The endogenous cannabinoid agonist, anandamide stimulates sensory nerves in guinea-pig airways

The endocannabinoid system and breathing

Recent changes in cannabis accessibility have provided adjunct therapies for patients across numerous disease states and highlights the urgency in understanding how cannabinoids and the endocannabinoid (EC) system interact with other physiological structures. The EC system plays a critical and modulatory role in respiratory homeostasis and pulmonary functionality. Respiratory control begins in the brainstem without peripheral input, and coordinates the preBötzinger complex, a component of the ventral respiratory group that interacts with the dorsal respiratory group to synchronize burstlet activity and drive inspiration. An additional rhythm generator: the retrotrapezoid nucleus/parafacial respiratory group drives active expiration during conditions of exercise or high CO₂. Combined with the feedback information from the periphery: through chemo- and baroreceptors including the carotid bodies, the cranial nerves, stretch of the diaphragm and intercostal muscles, lung tissue, and immune cells, and the cranial nerves, our respiratory system can fine tune motor outputs that ensure we have the oxygen necessary to survive and can expel the CO₂ waste we produce, and every aspect of this process can be influenced by the EC system. The expansion in cannabis access and potential therapeutic benefits, it is essential that investigations continue to uncover the underpinnings and mechanistic workings of the EC system. It is imperative to understand the impact cannabis, and exogenous cannabinoids have on these physiological systems, and how some of these compounds can mitigate respiratory depression when combined with opioids or other medicinal therapies. This review highlights the respiratory system from the perspective of central versus peripheral respiratory functionality and how these behaviors can be influenced by the EC system. This review will summarize the literature available on organic and synthetic cannabinoids in breathing and how that has shaped our understanding of the role of the EC system in respiratory homeostasis. Finally, we look at some potential future therapeutic applications the EC system has to offer for the treatment of respiratory diseases and a possible role in expanding the safety profile of opioid therapies while preventing future opioid overdose fatalities that result from respiratory arrest or persistent apnea.

Bradykinin and capsaicin induced airways obstruction in the guinea pig are platelet dependent

INTRODUCTION

Airways obstruction induced by intravenously administered bradykinin is abolished in guinea pigs treated with indomethacin, which has been shown to be, at least in part thromboxane dependent. As thromboxane is primarily generated from circulating platelets, we investigated whether airways obstruction induced by bradykinin, and other spasmogens, is platelet dependent and the role platelet aggregation played in this response.

METHODS

Guinea pigs were chronically treated with busulfan to induce thrombocytopenia. Total lung resistance was measured in anaesthetised and mechanically ventilated control and thrombocytopaenic animals to various stimuli that induce airways obstruction. In other experiments, platelet aggregation was assessed in vitro in response to the same stimuli: guinea pigs were anaesthetized, blood was collected and centrifuged to generate firstly platelet-rich plasma and then platelet-poor plasma. Platelets were resuspended in HEPES buffer and platelet aggregation was assessed.

RESULTS

Busulfan treatment significantly reduced the number of circulating platelets in guinea-pigs by 85.5%, but had no significant effect on the number of circulating leukocytes. Treatment with busulfan had no significant effect on bronchoconstriction induced by the direct acting spasmogens histamine or methacholine. However, platelet depletion significantly increased airways obstruction induced by Substance P, but caused a significant reduction in airways obstruction induced by bradykinin, bombesin or capsaicin (P < 0.05). None of these stimuli however were able to exhibit a direct effect on platelet aggregation in vitro. Moreover, busulfan did not significantly alter the contractility of guinea-pig isolated trachea in response to capsaicin.

CONCLUSION

Airways obstruction induced by bombesin, capsaicin and bradykinin is platelet dependent, but not secondary to platelet aggregation.

Exposure to Allergen Causes Changes in NTS Neural Activities after Intratracheal Capsaicin Application, in Endocannabinoid Levels and in the Glia Morphology of NTS

Allergen exposure may induce changes in the brainstem secondary neurons, with neural sensitization of the nucleus solitary tract (NTS), which in turn can be considered one of the causes of the airway hyperresponsiveness, a characteristic feature of asthma. We evaluated neurofunctional, morphological, and biochemical changes in the NTS of naive or sensitized rats. To evaluate the cell firing activity of NTS, in vivo electrophysiological experiments were performed before and after capsaicin challenge in sensitized or naive rats. Immunohistochemical studies, endocannabinoid, and palmitoylethanolamide quantification in the NTS were also performed. This study provides evidence that allergen sensitization in the NTS induced: (1) increase in the neural firing response to intratracheal capsaicin application, (2) increase of endocannabinoid anandamide and palmitoylethanolamide, a reduction of 2-arachidonoylglycerol levels in the NTS, (3) glial cell activation, and (4) prevention by a Group III metabotropic glutamate receptor activation of neural firing response to intratracheal application of capsaicin in both naïve and sensitized rats. Therefore, normalization of ovalbumin-induced NTS neural sensitization could open up the prospect of new treatments based on the recovery of specific brain nuclei function and for extensive studies on acute or long-term efficacy of selective mGlu ligand, in models of bronchial hyperreactivity.

Antitussive drugs--past, present, and future

Cough remains a serious unmet clinical problem, both as a symptom of a range of other conditions such as asthma, chronic obstructive pulmonary disease, gastroesophageal reflux, and as a problem in its own right in patients with chronic cough of unknown origin. This article reviews our current understanding of the pathogenesis of cough and the hypertussive state characterizing a number of diseases as well as reviewing the evidence for the different classes of antitussive drug currently in clinical use. For completeness, the review also discusses a number of major drug classes often clinically used to treat cough but that are not generally classified as antitussive drugs. We also reviewed a number of drug classes in various stages of development as antitussive drugs. Perhaps surprising for drugs used to treat such a common symptom, there is a paucity of well-controlled clinical studies documenting evidence for the use of many of the drug classes in use today, particularly those available over the counter. Nonetheless, there has been a considerable increase in our understanding of the cough reflex over the last decade that has led to a number of promising new targets for antitussive drugs being identified and thus giving some hope of new drugs being available in the not too distant future for the treatment of this often debilitating symptom.

The effects of cannabidiol on the antigen-induced contraction of airways smooth muscle in the guinea-pig

(-)-Δ(9)-Tetrahydrocannabinol has been demonstrated to have beneficial effects in the airways, but its psychoactive effects preclude its therapeutic use for the treatment of airways diseases. In the present study we have investigated the effects of (-)-cannabidiol, a non-psychoactive component of cannabis for its actions on bronchial smooth muscle in vitro and in vivo. Guinea-pig bronchial smooth muscle contractions induced by exogenously applied spasmogens were measured isometrically. In addition, contractile responses of bronchial smooth muscle from ovalbumin-sensitized guinea-pigs were investigated in the absence or presence of (-)-cannabidiol. Furthermore, the effect of (-)-cannabidiol against ovalbumin-induced airway obstruction was investigated in vivo in ovalbumin-sensitized guinea-pigs. (-)-Cannabidiol did not influence the bronchial smooth muscle contraction induced by carbachol, histamine or neurokinin A. In contrast, (-)-cannabidiol inhibited anandamide- and virodhamine-induced responses of isolated bronchi. A fatty acid amide hydrolase inhibitor, phenylmethanesulfonyl fluoride reversed the inhibitory effect of (-)-cannabidiol on anandamide-induced contractions. In addition, (-)-cannabidiol inhibited the contractile response of bronchi obtained from allergic guinea-pigs induced by ovalbumin. In vivo, (-)-cannabidiol reduced ovalbumin-induced airway obstruction. In conclusion, our results suggest that cannabidiol can influence antigen-induced airway smooth muscle tone suggesting that this molecule may have beneficial effects in the treatment of obstructive airway disorders.

