Posts tagged dopamine
Articles and news from the latest research reports.
Happiness lowers blood pressure
A synthetic gene module controlled by the happiness hormone dopamine produces an agent that lowers blood pressure. This opens up new avenues for therapies that are remote-controlled via the subsconscious.
The endogenous hormone dopamine triggers feelings of happiness. While its release is induced, among other things, by the “feel-good” classics sex, drugs or food, the brain does not content itself with a kick; it remembers the state of happiness and keeps wanting to achieve it again. Dopamine enables us to make the “right” decisions in order to experience even more moments of happiness.
Biological components reconnected
Now a team of researchers headed by ETH-Zurich professor Martin Fussenegger from the Department of Biosystems Science and Engineering (D-BSSE) in Basel has discovered a way to use the body’s dopamine system therapeutically. The researchers have created a new genetic module that can be controlled via dopamine. The feel-good messenger molecule activates the module depending on the dosage. In response to an increase in the dopamine level in the blood, the module produces the desired active agent.
The module consists of several biological components of the human organism, which are interconnected to form a synthetic signalling cascade. Dopamine receptors are found at the beginning of the cascade as sensors. A particular agent is produced as an end product: either a model protein called SEAP or ANP, a powerful vasodilator lowering blood pressure. The researchers placed these signal cascades in human cells, so-called HEK cells, around 100,000 of which were in turn inserted into capsules. These were then implanted in the abdomens of mice.
Contact with females activates module
These animals were subsequently exposed to situations that corresponded to their central reward system, such as sexual arousal, which a female mouse triggered in males, the injection of the drug methamphetamine or the drinking of golden syrup. In each case, the mouse brain responded with a “state of happiness”, the formation of dopamine and its release into the blood via the peripheral nervous system. In mice which received different concentrations of golden syrup, the “state of happiness” varied: the more the sugar was diluted, the smaller the amount of dopamine and thus the active agent that circulated in the blood. “This shows that dopamine does not merely switch our module on and off, but also that it responds based on the concentration of the happiness hormone,” says Fussenegger.
In another step, the scientists linked the dopamine sensor module to the production of the antihypertensive agent ANP and implanted the customised cells in the abdomens of hypertensive male mice. Contact with a female mouse triggered such feelings of happiness in the males that the dopamine-induced ANP production corrected the hypertension and the blood pressure even reached a normal level.
Serum dopamine linked to brain
Based on their experiments, the researchers were also able to demonstrate that dopamine is not only formed in the brain in corresponding feel-good situations, but also in nerves in the vegetative system, the so-called sympathetic nervous system, which are closely knit around blood vessels. The brain is interlinked with the rest of the body via the sympathetic nervous system, despite the fact that the brain is unable to release “its” dopamine directly into the circulation due to the blood-brain barrier. Dopamine receptors have also been known to exist in body tissue such as the kidneys, adrenalin glands or on blood vessels, as well as in the brain.
Dopamine, which circulates in the blood serum, regulates the breathing and the blood sugar balance. For a long time, it was thus assumed that the activities of brain and serum dopamine were connected. The fact that the ETH-Zurich researchers in Basel have now managed to demonstrate this connection deepens our understanding of the body’s reward system.
Eating as therapeutic input
Martin Fussenegger says that eating, for instance, can be seen as therapeutic input thanks to this module. “Using the gene network, we link up with the normal reward system,” he explains. Good food triggers feelings of happiness, which activate the module and intervene in a process that is normally only controlled by the subconscious. As a result, daily activities could be used for therapeutic interventions.
For the time being, however, the dopamine hypertension model is only a prototype. With their work, the scientists have proved that they can intervene in the body’s reward system as a result. Nonetheless, it is more than merely an idea or experiment in living cells. “It works in a mouse model that simulates a human disease and the components we used to produce the module also came from humans.” When and whether a treatment based on the happiness hormone will hit the market, however, remains uncertain. The development of prototypes into a marketable product takes years or even decades.
Further reading
Rössger K, Charpin-El-Hamri G & Fussenegger M. Reward-based hypertension control by a synthetic brain-dopamine interface. PNAS Early Edition, online 14th Oct. 2013.
New Strategy to Treat Multiple Sclerosis Shows Promise in Mice
Scientists at The Scripps Research Institute (TSRI) have identified a set of compounds that may be used to treat multiple sclerosis (MS) in a new way. Unlike existing MS therapies that suppress the immune system, the compounds boost a population of progenitor cells that can in turn repair MS-damaged nerve fibers.
One of the newly identified compounds, a Parkinson’s disease drug called benztropine, was highly effective in treating a standard model of MS in mice, both alone and in combination with existing MS therapies.
“We’re excited about these results, and are now considering how to design an initial clinical trial,” said Luke L. Lairson, an assistant professor of chemistry at TSRI and a senior author of the study, which is reported online in Nature on October 9, 2013.
Lairson cautioned that benztropine is a drug with dose-related adverse side effects, and has yet to be proven effective at a safe dose in human MS patients. “People shouldn’t start using it off-label for MS,” he said.
