Sensitivity to delta9-tetrahydrocannabinol is selectively enhanced in beta-arrestin2 -/- mice

Abstract

Little is known about the roles of beta-arrestins in the regulation of brain CB1 cannabinoid receptors. This study investigated the role of beta-arrestin2 in cannabinoid behavioral effects using beta-arrestin2 -/- mice and their wild-type counterparts. A variety of cannabinoid ligands from different chemical classes that exhibit a variety of efficacies for activation of CB1 receptors were investigated, including Delta-tetrahydrocannabinol, CP55940, methanandamide, JWH-073, and O-1812. Delta-tetrahydrocannabinol produced both greater antinociception and greater decreases in body temperature in beta-arrestin2 -/- compared with beta-arrestin2 +/+ mice. No significant differences were, however, present in either assay for the other CB1 agonists. Antagonist radioligand binding indicated no difference in the density of cannabinoid CB1 receptors in the cerebellum, cortex, or hippocampus of beta-arrestin2 +/+ and -/- mice. These data demonstrate that beta-arrestin2 may regulate cannabinoid CB1 receptor sensitivity in an agonist-specific manner.

FIGURE 1

Effect of water bath temperature on basal tail withdrawal latencies. The tips of the tails of beta-arrestin2+/+ and -/- mice were immersed in a water bath set to 50, 53 or 56°C and the time it took each animal to remove its tail from the water was measured. Values represent mean ± SEM; n = 10 mice in each group. 2-way ANOVA temperature versus genotype, p = 0.02; ***p < 0.001 by Bonferroni’s post-test.

FIGURE 2

Effects of CP55940 and THC in beta-arrestin2+/+ and -/- mice. Mice were assayed before and at different times after i.p. injection of either 1.0 or 0.3 mg/kg CP55940 (left) or 10 mg/kg THC or vehicle (right) for latency to tail withdrawal (above) and rectal temperature (below). Change in rectal temperature and %Maximum Possible Effect (%MPE) were calculated as described under Methods. Data shown are mean ± SEM with n = 5 for each genotype for CP55940; n = 6-9 for THC, and n = 6 for for vehicle. *p < 0.05, ** p < 0.01, ***p < 0.001, at the times marked, for beta-arrestin2+/+ versus -/- by Bonferroni’s post-test.

FIGURE 3

Effects of other cannabinoid agonists in beta-arrestin2+/+ and -/- mice. Mice were assayed before and at different times after i.p. injection of 50 and 250 mg/kg methanandamide (left) and 2.5 mg/kg O-1812 and 20 mg/kg JWH-015 (right) for latency to tail withdrawal (above) and rectal temperature (below). Data shown are mean ± SEM; n = 6 - 8 for each genotype for each drug.

FIGURE 4

Effect of CP55940 and vehicle (left) and THC (right) in the hot plate test of antinociception. %Maximum Possible Effect (%MPE) were calculated as described in Methods. Data shown are mean ± SEM with n = 5 for each group. *p < 0.05 at 90 min for beta-arrestin2+/+ versus -/- by Bonferroni’s posttest.

FIGURE 5

Cannabinoid CB₁ receptor binding in brain membranes from beta-arrestin2+/+ and -/- mice. Cerebellar membranes were prepared and incubated with various concentrations of [³H]SR141716A. Relative CB₁ receptor levels (inset) were also determined in membranes from cortex (Ctx), hippocampus (Hipp) and cerebellum (Cereb) by the amount of binding at a single [³H]SR141716A concentration (1 nM). Data shown are mean ± SEM for specific [³H]SR141716A binding (n = 3 or more). There were no differences between beta-arrestin2+/+ and -/-for any brain area (Student’s t-test, p > 0.05).