Role of the cannabinoid system in pain control and therapeutic implications for the management of acute and chronic pain episodes

Synthesis and pharmacological properties of new GABA uptake inhibitors

BACKGROUND

γ-Aminobutanoic acid (GABA) is the principal inhibitory neurotransmitter in the mammalian central nervous system. The identification and subsequent development of the GABA transport inhibitors which enhance the GABA-ergic transmission has shown the important role that GABA transporters play in the control of numerous functions of the nervous system. Compounds which inhibit GABA uptake are used as antiepileptic drugs (tiagabine - a selective GAT1 inhibitor), they are also being investigated for other indications, including treatment of psychosis, general anxiety, sleep disorders, drug addiction or acute and chronic pain.

METHODS

In this paper, the synthesis of 2-substituted-4-(1,3-dioxoisoindolin-2-ylo)-butanamides and 2-substituted-4-aminobutanoic acids derivatives is described. These compounds were tested in vitro for their ability to inhibit GABA uptake. The inhibitory potency towards murine plasma membrane GABA transporters (mGAT1-4) was performed as [(3)H]GABA uptake assay based on stably transfected HEK cells. Compound 18, which demonstrated the highest affinity for mGAT1-4 (pIC(50) ranged from 4.42 for mGAT1 to 5.07 for mGAT3), was additionally investigated in several behavioral tests in mice.

RESULTS

Compound 18 increased the locomotor activity (14-38%) and had anxiolytic-like properties in the four-plate test (ED(50) = 9.3 mg/kg). It did not show analgesic activity in acute pain model, namely the hot plate test, however, it was antinociceptive in the acetic acid-induced writhing test (ED(50) = 15.3 mg/kg) and in the formalin model of tonic pain. In the latter assay, it diminished nocifensive behavior in both phases and in the first (neurogenic) phase of this test the obtained ED(50) value (5.3 mg/kg) was similar to morphine (3.0 mg/kg).

CONCLUSION

Compound 18 exhibited significant anxiolytic-like properties and was antinociceptive in some models of pain in mice. Moreover, it did not impair animals' motor coordination in the chimney test. Some of the described pharmacological activities of compound 18 can be partly explained based on its affinity for plasma membrane GABA transporters.

Non-psychotropic analgesic drugs from the endocannabinoid system: "magic bullet" or "multiple-target" strategies?

The exploitation of preparations of Cannabis sativa to combat pain seems to date back to time immemorial, although their psychotropic effects, which are at the bases of their recreational use and limit their therapeutic use, are at least as ancient. Indeed, it has always been different to tease apart the unwanted central effects from the therapeutic benefits of Δ⁹-tetrahydrocannabinol (THC), the main psychotropic component of cannabis. The discovery of the cannabinoid receptors and of their endogenous ligands, the endocannabinoids, which, unlike THC, play a pro-homeostatic function in a tissue- and time-selective manner, offered the opportunity to develop new analgesics from synthetic inhibitors of endocannabinoid inactivation. The advantages of this approach over direct activation of cannabinoid receptors as a therapeutic strategy against neuropathic and inflammatory pain are discussed here along with its potential complications. These latter have been such that clinical success has been achieved so far more rapidly with naturally occurring THC or endocannabinoid structural analogues acting at a plethora of cannabinoid-related and -unrelated molecular targets, than with selective inhibitors of endocannabinoid enzymatic hydrolysis, thus leading to revisit the potential usefulness of "multi-target" versus "magic bullet" compounds as new analgesics.

The role of cannabinoids in prostate cancer: Basic science perspective and potential clinical applications

Prostate cancer is a global public health problem, and it is the most common cancer in American men and the second cause for cancer-related death. Experimental evidence shows that prostate tissue possesses cannabinoid receptors and their stimulation results in anti-androgenic effects. To review currently relevant findings related to effects of cannabinoid receptors in prostate cancer. PubMed search utilizing the terms “cannabis,” “cannabinoids,” “prostate cancer,” and “cancer pain management,” giving preference to most recent publications was done. Articles identified were screened for their relevance to the field of prostate cancer and interest to both urologist and pain specialists. Prostate cancer cells possess increased expression of both cannabinoid 1 and 2 receptors, and stimulation of these results in decrease in cell viability, increased apoptosis, and decreased androgen receptor expression and prostate-specific antigen excretion. It would be of interest to conduct clinical studies utilizing cannabinoids for patients with metastatic prostate cancer, taking advantage not only of its beneficial effects on prostate cancer but also of their analgesic properties for bone metastatic cancer pain.

