Quantitative study of degeneration and new growth of axons and synaptic endings in the chinchilla cochlear nucleus after acoustic overstimulation.

To determine if acoustic overstimulation altered synaptic connections in the cochlear nucleus, anesthetized adult chinchillas, with one ear protected by a silicone plug, were exposed for 3 hr to a 108-dB octave-band noise, centered at 4 kHz, and allowed to survive for periods up to 32 weeks. This exposure led to cochlear damage in the unprotected ear, mainly in the basal regions of the organ of Corti. The anterior part of the ipsilateral posteroventral cochlear nucleus consistently contained a band of degenerating axons and terminals, in which electron microscopic analysis revealed substantial losses of axons and synaptic terminals with excitatory and inhibitory cytology. The losses were significant after 1 week's survival and progressed for 16-24 weeks after exposure. By 24-32 weeks, a new growth of these structures produced a resurgence in the number of axons and terminals. The net number of excitatory endings fully recovered, but the quantity with inhibitory cytology was only partially recouped. Neuronal somata lost both excitatory and inhibitory endings at first and later recovered a full complement of excitatory but not inhibitory terminals. Dendrites suffered a net loss of both excitatory and inhibitory endings. Excitatory and inhibitory terminals with unidentified postsynaptic targets in the neuropil declined, then increased in number, with excitatory terminals exhibiting a greater recovery. These findings are consistent with a loss and regrowth of synaptic endings and with a reorganization of synaptic connections that favors excitation.