Posts tagged neuroscience
Posts tagged neuroscience
Autism spectrum disorder (ASD) is diagnosed when individuals exhibit characteristic behaviors that include repetitive actions, decreased social interactions, and impaired communication. Curiously, many individuals with ASD also suffer from gastrointestinal (GI) issues, such as abdominal cramps and constipation.
Using the co-occurrence of brain and gut problems in ASD as their guide, researchers at the California Institute Technology (Caltech) are investigating a potentially transformative new therapy for autism and other neurodevelopmental disorders.
The gut microbiota—the community of bacteria that populate the human GI tract—previously has been shown to influence social and emotional behavior, but the Caltech research, published online in the December 5 issue of the journal Cell, is the first to demonstrate that changes in these gut bacteria can influence autism-like behaviors in a mouse model.
"Traditional research has studied autism as a genetic disorder and a disorder of the brain, but our work shows that gut bacteria may contribute to ASD-like symptoms in ways that were previously unappreciated," says Professor of Biology Sarkis K. Mazmanian. "Gut physiology appears to have effects on what are currently presumed to be brain functions."
To study this gut–microbiota–brain interaction, the researchers used a mouse model of autism previously developed at Caltech in the laboratory of Paul H. Patterson, the Anne P. and Benjamin F. Biaggini Professor of Biological Sciences. In humans, having a severe viral infection raises the risk that a pregnant woman will give birth to a child with autism. Patterson and his lab reproduced the effect in mice using a viral mimic that triggers an infection-like immune response in the mother and produces the core behavioral symptoms associated with autism in the offspring.
In the new Cell study, Mazmanian, Patterson, and their colleagues found that the “autistic” offspring of immune-activated pregnant mice also exhibited GI abnormalities. In particular, the GI tracts of autistic-like mice were “leaky,” which means that they allow material to pass through the intestinal wall and into the bloodstream. This characteristic, known as intestinal permeability, has been reported in some autistic individuals. “To our knowledge, this is the first report of an animal model for autism with comorbid GI dysfunction,” says Elaine Hsiao, a senior research fellow at Caltech and the first author on the study.
To see whether these GI symptoms actually influenced the autism-like behaviors, the researchers treated the mice with Bacteroides fragilis, a bacterium that has been used as an experimental probiotic therapy in animal models of GI disorders.
The result? The leaky gut was corrected.
In addition, observations of the treated mice showed that their behavior had changed. In particular, they were more likely to communicate with other mice, had reduced anxiety, and were less likely to engage in a repetitive digging behavior.
"The B. fragilis treatment alleviates GI problems in the mouse model and also improves some of the main behavioral symptoms," Hsiao says. "This suggests that GI problems could contribute to particular symptoms in neurodevelopmental disorders."
With the help of clinical collaborators, the researchers are now planning a trial to test the probiotic treatment on the behavioral symptoms of human autism. The trial should begin within the next year or two, says Patterson.
"This probiotic treatment is postnatal, which means that the mother has already experienced the immune challenge, and, as a result, the growing fetuses have already started down a different developmental path," Patterson says. "In this study, we can provide a treatment after the offspring have been born that can help improve certain behaviors. I think that’s a powerful part of the story."
The researchers stress that much work is still needed to develop an effective and reliable probiotic therapy for human autism—in part because there are both genetic and environmental contributions to the disorder, and because the immune-challenged mother in the mouse model reproduces only the environmental component.
"Autism is such a heterogeneous disorder that the ratio between genetic and environmental contributions could be different in each individual," Mazmanian says. "Even if B. fragilis ameliorates some of the symptoms associated with autism, I would be surprised if it’s a universal therapy—it probably won’t work for every single case."
The Caltech team proposes that particular beneficial bugs are intimately involved in regulating the release of metabolic products (or metabolites) from the gut into the bloodstream. Indeed, the researchers found that in the leaky intestinal wall of the autistic-like mice, certain metabolites that were modulated by microbes could both easily enter the circulation and affect particular behaviors.
"I think our results may someday transform the way people view possible causes and potential treatments for autism," Mazmanian says.
