I1-imidazoline receptors. Definition, characterization, distribution, and transmembrane signaling

Central moxonidine on water and NaCl intake

In this study we investigated: (a) the effects of intracerebroventricular (i.c.v.) injections of moxonidine (an alpha2-adrenergic and imidazoline receptor agonist) on the ingestion of water and NaCl induced by 24 h of water deprivation; (b) the effects of i.c.v. injection of moxonidine on central angiotensin II (ANG II)- and carbachol-induced water intake; (c) the effects of the pre-treatment with i.c.v. idazoxan (an alpha2-adrenergic and imidazoline receptor antagonist) and RX 821002 (a selective alpha2-adrenergic antagonist) on the antidipsogenic action of central moxonidine. Male Holtzman rats had stainless steel cannulas implanted in the lateral cerebral ventricle. Intracerebroventricular injection of moxonidine (5 and 20 nmol/1 microl) reduced the ingestion of 1.5% NaCl solution (4.1 +/- 1.1 and 2.9 +/- 2.5 ml/2 h, respectively vs. control = 7.4 +/- 2.1 ml/2 h) and water intake (2.0 +/- 0.6 and 0.3 +/- 0.2 ml/h, respectively vs. control = 13.0 +/- 1.4 ml/h) induced by water deprivation. Intracerebroventricular moxonidine (5 nmol/1 microl) also reduced i.c.v. ANG II-induced water intake (2.8 +/- 0.9 vs. control = 7.9 +/- 1.7 ml/1 h) and i.c.v. moxonidine (10 and 20 nmol/1 microl) reduced i.c.v. carbachol-induced water intake (4.3 +/- 1.7 and 2.1 +/- 0.9, respectively vs. control = 9.2 +/- 1.0 ml/1 h). The pre-treatment with i.c.v. idazoxan (40 to 320 nmol/1 microl) abolished the inhibitory effect of i.c.v. moxonidine on carbachol-induced water intake. Intracerebroventricular idazoxan (320 nmol/1 microl) partially reduced the inhibitory effect of moxonidine on water deprivation-induced water intake and produced only a tendency to reduce the antidipsogenic effect of moxonidine on ANG II-induced water intake. RX 821002 (80 and 160 nmol/1 microl) completely abolished the antidipsogenic action of moxonidine on ANG II-induced water intake. The results show that central injections of moxonidine strongly inhibit water and NaCl ingestion. They also suggest the involvement of central alpha2-adrenergic receptors in the antidipsogenic action of moxonidine.

The I1-imidazoline receptor and its cellular signaling pathways

Two primary questions are addressed. First, do I1-imidazoline binding sites fulfill all the essential criteria for identification as a true receptor? Second, what are the cellular signaling pathways coupled to this novel receptor? I1-imidazoline binding sites show specificity in binding assays, linkage to physiologic functions, appropriate anatomic, and cellular and subcellular localization. Most important, binding affinities correlate with functional drug responses. I1-imidazoline binding sites meet several additional criteria identified with functional receptors: they show physiologic regulation and endogenous ligands and, most crucially, are coupled to cellular signaling events. A series of studies have identified cellular events triggered by I1-imidazoline receptor occupancy. This receptor is not coupled to conventional pathways downstream of heterotrimeric G-proteins, such as activation or inhibition of adenylyl or guanylyl cyclases, stimulation of inositol phospholipid hydrolysis, or induction of rapid calcium fluxes. The I1-imidazoline receptor is coupled to choline phospholipid hydrolysis, leading to the generation of diacylglyceride, arachidonic acid, and eicosanoids. Additional cellular responses include inhibition of Na+/H+ exchange and induction of genes for catecholamine synthetic enzymes. The signaling pathways linked to the I1-imidazoline receptor are similar to those of the interleukin family, implying that I1-receptors may belong to the family of neurocytokine receptors.

