Relationship between imidazoline and alpha2-adrenoceptors involved in the sympatho-inhibitory actions of centrally acting antihypertensive agents

Noradrenergic neuromodulation in ageing and disease

The locus coeruleus (LC) is a small brainstem structure located in the lower pons and is the main source of noradrenaline (NA) in the brain. Via its phasic and tonic firing, it modulates cognition and autonomic functions and is involved in the brain's immune response. The extent of degeneration to the LC in healthy ageing remains unclear, however, noradrenergic dysfunction may contribute to the pathogenesis of Alzheimer's (AD) and Parkinson's disease (PD). Despite their differences in progression at later disease stages, the early involvement of the LC may lead to comparable behavioural symptoms such as preclinical sleep problems and neuropsychiatric symptoms as a result of AD and PD pathology. In this review, we draw attention to the mechanisms that underlie LC degeneration in ageing, AD and PD. We aim to motivate future research to investigate how early degeneration of the noradrenergic system may play a pivotal role in the pathogenesis of AD and PD which may also be relevant to other neurodegenerative diseases.

Paroxysmal sympathetic hyperactivity during traumatic brain injury

Traumatic brain injury (TBI) is one of the leading causes of disability, morbidity, and mortality worldwide. Some of the more common etiologies of TBI include closed head injury, penetrating head injury, or an explosive blast head injury. Neuronal damage in TBI is related to both primary injury (caused by mechanical forces), and secondary injury (caused by the subsequent tissue and cellular damages). Recently, it has been well established that Paroxysmal Sympathetic Hyperactivity (PSH), also known as "Sympathetic Storm", is one of the main causes of secondary neuronal injury in TBI patients. The clinical manifestations of PSH include recurrent episodes of sympathetic hyperactivity characterized by tachycardia, systolic hypertension, hyperthermia, tachypnea with hyperpnea, and frank diaphoresis. Given the diverse manifestations of PSH and its notable impact on the outcome of TBI patients, we have comprehensively reviewed the current evidence and discussed the pathophysiology, clinical manifestations, time of onset and duration of PSH during TBI. This article reviews the different types of head injuries that most commonly lead to PSH, possible approaches to manage and minimize PSH complications in TBI and the current prognosis and outcomes of PSH in TBI patients.

Centrally acting antihypertensives change the psychogenic cardiovascular reactivity

Clonidine (CL) and Rilmenidine (RI) are among the most frequently prescribed centrally acting antihypertensives. Here, we compared CL and RI effects on psychogenic cardiovascular reactivity to sonant, luminous, motosensory, and vibrotactile stimuli during neurogenic hypertension. The femoral artery and vein of Wistar (WT - normotensive) and spontaneously hypertensive rats (SHR) were catheterized before (24 h interval) i.p. injection of vehicle (NaCl 0.9%, control - CT group), CL (10 µg/kg), or RI (10 µg/kg) and acute exposure to luminous (5000 lm), sonant (75 dB sudden tap), motor (180° cage twist), and air-jet (10 L/min - restraint and vibrotactile). Findings showed that: (i) CL or RI reduced the arterial pressure of SHR, without affecting basal heart rate in WT and SHR; (ii) different stimuli evoked pressor and tachycardic responses; (iii) CL and RI reduced pressor response to sound; (iv) CL or RI reduced pressor responses to luminous stimulus without a change in peak tachycardia in SHR; (v) cage twist increased blood pressure in SHR, which was attenuated by CL or RI; (vi) air-jet increased pressure and heart rate; (vii) CL or RI attenuated the pressor responses to air-jet in SHR while RI reduced the chronotropic reactivity in both strains. Altogether, both antihypertensives relieved the psychogenic cardiovascular responses to different stimuli. The RI elicited higher cardioprotective effects through a reduction in air-jet-induced tachycardia.

Responsiveness of α2-adrenoceptor/I1-imidazoline receptor in the rostral ventrolateral medulla to cardiovascular regulation is enhanced in conscious spontaneously hypertensive rat

Stimulation of α2-adrenoceptor/I1-imidazoline receptors in the rostral ventrolateral medulla decreases the blood pressure via sympathoinhibition. However, alteration of receptor responses in genetically hypertensive rats remains unclear. We examined cardiovascular responses of α2-adrenoceptor/I1-imidazoline receptor agonist and antagonists microinjected into the rostral ventrolateral medulla of conscious spontaneously hypertensive rats and normotensive Wistar Kyoto rats. Injection of 2-nmol clonidine-an α2-adrenoceptor/I1-imidazoline receptor agonist-unilaterally into the rostral ventrolateral medulla decreased the blood pressure, heart rate, and renal sympathetic nerve activity; the responses were significantly enhanced in spontaneously hypertensive rats than in Wistar Kyoto rats. Co-injection of 2-nmol 2-methoxyidazoxan (a selective α2-adrenoceptor antagonist) or 2-nmol efaroxan (an I1-receptor antagonist) with 2 nmol of clonidine attenuated the hypotensive and bradycardic effects of clonidine-only injection. Injection of 2-methoxyidazoxan alone increased the blood pressure and heart rate in spontaneously hypertensive rats, but not in Wistar Kyoto rats. These results suggest enhanced responsiveness of α2-adrenoceptor/I1-imidazoline receptors in the rostral ventrolateral medulla of spontaneously hypertensive rats.

Peripheral versus central effect of intravenous moxonidine on rat carotid sinus baroreflex-mediated sympathetic arterial pressure regulation

AIMS

Moxonidine is a centrally acting antihypertensive agent with a selectivity to I₁-imidazoline receptors higher than that to α₂-adrenergic receptors. The present study aimed to quantify a peripheral effect of moxonidine on carotid sinus baroreflex-mediated sympathetic arterial pressure (AP) regulation separately from its central effect.

MAIN METHODS

In eight anesthetized Wistar rats, changes in efferent sympathetic nerve activity (SNA) and AP in response to a carotid sinus pressure input were compared before and during an intravenous administration of moxonidine (100μgkg^-1 bolus followed by a continuous infusion at 200μg·kg^-1·h^-1).

