ATP is the direct source of energy for cellular activity. ADP may be converted into ATP or degraded to Ad (11
). Energy depletion may increase the need to sleep (12
). The ATP results for the control group () indicate that brain levels of ATP decreased with increasing SD time in the whole brain and 3 h of Rec sleep was insufficient to replenish the energy lost. In contrast to the SD-2.5 h control group, the ATP levels in the AHSLE-2.5 h group increased in the brainstem while they decreased in the cerebrum and cerebellum, which revealed the preferential provision of energy to the central area and indicated that energy deficiency in the cerebrum and cerebellum may establish the need to sleep. When AHSLE were administered, the ATP levels of the SD rats increased, which is different from the effect previously observed in freely behaving rats (4
); this may be due to state-related changes to the stress reactions of the rats.
Ad is a crucial neurotransmitter in the sleep system of the brain (13
); it accumulates in the brain and has been proposed as an important factor for homeostatic sleep (14
). In AHSL-treated SD rats, the observed increases in the concentrations of Ad in all regions of the brain reflected the bio-energetic stress of the animals. Furthermore, 3 h of Rec sleep was sufficient to reduce the sleep propensity signal (Ad) to the levels observed during normal wakefulness, following the administration of AHSLE.
The purine nucleoside, Ad, has been reported to be a candidate sleep factor, as systemic injections of Ad were observed to promote sleep and reduce wakefulness (15
). The concentrations of Ad have also been reported to increase with increased metabolism and neural activity (16
). In the current study, in SD rats, Ad accumulated as a product of increased metabolism/neural activity, to inhibit neural activity in various brain regions and was then consumed during the recovery of sleep.
The current study demonstrated that, during sleep deprivation, Ad accumulated in a site-specific and time-prolonged manner. In the brain stem and cerebellum, Ad levels in the AHSLE groups increased steadily and significantly during the SD phase, compared with basal and SD-2.5 h levels. By contrast, the cerebrum did not demonstrate an accumulation of Ad during the SD phase following the administration of AHSLE, indicating that, during prolonged wakefulness, Ad levels are not uniformly elevated by AHSLE throughout the brain. These findings support the hypothesis that an increase in the need to sleep enhances the release of cortical Ad. It indicates that the cerebellum and brainstem are critical sites of Ad action. Thus, these data revealed that during sleep deprivation, AHSLE stimulates Ad accumulation selectively in the cerebellum and, to a lesser extent, in the brainstem.
Regarding the effective components, we analyzed the composition of the essential oil of AHSL according to a previously described method (18
). Naturally occurring linalool may be transformed into a number of derivatives that are valuable to the flavour and fragrance industries (19
), and has demonstrated antidepressant- and sedative-like activity (21
). The sedative effect of linalool-rich essential oils is suggested to provoke a reduction in neuronal excitability (22
). Our analysis of the essential oil components of AHSL indicated a high linalool content, which may be the source of the sedative effects.
In conclusion, this site-specific accumulation of Ad during sleep deprivation indicates the physiological regulatory effect of AHSLE. Cumulative evidence suggests that AHSLE increases the levels of Ad in the brain during sleep deprivation, which has the potential to increase sleep. The sedative effects of AHSL may be attributed to their high linalool content.