Acute and chronic changes in dorsal horn innervation by primary afferents and descending supraspinal pathways after spinal cord injury

Sprouting of peptidergic nociceptive and descending supraspinal projections to the dorsal horn following spinal cord injury (SCI) has been proposed as a mechanism of neuropathic pain. To identify structural changes that could initiate or maintain SCI pain, we used a complete transection model in rats to examine how structural remodeling in the dorsal horn rostral to the lesion relates to distance from injury, laminar region, and duration of injury. The major classes of C-fiber primary afferents differed greatly in their susceptibility to structural and chemical changes and their ability to undergo plasticity. Peptidergic primary afferents showed a widespread loss throughout the dorsal horn of segments approaching the injury site. Some of this loss may have been due to decreased neuropeptide expression. The reduction in peptidergic fibers was transient, indicating compensatory sprouting and perhaps also increased neuropeptide expression within the cord. Nonpeptidergic afferents expressing GFRalpha1 were largely unaffected by SCI. In contrast, in GFRalpha2-expressing nonpeptidergic afferents SCI caused a permanent loss of dorsal horn innervation. Unexpectedly, GFRalpha2 was transiently induced throughout deeper laminae but this was not due to upregulation of GFRalpha2 in dorsal root ganglia. We also observed permanent sprouting of catecholamine terminals of supraspinal origin. This was restricted to the superficial laminae. Our results show that SCI caused a loss of sensory input as well as structural remodeling such that the balance of nociceptive inputs and descending modulation was permanently altered. These changes may contribute to mechanisms rostral to the site of SCI that trigger and maintain neuropathic pain.

Cocaine- and amphetamine-regulated transcript peptide (CART) is present in peptidergic C primary afferents and axons of excitatory interneurons with a possible role in nociception in the superficial laminae of the rat spinal cord

Cocaine- and amphetamine-regulated transcript peptides (CART) have been implicated in the regulation of several physiological functions, including pain transmission. A dense plexus of CART-immunoreactive fibres has been described in the superficial laminae of the spinal cord, which are key areas in sensory information and pain processing. In this study, we used antibody against CART peptide, together with markers for various types of primary afferents, interneurons and descending systems to determine the origin of the CART-immunoreactive axons in the superficial laminae of the rat spinal cord. Calcitonin gene-related peptide (CGRP), a marker for peptidergic primary afferents in the dorsal horn, was present in 72.6% and 34.8% of CART-immunoreactive axons in lamina I and II, respectively. The majority of these fibres also contained substance P (SP), while a few were somatostatin (SOM)-positive. The other subpopulation of CART-immunoreactive boutons in lamina I and II also expressed SP and/or SOM without CGRP, but contained vesicular glutamate transporter 2, which is present mainly in excitatory interneuronal terminals. Our data demonstrate that the majority of CART-immunoreactive axons in the spinal dorsal horn originate from peptidergic nociceptive primary afferents, while the rest arise from excitatory interneurons that contain SP or SOM. This strongly suggests that CART peptide can affect glutamatergic neurotransmission as well as the release and effects of SP and SOM in nociception and other sensory processes.

Potassium- and capsaicin-induced release of agmatine from spinal nerve terminals

Agmatine (decarboxylated arginine) was originally identified in the CNS as an imidazoline receptor ligand. Further studies demonstrated that agmatine antagonizes NMDA receptors and inhibits nitric oxide synthase. Intrathecally administered agmatine inhibits opioid tolerance and hyperalgesia evoked by inflammation, nerve injury, and intrathecally administered NMDA. These actions suggest an anti-glutamatergic role for agmatine in the spinal cord. We have previously reported that radiolabeled agmatine is transported into spinal synaptosomes in an energy- and temperature-dependent manner. In the present study, we demonstrate that agmatine is releasable from purified spinal nerve terminals upon depolarization. When exposed to either elevated potassium or capsaicin, tritiated agmatine (but not its precursor L-arginine or its metabolite putrescine) is released in a calcium-dependent manner. Control experiments confirmed that the observed release was specific to depolarization and not due to permeabilization of or degradation of synaptosomes. That capsaicin-evoked stimulation results in agmatine release implicates the participation of primary afferent nerve terminals. Radiolabeled agmatine also accumulates in purified spinal synaptosomal vesicles in a temperature-dependent manner, suggesting that the source of releasable agmatine may be vesicular in origin. These results support the proposal that agmatine may serve as a spinal neuromodulator involved in pain processing.

