Mediation of the hypotensive action of systemic clonidine in the rat by alpha 2-adrenoceptors

Agmatine induced NO dependent rat mesenteric artery relaxation and its impairment in salt-sensitive hypertension

l-Arginine and its decarboxylated product, agmatine are important mediators of NO production and vascular relaxation. However, the underlying mechanisms of their action are not understood. We have investigated the role of arginine and agmatine in resistance vessel relaxation of Sprague-Dawley (SD) and Dahl salt-sensitive hypertensive rats. Second or 3rd-order mesenteric arterioles were cannulated in an organ chamber, pressurized and equilibrated before perfusing intraluminally with agonists. The vessel diameters were measured after mounting on the stage of a microscope fitted with a video camera. The gene expression in Dahl rat vessel homogenates was ascertained by real-time PCR. l-Arginine initiated relaxations (EC50, 5.8±0.7mM; n=9) were inhibited by arginine decarboxylase (ADC) inhibitor, difluoromethylarginine (DFMA) (EC50, 18.3±1.3mM; n=5) suggesting that arginine-induced vessel relaxation was mediated by agmatine formation. Agmatine relaxed the SD rat vessels at significantly lower concentrations (EC50, 138.7±12.1μM; n=22), which was compromised by l-NAME (l-N(G)-nitroarginine methyl ester, an eNOS inhibitor), RX821002 (α-2 AR antagonist) and pertussis toxin (G-protein inhibitor). The agmatine-mediated vessel relaxation from high salt Dahl rats was abolished as compared to that from normal salt rats (EC50, 143.9±23.4μM; n=5). The α-2A AR, α-2B AR and eNOS mRNA expression was downregulated in mesenteric arterioles of high-salt treated Dahl hypertensive rats. These findings demonstrate that agmatine facilitated the relaxation via activation of α-2 adrenergic G-protein coupled receptor and NO synthesis, and this pathway is compromised in salt-sensitive hypertension.

Agmatine: biological role and therapeutic potentials in morphine analgesia and dependence

Agmatine is an amine that is formed by decarboxylation of L-arginine by the enzyme arginine decarboxylase (ADC) and hydrolyzed by the enzyme agmatinase to putrescine. Agmatine binds to several target receptors in the brain and has been proposed as a novel neuromodulator. In animal studies, agmatine potentiated morphine analgesia and reduced dependence/withdrawal. While the exact mechanism is not clear, the interactions with N-methyl-D-aspartate (NMDA) receptors, alpha2-adrenergic receptors, and intracellular cyclic adenosine monophosphate (cAMP) signaling have been proposed as possible targets. Like other monoamine transmitter molecules, agmatine is rapidly metabolized in the periphery and has poor penetration into the brain, which limits the use of agmatine itself as a therapeutic agent. However, the development of agmatinase inhibitors will offer a useful method to increase endogenous agmatine in the brain as a possible therapeutic approach to potentiate morphine analgesia and reduce dependence/withdrawal. This review provides a succinct discussion of the biological role/therapeutic potential of agmatine during morphine exposure/pain modulation, with an extensive amount of literature cited for further details.

Effect of agmatine on the development of morphine dependence in rats: potential role of cAMP system

Agmatine is an endogenous amine derived from arginine that potentiates morphine analgesia and blocks symptoms of naloxone-precipitated morphine withdrawal in rats. In this study, we sought to determine whether treatment with agmatine during the development of morphine dependence inhibits the withdrawal symptoms and that the effect is mediated by cAMP system. Exposure of rats to morphine for 7 days resulted in marked naloxone-induced withdrawal symptoms and agmatine treatment along with morphine significantly decreasing the withdrawal symptoms. The levels of cAMP were markedly increased in morphine-treated rat brain slices when incubated with naloxone and this increase was significantly reduced in rats treated with morphine and agmatine. The induction of tyrosine hydroxylase after morphine exposure was also reduced in locus coeruleus when agmatine was administered along with morphine. We conclude that agmatine reduces the development of dependence to morphine and that this effect is probably mediated by the inhibition of cAMP signaling pathway during chronic morphine exposure.

Recent advances in the identification of a 1- and a 2-adrenoceptor subtypes: therapeutic implications

The cloning of multiple subtypes of both alpha1- and alpha2-adrenoceptors has renewed interest in the therapeutic application of agents interacting with these receptors. Effort has primarily been directed towards the design of uroselective alpha1-adrenoceptor antagonists for the treatment of benign prostatic hyperplasia (BPH). Evidence is accumulating for the involvement of a novel alpha1-adrenoceptor, designated as alpha1L-adrenoceptor, in alpha1-adrenoceptor-mediated smooth muscle contraction in prostatic and other urogenital tissues. While several antagonists showing a high degree of uroselectivity in animal models have been identified, their clinical superiority over the currently available alpha1-adrenoceptor antagonists has not yet been demonstrated. It is possible that the interaction with alpha1-adrenoceptors, as yet uncharacterised subtypes, at non-prostatic sites contributes to the therapeutic activity of this drug class in BPH. The alpha1-adrenoceptor subtypes involved in the control of vascular tone are currently being evaluated, and the profile of interaction with the various alpha1-adrenoceptor subtypes may play a key role in the efficacy of cardiovascular drugs such as carvedilol. Alpha2-adrenoceptor agonists are now being employed for a variety of therapeutic applications, most involving actions on receptors within the central nervous system (CNS). These agents are useful in the treatment of hypertension, glaucoma, opiate withdrawal and attention deficit hyperactivity disorder (ADHD), and as analgesics and adjuncts to general anaesthesia. While subtype selectivity has not yet been applied to the design of new alpha2-adrenoceptor agonists for these applications, recent gene mutation/knock-out experiments have identified the alpha2-subtypes involved in some of these actions, and optimisation of a therapeutic profile may be possible. Furthermore, the design of agents combining affinities for multiple adrenoceptor subtypes, or the combination of a specific adrenoceptor affinity profile with another pharmacological action, may offer advantages over molecules selective for an individual adrenoceptor subtype.

Rat clonidine mydriasis model: imidazoline receptors are not involved

The clonidine mydriasis model in rats has been widely applied in preclinical research to characterize alpha(2)-adrenoceptor antagonistic properties of drugs. The present study was undertaken to pharmacologically determine if imidazoline I(1) receptors are also involved in this model system. Sigmoid dose-response curves for pupillary dilation were produced in pentobarbital anesthetized rats by intravenous administration of increasing doses of agonists (guanabenz for alpha(2)-adrenoceptors, clonidine for both alpha(2)-adrenoceptors and imidazoline I(1) receptors, and rilmenidine for imidazoline I(1) receptors). Two antagonists (RS 79948 for alpha(2)-adrenoceptors and efaroxan for imidazoline I(1) receptors) were used to antagonize the mydriasis elicited by those three agonists, with antagonistic potencies calculated. In additional experiments, we examined the effect of the selective imidazoline I(1) receptor antagonist, AGN 192403, on clonidine-induced mydriasis. The results showed that pupillary response curves elicited by guanabenz, clonidine and rilmenidine were competitively antagonized by both RS 79948 (0.03-1 mg/kg) and efaroxan (0.03-1 mg/kg) in a dose-related fashion. The potencies of either antagonist against the three agonists were not significantly different. AGN 192403 (5 mg/kg) did not significantly shift the clonidine mydriasis curve. These results suggest that imidazoline I(1) receptors are not functionally involved in the rat clonidine mydriasis model and support this in vivo system as a useful model for studies of alpha(2)-adrenoceptors.

Clonidine preconditioning decreases infarct size and improves neurological outcome from transient forebrain ischemia in the rat

Clonidine, a alpha(2)-adrenergic receptor agonist, has been demonstrated to be neuroprotective when administered during ischemia. It is not known whether clonidine can precondition brain against ischemia. We examined this possibility using a transient forebrain ischemia model. Rats received 40 microg/kg of clonidine intraperitoneally at 6, 18, 24 and 72 h as well as 1 week before the forebrain ischemia that was produced by bilateral common carotid arterial occlusion combined with hemorrhagic hypotension to mean arterial pressure 50 mm Hg for 30 min. They were intubated and ventilated with a gas mixture of 1.0% halothane in 30% O(2)/balance air during the procedure. Rats that received clonidine at 6, 18 and 24 h before the ischemia had significantly improved neurological deficit scores and reduced infarct sizes evaluated 3 days after the ischemia. A selective alpha(2)-adrenoceptor antagonist, yohimbine, abolished the neuroprotective effects of clonidine preconditioning. We conclude that there is time window for clonidine preconditioning to be neuroprotective and that alpha(2)-adrenoceptors are important in mediating clonidine preconditioning-induced neuroprotection.

