Dietary Gamma-Aminobutyric Acid (GABA) Induces Satiation by Enhancing the Postprandial Activation of Vagal Afferent Nerves

Abstract

Gamma-aminobutyric acid (GABA) is present in the mammalian brain as the main inhibitory neurotransmitter and in foods. It is widely used as a supplement that regulates brain function through stress-reducing and sleep-enhancing effects. However, its underlying mechanisms remain poorly understood, as it is reportedly unable to cross the blood-brain barrier. Here, we explored whether a single peroral administration of GABA affects feeding behavior as an evaluation of brain function and the involvement of vagal afferent nerves. Peroral GABA at 20 and 200 mg/kg immediately before refeeding suppressed short-term food intake without aversive behaviors in mice. However, GABA administration 30 min before refeeding demonstrated no effects. A rise in circulating GABA concentrations by the peroral administration of 200 mg/kg GABA was similar to that by the intraperitoneal injection of 20 mg/kg GABA, which did not alter feeding. The feeding suppression by peroral GABA was blunted by the denervation of vagal afferents. Unexpectedly, peroral GABA alone did not alter vagal afferent activities histologically. The coadministration of a liquid diet and GABA potentiated the postprandial activation of vagal afferents, thereby enhancing postprandial satiation. In conclusion, dietary GABA activates vagal afferents in collaboration with meals or meal-evoked factors and regulates brain function including feeding behavior.

Keywords: ERK; GABA; capsaicin-sensitive sensory nerves; dietary GABA; food intake; nodose ganglion; postprandial satiation; vagal afferents; vagotomy.

Figure 1

Plasma GABA concentrations in mice administered with peroral (po) vs. intraperitoneal (ip) GABA. Time course of GABA concentrations in the postcaval vein plasma after po administration of GABA (200 mg/kg; closed circle) or ip injection of GABA (200 mg/kg; open circle, 20 mg/kg; open triangle). n = 4–6. ^## p < 0.01 by one-way ANOVA followed by Dunnett’s test vs. 0 min in each group. GABA: Gamma-aminobutyric acid.

Figure 2

Po GABA administration immediately before the start of refeeding suppresses food intake in a dose-dependent manner without aversive behaviors. (A–C): Periodic food intake in response to the po administration of GABA at 2 (A), 20 (B), or 200 mg/kg (C) or saline immediately before refeeding in mice fasted overnight (16 h). Periodic food intake (kcal) for 0–0.5 h after GABA injection includes the energy from eaten chow and injected GABA. The amount of energy in GABA was calculated as 3.95 kcal/g. Therefore, the energy GABA administered at 2, 20, and 200 mg/kg to the mice was 0.163 ± 0.00473 (A), 1.69 ± 0.0392 (B), and 15.8 ± 0.219 (C) cal per mouse, respectively. * p < 0.05 by unpaired t-test. (D): Periodic food intake (left) and locomotor activity (right) in response to po administration of GABA 20 or 200 mg/kg immediately before refeeding in mice fasted overnight (16 h). * p < 0.05 by one-way ANOVA followed by Tukey’s test. n = 7. (E): In the taste aversion test, saccharine preference was measured 2 days after the injection of saline (po), GABA (200 mg/kg, po), or lithium chloride (LiCl, 3 mmol/kg, ip). ** p < 0.01 by one-way ANOVA followed by Tukey’s test. n = 6–12. (F): Ip injection of GABA at 20 mg/kg immediately before refeeding did not change food intake (left) or locomotor activity (right) in overnight-fasted mice. n = 6. GABA: Gamma-aminobutyric acid. N.S.: not significant.

Figure 3

Peroral GABA administration suppresses food intake via vagal afferents. (A): Peroral administration of GABA (200 mg/kg) immediately before refeeding significantly reduced food intake in intact mice fasted overnight (16 h). n = 6. (B): The feeding suppression by po GABA administration (200 mg/kg) immediately before refeeding was abolished in mice whose sensory nerves including vagal afferent nerves were denervated by systemic treatment with capsaicin. n = 6. (C,D): GABA (200 mg/kg, po) reduced cumulative milk-diet intake in sham-operated (C) but not subdiaphragmatic vagotomized mice (D). n = 5–8. * p < 0.05, ** p < 0.01 by unpaired t-test. po, peroral. GABA: Gamma-aminobutyric acid.