Psychophysics, Flare, and Neurosecretory Function in Human Pain Models: Capsaicin Versus Electrically Evoked Pain

Intradermal capsaicin injection (CAP) and electrical current stimulation (ES) are analyzed in respect to patterns and test-retest reliability of pain as well as sensory and neurosecretory changes. In 10 healthy subjects, 2x CAP (50 microg) and 2x ES (5 to 30 mA) were applied to the volar forearm. The time period between 2 identical stimulations was about 4 months. Pain ratings, areas of mechanical hyperalgesia, and allodynia were assessed. The intensity of sensory changes was quantified by using quantitative sensory testing. Neurogenic flare was assessed by using laser Doppler imaging. Calcitonin gene-related peptide (CGRP) release was quantified by dermal microdialysis in combination with an enzyme immunoassay. Time course and peak pain ratings were different between CAP and ES. Test-retest correlation was high (r > or = 0.73). Both models induced primary heat hyperalgesia and primary plus secondary pin-prick hyperalgesia. Allodynia occurred in about half of the subjects. Maximum flare sizes did not differ between CAP and ES, but flare intensities were higher for ES. Test-retest correlation was higher for flare sizes than for flare intensity. A significant CGRP release could only be measured after CAP. The different time courses of pain stimulation (CAP: rapidly decaying pain versus ES: pain plateau) led to different peripheral neurosecretory effects but induced similar central plasticity and hyperalgesia.

PERSPECTIVE

The present study gives a detailed overview of psychophysical and neurosecretory characteristics induced by noxious stimulation with capsaicin and electrical current. We describe differences, similarities, and reproducibility of these human pain models. These data might help to interpret past and future results of human pain studies using experimental pain.

Characteristics of Sensory Dorsal Root Ganglia Neurons Innervating the Lumbar Vertebral Body in Rats

Characteristics of sensory dorsal root ganglia (DRG) neurons innervating the L5 vertebral body were investigated in rats by using a retrograde neurotransport method, lectin affinity- and immuno-histochemistry to further elucidate the causes of diffuse pain suffered by some elderly patients in their back, lateral trunk, and iliac crest, after lumbar osteoporotic vertebral fracture. We used calcitonin gene-related peptide (CGRP) as a marker of small peptide-containing neurons and the glycoprotein binding the isolectin from Griffonia simplicifolia (IB4) as a marker of small non-peptide-containing neurons. Neurons innervating the L5 vertebral bodies, retrogradely labeled with fluoro-gold (FG), were distributed throughout DRGs from T13 to L6. The proportion of CGRP-immunoreactive (IR) FG-labeled neurons was 32%. The proportion of IB4-binding FG-labeled neurons was significantly smaller, at 4%. Other neurons that were non-CGRP-IR and non-IB4-binding were mostly large neurons, and they may transmit proprioception from vertebral bodies. Most neurons transmitting pain are CGRP-IR peptide-containing neurons. They may have a more significant role in pain sensation in the vertebral bodies as peptidergic DRG neurons.

PERSPECTIVE

This article shows that vertebral bodies are innervated by CGRP-IR neurons. CGRP-IR neurons may play a role in pain sensation through peptidergic DRG neurons. These findings contribute to an understanding of pain associated with the vertebral body such as tumor, infection, or osteoporotic fracture.

Mechano- and chemosensitivity of rat nodose neurones--selective excitatory effects of prostacyclin

Nodose ganglion sensory neurones exert a significant reflex autonomic influence. We contrasted their mechanosensitivity, excitability and chemosensitivity in response to the stable prostacyclin (PGI2) analogue carbacyclin (cPGI) in culture. Under current clamp conditions we measured changes in membrane potential (DeltamV) and action potential (AP) responses to mechanically induced depolarizations and depolarizing current injections before and after superfusion of cPGI (1 microM and 10 microM). Chemosensitivity was indicated by augmentation of AP firing frequency and increased maximum gain of AP frequency (max. dAP/dDeltamV), during superfusion with cPGI. Results indicate that two groups of neurones, A and B, are mechanosensitive (MS) and one group, C, is mechanoinsensitive (MI). Group A shows modest depolarization without AP generation during mechanical stimulation, and no increase in max. dAP/dDeltamV, despite a marked increase in electrical depolarization with cPGI. Group B shows pronounced mechanical depolarization accompanied by enhanced AP discharge with cPGI, and an increase in max. dAP/dDeltamV. Group C remains MI after cPGI but is more excitable and markedly chemosensitive (CS) with a pronounced enhancement of max. dAP/dDeltamV with cPGI. The effect of cPGI on ionic conductances indicates that it does not sensitize the mechanically gated depolarizing degenerin/epithelial Na+ channels (DEG/ENaC), but it inhibits two voltage-gated K+ currents, Maxi-K and M-current, causing enhanced AP firing frequency and depolarization, respectively. We conclude that MS nodose neurones may be unimodal MS or bimodal MS/CS, and that MI neurones are unimodal CS, and much more CS to cPGI than MS/CS neurones. We suggest that the known excitatory effect of PGI2 on baroreceptor and vagal afferent fibres is mediated by inhibition of voltage-gated K+ channels (Maxi-K and M-current) and not by an effect on mechanically gated DEG/ENaC channels.

