AFFERENT-PROJECTIONS TO THE RAT LOCUS-COERULEUS DEMONSTRATED BY RETROGRADE AND ANTEROGRADE TRACING WITH CHOLERA-TOXIN-B SUBUNIT AND PHASEOLUS-VULGARIS-LEUKOAGGLUTININ

The aim of this study was to examine the afferents to the rat locus co eruleus by means of retrograde and anterograde tracing experiments usi ng cholera-toxin B subunit and phaseolus leucoagglutinin. To obtain re liable injections of cholera-toxin B in the locus coeruleus, electroph ysiological recordings were made through glass micropipettes containin g the tracer and the noradrenergic neurons of the locus coeruleus were identified by their characteristic discharge properties. After iontop horetic injections of cholera-toxin B into the nuclear core of the loc us coeruleus, we observed a substantial number of retrogradely labeled cells in the lateral paragigantocellular nucleus and the dorsomedial rostral medulla (ventromedial prepositus hypoglossi and dorsal paragig antocellular nuclei) as previously described.(6) We also saw a substan tial number of retrogradely labeled neurons in (1) the preoptic area d orsal to the supraoptic nucleus, (2) areas of the posterior hypothalam us, (3) the Kolliker-Fuse nucleus, (4) mesencephalic reticular formati on. Fewer labeled cells were also observed in other regions including the hypothalamic paraventricular nucleus, dorsal raphe nucleus, median raphe nucleus, dorsal part of the periaqueductal gray, the area of th e noradrenergic A5 group, the lateral parabrachial nucleus and the cau doventrolateral reticular nucleus. No or only occasional cells were fo und in the cortex, the central nucleus of the amygdala, the lateral pa rt of the bed nucleus of the stria terminalis, the vestibular nuclei, the nucleus of the solitary tract or the spinal cord, structures which were previously reported as inputs to the locus coeruleus.(10,13) Con trol injections of cholera-toxin B were made in areas surrounding the locus coeruleus, including (1) Barrington's nucleus, (2) the mesenceph alic trigeminal nucleus, (3) a previously undefined area immediately r ostral to the locus coeruleus and medial to the mesencephalic trigemin al nucleus that we named the peri-mesencephalic trigeminal nucleus, an d (4) the medial vestibular nucleus lateral to the caudal tip of the l ocus coeruleus. These injections yielded patterns of retrograde labeli ng that differed from one another and also from that obtained with cho lera-toxin B injection sites in the locus coeruleus. These results ind icate that the area surrounding the locus coeruleus is divided into in dividual nuclei with distinct afferents. These results were confirmed and extended with anterograde transport of cholera-toxin B or phaseolu s leucoagglutinin. Injections of these tracers in the lateral paragiga ntocellular nucleus, preoptic area dorsal to the supraoptic nucleus, t he ventrolateral part of the periaqueductal gray, the Kolliker-Fuse nu cleus yielded a substantial to large number of labeled fibers in the n uclear core of the locus coeruleus. Anterograde transport of cholera-t oxin B or phaseolus leucoagglutinin from the posterior hypothalamic ar eas yielded a moderate to small number of labeled fibers in the nuclea r core of the locus coeruleus. These anterograde tracing experiments c onfirm that these areas send direct projections to the rat locus coeru leus. Importantly, fiber labeling from each of these areas was in most cases much denser in areas immediately surrounding the locus coeruleu s than in the locus coeruleus proper. In particular, the lamina and th e periaqueductal gray medial to the locus coeruleus where many dendrit es of locus coeruleus noradrenergic cells are located contained a larg e number of fibers. These data might indicate that a large number of t he afferents to the noradrenergic neurons of the locus coeruleus termi nate on dendrites outside the dense core of the nucleus. Further elect rophysiological as well as ultrastructural studies are necessary to te st this hypothesis.