Multiple actions of anandamide on neonatal rat cultured sensory neurones

Dual effect of anandamide on spinal nociceptive transmission in control and inflammatory conditions

Anandamide (AEA) is an important modulator of nociception in the spinal dorsal horn, acting presynaptically through Cannabinoid (CB1) and Transient receptor potential vanilloid (TRPV1) receptors. The role of AEA (1 µM, 10 µM, and 30 µM) application on the modulation of nociceptive synaptic transmission under control and inflammatory conditions was studied by recording miniature excitatory postsynaptic currents (mEPSCs) from neurons in spinal cord slices. Inhibition of the CB1 receptors by PF514273, TRPV1 by SB366791, and the fatty acid amide hydrolase (FAAH) by URB597 was used. Under naïve conditions, the AEA application did not affect the mEPSCs frequency (1.43±0.12 Hz) when all the recorded neurons were considered. The mEPSC frequency increased (180.0±39.2%) only when AEA (30 µM) was applied with PF514273 and URB597. Analysis showed that one sub-population of neurons had synaptic input inhibited (39.1% of neurons), the second excited (43.5%), whereas 8.7% showed a mixed effect and 8.7% did not respond to the AEA. With inflammation, the AEA effect was highly inhibitory (72.7%), while the excitation was negligible (9.1%), and 18.2% were not modulated. After inflammation, more neurons (45.0%) responded even to low AEA by mEPSC frequency increase with PF514273/URB597 present. AEA-induced dual (excitatory/inhibitory) effects at the 1st nociceptive synapse should be considered when developing analgesics targeting the endocannabinoid system. These findings contrast the clear inhibitory effects of the AEA precursor 20:4-NAPE application described previously and suggest that modulation of endogenous AEA production may be more favorable for analgesic treatments.

In Vitro Studies on Therapeutic Effects of Cannabidiol in Neural Cells: Neurons, Glia, and Neural Stem Cells

(‒)-Cannabidiol (CBD) is one of the major phytocannabinoids extracted from the Cannabis genus. Its non-psychoactiveness and therapeutic potential, partly along with some anecdotal—if not scientific or clinical—evidence on the prevention and treatment of neurological diseases, have led researchers to investigate the biochemical actions of CBD on neural cells. This review summarizes the previously reported mechanistic studies of the CBD actions on primary neural cells at the in vitro cell-culture level. The neural cells are classified into neurons, microglia, astrocytes, oligodendrocytes, and neural stem cells, and the CBD effects on each cell type are described. After brief introduction on CBD and in vitro studies of CBD actions on neural cells, the neuroprotective capability of CBD on primary neurons with the suggested operating actions is discussed, followed by the reported CBD actions on glia and the CBD-induced regeneration from neural stem cells. A summary section gives a general overview of the biochemical actions of CBD on neural cells, with a future perspective. This review will provide a basic and fundamental, but crucial, insight on the mechanistic understanding of CBD actions on neural cells in the brain, at the molecular level, and the therapeutic potential of CBD in the prevention and treatment of neurological diseases, although to date, there seem to have been relatively limited research activities and reports on the cell culture-level, in vitro studies of CBD effects on primary neural cells.

The naked mole-rat has a functional purinergic pain pathway despite having a non-functional peptidergic pain pathway

Naked mole-rats (Heterocephalus glaber) have adaptations within their pain pathway that are beneficial to survival in large colonies within poorly ventilated burrow systems, with lower O2 and higher CO2 ambient levels than ground-level environments. These adaptations ultimately lead to a partial disruption of the C-fiber pain pathway, which enables naked mole-rats to not feel pain from the acidosis associated with CO2 accumulation. One hallmark of this disruption is that naked mole-rats do not express neuropeptides, such as Substance P and calcitonin gene-related peptide in their cutaneous C-fibers, effectively making the peptidergic pain pathway hypofunctional. One C-fiber pathway that remains unstudied in the naked mole-rat is the non-peptidergic, purinergic pathway, despite this being a key pathway for inflammatory pain. The current study aimed to establish the functionality of the purinergic pathway in naked mole-rats and the effectiveness of cannabinoids in attenuating pain through this pathway. Cannabinoids can manage chronic inflammatory pain in both humans and mouse models, and studies suggest a major downstream role for the purinergic receptor, P2X3, in this treatment. Here we used Ca2+-imaging of cultured dorsal root ganglion neurons and in vivo behavioral testing to demonstrate that the P2X3 pathway is functional in naked mole-rats. Additionally, formalin-induced inflammatory pain was reduced by the cannabinoid receptor agonist, WIN55 (inflammatory, but not acute phase) and the P2X3 receptor antagonist A-317491 (acute and inflammatory phases). This study establishes that the purinergic C-fiber pathway is present and functional in naked mole-rats and that cannabinoid-mediated analgesia occurs in this species.

