Novel object recognition memory: measurement issues and effects of MDMA self-administration following short inter-trial intervals

Abstract

The present study was undertaken to examine effects of self-administered MDMA on novel object exploration (NOR) memory. Self-administration was conducted during daily 2 h tests that continued until a total of 165 mg/kg was self-administered (range = 13–41 days for individual rats). Control rats were placed in the self-administration boxes during daily sessions but did not receive any drug. One or 10 weeks following the last self-administration session, memory was assessed using a standard NOR task. When exploration time was used as the dependent measure for the control rats, there was no consistent pattern of change as a function of inter-trial interval (ITI) and exploration times failed to reveal decay in the function relating exploration to ITI. When number of approaches was examined as a function of ITI, however, there was a preference for the novel object following the short ITIs (1–15 min) and the function relating preference to ITI decayed with longer ITIs. When tested 7 days following the last self-administration session, rats that self-administered MDMA failed to demonstrate NOR even following the shortest ITI of 1 min. The data support the idea that MDMA self-administration produces cognitive deficits and are consistent with the idea that attentional processes become disrupted. There was, however, recovery of NOR memory when rats were tested following an extended drug-free period of 70 days. Thus, the deficits are transient and recovery was apparent.