Dexamethasone modifies the effect of Pseudomonas aeruginosa exotoxin A on hearing

In the present study, the protective effect of dexamethasone was analysed following exposure of the cochlea to Pseudomonas aeruginosa Exotoxin A (PaExoA). Four groups of albino Sprague-Dawley rats were used. 20 microl saline was instilled through the tympanic membrane into the round window niche (group A, n = 4); 1 microg/20 microl dexamethasone sodium 21-phosphate (dexamethasone) solution was instilled (group B, n = 4); 1 microg/20 microl PaExoA solution was initially instilled followed 1 h later by 20 microl saline (group C, n = 6); and 1 microg/20 microl PaExoA solution was initially instilled followed 1 h later by 1 microg/20 microl dexamethasone solution (group D, n = 6). Frequency-specific (4, 8, 10, 12, 16 and 20 kHz) auditory brainstem responses (ABR) were used to ascertain the threshold prior to exposure and 1, 2, 3, and 5 days and 1 and 2 weeks afterwards. No threshold change was observed in groups A and B, whereas the animals in groups C and D showed some threshold elevation, that in D being smaller than that in C. There was a significant difference at the frequencies 12, 16 and 20 kHz, 2 and 5 days after exposure. The intensity-latency (I-L) curve showed that in group D the cochlear component almost disappeared at high frequency one week after exposure. Our results indicate that dexamethasone can modify the effect of PaExoA caused by non-specific inflammation.