Some Prospective Alternatives for Treating Pain: The Endocannabinoid System and Its Putative Receptors GPR18 and GPR55

Background: Marijuana extracts (cannabinoids) have been used for several millennia for pain treatment. Regarding the site of action, cannabinoids are highly promiscuous molecules, but only two cannabinoid receptors (CB₁ and CB₂) have been deeply studied and classified. Thus, therapeutic actions, side effects and pharmacological targets for cannabinoids have been explained based on the pharmacology of cannabinoid CB₁/CB₂ receptors. However, the accumulation of confusing and sometimes contradictory results suggests the existence of other cannabinoid receptors. Different orphan proteins (e.g., GPR18, GPR55, GPR119, etc.) have been proposed as putative cannabinoid receptors. According to their expression, GPR18 and GPR55 could be involved in sensory transmission and pain integration.

Methods: This article reviews select relevant information about the potential role of GPR18 and GPR55 in the pathophysiology of pain.

Results: This work summarized novel data supporting that, besides cannabinoid CB₁ and CB₂ receptors, GPR18 and GPR55 may be useful for pain treatment.

Conclusion: There is evidence to support an antinociceptive role for GPR18 and GPR55.

Advances in the Physiology of GPR55 in the Central Nervous System

Background:

The G protein-coupled receptor 55 (GPR55) is a mammalian orphan receptor that awaits a formal classification. There are an increasing number of reports directed to know the physiology and pathophysiology of this receptor. Lamentably, its functions in the central nervous system (CNS) have been scarcely elucidated.

Methods:

A bibliographic search in PubMed database about GPR55 actions in the CNS was made. The information was grouped for brain structures to facilitate the interpretation. Finally, we constructed a schematic representation of the current knowledge about the potential participation of GPR55 in some physiological and pathophysiological events.

Results:

Seventy nine papers were included in the review. Only few of them showed data about GPR55 (mRNA/protein) expression in multiple brain areas. The rest showed findings in different preparations both in vitro and in vivo conditions that allowed us to speculate a potential activity of GPR55 in the different brain areas.

Conclusion:

GPR55 mRNA is expressed in several brain areas as the hippocampus, hypothalamus, frontal cortex and cerebellum; but due to the lack of information, only some speculative information about its function in these regions has been suggested. Therefore, this review provide relevant information to motivate further research about GPR55 physiology/pathophysiology in the CNS.

Cannabinoid CB1 and CB2 receptors differentially modulate L- and T-type Ca2+ channels in rat retinal ganglion cells

Endocannabinoid signaling system is involved in regulating multiple neuronal functions in the central nervous system by activating G-protein coupled cannabinoid CB1 and CB2 receptors (CB1Rs and CB2Rs). Growing evidence has shown that CB1Rs and CB2Rs are extensively expressed in retinal ganglion cells (RGCs). Here, modulation of L- and T-types Ca²⁺ channels by activating CB1Rs and CB2Rs in RGCs was investigated. Triple immunofluorescent staining showed that L-type subunit Ca_V1.2 was co-localized with T-type subunits (Ca_V3.1, Ca_V3.2 and Ca_V3.3) in rat RGCs. In acutely isolated rat RGCs, the CB1R agonist WIN55212-2 suppressed both peak and steady-state Ca²⁺ currents in a dose-dependent manner, with IC₅₀ being 9.6 μM and 8.4 μM, respectively. It was further shown that activation of CB1Rs by WIN55212-2 or ACEA, another CB1R agonist, significantly suppressed both L- and T-type Ca²⁺ currents, and shifted inactivation curve of T-type one toward hyperpolarization direction. While the effect on L-type Ca²⁺ channels was mediated by intracellular cAMP/protein kinase A (PKA), mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) and calcium/calmodulin-dependent protein kinase II (CaMKII) signaling pathways, only CaMKII signaling pathway was involved in the effect on T-type Ca²⁺ channels. Furthermore, CB65 and HU308, two specific CB2R agonists, significantly suppressed T-type Ca²⁺ channels, which was mediated by intracellular cAMP/PKA and CaMKII signaling pathways, but had no effect on L-type channels. These results imply that endogenous cannabinoids may modulate the excitability and the output of RGCs by differentially suppressing the activity of L- and T-type Ca²⁺ channels through activation of CB1Rs and CB2Rs. This article is part of the Special Issue entitled "A New Dawn in Cannabinoid Neurobiology".

