Evaluation of novel epibatidine analogs in the rat nicotine drug discrimination assay and in the rat chronic constriction injury neuropathic pain model.

Nicotine is the primary psychoactive component responsible for maintaining tobacco dependence in humans. Chronic pain is often a consequence of tobacco-related pathologies, and the development of a dual therapeutic that could treat chronic pain and tobacco dependence would be advantageous. Epibatidine reliably substitutes for nicotine in the drug discrimination assay, and is a potent analgesic, but has a side-effect profile that limits its therapeutic potential. Thus, considerable efforts to produce epibatidine derivatives are underway. Here we tested three epibatidine derivatives, 2'-fluoro-3'-(4-nitrophenyl)deschloroepibatidine (RTI-7527-102; i.e., RTI-102), 2'-fluorodeschloroepibatidine (RTI-7527-36; i.e., RTI-36), and 3'-(3″-dimethylaminophenyl)-epibatidine (RTI-7527-76; i.e., RTI-76) in both the rat nicotine drug discrimination assay as well as in the rat chronic constriction injury (CCI) of the sciatic nerve neuropathic pain model. Male and female Sprague-Dawley rats were trained on a fixed-ratio 10 schedule to discriminate nicotine (0.32 mg/kg base) from vehicle. All compounds dose-dependently substituted for nicotine, without significant decreases in response rates. In the discrimination assay the rank order potency was RTI-36 > nicotine > RTI-102 > RTI-76. Evidence suggests the α4ß2* subtype is particularly important to nicotine-related abuse potential. Thus, here we utilized the antagonist dihydro-ß-erythroidine (DHßE) to examine relative ß2 subunit contribution. DHßE (3.2 mg/kg, s.c.) antagonized the discriminative stimulus effects of nicotine. However, relative to antagonism of nicotine, DHßE produced less antagonism of RTI-102 and RTI-76 and greater antagonism of RTI-36. It is likely that at nicotinic receptor subunits RTI-102, RTI-76 and RTI-36 possess differing activity. To confirm that the full discriminative stimulus of these compounds was due to nAChR activity beyond the ß2 subunit, we examined these compounds in the presence of the non-selective nicotinic receptor antagonist mecamylamine. Mecamylamine (0.56 mg/kg, s.c.) pretreatment abolished nicotine-paired lever responding for all compounds. In a separate cohort, male and female Sprague-Dawley rats underwent CCI surgery and tested for CCI-induced mechanical allodynia via the von Frey assay. Each compound produced CCI-induced mechanical allodynia reversal. RTI-36 displayed higher potency than either RTI-102 or RTI-76. These novel epibatidine analogs may prove to be useful tools in the fight against nicotine dependence as well as novel neuropathic pain analgesics.

Cannabidiol and mitragynine exhibit differential interactive effects in the attenuation of paclitaxel-induced mechanical allodynia, acute antinociception, and schedule-controlled responding in mice.

