My name is Sanjana Venkataraman and I am a Senior majoring in Psychology. Apart from my work with Brain Matters, I enjoy photography and have been a part of the Flashpoint Photography Club. I also enjoy travel and singing. I have worked as a research assis- tant at the Rhodes Lab since I was a Freshman and enjoy doing research in Behavioural Neuroscience. My research experience and interest in playing an active role in the communication of Neuroscience motivated me to write for Brain Matters. There are numerous widely accepted short-term and longitudinal benefits of exercise. Established health benefits include the prevention of sev- eral diseases and illnesses, lowering the risk of cardiovascular disease, diabetes, and certain kinds of cancers (Clague and Bernstein, 2012; Chapman et al., 2013). Exercise also positively impacts mental health by reducing likelihoods of developing depression and anxi- ety, and generally improving overall quality of life. These positive effects of exercise make it a rich, constructive area of study in the fields of psycholo- gy and neuroscience (Mandolesi et. al., 2018). Moreover, physical exercise has been known to positively impact the process of acquiring knowledge, or cognition in humans as well as rodents. There is evidence to support the idea that voluntary exercise has aided in combating some biological symptoms of diseases associated with the degeneration of the brain and nervous system (Adlard, et al., 2005; Voss et al., 2013). Apart from studies that show improved cognitive ability in older adults, exercise has also shown to improve cognitive health across the lifespan (Voss et al., 2011). This has been further established by examining different regions of the brain and their relationship with physical activity. A diverse variety of research has examined the changes in brain regions associated with exercise, with the most outstanding being those that take place in the region widely known to be responsible for learning and memory, the hippocampus. Studies in humans have consistently corroborat- ed this, with randomized clinical trials demonstrating increased volume and blood flow to the hippocampus (Chad- dock–Heyman et al., 2016; Erickson et al., 2009). Across all brain regions that have been studied, the hippocampus is considered to be the most robust in its association with exercise. Hippocam- pal activity has also been correlated with the speed of running (Li et al., 2012; Chen et al., 2011). Neuronal in- dicators of hippocampal activity when mice voluntarily run on wheels have also raised the question of whether the activity generated by exercise is chiefly responsible for the observed survival of new cells in the dentate gyrus, a sub region of the hippocampus (Clark et al., 2010, 2011), adding another dimension to the improved cognitive effects of exercise. Given the abundance of research on exercise induced neuro- genesis and its effects on improved cognitive behaviour and function, it is important to scrutinize the crucial connection between the hippocampus and physical activity, so that we better understand the origins of the brain and behaviour related benefits of exer- cise. By doing so with recent and well established lines of research that have pervaded the neuroscientific commu- nity, upcoming research can be contex- tualized appropriately and appreciated for its relevance to our own daily lives. Neurogenesis is the growth of new neurons in the brain and has been studied substantively over the last two decades. Rodent models have consistently supported the relationship between exercise and neurogenesis, suggesting that exercise increases the total number of neurons in the hippo- campus by two to six-fold (Mustroph et al., 2012.; Rhodes et al., 2003). There are two primary hypoth- eses that address the origins of hippo- campal neurogenesis. The first one is the muscle hypothesis that postulates signals associated with this neuro- biological change originates from contracting muscles that are actively engaged during exercise (Wrann et. al. 2013). The second leading hypothesis, which will be the focus of this review, suggests that the signals are generated from within the brain itself. This hy- pothesis holds that interactions within the central nervous system rather than the peripheral are associated with increased hippocampal plasticity, neurogenesis, and improved cognitive performance. Running results in the imme- diate activation of the brain that lasts for the duration of the exercise. This can be seen through the lenses of theta oscillations and gamma oscillations as well as immediate early gene (IEG) induction. Immediate early genes are genes that are activated briefly in re- sponse to external stimuli thus becom- ing markers of neuronal activity. Theta oscillations are neural oscillations that occur in the brain and are best viewed through an electroencephalogram (EEG). Theta waves have been rigor- ously studied in relation to the hippo- campus. Hippocampal theta activity is observed during several different tasks, an important one being motor activity. Theta activations have been related to several different functions Origins of Exercise-Induced Neurogenesis 8 like memory, motor behavior, and attention. Studies have demon- strated the positive correlation be- tween voluntary movements and theta oscillation in the hippocampal forma- tion