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. 2024 Aug 20;121(34):e2404454121.
doi: 10.1073/pnas.2404454121. Epub 2024 Aug 15.

Thirst-driven hygrosensory suppression promotes water seeking in Drosophila

Li-An Chu  1   2   3 , Chu-Yi Tai  2 , Ann-Shyn Chiang  2   3   4   5   6
Affiliations

Thirst-driven hygrosensory suppression promotes water seeking in Drosophila

Li-An Chu et al. Proc Natl Acad Sci U S A. .

Abstract

Survival in animals relies on navigating environments aligned with physiological needs. In Drosophila melanogaster, antennal ionotropic receptors (IRs) sensing humidity changes govern hygrotaxis behavior. This study sheds light on the crucial role of IR8a neurons in the transition from high humidity avoidance to water-seeking behavior when the flies become thirsty. These neurons demonstrate a heightened calcium response toward high humidity stimuli in satiated flies and a reduced response in thirsty flies, modulated by fluctuating levels of the neuropeptide leucokinin, which monitors the internal water balance. Optogenetic activation of IR8a neurons in thirsty flies triggers an avoidance response similar to the moisture aversion in adequately hydrated flies. Furthermore, our study identifies IR40a neurons as associated with dry avoidance, while IR68a neurons are linked to moist attraction. The dynamic interplay among these neurons, each with opposing valences, establishes a preference for approximately 30% relative humidity in well-hydrated flies and facilitates water-seeking behavior in thirsty individuals. This research unveils the intricate interplay between sensory perception, neuronal plasticity, and internal states, providing valuable insights into the adaptive mechanisms governing hygrotaxis in Drosophila.

Keywords: Drosophila; IR8a; hygrosensory; leucokinin; thirsty.

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Conflict of interest statement

Competing interests statement:The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
Opposite valence between dry-sensing IR40a neurons and moist-sensing IR68a neurons in satiated flies under 30% RH comfort zone. (A) Satiated flies exhibit a preference for the 30% RH environment compared to other RH stimuli. (B) Flies exhibit a shift in preference, favoring 70% RH over 30% RH after more than 2 h of dehydration. (C) Optogenetic activation of dry-sensing IR40a neurons using yellow LED light triggers avoidance behavior. Control flies with Gal4 alone or CsChrimson alone distribute equally between two arms with blue and yellow LED, respectively (n = 4 for each mean in panels A and B). (D) Optogenetic activation of the moist-sensing IR68a neurons using yellow LED light induces attraction behavior in homozygous IR68a-Gal4;UAS-CsChrimson flies but not in control flies without all-trans retinal feeding or heterozygous IR68a-Gal4/UAS-CsChrimson flies (n = 8 for each mean in panels C and D). Error bar = SEM. **P < 0.01. ***P < 0.001. N.S., not significant.
Fig. 2.
Fig. 2.
Three distinct types of IRs are required for hygrotaxis preference to 30% RH in water-satiated flies. (A) Effects of RNAi-mediated downregulation of six different types of IRs in GH86-Gal4 neurons on hygrotaxis preference. (B) The anti-IR8a antibody detected expression in a subset of GH86-Gal4 neurons localized in the antennal sacculus. Additionally, IR40a-Gal4 and IR68a-Gal4 were expressed in two other subsets of sacculus neurons with distinct axonal terminals in different glomeruli of the antennal lobe. Red, anti-IR8a immunolabeling and cuticular autofluorescence. Blue, anti-DLG immunolabeling and cuticular autofluorescence. (C) Effects of blocking neurotransmission from IR40a, IR68a, or IR8a neurons using specific Gal4 lines driving the expression of temperature-sensitive mutant Shibiri (UAS-shits) at the restrictive temperature on hygrotaxis preference in satiated flies. Significance levels: **P < 0.01; ***P < 0.001. n = 8 for each mean in panels A and C.
Fig. 3.
Fig. 3.
Functional responses of hygrosensing IR neurons to changes in humidity. (A) Calcium responses in IR8a neurons to increased humidity. (B) Calcium responses in IR68a neurons to increased humidity. (C) Calcium responses in IR40a neurons to decreased humidity. In A–C, Dark bar and light bar indicate GCaMP responses (ΔF/F0) in water-satiated and thirsty flies (n = 4 to 7 for each mean). (D) Effects of optogenetic activation of IR8a neurons on hygrotaxis preferences. CsChrimson is used to active neurons in yellow light but not in blue light in both 30% and 70% RH. Further details on the calcium imaging and optogenetic experiments can be found in the Materials and Methods section. *P < 0.05; ***P < 0.001; N.S., not significant. n = 8 for each mean.
Fig. 4.
Fig. 4.
Lk suppresses moist-sensing IR8a neurons in thirsty flies. (A) In thirsty flies, RNAi-mediated downregulation of Lkr in IR8a neurons reverts hygrotaxis behavior from water attraction to water avoidance, similar to water-satiated flies. (B) Downregulation of Lkr in IR8a neurons in thirsty flies reverses the GCaMP responses to moist stimuli, reaching levels similar to those in satiated flies. (C) Direct application of Lk peptide inhibits the GCaMP response to a 70% RH moist stimulus from a 30% RH baseline in IR8a neurons. No inhibitory effect is observed when applying a scrambled Lk peptide or when applying the Lk peptide to Lk-downregulated IR8a neurons. (D) Optogenetic activation of IR8a neurons in thirsty flies within a high RH environment results in a decrease in moist attraction. (E) A model depicting the presynaptic sensory suppression caused by thirst-induced Lk neuropeptide signaling, leading to the reversion of hygrotaxis behavior from moist-avoidance in water-satiated flies to water-seeking in thirsty flies. Arrows indicate the valence of each neuron type under different internal states. The squire function indicates the functional response to stimuli under different internal states. Error bar = SEM. n = 8 for each mean in panels A, B, and D. **P < 0.01. ***P < 0.001. N.S., not significant.
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