Photoreceptors for immediate effects of light on circadian behavior

Abstract

Animals need to sharpen their behavioral output in order to adapt to a variable environment. Hereby, light is one of the most pivotal environmental signals and thus behavioral plasticity in response to light can be observed in diurnal animals, including humans. Furthermore, light is the main entraining signal of the clock, yet immediate effects of light enhance or overwrite circadian output and thereby mask circadian behavior. In Drosophila, such masking effects are most evident as a lights-on response in two behavioral rhythms - the emergence of the adult insect from the pupa, called eclosion, and the diurnal rhythm of locomotor activity. Here, we show that the immediate effect of light on eclosion depends solely on R8 photoreceptors of the eyes. In contrast, the increase in activity by light at night is triggered by different cells and organs that seem to compensate for the loss of each other, potentially to ensure behavioral plasticity.

Keywords: Biological sciences; Entomology; Neuroscience.

Graphical abstract

Figure 1

The immediate light effect on eclosion behavior (A) Eclosion pattern of Drosophila flies in 10 min intervals at the times around lights-on. Light elicits an immediate increase in eclosion. (B and C) The lights-on response is visible in flies perceiving a 20-min light pulse (B) 1 h before (−1) or (C) 1 h after (1) expected lights-on at CT (circadian time) 0. (D, D′) Wild-type CantonS (CS) flies show an immediate response to light (D), while flies in darkness lack the eclosion peak (D′). (D″) The third plot combines (D) and (D′) to visualize the immediate light response in comparison to the appropriate controls monitored in darkness. Gray bars: % eclosion in dark phase, yellow bars: % eclosion in light phase; dashed bars: % eclosion in controls. n = 384 (A), 516 (B), 524 (C), 649 (D), 636 (D′). See also Figure S1.

Figure 2

The immediate light effect on eclosion behavior requires the compound eyes and phospholipase C activity (A–E) Eclosion pattern in 10 min intervals at the times around CT0. Each plot visualizes the results for the experimental (gray, yellow bars) and control groups (dashed bars) as shown in Fig.1D-D″. (B, C) Flies without eyes (B, cli^eya) and impaired phospholipase C activity (C, norpA^p41) lack the lights-on response. (D, E) Flies lacking the rhodopsin (Rh) of the ocelli photoreceptor cells (D, rh2¹) or the photoprotein cryptochrome (E, cry⁰¹) respond to light. Gray bars: % eclosion in dark phase, yellow bars: % eclosion in light phase; dashed bars: % eclosion in darkness controls. n_exp, n_ctrl = 524, 544 (A); 502, 508 (B); 520, 539 (C); 604, 584 (D); 510, 556 (E). See also Figure S1.

Figure 3

The immediate light effect on eclosion behavior depends on R8 cells (A–J) Eclosion pattern in 10 min intervals at the times around CT0. Each plot visualizes the results for the experimental (gray, yellow bars) and control groups (dashed bars) as shown in Fig.1D-D″. (A–C) Twenty-minute blue (A, 455–475 nm, I = 3.6 W/m²), green (B, 510–545 nm, I = 2 W/m²), and red (C, 625–642 nm, I = 2.3 W/m²) light pulses elicit immediate eclosion. (D–F) The eclosion response to red light is visible in rh1 (D, ninaE¹⁷) but gone in rh6 mutants (E, rh6¹) and rh1, rh6 double mutants (F, ninaE¹; rh6¹). (G) In contrast, rh1,rh6 double mutants (ninaE¹; rh6¹) respond with an increase of eclosion to white light (I = 4.1 W/m²). (H) The quadruple mutant (rh5²; rh3¹, rh4¹, rh6¹), lacking all rhodopsins of the inner photoreceptors, shows no reaction to light. (I) The lights-on response in flies without R7 cells (sev^LY3). (J) Flies lacking Rh5 (rh5²) do not respond to light with increased eclosion. (K) Optogenetic activation of rh5-positive neurons with a 2 min blue light pulse (blue line; 455–475 nm, I = 3.41 μW/mm²) 1 h after expected lights-on elicits eclosion. (L) Control flies without Gal4 expression (w¹¹¹⁸; chop2^XXL) do not respond to the 2 min light pulse. n_exp, n_ctrl = 534, 1543 (A); 521, 1543 (B); 547, 1543 (C); 511, 512 (D); 579, 731 (E); 604, 567 (F); 561, 567 (G); 595, 518 (H), 525, 531 (I); 585, 506 (J); n = 516 (K), 515 (L). See also Figure S1.

Figure 4

The immediate effect of light on locomotor activity is visible in flies without functional eyes, photosensation in cry-positive cells, or ocelli (A–F) Activity pattern in 10 min intervals at the time around zeitgeber time (ZT) 0. First and second column show bar plots of mean ± SEM activity at the day the light pulse was applied (second column) and the activity of the same flies on the previous day (first column). The third column visualizes the comparison between the mean activity at ZT22 in 10 min intervals (0′–10′ and 10′–20′) during the light pulse (L) and the previous control day in darkness (D). Data are presented as mean (bar plot) and individual values (dots). (A) Control flies (w¹¹¹⁸) and (B) eyeless (cli^eya; cry^b) flies, (C) flies lacking cryptochrome (cry⁰¹), (D) flies without functional ocelli photoreceptors (rh2¹), and (E) flies without Rh5 and Rh6 (rh5²; rh6¹) increase activity in response to light at ZT22. (F) The light response is absent in flies that lack the histidine decarboxylase (hdc^JK910) and therefore histamine, the transmitter of photoreceptor cells. n = 27–32; asterisks denote level of significance: ^∗p ≤ 0.05, ^∗∗p ≤ 0.01, ^∗∗∗p ≤ 0.001, ^∗∗∗∗p ≤ 0.0001. See also Figure S3 and Table S1.