Blockade Of Α1 Research Articles

Adreno-melatonin receptor complexes control ion homeostasis and intraocular pressure - their disruption contributes to hypertensive glaucoma.

Often, glaucoma presents with elevated eye hydrostatic pressure, which is regulated by endogenous melatonin. Phenylephrine increases cytoplasmic [Ca2+ ], via α1 -adrenoceptor activation, that is detrimental in glaucoma. The aims of this study were (a) to elucidate the role of melatonin receptors in humour production and intraocular pressure (IOP) maintenance and (b) to identify glaucoma-relevant melatonin-adrenoceptor interactions. Biophysical and proximity ligation assays were performed to identify interactions in heterologous expression systems, in cell lines and in human eyes. Gs /Gi /Gq signalling was investigated in these systems and in cells producing aqueous humour. IOP was determined in a mice model of glaucoma. Retinography and topically pharmacological treatment were performed in control and in glaucomatous mice. α1 -adreno- and melatonin receptors form functional complexes in which the C-terminal tail of the adrenoceptor plays a role. Remarkably, activation of α1 -adrenoceptors in these complexes did not lead to cytosolic Ca2+ increases, suggesting Gs instead of Gq coupling is involved. The number of these complexes significantly decreased in models of glaucoma and, importantly, in human samples from glaucoma patients. This has led to hypothesize that melatonin, a hypotensive agent, plus blockade of α1 -adrenoceptors could normalize pressure in glaucoma. Remarkably, co-instillation of melatonin and prazosin, an α1 -adrenoceptor antagonist, resulted in long-term decreases in IOP in a well-established animal model of glaucoma. The findings are instrumental to understand the physiological function of melatonin in the eye and its potential to address eye pathologies by targeting melatonin receptors and their complexes.

The alpha- and beta-noradrenergic receptors blockade in the dorsal raphe nucleus impairs the panic-like response elaborated by medial hypothalamus neurons

Dorsal raphe nucleus (DRN) neurons are reciprocally connected to the locus coeruleus (LC) and send neural pathways to the medial hypothalamus (MH). The aim of this work was to investigate whether the blockade of α1-, α2- or β-noradrenergic receptors in the DRN or the inactivation of noradrenergic neurons in the LC modify defensive behaviours organised by MH neurons. For this purpose, Wistar male rats received microinjections of WB4101, RX821002, propranolol (α1-, α2- and β-noradrenergic receptor antagonists, respectively) or physiological saline in the DRN, followed 10 min later by MH GABAA receptor blockade. Other groups of animals received DSP-4 (a noradrenergic neurotoxin), physiological saline or only a needle insertion (sham group) into the LC, and 5 days later, bicuculline or physiological saline was administered in the MH. In all these cases, after MH treatment, the frequency and duration of defensive responses were recorded over 15 min. An anterograde neural tract tracer was also deposited in the DRN. DRN neurons send pathways to lateral and dorsomedial hypothalamus. Blockade of α1- and β-noradrenergic receptors in the DRN decreased escape reactions elicited by bicuculline microinjections in the MH. In addition, a significant increase in anxiety-like behaviours was observed after the blockade of α2-noradrenergic receptors in the DRN. LC pretreatment with DSP-4 decreased both anxiety- and panic attack-like behaviours evoked by GABAA receptor blockade in the MH. In summary, the present findings suggest that the norepinephrine-mediated system modulates defensive reactions organised by MH neurons at least in part via noradrenergic receptors recruitment on DRN neurons.

Noradrenergic signaling in the VTA modulates cocaine craving.