Synthesis and biological evaluation of new 2-(4,5-dihydro-1H-imidazol-2-yl)-3,4-dihydro-2H-1,4-benzoxazine derivatives

2-(4,5-Dihydro-1H-imidazol-2-yl)-3,4-dihydro-2H-1,4-benzoxazine derivatives and tricyclic analogues with a fused additional ring on the nitrogen atom of the benzoxazine moiety have been prepared and evaluated for their cardiovascular effects as potential antihypertensive agents. The imidazoline ring was generated by reaction of the corresponding ethyl ester with ethylenediamine. Affinities for imidazoline binding sites (IBS) I(1) and I(2) and alpha(1) and alpha(2) adrenergic receptors were evaluated as well as the effects on mean arterial blood pressure (MAP) and heart rate (HR) of spontaneously hypertensive rats. With few exceptions the most active compounds on MAP were those with high affinities for IBS and alpha(2) receptor. Among these, compound 4h was the most interesting and is now, together with its enantiomers, under complementary pharmacological evaluation.

Non-adrenergic exploratory behavior induced by moxonidine at mildly hypotensive doses

Moxonidine is a centrally-active imidazoline compound with preferential affinity for imidazoline receptors (IR) over alpha(2)-adrenoceptors (alpha(2)AR). Clinically, moxonidine has proven advantageous for treating hypertension over pure alpha(2)-adrenergic agonists (i.e., guanabenz) due to its lowered incidence of sedative side effects. The present experiments reveal divergent behavioral effects of low doses of moxonidine and guanabenz in C57Bl/6 mice in an exploratory arena. Low-dose moxonidine (0.05 mg kg(-1) i.p.) elicited an increase in novel object contacts (+36%) and more movement into central space (+56%; P<0.01) compared to saline-injected controls; whereas guanabenz induced only dose-responsive sedative-like behaviors in the same paradigm. Yet, the two agonists were indistinguishable in terms of blood pressure changes over a similar dose range (0.025-0.1 mg kg(-1) i.p.) in consciously free-moving mice (Delta mean+/-S.E.M.=-12.3+/-3.2 mm Hg for moxonidine versus -13.5+/-1.9 mm Hg for guanabenz). As expected of alpha(2)AR involvement, the sedative-like effects of guanabenz were completely blocked by pretreatment with the non-imidazoline alpha(2)AR-antagonist, SKF86466 (0.5 or 1.0 mg kg(-1) i.p.). However, the pro-exploratory effects of low doses of moxonidine (0.05 or 0.1 mg kg(-1)) were not antagonized by SKF86466. These results suggest that moxonidine acts preferentially through a non-adrenergic mechanism, possibly IR-mediated, to elicit pro-exploratory behavior.

Rilmenidine produces mydriasis in cats by stimulation of CNS alpha2-adrenoceptors

1. Experiments were undertaken to determine if the imidazoline/alpha2-adrenoceptor agonist, rilmenidine, would produce mydriasis in cats and, if so, to delineate its site of action and determine if this effect is mediated by imidazoline receptors or alpha2-adrenoceptors. 2. Rilmenidine produced dose-related pupillary dilator responses in pentobarbital anaesthetized cats that were independent of sympathetic innervation to the iris but were dependent upon intact parasympathetic neuronal tone. The ED50 for rilmenidine-induced pupillary dilation was approximately 200 microg kg(-1), i.v., and was sustained for at least 1 h. 3. The highly selective alpha2-adrenoceptor antagonist, RS-79948, administered either before or after rilmenidine, antagonized rilmenidine-induced mydriasis. Neuronally induced reflex inhibition of parasympathetic nerve activity was also inhibited by administration of RS-79948. 4. These results suggest that rilmenidine acts like clonidine to produce pupillary dilation by inhibition of parasympathetic tone to the iris sphincter and that this central nervous system parasympatho-inhibition is mediated by alpha2-adrenoceptors, rather than imidazoline receptors.

Rhythmic properties of neurons in the rostral ventrolateral medulla of the rat in vitro: effects of clonidine

The rostral ventrolateral medulla (RVLM) is thought to be the main central site for generation of tonic sympathetic activity. In the rat in vitro slice preparation, we used intracellular recordings to identify different populations of neurons in the RVLM: 43 spontaneously active neurons with regular (R) or irregular (I) patterns of spike firing and 10 silent neurons. The degree of regularity was quantified by the coefficient of variation (CV = SD/mean) of interspike interval durations, as well as by the rhythmic properties of the spike autospectrum and autocorrelation. The distribution of CVs was clustered: R and I neurons were defined as those with CVs </=12% (n = 21) or >12% (n = 22), respectively. The R-type and I-type neurons resemble the type II and type I neurons, respectively, which were previously characterized in the RVLM in vivo as barosensitive and bulbospinal. Both types may be important in generation of sympathetic tone. Clonidine (1-100 microM) was applied to 10 R-type neurons and 16 I-type neurons. The firing of 21/26 was depressed to the point of silence. However, 18/26 neurons were excited earlier in the perfusion. The later depression of firing occurred in both I and R neurons and in different cases was associated with either hyperpolarization or depolarization.

Effects of medullary alpha2-adrenoceptor blockade in the rat

The effect of alpha2-adrenoceptor blockade in the medulla was studied in pentobarbital anesthetized rats in which arterial blood pressure, heart rate and renal sympathetic nerve activity were analysed. Three series of experiments were performed: (1) i.c. administration of alpha2-adrenoceptor antagonists with different subtype affinities; (2) i.v. administration of methoxy-idazoxan to study its effects on neuronal activity into the rostral ventral medulla; (3) microinjections of methoxy-idazoxan in rostral ventral medulla and nucleus tractus solitarii. Methoxy-idazoxan (0.1-3 microg x kg(-1) i.c., n=5), but not saline, rauwolscine, BRL 44408 (2-[2H-(1,3,dihydroisoindol)methyl]-4,5dihydroimidazol) or ARC 239 (2-[2-(4-(2-methoxyphenyl)piperazin-1-yl)ethyl]-4,4-dimethyl-1,3-(2H,4H)-isoquilindione) (each at 10-100 microg x kg(-1) i.c., n=5-5-6-5, respectively), increased mean arterial blood pressure, heart rate and renal nerve activity (+19+/-6 mm Hg, +72+/-22 beats x min(-1), +43+/-9%) and blocked the sympatho-inhibitory action of clonidine (10 microg x kg(-1) i.v.). In further experiments, methoxy-idazoxan, BRL 44408 and the highest dose of rauwolscine i.c., reversed the clonidine-induced sympatho-inhibition (order of potency: methoxy-idazoxan>BRL4440>rauwolscine, n=6 each), whereas ARC 239 (n=5) or saline (n=7) did not. Methoxy-idazoxan i.v. (n=7, 10-100 microg x kg(-1)) increased the renal sympathetic nerve and rostral ventral medulla neuronal activity and the heart rate (+36+/-7%, +66+/-29% and +18+/-9 beats x min(-1)) without a significant effect on mean arterial blood pressure. Microinjection of methoxy-idazoxan (1 nmol/40 nl) into the rostral ventral medulla reversed the effect of clonidine microinjected into the same site (2 nmol/40 nl, n=5). In another group of rats (n=8), methoxy-idazoxan increased mean arterial blood pressure, heart rate and renal nerve activity (+16+/-2 mm Hg, +42+/-7 beats x min(-1), +24+/-5%) and blocked the effect of clonidine i.v. (10 microg x kg(-1)). Bilateral microinjections into the nucleus tractus solitarii (n=5) did not alter mean arterial blood pressure but decreased heart rate and sympathetic nerve activity (-30+/-16 beats x min(-1), -20+/-14%). Our results offer direct in vivo evidence for the main role of the alpha2A/D-adrenoceptors located in the ventral pressor area. The data show that the sympathy-excitatory effect of alpha2-adrenoceptor antagonists is due to the blockade of a tonic activation of these alpha2A/D-adrenoceptors present in the rostral ventral pressor area.

