Rationale

Oxytocin (OT), vasopressin (AVP) and 3,4 methylenedioxymethamphetamine (MDMA, ‘Ecstasy’) all increase social interaction in rats, perhaps by enhancing the rewarding value of social encounters.

Objectives

Here, we used the conditioned place preference (CPP) paradigm to assess the intrinsic rewarding effects of OT, AVP and MDMA, and whether these effects are enhanced by the presence of a conspecific, or a dynamic, tactile object (a tennis ball).

Methods

Adult male rats received conditioning sessions in a CPP apparatus twice a day (vehicle at 10 a.m., drug at 3 p.m.). Experiment 1 involved conditioning with OT (0.5 mg/kg, intraperitoneal (i.p.)), AVP (0.005 mg/kg, i.p.) or MDMA (5 mg/kg, i.p.). Experiments 2 and 3 involved conditioning with the same treatments but in the presence of a conspecific receiving the same treatment (social-CPP) or in the presence of a tennis ball (object-CPP), respectively. Conditioned place preference was assessed 24 h, 2 weeks and 4 weeks later.

Results

OT, AVP and MDMA did not produce a conventional CPP. However, when the conditioning environment also contained a conspecific both OT and MDMA induced a significant CPP lasting for at least 4 weeks. Rats given OT and MDMA also developed a more modest yet significant CPP for the environment where they encountered a tennis ball.

Conclusions

These results indicate that OT and MDMA can augment the rewarding effects of social interaction, but also interaction with a dynamic and tactile non-social object. AVP does not condition social- or object-CPPs and may promote social proximity by inducing generalized anxiety and defensive aggregation.