Enhanced hemeoxygenase activity in the rostral ventrolateral medulla mediates exaggerated hemin-evoked hypotension in the spontaneously hypertensive rat

Estrogen-mediated mitigation of cardiac oxidative stress in ovariectomized rats is associated with upregulated cardiac circadian clock Per2 and heart-specific miRNAs

AIM

Estrogen (E2) confers cardioprotection in premenopausal women and in models of menopause and its effects, mostly studied in female reproductive organs, vary on a circadian rhythm basis in relation to the circadian clock genes. However, it remains unknown if a similar circadian pattern exists in the female heart in a manner that explains, at least partly, the cardioprotective effect of E2. The aim of the present investigation was to determine if upregulation of the circadian clock Per2 and its regulated heart-specific miRNAs, and redox enzymes contribute to the E2-mediated cardioprotection in ovariectomized rats.

MAIN METHODS

Rats were subjected to ovariectomy (OVX) 2-weeks prior to a 2-week E2 treatment. On the last treatment day, hearts were collected every 4 h. for ex-vivo biochemical measurements. In parallel studies, telemetric mean arterial pressure (MAP) was obtained at the tissue collection times.

KEY FINDINGS

OVX + E2 rats exhibited lower body weight during daytime and MAP during day and night times, and their hearts exhibited: (1) higher Per2 protein abundance, cardioprotective miRNAs (miRNA1, miRNA133a, miRNA208a, miRNA499), mALDH2, and catalase; (2) lower reactive oxygen species, cardio-detrimental miRNA652, carbonyl, MDA and HO-1 levels. The reciprocal Per2/HO-1 relationship was more evident during the daytime and correlated with the upregulated cardioprotective miRNAs in OVX + E2 rats. Finally, cardiac Per2, heart-specific miRNAs and reactive oxygen species levels and redox enzymes activities were similar in normal female and OVX + E2 rats.

SIGNIFICANCE

Enhancement of cardiac Per2, redox enzymes and heart-specific miRNAs likely contribute to E2-mediated mitigation of cardiac oxidative stress in OVX rats.

Alcohol suppresses cardiovascular diurnal variations in male normotensive rats: Role of reduced PER2 expression and CYP2E1 hyperactivity in the heart

BACKGROUND AND AIMS

The molecular mechanism of the adverse effects of ethanol on diurnal cardiovascular regulation remains unknown. In separate studies, the cardiac circadian rhythm protein period-2 (PER2) confers cardioprotection and, in other organs, PER2 interaction with the ethanol-metabolizing enzyme CYP2E1 underlies, via heme oxygenase-1 (HO-1) upregulation, tissue injury/dysfunction. Here, we hypothesized that suppressed PER2 expression and elevated CYP2E1/HO-1 levels in the heart underlie the disrupted diurnal cardiovascular rhythm/function in alcohol-fed normotensive rats.

METHODS

In ethanol-fed (5%, w/v; 8 weeks) or isocaloric liquid diet-fed male rats, diurnal changes in blood pressure (BP), heart rate (HR), HR vagal variability index, root mean square of successive beat-to-beat differences in beat-interval duration (rMSSD), and cardiac function were measured by radiotelemetry and echocardiography followed by ex vivo molecular studies.

RESULTS

Radiotelemetry findings showed ethanol-evoked reductions in BP (during the dark cycle), rMSSD (during both cycles), and in diurnal differences in BP and rMSSD. Echocardiography findings revealed significant (p < 0.05) reductions in ejection fraction and fractional shortening (weeks 4-6) in the absence of cardiac remodeling (collagen content). Hearts of ethanol-fed rats exhibited higher (p < 0.05) CYP2E1 activity (50%) and HO-1 expression (63%), along with reduction (p < 0.05) in PER2 levels (29%), compared with the hearts of isocaloric diet-fed control rats.

CONCLUSIONS

Our novel findings implicate upregulations of CYP2E1/HO-1 and downregulation of the circadian rhythm cardioprotective protein PER2, in the heart, in the chronic deleterious diurnal cardiovascular effects of alcohol in male rats.