Sensory nerves and airway irritability

The lung, like many other organs, is innervated by a variety of sensory nerves and by nerves of the parasympathetic and sympathetic nervous systems that regulate the function of cells within the respiratory tract. Activation of sensory nerves by both mechanical and chemical stimuli elicits a number of defensive reflexes, including cough, altered breathing pattern, and altered autonomic drive, which are important for normal lung homeostasis. However, diseases that afflict the lung are associated with altered reflexes, resulting in a variety of symptoms, including increased cough, dyspnea, airways obstruction, and bronchial hyperresponsiveness. This review summarizes the current knowledge concerning the physiological role of different sensory nerve subtypes that innervate the lung, the factors which lead to their activation, and pharmacological approaches that have been used to interrogate the function of these nerves. This information may potentially facilitate the identification of novel drug targets for the treatment of respiratory disorders such as cough, asthma, and chronic obstructive pulmonary disease.

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.

Peripheral mechanisms II: the pharmacology of peripherally active antitussive drugs

Cough is an indispensable defensive reflex. Although generally beneficial, it is also a common symptom of diseases such as asthma, chronic obstructive pulmonary disease, upper respiratory tract infections, idiopathic pulmonary fibrosis and lung cancer. Cough remains a major unmet medical need and although the centrally acting opioids have remained the antitussive of choice for decades, they have many unwanted side effects. However, new research into the behaviour of airway sensory nerves has provided greater insight into the mechanisms of cough and new avenues for the discovery of novel non-opioid antitussive drugs. In this review, the pathophysiological mechanisms of cough and the development of novel antitussive drugs are reviewed.

Cannabinoid CB2 receptor activation prevents bronchoconstriction and airway oedema in a model of gastro-oesophageal reflux

Cannabinoids have been shown to inhibit sensory nerve activation in guinea-pigs and humans. Their effects are mediated by specific activation of two types of receptors, named CB(1) and CB(2). The purpose of this study was to investigate the effects of WIN 55,212-2, (R)-(+)-[2,3-dihydro-5methyl-3-[(4-morpholino)methyl]pyrrolo-[1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthyl)methanone, a non selective agonist of cannabinoid receptors, and JWH 133, (6aR,10aR)-3-(1,1-dimethylbutyl)-6a,7,10,10a-tetrahydro-6,6,9-trimethyl-6H-dibenzo[b,d]pyran a selective cannabinoid CB(2) receptor agonist, on the sensory nerve component of intraoesophageal (i.oe.) HCl-induced airway microvascular leakage and bronchoconstriction in guinea-pigs. We also tested the effect of WIN 55,212-2 on substance P-induced plasma extravasation and bronchoconstriction. Airway microvascular leakage and bronchoconstriction induced by i.oe. HCl was inhibited by the cannabinoid CB(1)/CB(2) agonist WIN 55,212-2 (0.3-3 mg/kg i.p.) in a dose-dependent manner (maximal inhibition at the dose of 3 mg kg(-1), P<0.01). The effect of WIN 55,212-2 was inhibited by a cannabinoid CB(2) receptor antagonist SR 144528, [N-[(1S)-endo-1,3,3-trimethylbicyclo[2,2,1] heptan-2yl]-5-(-4-chloro-3-methylphenyl)-1-(4-methylbenzyl)pyrazole-3-carboxamide], but not by a CB(1) receptor antagonist, SR 141716, [N-(piperidin-1yl)-5-(-4-chlorophenyl)-1-(2,4dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride]. The cannabinoid CB(2) agonist JWH 133 (0.3-3 mg/kg i.p.) mimicked the inhibitory effect of WIN 55,212-2 on HCl-induced microvascular leakage. Under similar conditions, WIN 55,212-2 (1 mg kg (-1) i.p.) was unable to counteract the airway microvascular leakage and bronchoconstriction induced by substance P. These results suggest that inhibition by WIN 55,212-2 of airway plasma extravasation and bronchoconstriction induced by i.oe. HCl instillation in guinea-pigs is mediated through cannabinoid CB(2) receptor activation.

TRPV1 antagonists as potential antitussive agents

Cough is an important defensive pulmonary reflex that removes irritants, fluids, or foreign materials from the airways. However, when cough is exceptionally intense or when it is chronic and/or nonproductive it may require pharmacologic suppression. For many patients, antitussive therapies consist of OTC products with inconsequential efficacies. On the other hand, the prescription antitussive market is dominated by older opioid drugs such as codeine. Unfortunately, "codeine-like" drugs suppress cough at equivalent doses that also often produce significant ancillary liabilities such as GI constipation, sedation, and respiratory depression. Thus, the discovery of a novel and effective antitussive drug with an improved side effect profile relative to codeine would fulfill an unmet clinical need in the treatment of cough. Afferent pulmonary nerves are endowed with a multitude of potential receptor targets, including TRPV1, that could act to attenuate cough. The evidence linking TRPV1 to cough is convincing. TRPV1 receptors are found on sensory respiratory nerves that are important in the generation of the cough reflex. Isolated pulmonary vagal afferent nerves are responsive to TRPV1 stimulation. In vivo, TRPV1 agonists such as capsaicin elicit cough when aerosolized and delivered to the lungs. Pertinent to the debate on the potential use of TRPV1 antagonist as antitussive agents are the observations that airway afferent nerves become hypersensitive in diseased and inflamed lungs. For example, the sensitivity of capsaicin-induced cough responses following upper respiratory tract infection and in airway inflammatory diseases such as asthma and COPD is increased relative to that of control responses. Indeed, we have demonstrated that TRPV1 antagonism can attenuate antigen-induced cough in the allergic guinea pig. However, it remains to be determined if the emerging pharmacologic profile of TRPV1 antagonists will translate into a novel human antitussive drug. Current efforts in clinical validation of TRPV1 antagonists revolve around various pain indications; therefore, clinical evaluation of TRPV1 antagonists as antitussive agents will have to await those outcomes.

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

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

The differential contractile responses to capsaicin and anandamide in muscle strips isolated from the rat urinary bladder

The contractile responses to capsaicin and anandamide, exogenous and endogenous agonists for transient receptor potential vanilloid receptor 1 (TRPV1), respectively, were investigated in muscle strips isolated from the rat urinary bladder. Capsaicin and anandamide produced concentration-dependent contractions of the muscle strips. The contractile response induced by capsaicin disappeared within approximately 20 min. In contrast, anandamide produced contractile responses lasting at least for 30 min. Capsaicin produced additive contractile responses in anandamide-treated muscle strips. The contractile response to anandamide was attenuated, but not abolished in strips desensitized by capsaicin. The response to capsaicin was abolished in the presence of a TRPV1 antagonist, N-(4-tertiarybutylphenyl)-4-(3-chlorphyridin-2-yl)tetrahydropyrazine-1(2H)-carbox-amide (BCTC), but not altered in the presence of either tetrodotoxin, atropine or indomethacin. In the presence of SR140333, a tachykinin NK(1) receptor antagonist or SR48968, an NK(2) receptor antagonist, the response to capsaicin was attenuated. The response to anandamide was partially attenuated in the presence of ONO8130, a prostanoid EP(1) receptor antagonist, URB597, a fatty-acid amide hydrolase inhibitor, BCTC, SR140333 or SR48968, and almost completely abolished by indomethacin. Neither tetrodotoxin, atropine, a cannabinoid CB(1) receptor antagonist, AM251, nor a cannabinoid CB(2) receptor antagonist, AM630, had any effect on the response to anandamide. These results indicate that capsaicin produces muscle contractions by stimulating the TRPV1 receptor, followed by release of neuropeptides that can activate tachykinin NK(1) and/or NK(2) receptors in the bladder and that the contractile response to anandamide is mediated at least in part by activation of prostanoid EP(1) receptors due to production of prostaglandins in addition to TRPV1 receptor activation.

