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

Neurotrophins in the lower urinary tract: becoming of age

The lower urinary tract (LUT) comprises a storage unit, the urinary bladder, and an outlet, the urethra. The coordination between the two structures is tightly controlled by the nervous system and, therefore, LUT function is highly susceptible to injuries to the neuronal pathways involved in micturition control. These injuries may include lesions to the spinal cord or to nerve fibres and result in micturition dysfunction. A common trait of micturition pathologies, irrespective of its origin, is an upregulation in synthesis and secretion of neurotrophins, most notably Nerve Growth Factor (NGF) and Brain Derived Neurotrophic Factor (BDNF). These neurotrophins are produced by neuronal and non-neuronal cells and exert their effects upon binding to their high-affinity receptors abundantly expressed in the neuronal circuits regulating LUT function. In addition, NGF and BDNF are present in detectable amounts in the urine of patients suffering from various LUT pathologies, suggesting that analysis of urinary NGF and BDNF may serve as likely biomarkers to be studied in tandem with other factors when diagnosing patients. Studies with experimental models of bladder dysfunction using antagonists of NGF and BDNF receptors as well as scavenging agents suggest that those NTs may be key elements in the pathophysiology of bladder dysfunctions. In addition, available data indicates that NGF and BDNF might constitute future targets for designing new drugs for better treatment of bladder dysfunction.

N-arachidonoyl-serotonin in the basolateral amygdala increases anxiolytic behavior in the elevated plus maze

CB(1) receptors in the amygdala have been shown to mediate learned and unlearned anxiety states, however, the role of amygdalar TRPV1 receptors remains unclear. In the present study we investigated the potential anxiolytic action of intra-basolateral amygdala (BLA) infusion of N-arachidonoyl-serotonin (AA-5-HT), a dual blocker of the endocannabinoid-inactivating enzyme, fatty acid amide hydrolase (FAAH), and a TRPV1 antagonist. Varying doses of AA-5-HT (0-0.5 nmol) were administered into the BLA prior to elevated plus maze testing. AA-5-HT significantly increased both time spent and number of entries into the open arms. Next, to determine whether the anxiolytic effects were the result of blocking FAAH, TRPV1, or whether a combined action was required, rats were given intra-BLA infusions of either 0.25 nmol AA-5-HT, 1.0 nmol capsazepine (CZP, a TRPV1 antagonist), 0.01 μg URB597 (a selective FAAH inhibitor), or vehicle. Again, AA-5-HT increased the time spent in the open arms as well as the number of open arm entries. In contrast, CZP and URB597 did not reliably alter plus maze performance. We then investigated the effects of co-administration of CZP (1.0 or 10.0 nmol) and URB597 (0.01 or 0.1 μg). At lower doses, co-injections significantly increased both open arm entries as well as the time spent in the open arms, compared to vehicle or either compound alone. While co-administration of the higher doses had no significant effect when compared to either vehicle or CZP treatment, we did observe that open arm activity was elevated in rats receiving combined CZP-URB597 treatment compared to URB597 alone. Overall, our findings indicate that simultaneous FAAH activity and TRPV1 activation are important with respect to the expression of unconditioned fear as mediated within the BLA.

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

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

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

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

Spinal anandamide produces analgesia in neuropathic rats: possible CB(1)- and TRPV1-mediated mechanisms

The endocannabinoid anandamide (AEA) activates also transient receptor potential vanilloid-1 (TRPV1) channels. However, no data exist on the potential role of spinal TRPV1 activation by AEA in neuropathic pain. We tested the effect of: 1) AEA (5-100 μg), alone or in the presence of an inhibitor of its hydrolysis, and 2) elevated levels of endogenous AEA (following inhibition of AEA hydrolysis), in CCI rats, and the involvement of TRPV1 or cannabinoid CB(1) receptors in the observed effects. Levels of AEA in the spinal cord of CCI rats were measured following all treatments. AEA (50 μg) displayed anti-allodynic and anti-hyperalgesic effects which were abolished by previous antagonism of TRPV1, but not CB(1), receptors. Depending on the administered dose, the selective inhibitor of AEA enzymatic hydrolysis, URB597 (10-100 μg), reduced thermal and tactile nociception via CB(1) or CB(1)/TRPV1 receptors. The anti-nociceptive effects of co-administered per se ineffective doses of AEA (5 μg) and URB597 (5 μg) was abolished by antagonism of CB(1), but not TRPV1, receptors. Spinal AEA levels were increased after CCI, slightly increased further by URB597, 10 μg i.t., and strongly elevated by URB597, 100 μg. Injection of AEA (50 μg) into the lumbar spinal cord led to its dramatic elevation in this tissue, whereas, when a lower dose was used (5 μg) AEA endogenous levels were elevated only in the presence of URB597 (5 μg). We suggest that spinal AEA reduces neuropathic pain via CB(1) or TRPV1, depending on its local concentration.

TRPV1, TRPA1, and CB1 in the isolated vagus nerve--axonal chemosensitivity and control of neuropeptide release

Vagal sensory afferents innervating airways and abdominal tissues express TRPV1 and TRPA1, two depolarizing calcium permeable ion channels playing a major role in sensing environmental irritants and endogenous metabolites which cause neuropeptide release and neurogenic inflammation. Here we have studied axonal chemosensitivity and control of neuropeptide release from the isolated rat and mouse vagus nerve by using prototypical agonists of these transduction channels - capsaicin, mustard oil and the specific endogenous activators, anandamide (methyl arachidonyl ethanolamide, mAEA), and acrolein, respectively. Capsaicin evoked iCGRP release from the rat vagus nerve with an EC₅₀ of 0.12 μM. Co-application of mAEA had a dual effect: nanomolar concentrations of mAEA (0.01 μM) significantly reduced capsaicin-evoked iCGRP release while concentrations ≥ 1 μM mAEA had sensitizing effects. Only 100 μM mAEA directly augmented iCGRP release by itself. In the mouse, 310 μM mAEA increased release in wildtype and TRPA1-/- mice which could be inhibited by capsazepine (10 μM) and was completely absent in TRPV1-/- mice. CB1-/- and CB1/CB2 double -/- mice equally displayed increased sensitivity to mAEA (100 μM) and a sensitizing effect to capsaicin, in contrast to wildtypes. Acrolein and mustard oil (MO)--at μM concentrations--induced a TRPA1-dependent iCGRP release; however, millimolar concentrations of mustard oil (>1mM) evoked iCGRP release by activating TRPV1, confirming recent evidence for TRPV1 agonism of high mustard oil concentrations. Taken together, we present evidence for functional expression of excitatory TRPV1, TRPA1, and inhibitory CB1 receptors along the sensory fibers of the vagus nerve which lend pathophysiological relevance to the axonal membrane and the control of neuropeptide release that may become important in cases of inflammation or neuropathy. Sensitization and possible ectopic discharge may contribute to the development of autonomic dysregulation in visceral tissues that are innervated by the vagus nerve.

Cannabinoid receptors as target for treatment of osteoporosis: a tale of two therapies

The central nervous system plays an important role in regulating bone metabolism in health and in disease with a number of neurotransmitters been reported to influence bone cell activity through a central relay. In keeping with this, recent studies demonstrated that endocannabinoids and their receptors are involved in the pathogenesis of osteoporosis. The endocannabinoids anandamide and 2-arachidonylglycerol are found in the skeleton and numerous studies also showed that bone cells express the cannabinoid receptors CB1 and CB2 and the orphan receptor GPR55. Pharmacological and genetic inactivation of CB1, CB2 and GPR55 in adult mice suppress bone resorption, increase bone mass and protect against bone loss, suggesting that inverse agonists/antagonists of these receptors may serve as anti-resorptive agents. In the ageing skeleton however CB1 and CB2 receptors have a protective effect against age-dependent bone loss in both male and female mice. CB1 receptor deficiency in aged mice results in accelerated age-dependent osteoporosis due to marked increase in bone resorption and significant reduction in bone formation coupled to enhanced adipocyte accumulation in the bone marrow compartment. Similar acceleration of bone loss was also reported in CB2 deficient mice of similar age but found to be associated with enhanced bone turnover. This review summarises in vitro and in vivo findings relating to the influence of cannabinoid ligands on bone metabolism and argues in favour of the exploitation of cannabinoid receptors as targets for both anabolic and anti-resorptive therapy for treatment of complex multifaceted bone diseases such as osteoporosis.