Carbon Monoxide Attenuates High Salt-Induced Hypertension While Reducing Pro-inflammatory Cytokines and Oxidative Stress in the Paraventricular Nucleus

Carbon monoxide (CO) presents anti-inflammatory and antioxidant activities as a new gaseous neuromessenger produced by heme oxygenase-1 (HO-1) in the body. High salt-induced hypertension is relevant to the levels of pro-inflammatory cytokines (PICs) and oxidative stress in the hypothalamic paraventricular nucleus (PVN). We explored whether CO in PVN can attenuate high salt-induced hypertension by regulating PICs or oxidative stress. Male Dahl Salt-Sensitive rats were fed high-salt (8% NaCl) or normal-salt (0.3% NaCl) diet for 4 weeks. CORM-2, ZnPP IX, or vehicle was microinjected into bilateral PVN for 6 weeks. High-salt diet increased the levels of MAP, plasma norepinephrine (NE), reactive oxygen species (ROS), and the expressions of COX2, IL-1β, IL-6, NOX2, and NOX4 significantly in PVN (p < 0.05), but decreased the expressions of HO-1 and Cu/Zn-SOD in PVN (p < 0.05). Salt increased sympathetic activity as measured by circulating norepinephrine, and increased the ratio of basal RSNA to max RSNA, in part by decreasing max RSNA. PVN microinjection of CORM-2 decreased the levels of MAP, NE, RSNA, ROS and the expressions of COX2, IL-1β, IL-6, NOX2, NOX4 significantly in PVN of hypertensive rat (p < 0.05), but increased the expressions of HO-1 and Cu/Zn-SOD significantly (p < 0.05), which were all opposite to the effects of ZnPP IX microinjected in PVN (p < 0.05). We concluded that exogenous or endogenous CO attenuates high salt-induced hypertension by regulating PICs and oxidative stress in PVN.

Restoration of Rostral Ventrolateral Medulla Cystathionine-γ Lyase Activity Underlies Moxonidine-Evoked Neuroprotection and Sympathoinhibition in Diabetic Rats

We recently demonstrated a fundamental role for cystathionine-γ lyase (CSE)-derived hydrogen sulfide (H₂S) in the cardioprotective effect of the centrally acting drug moxonidine in diabetic rats. Whether a downregulated CSE/H₂S system in the rostral ventrolateral medulla (RVLM) underlies neuronal oxidative stress and sympathoexcitation in diabetes has not been investigated. Along with addressing this question, we tested the hypothesis that moxonidine prevents the diabetes-evoked neurochemical effects by restoring CSE/H₂S function within its major site of action, the RVLM. Ex vivo studies were performed on RVLM tissues of streptozotocin (55 mg/kg, i.p.) diabetic rats treated daily for 3 weeks with moxonidine (2 or 6 mg/kg; gavage), H₂S donor sodium hydrosulfide (NaHS) (3.4 mg/kg, i.p.), CSE inhibitor DL-propargylglycine (DLP) (37.5 mg/kg, i.p.), a combination of DLP with moxonidine, or their vehicle. Moxonidine alleviated RVLM oxidative stress, neuronal injury, and increased tyrosine hydroxylase immunoreactivity (sympathoexcitation) by restoring CSE expression/activity as well as heme oxygenase-1 (HO-1) expression. A pivotal role for H₂S in moxonidine-evoked neuroprotection is supported by the following: 1) NaHS replicated the moxonidine-evoked neuroprotection, and the restoration of RVLM HO-1 expression in diabetic rats; and 2) DLP abolished moxonidine-evoked neuroprotection in diabetic rats, and caused RVLM neurotoxicity, reminiscent of a diabetes-evoked neuronal phenotype, in healthy rats. These findings suggest a novel role for RVLM CSE/H₂S/HO-1 in moxonidine-evoked neuroprotection and sympathoinhibition, and as a therapeutic target for developing new drugs for alleviating diabetes-evoked RVLM neurotoxicity and cardiovascular anomalies.

