Clonidine-displacing substance (CDS) and its putative imidazoline receptor. New leads for further divergence of alpha 2-adrenergic receptor activity

Imidazoleacetic acid-ribotide in vestibulo-sympathetic pathway neurons

Imidazole-4-acetic acid-ribotide (IAARP) is a putative neurotransmitter/modulator and an endogenous regulator of sympathetic drive, notably systemic blood pressure, through binding to imidazoline receptors. IAARP is present in neurons and processes throughout the CNS, but is particularly prevalent in regions that are involved in blood pressure control. The goal of this study was to determine whether IAARP is present in neurons in the caudal vestibular nuclei that participate in the vestibulo-sympathetic reflex (VSR) pathway. This pathway is important in modulating blood pressure upon changes in head position with regard to gravity, as occurs when humans rise from a supine position and when quadrupeds climb or rear. Sinusoidal galvanic vestibular stimulation was used to activate the VSR and cfos gene expression in VSR pathway neurons of rats. These subjects had previously received a unilateral FluoroGold tracer injection in the rostral or caudal ventrolateral medullary region. The tracer was transported retrogradely and filled vestibular neuronal somata with direct projections to the injected region. Brainstem sections through the caudal vestibular nuclei were immunostained to visualize FluoroGold, cFos protein, IAARP and glutamate immunofluorescence. The results demonstrate that IAARP is present in vestibular neurons of the VSR pathway, where it often co-localizes with intense glutamate immunofluorescence. The co-localization of IAARP and intense glutamate immunofluorescence in VSR neurons may represent an efficient chemoanatomical configuration, allowing the vestibular system to rapidly up- and down-modulate the activity of presympathetic neurons in the ventrolateral medulla, thereby altering blood pressure.

Imidazoleacetic acid-ribotide induces depression of synaptic responses in hippocampus through activation of imidazoline receptors

Imidazole-4-acetic acid-ribotide (IAA-RP), an endogenous agonist at imidazoline receptors (I-Rs), is a putative neurotransmitter/regulator in mammalian brain. We studied the effects of IAA-RP on excitatory transmission by performing extracellular and whole cell recordings at Schaffer collateral-CA1 synapses in rat hippocampal slices. Bath-applied IAA-RP induced a concentration-dependent depression of synaptic transmission that, after washout, returned to baseline within 20 min. Maximal decrease occurred with 10 μM IAA-RP, which reduced the slope of field extracellular postsynaptic potentials (fEPSPs) to 51.2 ± 5.7% of baseline at 20 min of exposure. Imidazole-4-acetic acid-riboside (IAA-R; 10 μM), the endogenous dephosphorylated metabolite of IAA-RP, also produced inhibition of fEPSPs. This effect was smaller than that produced by IAA-RP (to 65.9 ± 3.8% of baseline) and occurred after a further 5- to 8-min delay. The frequency, but not the amplitude, of miniature excitatory postsynaptic currents was decreased, and paired-pulse facilitation (PPF) was increased after application of IAA-RP, suggesting a principally presynaptic site of action. Since IAA-RP also has low affinity for α(2)-adrenergic receptors (α(2)-ARs), we tested synaptic depression induced by IAA-RP in the presence of α(2)-ARs, I(1)-R, or I(3)-R antagonists. The α(2)-AR antagonist rauwolscine (100 nM), which blocked the actions of the α(2)-AR agonist clonidine, did not affect either the IAA-RP-induced synaptic depression or the increase in PPF. In contrast, efaroxan (50 μM), a mixed I(1)-R and α(2)-AR antagonist, abolished the synaptic depression induced by IAA-RP and abolished the related increase in PPF. KU-14R, an I(3)-R antagonist, partially attenuated responses to IAA-RP. Taken together, these data support a role for IAA-RP in modulating synaptic transmission in the hippocampus through activation of I-Rs.

Imidazoleacetic acid-ribotide: an endogenous ligand that stimulates imidazol(in)e receptors

We identified the previously unknown structures of ribosylated imidazoleacetic acids in rat, bovine, and human tissues to be imidazole-4-acetic acid-ribotide (IAA-RP) and its metabolite, imidazole-4-acetic acid-riboside. We also found that IAA-RP has physicochemical properties similar to those of an unidentified substance(s) extracted from mammalian tissues that interacts with imidazol(in)e receptors (I-Rs). ["Imidazoline," by consensus (International Union of Pharmacology), includes imidazole, imidazoline, and related compounds. We demonstrate that the imidazole IAA-RP acts at I-Rs, and because few (if any) imidazolines exist in vivo, we have adopted the term "imidazol(in)e-Rs."] The latter regulate multiple functions in the CNS and periphery. We now show that IAA-RP (i) is present in brain and tissue extracts that exhibit I-R activity; (ii) is present in neurons of brainstem areas, including the rostroventrolateral medulla, a region where drugs active at I-Rs are known to modulate blood pressure; (iii) is present within synaptosome-enriched fractions of brain where its release is Ca(2+)-dependent, consistent with transmitter function; (iv) produces I-R-linked effects in vitro (e.g., arachidonic acid and insulin release) that are blocked by relevant antagonists; and (v) produces hypertension when microinjected into the rostroventrolateral medulla. Our data also suggest that IAA-RP may interact with a novel imidazol(in)e-like receptor at this site. We propose that IAA-RP is a neuroregulator acting via I-Rs.

The effects of imidazoline agents on the aggregation of human platelets

Clonidine (2-[(2,6-dichlorophenyl)amino]-2-imidazoline), an imidazoline alpha(2)-adrenoceptor agonist, is known to exert complex effects on human platelet aggregation distinct from those of the catecholamines, which are non-imidazoline alpha-adrenoceptor agonists. This study has investigated the aggregatory/anti-aggregatory effects of various imidazolines on human platelets. Blood samples were taken from normal volunteers and platelet aggregation was assessed by a turbidimetric method using a Chronolog aggregometer. Noradrenaline (2 microM) and adenosine diphosphate (1 microM) were used as aggregating agents. The results showed that, with the exception of moxonidine, all of the imidazoline agents used (with or without alpha(2)-adrenoceptor activity) were able to inhibit noradrenaline-induced platelet aggregation. Compared with the non-imidazoline alpha(2)-adrenergic antagonist, yohimbine, the rank order of potency was: efaroxan (IC50 = 3.07 x 10(-8) M) > idazoxan (IC50 = 1.74 x 10(-7) M) > tolazoline (IC50 = 3.90 x 10(-7) M) > clonidine (IC50 = 1.49 x 10(-6) M) congruent with antazoline (IC50 = 1.77 x 10(-6) M) > yohimbine (IC50 = 3.19 x 10(-6) M) > rilmenidine (IC50 = 1.27 x 10(-5) M) > moxonidine (IC50 > 10(-4) M). Clonidine-displacing substance (CDS), a putative endogenous ligand at imidazoline receptors, was found to inhibit noradrenaline-induced platelet aggregation. Harmane, norharmane and agmatine, putative candidates for the active principle of CDS, had no effect on noradrenaline-induced platelet aggregation. In contrast to noradrenaline-induced aggregation, ADP-induced platelet aggregation was neither potentiated nor inhibited by the imidazoline agents, with the exceptions of clonidine and moxonidine. In conclusion, most imidazoline agents effectively inhibit noradrenaline-induced human platelet aggregation. The lack of effect of moxonidine and the proposed endogenous ligands suggested this effect was mediated by an 'atypical' non-adrenoceptor imidazoline-binding site. The results indicated an anti-aggregatory role of imidazoline compounds on noradrenaline-induced human platelet aggregation. In addition, CDS might be an endogenous modulator that prevented platelet hyper-reactivity to catecholamine stimulation.

