Central control of opioid-induced mechanical hypersensitivity and tolerance in mice.

Repetitive use of morphine (MF) and other opioids can trigger two major pain-related side effects: opioid-induced hypersensitivity (OIH) and analgesic tolerance, which can be subclassified as mechanical and thermal. The central mechanisms underlying mechanical OIH/tolerance remain unresolved. Here, we report that a brain-to-spinal opioid pathway, starting from µ-opioid receptor (MOR)-expressing neuron in the lateral parabrachial nucleus (lPBNMOR+) via dynorphin (Dyn) neuron in the paraventricular hypothalamic nucleus (PVHDyn+) to κ-opioid receptor (KOR)-expressing GABAergic neuron in the spinal dorsal horn (SDHKOR-GABA), controls repeated systemic administration of MF-induced mechanical OIH and tolerance in mice. The above effect is likely mediated by disruption of dorsal horn gate control for MF-resistant mechanical pain via silencing of the Dyn-positive GABAergic neurons in the SDH (lPBNMOR+ → PVHDyn+ → SDHKOR-GABA → SDHDyn-GABA). Repetitive binding of MF to MORs during repeated MF administration disrupted the above circuits. Targeting the above brain-to-spinal opioid pathways rescued repetitive MF-induced mechanical OIH and tolerance.

Identification of brain-to-spinal circuits controlling the laterality and duration of mechanical allodynia in mice.

Mechanical allodynia (MA) represents one prevalent symptom of chronic pain. Previously we and others have identified spinal and brain circuits that transmit or modulate the initial establishment of MA. However, brain-derived descending pathways that control the laterality and duration of MA are still poorly understood. Here we report that the contralateral brain-to-spinal circuits, from Oprm1 neurons in the lateral parabrachial nucleus (lPBNOprm1), via Pdyn neurons in the dorsal medial regions of hypothalamus (dmHPdyn), to the spinal dorsal horn (SDH), act to prevent nerve injury from inducing contralateral MA and reduce the duration of bilateral MA induced by capsaicin. Ablating/silencing dmH-projecting lPBNOprm1 neurons or SDH-projecting dmHPdyn neurons, deleting Dyn peptide from dmH, or blocking spinal κ-opioid receptors all led to long-lasting bilateral MA. Conversely, activation of dmHPdyn neurons or their axonal terminals in SDH can suppress sustained bilateral MA induced by lPBN lesion.

Microglial Depletion does not Affect the Laterality of Mechanical Allodynia in Mice.

Mechanical allodynia (MA), including punctate and dynamic forms, is a common and debilitating symptom suffered by millions of chronic pain patients. Some peripheral injuries result in the development of bilateral MA, while most injuries usually led to unilateral MA. To date, the control of such laterality remains poorly understood. Here, to study the role of microglia in the control of MA laterality, we used genetic strategies to deplete microglia and tested both dynamic and punctate forms of MA in mice. Surprisingly, the depletion of central microglia did not prevent the induction of bilateral dynamic and punctate MA. Moreover, in dorsal root ganglion-dorsal root-sagittal spinal cord slice preparations we recorded the low-threshold Aß-fiber stimulation-evoked inputs and outputs of superficial dorsal horn neurons. Consistent with behavioral results, microglial depletion did not prevent the opening of bilateral gates for Aß pathways in the superficial dorsal horn. This study challenges the role of microglia in the control of MA laterality in mice. Future studies are needed to further understand whether the role of microglia in the control of MA laterality is etiology-or species-specific.

Targeting Peripheral µ-opioid Receptors or µ-opioid Receptor-Expressing Neurons Does not Prevent Morphine-induced Mechanical Allodynia and Anti-allodynic Tolerance.