or HPC (the hippocampus and related structures of the dentate gyrus and subiculum). In a running wheel experiment, the speed of running ini- tiation was directly related to the onset frequency of HPC theta oscillations (Bland and Oddie, 2001, Vanderwolf, 1969). Correlations between speed and hippocampal theta have been reported in several different studies. Li et al. (2012) reported that theta fre- quency is correlated with speed during the preparatory and initiation phases of wheel running while theta rhythms of middle frequency (6.5-9.5 Hz) are correlated with speed consistently throughout an entire wheel running episode. Hippocampal gamma waves mirror this speed-dependent effect. Gamma waves are a neural oscillatory pattern implicated in working memory and attention. Disruptions in gamma rhythm are commonly seen in disor- ders such as epilepsy and Alzheimer’s disease which further validate its significance and its role in exercise and hippocampal activity. Mice running on linear and Y-shaped paths showed speed modulated increases in gamma frequencies. Interestingly, theta-gam- ma coupling – the process whereby low frequency that oscillations modu- late high frequency gamma oscillations – was also observed to increase with speed (Chen et al., 2011; Ahmed and Mehta, 2012). Moreover, research into IEGs corroborates the phenomenon of acti- vation in the brain following an acute bout of exercise. IEG-positive cells in the dentate gyrus and other regions of the hippocampus are significantly positively correlated with average run- ning speed over an acute 90-minute period prior to euthanasia (Rhodes et al., 2003). A study using the IEG c-fos (an indicator for neuronal activity) demonstrated that rodents running on treadmills at higher speeds showed gr- eater c-fos expression or neuronal ac- tivity in sub-regions of the hippocam- pus, specifically the dentate gyrus and CA1 and CA3 areas (Lee et al., 2003). Wheel running is a commonly used model of exercise and has been implemented extensively in the study of exercise-induced neurogenesis. Early studies established that voluntary running in mice given access to wheels was enough to increase neurogenesis. Mice injected with BrdU, an analog for thymidine that incorporates itself into the cellular DNA and acts as a marker for neurogenesis, demonstrated twice as many surviving proliferating cells after being given access to a wheel for an extended period of time (Mustroph et al., 2012; van Praag et al., 1999). In a study by Clark et al. (2010), we see an even clearer direct connection between IEG expression and neurogenesis in wheel running mice. The study examined the induc- tion of the c-fos and neurogenesis in terms of cell proliferation and survival of new neurons in the dentate gyrus. Results indicated that cell survival at the 25-day mark after the last BrdU injection appeared to double in wheel running mice. Exercise-induced c-fos expression also appeared to elevate and attenuate in accordance with cell survival, corresponding with peak levels of neurogenesis in the initial days of running. It is worth noting that elevations in exercise-induced c-fos expression and cell survival coincid- ed with one another, adding another dimension to neuronal activity and The abundance of evidence in support of the central hypothesis makes it hard to dispute the role of endogenous brain activity in hippo- campal neurogenesis. Recent research has investi- gated a new perspective on exercise - brain interactions, focusing on the idea that factors released from the muscles in the peripheral nervous system themselves travel across the blood brain barrier, and form the basis for hippocampal neurogenesis. Among the initial molecules studied were insulin-like growth factor (IGF) and vascular endothelial growth factor (VEGF) (Rendeiro and Rhodes, 2018). The recently studied FNDC5, is a protein that has its expression regulated by PGC-1α, another protein released from contracting skeletal muscles. A study demonstrated that FNDC5, when delivered through peripheral injections, results in in- creased levels of the cleaved product of FNDC5, a protein called irisin (Wrann et al., 2013). Several other studies have corroborated the role of irisin/FNDC5 in the muscle hypothesis, some in conjunction with the neurotrophic factor brain derived neurotrophic fac- tor (BDNF) (Delezie and Handschin, 2018; Lourenco et al., 2018). BDNF, a member of the neurotrophin family, is among the most robustly researched factors that are associated with exercise induced hippocampal neurogenesis. BDNF has been demon- strated to peak in its expression a few Schematic coronal view of a rodent brain section outlining the hippocampus, and within it, the dentate gyrus, CA1 and CA3 regions. Adapted from “Reovirus Infection and Tissue Injury in the Mouse Central Nervous System Are Associated with Apoptosis by Oberhaus,” S. M. et al., 1997, Journal of Virology, 71(3), 2100–2106 Graphic illustrating the effect of exercise on the relationship between BDNF, FNDC5 and PGC-1α, and their collective influence on neuro- genesis and learning. Adapted from “Exercise Induces Hippocampal BDNF through a PGC-1α/FNDC5 Pathway,” Wrann, C. D. et al., 2013, Cell Metabolism, 18(5), 649–659 9 weeks after birth, during the change from embryonic phase to adult phase in neurogenesis in mice. It is also shown to increase through