Normal development and the effects of early rhizotomy on spinal systems in the rat

Abstract

The normal postnatal development of 4 spinal systems was examined in the dorsal horn of the rat spinal cord using histochemical and immunocytochemical techniques. We used thiamine monophosphatase (TMPase), a marker for dorsal root ganglion cells and their projections, a tachykinin, substance P (SP), which is provided by both dorsal root and intrinsic systems, and two markers for descending systems, serotonin (5-HT) and the synthesizing enzyme for noradrenalin, dopamine B-hydroxylase (DBH). The responses of each of these systems to unilateral dorsal lumbosacral rhizotomy on postnatal day 5 was then examined and quantified using image analysis methods to determine whether the extent of plasticity of spinal systems is different after a neonatal lesion than after a comparable lesion made in the adult. Each system differs in development, distribution, and in response to rhizotomy. TMPase is present in the dorsal horn on the day of birth (DPN0) and reaches adult levels of density by 5 days postnatal (DPN5). SP reaction product is present in a distribution similar to the adult in the dorsal horn on DPN0 and reaches adult levels of density by the second postnatal week. 5-HT is present in the dorsal horn on DPN0, shows a laminar distribution at DPN5, and acquires the adult distribution and density at the end of the second week. DBH is present in the dorsal horn on DPN0, acquires the adult distribution at DPN5 and adult levels of density at the end of the second postnatal week. Unilateral lumbosacral rhizotomy in 5 day old rats completely and permanently abolishes TMPase in the dorsal horn by 4 days postoperatively (4DPO). SP is decreased by 4 DPO (9 DPN) but recovers almost completely by 30 DPO. 5-HT is increased by 10 DPO and remains elevated thereafter. DBH is not changed postoperatively. There is shrinkage of lamina I and II by 10 DPO but the recovery of SP and the increase in density of 5-HT staining is proportionally greater than the extent of shrinkage. Therefore, shrinkage contributes to but does not entirely account for either the apparent recovery of SP staining or the increase in density of 5-HT staining. The responses of the TMPase, 5-HT and DBH systems to neonatal rhizotomy are very similar to the response to rhizotomy in adults and there is therefore no evidence for greater plasticity of these systems after neonatal rhizotomy than after adult rhizotomy. The SP systems show more rapid depletion and a greater and more rapid recovery than after adult deafferentation. SP containing spinal systems are therefore capable of greater plasticity following neonatal lesions than after adult lesions.