Endothelin Confers Protection against High Glucose-Induced Neurotoxicity via Alleviation of Oxidative Stress

Recent findings linked the inhibition in the neuromodulator peptide endothelin-1 (ET-1) level to the high glucose-evoked neurotoxicity. However, definitive neuroprotective role for ET-1 and the major neuronal ET (ET-3) against high glucose-evoked toxicity and the implicated neurochemical responses triggered by their ET-A and ET-B receptors remain unknown. Here, we tested the hypothesis that ET-B activation alleviates high glucose-evoked oxidative stress and cell death. High glucose (100 mM for 48 hours)-evoked cell death was associated with elevation in reactive oxygen species, inhibition of catalase activity, and a paradoxical upregulation of hemeoxygenase-1 expression along with ET-A and ET-B receptors were downregulated and upregulated, respectively. ET-1 or ET-3, in concentrations that had no effect on PC12 cell viability in normal glucose medium, alleviated all high glucose-evoked neurochemical responses, except for the reduction in ET-A receptor expression. Prior (4 hours) incubation with a selective ET-A (BQ123) or ET-B (BQ788) receptor blocker abrogated the neuroprotection conferred by ET-1 or ET-3. However, the ET-B receptor played a greater role because BQ788 abrogated the favorable ET-1- or ET-3-mediated reversal of the ERK1/2 phosphorylation and the inhibition in catalase activity caused by high glucose. These findings suggest that endothelin exerts ET-B receptor-dependent favorable redox and neuroprotective effects against high glucose-evoked oxidative damage and neurotoxicity.

Rostral Ventrolateral Medulla EP3 Receptor Mediates the Sympathoexcitatory and Pressor Effects of Prostaglandin E2 in Conscious Rats

Whereas few studies have dealt with the central sympathoexcitatory action of the inflammatory prostanoid prostaglandin E₂ (PGE₂), there is no information on the expression and cardiovascular function of different PGE₂ (EP) receptors in one of the major cardiovascular-regulating nuclei, the rostral ventrolateral medulla (RVLM). The current study aimed at filling this knowledge gap as well as elucidating the implicated molecular mechanisms. To achieve these goals, we showed the expression of EP2, EP3, and EP4 receptors in the RVLM and investigated their cardiovascular roles in conscious rats, ex vivo as well as in cultured PC12 cells. Intra-RVLM PGE₂ significantly increased blood pressure and sympathetic dominance (spectral analysis). Studies with selective EP receptor subtype agonists and antagonists showed that these PGE₂-evoked responses were only replicated by intra-RVLM activation of the EP3 receptor with its agonist sulprostone. The RVLM of PGE₂-treated rats exhibited increases in c-Fos expression and extracellular signal-regulated kinase 1/2 and neuronal nitric oxide synthase phosphorylation along with oxidative stress, and PGE₂ increased l-glutamate release in PC12 cells (surrogates of RVLM neurons). Abrogation of the PGE₂-evoked pressor and biochemical responses only occurred following EP3 receptor blockade (N-[(5-Bromo-2-methoxyphenyl)sulfonyl]-3-[2-(2-naphthalenylmethyl)phenyl]-2-propenamide, L-798106). These findings suggest the dependence of RVLM PGE₂-mediated sympathoexcitation/pressor response on local EP3 receptor signaling in conscious rats, and highlight central EP3 receptor blockade as a potential therapeutic modality for hypertension management.

