Anandamide regulates neuropeptide release from capsaicin-sensitive primary sensory neurons by activating both the cannabinoid 1 receptor and the vanilloid receptor 1 in vitro

Chronic exposure of sensory neurones to increased levels of nerve growth factor modulates CB1/TRPV1 receptor crosstalk

BACKGROUND

Anandamide (AEA) activates both cannabinoid CB(1) and TRPV1 receptors, which are expressed on cultured dorsal root ganglion neurones. Increased levels of nerve growth factor (NGF) are associated with chronic pain states.

EXPERIMENTAL APPROACH

The aim of this study was to compare of the effects of AEA on CB(1) receptor signalling and TRPV1-CB(1) crosstalk in low and high concentrations of NGF, using voltage-clamp electrophysiology and Fura-2 calcium imaging.

KEY RESULTS

Chronic exposure to high NGF (200 ng ml(-1)) as compared to low NGF (20 ng ml(-1)) increases the proportion of neurones that exhibit an inward current in response to AEA (1 microM), from 7 to 29%. In contrast, inhibition of voltage-gated calcium currents by AEA is not significantly different in low NGF (33+/-9%, compared to high NGF 28+/-6%). Crosstalk between CB and TRPV1 receptors is modulated by exposure to high NGF. In low NGF, exposure to the CB(1) receptor antagonist, SR141716A, (100 nM) increases the percentage of neurones in which AEA elicits an increase in [Ca(2+)](i), from 10 to 23%. In high NGF, the antagonist does not alter the percentage of responders (33 to 30%). In low NGF, exposure to the CB receptor agonist, WIN55 (1 microM) reduces capsaicin-mediated increases in [Ca(2+)](i) to 28+/-8% of control as compared to an enhancement to 172+/-26% of control observed in high NGF.

CONCLUSIONS AND IMPLICATIONS

We conclude that cannabinoid-mediated modulation of TRPV1 receptor activation is altered after exposure to high NGF.

Vanilloid receptor 1 (TRPV1/VR1) co-localizes with fatty acid amide hydrolase (FAAH) in retinal amacrine cells

Fatty acid amide hydrolase (FAAH) is the degradative enzyme for anandamide (AEA), an endogenous ligand for the vanilloid receptor (TRPV1) and cannabinoid receptor 1. As FAAH and TRPV1 are integral membrane proteins, FAAH activity could modulate the availability of AEA for TRPV1 activation. Previous studies in this laboratory reported an extensive endocannabinoid system in goldfish retina. Immunocytochemistry was used to determine the relative distributions of FAAH-immunoreactivity (IR) and TRPV1-IR in goldfish retina. Here, we show the first example in an intact neural system in which TRPV1-IR co-localizes in subpopulations of FAAH-immunoreactive neurons, in this case amacrine cells. These cells are rare and consist of three subtypes: 1. ovoid cell with granular-type dendrites restricted to sublamina a, 2. pyriform cell with smooth processes in sublamina b, and 3. fusiform cell with smooth processes that project to sublaminae a and b. The varied appearances of reaction product in the dendrites suggest different subcellular localization of TRPV1, and hence function of FAAH activity regarding TRPV1 stimulation among the cell types. Ovoid and pyriform amacrine cells, but not fusiform cells, labeled with GAD-IR and constituted subsets of GABAergic amacrine cells. TRPV1 amacrine cells, though rare, are represented in the ON, OFF and ON/OFF pathways of the retina. As TRPV1 stimulation increases intracellular calcium with numerous downstream effects, co-localization of TRPV1 and FAAH suggests an autoregulatory function for anandamide. Due to the rarity of these cells, the three vanilloid amacrine cell types may be involved in global effects rather than feature extraction, for example: sampling of ambient light or maintaining homeostasis.

Feedback mechanisms in the regulation of intracellular calcium ([Ca2+]i) in the peripheral nociceptive system: role of TRPV-1 and pain related receptors

Multimodal stimuli like heat, cold, bacterial or mechanical events are able to elicit pain, which is necessary to guarantee survival. However, the control of pain is of major clinical importance. The perception and transduction of pain is differentially modulated in the peripheral and central nervous system (CNS): while peripheral structures modulate these signals, the perception of pain occurs in the CNS. In recent years major advances have been made in the understanding of the processes which are involved in pain sensation. For the peripheral pain reception, the importance of specific pain receptors of the transition receptor pore (TRP)-family (e.g. the TRPV-1 receptor) has been analyzed. These receptors/channels are localized at the cell membrane of nociceptive neurones as well as in membranes of intracellular calcium stores like the endoplasmic reticulum. While the associated channel conducts different ions, a major proportion is calcium. Therefore, this review focuses on (1) the modulations of intracellular calcium ([Ca2+]i) initiated by the activation of pain receptors and (2) the consequences of [Ca2+]i changes for the processing of pain signals at the peripheral side. The possible interference of TRPV-1 induced [Ca2+]i modulations to the function of other membrane receptors and channels, like voltage gated calcium, sodium or potassium channels, or co-expressed CB1-receptors will be discussed. The latter interactions are of specific interest since the analgetic properties of endo- and exo-cannabinoids are mediated by CB1 receptors and their activation significantly modulates the calcium induced release of pain related transmitters. Furthermore, multiple cross links between different pain modulating intracellular pathways and their dependence on [Ca2+]i modulations will be illuminated. Overall, this review will summarize new insights resulting in the understanding of the prominent influence of [Ca2+]i for processes which are involved in pain sensation.

The pharmacology of the cannabinoid system--a question of efficacy and selectivity

Our knowledge of the function of the cannabinoid system in the body has been aided by the availability of pharmacological agents that affect its function. This has been achieved by the design of agents that either directly interact with the receptor (agonists and antagonist/inverse agonists) and agents that indirectly modulate the receptor output by changing the levels of the endogenous cannabinoids (endocannabinoids). In this review, examples of the most commonly used receptor agonists, antagonists/inverse agonists, and indirectly acting agents (anandamide uptake inhibitors, fatty acid amide hydrolase inhibitors, monoacylglycerol lipase inhibitors) are given, with particular focus upon their selectivity and, in the case of the directly acting compounds, efficacy. Finally, the links between the endocannabinoid and cyclooxygenase pathways are explored, in particular, with respect to agents whose primary function is to inhibit cyclooxygenase activity, but which also interact with the endocannabinoid system.

Sensing the air around us: the voltage-gated-like ion channel family

Ion channels are a complex set of proteins having many important physiologic and potentially pathologic roles. The flow of ions through these channels and the subsequent cellular depolarization can trigger complex mechanisms such as cardiac rhythm, hormone secretion, and numerous sensory experiences. The transient receptor potential (TRP) channels are an important means for multiple organ systems to interact with their environment. The various TRP channel subfamilies respond to voltage or to ligands such as G-protein coupled receptors. Their ability to sense temperature, pain, stretch, and osmolarity among others enables them to mediate responses such as smooth muscle contraction, cough, or sensation of pain.

Continuous infusion of the cannabinoid WIN 55,212-2 to the site of a peripheral nerve injury reduces mechanical and cold hypersensitivity

BACKGROUND AND PURPOSE

Cannabinoids have analgesic and anti-inflammatory properties but their use is limited by psychotropic activity at CNS receptors. Restricting cannabinoid delivery to peripheral tissues at systemically inactive doses offers a potential solution to this problem.

EXPERIMENTAL APPROACH

WIN 55,212-2 was continuously delivered to the site of a partial ligation injury to the sciatic nerve via a perineural catheter connected to a mini-osmotic pump implanted at the time of injury. Bilateral reflex limb withdrawal behaviour was measured in adult male Wistar rats in response to mechanical and cooling stimulation of the hind paw.

