Independence of, and interactions between, cannabinoid and opioid signal transduction pathways in N18TG2 cells

Cannabinoid 1 and mu-Opioid Receptor Agonists Synergistically Inhibit Abdominal Pain and Lack Side Effects in Mice

While effective in treating abdominal pain, opioids have significant side effects. Recent legalization of cannabis will likely promote use of cannabinoids as an adjunct or alternative to opioids, despite a lack of evidence. We aimed to investigate whether cannabinoids inhibit mouse colonic nociception, alone or in combination with opioids at low doses. Experiments were performed on C57BL/6 male and female mice. Visceral nociception was evaluated by measuring visceromotor responses (VMR), afferent nerve mechanosensitivity in flat-sheet colon preparations, and excitability of isolated DRG neurons. Blood oxygen saturation, locomotion, and defecation were measured to evaluate side effects. An agonist of cannabinoid 1 receptor (CB1R), arachidonyl-2'-chloroethylamide (ACEA), dose-dependently decreased VMR. ACEA and HU-210 (another CB1R agonist) also attenuated colonic afferent nerve mechanosensitivity. Additionally, HU-210 concentration-dependently decreased DRG neuron excitability, which was reversed by the CB1R antagonist AM-251. Conversely, cannabinoid 2 receptor (CB2R) agonists did not attenuate VMR, afferent nerve mechanosensitivity, or DRG neuron excitability. Combination of subanalgesic doses of CB1R and µ-opioid receptor agonists decreased VMR; importantly, this analgesic effect was preserved after 6 d of twice daily treatment. This combination also attenuated afferent nerve mechanosensitivity and DRG neuron excitability, which was inhibited by neuronal nitric oxide synthase and guanylate cyclase inhibitors. This combination avoided side effects (decreased oxygen saturation and colonic transit) caused by analgesic dose of morphine. Activation of CB1R, but not CB2R, decreased colonic nociception both alone and in synergy with µ-opioid receptor. Thus, CB1R agonists may enable opioid dose reduction and avoid opioid-related side effects.SIGNIFICANCE STATEMENT One of the most cited needs for patients with abdominal pain are safe and effective treatment options. The effectiveness of opioids in the management of abdominal pain is undermined by severe adverse side effects. Therefore, strategies to replace opioids or reduce the doses of opioids to suppress abdominal pain is needed. This study in mice demonstrates that cannabinoid 1 receptor (CB1R) agonists inhibit visceral sensation. Furthermore, a combination of subanalgesic doses of µ-opioid receptor agonist and CB1R agonist markedly reduce abdominal pain without causing the side effects of high-dose opioids. Thus, CB1R agonists, alone or in combination with low-dose opioids, may be a novel and safe treatment strategy for abdominal pain.

Unidirectional opioid-cannabinoid cross-tolerance in the modulation of social play behavior in rats

RATIONALE

The endocannabinoid and the endogenous opioid systems interact in the modulation of social play behavior, a highly rewarding social activity abundantly expressed in young mammals. Prolonged exposure to opioid or cannabinoid receptor agonists induces cross-tolerance or cross-sensitization to their acute behavioral effects.

OBJECTIVES AND METHODS

Behavioral and biochemical experiments were performed to investigate whether cross-tolerance or cross-sensitization occurs to the play-enhancing effects of cannabinoid and opioid drugs on social play behavior, and the possible brain substrate involved.

RESULTS

The play-enhancing effects induced by systemic administration of JZL184, which inhibits the hydrolysis of the endocannabinoid 2-AG, were suppressed in animals repeatedly pretreated with the opioid receptor agonist morphine. Conversely, acute morphine administration increased social play in rats pretreated with vehicle or with either JZL184 or the cannabinoid agonist WIN55,212-2. Acute administration of JZL184 increased the activation of both CB1 receptors and their effector Akt in the nucleus accumbens and prefrontal cortex, brain regions important for the expression of social play. These effects were absent in animals pretreated with morphine. Furthermore, only animals repeatedly treated with morphine and acutely administered with JZL184 showed reduced activation of CB1 receptors and Akt in the amygdala.

CONCLUSIONS

The present study demonstrates a dynamic opioid-cannabinoid interaction in the modulation of social play behavior, occurring in limbic brain areas strongly implicated in social play behavior. A better understanding of opioid-cannabinoid interactions in social play contributes to clarify neurobiological aspects of social behavior at young age, which may provide new therapeutic targets for social dysfunctions.

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

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

Revolution in GPCR signalling: opioid receptor heteromers as novel therapeutic targets: IUPHAR review 10

GPCRs can interact with each other to form homomers or heteromers. Homomers involve interactions with the same receptor type while heteromers involve interactions between two different GPCRs. These receptor-receptor interactions modulate not only the binding but also the signalling and trafficking properties of individual receptors. Opioid receptor heteromerization has been extensively investigated with the objective of identifying novel therapeutic targets that are as potent as morphine but without the side effects associated with chronic morphine use. In this context, studies have described heteromerization between the different types of opioid receptors and between opioid receptors and a wide range of GPCRs including adrenoceptors, cannabinoid, 5-HT, metabotropic glutamate and sensory neuron-specific receptors. Recent advances in the field involving the generation of heteromer-specific reagents (antibodies or ligands) or of membrane-permeable peptides that disrupt the heteromer interaction are helping to elucidate the physiological role of opioid receptor heteromers and the contribution of the partner receptor to the side effects associated with opioid use. For example, studies using membrane-permeable peptides targeting the heteromer interface have implicated μ and δ receptor heteromers in the development of tolerance to morphine, and heteromers of μ and gastrin-releasing peptide receptors in morphine-induced itch. In addition, a number of ligands that selectively target opioid receptor heteromers exhibit potent antinociception with a decrease in the side effects commonly associated with morphine use. In this review, we summarize the latest findings regarding the biological and functional characteristics of opioid receptor heteromers both in vitro and in vivo.

