Discriminative stimulus functions of methanandamide and delta(9)-THC in rats: tests with aminoalkylindoles (WIN55,212-2 and AM678) and ethanol

Abstract

Objective: The aim of the study was to characterize in vivo the aminoalkylindoles WIN55,212-2 (WIN) and AM678 (naphthalen-1-yl(1-pentyl-1H-indol-3-yl)methanone) as cannabinoid receptor (CB(1)R) ligands using drug discrimination. Tests also involved delta(9)-tetrahydrocannabinol (THC) and R-(+)-methanandamide (mAEA), a metabolically stable analog of the endogenous ligand anandamide, as well as the CB(1)R selective antagonist/inverse agonist rimonabant; tests with ethanol assessed pharmacological specificity. We used two different drug discriminations (mAEA and THC) allowing us to explore potential differences in CB(1)R activation which could be attributed to variations in their respective CB(1)R signaling mechanisms.

Methods: There were two concurrently trained groups of rats. One group discriminated between i.p. injected vehicle and 10 mg/kg mAEA. The other group was trained to discriminate between vehicle and 1.8 mg/kg THC.

Results: Dose generalization curves for AM678, WIN55,212-2, THC, and mAEA suggested the following rank order of potency: AM678 > WIN55,212-2 > or = THC > mAEA in both drug discrimination groups. Challenge by 1 mg/kg rimonabant resulted in shifts to the right of the generalization curves for the two aminoalkylindoles (4.4-fold for AM678 and 11.3-fold for WIN in the mAEA group, whereas for the THC group, the corresponding values were 13 and 2.6, respectively), suggesting surmountable antagonism. Ethanol did not generalize in either of the two groups, suggesting pharmacological specificity.

Conclusion: Data are congruent with the general observation that there is substantial overlap in the discriminative stimulus effects of CB(1)R ligands across different chemical classes. However, the quantitative differences in the interactions between the two aminoalkylindoles and rimonabant in the two discrimination groups suggest subtle variations in the ligand-receptor activation(s).

Fig. 1

Chemical structures of cannabinergic agonists used in study

Fig. 2

Generalization test results (top) and corresponding response rate data (bottom) for mAEA (R-(+)-methanandamide; n=10–11), Δ⁹-THC (n=10–20) as well as for ETOH (ethanol; n=9–18) in the mAEA (10 mg/kg)- versus vehicle-trained rats (left), and Δ⁹-THC (n=12) as well as for ETOH (ethanol; n=9–19) in the Δ⁹-THC (1.8 mg/kg)-versus vehicle-trained rats (right); sessions began 20 min after i.p. administration. The generalization results represent the mean (± SEM) percentage of lever presses on the drug (mAEA, top left; Δ⁹-THC, top right) appropriate lever out of the total number of lever presses emitted during a test session (Y-axis); doses examined in milligrams per kilogram (X-axis). Rate refers to the mean (± SEM) number of lever presses per second emitted during a test session (Y-axis); doses in milligrams per kilogram (X-axis). Dotted lines represent the ±95% confidence limits of vehicle control response rate determined from the initial six reinforcement cycles of the vehicle training sessions immediately preceding these tests; symbols outside the confidence limits are considered significantly different from control. Data points are based on one to two observations for each rat and were obtained on separate test days. Numbers within brackets indicate the number of rats responding (i.e., accumulating at least ten responses on either lever and thus obtaining at least one reinforcement) out of the total number used for the test. Test results are based on sessions of a maximum of six reinforcements (12 food pellets) or 20 min, whichever occurred first. V vehicle

Fig. 3

Generalization test results (top) and the corresponding response rate data (bottom) for WIN55,212-2 alone (open hexagons) and when combined with 1 mg/kg rimonabant (filled hexagons), 30 min after i.p. administration. Test results for the 10-mg/kg mAEA-versus vehicle-trained rats are shown in the two left panels (open hexagons; n=9–19; filled hexagons; n=8–18 except for the highest test dose where n=7), and the results for the 1.8-mg/kg Δ⁹-THC-versus vehicle-trained rats are depicted in the two right panels (open hexagons: n=9–21; filled hexagons: n=9–19). Drug lever responding results represent the mean (± SEM) percentage of lever presses on the drug (mAEA, top left; Δ⁹-THC, top right) appropriate lever out of the total number of lever presses emitted during a test session (Y-axis); doses examined in milligrams per kilogram (X-axis). Rate refers to the mean (± SEM) number of lever presses per second emitted during a test session (Y-axis); doses in milligrams per kilogram (X-axis). Other details are as described in the legend for Fig. 2

Fig. 4

Generalization test results (top) for AM678 alone (open triangles) and when combined with 1 mg/kg rimonabant (filled triangles) and the corresponding response rate data (bottom), 30 min after i.p. administration. Test results for the 10-mg/kg mAEA- versus vehicle-trained rats are shown in the two left panels (open triangles; n=8–18; filled triangles; n=8–10) and the results for the 1.8-mg/kg Δ⁹-THC- versus vehicle-trained rats are depicted in the two right panels (open triangles; n=20–21; filled triangles; n=10–21). Drug lever responding results represent the mean (± SEM) percentage of lever presses on the drug (mAEA, top left; Δ⁹-THC, top right) appropriate lever out of the total number of lever presses emitted during a test session (Y-axis); doses examined in milligrams per kilogram (X-axis). Rate refers to the mean (± SEM) number of lever presses per second emitted during a test session (Y-axis); doses in milligrams per kilogram (X-axis). Other details are as described in the legend for Fig. 2