Transneuronal changes of synaptic endings and nuclear chromatin in the trapezoid body following cochlear ablations in cats

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Abstract
This study demonstrates long‐lasting structural changes in highly specific synaptic endings, the calyces of Held, and in the principal cells, contacted by calyces, in the medial trapezoid nucleus following acoustic deafferentation of adult cats. The normal structure of the principal cell and its afferent axosomatic endings, including the calyx and other, smaller endings, was defined in rapid Golgi impregnations and electron micrographs. Each calyciferous axon, arising from the contralateral cochlear nucleus, forms a calyx, about 35 μm in diameter, around only one principal cell body; each principal cell receives only one calyx. In electron micrographs a calycine profile typically contains a central core of neurofilaments, surrounded by mitochondria; spherical vesicles gather at multiple, asymmetric synaptic complexes. Fifteen and 30 days after unilateral labyrinthectomy the contralateral calyces showed neurofilamentous hyperplasia; mitochondria and the now enlarged synaptic vesicles decreased in number. After 30 days the contralateral principal neurons showed increased condensation of nuclear chromatin. All of these changes diminished after 56 days and were gone by seven months, when the principal cell body had shrunk by 30%. The calyx, which exhibits strongly excitatory synaptic transmission spontaneously and during acoustic stimulation, could mediate significant trophic effects. Such effects are no doubt reflected in the extensive transneuronal changes after interruption of cochlear input. These changes must involve pre‐ and post‐synaptic alterations of protein metabolism, control of which may be linked to synaptic transmission.

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