AM-251 and rimonabant act as direct antagonists at mu-opioid receptors: implications for opioid/cannabinoid interaction studies

Mu-opioid and CB1-cannabinoid agonists produce analgesia; however, adverse effects limit use of drugs in both classes. Additive or synergistic effects resulting from concurrent administration of low doses of mu- and CB1-agonists may produce analgesia with fewer side effects. Synergism potentially results from interaction between mu-opioid receptors (MORs) and CB1 receptors (CB1Rs). AM-251 and rimonabant are CB1R antagonist/inverse agonists employed to validate opioid-cannabinoid interactions, presumed to act selectively at CB1Rs. Therefore, the potential for direct action of these antagonists at MORs is rarely considered. This study determined if AM-251 and/or rimonabant directly bind and modulate the function of MORs. Surprisingly, AM-251 and rimonabant, but not a third CB1R inverse agonist AM-281, bind with mid-nanomolar affinity to human MORs with a rank order of affinity (K(i)) of AM-251 (251 nM) > rimonabant (652 nM) > AM281 (2135 nM). AM-251 and rimonabant, but not AM-281, also competitively antagonize morphine induced G-protein activation in CHO-hMOR cell homogenates (K(b) = 719 or 1310 nM, respectively). AM-251 and rimonabant block morphine inhibition of cAMP production, while only AM-251 elicits cAMP rebound in CHO-hMOR cells chronically exposed to morphine. AM-251 and rimonabant (10 mg/kg) attenuate morphine analgesia, whereas the same dose of AM-281 produces little effect. Therefore, in addition to high CB1R affinity, AM-251 and rimonabant bind to MORs with mid-nanomolar affinity and at higher doses may affect morphine analgesia via direct antagonism at MORs. Such CB1-independent of these antagonists effects may contribute to reported inconsistencies when CB1/MOR interactions are examined via pharmacological methods in CB1-knockout versus wild-type mice.

Tetrahydrocannabinol (Delta 9-THC) Treatment in Chronic Central Neuropathic Pain and Fibromyalgia Patients: Results of a Multicenter Survey

Central neuropathic pain is difficult to treat, but delta 9-Tetrahydrocannabinol (delta 9-THC) may be a promising therapeutic agent. We administered in 172 patients on average 7.5 mg delta 9-THC over 7 months. Of these, 48 patients prematurely withdrew due to side effects, insufficient analgesia, or expense of therapy. Thus, 124 patients were assessed retrospectively in a multicenter telephone survey. Reported changes in pain intensity, recorded on a numeric rating scale (NRS), Pain Disability Index (PDI), Medical Outcomes Short-Form (SF-12), Quality of Life Impairment by Pain (QLIP), Hospital Anxiety Depression Scale (HADS), and amount of concomitant pain medication were recorded. Psychometric parameters (PDI, SF-12, QLIP, HADS) and pain intensity improved significantly during delta 9-THC treatment. Opioid doses were reduced and patients perceived THC therapy as effective with tolerable side effects. About 25% of the patients, however, did not tolerate the treatment. Therapy success and tolerance can be assessed by a transient delta 9-THC titration and its maintained administration for several weeks. The present survey demonstrates its ameliorating potential for the treatment of chronic pain in central neuropathy and fibromyalgia. A supplemental delta 9-THC treatment as part of a broader pain management plan therefore may represent a promising coanalgesic therapeutic option.

Do cannabinoids have a role in cancer pain management?

PURPOSE OF REVIEW

Historically cannabinoids have been used for both therapy and recreation, yet the elucidation of the endocannabinoid system and their chemistry has been relatively recent. Prohibition of cannabis has meant few clinical trials, especially in cancer pain. This review will consider previous animal and clinical data and assess more recent investigations of clinical effectiveness of cannabinoids in pain and specifically cancer pain.

RECENT FINDINGS

Meta-analyses based on historical studies question the utility of cannabinoids in pain due to modest analgesia and problematic central side effects. However, there has been a resurgence in clinical trials of cannabis extracts and analogues. New data have contributed to the understanding of how cannabinoids work and proposed how to obtain analgesia unfettered by adverse effects. Moreover, recent clinical trials have demonstrated the current role of cannabinoids may be to attain small but significant benefit in refractory chronic and cancer pain.

SUMMARY

Cannabinoids may be a useful addition to current analgesic treatments. The evidence supports a possible role for cannabinoids in refractory cancer pain. However, to realize the full potential of cannabinoids suggested by preclinical data, it is likely that peripheral CB1 or CB2 receptors or modulation of endocannabinoids will have to be targeted to achieve analgesia without dose limiting side effects.

