Two new studies hit the news this on Wednesday, both of which involve changing the emotional impact of memories.
The first was a joint project between MIT and Howard Hughes Medical Institute researchers. We'll start with the press release from MIT, a study that uses optogenetics (light stimulation) to alter emotional connections with memories:
Date: August 27, 2014
Source: Massachusetts Institute of Technology
Summary:
Most memories have some kind of emotion associated with them: Recalling the week you just spent at the beach probably makes you feel happy, while reflecting on being bullied provokes more negative feelings. A new study from neuroscientists reveals the brain circuit that controls how memories become linked with positive or negative emotions.
This image depicts the injection sites and the expression of the viral constructs in the two areas of the brain studied: the Dentate Gyrus of the hippocampus (middle) and the Basolateral Amygdala (bottom corners). Credit: Image courtesy of the researchers
Most memories have some kind of emotion associated with them: Recalling the week you just spent at the beach probably makes you feel happy, while reflecting on being bullied provokes more negative feelings.
A new study from MIT neuroscientists reveals the brain circuit that controls how memories become linked with positive or negative emotions. Furthermore, the researchers found that they could reverse the emotional association of specific memories by manipulating brain cells with optogenetics -- a technique that uses light to control neuron activity.
The findings, described in the Aug. 27 issue of Nature, demonstrated that a neuronal circuit connecting the hippocampus and the amygdala plays a critical role in associating emotion with memory. This circuit could offer a target for new drugs to help treat conditions such as post-traumatic stress disorder, the researchers say.
"In the future, one may be able to develop methods that help people to remember positive memories more strongly than negative ones," says Susumu Tonegawa, the Picower Professor of Biology and Neuroscience, director of the RIKEN-MIT Center for Neural Circuit Genetics at MIT's Picower Institute for Learning and Memory, and senior author of the paper.
The paper's lead authors are Roger Redondo, a Howard Hughes Medical Institute postdoc at MIT, and Joshua Kim, a graduate student in MIT's Department of Biology.
Shifting memories
Memories are made of many elements, which are stored in different parts of the brain. A memory's context, including information about the location where the event took place, is stored in cells of the hippocampus, while emotions linked to that memory are found in the amygdala.
Previous research has shown that many aspects of memory, including emotional associations, are malleable. Psychotherapists have taken advantage of this to help patients suffering from depression and post-traumatic stress disorder, but the neural circuitry underlying such malleability is not known.
In this study, the researchers set out to explore that malleability with an experimental technique they recently devised that allows them to tag neurons that encode a specific memory, or engram. To achieve this, they label hippocampal cells that are turned on during memory formation with a light-sensitive protein called channelrhodopsin. From that point on, any time those cells are activated with light, the mice recall the memory encoded by that group of cells.
Last year, Tonegawa's lab used this technique to implant, or "incept," false memories in mice by reactivating engrams while the mice were undergoing a different experience. In the new study, the researchers wanted to investigate how the context of a memory becomes linked to a particular emotion. First, they used their engram-labeling protocol to tag neurons associated with either a rewarding experience (for male mice, socializing with a female mouse) or an unpleasant experience (a mild electrical shock). In this first set of experiments, the researchers labeled memory cells in a part of the hippocampus called the dentate gyrus.
Two days later, the mice were placed into a large rectangular arena. For three minutes, the researchers recorded which half of the arena the mice naturally preferred. Then, for mice that had received the fear conditioning, the researchers stimulated the labeled cells in the dentate gyrus with light whenever the mice went into the preferred side. The mice soon began avoiding that area, showing that the reactivation of the fear memory had been successful.
The reward memory could also be reactivated: For mice that were reward-conditioned, the researchers stimulated them with light whenever they went into the less-preferred side, and they soon began to spend more time there, recalling the pleasant memory.
A couple of days later, the researchers tried to reverse the mice's emotional responses. For male mice that had originally received the fear conditioning, they activated the memory cells involved in the fear memory with light for 12 minutes while the mice spent time with female mice. For mice that had initially received the reward conditioning, memory cells were activated while they received mild electric shocks.
Next, the researchers again put the mice in the large two-zone arena. This time, the mice that had originally been conditioned with fear and had avoided the side of the chamber where their hippocampal cells were activated by the laser now began to spend more time in that side when their hippocampal cells were activated, showing that a pleasant association had replaced the fearful one. This reversal also took place in mice that went from reward to fear conditioning.
