Neuropathic pain and the endocannabinoid system in the dorsal raphe: pharmacological treatment and interactions with the serotonergic system

Alteration of serotonin release response in the central nucleus of the amygdala to noxious and non-noxious mechanical stimulation in a neuropathic pain model rat

Previously, we found that serotonin (5-HT) release in the central nucleus of the amygdala (CeA) of anesthetized rats decreases in response to innocuous stroking of the skin, irrespective of stimulus laterality, but increases in response to noxious pinching applied to a hindlimb contralateral to the 5-HT measurement site. The aim of the present study was to determine whether intra-CeA 5-HT release responses to cutaneous stimulation were altered in an animal model of neuropathic pain induced by ligation of the left L5 spinal nerve. In anesthetized neuropathic pain model rats, stroking of the left hindlimb increased 5-HT release in the CeA, whereas stroking of the right hindlimb decreased it. Meanwhile, pinching of the left hindlimb increased intra-CeA 5-HT release irrespective of stimulus laterality. In conclusion, the present study demonstrated that intra-CeA 5-HT release responses to cutaneous stimulation are altered in an animal model of neuropathic pain.

Participation of the cannabinoid system and the NO/cGMP/KATP pathway in serotonin-induced peripheral antinociception

It has already been shown that serotonin can release endocannabinoids at the spinal cord level, culminating in inhibition of the dorsal horn. At the peripheral level, cannabinoid receptors modulate primary afferent neurons by inhibiting calcium conductance and increasing potassium conductance. Studies have shown that after the activation of opioid receptors and cannabinoids, there is also the activation of the NO/cGMP/KATP pathway, inducing cellular hyperpolarization. In this study, we evaluated the participation of the cannabinoid system with subsequent activation of the NO/cGMP/KATP pathway in the peripheral antinociceptive effect of serotonin. The paw pressure test of mice was used in animals that had their sensitivity to pain increased due to an intraplantar injection of PGE2 (2 μg). Serotonin (250 ng/paw), administered locally in the right hind paw, induced antinociceptive effect. CB1 and CB2 cannabinoid receptors antagonists, AM251 (20, 40 and 80 μg) and AM630 (25, 50 and 100 μg), respectively, reversed the serotonin-induced antinociceptive effect. MAFP (0.5 μg), an inhibitor of the FAAH enzyme that degrades anandamide, and JZL184 (3.75 μg), an inhibitor of the enzyme MAGL that degrades 2-AG, as well as the VDM11 (2.5 μg) inhibitor of anandamide reuptake, potentiated the antinociceptive effect induced by a low dose (62. 5 ng) of serotonin. In the evaluation of the participation of the NO/cGMP/KATP pathway, the antinociceptive effect of serotonin was reversed by the administration of the non-selective inhibitor of NOS isoforms L-NOarg (12.5, 25 and 50 μg) and by the selective inhibitor for the neuronal isoform LNPA (24 μg), as well as by the soluble guanylate cyclase inhibitor ODQ (25, 50 and 100 μg). Among potassium channel blockers, only Glibenclamide (20, 40 and 80 μg), an ATP-sensitive potassium channel blocker, reversed the effect of serotonin. In addition, intraplantar administration of serotonin (250 ng) was shown to induce a significant increase in nitrite levels in the homogenate of the plantar surface of the paw of mice. Taken together, these data suggest that the antinociceptive effect of serotonin occurs by activation of the cannabinoid system with subsequent activation of the NO/cGMP/KATP pathway.

Electroacupuncture alleviates mechanical allodynia and anxiety-like behaviors induced by chronic neuropathic pain via regulating rostral anterior cingulate cortex-dorsal raphe nucleus neural circuit

AIMS

Epidemiological studies in patients with neuropathic pain have demonstrated a strong association between neuropathic pain and psychiatric conditions such as anxiety. Preclinical and clinical work has demonstrated that electroacupuncture (EA) effectively alleviates anxiety-like behaviors induced by chronic neuropathic pain. In this study, a potential neural circuitry underlying the therapeutic action of EA was investigated.

METHODS

The effects of EA stimulation on mechanical allodynia and anxiety-like behaviors in animal models of spared nerve injury (SNI) were examined. EA plus chemogenetic manipulation of glutamatergic (Glu) neurons projecting from the rostral anterior cingulate cortex (rACCGlu ) to the dorsal raphe nucleus (DRN) was used to explore the changes of mechanical allodynia and anxiety-like behaviors in SNI mice.

RESULTS

Electroacupuncture significantly alleviated both mechanical allodynia and anxiety-like behaviors with increased activities of glutamatergic neurons in the rACC and serotoninergic neurons in the DRN. Chemogenetic activation of the rACCGlu -DRN projections attenuated both mechanical allodynia and anxiety-like behaviors in mice at day 14 after SNI. Chemogenetic inhibition of the rACCGlu -DRN pathway did not induce mechanical allodynia and anxiety-like behaviors under physiological conditions, but inhibiting this pathway produced anxiety-like behaviors in mice at day 7 after SNI; this effect was reversed by EA. EA plus activation of the rACCGlu -DRN circuit did not produce a synergistic effect on mechanical allodynia and anxiety-like behaviors. The analgesic and anxiolytic effects of EA could be blocked by inhibiting the rACCGlu -DRN pathway.

CONCLUSIONS

The role of rACCGlu -DRN circuit may be different during the progression of chronic neuropathic pain and these changes may be related to the serotoninergic neurons in the DRN. These findings describe a novel rACCGlu -DRN pathway through which EA exerts analgesic and anxiolytic effects in SNI mice exhibiting anxiety-like behaviors.

5,7-Dimethoxycoumarin ameliorates vincristine induced neuropathic pain: potential role of 5HT3 receptors and monoamines

Vincristine is the drug of choice for Hodgkin's lymphoma, acute lymphoblastic leukemia, and non-Hodgkin lymphoma. Despite its significant anticancer effects, it causes dose-dependent neuropathy, leading to compulsive dose reduction. The available drugs used for vincristine-induced neuropathic pain (VINP) have a range of safety, efficacy, and tolerability issues prompting a search for new therapies. 5,7-Dimethoxycoumarin (5,7-DMC) also known as citropten, is a natural coumarin found in the essential oils of citrus plants such as lime, lemons, and bergamots, and it possesses both antidepressant and anti-inflammatory effects. This study was designed to investigate the possible analgesic and antiallodynic effects of 5,7-DMC in a murine model of VINP. Vincristine was administered to groups of BALB/c male mice (0.1 mg/kg intraperitoneally) once daily for 14 days to induce VINP. Thermal hyperalgesia and mechanical allodynia were quantified using the tail immersion test and von Frey filament application method. The levels of monoamine neurotransmitters and vitamin C in frontal cortical, striatal and hippocampal tissues, as well as the TNF-α level in plasma, were quantified using high performance liquid chromatography and ELISA respectively. On day 15 of the protocol, acute treatment with 5,7-DMC clearly reversed VINP thermal hyperalgesia, mechanical static allodynia, mechanical dynamic allodynia, and cold allodynia. The activity of 5,7-DMC against hyperalgesia and allodynia was inhibited by pretreatment with ondansetron but not naloxone, implicating a 5-HT3 receptor involvement. VINP vitamin C levels were restored by 5,7-DMC in the frontal cortex, and changes in serotonin, dopamine, adenosine, inosine and hypoxanthine levels caused by vincristine were reversed either fully or partially. Additionally, the vincristine-induced rise in hippocampal serotonin, dopamine, inosine and striatal serotonin was appreciably reversed by 5,7-DMC. 5,7-DMC also reversed the vincristine-induced increase in the plasma level of TNF-α. In negating the changes in the levels of some neurotransmitters in the brain caused by vincristine, 5,7-DMC showed stronger effects than gabapentin. It was concluded that, there is a potential role of 5-HT3 receptors and monoamines in the amelioration of VINP induced by 5,7-DMC, and the use of this compound warrants further investigation.

Serotonin and Dopamine Show Different Response Profiles to Acute Stress in the Nucleus Accumbens and Medial Prefrontal Cortex of Rats with Neuropathic Pain

The ability to adaptively guide behaviour requires the integration of external information with internal motivational factors. Decision-making capabilities can be impaired by acute stress and is often exacerbated by chronic pain. Chronic neuropathic pain patients often present with cognitive dysfunction, including impaired decision-making. The mechanisms underlying these changes are not well understood but may include altered monoaminergic transmission in the brain. In this study we investigated the relationships between dopamine, serotonin, and their metabolites in key brain regions that regulate motivated behaviour and decision-making. The neurochemical profiles of the medial prefrontal cortex, orbital prefrontal cortex, and nucleus accumbens were analysed using HPLC in rats that received a chronic constriction injury (CCI) of the right sciatic nerve and an acute stress (15-min restraint), prior to an outcome devaluation task. CCI alone significantly decreased dopamine but not serotonin concentrations in the medial prefrontal cortex. By contrast, restraint stress acutely increased dopamine in the medial prefrontal cortex, and the nucleus accumbens; and increased serotonin in the medial prefrontal cortex 2 h later. The sustained dopaminergic and serotonergic responses to acute stress highlight the importance of an animal's ability to mount an effective coping response. In addition, these data suggest that the impact of nerve injury and acute stress on outcome-devaluation occurs independently of dopaminergic and serotonergic transmission in the medial prefrontal cortex, orbital prefrontal cortex and nucleus accumbens of rats.