Moxonidine acting centrally inhibits airway reflex responses

We examined the role of I1-imidazoline (I1-IR) receptors in control of airway function, by testing the effects of systemic administration of the I1-IR agonist moxonidine on reflex responses of tracheal smooth muscle (TSM) tone to either lung deflation or mechanical stimulation of intrapulmonary rapidly adapting receptors. Experiments were performed in either alpha-chloralose anesthetized or decorticate, paralyzed, and mechanically ventilated beagle dogs. Moxonidine (10-100 micrograms/kg) administered via three different routes (femoral vein, muscular branch of superior thyroid artery, and vertebral artery) attenuated TSM responses to stimulation of airway sensory nerve fibers by two different ways and caused a decrease in arterial pressure and heart rate. These effects were dose dependent and were significantly reversed by efaroxan (an I1-IR and alpha 2-adrenergic blocker) administered via the vertebral artery. Intravertebral efaroxan abolished the hemodynamic effects of moxonidine. Intravenous moxonidine (10-100 micrograms/kg) did not alter airway smooth muscle responses to electrical stimulation of the peripheral vagus nerve. In addition, in vitro moxonidine (1-100 micrograms/ml) had no effect on contractile responses to increasing doses of acetylcholine. These findings indicate that moxonidine may act at a central site to suppress reflex airway constriction, even when given into the systemic circulation. Given the presence of I1-IR sites and alpha 2-adrenergic receptors in brain regions participating in airway reflexes, these receptor classes may be involved in brainstem control of the cholinergic outflow to the airways.

Plasma agmatine and platelet imidazoline receptors in depression

Plasma agmatine concentrations are elevated significantly in depressed patients compared to healthy controls. Treatment with the antidepressant bupropion normalized plasma agmatine levels. Correlational evidence is presented that a change in plasma agmatine levels may lead to similar changes in platelet I1 imidazoline receptors.

Pharmacologic characterization of imidazoline receptor proteins identified by immunologic techniques and other methods

Biochemical and pharmacologic evidence supports the heterogeneous nature of imidazoline receptors (IRs). However, only monoamine oxidase (MAO) (55- and 61-kD) isozymes have been identified as imidazoline binding site-containing proteins. Idazoxan-binding proteins of approximately 70- and approximately 45-kD of unknown amino acid sequences have been isolated from chromaffin cells and rat brain, respectively. Other proteins of approximately 27-30 to > 80 kD have been visualized by immunologic and photoaffinity labeling techniques in different tissues and species. The specific antiserum that recognizes the approximately 70-, approximately 45-, and approximately 29-kD IR proteins, but not MAO, was used to quantitate these proteins in the rat brain cortex. Treatments (7 days) with the I2-selective imidazoline drugs idazoxan (10 mg/kg), cirazoline (1 mg/kg), and LSL 60101 ([2-(2-benzofuranyl) imidazole; 10 mg/kg]) induced differential changes in these proteins: levels of the approximately 29-kD IR were increased by idazoxan and LSL 60101 (23%), levels of the approximately 45-kD protein only by cirazoline (44%), and those of the approximately 66-kD protein only by idazoxan (50%). These treatments also increased the densities of [3H]-idazoxan (I2) binding sites (32-42%). Chronic treatment with efaroxan, RX821002, and yohimbine (10 mg/kg), which possess very low affinity for I2-IRs, did not alter either their immunoreactivities or the density of I2 sites. Chronic treatment with MAO inhibitors clorgyline and phenelzine (10 mg/kg) and acute treatment with EEDQ (1.6 mg/kg, 6 h) induced decreases in the levels of these IR proteins (17-47%) and I2 sites (31-57%). Significant correlations were found when the mean percentage changes in immunoreactivity of IR proteins were related to the mean percentage changes in the density of I2 sites after treatment with the foregoing drug (r = 0.92, r = 0.69, and r = 0.75 for the approximately 29-, approximately 45-, and approximately 66-kD proteins, respectively). These results indicate that in the rat cerebral cortex, the I2 sites labeled by [3H]idazoxan are heterogeneous and that the related immunoreactive IR proteins contribute differently to the modulation of I2 sites after drug treatment.

Novel selective compounds for the investigation of imidazoline receptors

Over several years our group has sought to synthesize and identify selective ligands for imidazoline (I) receptors, in particular the I2 binding site. As a consequence, [3H]2-(2-benzofuranyl)-2-imidazoline (2BFI) has proved extremely useful for binding and autoradiographic studies. More recently we have synthesized a BU series of compounds and examined these for their affinities for both I1 and I2 binding sites. BU224 (2-(4,5-dihydroimidaz-2-yl)-quinoline) shows high affinity for I2 receptors with a Ki of 2.1 nM. BU226 (2-(4,5-dihydroimidaz-2-yl)-isoquinoline) demonstrated slightly higher affinity (Ki 1.4 nM) for I2 receptors, but overall BU224 displayed greater selectivity for I2 over I1 receptors (832-fold) than BU226 (380-fold). Both compounds showed low (microM) affinity for alpha 2-adrenoceptors. Given BU224's ability to cross the blood brain barrier, we predict that its in vivo effects are likely to be mediated via I2 receptors. Brain dialysis revealed BU224 to dose dependently (0-20 mg/kg i.p.) elevate basal noradrenaline in rat frontal cortex and basal dopamine in striatum. In a rat model of opiate withdrawal, behavioral studies showed that BU224 (10 mg/kg, s.c.) was able to reduce acute weight loss and diarrhea, but not the number of wet dog shakes associated with the withdrawal syndrome.

