A story about how my research trajectory changed
Photo by Nikita Kachanovsky on Unsplash
There are certain comments you don't want to hear during your Ph.D. qualifying exam. "You designed a project that would take 2 students at least 8 years each to complete… and that was just your first aim" is definitely on that list. After fielding questions and flipping through slides for over an hour and a half, I was gearing up to start the second half of my presentation, when one of my committee members stopped me. I had “run out of time”.
An abundance of overly ambitious ideas were still swirling in my head. Then, suddenly, I had to change my approach. How do you do that?
How do we change our minds.... and actions?
One region of the brain (the prefrontal cortex or PFC) is important for executive functions like anticipating, regulating, making decisions, working memory of task-relevant information, and cognitive flexibility. This final cognitive ability underlies the mental work we perform when deciding which of two streets to go down. But, cognitive flexibility is an ability we even use during highly emotional decisions, decisions based on trust (e.g., which mask to wear during a coronavirus pandemic), or ideology (e.g., which candidates to endorse). In true meta(cognitive) fashion, I used cognitive flexibility to start my dissertation research on, of all things, cognitive flexibility--a decision that was wrapped up in both the stress of wanting to do well on my qualifying exam and the vision for my future research trajectory. Although we know it is essential, the mechanisms underlying our ability to think and behave flexibly are commonly misunderstood, at times, to our detriment.
Cognitive flexibility, in its simplest form, is the ability to think about more than one concept simultaneously. Critically, however, we rely on our ability to not only consider two or more concepts but also switch between these concepts and adapt our behavior based on what’s happening around us. A more in-depth definition of cognitive flexibility is the ability of an individual to change cognitive patterns to adapt to changing external stimuli [1]. Throughout my doctoral research, I’ve investigated how a chemical called serotonin is involved in this ability. But first, here’s a little more background on how I chose my potential research question.
Concept 1: Serotonin plays a vital role during development
My research interests are a compilation of the scientific questions that I’ve had throughout my life. As a child, I started asking why we do certain things and feel certain emotions. In high school biology, I started wondering how plant and animal life develop. Later in my college and postbac years, I researched how nerve cells (neurons) become and stay neurons and how these cells use guidance cues to give them directions to the correct destination. I began my Ph.D. program in Neurobiology and Behavior at Columbia University broadly intending to study emotion and cognitive development, leading me to join the lab of Mark Ansorge, who broadly studies the developmental origins of neuropsychiatric disorders.
During development, all nerve cells need to divide and multiply, move to their correct locations throughout the brain, and set up shop--that is, start communicating with other nerve cells. One of the chemicals that serve as a signal shouting out “grow this way” or “set up shop here” early on is serotonin. The use of drugs that elevate serotonin levels around these cells during particularly sensitive developmental time periods can disrupt how nerve cells organize themselves and lead to effects later in life. These effects have been observed in the systems necessary for body sensations, vision, and hearing [2-5, 8]. The Ansorge lab and others have hypothesized and tested whether this is also true of the cognitive and emotional systems [6-8]. In fact, our lab identified a serotonin-sensitive period (in mice), corresponding to the third trimester in humans, that impacts the development of these systems to result in cognitive, anxiety, and depression-related behavior changes later in life. At the public health level, this work provides nuanced and long-term implications for the use of serotonin altering antidepressants during pregnancy on emotional system development [7].
