Plasticity of response properties of Inferior Colliculus neurons following
Acute Cochlear Damage.
Wang, Jian, Richard J. Salvi and Nicholas Powers.
Hearing Research Lab, 215 Parker Hall, State University of New York at
Buffalo, Buffalo, New York, 14214.
APStracts 2:0274N, 1995.
SUMMARY AND CONCLUSIONS
1. The discharge patterns of 40 neurons in the central nucleus of the inferior
colliculus (ICC) of the chinchilla were evaluated before and after acute
cochlea trauma from intense tone exposure. Single unit recordings were
obtained from neurons in the ICC contralateral to the sound-stimulated ear.
Cochlear trauma was induced with a short-duration (15-25 minutes), high-
intensity pure tone (95-115 dB SPL) at a frequency above the neuron's
characteristic frequency (CF). The aim of the traumatizing exposure was to
damage the peripheral sensory receptors associated with frequencies above the
neuron's CF. The damage was expected to attenuate inputs to the neural
circuits responsible for activating the inhibitory sidebands above CF. 2.
Three types of frequency-threshold tuning curves were observed before the
exposure: (1) open V tuning curves (55%) that became wider as sound intensity
increased; (2) level-tolerant tuning curves (37.5%) which maintained a very
narrow bandwidth even at high sound intensities; (3) upper-threshold tuning
curves (7.5%) in which excitatory responses were elicited at low and moderate
intensities, but not at high intensities. The traumatizing exposure caused a
dramatic widening of level-tolerant (80% of sample) and upper-threshold tuning
curves (100% of sample) at high sound intensities, but caused almost no change
in the low-threshold tip of the tuning curves. By contrast, tuning curves with
an open V configuration were generally unaffected (approximately 90% of
sample) by the traumatizing exposure. 3. Discharge rate-level functions in the
ICC were of two general types: (1) strongly nonmonotonic (60%) or (2)
saturating, monotonic (40%). The traumatizing exposure caused a significant
increase in the suprathreshold discharge rates in 70% of all neurons studied.
Among the neurons with strongly nonmonotonic discharge rate-level functions,
93% showed a significant increase in discharge rate. 4. The post-stimulus time
histograms (PSTH) to tone bursts were of three main types: (1) onset, (2)
pauser, and (3) sustained responders. The traumatizing exposure had almost no
effect on the PSTH of onset or sustained responders. However, pause PSTH
frequently (75%) showed a significant decrease in the pause duration and an
increase in the sustained discharge rate following the pause after the
exposure. 5. The results suggest that the response properties of neurons with
extremely narrow tuning curves and nonmonotonic discharge rate-level functions
are shaped by an inhibitory circuit that is activated by frequencies above
the high frequency flank of the tuning curve. This inhibitory circuit
modifies the excitatory response in the following ways: (1) it narrows the
excitatory response area at suprathreshold intensities particularly at
frequencies below CF (2) it alters the shape of the discharge rate-level
function by suppressing the discharge rates at suprathreshold intensities,
and (3) it modifies the temporal discharge pattern of the pause PSTH by
suppressing neural activity that occurs after the onset response. The effects
of these inhibitory inputs can be reduced (disinhibition) by damaging the
sensory cells in the inner ear that activate this circuit. The exact locus of
the inhibitory circuit(s) is unknown, but may involve inhibitory inputs
located at the level of the cochlear -nucleus and/or at levels up to the
inferior colliculus.
Received 10 April 1995; accepted in final form 23 August 1995.
APS Manuscript Number J236-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 September 1995.