KEY RESULTS

Compared with vehicle treatment, WIN 55,212-2 (1.4 microg microl(-1) hr(-1)) reduced hypersensitivity to stimuli applied to the injured limb at 2, 4 and 6 days after injury. The effects of WIN 55,212-2 (0.6-2.8 microg microl(-1) hr(-1)) were dose-dependent. Estimated EC(50) values for reduction in mean responses to mechanical and cooling stimulation (day 4 post-surgery) were 1.55 (95% C.I, [1.11-2.16]) microg microl(-1) hr(-1) and 1.52 (95% C.I, [1.07-2.18]) microg microl(-1) hr(-1), respectively. When delivered to the contralateral side to injury, WIN 55,212-2 (1.4 or 2.8 microg microl(-1) hr(-1)) did not significantly affect nerve injury-associated hypersensitivity. Co-perineural application of a CB(1) receptor antagonist SR141716a and WIN 55,212-2 prevented the effects of WIN 55,212-2 on hypersensitivity. Co-application of CB(2) receptor antagonist SR144528 reversed WIN 55,212-2's effect on mechanical hypersensitivity on day 2 only.

CONCLUSIONS AND IMPLICATIONS

These data support a peripheral antihyperalgesic effect of WIN 55,212-2 when delivered directly to the site of a nerve injury at systemically inactive doses.

Involvement of cannabinoid (CB1)-receptors in the development and maintenance of opioid tolerance

Sustained exposure to opioid agonists such as morphine increases levels of calcitonin gene-related peptide (CGRP) in the spinal dorsal horn, a response implicated in the development of opioid tolerance and physical dependence. Recent evidence suggests that both the opioid-induced increase in CGRP and the development of opioid physical dependence are suppressed by blockade of spinal cannabinoid (CB1)-receptors. The present study examined whether CB1-receptor activity also has a role in the development of opioid tolerance. In rats implanted with spinal catheters, repeated acute injections of morphine (15 microg) delivered over 4 h resulted in a rapid decline of thermal and mechanical antinociception and a significant loss of analgesic potency, reflecting development of acute opioid tolerance. In another set of experiments, chronic administration of spinal morphine (15 microg) once daily for 5 days produced a similar loss of analgesic effect and a marked increase in CGRP-immunoreactivity in the superficial laminae of the dorsal horn. Consistent with the in vivo findings, primary cultures of adult dorsal root ganglion (DRG) neurons exposed to morphine for 5 days showed a significant increase in the number of CGRP-immunoreactive neurons. Co-administration of acute or chronic morphine with a CB1-receptor antagonist/inverse agonist, 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide (AM-251), inhibited the development of both acute and chronic analgesic tolerance. In animals already exhibiting tolerance to morphine, intervention with AM-251 restored morphine analgesic potency. Co-administration with AM-251 attenuated the morphine-induced increase in CGRP-immunoreactivity in the spinal cord and in DRG cultured neurons. Collectively, the results of this study suggest that activity of endocannabinoids, mediated via CB1-receptors, contributes to both the development and maintenance of opioid tolerance by influencing the opioid-induced increase in spinal CGRP.

Characterization of the vasorelaxation to methanandamide in rat gastric arteries

In the present study, the relaxant effect of the cannabinoid methanandamide was explored in rat gastric arteries. Since in some vessels cannabinoids have been shown to release calcitonin gene-related peptide (CGRP) from perivascular nerves, the influence of methanandamide was compared with that of exogenous CGRP. Methanandamide and CGRP elicited concentration-dependent, endothelium-independent relaxations. Methanandamide-induced relaxations were unaffected by the CB1 receptor antagonist AM251, the CB2 receptor antagonists AM630 and SR144528, and combined pre-exposure to AM251 and SR144528. Pre-exposure to O-1918, an antagonist of a novel nonCB1/nonCB2 cannabinoid receptor, did not influence the relaxations to methanandamide. Capsaicin or capsazepine treatment slightly inhibited methanandamide-induced relaxations. Preincubation with 30 mmol/L extracellular K+ or 3 mmol/L TEA had no significant effect on the responses elicited by methanandamide, but reduced CGRP-induced relaxations. Relaxation to 10(-5) mol/L methanandamide was significantly blunted by Bay K8644 and by preincubation with nifedipine. Furthermore, 10(-5) mol/L methanandamide significantly inhibited CaCl2-induced contractions in norepinephrine-stimulated vessels previously depleted of intra- and extracellular Ca2+. Finally, preincubation with 10(-5) mol/L methanandamide almost completely abolished high K+-induced contractions. These findings suggest that the vasorelaxant action of methanandamide in rat gastric arteries is not mediated by stimulation of known cannabinoid receptors and only partly related to stimulation of TRPV1 receptors on perivascular nerves. At high concentrations, methanandamide might induce relaxation by reducing calcium entry into the smooth muscle cells.

The role of Transient Receptor Potential Vanilloid 1 (TRPV1) channels in pancreatitis

Premature activation of digestive enzymes within the pancreas which leads to autodigestion of the gland is an early step in the pathogenesis of pancreatitis. Pancreatic injury is followed by other manifestations of inflammation including plasma extravasation, edema, and neutrophil infiltration which constitute the features of pancreatitis. Recent studies indicate that neural innervation of the pancreas may play an important role in the initiation and maintenance of the inflammatory response to injury. The pancreas is innervated by vagal, sympathetic and parasympathetic neurons, as well as sensory neurons. Activation of pancreatic primary sensory neurons causes the release of inflammatory neuropeptides both in the spinal cord to signal pain and in the pancreas itself where they produce plasma extravasation and neutrophil infiltration. Recent studies indicate that primary sensory neurons of the pancreas express transient receptor potential V1 (TRPV1) channels whose activation induces pancreatic inflammation. Moreover, blockade of these TRP channels significantly ameliorates experimental pancreatitis. This review describes our current understanding of the role of TRPV1 channels in pancreatitis and illustrates how this mechanism might be used to direct future treatments of pancreatic diseases.

Biochemistry and pharmacology of endovanilloids

Endovanilloids are defined as endogenous ligands and activators of transient receptor potential (TRP) vanilloid type 1 (TRPV1) channels. The first endovanilloid to be identified was anandamide (AEA), previously discovered as an endogenous agonist of cannabinoid receptors. In fact, there are several similarities, in terms of opposing actions on the same intracellular signals, role in the same pathological conditions, and shared ligands and tissue distribution, between TRPV1 and cannabinoid CB(1) receptors. After AEA and some of its congeners (the unsaturated long chain N-acylethanolamines), at least 2 other families of endogenous lipids have been suggested to act as endovanilloids: (i) unsaturated long chain N-acyldopamines and (ii) some lipoxygenase (LOX) metabolites of arachidonic acid (AA). Here we discuss the mechanisms for the regulation of the levels of the proposed endovanilloids, as well as their TRPV1-mediated pharmacological actions in vitro and in vivo. Furthermore, we outline the possible pathological conditions in which endovanilloids, acting at sometimes aberrantly expressed TRPV1 receptors, might play a role.

Cannabinoids and pain

Convincing evidence from preclinical studies demonstrates that cannabinoids can reduce pain responses in a range of inflammatory and neuropathic pain models. The anatomical and functional data reveal cannabinoid receptor-mediated analgesic actions operating at sites concerned with the transmission and processing of nociceptive signals in brain, spinal cord and the periphery. The precise signalling mechanisms by which cannabinoids produce analgesic effects at these sites remain unclear; however, significant clues point to cannabinoid modulation of the functions of neurone and immune cells that mediate nociceptive and inflammatory responses. Intracellular signalling mechanisms engaged by cannabinoid receptors-like the inhibition of calcium transients and adenylate cyclase, and pre-synaptic modulation of transmitter release-have been demonstrated in some of these cell types and are predicted to play a role in the analgesic effects of cannabinoids. In contrast, the clinical effectiveness of cannabinoids as analgesics is less clear. Progress in this area requires the development of cannabinoids with a more favourable therapeutic index than those currently available for human use, and the testing of their efficacy and side-effects in high-quality clinical trials.