Regulatory effects of anandamide on intracellular Ca(2+) concentration increase in trigeminal ganglion neurons

Activation of cannabinoid receptor type 1 on presynaptic neurons is postulated to suppress neurotransmission by decreasing Ca²⁺ influx through high voltage-gated Ca²⁺ channels. However, recent studies suggest that cannabinoids which activate cannabinoid receptor type 1 can increase neurotransmitter release by enhancing Ca²⁺ influx in vitro. The aim of the present study was to investigate the modulation of intracellular Ca²⁺ concentration by the cannabinoid receptor type 1 agonist anandamide, and its underlying mechanisms. Using whole cell voltage-clamp and calcium imaging in cultured trigeminal ganglion neurons, we found that anandamide directly caused Ca²⁺ influx in a dose-dependent manner, which then triggered an increase of intracellular Ca²⁺ concentration. The cyclic adenosine and guanosine monophosphate-dependent protein kinase systems, but not the protein kinase C system, were involved in the increased intracellular Ca²⁺ concentration by anandamide. This result showed that anandamide increased intracellular Ca²⁺ concentration and inhibited high voltage-gated Ca²⁺ channels through different signal transduction pathways.

Activation of CB1 inhibits NGF-induced sensitization of TRPV1 in adult mouse afferent neurons

Transient receptor potential vanilloid 1 (TRPV1)-containing afferent neurons convey nociceptive signals and play an essential role in pain sensation. Exposure to nerve growth factor (NGF) rapidly increases TRPV1 activity (sensitization). In the present study, we investigated whether treatment with the selective cannabinoid receptor 1 (CB1) agonist arachidonyl-2'-chloroethylamide (ACEA) affects NGF-induced sensitization of TRPV1 in adult mouse dorsal root ganglion (DRG) afferent neurons. We found that CB1, NGF receptor tyrosine kinase A (trkA), and TRPV1 are present in cultured adult mouse small- to medium-sized afferent neurons and treatment with NGF (100ng/ml) for 30 min significantly increased the number of neurons that responded to capsaicin (as indicated by increased intracellular Ca(2 +) concentration). Pretreatment with the CB1 agonist ACEA (10nM) inhibited the NGF-induced response, and this effect of ACEA was reversed by a selective CB1 antagonist. Further, pretreatment with ACEA inhibited NGF-induced phosphorylation of AKT. Blocking PI3 kinase activity also attenuated the NGF-induced increase in the number of neurons that responded to capsaicin. Our results indicate that the analgesic effect of CB1 activation may in part be due to inhibition of NGF-induced sensitization of TRPV1 and also that the effect of CB1 activation is at least partly mediated by attenuation of NGF-induced increased PI3 signaling.

Anandamide produced by Ca(2+)-insensitive enzymes induces excitation in primary sensory neurons

The endogenous lipid agent N-arachidonoylethanolamine (anandamide), among other effects, has been shown to be involved in nociceptive processing both in the central and peripheral nervous systems. Anandamide is thought to be synthesised by several enzymatic pathways both in a Ca(2+)-sensitive and Ca(2+)-insensitive manner, and rat primary sensory neurons produce anandamide. Here, we show for the first time, that cultured rat primary sensory neurons express at least four of the five known Ca(2+)-insensitive enzymes implicated in the synthesis of anandamide, and that application of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-arachidonoyl, the common substrate of the anandamide-synthesising pathways, results in anandamide production which is not changed by the removal of extracellular Ca(2+). We also show that anandamide, which has been synthesised in primary sensory neurons following the application of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-arachidonoyl induces a transient receptor potential vanilloid type 1 ion channel-mediated excitatory effect that is not inhibited by concomitant activation of the cannabinoid type 1 receptor. Finally, we show that sub-populations of transient receptor potential vanilloid type 1 ion channel-expressing primary sensory neurons also express some of the putative Ca(2+)-insensitive anandamide-synthesising enzymes. Together, these findings indicate that anandamide synthesised by primary sensory neuron via a Ca(2+)-insensitive manner has an excitatory rather than an inhibitory role in primary sensory neurons and that excitation is mediated predominantly through autocrine signalling. Regulation of the activity of the Ca(2+)-insensitive anandamide-synthesising enzymes in these neurons may be capable of regulating the activity of these cells, with potential relevance to controlling nociceptive processing.

Anandamide reduces intracellular Ca2+ concentration through suppression of Na+/Ca2+ exchanger current in rat cardiac myocytes

Purpose

Anandamide, one of the endocannabinoids, has been reported to exhibit cardioprotective properties, particularly in its ability to limit the damage produced by ischemia reperfusion injury. However, the mechanisms underlying the effect are not well known. This study is to investigate whether anandamide alter Na⁺/Ca²⁺ exchanger and the intracellular free Ca²⁺ concentration ([Ca²⁺]_i).

Methods

Na⁺/Ca²⁺ exchanger current (I_NCX) was recorded and analysed by using whole-cell patch-clamp technique and [Ca²⁺]_i was measured by loading myocytes with the fluorescent Ca²⁺ indicator Fura-2/AM.

