The ability to learn associations between events is critical for survival, but it has not been clear how different pieces of information stored in memory may be linked together by populations of neurons. In a study published April 2nd in Cell Reports, synchronous activation of distinct neuronal ensembles caused mice to artificially associate the memory of a foot shock with the unrelated memory of exploring a safe environment, triggering an increase in fear-related behavior when the mice were re-exposed to the non-threatening environment. The findings suggest that co-activated cell ensembles become wired together to link two distinct memories that were previously stored independently in the brain.
“Memory is the basis of all higher brain functions, including consciousness, and it also plays an important role in psychiatric diseases such as post-traumatic stress disorder,” says senior study author Kaoru Inokuchi of the University of Toyama. “By showing how the brain associates different types of information to generate a qualitatively new memory that leads to enduring changes in behavior, our findings could have important implications for the treatment of these debilitating conditions.”
Recent studies have shown that subpopulations of neurons activated during learning are reactivated during subsequent memory retrieval, and reactivation of a cell ensemble triggers the retrieval of the corresponding memory. Moreover, artificial reactivation of a specific neuronal ensemble corresponding to a pre-stored memory can modify the acquisition of a new memory, thereby generating false or synthetic memories. However, these studies employed a combination of sensory input and artificial stimulation of cell ensembles. Until now, researchers had not linked two distinct memories using completely artificial means.
With that goal in mind, Inokuchi and Noriaki Ohkawa of the University of Toyama used a fear-learning paradigm in mice followed by a technique called optogenetics, which involves genetically modifying specific populations of neurons to express light-sensitive proteins that control neuronal excitability, and then delivering blue light through an optic fiber to activate those cells. In the behavioral paradigm, one group of mice spent six minutes in a cylindrical enclosure while another group explored a cube-shaped enclosure, and 30 minutes later, both groups of mice were placed in the cube-shaped enclosure, where a foot shock was immediately delivered. Two days later, mice that were re-exposed to the cube-shaped enclosure spent more time frozen in fear than mice that were placed back in the cylindrical enclosure.
The researchers then used optogenetics to reactivate the unrelated memories of the safe cylinder-shaped environment and the foot shock. Stimulation of neuronal populations in memory-related brain regions called the hippocampus and amygdala, which were activated during the learning phase, caused mice to spend more time frozen in fear when they were later placed back in the cylindrical enclosure, as compared with stimulation of neurons in either the hippocampus or amygdala, or no stimulation at all.
The findings show that synchronous activation of distinct cell ensembles can generate artificial links between unrelated pieces of information stored in memory, resulting in long-lasting changes in behavior. “By modifying this technique, we will next attempt to artificially dissociate memories that are physiologically connected,” Inokuchi says. “This may contribute to the development of new treatments for psychiatric disorders such as post-traumatic stress disorder, whose main symptoms arise from unnecessary associations between unrelated memories.”
More information: Cell Reports, Ohkawa et al.: “Artificial Association of Pre-Stored Information to Generate a Qualitatively New Memory” www.cell.com/cell-reports/abst… 2211-1247(15)00270-3
The finding could mean recollections are more enduring than expected and disrupt plans for PTSD treatments.
As intangible as they may seem, memories have a firm biological basis. According to textbook neuroscience, they form when neighboring brain cells send chemical communications across the synapses, or junctions, that connect them. Each time a memory is recalled, the connection is reactivated and strengthened. The idea that synapses store memories has dominated neuroscience for more than a century, but a new study by scientists at the University of California, Los Angeles, may fundamentally upend it: instead memories may reside inside brain cells. If supported, the work could have major implications for the treatment of post-traumatic stress disorder (PTSD), a condition marked by painfully vivid and intrusive memories.
More than a decade ago scientists began investigating the drug propranolol for the treatment of PTSD. Propranolol was thought to prevent memories from forming by blocking production of proteins required for long-term storage. Unfortunately, the research quickly hit a snag. Unless administered immediately after the traumatic event, the treatment was ineffective. Lately researchers have been crafting a work-around: evidence suggests that when someone recalls a memory, the reactivated connection is not only strengthened but becomes temporarily susceptible to change, a process called memory reconsolidation. Administering propranolol (and perhaps also therapy, electrical stimulation and certain other drugs) during this window can enable scientists to block reconsolidation, wiping out the synapse on the spot.
The possibility of purging recollections caught the eye of David Glanzman, a neurobiologist at U.C.L.A., who set out to study the process in Aplysia, a sluglike mollusk commonly used in neuroscience research. Glanzman and his team zapped Aplysia with mild electric shocks, creating a memory of the event expressed as new synapses in the brain. The scientists then transferred neurons from the mollusk into a petri dish and chemically triggered the memory of the shocks in them, quickly followed by a dose of propranolol.
Initially the drug appeared to confirm earlier research by wiping out the synaptic connection. But when cells were exposed to a reminder of the shocks, the memory came back at full strength within 48 hours. “It was totally reinstated,” Glanzman says. “That implies to me that the memory wasn’t stored in the synapse.” The results were recently published in the online open-access journal eLife.
If memory is not located in the synapse, then where is it? When the neuroscientists took a closer look at the brain cells, they found that even when the synapse was erased, molecular and chemical changes persisted after the initial firing within the cell itself. The engram, or memory trace, could be preserved by these permanent changes. Alternatively, it could be encoded in modifications to the cell’s DNA that alter how particular genes are expressed. Glanzman and others favor this reasoning.
