Interconnections between the rat dorsal raphe and the locus coeruleus nuclei demonstrated by anterograde tracing with phaseolus vulgaris leucoagglutinin

The projections from the dorsal raphe (DR) to the locus coeruleus (LC) or vice versa were analyzed in the rat using an anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA‐L) combined with serotonin (5‐hydroxytryptamine, 5‐HT) or dopamine‐beta‐hydroxylase (DBH) immunostaining. Following the injection of PHA‐L into the middle DR, DR‐originating fibers with varicosities have contacted DBH‐immunolabeled cells in the rostral, middle, and caudal LC. Axon terminals were also observed in the subcoeruleus nucleus. When the PHA‐L injection was confined within the caudal DR, axonal fibers with varicosities were observed mainly at the rostral pole of the LC. Following the injection of PHA‐L into the caudal, principal LC, labeled fibers with varicosities have contacted 5‐HT‐immunolabeled neurons at dorsomedial, ventromedial, lateral wing, and caudal sub‐divisions of the DR. The present anterograde study suggests that the DR or the LC nuclei communicate with each other in order to perform a variety of functions including vigilance, analgesia, and stress responses.