Local administration of delta9-tetrahydrocannabinol attenuates capsaicin-induced thermal nociception in rhesus monkeys: a peripheral cannabinoid action.

RATIONALE: Cannabinoids can reduce nociceptive responses by acting on peripheral cannabinoid receptors in rodents.
OBJECTIVES: The study was conducted to evaluate the hypothesis that local administration of delta9-tetrahydrocannabinol (delta9-THC) can attenuate capsaicin-induced nociception in rhesus monkeys.
METHODS: Capsaicin (100 microg) was applied locally in the tail of rhesus monkeys to evoke a nociceptive response, thermal allodynia, in normally innocuous 46 degrees C water. delta9-THC (10-320 microg) was coadministered with capsaicin in the tail to assess local antinociceptive effects. In addition, a local antagonism study was performed to confirm the selectivity of delta9-THC action.
RESULTS: delta9-THC dose-dependently inhibited capsaicin-induced allodynia. This local antinociception was antagonized by small doses (10-100 microg) of the cannabinoid CB1 antagonist, SR141716A, applied in the tail. However, 100 microg SR141716A injected subcutaneously in the back did not antagonize local delta9-THC.
CONCLUSIONS: These results indicate that the site of action of locally applied delta9-THC is in the tail. It provides functional evidence that activation of peripheral cannabinoid CB1 receptors can attenuate capsaicin-induced thermal nociception in non-human primates and suggests a new approach for cannabinoids in pain management.