Abstract

Curcumin, the major constituent of the spice tumeric produces a plethora of biological actions that have translated in vivo into behavioral and neurochemical effects in rodents that are also produced by clinically-used antidepressants. The present study was designed to provide a systematic replication of prior behavioral, pharmacological, and neurochemical experiments. In particular, the ability of curcumin to engender anti-immobility effects in the mouse forced-swim assay was established. Although prior work had shown curcumin to function as an inhibitor of the monoamine metabolizing enzyme, monoamine oxidase (MAO), neither MAOA nor MAOB was inhibitied by curcumin in the present study. Curcumin had also been reported previously to function as a cannabinoid CB1 receptor inverse agonist/antagonist. However, in our hands, curcumin did not potently alter GTP-γ-35S binding indicative of functional CB1 antagonism (Kb = 2080 nM). Moreover, curcumin was not able to prevent the hypothermic effects of the cannabinoid receptor agonist (-)-cis-3-[2-Hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol (CP 55,940). Nonetheless, the anti-immobility effects of curcumin did not occur in CB1 -/- mice. Finally, a broad array of protein receptors and enzymes were evaluated in vitro for their potential interaction with and/or functional engagement with curcumin. Of the more than 100 targets screened, curcumin had very low potency in most. Of those targets with appreciable activity, curcumin had affinities for the human cloned muscarinic receptor subtypes (Ki = 1.3-3.1 uM). Moreover, the plasma and brain levels of curcumin at behaviorally-active doses were below quantitative limits. Given these findings, it is concluded that the prominent antidepressant-like behavioral effects of curcumin, replicated here and in multiple acute and chronic rodent models detailed in the literature, are the result of as yet undisclosed mechanisms of action. The scientific and patient communities await the full scale clinical evaluation of a sufficiently bioavailable curcumin analog in major depressive disorder.

Keywords: Curcumin, antidepressant, forced-swim, monoamine oxidase, CB1 receptor, mice.