Functional role of cannabinoid receptors in urinary bladder

Cannabinoids, the active components of Cannabis sativa (maijuana), and their derivatives produce a wide spectrum of central and peripheral effects, some of which may have clinical applications. The discovery of specific cannabinoid receptors and a family of endogenous ligands of those receptors has attracted much attention to the general cannabinoid pharmacology. In recent years, studies on the functional role of cannabinoid receptors in bladder have been motivated by the therapeutic effects of cannabinoids on voiding dysfunction in multiple sclerosis patients. In this review, we shall summarize the literature on the expression of cannabinoid receptors in urinary bladder and the peripheral influence of locally and systemically administered cannabinoids in the bladder. The ongoing search for cannabinoid-based therapeutic strategies devoid of psychotropic effects can be complemented with local delivery into bladder by the intravesical route. A greater understanding of the role of the peripheral CB₁ and CB₂ receptor system in lower urinary tract is necessary to allow the development of new treatment for pelvic disorders.

The endovanilloid/endocannabinoid system: a new potential target for osteoporosis therapy

Human osteoclasts express functional TRPV1 channels, CB1/CB2 cannabinoid receptors and endocannabinoid/endovanilloid synthetic/catabolic enzymes. Pharmacologic manipulation of this system can modulate osteoclast activity. Here, through multidisciplinary approaches, we demonstrate that enzymes and receptors of the endocannabinoid/endovanilloid system are differently expressed in osteoclasts from menopausal women without or with osteoporosis. We report that in osteoclasts from osteoporotic patients, TRPV1 channels are upregulated and, if persistently stimulated with resiniferatoxin, become clustered to the plasma membrane while inducing a massive over-expression of CB2 receptors. By providing new evidence for a critical functional cross-talk between CB2 and TRPV1 receptors in osteoporosis, we speculate that TRPV1 desensitization, or its enhanced trafficking, together with TRPV1 agonist-induced CB2 receptor overexpression, might be critical to minimize calcium entry in osteoclasts, which could be in turn responsible of cell over-activation and higher bone resorption. Our data pave the way to the use of TRPV1 agonist together with CB2 agonists or CB1 antagonists in osteoporosis.

International Union of Basic and Clinical Pharmacology. LXXIX. Cannabinoid receptors and their ligands: beyond CB₁ and CB₂

There are at least two types of cannabinoid receptors (CB(1) and CB(2)). Ligands activating these G protein-coupled receptors (GPCRs) include the phytocannabinoid Δ(9)-tetrahydrocannabinol, numerous synthetic compounds, and endogenous compounds known as endocannabinoids. Cannabinoid receptor antagonists have also been developed. Some of these ligands activate or block one type of cannabinoid receptor more potently than the other type. This review summarizes current data indicating the extent to which cannabinoid receptor ligands undergo orthosteric or allosteric interactions with non-CB(1), non-CB(2) established GPCRs, deorphanized receptors such as GPR55, ligand-gated ion channels, transient receptor potential (TRP) channels, and other ion channels or peroxisome proliferator-activated nuclear receptors. From these data, it is clear that some ligands that interact similarly with CB(1) and/or CB(2) receptors are likely to display significantly different pharmacological profiles. The review also lists some criteria that any novel "CB(3)" cannabinoid receptor or channel should fulfil and concludes that these criteria are not currently met by any non-CB(1), non-CB(2) pharmacological receptor or channel. However, it does identify certain pharmacological targets that should be investigated further as potential CB(3) receptors or channels. These include TRP vanilloid 1, which possibly functions as an ionotropic cannabinoid receptor under physiological and/or pathological conditions, and some deorphanized GPCRs. Also discussed are 1) the ability of CB(1) receptors to form heteromeric complexes with certain other GPCRs, 2) phylogenetic relationships that exist between CB(1)/CB(2) receptors and other GPCRs, 3) evidence for the existence of several as-yet-uncharacterized non-CB(1), non-CB(2) cannabinoid receptors; and 4) current cannabinoid receptor nomenclature.

The role of sex steroid hormones, cytokines and the endocannabinoid system in female fertility

BACKGROUND

Marijuana, the most used recreational drug, has been shown to have adverse effects on human reproduction. Endogenous cannabinoids (also called endocannabinoids) bind to the same receptors as those of Δ(9)-tetrahydrocannabinol (THC), the psychoactive component of Cannabis sativa. The most extensively studied endocannabinoids are anandamide (N-arachidonoylethanolamine, AEA) and 2-arachidonoylglycerol. The endocannabinoids, their congeners and the cannabinoid receptors, together with the metabolic enzymes and putative transporters form the endocannabinoid system (ECS). In this review, we summarize current knowledge about the relationships of ECS, sex steroid hormones and cytokines in female fertility, and underline the importance of this endocannabinoid-hormone-cytokine network.

METHODS

Pubmed and the Web of Science databases were searched for studies published since 1985, looking into the ECS, sex hormones, type-1/2 T-helper (Th1/Th2) cytokines, leukaemia inhibitory factor, leptin and reproduction.

RESULTS

The ECS plays a pivotal role in human reproduction. The enzymes involved in the synthesis and degradation of endocannabinoids normalize levels of AEA for successful implantation. The AEA degrading enzyme (fatty acid amide hydrolase) activity as well as AEA content in blood may potentially be used for the monitoring of early pregnancies. Progesterone and oestrogen are involved in the maintenance of endocannabinoid levels. The ECS plays an important role in the immune regulation of human fertility.

CONCLUSIONS

The available studies suggest that tight control of the endocannabinoid-hormone-cytokine network is required for successful implantation and early pregnancy maintenance. This hormone-cytokine network is a key element at the maternal-foetal interface, and any defect in such a network may result in foetal loss.

The endogenous cannabinoid anandamide inhibits transient receptor potential vanilloid type 1 receptor-mediated currents in rat cultured primary sensory neurons

The activity of the transient receptor potential vanilloid type 1 ion channel (TRPV1) that is expressed by the great majority of polymodal nociceptors is pivotal for the development of inflammatory heat hyperalgesia. The responsiveness of TRPV1 is regulated by a series of intracellular signalling molecules including the cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA); increased or decreased PKA activity results in TRPV1 sensitisation or desensitisation, respectively. Activation of the cannabinoid 1 (CB1) receptor that is expressed by the majority of the TRPV1-expressing primary sensory neurons reduces PKA activity. Therefore, here we studied whether activation of the CB1 receptor resulted in reduced TRPV1-mediated responses in cultured rat primary sensory neurons. We found that TRPV1-mediated whole-cell currents were significantly reduced respectively, by 50% and 25% by 10 nM and 30 nM of the endogenous CB1 receptor agonist, anandamide. The PKA inhibitor, H89 (10 microM) also had a significant inhibitory effect on TRPV1-mediated currents ( approximately 70%). These findings suggest that activation of the CB1 receptor can reduce the activity of TRPV1 in primary sensory neurons, and that this inhibitory effect could be mediated through the reduction of PKA-mediated phosphorylation of TRPV1.

