An electron microscope autoradiographic study of regenerating locus coeruleus fibers into iris tissue implants in the...

Following interruption, fibers from the locus coeruleus have been shown to undergo axonal regeneration and invade peripheral tissue implants in the rat brain. The aim of the present study was to analyze the ultrastructural features of this phenomenon using electron microscope autoradiography. For this, iris tissue implants were inserted in the region of the dorsal tegmental bundle (DTB) in the mesencephalon of unilaterally sympathectomized rats. Approximately 2 weeks later, rats were injected with [3H]-leucine in the ipsilateral locus coeruleus and sacrificed 2 days later. The tissue was prepared for EM autoradiography and studied after an incubation period of at least 12 weeks. The results showed that regenerating fibers from the locus coeruleus were most dense in regions of implant proximal to the DTB. Within the implants, less than one-half of regenerating axons present were labeled and, hence, of locus coeruleus origin. The labeled fibers were characteristically thin and unmyelinated. At this relatively short survival time, there was no evidence that the regenerating locus coeruleus axons formed synaptic specializations or contacts with the surrounding iris neuropil. Thus, while these observations confirm previous reports describing the regenerative capacity of central monoaminergic axons, they fail to provide anatomical evidence of the establishment of functional interactions between incoming fibers and the target tissue.