THC Produced Greater Regional Desensitization Than WIN in an Escalating-Dose Mouse Comparison
Repeated THC and WIN 55,212-2 both produced cannabinoid adaptation in mice, but THC caused greater signaling desensitization in some regions. The comparison cautions against assuming that a higher-efficacy agonist must always produce greater adaptation.
The regimens used escalating experimental doses
Mice received escalating doses for 15 days, reaching maximally tolerated experimental doses. THC was characterized as a partial agonist and WIN as a full agonist for G protein activation. This design was not a comparison of equal consumer doses or ordinary cannabis and synthetic-cannabinoid exposure patterns.
Binding and signaling results were different comparisons
Both treatments produced behavioral tolerance, reduced agonist-stimulated G protein activation and decreased receptor binding. The binding decrease did not differ between treatment groups, whereas THC caused greater desensitization in some regions. Similar receptor downregulation therefore did not establish equal signaling adaptation.
Even signaling changes differed by region
In hippocampus, reduced maximal agonist effect explained the signaling decrease in both groups. In substantia nigra after THC, maximal effect decreased and the concentration-response curve also shifted. These distinguish changes in the response ceiling from changes in apparent sensitivity.
The findings support a role for agonist properties within a broader exposure-dependent process. They do not rank the real-world safety of THC and WIN or establish that partial agonists are generally more damaging. Interpretation requires retaining the escalating regimen, regional measurements and difference between receptor availability and functional responsiveness.
Related: interpreting cannabis tolerance claims.
Research cited
Sim-Selley LJ; Martin BR. Effect of chronic administration of R-(+)-[2,3-Dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone mesylate (WIN55,212-2) or delta(9)-tetrahydrocannabinol on cannabinoid receptor adaptation in mice. The Journal of pharmacology and experimental therapeutics (2002). doi:10.1124/jpet.102.035618. Findings discussed from the study abstract.