TRPV1: a target for next generation analgesics

Transient Receptor Potential Vanilloid 1 (TRPV1) is a Ca²⁺ permeant non-selective cation channel expressed in a subpopulation of primary afferent neurons. TRPV1 is activated by physical and chemical stimuli. It is critical for the detection of nociceptive and thermal inflammatory pain as revealed by the deletion of the TRPV1 gene. TRPV1 is distributed in the peripheral and central terminals of the sensory neurons and plays a role in initiating action potentials at the nerve terminals and modulating neurotransmitter release at the first sensory synapse, respectively. Distribution of TRPV1 in the nerve terminals innervating blood vessels and in parts of the CNS that are not subjected to temperature range that is required to activate TRPV1 suggests a role beyond a noxious thermal sensor. Presently, TRPV1 is being considered as a target for analgesics through evaluation of different antagonists. Here, we will discuss the distribution and the functions of TRPV1, potential use of its agonists and antagonists as analgesics and highlight the functions that are not related to nociceptive transmission that might lead to adverse effects.

Inhibitors of monoacylglycerol lipase, fatty-acid amide hydrolase and endocannabinoid transport differentially suppress capsaicin-induced behavioral sensitization through peripheral endocannabinoid mechanisms

Monoacylglycerol lipase (MGL) and fatty-acid amide hydrolase (FAAH) degrade the endocannabinoids 2-arachidonoylglycerol (2-AG) and anandamide (AEA), respectively. Pharmacological inhibition of these enzymes in the periphery may elucidate the role of endocannabinoids in controlling nociceptive transmission. We compared effects of the MGL inhibitor JZL184, the FAAH inhibitor URB597, and the endocannabinoid uptake inhibitor VDM11, administered locally in the paw, on behavioral hypersensitivities produced by capsaicin, the pungent ingredient in hot chili peppers. Intradermal capsaicin (10 microg i.pl.) produced nocifensive behavior, thermal hyperalgesia, and mechanical allodynia in rats. JZL184 (100 microg i.pl.) suppressed capsaicin-induced nocifensive behavior and thermal hyperalgesia without altering capsaicin-evoked mechanical allodynia. Effects of JZL184 were blocked by either the CB(1) antagonist AM251 (80 microg i.pl.) or the CB(2) antagonist AM630 (25 microg i.pl.). URB597 (75 microg i.pl.) suppressed capsaicin-induced mechanical allodynia without altering capsaicin-evoked thermal hyperalgesia or nocifensive behavior. Effects of URB597 were blocked by AM251 (80 microg i.pl.), but not by AM630 (25 microg i.pl.). VDM11 (100 microg i.pl.) suppressed capsaicin-evoked hypersensitivity for all three dependent measures (nocifensive behavior, thermal hyperalgesia, and mechanical allodynia), suggesting an additive effect following putative elevation of both AEA and 2-AG. The VDM11-induced suppression of capsaicin-evoked nocifensive behavior and thermal hyperalgesia was blocked by either AM251 (80 microg i.pl.) or AM630 (25 microg i.pl.), as observed with JZL184. The VDM11-induced suppression of capsaicin-evoked mechanical allodynia was blocked by AM251 (25 microg i.pl.) only, as observed with URB597. Thus, peripheral inhibition of enzymes hydrolyzing 2-AG and AEA suppresses capsaicin-evoked behavioral sensitization with distinct patterns of pharmacological specificity and in a non-overlapping and modality-specific manner. Modulation of endocannabinoids in the periphery suppressed capsaicin-evoked nocifensive behavior and thermal hyperalgesia through either CB(1) or CB(2) receptor mechanisms but suppressed capsaicin-evoked mechanical allodynia through CB(1) mechanisms only. Inhibition of endocannabinoid transport was more effective in suppressing capsaicin-induced sensitization compared to inhibition of either FAAH or MGL alone. These studies are the first to unveil the effects of pharmacologically increasing peripheral endocannabinoid levels on capsaicin-induced behavioral hypersensitivities. Our data suggest that 2-AG, the putative product of MGL inhibition, and AEA, the putative product of FAAH inhibition, differentially suppress capsaicin-induced nociception through peripheral cannabinoid mechanisms.

The TRPV1 ion channel antagonist capsazepine inhibits osteoclast and osteoblast differentiation in vitro and ovariectomy induced bone loss in vivo

The vanilloid type 1 ion channel (TRPV1) is known to play an important role in the regulation of pain and inflammation. Pharmacological ligands of TRPV1 regulate human osteoclast formation in vitro, but the effects of these agents on osteoblast function have not been studied and their effects on bone loss in vivo are unknown. Here we examined the effects of the TRPV1 ion channel antagonist capsazepine on mouse osteoclast and osteoblast differentiation in vitro and ovariectomy induced bone loss in vivo. Capsazepine inhibited osteoclast formation and bone resorption in a dose dependent manner in bone marrow-osteoblast co-cultures and RANKL generated osteoclast cultures, whereas the TRPV1 agonist capsaicin enhanced RANKL and M-CSF stimulated osteoclast formation. Capsazepine also suppressed RANKL induced IkappaB and ERK1/2 phosphorylation and caused apoptosis of mature osteoclasts and also inhibited alkaline phosphatase activity and bone nodule formation in calvarial osteoblast cultures. Studies in vivo showed that capsazepine (1mg/kg/day) inhibited ovariectomy induced bone loss in mice and histomorphometric analysis showed inhibitory effects on indices of bone resorption and bone formation. We conclude that pharmacological blockade of TRPV1 ion channels by capsazepine inhibits osteoclastic bone resorption and protects against ovariectomy induced bone loss in mice, but also inhibits osteoblast activity and bone formation.

Cannabinoids and the gut: new developments and emerging concepts

Cannabis has been used to treat gastrointestinal (GI) conditions that range from enteric infections and inflammatory conditions to disorders of motility, emesis and abdominal pain. The mechanistic basis of these treatments emerged after the discovery of Delta(9)-tetrahydrocannabinol as the major constituent of Cannabis. Further progress was made when the receptors for Delta(9)-tetrahydrocannabinol were identified as part of an endocannabinoid system, that consists of specific cannabinoid receptors, endogenous ligands and their biosynthetic and degradative enzymes. Anatomical, physiological and pharmacological studies have shown that the endocannabinoid system is widely distributed throughout the gut, with regional variation and organ-specific actions. It is involved in the regulation of food intake, nausea and emesis, gastric secretion and gastroprotection, GI motility, ion transport, visceral sensation, intestinal inflammation and cell proliferation in the gut. Cellular targets have been defined that include the enteric nervous system, epithelial and immune cells. Molecular targets of the endocannabinoid system include, in addition to the cannabinoid receptors, transient receptor potential vanilloid 1 receptors, peroxisome proliferator-activated receptor alpha receptors and the orphan G-protein coupled receptors, GPR55 and GPR119. Pharmacological agents that act on these targets have been shown in preclinical models to have therapeutic potential. Here, we discuss cannabinoid receptors and their localization in the gut, the proteins involved in endocannabinoid synthesis and degradation and the presence of endocannabinoids in the gut in health and disease. We focus on the pharmacological actions of cannabinoids in relation to GI disorders, highlighting recent data on genetic mutations in the endocannabinoid system in GI disease.

Cannabinoid system in the skin - a possible target for future therapies in dermatology

Cannabinoids and their derivatives are group of more than 60 biologically active chemical agents, which have been used in natural medicine for centuries. The major agent of exogenous cannabinoids is Delta(9)-tetrahydrocannabinol (Delta(9)-THC), natural psychoactive ingredient of marijuana. However, psychoactive properties of these substances limited their use as approved medicines. Recent discoveries of endogenous cannabinoids (e.g. arachidonoylethanolamide, 2-arachidonoylglycerol or palmithyloethanolamide) and their receptors initiated discussion on the role of cannabinoid system in physiological conditions as well as in various diseases. Based on the current knowledge, it could be stated that cannabinoids are important mediators in the skin, however their role have not been well elucidated yet. In our review, we summarized the current knowledge about the significant role of the cannabinoid system in the cutaneous physiology and pathology, pointing out possible future therapeutic targets.

