Endogenous angiotensin II in the paraventricular nucleus regulates arterial pressure during hypotension in rat, a single-unit study

In the parvocellular part of paraventricular nucleus, glutamatergic and GABAergic neurons mediate cardiovascular responses to AngII

Angiotensinergic, GABAergic, and glutamatergic neurons are present in the parvocellular region of the paraventricular nucleus (PVNp). It has been shown that microinjection of AngII into the PVNp increases arterial pressure (AP) and heart rate (HR). The presence of synapses between the angiotensinergic, GABAergic, and glutamatergic neurons has been shown in the PVNp. In this study, we investigated the possible interaction between these three systems of the PVNp for control of AP and HR. All drugs were bilaterally (100 nl/side) microinjected into the PVNp of urethane-anesthetized rats, and AP and HR were recorded continuously. Microinjection of AngII into the PVNp produced pressor and tachycardia responses. Pretreatment of PVNp with AP5 or CNQX, glutamatergic NMDA and AMPA receptors antagonists, attenuated the responses to AngII. Pretreatment of PVNp with bicuculline greatly attenuated the pressor and tachycardia responses to AngII. In conclusion, this study provides the first evidence that pressor and tachycardia responses to microinjection of AngII into the PVNp are partly mediated by both NMDA and non-NMDA receptors of glutamate. Activation of glutamatergic neurons by AngII stimulates the sympathoexcitatory neurons. We also showed that the responses to AngII were strongly mediated by GABA_A receptors, probably through activation of GABAergic neurons, which in turn inhibit sympathoinhibitory neurons.

Hypothalamic paraventricular nucleus augments baroreflex sensitivity, role of angiotensin II

The hypothalamic paraventricular nucleus (PVN) is an important brain region involved in control of the cardiovascular system. Direct injection of angiotensin II (AngII) into the PVN produces a short or long pressor response. This study was performed in anesthetized rats to find whether the parvocellular part of the paraventricular nucleus (PVNp) affects the baroreflex. And if so, what is the effect of AngII injected into the PVNp on the baroreflex? Drugs were microinjected into the PVNp while blood pressure and heart rate were recorded continuously. We found that microinjection of AT1 and AT2 receptor antagonists into the PVNp region did not affect the baseline mean arterial pressure (MAP) and heart rate (HR) indicating that under normal conditions AngII may not provide tonic activity, at least in anaesthetized animals. Bilateral microinjections of a synaptic blocker (CoCl₂) into the PVNp attenuated the baroreflex gains in responses to loading and unloading of baroreceptors, indicating that PVNp is involved in the baroreflex rate component. Microinjection of AngII into the PVNp increased MAP and HR. However, AngII slightly attenuated the baroreflex rate component using its two receptors AT1 and AT2. Collectively, these findings suggest that the PVNp as a whole is involved in the baroreflex. But AngII attenuates the heart rate response of the baroreflex through AT1 and AT2 receptors.

Central Nucleus of Amygdala Mediate Pressor Response Elicited by Microinjection of Angiotensin II into the Parvocellular Paraventricular Nucleus in Rats

BACKGROUND

The Paraventricular Hypothalamic Nucleus (PVN) coordinates autonomic and neuroendocrine systems to maintain homeostasis. Microinjection of angiotensin II (AngII) into the PVN has been previously shown to produce pressor and bradycardia responses. Anatomical evidence has indicated that a substantial proportion of PVN neurons is connected with the neurons in the central amygdala (CeA). The present study aimed to examine the possible contribution of the CeA in cardiovascular responses evoked by microinjection of AngII into the parvocellular portion of PVN (PVNp) before and after microinjection of cobalt chloride (CoCl₂) into the CeA.

METHODS

The experiments were conducted at the Department of Physiology of Shiraz University of Medical Sciences, from April 2019 to November 2019. There were two groups of 21 eight-week-old urethane anesthetized male rats, namely saline (n=9 rats) and AngII (n=12 rats) groups. Drugs (100 nL) were microinjected via a single-glass micropipette into the PVNp and CeA. Their blood pressure (BP) and heart rate (HR) were recorded throughout the experiments. The mean arterial pressure (MAP) and heart rate (HR) were compared to the pre-injection values using paired t test, and to those of the saline group using independent t test.