Inhibition by clonidine of the carbachol-induced tension development and nonselective cationic current in guinea pig ileal myocytes

Effects of clonidine, an imidazoline derivative as well as alpha2-adrenoceptor agonist, on carbachol (CCh)-evoked contraction in guinea pig ileal smooth muscle were studied using isometric tension recording. To investigate the cellular mechanisms of the inhibitory effect of clonidine, its effects on CCh-evoked nonselective cationic current (I(CCh)), voltage-dependent Ca2+ current (I(Ca)) and voltage-dependent K+ current (I(K)) was also studied using patch-clamp recording techniques in single ileal cells. Clonidine inhibited the contraction evoked by CCh (1 microM) in a concentration-dependent manner with an IC50 valve of 61.7 +/- 2.5 microM. High K+ (40 mM)-evoked contraction was only slightly inhibited even when clonidine was used at 300 microM. Externally applied clonidine inhibited I(CCh) dose-dependently with an IC50 of 42.0 +/- 2.6 microM. When applied internally via patch pipettes, clonidine was without effect. An I(CCh)-like current induced by GTPgammaS was also inhibited by bath application of clonidine. None of KU14R and BU224, both imidazoline receptor blockers, and yohimbine, an alpha2-adrenergic blocker, significantly affects the inhibitory effect of clonidine on I(CCh). Clonidine (300 microM) only slightly decreased membrane currents flowing through voltage-gated Ca2+ channels or K+ channels. These data indicate that clonidine relaxes smooth muscle contraction produced by muscarinic receptor activation and suggest that the effect of clonidine seems due mainly to inhibition of I(CCh) via acting directly on the involved cationic channel.

Imidazoline and alpha(2a)-adrenoceptor binding sites in postmenopausal women before and after estrogen replacement therapy

BACKGROUND

Platelet alpha(2A)-adrenoceptors (alpha(2A)AR) and imidazoline binding sites (subtype I(1)) have been proposed as peripheral markers of brain stem receptors that mediate sympathetic outflow and are reported to be elevated in major depression.

METHODS

In our study, p[(125)I]-iodoclonidine was used to assess platelet alpha(2A)AR and I(1) binding sites in healthy postmenopausal women (n = 34) compared with healthy women of reproductive age (n = 26). Receptor determinations were repeated in 19 postmenopausal women following 59-60 days of estrogen replacement therapy (ERT; 0.1 mg estradiol transdermal patches).

RESULTS

I(1) binding sites were twofold higher in platelets of postmenopausal women compared with women of reproduction age but were down-regulated (normalized) after 59-60 days of ERT. All other binding parameters, including platelet alpha(2A)AR density, were not different between groups nor were they changed after ERT. Platelet I(1) densities after 59-60 days of ERT were positively correlated with plasma luteinizing hormone concentrations.

CONCLUSIONS

It is suggested that increased imidazoline binding sites might be associated with mood and behavioral changes in postmenopausal women.

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.

Imidazoline recognition sites and stomach function

Radioligand binding experiments carried out in cell membranes from rat and human stomach revealed the existence of non-adrenoceptor [3H]clonidine and [3H]idazoxan binding sites and of [3H]DTG (1,2-di-(2-tolyl)guanidine) binding sites. In rat stomach, specific binding was inhibited by imidazolines and guanidines and by non-imidazoline sigma-site ligands, respectively, at different rank orders of affinity, suggesting the existence of non-I1/non-I2 [3H]clonidine binding sites, I2-imidazoline binding sites as well as sigma 2-like-sites. These sites are not directly related to a postsynaptic contractile effect on rat gastric smooth muscle or to acid release from isolated gastric glands. Finally, we demonstrated that the gastric pathogen Helicobacter pylori is able to form and to release the endogenous imidazoline receptor ligand agmatine and that considerable amounts of agmatine are present in human gastric juice. The quantities of agmatine were higher in gastric juice from H. pylori-positive than H. pylori-negative patients.

Clonidine-displacing substance and its putative role in control of insulin secretion: a minireview

1. Imidazoline-binding sites, or I-sites, are a class of recently defined nonadrenoceptor recognition sites whose most potent ligands are imidazolines and related compounds. 2. The pancreatic islet beta-cell I-site, which mediates imidazoline-induced stimulation of insulin release, appears to be the first site to be pharmacologically defined with selective agonists and antagonists. 3. The natural ligand for imidazoline recognition sites is still unknown. The strongest candidate is clonidine-displacing substance (CDS), originally identified in extracts of rat and bovine brain. However, the bioactive molecule has not been identified definitively. Agmatine, a decarboxylated derivative of arginine, also binds to both I-sites and alpha2-adrenoceptors (Li et al., 1994), and is, by definition, a CDS molecule. 4. In the endocrine pancreas, agmatine is a weak insulin secretagogue, which induces a slowly developing secretory response. However, this profile does not correlate with interaction at the islet I-site, and thus agmatine is unlikely to be an endogenous secretagogue acting functionally at the islet I-site. 5. Crude preparations of CDS from rat brain can potentiate glucose-induced insulin release and reverse the effects of diazoxide in rat and human islets of Langerhans. These two effects are also subject to blockade by the imidazoline antagonists RX801080 and KU14R. Furthermore, islets that were desensitized to the effects of the imidazoline secretagogue efaroxan (after 18-hr culture with imidazoline) were refractory to the actions of CDS. 6. Overall, CDS displays many characteristics expected of an endogenous regulator of insulin secretion acting through the islet beta-cell imidazoline site. This evidence strengthens the hypothesis that the islet beta-cell imidazoline site mediating control of insulin release in the endocrine pancreas is a biologically relevant receptor. Furthermore, a physiological role of CDS in the endocrine pancreas cannot be excluded.

Pharmacology and function of imidazole 4-acetic acid in brain

1. Imidazole 4-acetic acid (IMA) is a naturally occurring metabolite in brain, although it is unclear what biochemical pathways are involved in its biosynthesis and breakdown. Some evidence, however, suggests that IMA is an oxidation product of histamine. 2. The compound has pronounced neuropharmacological properties, many of which are consistent with an activation of GABA(A) receptors. Indeed, IMA is able to displace [3H]GABA from GABA(A) sites in a potent manner. 3. IMA displays definite partial agonist characteristics as an enhancer of benzodiazepine binding to the GABA(A) receptor complex in membrane preparations. In addition, it has an affinity for GABA(C) receptors, where it seems to act as an antagonist, and perhaps as a weak partial agonist. A third recognition site for IMA in brain is the I1-imidazoline receptor. 4. Parenteral administration to experimental animals leads to a sleep-like state which can often be accompanied by seizures. In addition, central application of IMA has been associated with a dose-related reduction in arterial pressure and sympathetic nervous discharge. 5. No specific receptor site or uptake system for IMA has yet been discovered, adding uncertainty to its role in central nervous system function. Yet the possibility cannot be overlooked that IMA plays a role in regulating blood pressure.

'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.