Biochemistry and pharmacology of endovanilloids

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

TRPV1 antagonists attenuate antigen-provoked cough in ovalbumin sensitized guinea pigs

We examined the molecular pharmacology and in vivo effects of a TRPV1 receptor antagonist, N-(4-Tertiarybutylphenyl)-4(3-cholorphyridin-2-yl)-tetrahydro-pyrazine1(2H) - carboxamide (BCTC) on the guinea pig TRPV1 cation channel. BCTC antagonized capsaicin-induced activation and PMA-mediated activation of guinea pig TRPV1 with IC50 values of 12.2 +/- 5.2 nM, and 0.85 +/- 0.10 nM, respectively. In addition, BCTC (100 nM) completely blocked the ability of heterologously expressed gpTRPV1 to respond to decreases in pH. Thus, BCTC is able to block polymodal activation of gpTRPV1. Furthermore, in nodose ganglia cells, capsaicin induced Ca2+ influx through TRPV1 channel was inhibited via BCTC in a concentration dependent manner. In in vivo studies capsaicin (10 - 300 muM) delivered by aerosol to the pulmonary system of non-sensitized guinea pigs produced an increase in cough frequency. In these studies, the tussigenic effects of capsaicin (300 muM) were blocked in a dose dependent fashion when BCTC (0.01-3.0 mg/kg, i.p.) was administered 30 minutes before challenge. The high dose of BCTC (3.0 mg/kg, i.p) produced a maximum inhibition of capsaicin-induced cough of 65%. We also studied the effects of BCTC (0.03 and 3.0) when administered 60 minutes before capsaicin. Under these conditions, BCTC (3.0 mg/kg, i.p) produced a maximum decrease in capsaicin-induced cough of 31%. In ovalbumin passively sensitized guinea pigs, we found that BCTC (1 and 3 mg/kg, i.p.) attenuated antigen ovalbumin (0.3%) cough responses by 27% and 60%, respectively. We conclude that TRPV1 channel activation may play role in cough mediated by antigen in sensitized guinea pigs. Our results supports increasing evidence that TRPV1 may play a role in the generation of the cough response.

Effect of olvanil and anandamide on vagal C-fiber subtypes in guinea pig lung

Certain fatty acid amides such as anandamide (AEA) and olvanil are agonists for the transient receptor potential, vanilloid-1 (TRPV1) receptor, but have been found to activate TRPV1-containing C-fibers in some tissues but not others. We used extracellular recording and whole-cell patch clamp techniques to investigate the effect of olvanil and AEA on different types of vagal C-fibers innervating the same tissue, namely jugular and nodose vagal C-fibers in guinea pig lungs. A 30 s exposure to AEA and olvanil caused action potential discharge in all nodose C-fiber innervating lung but failed to activate jugular C-fibers innervating lung and airways. The activation of nodose C-fibers was blocked by the TRPV1 antagonist iodo-resiniferatoxin. In whole-cell patch clamp recordings of dissociated nodose and jugular capsaicin-sensitive neurons labeled from lungs and airways, olvanil induced large TRPV1-dependent inward currents in cell bodies of both nodose and jugular ganglion neurons. Prolonged exposure (up to 5 min) to olvanil caused action potential discharge in jugular C-fiber innervating lung but the onset latency was four times longer in jugular than in nodose C-fibers. The onsets of capsaicin response in nodose and jugular C-fibers were not different. Decreasing the tissue temperature to 25 degrees C increased the onset latency of olvanil-induced activation of nodose C-fibers 2-3-fold, but did not effect the latency of the capsaicin response. Capsaicin, olvanil, and AEA stimulate jugular C-fibers leading to tachykinergic contractions of isolated bronchi. The time to reach half-maximum is more than four times longer for olvanil and AEA, as compared to capsaicin in evoking contractions. We conclude that brief exposure to certain fatty acid amides, such as AEA and olvanil activate nodose but not jugular C-fiber terminals in the lungs. We hypothesize that this is because the nodose C-fiber terminals are equipped with a temperature-dependent mechanism for effectively and rapidly transporting the TRPV1 agonists so that they gain access to the intracellular binding sites on TRPV1. This transport mechanism may be differently expressed in two distinct subtypes of pulmonary C-fiber terminals innervating the same tissue.

The transient receptor potential vanilloid 1: role in airway inflammation and disease

The transient receptor potential vanilloid 1 (TRPV1) is an excitatory cation channel, rather selectively expressed in a subpopulation of nociceptive, primary sensory neurons that promote neurogenic inflammation via neuropeptide release. TRPV1 is activated by noxious temperature, low extracellular pH and diverse lipid derivatives, and is uniquely sensitive to vanilloid molecules, including capsaicin. TRPV1 expression and sensitivity is highly regulated by diverse G protein-coupled and tyrosine kinase receptors. Other exogenous or endogenous chemical agents, including reactive oxygen species, ethanol and hydrogen sulphide sensitize/activate TRPV1. In the airways, TRPV1 agonists cause cough, bronchoconstriction, microvascular leakage, hyperreactivity and hypersecretion. Patients with asthma and chronic obstructive pulmonary disease are more sensitive to the tussive effect of TRPV1 agonists and TRPV1 activation may contribute to respiratory symptoms caused by acidic media present in the airways during asthma exacerbation, gastroesophageal reflux induced asthma or in other conditions. TRPV1 antagonists may be useful in the treatment of these diseases.

Cannabinoid receptor agonists inhibit Ca(2+) influx to synaptosomes from rat brain

We examined the effects of cannabinoid receptor agonists on (45)Ca(2+) uptake in rat brain synaptosomes. A cannabinoid receptor agonist, (R)-(+)-[2,3-dihydro-5-methyl-3-[(4-merpholino)methyl]pyrrolo-[1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthyl)methanone (WIN 55212-2) dose-dependently inhibited (45)Ca(2+) uptake in rat synaptosomes. Only an endogenous cannabinoid receptor agonist, anandamide, dose-dependently inhibited (45)Ca(2+) uptake in rat synaptosomes, but not an endogenous cannabinoid receptor agonist, palmitoylethanolamide. Only a cannabinoid CB1 antagonist, [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamidehydrochloride] (SR 141716A), reversed the inhibitory effect of these WIN 55212-2 and anandamide on (45)Ca(2+) uptake in rat synaptosomes, but not a cannabinoid CB2 receptor antagonist, [N-[(1S)-endo-1,3,3-trimethylbicyclo[2.2.1]heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)pyrazole-3-carboxamide] (SR 144528). The inhibitory effects of WIN 55212-2 and anandamide on (45)Ca(2+) uptake in rat synaptosomes were reversed by the pretreatment of a voltage-sensitive A-type K(+) channel blocker, dendrotoxin, but no other type of K(+) channel blockers, i.e. iberiotoxin, charybdotoxin or glibenclamide. These findings suggest that cannabinoid receptors inhibit Ca(2+) influx into rat brain nerves via the activation of CB1 receptors and the opening of voltage-sensitive A-type K(+) channels.

Endogenous cannabinoid receptor agonists inhibit neurogenic inflammations in guinea pig airways

BACKGROUND

Although neurogenic inflammation via the activation of C fibers in the airway must have an important role in the pathogenesis of asthma, their regulatory mechanism remains uncertain.

OBJECTIVE

The pharmacological profiles of endogenous cannabinoid receptor agonists on the activation of C fibers in airway tissues were investigated and the mechanisms how cannabinoids regulate airway inflammatory reactions were clarified.

METHODS

The effects of endogenous cannabinoid receptor agonists on electrical field stimulation-induced bronchial smooth muscle contraction, capsaicin-induced bronchoconstriction and capsaicin-induced substance P release in guinea pig airway tissues were investigated. The influences of cannabinoid receptor antagonists and K+ channel blockers to the effects of cannabinoid receptor agonists on these respiratory reactions were examined.

RESULTS

Both endogenous cannabinoid receptor agonists, anandamide and palmitoylethanolamide, inhibited electrical field stimulation-induced guinea pig bronchial smooth muscle contraction, but not neurokinin A-induced contraction. A cannabinoid CB2 antagonist, SR 144528, reduced the inhibitory effect of endogenous agonists, but not a cannabinoid CB1 antagonist, SR 141716A. Inhibitory effects of agonists were also reduced by the pretreatment of large conductance Ca2+ -activated K+ channel (maxi-K+ channel) blockers, iberiotoxin and charybdotoxin, but not by other K+ channel blockers, dendrotoxin or glibenclamide. Anandamide and palmitoylethanolamide blocked the capsaicin-induced release of substance P-like immunoreactivity from guinea pig airway tissues. Additionally, intravenous injection of palmitoylethanolamide dose-dependently inhibited capsaicin-induced guinea pig bronchoconstriction, but not neurokinin A-induced reaction. However, anandamide did not reduce capsaicin-induced guinea pig bronchoconstriction.