Results

We found that I_NCX was enhanced significantly after perfusion with simulated ischemic external solution; [Ca²⁺]_i was also significantly increased by simulated ischemic solution. The reversal potential of I_NCX was shifted to negative potentials in simulated ischemic external solution. Anandamide (1–100 nM) failed to affect I_NCX and [Ca²⁺]_i in normal solution. However, anandamide (1–100 nM) suppressed the increase in I_NCX in simulated ischemic external solution concentration-dependently and normalized I_NCX reversal potential. Furthermore, anandamide (100 nM) significantly attenuated the increase in [Ca²⁺]_i in simulated ischemic solution. Blocking CB1 receptors with the specific antagonist AM251 (500 nM) failed to affect the effects of anandamide on I_NCX and [Ca²⁺]_i in simulated ischemic solution. CB2 receptor antagonist AM630 (100 nM) eliminated the effects of anandamide on I_NCX and [Ca²⁺]_i in simulated ischemic solution, and CB2 receptor agonist JWH133 (100 nM) simulated the effects of anandamide that suppressed the increase in I_NCX and [Ca²⁺]_i in simulated ischemic solution. In addition, pretreatment with the Gi/o-specific inhibitor pertussis toxin (PTX, 500 ng/ml) eliminated the effects of anandamide and JWH133 on I_NCX in simulated ischemic solution.

Conclusions

Collectively, these findings suggest that anandamide suppresses calcium overload through inhibition of I_NCX during perfusion with simulated ischemic solution; the effects may be mediated by CB2 receptor via PTX-sensitive Gi/o proteins. This mechanism is importantly involved in the anti-ischemia injury caused by endocannabinoids.

Off-target cannabinoid effects mediated by GPR55

Given the vast therapeutic potential of the endocannabinoid system, the revelation of a novel cannabinoid-sensitive target was treated with great excitement. The orphan G-protein coupled receptor 55 (GPR55) was initially touted as a novel cannabinoid target in early industrial patent literature. Consequently, numerous studies have revealed GPR55 expression in a diverse array of cells and tissues, regulating various physiological and pathological processes. Although a confusing cannabinoid profile has prevented its classification as a cannabinoid receptor, the therapeutic potential of the receptor cannot be denied, with roles in cancer progression, bone resorption and analgesia. This commentary aims to summarize GPR55 expression data and speculate on potential therapeutic exploitation of this enigmatic orphan receptor.

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.

Arachidonoyl ethanolamide (AEA)-induced apoptosis is mediated by J-series prostaglandins and is enhanced by fatty acid amide hydrolase (FAAH) blockade

The endocannabinoid arachidonoyl ethanolamide (AEA) is a potent inducer of tumor cell apoptosis however its mechanism of cytotoxicity is unclear. A previous report from our laboratory showed that AEA induced cell death in a cyclooxygenase-2 (COX-2)-dependent manner and in this report our data indicate that AEA-induced apoptosis is mediated by COX-2 metabolic products of the J-series. In experiments conducted with JWF2 keratinocytes which over-express COX-2, AEA caused a concentration-regulated increase in J-series prostaglandin production and apoptosis. Similarly, cell treatment with exogenously added J-series prostaglandins (15-deoxy, Δ(12,14) PGJ(2) and PGJ(2)) induced apoptosis. AEA-induced apoptosis was inhibited by the antioxidant, N-acetyl cysteine, indicating that reactive oxygen species generation was required for apoptosis. Using antagonists of cannabinoid receptor 1, cannabinoid receptor 2, or transient receptor potential cation channel, subfamily V, member 1, it was observed that cannabinoid receptor inhibition did not block AEA-mediated cell death. In contrast, an inhibitor of fatty acid amide hydrolase (FAAH) potentiated AEA-induced J-series PG synthesis and apoptosis. These results suggest that the metabolism of AEA to J-series PGs regulates the induction of apoptosis in cells with elevated COX-2 levels. Our data further indicate that the proapoptotic activity of AEA can be enhanced by combining it with an inhibitor of FAAH. As such, AEA may be an effective agent to eliminate tumor cells that over-express COX-2.

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.

Inactivation of Anandamide Signaling: A Continuing Debate

Since the first endocannabinoid anandamide was identified in 1992, extensive research has been conducted to characterize the elements of the tightly controlled endocannabinoid signaling system. While it was established that the activity of endocannabinoids are terminated by a two-step process that includes cellular uptake and degradation, there is still a continuing debate about the mechanistic role of these processes in inactivating anandamide signals.