Selective action of noradrenaline and serotonin on neurones of the spinal superficial dorsal horn in the rat

The superficial dorsal horn of the spinal cord (SDH; laminae I and II) receives strong input from thin primary afferent fibres and is involved in nociception, pain, temperature sensing and other experiences. The SDH also is the target of serotonergic and adrenergic projections from the brain stem. The interaction between descending pathways that utilize particular mediators and the neurone population of the SDH is poorly understood. To explore this issue, in rat spinal cord slices during whole-cell recordings from identified SDH neurones, noradrenaline (NA) or serotonin (5HT) were briefly applied in the superfusing artificial cerebrospinal fluid. The action of these agents proved specifically related to the type of SDH neurone and its dorsal-root afferent input. Vertical, radial and tonic central lamina II cells consistently expressed outward current to both NA and 5HT, but transient central and Substance P (SP)-insensitive lamina I cells were unaffected directly by either NA or 5HT. Extended islet cells responded with outward current to NA and inward current to 5HT. Lamina I SP-sensitive cells expressed an outward current regularly to NA. 5HT had inhibitory effects on Adelta and C fibre input to all types of SDH neurones. NA inhibited C fibre input to transient central neurones. The present results support the idea that descending systems may have multiple functions, including but not limited to nociceptive modulation.

Lowering of interstitial fluid pressure after neurogenic inflammation in mouse skin is partly dependent on mast cells

Neurogenic inflammation is known to induce lowering of interstitial fluid pressure (Pif) in mouse skin. This study examined the possible role of mast cell activation secondary to neuropeptide release in lowering of Pifby using KitW/ KitW-vmice, which are devoid of mast cells, including connective tissue mast cells (CTMCs). Pifwas measured in paw skin of anesthetized (fentanyl-fluanison and midazolam, 1:1) mice with glass capillaries connected to a servo-controlled counterpressure system. In contrast to wild-type mice, intravenous administration of mast cell-activating compound 48/80 induced no lowering of Pifin KitW/ KitW-vmice. Intravenous challenge with substance P (SP), calcitonin gene-related peptide (CGRP), or capsaicin induced a significant ( P < 0.05) lowering of Pifin wild-type mice to −2.16 ± 0.28, −1.96 ± 0.11, and −2.22 ± 0.19 mmHg, respectively, compared with vehicle (−0.49 ± 0.11 mmHg). In KitW/ KitW-vmice the Pifresponse to SP was completely abolished (−0.53 ± 0.32 mmHg) while the response to CGRP and capsaicin was attenuated (−1.33 ± 0.13 and −1.42 ± 0.13 mmHg, respectively) although significantly ( P < 0.05) lowered compared with vehicle. Immunohistochemical analysis revealed no difference in distribution or density of SP- and CGRP-immunoreactive fibers in paws of KitW/ KitW-vcompared with wild-type mice. We conclude that lowering of Pifnormally depends on mast cells. However, the sensory nerves can also elicit a lowering of Pifthat is independent of mast cells.

What lies beneath the surface of the itch in adults?

Pruritus is synonymous with itching. Many medical conditions are complicated by chronic pruritus compromising the patient's quality of life. The majority of pruritic stimuli are transmitted through C fibers into the lateral spinothalamic tract and then into the somatic sensory cortex where the itching is detected. Histamine, substance P, and tumor necrosis factor a play significant roles in the perception of pruritus. Medical conditions in adults with significant pruritus will be defined in this review.