CONCLUSIONS

These findings suggest that endogenous cannabinoid receptor agonists inhibit the activation of C fibers via cannabinoid CB2 receptors and maxi-K+ channels in guinea pig airways.

Lipids as regulators of the activity of transient receptor potential type V1 (TRPV1) channels

After 7 years from its cloning, the transient receptor potential vanilloid type-1 (TRPV1) channel remains the sole membrane receptor mediating the pharmacological effects of the hot chilli pepper pungent component, capsaicin, and of the Euphorbia toxin, resiniferatoxin. Yet, this ion channel represents one of the most complex examples of how the activity of a protein can be regulated. Among the several chemicophysical stimuli that can modulate TRPV1 permeability to cations, endogenous lipids appear to play a major role, either as allosteric effectors or as direct agonists, or both. Furthermore, the capability of some mediators, such as the endocannabinoid anandamide, or the eicosanoid precursors 12- and 5-hydroperoxy-eicosatetraenoic acids, to activate TRPV1 receptors provides a striking example of the "site-dependent" and "metabolic" functional plasticity, respectively, typical of bioactive lipids. In this article, the multi-faceted and most recently discovered aspects of TRPV1 regulation are reviewed, with particular emphasis on the interaction between these membrane channels and some lipid molecules.

Cannabinoid receptor agonists inhibit sensory nerve activation in guinea pig airways

We examined the effects of cannabinoid receptor agonists on various respiratory reactions induced by the activation of capsaicin-sensitive afferent sensory nerves (C-fibers). (R)-(+)-[2,3-dihydro-5-methyl-3-[(4-merpholino)methyl]pyrrolo-[1,2,3-de]-1,4-benzoxazin-6-yl](1-naphthyl)methanone (WIN 55212-2) dose-dependently inhibited electrical field stimulation- and capsaicin-induced guinea pig bronchial smooth muscle contraction, but not the neurokinin A-induced contraction. A cannabinoid CB2 receptor antagonist, [N-[(1S)-endo-1,3,3-trimethylbicyclo[2.2.1]heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)pyrazole-3-carboxamide] (SR 144528), reduced the inhibitory effect of WIN 55212-2, but not a cannabinoid CB1 antagonist, [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamidehydrochloride] (SR 141716A). A cannabinoid CB2 agonist, JWH 133, also inhibited electrical field stimulation-induced guinea pig bronchial smooth muscle contraction and its inhibitory effect was blocked by SR 144528. The inhibitory effect of WIN 55212-2 on electrical field stimulation-induced bronchial contraction was reduced by the pretreatment of large conductance Ca(2+)-activated K+ channel (Maxi-K+ channel) blockers, iberiotoxin and charybdotoxin, but not other K+ channel blockers, dendrotoxin or glibenclamide. A Maxi-K+ channel opener, 1-(2'-hydroxy-5'-trifluoromethylphenyl)-5-trifluoromethyl-2(3H)benzimidazolone (NS1619), inhibited bronchial contraction induced by electrical field stimulation. WIN 55212-2 and JWH 133 blocked the capsaicin-induced release of substance P-like immunoreactivity from guinea pig airway tissues. These findings suggest that WIN 55212-2 inhibit the activation of C-fibers via cannabinoid CB2 receptors and Maxi-K+ channels in guinea pig airways.

Cannabidiol lacks the vanilloid VR1-mediated vasorespiratory effects of capsaicin and anandamide in anaesthetised rats

The results of vasorespiratory studies in rats anaesthetised with pentobarbital show that (+/-) cannabidiol, a cannabinoid that lacks psychotropic actions and is inactive at cannabinoid (CB) receptors, does not affect respiration or blood pressure when injected (1-2000 microg; 3.2-6360 nmol i.a.). Cannabidiol in doses up to 2 mg (6360 nmol) i.a. or i.v. did not affect the fall in mean blood pressure or the increase in ventilation (respiratory minute volume) caused by capsaicin and high doses of anandamide, responses that are mediated by activation of vanilloid VR1 (TRPV1) receptors in this species. Similar results were obtained with (-) cannabidiol (30-100 microg i.a.; 95-318 nmol). It has previously been shown using human embryonic kidney (HEK) cells over-expressing vanilloid human VR1 (hVR1) receptors that cannabidiol is a full agonist at vanilloid VR1 receptors in vitro. However, in the intact rat cannabidiol lacked vanilloid VR1 receptor agonist effects. We conclude that there are substantial functional differences between human and rat vanilloid VR1 receptors with respect to the actions of cannabidiol as an agonist at vanilloid VR1 receptors. Studies in vivo show that cannabidiol lacks any significant effect on mean blood pressure or respiratory minute volume when injected i.a. or i.v., and that this cannabinoid does not modulate the vanilloid VR1 receptor-mediated cardiovascular and ventilatory changes reflexly evoked by capsaicin or anandamide in rats anaesthetised with pentobarbital.

Differential effects of the sleep-inducing lipid oleamide and cannabinoids on the induction of long-term potentiation in the CA1 neurons of the rat hippocampus in vitro

Cannabinoids have been shown to impair cognition in vivo and block long-term potentiation (LTP), a candidate experimental model of learning and memory in vitro, via cannabinoid receptor (CB1) activation. cis-Oleamide (cOA) is an endogenous sleep-inducing lipid with putative cannabinomimetic properties. We hypothesise that cOA is cannabinomimetic and perform a comparative study with synthetic and endogenous cannabinoids on their effects on synaptic conditioning via two different patterns of stimulation in the hippocampal slice. CB1 agonists, R(+)-WIN55212-2 and anandamide, but not cOA blocked high frequency stimulation (HFS)-LTP. R(+)-WIN55212-2 and cOA (stereoselectively) attenuated responses to theta-burst-LTP, while anandamide did not. The anandamide transport inhibitor, AM404, attenuated HFS-LTP, an effect reversed by the CB1 receptor antagonist SR141716A but not mimicked by the vanilloid receptor agonist capsaicin. TFNO, an inhibitor of fatty acid amide hydrolase (FAAH), the enzyme responsible for degrading anandamide, failed to block HFS-LTP alone or in combination with cOA. On the contrary, this combination was as effective as cOA on its own in attenuating theta-burst-LTP. cOA effects on theta-burst-LTP were prevented in the presence of the GABA(A) receptor blocker picrotoxin, but not by pretreatment with SR141716A. These findings suggest that cOA neither directly activates CB1 receptors nor acts via the proposed "entourage" effect [Nature 389 (1997) 25] to increase titres of anandamide through FAAH inhibition. The selective effects of cOA on theta-burst-conditioning may reflect modulation of GABAergic transmission. Anandamide uptake inhibition, but not blockade of FAAH, effectively increases synaptic concentrations of endocannabinoids.

Activation of bronchopulmonary vagal afferent nerves with bradykinin, acid and vanilloid receptor agonists in wild-type and TRPV1-/- mice

The vanilloid receptor TRPV1 (formerly VR1) has been implicated in the activation of nociceptive sensory nerves by capsaicin, noxious heat, protons, bradykinin, cannabinoids such as anandamide, and certain metabolites of arachidonic acid. Using TRPV1 knockout mouse (TRPV1-/-) we address the question of whether TRPV1 is obligatory for action potential discharge in vagal C-fibre terminals evoked by capsaicin, anandamide, acid and bradykinin. The response of a defined subtype of the vagal afferent bronchopulmonary C-fibres (conduction velocity < 0.7 ms(-1)) to the putative TRPV1 activators was studied in vitro in the mouse isolated/perfused lung-nerve preparation. Capsaicin (1 microm) evoked action potential discharge of approximately 90% (28/31) of C-fibres in the TRPV1+/+ mice, but failed to activate bronchopulmonary C-fibres in TRPV1-/- animals (n = 10). Anandamide (3-100 microm) induced concentration-dependent activation of capsaicin-sensitive TRPV1+/+ C-fibres with a threshold of 3-10 microm, but failed to evoke substantive discharge in TRPV1-/- C-fibres. In the TRPV1+/+ mice, the B2 receptor-mediated activation by bradykinin (1 microm) was restricted to the capsaicin-sensitive C-fibres. Bradykinin was effective in evoking B2 receptor-mediated action potential discharge in TRPV1-/- C-fibres, but the response was significantly (P < 0.05) less persistent than in TRPV1+/+ C-fibres. Exposing the tissue to acid (pH = 5) excited both TRPV1+/+ and TRPV1-/- C-fibres. We conclude that TRPV1 is obligatory for vagal C-fibre activation by capsaicin and anandamide. By contrast, whereas TRPV1 may have a modulatory role in bradykinin and acid-induced activation of bronchopulmonary C-fibres, it is not required for action potential discharge evoked by these stimuli.

