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Abstract
In the rat, the superior olivary complex contains lateral and medial olivocochlear
neurones, which respectively innervate two separate targets within the cochlea; the
auditory afferents contacting the inner hair cells and the outer hair cells themselves.
Previous double label immunohistochemical studies have shown that both lateral and
medial olivocochlear neurones are contacted by noradrenergic nerve endings, and electrophysiological
studies on in-vitro rat brain slices have demonstrated that noradrenaline exerts a
direct, predominantly excitatory effect on medial olivocochlear neurones. In this
paper, we have investigated the origin of the noradrenergic input to the superior
olivary complex (SOC). A retrograde tracer, Fluorogold, was used to map the inputs
to the SOC, and this was combined with immunofluorescent staining for dopamine-beta-hydroxylase
(DbetaH) to identify which of the afferent inputs was noradrenergic. These experiments
showed small numbers of neurones double-stained for both Fluorogold and DbetaH in
the A6 cell group (the locus coeruleus). In the A7 cell group, within and medial to
the lateral lemniscus, numerous Fluorogold labelled and DbetaH positive neurones were
found, but no neurones were seen that were double-labelled. In none of the other major
noradrenergic cell groups were labelled Fluorogold neurones ever detected. To confirm
the results obtained by retrograde tracer injections, anterograde tracer injections
with biotinylated dextran amine were made in the locus coeruleus. This resulted in
labelled fibres within all subdivisions of the superior olivary complex. These experiments
indicate that the noradrenergic input to the olivocochlear neurones originates solely
from the locus coeruleus. The small numbers of double-labelled neurones found in the
locus coeruleus indicate a very divergent non-selective noradrenergic input to the
SOC.