Modulation of motor and sensory pathways of the peristaltic reflex by cannabinoids

Cannabinoids have long been known to be potent inhibitors of intestinal and colonic propulsion. This effect has generally been attributed to their ability to prejunctionally inhibit release of acetylcholine from excitatory motor neurons that mediate, in part, the ascending contraction phase of the peristaltic reflex. In the present study we examined the effect of cannabinoids on the other transmitters known to participate in the peristaltic reflex using a three-compartment preparation of rat colon that allows separation of ascending contraction, descending relaxation, and the sensory components of the reflex. On addition to the orad motor compartment, anandamide decreased and AM-251, a CB-1 antagonist, increased ascending contraction and the concomitant substance P (SP) release. Similarly, on addition to the caudad motor compartment, anandamide decreased and AM-251 increased descending relaxation and the concomitant vasoactive intestinal peptide (VIP) release. On addition to the central sensory compartment, anandamide decreased and AM-251 increased both ascending contraction and SP release orad, and descending relaxation and VIP release caudad. This suggested a role for CB-1 receptors in modulation of sensory transmission that was confirmed by the demonstration that central addition of anandamide decreased and AM-251 increased release of the sensory transmitter, calcitonin gene-related peptide (CGRP). We conclude that the potent antipropulsive effect of cannabinoids is the result of inhibition of both excitatory cholinergic/tachykininergic and inhibitory VIPergic motor neurons that mediate ascending contraction and descending relaxation, respectively, as well as inhibition of the intrinsic sensory CGRP-containing neurons that initiate the peristaltic reflex underlying propulsive motility.

Altered responses of dopamine D3 receptor null mice to excitotoxic or anxiogenic stimuli: Possible involvement of the endocannabinoid and endovanilloid systems

Dopamine and the endocannabinoids, anandamide and 2-arachidonoylglycerol, interact at several levels in the brain, with the involvement of both cannabinoid CB(1) receptors and transient receptor potential vanilloid type-1 (TRPV1) channels (which are alternative anandamide receptors). Using pharmacological, immunohistochemical and analytical approaches, we investigated the response of dopamine D(3) receptor null (D3R((-/-))) mice in models of epilepsy and anxiety, in relation to their brain endocannabinoid and endovanilloid tone. Compared to wild-type mice, D3R((-/-)) mice exhibited a delayed onset of clonic seizures, enhanced survival time, reduced mortality rate and more sensitivity to anticonvulsant effects of diazepam after intraperitoneal administration of picrotoxin (7 mg/kg), and a less anxious-like behaviour in the elevated plus maze test. D3R((-/-)) mice also exhibited different endocannabinoid and TRPV1, but not CB(1), levels in the hippocampus, nucleus accumbens, amygdala and striatum. Given the role played by CB(1) and TRPV1 in neuroprotection and anxiety, and based on data obtained here with pharmacological tools, we suggest that the alterations of endocannabinoid and endovanilloid tone found in D3R((-/-)) mice might account for part of their altered responses to excitotoxic and anxiogenic stimuli.

Capsaicin-sensitive primary sensory neurons in the mouse express N-Acyl phosphatidylethanolamine phospholipase D

Our previous finding, that the capsaicin- and KCl-induced Ca(2+)-dependent production of the intra- and intercellular signaling molecule N-arachidonoyl ethanolamine (anandamide) in cultured primary sensory neurons could be abolished and reduced by approximately 2/3 by capsaicin-induced degeneration of capsaicin-sensitive neurons, respectively suggests that a major sub-population of capsaicin-sensitive cells together with a group of non-capsaicin-sensitive cells should express enzymes involved in Ca(2+)-dependent anandamide synthesis. N-acyl phosphotidylethanolamine phospholipase D (NAPE-PLD) is known to be involved in Ca(2+)-dependent anandamide production. Hence, here, we used reverse transcriptase and quantitative real time polymerase chain reaction to study NAPE-PLD expression in dorsal root ganglia and to clarify the sub-population of cells expressing this enzyme. Cultures prepared from mouse dorsal root ganglia were grown either in the absence or presence of the neurotoxin, capsaicin (10 muM) overnight. We report, that NAPE-PLD is expressed both in dorsal root ganglia and cultures prepared from dorsal root ganglia and grown in the absence of capsaicin. Furthermore, we also report that capsaicin application downregulates the expression of NAPE-PLD as well as the capsaicin receptor, transient receptor potential vanilloid type 1 ion channel, by about 70% in the cultures prepared from dorsal root ganglia. These findings indicate that a major sub-population of capsaicin-sensitive primary sensory neurons expresses NAPE-PLD, and suggest that NAPE-PLD is expressed predominantly by capsaicin-sensitive neurons in dorsal root ganglia. These data also suggest that NAPE-PLD might be a target to control the activity and excitability of a major sub-population of nociceptive primary sensory neurons.

Localization of the endocannabinoid-degrading enzyme fatty acid amide hydrolase in rat dorsal root ganglion cells and its regulation after peripheral nerve injury

Fatty acid amide hydrolase (FAAH) is a degradative enzyme for a group of endogenous signaling lipids that includes anandamide (AEA). AEA acts as an endocannabinoid and an endovanilloid by activating cannabinoid and vanilloid type 1 transient receptor potential (TRPV1) receptors, respectively, on dorsal root ganglion (DRG) sensory neurons. Inhibition of FAAH activity increases AEA concentrations in nervous tissue and reduces sensory hypersensitivity in animal pain models. Using immunohistochemistry, Western blotting, and reverse transcription-PCR, we demonstrate the location of the FAAH in adult rat DRG, sciatic nerve, and spinal cord. In naive rats, FAAH immunoreactivity localized to the soma of 32.7 +/- 0.8% of neurons in L4 and L5 DRG. These were small-sized (mean soma area, 395.96 +/- 5.6 mum(2)) and predominantly colabeled with peripherin and isolectin B4 markers of unmyelinated C-fiber neurons; 68% colabeled with antibodies to TRPV1 (marker of nociceptive DRG neurons), and <2% colabeled with NF200 (marker of large myelinated neurons). FAAH-IR was also present in small, NF200-negative cultured rat DRG neurons. Incubation of these cultures with the FAAH inhibitor URB597 increased AEA-evoked cobalt uptake in a capsazepine-sensitive manner. After sciatic nerve axotomy, there was a rightward shift in the cell-size distribution of FAAH-immunoreactive (IR) DRG neurons ipsilateral to injury: FAAH immunoreactivity was detected in larger-sized cells that colabeled with NF200. An ipsilateral versus contralateral increase in both the size and proportion of FAAH-IR DRG occurred after spinal nerve transection injury but not after chronic inflammation of the rat hindpaw 2 d after injection of complete Freund's adjuvant. This study reveals the location of FAAH in neural tissue involved in peripheral nociceptive transmission.

Peripheral and central sites of action for the non-selective cannabinoid agonist WIN 55,212-2 in a rat model of post-operative pain

BACKGROUND AND PURPOSE

Activation of cannabinoid (CB) receptors decreases nociceptive transmission in inflammatory or neuropathic pain states. However, the effects of CB receptor agonists in post-operative pain remain to be investigated. Here, we characterized the anti-allodynic effects of WIN 55,212-2 (WIN) in a rat model of post-operative pain.

