Multifactorial control of water and saline intake: Role of α2-adrenoceptors

Water and saline intake is controlled by several mechanisms activated during dehydration. Some mechanisms, such as the production of angiotensin II and unloading of cardiovascular receptors, activate both behaviors, while others, such as the increase in blood osmolality or sodium concentration, activate water, but inhibit saline intake. Aldosterone probably activates only saline intake. Clonidine, anα2-adrenergic agonist, inhibits water and saline intake induced by these mechanisms. One model to describe the interactions between these multiple mechanisms is a wire-block diagram, where the brain circuit that controls each intake is represented by a summing point of its respective inhibiting and activating factors. The α2-adrenoceptors constitute an inhibitory factor common to both summing points.

Catecholamine effects on human melanoma cells evoked by α1-adrenoceptors

The biological effects of catecholamines in mammalian pigment cells are poorly understood. Our previous results showed the presence of α1-adrenoceptors in SK-Mel 23 human melanoma cells. The aims of this work were to (1) characterize catecholamine effects on proliferation, tyrosinase activity and expression, (2) identify the α1- adrenoceptor subtypes, and (3) verify whether chronic norepinephrine (NE) treatment modified the types and/or pharmacological characteristics of adrenoceptors present in SK-Mel 23 human melanoma cells. Cells treated with the aradrenergic agonist, phenylephrine (PHE, 10-5 or 10-4 M), for 24-72 h, exhibited decreased cell proliferation and enhanced tyrosinase activity, but unaltered tyrosinase expression as compared with the control. The proliferation and tyrosinase activity responses were inhibited by the α1-adrenergic antagonist prazosin, suggesting they were evoked by α1-adrenoceptors. The presence of actinomycin D, a transcription inhibitor, did not diminish PHE-induced effects. RT-PCR assays, followed by cloning and sequencing, demonstrated the presence of α1A- and α1B-adrenoceptor subtypes. NE-treated cells (24 or 72 h) were used in competition assays, and showed no significant change in the competition curves of α1-adrenoceptors as compared with control curves. Other adrenoceptor subtypes were not identified in these cells, and NE pretreatment did not induce their expression. In conclusion, the activation of SK-Mel 23 human melanoma α1- radrenoceptors elicit biological effects, such as proliferation decrease and tyrosinase activity increase. Desensitization or expression of other adrenoceptor subtypes after chronic NE treatment were not observed.

Activation of central aα-adrenoceptors mediates salivary gland vasoconstriction

Objective: Peripheral treatment with the cholinergic agonist pilocarpine increases salivary gland blood flow and induces intense salivation that is reduced by the central injection of moxonidine (aα-adrenoceptors/ imidazoline agonist). In the present study, we investigated the effects of the intracerebroventricular (i.c.v.) injection of pilocarpine alone or combined with moxonidine also injected i.c.v. On submandibular/sublingual gland (SSG) vascular resistance. In addition, the effects of these treatments on arterial pressure, heart rate and on mesenteric and hindlimb vascular resistance were also tested. Design: Male Holtzman rats with stainless steel cannula implanted into lateral ventricle and anaesthetized with urethane + α-chloralose were used. Results: Pilocarpine (500 nmol/1 μl) injected i.c.v. Reduced SSG vascular resistance and increased arterial pressure, heart rate and mesenteric vascular resistance. Contrary to pilocarpine alone, the combination of moxonidine (20 nmol/1 μl) and pilocarpine injected i.c.v. Increased SSG vascular resistance, an effect abolished by the pre-treatment with the α2-adrenoceptor antagonist yohimbine (320 nmol/2 μl). The increase in arterial pressure, heart rate and mesenteric resistance was not modified by the combination of moxonidine and pilocarpine i.c.v. Conclusion: These results suggest that the activation of central α2- adrenoceptors may oppose to the effects of central cholinergic receptor activation in the SSG vascular resistance. © 2012 Elsevier Ltd. All rights reserved.

Differential phosphorylation, desensitization, and internalization of α1A-adrenoceptors activated by norepinephrine and oxymetazoline