Antibodies to probe endogenous G protein-coupled receptor heteromer expression, regulation, and function

Over the last decade an increasing number of studies have focused on the ability of G protein-coupled receptors to form heteromers and explored how receptor heteromerization modulates the binding, signaling and trafficking properties of individual receptors. Most of these studies were carried out in heterologous cells expressing epitope tagged receptors. Very little information is available about the in vivo physiological role of G protein-coupled receptor heteromers due to a lack of tools to detect their presence in endogenous tissue. Recent advances such as the generation of mouse models expressing fluorescently labeled receptors, of TAT based peptides that can disrupt a given heteromer pair, or of heteromer-selective antibodies that recognize the heteromer in endogenous tissue have begun to elucidate the physiological and pathological roles of receptor heteromers. In this review we have focused on heteromer-selective antibodies and describe how a subtractive immunization strategy can be successfully used to generate antibodies that selectively recognize a desired heteromer pair. We also describe the uses of these antibodies to detect the presence of heteromers, to study their properties in endogenous tissues, and to monitor changes in heteromer levels under pathological conditions. Together, these findings suggest that G protein-coupled receptor heteromers represent unique targets for the development of drugs with reduced side-effects.

Impact of efficacy at the μ-opioid receptor on antinociceptive effects of combinations of μ-opioid receptor agonists and cannabinoid receptor agonists

Cannabinoid receptor agonists, such as Δ(9)-tetrahydrocannabinol (Δ(9)-THC), enhance the antinociceptive effects of μ-opioid receptor agonists, which suggests that combining cannabinoids with opioids would improve pain treatment. Combinations with lower efficacy agonists might be preferred and could avoid adverse effects associated with large doses; however, it is unclear whether interactions between opioids and cannabinoids vary across drugs with different efficacy. The antinociceptive effects of μ-opioid receptor agonists alone and in combination with cannabinoid receptor agonists were studied in rhesus monkeys (n = 4) using a warm water tail withdrawal procedure. Etorphine, fentanyl, morphine, buprenorphine, nalbuphine, Δ(9)-THC, and CP 55,940 (2-[(1R,2R,5R)-5-hydroxy-2-(3-hydroxypropyl) cyclohexyl]-5-(2-methyloctan-2-yl)phenol) each increased tail withdrawal latency. Pretreatment with doses of Δ(9)-THC (1.0 mg/kg) or CP 55,940 (0.032 mg/kg) that were ineffective alone shifted the fentanyl dose-effect curve leftward 20.6- and 52.9-fold, respectively, and the etorphine dose-effect curve leftward 12.4- and 19.6-fold, respectively. Δ(9)-THC and CP 55,940 shifted the morphine dose-effect curve leftward only 3.4- and 7.9-fold, respectively, and the buprenorphine curve only 5.4- and 4.1-fold, respectively. Neither Δ(9)-THC nor CP 55,940 significantly altered the effects of nalbuphine. Cannabinoid receptor agonists increase the antinociceptive potency of higher efficacy opioid receptor agonists more than lower efficacy agonists; however, because much smaller doses of each drug can be administered in combinations while achieving adequate pain relief and that other (e.g., abuse-related) effects of opioids do not appear to be enhanced by cannabinoids, these results provide additional support for combining opioids with cannabinoids to treat pain.

Disease-specific heteromerization of G-protein-coupled receptors that target drugs of abuse

Drugs of abuse such as morphine or marijuana exert their effects through the activation of G-protein-coupled receptors (GPCRs), the opioid and cannabinoid receptors, respectively. Moreover, interactions between either of these receptors have been shown to be involved in the rewarding effects of drugs of abuse. Recent advances in the field, using a variety of approaches, have demonstrated that many GPCRs, including opioid, cannabinoid, and dopamine receptors, can form associations between different receptor subtypes or with other GPCRs to form heteromeric complexes. The formation of these complexes, in turn, leads to the modulation of the properties of individual protomers. The development of tools that can selectively disrupt GPCR heteromers as well as monoclonal antibodies that can selectively block signaling by specific heteromer pairs has indicated that heteromers involving opioid, cannabinoid, or dopamine receptors may play a role in various disease states. In this review, we describe evidence for opioid, cannabinoid, and dopamine receptor heteromerization and the potential role of GPCR heteromers in pathophysiological conditions.

Opioid receptor heteromers in analgesia

Opiates such as morphine and fentanyl, a major class of analgesics used in the clinical management of pain, exert their effects through the activation of opioid receptors. Opioids are among the most commonly prescribed and frequently abused drugs in the USA; however, the prolonged use of opiates often leads to the development of tolerance and addiction. Although blockade of opioid receptors with antagonists such as naltrexone and naloxone can lessen addictive impulses and facilitate recovery from overdose, systemic disruption of endogenous opioid receptor signalling through the use of these antagonistic drugs can have severe side effects. In the light of these challenges, current efforts have focused on identifying new therapeutic targets that selectively and specifically modulate opioid receptor signalling and function so as to achieve analgesia without the adverse effects associated with chronic opiate use. We have previously reported that opioid receptors interact with each other to form heteromeric complexes and that these interactions affect morphine signalling. Since chronic morphine administration leads to an enhanced level of these heteromers, these opioid receptor heteromeric complexes represent novel therapeutic targets for the treatment of pain and opiate addiction. In this review, we discuss the role of heteromeric opioid receptor complexes with a focus on mu opioid receptor (MOR) and delta opioid receptor (DOR) heteromers. We also highlight the evidence for altered pharmacological properties of opioid ligands and changes in ligand function resulting from the heteromer formation.