EXPERIMENTAL APPROACH

WIN 55,212-2 was characterized in radioligand binding and in vitro functional assays at rat and human CB(1) and CB(2) receptors. Analgesic activity and site(s) of action of WIN were assessed in the skin incision-induced post-operative pain model in rats; receptor specificity was investigated using selective CB(1) and CB(2) receptor antagonists.

KEY RESULTS

WIN 55,212-2 exhibited non-selective affinity and agonist efficacy at human and rat CB(1) versus CB(2) receptors. Systemic administration of WIN decreased injury-induced mechanical allodynia and these effects were reversed by pretreatment with a CB(1) receptor antagonist, but not with a CB(2) receptor antagonist, given by systemic, intrathecal and supraspinal routes. In addition, peripheral administration of both CB(1) and CB(2) antagonists blocked systemic WIN-induced analgesic activity.

CONCLUSIONS AND IMPLICATIONS

Both CB(1) and CB(2) receptors were involved in the peripheral anti-allodynic effect of systemic WIN in a pre-clinical model of post-operative pain. In contrast, the centrally mediated anti-allodynic activity of systemic WIN is mostly due to the activation of CB(1) but not CB(2) receptors at both the spinal cord and brain levels. However, the increased potency of WIN following i.c.v. administration suggests that its main site of action is at CB(1) receptors in the brain.

Sensitization of capsaicin-sensitive lung vagal afferents by anandamide in rats: role of transient receptor potential vanilloid 1 receptors

Anandamide (AEA), an arachidonic acid derivative produced during inflammatory conditions, is an endogenous agonist of both transient receptor potential vanilloid 1 (TRPV1) receptors and cannabinoid CB1 receptors. Sensitization of capsaicin-sensitive lung vagal afferent (CSLVA) fibers by chemical mediators is important in the pathogenesis of hyperreactive airway diseases. We investigated the effect of the intravenous infusion of AEA (2 mg·kg−1·ml−1, 0.5 ml/min for 2 min) on the sensitivity of CSLVA fibers to chemical and mechanical stimulation in anesthetized rats. In artificially ventilated rats, AEA infusion only mildly elevated the baseline activity of CSLVA fibers. However, CSLVA fiber responses to right atrial injection of capsaicin, AEA, or adenosine and to lung inflation (tracheal pressure = 30 cmH2O) were all markedly potentiated during AEA infusion, which reverted 20 min after termination of the infusion. The potentiating effect on the sensitivity of CSLVA fibers to adenosine injection or lung inflation was completely blocked by pretreatment with capsazepine (a TRPV1 receptor antagonist) but was unaffected by pretreatment with AM281 (a CB1 receptor antagonist). In spontaneously breathing rats, right atrial injection of adenosine evoked an apneic response that is presumably mediated through CSLVA fibers. Similarly, the adenosine-evoked apneic response was potentiated during AEA infusion, and this potentiating effect was also completely prevented by pretreatment with capsazepine. These results suggest that AEA infusion at the dose tested produces a mild activation of TRPV1 receptors and this nonspecifically increases CSLVA fiber sensitivity to chemical and mechanical stimulation.

Excitation of cutaneous C nociceptors by intraplantar administration of anandamide

Anandamide has been characterized as both an endocannabinoid and endovanilloid. Consistent with its actions as an endovanilloid, previous studies have demonstrated that anandamide can excite primary sensory neurons in vitro via transient receptor potential vanilloid type one (TRPV1) receptors. In the present study, we sought to determine if anandamide excited cutaneous C nociceptors in vivo and if this excitation correlated with nocifensive behaviors. Using teased-fiber electrophysiological methods in the rat, C nociceptors isolated from the tibial nerve with receptive fields (RFs) on the plantar surface of the hindpaw were studied. Injection of anandamide into the RF dose-dependently excited nociceptors at doses of 10 and 100 microg. The TRPV1 receptor antagonists, capsazepine or SB 366791, were applied to the RF to determine if excitation by anandamide was mediated through TRPV1 receptors. Intraplantar injection of either capsazepine (10 microg) or SB 366791 (3 microg) attenuated the excitation produced by 100 microg anandamide. We also determined whether excitation of C nociceptors by anandamide was associated with nocifensive behaviors. Intraplantar injection of 100 microg anandamide produced nocifensive behaviors that were attenuated by pre-treatment with either capsazepine or SB 366791. Furthermore, we determined if intraplantar injection of anandamide altered withdrawal responses to radiant heat. Neither intraplantar injection of anandamide nor vehicle produced antinociception or hyperalgesia to radiant heat. Our results indicate that anandamide excited cutaneous C nociceptors and produced nocifensive behaviors via activation of TRPV1 receptors.

The endovanilloid/endocannabinoid system in human osteoclasts: possible involvement in bone formation and resorption

Recent studies suggest a role for the endocannabinoid/endovanilloid anandamide in the regulation of bone resorption/formation balance in mice. Here, we examined the co-expression of the transient receptor potential vanilloid type 1 (TRPV1) and the cannabinoid CB1/CB2 receptors together with N-acylphosphatidylethanolamine-hydrolizing phospholipase D (NAPE-PLD) and fatty acid amide hydrolase (FAAH), the two enzymes responsible of the synthesis and catabolism of anandamide respectively, in human osteoclasts. Co-expression of TRPV1, CB1/CB2, NAPE-PLD and FAAH was found in both human osteoclast cultures and in native osteoclasts from human bone biopsies. Moreover, agonist-evoked calcium entry indicated that the TRPV1 receptor is functionally active in vitro. Consistently, biomolecular and functional experiments showed that resiniferatoxin (RTX), a selective TRPV1 receptor agonist, increased the expression and the activity of TRAP and cathepsin K, two specific osteoclast biomarkers. The evidence that cannabinoid and vanilloid receptors are co-expressed in human osteoclasts suggests that they might cross-talk to modulate the intrinsic balance of bone mineralization and resorption by different actions of anandamide through TRPV1 and cannabinoid receptors. The presence of the endocannabinoid/endovanilloid proteins in human osteoclasts will likely have implications for the management of bone demineralization associated syndrome (i. e. osteoporosis).

Deletion of vanilloid receptor (TRPV1) in mice alters behavioral effects of ethanol

The vanilloid receptor TRPV1 is activated by ethanol and this may be important for some of the central and peripheral actions of ethanol. To determine if this receptor has a role in ethanol-mediated behaviors, we studied null mutant mice in which the Trpv1 gene was deleted. Mice lacking this gene showed significantly higher preference for ethanol and consumed more ethanol in a two-bottle choice test as compared with wild type littermates. Null mutant mice showed shorter duration of loss of righting reflex induced by low doses of ethanol (3.2 and 3.4 g/kg) and faster recovery from motor incoordination induced by ethanol (2 g/kg). However, there were no differences between null mutant and wild type mice in severity of ethanol-induced acute withdrawal (4 g/kg) or conditioned taste aversion to ethanol (2.5 g/kg). Two behavioral phenotypes (decreased sensitivity to ethanol-induced sedation and faster recovery from ethanol-induced motor incoordination) seen in null mutant mice were reproduced in wild type mice by injection of a TRPV1 antagonist, capsazepine (10 mg/kg). These two ethanol behaviors were changed in the opposite direction after injection of capsaicin, a selective TRPV1 agonist, in wild type mice. The studies provide the first evidence that TRPV1 is important for specific behavioral actions of ethanol.

