Search

Kainic acid (KA) selectively damages afferent synapses that innervate, in chickens, mainly tall hair cells. To better understand the nature of KA-induced excitotoxic damage to the cochlear afferent neurons, KA, at two different concentrations (0.3 or 5 mM), was injected directly into the inner ear of adult chickens. Pathologic changes in the afferent nerve ending and cell body were evaluated with light and transmission electron microscopy at various time points after KA application. The compound action potential (CAP) and cochlear microphonic (CM) potential were recorded to monitor the physiologic status of the afferent neurons and hair cells, respectively. Hair cell morphology and function were essentially normal after KA treatment. However, afferent synapses beneath tall hair cells were swollen within 30 minutes after KA at both low (KA-L) and high (KA-H) doses. In the KA-L group, the swelling disappeared within 1 day and the morphology of the postsynaptic region returned to near normal condition. In the KA-H group, by contrast, the vacant region beneath tall hair cells remained evident even 20 weeks after KA. The number of cochlear ganglion neurons in the KA-H group decreased progressively from 1 to 8-20 weeks, whereas hair cells in the basilar papilla remained morphologically intact out to 20 weeks after KA. There was no significant change in neuron number in the KA-L group. Temporal changes in the CAP amplitude paralleled the anatomic changes, although the CAP only partially recovered. These results suggest that KA induces partially reversible damage to cochlear afferent neurons with low KA concentration; above this level, KA triggers irreversible, progressive neurodegeneration., (Copyright 2001 Wiley-Liss, Inc.)

The aim of the present study was to examine the role of the olivocochlear system in auditory processing by examining the long-term effects of cochlear de-efferentation on auditory nerve response properties in adult chinchillas. Spontaneous rates, response thresholds, tuning curves, discharge rate-level functions, and adaptation of single auditory nerve fibers were measured in chinchillas with complete cochlear de-efferentation produced by sectioning the olivocochlear bundle in the internal auditory meatus. De-efferentation was verified as successful on the basis of acetylcholinesterase staining of surface preparations of the organ of Corti. Following chronic de-efferentation, there was a striking decrease in spontaneous rate, consistent with earlier observations in cats. In addition, the present study shows that complete de-efferentation results in: (1) increased driven discharge rates and decreased dynamic range of discharge rate-level functions, (2) larger onset-to-steady state ratio of discharge rate at moderate intensities, and (3) a hypersensitive tail of the tuning curve. These effects, largely confined to neurons that were most sensitive to frequencies between 2-8 kHz, indicate that the cochlear efferent system is important in maintaining normal function (e.g., frequency and intensity selectivity) of the auditory periphery by modulating auditory nerve fiber response properties.