The effect of moxonidine on feeding and body fat in obese Zucker rats: role of hypothalamic NPY neurones

The antihypertensive agent moxonidine, an imidazoline Ii-receptor agonist, also induces hypophagia and lowers body weight in the obese spontaneously hypertensive rat, but the central mediation of this action and the neuronal pathways that moxonidine may interact with are not known. We studied whether moxonidine has anti-obesity effects in the genetically-obese and insulin-resistant fa/fa Zucker rat, and whether these are mediated through inhibition of the hypothalamic neuropeptide Y (NPY) neurones. Lean and obese Zucker rats were given moxonidine (3 mg kg(-1) day(-1)) or saline by gavage for 21 days. Moxonidine decreased food intake throughout by 20% in obese rats (P<0.001) and by 8% in lean rats (P<0.001), and reduced weight gain that final body weight was 15% lower in obese (P<0.001) and 7% lower in lean (P<0.01) rats than their untreated controls. Plasma insulin and leptin levels were decreased in moxonidine-treated obese rats (P<0.01 and P<0.05), but unchanged in treated lean rats. Uncoupling protein-1 gene expression in brown adipose tissue was stimulated by 40-50% (P< or =0.05) in both obese and lean animals given moxonidine. Obese animals given moxonidine showed a 37% reduction in hypothalamic NPY mRNA levels (P = 0.01), together with significantly increased NPY concentrations in the paraventricular nucleus (P<0.05), but no changes in the arcuate nucleus or other nuclei; this is consistent with reduced NPY synthesis in the arcuate nucleus and blocked release of NPY in the paraventricular nucleus. In lean animals, moxonidine did not affect NPY levels or NPY mRNA. The hypophagic, thermogenic and anti-obesity effects of moxonidine in obese Zucker rats may be partly due to inhibition of the NPY neurones, whose inappropriate overactivity may underlie obesity in this model.

Affinity isolation of imidazoline binding proteins from rat brain using 5-amino-efaroxan as a ligand

We have employed an amino derivative of the imidazoline ligand, efaroxan, to isolate imidazoline binding proteins from solubilised extracts of rat brain, by affinity chromatography. A number of proteins were specifically retained on the affinity column and one of these was immunoreactive with an antiserum raised against the ion conducting pore component of the ATP-sensitive potassium channel. Patch clamp experiments confirmed that, like its parent compound, amino-efaroxan blocks ATP-sensitive potassium channels in human pancreatic beta-cells and can stimulate the insulin secretion from these cells. The results reveal that a member of the ion conducting pore component family is strongly associated with imidazoline binding proteins in brain and in the endocrine pancreas.

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.

alpha-2 and imidazoline receptor agonists. Their pharmacology and therapeutic role

Clonidine has proved to be a clinically useful adjunct in clinical anaesthetic practice as well as in chronic pain therapy because it has both anaesthetic and analgesic-sparing activity. The more selective alpha-2 adrenoceptor agonists, dexmedetomidine and mivazerol, may also have a role in providing haemodynamic stability in patients who are at risk of peri-operative ischaemia. The side-effects of hypotension and bradycardia have limited the routine use of alpha-2 adrenoceptor agonists. Investigations into the molecular pharmacology of alpha-2 adrenoceptors have elucidated their role in the control of wakefulness, blood pressure and antinociception. We discuss the pharmacology of alpha-2 adrenoceptors and their therapeutic role in this review. The alpha-2 adrenoceptor agonists are agonists at imidazoline receptors which are involved in central blood pressure control. Selective imidazoline agonists are now available for clinical use as antihypertensive agents and their pharmacology is discussed.