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.

Respective contributions of alpha-adrenergic and non-adrenergic mechanisms in the hypotensive effect of imidazoline-like drugs

The hypotensive effect of imidazoline-like drugs, such as clonidine, was first attributed to the exclusive stimulation of central alpha2-adrenoceptors (alpha2ARs). However, a body of evidence suggests that non-adrenergic mechanisms may also account for this hypotension. This work aims (i) to check whether imidazoline-like drugs with no alpha2-adrenergic agonist activity may alter blood pressure (BP) and (ii) to seek a possible interaction between such a drug and an alpha2ARs agonist alpha-methylnoradrenaline (alpha-MNA). We selected S23515 and S23757, two imidazoline-like drugs with negligible affinities and activities at alpha2ARs but with high affinities for non-adrenergic imidazoline binding sites (IBS). S23515 decreased BP dose-dependently (-27+/-5% maximal effect) when administered intracisternally (i.c.) to anaesthetized rabbits. The hypotension induced by S23515 (100 microg kg(-1) i.c.) was prevented by S23757 (1 mg kg(-1) i.c.) and efaroxan (10 microg kg(-1) i.c.), while these compounds, devoid of haemodynamic action by themselves, did not alter the hypotensive effect of alpha-MNA (3 and 30 microg kg(-1) i.c.). Moreover, the alpha2ARs antagonist rauwolscine (3 microg kg(-1) i.c.) did not prevent the effect of S23515. Finally, whilst 3 microg kg(-1) of S23515 or 0.5 microg kg(-1) of alpha-MNA had weak hypotensive effects, the sequential i.c. administration of these two drugs induced a marked hypotension (-23+/-2%). These results indicate that an imidazoline-like drug with no alpha2-adrenergic properties lowers BP and interacts synergistically with an alpha(ARs agonist.

Characterization of mechanisms involved in presynaptic inhibition of sympathetic pressor effects induced by some 5-HT1 receptor antagonists

1. In a previous study, we showed that the presynaptic inhibitory action of 5-hydroxytryptamine receptor agonists on sympathetic pressor effects obtained in the pithed rats were mainly mediated by activation of 5-HT1A and 5-HT1D receptor subtypes. At the time, we observed that some 5-HT1 receptors antagonists - WAY 100,635 and NAN-190 (both 5-HT1A receptor antagonists), methiothepin (a 5-HT1,2,5,6,7 receptor antagonist) and spiperone (a 5-HT1,2 receptor antagonist) - reduced per se the pressor effects obtained by electrical stimulation. The aim of the present work was to investigate the mechanism participating in this inhibitory effect. 2. The inhibition induced by WAY 100,635 (1000 microg kg-1, i.v.) was blocked after i.v. treatment with idazoxan, an alpha2-adrenoceptor antagonist (300 and 1000 microg kg-1) and was not modified after i.v. treatment with propranolol, a beta-adrenoceptor antagonist (1000 microg kg-1) and sulpiride, a D2 receptor antagonist (1000 microg kg-1). The inhibition induced by spiperone (500 microg kg-1 i.v.) was significantly blocked by sulpiride (1000 microg kg-1) and was not modified by idazoxan or propranolol. 3. Sulpiride (1000 microg kg-1) partially blocked the inhibition induced by methiothepin (50 microg kg-1 i.v.). Only pretreatment with idazoxan (300 microg kg-1) modified the inhibition induced by NAN-190 (100 microg kg-1 i.v.), such inhibition increasing after intravenous administration of idazoxan. 4. All the antagonists used in our experiments failed to inhibit the pressor responses elicited by i.v. noradrenaline administration. 5. The above results suggest that the inhibitory effects of these 5-HT1 receptor antagonists are presynaptic in nature, but not related to the blockade of 5-HT1 receptors subtypes. The simultaneous activation or inhibition of other receptor systems could explain the inhibition produced by each 5-HT1 receptor antagonist studied.

Inhibition of abnormal hindquarter vascular tone in spontaneously hypertensive rats with chlorpromazine

The presence of an abnormal sympathetic vascular tone is assumed in the hindquarters of spontaneously hypertensive rats (SHR) on the basis that ganglionic blockade decreases hindquarter vascular resistance (HQR) in them but not in normotensive control rats (NCR). Hindquarter blood flow (HQF) was observed with an electromagnetic flow probe implanted around the terminal aorta in SHR and NCR in the conscious state. Mean arterial pressure (AP) was also recorded with an indwelling catheter. HQR was calculated as AP divided by HQF. Intravenous bolus injection of chlorpromazine-HCl at 0.5 mg/kg significantly decreased HQR in SHR but not in NCR. Thereafter, in SHR, ganglionic blockade with hexamethonium bromide did not decrease HQR further. Chlorpromazine given to SHR after ganglionic blockade did not decrease HQR either. These findings indicate that the abnormal hindquarter tone in SHR was inhibited by chlorpromazine. It is suggested that dopaminergic neurons are involved in the hindquarter sympathetic tone generation.

NaCl-induced hypertensive rat model of non-insulin-dependent diabetes: role of sympathetic modulation

Systemic hypertension is common in individuals with non-insulin-dependent diabetes (NIDD) and, in this population, markedly increases the risk for cardiovascular complications. The aims of this study were to develop a rat model of combined NaCl-induced hypertension and NIDD, and to determine the contribution of the sympathetic nervous system to the development of the manifest hypertension. Two-day old male Wistar-Kyoto rats were injected with either streptozotocin (90 mg/kg, ip; NIDD) or vehicle (citrate buffer; control). At 4 weeks of age, the animals underwent either a right nephrectomy or a sham operation. Animals in each group were further subdivided, with one group maintained on normal (0.72 %) NaCl diet whereas the other was placed on a high (8%)-NaCl diet. At 6 months of age, diabetes was confirmed by glucose tolerance testing. Hemodynamic parameters were measured in the freely moving animal (ia) before and after the administration of prazosin (peripheral alpha1-adrenergic antagonist, iv) or clonidine (central alpha2-adrenergic agonist). The NIDD rat displayed a higher (P < .05) blood glucose concentration than the nondiabetic control rat during the glucose tolerance test. Elevated dietary NaCl significantly increased mean arterial pressure (MAP) in the uninephrectomized, but not the sham-operated groups. Acute administration of prazosin resulted in a significantly greater reduction in MAP of both hypertensive groups than of their normotensive counterparts. Moreover, clonidine caused a significant reduction in MAP of the hypertensive control rat but not in the normotensive controls. By contrast, both the hypertensive NIDD and the normotensive NIDD rats showed a similar reduction in MAP in response to clonidine administration. The data suggest that the combination of uninephrectomy and dietary NaCl excess confers hypertension on the NIDD rat. Moreover, enhancement of the sympathetic pathway plays an important role in the regulation of arterial pressure in the hypertensive NIDD rat.

alpha(2)-adrenergic receptors are not required for central anti-hypertensive action of moxonidine in mice

In the mouse medulla oblongata, we characterized binding properties and functional responses of two recognition sites for imidazoline compounds: I(1)-imidazoline and alpha(2)-adrenergic receptors. The mouse medulla expresses a higher density of I(1)-receptors than in the rat, whereas alpha(2)-receptor densities were similar between the two species. In anesthetized, ventilated and paralyzed mice, we tested the hypotensive actions of the I(1)/alpha(2) agonist moxonidine, determined its central site of its actions, and the relative roles of I(1) and alpha(2)-receptors. Experiments were performed in C(57)Bl(6) wild type and alpha(2A)-adrenergic receptor deficient mice. In both types of mice, neuronal activation within the rostral ventrolateral medulla (RVLM) region by glutamate microinjection elicited increases in arterial pressure. Moxonidine (0.5 nmol/site/10 nl) microinjected bilaterally into this vasopressor region decreased arterial pressure by 30% and heart rate by 11% in wild type mice. Efaroxan, the I(1)/alpha(2) antagonist (0.4 nmol) when microinjected into the RVLM elevated blood pressure itself and abolished the action of moxonidine, whereas alpha(2)-blockade with SK&F 86466 had no significant effect on blood pressure and did not attenuate moxonidine's effect. To more definitively test the role of alpha(2)-adrenergic receptors in the action of moxonidine, moxonidine was microinjected into the RVLM of alpha(2A)-adrenergic deficient mice. The decreases in arterial pressure were nearly identical to those of wild type mice, whereas bradycardia was attenuated. Thus, in the mouse moxonidine acts within the RVLM region to lower arterial pressure mainly through the I(1)-imidazoline receptor independent of alpha(2)-adrenergic receptors.

