The discerning fruit fly: Linking brain-cell activity and behavior in smell recognition

Behind the common expression “you can’t compare apples to oranges” lies a fundamental question of neuroscience: How does the brain recognize that apples and oranges are different? A group of neuroscientists at Cold Spring Harbor Laboratory (CSHL) has published new research that provides some answers.

In the fruit fly, the ability to distinguish smells lies in a region of the brain called the mushroom body (MB). Prior research has demonstrated that the MB is associated with learning and memory, especially in relation to the sense of smell, also known as olfaction.

CSHL Associate Professor Glenn Turner and colleagues have now mapped the activity of brain cells in the MB, in flies conditioned to have Pavlovian behavioral responses to different odors. Their results, outlined in a paper published today by the Journal of Neuroscience, suggest that the activity of a remarkably small number of neurons — as few as 25 — is required to be able to distinguish between different odors.

They also found that a similarly small number of nerve cells are involved in grouping alike odors. This means, for instance, that “if you’ve learned that oranges are good, the smell of a tangerine will also get you thinking about food,” says Robert Campbell, a postdoctoral researcher in the Turner lab and lead author on the new study.

These intriguing new findings are part of a broad effort in contemporary neuroscience to determine how the brain, easily the most complex organ in any animal, manages to make a mass of raw sensory data intelligible to the individual — whether a person or a fly — in order to serve as a basis for making vital decisions.

Looking closely at Kenyon cells

The neurons in the fly MB are known as Kenyon cells, named after their discoverer, the neuroscientist Frederick Kenyon, who was the first person to stain and visualize individual neurons in the insect brain. Kenyon cells receive sensory inputs from organs that perceive smell, taste, sight and sound. This confluence of sensory input in the MB is important for memory formation, which comes about through a linking of different types of information.

Kenyon cells make up only about 4% of the entire fly brain and are extremely sensitive to inputs triggered by odors, in which only two connections between neurons, called synapses, separate them from the receptor cells at the “front end” of the olfactory system.

But in contrast to other regions of the brain, such as the vertebrate hippocampus, the sensory responses in the MB are few in number and relatively weak. It is the sparseness of the signals in the Kenyon cell neurons that makes studying memory formation in flies so promising to Turner and his team. “We set out to learn if these signals were really informative to the animal’s learning and memory with regard to smell,” Turner says.

In particular, Turner’s group wanted to see if they could link these signals with actual behavior in flies. The team used an imaging technique that allowed them to view the responses of over 100 Kenyon cells at a time and, importantly, quantify their results. They found that even the very sparse responses in these cells that are triggered by odors provide a large amount of information about odor identity. Turner suspects the very selectiveness of the response helps in the accurate formation and recall of memories.

When the researchers used two odors blended together in a series of increasingly similar concentrations, they found that two very similar smells could be distinguished as a result of the activity of as few as 25 Kenyon cells. This correlated well with the behavior of the flies: when brain activity suggested the flies had difficulty discerning the odors, their behavior also showed they could not choose between them.

The activity of these cells also accounts for flies’ ability to discern novel odors and group them together. This was determined in a “generalization” test, in which the degree to which flies learned a generalized aversion to unfamiliar test odors could be predicted based upon the relatively similar activity patterns of Kenyon cells that the odors induced.

“Being able to do this type of ‘mind-reading’ means we really understand what signals these activity patterns are sending,” says Turner. Ultimately, he and colleagues hope to be able to relate their findings in the fly brain with the operation of the brain in mammals.

Memory-Boosting Chemical Is Identified in Mice

Memory improved in mice injected with a small, drug-like molecule discovered by UCSF San Francisco researchers studying how cells respond to biological stress.

The same biochemical pathway the molecule acts on might one day be targeted in humans to improve memory, according to the senior author of the study, Peter Walter, PhD, UCSF professor of biochemistry and biophysics and a Howard Hughes Investigator.

The discovery of the molecule and the results of the subsequent memory tests in mice were published in eLife, an online scientific open-access journal, on May 28, 2013.

In one memory test included in the study, normal mice were able to relocate a submerged platform about three times faster after receiving injections of the potent chemical than mice that received sham injections.

The mice that received the chemical also better remembered cues associated with unpleasant stimuli – the sort of fear conditioning that could help a mouse avoid being preyed upon.

Notably, the findings suggest that despite what would seem to be the importance of having the best biochemical mechanisms to maximize the power of memory, evolution does not seem to have provided them, Walter said.

“It appears that the process of evolution has not optimized memory consolidation; otherwise I don’t think we could have improved upon it the way we did in our study with normal, healthy mice,” Walter said.

