Histamine is a transmitter to maintain tonic firing of mesopontine tegmental cholinergic neurons during wakefulness

In unanesthetized, head-restrained rats and cats, single neuronal activity was recorded from cholinergic neurons in the mesopontine tegmentum, and effects of iontophoretically-applied histamine on them were examined. The cholinergic neurons were classified into subgroups which showed different firing pattern across sleep-wakefulness conditions. Action of histamine was excitatory on virtually all cholinergic neurons active during both wakefulness and paradoxical sleep (activity during wakefulness was tonic, while that during paradoxical sleep was either tonic or bursting). Tonic activity during wakefulness, but not that during paradoxical sleep, was suppressed by application of an H, receptor antagonist. Cholinergic neurons of another subgroup, specifically active during paradoxical sleep and silent during wakefulness, were not affected by histamine. The results suggest that histamine is a major excitatory input to cholinergic neurons to maintain tonic firing during wakefulness.