Anxiolytic effects in mice of a dual blocker of fatty acid amide hydrolase and transient receptor potential vanilloid type-1 channels

The endocannabinoid-inactivating enzyme, fatty acid amide hydrolase (FAAH), and the transient receptor potential vanilloid type-1 (TRPV1) channel are new targets for the development of anxiolytic drugs. We studied the effect on anxiety-like behavior in the elevated plus maze of a dual FAAH/TRPV1 blocker, N-arachidonoyl-serotonin (AA-5-HT). In male C57BL/6J mice, acute intraperitoneal administration of AA-5-HT (0.1-2.5 mg/kg) increased both the time spent and the number of entries in the open arm, while being inactive at the highest dose tested (5 mg/kg). AA-5-HT was more potent than selective blockers of FAAH or TRPV1 (URB597 and SB366791, respectively). In male Swiss mice, AA-5-HT had to be administered chronically to observe an anxiolytic effect at an intermediate dose (2.5 mg/kg), the highest dose (5 mg/kg) being anxiogenic, and 1 mg/kg being ineffective. In both strains, the anxiolytic effects of AA-5-HT were paralleled by elevation of brain endocannabinoid levels and were reversed by per se inactive doses of the cannabinoid receptors of type-1 (CB(1)) receptor antagonist AM251, or the TRPV1 agonist, olvanil. Immunohistochemical localization of CB(1) and TRPV1 receptors was observed in mouse prefrontal cortex, nucleus accumbens, amygdala, and hippocampus. Simultaneous 'indirect' activation of CB(1) receptors following FAAH inhibition, and antagonism at TRPV1 receptors might represent a new therapeutic strategy against anxiety.

The case for the development of novel analgesic agents targeting both fatty acid amide hydrolase and either cyclooxygenase or TRPV1

Although the dominant approach to drug development is the design of compounds selective for a given target, compounds targeting more than one biological process may have superior efficacy, or alternatively a better safety profile than standard selective compounds. Here, this possibility has been explored with respect to the endocannabinoid system and pain. Compounds inhibiting the enzyme fatty acid amide hydrolase (FAAH), by increasing local endocannabinoid tone, produce potentially useful effects in models of inflammatory and possibly neuropathic pain. Local increases in levels of the endocannabinoid anandamide potentiate the actions of cyclooxygenase inhibitors, raising the possibility that compounds inhibiting both FAAH and cyclooxygenase can be as effective as non-steroidal anti-inflammatory drugs but with a reduced cyclooxygenase inhibitory 'load'. An ibuprofen analogue active in models of visceral pain and with FAAH and cyclooxygenase inhibitory properties has been identified. Another approach, built in to the experimental analgesic compound N-arachidonoylserotonin, is the combination of FAAH inhibitory and transient receptor potential vanilloid type 1 antagonist properties. Although finding the right balance of actions upon the two targets is a key to success, it is hoped that dual-action compounds of the types illustrated in this review will prove to be useful analgesic drugs.

Targeting fatty acid amide hydrolase (FAAH) to treat pain and inflammation

The endogenous cannabinoid N-arachidonoyl ethanolamine (anandamide; AEA) produces most of its pharmacological effects by binding and activating CB(1) and CB(2) cannabinoid receptors within the CNS and periphery. However, the actions of AEA are short lived because of its rapid catabolism by fatty acid amide hydrolase (FAAH). Indeed, FAAH knockout mice as well as animals treated with FAAH inhibitors are severely impaired in their ability to hydrolyze AEA as well as a variety of noncannabinoid lipid signaling molecules and consequently possess greatly elevated levels of these endogenous ligands. In this mini review, we describe recent research that has investigated the functional consequences of inhibiting this enzyme in a wide range of animal models of inflammatory and neuropathic pain states. FAAH-compromised animals reliably display antinociceptive and anti-inflammatory phenotypes with a similar efficacy as direct-acting cannabinoid receptor agonists, such as Delta(9)-tetrahydrocannabinol (THC), the primary psychoactive constituent of Cannabis sativa. Importantly, FAAH blockade does not elicit any apparent psychomimetic effects associated with THC or produce reinforcing effects that are predictive of human drug abuse. The beneficial effects caused by FAAH blockade in these models are predominantly mediated through the activation of CB(1) and/or CB(2) receptors, though noncannabinoid mechanisms of actions can also play contributory or even primary roles. Collectively, the current body of scientific literature suggests that activating the endogenous cannabinoid system by targeting FAAH is a promising strategy to treat pain and inflammation but lacks untoward side effects typically associated with Cannabis sativa.

Constitutive activity at the cannabinoid CB1 receptor is required for behavioral response to noxious chemical stimulation of TRPV1: antinociceptive actions of CB1 inverse agonists

The potential modulation of TRPV1 nociceptive activity by the CB(1) receptor was investigated here using CB(1) wild-type (WT) and knock-out (KO) mice as well as selective CB(1) inverse agonists. No significant differences were detected in baseline thermal thresholds of ICR, CB(1)WT or CB(1)KO mice. Intraplantar capsaicin produced dose- and time-related paw flinch responses in ICR and CB(1)WT mice and induced plasma extravasation yet minimal responses were seen in CB(1)KO animals with no apparent differences in TRPV1 channel expression. Capsaicin-evoked CGRP release from spinal cord tissue and capsaicin-evoked action potentials on isolated skin-nerve preparation were significantly decreased in CB(1)KO mice. Pretreatment with intraplantar galanin and bradykinin, compounds known to sensitize TRPV1 receptors, restored capsaicin-induced flinching in CB(1)KO mice. The possibility that constitutive activity at the CB(1) receptor is required to maintain the TRPV1 receptor in a "sensitized" state was tested using CB(1) inverse agonists. The CB(1) inverse agonists elicited concentration-related inhibition of capsaicin-induced calcium influx in F-11 cells and produced dose-related inhibition of capsaicin-induced flinching in ICR mice. These data suggest that constitutive activity at the CB(1) receptor maintains the TRPV1 channel in a sensitized state responsive to noxious chemical stimuli. Treatment with CB(1) inverse agonists may promote desensitization of the channel resulting in antinociceptive actions against chemical stimulus modalities. These studies propose possible therapeutic exploitation of a novel mechanism providing pain relief by CB(1) inverse agonists.

Chemical synthesis, pharmacological characterization, and possible formation in unicellular fungi of 3-hydroxy-anandamide

The fungal pathogen Candida albicans transforms arachidonic acid (AA) into 3-hydroxyarachidonic acid [3R-HETE], and we investigated if its nonpathogenic and 3R-HETE-producing close relative, Dipodascopsis uninucleata, could similarly transform the endocannabinoid/endovanilloid anandamide into 3-hydroxyanandamide (3-HAEA). We found that D. uninucleata converts anandamide into 3-HAEA, and we therefore developed an enantiodivergent synthesis for this compound to study its pharmacological activity. Both enantiomers of 3-HAEA were as active as anandamide at elevating intracellular Ca2+ via TRPV1 receptors overexpressed in HEK-293 cells, while a approximately 70-90-fold and approximately 45-60-fold lower affinity at cannabinoid CB1 and CB2 receptors was instead observed. Patch clamp recordings showed that 3R-HAEA activates a TRPV1-like current in TRPV1-expressing HEK-293 cells. Thus, 3R-HETE-producing yeasts might convert anandamide released by host cells at the site of infection into 3R-HAEA, and this event might contribute to the inflammatory and algogenous responses associated to fungal diseases.

