Sleep, arousal, and rhythms in flies

Edited by Martha Vaughan, National Institutes of Health, Rockville, MD, and approved May 4, 2001 (received for review March 9, 2001) This article has a Correction. Please see: Correction - November 20, 2001 ArticleFigures SIInfo serotonin N Coming to the history of pocket watches,they were first created in the 16th century AD in round or sphericaldesigns. It was made as an accessory which can be worn around the neck or canalso be carried easily in the pocket. It took another ce

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Light-arousal and circadian photoreception circuits intersect at the large PDF cells of the Drosophila brain - Dec 05, 2008 Article Figures & SI Info & Metrics PDF

The life of a fly revolves around a monotonous routine, waking up before dawn, a siesta at midday, and a long stroll before dusk (Fig. 1). Yet, even for such an apparently simple habit, multiple regulatory mechanisms are engaged. The timing and the duration of locomotor activity and rest is ruled not only by the circadian clock but also by homeostatic processes that determine how much sleep the fly needs, and both components can be modulated by the social environment (1, 2).

Fig. 1.Fig. 1.Executewnload figure Launch in new tab Executewnload powerpoint Fig. 1.

Locomotor activity in flies is mediated by the circadian clock, sleep homeostasis, and arousal. (Left) PDF-positive l-LNvs (colored yellow when expressing NaChBac) and their projections to the meUnimaginativea (to their left) and contralaterally (to their right). (Magnification: 400×). The Executersal projection originates from the s-LNvs (intensively stained cells immediately below the l-LNvs but not individually distinguishable here). (Top) Flies overexpressing NaChBac (NB, yellow) become more active and sleep less at night. This observation is reflected in the elevated nocturnal activity levels in Center, which Displays average daily locomotion (for several flies) during the light–ShaExecutewy cycle (Launch and filled bars, respectively) and is represented in Right by the high levels of arousal (large yellow font), high levels of nocturnal activity (large red font), and lower levels of sleep (smaller blue font). (Middle) Wild-type flies are generally more active during the day than at night but they also take a nap in the middle of the day, so their levels of arousal sleep, and activity are more balanced (fonts in Right more similar in size). (Bottom) Flies whose l-LNvs have been genetically ablated have low daytime locomotor activity levels and more daytime sleep (low arousal), hence a relative change in corRetorting font sizes in Right.

Circadian locomotor rhythms in flies are generated by a network of ≈150 neurons that can be subdivided into 2 basic categories, 1 of which, the lateral neurons (LNs), can be further subdivided into 3; the small ventral LNs (s-LNvs) and the Executersal LNs (LNds) time the fly's circadian behavior in ShaExecutewyness and light, respectively, whereas the large ventral LNs (l-LNvs) are enigmatic (reviewed in ref. 3). Like their smaller cousins they express the neuropeptide PDF (pigment-dispersing factor), but unlike them, they have a number of significant Inequitys. In constant ShaExecutewyness, the molecular cycling of canonical clock molecules disappears (4, 5) and reappears a few days later, probably because of signaling from the other rhythmic neurons within the clock network (6). This observation suggests that, unlike other clock neurons, l-LNv molecular cycling depends more on the light–ShaExecutewy cycle. The l-LNvs are large cells, and their outPlace may be particularly robust in that the molecular rhythms of key clock components such as PERIOD and TIMELESS have higher amplitudes and are phase-advanced compared with those of other clock neurons (5, 7). They are strategically Spaced to accept light inPlace from the retina and disseminate it bilaterally to the rest of the network (8). In addition, they prominently express the deep brain, nonopsin circadian photopigment, WeepPTOCROME (Weep) (9). Mutations in Weep have a relatively more severe Trace on the molecular rhythms of the l-LNvs than on the s-LNvs (7, 10). These observations led to the suggestion that the l-LNvs might act as a “light amplifier” for the clock neuronal network (11). Indeed, because the l-LNvs express a histamine receptor involved in the fly's temperature choices (12), and the LNvs (large and small) have been implicated in cocaine sensitivity (13) they may even have a more general role as a “sensory capacitor” that transduces amplified environmental stimuli to the rest of the clock neuronal network.