Concept 2: Serotonin plays a vital role during adulthood
Although serotonin has a developmental role as a nerve cell’s traffic light, later in life it also functions as a brilliant communication device through its role as a modulatory “neurotransmitter.” Though I was intent on focusing on development, I found the adulthood roles of serotonin fascinating. Going back to my childhood questions, I wanted to know what drives our feelings and actions. Before joining the lab, I spent my first year rotating in labs investigating the function of serotonin later on in life, particularly how the neurotransmitter regulates emotion and promotes gut motility. Here’s a brief list of what serotonin does:
Mood: Serotonin in the brain is typically thought to regulate anxiety, happiness, and mood. (Think Prozac, Zoloft, and other selective serotonin reuptake inhibitors or SSRIs) [9]
Bowel movements: Serotonin is primarily manufactured in the body's stomach and intestines (not brain!) Serotonin sends a signal to the muscles there to tense up (contract) and squeeze. I’ll leave the rest to your imagination. [10]
Nausea: Serotonin is part of the reason why you become nauseated. Production of serotonin ramps up to push out noxious or upsetting food more quickly in diarrhea (Nope, I didn’t leave it to your imagination). Serotonin also increases in the blood, which stimulates the part of the brain that controls nausea. Drugs (e.g. Ondansetron) that block a specific serotonin receptor subtype prevent nausea and vomiting caused by cancer chemotherapy, radiation therapy, and surgery. [11]
Sleep: Serotonin can be used by the brain to make melatonin, commonly thought of as the sleep hormone. On its own accord, serotonin is responsible for stimulating the parts of the brain that control the sleep-wake cycle through action on specific serotonin receptors. [12]
Bone health: Significantly high levels of serotonin in the bones can lead to osteoporosis, which occurs when bone loss is too great. [13]
Sexual function: Low levels of serotonin are associated with increased libido, while increased serotonin levels are associated with reduced libido. This may have relevance especially for women who take drugs that elevate serotonin e.g., SSRIs (but there’s some hope [14])
Blood clotting: Blood platelets (cells that help prevent bleeding) release serotonin to help heal wounds. Serotonin can act on the platelets themselves as a “glue” keeping proteins on blood platelets stuck together. Additionally, serotonin can cause the narrowing of blood vessels throughout the body but, in small arteries this narrowing can help form blood clots. (15) (I once was a postbac at the National Heart Lung & Blood Institute (NHLBI), so I have a soft spot for serotonin’s role in the blood.)
Migraine: On the flip side, low serotonin levels are thought to dilate blood vessels. Through other downstream mechanisms, lower serotonin concentrations are thought to affect pain signaling to initiate a migraine. [16]
See references for a selection of recent articles and reviews of the topics above [9-16].
Considering the evidence... I change my mind
Photo by Daniel Öberg on Unsplash
I have told you about two (really cool) stories of serotonin activity during different time points: (1) early development and (2) in everyday functions through adulthood. While I was intent on continuing my work in development, I also wanted to dig deeper into the roles of serotonin on cognitive behavior (which is not on the list). Thus, during my qualifying exam, I presented both under the joint title “Developmental serotonin signaling impacts medial prefrontal cortex circuitry to impact adult behavior.”
As I indicated in the beginning, cognitive flexibility is not just holding two concepts in your mind, but changing your mind adaptively with the environment. I wanted to work on both projects, but had not actually considered the time required to complete both. Given the time constraints expressed by my committee member, I narrowed my project considerably. I have since pruned further, in collaboration with my committee and PI, as I read more about the field and built on data from our lab and others.
I am sometimes still tempted to expand my project, but because my objectives are clear I am motivated to stay on track. The research question underlying my first aim is: Does serotonin release in the medial prefrontal cortex play a role in cognitive flexibility?
Where am I now? My dissertation years
I sometimes joke that my dissertation research involves putting mice in boxes with the hopes that one day they will tell me what they are thinking. In some ways, I'm not joking.
The primary experiment I run is a cognitive flexibility task. Although I cannot ask my mice directly, “How did you distinguish between two choices to get the reward? Are you using previous associations or forming new ones?” I can observe their behavior to determine how they are solving the problem.
I use a simplified version of a two-choice digging task. First, we train the mice to dig for a reward: "Hey mouse, there are these two weird, but identical clay dishes filled with bedding, and if you dig to the bottom of it, you will get a delicious treat--a piece of a honey nut Cheerio.” (They love these!) The association they learn is digging in this dish = reward. No matter what.
Once the mice are able to quickly dig and find the reward, I introduce a rule -- where only one pot is rewarded. The rule involves compound stimuli, which here are combinations of odor and texture cues. Examples are cinnamon vs. paprika (odors), and paper crinkles vs. small beads (textures). We mix one of the two odors with one of the two beddings to get 4 compound stimulus pairs. We introduce two of the dishes at a time and ask the mice to choose. If, for instance, the rule is odor-based (i.e. cinnamon), each trial that the mouse digs in the cinnamon-scented dish it gets a cheerio bit regardless of bedding type. Mice are pretty good at this and reach the criterion we set within a dozen or so trials.