Anandamide and vanilloid TRPV1 receptors

A large body of evidence now exists to substantiate that the endocannabinoid, anandamide, activates TRPV1 receptors. It is a low intrinsic efficacy TRPV1 agonist that behaves as a partial agonist in tissues with a low receptor reserve, while in tissues with high receptor reserve and in circumstances associated with certain disease states, it behaves as a full agonist. The efficacy of anandamide as a TRPV1 agonist is influenced by a succession of factors including receptor reserve, phosphorylation, metabolism and uptake, CB1 receptor activation, voltage, temperature, pH and bovine serum albumin. There are indications that the endocannabinoid system may play a role in the modulation of TRPV1 receptor activation. The activation of TRPV1 receptors by anandamide has potential implications in the treatment of inflammatory, respiratory and cardiovascular disorders. The relative importance of anandamide as a physiological and/or pathophysiological TRPV1 receptor agonist in comparison to other potential candidates has yet to be revealed.

Pharmacology of vagal afferent nerve activity in guinea pig airways

The excitability and activity of vagal afferent nerves innervating the airways can be pharmacologically increased and decreased. Autacoids released as a result of airway inflammation can lead to substantial increases in afferent nerve activity, consequently altering pulmonary reflex physiology. In a manner analogous to hyperalgesia associated with inflammation in the somato-sensory system, increases in vagal afferent nerve activity in inflamed airways may lead to a heightened cough reflex, and increases in autonomic activity in the airways. These effects may contribute to many of the symptoms of inflammatory airway disease. Here we provide a brief overview of some of the mechanisms by which the afferent activity in airway nerves can be pharmacologically modified.

Cannabinoid modulation of peripheral autonomic and sensory neurotransmission

Cannabinoids are cell membrane-derived signalling molecules that are released from nerves, blood cells and endothelial cells, and have diverse biological effects. They act at two distinct types of G-protein-coupled receptors, cannabinoid CB(1) and CB(2) receptors. Cannabinoid CB(1) receptors are highly localised in the central nervous system and are also found in some peripheral tissues, and cannabinoid CB(2) receptors are found outside the central nervous system, in particular in association with immune tissues. Novel actions of cannabinoids at non-CB(1) non-CB(2) cannabinoid-like receptors and vanilloid VR1 receptors have also recently been described. There is growing evidence that, among other roles, cannabinoids can act at prejunctional sites to modulate peripheral autonomic and sensory neurotransmission, and the present article is aimed at providing an overview of this. Inhibitory cannabinoid CB(1) receptors are expressed on the peripheral terminals of autonomic and sensory nerves. The role of cannabinoid receptor ligands in modulation of sensory neurotransmission is complex, as certain of these (anandamide, an "endocannabinoid", and N-arachidonoyl-dopamine, an "endovanilloid") also activate vanilloid VR1 receptors (coexpressed with cannabinoid CB(1) receptors), which excites sensory nerves and causes a release of sensory neurotransmitter. The fact that the activities of anandamide and N-arachidonoyl-dopamine span two distinct receptor families raises important questions about cannabinoid/vanilloid nomenclature, and as both compounds are structurally related to the archetypal vanilloid capsaicin, all three are arguably members of the same family of signalling molecules. Anandamide is released from nerves, but unlike classical neurotransmitters, it is not stored in and released from nerve vesicles, but is released on demand from the nerve cell membrane. In the central nervous system, cannabinoids function as retrograde signalling molecules, inhibiting via presynaptic cannabinoid CB(1) receptors the release of classical transmitter following release from the postsynaptic cell. At the neuroeffector junction, it is more likely that cannabinoids are released from prejunctional sites, as the neuroeffector junction is wide in some peripheral tissues and cannabinoids are rapidly taken up and inactivated. Understanding the actions of cannabinoids as modulators of peripheral neurotransmission is relevant to a variety of biological systems and possibly their disorders.

Activation of capsaicin-sensitive primary sensory neurones induces anandamide production and release

The inhibitory cannabinoid 1 receptor and the excitatory vanilloid receptor 1, both of which are responsive to the endogenous ligand anandamide, are co-expressed on a subpopulation of primary sensory neurones. We report that activation of the cannabinoid 1 receptor/vanilloid receptor 1-co-expressing primary sensory neurones induces the production and release of anandamide. Application of capsaicin (3 nm-1 micro m) to cultured primary sensory neurones evoked calcitonin gene-related peptide release, which was significantly increased by the selective cannabinoid 1 receptor antagonist, SR141716A (200 nm). Mass spectrometric analyses of the extracellular solution revealed that exposure to capsaicin (10 nm or 100 nm) enhanced the anandamide concentration of the medium from less then 0.05 pmol/ micro L to more then 2 pmol/ micro L. Depolarization of the neurones with 50 mm KCl also enhanced the anandamide content of the buffer. Both the capsaicin- and KCl-induced anandamide release depended on extracellular Ca2+. Prolonged treatment of the cultures with capsaicin (10 micro m) reduced both the capsaicin- and KCl-induced anandamide release. These findings indicate that activation of capsaicin-sensitive primary sensory neurones evokes anandamide production and release, and that anandamide might be a key endogenous regulator of the excitability of these neurones.

R+-methanandamide inhibits tracheal response to endogenously released acetylcholine via capsazepine-sensitive receptors

The effects of cannabinoid drugs on the cholinergic response evoked by electrical field stimulation (0.2 ms pulse width, 20 V amplitude, 10 Hz, 7.5 s train duration) in guinea-pig tracheal preparations were investigated. The stable analogue of the endocannabinoid anandamide, R(+)-methanandamide (10(-7)-10(-4) M), produced a dose-dependent inhibition (up to 27+/-5% of control) of electrical field stimulation-mediated atropine-sensitive response. This effect was not blocked by the selective cannabinoid CB(1) receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3 carboxamide hydrochloride (SR 141716A; 10(-6) M), and was not reproduced with the cannabinoid CB(1)/CB(2) receptor agonist R(+)-[2,3-dihydro-5-methyl-[(morpholinyl)methyl]pyrrolo [1,2,3-de]-1,4-benzoxazin-6-yl]-(1-naphthalenyl)methanone mesylate) (WIN 55,212-2; 10(-8)-10(-5) M) or the cannabinoid CB(2) receptor selective agonist 1-propyl-2-methyl-3-(1-naphthoyl)indole (JWH-015; 10(-8)-10(-5) M); it was, on the contrary, antagonized by the vanilloid antagonist 2-[2-(4-chlorophenyl)ethyl-amino-thiocarbonyl]-7,8-dihydroxy-2,3,4,5-tetrahydro-1H-2 benzazepine (capsazepine; 10(-6) M). At the postjunctional level, neither R(+)-methanandamide nor WIN 55,212-2 nor JWH-015 did affect tracheal contractions induced by exogenous acetylcholine (10(-6) M). An inhibitory vanilloid receptor-mediated effect on the cholinergic response evoked by electrical stimulation was confirmed with the vanilloid agonist capsaicin, at doses (3-6 x 10(-8) M) which poorly influenced the basal smooth muscle tone of trachea. In conclusion, our data indicate that in guinea-pig trachea (a) neither CB(1) nor CB(2) cannabinoid receptor-mediated modulation of acetylcholine release occurs; (b) vanilloid VR1-like receptors appear involved in R(+)-methanandamide inhibitory activity on the cholinergic response to electrical field stimulation.