Enhanced vascular PI3K/Akt-NOX signaling underlies the peripheral NMDAR-mediated pressor response in conscious rats

The molecular mechanisms for peripheral N-methyl-D-aspartate receptor (NMDAR)-mediated vascular oxidative stress and pressor response are not known. We conducted integrative (in vivo) and ex vivo biochemical studies to test the hypothesis that reactive oxygen species (ROS)-dependent calcium influx, triggered by the activation of vascular kinases, underlies the NMDAR-mediated pressor response. Pharmacological inhibition of phosphoinositide 3-kinase (PI3K)/Akt (wortmannin, 15 μg/kg), protein kinase C (chelerythrine: 5 mg/kg, intravenous), Ca²⁺ influx (nifedipine, 0.35 or 0.75 mg/kg), or NADPH oxidase (NOX: apocynin, 5 mg/kg) attenuated the peripheral NMDAR-mediated pressor response in conscious male Sprague-Dawley rats. NMDAR activation enhanced the phosphorylation of Akt, ERK1, JNK and p38 (Western blot), and NOX activity in vascular tissues collected during the pressor response caused by NMDA infusion (180 μg·kg⁻¹·min⁻¹, 30 minutes). Furthermore, ex vivo studies showed that wortmannin, chelerythrine, or apocynin abrogated the NMDAR-mediated vascular nitric oxide (NO) and ROS generation and NOX activation in the vasculature. These findings implicate vascular PI3K/Akt-protein kinase C signaling in the peripheral NMDAR-mediated increases in vascular NO and NOX activation (ROS), which ultimately lead to calcium influx and pressor response in conscious rats.

Neuronal nitric oxide synthase-dependent elevation in adiponectin in the rostral ventrolateral medulla underlies g protein-coupled receptor 18-mediated hypotension in conscious rats

Direct activation of the endocannabinoid receptor G protein-coupled receptor 18 (GPR18) in the rostral ventrolateral medulla (RVLM) of conscious rats by abnormal cannabidiol (Abn CBD; trans-4-[3-methyl-6-(1-methylethenyl)-2-cyclohexen-1-yl]-5-pentyl-1,3-benzenediol) elevates local nitric oxide (NO) and adiponectin (ADN) levels and reduces oxidative stress and blood pressure (BP). However, the molecular mechanisms for GPR18-mediated neurochemical responses, including the nitric oxide synthase isoform that generates NO, and their potential causal link to the BP reduction are not known. We hypothesized that GPR18-mediated enhancement of Akt, extracellular signal-regulated kinase 1/2 (ERK1/2), and neuronal nitric oxide synthase (nNOS) phosphorylation, triggered by a reduction in cAMP, accounts for the NO/ADN-dependent reductions in RVLM oxidative stress and BP. Intra-RVLM GPR18 activation (Abn CBD; 0.4 μg) enhanced RVLM Akt, ERK1/2, and nNOS phosphorylation as well as ADN levels during the hypotensive response. Prior GPR18 blockade with O-1918 (1,3-dimethoxy-5-methyl-2-[(1R,6R)-3-methyl-6-(1-methylethenyl)-2-cyclohexen-1-yl]benzene) produced the opposite effects and abrogated Abn CBD-evoked neurochemical and BP responses. Pharmacological inhibition of RVLM phosphoinositide 3-kinase (PI3K)/Akt (wortmannin), ERK1/2 (PD98059 [2-​(2-​amino-​3-​methoxyphenyl)-​4H-​1-​benzopyran-​4-​one]), or nNOS (N(ω)-propyl-l-arginine), or activation of adenylyl cyclase (forskolin) virtually abolished intra-RVLM Abn CBD-evoked hypotension and the increases in Akt, ERK1/2, and nNOS phosphorylation and in ADN levels in the RVLM. Our pharmacological and neurochemical findings support a pivotal role for PI3K, Akt, ERK1/2, nNOS, and adenylyl cyclase, via modulation of NO, ADN, and cAMP levels, in GPR18 regulation of the RVLM redox state and BP in conscious rats.

