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Rationale: Elevated dopamine function is thought to play a key role in both the rewarding effects of addictive drugs and the pathophysiology of schizophrenia. Accumulating epidemiological evidence indicates that cannabis use is a risk factor for the development of schizophrenia. However, human neurochemical imaging studies that examined the impact of ∆9-tetrahydrocannabinol (THC), the main psychoactive component in cannabis, on striatal dopamine release have provided inconsistent results. Objectives: The objective of this study is to assess the effect of a THC challenge on human striatal dopamine release in a large sample of healthy participants. Methods: We combined human neurochemical imaging data from two previous studies that used [C]raclopride positron emission tomography (PET) ( n = 7 and n = 13, respectively) to examine the effect of THC on striatal dopamine neurotransmission in humans. PET images were re-analysed to overcome differences in PET data analysis. Results: THC administration induced a significant reduction in [C]raclopride binding in the limbic striatum (−3.65 %, from 2.39 ± 0.26 to 2.30 ± 0.23, p = 0.023). This is consistent with increased dopamine levels in this region. No significant differences between THC and placebo were found in other striatal subdivisions. Conclusions: In the largest data set of healthy participants so far, we provide evidence for a modest increase in human striatal dopamine transmission after administration of THC compared to other drugs of abuse. This finding suggests limited involvement of the endocannabinoid system in regulating human striatal dopamine release and thereby challenges the hypothesis that an increase in striatal dopamine levels after cannabis use is the primary biological mechanism underlying the associated higher risk of schizophrenia. [ABSTRACT FROM AUTHOR]

Abstract: Accumulating evidence suggests involvement of the endocannabinoid system in the pathophysiology of schizophrenia, which signifies a potential application for this system in the treatment of this disorder. However, before new research can focus on potential treatments that work by manipulating the endocannabinoid system, it needs to be elucidated how this system is involved in symptoms of schizophrenia. Here we review human studies that investigated acute effects of cannabis or ∆9-tetrahydrocannabinol (THC) on brain functions that are implicated in schizophrenia. Results suggest that the impact of THC administration depends on the difficulty of the task performed. Impaired performance of cognitive paradigms is reported on more challenging tasks, which is associated with both activity deficits in temporal and prefrontal areas and a failure to deactivate regions of the default mode network. Comparable reductions in prefrontal activity and impairments in deactivation of the default mode network are seen in patients during performance of cognitive paradigms. Normal performance levels after THC administration demonstrated for less demanding tasks are shown to be related to either increased neural effort in task-specific regions (‘neurophysiological inefficiency’), or recruitment of alternative brain areas, which suggests a change in strategy to meet cognitive demands. Particularly a pattern of performance and brain activity corresponding with an inefficient working memory system is consistently demonstrated in patients. These similarities in brain function between intoxicated healthy volunteers and schizophrenia patients provide an argument for a role of the endocannabinoid system in symptoms of schizophrenia, and further emphasize this system as a potential novel target for treatment of these symptoms. [Copyright &y& Elsevier]