While the human brain is in a resting state, patterns of neuronal activity which are associated to specific memories may spontaneously reappear. Such recurrences contribute to memory consolidation – i.e. to the stabilization of memory contents. Scientists of the DZNE and the University of Bonn are reporting these findings in the current issue of The Journal of Neuroscience. The researchers headed by Nikolai Axmacher performed a memory test on a series of persons while monitoring their brain activity by functional magnetic resonance imaging (fMRI). The experimental setup comprised several resting states including a nap inside a neuroimaging scanner. The study indicates that resting periods can generally promote memory performance.
Depending on one’s mood and activity different regions are active in the human brain. Perceptions and thoughts also influence this condition and this results in a pattern of neuronal activity which is linked to the experienced situation. When it is recalled, similar patterns, which are slumbering in the brain, are reactivated. How this happens, is still largely unknown.
The prevalent theory of memory formation assumes that memories are stored in a gradual manner. At first, the brain stores new information only temporarily. For memories to remain in the long term, a further step is required. „We call it consolidation“, Dr. Nikolai Axmacher explains, who is a researcher at the Department of Epileptology of the University of Bonn and at the Bonn site of the DZNE. “We do not know exactly how this happens. However, studies suggest that a process we call reactivation is of importance. When this occurs, the brain replays activity patterns associated with a particular memory. In principle, this is a familiar concept. It is a fact that things that are actively repeated and practiced are better memorized. However, we assume that a reactivation of memory contents may also happen spontaneously without there being an external trigger.”
A memory test inside the scanner
Axmacher and his team tested this hypothesis in an experiment that involved ten healthy participants with an average age of 24 years. They were shown a series of pictures, which displayed – among other things – frogs, trees, airplanes and people. Each of these pictures was associated with a white square as a label at a different location. The subjects were asked to memorize the position of the square. At the end of the experiment all images were shown again, but this time without the label. The study participants were then asked to indicate with a mouse cursor where the missing mark was originally located. Memory performance was measured as the distance between the correct and the indicated position.
“This is an associative task. Visual and spatial perceptions have to be linked together”, the researcher explains. “Such tasks involve several brain regions. These include the visual cortex and the hippocampus, which takes part in many memory processes.”
Brain activity was recorded by fMRI during the entire experiment, which lasted several hours and included resting periods and a nap inside the neuroimaging scanner.
Recurrent brain patterns increased the accuracy
For data processing a pattern recognition algorithm was trained to look for similarities between neuronal patterns observed during initial encoding and patterns appearing at later occasions. “This method is complex, but quite effective”, Axmacher says. “Analysis showed that neuronal activity associated with images that were shown initially did reappear during subsequent resting periods and in the sleeping phase.”
Memory performance correlated with the replay of neuronal activity patterns. “The more frequently a pattern had reappeared, the more accurate test participants could label the corresponding image”, Axmacher summarizes the findings. “These results support our assumption that neural patterns can spontaneously reappear and that they promote the formation of long-lasting memory contents. There was already evidence for this from animal studies. Our experiment shows that this phenomenon also happens in humans.”
Memory performance benefits from resting periods
The study indicates that resting periods can generally foster memory performance. “Though, our data did not show whether sleeping had a particular effect. This may be due to the experimental setup, which only allowed for a comparatively short nap”, Axmacher reckons. “By contrast, night sleep is considered to be beneficial for the consolidation of memory contents. But it usually takes many hours and includes multiple transitions between different stages of sleep. However, other studies suggest that even short naps may positively affect memory consolidation.”
An objective look at memory contents
It is up to speculation whether the recurring brain patterns triggered conscious memories or whether they remained below the threshold of perception. “I think it is reasonable to assume that during resting periods the test participants let their mind wander and that they recalled images they had just seen before. But this is a matter of subjective perception of the test participants. That’s something we did not look at because it is not essential for our investigation“, Axmacher says. “The strength of our approach lies rather in the fact that we look at memory contents from the outside, in an objective manner. And that we can evaluate them by pattern recognition. This opens ways to many questions of research. For example, brain patterns that reoccur spontaneously are also of interest in the context of experimental dream research.”
As you step outdoors into the bright sunshine, your pupils automatically contract. Scientists from the Australian Centre of Excellence in Vision Science (ACEVS) based at The Australian National University (ANU) are making use of how this ‘pupil reflex’ is connected to the brain as a potential new way of testing the severity of multiple sclerosis (MS).