The chronic use of morphine and other opioids is associated with opioid-induced hypersensitivity (OIH) and analgesic tolerance. Among the different forms of OIH and tolerance, the opioid receptors and cell types mediating opioid-induced mechanical allodynia and anti-allodynic tolerance remain unresolved. Here we demonstrated that the loss of peripheral µ-opioid receptors (MORs) or MOR-expressing neurons attenuated thermal tolerance, but did not affect the expression and maintenance of morphine-induced mechanical allodynia and anti-allodynic tolerance. To confirm this result, we made dorsal root ganglia-dorsal roots-sagittal spinal cord slice preparations and recorded low-threshold Aß-fiber stimulation-evoked inputs and outputs in superficial dorsal horn neurons. Consistent with the behavioral results, peripheral MOR loss did not prevent the opening of Aß mechanical allodynia pathways in the spinal dorsal horn. Therefore, the peripheral MOR signaling pathway may not be an optimal target for preventing mechanical OIH and analgesic tolerance. Future studies should focus more on central mechanisms.

Embryonic Ketamine Produces a Downregulation of Prefrontal Cortex NMDA Receptors and Anxiety-Like Behavior in Adult Offspring.

Previous studies have focused on the effects of N-methyl-D-aspartate receptor (NMDAR) blockade on neonates, but little is known about the effect of the embryonic NMDAR blockade on offspring, especially the long-lasting effect, on behavior in adulthood. Here, pregnant rats at E14 were treated with ketamine for 5 successive days and undergone multiple behavior tests, electrophysiology experiment, and Western blotting analysis to detect the alterations in their offspring. We found that embryonic ketamine treatment induced anxiety-like behavior in adulthood (8-week old) offspring. At the same period, we observed an attenuation of NMDA-evoked current as well as decreased NR2A and NR2B membrane expression in the prefrontal cortex (PFC), but not in the hippocampus or amygdala. Selective inhibition experiments with NR2A or NR2B specific antagonists suggested that embryonic ketamine treatment induced NMDAR current attenuation was likely mediated by changes in NR2A subunit. Moreover, at the 4-week time point, NMDA-evoked current was unchanged in PFC, but enhanced in hippocampal CA1 area, which may be caused by the over expression of NR2B in the hippocampus at 4-week time. Furthermore, NR2B knockdown, by using NR2B-shRNA lentivirus, in the hippocampal CA1 area at 3-4-week of age significantly rescued the decrease in NR2A expression in the PFC and anxiety-like behavior observed at 8-week adult offspring rats. In conclusion, our results suggested that embryonic ketamine treatment induced anxiety-like behavior and the downregulation of NMDAR function in PFC in the adulthood period of offspring, which might result from the enhanced function of NMDARs in the hippocampus at the 4-week juvenile time point.

Developmental exposure to mercury chloride impairs social behavior in male offspring dependent on genetic background and maternal autoimmune environment.

To date, the connection between inorganic mercury (Hg) and social behavior remains incompletely understood. The aim of this study was to investigate the influence of maternal autoimmunity by inorganic Hg (Hg2+) exposure on social behavior of offspring. Wild-type (WT) and immunoglobulin deficient (Ig-/-) B10.S dams fertilized by male WT B10.S or SJL mice were treated with 50 µM Hg chloride (HgCl2). Non-pregnant female WT B10.S mice were used to investigate factors regulating HgCl2-induced autoimmunity to brain. HgCl2 selectively impaired social behavior in male offspring, but not female offspring from WT B10.S dams × male SJL, in that only male offspring displayed reduced time distribution with the stranger mouse, decreased sniffing to the stranger mouse and increased self-grooming. HgCl2 did not disrupt social behavior of male or female offspring from WT B10.S dams × male WT B10.S or Ig-/- B10.S dams × male SJL. The offspring from WT and Ig-/- B10.S dams × male SJL had equivalent autoimmunity to brain antigens during HgCl2 exposure, indicating that maternal, but not offspring-derived anti-brain antibodies (Ab) impaired social behavior of the offspring. Non-pregnant WT B10.S mice treated with HgCl2 had increased anti-brain Ab dependent on increase in CD4 T cell activation and IFNγ signaling to macrophages. IFNγ interaction with macrophages drove B cells and plasma cells to produce IgG. Therefore, HgCl2 selectively impaired social behavior in males with certain genetic background via maternally derived anti-brain Ab production, thus providing a novel insight into our current understanding of Hg toxicity.