Determination of agmatine in brain and plasma using high-performance liquid chromatography with fluorescence detection

Decarboxylated arginine, agmatine, is a neurotransmitter candidate for imidazoline receptors. A method is described to measure agmatine in rat brain and human plasma by isocratic high-performance liquid chromatography (HPLC) with fluorescence detection and o-phthalaldehyde derivatization. Quantitation is based on the method of additions of internal agmatine spikes. This assay has sensitivity in the low picomole range and a detection limit of 100 fmol. The correlation coefficient for the agmatine standard curve was 0.999+/-0.001 S.D., and intra- and inter-assay C.V.s were less than 8%. The accuracy of our isocratic method compared favorably with a gradient HPLC protocol, originally developed for bacterial agmatine, which we modified for use with tissues. Agmatine concentrations in rat brain were proportioned similarly to the regional distribution of imidazoline-1 receptors. These methods can be used as reliable research tools in various biological samples.

The effect of the putative endogenous imidazoline receptor ligand, clonidine-displacing substance, on insulin secretion from rat and human islets of Langerhans

1. The effects of a rat brain extract containing clonidine-displacing substance (CDS), a putative endogenous imidazoline receptor ligand, on insulin release from rat and human isolated islets of Langerhans were investigated. 2. CDS was able to potentiate the insulin secretory response of rat islets incubated at 6 mM glucose, in a dose-dependent manner. The magnitude of this effect was similar to that in response to the well-characterized imidazoline secretagogue, efaroxan. 3. CDS, like other imidazoline secretagogues, was also able to reverse the inhibitory action of diazoxide on glucose-induced insulin release, in both rat and human islets. 4. These effects of CDS on secretion were reversed by the imidazoline secretagogue antagonists, RX801080 and the newly defined KU14R, providing the first evidence that imidazoline-mediated actions of CDS can be blocked by specific imidazoline antagonists. 5. The effects of CDS on insulin secretion were unaffected when the method of preparation involved centri-filtration through a 3,000 Da cut-off membrane or when the extract was treated with protease. These results confirm that the active principle is of low molecular weight and is not a peptide. 6. Overall, the data suggest that CDS behaves as a potent endogenous insulin secretagogue acting at the islet imidazoline receptor.

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.

An antiserum to idazoxan recognizes an immunoreactive substance in human serum and cerebral spinal fluid which is not agmatine

A polyclonal antibody was generated in rabbits to an idazoxan-albumin antigen. The anti-idazoxan antiserum had high affinity for unconjugated 3H-idazoxan (Kd of 19.8 nM) in a radio-immunoassay (RIA). Of various drugs and native molecules only idazoxan potently (Ki of 24 nM) inhibited 3H-idazoxan binding to the anti-idazoxan antibody. A few drugs weakly inhibited 3H-idazoxan binding (IC50 > 605 microM) with rank order of UK 14304 > guanabenz > cirazoline > amiloride > naphazoline. Neither agmatine, an endogenous clonidine displacing substance (CDS), catecholamines or imidazoles inhibited the binding of 3H-idazoxan to the anti-idazoxan antibody. The anti-idazoxan RIA was 4-6 fold more sensitive than an antibody to para-amino clonidine. The CDS detected by ligand displacement from bovine brain dose-dependently inhibited 3H-idazoxan binding. This immunoreactive (ir-) CDS activity was present in human (0.9-4.1 U/ml) and rat sera (1-2 U/ml) and in the cerebro-spinal fluid of eight patients with serious disease of the central nervous system, but not in controls. We conclude: (1) an anti-idazoxan RIA is a sensitive, selective and clinically applicable RIA for measuring ir-CDS; (2) ir-CDS is not agmatine; (3) CDS represents a family of endogenous ligands for imidazoline receptors including ir-CDS and agmatine.

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.

Agmatine does not have activity at alpha 2-adrenoceptors which modulate the firing rate of locus coeruleus neurones: an electrophysiological study in rat

Agmatine (decarboxylated arginine) has been proposed as an endogenous ligand for non-adrenoceptor, imidazoline binding sites, but also binds to alpha 2-adrenoceptors. The interaction of agmatine with alpha 2-adrenoceptors was evaluated by studying the effect of agmatine on the firing rate of locus coeruleus (LC) neurones using extracellular recordings in anesthetized rats and rat brain slices. In vivo, local application of agmatine into the LC caused a slight and short-lasting increase in cell firing rate (P < 0.005). In vitro, agmatine failed to change the firing rate of LC neurones nor did it antagonize the inhibitory effect of noradrenaline on these cells. Since alpha 2-adrenoceptors are known to inhibit the firing of LC cells, we conclude that agmatine does not have agonist or antagonist properties at alpha 2-adrenoceptors of these neurones.

Platelet I1-imidazoline binding sites are elevated in depression but not generalized anxiety disorder

Depressed patients have been reported to have a higher than normal density of platelet binding sites for 3H-clonidine, an alpha 2-adrenoceptor agonist. Paradoxically, other studies using 3H-alpha 2, antagonists have found no differences from controls. Because 3H-clonidine interacts with platelet alpha 2-adrenoceptors to form G-protein complexes, whereas 3H-alpha 2-antagonists bind with uncoupled receptors, an elevation in G-protein coupling might explain this paradox. Another possibility is that depression might be associated with increased non-adrenergic I1-imidazoline binding sites, which are also clonidine sensitive. To distinguish these possibilities, we utilized p125I-clonidine to measure density (Bmax) and affinity (KD) of platelet G-protein coupled alpha 2-adrenoceptors as well as platelet I1 binding sites, and compared diagnostic groups of major depressive disorder (MDD), generalized anxiety disorder (GAD) and healthy subjects. Specific inhibition of binding by norepinephrine (NE = 10 microM) was used to selectively quantify alpha 2-adrenoceptors, whereas inhibition by 10 microM moxonidine (a > 100-fold selective I1 ligand) quantified I1 binding sites under a NE mask. I1 sites were found to be markedly elevated by, on average, +136% in MDD patients (p = .0007), whereas there was only a marginal increase in alpha 2-adrenoceptor Bmax values in MDD patients (p = .08; GAD and healthy subjects did not differ). Treatment of MDD patients for 6-8 weeks with desipramine downregulated I1 sites as well as alpha 2-adrenoceptors. Positive correlations were also noted for both sites: (a) between Bmax values and the severity of depression (using the Hamilton Depression Rating Scale); and (b) between end-of-treatment plasma desipramine concentrations and the extent of downregulation in Bmax values when subject groups were pooled. None of the binding parameters was associated with plasma catecholamine concentrations. The results suggest that an increased density of platelet I1 binding sites may partially explain the utility of radiolabeled clonidine as a potential biological marker for depressive illness, although an additional increase in G-protein coupling cannot be excluded.