Inhaled Cannabis Suppresses Chemotherapy-Induced Neuropathic Nociception by Decoupling the Raphe Nucleus: A Functional Imaging Study in Rats

BACKGROUND

Efficacy of inhaled cannabis for treating pain is controversial. Effective treatment for chemotherapy-induced neuropathy represents an unmet medical need. We hypothesized that cannabis reduces neuropathic pain by reducing functional coupling in the raphe nuclei.

METHODS

We assessed the impact of inhalation of vaporized cannabis plant (containing 10.3% Δ⁹-tetrahydrocannabinol/0.05% cannabidiol) or placebo cannabis on brain resting-state blood oxygen level-dependent functional connectivity and pain behavior induced by paclitaxel in rats. Rats received paclitaxel to produce chemotherapy-induced peripheral neuropathy or its vehicle. Behavioral and imaging experiments were performed after neuropathy was established and stable. Images were registered to, and analyzed using, a 3D magnetic resonance imaging rat atlas providing site-specific data on more than 168 different brain areas.

RESULTS

Prior to vaporization, paclitaxel produced cold allodynia. Inhaled vaporized cannabis increased cold withdrawal latencies relative to prevaporization or placebo cannabis, consistent with Δ⁹-tetrahydrocannabinol-induced antinociception. In paclitaxel-treated rats, the midbrain serotonergic system, comprising the dorsal and median raphe, showed hyperconnectivity to cortical, brainstem, and hippocampal areas, consistent with nociceptive processing. Inhalation of vaporized cannabis uncoupled paclitaxel-induced hyperconnectivity patterns. No such changes in connectivity or cold responsiveness were observed following placebo cannabis vaporization.

CONCLUSIONS

Inhaled vaporized cannabis plant uncoupled brain resting-state connectivity in the raphe nuclei, normalizing paclitaxel-induced hyperconnectivity to levels observed in vehicle-treated rats. Inhaled vaporized cannabis produced antinociception in both paclitaxel- and vehicle-treated rats. Our study elucidates neural circuitry implicated in the therapeutic effects of Δ⁹-tetrahydrocannabinol and supports a role for functional imaging studies in animals in guiding indications for future clinical trials.

Targeting Peripherally Restricted Cannabinoid Receptor 1, Cannabinoid Receptor 2, and Endocannabinoid-Degrading Enzymes for the Treatment of Neuropathic Pain Including Neuropathic Orofacial Pain

Neuropathic pain conditions including neuropathic orofacial pain (NOP) are difficult to treat. Contemporary therapeutic agents for neuropathic pain are often ineffective in relieving pain and are associated with various adverse effects. Finding new options for treating neuropathic pain is a major priority in pain-related research. Cannabinoid-based therapeutic strategies have emerged as promising new options. Cannabinoids mainly act on cannabinoid 1 (CB1) and 2 (CB2) receptors, and the former is widely distributed in the brain. The therapeutic significance of cannabinoids is masked by their adverse effects including sedation, motor impairment, addiction and cognitive impairment, which are thought to be mediated by CB1 receptors in the brain. Alternative approaches have been developed to overcome this problem by selectively targeting CB2 receptors, peripherally restricted CB1 receptors and endocannabinoids that may be locally synthesized on demand at sites where their actions are pertinent. Many preclinical studies have reported that these strategies are effective for treating neuropathic pain and produce no or minimal side effects. Recently, we observed that inhibition of degradation of a major endocannabinoid, 2-arachydonoylglycerol, can attenuate NOP following trigeminal nerve injury in mice. This review will discuss the above-mentioned alternative approaches that show potential for treating neuropathic pain including NOP.

Transmodulation of Dopaminergic Signaling to Mitigate Hypodopminergia and Pharmaceutical Opioid-Induced Hyperalgesia

Neuroscientists and psychiatrists working in the areas of "pain and addiction" are asked in this perspective article to reconsider the current use of dopaminergic blockade (like chronic opioid agonist therapy), and instead to consider induction of dopamine homeostasis by putative pro-dopamine regulation. Pro-dopamine regulation could help pharmaceutical opioid analgesic agents to mitigate hypodopaminergia-induced hyperalgesia by inducing transmodulation of dopaminergic signaling. An optimistic view is that early predisposition to diagnosis based on genetic testing, (pharmacogenetic/pharmacogenomic monitoring), combined with appropriate urine drug screening, and treatment with pro-dopamine regulators, could conceivably reduce stress, craving, and relapse, enhance well-being and attenuate unwanted hyperalgesia. These concepts require intensive investigation. However, based on the rationale provided herein, there is a good chance that combining opioid analgesics with genetically directed pro-dopamine-regulation using KB220 (supported by 43 clinical studies). This may become a front-line technology with the potential to overcome, in part, the current heightened rates of chronic opioid-induced hyperalgesia and concomitant Reward Deficiency Syndrome (RDS) behaviors. Current research does support the hypothesis that low or hypodopaminergic function in the brain may predispose individuals to low pain tolerance or hyperalgesia.

Metabotropic Glutamate Receptor 5 and 8 Modulate the Ameliorative Effect of Ultramicronized Palmitoylethanolamide on Cognitive Decline Associated with Neuropathic Pain

This study investigated whether metabotropic glutamate receptor (mGluR) 5 and 8 are involved in the effect of ultramicronizedpalmitoylethanolamide (um-PEA) on the cognitive behavior and long term potentiation (LTP) at entorhinal cortex (LEC)-dentate gyrus (DG) pathway in mice rendered neuropathic by the spare nerve injury (SNI). SNI reduced discriminative memory and LTP. Um-PEA treatment started after the development of neuropathic pain had no effects in sham mice, whereas it restored cognitive behavior and LTP in SNI mice. 2-Methyl-6-(phenylethynyl) pyridine (MPEP), a selective mGluR5 antagonist, improved cognition in SNI mice and produced a chemical long term depression of the field excitatory postsynaptic potentials (fEPSPs) in sham and SNI mice. After theta burst stimulation (TBS) MPEP restored LTP in SNI mice. In combination with PEA, MPEP antagonized the PEA effect on discriminative memory and decreased LTP in SNI mice. The (RS)-4-(1-amino-1-carboxyethyl)phthalic acid (MDCPG), a selective mGluR8 antagonist, did not affect discriminative memory, but it induced a chemical LTP and prevented the enhancement of fEPSPs after TBS in SNI mice which were treated or not treated with PEA. The effect of PEA on LTP and cognitive behavior was modulated by mGluR5 and mGluR8. In particular in the SNI conditions, the mGluR5 blockade facilitated memory and LTP, but prevented the beneficial effects of PEA on discriminative memory while the mGluR8 blockade, which was ineffective in itself, prevented the favorable action of the PEA on LTP. Thus, although their opposite roles (excitatory/inhibitory of the two receptor subtypes on the glutamatergic system), they appeared to be required for the neuroprotective effect of PEA in conditions of neuropathic pain.

Motor cortex and pain control: exploring the descending relay analgesic pathways and spinal nociceptive neurons in healthy conscious rats

Motor cortex stimulation (MCS) is an effective therapy for refractory neuropathic pain. MCS increases the nociceptive threshold in healthy rats via endogenous opioids, inhibiting thalamic nuclei and activating the periaqueductal gray. It remains unclear how the motor cortex induces top-down modulation of pain in the absence of persistent pain. Here, we investigated the main nuclei involved in the descending analgesic pathways and the spinal nociceptive neurons in rats that underwent one session of MCS and were evaluated with the paw pressure nociceptive test. The pattern of neuronal activation in the dorsal raphe nucleus (DRN), nucleus raphe magnus (NRM), locus coeruleus (LC), and dorsal horn of the spinal cord (DHSC) was assessed by immunoreactivity (IR) for Egr-1 (a marker of activated neuronal nuclei). IR for serotonin (5HT) in the DRN and NRM, tyrosine hydroxylase (TH) in the LC, and substance P (SP) and enkephalin (ENK) in the DHSC was also evaluated. MCS increased the nociceptive threshold of the animals; this increase was accompanied by activation of the NRM, while DRN activation was unchanged. However, cortical stimulation induced an increase in 5HT-IR in both serotonergic nuclei. MCS did not change the activation pattern or TH-IR in the LC, and it inhibited neuronal activation in the DHSC without altering SP or ENK-IR. Taken together, our results suggest that MCS induces the activation of serotonergic nuclei as well as the inhibition of spinal neurons, and such effects may contribute to the elevation of the nociceptive threshold in healthy rats. These results allow a better understanding of the circuitry involved in the antinociceptive top-down effect induced by MCS under basal conditions, reinforcing the role of primary motor cortex in pain control.