Memantine selectively prevented the induction of dynamic allodynia by blocking Kir2.1 channel and inhibiting the activation of microglia in spinal dorsal horn of mice in spared nerve injury model.

BACKGROUND: Memantine is one of the important clinical medications in treating moderate to severe Alzheimer disease. The effect of memantine on preventing or treating punctate allodynia has been thoroughly studied but not on the induction of dynamic allodynia. The aim of this study is to investigate whether memantine could prevent the induction of dynamic allodynia and its underlying spinal mechanisms. RESULTS: (1) In in vivo spared nerve injury pain model, pretreatment with memantine at a lower dose (10 nmol, intrathecal; memantine-10) selectively prevented the induction of dynamic allodynia but not the punctate allodynia. (2) Pretreatment with either MK801-10 (MK801-10 nmol, intrathecal) or higher dose of memantine (30 nmol, intrathecal; memantine-30) prevented the induction of both dynamic and punctate allodynia. (3) Memantine-10 showed significant effect on the inhibition of the spared nerve injury-induced overactivation of microglia in spinal dorsal horn. (4) In contrast, in complete freund's adjuvant (CFA) model, memantine-10 neither affected the CFA injection-induced activation of microglia in spinal dorsal horn nor the induction of dynamic allodynia. (5) Immunohistological studies showed Kir2.1 channel distributed widely and co-localized with microglia in the spinal dorsal horn of mice. (6) Pretreatment with either minocycline, a microglia inhibitor, or ML133, a Kir2.1 inhibitor, both selectively prevented the overactivation of microglia in spinal dorsal horn and the induction of dynamic allodynia following spared nerve injury. CONCLUSION: The selective inhibitory effect on the induction of dynamic allodynia in spared nerve injury model by low dose of the memantine (memantine-10) was tightly correlated with the blockade of microglia Kir2.1 channel to suppress the microglia activation.

Mercury impact on hematopoietic stem cells is regulated by IFNγ-dependent bone marrow-resident macrophages in mice.

Although immunotoxic effects of mercury (Hg) have been extensively investigated, the influence of Hg on hematopoietic stem cells (HSC) remains elusive. The aim of this study was to investigate the effects of Hg on HSC. B10.S (H-2s) and DBA/2 mice (H-2d) were treated with Hg chloride (25, 50 or 100 µM HgCl2) or methyl Hg (1.25, 3.75 or 6.25 µM MeHg) via drinking water for 4 weeks, and thereafter, HSC in the bone marrow (BM) were evaluated. The number of HSC in B10.S mice was increased after treatment with 50 µM HgCl2 and decreased after treatment with 100 µM HgCl2; the number of HSC in DBA/2 mice was reduced after treatment with 50 µM HgCl2 and unaffected after treatment with 25 µM HgCl2. These effects from the HgCl2 treatments were associated with alterations of HSC proliferation, IFNγ expression and BM-resident macrophages. In vivo neutralization of IFNγ diminished the HgCl2-driven HSC proliferation, and in vivo replenishment of recombinant IFNγ eliminated the HgCl2 suppression of HSC proliferation and allowed HgCl2 enhancement of proliferation, suggesting a pivotal role of IFNγ in HSC proliferation regulated by HgCl2. In vivo depletion of macrophages and an in vitro co-culture assay indicated that BM-resident macrophages promoted HSC proliferation during HgCl2 exposure. Furthermore, the induction of BM-resident macrophages was critically dependent on IFNγ. In contrast, MeHg did not influence HSC in B10.S or DBA/2 mice. Collectively, HgCl2, but not MeHg, affects HSC through regulating IFNγ-dependent BM-resident macrophages in mice. These findings reveal a previously unknown toxicity of Hg.

