Central GPR109A Mediates Neuronal Oxidative Stress and Pressor Response in Conscious Rats
The primary goal of this study was to characterize the role of GPR109A in the rostral ventrolateral medulla (RVLM) in blood pressure (BP) regulation and to elucidate the mechanisms involved in the hypertensive response elicited by central GPR109A activation. The central hypothesis of this study was "central GPR109A activation causes neuronal oxidative stress and pressor response via local glutamate/prostaglandins release". The data provide the first evidence for GPR109A expression in the RVLM, the major cardiovascular regulatory nucleus of the brainstem, and in pheochromocytoma cell line (PC12 cells), used as surrogates of the RVLM neurons. GPR109A co-localization was evident in tyrosine hydroxylase (TH)-expressing neurons and in PC12 cells. The anti-hyperlipidemic drug, nicotinic acid (NA) a known GPR109A agonist that activate the receptor and Ca2+-dependently release prostaglandins (PGs), was used in the study. Our findings demonstrated that intra-RVLM activation of GPR109A receptors with NA produced a robust dose-dependent glutamate-like elevation in sympathetic tone and BP in normotensive conscious male Sprague Dawley (SD) rats. The pressor response was abolished by prior blockade of the NMDA glutamate receptor (NMDAR) using 2-amino-5-phosphonopentanoic acid (AP5) or the prostanoid EP3 receptor (EP3R) using N-[(5-Bromo-2-methoxyphenyl) sulfonyl]-3-[2-(2-naphthalenylmethyl)phenyl]-2-propenamide (L-798106). Further, the NA pressor response was exacerbated by a prior application of the glutamate uptake inhibitor, L-trans-Pyrrolidine-2,4-dicarboxylic acid (PDC). Ex vivo studies revealed that intra-RVLM GPR109A activation (NA; 20[mu]g) increased local prostaglandin E2 (PGE2) levels, enhanced RVLM ERK1/2 and nNOS phosphorylation and increased c-Fos immunoreactivity. Further, NA induced oxidative stress in the RVLM of NA-treated rats (increased ROS levels and NADPH oxidase activity and decreased catalase activity). Prior EP3R blockade (L-798106) abrogated the biochemical and the pressor response caused by intra-RVLM NA. NMDAR (AP5) or EP3R blockade similarly abolished NA-mediated pressor response, indicating the involvement of both glutamate and PGE2 in this effect. Selective inhibition of RVLM nNOS (N[omega]-propyl-L-arginine; NPLA) abolished the intra-RVLM NA-evoked pressor response. Further, NPLA abrogated the GPR109A-mediated increases in RVLM nNOS phosphorylation and c-Fos and ROS levels. Our in vitro (PC12) studies supported and extended the in-vivo findings as NA increased Ca2+, PGE2, L-glutamate and NO levels as well as ROS levels in cultured PC12 cells. The increase in L-glutamate level is likely mediated by PGE2/EP3R because L-798106 attenuated NA or PGE2-evoked L-glutamate release. The above-mentioned effects are mediated via GPR109A because the use of the inactive isomer, IsoNA failed to produce any hemodynamic or biochemical changes. Further, NA failed to increase Ca2+ or L-glutamate levels in PC12 cells following siRNA-evoked GPR109A knockdown. Collectively, these studies provide insight into identifying the role of central GPR109A activation in cardiovascular regulation in conscious animals and the potential mechanisms involved in this effect.
Rezq, Samar. (May 2016). Central GPR109A Mediates Neuronal Oxidative Stress and Pressor Response in Conscious Rats (Doctoral Dissertation, East Carolina University). Retrieved from the Scholarship. (http://hdl.handle.net/10342/5389.)
Rezq, Samar. Central GPR109A Mediates Neuronal Oxidative Stress and Pressor Response in Conscious Rats. Doctoral Dissertation. East Carolina University, May 2016. The Scholarship. http://hdl.handle.net/10342/5389. October 20, 2018.
Rezq, Samar, “Central GPR109A Mediates Neuronal Oxidative Stress and Pressor Response in Conscious Rats” (Doctoral Dissertation., East Carolina University, May 2016).
Rezq, Samar. Central GPR109A Mediates Neuronal Oxidative Stress and Pressor Response in Conscious Rats [Doctoral Dissertation]. Greenville, NC: East Carolina University; May 2016.
East Carolina University