Palmitoylethanolamide, endocannabinoids and related cannabimimetic compounds in protection against tissue inflammation and pain: Potential use in companion animals

Endocannabinoids have analgesic/anti-inflammatory properties. The biology of endocannabinoids, their receptors, signalling mechanisms and role in the regulation of physiological processes have been extensively reviewed. This review focuses on the role of palmitoylethanolamide (PEA), an endogenous fatty acid amide analogue of the endocannabinoid anandamide, in tissue protective mechanisms. PEA was first identified almost five decades ago in lipid extracts of various natural products, and its anti-inflammatory and antinociceptive effects were established later. Evidence exists that PEA is synthesised during inflammation and tissue damage and a number of beneficial effects, including the relief of inflammation and pruritus, have been shown to be useful in the control of neurogenic and neuropathic pain. The postulated hypotheses as to the mode of action of PEA include a possible local autacoid-like mediator activity regulating mast-cell activity and putative activation of cannabinoids and vanilloid TRPV1 receptors via "entourage" effects. The large number of scientific investigations into the effects of PEA and PEA-related compounds has given rise to new therapeutic opportunities. In spite of the multitude of therapies currently employed to control inflammation, pain, pruritus and tissue damage, the possibility of using a natural compound, such as PEA to manipulate endogenous protective mechanisms may be considered a beneficial novel therapeutic strategy in veterinary medicine.

Effects of inhibition of fatty acid amide hydrolase vs. the anandamide membrane transporter on TRPV1-mediated calcium responses in adult DRG neurons; the role of CB1receptors

The aim of the present study was to investigate the relationship between TRPV1 stimulation and endocannabinoid-driven CB(1) receptor-mediated inhibition of activity in adult rat dorsal root ganglion (DRG) neurons, a model of primary afferent nociceptors. Calcium-imaging studies were performed to compare the effects of the fatty acid amide hydrolase (FAAH) inhibitor URB597 (1 microm) vs. the anandamide (AEA) uptake inhibitor UCM707 (1 microm) on capsaicin (100 nm) and N-arachidonoyl dopamine (NADA; 1 microm)-evoked changes in intracellular calcium [Ca(2+)](i) in DRG neurons. The ability of the CB(1) receptor antagonist AM251 (1 microm) to modulate the effects of URB597 and UCM707 was also determined. Suprafusion of NADA and capsaicin evoked robust increases in [Ca(2+)](i) in DRG neurons (89 +/- 4% and 132 +/- 6% of the depolarizing KCl response, respectively). Co-incubation with URB597 significantly attenuated both NADA and capsaicin-evoked increases in [Ca(2+)](i) (39 +/- 3% and 79 +/- 4% of KCl response, respectively). Similarly, co-incubation with UCM707 significantly attenuated both NADA and capsaicin-evoked increases in [Ca(2+)](i) (59 +/- 7% and 72 +/- 4% of KCl response, respectively). The CB(1) receptor antagonist AM251 significantly attenuated the effects of URB597 on NADA-evoked increases in [Ca(2+)](i) but not the effects of URB597 on capsaicin-evoked increases in [Ca(2+)](i). By contrast, AM251 significantly attenuated the inhibitory effects of UCM707 on both NADA and capsaicin-evoked increases in [Ca(2+)](i.) These data suggest that transport of both NADA and capsaicin into DRG neurons and the subsequent activation of TRPV1 is partly governed by FAAH-dependent mechanisms as well as via the putative AEA membrane transporter.

Spinal modulation of calcitonin gene-related peptide by endocannabinoids in the development of opioid physical dependence

Studies implicate endocannabinoids in the acute and chronic actions of opioid drugs, including the genesis of physical dependence. Previous evidence suggests that spinal release of calcitonin gene-related peptide (CGRP) and activation of its receptors contribute to opioid physical dependence. The release of CGRP at the spinal level is modulated by cannabinoid (CB1)-receptors. Thus, this study examined whether CB1-receptor activity mediates changes in CGRP underlying development of opioid physical dependence. Systemic morphine administration for 5-days elevated CGRP-immunoreactivity in the rat spinal dorsal horn. In situ hybridization of dorsal root ganglion (DRG) neurons revealed an increase in CGRP mRNA during initial (day 1-3) but not later phase (day 4-5) of morphine treatment. CGRP-immunoreactivity in DRG neurons, however, was increased in the later phase of morphine treatment. Naloxone challenge to morphine-treated animals precipitated an intense withdrawal syndrome that depleted CGRP-immunoreactivity and increased Fos expression in the dorsal horn. The Fos-response primarily occurred in neurons that expressed CGRP receptor component protein (RCP) suggesting CGRP activity contributes to neuronal activation during precipitated withdrawal. Spinal slices obtained from morphine-treated animals showed higher levels of CGRP release than from saline controls. Intrathecal co-administration of CB1-receptor antagonists, AM-251 or SR141716A, with daily morphine attenuated the behavioral manifestations of withdrawal. Treatment with AM-251 also reduced the depletion of CGRP, suppressed Fos-induction, and prevented the increase in capsaicin-evoked spinal CGRP release. Altogether, this study suggests that endocannabinoid activity, expressed via CB1-receptors, contributes to the induction of opioid physical dependence through spinal modulation of CGRP.

Methanandamide hyperpolarizes gastric arteries by stimulation of TRPV1 receptors on perivascular CGRP containing nerves

Endogenous as well as synthetic cannabinoids have potent vasodilatory actions in a variety of vascular preparations. Their precise mechanism of action is as yet unclear, but several studies point to the activation of type 1 vanilloid (TRPV1) receptors on primary afferent perivascular nerves, stimulating the release of calcitonin gene related peptide (CGRP). Given the documented gastroprotective function of these nerves, and the various gastrointestinal effects reported for cannabinoids, we explored a possible link between these systems in the gastric circulation by comparing responses of small gastric arteries to cannabinoids and to calcitonin gene related peptide using conventional microelectrode techniques. Exposure of small gastric arteries to the stable endocannabinoid analogue methanandamide caused a hyperpolarization of the vascular smooth muscle cells, which was completely abolished by the vanilloid receptor antagonist capsazepine (P < 0.01). Exposure to exogenous calcitonin gene related peptide evoked fully reproducible (P > 0.05) hyperpolarizations with similar time course, unaffected by capsazepine. Preincubation with glibenclamide, an inhibitor of ATP-sensitive potassium (KATP) channels, reversed both responses to methanandamide (P < 0.01) and calcitonin gene related peptide (P < 0.05). Similar results were found in rat mesenteric arteries. These findings show that cannabinoids stimulate TRPV1 receptors, presumably causing the release of calcitonin gene related peptide, which hyperpolarizes the smooth muscle cells by activation of KATP channels. Because membrane hyperpolarization is a powerful mediator of vasorelaxation, this novel pathway might prove to be an important mechanism affording gastroprotection.

Site-specific increases in peripheral cannabinoid receptors and their endogenous ligands in a model of neuropathic pain

Selective activation of the peripheral cannabinoid receptor 1 (CB1R) has been shown to suppress neuropathic pain symptoms in rodents. However, relatively little is known about changes in CB1R and its endogenous ligands during development or maintenance of neuropathic pain. Using immunohistochemistry, Western blot, real-time reverse transcription polymerase chain reaction, as well as liquid chromatography/mass spectrometry, we studied the changes in CB1Rs and endocannabinoids N-arachidonoylethanolamine/anandamide (AEA) and 2-arachidonoylglycerol (2-AG) in rat lumbar (L4 and L5) dorsal root ganglia (DRG) after neuropathic pain induction (L5 spinal nerve ligation: SNL). Immunohistochemistry revealed that in control rats, CB1R is expressed in the majority (76-83%) of nociceptive neurons as indicated by co-labeling with isolectin B4 (IB4) or antibodies recognizing transient receptor potential vanilloid (TRPV1), calcitonin gene related peptide (CGRP), and the NR2C/2D subunits of the N-methyl-D-aspartate receptor. After L5 SNL, CB1R mRNA and protein increases in the ipsilateral uninjured L4 DRG whereas the percentages of CB1R immunoreactive (CB1R-ir) neurons remain unchanged in L4 and L5 DRG. However, for these CB1R-ir neurons, we observe significant increases in percentage of TRPV1-ir cells in ipsilateral L4 DRG, and decreases in percentage of IB4- and CGRP-co-labeled cells in ipsilateral L5 DRG. Levels of both AEA and 2-AG increase significantly only in the ipsilateral L5 DRG. These results are consistent with the preserved analgesic effects of cannabinoids in neuropathic pain and provide a rational framework for the development of peripherally acting endocannabinoid-based therapeutic interventions for neuropathic pain.