Dr Eman Ali and her ACEVS colleagues have used an instrument they are developing to accurately measure the pupil responses of MS patients and have found that the pupils of MS sufferers respond appreciably slower. The finding opens the door to a simple and quick way of tracking the severity of MS over time: the slower the response, the worse the MS.
“Our instrument uses special patterns of flashing lights that the patient looks at for four minutes,” says Professor Ted Maddess, a vision scientist at ANU who is head of the ACEVS team.
“We use infrared cameras to measure light-induced changes in the diameters of both pupils, and with computer tracking we can measure the diameter to within a micrometre 30 times a second.
“We have just published the results of our study of 85 MS patients, and we find that in MS patients the pupil response is about 25 milliseconds slower than in our control group. Although the study is preliminary, we believe the test has good potential in individual patients because it can precisely measure the speed of their response to within a millisecond.
“So instead of an expensive MRI to track the condition, the new method gives an accurate readout after just a few minutes. That quick and easy test might, in the future, allow MS patients to be assessed on the spot and have their medication adjusted accordingly,” he says.
MS is a potentially devastating neurological condition affecting the myelin sheath of nerve fibres, leading to sensory disturbances and muscle weakness. Vision, speech, and walking are most often affected, and pain can occur. Puzzlingly, MS affects different people in different ways, but the condition inexorably gets worse with age and there is currently no cure. Some patients experience acute, inflammatory attacks while others don’t.
“MS is the most common neurological disability in adults, with about 12,000 sufferers in Australia,” says Professor Maddess. “Although it seems to be some sort of immune disorder, its cause is still obscure.
“There are many puzzling aspects to MS, and there are many theories,” he says. “But our main aim in this work was just to find a way of accurately monitoring the progression of the disease, a single measure that relates to the degree of disability. MRI is good for giving insight into the inflammation associated with episodic attacks, but it’s not so good at monitoring the chronic decline that’s always going on.
“If we can use our pupil measurements to monitor the decline, we might be in a better position to adjust medications, which often have unpleasant side-effects.”
The instrument to measure the pupil responses is the same one which has also been shown to be helpful in diagnosing vision loss in glaucoma, diabetes, and age-related macular degeneration. The device was developed by Professor Maddess together with Associate Professor Andrew James and other ACEVS team members. Under the name TrueField, it is being commercially developed by an Australian company, Seeing Machines, which plans to sell it as a multipurpose medical diagnostic instrument.
TrueField has already received American FDA clearance, and Professor Maddess is hopeful it might, after some more research, also find a role in monitoring MS. He believes it has good prospects of reducing the high treatment costs associated with the disease.
The paper by Dr Ali and colleagues, “Pupillary response to sparse multifocal stimuli in multiple sclerosis patients”, is available online in the Multiple Sclerosis Journal.
New research from Karolinska Institutet shows that omega-3 fatty acids in dietary supplements can cross the blood brain barrier in people with Alzheimer’s disease, affecting known markers for both the disease itself and inflammation. The findings are presented in the Journal of Internal Medicine, and strengthen the evidence that omega-3 may benefit certain forms of this seriously debilitating disease.
"Earlier population studies indicate that omega-3 can protect against Alzheimer’s disease, which makes it interesting to study the effects of dietary supplements containing this group of fatty acids in patients who have already developed the disease," says the study’s lead author Dr Yvonne Freund-Levi.
Omega-3 and other essential polyunsaturated fatty acids accumulate in the central nervous system (CNS) during gestation. It has been assumed that these acids are continually replaced throughout life, but little is known about how this occurs and whether changes in diet can affect the transport of important fatty acids across the blood-brain barrier. The blood-brain barrier serves to protect the brain from harmful chemicals existing naturally in the blood, but also blocks the delivery of drug substances to the brain.
Several diseases can affect the fatty acid profile of the CNS; in patients with Alzheimer’s disease, for example, previous research has observed lower than normal brain concentrations of docosahexaenoic acid (DHA), an omega-3 fatty acid.
In the present study, part of the larger OmegAD project, scientists examined whether omega-3 dietary supplements change the fatty acid profile of the CNS in patients with mild Alzheimer’s disease. Thirty-three patients participated in the study, 18 of whom received a daily omega-3 supplement and 15 a placebo for six months. The results show that the first group had higher levels of both DHA and eicosapentaenoic acid (EPA, another omega-3 fatty acid) in their cerebrospinal fluid (which surrounds the CNS) and blood. No such change was seen in the placebo group.