KEY FINDINGS

Moxonidine significantly narrowed the range of the AP response (55.3±5.8 to 39.1±6.1mmHg, P<0.05) without changing the minimum AP (77.2±6.4 to 80.7±5.1mmHg, not significant). In the neural arc, moxonidine reduced the minimum SNA (56.6±5.9 to 29.7±6.2%, P<0.05) without affecting the range of the SNA response (45.3±5.5 to 40.2±5.0%, not significant). In the peripheral arc, moxonidine increased the intercept (3.0±8.5 to 51.1±7.2mmHg, P<0.01) and reduced the slope (1.28±0.06 to 0.92±0.15mmHg/%, P<0.05).

SIGNIFICANCE

Moxonidine increased AP at any given SNA, suggesting that the peripheral vasoconstrictive effect is stronger than generally recognized. The peripheral vasoconstrictive effect of moxonidine may partly offset the vasodilatory effect attained by centrally-mediated sympathoinhibition.

Circulatory effect of TCS-80, a new imidazoline compound, in rats

BACKGROUND

Synthesis and hypotensive properties of centrally acting imidazoline agents: 1-[(imidazolidin-2-yl)imino]-1H-indazole (Marsanidine) and 7-chloro-1-[(4,5-dihydro-1H-imidazol-2-yl)methyl]-1H-indazole (TCS-80) were tested in rats. We have recently synthesized two novel Marsanidine analogues which decrease blood pressure and heart rate in rats: 1-[(4,5-dihydro-1H-imidazol-2-yl)methyl]-1H-indole (TCS-54), and 7-chloro-1-[(4,5-dihydro-1H-imidazol-2-yl)methyl]-1H-indole (TCS-213). Among all these analogues, compound TCS-80 exhibits the highest affinity to I1-imidazoline receptors and the lowest α2/I1 selectivity ratio. The observed cardiovascular effects of the compounds might be mediated through α2-adrenergic and I1-imidazoline receptors and subsequent decrease of the symphathetic nerve activity. The present studies were performed to determine whether α2-adrenergic and/or I1-imidazoline receptors are involved in the decrease of blood pressure and heart rate induced by Marsanidine, TCS-54, TCS-80, and TCS-213 in rats.

METHODS

Anesthetized rats were infused iv with the tested compounds and selective α2-adrenoceptor antagonist, RX821002, or nonselective α2-adrenergic/I1-imidazoline receptor antagonist, Efaroxan. The mean arterial blood pressure and heart rate were monitored directly and continuously throughout the experiment.

RESULTS

Efaroxan inhibited the hypotensive effect of TCS-80 stronger than RX821002. The degree of inhibition of the hypotensive effect of the remaining compounds was similar for both antagonists. The presence of Efaroxan and RX821002 diminished the heart rate decrease induced by all compounds administration, though the influence on the maximal chronotropic effect was attenuated significantly in the TCS-80 and TCS-213 treated animals only.

CONCLUSION

Our results indicate that hypotensive and negative chronotropic activities of all tested compounds are mediated by both the α2-adrenergic and I1-imidazoline receptors. Moreover, the circulatory effect of TCS-80 might be mediated to relatively higher degree by the I1-imidazoline receptors than by the α2-adrenergic ones.

GABA mechanisms of the nucleus of the solitary tract regulates the cardiovascular and sympathetic effects of moxonidine

The antihypertensive drugs moxonidine and clonidine are α2-adrenoceptor and imidazoline (I1) agonists. Previous results from our laboratory have shown that moxonidine can act in the commissural nucleus of the solitary tract (commNTS). In addition, some studies have shown that GABA or glutamate receptor blockade in the RVLM blunted the hypotension produced by these antihypertensive agents in spontaneously hypertensive rats. Therefore, in the present study we verify whether the cardiovascular and sympathetic effects produced by moxonidine in the commNTS are dependent on GABAergic or glutamatergic mechanisms. Mean arterial pressure (MAP) and splanchnic sympathetic nerve activity (sSNA) were recorded in urethane-anesthetized, and artificially-ventilated male Wistar rats (250-350 g). Injection of the GABAA antagonist bicuculline (25 pmol/50 nL) into the commNTS reduced the hypotension as well as the sympathoinhibition elicited by moxonidine. Prior injection of the glutamate receptor antagonist kynurenic acid (2.5 nmol/50 nL) into the commNTS was not effective in reducing the hypotension and sympathoinhibition elicited by moxonidine. Therefore, we conclude that the hypotensive and sympathoinhibitory effects elicited by microinjection of moxonidine into the commNTS are dependent on GABA receptors, but not ionotropic glutamate receptors.

Distinct association between the antagonistic jaw muscle activity levels and cardiac activity during chewing and NREM sleep in the freely moving guinea pigs

The aim of this study was to investigate the changes of the association between cardiac activity and the electromyographic (EMG) level of the antagonistic jaw muscles during chewing and NREM sleep in guinea pigs after systemic clonidine injections. Ten animals were prepared for chronic experiments to monitor sleep, cardiac activity and EMG activity of jaw-closing masseter (MAS) and jaw-opening anterior belly of digastric (ADG) muscles. The recordings were made for ten hours with the injections of saline or clonidine (10 μg/kg, i.p.). Integrated EMG activity of the two muscles and mean heart rate (mHR) were calculated for every 10-s epoch. During the two hours after clonidine injection, the duration of REM sleep and mHR were significantly reduced. During chewing, the high EMG activity level of the two muscles and the activity ratio between the two muscles were not modified although mHR was decreased. During NREM sleep, after clonidine injection, the low EMG activity level at baseline was further decreased by 20-30% in parallel to a decrease of mHR although the heterogeneity of the activity ratio remained unaltered. The results suggest that the maintenance of the activity level for the antagonistic jaw muscles are regulated by the distinct physiological mechanisms reflecting the behavioral states during conscious chewing and unconscious NREM sleep.

In-vitro study of the effect of anti-hypertensive drugs on placental hormones and angiogenic proteins synthesis in pre-eclampsia

INTRODUCTION

Antihypertensive drugs lower the maternal blood pressure in pre-eclampsia (PE) by direct or central vasodilatory mechanisms but little is known about the direct effects of these drugs on placental functions.

OBJECTIVE

The aim of our study is to evaluate the effect of labetolol, hydralazine, α-methyldopa and pravastatin on the synthesis of placental hormonal and angiogenic proteins know to be altered in PE.