CB2 receptor-mediated antihyperalgesia: possible direct involvement of neural mechanisms

In mouse the cannabinoid receptor 2 (CB2) agonists L768242 and (+)-AM1241, at doses of 30 mg/kg i.p. and 1 and 3 mg/kg i.v., respectively, reduced the second phase of nocifensive behaviors elicited by formalin intraplantar injection. This effect was counteracted by the selective CB2 antagonist SR144528 (1 mg/kg i.p.). In rat (+)-AM1241 (3 and 6 mg/kg i.v.) and L768242 (30 mg/kg i.p.) reduced allodynia elicited by L5-L6 spinal nerve ligation. SR144528 reverted these effects, supporting a CB2-mediated action. To clarify the mechanisms underlying these effects we investigated CB2 gene expression and function in the nervous system. CB2 mRNA was expressed in spinal cord and dorsal root ganglia (DRG) of both sham and neuropathic rats and was up-regulated in the ipsilateral spinal cord of neuropathic rats. Expression studies demonstrated the presence of CB2 mRNA in culture of spinal cord microglia. A biomarker, CGRP, was used to investigate modulation of DRG primary afferents by CB2 agonists. Both L768242 and (+)-AM1241 dose dependently (EC50 of 3.6 and 4.5 nM, respectively) reduced capsaicin-induced calcitonin gene-related peptide (CGRP) release. Coadministration of SR144528 resulted in a rightforward shift (pKB 8.1 and 8.2 for (+)-AM1241 and L768242, respectively) of the dose-response curve. Experiments on capsaicin-induced CGRP release in tissue from CB1-/- mice ruled out a CB1-mediated effect. These results confirm that CB2 is present in the central nervous system and suggest that CB2 agonists may elicit their analgesic effect by acting not only at non-neuronal peripheral sites but also at neural level, making CB2 an attractive target for chronic pain treatment.

Induction of CB1 cannabinoid receptor by inflammation in primary afferent neurons facilitates antihyperalgesic effect of peripheral CB1 agonist

Cannabinoids act on various regions in the nervous system to modulate neuronal activity including nociception. Here, we investigated CB1 receptor expression in primary afferent neurons in the dorsal root ganglion (DRG) and the efficacy of a local (intraplantar) application of the selective CB1 agonist, 2-arachidonyl-2-chloroethylamide (ACEA), on inflammatory thermal hyperalgesia. In situ hybridization showed normal CB1 mRNA expression in 28% of DRG neurons. Peripheral inflammation by CFA (complete Freund's adjuvant) significantly increased the ratio of CB1 mRNA-positive neurons to 43%, primarily with increase in NF200-negative C-fiber nociceptors. Furthermore, CB1 and TRPV1 (transient potential receptor vanilloid subtype-1) co-localization was increased from 41% before inflammation to 67% two days after inflammation. Inflammation also increased CB1 immunoreactivity in DRG neurons and in nerve fibers of the hindpaw dermis, indicating increased CB1 transport from the cell body to the peripheral nerve. The intraplantar application of ACEA attenuated CFA-induced thermal hyperalgesia. The antinociceptive properties of ACEA became more prominent at 2 days after inflammation, compared with those in non-inflamed and inflamed animals at 8 h. These results suggest that CB1 expression in primary afferent neurons is increased by inflammation and that the subsequent increase in CB1 transport to peripheral axons contributes to the increased antihyperalgesic efficacy of locally administered CB1 agonist.

Activity and expression of the vanilloid receptor 1 (TRPV1) is altered by long-term diabetes in epineurial arterioles of the rat sciatic nerve

BACKGROUND

Epineurial arterioles of the sciatic nerve are innervated by sensory nerves containing calcitonin gene-related peptide (CGRP). We postulated that treating these resistance vessels with capsaicin would cause the release of endogenous CGRP and vascular relaxation.

METHODS

Videomicroscopy was used to examine the effect of capsaicin and neuropeptides on vascular reactivity of epineurial arterioles from control and streptozotocin-induced diabetic rats. Using immunohistochemistry, we examined expression of neuropeptide Y (NPY) and vanilloid receptor 1 in epineurial arterioles.

RESULTS

Instead of relaxation, capsaicin was found to cause a concentration-dependent vasoconstriction in epineurial arterioles. The effect of capsaicin was transient, refractory, blocked by capsazepine and duplicated by resiniferatoxin. When examining potential candidates for the mediation of capsaicin-induced constriction, we found that vasopressin (VP), NPY, serotonin (5HT) and endothelin (ET), but not neurokinin A or substance P, caused a concentration-dependent vasoconstriction of epineurial arterioles. Epineurial arterioles express NPY and receptor antagonists to NPY significantly decreased capsaicin-induced vasoconstriction. In long-term diabetic rats, vasoconstriction to capsaicin was significantly attenuated. However, long-term diabetes did not impair vasoconstriction of epineurial arterioles to exogenous VP, NPY, 5HT or ET. Examining the expression of vanilloid receptor 1 in epineurial arterioles from control and long-term diabetic rats, we found that immunoreactivity for vanilloid receptor 1 was decreased by diabetes.

CONCLUSIONS

These studies suggest that long-term diabetes causes vascular dysfunction in epineurial arterioles of the sciatic nerve that includes a decrease in capsaicin-induced vasoconstriction that is likely due to a decrease in the expression of vanilloid receptor 1.

Peripheral and central distribution of TRPV1, substance P and CGRP of rat corneal neurons

The rat corneal neurons expressing vanilloid receptor TRPV1, substance P (SP) and calcitonin-gene-related peptide (CGRP) were examined. In the cornea, some TRPV1-immunoreactive nerve fibers displayed either SP- or CGRP immunoreactivity also. For observing corneal neuronal elements in the trigeminal ganglion (TG) and in the medulla oblongata, retrograde and anterograde cholera toxin subunit B (CTB) tracing methods combining with triple immunofluorescence technique were performed. The corneal neuronal somata were located in the ophthalmic division of the TG; 37% of them were immunoreactive for TRPV1. One third and three quarters of the corneal TRPV1-immunoreactive neurons co-expressed SP and CGRP, respectively. All of SP-immunoreactive corneal neurons exhibited TRPV1 immunoreactivity. They were predominantly medium-sized (mean +/- SE = 638.2 +/- 49.5 microm(2)) and significantly larger than SP-immunoreactive and TRPV1-immunonegative neurons in the ophthalmic division of the TG. The central projection fibers of corneal neurons co-expressing TRPV1 with SP and CGRP were observed at the subnucleus interpolaris/caudalis transition within trigeminal nucleus. The present study suggests that TRPV1 of the corneal neurons works in close relation to SP and CGRP both in the cornea and CNS for healing and nociceptive transduction.

A brief history of cannabinoid and endocannabinoid pharmacology as inspired by the work of British scientists

British scientists have played a leading role in the long history of cannabinoid and endocannabinoid research. Such research has progressed from the first crucial evaluation of the medicinal properties of Cannabis sativa in the Western world to pioneering studies of the chemical constituents of this plant, the development of in vitro biological assays to study cannabinoids, the identification of the mechanism of action of cannabinoids, the discovery of endocannabinoids and the assessment of their therapeutic implications. Stemming from the many innovative ideas and achievements of these researchers, I provide a personal view of where these studies have led us thus far and where they are likely to take us in the future.