Molecular characterization and isolation of a 45-kilodalton imidazoline receptor protein from the rat brain

Imidazoli(di)nes bind to molecular entities different from alpha 2-adrenoceptors: the so-called imidazoline receptors (IRs). Two main types of IRs have been described, the clonidine- and the idazoxan-preferring types, as well as other IRs whose pharmacological properties do not fit either type, but little is known about the molecular features of these receptors. In this study, IR proteins have been solubilized from the rat brain, using the zwitterionic detergent CHAPS, and analyzed by pharmacological and immunological means two of the four peak discriminated by gel filtration chromatography using [3H]idazoxan binding and a specific antibody. The IR eluted in the first peak accounted for 80% of the specific binding of [3H]idazoxan to solubilized brain membranes, and its pharmacological features corresponded to the non-adrenoceptor component of [3H]idazoxan binding in rat brain native membranes. The elution volume of this peak corresponded to a 130-140-kDa protein, but immunoblot analysis with a specific anti-IR antiserum showed the presence of a approximate 45-kDa IR protein, suggesting that this receptor is either an oligomeric protein complex or that it is associated with other proteins. This result was in agreement with the isolation and immunodetection of a 45-kDa peptide by affinity chromatography, which supported the relationship between this protein and a rat brain imidazoline binding site. The second peak, accounting for 15% of the specific binding of [3H]idazoxan to solubilized membranes, had a Mr of approximately 65-70,000, as determined by gel filtration chromatography and immunoblotting.(ABSTRACT TRUNCATED AT 250 WORDS)

The prejunctional inhibitory effect of alpha-methylnoradrenaline in the rat vas deferens is not mediated via alpha 2-adrenoceptors

In field-stimulated rat vas deferens, clonidine (10(-9)-10(-7) M) and alpha-methylnoradrenaline (10(-7)-3 x 10(-5) M) concentration-dependently inhibited twitch response to electrical field stimulation. The inhibitory effects of clonidine and alpha-methylnoradrenaline were similarly reduced by phenoxybenzamine (10(-6) M). However, idazoxan (3 x 10(-7) M) antagonized clonidine but not alpha-methylnoradrenaline-induced responses. The inhibitory effect of alpha-methylnoradrenaline was also not antagonized by phentolamine (3 x 10(-7) M), SK&F 104856 (2-vinyl-7-chloro-3,4,5,6-tetrahydro-4-methylthienol[4,3,2 ef][3]benzazepine) 3 x 10(-6) M) or yohimbine 3 x 10(-7) M). These antagonists however, attenuated the twitch-inhibiting effect of clonidine. The -logKB values were 8.02 +/- 0.05, 6.91 +/- 0.10, and 7.58 +/- 0.07 for phentolamine, SK&F 104856 and yohimbine respectively. In-vivo treatment of rats with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (6 mg kg-1), to inactivate prejunctional alpha 2-adrenoceptors, attenuated clonidine, but not alpha-methylnoradrenaline-induced responses. It was concluded that adrenoceptors are not involved in the prejunctional inhibitory effects of alpha-methylnoradrenaline in the rat vas deferens.

Clonidine and cirazoline inhibit activation of nicotinic channels in PC-12 cells

Clonidine and cirazoline bind with high affinity to a nonadrenergic site in the brain stem, the so-called imidazoline I1 receptor. Our aim was to determine the mechanism by which these receptors act and their possible linkage to signal-transducing heterotrimeric G-proteins. We examined the effects of clonidine and cirazoline on PC-12 cells, a neuronal cell line that is reported to possess the I1 site and have no alpha 2-adrenoceptors. In undifferentiated PC-12 cells loaded with the Ca2+ indicator dye fura-2, clonidine and cirazoline (10-100 microM) inhibited the increase in [Ca2+]i produced by nicotine (10 microM). This inhibition was not reversed by yohimbine (100 microM), and adrenaline and BHT 920 were ineffective at 100 microM. This effect was not inhibited by pretreatment with pertussis toxin (24 hours, 100 ng/ml) and not modulated by pretreatment with IBMX (100 microM). The nicotine-induced increase in [Ca2+]i is apparently due to Ca2+ entering via the intrinsic ion channel of the nicotinic acetylcholine receptor. Clonidine and cirazoline inhibited the inward current produced by nicotine (10 microM) as measured by the whole cell patch-clamp technique in differentiated PC-12 cells, recorded at a holding potential of -60 mV. In agreement with the results found with fura-2, inhibition of inward current was concentration dependent and not blocked by yohimbine (100 microM) or mimicked by adrenaline (100 microM). Pretreatment of PC-12 cells with pertussis toxin or infusion of GDP-beta-S (2 mM) via the patch pipette did not alter the inhibition of the nicotine-induced inward current by clonidine or cirazoline. Clonidine and cirazoline, but not adrenaline, displayed [3H]phencyclidine from Torpedo electroplaque membranes enriched in nicotinic acetylcholine receptors in a concentration-dependent manner (10-100 microM). Taken together, these results suggest that clonidine and cirazoline inhibit Na+ and Ca2+ entry through the nicotinic acetylcholine receptor via a nonadrenergic mechanism that is independent of G-proteins and cyclic nucleotides, presumably by direct blockade of the intrinsic ion channel of the nicotinic acetylcholine receptor.

The relationship between the adrenoceptor and nonadrenoceptor-mediated effects of imidazoline- and imidazole-containing compounds

This article brings together work on imidazoline or imidazole-containing compounds concerned with the pharmacology of alpha-adrenoceptors, principally on smooth muscle, to illustrate how imidazolines have contributed to the subclassification of alpha-adrenoceptors and how, against this background, attempts have been made to use this knowledge to uncover "nonadrenoceptor"-mediated biological effects of previously uncharacterized compounds, notably imidazole-containing dipeptides and "clonidine displacing substance" (CDS). Recent data are included on (1) the pharmacology of UK-14304, (2) nonadrenoceptor actions of phentolamine, (3) the pharmacology of tissue extracts containing imidazole-containing dipeptides and CDS activity, and (4) ligand binding data at I1 and I2 sites.

Endogenous ligands of imidazoline receptors: classic and immunoreactive clonidine-displacing substance and agmatine

1. There are several endogenous ligands that bind to I-receptors of both the I1 and I2 subclass. These include: (a) classic CDS, a partially purified entity isolated by the criteria that it displaces binding ligands to alpha 2- and I-receptors; (b) immunoreactive (ir)-CDS, a moiety that binds to antibodies raised against clonidine, para-amino-clonidine, or idazoxan; and (c) agmatine. 2. Classic-CDS, not yet defined structurally, binds to I1, I2, and alpha 2-adrenergic receptors, is neither a peptide nor a catecholamine, and has purportedly a molecular weight of 588 Da. By ligand binding assays, it was found in brain, serum, CSF, and placenta and in a neural-glial cell line. Partially purified classic CDS is bioactive. Like clonidine, it contracts aorta and vas deferens and inhibits platelet aggregation, effects largely attributable to agonism at alpha 2-adrenergic receptors. Unlike clonidine, it contracts rat gastric fundus and releases catecholamines from chromaffin cells, effects attributable to actions at I-receptors. Injected into the RVL, classic CDS alters arterial pressure, but the direction of change of pressure has differed between groups of investigators. However, in the absence of structure, it is possible that ligand binding and bioactivity may be attributable to different molecules. 3. Ir-CDS, also of unknown structure, is a material(s) that binds to antibodies raised against clonidine, PAC, or idazoxan. Ir-CDS, measured by radioimmunoassay, is unevenly distributed in brain with highest concentrations in the hypothalamus, midbrain, and dorsal medulla. It is contained in the gastric fundus, adrenal gland, heart, kidney, and serum in amounts substantially higher than found in brain. Ir-CDS may be elevated in the serum of some patients with hypertension and in the CSF of patients with structural brain disease. The concentration of ir-CDS and bioactivity on gastric fundus directly correlates, suggesting that it may share similarities with classic-CDS. However, until the structure of classic and ir-CDS is determined, the possibility that ligand binding and antibody recognition are properties of different molecules must be considered. 4. Agmatine (decarboxylated arginine) is the only endogenous molecule that, like CDS, binds to alpha 2- and I-receptors of both classes. It and its biosynthetic enzyme arginine decarboxylase are present in brain, and agmatine is widely distributed throughout the body. However, the distribution of agmatine and ir-CDS differs, whereas the biological actions of agmatine do not mimic those of classic CDS. Its presence raises the possibility of an alternative pathway for polyamine biosynthesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular and physiological properties of clonidine-displacing substance

