Rationale: Dysregulation of hepatic triglyceride (TG)-rich very low-density lipoproteins (VLDL-TG) in obesity and type 2 diabetes contributes to the dyslipidemia that leads to cardiovascular morbidity. The central nervous system (CNS), particularly the hypothalamus, regulates hepatic lipid metabolism. Although the underlying neurocircuitry remains elusive, glycine has been documented to enhance CNS N-methyl-D-aspartate (NMDA) receptor-mediated transmission. Objective: We tested the hypothesis

more »

... at glycine regulates hepatic VLDL-TG secretion by potentiating NMDA receptor-mediated transmission in the CNS. Methods and Results: Using 10-hour fasted male Sprague-Dawley rats implanted with stereotaxic cannulae into an extrahypothalamic region termed the dorsal vagal complex (DVC) and vascular catheters to enable direct DVC infusion and blood sampling, respectively, the rate of hepatic VLDL-TG secretion was measured following tyloxapol (an inhibitor of lipoprotein lipase) injection. Direct DVC infusion of glycine lowered VLDL-TG secretion, whereas NMDA receptor blocker MK-801 fully negated glycine's effect. NR1 subunit of NMDA receptor antagonist 7-chlorokynurenic acid, adenoviral injection of NR1 short hairpin RNA (shRNA), and hepatic vagotomy also nullified glycine's effect. Finally, DVC glycine normalized the hypersecretion of VLDL-TG induced by high-fat feeding. Conclusions: Molecular and pharmacological inhibition of the NR1-containing NMDA receptors in the DVC negated the ability of glycine to inhibit hepatic secretion of VLDL-TG in vivo. Importantly, the hypersecretion of VLDL-TG from the liver induced by a model of high-fat feeding was restored by the hepatic lipid control of CNS glycine sensing. These findings collectively suggest that glycine or glycine analogues may have therapeutic benefits in lowering plasma lipid levels in diabetes and obesity by triggering the CNS. (Circ Res. 2012;110:00-00.) Key Words: glycine Ⅲ NMDA receptors Ⅲ CNS Ⅲ hepatic lipid metabolism Ⅲ obesity H ypertriglyceridemia and overproduction and secretion of VLDL-TG are key contributors to the metabolic syndrome and atherogenic dyslipidemia. 1-3 Although circulating insulin and free fatty acids (FFA) regulate hepatic VLDL-TG secretion, 4,5 the underlying regulatory mechanisms remain unclear. Given that diabetes and obesity are associated with hypertriglyceridemia and increased hepatic secretion and production of VLDL-TG, 1-3 it is important to elucidate the regulatory mechanisms of hepatic VLDL-TG secretion in a normal setting with the hope to discover novel therapeutic molecules to lower blood lipid levels in obesity and diabetes. In addition to the array of circulating factors that directly regulate hepatic lipid metabolism, the hypothalamic region of the brain senses circulating nutrients and hormones to regulate peripheral lipid homeostasis 6 -10 and blood pressure. 11 For example, nutrients such as glucose signal within the hypothalamus to inhibit VLDL-TG secretion through the hepatic vagus. 12 Consistent with the fact that nutrients in the brain lowers appetite through the suppression of neuropeptide Y, 13 direct administration of neuropeptide Y into the hypothalamus conversely stimulates VLDL-TG secretion. 9 Central neuropeptide Y signaling also negates the ability of circulating insulin to inhibit VLDL-TG secretion, 6 whereas hormones such as insulin, 8 ghrelin, 7 melanocortins, 14,15 and leptin 16,17 trigger hypothalamic signaling cascades to modulate peripheral lipid profiles. In spite of the fact that the Original