The endocannabinoid anandamide inhibits potassium conductance in rat cortical astrocytes

Endocannabinoids are a family of endogenous signaling molecules that modulate neuronal excitability in the central nervous system (CNS) by interacting with cannabinoid (CB) receptors. In spite of the evidence that astroglial cells also possess CB receptors, there is no information on the role of endocannabinoids in regulating CNS function through the modulation of ion channel-mediated homeostatic mechanisms in astroglial cells. We provide electrophysiological evidence that the two brain endocannabinoids anandamide (AEA) and 2-arachidonylglycerol (2-AG) markedly depress outward conductance mediated by delayed outward rectifier potassium current (IK(DR)) in primary cultured rat cortical astrocytes. Pharmacological experiments suggest that the effect of AEA does not result from the activation of known CB receptors. Moreover, neither the production of AEA metabolites nor variations in free cytosolic calcium are involved in the negative modulation of IK(DR). We show that the action of AEA is mediated by its interaction with the extracellular leaflet of the plasma membrane. Similar experiments performed in situ in cortical slices indicate that AEA downregulates IK(DR) in complex and passive astroglial cells. Moreover, IK(DR) is also inhibited by AEA in NG2 glia. Collectively, these results support the notion that endocannabinoids may exert their modulation of CNS function via the regulation of homeostatic function of the astroglial syncytium mediated by ion channel activity.

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.

A decrease in anandamide signaling contributes to the maintenance of cutaneous mechanical hyperalgesia in a model of bone cancer pain

Tumors in bone are associated with pain in humans. Data generated in a murine model of bone cancer pain suggest that a disturbance of local endocannabinoid signaling contributes to the pain. When tumors formed after injection of osteolytic fibrosarcoma cells into the calcaneus bone of mice, cutaneous mechanical hyperalgesia was associated with a decrease in the level of anandamide (AEA) in plantar paw skin ipsilateral to tumors. The decrease in AEA occurred in conjunction with increased degradation of AEA by fatty acid amide hydrolase (FAAH). Intraplantar injection of AEA reduced the hyperalgesia, and intraplantar injection of URB597, an inhibitor of FAAH, increased the local level of AEA and also reduced hyperalgesia. An increase in FAAH mRNA and enzyme activity in dorsal root ganglia (DRG) L3-L5 ipsilateral to the affected paw suggests DRG neurons contribute to the increased FAAH activity in skin in tumor-bearing mice. Importantly, the anti-hyperalgesic effects of AEA and URB597 were blocked by a CB1 receptor antagonist. Increased expression of CB1 receptors by DRG neurons ipsilateral to tumor-bearing limbs may contribute to the anti-hyperalgesic effect of elevated AEA levels. Furthermore, CB1 receptor protein-immunoreactivity as well as inhibitory effects of AEA and URB597 on the depolarization-evoked Ca(2+) transient were increased in small DRG neurons cocultured with fibrosarcoma cells indicating that fibrosarcoma cells are sufficient to evoke phenotypic changes in AEA signaling in DRG neurons. Together, the data provide evidence that manipulation of peripheral endocannabinoid signaling is a promising strategy for the management of bone cancer pain.

Modulation of sensory neuron potassium conductances by anandamide indicates roles for metabolites

BACKGROUND AND PURPOSE

The endogenous cannabinoid anandamide (AEA) acts at cannabinoid (CB(1)) and vanilloid (TRPV(1)) receptors. AEA also shows antinociceptive properties; although the underlying mechanism for this is not fully understood, both CB(1) and TRPV(1) may be involved. Voltage-activated Ca(2+) channels in rat-cultured dorsal root ganglion (DRG) neurons are modulated by AEA. However, AEA in different populations of neurons enhanced or attenuated KCl-evoked Ca(2+) influx; these effects were linked with soma size. The aim of this study was to determine how AEA or its metabolites might produce these variable responses.

EXPERIMENTAL APPROACH

The whole cell patch-clamp technique and fura-2 Ca(2+) imaging were used to characterize the actions of AEA on action potential firing and voltage-activated K(+) currents and to determine whether AEA metabolism plays any role in its effects on cultured DRG neurons.

KEY RESULTS

AEA attenuated multiple action potential firing evoked by 300 ms depolarizing current commands in a subpopulation of DRG neurons. Application of 1 microM AEA attenuated voltage-activated K(+) currents and the recovery of KCl-evoked Ca(2+) transients. The insensitivity of these responses to the CB(1) receptor antagonist rimonabant (100 nM) and preincubation of DRG neurons with pertussis toxin suggested that these actions are not CB(1) receptor-mediated. Preincubating DRG neurons with the fatty acid amide hydrolase (FAAH) inhibitor phenylmethylsulphonyl fluoride (PMSF) attenuated the inhibitory actions of AEA on K(+) currents and Ca(2+) influx.

CONCLUSION AND IMPLICATIONS

These data suggest that the products of AEA metabolism by FAAH contribute to the attenuation of K(+) conductances and altered excitability of cultured sensory neurons.

The complications of promiscuity: endocannabinoid action and metabolism

In this review, we present our understanding of the action and metabolism of endocannabinoids and related endogenous molecules. It is clear that the interactions between the multiple endocannabinoid-like molecules (ECLs) are highly complex, both at the level of signal transduction and metabolism. Thus, ECLs are a group of ligands active at 7-transmembrane and nuclear receptors, as well as transmitter-gated and ion channels. ECLs and their metabolites can converge on common endpoints (either metabolic or signalling) through contradictory or reinforcing pathways. We highlight the complexity of the endocannabinoid system, based on the promiscuous nature of ECLs and their metabolites, as well as the synthetic modulators of the endocannabinoid system.

