The effect of prednisolone and non-steroidal anti-inflammatory agents on the normal and noise-damaged guinea pig inner ear.

The effect of anti-inflammatory agents, such as the synthetic glucocorticoid prednisolone, diclofenac sodium, and histamine H1-receptor antagonist, was studied in unexposed and noise-exposed (broad-band noise, bandwidth 1-12 kHz, 106 dB SPL, 30 min) guinea pigs. The results were compared with the results obtained from no treatment and with isotonic saline (placebo) therapy. The cochlear blood flow (CoBF) and the partial oxygen pressure in the perilymph (PL-pO2) were continuously and simultaneously recorded over a period of 210 min. In addition, cochlear microphonics (CMs), compound action potentials of the auditory nerve (CAPs), and auditory brain stem responses (ABRs) were registered. Noise-induced hearing loss paralleled a decrease of PL-pO2. Both were found to occur before evidence of reduced CoBF. PL-pO2 and CoBF progressively declined post-exposure, while CMs, CAPs, and ABRs did not further deteriorate nor showed signs of recovery up to 180 min after cessation of noise. Treatment started 60 min post-exposure, or after 90 min without manipulation and was then further studied for 120 min. In the unexposed animals, diclofenac sodium and prednisolone induced a significant decline of PL-pO2, while CoBF, CMs, CAPs, and ABRs revealed no change. Isotonic saline did not influence the measured parameters. After infusion of the histamine H1-receptor antagonist, a significant decrease of CoBF together with blood pressure and CMs was observed, while PL-pO2, CAPs, and ABRs showed no change. In the noise-exposed animals, diclofenac sodium induced partial restoration of CM and CAP amplitudes and full restoration of ABRs. Following a high dose of prednisolone (25 mg), partial restoration of CMs and full restoration of CAPs and ABRs were registered. This effect was significantly less pronounced following a low dose of prednisolone (2.5 mg). Restoration of CMs, CAPs, and ABRs was immediate (i.e. 50 min after infusion) and remained stable for another 60 min until the end of the recording period. The histamine H1-receptor antagonist and isotonic saline did not influence CMs, CAPs, and ABRs. None of the applied drugs resulted in relief of progressive noise-induced cochlear hypoxia and post-traumatic ischemia. These findings indicate direct cellular effects of prednisolone and diclofenac sodium in the cochlea taking into account no blood flow and oxygenation. The possible mechanisms involved are discussed.