Behavioral assessment of facial pain in rats: face grooming patterns after painful and non-painful sensory disturbances in the territory of the rat's infraorbital nerve.

Noxious stimulation of the rat's face evokes intense face grooming with face wash strokes almost exclusively directed to the stimulated area (e.g. Clavelou et al., Neurosci. Lett., 14 (1989) 3263-3270). Similar asymmetric face grooming behavior has been observed after transection (Berridge and Fentress, J. Neurosci., 6 (1986) 325-330) and chronic constriction of the infraorbital nerve (Vos et al., J. Neurosci., 14 (1994) 2708-2723). In the present study, the relation between unilateral facial pain and asymmetric face grooming was experimentally studied in normal, intact rats: face grooming patterns evoked by non-painful sensory disturbances in the territory of the infraorbital nerve (i.c. unilateral vibrissae clipping, anesthetic infraorbital nerve blockade, application of mineral oil on vibrissae) were compared to those evoked by noxious facial stimulation (s.c. formalin injection in mystacial pad) and those observed in unstimulated control rats, using video-analysis. Only formalin-injected rats displayed significantly more face grooming activity directed to the affected infraorbital nerve territory than unstimulated control rats. Non-painful sensory disturbances (especially mineral oil application) induced an initial bout of directed face grooming; this response was transient and short-lasting. These observations suggest that directed face grooming can be used as a sign of unilateral facial pain in freely moving rodents; unilateral non-painful facial sensory disturbances do not lead to intense and persistent directed face grooming.