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 μM, 50 μM or 10 mM). Pyrilamine and cimetidine were applied at 50 μM each. Results: histamine increased the CAP at 10 and 50 μM without any significant effects on the CM. The effects of histamine at 50 μM were suppressed by the 50-μM 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.