A New Approach
An autoimmune disease of the brain and spinal cord, MS currently affects more than half a million people in North America and Europe, and more than two million worldwide. Its precise triggers are unknown, but certain infections and a lack of vitamin D are thought to be risk factors. The disease is much more common among those of Northern European heritage, and occurs about twice as often in women as in men.
In MS, immune cells known as T cells infiltrate the upper spinal cord and brain, causing inflammation and ultimately the loss of an insulating coating called myelin on some nerve fibers. As nerve fibers lose this myelin coating, they lose their ability to transmit signals efficiently, and in time may begin to degenerate. The resulting symptoms, which commonly occur in a stop-start, “relapsing-remitting” pattern, may include limb weakness, numbness and tingling, fatigue, vision problems, slurred speech, memory difficulties and depression, among other problems.
Current therapies, such as interferon beta, aim to suppress the immune attack that de-myelinates nerve fibers. But they are only partially effective and are apt to have significant adverse side effects.
In the new study, Lairson and his colleagues decided to try a complementary approach, aimed at restoring a population of progenitor cells called oligodendrocytes. These cells normally keep the myelin sheaths of nerve fibers in good repair and in principle could fix these coatings after MS damages them. But oligodendrocyte numbers decline sharply in MS, due to a still-mysterious problem with the stem-like precursor cells that produce them. “Oligodendrocyte precursor cells (OPCs) are present during progressive phases of MS, but for unknown reasons don’t mature into functional oligodendrocytes,” Lairson said.
A 100,000-Molecule Screen
Using a sophisticated small-molecule screening laboratory that TSRI manages in conjunction with the California Institute of Regenerative Medicine and in collaboration with the California Institute for Biomedical Research (Calibr), Lairson and his team screened a library of about 100,000 diverse compounds for any that could potently induce OPCs to mature or “differentiate.”
Several compounds scored well as OPC differentiation-inducers. Most were compounds of unknown activity —but one, benztropine, had been well characterized and indeed was already FDA-approved for treating Parkinson’s disease. “That was a surprise, and it meant that we could move forward relatively quickly in testing it,” said graduate student Vishal A. Deshmukh, first author of the paper who performed most of these experiments.
With the help of Brian R. Lawson, a senior author of the paper and assistant professor of immunology at TSRI, and his colleague Research Associate Virginie Tardif, Deshmukh set up tests of benztropine in mice with an induced MS-like autoimmune disease—a model commonly used for testing prospective MS drugs.
In these tests, benztropine showed a powerful ability to prevent autoimmune disease and also was effective in treating it after symptoms had arisen—virtually eliminating the disease’s ability to relapse. Although benztropine on its own worked about as well as existing treatments, it also showed a remarkable ability to complement these existing treatments, in particular two first-line immune-suppressant therapies, interferon-beta and fingolimod.
“Adding even a suboptimal level of benztropine effectively allowed us, for example, to cut the dose of fingolimod by 90%—and achieve the same disease-modifying effect as a normal dose of fingolimod,” said Lawson. “In a clinical setting that dose-lowering could translate into a big reduction in fingolimod’s potentially serious side effects.”
In further analyses, the researchers confirmed that benztropine works against disease in this mouse model by boosting the population of mature oligodendrocytes, which in turn restore the myelin sheaths of damaged nerves—even as the immune attack continues. “The benztropine-treated mice showed no change in the usual signs of inflammation, yet their myelin was mostly intact, suggesting that it was probably being repaired as rapidly as it was being destroyed,” said Lawson.
Benztropine is known to have multiple specific effects on brain cells, including the blocking of activity at acetylcholine and histamine receptors and a boosting of activity at dopamine receptors. But Lairson and his colleagues found evidence that the drug stimulates OPCs to differentiate mainly by blocking M1 or M3 acetylcholine receptors on these cells.
In addition to setting up initial clinical trials, Lairson and his team hope to learn more about how benztropine induces OPC maturation, and how its molecular structure might be optimized for this purpose. “We’re also looking at some of the other, relatively unknown molecules that we identified in our initial screen, to see if any of those has better clinical potential than benztropine,” he said.
“This work, like our previous studies with hematopoietic and mesenchymal stem cells, illustrates the power of small molecules to control stem and precursor cells in ways that may ultimately lead to a new generation of drugs for regenerative medicine,” said Peter G. Schultz, the Scripps Family Chair Professor in the Department of Chemistry at TSRI and one of the study’s senior authors.
Seeking new methods to treat heroin addiction
“Heroin itself is an inactive substance,” explains Jørg Mørland, Norwegian forensic medicine and toxicology researcher. “The substances that heroin forms in the body are mainly what enter the brain and cause the narcotic effects.”
The heroin high and feelings of pain relief manifest themselves almost immediately after the drug has been injected. Yet it was shown many years ago that heroin is inactive at the opioid receptors in the brain.
So what is it about heroin that brings about such a pronounced effect? A number of research projects funded under the Programme on Alcohol and Drug Research (RUSMIDDEL) at the Research Council of Norway may help to solve the mystery.
