Effects of imidazoline antihypertensive drugs on sympathetic tone and noradrenaline release in the prefrontal cortex

Presynaptic Adrenoceptors

Presynaptic α2-adrenoceptors are localized on axon terminals of many noradrenergic and non-noradrenergic neurons in the peripheral and central nervous systems. Their activation by exogenous agonists leads to inhibition of the exocytotic release of noradrenaline and other transmitters from the neurons. Most often, the α2A-receptor subtype is involved in this inhibition. The chain of molecular events between receptor occupation and inhibition of the exocytotic release of transmitters has been determined. Physiologically released endogenous noradrenaline elicits retrograde autoinhibition of its own release. Some clonidine-like α2-receptor agonists have been used to treat hypertension. Dexmedetomidine is used for prolonged sedation in the intensive care; It also has a strong analgesic effect. The α2-receptor antagonist mirtazapine increases the noradrenaline concentration in the synaptic cleft by interrupting physiological autoinhibion of release. It belongs to the most effective antidepressive drugs. β2-Adrenoceptors are also localized on axon terminals in the peripheral and central nervous systems. Their activation leads to enhanced transmitter release, however, they are not activated by endogenous adrenaline.

Moxonidine ameliorates cardiac injury in rats with metabolic syndrome by regulating autophagy

AIMS

Reduced cardiac autophagy, ischemic injury, sympathetic overactivity, and apoptosis all contribute to metabolic syndrome (MetS)-associated cardiovascular risks. NR4A2, an orphan nuclear receptor NR4A family member, induces autophagy while suppressing apoptosis in myocardial infarction. Moxonidine, a sympathoinhibitor imidazoline1 receptor (I1R) agonist, has beneficial metabolic and hemodynamic effects; however, whether autophagy and/or NR4A2 signaling are involved in moxonidine's cardiovascular effects via I1R activation, is unknown, and is the aim of this study.

MATERIALS AND METHODS

To induce MetS, rats were fed 3 % salt in their diet and 10 % fructose in their drinking water for 12 weeks. MetS-rats were given either moxonidine (6 mg/kg/day, gavage), efaroxan (I1R antagonist, 0.6 mg/kg/day, i.p), both treatments, or vehicles for the last two weeks. Blood pressure, lipid profile, and glycemic control were evaluated. Histopathological examination, circulating cardiac troponin I (c-TnI), proinflammatory interleukin-6 (IL-6), apoptosis (active caspase-3 and Fas-immunostaining), interstitial fibrosis [transforming growth factor-β1 (TGF-β1), Mallory's trichrome staining], and extracellular matrix remodeling [matrix metalloproteinase-9 (MMP-9)], were used to assess cardiac pathology. Cardiac NR4A2 and its downstream factor, p53, as well as autophagic flux markers, SQSTM1/p62, LC3, and Beclin-1 were also determined.

KEY FINDINGS

Moxonidine significantly ameliorated MetS-induced metabolic and hemodynamic derangements and the associated cardiac pathology. Moxonidine restored NR4A2 and p53 myocardial levels and enhanced autophagic flux via modulating SQSTM1/p62, LC3, and Beclin-1. Efaroxan reversed the majority of the moxonidine-induced improvements.

SIGNIFICANCE

The current study suggests that autophagy modulation via I1R activation is involved in moxonidine-mediated cardiac beneficial effects in MetS.

Validation of the xylazine/ketamine anesthesia test as a predictor of the emetic potential of pharmacological compounds in rats

The xylazine/ketamine anesthesia test is widely used as a predictor of the emetic potential of pharmacological compounds in rats. An emetic reflex is usually triggered by the emetic center, which is populated with many different chemoreceptors. Inhibition of the α2 adrenergic receptor (α2 receptor) is involved in the initiation of the emetic reflex, and this is the key mechanism behind the xylazine/ketamine anesthesia test. In this study, we attempt to validate this test as a predictor of the emetic potential of pharmacological compounds. Furthermore, it was investigated whether an anti-emetic potential of pharmacological compounds could be assessed within this test as well. Rats were anesthetized with a combination of low doses of ketamine and xylazine, and subsequently treated with PDE4 inhibitor rolipram, α2 receptor antagonist yohimbine, α2 receptor agonist clonidine, tricyclic antidepressant imipramine, D2-receptor antagonist haloperidol, or 5-HT3 receptor antagonist (and anti-emetic drug) ondansetron. We were able to successfully reproduce the reduction in anesthesia time after rolipram or yohimbine treatment, as found in previous studies and has been suggested to be indicative of emetic properties of these treatments is humans. Furthermore, clonidine shortened anesthesia duration whereas imipramine and haloperidol lengthened anesthesia duration. Ondansetron was unable to rescue the reduction in duration of anesthesia induced by either rolipram or yohimbine. Altogether, the xylazine/ketamine anesthesia test is a reliable measure for α2 receptor antagonism. However, it may not be appropriate to assess emesis independent of this mechanism.

