Histamine excites rat cerebellar granule cells in vitro through H1 and H2 receptors.

The effects of histamine on the firing of cerebellar granule cells were investigated in vitro. Histamine predominantly produced excitatory (117/123, 95.1%) and in a few cases inhibitory (6/123, 4.9%) responses in granule cells. The histamine-induced excitation was not blocked by perfusing the slice with low Ca2+/high Mg2+ medium, supporting a direct postsynaptic action of histamine. The H1 receptor antagonists triprolidine and chlorpheniramine significantly diminished the histamine-induced excitation, but the H2 receptor antagonist ranitidine did not significantly reduce the excitation. On the other hand, the H2 receptor agonist dimaprit could elicit a weak excitation of granule cells. This dimaprit-induced excitation was blocked by ranitidine but not triprolidine. These results reveal that the excitatory effect of histamine on cerebellar granule cells is mediated by both H1 and H2 receptors with a predominant contribution of H1 receptors. The relevance of these findings to the possible function of the hypothalamocerebellar histaminergic fibers in cerebellum is discussed.