Synergistic centrally mediated cardiovascular effects of a kappa opioid agonist and an alpha2-adrenoceptor agonist

In this study, we determined possible additive and synergistic centrally mediated hypotensive and bradycardic effects of U-62,066E, a nonpeptide kappa opioid agonist acting on the hippocampal formation (HF), and guanabenz, an alpha2-adrenoceptor agonist acting on the rostral ventrolateral medulla (RVLM), the nucleus tractus solitarius (NTS), or the locus coeruleus (LC). The drugs were microinjected at various doses into these areas of alpha-chloralose-anesthetized Sprague-Dawley rats. There were synergistic hypotensive and bradycardic effects between low, noneffective doses of U-62,066E acting on the HF and guanabenz acting simultaneously on the RVLM. Higher doses of each agent, which themselves caused hypotension and bradycardia acting on each brain area alone, did not lead to synergistic effects when the drugs were injected simultaneously into those areas. There were no synergistic effects between U-62,066E acting on the HF and guanabenz acting on the NTS or the LC.

Drugs acting on imidazoline receptors: a review of their pharmacology, their use in blood pressure control and their potential interest in cardioprotection

Drugs acting within the autonomic nervous system are of particular interest when autonomic abnormalities are implicated in the development and maintenance of various cardiovascular pathologies. For example, it has been documented that in the early stages of hypertensive disease, i.e. hyperkinetic borderline hypertension, a sympathetic hyperactivity associated with a decreased parasympathetic activity results in increased cardiac output and heart rate. Several classes of drugs acting within the central, as well as the peripheral, autonomic nervous system are very efficient in treating hypertensive disease. One class - the second generation of a group of centrally acting drugs selective for imidazoline receptors - has proved beneficial in this respect, because drugs in this class are well tolerated and have interesting additional effects such as their antiarrhythmic action. Rilmenidine and moxonidine are the lead compounds of this class of drugs. Rilmenidine and moxonidine both proved more selective for cerebral imidazoline receptors than the reference drug, clonidine. It was suggested that this selectivity, attributable to their lower affinity for alpha2-adrenoceptors, explains the low incidence of adverse effects (including sedation) associated with these drugs. In addition, potentially beneficial actions on cardiac dysrythmias and congestive heart failure enlarge the therapeutic potential of the second generation of imidazoline-related drugs. This review focuses on the main pharmacological and clinical properties of rilmenidine and moxonidine, paying particular attention not only to their efficacy in hypertension but also to other potential cardiovascular indications.

Method for comparison of the hemodynamic effects of equi-antinociceptive oral doses of drugs in anesthetized rats

In a typical flowchart for discovery of novel analgesic (or other) agents, a critical path often involves maximization of the separation of the therapeutic endpoint from known adverse-effect (AE) endpoint(s). Although strategies can easily be designed for in vitro paradigms such as high-throughput screening, extension to in vivo testing can represent a major obstacle to the rapid progression to the next step in development. The problem can be particularly acute when the assessment is required for oral dosing, and when it is not known if the therapeutic and AE mechanism(s) of action are the same. As a case in point, alpha(2)-adrenoceptor (alpha(2)-AR) agonists have potential therapeutic use as analgesics, but they also produce cardiovascular (CV) effects. However, whether the two effects are inexorably linked has not been resolved, particularly for oral administration. The present study used a novel method for comparing the CV effects produced by alpha(2)-AR agonists given by intraduodenal administration to anesthetized rats at fixed ratios of the oral antinociceptive ED(50) dose of each agonist. The technique provided a useful screen of compounds. In addition,there was no correlation between CV endpoints and alpha(2A)-AR affinity, suggesting that oral alpha(2)-AR-mediated analgesia and CV effects might be separable or that other mechanisms might be involved.

Vascular areas where clonidine induces vasodilation in hypertensive and normotensive rats

The aim of this study was to find vascular areas where clonidine decreases the regional vascular resistance when this drug lowers arterial pressure in conscious spontaneously hypertensive rats and normotensive control rats. Arterial pressure was observed with an indwelling catheter at a carotid. Blood flow was measured with an electromagnetic flow probe implanted around the renal artery or the superior mesenteric artery. Regional vascular resistance was calculated as arterial pressure divided by blood flow. Intravenous bolus injection of clonidine at a dose to decrease arterial pressure decreased renal resistance and superior mesenteric resistance. Quantitatively, the combined effect of the decrease in these two resistances was enough to account for the decrease in arterial pressure. Although clonidine is thought to inhibit sympathetic nerve activity centrally, the above vasodilator effect is not ascribable to this inhibitory mechanism: Sympathetic activity to be inhibited does not seem to be present in the superior mesenteric area and clonidine similarly decreased renal vascular resistance even after renal denervation.

Imidazoline receptors: from discovery to antihypertensive therapy (facts and doubts)

The hypothesis and indirect evidence of imidazoline receptors has been promoted since some 15 years ago and it gave a substantial impetus for research in this field, resulting in a better understanding of neuronal and cardiovascular regulatory processes. The nomenclature of the imidazoline receptors has been accepted by international forums but no direct proof for the existence of these receptors has been published. Authors summarise the most important available data, including facts and doubts as far as the discovery, characterisation, and function of imidazoline receptors and their subtypes, the differences between imidazoline receptors and alpha-2 adrenoceptors, and also on their participation in regulatory processes.

Mechanism of the sympathoinhibition produced by the clonidine-like drugs rilmenidine and moxonidine

The mechanism of the sympathoinhibition produced by two new derivatives of clonidine, rilmenidine and moxonidine, was studied. One aim was to determine the receptor responsible for the central sympathoinhibition by these drugs. Rilmenidine and moxonidine were injected into the cisterna cerebellomedullaris. They decreased blood pressure and the plasma noradrenaline concentration. After rilmenidine and moxonidine, two selective alpha 2-adrenoceptor antagonists (devoid of affinity for I1 binding sites), yohimbine and SK&F86466, were given intracisternally. They completely counteracted the hypotensive effects of rilmenidine and moxonidine, indicating that alpha 2-adrenoceptors are involved in the central sympathoinhibition produced by these drugs. The other aim was to determine if peripheral presynaptic inhibition of noradrenaline release from postganglionic sympathetic neurons contributes to the overall reduction of sympathetic tone. Rilmenidine and moxonidine were injected i.v. in pithed rabbits with electrically stimulated sympathetic outflow. They dose-dependently lowered blood pressure and the plasma noradrenaline concentration and inhibited stimulation-evoked cardioacceleration. Moreover, the doses necessary for these peripheral effects were identical to the doses that reduce the sympathetic nerve firing rate and blood pressure in conscious rabbits. These observations indicate that peripheral presynaptic inhibition of noradrenaline release from postganglionic sympathetic neurons contributes to the overall reduction of sympathetic tone produced by rilmenidine and moxonidine in intact animals.