Loss of response on repetitive drug exposure (i.e., tachyphylaxis) is a particular problem for the vasoconstrictor effects of medications containing oxymetazoline (OXY), an α1-adrenoceptor (AR) agonist of the imidazoline class. One cause of tachyphylaxis is receptor desensitization, usually accompanied by phosphorylation and internalization. It is well established that a1A-ARs are less phosphorylated, desensitized, and internalized on exposure to the phenethylamines norepinephrine (NE), epinephrine, or phenylephrine (PE) than are the a1B and a1D subtypes. However, here we show in human embryonic kidney-293 cells that the low-efficacy agonist OXY induces G protein-coupled receptor kinase 2-dependent a1A-AR phosphorylation, followed by rapid desensitization and internalization (∼40% internalization after 5 minutes of stimulation), whereas phosphorylation of α1A-ARs exposed to NE depends to a large extent on protein kinase C activity and is not followed by desensitization, and the receptors undergo delayed internalization (∼35% after 60 minutes of stimulation). Native α1A-ARs from rat tail artery and vas deferens are also desensitized by OXY, but not by NE or PE, indicating that thisproperty of OXY is not limited to recombinant receptors expressed in cell systems. The results of the present study are clearly indicative of agonist-directed a1A-AR regulation. OXY shows functional selectivity relative to NE and PE at a1A-ARs, leading to significant receptor desensitization and internalization, which is important in view of the therapeutic vasoconstrictor effects of this drug and the varied biologic process regulated by α1A-ARs. Copyright © 2013 by The American Society for Pharmacology and Experimental Therapeutics.

Cross-talk with β2-adrenoceptors enhances ligand affinity properties from endothelial alpha1D-adrenoceptors that mediates carotid relaxation

Objectives Our main objectives were to investigate the affinity properties of endothelial and muscular α1D-adrenoceptors and to characterize the cross-talk between endothelial α1D- adrenoceptors and β2-adrenoceptors in rat carotid. Methods Relaxation and contraction concentration-response curves for phenylephrine (α1-adrenergic agonist) were obtained in carotid rings in absence or presence of increasing concentrations of BMY7378 (α 1D-adrenergic antagonist), combined or not with increasing concentration of ICI-118,551 (β2-adrenergic antagonist). Schild analysis was used to estimate the affinity constant from pA2 values of BMY7378. Key Findings BMY7378 produced an unsurmountable antagonism on phenylephrine-induced relaxation but a surmountable antagonism on phenylephrine-induced contraction. BMY7378 potency was higher in inhibiting the relaxation than the contraction induced by phenylephrine because the rightward shifts induced by BMY7378 were greater in the relaxation. The apparent pA 2 value for BMY7378 in phenylephrine-induced relaxation was greater than in contraction. When combined with ICI-118,551, BMY7378 yielded a surmountable antagonism on phenylephrine-induced relaxation and presented a pA2 value similar to that obtained in phenylephrine-induced contraction. Conclusions Endothelial α1D-adrenoceptors, which mediates rat carotid relaxation, present high ligand affinity because of the cross-talk with β2-adrenoceptors, which explains the higher potency of phenylephrine in inducing relaxation than contraction and the atypical unsurmountable antagonism produced by BMY7378 on phenylephrine-induced relaxation. © 2013 Royal Pharmaceutical Society.

Role of α2-adrenoceptors in the lateral parabrachial nucleus in the control of body fluid homeostasis

Central α2-adrenoceptors and the pontine lateral parabrachial nucleus (LPBN) are involved in the control of sodium and water intake. Bilateral injections of moxonidine (α2-adrenergic/imidazoline receptor agonist) or noradrenaline into the LPBN strongly increases 0.3 M NaCl intake induced by a combined treatment of furosemide plus captopril. Injection of moxonidine into the LPBN also increases hypertonic NaCl and water intake and reduces oxytocin secretion, urinary sodium, and water excreted by cell-dehydrated rats, causing a positive sodium and water balance, which suggests that moxonidine injected into the LPBN deactivates mechanisms that restrain body fluid volume expansion. Pretreatment with specific α2-adrenoceptor antagonists injected into the LPBN abolishes the behavioral and renal effects of moxonidine or noradrenaline injected into the same area, suggesting that these effects depend on activation of LPBN α2-adrenoceptors. In fluid-depleted rats, the palatability of sodium is reduced by ingestion of hypertonic NaCl, limiting intake. However, in rats treated with moxonidine injected into the LPBN, the NaCl palatability remains high, even after ingestion of significant amounts of 0.3 M NaCl. The changes in behavioral and renal responses produced by activation of α2-adrenoceptors in the LPBN are probably a consequence of reduction of oxytocin secretion and blockade of inhibitory signals that affect sodium palatability. In this review, a model is proposed to show how activation of α2-adrenoceptors in the LPBN may affect palatability and, consequently, ingestion of sodium as well as renal sodium excretion.

