There is now growing evidence that cannabis use causes a small but reliable increase in the chance of developing psychosis. Traditionally, this was explained by the drug increasing dopamine levels in the brain but a new study shortly to be published in NeuroImage suggests that the active ingredient in cannabis doesn’t effect this important neurotransmitter.

Despite some dissenting voices, disruption to the mesolimbic dopamine pathway is widely thought to be the key problem in the development of delusions, hallucinations and the other psychotic symptoms commonly diagnosed as schizophrenia.

This has led to the assumption that the small increased risk of psychosis reliably associated with cannabis use is due to the drug increasing dopamine levels in a deep brain structure called the striatum.

In itself, this is partly based on another assumption – the virtual mantra of recreational drug research that ‘all drugs of abuse increase dopamine levels in the reward system’ of which the striatum is a part.

This new study, led by neuroscientist Paul Stokes, tested dopamine levels by using a type of PET brain scan where participants are injected with a radioactive tracer that binds to free dopamine receptors. Higher dopamine levels will mean that there are less free dopamine receptors and, therefore, lower tracer levels.

Participants were tested twice, once when given placebo and once when given a dose of pure THC – one of the most important active ingredients in cannabis. The dose was designed to be roughly equivalent to the amount you might absorb from a single joint.

The researchers found no difference in dopamine levels between the THC and the sugar pill, even though the participants clearly reported the effects of the drug.

Although they only tested 13 participants, this is the largest study of its kind so far. These type of neurotransmitter tracer studies are know to produce conflicting results at times, so further experiments will be needed to be sure of the result.

But if it is the case that cannabis does not cause a significant increase in dopamine levels, this will mean our ideas about cannabis and psychosis will need a rethink.

It also shakes up the idea common idea that all recreational drugs are pleasurable because they affect the ‘dopamine reward system’.

Link to PubMed entry for the ‘in press’ study.