Exposure to drug-associated cues evokes drug-seeking behavior and is regarded as a major cause of relapse. Conditional stimulus upregulates noradrenaline (NA) system activity, but the drug-seeking behavior depends particularly on phasic dopamine signaling downstream from the ventral tegmental area (VTA). The VTA dopamine-ergic activity is regulated via the signaling of alpha1 -adrenergic and alpha2 -adrenergic receptors (α1 -ARs and α2 -ARs); thus, the impact of the conditional stimulus on drug-seeking behavior might involve NAergic signaling in the VTA. To date, the role of VTA ARs in regulating cocaine seeking was not studied. We found that cocaine seeking under extinction conditions in male Sprague-Dawley rats was attenuated by intra-VTA prazosin or terazosin-two selective α1 -AR antagonists. In contrast, cocaine seeking was facilitated by intra-VTA administration of the selective α1 -AR agonist phenylephrine as well as α2 -AR antagonist RX 821002, whereas the selective β-AR antagonist propranolol had no effects. In addition, blockade of α1 -AR in the VTA prevented α2 -AR antagonist-induced enhancement of cocaine seeking. Importantly, the potential non-specific effects of the VTA AR blockade on cocaine seeking could be excluded, because none of the AR antagonists influenced sucrose seeking under extinction conditions or locomotor activity in the open field test. These results demonstrate that NAergic signaling potently and selectively regulates cocaine seeking during early cocaine withdrawal via VTA α1 -AR and α2 -AR but not β-AR. Our findings provide new insight into the NAergic mechanisms that underlie cocaine craving.

Dissociable effects of systemic and orbitofrontal administration of adrenoceptor antagonists on yohimbine-induced motor impulsivity

The α2-adrenoceptor antagonist, yohimbine, is commonly used as a pharmacological stressor. Its behavioural effects are typically attributed to elevated noradrenaline release via blockade of central, inhibitory autoreceptors. We have previously reported that yohimbine increases motor impulsivity in rats on the five-choice serial reaction time task (5CSRTT), a cognitive behavioural assessment which measures motor impulsivity and visuospatial attention. Furthermore, this effect depended on cyclic adenomonophosphate (cAMP) signalling via cAMP response element binding (CREB) protein in the orbitofrontal cortex (OFC). However, the role of specific adrenoceptors in this effect is not well-characterised. We therefore investigated whether the pro-impulsive effects of systemic yohimbine could be reproduced by direct administration into the OFC, or attenuated by intra-OFC or systemic administration of prazosin and propranolol—antagonists at the α1- and β-adrenoceptor, respectively. Male Long-Evans rats were trained on the 5CSRTT and implanted with guide cannulae aimed at the OFC. Systemically administered α1- or β-adrenoceptor antagonists attenuated yohimbine-induced increases in premature responding. In contrast, local infusion of yohimbine into the OFC reduced such impulsive responding, while blockade of α1- or β-adrenoceptors within the OFC had no effect on either basal or yohimbine-stimulated motor impulsivity. Direct administration of selective antagonists at the α1-, α2- or β-adrenoceptor into the OFC therefore produce clearly dissociable effects from systemic administration. Collectively, these data suggest that the pro-impulsivity effect of yohimbine can be modulated by adrenergic signalling in brain areas outside of the OFC, in addition to non-adrenergic signalling pathways within the OFC.

Growth restriction induced by chronic prenatal hypoxia affects breathing rhythm and its pontine catecholaminergic modulation

Impaired transplacental supply of oxygen leads to intrauterine growth restriction, one of the most important causes of perinatal mortality and respiratory morbidity. Breathing rhythm depends on the central respiratory network modulated by catecholamines. We investigated the impact of growth restriction, using prenatal hypoxia, on respiratory frequency, on central respiratory-like rhythm, and on its catecholaminergic modulation after birth. At birth, respiratory frequency was increased and confirmed in en bloc medullary preparations, where the frequency of the fourth cervical (C4) ventral root discharge was increased, and in slice preparations containing the pre-Bötzinger complex with an increased inspiratory rhythm. The inhibition of C4 burst discharge observed in pontomedullary preparations was stronger in the growth-restricted group. These results cannot be directly linked by the tyrosine hydroxylase activity increase of A1/C1 and A2/C2 cell groups in the medulla since blockade of α1- and α2-adrenergic receptors did not abolish the difference between both groups. However, in pontomedullary preparations, the stronger inhibition of C4 burst discharge is probably supported by an increased inhibition of A5, a respiratory rhythm inhibitor pontine group of neurons displaying increased tyrosine hydroxylase activity, because blockade of α2-adrenergic receptors abolished the difference between the two groups. Taken together, these results indicate that growth restriction leads to a perturbation of the breathing frequency, which finds, at least in part, its origin in the modification of catecholaminergic modulation of the central breathing network.