DESIGN

Placental villous explants from late onset PE (n = 3) and normotensive controls (n = 6) were cultured for 3 days at 10 and 20% oxygen (O2) with variable doses anti-hypertensive drugs. The levels of activin A, inhibin A, human Chorionic Gonadotrophin (hCG), soluble fms-like tyrosine kinase-1 (sFlt-1) and soluble endoglin (sEng) were measured in explant culture media on day 1, 2 and 3 using standard immunoassays. Data at day 1 and day 3 were compared.

RESULTS

Spontaneous secretion of sEndoglin and sFlt-1 were higher (p < 0.05) in villous explants from PE pregnancies compared to controls. There was a significant time dependent decrease in the secretion of sFlt-1 and sEndoglin in PE cases, which was seen only for sFlt-1 in controls. In both PE cases and controls the placental protein secretions were not affected by varying doses of anti-hypertensive drugs or the different O2 concentration cultures, except for Activin, A which was significantly (p < 0.05) higher in controls at 10% O2.

INTERPRETATION

Our findings suggest that the changes previously observed in maternal serum hormones and angiogenic proteins level after anti-hypertensive treatment in PE could be due to a systemic effect of the drugs on maternal blood pressure and circulation rather than a direct effect of these drugs on placental biosynthesis and/or secretion.

ATP in the locus coeruleus as a modulator of cardiorespiratory control in unanaesthetized male rats

Locus coeruleus (LC) noradrenergic neurons are chemosensitive to CO2 and pH in mammals and amphibians and are involved in the CO2-related drive to breathe. Purinergic neuromodulation in the LC is of particular interest because ATP acts as a neuromodulator in brainstem regions involved in cardiovascular and respiratory regulation, such as the LC. ATP acting on LC P2 receptors influences the release of noradrenaline. Thus, the goal of the present study was to investigate the role of LC purinergic neuromodulation of ventilatory and cardiovascular responses in normocapnic and hypercapnic conditions in unanaesthetized male Wistar rats. We assessed the purinergic modulation of cardiorespiratory systems by microinjecting an ATP P2X receptor agonist [α,β-methylene ATP (α,β-meATP), 0.5 or 1 nmol in 40 nl] and two non-selective P2 receptor antagonists [pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), 0.5 or 1 nmol in 40 nl; and suramin, 1 nmol in 40 nl] into the LC. Pulmonary ventilation (measured by plethysmography), mean arterial pressure (MAP) and heart rate (HR) were determined before and after unilateral microinjection (40 nl) of α,β-meATP, PPADS, suramin or 0.9% saline (vehicle) into the LC. These measurements were made during a 60 min exposure to normocapnic conditions or a 30 min exposure to 7% CO2. Subsequently, animals undergoing pharmacological treatment were subjected to a 30 min exposure to normocapnic conditions as a recovery period. In normocapnic conditions, α,β-meATP did not affect any parameter, whereas PPADS decreased respiratory frequency and increased MAP and HR. Suramin increased MAP and HR but did not change ventilation. Moreover, hypercapnic conditions induced an increase in ventilation and a decrease in HR in all groups. In hypercapnic conditions, α,β-meATP increased ventilation but did not change cardiovascular parameters, whereas PPADS increased MAP but did not alter ventilation, and suramin increased both ventilation and MAP. Thus, our data suggest that purinergic signalling, specifically through P2 receptors, in the LC plays an important role in cardiorespiratory control in normocapnic and hypercapnic conditions in unanaesthetized rats.

Functional neuroanatomy of the central noradrenergic system

The central noradrenergic neurone, like the peripheral sympathetic neurone, is characterized by a diffusely arborizing terminal axonal network. The central neurones aggregate in distinct brainstem nuclei, of which the locus coeruleus (LC) is the most prominent. LC neurones project widely to most areas of the neuraxis, where they mediate dual effects: neuronal excitation by α₁-adrenoceptors and inhibition by α₂-adrenoceptors. The LC plays an important role in physiological regulatory networks. In the sleep/arousal network the LC promotes wakefulness, via excitatory projections to the cerebral cortex and other wakefulness-promoting nuclei, and inhibitory projections to sleep-promoting nuclei. The LC, together with other pontine noradrenergic nuclei, modulates autonomic functions by excitatory projections to preganglionic sympathetic, and inhibitory projections to preganglionic parasympathetic neurones. The LC also modulates the acute effects of light on physiological functions ('photomodulation'): stimulation of arousal and sympathetic activity by light via the LC opposes the inhibitory effects of light mediated by the ventrolateral preoptic nucleus on arousal and by the paraventricular nucleus on sympathetic activity. Photostimulation of arousal by light via the LC may enable diurnal animals to function during daytime. LC neurones degenerate early and progressively in Parkinson's disease and Alzheimer's disease, leading to cognitive impairment, depression and sleep disturbance.

Imidazoline antihypertensive drugs: selective i(1) -imidazoline receptors activation

Involvement of imidazoline receptors (IR) in the regulation of vasomotor tone as well as in the mechanism of action of some centrally acting antihypertensives has received tremendous attention. To date, pharmacological studies have allowed the characterization of three main imidazoline receptor classes, the I(1) -imidazoline receptor which is involved in central inhibition of sympathetic tone to lower blood pressure, the I(2) -imidazoline receptor which is an allosteric binding site of monoamine oxidase B (MAO-B), and the I(3) -imidazoline receptor which regulates insulin secretion from pancreatic β-cells. All three imidazoline receptors represent important targets for cardiovascular research. The hypotensive effect of clonidine-like centrally acting antihypertensives was attributed both to α(2) -adrenergic receptors and nonadrenergic I(1) -imidazoline receptors, whereas their sedative action involves activation of only α(2) -adrenergic receptors located in the locus coeruleus. Since more selective I(1) -imidazoline receptors ligands reduced incidence of typical side effects of other centrally acting antihypertensives, there is significant interest in developing new agents with higher selectivity and affinity for I(1) -imidazoline receptors. The selective imidazoline receptors agents are also more effective in regulation of body fat, neuroprotection, inflammation, cell proliferation, epilepsy, depression, stress, cell adhesion, and pain. New agonists and antagonists with high selectivity for imidazoline receptor subtypes have been recently developed. In the present review we provide a brief update to the field of imidazoline research, highlighting some of the chemical diversity and progress made in the theoretical studies of imidazoline receptor ligands.

