An internal thermal sensor controlling temperature preference in Drosophila

Abstract

Animals from flies to humans are able to distinguish subtle gradations in temperature and show strong temperature preferences. Animals move to environments of optimal temperature and some manipulate the temperature of their surroundings, as humans do using clothing and shelter. Despite the ubiquitous influence of environmental temperature on animal behaviour, the neural circuits and strategies through which animals select a preferred temperature remain largely unknown. Here we identify a small set of warmth-activated anterior cell (AC) neurons located in the Drosophila brain, the function of which is critical for preferred temperature selection. AC neuron activation occurs just above the fly's preferred temperature and depends on dTrpA1, an ion channel that functions as a molecular sensor of warmth. Flies that selectively express dTrpA1 in the AC neurons select normal temperatures, whereas flies in which dTrpA1 function is reduced or eliminated choose warmer temperatures. This internal warmth-sensing pathway promotes avoidance of slightly elevated temperatures and acts together with a distinct pathway for cold avoidance to set the fly's preferred temperature. Thus, flies select a preferred temperature by using a thermal sensing pathway tuned to trigger avoidance of temperatures that deviate even slightly from the preferred temperature. This provides a potentially general strategy for robustly selecting a narrow temperature range optimal for survival.

Figure 1. dTrpA1 is required for warmth avoidance

a, Distribution of animals on thermal gradient. b, Fraction of animals in 18–22°C (blue) and 28–32°C (red) regions of thermal gradient. Data are mean +/− SEM. n= number of assays. ** P≤0.0001 compared to wild type (unpaired t-test).

Figure 2. AC neurons are thermosensors

a, AC (arrowhead), LC (arrow) and VC (double arrow) neurons. b, dTRPA1 minigene expression. c, AC locations (antennae removed). d, AC projections. e, Left, AC processes labeled using dTrpA1^SH-Gal4;UAS-myr:RFP. Also labeled are cells that express dTrpA1^SH-Gal4, but do not detectably express dTRPA1 protein [also see a]. Right, camera-lucida-style outline of AC projections. f–g, AC projections to AL (f) and SOG (g). h–j, G-CaMP-labeled AC’s. Two AC’s sometimes imaged simultaneously (h). k–l, Warmth-responsive G-CaMP fluorescence of AC’s. m, Maximum F/F each AC imaged. AL, antennal lobe; SOG, subesophageal ganglion; SLPR, superior lateral protocerebrum; eso (asterisk), esophagus.

Figure 3. AC neurons are necessary for warmth avoidance

a, AC-specific knockdown of dTRPA1 protein expression in dTrpA1 ^SH-Gal4;UAS-GFP;UAS-dTrpA1^RNAi animals: dTRPA1 is expressed in LC (arrow) and VC (double arrow), but not AC neurons (arrowhead). GFP marks dsRNA-expressing cells. Two left panels show adult brain and right hand panels show close-ups of specific cells. b, Distribution of indicated genotypes along thermal gradient. c, Fraction of RNAi animals in 28–32°C region of gradient. d,e, Animal distributions along thermal gradient. ab, unilateral ablation. ab, bilateral ablation. f, Fraction of animals in 18–22°C (blue) and 28–32°C (red) regions of gradient.

Figure 4. dTRPA1 is a warmth sensor

a, Flies expressing dTRPA1 in all neurons (c155-Gal4;UAS-dTRPA1) are incapacitated after 60 sec at 35°C, but recover at 23°C. Gal4 control, c155-Gal4. UAS control, UAS-dTRPA1. Ectopic dTRPA1, c155-Gal4;UAS-dTRPA1. Five experiments/genotype, 15 flies/experiment, SEM’s=0. b, c. Warming (above ~25°C) stimulates transmission at neuromuscular junction in c155-Gal4;UAS-dTRPA1. d–g, dTRPA1 and agTRPA1 are warmth-activated in oocytes (n>14 each). d,e Warmth-evoked currents in dTRPA1 or agTRPA1-expressing oocytes (−60mV). Oocytes were injected with BAPTA 30 min prior to recording, minimizing cytosolic Calcium elevations. RR: 50 micromolar Ruthenium Red. f,g Current-voltage relationships of dTRPA1 and agTRPA1 at indicated temperatures.