Neuronal adenosine A2A receptors signal ergogenic effects of caffeine

Abstract

Caffeine is one of the most used ergogenic aid for physical exercise and sports. However, its mechanism of action is still controversial. The adenosinergic hypothesis is promising due to the pharmacology of caffeine, a nonselective antagonist of adenosine A₁ and A_2A receptors. We now investigated A_2AR as a possible ergogenic mechanism through pharmacological and genetic inactivation. Forty-two adult females (20.0 ± 0.2 g) and 40 male mice (23.9 ± 0.4 g) from a global and forebrain A_2AR knockout (KO) colony ran an incremental exercise test with indirect calorimetry (V̇O₂ and RER). We administered caffeine (15 mg/kg, i.p., nonselective) and SCH 58261 (1 mg/kg, i.p., selective A_2AR antagonist) 15 min before the open field and exercise tests. We also evaluated the estrous cycle and infrared temperature immediately at the end of the exercise test. Caffeine and SCH 58621 were psychostimulant. Moreover, Caffeine and SCH 58621 were ergogenic, that is, they increased V̇O₂max, running power, and critical power, showing that A_2AR antagonism is ergogenic. Furthermore, the ergogenic effects of caffeine were abrogated in global and forebrain A_2AR KO mice, showing that the antagonism of A_2AR in forebrain neurons is responsible for the ergogenic action of caffeine. Furthermore, caffeine modified the exercising metabolism in an A_2AR-dependent manner, and A_2AR was paramount for exercise thermoregulation.

Figure 1

Effects of SCH 58261 (1 mg/kg, i.p.) on locomotion (A), ergospirometry (B–E), and thermoregulation (F–H) of wild type male and female mice. (A) SCH 58261 was psychostimulant only in males. (B) The dotted line represents the V̇O₂max of the DMSO group. Ergospirometry increased V̇O₂ (B), running power (B), and metabolic rate (E) until the animals reached fatigue. SCH 58261 was ergogenic in both sexes, as it increased V̇O₂max (C) and running power (D). The animals presented exercise-induced core and tail hyperthermia (H), which was not 58261 modified by SCH (F–G). Sex was a significant factor in decreasing maximum responses to V̇O₂ (C) and running power (D), and increasing core and tail temperature in females. Data are presented as mean ± SEM. N = 8–9 animals/group for 12 independent experiments. *P < 0.05 vs. DMSO (Two-way ANOVA followed by Newman-Keuls post hoc test). # P < 0.05 vs. rest (Repeated measures ANOVA followed by Bonferroni post hoc test). DMSO dimethyl sulfoxide. Rec recovery. RER Respiratory Exchange Ratio. V̇O₂ oxygen consumption.

Figure 2

Effects of caffeine (15 mg/kg, i.p.) in wild type and global A_2AR KO mice on locomotion (A), ergospirometry (B–E), and thermoregulation (F–G) of female mice. (A) Caffeine displayed a psychostimulant effect in the open field in wild type mice, but not in A_2AR KO mice. Ergospirometry increased V̇O₂ (B), running power (B), and metabolic rate (E) until the animals reached fatigue. The dotted line represents the V̇O₂max of the wild type-saline group (B). Caffeine increased V̇O₂max (C) and running power (D) of wild type, but not A_2AR KO mice. Exercise test induced hyperthermia, which was not affected by caffeine in wild type mice, whereas caffeine caused a hypothermic response in A_2AR KO mice (F and G). Genotype was a significant factor for V̇O₂max (C), running power (D), resting (F), and recovery (F' and G') temperatures. Data are described as mean ± SEM. N = 8–9 animals/group for 12 independent experiments. *P < 0.05 vs. saline and # P < 0.05 vs. caffeine (Two-way ANOVA followed by Newman-Keuls post hoc test). A_2AR—adenosine A_2A receptor. KO—knockout. Rec recovery. RER Respiratory Exchange Ratio. V̇O₂ oxygen consumption.

Figure 3

Effects of caffeine (15 mg/kg, i.p.) in wild type and forebrain A_2AR KO mice on motor behavior (A), ergospirometry (B–E), and thermoregulation (F, G) of male mice. (A) Caffeine displayed a psychostimulant effect in the open field in wild type mice, but not in forebrain-A_2AR KO mice. Ergospirometry increased V̇O₂ (B), running power (B), and metabolic rate (E) until the animals reached fatigue. The dotted line represents the V̇O₂max of the wild type-saline group (B). Caffeine increased V̇O₂max (C) and running power (D) of wild type, but not forebrain A_2AR KO mice. Resting core (F) and tail (G) temperature and tail heating (G') were similar between groups. Data are presented as mean ± SEM. N = 8–9 animals/group for 12 independent experiments. *P < 0.05 vs. saline and ^#P < 0.05 vs. caffeine (Two-way ANOVA followed by Newman-Keuls post hoc test). A_2AR adenosine A_2A receptor, fb forebrain, KO knockout, Rec recovery, RER respiratory exchange ratio. V̇O₂ oxygen consumption.