Effect of grafting order on innervation of spinal cord transplants by grafted locus coeruleus neurons in oculo

Abstract

Fetal brain stem containing locus coeruleus/subcoeruleus (hereafter referred to as LC) and thoracic spinal cord (SC) were sequentially allografted into the anterior eye chamber of adult Sprague-Dawley albino rats creating two groups: (1) LC graft followed after 11 weeks by an SC graft (LC-SC); (2) SC graft followed after 11 weeks by an LC graft (SC-LC). The cografts were allowed to mature in oculo for 15-18 months. After sacrifice, the grafts were processed for the immunohistochemical localization of tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (D beta H) and the extent of fiber ingrowth into the SC graft was measured using computerized image analysis. TH- and D beta H-immunoreactive fibers were found to innervate the SC cograft in five of the six SC-LC graft combinations. The innervation was abundant, but uneven in distribution. The average density of TH-immunoreactive fibers (derived from the LC graft) was 8.29% of the total cross-sectional area of the SC graft neuropil. In contrast, the innervation of the SC graft neuropil was very sparse in the LC-SC graft combinations, with an uneven distribution and an average density of TH-immunoreactive fibers in the SC graft of only 2.28% of the cross-sectional area of the SC graft neuropil. The results support earlier studies of intraspinal grafting of LC neurons, in that embryonic LC neurons upon grafting and during ontogenetic fiber growth are capable of innervating mature spinal cord neuropil devoid of its normal catecholaminergic innervation. However, embryonic spinal cord tissue is a poor stimulant to reinitiate terminal fiber growth from mature LC neurons, in contrast to several other LC target areas such as the cerebral cortex and hippocampus.