Mechanisms Underlying the Placebo Effect in Both Histaminergic and Non-histaminergic Itch

Pain and Itch Neuromodulation: Effects of Offset Analgesia, Placebo, and Nocebo and Topographical Distribution of Itch and Pain Receptors

The study consists of one experimental session of approximately 1½ hours. The participant will fill in a series of questionnaires about how you perceive itch and pain, and about some traits of your personality, such as depression, anxiety and stress. Afterwards, two areas will be selected on each forearm of the subjects. The four areas will be treated with the same vehicle cream for 20 minutes, in order to test the placebo effect. At the end of the cream application, two areas will be randomly treated with histamine while cowhage will be applied on the other two areas. During the 10 minutes of the application of the pruritogens, itch and pain will be continuously monitored using a Visual Analog Scale.

Inclusion Criteria:

• Healthy men and women
• 18-60 years
• Speak and understand English

Exclusion Criteria:

• Pregnancy or lactation
• Drug addiction defined as any use of cannabis, opioids, or other drugs
• Previous or current history of neurological, immunological, musculoskeletal, cardiac disorder or psychiatric diagnoses that may affect the results (e.g., neuropathy, muscular pain in the upper extremities, anxiety, depression, schizophrenia, etc.)
• Moles, wounds, scars, or tattoos in the area to be treated or tested
• Current use of medications that may affect the trial such as antihistamines and pain killers
• Skin diseases
• Consumption of alcohol or painkillers 24 hours before the study days and between these
• Acute or chronic pain and itch
• Participation in other trials within 1 week of study entry (4 weeks in the case of pharmaceutical studies)
• Contraindications to capsaicin, including intolerance to chili or burns or wounds at the application site
• The subject is assessed as unable to engage in the necessary cooperation required by the study