9/26 - Chaudhury/Tye

              Chaudhury et al. and Tye et al. highlighted several key issues in the ways both the research community and society at large can sometimes view depression and its treatment. Depression is often seen is often used an umbrella terms that describes a wide array of contrasting symptoms that can arise from starkly different conditions. This is in part evidenced by the fact that broad-sweeping antidepressants have been used for years without specific knowledge into the exact mechanism these drugs are working on. Many individuals suffering with depression simply cycle through antidepressants until they find one that works for their specific presentation of depression. These research articles use targeted methods, both optogenetic stimulation and electrophysiology, to isolated specific neural circuitries. By doing research that is so explicitly looking at one thing, these articles show that there is benefit to figuring out exactly what mechanisms are at play in different facets of depression. Research like this could lead to more targeted and specific treatment of depression.

              I also found the use of optogenetic phasic stimulation in Chaudbury at all particularly interesting for the topic of specificity in treatment of mental health. At the very least, optogenetic stimulation can and has been incredibly useful in improving animal models of mental health. As stated above, depression and other related illnesses are complex and highly dependent on condition. The ability to instantly create specific conditions and isolate precise neural circuitries provides the most accurate as possible model of mental illness. Furthermore, optogenetics could provide an innovative new treatment option for various mental illnesses, especially in terms of acute treatment of patients with known disorders.

Tye vs Chaudhury Comparison - Joe

Chaudhury vs Tye Papers

Seminar in Biological Psychology

Dr. Rebecca Shansky

Fall 2016

    The most striking facet of these two pieces of work is that they were both published in Nature in the same year, and both papers featured Karl Deisseroth, a pioneer in the optogenetics field. Furthermore, the papers published arguably contradictory information — although that was accounted for in the discussion portions of the respective letters.

    Chaudhury et al took a social defeat approach to the problem. They asked, if we subject mice to a behavioral paradigm where they experience enough stress to remain resilient but have undergone considerable stress, and also optically activate a specific subset of neurons in the ventral tegmental area (VTA) during the same time, what behavior will these mice manifest? Will they remain resilient? Will they exhibit social defeat? And thus, what is the function of this particular subset of neurons? According to previous literature, it had been well established that the dopamine (DA) neurons in the VTA had been shown to play a role in reward behavior, and to be implicated in disorders such as anxiety and depression. Thus, they tackled this issue by utilizing a tyrosine hydroxylase (TH) - cre transgenic mouse, and virally expressing channelrhodopsin (ChR2) or halorhodopsin (NpHR) in TH-positive cells, using a cre-dependent opsin virus (TH is a precursor for catecholamines such as dopamine).

    They variably stimulated these dopamine cells in the VTA (tonically and phasically), to observe that phasic — and not tonic — stimulation had a strong effect on a mouse’s motivational and hedonic state after experiencing subthreshold social defeat. Furthermore, they showed that in a mouse that undergoes subthreshold social defeat without optogenetic stimulation, susceptibility can still be induced by phasic stimulation during social interaction or sucrose preference assessments.

    After this, they moved away from a transgenic approach to use a pathway-specific approach using a retrograde cre virus. They knew that the nucleus accumbens (NAc) had been implicated in the reward pathway, and so they specifically expressed ChR2 in VTA->NAc direct projections. The issue with this technique (compared to the last one) is the loss of cell-type specificity. The beginning experiments were based on the finding that the specific firing pattern induced in the specific neuronal cell type (VTA DA neurons) induced a susceptibility phenotype, but this was not as directly addressed in this next experiment. Nonetheless, they found that increased phasic firing of VTA->NAc projections accompanied the susceptibility phenotype in both measures (social interaction and sucrose preference), and conversely, inactivation of this projection showed an increase in the resilience phenotype. They followed up with an experiment specifically targeting the VTA->mPFC projections and found that there was no change in the behavioral phenotype in sucrose preference, but there was a change in social interaction when the projection was inhibited. This brings up the point that these two tests may not be the same measure for social defeat. What is the best animal correlate of human depression? Of human anxiety?