Respiratory actions of vanilloid receptor agonists in the nucleus of the solitary tract: comparison of resiniferatoxin with non-pungent agents and anandamide

1. Activation of vanilloid receptors on sensory nerve terminals in the commissural nucleus of the solitary tract (cNTS) of rats with capsaicin, produces respiratory slowing. In this study, we used microinjection techniques employing pungent and non-pungent vanilloids to further characterize vanilloid receptors in the cNTS. 2. Microinjection of the pungent vanilloid, resiniferatoxin (RTX), into the cNTS of urethane-anaesthetized rats, dose-dependently reduced respiratory rate without affecting tidal volume. RTX was 20 fold more potent at slowing respiration ( approximately ED(50), 100 pmol) than capsaicin ( approximately ED(50), 2 nmol). Doses of RTX greater than 100 pmol caused either irregular (dyspnoeic) breathing or terminal apnoea (>250 pmol). The respiratory slowing response to RTX (75 pmol), was dose-dependently attenuated by injecting RTX (but not vehicle) into the same site 60 min earlier. 3. The non-pungent phorbol derivative of RTX, phorbol 12-phenylacetete 13-acetate 20-homovanillate (PPAHV, 0.1-1 nmol), also slowed respiration (ED(50), approximately 1 nmol) and almost abolished response to RTX (75 pmol) injected into the same site 60 min later. 4. In contrast to RTX, PPAHV and capsaicin, the putative endogenous vanilloid receptor agonist, arachidonyl ethanolamide (AEA), and non-pungent capsaicin derivative, olvanil, had no direct effect on respiration. However, both AEA and olvanil dose-dependently reduced the respiratory response to injection of RTX (75 pmol) 60 min later into the same site (EC(50)s, for AEA and olvanil, approximately 2 and 0.2 nmol, respectively). 5. These studies suggest that both pungent and non-pungent vanilloids interact with vanilloid receptors in the cNTS. However, whereas RTX and PPAHV activate and subsequently desensitize vanilloid receptors on sensory nerve terminals in the cNTS, olvanil and AEA fail to activate despite readily desensitizing responses to RTX in this region.

Characterization of the vanilloid receptor 1 antagonist iodo-resiniferatoxin on the afferent and efferent function of vagal sensory C-fibers

The effect of iodo-resiniferatoxin (I-RTX) on efferent function (tachykinergic contractions of airway smooth muscle) and afferent function (action potential discharge) of vagal C-fibers mediated by vanilloid receptor 1 (VR1) activation was studied in an isolated guinea pig airway preparation. I-RTX (1 microM) had no VR1 agonist activity in either the afferent or efferent assays. I-RTX (30 nM-1 microM) shifted the resiniferatoxin and capsaicin concentration-response curves for neurokinin-mediated contractions rightward but did not inhibit the maximum response. The pK(B) value calculated from 0.3 microM I-RTX against resiniferatoxin and capsaicin was 7.3 +/- 0.2 and 6.8 +/- 0.2, respectively, showing 10 to 30 times higher potency compared with capsazepine. The slope of Schild plot from the resiniferatoxin efferent studies deviated from unity (~0.6), suggesting complex interactions at VR1 binding site(s). This notion was further supported by lack of additional inhibitory effect of 1 microM I-RTX on capsaicin-evoked contractions compared with 0.3 microM I-RTX. Concentrations of I-RTX up to 1 microM had no effect on trypsin-induced neurokinin-mediated contractions, nor neurokinin A-induced contractions of guinea pig trachea. However, nonselective effects on airway smooth muscle contractions were noted with 10 microM I-RTX. In both afferent and efferent studies I-RTX (30 nM-1 microM) caused a substantial delay of the response to capsaicin. This led to an apparent increase in potency in experiments where the agonist was applied transiently, with insufficient time to reach equilibrium. I-RTX inhibited contractions induced by anandamide and action potential discharge induced by low pH, showing that the I-RTX-antagonism of VR1 does not strictly depend on the vanilloid nature of the agonist.

Anandamide induces cough in conscious guinea-pigs through VR1 receptors

1. Endogenous neuronal lipid mediator anandamide, which can be synthesized in the lung, is a ligand of both cannabinoid (CB) and vanilloid receptors (VR). The tussigenic effect of anandamide has not been studied. The current study was designed to test the direct tussigenic effect of anandamide in conscious guinea-pigs, and its effect on VR1 receptor function in isolated primary guinea-pig nodose ganglia neurons. 2. Anandamide (0.3-3 mg.ml(-1)), when given by aerosol, induced cough in conscious guinea-pigs in a concentration dependent manner. When guinea-pigs were pretreated with capsazepine, a VR1 antagonist, the anandamide-induced cough was significantly inhibited. Pretreatment with CB1 (SR 141716A) and CB2 (SR 144528) antagonists had no effect on anandamide-induced cough. These results indicate that anandamide-induced cough is mediated through the activation of VR1 receptors. 3. Anandamide (10-100 micro M) increased intracellular Ca(2+) concentration estimated by Fluo-4 fluorescence change in isolated guinea-pig nodose ganglia cells. The anandamide-induced Ca(2+) response was inhibited by two different VR1 antagonists: capsazepine (1 micro M) and iodo-resiniferatoxin (I-RTX, 0.1 micro M), indicating that anandamide-induced Ca(2+) response was through VR1 channel activation. In contrast, the CB1 (SR 141716A, 1 micro M) and CB2 (SR 144528, 0.1 micro M) receptor antagonists had no effect on Ca(2+) response to anandamide. 4. In conclusion, these results provide evidence that anandamide activates native vanilloid receptors in isolated guinea-pig nodose ganglia cells and induces cough through activation of VR1 receptors.

Cannabinoid modulation of sensory neurotransmission via cannabinoid and vanilloid receptors: roles in regulation of cardiovascular function

Capsaicin-sensitive sensory nerves are widely distributed in the cardiovascular system. They are activated by a variety of physical and chemical stimuli, characteristically by capsaicin acting via the vanilloid receptor VR1, and have a role in the regulation of peripheral vascular resistance and maintenance of homeostasis via their afferent and efferent functions. Cannabinoids, a recently discovered family of extracellular signalling molecules, can act at cannabinoid (CB) receptors expressed on sensory nerves, to cause inhibition of sensory neurotransmitter release. There is recent evidence, however, that anandamide, an endogenous cannabinoid, can activate VR1, coexpressed with CB receptors on the same sensory nerve terminals, causing a release of sensory neurotransmitter, vasorelaxation and hypotension. Hence, anandamide can elicit opposite actions, inhibition via CB receptors and excitation via VR1, on sensory neurotransmission. The possible biological significance of this is discussed.

Anandamide is a partial agonist at native vanilloid receptors in acutely isolated mouse trigeminal sensory neurons

1. The endogenous fatty acid anandamide (AEA) is a partial agonist at cannabinoid CB1 receptors and has been reported to be a full agonist at the recombinant vanilloid receptor, VR1. 2. Whole cell voltage clamp techniques were used to examine the efficacy of AEA and related analogues methanandamide and N-(4-hydroxyphenyl)-arachidonylamide (AM404) at native VR1 receptors in acutely isolated mouse trigeminal neurons. 3. Superfusion of the VR1 agonist capsaicin onto small trigeminal neurons voltage clamped at +40 mV produced outward currents in most cells, with a pEC(50) of 6.3+/-0.1 (maximum currents at 10-30 micro M). 4. AEA produced outward currents with a pEC(50) of 5.6+/-0.1. Maximal AEA currents (30-100 micro M) were 38+/-2% of the capsaicin maximum. AEA currents were blocked by the VR1 antagonist capsazepine (30 micro M), but unaffected by the CB1 antagonist SR141716A (1 micro M). 5. Methanandamide and AM404 were less potent than AEA at activating VR1. Methanandamide (100 micro M) produced currents 37+/-6% of the capsaicin maximum, the highest concentration of AM404 tested (100 micro M) produced currents that were 55+/-9% of the capsaicin maximum. 6. Capsazepine abolished the currents produced by AM404 (100 micro M) and strongly attenuated (>70%) those produced by methanandamide (100 micro M). 7. Co-superfusion of AEA (30 micro M, methanandamide (100 micro M) or AM404 (100 micro M) with capsaicin (3 micro M) resulted in a significant reduction of the capsaicin current. 8. These data indicate that AEA, methanandamide and AM404 activate native VR1 receptors, but that all three compounds are partial agonists when compared with capsaicin.

