Effects of MDMA Injections on the Behavior of Socially-Housed Long-Tailed Macaques (Macaca fascicularis).

MDMA, known to increase prosocial behaviors in humans and rodents, was tested on three young male long-tailed macaques. Subcutaneous injections of 1.0, 1.5, or 2.0 mg/kg were given, and over 200 hours of behavioral data were recorded using an automatic 3D tracking system. At 1.5 mg/kg only, MDMA significantly increased social grooming, confirming a prosocial effect in macaques. At 1.5 and 2.0 mg/kg, foraging decreased, consistent with an anorexigenic effect. At 2.0 mg/kg, locomotion increased, matching its stimulant property. At 1.0 mg/kg, object play increased, possibly reflecting reduced inhibition or increased motivation. These results support MDMA's effectiveness for studying primate social behavior neurobiology.

Study at a glance

Abstract

3,4-methylenedioxy-N-methyl amphetamine (MDMA) is one of the few known molecules to increase human and rodent prosocial behaviors. However, this effect has never been assessed on the social behavior of non-human primates. In our study, we subcutaneously injected three different doses of MDMA (1.0, 1.5 or 2.0mg/kg) to a group of three, socially housed, young male long-tailed macaques. More than 200 hours of behavioral data were recorded, during 68 behavioral sessions, by an automatic color-based video device that tracked the 3D positions of each animal and of a toy. This data was then categorized into 5 exclusive behaviors (resting, locomotion, foraging, social contact and object play). In addition, received and given social grooming was manually scored. Results show several significant dose-dependent behavioral effects. At 1.5mg/kg only, MDMA induces a significant increase in social grooming behavior, thus confirming the prosocial effect of MDMA in macaques. Additionally, at 1.5 and 2.0 mg/kg MDMA injection substantially decreases foraging behavior, which is consistent with the known anorexigenic effect of this compound. Furthermore, at 2.0 mg/kg MDMA injection induces an increase in locomotor behavior, which is also in accordance with its known stimulant property. Interestingly, MDMA injected at 1.0mg/kg increases the rate of object play, which might be interpreted as a decrease of the inhibition to manipulate a unique object in presence of others, or, as an increase of the intrinsic motivation to manipulate this object. Together, our results support the effectiveness of MDMA to study the complex neurobiology of primates' social behaviors.