Receptor heteromerization expands the repertoire of cannabinoid signaling in rodent neurons

A fundamental question in G protein coupled receptor biology is how a single ligand acting at a specific receptor is able to induce a range of signaling that results in a variety of physiological responses. We focused on Type 1 cannabinoid receptor (CB₁R) as a model GPCR involved in a variety of processes spanning from analgesia and euphoria to neuronal development, survival and differentiation. We examined receptor dimerization as a possible mechanism underlying expanded signaling responses by a single ligand and focused on interactions between CB₁R and delta opioid receptor (DOR). Using co-immunoprecipitation assays as well as analysis of changes in receptor subcellular localization upon co-expression, we show that CB₁R and DOR form receptor heteromers. We find that heteromerization affects receptor signaling since the potency of the CB₁R ligand to stimulate G-protein activity is increased in the absence of DOR, suggesting that the decrease in CB₁R activity in the presence of DOR could, at least in part, be due to heteromerization. We also find that the decrease in activity is associated with enhanced PLC-dependent recruitment of arrestin3 to the CB₁R-DOR complex, suggesting that interaction with DOR enhances arrestin-mediated CB₁R desensitization. Additionally, presence of DOR facilitates signaling via a new CB₁R-mediated anti-apoptotic pathway leading to enhanced neuronal survival. Taken together, these results support a role for CB₁R-DOR heteromerization in diversification of endocannabinoid signaling and highlight the importance of heteromer-directed signal trafficking in enhancing the repertoire of GPCR signaling.

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.

CB1 receptors modulate the intake of a sweetened-fat diet in response to μ-opioid receptor stimulation of the nucleus accumbens

Previous research has demonstrated that concurrent systemic administration of CB(1) cannabinoid and mu-opioid receptor agonists increases feeding in rats. However, the possible neural loci of this cooperative effect have yet to be identified. These studies tested whether the nucleus accumbens shell may be one site of the interactive effects of opioid and cannabinoid ligands on feeding. Injection of the mu-opioid agonist DAMGO (at 0, 0.025, 0.25, or 2.5 µg/0.5 µl/side) directly into the rat nucleus accumbens shell increased feeding on a sweetened-fat diet, and this effect was blocked by pretreatment with either the mu-opioid antagonist naltrexone (20 µg/0.5 µl/side) or the CB(1) antagonist SR141716 (0.5 µg/0.5 µl/side). Activation of nucleus accumbens shell CB(1) receptors with WIN55212-2 alone (at 0.1 or 0.5 µg/0.5 µl/side) had no apparent effect on food intake. However, local injections of the low dose of DAMGO (.025 µg/0.5 µl/side) in this region along with WIN55212-2 (at 0.25 or 0.50 µg/0.5 µl/side) increased feeding above that induced by DAMGO alone. These data suggest an important modulatory role for cannabinoid receptors in the expression of feeding behaviors in response to mu-opioid receptor activation of the nucleus accumbens shell.

Signal transduction via cannabinoid receptors

The endocannabinoids anandamide and 2-arachidonoylglycerol are lipid mediators that signal via CB(1) and CB(2) cannabinoid receptors and Gi/o-proteins to inhibit adenylyl cyclase and stimulate mitogen-activated protein kinase. In the brain, CB(1) receptors interact with opioid receptors in close proximity, and these receptors may share G-proteins and effector systems. In the striatum, CB(1) receptors function in coordination with D(1) and D(2) dopamine receptors, and combined stimulation of CB(1)-D(2) receptor heteromeric complexes promotes a unique interaction to stimulate cAMP production. CB(1) receptors also trigger growth factor receptor signaling cascades in cells by engaging in cross-talk or interreceptor signal transmission with the receptor tyrosine kinase (RTK) family. Mechanisms for CB(1) receptor-RTK transactivation can include stimulation of signal transduction pathways regulated by second messengers such as phospholipase C, metalloprotease cleavage of membrane-bound precursor proteins such as epidermal growth factor which activate RTKs, RTK autophosphorylation, and recruitment of non-receptor tyrosine kinases. CB(1) and CB(2) receptors are expressed in peripheral tissues including liver and adipose tissue, and are induced in pathological conditions. Novel signal transduction resulting from endocannabinoid regulation of AMP-regulated kinase and peroxisome proliferator-activated receptors have been discovered from studies of hepatocytes and adipocytes. It can be predicted that drug discovery of the future will be based upon these novel signal transduction mechanisms for endocannabinoid mediators.

Cannabinoid-opioid interactions during neuropathic pain and analgesia

Opiates and exogenous cannabinoids, both potent analgesics used for the treatment of patients with neuropathic pain, bind to and activate class A G-protein-coupled receptors (GPCRs). Several lines of evidence have recently suggested that opioid and cannabinoid receptors can functionally interact in the central nervous system (CNS). These interactions may be direct, such as through receptor heteromerization, or indirect, such as through signaling cross-talk that includes agonist-mediated release and/or synthesis of endogenous ligands that can activate downstream receptors. Interactions between opioid and cannabinoid receptors may mediate many of the behavioral phenomena associated with the use of these drugs, including the production of acute antinociception and the development of tolerance and cross-tolerance to the antinociceptive effects of opioid and cannabinoid-specific ligands. This review summarizes behavioral, anatomical, and molecular data characterizing these interactions during the development of neuropathic pain and during antinociceptive treatment with these drugs alone or in combination. These studies are critical for understanding how the receptor systems involved in pain relief are altered during acute or chronic pain, and for designing better antinociceptive drug therapies, such as the combined use of opioid and cannabinoid receptor agonists or selective activation of receptor heteromers, that directly target the altered neurophysiology of patients experiencing pain.

Is withdrawal hyperalgesia in morphine-dependent mice a direct effect of a low concentration of the residual drug?