Cannabinoids Modulate Neuronal Activity and Cancer by CB1 and CB2 Receptor-Independent Mechanisms

Cannabinoids include the active constituents of Cannabis or are molecules that mimic the structure and/or function of these Cannabis-derived molecules. Cannabinoids produce many of their cellular and organ system effects by interacting with the well-characterized CB1 and CB2 receptors. However, it has become clear that not all effects of cannabinoid drugs are attributable to their interaction with CB1 and CB2 receptors. Evidence now demonstrates that cannabinoid agents produce effects by modulating activity of the entire array of cellular macromolecules targeted by other drug classes, including: other receptor types; ion channels; transporters; enzymes, and protein- and non-protein cellular structures. This review summarizes evidence for these interactions in the CNS and in cancer, and is organized according to the cellular targets involved. The CNS represents a well-studied area and cancer is emerging in terms of understanding mechanisms by which cannabinoids modulate their activity. Considering the CNS and cancer together allow identification of non-cannabinoid receptor targets that are shared and divergent in both systems. This comparative approach allows the identified targets to be compared and contrasted, suggesting potential new areas of investigation. It also provides insight into the diverse sources of efficacy employed by this interesting class of drugs. Obtaining a comprehensive understanding of the diverse mechanisms of cannabinoid action may lead to the design and development of therapeutic agents with greater efficacy and specificity for their cellular targets.

Antihyperalgesic Activities of Endocannabinoids in a Mouse Model of Antiretroviral-Induced Neuropathic Pain

Background: Nucleoside reverse transcriptase inhibitors (NRTIs) are the cornerstone of the antiretroviral therapy for human immunodeficiency virus/acquired immune deficiency syndrome (HIV/AIDS). However, their use is sometimes limited by the development of a painful sensory neuropathy, which does not respond well to drugs. Smoked cannabis has been reported in clinical trials to have efficacy in relieving painful HIV-associated sensory neuropathy. Objectives: The aim of this study was to evaluate whether the expression of endocannabinoid system molecules is altered during NRTI-induced painful neuropathy, and also whether endocannabinoids can attenuate NRTI-induced painful neuropathy. Methods: BALB/c mice were treated with 25 mg/kg of 2',3'-dideoxycytidine (ddC, zalcitabine), a NRTI, to induce thermal hyperalgesia. The expression of endocannabinoid system molecules was evaluated by real time polymerase chain reaction in the brain, spinal cord and paw skin at 6 days post ddC administration, a time point when mice had developed thermal hyperalgesia. The effects of the endocannabinoids, N-arachidonoyl ethanolamine (AEA) and 2-arachidonoyl glycerol (2-AG), the cannabinoid type 1 (CB1) receptor antagonist AM 251, CB2 receptor antagonist AM 630, and G protein-coupled receptor 55 (GPR55) antagonists ML193 and CID 16020046 on ddC-induced thermal hyperalgesia were evaluated using the hot plate test. Results: ddC treatment resulted in thermal hyperalgesia and increased transcripts of the synthesizing enzyme Plcβ1 and decreased Daglβ in the paw skins, but not Napepld, and Daglα compared to vehicle treatment. Transcripts of the inactivating enzymes Faah and Mgll were downregulated in the brain and/or paw skin but not in the spinal cord of ddC-treated mice. Both AEA and 2-AG had antihyperalgesic effects in mice with ddC-induced thermal hyperalgesia, but had no effect in ddC-naïve mice. The antihyperalgesic activity of AEA was antagonized by AM251 and AM630, whereas the activity of 2-AG was antagonized by AM251, ML193 and CID 16020046, but not by AM630. Conclusion: These data show that ddC induces thermal hyperalgesia, which is associated with dysregulation of the mRNA expression of some endocannabinoid system molecules. The endocannabinoids AEA and 2-AG have antihyperalgesic activity, which is dependent on cannabinoid receptor and GPR55 activation. Thus, agonists of cannabinoid receptors and GPR55 could be useful therapeutic agents for the management of NRTI-induced painful sensory neuropathy.

Prophylactic treatment with the tricyclic antidepressant desipramine prevents development of paclitaxel-induced neuropathic pain through activation of endogenous analgesic systems

Neuropathic pain impacts approximately 3-4.5% of the global population and remains an unresolved health problem. The management of neuropathic pain has two distinct goals-prevention of development and control of established neuropathic pain. We examined the impact of both prophylactic and therapeutic treatments with the tricyclic antidepressant desipramine on the development and maintenance of toxic neuropathic pain induced by the chemotherapeutic agent paclitaxel. We also investigated the involvement of endogenous analgesic (i.e., endogenous opioid and endocannabinoid) systems in the antinociceptive actions of desipramine in these two distinct phases of neuropathic pain. Chronic subcutaneous infusion of desipramine via osmotic pumps suppressed both the development and maintenance of paclitaxel-induced neuropathic pain. However, only prophylactic desipramine treatment blocked the development of neuropathic pain throughout the three month observation interval; neuropathic pain did not return. The opioid receptor antagonist naloxone blocked the antinociceptive effects of both prophylactic and therapeutic desipramine treatments throughout the entire timecourse of desipramine-induced antinociception. By contrast, cannabinoid CB₁ and CB₂ receptor antagonists partially attenuated the antinociceptive actions of desipramine in a manner that was restricted to the development phase of paclitaxel-induced neuropathic pain only. Paclitaxel decreased cell viability in TMD231 tumor cells in an MTT assay in vitro. Notably, desipramine (1nM-1μM) alone did not alter tumor cell viability and did not prevent the cytotoxic effects of paclitaxel under identical conditions. The highest concentration of desipramine (10μM) reduced tumor cell viability alone and enhanced the cytotoxic effects of paclitaxel. Our study identifies a previously unrecognized preemptive analgesic strategy that prevents development of paclitaxel-induced neuropathic pain, and also dissects receptor mechanisms underlying desipramine-induced antinociceptive effects. This information may be applied to improve current therapeutic strategies with the goal of preventing and managing neuropathic pain induced by chemotherapeutic treatment.

Are cannabidiol and Δ(9) -tetrahydrocannabivarin negative modulators of the endocannabinoid system? A systematic review

Based upon evidence that the therapeutic properties of Cannabis preparations are not solely dependent upon the presence of Δ(9) -tetrahydrocannabinol (THC), pharmacological studies have been recently carried out with other plant cannabinoids (phytocannabinoids), particularly cannabidiol (CBD) and Δ(9) -tetrahydrocannabivarin (THCV). Results from some of these studies have fostered the view that CBD and THCV modulate the effects of THC via direct blockade of cannabinoid CB1 receptors, thus behaving like first-generation CB1 receptor inverse agonists, such as rimonabant. Here, we review in vitro and ex vivo mechanistic studies of CBD and THCV, and synthesize data from these studies in a meta-analysis. Synthesized data regarding mechanisms are then used to interpret results from recent pre-clinical animal studies and clinical trials. The evidence indicates that CBD and THCV are not rimonabant-like in their action and thus appear very unlikely to produce unwanted CNS effects. They exhibit markedly disparate pharmacological profiles particularly at CB1 receptors: CBD is a very low-affinity CB1 ligand that can nevertheless affect CB1 receptor activity in vivo in an indirect manner, while THCV is a high-affinity CB1 receptor ligand and potent antagonist in vitro and yet only occasionally produces effects in vivo resulting from CB1 receptor antagonism. THCV has also high affinity for CB2 receptors and signals as a partial agonist, differing from both CBD and rimonabant. These cannabinoids illustrate how in vitro mechanistic studies do not always predict in vivo pharmacology and underlie the necessity of testing compounds in vivo before drawing any conclusion on their functional activity at a given target.

Receptome: Interactions between three pain-related receptors or the "Triumvirate" of cannabinoid, opioid and TRPV1 receptors

A growing amount of data demonstrates the interactions between cannabinoid, opioid and the transient receptor potential (TRP) vanilloid type 1 (TRPV1) receptors. These interactions can be bidirectional, inhibitory or excitatory, acute or chronic in their nature, and arise both at the molecular level (structurally and functionally) and in physiological processes, such as pain modulation or perception. The interactions of these three pain-related receptors may also reserve important and new therapeutic applications for the treatment of chronic pain or inflammation. In this review, we summarize the main findings on the interactions between the cannabinoid, opioid and the TRPV1 receptor regarding to pain modulation.

