Because of glial Tim-Gal4 expression ( 106), repo-Gal80 was always included with this driver.
Bruchpilot (Brp), a presynaptic protein, helps visualize the brain. ( E) Left: Circadian neurons visualized with GFP driven by Tim-Gal4. ( D) Schematic responsiveness to light depends on time of day. Independent wild-type cohorts tested at each point. ( C) Reactivity to light across 24 hours. Bottom, averaged activities of all wild-type animals top, representative raster plots of individuals ( n = 1 per row). Twenty minutes of baseline, 60 min of light exposure. ( B) Locomotor responsiveness to light at 2 p.m. ( A) Locomotor activity (15-min bins) of wild-type flies ( w +iso31) during entrainment, baseline, and testing with 1-hour light pulse. RESULTS Locomotor reactivity to light depends on time of day The experimental paradigm we established provides a new way to study timekeeping, as well as how expectations are encoded and evaluated. Two functionally opposed neuronal populations use cellular remodeling as a strategy to organize slowly shifting internal predictions. The internal sense of daytime versus nighttime, against which light is evaluated, is generated by a microcircuit within the network of circadian neurons.
Using a paradigm in which flies report their time-of-day estimates, we found that the circadian system assists in evaluating light conditions relative to time of day, and that mismatch between prediction and reality shapes behavioral responsiveness to light. What if conditions in the environment suddenly changed and no longer matched the expectation set by the clock? A mismatch between reality and prediction (bright light during the night or darkness during daytime) could be interpreted as an error signal and lead to behavioral modification.
Predictability, enabled by the circadian clock, needs to be balanced with flexibility. For example, it instructs the timing of cell division ( 7) and hormone secretion ( 8) and enables animals to seek food and shelter before nightfall ( 9). The clock organizes various physiological processes on a ~24-hour scale ( 6). The day-night oscillations are represented on the cellular level by the circadian clock, a molecular program shaped by environmental light-key clock proteins are light sensitive, so that under light-oscillating conditions their levels rise and fall rhythmically ( 5). We propose that a dynamic model of environmental light resides in the shifting connectivities of the LNv-DN1a circuit, which helps animals evaluate ongoing conditions and choose a behavioral response. Switching between the two states requires daily remodeling of LNv and DN1a axons such that the maximum presynaptic area in one population coincides with the minimum in the other. The internal daytime-nighttime context is generated by two interconnected and functionally opposing populations of circadian neurons-LNvs generating the daytime state and DN1as generating the nighttime state. We found that light elicits an acute increase in locomotion (startle) that is modulated in a time-of-day–dependent manner: Startle is potentiated during the nighttime, when light is unexpected, but is suppressed during the daytime. We studied how sensory information can be contextualized, by examining light-evoked locomotor responsiveness of Drosophila relative to time of day. Behavioral responsiveness to external stimulation is shaped by context.
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