Macrophage-related chronic inflammation in adipose tissues is associated with the development of insulin resistance and glucose intolerance, leading to the development of obesity and T2DM. Our previous studies, and those of others, have shown that GABA, through the activation of GABAA
-R, inhibits inflammation in mouse models of autoimmune diseases 
. In this study, we examined the impact of oral treatment with GABA on the development of HFD-induced glucose intolerance and insulin resistance as well as obesity in mice. We found that oral treatment with GABA did not modulate the amount of food intake and water consumption by mice, consistent with our previous findings 
. Given that GABA has little or no ability to pass through the blood brain barrier, it is not surprising that activation of peripheral GABA receptors did not alter calorie intake in mice.
Furthermore, while most of the HFD-fed control mice displayed a significant increase in body weights, the mice that had been fed with HFD and GABA had significantly less gain in body weights over the observation period. Over-consumption of calories can cause obesity and lead to the development of insulin resistance and T2DM 
. We found that oral treatment with GABA significantly improved glucose intolerance and insulin resistance in HFD-fed mice. Evidentially, the levels of fasting blood glucose, blood glucose at 2 hours post-glucose challenge, and the relative levels of blood glucose post-insulin treatment in the GABA-treated mice were significantly lower than that of the control mice. More importantly, after the establishment of obesity and T2DM, oral treatment with GABA significantly reduced the gain in body weight and improved glucose tolerance and insulin sensitivity in mice. These novel data demonstrated that oral treatment with GABA not only prevented the HFD-induced obesity and T2DM development, but also inhibited the progression of obesity and T2DM in mice. Given that GABA is safe for human consumption, GABA may be valuable for the prevention and treatment of obesity and T2DM in the clinic.
Recent studies have shown that adipose tissue-related chronic inflammation contributes to the development of insulin resistance, a key component of metabolic syndrome, and leads to the development of obesity and T2DM as well as other metabolic disorder-related diseases 
. To understand the potential mechanisms underlying the therapeutic effect of GABA in inhibiting the HFD-induced obesity and T2DM, we examined the mass of adipose tissues and inflammatory infiltrates. In comparison with that in the control mice, oral treatment with GABA significantly reduced the total amounts of epididymal fat tissues and the size of adipocytes in the HFD-fed mice. Because macrophages are predominant players in the development of chronic inflammation in adipose tissues 
, we further characterized the frequency of macrophage infiltrates in the epididymal fat tissues. We found that oral treatment with GABA significantly decreased the frequency of macrophage infiltrates in adipose tissues of the HFD-fed mice. More importantly, GABAA
receptor subunits are expressed by adipocytes and macrophages, and GABAA
receptor agonists can inhibit macrophage activation 
. It is possible that GABA, through the GABAA
receptors, inhibits macrophage activation and migration, reducing chronic inflammation in adipose tissues. Given that inflammatory adipocytes can produce adipokines that recruit inflammatory infiltrates, such as macrophages, GABA may alternatively inhibit adipogenesis and adipokine production, indirectly limiting the macrophage infiltration into adipose tissue in the HFD-fed mice. Recent studies have highlighted the importance of glucagon in regulating glucose homeostasis 
. Physiologically, GABA acts synergistically with insulin to inhibit the secretion of glucagon by the pancreatic α-cells, and reverse insulin-deficiency-related hyperglycemia in mice 
. Although we can not completely exclude the possible effect of GABA-mediated inhibition of glucagon secretion, our data from islet transplant suggest that treatment with GABA alone has little ability to reverse T1D in diabetic NOD mice 
. Hence, the effect of GABA-mediated inhibition of glucagon secretion may be minor in inhibiting HFD-induced glucose intolerance, insulin resistance, and macrophage-related inflammation. Conceivably, GABA, through its receptors, on macrophages and adipocytes, improves glucose tolerance and insulin sensitivity in our experimental model. We are interested in further investigating how activation of GABAA
receptors modulates adipogenesis and macrophage activation and migration as well as adipokine and cytokine production by adipocytes.
Notably, Tregs are potent inhibitors for macrophage activation and function 
. Tregs are negative regulators of obesity-related insulin resistance and T2DM in mice 
. We found that oral treatment with GABA significantly increased the frequency of splenic Tregs in C57BL/6 mice, indicating that GABA promoted Treg proliferation and maturation in vivo
. Our previous study has shown that GABA, through the GABAA
receptors, induces effector T cell cycle arrest, consistent with the notion that engagement of GABAA
receptors induces the hyperpolarization of membrane potentials, which may inhibit the TCR-related signaling 
. The significantly increased frequency of splenic Tregs by GABA treatment suggests that GABA, through the GABAA
receptors, may promote the depolarization of membrane potentials by opening the voltage-dependent calcium channel and enhancing the TCR-triggering calcium-dependent downstream survival and growth signaling 
. Given that Tregs are potent inhibitors of inflammation and insulin resistance the GABA-induced increase in Tregs may also inhibit macrophage infiltration and related chronic inflammation, contributing to the therapeutic effect of GABA in inhibiting the HFD-induced obesity and T2DM.
In summary, our data demonstrated that oral treatment with GABA inhibited the HFD-induced obesity and improved glucose intolerance and insulin sensitivity, even after the establishment of obesity and T2DM in mice. Furthermore, oral treatment with GABA reduced the HFD-induced adipocyte hypertrophy and adipose tissue mass, accompanied by significantly reduced macrophage infiltrates in the adipose tissues. Furthermore, we found that GABA treatment increased the frequency of splenic Tregs in mice. Apparently, GABA, through its GABAA-Rs on adipocytes, macrophages and T cells, inhibited chronic inflammation in adipose tissues, leading to the improvement of glucose tolerance and insulin sensitivity in HFD-fed mice. Given that GABA mainly acts on the peripheral GABA receptors and is safe for human consumption, GABA and other GABAA-R agonists may be valuable for the prevention and treatment of obesity and T2DM in the clinic.