High Times for Painful Blues: The Endocannabinoid System in Pain-Depression Comorbidity

Depression and pain are two of the most debilitating disorders worldwide and have an estimated cooccurrence of up to 80%. Comorbidity of these disorders is more difficult to treat, associated with significant disability and impaired health-related quality of life than either condition alone, resulting in enormous social and economic cost. Several neural substrates have been identified as potential mediators in the association between depression and pain, including neuroanatomical reorganization, monoamine and neurotrophin depletion, dysregulation of the hypothalamo-pituitary-adrenal axis, and neuroinflammation. However, the past decade has seen mounting evidence supporting a role for the endogenous cannabinoid (endocannabinoid) system in affective and nociceptive processing, and thus, alterations in this system may play a key role in reciprocal interactions between depression and pain. This review will provide an overview of the preclinical evidence supporting an interaction between depression and pain and the evidence supporting a role for the endocannabinoid system in this interaction.

Enhanced serotonin and mesolimbic dopamine transmissions in a rat model of neuropathic pain

In humans, affective consequences of neuropathic pain, ranging from depression to anxiety and anhedonia, severely impair quality of life and are a major disease burden, often requiring specific medications. Depressive- and anxiety-like behaviors have also been observed in animal models of peripheral nerve injury. Dysfunctions in central nervous system monoamine transmission have been hypothesized to underlie depressive and anxiety disorders in neuropathic pain. To assess whether these neurons display early changes in their activity that in the long-term might lead to chronicization, maladaptive plasticity and affective consequences, we carried out in vivo extracellular single unit recordings from serotonin neurons in the dorsal raphe nucleus (DRN) and from dopamine neurons in ventral tegmental area (VTA) in the spared nerve injury (SNI) model of neuropathic pain in rats. Extracellular dopamine levels and the expression of dopamine D1, D2 receptors and tyrosine hydroxylase (TH) were measured in the nucleus accumbens. We report that, two weeks following peripheral nerve injury, discharge rate of serotonin DRN neurons and burst firing of VTA dopamine cells are enhanced, when compared with sham-operated animals. We also observed higher extracellular dopamine levels and reduced expression of D2, but not D1, receptors and TH in the nucleus accumbens. Our study confirms that peripheral neuropathy induces changes in the serotonin and dopamine systems that might be the early result of chronic maladaptation to persistent pain. The allostatic activation of these neural systems, which mirrors that already described as a consequence of stress, might lead to depression and anxiety previously observed in neuropathic animals but also an attempt to cope positively with the negative experience.

Comprehensive Review of Medicinal Marijuana, Cannabinoids, and Therapeutic Implications in Medicine and Headache: What a Long Strange Trip It's Been …

BACKGROUND

The use of cannabis, or marijuana, for medicinal purposes is deeply rooted though history, dating back to ancient times. It once held a prominent position in the history of medicine, recommended by many eminent physicians for numerous diseases, particularly headache and migraine. Through the decades, this plant has taken a fascinating journey from a legal and frequently prescribed status to illegal, driven by political and social factors rather than by science. However, with an abundance of growing support for its multitude of medicinal uses, the misguided stigma of cannabis is fading, and there has been a dramatic push for legalizing medicinal cannabis and research. Almost half of the United States has now legalized medicinal cannabis, several states have legalized recreational use, and others have legalized cannabidiol-only use, which is one of many therapeutic cannabinoids extracted from cannabis. Physicians need to be educated on the history, pharmacology, clinical indications, and proper clinical use of cannabis, as patients will inevitably inquire about it for many diseases, including chronic pain and headache disorders for which there is some intriguing supportive evidence.

OBJECTIVE

To review the history of medicinal cannabis use, discuss the pharmacology and physiology of the endocannabinoid system and cannabis-derived cannabinoids, perform a comprehensive literature review of the clinical uses of medicinal cannabis and cannabinoids with a focus on migraine and other headache disorders, and outline general clinical practice guidelines.

CONCLUSION

The literature suggests that the medicinal use of cannabis may have a therapeutic role for a multitude of diseases, particularly chronic pain disorders including headache. Supporting literature suggests a role for medicinal cannabis and cannabinoids in several types of headache disorders including migraine and cluster headache, although it is primarily limited to case based, anecdotal, or laboratory-based scientific research. Cannabis contains an extensive number of pharmacological and biochemical compounds, of which only a minority are understood, so many potential therapeutic uses likely remain undiscovered. Cannabinoids appear to modulate and interact at many pathways inherent to migraine, triptan mechanisms ofaction, and opiate pathways, suggesting potential synergistic or similar benefits. Modulation of the endocannabinoid system through agonism or antagonism of its receptors, targeting its metabolic pathways, or combining cannabinoids with other analgesics for synergistic effects, may provide the foundation for many new classes of medications. Despite the limited evidence and research suggesting a role for cannabis and cannabinoids in some headache disorders, randomized clinical trials are lacking and necessary for confirmation and further evaluation.

Alterations in the anandamide metabolism in the development of neuropathic pain

Endocannabinoids (EC), particularly anandamide (AEA), released constitutively in pain pathways might be accountable for the inhibitory effect on nociceptors. Pathogenesis of neuropathic pain may reflect complex remodeling of the dorsal root ganglia (DRGs) and spinal cord EC system. Multiple pathways involved both in the biosynthesis and degradation of AEA have been suggested. We investigated the local synthesis and degradation features of AEA in DRGs and spinal cord during the development and maintenance of pain in a model of chronic constriction injury (CCI). All AEA synthesis and degradation enzymes are present on the mRNA level in DRGs and lumbar spinal cord of intact as well as CCI-treated animals. Deregulation of EC system components was consistent with development of pain phenotype at days 3, 7, and 14 after CCI. The expression levels of enzymes involved in AEA degradation was significantly upregulated ipsilateral in DRGs and spinal cord at different time points. Expression of enzymes of the alternative, sPLA2-dependent and PLC-dependent, AEA synthesis pathways was elevated in both of the analyzed structures at all time points. Our data have shown an alteration of alternative AEA synthesis and degradation pathways, which might contribute to the variation of AEA levels and neuropathic pain development.

Endocannabinoids in the brainstem modulate dural trigeminovascular nociceptive traffic via CB1 and "triptan" receptors: implications in migraine

Activation and sensitization of trigeminovascular nociceptive pathways is believed to contribute to the neural substrate of the severe and throbbing nature of pain in migraine. Endocannabinoids, as well as being physiologically analgesic, are known to inhibit dural trigeminovascular nociceptive responses. They are also involved in the descending modulation of cutaneous-evoked C-fiber spinal nociceptive responses from the brainstem. The purpose of this study was to determine whether endocannabinoids are involved in the descending modulation of dural and/or cutaneous facial trigeminovascular nociceptive responses, from the brainstem ventrolateral periaqueductal gray (vlPAG). CB1 receptor activation in the vlPAG attenuated dural-evoked Aδ-fiber neurons (maximally by 19%) and basal spontaneous activity (maximally by 33%) in the rat trigeminocervical complex, but there was no effect on cutaneous facial receptive field responses. This inhibitory vlPAG-mediated modulation was inhibited by specific CB1 receptor antagonism, given via the vlPAG, and with a 5-HT1B/1D receptor antagonist, given either locally in the vlPAG or systemically. These findings demonstrate for the first time that brainstem endocannabinoids provide descending modulation of both basal trigeminovascular neuronal tone and Aδ-fiber dural-nociceptive responses, which differs from the way the brainstem modulates spinal nociceptive transmission. Furthermore, our data demonstrate a novel interaction between serotonergic and endocannabinoid systems in the processing of somatosensory nociceptive information, suggesting that some of the therapeutic action of triptans may be via endocannabinoid containing neurons in the vlPAG.

Neural Circuit in the Dorsal Raphe Nucleus Responsible for Cannabinoid-Mediated Increases in 5-HT Efflux in the Nucleus Accumbens of the Rat Brain

In vivo microdialysis was used in this study to reveal the role of cannabinoids in regulating serotonin (5-HT) efflux in the nucleus accumbens (NAcc) and dorsal raphe nucleus (DRN). The cannabinoid CB1 receptor agonists WIN55212-2 and CP55940 systematically administered to rats caused significant increases in 5-HT efflux in the NAcc but failed to have an effect in the DRN. To reveal mechanisms underlying regionally selective responses, we tested the hypothesis that cannabinoids have both direct and indirect effects on 5-HT efflux, depending on the location of CB1 receptors in the neural circuit between DRN and NAcc. We showed that the direct effect of cannabinoids caused a reduction in 5-HT efflux whereas the indirect effect resulted in an increase. Furthermore, the indirect effect was blocked by the GABA_A receptor antagonist bicuculline in the DRN, suggesting that the action is likely due to a presynaptic inhibition on GABAergic activity that exerts a tonic influence on neuronal circuits regulating 5-HT efflux. Involvement of GABAergic neurons was confirmed by measuring changes in GABA efflux. Taken together, our study suggests that cannabinoids may have direct and indirect effects on the 5-HT regulatory circuits, resulting in regionally selective changes of 5-HT efflux in the brain.

