Modification of serotonergic immunoreactive pattern in the dorsal horn of the rat spinal cord following dorsal root rhizotomy.

We investigated by immunocytochemistry the reorganisation of serotonergic endings in the dorsal horn of the rat spinal cord following thoracolumbar unilateral rhizotomy. The extent of the removal of primary afferent fibres was testified 2 weeks and 4 months after surgery by the disappearance of almost all calcitonin gene-related peptide (CGRP) immunoreactivity in the dorsal horn. Two weeks after surgery, serotonin immunoreactivity was severely reduced within the whole dorsal horn. Then, after 4 months, we observed a regrowth of serotonergic axons giving rise to an immunoreactive pattern close to that of intact animals, except in lamina IIi which was spanned by growing fibres. Thus, the removal of putative targets of serotonergic axons in the dorsal horn induces a transient loss of immunoreactivity, which illustrates the interaction between primary afferents and descending bulbo-spinal fibres.