“Gaining a thorough understanding of the effects of heroin and of the neurobiological mechanisms involved will be a valuable basis for the development of new treatments for addiction,” states Jørg Mørland, who is the project manager of an ongoing project on this important subject, the most recent in a long line of such Norwegian projects which he has headed.
Dr Mørland is a senior researcher at the Norwegian Institute of Public Health and Professor emeritus at the University of Oslo. Through studies on rats and mice, he and his colleagues have come up with new findings that may be significant to the development of new treatment methods.
Heroin metabolises rapidly
One widely-held theory has been that heroin passes quickly into the brain where it is converted into morphine, and that what users are actually experiencing are the effects of morphine. As it turns out, however, heroin undergoes a number of important transformations on its way to the brain. Just a few minutes after injection, the conversion of heroin into the metabolite 6-MAM is almost complete.
“Our research shows that it is primarily 6-MAM that crosses the blood-brain barrier and that heroin as such only enters the brain to a small degree. Thirty minutes after injecting heroin, 6-MAM is the predominant substance both in the blood and in the brain,” Dr Mørland explains.
The presence of 6-MAM also results in a sharp increase in the signalling molecule, dopamine, in certain areas of the brain. This plays a pivotal role in the rewarding effect.
“This points towards 6-MAM as the main substance behind all the acute effects of heroin,” says Dr Mørland.
“After about an hour, most of the 6-MAM has been converted into morphine. Morphine acts rapidly on the body and is the dominant component for the next hours, but from six to twelve hours after injection the effects observed are mostly consequences of a metabolite formed from morphine, morphin-6-glucuronide.
Looking for a new treatment
“We are working to understand the roles of all these metabolites and to investigate potential treatments to counter their effects,” Dr Mørland states.
The current approach to treating heroin addiction in Norway is pharmacotherapy – using methadone, subutex or subuxone. These are synthetic substances that all work in the same way as heroin, however, and are addictive in their own right.
“The treatment method involves administering these substances over the course of a day to reduce the rewarding effect. The intent is to diminish the patient’s preoccupation with finding heroin in order to lead a more normal life,” Dr Mørland points out.
Researchers at the Norwegian Centre for Addiction Research (SERAF) in Oslo are examining sustained-release naltrexone – a non-addictive opioid antagonist that blocks the effects of opiates in the brain. Dr Mørland is hopeful that his research will make it possible to affect opiates even before they reach the brain.
An opiate roadblock
“It may be possible to block these substances from ever entering the brain, thereby modifying the effect of heroin,” Dr Mørland adds.
As part of a new project, he and his colleagues will study the effect of a 6-MAM antibody developed by a Norwegian company. The antibody binds to the 6-MAM in the blood, making the 6-MAM molecule too large to cross the blood-brain barrier.
“If we succeed in getting this antibody to work it could block much – and maybe even all – of the effect of heroin,” the researcher concludes.
New Study Shows How ICU Ventilation May Trigger Mental Decline
Researchers from Penn Medicine and University of Oviedo Identify Molecular Pathway Linking ICU Ventilation to Brain Damage
At least 30 percent of patients in intensive care units (ICUs) suffer some form of mental dysfunction as reflected in anxiety, depression, and especially delirium. In mechanically-ventilated ICU patients, the incidence of delirium is particularly high, about 80 percent, and may be due in part to damage in the hippocampus, though how ventilation is increasing the risk of damage and mental impairment has remained elusive.
Now, a new study published in the American Journal of Respiratory and Critical Care Medicine fromresearchers at the University of Oviedo in Spain, St. Michael’s Hospital in Toronto, Canada, and the Perelman School of Medicine at the University Pennsylvaniafound a molecular mechanism that may explain the connection between mechanical ventilation and hippocampal damage in ICU patients.
The investigators, including Adrian González-López, PhD, in the laboratory of Guillermo M. Albaiceta, MD, PhD at the University of Oviedo , and co-authored by Konrad Talbot, PhD, an assistant research professor in Neurobiology in the Department of Psychiatry at Penn Medicine, began by studying the hippocampus in control mice and in mice on low or high-pressure mechanical ventilation for 90 minutes. Compared to the controls, those on either low- or high-pressure ventilation showed evidence of neuronal cell death in the hippocampus, as a result of a cell suicide program called apoptosis.
Searching for the molecular cause of the ventilation-induced apoptosis, the team discovered that a well-known apoptosis trigger had been set off in the hippocampus of the ventilated animals. That trigger is dopamine-induced suppression of a molecule known as Akt, which normally acts to prevent neuronal apoptosis. Akt suppression was clearly evident in the hippocampus of the ventilated mice and was associated with a hyperdopaminergic state (increased levels of dopamine) in that brain area. The ventilated mice had elevated gene expression of the enzyme tyrosine hydroxylase, which is critical in synthesizing dopamine. The resulting rise in dopamine increases the strength of dopamine receptor activation in the hippocampus.
The investigators hypothesized that ventilation-induced apoptosis in the hippocampus was at least partly mediated by elevated activation of dopamine receptors in that brain area. This was confirmed by showing that pretreatment of mice with type 2 (D2) dopamine receptor blockers injected into the ventricles of the brain significantly reduced ventilation-induced apoptosis in the hippocampus.