Effect of delta-9-tetrahydrocannabinol on behavioral despair and on pre- and postsynaptic serotonergic transmission

Preclinical and clinical studies suggest that direct and indirect cannabinoid agonists, including enhancers of endocannabinoids, engender stress-relieving, anxiolytic and antidepressant effects, mediated by central CB(1) receptors (CB(1)Rs). The effect of the main pharmacologically active principle in cannabis, (-)-trans-Δ(9)-tetrahydrocannabinol (delta-9-THC), on depressive behavior and on the serotonin (5-HT) system, which is implicated in the mechanism of action of antidepressants, has not been extensively clarified. Here, we showed that repeated (5 days), but not single (acute) intraperitoneal (ip) treatment with delta-9-THC (1mg/kg) exerts antidepressant-like properties in the rat forced swim test (FST). This effect was CB(1)R-dependent because it was blocked by the CB(1)R antagonist rimonabant (1mg/kg, ip). Using in vivo electrophysiology, we demonstrated that delta-9-THC modulated dorsal raphe (DR) 5-HT neuronal activity through a CB(1)R-dependent mechanism. Acute intravenous delta-9-THC administration (0.1-1.5mg/kg) elicited a complex response profile, producing excitatory, inhibitory and inert responses of 5-HT neurons. Only excitatory responses were blocked by rimonabant. Finally, repeated but not single delta-9-THC administration (1mg/kg, ip) enhanced tonic 5-HT(1A) receptor activity in the hippocampus, a postsynaptic event commonly elicited by standard antidepressants. These results suggest that delta-9-THC, like other CB(1)R agonists and endocannabinoid enhancers, may possess antidepressant properties at low doses, and could modulate 5-HT transmission in the DR and hippocampus as standard antidepressants such as selective serotonin reuptake inhibitors.

Involvement of descending serotonergic and noradrenergic pathways in CB1 receptor-mediated antinociception

Cannabinoids produce antinociceptive and antihyperalgesic effects mainly through activation of the inhibitory CB1 receptors. The demonstration that antinociceptive effects of systemic cannabinoids are significantly diminished following surgical dorsolateral funiculus lesion provides evidence that supraspinal sites and descending pain modulatory pathways play crucial roles in systemic cannabinoid analgesia. In this review, we will firstly provide a background, brief overview of descending modulatory pathways followed by descending pathways implicated in cannabinoid analgesia. We will then describe the recent evidence of the involvement of descending serotonergic and noradrenergic pathways in CB1 receptor-mediated antinociception. This review will provide evidences that systemically administered cannabinoids reinforce the descending serotonergic and noradrenergic pathways to produce acute antinociceptive effects via spinal 5-HT7, 5-HT2A and alpha-2 adrenoceptors activation.

The function of alpha-2-adrenoceptors in the rat locus coeruleus is preserved in the chronic constriction injury model of neuropathic pain

RATIONALE

Peripheral neuropathic pain is a chronic condition that may produce plastic changes in several brain regions. The noradrenergic locus coeruleus (LC) is a crucial component of ascending and descending pain pathways, both of which are frequently compromised after nerve injury.

OBJECTIVES

The objective of the study was to examine whether chronic constriction injury (CCI), a model of neuropathic pain, alters noradrenergic activity in the rat LC.

METHODS

Activity in the LC was assessed by electrophysiology and microdialysis, while protein expression was monitored in western blots and by immunohistochemistry.

RESULTS

The pain threshold had dropped in injured rats 7 days after inducing neuropathy. While alpha-2-adrenoceptors mediate activity in the LC and in its terminal areas, no alterations in either spontaneous neuronal activity or extracellular noradrenaline levels were observed following CCI. Moreover, alpha-2-adrenoceptor activity in the LC of CCI rats remained unchanged after systemic administration of UK14,304, RX821002 or desipramine. Accordingly, extracellular noradrenaline levels in the LC were similar in CCI and control animals following local administration of clonidine or RX821002. In addition, there were no changes in the expression of the alpha-2-adrenoceptors, Gαi/z subunits or the regulators of G-protein signaling. However, pERK1/2 (phosphorylated extracellular signal-regulated kinases 1/2) expression augmented in the spinal cord, paragigantocellularis nucleus (PGi) and dorsal raphe nucleus (DRN) following CCI.

CONCLUSIONS

Neuropathic pain is not accompanied by modifications in tonic LC activity after the onset of pain. This may indicate that the signals from the PGi and DRN, the excitatory and inhibitory afferents of the LC, cancel one another out.

Altering endocannabinoid neurotransmission at critical developmental ages: impact on rodent emotionality and cognitive performance

The endocannabinoid system shows functional activity from early stages of brain development: it plays an important role in fundamental developmental processes such as cell proliferation, migration, and differentiation, thus shaping brain organization during pre- and postnatal life. Cannabis sativa preparations are among the illicit drugs most commonly used by young people, including pregnant women. The developing brain can be therefore exposed to cannabis preparations during two critical periods: first, in offspring of cannabis-using mothers through perinatal and/or prenatal exposure; second, in adolescent cannabis users during neural maturation. In the last decade, it has become clear that the endocannabinoid system critically modulates memory processing and emotional responses. Therefore, it is well possible that developmental exposure to cannabinoid compounds induces enduring changes in behaviors and neural processes belonging to the cognitive and emotional domains. We address this issue by focusing on rodent studies, in order to provide a framework for understanding the impact of cannabinoid exposure on the developing brain.

Putative role of endocannabinoid signaling in the etiology of depression and actions of antidepressants

In the last few years, there have been several advances in the determination of the role of the endocannabinoid system in the etiology of depression and the functional actions of antidepressant drugs. Specifically, a deficiency in endocannabinoid signaling is sufficient to produce a "depressive-like" phenotype at the preclinical level (including changes in rewarding, emotional and cognitive behavior and biological changes such as increased HPA axis activity, impaired stress adaptation, reduced neurogenesis and altered serotonin negative feedback), and capable of inducing symptoms of depression in humans at a clinical level. In line with these findings, clinical populations diagnosed with depression are found to have reduced levels of circulating endocannabinoids and preclinical models of depression reveal a deficit in central endocannabinoid signaling. Moreover, facilitation of endocannabinoid signaling is sufficient to produce all of the behavioral and biochemical effects of conventional antidepressant treatments. Further, many forms of antidepressant treatments significantly alter endocannabinoid signaling, and in some of these cases this recruitment of endocannabinoid signaling is involved in the neuroadaptive effects of these treatments. Ultimately, these data present a compelling picture of the putative role of the endocannabinoid system in the processes subserving both the development and treatment of depression.

Effects of COX-2 inhibition on spinal nociception: the role of endocannabinoids

BACKGROUND AND PURPOSE

Recent studies suggest that the effects of cyclooxygenase-2 (COX-2) inhibition are mediated by cannabinoid receptor activation. However, some non-steroidal anti-inflammatory drugs inhibit the enzyme fatty acid amide hydrolase, which regulates levels of some endocannabinoids. Whether COX-2 directly regulates levels of endocannabinoids in vivo is unclear. Here, the effect of the COX-2 inhibitor nimesulide, which does not inhibit fatty acid amide hydrolase, on spinal nociceptive processing was determined. Effects of nimesulide on tissue levels of endocannabinoids and related compounds were measured and the role of cannabinoid 1 (CB(1)) receptors was determined.

EXPERIMENTAL APPROACH

Effects of spinal and peripheral administration of nimesulide (1-100 microg per 50 microL) on mechanically evoked responses of rat dorsal horn neurones were measured, and the contribution of the CB(1) receptor was determined with the antagonist AM251 (N-(piperidin-1-yl)-5-(-4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide), in anaesthetized rats. Effects of nimesulide on spinal levels of endocannabinoids and related compounds were quantified using liquid chromatography-tandem mass spectrometry.

KEY RESULTS

Spinal, but not peripheral, injection of nimesulide (1-100 microg per 50 microL) significantly reduced mechanically evoked responses of dorsal horn neurones. Inhibitory effects of spinal nimesulide were blocked by the CB(1) receptor antagonist AM251 (1 microg per 50 microL), but spinal levels of endocannabinoids were not elevated. Indeed, both anandamide and N-oleoylethanolamide (OEA) were significantly decreased by nimesulide.

CONCLUSIONS AND IMPLICATIONS

Although the inhibitory effects of COX-2 blockade on spinal neuronal responses by nimesulide were dependent on CB(1) receptors, we did not detect a concomitant elevation in anandamide or 2-AG. Further understanding of the complexities of endocannabinoid catabolism by multiple enzymes is essential to understand their contribution to COX-2-mediated analgesia.

