The cholinergic forebrain arousal system acts directly on the circadian pacemaker

Abstract

Sleep and wake states are regulated by a variety of mechanisms. One such important system is the circadian clock, which provides temporal structure to sleep and wake. Conversely, changes in behavioral state, such as sleep deprivation (SD) or arousal, can phase shift the circadian clock. Here we demonstrate that the level of wakefulness is critical for this arousal resetting of the circadian clock. Specifically, drowsy animals with significant power in the 7- to 9-Hz band of their EEGs do not exhibit phase shifts in response to a mild SD procedure. We then show that treatments that both produce arousal and reset the phase of circadian clock activate (i.e., induce Fos expression in) the basal forebrain. Many of the activated cells are cholinergic. Using retrograde tract tracing, we demonstrate that cholinergic cells activated by these arousal procedures project to the circadian clock in the suprachiasmatic nuclei (SCN). We then demonstrate that arousal-induced phase shifts are blocked when animals are pretreated with atropine injections to the SCN, demonstrating that cholinergic activity at the SCN is necessary for arousal-induced phase shifting. Finally, we demonstrate that electrical stimulation of the substantia innominata of the basal forebrain phase shifts the circadian clock in a manner similar to that of our arousal procedures and that these shifts are also blocked by infusions of atropine to the SCN. These results establish a functional link between the major forebrain arousal center and the circadian system.