Researchers
say long-term marijuana use lowers dopamine levels in the brain, which could
explain why some users develop mental illness..
The study says there is sufficient
evidence to suggest marijuana, or cannabis, reduces levels of dopamine in the
brain - a neurotransmitter that plays a key role in learning, movement,
motivation, emotion, and reward.
Low dopamine levels have been associated with mood
changes, fatigue, depression,
and lack of motivation; dopamine deficiency is present in a number of
neurological conditions, including Parkinson's disease and attention deficit hyperactivity
disorder (ADHD).
Study leader Prof. Oliver Howes, of the Medical Research
Council (MRC) Clinical Sciences Center at Imperial College London in the United
Kingdom, and team recently published their results in the journal Nature.
According to the 2014 National Survey on Drug Use and
Health, there are around 22.2 million marijuana users in the United States,
making it the most commonly used illicit drug in the country.
Long-term marijuana use has been linked to a number of mental
health conditions,
including schizophrenia, anxiety, and depression, but the mechanisms
underlying this association have been unclear.
Given
the increased legalization of marijuana for medicinal and recreational
purposes, researchers are keen to learn more about how the drug affects the
brain.
For this latest study, Prof.
Howes and team conducted a review of numerous studies investigating how the
primary psychoactive compound in marijuana - tetrahydrocannabinol (THC) -
affects the brain.
THC exposure
has long-term effects on the dopamine system
According
to the researchers, there is now "substantial evidence" in animal and
human studies that long-term exposure to THC leads to a decrease in levels of
dopamine.
"The
available evidence indicates that THC exposure produces complex, diverse and
potentially long-term effects on the dopamine system," the authors
explain. "These include increased nerve firing and dopamine release in
response to acute THC, and dopaminergic blunting associated with long-term
use."
The
team believes this effect may explain why people who engage in long-term
marijuana use are at increased risk for mental health problems.
In
animal models, current research shows that marijuana use initially raises
dopamine levels, fueling a sense of reward, which the team says may offer an
explanation for why some users become addicted to the drug.
However,
the authors point to some limitations in this area. "Fundamentally, animal
studies are too short, and don't give cannabis repeatedly or in combination
with other substances," notes Prof. Howes.
Addressing the
gaps in marijuana research
The
researchers also noticed some other gaps in research, such as studies assessing
what happens to the dopamine system when marijuana use is ceased.
What
is more, the team notes it is important to learn more about how marijuana use
affects brain development, as some women may use the drug in early pregnancy,
before realizing they are expecting.
"Given
the increasing use of cannabis, particularly in young people and women who may
be pregnant, animal studies are needed to understand the effects of long-term
cannabis use on the developing brain in a controlled way that is not possible
in human studies." says Prof. Howes.
"These
studies also need to use techniques that can be translated into human studies,
and to better represent human patterns of use."
While
further investigation into the effects of marijuana is clearly warranted, the
researchers believe their current study helps broaden our understanding.
"The changing patterns of
cannabis use, including 'cannavaping' and edible products, mean it's vital that
we understand the long-term effects of cannabis on the brain.
This new research helps to explain
how some people get addicted to cannabis, by showing that one of its main
components, called THC, alters a delicate balance of brain chemicals."
Co-author
Dr. Michael Bloomfield, Clinical Sciences Centre, Imperial College London
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