FOS-LIKE IMMUNOREACTIVITY IN THE RAT SPINAL-CORD INDUCED BY FORMALIN INJECTION IN THE FORELIMB TO GAUGE POSSIBLE PLASTICITY OF PRIMARY AFFERENT-FIBERS FOLLOWING PARTIAL DEAFFERENTATION

The aim of this study was to gauge the possible sprouting of primary a fferent fibers following dorsal rhizotomies using Fos-like immunoreact ivity (Fos-L!) in order to label neurons activated by formalin injecti on into the rat forepaw. To assess the functional consequences of poss ible sprouting of fine diameter primary afferents, we monitored the be havioral responses and to visualize fine afferent fiber sprouting, we examined calcitonin gene-related peptide-like immunoreactivity (CGRP-L I) in the superficial laminae of the dorsal horn. Two types of dorsal rhizotomies and two delays post-rhizotomy were used: one dorsal root, C7 (C7 cut) or three consecutive dorsal roots, C6, C7 and C8 (C6, C7, C8 cut), were sectioned on the right side three months or 7 days befor e the day when formalin was injected in the extremity of the ipsilater al forelimb. Control animals had no rhizotomy but received the formali n injection. In control animals, following the formalin injection the greatest number of Fos-LI neurons was encountered in the superficial l aminae and in the neck of the dorsal horn of segments C5-T1. In the C7 cut group, the number of Fos-LI neurons was slightly decreased in all segments 7 days after the lesion whereas it was slightly increased 3 months after the lesion as compared to 7 days. In C6, C7 C8 cut group, the number of Fos-LI significantly decreased (90% of the control valu es) 7 days after the lesion, but after three months, it significantly increased in segments C7 and C8 as compared to 7 days. In parallel in this latter group, a marked depletion of CGRP-LI fibers was observed i n the medial part of the superficial laminae at 7 days whereas a clear increase in CGRP-LI occured in the same region at 3 months. Behaviora l observations showed a slight decrease in the licking time induced by the formalin injection in the C7 cut group both at 7 days and 3 month s after the lesion as compared to the control group. The significant d ecrease of this behavior observed in C6, C7, C8 cut group at 7 days wa s not changed after 3 months. The increase in the number of Fos-LI neu rons after 3 months in the C6, C7, C8 cut group is discussed in terms of collateral sprouting of thin primary afferent fibers and/or central compensatory mechanisms in response to peripheral deafferentation. Ou r data favor the first hypothesis, and in addition, support the use of the Fos-LI technique to assess the functional post-synaptic changes a t the dorsal horn level.