Neuroscience

Young adults who used marijuana only recreationally showed significant abnormalities in two key brain regions that are important in emotion and motivation, scientists report. The study was a collaboration between Northwestern Medicine® and Massachusetts General Hospital/Harvard Medical School.

This is the first study to show casual use of marijuana is related to major brain changes. It showed the degree of brain abnormalities in these regions is directly related to the number of joints a person smoked per week. The more joints a person smoked, the more abnormal the shape, volume and density of the brain regions.

"This study raises a strong challenge to the idea that casual marijuana use isn’t associated with bad consequences," said corresponding and co-senior study author Hans Breiter, M.D. He is a professor of psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine and a psychiatrist at Northwestern Memorial Hospital.

"Some of these people only used marijuana to get high once or twice a week," Breiter said. "People think a little recreational use shouldn’t cause a problem, if someone is doing OK with work or school. Our data directly says this is not the case."

The study will be published April 16 in the Journal of Neuroscience.

Scientists examined the nucleus accumbens and the amygdala — key regions for emotion and motivation, and associated with addiction — in the brains of casual marijuana users and non-users. Researchers analyzed three measures: volume, shape and density of grey matter (i.e., where most cells are located in brain tissue) to obtain a comprehensive view of how each region was affected.

Both these regions in recreational pot users were abnormally altered for at least two of these structural measures. The degree of those alterations was directly related to how much marijuana the subjects used.

Of particular note, the nucleus acccumbens was abnormally large, and its alteration in size, shape and density was directly related to how many joints an individual smoked.

"One unique strength of this study is that we looked at the nucleus accumbens in three different ways to get a detailed and consistent picture of the problem," said lead author Jodi Gilman, a researcher in the Massachusetts General Center for Addiction Medicine and an instructor in psychology at Harvard Medical School. "It allows a more nuanced picture of the results."

Examining the three different measures also was important because no single measure is the gold standard. Some abnormalities may be more detectable using one type of neuroimaging analysis method than another. Breiter said the three measures provide a multidimensional view when integrated together for evaluating the effects of marijuana on the brain.

"These are core, fundamental structures of the brain," said co-senior study author Anne Blood, director of the Mood and Motor Control Laboratory at Massachusetts General and assistant professor of psychiatry at Harvard Medical School. "They form the basis for how you assess positive and negative features about things in the environment and make decisions about them."

Through different methods of neuroimaging, scientists examined the brains of young adults, ages 18 to 25, from Boston-area colleges; 20 who smoked marijuana and 20 who didn’t. Each group had nine males and 11 females. The users underwent a psychiatric interview to confirm they were not dependent on marijuana. They did not meet criteria for abuse of any other illegal drugs during their lifetime.

The changes in brain structures indicate the marijuana users’ brains are adapting to low-level exposure to marijuana, the scientists said.

The study results fit with animal studies that show when rats are given tetrahydrocannabinol (THC) their brains rewire and form many new connections. THC is the mind-altering ingredient found in marijuana.

"It may be that we’re seeing a type of drug learning in the brain," Gilman said. "We think when people are in the process of becoming addicted, their brains form these new connections."

In animals, these new connections indicate the brain is adapting to the unnatural level of reward and stimulation from marijuana. These connections make other natural rewards less satisfying.

"Drugs of abuse can cause more dopamine release than natural rewards like food, sex and social interaction," Gilman said. "In those you also get a burst of dopamine but not as much as in many drugs of abuse. That is why drugs take on so much salience, and everything else loses its importance."

The brain changes suggest that structural changes to the brain are an important early result of casual drug use, Breiter said. “Further work, including longitudinal studies, is needed to determine if these findings can be linked to animal studies showing marijuana can be a gateway drug for stronger substances,” he noted.

Because the study was retrospective, researchers did not know the THC content of the marijuana, which can range from 5 to 9 percent or even higher in the currently available drug. The THC content is much higher today than the marijuana during the 1960s and 1970s, which was often about 1 to 3 percent, Gilman said.

Marijuana is the most commonly used illicit drug in the U.S. with an estimated 15.2 million users, the study reports, based on the National Survey on Drug Use and Health in 2008. The drug’s use is increasing among adolescents and young adults, partially due to society’s changing beliefs about cannabis use and its legal status.