An endogenous small molecular mass compound, termed clonidine-displacing substance (CDS), has been isolated and purified from bovine brain. The estimated level of CDS in bovine brain is 400-1,000 units/wet brain, with 1 unit of activity calculated to be approximately 1-2 ng. It is present in human serum, urine, and cerebrospinal fluid. The isolation procedure consists of initial aqueous and methanolic extractions followed by a series of HPLC chromatography steps (reverse phase and TSK sizing columns). The reverse-phase chromatography of CDS extracted under identical conditions from bovine brain and human serum show similar retention times. The final chromatography step gives a single active peak with a distinct ultraviolet spectrum, a single molecular peak m/z 587.8 +/- 2 in plasma desorption mass spectrometry (PDMS), and a unique pharmacological and physiological profile. Clonidine-displacing substance does not partition into organic solvents and it is ninhydrin and fluorescamine negative. All of these molecular properties clearly distinguish CDS from agmatine, an endogenous 130-dalton compound of far greater abundance which displays lower affinity for p-aminoclonidine-labeled sites in rat brain membranes. The ultraviolet spectrum of CDS consists of two aromatic peaks at 224 and 276 nm, whereas agmatine is an aliphatic substance with no ultraviolet absorbance. Like many antihypertensive drugs of the guanidine and imidazoline family of compounds, CDS recognizes alpha 2-adrenergic receptors, clonidine sites (IR-I1), and imidazoline sites (IR-I2). A good correlation exists between the affinities of various imidazoline/guanidine type ligands for IR-I2 in both human placenta and rat liver membranes which can be accurately determined because both tissues lack IR-I1 and alpha 2-adrenergic receptors. There is no correlation in the affinities of these ligands for IR-I2 of human-placental versus alpha 2-adrenergic receptors of human platelets. By uncovering the role of CDS in the central nervous system we will be able to understand the coupling of IRs to neurotransmission and, in turn, to changes in arterial pressure.

Imidazoline receptors in vascular smooth muscle and endothelial cells

We sought to determine if smooth muscle and endothelial cells of blood vessels express imidazoline receptors. Membranes of cultured smooth muscle cells specifically bind with high affinity to alpha 2-adrenergic ligands, [3H]p-aminoclonidine, [3H]rauwolscine, and [3H]idazoxan. All of [3H]rauwolscine and [3H]p-aminoclonidine but less than 10% of [3H]idazoxan binding was displaced by 10 microM epinephrine, indicating a nonadrenergic binding site for [3H]idazoxan. [3H]Idazoxan binding was inhibited with a rank order of potency: cirazoline > idazoxan > naphazoline >> guanabenz > amiloride > clonidine = phentolamine. Agmatine, an endogenous ligand for I-receptors, inhibited binding with a Ki of 240 +/- 25 nM. The binding of [3H]idazoxan to membranes of pulmonary artery endothelial cells was to both alpha 2-adrenergic and imidazoline receptors. Cultured smooth muscle cells, as well as rat carotid arterioles, were specifically immunostained by antibodies to an I-receptor-associated protein. We conclude that vascular smooth muscle and endothelial cells express not only alpha 2-adrenergic receptors but also I-receptors of the I2 subclass with high affinity for agmatine. Since serum contains an endogenous ligand for I-receptors, possibly agmatine, the results suggest the presence of a novel receptor mechanism on vascular smooth muscle which may regulate vascular tone.

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.

Why imidazoline receptor modulator in the treatment of hypertension?

The influence of the sympathetic nervous system on blood pressure control was impressively demonstrated in 1940 by bilateral excision of sympathetic nerve fibers. Thereafter, the first generation of drugs lowering blood pressure by central modulation of the sympathetic outflow through alpha 2-adrenoceptor for stimulation, such as alpha-methyldopa, guanabenz, clonidine, and guanfacine, were marketed. However, these compounds were often tolerated poorly, because they caused orthostatic hypotension, sedation, tachycardia or bradycardia, dry mouth, and reduced cardiac output. The mode of action of the second generation centrally acting antihypertensive drugs moxonidine and rilmenidine is different from that of the first generation compounds (e.g., clonidine). Contrary to clonidine, the newer drugs bind more selectively to I1-imidazoline receptors rather than to alpha 2-adrenoceptors where first-generation drugs act. The high affinity and selectivity of these two drugs for this recently discovered new receptor class make it possible to discriminate between I1-imidazoline receptor-mediated blood pressure lowering, on the one hand, and alpha 2-adrenoceptor-mediated side effects, on the other. Discrimination of the two effects was substantiated either by studies using moxonidine alone or in interaction experiments with I1-imidazoline receptor or alpha 2-adrenoceptor antagonists. The high selectivity of moxonidine at the I1-imidazoline receptor allows discrimination between alpha 2-adrenoceptors and I1-imidazoline receptors and is reflected in man by the relatively low incidence of adverse drug events during moxonidine treatment. Concentration of endazoline, a specific mediator of I1-imidazoline receptors, is elevated in some patients with essential hypertension. Modulation of I1-imidazoline receptors by moxonidine could be interpreted as antagonism with regard to the endogenous agonistic effect of the endogenous "transmitter" endazoline. On the other hand, moxonidine acted directly as an agonist at the putative I1-imidazoline receptor. Therefore, to clear the ground, characterization as well as physiological function of the mediator for imidazoline receptors seems essential. The therapeutic relevance of using drugs selective for I1-imidazoline receptors for blood pressure reduction in hypertensive patients is substantiated by the finding that in human rostral ventrolateral medulla (RVLM), which is essential in central blood pressure regulation, the relation between alpha 2-adrenoceptors and I1-imidazoline receptors is about one to ten (1:10). Reduction of a long-lasting sympathetic overdrive may avoid the deteriorating effects on the heart and peripheral circulation. These recent findings give a rational explanation for the very low incidence of sedation and the absence of respiratory depression, orthostatic hypotension, and rebound hypertension that banned the former central acting antihypertensive drugs from first-line treatment despite the advantages of central mediated blood pressure control.

Comparison of the interaction of agmatine and crude methanolic extracts of bovine lung and brain with alpha 2-adrenoceptor binding sites

