We finally understood how the placebo effect works

The placebo effect is a therapeutic process without specific efficacy but acting on the patient through psychological and physiological mechanisms. Its effectiveness has been confirmed in numerous studies and has intrigued researchers for a long time. Today, a new study published in the journal Nature specifies the neural circuits activated in the brain areas involved in this process. Thus, not only does this work help to understand how the placebo works, but it also opens up avenues on how to manipulate it.

For their research, scientists from the University of North Carolina (USA) worked on mice. The latter were placed in a space made up of two easily recognizable rooms. For three days, the floor of the two rooms was heated to 30°, a comfortable temperature for the mice, who could come and go as they pleased. Then, for three days also, the floor of one of the rooms was heated to 48°C, a temperature far too high for their small paws. During this period, the mice learned to associate one room with pain and the other with the absence of pain.

Then, the temperature was raised to 48°C in both chambers. The researchers were then able to observe that the mice showed fewer signs of pain in the supposedly painless chamber than in the other. This placebo effect lasted for at least a week.

“Treat people differently”

In their analyses, the researchers identified a neural circuit with neurons running from the anterior cingulate cortex in the prefrontal cortex (an area associated with pain) to a structure in the brainstem (under the brain). The latter, nicknamed the pons, connects to the cerebellum, a structure involved in learning and conditioning.

During the conditioning phase, these neurons connecting the anterior cingulate cortex and the brainstem bridge were gradually activated. They then activated neurons in the bridge expressing opiate receptors. They can therefore also be activated by endogenous opioids such as endorphins which have an analgesic effect. These neurons end up activating others in the cerebellum, the Purkinje cells.

« We need better ways to treat chronic pain, including treatments without harmful side effects or addictive effects.. We believe our results open the door to the possibility of targeting this new neural pathway for pain to treat people differently, but potentially more effectively,” enthuses Greg Scherrer, author of the study, in a communiqué.

More effective in treating pain?

According to the newspaper Medicine/Scienceit is in the treatment of pain that the response rate of a placebo is the highest. “In a study of more than 1,000 patients, treated for pain of various types, the efficacy rate is thus 35% on average,” it is written. “It is the experimental pains in the volunteer subject that are the least sensitive to the placebo effect, while the pains linked to an illness related to anxiety (angina pectoris, for example) are the most sensitive (up to 90% response).”