Characterization of the anandamide induced depolarization of guinea-pig isolated vagus nerve

1. There is considerable interest in elucidating potential endogenously derived agonists of the vanilloid receptor and the role of anandamide in this regard has received considerable attention. In the present study, we have used an electrophysiological technique to investigate the mechanism of activation of vanilloid receptors in an isolated vagal preparation. 2. Both capsaicin and anandamide depolarized de-sheathed whole vagal nerve preparations that was antagonized by the VR1 antagonist, capsazepine (P<0.05) whilst this response was unaltered by the cannabinoid (CB1) selective antagonist SR141716A or the CB2 selective antagonist, SR144528, thereby ruling out a role for cannabinoid receptors in this response. 3. The PKC activator, phorbol-12-myristate-13-acetate (PMA) augmented depolarization to both anandamide and capsaicin and this response was significantly inhibited with the PKC inhibitor, bisindolylmaleimide (BIM) (P<0.05). 4. The role of lipoxygenase products in the depolarization to anandamide was investigated in the presence of the lipoxygenase inhibitor, 5,8,11-Eicosatriynoic acid (ETI). Depolarization to anandamide and arachidonic acid was significantly inhibited in the presence of ET1 (P<0.05). However, in the absence of calcium depolarization to anandamide was not inhibited by ETI. 5. Using confocal microscopy we have demonstrated the presence of vanilloid receptors on both neuropeptide containing nerves and nerves that did not stain for sensory neuropeptides. 6. These results demonstrate that anandamide evokes depolarization of guinea-pig vagus nerve, following activation of vanilloid receptors, a component of which involves the generation of lipoxygenase products. Furthermore, these receptors are distributed in both neuropeptide and non-neuropeptide containing nerves.

Mechanisms underlying tissue selectivity of anandamide and other vanilloid receptor agonists

Anandamide acts as a full vanilloid receptor agonist in many bioassay systems, but it is a weak activator of primary afferents in the airways. To address this discrepancy, we compared the effect of different vanilloid receptor agonists in isolated airways and mesenteric arteries of guinea pig using preparations containing different phenotypes of the capsaicin-sensitive sensory nerve. We found that anandamide is a powerful vasodilator of mesenteric arteries but a weak constrictor of main bronchi. These effects of anandamide are mediated by vanilloid receptors on primary afferents and do not involve cannabinoid receptors. Anandamide also contracts isolated lung strips, an effect caused by the hydrolysis of anandamide and subsequent formation of cyclooxygenase products. Although capsaicin is equally potent in bronchi and mesenteric arteries, anandamide, resiniferatoxin, and particularly olvanil are significantly less potent in bronchi. Competition experiments with the vanilloid receptor antagonist capsazepine did not provide evidence of vanilloid receptor heterogeneity. Arachidonoyl-5-methoxytryptamine (VDM13), an inhibitor of the anandamide membrane transporter, attenuates responses to olvanil and anandamide, but not capsaicin and resiniferatoxin, in mesenteric arteries. VDM13 did not affect responses to these agonists in bronchi, suggesting that the anandamide membrane transporter is absent in this phenotype of the sensory nerve. Computer simulations using an operational model of agonism were consistent, with differences in intrinsic efficacy and receptor content being responsible for the remaining differences in agonist potency between the tissues. This study describes differences between vanilloid receptor agonists regarding tissue selectivity and provides a conceptual framework for developing tissue-selective vanilloid receptor agonists devoid of bronchoconstrictor activity.

Cough: potential pharmacological developments

Cough is an important defensive reflex of the upper airway and is also a very common symptom of respiratory disease. Cough following an upper respiratory viral infection is transient, and persistent cough is associated with a whole range of conditions, such as asthma, rhino-sinusitis and gastro-oesophageal reflux. Treatment directed at these conditions may improve the associated cough. There is often a need, however, to control cough itself whatever the cause. The most effective drugs in this class are the opioids, such as morphine, codeine or pholcodeine, but at effective doses they have side effects including drowsiness, nausea, constipation and physical dependence. Investigations into the cough reflex and into the potential mechanisms of sensitised cough reflex have uncovered several potential targets for novel drugs. New opioids apart from mu-agonists such as kappa- and delta -receptor agonists, have been developed, in addition to non-opioids such as nociceptin. Neurokinin receptor antagonists, bradykinin receptor antagonists, vanniloid receptor VR-1 antagonists may be beneficial by blocking effects of tachykinins and sensory nerve activation. Local anaesthetics, blockers of sodium-dependent channels and maxi-K Ca2+-dependent channel activators of afferent nerves are inhibitors of the cough reflex. Some of these novel agents may act centrally or peripherally or at both sites as antitussives. Large scale trials of these novel compounds have not been carried out in cough in man but there is a serious need for more effective antitussives devoid of side effects.

The effect of cannabinoids on capsaicin-evoked calcitonin gene-related peptide (CGRP) release from the isolated paw skin of diabetic and non-diabetic rats

Sensory neural dysfunction is common in patients with peripheral neuropathy, a major complication of diabetes mellitus. In animal models of inflammatory and neuropathic pain cannabinoids potently attenuate pain behaviour, cannabinoid (CB) receptors located on nociceptive primary afferent neurones being important in their anti-hyperalgesic actions. A key measure of sensory neurone function is stimulus-evoked neuropeptide release. We investigated the effect of cannabinoid on capsaicin-evoked release of calcitonin gene-related peptide (CGRP) from the rat paw skin in vitro, comparing non-diabetic and streptozotocin-induced diabetic animals. Diabetes caused a greater than two-fold increase in basal and capsaicin-evoked CGRP release. The synthetic CB(1)/CB(2) receptor agonist, CP55940 (100 nM), inhibited capsaicin-evoked CGRP release in both non-diabetic (30.92+/-7.69%, P<0.05) and diabetic animals (37.82+/-9.85%, P<0.05). The CB(1) receptor antagonist SR141716A (100 nM), but not the CB(2) receptor antagonist SR144528 (100 nM), significantly attenuated the inhibitory action of CP55940. The endogenous cannabinoid, anandamide (100 nM) inhibited capsaicin-evoked CGRP release in non-diabetic animals (28.88+/-7.12%, P<0.05) but neither the CB(1) nor the CB(2) receptor antagonist attenuated this action of anandamide. Anandamide (100 nM) did not significantly inhibit capsaicin-evoked CGRP release from the paw skin of diabetic animals, but it did produce a small stimulation of CGRP release at high concentrations (10 microM). These data suggest that peripheral CB(1) receptors mediate inhibition of capsaicin-evoked neuropeptide release from the paw skin of both non-diabetic and diabetic animals. However, pathological changes in the diabetic animals appear to preclude the non-CB(1) receptor mediated inhibitory action of the endogenous cannabinoid, anandamide.

