapentin (up to 100 microM) is ordinarily unable to affect release of glutamate from the trigeminal slice. 3. The uptake of glutamate in Sp5C punches from streptozotocin-treated rats was reduced under normal glucose conditions (41.7% of control), whereas high glucose restored uptake to normal (84.7% of control). 4. The addition of 1 microm substance P potentiated the evoked release of glutamate in both normal (40% increase) and high glucose (28%), and this was blocked by gabapentin (30 microM) in both conditions. It is interesting to speculate that this ability of gabapentin to reduce the release of glutamate in the trigeminal nucleus after streptozotocin treatment may be of relevance to the antihyperalgesic-allodynic actions of the drug.

Characterization of wide dynamic range neurons in the deep dorsal horn of the spinal cord in preprotachykinin-a null mice in vivo

We previously reported that mice with a deletion of the preprotachykinin-A (pptA) gene, from which substance P (SP) and neurokinin A (NKA) are derived, exhibit reduced behavioral responses to intense stimuli, but that behavioral hypersensitivity after injury is unaltered. To understand the contribution of SP and NKA to nociceptive transmission in the spinal cord, we recorded single-unit activity from wide dynamic range neurons in the lamina V region of the lumbar dorsal horn of urethane-anesthetized wild-type and ppt-A null mutant (-/-) mice. We found that intensity coding to thermal stimuli was largely preserved in the ppt-A -/- mice. Neither the peak stimulus-evoked firing nor the neuronal activity during the initial phase (0-4 s) of the 41-49 degrees C thermal stimuli differed between the genotypes. However, electrophysiological responses during the late phase of the stimulus (5-10 s) and poststimulus (11-25 s) were significantly reduced in ppt-A -/- mice. To activate C-fibers and to sensitize the dorsal horn neurons we applied mustard oil (MO) topically to the hindpaw. We found that neither total MO-evoked activity nor sensitization to subsequent stimuli differed between the wild-type and ppt-A -/- mice. However, the time course of the sensitization and the magnitude of the poststimulus discharges were reduced in ppt-A -/- mice. We conclude that SP and/or NKA are not required for intensity coding or sensitization of nociresponsive neurons in the spinal cord, but that these peptides prolong thermal stimulus-evoked responses. Thus whereas behavioral hypersensitivity after injury is preserved in ppt-A -/- mice, our results suggest that the magnitude and duration of these behavioral responses would be reduced in the absence of SP and/or NKA.

Participation of AMPA- and NMDA-type excitatory amino acid receptors in the spinal reflex transmission, in rat

Classical in vitro and in vivo models and electrophysiological techniques were used to investigate the role of AMPA- and NMDA-type glutamate receptors in various components of spinal segmental reflex potentials. In the rat hemisected spinal cord preparation, the AMPA antagonists NBQX and GYKI 52466 abolished the monosynaptic reflex (MSR) potential but caused only partial inhibition of the motoneuronal population EPSP. NMDA antagonists had no noticeable effect on the MSR in normal medium, but markedly depressed the late part of EPSP. However, an NMDA receptor antagonist sensitive monosynaptic response was recorded in magnesium-free medium at complete blockade of the AMPA receptors. In spinalized rats, the AMPA antagonists completely blocked all components of the dorsal root stimulation evoked potential. MK-801 (2mg/kg, i.v.) reduced monosynaptic responses in a frequency dependent way, with no effect at 0.03 Hz and 22% inhibition at 0.25 Hz. The reduction of the di- and polysynaptic reflex components was about 30% and did not depend on stimulation frequency. Long-latency reflex discharge responses, especially when evoked by train stimulation, were more sensitive to MK-801 than the polysynaptic reflex. These results suggest that glutamate activates MSR pathways through AMPA receptors. However, under certain conditions, NMDA receptors can modulate this transmission through plastic changes in the underlying neuronal circuits. AMPA and NMDA receptors play comparable roles in the mediation of longer latency reflex components.

Calcitonin gene-related peptide regulates expression of neurokinin1 receptors by rat spinal neurons

Although neurokinin 1 (NK1) receptors contribute to hyperalgesia, and their expression is increased in the spinal cord during peripheral inflammation, little is known regarding the signaling molecules and the second messenger pathways that they activate in regulating the expression of the NK1 receptor gene. Because the promoter region of the NK1 receptor contains a cAMP response element (CRE), we tested the hypothesis that calcitonin gene-related peptide (CGRP) regulates the expression of NK1 receptors via a pathway involving activation of the transcription factor cAMP response element binding protein (CREB). Experiments were conducted on primary cultures of neonatal rat spinal neurons. Treatment of cultures with CGRP for 8-24 hr increased (125)I-substance P binding on spinal neurons; the increase in binding was preceded by an elevation in NK1 receptor mRNA. The CGRP-induced change in (125)I-substance P binding was concentration-dependent and was inhibited by the antagonist CGRP(8-37). CGRP increased phosphorylated CREB immunoreactivity and CRE-dependent transcription in neurons, indicating the involvement of the transcription factor CREB. Evidence that CGRP increased cAMP levels in spinal neurons and that the protein kinase A inhibitor H89 attenuated CGRP-induced CRE-dependent transcription suggests that the intracellular pathway stimulated by CGRP leads to activation of protein kinase A. Collectively these data define a role for CGRP as a signaling molecule that induces expression of NK1 receptors in spinal neurons. The data provide evidence that a neuropeptide receptor controls gene expression in the CNS and add another dimension to understanding the cotransmission of substance P and CGRP by primary afferent neurons.