Recent studies have focused on the role of endogenous opioids on central sensitization. Central sensitization is known to be impaired or altered in chronic pain conditions, as fibromyalgia or chronic tension headache.

Animal studies have shown reinstatement of mechanical hypersensitivity following naloxone administration after resolution of an injury. This suggests latent sensitization.

In the present study, investigators hypothesize that naloxone (2 mg/kg) can reinstate secondary hyperalgesia 168 hours after a first-degree burn-injury. Investigators aim therefore to show that latent sensitization is present in humans and is modulated by endogenous opioids.

Inclusion Criteria:

• healthy man
• written informed consent
• ASA 1-2
• BMI 18 < BMI < 30
• normal ultrasound examination of the heart
• normal ECG
• urin sample without traces of opioids

Exclusion Criteria:

• volunteers, who do not understand the Danish language
• participation in another experimental trial in the previous 60 days
• nerve damage or skin lesions in the assessment areas
• neurological or psychiatric condition
• use of psycho-active drugs
• abuse of alcohol or drugs
• chronic pain
• regular use of pain-killers (> 1 a week)
• allergy against morphine or other opioids (including naloxone)
• use of prescription drugs 1 week prior to the trial
• use of over-the-counter medication 48 hours prior to the trial
• urin sample with traces of opioids
• volunteer is not suitable for the trial according to the investigator's consideration