Low dose combination of morphine and delta9-tetrahydrocannabinol circumvents antinociceptive tolerance and apparent desensitization of receptors

Abstract

Morphine and delta9-tetrahydrocannabinol (THC) produce antinociception via mu opioid and cannabinoid CB1 receptors, respectively, located in central nervous system (CNS) regions including periaqueductal gray and spinal cord. Chronic treatment with morphine or THC produces antinociceptive tolerance and cellular adaptations that include receptor desensitization. Previous studies have shown that administration of combined sub-analgesic doses of THC+morphine produced antinociception in the absence of tolerance. The present study assessed receptor-mediated G-protein activity in spinal cord and periaqueductal gray following chronic administration of THC, morphine or low dose combination. Rats received morphine (escalating doses from 1 to 6x75 mg s.c. pellets or s.c. injection of 100 to 200 mg/kg twice daily), THC (4 mg/kg i.p. twice daily) or low dose combination (0.75 mg/kg each morphine (s.c) and THC (i.p.) twice daily) for 6.5 days. Antinociception was measured in one cohort of rats using the paw pressure test, and a second cohort was assessed for agonist-stimulated [35S]GTPgammaS binding. Chronic administration of morphine or THC produced antinociceptive tolerance to the respective drugs, whereas combination treatment did not produce tolerance. Administration of THC attenuated cannabinoid CB1 receptor-stimulated G-protein activity in both periaqueductal gray and spinal cord, and administration of morphine decreased mu opioid receptor-stimulated [35S]GTPgammaS binding in spinal cord or periaqueductal gray, depending on route of administration. In contrast, combination treatment did not alter cannabinoid CB1 receptor- or mu opioid receptor-stimulated G-protein activity in either region. These results demonstrate that low dose THC-morphine combination treatment produces antinociception in the absence of tolerance or attenuation of receptor activity.

Fig. 1

The effects of chronic morphine pellet implantation (6 x 75 mg pellets)(A), chronic THC (4 mg/kg i.p, twice daily) (B), low dose combination (0.75 mg/kg each morphine (s.c) and THC (i.p.) twice daily)(C) or morphine injection (100, 150 and 200 mg/kg (s.c.) twice daily) (D) in the paw withdrawal test over a 7-day period. Data are mean percent MPE ± S.E., with each point representing 8–10 rats in the paw pressure test. *, **,***: p < 0.05, 0.01 or 0.001, respectively, different from vehicle treated mice determined by ANOVA with post-hoc Newman-Keuls test. ^†,††: p < 0.05, 0.01, respectively, different from day 1 within the same treatment group determined by repeated measures ANOVA with post-hoc Dunnett’s test.

Fig. 1

The effects of chronic morphine pellet implantation (6 x 75 mg pellets)(A), chronic THC (4 mg/kg i.p, twice daily) (B), low dose combination (0.75 mg/kg each morphine (s.c) and THC (i.p.) twice daily)(C) or morphine injection (100, 150 and 200 mg/kg (s.c.) twice daily) (D) in the paw withdrawal test over a 7-day period. Data are mean percent MPE ± S.E., with each point representing 8–10 rats in the paw pressure test. *, **,***: p < 0.05, 0.01 or 0.001, respectively, different from vehicle treated mice determined by ANOVA with post-hoc Newman-Keuls test. ^†,††: p < 0.05, 0.01, respectively, different from day 1 within the same treatment group determined by repeated measures ANOVA with post-hoc Dunnett’s test.

Fig. 1

The effects of chronic morphine pellet implantation (6 x 75 mg pellets)(A), chronic THC (4 mg/kg i.p, twice daily) (B), low dose combination (0.75 mg/kg each morphine (s.c) and THC (i.p.) twice daily)(C) or morphine injection (100, 150 and 200 mg/kg (s.c.) twice daily) (D) in the paw withdrawal test over a 7-day period. Data are mean percent MPE ± S.E., with each point representing 8–10 rats in the paw pressure test. *, **,***: p < 0.05, 0.01 or 0.001, respectively, different from vehicle treated mice determined by ANOVA with post-hoc Newman-Keuls test. ^†,††: p < 0.05, 0.01, respectively, different from day 1 within the same treatment group determined by repeated measures ANOVA with post-hoc Dunnett’s test.

Fig. 1

The effects of chronic morphine pellet implantation (6 x 75 mg pellets)(A), chronic THC (4 mg/kg i.p, twice daily) (B), low dose combination (0.75 mg/kg each morphine (s.c) and THC (i.p.) twice daily)(C) or morphine injection (100, 150 and 200 mg/kg (s.c.) twice daily) (D) in the paw withdrawal test over a 7-day period. Data are mean percent MPE ± S.E., with each point representing 8–10 rats in the paw pressure test. *, **,***: p < 0.05, 0.01 or 0.001, respectively, different from vehicle treated mice determined by ANOVA with post-hoc Newman-Keuls test. ^†,††: p < 0.05, 0.01, respectively, different from day 1 within the same treatment group determined by repeated measures ANOVA with post-hoc Dunnett’s test.

Fig. 2

Effect of chronic morphine pellet implantation (6 x 75 mg pellets) on DAMGO-stimulated [³⁵S]GTPγS binding. Membranes from spinal cord (A) or periaqueductal gray (B) of vehicle or morphine pellet-implanted rats were incubated with 0.05 nM [³⁵S]GTPγS, 30 μM GDP and the indicated concentrations of DAMGO. Data are mean percent control stimulation ± S.E from 4–7 rats per treatment group.

Fig. 3

Effect of chronic THC treatment (4 mg/kg i.p, twice daily) on WIN55,212-2-stimulated [³⁵S]GTPγS binding. Membranes from spinal cord (A) or periaqueductal gray (B) of vehicle or THC treated rats were incubated with 0.05 nM [³⁵S]GTPγS, 30 μM GDP and the indicated concentrations of WIN55,212-2. Data are mean percent control stimulation ± S.E from 4–7 rats per treatment group.