The claustrum is a telencephalic nucleus that likely dates back to the common ancestor of amniotes. It is one of the most highly connected regions of the mammalian brain, with massive reciprocal projections to the cerebral cortex and certain subcortical regions. The function of this “brain hub” still remains elusive but several hypotheses emerged, such as the claustrum being crucial for some attentional processes and cognitive control, but also for the modulation of cortical slow-wave synchronization during sleep and quiet wakefulness (Narikiyo, Nat.Neurosci 2020; Marriott, Cell Rep. 2024). Using a brain slice and whole-cell patch-clamp approach, we found a sleep-like 0.3-1Hz oscillation in the claustrum of mice when applying light cholinergic modulation to activate claustro-cortical networks. Claustral glutamatergic principal cells undergoing these cholinergic-induced oscillations receive tonic excitatory inputs while also receiving rhythmic chloride inhibitory inputs. We propose that local GABAergic interneurons could thus drive the synchronized oscillation of principal cells by oscillating in an anti-phase manner with them. So far, our investigation of those claustral inhibitory neurons’ activity suggests that most subtypes oscillate in phase with principal cells and are mainly driven by rhythmic excitatory inputs. However, we also discovered a few claustral cells with intrinsic cholinergic-dependent properties, which further supports our hypothesis that the cholinergic-induced oscillation is generated inside the claustrum. As parts of the cortex preserved in our slices can generate both tonic excitation and slow-wave oscillations under cholinergic modulation, we are currently investigating through extracellular recordings the impact of cortical activity on the claustral oscillations.