Stimulation of pulmonary vagal C-fibres by anandamide in anaesthetized rats: role of vanilloid type 1 receptors

This study was carried out to determine the effect of intravenous injection of anandamide on pulmonary C-fibre afferents and the cardiorespiratory reflexes. In anaesthetized, spontaneously breathing rats, intravenous bolus injection of anandamide near the right atrium immediately elicited the pulmonary chemoreflex responses, characterized by apnoea, bradycardia and hypotension. After perineural treatment of both cervical vagi with capsaicin to block the conduction of C-fibres, anandamide no longer evoked these reflex responses. In open-chest, and artificially ventilated rats, anandamide injection evoked an abrupt and intense discharge in vagal pulmonary C-fibres in a dose-dependent manner. After injection of the high dose, the fibre discharge generally started within 1 s, reached a peak in approximately 2 s, and returned to baseline within 7 s. The stimulation of C-fibres by anandamide was completely and reversibly blocked by pretreatment with capsazepine, a competitive antagonist of the vanilloid type 1 receptor. Anandamide (0.4 mg kg(-1)) stimulated approximately 93 % of pulmonary C-fibres that were activated by capsaicin at a much lower dose (0.6 microg kg(-1)); the response to anandamide showed similar intensity, but had slightly longer latency and duration than that to capsaicin. In conclusion, intravenous bolus injection of anandamide evokes a consistent and distinct stimulatory effect on pulmonary C-fibre terminals, and this effect appears to be mediated through an activation of the vanilloid type 1 receptor.

Anandamide receptors

Anandamide (N -arachidonoyl-ethanolamine, AEA) was the first endogenous ligand of cannabinoid receptors to be discovered. Yet, since early studies, AEA appeared to exhibit also some effects that were not mediated by cannabinoid CB(1) or CB(2) receptors. Indeed, AEA exerts some behavioral actions also in mice with genetically disrupted CB(1) receptors, whereas in vitro it is usually a partial agonist at these receptors and a weak activator of CB(2) receptors. Nevertheless, several pharmacological effects of AEA are mediated by CB(1) receptors, which, by being coupled to G-proteins, can be seen as AEA "metabotropic" receptors. Furthermore, at least two different, and as yet uncharacterized, G-protein-coupled AEA receptors have been suggested to exist in the brain and vascular endothelium, respectively. AEA is also capable of directly inhibiting ion currents mediated by L-type Ca(2+) channels and TASK-1 K(+) channels. However, to date the only reasonably well characterized, non-cannabinoid site of action for AEA is the vanilloid receptor type 1 (VR1), a non-selective cation channel gated also by capsaicin, protons and heat. VR1 might be considered as an AEA "ionotropic" receptor and, under certain conditions, mediates effects ranging from vasodilation, broncho-constriction, smooth muscle tone modulation and nociception to stimulation of hippocampal pair-pulse depression, inhibition of tumor cell growth and induction of apoptosis.

Pharmacology of airway afferent nerve activity

Afferent nerves in the airways serve to regulate breathing pattern, cough, and airway autonomic neural tone. Pharmacologic agents that influence afferent nerve activity can be subclassified into compounds that modulate activity by indirect means (e.g. bronchial smooth muscle spasmogens) and those that act directly on the nerves. Directly acting agents affect afferent nerve activity by interacting with various ion channels and receptors within the membrane of the afferent terminals. Whether by direct or indirect means, most compounds that enter the airspace will modify afferent nerve activity, and through this action alter airway physiology.

Anandamide induces cardiovascular and respiratory reflexes via vasosensory nerves in the anaesthetized rat

1. We tested the hypothesis that sensory nerves innervating blood vessels play a role in the local and systemic regulation of the cardiovascular and respiratory (CVR) systems. We measured CVR reflexes evoked by administration of anandamide (86 - 863 nmoles) and capsaicin (0.3 - 10 nmoles) into the hindlimb vasculature of anaesthetized rats. 2. Anandamide and capsaicin each caused a rapid dose-dependent reflex fall in blood pressure and an increase in ventilation when injected intra-arterially into the hindlimb. 3. Action of both agonists at the vanilloid receptor (VR1) on perivascular sensory nerves was investigated using capsazepine (1 mg kg(-1) i.a.) a competitive VR1 antagonist, ruthenium red (1 mg kg(-1) i.a.), a non-competitive antagonist at VR1, or a desensitizing dose of capsaicin (200 nmoles i.a.). The cannabinoid receptor antagonist SR141716 (1 mg kg(-1) i.a.) was used to determine agonist activity at the CB(1) receptor. 4. Capsazepine, ruthenium red, or acute VR1 desensitization by capsaicin-pretreatment, markedly attenuated the reflex CVR responses evoked by anandamide and capsaicin (P< 0.05; paired Student's t-test). Blockade of CB(1) had no significant effect on the responses to anandamide. 5. Local sectioning of the femoral and sciatic nerves attenuated CVR responses to anandamide and capsaicin (P< 0.05). Vagotomy or carotid sinus sectioning had no significant effect on anandamide- or capsaicin-induced responses. 6. These data demonstrate that both the endogenous cannabinoid, anandamide, and the vanilloid, capsaicin, evoke CVR reflexes when injected intra-arterially into the rat hindlimb. These responses appear to be mediated reflexly via VR1 located on sensory nerve endings within the hindlimb vasculature.

A possible role of lipoxygenase in the activation of vanilloid receptors by anandamide in the guinea-pig bronchus

1. In the absence of indomethacin, anandamide did not contract the guinea-pig bronchus at concentrations up to 100 microM. In the presence of indomethacin (10 microM), anandamide induced concentration-related contractions with a pEC(50) value of 5.18+/-0.11. It was significantly less potent than capsaicin (pEC(50) 7.01+/-0.1). The anandamide uptake inhibitor AM404, produced only a 14.1+/-3.22% contraction at 100 microM. All experiments were conducted in the presence of PMSF (20 microM). 2. The vanilloid receptor antagonist, capsazepine (10 microM), significantly attenuated the contractile effect of anandamide, the response to 100 microM anandamide being 40.53+/-7.04% in the presence of vehicle and 1.57+/-8.93% in the presence of 10 microM capsazepine. The contractile actions of anandamide and AM404 were markedly enhanced by the peptidase inhibitor thiorphan. 3. The log concentration-response curve of anandamide was unaltered by the CB1 receptor antagonist, SR141716A. The pEC(50) values for anandamide were 4.88+/-0.08 and 5.17+/-0.19 in the presence of vehicle and SR141716A (1 microM) respectively. 4. The lipoxygenase inhibitors 5,8,11,14-eicosatetraynoic acid (ETYA) and 5,8,11 eicosatriynoic acid (ETI) reduced the effect of 100 microM anandamide from 34.7+/-1.9% (vehicle) to 7.7+/-5% (ETYA, 10 microM) and from 41.85+/-4.25% (n=6) (vehicle) to 10.31+/-3.54 (n=6) (ETI, 20 microM). Neither inhibitor significantly affected contraction of the tissue by substance P. 5. This study provides evidence that anandamide acts on vanilloid receptors in the guinea-pig isolated bronchus. These data raise the possibility that the contractile action of anandamide may be due, at least in part, to lipoxygenase metabolites of this fatty acid amide that are vanilloid receptor agonists.

The vanilloid receptor (VR1)-mediated effects of anandamide are potently enhanced by the cAMP-dependent protein kinase

The endogenous cannabinoid receptor ligand, anandamide (AEA), is a full agonist of the vanilloid receptor type 1 (VR1) for capsaicin. Here, we demonstrate that the potency and efficacy of AEA at VR1 receptors can be significantly increased by the concomitant activation of protein kinase A (PKA). In human embryonic kidney (HEK) cells over-expressing human VR1, AEA induces a rise in cytosolic Ca(2+) concentration that is mediated by this receptor. The EC(50) for this effect was decreased five-fold in the presence of forskolin (FRSK, 1-5 microM) or the cAMP analogue, 8-Br-cAMP (10-100 microM). The effects of 8-Br-cAMP and FRSK were blocked by a selective PKA inhibitor. The FRSK (10 nM) also potently enhanced the sensory neurone- and VR1-mediated constriction by AEA of isolated guinea-pig bronchi, and this effect was abolished by a PKA inhibitor. In rat dorsal root ganglia slices, AEA-induced release of substance P, an effect mediated by VR1 activation, was enhanced three-fold by FRSK (10 nM). Thus, the ability of AEA to stimulate sensory VR1, with subsequent neuropeptide release, appears to be regulated by the state of activation of PKA. This observation supports the hypothesis that endogenous AEA might stimulate VR1 under certain pathophysiological conditions.