Spinal endocannabinoids and CB1 receptors mediate C-fiber-induced heterosynaptic pain sensitization

Genome-wide expression analysis of Ptf1a- and Ascl1-deficient mice reveals new markers for distinct dorsal horn interneuron populations contributing to nociceptive reflex plasticity

Inhibitory interneurons of the spinal dorsal horn play critical roles in the processing of noxious and innocuous sensory information. They form a family of morphologically and functionally diverse neurons that likely fall into distinct subtypes. Traditional classifications rely mainly on differences in dendritic tree morphology and firing patterns. Although useful, these markers are not comprehensive and cannot be used to drive specific genetic manipulations targeted at defined subsets of neurons. Here, we have used genome-wide expression profiling of spinal dorsal horns of wild-type mice and of two strains of transcription factor-deficient mice (Ptf1a(-/-) and Ascl1/Mash1(-/-) mice) to identify new genetic markers for specific subsets of dorsal horn inhibitory interneurons. Ptf1a(-/-) mice lack all inhibitory interneurons in the dorsal horn, whereas only the late-born inhibitory interneurons are missing in Ascl1(-/-) mice. We found 30 genes that were significantly downregulated in the dorsal horn of Ptf1a(-/-) mice. Twenty-one of those also showed reduced expression in Ascl1(-/-) mice. In situ hybridization analyses of all 30 genes identified four genes with primarily non-overlapping expression patterns in the dorsal horn. Three genes, pDyn coding the neuropeptide dynorphin, Kcnip2 encoding a potassium channel associated protein, and the nuclear receptor encoding gene Rorb, were expressed in Ascl1-dependent subpopulations of the superficial dorsal horn. The fourth gene, Tfap2b, encoding a transcription factor, is expressed mainly in a Ascl1-independent subpopulation of the deep dorsal horn. Functional experiments in isolated spinal cords showed that the Ascl1-dependent inhibitory interneurons are key players of nociceptive reflex plasticity.

Sphingosine lysolipids in the CNS: endogenous cannabinoid antagonists or a parallel pain modulatory system?

A significant number of patients experience chronic pain and the intractable side effects of currently prescribed pain medications. Recent evidence indicates important pain-modulatory roles for two classes of G-protein-coupled receptors that are activated by endogenous lipid ligands, the endocannabinoid (eCB) and sphingosine-1-phosphate (S1P) receptors, which are widely expressed in both the immune and nervous systems. In the central nervous system (CNS), CB1 cannabinoid and S1P1 receptors are most abundantly expressed and exhibit overlapping anatomical distributions and similar signaling mechanisms. The eCB system has emerged as a potential target for treatment of chronic pain, but comparatively little is known about the roles of S1P in pain regulation. Both eCB and S1P systems modulate pain perception via the central and peripheral nervous systems. In most paradigms studied, the eCB system mainly inhibits pain perception. In contrast, S1P acting peripherally at S1P1 and S1P3 receptors can enhance sensitivity to various pain stimuli or elicit spontaneous pain. However, S1P acting at S1P1 receptors and possibly other targets in the CNS can attenuate sensitivity to various pain stimuli. Interestingly, other endogenous sphingolipid derivatives might play a role in central pain sensitization. Moreover, these sphingolipids can also act as CB1 cannabinoid receptor antagonists, but the physiological relevance of this interaction is unknown. Overall, both eCB and sphingolipid systems offer promising targets for the treatment of chronic pain. This review compares and contrasts the eCB and S1P systems with a focus on their roles in pain modulation, and considers possible points of interaction between these systems.

Differential modulation of nociceptive versus non-nociceptive synapses by endocannabinoids

Background

Although a number of clinical and preclinical studies have demonstrated analgesic effects of cannabinoid treatments, there are also instances when cannabinoids have had no effect or even exacerbated pain. The observed pro-nociceptive effects appear to be due to cannabinoid-induced disinhibition of afferent synaptic input to nociceptive circuits. To better understand how cannabinoid-mediated plasticity can have both pro- and anti-nociceptive effects, we examined the possibility that cannabinoids differentially modulate nociceptive vs. non-nociceptive synapses onto a shared postsynaptic target. These experiments were carried out in the central nervous system (CNS) of the medicinal leech, in which it is possible to intracellularly record from presynaptic nociceptive (N-cell) or pressure-sensitive (P-cell) neurons and their shared postsynaptic targets.

Results

The endocannabinoid 2-arachidonoyl glycerol (2AG) elicited significant long-lasting depression in nociceptive (N-cell) synapses. However, non-nociceptive (P-cell) synapses were potentiated following 2AG treatment. 2AG-induced potentiation of non-nociceptive synapses was blocked by the TRPV antagonist SB366791, suggesting involvement of the same TRPV-like receptor that has already been shown to mediate endocannabinoid-dependent depression in nociceptive inputs. Treatment with the GABA receptor antagonist bicuculline also blocked 2AG-induced potentiation, consistent with the idea that increased synaptic signaling was the result of endocannabinoid-mediated disinhibition. Interestingly, while bicuculline by itself increased non-nociceptive synaptic transmission, nociceptive synapses were depressed by this GABA receptor antagonist indicating that nociceptive synapses were actually excited by GABAergic input. Consistent with these observations, GABA application depolarized the nociceptive afferent and hyperpolarized the non-nociceptive afferent.

Conclusions

These findings show that endocannabinoids can differentially modulate nociceptive vs. non-nociceptive synapses and that GABAergic regulation of these synapses plays an important role in determining whether endocannabinoids have a potentiating or depressing effect.

Spinal administration of the monoacylglycerol lipase inhibitor JZL184 produces robust inhibitory effects on nociceptive processing and the development of central sensitization in the rat

BACKGROUND AND PURPOSE

The cannabinoid receptor-mediated analgesic effects of 2-arachidonoylglycerol (2-AG) are limited by monoacylglycerol lipase (MAGL). 4-nitrophenyl 4-[bis (1,3-benzodioxol-5-yl) (hydroxy) methyl] piperidine-1-carboxylate (JZL184) is a potent inhibitor of MAGL in the mouse, though potency is reportedly reduced in the rat. Here we have assessed the effects of spinal inhibition of MAGL with JZL184 on nociceptive processing in rats.

EXPERIMENTAL APPROACH

In vivo spinal electrophysiological assays in anaesthetized rats were used to determine the effects of spinal administration of JZL184 on spinal nociceptive processing in the presence and absence of hindpaw inflammation. Contributions of CB(1) receptors to these effects was assessed with AM251. Inhibition of 2-oleoylglycerol hydrolytic activity and alterations of 2-AG in the spinal cord after JZL 184 were also assessed.

KEY RESULTS

Spinal JZL184 dose-dependently inhibited mechanically evoked responses of wide dynamic range (WDR) neurones in naïve anaesthetized rats, in part via the CB(1) receptor. A single spinal administration of JZL184 abolished inflammation-induced expansion of the receptive fields of spinal WDR neurones. However, neither spinal nor systemic JZL184 altered levels of 2-AG, or 2-oleoylglycerol hydrolytic activity in the spinal cord, although JZL184 displayed robust inhibition of MAGL when incubated with spinal cord tissue in vitro.

CONCLUSIONS AND IMPLICATIONS

JZL184 exerted robust anti-nociceptive effects at the level of the spinal cord in vivo and inhibited rat spinal cord MAGL activity in vitro. The discordance between in vivo and in vitro assays suggests that localized sites of action of JZL184 produce these profound functional inhibitory effects.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.167.issue-8.