The role of central noradrenergic dysregulation in anxiety disorders: evidence from clinical studies

The nature of the noradrenergic dysregulation in clinical anxiety disorders remains unclear. In panic disorder, the predominant view has been that central noradrenergic neuronal networks and/or the sympathetic nervous system was normal in patients at rest, but hyper-reactive to specific stimuli, for example carbon dioxide. These ideas have been extended to other anxiety disorders, which share with panic disorder characteristic subjective anxiety and physiological symptoms of excess sympathetic activity. For example, Generalized Anxiety Disorder is characterized by chronic free-floating anxiety, muscle tension, palpitation and insomnia. It has been proposed that there is chronic central hypersecretion of noradrenaline in Generalized Anxiety Disorder, with consequent hyporesponsiveness of central post-synaptic receptors. With regards to other disorders, it has been suggested that there is noradrenergic involvement or derangement, but a more specific hypothesis has not been enunciated. This paper reviews the evidence for noradrenergic dysfunction in anxiety disorders, derived from indirect measures of noradrenergic function in clinical populations.

Functional neuroanatomy of the noradrenergic locus coeruleus: its roles in the regulation of arousal and autonomic function part I: principles of functional organisation

The locus coeruleus (LC) is the major noradrenergic nucleus of the brain, giving rise to fibres innervating extensive areas throughout the neuraxis. Recent advances in neuroscience have resulted in the unravelling of the neuronal circuits controlling a number of physiological functions in which the LC plays a central role. Two such functions are the regulation of arousal and autonomic activity, which are inseparably linked largely via the involvement of the LC. The LC is a major wakefulness-promoting nucleus, resulting from dense excitatory projections to the majority of the cerebral cortex, cholinergic neurones of the basal forebrain, cortically-projecting neurones of the thalamus, serotoninergic neurones of the dorsal raphe and cholinergic neurones of the pedunculopontine and laterodorsal tegmental nucleus, and substantial inhibitory projections to sleep-promoting GABAergic neurones of the basal forebrain and ventrolateral preoptic area. Activation of the LC thus results in the enhancement of alertness through the innervation of these varied nuclei. The importance of the LC in controlling autonomic function results from both direct projections to the spinal cord and projections to autonomic nuclei including the dorsal motor nucleus of the vagus, the nucleus ambiguus, the rostroventrolateral medulla, the Edinger-Westphal nucleus, the caudal raphe, the salivatory nuclei, the paraventricular nucleus, and the amygdala. LC activation produces an increase in sympathetic activity and a decrease in parasympathetic activity via these projections. Alterations in LC activity therefore result in complex patterns of neuronal activity throughout the brain, observed as changes in measures of arousal and autonomic function.

Structure and inhibition of human diamine oxidase

Humans have three functioning genes that encode copper-containing amine oxidases. The product of the AOC1 gene is a so-called diamine oxidase (hDAO), named for its substrate preference for diamines, particularly histamine. hDAO has been cloned and expressed in insect cells and the structure of the native enzyme determined by X-ray crystallography to a resolution of 1.8 A. The homodimeric structure has the archetypal amine oxidase fold. Two active sites, one in each subunit, are characterized by the presence of a copper ion and a topaquinone residue formed by the post-translational modification of a tyrosine. Although hDAO shares 37.9% sequence identity with another human copper amine oxidase, semicarbazide sensitive amine oxidase or vascular adhesion protein-1, its substrate binding pocket and entry channel are distinctly different in accord with the different substrate specificities. The structures of two inhibitor complexes of hDAO, berenil and pentamidine, have been refined to resolutions of 2.1 and 2.2 A, respectively. They bind noncovalently in the active-site channel. The inhibitor binding suggests that an aspartic acid residue, conserved in all diamine oxidases but absent from other amine oxidases, is responsible for the diamine specificity by interacting with the second amino group of preferred diamine substrates.

Effect of antihypertensive therapy with alpha methyldopa on levels of angiogenic factors in pregnancies with hypertensive disorders

Background

Antihypertensive drugs are believed to lower blood pressure in pre-eclampsia by direct or central vasodilatory mechanisms. However, they could also act by decreasing production of anti-angiogenic proteins involved in the pathophysiology of hypertension and proteinuria in pre-eclampsia (PE). The aim of our study was to evaluate the impact of antihypertensive therapy with alpha methyldopa on maternal circulating levels and placental production of soluble fms-like tyrosine kinase 1 (sFlt-1), soluble endoglin (sEng), vascular endothelial growth factor (VEGF) and placental growth factor (PlGF) in hypertensive disorders of pregnancy.

Methodology/Principal Findings

In a study conducted at University College Hospital and the Homerton University Hospital in London, we recruited 51 women with PE, 29 with gestational hypertension (GH), and 80 matched normotensive controls. Eight (16%) of the women with PE had severe disease. Placental samples were obtained from a further 48 women (14 PE, 10 GH and 24 matched controls). Serum levels of angiogenic factors were measured before and 24–48 hours after commencing antihypertensive therapy with alpha methyldopa for clinical indications. The same parameters were measured in placental extracts. In both PE (P<0.0001) and GH (P<0.05), serum sFlt-1 was increased and PlGF reduced at all gestations (P<0.001) compared to controls. Serum sEng levels were also increased in PE. Placental concentration of sFlt-1 and sEng was significantly higher in women with PE compared to controls and women with GH (P<0.0001). The concentration of PlGF was significantly lower in the placental tissue of women with PE compared to GH (P = 0.008). Antihypertensive treatment was associated with a significant fall in serum and placental content of sFlt1 and sEng in PE only.

Conclusions

Our data suggest that alpha methyldopa may have a specific effect on placental and/or endothelial cell function in pre-eclampsia patients, altering angiogenic proteins.