A recent Northwestern study showed chronic use of marijuana was linked to brain abnormalities. “With the findings of these two papers,” Breiter said, “I’ve developed a severe worry about whether we should be allowing anybody under age 30 to use pot unless they have a terminal illness and need it for pain.”

Past research has long indicated that depression is a big risk factor for memory loss in aging adults. But it is still unclear exactly how the two issues are related and whether there is potential to slow memory loss by fighting depression.

A preliminary study conducted by researchers from the University of Rochester School of Medicine and Dentistry and the School of Nursing, and published in the 42nd edition of Psychoneuroendocrinology in April, delves more deeply into the relationship between depression and memory loss, and how this connection may depend on levels of insulin-like growth factor, or IGF-1.

Prior research has shown that IGF-1, a hormone that helps bolster growth, is important for preserving memory, especially among older adults.

The collaborative study found that people with lower cognitive ability were more likely to have had higher depressive symptoms if they also had low levels of IGF-1. Reversely, participants with high levels of IGF-1 had no link between depressive symptoms and memory.

Senior author Kathi L. Heffner, Ph.D., assistant professor in the School of Medicine and Dentistry’s Department of Psychiatry, had originally examined possible associations between IGF-1 and memory in a sample of 94 healthy older adults, but couldn’t find strong or consistent evidence.

Heffner then approached the study’s lead author Feng (Vankee) Lin, Ph.D, R.N., assistant professor at the School of Nursing, for input because of her expertise in cognitive aging. Lin is a young nurse researcher whose collaborative work focuses on developing multi-model interventions to slow the progression of cognitive decline in at-risk adults, and reduce their risk of developing dementia and Alzheimer’s disease.

“Vankee spearheaded the idea to examine the role of depressive symptoms in these data, which resulted in the interesting link,” Heffner said.

The association discovered between memory loss, depression and IGF-1 means that IGF-1 could be a very promising factor in protecting memory, Lin said.

“IGF-1 is currently a hot topic in terms of how it can promote neuroplasticity and slow cognitive decline,” Lin said. “Depression, memory and the IGF-1 receptor are all located in a brain region which regulates a lot of complicated cognitive ability. As circulating IGF-1 can pass through the blood-brain barrier, it may work to influence the brain in a protective way.”

Lin said more data studies are needed of people with depression symptoms and those with Alzheimer’s disease, but this study opens an important door for further research on the significance of IGF-1 levels in both memory loss and depression.

“It really makes a lot of sense to further develop this study,” Lin said. “If this could be a way to simultaneously tackle depression while preventing cognitive decline it could be a simple intervention to implement.”

Heffner said that clinical trials are underway to determine whether IGF-1 could be an effective therapeutic agent to slow or prevent cognitive decline in people at risk.

“Cognitive decline can also increase risk for depressive symptoms, so if IGF-1 protects people from cognitive decline, this may translate to reduced risk for depression as well,” Heffner said.

Working with human neurons and fruit flies, researchers at Johns Hopkins have identified and then shut down a biological process that appears to trigger a particular form of Parkinson’s disease present in a large number of patients. A report on the study, in the April 10 issue of the journal Cell, could lead to new treatments for this disorder.

“Drugs such as L-dopa can, for a time, manage symptoms of Parkinson’s disease, but as the disease worsens, tremors give way to immobility and, in some cases, to dementia. Even with good treatment, the disease marches on,” says Ted Dawson, M.D., Ph.D., professor of neurology and director of the Johns Hopkins Institute for Cell Engineering, Dawson says the new research builds on a growing body of knowledge about the origins of Parkinson’s disease, whose symptoms appear when dopamine-producing nerve cells in the brain degenerate. Further evidence for a role of genetics in Parkinson’s disease appeared a decade ago when researchers identified key mutations in an enzyme known as leucine-rich repeat kinase 2, or LRRK2 — pronounced “lark2.” When that enzyme was cloned, Dawson, together with his wife and longtime collaborator Valina Dawson, Ph.D., professor of neurology and member of the Institute for Cell Engineering, discovered that LRRK2 was a kinase, a type of enzyme that transfers phosphate groups to proteins and turns proteins on or off to change their activity.