The memory-boosting chemical was singled out from among 100,000 chemicals screened at the Small Molecule Discovery Center at UCSF for their potential to perturb a protective biochemical pathway within cells that is activated when cells are unable to keep up with the need to fold proteins into their working forms.

However, UCSF postdoctoral fellow Carmela Sidrauski, PhD, discovered that the chemical acts within the cell beyond the biochemical pathway that activates this unfolded protein response, to more broadly impact what’s known as the integrated stress response. In this response, several biochemical pathways converge on a single molecular lynchpin, a protein called eIF2 alpha.

Scientists have known that in organisms ranging in complexity from yeast to humans different kinds of cellular stress — a backlog of unfolded proteins, DNA-damaging UV light, a shortage of the amino acid building blocks needed to make protein, viral infection, iron deficiency — trigger different enzymes to act downstream to switch off eIF2 alpha.

“Among other things, the inactivation of eIF2 alpha is a brake on memory consolidation,” Walter said, perhaps an evolutionary consequence of a cell or organism becoming better able to adapt in other ways.

Turning off eIF2 alpha dials down production of most proteins, some of which may be needed for memory formation, Walter said. But eIF2 alpha inactivation also ramps up production of a few key proteins that help cells cope with stress.

Study co-author Nahum Sonenberg, PhD, of McGill University previously linked memory and eIF2 alpha in genetic studies of mice, and his lab group also conducted the memory tests for the current study.

The chemical identified by the UCSF researchers is called ISRIB, which stands for integrated stress response inhibitor. ISRIB counters the effects of eIF2 alpha inactivation inside cells, the researchers found.

“ISRIB shows good pharmacokinetic properties [how a drug is absorbed, distributed and eliminated], readily crosses the blood-brain barrier, and exhibits no overt toxicity in mice, which makes it very useful for studies in mice,” Walter said. These properties also indicate that ISRIB might serve as a good starting point for human drug development, according to Walter.

Walter said he is looking for scientists to collaborate with in new studies of cognition and memory in mouse models of neurodegenerative diseases and aging, using ISRIB or related molecules.

In addition, chemicals such as ISRIB could play a role in fighting cancers, which take advantage of stress responses to fuel their own growth, Walter said. Walter already is exploring ways to manipulate the unfolded protein response to inhibit tumor growth, based on his earlier discoveries.

At a more basic level, Walter said, he and other scientists can now use ISRIB to learn more about the role of the unfolded protein response and the integrated stress response in disease and normal physiology.

Video Gamers Really Do See More

Hours spent at the video gaming console not only train a player’s hands to work the buttons on the controller, they probably also train the brain to make better and faster use of visual input, according to Duke University researchers.

“Gamers see the world differently,” said Greg Appelbaum, an assistant professor of psychiatry in the Duke School of Medicine. “They are able to extract more information from a visual scene.”

It can be difficult to find non-gamers among college students these days, but from among a pool of subjects participating in a much larger study in Stephen Mitroff’s Visual Cognition Lab at Duke, the researchers found 125 participants who were either non-gamers or very intensive gamers. 

Each participant was run though a visual sensory memory task that flashed a circular arrangement of eight letters for just one-tenth of a second. After a delay ranging from 13 milliseconds to 2.5 seconds, an arrow appeared, pointing to one spot on the circle where a letter had been. Participants were asked to identify which letter had been in that spot.

At every time interval, intensive players of action video games outperformed non-gamers in recalling the letter.

Earlier research by others has found that gamers are quicker at responding to visual stimuli and can track more items than non-gamers. When playing a game, especially one of the “first-person shooters,” a gamer makes “probabilistic inferences” about what he’s seeing — good guy or bad guy, moving left or moving right — as rapidly as he can. 

Appelbaum said that with time and experience, the gamer apparently gets better at doing this. “They need less information to arrive at a probabilistic conclusion, and they do it faster.” 

Both groups experienced a rapid decay in memory of what the letters had been, but the gamers outperformed the non-gamers at every time interval. 

The visual system sifts information out from what the eyes are seeing, and data that isn’t used decays quite rapidly, Appelbaum said. Gamers discard the unused stuff just about as fast as everyone else, but they appear to be starting with more information to begin with.

The researchers examined three possible reasons for the gamers’ apparently superior ability to make probabilistic inferences. Either they see better, they retain visual memory longer or they’ve improved their decision-making. 

Looking at these results, Applebaum said, it appears that prolonged memory retention isn’t the reason. But the other two factors might both be in play — it is possible that the gamers see more immediately, and they are better able make better correct decisions from the information they have available.

To get at this question, the researchers will need more data from brainwaves and MRI imagery to see where the brains of gamers have been trained to perform differently on visual tasks.