Withdrawal of opioid drugs leads to a cluster of unpleasant symptoms in dependent subjects. These symptoms are stimulatory in nature and oppose the acute, inhibitory effects of opiates. The conventional theory that explains the opioid withdrawal syndrome assumes that chronic usage of opioid drugs activates compensatory mechanisms whose stimulatory effects are revealed upon elimination of the inhibitory opioid drug from the body. Based on previous studies that show a dose-dependent dual activity of opiates, including pain perception, we present here an alternative explanation to the phenomenon of withdrawal-induced hyperalgesia. According to this explanation, the residual low concentration of the drug that remains after cessation of its administration elicits the stimulatory withdrawal hyperalgesia. The goal of the present study was to test this hypothesis. In the present study we rendered mice dependent on morphine by a daily administration of the drug. Cessation of morphine application elicited withdrawal hyperalgesia that was completely blocked by a high dose of the opiate antagonist naloxone (100 mg/kg). Similarly, naloxone (2 mg/kg)-induced withdrawal hyperalgesia was also blocked by 100 mg/kg of naloxone. The blockage of withdrawal hyperalgesia by naloxone suggested the involvement of opioid receptors in the phenomenon and indicated that withdrawal hyperalgesia is a direct effect of a residual, low concentration of morphine. Acute experiments that show morphine- and naloxone-induced hyperalgesia further verified our hypothesis. Our findings offer a novel, alternative approach to opiate detoxifications that may prevent withdrawal symptoms by a complete blockage of the opioid receptors using a high dose of the opioid antagonist.

Modulation of extracellular signal-regulated kinase (ERK) by opioid and cannabinoid receptors that are expressed in the same cell

In the present study we investigated the signal transduction pathways leading to the activation of extracellular signal-regulated kinase (ERK) by opioid or cannabinoid drugs, when their receptors are coexpressed in the same cell-type. In N18TG2 neuroblastoma cells, the opioid agonist etorphine and the cannabinoid agonist CP-55940 induced the phosphorylation of ERK by a similar mechanism that involved activation of delta-opioid receptors or CB1 cannabinoid receptors coupled to Gi/Go proteins, matrix metalloproteases, vascular endothelial growth factor (VEGF) receptors and MAPK/ERK kinase (MEK). In HEK-293 cells, these two drugs induced the phosphorylation of ERK by separate mechanisms. While CP-55940 activated ERK by transactivation of VEGFRs, similar to its effect in N18TG2 cells, the opioid agonist etorphine activated ERK by a mechanism that did not involve transactivation of a receptor tyrosine kinase. Interestingly, the activation of ERK by etorphine was resistant to the inhibition of MEK, suggesting the possible existence of a novel, undescribed yet mechanism for the activation of ERK by opioids. This mechanism was found to be specific to etorphine, as activation of ERK by the micro-opioid receptor (MOR) agonist DAMGO ([D-Ala(2), N-Me-Phe(4), Gly(5)-ol] enkephalin) was mediated by MEK in these cells, suggesting that etorphine and DAMGO activate distinct, ligand-specific, conformations of MOR. The characterization of cannabinoid- and opioid-induced ERK activation in these two cell-lines enables future studies into possible interactions between these two groups of drugs at the level of MAPK signaling.

Role of cannabinoid CB1 receptors and Gi/o protein activation in the modulation of synaptosomal Na+,K+-ATPase activity by WIN55,212-2 and delta(9)-THC

In the present study, we evaluated the effects of the synthetic cannabinoid receptor agonist (R)-(+)-[2,3-Dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN55,212-2) and the active component of Cannabis delta-9-tetrahydrocannabinol (triangle up(9)-THC) on Na(+),K(+)-ATPase activity in synaptosomal mice brain preparation. Additionally, the potential exogenous cannabinoids and endogenous opioid peptides interaction as well as the role of G(i/o) proteins in mediating Na(+),K(+)-ATPase activation were also explored. The ouabain-sensitive Na(+),K(+)-ATPase activity was measured in whole-brain pure intact synaptosomes (obtained by Percoll gradient method) of female CF-1 mice and was calculated as the difference between the total and the ouabain (1 mM)-insensitive Na(+),K(+)-ATPase activities. Incubation in vitro of the synaptosomes with WIN55,212-2 (0.1 pM-10 microM) or triangle up(9)-THC (0.1 pM-0.1 microM), in a concentration-dependent manner, stimulated ouabain-sensitive Na(+),K(+)-ATPase activity. WIN55,212-2 was less potent but more efficacious than triangle up(9)-THC. N-(Piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM-251) (10 nM), a CB(1) cannabinoid receptor selective antagonist, had not effect per se but antagonized the enhancement of Na(+),K(+)-ATPase activity induced by both, WIN55,212-2 and triangle up(9)-THC. AM-251 produced a significant reduction in the E(max) of cannabinoid-induced increase in Na(+),K(+)-ATPase activity, but did not significantly modify their EC(50). On the other hand, co-incubation with naloxone (1 microM), an opioid receptor antagonist, did not significantly modify the effect of WIN55,212-2 and completely failed to modify the effect of triangle up(9)-THC on synaptosomal Na(+),K(+)-ATPase. Finally, pre-incubation with 0.5 microg of pertussis toxin (G(i/o) protein blocker) completely abolished the enhancement of ouabain-sensitive Na(+),K(+)-ATPase activity induced by WIN55,212-2. A lower dose, 0.25 microg, decreased the E(max) of WIN55,212-2 by 70% but did not significantly affect its EC(50). These results suggest that WIN55212-2 and triangle up(9)-THC indirectly enhance Na(+),K(+)-ATPase activity in the brain by activating cannabinoid CB(1) receptors in a naloxone-insensitive manner. In addition, the effect of WIN55,212-2 on neuronal Na(+),K(+)-ATPase is apparently due to activation of G(i/o) proteins.

