Evidence for an ascending inhibitory histaminergic pathway to the cerebral cortex

Abstract

Metiamide is a specific histamine antagonist in rat cerebral cortex. Since histamine levels and L-histidine decarboxylase (EC 4.1.1.22) activity in cerebral cortex decrease following disruption of the ipsilateral medial forebrain bundle (MFB), whether iontophoretically applied metiamide antagonized the inhibition of deep cerebral cortical neurons produced by stimulation of the MFB was investigated. In rats anesthetized with a mixture of methoxyflurane, nitrous oxide and O2, stimulation of the ipsilateral MFB or the cortical surface with iontophoretically applied histamine depressed the firing of cortical neurons. Metiamide antagonized the histamine-induced depression and reduced the duration of inhibition produced by MFB stimulation. It did not alter the inhibition induced by the cortical surface stimulation. A histaminergic pathway ascending through the MFB may inhibit rat cerebral cortical neurons.