Insensitivity of the Chicken Embryo to the Ototoxicity of Aminoglycoside Antibiotics and a Loop Diuretic

Abstract

Guinea pigs are routinely used in the histological evaluation of the cochlea as a method of testing for ototoxicity, but the procedures are very time-consuming. Because the avian cochlea is easier to examine and newly hatched chicks are sensitive to the ototoxic effects of gentamicin, birds may be useful in testing for ototoxicity. The use of chicken embryos would be even better for testing, but whether or not chicken embryos are sensitive to ototoxicants is unknown. In an attempt to determine whether or not chicken embryos may be used instead ofguinea pigs in screening tests for ototoxicity, aminoglycoside antibiotics and a loop diuretic, ethacrynic acid, were administered to chicken embryos. A maximum-tolerated dose of gentamicin, kanamycin, streptomycin, ethacrynic acid, or a combination of gentamicin and ethacrynic acid was administered to fertile eggs of White Leghorn chickens on incubation days 10-17. To compare the effect of route ofexposure on ototoxicity, gentamicin was administered by injection into the allantoic space, yolk sac, and air cell as well as by submerging the egg in gentamicin solution. With the preferred air cell route the effects of the ototoxic drugs kanamycin, streptomycin, ethacrynic acid, and a combination of ethacrynic acid and gentamicin were compared. On incubation day 18, cochleas were removed from the chicken embryos. Serial sections of these avian cochleas were examined and hair cells were counted. No significant difference was seen between the number of hair cells in cochleas ofcontrol chicken embryos and those from chicken embryos treated with drugs. Therefore, the chicken embryo appears to be insensitive to the ototoxicity of aminoglycoside antibiotics and a loop diuretic.

Regenerated Hair Cells Become Functional during Continuous Administration of Kanamycin

“…The present results demonstrating functional development of regenerated hair cells during prolonged KM intoxication further support our hypothesis that regenerated hair cells are aminoglycoside resistant in their early stages of development [Hashino, 1996;Hashino and Salvi, 1996]. This hypothesis is consistent with a study reporting that the chick embryonic cochlea is insensitive to aminoglycoside ototoxicity [Fikes et al, 1994]. The reason why immature, regenerated hair cells are aminoglycoside resistant still remains unclear and needs further investigation.…”

Development of Cochlear Sensitivity to Aminoglycoside Antibiotics

“…Similar results have been found in the mouse 5 -32 and the chicken. 8 The experimental literature discussed above suggests that human fetal sensitivity to ototoxicity may commence around, or just before, the onset of cochlear function, between 19 and 27 weeks' gestation. 10 · 11 The results of the present study suggest that sensitivity to fetal aminoglycosides develops rapidly.…”

Effects of Prolonged Kanamycin Administration on Cochlear Anatomy and Auditory Brainstem Response Thresholds in Chickens

“…The finding that kanamycin accumulates in original but not in immature regenerated hair cells 3 suggests that the resistance of regenerated hair cells to kanamycin may be a consequence of altered and/or dysfunctional drug uptake pathways. Based on the finding that kanamycin resistance in regenerated hair cells was limited to those in the immature stage, and given that hair cells in the basilar papilla of the embryonic chick are also kanamycin insensitive, 10 we are inclined to agree that resistance may be associated with the lack of structural development within immature hair cells. 5 It is probable that structural development within mature regenerated hair cells is complete, thus enabling kanamycin to enter and exert its cytotoxic effects.…”