Influence of 22-day treatment on the anticonvulsant properties of cannabinoids.

Mice were given delta-9-tetrahydrocannabinol (delta-9-THC) cannabidiol (CBD) or phenytoin (PHT) daily for 22 days. Drug activity was measured weekly in three different anticonvulsant tests: the maximal electroshock threshold, the 60-Hz-electroshock threshold and the 6-Hz-electroshock threshold. In order to correlate potential pharmacodynamic and pharmacokinetic changes resulting from repeated treatment, brain-drug concentrations were determined at each test time. The results from the delta-9-THC study indicate that, although tolerance developed in all three tests, there were no changes in the brain-drug concentration. For CBD the pharmacodynamics were strikingly different: an increase in sensitivity to the drug developed in two of the tests, tolerance in only one test. Here again, there were no changes in brain-drug concentrations. The results of the PHT study differed from both the cannabinoids, for tolerance developed in one test, an increase in sensitivity in one test, and the activity was unchanged in the third test. Again, the brain concentrations remained constant throughout. The results demonstrate that both tolerance and increased sensitivity can develop concomitantly with anticonvulsant effects of the cannabinoids and PHT, and that these modifications in drug activity appear to result from cellular or functional rather than dispositional changes.