Role of CGRP and GABA in the hypotensive effect of intrathecally administered anandamide to anesthetized rats

In urethane-anesthetized rats the intrathecal (i.t.) injection of 100 nmol anandamide produced a hypotensive effect (-19.3+/-1.6 mm Hg; n=6) that was mimicked by i.t. administration of 0.25 nmol calcitonin gene-related peptide (CGRP; -26.2+/-1.8 mm Hg, n=4). Both effects were antagonized either by the CGRP receptor antagonist CGRP(8-37) (5 nmol; i.t.) or by the gamma-aminobutyric acid (GABA)(A) receptor antagonist bicuculline (8.8 nmol, i.t) or by the GABA(B) receptor antagonist 2-hydroxy saclofen (110 nmol; i.t.). On the contrary, blockade of spinal CGRP receptors by CGRP(8-37) did not modify the hypotensive response to either the GABA(A)-receptor agonist muscimol (8.8 nmol; i.t.) or the GABA(B)-receptor agonist baclofen (100 nmol; i.t). This result suggests a unidirectional effect of CGRP on the GABAergic system. The response to anandamide remained unaltered after acute inhibition of nitric oxide (NO) synthase activity by either i.t. (1 micromol) or i.v. (10 mg/kg) injection of N(G)-nitro-L-arginine methyl ester (L-NAME), but increased significantly after long-term L-NAME administration (70 mg/kg/day; four weeks; p.o.), thus suggesting compensatory changes in cardiovascular homeostasis. It is proposed that the hypotensive effect of anandamide in urethane-anesthetized rats could involve the release of CGRP followed by the release of GABA in the spinal cord. NO does not appear to have a direct participation in the spinal mechanisms involved in the decrease of the blood pressure caused by anandamide.

[Cannabinoids--signal transduction and mode of action]

The therapeutic use of cannabinoids, the components of cannabis sativa L., was investigated in numerous researches in detail. Animal studies revealed that cannabinoid receptor agonists alter pain-associated behaviour, have immune-suppressive properties, suppress tumor growth, modulate sensitisation processes and influence memory and learning. Those effects are mediated by two membrane-bound cannabinoid receptors and as mechanisms of signal transduction blockade of ion channels, inhibition of adenylate cyclase and retrograde inhibition of neurotransmitter release are currently being discussed. In clinical studies oral administration of cannabinoids indicated beneficial results during the therapy of multiple sclerosis, weight loss, nausea and vomiting due to chemotherapy, and intractable pruritus. However, therapy of chronic pain conditions revealed conflicting results and unequivocal success could not have been delivered due to unwanted side effects. Further multicentre studies are required to estimate cannabinoids as novel therapeutic tools for the treatment of chronic pain.

Immunohistochemical localization of cannabinoid type 1 and vanilloid transient receptor potential vanilloid type 1 receptors in the mouse brain

Cannabinoid type 1 receptors and transient receptor potential vanilloid type 1 channels have been proposed to act as metabotropic and ionotropic receptors, respectively, for two classes of endogenous polyunsaturated fatty acid amides, the acylethanolamides and the acyldopamides. Furthermore, we and others have shown that functional crosstalk occurs between these two receptors when they are expressed in the same cell. Although demonstrated in sensory neurons of the dorsal root ganglia, spinal cord and myenteric neurons, co-expression of cannabinoid type 1 and transient receptor potential vanilloid type 1 has not yet been studied in the brain. In the present study, we addressed this issue by using commercially available specific antibodies whose specificity was confirmed by data obtained with brains from cannabinoid type 1(-/-) and transient receptor potential vanilloid type 1(-/-) mice. Double cannabinoid type 1/transient receptor potential vanilloid type 1 immunofluorescence and single cannabinoid type 1 or transient receptor potential vanilloid type 1 avidin-biotin complex immunohistochemistry techniques were performed and both methods used point to the same results. Cannabinoid type 1/transient receptor potential vanilloid type 1 expression was observed in the hippocampus, basal ganglia, thalamus, hypothalamus, cerebral peduncle, pontine nuclei, periaqueductal gray matter, cerebellar cortex and dentate cerebellar nucleus. In particular, in the hippocampus, cannabinoid type 1/transient receptor potential vanilloid type 1 expression was detected on cell bodies of many pyramidal neurons throughout the CA1-CA3 subfields and in the molecular layer of dentate gyrus. In the cerebellar cortex, expression of cannabinoid type 1/transient receptor potential vanilloid type 1 receptors was found surrounding soma and axons of the vast majority of Purkinje cell bodies, whose cytoplasm was found unstained for both receptors. Cannabinoid type 1 and transient receptor potential vanilloid type 1 immunoreactivity was also detected in: a) the globus pallidus and substantia nigra, in which some intensely transient receptor potential vanilloid type 1 immunopositive cell bodies were found in dense and fine cannabinoid type 1/transient receptor potential vanilloid type 1 positive and cannabinoid type 1 positive nerve fiber meshworks, respectively; b) the cytoplasm of thalamic and hypothalamic neurons; and c) some neurons of the ventral periaqueductal gray. These data support the hypothesis of a functional relationship between the two receptor types in the CNS.

Effects of cannabinoids on neurotransmission

The CB1 cannabinoid receptor is widely distributed in the central and peripheral nervous system. Within the neuron, the CB1 receptor is often localised in axon terminals, and its activation leads to inhibition of transmitter release. The consequence is inhibition of neurotransmission via a presynaptic mechanism. Inhibition of glutamatergic, GABAergic, glycinergic, cholinergic, noradrenergic and serotonergic neurotransmission has been observed in many regions of the central nervous system. In the peripheral nervous system, CB1 receptor-mediated inhibition of adrenergic, cholinergic and sensory neuroeffector transmission has been frequently observed. It is characteristic for the ubiquitous operation of CB1 receptor-mediated presynaptic inhibition that antagonistic components of functional systems (for example, the excitatory and inhibitory inputs of the same neuron) are simultaneously inhibited by cannabinoids. Inhibition of voltage-dependent calcium channels, activation of potassium channels and direct interference with the synaptic vesicle release mechanism are all implicated in the cannabinoid-evoked inhibition of transmitter release. Many presynaptic CB1 receptors are subject to an endogenous tone, i.e. they are constitutively active and/or are continuously activated by endocannabinoids. Compared with the abundant data on presynaptic inhibition by cannabinoids, there are only a few examples for cannabinoid action on the somadendritic parts of neurons in situ.

Review article: endocannabinoids and their receptors in the enteric nervous system

The therapeutic actions of cannabinoids have been known for centuries. In the last 25 years this area of research has grown exponentially with the discovery of specific cannabinoid receptors and endogenous ligands. In the enteric nervous system of gastrointestinal tract, cannabinoid receptors are located on enteric nerve terminals where they exert inhibitory actions on neurotransmission to reduce motility and secretion. Endogenous cannabinoids are present in the enteric nervous system, as are the degradative enzymes necessary to inhibit their action. The cellular mechanism of action of endocannabinoids has not been established in the enteric nervous system. Endocannabinoids not only act at cannabinoid receptors, but potentially also at vanilloid and 5-HT3 receptors, both of which are expressed in the gastrointestinal tract. The interactions between endocannabinoids and these other important receptor systems have not been extensively investigated. A greater understanding of the endocannabinoid system in the enteric nervous system could lead to advances with important therapeutic potential in the treatment of gastrointestinal disorders such as irritable bowel syndrome, inflammatory bowel disease, secretory diarrhoea and gastro-oesophageal reflux disease.

Inflammatory mediators convert anandamide into a potent activator of the vanilloid type 1 transient receptor potential receptor in nociceptive primary sensory neurons

The endogenous ligand, anandamide activates at least two receptors on nociceptors; the excitatory vanilloid type 1 transient receptor potential receptor, the activity of which is indispensable for the development and maintenance of inflammatory heat hyperalgesia, and the inhibitory cannabinoid 1 receptor, the activity of which reduces that pathological pain sensation. Recent data are equivocal on whether increasing anandamide levels at the peripheral terminals of nociceptors in pathological conditions increases or decreases inflammatory heat hyperalgesia. Here, by using the cobalt-uptake technique we examined whether vanilloid type 1 transient receptor potential receptor activity evoked by 10 nM-100 microM anandamide is increased or decreased in inflammatory conditions. An inflammatory milieu for cultured rat primary sensory neurons was established by incubating the cells in the presence of the inflammatory mediators, bradykinin and prostaglandin E2. Anandamide, similarly to the archetypical vanilloid type 1 transient receptor potential receptor agonist, capsaicin induced concentration-dependent cobalt-uptake in a proportion of neurons. However, the potency of anandamide was significantly lower than that of capsaicin. While pre-incubation of cultures with bradykinin and prostaglandin E2 alone did not evoke cobalt-entry, the inflammatory mediators potentiated the effect of both capsaicin and anandamide. Application of the competitive vanilloid type 1 transient receptor potential receptor antagonist, capsazepine, or inhibitors of protein kinase A, protein kinase C or phospholipase C inhibited the anandamide-evoked cobalt-uptake both in the presence and absence of bradykinin and prostaglandin E2. These findings show that inflammatory mediators significantly increase the excitatory potency and efficacy of anandamide on vanilloid type 1 transient receptor potential receptor, thus, increasing the anandamide concentration in, or around the peripheral terminals of nociceptors might rather evoke than decrease inflammatory heat hyperalgesia.