Modulation of neuropathic-pain-related behaviour by the spinal endocannabinoid/endovanilloid system

Neuropathic pain refers to chronic pain that results from injury to the nervous system. The mechanisms involved in neuropathic pain are complex and involve both peripheral and central phenomena. Although numerous pharmacological agents are available for the treatment of neuropathic pain, definitive drug therapy has remained elusive. Recent drug discovery efforts have identified an original neurobiological approach to the pathophysiology of neuropathic pain. The development of innovative pharmacological strategies has led to the identification of new promising pharmacological targets, including glutamate antagonists, microglia inhibitors and, interestingly, endogenous ligands of cannabinoids and the transient receptor potential vanilloid type 1 (TRPV1). Endocannabinoids (ECs), endovanilloids and the enzymes that regulate their metabolism represent promising pharmacological targets for the development of a successful pain treatment. This review is an update of the relationship between cannabinoid receptors (CB1) and TRPV1 channels and their possible implications for neuropathic pain. The data are focused on endogenous spinal mechanisms of pain control by anandamide, and the current and emerging pharmacotherapeutic approaches that benefit from the pharmacological modulation of spinal EC and/or endovanilloid systems under chronic pain conditions will be discussed.

Why do cannabinoid receptors have more than one endogenous ligand?

The endocannabinoid system was revealed following the understanding of the mechanism of action of marijuana's major psychotropic principle, Δ(9)-tetrahydrocannabinol, and includes two G-protein-coupled receptors (GPCRs; the cannabinoid CB1 and CB2 receptors), their endogenous ligands (the endocannabinoids, the best studied of which are anandamide and 2-arachidonoylglycerol (2-AG)), and the proteins that regulate the levels and activity of these receptors and ligands. However, other minor lipid metabolites different from, but chemically similar to, anandamide and 2-AG have also been suggested to act as endocannabinoids. Thus, unlike most other GPCRs, cannabinoid receptors appear to have more than one endogenous agonist, and it has been often wondered what could be the physiological meaning of this peculiarity. In 1999, it was proposed that anandamide might also activate other targets, and in particular the transient receptor potential of vanilloid type-1 (TRPV1) channels. Over the last decade, this interaction has been shown to occur both in peripheral tissues and brain, during both physiological and pathological conditions. TRPV1 channels can be activated also by another less abundant endocannabinoid, N-arachidonoyldopamine, but not by 2-AG, and have been proposed by some authors to act as ionotropic endocannabinoid receptors. This article will discuss the latest discoveries on this subject, and discuss, among others, how anandamide and 2-AG differential actions at TRPV1 and cannabinoid receptors contribute to making this signalling system a versatile tool available to organisms to fine-tune homeostasis.

Dynamic changes to the endocannabinoid system in models of chronic pain

The analgesic effects of cannabinoid ligands, mediated by CB1 receptors are well established. However, the side-effect profile of CB1 receptor ligands has necessitated the search for alternative cannabinoid-based approaches to analgesia. Herein, we review the current literature describing the impact of chronic pain states on the key components of the endocannabinoid receptor system, in terms of regionally restricted changes in receptor expression and levels of key metabolic enzymes that influence the local levels of the endocannabinoids. The evidence that spinal CB2 receptors have a novel role in the modulation of nociceptive processing in models of neuropathic pain, as well as in models of cancer pain and arthritis is discussed. Recent advances in our understanding of the spinal location of the key enzymes that regulate the levels of the endocannabinoid 2-AG are discussed alongside the outcomes of recent studies of the effects of inhibiting the catabolism of 2-AG in models of pain. The complexities of the enzymes capable of metabolizing both anandamide (AEA) and 2-AG have become increasingly apparent. More recently, it has come to light that some of the metabolites of AEA and 2-AG generated by cyclooxygenase-2, lipoxygenases and cytochrome P450 are biologically active and can either exacerbate or inhibit nociceptive signalling.

Visceral and somatic sensory function in functional dyspepsia

BACKGROUND

Visceral hypersensitivity is one of the proposed underlying mechanisms in functional dyspepsia (FD). It is not clear whether visceral hypersensitivity in FD is a manifestation of a central sensitization also encompassing somatic sensitization. Transient receptor potential vanilloid-1 (TRPV(1)) pathways are involved in gastric mechanosensory physiology and the TRPV(1) receptor agonist, capsaicin, has been used as a chemical stimulant.

METHODS

In this double-blind, randomized study we evaluated both visceral and somatic sensory function in 34 FD patients and 42 healthy controls using quantitative sensory testing. Visceral pain sensitivity was assessed using a validated gastric pain model with oral capsaicin capsule titration and somatic pain sensitivity was determined by foot heat and hand electric stimulation.

KEY RESULTS

The median capsaicin dose required to attain moderate pain was 0.5mg in FD and 1mg in controls (P = 0.03). At these doses, mean pain intensities on a 0-100 visual analog scale were greater for FD than controls [56.9 (95% confidence intervals, 52.2-61.5) vs 45.1 (41.6-48.6), resp.] (P = 0.005). Overall, mean somatic sensory and pain thresholds were similar in FD and control groups, but in a subgroup of FD pain hypersensitivity was seen on the hand and on the foot at different stimulation thresholds.

CONCLUSIONS & INFERENCES

A majority of patients with FD have visceral chemo-hypersensitivity involving TRPV(1) pathways. A substantial subgroup also has somatic hypersensitivity as evidence of central sensitization.

Changes of blood endocannabinoids during anaesthesia: a special case for fatty acid amide hydrolase inhibition by propofol?

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

• Available data from animal studies suggest that the narcotic drug propofol interacts with the endocannabinoid system. Inhibition of enzymatic degradation of anandamide could explain some of the characteristics of propofol. Direct measurements have not been reported yet in humans.

WHAT THIS STUDY ADDS

• Propofol does not change the time course of anandamide plasma concentrations during anaesthesia. Furthermore, propofol does not inhibit fatty acid amide hydrolase activity ex vivo or in vitro. Thus, specific characteristics of the narcotic drug propofol cannot be explained by peripheral inhibition of anandamide degradation in humans.

AIMS

The aim of our study was to describe the time course of endocannabinoids during different anaesthesia protocols in more detail, and to challenge the hypothesis that propofol acts as a FAAH inhibitor.

METHODS

Endocannabinoids were measured during the first hour of anaesthesia in 14 women and 14 men undergoing general anaesthesia with propofol and in 14 women and 14 men receiving thiopental/sevoflurane. We also incubated whole human blood samples ex vivo with propofol and the known FAAH inhibitor oloxa and determined FAAH enzyme kinetics.

RESULTS

Plasma anandamide decreased similarly with propofol and thiopental/sevoflurane anaesthesia, and reached a nadir after 10 min. Areas under the curve for anandamide (mean and 95% CI) were 53.3 (47.4, 59.2) nmol l(-1) 60 min with propofol and 48.5 (43.1, 53.8) nmol l(-1) 60 min with thiopental/sevoflurane (P= NS). Anandamide and propofol plasma concentrations were not correlated at any time point. Ex vivo FAAH activity was not inhibited by propofol. Enzyme kinetics (mean ± SD) of recombinant human FAAH were K(m) = 16.9 ± 8.8 µmol l(-1) and V(max) = 44.6 ± 15.8 nmol mg(-1) min(-1) FAAH without, and K(m) = 16.6 ± 4.0 µmol l(-1) and V(max) = 44.0 ± 7.6 nmol mg( 1 ) min(-1) FAAH with 50 µmol l(-1) propofol (P= NS for both).

CONCLUSIONS

Our findings challenge the idea that propofol anaesthesia and also propofol addiction are directly mediated by FAAH inhibition, but we cannot exclude other indirect actions on cannabinoid receptors.

