OH NO!- Comorbidity!

Autism Spectrum Disorder (ASD) is a neurodevelopmental condition that is known for its challenges in social situations such as social communication, repetitive behaviors, and problems with social cues. Those with ASD have difficulty expressing and understanding emotions, however this expands beyond the normal symptomology

Comorbidity is an additional disease on top of a primary diagnosis. They are more common in those with ASD than those without. Research shows a high rate of comorbidities- about 70% of those with ASD have co-occuring conditions that blur diagnosis and treatment for patients with ASD. Symptoms that develop with comorbidities add to the original symptoms of ASD making a healthy life difficult to have.

Genetic complexity

Autism does not have a single genetic cause. The developmental disorder is influenced by a multitude of factors. Primarily, multiple genetic factors like genetic mutations or copy number variations (CNVs). CNVs are a type of structural variation in the genome where DNA is deleted or duplicated. This changes normal function of genes and changes the number of gene copies present therefore, leading to developmental issues.

Fragile X Chromosome (FXS)

FXS is one condition that is associated with autism that is linked with DA dysfunction. It is the number 1 inherited cause of a wide range of intellectual disabilities. 1 in 3 individuals with ASD have FXS. This condition results from the repetition and mutation of the gene FMR1 which null mutants have significant increase in the synthesis of DA and 5-HT which is another name for serotonin- a neurotransmitter in charge of various functions including mood and helping out the nervous system.

In Figure 2, a typical chromosome the FMR1 gene will repeat just enough to maintain regulation for CGG repeat sequence, which will not interfere with functioning of the gene, however, a fragile X chromosome will exceed the number of regulated CGG repeat sequence and the FMR1 gene will slice, leaving an absence of the gene. Without the FMR1 gene neurons struggle to make synaptic connections in the brain.

When working with CNV’s you’re working with a lot of DNA! CNV’s make large changes to genetic material. This is one of the reasons why they are common in people who are neurodivergent. These genetic risk factors result in subtypes that  ultimately have a cascade of behavioral symptoms. For example, deletion of specific gene like 16p11.2 can result in delayed speech and struggling with social intelligence. So rather than focus on one gene, like finding a needle in a haystack, its better to try and adapt and understand the subtypes and comorbities of ASD.

The Role of Dopamine and Dopamine Transporter DAT

Dopamine (DA) is a widely known neurotransmitter that influences an array of functions within the brain. As one of the top three numerous neurotransmitters found in the brain, dysfunction of this molecule can lead to neurodevelopemental and psychiatric disorders including Autism. Too much or too little dopamine in those with ASD cause symptoms such as issues with sensory processing, repetitive behaviors, motor control and social reward processing.

There are two primary dopamine receptors, D1 and D2. These receptors are critical for managing the effects that dopamine has on the brain. Dysfunction of these receptors has been proven to be linked in ASD. These two receptors contribute mainly to behavioral disruptions that are present in ASD such as, social deficits, behavioral flexibility, and cognitive attention.

D1 vs D2

D1 Receptors: During activation D1 receptors  are primarily known for their synaptic plasticity, the strength in connection between neurons, and facilitating excitatory signaling, which is increasing neural activity. Synaptic plasticity and facilitating excitatory signaling regulates executive function and cognitive flexibility. They’re crucial for learning and memory. Dysfunction in D1 leads to deficits in communication and attention.

D2 Receptors: Unlike D1 receptors, D2 is mainly involved  inhibitory signaling-which is the decrease of neural activity- and involvement with motor control and coordination, in ASD, an overactive reward system leads to repetitive behaviors. In this situation the brains reward pathways are eing hyperstimulated and overly sensitive. When D2 receptors are not functioning there is no ‘manager’ to modulate motor control and coordination within movements.

DAT is the dopamine transporter in charge of regulating dopamine. The process in which dopamine levels are regulated from the synapse back into the neuron This means a dysfunction in DAT is a dysfunction in dopamine signaling. DA signaling drives behavioral activation which increases with reward rate.

What does This Have to do With Comorbidities? 

