Cardiovascular effects of agmatine within the rostral ventrolateral medulla are similar to those of clonidine in anesthetized rats

Baroreceptor Modulation of the Cardiovascular System, Pain, Consciousness, and Cognition

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain cardiovascular homeostasis by coordinating the responses to external and internal environmental stressors. While it is well known that carotid and cardiopulmonary baroreceptors modulate sympathetic vasomotor and parasympathetic cardiac neural autonomic drive, to avoid excessive fluctuations in vascular tone and maintain intravascular volume, there is increasing recognition that baroreceptors also modulate a wide range of non-cardiovascular physiological responses via projections from the nucleus of the solitary tract to regions of the central nervous system, including the spinal cord. These projections regulate pain perception, sleep, consciousness, and cognition. In this article, we summarize the physiology of baroreceptor pathways and responses to baroreceptor activation with an emphasis on the mechanisms influencing cardiovascular function, pain perception, consciousness, and cognition. Understanding baroreceptor-mediated effects on cardiac and extra-cardiac autonomic activities will further our understanding of the pathophysiology of multiple common clinical conditions, such as chronic pain, disorders of consciousness (e.g., abnormalities in sleep-wake), and cognitive impairment, which may result in the identification and implementation of novel treatment modalities. © 2021 American Physiological Society. Compr Physiol 11:1373-1423, 2021.

Investigation of the Role of AT2 Receptors in the Nucleus Tractus Solitarii of Normotensive Rats in Blood Pressure Control

Aim

The nucleus tractus solitarii (NTS) densely expresses angiotensin II type 2 receptors (AT2R), which are mainly located on inhibitory gamma-aminobutyric acid (GABA) neurons. Central AT2R stimulation reduces blood pressure, and AT2R stimulation in the rostral ventrolateral medulla (RVLM), mediates a hypotensive response through a GABAergic mechanism. We aimed to test the hypothesis that an AT2R mediated inhibition of the GABA release within the NTS might be involved in this hypotensive response, by assessing possible alterations in blood pressure and heart rate, as well as in GABA levels in normotensive Wistar rats.

Methods

In vivo microdialysis was used for measurement of extracellular GABA levels and for perfusion of the selective AT2R agonist, Compound 21, within the NTS. Our set-up allowed to determine simultaneously the excitatory glutamate dialysate levels. The mean arterial pressure and heart rate responses were monitored with a pressure transducer.

Results

Local perfusion of Compound 21 into the NTS did not modify blood pressure and heart rate, nor glutamate and GABA levels compared to baseline concentrations. A putative effect was also not unmasked by concomitant angiotensin II type 1 receptor blockade with candesartan. Positive control experiments confirmed that the experimental set up had enough sensitivity to detect a reduction in GABA dialysate levels and blood pressure.

Conclusion

The results did not provide evidence for a role of the AT2R within the NTS in the control of blood pressure, nor for an interaction with local GABAergic signaling in normotensive rats.

Cardiovascular Effects Mediated by Imidazoline Drugs: An Update

OBJECTIVE

Clonidine is a centrally acting antihypertensive drug. Hypotensive effect of clonidine is mediated mainly by central α2-adrenoceptors and/or imidazoline receptors located in a complex network of the brainstem. Unfortunately, clonidine produces side effects such as sedation, mouth dry, and depression. Moxonidine and rilmenidine, compounds of the second generation of imidazoline drugs, with fewer side effects, display a higher affinity for the imidazoline receptors compared with α2-adrenoceptors. The antihypertensive action of these drugs is due to inhibition of the sympathetic outflow primarily through central I1-imidazoline receptors in the RVLM, although others anatomical sites and mechanisms/receptors are involved. Agmatine is regarded as the endogenous ligand for imidazoline receptors. This amine modulates the cardiovascular function. Indeed, when administered in the RVLM mimics the hypotension of clonidine.

RESULTS

Recent findings have shown that imidazoline drugs also exert biological response directly on the cardiovascular tissues, which can contribute to their antihypertensive response. Currently, new imidazoline receptors ligands are in development.

CONCLUSION

In the present review, we provide a brief update on the cardiovascular effects of clonidine, moxonidine, rilmenidine, and the novel imidazoline agents since representing an important therapeutic target for some cardiovascular diseases.