Cardiovascular effects of rilmenidine, moxonidine and clonidine in conscious wild-type and D79N alpha2A-adrenoceptor transgenic mice

1. We investigated the cardiovascular effects of rilmenidine, moxonidine and clonidine in conscious wild-type and D79N alpha2A-adrenoceptor mice. The in vitro pharmacology of these agonists was determined at recombinant (human) alpha2-adrenoceptors and at endogenous (dog) alpha2A-adrenoceptors. 2. In wild-type mice, rilmenidine, moxonidine (100, 300 and 1000 microg kg(-1), i.v.) and clonidine (30, 100 and 300 microg kg(-1), i.v.) dose-dependently decreased blood pressure and heart rate. 3. In D79N alpha2A-adrenoceptor mice, responses to rilmenidine and moxonidine did not differ from vehicle control. Clonidine-induced hypotension was absent, but dose-dependent hypertension and bradycardia were observed. 4. In wild-type mice, responses to moxonidine (1 mg kg(-1), i.v.) were antagonized by the non-selective, non-imidazoline alpha2-adrenoceptor antagonist, RS-79948-197 (1 mg kg(-1), i.v.). 5. Affinity estimates (pKi) at human alpha2A-, alpha2B- and alpha2C-adrenoceptors, respectively, were: rilmenidine (5.80, 5.76 and 5.33), moxonidine (5.37, <5 and <5) and clonidine (7.21, 7.16 and 6.87). In a [35S]-GTPgammaS incorporation assay, moxonidine and clonidine were alpha2A-adrenoceptor agonists (pEC50/intrinsic activity relative to noradrenaline): moxonidine (5.74/0.85) and clonidine (7.57/0.32). 6. In dog saphenous vein, concentration-dependent contractions were observed (pEC50/intrinsic activity relative to noradrenaline): rilmenidine (5.83/0.70), moxonidine (6.48/0.98) and clonidine (7.22/0.83). Agonist-independent affinities were obtained with RS-79948-197. 7. Thus, expression of alpha2A-adrenoceptors is a prerequisite for the cardiovascular effects of moxonidine and rilmenidine in conscious mice. There was no evidence of I1-imidazoline receptor-mediated effects. The ability of these compounds to act as alpha2A-adrenoceptor agonists in vitro supports this conclusion.

'Seeing through a glass darkly': casting light on imidazoline 'I' sites

Although imidazoline sites have been the subject of research for several years, there is still controversy about their structure, diversity and physiology. The I1 site is thought to exist principally as a binding site and is widely purported to play a role in controlling systemic blood pressure, although this is still unclear. The majority of I2 sites are widely accepted as being allosteric sites on monoamine oxidase; however, even with selective ligands, their exact function remains to be determined. A putative I3 site modulates insulin secretion and could represent the first functional site to be pharmacologically defined with selective agonists and antagonists. The structure and relevance of the proposed endogenous ligand 'clonidine-displacing substance' remains elusive. A potential candidate for this substance is agmatine; however, although it is capable of displacing bound clonidine from imidazoline sites, it lacks the functionality ascribed to the clonidine-displacing substance. In this review, Richard M. Eglen and colleagues assess our knowledge of imidazoline sites in the light of recent data.

Contribution of alpha 2-adrenoceptors in caudal ventrolateral medulla to cardiovascular regulation in rat

The inhibitory action of alpha 2-agonists on the cardiovascular neurons has been elucidated in the rostral ventrolateral medulla (RVLM) but not in the caudal ventrolateral medulla (CVLM). Our study aimed to clarify whether microinjection of clonidine into the CVLM elicits any cardiovascular effect and whether endogenous alpha 2-adrenoceptor-mediated mechanisms contribute to the tonic activity of the CVLM neurons. In male Sprague-Dawley rats (7-9 wk old, 270-320 g) anesthetized with urethan, unilateral microinjection of 8 nmol of clonidine into the CVLM (n = 10) increased mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) by 12.1 +/- 1.8 mmHg (mean +/- SE, P < 0.01) and 25.8 +/- 4.8% (P < 0.01), while heart rate (HR) remained unaltered. Unilateral microinjection of 2 nmol of SKF-86466, a selective blocker of the alpha 2-adrenoceptors, into the CVLM (n = 10) decreased MAP, HR, and RSNA (-11.6 +/- 2.6 mmHg, -26 +/- 7 beats/min, and -15.3 +/- 1.7%, respectively, P < 0.01 for each). Artificial cerebrospinal fluid caused neither a cardiovascular effect nor a sympathetic response. Prior injection of SKF-86466 into the ipsilateral CVLM attenuated the effects of clonidine. Bilateral microinjection of muscimol into the RVLM abolished the effects of both clonidine and SKF-86466 injected into the CVLM. The pressor and sympathoexcitatory effects of clonidine injected into the CVLM suggest a neuroinhibitory action of the drug on the CVLM neurons. In addition, the depressor and sympathoinhibitory effects of SKF-86466 injected into the CVLM indicated that activation of alpha 2-adrenoceptors by endogenous ligand inhibits CVLM neurons. The effects of clonidine and the alpha 2-adrenoceptor antagonist in the CVLM require the integrity of the RVLM.

The I1-imidazoline-binding site is a functional receptor mediating vasodepression via the ventral medulla

I1-imidazoline-binding sites fulfill all essential criteria for identification as receptors, including specificity of binding, association with physiological functions, appropriate anatomic and cellular and subcellular localization, and specific cell signaling pathways. Moreover, binding affinities correlate with functional drug responses. The evidence linking I1 receptors to vasodepression includes expression in RVLM and consistent correlations between vasodepressor potency in humans and animals and I1 binding affinity. Some I1 agonists are antagonists at alpha 2-adrenergic receptors (alpha 2AR), and these elicit vasodepression in RVLM. Potent alpha 2-agonists with phenylethylamine or guanidine structures are inactive in RVLM, yet highly effective in nucleus of the solitary tract, a region with well-defined alpha 2-mediated vasodepressor responses. Selective I1 agonists are used clinically to lower blood pressure with minimal alpha 2-mediated sedation. Moreover, when microinjected into the RVLM only antagonists active at I1 receptors can block the vasodepressor action of either local or systemic imidazolines. RVLM alpha 2-blockade has no effect. Some reports appear to conflict with the I1 receptor hypothesis; but these reports often make incorrect assumptions regarding drug specificity, overlook systemic effects of alpha 2-antagonists, or inappropriately analyze data. Blockade of gamma-aminobutyric acid (GABA) receptors blocks the vasodepressor action of imidazolines, implying a multisynaptic pathway. Thus imidazolines act via I1 receptors in RVLM to lower blood pressure, although alpha 2AR are also important, especially in NTS.

The imidazoline receptor in control of blood pressure by clonidine and allied drugs

Clonidine, moxonidine, and rilmenidine are centrally acting antihypertensive agents that lower arterial pressure by inhibiting the tonic activity of sympathoexcitatory neurons in the rostral ventrolateral medulla. Competing hypotheses have been put forward by different investigators to explain the sympathoinhibition evoked by "imidazoline drugs": either via central actions at alpha 2-adrenergic receptors or novel I1-imidazoline receptors. These different perspectives are presented in the accompanying reviews.

Is the hypotensive effect of clonidine and related drugs due to imidazoline binding sites?

Clonidine and related alpha 2-adrenergic receptor (alpha 2AR) agonists lower arterial pressure primarily by an action within the central nervous system. These drugs also have varying degrees of affinity for other cellular components called nonadrenergic imidazoline binding sites (NAIBS). For over 20 years, the alpha 2AR agonist activity of clonidine-like drugs was thought to account for their therapeutic effects (alpha 2 theory). However, several groups have recently proposed a competing "imidazoline theory" according to which the hypotensive effect of clonidine-like drugs would in fact owe more to their affinity for one type of NAIBS, called I1 receptors. The alpha 2-theory is strongly supported by four main types of congruent data. First, the hypotensive effect of systemically administered clonidine is blocked by alpha 2AR antagonists that are without affinity for I1 NAIBs. Second, the hypotensive effect of intravenous clonidine is absent in genetically engineered mice in which a defective alpha 2AAR has been substituted for the normal one. Third, the sympatholytic effect of clonidine is consistent with the presence of conventional inhibitory alpha 2ARs on sympathetic preganglionic neurons and on their main excitatory inputs in the medulla oblongata. Fourth, the first I1 ligand without affinity for alpha 2ARs was found to be biologically inactive. The imidazoline theory is supported by a limited repertoire of whole animal "in vivo" pharmacological experiments that remain open to a wide range of interpretations. In conclusion, the bulk of the evidence strongly supports a largely predominant role of alpha 2AR mechanisms in the action of most clonidine-like agents at therapeutically relevant doses or concentrations. Even the small pharmacological differences between these agents cannot yet be linked with certainty to their relative affinity for I1 NAIBS.