1. In the present study we have evaluated whether alpha 2-adrenoceptor binding sites on bovine cerebral cortex membranes labelled by [3H]-clonidine, [3H]-idazoxan and [3H]-RX-821002 can distinguish between known agonists and antagonists. This model has then been used to compare the binding profiles of the putative non-catecholamine, clonidine-displacing substance (CDS), agmatine and crude methanolic extracts of bovine lung and brain. 2. Saturation studies carried out in the presence and absence of noradrenaline, 10 mumol 1(-1), revealed that the maximum number of binding sites on bovine cerebral cortex membranes for [3H]-idazoxan and [3H]-RX-821002 were approximately 60-80% greater than those for [3H]-clonidine (62.6 fmol mg-1 protein). Rauwolscine, the selective alpha 2-adrenoceptor antagonist, was approximately 100 fold more potent against each of the ligands than the selective alpha 1-adrenoceptor diastereoisomer, corynanthine. Also, the pKi value for the selective alpha 1-adrenoceptor prazosin against each ligand was less than 6. 3. Adrenaline, UK-14034, rauwolscine, corynanthine, RX-811059 and prazosin produced concentration-dependent inhibition of binding of all three 3H-ligands. The agonists, adrenaline and UK-14304, were approximately 5 and 10 fold less potent against [3H]-idazoxan and [3H]-RX-821002, respectively, than against [3H]-clonidine. In marked contrast, the antagonists, rauwolscine, corynanthine, RX-811059 and prazosin exhibited a different profile, being approximately 2-3 fold more potent against sites labelled by [3H]-RX-821002 and [3H]-idazoxan compared to sites labelled by [3H]-clonidine. 4. Agmatine and histamine produced a concentration-dependent displacement of [3H]-clonidine, [3H]-idazoxan and [3H]-RX-821002 binding to bovine cerebral cortex membranes. The pKi values for agmatine and histamine were independent of the 3H-ligand employed, approximately 4.8 and 4.5,respectively.5. Crude methanolic extracts of bovine brain and lung produced a concentration-dependent inhibition of [3H]-clonidine binding to bovine cerebral cortex membranes (>90%). Based on the volume of the extract that caused 50% inhibition of [3H]-clonidine binding, bovine lung contains 3 fold more CDS than bovine brain. Both extracts were at least 5 fold more potent against a2-adrenoceptor sites labelled by[3H]-clonidine than those labelled by [3H]-idazoxan and [3H]-RX-821002.6. All three 3H-ligands label the same population of alpha2-adrenoceptor binding sites on bovine cerebral cortex membranes, but [3H]-clonidine appears to label selectively the 'agonist' state of the sites: for which known agonists, adrenaline and UK-14304, exhibit a higher affinity. Our results indicate that neither agmatine nor histamine can account for the CDS activity present in crude extracts of bovine brain and lung. Moreover, these extracts appear to possess a binding profile similar to that of adrenaline and UK-14304, suggesting that they may possess agonist activity.

Effects of three alpha 2-adrenoceptor agonists, rilmenidine, UK 14304 and clonidine on bradykinin- and substance P-induced airway microvascular leakage in guinea-pigs

The effects of three alpha 2-adrenoceptor agonists, clonidine, rilmenidine and UK 14304 on the increase of microvascular permeability induced by bradykinin or substance P in guinea-pigs airways have been studied in vivo. Extravasation of intravenously (i.v.) injected Evans blue dye was used as index of permeability. The effects of the three alpha 2-adrenoceptor agonists on the contraction induced by bradykinin (0.3 micrograms.kg-1, i.v.) and substance P (0.3 micrograms.kg-1, i.v.) were also studied on the isolated guinea-pig trachea. The increase of plasma exudation induced by bradykinin (0.3 micrograms.kg-1, i.v.) was inhibited partially by rilmenidine and UK 14304 (20 micrograms and 100 micrograms, intratracheally) respectively. These two substances had no action on the effects of substance P. The effects of rilmenidine and UK 14304 were abolished by alpha 2-blockers (idazoxan 1mg.kg-1 i.v. and RX 821001 100 micrograms.kg-1, i.v.), but they were not altered by the alpha 1-blocker prazosin (30 micrograms.kg-1, i.v.). Under similar conditions, clonidine or the alpha 1-adrenoceptor agonist methoxamine were without significant effects. In vitro, rilmenidine and UK 14304 inhibited partially the contractile effects of bradykinin but not those of substance P. To conclude, both rilmenidine and UK 14304 inhibit the bradykinin-induced increase of vascular permeability in the airways, and they probably do so on peptidergic nerve endings at the prejunctional level since these substances are without effect on substance P. The absence of activity of clonidine in our study might be due to a difference in spectrum of action on the several types of alpha 2-adrenergic or imidazole receptors.

Excess catecholamines and the metabolic syndrome: should central imidazoline receptors be a therapeutic target?

A sympathetic overactivity plays a major role in the pathogenesis of cardiovascular diseases in Westernized affluent societies. Of importance is an increased caloric intake and psychosocial stress which are associated with a raised central sympathetic outflow and unfavourable changes in metabolic parameters. Normalization of central sympathetic outflow could thus be a major therapeutic target. The newly developed antihypertensive drugs moxonidine and rilmenidine reduce the excitatory activity of neurons of the rostral ventrolateral medulla (RVLM) via binding to imidazoline receptors. Using radio telemetry, it is shown that, in contrast to the first generation centrally acting drug clonidine, moxonidine did not result in rebound of blood pressure after drug withdrawal in rats with spontaneous hypertension. In accordance, moxonidine is characterized by a low affinity for alpha-adrenoceptors and exhibits few side-effects. It is proposed that normalization of central sympathetic outflow represents a causal approach for improving crucial features of the metabolic syndrome.

The effects of phenelzine and other monoamine oxidase inhibitor antidepressants on brain and liver I2 imidazoline-preferring receptors

1. The binding of [3H]-idazoxan in the presence of 10(-6) M (-)-adrenaline was used to quantitate I2 imidazoline-preferring receptors in the rat brain and liver after chronic treatment with various irreversible and reversible monoamine oxidase (MAO) inhibitors. 2. Chronic treatment (7-14 days) with the irreversible MAO inhibitors, phenelzine (1-20 mg kg-1, i.p.), isocarboxazid (10 mg kg-1, i.p.), clorgyline (3 mg kg-1, i.p.) and tranylcypromine (10 mg kg-1, i.p.) markedly decreased (21-71%) the density of I2 imidazoline-preferring receptors in the rat brain and liver. In contrast, chronic treatment (7 days) with the reversible MAO-A inhibitors, moclobemide (1 and 10 mg kg-1, i.p.) or chlordimeform (10 mg kg-1, i.p.) or with the reversible MAO-B inhibitor Ro 16-6491 (1 and 10 mg kg-1, i.p.) did not alter the density of I2 imidazoline-preferring receptors in the rat brain and liver; except for the higher dose of Ro 16-6491 which only decreased the density of these putative receptors in the liver (38%). 3. In vitro, phenelzine, clorgyline, 3-phenylpropargylamine, tranylcypromine and chlordimeform displaced the binding of [3H]-idazoxan to brain and liver I2 imidazoline-preferring receptors from two distinct binding sites. Phenelzine, 3-phenylpropargylamine and tranylcypromine displayed moderate affinity (KiH = 0.3-6 microM) for brain and liver I2 imidazoline-preferring receptors; whereas chlordimeform displayed high affinity (KiH = 6 nM) for these receptors in the two tissues studied, Clorgyline displayed very high affinity for rat brain (KiH = 40 pM) but not for rat liver I2 imidazoline-preferring receptors (KiH = 169 nM). 4. Preincubation of cortical or liver membranes with phenelzine (10-4 M for 30 min) did not alter the total density of I2 imidazoline-preferring receptors, indicating that this irreversible MAO inhibitor does not irreversibly bind to I2 imidazoline-preferring receptors. In contrast, preincubation with 10-6 Mclorgyline reduced by 40% the Bmax of [3H]-idazoxan to brain and liver I2 imidazoline-preferring receptors.5. Chronic treatment (7 days) with the inducers of cytochrome P-450 enzymes phenobarbitone (40 or 80 mg kg-1, i.p.), 3-methylcholanthrene (20 mg kg-1, i.p.) or 2-methylimidazole (40 mg kg-1, i.p.) did not alter the binding parameters of [3H]-idazoxan to brain and liver 12 imidazoline-preferring receptors.The compound SKF 525A, a potent inhibitor of cytochrome P-450 enzymes which forms a tight but reversible complex with the haemoprotein, completely displaced with moderate affinity (KiH = 2-10 microM)the specific binding of [3H]-idazoxan to brain and liver 12 imidazoline-preferring receptors. Preincubation of total liver homogenates with 3 x 10-4 M phenelzine in the presence of 10-3 M NADH, a treatment that irreversibly inactivates the haeme group of cytochrome P-450, did not reduce the density of liver I2 imidazoline-preferring receptors. These results discounted a possible interaction of [3H]-idazoxan with the haeme group of cytochrome P-450 enzymes.6. Together the results indicate that the down-regulation of I2 imidazoline-preferring receptors is associated with an irreversible inactivation of MAO (at least in the brain) that is not related either to the affinity of the MAO inhibitors for I2 imidazoline-preferring receptors or to an irreversible binding to these putative receptors. These findings indicate a novel effect of irreversible MAO inhibitors in the brain and suggest a new target for these compounds that could be of relevance in the treatment of depression, a disease in which an increased density of brain I2 imidazoline-preferring receptors has been reported.