How mechanical ventilation manages to affect the hippocampus was answered by experiments on mice in which the vagus cranial nerve connecting the lungs with the brain was severed. In these mice, mechanical ventilation had virtually no effect on levels of the dopamine-synthesizing enzyme or on apoptosis in the hippocampus.
The investigators then studied the consequences of ventilation and elevated hippocampal dopamine on dysbindin-1, a protein known to affect levels of cell surface D2 dopamine receptors, cognition, and possibly the risk of psychosis. High-pressure ventilation in mice caused an increase in gene expression of dysbindin-1C, and later, in protein levels of dysbindin-1C. Dopamine alone had similar effects on dysbindin-1C in hippocampal slice preparations, effects that were inhibited by D2 receptor blockers.
Since dysbindin-1 can lower cell-surface D2 receptors and protect against apoptosis, the authors speculate that increased dysbindin-1C expression in the ventilated mice may reflect compensatory responses to ventilation-induced hippocampal apoptosis. That possibility applies to ICU cases given the additional finding by the authors that total dysbindin-1 was increased in hippocampal neurons of ventilated compared to non-ventilated humans who died in the ICU.
The findings could lead to new therapeutic uses of established drugs and targets for new drugs that activate a molecular pathway mediating adverse effects of ICU ventilation on brain function.
“The results prove the existence of a pathogenic mechanism of lung stretch-induced hippocampal apoptosis that could explain the development of neurobehavioral disorders in patients exposed to mechanical ventilation,” the authors write. One of the coauthors, Dr. Talbot, adds: “The study indicates the need to reevaluate use of D2 receptor antagonists in minimizing the negative cognitive effects of mechanical ventilation in ICU patients and to evaluate the novel possibility that elevation in dysbindin-1C expression can also reduce those effects.”
The corresponding author, Dr. Albaiceta, offered a look at future research on this topic: “Now that we have established the mouse model, we are mainly looking for therapeutic approaches aimed at avoiding the vagal activation caused by mechanical ventilation and therefore prevent the deleterious effects observed in the hippocampus,” he said. “We are also interested in studying the relationship between the different described gene polymorphisms of dysbindin, Akt, and type 2 dopamine receptor versus the incidence of neurological disorders in patients on ventilation in ICUs. This could help us to identify susceptible individuals to in which a preventive treatment could be effective.”
(Source: uphs.upenn.edu)
Why don’t apes have musical talent, while humans, parrots, small birds, elephants, whales, and bats do? Matz Larsson, senior physician at the Lung Clinic at Örebro University Hospital, attempts to answer this question in the scientific publication Animal Cognition.
In his article, he asserts that the ability to mimic and imitate things like music and speech is the result of the fact that synchronised group movement quite simply makes it possible to perceive sounds from the surroundings better.
The hypothesis is that the evolution of vocal learning, that is musical traits, is influenced by the need of a species to deal with the disturbing sounds that are created in connection with locomotion. These sounds can affect our hearing only when we move.
“When several people with legs of roughly the same length move together, we tend to unconsciously move in rhythm. When our footsteps occur simultaneously, a brief interval of silence occurs. In the middle of each stride we can hear our surroundings better. It becomes easier to hear a pursuer, and perhaps easier to conduct a conversation as well,” explains Larsson.
A behaviour that has survival value tends to produce dopamine, the “reward molecule”. In dangerous terrain, this could result in the stimulation of rhythmic movements and enhanced listening to surrounding sounds in nature. If that kind of synchronized behaviour was rewarding in dangerous environments it may as well have been rewarding for the brain in relative safety, resulting in activities such as hand- clapping, foot-stamping and yelping around the campfire. From there it is just a short step to dance and rhythm. The hormone dopamine flows when we listen to music.
Researchers Pinpoint Molecular Path that Makes Antidepressants Act Quicker in Mouse Model
Understanding alternate pathways for how mental meds work could lead to faster-acting drug targets
The reasons behind why it often takes people several weeks to feel the effect of newly prescribed antidepressants remains somewhat of a mystery – and likely, a frustration to both patients and physicians.
(Image: Mouse hippocampus expressing the Cre- virus. Credit: Julie Blendy, PhD; Brigitta Gunderson, PhD; Perelman School of Medicine, University of Pennsylvania)
Julie Blendy, PhD, professor of Pharmacology, at the Perelman School of Medicine, University of Pennsylvania; Brigitta Gunderson, PhD, a former postdoctoral fellow in the Blendy lab, and colleagues, have been working to find out why and if there is anything that can be done to shorten the time in which antidepressants kick in.
“Our goal is to find ways for antidepressants to work faster,” says Blendy.
The proteins CREB and CREM are both transcription factors, which bind to specific DNA sequences to control the “reading” of genetic information from DNA to messenger RNA (mRNA). Both CREB and CREM bind to the same 8-base-pair DNA sequence in the cell nucleus. But, the comparative influence of CREM versus CREB on the action of antidepressants is a “big unknown,” says Blendy.