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.

Inhibition of [3H]batrachotoxinin A-20alpha-benzoate binding to sodium channels and sodium channel function by endocannabinoids

A number of putative endocannabinoids were found to modify the binding of [(3)H]batrachotoxinin A-20alpha-benzoate ([(3)H]BTX-B) to site 2 on voltage-gated sodium channels of mouse brain and achieve functional inhibition of sodium channels in vitro. 2-Arachidonoyl-glycerol (2-AG), arachidonoyl glycerol ether (AGE), N-arachidonoyl-dopamine (NADA) gave almost complete inhibition of [(3)H]BTX-B binding with IC(50) values of 90.4, 51.2 and 20.7 microM, respectively. The CB1 receptor antagonist AM251 (2 microM) had no effect on the displacement of radioligand by these endocanabinoids. Arachidonoyl-glycine (A-Gly) and arachidonoyl-GABA (A-GABA) were apparently less effective inhibitors of [(3)H]BTX-B binding giving 14.8+/-2.2 and 23.9+/-4.8% inhibition at 100 microM. Phenylmethanesulphonylfluoride (PMSF) did not alter the inhibitory effects of 2-AG, AGE, NADA and A-Gly on binding, but the efficacy of 100 microM A-GABA was increased by 60.3+/-6.3% (P<0.05). Scatchard analyses showed that 2-AG, AGE and NADA reduce the binding of [(3)H]BTX-B by lowering B(max) although increases in K(D) were also evident for AGE and NADA. Our kinetic experiments found that 2-AG, AGE and NADA increase the dissociation velocity of radioligand from site 2 on sodium channels demonstrating that these endocannabinoids operate as allosteric inhibitors of [(3)H]BTX-B binding. 2-AG, AGE and NADA inhibited veratridine-dependent (TTX-suppressible) depolarization of the plasma membrane of synaptoneurosomes at low micromolar concentrations and again the capacities of A-Gly and A-GABA to inhibit this response were less pronounced. The three most effective endocannabinoids (2-AG, AGE and NADA) were then examined in a synaptosomal transmitter release assay where they were observed to inhibit sodium channel- (veratridine-dependent) release of l-glutamate and GABA in the low micromolar range. These effects also occurred through a mechanism that was not influenced by 2 microM AM251. It is concluded that direct inhibition of sodium channel function leading to reduced neuronal excitation and depression of presynaptic release of amino acid transmitters is a property shared by several endocannabinoids.

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.

Interactions of cannabidiol with endocannabinoid signalling in hippocampal tissue

The phytocannabinoid cannabidiol (CBD) possesses no psychotropic activity amid potentially beneficial therapeutic applications. We here characterized interactions between CBD (1 microM) and the endocannabinoid system in cultured rat hippocampal cells. The CBD-induced Ca2+ rise observed in neurons and glia was markedly reduced in the presence of the endogenous cannabinoid anandamide in neurons, with no alteration seen in glia. Neuronal CBD responses were even more reduced in the presence of the more abundant endocannabinoid 2-arachidonyl glycerol, this action was maintained in the presence of the CB1 receptor antagonist AM281 (100 nM). Neuronal CBD responses were also reduced by pre-exposure to glutamate, expected to increase endocannabinoid levels by increasing in [Ca2+]i. Application of AM281 at 1 microM elevated CBD-induced Ca2+ responses in both cell types, further confirming our finding that endocannabinoid-mediated signalling is negatively coupled to the action of CBD. However, upregulation of endogenous levels of endocannabinoids via inhibition of endocannabinoid hydrolysis (with URB597 and MAFP) could not be achieved under resting conditions. Because delta9-tetrahydrocannabinol did not mimic the endocannabinoid actions, and pertussis toxin treatment had no effect on CBD responses, we propose that the effects of AM281 were mediated via a constitutively active signalling pathway independent of CB1 signalling. Instead, signalling via G(q/11) and phospholipase C appears to be negatively coupled to CBD-induced Ca2+ responses, as the inhibitor U73122 enhanced CBD responses. Our data highlight the interaction between exogenous and endogenous cannabinoid signalling, and provide evidence for the presence of an additional pharmacological target, sensitive to endocannabinoids and to AM281.

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.

A pyridinium derivative from Red Sea soft corals inhibited voltage-activated potassium conductances and increased excitability of rat cultured sensory neurones

BACKGROUND

Whole cell patch clamp recording and intracellular Ca2+ imaging were carried out on rat cultured dorsal root ganglion (DRG) neurones to characterize the actions of crude extracts and purified samples from Red Sea soft corals. The aim of the project was to identify compounds that would alter the excitability of DRG neurones.