Altered connections
The researchers then performed the same set of experiments but labeled memory cells in the basolateral amygdala, a region involved in processing emotions. This time, they could not induce a switch by reactivating those cells -- the mice continued to behave as they had been conditioned when the memory cells were first labeled.
This suggests that emotional associations, also called valences, are encoded somewhere in the neural circuitry that connects the dentate gyrus to the amygdala, the researchers say. A fearful experience strengthens the connections between the hippocampal engram and fear-encoding cells in the amygdala, but that connection can be weakened later on as new connections are formed between the hippocampus and amygdala cells that encode positive associations.
"That plasticity of the connection between the hippocampus and the amygdala plays a crucial role in the switching of the valence of the memory," Tonegawa says.
These results indicate that while dentate gyrus cells are neutral with respect to emotion, individual amygdala cells are precommitted to encode fear or reward memory. The researchers are now trying to discover molecular signatures of these two types of amygdala cells. They are also investigating whether reactivating pleasant memories has any effect on depression, in hopes of identifying new targets for drugs to treat depression and post-traumatic stress disorder.
David Anderson, a professor of biology at the California Institute of Technology, says the study makes an important contribution to neuroscientists' fundamental understanding of the brain and also has potential implications for treating mental illness.
"This is a tour de force of modern molecular-biology-based methods for analyzing processes, such as learning and memory, at the neural-circuitry level. It's one of the most sophisticated studies of this type that I've seen," he says.
The research was funded by the RIKEN Brain Science Institute, Howard Hughes Medical Institute, and the JPB Foundation.
Story Source:
The above story is based on materials provided by Massachusetts Institute of Technology. The original article was written by Anne Trafton. Note: Materials may be edited for content and length.
Journal Reference:
Redondo RL, Kim J, Arons AL, Ramirez S, Liu X, Tonegawa S. (2014, Aug 27). Bidirectional switch of the valence associated with a hippocampal contextual memory engram. Nature; DOI: 10.1038/nature13725
* * * * *
Here is the abstract for the
Nature article, which is pay-walled, of course.
Roger L. Redondo, Joshua Kim, Autumn L. Arons, Steve Ramirez, Xu Liu & Susumu Tonegawa
Nature (2014) doi:10.1038/nature13725 Published online 27 August 2014
The valence of memories is malleable because of their intrinsic reconstructive property1. This property of memory has been used clinically to treat maladaptive behaviours2. However, the neuronal mechanisms and brain circuits that enable the switching of the valence of memories remain largely unknown. Here we investigated these mechanisms by applying the recently developed memory engram cell- manipulation technique3, 4. We labelled with channelrhodopsin-2 (ChR2) a population of cells in either the dorsal dentate gyrus (DG) of the hippocampus or the basolateral complex of the amygdala (BLA) that were specifically activated during contextual fear or reward conditioning. Both groups of fear-conditioned mice displayed aversive light-dependent responses in an optogenetic place avoidance test, whereas both DG- and BLA-labelled mice that underwent reward conditioning exhibited an appetitive response in an optogenetic place preference test. Next, in an attempt to reverse the valence of memory within a subject, mice whose DG or BLA engram had initially been labelled by contextual fear or reward conditioning were subjected to a second conditioning of the opposite valence while their original DG or BLA engram was reactivated by blue light. Subsequent optogenetic place avoidance and preference tests revealed that although the DG-engram group displayed a response indicating a switch of the memory valence, the BLA-engram group did not. This switch was also evident at the cellular level by a change in functional connectivity between DG engram-bearing cells and BLA engram-bearing cells. Thus, we found that in the DG, the neurons carrying the memory engram of a given neutral context have plasticity such that the valence of a conditioned response evoked by their reactivation can be reversed by re-associating this contextual memory engram with a new unconditioned stimulus of an opposite valence. Our present work provides new insight into the functional neural circuits underlying the malleability of emotional memory.
References:
- Pavlov, I. P. Conditioned Reflexes: An Investigation of the Physiological Activity of the Cerebral Cortex (Oxford Univ. Press, 1927)
- Wolpe, J. Psychotherapy by Reciprocal Inhibition (Stanford Univ. Press, 1958)
- Liu, X. et al. Optogenetic stimulation of a hippocampal engram activates fear memory recall. Nature 484, 381–385 (2012)
- Ramirez, S. et al. Creating a false memory in the hippocampus. Science 341, 387–391 (2013)
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The second study comes from researchers at Harvard University who are using Xenon gas to remove the emotional context from traumatic memories. This is a murine study, but the results suggest further research will be coming.
