Mitraciliatine, an alkaloid discovered in M. speciosa (Kratom in Thai), acts as a partial agonist for the μ-opioid receptor (MOR) and an agonist for the κ-opioid receptor (KOR), demonstrating selectivity for these receptors over the δ-opioid receptor (DOR) with EC50 values of 228, 218, and >1,000 nM respectively in mouse receptors through a GTPγS binding assay. When administered intracerebroventricularly (i.c.v.) at 100 nmol/animal, mitraciliatine increases latency to withdrawal in the warm water tail withdrawal assay in mice, an effect reversable by MOR knockout but unaffected by KOR knockout, distinguishing it from morphine by not causing hyperlocomotion or respiratory depression.