RESULTS

Crude extracts of Sarcophyton glaucum and Lobophyton crassum attenuated spike frequency adaptation causing DRG neurones to switch from firing single action potentials to multiple firing. The increase in excitability was associated with enhanced KCl-evoked Ca2+ influx. The mechanism of action of the natural products in the samples from the soft corals involved inhibition of voltage-activated K+ currents. An active component of the crude marine samples was identified as 3-carboxy-1-methyl pyridinium (trigonelline). Application of synthetic 3-carboxy-1-methyl pyridinium at high concentration (0.1 mM) also induced multiple firing and reduced voltage-activated K+ current. The changes in excitability of DRG neurones induced by 3-carboxy-1-methyl pyridinium suggest that this compound contributes to the bioactivity produced by the crude extracts from two soft corals.

CONCLUSION

Sarcophyton glaucum and Lobophyton crassum contain natural products including 3-carboxy-1-methyl pyridinium that increase the excitability of DRG neurones. We speculate that in addition to developmental control and osmoregulation these compounds may contribute to chemical defenses.

Receptor-independent actions of cannabinoids on cell membranes: Focus on endocannabinoids

Cannabinoids are a structurally diverse group of mostly lipophilic molecules that bind to cannabinoid receptors. In fact, endogenous cannabinoids (endocannabinoids) are a class of signaling lipids consisting of amides and esters of long-chain polyunsaturated fatty acids. They are synthesized from lipid precursors in plasma membranes via Ca(2+) or G-protein-dependent processes and exhibit cannabinoid-like actions by binding to cannabinoid receptors. However, endocannabinoids can produce effects that are not mediated by these receptors. In pharmacologically relevant concentrations, endocannabinoids modulate the functional properties of voltage-gated ion channels including Ca(2+) channels, Na(+) channels, various types of K(+) channels, and ligand-gated ion channels such as serotonin type 3, nicotinic acetylcholine, and glycine receptors. In addition, modulatory effects of endocannabinoids on other ion-transporting membrane proteins such as transient potential receptor-class channels, gap junctions and transporters for neurotransmitters have also been demonstrated. Furthermore, functional properties of G-protein-coupled receptors for different types of neurotransmitters and neuropeptides are altered by direct actions of endocannabinoids. Although the mechanisms of these effects are currently not clear, it is likely that these direct actions of endocannabinoids are due to their lipophilic structures. These findings indicate that additional molecular targets for endocannabinoids exist and that these targets may represent novel sites for cannabinoids to alter either the excitability of the neurons or the response of the neuronal systems. This review focuses on the results of recent studies indicating that beyond their receptor-mediated effects, endocannabinoids alter the functions of ion channels and other integral membrane proteins directly.

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.

Acute actions of marine toxin latrunculin A on the electrophysiological properties of cultured dorsal root ganglion neurones

The effects of latrunculin A, isolated from the nudibranch Chromodoris sp., on the excitability of neonatal rat cultured dorsal root ganglion neurones were investigated using patch-clamp recording and Ca(2+) imaging techniques. Under current-clamp conditions, acute application of latrunculin A (100 microM) reversibly induced multiple action potential firing and significantly increased action potential duration. No significant effects on action potential peak amplitude, threshold of action potential firing, resting membrane potential and input resistance were observed. Under voltage-clamp conditions, significant and dose-dependent suppression of K(+) current was seen with 10-100 microM latrunculin A. Additionally, a significant difference between inhibition of the current measured at the peak and the end of a 100 ms voltage step was seen with 100 microM latrunculin A. Fura-2 fluorescence Ca(2+) imaging revealed that latrunculin A (100 microM) significantly inhibited Ca(2+) transients evoked by KCl-induced depolarisation in all neurones. In 36% of DRG neurones, latrunculin A alone had no effect on intracellular Ca(2+). In 64% of neurones, latrunculin A alone evoked a transient rise in intracellular Ca(2+). Moreover, latrunculin A (10-100 microM) significantly inhibited the mean high voltage-activated Ca(2+) current. The effects of latrunculin A on action potential firing and K(+) currents were attenuated by intracellular phalloidin, an indication that these effects are mediated through actin disruption.

Anandamide as an intracellular messenger regulating ion channel activity

The endocannabinoid anandamide (N-arachidonoylethanolamine) was proposed to be an extracellular retrograde messenger, which regulates excitability of neurons by cannabinoid CB1 receptor-dependent inhibition of neurotransmitter release. Recent findings indicate that the neuromodulatory actions of anandamide might be more complex. Anandamide has been shown to directly modulate various ion channels, such as alpha7-nicotinic acetylcholine receptors, T-type Ca2+ channels, voltage-gated and background K+-channels and Transient Receptor Potential Vanilloid type 1 (TRPV1) channels. The binding site of anandamide at some of these ion channels appears to be intracellular or at the bilayer interface. This rises the intriguing possibility that anandamide, prior to its release into the synaptic cleft, may regulate ion homeostasis and excitability of neurons as an intracellular modulator of ion channels independent of its action at cannabinoid CB1 receptors. This possibility might extend the concept of anandamide as an endocannabinoid retrograde messenger and may have profound implications for its role in neurotransmission and neuronal function. Here, we will review the evidence for this hypothesis.

