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McCann UD, Szabo Z, Vranesic M, Palermo M, Mathews WB, Ravert HT, Dannals RF, Ricaurte GA. 
“Positron emission tomographic studies of brain dopamine and serotonin transporters in abstinent (+/-)3,4-methylenedioxymethamphetamine ('ecstasy') users: relationship to cognitive performance”. 
Psychopharmacology (Berl). 2008 Oct 16;200(3):439-50.

BACKGROUND: (+/-)3,4-Methylenedioxymethamphetamine (MDMA, "ecstasy") is a recreational drug and brain serotonin (5-HT) neurotoxin. Under certain conditions, MDMA can also damage brain dopamine (DA) neurons, at least in rodents. Human MDMA users have been found to have reduced brain 5-HT transporter (SERT) density and cognitive deficits, although it is not known whether these are related. This study sought to determine whether MDMA users who take closely spaced sequential doses, which engender high plasma MDMA concentrations, develop DA transporter (DAT) deficits, in addition to SERT deficits, and whether there is a relationship between transporter binding and cognitive performance.

MATERIALS AND METHODS: Sixteen abstinent MDMA users with a history of using sequential MDMA doses (two or more doses over a 3- to 12-h period) and 16 age-, gender-, and education-matched controls participated. Subjects underwent positron emission tomography with the DAT and SERT radioligands, [11C]WIN 35,428 and [11C]DASB, respectively. Subjects also underwent formal neuropsychiatric testing.

RESULTS: MDMA users had reductions in SERT binding in multiple brain regions but no reductions in striatal DAT binding. Memory performance in the aggregate subject population was correlated with SERT binding in the dorsolateral prefrontal cortex, orbitofrontal cortex, and parietal cortex, brain regions implicated in memory function. Prior exposure to MDMA significantly diminished the strength of this relationship. CONCLUSIONS: Use of sequential MDMA doses is associated with lasting decreases in brain SERT, but not DAT. Memory performance is associated with SERT binding in brain regions involved in memory function. Prior MDMA exposure appears to disrupt this relationship. These data are the first to directly relate memory performance to brain SERT density.
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Apr 17, 2011 1:59
Cannabis Use Correlated with Some Cognitive Improvements #

Although this research was targeted at studying the dopamine transporter, serotonin transporter, and cognitive deficits associated with MDMA/Ecstasy use, they also reported on correlations with cannabis use. McCann et al. wrote: 'Interestingly, significant correlations were found on three tests, but these differed from those found to be related to MDMA use. Also, in two of the three tests, correlations were paradoxical (i.e., marijuana use was associated with improved performance). In particular, the total number of lifetime exposures was negatively associated with immediate recall on one test of the Wechsler Memory Scale-III (story A recall; r=−0.53, p=0.05) and no measures of delayed recall. In addition, lifetime duration of marijuana use was associated with improved performance on the Rey Auditory Verbal Learning Test (r=0.54, p=0.04) and the Finger Tapping Test (nondominant hand; r=0.64, p=0.01). There was no overlap in the tasks found to be associated with MDMA use and marijuana use.'
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