Efferent projections of different subpopulations of central noradrenaline neurons.

Early anatomical studies of the projections of central noradrenergic (NA) neurons led to the widely accepted view of NA cells as a class of diffusely projecting neurons. This view greatly influenced the formulation of numerous hypotheses about the functional role of these neurons in the central nervous system (CNS). With the introduction of transmitter-specific retrograde and anterograde transport methods, two powerful tools became available to rigorously re-examine whether the projections of NA neurons are diffuse or topographically organized. This article summarizes some of the results of these studies in which retrograde transport of fluorescent tracers and anterograde transport of the lectin Phaseolus vulgaris leucoagglutinin (PHA-L), respectively, were combined with immunohistochemical identification of NA neurons and their projections. The results of these studies revealed a remarkable degree of specificity in the projections of different subgroups of NA neurons. In the rat CNS, the differential distribution of NA axons of the locus coeruleus (LC) and non-coerulean NA cells is particularly striking in the spinal cord and brainstem. In these regions, NA axons of the LC are primarily distributed to sensory nuclei while NA axons of non-coerulean NA neurons are distributed to motor nuclei. The results support the proposition that NA neurons can be divided into subgroups which differ in their connections and hence represent separated anatomical entities with different functional capacities.