Descending projections from the prefrontal cortex to the locus coeruleus of the rat

The fiber projection from the prefrontal cortex to the locus coeruleus (LC) in the periventricular region was analyzed in rat using anterograde and retrograde tracing methods. Following injection of an anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA‐L), into prelimbic and infralimbic regions of the medial prefrontal cortex, labeled axonal fibers with varicosities were observed bilaterally within the LC, with ipsilateral predominance. Terminal labeling was also observed in the region medial to the nucleus at rostral to middle levels of the LC, whereas axonal labeling in the caudal LC was minimal. Anterogradely‐labeled axonal fibers were not found in the subcoerulear region. A retrograde tracer, gold‐conjugated and inactivated wheatgerm‐agglutinin horseradish‐peroxidase (WGA‐apo‐HRP‐gold), was injected into several rostro‐caudal levels of the LC. Majority of retrogradely‐labeled cells were observed in the prelimbic or infralimbic regions of the medial prefrontal cortex when the injections were made into rostral to middle levels of the LC. Only a few cells were observed in cingulate, dorsal peduncular, orbital, or insular cortices. The present findings suggest that the nucleus LC receives restricted, excitatory inputs from cognitive, emotional, and autonomic centers of the cerebral cortex and might secondarily have influences on widespread brain regions via its diversified monoaminergic innervation.