Sustainable Development Goals

Abstract/Objectives

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.

Results/Contributions

In Drosophila melanogaster, humidity preference dynamically shifts depending on hydration state. Satiated flies prefer environments with ~30% relative humidity (RH), avoiding overly moist areas, while thirsty flies increasingly prefer higher humidity (~70% RH), especially after 3 hours of dehydration. This behavioral switch is mediated by distinct hygrosensory neurons: IR40a neurons promote avoidance of dry air, IR68a neurons attract flies to moist air, and IR8a neurons—newly identified in this study—drive avoidance of high humidity when flies are well-hydrated.

Through RNAi screening and behavioral assays using a humidity-controlled T-maze, the researchers discovered that IR8a neurons are crucial for maintaining humidity preference in hydrated flies. Downregulating IR8a expression eliminated the preference for 30% RH, indicating its role in moist-avoidance behavior. Calcium imaging revealed that IR8a neurons show high activation in response to humidity in hydrated flies, but this response is significantly suppressed in thirsty flies. In contrast, IR68a neurons consistently respond positively to humidity, with enhanced sensitivity in dehydrated conditions.

Optogenetic activation experiments confirmed that IR8a neuron stimulation elicits avoidance behavior in hydrated flies, regardless of actual humidity levels. However, in thirsty flies, this moist-avoidance behavior is suppressed due to inhibition of IR8a neurons by the neuropeptide leucokinin (Lk), which is released in response to internal osmolarity changes. Application of Lk peptide mimicked this inhibition, while blocking Lk receptors in IR8a neurons restored avoidance behavior, even in thirsty flies.

Together, these findings reveal that IR8a neurons function as a dynamic behavioral switch: they are active and aversive under normal hydration, but are suppressed under dehydration to allow for water-seeking behavior. The study establishes a three-neuron model: IR40a (dry avoidance), IR68a (moist attraction), and IR8a (moist avoidance when hydrated), integrating external humidity cues with internal thirst states to produce adaptive behavior in Drosophila.

Keywords

Drosophila, hygrosensation, IR8a neurons, leucokinin, water seeking, thirst humidity preference, optogenetics

References

1. https://www.pnas.org/doi/abs/10.1073/pnas.2404454121

Thirst-driven hygrosensory suppression promotes water seeking in Drosophila

Contact Information

江安世教授
aschiang@life.nthu.edu.tw