α1- and α2-adrenergic receptors in the retrotrapezoid nucleus differentially regulate breathing in anesthetized adult rats.

Norepinephrine (NE) is a potent modulator of breathing that can increase/decrease respiratory activity by α1-/α2-adrenergic receptor (AR) activation, respectively. The retrotrapezoid nucleus (RTN) is known to contribute to central chemoreception, inspiration, and active expiration. Here we investigate the sources of catecholaminergic inputs to the RTN and identify respiratory effects produced by activation of ARs in this region. By injecting the retrograde tracer Fluoro-Gold into the RTN, we identified back-labeled catecholaminergic neurons in the A7 region. In urethane-anesthetized, vagotomized, and artificially ventilated male Wistar rats unilateral injection of NE or moxonidine (α2-AR agonist) blunted diaphragm muscle activity (DiaEMG) frequency and amplitude, without changing abdominal muscle activity. Those inhibitory effects were reduced by preapplication of yohimbine (α2-AR antagonist) into the RTN. Conversely, unilateral RTN injection of phenylephrine (α1-AR agonist) increased DiaEMG amplitude and frequency and facilitated active expiration. This response was blocked by prior RTN injection of prazosin (α1-AR antagonist). Interestingly, RTN injection of propranolol (β-AR antagonist) had no effect on respiratory inhibition elicited by applications of NE into the RTN; however, the combined blockade of α2- and β-ARs (coapplication of propranolol and yohimbine) revealed an α1-AR-dependent excitatory response to NE that resulted in increase in DiaEMG frequency and facilitation of active expiration. However, blockade of α1-, α2-, or β-ARs in the RTN had minimal effect on baseline respiratory activity, on central or peripheral chemoreflexes. These results suggest that NE signaling can modulate RTN chemoreceptor function; however, endogenous NE signaling does not contribute to baseline breathing or the ventilatory response to central or peripheral chemoreceptor activity in urethane-anesthetized rats.

Brain Stem Catecholamines Circuitry: Activation by Alcohol and Role in the Hypothalamic-Pituitary-Adrenal Response to this Drug

Although the stimulatory effect of alcohol on the rat hypothalamic-pituitary-adrenal (HPA) axis is well known, the mechanisms underlying this influence remain poorly understood. In the present study, we tested the hypothesis that brain catecholamines play an important role in this response. As expected, the acute intragastric administration of alcohol to adult male rats elevated plasma adrenocorticotrophic hormone (ACTH) levels and activated hypothalamic corticotrophin-releasing factor neurones. Novel findings pertain to the effect of alcohol on, and the role played by, brain adrenergic circuits. We first observed that alcohol up-regulated c-fos signals in the locus coeruleus, the main noradrenergic brain cell group; and that it activated (nor)adrenergic medullary cells (A1-A2/C1-C3). Evidence for the role played by these catecholaminergic circuits then came from the observation that blockade of α(1) -, but not β-, adrenergic receptors interfered with alcohol-induced ACTH secretion; and that depletion of catecholaminergic input to the paraventricular nucleus (PVN) by the toxin 6-hydroxydopamine significantly decreased the ACTH response to alcohol. Finally, destruction of the A1-A2/C1-C3 region with the immunotoxin anti-dopamine-B-hydroxylase-saporin interfered with the catecholaminergic input to the PVN. Collectively, our work extends our knowledge of the ability of this drug to up-regulate catecholamine circuitry in the rat brain. It also shows that medullary catecholamine innervation of the hypothalamus plays an important role in modulating the stimulatory effect of alcohol on the HPA axis, an effect exerted through activation of α(1) -adrenergic receptors.