Sphingosine-1-phosphate receptors as emerging targets for treatment of pain

Lysolipids are important mediators of cellular communication in multiple physiological processes. Sphingosine-1-phosphate (S1P) is a major lysolipid in many organs, including the central nervous system (CNS). This commentary discusses recent findings on the role of S1P in regulating pain perception, and highlights advances and challenges in the field. S1P interacts with multiple cellular targets, including G-protein-coupled receptors. Known S1P receptors include five types, four of which are expressed in the CNS (S1P(1,2,3,5)) where they are localized on neurons and glia. S1P receptor-mediated G-protein activation has been demonstrated throughout the CNS, including regions that regulate nociception. S1P receptors couple to multiple G-proteins to produce various intracellular responses, and can mediate both excitatory and inhibitory neuromodulation, depending on the receptor type and cellular context. Both antinociceptive and pro-nociceptive effects of S1P have been reported, and both actions can involve S1P(1) receptors. Current evidence suggests that antinociception is mediated by CNS neurons, whereas pro-nociception is mediated by primary afferent neurons or immune cells in the periphery, or CNS glia. Nonetheless, peripheral administration of the S1P(1,3,4,5) agonist pro-drug, FTY720, produces antinociception. FTY720 is approved to treat multiple sclerosis, and produces potent anti-inflammatory effects, which suggests potential utility for painful autoimmune diseases. Furthermore, evidence suggests that the S1P system interacts with other pain-modulatory systems, such as endogenous cannabinoid and opioid systems, and putative novel sphingolipid targets in the CNS. These findings suggest that drugs targeting the S1P system could be developed as novel analgesics, either as monotherapy or potential adjuncts to established analgesics.

Endocannabinoid-dependent plasticity at spinal nociceptor synapses

Neuroplastic changes at the spinal synapses between primary nociceptors and second order dorsal horn neurons play key roles in pain and analgesia. NMDA receptor-dependent forms of long-term plasticity have been studied extensively at these synapses, but little is known about possible contributions of the endocannabinoid system. Here, we addressed the role of cannabinoid (CB)1 receptors in activity-dependent plasticity at these synapses. We report that conditional low-frequency stimulation of high-threshold primary sensory nerve fibres paired with depolarisation of the postsynaptic neuron evoked robust long-term depression (LTD)of excitatory synaptic transmission by about 40% in the vast majority (90%) of recordings made in wild-type mice. When recordings were made from global or nociceptor-specific CB(1) receptor-deficient mice (CB(1) (−/− ) mice and sns-CB(1)(−/−) mice), the portion of neurons exhibiting LTD was strongly reduced to about 25%. Accordingly, LTD was prevented to a similar extent by the CB1 receptor antagonist AM251 and mimicked by pharmacological activation of CB1 receptors. In a subset of neurons with EPSCs of particularly high stimulation thresholds, we furthermore found that the absence of CB(1) receptors in CB(1)(−/−) and sns-CB(1)(−/−) mice converted the response to the paired conditioning stimulation protocol from LTD to long-term potentiation (LTP). Our results identify CB1 receptor-dependent LTD as a form of synaptic plasticity previously unknown in spinal nociceptors. They furthermore suggest that prevention of LTP may be a second hither to unknown function of CB1 receptors in primary nociceptors. Both findings may have important implications for our understanding of endogenous pain control mechanisms and of analgesia evoked by cannabinoid receptor agonists.

Cannabinoid receptors: nomenclature and pharmacological principles

The CB1 and CB2 cannabinoid receptors are members of the G protein-coupled receptor (GPCR) family that are pharmacologically well defined. However, the discovery of additional sites of action for endocannabinoids as well as synthetic cannabinoid compounds suggests the existence of additional cannabinoid receptors. Here we review this evidence, as well as the current nomenclature for classifying a target as a cannabinoid receptor. Basic pharmacological definitions, principles and experimental conditions are discussed in order to place in context the mechanisms underlying cannabinoid receptor activation. Constitutive (agonist-independent) activity is observed with the overexpression of many GPCRs, including cannabinoid receptors. Allosteric modulators can alter the pharmacological responses of cannabinoid receptors. The complex molecular architecture of each of the cannabinoid receptors allows for a single receptor to recognize multiple classes of compounds and produce an array of distinct downstream effects. Natural polymorphisms and alternative splice variants may also contribute to their pharmacological diversity. As our knowledge of the distinct differences grows, we may be able to target select receptor conformations and their corresponding pharmacological responses. Importantly, the basic biology of the endocannabinoid system will continue to be revealed by ongoing investigations.

Gating the polarity of endocannabinoid-mediated synaptic plasticity by nitric oxide in the spinal locomotor network

The final motor output underlying behavior arises from an appropriate balance between excitation and inhibition within neural networks. Retrograde signaling by endocannabinoids adapts synaptic strengths and the global activity of neural networks. In the spinal cord, endocannabinoids are mobilized postsynaptically from network neurons and act retrogradely on presynaptic cannabinoid receptors to potentiate the locomotor frequency. However, it is still unclear whether mechanisms exist within the locomotor networks that determine the sign of the modulation by cannabinoid receptors to differentially regulate excitation and inhibition. In this study, using the lamprey spinal cord in vitro, we first report that 2-AG (2-arachidonyl glycerol) is mobilized by network neurons and underlies a form of modulation that is embedded within the locomotor networks. We then show that the polarity of the endocannabinoid modulation is gated by nitric oxide to enable simultaneously potentiation of excitation and depression of inhibition within the spinal locomotor networks. Our results suggest that endocannabinoid and nitric oxide systems interact to mediate inversion of the polarity of synaptic plasticity within the locomotor networks. Thus, endocannabinoid and nitric oxide shift in the excitation-inhibition balance to set the excitability of the spinal locomotor network.

Characterisation of cannabinoid 1 receptor expression in the perikarya, and peripheral and spinal processes of primary sensory neurons

The cannabinoid 1 (CB1) receptor is expressed by a sub-population of primary sensory neurons. However, data on the neurochemical identity of the CB1 receptor-expressing cells, and CB1 receptor expression by the peripheral and central terminals of these neurons are inconsistent and limited. We characterised CB1 receptor expression in dorsal root ganglia (DRG) and spinal cord at the lumbar 4-5 level, as well as in the urinary bladder and glabrous skin of the hindpaw. About 1/3 of DRG neurons exhibited immunopositivity for the CB1 receptor, the majority of which showed positivity for the nociceptive markers calcitonin gene-related peptide (CGRP) or/and Griffonia (bandeiraea) simplicifolia IB4 isolectin-binding. Virtually all CB1 receptor-immunostained fibres showed immunopositivity for CGRP in the skin, while very few did in the urinary bladder. No CB1 receptor-immunopositive nerve fibres were IB4 positive in either peripheral tissue. Spinal laminae I and II-outer showed the highest density of CB1 receptor-immunopositive punctae, the majority of which showed positivity for CGRP or/and IB4 binding. These data indicate that a major sub-population of nociceptive primary sensory neurons expresses CB1 receptors that are transported to both peripheral and central terminals of these cells. Therefore, the present data suggest that manipulation of endogenous CB1 receptor agonist levels in these areas may significantly reduce nociceptive input into the spinal cord.

Cannabinoids suppress inflammatory and neuropathic pain by targeting α3 glycine receptors

Systemic and intrathecal administration of derivatives of a nonpsychoactive component of marijuana significantly suppresses chronic inflammatory and neuropathic pain, without causing analgesic tolerance, in several rodent models.

Certain types of nonpsychoactive cannabinoids can potentiate glycine receptors (GlyRs), an important target for nociceptive regulation at the spinal level. However, little is known about the potential and mechanism of glycinergic cannabinoids for chronic pain treatment. We report that systemic and intrathecal administration of cannabidiol (CBD), a major nonpsychoactive component of marijuana, and its modified derivatives significantly suppress chronic inflammatory and neuropathic pain without causing apparent analgesic tolerance in rodents. The cannabinoids significantly potentiate glycine currents in dorsal horn neurons in rat spinal cord slices. The analgesic potency of 11 structurally similar cannabinoids is positively correlated with cannabinoid potentiation of the α3 GlyRs. In contrast, the cannabinoid analgesia is neither correlated with their binding affinity for CB1 and CB2 receptors nor with their psychoactive side effects. NMR analysis reveals a direct interaction between CBD and S296 in the third transmembrane domain of purified α3 GlyR. The cannabinoid-induced analgesic effect is absent in mice lacking the α3 GlyRs. Our findings suggest that the α3 GlyRs mediate glycinergic cannabinoid-induced suppression of chronic pain. These cannabinoids may represent a novel class of therapeutic agents for the treatment of chronic pain and other diseases involving GlyR dysfunction.