Atorvastatin increases lipopolysaccharide-induced expression of tumour necrosis factor-α-induced protein 8-like 2 in RAW264.7 cells

RAW264.7 cells are one of the major sources of productive inflammatory biomediators, including tumour necrosis factor-α (TNF-α) and interleukin (IL)-6. TNF-α-induced protein 8-like 2 (TIPE2) is an essential negative regulator of Toll-like and T-cell receptors, and the selective expression in the immune system prevents hyper-responsiveness and maintains immune homeostasis. The aim of the present study was to investigate whether atorvastatin upregulates the expression of TIPE2 and further regulates the inflammatory response and oxidation emergency response in RAW264.7 cells. RAW264.7 cells were incubated in Dulbecco’s modified Eagle’s medium containing lipopolysaccharide (LPS) in the presence or absence of atorvastatin. Following incubation, the medium was collected and the levels of TNF-α and IL-6 were measured using an enzyme-linked immunosorbent assay. The cells were harvested, and the mRNA and protein expression levels of TIPE2, macrophage migration inhibitory factor (MIF), IκB and nuclear factor (NF-κB)-κB were analysed using quantitative polymerase chain reaction and western blotting analysis, respectively, the expression of NOS, COX-2 and HO-1 protein were essayed by western blotting analysis, NO and ROS activities were determined. The results revealed that LPS increased the mRNA and protein expression levels of TIPE2, MIF and NF-κB, as well as the production of IL-6 and TNF-α, in a dose and time dependent manner in RAW264.7 cells. Meanwhile, LPS enhanced the expression of NOS and COX-2 protein, blocked HO-1 protein expression, increased NO and PGE2 production and ROS activity in a dose dependent manner in RAW264.7 cells. Atorvastatin significantly increased LPS induced expression of TIPE2, downregulated the expression of NOS, COX-2, MIF and NF-κB and the production of PGE2, NO, IL-6 and TNF-α in a time and dose dependent manner, and increased HO-1 protein expression, reduced ROS production in a dose dependent manner. The observations indicated that atorvastatin upregulated LPS induced expression of TIPE2 and consequently inhibited MIF, NF-κB, NOS and COX-2 expression and the production of NO, PGE2, TNF-α and IL-6, increased HO-1 expression, and inhibited ROS activity in cultured RAW264.7 cells.

The novel endocannabinoid receptor GPR18 is expressed in the rostral ventrolateral medulla and exerts tonic restraining influence on blood pressure

Systemic administration of the G-protein-coupled receptor 18 (GPR18) agonist abnormal cannabidiol (Abn CBD) lowers blood pressure (BP). Whether GPR18 is expressed in the central nervous system (CNS) and plays a role in BP control is not known despite the abundance of the GPR18 ligand N-arachidonoyl glycine (NAGly) in the CNS. Therefore, we first determined whether GPR18 is expressed in the presympathetic tyrosine hydroxylase (TH) immunoreactive (ir) neurons of the brainstem cardiovascular regulatory nuclei. Second, we investigated the impact of GPR18 activation and blockade on BP and heart rate (HR) and neurochemical modulators of sympathetic activity and BP. Immunofluorescence findings revealed GPR18 expression in TH-ir neurons in the rostral ventrolateral medulla (RVLM). Intra-RVLM GPR18 activation (Abn CBD) and blockade (O-1918, 1,3-dimethoxy-5-methyl-2-[(1R,6R)-3-methyl-6-(1-methylethenyl)-2-,cyclohexen-1-yl]benzene) elicited dose-dependent reductions and elevations in BP, respectively, along with respective increases and decreases in HR in conscious male Sprague-Dawley rats. RVLM GPR18 activation increased neuronal adiponectin (ADN) and NO and reduced reactive oxygen species (ROS) levels, and GPR18 blockade reduced neuronal ADN and increased oxidative stress (i.e., ROS) in the RVLM. Finally, we hypothesized that the negligible hypotensive effect caused by the endogenous GPR18 ligand NAGly could be due to concurrent activation of CB(1)R in the RVLM. Our findings support this hypothesis because NAGly-evoked hypotension was doubled after RVLM CB(1)R blockade (SR141716, rimonabant). These findings are the first to demonstrate GPR18 expression in the RVLM and to suggest a sympathoinhibitory role for this receptor. The findings yield new insight into the role of a novel cannabinoid receptor (GPR18) in central BP control.

