Altered sleep latency and arousal regulation in mice lacking norepinephrine.

Latency to sleep and the amount of sensory stimulation required to awaken an animal are measures of arousal threshold, which are ultimately modulated by an arousal regulation system involving many brain areas. Among these brain areas and network connections are wake-promoting nuclei of the brainstem and their corresponding neurotransmitters, including norepinephrine (NE). In this study, we used mice that are unable to produce NE to study its role in regulating sleep latency after a variety of interventions, and to study arousal from sleep after sleep deprivation (SD). Sleep latency was measured after gentle awakening or after injections of saline, caffeine or modafinil. Sleep latency was also measured before and after partial restoration of NE pharmacologically. Arousal threshold was measured by recording the number of decibels of white noise required to wake each mouse from NREM sleep after 0, 3 and 3 + 3 h SD (3 h SD followed by < 2 min sleep, followed by an additional 3 h SD). Results showed that when mice were awakened without being touched, there were no differences in sleep latency between the genotypes. However, after an injection of saline, the control mice increased their sleep latency, whereas the NE-deficient mice did not. There were no group differences in sleep latency after treatment with either stimulant. The sleep latency difference between the genotypes was ameliorated by partial restoration of NE. The arousal threshold experiments revealed that significantly more noise was required to wake the NE-deficient mice after 3 and 3 + 3 h of SD. These findings show that mice lacking NE fall asleep more rapidly only after a mild stressor, such as an intraperitoneal injection. NE-deficient mice are also more difficult to wake up using audio stimulation after SD. The results presented here suggest that NE promotes wakefulness during transitions between sleep and wake under conditions involving mild stress and SD, but not under baseline circumstances.