Making memories

Modifying the activity of neuronal networks that encode spatial memories leads to the formation of an incorrect fear memory in mice

The formation and retrieval of memories allows all kinds of organisms, including humans, to learn and thrive in their environment. Yet our memories are not always accurate, and mistaken remembrances can have important consequences, such as in the justice system and in our navigation of the world. Susumu Tonegawa, Steve Ramirez, Xu Liu and colleagues at the RIKEN-MIT Center for Neural Circuit Genetics, have gained insight into the creation of mistaken memories by using light activation of neurons to generate an incorrect fear memory in mice.

The researchers allowed mice to explore a novel location and used genetic techniques to label neurons in the hippocampus—a part of the brain linked to spatial memory—that were activated in the process with a special channel called channelrhodopsin-2. The cells that expressed this channel could then be artificially activated by light. In this way, the researchers were able to reactivate neurons that fired in that particular location, even if the mice were no longer there.

They then moved the mice to another location where they were exposed to foot shocks, causing the mice to exhibit immobility, a fear behavior. At the same time, the researchers used light to activate the channelrhodopsin-2-expressing neurons that had fired in the first location.

When Tonegawa and his colleagues moved the animals to a third location, they did not show fear behavior. Yet when the mice went back to the first location, where they had never experienced a foot shock, the mice now exhibited prominent freezing behavior. The researchers had generated a ‘false memory’ in the mice of foot shocks in a location in which they had never been exposed to them.

The researchers showed that light reactivation of neuronal networks in the central area of the hippocampus, called the dentate gyrus, could create false memories, while reactivation of the outer ‘CA1’ area of the hippocampus could not. Tonegawa and his colleagues suggest that this is because mouse exploration of different locations leads to activation of more overlapping neuronal networks in the CA1 than in the dentate gyrus. “This may reflect the fundamental differences of how memories are encoded in these two regions,” explains Liu.

The findings provide insight into how the brain encodes and processes memories and could one day lead to treatments for post-traumatic stress disorder. “Our work may also have implications for situations where patients mix reality with their own imaginations, such as in schizophrenia,” says Liu.

Study Expands Concerns About Anesthesia’s Impact on the Brain

As pediatric specialists become increasingly aware that surgical anesthesia may have lasting effects on the developing brains of young children, new research suggests the threat may also apply to adult brains.

Researchers from Cincinnati Children’s Hospital Medical Center report June 5 the Annals of Neurology that testing in laboratory mice shows anesthesia’s neurotoxic effects depend on the age of brain neurons – not the age of the animal undergoing anesthesia, as once thought.

Although more research is needed to confirm the study’s relevance to humans, the study suggests possible health implications for millions of children and adults who undergo surgical anesthesia annually, according to Andreas Loepke, MD, PhD, a physician and researcher in the Department of Anesthesiology.

“We demonstrate that anesthesia-induced cell death in neurons is not limited to the immature brain, as previously believed,” said Loepke. “Instead, vulnerability seems to target neurons of a certain age and maturational stage. This finding brings us a step closer to understanding the phenomenon’s underlying mechanism”.

New neurons are generated abundantly in most regions of the very young brain, explaining why previous research has focused on that developmental stage. In a mature brain, neuron formation slows considerably, but extends into later life in dentate gyrus and olfactory bulb.

The dentate gyrus, which helps control learning and memory, is the region Loepke and his research colleagues paid particular attention to in their study. Also collaborating were researchers from the University of Cincinnati College of Medicine and the Children’s Hospital of Fudan University, Shanghai, China.

Researchers exposed newborn, juvenile and young adult mice to a widely used anesthetic called isoflurane in doses approximating those used in surgical practice. Newborn mice exhibited widespread neuronal loss in forebrain structures – confirming previous research – with no significant impact on the dentate gyrus. However, the effect in juvenile mice was reversed, with minimal neuronal impact in the forebrain regions and significant cell death in the dentate gyrus.

The team then performed extensive studies to discover that age and maturational stage of the affected neurons were the defining characteristics for vulnerability to anesthesia-induced neuronal cell death. The researchers observed similar results in young adult mice as well.

Research over the past 10 years has made it increasingly clear that commonly used anesthetics increase brain cell death in developing animals, raising concerns from the Food and Drug Administration, clinicians, neuroscientists and the public. As well, several follow-up studies in children and adults who have undergone surgical anesthesia show a link to learning and memory impairment.

Cautioning against immediate application of the current study’s findings to children and adults undergoing anesthesia, Loepke said his research team is trying to learn enough about anesthesia’s impact on brain chemistry to develop protective therapeutic strategies, in case they are needed. To this end, their next step is to identify specific molecular processes triggered by anesthesia that lead to brain cell death.