Figure 4

A single po administration of GABA does not alter ERK1/2 phosphorylation in the nodose ganglia (NGs), nucleus tractus solitarius (NTS), or area postrema (AP). A–D: Left NG (A,B) and medial NTS and AP in the medulla (C,D) sections immunostained for pERK1/2 30 min after po administration of 200 mg/kg GABA or saline to overnight-fasted mice. (E,F): Extended pictures of red squares in (C) to (D), respectively. Yellow arrowheads indicate pERK1/2-positive neurons. Scale bar, 100 µm. (G–I): The incidence of pERK1/2-positive neurons in the left and right NG neurons (G) and the number of them in the bilateral NTS neurons (H). The fluorescence intensity of pERK1/2-immunoreactivity per unit area in AP is indicated in I. n = 4. DMV, dorsal motor nucleus of vagus nerves. po, peroral. GABA: Gamma-aminobutyric acid. Dotted line: borderline indicating brain region.

Figure 5

The preadministration of GABA 30 min before refeeding did not alter food intake. A single po administration of GABA at 200 mg/kg 30 min before refeeding failed to decrease food intake in overnight-fasted mice. n = 12. At −0.5 h, the value of saline group is 0 and that of GABA is 0.0174 ± 0.000278 kcal, which are the energy values of the injected solution. po, peroral. GABA: Gamma-aminobutyric acid.

Figure 6

The coadministration of liquid diet EnsureH and GABA potentiates the postprandial activation of neurons in nodose ganglia (NGs), nucleus tractus solitarius (NTS), and area postrema (AP). (A–F): Left NG (A–C) and medial NTS and AP in the medulla (D–F) sections immunostained for ERK1/2 phosphorylation 30 min after po administration of EnsureH (40 mL/kg), EnsureH + GABA (40 mL/kg and 200 mg/kg), or saline (40 mL/kg) in overnight-fasted mice. Scale bar, 100 µm. (G–I): The incidence of pERK1/2-positive neurons in the left and right NG neurons (G) and the number of them in the bilateral NTS neurons (H). The fluorescence intensity of pERK1/2-immunoreactivity per unit area in AP is indicated in I. * p < 0.05, ** p < 0.01 by one-way ANOVA followed by Tukey’s test. n = 4. DMV, dorsal motor nucleus of vagus nerves. po, peroral. GABA: Gamma-aminobutyric acid. Dotted line: borderline indicating brain region. EnsureH+, po administration of EnsureH (gray columns). GABA+, po administration of EnsureH containing GABA (black columns). EnsureH– and GABA–, po administration of saline (white columns).

Figure 7

The coadministration of liquid diet EnsureH and GABA markedly prevents overeating through sensory nerves including vagal afferents. (A,B): Po administration of only GABA (200 mg/kg, 40 mL/kg) 30 min before refeeding, compared with saline (40 mL/kg, po), did not alter food intake in intact mice fasted overnight. The preadministration of only EnsureH (40 mL/kg) by oral gavage 30 min before refeeding did not alter food intake (B), thereby increasing cumulative food intake and inducing overeating until 6 h after refeeding (A). The coadministration of EnsureH (40 mL/kg) and GABA (200 mg/kg) markedly suppressed food intake for 0–0.5 h after refeeding (B) and thereby prevented overeating (A). n = 5–6. (C): In capsaicin-treated mice, the simultaneous administration of EnsureH (40 mL/kg) and GABA (200 mg/kg) failed to decrease food intake. n = 11. * p < 0.05, ** p < 0.01 by one-way ANOVA followed by Tukey’s test. GABA: Gamma-aminobutyric acid.