Various effects of angiotensin II on amygdaloid neuronal activity in normotensive control and hypertensive transgenic [TGR(mREN-2)27] rats

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.

Brain Angiotensinergic Regulation of the Immune System: Implications for Cardiovascular and Neuroendocrine Responses

OBJECTIVE

The Renin-Angiotensin-Aldosterone System (RAAS) plays a major role in the regulation of cardiovascular functions, water and electrolytic balance, and hormonal responses. We perform a review of the literature, aiming at providing the current concepts regarding the angiotensin interaction with the immune system in the brain and the related implications for cardiovascular and neuroendocrine responses.

METHODS

Appropriate keywords and MeSH terms were identified and searched in Pubmed. Finally, references of original articles and reviews were examined.

RESULTS

Angiotensin II (ANG II), beside stimulating aldosterone, vasopressin and CRH-ACTH release, sodium and water retention, thirst, and sympathetic nerve activity, exerts its effects on the immune system via the Angiotensin Type 1 Receptor (AT 1R) that is located in the brain, pituitary, adrenal gland, and kidney. Several actions are triggered by the binding of circulating ANG II to AT 1R into the circumventricular organs that lack the Blood-Brain-Barrier (BBB). Furthermore, the BBB becomes permeable during chronic hypertension thereby ANG II may also access brain nuclei controlling cardiovascular functions. Subfornical organ, organum vasculosum lamina terminalis, area postrema, paraventricular nucleus, septal nuclei, amygdala, nucleus of the solitary tract and retroventral lateral medulla oblongata are the brain structures that mediate the actions of ANG II since they are provided with a high concentration of AT 1R. ANG II induces also T-lymphocyte activation and vascular infiltration of leukocytes and, moreover, oxidative stress stimulating inflammatory responses via inhibition of endothelial progenitor cells and stimulation of inflammatory and microglial cells facilitating the development of hypertension.

CONCLUSION

Besides the well-known mechanisms by which RAAS activation can lead to the development of hypertension, the interactions between ANG II and the immune system at the brain level can play a significant role.

The Potential Therapeutic Capacity of Inhibiting the Brain Renin-Angiotensin System in the Treatment of Co-Morbid Conditions in Epilepsy

Epilepsy is one of the most prevalent neurological diseases and although numerous novel anticonvulsants have been approved, the proportion of patients who are refractory to medical treatment of seizures and have progressive co-morbidities such as cognitive impairment and depression remains at about 20-30%. In the last decade, extensive research has identified a therapeutic capacity of the components of the brain renin-angiotensin system (RAS) in seizure- and epilepsy-related phenomena. Alleviating the activity of RAS in the central nervous system is considered to be a potential adjuvant strategy for the treatment of numerous detrimental consequences of epileptogenesis. One of the main advantages of RAS is associated with its modulatory influence on different neurotransmitter systems, thereby exerting a fine-tuning control mechanism for brain excitability. The most recent scientific findings regarding the involvement of the components of brain RAS show that angiotensin II (Ang II), angiotensin-converting enzyme (ACE), Ang II type 1 (AT₁) and type 2 (AT₂) receptors are involved in the control of epilepsy and its accompanying complications, and therefore they are currently of therapeutic interest in the treatment of this disease. However, data on the role of different components of brain RAS on co-morbid conditions in epilepsy, including hypertension, are insufficient. Experimental and clinical findings related to the involvement of Ang II, ACE, AT₁, and AT₂ receptors in the control of epilepsy and accompanying complications may point to new therapeutic opportunities and adjuvants for the treatment of common co-morbid conditions of epilepsy.

