'Flies on a treadmill' - Investigating sleep using the fruit fly model

23rd Jul 2018

The ‘Bright Brains’ newsletter is composed and edited by students, postdocs and early career members of the BNA. In the Summer 2018 Bulletin edition, undergraduate student Francesco Monaca reported on research investigating the mechanisms behind sleep using the drosophila model.

The mystery in question is sleep. Fruit flies (Drosophila melanogaster) placed on a spherical treadmill are offering insights into the mechanisms regulating this marvellous yet poorly understood biological process. Fruit flies have already played a paramount role in elucidating the circadian timekeeping system. The circadian clock dictates when we should go to sleep, according to environmental cues. The same model organism is now being studied to shed light onto the sleep homeostat, a second ‘controller’ which might explain why we need to sleep in the first place.

In Drosophila, a population of dopaminergic neurons projecting to the ‘dorsal fan-shaped body’ (dFB) of the central complex (a region running across the midline of the insect brain) has been observed to induce sleep when stimulated (1). These neurons are electrically active and inactive in sleep-deprived and rested flies, respectively. It is therefore plausible to believe that dFB neurons effectively act as a switch between quiescent and active states, with Dop1R2 receptors mediating the arousing effects of dopamine.

To test this idea and characterise the mechanisms underlying the dopamine-modulated switch, the behaviour of head-fixed experimental flies on treadmills was studied while wake-promoting signals resulting in dopamine release were delivered via optogenetics (2). The behavioural mark indicating that flies transitioned from sleep to wakefulness was a period of locomotor activity after at least five minutes of inactivity.

Interestingly, this research highlighted that optogenetic stimulation of dFB neurons resulted in their transient hyperpolarisation and concomitant awakening of flies. Both effects were mediated by dopamine interacting with Dop1R2 receptors.

Surprisingly, while single dopamine pulses silenced dFB neurons temporarily, prolonged dopamine supply switched these neurones to the OFF (inactive) state, in which they remained even in the absence of transmitter. The speed of transition between ON and OFF states suggested that the translocation of ion channels to the plasma membrane could effectively be the mechanism underlying this switch, accounting for the increased potassium conductances and subsequent hyperpolarisation of dFB neurones observed when flies wake up.

Two main types of channels are expressed in dFB neurons in their ON, electrically active state, namely Shaker and Shab. Currents associated with these two channels are downregulated when cells are switched to their OFF state by dopamine, whereas voltage-independent leak currents are upregulated through a channel termed Sandman.

Therefore, in response to dopamine, Sandman is internalised within the plasma membrane and its hyperpolarising current, along with the attenuation of Shaker and Shab, is responsible for the transition of dFB neurons into OFF state, triggering awakening of flies. The next big step for sleep researchers would now be understanding the molecular players influencing this homeostatic switch.

If you have any comments or questions, you can find Francesco on Twitter (@Monaca97)

References:

1. Donlea, J. M., Pimentel, D. & Miesenböck, G. (2014) Neuronal machinery of sleep homeostasis in Drosophila. Neuron, 81, 860–87.
2. Pimentel, D., Donlea, J. M., Talbot, C.B., Song, S.M., Thurston, A.J.F. & Miesenböck, G. (2016) Operation of a homeostatic sleep switch. Nature, 536, 333-337.

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