Phospholipase Cbeta 3 mediates the scratching response activated by the histamine H1 receptor on C-fiber nociceptive neurons

Phospholipase Cbeta (PLCbeta) isozymes represent a family of molecules that link G protein-coupled receptors (GPCRs) to an intracellular signaling network. Here, we investigated the function of PLCbeta isozymes in sensory neurons by using mutant mice deficient for specific PLCbeta family members. Expression analysis indicated that PLCbeta3, one of the four isoforms, is predominantly expressed in a subpopulation of C-fiber nociceptors. A subset of these neurons expressed the histamine H1 receptor. Ca(2+) imaging studies revealed that PLCbeta3 specifically mediates histamine-induced calcium responses through the histamine H1 receptor in cultured sensory neurons. In line with this, we found that PLCbeta3(-/-) mice showed significant defects in scratching behavior induced by histamine; histamine-trifluoromethyl-toluidine (HTMT), a selective H1 agonist; and compound 48/80, a mast cell activator. These results demonstrate that PLCbeta3 is required to mediate "itch" sensation in response to histamine acting on the histamine H1 receptor in C-fiber nociceptive neurons.

Vesicular release of glutamate from unmyelinated axons in white matter

Directed fusion of transmitter-laden vesicles enables rapid intercellular signaling in the central nervous system and occurs at synapses within gray matter. Here we show that action potentials also induce the release of glutamate from axons in the corpus callosum, a white matter region responsible for interhemispheric communication. Callosal axons release glutamate by vesicular fusion, which induces quantal AMPA receptor-mediated currents in NG2(+) glial progenitors at anatomically distinct axo-glial synaptic junctions. Glutamate release from axons was facilitated by repetitive stimulation and could be inhibited through activation of metabotropic autoreceptors. Although NG2(+) cells form associations with nodes of Ranvier in white matter, measurements of conduction velocity indicated that unmyelinated fibers are responsible for glutamatergic signaling with NG2(+) glia. This activity-dependent secretion of glutamate was prevalent in the developing and mature mouse corpus callosum, indicating that axons within white matter both conduct action potentials and engage in rapid neuron-glia communication.

Morphine acts via μ-opioid receptors to enhance spinal regeneration and synaptic reconstruction of primary afferent fibers injured by sciatic nerve crush

The present study investigated whether morphine can promote regeneration and synaptic reconstruction of the terminals of injured primary afferent fibers in lamina II of the spinal cord in rats following sciatic nerve injury. Fluoride-resistant acid phosphatase (FRAP)-positive terminals in lamina II of the L4 spinal segment after sciatic nerve injury were assessed after treatment with vehicle, morphine, and naloxone plus morphine. Under the electron microscope, types I and II complex terminals of unmyelinated afferent fibers from the dorsal root, simple terminals of interneuronal axons, and terminals of descending axons at lamina II of the L4 spinal segment were documented in the different groups after injury. FRAP-positive terminals in lamina II were depleted after sciatic nerve injury in the vehicle group. Treatment with morphine increased the numbers of FRAP-positive terminals, and this was prevented by naloxone. The present study demonstrates that morphine may promote the regeneration and synaptic reconstruction of the terminals of injured primary unmyelinated afferent fibers in lamina II of spinal cord, by a process mediated by mu-opioid receptors.

Stimulation of mu and delta opioid receptors induces hyperalgesia while stimulation of kappa receptors induces antinociception in the hot plate test in the naked mole-rat (Heterocephalus glaber)

The antinociceptive effects of highly selective mu (DAMGO), delta (DPDPE) and kappa (U-50488 and U-69593) opioid agonists were evaluated following intraperitoneal (i.p.) administration in the naked mole-rat. A hot plate test set at 60 degrees C was used as a nociceptive test and the latency to the stamping of the right hind paw (response latency) was used as the end-point. DAMGO (5-10 mg/kg) and DPDPE (2.5-5 mg/kg) caused a naloxone-reversible significant decrease in the mean response latency. Subcutaneous injection of naloxonazine (20 mg/kg) 24h prior to the administration of DAMGO (5 mg/kg) also blocked the reduction in the response latency observed when DAMGO was injected alone. On the contrary, U-50488 (2.5-5 mg/kg) or U-69593 (0.08 or 0.1 mg/kg) caused a naloxone-reversible significant increase in the mean response latency. These results showed that activation of mu or delta receptors caused hyperalgesia, whereas activation of kappa receptors caused antinociception in the hot plate test in naked mole-rat. This suggests that mu and delta receptors modulate thermal pain in a different way than kappa receptors in the naked mole-rat. It is not possible at the moment to point out how they modulate thermal pain as little is known about the neuropharmacology of the naked mole-rat.