Imidazoline binding sites and receptors in cardiovascular tissue

1. Imidazoline binding sites and receptors and their endogenous ligands have been identified in cardiovascular tissue of various species including human beings. 2. I2- (but only exceptionally I1-)imidazoline binding sites have been shown to exist on cardiac myocytes and vascular smooth muscle cells; at present, their functional role is unknown. 3. The sympathetic nerves supplying the cardiovascular system are endowed with presynaptic inhibitory imidazoline receptors that may become of therapeutic relevance as targets of drugs. 4. ATP-sensitive K+ channels present in heart and blood vessels can be blocked by several imidazolines and guanidines; hence, those drugs can interfere with the cardioprotective effects resulting from K(ATP) channel activation by a decrease in the endogenous ligand ATP or by drugs. 5. Imidazoline derivatives exhibit antiarrhythmic properties that are due to a reduction of sympathetic tone by central and peripheral mechanisms and to blockade of postsynaptic alpha2-adrenoceptors in the heart and coronary arteries. 6. Agmatine and clonidine-displacing substance, which are endogenous ligands at imidazoline and alpha2-receptors, are present in the blood serum and appear to participate in vascular smooth muscle proliferation and blood pressure regulation.

Stimulation of locus coeruleus neurons by non-I1/I2-type imidazoline receptors: an in vivo and in vitro electrophysiological study

1. Imidazoline binding sites have been reported to be present in the locus coeruleus (LC). To investigate the role of these sites in the control of LC neuron activity, we studied the effect of imidazolines using in vivo and in vitro single-unit extracellular recording techniques. 2. In anaesthetized rats, local (27 pmoles) and systemic (1 mg kg(-1), i.v.) administrations of 2-(2-benzofuranyl)-2-imidazoline (2-BFI), a selective I-imidazoline receptor ligand, increased the firing rate of LC cells (maximal increase: 22+/-5%, P<0.001 and 16+/-7%, P<0.001 respectively). Chronic pretreatment with the irreversible monoamine oxidase inhibitor clorgyline (3 mg kg(-1), i.p., every 12 h for 14 days) abolished this effect. 3. In rat midpontine brain slices containing the LC, bath application (1 mM) of the imidazolines 2-BFI, 2-(4,5-dihydroimidaz-2-yl)-quinoline (BU224), idazoxan, efaroxan, phentolamine and (2-2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline (RX821002) reversibly stimulated LC cells. The maximal effect was approximately 90% except for RX821002 and efaroxan which induced smaller maximal effects (approximately 58% and approximately 35% respectively). Simultaneous application of idazoxan and 2BFI did not lead to additive effects. 4. Bath application of the alpha2-adrenoceptor antagonists, yohimbine (1 - 10 microM) and N-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ) (10 microM), failed to modify LC activity. The irreversible blockade of alpha2-adrenoceptors with EEDQ (10 microM) did not alter the effect of idazoxan or that of efaroxan. Previous application of clorgyline (10 microM) did not modify the excitatory effect of 2-BFI or efaroxan. 5. Changes in the pH of the bathing solution (6.84-7.84) did not influence the effect caused by idazoxan. Bath application of 2-BFI (1 mM) reversed the inhibition induced by diazoxide (300 microM), an ATP-sensitive K+ channel opener, whereas application of glibenclamide (3 microM), an ATP-sensitive K+ channel blocker, partially blocked the effect of 2-BFI. 6. This study shows that imidazoline compounds stimulate the firing rate of LC neurons. This effect is not mediated by alpha2-adrenoceptors nor by I1 or I2-imidazoline receptors but involves a different subtype of imidazoline receptor. Our results indicate that this receptor is located extracellularly and modulates ATP-sensitive K+ channels.

Investigation of I1-imidazoline receptors using microphysiometry and molecular modelling

