The Cortical Column

It’s been known that cannabis greatly increases the risk of developing paranoid schizophrenia since 1987, when Lancet published his article, “Cannabis and schizophrenia. A longitudinal study of Swedish conscripts.” In a Nature review published today, titled “Cannabis, the mind and society: the hash realities” (subscription required), Murray et al., explain the leading hypothesis behind this increased risk:

How might cannabis cause psychotic symptoms? Acutely psychotic patients show dopamine sensitization. For example, they release excessive striatal dopamine in response to an amphetamine challenge, and the degree of dopamine release correlates positively with the severity of the psychotic symptoms. The probable mechanism is the increased dopamine resulting in increased attention and excessive significance (salience) being attributed to everyday stimuli. In this way, an unexpected sound, the comments of a TV newsreader or eye contact with a stranger, for example, are transformed from trivial everyday occurrences into highly salient events of great personal meaning to the psychotic individual. Delusions can be understood as an attempt to explain these experiences and resolve the resultant perplexity, confusion and dysphoria.

Cannabis markedly increases dopaminergic neuronal firing, including burst-firing, and increases the release of dopamine at terminal fields in the striatum. It is tempting therefore to suggest that this is the mechanism by which it exerts its psychotogenic effects. No investigations have directly tested this in humans, although in one imaging study a subject broke the protocol by smoking cannabis during a pause between imaging sessions, and the resulting brain scans showed evidence that suggested the occurrence of a cannabis-induced increase in synaptic dopaminergic activity.

The article also presents a world map showing survey results on personal cannabis use. The survey found that greater than 8% of much of the western world has used cannabis in the last year, and that as much as 14% of cases of schizophrenia may be partly due to cannabis use. The lifetime prevalence of schizophrenia is though to be between .5% and 1%, although it varies widely.