Why is the hypotensive effect of clonidine greater in hypertensive rats?

The original aim of this study was to observe whether the depressor drug clonidine inhibited the abnormal hindquarter tone in spontaneously hypertensive rats (SHR). In conscious SHR and normotensive control rats (NCR), hindquarter (terminal aortic) blood flow was observed with an implanted electromagnetic flow probe and mean arterial pressure with an indwelling catheter. Twenty minutes after intravenous injection of clonidine (5 micrograms/kg) when arterial pressure reached a steady lower level, hindquarter resistance (HQR), calculated as mean arterial pressure divided by hindquarter flow, did not decrease in SHR. Thus we were unable to obtain evidence for an inhibitory effect of clonidine on the abnormal hindquarter tone in SHR. In NCR, HQR increased significantly by clonidine. The decrease in arterial pressure on clonidine was greater in SHR than in NCR, presumably because the increase in HQR partially offset the hypotensive effect in NCR. It seems that the increase in HQR in NCR was induced by a reflexive excitation of regional sympathetic vasoconstrictor fibers, which, being the final common path for the abnormal hindquarter tone also, were already being excited in SHR before clonidine administration. This point was quantitatively verified.

Atipamezole-precipitated clonidine withdrawal induces c-Fos expression in rat central nervous system

We sought to identify a clonidine withdrawal syndrome in conscious rats by investigating the effects of a single injection of the specific alpha2-adrenergic antagonist atipamezole (1.5 mg/kg i.p.) after chronic treatment with the alpha2-adrenergic agonist clonidine (200 microg/kg per day via osmotic mini-pump for 7-10 days). Rats treated chronically with clonidine followed by atipamezole injection (clonidine-atipamezole) demonstrated dramatic behavioral effects including shaking, vigorous digging, and whole-body seizure-like movements. Control groups (saline-saline, clonidine-saline and saline-atipamezole) showed no overt unusual behavioral effects following injection. The brains of the clonidine-atipamezole group showed massive c-Fos expression (especially in di- and telencephalon) while the other groups showed either background levels of c-Fos-immunopositive cells (saline-saline and clonidine-saline groups) or a slight increase over background in selected areas (saline-atipamezole group). Maps of c-Fos-immunolabeled cells were generated at five representative coronal planes for each treatment group. C-Fos-immunopositive cells were counted in three representative brainstem structures (locus coeruleus, nucleus of the solitary tract, rostral ventrolateral medulla (RVL)) and in three regions of the thoracic spinal cord (dorsal horn, intermediate zone and ventral horn). In the three brainstem structures the number of c-Fos-positive cells was elevated 8-10-fold in the clonidine-atipamezole group compared to the other groups. No other treatment group was significantly different from the saline-saline group. An increased number of c-Fos-positive neurons was also noted in the dorsal horn and intermediate layers of the thoracic spinal cord in the clonidine-atipamezole group compared to a sham-operated atipamezole-injected group. In the RVL, 59% of c-Fos-positive cells contained alpha2A-adrenergic receptor-like immunoreactivity in clonidine-atipamezole treated (withdrawing) rats. In addition, one-third of the tyrosine hydroxylase (TH)-immunopositive cells in RVL were also c-Fos-positive in clonidine withdrawing rats where no TH-positive cells were also c-Fos-positive in RVL of control groups. Atipamezole injected 10 min after a single injection of clonidine (200 microg/kg, i.p.) produced no behavioral effect and did not increase c-Fos expression in brainstem. Injection of the opiate antagonist naltrexone (100 mg/kg, i.p.) in rats chronically treated with clonidine did not elicit behavioral effects or result in increased c-Fos expression in brainstem. In conclusion, administration of the selective alpha2-antagonist atipamezole to rats chronically treated with the alpha2-adrenergic agonist clonidine triggers a powerful withdrawal syndrome associated with massive CNS expression of c-Fos protein. The intensity of the withdrawal syndrome indicates that chronic exposure to alpha2-adrenergic receptor agonists produces strong dependence.

Cardiovascular effects of central administration of clonidine in conscious cats

The effects on arterial blood pressure and heart rate after an intracerebroventricular (i.c.v.) administration of clonidine were investigated using conscious normotensive cats. Injection of clonidine (5-10 microg; 5 microl; i.c.v.) elicited a decrease in mean arterial pressure (MAP) and heart rate (HR) in a dose-dependent manner. The highest dose of 10 microg of clonidine decreased MAP and HR by 39 +/- 3 mmHg and 74 +/- 5 b.p.m., respectively (n = 7). Pretreatment with yohimbine, the alpha2-adrenoceptor antagonist (8 microg; 5 microl; i.c.v.) blocked the cardiovascular responses to a subsequent i.c.v. injection of 10 microg clonidine (n = 7). Furthermore, preadministration of cimetidine (100 microg; 5 microl; i.c.v.), the H2 histamine receptor antagonist with imidazoline receptor activating properties, prevented the decreases in MAP and HR to a subsequent i.c.v. injection of 10 microg clonidine (n = 7). By contrast, pretreatment with the specific I1 imidazoline receptor blocker, efaroxan (100-500 microg; 5 microl; i.c.v.), failed to inhibit the cardiovascular effects of an i.c.v. administration of 10 microg clonidine (n = 7). These results suggest that the effects of centrally administered clonidine on MAP and HR are probably not mediated through activation of the I1 subtype of imidazoline receptors in conscious cats. However, the cardiovascular effects elicited by i.c.v. administration of clonidine appear to result from stimulation of central alpha2-adrenergic or the H2 histaminergic-like receptors.

Modulation of catecholamine release by alpha 2-adrenoceptors and I1-imidazoline receptors in rat brain

The physiological and pharmacological effects of imidazoli(di)ne derivatives, such as clonidine, have been related not only to the interaction with alpha 2-adrenoceptors but also to their activity on non-adrenoceptor sites termed imidazoline receptors. The modulation of catecholamine release by imidazoline drugs was studied by monitoring extracellular levels of norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) with microdialysis in cingulate cortex of rats, with or without irreversible alpha 2-adrenoceptor blockade. NE and DA levels were in the 1 nM range whereas DOPAC and HIVA levels were approximately equal to 100 nM. NE and DA levels were increased when the uptake blocker desipramine (1 microM) or KCl (100 mM) were added to the perfusion medium. Clonidine induced a dose-dependent (0.3-1.2 mg/kg i.p.) decrease in NE (max 61%) and DA (max 40+) levels that was reversed by the alpha 2-adrenoceptor antagonist RX821002. After alpha 2-adrenoceptor irreversible blockade with the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), [3H]clonidine binding to alpha 2-adrenoceptors was reduced by 94 +/- 1%. Under such conditions, clonidine elicited a paradoxical dose-dependent (0.6-2.4 mg/kg i.p.) increase of NE (max 56%) without modifications in DA, DOPAC and HVA levels. The stimulatory effect of clonidine was prevented by the imidazoline receptor antagonist idazoxan (10 mg/kg i.p.) but not by RX821002 (5 mg/kg i.p.). In rats pretreated with EEDQ, cirazoline (I1/I2-imidazoline receptor agonist), moxonidine (I1-imidazoline receptor agonist), but not guanabenz (I2-imidazoline receptor agonist) (1.2-2.4 mg/kg i.p.) elicited an increase of NE levels in a similar manner to clonidine (11-82%). Idazoxan also abolished these responses to cirazoline or moxonidine. In contrast to systemic administration, local perfusion of clonidine (10-100 microM) through the microdialysis probe under alpha 2-adrenoceptor alkylating conditions, did not modify extracellular levels of NE and DA suggesting an indirect mechanism. The results demonstrate that clonidine and related imidazoli(di)ne drugs are able not only to inhibit NE release in rat cerebral cortex involving an alpha 2-adrenoceptor mechanism, but also to induce a paradoxical NE release through an indirect extracortical mechanism. The findings evidence that the indirect modulation of NE levels by imidazoline drugs is mainly due to a functional activity on I1-imidazoline receptors.

