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Long-term cannabinoid administration produces region-dependent CB 1 receptor desensitization and down-regulation. This study examined the time course for normalization of CB 1 receptors and G-protein activation using 35 S]GTP␥S binding were decreased in both regions 1 day after treatment. WIN55,212-2-stimulated G-protein activation in striatum/GP returned to control level at 3 days after cessation of treatment with either drug but did not return to control level in hippocampus until 14 days. CB 1 receptor binding did not recover to control levels until day 7 or 14 after treatment in striatum/GP and hippocampus, respectively. The mechanism of CB 1 binding site down-regulation was investigated after long-term ⌬ 9 -THC treatment. Analysis of CB 1 receptor mRNA in hippocampus and striatum/GP showed that transcriptional regulation could not explain prolonged recovery rates from CB 1 receptor down-regulation. In contrast, CB 1 receptor protein, as determined by immunoblot analysis, matched the down-regulation and recovery rates of CB 1 receptor binding sites relatively closely. These data demonstrate that cannabinoid-induced decreases in CB 1 receptor function persist for relatively long time periods after cessation of long-term drug treatment and that CB 1 receptor signaling recovers more quickly in striatum/GP than hippocampus. Moreover, down-regulation of CB 1 receptor binding sites does not seem to result mainly from transcriptional regulation, suggesting that adaptive regulation of CB 1 receptors in brain primarily occurs at the protein level.

A number of side‐chain analogues of Δ8‐THC were tested in GTPγS binding assay in rat cerebellar membranes. O‐1125, a saturated side‐chain compound stimulated GTPγS binding with an Emax of 165.0%, and an EC50 of 17.4 nM. O‐1236, O‐1237 and O‐1238, three‐enyl derivatives containing a cis carbon‐carbon double bond in the side‐chain, stimulated GTPγS binding, acting as partial agonists with Emax values ranging from 51.3–87.5% and EC50 values between 4.4 and 29.7 nM. The stimulatory effects of O‐1125, O‐1236, O‐1237 and O‐1238 on GTPγS binding were antagonized by the CB1 receptor antagonist SR 141716A. The KB values obtained ranged from 0.11–0.21 mM, suggesting an action at CB1 receptors. Five‐ynyl derivatives (O‐584, O‐806, O‐823, O‐1176 and O‐1184), each containing a carbon‐carbon triple bond in the side‐chain, did not stimulate GTPγS binding and were tested as potential cannabinoid receptor antagonists. Each ‐ynyl compound antagonized the stimulatory effects of four cannabinoid receptor agonists on GTPγS binding. The KB values obtained, all found to be in the nanomolar range, did not differ between agonists or from cerebellar binding affinity. In conclusion, alterations of the side‐chain of the classical cannabinoid structure may exert a large influence on affinity and efficacy at the CB1 receptor. Furthermore, this study confirms the ability of the GTPγS binding assay to assess discrete differences in ligand efficacies which potentially may not be observed using alternative functional assays, thus providing a unique tool for the assessment of the molecular mechanisms underlying ligand efficacies. British Journal of Pharmacology (1999) 126, 1575–1584; doi:

Heterozygous CB1 receptor knockout mice were used to examine the effect of reduced CB1 receptor density on G-protein activation in membranes prepared from four brain regions: cerebellum, hippocampus, striatum/globus pallidus (striatum/GP) and cingulate cortex. Results showed that CB1 receptor levels were approximately 50% lower in heterozygous mice in all regions examined. However