The locus coeruleus(LC)is one of the essential chemoregulatory and sleep-wake(S-W)modulating cen-ters in the brain.LC neurons remain highly active during wakefulness,and some implicitly become silent during rapid eye movement(REM)sleep.LC neurons are also involved in CO2-dependent modulation of the respiratory drive.Acid-sensing ion channels(ASICs)are highly expressed in some brainstem chemosensory breathing regulatory areas,but their localization and functions in the LC remain unknown.Mild hypercapnia increases the amount of non-REM(NREM)sleep and the number of REM sleep episodes,but whether ASICs in the LC modulate S-W is unclear.Here,we investigated the presence of ASICs in the LC and their role in S-W modulation and the state transition from NREM to REM sleep.Male Wistar rats were surgically prepared for chronic polysomnographic recordings and drug microin-jections into the LC.The presence of ASIC-2 and ASIC-3 in the LC was immunohistochemically characterized.Microinjections of amiloride(an ASIC blocker)and APETx2(a blocker of ASIC-2 and-3)into the LC significantly decreased wakefulness and REM sleep,but significantly increased NREM sleep.Mild hypercapnia increased the amount of NREM and the number of REM episodes.However,APETx2 microinjection inhibited this increase in REM frequency.These results suggest that the ASICs of LC neurons modulate S-W,indicating that ASICs could play an important role in vigilance-state transition.A mild increase in CO2 level during NREM sleep sensed by ASICs could be one of the determinants of state transition from NREM to REM sleep.