Chemogenetic inhibition of Phox2-expressing neurons in the commissural NTS decreases blood pressure in anesthetized spontaneously hypertensive rats

Interruption of the activity of neurons in the commissural portion of the nucleus of the solitary tract (cNTS) decreases blood pressure (BP) in experimental models of hypertension, such as the spontaneously hypertensive (SH) rat. To examine whether PHOX2B expressing cNTS neurons are involved in maintaining the elevated BP, we used replication-deficient viruses with a modified Phox2 binding site promoter to express the inhibitory chemogenetic allatostatin receptor or green fluorescent protein in the cNTS. Following administration of allatostatin, we observed a depressor and bradycardic response in anesthetized SH rats that expressed the allatostatin receptor. Injection of allatostatin did not affect BP or heart rate (HR) in control SH rats expressing green fluorescent protein in the cNTS. Immunohistochemistry showed that the majority of transduced cNTS neurons were PHOX2B-immunoreactive and some also expressed tyrosine hydroxylase. We conclude that in anesthetized SH rat, the Phox2B expressing cNTS neurons maintain elevated BP.

Deletion of Orphan G Protein-Coupled Receptor GPR37L1 in Mice Alters Cardiovascular Homeostasis in a Sex-Specific Manner

GPR37L1 is a family A orphan G protein-coupled receptor (GPCR) with a putative role in blood pressure regulation and cardioprotection. In mice, genetic ablation of Gpr37l1 causes sex-dependent effects; female mice lacking Gpr37l1 (GPR37L1^-/-) have a modest but significant elevation in blood pressure, while male GPR37L1^-/- mice are more susceptible to cardiovascular dysfunction following angiotensin II-induced hypertension. Given that this receptor is highly expressed in the brain, we hypothesize that the cardiovascular phenotype of GPR37L1^-/- mice is due to changes in autonomic regulation of blood pressure and heart rate. To investigate this, radiotelemetry was employed to characterize baseline cardiovascular variables in GPR37L1^-/- mice of both sexes compared to wildtype controls, followed by power spectral analysis to quantify short-term fluctuations in blood pressure and heart rate attributable to alterations in autonomic homeostatic mechanisms. Additionally, pharmacological ganglionic blockade was performed to determine vasomotor tone, and environmental stress tests were used to assess whether cardiovascular reactivity was altered in GPR37L1^-/- mice. We observed that mean arterial pressure was significantly lower in female GPR37L1^-/- mice compared to wildtype counterparts, but was unchanged in male GPR37L1^-/- mice. GPR37L1^-/- genotype had a statistically significant positive chronotropic effect on heart rate across both sexes when analyzed by two-way ANOVA. Power spectral analysis of these data revealed a reduction in power in the heart rate spectrum between 0.5 and 3 Hz in female GPR37L1^-/- mice during the diurnal active period, which indicates that GPR37L1^-/- mice may have impaired cardiac vagal drive. GPR37L1^-/- mice of both sexes also exhibited attenuated depressor responses to ganglionic blockade with pentolinium, indicating that GPR37L1 is involved in maintaining sympathetic vasomotor tone. Interestingly, when these mice were subjected to aversive and appetitive behavioral stressors, the female GPR37L1^-/- mice exhibited an attenuation of cardiovascular reactivity to aversive, but not appetitive, environmental stimuli. Together, these results suggest that loss of GPR37L1 affects autonomic maintenance of blood pressure, giving rise to sex-specific cardiovascular changes in GPR37L1^-/- mice.

Brain Angiotensin Type-1 and Type-2 Receptors in Physiological and Hypertensive Conditions: Focus on Neuroinflammation

PURPOSE OF REVIEW

To review recent data that suggest opposing effects of brain angiotensin type-1 (AT1R) and type-2 (AT2R) receptors on blood pressure (BP). Here, we discuss recent studies that suggest pro-hypertensive and pro-inflammatory actions of AT1R and anti-hypertensive and anti-inflammatory actions of AT2R. Further, we propose mechanisms for the interplay between brain angiotensin receptors and neuroinflammation in hypertension.