Sympathoexcitation of moxonidine in the caudal ventrolateral medulla is dependent on I1-imidazoline receptors in anesthetized rats

Moxonidine is a second-generation centrally acting antihypertensive drug that has a high affinity for I(1)-imidazoline receptors (I(1)R). The caudal ventrolateral medulla (CVLM), an important region involved in cardiovascular activity, contains binding sites for centrally acting drugs. Our study aimed to determine the effects of moxonidine injected into the CVLM on cardiovascular activity in anesthetized rats. Unilateral microinjection of moxonidine (0.4 and 4 nmol) into the CVLM dose-dependently increased blood pressure (BP) by 8+/-2 and 18+/-2 mmHg and renal sympathetic nerve activity (RSNA) by 19+/-3 and 48+/-5% without modifying heart rate. Microinjection of the I(1)R/alpha(2)-adrenoceptor antagonist efaroxan (4 nmol) into the CVLM produced significant decreases in baseline BP and RSNA, but also completely abolished the increases in BP (2+/-1 versus 18+/-2 mmHg, P<0.01) and RSNA (3+/-2 versus 45+/-10%, P<0.01) evoked by subsequent injection of moxonidine (4 nmol). However, prior injection of yohimbine (500 pmol), a selective antagonist of alpha(2)-adrenoceptors, into the CVLM had no significant (P>0.05) effect on the moxonidine-induced increase in BP (18+/-2 versus 17+/-3 mmHg) and RSNA (45+/-10 versus 42+/-7%). The current data suggest that moxonidine injection into the CVLM has an excitatory effect on cardiovascular activity, which is mediated by an I(1)R dependent mechanism.

Contribution of imidazoline receptors and alpha2-adrenoceptors in the rostral ventrolateral medulla to sympathetic baroreflex inhibition by systemic rilmenidine

OBJECTIVES

To determine whether the hypotensive and sympathetic baroreflex inhibition by rilmenidine administered systemically are mediated via imidazoline receptors in the rostral ventrolateral medulla (RVLM).

METHODS

Initial dose-response curves to rilmenidine were determined in urethane anaesthetized rabbits. Effects of a single intravenous dose of rilmenidine (445 microg/kg) on the renal sympathetic nerve activity (RSNA) baroreflex were examined before and after microinjection into the RVLM of the mixed imidazoline/alpha2-adrenoceptor antagonist idazoxan and the alpha2-adrenoceptor antagonist 2-methoxyidazoxan (2-MI).

RESULTS

Intravenous administration of rilmenidine lowered mean arterial pressure and RSNA, inhibited the RSNA baroreflex range by 33% and shifted the baroreflex curve to the left. Idazoxan injected into the RVLM reversed the hypotension and completely restored the baroreflex curve at doses that did not affect the hypotension produced by the selective alpha2-adrenoceptor agonist alpha-methylnoradrenaline. The alpha2-adrenoceptor antagonist, 2-MI also reversed the rilmenidine sympatho-inhibition suggesting that alpha2-adrenoceptors are activated as well.

CONCLUSIONS

The results of the present study show that the hypotensive and sympatho-inhibitory actions of systemic rilmenidine are primarily mediated via imidazoline receptors in the RVLM. However, alpha2-adrenoceptors are also involved, probably as a direct result of the imidazoline receptor action.

Antihypertensive responses elicited by central moxonidine in rats: possible role of nitric oxide

In the present study, we investigated the effects of pretreatment with NG-nitro-L-arginine methyl ester (L-NAME) (nitric oxide synthase inhibitor) injected intravenously (IV) on the hypotension, bradycardia, and vasodilation produced by moxonidine (alpha2-adrenergic/imidazoline receptor agonist) injected into the fourth brain ventricle (4th V) in rats submitted to acute hypertension that results from baroreflex blockade by bilateral injections of kynurenic acid (kyn, glutamatergic receptor antagonist) into the nucleus of the solitary tract (NTS) or in normotensive rats. Male Wistar rats (n=5 to 7/group) anesthetized with IV urethane (1.0 g kg(-1) of body weight) and alpha-chloralose (60 mg kg(-1) of body weight) were used. Bilateral injections of kyn (2.7 nmol 100 nL(-1)) into the NTS increased baseline mean arterial pressure (148 +/- 11 mm Hg, vs. control: 102 +/- 4 mm Hg) and baseline heart rate (417 +/- 11 bpm, vs. control: 379 +/- 6 bpm). Moxonidine (20 nmol microL(-1)) into the 4th V reduced mean arterial pressure and heart rate to similar levels in rats treated with kyn into the NTS (68 +/- 9 mm Hg and 359 +/- 7 bpm) or in control normotensive rats (66 +/- 7 mm Hg and 362 +/- 8 bpm, respectively). The pretreatment with L-NAME (25 micromol kg, IV) attenuated the hypotension produced by moxonidine into the 4th V in rats treated with kyn (104 +/- 6 mm Hg) or in normotensive rats (95 +/- 8 mm Hg), without changing bradycardia. Moxonidine into the 4th V also reduced renal, mesenteric, and hindquarter vascular resistances in rats treated or not with kyn into the NTS and the pretreatment with L-NAME IV reduced these effects of moxonidine. Therefore, these data indicate that nitric oxide mechanisms are involved in hypotension and mesenteric, renal, and hindquarter vasodilation induced by central moxonidine in normotensive and in acute hypertensive rats.

Site-dependent inhibition of neuronal c-jun in the brainstem elicited by imidazoline I1 receptor activation: Role in rilmenidine-evoked hypotension

Clonidine (a mixed alpha2-adrenoceptor and imidazoline I1 receptor agonist)-evoked hypotension was associated with dissimilar reductions in c-jun gene expression in the rostral ventrolateral medulla (RVLM) and the nucleus tractus solitarius (NTS) in normotensive rats. In the present study, we investigated the relative contribution of the alpha2-adrenoceptor vs. the imidazoline I1 receptor to the reduction in c-jun gene expression in these two brainstem areas. In conscious spontaneously hypertensive rats (SHRs), equihypotensive doses of three centrally acting hypotensive drugs with different selectivity for the two receptors were administered intracisternally (4 microl) to limit their actions to the brain. As a control, a similar hypotensive response was elicited by i.v. hydralazine. Clonidine (0.5 microg), or alpha-methylnorepinephrine (alpha-MNE, 4 microg), a highly selective alpha2-adrenoceptor agonist, similarly reduced c-jun mRNA expression in the NTS and rostral ventrolateral medulla. In contrast, a similar hypotensive response (-37+/-3.5 mm Hg) caused by the selective imidazoline I1 receptor agonist rilmenidine (25 microg) was associated with reduction in c-jun mRNA expression in the rostral ventrolateral medulla, but not in the NTS. Further, intra-rostral ventrolateral medulla rilmenidine (40 nmol) reduced c-Jun protein expression in rostral ventrolateral medulla and blood pressure and both responses were antagonized by selective imidazoline I1 receptor (efaroxan, 4 nmol), but not alpha2-adrenoceptor (SK&F 86466, 10 nmol) blockade. These results suggest: (1) the c-jun containing neurons in the brainstem are involved in the centrally mediated hypotension elicited by centrally acting antihypertensive agents, and (2) the alpha2-adrenoceptor modulates c-jun gene expression in the NTS and rostral ventrolateral medulla implicated in centrally mediated hypotension, and (3) the imidazoline I1 receptor mediated inhibition of c-jun gene expression in the rostral ventrolateral medulla, but not in the NTS, contributes to the centrally mediated hypotension by the second generation drugs.

