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Brain response to cannabis

Brain imaging studies have allowed scientists to differentiate the effect on the brain of specific chemicals found in cannabis. The findings, published in the January issue of the Archives of General Psychiatry, provide the first experimental evidence of the effect cannabis has on the importance people attach to things they perceive and help explain why some more potent forms of the drugs are more strongly linked to psychosis.

Dr Sagnik Bhattacharyya and Professor Philip McGuire led the research at the Institute of Psychiatry at King’s College London. The study looked at 15 healthy men, who were occasional cannabis users, to examine the effects of the two main components of cannabis, Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD) on regional brain function.

The authors used functional MRI imaging to study each participant on three occasions after administration of THC, CBD or a placebo. They then performed a visual oddball detection task so that researchers could understand the importance each individual attached to specific stimuli.

THC significantly weakened the activation of the striatum and increased the activation of the lateral prefrontal cortex. The effect in the striatum was a result of THC increasing individuals’ response to normally insignificant stimuli, and decreasing its response to significant stimuli. The findings help explain why smoking cannabis can result in feelings of paranoia, or in the most extreme cases, psychotic episodes, as individuals attach special importance or meaning to normally insignificant experiences or stimuli.

However, cannabidiol (CBD),the other major ingredient in cannabis, had the opposite effect on the brain. CBD increased the response of the left caudate, an area of the brain weakened by THC.

Whilst the results are still preliminary, the findings suggest that psychotic symptoms may develop through individuals attaching inappropriate prominence to insignificant experiences or stimuli.

Dr Bhattacharyya says: ‘Our findings help shed further light on the mental and public health consequencesof cannabis.  The increasing prevalence in Europe and the UK of more potent forms of cannabis with high levels of THC is worrying, as there is a strong evidence of a link between THC and psychosis in some individuals. However, there may be ingredients in cannabis, such as CBD, which could prove to be useful treatments for psychiatric conditions, but more research is needed here.’

The work was supported by a Joint Medical Research Council/Priory Clinical researchtraining fellowship from the Medical Research Council (UK) to Dr Bhattacharyya, a grant from the Psychiatry Research Trust (UK) and a Conselho Nacional de Desenvolvimento Científico e Tecnológico (Brazil).

For full paper: Bhattacharyya et al. ‘Induction of psychosis by Δ9-tetrahydrocannabinol reflects modulation of prefrontal and striatal function during attention salience processing’ Archives of General Psychiatry (January 2012) doi:10.1001/archgenpsychiatry.2011.161

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