Cadmium Activates Noncanonical Wnt Signaling to Impair Hematopoietic Stem Cell Function in Mice.

Cadmium (Cd) is a toxic heavy metal that impairs the development of hematopoietic stem cells (HSCs) in mice, yet the mechanism of how Cd influences HSC remains elusive. Herein, we show that Cd activated non-canonical Wnt signaling pathway to impair HSC function in mice. After exposure to 10 ppm Cd chloride (CdCl2) via drinking water for 3 months, C57BL/6 mice displayed aberrant HSC function, in that HSC from Cd-treated mice were less efficient in rescue of lethally irradiated hosts and less competitive under mixed chimeric condition. Further analyses indicated that the small GTPase cdc42 was activated and its distribution pattern was depolarized in HSC by Cd exposure, and inhibition of cdc42 by casin, a selective chemical inhibitor, recovered the HSC capacity in rescue assay and their potential for lymphopoiesis under competitive mixed chimeric assay. Cd interaction with HSC was sufficient to promote noncanonical Wnt signaling pathway, but not canonical Wnt signaling pathway, to drive cdc42 activation and further increase the expression of C/EBPα and decrease the expression of Hhex. Moreover, Cd-induced activation of non-canonical Wnt signaling pathway in HSC did not persist long-termly in the presence of a normal niche without Cd, in that the elevated non-canonical Wnt signaling by Cd was diminished in HSC in the BM of normal recipients receiving purified HSC from Cd-treated mice after 6 months posttransplantation. Taken together, our study suggests that Cd activates cdc42 of noncanonical Wnt signaling pathway to impair HSC function, a previously unknown mechanism for Cd toxicity on HSC.

Spinal Circuits Transmitting Mechanical Pain and Itch.

In 1905, Henry Head first suggested that transmission of pain-related protopathic information can be negatively modulated by inputs from afferents sensing innocuous touch and temperature. In 1965, Melzak and Wall proposed a more concrete gate control theory of pain that highlights the interaction between unmyelinated C fibers and myelinated A fibers in pain transmission. Here we review the current understanding of the spinal microcircuits transmitting and gating mechanical pain or itch. We also discuss how disruption of the gate control could cause pain or itch evoked by innocuous mechanical stimuli, a hallmark symptom for many chronic pain or itch patients.

Identification of spinal circuits involved in touch-evoked dynamic mechanical pain.

Mechanical hypersensitivity is a debilitating symptom for millions of chronic pain patients. It exists in distinct forms, including brush-evoked dynamic and filament-evoked punctate hypersensitivities. We reduced dynamic mechanical hypersensitivity induced by nerve injury or inflammation in mice by ablating a group of adult spinal neurons defined by developmental co-expression of VGLUT3 and Lbx1 (VT3Lbx1 neurons): the mice lost brush-evoked nocifensive responses and conditional place aversion. Electrophysiological recordings show that VT3Lbx1 neurons form morphine-resistant polysynaptic pathways relaying inputs from low-threshold Aß mechanoreceptors to lamina I output neurons. The subset of somatostatin-lineage neurons preserved in VT3Lbx1-neuron-ablated mice is largely sufficient to mediate morphine-sensitive and morphine-resistant forms of von Frey filament-evoked punctate mechanical hypersensitivity. Furthermore, acute silencing of VT3Lbx1 neurons attenuated pre-established dynamic mechanical hypersensitivity induced by nerve injury, suggesting that these neurons may be a cellular target for treating this form of neuropathic pain.

Gate control of mechanical itch by a subpopulation of spinal cord interneurons.