Stress-induced epinephrine levels compromise murine dermal fibroblast activity through β-adrenoceptors

Stress-induced catecholamine impairs the formation of granulation tissue acting directly in fibroblast activity; however, the mechanism by which high levels of catecholamines alter the granulation tissue formation is still unclear. Thus, the aim of this study was to investigate how high levels of epinephrine compromise the activity of murine dermal fibroblasts. Dermal fibroblasts isolated from the skin of neonatal Swiss mice were preincubated with α- or β-adrenoceptor antagonists. Thereafter, cells were exposed to physiologically elevated levels of epinephrine or epinephrine plus α- or β-adrenoceptor antagonists, and fibroblast activity was evaluated. The blockade of β1- and β2-adrenoceptors reversed the increase in fibroblast proliferation, ERK 1/2 phosphorylation, myofibroblastic differentiation and the reduction of collagen deposition induced by epinephrine. In addition, the blockade of β3-adrenoceptors reversed the increase in fibroblast proliferation and nitric oxide synthesis as well as the reduction of fibroblast migration, AKT phosphorylation and active matrix metalloproteinase-2 expression induced by epinephrine. However, the blockade of α1- and α2-adrenoceptors did not alter the effects of epinephrine on the activity of murine dermal fibroblasts. In conclusion, high levels of epinephrine directly compromise the activity of neonatal mouse skin fibroblasts through the activation of β1-, β2- and β3-adrenoceptors, but not through α1- and α2-adrenoceptors.

Stimulation of α2-adrenergic receptors in the central nucleus of the amygdala attenuates stress-induced reinstatement of nicotine seeking in rats

Tobacco addiction is a chronic disorder that is characterized by craving for tobacco products, withdrawal upon smoking cessation, and relapse after periods of abstinence. Previous studies demonstrated that systemic administration of α2-adrenergic receptor agonists attenuates stress-induced reinstatement of drug seeking in rats. The aim of the present experiments was to investigate the role of noradrenergic transmission in the central nucleus of amygdala (CeA) in stress-induced reinstatement of nicotine seeking. Rats self-administered nicotine for 14–16 days and then nicotine seeking was extinguished by substituting saline for nicotine. The effect of the intra-CeA infusion of the α2-adrenergic receptor agonists clonidine and dexmedetomidine, the nonselective β1/β2-adrenergic receptor antagonist propranolol, and the α1-adrenergic receptor antagonist prazosin on stress-induced reinstatement of nicotine seeking was investigated. In all the experiments, exposure to footshocks reinstated extinguished nicotine seeking. The administration of clonidine or dexmedetomidine into the CeA attenuated stress-induced reinstatement of nicotine seeking. The administration of propranolol or prazosin into the CeA did not affect stress-induced reinstatement of nicotine seeking. Furthermore, intra-CeA administration of clonidine or dexmedetomidine did not affect operant responding for food pellets. This suggests that the effects of clonidine and dexmedetomidine on stress-induced reinstatement of nicotine seeking were not mediated by motor impairments or sedation. Taken together, these findings indicate that stimulation of α2-adrenergic receptors, but not blockade of α1 or β-adrenergic receptors, in the CeA attenuates stress-induced reinstatement of nicotine seeking. These findings suggest that α2-adrenergic receptor agonists may at least partly attenuate stress-induced reinstatement of nicotine seeking by stimulating α2-adrenergic receptors in the CeA.

Role of adrenoceptor-linked signaling pathways in the regulation of CYP1A1 gene expression

