We may soon be able to target pain relief to the affected area of the body using the placebo effect by simply stimulating our brains. Because at its base, in our brainstem, tiny areas have precisely this selective analgesic capacity, describes a new study published in Science.
Ninety-three people received moderately painful stimuli on their cheeks, forearms, or calves. Just beforehand, the researchers applied either petroleum jelly or a pain-relieving cream... Or at least, that's what they thought! In reality, both creams, although different in appearance, were petroleum jelly. Yet, the brains of half of the participants activated differently when they thought they had received the pain-relieving cream. This is precisely what the scientists were looking for: to observe the placebo effect in action.
A pain circuit of a few cubic millimeters
It's in the brainstem that everything happens, in an area 10 to 12 millimeters long and 3 in diameter, called the periaqueductal gray matter (or PAGM), which itself communicates with another just below called the medulla oblongata. The pathway connecting cortex – SGPA – medulla oblongata – spinal cord or spinal trigeminal nucleus is considered the main descending pathway by which humans can directly modulate pain", explains to Science and Future First author Lewis Crawford, a neuroscientist at the University of Sydney, Australia.
Under the influence of small proteins – peptides – produced by SGPA, specialized cells in the medulla oblongata inhibit or amplify the downstream pain signal. A circuit that was not previously suspected could act locally, on a specific part of the body. We show that this well-known circuit contains an approximate map of the body allowing the elicitation of region-specific analgesia." says Lewis Crawford.
Read alsoPlacebo effect: we finally know how it works
A "map" of the placebo effect according to the origin of the pain
And indeed, within the medulla oblongata and SGPA, the researchers found that the placebo effects generated when pain was applied to the face, arms, or legs activated three different sections. The area related to the face is spaced approximately 2 mm from those of the arms and legs, themselves spaced 0.5 to 1.0 mm apart", explains Lewis Crawford. Brain subunits so small that their study required cutting-edge equipment, namely a functional MRI creating a magnetic field of 7 teslas, allowing the resolution to be increased from 8 (with a 3 Tesla fMRI) to 1.2 cubic millimeters.
From an evolutionary perspective, localized processing of pain signals based on the affected limb makes sense. It can reduce pain to optimize the body's response to a threat, and it can produce different behaviors based on the location of the pain. Facial pain usually comes from a predator in front of the animal and instinctively triggers fight behavior. Body pain, on the other hand, usually comes from behind and triggers a flight response.", explains Lewis Crawford. This is the famous fight or flight response induced in cases of stress. " Body pain activates the caudal part (back, editor's note) of the SGPA and that stimulation of the latter causes flight reactions. Conversely, pain in the face activates the rostral part (before, editor's note) of the SGPA and the stimulation of the latter causes fighting behaviors. »
Read alsoPain is subjective, so how do you know if it's worrying?
Better still, these tiny, targeted pain-relieving areas within the SGPA usually act on the medulla oblongata – and therefore on pain – by means of non-opioid peptides. I think what excites us so much about this work is that the SGPA activity that we're seeing is not typically associated with opioid signaling, which suggests that the kind of placebo analgesia that we're triggering isn't mediated by them, isn't producing effects. systemic (over the whole body, editor's note) on perceived pain and is very specific to the body site and, by extension, to the perceived pain", summarizes Lewis Crawford. It is rather the cannabinoid system, with a more localized action, which would be involved.
Playing on the placebo effect to relieve chronic pain
This work may have important implications for the treatment of chronic pain, by directly stimulating the correct brain area to induce a localized placebo effect. For example, there are new techniques such as focused ultrasound that can stimulate the brain with a resolution of a few millimeters.", develops Lewis Crawford. " If we can determine which part of the cerebral cortex projects to the rostral or caudal part of the SGPA, we will then be able to specifically stimulate a facial or body analgesic circuit. This could revolutionize personalized treatment of chronic pain.. »
English 
French