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

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

Keine synergistische Wirkung der Kombination von ∆9-Tetrahydrocannabinol und Piritramid bei postoperativen Schmerzen

BACKGROUND

It is concluded from animal experiments that cannabinoid receptor and mu-opioid receptor agonists act synergistically with respect to antinociception. In order to demonstrate this effect under clinical conditions, we conducted a randomized double blind trial with patients after radical prostatectomy.

PATIENTS AND METHODS

From the evening before the operation until the morning of the second postoperative day, all patients received eight oral doses of either placebo or 5 mg Delta(9)-tetrahydrocannabinol (dronabinol). Postoperatively patients had access to patient-controlled analgesia with the micro-opioid agonist piritramide for 48 h. We expected patients receiving dronabinol to require significantly less piritramide compared to patients on placebo.

RESULTS

The consumption of piritramide was recorded in 100 patients after radical retropubic prostatectomy with regional lymphadenectomy. Patients in the placebo group consumed 74 mg (median), interquartile range (IQR) 44-90 mg, patients in the verum group consumed 54 mg (median) IQR 46-88 mg. The difference between groups was not statistically significant. Plasma concentrations of Delta(9)-THC were measurable in all patients in the verum group. The levels (median) were 1.5 ng/ml (IQR 0.6-2.3), 1.3 ng/ml (IQR 0.5-2.2) and 1.9 ng/ml (IQR 0.8-2.7) on the day of operation, the first and second postoperative day, respectively.

CONCLUSION

We found neither a synergistic nor even an additive antinociceptive interaction between Delta(9)-tetrahydrocannabinol and the micro-opioid agonist piritramide in a setting of acute postoperative pain.

Cannabinoid receptor signaling

The cannabinoid receptor family currently includes two types: CB1, characterized in neuronal cells and brain, and CB2, characterized in immune cells and tissues. CB1 and CB2 receptors are members of the superfamily of seven-transmembrane-spanning (7-TM) receptors, having a protein structure defined by an array of seven membrane-spanning helices with intervening intracellular loops and a C-terminal domain that can associate with G proteins. Cannabinoid receptors are associated with G proteins of the Gi/o family (Gi1, 2 and 3, and Go1 and 2). Signal transduction via Gi inhibits adenylyl cyclase in most tissues and cells, although signaling via Gs stimulates adenylyl cyclase in some experimental models. Evidence exists for cannabinoid receptor-mediated Ca2+ fluxes and stimulation of phospholipases A and C. Stimulation of CB1 and CB2 cannabinoid receptors leads to phosphorylation and activation of p42/p44 mitogen-activated protein kinase (MAPK), p38 MAPK and Jun N-terminal kinase (JNK) as signaling pathways to regulate nuclear transcription factors. The CB1 receptor regulates K+ and Ca2+ ion channels, probably via Go. Ion channel regulation serves as an important component of neurotransmission modulation by endogenous cannabinoid compounds released in response to neuronal depolarization. Cannabinoid receptor signaling via G proteins results from interactions with the second, third and fourth intracellular loops of the receptor. Desensitization of signal transduction pathways that couple through the G proteins probably entails phosphorylation of critical amino acid residues on these intracellular surfaces.

Molecular mechanisms involved in the asymmetric interaction between cannabinoid and opioid systems

The aim of this work was to study the mechanism of cross-modulation between cannabinoid and opioid systems for analgesia during acute and chronic exposure. Acute coadministration of ineffectual subanalgesic doses of the synthetic cannabinoid CP-55,940 (0.2 mg/kg i.p.) and morphine (2.5 mg/kg i.p.) resulted in significant antinociception. In chronic studies, a low dose of CP-55,940 (0.2 mg/kg, i.p.) that per se did not induce analgesia in naive animals produced a significant degree of antinociception in rats made tolerant to morphine, whereas in rats made tolerant to CP-55,940, morphine challenge did not produce any analgesic response. To identify the mechanism of these asymmetric interactions during chronic treatment, we investigated the functional activity of cannabinoid and mu opioid receptors and their effects on the cyclic AMP (cAMP) cascade. Autoradiographic-binding studies indicated a slight but significant reduction in cannabinoid receptor levels in the hippocampus and cerebellum of morphine-tolerant rats, whereas CP-55,940-stimulated [35S]GTPgammaS binding showed a significant decrease in receptor/G protein coupling in the limbic area. In CP-55,940 exposed rats, mu opioid receptor binding was significantly raised in the lateral thalamus and periaqueductal gray (PAG), with an increase in DAMGO-stimulated [35S]GTPgammaS binding in the nucleus accumbens. Finally, we tested the cAMP system's responsiveness to the cannabinoid and opioid in the striatum and dorsal mesencephalon. In vivo chronic morphine did not affect CP-55,940's ability to inhibit forskolin-stimulated cAMP production in vitro and actually induced sensitization in striatal membranes. In contrast, in vivo chronic CP-55,940 desensitized DAMGO's efficacy in inhibiting forskolin-stimulated cAMP production in vitro. The alterations to the cAMP system seem to mirror the behavioral responses, indicating that the two systems may interact at the postreceptor level. This might open up new therapeutic opportunities for relief of chronic pain through cannabinoid-opioid coadministration.