Light mechanical stimulation of hairy skin can induce a form of itch known as mechanical itch. This itch sensation is normally suppressed by inputs from mechanoreceptors; however, in many forms of chronic itch, including alloknesis, this gating mechanism is lost. Here we demonstrate that a population of spinal inhibitory interneurons that are defined by the expression of neuropeptide Y::Cre (NPY::Cre) act to gate mechanical itch. Mice in which dorsal NPY::Cre-derived neurons are selectively ablated or silenced develop mechanical itch without an increase in sensitivity to chemical itch or pain. This chronic itch state is histamine-independent and is transmitted independently of neurons that express the gastrin-releasing peptide receptor. Thus, our studies reveal a dedicated spinal cord inhibitory pathway that gates the transmission of mechanical itch.

Identification of spinal circuits transmitting and gating mechanical pain.

Pain information processing in the spinal cord has been postulated to rely on nociceptive transmission (T) neurons receiving inputs from nociceptors and Aß mechanoreceptors, with Aß inputs gated through feed-forward activation of spinal inhibitory neurons (INs). Here, we used intersectional genetic manipulations to identify these critical components of pain transduction. Marking and ablating six populations of spinal excitatory and inhibitory neurons, coupled with behavioral and electrophysiological analysis, showed that excitatory neurons expressing somatostatin (SOM) include T-type cells, whose ablation causes loss of mechanical pain. Inhibitory neurons marked by the expression of dynorphin (Dyn) represent INs, which are necessary to gate Aß fibers from activating SOM(+) neurons to evoke pain. Therefore, peripheral mechanical nociceptors and Aß mechanoreceptors, together with spinal SOM(+) excitatory and Dyn(+) inhibitory neurons, form a microcircuit that transmits and gates mechanical pain. PAPERCLIP:

Activation of extrasynaptic GABA(A) receptors inhibits cyclothiazide-induced epileptiform activity in hippocampal CA1 neurons.

Extrasynaptic GABA(A) receptors (GABA(A)Rs)-mediated tonic inhibition is reported to involve in the pathogenesis of epilepsy. In this study, we used cyclothiazide (CTZ)-induced in vitro brain slice seizure model to explore the effect of selective activation of extrasynaptic GABA(A)Rs by 4,5,6,7-tetrahydroisoxazolo[5,4-c] pyridine-3-ol (THIP) on the CTZ-induced epileptiform activity in hippocampal neurons. Perfusion with CTZ dose-dependently induced multiple epileptiform peaks of evoked population spikes (PSs) in CA1 pyramidal neurons, and treatment with THIP (5 µmol/L) significantly reduced the multiple PS peaks induced by CTZ stimulation. Western blot showed that the δ-subunit of the GABA(A)R, an extrasynaptic specific GABA(A)R subunit, was also significantly down-regulated in the cell membrane 2 h after CTZ treatment. Our results suggest that the CTZ-induced epileptiform activity in hippocampal CA1 neurons is suppressed by the activation of extrasynaptic GABA(A)Rs, and further support the hypothesis that tonic inhibition mediated by extrasynaptic GABA(A)Rs plays a prominent role in seizure generation.

(R)-alpha-methylhistamine suppresses inhibitory neurotransmission in hippocampal CA1 pyramidal neurons counteracting propofol-induced amnesia in rats.