RESULTS

Microinjection of AngII into the PVNp produced pressor response (P<0.0001) with no significant changes in HR (P=0.70). Blockade of CeA with CoCl₂ attenuated the pressor response to microinjection of AngII into the PVNp (P<0.001).

CONCLUSION

In the PVNp, Ang II increased the rats' blood pressure. This response was in part mediated by the CeA. Our study suggested that these two nuclei cooperate to perform their cardiovascular functions.

Controlled Hemorrhage Sensitizes Angiotensin II-Elicited Hypertension through Activation of the Brain Renin-Angiotensin System Independently of Endoplasmic Reticulum Stress

Hemorrhagic shock is associated with activation of renin-angiotensin system (RAS) and endoplasmic reticulum stress (ERS). Previous studies demonstrated that central RAS activation produced by various challenges sensitizes angiotensin (Ang) II-elicited hypertension and that ERS contributes to the development of neurogenic hypertension. The present study investigated whether controlled hemorrhage could sensitize Ang II-elicited hypertension and whether the brain RAS and ERS mediate this sensitization. Results showed that hemorrhaged (HEM) rats had a significantly enhanced hypertensive response to a slow-pressor infusion of Ang II when compared to sham HEM rats. Treatment with either angiotensin-converting enzyme (ACE) 1 inhibitor, captopril, or ACE2 activator, diminazene, abolished the HEM-induced sensitization of hypertension. Treatment with the ERS agonist, tunicamycin, in sham HEM rats also sensitized Ang II-elicited hypertension. However, blockade of ERS with 4-phenylbutyric acid in HEM rats did not alter HEM-elicited sensitization of hypertension. Either HEM or ERS activation produced a greater reduction in BP after ganglionic blockade, upregulated mRNA and protein expression of ACE1 in the hypothalamic paraventricular nucleus (PVN), and elevated plasma levels of Ang II but reduced mRNA expression of the Ang-(1-7) receptor, Mas-R, and did not alter plasma levels of Ang-(1-7). Treatment with captopril or diminazene, but not phenylbutyric acid, reversed these changes. No treatments had effects on PVN protein expression of the ERS marker glucose-regulated protein 78. The results indicate that controlled hemorrhage sensitizes Ang II-elicited hypertension by augmenting RAS prohypertensive actions and reducing RAS antihypertensive effects in the brain, which is independent of ERS mechanism.

Another controller system for arterial pressure. AngII-vasopressin neural network of the parvocellular paraventricular nucleus may regulate arterial pressure during hypotension

Angiotensin II (AngII) immunoreactive cells, fibers and receptors, were found in the parvocelluar region of paraventricular nucleus (PVNp) and AngII receptors are present on vasopressinergic neurons. However, the mechanism by which vasopressin (AVP) and AngII may interact to regulate arterial pressure is not known. Thus, we tested the cardiovascular effects of blockade of the AngII receptors on AVP neurons and blockade of vasopressin V1a receptors on AngII neurons. We also explored whether the PVNp vasopressin plays a regulatory role during hypotension in anesthetized rat or not. Hypovolemic-hypotension was induced by gradual bleeding from femoral venous catheter. Either AngII or AVP injected into the PVNp produced pressor and tachycardia responses. The responses to AngII were blocked by V1a receptor antagonist. The responses to AVP were partially attenuated by AT1 antagonist and greatly attenuated by AT2 antagonist. Hemorrhage augmented the pressor response to AVP, indicating that during hemorrhage, sensitivity of PVNp to vasopressin was increased. By hemorrhagic-hypotension and bilateral blockade of V1a receptors of the PVNp, we found that vasopressinergic neurons of the PVNp regulate arterial pressure towards normal during hypotension. Taken together these findings and our previous findings about angII (Khanmoradi and Nasimi, 2017a) for the first time, we found that a mutual cooperative system of angiotensinergic and vasopressinergic neurons in the PVNp is a major regulatory controller of the cardiovascular system during hypotension.