Serotonergic System: What is it and is it related to our Cognition?


In popular discourse, serotonin is often called a happy hormone, with phrases like 'serotonin boost' tend to mean to get or be happy. A definition of serotonin that is considered very dry by many people is that serotonin is a monoamine neurotransmitter. It is called 5-hydroxytryptamine as people with an interest in chemistry, biology and other allied sciences would know. The purpose of this article is not to get into the chemical composition details of serotonin, which can be discussed later.


For this article, first, let’s break down the definition of serotonin-mono as we all know means one, the amine is a type of compound that is derived from ammonia, and neurotransmitter is a chemical that is present in each one of us, it helps to transfer messages and signals within and between neurons (neurons are nerve cell, many neurons come together to form one nerve cell). Now, this is where it starts getting interesting; our bodies constantly transmit messages with the help of these neurotransmitters. Imagine how electricity travels through wires, similarly, our body is like this huge powerhouse, constantly transmitting and receiving these electrochemical signals. Without these, we might not be able to do most of our day-to-day activities or even engage in thinking.


Serotonin, however, is also a hormone, so it kind of acts double duty as a neurotransmitter and a hormone. Serotonin is the primary hormone responsible for mood stabilization, emotions of well-being, and happiness (hence the popular connotation of ‘happy hormone’). In fact, most serotonin is found outside the central nervous system(CNS). Almost all of the 15 serotonin receptors are present both outside and inside the brain.

The functions of serotonin, or the serotonergic system in general, are so vast that even medical science has not been able to fully explore them even after 70 years of its discovery. We are talking about functions like consciousness, attention, cognition, and emotion as well as bone metabolism, cardiovascular health, eye health, blood clotting, sleep, and neurological disorders.

As it is evident, serotonin really is a very important component for our bodies and our brains, a slight change here and there can change the way we think, perceive, or function in our day-to-day lives. It is far more than just a happy hormone. And since it has such a wide range of functions, drugs which target the serotonergic system often come with a lot of side effects (Berger et al. 2009).


In Cognition

In a study done by Ligaya, K. et al to study animals’ learning processes and speed, the researchers exposed mice to a learning task, in which the aim was to find water. They had to poke either a water-dispenser on their left or a water-dispenser on their right, which would either dispense water or not. What the researchers found was that there were differences in the intervals in which the mice tended to wait before making their next try.


Sometimes they would try repeatedly in a very short span of time, other times they would wait longer. The first strategy employed the working memory(the decision was made based on their immediate experience), and the latter employed long-term memory (using the knowledge that was built over time rather than immediate experiences). When serotonin neurons were artificially triggered using light (optogenetics), mice were able to alter their behavior more quickly in a setting. The researchers concluded that higher serotonin levels amplified how effectively and effortfully the animals learned from previous experiences or knowledge gathered over time. This, however, only applied to choices made following longer waiting intervals which shows that the choice that the mice made depended on their activation of serotonergic neurons, it affected their rate of learning by making them employ the long-term memory (a sign of higher cognitive function).


What does this tell us? This tells us that serotonin boosts brain plasticity by influencing the rate of learning. It might seem that since the study was done on mice, it might have lesser validity. But that is how most scientific experiments proceed and experiments on animals are still used to shed light on human anatomy and behavior. Besides the aforementioned study, healthy volunteer studies have provided support, of varying strength, for the role of 5-HT (serotonin) in a number of human cognitive functions. The most robust evidence is available for human episodic memory functions and particularly the detrimental effects of the low brain 5-HT on long-term memory(just like in the mouse study).


What happens to cognition when serotonin lowers


Now, the question arises against what happens to our cognition when the serotonin level lowers in our body. According to new findings, loss of serotonin may drive cognitive decline rather than simply being a by-product of memory problems, showing a much greater role of serotonin than previously predicted.


The John Hopkins University study that is being referred to in the previous sentence, included 28 adult participants with mild cognitive impairment (MCI) and 28 healthy, cognitively normal, demographically matched control persons who underwent various neuroimaging studies. The results that were found have implications on the whole discourse of serotonin and cognition. In comparison with control participants, for patients with MCI, levels of serotonin transporters were lower in cortical and limbic areas that are typically affected by pathologic processes in Alzheimer's. Levels were also lower in sensory and motor areas, the striatum, and the thalamus, which are relatively spared in AD.


A brain showing decreases in serotonin transporters (blue) in the whole mild cognitive impairment group compared to the whole healthy control group.