“Surgery is often vital to save lives or maintain quality of life and usually cannot be performed without general anesthesia,” Loepke said. “Physicians should carefully discuss with patients, parents and caretakers the risks and benefits of procedures requiring anesthetics, as well as the known risks of not treating certain conditions.”

Loepke is also collaborating with researchers from the Pediatric Neuroimaging Research Consortium at Cincinnati Children’s Hospital Medical Center to examine anesthesia’s impact on children’s brain using non-invasive magnetic resonance imaging (MRI) technology.

The neuroscience of finding your lost keys

Ever find yourself racking your brain on a Monday morning to remember where you put your car keys?

When you do find those keys, you can thank the hippocampus, a brain region responsible for storing and retrieving memories of different environments-such as that room where your keys were hiding in an unusual spot.

Now, scientists have helped explain how the brain keeps track of the incredibly rich and complex environments people navigate on a daily basis. They discovered how the dentate gyrus, a subregion of the hippocampus, helps keep memories of similar events and environments separate, a finding they reported March 20 in eLife. The findings, which clarify how the brain stores and distinguishes between memories, may also help identify how neurodegenerative diseases, such as Alzheimer’s disease, rob people of these abilities.

"Everyday, we have to remember subtle differences between how things are today, versus how they were yesterday - from where we parked our car to where we left our cellphone," says Fred H. Gage, senior author on the paper and the Vi and John Adler Chair for Research on Age-Related Neurodegenerative Disease at Salk. "We found how the brain makes these distinctions, by storing separate ‘recordings’ of each environment in the dentate gyrus."

The process of taking complex memories and converting them into representations that are less easily confused is known as pattern separation. Computational models of brain function suggest that the dentate gyrus helps us perform pattern separation of memories by activating different groups of neurons when an animal is in different environments.

However, previous laboratory studies found that in fact the same populations of neurons in the dentate gyrus are active in different environments, and that the way the cells distinguished new surroundings was by changing the rate at which they sent electrical impulses. This discrepancy between theoretical predictions and laboratory findings has perplexed neuroscientists and obscured our understanding of memory formation and retrieval.

To explore this mystery more deeply, the Salk scientists compared the functioning of the mouse dentate gyrus and another region of the hippocampus, known as CA1, using laboratory techniques for tracking the activity of neurons at multiple time points.

First, the researchers took mice from their original chamber and placed them in a novel chamber to learn about a new environment (episode 1). Meanwhile, they recorded which hippocampal neurons were active as the animals responded to their new surroundings. Subsequently, the mice were either returned to that same novel chamber to measure memory recall or to a slightly modified chamber to measure discrimination (episode 2). The active neurons in episode 2 were also labeled in order to determine if the neurons activated in episode 1 were used in the same way for recall and for discrimination of small differences between environments.

When the researchers compared the neural activity during the two episodes, they found that the dentate gyrus and CA1 sub-regions functioned differently. In CA1, the same neurons that were active during the initial learning episode were also active when the mice retrieved the memories. In the dentate gyrus, however, distinct groups of cells were active during the learning episodes and retrieval. Also, exposing the mice to two subtly different environments activated two distinct groups of cells in the dentate gyrus.

"This finding supported the predictions of theoretical models that different groups of cells are activated during exposure to similar, but distinct, environments," says Wei Deng, a Salk postdoctoral research and first author on the paper. "This contrasts with the findings of previous laboratory studies, possibly because they looked at different sub-populations of neurons in the dentate gyrus."

The Salk researchers’ findings suggest that recalling a memory-such as the location of missing keys-does not always involve reactivation of the same neurons that were active during encoding. More importantly, the results indicate that the dentate gyrus performs pattern separation by using distinct populations of cells to represent similar but non-identical memories.

The findings help clarify the mechanisms that underpin memory formation and shed light on systems that are disrupted by injuries and diseases of the nervous system.

Portion of Hippocampus Found to Play Role in Modulating Anxiety

Columbia University Medical Center (CUMC) researchers have found the first evidence that selective activation of the dentate gyrus, a portion of the hippocampus, can reduce anxiety without affecting learning. The findings suggest that therapies that target this brain region could be used to treat certain anxiety disorders, such as panic disorder and post-traumatic stress syndrome (PTSD), with minimal cognitive side effects. The study, conducted in mice, was published in the online edition of the journal Neuron.

The dentate gyrus is known to play a key role in learning. Some evidence suggests that the structure also contributes to anxiety. “But until now no one has been able to figure out how the hippocampus could be involved in both processes,” said senior author Rene Hen, PhD, professor of neuroscience and pharmacology (in psychiatry) at CUMC.