Effect of olvanil and anandamide on vagal C-fiber subtypes in guinea pig lung

Certain fatty acid amides such as anandamide (AEA) and olvanil are agonists for the transient receptor potential, vanilloid-1 (TRPV1) receptor, but have been found to activate TRPV1-containing C-fibers in some tissues but not others. We used extracellular recording and whole-cell patch clamp techniques to investigate the effect of olvanil and AEA on different types of vagal C-fibers innervating the same tissue, namely jugular and nodose vagal C-fibers in guinea pig lungs. A 30 s exposure to AEA and olvanil caused action potential discharge in all nodose C-fiber innervating lung but failed to activate jugular C-fibers innervating lung and airways. The activation of nodose C-fibers was blocked by the TRPV1 antagonist iodo-resiniferatoxin. In whole-cell patch clamp recordings of dissociated nodose and jugular capsaicin-sensitive neurons labeled from lungs and airways, olvanil induced large TRPV1-dependent inward currents in cell bodies of both nodose and jugular ganglion neurons. Prolonged exposure (up to 5 min) to olvanil caused action potential discharge in jugular C-fiber innervating lung but the onset latency was four times longer in jugular than in nodose C-fibers. The onsets of capsaicin response in nodose and jugular C-fibers were not different. Decreasing the tissue temperature to 25 degrees C increased the onset latency of olvanil-induced activation of nodose C-fibers 2-3-fold, but did not effect the latency of the capsaicin response. Capsaicin, olvanil, and AEA stimulate jugular C-fibers leading to tachykinergic contractions of isolated bronchi. The time to reach half-maximum is more than four times longer for olvanil and AEA, as compared to capsaicin in evoking contractions. We conclude that brief exposure to certain fatty acid amides, such as AEA and olvanil activate nodose but not jugular C-fiber terminals in the lungs. We hypothesize that this is because the nodose C-fiber terminals are equipped with a temperature-dependent mechanism for effectively and rapidly transporting the TRPV1 agonists so that they gain access to the intracellular binding sites on TRPV1. This transport mechanism may be differently expressed in two distinct subtypes of pulmonary C-fiber terminals innervating the same tissue.

Expression of hyperpolarization-activated cyclic nucleotide-gated cation channels in rat dorsal root ganglion neurons innervating urinary bladder

Afferent pathways innervating the urinary bladder consist of myelinated Adelta- and unmyelinated C-fibers, the neuronal cell bodies of which correspond to medium and small-sized cell populations of dorsal root ganglion (DRG) neurons, respectively. Since hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel currents have been identified in various peripheral sensory neurons, we examined the expression of isoforms of HCN channels in the L6-S1 spinal cord and bladder afferent neurons from L6-S1 DRG in rats. Among HCN-1, HCN-2 and HCN-4 channel subtypes, positive staining with HCN-2 antibodies was found in the superficial dorsal horn of the spinal cord and small- and medium-sized unidentified DRG neurons. In dye-labeled bladder afferent neurons, HCN-2-positive cells were found in approximately 60% of neurons, and HCN-2 was expressed in both small- and medium-sized neurons with a higher ratio (expression ratio: 61% and 50% of neurons, respectively) compared with unidentified DRG neurons, in which the HCN expression ratio was 47% and 21% of small- and medium-sized cells, respectively. These results suggest that HCN-2 is the predominant subtype of HCN channels, which can control neuronal excitability, in small-sized C-fiber and medium-sized Adelta fiber DRG neurons including bladder afferent neurons, and might modulate activity of bladder afferent pathways controlling the micturition reflex.

Local and descending circuits regulate long-term potentiation and zif268 expression in spinal neurons

Long-term potentiation (LTP), a use dependent long-lasting modification of synaptic strength, was first discovered in the hippocampus and later shown to occur in sensory areas of the spinal cord. Here we demonstrate that spinal LTP requires the activation of a subset of superficial spinal dorsal horn neurons expressing the neurokinin-1 receptor (NK1-R) that have previously been shown to mediate certain forms of hyperalgesia. These neurons participate in local spinal sensory processing, but are also the origin of a spino-bulbo-spinal loop driving a 5-hydroxytryptamine 3 receptor (5HT3-R)- mediated descending facilitation of spinal pain processing. Using a saporin-substance P conjugate to produce site-specific neuronal ablation, we demonstrate that NK1-R expressing cells in the superficial dorsal horn are crucial for the generation of LTP-like changes in neuronal excitability in deep dorsal horn neurons and this is modulated by descending 5HT3-R-mediated facilitatory controls. Hippocampal LTP is associated with early expression of the immediate-early gene zif268 and knockout of the gene leads to deficits in long-term LTP and learning and memory. We found that spinal LTP is also correlated with increased neuronal expression of zif268 in the superficial dorsal horn and that zif268 antisense treatment resulted in deficits in the long-term maintenance of inflammatory hyperalgesia. Our results support the suggestion that the generation of LTP in dorsal horn neurons following peripheral injury may be one mechanism whereby acute pain can be transformed into a long-term pain state.