Contribution of amygdala to the pressor response elicited by microinjection of angiotensin II into the bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BST) is part of the limbic system located in the rostral forebrain. BST is involved in behavioral, neuroendocrine and autonomic functions, including cardiovascular regulation. The amygdala, plays an important role in mediating the behavioral and physiological responses associated with fear and anxiety, including cardiovascular responses. In a previous study, we showed that microinjection of AngII into the BST produced a pressor and two types of single-unit responses in the BST, short excitatory and long inhibitory. This study was performed to find possible involvement of amygdala in cardiovascular responses elicited by microinjection of AngII into the BST, using blockade of the central nucleus of amygdala (CeA) and single unit recording from the CeA, while injecting AngII into the BST in anesthetized rat. Blockade of CeA attenuated the pressor response to microinjection of AngII into the BST. Eighty-six AngII microinjections were given into the BST and 198 single unit responses were recorded from CeA simultaneously, from which 89 showed a short duration excitatory response and 109 showed no responses. In conclusion, microinjection of AngII into the BST produces a short excitatory single unit response in the CeA, resulting in contribution of amygdala to the resulted pressor response. Taken together, our study and previous studies suggest a plausible hypothesis that these two nuclei perform their cardiovascular functions in cooperation with each other.

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

The hypothalamic paraventricular nucleus (PVN) controls cardiovascular regulation through vasopressin and sympathetic system. The PVN contains angiotensin II (AngII) and AngII receptors. We have already shown that microinjection of AngII into PVN produced a pressor response concomitant with an increase in firing rate of some PVN neurons. This study was performed to find if PVN AngII plays a regulatory function during hypotension. Hypovolemic-hypotension was induced and the possible role of the PVN AngII in returning arterial pressure toward normal was assessed by monitoring cardiovascular response and single-unit activity of the PVN neurons. Hemorrhage augmented the pressor, tachycardic and single-unit responses to AngII. After-hemorrhage injection of PD123319, an AT2 antagonist, into PVN resulted in a significant decrease in firing rate of some neurons, indicating that AngII was released into the PVN due to hemorrhage. Using single-unit recording, we found that PVN receives electrical signals from baroreceptors and from circulating AngII through circumventricular organs. In addition, by producing hemorrhagic-hypotension and bilateral blockade of AT2 receptors of the PVN, we found that AngII regulates arterial pressure toward normal during hypotension. So for the first time, it was verified that brain renin-angiotensin system is also a major regulatory system of the cardiovascular system.

The effect of angiotensin II microinjection into the bed nucleus of the stria terminalis on serum lipid peroxidation and nitric oxide metabolite levels

BACKGROUND

Overactivity of renin-angiotensin system is involved in the pathophysiology of renal and cardiovascular diseases. It is suggested that endothelial cells can release nitric oxide (NO) and reactive oxygen species in response to angiotensin II (Ang II). Angiotensin type 1 (AT1) receptor of Ang II has been found in the bed nucleus of the stria terminalis (BST). BST is involved in autonomic function. This study was performed to find the role of central Ang II in serum lipid peroxidation product and in releasing NO into circulation.

MATERIALS AND METHODS

Twenty-one catheterized rats were placed in stereotaxic instrument. A hole was drilled above BST. In the control group, saline 0.9% (100 nl) was microinjected into the BST. In the second group, Ang II (100 μM, 100-150 nl) was microinjected into the BST. In the third group losartan (an AT1 antagonist) was microinjected (100 μM, 200 nl) before Ang II into the BST. Systolic blood pressure was recorded. The NO metabolite (nitrite) and malondialdehyde (MDA) were measured in the rat's serum.

RESULTS

The data indicated that microinjection of Ang II into the BST produced a pressor response (P < 0.0001). It also increased MDA and nitrite levels of the serum significantly (P < 0.001, P < 0.0001). Pretreatment with losartan before Ang II microinjection attenuated serum's levels of MDA and nitrite (P < 0.001, P < 0.0001).

CONCLUSION

Our findings suggest that central effect of Ang II on blood pressure is accompanied with increased levels of MDA and nitrite in the circulation.

Cardiovascular and single-unit responses to microinjection of angiotensin II into the bed nucleus of the stria terminalis in rat

The bed nucleus of the stria terminalis (BST) is part of the limbic system located in the rostral forebrain. BST is involved in behavioral, neuroendocrine and autonomic functions, including cardiovascular regulation. The angiotensin II (Ang II) receptor, AT1, was found in the BST, however its effects on the cardiovascular system and on single-unit responses have not been studied yet. In the present study, Ang II was microinjected into the BST of anesthetized rats and cardiovascular and single-unit responses were recorded simultaneously. Furthermore the responses were re-tested after the microinjection of a blocker of the AT1 receptor, losartan, into the BST. We found that microinjection of Ang II into the BST produced a pressor response of 11±1mmHg for a duration of 2-8min. Ang II had no consistent effect on heart rate. It also produced two types of single-unit responses in the BST, short excitatory and long inhibitory. Blockade of AT1 receptors abolished both the cardiovascular and single-unit responses, indicating that the responses were mediated through AT1 receptors. These findings imply that Ang II may be utilized as a neurotransmitter and may play a role in returning blood pressure toward normal during hypotension.