CREB, and CREM to some degree, has been implicated in the pathophysiology of depression, as well as in the efficacy of antidepressants. However, whenever CREB is deleted, CREM is upregulated, further complicating the story.
Therefore, how an antidepressant works on the biochemistry and behavior in a mouse in which the CREB protein is deleted only in the hippocampus versus a wild type mouse in which CREM is overexpressed let the researchers tease out the relative influence of CREB and CREM on the pharmacology of an antidepressant. They saw the same results in each type of mouse line – increased nerve-cell generation in the hippocampus and a quicker response to the antidepressant. Their findings appear in the Journal of Neuroscience.
“This is the first demonstration of CREM within the brain playing a role in behavior, and specifically in behavioral outcomes, following antidepressant treatment,” says Blendy.
A Flood of Neurotransmitters
Antidepressants like SSRIs, NRIs, and older tricyclic drugs work by causing an immediate flood of neurotransmitters like serotonin, norepinephrine, and in some cases dopamine, into the synaptic space. However, it can take three to four weeks for patients to feel changes in mental state. Long-term behavioral effects of the drugs may take longer to manifest themselves, because of the need to activate CREB downstream targets such as BDNF and trkB, or as of yet unidentified targets, which could also be developed as new antidepressant drug targets.
The Penn team compared the behavior of the control, wild-type mice to the CREB mutant mice using a test in which the mouse is trained to eat a treat – Reese’s Pieces, to be exact – in the comfort of their home cage. The treat-loving mice are then placed in a new cage to make them anxious. They are given the treat again, and the time it takes for the mouse to approach the treat is recorded.
Animals that receive no drug treatment take a long time to venture out into the anxious environment to retrieve the treat, however, if given an antidepressant drug for at least three weeks, the time it takes a mouse to get the treat decreases significantly, from about 400 seconds to 100 seconds. In mice in which CREB is deleted or in mice in which CREM is upregulated, this reduction happens in one to two days versus the three weeks seen in wild-type mice.
The accelerated time to approach the treat in mice on the medication was accompanied by an increase in new nerve growth in the hippocampus.
“Our results suggest that activation of CREM may provide a means to accelerate the therapeutic efficacy of current antidepressant treatment,” says Blendy. Upregulation of CREM observed after CREB deletion, appears to functionally compensate for CREB loss at a behavioral level and leads to maintained or increased expression of some CREB target genes. The researchers’ next step is to identify any unique CREM target genes in brain areas such as the hippocampus, which may lead to the development of faster-acting antidepressants.
(Source: uphs.upenn.edu)
Cell transplants may be a novel treatment for schizophrenia
Rodent research suggests feasibility of restoring neuron function
Research from the School of Medicine at The University of Texas Health Science Center at San Antonio suggests the exciting possibility of using cell transplants to treat schizophrenia.
Cells called “interneurons” inhibit activity within brain regions, but this braking or governing function is impaired in schizophrenia. Consequently, a group of nerve cells called the dopamine system go into overdrive. Different branches of the dopamine system are involved in cognition, movement and emotions.
“Since these cells are not functioning properly, our idea is to replace them,” said study senior author Daniel Lodge, Ph.D., assistant professor of pharmacology in the School of Medicine.
Transplant restored normal function
Dr. Lodge and lead author Stephanie Perez, graduate student in his laboratory, biopsied tissue from rat fetuses, isolated cells from the tissue and injected the cells into a brain center called the hippocampus. This center regulates the dopamine system and plays a role in learning, memory and executive functions such as decision making. Rats treated with the transplanted cells have restored hippocampal and dopamine function.
Stem cells are able to become different types of cells, and in this case interneurons were selected. “We put in a lot of cells and not all survived, but a significant portion did and restored hippocampal and dopamine function back to normal,” Dr. Lodge said.
‘You can essentially fix the problem’
Unlike traditional approaches to treating schizophrenia, such as medications and deep-brain stimulation, transplantation of interneurons potentially can produce a permanent solution. “You can essentially fix the problem,” Dr. Lodge said. “Ultimately, if this is translated to humans, we want to reprogram a patient’s own cells and use them.”
After meeting with other students, Perez brought the research idea to Dr. Lodge. “The students have journal club, and somebody had done a similar experiment to restore motor deficits and had good results,” Perez said. “We thought, why can’t we use it for schizophrenia and have good results, and so far we have.”
The study is in Molecular Psychiatry.
(Source: uthscsa.edu)
Researchers develop new model to study schizophrenia and other neurological conditions
Schizophrenia is one of the most devastating neurological conditions, with only 30 percent of sufferers ever experiencing full recovery. While current medications can control most psychotic symptoms, their side effects can leave individuals so severely impaired that the disease ranks among the top ten causes of disability in developed countries.
Now, in this week’s issue of the Proceedings of the National Academy of Sciences, Thomas Albright and Ricardo Gil-da-Costa of the Salk Institute for Biological Studies describe a model system that completes the bridge between cellular and human studies of schizophrenia, an advance that should help speed the development of therapeutics for schizophrenia and other neurological disorders.