Eric R. Kandel, a neuroscientist at Columbia University and recipient of the 2000 Nobel Prize in Physiology or Medicine for his work on memory, cautions that the study’s results were observed in the first 48 hours after treatment, a time when consolidation is still sensitive.
Though preliminary, the results suggest that for people with PTSD, pill popping will most likely not eliminate painful memories. “If you had asked me two years ago if you could treat PTSD with medication blockade, I would have said yes, but now I don’t think so,” Glanzman says. On the bright side, he adds, the idea that memories persist deep within brain cells offers new hope for another disorder tied to memory: Alzheimer’s.
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 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.”
Journal reference: Abstract for “Arc in the nucleus regulates PML-dependent GluA1 transcription and homeostatic plasticity” by Erica Korb, Carol L Wilkinson, Ryan N Delgado, Kathryn L Lovero and Steven Finkbeiner in Nature Neuroscience. Published online June 9 2013 doi:10.1038/nn.3429
Dendrites of a nerve cell in brain appear like branches of a tree. Left: A patch clamp pipette injects fluorescent dye into the cell. (Credit: Image courtesy of Technische Universitaet Muenchen)
Pioneering a novel microscopy method, neuroscientist Arthur Konnerth and colleagues from the Technische Universitaet Muenchen (TUM) have shown that individual neurons carry out significant aspects of sensory processing: specifically, in this case, determining which direction an object in the field of view is moving. Their method makes it possible for the first time to observe individual synapses, nerve contact sites that are just one micrometer in size, on a single neuron in a living mammalian brain.
Focusing on neurons known to play a role in processing visual signals related to movement, Konnerth’s team discovered that an individual neuron integrates inputs it receives via many synapses at once into a single output signal — a decision, in essence, made by a single nerve cell. The scientists report these results in the latest issue of the journal Nature. Looking ahead, they say their method opens a new avenue for exploration of how learning functions at the level of the individual neuron.
When light falls on the retina of the human eye, it hits 126 million sensory cells, which transform it into electrical signals. Even the smallest unit of light, a photon, can stimulate one of these sensory cells. As a result, enormous amounts of data have to be processed for us to be able to see. While the processing of visual data starts in the retina, the finished image only arises in the brain or, to be more precise, in the visual cortex at the back of the cerebrum. Scientists working with Arthur Konnerth — professor of neurophysiology at TUM and Carl von Linde Senior Fellow at the TUM Institute for Advanced Study — are interested in a certain kind of neuron in the visual cortex that fires electrical signals when an object moves in front of our eyes — or the eyes of a mouse.
When a mouse is shown a horizontal bar pattern in motion, specific neurons in its visual cortex consistently respond, depending on whether the movement is from bottom to top or from right to left. The impulse response pattern of these “orientation” neurons is already well known. What was not previously known, however, is what the input signal looks like in detail. This was not easy to establish, as each of the neurons has a whole tree of tiny, branched antennae, known as dendrites, at which hundreds of other neurons “dock” with their synapses.
To find out more about the input signal, Konnerth and his colleagues observed a mouse in the act of seeing, with resolution that goes beyond a single nerve cell to a single synapse. They refined a method called two-photon fluorescence microscopy, which makes it possible to look up to half a millimeter into brain tissue and view not only an individual cell, but even its fine dendrites. Together with this microscopic probe, they conducted electrical signals to individual dendrites of the same neuron using tiny glass pipettes (patch-clamp technique). “Up to now, similar experiments have only been carried out on cultured neurons in Petri dishes,” Konnerth says. “The intact brain is far more complex. Because it moves slightly all the time, resolving individual synaptic input sites on dendrites was extremely difficult.”
The effort has already rewarded the team with a discovery. They found that in response to differently oriented motions of a bar pattern in the mouse’s field of vision, an individual orientation neuron receives input signals from a number of differently oriented nerve cells in its network of connections but sends only one kind of output signal. “And this,” Konnerth says, “is where things get really exciting.” The orientation neuron only sends output signals when, for example, the bar pattern moves from bottom to top. Evidently the neuron weighs the various input signals against each other and thus reduces the glut of incoming data to the most essential information needed for clear perception of motion.
In the future, Konnerth would like to extend this research approach to observation of the learning process in an individual neuron. Neuroscientists speculate that a neuron might be caught in the act of learning a new orientation. Many nerve endings practically never send signals to the dendritic tree of an orientation neuron. Presented with visual input signals that represent an unfamiliar kind of movement, formerly silent nerve endings may become active. This might alter the way the neuron weighs and processes inputs, in such a way that it would change its preferred orientation; and the mouse might learn to discern certain movements better or more rapidly. “Because our method enables us to observe, down to the level of a single synapse, how an individual neuron in the living brain is networked with others and how it behaves, we should be able to make a fundamental contribution to understanding the learning process,” Konnerth asserts. “Furthermore, because here at TUM we work closely with physicists and engineers, we have the best possible prospects for improving the spatial and temporal resolution of the images.”
This work was supported by grants from Deutsche Forschungsgemeinschaft (DFG) and Friedrich-Schiedel-Stiftung.
Provided by Technische Universitaet Muenchen.
Hongbo Jia, Nathalie L. Rochefort, Xiaowei Chen, Arthur Konnerth. Dendritic organization of sensory input to cortical neurons in vivo. Nature, 2010; 464 (7293): 1307 DOI: 10.1038/nature08947