The Deakin/Graeff hypothesis: focus on serotonergic inhibition of panic

The Deakin/Graeff hypothesis proposes that different subpopulations of serotonergic neurons through topographically organized projections to forebrain and brainstem structures modulate the response to acute and chronic stressors, and that dysfunction of these neurons increases vulnerability to affective and anxiety disorders, including panic disorder. We outline evidence supporting the existence of a serotonergic system originally discussed by Deakin/Graeff that is implicated in the inhibition of panic-like behavioral and physiological responses. Evidence supporting this panic inhibition system comes from the following observations: (1) serotonergic neurons located in the 'ventrolateral dorsal raphe nucleus' (DRVL) as well as the ventrolateral periaqueductal gray (VLPAG) inhibit dorsal periaqueductal gray-elicited panic-like responses; (2) chronic, but not acute, antidepressant treatment potentiates serotonin's panicolytic effect; (3) contextual fear activates a central nucleus of the amygdala-DRVL/VLPAG circuit implicated in mediating freezing and inhibiting panic-like escape behaviors; (4) DRVL/VLPAG serotonergic neurons are central chemoreceptors and modulate the behavioral and cardiorespiratory response to panicogenic agents such as sodium lactate and CO2. Implications of the panic inhibition system are discussed.

Fear-potentiated behaviour is modulated by central amygdala angiotensin II AT1 receptors stimulation

Central nucleus of the amygdala (CeA) is one of the most important regulatory centres for the emotional processes. Among the different neurotransmitter systems present in this nucleus, AT1 receptors have been also found, but their role in the generation and modulation of emotions is not fully understood. The present work evaluated the effect of intra-amygdalar injection of losartan (AT1 receptor antagonist) and angiotensin II (Ang II) in the anxiety state induced by fear-potentiated plus maze in male Wistar rats. Fear in the elevated plus maze can be potentiated by prior inescapable footshock stress. The decrease in the time spent in the open arms induced by the inescapable footshock was totally prevented by losartan (4 pmol) administration in CeA. It was also found that Ang II (48 fmol) administration decreased the time spent in the open arms in animals with or without previous footshock exposure. The locomotor activity and grooming behaviour were also evaluated. The results obtained from the different parameters analyzed allowed us to conclude that the Ang II AT1 receptors in CeA are involved in the anxiety state induced by stress in the fear-potentiated plus-maze behaviour.

Angiotensin II's role in sodium lactate-induced panic-like responses in rats with repeated urocortin 1 injections into the basolateral amygdala: amygdalar angiotensin receptors and panic

Rats treated with three daily urocortin 1 (UCN) injections into the basolateral amygdala (BLA; i.e., UCN/BLA-primed rats) develop prolonged anxiety-associated behavior and vulnerability to panic-like physiological responses (i.e., tachycardia, hypertension and tachypnea) following intravenous infusions of 0.5 M sodium lactate (NaLac, an ordinarily mild interoceptive stressor). In these UCN-primed rats, the osmosensitive subfornical organ (SFO) may be a potential site that detects increases in plasma NaLac and mobilizes panic pathways since inhibiting the SFO blocks panic following NaLac in this model. Furthermore, since SFO neurons synthesize angiotensin II (A-II), we hypothesized that the SFO projects to the BLA and releases A-II to mobilizing panic responses in UCN/BLA-primed rats following NaLac infusions. To test this hypothesis, rats received daily bilateral injections of UCN or vehicle into the BLA daily for 3 days. Five to seven days following the intra-BLA injections, we microinjected either the nonspecific A-II type 1 (AT1r) and 2 (AT2r) receptor antagonist saralasin, or the AT2r-selective antagonist PD123319 into the BLA prior to the NaLac challenge. The UCN/BLA-primed rats pre-injected with saralasin, but not PD123319 or vehicle, had reduced NaLac-induced anxiety-associated behavior and panic-associated tachycardia and tachypnea responses. We then confirmed the presence of AT1rs in the BLA using immunohistochemistry which, combined with the previous data, suggest that A-II's panicogenic effects in the BLA is AT1r dependent. Surprisingly, the SFO had almost no neurons that directly innervate the BLA, which suggests an indirect pathway for relaying the NaLac signal. Overall these results are the first to implicate A-II and AT1rs as putative neurotransmitter-receptors in NaLac induced panic-like responses in UCN/BLA-primed rats.