Moreover, they also found that levels of DHA correlated directly with the degree of change in Alzheimer’s disease and inflammatory markers in the cerebrospinal fluid. Researchers in the field have long been interested in this link between Alzheimer’s disease and inflammation, but attempts to treat the disease using traditional anti-inflammatory drugs have failed to produce any improvements in memory function.
"In animals, DHA dietary supplements can lead to an increase in DHA concentrations in the CNS," says Professor Jan Palmblad, who initiated the study. "Here we show that the same applies to humans, which suggests that omega-3 fatty acids in dietary supplements cross the blood-brain barrier. However, much work remains to be done before we know how these fatty acids can be used in the treatment of Alzheimer’s disease to halt memory loss."
Even with today’s technology, it still takes both a male and a female to make a baby. But is it important for both parents to raise that child? Many studies have outlined the value of a mother, but few have clearly defined the importance of a father, until now. New findings from the Research Institute of the McGill University Health Centre (RI-MUHC) show that the absence of a father during critical growth periods, leads to impaired social and behavioural abilities in adults. This research, which was conducted using mice, was published today in the journal Cerebral Cortex. It is the first study to link father absenteeism with social attributes and to correlate these with physical changes in the brain.
“Although we used mice, the findings are extremely relevant to humans,” says senior author Dr. Gabriella Gobbi, a researcher of the Mental Illness and Addiction Axis at the RI-MUHC and an associate professor at the Faculty of Medicine at McGill University. “We used California mice which, like in some human populations, are monogamous and raise their offspring together.”
“Because we can control their environment, we can equalize factors that differ between them,” adds first author, Francis Bambico, a former student of Dr. Gobbi at McGill and now a post-doc at the Centre for Addiction and Mental Health (CAMH) in Toronto. “Mice studies in the laboratory may therefore be clearer to interpret than human ones, where it is impossible to control all the influences during development.”
Dr. Gobbi and her colleagues compared the social behaviour and brain anatomy of mice that had been raised with both parents to those that had been raised only by their mothers. Mice raised without a father had abnormal social interactions and were more aggressive than counterparts raised with both parents. These effects were stronger for female offspring than for their brothers. Females raised without fathers also had a greater sensitivity to the stimulant drug, amphetamine.
“The behavioural deficits we observed are consistent with human studies of children raised without a father,” says Dr. Gobbi, who is also a psychiatrist at the MUHC. “These children have been shown to have an increased risk for deviant behaviour and in particular, girls have been shown to be at risk for substance abuse. This suggests that these mice are a good model for understanding how these effects arise in humans.”
In pups deprived of fathers, Dr. Gobbi’s team also identified defects in the mouse prefrontal cortex, a part of the brain that helps control social and cognitive activity, which is linked to the behaviourial deficits.
“This is the first time research findings have shown that paternal deprivation during development affects the neurobiology of the offspring,” says Dr. Gobbi. These results should incite researchers to look more deeply into the role of fathers during critical stages of growth and suggest that both parents are important in children’s mental health development.
After capturing the first brain images of two alert, unrestrained dogs last year, researchers at Emory University have confirmed their methods and results by replicating them in an experiment involving 13 dogs.
The research, published by the Public Library of Science One (PLOS One), showed that most of the dogs had a positive response in the caudate region of the brain when given a hand signal indicating they would receive a food treat, as compared to a different hand signal for “no treat.”
“Our experiment last year was really a proof of concept, demonstrating that dogs could be trained to undergo successful functional Magnetic Resonance Imaging (fMRI),” says the lead researcher Gregory Berns, director of Emory’s Center for Neuropolicy. “Now we’ve shown that the initial study wasn’t a fluke: Canine fMRI is reliable and can be done with minimal stress to the dogs. We have laid the foundation for exploring the neural biology and cognitive processes of man’s best, and oldest, friend.”
Co-authors of the paper include Andrew Brooks, a post-doctoral fellow at the Center for Neuropolicy, and Mark Spivak, a dog trainer and the owner of Comprehensive Pet Therapy.
Both the initial experiment and the more recent one involved training the dogs to acclimatize to an fMRI machine. The task requires dogs to cooperatively enter the small enclosure of the fMRI scanner and remain completely motionless despite the noise and vibration of the machine.
