THC Pain-Response Tolerance Occurred Without Increased Measured PKA Activity in Mice
Mice developed substantial tolerance to THC's antinociceptive effects without a detected increase in PKA activity in the brain regions and spinal cord tested. A positive control showed that the assay could detect increased activity, strengthening the specific null finding.
Behavior and enzyme activity were measured separately
The investigators created two levels of THC antinociceptive tolerance through repeated dosing. They then directly measured basal and total PKA activity in cytosolic and particulate fractions from the striatum, hippocampus, cerebellum, cortex and lumbar spinal cord. These tissue measurements were distinct from the behavioral pain-response tests.
Why the positive control mattered
A cAMP analogue increased PKA activity in several sampled regions following experimental administration. That comparison addressed whether the assay was capable of detecting an increase. With chronic THC, however, the study did not find increased PKA activity in the tested tissues despite the behavioral tolerance.
What the null result does and does not exclude
The finding challenges a simple account in which THC tolerance necessarily produces a measurable overall rise in PKA activity throughout these samples. It does not establish that PKA has no role under any condition. Particular cells, compartments, times or downstream interactions could present questions beyond the measurements reported in the abstract.
A mechanistic assessment should distinguish an intervention that changes behavior from a direct measurement of enzyme activity. This mouse study supports neither a universal PKA explanation nor a universal exclusion. It shows that marked behavioral tolerance and unchanged measured enzyme activity can coexist.
Related: interpreting cannabis tolerance claims.
Research cited
Dalton GD; Smith FL; Smith PA; Dewey WL. Chronic Delta9-tetrahydrocannabinol treatment produces antinociceptive tolerance in mice without altering protein kinase A activity in mouse brain and spinal cord. Biochemical pharmacology (2005). doi:10.1016/j.bcp.2005.04.008. Findings discussed from the study abstract.