Over the years, it was found that blocking kinase activity in mutated LRRK2 halted degeneration, while enhancing it made things worse. But nobody knew what proteins LRRK2 was acting on.

"For nearly a decade, scientists have been trying to figure out how mutations in LRRK2 cause Parkinson’s disease," said Margaret Sutherland, Ph.D., a program director at the National Institute of Neurological Disorders and Stroke. "This study represents a clear link between LRRK2 and a pathogenic mechanism linked to Parkinson’s disease."

Dawson went fishing for the right proteins using LRRK2 as bait. When his team began to identify those proteins, Dawson says they were surprised to discover that many were linked to the cellular machinery, like ribosomes, that make proteins. Nobody, says Dawson, suspected that LRRK2 might be involved at such a basic level as protein manufacture.

Unsure if they were right, the team then tested the proteins they identified to see which of them, if any, LRRK2 could add phosphate groups to. They came up with three ribosomal protein candidates — s11, s15 and s27. They then altered each ribosomal protein to see what would happen. It turned out that mutating s15 in a manner that blocked LRRK2 phosphorylation protected nerve cells taken from rats, humans and fruit flies from death. In other words, s15 appeared to be the much sought-after target of LRRK2, Dawson says.

"When you go fishing, you want to catch fish. We just happened to catch a big one,” Dawson says.

With the protein now identified, Dawson’s team is tackling further experiments to find out how excess protein production causes dopamine neurons to degenerate. And they want to see what happens when they block LRRK2 from phosphorylating the s15 protein in mice, to build on their findings from fruit flies and nerve cells grown in a dish.

“There’s a big chasm between animal disease models and human treatments,” says Ian Martin, Ph.D., a neuroscientist in Dawson’s lab and the lead author on the paper. “But it’s exciting. I think it definitely could turn into something real, hopefully in my lifetime.”

How nerve cells flexibly adapt to acoustic signals: Depending on the input signal, neurons generate action potentials either near or far away from the cell body. This flexibility improves our ability to localize sound sources.

(Image caption: A neuron in the brain stem, that processes acoustic information. Depending on the situation, the cell generates action potentials in the axon (thin process) either close to or far from the body. Photo: Felix Felmy)

In order to process acoustic information with high temporal fidelity, nerve cells may flexibly adapt their mode of operation according to the situation. At low input frequencies, they generate most outgoing action potentials close to the cell body. Following inhibitory or high frequency excitatory signals, the cells produce many action potentials more distantly. This way, they are highly sensitive to the different types of input signals. These findings have been obtained by a research team headed by Professor Christian Leibold, Professor Benedikt Grothe, and Dr. Felix Felmy from the LMU Munich and the Bernstein Center and the Bernstein Focus Neurotechnology in Munich, who used computer models in their study. The researchers report their results in the latest issue of The Journal of Neuroscience.

Did the bang come from ahead or from the right? In order to localize sound sources, nerve cells in the brain stem evaluate the different arrival times of acoustic signals at the two ears. Being able to detect temporal discrepancies of up to 10 millionths of a second, the neurons have to become excited very quickly. In this process, they change the electrical voltage that prevails on their cell membrane. If a certain threshold is exceeded, the neurons generate a strong electrical signal — a so-called action potential — which can be transmitted efficiently over long axon distances without weakening. In order to reach the threshold, the input signals are summed up. This is achieved easier, the slower the nerve cells alter their electrical membrane potential.

Input signals are optimally processed
These requirements — rapid voltage changes for a high temporal resolution of the input signals, and slow voltage changes for an optimal signal integration that is necessary for the generation of an action potential — represent a paradoxical challenge for the nerve cell. “This problem is solved by nature by spatially separating the two processes. While input signals are processed in the cell body and the dendrites, action potentials are generated in the axon, a cell process,” says Leibold, leader of the study. But how sustainable is the spatial separation?