Only those dogs that willingly cooperated were involved in the experiments. The canine subjects were given harmless fMRI brain scans while they watched a human giving hand signals that the dogs had been trained to understand. One signal indicated that the dog would receive a hot dog for a treat. The other hand signal meant that the dog would not receive a hot dog.
The most recent experiment involved the original two dogs, plus 11 additional ones, of varying breeds. Eight out of the 13 showed the positive caudate response for the hand signal indicating they were going to receive a hot dog.
The caudate sits above the brain stem in mammals and has the highest concentration of dopamine receptors, which are implicated in motivation and pleasure, among other neurological processes.
“We know that in humans, the caudate region is associated with decision-making, motivation and processing emotions,” Berns says.
As a point of reference, the researchers compared the results to a similar experiment Berns had led 10 years previously involving humans, in which the subjects pressed a button when a light appeared, to get a squirt of fruit juice.
Eleven of 17 humans involved in that experiment showed a positive response in the caudate region that was similar to the positive response of the dogs. “Our findings suggest that the caudate region of the canine brain behaves similarly to the caudate of the human brain, under similar circumstances,” Berns says.
Six of the dogs involved in the experiment had been specially bred and trained to assist disabled people as companion animals, and two of the dogs (including one of the service dogs) had worked as therapy dogs, used to help alleviate stress in people in hospitals or nursing homes. All of the service/therapy dogs showed a greater level of positive caudate activation for the hot dog signal, compared to the other dogs.
“We don’t know if the service dogs and therapy dogs showed this difference because of genetics, or because of the environment in which they were raised, but we hope to find out in future experiments,” Berns says. “This may be the first hint of how the brains of dogs with different temperaments and personalities differ.”
He adds: “I don’t think it was because they liked hot dogs more. I saw no evidence of that. None of the dogs turned down the hot dogs.”
One limitation of the experiments is the small number of subjects and the selectivity of the dogs involved, since only certain dogs can be trained to do the experiments, Berns says.
“We’re expanding our cohort to include more dogs and more breeds,” Berns says. “As the dogs get more accustomed to the process, we can conduct more complicated experiments.”
Plans call for comparing how the canine brain responds to hand signals coming from the dog’s owner, a stranger and a computer. Another experiment already under way is looking at the neural response of dogs when they are exposed to scents of members of their households, both humans and other dogs, and unfamiliar humans and dogs.
“Ultimately, our goal is to map out canine cognitive processes,” says Berns, who recently published a book entitled “How Dogs Love Us: A Neuroscientist and His Adopted Dog Decode the Canine Brain.”
Even in an increasingly technical era, the role of dogs has not diminished, Berns says. In addition to being popular pets, he notes that dogs are important in the U.S. military, in search-and-rescue missions, as assistants for the disabled and as therapeutic stress relievers for hospital patients and others.
“Dogs have been a part of human society for longer than any other animal,” Berns says. He cites a genetic analysis recently published in Science suggesting that the domestication of dogs goes back 18,000 to 32,000 years, preceding the development of agriculture some 10,000 years ago.
“Most neuroscience studies on animals are conducted to serve as models for human disease and brain functions,” Berns says. “We’re not studying canine cognition to serve as a model for humans, but what we learn about the dog brain may also help us understand more about how our own brains evolved.”
Turns out the crocodile can be a shrewd hunter himself. A University of Tennessee, Knoxville, researcher has found that some crocodiles use lures to hunt their prey.
Vladimir Dinets, a research assistant professor in the Department of Psychology, is the first to observe two crocodilian species—muggers and American alligators—using twigs and sticks to lure birds, particularly during nest-building time.
The research is published in the current edition of Ethology, Ecology and Evolution. Dinets’ research is the first report of tool use by any reptiles, and also the first known case of predators timing the use of lures to a seasonal behavior of the prey—nest-building.
Dinets first observed the behavior in 2007 when he spotted crocodiles lying in shallow water along the edge of a pond in India with small sticks or twigs positioned across their snouts. The behavior potentially fooled nest-building birds wading in the water for sticks into thinking the sticks were floating on the water. The crocodiles remained still for hours and if a bird neared the stick, they would lunge.
