Oral administration of γ-aminobutyric acid affects heat production in a hot environment in resting humans

Eight male participants drank a 200-ml sports drink with 1 g of GABA (trial G) or without GABA (trial C), then rested for 30 minutes in a sitting position in a hot environment (ambient air temperature 33°C, relative humidity 50%).We found that changes in esophageal temperature from before drinking the sports drink were lower in trial G than in trial C (-0.046 ± 0.079°C vs 0.001 ± 0.063°C; P < 0.05), with lower heat production calculated by oxygen consumption (41 ± 5 W/m2 vs 47 ± 8 W/m2; P < 0.05).In this study, we have demonstrated that a single oral administration of GABA induced a larger decrease in body core temperature compared to a control condition during rest in a hot environment and that this response was concomitant with a decrease in total heat production.γ-aminobutyric acid (GABA) is an amino acid that is widely distributed throughout the central nervous system (CNS) and is the most important depressive neurotransmitter [1]. GABA has important roles concerning temperature regulation in the hypothalamus. In experimental animals, it has been reported that central pharmacological stimulation of GABA in the dorsomedial hypothalamus (DMH) and the posterior hypothalamus (PH) inhibits heat production [2,3]. On the contrary, it has been reported that central pharmacological stimulation of GABA in the preoptic (PO) area and anterior hypothalamus (AH) increases heat production [4,5]. On the basis of these reports, it is assumed that central GABA stimulation to the hypothalamic region has some practical effect on temperature regulation.Few studies have examined the influence of systemic GABA stimulation on temperature regulation in either experimental animals or humans because the blood-brain barrier is impermeable to GABA [6] and it has long been thought that systemic administration of GABA cannot affect GABA's availability in the CNS [7,8]. However, it has been suggested that GABA could access certain areas of the brain that lack the blood-brain barrier [9-11], su