Histamine and related substances influence neurotransmission in the semicircular canal

Histamine and other imidazole-containing substances were found to increase ampullar nerve afferent firing rate while both H1 and H2 histamine antagonists effectively inhibited ampullar nerve activity. A specific inhibitor of histidine decarboxylase, the enzyme which catalyses the synthesis of histamine, reduced ampullar nerve firing in a dose-dependent manner. These observations suggest a physiological role for histamine in the inner ear. Maintenance of a response to histamine after de-efferentation of the crista ampullaris supports the hypothesis that the site of action is the hair cell; antagonism of the histamine response by a cholinergic antagonist, atropine, and antagonism of a cholinergically mediated facilitation by the histaminergic antagonist pyrilamine, indicate that the site of action may involve the acetylcholine receptor complex on the crista ampullaris hair cells. The observation that imidazole-containing compounds cause significant effects on semicircular canal neurotransmission provides an important finding with regard to the site of action of antihistamines used for the treatment of vertigo and motion sickness.