In their study, the researchers measured the axons’ geometry and the threshold of the corresponding cells and then constructed a computer model that allowed them to investigate the effectiveness of this spatial separation. The researchers’ model predicts that depending on the situation, neurons produce action potentials with more or less proximity to the cell body. For high frequency or inhibitory input signals, the cells will shift the location from the axon’s starting point to more distant regions. In this way, the nerve cells ensure that the various kinds of input signals are optimally processed — and thus allow us to perceive both small and large acoustic arrival time differences well, and thereby localize sounds in space.

A research led by the Research Institute Vall d’Hebron (VHIR), in which the University of Valencia participated, has shown that pathological forms of the α-synuclein protein present in deceased patients with Parkinson’s disease are able to initiate and spread in mice and primates the neurodegenerative process that typifies this disease. The discovery, published in the March cover of Annals of Neurology, opens the door to the development of new treatments that allow to stop the progression of Parkinson’s disease, aimed at blocking the expression, the pathological conversion and the transmission of this protein.

Recent studies have shown that synthetic forms of α-synuclein are toxic for the neurons, both in vitro (cell culture) and in vivo (mice), which can spread from one cell to another. However, until now it was not known if this pathogenic protein synthetic capacity could be extended to the pathological human protein found in patients with Parkinson and, therefore, whether it was relevant for the disease in humans.

In the present study, led by Doctor Miquel Vila, from the group of Neurodegenerative Diseases of the VHIR and CIBERNED member, and in which two other groups of CIBERNED have also participated (the lead by Doctor Isabel Fariñas, University of Valencia, and the led by Doctor José Obeso, CIMA-University of Navarra), as well as a group from the University of Bordeaux in France (Doctor Erwan Bezard), the researchers extracted α-synuclein aggregates of brains of dead patients because of the Parkinson’s disease to inject them into the brains of rodents and primates.

Four months after the injection into mice, and nine months after the injection into monkeys, these animals began to present degeneration of dopaminergic neurons and intracellular cumulus of α-synuclein pathology in these cells, as occurs in the Parkinson’s disease. Months later, the animals also showed cumulus of this protein in other brain remote areas, with a pattern of similar extension to that observed in the brains of patients after years of disease evolution.

According to Doctor Vila, these results indicate that “the pathological aggregates of this protein obtained from patients with the Parkinson’s disease have the ability to initiate and extend the neurodegenerative process that typifies the Parkinson’s disease in mice and primates”. A discovery that, he adds, “provides new insights about the possible mechanisms of initiation and progression of the disease and opens the door to new therapeutic opportunities”. Therefore, the next step is to find out how to stop the progression and spread of the disease, by blocking the transmission of cell to cell of the α-synuclein, as well as regulating the levels of expression and stopping the pathological conversion of this protein.

The Parkinson’s disease

The Parkinson’s disease is the second most common neurodegenerative disease after the Alzheimer’s disease. It is characterized by progressive loss of neurons that produce dopamine in a brain region (the substantia nigra of the ventral midbrain) and the presence in these cells of pathological intracellular aggregates of the α-synuclein protein, called Lewy bodies. The loss of brain dopamine as a consequence of neuronal death results in the typical motor manifestations of the disease, such as muscle stiffness, tremors and slow movement.

The most effective treatment for this disease is the levodopa, a palliative drug that allows to restore the missing dopamine. However, as the disease progresses, the pathological process of neurodegeneration and accumulation of α-synuclein progressively extends beyond the ventral midbrain to other brain areas. As a result, there is a progressive worsening of the patient and the emergence of non-motor clinical manifestations unresponsive to dopaminergic drugs. There is currently no treatment that avoids, delays or halts the progressive evolution of the neurodegenerative process.

Research linked to stress in mice confirms blood-brain comparison is valid

Johns Hopkins researchers say they have confirmed suspicions that DNA modifications found in the blood of mice exposed to high levels of stress hormone — and showing signs of anxiety — are directly related to changes found in their brain tissues.

The proof-of-concept study, reported online ahead of print in the June issue of Psychoneuroendocrinology, offers what the research team calls the first evidence that epigenetic changes that alter the way genes function without changing their underlying DNA sequence — and are detectable in blood — mirror alterations in brain tissue linked to underlying psychiatric diseases.

The new study reports only on so-called epigenetic changes to a single stress response gene called FKBP5, which has been implicated in depression, bipolar disorder and post-traumatic stress disorder. But the researchers say they have discovered the same blood and brain matches in dozens more genes, which regulate many important processes in the brain.