gene expression in the hippocampus from treadmill and aerobic exercise (Liu and Nusslock, 2018). In wheel running mice, levels of BDNF mRNA within the dentate gyrus of the hippocampus increased within a few days of exercise itself (Neeper et al, 1995). Further, hippocampal plas- ticity is also seen as an outcome of the administration of AICAR. AICAR can be defined as an agonist of the enzyme AMP-activated protein kinase, (AMPK) which works by restoring cel- lular energy levels, i.e. ATP, when they are low by transporting glucose-oxi- dizing fatty acids in response to muscle contractions such as those that occur during exercise (Schimmack et al., 2006). In a study by Guerrieri and van Praag (2015), mice were administered with AICAR while remaining seden- tary or tasked to run with a vehicle (control substance) injection for vary- ing periods of time. Mice in the group that were administered with AICAR and that were made to run for 14 days showed significant increases in AMPK levels compared to the control group. Furthermore, both 7 and 14-day run- ning groups showed a higher number of BrdU positive cells compared to controls, indicating neurogenesis (Guerrieri and van Praag, 2015). AMPK is also implicated in research aiming to establish the role of the myokine Cathepsin B (CTSB) in exercise - brain interactions. In a simulation of exercise conditions, AICAR was administered in vitro to muscle cells. CTSB protein levels in the cultured cells increased significantly at 6 hour and 12 hour time points when treated with AICAR as compared to controls. The same study also reported elevated levels of CTSB plasma levels due to running as well as elevated CTSB expression in the hippocam- pus (Moon et al., 2016). The extent to which several of these factors, when isolated or operating in combination with one another can solely be con- sidered responsible for the benefits of running and exercise is very much in dispute. More research in this area is needed for a cause-and-effect relation- ship to be formed between circulating myokines and effects of exercise (Ren- deiro and Rhodes, 2018). Having now established the consistency with which we see neu- rogenesis due to wheel running and myokines, as well as concomitant with hippocampal activity, it is important to revisit one of the driving forces behind this area of study. Neurogenesis has several important implications for be- haviours demonstrating learning and memory and cognitive performance in general. Tasks like Novel Environment Exploration and the Morris water maze form an important part of the repertoire of measures that aim to as- sess learning and memory in rodents. Mice with running wheels had twice the number of new neurons compared to sedentary mice and hence displayed a two-fold increase in the number of Zif268+ (protein product of an IEG) cells following tasks like Novel envi- ronment exploration and the Morris Water Maze, both of which are tests to gauge spatial learning and mem- ory. This suggests that the neurons generated by running are recruited in hippocampus-engaging tasks and be- haviours. Therefore, wheel running-in- duced neurogenesis can potentially play a functional role in behaviours that engage the hippocampus (Clark et al., 2012). These findings with improved mea- surable behaviour are also support- ed by studies exploring the subject from a muscle-brain axis perspective. Wild type mice treated with AICAR demonstrate improved spatial mem- ory (Kobilo et al., 2011). The study by Moon et al. (2016) found that CTSB knockout mice that were made to run did not display improvements in spa- tial memory as gauged by the Morris Water Maze task. Recent advances in molecular biology, such as optogenetics (the use of light to manipulate cellular activity through micro-LED implants) and cel- lular tracing techniques, have enabled us to identify and localize cells within the hippocampus that play significant roles in memory. Furthermore, cells responsible for particular brain func- tions like those relating to memory, can be manipulated to recapitulate an experience as if it were happening in real time (Liu et. al., 2014). Artificial recapitulation of complex experiences like exercise would allow us to assess the role of brain activity in neurogene- sis. Moreover, attempts are being made to isolate the main physiologi- cal constituents of exercise. The novel e-stim model attempted to evaluate the extent to which electric stimulation of hindlimb muscles in mice while they are anaesthetized or do not have their hippocampus activated, is sufficient to increase hippocampal neurogene- sis. Interestingly, there was a greater proportion of BrdU positive cells in the dentate gyrus of the e-stim mice compared to a control group that was given a sham treatment, but the cells were identified as astrocytes, cells that support and nourish neurons (Gard- ner et al., 2020). This brings us closer to delineating the causal mechanisms that are at play and other processes like astrogliogenesis (formation of new astrocytes) that may occur as a result of exercise. The growing body of research on the relationship between exercise like hippocampal activation as well as i) Representative images of IEG cfos indicating neuronal activity in the dentate gyrus during control, mild, moderate and severe treadmill exercise. 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