    Tye et al took a similar approach, also using TH-cre mice to optogenetically stimulate or inhibit DA neurons in the VTA (and furthermore, to specifically target VTA->NAc projections), but they took different behavioral approaches. Instead of using a social defeat model, Tye et al utilized a chronic mild stress model. Both papers addressed that a potential reason for the differential findings may stem from a longstanding idea that chronic mild stress and social defeat recruit the same neural circuits in an opposing manner.

    Tye et al found that activation of VTA DA neurons after chronic mild stress induces a resilience phenotype, while inactivation of that ensemble produces the opposing phenotype. Again, this may be due to the behavioral approach.

    A good control that they used was the open field test along with optogenetic stimulation, which helped say that the behavioral effects they were seeing were indeed due to motivational changes and not simply to locomotor changes. Another good control was the inactivation of glutamatergic neurons along with dopamine neurons in the NAc, which held a similar purpose — to say that the dopaminergic cells in the NAc were recruited in the manifestation of this phenotype.

    A clever experimental approach they took, however, was to pharmacologically inactivate NAc neurons and see that the behavioral effects of VTA activation were nullified. That is to say that there is something about the connectivity of VTA->NAc circuits that may affect behavior. To probe the causality of VTA activation, they concurrently activated VTA DA neurons and recorded from the NAc in vivo during a stressful task (the forced swim test). This was a great way to follow up the claim that the VTA->NAc pathway may indeed be leading to behavioral changes related to motivation.

    In the end, they spike sorted the waveforms they had accumulated during the in vivo recordings, and the issue of this sorting technique is that it is usually done manually and is not perfect. Moreover, they found that all the possible responses a cell could have had, they found a cell that did. In other words, a neuron could have responded to VTA activation, could have preceded an increase in escape-related behaviors, or both. And they found cells that exhibited all 3 properties.

    The two papers highlight a key talking point in behavioral neuroscience: not all behavioral tests that test for disorders are necessarily the same, or test for the same thing at all. And so interpretations of the data must be taken carefully.

Note: Tye et al used 500nL for all their viral injections. That’s a lot! Did they not worry about diffusion of the virus/aspecific infection to neighboring regions of the brain?

Chaudhury vs Tye: Complexities in Stress-Induced Depression Models

As I was reading this week's papers, I found it most interesting that they seemed to reference each other fairly often in terms of their respective choices of stress paradigms. Both papers address the limitations of their respective stress-induced depression models. It is unclear which set of models is  more viable since VTA dopamine neurons seem to react differently to stress depending on its severity. In this way, the changes in these neurons are "context dependent". This brings about the question of how one might measure the severity of a particular stress paradigm. How do scientists decide whether a source of stress is "mild" or "severe"? One thing I would have liked to have seen done by these researchers is measure levels of cortisol in animals that underwent chronic mild stress as well as the social defeat paradigm. Do both paradigms increase cortisol to the same extent? If they do increase cortisol, how long does the increased cortisol level persist after the stress is removed? Perhaps this can give more insight as to how the different stress paradigms are affecting the animals on another physiological scale. I also found it interesting that both papers agreed that the ability to make rapid changes in this neural circuitry was important in a clinical aspect. Since most anti-depressant medications take 2-3 weeks to take effect, a faster-acting drug would be highly advantageous in treating high risk patients. Both papers briefly mention that ketamine might offer a solution in this way.

I found the approach of the Tye et al paper to be particularly confusing. They exposed their experimental animals to a chronic mild stress paradigm and then tested their motivation using the tail suspension test (TST) and forced swim test (FST). Both the TST and FST seem to be situations that would induce some level of stress in the animals. However, the researchers did not test if the stress from these tests themselves would change VTA dopamine neuron function. This leads me to return to my previous idea about cortisol levels. If cortisol levels were measured after the TST/ FST in naive mice, it may be possible to determine how stressful they are in relation to the chronic mild stress paradigm to eliminate any confounding variables.