BACKGROUND: For many chemotherapy patients peripheral neuropathy is a debilitating side effect. Mitragyna speciosa (kratom) contains the alkaloid mitragynine (MG), which produces analgesia in multiple preclinical pain models. In humans, anecdotal reports suggest cannabidiol (CBD) may enhance kratom-related analgesia. We examined the interactive activity of MG and CBD in a mouse chemotherapy-induced peripheral neuropathy (CIPN) model. We also examined MG + CBD in acute antinociception and schedule-controlled responding assays, as well as examined underlying receptor mechanisms. METHODS: Male and female C57BL/6J mice received a cycle of intraperitoneal (ip) paclitaxel injections (cumulative dose 32 mg/kg). The von Frey assay was utilized to assess CIPN allodynia. In paclitaxel-naïve mice, schedule-controlled responding for food was conducted under a fixed ratio (FR)-10, and hot plate antinociception was examined. RESULTS: MG dose-relatedly attenuated CIPN allodynia (ED50 102.96 mg/kg, ip), reduced schedule-controlled responding (ED50 46.04 mg/kg, ip), and produced antinociception (ED50 68.83 mg/kg, ip). CBD attenuated allodynia (ED50 85.14 mg/kg, ip) but did not decrease schedule-controlled responding or produce antinociception. Isobolographic analysis revealed 1:1, 3:1 MG + CBD mixture ratios additively attenuated CIPN allodynia. All combinations decreased schedule-controlled responding and produced antinociception. WAY-100635 (serotonin 5-HT1A receptor antagonist) pretreatment (0.01 mg/kg, ip) antagonized CBD anti-allodynia. Naltrexone (pan opioid receptor antagonist) pretreatment (0.032 mg/kg, ip) antagonized MG anti-allodynia and acute antinociception but produced no change in MG-induced decreased schedule-controlled behavior. Yohimbine (α2 receptor antagonist) pretreatment (3.2 mg/kg, ip) antagonized MG anti-allodynia and produced no change in MG-induced acute antinociception or decreased schedule-controlled behavior. CONCLUSIONS: Although more optimization is needed, these data suggest CBD combined with MG may be useful as a novel CIPN therapeutic.

Evaluation of the terpenes ß-caryophyllene, α-terpineol, and γ-terpinene in the mouse chronic constriction injury model of neuropathic pain: possible cannabinoid receptor involvement.

Pain is one of the most common reasons to seek medical attention, and chronic pain is a worldwide epidemic. Anecdotal reports suggest cannabis may be an effective analgesic. As cannabis contains the terpenes α-terpineol, ß-caryophyllene, and γ-terpinene, we hypothesized these terpenes would produce analgesia in a mouse model of neuropathic pain. We used the chronic constriction injury of the sciatic nerve mouse model, which produces mechanical allodynia, assessed via the von Frey assay, as well as thermal hyperalgesia assessed via the hotplate assay. Compounds were further assessed in tests of locomotor activity, hypothermia, and acute antinociception. Each terpene produced dose-related reversal of mechanical allodynia and thermal hyperalgesia. Thermal hyperalgesia displayed higher sensitivity to the effects of each terpene than mechanical allodynia, and the rank order potency of the terpenes was α-terpineol > ß-caryophyllene > γ-terpinene. To examine the involvement of cannabinoid receptors, further tests were conducted in mice lacking either functional cannabinoid type 1 receptors (CB1R (-/-)) or cannabinoid type 2 receptors (CB2R (-/-)). Compared to wild type mice, CB1R (-/-) mice treated with α-terpineol displayed a 2.91-fold decrease in potency to reverse mechanical allodynia; in CB2R (-/-) mice, the potency of α-terpineol was decreased 11.73-fold. The potency of ß-caryophyllene to reverse mechanical allodynia decreased 1.80-fold in CB2R (-/-) mice. Each terpene produced a subset of effects in tests of locomotor activity, hypothermia, and acute antinociception. These findings suggest α-terpineol, ß-caryophyllene, and γ-terpinene may have differential cannabinoid receptor activity and a pharmacological profile that may yield new efficacious analgesics.

Characterization of a mouse neuropathic pain model caused by the highly active antiviral therapy (HAART) Stavudine.