Alpha 2A-adrenoceptors mediate activation of non-selective cation channels via Gi-proteins in human erythroleukaemia (HEL) cells. No evidence for a functional role of imidazoline receptors in modulating calcium

Human erythroleukaemia (HEL) cells were investigated to characterize their alpha 2-adrenoceptor and imidazoline receptor sites. Membranes from HEL cells bound [3H]2-(2-methoxy-1, 4-benzodioxan-2yl)-2-imidazoline ([3H]RX821002) in a saturable and specific manner with a KD of 0.64 +/- 0.07 nM and a Bmax of 126 +/- 4 fmol/mg protein. [3H]RX821002 was displaced from HEL membranes by adrenergic drugs with the order of potency being yohimbine approximately oxymetazoline >> prazosin = 2-[2-[4-(o-methoxyphenyl)piperazin-1-yl]ethyl]-4,4-dimethyl- 1,3(2H,4H)-isochinolindione HCl (ARC 239), consistent with this site being an alpha 2A-adrenoceptor. HEL membranes also bound [3H]idazoxan in the presence of adrenaline to block alpha 2-adrenoceptors. This binding was saturable and specific with a KD of 3.5 +/- 1.0 nM and a Bmax of 31 +/- 6 fmol/mg protein. Adrenergic drugs from both the phenylethylamine and imidazoline classes increased high-affinity GTPase activity, an index of activation of regulatory heterotrimeric guanine-nucleotide binding proteins (G-proteins), and produced increases in cytosolic free calcium concentration ([Ca2+]i). The effects of these agonists in both systems were abolished by pertussis toxin pretreatment, and oxymetazoline and clonidine were antagonists. The potency of adrenergic drugs to inhibit 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304)-induced increases in [Ca2+]i was yohimbine approximately oxymetazoline >> ARC 239, consistent with the binding data and an action at alpha 2A-adrenoceptors. No evidence was found for a role of imidazoline receptors in stimulating G-proteins or modulating [Ca2+]i. The adrenergic agonist-induced increases in [Ca2+]i were due to both release of Ca2+ from intracellular stores and entry of extracellular Ca2+. Ca2+ entry was blocked by 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenylethyl)-1H- imidazole hydrochloride (SKF 96365), but not by nitrendipine. Adrenaline also stimulated Mn2+ entry in HEL cells. Taken together, these results suggest that HEL cells have alpha 2A-adrenoceptors that activate non-selective cation channels via pertussis toxin-sensitive G-proteins, i.e. Gi-proteins.

Imidazolines stimulate release of insulin from RIN-5AH cells independently from imidazoline I1 and I2 receptors

The effect on insulin release of efaroxan, an alpha 2-adrenoceptor antagonist and a highly potent drug at imidazoline I1 receptors, and the effects of seven other imidazoline compounds selective for the imidazoline I1 or I2 receptors, were studied in the rat insulinoma cell line RIN-5AH. The cells released insulin in response to glucose (0.3-10 mM), and efaroxan (100 microM) potentiated glucose-induced insulin release. (-)-Adrenaline completely displaced the binding of [125I]p-iodoclonidine to membranes of RIN-5AH cells, indicating that these cells do not express imidazoline I1 receptors. Cirazoline and idazoxan (100 microM), both highly potent drugs at imidazoline I2 receptors, and the guanidines guanoxan and amiloride (200 microM), also promoted insulin release from RIN-5AH cells. Irreversible blockade of imidazoline I2 receptors with 10 microM clorgyline did not prevent the stimulatory effects of cirazoline or idazoxan; however, these compounds completely reversed the inhibition by diazoxide (250 microM), an opener of ATP-dependent K+ channels (K+ATP channels), of glucose-induced insulin release. These data indicate that the imidazoline/guanidine compounds promote insulin release from RIN-5AH cells, by interacting with a novel binding site related to K+ATP channels that does not represent any of the known imidazoline I1 or I2 receptors.

The imidazoline site involved in control of insulin secretion: characteristics that distinguish it from I1- and I2-sites

1. The nature of the binding site mediating the insulin secretagogue activity of certain imidazoline compounds remains unclear and the pharmacology of the I1- and I2-imidazoline sites, described in many tissues, does not correlate with the observed responses to imidazolines in islets. In the present paper, we describe further results which support the concept that the islet imidazoline site may represent a novel subtype of imidazoline receptor. 2. Culture of rat isolated islets in the presence of imidazoline secretagogues (either efaroxan or phentolamine) resulted in loss of responsiveness on subsequent re-exposure to these agents. However, culture of islets with either idazoxan or UK14,304 (imidazoline ligands that do not stimulate insulin secretion) did not lead to any loss of response when the islets were subsequently exposed to efaroxan. By contrast, islets cultured with UK14,304 (a potent alpha 2-adrenoceptor agonist), displayed loss of sensitivity to noradrenaline, consistent with down-regulation of alpha 2-adrenoceptors. 3. In order to characterize the imidazoline site further, radioligand binding studies were performed in membranes from RINm5F insulinoma cells using [3H]-RX821002, an imidazoline insulin secretagogue that does not interact significantly with imidazoline sites in other tissues. [3H]-RX821002 labelled alpha 2-adrenoceptors with high affinity (2.01 +/- 0.7 nM) but also labelled a second, non-adrenoceptor site with much lower affinity. 4. Under conditions of alpha 2-adrenoceptor blockade (in the presence of adrenaline), efaroxan displaced [3H]-RX821002 binding to the low affinity site, in a dose-dependent manner. Competition studies employing additional imidazoline compounds of varying secretagogue activity revealed that the pharmacological profile of the low affinity site correlates well with that observed in secretion experiments.5. The results obtained from the down-regulation experiments with isolated islets and from the radioligand binding studies suggest that the low affinity [3H]-RX821002 binding site may represent the functional receptor responsible for the secretagogue activity of imidazoline compounds in the endocrine pancreas and that it has a pharmacological profile distinct from those of I,- and 12-sites.