Animal models of tic disorders: a translational perspective

Tics are repetitive, sudden movements and/or vocalizations, typically enacted as maladaptive responses to intrusive premonitory urges. The most severe tic disorder, Tourette syndrome (TS), is a childhood-onset condition featuring multiple motor and at least one phonic tic for a duration longer than 1 year. The pharmacological treatment of TS is mainly based on antipsychotic agents; while these drugs are often effective in reducing tic severity and frequency, their therapeutic compliance is limited by serious motor and cognitive side effects. The identification of novel therapeutic targets and development of better treatments for tic disorders is conditional on the development of animal models with high translational validity. In addition, these experimental tools can prove extremely useful to test hypotheses on the etiology and neurobiological bases of TS and related conditions. In recent years, the translational value of these animal models has been enhanced, thanks to a significant re-organization of our conceptual framework of neuropsychiatric disorders, with a greater focus on endophenotypes and quantitative indices, rather than qualitative descriptors. Given the complex and multifactorial nature of TS and other tic disorders, the selection of animal models that can appropriately capture specific symptomatic aspects of these conditions can pose significant theoretical and methodological challenges. In this article, we will review the state of the art on the available animal models of tic disorders, based on genetic mutations, environmental interventions as well as pharmacological manipulations. Furthermore, we will outline emerging lines of translational research showing how some of these experimental preparations have led to significant progress in the identification of novel therapeutic targets for tic disorders.

Brain nitric oxide production by a proline-rich decapeptide from Bothrops jararaca venom improves baroreflex sensitivity of spontaneously hypertensive rats

Baroreflex sensitivity is disturbed in many people with cardiovascular diseases such as hypertension. Brain deficiency of nitric oxide (NO), which is synthesized by NO synthase (NOS) in the citrulline-NO cycle (with argininosuccinate synthase (ASS) activity being the rate-limiting step), contributes to impaired baroreflex. We recently showed that a decapeptide isolated from Bothrops jararaca snake venom, denoted Bj-PRO-10c, exerts powerful and sustained antihypertensive activity. Bj-PRO-10c promoted vasodilatation dependent on the positive modulation of ASS activity and NO production in the endothelium, and also acted on the central nervous system, inducing the release of GABA and glutamate, two important neurotransmitters in the regulation of autonomic systems. We evaluated baroreflex function using the regression line obtained by the best-fit points of measured heart rate (HR) and mean arterial pressure (MAP) data from spontaneously hypertensive rats (SHRs) treated with Bj-PRO-10c. We also investigated molecular mechanisms involved in this effect, both in vitro and in vivo. Bj-PRO-10c mediated an increase in baroreflex sensitivity and a decrease in MAP and HR. The effects exerted by the peptide include an increase in the gene expression of endothelial NOS and ASS. Bj-PRO-10c-induced NO production depended on intracellular calcium fluxes and the activation of a G(i/o)-protein-coupled metabotropic receptor. Bj-PRO-10c induced NO production and the gene expression of ASS and endothelial NOS in the brains of SHRs, thereby improving baroreflex sensitivity. Bj-PRO-10c may reveal novel approaches for treating diseases with impaired baroreflex function.

