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

Abstract

The aim of this study was to gauge the possible sprouting of primary afferent fibers following dorsal rhizotomies using Fos-like immunoreactivity (Fos-LI) in order to label neurons activated by formalin injection into the rat forepaw. To assess the functional consequences of possible sprouting of fine diameter primary afferents, we monitored the behavioral responses and to visualize fine afferent fiber sprouting, we examined calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) 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 before the day when formalin was injected in the extremity of the ipsilateral forelimb. Control animals had no rhizotomy but received the formalin injection. In control animals, following the formalin injection the greatest number of Fos-LI neurons was encountered in the superficial laminae 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 values) 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 in 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. Behavioral 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 months after the lesion as compared to the control group. The significant decrease of this behavior observed in C6, C7, C8 cut group at 7 days was not changed after 3 months. The increase in the number of Fos-LI neurons 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. Our data favor the first hypothesis, and in addition, support the use of the Fos-LI technique to assess the functional post-synaptic changes at the dorsal horn level.