Differential distribution of diacylglycerol lipase-alpha and N-acylphosphatidylethanolamine-specific phospholipase d immunoreactivity in the superficial spinal dorsal horn of rats

It is generally accepted that the endocannabinoid system plays important roles in spinal pain processing. Although it is documented that cannabinoid-1 receptors are strongly expressed in the superficial spinal dorsal horn, the cellular distribution of enzymes that can synthesize endocannabinoid ligands is less well studied. Thus, using immunocytochemical methods at the light and electron microscopic levels, we investigated the distribution of diacylglycerol lipase-alpha (DGL-α) and N-acylphosphatidylethanolamine-specific phospholipase D (NAPE-PLD), enzymes synthesizing the endocannabinoid ligands, 2-arachidonoylglycerol (2-AG) and anandamide, respectively. Positive labeling was revealed only occasionally in axon terminals, but dendrites displayed strong immunoreactivity for both enzymes. However, the dendritic localization of DGL-α and NAPE-PLD showed a remarkably different distribution. DGL-α immunolabeling in dentrites was always revealed at membrane compartments in close vicinity to synapses. In contrast to this, dendritic NAPE-PLD labeling was never observed in association with synaptic contacts. In addition to dendrites, a substantial proportion of astrocytic (immunoreactive for GFAP) and microglial (immunoreactive for CD11b) profiles were also immunolabeled for both DGL-α and NAPE-PLD. Glial processes immunostained for DGL-α were frequently found near to synapses in which the postsynaptic dendrite was immunoreactive for DGL-α, whereas NAPE-PLD immunoreactivity on glial profiles at the vicinity of synapses was only occasionally observed. Our results suggest that both neurons and glial cells can synthesize and release 2-AG and anandamide in the superficial spinal dorsal horn. The 2-AG can primarily be released by postsynaptic dendrites and glial processes adjacent to synapses, whereas anandamide can predominantly be released from nonsynaptic dendritic and glial compartments.

Chronic pain states: pharmacological strategies to restore diminished inhibitory spinal pain control

Potentially noxious stimuli are sensed by specialized nerve cells named nociceptors, which convey nociceptive signals from peripheral tissues to the central nervous system. The spinal dorsal horn and the trigeminal nucleus serve as first relay stations for incoming nociceptive signals. At these sites, nociceptor terminals contact a local neuronal network consisting of excitatory and inhibitory interneurons as well as of projection neurons. Blockade of neuronal inhibition in this network causes an increased sensitivity to noxious stimuli (hyperalgesia), painful sensations occurring after activation of non-nociceptive fibers (allodynia), and spontaneous pain felt in the absence of any sensory stimulation. It thus mimics the major characteristics of chronic pain states. Diminished inhibitory pain control in the spinal dorsal horn occurs naturally, e.g., through changes in the function of inhibitory neurotransmitter receptors or through altered chloride homeo-stasis in the course of inflammation or nerve damage. This review summarizes our current knowledge about endogenous mechanisms leading to diminished spinal pain control and discusses possible ways that could restore proper inhibition through facilitation of fast inhibitory neurotransmission.

Multiple functions of endocannabinoid signaling in the brain

Despite being regarded as a hippie science for decades, cannabinoid research has finally found its well-deserved position in mainstream neuroscience. A series of groundbreaking discoveries revealed that endocannabinoid molecules are as widespread and important as conventional neurotransmitters such as glutamate or GABA, yet they act in profoundly unconventional ways. We aim to illustrate how uncovering the molecular, anatomical, and physiological characteristics of endocannabinoid signaling has revealed new mechanistic insights into several fundamental phenomena in synaptic physiology. First, we summarize unexpected advances in the molecular complexity of biogenesis and inactivation of the two endocannabinoids, anandamide and 2-arachidonoylglycerol. Then, we show how these new metabolic routes are integrated into well-known intracellular signaling pathways. These endocannabinoid-producing signalosomes operate in phasic and tonic modes, thereby differentially governing homeostatic, short-term, and long-term synaptic plasticity throughout the brain. Finally, we discuss how cell type- and synapse-specific refinement of endocannabinoid signaling may explain the characteristic behavioral effects of cannabinoids.

Fast synaptic inhibition in spinal sensory processing and pain control

The two amino acids GABA and glycine mediate fast inhibitory neurotransmission in different CNS areas and serve pivotal roles in the spinal sensory processing. Under healthy conditions, they limit the excitability of spinal terminals of primary sensory nerve fibers and of intrinsic dorsal horn neurons through pre- and postsynaptic mechanisms, and thereby facilitate the spatial and temporal discrimination of sensory stimuli. Removal of fast inhibition not only reduces the fidelity of normal sensory processing but also provokes symptoms very much reminiscent of pathological and chronic pain syndromes. This review summarizes our knowledge of the molecular bases of spinal inhibitory neurotransmission and its organization in dorsal horn sensory circuits. Particular emphasis is placed on the role and mechanisms of spinal inhibitory malfunction in inflammatory and neuropathic chronic pain syndromes.

Peripheral FAAH inhibition causes profound antinociception and protects against indomethacin-induced gastric lesions

Fatty-acid amide hydrolase (FAAH) catalyzes the intracellular hydrolysis of the endocannabinoid anandamide and other bioactive lipid amides. In the present study, we conducted a comparative characterization of the effects of the newly identified brain-impermeant FAAH inhibitor, URB937 ([3-(3-carbamoylphenyl)-4-hydroxy-phenyl] N-cyclohexylcarbamate), in various rodent models of acute and persistent pain. When administered by the oral route in mice, URB937 was highly active (median effective dose, ED(50), to inhibit liver FAAH activity: 0.3mgkg(-1)) and had a bioavailability of 5.3%. The antinociceptive effects of oral URB937 were investigated in mouse models of acute inflammation (carrageenan), peripheral nerve injury (chronic sciatic nerve ligation) and arthritis (complete Freund's adjuvant). In all models, URB937 was as effective or more effective than standard analgesic and anti-inflammatory drugs (indomethacin, gabapentin, dexamethasone) and reversed pain-related responses (mechanical hyperalgesia, thermal hyperalgesia, and mechanical allodynia) in a dose-dependent manner. ED(50) values ranged from 0.2 to 10mgkg(-1), depending on model and readout. Importantly, URB937 was significantly more effective than two global FAAH inhibitors, URB597 and PF-04457845, in the complete Freund's adjuvant model. The effects of a combination of URB937 with the non-steroidal anti-inflammatory agent, indomethacin, were examined in the carrageenan and chronic sciatic nerve ligation models. Isobolographic analyses showed that the two compounds interacted synergistically to attenuate pain-related behaviors. Furthermore, URB937 reduced the number and severity of gastric lesions produced by indomethacin, while exerting no ulcerogenic effect when administered alone. The results indicate that the peripheral FAAH inhibitor URB937 is more effective than globally active FAAH inhibitors at inhibiting inflammatory pain. Our findings further suggest that FAAH and cyclooxygenase inhibitors interact functionally in peripheral tissues, to either enhance or hinder each other's actions.