The central nervous system as target for antihypertensive actions of a proline-rich peptide from Bothrops jararaca venom

Pyroglutamyl proline-rich oligopeptides, present in the venom of the pit viper Bothrops jararaca (Bj-PROs), are the first described naturally occurring inhibitors of the angiotensin I-converting enzyme (ACE). The inhibition of ACE by the decapeptide Bj-PRO-10c (<ENWPHPQIPP) and other Bj-PROs was classically used to explain the pharmacological effects of these venom peptides in mammals resulting in a decrease of blood pressure. Recent studies, however, suggest that ACE inhibition alone is not sufficient for explaining the antihypertensive actions exerted by these peptides. In this study, we show that intracerebroventricular injection of Bj-PRO-10c induced a significant reduction of mean arterial pressure (MAP) together with a decrease of heart rate (HR) in spontaneously hypertensive rats, indicating that Bj-PRO-10c may act on the central nervous system. In agreement with its supposed neuronal action, this peptide dose-dependently evoked elevations of intracellular calcium concentration ([Ca(2+)](i)) in primary culture from postnatal rat brain. The N-terminal sequence of the peptide was not essential for induction of calcium fluxes, while any changes of C-terminal Pro or Ile residues affected Bj-PRO-10c's activity. Using calcium imaging by confocal microscopy and fluorescence imaging plate reader analysis, we have characterized Bj-PRO-10c-induced [Ca(2+)](i) transients in rat brain cells as being independent from bradykinin-mediated effects and ACE inhibition. Bj-PRO-10c induced pertussis toxin-sensitive G(i/o)-protein activity mediated through a yet unknown receptor, influx and liberation ofcalcium from intracellular stores, as well as reduction of intracellular cAMP levels. Bj-PRO-10c promoted glutamate and GABA release that may be responsible for its antihypertensive activity and its effect on HR.

Intravenous administration of clonidine reduces intraocular pressure and alters ocular blood flow

AIM

To evaluate the effect of intravenously administered clonidine on ocular blood flow in healthy volunteers.

METHODS

A randomised, double-masked, placebo-controlled, two-way crossover study was performed in 12 healthy young volunteers. Clonidine (0.2 microg/kg/min) or placebo was administered intravenously over 10 minutes. The effects of clonidine were studied at baseline and up to 150 minutes after infusion. Ocular haemodynamics were measured using laser Doppler flowmetry, laser Doppler velocimetry and a retinal vessel analyser.

RESULTS

Clonidine significantly decreased mean arterial pressure (MAP) and intraocular pressure (IOP). Calculated ocular perfusion pressure decreased significantly by -8.7+/-8.7% after infusion of clonidine (p<0.01 vs placebo). Retinal arterial diameters increased by +4.4+/-2.7% (p = 0.012 vs placebo), whereas no significant change was observed in retinal veins. Red blood cell velocity decreased by -16+/-14% (p<0.01 vs placebo) after infusion of clonidine. Hence, calculated retinal blood flow decreased by -14+/-12% (p = 0.033 vs placebo). Choroidal blood flow increased by +18+/-19% (p<0.01 vs placebo) and optic nerve head blood flow increased by +16+/-23% (p = 0.046 vs placebo) 30 minutes after administration of clonidine but both returned to baseline thereafter.

CONCLUSION

The short-time increase in choroidal and optic nerve head blood flow indicates a transient vasodilatory effect of clonidine due to an unknown mechanism. The decrease in retinal blood flow indicates clonidine-induced vasoconstriction in the retinal microvasculature.

Low Catechol-O-methyltransferase Activity in the Brain and Blood Pressure Regulation

Catecholamines (CAs) are important hormones in regulating blood pressure both in centrally and peripheral sympathetic nerve endings. Production of CAs, release and inactivation are three components to regulate CAs level. We have reported that the inactivation of CAs by catechol-O-methyltransferase (COMT) in the liver is important in high blood pressure in spontaneously hypertensive rats (SHR). In the present study, we investigated central role of COMT in hypertension. We investigated COMT activities in cerebral cortex, cerebellum, hippocampus, brain stem, hypophysis, and hypothalamus of SHR and Wistar-Kyoto (WKY) rats. COMT activities were assessed by measuring normetanephrine with the use of norepinephrine as an endogenous substrate. Membrane-bound COMT activities in cerebral cortex were significantly reduced in SHR (19.1+/-1.8 pmol/min/mg protein) compared with WKY rats (25.0+/-3.3 pmol/min/mg protein). The ratio of concentrations of normetanephrine/norepinephrine in cerebral cortex was also lower in SHR than in WKY rats. Our results suggest that there is an association between MB-COMT in cerebral cortex and blood pressure regulation.

