Serotonin And Autism

Understanding serotonin and autism

What is serotonin 

Serotonin is a neurotransmitter (synthesised by neurons, and functions to send messages to other neurons or receptors. After the message is sent, the neurotransmitter is reabsorbed by the body) in your brain.¹ 

Also, serotonin acts as a hormone in the gut. It is produced by the cell of the gastrointestinal tract. It helps stimulate the muscles of the gut so that food can be digested properly and it protects from gastrointestinal diseases such as irritable bowel syndrome  (IBS).¹

Serotonin plays a major role in our body including:¹

1. Influences learning, memory, and executive function: the regions of the brain that are responsible  for learning and memory contain a lot of serotonin . Neurons release serotonin when you are learning and when you are need to remember something
2. Mood control: specific serotonin receptors in the brain might be responsible for specific moods and behaviours, so serotonin levels may dictate how you respond to things that happen to you
3. Depression and instability: low serotonin depletes your energy
4. Mental decline: as mentioned above, serotonin plays a role in learning and memory. Studies show that not having enough serotonin can cause problems with processing and memory. Individuals with Alzheimer’s disease and dementia tend to have low levels of serotonin in their brains
5. Inhibits sexual activity and libido: high levels of serotonin indicate low libido and low level of serotonin indicate high libido according to the receptor of the brain that is activated. This explains why sexual dysfunction is a common side effect of antidepressants that target serotonin. This side effect is less common with drugs that increase dopamine activity
6. Aids in blood clotting 
7. Regulates sleep cycle: our body uses serotonin to make melatonin, which influences  the timing of circadian rhythm
8. Influences appetite and digestion

What is autism 

Autism - also referred to as autism spectrum disorder (ASD)-  is a group of conditions characterised by some degree of difficulty with social communication and interaction. Other characteristics include atypical patterns of behaviours and activities, such as difficulty in transitioning from one activity to another, unusual reactions to sensations, and focus on details.²

ASD involves persistent challenges with social communication, repetitive behaviour,  and restricted interests. Old parents,specific genetic conditions, and certain medications during pregnancy are risk factors for autism.³

Social communication deficits in autism include:

• Difficulty appreciating their own and others emotion
• Decreased sharing of interests with others
• Scripted speech
• Difficulty making friends
• Aversion to maintaining eye contact

Restricted interested and repetitive behaviours include:³

• Extreme difficulty coping with change
• Overly focused on niche subjects to the exclusion of other
• Expecting others to be equally interested in those subjects
• Sensory hypersensitivity, such as aversion to loud sounds 
• Stereotypical movements, e.g hand flapping, spinning, rocking
• Arranging things in a very particular manner
• Difficulty tolerating changes in routine and new experiences

The relationship between serotonin and autism

Serotonin and serotonin transporter levels in autistic children

Levels of serotonin and serotonin transporter are increased in autistic children. They are considered biomarkers for autism and have a role in autism pathogenesis so they are used to diagnose autism severity. The increase of both serotonin and serotonin transporter is associated with the severity of autism. If the levels are very high, then autism is severe.⁴ 

Hyper serotonergic during pregnancy is considered a potential environmental factor in the prevalence of autism spectrum  disorder. Selective serotonin reuptake inhibitors(SSRIs) which are used to treat depression lead to increased serotonin in the brain. Studies show that use of SSRIs during pregnancy increases the risk of autism in offspring.⁵ 

Molecular imaging studies of serotonin in ASD become crucial evidence of serotonergic abnormalities in the CNS of autistic individuals. One study discovered asymmetric serotonin synthesis in the dentate nucleus of the cerebellum, thalamus, and frontal cortex of autistic children. In the frontal cortex and thalamus, there was a decrease in serotonin synthesis, but in the dentate nucleus there was increased serotonin synthesis. Enhancement of capacity of brain serotonin synthesis is disrupted during childhood in autistic children.⁵ 

Children with ASD commonly have deficient levels of vitamin D and low serotonin level in the brain. The behavioural characteristics similar to ASD is due to depletion of both tryptophan and serotonin. Vitamin D deficiency leads to reduced serotonin concentrations in the brain by decreasing the synthesis of tryptophan hydroxylase 2, impacting the neuronal wiring and structure of the brain.⁶ 

Summary 

Serotonin is a neurotransmitter (synthesised by neurons, and functions to send messages to other neurons or receptors. After the message is sent, the neurotransmitter is reabsorbed by the body) in your brain. Also, serotonin acts as a hormone in the gut. Serotonin plays major role in influencing learning, memory, and executive function, mood control, depression and instability , inhibits sexual activity and libido, aids in blood clotting, and regulating sleep cycle. 

Autism - also referred to as autism spectrum disorder (ASD) - is a group of conditions characterised by some degree of difficulty with social communication and interaction. Other characteristics include atypical patterns of behaviours and activities, such as difficulty in transitioning from one activity to another, unusual reactions to sensations, and focus on details. Social communication deficits in autism include difficulty appreciating their own and others emotion, decreased sharing of interests with others, scripted speech, difficulty making friends, and aversion to maintaining eye contact. 

Levels of serotonin and serotonin transporter were increased in autistic children. They are considered biomarkers for autism and they have a role in autism pathogenesis so they are used to diagnose autism severity. Hyper serotonergic during pregnancy is considered a potential environmental factor in the prevalence of autism spectrum  disorder.