Angiotensin II and CRF receptors in the central nucleus of the amygdala mediate hemodynamic response variability to cocaine in conscious rats

Stress or cocaine evokes either a large increase in systemic vascular resistance (SVR) or a smaller increase in SVR accompanied by an increase in cardiac output (designated vascular and mixed responders, respectively) in Sprague-Dawley rats. We hypothesized that the central nucleus of the amygdala (CeA) mediates this variability. Conscious, freely-moving rats, instrumented for measurement of arterial pressure and cardiac output and for drug delivery into the CeA, were given cocaine (5 mg/kg, iv, 4-6 times) and characterized as vascular (n=15) or mixed responders (n=10). Subsequently, we administered cocaine after bilateral microinjections (100 nl) of saline or selective agents in the CeA. Muscimol (80 pmol), a GABA(A) agonist, or losartan (43.4 pmol), an AT(1) receptor antagonist, attenuated the cocaine-induced increase in SVR in vascular responders, selectively, such that vascular responders were no longer different from mixed responders. The corticotropin releasing factor (CRF) antagonist, alpha-helical CRF(9-41) (15.7 pmol), abolished the difference between cardiac output and SVR in mixed and vascular responders. We conclude that greater increases in SVR observed in vascular responders are dependent on AT(1) receptor activation and, to a lesser extent on CRF receptors. Therefore, AT(1) and CRF receptors in the CeA contribute to hemodynamic response variability to intravenous cocaine.

Angiotensin-(1-7)-induced plasticity changes in the lateral amygdala are mediated by COX-2 and NO

It is known from studies outside the brain that upon binding to its receptor, angiotensin-(1-7) elicits the release of prostanoids and nitric oxide (NO). Cyclooxygenase (COX) is a key enzyme that converts arachidonic acid to prostaglandins. Since there are no data available so far on the role of COX-2 in the amygdala, in a first step we demonstrated that the selective COX-2 inhibitor NS-398 significantly reduced the probability of long-term potentiation (LTP) induction in the lateral nucleus of the amygdala. Similarly, in COX-2(-/-) mice, LTP induced by external capsule (EC) stimulation was impaired. Second, we evaluated the action of angiotensin-(1-7) in the amygdala. In wild-type mice, angiotensin-(1-7) increased LTP. This LTP-enhancing effect of Ang-(1-7) was not observed in COX-2(+/-) mice. However, in COX-2(-/-) mice, Ang-(1-7) caused an enhancement of LTP similar to that in wild-type mice. The NO synthetase inhibitor L-NAME blocked this angiotensin-(1-7)-induced increase in LTP in COX-2(-/-) mice. Low-frequency stimulation of external capsule fibers did not cause long-term depression (LTD) in drug-free and angiotensin-(1-7)-treated brain slices in wild-type mice. In contrast, in COX-2(-/-) mice, angiotensin-(1-7) caused stable LTD. Increasing NO concentration by the NO-donor SNAP also caused LTD in wild-type mice. Our study shows for the first time that LTP in the amygdala is dependent on COX-2 activity. Moreover, COX-2 is involved in the mediation of angiotensin-(1-7) effects on LTP. Finally, it is recognized that there is a molecular cross-talk between COX-2 and NO that may regulate synaptic plasticity.