Xenon gas is already being used for general anesthetic with fewer side effects and actually providing some cardioprotection and neuorprotection. From
Wikipedia:
Xenon is a high-affinity glycine-site NMDA receptor antagonist.[129] However, xenon distinguishes itself from other clinically used NMDA receptor antagonists in its lack of neurotoxicity and its ability to inhibit the neurotoxicity of ketamine and nitrous oxide.[130][131] Unlike ketamine and nitrous oxide, xenon does not stimulate a dopamine efflux from the nucleus accumbens.[132]
First up the press release from Harvard (the
Harvard Gazette) and then the abstract and introduction from
PLOS ONE, the open access publication platform for science.
Xenon exposure may be potential new treatment for people with PTSD
August 27, 2014 | Editor's Pick
By Scott O’Brien, McLean Hospital Communications
Researchers at Harvard-affiliated McLean Hospital are reporting that xenon gas, used in humans for anesthesia and diagnostic imaging, has the potential to become a treatment for post-traumatic stress disorder (PTSD) and other memory-related disorders.
“In our study, we found that xenon gas has the capability of reducing memories of traumatic events,” said Edward G. Meloni, assistant psychologist at McLean and an assistant professor of psychiatry at Harvard Medical School (HMS). “It’s an exciting breakthrough.”
In the study, published in the current issue of PLOS ONE, Meloni and HMS Associate Professor of Psychiatry Marc J. Kaufman, director of the Translational Imaging Laboratory at McLean, examined whether a low concentration of xenon gas could interfere with a process called reconsolidation — a state in which reactivated memories become susceptible to modification. “We know from previous research that each time an emotional memory is recalled, the brain actually re-stores it as if it were a new memory. With this knowledge, we decided to see whether we could alter the process by introducing xenon gas immediately after a fear memory was reactivated,” explained Meloni.
Statistics show an increase in PTSD diagnoses among the military. Harvard researchers are investigating a potential breakthrough that would treat symptoms associated with PTSD. Credit: Congressional Research Service PTSD data/McLean Hospital
The investigators used an animal model of PTSD called fear conditioning to train rats to be afraid of environmental cues that were paired with brief foot shocks. Reactivating the fearful memory was done by exposing the rats to those same cues and measuring their freezing response as a readout of fear. “We found that a single exposure to the gas, which is known to block NMDA receptors involved in memory formation in the brain, dramatically and persistently reduced fear responses for up to two weeks. It was as though the animals no longer remembered to be afraid of those cues,” said Meloni.
Meloni points out that the inherent properties of a gas such as xenon make it especially attractive for targeting dynamic processes like memory reconsolidation. “Unlike other drugs or medications that may also block NMDA receptors involved in memory, xenon gets in and out of the brain very quickly. This suggests that xenon could be given at the exact time the memory is reactivated, and for a limited amount of time, which may be key features for any potential therapy used in humans.”
“The fact that we were able to inhibit remembering of a traumatic memory with xenon is very promising because it is currently used in humans for other purposes, and thus it could be repurposed to treat PTSD,” added Kaufman.
For these investigators, several questions remain to be addressed with further testing. “From here we want to explore whether lower xenon doses or shorter exposure times would also block memory reconsolidation and the expression of fear. We’d also like to know if xenon is as effective at reducing traumatic memories from past events, so-called remote memories, versus the newly formed ones we tested in our study.”
Meloni and Kaufman indicate that future studies are planned to test whether the effects of xenon in rats that they saw in their study translate to humans. Given that intrusive re-experiencing of traumatic memories — including flashbacks, nightmares, and distress and physiological reactions induced by with trauma reminders — is a hallmark symptom for many who suffer from PTSD, a treatment that alleviates the impact of those painful memories could provide welcome relief.
The study may be viewed on the PLOS ONE website.