"Part of the terror of schizophrenia is that the brain can’t properly integrate sensory information, so the world is a disorientating series of unrelated bits of input," says Albright, the Conrad T. Prebys Chair in Vision Research. "We’ve created a model that tests the ability to do sensory integration, which should be extremely useful for pharmaceutical research."
Currently, over 1.1 percent of the world’s population has schizophrenia, with an estimated three million individuals in the United States alone. The economic cost is high: In 2002, Americans spent nearly $63 billion on treatment and managing disability. The emotional cost is higher still: Ten percent of those with schizophrenia are driven to commit suicide by the burden of coping with the disease.
Initially, it was thought that excessive amounts of the neurotransmitter dopamine caused psychotic symptoms, and indeed, current anti-psychotic drugs work by blocking dopamine from entering brain cells. But nearly all of these drugs have severe cognitive side effects, which led researchers to speculate that some other mechanism must also be involved.
A major clue to understanding schizophrenia came with the development of phencyclidine (PCP) in 1956. It was intended to keep patients safely asleep during surgeries, but many woke up with symptoms similar to those experienced by people with schizophrenia, including hallucinations and the disorientation of feeling “dissociated” from their limbs, resulting in PCP being abandoned for clinical purposes. A decade later, it was replaced by a derivative called ketamine. At doses high enough to put patients to sleep, ketamine is an effective anesthetic. At lower doses, it temporarily produces the same schizophrenia-like effects as PCP.
The two drugs are part of a class called N-methyl-D-aspartate receptor antagonists. Essentially, they work by gumming up the mechanism by which glutamate, the main excitatory neurotransmitter, would enter brain cells. Thus, it is clear that dopamine dysfunction accounts for some of the symptoms of psychosis, although that is probably not the full story.
"While dopamine has limited reach in the brain, any dysfunction in glutamate would be expected to have the sort of widespread effects we see in the perceptual disorders associated with schizophrenia," says Albright. "Nevertheless, which neurotransmitter was primary to these disorders—glutamate or dopamine—has been argued about for years."
Standing in the way of a definitive answer was a researcher’s Catch-22: Many experiments designed to understand cognitive disorders such as schizophrenia or Alzheimer’s require a participant’s conscious attention-yet these disorders interfere with attention.
To get around this, scientists turned to electroencephalograms (EEGs), which can be used to detect changes in cases where a subject is not consciously paying attention to a stimulus, by recording the brain’s electrical signals through electrodes placed in a scalp cap. In one test, a series of tones is played, but an “oddball” tone breaks the pattern in the sequence. A healthy brain can still easily spot the differences, even if a participant is concentrating on another task, such as reading a magazine.
"The test works because the brain is a prediction machine-it’s built to anticipate what should come next," says Albright. "If you have healthy working memory, you should be able to perceive a pattern and notice when something violates it, but patients suffering from some mental health disorders lack that basic ability."
In their latest research, Albright’s team detected the difference through two signals, event-related brain potentials called mismatch negativity (MMN) and P3. The MMN reflects differential brain activity to the detected oddball tone, below the level of conscious awareness. P3 picks up the next phase: a subject’s attention orientation to the oddball tone.
Still, a gap in understanding remained. While scientists could do cellular work in animal models on the role of dopamine versus glutamate, and they could do EEGs in human beings, a bridge between the two remained elusive. Such a bridge can help scientists understanding of how healthy and disordered brains work from the cellular level all the way to the multiple interactions between brain areas. Moreover, it can enable pre-clinical and clinical trials linking cellular and systems levels for successful therapeutic avenues.
Gil-da-Costa has at last crossed the bridge by crafting the first non-invasive scalp EEG setup that records accurately from the brains of non-human primates, with the same proportional density of electrodes as a human cap and no distortions in signal caused by an incorrect fit. This setup allows him to get accurate measurements of MMN and P3, with the same protocols that are followed in humans. As a result, the lab has come closer than ever before to untangling the roles of dopamine and glutamate.
"While rodents are essential for understanding mechanisms at a cellular or molecular level, at a higher cognitive level, the best you could do was a sort of rough analogy. Now, finally, we can have a one-to-one correspondence," says Gil-da-Costa. "For sensory integration, our findings with this model support the glutamate hypothesis."
Pharmaceutical companies are interested in the model, because of the potential for more precise testing and the universality of the MMN/P3 assays. “These brain makers are the same across dozens of neurological diseases, as well as brain trauma, so you can test potential therapies not just for schizophrenia, but for conditions such as Parkinson’s, Alzheimer’s, bi-polar disorder, and traumatic brain injuries,” says Gil-da-Costa. “We hope this will help begin a new era in neurological therapeutics.”
(Source: salk.edu)
How the Brain Remembers Pleasure and Its Implications for Addiction
Key details of the way nerve cells in the brain remember pleasure are revealed in a study by University of Alabama at Birmingham (UAB) researchers published today in the journal Nature Neuroscience. The molecular events that form such “reward memories” appear to differ from those created by drug addiction, despite the popular theory that addiction hijacks normal reward pathways.
