Brain AT(2) receptor mediate vasodepressor response to footshocks: role of kinins and nitric oxide

The intricacies of the renin-angiotensin-system in metabolic regulation

Over recent years, the renin-angiotensin-system (RAS), which is best-known as an endocrine system with established roles in hydromineral balance and blood pressure control, has emerged as a fundamental regulator of many additional physiological and pathophysiological processes. In this manuscript, we celebrate and honor Randall Sakai's commitment to his trainees, as well as his contribution to science. Scientifically, Randall made many notable contributions to the recognition of the RAS's roles in brain and behavior. His interests, in this regard, ranged from its traditionally-accepted roles in hydromineral balance, to its less-appreciated functions in stress responses and energy metabolism. Here we review the current understanding of the role of the RAS in the regulation of metabolism. In particular, the opposing actions of the RAS within adipose tissue vs. its actions within the brain are discussed.

Reporter mouse strain provides a novel look at angiotensin type-2 receptor distribution in the central nervous system

Angiotensin-II acts at its type-1 receptor (AT1R) in the brain to regulate body fluid homeostasis, sympathetic outflow and blood pressure. However, the role of the angiotensin type-2 receptor (AT2R) in the neural control of these processes has received far less attention, largely because of limited ability to effectively localize these receptors at a cellular level in the brain. The present studies combine the use of a bacterial artificial chromosome transgenic AT2R-enhanced green fluorescent protein (eGFP) reporter mouse with recent advances in in situ hybridization (ISH) to circumvent this obstacle. Dual immunohistochemistry (IHC)/ISH studies conducted in AT2R-eGFP reporter mice found that eGFP and AT2R mRNA were highly co-localized within the brain. Qualitative analysis of eGFP immunoreactivity in the brain then revealed localization to neurons within nuclei that regulate blood pressure, metabolism, and fluid balance (e.g., NTS and median preoptic nucleus [MnPO]), as well as limbic and cortical areas known to impact stress responding and mood. Subsequently, dual IHC/ISH studies uncovered the phenotype of specific populations of AT2R-eGFP cells. For example, within the NTS, AT2R-eGFP neurons primarily express glutamic acid decarboxylase-1 (80.3 ± 2.8 %), while a smaller subset express vesicular glutamate transporter-2 (18.2 ± 2.9 %) or AT1R (8.7 ± 1.0 %). No co-localization was observed with tyrosine hydroxylase in the NTS. Although AT2R-eGFP neurons were not observed within the paraventricular nucleus (PVN) of the hypothalamus, eGFP immunoreactivity is localized to efferents terminating in the PVN and within GABAergic neurons surrounding this nucleus. These studies demonstrate that central AT2R are positioned to regulate blood pressure, metabolism, and stress responses.

Nitric oxide impacts on angiotensin AT2 receptor modulation of high-pressure baroreflex control of renal sympathetic nerve activity in anaesthetized rats

Aim

Nitric oxide (NO) interacts with the local brain renin-angiotensin system to modulate sympathetic outflow and cardiovascular homoeostasis. This study investigated whether NO influenced the ability of angiotensin AT2 receptor activation to modify the high-pressure baroreceptor regulation of renal sympathetic nerve activity (RSNA) and heart rate (HR).

Methods

Anaesthetized (chloralose/urethane) rats were prepared to allow generation of baroreflex gain curves for RSNA or HR following intracerebroventricular (I.C.V.) CGP42112 (AT2 receptor agonist), PD123319 (AT2 receptor antagonist) or losartan (AT1 receptor antagonist), and then in combination with L-NAME (NO synthase inhibitor).

Results

I.C.V. PD123319, CGP42112, and Losartan did not change baseline mean arterial pressure, HR or RSNA. Baroreflex sensitivities for RSNA and HR were increased following AT2 receptor activation with CGP42112 by 112 and 157%, respectively, but were reduced following PD123319 by 20% (all P < 0.05). L-NAME alone increased baroreflex sensitivity for both RSNA and HR, by 62 and 158%, respectively, but when co-infused with either CGP42112 or PD123319, the baroreflex sensitivity fell to values comparable to those obtained during I.C.V. saline infusion. The baroreflex sensitivities for RSNA and HR were increased by losartan by 92% and 192%, respectively, but in the presence of L-NAME were no different from those obtained during I.C.V. saline infusion.

Conclusion

There is an important facilitatory role for AT2 receptors in the high-pressure baroreflex regulation of RSNA and HR which is dependent on a functional NO/NOS system. Conversely, AT1 receptors have an inhibitory effect on the baroreflex, an action that relies on a tonic inhibition of NO.

Angiotensin receptor blockade in the anterior hypothalamic area inhibits stress-induced pressor responses in rats

Central angiotensin systems are involved in expression of pressor responses induced by immobilization stress. In this study, we examined whether angiotensin receptors in the anterior hypothalamic area are involved in the pressor response during stress exposure in rats. Intracerebroventricular injections of the angiotensin AT1-receptor antagonist losartan (6.5 and 22 nmol) attenuated pressor responses to immobilization stress dose-dependently. Injections of losartan (0.065 and 0.22 nmol) into the anterior hypothalamic area also suppressed the stress-induced pressor response dose-dependently, whereas intraventricular injection of losartan (2.2 nmol) did not affect it. Immobilization stress caused increases in plasma catecholamine levels. The stress-induced increase of plasma catecholamine levels was also inhibited by angiotensin receptor blockade in the anterior hypothalamic area. The present results suggest that angiotensin receptors in the anterior hypothalamic area are involved in expression of the pressor response and sympathetic activation induced by immobilization stress.

Roles of central interleukin-1 on stress-induced-hypertension and footshock-induced-analgesia in rats

Central interleukin-1 (IL-1) plays an important role in mediating the neural, endocrine, and behavioral responses to stressors. Here we tested whether central IL-1 is involved in stress-induced hypertension or footshock (FS)-induced-analgesia. We observed that: (1) intracerebral ventricular injection of (ICV) IL-1beta induced pressor responses; (2) hypertension induced by IL-1beta was blocked by ICV an IL-1 antagonist, IL-1ra; (3) ICV IL-1ra attenuated the pressor response induced by FS but intravenous injection of IL-1ra did not significantly reduce this response; (4) the hypertensive response to conditioned fear stimuli was reversed by ICV IL-1ra; (5) FS-induced-analgesia was attenuated by ICV IL-1ra and this effect disappeared 15 min after ICV IL-1ra. These results suggest that both the pressor response to FS or conditioned fear stimuli and short lasting analgesia induced by FS are mediated by central IL-1.