“It turns out that different parts of the dentate gyrus have somewhat different functions, with the dorsal portion largely dedicated to learning and the ventral portion dedicated to anxiety,” said lead author Mazen A. Kheirbek, PhD, a postdoctoral fellow in neuroscience at CUMC.

To examine the role of the dentate gyrus in learning and anxiety, the investigators used a state-of-the-art technique called optogenetics, in which light-sensitive proteins, or opsins, are genetically inserted into neurons in the brains of mice. Neurons with these genes can then be selectively activated or silenced through the application of light (via a fiber-optic strand), allowing researchers to study the function of the cells in real time. Previously, the only way to study the dentate gyrus was to silence portions of it using such long-term manipulations as drugs or lesions, techniques that yielded conflicting results.

In the current study, opsins were inserted into dentate gyrus granule cells (the principal cells of the dentate gyrus). The researchers then activated or silenced the ventral or dorsal portions of the dentate gyrus for three minutes at a time, while the mice were subjected to two well-validated anxiety tests (the elevated plus maze and the open field test).

“Our main findings were that elevating cell activity in the dorsal dentate gyrus increased the animals’ desire to explore their environment. But this also disrupted their ability to learn. Elevating activity in the ventral dentate gyrus lowered their anxiety, but had no effect on learning,” said Dr. Kheirbek. The effects were completely reversible — that is, when the stimulation was turned off, the animals returned to their previous anxiety levels.

“The therapeutic implication is that it may be possible to relieve anxiety in people with anxiety disorders by targeting the ventral dentate gyrus, perhaps with medications or deep-brain stimulation, without affecting learning,” said Dr. Hen, who is also director of the Division of Integrative Neuroscience, the New York State Psychiatric Institute, and a member of The Kavli Institute for Brain Science. “Given the immediate behavioral impact of such manipulations, these strategies are likely to work faster than current treatments, such as serotonin reuptake inhibitors.”

According to Dr. Hen, such an intervention would probably work best in people with panic disorder or PTSD. “There is evidence that people with these anxiety disorders tend to have a problem with pattern separation — the ability to distinguish between similar experiences,” he said. “In other words, they overgeneralize, perceiving minor threats to be the same as major ones, leading to a heightened state of anxiety. Such patients could conceivably benefit from therapies that fine-tune hippocampal activity.”

Dr. Hen and his team are currently exploring strategies aimed at modulating the activity of the ventral dentate gyrus by stimulating neurogenesis in the ventral dentate gyrus. “Indeed the dentate gyrus is one of the few areas in the adult brain where neurons are continuously produced, a phenomenon termed adult hippocampal neurogenesis,” added Dr. Hen.

(Image: Catherine E. Myers, Memory Loss and the Brain)

Every science writer loves a good challenge to dogma. I wish I had been in the working world in the spring of 1992, when one such intellectual overhaul happened in neuroscience. The dogma: Neurons, unlike most of the body’s cells, can’t be replenished. You’re born with just 100 billion of them and you better use them wisely. The challenge: Samuel Weiss and Brent Reynolds reported in Science that brain tissue taken from adult mice could be chemically coaxed into making new neurons.

“It left us speechless,” Weiss told the New York Times. Everybody else was pretty stunned, too. Over the next six years, other researchers confirmed that this so-called neurogenesis happens in the adult hippocampus of many animals, including tree shrews, marmosets, Old World monkeys and people. Today, more than two decades since the splashy Science report, adult neurogenesis is a bona fide subfield, with hundreds of labs studying it around the world.

But after all this time, researchers still don’t really know what it’s for. Studies have uncovered a wide variety of environmental stimuli — what you might think of as inputs — that affect neurogenesis in the dentate gyrus, a part of the hippocampus. Running and antidepressants can ramp up neurogenesis, for example, while stress, social isolation, sleep deprivation and aging can shut it down. Scientists have also looked at the outputs of neurogenesis, showing that a boost of new neurons may be important for exploratory behavior and certain kinds of learning, such as navigating a new space. But how do the inputs lead to the outputs?

“I like to think of the dentate as an association machine,” says Sam Pleasure, a neuroscientist at the University of California, San Francisco. All day long, he says, we’re walking around the world trying to associate various sensations and emotions — big dog with fangs, small screaming toddler, perilous traffic intersection — so that we can remember them later. “All these stimuli are happening and converge on this circuit, and they somehow affect how new neurons are recruited into the circuit, and that ends up coming out as the ability to form new memories.” But how it all works on the molecular level is a black box.

Two papers published in Cell Stem Cell [1 , 2]open that box a little bit. They identify molecular inhibitors — what Pleasure calls “wet blankets” — that turn off neurogenesis in certain contexts.

Opening the Black Box of Neurogenesis by Virginia Hughes