The effect of MDMA self-administration on MDMA-produced hyperactivity and c-fos expression
Background: MDMA preferentially releases serotonin (5HT) but following repeated exposure there is a decrease in this MDMA-produced effect. At the same time, some studies suggest an increase in MDMA-produced dopamine (DA) release following repeated exposure. The sensitised DA response is often accompanied by sensitisation of MDMA-produced locomotor activity. Because DAergic mechanisms have been implicated in the positively reinforcing properties of MDMA, these neuroadaptations might be relevant to MDMA self-administration. Objectives: The main objective of this study was to determine whether MDMA self-administration and non-contingent MDMA exposure differentially affected the development of sensitisation to MDMA-produced hyperactivity. Additionally, the relationship between MDMA-produced hyperactivity and changes in c-fos expression in DA terminal regions was determined. Methods: Triads of rats were designated ‘master’, ‘yoked MDMA’, or ‘yoked saline’. Lever press responding by the master rat resulted in an intravenous infusion of MDMA for both the master rat and the yoked MDMA rat, as well as an equal infusion of vehicle for the yoked control rat. Daily tests continued until a total of 350 mg/kg MDMA had been self-administered. Three days following the last self-administration session, forward and vertical locomotion produced by MDMA (5.0 mg/kg, i.p) were measured during a 2 hr test. Rats were sacrificed immediately following the behavioural test, and c-fos immunohistochemistry was measured. Results: Repeated MDMA exposure resulted in sensitised forward and vertical locomotor activity. Sensitisation of the increase in forward locomotion was produced only in rats that self-administered MDMA; non-contingent MDMA administration failed to sensitise this behavioural response. In contrast, sensitisation to MDMA-produced vertical activity was produced following both contingent and non-contingent MDMA exposure. C-fos expression was reduced in ventrolateral, and ventromedial areas of the dorsal striatum, as well as the infralimbic cortex, after MDMA exposure, regardless of whether the exposure was via self-administration or yoked administration. A selective decrease in c-fos expression in the nucleus accumbens (NAc) core and the cingulate cortex was produced by MDMA self-administration. There was a negative correlation between MDMA-produced forward locomotor activity and MDMA-produced c-fos expression in the NAc core, cingulate cortex and infralimbic cortex. A negative correlation between rearing activity and MDMA-produced c-fos expression in the NAc core, NAc shell, cingulate cortex, and infralimbic cortex was also found. Conclusions: These data provide evidence of behavioural sensitisation as a result of repeated MDMA exposure. Furthermore, MDMA-produced behavioural sensitisation was associated with a decrease in c-fos expression that was evident in the NAc and prefrontal cortex. Finally, region-specific changes in c-fos expression suggest an important role of neuroadaptations in the NAc core and the infralimbic cortex as a consequence of MDMA self-administration.