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      Hypothalamic Neuropeptide Y (NPY) Controls Hepatic VLDL-Triglyceride Secretion in Rats via the Sympathetic Nervous System

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          Abstract

          Excessive secretion of triglyceride-rich very low-density lipoproteins (VLDL-TG) contributes to diabetic dyslipidemia. Earlier studies have indicated a possible role for the hypothalamus and autonomic nervous system in the regulation of VLDL-TG. In the current study, we investigated whether the autonomic nervous system and hypothalamic neuropeptide Y (NPY) release during fasting regulates hepatic VLDL-TG secretion. We report that, in fasted rats, an intact hypothalamic arcuate nucleus and hepatic sympathetic innervation are necessary to maintain VLDL-TG secretion. Furthermore, the hepatic sympathetic innervation is necessary to mediate the stimulatory effect of intracerebroventricular administration of NPY on VLDL-TG secretion. Since the intracerebroventricular administration of NPY increases VLDL-TG secretion by the liver without affecting lipolysis, its effect on lipid metabolism appears to be selective to the liver. Together, our findings indicate that the increased release of NPY during fasting stimulates the sympathetic nervous system to maintain VLDL-TG secretion at a postprandial level.

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          Fasting induces a large, leptin-dependent increase in the intrinsic action potential frequency of orexigenic arcuate nucleus neuropeptide Y/Agouti-related protein neurons.

          Kanji Takahashi, Roger Cone (2005)
          The neuropeptide Y (NPY)/Agouti-related protein (AgRP) neurons of the hypothalamic arcuate nucleus are thought to promote feeding. Here, we demonstrate that feeding state in vivo, through a leptin-dependent process, induces large and persistent changes in the electrophysiological activity of these neurons as measured extracellularly in vitro. Consistent with an orexigenic role, fasting induced a 4-fold increase in the basal action potential frequency of NPY/AgRP neurons. Leptin, when injected into fasted wild-type mice, induced a dose- and time-dependent decrease in spike frequency, which approached fed levels 2-3 h post treatment. In leptin-deficient (lep(ob)/lep(ob)) and leptin receptor-deficient (lepr(db)/lepr(db)) mice, NPY/AgRP spike frequency was not significantly increased by fasting, and even in mutant mice fed ad libitum, spike frequency was at least as high as in fasted wild-type mice. All recordings included GABA(A) and ionotropic glutamate receptor antagonists, suggesting that expression of this modulation is potentially intrinsic and not synaptically dependent. Recorded neurons were unambiguously identified using NPY-Sapphire transgenic mice. This is a remarkably straightforward example of a very robust in vitro electrophysiogical effect produced by a simple behavioral manipulation, food restriction.
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            The central melanocortin system directly controls peripheral lipid metabolism.

            Brigitte Schrott, Ildiko Szanto, Paul T. Pfluger … (2007)
            Disruptions of the melanocortin signaling system have been linked to obesity. We investigated a possible role of the central nervous melanocortin system (CNS-Mcr) in the control of adiposity through effects on nutrient partitioning and cellular lipid metabolism independent of nutrient intake. We report that pharmacological inhibition of melanocortin receptors (Mcr) in rats and genetic disruption of Mc4r in mice directly and potently promoted lipid uptake, triglyceride synthesis, and fat accumulation in white adipose tissue (WAT), while increased CNS-Mcr signaling triggered lipid mobilization. These effects were independent of food intake and preceded changes in adiposity. In addition, decreased CNS-Mcr signaling promoted increased insulin sensitivity and glucose uptake in WAT while decreasing glucose utilization in muscle and brown adipose tissue. Such CNS control of peripheral nutrient partitioning depended on sympathetic nervous system function and was enhanced by synergistic effects on liver triglyceride synthesis. Our findings offer an explanation for enhanced adiposity resulting from decreased melanocortin signaling, even in the absence of hyperphagia, and are consistent with feeding-independent changes in substrate utilization as reflected by respiratory quotient, which is increased with chronic Mcr blockade in rodents and in humans with loss-of-function mutations in MC4R. We also reveal molecular underpinnings for direct control of the CNS-Mcr over lipid metabolism. These results suggest ways to design more efficient pharmacological methods for controlling adiposity.
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              Suprachiasmatic GABAergic inputs to the paraventricular nucleus control plasma glucose concentrations in the rat via sympathetic innervation of the liver.