To see if the stick-displaying was a form of clever predation, Dinets and his colleagues performed systematic observations of the reptiles for one year at four sites in Louisiana, including two rookery and two nonrookery sites. A rookery is a bird breeding ground. The researchers observed a significant increase in alligators displaying sticks on their snouts from March to May, the time birds were building nests. Specifically, the reptiles in rookeries had sticks on their snouts during and after the nest-building season. At non-rookery sites, the reptiles used lures during the nest-building season.
"This study changes the way crocodiles have historically been viewed," said Dinets. "They are typically seen as lethargic, stupid and boring but now they are known to exhibit flexible multimodal signaling, advanced parental care and highly coordinated group hunting tactics."
The observations could mean the behavior is more widespread within the reptilian group and could also shed light on how crocodiles’ extinct relatives—dinosaurs—behaved.
"Our research provides a surprising insight into previously unrecognized complexity of extinct reptile behavior," said Dinets. "These discoveries are interesting not just because they show how easy it is to underestimate the intelligence of even relatively familiar animals, but also because crocodilians are a sister taxon of dinosaurs and flying reptiles."
Dinets collaborated with J.C and J.D. Brueggen from the St. Augustine Alligator Farm Zoological Park in St. Augustine, Fla. More of his crocodile research can be found in his book “Dragon Songs.”
A UW-Madison research team reports today that the brain can produce and release estrogen — a discovery that may lead to a better understanding of hormonal changes observed from before birth throughout the entire aging process.
The new research shows that the hypothalamus can directly control reproductive function in rhesus monkeys and very likely performs the same action in women.
Scientists have known for about 80 years that the hypothalamus, a region in the brain, is involved in regulating the menstrual cycle and reproduction. Within the past 40 years, they predicted the presence of neural estrogens, but they did not know whether the brain could actually make and release estrogen.
Most estrogens, such as estradiol, a primary hormone that controls the menstrual cycle, are produced in the ovaries. Estradiol circulates throughout the body, including the brain and pituitary gland, and influences reproduction, body weight, and learning and memory. As a result, many normal functions are compromised when the ovaries are removed or lose their function after menopause.
"Discovering that the hypothalamus can rapidly produce large amounts of estradiol and participate in control of gonadotropin-releasing hormone neurons surprised us," says Ei Terasawa, professor of pediatrics at the UW School of Medicine and Public Health and senior scientist at the Wisconsin National Primate Research Center. "These findings not only shift the concept of how reproductive function and behavior is regulated but have real implications for understanding and treating a number of diseases and disorders."
For diseases that may be linked to estrogen imbalances, such as Alzheimer’s disease, stroke, depression, experimental autoimmune encephalomyelitis and other autoimmune disorders, the hypothalamus may become a novel area for drug targeting, Terasawa says. “Results such as these can point us in new research directions and find new diagnostic tools and treatments for neuroendocrine diseases.”
The study, published today in the Journal of Neuroscience, “opens up entirely new avenues of research into human reproduction and development, as well as the role of estrogen action as our bodies age,” reports the first author of the paper, Brian Kenealy, who earned his Ph.D. this summer in the Endocrinology and Reproductive Physiology Program at UW-Madison. Kenealy performed three studies. In the first experiment, a brief infusion of estradiol benzoate administered into the hypothalamus of rhesus monkeys that had surgery to remove their ovaries rapidly stimulated GnRH release. The brain took over and began rapidly releasing this estrogen in large pulsing surges.
In the second experiment, mild electrical stimulation of the hypothalamus caused the release of both estrogen and GnRH (thus mimicking how estrogen could induce a neurotransmitter-like action). Third, the research team infused letrazole, an aromatase inhibitor that blocks the synthesis of estrogen, resulting in a lack of estrogen as well as GnRH release from the brain. Together, these methods demonstrated how local synthesis of estrogen in the brain is important in regulating reproductive function.
The reproductive, neurological and immune systems of rhesus macaques have proven to be excellent biomedical models for humans over several decades, says Terasawa, who focuses on the neural and endocrine mechanisms that control the initiation of puberty. “This work is further proof that these animals can teach us about so many basic functions we don’t fully understand in humans.”
Leading up to this discovery, Terasawa said, recent evidence had shown that estrogen acting as a neurotransmitter in the brain rapidly induced sexual behavior in quails and rats. Kenealy’s work is the first evidence of this local hypothalamic action in primates, and in those that don’t even have ovaries.
