The effects of histamine and its antagonists on the cochlear microphonic and the compound action potential of the guinea pig

OBJECT: we studied the effects of histamine, the H1 receptor antagonist pyrilamine, and the H2 receptor antagonist cimetidine on the cochlear potential of guinea pigs (cochlear microphonic, CM; compound action potential, CAP). METHODS: histamine was applied into the cochlear perilymph at three different dosages (10 microM, 50 microM or 10 mM). Pyrilamine and cimetidine were applied at 50 microM each. RESULTS: histamine increased the CAP at 10 and 50 microM without any significant effects on the CM. The effects of histamine at 50 microM were suppressed by the 50-microM of pyrilamine and cimetidine. At 10 mM of histamine, CAP and CM amplitudes were significantly decreased. CONCLUSION: in low concentrations, histamine may act as an extracellular signal on inner hair cells (IHCs) or it may stimulate the afferent nerve by binding to their H1 and H2 receptors. A possible explanation for the inhibitory effects of histamine at 10-mM dosage was apparently found in that the effects of the high concentration may be supraphysiological; and furthermore, there is a difference in the mechanism by which histamine exerts its effects mediated by the histamine receptors on the cochlea.