Since the serotonin system is affected in the early stages, before memory problems are severe enough to meet the criteria for dementia, interventions can be made to target this stage of cognitive impairment. That is, if the serotonergic system is targeted at the right time, by preventing the loss of serotonin or introducing a substitute neurotransmitter, we could slow or stop the progression of Alzheimer’s disease and perhaps other dementias. This finding can radically change how we treat dementia.


Conclusion


Linkages have definitely been established between serotonin and cognition. However, caution needs to be exercised while claiming causality. This is because, according to Schmitt et al (2006), the effects generated are quite selective, and it may be easier to induce performance decrements than to enhance cognitive performance in healthy volunteers who have a close to optimal performance level. Also, relatively few serotonin challenge studies have been performed, using varying pharmacological manipulations, dosages, methods of administration, and treatment regimes, which all can affect the actual level of central serotonin.


Most theories of the causes of psychological disorders involve serotonin, and as a result, medication to regulate serotonin has been developed, like SSRIs (selective serotonin reuptake inhibitors). But, as mentioned previously in the last paragraphs, these are merely theories and the case is similar for most other functions and effects of serotonin- we know it is involved but we do not know exactly how.


Lastly, as it might be evident from this article, we are only still scratching the surface of this neurotransmitter hormone, and further research might give us results of tremendous importance, which can be used to better human living.

 

Unwired India is a neurotech-startup that aims at integrating state-of-the-art research and developments in STEM, for catalyzing the transition of Neuroscience to Neurotechnology. We develop avant-garde non-invasive neurostimulation products used to solve some of the world’s most critical global issues and challenges. Our mission is to take cutting-edge brain research directly into the lives and homes of people, thereby fostering a unique culture of sustainable neuroscience and scientific literacy in India.

  • Founded in 2020, we are the pioneers of Nootropics and non-invasive Neurotechnology devices in the country, and offer so much more than high-quality, delicious Brain Nutrition products for daily cognitive support; a full-service health and fitness startup that has become an important part of the local community, here in New Delhi, India.

  • We develop non-GMO, all-natural nootropic (smart-drug) formulations, Himalayan herb blends, and specialized amino-nutraceutical interventions and supplements for enhanced brain function, cognition, neuroinflammation, and neurodegeneration.

 

A friendly reminder: We've done our research, but you should too! Check our sources against your own and always exercise sound judgment.



https://clinicalconnection.hopkinsmedicine.org/news/new-evidence-links-serotonin-and-mild-cognitive-impairment

https://www.psycom.net/serotonin

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5864293/pdf/nihms949956.pdf 

https://www.sciencedirect.com/science/article/abs/pii/S0969996117301109

https://www.medscape.com/viewarticle/884572

Iigaya, K., Fonseca, M.S., Murakami, M. et al. (2018). An effect of serotonergic stimulation on learning rates for rewards apparent after long intertrial intervals. Nat Commun 9, 2477 . https://doi.org/10.1038/s41467-018-04840-2

Schmitt, Jeroen & Wingen, Marleen & Ramaekers, Johannes & Evers, E & Riedel, Wim. (2006). Serotonin and Human Cognitive Performance. Current pharmaceutical design. 12. 2473-86. 10.2174/138161206777698909. 

Frederick S. Barrett, Jin Hui Joo, Najlla Nassery, Alena Savonenko, Devin J. Sodums, Christopher M. Marano, Cynthia A. Munro, Jason Brandt, Michael A. Kraut, Yun Zhou, Dean F. Wong, Clifford I. Workman. Molecular imaging of serotonin degeneration in mild cognitive impairment, Neurobiology of Disease, Volume 105, 2017, Pages 33-41, ISSN 0969-9961,https://doi.org/10.1016/j.nbd.2017.05.007.

Image Credits: Gwen Smith Labs and Photo by Anna Shvets from Pexels.com .


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About the Author


Priyambada Kashyap


Program Trainee, The Aryabhata-Shankara Neurotech Society (Unwired India)


Hailing from Assam, India, Priyambada Kashyap is a neurotech aspirant, who feels her life should have a perfect balance of everything in the universe from dance to academics. Pursuing her major in Psychology, every step of the way, she wonders about the various ways humans are influenced in their daily lives, as well as how embedded we really are in our social context. Her fascination lies in researching various psychological phenomena around counseling psychology, social psychology, biopsychology, educational psychology, political science, history, political psychology, and even a tad bit of philosophy.







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