RECENT FINDINGS

The renin-angiotensin system (RAS) plays an important role in regulating cardiovascular physiology. This includes brain AT1R and AT2R, both of which are expressed in or adjacent to brain regions that control BP. Activation of AT1R within those brain regions mediate increases in BP and cause neuroinflammation, which augments the BP increase in hypertension. The fact that AT1R and AT2R have opposing actions on BP suggests that AT1R and AT2R may have similar opposing actions on neuroinflammation. However, the mechanisms by which brain AT1R and AT2R mediate neuroinflammatory responses remain unclear. The interplay between brain angiotensin receptor subtypes and neuroinflammation exacerbates or protects against hypertension.

Brain angiotensin type-1 and type-2 receptors: cellular locations under normal and hypertensive conditions

Brain angiotensin-II (Ang-II) type-1 receptors (AT1Rs), which exert profound effects on normal cardiovascular, fluid, and metabolic homeostasis, are overactivated in and contribute to chronic sympathoexcitation and hypertension. Accumulating evidence indicates that the activation of Ang-II type-2 receptors (AT2Rs) in the brain exerts effects that are opposite to those of AT1Rs, lowering blood pressure, and reducing hypertension. Thus, it would be interesting to understand the relative cellular localization of AT1R and AT2R in the brain under normal conditions and whether this localization changes during hypertension. Here, we developed a novel AT1aR-tdTomato reporter mouse strain in which the location of brain AT1aR was largely consistent with that determined in the previous studies. This AT1aR-tdTomato reporter mouse strain was crossed with our previously described AT2R-eGFP reporter mouse strain to yield a novel dual AT1aR/AT2R reporter mouse strain, which allowed us to determine that AT1aR and AT2R are primarily localized to different populations of neurons in brain regions controlling cardiovascular, fluid, and metabolic homeostasis. Using the individual AT1aR-tdTomato reporter mice, we also demonstrated that during hypertension induced by the administration of deoxycorticosterone acetate-salt, there was no shift in the expression of AT1aR from neurons to microglia or astrocytes in the paraventricular nucleus, a brain area important for sympathetic regulation. Using AT2R-eGFP reporter mice under similar hypertensive conditions, we demonstrated that the same was true of AT2R expression in the nucleus of the solitary tract (NTS), an area critical for baroreflex control. Collectively, these findings provided a novel means to assess the colocalization of AT1R and AT2R in the brain and a novel view of their cellular localization in hypertension.

Acute Pressor Response to Psychosocial Stress Is Dependent on Endothelium-Derived Endothelin-1

Background

Acute psychosocial stress provokes increases in circulating endothelin‐1 (ET‐1) levels in humans and animal models. However, key questions about the physiological function and cellular source of stress‐induced ET‐1 remain unanswered. We hypothesized that endothelium‐derived ET‐1 contributes to the acute pressor response to stress via activation of the endothelin A receptor.

Methods and Results

Adult male vascular endothelium‐specific ET‐1 knockout mice and control mice that were homozygous for the floxed allele were exposed to acute psychosocial stress in the form of cage switch stress (CSS), with blood pressure measured by telemetry. An acute pressor response was elicited by CSS in both genotypes; however, this response was significantly blunted in vascular endothelium‐specific ET‐1 knockout mice compared with control mice that were homozygous for the floxed allele. In mice pretreated for 3 days with the endothelin A antagonist, ABT‐627, or the dual endothelin A/B receptor antagonist, A‐182086, the pressor response to CSS was similar between genotypes. CSS significantly increased plasma ET‐1 levels in control mice that were homozygous for the floxed allele. CSS failed to elicit an increase in plasma ET‐1 in vascular endothelium‐specific ET‐1 knockout mice. Telemetry frequency domain analyses suggested similar autonomic responses to stress between genotypes, and isolated resistance arteries demonstrated similar sensitivity to α₁‐adrenergic receptor‐mediated vasoconstriction.

