Mesembrine is an alkaloid present in Sceletium tortuosum. It has been shown to be an extremely potent serotonin reuptake inhibitor, active at dosages as low as 100μg (micrograms). It has also been shown to be a PDE4-inhibitor. Mesembrine may contribute to the antidepressant effects of kanna.
1. Smith M, Crouch N, Gericke N, Hirst M (1996). "Psychoactive constituents of the genus Sceletium N.E.Br. and other Mesembryanthemaceae: a review". J Ethnopharmacol 50 (3): 119–30. PMID 8691846.
2. Pharmaceutical compositions containing mesembrine and related compounds. U.S. Patent 6,288,104 (PDF)
3. Mesembrine - an inhibitor of PDE4 that follows structure-activity relationship to rolipram. 
Selective Phosphodiesterase inhibitor