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

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

Inhibitory interaction of cannabinoid CB1 receptor and dopamine D2 receptor agonists on voltage-gated currents of goldfish cones

Dopamine is a light-adaptive signal that desensitizes the retina, while cannabinoids reportedly increase photosensitivity. The presynaptic membrane of goldfish retinal cones has dopamine D2 receptors and cannabinoid CB1 receptors. This work focused on whether dopamine D2 receptor agonist quinpirole and cannabinoid CB1 receptor agonist WIN 55212-2 (WIN) interacted to modulate voltage-dependent membrane currents of cones. A conventional patch-clamp method was used to record depolarization evoked whole-cell outward currents (Iout) and an inward calcium current (ICa) from the inner segment of cones in goldfish retinal slices. WIN had biphasic actions: low concentrations (<1 μM) increased the currentsviaGs, while higher concentrations (>1 μM) decreased the currentsviaGi/Go. Neither dopamine nor the D2 agonist quinpirole (1–20 μM) had a significant effect on eitherIoutorICa. Quinpirole at 50 μM had a mild suppressive (∼20%) effect onIout. However, quinpirole (<10 μM) completely blocked the enhancement of both currents seen with 0.7 μM WIN. The effect of quinpirole was blocked by sulpiride and by pertussis toxin, indicating that quinpirole was actingviaa D2 receptor-Gi/o coupled mechanism. The suppressive action of 50 μM quinpirole (∼20%) was not additive with the suppressive effect of 3 μM WIN (∼40%). D2 agonistsviaGi/o oppose the action of low concentrations of CB1 agonists actingviaGs to modulate cone membrane currents, suggesting a role in shaping the cone light response and/or sensitivity to changes in ambient light conditions. The nonadditive effect of high concentrations of WIN and quinpirole suggests that both decrease membrane currentsviathe same transduction pathway, Gi/Go protein kinase A (PKA).

Cannabinoids and opioids share cAMP pathway in rat splenocytes

In the present work we investigated on rat splenocytes long-term interactions between opioid and cannabinoid drugs in terms of a common regulation of cAMP intracellular pathway. Both morphine and the synthetic cannabinoid compound CP-55,940 inhibited in a concentration-dependent manner the intracellular cAMP level in splenocytes stimulated by forskolin. The in vitro combination of submaximal concentrations of the two drugs did not yield any additive effect on the inhibition induced by the two drugs. In splenocytes taken from rats chronically treated with CP-55,940 (0.2 mg/kg i.p., twice a day for 4.5 days) or morphine (5 mg/kg s.c., twice a day for 6.5 days) and in vitro exposed to either CP-55,940 or morphine, it was found a desensitisation and cross-desensitisation to the inhibitory effects on cAMP production induced by the two drugs. Binding experiments on the cannabinoid receptors level in spleen coronal sections after in vivo chronic administration of morphine, revealed that there was no changes in the binding of [H3]-CP-55,940. Thus, these results strengthen the hypothesis of cAMP as part of the common intracellular pathway shared by opiates and cannabinoids at immune cell level.

Multiple mechanisms of CB1 cannabinoid receptors regulation

Agonist-induced regulation of cannabinoid CB1 receptors was examined in HEK-293 cells transfected with CB1 receptors and in neuroblastoma N18TG2 cells that naturally express CB1 receptors. In HEK-293 cells, CB1 receptors internalization proceeded, in parallel, via clathrin-coated pits and caveolae. Simultaneous disruption of both pathways induced compensatory endocytic mechanism(s). In N18TG2 cells, endocytosis was not mediated by caveolae-like membrane domains. Heterologous, opioid-induced, downregulation of CB1 receptors was evident in HEK-293 but not N18TG2 cells. The data demonstrate the existence of multiple pathways of CB1 receptors regulation.

Chronic morphine modulates the contents of the endocannabinoid, 2-arachidonoyl glycerol, in rat brain

Opioids and cannabinoids are among the most widely consumed drugs of abuse in humans and the phenomena of cross-tolerance or mutual potentiation have been demonstrated between the two drugs. Several authors have suggested that both drugs share common links in their molecular mechanisms of action, although this has been a matter of controversy. Furthermore, no data exist on the possible adaptive changes in the contents of arachidonoylethanolamide (anandamide, AEA) and 2-arachidonoylglycerol (2-AG), the two major endogenous ligands for cannabinoid receptors, in morphine-tolerant rats. In the present work, we investigated the alterations in cannabinoid receptor functionality and endocannabinoid levels in rats chronically treated with morphine (5 mg/kg, s.c., twice a day for 5 days). Autoradiographic-binding studies using [(3)H]CP-55 940 revealed a slight but significant reduction in cannabinoid receptor level in the cerebellum and hippocampus of morphine-tolerant rats, while CP-55 940-stimulated [(35)S]GTPgammaS binding showed a strong decrease (40%) in receptor/G protein coupling in the limbic area of these animals. Moreover, in the same brain regions we measured, by isotope-dilution gas chromatography/mass spectrometry, the contents of AEA and 2-AG. Chronic morphine exposure produced a strong reduction in 2-AG contents without changes in AEA levels in several brain regions (ie striatum, cortex, hippocampus, limbic area, and hypothalamus). These findings clearly demonstrate that prolonged activation of opioid receptors could alter the cannabinoid system, in terms of both receptor functionality and endocannabinoid levels, and suggest the involvement of this system, alone or in combination with other mediators, in the phenomenon of morphine tolerance.

Long-term interactions between opioid and cannabinoid agonists at the cellular level: cross-desensitization and downregulation

In the present study we investigated long-term interactions between opioid and cannabinoid drugs at several steps along their cellular signal transduction pathways. For this purpose we co-transfected HEK-293 and COS-7 cells with delta-opioid (DOR) and CB1-cannabinoid receptors, and examined the effect of prolonged exposure to either opioid (etorphine) or cannabinoid (DALN) agonists on DOR and CB-1 receptor density and on the ability of subsequent application of the agonists to activate G-proteins (as measured by [35S]GTPgammaS binding) and to inhibit cAMP production. In HEK-293 cells, etorphine induced both homologous and heterologous desensitization, while DALN induced only homologous desensitization. This asymmetric cross-desensitization coincided with asymmetric cross downregulation: etorphine downregulated the binding of the cannabinoid ligand [3H]CP55,940, while DALN failed to reduce the binding of the opioid ligand [3H]diprenorphine. In contrast to the asymmetric desensitization in HEK-293 cells, COS-7 cells presented a two-way cross-desensitization between opioid and cannabinoid agonists, and DALN downregulated the binding of [3H]diprenorphine in these cells. Thus, a complete correlation was found between downregulation and reduction in cell responsiveness ('desensitization'). Moreover, when opioid downregulation in HEK-293 cells was inhibited by either hypertonic sucrose solution or protein kinase inhibitors, desensitization was suppressed to the same extent. These results suggest that, under the present experimental conditions, the reduction in cell responsiveness resulted primarily from downregulation of the receptors.

