Breakdown of the major sleep-promoting
neurotransmitter, γ-
aminobutyric acid (
GABA), in the
GABA shunt generates catabolites that may enter the tricarboxylic acid cycle, but it is unknown whether catabolic by-products of the
GABA shunt actually support metabolic homeostasis. In Drosophila, the loss of the specific
enzyme that degrades
GABA,
GABA transaminase (GABAT), increases sleep, and we show here that it also affects metabolism such that flies lacking GABAT fail to survive on
carbohydrate media. Expression of GABAT in neurons or glia rescues this phenotype, indicating a general metabolic function for this
enzyme in the brain. As
GABA degradation produces two catabolic products,
glutamate and
succinic semialdehyde, we sought to determine which was responsible for the metabolic phenotype. Through genetic and pharmacological experiments, we determined that
glutamate, rather than
succinic semialdehyde, accounts for the metabolic phenotype of gabat mutants. This is supported by biochemical measurements of catabolites in wild-type and mutant animals. Using in vitro labeling assays, we found that inhibition of GABAT affects energetic pathways. Interestingly, we also observed that
gaba mutants display a general disruption in bioenergetics as measured by altered levels of tricarboxylic acid cycle intermediates,
NAD(+)/
NADH, and
ATP levels. Finally, we report that the effects of GABAT on sleep do not depend upon
glutamate, indicating that GABAT regulates metabolic and sleep homeostasis through independent mechanisms. These data indicate a role of the
GABA shunt in the development of metabolic risk and suggest that
neurological disorders caused by altered
glutamate or
GABA may be associated with metabolic disruption.