I. Cytoarchitecture of the Locus Coeruleus area
The cytoarchitecture of the LC area has been described previously 25,
66. Nevertheless, in the course
of our study of LC afferents, we found additional undefined landmarks
in this area. To study the organization of the LC area, 20µm frontal
LC sections taken every 100µm from control rat brains were stained
with neutral red and mounted in rostro-caudal order. At its more caudal
tip, the LC is separated medially from the fourth ventricle by a 30µm
thick cell poor lamina and the 20 µm thick ependyma. Laterally and
ventrally, the nucleus is bordered by the MVe. Dorsally, the LC is bordered
by the brachium conjunctivum (Fig. 2F) . Slightly
rostrally, the medially located cell poor lamina is thicker (approximately
100µm) and the 5Me appears laterally. At this level, the 5Me is
separated from the LC by an area containing few cells in white matter
(Fig. A HREF="../figures/fig2b.php">2B) that appears to be a rostral
extension of the MVe. Slightly more rostrally, at the level of the maximum
extension of the LC, this region becomes thinner and the 5Me is very close
to the lateral border of the LC. Medially, the LC is separated from the
ventricle by a large undefined area that appears to be part of the pontine
periaqueductal gray. At this level and more rostrally, the LC is bordered
dorsally by the medial parabrachial nucleus (Fig. 3C).
More rostrally, the ventral part of the LC progressively becomes less
compact and then disappears and only a dorsal collection of LC cells remains
(Fig. 3C). Ventral to them, two groups of cells
appear with a morphology distinct from the adjacent periaqueductal gray.
One is located just ventral to the few LC cells and medial to the 5Me
and is composed of small ovoid cells. The second is located more medially
and ventrally and is composed of slightly larger ovoid cells. The first
group has not been previously defined. Therefore, because of its localization
just medial to the 5Me, we denote this as the Peri-5Me nucleus (Fig. 3C).
The second group of cells corresponds to t he previously defined Barrington's
nucleus (Bar) 8,59.
With regard to the LC noradrenergic neurons, it is noteworthy that a large
number of processes (apparently dendrites) were found outside the nuclear
core of the LC in select peri-LC regions, particularly in the lamina between
the LC and the fourth ventricle and also in the a reas rostromedial and
caudodorsal to the LC nucleus proper. Note that LC dendrites did not appear
to be extensive lateral to the LC nucleus proper. This topographically
specific distribution of presumed LC dendrites is similar to that previously
reported in a more extensive analysis 22,61.