In the 21st century, anxiety has become a prevalent mental disorder affecting 14% of the global population per year (Bystritsky et al., 2013). Anxiety comes in many forms—generalized panic disorder (GAD), social anxiety disorder (SAD), obsessive compulsive disorder (OCD), post-traumatic stress disorder (PTSD)—but all of them are essentially the anticipation of fearful situations that do not exist in reality, and all of them interfere with a person’s daily functioning  (Miller 2024).  Anxiety tends to affect women more, as well as people in developed countries more (Miller 2024). In the brain, the neurobiology of anxiety centers around the amygdala, the hippocampus, the ventromedial prefrontal cortex (vmPFC), and the anterior cingulate cortex (ACC). Anxiety research has shifted from simply identifying that they exist to studying them directly using neuroimaging technology. However, the research still remains incomplete because of a lack of clinical trials (Bystritsky et al., 2013). This review reflects the lack of clinical trials, and will focus on animal model studies in the amygdala, and the connection of anxiety to glial cells.

Glia provide chemical and physical support to neurons, and are involved in nervous system regulation, signal transmission, synapse formation, and neuroplasticity (Sild et al., 2017). They come in multiple forms in the central nervous system (CNS): oligodendrocytes myelinate neuronal axons, microglia fight infection, and astrocytes ensure transmission between synapses is smooth. Dysfunction of glial cells is postulated to be a critical factor contributing to mood-disorders like anxiety, which has been proved with animal models (Mayegowda & Thomas, 2019). Glia specifically maintain the neurotransmitter balance for the excitatory neurotransmitter glutamate and the inhibitory neurotransmitter GABA (Mayegowda & Thomas, 2019). Under chronic stress conditions, glia undergo remodeling and alter the neurochemistry of the brain in ways that can contribute to anxiety-like behavior. For example, chronic stress exposure has been proven to contribute to the formation of new dendritic spines in the basolateral nucleus of the amygdala, and consequently, the development of anxiety-like behavior (Mitra et al., 2005). Furthermore, dysfunctional glutamate release can also contribute to anxiety, as it plays a significant role in the neurochemistry of stress (Mayegowda & Thomas, 2019). This paper postulates that despite glia not directly contributing to signal transmission, that gene knockout in glia can contribute to anxiety, and as such, also poses a possible avenue of treatment.

In 2015, researchers studied the effects of knocking out oligodendrocyte gene (Olig2) on cortical glutamate levels and the behavior of juvenile mice. They did so by breeding Olig2 knockout mice and a control group. Assays included protein magnetic resonance spectroscopy to study the neurochemicals in the cortex, an enzymatic assay to determine glutamate levels, transmission electron microscopy to study the glutamatergic synapses, and three behavioral tests—an open field test, elevated plus-maze, and cliff avoidance reaction test—to measure anxiety (Chen et al., 2015). They were studying stress and its relation to mood disorders; stress plays a major role in anxiety disorders, and many patients pinpoint disorder onset or triggers to stressful events (Bystritsky et al., 2013). They found that Olig2 knockout mice demonstrated a statistically significant increase in glutamate and in synaptic vesicles within the excitatory presynaptic areas of the cortex compared to wildtype mice. Behaviorally, the mice showed increased anxiety and an increase in impulsive behavior—this demonstrates that knockout of glial genes can impact anxiety behavior in the brain (Chen et al., 2015). This was a revolutionary idea, because the idea of altering glia genetically in order to later impact anxiety has not yet been explored in detail. Though it is impossible to replicate this experiment exactly in humans, since transcription of the Olig2 gene would have to be altered later in life, it is still a possible area of research.

Building off of this paper, a separate study in 2021 looked at knockout of a specific gene, Period2 (Per2), only in glial cells (Martini et al., 2021). Per2’s widely known role is in forming a transcriptional complex with another gene CRY, and inhibiting its own transcription by inhibiting the BMAL/CLOCK transcriptional complex as part of the sleep/wake cycle. However, this study found a lesser known role that Per2 plays in glial cells: balancing mood-related behavior. In the study, they generated mice lacking Per2 via two methods—cross-breeding Per2 floxed with with a Cre mouse line, and deleting Per2 in glial fibrillary acidic protein (GFAP) via Cre to exclude development contributions—to create two models, GPer2 and vGPer2 (Martini et al., 2021). They were assessed for anxiety behavior using the FST and O-maze tests, and they found that after knockout, both strains of mice showed reduced anxiety and spent more time in the open parts of mazes compared to control animals (Martini et al., 2021). At the cellular level in the hypothalamus, inhibitory GABA transporter 1 mRNA was decreased in GPer2 mice, and in the nucleus accumbens, GABA transporter 2 and dopamine receptor 3 mRNA were increased. Interestingly, animals with Per2 deleted only in the glia of the nucleus accumbens and not the hypothalamus did not show reduced anxiety: thus, Per2 in the nucleus accumbens glia is not as important as Per2 in the hypothalamus (Martini et al., 2021). While only recently studied in mice, this study demonstrates that glial genetic knockout does indeed pose as a possible avenue of treatment. Future studies could look at possible Per2 knockout in hypothalamus glial cells if the changes in GABA and Dopamine receptor levels have no side effects.

Glial research is a fascinating field of study; it is only recently being explored as a possible avenue of treatment for mood disorders like depression and anxiety, instead of traditional, neuronal research. There is so much about glia that is undiscovered, as explored here; genetic knockout only on glia is barely explored, but there is a lot of potential. In the future, more knockout studies on different genes in glia could be explored, and eventually, possible treatments or drugs that cause a similar effect in humans. Regardless of what happens, it is heartening to know that anxiety research has expanded from simply neuronal research to glial studies as well, and that so far, they are promising.

Sources

1. Bystritsky, A., Khalsa, S. S., Cameron, M. E., & Schiffman, J. (2013). Current diagnosis and treatment of anxiety disorders. P & T : A Peer-Reviewed Journal for Formulary Management, 38(1), 30–57.
2. Mayegowda, S. B., & Thomas, C. (2019). Glial pathology in neuropsychiatric disorders: a brief review. Journal of Basic and Clinical Physiology and Pharmacology, 30(4). https://doi.org/10.1515/jbcpp-2018-0120
3. Sild, M., Ruthazer, E. S., & Booij, L. (2017). Major depressive disorder and anxiety disorders from the glial perspective: Etiological mechanisms, intervention and monitoring. Neuroscience & Biobehavioral Reviews, 83, 474–488. https://doi.org/10.1016/j.neubiorev.2017.09.014
4. Mitra, R., Vyas, A., Chatterjee, G., & Chattarji, S. (2005). Chronic-stress induced modulation of different states of anxiety-like behavior in female rats. Neuroscience Letters, 383(3), 278–283. https://doi.org/10.1016/j.neulet.2005.04.037
5. Chen, X., Zhang, W., Li, T., Guo, Y., Tian, Y., Wang, F., Liu, S., Shen, H.-Y., Feng, Y., & Xiao, L. (2015). Impairment of Oligodendroglia Maturation Leads to Aberrantly Increased Cortical Glutamate and Anxiety-Like Behaviors in Juvenile Mice. Frontiers in Cellular Neuroscience, 9. https://doi.org/10.3389/fncel.2015.00467
6. Martini, T., Ripperger, J. A., Stalin, J., Kores, A., Stumpe, M., & Albrecht, U. (2021). Deletion of the clock gene Period2 (Per2) in glial cells alters mood-related behavior in mice. Scientific Reports, 11(1), 12242. https://doi.org/10.1038/s41598-021-91770-7