The antinociceptive interaction of anandamide and adenosine at the spinal level

Both anandamide and adenosine have significant roles in pain mechanisms, but no data are available concerning their interaction at the spinal level. The goal of this study was to determine how adenosine and the adenosine receptor antagonist caffeine affect the antinociceptive effect of anandamide. The pain sensitivity was assessed by the acute tail-flick test and by paw withdrawal test after carrageenan-induced inflammation. The substances were administered intrathecally to male Wistar rats. Anandamide alone (1, 30 and 100 microg) dose-dependently decreased the hyperalgesia, however it had low potency in the tail-flick test. Neither adenosine (100 microg) nor caffeine (400 microg) alone changed the pain sensitivity markedly. Their combination caused a short-lasting antihyperalgesia, but it did not influence the tail-flick latency. Both adenosine and caffeine decreased the antihyperalgesic potential of 100 microg anandamide, while adenosine-caffeine pretreatment temporarily enhanced its effect. As regards acute heat pain sensitivity, no combination with anandamide influenced the effect of anandamide. These findings provide new data concerning the interaction between two endogenous ligands and caffeine. Since these substances may exert effects on several receptors and/or systems, their interaction in vivo must be very complex and the net outcome after their coadministration could not been predicted from the in vitro results.

Gastrointestinal endocannabinoid system: multifaceted roles in the healthy and inflamed intestine

1. The endogenous cannabinoid (endocannabinoid) system is emerging as a key modulator of intestinal physiology, influencing motility, secretion, epithelial integrity and immune function in the gut, in addition to influencing satiety and emesis. 2. Accumulating evidence suggests that the endocannabinoid system may play a pivotal role in the pathophysiology of gastrointestinal disease, particularly in the light of recent studies demonstrating an effect of endocannabinoids on the development of experimental inflammation and linkages with functional clinical disorders characterized by altered motility. 3. The predominant endocannabinoids, anandamide and 2-arachidonoylglycerol, not only mediate their effects via two recognized cannabinoid receptor subtypes, namely CB(1) and CB(2), but emerging evidence now shows they are also substrates for cyclo-oxygenase (COX)-2, generating a distinct and novel class of prostaglandin ethanolamides (prostamides) and prostaglandin glycerol esters. These compounds are bioactive and may mediate an array of biological effects distinct to those of conventional prostanoids. 4. The effects of prostamides on gastrointestinal motility, secretion, sensation and immune function have not been characterized extensively. Prostamides may play an important role in gastrointestinal inflammation, particularly given the enhanced expression of both COX-2 and endocannabinoids that occurs in the inflamed gut. 5. Further preclinical studies are needed to determine the therapeutic potential of drugs targeting the endocannabinoid system in functional and inflammatory gut disorders, to assist with the determination of feasibility for clinical translation.

Dual effect of anandamide on rat placenta nitric oxide synthesis

Anandamide (AEA) has been reported to have pleiotropic effects on reproduction, but the mechanism by which it exerts these effects is unclear. The aim of this study is to characterize rat placental endocannabinoid system and to analyze the possible functional role of AEA in the regulation of NO levels in rat placenta during pregnancy. We found that cannabinoids receptors (CB1 and CB2), FAAH and TRPV1 were expressed in chorio-allantoic placenta. NOS activity peaked at day 13 and decreased with progression of pregnancy. Both exogenous and endogenous AEA significantly decreased NOS activity. Although pre-incubation with AM251 (CB1 antagonist) or AM630 (CB2 antagonist) had no effect, co-incubation with both antagonists induced NOS activity. Furthermore, pre-incubation with exogenous AEA and both antagonists resulted in the induction of placental NOS activity and this effect was reverted with capsazepine (selective TRPV1 antagonist). Additionally, the enhanced NO synthesis caused by capsaicin was abrogated by co-treatment with capsazepine, illustrating that NOS activity could be modulated by TRPV1. Finally, the inhibition of TRPV1 receptor by capsazepine caused a significant fall in NOS activity. These data support the concept that AEA modulates NO levels by two independent pathways: (1) diminishing the NOS activity via CBs; and (2) stimulating NO synthesis via TRPV1. We hypothesized that AEA have an important implication in the normal function of placental tissues.

The role of TRPV1 receptors in the antinociceptive effect of anandamide at spinal level

While it is well known that the endogenous cannabinoid receptor ligand anandamide also activates the transient receptor potential vanilloid1 (TRPV1) receptors, there has been no in vivo study indicating the role of the TRPV1 receptors in the antinociceptive effect of anandamide at spinal level. The goal of this study was to determine the effect of inhibition of TRPV1 receptors by capsazepine on the antinociceptive potency of anandamide after intrathecal administration. Anandamide alone (1, 30 or 100 microg) dose-dependently decreased carrageenan-induced thermal hyperalgesia, however, the highest dose caused temporary excitation and vocalization, suggesting the pain-inducing potential of anandamide. Capsazepine (10 or 20 microg) by itself did not change the pain sensitivity markedly, but the lower dose increased it, and the higher dose decreased the antinociceptive effect of 30 microg anandamide. Furthermore, both doses of capsazepine decreased the efficacy of the largest dose of anandamide. These results show that TRPV1 receptor activation plays a substantial role in the antinociceptive effects of anandamide at spinal level. The effect of the inhibition on TRPV1 receptors depended on the dose applied. We presume that coactivation of the cannabinoid and TRPV1 receptors by anandamide provides elevated antinociception through the release of antinociceptive endogenous ligands at spinal level.

Transient receptor potential vanilloid type 1 channels act as mechanoreceptors and cause substance P release and sensory activation in rat kidneys

Stimulation of capsaicin receptors results in an increase in afferent renal nerve activity (ARNA), but it is unclear how capsaicin contributes to sensory activation intrarenally. Here, we studied the relationships between capsaicin receptor activation, substance P (SP) release, and the sensory response in the rat renal pelvis. Immunoblots showed that one of the capsaicin receptors, transient receptor potential vanilloid type 1 channel (TRPV1), was found in various renal tissues and was especially abundant in the renal pelvis, where most sensory nerve fibers originate. Interestingly, immunolabeling showed colocalization of TRPV1, SP, and the panneuronal marker PGP9.5 in the renal pelvis. Electrophysiological recordings showed that SP and capsaicin activated the same mechanosensitive ARNA in a single-unit preparation. Intrapelvic administration of capsaicin or a specific TRPV1 agonist, resiniferatoxin, resulted in a dose-dependent increase in multi-unit ARNA and SP release, and these effects were blocked by the TRVP1 blocker capsazepine. Inhibition of the SP receptor by L-703,606 largely prevented capsaicin- or resiniferatoxin-induced ARNA. Capsazepine also prevented intrapelvic pressure (IPP)-dependent ARNA activation and contralateral diuresis/natriuresis in the renorenal reflex at an IPP of 20 mmHg, but had no effect at an IPP of 50 mmHg. These data indicate that TRPV1, a low-pressure baroreceptor, is present in the renal pelvis and exclusively regulates neuropeptide release from primary renal afferent C-fibers in response to mechanostimulation.

Actions of 3-methyl-N-oleoyldopamine, 4-methyl-N-oleoyldopamine and N-oleoylethanolamide on the rat TRPV1 receptor in vitro and in vivo

N-oleoyldopamine (OLDA) has been identified as an agonist of the transient receptor potential vanilloid type 1 (TRPV1) receptor. A related fatty acid amide, N-oleoylethanolamide (OEA), was found to excite sensory neurons and produce visceral hyperalgesia via activation of the TRPV1 receptor, however, a recent study described this agent as an antinociceptive one. The aim of the present paper was to characterize two newly synthesized derivatives of N-oleoyldopamine, 3-methyl-N-oleoyldopamine (3-MOLDA) and 4-methyl-N-oleoyldopamine (4-MOLDA) as well as OEA with regard to their effects on the TRPV1 receptor. Radioactive 45Ca2+ uptake was measured in HT5-1 cells transfected with the rat TRPV1 receptor and intracellular Ca2+ concentration was monitored by fura-2 microfluorimetry in cultured trigeminal sensory neurons. Thermonociception was assessed by determining the behavioral noxious heat threshold in rats. 3-MOLDA induced 45Ca2+ uptake in a concentration-dependent manner, whereas 4-MOLDA and OEA were without effect. 4-MOLDA and OEA, however, concentration-dependently reduced the 45Ca2+ uptake-inducing effect of capsaicin. In trigeminal sensory neurons, 3-MOLDA caused an increase in intracellular Ca2+ concentration and this effect exhibited tachyphylaxis upon repeated application. Again, 4-MOLDA and OEA failed to alter intracellular Ca2+ levels. Upon intraplantar injection, 3-MOLDA caused an 8-10 degrees C drop of the noxious heat threshold in rats which was inhibited by the TRPV1 receptor antagonist iodo-resiniferatoxin. 4-MOLDA and OEA failed to alter the heat threshold but inhibited the threshold drop induced by the TRPV1 receptor agonist resiniferatoxin. These data show that 3-MOLDA behaves as an agonist, whereas 4-MOLDA and OEA appear to be antagonists, at the rat TRPV1 receptor.

