Non-cannabinoid CB1, non-cannabinoid CB2 antinociceptive effects of several novel compounds in the PPQ stretch test in mice.

The analgesic and anti-hyperalgesic effects of cannabinoid- and vanilloid-like compounds, plus the fatty acid amide hydrolase (FAAH) inhibitor Cyclohexylcarbamic acid 3'-carbamoyl-biphenyl-3-yl ester (URB597), and acetaminophen, were evaluated in the phenyl-p-quinone (PPQ) pain model, using different routes of administration in combination with opioid and cannabinoid receptor antagonists. All the compounds tested produced analgesic effects. Delta(9)-tetrahydrocannabinol (Delta(9)-THC) and (R)-(+)-arachidonyl-1'-hydroxy-2'-propylamide ((R)-methanandamide) were active by three routes of administration: i.p., s.c. and, p.o. Delta(9)-THC produced ED(50)s of 2.2 mg/kg (0.3-15.6) i.p., 9 mg/kg (4.3-18.9) s.c., and 6.4 mg/kg (5.5-7.6) p.o. Similarly, (R)-methanandamide yielded ED(50)s of 2.9 mg/kg (1-8) i.p., 11 mg/kg (7-17) s.c., and 11 mg/kg (0.9-134) p.o. N-vanillyl-arachidonyl-amide (arvanil) was active by two routes, producing ED(50)s of 4.7 mg/kg (3.0-7.4) s.c. and 0.06 mg/kg (0.02-0.2) i.p. Palmitoylethanolamide, URB597, and acetaminophen were active i.p., resulting in ED(50)s of 3.7 mg/kg (3.2-4.2), 22.9 mg/kg (11.1-47.2), and 160 mg/kg (63-405), respectively. None of the cannabinoid or opioid receptor antagonists tested blocked the compounds evaluated, with two exceptions: the antinociceptive effects of Delta(9)-THC and URB597 were completely blocked by SR141716A, a cannabinoid CB(1) receptor antagonist. Western immunoassays performed using three opioid receptor antibodies, a cannabinoid CB(1) receptor antibody and a transient receptor potential vanilloid type 1(TRPV(1)) receptor antibody, yielded no change in receptor protein levels after short-term arvanil, (R)-methanandamide or Delta(9)-THC administration. These data suggest that all the compounds tested, except Delta(9)-THC and URB597, produced analgesia via a non-cannabinoid CB(1), non-cannabinoid CB(2) pain pathway not yet identified.

Enhancement of transdermal fentanyl and buprenorphine antinociception by transdermal delta9-tetrahydrocannabinol.

Previous studies have demonstrated that delta9-tetrahydrocannabinol (THC) enhances the antinociceptive potency of many opioids administered by a variety of different routes of administration. We hypothesized that THC would enhance fentanyl or buprenorphine analgesia via the transdermal route of administration. THC was first demonstrated to enhance opioid antinociception when both drugs were administered parenterally in a hairless guinea pig model using the pin prick test. A low dose of THC (50 mg/kg, i.p.) produced no antinociception. However, THC enhanced the potency of s.c. fentanyl by 6.7-fold, and s.c. buprenorphine in a non-parallel fashion. For the transdermal studies, THC, fentanyl or buprenorphine was applied by pipette to the skin of the dorsum between the fore- and hind-flanks and covered with individual Tegederm patches. THC (400 mg/kg) produced no antinociception. However, THC enhanced fentanyl's potency by 3.7-fold at 2-h, and 5.8-fold at 4-h. Buprenophine's potency was increased 8.2-fold at 2-h and 7.2-fold at 4-h when co-administered with THC. These results indicate that the enhancement of transdermal opioids by THC could lead to the design of an effective combination analgesic patch.

Mu and delta opioid-stimulated [35S]GTP gamma S binding in brain and spinal cord of polyarthritic rats.

