Oxygen consumption rate of flatworms under the influence of wake‐ and sleep‐promoting neurotransmitters

Abstract

Flatworms are among the best studied animal models for regeneration; however, they also represent an emerging opportunity to investigate other biological processes as well. For instance, flatworms are nocturnal and sleep during the day, a state that is regulated by sleep/wake history and the action of the sleep‐promoting neurotransmitter gamma‐aminobutyric acid (or GABA). Sleep is widespread across the animal kingdom, where it serves many nonexclusive functions. Notably, sleep saves energy by reducing metabolic rate and by not doing something more energetically taxing. Whether the conservation of energy is apparent in sleeping flatworms is unclear. We measured the oxygen consumption rate (OCR) of flatworms dosed with either ( 1) GABA ( n = 29) which makes flatworms inactive or ( 2) dopamine ( n = 20) which stimulates flatworms to move, or ( 3) day and night neurotransmitter‐free controls ( n = 28 and 27, respectively). While OCR did not differ between the day and night, flatworms treated with GABA used less oxygen than those treated with dopamine, and less than the day‐time control. Thus, GABA affected flatworm physiology, ostensibly by enforcing energy‐conserving sleep. Evidence that dopamine increased metabolism was less strong. This work broadens our understanding of flatworm physiology and expands the phylogenetic applicability of energy conservation as a function of sleep.

Abstract

In the free‐living platyhelminth flatworm, Girardia tigrina, the sleep‐promoting neurotransmitter, gamma‐aminobutyric acid (or GABA), decreases oxygen consumption rate relative to the wake‐promoting neurotransmitter, dopamine, and also a neurotransmitter‐free control. This metabolic rate study builds on the behavioral and biochemical characterization of sleep in flatworms.

Research Highlights

While flatworms are known to sleep, it has been unclear whether restful flatworms reap the metabolic benefits of this quiescent state. Using neurotransmitters that regulate sleep and wakefulness, we show that sleep‐promoting gamma‐aminobutyric acid reduces oxygen consumption in flatworms.