Neuronal Norepinephrine Responses of the Rostral Ventrolateral Medulla and Nucleus Tractus Solitarius Neurons Distinguish the I1- from the α2-Receptor-Mediated Hypotension in Conscious SHRs

We tested the hypothesis that the I1 receptor mediates the reduction in rostral ventrolateral medulla (RVLM) neuronal norepinephrine (NE; index of sympathetic activity) that leads to hypotension independent of other brainstem areas or the alpha2-adrenergic receptor. To this end, we developed a model that permitted measurement of real-time changes in neuronal NE in the RVLM or nucleus tractus solitarius (NTS) along with blood pressure and heart rate in the conscious SHR in response to localized microinjections of selective I1 (rilmenidine) or alpha2-adrenergic (alpha-methylnorepinephrine; alpha-MNE) agonist versus the mixed I1/alpha2 agonist clonidine. To further support the hypothesis, we investigated the effects of localized selective alpha2- (SK&F86466) or I1 (efaroxan) blockade on the reductions in neuronal NE and blood pressure elicited by intra-RVLM rilmenidine. In the latter experiment, changes in RVLM neuronal c-Fos (another marker of sympathetic neural activity) were also investigated. Intra-RVLM rilmenidine (40 nmol) or clonidine (1 nmol) similarly reduced RVLM NE and blood pressure; these responses were approximately 2-fold greater than those elicited by the pure alpha2-adrenergic agonist alpha-MNE (10 nmol). By contrast, intra-NTS rilmenidine or clonidine had no effect on NTS NE or blood pressure versus significant reductions in both parameters by alpha-MNE. Intra-RVLM rilmenidine decreased c-Fos expression, and these responses were abolished by efaroxan but not by SK&F 86466. These findings suggest: (1) in the RVLM, I1-receptor signaling suppresses cardiovascular neuron activity, which leads to lowering of blood pressure; (2) although the alpha2-adrenergic receptor in the RVLM serves a similar role, it does not exert a tonic neuronal inhibitory effect and is not essential, as a downstream signaling entity, for the I1-evoked neurobiological effects in the brainstem. The potential confounding effects of anesthetics on the I1 and/or alpha2 receptor-mediated neuronal and cardiovascular responses were circumvented in the present study.

Search for an endogenous cannabinoid-mediated effect in the sympathetic nervous system

Activation of CB(1) cannabinoid receptors by exogenous agonists causes presynaptic inhibition of neurotransmitter release from axon terminals. In the central nervous system, presynaptic CB(1) receptors can also be activated by endogenous cannabinoids (endocannabinoids) released from postsynaptic neurons. Except in the vas deferens, there is no indication of endocannabinoid-mediated presynaptic inhibition in the sympathetic nervous system. The aim of the present study was to search for such inhibition in pithed rats. Artificial sympathetic tone was established by continuous electrical stimulation of preganglionic sympathetic axons. The CB(1) cannabinoid receptor antagonist rimonabant (0.5 and 2 mg kg(-1) i.v.) did not change blood pressure, heart rate or plasma noradrenaline concentration. Since activation of Galpha(q/11) protein-coupled receptors enhances endocannabinoid synthesis in the central nervous system, we attempted to stimulate endocannabinoid production by infusion of arginine vasopressin and phenylephrine (both activate Galpha(q/11) protein-coupled receptors). Rimonabant (2 mg kg(-1) i.v.) did not change blood pressure, heart rate or plasma noradrenaline concentration during infusion of phenylephrine or vasopressin. In the final series of experiments we verified that an exogenous cannabinoid agonist produces sympathoinhibition. The synthetic CB(1)/CB(2) receptor agonist WIN55212-2 (0.1 and 1 mg kg(-1) i.v.) markedly lowered blood pressure and plasma noradrenaline concentration in pithed rats with electrically stimulated sympathetic outflow. In contrast, in pithed rats with a pressor infusion of noradrenaline, WIN55212-2 did not change blood pressure or heart rate. The results verify that activation of peripheral presynaptic CB(1) receptors inhibits noradrenaline release from sympathetic nerve terminals. The lack of effect of the CB(1) receptor antagonist rimonabant indicates that, even under conditions favouring endocannabinoid synthesis, endocannabinoid-mediated presynaptic inhibition is not operating in the sympathetic nervous system of the pithed rat.

