Dose-Related Differences in the Regional Pattern of Cannabinoid Receptor Adaptation and in Vivo Tolerance Development to Δ9- Tetrahydrocannabinol

Chronic treatment with Δ9-tetrahydrocannabinol (THC) produces tolerance to cannabinoid-mediated behaviors and region-specific adaptation of brain cannabinoid receptors. However, the relationship between receptor adaptation and tolerance is not well understood, and the dose-response relationship of THC-induced cannabinoid receptor adaptation is unknown. This study assessed cannabinoid receptor function in the brain and cannabinoid-mediated behaviors after chronic treatment with different dosing regimens of THC. Mice were treated twice per day for 6.5 days with the following: vehicle, 10 mg/kg THC, or escalating doses of 10 to 20 to 30 or 10 to 30 to 60 mg/kg THC. Tolerance to cannabinoid-mediated locomotor inhibition, ring immobility, antinociception, and hypothermia was produced by both ramping THC-dose paradigms. Administration of 10 mg/kg THC produced less tolerance development, the magnitude of which depended upon the particular behavior. Decreases in cannabinoid-mediated G-protein activation, which varied with treatment dose and region, were observed in autoradiographic and membrane guanosine 5′-O-(3-[35S]thio)triphosphate ([35S]GTPγS)-binding assays in brains from THC-treated mice. Agonist-stimulated [35S]GTPγS binding was reduced in the hippocampus, cingulate cortex, periaqueductal gray, and cerebellum after all treatments. Decreased agonist-stimulated [35S]GTPγS binding in the caudate-putamen, nucleus accumbens, and preoptic area occurred only after administration of 10 to 30 to 60 mg/kg THC, and no change was found in the globus pallidus or entopeduncular nucleus after any treatment. Changes in the CB1 receptor Bmax values also varied by region, with hippocampus and cerebellum showing reductions after all treatments and striatum/globus pallidus showing effects only at higher dosing regimens. These results reveal that tolerance and CB1 receptor adaptation exhibit similar dosedependent development, and they are consistent with previous studies demonstrating less cannabinoid receptor adaptation in striatal circuits. Cannabinoids are used for their psychoactive effects and for therapeutic treatment of nausea/ emesis and cachexia. Previous studies also suggest that cannabinoids may have clinical potential for the treatment of pain and degenerative disorders (Piomelli et al., 2000; van der Stelt and Di Marzo, 2003). Acute administration of cannabinoids produces antinociception, locomotor inhibition, hypothermia, and impairment of short-term memory (Howlett et al., 2002). Δ9-Tetrahydrocannabinol (THC) and other cannabinoids produce their psychoactive and behavioral effects via activation of CB1 receptors in the central nervous system (CNS) (Ledent et al., 1999). Recent reports indicate that CB2 and novel cannabinoid receptors might Address correspondence to: Dr. Laura Sim-Selley, Department of Pharmacology and Toxicology, Virginia Commonwealth University, Box 980524, 1112 East Clay St., Richmond, VA 23298. E-mail: [email protected]. NIH Public Access Author Manuscript J Pharmacol Exp Ther. Author manuscript; available in PMC 2009 February 9. Published in final edited form as: J Pharmacol Exp Ther. 2008 February ; 324(2): 664–673. doi:10.1124/jpet.107.130328. N IH PA Athor M anscript N IH PA Athor M anscript N IH PA Athor M anscript exist in the CNS (Mackie and Stella, 2006), but their role is unclear. CB1 receptors are widely distributed in the brain where they exhibit a predominantly presynaptic location and modulate neurotransmitter release (Schlicker and Kathmann, 2001). High levels of CB1 receptors are found in the hippocampus, cerebellum, and basal ganglia (Herkenham et al., 1991), consistent with effects on memory and motor function, respectively. Low to moderate levels of CB1 receptors are distributed in the hypothalamus and periaqueductal gray (PAG) (Herkenham et al., 1991), consistent with effects on temperature, feeding, and pain. CB1 receptors are also found in the mesocorticolimbic pathway, which probably contributes to the reinforcing effects of cannabinoids and modulation of reinforcement produced by other psychoactive drugs (van der Stelt and Di Marzo, 2003; Le Foll and Goldberg, 2005). The intracellular effects of CB1 receptor activation are produced primarily via activation of inhibitory G-proteins, resulting in inhibition