ASD and its comorbid conditions can help research within dopamine receptors to understand where dysregulation occurs. ASD contributes to neurological development of existing disorders as behavior, cognition, and emotion are all affected. Comorbidities are harmful to an individuals quality of life as with autism, it can be difficult to express how you are feeling. If we evaluate dopaminergic dysfunction we can understand core symptomology and be closer to the answer.

Al-Beltagi M. (2021). Autism medical comorbidities. World journal of clinical pediatrics, 10(3), 15–28. https://doi.org/10.5409/wjcp.v10.i3.15

DiCarlo, G. E., & Wallace, M. T. (2022, February). Modeling dopamine dysfunction in autism spectrum disorder: From invertebrates to vertebrates. Neuroscience and biobehavioral reviews. https://pmc.ncbi.nlm.nih.gov/articles/PMC8792250/

Artstract created by Ren Lind

There are many different theories as to the causes of Autism, however, Fragile X Syndrome is a direct cause for an estimated 2-6% of Autism cases. [1] Theories for other causes include genetics, environmental factors, the communication between neurons being disrupted, and others, but today we’ll be focusing on Fragile X.

Fragile X Background

Fragile X Syndrome (FXS) is an inherited genetic disorder. It gets its name from the X chromosome appearing “fragile” compared to a typical X chromosome, as figure 1 depicts. 

FXS individuals have a mutation on the FMR1 gene. This gene is responsible for making a protein that manages and develops synapses between neurons. Synapses are where communication between neurons gets passed along and important messages can be spread to the brain, or the body so bodily processes can happen. 

This gene mutation can lead to behavioral and social challenges, intellectual deficits, alterations in physical features, anxiety, and delayed speech and learning in childhood, among other symptoms. Typically, FXS is diagnosed between 12 months and 3.5 years old, but it can be diagnosed later in life. Symptoms of FXS greatly overlap with symptoms presented in Autism Spectrum Disorder. 

Autism Background

Autism Spectrum Disorder (ASD) is characterized by social deficits and repetitive motions or interests throughout life. [4] Symptoms such as social anxiety, avoiding eye contact, social impairment, and other diagnostic symptoms overlap between FXS and ASD. 

FXS causes Autism because the symptoms presented can be the same, depending on which symptoms the individual with FXS displays. It’s important to note that not all people with FXS will be diagnosed with ASD because they may have different symptoms than those of Autism. However, about 60% of males with FXS have Autism, and 20% of females with FXS have Autism. [5]

ADHD and Seizures Co-occurring 

Interestingly, there are also comorbidities of Attention-Deficit/Hyperactivity Disorder (ADHD) and seizure for people with Fragile X and Autism. Around 50% of people with Fragile X and ASD also have ADHD. [6]

Summary

Autism has many theorized causes and risk factors, but Fragile X Syndrome is a confirmed cause for a small percentage of Autism cases. This occurs because Fragile X can present the same way as Autism, creating an overlap between the conditions, however, not every person with Fragile X will have the same symptoms as Autism. 

Resources

[1] Rajaratnam, A., Shergill, J., Salcedo-Arellano, M., Saldarriaga, W., Duan, X., & Hagerman, R. (2017). Fragile X syndrome and fragile X-associated disorders. F1000Research, 6, 2112. https://doi.org/10.12688/f1000research.11885.1

[2] Image from https://healthjade.net/fragile-x-syndrome/

[3] Rajaratnam, A., Shergill, J., Salcedo-Arellano, M., Saldarriaga, W., Duan, X., & Hagerman, R. (2017). Fragile X syndrome and fragile X-associated disorders. F1000Research, 6, 2112. https://doi.org/10.12688/f1000research.11885.1

[4] Dicarlo, G., Wallace, M. (2022). Modeling dopamine disfunction in autism spectrum disorder: from invertebrates and vertebrates. Neuroscience and Biobehavioral Reviews, 133. https://doi.org/10.1016/j.neubiorev.2021.12.017

[5, 6, 7] Rajaratnam, A., Shergill, J., Salcedo-Arellano, M., Saldarriaga, W., Duan, X., & Hagerman, R. (2017). Fragile X syndrome and fragile X-associated disorders. F1000Research, 6, 2112. https://doi.org/10.12688/f1000research.11885.1