Brain circuits have evolved to encourage behaviors proven to help our species survive by attaching pleasure to them. Eating rich food tastes good because it delivers energy and sex is desirable because it creates offspring. The same systems also connect in our mind’s environmental cues with actual pleasures to form reward memories.
This study in rats supports the idea that the mammalian brain features several memory types, each using different circuits, with memories accessed and integrated as needed. Ancient memory types include those that remind us what to fear, what to seek out (reward), how to move (motor memory) and navigate (place memory). More recent developments enable us to remember the year Columbus sailed and our wedding day.
“We believe reward memory may serve as a good model for understanding the molecular mechanisms behind many types of learning and memory,” said David Sweatt, Ph.D., chair of the UAB Department of Neurobiology, director of the Evelyn F. McKnight Brain Institute at UAB and corresponding author for the study. “Our results provide a leap in the field’s understanding of reward-learning mechanisms and promise to guide future attempts to solve related problems such as addiction and criminal behavior.”
The study is the first to illustrate that reward memories are created by chemical changes that influence known memory-related genes in nerve cells within a brain region called the ventral tegmental area, or VTA. Experiments that blocked those chemical changes — a mix of DNA methylation and demethylation — in the VTA prevented rats from forming new reward memories.
Methylation is the attachment of a methyl group (one carbon and three hydrogens) to a DNA chain at certain spots (cytosine bases). When methylation occurs near a gene or inside a gene sequence, it generally is thought to turn the gene off and its removal is thought to turn the gene on. This back-and-forth change affects gene expression without changing the code we inherit from our parents. Operating outside the genetic machinery proper, epigenetic changes enable each cell type to do its unique job and to react to its environment.
Furthermore, a stem cell in the womb that becomes bone or liver cells must “remember” its specialized nature and pass that identity to its descendants as they divide and multiply to form organs. This process requires genetic memory, which largely is driven by methylation. Note, most nerve cells do not divide and multiply as do other cells. They can’t, according to one theory, because they put their epigenetic mechanisms to work making actual memories.
Natural pleasure versus addiction
The brain’s pleasure center is known to proceed through nerve cells that signal using the neurochemical dopamine and generally is located in the VTA. Dopaminergic neurons exhibit a “remarkable capacity” to pass on pleasure signals. Unfortunately, the evolutionary processes that attached pleasure to advantageous behaviors also accidentally reinforced bad ones.
Addiction to all four major classes of abused drugs — psychostimulants, opiates, ethanol and nicotine — has been linked to increased dopamine transmission in the same parts of the brain associated with normal reward processing. Cues that predict both normal reward and effects of cocaine or alcohol also make dopamine nerve cells fire as do the experiences they recall. That had led to idea that drug addiction must take over normal reward-memory nerve pathways.
Along those lines, past research has argued that dopamine-producing neurons in the VTA — and in a region that receives downstream dopamine signals from the VTA called the nucleus accumbens (NAC) — both were involved in natural reward and drug-addiction-based memory formation. While that may true to some extent, this study revealed that blocking methylation in the VTA with a drug stopped the ability of rats to attach rewarding experiences to remembered cues but doing so in the NAC did not.
“We observed an important distinction, not in circuitry, but instead in the epigenetic regulation of that circuitry between natural reward responses and those that occur downstream with drugs of abuse or psychiatric illness,” said Jeremy Day, Ph.D., a post-doctoral scholar in Sweatt’s lab and first author for this study. “Although drug experiences may co-opt normal reward mechanisms to some extent, our results suggest they also may engage entirely separate epigenetic mechanisms that contribute only to addiction and that may explain its strength.”
To investigate the molecular and epigenetic changes in the VTA, researchers took their cue from 19th century Russian physiologist Ivan Pavlov, who was the first to study the phenomenon of conditioning. By ringing a bell each day before giving his dogs food, Pavlov soon found that the dogs would salivate at the sound of the bell.
In this study, rats were trained to associate a sound tone with the availability of sugar pellets in their feed ports. This same animal model has been used to make most discoveries about how human dopamine neurons work since the 1990s, and most approved drugs that affect the dopamine system (e.g. L-Dopa for Parkinson’s) were tested in it before being cleared for human trials.
To separate the effects of memory-related brain changes from those arising from the pleasure of the eating itself, the rats were separated into three groups. Rats in the “CS+” rats got sugar pellets each time they heard a sound cue. The “CS–” group heard the sound the same number of times and received as many sugar pellets — but never together. A third tone-only group heard the sounds but never received sugar rewards.
Rats that always received sugar with the sound cue were found to poke their feed ports with their noses at least twice as often during this cue as control rats after three, 25-sound-cue sessions. Nose pokes are an established measure of the degree to which a rat has come to associate a cue with the memory of a tasty treat.
The team found that those CS+ rats (sugar paired with sound) that were better at forming reward memories had significantly higher expression of the genes Egr1 and Fos than control rats These genes are known to regulate memory in other brain regions by fine-tuning the signaling capacity of the connections between nerve cells. In a series of experiments, the team next revealed the methylation and demethylation pattern that drove the changes in gene expression seen as memories formed.