Scientists Map Process by Which Brain Cells Form Long-Term Memories

Scientists at the Gladstone Institutes have deciphered how a protein called Arc regulates the activity of neurons – providing much-needed clues into the brain’s ability to form long-lasting memories.

These findings, reported Sunday in Nature Neuroscience, also offer newfound understanding as to what goes on at the molecular level when this process becomes disrupted.

Led by Gladstone senior investigator Steve Finkbeiner, MD, PhD, this research delved deep into the inner workings of synapses. Synapses are the highly specialized junctions that process and transmit information between neurons. Most of the synapses our brain will ever have are formed during early brain development, but throughout our lifetimes these synapses can be made, broken and strengthened. Synapses that are more active become stronger, a process that is essential for forming new memories.

However, this process is also dangerous, as it can overstimulate the neurons and lead to epileptic seizures. It must therefore be kept in check.

Neuroscientists recently discovered one important mechanism that the brain uses to maintain this important balance: a process called “homeostatic scaling.” Homeostatic scaling allows individual neurons to strengthen the new synaptic connections they’ve made to form memories, while at the same time protecting the neurons from becoming overly excited. Exactly how the neurons pull this off has eluded researchers, but they suspected that the Arc protein played a key role.

“Scientists knew that Arc was involved in long-term memory, because mice lacking the Arc protein could learn new tasks, but failed to remember them the next day,” said Finkbeiner, who is also a professor of neurology and physiology at UC San Francisco, with which Gladstone is affiliated. “Because initial observations showed Arc accumulating at the synapses during learning, researchers thought that Arc’s presence at these synapses was driving the formation of long-lasting memories.”

But Finkbeiner and his team thought there was something else in play.

The Role of Arc in Homeostatic Scaling

In laboratory experiments, first in animal models and then in greater detail in the petri dish, the researchers tracked Arc’s movements. And what they found was surprising.

“When individual neurons are stimulated during learning, Arc begins to accumulate at the synapses – but what we discovered was that soon after, the majority of Arc gets shuttled into the nucleus,” said Erica Korb, PhD, the paper’s lead author who completed her graduate work at Gladstone and UCSF.

“A closer look revealed three regions within the Arc protein itself that direct its movements: one exports Arc from the nucleus, a second transports it into the nucleus, and a third keeps it there,” she said. “The presence of this complex and tightly regulated system is strong evidence that this process is biologically important.”

In fact, the team’s experiments revealed that Arc acted as a master regulator of the entire homeostatic scaling process. During memory formation, certain genes must be switched on and off at very specific times in order to generate proteins that help neurons lay down new memories.  From inside the nucleus, the authors found that it was Arc that directed this process required for homeostatic scaling to occur. This strengthened the synaptic connections without overstimulating them – thus translating learning into long-term memories. 

Implications for a Variety of Neurological Diseases

“This discovery is important not only because it solves a long-standing mystery on the role of Arc in long-term memory formation, but also gives new insight into the homeostatic scaling process itself – disruptions in which have already been implicated in a whole host of neurological diseases,” said Finkbeiner. “For example, scientists recently discovered that Arc is depleted in the hippocampus, the brain’s memory center, in Alzheimer’s disease patients. It’s possible that disruptions to the homeostatic scaling process may contribute to the learning and memory deficits seen in Alzheimer’s.”

Dysfunctions in Arc production and transport may also be a vital player in autism. For example, the genetic disorder Fragile X syndrome – a common cause of both mental retardation and autism, directly affects the production of Arc in neurons.

“In the future,” added Dr. Korb, “we hope further research into Arc’s role in human health and disease can provide even deeper insight into these and other disorders, and also lay the groundwork for new therapeutic strategies to fight them.”

(Image: Wikimedia)

Weird: Nuclear Bomb Tests Reveal Adults Grow New Brain Cells

Aboveground nuclear bomb testing in the 1950s and 1960s inadvertently gave modern scientists a way to prove the adult brain regularly creates new neurons, research reveals.

Researchers used to believe that the brain changed little once it finished maturing. That view is now considered out of date, as studies have revealed how changeable — or plastic — the adult brain can be.

Much of this plasticity is related to the brain’s organization; brain cells can alter their connections and communications with other brain cells. What has been less clear is whether, and to what extent, the human brain grows brand-new neurons in adulthood.

“There was a lot in the literature showing there was neurogenesis in rodents and every animal studied,” said study researcher Kirsty Spalding, a biologist at the Karolinska Institute in Sweden, “But there was very little evidence of whether this happens in humans.”