BACKGROUND: Propofol is a short-acting, intravenous general anesthetic that is widely used in clinical practice for short procedures; however, it causes depressed cognitive function for several hours thereafter. (R)-alpha-methylhistamine (RAMH), a selective histamine H3 receptor agonist, can enhance memory retention and attenuates memory impairment in rats. In this study, we investigated whether RAMH could rescue propofol-induced memory deficits and the underlying mechanisms partaking in this process. METHODS: In the modified Morris water maze (MWM) test, rats were randomized into the following groups: control, propofol (25 mg/kg, i.p., 30 min before training), RAMH (10 mg/kg, i.p., 60 min before training), and propofol plus RAMH. All randomized rats were subjected to 2 days of training, and a probe test was conducted on day 3. Field excitatory postsynaptic potentials were recorded from CA1 neurons in rat hippocampal slices, and long-term potentiation (LTP) was induced by either theta-burst stimulation (TBS) or high-frequency tetanic stimulation (HFS). Spontaneous and miniature inhibitory (sIPSCs, mIPSCs) or excitatory (sEPSCs, mEPSCs) postsynaptic currents were recorded from CA1 pyramidal neurons by whole-cell patch clamp. RESULTS: In the MWM task, propofol injection significantly impaired spatial memory retention. Pretreatment with RAMH reversed propofol-induced memory retention. In hippocampal CA1 slices, propofol perfusion markedly inhibited TBS- but not HFS-induced LTP. Co-perfusion of RAMH reversed the inhibitory effect of propofol on TBS-induced LTP reduction. Furthermore, in hippocampal CA1 pyramidal neurons, RAMH significantly suppressed the frequency but not the amplitude of sIPSCs and mIPSCs and had little effects on both the frequency and amplitude of sEPSCs and mEPSCs. CONCLUSIONS: Our results suggest that RAMH, by inhibiting presynaptic GABAergic neurotransmission, suppresses inhibitory neurotransmission in hippocampal CA1 pyramidal neurons, which in turn reverses inhibition of CA1 LTP and the spatial memory deficits induced by propofol in rats.

Role of P2X7 receptor-mediated IL-18/IL-18R signaling in morphine tolerance: multiple glial-neuronal dialogues in the rat spinal cord.

UNLABELLED: The glial function in morphine tolerance has been explored, but its mechanisms remain unclear. Our previous study has showed that microglia-expressed P2X7 receptors (P2X7R) contribute to the induction of tolerance to morphine analgesia in rats. This study further explored the potential downstream mechanisms of P2X7R underlying morphine tolerance. The results revealed that the blockade of P2X7 receptor by P2X7R antagonist or targeting small interfering RNA (siRNA) reduced tolerance to morphine analgesia in the pain behavioral test and spinal extracellular recordings in vivo and whole-cell recording of the spinal cord slice in vitro. Chronic morphine treatment induced an increase in the expression of interleukin (IL)-18 by microglia, IL-18 receptor (IL-18R) by astrocytes, and protein kinase Cγ (PKCγ) by neurons in the spinal dorsal horn, respectively, which was blocked by a P2X7R antagonist or targeting siRNA. Chronic morphine treatment also induced an increased release of D-serine from the spinal astrocytes. Further, both D-amino acid oxygenase (DAAO), a degrading enzyme of D-serine, and bisindolylmaleimide α (BIM), a PKC inhibitor, attenuated morphine tolerance. The present study demonstrated a spinal mechanism underlying morphine tolerance, in which chronic morphine triggered multiple dialogues between glial and neuronal cells in the spinal cord via a cascade involving a P2X7R-IL-18-D-serine-N-methyl-D-aspartate receptor (NMDAR)-PKCγ-mediated signaling pathway. PERSPECTIVE: The present study shows that glia-neuron interaction via a cascade (P2X7R-IL-18-D-serine-NMDAR-PKCγ) in the spinal cord plays an important role in morphine tolerance. This article may represent potential new therapeutic targets for preventing morphine analgesic tolerance in clinical management of chronic pain.

Involvement of ryanodine receptors in tetanic sciatic stimulation-induced long-term potentiation of spinal dorsal horn and persistent pain in rats.

Tetanic stimulation of the sciatic nerve induces long-term potentiation (LTP) of C-fiber-evoked field potentials in the spinal dorsal horn and persistent pain, suggesting that spinal LTP may be a substrate for central sensitization of the pain pathway. However, its cellular mechanism remains unclear. The present study provides electrophysiological and behavioral evidence for the involvement of ryanodine receptor (RyR) in the induction of spinal LTP and persistent pain in rats. The specific inhibitor of ryanodine receptor, ryanodine and dantrolene, dose dependently blocked the induction, but not maintenance, of spinal LTP and reduced persistent pain behaviors induced by tetanic sciatic stimulation. Both cyclic ADP ribose (cADPR), an endogenous agonist of RyR, and (±)-1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluromethyl)-phenyl]-3-pyridine carboxylic acid methyl ester (Bay K 8644), an agonist of L-type calcium channel, attenuated ryanodine-induced inhibition. Immunohistochemistry and electron microscopic observation showed that RyR subtypes RyR1 and RyR3 were located in the spinal dorsal horn. The results suggest that RyRs are involved in synaptic plasticity of the spinal pain pathway and may be a novel target for treating pain. © 2012 Wiley Periodicals, Inc.

