Immunohistochemical evidence for the adrenergic medullary longitudinal bundle as a major ascending pathway to the locus coeruleus.

The present study describes the first experimental evidence for a direct projection of the medulla oblongata adrenergic (Ad) neurons to the rat pontine structures by using the indirect immunoperoxidase technique. Three weeks after unilateral electrolytic lesion of the longitudinal axon bundle in the medulla oblongata, the morphological changes of the phenylethanolamine-N-methyltransferase (PNMT)-immunoreactive (IR) structures located in the rat brainstem have been analysed. In the lesioned rats we observed a decrease in the number of PNMT-IR structures in virtually all regions of the brainstem ipsilateral to the lesion, especially in the locus coeruleus (LC). These results indicate that the PNMT-IR terminal-like fibers of the LC are derived from the ipsilateral medulla oblongata Ad neurons and are mainly provided by the longitudinal axon bundle.