The CNS renin-angiotensin system

The renin-angiotensin system (RAS) is one of the best-studied enzyme-neuropeptide systems in the brain and can serve as a model for the action of peptides on neuronal function in general. It is now well established that the brain has its own intrinsic RAS with all its components present in the central nervous system. The RAS generates a family of bioactive angiotensin peptides with variable biological and neurobiological activities. These include angiotensin-(1-8) [Ang II], angiotensin-(3-8) [Ang IV], and angiotensin-(1-7) [Ang-(1-7)]. These neuroactive forms of angiotensin act through specific receptors. Only Ang II acts through two different high-specific receptors, termed AT1 and AT2. Neuronal AT1 receptors mediate the stimulatory actions of Ang II on blood pressure, water and salt intake, and the secretion of vasopressin. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic to AT1 receptors. Among the many potential effects mediated by stimulation of AT2 are neuronal regeneration after injury and the inhibition of pathological growth. Ang-(1-7) mediates its antihypertensive effects by stimulating the synthesis and release of vasodilator prostaglandins and nitric oxide and by potentiating the hypotensive effects of bradykinin. New data concerning the roles of Ang IV and Ang-(1-7) in cognition also support the existence of complex site-specific interactions between multiple angiotensins and multiple receptors in the mediation of important central functions of the RAS. Thus, the RAS of the brain is involved not only in the regulation of blood pressure, but also in the modulation of multiple additional functions in the brain, including processes of sensory information, learning, and memory, and the regulation of emotional responses.

Valsartan abolishes most of the memory-improving effects of intracerebroventricular angiotensin II in rats

This study explores behavioral effects of angiotensin II (Ang II) and a potent AT(1) receptor inhibitor valsartan ((S)-N-valeryl-N-{[2'-(1H-tetrazol-5-yl)biphenyl-4-yl]-methyl}-valine). Male Wistar rats (160-180 g) were administered valsartan (10 mg/kg) orally followed, 2 hr later, by Ang II (1 nmol) given intracerebroventricularly (i.c.v., right lateral ventricle). Then 15 min later rats underwent behavioral testing: acquisition of conditioned avoidance responses (CARs), recall of a passive avoidance behavior, open field, elevated "plus" maze, and "chimney" test. Object recognition was tested 60 min after the i.c.v. injections. In addition, effect of valsartan on Ang II stimulated drinking of water was tested. We found that valsartan did not modify the Ang II facilitation of CARs acquisition but abolished the Ang II improvement of memory retrieval and consolidation. The lack of effect of our treatments on the rats' motor activity in the open field makes unspecific contribution of the drug-induced performance changes to the cognitive tests improbable. The anxiogenic action of Ang II, decreased by valsartan, makes an unspecific influence of anxiety possible. The prevention of Ang II increase of drinking by orally given valsartan confirmed effective blockade of the brain AT(1) receptors by the drug.

IN CONCLUSION

valsartan appears to affect cognitive effects of i.c.v. Ang II in rats in a similar way to losartan; anxiolytic activity of valsartan appears to be slightly weaker than that of losartan.

Angiotensin-(1–7) enhances LTP in the hippocampus through the G-protein-coupled receptor Mas

The renin-angiotensin system not only plays a critical role in blood pressure control but is also involved in learning and memory mechanisms. In addition to angiotensin (Ang) II, Ang-(1-7) may also have important biological activities in the brain. Here, we show for the first time that Ang-(1-7) enhances long-term potentiation (LTP) in the CA1 region of the hippocampus. Our studies with AT1 receptor antagonists and selective Ang-(1-7) receptor antagonists demonstrate the existence of a distinct Ang-(1-7) receptor in the brain, the G-protein-coupled receptor Mas, encoded by the Mas protooncogene. We also show that the genetic deletion of this receptor abolishes the Ang-(1-7)-induced enhancement of LTP. Thus, we firstly demonstrate that Ang-(1-7) influences the induction of LTP in limbic structures implicating its distinct function in learning and memory mechanisms; secondly, we have identified Mas as a functional receptor for Ang-(1-7) in the brain.