Tantalizing clues

Scientists had reason to believe it does. In adult mice, the hippocampus, a structure deep in the brain involved in memory and navigation, turns over cells all the time. Some of the biological markers linked to this turnover are seen in the human hippocampus. But the only direct evidence of new brain cells forming in the region came from a 1998 study in which researchers looked at the brains of five people who had been injected with a compounded called BrdU that cells take up into their DNA. (The compound was once used in experimental cancer studies, but is not used anymore for safety reasons.)

The BrdU study revealed that neurons in the hippocampuses of the participants contained the compound in their DNA, indicating these brain cells had formed after the injections. The oldest person in the study was 72, suggesting new neuron creation, known as neurogenesis, continues well into old age.

The 1998 study was the only direct evidence of such neurogenesis in the human hippocampus, however. Spalding and her colleagues wanted to change that. Ten years ago, they began a project to track the age of neurons in the human brain using an unusual tool: spare molecules left over from Cold War-era nuclear bomb tests.

Learning to love the bomb

Between 1945 and 1962, the United States conducted hundreds of aboveground nuclear bomb tests. These tests largely stopped with the Limited Test Ban Treaty of 1963, but their effects remained in the atmosphere. The neutrons sent flying by the bombs reacted with nitrogen in the atmosphere, creating a spike in carbon 14, an isotope (or variation) of carbon.

This carbon 14, in turn, did what carbon in the atmosphere does. It combined with oxygen to form carbon dioxide, and was then taken in by plants, which use carbon dioxide in photosynthesis. Humans ate some of these plants, along with some of the animals that also ate these plants, and the carbon 14 inside ended up in their bodies.

When a cell divides, it uses this carbon 14, integrating it into the DNA of the new cells that are forming. Carbon 14 decays over time at a known rate, so scientists can pinpoint from that decay exactly when the new cells were born.

Over the past decade, Spalding and her colleagues have used the technique in a variety of cells, including fat cells, refining it along the way until it became sensitive enough to measure tiny amounts of carbon 14 in small hippocampus samples. The researchers collected samples, with family permission, from autopsies in Sweden.

They found the tantalizing 1998 evidence was correct: Human hippocampuses do grow new neurons. In fact, about a third of the brain region is subject to cell turnover, with about 700 new neurons being formed each day in each hippocampus (humans have two, a mirror-image set on either side of the brain). Hippocampus neurons die each day, too, keeping the overall number more or less in balance, with some slow loss of cells with aging, Spalding said.

This turnover occurs at a ridge in the hippocampus known as the dentate gyrus, a spot known to contribute to the formation of new memories. Researchers aren’t sure what the function of this constant renewal is, but it could relate to allowing the brain to cope with novel situations, Spalding told LiveScience.

“Neurogenesis gives a particular kind of plasticity to the brain, a cognitive flexibility,” she said.

Spalding and her colleagues had used the same techniques in other regions of the brain, including the cortex, the cerebellum and the olfactory bulb, and found no evidence of newborn neurons being integrated into those areas. The researchers now plan to study whether there are any links between neurogenesis and psychiatric conditions such as depression.

The new findings are detailed in the journal Cell.

New neuron formation could increase capacity for new learning, at the expense of old memories

Cause of infantile amnesia revealed

New research presented today shows that formation of new neurons in the hippocampus - a brain region known for its importance in learning and remembering - could cause forgetting of old memories by causing a reorganization of existing brain circuits. Drs. Paul Frankland and Sheena Josselyn, both from the Hospital for Sick Children in Toronto, argue this reorganization could have the positive effect of clearing old memories, reducing interference and thereby increasing capacity for new learning. These results were presented at the 2013 Canadian Neuroscience Meeting, the annual meeting of the Canadian Association for Neuroscience - Association Canadienne des Neurosciences (CAN-ACN).

Researchers have long known of the phenomenon of infantile amnesia: This refers to the absence of long-term memory of events occurring within the first 2-3 years of life, and little long-term memories for events occurring until about 7 years of age. Studies have shown that though young children can remember events in the short term, these memories do not persist. This new study by Frankland and Josselyn shows that this amnesia is associated with high levels of new neuron production - a process called neurogenesis - in the hippocampus, and that more permanent memory formation is associated with a reduction in neurogenesis.

Dr. Frankland and Dr. Josselyn’s approach was to look at retention of memories in young mice in which they suppressed the usual high levels of neurogenesis in the hippocampus (thereby replicating the circuit stability normally observed in adult mice), but also in older mice in which they stimulated increased neurogenesis (thereby replicating the conditions normally seen in younger mice). Dr. Frankland was able to show a causal relationship between a reduction in neurogenesis and increased remembering, and the converse, decreased remembering when neurogenesis increased.

Dr. Frankland concludes: ” Why infantile amnesia exists has long been a mystery. We think our new studies begin to explain why we have no memories from our earliest years.”