Human
pain models invoking central sensitization, one of the key mechanisms of
chronic pain, may be useful for characterizing new
analgesics. A new model of electrical
hyperalgesia can detect the efficacy of several
analgesic mechanisms. Because IV
adenosine can alleviate
neuropathic pain, we investigated its effect on experimental sensitization. This was a double-blinded, randomized, two-period crossover study in 20 healthy volunteers. Current pulses (0.5 ms; 1 Hz) were applied intracutaneously to achieve
pain rating of approximately 5 on a 0-10 numeric rating scale.
Pain, areas of pinprick
hyperalgesia, and
tactile allodynia were assessed during the 2.5-h stimulation period.
Adenosine (50 microg. kg(-1). min(-1)) and placebo were infused IV over 60 min. Additional testing was performed 24 h after each treatment.
Adenosine reduced the area of pinprick
hyperalgesia during the infusion compared with placebo; there was no significant effect on
tactile allodynia or
pain rating. The effect on
hyperalgesia developed over 15 min and was significant (P < or = 0.05) for the rest of the infusion period. There was no difference between treatments at 24 h. Thus, in accordance with reports on
neuropathic pain,
adenosine reduced central sensitization in the human model of electrical
hyperalgesia. However,
adenosine did not have the long-term effects seen in patients. The model can investigate mechanisms of drugs for the treatment of
chronic pain.