This is great, but what I really want to ask my mice is: “How do you change your thinking when the rule changes?”. Knowing how they adapt once the previous association no longer leads to a guaranteed reward may provide valuable insights for cognitive flexibility in humans, and have implications for patients with cognitive impairments. So, I switch things up.
I can change the rules in two ways: within the dimension (e.g., Odor→ Odor or cinnamon to paprika) or between dimensions (e.g., Odor→ Texture or cinnamon to paper bedding).
To test the role of serotonin in this activity, I use a combined optical and genetic technique that allows me to use laser light to activate the release of serotonin in a small part of the medial prefrontal cortex (prelimbic cortex). What I have found so far is that serotonin released in this subregion leads to fewer mistakes across trials of the task when the rules are changed between dimensions (O→T), but not within the dimension (O→O) (probably because this requires a different brain region). We are still uncovering the role of serotonin in this region of the medial prefrontal cortex, but my results may reveal a target for treatment benefiting patients with cognitive impairments due to prefrontal cortex dysfunction. This could lead to treatment of cognitive symptoms in neuropsychiatric disorders such as attention deficit disorders, schizophrenia, depression, anxiety, and Parkinson’s Disease. All in all, I place my work in the context of the list of roles serotonin plays above: We see that serotonin is not just used for emotional, autonomic, and unconscious aspects, but also in every day conscious decisions and cognitive flexibility behaviors [17].
Where are we now? Cognitive flexibility, pandemics, and elections
The decisions we make require cognitive flexibility, whether human or mouse. As humans, though, our agency allows us to make decisions that can be more life-altering than attaining a sugary reward in a lab experiment. We can affect the lives of others around the world by deciding whether to wear a mask during a viral pandemic and whether to check a box on an election ballot. I think we are encouraged to go with “our gut” and make emotion-based decisions, but I believe my mice would say otherwise.
I am not telling anyone to sniff their ballot choices, but in watching my mice correctly master a task with no prior knowledge of the rules, I’ve learned that it really does take a moment of hesitation. A moment of “wait a minute… I thought I knew what was going on, but I need to re-think this.”
The majority* of my mice display this behavior during the rule change (*data based on qualitative observation and not quantitative measurements). Some are dramatic. They hem and haw. They walk from the left side of the arena to the right over and over stopping periodically to sniff the correct and incorrect scented bedding combos. Sometimes they’ll go back and pause in the starting area briefly only to pace back and forth again, as if they are retracing their steps. They are not just highly engaged in the task, they are genuinely weighing their two options. This is in opposition to the mice that run to the choice they “know is right,” only to make what are called perseverative errors (i.e. digging in the previously rewarded, but now incorrect and unrewarded dish).
Like most people, I’m sure, my mind has been pretty “made up”. I have my ballot checking pen and my favorite masks in tow. But, in thinking about my experiments and mice, I’ve learned that it is important to keep your reward in mind. For me, “cheerios” are (1) health and safety for myself and neighbors, especially during this COVID-19 pandemic and (2) in thinking about this election cycle and beyond, increased access and opportunity to physical and mental healthcare, employment, financial security, housing, education, and social and climate justice. An additional “cheerio” is (3) graduating with my doctoral degree (!!).
In moving towards my “cheerios”, I have had to exhibit cognitive flexibility and change my behavior accordingly. I used to believe masks were unnecessary, but now I wear one every time I leave my house. I also went from never taking it off outside the house to finding times when I am alone or far enough away from people that a mask isn't necessary. I have also had to change my mind on who to support in the national political race--I bubbled in a name I hadn’t during the primary. Finally, I told you the story of changing my doctoral research trajectory and anticipate that I’ll need to adapt again before completion.
We use cognitive flexibility to think about multiple concepts and weigh the outcomes of these choices each day. Certainly the stress of surviving (especially during this pandemic) can negatively affect this ability [18, 19], but I believe it is valuable to understand what cognitive flexibility is and how we can use it to improve our decisions and behaviors for a better tomorrow. If, like me, you are attempting to procure a doctoral degree, I hope you also use cognitive flexibility to adapt alongside your research trajectory on the path to your degree.
Edited by: Lara Boyle and Nainika Roy
References
Cognitive flexibility definition
Dennis, John P., and Jillon S. Vander Wal. "The cognitive flexibility inventory: Instrument development and estimates of reliability and validity." Cognitive Therapy and Research 34.3 (2010): 241-253.