Central cardiovascular actions of agmatine, a putative clonidine-displacing substance, in conscious rabbits

Agmatine, an endogenous clonidine-displacing substance, has been shown to have an affinity for both alpha 2-adrenoceptors and imidazoline receptors (IR). In conscious rabbits, we have examined the cardiovascular effects of agmatine and its interaction with clonidine, a presumed agonist and 2-methoxyidazoxan, an antagonist at alpha 2-adrenoceptors. We have also examined the effect of agmatine on agents having high affinity for I1-imidazoline receptors namely moxonidine (agonist) and efaroxan (antagonist). Initial dose-response studies showed that agmatine administered in low doses (0.01-10 micrograms/kg) into the fourth ventricle did not change mean arterial pressure but did produce a dose-dependent bradycardia (maximum -16 +/- 3 beats/min). A higher dose of 100 micrograms/kg produced an adverse reaction in the conscious animals accompanied by a marked increase in mean arterial pressure and a reversal of the bradycardia. This is in contrast to the effects of fourth ventricular clonidine and moxonidine, which caused a dose-dependent fall in both mean arterial pressure and heart rate. Agmatine when administered at the highest well-tolerated dose of 10 micrograms/kg did not further alter the clonidine-induced hypotension but produced a greater bradycardia (-12 +/- 4 beats/min clonidine; -29 +/- 4 beats/min clonidine plus agmatine; p < 0.05). Similarly, the hypotension induced by moxonidine was not altered by agmatine but heart rate was reduced after the addition of agmatine (p < 0.01). Efaroxan and 2-methoxy-idazoxan, at doses which produced no effects when given alone, similarly reversed the fall in heart rate elicited by agmatine and caused a small but significant rise in mean arterial pressure. We have previously shown that the doses of these antagonists used in this study produce an equal reversal of the bradycardia induced by fourth ventricular alpha-methyldopa (alpha 2-adrenoceptor agonist) and clonidine and hence have similar alpha 2-adrenoceptor blocking effects. Our results show that agmatine produces bradycardia as does moxonidine and clonidine but does not mimic or block the hypotensive responses to these agents. These findings do not support the hypothesis that agmatine is an endogenous ligand for IR. However, the bradycardia induced by agmatine may be mediated via alpha 2-adrenoceptors since it was equally blocked by efaroxan and 2-methoxy-idazoxan. Thus while alpha 2-adrenoceptor actions of agmatine on heart rate are evident at relatively low doses, the reason for the lack of alpha 2-adrenoceptor mediated hypotension is not known.

The I1-imidazoline receptor: from binding site to therapeutic target in cardiovascular disease

OBJECTIVE

To review previous work and present additional evidence characterizing the I1-imidazoline receptor and its role in cellular signaling, central cardiovascular control, and the treatment of metabolic syndromes. Second-generation centrally-acting antihypertensives inhibit sympathetic activity mainly via imidazoline receptors, whereas first-generation agents act via alpha2-adrenergic receptors. The I1 subtype of imidazoline receptor resides in the plasma membrane and binds central antihypertensives with high affinity.

METHODS AND RESULTS

Radioligand binding assays have characterized I1-imidazoline sites in the brainstem site of action for these agents in the rostral ventrolateral medulla. Binding affinity at I1-imidazoline sites, but not at other classes of imidazoline binding sites, correlates closely with the potency of central antihypertensive agents in animals and in human clinical trials. The antihypertensive action of systemic moxonidine is eliminated by the I1/alpha2-antagonist efaroxan, but not by selective blockade of alpha2-adrenergic receptors. Until now, the cell signaling pathway coupled to I1-imidazoline receptors was unknown. Using a model system lacking alpha2-adrenergic receptors (PC12 pheochromocytoma cells) we have found that moxonidine acts as an agonist at the cell level and I1-imidazoline receptor activation leads to the production of the second messenger diacylglycerol, most likely through direct activation of phosphatidylcholine-selective phospholipase C. The obese spontaneously hypertensive rat (SHR; SHROB strain) shows many of the abnormalities that cluster in human syndrome X, including elevations in blood pressure, serum lipids and insulin. SHROB and their lean SHR littermates were treated with moxonidine at 8 mg/kg per day. SHROB and SHR treated with moxonidine showed not only lowered blood pressure but also improved glucose tolerance and facilitated insulin secretion in response to a glucose load. Because alpha2-adrenergic agonists impair glucose tolerance, I1-imidazoline receptors may contribute to the multiple beneficial effects of moxonidine treatment.

CONCLUSION

The I1-imidazoline receptor is a specific high-affinity binding site corresponding to a functional cell-surface receptor mediating the antihypertensive actions of moxonidine and other second-generation centrally-acting agents, and may play a role in countering insulin resistance in an animal model of metabolic syndrome X.

A comparative study of the reversal by different alpha 2-adrenoceptor antagonists of the central sympatho-inhibitory effect of clonidine

1. The recovery of the clonidine-induced hypotension, bradycardia and sympatho-inhibition produced by several putative alpha 2-adrenoceptor antagonists was investigated in pentobarbitone anaesthetized rats. The activity of four substances containing an imidazoline structure: idazoxan, methoxy-idazoxan, BRL44408 and atipamezole was compared with the effect of fluparoxan, yohimbine and L-657,743; in addition the effect of the alpha 1-adrenoceptor antagonist, prazosin, was also studied. 2. Prazosin (0.03-1 mg kg-1, i.v.) failed to alter the sympatho-inhibitory and hypotensive effects of clonidine (10 micrograms kg-1, i.v.). L-657,743 (0.01-1 mg kg-1, i.v.) induced a recovery of blood pressure, heart rate and renal sympathetic nerve activity. Yohimbine (0.03-3 mg kg-1, i.v.) completely reversed the sympatho-inhibitory effect of clonidine but did not alter its hypotensive effect. 3. The four imidazoline drugs: idazoxan (10-300 micrograms kg-1, i.v.), methoxy-idazoxan (1-100 micrograms kg-1, i.v.), BRL44408 (0.1-3 mg kg-1, i.v.) and atipamezole (0.03-1 mg kg-1, i.v.) and fluparoxan (10-300 micrograms kg-1, i.v.) reversed the clonidine-induced hypotension but produced only a partial recovery of the renal sympathetic nerve activity and of the heart rate. After pretreatment with prazosin (0.1 mg kg-1, i.v.), the recovery of the sympathetic nerve activity elicited by these compounds was significantly higher. In hexamethonium (10 mg kg-1, i.v.) pretreated rats, these five drugs induced dose-related hypertension which was reduced by pretreatment with prazosin (0.1 mg kg-1, i.v.). 4. Our results indicate that the putative alpha 2-adrenoceptor antagonists idazoxan, methoxy-idazoxan, BRL44408, atipamezole and fluparoxan also have a peripheral hypertensive effect which is mediated through activation of vascular alpha 1-adrenoceptors; this property of the compounds may be partly responsible for the reversal of the hypotensive action of clonidine. Considering the structure and the affinities of the drugs tested, our data indirectly suggest that alpha 2A-adrenoceptors may be implicated in the central sympatho-inhibitory effects of clonidine.

Involvement of alpha 2-adrenoceptors in the cardiovascular effects of moxonidine

The central sympathoinhibition caused by moxonidine has been explained by activation of alpha 2-adrenoceptors on the one hand, and by an action at imidazoline I1 receptors on the other hand. In order to examine these possibilities, effects of moxonidine were compared with those of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304), an alpha 2-adrenoceptor agonist with very low affinity for I1 receptors, in conscious rabbits. The interaction with yohimbine, an alpha 2-adrenoceptor antagonist with very low affinity for imidazoline I1 receptors, was also studied. Moxonidine 3-100 micrograms kg-1 and UK 14304 1-30 micrograms kg-1 i.v. elicited similar effects: they decreased arterial blood pressure after a transient increase, decreased renal sympathetic nerve activity (recorded with chronically implanted electrodes), decreased heart rate and decreased the plasma noradrenaline concentration. Yohimbine given i.v. antagonized the effects of moxonidine and of UK 14304 in a similar manner. Yohimbine injected into the cisterna magna (i.c.) prevented the hypotension but did not change the decrease in plasma noradrenaline and heart rate, again in the case of both moxonidine and UK 14304. The agreement of the effect patterns of moxonidine and UK 14304, and the similar antagonism of yohimbine against either drug, demonstrate involvement of alpha 2-adrenoceptors in their central sympathoinhibitory action. The resistance of the bradycardia and the plasma noradrenaline fall against yohimbine i.c. indicates a contribution of presynaptic alpha 2-adrenergic inhibition of transmitter release from postganglionic sympathetic neurons to the overall reduction of sympathetic tone.

