Nonevoked facial pain in rats following infraorbital nerve injury: a parametric analysis.

Unilateral chronic constriction injury to the infraorbital nerve (IoN-CCI) induces an increase in face-grooming behavior that is not part of normal body grooming (i.e., "isolated face grooming"). Despite the validity of isolated face grooming as a measure of spontaneous neuropathic pain, variation between rats in postoperative face grooming has limited its usefulness. We examined whether performing bilateral rather than unilateral IoN surgery could induce a more stable face-grooming behavior. It was found that bilaterally ligated rats performed a significantly greater amount of isolated face grooming (approximately four- to fivefold) when compared with unilaterally ligated rats. However, this effect was accompanied by an equivalent increase in between-subjects variation. No significant difference in face grooming during body grooming was found between bilaterally and unilaterally ligated rats. Rats were observed in two different sizes of observation cages; also, in addition to the amount of time spent on face grooming, the number and average duration of the face-grooming episodes were recorded. The effects of IoN surgery are caused by increases in the number of episodes. The effects of cage size were mostly related to differences in episode duration; that is, rats performed longer face-grooming episodes in the small compared to the large observation cage. The present data confirm previous reports that isolated face grooming is a more sensitive measure than the total amount of face grooming. Bilateral IoN surgery does not seem to offer a more stable outcome measure; increases in isolated face grooming were, however, more clearly observed in the small cage.