of adenylyl cyclase, activation of A-type and inwardly rectifying K+ channels, and inhibition of Nand P/Q-type Ca2+ channels (Howlett et al., 2002). Preclinical studies have shown that tolerance to cannabinoid-mediated behaviors developed after repeated cannabinoid administration (Carlini, 1968; McMillan et al., 1971). Tolerance to cannabinoid-mediated effects was also found in humans after chronic marijuana use (Jones et al., 1976, 1981). It is of interest that the characteristics of tolerance can vary with regard to time and magnitude in a behavior-specific manner. For example, tolerance to cannabinoidmediated hypoactivity developed more slowly than tolerance to certain other effects, such as hypothermia (Dewey, 1986; Whitlow et al., 2003). Tolerance to cannabinoid-mediated effects in operant tests also developed more slowly than to hypothermia or analgesia (De Vry et al., 2004). The rate of recovery of tolerance also varied by behavior, with tolerance to cannabinoidmediated hypomotility disappearing more quickly than for antinociception (Bass and Martin, 2000). Differences in the development of tolerance to cannabinoid-mediated effects have also been reported in humans (Jones et al., 1981; Haney et al., 1999; Hart et al., 2002). Comparison of THC-mediated effects in frequent versus infrequent marijuana users revealed differential tolerance to various subjective measures (e.g., greater tolerance to sedation than “high”) and less tolerance to physiological and psychomotor effects compared with subjective effects (Jones et al., 1976, 1981; Kirk and de Wit, 1999; Hart et al., 2002). Although it is difficult to compare clinical studies, it is clear that greater tolerance developed after administration of higher doses of THC and longer treatment duration (Jones et al., 1976, 1981; Hart et al., 2002). However, the mechanistic basis for these observations is not known. Repeated cannabinoid administration produces alterations in cannabinoid receptors that include receptor down-regulation and desensitization of receptor-mediated G-protein activation and second messenger effects (reviewed by Sim-Selley, 2003). In fact, downregulation of CB1 receptors has recently been found in the brains of human cannabis users (Villares, 2007). Studies have consistently found that cannabinoid receptor adaptation varies by brain region (reviewed by Sim-Selley, 2003). For example, desensitization of receptormediated G-protein activity is smaller in magnitude and develops more slowly in the basal ganglia (BG), especially the globus pallidus (GP), entopeduncular nucleus, and substantia nigra, compared with the hippocampus or cerebellum (Breivogel et al., 1999). Treatment paradigms have been developed to examine whether parameters that affect tolerance, such as treatment duration, affect cannabinoid receptor adaptation. Time-course studies have revealed that cannabinoid receptor down-regulation and desensitization were generally greater with increasing treatment duration, although the time course of adaptation varied by brain region (Breivogel et al., 1999). The dose effect of the treatment drug on receptor adaptation is not as well understood. Oviedo et al. (1993) reported that administration of increasing doses of CP55,940 produced greater decreases in CB1 agonist binding in several forebrain nuclei. Regional differences in the pattern of down-regulation seemed to be present, but they were difficult to assess because analysis was limited to the striatum and adjacent structures. The majority of chronic cannabinoid studies in the literature administer either 1) the threshold dose McKinney et al. Page 2 J Pharmacol Exp Ther. Author manuscript; available in PMC 2009 February 9. N IH PA Athor M anscript N IH PA Athor M anscript N IH PA Athor M anscript to produce tolerance (low dose) or 2) maximal dose to produce greater receptor adaptations (high dose). Therefore, it is unclear whether regional differences in adaptation reflect differential sensitivity to THC (e.g., are dose-dependent) or differential mechanisms of cannabinoid receptor adaptation. Interpretation is further complicated by differences in the cannabinoid drug administered, dose, and treatment duration between studies. This study was designed to directly examine regional differences in adaptation of cannabinoid receptors after administration of varying doses of THC and to assess tolerance produced by these administration paradigms. Materials and Methods