Brain stem adenosine receptors modulate centrally mediated hypotensive responses in conscious rats: A review

Adenosine is implicated in the modulation of cardiovascular responses either at the peripheral or at central level in experimental animals. However, there are no dedicated reviews on the involvement of adenosine in mediating the hypotensive response of centrally administered clonidine in general and specifically in aortically barodenervated rats (ABD). The conscious ABD rat model exhibits surgically induced baroreflex dysfunction and exaggerated hypotensive response, compared with conscious sham-operated (SO) rats. The current review focuses on, the role of adenosine receptors in blood pressure (BP) regulation and their possible crosstalk with other receptors e.g. imidazoline (I₁) and alpha (α_2A) adrenergic receptor (AR). The former receptor is a molecular target for clonidine, whose hypotensive effect is enhanced approx. 3-fold in conscious ABD rats. We also discussed how the balance between the brain stem adenosine A₁ and A_2A receptors is regulated by baroreceptors and how such balance influences the centrally mediated hypotensive responses. The use of the ABD rat model yielded insight into the downstream signaling cascades following clonidine-evoked hypotension in a surgical model of baroreflex dysfunction.

Characterization of imidazoline receptors in blood vessels for the development of antihypertensive agents

It has been indicated that activation of peripheral imidazoline I2-receptor (I-2R) may reduce the blood pressure in spontaneously hypertensive rats (SHRs). Also, guanidinium derivatives show the ability to activate imidazoline receptors. Thus, it is of special interest to characterize the I-2R using guanidinium derivatives in blood vessels for development of antihypertensive agent(s). Six guanidinium derivatives including agmatine, amiloride, aminoguanidine, allantoin, canavanine, and metformin were applied in this study. Western blot analysis was used for detecting the expression of imidazoline receptor in tissues of Wistar rats. The isometric tension of aortic rings isolated from male rats was also estimated. The expression of imidazoline receptor on rat aorta was identified. However, guanidinium derivatives for detection of aortic relaxation were not observed except agmatine and amiloride which induced a marked relaxation in isolated aortic rings precontracted with phenylephrine or KCl. Both relaxations induced by agmatine and amiloride were attenuated by glibenclamide at concentration enough to block ATP-sensitive potassium (KATP) channels. Meanwhile, only agmatine-induced relaxation was abolished by BU224, a selective antagonist of imidazoline I2-receptors. Taken together, we suggest that agmatine can induce vascular relaxation through activation of peripheral imidazoline I2-receptor to open KATP channels. Thus, agmatine-like compound has the potential to develop as a new therapeutic agent for hypertension in the future.

Antihypertensive action of allantoin in animals

The agonists of imidazoline I-1 receptors (I-1R) are widely used to lower blood pressure. It has been indicated that guanidinium derivatives show an ability to activate imidazoline receptors. Also, allantoin has a chemical stricture similar to guanidinium derivatives. Thus, it is of special interest to characterize the effect of allantoin on I-1R. In conscious male spontaneous hypertensive rats (SHRs), mean blood pressure (MBP) was recorded using the tail-cuff method. Furthermore, the hemodynamic analyses in catheterized rats were applied to measure the actions of allantoin in vivo. Allantoin decreased blood pressures in SHRs at 30 minutes, as the most effective time. Also, this antihypertensive action was shown in a dose-dependent manner from SHRs treated with allantoin. Moreover, in anesthetized rats, allantoin inhibited cardiac contractility and heart rate as showing in hemodynamic dP/dt max significantly. Also, the peripheral blood flow was markedly increased by allantoin. Both actions were diminished by efaroxan at the dose sufficient to block I-1R. Thus, we suggest that allantoin, as I-1R agonist, has the potential to develop as a new therapeutic agent for hypertension in the future.