Both alpha(1)- and alpha(2)-adrenoceptors in the Insular Cortex Are Involved in the Cardiovascular Responses to Acute Restraint Stress in Rats

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Involvement of alpha(1B)-adrenoceptors in the anti-immobility effect of imipramine in the tail suspension test

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Role of central α1- and α2-adrenoceptors on the dipsogenic and cardiovascular effect of angiotensin II

We investigated the effects of previous central treatment with prazosin (an α1-adrenoceptor antagonist) or clonidine (an α2-adrenoceptor agonist) on the dipsogenic, pressor and tachycardic responses produced by intracerebroventricular (ICV) injection of angiotensin II (AII) in conscious rats. Holtzman rats with a chronic cannula implanted in the lateral ventricle were tested for dipsogenic and cardiovascular (arterial pressure and heart rate) responses in separate experiments. Previous ICV treatment with clonidine (20, 40, 80 and 120 nmol) abolished the pressor, tachycardic and dipsogenic effects of ICV AII. After all doses of prazosin (40, 80 and 120 nmol), AII induced bradycardic responses, but only the 80 and 120 nmol doses of prazosin reduced the pressor responses to AII. Prazosin produced no alteration in the dipsogenic effect of AII. The results show that the periventricular α1-adrenoceptors are involved only in the cardiovascular responses produced by central AII, whereas clonidine acting through α2-adrenergic and/or imidazole receptors can modulate all actions of AII. © 1990.

Role of alpha1 and alpha2-adrenoceptors of the lateral hypothalamic area on urinary excretion caused by centrally administered angiotensin II

To determine whether central α1 and α2-adrenergic mechanisms are involved in urinary sodium and potassium excretion and urine volume induced by angiotensin II (ANGII), these renal parameters were measured in volume-expanded Holtzman rats with cannulas implanted into lateral ventricle (LV) and lateral hypothalamus (LH). The injection of ANGII into LV in rats with volume expansion reduced the sodium, potassium and urine excretion in comparison to the control injections of isotonic saline, whereas prazosin (α1 antagonist) potentiated these effects. Clonidine (α2 agonist) and yohimbine (α2 antagonist) injected into LH previous to injection of ANGII into LV also abolished the inhibitory effect of ANGII. These results suggest that the discharge of central alpha-adrenergic receptors has dual inhibitory and excitatory effect on antinatriuretic, antikaliuretic and antidiuretic effect induced by central ANGII in volume-expanded rats. © 1995.

Role of the alfa1- and alfa2-adrenoceptors of the lateral hypothalamus in the dipsogenic response to central angiotensin II in rats

The present experiments were conducted to investigate the role of the α1- and α2-adrenergic receptors of the lateral hypothalamus (LH) on the drinking response elicited by intracerebroventricular (i.c.v) injections of carbachol and angiotensin II (AII) in rats. Clonidine (an α2-adrenergic agonist) injected into the LH produced a dose-dependent reduction of the drinking responses elicited by i.c.v. administration of carbachol and AII. The α1-adrenergic agonist phenylephrine injected into the LH reduced the dipsogenic response to i.c.v. AII, but not to carbachol. Injection of yohimbine (an α2-adrenergic antagonist) and prazosin (an α1-adrenergic antagonist) into the LH also reduced the water intake produced by i.c.v. injection of AII. Previous injection of α1- or α2-adrenergic antagonists into the LH increased the antidipsogenic effect of clonidine or phenylephrine injected into the same area on the water intake induced by i.c.v. AII. These results show that the α1- and α2-adrenergic receptors of the LH are involved in the control of drinking responses elicited by i.c.v. injection of AII in rats. They also show that clonidine, but not phenylephrine, suppresses the drinking induced by i.c.v. carbachol. The data suggest that the discharge of central α-adrenergic receptors has a dual (inhibitory and excitatory) effect on water intake induced by central AII. © 1991.

Participation of α2 -adrenoceptors in sodium appetite inhibition during sickness behaviour following administration of lipopolysaccharide

Sickness behaviour, a syndrome characterized by a general reduction in animal activity, is part of the active-phase response to fight infection. Lipopolysaccharide (LPS), an effective endotoxin to model sickness behaviour, reduces thirst and sodium excretion, and increases neurohypophysial secretion. Here we review the effects of LPS on thirst and sodium appetite. Altered renal function and hydromineral fluid intake in response to LPS occur in the context of behavioural reorganization, which manifests itself as part of the syndrome. Recent data show that, in addition to its classical effect on thirst, non-septic doses of LPS injected intraperitoneally produce a preferential inhibition of intracellular thirst versus extracellular thirst. Moreover, LPS also reduced hypertonic NaCl intake in sodium-depleted rats that entered a sodium appetite test. Antagonism of α2 -adrenoceptors abolished the effect of LPS on sodium appetite. LPS and cytokine transduction potentially recruit brain noradrenaline and α2 -adrenoceptors to control sodium appetite and sickness behaviour.

