Vestibular ototoxicity in the chick: effects of streptomycin on equilibrium and on ampullary dark cells.

Starting a week after they were hatched, chicks received daily subcutaneous injections of streptomycin sulfate for 15 or 30 days at one of three dosages: 400, 800, or 1,200 mg/kg body weight. During the period of administration, the chicks were weighed, examined for signs of systemic intoxication, and tested for impairment of equilibrium. At intervals some birds from each group were sacrificed and the end-organs of the semicircular canals were examined for damage. After the fifteenth injection, the weights of the control and experimental chicks were similar. By comparison, the chicks that received streptomycin injections showed varying degrees of impairment of equilibrium. First, some birds in the three experimental groups began to tremble at least slightly by the third injections, but others, particularly at the highest dosage (1,200 mg/kg body weight), trembled severely by the fifth injections. However, trembling began to subside in the lowest-dosage (400 mg/kg body weight) group by the fourteenth injection. Second, the chicks' ability to perch on dowels, either hooded or unhooded, and their ability to perch on the investigator's fingers in the dark deteriorated. Perching performances on the dowel deteriorated conspicuously only at higher dosages, while changes in perching on the finger were detected earlier and at lower dosages. Streptomycin damaged dark cells before other cell types. The cuboidal dark cells were most sensitive, followed by the pyriform cells. The eminential cells were least sensitive. Although the hair cells were functionally damaged by either the primary or the secondary actions of streptomycin, as evidenced by the chicks' early impairment of equilibrium, they showed distinct cytologic lesions later than did the dark cells.