Role of rostral ventrolateral medullary ERK/JNK/p38 MAPK signaling in the pressor effects of ethanol and its oxidative product acetaldehyde

BACKGROUND

We tested the hypothesis that alterations of the phosphorylation/dephosphorylation profile of mitogen-activated protein kinases (MAPKs) in the rostral ventrolateral medulla (RVLM) underlie the pressor response elicited by ethanol (EtOH) microinjection into the RVLM of spontaneously hypertensive rats (SHRs). The studies were extended to determine whether acetaldehyde (ACA), the primary oxidative product of EtOH, replicates the molecular effects of EtOH within the RVLM and the consequent pressor response.

METHODS

Effects of EtOH or ACA on blood pressure (BP) were evaluated in the absence or presence of selective JNK (SP600125), ERK (PD98059), p38 (SB203580), or ser/thr phosphatases (okadaic acid [OKA]) inhibitor.

RESULTS

Intra-RVLM EtOH (10 μg/rat) or ACA (2 μg/rat) caused a similar ERK2-dependent pressor response because EtOH or ACA-evoked increases in BP and in RVLM p-ERK2 level were abolished after pharmacologic inhibition of ERK phosphorylation. SP600125 abrogated the pressor action of EtOH, but not ACA, thus implicating JNK in EtOH action on BP. Despite EtOH enhancement of p38 phosphorylation, pharmacological studies argued against a causal role for this kinase in EtOH-evoked pressor response. RVLM phosphatase catalytic activity was not influenced by EtOH or ACA. Interestingly, pharmacologic phosphatase inhibition (OKA), which increased RVLM p-ERK2 and BP, abrogated the pressor effect of subsequently administered EtOH or ACA.

CONCLUSIONS

Enhancement of RVLM ERK2 phosphorylation constitutes a major molecular mechanism for the pressor response elicited by intra-RVLM EtOH or its metabolite, ACA, in conscious SHRs. Further, RVLM kinases dephosphorylation does not contribute to intra-RVLM EtOH- or ACA-evoked pressor response.

Differential modulation by vascular nitric oxide synthases of the ethanol-evoked hypotension and autonomic dysfunction in female rats

We recently reported that chronic exposure to ethanol lowers blood pressure (BP) via altering cardiac contractility and autonomic control in female rats. In this investigation we conducted pharmacological and molecular studies to elucidate the role of constitutive and inducible nitric oxide synthase (NOS) in these hemodynamic effects of ethanol. Changes caused by selective inhibition of eNOS [N(5)-(1-iminoethyl)-l-ornithine; l-NIO], nNOS (N(ω)-propyl-l-arginine; NPLA), or iNOS (1400W) in BP, heart rate (HR), myocardial contractility index (dP/dt(max)), and power spectral indices of hemodynamic variability were evaluated in telemetered female rats receiving ethanol (5%, w/v) or control liquid diet for 8 weeks. Ethanol increased plasma nitrite/nitrate (NOx) and enhanced the phosphorylation of eNOS and nNOS, but not iNOS, in the tail artery. Ethanol also reduced BP, +dP/dt(max), low-frequency bands of interbeat intervals (IBI(LF), 0.25-0.75 Hz) and IBI(LF/HF) ratio while high-frequency bands (IBI(HF), 0.75-3 Hz) were increased, suggesting parasympathetic overactivity. l-NIO (20 mg/kg i.p.) caused greater increases in BP in control than in ethanol-fed rats but elicited similar reductions in IBI(LF/HF) and +dP/dt(max) both groups. NPLA (1 mg/kg i.p.) caused minimal effects in control rats but exacerbated the reductions in BP, +dP/dt(max), and IBI(LF/HF) in ethanol-fed rats. No hemodynamic modifications were caused by 1400W (5 mg/kg i.p.) in either rat group. Together, these findings suggest that nNOS acts tonically to offset the detrimental cardiovascular actions of ethanol in female rats, and the enhanced vascular NO bioavailability may explain the blunted l-NIO evoked pressor response in ethanol-fed rats.

