Teens who were heavy marijuana users – smoking it daily for about three years – had an abnormally shaped hippocampus and performed poorly on long-term memory tasks, reports a new Northwestern Medicine study.
The hippocampus is important to long-term memory (also known as episodic memory), which is the ability to remember autobiographical or life events. The brain abnormalities and memory problems were observed during the individuals’ early twenties, two years after they stopped smoking marijuana. Young adults who abused cannabis as teens performed about 18 percent worse on long-term memory tests than young adults who never abused cannabis.
"The memory processes that appear to be affected by cannabis are ones that we use every day to solve common problems and to sustain our relationships with friends and family," said senior author Dr John Csernansky, the Lizzie Gilman professor and chair of psychiatry and behavioural sciences at Northwestern University Feinberg School of Medicine and Northwestern Memorial Hospital.
The study is among the first to say the hippocampus is shaped differently in heavy marijuana smokers and the different looking shape is directly related to poor long-term memory performance. Previous studies of cannabis users have shown either the oddly shaped hippocampus or poor long-term memory but none have linked them. Previous research by the same Northwestern team showed poor short-term and working memory performance and abnormal shapes of brain structures in the sub-cortex including the striatum, globus pallidus and thalamus.
"Both our recent studies link the chronic use of marijuana during adolescence to these differences in the shape of brain regions that are critical to memory and that appear to last for at least a few years after people stop using it," said lead study author Matthew Smith, assistant professor of psychiatry and behavioural sciences at the Feinberg School of Medicine.
The longer the individuals were chronically using marijuana, the more abnormal the shape of their hippocampus, the study reports. The findings suggest that these regions related to memory may be more susceptible to the effects of the drug the longer the abuse occurs. The abnormal shape likely reflects damage to the hippocampus and could include the structure’s neurons, axons or their supportive environments.
"Advanced brain mapping tools allowed us to examine detailed and sometimes subtle changes in small brain structures, including the hippocampus," said Lei Wang, also a senior study author and an assistant professor of psychiatry and behavioural sciences at Feinberg. The scientists used computerised programs they developed with collaborators that performed fine mappings between structural MRIs of different individuals' brains. Subjects took a narrative memory test in which they listened to a series of stories for about one minute, then were asked to recall as much content as possible 20 to 30 minutes later. The test assessed their ability to encode, store, and recall details from the stories.
The groups in the study started using marijuana daily between 16 to 17 years of age for about three years. At the time of the study, they had been marijuana free for about two years. A total of 97 subjects participated, including matched groups of healthy controls, subjects with a marijuana use disorder, schizophrenia subjects with no history of substance use disorders, and schizophrenia subjects with a marijuana use disorder. The subjects who used marijuana did not abuse other drugs.
The study also found that young adults with schizophrenia who abused cannabis as teens performed about 26% more poorly on memory tests than young adults with schizophrenia who never abused cannabis. But because the study results examined one point in time, a longitudinal study is needed to definitively show if marijuana is responsible for the observed differences in the brain and memory impairment, Smith said.
"It is possible that the abnormal brain structures reveal a pre-existing vulnerability to marijuana abuse," Smith said. "But evidence that the longer the participants were abusing marijuana, the greater the differences in hippocampus shape suggests marijuana may be the cause."
Also, the first study to examine the use of cannabis in the context of daily life among people with bipolar disorder has shown how the drug is linked to increases in both manic and depressive symptoms.
Dr Elizabeth Tyler of the Spectrum Centre for Mental Health Research at Lancaster University led the study, with Professor Steven Jones and colleagues from the University of Manchester, Professor Christine Barrowclough, Nancy Black and Lesley-Anne Carter.
Tyler said: "One theory that is used to explain high levels of drug use is that people use cannabis to self-medicate their symptoms of bipolar disorder. The study looked at people diagnosed with bipolar disorder but who were not experiencing a depressive or manic episode during the six days the research was carried out."
Each participant completed a paper diary about their emotional state and drug use at several random points daily over a period of week. This enabled people to log their daily experiences in the moment before they forgot how they were feeling.
An individual with experience of bipolar disorder and cannabis use commented: "I do smoke a small amount to lift my mood and make myself slightly manic but it also lifts my mood and switches me into a different mind-set. I do not use weed to manage depression as it can make it worse, making me anxious and paranoid. I have found though that if I have smoked more excessively it can make me feel depressed for days afterwards".
The study found that the odds of using cannabis increased when individuals were in a good mood. Cannabis use was also associated with an increase in positive mood, manic symptoms and paradoxically an increase in depressive symptoms, but not in the same individuals.
Tyler said: "The findings suggest that cannabis is not being used to self-medicate small changes in symptoms within the context of daily life. However, cannabis use itself may be associated with both positive and negative emotional states. We need to find out whether these relationships play out in the longer term as this may have an impact on a person’s course of bipolar disorder."
[link url="http://www.northwestern.edu/newscenter/stories/2015/03/teen-cannabis-users-have-poor-long-term-memory-in-adulthood.html"]Northwestern University material[/link]
[link url="http://onlinelibrary.wiley.com/doi/10.1002/hipo.22427/full"]Hippocampus abstract[/link]
[link url="http://www.lancaster.ac.uk/news/articles/2015/how-cannabis-use-affects-people-with-bipolar-disorder/"]Lancaster University material[/link]
[link url="http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0118916"]PLOS One abstract[/link]