Involvement of peripheral presynaptic inhibition in the reduction of sympathetic tone by moxonidine, rilmenidine and UK 14304

We studied the possibility that presynaptic inhibition of transmitter release from postganglionic sympathetic neurons contributes to the overall reduction of sympathetic tone produced by moxonidine, rilmenidine and 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline tartrate (UK 14304). In pithed rabbits without electric stimulation, moxonidine, rilmenidine and UK 14304 caused a long-lasting, > 10 min, increase in arterial pressure. Heart rate was not changed. In pithed rabbits in which sympathetic tone was created by electric stimulation through the pithing rod (2 Hz), the same doses of moxonidine, rilmenidine and UK 14304 caused only a brief, < 10 min, blood pressure rise. Heart rate was decreased, as were the plasma concentrations of noradrenaline and adrenaline. Dose-response curves for the effects on the plasma noradrenaline concentration (stimulated pithed rabbits) were compared with previously obtained dose-response curves for depression of renal sympathetic nerve activity (conscious rabbits). For each drug, the curve describing peripheral presynaptic inhibition and the curve describing central sympathoinhibition were very similar. Both the power and the dose dependence of the peripheral inhibitory effect support its contribution to the overall decrease in sympathetic tone produced by clonidine-like drugs in intact animals. The peripheral effect in all likelihood consists in activation of presynaptic alpha 2-autoreceptors. The agreement of the dose-response curves for the peripheral and for the central effect supports the view that the central effect, like the peripheral one, is mediated through alpha 2-adrenoceptors.

Mechanism of sympathoinhibition by imidazolines

The aim of the present study is to characterize the cardiovascular effects of rilmenidine and moxonidine, two recently developed centrally acting antihypertensive drugs. Rilmenidine and moxonidine are alpha 2-adrenoceptor agonists and, in addition, possess affinity for imidazoline (I1)-receptors. To determine if alpha 2- or I1-receptors are involved in sympathoinhibition, rilmenidine and moxonidine were compared with UK 14304, an alpha 2-agonist devoid of affinity for I1-receptors, and antagonism by the "pure" alpha 2-adrenoceptor antagonists yohimbine, SK&F86466, and RX821002 was studied. When injected intravenously into conscious rabbits, rilmenidine and moxonidine, on the one hand, and UK 14304, on the other, elicited a similar pattern of effects. Thus, transient hypertension was followed by long-lasting hypotension accompanied by a decrease in heart rate, renal sympathetic nerve firing rate, and plasma norepinephrine concentration. The effects of rilmenidine, moxonidine, and UK 14304 were antagonized by intravenously administered yohimbine, SK&F86466, and RX821002. The effects of moxonidine and UK 14304 were also prevented by yohimbine injected into the cisterna magna. Altogether, the degree of antagonism of the effects of rilmenidine and moxonidine did not differ from the degree of antagonism of the effects of UK 14304. Rilmenidine, moxonidine, and UK 14304 were also given to pithed rabbits in which a constant sympathetic tone was maintained by electrical stimulation of the sympathetic nerves. At doses that caused sympathoinhibition in conscious rabbits, they lowered the plasma norepinephrine concentration markedly. Our experiments show by direct measurement of sympathetic nerve activity and plasma norepepinephrine concentration that rilmenidine, moxonidine, and UK 14304 cause sympathoinhibition. As a consequence, blood pressure and heart rate decrease. The simplest interpretation of the blockade of central sympathoinhibition by the selective alpha 2-adrenoceptor antagonists is that rilmenidine, moxonidine, and UK 14304 primarily activated alpha 2-adrenoceptors. The decrease in plasma norepinephrine in pithed rabbits indicates peripheral presynaptic alpha 2-adrenoceptor-mediated inhibition of norepinephrine release per action potential from postganglionic sympathetic axons and suggests a contribution of this mechanism to the overall reduction in sympathetic tone.

Possible structural and functional relationships between imidazoline receptors and alpha 2-adrenoceptors

Although it is now well established that imidazoline receptors and alpha 2-adrenoceptors are discrete entities with distinct endogenous ligands, the two receptor classes apparently have several common features. While the catecholamines stimulate alpha 2-adrenoceptors but not imidazoline receptors, agmatine, a guanidine analog that may be an endogenous imidazoline receptor ligand, can interact with both I1 and I2 imidazoline receptors as well as alpha 2-adrenoceptors, although, interestingly, other guanidines such as guanabenz are highly selective for alpha 2-adrenoceptors versus I1 receptors. Most I1 receptor agonists such as moxonidine, rilmenidine, and clonidine can also stimulate alpha 2-adrenoceptors, and the same physiological response is produced by activation of central I1 receptors and alpha 2-adrenoceptors, but their anatomical locations differ. The imidazoline idazoxan is an antagonist at I1, I2, and alpha 2-receptors, but minor structural alterations of idazoxan can result in molecules with selectivity for either alpha 2-adrenoceptors or imidazoline receptors. The precise mode of interaction of imidazoline agonists and antagonists with the alpha 2-adrenoceptor is not yet understood, and structures of the imidazoline receptors are still unknown. Nevertheless, the fact that many agents can stimulate or block both receptor classes, combined with the fact that alpha 2-adrenoceptors and I1 receptors can mediate identical physiological responses, suggests that many common structural features may be present.

Importance of imidazoline receptors in the cardiovascular actions of centrally acting antihypertensive agents

Increasing evidence indicates that the hypotensive effect of centrally acting antihypertensive drugs is not due to stimulation of alpha 2-adrenoceptors but to action on imidazoline receptors (IR). This has led to the development and recent clinical use of second generation agents such as rilmenidine and moxonidine that possess a much greater selectivity toward these nonadrenergic receptors. However, relatively few studies have examined the role of these receptors in conscious animals or have adequately accounted for the alpha 2-adrenoceptor antagonist properties of IR antagonists such as idazoxan. We have taken the approach of initially calibrating the alpha 2-adrenoceptor antagonist potency of intracisternally (ic) administered idazoxan and the IR-1 receptor antagonist efaroxan against 2-methoxyidazoxan, a highly selective alpha 2-adrenoceptor antagonist with little or no imidazoline antagonist effect. This was done using alpha-methyldopa, a hypotensive agent affecting only alpha 2-adrenoceptors. Thus, we chose doses of the antagonists with equal alpha 2-adrenoceptor blocking action such that differences in the ability of idazoxan or efaroxan compared to 2-methoxy-idazoxan to reverse the hypotension produced by rilmenidine, moxonidine, or clonidine indicate an interaction with IR. By this method we found that the hypotensive effects of rilmenidine and moxonidine at moderate intracisternal doses were more readily reversed by the imidazoline antagonists than by 2-methoxy-idazoxan, indicating that IR were largely responsible for their hypotensive actions. By contrast, clonidine's effects were equally reversed by all antagonists, suggesting interaction mainly with alpha 2-adrenoceptors. In conscious rabbits with chronic renal sympathetic nerve electrodes we examined the effect of rilmenidine and alpha-methyldopa on the renal sympathetic baroreflex. Both drugs reduced renal sympathetic nerve activity and sympathetic baroreflex responses, but only the effect of rilmenidine was preferentially reversed by idazoxan. Thus, both IR and central alpha 2-adrenoceptor receptors can influence the renal baroreflex, but the former are relatively more important for the actions of rilmenidine. We recently examined the possible sites of action of rilmenidine in anesthetized rabbits and showed that sixfold lower doses were required to reduce blood pressure when the drug was injected into the rostral ventrolateral medulla compared to intracisternal administration. At this site rilmenidine also reduced renal sympathetic tone and inhibited renal sympathetic baroreflex responses. By contrast, rilmenidine was relatively ineffective when injected into the nucleus of the solitary tract. These experiments support the view that rilmenidine acts primarily at IR in the rostral ventrolateral medulla to reduce sympathetic tone and modulate sympathetic baroreflexes.

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.