The Effects of General Anaesthesia and Light on Behavioural Rhythms and GABAA Receptor Subunit Expression in the Mouse SCN.

General anaesthesia (GA) is known to affect the circadian clock. However, the mechanisms that underlie GA-induced shifting of the clock are less well understood. Activation of γ-aminobutyric acid (GABA)-type A receptors (GABAAR) in the suprachiasmatic nucleus (SCN) can phase shift the clock and thus GABA and its receptors represent a putative pathway via which GA exerts its effect on the clock. Here, we investigated the concurrent effects of the inhalational anaesthetic, isoflurane, and light, on mouse behavioural locomotor rhythms and on α1, ß3, and γ2 GABAAR subunit expression in the SCN of the mouse brain. Behavioural phase shifts elicited by exposure of mice to four hours of GA (2% isoflurane) and light (400 lux) (n = 60) were determined by recording running wheel activity rhythms in constant conditions (DD). Full phase response curves for the effects of GA + light on behavioural rhythms show that phase shifts persist in anaesthetized mice exposed to light. Daily variation was detected in all three GABAAR subunits in LD 12:12. The γ2 subunit expression was significantly increased following GA in DD (compared to light alone) at times of large behavioural phase delays. We conclude that the phase shifting effect of light on the mouse clock is not blocked by GA administration, and that γ2 may potentially be involved in the phase shifting effect of GA on the clock. Further analysis of GABAAR subunit expression in the SCN will be necessary to confirm its role.

Development of the Molecular Circadian Clock and Its Light Sensitivity in Drosophila Melanogaster.

The importance of the circadian clock for the control of behavior and physiology is well established but how and when it develops is not fully understood. Here the initial expression pattern of the key clock gene period was recorded in Drosophila from embryos in vivo, using transgenic luciferase reporters. PERIOD expression in the presumptive central-clock dorsal neurons started to oscillate in the embryo in constant darkness. In behavioral experiments, a single 12-h light pulse given during the embryonic stage synchronized adult activity rhythms, implying the early development of entrainment mechanisms. These findings suggest that the central clock is functional already during embryogenesis. In contrast to central brain expression, PERIOD in the peripheral cells or their precursors increased during the embryonic stage and peaked during the pupal stage without showing circadian oscillations. Its rhythmic expression only initiated in the adult. We conclude that cyclic expression of PERIOD in the central-clock neurons starts in the embryo, presumably in the dorsal neurons or their precursors. It is not until shortly after eclosion when cyclic and synchronized expression of PERIOD in peripheral tissues commences throughout the animal.

The functional changes of the circadian system organization in aging.

The circadian clock drives periodic oscillations at different levels of an organism from genes to behavior. This timing system is highly conserved across species from insects to mammals and human beings. The question of how the circadian clock is involved in the aging process continues to attract more attention. We aim to characterize the detrimental impact of aging on the circadian clock organization. We review studies on different components of the circadian clock at the central and periperal levels, and their changes in aged rodents and humans, and the fruit fly Drosophila. Intracellular signaling, cellular activity and intercellular coupling in the central pacemaker have been found to decline with advancing age. Evidence of degradation of the molecular clockwork reflected by clock gene expression in both central and peripheral oscillators due to aging is inadequate. The findings on age-associated molecular and functional changes of peripheral clocks are mixed. We conclude that aging can affect the circadian clock organization at various levels, and the impairment of the central network may be a fundamental mechanism of circadian disruption seen in aged species.

Clock gene expression and locomotor activity predict death in the last days of life in Drosophila melanogaster.

The importance of the circadian clock for the regulation of behaviour and physiology, and the molecular control of these rhythms by a set of clock genes are well defined. The circadian clock deteriorates with advancing age but the mechanism underlying is unclear. Here we recorded the expression of two key clock genes in young, middle-aged and old Drosophila using transgenic luciferase lines reporting period and timeless in vivo. We report a novel marker of imminent death in the expression of TIMELESS. In the days immediately preceding death TIMELESS expression increased to at least 150% of previous acrophase values (88.0% of n = 217) and lost circadian rhythmicity, which predicted death equally well in flies of different ages and under light and temperature cycles. We suggest this transient aberrant clock-gene expression is central to the mechanism of the disturbance in circadian behaviour before death (82.7% of n = 342). We also find that PERIOD expression in central-clock neurons remained robust with age, however PERIOD and TIMELESS in peripheral clocks showed a reduction in both expression level and rhythmicity. In conclusion, as flies age the molecular clock gradually declines at the peripheral level but continues to function at the central until days before death.