Central effects of the cannabinoid receptor agonist WIN55212-2 on respiratory and cardiovascular regulation in anaesthetised rats

1 The primary aim was to study the central respiratory effects of cannabinoids (CB). To this end, the cannabinoid receptor agonist WIN55212-2 was injected into the cisterna magna of urethane-anaesthetised rats and changes in respiratory parameters were observed. The secondary aim was to observe the centrally elicited cardiovascular actions of WIN55212-2. Involvement of opioid mechanisms in the central effects of WIN55212-2 was also studied. 2 Intracisternal (i.c.) application of WIN55212-2 (1, 3, 10 and 30 microg kg(-1)) dose-dependently decreased the respiratory rate and minute volume. Tidal volume was slightly increased, whereas peak inspiratory flow remained unchanged. In addition, WIN55212-2 increased mean arterial pressure and the plasma noradrenaline concentration and decreased heart rate. 3 I.c. injection of WIN55212-3 (1, 3, 10 and 30 microg kg(-1)), an enantiomer of WIN55212-2 lacking affinity for cannabinoid receptors, elicited no effects. All effects of WIN55212-2 were prevented by the CB1 receptor antagonist SR141716 (2 mg kg(-1) i.v.). I.c. administration of the opioid receptor agonist DAMGO (0.1, 0.3, 1 and 3 microg kg(-1)) markedly lowered the respiratory rate, tidal volume, minute volume and peak inspiratory flow. These effects were attenuated by the opioid receptor antagonist naloxone (0.2 mg kg(-1) i.v.). In contrast, naloxone did not affect the respiratory and cardiovascular effects of i.c. administered WIN55212-2. 4 Our results show that activation of CB1 cannabinoid receptors in the brain stem depresses respiration and enhances sympathetic tone and cardiac vagal tone. Opioid mechanisms are not involved in these central cannabinoid effects.

Moxonidine and rilmenidine injected into the medial septal area reduces the salivation induced by pilocarpine

We determined the effects of moxonidine and rilmenidine 20 nmol (alpha(2)-adrenergic and imidazoline receptor agonists) injected into the medial septal area (MSA) on the pilocarpine-induced salivation, when injected intraperitoneally (i.p.), of male Holtzman rats weighing 250-300 g, with stainless-steel cannula implanted into the MSA. The rats were anesthetized with zoletil 50 mg kg(-1) b.wt. (tiletamine chloridrate 125.0 mg and zolazepan chloridrate 125.0 mg) into quadriceps muscle intramuscularly (IM), saliva was collected using pre-weighed small cotton balls inserted in the animal's mouth. The pre-treatment with moxonidine injected into the MSA reduced the salivation induced by pilocarpine (1 mg kg(-1)) injected i.p. (12+/-3 mg min(-1)) vs. control (99+/-9 mg min(-1)). The pre-treatment with rilmenidine 40 nmol also reduced the salivation induce by pilocarpine injected i.p. (20+/-5 mg min(-1)) vs. control (94+/-7 mg min(-1)). Idazoxan 40 nmol (imidazoline receptor antagonist) injected into the MSA previous to moxonidine and rilmenidine partially blocked the effect of moxonidine and totally blocked the rilmenidine effect in pilocarpine-induced salivation injected i.p. (60+/-8 and 95+/-10 mg min(-1), respectively). Yohimbine 40 nmol (alpha(2)-adrenergic receptor antagonist) injected into the MSA previously to moxonidine and rilmenidine partially blocked the moxonidine effect but produced no change on the rilmenidine effect on i.p. pilocarpine-induced salivation (70+/-6 and 24+/-6 mg min(-1), respectively). Injection of these alpha(2)-adrenergic and imidazoline agonists and antagonists agents i.p. produced no change on i.p. pilocarpine-induced salivation. These results show that central, but not peripheral, injection of alpha(2)-adrenergic and imidazoline agonists' agents inhibit pilocarpine-induced salivation. Idazoxan, an imidazoline receptor antagonist, totally inhibits the rilmenidine effect and partially inhibits the moxonidine effect on pilocarpine-induced salivation. Yohimbine produced no change on rilmenidine effect but partially inhibited the moxonidine effect. Both of these antagonists when injected into the MSA previous to pilocarpine i.p. potentiated the sialogogue effect of pilocarpine. The results suggest that alpha(2)-adrenergic/imidazoline receptor of the MSA when stimulated blocked pilocarpine-induced salivation in rats when injected intraperitonially. These receptors of the medial septal area have an inhibitory mechanism on salivary secretion.