“Many human studies rely on the assumption that disease-relevant epigenetic changes that occur in the brain — which is largely inaccessible and difficult to test — also occur in the blood, which is easily accessible,” says study leader Richard S. Lee, Ph.D., an instructor in the Department of Psychiatry and Behavioral Sciences at the Johns Hopkins University School of Medicine. “This research on mice suggests that the blood can legitimately tell us what is going on in the brain, which is something we were just assuming before, and could lead us to better detection and treatment of mental disorders and for a more empirical way to test whether medications are working.”

For the study, the Johns Hopkins team worked with mice with a rodent version of Cushing’s disease, which is marked by the overproduction and release of cortisol, the primary stress hormone also called glucocorticoid. For four weeks, the mice were given different doses of stress hormones in their drinking water to assess epigenetic changes to FKBP5. The researchers took blood samples weekly to measure the changes and then dissected the brains at the end of the month to study what changes were occurring in the hippocampus as a result of glucocorticoid exposure. The hippocampus, in both mice and humans, is vital to memory formation, information storage and organizational abilities.

The measurements showed that the more stress hormones the mice got, the greater the epigenetic changes in the blood and brain tissue, although the scientists say the brain changes occurred in a different part of the gene than expected. This was what made finding the blood-brain connection very challenging, Lee says.

Also, the more stress hormone, the more RNA from the FKBP5 gene was expressed in the blood and brain, and the greater the association with depression. However, it was the underlying epigenetic changes that proved to be more robust. This is important, because while RNA levels may return to normal after stress hormone levels decrease or change due to small fluctuations in hormone levels, epigenetic changes persist, reflect overall stress hormone exposure and predict how much RNA will be made when stress hormone levels increase.

The team of researchers used an epigenetic assay previously developed in their laboratory that requires just one drop of blood to accurately assess overall exposure to stress hormone over 30 days. Elevated levels of stress hormone exposure are considered a risk factor for mental illness in humans and other mammals.

When you throw a wild pitch or sing a flat note, it could be that your basal ganglia made you do it. This area in the middle of the brain is involved in motor control and learning. And one reason for that errant toss or off-key note may be that your brain prompted you to vary your behavior to help you learn, from trial-and-error, to perform better.

But how does the brain do this, how does it cause you to vary your behavior?

Along with researchers from the University of California, San Francisco, Indian Institute of Science Education and Research and Duke University, Professor Sarah Woolley, Department of Biology, investigated this question in songbirds, which learn their songs during development in a manner similar to how humans learn to speak. In particular, songbirds memorize the song of their father or tutor, then practice that song until they can produce a similar song.

“As adults, they continue to produce this learned song, but what’s interesting is that they keep it just a little bit variable” says Woolley. “The variability isn’t a default, it isn’t that they can’t produce a better version, they can — in particular when they sing to a female. So when they sing alone and their song is variable it’s because they are actively making it that way.”  

The team used this change in the variability of the song to look at how the activity of single cells in different parts of the brain altered their activity depending on the social environment.

“We found that the social modulation of variability emerged within the basal ganglia, a brain area known to be important for learning and producing movements not only in birds but also in mammals, including humans” says Woolley. “This indicates that one way that the basal ganglia may be important in motor learning across species is through its involvement in generating variability.”

The researchers studied song birds because they have a cortical-basal ganglia circuit that is specific for singing. In contrast, for most behaviors in other species, the cortical-basal ganglia cells and circuits that are important for particular behaviors, like learning to walk, may be situated right next to, or even intermingled with cells and circuits important for other behaviors. “The evolution in songbirds of an identifiable circuit for a single complex behavior gives us a tremendous advantage as we try to parse out exactly what these parts of the brain do and how they do it,” says Woolley.  

Useful for Parkinson’s disease

The basal ganglia is dramatically affected in illnesses such as Parkinson’s and Huntington disease. The team’s findings may eventually be relevant to understanding changes to learning and flexibility in movement that occur in those diseases.  

“These are the kind of questions that we are now starting to pursue in the lab: how variability is affected when you radically manipulate the system akin to what happens during disease”, says Woolley.