[8] Dicarlo, G., Wallace, M. (2022). Modeling dopamine disfunction in autism spectrum disorder: from invertebrates and vertebrates. Neuroscience and Biobehavioral Reviews, 133. https://doi.org/10.1016/j.neubiorev.2021.12.017

The human body is a finely tuned machine, constantly regulating pain, mood, metabolism, and more to maintain balance. One important system responsible for this regulation is the endocannabinoid system (ECS), a network of cannabinoid receptors(CB1 and CB2) , endocannabinoids (Anandamide (AEA) and 2-Arachidonoylglycerol (2-AG) ) , and enzymes (FAAH and MAGL) that work together to keep everything running smoothly. This means that the ECS helps regulate things like pain perception, mood, immune function, and metabolism. It’s constantly monitoring the body’s needs and adjusting accordingly, ensuring that everything stays in balance. [1]

• Endocannabinoids (AEA and 2-AG) are produced when needed and act locally to help regulate these functions.
• Cannabinoid Receptors (CB1 and CB2) are activated by these endocannabinoids to trigger specific responses.
• Enzymes (FAAH and MAGL) break down these molecules once they’ve done their job, ensuring the ECS doesn’t overdo it.

But maintaining balance isn’t as simple as turning functions on and off. If the ECS activated all its pathways randomly or continuously, it could lead to dysfunction instead of stability. That’s why it relies on selective signaling, only activating specific pathways when needed to ensure precise control.

Therefore, understanding how the ECS’s selective signaling works is essential for improving health. By learning how to support this system, we can regulate vital functions more effectively while avoiding unwanted side effects, helping the body stay in perfect balance.

So, how does the ECS know when and where to act? It carefully monitors the body’s needs and responds accordingly. Without selective signaling, ECS pathways would activate constantly and unpredictably, disrupting balance rather than maintaining it.

What Is Selective Signaling and Why Does It Matter?

Selective signaling is the ability of the ECS to target specific pathways or receptors at just the right time and place [2]. It’s like a light switch, when you need light in a room, you flip the switch, and it turns on just the right amount. If the light was on everywhere, all at once, it would be too bright and overwhelming. 

The ECS uses selective signaling to activate only the receptors that are needed in specific tissues, reducing unnecessary effects elsewhere. This process is achieved through factors such as GPCR signalling and β-Arrestin Pathway.

The GPCR Pathway in the Endocannabinoid System (ECS)

CB1 and CB2 receptors are G-protein coupled receptors (GPCRs) that help control pain, mood, and immune function [3]. When endocannabinoids like  AEA or 2-AG attach to these receptors, they start a Gi/o protein signaling process, which leads to:

• Blocking adenylate cyclase (AC) → Lowers cAMP levels, slowing down cell activity.
• Reducing protein kinase A (PKA) activation → Less activation of other proteins inside the cell.
• Decreasing neurotransmitter release → Less glutamate, GABA, and dopamine are sent between brain cells.

But the effects of CB1 and CB2 activation depend on where they are located in the body. CB1 receptors in the brain and nervous system help regulate pain, mood, and neurotransmission by:

• Reducing glutamate & GABA release, altering pain perception and mood (leading to relaxation or, in some cases, anxiety).[4]
• Changing dopamine levels, which influences pleasure, motivation, and addiction.
• Activating potassium (K+) channels and blocking calcium (Ca2+) channels, making neurons less excitable—reducing pain but sometimes causing drowsiness.

Meanwhile, CB2 receptors in the immune system and peripheral tissues focus on reducing inflammation and regulating immune responses by:

• Lowering immune cell activity, which helps control inflammation.
• Modulating chronic pain and autoimmune diseases, calming an overactive immune system.

Therefore, selective signaling is essential for ensuring these processes remain controlled and beneficial. Without it, the ECS could become overactive or unbalanced. 