The study demonstrated that reward-related experiences caused both types of DNA methylation known to regulate gene expression.
One type involves attaching methyl groups to pieces of DNA called promoters, which reside immediately upstream of individual gene sequences (between genes), that tell the machinery that follows genetic instructions to “start reading here.” The attachment of a methyl group to a promoter generally interferes with this and silences a nearby gene. However, ancient organisms such as plants and insects have less methylation between their genes, and more of it within the coding regions of the genes themselves (within gene bodies). Such gene-body methylation has been shown to encourage rather than silence gene expression.
Specifically, the team reported that two sites in the promoter for Egr1 gene were demethylated during reward experiences and, to a greater degree, in rats that associated the sugar with the sound cue. Conversely, spots within the gene body of both Egr1 and Fos underwent methylation as reward memories formed.
“When designing therapeutic treatments for psychiatric illness, addictions or memory disorders, you must profoundly understand the function of the biological systems you’re working with,” Day said. “Our field has learned from experience that attempts to treat addiction with something that globally impairs normal reward perception or reward memories do not succeed. Our study suggests the possibility that future treatments could dial down drug addiction or mental illness without affecting normal rewards.”
(Image: Corbis)
Why One Cream Cake Leads to Another
Continuously eating fatty foods perturbs communication between the gut and brain, which in turn perpetuates a bad diet.
A chronic high-fat diet is thought to desensitize the brain to the feeling of satisfaction that one normally gets from a meal, causing a person to overeat in order to achieve the same high again. New research published today (August 15) in Science, however, suggests that this desensitization actually begins in the gut itself, where production of a satiety factor, which normally tells the brain to stop eating, becomes dialed down by the repeated intake of high-fat food.
“It’s really fantastic work,” said Paul Kenny, a professor of molecular therapeutics at The Scripps Research Institute in Jupiter, Florida, who was not involved in the study. “It could be a so-called missing link between gut and brain signaling, which has been something of a mystery.”
While pork belly, ice cream, and other high-fat foods produce an endorphin response in the brain when they hit the taste buds, according to Kenny, the gut also sends signals directly to the brain to control our feeding behavior. Indeed, mice nourished via gastric feeding tubes, which bypass the mouth, exhibit a surge in dopamine—a neurotransmitter promoting reinforcement in the brain’s reward circuitry—similar to that experienced by those eating normally.
This dopamine surge occurs in response to feeding in both mice and humans. But evidence suggests that dopamine signaling in the brain is deficient in obese people. Ivan de Araujo, a professor of psychiatry at the Yale School of Medicine, has now discovered that obese mice on a chronic high-fat diet also have a muted dopamine response when receiving fatty food via a direct tube to their stomachs.
To determine the nature of the dopamine-regulating signal emanating from the gut, Araujo and his team searched for possible candidates. “When you look at animals chronically exposed to high-fat foods, you see high levels of almost every circulating factor—leptin, insulin, triglycerides, glucose, et cetera,” he said. But one class of signaling molecule is suppressed. Of these, Araujo’s primary candidate was oleoylethanolamide. Not only is the factor produced by intestinal cells in response to food, he said, but during chronic high-fat exposure, “the suppression levels seemed to somehow match the suppression that we saw in dopamine release.”
Araujo confirmed oleoylethanol’s dopamine-regulating ability in mice by administering the factor via a catheter to the tissues surrounding their guts. “We discovered that by restoring the baseline level of [oleoylethanolamide] in the gut … the high-fat fed animals started having dopamine responses that were indistinguishable from their lean counterparts.”
The team also found that oleoylethanolamide’s effect on dopamine was transmitted via the vagus nerve, which runs between the brain and abdomen, and was dependent on its interaction with a transcription factor called PPAR-a.
Oleoylethanolamide levels are also reduced in fasting animals and increase in response to eating, communicating with the brain to stop further consumption once the belly is full. Indeed, oleoylethanolamide is a known satiety factor. Therefore, when chronic consumption of high-fat food diminishes its production, the satisfaction signal is not achieved, and the brain is essentially “blind to the presence of calories in the gut,” said Araujo, and thus demands more food.
It is not clear why a chronic high-fat diet suppresses the production of oleoylethanolamide. But once the vicious cycle starts, it is hard to break because the brain is receiving its information subconsciously, said Daniele Piomelli, a professor at the University of California, Irvine, and director of drug discovery and development at the Italian Institute of Technology in Genoa.
“We eat what we like, and we think we are conscious of what we like, but I think what this [paper] and others are indicating is that there is a deeper, darker side to liking—a side that we’re not aware of,” Piomelli said. “Because it is an innate drive, you can not control it.” Put another way, even if you could trick your taste buds into enjoying low-fat yogurt, you’re unlikely to trick your gut.
The good news, however, is that “there is no permanent impairment in the [animals’] dopamine levels,” Araujo said. This suggests that if drugs could be designed to regulate the oleoylethanolamide–to-PPAR-a pathway in the gut, Kenny added, it could have “a huge impact on people’s ability to control their appetite.”
(Source: the-scientist.com)