Enhanced vascular neuronal nitric-oxide synthase-derived nitric-oxide production underlies the pressor response caused by peripheral N-methyl-D-aspartate receptor activation in conscious rats

Although the N-methyl-D-aspartate (NMDA) receptor (NMDAR) obligatory unit NMDAR1 is expressed in the vasculature and myocardium, the impact of peripheral NMDAR activation on blood pressure (BP) has received little attention. We demonstrate, for the first time, dose-related pressor responses elicited by systemic NMDA (125, 250, 500, and 1000 μg/kg) in conscious rats. The pressor response was peripheral NMDAR-mediated because: 1) it persisted after ganglion blockade (hexamethonium; 5 mg/kg i.v.); 2) it was attenuated by the selective NMDAR blocker DL-2-amino-5-phosphonopentanoic acid (5 mg/kg, i.v.) or the glycine/NMDAR antagonist R-(+)-3-amino-1-hydroxypyrrolid-2-one [R-(+)-HA-966; 10 mg/kg i.v.]; and 3) NMDA (1.25-10 mM) increased contractile force of rat aorta in vitro. It is noteworthy that ex vivo studies revealed enhanced nitric oxide (NO) and reactive oxygen species (ROS) generation in vascular tissues collected at the peak of the NMDAR-mediated pressor response. Pharmacological, ex vivo, and in vitro findings demonstrated attenuation of the NMDAR-mediated increases in BP and vascular NO and ROS by the nonselective NO synthase (NOS) inhibitor N(ω)-nitro-L-arginine methyl ester hydrochloride (10 mg/kg i.v.) or the neuronal NOS (nNOS) inhibitor N(ω)-propyl-L-arginine hydrochloride (150 μg/kg i.p.) but not by the endothelial NOS inhibitor N(5)-(1-iminoethyl)-L-ornithine (4 or 10 mg/kg i.v.). Furthermore, R-(+)-HA-966 attenuated NMDA-evoked generation of vascular NO and ROS. The findings suggest a pivotal role for enhanced vascular nNOS-derived NO in ROS generation and in the subsequent pressor response elicited by peripheral NMDAR in conscious rats.

Enhancement of rostral ventrolateral medulla neuronal nitric-oxide synthase-nitric-oxide signaling mediates the central cannabinoid receptor 1-evoked pressor response in conscious rats

Our recent studies implicated brainstem GABAergic signaling in the central cannabinoid receptor 1 (CB(1)R)-mediated pressor response in conscious rats. Given the well established link between neuronal nitric-oxide synthase (nNOS)/nitric oxide (NO) signaling and GABAergic transmission in brainstem cardiovascular regulating areas, we elucidated the role of nNOS-generated NO in the central CB(1)R-elicited pressor response. Compared with vehicle, intracisternal (i.c.) microinjection of the CB(1)R agonist (R)-(+)-[2,3-dihydro-5-methyl-3[(4-morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl) methanone mesylate (WIN55212-2) (15 μg/rat) significantly enhanced nNOS phosphorylation as well as the total nitrate and nitrite content in the rostral ventrolateral medulla (RVLM) at 5, 10, and 30 min, which paralleled the elicited pressor response. These findings were corroborated by: 1) the parallel dose-related increases in blood pressure and RVLM-NO levels, measured in real time by in vivo electrochemistry, elicited by intra-RVLM WIN55212-2 (100, 200, or 300 pmol /80 nl; n = 5) in conscious rats; and 2) the significantly higher phosphorylated nNOS (p-nNOS) levels in the WIN55212-2-injected RVLM compared with the contralateral RVLM. Subsequent neurochemical studies showed that WIN55212-2 (15 μg/rat i.c.) significantly increased the number and percentage of neurons immunostained for nNOS (nitroxidergic neurons) and c-Fos (marker of neuronal activity) within the RVLM. The increases in blood pressure and the neurochemical responses elicited by intracisternal WIN55212-2 were attenuated by prior central CB(1)R blockade by N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251; 30 μg/rat i.c.) or selective nNOS inhibition by N(ω)-propyl-(L)-arginine (1 μg/rat i.c.). These findings implicate RVLM p-nNOS/NO signaling as a molecular mechanism in the central CB(1)R-evoked pressor effect in conscious rats.