9/26 VTA DA Neurons. Tye et al and Chaudhury et al

Summary of Tye et al:

In this study, they hypothesized that VTA DA neurons were involved in the neural circuit responsible for depression-like behaviors. The main findings were that 1) selectively inhibiting VTA DA neurons produces many depression-like behaviors, 2) the depression-like phenotype produced from CMS can be reversed with phasic activation of VTA DA neurons, 3) activation of NAc DA receptors are necessary to express baseline escape-related behavior, and 4) NAc neurons regulate escape-related behavior and phasic activation of VTA DA neurons. 

Questions/Comments for Tye et al:

Why did they use mice AND rats? During the electrophysiological portion of their study, they mentioned that the equipment was too cumbersome for mice and opted to use rats because their escape-related behavior wouldn't be affected as much. So now their neural representation data is from rats while the majority of their behavioral assays were performed on mice. They could have just used rats from the beginning. (In their defense, rats are likely more expensive to house, but that's the only reason I can think of that they'd use 2 different species)

I liked how in the conclusion, they included how other studies also using CMS show a decreased rate of DA neuron firing while studies that use social-defeat models (more severe stress) show an increased rate of firing; it just shows that they've looked at a wider assortment of studies that used different animal models that also looked at the link between depression-like symptoms and DA neurons.

Summary of Chaudhury et al:

In this study, they also looked at VTA DA neurons but instead focused on VTA DA neuron firing rate/firing type, 2 different VTA pathways (NAc and mPFC), and used a social-defeat model instead of CMS paradigm. They hypothesized that increased phasic firing of VTA DA neurons produced a depressive-like (susceptible) phenotype in mice when exposed to social-defeat stress, specifically the VTA-NAc pathway. The main findings were that 1) only phasic firing of VTA DA neurons produced depressive-like behavior in the social interaction and sucrose preference tests, 2) phasic firing of VTA DA neurons changed previously resilient mice to the susceptible phenotype, 3) activation of DA neuronal phasic firing in the VTA-NAc pathway produce susceptibility while inhibition of DA phasic firing in the same pathway produces resilience (the effects of activation/inhibition of DA phasic firing in the VTA-mPFC are reversed).

Questions/Comments for Chaudhury et al:

I'm not really sure how I feel about the social interaction test. I've never really liked the idea of it as an indicator of depression-/anxiety-like/avoidance behaviors. I understand why they'd use it though since they're also using the social-defeat model. The bar graphs for the social interaction tests (specifically Fig 1e, 2c, and 3c) show that when there's no other mouse in the cage, all 3 groups approach the interaction zone with the same frequency. When another mouse IS present, only the eYFP (phasic) and CHr2 (tonic) groups show a significant increase in time spent in the interaction zone. The CHr2 (phasic) mice, however, spend the same amount of time in the interaction zone whether a mouse is present or not. If the amount of time spent in the interaction zone was significantly LESS when a mouse IS present, then I would agree that it's a reliable indicator of avoidance behavior. 

In general, I don't think I was really "sold" on this paper. Toward the end, they claimed "Our study establishes a direct link between VTA DA-neuronal firing patterns and susceptibility to a depression-related phenotype", to which I responded with eyebrow-raising. 

9/26 VTA DA neurons and depression

The two papers being discussed this week, Tye et al and Chaundhury et al, both explore the effects of controlling midbrain dopamine neurons on depression-related behaviors. However, they bring about differences in results, which can be attributed to the models used. Tye et al showed that chronic mild stressors (CMS) decreased firing of dopamine neurons in the VTA, whereas Chaundhury et al showed that social defeat (analogous to more severe stressors) increased the firing of dopamine neurons in the VTA. I found this to be an interesting observation because it shows the complexity of the dopaminergic system in the midbrain and how multiple experiments showing different stressors are needed to show the varying effects of dopamine on depression. Thus, you cannot read Tye et al, for example, and simply say that increased dopamine produces anti-depressant-like behaviors because it differs based on the type of stress. The study of depression can be very problematic in this sense and requires immense time, care and research.

