Cochlear microphonics in the jaundiced Gunn rat.
BACKGROUND
Bilirubin toxicity causes brain damage and deafness. Brainstem auditory areas are damaged, but the effects of bilirubin toxicity on the peripheral auditory system are less well defined. As a first step in the study of cochlear physiology, we performed studies of scalp-derived cochlear microphonic (CM) responses in the jaundiced (jj) Gunn rat model of acute bilirubin toxicity.


METHODS
CMs were obtained from scalp needle electrodes in response to acoustically delayed 500-, 1,000-, 2,000-, 4,000-, 6,000-, and 8,000-Hz tonebursts delivered by overhead speakers, and brainstem auditory-evoked potentials (BAEPs) were obtained to clicks. At 18 days of age, one nonjaundiced (Nj) and two jj Gunn rats in each of four litters were anesthetized. CMs and BAEPs were performed before and 4 hours after either (1) sulfonamide was injected into jj rats to produce acute bilirubin toxicity, (2) jj controls were injected with saline, or (3) Nj controls were given sulfonamide. In a second experiment, 16-day-old jj-sulfa and Nj-saline littermates were studied with insert speakers at 60 and 75 dB hearing level (HL) at baseline and 6 hours later, and CM amplitude was analyzed with a fast fourier transformation.


RESULTS
No statistically significant differences were found by repeated measures analysis of variance in the CMs in either experiment between groups or after sulfonamide, despite BAEP changes of decreased amplitude of waves II and III and increased latency of I-II and I-III interwave intervals in jjs given sulfa.


CONCLUSIONS
Alterations of CM after acute bilirubin toxicity did not occur at a time when there was electrophysiologic evidence of brainstem dysfunction. Although more subtle effects might be detected with a larger sample studied at longer intervals after the insult, CM seems insensitive or less sensitive to acute bilirubin toxicity than brainstem auditory function. These results suggest that retrocochlear tests of central auditory function may be more sensitive to the effects of hyperbilirubinemia than tests of peripheral auditory function in humans.
Shapiro
S M
SM
Department of Neurology, Medical College of Virginia, Virginia Commonwealth University, Richmond.
Te Selle
M E
ME
D000161:Acoustic Stimulation; D000818:Animals; D001663:Bilirubin / Q000097:blood / Q000633:toxicity; D003055:Cochlear Microphonic Potentials / Q000187:drug effects / Q000502:physiology*; D016057:Evoked Potentials, Auditory, Brain Stem / Q000187:drug effects / Q000502:physiology*; D005260:Female; D005583:Fourier Analysis; D006932:Hyperbilirubinemia / Q000503:physiopathology; D007565:Jaundice / Q000503:physiopathology*; D008297:Male; D051381:Rats; D011911:Rats, Gunn; D011930:Reaction Time / Q000187:drug effects / Q000502:physiology; D012815:Signal Processing, Computer-Assisted; D013449:Sulfonamides / Q000494:pharmacology
D013449:Sulfonamides; D001663:Bilirubin