Distribution of Gentamicin to the Cochlea of the Chicken Embryo

Aminoglycoside antibiotics are ototoxic in mammals and birds, including recently hatched chicks, but chicken embryos are insensitive to the ototoxicity ofgentamicin, kanamycin, and streptomycin. To determine whether or not the insensitivity is due to a lack ofantibiotic distribution to the avian cochlea, the distribution of gentamicin to the cochlea of the White Leghorn chicken embryo was compared to the distribution to the cochlea of the recently hatched White Leghorn chick. Fertile eggs were injected with a maximally tolerated dose of gentamicin sulfate (0.1 mg/e&day) on incubation days 10-18, and the chicks were injected subcutaneously with either 5 mg (non-ototoxic) or 100 mg (ototoxic) gentamicin sulfatelkg body weight on days 1-9 after hatching. Gentamicin sulfate was histochemically detected within the basilar papilla (the avian equivalent of the organ of Coni) in all treated chicken embryos and chicks by 1 day after the first injection, and the staining was intense after 3 days of treatment. By ultrastructural immunocytochemistry, mild, diffuse labeling for gentamicin sulfate was detected within the endoplasmic reticulum of short and tall hair cells of chicken embryos by incubation day 17. Moderate labeling of gentamicin sulfate was detected in the infracuticular region of lysosomes of hair cells in chicks receiving 5 treatments of gentamicin sulfate at 5.0 mg/ kg body weight and after 1 treatment of gentamicin sulfate at 100 mg/kg body weight. There was labeling of phosphatidylinositol 4,5-bisphosphate (PIP,), believed to be an essential factor in the ototoxicity of gentamicin in the stereocilia, cuticular plate, nucleus, endoplasmic reticulum, cell walls, and tight junctions of hair cells in all control and treated chicken embryos and hatched chicks at each time point with little variation in the amount. In hatched chicks from the high-dose group, short and tall hair cell loss was detected after 3 doses ofgentamicin sulfate, and degeneration of dark cells in the tegmentum vasculosum was observed after 5 doses. No hair cell loss and no ultrastructural evidence of degeneration, accumulation of gentamicin sulfate, or an increase in the numbers of lysosomes were detected in hair cells from chicken embryos. Although there was no evidence of drug-induced ototoxicity, PIPz and gentamicin were detected in the hair cells from chicken embryos. Therefore, the intracellular concentration of gentamicin was either insufficient to induce toxicity or the hair cells were still functionally undeveloped and could not be affected by the gentamicin.