Brain angiotensin and anxiety-related behavior: The transgenic rat TGR(ASrAOGEN)680

The transgenic rat TGR(ASrAOGEN)680, characterized by a transgene-producing antisense RNA against angiotensinogen in the brain, provides an opportunity to study the behavioral effects of angiotensin. While exposed to the elevated plus-maze (EPM) and the light/dark box, TGR(ASrAOGEN)680 rats showed more signs of anxiety compared to parental Sprague-Dawley (SD) rats. In the EPM, they made fewer entries into the open arms, spent less time there and more time on the closed arms. Head dips were reduced and U-turns were increased. In the light/dark box, the latency to the first re-entry into the light compartment was higher in TGR(ASrAOGEN)680. They displayed more SAP out from the dark and a reduced number of transitions between the two compartments. In the social interaction test, active social contacts were reduced, further suggesting an anxious phenotype. Although there was no transgenic effect on distance traveled in the open field, the more anxious TGR(ASrAOGEN)680 spent less time in the inner zone. Self-grooming was increased in TGR(ASrAOGEN)680 during exposure to the EPM and the open field, but was decreased in the social interaction test. In TGR(ASrAOGEN)680, tissue content of 5-HT and its metabolite 5-HIAA was lower in the hippocampus, frontal, and parietal cortex. HIAA and 5-HIAA/5-HT ratios were reduced in the hypothalamus, striatum, and septum. In the open field, the anxiogenic effect of the 5-HT2C/1B receptor agonist mCPP (0.5-1 mg/kg IP) was more pronounced in TGR(ASrAOGEN)680. The data suggest an anxious phenotype in rats with low brain angiotensinogen, possibly related to secondary dysfunctions of the brain serotonergic system.

The medial amygdaloid area is involved in activation of angiotensin II-sensitive neurons in the anterior hypothalamic area

We have previously reported that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins in rats and that the activities of these AHA angiotensin II-sensitive neurons are enhanced in spontaneously hypertensive rats. It is suggested that there exist neural projections from the medial amygdala to the AHA in rats. In this study, we examined whether neurons in the medial amygdaloid area (MeA) are involved in the activation of AHA angiotensin II-sensitive neurons. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Microinjection of glutamate into the MeA caused an increase in the firing rate of AHA angiotensin II-sensitive neurons. The glutamate-induced increase of firing rate was inhibited by pressure application of the AT1 receptor antagonist losartan onto AHA angiotensin II-sensitive neurons. The microinjection of glutamate into the central amygdaloid area also increased the firing rate of AHA angiotensin II-sensitive neurons, but the glutamate-induced increase of firing rate was not affected by pressure application of losartan onto AHA angiotensin II-sensitive neurons. The microinjection of corticotropin-releasing factor (CRF) into the MeA also increased the firing rate of AHA angiotensin II-sensitive neurons, but the CRF-induced increase of firing rate was not inhibited by pressure application of losartan onto AHA angiotensin II-sensitive neurons. Repeated microinjection of glutamate into the MeA caused an increase in the release of angiotensins in the AHA. These findings indicate that neurons in the MeA are involved in the activation of AHA angiotensin II-sensitive neurons. It seems likely that the activation of AHA angiotensin II-sensitive neurons induced by glutamate but not CRF is partly mediated via the release of angiotensins at AHA angiotensin II-sensitive neuron levels.

Angiotensin peptides modulatory system: how is it implicated in the control of seizure susceptibility?

Accumulated studies support the concept that angiotensin peptides, ANG II, ANG III, and ANG IV act as neurotransmitters or neuromodulators in specific neuronal pathways in the brain stem, the hypothalamus, and the forebrain. They have been implicated in the regulation of several physiological processes, particularly in excitable brain structures that express high concentration of their receptors. With the help of pharmacological approaches it was shown that angiotensin peptides appear to be anticonvulsant in a variety of experimental seizure models. Thus, ANG II increases the threshold for pentylenetetrazol (PTZ)-, bicuculline-and picrotoxin-induced seizures in mice. It also attenuates the intensity of clonic seizures evoked by PTZ and 3-mercaptopropionic acid and is effective in the maximal electroshock test. Furthermore, ANG II, ANG III, and ANG IV protect against the clonic convulsions in the PTZ kindling model of epilepsy in mice. From the accumulated results it could be assumed that the angiotensin peptides appear to realize their effects acting directly on their receptors (AT(1), AT(2) and AT(4)) and through close interaction with different neurotransmitter/neuromodulator systems as dopamine (DA)-, gamma-aminobutyric acid (GABA)-and adenosine. This may contribute to a new potential use of angiotensin drugs either alone or in combination with other neuroprotective agents acting through the above mentioned systems, thus providing a more rational strategy for the treatment of neurodegenerative disorders such as epilepsy.