Attenuated renal response to moxonidine and rilmenidine in one kidney-one clip hypertensive rats

1. I1 non-adrenoceptor, imidazoline receptor agonists, such as moxonidine, increase urine flow rate and sodium excretion following infusion into the renal artery. The functions of these agonists in genetic and acquired models of hypertension have not been determined. 2. We therefore studied the renal effects of two known non-adrenoceptor, imidazoline receptor agonists, rilmenidine and moxonidine, in 1K-1C hypertensive and 1K-sham normotensive rats. Rilmenidine (0, 3, 10, 30 nmol kg-1 min-1) or moxonidine (0, 1, 3, 10 nmol kg-1 min-1) was infused directly into the renal artery (30 gauge needle) of 1K-sham normotensive and 1K-1C hypertensive rats. 3. In 1K-sham normotensive rats, rilmenidine and moxonidine produced dose related increases in urine flow rate, sodium excretion and osmolar clearance. Both rilmenidine and moxonidine failed to increase urine flow rate, sodium excretion and osmolar clearance in 1K-1C hypertensive rats to the same extent as in 1K-sham animals. At comparable doses, rilmenidine had no effect, while moxonidine (3 and 10 nmol kg-1 min-1) did result in a small increase in urine volume and osmolar clearance which was less than that observed in the 1K sham control animals. 4. These studies indicate that the renal effects of non-adrenoceptor, imidazoline receptor stimulation are diminished in 1K-1C hypertensive rats compared with 1K-sham normotensive rats. Whether this decrease in activity of the natriuretic non-adrenoceptor, imidazoline receptors contributes to the increase in blood pressure in the 1K-1C acquired model of hypertension remains to be determined.

Historic aspects in the identification of the I1 receptor and the pharmacology of imidazolines

The central nervous system is involved in the control of arterial blood pressure. Stimulation of central alpha 2-adrenoceptors in the nucleus tractus solitarii (NTS) decreases sympathetic outflow, resulting in a fall in arterial blood pressure. One of the first antihypertensive substances with actions on the alpha 2-adrenoceptors of the NTS was alpha-methylnoradrenaline. Later on the imidazoline clonidine was developed for which numerous effects, mediated by alpha 2-adrenoceptors, in the CNS could be demonstrated. Since the centrally acting alpha 2-adrenoceptor agonists possess severe side effects, the development of more specific and selective centrally acting imidazolines resulted in the derivatives moxonidine and rilmenidine. The effects of the "second-generation imidazolines" could not be fully understood as alpha 2-adrenoceptor agonists. In the meantime, the rostral ventrolateral medulla (RVLM) has been identified as the site of action of the imidazolines and an I1-imidazoline binding site was characterized in this region. For the antihypertensive action of the imidazolines, agonism at the I1-imidazoline subtype seems to be responsible. In addition, an acid- and heat-stable endogenous substance, called clonidine displacing substance (CDS), was reported to bind at the putative I receptor. In 1992 a receptor protein for I receptors (70 kD) could be separated that is different from that of alpha 2-adrenoceptors. However, up to now we are still lacking the amino-acid sequence of the I receptor and its second messenger system.

Agmatine: an endogenous clonidine-displacing substance in the brain

Clonidine, an antihypertensive drug, binds to alpha 2-adrenergic and imidazoline receptors. The endogenous ligand for imidazoline receptors may be a clonidine-displacing substance, a small molecule isolated from bovine brain. This clonidine-displacing substance was purified and determined by mass spectroscopy to be agmatine (decarboxylated arginine), heretofore not detected in brain. Agmatine binds to alpha 2-adrenergic and imidazoline receptors and stimulates release of catecholamines from adrenal chromaffin cells. Its biosynthetic enzyme, arginine decarboxylase, is present in brain. Agmatine, locally synthesized, is an endogenous agonist at imidazoline receptors, a noncatecholamine ligand at alpha 2-adrenergic receptors and may act as a neurotransmitter.

Antagonism of levcromakalim by imidazoline- and guanidine-derivatives in rat portal vein: involvement of the delayed rectifier

1. In rat whole portal veins, guanabenz (100 nM to 10 microM) and antazoline (100 nM to 100 microM) each increased the amplitude, frequency and duration of spontaneous contractions. In addition, guanabenz (30 microM) and antazoline (30 microM) each antagonized the ability of levcromakalim (3 nM to 10 microM) to inhibit the spontaneous contractions of this tissue. 2. Whole-cell voltage-clamp recordings were made from freshly-isolated rat portal vein cells dispersed by a collagenase/pronase enzyme treatment. The ability of several agents (antazoline, cirazoline, clonidine, guanabenz and phentolamine, each containing an imidazoline or guanidine moiety), to modulate potassium (K) currents and to inhibit the actions of levcromakalim was investigated. 3. Antazoline, cirazoline, clonidine, guanabenz and phentolamine (each at a concentration of 30 microM) had little effect on control non-inactivating currents but inhibited the delayed-rectifier current, IK(V). 4. Levcromakalim (1 microM) induced a non-inactivating current, IK(ATP), and also inhibited the delayed rectifier current, IK(V). 5. Glibenclamide (1 microM) had no effect on control delayed rectifier or non-inactivating currents, but it inhibited the simultaneous induction of IK(ATP) and reduction of IK(V) produced by levcromakalim (1 microM). 6. Antazoline, cirazoline, clonidine and guanabenz (each at a concentration of 30 microM) prevented the induction of IK(ATP) by levcromakalim (1 microM). Phentolamine (30 microM) and clonidine (30 microM) each inhibited the IK(ATP) generated by levcromakalim (1 microM). 7. It is concluded that a variety of agents which possess either an imidazoline (antazoline, cirazoline, clonidine and phentolamine) or a guanidine (guanabenz) moiety within their structure inhibit the delayed rectifier current, IK(V). This action may thus be mediated via a so-called non-adrenoceptor imidazoline binding site. Furthermore, the ability of these ligands to inhibit IK(V) and to antagonize both the induction of IK(ATP) and the vasorelaxation produced by levcromakalim is consistent with the view that the channel (KATP) which underlies IK(ATP) is a voltage-insensitive state of the delayed rectifier K-channel (Kv).

Opposite age-dependent changes of alpha 2A-adrenoceptors and nonadrenoceptor [3H]idazoxan binding sites (I2-imidazoline sites) in the human brain: strong correlation of I2 with monoamine oxidase-B sites

In the postmortem human brain (27 specimens of frontal cortex, Brodmann area 9), the specific binding of the antagonists [3H]RX 821002 (2-methoxyidazoxan) to alpha 2A-adrenoceptors and that of [3H]idazoxan to I2-imidazoline sites (a nonadrenoceptor mitochondrial site) were determined in parallel to study the effect of aging (range, 4-89 years) on both brain proteins. The density of alpha 2A-adrenoceptors and age were negatively correlated (r = -0.71; p < 0.001). In contrast, the density of I2-imidazoline sites was positively correlated with aging (r = 0.59; p < 0.005). The ratio of receptor densities (alpha 2A/I2) also showed a marked negative correlation with age (r = -0.76; p < 0.001). In an age-selected group (range, 10-89 years), the density of monoamine oxidase (MAO)-B sites labeled by [3H]Ro 19-6327 (lazabemide) also showed a positive correlation with age (r = 0.80; p < 0.005). In these subjects, the density of I2-imidazoline sites correlated well with the density of MAO-B sites (r = 0.70; p < 0.005). The ratio of the density of these sites (MAO-B/I2) did not correlate with the age of the subject at death (r = -0.15). In the human frontal cortex, idazoxan displayed very low affinity (Ki = 89 microM) against the binding of [3H]Ro 19-6327 to MAO-B, which discounted a direct interaction of [3H]idazoxan with the active center of the enzyme and indicated that the I2-imidazoline site cannot be identified with MAO-B.(ABSTRACT TRUNCATED AT 250 WORDS)