The differential effects of halothane and isoflurane on windup of dorsal horn neurons selected in unanesthetized decerebrated rats

Halothane and isoflurane, in the peri-minimum alveolar anesthetic concentration (MAC) range, exert differential effects on spinal nociceptive neurons, whereby halothane further depresses their responses from 0.8 to 1.2 MAC, whereas isoflurane does not. We presently investigated if these anesthetics differentially affect windup, the progressive increase in neuronal responses to repetitive noxious stimuli, over a broad concentration range from 0 to 1.2 MAC. In decerebrated rats, single-unit recordings were made from dorsal horn neurons exhibiting windup to 20 1-Hz C-fiber strength electrical stimuli. Halothane and isoflurane (0, 0.4, 0.8, and 1.2 MAC) were tested in a counterbalanced crossover protocol. Increasing halothane and isoflurane from 0 to 1.2 MAC progressively suppressed the response to the first stimulus, as well as summed responses to all stimuli (to 34% +/- 8% and 50% +/- 8%, respectively; P < 0.05). Absolute windup (summed response minus 20x the first response) was suppressed by both anesthetics from 0 to 0.8 MAC, with further depression by halothane but not isoflurane at 1.2 MAC. Responses of neurons isolated at 0 MAC were partially, but never totally, depressed at 0.8 MAC. The dose-dependent suppression of windup is consistent with reduced temporal summation of pain. Further depression at 1.2 MAC halothane, but not isoflurane, suggests different sites of immobilizing action for these two anesthetics. Immobility seems to not be mediated by severe anesthetic depression of a subpopulation of nociceptive neurons.

Heterosynaptic modulation of the dorsal root potential in the turtle spinal cord in vitro

In the somatosensory system, the flow of sensory information is regulated at early stages by presynaptic inhibition. Recent findings have shown that the mechanisms generating the primary afferent depolarization (PAD) associated with presynaptic inhibition are complex, with some components mediated by a non-spiking mechanism. How sensory inputs carried by neighbouring afferent fibres interact to regulate the generation of PAD, and thus presynaptic inhibition, is poorly known. Here, we investigated the interaction between neighbouring primary afferents for the generation of PAD in an in vitro preparation of the turtle spinal cord. To monitor PAD we recorded the dorsal root potential (DRP), while the simultaneous cord dorsum potential (CDP) was recorded to assess the population postsynaptic response. We found that the DRP and the CDP evoked by a primary afferent test stimulus was greatly reduced by a conditioning activation of neighbouring primary afferents. This depression had early and late components, mediated in part by GABAA and GABAB receptors, since they were reduced by bicuculline and SCH 50911 respectively. However, with the selective stimulation of C and Adelta fibres in the presence of TTX, the early and late depression of the DRP was replaced by facilitation of the GABAergic and glutamatergic components of the TTX-resistant DRP. Our findings suggest a subtle lateral excitatory interaction between primary afferents for the generation of PAD mediated by a non-spiking mechanism that may contribute to shaping of information transmitted by C and Adelta fibres in a spatially confined scale in analogy with the retina and olfactory bulb.

Clonidine depresses LTP of C-fiber evoked field potentials in spinal dorsal horn via NO-cGMP pathway

Clonidine, a specific alpha2-adrenergic receptor agonist, has been found to be effective for the treatment of neuropathic pain, the mechanism underlying the effect is, however, not well understood. Here, the effect of clonidine on long-term potentiation (LTP) of C-fiber evoked field potentials in spinal dorsal horn, which is a synaptic model of injury-induced hyperalgesia, was investigated. LTP of C-fiber evoked field potentials was recorded in the superficial layers of spinal dorsal horn in anesthetized adult Sprague-Dawley rats. Clonidine and other substances were applied locally at the recording spinal segments before or after LTP induction by tetanic stimulation. We found that (1) Clonidine completely blocked LTP induction, when applied 30 min before tetanic stimulation and depressed spinal LTP, when applied 30 min and 3 h after LTP induction. (2) The inhibitory effect of clonidine on spinal LTP had two phases: a fast phase lasting for about 3.5 h and a slow phase persisting for the rest time of experiments (up to 8 h after drug). (3) Spinal clonidine at low dose (10.7 micro g/100 micro l) depressed spinal LTP but not C-fiber baseline response and at higher dose (107 micro g/100 micro l) depressed both of them. (4) Pretreatment with alpha2-adrenergic receptor antagonist yohimbine completely blocked the inhibitory effect of clonidine. (5) Pretreatment with muscarinic receptor antagonist atropine, nitric oxide synthesis inhibitor l-NNA or cGMP inhibitor ODQ depressed the fast phase inhibition significantly and abolished the slow phase inhibition completely. These results suggest that clonidine may exert analgesic effect by depressing the synaptic plasticity in spinal dorsal horn, via activation of muscarinic receptor-NO-cGMP pathway.

