The distribution and orientation of noradrenergic fibers in neocortex of the rat: An immunofluorescence study

Abstract A homologous antiserum directed against rat dopamine‐β‐hydroxylase (DBH) was employed in the indirect immunofluorescence technique of Coons ('58) to describe the distribution and orientation of noradrenergic (NA) axons in the cerebral neocortex of the adult albino rat. DBH is the terminal enzyme in the biosynthesis of noradrenaline and is a sensitive anatomic marker for NA neurons and their processes. Cryostat sections in three standard planes (coronal, sagittal and tangential) taken from four cortical fields (somatosensory, motor, visual and anterior cingulate) were processed for immunofluorescence, and adjacent sections were stained with a Nissl method to relate the pattern of NA axons to specific cortical laminae. Noradrenaline levels and DBH activity were measured in the same cortical fields by radioenzymatic assays. A rich network of DBH immunoreactive axons was observed throughout all cortical layers in each of the areas examined. The pattern of NA distribution is not diffuse, but is characterized by a geometric orderliness that is uniform throughout the neocortex, with only minor regional variations. In layer I the DBH‐positive fibers are predominantly parallel to the pial surface and thus form a tangential grid‐like pattern. Layers II and III are characterized by straight, radial fibers that rarely extend through layer IV to the infragranular layers. There appears to be a NA terminal field in layers IV and V where there is a network of short, oblique or tortuous axon segments, especially prominent in lateral neocortex. Layer VI is unique in that the prevailing orientation of NA fibers is in the anterior‐posterior direction. This band of fibers does not extend into the sub‐cortical white matter, where very few NA fibers are seen. The strong tangential projection of the NA neurons, particularly in layers I and VI, emerges as a dominant feature of this afferent system and appears to be superimposed on the predominantly vertical organization of the cortex. This tangential pattern of organization may allow the locus coeruleus to modulate the activity of adjacent columns synchronously over a vast expanse of cerebral cortex.