06:35pm Friday 10 July 2020

Long-term cannabis use may blunt the brain’s motivation system

Researchers found that dopamine levels in a part of the brain called the striatum were lower in people who smoke more cannabis and those who began taking the drug at a younger age.

They suggest this finding could explain why some cannabis users appear to lack motivation to work or pursue their normal interests.

The study, by scientists at Imperial College London, UCL and King’s College London, was funded by the Medical Research Council and published in the journal Biological Psychiatry.

The researchers used PET brain imaging to look at dopamine production in the striatum of 19 regular cannabis users and 19 non-users of matching age and sex.

The cannabis users in the study had all experienced psychotic-like symptoms while smoking the drug, such as experiencing strange sensations or having bizarre thoughts like feeling as though they are being threatened by an unknown force.

The researchers expected that dopamine production might be higher in this group, since increased dopamine production has been linked with psychosis. Instead, they found the opposite effect.

The cannabis users in the study had their first experience with the drug between the ages of 12 and 18. There was a trend for lower dopamine levels in those who started earlier, and also in those who smoke more cannabis. The researchers say these findings suggest that cannabis use may be the cause of the difference in dopamine levels.

The lowest dopamine levels were seen in users who meet diagnostic criteria for cannabis abuse or dependence, raising the possibility that this measure could provide a marker of addiction severity.

Previous research has shown that cannabis users have a higher risk of mental illnesses that involve repeated episodes of psychosis, such as schizophrenia.

“It has been assumed that cannabis increases the risk of schizophrenia by inducing the same effects on the dopamine system that we see in schizophrenia, but this hasn’t been studied in active cannabis users until now,” said Dr Michael Bloomfield, from the Institute of Clinical Sciences at Imperial, who led the study.

“The results weren’t what we expected, but they tie in with previous research on addiction, which has found that substance abusers – people who are dependent on cocaine or amphetamine, for example – have altered dopamine systems.

“Although we only looked at cannabis users who have had psychotic-like experiences while using the drug, we think the findings would apply to cannabis users in general, since we didn’t see a stronger effect in the subjects who have more psychotic-like symptoms. This needs to be tested though.

“It could also explain the ‘amotivational syndrome’ which has been described in cannabis users, but whether such a syndrome exists is controversial.”

Other studies have looked at dopamine release in former cannabis users and not seen differences with people who haven’t taken cannabis, suggesting that the effects seen in this study are likely to be reversible.

For further information please contact:

Sam Wong
Research Media Officer
Imperial College London
Email: [email protected]
Tel: +44(0)20 7594 2198
Out of hours duty press officer: +44(0)7803 886 248

Notes to editors

1. M Bloomfield et al. ‘Dopaminergic Function in Cannabis Users and its Relationship to Cannabis-Induced Psychotic Symptoms’ Biological Psychiatry (2013) http://dx.doi.org/10.1016/j.biopsych.2013.05.027

2. About Imperial College London

Consistently rated amongst the world’s best universities, Imperial College London is a science-based institution with a reputation for excellence in teaching and research that attracts 14,000 students and 6,000 staff of the highest international quality. Innovative research at the College explores the interface between science, medicine, engineering and business, delivering practical solutions that improve quality of life and the environment – underpinned by a dynamic enterprise culture.

Since its foundation in 1907, Imperial’s contributions to society have included the discovery of penicillin, the development of holography and the foundations of fibre optics. This commitment to the application of research for the benefit of all continues today, with current focuses including interdisciplinary collaborations to improve global health, tackle climate change, develop sustainable sources of energy and address security challenges.

In 2007, Imperial College London and Imperial College Healthcare NHS Trust formed the UK’s first Academic Health Science Centre. This unique partnership aims to improve the quality of life of patients and populations by taking new discoveries and translating them into new therapies as quickly as possible.

Website: www.imperial.ac.uk

3. About the Medical Research Council
Over the past century, the Medical Research Council has been at the forefront of scientific discovery to improve human health. Founded in 1913 to tackle tuberculosis, the MRC now invests taxpayers’ money in some of the best medical research in the world across every area of health. Twenty-nine MRC-funded researchers have won Nobel prizes in a wide range of disciplines, and MRC scientists have been behind such diverse discoveries as vitamins, the structure of DNA and the link between smoking and cancer, as well as achievements such as pioneering the use of randomised controlled trials, the invention of MRI scanning, and the development of a group of antibodies used in the making of some of the most successful drugs ever developed.
Today, MRC-funded scientists tackle some of the greatest health problems facing humanity in the 21st century, from the rising tide of chronic diseases associated with ageing to the threats posed by rapidly mutating micro-organisms. www.mrc.ac.uk The MRC Centenary Timeline chronicles 100 years of life-changing discoveries and shows how our research has had a lasting influence on healthcare and wellbeing in the UK and globally, right up to the present day. www.centenary.mrc.ac.uk

4. About UCL (University College London) Founded in 1826, UCL was the first English university established after Oxford and Cambridge, the first to admit students regardless of race, class, religion or gender and the first to provide systematic teaching of law, architecture and medicine.
We are among the world’s top universities, as reflected by our performance in a range of international rankings and tables. According to the Thomson Scientific Citation Index, UCL is the second most highly cited European university and the 15th most highly cited in the world.
UCL has nearly 25,000 students from 150 countries and more than 9,000 employees, of whom one third are from outside the UK. The university is based in Bloomsbury in the heart of London, but also has two international campuses – UCL Australia and UCL Qatar. Our annual income is more than £800 million.
www.ucl.ac.uk | Follow us on Twitter @uclnews | Watch our YouTube channel YouTube.com/UCLTV

5. About King’s College London:
King’s College London is one of the top 30 universities in the world (2012/13 QS international world rankings), and was The Sunday Times ‘University of the Year 2010/11’, and the fourth oldest in England. A research-led university based in the heart of London, King’s has more than 25,000 students (of whom more than 10,000 are graduate students) from nearly 140 countries, and more than 6,500 employees. King’s is in the second phase of a £1 billion redevelopment programme which is transforming its estate.
King’s has an outstanding reputation for providing world-class teaching and cutting-edge research. In the 2008 Research Assessment Exercise for British universities, 23 departments were ranked in the top quartile of British universities; over half of our academic staff work in departments that are in the top 10 per cent in the UK in their field and can thus be classed as world leading. The College is in the top seven UK universities for research earnings and has an overall annual income of nearly £525 million (year ending 31 July 2011).
King’s has a particularly distinguished reputation in the humanities, law, the sciences (including a wide range of health areas such as psychiatry, medicine, nursing and dentistry) and social sciences including international affairs. It has played a major role in many of the advances that have shaped modern life, such as the discovery of the structure of DNA and research that led to the development of radio, television, mobile phones and radar.
King’s College London and Guy’s and St Thomas’, King’s College Hospital and South London and Maudsley NHS Foundation Trusts are part of King’s Health Partners. King’s Health Partners Academic Health Sciences Centre (AHSC) is a pioneering global collaboration between one of the world’s leading research-led universities and three of London’s most successful NHS Foundation Trusts, including leading teaching hospitals and comprehensive mental health services. For more information, visit: www.kingshealthpartners.org.
The College is in the midst of a five-year, £500 million fundraising campaign – World questions|King’s answers – created to address some of the most pressing chall


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