Serotonin during development:
Body Sensation (Somatosensory System):
Cases O, Vitalis T, Seif I, De Maeyer E, Sotelo C, Gaspar P. Lack of barrels in the somatosensory cortex of monoamine oxidase A–deficient mice: role of a serotonin excess during the critical period. Neuron. 1996 Feb 1;16(2):297-307.
Luo, Xiaoyan, Antonio M. Persico, and Jean M. Lauder. "Serotonergic regulation of somatosensory cortical development: lessons from genetic mouse models." Developmental Neuroscience 25.2-4 (2003): 173-183.
Seeing (Visual System):
Gu, Qiang. "Serotonin involvement in plasticity of the visual cortex." Monoaminergic modulation of cortical excitability. Springer, Boston, MA, 2007. 113-124.
Hearing (Auditory System):
Hurley, L. M., and I. C. Hall. "Context-dependent modulation of auditory processing by serotonin." Hearing Research 279.1-2 (2011): 74-84.
Emotional (Work from my lab):
Rebello, Tahilia J., et al. "Postnatal day 2 to 11 constitutes a 5-HT-sensitive period impacting adult mPFC function." Journal of Neuroscience 34.37 (2014): 12379-12393.
Gingrich, Jay A., et al. "New insights into how serotonin selective reuptake inhibitors shape the developing brain." Birth Defects Research 109.12 (2017): 924-932.
Various functions:
Teissier, Anne, Mariano Soiza-Reilly, and Patricia Gaspar. "Refining the role of 5-HT in postnatal development of brain circuits." Frontiers in cellular neuroscience 11 (2017): 139.
Serotonin as a neurotransmitter (recent articles):
Mood:
Rancans, Elmars, et al. "Intravenous vortioxetine to accelerate onset of effect in major depressive disorder: a 7-day randomized, double-blind, placebo-controlled exploratory study." International Clinical Psychopharmacology (2020). (<-- This study is disappointing - the SSRI did not shorten the time it takes to alleviate the patients’ depressive symptoms)
Constipation:
Vijayvargiya, Priya, and Michael Camilleri. "Use of prucalopride in adults with chronic idiopathic constipation." Expert Review of Clinical Pharmacology 12.7 (2019): 579-589.
Nausea & Vomiting:
Zarkadas, Eleftherios, et al. "The binding of palonosetron and other antiemetic drugs to the serotonin 5-HT3 receptor." Structure (2020).
Sleep/Wake (Arousal):
Kaur, Satvinder, et al. "Role of serotonergic dorsal raphe neurons in hypercapnia-induced arousals." Nature Communications 11.1 (2020): 1-15.
Bone Healing:
Kumar, Manoj, et al. "Effect of selective serotonin reuptake inhibitors on markers of bone loss." Psychiatry research 276 (2019): 39-44.
Libido:
Fooladi, Ensieh, et al. "Testosterone Improves Antidepressant‐Emergent Loss of Libido in Women: Findings from a Randomized, Double‐Blind, Placebo‐Controlled Trial." The Journal of Sexual Medicine 11.3 (2014): 831-839. (<--This study is encouraging.)
Blood Coagulation:
Bader, Michael. "Serotonylation: serotonin signaling and epigenetics." Frontiers in Molecular Neuroscience 12 (2019): 288. (<--Interesting review with a bit of history)
Migraine:
Burch, Rebecca. "Antidepressants for preventive treatment of migraine." Current treatment options in neurology 21.4 (2019): 18.
Serotonin & Cognitive Flexibility
Amodeo, Dionisio A., et al. "Acute serotonin 2A receptor activation impairs behavioral flexibility in mice." Behavioural Brain Research (2020): 112861.
Cognitive flexibility & stress
Kruczek, Agnieszka, M. Basińska, and Martyna Janicka. "Cognitive flexibility and flexibility in coping in nurses-the moderating role of age, seniority, and the sense of stress." International Journal of Occupational Medicine & Environmental Health 33.4 (2020): 507-521.
Hurtubise, Jessica L., and John G. Howland. "Effects of stress on behavioral flexibility in rodents." Neuroscience 345 (2017): 176-192.
