Hyperalgesia and Pain

Single site, exploratory, comparative, double-blind, randomised, cross-over study. Three physicians will be involved in the conduct of this study: * Physician 1 will include and randomise the subjects, prepare and administer the study products (namely Investigational Medicinal Products (IMPs)) in an open manner and will assess subject safety. * Physician 2, kept IMP blinded, will perform all evaluation measurements and sedation assessments during all experimental sessions. Physician 2 will always be the same person throughout the whole study period. * Physician 3 will be available if needed to reinforce subject safety Design: A group of 20 assessable subjects who will undergo 4 randomised experimental sessions of CCES on 4 separate study days, 2 weeks apart preceded by a 2 week period between selection and first experimental session and followed by a 1 week period between last experimental session and study end. After the selection visit, each visit corresponds to one of the 4 experimental sessions named A, B, C, D differing by the gas mixture inhaled and the intravenous medicine administered. The order of these experimental sessions A, B, C, D, will be assigned according to a pre-established list that the investigator will not know. Each experimental session includes a set-up-phase followed by an evaluation phase. The set up phase will last 20 mn during each an electrical stimulation will generate a spontaneous pain: every 2 minutes the electrical stimulation will increase and the subject will be asked to rate the pain on a visual scale from 0 to 100. When a stable levels of pain intensity will be reached, the regimen of the electrical stimulation will be kept constant up to the end of the experimental session. The Evaluation phase will last 160 mn : the electrical stimulation will be constant during 160 mn as detailed above. During this phase the subject will inhale a gas mixture with a face mask during 60 minutes combined with an intravenous administration of medicine during 30 mn starting at the same time. Every 5 minutes the subject will be asked to rate the pain and at predefined times the spontaneous pain and the area of allodynia and hyperalgesia will be measured and recorded as well as safety parameters (transcutaneous oxygen saturation, respiratory parameters, sedation score, blood pressure and heart rate). The 4 experimental sessions named A, B, C, D differ by the gas mixture inhaled and the intravenous medicine administered. For the placebo session, inhaled gas mixture will be 50%/50% N2/O2 and intravenous administration will be an isotonic saline solution. For the three other sessions, inhaled gas mixture will differ from a session to the other one (35%/15%/50% N2O/N2/O2 or 50%/50% N2/O2 or 50%/50% N2O/O2), and remifentanil will be administered intravenously at each of these sessions.