Conclusions

These findings specify that acute stress‐induced activation of endothelium‐derived ET‐1 and subsequent endothelin A receptor activation is a novel mediator of the blood pressure response to acute psychosocial stress.

Intracellular NAMPT-NAD+-SIRT1 cascade improves post-ischaemic vascular repair by modulating Notch signalling in endothelial progenitors

AIMS

Intracellular nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme for nicotinamide adenine dinucleotide (NAD(+)) biosynthesis. This study investigated the role of NAMPT-mediated NAD(+) signalling in post-ischaemic vascular repair.

METHODS AND RESULTS

Mouse hind-limb ischaemia up-regulated NAMPT expression and NAD(+) level in bone marrow (BM). Pharmacological inhibition of NAMPT by a chemical inhibitor FK866 impaired the mobilization of endothelial progenitor cells (EPCs) from BM upon ischaemic stress. Transgenic mice overexpressing NAMPT (Tg mice), but not H247A-mutant dominant-negative NAMPT (DN-Tg mice), exhibited enhanced capillary density, increased number of proliferating endothelial cells, improved blood flow recovery, and augmented collateral arterioles in the ischaemic limb. In cultured BM-derived EPCs, inhibition of NAMPT suppressed proliferation, migration, and tube formation, whereas overexpression of NAMPT induced opposite effects. The promoting effects of NAMPT on EPCs were abolished by silencing of sirtuin 1 (SIRT1), rather than silencing of SIRT2-7. Overexpression of NAMPT led to a SIRT1-depedent enhancement of Notch-1 intracellular domain deacetylation, which inhibited Delta-like ligand-4 (DLL4)-Notch signalling and thereby up-regulated of VEGFR-2 and VEGFR-3. Injection of recombinant VEGF induced a more pronounced EPC mobilization in Tg, but not in DN-Tg, mice. Furthermore, overexpression of NAMPT down-regulated Fringe family glycosyltransferases in a SIRT1-dependent manner, which rendered Notch more sensitive to the pro-angiogenic ligand Jagged1 rather than the anti-angiogenic ligand DLL4.

CONCLUSIONS

These results demonstrate that intracellular NAMPT-NAD(+)-SIRT1 cascade improves post-ischaemic neovascularization. The modulation of Notch signalling may contribute to the enhanced post-ischaemic neovascularization.

Dual implantation of a radio-telemeter and vascular access port allows repeated hemodynamic and pharmacological measures in conscious lean and obese rats

Expansion of physiological knowledge increasingly requires examination of processes in the normal, conscious state. The current study describes a novel approach combining surgical implantation of radio-telemeters with vascular access ports (VAPs) to allow repeated hemodynamic and pharmacological measures in conscious rats. Dual implantation was conducted on 16-week-old male lean and obese Zucker rats. Continued viability one month after surgery was observed in 67% of lean and 44% of obese animals, giving an overall 54% completion rate. Over the five-week measurement period, reliable and reproducible basal mean arterial pressure and heart rate measures were observed. VAP patency and receptor-independent vascular reactivity were confirmed by consistent hemodynamic responses to sodium nitroprusside (6.25 µg/kg). Acutely, minimal hemodynamic responses to repeated bolus administration of 0.2 mL saline indicated no significant effect of increased blood volume or administration stress, making repeated acute measures viable. Similarly, repeated administration of the β-adrenoceptor agonist dobutamine (30 µg/kg) at 10 min intervals resulted in reproducible hemodynamic changes in both lean and obese animals. Therefore, our study demonstrates that this new approach is viable for the acute and chronic assessment of hemodynamic and pharmacological responses in both lean and obese conscious rats. This technique reduces the demand for animal numbers and allows hemodynamic measures with minimal disruption to animals' welfare, while providing reliable and reproducible results over several weeks. In conclusion, dual implantation of a radio-telemeter and VAP introduces a valuable technique for undertaking comprehensive studies involving repeated pharmacological tests in conscious animals to address important physiological questions.