The present study was designed to investigate the effect of a methanolic extract of Morinda citrifolia Linn. fruit (MMC) on the rewarding effect of heroin in the rat conditioned place preference (CPP) paradigm and naloxone-precipitated withdrawal in mice. In the first experiment, following a baseline preference test (preconditioning score), the rats were subjected to conditioning trials with five counterbalanced escalating doses of heroin versus saline followed by a preference test conducted under drug-free conditions (post-conditioning score) using the CPP test. Meanwhile, in the second experiment, withdrawal jumping was precipitated by naloxone administration after heroin dependence was induced by escalating doses for 6 days (3×/ day). The CPP test results revealed that acute administration of MMC (1, 3, and 5 g/kg body weight (bw), p.o.), 1 h prior to the CPP test on the 12th day significantly reversed the heroin-seeking behavior in a dose-dependent manner, which was similar to the results observed with a reference drug, methadone (3 mg/kg bw, p.o.). On the other hand, MMC (0.5, 1, and 3 g/kg bw, p.o.) did not attenuate the heroin withdrawal jumps precipitated by naloxone. These findings suggest that the mechanism by which MMC inhibits the rewarding effect of heroin is distinct from naloxone-precipitated heroin withdrawal.
Mitragynine is the major alkaloid found in the leaves of M. speciosa Korth (Rubiaceae), a plant that is native to Southeast Asia. This compound has been used, either traditionally or recreationally, due to its psychostimulant and opioid-like effects. Recently, mitragynine has been shown to exert conditioned place preference (CPP), indicating the rewarding and motivational properties of M. speciosa. Here, the involvement of GABAB receptors in mediating mitragynine reward is studied using a CPP paradigm in rats. First, we examined the effects of GABAB receptor agonist baclofen (1.25, 2.5 and 5 mg/kg) on the acquisition of mitragynine (10 mg/kg)-induced CPP. Second, the involvement of GABAB receptors in the expression of mitragynine-induced CPP was tested. We found that the acquisition of mitragynine-induced CPP could be blocked by higher doses (2.5 and 5 mg/kg) of baclofen. Baclofen at a high dose inhibited locomotor activity and caused a CPP. Furthermore, we found that baclofen (2.5 and 5 mg/kg) also blocked the expression of mitragynine-induced CPP. These findings suggest that both, the acquisition and expression of mitragynine's reinforcing properties is controlled by the GABAB receptor.
Mitragynine is the main psychoactive ingredient of the herbal drug preparation Kratom (Ketum), derived from the plant Mitragyna speciosa. Kratom is a widely abused drug in Southeast Asian and has a psychostimulant profile at low-medium doses, while high doses have opioidergic effects. Mitragynine was shown to possess opiate receptor affinity. However, its role in the behavioural effects of mitragynine is unclear. Here we asked whether the reinforcing effects of mitragynine are mediated by opiate receptors using a conditioned place preference (CPP) paradigm in rats. In the first experiment we tested the effects of the opiate receptor antagonist naloxone (0.1, 0.3 and 1.0mg/kg) on the acquisition of mitragynine (10mg/kg)-induced CPP. In the second experiment, we tested the involvement of opiate receptors in the expression of mitragynine-induced CPP in rats. We found that naloxone suppresses the acquisition of mitragynine-induced CPP. This effect was already evident at a dose of naloxone (0.1mg/kg) which, by itself, had no conditioned place aversion (CPA) effect. Higher doses of naloxone induced a CPA and blocked mitragynine-induced CPP. In contrast, naloxone had no effect on the expression of mitragynine-induced CPP. These findings suggest that the acquisition, but not the expression of the reinforcing effects of mitragynine is mediated by opiate receptors.