Week 5: Herry & Courtin

Like many others in the class, I had difficulty with both papers this week. Despite my hypothetical interest in neuronal circuits, I find the concepts involved quite challenging. Thus, I expect many of my questions and observations to be misguided or upon false pretense. Nevertheless, here we go.

While I found the Herry et al. paper to be far less confusing and much more linear than the Courtin et al. paper, I still was not one hundred percent on board with their experimental design. I did not always feel that their results answered the exact questions they posed, and overall, I feel that they are asking too much of their results in terms of conceptual extensions. For instance, concluding that BA inactivation prevents behavioral transitions after observing disruptions in behavior after inhibiting the circuits using muscimol doesn’t feel sufficient to me. Although the authors do concede that muscimol is indiscriminate in inhibiting neuronal activity, I think the conclusion is too great a jump – what about all the interneurons outside the primary circuits being studied that also get inhibited? Why use muscimol in the first place and not a DREADD or opto? I understand that such experimental measures rely heavily on resources like time and money, but in this case, I don’t feel the researchers actually accomplish much by inactivating the BA with muscimol.

I struggle to comment on the Courtin et al. paper… Despite spending a large chunk of time trying to understand the basic concepts of the paper, I cannot say I have any insights to share. I am, however, prepared for class tomorrow with questions.

Week 5

            The 2008 Herry et al paper was our first look at extinction in relation to the fear circuitry and optogenetic manipulations that we had seen up to this point. The goal of the paper was to show that two distinct populations of neurons, extinction and fear, are at work regulating the response to conditioned stimuli during fear paradigms.

            I thought that the experimental design was lacking at times as they made broad claims and backed up with little evidence. The first fault I had was with their explanation that these fear and extinction neurons were two totally separate neuronal populations. The questions they posed was a good one, whether individual extinction neurons can function as fear neurons later with a different CS perhaps. The problem was when “demonstrating” that this was not the case and they are in fact two separate classes of neurons, all they showed was a classic fear paradigm. In the example, it stated that extinction neurons responded to the extinguished CS but not the unextinguished CS (obviously) and that the fear neurons responded to the unextinguished CS and not the extinguished CS (also obviously). This does not prove their point that these two neurons cannot later become a extinction neuron or fear neuron after having played the other role prior. This is later remedied a little with their physiological data showing that the two different types of neurons do have some distinct differences. This helped the validity of their assertions much more than their designed experiment made it seem. After these facts, and a few other procedural mishaps such as using multiple contexts to condition and extinguish fears, the paper made a valid case for microarchitecture of these cells providing a switch of behavior when confronting fear stimuli.

            The second paper from Courtin et al was much more thorough in its language and figures providing data for their identification of subpopulations of neurons from type 2 INs the PVINs. These figures made this paper easier to follow and the story came across more convincingly. The synchronicity of the neurons firing being the defining feature of information encoding makes it a prime target to explore when attempting to find see behavioral expression come from neuronal circuits. Their findings that synchronization of neurons could elicit a sustained fear response and that resetting to the optimal firing relationship could be used as a sort of therapy was exciting.

Week 5: Herry and Courtin

This week, I started by reading the Herry paper, which identified distinct fear and extinction neurons in the BLA. I was most interested by the findings summarized in figure 3, where researchers were able to outline the order of the events leading up to extinction: extinction neuron activity increasing, followed by fear neuron activity decreasing, followed by visible behavioral changes (decreased freezing). I think it would have been interesting if researchers used the methods to take the findings in figure 4 a step further, and analyze the sequence of events during fear renewal as well. Overall, I think the order of experiments made a lot of sense and they all tied together really nicely, with the exception of figure 6, which felt a bit out of place to me. It seems like a great starting point for a completely new paper. In that case, I would be more interested in seeing how targeted inhibition of either fear or extinction neurons at various points throughout the experiment would affect fear conditioning, extinction recall, or fear renewal. I am curious about the location of these two distinct neuronal populations in the BLA (are they clustered together or evenly distributed?). Finally, I would love to see how activity of these neurons deviates from the norm in PTSD patients. Are their extinction neurons less active, are their fear neurons more active, is it a combination of the two, is it circuitry damage between these neurons and the brain regions they communicate with, or something else entirely?

The Courtin paper was far more challenging for me, mostly due to my initial unfamiliarity with the significance of theta firing and the importance of neuronal synchronization. While I did eventually come to understand their findings, I am still a bit perplexed about the importance of synchronized neuronal firing. I wish we could have seen some side-by-side data of theta phase resetting and freezing behavior. Specifically, I would have liked to see how turning the light responsible for the inhibition of PVIN on and off would impact freezing behavior in a fear conditioned mouse (in figure 4).

Week 5

I had a lot of issues with the Harry et al. 2008 paper. While it is most likely a function of my lack of knowledge of fear/fear research, I still felt myself feeling as though this paper didn't really address anything novel. First and foremost, while I am convinced that different neurons are activated during a fear or extinction behavior, I thought their results did NOT address "the question of whether individual extinction neurons can function as fear neurons for another CS". I do not think that two CS's would be enough to answer the question. Of course the fear neurons activated during CS2 weren't activated during the extinct CS1, and vise versa - but there could be plausibly hundreds of fear memories in a life time and how is this evidence to suggest that an extinction neuron for CSX couldn't be a fear neuron for CSY? While later in the paper their morphological and connectivity data do lend weight to the distinction of individual neuronal populations for the two behaviors, their CS1/CS2 data was week. While perhaps out of reach then (and maybe even out of reach now, I'm not sure), to lock down this idea they could have used this paradigm to identify the extinction neurons for one CS and see if activating them during a different fear CS could program a fear response.