Enhancement of bupivacaine local anesthesia with the potassium channel blocker ibutilide

In some clinical settings it is necessary to inject large volumes of local anesthetic--and consequently very high doses--in order to provide an adequate level of block. Subsequent absorption of these high doses, or inadvertent intravenous administration of even small doses, has led to systemic toxicity. Thus, it is desirable to develop adjuvants that are inert alone, but would enhance the potency and/or efficacy of local anesthetics to improve their safety. Adelta/C fibers possess K(+) channels identified as sustained delayed rectifier type K(DR) currents and transient A-type K(A) currents. In the heart, the class III antiarrhythmic drug ibutilide blocks the cardiac component of the rapid delayed rectifying K(+) current (IKr). Experiments were conducted to determine whether co-administration of the K(+) channel blocker ibutilide would enhance the local anesthetic bupivacaine in mice. After injecting bupivacaine mixed with vehicle or ibutilide in the popliteal region of mice, paw withdrawal latencies were determined by applying the plantar aspect of a single hind-paw to the surface a 55 degrees C hot-plate device. 0.5% Bupivacaine+ibutilide (7.8x10(-5) M) elicited significantly longer hot-plate latencies than 0.5% bupivacaine+vehicle. In addition, bupivacaine was 2.6-fold more potent when co-administered with ibutilide rather than vehicle. Epinephrine extends the tissue concentrations of local anesthetics by inducing localized vasoconstriction. Epinephrine augmented the enhancement by ibutilide of bupivacaine's potency by 6.8-fold. In summary, ibutilide may enhance the effects of bupivacaine by blocking K(+) channels on sensory nociceptive nerves.

Loss of spinal substance P pain transmission under the condition of LPA1 receptor-mediated neuropathic pain

Among various machineries occurring in the experimental neuropathic pain model, there exists the loss of pain transmission through C-fiber neurons as well as the hypersensitivity through A-fibers. The current study reveals that molecular machineries underlying the latter hypersensitivity are derived from the events through LPA₁ receptor and its downstream RhoA-activation following peripheral nerve injury. The loss of C-fiber responses, which are mediated by spinal substance P (SP) pain transmission was observed with the nociceptive flexor responses by intraplantar injection of SP in nerve-injured mice. The immunohistochemistry revealed that SP signal in the dorsal horn was markedly reduced in such mice. All these changes were completely abolished in LPA₁^-/- mice or by the pretreatment with BoNT/C3, a RhoA inhibitor. In addition, the loss of C-fiber responses and the down-regulation of spinal SP signal induced by single intrathecal LPA injection were also abolished in such treatments. All these results suggest that the loss of pain transmission through polymodal C-fiber neurons is also mediated by the LPA₁ activation following nerve injury.

Intense isolectin-B4 binding in rat dorsal root ganglion neurons distinguishes C-fiber nociceptors with broad action potentials and high Nav1.9 expression