Modulation of the baroreceptor reflex by alpha 2A-adrenoceptors: a study in alpha 2A knockout mice

1. Our objective was to determine whether alpha(2A)-adrenoceptors modulate the baroreceptor reflex. The efficacy of the reflex was evaluated by measuring the spontaneous blood pressure and heart rate variability at rest and the heart rate responses to evoked changes in blood pressure. Experiments were carried out in conscious, unrestrained, and anaesthetized alpha(2A)-adrenoceptor-deficient (alpha(2A)-KO) mice and WT mice. 2. In conscious alpha(2A)-KO mice, the spontaneous blood pressure variability was greater, and the spontaneous heart rate variability was lower than in conscious WT mice. This was also observed in anaesthetized animals. 3. The reflex bradycardia after intravenous injection of phenylephrine was greatly attenuated in conscious alpha(2A)-KO compared to conscious WT mice; the baroreceptor reflex gain (ratio maximal change in heart rate/maximal change in mean arterial pressure) was decreased by 40%. 4. Similar results were obtained when reflex bradycardia was elicited by intra-arterial volume loading of conscious WT and alpha(2A)-KO mice. The baroreceptor reflex gain upon volume loading was also low in anaesthetized alpha(2A)-KO mice. 5. The reflex tachycardia evoked by intravenous sodium nitroprusside injection was also significantly less in alpha(2A)-KO mice as compared to WT, conscious as well as anaesthetized; the baroreceptor reflex gains were decreased by 50 and 65%, respectively. 6. Direct stimulation of cardiac beta-adrenoceptors by the agonist isoprenaline produced similar cardioacceleration in alpha(2A)-KO and WT animals. 7. Our results show that the baroreceptor reflex function is impaired in mice lacking alpha(2A)-adrenoceptors. We conclude that central alpha(2A)-adrenoceptors facilitate the reflex response to both loading and unloading of the arterial baroreceptors.

The peripheral sympathetic nervous system is the major target of cannabinoids in eliciting cardiovascular depression

Our objective was to identify the sites of interaction of cannabinoids with cardiovascular sympathetic regulation in the rat. Effects on sympathetic tone were first determined in anaesthetised animals following i.v. administration of the drugs. Central effects were evaluated in anaesthetised rats receiving microinjections of cannabinoids into brain stem nuclei. Peripheral effects were identified in pithed rats with electrically stimulated sympathetic outflow. In anaesthetised and artificially ventilated rats, i.v. injection of the cannabinoid agonists WIN55212-2 and CP55940 decreased mean arterial pressure, heart rate and the plasma noradrenaline concentration. These effects were antagonized by the CB(1) cannabinoid receptor antagonist SR141716A. The bradycardia was abolished by the muscarinic acetylcholine receptor antagonist methylatropine. The decreases in mean arterial pressure and heart rate caused by cannabinoids in ventilated rats were much less pronounced than in spontaneously breathing rats. Microinjection of WIN55212-2 into the nucleus tractus solitarii had no effect. Microinjected into the rostral ventrolateral medulla oblongata, WIN55212-2 lowered mean arterial pressure slightly without changing other parameters. In pithed rats, WIN55212-2 inhibited the increases in mean arterial pressure, heart rate and the plasma noradrenaline concentration evoked by electrical stimulation of the sympathetic outflow. Our results show that activation of CB(1) cannabinoid receptors induces sympathoinhibition and enhancement of cardiac vagal tone, leading to hypotension and bradycardia. Presynaptic inhibition of noradrenaline release from terminals of postganglionic sympathetic neurons is the major component of the sympathoinhibition, but an effect in the rostral ventrolateral medulla oblongata may also contribute. The cannabinoid-evoked cardiovascular depression depends strongly on the respiratory state of the animals.