Cannabinoid CB(1) receptor blockade enhances the protective effect of melanocortins in hemorrhagic shock in the rat

Activation of peripheral cannabinoid CB(1) receptors contributes to hemorrhagic hypotension, and endocannabinoids produced by macrophages and platelets may be mediators of this effect. A number of studies have provided evidence that functional links exist in the mechanisms of action of cannabinoids and opioid peptides; and opioids too play an important role in the pathophysiology of hemorrhagic hypotension and shock. On the other hand, melanocortin peptides, which are the main endogenous functional antagonists of opioid peptides, have an antishock effect in animals and humans. Thus, we investigated whether an interaction exists between endocannabinoids and the endogenous opioid/antiopioid system also in a condition of hemorrhagic shock and, particularly, whether the blockade of cannabinoid CB(1) receptors potentiates the antishock effect of melanocortins. Urethane-anesthetized rats were stepwise bled until mean arterial pressure decreased to, and stabilized at, 21-23 mm Hg. In this model of hemorrhagic shock, which caused the death of all control rats within 30 min after vehicle (tween 80, 5% in saline) injection, the intravenous (i.v.) bolus injection of the cannabinoid CB(1) receptor antagonist N-piperidino-5-[4-chlorophenyl]-1-[2,4 dichlorophenyl]-4-methyl-3-pyrazolecarboxamide (SR141716A) increased mean arterial pressure, pulse pressure, respiratory rate and survival rate in a dose-related manner (0.1-3 mg/kg), an almost complete recovery of mean arterial pressure, pulse pressure and respiratory rate, and 100% survival at the end of the observation period (2 h), occurring with the dose of 3 mg/kg. The melanocortin ACTH-(1-24) (adrenocorticotropin) also produced in a dose-related manner (0.02-0.16 mg/kg i.v.) a restoration of cardiovascular and respiratory functions, and increased survival rate, an almost complete recovery and 100% survival at the end of the observation period (2 h) occurring with the dose of 0.16 mg/kg. When a subactive dose of SR141716A (0.2 mg/kg; 30% survival) was associated with a subactive dose of ACTH-(1-24) (0.02 mg/kg; 12% survival), a complete reversal of the shock condition was obtained with 100% survival at the end of the 2-h observation period. The present results show that the concurrent inhibition of both endogenous opioid and cannabinoid systems produces a reversal of hemorrhagic shock more effective than that produced by the inhibition of either of them. These data suggest that functional interactions between endocannabinoids and opioid/antiopioid are at work also in the pathophysiology of hemorrhagic shock.

Signal transduction interactions between CB1 cannabinoid and dopamine receptors in the rat and monkey striatum

Signal transduction interactions between the CB1 cannabinoid and 1 and D2 dopamine receptor systems were studied in rat (Sprague Dawley) and monkey (Macaca fascilaris) striatal membranes. The D2 agonist quinelorane inhibited forskolin (10 microM)-stimulated adenylyl cyclase in a dose-dependent manner (26% and 20% maximal inhibition; EC50 = 2 and 0.5 microM, in rats and monkeys, respectively) and maximal inhibition was completely blocked by the D2 antagonist sulpiride (10 microM). The CB1 agonist desacetyllevonantradol inhibited forskolin-stimulated adenylyl cyclase (18% and 36% maximal inhibition; EC50 = 160 and 73 nM, in rats and monkeys, respectively) and the CB1 antagonist SR141716A (10 microM) completely blocked the maximal inhibition. Combined addition of > EC(90) concentrations of quinelorane (10, 30 microM) and desacetyllevonantradol (1 microM) resulted in no greater inhibition than that produced by either drug alone, indicative of signal transduction convergence between the D2 and CB1 receptor systems. The D1 agonist 6-Br-APB (3-allyl-6-bromo-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepin) produced a dose-dependent stimulation of adenylyl cyclase (45% and 26% stimulation; EC50 = 24 and 32 nM, in rat and monkey, respectively), and maximal stimulation was completely blocked by the D1 antagonist SCH23390 (1 microM). D1 agonist-stimulated activity could be inhibited to basal levels with desacetyllevonantradol (1 microM), indicative of D1 and CB1 signal transduction convergence. The data suggest that CB1 receptors are co-localized with D1 or D2 receptors on the same population of striatal membranes and can interact at the level of G-protein/adenylyl cyclase signal transduction. Similar results obtained with both rat and monkey membranes indicate that striatal dopamine and cannabinoid interactions are conserved for these two species.

Comparative characterization in the rat of the interaction between cannabinoids and opiates for their immunosuppressive and analgesic effects

In the present work, we investigated in the rat the possibility of functional interaction between opiate and cannabinoid systems at immune level comparatively with the central nervous system (CNS). Moderate analgesic doses of the synthetic cannabinoid compound CP-55,940 (0.2 mg/kg, i.p.) and morphine (5 mg/kg, s.c.) significantly inhibited the ConA-induced splenocyte proliferation and natural killer (NK) cytolytic activity. The acute co-administration of the two drugs resulted in an enhancement of antinociception while they did not yield any additive inhibition of the immune parameters. The CB1 cannabinoid receptor antagonist N-(Piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A; 3 mg/kg, i.p.) and the CB2 receptor antagonist N-[(1S)-endo-1,3,3-trimethhyl bicyclo[2.2.1]heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide (SR144528; 3 mg/kg, i.p.) did not block the central nor the immune effects of morphine; similarly, the opioid receptor antagonist naloxone did not attenuate CP-55,940-induced effects. Animals tolerant to CP-55,940-induced (0.2 mg/kg, i.p.; twice a day for 4 days) or morphine-induced analgesia (5 mg/kg, s.c.; twice a day for 6 days) also developed tolerance to their acute immunosuppressive effects. Concomitantly, animals became cross-resistant to the immunosuppressive effects while an asymmetric cross-tolerance developed for analgesia. Our data demonstrated the existence of an interaction between cannabinoids and opiates at the immune level that differs from the interaction present in the CNS.