"The discovery that the primate brain can make estrogen is key to a better understanding of hormonal changes observed during every phase of development, from prenatal to puberty, and throughout adulthood, including aging," Kenealy says.
University of Utah and German biologists discovered how nerve cells recycle tiny bubbles or “vesicles” that send chemical nerve signals from one cell to the next. The process is much faster and different than two previously proposed mechanisms for recycling the bubbles.
Researchers photographed mouse brain cells using an electron microscope after flash-freezing the cells in the act of firing nerve signals. That showed the tiny vesicles are recycled to form new bubbles only one-tenth of a second after they dump their cargo of neurotransmitters into the gap or “synapse” between two nerve cells or neurons.
“Without recycling these containers or ‘synaptic vesicles’ filled with neurotransmitters, you could move once and stop, think one thought and stop, take one step and stop, and speak one word and stop,” says University of Utah biologist Erik Jorgensen, senior author of the study in the Dec. 4 issue of the journal Nature.
“A fast nervous system allows you to think and move. Recycling synaptic vesicles allows your brain and muscles to keep working longer than a couple of seconds,” says Jorgensen, a distinguished professor of biology. “This process also may protect neurons from neurodegenerative diseases like Lou Gehrig’s disease and Alzheimer’s. So understanding the process may give us insights into treatments someday.”
A brain cell maintains a supply of 300 to 400 vesicles to send chemical nerve signals, using up to several hundred per second to release neurotransmitters, says the study’s first author, postdoctoral fellow Shigeki Watanabe.
Recycling vesicles is called “endocytosis.” Jorgensen and Watanabe named the process they observed “ultrafast endocytosis.” They showed it takes one-tenth of a second for a vesicle to be recycled, and such recycling occurs on the edge of “active zone” – the place on the end of the nerve cell where the vesicles first unload neurotransmitters into the synapse between brain cells.
“It’s like Whac-A-Mole: one vesicle goes down (fuses and unloads) and another pops up someplace else,” Jorgensen says.
Jorgensen believes ultrafast endocytosis is the most common way of recycling vesicles, but says the study doesn’t disprove two other, long-debated hypotheses:
– “Kiss-and-run endocytosis,” which supposedly takes one second, with a vesicle just “kissing” the inside of its nerve cell, dumping its neurotransmitters outside and “running” by detaching to reform a recycled vesicle in the same part of the active zone.
– Clathrin-mediated endocytosis,” which purportedly takes 20 seconds and occurs away from the active zone, at a point where a protein named clathrin assembles itself into a soccer-ball-shaped scaffold that forms a new vesicle or bubble.
Earlier this year, Jorgensen, Watanabe and colleagues published a related study in the journal eLife revealing that ultrafast endocytosis occurs in nematode worms. The new study of hippocampal brain cells from mice “tells us that mammals – and thus humans – do it the same way,” Jorgensen says. “The two papers together identify a process never previously seen – much faster than has been measured before.”
Jorgensen and Watanabe conducted the study with M. Wayne Davis, a University of Utah research assistant professor of biology; and technician Berit Söhl-Kielczynski and neuroscientists Christian Rosenmund, Benjamin Rost and Marcial Camacho-Pérez, all of Germany’s Charity University Medicine Berlin.
The study was funded by the National Institutes of Health, the European Research Council and the German Research Council. Jorgensen also is funded by his status as a Howard Hughes Medical Institute investigator and an Alexander von Humboldt Scholar.
Machine Gun Analogy for Vesicle Recycling
The process of a vesicle fusing to the nerve cell’s wall from the inside, then releasing neurotransmitters into the synapse is known as “exocytosis.” An analogy might be a bubble rising from boiling soup and releasing steam. The liquid part of the bubble fuses with the liquid in the soup, sooner or later to arise in another bubble.
The 2013 Nobel Prize in Physiology or Medicine went to three scientists who discovered key aspects of vesicle transport of cargo and exocytosis in nerve and other cells: which genes are required for vesicle transport, how vesicles deliver cargo to the correct locations, and how vesicles in brain cells release neurotransmitters to send a signal to the next brain neuron.
Jorgensen, Shigeki and colleagues studied the next step, endocytosis: how the membrane that forms vesicles (and nerve cell walls) is recycled to form new vesicles.