BACKGROUND: Although highly active antiviral therapies (HAART) exert control over viral replication in persons with Acquired Immunodeficiency Syndrome (AIDS), neuropathic pain is a side effect. Symptoms include hyperalgesia and allodynia. Stavudine, also known as D4T, is a HAART used to treat Human Immunodeficiency Virus (HIV). This study examined the extent to which D4T produces neuropathic pain and examined pharmacological management with a standard opioid analgesic. METHODS: Male and female C57BL/6 J mice were injected intraperitoneally with one dose of vehicle or D4T (10-56 mg/kg). Mice were tested through day 92 post injection for mechanical allodynia, assessed with von Frey filaments, and thermal hyperalgesia, assessed via the hotplate test. Separate cohorts received vehicle or 56 mg/kg D4T, the presence of allodynia and thermal hyperalgesia confirmed, and mice received intraperitoneal vehicle, morphine, or 0.032 mg/kg naltrexone + morphine. RESULTS: D4T produced dose- and time-dependent mechanical allodynia and thermal hyperalgesia. The smallest effective D4T dose was 17.8 mg/kg. This dose produced mechanical allodynia but not thermal hyperalgesia. Larger D4T doses (32 and 56 mg/kg) produced mechanical allodynia and thermal hyperalgesia lasting 92 days. Morphine dose-dependently alleviated both mechanical allodynia and thermal hyperalgesia in D4T-treated mice with ED50 values of 4.4 and 1.2 mg/kg, respectively. Naltrexone produced a rightward shift of the morphine dose-response function, i.e., increased the ED50 value of morphine by at least 3.8-fold. CONCLUSION: Stavudine produced neuropathic pain as a function of dose and time in mice. Opioid analgesics appear to be effective in alleviating neuropathic pain in a D4T-induced mouse model.

Untapped endocannabinoid pharmacological targets: Pipe dream or pipeline?

It has been established that the endogenous cannabinoid (endocannabinoid) system plays key modulatory roles in a wide variety of pathological conditions. The endocannabinoid system comprises both cannabinoid receptors, their endogenous ligands including 2-arachidonoylglycerol (2-AG), N-arachidonylethanolamine (anandamide, AEA), and enzymes that regulate the synthesis and degradation of endogenous ligands which include diacylglycerol lipase alpha (DAGL-α), diacylglycerol lipase beta (DAGL-ß), fatty acid amide hydrolase (FAAH), monoacylglycerol lipase (MAGL), α/ß hydrolase domain 6 (ABHD6). As the endocannabinoid system exerts considerable involvement in the regulation of homeostasis and disease, much effort has been made towards understanding endocannabinoid-related mechanisms of action at cellular, physiological, and pathological levels as well as harnessing the various components of the endocannabinoid system to produce novel therapeutics. However, drug discovery efforts within the cannabinoid field have been slower than anticipated to reach satisfactory clinical endpoints and raises an important question into the validity of developing novel ligands that therapeutically target the endocannabinoid system. To answer this, we will first examine evidence that supports the existence of an endocannabinoid system role within inflammatory diseases, neurodegeneration, pain, substance use disorders, mood disorders, as well as metabolic diseases. Next, this review will discuss recent clinical studies, within the last 5 years, of cannabinoid compounds in context to these diseases. We will also address some of the challenges and considerations within the cannabinoid field that may be important in the advancement of therapeutics into the clinic.

Sex differences and the endocannabinoid system in pain.

Cannabis use has been increasing in recent years, particularly among women, and one of the most common uses of cannabis for medical purposes is pain relief. Pain conditions and response to analgesics have been demonstrated to be influenced by sex, and evidence is emerging that this is also true with cannabinoid-mediated analgesia. In this review we evaluate the preclinical evidence supporting sex differences in cannabinoid pharmacology, as well as emerging evidence from human studies, both clinical and observational. Numerous animal studies have reported sex differences in the antinociceptive response to natural and synthetic cannabinoids that may correlate to sex differences in expression, and function, of endocannabinoid system components. Female rodents have generally been found to be more sensitive to the effects of Δ9-THC. This finding is likely a function of both pharmacokinetic and pharmacodynamics factors including differences in metabolism, differences in cannabinoid receptor expression, and influence of ovarian hormones including estradiol and progesterone. Preclinical evidence supporting direct interactions between sex hormones and the endocannabinoid system may translate to sex differences in response to cannabis and cannabinoid use in men and women. Further research into the role of sex in endocannabinoid system function is critical as we gain a deeper understanding of the impact of the endocannabinoid system in various disease states, including chronic pain.