Alpha2-adrenoceptor agents as well as stress affect the activity of several hepatic monoxygenases including those related to CYP1A enzymes. This study was therefore designed to assess the role of central and/or peripheral catecholamines and, in particular, of adrenoceptors in the regulation of B(α)P-induced cytochrome CYP1A1 expression. In order to discriminate the role of central from that of peripheral catecholamines in the regulation of CYP1A1 induction, the effect of central and peripheral catecholamine depletion using reserpine versus only peripheral catecholamine depletion using guanethidine was assessed. By using selected agonists and antagonists, the role of alpha and beta-adrenoceptors in the regulation of CYP1A1 induction was evaluated. The results showed that the central catecholaminergic system has a negative regulatory effect on 7-ethoxyresorufin O-deethylase (EROD) inducibility by benzo(α)pyrene (B(α)P), and that this may be mediated via α1-, α2- and β-adrenoceptors. Specifically, stimulation of α2-adrenoceptors with dexmedetomidine and blockade of α1- or β-adrenoceptors with prazosin or propranolol respectively, resulted in a further increase of EROD inducibility. Adrenoceptors were found to be involved in the regulation of the CYP1A1 gene at mRNA level. Both, reduced noradrenaline release in central nervous system induced with dexmedetomidine and central catecholamine depletion, as well as blockade of central α1-adrenoceptors induced with prazosin, all were associated with up-regulation of CYP1A1 expression. In contrast, stimulation of central beta-adrenoceptors with isoprenaline resulted in a down-regulation of CYP1A1 expression. Our observations indicate that drugs, which stimulate or block adrenoceptors and catecholamine release may lead to complications in drug therapy and modulate the toxicity or carcinogenicity of drugs that are substrates for the CYP1A1.

Insulin secretion and glucose kinetics during exercise with and without pharmacological alpha(1)- and alpha(2)-receptor blockade.

The mechanism behind exercise-induced decreases in plasma insulin concentrations was examined in eight healthy young men. In addition, the influence of specific alpha(1)- and alpha(2)-adrenoceptor blockade on glucose kinetics during exercise was studied. To test the hypothesis that exercise-induced decreases in insulin secretion are mediated via alpha(2)-adrenoceptors, all subjects exercised for 60 min on separate occasions under four conditions: with and without alpha(1)-receptor blockade (1 mg prazosin) and with and without or alpha(2)-receptor blockade (15 mg yohimbine). Glucose kinetics were measured using [3-(3)H]glucose. During exercise with alpha(2)-receptor blockade, the insulin concentration initially increased (first 20 min) then decreased, whereas it continually decreased in the corresponding control experiment. The C-peptide concentration did not change during exercise with alpha(2)-receptor blockade but decreased in the control experiment. During exercise with alpha(1)-receptor blockade and corresponding control experiments, insulin and C-peptide levels always decreased. With alpha(1)-receptor blockade, the glucose concentration increased (first 30 min) and then decreased, whereas it slightly decreased in all other experiments. In addition, with alpha(1)-receptor blockade, the glucose rate of appearance (Ra) increased rapidly (because of higher catecholamine concentrations in alpha(1)-receptor blockade versus control) and the glucose rate of disappearance (Rd) was higher compared with control. During exercise with alpha(2)-receptor blockade, the Ra and Rd were always lower compared with control. Therefore, we conclude that exercise-induced decreases in insulin secretion are mediated via alpha(2)-adrenoceptors and that blockade of alpha(1)- and alpha(2)-adrenoceptors during exercise elicits opposite responses in glucose Ra and Rd.

Survival and mitogenesis of neuroepithelial cells are influenced by noradrenergic but not cholinergic innervation in cultured embryonic rat neopallium

α-Adrenoreceptors are present in the ventricular (VZ) and subventricular (SVZ) zones of the mammalian embryonic forebrain, and contribute towards cell cycle controls in the germinal neuroepithelium [Pabbathi et al., Brain Res. 760 (1997) 22–33.]. Since noradrenaline-containing fibres from the locus coeruleus (LC) and other brainstem nuclei innervate many parts of the CNS from an early stage of embryogenesis, we have used heterochronic cocultures to investigate whether noradrenergic innervation may be the source of trophic support. Dorsal neopallium from E13 rat embryo was cultured in proximity to E15 rostral pons (RP), enabling rapid innervation of the neuroepithelium by differentiated noradrenergic neurons of the LC. The projection of interconnecting fibres into germinal areas of the cortex was established in vitro by retrograde tracing and dopamine β-hydroxylase (DBH) immunocytochemistry. When functional innervation was prevented by transection of axons connecting the explants, the number of apoptotic (TUNEL-positive) cells in the neopallium was increased. Bromodeoxyuridine (BrdU) double-labelling confirmed that germinal cells were amongst those which underwent apoptosis. When cortical explants were innervated by a source of non-monoaminergic (cholinergic) axons from the E18 basal forebrain diagonal band/septum complex (DS), numbers of apoptotic cells were comparable to those in non-innervated cultures. α1 and α2 adrenoreceptor antagonists included in the medium of cortical cocultures innervated by noradrenergic axons reversed the survival-promoting effect of innervation. Blockade of α1 but not of α2 receptors also reduced the numbers of S-phase nuclei in the cortex. The results provide evidence that local delivery of neurotransmitter from embryonic noradrenergic axons terminating in the neocortex can regulate survival and proliferation of neuroepithelial cells.