Effect of Clonidine on Cardiac Norepinephrine Spillover in Isolated Rat Heart

The purpose of this study is to determine the effect of clonidine on cardiac norepinephrine spillover utilizing an isolated rat heart preparation with attached cardiac sympathetic nerves. Following a 20-minute stabilization period, the sympathetic ganglion for each heart preparation was electrically stimulated with 10V and 2 Hz for 30 seconds (S1: 60 pulses). Heart rate, left ventricular developed pressure, and coronary perfusion pressure was allowed to return to baseline and the perfusate was randomly switched to Krebs buffer containing one of two treatments: placebo or clonidine (1 microM). After 10 minutes of treatment, the sympathetic ganglion was again electrically stimulated with 10V and 2 Hz for 30 seconds (S2: 60 pulses). The perfusate exiting the heart before, during, and after each electrical stimulation was collected for the determination of cardiac norepinephrine spillover. Clonidine administration significantly reduced cardiac norepinephrine spillover by approximately 50% (P < 0.05) and was associated with a 36% reduction in heart rate (P < 0.05). These findings provide evidence that clonidine can directly suppress NE spillover from cardiac sympathetic nerve terminals. Thus, suppression of cardiac NE by clonidine may be due to stimulation of presynaptic alpha2-adrenergic receptors or imidazoline subtype I receptors located on cardiac sympathetic nerve terminals. Results from our study demonstrate a reduction in cardiac NE spillover by clonidine and provide additional evidence that it can directly suppress peripheral sympathetic activity in that our results were obtained utilizing an isolated perfused heart preparation with attached cardiac sympathetic nerves devoid of any CNS input.

Importance of imidazoline-preferring receptors in the cardiovascular actions of chronically administered moxonidine, rilmenidine and clonidine in conscious rabbits

OBJECTIVE

To determine the involvement of central imidazoline receptors in the cardiovascular actions of the chronically administered antihypertensive agents moxonidine, rilmenidine and clonidine.

DESIGN AND METHODS

In 21 rabbits with implanted fourth-ventricular catheters, we investigated the central effects of three cumulative doses of an I(1)-imidazoline/alpha(2)-adrenoceptor antagonist, efaroxan, and of an alpha(2)-adrenoceptor antagonist, 2-methoxyidazoxan (2-MI), on the changes in blood pressure and heart rate (HR) elicited by chronic subcutaneous administration of moxonidine, rilmenidine and clonidine, after 1 and 3 weeks of treatment. A low, medium and high dose of 2-MI was matched to three doses of efaroxan, such that each produced equal reversal of the hypotension induced by fourth-ventricular alpha-methyldopa and hence produced a similar degree of alpha(2)-adrenoceptor blockade.

RESULTS

Clonidine and moxonidine, at doses of 1 mg/kg per day, and rilmenidine at 5 mg/kg per day, produced sustained reductions in mean arterial pressure of 13 +/- 3, 15 +/- 2 and 13 +/- 2 mmHg, respectively over the 3-week treatment period, but did not alter HR. Central administration of efaroxan on day 9 and day 23 of treatment produced a greater increase in blood pressure than did 2-MI with all three antihypertensive agents. Blood pressure reached levels that were significantly above the original control values. By contrast, the alpha(2)-adrenoceptor antagonist 2-MI only induced a rebound blood pressure effect in clonidine- and to a lesser extent in rilmenidine-treated rabbits. Both efaroxan and 2-MI produced a similar degree of tachycardia in moxonidine-, rilmenidine- and clonidine-treated animals.(2)

CONCLUSIONS

The greater effect of efaroxan compared to the alpha(2)-adrenoceptor antagonist 2-MI suggests that the hypotension induced by chronic subcutaneous administration of moxonidine, rilmenidine and clonidine is mediated predominantly via an action on central imidazoline receptors. Furthermore, all agents showed a propensity to produce rebound hypertension with imidazoline receptor blockade. However, only clonidine showed a rebound phenomenon when challenged by acute central alpha(2)-adrenoceptor blockade

Serotonin(3) receptor stimulation in the nucleus tractus solitarii activates non-catecholaminergic neurons in the rat ventrolateral medulla

The present study was performed to determine whether or not the increased arterial pressure triggered by 5-HT(3) receptor stimulation in the nucleus tractus solitarii and underlain by a sympathoexcitation is associated with the activation of ventromedullary cells known to be involved in vascular regulation, i.e. the C1 and A1 catecholaminergic cells. For this purpose, double immunohistochemical labeling for tyrosine hydroxylase and c-fos protein was performed all along the ventrolateral medulla after microinjection of 1-(m-chlorophenyl)-biguanide, a selective and potent 5-HT(3) receptor agonist, into the nucleus tractus solitarii of alpha-chloralose/urethane-anaesthetized rats. This treatment produced a significant elevation of arterial pressure ( approximately +35 mm Hg). Concomitantly, a significant increase in the number of c-fos expressing neurons was observed in the rostral ventrolateral medulla (+63%), in particular in its most anterior part (+78%), and in the medullary region surrounding the caudal part of the facial nucleus (+91%). Retrograde labeling with gold-horseradish peroxidase complex showed that at least some of these activated c-fos expressing cells project to the spinal cord. However, the number of double-stained neurons, i.e. c-fos and tyrosine hydroxylase positive neurons, did not increase at any level of the ventrolateral medulla. In contrast, under the same alpha-chloralose/urethane anesthesia, systemic infusion of sodium nitroprusside appeared to produce a hypotension and a marked increase in the density of such double c-fos and tyrosine hydroxylase expressing cells in the rostral ventrolateral medulla and the caudal medullary region surrounding the caudal part of the facial nucleus. These data indicate that medullary catecholaminergic C1 and A1 neurons are not involved in the pressor effect elicited by 5-HT(3) receptor stimulation in the nucleus tractus solitarii. However, this 5-HT(3) receptor-mediated effect is clearly associated with the excitation of (non-catecholaminergic) neurons within the pressor region of the ventral medulla.

