Week 3: Ramirez and Memory

Both papers this week are from Steve Ramirez from the Tonegawa Lab at MIT Picower Institute. The 2013 paper laid the groundwork for the 2015 paper for the methodology and further of the 2013 results. 

            In the 2013 paper, Ramirez sought to investigate if light-activation of contextual memory in the dentate gyrus or CA1 could serve as a functional conditioned stimulus in fear conditioning. To do this, he utilized a cFos-tTA transgenic mouse line. These animals were then injected with a TRE-ChR2-MCherry into the DG or CA1. Without the presence of doxycycline, cells that are recently activated and producing cfos then drives expression of the virus. So, using this tool they would expose an animal to a context with a Dox-off diet and then put the animals back on Dox. This labeled DG or CA1 cells that are involved in the engram of contextual memory for that particular context. Overall, they were able to show that stimulating the context A cells during context B fear conditioning, they were able to induce freezing in context A without the need for conditioning in context A. This is a false memory given that the animals never learned a negative association with context A, but are exhibiting fear behaviors in said condition anyway from just activating context A cells during fear conditioning. They repeated this in the CA1 and found no change in freezing. They also tested fear recall in a novel condition D. They did the same behavioral paradigm as before, but also tested freezing in a new context D while stimulating the context A cells and were able to induce freezing and recall of fear memory. They continued to test the effects of this induced false memory on behavior in conditioned place avoidance. They labeled cells with their cfos transgenic line and were able to induce avoidance of that labeled-box by stimulating those corresponding labeled cells during fear conditioning. 

            In this paper, they propose a sort of competitive conditioning. This means that memories of each individual conditioned stimuli are acquired less strongly when compared to presented alone. It could be that the light-activation of DG cells encoding for context A is interfering with the fear memory of context B in some way. It could also be that there is an additive effect on fear.

            In their 2015 paper, they continued utilizing their labeling tools and furthered exploring the role of DG cell activation in affecting behavior. They wanted to study how memories of a positive episode can be useful for the treatment and understanding of depression. They utilized their Cfos-tTA system with Dox-on/Dox-off to mark recently active engram cells of, particularly valenced memory. They exposed mice to either a positive, neutral, or negative episode while Dox-off and then submitted then all to chronic immobilization stress except for one group of positive memory mice. They then put the animals through various behavioral tests while stimulating the cells labeled from the valenced memory. In stressed animals, light-stimulation of cells from the positive memory increased struggle time, increased sucrose preference, and decreased latency to feed. Neutral and negative memory cell stimulation did not have the same effect. Furthering, they tested for regions that could be implicated in this behavior reversal. Cfos revealed that the NAcc, shell lateral septum, BLA, CeA, and various regions of the hypothalamus were active but the mPFC was not. They studied the circuit of the NAcc-BLA-Hippocampus and found that light-induced activation of DG cells also activated BLA and NAcc cells. When they blocked BLA terminals on the NAcc, they were able to block the light-induced rescue. Blocking terminals of the mPFC onto the NAcc had no blockade on behavior. They further studied the effect of chronic and acute. They found that chronic stimulation of 5 days can induce a reversal similar to unstressed groups compared to 1-day and no stimulation. Decreased neurogenesis was found in all groups except those that experience the 5-day stimulation, or positive experience, or unstressed groups. 

            From this paper, I would be intrigued if they could induce a chronic stress model by only chronically activating negative memory cells. It’d be interesting to see if they could also negatively induce memories and produce a model of chronic stress with only one day of immobilization. Ramirez also shows that exposure to a naturally rewarding behavior is not sufficient for this rescue but that stimulation of positive memory cells is necessary. They propose that depression-related readouts can be ameliorated by activating cells in the DG associated with a positive experience, but I’m wondering if this could be thought of more as a priming effect. Given that the animal experiences a positive interaction prior to chronic stress, I’d be interested to see if the animal were to experience all valenced experience states but only experience the stimulation of labeled cells from the positive memory. This could then be a control for the priming effect to see if exposure to a positive memory prior to chronic stress can impact on the behavioral readout. Finally, I’d be interested to see if affecting particular interneuron subtypes in this paradigm would affect behavior. Given that anxiety-related behaviors rely on complex amygdala circuitry, I’d be interested to see that if PV interneurons could be responsible for this glutamate activity from the BLA. Disinhibition and general inhibition play a big role in functioning microcircuits for fear. Inhibiting PV cells may prevent glutamatergic activity by not inhibiting nearby SST cells. I’d be interested to see if there could be an investigation on the role of microcircuits for this particular behavioral pathway.

They also propose that because this effect circumvents the time needed for plastic remodeling to occur following antidepressant usage, that it presents a new avenue for therapeutic intervention. I am a little hesitant to see the therapeutic of this finding. While it’s true that chronic cell stimulation circumvents the time of activation for antidepressants, I’m not sure how one would functionally target engram cells in humans without the use of cfos mediated labeling. There are not many clinically relevant effectors that are being used in research like this and optogenetic stimulation of a group of cells is difficult to replicate in humans. I feel that it’s a large leap for the authors to include that these results can be clinically relevant when it seems near impossible to stimulate these small groups of DG cells in humans.