Rodent Gnawing Triggers Dopamine Release Through a Newly Discovered Neural Circuit
University of Michigan
Rats gnaw. Constantly. Their incisors grow continuously throughout their lives, and gnawing keeps teeth at functional length and jaws properly aligned. Ask a textbook why they do it, and you will get a straightforward answer: mechanical necessity. The teeth grow, so the animal grinds them down. Simple reflex, simple explanation.
That explanation, it turns out, is incomplete.
A clue from overgrown teeth
Bo Duan, an associate professor at the University of Michigan's Department of Molecular, Cellular and Developmental Biology, studies neural circuits underlying touch and sensation in mouse models. His team began noticing something unexpected: some of their experimental animals were developing abnormally long teeth. Certain manipulations of neural pathways seemed to reduce gnawing behavior, and without sufficient gnawing, the ever-growing incisors became overgrown.
If gnawing were purely a mechanical reflex driven by tooth contact with hard surfaces, disrupting neural circuits elsewhere in the brain should not have affected it. The observation suggested that something beyond reflexive grinding was sustaining the behavior.
Duan partnered with Joshua Emrick, an assistant professor at the U-M School of Dentistry and a dentist-scientist who studies oral and craniofacial sensation. Together, they followed the clue.
Two pathways from a single junction
Their investigation, published in the journal Neuron, traced the neural circuitry from the teeth to the brain. Touch-sensitive neurons in the periodontal tissue, the tissue surrounding the teeth, send signals to a brainstem junction. From there, the signal splits into two distinct pathways.
One pathway runs to motor neurons that control jaw movement and tooth positioning. That is the mechanical component, the part that physically executes the gnawing motion. The other pathway extends into the midbrain, where it activates dopamine-producing neurons. Dopamine, the neurotransmitter most associated with reward and motivation, effectively tells the animal that gnawing feels good and is worth repeating.
The dual-pathway structure means gnawing is both a motor behavior and a motivated one. The animal does not just gnaw because its teeth touch something hard; it gnaws because its brain rewards it for doing so.
Blocking motivation without blocking mechanics
To confirm the two pathways are functionally distinct, the researchers selectively blocked the dopamine motivation pathway while leaving the sensory-motor pathway intact. The result: mice could still gnaw, and the reflexive component still worked, but they gnawed less efficiently. Without the motivational drive, the behavior was not sustained at the level needed to maintain healthy tooth length.
"Without the motivation, it's just not very efficient," Duan said. "So the motivation part is very important."
This finding reframes gnawing from a passive maintenance behavior into an actively reinforced habit, driven by the same dopamine reward circuitry that underlies many motivated behaviors across mammals.
From rodent teeth to human grinding
The word "rodent" comes from the Latin "rodens," meaning "gnawing." Rodents are defined by their ever-growing incisors and their compulsion to wear them down. But the dopamine reward pathway the team identified may not be unique to animals with continuously growing teeth.
"Even though human teeth stop growing, the brain mechanisms that drive repetitive oral behaviors may still be operating," Duan said. Human conditions that involve repetitive oral activity already have known connections to dopamine regulation. Bruxism, the involuntary grinding or clenching of teeth that affects roughly 8% to 31% of the general population, has been linked to dopamine system dysfunction. Patients with Parkinson's disease who receive long-term dopamine precursor treatment can develop bruxism as a side effect.
Malocclusion, the misalignment of upper and lower teeth, occurs at higher-than-average rates in people with conditions affecting motivation and behavior, including autism and depression. The newly discovered circuit provides a biological mechanism that could help explain these associations.
"Now we have the evidence for a biological circuitry link that may be contributing," Emrick said.
Practical limits and open questions
The circuit was identified in mice, and its existence in humans has not been confirmed. While the brainstem structures involved are broadly conserved across mammals, the specific connectivity and functional significance could differ in primates. The study also does not establish whether manipulating this pathway in humans would have therapeutic effects on conditions like bruxism.
Current treatments for bruxism include mouth guards (which address symptoms, not causes) and botulinum toxin injections into jaw muscles (which reduce grinding force but do not target the underlying neural drive). If the reward pathway identified in this study operates in humans, it could offer a more targeted approach. But translating a mouse neural circuit discovery into a human therapy requires extensive intermediate work, including identifying the homologous pathway in humans, developing drugs or interventions that can selectively modulate it, and conducting clinical trials.
The researchers note that the circuit could also explain oral behaviors in other mammals. Dogs chew bones; many species maintain oral muscle tone through repetitive jaw activity. Whether all of these behaviors tap the same sensory-to-dopamine pathway is an open question the team plans to investigate.
A broader principle at work
Duan suggested the finding may reflect something more general about how the brain sustains maintenance behaviors. "We think this may represent a more general principle," he said. "Understanding how these circuits are organized could eventually help us target them when the behavior becomes maladaptive." The brain, it appears, does not leave essential maintenance tasks to reflexes alone. It backs them up with reward.