When rodents gnaw, it is not a purely mechanical process. A study shows that a previously unknown reward circuit in the brain is activated by the stimuli on the gums and provides motivation. On the one hand, the results explain how repetitive behaviors are maintained over a longer period of time. On the other hand, they can possibly also be transmitted to people and could provide clues as to why some people bite their fingernails or grind their teeth.
In rodents, incisors grow throughout life. If they are not continually worn down by nibbling on hard objects, they become so long that affected animals can no longer eat and eventually starve. Gnawing is therefore vital beyond just eating food. But what makes mice, guinea pigs and the like voluntarily gnaw on everything that comes within their reach?
Reward system activated
A team led by Xin-Yu Su from the University of Michigan has now found an answer to this question. “We discovered that gnawing is a motivated behavior,” explains Su’s colleague Bo Duan. “There is a defined neural circuit that connects sensory input from the teeth to dopamine neurons in the midbrain. This shows us that even very basic maintenance behaviors are actively reinforced by the brain.”
Using mice, the researchers traced the nerve connections that are activated when gnawing. Accordingly, the touch sensors in the gums send signals to two different neural pathways. One leads to motor neurons that control jaw movements and move the incisors into the correct position. But beyond this mechanical component, there is also a motivational component: the second nerve pathway leads to the midbrain, where it ensures the release of dopamine. When the mouse gnaws, its reward system becomes active.
To find out what role this circuit plays in maintaining gnawing behavior, Su and his team blocked the corresponding nerve connections in some experimental mice. The animals were still able to gnaw using purely motor skills, but they apparently lacked desire and stamina. They gnawed less than their counterparts, their teeth grew excessively and their jaws shifted sideways because the teeth, which were too long, would no longer fit into the small mouse mouth. “This confirms that dopamine release is essential for maintaining gnawing motivation,” the researchers write.
Similar circuits in humans?
From the researchers’ perspective, the results could also be relevant beyond rodents. Even though the teeth of most other mammals do not grow indefinitely, it is important to keep the jaw muscles well trained when it comes to eating. A reward circuit that increases motivation for chewing movements could therefore also have become established in other animal species, including humans. “For example, people like to chew chewing gum, but sometimes they also exhibit harmful habits such as grinding their teeth or compulsively biting their nails,” explain Su and his colleagues. “These behaviors also involve forceful tooth contact and are often modulated by internal states such as stress or excitement.”
In future studies, the team would like to find out whether similar sensory reward pathways are actually active in other animals and people. “We think this could be a more general principle,” says Duan. “Understanding how these circuits are organized could ultimately help us target them when behavior becomes maladaptive.”
Source: Xin-Yu Su (University of Michigan, Ann Arbor, USA) et al., Neuron, doi: 10.1016/j.neuron.2026.01.021