Enhanced catabolism to acetaldehyde in rostral ventrolateral medullary neurons accounts for the pressor effect of ethanol in spontaneously hypertensive rats

We have previously shown that ethanol microinjection into the rostral ventrolateral medulla (RVLM) elicits sympathoexcitation and hypertension in conscious spontaneously hypertensive rats (SHRs) but not in Wistar-Kyoto (WKY) rats. In this study, evidence was sought to implicate the oxidative breakdown of ethanol in this strain-dependent hypertensive action of ethanol. Biochemical experiments revealed significantly higher catalase activity and similar aldehyde dehydrogenase (ALDH) activity in the RVLM of SHRs compared with WKY rats. We also investigated the influence of pharmacological inhibition of catalase (3-aminotriazole) or ALDH (cyanamide) on the cardiovascular effects of intra-RVLM ethanol or its metabolic product acetaldehyde in conscious rats. Compared with vehicle, ethanol (10 μg/rat) elicited a significant increase in blood pressure in SHRs that lasted for the 60-min observation period but had no effect on blood pressure in WKY rats. The first oxidation product, acetaldehyde, played a critical role in ethanol-evoked hypertension because 1) catalase inhibition (3-aminotriazole treatment) virtually abolished the ethanol-evoked pressor response in SHRs, 2) intra-RVLM acetaldehyde (2 μg/rat) reproduced the strain-dependent hypertensive effect of intra-RVLM ethanol, and 3) ALDH inhibition (cyanamide treatment) uncovered a pressor response to intra-RVLM acetaldehyde in WKY rats similar to the response observed in SHRs. These findings support the hypothesis that local production of acetaldehyde, due to enhanced catalase activity, in the RVLM mediates the ethanol-evoked pressor response in SHRs.

Differential modulation of brainstem phosphatidylinositol 3-kinase/Akt and extracellular signal-regulated kinase 1/2 signaling underlies WIN55,212-2 centrally mediated pressor response in conscious rats

Our recent study demonstrated that central cannabinoid receptor 1 (CB₁R) activation caused dose-related pressor response in conscious rats, and reported studies implicated the brainstem phosphatidylinositol 3-kinase (PI3K)/Akt-extracellular signal-regulated kinase 1/2 (ERK1/2) pathway in blood pressure control. Therefore, in this study, we tested the hypothesis that the modulation of brainstem PI3K/Akt-ERK1/2 signaling plays a critical role in the central CB(1)R-mediated pressor response. In conscious freely moving rats, the pressor response elicited by intracisternal (i.c.) (R)-(+)-[2,3-dihydro-5-methyl-3[(4-morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl) methanone mesylate salt (WIN55,212-2) (15 μg) was associated with significant increases in ERK1/2 phosphorylation in the rostral ventrolateral medulla (RVLM) and the nucleus tractus solitarius (NTS). In contrast, Akt phosphorylation was significantly reduced in the same neuronal pools. Pretreatment with the selective CB₁R antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) (30 μg i.c.) attenuated the neurochemical responses elicited by central CB₁R activation. Furthermore, pretreatment with the ERK/mitogen-activated protein kinase kinase inhibitor 2'-amino-3'-methoxyflavone (PD98059) (5 μg i.c.) abrogated WIN55,212-2-evoked increases in blood pressure and neuronal ERK1/2 phosphorylation but not the reduction in Akt phosphorylation. On the other hand, prior PI3K inhibition with wortmannin (0.4 μg i.c.) exacerbated the WIN55,212-2 (7.5 and 15 μg i.c.) dose-related increases in blood pressure and ERK1/2 phosphorylation in the RVLM. The present neurochemical and integrative studies yield new insight into the critical role of two brainstem kinases, PI3K and ERK1/2, in the pressor response elicited by central CB₁R activation in conscious rats.