The molecular identity and structure of imidazoline receptors is still poorly understood. For example the I1-imidazoline binding site (I1-site) is localised to the plasma membrane, but it is not clear if this represents a conventional receptor. The I1-site reportedly has both high and low affinity binding states. Again it is not clear if these sites represent different states of the same receptor, or distinct molecular entities. The signal transduction mechanisms of I1-imidazoline receptors are beginning to be unravelled. There is clear evidence that ligands with high affinity for I1-sites stimulate phosphatidylcholine-selective phospholipase C in the rat adrenal medullary tumour cell line PC-12, but this may not be the case in all cell types. We investigated the possible role of this novel pathway in bovine adrenal medullary cells. Radioligand binding studies with [3H]clonidine confirmed the presence of I1-sites in membranes from these cells. Using microphysiometry, a recently developed technique for determining cellular activation, the extracellular acidification rates of cultured bovine adrenal medullary cells were unaffected by a number of imidazolines considered to be agonists at the I1-site. This suggests that there is no I1-site mediated stimulation of phosphatidylcholine specific phospholipase C in these cells. However, nicotine-stimulated increases in extracellular acidification were blocked by 100 microM clonidine. Ion channels have been suggested as another possible I1-imidazoline 'receptor' family, and may represent the low affinity I1-site detected in binding studies. I1-Site ligands can be shown to bind to, or block, several members of the ligand-gated ion channel superfamily, including the 5HT3, K+ATP, NMDA and nicotinic acetylcholine receptors. The I1-site ligands appear to be binding to, and acting at, the previously described phencyclidine binding site in these channels. Furthermore, molecular modelling suggests that I1-site selective ligands share a common three-dimensional structure with phencyclidine, and that I2-site selective ligands do not have this structure. This suggests that a phencyclidine-binding site motif may represent a novel site of action for I1-site ligands, and a search for receptors based on this motif may reveal novel imidazoline 'receptors'.

Isolation and partial structure determination of a clonidine-displacing substance from bovine lung and brain

A large scale extraction and isolation method was developed for the purification of clonidine-displacing substance (CDS) activity from bovine lung or brain. This optimised method used direct freeze drying of tissue, hexane removal of lipids, and methanol extraction of CDS activity. Using a bioassay directed isolation strategy a new CDS compound was purified from an extract of bovine lung. The isolation strategy involved subsequent steps of flash C-18 chromatography, ion exchange, size exclusion, and C-18 HPLC. An HPLC detection method was developed and applied to show that the new CDS is present in both lung and brain tissue. Spectroscopic data for this new CDS indicates that it is related to guanosine, but is not noradrenaline, adrenaline, histamine, agmatine, guanosine, GMP, GDP or GTP.

Arachidonic acid release from PC12 pheochromocytoma cells is regulated by I1-imidazoline receptors

Rat PC 12 pheochromocytoma cells lack alpha2-adrenergic receptors but express plasma membrane I1-imidazoline receptors. In response to the I1-agonist moxonidine, diglycerides are generated via phosphatidylcholine-selective phospholipase C, and prostaglandin E2 is released. This report characterizes I-receptor-mediated release of arachidonic acid, the precursor to the prostaglandins. PC12 cells were incubated with [3H]arachidonic acid for 24 h and superfused with 0.01% bovine serum albumin in Krebs' physiological buffer at 1 ml/min. Calcium ionophore increased arachidonic acid release only marginally, implying that in PC12 cells arachidonic acid release is not driven by calcium. The I1-agonist moxonidine at concentrations between 10 nM and 1.0 microM rapidly elicited up to two-fold increases in [3H]arachidonic acid release. Guanabenz, a potent alpha2-agonist and I2-ligand, had no effect. The selective I1-antagonist efaroxan blocked the action of moxonidine. The phospholipase A2 inhibitor aristolochic acid had no effect, suggesting that arachidonic acid release may be through an indirect pathway, possibly involving diglycerides. Thus, I1-imidazoline receptors in PC12 cells are coupled to arachidonic acid release through an as yet unknown pathway.

Characterization of a partial cDNA clone detected by imidazoline receptor-selective antisera

A cDNA clone has been isolated from a human hippocampal cDNA expression library by relying on the selectivity of two antisera that are specific for imidazoline binding proteins. A 1789 bp cDNA clone was sequenced and shown to contain a single open-reading frame that predicts a 66 kDa polypeptide, but it is truncated based on its lack of a stop codon and poly-A+ tail. Two regions of homology exist for the predicted amino acid sequence in common with chromogranin-A and B proteins, a zinc finger protein, and the ryanodine receptor. Northern blot analyses of poly-A+ mRNA from 36 human tissues indicated two differentially expressed transcripts of 6.0 and 9.5 kb. The 6.0 kb mRNA form was enriched in brain and endocrine tissues as compared to other tissues, but not in strict concordance with I1-imidazoline binding sites. The highest overall amounts of the combined transcripts were found in pituitary. In situ hybridization histochemistry revealed an enrichment of the message in neuronal cell bodies of the rat hippocampus and cerebellar cortex. This clone has some of the properties expected of an imidazoline receptor.