TRPV1 in brain is involved in acetaminophen-induced antinociception

BACKGROUND

Acetaminophen, the major active metabolite of acetanilide in man, has become one of the most popular over-the-counter analgesic and antipyretic agents, consumed by millions of people daily. However, its mechanism of action is still a matter of debate. We have previously shown that acetaminophen is further metabolized to N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z -eicosatetraenamide (AM404) by fatty acid amide hydrolase (FAAH) in the rat and mouse brain and that this metabolite is a potent activator of transient receptor potential vanilloid 1 (TRPV(1)) in vitro. Pharmacological activation of TRPV(1) in the midbrain periaqueductal gray elicits antinociception in rats. It is therefore possible that activation of TRPV(1) in the brain contributes to the analgesic effect of acetaminophen.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that the antinociceptive effect of acetaminophen at an oral dose lacking hypolocomotor activity is absent in FAAH and TRPV(1) knockout mice in the formalin, tail immersion and von Frey tests. This dose of acetaminophen did not affect the global brain contents of prostaglandin E(2) (PGE(2)) and endocannabinoids. Intracerebroventricular injection of AM404 produced a TRPV(1)-mediated antinociceptive effect in the mouse formalin test. Pharmacological inhibition of TRPV(1) in the brain by intracerebroventricular capsazepine injection abolished the antinociceptive effect of oral acetaminophen in the same test.

CONCLUSIONS

This study shows that TRPV(1) in brain is involved in the antinociceptive action of acetaminophen and provides a strategy for developing central nervous system active oral analgesics based on the coexpression of FAAH and TRPV(1) in the brain.

The Role of Cannabinoid Receptors in the Descending Modulation of Pain

The endogenous antinociceptive descending pathway represents a circuitry of the supraspinal central nervous system whose task is to counteract pain. It includes the periaqueductal grey (PAG)-rostral ventromedial medulla (RVM)-dorsal horn (DH) axis, which is the best characterized pain modulation system through which pain is endogenously inhibited. Thus, an alternative rational strategy for silencing pain is the activation of this anatomical substrate. Evidence of the involvement of cannabinoid receptors (CB) in the supraspinal modulation of pain can be found in several studies in which intra-cerebral microinjections of cannabinoid ligands or positive modulators have proved to be analgesic in different pain models, whereas cannabinoid receptor antagonists or antisense nucleotides towards CB1 receptors have facilitated pain. Like opioids, cannabinoids produce centrally-mediated analgesia by activating a descending pathway which includes PAG and its projection to downstream RVM neurons, which in turn send inhibitory projections to the dorsal horn of the spinal cord. Indeed, several studies underline a supraspinal regulation of cannabinoids on γ-aminobutyric acid (GABA) and glutamate release which inhibit and enhance the antinociceptive descending pathway, respectively. Cannabinoid receptor activation expressed on presynaptic GABAergic terminals reduces the probability of neurotransmitter release thus dis-inhibiting the PAG-RVM-dorsal horn antinociceptive pathway. Cannabinoids seem to increase glutamate release (maybe as consequence of GABA decrease) and to require glutamate receptor activation to induce antinociception. The consequent outcome is behavioral analgesia, which is reproduced in several pain conditions, from acute to chronic pain models such as inflammatory and neuropathic pain. Taken together these findings would suggest that supraspinal cannabinoid receptors have broad applications, from pain control to closely related central nervous system diseases such as anxiety and depression.

Fatty acid amide hydrolase and monoacylglycerol lipase inhibitors produce anti-allodynic effects in mice through distinct cannabinoid receptor mechanisms

The endocannabinoids anandamide and 2-arachidonoylglycerol are predominantly regulated by the respective catabolic enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL). Inhibition of these enzymes elevates endocannabinoid levels and attenuates neuropathic pain. In the present study, CB₁ and CB₂ receptor-deficient mice were subjected to chronic constriction injury (CCI) of the sciatic nerve to examine the relative contribution of each receptor for the anti-allodynic effects of the FAAH inhibitor, PF-3845, and the MAGL inhibitor, JZL184. CCI caused marked hypersensitivity to mechanical and cold stimuli, which was not altered by deletion of either the CB₁ or CB₂ receptor, but was attenuated by gabapentin, as well as by each enzyme inhibitor. Whereas PF-3845 lacked anti-allodynic efficacy in both knockout lines, JZL184 did not produce anti-allodynic effects in CB₁ (-/-) mice, but retained its anti-allodynic effects in CB₂ (-/-) mice. These data indicate that FAAH and MAGL inhibitors reduce nerve injury-related hyperalgesic states through distinct cannabinoid receptor mechanisms of action. In conclusion, although endogenous cannabinoids do not appear to play a tonic role in long-term expression of neuropathic pain states, both FAAH and MAGL represent potential therapeutic targets for the development of pharmacological agents to treat chronic pain resulting from nerve injury.

PERSPECTIVE

This article presents data addressing the cannabinoid receptor mechanisms underlying the anti-allodynic actions of endocannabinoid catabolic enzyme inhibitors in the mouse sciatic nerve ligation model. Fatty acid amide hydrolase and monoacylglycerol lipase inhibitors reduced allodynia through distinct cannabinoid receptor mechanisms. These enzymes offer potential targets to treat neuropathic pain.

Quantification of brain endocannabinoid levels: methods, interpretations and pitfalls

Endocannabinoids play an important role in a diverse range of neurophysiological processes including neural development, neuroimmune function, synaptic plasticity, pain, reward and affective state. This breadth of influence and evidence for altered endocannabinoid signalling in a variety of neuropathologies has fuelled interest in the accurate quantification of these lipids in brain tissue. Established methods for endocannabinoid quantification primarily employ solvent-based lipid extraction with further sample purification by solid phase extraction. In recent years in vivo microdialysis methods have also been developed for endocannabinoid sampling from the brain interstitial space. However, considerable variability in estimates of endocannabinoid content has led to debate regarding the physiological range of concentrations present in various brain regions. This paper provides a critical review of factors that influence the quantification of brain endocannabinoid content as determined by lipid extraction from bulk tissue and by in vivo microdialysis. A variety of methodological issues are discussed including analytical approaches, endocannabinoid extraction and purification, post-mortem changes in brain endocannabinoid content, cellular reactions to microdialysis probe implantation and caveats related to lipid sampling from the extracellular space. The application of these methods for estimating brain endocannabinoid content and the effects of endocannabinoid clearance inhibition are discussed. The benefits, limitations and pitfalls associated with each approach are emphasized, with an eye toward the appropriate interpretation of data gathered by each method.

Subtype-specific changes in 5-HT receptor-mediated modulation of C fibre-evoked spinal field potentials are triggered by peripheral nerve injury

Neurotransmitter serotonin (5-HT) released from descending pain modulation pathways to the dorsal horn is crucial to spinal nociception processing. This study sought to gain insight into the modulatory roles of specific serotonin receptor subtypes in experimentally induced neuropathic pain. In rats subjected to spinal nerve ligation (SNL) surgery, we recorded field potentials evoked in the spinal dorsal horn by C fibre-input, during spinal superfusion with subtype-selective drugs. In neuropathic rats, subtype 5-HT1A agonist 8-OH-DPAT (100 nM) was found to potently depress evoked field potentials, as opposed to 5-HT2A or 5-HT2B subtype agonists TCB-2 (100 nM) or BW 723C86 (1 microM), respectively, which consistently enhanced evoked potentials. All three failed to alter spinal field potentials in sham operated rats. CP 94253 (1 microM), WAY 161503 (1 mM) or SR 57227 (at 1 microM in SNL rats, and 100 microM in sham rats), selective agonists for 5-HT1B, 5-HT2C and 5-HT3 receptors, respectively, significantly depressed evoked field potentials in both animal groups. The 5-HT4 agonist RS 67333 (1 microM) was depressant only in sham operated animals. Only after SNL, spinal superfusion with 5-HT1A- or 5-HT1B receptor-antagonists (S)-WAY 100135 (100 microM) or SB 224289 (100 microM), respectively, disinhibited C fibre-evoked potentials, whereas 5-HT2A or 5-HT2B receptor-antagonists 4F 4PP (100 microM) or SB 204741 (100 microM) depressed evoked potentials, suggesting tonic activity of all four subtypes as a consequence of experimental nerve injury. The present findings reveal profound subtype-specific changes in the functional modulatory activities of spinal serotonin receptors following peripheral nerve injury. In particular, spinal hyperexcitation promoted by receptors 5-HT2A and 5-HT2B is suggested as a novel pathogenic pathway contributing to neuropathic pain.

The antinociceptive effect of acetylsalicylic acid is differently affected by a CB1 agonist or antagonist and involves the serotonergic system in rats

AIMS

Combinations of non-steroidal anti-inflammatory drugs (NSAIDs) and cannabinoids are promising because of their potential synergistic effects in analgesia, resulting in a reduction in dosage and minimizing adverse reactions. The analgesic effect of acetylsalicylic acid (ASA), probably due to a central mechanism, also implicates changes in the central monoaminergic system. Therefore, we decided to evaluate the antinociceptive interaction between the CB(1) receptor agonist, HU210, and ASA in tests involving central pain in rats as well as the implication of the central serotonergic system thereon.