A Role for the Cannabinoid 1 Receptor in Neuronal Differentiation of Adult Spinal Cord Progenitors in vitro is Revealed through Pharmacological Inhibition and Genetic Deletion

In contrast to the adult brain, the adult spinal cord is a non-neurogenic environment. Understanding how to manipulate the spinal cord environment to promote the formation of new neurons is an attractive therapeutic strategy for spinal cord injury and disease. The cannabinoid 1 receptor (CB1R) has been implicated as a modulator of neural progenitor cell proliferation and fate specification in the brain; however, no evidence exists for modulation of adult spinal cord progenitor cells. Using adult rat spinal cord primary cultures, we demonstrated that CB1R antagonism with AM251 significantly decreased the number of Nestin(+) cells, and increased the number of βIII tubulin(+) and DCX(+) cells, indicative of neuronal differentiation. AM251’s effect was blocked by co-application of the CB1R agonists, WIN 55, 212-2, or ACEA. Consistent with our hypothesis, cultures, and spinal cord slices derived from CB1R knock-out (CB1−/−) mice had significantly higher levels of DCX(+) cells compared to those derived from wild type (CB1+/+) mice, indicative of enhanced neuronal differentiation in CB1−/− spinal cords. Moreover, AM251 promoted neuronal differentiation in CB1+/+, but not in CB1−/− cultures. Since CB1R modulates synaptic transmission, and synaptic transmission has been shown to influence progenitor cell fate, we evaluated whether AM251-induced neuronal differentiation was affected by chronic inactivity. Either the presence of the voltage-dependent sodium channel blocker tetrodotoxin (TTX), or the removal of mature neurons, inhibited the AM251-induced increase in DCX(+) cells. In summary, antagonism or absence of CB1R promotes neuronal differentiation in adult spinal cords, and this action appears to require TTX-sensitive neuronal activity. Our data suggest that the previously detected elevated levels of endocannabinoids in the injured adult spinal cord could contribute to the non-neurogenic environment and CB1R antagonists could potentially be used to enhance replacement of damaged neurons.

Multiple mechanisms of microglia: a gatekeeper's contribution to pain states

Microglia are gatekeepers in the CNS for a wide range of pathological stimuli and they blow the whistle when things go wrong. Collectively, microglia form a CNS tissue alarm system (Kreutzberg's "sensor of pathology"), and their involvement in physiological pain is in line with this function. However, pathological neuropathic pain is characterized by microglial activation that is unwanted and considered to contribute to or even cause tactile allodynia, hyperalgesia and spontaneous pain. Such abnormal microglial behavior seems likely due to an as yet ill-understood disturbance of microglial functions unrelated to inflammation. The idea that microglia have roles in the CNS that differ from those of peripheral macrophages has gained momentum with the discovery of their separate, pre-hematopoietic lineage during embryonic development and their direct interactions with synapses.

Pain and itch: insights into the neural circuits of aversive somatosensation in health and disease

Although pain and itch are distinct sensations, most noxious chemicals are not very specific to one sensation over the other, and recent discoveries are revealing that Trp channels function as transducers for both. A key difference between these sensations is that itch is initiated by irritation of the skin, whereas pain can be elicited from almost anywhere in the body; thus, itch may be encoded by the selective activation of specific subsets of neurons that are tuned to detect harmful stimuli at the surface and have specialized central connectivity that is specific to itch. Within the spinal cord, cross-modal inhibition between pain and itch may help sharpen the distinction between these sensations. Moreover, this idea that somatosensory modalities inhibit one another may be generalizable to other somatosensory subtypes, such as cold and hot. Importantly, just as there are inhibitory circuits in the dorsal horn that mediate cross-inhibition between modalities, it appears that there are also excitatory connections that can be unmasked upon injury or in disease, leading to abnormally elevated pain states such as allodynia. We are now beginning to understand some of this dorsal horn circuitry, and these discoveries are proving to be relevant for pathological conditions of chronic pain and itch.

Centrally mediated antinociceptive effects of cannabinoid receptor ligands in rat models of nociception

The endogenous nonapeptide hemopressin (HE) demonstrates potent block of the cannabinoid subtype-1 (CB1) receptor in vitro and robust antinociception in vivo. The current study evaluated the effects of centrally administered HE in mechanistically distinct pre-clinical rat models of pain-the hot plate test and the hind paw formalin test. The non-subtype selective CB receptor agonist WIN 55,212-2 was tested concurrently as a positive control. In the hot plate test, neither intrathecal (i.t.) HE nor WIN 55,212-2 significantly altered the latency to respond to noxious heat. By contrast, i.t. HE and WIN 55,212-2 significantly reduced pain-related behaviors in the formalin test. Possible HE functionality as a CB1 receptor antagonist at the spinal level was evaluated in the formalin test. Intrathecal pretreatment with HE did not attenuate the antinociceptive effect of i.t. WIN 55,212-2. However, pretreatment with the CB1 receptor antagonist rimonabant did; i.t. rimonabant pretreatment was not antinociceptive. Potential supraspinal antinociceptive activity of HE was also evaluated. Whereas intracerebroventricular (i.c.v.) injection of WIN 55,212-2 reduced pain-related behaviors in the formalin test, interestingly, i.c.v. HE increased behaviors. In the current study, an antinociceptive effect with the CB receptor ligand HE was obtained under the specific condition of tissue injury and not in the uninjured state. Thus, HE could be a useful analgesic peptide with a novel spinal mechanism of action.

Molecular sites for the positive allosteric modulation of glycine receptors by endocannabinoids

Glycine receptors (GlyRs) are transmitter-gated anion channels of the Cys-loop superfamily which mediate synaptic inhibition at spinal and selected supraspinal sites. Although they serve pivotal functions in motor control and sensory processing, they have yet to be exploited as drug targets partly because of hitherto limited possibilities for allosteric control. Endocannabinoids (ECs) have recently been characterized as direct allosteric GlyR modulators, but the underlying molecular sites have remained unknown. Here, we show that chemically neutral ECs (e.g. anandamide, AEA) are positive modulators of α₁, α₂ and α₃ GlyRs, whereas acidic ECs (e.g. N-arachidonoyl-glycine; NA-Gly) potentiate α₁ GlyRs but inhibit α₂ and α₃. This subunit-specificity allowed us to identify the underlying molecular sites through analysis of chimeric and mutant receptors. We found that alanine 52 in extracellular loop 2, glycine 254 in transmembrane (TM) region 2 and intracellular lysine 385 determine the positive modulation of α₁ GlyRs by NA-Gly. Successive substitution of non-conserved extracellular and TM residues in α₂ converted NA-Gly-mediated inhibition into potentiation. Conversely, mutation of the conserved lysine within the intracellular loop between TM3 and TM4 attenuated NA-Gly-mediated potentiation of α₁ GlyRs, without affecting inhibition of α₂ and α₃. Notably, this mutation reduced modulation by AEA of all three GlyRs. These results define molecular sites for allosteric control of GlyRs by ECs and reveal an unrecognized function for the TM3-4 intracellular loop in the allosteric modulation of Cys-loop ion channels. The identification of these sites may help to understand the physiological role of this modulation and facilitate the development of novel therapeutic approaches to diseases such as spasticity, startle disease and possibly chronic pain.

