DEVELOPMENT OF ECTOPIC PROJECTIONS FROM THE VENTRAL COCHLEAR NUCLEUS TO THE SUPERIOR OLIVARY COMPLEX-INDUCED BY NEONATAL ABLATION OF THE CONTRALATERAL COCHLEA
Lm. Kitzes et al., DEVELOPMENT OF ECTOPIC PROJECTIONS FROM THE VENTRAL COCHLEAR NUCLEUS TO THE SUPERIOR OLIVARY COMPLEX-INDUCED BY NEONATAL ABLATION OF THE CONTRALATERAL COCHLEA, Journal of comparative neurology, 353(3), 1995, pp. 341-363
The ability of an animal to localize a sound in space requires the pre
cise innervation of the superior olivary complex by the ventral cochle
ar nuclei on each side of the lower brainstem. This precise pattern of
innervation could require an immutable recognition of appropriate tar
gets by afferent processes arising from these nuclei. This possibility
was investigated by destroying one cochlea of gerbil pups (Meriones u
nguiculatus) on the second postnatal day and assessing the projections
from the ventral cochlear nucleus (VCN) on the unablated side to the
superior olivary complex during the subsequent 2 weeks and after the a
nimals had reached maturity. A crystal of -dioctadecyl-3,3,3',3'-tetra
methylindocarbocyanine perchlorate (DiI) was inserted into VCN on the
unablated side in animals ranging in age from 3 to 14 days. To assess
the permanence of any altered pattern of innervation, horseradish pero
xidase was injected into VCN on the unablated side in adult, neonatall
y ablated animals. Finally, electrophysiological responses to acoustic
stimuli delivered to the ear on the unablated side were recorded in t
he superior olivary complex of adult animals to assess whether altered
innervation patterns were functional. Normative data were derived fro
m our accompanying study of the development of VCN projections to the
superior olivary complex in normal gerbils (Kil et al., this issue). W
hereas VCN normally projects to the lateral aspect of the ipsilateral
medial superior olive and to the medial aspect of the contralateral me
dial superior olive in control animals, in experimental animals VCN on
the unablated side projects to both sides of these nuclei. Whereas in
the gerbil, VCN normally projects only to the hilar area and to the v
entrolateral limb of the contralateral lateral superior olive, in expe
rimental animals VCN on the unablated side projects throughout this nu
cleus. This induced projection is specific in that the efferents to ea
ch limb of the contralateral nucleus are linked to the normal projecti
on to the homotopic region of the ipsilateral nucleus. Whereas VCN inn
ervates the contralateral medial nucleus of the trapezoid body in cont
rol animals, in experimental animals VCN on the unablated side provide
s calyces of Held in the ipsilateral nucleus as well. The induced proj
ections to these three major subnuclei of the superior olivary complex
first appear within 24 hours of the cochlear ablation and continue to
develop over at least the subsequent 11 days. Thus, prior to the day
when the cochlea becomes functional, VCN has established specific ecto
pic projections to loci normally innervated by VCN on the ablated side
. All induced ectopic projections observed in neonatal animals were al
so present in neonatally ablated adult animals, indicating their perma
nence. As in the case of the normal projections from VCN to these subn
uclei, all labeled synapses contained round vesicles and formed asymme
tric synaptic junctional complexes with dendrites and somata in these
target nuclei. Electrophysiological responses of single units in the i
psilateral medial nucleus of the trapezoid body demonstrated that the
induced calyces of Held in this structure are quite functional in adul
t animals. (C) 1995 Wiley-Liss, Inc.