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Scratching the head or rubbing the hands repeatedly is common, unconscious behaviour when people are facing stress. Neuroscientists from the School of Biomedical Sciences and the Gerald Choa Neuroscience Centre of the Faculty of Medicine at The Chinese University of Hong Kong (CU Medicine) have discovered a mammalian brain circuitry underlying our ability to generate adaptive responses when facing stress with strong negative emotions. The result of the study was recently published in the renowned international scientific journal Nature Communications.

Stress-associated repetitive behaviour is common in animals including humans

Stress is common in life. Clearly, we are stressed in an aversive or threatening environment. We could also feel stressed when we have to handle multiple tasks at the same time. Even if we are just confined to a limited space for some time, we could develop stress. Prolonged exposure to these and other types of stressors that cause emotional upheaval could be harmful if we cannot respond appropriately. Therefore, whether we can handle stress efficiently could make a difference to our mental well-being and therefore health.

Stress in animals, including humans, often results in apparently irrelevant, repetitive behaviour, also known as displacement activities. For example, a candidate during an interview may scratch the head, bite the lips, or rub the hands unconsciously but compulsively. Why do we exhibit such behaviour? Does it serve any function? What is the origin and mechanism in the brain that underlies these responses? Solving these questions is critical in understanding our capability and the neural basis of stress management.

A circuitry in the limbic system responsible for generating response to stress

The neural pathways that convey emotional stress signals that in turn generate repetitive behaviour are not clearly known, and are likely to be complex. This study led by Prof. Ya KE and Prof. Wing Ho YUNG from the School of Biomedical Sciences and the Gerald Choa Neuroscience Centre at CU Medicine, has provided an important clue in answering this critical question.

Based on a combination of cutting-edge techniques in neuroscience, the research team identified in the rat a previously unknown circuitry in the limbic system, an important emotion processing system in the brain, that mediates stress-induced repetitive grooming behaviour. This circuitry is specific to emotional stress in the rat model, such as exposure to a threatening environment and long-term body constraint, but not to physical stress like wetting the body.

Taking advantage of powerful techniques known as optogenetics and chemogenetics, the research team could manipulate this circuitry precisely while sparing other pathways of the brain. Instant repetitive behaviour of the animals could be triggered following activation of this circuit. On the other hand, inhibiting this circuit largely prevented the repetitive behaviour that otherwise could be evoked by emotional stress.

Repetitive behaviour is important in stress management

One intriguing discovery in this study is that the animals apparently ‘feel’ good when this neural circuit is activated. This was revealed in experiments in which the rats were first put into two connected chambers and this circuit would be activated only when the animal entered one but not the other chamber. The team found that the rats remembered the treatment and would later spend much more time in the chamber associated with activation of the circuit. This suggests that activation of this specific circuit could help soothe the animals and so reduce their stress.

Prof. Ya KE remarked, “This proved that the circuit we discovered in the current study likely represents an important system in the brain that evaluates real or perceived stressors and also helps to manage stress. It also implies that trying to suppress such repetitive behaviour may not be necessary as it is considered beneficial.”

On the other hand, Prof. Wing Ho YUNG, noted, “Since our normal response to stress could go awry in some neuropsychiatric disorders like obsessive compulsive disorders or developmental disorders like autism in which excessive repetitive behaviour is common, our findings not only uncover a limbic circuit that plays a significant role in emotional stress response but also provide a basis for probing the origin of malfunctions that could lead to abnormal repetitive behaviour exhibited in different brain disorders.”