Clonidine toxicity revisited

The incidence of clonidine overdose is increasing, yet there is a paucity of new information regarding treatment options for clonidine toxicity. Reported treatment approaches vary widely, demonstrating the lack of science on which current treatment is based. Available research needs to be reassessed. Neurotransmitters, receptors, endogenous opioids, and baseline sympathetic tone determine the clinical response to clonidine as well as the potential response to drug therapy following clonidine overdose. This article reviews aspects of clonidine toxicity that need to be further investigated. Multicenter research trials will be required to evaluate new treatment options.

The chronic effect of rilmenidine on heart rate variability in patients with mild hypertension

The purpose of this study was to evaluate the chronic effect of rilmenidine on time domain indexes of heart rate variability in patients with mild hypertension. Twenty patients (12 males, eight females; mean age, 47 yr; age range, 38-55 yr), with untreated and newly diagnosed mild hypertension were studied. There was no evidence of diseases other than hypertension. All patients received 1 mg of rilmenidine once daily. If the diastolic blood pressure was still greater than 90 mm Hg after 4 weeks of active treatment, the dose was increased to 2 mg once daily. Twenty-four hour ambulatory electrocardiograms were recorded before, and 4 and 12 weeks after the start of therapy. Time domain parameters of heart rate variability were calculated. Rilmenidine therapy determined a marked decrease in blood pressure. At 4 weeks, rilmenidine induced a significant reduction in systolic and diastolic blood pressure and a further reduction was observed after 12 weeks. At 4 and 12 weeks, time domain parameters of heart rate variability and heart rate were not significantly different in the data obtained before therapy. In conclusion, this study demonstrated that the administration of rilmenidine to patients with mild essential hypertension induced significant reductions in blood pressure, without any significant changes in time domain parameters of heart rate variability.

Imidazoline antihypertensive drugs: a critical review on their mechanism of action

It was long thought that the prototypical centrally acting antihypertensive drug clonidine lowers sympathetic tone by activating alpha(2)-adrenoceptors in the brain stem. Supported by the development of two new centrally acting drugs, rilmenidine and moxonidine, the imidazoline hypothesis evolved recently. It assumes the existence of a new group of receptors, the imidazoline receptors, and attributes the sympathoinhibition to activation of I(1) imidazoline receptors in the medulla oblongata. This review analyzes the mechanism of action of clonidine-like drugs, with special attention given to the imidazoline hypothesis. Two conclusions are drawn. The first is that the arguments against the imidazoline hypothesis outweigh the observations that support it and that the sympathoinhibitory effects of clonidine-like drugs are best explained by activation of alpha(2)-adrenoceptors. The second conclusion is that this class of drugs lowers sympathetic tone not only by a primary action in cardiovascular regulatory centres in the medulla oblongata. Peripheral presynaptic inhibition of transmitter release from postganglionic sympathetic neurons contributes to the overall sympathoinhibition.

Hypotensive and sedative effects of clonidine injected into the rostral ventrolateral medulla of conscious rats

We examined the effects of clonidine injected unilaterally into the rostral ventrolateral medulla (RVLM) of conscious, unrestrained rats. We also examined whether the local alpha(2)-adrenoceptor mechanism contributed to the action of clonidine injected into the RVLM. Injection of clonidine but not vehicle solution significantly decreased the mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) in conscious, unrestrained rats as well as in propofol-anesthetized rats. The frequency of natural behavior was significantly lower after clonidine injection than after vehicle injection. The depressor and sympathoinhibitory responses were significantly larger in the propofol-anesthetized rats than in the conscious rats. Coinjection of a selective alpha(2)-adrenoceptor antagonist, 2-methoxyidazoxan, with clonidine into the RVLM significantly attenuated the depressor, bradycardiac, sympathoinhibitory, and sedative effects of clonidine injected alone. In conclusion, clonidine injected into the RVLM decreased MAP, HR, and RSNA and caused sedation in conscious, unrestrained rats. The action of clonidine in the RVLM was at least partly mediated by alpha(2)-adrenoceptor mechanisms.

Activation of alpha(2)-receptors in the rostral ventrolateral medulla evokes natriuresis by a renal nerve mechanism

The contribution of alpha(2)-receptor mechanisms in the rostral ventrolateral medulla (RVLM) in mediating the enhanced renal excretory responses evoked by the intravenous infusion of the alpha(2)-receptor agonist xylazine was examined in ketamine-anesthetized rats. In ketamine-anesthetized rats, the bilateral microinjection of the alpha(2)-receptor antagonist yohimbine into the RVLM significantly reduced the enhanced levels of urine flow rate (V) and urinary sodium excretion (UNaV) produced by xylazine. In contrast, microinjection of yohimbine into the RVLM of chronically bilaterally renal-denervated rats significantly reduced the xylazine-evoked diuretic, but not natriuretic, response. In separate ketamine-anesthetized rats, intravenous xylazine infusion produced a near complete inhibition of renal sympathetic nerve activity (RSNA). The subsequent microinjection of yohimbine into the RVLM reversed this neural response and concurrently decreased V and UNaV. Together, these results indicate that during intravenous infusion, xylazine activates alpha(2)-receptor mechanisms in the RVLM to selectively promote urinary sodium excretion by a renal nerve-dependent pathway. In contrast, activation of alpha(2)-receptor in the RVLM affects the renal handling of water by a pathway independent of the renal nerves. This latter pathway may involve an interaction with other brain regions involved in antidiuretic hormone release (e.g., paraventricular nucleus of the hypothalamus).