* * * * *
Edward G. Meloni, Timothy E. Gillis, Jasmine Manoukian, Marc J. Kaufman
Abstract
Xenon (Xe) is a noble gas that has been developed for use in people as an inhalational anesthestic and a diagnostic imaging agent. Xe inhibits
glutamatergic N-methyl-D-aspartate (NMDA) receptors involved in learning
and memory and can affect synaptic plasticity in the amygdala and
hippocampus, two brain areas known to play a role in fear conditioning
models of post-traumatic stress disorder (PTSD). Because glutamate
receptors also have been shown to play a role in fear memory
reconsolidation – a state in which recalled memories become susceptible
to modification – we examined whether Xe administered after fear memory
reactivation could affect subsequent expression of fear-like behavior
(freezing) in rats. Male Sprague-Dawley rats were trained for contextual
and cued fear conditioning and the effects of inhaled Xe (25%, 1 hr) on
fear memory reconsolidation were tested using conditioned freezing
measured days or weeks after reactivation/Xe administration. Xe
administration immediately after fear memory reactivation significantly
reduced conditioned freezing when tested 48 h, 96 h or 18 d after
reactivation/Xe administration. Xe did not affect freezing when
treatment was delayed until 2 h after reactivation or when administered
in the absence of fear memory reactivation. These data suggest that Xe
substantially and persistently inhibits memory reconsolidation in a
reactivation and time-dependent manner, that it could be used as a new
research tool to characterize reconsolidation and other memory
processes, and that it could be developed to treat people with PTSD and
other disorders related to emotional memory.
Full Citation:
Meloni
EG, Gillis TE, Manoukian J, Kaufman MJ. (2014. Aug 27). Xenon Impairs
Reconsolidation of Fear Memories in a Rat Model of Post-Traumatic Stress
Disorder (PTSD). PLoS ONE 9(8):
e106189.
doi:10.1371/journal.pone.0106189
Introduction
Mitigation of persistent, intrusive, traumatic memories experienced by people with post-traumatic stress disorder (PTSD) remains a key therapeutic challenge [1]. Behavioral treatments such as extinction training – administered alone or in combination with cognitive-enhancing drugs (e.g. d-cycloserine) – attempt to inhibit underlying traumatic memories by facilitating a new set of learning contingencies, but often achieve limited success [2]. Another learning and memory phenomenon known as reconsolidation, a process by which reactivated (retrieved) memories temporarily enter a labile state (the reconsolidation window), has been studied to determine whether drug or behavioral interventions can prevent a traumatic memory trace from being re-incorporated back into the neural engram, inhibiting the memory [3]–[6]. Several chemical agents have been found to inhibit fear memory reconsolidation in animals [7] but unfortunately do not translate well to humans, limiting their clinical use. They either are toxic (e.g. protein synthesis inhibitors), induce unwanted side effects, are slow acting such that brain drug concentrations peak outside of the reconsolidation window, or are slowly eliminated such that they interfere with later onset memory processes including extinction [8]. A recent human study documented that a single electroconvulsive therapy (ECT) treatment administered to unipolar depressed subjects immediately after emotional memory reactivation disrupted reconsolidation, confirming that reconsolidation occurs in humans and that it can be inhibited by a brief treatment [9]. While ECT is indicated for therapeutic use in people with treatment-resistant major depression, it may not be a viable treatment for other clinical populations. Thus, there is a significant unmet need for a minimally invasive, safe and well-tolerated treatment that can be used clinically to inhibit fear memory reconsolidation in people with PTSD.
The noble gas xenon (Xe) inhibits glutamatergic N-methyl-D-aspartate (NMDA) receptors [10] known to play a role in memory reconsolidation [11]. Xe reduces NMDA-mediated synaptic currents and neuronal plasticity in the basolateral amygdala and CA1 region of the hippocampus [12], [13]; these brain areas are involved in Pavlovian fear conditioning, an animal model of PTSD used to elucidate learning and memory processes, including reconsolidation [14]–[16]. Xe already is used in humans at high concentration (>50%) as an anesthetic and at subsedative concentration (28%) as a diagnostic imaging agent; in both applications, Xe has excellent safety/side effect profiles and is well tolerated [17]–[19]. Further, NMDA receptor glycine antagonists like Xe [10] do not appear to have significant abuse liability and do not induce psychosis [20], [21], consistent with clinical experience [18], [19]. Thus, Xe has a number of favorable properties that might be beneficial for treating fear memory disorders. As fear memory reconsolidation is an “evolutionarily conserved memory-update mechanism” [5], we evaluated in rats whether administering a subsedative concentration of Xe (maximum concentration 25%, 1 h) via inhalation following conditioned fear memory reactivation could reduce subsequent expression of fear-like behavior. Here, we report that Xe impaired reconsolidation of fear memory demonstrated as a reduction in conditioned freezing, a behavioral readout used to measure fear in animals.