MAIN METHODS

The selective CB(1) antagonist SR141716A and the potent cannabinoid agonist HU210 were evaluated alone and in combination with ASA in both algesimetric tests (hot-plate and formalin tests) and for 5-HT activity and 5-HT(2) receptor density and affinity.

KEY FINDINGS

ASA or HU210 alone showed a dose-dependent effect in both tests. HU210, at an inactive dose, significantly increased the antinociceptive effect of the sub-active dose of ASA. SR141716A (1.5mg/kgi.p.) was ineffective per se and failed to modify antinociception induced by the HU210 plus ASA combination in either test. HU210 plus ASA significantly decreased the 5-HIAA/5-HT ratio and the 5-HT(2) receptor density in the frontal cortex, changes not antagonized by SR141716A.

SIGNIFICANCE

The present study provides evidence that mutual potentiation of the antinociceptive effects of HU210 and ASA may, at least partly, depend on serotonergic mechanisms, with an indirect participation of cannabinodiergic mechanism. In conclusion, combinations of low doses of cannabinoids and NSAIDs may be of interest from the therapeutic point of view.

Chronic exposure to cannabinoids during adolescence but not during adulthood impairs emotional behaviour and monoaminergic neurotransmission

The pathophysiological neural mechanism underlying the depressogenic and anxiogenic effects of chronic adolescent cannabinoid use may be linked to perturbations in monoaminergic neurotransmission. We tested this hypothesis by administering the CB(1) receptor agonist WIN55,212-2, once daily for 20 days to adolescent and adult rats, subsequently subjecting them to tests for emotional reactivity paralleled by the in vivo extracellular recordings of serotonergic and noradrenergic neurons. Chronic adolescent exposure but not adult exposure to low (0.2 mg/kg) and high (1.0 mg/kg) doses led to depression-like behaviour in the forced swim and sucrose preference test, while the high dose also induced anxiety-like consequences in the novelty-suppressed feeding test. Electrophysiological recordings revealed both doses to have attenuated serotonergic activity, while the high dose also led to a hyperactivity of noradrenergic neurons only after adolescent exposure. These suggest that long-term exposure to cannabinoids during adolescence induces anxiety-like and depression-like behaviours in adulthood and that this may be instigated by serotonergic hypoactivity and noradrenergic hyperactivity.

The endocannabinoid system and pain

The therapeutic potential of cannabinoids has been the topic of extensive investigation following the discovery of cannabinoid receptors and their endogenous ligands. Cannabinoid receptors and their endogenous ligands are present at supraspinal, spinal and peripheral levels. Cannabinoids suppress behavioral responses to noxious stimulation and suppress nociceptive processing through activation of cannabinoid CB(1) and CB(2) receptor subtypes. Endocannabinoids, the brain's own cannabis-like substances, share the same molecular target as Delta(9)-tetrahydrocannabinol, the main psychoactive component in cannabis. Endocannabinoids serve as synaptic circuit breakers and regulate multiple physiological and pathological conditions, e.g. regulation of food intake, immunomodulation, inflammation, analgesia, cancer, addictive behavior, epilepsy and others. This review will focus on uncovering the roles of anandamide and 2-arachidonoylglycerol, the two best characterized endocannabinoids identified to date, in controlling nociceptive responding. The roles of anandamide and 2-arachidonoylglycerol, released under physiological conditions, in modulating nociceptive responding at different levels of the neuraxis will be emphasized in this review. Effects of modulation of endocannabinoid levels through inhibition of endocannabinoid hydrolysis and uptake is also compared with effects of exogenous administration of synthetic endocannabinoids in acute, inflammatory and neuropathic pain models. Finally, the therapeutic potential of the endocannabinoid signaling system is discussed in the context of identifying novel pharmacotherapies for the treatment of pain.

Effect of the CB(1) receptor antagonists rimonabant and AM251 on the firing rate of dorsal raphe nucleus neurons in rat brain slices

BACKGROUND AND PURPOSE

Previous studies have suggested a regulation of 5-hydroxytryptamine (5-HT) neurons by the endocannabinoid system. The aim of our work was to examine the effect of two CB(1) receptor antagonists, SR141716A (rimonabant, Sanofi-Synthélabo Recherche, Montpellier, France) and N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251, Tocris Cookson, Bristol, UK), on the firing rate of dorsal raphe nucleus (DRN) neurons.

EXPERIMENTAL APPROACH

Single-unit extracellular recordings were performed to study the effect of CB(1) receptor antagonists in slices of the DRN from rat brain.

KEY RESULTS

Rimonabant (1 microM) and AM251 (1 microM) decreased the firing rate of about 50% of all the recorded DRN 5-HT cells. The GABA(A)receptor antagonist picrotoxin (20 microM) (Sigma) prevented and also reversed the inhibitory effect of rimonabant (1 microM) and AM251 (1 microM), suggesting that CB(1) receptors regulate 5-HT neurons through the GABAergic system. However, the CB(1)/CB(2) receptor agonist R-(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)-methyl]pyrrolol[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl) methanone mesylate salt (10 microM) (WIN55212-2, Sigma, St. Louis, MO, USA) failed to change the firing activity of non-5-HT (presumably GABAergic) neurons in the DRN. The endocannabinoid N-(2-hydroxyethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (anandamide, Tocris Cookson) (10 microM) also inhibited the firing activity of a number of 5-HT neurons, but this inhibition was not blocked by rimonabant (1 microM) or AM251 (1 microM), and the stable analogue R-(+) N-(2-hydroxy-1methylethyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (methanandamide, Tocris Cookson) (10 microM) did not mimic this effect. The selective CB(1) receptor agonist arachidonoyl-2-chloroethylamide (ACEA) (1 microM) only slightly increased the firing rate of DRN 5-HT cells.

CONCLUSIONS AND IMPLICATIONS

These results suggest a tonic/constitutive regulation of DRN 5-HT neurons by the endocannabinoid system, which may occur through a CB(1) receptor-mediated inhibition of the GABAergic system. The inhibitory effect of anandamide may be mediated through a CB(1) receptor-independent mechanism.

Monoaminergic neurotransmission contributes to cannabinoid-induced activation of the hypothalamic-pituitary-adrenal axis

Administration of high doses of cannabinoid CB(1) receptor agonists activates the hypothalamic-pituitary-adrenal (HPA) axis; however, the mechanism by which this occurs has not been well characterized. Both monoaminergic and glutamatergic neurotransmission are known to activate the HPA axis and cannabinoids have been found to modify levels of these neurotransmitters. Employing pharmacological antagonists to specific serotonergic, noradrenergic and glutamatergic receptor subtypes, we examined whether activation of these receptors is involved in the ability of a high dose of a cannabinoid CB(1) receptor agonist to activate the HPA axis. We characterized a robust induction of corticosterone secretion following administration of a 100 microg/kg dose of HU-210, a potent cannabinoid CB(1) receptor agonist. Pre-treatment with antagonists to the serotonergic type 1A (5-HT(1A); WAY100635; 0.5mg/kg) and 5-HT(2A/2C) (ketanserin; 1mg/kg) receptors significantly attenuated the HU-210-induced increase in corticosterone secretion. Similarly, the increase in corticosterone secretion following HU-210 administration was significantly reduced by pre-treatment with antagonists to the alpha(1)-adrenoceptor (prazosin; 1mg/kg) and beta-adrenoceptor (propanolol; 2.5mg/kg). However, pre-treatment with antagonists to the NMDA (MK-801; 0.1mg/kg) and AMPA/Kainate (DNQX; 10mg/kg) receptors did not modify activation of adrenocortical secretion evoked by HU-210. These data suggest that acute administration of exogenous cannabinoid ligands activates the HPA axis indirectly through an increase in serotonergic and noradrenergic neurotransmission.

Cannabinoids as pharmacotherapies for neuropathic pain: from the bench to the bedside

Neuropathic pain is a debilitating form of chronic pain resulting from nerve injury, disease states, or toxic insults. Neuropathic pain is often refractory to conventional pharmacotherapies, necessitating validation of novel analgesics. Cannabinoids, drugs that share the same target as Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the psychoactive ingredient in cannabis, have the potential to address this unmet need. Here, we review studies evaluating cannabinoids for neuropathic pain management in the clinical and preclinical literature. Neuropathic pain associated with nerve injury, diabetes, chemotherapeutic treatment, human immunodeficiency virus, multiple sclerosis, and herpes zoster infection is considered. In animals, cannabinoids attenuate neuropathic nociception produced by traumatic nerve injury, disease, and toxic insults. Effects of mixed cannabinoid CB(1)/CB(2) agonists, CB(2) selective agonists, and modulators of the endocannabinoid system (i.e., inhibitors of transport or degradation) are compared. Effects of genetic disruption of cannabinoid receptors or enzymes controlling endocannabinoid degradation on neuropathic nociception are described. Specific forms of allodynia and hyperalgesia modulated by cannabinoids are also considered. In humans, effects of smoked marijuana, synthetic Delta(9)-THC analogs (e.g., Marinol, Cesamet) and medicinal cannabis preparations containing both Delta(9)-THC and cannabidiol (e.g., Sativex, Cannador) in neuropathic pain states are reviewed. Clinical studies largely affirm that neuropathic pain patients derive benefits from cannabinoid treatment. Subjective (i.e., rating scales) and objective (i.e., stimulus-evoked) measures of pain and quality of life are considered. Finally, limitations of cannabinoid pharmacotherapies are discussed together with directions for future research.

