Sustained hyperalgesia can be induced in the rat by a single formalin injection and depends on the initial...

The present study was undertaken to find out if a hyperalgesia can be observed 1 week after a conditioning stimulation of the orofacial area of the rat. Sprague-Dawley rats received a left infraorbital infiltration with either bupivacaine or saline and then, 30 min thereafter, an injection of either saline or 10% formalin solution in the left upper lip. Four groups of animals were thus made up depending on their conditioning treatment. Seven days later, an algesimetric test initiated by a contralateral orofacial formalin injection was carried out. The duration of lip rubbing was significantly increased in formalin-conditioned groups. The hyperalgesia observed at 7 days was suppressed by an infraorbital nerve block carried out at the time of the conditioning stimulus. These results tend to indicate that a nociceptive message of short duration induces a trace in the central nervous system which can be retained for 1 week.