              Andries Kalsbeek, Susanne la Fleur, Caroline Van Heijningen … (2004)
              Daily peak plasma glucose concentrations are attained shortly before awakening. Previous experiments indicated an important role for the biological clock, located in the suprachiasmatic nuclei (SCN), in the genesis of this anticipatory rise in plasma glucose concentrations by controlling hepatic glucose production. Here, we show that stimulation of NMDA receptors, or blockade of GABA receptors in the paraventricular nucleus of the hypothalamus (PVN) of conscious rats, caused a pronounced increase in plasma glucose concentrations. The local administration of TTX in brain areas afferent to the PVN revealed that an important part of the inhibitory inputs to the PVN was derived from the SCN. Using a transneuronal viral-tracing technique, we showed that the SCN is connected to the liver via both branches of the autonomic nervous system (ANS). The combination of a blockade of GABA receptors in the PVN with selective removal of either the sympathetic or parasympathetic branch of the hepatic ANS innervation showed that hyperglycemia produced by PVN stimulation was primarily attributable to an activation of the sympathetic input to the liver. We propose that the daily rise in plasma glucose concentrations is caused by an SCN-mediated withdrawal of GABAergic inputs to sympathetic preautonomic neurons in the PVN, resulting in an increased hepatic glucose production. The remarkable resemblance of the presently proposed control mechanism to that described previously for the control of daily melatonin rhythm suggests that the GABAergic control of sympathetic preautonomic neurons in the PVN is an important pathway for the SCN to control peripheral physiology.
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                Author and article information

                Journal
                Journal ID (nlm-ta): Diabetes
                Journal ID (iso-abbrev): Diabetes
                Journal ID (hwp): diabetes
                Journal ID (pmc): diabetes
                Journal ID (publisher-id): Diabetes
                Title: Diabetes
                Publisher: American Diabetes Association
                ISSN (Print): 0012-1797
                ISSN (Electronic): 1939-327X
                Publication date (Print): May 2012
                Publication date (Electronic): 13 April 2012
                Volume: 61
                Issue: 5
                Pages: 1043-1050
                Affiliations
                [1] 1Department of Endocrinology and Metabolism, Laboratory of Endocrinology, Academic Medical Center, University of Amsterdam, Amsterdam, the Netherlands
                [2] 2Department of Hypothalamic Integration Mechanisms, Netherlands Institute for Neuroscience, Royal Netherlands Academy of Arts and Science, Amsterdam, the Netherlands
                [3] 3Department of Neurobiology, Tongji Medical College of Huazhong University of Science and Technology, Hubei, China
                [4] 4Department of Clinical Chemistry, Laboratory of Endocrinology, Academic Medical Center, University of Amsterdam, Amsterdam, the Netherlands
                Author notes
                Corresponding author: Eveline Bruinstroop, e.bruinstroop@ 123456amc.uva.nl .
                Article
                Publisher ID: 1142
                DOI: 10.2337/db11-1142
                PMC ID: 3331766
                PubMed ID: 22461566
                SO-VID: 8fae2cb2-c6d9-46dd-9d7b-7c12bd015049
                Copyright © © 2012 by the American Diabetes Association.
                License:

                Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. See http://creativecommons.org/licenses/by-nc-nd/3.0/ for details.

                History
                Date received : 17 August 2011
                Date accepted : 13 January 2012
                Categories
                Subject: Metabolism

                ScienceOpen disciplines: Endocrinology & Diabetes
                Data availability:
                ScienceOpen disciplines: Endocrinology & Diabetes

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