Cannabis rewires the brain

Is smoking cannibis linked to psychosis? And how it can rewire the brain making it susceptible to depression and apathy.
18 September 2014

Interview with 

Michael Bloomfield, Imperial College London


New research has shown that long-term marijuana use rewires the brain, making it less sensitive to a feel-good chemical called dopamine, and leaving users at risk of becoming depressed and demotivated and at risk of psychosis. Hannah Critchlow visited the British Association of Psychopharmacology 2014 meeting to speak with Michael Bloomfield, from Imperial College London...

Michael -   We know that people who smoke lots and lots of Cannabis and particularly, during adolescence when they're teenagers and more at risk of having mental health problems later on in life. 

The one that we're particularly worried about is something called Schizophrenia.  This is a potentially devastating mental illness where people can for example, hear frightening voices and become quite paranoid, become quite scared.  It's very important to understand how that increase in risk from smoking Cannabis and Schizophrenia is happening.  So, based on research using a brain scanning technique called Positron Emission Tomography affirm our group in London which is led by Dr. Oliver Howes.  We know that people who have Schizophrenia have a tendency to make large amounts of the brain chemical dopamine.  So, we wanted to find out if people who smoke lots of cannabis also have the same imbalance, so also had lots of dopamine to see if that could potentially explain how smoking lots of cannabis increases risk of Schizophrenia.

Hannah -   I think it's estimated that something along the lines of 6% of the UK population aged between 16 and 24 will regularly smoke cannabis.  What proportion of those will then go on to develop Schizophrenia?  What's the risk? What's the link between cannabis and Schizophrenia?

Michael -   There's some debate between scientists about how much it increases the risk.  Most of studies tend to agree that it about roughly doubles the chance of getting Schizophrenia.  I think the key thing that's really, really important is that the risk is most elevated at its highest for people who smoke cannabis very regularly, particularly during their teenage years.  That's the time that it's most risky.

Hannah -   Going back now to the PET scanning that you did looking at this dopamine chemical in the brain...

Michael -   Dopamine is a really interesting chemical in the brain that does lots and lots of things.  One of the things that it does is send a signal within your brain when something exciting potentially is about to happen, something rewarding.  And so, it's involved in motivation.  Now, we found within the cannabis users that there was a correlation so a relationship between their motivation levels and their dopamine levels.  What we found was that the lower their dopamine levels were, the more unmotivated that they felt.

Hannah -   So, your brain lights up with dopamine and reward when you're feeling things of pleasure and when you're getting keyed up to be motivated with something.  Cannabis actually decreases the amount of dopamine that's in your brain.  Is that right?

Michael -   What we think is happening is that if you smoke cannabis that it probably releases a bit of dopamine.  When people smoke lots and lots of cannabis over time, the same with recreational drugs that people can use, that the dopamine system can become used to being stimulated.  And so, it tries to adapt.  It tries to respond to this by probably lowering the amount of dopamine that it makes.

Hannah -   How much cannabis would you have to smoke?  I mean, from your studies, you're only looking at 19 smokers of cannabis.  So, in comparing them against those that didn't smoke cannabis.  So, you're only looking at 19 people, but how much cannabis do you have to smoke in order rewire and alter your dopamine, kind of rewards pathways in the brain?

Michael -   In our study, all of the cannabis users were quite heavy cannabis users.  So, most cannabis in the UK is sold as 1/8 which is an 8th of an ounce.  That's roughly 3.5 grams.  The cannabis users in our study was smoking an 8th.  It would last them roughly half a week and most of them are smoking a quarter on an ounce of cannabis a week which is quite a lot.  I think the other really interesting thing is, presently, we're understanding a bit more about the different chemicals in cannabis.  So, there are probably almost 100 different chemicals in cannabis.  Depending on the balance between these, that they probably have different effects on the brain.  So, there's one called THC which is the main one and another one called CBD.  What we think is really important is it's the balance between these as to the effects that they have on the brain in the short term, but also the long term.

Hannah -   So, bottom line then in your studies at least, regular cannabis use - so, at least a few times a week - can actually decrease or desensitise your brain's reward pathways so that you might get feelings of apathy and lack of motivation.  And that's what we also see with people that smoke cannabis regularly.  You do stereotype them as kind of being quite lacking in motivation.

Michael -   Certainly, that's what we believe based on the findings that we have.  I think the story is beginning to fit together.  Almost half of young people have tried cannabis at some points in their lives and I think that smoking lots of cannabis over a prolonged period of time does seem to have this effect.  There is some work that's been done in the past that's looked at educational outcomes.  It's found that people who smoke lots and lots of cannabis and importantly, carry on smoking lots of cannabis can affect how they do at school or a university or in their work life.  this might tie into that quite well.  I think the other thing as well is that there's some evidence that people who smoke lots of cannabis regularly are more likely to get depressed.  This is a bit more controversial because it's very difficult to tease out if cannabis is making people depressed or if people who are depressed for example have low mood are more likely to smoke cannabis to try and feel better about it.  but also, we know that this chemical dopamine is probably involved in depression as well.  One of the key symptoms of depression is not being able to enjoy things.  And that's if anything, the one that people find the most upsetting as they're not able to enjoy things that they used to enjoy.  I think it may tie in with that as well, but I think again, we need to do more research into how all these things affect dopamine and that affects how we feel.

Hannah -   So, those people who have smoked cannabis regularly and might feel that they changed or altered their dopamine reward pathways in their brain.  If they stopped smoking now, would the brain rebalance itself and retweak itself so that they could feel feelings of joy and reward, and motivation later on.

Michael -   Some of the studies that had been done have looked at people who used to smoke cannabis.  In those studies, they've only found very, very small effects if any, on the dopamine system.  So, what we think happens is that after a period of abstinence that the brain get back to normal again.  And so, I think if people are worried about the amounts of cannabis that they're smoking, if it's having negative effects on them and my advice would be to either stop or at least cut down the amount of cannabis that they're smoking.  If they need help with this then it's about time they see their general practitioner.

Hannah -   And going back to patients with Schizophrenia or psychosis, so patients with Schizophrenia have increased amounts of dopamine and cannabis seems to decrease in the amount of dopamine that your brain is sensitive to.  In that case, people with Schizophrenia or people that have a predisposition to Schizophrenia in some ways self-medicating with cannabis.

Michael -   That's a really, really interesting question.  I think that's certainly a possibility and we need to do quite a bit more work to tease out these really important points.  In our study, all of the cannabis users, although they experienced a temporary increase in psychotic-like experiences - so these are strange experiences all of us can have in everyday life - none of them actually had Schizophrenia.  So, I think a really good study would be to look at people who did have Schizophrenia and who wants to smoke cannabis.


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