BEHAVIORAL EVIDENCE OF TRIGEMINAL NEUROPATHIC PAIN FOLLOWING CHRONIC CONSTRICTION INJURY TO THE RATS INFRAORBITAL NERVE

Video recordings of free behavior and responses to mechanical facial s timulation were analyzed to assess whether chronic constriction injury (CCI) to the rat's infraorbital nerve (IoN) results in behavioral alt erations indicative of neuropathic pain. A unilateral CCI was produced by placing loose chromic gut ligatures around the IoN. After CCI to t he IoN, rats exhibited changes in both non-evoked and evoked behavior. Behavioral changes developed in two phases. Early after CCI (postoper ative days 1-15), rats showed increased face-grooming activity with fa ce-wash strokes directed to the injured nerve territory, while the res ponsiveness to stimulation of this area was decreased. Later after CCI (postoperative days 15-130), the prevalence of asymmetric face groomi ng was reduced but remained significantly increased compared to contro l rats. The early hyporesponsiveness was abruptly replaced by an extre me hyperresponsiveness: all stimulus intensities applied to the injure d nerve territory evoked the ''maximal'' response (brisk head withdraw al, avoidance behavior plus directed face grooming). This response was never observed in control rats. Concurrently, IoN ligation rats showe d a limited increase in the responsiveness to stimulation of the contr alateral IoN territory, and around postoperative days 30-40 the respon siveness to stimulation of facial areas outside the IoN territories al so increased. The hyperresponsiveness to stimulation of the ligated Io N territory slightly decreased from 60 d postoperative. Throughout the study, IoN ligation rats showed decreased exploratory behavior, displ ayed more freezing-like behavior, had a slower body weight gain, and a higher defecation rate, compared to control rats. The behavioral alte rations observed after CCI to the IoN are indicative of severe sensory disturbances within the territory of the injured nerve: mechanical al lodynia develops after a period of relative hypo-/anesthesia during wh ich behavioral signs of recurrent spontaneous, aversive (possibly pain ful) sensations (paresthesias/dysesthesias) are maximal.