In vitro photo-release of a TRPV1 agonist

Intracellular photolysis of a novel 'caged' capsaicin analogue results in in vitro activation of the capsaicin receptor TRPV1.

Novel compounds that interact with both leukotriene B4 receptors and vanilloid TRPV1 receptors

The aim of this study was to investigate the interaction of a series of novel compounds with leukotriene B(4) receptors (BLT) and vanilloid receptor (TRPV1). First, we characterized leukotriene B(4) (LTB(4)) ethanolamide. In guinea pig isolated lung parenchyma, LTB(4) ethanolamide antagonized the contractile action of LTB(4) with an apparent K(B) value of 7.28 nM. Using a Boyden chamber assay, we demonstrated that this compound stimulated human neutrophil migration in a similar manner to LTB(4) but with lower efficacy. In rat TRPV1 (rTRPV1)-expressing Chinese hamster ovary (CHO) cells and dorsal root ganglion (DRG) neurons, LTB(4) and LTB(4) ethanolamide acted as low-efficacy agonists, increasing intracellular calcium concentration ([Ca(2+)](i)) in a capsazepine-sensitive manner. These results prompted us to hypothesize that a molecule may possess pharmacophores such that it is capable of dual antagonism of BLT and TRPV1 receptors. Two novel compounds, N-[2-fluoro-4-[3-(11 hydroxyheptadec-8-enyl)-thioureiomethyl]-phenyl]-methanesulfonamide (O-3367) and N-[4-[3-(11 hydroxyheptadec-8-enyl)-thioureio-methyl]-phenyl]-methanesulfonamide (O-3383), were synthesized. In human neutrophils, both compounds acted as antagonists, significantly attenuating the BLT receptor-mediated ability of LTB(4) to induce migration, with pIC(50) values of 7.22 +/- 0.17 and 5.95 +/- 0.16, respectively. In rTRPV1-expressing CHO cells, they caused a significant rightward shift in the log concentration-response curve for the TRPV1 receptor agonist capsaicin (3-methoxy-4-hydroxy)benzyl-8-methyl-6-nonenamide). In DRG neurons O-3367 significantly attenuated the capsaicin-induced increases in [Ca(2+)](i) with a pIC(50) value of 5.94 +/- 0.004. O-3367 and O-3383 represent novel structural templates for generating compounds possessing dual antagonism at BLT and TRPV1 receptors. In view of the crucial role of both TRPV1 and BLT receptors in the pathophysiology of inflammatory conditions, such compounds may betoken a novel class of highly effective therapeutics.

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.

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.

Endogenous unsaturated C18 N-acylethanolamines are vanilloid receptor (TRPV1) agonists

The endogenous C18 N-acylethanolamines (NAEs) N-linolenoylethanolamine (18:3 NAE), N-linoleoylethanolamine (18:2 NAE), N-oleoylethanolamine (18:1 NAE), and N-stearoylethanolamine (18:0 NAE) are structurally related to the endocannabinoid anandamide (20:4 NAE), but these lipids are poor ligands at cannabinoid CB(1) receptors. Anandamide is also an activator of the transient receptor potential (TRP) vanilloid 1 (TRPV(1)) on primary sensory neurons. Here we show that C18 NAEs are present in rat sensory ganglia and vascular tissue. With the exception of 18:3 NAE in rat sensory ganglia, the levels of C18 NAEs are equal to or substantially exceed those of anandamide. At submicromolar concentrations, 18:3 NAE, 18:2 NAE, and 18:1 NAE, but not 18:0 NAE and oleic acid, activate native rTRPV(1) on perivascular sensory nerves. 18:1 NAE does not activate these nerves in TRPV(1) gene knock-out mice. Only the unsaturated C18 NAEs elicit whole cell currents and fluorometric calcium responses in HEK293 cells expressing hTRPV(1). Molecular modeling revealed a low energy cluster of U-shaped unsaturated NAE conformers, sharing several pharmacophoric elements with capsaicin. Furthermore, one of the two major low energy conformational families of anandamide also overlaps with the cannabinoid CB(1) receptor ligand HU210, which is in line with anandamide being a dual activator of TRPV(1) and the cannabinoid CB(1) receptor. This study shows that several endogenous non-cannabinoid NAEs, many of which are more abundant than anandamide in rat tissues, activate TRPV(1) and thus may play a role as endogenous TRPV(1) modulators.

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 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.