Low dose combination of morphine and Δ9-tetrahydrocannabinol circumvents antinociceptive tolerance and apparent desensitization of receptors

https://doi.org/10.1016/j.ejphar.2007.06.001Get rights and content

Abstract

Morphine and ▵9-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 6 × 75 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]GTPγS 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]GTPγS 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.

Introduction

Both opioids and cannabinoids produce analgesia, hypothermia, sedation, hypotension, slowed intestinal motility, immunomodulation and motor depression (Holtzman et al., 1969, Bloom and Dewey, 1978). These effects are mediated by distinct opioid and cannabinoid receptors that are G-protein coupled and primarily activate Gi/Go proteins (Childers, 1991, Howlett et al., 2002). Each receptor system includes multiple receptor subtypes; most clinically relevant analgesic opioids activate mainly mu opioid receptors and the CNS effects of cannabinoids are produced primarily by cannabinoid CB1 receptors. Mu opioid and cannabinoid CB1 receptors modulate a similar profile of effectors, including adenylyl cyclase, cAMP-dependent protein kinase (PKA), mitogen-activated protein kinase (MAPK), and voltage-dependent potassium and calcium channels (Childers, 1991, Howlett et al., 2002). Thus, the major intracellular effects of mu opioid or cannabinoid CB1 receptor activation include decreased calcium and cAMP levels, which leads to inhibition of neurotransmitter release.

Opioids are considered highly effective analgesic agents, whereas clinical use of cannabinoids for treatment of pain remains controversial. A major limitation to the use of opioid analgesics is the development of tolerance and dependence with chronic use (Ellison, 1993). These adaptive changes have also been observed following chronic cannabinoid administration (Sim-Selley, 2003, Lichtman and Martin, 2005). At the cellular level, adaptation of G-protein-coupled receptors to chronic agonist administration occurs in two closely related steps: 1) uncoupling of receptor from G-protein activation (desensitization) and 2) internalization leading to receptor recycling or downregulation (Zhang et al., 1997). Chronic opioid administration produces region-specific desensitization of mu opioid receptors in the brain, with brainstem nuclei showing the greatest response (Noble and Cox, 1996, Sim et al., 1996b, Sim-Selley, 2000). Mu opioid receptor downregulation in brain occurs with certain chronic administration paradigms, but does not appear to be required for mu opioid receptor adaptation following chronic morphine treatment (reviewed in Sim-Selley, 2005). Chronic administration of cannabinoids, such as THC, produces cannabinoid CB1 receptor desensitization and downregulation (reviewed in Sim-Selley, 2003). Although region-specific changes are found in cannabinoid CB1 receptor adaptation, most CNS regions exhibit both responses following chronic cannabinoid treatment.

We have previously reported that simultaneous administration of sub-analgesic doses of morphine and THC in a 1:1 ratio (combination treatment) produces an analgesic effect equal to that of higher doses of either drug alone (Smith et al., 1998, Cichewicz et al., 1999) via a synergistic interaction (Cichewicz and McCarthy, 2003). We hypothesize that administration of the low dose combination of THC and morphine circumvents the development of analgesic tolerance while producing similar analgesic efficacy as a higher dose of morphine or THC alone. The current studies investigated the mechanisms involved in prevention of tolerance to low dose morphine–THC combination by examining receptor-mediated G-protein activity using agonist-stimulated [35S]GTPγS binding. The goal of this study was to characterize the effects of THC, morphine, or the combination of the two drugs on G-protein activation by mu opioid and cannabinoid CB1 receptors. Studies were conducted in membranes prepared from periaqueductal gray or spinal cord because these regions have been implicated in opioid- and cannabinoid-mediated antinociception (Yaksh, 1981, Basbaum and Fields, 1984, Lichtman and Martin, 1991, Lichtman et al., 1996).

Section snippets

Animals

Male Sprague Dawley rats (Harlan Laboratories, Indianapolis IN) weighing approximately 325–350 g were housed two per cage in animal quarters maintained at 22 +/− 2 °C on a 12 h light/dark cycle. Food and water were available ad libitum. The rats were brought to the test room prior to testing and were allowed to acclimate and recover from transport. 8–10 rats were used for each treatment group.

Morphine

Rats were rendered tolerant by subcutaneous (s.c.) implantation of morphine pellets (75 mg). Rats were

Paw withdrawal

Rats were treated chronically with morphine, THC or a low dose combination of the two drugs that produced 80% MPE, as described in Methods, and paw withdrawal thresholds were obtained on days 1, 3, 5, and 7. The results of paw withdrawal testing in high dose morphine-treated rats are shown in Fig. 1A. The rats implanted with 75 mg morphine pellets exhibited significant antinociception compared to the placebo-pellet implanted rats on day 1 (p < 0.001, ANOVA with post-hoc Newman–Keuls test). By day

Discussion

Morphine and THC are effective in producing analgesia, but can produce dose-related side-effects. Moreover, long-term administration of therapeutic doses of either morphine or THC results in the development of tolerance and physical dependence, thus reducing analgesic potency and necessitating the administration of higher drug doses which are associated with increased side-effects. Therefore, low dose combination therapy might offer a safer, more effective alternative than either drug alone by

Acknowledgements

The authors thank Michael P. Cassidy for excellent technical assistance. These studies were supported by USPHS grants DA14277 (LJS), DA107710 (DES) and DA05274 (DES, SPW) from the National Institute on Drug Abuse.

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