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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Science. Author manuscript; available in PMC 2010 April 7.
Published in final edited form as:
PMCID: PMC2850598
NIHMSID: NIHMS183680

Use-Dependent Plasticity in Clock Neurons Regulates Sleep Need in Drosophila

Abstract

Sleep is important for memory consolidation and is responsive to waking experience. Clock circuitry is uniquely positioned to coordinate interactions between processes underlying memory and sleep need. Flies increase sleep both after exposure to an enriched social environment and after protocols that induce long-term memory. We found that flies mutant for rutabaga, period, and blistered were deficient for experience-dependent increases in sleep. Rescue of each of these genes within the ventral lateral neurons (LNVs) restores increased sleep after social enrichment. Social experiences that induce increased sleep were associated with an increase in the number of synaptic terminals in the LNV projections into the medulla. The number of synaptic terminals was reduced during sleep and this decline was prevented by sleep deprivation.

Although sleep is a process that is necessary for survival, the functions of sleep are unknown (1, 2). Sleep is regulated by circadian influences and is important for consolidation of long-term memory (LTM) (35). Additionally, LTM is modulated by circadian mechanisms (6, 7). Because the relationship between sleep, memory, and circadian rhythms seem to be phylogenetically conserved, Drosophila can be used to explain mechanisms that coordinate these processes. Drosophila show an increase in daytime sleep after exposure to socially enriched environments (5). Similarly, an increase in sleep after courtship conditioning is necessary for LTM (5).

Increased sleep after social enrichment is dependent upon genes that are required for learning and memory, including genes that alter cyclic adenosine monophosphate signaling (5). Although newly eclosed flies that are mutant for the adenylyl cyclase rutabaga (rut2080) show increased sleep after social enrichment, 3- 4-day-old adult rut mutants do not respond to changes in the social environment (fig. S1). Elevating wild-type rut in adult flies with an RU486-inducible driver rescued experience-dependent increases in sleep in adult rut mutants (fig. S1); vehicle-treated siblings showed no increase in sleep (fig. S1). To identify circuits that mediate experience-dependent increases in sleep, we used a series of GAL4 lines to drive wild-type rut expression in brain circuits (Fig. 1A). Figure S2 illustrates the data analysis used to quantify each GAL4 rescue. Expression of UAS-rut using pdf-GAL4 restored the increase in daytime sleep and daytime sleep-bout duration, although to a lesser extent than GSelav (Fig. 1, B to E). The expression pattern of pdf-GAL4 is limited to the ventral lateral neurons (LNVs), a group of clock neurons that express pigment-dispersing factor (pdf) (8, 9). Although pdf is the only known output from the LNVs, flies mutant for pdf show a wild-type increase in sleep (fig. S3).

Fig. 1
Clock cells regulate experience-dependent increases in sleep. (A) Data for each GAL4 line (GAL4>/+:rut2080/+;;UAS-rut/+) is compared to its parental line (GAL4/+:rut2080/+). No increase in Δ daytime sleep is observed in the absence of ...

Given this role of clock cells, we examined the clock gene period (per), which is expressed in the LNVs and is required for LTM (6). Rescue of wild-type per using a 7.2-kb fragment of the per genomic sequence (per01; per+7.2-2) restored expression of PER at CT0 within the LNVsaswellasthedorsal lateral neurons, LNDs (Fig. 1F); mutant flies carrying a null mutation, per01, expressed no PER (Fig. 1G). Although per01 mutants showed no increase in sleep after social enrichment, per01;per+7.2-2 flies displayed normal experience-dependent increases in sleep (Fig. 1H). per01 mutants have no LTM when tested 48 hours after training and only show a transient increase in sleep (Fig. 1, I and J). per01;per+7.2-2 flies displayed LTM (Fig. 1K) and increases in sleep (Fig. 1, L and M). Although per levels are low in mutants for Clock and cycle, both acquire LTM (6) and increase sleep after social enrichment (5). Thus, only a very small amount of per may be required to support increased sleep and LTM.

To further investigate the role of synaptic plasticity in clock cells, we used the Drosophila homolog for serum response factor (SRF), blistered (bs). In mice, SRF is essential for activity-induced gene expression and plays an important role in synaptic long-term potentiation (10) and in contextual habituation (11). bs retains a 93% identity with SRF within the DNA-binding MCM1-ARG80-Agamous-Deficiens-SRF (MADS) domain (12). Social enrichment elevated the transcription of bs in wild-type Canton-S (Cs) flies (Fig. 2A). Mutants carrying a P element inserted into the bs gene (P{GAL4}bs1348) do not increase sleep after social enrichment (Fig. 2B). This deficit was also found in flies carrying either of two other mutant alleles for bs (bs2 and bs3) and was present in flies that are homozygous for mutant bs alleles and flies that have been outcrossed to either Cs or to flies carrying the In(2LR)Px4 deficiency (Fig. 2C). The P-element insertion in bs1348 preserves the MADS domain; similar N-terminal truncated mutant SRF acts as dominant negative (13). BS is expressed throughout the brain, including pdf-expressing LNVs (Fig. 2, D to F). When UAS-egfp was driven by P{GAL4}bs1348, expression was restricted to a small number of neurons, including the LNVs (Fig. 2, G and I). Expression of bs using P{GAL4}bs1348 to drive either of two wild-type bs (UAS-bs) constructs rescued experience-dependent increases in sleep (Fig. 2J). Moreover, inducing bs expression within the LNVsusing pdf-GAL4 increased sleep after social enrichment (Fig. 2K).