To illustrate the three possible mechanisms for recycling vesicles, Jorgensen compares vesicles with machine gun shells.
“You are fusing vesicles to the nerve cell membrane and expelling the neurotransmitter contents at extremely high rates,” he says. “The synapse will use up its ‘ammo’ very quickly at these rates, so the cell needs to refill the empty shells.”
Clathrin-mediated vesicle recycling is like “remaking the shell from scratch,” he says, while kiss-and-run endocytosis is like picking up every empty shell casing and refilling them one at a time.
“Ultrafast endocytosis allows the synapse to whip up all of the empty shells by the handful, fill them, and put them back in line at incredibly fast rates so the machine gun never runs out of ammo,” Jorgensen says.
Flash and Freeze for Nerve Cells in Action
Shigeki, Jorgensen and colleagues developed a method to photograph the tiny vesicles inside a nerve cell as the bubbles moved to the end of the cell, fused with the cell membrane, dumped their load of neurotransmitters into the gap or “synapse” between nerve cells, and then were recycled to reappear as new bubbles inside the nerve cell.
“We found a way to look at this process on a timescale that no one ever looked at before,” Watanabe says.
First, the researchers grew hundreds of brain cells from the mouse hippocampus – the often-studied part of the brain required for memory formation – on quarter-inch-wide sapphire disks placed in petri dishes with growth medium.
They added an algae gene to mouse brain cells that made the neurons produce an “ion channel” – basically a switch – that is stimulated by light instead of electricity. Then the brain cells were placed in a super-cold, high-pressure chamber, at 310 degrees below zero Fahrenheit and pressure 2,000 times greater than Earth’s atmosphere at sea level.
A wire cannot be routed into the chamber, which is why the cells were genetically programmed to be stimulated by light. The researchers flashed blue light on the mouse brain cells, making them “fire” neurotransmitter nerve signals. At the same time, the firing neurons were frozen with a blast of liquid nitrogen. To catch neurons in all stages of firing, the nerve cells were frozen at various times after the flash of blue light: 15, 30 and 100 milliseconds and one, three and 10 seconds.
“We built a new device to capture neurons performing fast behaviors,” Jorgensen says. “It stops all motion in the cell – even membranes in the act of fusing.
“We call it flash and freeze,” Watanabe says.
Next, the sapphire disks with neurons were put into liquid epoxy, which hardened and then were thin-sliced so the neurons could be photographed under an electron microscope. The ultrafast formation of recycled vesicles was visible.
“You see the outline of the membrane,” Jorgensen says. “You see the bubbles or vesicles in different stages of formation.”
Watanabe says about 3,000 mouse brain cell synapses were flashed, frozen and analyzed during the study. About 20 percent of the nerve cells had been fired and showed signs that nerve vesicles were being recycled.
In a new paper published in the current issue of Neuron, McLean Hospital and Harvard Medical School researchers report that increased activity in the medial prefrontal cortex (mPFC) of the brain is linked to decreased activity in the amygdala, the portion of the brain used in the creation of memories of events that scared those exposed.
According to author Vadim Bolshakov, PhD, director of the Cellular Neurobiology Laboratory at McLean and professor at Harvard Medical School, this finding is significant in that it could lead to better methods to prevent PTSD.
"A single exposure to something traumatic or scary can be enough to create a fear memory—causing someone to expect and be afraid in similar situations in the future," said Bolshakov. "What we’re seeing is that we may one day be able to prevent those fear memories."
Bolshakov and his colleagues tested their theory using animal models. Dividing the mice into two groups, some were taught to fear an auditory stimulus while in others fear memory was extinguished Increased activation of mPFC in extinguished animals led to inhibition of the amygdala and significant decreases in fear responses.
"For example, if a sound ended with an extremely loud shriek, a subject would come to expect that scary noise at the end of the sound," explained Bolshakov. "What we found was when we suppressed the fear memory by decreasing activity in the amygdala, the subjects were not afraid of the end of the auditory stimulus any longer."
Bolshakov notes that this work could have serious implications for the treatment of a number of conditions including PTSD.
"While there is still a great deal of research that needs to be done before our work can be translated to clinical trials, what we are showing has the potential to ensure that individuals exposed to trauma were not haunted by the conditions surrounding their initial stressor."