Kratom pharmacology: Clues from planarians exposed to mitragynine

Mitragynine (MG), the most prevalent bioactive alkaloid in kratom, displays nanomolar affinity for mu, kappa and delta opioid receptors and produces opioid-dependent antinociception and dependence in rats. Here, using a battery of behavioral assays, we investigated MG effects in planarians. Acute MG exposure (< 100 mu M) did not affect planarian motility or environmental preference, but reduced motility was detected during abstinence from chronic MG (1, 10 mu M). MG (10 mu M) produced place conditioning effects that were reduced by naltrexone (10 mu\M). These results suggest that MG produces opioid-sensitive reinforcing effects in planarians and MG pharmacology is conserved across different species.