神経学：プラセボによる疼痛緩和と関連する脳回路がマウスで発見される

A neural circuit that may underpin pain relief caused by the placebo effect is identified using mouse models and reported in Nature this week.

The placebo effect has a role in pain relief. When an individual anticipates pain relief, their perception of that pain lowers in a process known as placebo analgesia. Previous research has indicated that placebo analgesia is correlated with activity in the anterior cingulate cortex, a region of the brain also associated with feelings of pain. However, the biological mechanisms that underlie this phenomenon remain poorly understood.

Grégory Scherrer and colleagues designed a mouse model of placebo analgesia to study how pain relief is mediated. Mice were conditioned to associate two chambers with different floor temperatures, one at a comfortable heat and another that was very hot. The conditioned mice spent more time on the cooler floor after exposure to the hotter floor, indicating that they anticipated pain relief, and performed fewer pain-relief behaviours, such as licking their feet.

In a follow-up genetic analysis of the mouse brains, the researchers identified a pathway associated with pain-relief behaviours between the rostral anterior cingulate cortex and the pontine nuclei, a brain region not previously thought to have a role in pain. Scherrer and colleagues also note that the pontine nuclei contain an abundance of opioid receptors, evidence that indicates the region’s potential role in pain tolerance. They also identified a cluster of cells in the cerebellum that may encode the expectation of pain relief in the rostral anterior cingulate cortex and pontine nuclei pathway.

The authors note that this cerebral pathway could be stimulated using drugs, electrodes or cognitive behavioural therapies to induce pain relief in individuals.