Blockade of GABA(B) receptors facilitates evoked neurotransmitter release at spinal dorsal horn synapse

Metabotropic GABA type B (GABA(B)) receptors are abundantly expressed in the rat spinal dorsal horn. Activation of GABA(B) receptors by exogenous agonists inhibits synaptic transmission, which is believed to underlie the GABA(B) receptor-mediated analgesia. However, little effort has been made to test whether endogenous GABA might also mediate inhibition by acting on GABA(B) receptors. In this study, whole-cell recording techniques were employed to study the effect of endogenous GABA on GABA(B) receptors in substantia gelatinosa (SG) neurons in adult rat spinal cord slices. In current-clamp mode, blockade of GABA(B) receptors by their selective antagonist 3-[[[(3,4-dichlorophenyl)methyl]amino]propyl] (diethoxy-methyl) phosphinic acid (CGP 52432) facilitated presynaptic stimulation-induced action potential discharge and increased amplitude of postsynaptic potentials (PSPs), meaning a GABA(B) receptor-mediated inhibition of SG neuron excitability. In voltage-clamp mode, blockade of GABA(B) receptors increased the amplitude of evoked excitatory postsynaptic currents (eEPSCs) and decreased paired-pulse ratio, indicating a presynaptic CGP 52432 action. Primary afferent Aδ or C fiber-evoked EPSCs were also facilitated by CGP 52432 application. Amplitudes of evoked GABAergic and glycinergic inhibitory postsynaptic currents (eIPSCs) were enhanced by GABA(B) receptor blockade. The facilitation of amplitude persisted in the presence of a specific GABA transporter 1 (GAT-1) blocker, tiagabine, or GAT-2/3 blocker SNAP5114. However, blockade of GABA(B) receptors had no effect on action potential-independent miniature EPSCs (mEPSCs), miniature IPSCs (mIPSCs), or membrane conductance. Taken together, these results suggest that endogenous GABA modulates evoked synaptic transmission in SG neurons by acting on GABA(B) receptors. This GABA(B) receptor-mediated homeostatic regulation of neuronal excitability and neurotransmitter release might contribute to modulation of nociception in spinal dorsal horn.

Activation of spinal and supraspinal cannabinoid-1 receptors leads to antinociception in a rat model of neuropathic spinal cord injury pain

Activation of CNS cannabinoid subtype-1 (CB1) receptors has been shown to mediate the antinociceptive and other effects of systemically administered CB receptor agonists. The endogenous peptide CB receptor ligand hemopressin (HE) has previously demonstrated an antinociceptive effect in rats with a hind paw inflammation, without exhibiting characteristic CB1 receptor-mediated side-effects. The current study evaluated the effect of intrathecal (i.t.) and intracerebroventricular (i.c.v.) injection of HE in a rat model of neuropathic spinal cord injury (SCI) pain. The non-subtype selective CB receptor agonist WIN 55,212-2 was also centrally administered in SCI rats as a comparator. Four weeks following an acute compression of the mid-thoracic spinal cord, rats displayed markedly decreased hind paw withdrawal thresholds, indicative of below-level neuropathic pain. Central administration of WIN 55,212-2 significantly increased withdrawal thresholds, whereas HE did not. Hemopressin has been reported to block CB1 receptors in vitro, similar to the CB1 receptor antagonist rimonabant. Pretreatment with rimonabant completely blocked the antinociceptive effect of centrally administered WIN 55,212-2, but pretreatment with HE did not. While the data confirm that activation of either supraspinal or spinal CB1 receptors leads to significant antinociception in SCI rats, the current data do not support an antinociceptive effect from an acute blockade of central CB1 receptors, HE's putative antinociceptive mechanism, in neuropathic SCI rats. Although such a mechanism could be useful in other models of pain with a significant inflammatory component, the current data indicate that activation of CB1 receptors is needed to ameliorate neuropathic SCI pain.

mGluR1, but not mGluR5, activates feed-forward inhibition in the medial prefrontal cortex to impair decision making

Cognitive flexibility depends on the integrity of the prefrontal cortex (PFC). We showed previously that impaired decision making in pain results from amygdala-driven inhibition of medial PFC neurons, but the underlying mechanisms remain to be determined. Using whole cell patch clamp in rat brain slices and a cognitive behavioral task, we tested the hypothesis that group I metabotropic glutamate receptors (mGluRs) activate feed-forward inhibition to decrease excitability and output function of PFC pyramidal cells, thus impairing decision making. Polysynaptic inhibitory postsynaptic currents (IPSCs) and monosynaptic excitatory postsynaptic currents (EPSCs) were evoked in layer V pyramidal cells by stimulating presumed amygdala afferents. An mGluR1/5 agonist [(S)-3,5-dihydroxyphenylglycine, DHPG] increased synaptic inhibition more strongly than excitatory transmission. The facilitatory effects were blocked by an mGluR1 [(S)-(+)-α-amino-4-carboxy-2-methylbenzeneacetic acid, LY367385], but not mGluR5, antagonist, 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine. IPSCs were blocked by bicuculline and decreased by 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide disodium salt (NBQX). Facilitation of synaptic inhibition by DHPG was glutamate driven because it was blocked by NBQX. DHPG increased frequency but not amplitude of spontaneous IPSCs; consistent with action potential-dependent synaptic inhibition, tetrodotoxin (TTX) prevented the facilitatory effects. DHPG decreased synaptically evoked spikes (E-S coupling) and depolarization-induced spiking [frequency-current (f-I) relationship]. This effect was indirect, resulting from glutamate-driven synaptic inhibition, because it persisted when a G protein blocker was included in the pipette but was blocked by GABA(A) receptor antagonists and NBQX. In contrast, DHPG increased E-S coupling and f-I relationships in mPFC interneurons through a presynaptic action, further supporting the concept of feed-forward inhibition. DHPG also impaired the ability of the animals to switch strategies in a decision-making task; bicuculline restored normal decision making, whereas a GABA(A) receptor agonist (muscimol) mimicked the decision-making deficit. The results show that mGluR1 activates feed-forward inhibition of PFC pyramidal cells to impair cognitive functions.

Cannabinoid potentiation of glycine receptors contributes to cannabis-induced analgesia

Cannabinoids enhance the function of glycine receptors (GlyRs). However, little is known about the mechanisms and behavioral implication of cannabinoid-GlyR interaction. Using mutagenesis and NMR analysis, we have identified a serine at 296 in the GlyR protein critical for the potentiation of I(Gly) by Δ(9)-tetrahydrocannabinol (THC), a major psychoactive component of marijuana. The polarity of the amino acid residue at 296 and the hydroxyl groups of THC are critical for THC potentiation. Removal of the hydroxyl groups of THC results in a compound that does not affect I(Gly) when applied alone but selectively antagonizes cannabinoid-induced potentiating effect on I(Gly) and analgesic effect in a tail-flick test in mice. The cannabinoid-induced analgesia is absent in mice lacking α3GlyRs but not in those lacking CB1 and CB2 receptors. These findings reveal a new mechanism underlying cannabinoid potentiation of GlyRs, which could contribute to some of the cannabis-induced analgesic and therapeutic effects.

Role of cannabinoids in the treatment of pain and (painful) spasticity

Both the discovery of the endocannabinoid system (ECS) and its role in the control of pain and habituation to stress, as well as the significant analgesic and antihyperalgesic effects in animal studies, suggest the usefulness of cannabinoids in pain conditions. However, in human experimental or clinical trials, no convincing reduction of acute pain, which may be caused by a pronociceptive, ECS-triggered mechanism on the level of the spinal cord, has been demonstrated. In contrast, in chronic pain and (painful) spasticity, an increasing number of randomized, double-blind, placebo-controlled studies have shown the efficacy of cannabinoids, which is combined with a narrow therapeutic index. Patients with unsatisfactory response to other methods of pain therapy and who were characterized by failed stress adaptation particularly benefited from treatment with cannabinoids. None of the attempts to overcome the disadvantage of the narrow therapeutic index, either by changing the route of application or by formulating balanced cannabinoid preparations, have resulted in a major breakthrough. Therefore, different methods of administration and other types of cannabinoids, such as endocannabinoid modulators, should be tested in future trials.

Genetic evidence for involvement of neuronally expressed S1P₁ receptor in nociceptor sensitization and inflammatory pain

Sphingosine-1-phosphate (S1P) is a key regulator of immune response. Immune cells, epithelia and blood cells generate high levels of S1P in inflamed tissue. However, it is not known if S1P acts on the endings of nociceptive neurons, thereby contributing to the generation of inflammatory pain. We found that the S1P₁ receptor for S1P is expressed in subpopulations of sensory neurons including nociceptors. Both S1P and agonists at the S1P₁ receptor induced hypersensitivity to noxious thermal stimulation in vitro and in vivo. S1P-induced hypersensitivity was strongly attenuated in mice lacking TRPV1 channels. S1P and inflammation-induced hypersensitivity was significantly reduced in mice with a conditional nociceptor-specific deletion of the S1P₁ receptor. Our data show that neuronally expressed S1P₁ receptors play a significant role in regulating nociceptor function and that S1P/S1P₁ signaling may be a key player in the onset of thermal hypersensitivity and hyperalgesia associated with inflammation.

