Review: AT1-receptors in the central nervous system

Role of the angiotensin type 1 receptor in modulating the carotid chemoreflex in an ovine model of renovascular hypertension

OBJECTIVE

The carotid body has been implicated as an important mediator and putative target for hypertension. Previous studies have indicated an important role for angiotensin II in mediating carotid body function via angiotensin type-1 receptors (AT1R); however, their role in modulating carotid body function during hypertension is unclear.

METHODS

Using a large preclinical ovine model of renovascular hypertension, we hypothesized that acute AT1R blockade would lower blood pressure and decrease carotid body-mediated increases in arterial pressure. Adult ewes underwent either unilateral renal artery clipping or sham surgery. Two weeks later, flow probes were placed around the contralateral renal and common carotid arteries.

RESULTS

In both hypertensive and sham animals, carotid body stimulation using potassium cyanide caused dose-dependent increases in mean arterial pressure but a reduction in renal vascular conductance. These responses were not different between groups. Infusion of angiotensin II led to an increase in arterial pressure and reduction in renal blood flow. The sensitivity of the renal vasculature to angiotensin II was significantly attenuated in hypertension compared with the sham animals. Systemic inhibition of the AT1R did not alter blood pressure in either group. Interestingly carotid body-evoked arterial pressure responses were attenuated by AT1R blockade in renovascular hypertension but not in shams.

CONCLUSION

Taken together, our findings indicate a decrease in vascular reactivity of the non-clipped kidney to angiotensin II in hypertension. The CB-evoked increase in blood pressure in hypertension is mediated in part, by the AT1R. These findings indicate a differential role of the AT1R in the carotid body versus the renal vasculature.

Anatomical Organization of the Rat Subfornical Organ

The subfornical organ (SFO) is a sensory circumventricular organ located along the anterodorsal wall of the third ventricle. SFO lacks a complete blood-brain barrier (BBB), and thus peripherally-circulating factors can penetrate the SFO parenchyma. These signals are detected by local neurons providing the brain with information from the periphery to mediate central responses to humoral signals and physiological stressors. Circumventricular organs are characterized by the presence of unique populations of non-neuronal cells, such as tanycytes and fenestrated endothelium. However, how these populations are organized within the SFO is not well understood. In this study, we used histological techniques to analyze the anatomical organization of the rat SFO and examined the distribution of neurons, fenestrated and non-fenestrated vasculature, tanycytes, ependymocytes, glia cells, and pericytes within its confines. Our data show that the shell of SFO contains non-fenestrated vasculature, while fenestrated capillaries are restricted to the medial-posterior core region of the SFO and associated with a higher BBB permeability. In contrast to non-fenestrated vessels, fenestrated capillaries are encased in a scaffold created by pericytes and embedded in a network of tanycytic processes. Analysis of c-Fos expression following systemic injections of angiotensin II or hypertonic NaCl reveals distinct neuronal populations responding to these stimuli. Hypertonic NaCl activates ∼13% of SFO neurons located in the shell. Angiotensin II-sensitive neurons represent ∼35% of SFO neurons and their location varies between sexes. Our study provides a comprehensive description of the organization of diverse cellular elements within the SFO, facilitating future investigations in this important brain area.

New insights into the importance of aminopeptidase A in hypertension

The renin-angiotensin system (RAS) plays an important role in the maintenance of normal blood pressure and the etiology of hypertension; however, minimal attention has been paid to the degradation of the effector peptide, angiotensin II (AngII). Since aminopeptidase A (APA)-deficient mice develop hypertension APA appears to be an essential enzyme in the control of blood pressure via degradation of AngII. The robust hypertension seen in the spontaneously hypertensive rat (SHR) is due to activation of the RAS, and an accompanying decrease in kidney APA. Changes in APA have also been measured during the activation of the RAS in the Goldblatt hypertension model and Dahl salt-sensitive (DSS) rat. The DSS rat shows an elevation in renal APA activity at the onset of hypertension suggesting a protective role against elevations in circulating AngII, followed by decreased APA activity with advancing hypertension. Changes seen in human maternal serum APA activity during preeclampsia are similar to changes measured in renal APA in the DSS rat model. APA activity is higher than during normal pregnancy at the onset of preeclampsia, and with advancing preeclampsia (severe preeclampsia) declines below that seen during normal pregnancy. Serum APA activity is also increased during hormone replacement therapy (HRT), perhaps in reaction to elevated levels of AngII. Thus, it appears important to consider the relationship among activation of the RAS, circulating levels of AngII, and the availability of APA in hypertensive disorders.

Angiotensin receptor subtype mediated physiologies and behaviors: new discoveries and clinical targets

The renin-angiotensin system (RAS) mediates several classic physiologies including body water and electrolyte homeostasis, blood pressure, cyclicity of reproductive hormones and sexual behaviors, and the regulation of pituitary gland hormones. These functions appear to be mediated by the angiotensin II (AngII)/AT(1) receptor subtype system. More recently, the angiotensin IV (AngIV)/AT(4) receptor subtype system has been implicated in cognitive processing, cerebroprotection, local blood flow, stress, anxiety and depression. There is accumulating evidence to suggest an inhibitory influence by AngII acting at the AT(1) subtype, and a facilitory role by AngIV acting at the AT(4) subtype, on neuronal firing rate, long-term potentiation, associative and spatial learning, and memory. This review initially describes the biochemical pathways that permit synthesis and degradation of active angiotensin peptides and three receptor subtypes (AT(1), AT(2) and AT(4)) thus far characterized. There is vigorous debate concerning the identity of the most recently discovered receptor subtype, AT(4). Descriptions of classic and novel physiologies and behaviors controlled by the RAS are presented. This review concludes with a consideration of the emerging therapeutic applications suggested by these newly discovered functions of the RAS.

Influence of brain angiotensin on thermoregulation and hydromineral balance during pregnancy in rats

During mammalian pregnancy, body temperature decreases and there are changes in fluid and electrolyte balance. Angiotensin signaling mechanisms in the brain have been shown to influence thermoregulation and body fluid balance in the nonpregnant state. We hypothesized that brain angiotensin is also implicated in adjusting these physiological systems in the pregnant rat. We compared core temperature and fluid regulation in three groups of pregnant rats: untreated rats, rats receiving continuous infusion of an AT1 antagonist candesartan (5 μg·kg−1·day−1) into a lateral cerebral ventricle to block brain AT1 receptors, and rats receiving vehicle [artificial cerebrospinal fluid (aCSF)] vehicle. Untreated and aCSF-treated rats showed a decrease in colonic temperature (−0.5 and −0.8°C respectively) by day 20 of gestation. However, rats treated with candesartan had increased colonic temperature compared with baseline (+0.9°C), and their temperature was significantly higher on days 7 ( P < 0.05), 17 ( P < 0.05), and 20 ( P < 0.001) compared with the other groups (aCSF and untreated). Daily food and water intakes and body weight were not different between the three groups. Similarly, litter sizes and pup weights were equal in all groups. Finally, the expected decreases in plasma Na+ and osmolality during pregnancy were equivalent in all groups. This study suggests that brain angiotensin mediates the progressive decrease in body temperature that occurs during pregnancy. However, the changes in fluid balance associated with pregnancy are not dependent on brain angiotensin.