Cardiac output in rats of different ages during blockade of α1 and β-adrenoceptorsand β-adrenoceptors

In acute experiments on rats of different ages (21, 30, 42 and 70 days), the α1-adrenoblocker prazosin increased the heart rate (HR), and the β-blocker propranolol (Obsidan) infused after prazosin reduced this parameter. Prazosin decreased both stroke volume and cardiac output, blockade of β-adrenoceptors further decreased these parameters. It is concluded that both α1- and β-adrenoceptors are involved in the sympathetic regulation of cardiac output in developing animals.

The Role of Conditioned Taste Aversion in the Hypophagia Induced by Intraperitoneal Epinephrine and Glucose

It has been repeatedly shown that relatively high doses of epinephrine (E) and glucose (G) injected intraperitoneally (ip) produce hypophagia in fasted rats. In the present work we used a conditioned taste aversion (CTA) paradigm in order to test whether this effect could be due to "malaise." We determined the effect on food intake and saccharin preference with the following treatments: (a) E ip 100 and 250 μg/kg; (b) E ip 250 μg/kg with or without previous α1 plus β adrenergic blockade; (c) G ip 3.5 and 4 g/kg. Both doses of E significantly reduced food intake more than 75% but only the high dose produced a significant (50%) reduction in saccharin preference. Blockade of α1 and β adrenergic receptors completely suppressed the E-induced hypophagia but attenuated only slightly the taste aversivon effect. Both doses of G decreased food intake but only the high dose reduced saccharin preference; part of these effects would appear to be due to the high osmolarity of the solution. The present results indicate that E and G may induce CTA in our experimental conditions. However, their hypophagic and aversive effects seem to be elicited by different mechanisms.

Stereochemical requirements for activation and blockade of α1- and α2- adrenoceptors

Stereochemical requirements for activation and blockade of α1- and α2- adrenoceptors

Affective disorders: The catecholamine hypothesis revisited

The α2-adrenoceptor antagonist, yohimbine, is commonly used as a pharmacological stressor. Its behavioural effects are typically attributed to elevated noradrenaline release via blockade of central, inhibitory autoreceptors. We have previously reported that yohimbine increases motor impulsivity in rats on the five-choice serial reaction time task (5CSRTT), a cognitive behavioural assessment which measures motor impulsivity and visuospatial attention. Furthermore, this effect depended on cyclic adenomonophosphate (cAMP) signalling via cAMP response element binding (CREB) protein in the orbitofrontal cortex (OFC). However, the role of specific adrenoceptors in this effect is not well-characterised. We therefore investigated whether the pro-impulsive effects of systemic yohimbine could be reproduced by direct administration into the OFC, or attenuated by intra-OFC or systemic administration of prazosin and propranolol—antagonists at the α1- and β-adrenoceptor, respectively. Male Long-Evans rats were trained on the 5CSRTT and implanted with guide cannulae aimed at the OFC. Systemically administered α1- or β-adrenoceptor antagonists attenuated yohimbine-induced increases in premature responding. In contrast, local infusion of yohimbine into the OFC reduced such impulsive responding, while blockade of α1- or β-adrenoceptors within the OFC had no effect on either basal or yohimbine-stimulated motor impulsivity. Direct administration of selective antagonists at the α1-, α2- or β-adrenoceptor into the OFC therefore produce clearly dissociable effects from systemic administration. Collectively, these data suggest that the pro-impulsivity effect of yohimbine can be modulated by adrenergic signalling in brain areas outside of the OFC, in addition to non-adrenergic signalling pathways within the OFC.