β-Arrestin Pathway and CB1 Receptor Regulation

When CB1 receptors are overstimulated, such as with excessive THC use, the β-arrestin pathway helps regulate their activity to prevent overstimulation and build-up of tolerance. [5] This happens through three key processes:

1. Desensitization: β-arrestin attaches to the CB1 receptor, blocking it from sending signals and reducing its activity.
2. Internalization: The receptor is pulled inside the cell through clathrin-coated vesicles, making it temporarily inactive.
3. Downregulation and Tolerance: If CB1 receptors stay internalized for too long, they may be broken down instead of recycled, leading to fewer receptors available for activation. This makes the body less responsive to THC over time, requiring higher doses to achieve the same effects.

Figure 1: Shows how GPCR signaling is regulated and how β-arrestin affects CB1 receptors. (a) When an agonist binds, GPCRs activate G proteins. (b) GRK phosphorylates the receptor, allowing β-arrestin to bind and stop signaling (desensitization). (c) β-arrestin helps remove the receptor through clathrin-coated vesicles for recycling or breakdown. With too much THC, CB1 receptors may be broken down instead of reused, reducing their numbers and leading to tolerance. [6]

But, if CB1 receptors are overstimulated too often, this desensitization can lead to tolerance, meaning the user will need more of the substance to experience the same effect. Over time, this could increase the risk of dependence. Since excessive β-arrestin activation plays an important role in this process, researchers are looking for ways to fine-tune CB1 receptor signaling.

This is where ligand bias comes in. Instead of activating all pathways equally, ligand bias allows for more accurate control, favoring beneficial signaling while minimizing unwanted effects.

Ligand Bias: Activating the Right Pathway

CB1 receptors don’t always respond the same way to different molecules. Some activate the G-protein pathway, which helps with pain relief, while others activate the β-arrestin pathway as shown in figure 2, which can lead to tolerance and side effects.

• G-protein signaling → Helps with pain & inflammation
• β-arrestin pathway → Leads to tolerance & side effects

This idea, called ligand bias (biased agonism), is helping scientists develop better treatments. By creating drugs that only activate the helpful G-protein pathway while avoiding too much β-arrestin activation, we can improve pain relief and neuroprotection without causing tolerance or unwanted effects.

Figure 2. Ligand-Biased Signaling in CB1 and CB2 Receptors. This figure shows ligand-biased signaling in CB1 and CB2 receptors, where ligands primarily activate either the G protein pathway or the β-arrestin pathway, leading to different cellular responses. Antagonists block both pathways by preventing ligand binding.[7]

Selective Signaling in Neurodegenerative Diseases 

Neurodegenerative diseases like multiple sclerosis (MS), Huntington’s disease (HD), and Alzheimer’s disease (AD)are linked to problems in the endocannabinoid system (ECS). The CB1 receptor plays an important role in protecting the brain, but if it is overused, the body can build tolerance, making treatments less effective.

Multiple Sclerosis (MS)

✔ CB1 activation helps reduce brain inflammation, muscle stiffness, and pain.
✔ Sativex (a THC-CBD spray) improves movement and reduces symptoms in MS patients.[8]
✔ According to the paper “Cannabinoid Receptors in the Central Nervous System: Their Signaling and Roles in Disease” studies on CB1-deficient mice show increased brain damage, proving that CB1 protects neurons. [9]

Huntington’s Disease (HD)

✔ CB1 receptors start disappearing early in the disease, even before symptoms appear.
✔ Losing CB1 receptors makes movement problems worse and leads to faster brain cell damage.
✔ CB1 activation increases BDNF (Brain-Derived Neurotrophic Factor), which helps protect brain cells and supports neuron survival. [9]

Alzheimer’s Disease (AD) 

✔ CB1 activation helps clear harmful β-amyloid buildup, which is linked to memory loss in AD.
✔ Mice without CB1 receptors have worse memory problems, showing that CB1 is needed for brain function.
✔ CBD helps protect the brain by reducing tau buildup, which contributes to neuron damage. [9]

In diseases like MS, HD, and AD, selective CB1 activation can help protect brain cells and improve symptoms. However, too much activation can lead to tolerance, making treatments less effective over time.Therefore, Selective signaling plays an important role in ensuring that CB1 receptors are activated only when and where needed, allowing scientists to develop more effective therapies with fewer side effects.

What Happens When Selective Signaling Fails?