Polyarthritis induced by inoculation with complete Freund's adjuvant alters opioid peptides, but does not affect opioid receptor binding. This study was conducted to measure mu and delta opioid receptor-stimulated G-protein activity in brain and spinal cord of rats 19 days after injection of complete Freund's adjuvant or vehicle. Mu and delta opioid-stimulated [35S]GTPgammaS binding measured autoradiographically in caudate-putamen, medial thalamus and periaqueductal gray was unchanged in polyarthritic rats. Delta opioid-stimulated [35S]GTPgammaS binding was significantly decreased in the spinal cord of polyarthritic rats, whereas mu opioid-stimulated activity was unchanged. These data reveal that the functional activity of delta opioid receptors in the spinal cord is altered in polyarthritis.

Synergistic interactions between cannabinoid and opioid analgesics.

Cannabinoids and opioids both produce analgesia through a G-protein-coupled mechanism that blocks the release of pain-propagating neurotransmitters in the brain and spinal cord. However, high doses of these drugs, which may be required to treat chronic, severe pain, are accompanied by undesirable side effects. Thus, a search for a better analgesic strategy led to the discovery that delta 9-tetrahydrocannabinol (THC), the major psychoactive constituent of marijuana, enhances the potency of opioids such as morphine in animal models. In addition, studies have determined that the analgesic effect of THC is, at least in part, mediated through delta and kappa opioid receptors, indicating an intimate connection between cannabinoid and opioid signaling pathways in the modulation of pain perception. A host of behavioral and molecular experiments have been performed to elucidate the role of opioid receptors in cannabinoid-induced analgesia, and some of these findings are presented below. The aim of such studies is to develop a novel analgesic regimen using low dose combinations of cannabinoids and opioids to effectively treat acute and chronic pain, especially pain that may be resistant to opioids alone.

Antinociceptive synergy between delta(9)-tetrahydrocannabinol and opioids after oral administration.

The analgesic effects of opioids, such as morphine and codeine, in mice are enhanced by oral administration of the cannabinoid delta(9)-tetrahydrocannabinol (delta(9)-THC). However, isobolographic analysis has never been done to confirm a synergy between delta(9)-THC and morphine or codeine via oral routes of administration. To determine the nature of the interaction between these drugs for pain relief and extend previous experimental results, we performed an isobolographic analysis to evaluate for additivity or synergy in the tail-flick test. Fixed-ratio combinations of delta(9)-THC with either morphine or codeine were tested for antinociceptive effects. The experimentally derived ED(50) for each combination was compared with the theoretical additive ED(50), using an isobolographic analysis. All of the fixed-ratio combinations tested produced greater antinociception (synergy) than predicted from simple additivity. These findings suggest that the use of a low-dose combination of analgesics is a valid and effective approach for the treatment of pain and necessitates further study.

Modulation of oral morphine antinociceptive tolerance and naloxone-precipitated withdrawal signs by oral Delta 9-tetrahydrocannabinol.

Previous studies have demonstrated a functional interaction between cannabinoid and opioid systems in the development and expression of morphine tolerance and dependence. In these experiments, we examined the effect of a low oral dose of Delta 9-tetrahydrocannabinol (Delta 9-THC) on the development of oral morphine tolerance and the expression of naloxone-precipitated morphine withdrawal signs of jumping and diarrhea in ICR mice. Chronic treatment with high-dose oral morphine produced a 3.12-fold antinociceptive tolerance. Tolerance to morphine was prevented in groups receiving a daily cotreatment with a nonanalgetic dose (20 mg/kg p.o.) of Delta 9-THC, except when challenged with a very high dose of morphine. The chronic coadministration of low-dose Delta 9-THC also reduced naloxone-precipitated (1 mg/kg s.c.) platform jumping by 50% but did not reduce diarrhea. In separate experiments, mice treated chronically with high-dose morphine p.o. were not cross-tolerant to Delta 9-THC; in fact, these morphine-tolerant mice were more sensitive to the acute antinociceptive effects of Delta 9-THC. Delta 9-THC (20 mg/kg p.o.) also reduced naloxone-precipitated jumping but not diarrhea when administered acutely to morphine-tolerant mice. These results represent the first evidence that oral morphine tolerance and dependence can be circumvented by coadministration of a nonanalgetic dose of Delta 9-THC p.o. In summary, cotreatment with a combination of morphine and Delta 9-THC may prove clinically beneficial in that long-term morphine efficacy is maintained.