Enhanced inhibitory synaptic transmission in the spinal dorsal horn mediates antinociceptive effects of TC-2559.

BACKGROUND: TC-2559 is a selective α4ß2 subtype of nicotinic acetylcholine receptor (nAChR) partial agonist and α4ß2 nAChR activation has been related to antinociception. The aim of this study is to investigate the analgesic effect of TC-2559 and its underlying spinal mechanisms. RESULTS: 1) In vivo bioavailability study: TC-2559 (3 mg/kg) had high absorption rate in rats with maximal total brain concentration reached over 4.6 µM within first 15 min after administration and eliminated rapidly with brain half life of about 20 min after injection. 2) In vivo behavioral experiments: TC-2559 exerts dose dependent antinociceptive effects in both formalin test in mice and chronic constriction injury (CCI) model in rats by activation of α4ß2 nAChRs; 3) Whole-cell patch-clamp studies in the superficial dorsal horn neurons of the spinal cord slices: perfusion of TC-2559 (2 µM) significantly increased the frequency, but not amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs). The enhancement of sIPSCs was blocked by pre-application of DHßE (2 µM), a selective α4ß2 nicotinic receptor antagonist. Neither the frequency nor the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) of spinal dorsal horn neurons were affected by TC-2559. CONCLUSIONS: Enhancement of inhibitory synaptic transmission in the spinal dorsal horn via activation of α4ß2 nAChRs may be one of the mechanisms of the antinociceptive effects of TC-2559 on pathological pain models. It provides further evidence to support the notion that selective α4ß2 subtype nAChR agonist may be developed as new analgesic drug for the treatment of neuropathic pain.

Ryanodine receptors contribute to the induction of nociceptive input-evoked long-term potentiation in the rat spinal cord slice.

BACKGROUND: Our previous study demonstrated that nitric oxide (NO) contributes to long-term potentiation (LTP) of C-fiber-evoked field potentials by tetanic stimulation of the sciatic nerve in the spinal cord in vivo. Ryanodine receptor (RyR) is a downstream target for NO. The present study further explored the role of RyR in synaptic plasticity of the spinal pain pathway. RESULTS: By means of field potential recordings in the adult male rat in vivo, we showed that RyR antagonist reduced LTP of C-fiber-evoked responses in the spinal dorsal horn by tetanic stimulation of the sciatic nerve. Using spinal cord slice preparations and field potential recordings from superficial dorsal horn, high frequency stimulation of Lissauer's tract (LT) stably induced LTP of field excitatory postsynaptic potentials (fEPSPs). Perfusion of RyR antagonists blocked the induction of LT stimulation-evoked spinal LTP, while Ins(1,4,5)P3 receptor (IP(3)R) antagonist had no significant effect on LTP induction. Moreover, activation of RyRs by caffeine without high frequency stimulation induced a long-term potentiation in the presence of bicuculline methiodide and strychnine. Further, in patch-clamp recordings from superficial dorsal horn neurons, activation of RyRs resulted in a large increase in the frequency of miniature EPSCs (mEPSCs). Immunohistochemical study showed that RyRs were expressed in the dorsal root ganglion (DRG) neurons. Likewise, calcium imaging in small DRG neurons illustrated that activation of RyRs elevated [Ca(2+)]i in small DRG neurons. CONCLUSIONS: These data indicate that activation of presynaptic RyRs play a crucial role in the induction of LTP in the spinal pain pathway, probably through enhancement of transmitter release.