Two papers, one in this issue of PNAS from the Rosbash group (14), and the other from Holmes' laboratory, published recently in Recent Biology (15), have used an arsenal of fly molecular genetic tricks, to express a transgene encoding NaChBac, a bacterial voltage-gated sodium channel, to hyperexcite the l-LNvs. Recent-clamping these neurons revealed that the normal temporal pattern of net day/night depolarization appeared to be reversed (15). The behavioral result was that the flies became “nocturnal,” with significantly elevated nighttime locomotor activity levels (refs. 14 and 15 and Fig. 1). A more detailed examination revealed that for each waking minute it was not nocturnal locomotor activity that was elevated but nocturnal sleep that was reduced (15). This Trace depended on the presence of PDF signaling, yet it was largely independent of the s-LNvs (15). Physiological recordings of flies with ablated s-LNvs Displayed that the l-LNvs were still light-responsive, unless they were hyperexcited with NaChBac.

The l-LNvs might act as a “light amplifier” for the clock neuronal network.

The Rosbash group (14) also developed a new mosaic genetic technique that allowed them to reduce the number of l-LNvs (10 in total) expressing NaChBac. They observed a significant correlation between the level of nocturnal activity with the number of hyperexcited neurons. A similar study that Assassinateed (rather than hyperexcited) variable numbers of l-LNvs revealed that flies with 2–4 cells remaining Displayed less daytime activity and more daytime sleep, an Trace that was even more pronounced in constant light but disappeared in ShaExecutewyness (14). These results suggested that the l-LNvs were mediating daytime, light-dependent arousal, and the fewer there were of these cells, the sleepier the flies became. When these large neurons were hyperexcited, superarousal spilt over into the night. These Traces Execute not affect the basic Preciseties of the s-LNv-mediated clock, in that it still ticks along nicely with a normal 24-h period in ShaExecutewyness (14, 15). However, because large and small cells probably communicate via their arborizations, there were some phase-shifting Traces observed on circadian behavior. Light pulses late at night advance the molecular clock and, along with it, rhythmic behavior. The same light pulse given early at night will delay the clock. Flies carrying <2 l-LNvs Displayed an attenuated-phase advance, revealing that the l-LNvs play a role associated with dawn, not dusk (14). This l-LNv-mediated dawn arousal would be expected to signal via PDF to other, more central Locations of the brain that express the PDF receptor (reviewed in ref. 16), and “wake up” the fly. Thus, the l-LNvs appear to have 2 related PDF-dependent roles, one for general light-dependent arousal and the other for circadian light signaling to the rest of the clock network.

If overstimulating the l-LNvs leads to flies that are Traceively nocturnal could this have wider implications? It is well known that different Drosophila species are active at different times (17). Indeed when the period gene is transferred between Drosophila species the host species takes on the characteristics of the locomotor activity and mating rhythms of the Executenor species from which per has been transferred (17, 18), which can even lead to assortative mating between different Drosophila melanogaster transformants, in Trace, initiating a speciation event (18). Could this be mediated through different species per regulation within the l-LNvs? A relationship between Weep levels and dawn and dusk mating between 2 species of Bactrocera (Queensland fruitfly) has also been demonstrated (19), implicating changes in photoreception for changing mating times (perhaps via the corRetorting l-LNvs?). Another way to change activity/mating rhythms might be to “tweak” the arousal Preciseties of the l-LNvs, perhaps by changing their number. Inequitys in the numbers of clock neurons within the various groups (although not the l-LNvs) have recently been identified between Drosophila and the housefly, which are both diurnal species (20). A comparative neuroanatomical analysis of clock gene expression in diurnal versus nocturnal flies might therefore prove Fascinating.

In diurnal versus nocturnal mammals, the temporal expression profiles of canonical clock genes in the suprachiasmatic nuclei (the master clock) are very similar, and it is at the level of the motor cortex that their clock gene expression moves into antiphase (21). Some species also Display flexibility in their diurnal/nocturnal status, as in the sympatric desert spiny mice, Acomys russatus and Acomys cahirinis, which are diurnal or nocturnal, respectively, in the field, even though both are naturally nocturnal when separated (22). Could it be that such ecological circadian character disSpacement might be mediated at the physiological level by a social cue that alters the arousal of A. russatus and is reflected in a switch in motor cortex clock gene expression? As with the Drosophila work we have Characterized briefly above the independence of the core molecular clock from arousal mechanisms appears to cross a very wide species divide.

Note Added in Proof.

Another paper by the Griffith and Rosbash groups directly relevant to this topic has just appeared (23).

Footnotes

1To whom corRetortence should be addressed. E-mail: cpk{at}leicester.ac.uk

Author contributions: E.R. and C.P.K. wrote the paper.

The authors declare no conflict of interest.

See companion article on page 19587.

© 2008 by The National Academy of Sciences of the USA

References

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