Role of central cannabinoid receptor GPR18 in cardiovascular regulation
The primary goal of this study was to characterize the role of GPR18 in the rostral ventrolateral medulla (RVLM) in blood pressure (BP) regulation and elucidate the mechanisms involved in central GPR18 mediated hypotensive response. The central hypothesis of this study was "central GPR18 signaling exerts a tonic sympathoinhibitory effect". The data provide the first evidence for expression of GPR18 in the RVLM, the cardiovascular regulatory nuclei of the brainstem and its co-localization in tyrosine hydroxylase (TH)-expressing neurons as well as in RVLM neurons expressing cannabinoid₁ receptors (CB₁R). Also intra-RVLM activation of GPR18 receptors with abnormal cannabidiol (Abn CBD) produced a dose-dependent reduction in BP in conscious male Sprague Dawley rats whereas blockade of those receptors with 1, 3-dimethoxy-5-methyl-2-[(1R, 6R)-3-methyl-6-(1-methylethenyl)-2-cyclohexen-1-yl] benzene (O-1918) increased BP and abrogated the Abn CBD-evoked reduction in BP. Additional studies tested the hypothesis that the negligible hypotensive effect caused by the endogenous GPR18 ligand n-arachidonoyl glycine (NAGly) could be due to concurrent activation of CB₁R in the RVLM. Our findings supported this hypothesis because NAGly-evoked hypotension was doubled following RVLM CB₁R blockade (SR141716). Ex-vivo studies revealed that intra-RVLM GPR18 activation (Abn CBD; 0.4 [mu]g) enhanced RVLM Akt, ERK1/2 and nNOS phosphorylation and increased adiponectin (ADN) levels, during the hypotensive response. In contrast, prior GPR18 receptor blockade (O-1918) produced the opposite effects, and abrogated Abn CBD-evoked responses in conscious Sprague Dawley rats. Inhibition of RVLM PI3K/Akt (wortmannin), ERK1/2 (PD98059) or nNOS (N[omega]-propyl-L-arginine, NPLA) or activation of adenylyl cyclase (forskolin) virtually abolished the intra-RVLM Abn CBD-evoked hypotension. Further, wortmannin, PD98059, NPLA or forskolin abrogated the GPR18-mediated increases in RVLM Akt, ERK1/2, nNOS phosphorylation and ADN levels, along with increased ROS generation. Our in-vitro studies show that GPR18 is co-expressed with CB1R in nPC12 cells. Our in-vitro studies are in line with our in-vivo findings in normotensive rats indicating that GPR18 signals through the PI3K/Akt-ERK1/2-nNOS/ADN pathway. Additionally, our confocal imaging findings show that GPR18 is associated with lipid rafts (LRs). Furthermore, activation of GPR18 (Abn CBD/ NAGly) displaces it from the LRs and this response was abrogated by prior GPR18 blockade (O-1918). Furthermore, cholesterol depletion by methyl-[beta]-cyclodextrin (M[beta]CD) enhanced GPR18 signaling and uncovered O-1918 blockade in nPC12 cells. Collectively, these studies provide insight into identifying the potential signaling pathway(s) triggered by central GPR18 activation in conscious animals and the potential role of lipid rafts in modulating GPR18 signaling in-vitro
Penumarti, Anusha. (January 2014). Role of central cannabinoid receptor GPR18 in cardiovascular regulation (Doctoral Dissertation, East Carolina University). Retrieved from the Scholarship. (http://hdl.handle.net/10342/4429.)
Penumarti, Anusha. Role of central cannabinoid receptor GPR18 in cardiovascular regulation. Doctoral Dissertation. East Carolina University, January 2014. The Scholarship. http://hdl.handle.net/10342/4429. June 19, 2018.
Penumarti, Anusha, “Role of central cannabinoid receptor GPR18 in cardiovascular regulation” (Doctoral Dissertation., East Carolina University, January 2014).
Penumarti, Anusha. Role of central cannabinoid receptor GPR18 in cardiovascular regulation [Doctoral Dissertation]. Greenville, NC: East Carolina University; January 2014.
East Carolina University