Physiological significance of the KCNQ4-mediated M-current in the pedunculopontine nucleus of the reticular activating system

Abstract

The pedunculopontine nucleus (PPN) is part of the reticular activating system (RAS) which is associated with sleep regulation. The PPN has cholinergic and non-cholinergic neurons. A hallmark of the PPN-cholinergic neurons is the M-current, a slowly activating, non-inactivating voltage-gated potassium current. KCNQ2 to 5 subunit alone or in combination are responsible for the M-current. Our aim was to investigate the contribution of the KCNQ4 subunit to PPN neuronal function. We used a transgenic mouse model for KCNQ4 (knock-out (KO)) and one with fluorescent-labeled cholinergic neurons (tdTomatoStop+ChAT::Cre). We analyzed KCNQ4 expression by real-time PCR and its localization using immunofluorescence. We also studied the M-current by electrophysiology on brain slices, the contribution of KCNQ4 to neuronal activity and its influence on circadian rhythm. We found a weak mRNA expression of KCNQ4 in PPN and the protein was located only on cholinergic neurons of the external limits of the nucleus. M-current was present in most of cholinergic neurons in WT animals, but absent in 40% of them in the KO ones. These last also exhibited behavioral alterations in the activity cycles showing a 5-hour increase and a higher sensitivity to changes in the light/darkness cycles. In summary, we found that only a subpopulation of PPN cholinergic neurons have KCNQ4-dependent M-current and this subunit contributes to modulate the circadian rhythm through the activity of the RAS system.