Characterisation and localisation of [3H]2-(2-benzofuranyl)-2-imidazoline binding in rat brain: a selective ligand for imidazoline I2 receptors

In rat whole brain homogenates, saturation binding analysis revealed that both [3H]2-BFI (2-(2-benzofuranyl)-2-imidazoline) and [3H]idazoxan (in the presence of 5 microM rauwolscine) bound with high affinity to an apparent single population of sites. However, the Kd for [3H]2-BFI (1.74+/-0.14 nM) was significantly (P < 0.01) less than that for [3H]idazoxan (10.4+/-2.68 nM). In competition studies idazoxan, 2-BFI, BU224 (2-(4,5-dihydroimidaz-2-yl)-quinoline), amiloride and guanabenz displayed high affinity (Ki values = 7.32, 1.71, 2.08, 21.80 and 14.90 nM, respectively) for 70-80% of sites, and low microM affinity for the remaining 20-30% of sites labelled by [3H]2-BFI. In contrast, several alpha2-adrenoceptor, imidazoline I1 receptor and histamine receptor ligands exhibited only micromolar affinity for the [3H]2-BFI labelled site. Quantitative receptor autoradiography revealed high binding by [3H]2-BFI to discrete brain nuclei, notably the area postrema, interpeduncular nucleus, arcuate nucleus, mammillary peduncle, ependyma and pineal gland. These data indicate that [3H]2-BFI recognises imidazoline I2 receptors in rat brain with higher affinity and selectivity than [3H]idazoxan and thus represents a superior radioligand to [3H]idazoxan for the study of imidazoline I2 receptors.

Antagonist precipitated clonidine withdrawal in rat: effects on locus coeruleus neurons, sympathetic nerves and cardiovascular parameters

The goal of the present study was to examine the effect of clonidine withdrawal on the neural control of blood pressure. Rats were treated for 7-13 days with clonidine via osmotic minipumps (200 microg kg(-1) day(-1), s.c.). Controls received saline or were sham operated. Withdrawal was precipitated by the alpha2-adrenergic receptor (alpha2-AR) antagonist atipamezole. Most experiments were done under halothane anesthesia. Chronic treatment with clonidine did not change mean arterial pressure (MAP) or heart rate (HR) but raised femoral artery resistance and the activity of locus coeruleus neurons slightly. Atipamezole given to rats treated chronically with clonidine produced the following effects: no change in MAP, severe tachycardia, sustained increase in splanchnic sympathetic nerve discharge (SND; +75 +/- 13%), transient increase in lumbar SND (+23 +/- 7%), ON-OFF activity pattern in the locus coeruleus (LC). The ON phase of LC activity was synchronized with upswings of SND and with small changes in MAP. A second alpha2-AR antagonist, methoxyidazoxan, produced effects identical to those of atipamezole. Atipamezole given to control rats produced no effect on MAP, HR, SND or LC activity. Atipamezole reversed the hypotension, sympathoinhibition and bradycardia produced by acute administration of clonidine. In awake rats treated chronically with clonidine, atipamezole did not change MAP but produced arterial pressure lability and tachycardia. In conclusion, under anesthesia, selective alpha2-AR antagonists elicit a clonidine withdrawal syndrome that displays autonomic characteristics reminiscent of the spontaneous withdrawal syndrome found in awake rats. The most prominent features of this syndrome are tachycardia, sympathoactivation, lack of hypertension and an oscillating activity pattern of brainstem neurons leading to abrupt changes in SND and in MAP.

I1-imidazoline receptors and cholinergic outflow to the airways

We examined the role of I1-imidazoline receptors in the control of airway function, by testing the effects of systemic administration of the I1-imidazoline agonist moxonidine on reflex responses of tracheal smooth muscle (TSM) tone to either lung deflation or mechanical stimulation of intrapulmonary rapidly adapting receptors. Experiments were performed in either alpha-chloralose anaesthetized or decorticate, paralyzed and mechanically ventilated beagle dogs. Moxonidine (10-100 microg/kg) administered via three different routes (the femoral vein, muscular branch of superior thyroid artery, and vertebral artery) attenuated TSM responses to stimulation of airway sensory nerve fibers by two different ways, and caused a decrease in arterial pressure and heart rate. These effects were dose-dependent, and were significantly reversed by efaroxan (an I1-imidazoline and alpha2-adrenergic blocker) administered via the vertebral artery. Intravertebral efaroxan abolished the hemodynamic effects of moxonidine. Intravenous moxonidine (10-100 microg/kg) did not alter airway smooth muscle responses to electrical stimulation of the peripheral vagus nerve. In addition, in vitro moxonidine (1-100 microg/ml) had no effect on contractile responses to increasing doses of acetylcholine. These findings indicate that moxonidine may act at a central site to suppress reflex airway constriction, even when given into the systemic circulation. Given the presence of I1-imidazoline sites and alpha2-adrenergic receptors in brain regions participating in airway reflexes, these receptor classes may be involved in brainstem control of the cholinergic outflow to the airways.