            Last week’s discussion on the effects of serotonin and norepinephrine in the hippocampus on depression and this week’s discussion on the effects of dopamine in the midbrain on depression further show the complexity of the mechanisms of depression and make me want to learn more about these different processes and if they are at all related. In this week’s papers, the effects of modulating dopamine firing on depressive behavior occurred rather quickly, as compared to last week’s slow-acting antidepressant effects on neurogenesis and neural plasticity. Since current antidepressant use on humans is slower acting and can take weeks, researchers should develop more experiments on altering the dopamine system (such as this week’s papers) so that effects can occur swifter in humans. Further exploration in this subject is extremely important because of the complexity of depression and the fact that millions of people are affected by it.  

9/26 Tye/Chaudhury

Both papers pointed out that tests measuring depression-like symptoms aren’t always helpful in studying depression. The Chaudhury paper said that chronic mild stress (CMS) paradigms and physically aversive stimuli inhibit ventral tegmental area (VTA) dopamine neurons, whereas more severe stressors and severe social stressors will increase neuron activity. Because of this, both papers agree that a different method must be used to study depression-like behaviors.

Instead, they suggest studying circuits that are translational between rodents and humans. Both papers researched the VTA-NAc pathway by stimulating dopamine neurons using optogenetics. This has shown to be an important pathway in depression, especially relating to susceptibility vs. resilience. However, they differed by the stressor used to cause the depression-like behaviors.

Tye et al. temporally excited VTA dopamine neurons using channelrhodopsin-2 (ChR2) in a phasic firing pattern. When combined with a twelve-week CMS paradigm, illumination increased kicking and swimming behavior in ChR2 animals. They found that locomotion wasn’t increased in an open field test (OFT), so the increase in movement was from VTA dopamine activation, not just a general increase in locomotion.

Chaudhury et al. investigated the use of optogenetics in resilient vs. susceptible animals that were subjected to a ten-day social defeat stress paradigm. They used ChR2 to phasically induce VTA dopamine neuron firing. Optical phasic firing stimulation of the VTA-NAc dopamine neuronal pathway induced the depression-like phenotype in socially stressed mice. By inhibiting the VTA-NAc pathway by using a halorhodopsin (NpHR), the mice subjected to social stress became more resilient. They discovered that susceptible mice have increased dopamine neuron firing in this circuit.

The results of the two papers suggests that further research should be done on brain circuits to treat depression, and optogenetics seems like a valuable way to approach it.

9/19 Santarelli and Bessa

Santarelli et al. studied the role of anti-depressant induced neurogenesis in treating depression-like behaviors. However, the research from Bessa et al. suggests that anti-depressants treat depression through neuronal remodeling, rather than neurogenesis.

To determine the effects of antidepressants on depression-like behaviors in 5-HT_1a receptor knockout mice, Santarelli et al. used a novelty suppressed feeding test . Bessa et al. subjected Wistar rats to an unpredictable chronic mild stress (CMS) paradigm and used the sucrose preference, forced swim, and novelty suppressed feeding tests to assess depression-like behavior in Wistar rats. They found that the paradigm increased depression-like behaviors, but the symptoms were relieved after 1 week with imipramine and after 2 weeks with fluoxetine.  

Santarelli et al. used knockout mice to model depression. In the discussion, the authors acknowledge the problems with using knockout mice; the absence of the 5-HT_1A receptor during development causes anxious behavior. This could effect the results of their experiment, because the mice’s behaviors could be because of the developmental deficit instead of the depression model. Bessa et al. used the CMS to induce depression-like symptoms in rats. This method has a better translational value to humans than knockout mice. The authors stated that the loss in hippocampal and PFC volume is similar to the reduction in volume that depressed humans experience. In both rats and humans, the atrophy can be reversed by chronic antidepressant treatment. This supports the idea that antidepressants work through neuronal remodeling instead of neurogenesis. It is important for anti-depressant researchers to do studies that are translational to humans so we can create more effective treatments for patients with depression.