Pharmacological and therapeutic secrets of plant and brain (endo)cannabinoids

Research on the chemistry and pharmacology of cannabinoids and endocannabinoids has reached enormous proportions, with approximately 15,000 articles on Cannabis sativa L. and cannabinoids and over 2,000 articles on endocannabinoids. The present review deals with the history of the Cannabis sativa L. plant, its uses, constituent compounds and their biogeneses, and similarity to compounds from Radula spp. In addition, details of the pharmacology of natural cannabinoids, as well as synthetic agonists and antagonists are presented. Finally, details regarding the pioneering isolation of the endocannabinoid anandamide, as well as the pharmacology and potential therapeutic uses of endocannabinoid congeners are presented.

Pathological and protective roles of glia in chronic pain

Glia have emerged as key contributors to pathological and chronic pain mechanisms. On activation, both astrocytes and microglia respond to and release a number of signalling molecules, which have protective and/or pathological functions. Here we review the current understanding of the contribution of glia to pathological pain and neuroprotection, and how the protective, anti-inflammatory actions of glia are being harnessed to develop new drug targets for neuropathic pain control. Given the prevalence of chronic pain and the partial efficacy of current drugs, which exclusively target neuronal mechanisms, new strategies to manipulate neuron-glia interactions in pain processing hold considerable promise.

Cannabinoid CB1 receptors are expressed in the mouse urinary bladder and their activation modulates afferent bladder activity

Pharmacological studies have indirectly shown the possible presence of cannabinoid receptors in the urinary bladder and their potential role in reducing bladder inflammatory pain. However, the localization of cannabinoid receptors in the urinary bladder remains unknown and there are no published data on the effects of cannabinoids on the sensory system of the bladder. The present study was performed to evaluate the expression of the cannabinoid CB(1) receptors in the mouse urinary bladder and to assess their co-localization with the purinergic P2X(3) receptor, a major player in the transduction of sensory events in the bladder. Also, the effect of intravesical administration of a cannabinoid agonist on the electrical activity of bladder afferent fibers was studied. The expression of mRNA coding for CB(1) receptor was assessed by reverse transcriptase-polymerase chain reaction (RT-PCR). Immunofluorescence experiments were performed to study CB(1) and P2X(3) protein expression in the bladder. The electrical activity of bladder afferent fibers was recorded using an ex vivo bladder-nerve preparation. Mechanical stimulation of the bladder was performed by a controlled slow inflation with an external pump. A bolus of a cannabinoid agonist (AZ12646915) was administered intravesically prior to a second inflation. Afferent activity was measured before and after administration of the cannabinoid compound or its vehicle. The effects of CB(1) receptor antagonist (AM251) on the AZ12646915 response were also analyzed. Cannabinoid receptor CB(1) mRNA was detected in the urinary bladder of the mouse. The protein was found in the urothelium, as well as in nerve fibers. CB(1) and P2X(3) receptors were found to be co-expressed in urothelial cells and in some nerve fibers. In addition, intravesical administration of a cannabinoid receptor agonist reduced the mechanically-evoked activity of bladder afferents in the pelvic nerve. This effect was abolished by the previous administration of the CB(1) antagonist AM251. These data demonstrate the presence of cannabinoid CB(1) receptor mRNA and the protein in the mouse urinary bladder. CB(1) and P2X(3) protein co-localization supports the hypothesis of an interaction between the cannabinoid and the purinergic systems in the transduction of sensory information in the urinary bladder. Finally, the reduction of nerve activity induced by cannabinoid-receptor activation implicates CB(1) receptors in the peripheral modulation of bladder afferent information.

Targeting cannabinoid agonists for inflammatory and neuropathic pain

The cannabinoid receptors CB(1) and CB(2) are class A G-protein-coupled receptors. It is well known that cannabinoid receptor agonists produce relief of pain in a variety of animal models by interacting with cannabinoid receptors. CB(1) receptors are located centrally and peripherally, whereas CB(2) receptors are expressed primarily on immune cells and tissues. A large body of preclinical data supports the hypothesis that either CB(2)-selective agonists or CB(1) agonists acting at peripheral sites, or with limited CNS exposure, will inhibit pain and neuroinflammation without side effects within the CNS. There has been a growing interest in developing cannabinoid agonists. Many new cannabinoid ligands have been synthesized and studied covering a wide variety of novel structural scaffolds. This review focuses on the present development of cannabinoid agonists with an emphasis on selective CB(2) agonists and peripherally restricted CB(1) or CB(1)/CB(2) dual agonists for treatment of inflammatory and neuropathic pain.