Noise-induced hyperactivity in the inferior colliculus: its relationship with hyperactivity in the dorsal cochlear nucleus.

Intense noise exposure causes hyperactivity to develop in the mammalian dorsal cochlear nucleus (DCN) and inferior colliculus (IC). It has not yet been established whether the IC hyperactivity is driven by hyperactivity from extrinsic sources that include the DCN or instead is maintained independently of this input. We have investigated the extent to which IC hyperactivity is dependent on input from the contralateral DCN by comparing recordings of spontaneous activity in the IC of noise-exposed and control hamsters before and after ablation of the contralateral DCN. One group of animals was binaurally exposed to intense sound (10 kHz, 115 dB SPL, 4 h), whereas the control group was not. Both groups were studied electrophysiologically 2-3 wk later by first mapping spontaneous activity along the tonotopic axis of the IC to confirm induction of hyperactivity. Spontaneous activity was then recorded at a hyperactive IC locus over two 30-min periods, one with DCNs intact and the other after ablation of the contralateral DCN. In a subset of animals, activity was again mapped along the tonotopic axis after the time course of the activity was recorded before and after DCN ablation. Following recordings, the brains were fixed, and histological evaluations were performed to assess the extent of DCN ablation. Ablation of the DCN resulted in major reductions of IC hyperactivity. Levels of postablation activity in exposed animals were similar to the levels of activity in the IC of control animals, indicating an almost complete loss of hyperactivity in exposed animals. The results suggest that hyperactivity in the IC is dependent on support from extrinsic sources that include and may even begin with the DCN. This finding does not rule out longer term compensatory or homeostatic adjustments that might restore hyperactivity in the IC over time.

Stimulus-dependent changes in optical responses of the dorsal cochlear nucleus using voltage-sensitive dye.

Optical imaging with voltage-sensitive dye was used to examine the spatiotemporal dynamics of stimulus-driven activity on the surface of the dorsal cochlear nucleus (DCN). Stimulation with tones at low to moderate levels produced localized regions of activation that were most commonly elongated rostrocaudally. The size of these activation areas expanded with increases in sound level, while their centers shifted from the lateral direction to the medial direction with increases in stimulus frequency. In contrast to the tonotopic patterns of activation evoked by tones, electrical stimulation of the DCN surface resulted in bands of activation that were elongated along the medial-lateral axis; response latencies increased with distance along these bands from the point of stimulation. Shifting the site of electrical stimulation from the rostral direction to the caudal direction induced corresponding shifts in the rostrocaudal location of the activation band; moving the electrode tip to subsurface depths resulted in loss of the elongated band. Transecting the DCN along the rostrocaudal axis midway between its medial and lateral extremities blocked propagation of the response to the half of the DCN distal to but not proximal to the stimulating electrode. The results suggest that the two modes of stimulation activated two distinct populations of neurons, one involving primarily tonotopically organized cells and the other crossing these tonotopic zones and likely representing the activation of parallel fibers. These results reveal a number of new features in the spatial patterns of tone-elicited activation that are not readily predicted by responses recorded electrophysiologically.