Santarelli et al. irradiated the SGZ of the mice to decrease neurogenesis while administering antidepressants.  However, Bessa et al. pointed out that irradiation requires a recovery period before the mice can be given antidepressants, which might affect the results of the experiment. Instead of irradiation, Bessa et al. used MAM, a drug that reduces neurogenesis without any noticeable health deficits.  

Santarelli vs Bessa: A Comparison of Methodology

While reading the papers from Santarelli et al and Bessa et al, I found several intriguing differences in the approach they took to their experiments. The paper from Santarelli et al that was published previous to the Bessa paper, takes a “top-down” approach to answering the question of whether or not neurogenesis is required for the behavioral effects of anti-depressants. They manipulated the 5HT1a receptor and then measured its effects on BrdU staining, latency to feed in novelty suppressed feeding (NSF), and the efficacy of antidepressant treatments. In this way, they picked a molecular mechanism, which they predicted may simulate a depressive phenotype. On the other hand, Bessa et al induced a depressive phenotype by exposing their animals to a chronic mild stress paradigm and tested its effects on many factors such as anhedonia, learned helplessness, NSF, neurogenesis, and dendritic morphology. In some ways, I think that the Bessa paper’s approach is probably a more accurate model of human depression. We don’t fully understand the complex mechanisms of depression in humans, so to assume that a 5HT1a knockout mouse will achieve the same experimental conditions seems like a huge generalization. I also assume that humans with depression don’t have a full knockout of their 5HT1a receptors, since that would have most likely have implications beyond mood regulation.

Another difference between the two papers that caught my eye was the differing approaches in blocking neurogenesis. The Santarelli paper used x-ray radiation which is “likely to induce inflammation and requires a period of recovery before antidepressants can be administered,” according to Bessa et al. They did in fact begin antidepressant treatment concurrently with the x-ray radiation which may have created a confounding variable of the effects of inflammation on the efficacy of antidepressant drugs. The Bessa paper, however, chose to use a drug-based approach by administering methylazoxymethanol (MAM), a cytostatic agent that arrests cell division. In this way, they avoided the issue of the effects of inflammation in the brain, however, their approach was not localized to the SGZ. They did attempt to control the damage done on other areas of the body by keeping the dosage low, but it is difficult to tell if there are subtle effects from this deficit in mitosis. The Santarelli approach did offer some specificity by using lead plates to somewhat limit the area of exposure to the radiation. Because of the limitations of science and technology, scientists sometimes have to "pick their battles" when choosing the methods with which they manipulate their experiments. In these cases, we must ask ourselves if the data in these papers is really comparable because of the severe difference in their methodology.

9/19 - Santarelli/Bessa

Anti-depressant use is a critically important topic that intersects public health and scientific research communities. Given the gravity of the implications of anti-depressant use, it is interesting that research on many of the underlying mechanisms of the drugs lacks consensus from the scientific community. The papers we read this week provide a great window into the somewhat backwards nature of research into why anti-depressants do what they do.

The contrast between Santarelli et. al. and Bessa et. al.  highlighted the importance of clearly delineating every step of the experimental process when writing a scientific paper. Bessa et. al. used individual sections to explain every new item in the study in precise detail as it was presented. While Santarelli et. al. did provide clarity, it was not as clearly delineated, making the article a heavier read. Particularly little attention was given by Santarelli et. al. to describing methods of measuring behavioral change in animal subjects. Without context, methods of behavior measurement in animal models can seem more subjective than is often the standard in scientific research. Bessa et. al. did a superior job in clearly outlining exactly what was done throughout the paper, but particularly in this regard.

With conflicting results, these studies could provide a great jumping off point for neuroimaging studies on the topic of neurogenesis in anti-depressant use. Cerebral blood volume (CBV) measurement and spectroscopy of biomarkers for neuroprogenitor cells have been shown to be useful in identification of adult neurogenesis in animals and are easily translatable to humans. Use of these methods could eliminate the potential post-mortem changes to the brain. Overall, there is great need for further research into this topic as it is of great consequence to the population at large.