Capsaicin- and anandamide-induced gastric acid secretion via vanilloid receptor type 1 (TRPV1) in rat brain

The activation of transient receptor potential vanilloid receptor 1 (TRPV1) by capsaicin in rat brain stimulates gastric acid secretion via tachykinin NK2 receptors and the vagus cholinergic nerve, but the involvement of other receptor systems has not been elucidated. We investigated the role of the glutamate and gamma-amino-butyric acid (GABA) receptor systems on the capsaicin response. Gastric acid secretion stimulated by the injection of capsaicin (30 nmol) into the lateral cerebroventricle (i.c.v.) was inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, an antagonist of non-N-methyl-D-aspartate (non-NMDA) receptors, 10.9 nmol, i.c.v.) and bicuculline (a GABA(A) receptor antagonist, 222 microg kg(-1) 10 min(-1), i.v. infusion). Secretion stimulated by the injection of capsaicin (50 nmol) into the fourth cerebroventricle was inhibited by CNQX and bicuculline. I.c.v. injection of anandamide (an endogenous ligand of TRPV1 and cannabinoid receptors, 30 and 100 nmol) stimulated gastric acid secretion, and the response was inhibited by an antagonist of TRPV1 and in the capsaicin-treated rats, but not by an antagonist of cannabinoid receptors. In conclusion, the TRPV1 system, which is activated by capsaicin and anandamide, is preferentially coupled with non-NMDA and GABA(A) receptor systems in the brain and stimulates gastric acid secretion in rats.

The endocannabinoid system in the physiology and pathophysiology of the gastrointestinal tract

Numerous investigations have recently demonstrated the important roles of the endocannabinoid system in the gastrointestinal (GI) tract under physiological and pathophysiological conditions. In the GI tract, cannabinoid type 1 (CB1) receptors are present in neurons of the enteric nervous system and in sensory terminals of vagal and spinal neurons, while cannabinoid type 2 receptors are located in immune cells. Activation of CB1 receptors was shown to modulate several functions in the GI tract, including gastric secretion, gastric emptying and intestinal motility. Under pathophysiological conditions induced experimentally in rodents, the endocannabinoid system conveys protection to the GI tract (e.g. from inflammation and abnormally high gastric and enteric secretions). Such protective activities are largely in agreement with anecdotal reports from folk medicine on the use of Cannabis sativa extracts by subjects suffering from various GI disorders. Thus, the endocannabinoid system may serve as a potentially promising therapeutic target against different GI disorders, including frankly inflammatory bowel diseases (e.g. Crohn's disease), functional bowel diseases (e.g. irritable bowel syndrome) and secretion- and motility-related disorders. As stimulation of this modulatory system by CB1 receptor agonists can lead to unwanted psychotropic side effects, an alternative and promising avenue for therapeutic applications resides in the treatment with CB1 receptor agonists that are unable to cross the blood-brain barrier, or with compounds that inhibit the degradation of endogenous ligands (endocannabinoids) of CB1 receptors, hence prolonging the activity of the endocannabinoid system.

Anandamide acts as an intracellular messenger amplifying Ca2+ influx via TRPV1 channels

The endocannabinoid anandamide is able to interact with the transient receptor potential vanilloid 1 (TRPV1) channels at a molecular level. As yet, endogenously produced anandamide has not been shown to activate TRPV1, but this is of importance to understand the physiological function of this interaction. Here, we show that intracellular Ca2+ mobilization via the purinergic receptor agonist ATP, the muscarinic receptor agonist carbachol or the Ca(2+)-ATPase inhibitor thapsigargin leads to formation of anandamide, and subsequent TRPV1-dependent Ca2+ influx in transfected cells and sensory neurons of rat dorsal root ganglia (DRG). Anandamide metabolism and efflux from the cell tonically limit TRPV1-mediated Ca2+ entry. In DRG neurons, this mechanism was found to lead to TRPV1-mediated currents that were enhanced by selective blockade of anandamide cellular efflux. Thus, endogenous anandamide is formed on stimulation of metabotropic receptors coupled to the phospholipase C/inositol 1,4,5-triphosphate pathway and then signals to TRPV1 channels. This novel intracellular function of anandamide may precede its action at cannabinoid receptors, and might be relevant to its control over neurotransmitter release.

The spinal basis of opioid tolerance and physical dependence: Involvement of calcitonin gene-related peptide, substance P, and arachidonic acid-derived metabolites

Chronic opioid use in the management of pain is limited by development of analgesic tolerance and physical dependence. The mechanisms underlying tolerance-dependence are not entirely clear, however, recent evidence suggests that spinal adaptations leading to increased activity of sensory neuropeptides (calcitonin gene-related peptide (CGRP), substance P) and their downstream signaling messengers derived from metabolism of arachidonic acid: prostaglandins (PG), lipoxygenase (LOX) metabolites, and endocannabinoids, plays an important role in this phenomenon. In this communication we review the evidence implicating these factors in the induction and expression of opioid tolerance and physical dependence at the spinal level.

Anandamide-evoked activation of vanilloid receptor 1 contributes to the development of bladder hyperreflexia and nociceptive transmission to spinal dorsal horn neurons in cystitis

The role of anandamide in the development of inflammatory hyperalgesia and visceral hyperreflexia was studied in the rat urinary bladder. Animals were given intraperitoneal cyclophosphamide injection, which evokes painful hemorrhagic cystitis accompanied by increased bladder reflex activity. The vanilloid receptor 1 [transient receptor potential vanilloid 1 (TRPV1)] antagonist capsazepine, applied onto the serosal surface of bladders, significantly reduced the hyperreflexia. Mass spectrometric analysis revealed that cyclophosphamide injection significantly and persistently increased the anandamide content of bladder tissues. The increase in the anandamide content paralleled the development of reflex hyperactivity. Anandamide (1-100 microm), applied onto the serosal surface of naive bladders, increased the reflex activity in a concentration-dependent manner. Repeated anandamide applications did not produce desensitization of the response. The anandamide-evoked effect was blocked by capsazepine or by instillation of resiniferatoxin, the ultrapotent TRPV1 agonist, into the bladders 24 hr before the anandamide challenge. The cannabinoid 1 receptor antagonist SR141716A [N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methylpyrazole-3-carboxamide] significantly increased the potency of anandamide in enhancing bladder reflex activity in naive but not in cyclophosphamide-injected animals. Application of the fatty acid amide hydrolyze inhibitor palmitoylisopropylamine onto the serosal surface of bladders also increased the reflex activity both in naive and cyclophosphamide-injected rats. This latter effect in naive animals was blocked by capsazepine and by resiniferatoxin pretreatment. Finally, intravesical instillation of anandamide (50 microm) increased c-fos expression in the spinal cord, which was reduced by capsazepine or by resiniferatoxin pretreatment. These results suggest that anandamide, through activating TRPV1, contributes to the development of hyperreflexia and hyperalgesia during cystitis.