Corticosterone mediates reciprocal changes in CB 1 and TRPV1 receptors in primary sensory neurons in the chronically stressed rat

BACKGROUND & AIMS

Chronic stress is associated with visceral hyperalgesia in functional gastrointestinal disorders. We investigated whether corticosterone plays a role in chronic psychological stress-induced visceral hyperalgesia.

METHODS

Male rats were subjected to 1-hour water avoidance (WA) stress or subcutaneous corticosterone injection daily for 10 consecutive days in the presence or absence of corticoid-receptor antagonist RU-486 and cannabinoid-receptor agonist WIN55,212-2. The visceromotor response to colorectal distension was measured. Receptor protein levels were measured and whole-cell patch-clamp recordings were used to assess transient receptor potential vanilloid type 1 (TRPV1) currents in L6-S2 dorsal root ganglion (DRG) neurons. Mass spectrometry was used to measure endocannabinoid anandamide content.

RESULTS

Chronic WA stress was associated with visceral hyperalgesia in response to colorectal distension, increased stool output and reciprocal changes in cannabinoid receptor 1 (CB1) (decreased) and TRPV1 (increased) receptor expression and function. Treatment of WA stressed rats with RU-486 prevented these changes. Control rats treated with serial injections of corticosterone in situ showed a significant increase in serum corticosterone associated with visceral hyperalgesia, enhanced anandamide content, increased TRPV1, and decreased CB1 receptor protein levels, which were prevented by co-treatment with RU-486. Exposure of isolated control L6-S2 DRGs in vitro to corticosterone reproduced the changes in CB1 and TRPV1 receptors observed in situ, which was prevented by co-treatment with RU-486 or WIN55,212-2.

CONCLUSIONS

These results support a novel role for corticosterone to modulate CB1 and TRPV1-receptor pathways in L6-S2 DRGs in the chronic WA stressed rat, which contributes to visceral hyperalgesia observed in this model.

Mitochondrial reactive oxygen species are activated by mGluR5 through IP3 and activate ERK and PKA to increase excitability of amygdala neurons and pain behavior

Reactive oxygen species (ROS) such as superoxide are emerging as important signaling molecules in physiological plasticity but also in peripheral and spinal cord pain pathology. Underlying mechanisms and pain-related ROS signaling in the brain remain to be determined. Neuroplasticity in the amygdala plays a key role in emotional-affective pain responses and depends on group I metabotropic glutamate receptors (mGluRs) and protein kinases. Using patch-clamp, live-cell imaging, and behavioral assays, we tested the hypothesis that mitochondrial ROS links group I mGluRs to protein kinase activation to increase neuronal excitability and pain behavior. Agonists for mGluR1/5 (DHPG) or mGluR5 (CHPG) increased neuronal excitability of neurons in the laterocapsular division of the central nucleus of the amygdala (CeLC). DHPG effects were inhibited by an mGluR5 antagonist (MTEP), IP(3) receptor blocker (xestospongin C), or ROS scavengers (PBN, tempol), but not by an mGluR1 antagonist (LY367385) or NO synthase inhibitor (l-NAME). Tempol inhibited the effects of IP(3) but not those of a PKC activator, indicating that ROS activation was IP(3) mediated. Live-cell imaging in CeLC-containing brain slices directly showed DHPG-induced and synaptically evoked mitochondrial superoxide production. DHPG also increased pain-related vocalizations and spinal reflexes through a mechanism that required mGluR5, IP(3), and ROS. Combined application of inhibitors of ERK (U0126) and PKA (KT5720) was necessary to block completely the excitatory effects of a ROS donor (tBOOH). A PKC inhibitor (GF109203X) had no effect. Antagonists and inhibitors alone did not affect neuronal excitability. The results suggest an important role for the novel mGluR5- IP(3)-ROS-ERK/PKA signaling pathway in amygdala pain mechanisms.

Pain-related increase of excitatory transmission and decrease of inhibitory transmission in the central nucleus of the amygdala are mediated by mGluR1

Neuroplasticity in the central nucleus of the amygdala (CeA), particularly its latero-capsular division (CeLC), is an important contributor to the emotional-affective aspects of pain. Previous studies showed synaptic plasticity of excitatory transmission to the CeLC in different pain models, but pain-related changes of inhibitory transmission remain to be determined. The CeLC receives convergent excitatory inputs from the parabrachial nucleus in the brainstem and from the basolateral amygdala (BLA). In addition, feedforward inhibition of CeA neurons is driven by glutamatergic projections from the BLA area to a cluster of GABAergic neurons in the intercalated cell masses (ITC). Using patch-clamp in rat brain slices we measured monosynaptic excitatory postsynaptic currents (EPSCs) and polysynaptic inhibitory currents (IPSCs) that were evoked by electrical stimulation in the BLA. In brain slices from arthritic rats, input-output functions of excitatory synaptic transmission were enhanced whereas inhibitory synaptic transmission was decreased compared to control slices from normal untreated rats. A non-NMDA receptor antagonist (NBQX) blocked the EPSCs and reduced the IPSCs, suggesting that non-NMDA receptors mediate excitatory transmission and also contribute to glutamate-driven feed-forward inhibition of CeLC neurons. IPSCs were blocked by a GABAA receptor antagonist (bicuculline). Bicuculline increased EPSCs under normal conditions but not in slices from arthritic rats, which indicates a loss of GABAergic control of excitatory transmission. A metabotropic glutamate receptor subtype 1 (mGluR1) antagonist (LY367385) reversed both the increase of excitatory transmission and the decrease of inhibitory transmission in the arthritis pain model but had no effect on basal synaptic transmission in control slices from normal rats. The inhibitory effect of LY367385 on excitatory transmission was blocked by bicuculline suggesting the involvement of a GABAergic mechanism. An mGluR5 antagonist (MTEP) inhibited both excitatory and inhibitory transmission in slices from normal and from arthritic rats. The analysis of spontaneous and miniature EPSCs and IPSCs showed that mGluR1 acted presynaptically whereas mGluR5 had postsynaptic effects. In conclusion, mGluR1 rather than mGluR5 can account for the pain-related changes of excitatory and inhibitory synaptic transmission in the CeLC through a mechanism that involves inhibition of inhibitory transmission (disinhibition).

Cannabinoid subtype-2 receptors modulate the antihyperalgesic effect of WIN 55,212-2 in rats with neuropathic spinal cord injury pain

BACKGROUND CONTEXT

There is increasing evidence for a role of the cannabinoid (CB) system in the development of neuropathic pain (NP) after spinal cord injury (SCI). The nonspecific CB₁ and CB₂ receptor agonists, WIN 55, 212-2 (WIN), have previously been shown to alleviate both mechanical and thermal hyperalgesia (TH) after peripheral nerve injury.

PURPOSE

The present study was designed to identify the CB receptors involved in the antihyperalgesic effect of WIN by using selective antagonists for CB₁ and CB₂ receptors.

STUDY DESIGN

This is an in vivo and behavioral study using a moderate T9 contusion SCI. After injury, TH of the hind paws was measured on postinjury days 21 through 42.