Without selective signaling, the body could suffer from several issues:

• Increased side effects: If CB1 receptors are overstimulated in the brain, it could lead to memory loss, confusion, and impaired motor function. Overstimulation of CB2 receptors might suppress the immune system too much, making us vulnerable to infections.
• Uncontrolled pain and inflammation: Without selective signaling, the body could either feel too much pain in some areas or too little in others. Pain relief might not be targeted properly, leaving some areas of the body still suffering.
• Tolerance and receptor burnout: With constant activation of CB1 receptors, they can become desensitized. This means a person may need higher doses to achieve the same effect, which increases the risk of addiction and worsens the overall response.
• Mood disruptions: Overactivation of the ECS could lead to psychological side effects like anxiety, paranoia, or depression.

Therefore, without selective signaling, the ECS would lose its ability to finely tune processes, and the body would suffer from overactivation or underactivation of critical pathways.

The Bottom Line: Balance Is Key

And the endocannabinoid system (ECS) plays a vital role in regulating key functions like pain, mood, and immune response. It’s like the body’s internal balancing act, making sure everything runs smoothly. But here’s the catch: when the ECS doesn’t work properly, it can lead to serious issues, from chronic pain to mood swings and even disorders like anxiety or inflammation. Without proper regulation, things can get out of control fast!

Therefore, by understanding how selective signaling works in the ECS, we can create better treatments that target exactly what’s needed without all the side effects. So, why should you care? Because the ECS affects everyone! Whether you’re dealing with pain, stress, or just want to stay healthy as you age, learning more about how the ECS works could lead to safer, smarter treatments for a better life. And who wouldn’t want that?

Footnotes

[1] Lu, H. C., & Mackie, K. (2016). An Introduction to the Endogenous Cannabinoid System. Biological psychiatry, 79(7), 516–525. https://doi.org/10.1016/j.biopsych.2015.07.028

[2] Bosier, B., Muccioli, G. G., Hermans, E., & Lambert, D. M. (2010). Functionally selective cannabinoid receptor signalling: therapeutic implications and opportunities. Biochemical pharmacology, 80(1), 1–12. https://doi.org/10.1016/j.bcp.2010.02.013

[3] Howlett, A. C., & Abood, M. E. (2017). CB1 and CB2 Receptor Pharmacology. Advances in pharmacology (San Diego, Calif.), 80, 169–206. https://doi.org/10.1016/bs.apha.2017.03.007

[4] Patel, S., & Hillard, C. J. (2009). Role of endocannabinoid signaling in anxiety and depression. Current topics in behavioral neurosciences, 1, 347–371. https://doi.org/10.1007/978-3-540-88955-7_14

[5] Nguyen, P. T., Schmid, C. L., Raehal, K. M., Selley, D. E., Bohn, L. M., & Sim-Selley, L. J. (2012). β-arrestin2 regulates cannabinoid CB1 receptor signaling and adaptation in a central nervous system region-dependent manner. Biological psychiatry, 71(8), 714–724. https://doi.org/10.1016/j.biopsych.2011.11.027

[6] Whalen, E. J., Rajagopal, S., & Lefkowitz, R. J. (2011). Therapeutic potential of β-arrestin- and G protein-biased agonists. Trends in Molecular Medicine, 17(3), 126-139. https://doi.org/10.1016/j.molmed.2010.11.004

[7] Ye, L., Cao, Z., Wang, W., & Zhou, N. (2019). New Insights in Cannabinoid Receptor Structure and Signaling. Current Molecular Pharmacology, 12(3), 239–248. https://doi.org/10.2174/1874467212666190215112036

[8] Russo, M., Calabrò, R. S., Naro, A., Sessa, E., Rifici, C., D’Aleo, G., Leo, A., De Luca, R., Quartarone, A., & Bramanti, P. (2015). Sativex in the management of multiple sclerosis-related spasticity: role of the corticospinal modulation. Neural plasticity, 2015, 656582. https://doi.org/10.1155/2015/656582

[9] Kendall, D. A., & Yudowski, G. A. (2017). Cannabinoid Receptors in the Central Nervous System: Their Signaling and Roles in Disease. Frontiers in Cellular Neuroscience, 10(294). https://doi.org/10.3389/fncel.2016.00294