The role of the vanilloid (capsaicin) receptor (TRPV1) in physiology and pathology

The cloning of the vanilloid receptor 1 opened a floodgate for discoveries regarding the function of this complex molecule. It has been found that, in addition to heat, protons and vanilloids, this receptor also responds to various endogenous ligands. Furthermore, it has been also emerged that, through associations with other molecules, the vanilloid receptor 1 plays an important role in the integration of various stimuli and modulation of cellular excitability. Although, originally, the vanilloid receptor 1 was associated with nociceptive primary afferent fibres, it has been gradually revealed that it is broadly expressed in the brain, epidermis and visceral cells. The expression pattern of the vanilloid receptor 1 indicates that it could be involved in various physiological functions and in the pathomechanisms of diverse diseases. Here, we summarise the molecular, pharmacological and physiological characteristics, and putative functions, of the vanilloid receptor 1, and discuss the therapeutic potential of this molecule.

The biosynthesis, fate and pharmacological properties of endocannabinoids

The finding of endogenous ligands for cannabinoid receptors, the endocannabinoids, opened a new era in cannabinoid research. It meant that the biological role of cannabinoid signalling could be finally studied by investigating not only the pharmacological actions subsequent to stimulation of cannabinoid receptors by their agonists, but also how the activity of these receptors was regulated under physiological and pathological conditions by varying levels of the endocannabinoids. This in turn meant that the enzymes catalysing endocannabinoid biosynthesis and inactivation had to be identified and characterized, and that selective inhibitors of these enzymes had to be developed to be used as (1) probes to confirm endocannabinoid involvement in health and disease, and (2) templates for the design of new therapeutic drugs. This chapter summarizes the progress achieved in this direction during the 12 years following the discovery of the first endocannabinoid.

Anandamide acts as a vasodilator of dural blood vessels in vivo by activating TRPV1 receptors

Migraine pathophysiology is believed to involve the release of neuropeptides via the activation of trigeminal afferents that innervate the cranial vasculature. Anandamide, the endogenous ligand to the cannabinoid receptor, is able to inhibit neurogenic dural vasodilatation, calcitonin gene-related peptide (CGRP)-induced and nitric oxide-induced dural vessel dilation in the intravital microscopy model. In an in vitro setting anandamide is also able to activate the vanilloid type 1 (TRPV1) receptor and cause vasodilation, via the release of CGRP. In this study we used intravital microscopy to study whether anandamide behaves as a TRPV1 receptor agonist in the trigeminovascular system. We examined if anandamide-induced dural vasodilation involves CGRP release that can be reversed by the CGRP receptor antagonist, CGRP(8-37), and whether like capsaicin the anandamide effect could be reversed by the TRPV1 receptor antagonist, capsazepine. Anandamide 1 (19+/-9%, n=12), 3 (29+/-5%, n=37), 5 (74+/-7%, n=13) and 10 mg kg(-1) (89+/-18%, n=6) was able to cause a dose-dependent increase in dural vessel diameter. Capsazepine (3 mg kg(-1), t(5)=6.2, P<0.05) and CGRP(8-37) (300 micrograms kg(-1), t(6)=11.1, P<0.05) attenuated the anandamide-induced dural vessel dilation when compared to control (Student's paired t-test). AM251 (3 mg kg(-1)), a cannabinoid type 1 (CB(1)) receptor antagonist, was unable to reverse this anandamide-induced dilation. The study demonstrates that anandamide acts as a TRPV1 receptor agonist in the trigeminovascular system, activating TRPV1 receptors that promote CGRP release and cause vasodilation independent of any action at the CB(1) receptor. Anandamide has been shown previously to inhibit trigeminovascular neurons and prevent vasodilation, through an action at CB(1) receptors.

Endovanilloids. Putative endogenous ligands of transient receptor potential vanilloid 1 channels

Endovanilloids are defined as endogenous ligands of the transient receptor potential vanilloid type 1 (TRPV1) protein, a nonselective cation channel that belongs to the large family of TRP ion channels, and is activated by the pungent ingredient of hot chilli peppers, capsaicin. TRPV1 is expressed in some nociceptor efferent neurons, where it acts as a molecular sensor of noxious heat and low pH. However, the presence of these channels in various regions of the central nervous system, where they are not likely to be targeted by these noxious stimuli, suggests the existence of endovanilloids. Three different classes of endogenous lipids have been found recently that can activate TRPV1, i.e. unsaturated N-acyldopamines, lipoxygenase products of arachidonic acid and the endocannabinoid anandamide with some of its congeners. To classify a molecule as an endovanilloid, the compound should be formed or released in an activity-dependent manner in sufficient amounts to evoke a TRPV1-mediated response by direct activation of the channel. To control TRPV1 signaling, endovanilloids should be inactivated within a short time-span. In this review, we will discuss, for each of the proposed endogenous ligands of TRPV1, their ability to act as endovanilloids in light of the criteria mentioned above.

Immunosuppressive activity of endovanilloids: N-arachidonoyl-dopamine inhibits activation of the NF-kappa B, NFAT, and activator protein 1 signaling pathways

Endogenous N-acyl dopamines such as N-arachidonoyldopamine (NADA) and N-oleoyldopamine have been recently identified as a new class of brain neurotransmitters sharing endocannabinoid and endovanilloid biological activities. As endocannabinoids show immunomodulatory activity, and T cells play a key role in the onset of several diseases that affect the CNS, we have evaluated the immunosuppressive activity of NADA and N-oleoyldopamine in human T cells, discovering that both compounds are potent inhibitors of early and late events in TCR-mediated T cell activation. Moreover, we found that NADA specifically inhibited both IL-2 and TNF-alpha gene transcription in stimulated Jurkat T cells. To further characterize the inhibitory mechanisms of NADA at the transcriptional level, we examined the DNA binding and transcriptional activities of NF-kappaB, NF-AT, and AP-1 transcription factors in Jurkat cells. We found that NADA inhibited NF-kappaB-dependent transcriptional activity without affecting either degradation of the cytoplasmic NF-kappaB inhibitory protein, IkappaBalpha, or DNA binding activity. However, phosphorylation of the p65/RelA subunit was clearly inhibited by NADA in stimulated cells. In addition, NADA inhibited both binding to DNA and the transcriptional activity of NF-AT and AP-1, as expected from the inhibition of NF-AT1 dephosphorylation and c-Jun N-terminal kinase activation in stimulated T cells. Finally, overexpression of a constitutively active form of calcineurin demonstrated that this phosphatase may represent one of the main targets of NADA. These findings provide new mechanistic insights into the anti-inflammatory activities of NADA and highlight their potential to design novel therapeutic strategies to manage inflammatory diseases.

Activation of peripheral cannabinoid receptors attenuates cutaneous hyperalgesia produced by a heat injury

Accumulating evidence suggests that cannabinoids can produce antinociception through peripheral mechanisms. In the present study, we determined whether cannabinoids attenuated existing hyperalgesia produced by a mild heat injury to the glabrous hindpaw and whether the antihyperalgesia was receptor-mediated. Anesthetized rats received a mild heat injury (55 degrees C for 30 s) to one hindpaw. Fifteen minutes after injury, animals exhibited hyperalgesia as evidenced by lowered withdrawal latency to radiant heat and increased withdrawal frequency to a von Frey monofilament (200 mN force) delivered to the injured hindpaw. Separate groups of animals were then treated with an intraplantar (i.pl.) injection of vehicle or the cannabinoid receptor agonist WIN 55,212-2 at doses of 1, 10, or 30 microg in 100 microl. WIN 55,212-2 attenuated both heat and mechanical hyperalgesia dose-dependently. The inactive enantiomer WIN 55,212-3 did not alter mechanical or heat hyperalgesia, suggesting the effects of WIN 55,212-2 were receptor-mediated. The CB1 receptor antagonist AM 251 (30 microg) co-injected with WIN 55,212-2 (30 microg) attenuated the antihyperalgesic effects of WIN 55,212-2. The CB2 receptor antagonist AM 630 (30 microg) co-injected with WIN 55,212-2 attenuated only the early antihyperalgesic effects of WIN 55,212-2. I.pl. injection of WIN 55,212-2 into the contralateral paw did not alter the heat-injury induced hyperalgesia, suggesting that the antihyperalgesia occurred through a peripheral mechanism. These data demonstrate that cannabinoids primarily activate peripheral CB1 receptors to attenuate hyperalgesia. Activation of this receptor in the periphery may attenuate pain without causing unwanted side effects mediated by central CB1 receptors.