Endocannabinoids suppress excitatory synaptic transmission to dorsal raphe serotonin neurons through the activation of presynaptic CB1 receptors

Endocannabinoid signaling in the dorsal raphe (DR) has recently been implicated in the regulation of anxiety and depression. However, the cellular mechanisms by which endocannabinoids (eCBs) regulate the excitability of DR 5-hydroxytryptamine (serotonin; 5-HT) neurons remain poorly understood. In the present study, using whole-cell recording from DR 5-HT neurons, we examined the effects of eCBs on glutamatergic synapses in the DR. We found that the eCB anandamide decreased the amplitude of evoked excitatory postsynaptic currents (eEPSCs). This effect was blocked by CB(1) receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM 251) and mimicked by (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN 55,212-2), a CB(1) receptor agonist. The inhibition of eEPSC amplitude was associated with an increase in the paired-pulse ratio and coefficient of variance. Activation of CB(1) receptors also reduced the frequency, but not the amplitude, of miniature excitatory postsynaptic currents, indicating that eCBs inhibit glutamate release in the DR. In addition, we found that depolarization of DR 5-HT neurons induced a transient inhibition of the amplitude of eEPSCs, termed depolarization-induced suppression of excitation (DSE). The induction of DSE required an increase in postsynaptic intracellular calcium and was due to a decrease in glutamate release. Furthermore, pharmacological studies showed that blockade of CB(1) receptors with AM 251 abolished the DSE. In contrast, activation of CB(1) receptors with WIN 55,212-2 mimicked and occluded the DSE, indicating that depolarization of DR 5-HT neurons triggers eCB release, which in turn mediates the DSE. Together, these results indicate that eCBs play a role in modulating glutamatergic synaptic transmission to DR 5-HT neurons.

Central antinociception induced by mu-opioid receptor agonist morphine, but not delta- or kappa-, is mediated by cannabinoid CB1 receptor

BACKGROUND AND PURPOSE

It has been demonstrated that cannabinoids evoke the release of endogenous opioids to produce antinociception; however, no information exists regarding the participation of cannabinoids in the antinociceptive mechanisms of opioids. The aim of the present study was to determine whether endocannabinoids are involved in central antinociception induced by activation of mu-, delta- and kappa-opioid receptors.

EXPERIMENTAL APPROACH

Nociceptive threshold to thermal stimulation was measured according to the tail-flick test in Swiss mice. Morphine (5 microg), SNC80 (4 microg), bremazocine (4 microg), AM251 (2 and 4 microg), AM630 (2 and 4 microg) and MAFP (0.1 and 0.4 microg) were administered by the intracerebroventricular route.

KEY RESULTS

The CB(1)-selective cannabinoid receptor antagonist AM251 completely reversed the central antinociception induced by morphine in a dose-dependent manner. In contrast, the CB(2)-selective cannabinoid receptor antagonist AM630 did not antagonize this effect. Additionally, the administration of the anandamide amidase inhibitor, MAFP, significantly enhanced the antinociception induced by morphine. In contrast, the antinociceptive effects of delta- and kappa-opioid receptor agonists were not affected by the cannabinoid antagonists. The antagonists alone caused no hyperalgesic or antinociceptive effects.

CONCLUSIONS AND IMPLICATIONS

The results provide evidence for the involvement of cannabinoid CB(1) receptors in the central antinociception induced by activation of mu-opioid receptors by the agonist morphine. The release of endocannabinoids appears not to be involved in central antinociception induced by activation of kappa- and delta-opioid receptors.

Role of endocannabinoids in 5-HT2 receptor-mediated effects

Endocannabinoids are lipid retrograde messengers that can be released by postsynaptic depolarization and/or activation of certain metabotropic receptors. We review a recent report that activation of metabotropic 5-HT2 receptors by endogenous serotonin induces the release of endocannabinoids in the olivary nucleus and suppresses glutamatergic input through a presynaptic action. This serotonin-endocannabinoid interaction has implications in the pathophysiology of pain and mental illness and raises the possibility that drugs targeting the 5-HT2 receptor may act by modulating endocannabinoid release.

Evidence for a novel functional role of cannabinoid CB(2) receptors in the thalamus of neuropathic rats

Cannabinoid CB₁ receptors have analgesic effects in models of neuropathic pain, but can also produce psychoactive side-effects. A supraspinal location of CB₂ receptors has recently been described. CB₂ agonists are also antinociceptive, although the functional role of supraspinal CB₂ receptors in the control of nociception is unknown. Herein, we provide evidence that CB₂ receptors in the thalamus play a functional role in the modulation of responses of neurons in the ventral posterior nucleus (VPL) of the thalamus in neuropathic, but not sham-operated, rats. Spontaneous and mechanically evoked activity of VPL neurons was recorded with a multichannel electrode array in anaesthetized spinal nerve-ligated (SNL) rats and compared to sham-operated rats. Intra-VPL administration of the CB₂ agonist JWH-133 (30 ng in 500 nL) significantly reduced spontaneous (P < 0.05), non-noxious (P < 0.001) and noxious (P < 0.01) mechanically evoked responses of VPL neurons in SNL rats, but not in sham-operated rats. Inhibitory effects of JWH-133 on spontaneous (P < 0.01) and noxious-evoked (P < 0.001) responses of neurons were blocked by the CB₂ antagonist SR144528. Local administration of SR144528 alone did not alter spontaneous or evoked responses of VPL neurons, but increased burst activity of VPL neurons in SNL rats. There were, however, no differences in levels of the endocannabinoids anandamide and 2AG in the thalamus of SNL and sham-operated rats. These data suggest that supraspinal CB₂ receptors in the thalamus may contribute to the modulation of neuropathic pain responses.

Differential involvement of opioidergic and serotonergic systems in the antinociceptive activity of N-arachidonoyl-phenolamine (AM404) in the rat: comparison with paracetamol

It is recognized that paracetamol undergoes a metabolic transformation to N-arachydonylphenolamine (AM404), a CB(1) receptor ligand and anandamide uptake inhibitor. Using hot-plate and paw pressure tests, we decided to establish whether AM404 may act through opioidergic and serotonergic mechanisms. Thus, we pretreated rats with opioid mu(1) (naloxonazine) and kappa (MR2266) or 5-HT(1A) (NAN-190), 5-HT(2) (ketanserin), and 5-HT(3) (ondansetron) receptor antagonists. We investigated the possible changes in 5-hydroxyindoleacetic acid/serotonin ratio in the frontal cortex and pons. The antinociceptive effect of AM404 (10 mg/kg, intraperitoneally) or paracetamol (400 mg/kg, intraperitoneally) in either test was abolished by naloxonazine or MR2266. Ondansetron prevented AM404 activity; NAN-190 and ketanserin were ineffective. Ketanserin antagonized paracetamol activity; NAN-190 and ondansetron were inactive. AM404 did not change serotonergic activity, while paracetamol decreased serotonin turnover. The diverse antinociceptive potency of the compounds might be explained by the different influence on the serotonergic system, despite a similar involvement of opioidergic one.

Actions of the endocannabinoid transport inhibitor AM404 in neuropathic and inflammatory pain models

1. Although cannabinoid receptor agonists have analgesic activity in chronic pain states, they produce a spectrum of central cannabinoid CB(1) receptor-mediated motor and psychotropic side-effects. The actions of endocannabinoids, such as anandamide, are terminated by uptake and subsequent intracellular enzymatic degradation. In the present study, we examined the effect of acute administration of the anandamide transport inhibitor AM404 in rat models of chronic neuropathic and inflammatory pain. 2. Systemic administration of AM404 (10 mg/kg) reduced mechanical allodynia in the partial sciatic nerve ligation (PNL) model of neuropathic pain, but not in the complete Freund's adjuvant (CFA) model of inflammatory pain. 3. The effect of AM404 in the PNL model was abolished by coapplication with the selective cannabinoid CB(1) receptor antagonist AM251 (1 mg/kg). AM404 did not produce a reduction in motor performance in either the PNL or CFA models. 4. These findings suggest that acute administration of AM404 reduces allodynia in a neuropathic pain model via cannabinoid CB(1) receptor activation, without causing the undesirable motor disruption associated with cannabinoid receptor agonists.