The effect of repeated administration of delta9-tetrahydrocannabinol on serotonin metabolism in the rat brain.

The level of serotonin (5-HT) was increased in the whole rat brain as well as in the hypothalamus plus midbrain region at 0.5 hr after the fifth or sixth daily dose of delta9-tetrahydrocannabinol (THC), 20.0 mg/kg, i.p., respectively. A decreased rate of 5-HT synthesis was also observed. A slight development of tolerance was indicated by the fact that elevation of 5-HT was smaller than that seen after a single dose (1).