The vanilloid receptor-1 is expressed in rat spinal dorsal horn astrocytes

The vanilloid receptor-1 (TRPV1), expressed by nociceptive fibers, is a transducer of thermal and chemical nociceptive messages. However, endogenous ligands excite TRPV1 receptors localized on central nociceptive terminals and interneurons. Using immunocytochemistry at the ultrastructural level, we show that TRPV1 is also expressed in spinal glial cells characterized as astrocyte by double labeling with glial fibrillary acid protein. Quantification of the labeling shows that the most numerous labeling is neuronal and that 7% of the total TRPV1 labeling is localized in astrocytes. The total absence of staining in TRPV1 knock out mice strongly suggests that true TRPV1 protein is present in astrocytes. The localization of TRPV1-containing astrocytes apposed to nociceptive C-terminals suggests that they may be involved in the control of pain transmission.

Biochemical pharmacology of the vanilloid receptor TRPV1. An update

There is mounting evidence that the vanilloid (capsaicin) receptor; transient receptor potential channel, vanilloid subfamily member 1 (TRPV1), is subjected to multiple interacting levels of control. The first level is by reversible phosphorylation catalyzed by intrinsic kinases (e.g. protein kinase A and C) and phosphatases (e.g. calcineurin), which plays a pivotal role in receptor sensitization vs. tachyphylaxis. In addition, this mechanism links TRPV1 to intracellular signaling by various important endogenous as well as exogenous substances such as bradykinin, ethanol, nicotin and insulin. It is not clear, however, whether phosphorylation per se is sufficient to liberate TRPV1 under the inhibitory control of phosphatydylinositol-4,5-bisphosphate. The second level of control is by forming TRPV1 heteromers and their association with putative regulatory proteins. The next level of regulation is by subcellular compartmentalization. The membrane form of TRPV1 functions as a nonselective cation channel. On the endoplasmic reticulum, TRPV1 is present in two differentially regulated forms, one of which is inositol triphosphate-dependent whereas the other is not. These three TRPV1 compartments provide a versatile regulation of intracellular Ca(2+) levels. Last, there is a complex and poorly understood regulation of TRPV1 activity via control of gene expression. Factors that downregulate TRPV1 expression include vanilloid treatment and growth factor (notably, nerve growth factor) deprivation. By contrast, TRPV1 appears to be upregulated during inflammatory conditions. Interestingly, following experimental nerve injury and in animal models of diabetic neuropathy TRPV1 is present on neurons that do not normally express TRPV1. Combined, these findings imply an important role for aberrant TRPV1 expression in the development of neuropathic pain and hyperalgesia. In humans, disease-related changes in TRPV1 expression have already been described (e.g. inflammatory bowel disease and irritable bowel syndrome). The mechanisms that regulate TRPV1 gene expression under pathological conditions are unknown but a better understanding of these pathways has obvious implications for rational drug development.

Multiple actions of anandamide on neonatal rat cultured sensory neurones

1. We have investigated the effects of the endocannabinoid anandamide (AEA) on neuronal excitability and vanilloid TRPV1 receptors in neonatal rat cultured dorsal root ganglion neurones. 2. Using whole-cell patch-clamp electrophysiology, we found that AEA inhibits high-voltage-activated Ca(2+) currents by 33+/-9% (five out of eight neurones) in the absence of the CB(1) receptor antagonist SR141716A (100 nM) and by 32+/-6% (seven out of 10 neurones) in the presence of SR141716A. 3. Fura-2 fluorescence Ca(2+) imaging revealed that AEA produced distinct effects on Ca(2+) transients produced by depolarisation evoked by 30 mM KCl. In a population of neurones of larger somal area (372+/-20 microM(2)), it significantly enhanced Ca(2+) transients (80.26+/-13.12% at 1 microM), an effect that persists after pertussis toxin pretreatment. In a population of neurones of smaller somal area (279+/-18 microM(2)), AEA significantly inhibits Ca(2+) transients (30.75+/-3.54% at 1 microM), an effect that is abolished by PTX pretreatment. 4. Extracellular application of 100 nM AEA failed to evoke TRPV1 receptor inward currents in seven out of eight neurones that responded to capsaicin (1 microM), with a mean inward current of -0.94+/-0.21 nA. In contrast, intracellular application of 100 nM AEA elicited robust inward currents in approximately 62% of neurones, the mean population response was -0.85+/-0.21 nA. When AEA was applied to the intracellular environment with capsazepine (1 microM), the mean population inward current was -0.01+/-0.01 nA. Under control conditions, mean population current fluctuations of -0.09+/-0.05 nA were observed.

Modulation of trigeminal sensory neuron activity by the dual cannabinoid-vanilloid agonists anandamide, N-arachidonoyl-dopamine and arachidonyl-2-chloroethylamide

1. Peripheral cannabinoids have been shown to suppress nociceptive neurotransmission in a number of behavioral and neurophysiological studies. It is not known, however, whether cannabinoids exert this action through direct interactions with nociceptors in the periphery and/or if other processes are involved. To gain a better understanding of the direct actions of cannabinoid-vanilloid agonists on sensory neurons, we examined the effects of these compounds on trigeminal ganglion (TG) neurons in vitro. 2. AEA (EC(50)=11.0 microM), NADA (EC(50)=857 nM) and arachidonyl-2-chloroethylamide ACEA (EC(50)=14.0 microM) each evoked calcitonin gene-related peptide (CGRP) release from TG neurons. The TRPV1 antagonists iodo-resiniferatoxin (I-RTX) and capsazepine (CPZ) each obtunded AEA-, NADA-, ACEA- and capsaicin (CAP)-evoked CGRP release with individually equivalent IC(50)'s for each of the compounds (I-RTX IC(50) range=2.6-4.0 nM; CPZ IC(50) range=523-1140 microM). 3. The pro-inflammatory mediator prostaglandin E(2) significantly increased the maximal effect of AEA-evoked CGRP release without altering the EC(50). AEA, ACEA and CAP stimulated cAMP accumulation in TG neurons in a calcium- and TRPV1-dependent fashion. Moreover, the protein kinase inhibitor staurosporine significantly inhibited AEA- and CAP-evoked CGRP release. 4. The pungency of AEA, NADA, ACEA and CAP in the rat eye-wipe assay was also assessed. Interestingly, when applied intraocularly, NADA or CAP each produced nocifensive responses, while AEA or ACEA did not. 5. Finally, the potential inhibitory effects of these cannabinoids on TG nociceptors were evaluated. Neither AEA nor ACEA decreased CAP-evoked CGRP release. Furthermore, neither of the cannabinoid receptor type 1 antagonists SR141716A nor AM251 had any impact on either basal or CAP-evoked CGRP release. AEA also did not inhibit 50 mM K(+)-evoked CGRP release and did not influence bradykinin-stimulated inositol phosphate accumulation. 6. We conclude that the major action of AEA, NADA and ACEA on TG neurons is excitatory, while, of these, only NADA is pungent. These findings are discussed in relation to our current understanding of interactions between the cannabinoid and vanilloid systems and nociceptive processing in the periphery.

The endocannabinoid system: a general view and latest additions

After the discovery, in the early 1990s, of specific G-protein-coupled receptors for marijuana's psychoactive principle Delta(9)-tetrahydrocannabinol, the cannabinoid receptors, and of their endogenous agonists, the endocannabinoids, a decade of investigations has greatly enlarged our understanding of this altogether new signalling system. Yet, while the finding of the endocannabinoids resulted in a new effort to reveal the mechanisms regulating their levels in the brain and peripheral organs under physiological and pathological conditions, more endogenous substances with a similar action, and more molecular targets for the previously discovered endogenous ligands, anandamide and 2-arachidonoylglycerol, or for some of their metabolites, were being proposed. As the scenario becomes subsequently more complicated, and the experimental tasks to be accomplished correspondingly more numerous, we briefly review in this article the latest 'additions' to the endocannabinoid system together with earlier breakthroughs that have contributed to our present knowledge of the biochemistry and pharmacology of the endocannabinoids.