Fos expression in the medullary dorsal horn of the rat after chronic constriction injury to the infraorbital nerve

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Abstract
Chronic constriction injury to the rat's infraorbital nerve (IoN‐CCI) induces asymmetric face grooming directed to the injured nerve territory and, beginning at 7‐‐12 days postoperative, hyperresponsiveness to mechanical stimulation in this territory (B. P. Vos, A. M. Strassman, and R. J. Maciewicz, 1994, J. Neurosci. 14:2708‐‐2723). To examine central mechanisms involved in these behavioral alterations, changes in nonevoked and mechanical stimulationevoked fos‐like immunoreactivity (fos‐LI) following IoN‐CCI were quantified in the medullary dorsal horn.Following the appearance of hyperresponsiveness in IoN‐CCI rats, experimental and matched sham‐operated rats were anesthetized with urethane and received either no stimulation or repeated stimulation with either a 2‐ or 15‐g von Frey hair applied to the hairy skin between vibrissae B3‐4/C3‐4 on the operated side.Unstimulated IoN‐CCI rats had increased fos‐LI in laminae I‐IV of the ipsilateral medullary dorsal horn. In both groups, mechanical stimulation produced a distinct pattern of fos‐LI in the ipsilateral medullary dorsal horn, the quantity of which was related to stimulus intensity. For both stimulus intensities, the total amount and the rostrocaudal spread of evoked fos‐LI were significantly larger in IoN‐CCI rats. In IoN‐CCI rats, stimulation‐evoked increases in fos‐LI were proportionally larger in laminae I‐II than in III‐IV. This laminar effect was also present in sham‐operated rats but only for 15‐g stimulation. Neither condition nor stimulus intensity affected fos‐LI in the contralateral medullary dorsal horn. Positive correlations were found between the behavioral parameters of increased trigeminal nociceptive activity and the total amount of fos‐LI in the ipsilateral medullary dorsal horn.The results demonstrate that IoN‐CCI induces significant alterations in the central processing of afferent signals, which may underlie behavioral manifestations of increased nociceptive activity.

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