Putative agmatinase inhibitor for hypoxic-ischemic new born brain damage

Agmatine is an endogenous brain metabolite, decarboxylated arginine, which has neuroprotective properties when injected intraperitoneally (i.p.) into rat pups following hypoxic-ischemia. A previous screen for compounds based on rat brain lysates containing agmatinase with assistance from computational chemistry, led to piperazine-1-carboxamidine as a putative agmatinase inhibitor. Herein, the neuroprotective properties of piperazine-1-carboxamidine are described both in vitro and in vivo. Organotypic entorhinal-hippocampal slices were firstly prepared from 7-day-old rat pups and exposed in vitro to atmospheric oxygen depletion for 3 h. Upon reoxygenation, the slices were treated with piperazine-1-carboxamidine or agmatine (50 μg/ml agents), or saline, and 15 h later propidium iodine was used to stain. Piperazine-1-carboxamidine or agmatine produced substantial in vitro protection compared to post-reoxygenated saline-treated controls. An in vivo model involved surgical right carotid ligation followed by exposure to hypoxic-ischemia (8 % oxygen) for 2.5 h. Piperazine-1-carboxamidine at 50 mg/kg i.p. was given 15 min post-reoxygenation and continued twice daily for 3 days. Cortical agmatine levels were elevated (+28.5 %) following piperazine-1-carboxamidine treatment with no change in arginine or its other major metabolites. Histologic staining with anti-Neun monoclonal antibody also revealed neuroprotection of CA1-3 layers of the hippocampus. Until endpoint at 22 days of age, no adverse events were observed in treated pups' body weights, rectal temperatures, or prompted ambulation. Piperazine-1-carboxamidine therefore appears to be a neuroprotective agent of a new category, agmatinase inhibitor.

Changes of imidazoline receptors in spontaneously hypertensive rats

The role of imidazoline receptors in the regulation of vascular function remains unclear. In this study, we evaluated the effect of agmatine, an imidazoline receptor agonist, on systolic blood pressure (SBP) in spontaneously hypertensive rats (SHRs) and investigated the expressions of imidazoline receptors by Western blot. The isometric tension of aortic rings isolated from male SHRs was also estimated. Agmatine decreased SBP in a dose-dependent manner in SHRs but not in the normal group [Wistar-Kyoto (WKY) rats]. This reduction in SBP in SHRs was abolished by BU224, a selective antagonist of imidazoline I(2) -receptors. Higher expression of imidazoline receptors in SHR was observed. Moreover, agmatine-induced relaxation in isolated aortic rings precontracted with phenylephrine or KCl. This relaxation was also abolished by BU224 but was not modified by efaroxan, an imidazoline I(1) -receptor antagonist. Agmatine-induced relaxation was also attenuated by PNU 37883, a selective blocker of vascular ATP-sensitive potassium (K(ATP) ) channels. Additionally, vasodilatation by agmatine was reduced by an inhibitor of protein kinase A (PKA). We suggest that agmatine can lower blood pressure in SHRs through activation of the peripheral imidazoline I(2) -receptor, which is expressed more highly in SHRs.

Central Adenosine Signaling Plays a Key Role in Centrally Mediated Hypotension in Conscious Aortic Barodenervated Rats

We tested the hypothesis that clonidine-evoked hypotension is dependent on central adenosinergic pathways. Five groups of male, conscious, aortic baroreceptor-denervated (ABD) rats received clonidine (10 microg/kg i.v.) 30 min after i.v. 1) saline, 2) theophylline (10 mg/kg), or 3) 8-(p-sulfophenyl)theophylline (8-SPT) (2.5 mg/kg) or 1 h after i.p. 4) dipyridamole (5 mg/kg) or 5) an equal volume of sesame oil. Blockade of central (theophylline) but not peripheral (8-SPT) adenosine receptors abolished clonidine hypotension. In contrast, dipyridamole substantially enhanced the bradycardic response to clonidine. In additional groups, intracisternal (i.c.) dipyridamole (150 microg) and 8-SPT (10 microg) enhanced and abolished, respectively, clonidine (0.6 microg i.c.)-evoked hypotension. Because clonidine is a mixed I1/alpha2 agonist, we also investigated whether adenosine signaling is linked to the I1 or the alpha2A receptor by administering the selective I1 (rilmenidine, 25 microg) or alpha2A [alpha-methylnorepinephrine (alpha-MNE), 4 microg] agonist 30 min after central adenosine receptor blockade (8-SPT; 10 microg i.c.) or artificial cerebrospinal fluid. The hypotensive response elicited by rilmenidine or alpha-MNE was abolished in 8-SPT-pretreated rats. To delineate the role of the adenosine A2A receptor in clonidine-evoked hypotension, i.c. clonidine (0.6 microg) was administered 30 min after central adenosine receptor A2A blockade [5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-epsilon]-1,2,4-triazolo[1,5-c]-pyrimidine (SCH58261); 150 microg i.c.]. The latter virtually abolished the hypotensive and bradycardic responses elicited by clonidine. In conclusion, central adenosine A2A signaling plays a key role in clonidine-evoked hypotension in conscious aortic barodenervated rats.