Differential anxiolytic effect of enalapril and losartan in normotensive and renal hypertensive rats

The effect of angiotensin-converting enzyme (ACE) inhibitor enalapril (EPL) (2 and 4 mg/kg), angiotensin (AT) II receptor antagonist losartan (LRN) (5 and 10 mg/kg), and anxiolytic drug diazepam (DZP) (0.5 mg/kg) on anxiety parameters were evaluated in experimentally induced renal hypertensive rats (RHR). Renal hypertension was induced in Wistar strain male albino rats weighing 200-250 g by following the method of Goldblatt. The animals having systolic blood pressure more than 180-210 mm Hg were subjected to open-field exploratory behaviour, elevated plus maze behaviour, and social interaction tests of anxiety. The RHR showed hyperactivity in open-field behaviour and anxiogenicity in elevated plus maze and social interaction tests. Losartan (5 and 10 mg/kg) and DZP (0.5 mg/kg) significantly attenuated the hyperactivity and anxiogenic behaviour in experimentally induced hypertensive rats and induced anxiolysis in normotensive rats (NTR). Enalapril reversed the hypertension-induced alteration only at higher dose (4 mg/kg) and failed to show any effect in NTR. It can be concluded that renin angiotensin aldosterone system (RAAS) has a significant role on behaviour, and LRN has shown better effect in reversing the hyperactivity and anxiogenicity in the experimentally induced hypertensive rats, indicating a possible role of AT receptor in the mediation of anxiolysis.

Effects of angiotensin II microinjected into medial amygdala on male sexual behavior in rats

Research was undertaken to study the role of central angiotensin in the modulation of male sexual behavior, testing the effect of angiotensin II (Ang II) injections into the medial amygdaloid nucleus (MeA). The sexual behavior of adult male Wistar rats was evaluated, 15 min after bilateral intra-amygdaloid microinjection (0.3 microl) of saline and 5 doses of Ang II: 10; 25; 50; 100, and 150 fmol. The effects of the Ang II receptor blockade were also studied. We tested the effect of coinjection of Ang II (50 fmol) with the AT1 antagonist, losartan (20 pmol) and the AT2 antagonist, CGP 42112 (1 pmol). Ang II inhibited sexual behavior and this inhibition was prevented by the coinjection of AT1 antagonist, losartan, or the AT2 antagonist, CGP 42112. Results show that Ang II has a powerful effect on male sexual behavior, which may be mediated by both AT1 and AT2 receptors.

Chronotropic action of angiotensin II in neurons via protein kinase C and CaMKII

Angiotensin II (Ang II) plays an important role in the central control of blood pressure and baroreflexes. These effects are initiated by stimulation of Ang II type 1 (AT(1)) receptors on neurons within the hypothalamus and brain stem, and involve increasing the activity of noradrenergic, substance P, and glutamatergic pathways. The goal of this study is to investigate the intracellular signaling molecules, which are involved in mediating the Ang II-induced increases in neuronal activity. Using neurons in primary culture from newborn rat hypothalamus and brain stem, we have previously determined that Ang II elicits an AT(1) receptor-mediated inhibition of delayed rectifier K(+) current, a stimulation of Ca(2+) current, and a consequent increase in firing rate. In the present study we have demonstrated that this chronotropic action of Ang II in neuronal cultures involves activation of Ca(2+)-dependent signaling molecules. The Ang II-induced increase in firing rate was abolished by inhibition of phospholipase C with U73122 (10 micromol/L), and was attenuated by the protein kinase C inhibitor calphostin C (10 micromol/L) or by the calcium/calmodulin-dependent kinase II (CaMKII) inhibitor KN-93 (10 micromol/L). A combination of calphostin C and KN-93 completely inhibited this Ang II action. These results indicate that the AT(1) receptor-mediated increase in neuronal firing rate involves activation of both PKC and CaMKII, and suggest that these enzymes are potential targets for manipulating the central actions of Ang II.