Role of α1-, and α2- and β-adrenoceptors of the median preoptic area on the water intake, renal excretion, and arterial pressure induced by ANG II

The present experiments were conducted to investigate the role of the α1-, α2- and β-adrenergic receptors of the median preoptic area (MnPO) on the water intake and urinary electrolyte excretion, elicited by central injections of angiotensin II (ANG II). Prazosin (an α1-adrenergic receptor antagonist) and yohimbine (an α2-adrenergic receptor antagonist) antagonized the water ingestion, Na +, K +, and urine excretion induced by ANG II. Administration of propranolol, a β-adrenergic receptor antagonist increased the Na +, K +, and urine excretion induced by ANG II. Previous treatment with prazosin and yohimbine reduced the pressor responses to ANG II. These results suggest that the adrenergic neurotransmission in the MnPO may actively participate in ANG II-induced dipsogenesis, natriuresis, kaliuresis, diuresis and pressor responses in a process that involves α1-, α2-, and β-adrenoceptors.

Role of alfa2-adrenoceptors in the lateral parabrachial nucleus in the control of body fluid homeostasis

Central α2-adrenoceptors and the pontine lateral parabrachial nucleus (LPBN) are involved in the control of sodium and water intake. Bilateral injections of moxonidine (α2-adrenergic/imidazoline receptor agonist) or noradrenaline into the LPBN strongly increases 0.3 M NaCl intake induced by a combined treatment of furosemide plus captopril. Injection of moxonidine into the LPBN also increases hypertonic NaCl and water intake and reduces oxytocin secretion, urinary sodium, and water excreted by cell-dehydrated rats, causing a positive sodium and water balance, which suggests that moxonidine injected into the LPBN deactivates mechanisms that restrain body fluid volume expansion. Pretreatment with specific α2-adrenoceptor antagonists injected into the LPBN abolishes the behavioral and renal effects of moxonidine or noradrenaline injected into the same area, suggesting that these effects depend on activation of LPBN α2-adrenoceptors. In fluid-depleted rats, the palatability of sodium is reduced by ingestion of hypertonic NaCl, limiting intake. However, in rats treated with moxonidine injected into the LPBN, the NaCl palatability remains high, even after ingestion of significant amounts of 0.3 M NaCl. The changes in behavioral and renal responses produced by activation of α2-adrenoceptors in the LPBN are probably a consequence of reduction of oxytocin secretion and blockade of inhibitory signals that affect sodium palatability. In this review, a model is proposed to show how activation of α2-adrenoceptors in the LPBN may affect palatability and, consequently, ingestion of sodium as well as renal sodium excretion.

beta-ADRENOCEPTORS IN THE MEDIAL AMYGDALOID NUCLEUS MODULATE THE TACHYCARDIAC RESPONSE TO RESTRAINT STRESS IN RATS

In the present study, we investigated the involvement of beta-adrenoceptors in the medial amygdaloid nucleus (MeA) in cardiovascular responses evoked in rats submitted to an acute restraint stress. We first pretreated Wistar rats with the nonselective beta-adrenoceptor antagonist propranolol microinjected bilaterally into the MeA (10, 15, and 20 nmol/100 nL) 10 min before exposure to acute restraint. The pretreatment with propranolol did not affect the blood pressure (BP) increase evoked by restraint. However, it increased the tachycardiac response caused by acute restraint when animals were pretreated with a dose of 15 nmol, without a significant effect on the BP response. This result indicates that beta-adrenoceptors in the MeA have an inhibitory influence on restraint-evoked heart rate (HR) changes. Pretreatment with the selective beta(2)-adrenoceptor antagonist ICI 118,551 (10, 15, and 20 nmol/100 nL) significantly increased the restraint-evoked tachycardiac response after doses of 15 and 20 nmol, an effect that was similar to that observed after the pretreatment with propranolol at a dose of 15 nmol, without a significant effect on the BP response. Pretreatment of the MeA with the selective beta(1)-adrenoceptor antagonist CGP 20712 (10, 15, and 20 nmol/100 nL) caused an opposite effect on the HR response, and a significant decrease in the restraint-evoked tachycardia was observed only after the dose of 20 nmol, without a significant effect on the BP response. Because propranolol is an equipotent antagonist of both beta(1) and beta(2)-adrenoceptors, and opposite effects were observed after the treatment with the higher doses of the selective antagonists ICI 118,551 and CGP 20712, the narrow window in the dose-response to propranolol could be explained by a functional antagonism resulting from the simultaneous inhibition of beta(1) and beta(2)-adrenoceptors by the treatment with propranolol. The present results suggest that beta(2)-adrenoceptors have an inhibitory influence on the restraint-evoked tachycardiac response, whereas beta(1)-adrenoceptors have a facilitatory influence on the restraint-evoked tachycardiac response. (c) 2012 IBRO. Published by Elsevier Ltd. All rights reserved.