METHODS

Sprague-Dawley rats underwent a contusion SCI using the Multicenter Animal Spinal Cord Injury Study (MASCIS) weight-drop impactor, which induced a moderate T9 SCI. Only animals showing consistent plantar stepping and consistent forelimb and hind limb coordination (Basso, Beattie, and Bresnahan score=15) were tested for TH. Animals exhibiting decreased withdrawal latency time, indicating TH, on or before Day 42, were selected for pharmacological intervention. Animals not exhibiting TH did not receive pharmacological intervention and were sacrificed. Rats underwent hind paw testing before any drug administration (after injury), 45 minutes after selective CB antagonist (AM 251 or AM 630) administration (postantagonist) and again 45 minutes after WIN administration (post-WIN). There were a total of seven treatment groups: saline vehicle control; Dimethyl sulfoxide (DMSO) vehicle control; low-dose WIN (0.2 mg/kg); and high-dose WIN (2.0 mg/kg); AM 251 (3 mg/kg) and AM 630 (1 mg/kg) were given subcutaneously in a total volume of 0.5 mL. Followed by intraperitoneal injection of WIN after each antagonist, sham-operated rats repeated pharmacological intervention used with treatment Groups 5 and 6.

RESULTS

Thermal hyperalgesia was significantly ameliorated in a dose-dependent manner with systemically administered WIN. Cannabinoid receptor Type 1 antagonist AM 251 pretreatment did not affect the antihyperalgesic effect of WIN. By contrast, pretreatment with the CB₂ receptor antagonist AM 630 significantly attenuated the effect of WIN.

CONCLUSION

Taken together, these results suggest a role of the CB₂ receptor in modulating SCI-induced TH. Selective activation of the CB₂ receptor could potentially lead to analgesic effects on NP while avoiding psychotropic side effects in patients with SCI.

Targeting TRPV1 as an alternative approach to narcotic analgesics to treat chronic pain conditions

In spite of intense research efforts and after the dedicated Decade of Pain Control and Research, there are not many alternatives to opioid-based narcotic analgesics in the therapeutic armamentarium to treat chronic pain conditions. Chronic opioid treatment is associated with sedation, tolerance, dependence, hyperalgesia, respiratory depression, and constipation. Since the affective component is an integral part of pain perception, perhaps it is inevitable that potent analgesics possess the property of impacting pain pathways in the supraspinal structures. The question still remains to be answered is that whether a powerful analgesic can be devoid of narcotic effect and addictive potentials. Local anesthetics are powerful analgesics for acute pain by blocking voltage-gated sodium channels that are involved in generation and propagation of action potentials. Antidepressants and anticonvulsants have proven to be useful in the treatment of certain modalities of pain. In neuropathic pain conditions, the complexity arises because of the notion that neuronal circuitry is altered, as occurs in phantom pain, in that pain is perceived even in the absence of peripheral nociceptive inputs. If the locus of these changes is in the central nervous system, commonly used analgesics may not be very useful. This review focuses on the recent advances in nociceptive transmission and nociceptive transient receptor potential vanilloid 1 channel as a target for treating chronic pain conditions with its agonists/antagonists.

Reactive oxygen species are involved in group I mGluR-mediated facilitation of nociceptive processing in amygdala neurons

Recent biochemical and behavioral data implicate reactive oxygen species (ROS) in peripheral and spinal pain mechanisms. However, pain-related functions of ROS in the brain and mechanisms of pain-related ROS activation remain to be determined. Our previous studies showed that the amygdala plays a key role in emotional-affective pain responses and pain modulation. Hyperactivity of amygdala neurons in an animal pain model depends on group I metabotropic glutamate receptors (subtypes mGluR1 and mGluR5), but their signaling pathway remains to be determined. Here we tested the hypothesis that activation of group I mGluRs increases nociceptive processing in amygdala neurons through a mechanism that involves ROS. Extracellular single-unit recordings were made from neurons in the laterocapsular division of the central nucleus of the amygdala (CeLC) in anesthetized adult male rats. Administration of a group I mGluR agonist (DHPG) into the CeLC by microdialysis increased the responses to innocuous and noxious somatosensory (knee joint compression) and visceral (colorectal distention [CRD]) stimuli. A ROS scavenger (PBN) and a superoxide dismutase mimetic (TEMPOL) reversed the facilitatory effects of DHPG. An mGluR5 antagonist (MPEP) also inhibited the effects of DHPG on the responses to innocuous and noxious somatosensory and visceral stimuli, whereas an mGluR1 antagonist (LY367385) decreased only the responses to visceral stimulation. The results show for the first time that ROS mediate group I mGluR-induced facilitation of nociceptive processing in amygdala neurons. The antagonist data may suggest differential contributions of subtypes mGluR1 and mGluR5 to the processing of somatosensory and visceral nociceptive information in the amygdala.

Endocannabinoid regulation of spinal nociceptive processing in a model of neuropathic pain

Models of neuropathic pain are associated with elevated spinal levels of endocannabinoids (ECs) and altered expression of cannabinoid receptors on primary sensory afferents and post-synaptic cells in the spinal cord. We investigated the impact of these changes on the spinal processing of sensory inputs in a model of neuropathic pain. Extracellular single-unit recordings of spinal neurones were made in anaesthetized neuropathic and sham-operated rats. The effects of spinal administration of the cannabinoid CB(1) receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) and the cannabinoid receptor type 2 (CB(2)) receptor antagonist N-[(1S)-endo-1,3,3-trimethylbicycloheptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide (SR144528) on mechanically-evoked responses of spinal neurones were determined. The effects of spinal administration of (5Z,8Z11Z,14Z)-N-(3-furanylmethyl)-5,8,11,14-eicosatetraenamide (UCM707), which binds to CB(2) receptors and alters transport of ECs, on evoked responses of spinal neurones and spinal levels of ECs were also determined. The cannabinoid CB(1) receptor antagonist AM251, but not the CB(2) receptor antagonist, significantly facilitated 10-g-evoked responses of spinal neurones in neuropathic, but not sham-operated, rats. Spinal administration of UCM707 did not alter spinal levels of ECs but did significantly inhibit mechanically-evoked responses of neurones in neuropathic, but not sham-operated, rats. Pharmacological studies indicated that the selective inhibitory effects of spinal UCM707 in neuropathic rats were mediated by activation of spinal CB(2) receptors, as well as a contribution from transient receptor potential vanilloid 1 (TRPV1) channels. This work demonstrates that changes in the EC receptor system in the spinal cord of neuropathic rats influence the processing of sensory inputs, in particular low-weight inputs that drive allodynia, and indicates novel effects of drugs acting via multiple elements of this receptor system.

Cannabinoid CB1 receptor facilitation of substance P release in the rat spinal cord, measured as neurokinin 1 receptor internalization

The contribution of CB1 receptors in the spinal cord to cannabinoid analgesia is still unclear. The objective of this study was to investigate the effect of CB1 receptors on substance P release from primary afferent terminals in the spinal cord. Substance P release was measured as neurokinin 1 (NK1) receptor internalization in lamina I neurons. It was induced in spinal cord slices by dorsal root stimulation and in live rats by a noxious stimulus. In spinal cord slices, the CB1 receptor antagonists AM251, AM281 and rimonabant partially but potently inhibited NK1 receptor internalization induced by electrical stimulation of the dorsal root. This was due to an inhibition of substance P release and not of NK1 receptor internalization itself, because AM251 and AM281 did not inhibit NK1 receptor internalization induced by exogenous substance P. The CB1 receptor agonist ACEA increased NK1 receptor internalization evoked by dorsal root stimulation. The effects of AM251 and ACEA cancelled each other. In vivo, AM251 injected intrathecally decreased NK1 receptor internalization in spinal segments L5 and L6 induced by noxious hind paw clamp. Intrathecal AM251 also produced analgesia to radiant heat stimulation of the paw. The inhibition by AM251 of NK1 receptor internalization was reversed by antagonists of mu-opioid and GABA(B) receptors. This indicates that CB1 receptors facilitate substance P release by inhibiting the release of GABA and opioids next to primary afferent terminals, producing disinhibition. This results in a pronociceptive effect of CB1 receptors in the spinal cord.