Ultrastructural localization of the CB1 cannabinoid receptor in mu-opioid receptor patches of the rat Caudate putamen nucleus

Cannabinoids and opioids are widely consumed drugs of abuse that produce motor depression, in part via respective activation of the cannabinoid subtype 1 receptor (CB1R) and the mu-opioid receptor (muOR), in the striatal circuitry originating in the caudate putamen nucleus (CPN). Thus, the CB1R and muOR may show similar targeting in the CPN. To test this hypothesis, we examined the electron microscopic immunocytochemical labeling of CB1R and muOR in CPN patches of rat brain. Of the CB1R-labeled profiles, 34% (588) were dendrites, presumably arising from spiny as well as aspiny-type somata, which also contained CB1R immunoreactivity. In dendrites, CB1R often was localized to nonsynaptic and synaptic plasma membranes, particularly near asymmetric excitatory-type junctions. Almost one-half of the CB1R-labeled dendrites contained muOR immunoreactivity, whereas only 20% of all muOR-labeled dendrites expressed CB1R. Axons and axon terminals as well as abundant glial processes also showed plasmalemmal CB1R and were mainly without muOR immunoreactivity. Many CB1R-labeled axon terminals were small and without recognizable synaptic junctions, but a few also formed asymmetric, or more rarely symmetric, synapses. The CB1R-labeled glial processes were often perivascular or perisynaptic, surrounding asymmetric excitatory-type axospinous synapses. Our results show that in CPN patches CB1R and muOR are targeted strategically to some of the same postsynaptic neurons, which may account for certain similarities in motor function. Furthermore, they also provide evidence that CB1R may play a major role in the modulation of presynaptic transmitter release and glial functions that are unaffected in large part by opioids active at muOR in CPN.

Opioid and cannabinoid receptors share a common pool of GTP-binding proteins in cotransfected cells, but not in cells which endogenously coexpress the receptors

1. Opioid (mu, delta, kappa) and cannabinoid (CB1, CB2) receptors are coupled mainly to Gi/Go GTP-binding proteins. The goal of the present study was to determine whether different subtypes of opioid and cannabinoid receptors, when coexpressed in the same cell, share a common reservoir, or utilize different pools, of G proteins. 2. The stimulation of [35S]GTPgammaS binding by selective opioid and cannabinoid agonists was tested in transiently transfected COS-7 cells, as well as in neuroblastoma cell lines. In COS-7 cells, cotransfection of mu- and delta-opioid receptors led to stimulation of [35S]GTPgammaS binding by either mu-selective (DAMGO) or delta-selective (DPDPE) agonists. The combined effect of the two agonists was similar to the effect of either DAMGO or DPDPE alone, suggesting the activation of a common G-protein reservoir by the two receptor subtypes. 3. The same phenomenon was observed when COS-7 cells were cotransfected with CB1 cannabinoid receptors and either mu- or delta-opioid receptors. 4. On the other hand, in N18TG2 neuroblastoma cells, which endogenously coexpress CB1 and delta-opioid receptors, as well as in SK-N-SH neuroblastoma cells, which coexpress mu- and delta-opioid receptors, the combined effects of the various agonists (the selective cannabinoid DALN and the selective opioids DPDPE and DAMGO) were additive, implying the activation of different pools of G proteins by each receptor subtype. 5. These results suggest a fundamental difference between native and artificially transfected cells regarding the compartmentalization of receptors and GTP-binding proteins.

Chronic delta-9-tetrahydrocannabinol treatment increases cAMP levels and cAMP-dependent protein kinase activity in some rat brain regions

When Delta(9)-tetrahydrocannabinol (Delta(9)-THC,15 mg/kg) was injected intraperitoneally twice a day for 6 days, tolerance to its analgesic effect appeared to be complete. Chronic exposure to Delta(9)-THC caused a significant reduction in CB1 receptor binding in all brain areas that contain this receptor. Cannabinoid receptor density was markedly reduced in the cerebellum (52%), hippocampus (40%) and globus pallidum (47%) compared to 30% in the cortex and striatum. Chronic exposure enhanced the cAMP pathway, as shown by the significant increase of cAMP levels and PKA activity in the areas with receptor down-regulation (cerebellum, striatum and cortex). We propose that the increase in cAMP cascade is part of the biochemical basis of cannabinoid tolerance.

Pharmacological and biochemical interactions between opioids and cannabinoids

Opioids and cannabinoids are among the most widely consumed drugs of abuse in humans. A number of studies have shown that both types of drugs share several pharmacological properties, including hypothermia, sedation, hypotension, inhibition of both intestinal motility and locomotor activity and, in particular, antinociception. Moreover, phenomena of cross-tolerance or mutual potentiation of some of these pharmacological effects have been reported. In recent years, these phenomena have supported the possible existence of functional links in the mechanisms of action of both types of drugs. The present review addresses the recent advances in the study of pharmacological interactions between opioids and cannabinoids, focusing on two aspects: antinociception and drug addiction. The potential biochemical mechanisms involved in these pharmacological interactions are also discussed together with possible therapeutic implications of opioid-cannabinoid interactions.