Rostral ventral medulla 5-HT1A receptors selectively inhibit the somatosympathetic reflex

The role of the 5-hydroxytryptamine (5-HT1A) receptors in the rostral ventrolateral medulla (RVLM) on somatosympathetic, baroreceptor, and chemoreceptor reflexes was examined in anesthetized rats. Microinjection of the selective 5-HT1A agonist 8-hydroxy-di-n-propylamino tetralin (8-OH-DPAT) decreased arterial blood pressure and splanchnic sympathetic nerve activity (SNA). Electrical stimulation of the hindlimb evoked early and late excitatory sympathetic responses. Bilateral microinjection in the RVLM of 8-OH-DPAT markedly attenuated both the early and late responses. This potent inhibition of the somatosympathetic reflex persisted even after SNA and arterial blood pressure returned to preinjection levels. Preinjection of the selective 5-HT1A antagonist NAN-190 in the RVLM blocked the sympathoinhibitory effect of 8-OH-DPAT and attenuated the inhibitory effect on the somatosympathetic reflex. 8-OH-DPAT injected in the RVLM did not affect baroreceptor or chemoreceptor reflexes. Our findings suggest that activation of 5-HT1A receptors in the RVLM exerts a potent, selective inhibition on the somatosympathetic reflex.

Relative importance of rostral ventrolateral medulla in sympathoinhibitory action of rilmenidine in conscious and anesthetized rabbits

The pressor region of the rostral ventrolateral medulla (RVLM) is a critical site in the sympathoinhibitory action of imidazoline receptor agonists as shown by studies in anesthetized animals. The aim of this study was to compare the importance of the RVLM in mediating the inhibitory action of rilmenidine on renal sympathetic nerve activity (RSNA) and arterial pressure in urethane-anesthetized rabbits (n = 11) and in conscious, chronically instrumented rabbits (n = 6). Bilateral microinjection of rilmenidine (4 nmol in 100 nl) into the RVLM caused a greater decrease in resting arterial pressure in anesthetized animals (-19 mm Hg) than in conscious animals (-8 mm Hg). By contrast, the decrease in resting RSNA evoked by rilmenidine was similar in conscious (-27%) and anesthetized (-36%) rabbits. Furthermore, rilmenidine microinjection into the RVLM was equally effective in inhibiting the RSNA baroreflex in both groups of animals. The upper plateau of the RSNA baroreflex decreased by 37% and 42%, and gain decreased by 41% and 44% after rilmenidine treatments in conscious and anesthetized rabbits, respectively. We conclude that the RVLM plays an equally important role in the inhibitory action of rilmenidine on RSNA in conscious and anesthetized rabbits either at rest or during baroreflex responses. A relatively moderate effect of rilmenidine on arterial pressure in conscious, chronically instrumented rabbits may relate to a lower level of sympathetic drive compared with anesthetized animals.

Single-cell RT-PCR as a tool to study gene expression in central and peripheral autonomic neurones

In studies of the central and peripheral autonomic nervous system, it has become increasingly important to be able to investigate mRNA expression patterns within specific neuronal populations. Traditionally, the identification of mRNA species in discrete populations of cells has relied upon in situ hybridization. An alternative, relatively simple procedure is 'multiplex' reverse transcription-polymerase chain reaction (RT-PCR), conducted on single neurons after their in vitro isolation. Multiplex single-cell RT-PCR can be used to examine the expression of multiple genes within individual cells, and can be combined with electrophysiological, pharmacological and anatomical (retrograde labelling) studies. This review focuses on a number of key aspects of this approach, methodology, and both the advantages and the limitations of the technique. We also provide specific examples of work performed in our laboratory, examining the expression of alpha 2-adrenergic receptors in catecholaminergic cells of the rat brainstem and adrenal medulla. The application of single-cell RT-PCR to future studies of the autonomic nervous system will hopefully provide information on how physiological and pathological conditions affect gene expression in autonomic neurones.

Prototypical imidazoline-1 receptor ligand moxonidine activates alpha2-adrenoceptors in bulbospinal neurons of the RVL

Moxonidine is an antihypertensive drug that lowers sympathetic vasomotor tone by stimulating either alpha2-adrenergic (alpha2-AR) or imidazoline I1 receptors within the rostral ventrolateral medulla (RVL). In this study, we investigated the effects of moxonidine (10 microM) on RVL neurons in brain stem slices of neonatal rats. We recorded mainly from retrogradely labeled RVL bulbospinal neurons (putative presympathetic neurons) except for some extracellular recordings. Prazosin was used to block alpha1-adrenoceptors. Moxonidine inhibited the extracellularly recorded discharges of all spontaneously active RVL neurons tested (bulbospinal and unidentified). This effect was reversed or blocked by the selective alpha2-AR antagonist SKF 86466 (10 microM). In contrast, the I1 imidazoline ligand AGN 192403 (10 microM) had no effect on the spontaneous activity. In whole cell recordings (holding potential -70 mV), moxonidine produced a small and variable outward current (mean 7 pA). This current was observed in both tyrosine hydroxylase-immunoreactive and other bulbospinal neurons and was blocked by SKF 86466. Excitatory postsynaptic currents (EPSCs) evoked by focal electrical stimulation were isolated by incubation with gabazine and strychnine, and inhibitory postsynaptic currents (IPSCs) were isolated with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Moxonidine reduced the amplitude of the evoked EPSCs (EC(50) = 1 microM; 53% inhibition at 10 microM) but not their decay time constant (5.6 ms). The effect of moxonidine on EPSCs persisted in barium (300 microM) and was reduced approximately 80% by SKF 86466. Moxonidine also reduced the amplitude of evoked IPSCs by 63%. In conclusion, moxonidine inhibits putative RVL presympathetic neurons both presynaptically and postsynaptically. All observed effects in the present study are consistent with an alpha2-AR agonist activity of moxonidine.

Recent advances in imidazoline receptor research: ligands--localization and isolation--signaling--functional and clinical studies

In this article we outline the highlights of this special issue of the journal containing a series of articles covering many aspects of current interest in the field of imidazoline receptor research. This volume is the result of an international symposium held in September 1997 in Melbourne as an official satellite of the inaugural meeting of the International Society of Autonomic Neurosciences held in Cairns, Australia. A wide range of topics relating to imidazoline receptors were canvassed, including endogenous and synthetic ligands, identification and localisation of binding sites, putative transduction mechanisms and experimental and clinical functional studies.