Fig. 2
blistered regulates experience-dependent increases in sleep. (A) bs transcripts are significantly elevated in socially enriched Cs flies compared with their isolated siblings (P = 0.03). (B) Flies homozygous for the P{GAL4}bs1348 insertion do not respond ...

To establish whether expression of bs is required for LTM, we tested flies carrying the P{GAL4}bs1348 mutant allele using courtship conditioning. Although P{GAL4}bs1348/+ flies acquire short-term memory (Fig. 2L), LTM was impaired (Fig. 2M, left). Rescue of wild-type bs using P{GAL4}bs1348 restored LTM (Fig. 2M, right). Next, we used the GAL4 repressor cry-GAL80toblock UAS-bs expression within the LNs. Although UAS-bs/+;cry-gal80/+ control flies showed significant courtship suppression (Fig. 2N, left), P{GAL4}bs1348/UAS-bs;cry-GAL80/+ flies had no LTM (Fig. 2N, right), which suggests a role for the LNs, although we cannot exclude a role for the dorsal neurons (DNs). Although SRF deletion in mouse forebrain results in neurons with abnormal morphology (14), the morphology of LNVs in mutant P{GAL4}bs1348/+ flies (fig. S4A) did not differ from that of LNVsin P{GAL4}bs1348/UAS-bs rescue flies (fig. S4B). All three mutants for bs had intact circadian rhythms and showed anticipatory activity before light-dark transitions; only bs3 flies show an altered period under constant darkness (fig. S5, A to F). These findings suggest that there are no developmental abnormalities in the LNVsin bs mutants.

Hypomorphic alleles for bs prevent proper wing development through interactions with Epidermal growth factor receptor (Egfr) signaling (1517). Because Egfr alters sleep in Drosophila (18), interactions between bs and Egfr may regulate responses to social experience. After social enrichment, transcription of Egfr was significantly elevated in Cs flies (Fig. 3A). The Egfr genomic sequence contains several CC(A/T)6GG CArG elements that can be bound by bs to promote transcription (Fig. 3B), and transcription of Egfr was significantly reduced in bs mutants (data not shown). Thus, we used P{GAL4}bs1348 to drive expression of a constitutively active Egfr construct (UAS-Egfr*). Although P{GAL4}bs1348/+ mutants showed no change in sleep after social enrichment (Fig. 3C), activation of Egfr in P{GAL4}bs1348/+;UAS-Egfr*/+ flies increased sleep (Fig. 3, D and E). Conversely, the expression of a dominant-negative construct for Egfr (UAS-EgfrDN) using pdf-GAL4 prevented increases in sleep after social enrichment, but parental controls (pdf-GAL4/+ and UAS-EgfrDN/+) were wild-type (Fig. 3F).

Fig. 3
Egfr mediates experience-dependent sleep. (A) Transcription of Egfr is significantly elevated in socially enriched Cs flies compared with their isolated siblings, as measured by quantitative polymerase chain reaction. (B) Genomic Egfr sequence contains ...

A recent theory proposes that a function of sleep is to downscale synaptic connections (19). Moreover, structural plasticity can be induced by environmental manipulation in Drosophila (20). To quantify the effect of social enrichment on the number of post-synaptic terminals in LNV projections, we used pdf-GAL4 to drive expression of a green fluorescent protein (GFP)–tagged construct of the postsynaptic protein discs-large (UAS-dlgWT-gfp). After 5 days of social enrichment, LNV projections into the medulla of pdf-GAL4/+;;UAS-dlgWT-gfp/+ flies contained significantly more GFP-positive terminals (Fig. 4, A to C). Although we have not demonstrated that the labeled synaptic terminals are functional, these tools have been used to quantify synapses (21). The expression of the UAS-dlgWT-GFP marker did not alter synaptic function in a wild-type background (20) and did not prevent the increase in sleep when expressed using pdf-GAL4 after social enrichment (fig. S6). To determine the effect of waking on synapse number, socially isolated pdf-GAL4/+;;UAS-dlgWT-gfp/+ flies and their enriched siblings either were allowed to sleep ad libitum or were sleep deprived for 48 hours after social enrichment. Although the number of dlg-GFP positive terminals remained elevated in sleep-deprived socially enriched flies, terminal number was significantly reduced in siblings that were allowed to sleep (Fig. 4D). Similarly, the number of presynaptic terminals in LNV projections into the medulla using a GFP-tagged construct of the presynaptic protein synaptobrevin (UAS-VAMP-GFP) in pdf-GAL4/+;UAS-VAMP-GFP/+ flies was increased (Fig. 4, E to G). After 48 hours of recovery, socially enriched pdf-GAL4/+;UAS-VAMP-GFP/+ flies had a reduced number of VAMP-GFP–positive presynaptic terminals relative to their sleep-deprived siblings (Fig. 4H). A recent study has reported a clock-dependent remodeling in the axonal terminals of the PDF circuit that is highest during the day (22). Recent data indicates that hyperexcitation of a subset of the LNVs suppresses sleep in Drosophila (2325). Together with our results, these data suggest that the PDF circuit is well suited to test the hypothesis that sleep acts to downscale synaptic connections that are potentiated during waking experience.

Fig. 4
LNV synapse number in medulla. (A and B) LNV projections in socially enriched pdf-GAL4/+;;UAS-dlgWT-gfp/+ flies contain more GFP-positive terminals than their socially isolated siblings when evaluated using confocal microscopy. (C) Relative quantification ...

Supplementary Material

supplementary data

Acknowledgments

We thank M. Thimgan, W. Vanderheyden, P. Gray, and P. Taghert for comments. This study was funded by NIH grants R01-NS051305-01A1 and T32-GM008151.

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