Binding to isolectin-B4 (IB4) and expression of tyrosine kinase A (trkA) (the high-affinity NGF receptor) have been used to define two different subgroups of nociceptive small dorsal root ganglion (DRG) neurons. We previously showed that only nociceptors have high trkA levels. However, information about sensory and electrophysiological properties in vivo of single identified IB4-binding neurons, and about their trkA expression levels, is lacking. IB4-positive (IB4+) and small dark neurons had similar size distributions. We examined IB4-binding levels in >120 dye-injected DRG neurons with sensory and electrophysiological properties recorded in vivo. Relative immunointensities for trkA and two TTX-resistant sodium channels (Nav1.8 and Nav1.9) were also measured in these neurons. IB4+ neurons were classified as strongly or weakly IB4+. All strongly IB4+ neurons were C-nociceptor type (C-fiber nociceptive or unresponsive). Of 32 C-nociceptor-type neurons examined, approximately 50% were strongly IB4+, approximately 20% were weakly IB4+ and approximately 30% were IB4-. Adelta low-threshold mechanoreceptive (LTM) neurons were weakly IB4+ or IB4-. All 33 A-fiber nociceptors and all 44 Aalpha/beta-LTM neurons examined were IB4-. IB4+ compared with IB4- C-nociceptor-type neurons had longer somatic action potential durations and rise times, slower conduction velocities, more negative membrane potentials, and greater immunointensities for Nav1.9 but not Nav1.8. Immunointensities of IB4 binding in C-neurons were positively correlated with those of Nav1.9 but not Nav1.8. Of 23 C-neurons tested for both trkA and IB4, approximately 35% were trkA+/IB4+ but with negatively correlated immunointensities; 26% were IB4+/trkA-, and 35% were IB4-/trkA+. We conclude that strongly IB4+ DRG neurons are exclusively C-nociceptor type and that high Nav1.9 expression may contribute to their distinct membrane properties.

Differential Expression of Capsaicin-, Menthol-, and Mustard Oil-Sensitive Receptors in Naive Rat Geniculate Ganglion Neurons

The roles of capsaicin, menthol, and mustard oils and their receptors in geniculate ganglion (GG) neurons still remain to be elucidated. These receptors belong to the transient receptor potential (TRP) family. Capsaicin-, menthol-, and mustard oil-sensitive receptors are TRPV1, TRPM8, and TRPA1, respectively. The present study aimed to investigate the expression of TRPV1, TRPM8, and TRPA1 in naive rat GG neurons. Furthermore, we examined whether these TRP-expressing GG neurons are myelinated A-fiber or unmyelinated C-fiber neurons. Firstly, using reverse transcription-polymerase chain reaction, TRPV1 mRNA and TRPA1 mRNA were distinctly detected in the naive GG. TRPM8 mRNA was faintly detected. Secondly, using in situ hybridization, TRPV1 mRNA- or TRPA1 mRNA-labeled neurons (signal/noise ratio >or= 10) were observed in 15-20% of GG neurons. Few neurons were labeled by TRPM8 mRNA. Thirdly, neurofilament 200 (NF200) protein, a marker of mylinated A-fiber neurons, was detected in 57% of naive GG neurons. Coexpression of TRPV1 mRNA or TRPA1 mRNA with NF200 was detected in 10% of GG neurons. The present study confirmed the expression of the TRP receptors in the naive GG. The possible roles of TRP receptors in naive GG neurons in somatosensory or gustatory function were suggested.

Evidence for both adenosine A1 and A2A receptors activating single vagal sensory C-fibres in guinea pig lungs

We addressed the hypothesis that single vagal afferent C-fibres can be stimulated via either the adenosine A1 or A2A receptor subtypes. The effect of adenosine on the nerve terminals of vagal sensory nerve subtypes was evaluated in an ex vivo perfused guinea pig lung preparation using extracellular recording techniques. Adenosine (10 microm) consistently evoked action potential discharge in lung C-fibre terminals arising from the nodose ganglia, but failed to evoke action potential discharge in most jugular ganglion C-fibres. Adenosine also failed to activate stretch-sensitive nodose A-fibres in the lungs. The selective A1 antagonist DPCPX (0.1 microm) or the selective A2A antagonist SCH 58261 (0.1 microm) partially inhibited the nodose C-fibre activation by adenosine, and the combination of both antagonists almost completely inhibited the response. The adenosine-induced action potential discharge in nodose C-fibres was mimicked by either the selective A1 agonist CCPA (1 microm) or the selective A2A agonist CGS 21680 (1 microm). Single cell PCR techniques revealed that adenosine A1 and A2A receptor mRNA was expressed in individual nodose neurons retrogradely labelled from the lungs. The gramicidin-perforated patch clamp technique on neurons retrogradely labelled from the lungs was employed to study the functional consequence of adenosine receptor agonists directly on neuronal membrane properties. Both the selective A1 agonist CCPA (1 microm) and the selective A2A agonist CGS 21680 (1 microm) depolarized the airway-specific, capsaicin-sensitive, nodose neurons to action potential threshold. The data support the hypothesis that adenosine selectively depolarizes vagal nodose C-fibre terminals in the lungs to action potential threshold, by stimulation of both adenosine A1 and A2A receptor subtypes located in the neuronal membrane.