The oral activity of delta'-tetrahydrocannabinol and its dependence on prostaglandin E2.

1 delta'-trans-Tetrahydrocannabinol (THC) is more active orally in mice than previously thought, as cataleptic responses occur at doses from 0.06 mg/kg upwards, with peak activity at 2 to 4 h after dosing. These doses and peaks correspond well with the effects in man. 2 Comparison with chlorpromazine in mice shows that chlorpromazine and THC are equipotent as cataleptics during the first 2 h after dosing; thereafter the THC activity increases to a peak when it is 5.67 times as active as chlorpromazine. 3 The cataleptic effect of THC is abolished by aspirin, indomethacine, diffunisal and phenylbutazone which inhibit the biosynthesis of prostaglandins and is restored by exogenous prostaglandin E2 (PGE2) but not PGE1 and PGF2 alpha. This suggests that the effect of THC depends upon the presence of PGE2. 4 In contrast, the cataleptic effect of chlorpromazine is not affected by pretreatment with aspirin. 5 THC is very much less active intraperitoneally than orally; our results suggest this is not due to poor absorption or extraction into fat depots. 6 Cannabidiol has no cataleptic effect.