The effects of furosemide on distortion product otoacoustic emissions and
on neuronal responses in the anteroventral cochlear nucleus.
Rubsamen, Rudolf, David M. Mills, Edwin W Rubel, Virginia Merrill Bloedel.
Hearing Research Center, University of Washington, Seattle, WA 98195
(USA).
APStracts 2:0174N, 1995.
SUMMARY AND CONCLUSIONS
1. The objective of this study was to precisely evaluate the relationship
between the threshold of neurons in the anteroventral cochlear nucleus (AVCN)
and the properties of distortion product otoacoustic emissions (DPOAEs).
Response areas of multiunit clusters in the AVCN and DPOAEs in the ear canal
were measured alternately in the adult gerbil during furosemide induced
changes of the endocochlear potential (EP). Stimulus frequencies of the probe
tones for DPOAE measurement were in the range of f 1 =1.7-7.6 kHz and f 2 =
2.0-9.0 kHz; the ratio f 2 /f 1 was always 1.18. Stimulus amplitudes were
varied in 5 dB steps from 30 to 80 dB SPL, with either equal amplitudes (L l =
L 2 ) or unequal, with L l set 10 dB above L 2 . Multiunit response areas were
determined from cluster responses to a series of 100 ms tone bursts presented
with a pseudorandom sequence in frequency and intensity. 2. Changes in the
multiunit discharge properties after 50-75 mg/kg furosemide injection were as
follows: The best frequency (BF) threshold increased from initial values in
the range of 20-30 dB SPL to 50-80 dB SPL at 10-20 minutes post-injection,
then recovered fully by 60-90 min. The spontaneous discharge activity
decreased to zero before any changes in the FTC were observed and did not
return to initial values for several hours. Likewise, total discharge rates of
stimulus elicited responses were reduced and tended to stay reduced even after
BF threshold had fully recovered. 3. From the DPOAE measurements, the changes
observed in the cubic distortion tone (CDT, 2f 1 -f 2 ) emission after
furosemide injection were as follows: At high levels of the probe tones,
changes in the emission intensities generally stayed within a 10 dB range. The
CDT amplitudes for low stimulus levels, however, were typically reduced by up
to 40 dB, but recovered (depending on the furosemide dosage) by about 60-90
min. 4. At low to moderate stimulus levels of 40-60 dB SPL there was a near
perfect, minute by minute covariation of the ear canal CDT amplitude and the
BF threshold measured in the AVCN. A 10 dB increase in threshold was
associated with a 5-7 dB decrease in the CDT emission. 5. The optimum stimulus
parameter set for the non-invasive estimation of cochlear performance from the
CDT response was for stimulus amplitudes L l = 50, L 2 = 40 dB SPL. 6. This
experiment demonstrates that CDT emissions at low stimulus levels are very
good predictors of the thresholds of cochlear afferents, but this validity is
lost for BF thresholds greater than about 60-70 dB SPL. 7. The ear canal CDT
amplitude is better correlated with the BF threshold sensitivity of neuronal
response areas in the AVCN than with the spontaneous discharge rate or
absolute above-threshold discharge rates.
Received 20 August 1994; accepted in final form 31 May 1995.
APS Manuscript Number J437-3.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 July 1995.