Methanandamide activation of a novel current in mouse trigeminal ganglion sensory neurons in vitro

Anandamide is an endogenous agonist for cannabinoid receptors and produces analgesia by acting at these receptors in several sites in the brain and peripheral nervous system. Anandamide is also an agonist at the TRPV1 receptor, a protein that serves as an important integrator of noxious stimuli in sensory neurons. Although anandamide actions at CB1 and TRPV1 receptors can explain many of its effects on sensory neurons, some apparently CB1- and TRPV1-independent effects of anandamide have been reported. To explore possible mechanisms underlying these effects we examined the actions of the stable anandamide analog methanandamide on the membrane properties of trigeminal ganglion neurons from mice with TRPV1 deleted. We found that methanandamide and anandamide activate a novel current in a subpopulation of small trigeminal ganglion neurons. Methanandamide activated the current (EC(50) 2 microM) more potently than it activates TRPV1 under the same conditions. The methanandamide-activated current reverses at 0 mV and does not inactivate at positive potentials but declines rapidly at negative membrane potentials. Activation of the current is not mediated via cannabinoid receptors and does not appear to involve G proteins. The phytocannabinoid Delta(9)-tetrahydrocannabinol, the endocannabinoid-related molecules N-arachidonoyl dopamine and N-arachidonoyl glycine and the non-specific TRPV channel activator 2-aminoethoxydiphenyl borate do not mimic the effects of methanandamide. The molecular identity of the current remains to be established, but we have identified a potential new effector for endocannabinoids in sensory neurons, and activation of this current may underlie some of the previously reported CB1 and TRPV1-independent effects of these compounds.

Presynaptic lonotropic receptors

The release of transmitters through vesicle exocytosis from nerve terminals is not constant but is subject to modulation by various mechanisms, including prior activity at the synapse and the presence of neurotransmitters or neuromodulators in the synapse. Instantaneous responses of postsynaptic cells to released transmitters are mediated by ionotropic receptors. In contrast to metabotropic receptors, ionotropic receptors mediate the actions of agonists in a transient manner within milliseconds to seconds. Nevertheless, transmitters can control vesicle exocytosis not only via slowly acting metabotropic, but also via fast acting ionotropic receptors located at the presynaptic nerve terminals. In fact, members of the following subfamilies of ionotropic receptors have been found to control transmitter release: ATP P2X, nicotinic acetylcholine, GABA(A), ionotropic glutamate, glycine, 5-HT(3), andvanilloid receptors. As these receptors display greatly diverging structural and functional features, a variety of different mechanisms are involved in the regulation of transmitter release via presynaptic ionotropic receptors. This text gives an overview of presynaptic ionotropic receptors and briefly summarizes the events involved in transmitter release to finally delineate the most important signaling mechanisms that mediate the effects of presynaptic ionotropic receptor activation. Finally, a few examples are presented to exemplify the physiological and pharmacological relevance of presynaptic ionotropic receptors.

Expression of Cannabinoid CB1 Receptors in Models of Diabetic Neuropathy

A clearer understanding of the mechanisms underlying the development and progression of diabetic neuropathy is likely to indicate new directions for the treatment of this complication of diabetes. In the present study we investigated the expression of cannabinoid CB(1) receptors in models of diabetic neuropathy. PC12 cells were differentiated into a neuronal phenotype with nerve growth factor (NGF) (50 ng/ml) in varying concentrations of glucose (5.5-50 mM). CB(1) receptor expression was studied at the mRNA level by reverse transcriptase-polymerase chain reaction (RT-PCR) and at the protein level via immunohistochemical and Western blot analysis. CB(1) expression was also compared in dorsal root ganglia (DRG) removed from streptozotocin-induced diabetic rats versus control animals. Total neurite length induced by NGF was reduced in cells cultured in 20 to 50 mM glucose at day 6 (P < 0.01 versus 5.5 mM; n = 6). Cell viability assays conducted in parallel on day 6 confirmed that the total cell numbers were not significantly different among the various glucose concentrations (P = 0.86; n = 12). RT-PCR, immunohistochemical, and Western blot analysis all revealed down-regulation of the CB(1) receptor in cells treated with high glucose (P < 0.05; n = 4-5 for each), and in DRG removed from diabetic rats compared with controls (P < 0.01; n = 5 for immunohistochemistry, and n = 3 for Western blot). These results suggest that high glucose concentrations are associated with decreased expression of CB(1) receptors in nerve cells. Given the neuroprotective effect of cannabinoids, a decline in CB(1) receptor expression may contribute to the neurodegenerative process observed in diabetes.

Inhibition of fatty acid amide hydrolase produces analgesia by multiple mechanisms

1 The reversible fatty acid amide hydrolase (FAAH) inhibitor OL135 reverses mechanical allodynia in the spinal nerve ligation (SNL) and mild thermal injury (MTI) models in the rat. The purpose of this study was to investigate the role of the cannabinoid and opioid systems in mediating this analgesic effect. 2 Elevated brain concentrations of anandamide (350 pmol g(-1) of tissue vs 60 pmol g(-1) in vehicle-treated controls) were found in brains of rats given OL135 (20 mg kg(-1)) i.p. 15 min prior to 20 mg kg(-1) i.p. anandamide. 3 Predosing rats with OL135 (2-60 mg kg(-1) i.p.) 30 min before administration of an irreversible FAAH inhibitor (URB597: 0.3 mg kg(-1) intracardiac) was found to protect brain FAAH from irreversible inactivation. The level of enzyme protection was correlated with the OL135 concentrations in the same brains. 4 OL135 (100 mg kg(-1) i.p.) reduced by 50% of the maximum possible efficacy (MPE) mechanical allodynia induced by MTI in FAAH(+/+)mice (von Frey filament measurement) 30 min after dosing, but was without effect in FAAH(-/-) mice. 5 OL135 given i.p. resulted in a dose-responsive reversal of mechanical allodynia in both MTI and SNL models in the rat with an ED(50) between 6 and 9 mg kg(-1). The plasma concentration at the ED(50) in both models was 0.7 microM (240 ng ml(-1)). 6 In the rat SNL model, coadministration of the selective CB(2) receptor antagonist SR144528 (5 mg kg(-1) i.p.), with 20 mg kg(-1) OL135 blocked the OL135-induced reversal of mechanical allodynia, but the selective CB(1) antagonist SR141716A (5 mg kg(-1) i.p.) was without effect. 7 In the rat MTI model neither SR141716A or SR144528 (both at 5 mg kg(-1) i.p.), or a combination of both antagonists coadministered with OL135 (20 mg kg(-1)) blocked reversal of mechanical allodynia assessed 30 min after dosing. 8 In both the MTI model and SNL models in rats, naloxone (1 mg kg(-1), i.p. 30 min after OL135) reversed the analgesia (to 15% of control levels in the MTI model, to zero in the SNL) produced by OL135.