Study on the recovery processes of the tactile evoked potential after infraorbital nerve crushing in rats

The changes in latency and amplitude of the cranial evoked potentials (EP) generated by mechanically stimulating the whiskers were measured in adult rats, in which the unilateral infraorbital nerve innervating the whiskers was crushed.1. In normal rats with intact infraorbital nerves, disphasic EPs were recorded subcutaneously from the contra and ipsilateral hemisphere. Latency of the contralateral waves was always shorter than that of the ipsilateral ones.2. Immediately after crushing the unilateral infraorbital nerve, latency of the contralateral EPs evoked by stimulating the crushed side whiskers was prolonged with a significant correlation between the positive and negative component of the EPs. Latency of the EPs simulataneously recorded from the ipsilateral side showed no significant changes. As a result, in some cases latency of the ipsilateral EPs became shorter than that of the contralateral ones. Most of these changes were restored within 3 weeks.3. The contralateral positive and negative EPs evoked by stimulating the crushed side whiskers showed immediate amplitude reduction. Positive EPs recorded from the hemisphere ipsilateral to the stimulation was consistently reduced in amplitude, while in the negative component, increases and decreases in amplitude were found to occur at roughly the same frequency.In the ipsilateral evoked positive component, the amount of amplitude reduction tended to show a positive correlation with the length of time needed to recover control value.4. The ratio of the latency after crushed side stimulation to that after intact side stimulation (C/I ratio) was increased significantly for one week after the nerve crush. The ratio of amplitude of the negative component to the positive component of individual EPs (N/P ratio) was found to be smaller for one week in the EPs evoked on the contralaterral hemisphere by stimulating the crushed side whiskers. The N/P ratio of the EPs on the ipsilateral hemisphere by stimulating the crushed side whiskers was increased at the 1st week after the crushing.These changes returned to normal at the 2nd week. These results indicated that reliable parameters to predict recovery time course of the tactile function after infraorbital nerve crushing would be (1) the N/P ratio of amplitude of the EPs evoked simultaneously on both sides of the head following stimulation of the crushed side whiskers, and (2) the C/I ratio of latency of the EPs evoked through the crossed pathway on the contralateral hemisphere by stimulating the crushed or intact side whiskers.