Discharge properties of presumed cholinergic and noncholinergic laterodorsal tegmental neurons related to cortical activation in non-anesthetized mice

We have recorded, for the first time, in non-anesthetized, head-restrained mice, a total of 339 single units in and around the laterodorsal (LDT) and sublaterodorsal (SubLDT) tegmental nuclei, which are located, respectively, in, or beneath, the periaqueductal gray and contain cholinergic neurons. The recordings were made during the complete wake-sleep cycle including wakefulness (W), slow-wave sleep (SWS), and paradoxical (or rapid eye movement) sleep (PS). The tegmental neurons displayed either a biphasic narrow or triphasic broad action potential. Seventy-six LDT or SubLDT neurons characterized by their triphasic long-duration action potentials were judged to be cholinergic and this was verified in anesthetized mice using neurobiotin juxtacellular labeling combined with choline acetyltransferase immunohistochemistry of the recorded cell. The 76 presumed cholinergic neurons discharged tonically at the highest rate during W and PS (W/PS-active neurons) as either single isolated spikes or clusters of two to five spikes, and 26 of them discharged selectively during W and PS, these W/PS-selective neurons being found mainly in the SubLDT. The clustering discharge was particularly prominent during PS, when it was associated with an obvious phasic change in the cortical electroencephalogram (EEG), and during waking periods, when it was accompanied by abrupt body movements. During the transition from sleep to waking, the cholinergic W/PS-selective neurons and the LDT or SubLDT noncholinergic W-selective neurons showed firing before the onset of W, while, at the transition from waking to sleep, they ceased firing before sleep onset. At the transition from SWS to PS, all the cholinergic neurons exhibited a significant increase in discharge rate before the onset of PS. The present study in mice supports the view that cholinergic and noncholinergic LDT and SubLDT neurons play an important role in tonic and phasic processes of arousal and cortical EEG activation occurring during W or PS, as well as in the sleep/waking switch.