Additionally in regards to the first paper, I was confused as to the significance of the findings presented in the "Rapid reversal of activity during fear renewal" section. It was my understanding that extinction takes place in the same context as the fear conditioning, and in this way one can be assured that the memory is actually "extinct" and they aren't just associating the tone in a safer context. I'm not understanding how one can claim the fear is extinguished if putting them back in the fear conditioning context causes the fear response to return. I believe this issue with the paper is more due to lack of understanding of the fear/extinction paradigm however.

All in all I definitely appreciated the connectivity data presented in the Harry et. al 2008 paper. I thought that showing that these specific neurons which are activated in a distinct behavior (that is mutually exclusive with the other behavior/set of neurons) have differential connectivity strengthen every other aspect of their results, as merely showing a physiological difference made the dichotomy of two populations much more plausible.

Week 5

These weeks papers were significantly more difficult to read and understand compared to previous weeks’ papers. Maybe this had to do with my lack of knowledge on neuronal activity, reading spikes, or understanding theta oscillations, but I have been left perplexed by these articles. I thought after talking at length about fear conditioning, extinction, and renewal I would have had a better foundation for these papers but I still was at a loss. I am glad that I had at least the experimental designs down and packed but unfortunately got lost with the actual results and data collected in both papers.  The Herry et al. paper was easier to follow as they provided more detail on exactly what they were studying, how they studied it, and why the results supported their hypothesis. I think something interesting that they can also test is the circuit between the Prefrontal Cortex, Intercalated Cell Masses, and extinction neurons in the BLA. Since Intercalated Cell Masses inhibit fear neurons, they could also have a role in changing behavioral states during fear extinction and renewal. What I liked most about this paper is that the experiments seemed to build off each other. First, they looked for which neurons were encoded in fear conditioning, extinction, and renewal. Once they found these neurons, they questioned when are these neurons activated in relation to behavioral changes. They also tested different paradigms and location of the fear and extinction neurons in relation to activity. What I thought was the most interesting experiment was testing necessity of the BA in fear extinction. As for the Courtin et al. paper, I really hope the presenters in class tomorrow and Professor Shansky can go through this one slowly as I really do not understand it after reading through a couple of times. I thought this would be the easier paper to follow because they use optogenetics but I had a lot of difficulty with it. 

Week 5

This week I felt that the papers were the most challenging we had read so far. However, I think because we read other papers that utilised fear conditioning experiments, it was a little bit easier to not have to learn a new test or experimental design.

The first paper I read was Herry et al.'s Switching on and off fear by distinct neuronal circuits. Towards the beginning, I was having a hard time understanding the overall experiment and how each section was relevant to their hypothesis. However, by the end of the paper, I began to understand the larger picture and how the experiments worked to prove how the two circuits were distinct and integrated into the PFA/hippocampal network. Based on my limited research background, scarcer in neuroscience, I had a hard time understanding a lot of the figures because I haven't had a lot of exposure to circuit and neuronal activation research. Because of this, I relied heavily on the text, which was organized nicely in the different sub-titled sections that made it easier to follow. One way I thought this paper could have been stronger was if they could selectively inhibit fear or extinction neurons, as opposed to the muscimol inhibiting the entire BA. I think isolating these neuronal circuits could give clarity to its role in fear reactions and extinction. In addition, (and I'm not sure how this would be done) but I think if you could severe the connections between the hippocampus or the PFC and the basolateral nuclei, it would be interesting to see how this severed connection impacts fear extinction and/or renewal. Finally, it would be interesting to see if different types of fear memories acted upon different circuits. I know that one part of the paper distinguished between two different conditioned stimuli and the ability for the circuits to discriminate between each. But I think it would supplement the paper to include a different type of fear test to see if these fear and extinction circuits follow the same pathways, are maybe clustered together in a different area, or also intermixed between the already-established neurons.

As for Courtin et al.'s Prefrontal parvalbumin interneurons shape neuronal activity to drive fear expression, I had a very hard time following the experiments and the overall message of the paper. This may be due to my lack of knowledge on parvalbumin interneurons, as well as understanding of theta oscillations. One (of many) questions that I had at the end of this paper was what is the point of phase resetting? While they gave various notes on it in the very end of the paper, I wasn't really connecting how, cognitively, phase resetting is important.

Week 5: Herry and Courtin

Out of both of these articles I enjoyed reading and analyzing the article written in 2008 significantly more. While the 2014 article was shorter and more compact, it was difficult for me to wrap my head around the theta oscillations associated with this analysis. This may be due to the fact that I have learned significantly less of this in school compared to other methods. I think that some background knowledge on this reading may be necessary in order to fully understand the implications this research has. I’m hoping that the people presenting this paper are able to explain it in a clearer capacity than this article did. Since it was published in Nature, I’m sure the editors assumed a lot of background knowledge by the readers and took out pieces that weren’t integral to the overall results.

Although I enjoyed the 2008 article significantly more, there were still some questions left unanswered in this article that I think when answered, would make it even stronger. For example, the author states that the largest decrease seen in fear behavior during extinction training was seen in between the third and fourth blocks. This statement immediately made me as why this could be. The author hypothesizes that there could be some sort of consolidation method that occurs overnight, but I wish more care was taken into determining what that mechanism was. I’m sure a question like this will take an entirely new research paper to explore, but the underlying mechanism behind this consolidation process may hint at what occurs during the on/off switching of fear. When this behavior is reactivated do these consolidated neurons then reactivate? Or is it a completely separate system? I think this would be an interesting area to explore and could add a lot more information to this article.