Pain, Sleep and Gut Microbiota

The participants will be screened for inclusion and exclusion criteria and will be asked to fill the following questionnaires : Pittsburgh Sleep Quality Index, State and Trait Anxiety Questionnaire, Pain Catastrophizing Scale Fear of Pain Questionnaire, Perceived Stress Scale, Beck Depression Inventory, and questionnaires to assess their health status and the use of medications. A blood sample (to measure pro- and anti-inflammatory cytokines and perform metabolomic analyses) will be obtained. Enrolled participants will be given a wearable actimeter to assess daily total sleep duration during one week before the first experimental session. They will also be requested to complete daily a sleep diary during one week before each experimental session, and a food diary before the first experimental session. A fecal sample (to assess gut microbiota composition and perform metabolomic analyses) will be collected between the screening visit and the first experimental session.

The extent of neurogenic inflammation induced by topical capsaicin will be used as a measure of the susceptibility to sensitize at peripheral level. A solution of capsaicin will be applied to the skin of the left or right volar forearm for 30 minutes. The capsaicin-induced neurogenic inflammation can be quantified by assessing the intensity, extent and duration of the capsaicin-induced cutaneous flare response using thermal infrared imaging. Using heat stimuli delivered to the treated skin and surrounding skin, the intensity and duration of the capsaicin-induced hyperalgesia will also be assessed. The experimental session will last approximately two hours.

The extent of secondary mechanical hyperalgesia induced by high-frequency electrical stimulation (HFS) of the skin of the left or right volar forearm will be used as a measure of the susceptibility to develop central sensitization. The stimulation will be delivered using a multi-pin electrode designed to preferentially activate epidermal free nerve endings. The strength, spatial extent and duration of the HFS-induced changes in pinprick sensitivity will be characterized by using calibrated mechanical pinprick stimuli. The experimental session will last approximately one hour.