A new study has found evidence of a brain link that could explain why Tourette syndrome often overlaps with other neuropsychiatric conditions such as obsessive-compulsive disorder (OCD), and potentially lead to better treatments.
Researchers led by a team at Kobe University in Japan used a mouse model of tic-like movements to investigate the brain circuitry underlying the involuntary movements that characterize Tourette's.
"We believe that the neuronal circuit we found plays a crucial role as a bridge connecting brain regions that were previously thought to act independently from each other," says neurophysiologist Yoshihisa Tachibana, from Kobe University.
Via a combination of neuron-tracing techniques, an active pathway between a major movement-control region of the brain (the basal ganglia) and a key emotion-processing hub (the insular cortex) was discovered.
Importantly, this pathway goes through the thalamus, a major relay station of the brain, and specifically via intralaminar thalamic nuclei – nodes responsible for routing neuron activity to the right spots.
The researchers tested the pathway using a bespoke drug to block the thalamus-to-insular connection. This caused the tics in the mice to become much less intense while dampening relevant signals in the brain, further indicating the pathway's significance.
While earlier research had found that the insular cortex, a brain region connected to OCD, was involved in Tourette's and possibly related to involuntary tics, the brain wiring carrying the disrupted movement signals hadn't previously been established.
A previous study showed that a dental mouthguard designed to activate jaw muscles can ease Tourette's tics, via the insular cortex – findings that partly inspired this new study. Now that more of the underlying neuronal wiring has been mapped, researchers can look for ways to target it.
We know that Tourette syndrome and OCD often occur together – along with other neuropsychiatric conditions such as autism and ADHD, which suggests connected brain regions.
Tourette's also comes with the buildup of 'premonitory' mental urges, which precede tics and are usually relieved by them, suggesting Tourette syndrome isn't entirely about movement.
This new brain-wiring discovery suggests a mechanism by which abnormal motor signals could spill into brain regions involved in emotional processing, and the authors think it goes some way toward explaining the overlap between Tourette's and other conditions.
"Abnormal insular cortex activity in patients with Tourette syndrome may contribute not only to motor symptoms such as tics but also to premonitory urges and comorbid psychiatric disorders, including OCD, ADHD, and ASD," the team writes.
"However, because rodents cannot report premonitory urges, the present model should be interpreted as capturing selected features of tic-like behaviors."
A better understanding of what's going on in the brain could help in the development of treatments, further down the line. Deep brain stimulation of the thalamus is already used to treat severe Tourette's, but before now, experts haven't been entirely sure why this works effectively.
"Tic disorders, such as Tourette syndrome, are among the most common neuropsychiatric conditions and have a significant impact on children's lives," says Tachibana.
"However, their underlying neuronal mechanisms remain poorly understood."
Every new piece of research published on Tourette's and OCD builds up more of the overall picture when it comes to what's happening in the brain on the most fundamental level, and why unexplained urges and involuntary actions take over.
It's going to take some time to work out how this brain relay hub could be manipulated to manage the thoughts and movements associated with Tourette's – this specific brain link has only just been discovered – but the potential is there.
One possibility is that a similar calming of the basal-ganglia-to-insular-cortex pathway, perhaps using ultrasound, could bring tics under control. However, this research needs to expand beyond mouse models into human patients first.
Related: Several Psychiatric Disorders Share The Same Root Cause, Study Suggests
"The currently used deep brain stimulation is invasive and comes with a risk of complications," says Tachibana.
"On the other hand, targeted intervention of the neuronal processing in the circuit we identified, such as ultrasound neuromodulation, may be options for the development of less invasive and safer treatment methods."
The research has been published in Cell Reports.