RESUMO
BACKGROUND: Made as a tea, the Thai traditional drug "kratom" reportedly possesses pharmacological actions that include both a coca-like stimulant effect and opium-like depressant effect. Kratom has been used as a substitute for opium in physically-dependent subjects. The objective of this study was to evaluate the antinociception, somatic and physical dependence produced by kratom tea, and then assess if the tea ameliorated withdrawal in opioid physically-dependent subjects. METHODS: Lyophilized kratom tea (LKT) was evaluated in C57BL/6J and opioid receptor knockout mice after oral administration. Antinociceptive activity was measured in the 55 °C warm-water tail-withdrawal assay. Potential locomotor impairment, respiratory depression and locomotor hyperlocomotion, and place preference induced by oral LKT were assessed in the rotarod, Comprehensive Lab Animal Monitoring System, and conditioned place preference assays, respectively. Naloxone-precipitated withdrawal was used to determine potential physical dependence in mice repeatedly treated with saline or escalating doses of morphine or LKT, and LKT amelioration of morphine withdrawal. Data were analyzed using one- and two-way ANOVA. RESULTS: Oral administration of LKT resulted in dose-dependent antinociception (≥1 g/kg, p.o.) absent in mice lacking the mu-opioid receptor (MOR) and reduced in mice lacking the kappa-opioid receptor. These doses of LKT did not alter coordinated locomotion or induce conditioned place preference, and only briefly reduced respiration. Repeated administration of LKT did not produce physical dependence, but significantly decreased naloxone-precipitated withdrawal in morphine dependent mice. CONCLUSIONS: The present study confirms the MOR agonist activity and therapeutic effect of LKT for the treatment of pain and opioid physical dependence.