By Courtney Bricker-Anthony
January 28, 2016
|Carissa Cascio, Ph.D.|
“A better understanding of how serotonin influences sensory processing may help us to eventually design new treatments for sensory difficulties,” she said.
When people think about autism spectrum disorder, the first characteristics that often come to mind are social deficits, such as a lack of communication and avoidance of eye contact, and restricted, repetitive behaviors such as rocking back and forth and repeating phrases. Even clinical definitions of ASD have historically emphasized these features.
However, the study published by Cascio and her colleagues in Research in Autism Spectrum Disorders, sought to shed light upon a commonly overlooked feature of ASD: sensory dysfunction.
Sensory dysfunction in ASD is broadly classified as either hyper- or hyporesponsiveness — that is, unusually strong reactions to touch, sound or visual stimuli, or diminished or absent reactions to stimuli.
“There is speculation that these behaviors serve an adaptive purpose with regard to sensory input, either to limit sensory input in an environment that is overwhelming, or to increase a specific kind of sensory/sensorimotor input,” Cascio said.
Serotonin is also heavily associated with ASD; an estimated 30 percent of children with ASD have a condition called hyperserotonemia, in which serotonin levels are elevated in whole blood.
“It’s a strong possibility that hyperserotonemia contributes to some of the medical issues seen commonly in individuals with ASD,” Cascio said.
She added that researchers are currently focusing on how the serotonin transporter, SERT, contributes to ASD.
SERT regulates serotonin levels in the blood as well as in the brain. Due to genetic variation, SERT comes in either a short or high-expressing long allele. The longer allele of SERT is associated with anxiety and emotional regulation, but its relationship with ASD is not clear.
Given that serotonin has a role in the development of a brain area responsible for tactile processing, Cascio and her collaborators hypothesized that high SERT expression was associated with tactile hyper-responsiveness in ASD.
In order to investigate this idea, Cascio and her colleagues explored genetic variations in SERT and sensory function in both children with ASD and typically developing children. The researchers used the Autism Diagnostic Observation Schedule to confirm ASD diagnoses and parent/caregiver questionnaires to determine if the children exhibited characteristics of hyper- or hypo-responsiveness.
The authors also collected saliva samples and performed genotyping to discern whether the children expressed the long or short SERT allele.
Among the children with ASD, tactile hyperresponsiveness and high SERT expression were significantly correlated, which supported the authors’ hypothesis.
With increasing age, children with ASD exhibited fewer hyporesponsive behaviors, which was in agreement with previous research. Cascio and colleagues also uncovered a strong relationship between tactile hyperresponsiveness and repetitive behaviors.
While the distinct mechanisms underlying hyper- and hyporesponsiveness in ASD are currently unclear, serotonin is a potential modulator of hyperresponsiveness, and that provides a potentially fertile ground for future research, Cascio said.
The research was supported by NIH grants MH090232 and MH094604.