Carotid body I1-imidazoline receptors: binding, visualization and modulatory function

The carotid body is influenced by many neurotransmitter receptors. A novel receptor specific for imidazolines has been implicated in cardiorespiratory regulation in the brain. To test for both I1-imidazoline and alpha2-adrenergic receptors, which also recognize imidazolines, specific [125I]p-iodoclonidine binding to carotid body membranes was characterized. The specific alpha2-agents epinephrine (100 microM) or SK&F 86466 (10 microM) inhibited only a portion of specific [125I]p-iodoclonidine binding in both cat and rabbit carotid bodies, indicating the presence of I1-imidazoline as well as alpha2-adrenergic sites. The distribution of [125I]p-iodoclonidine binding sites was visualized autoradiographically. The cat carotid body was intensely labeled by [125I]p-iodoclonidine, with both I1-imidazoline and alpha2-adrenergic sites expressed. The relevance of I1-imidazoline receptors in modulation of chemosensory discharge was determined in seven cats after alpha2-adrenergic blockade. Clonidine (100 microg/kg) facilitated chemosensory activity particularly under hypoxia. We conclude that I1-imidazoline receptors are expressed within the carotid body and may potentiate chemosensory discharge, in contrast to the inhibitory action of alpha2-adrenergic receptors.

Activation of L-type calcium channel by tolazoline derivatives: role of isothiocyanate moiety

Studies have investigated the pharmacologic mechanism of 2-(4'-isothiocyanatobenzyl) imidazoline (IBI) and analogs for interaction with imidazoline receptors (IRs), alpha-adrenergic receptors (alpha-ARs), and calcium channels in cardiovascular muscle systems. IBI differs from tolazoline by substitution of an electrophilic isothiocyanato (NCS) group. Unlike tolazoline, which is a partial alpha-AR agonist, IBI produced an irreversible, slow-onset, and sustained contraction of rat aorta with an median effective concentration (EC50) value of 5 microM, and a maximal contraction (116%) greater than that of phenylephrine (100%) and tolazoline (59%). The IBI-induced contractions were dependent on calcium channels and independent of alpha-ARs or IRs. Similarly, structure-activity relation studies in rat aortic smooth muscles on a series of synthesized IBI analogs indicated that NCS analogs, but not those without the NCS group, exhibited effects by a non-alpha-AR, non-IR, but a calcium channel-dependent mechanism. Thus the presence of an intact IBI ring in these analogs is not a requirement for these activities. Further, IBI inhibited dihydropyridine (DHP, [3H]PN 200-110 and [3H]Bay K 8644) binding to L-type calcium channels of T-tubule membranes in rabbit skeletal muscle. In contrast to nifedipine, IBI and NCS derivatives (nifedipine-NCS, naphazoline-NCS) only partially (50-88%) displaced specific binding of these radioligands. A single site of noncooperative interaction was observed for nifedipine (nH = 0.97), whereas tolazoline-NCS (IBI, nH = 1.46) and nifedipine-NCS (nH = 1.37) exhibited a positive cooperativity in binding to DHP sites. These receptor-binding data indicate that NCS derivatives bind to L-type calcium channels and interact allosterically with DHP-binding sites. Direct binding of the NCS group to specific nucleophilic protein sites of the calcium channel may be responsible for its activation and the subsequent contractile effects of IBI.

Co-detection by two imidazoline receptor protein antisera of a novel 85 kilodalton protein

Imidazoline receptors (I-receptors) are considered as potential therapeutic targets for a spectrum of stress-induced illnesses. Yet, I-receptors remain poorly defined at the molecular level. In this study, candidate imidazoline receptor proteins were compared using two imidazoline receptor-selective antisera of diverse origins. One antiserum was derived from affinity-purified imidazoline-binding protein. The second antiserum was produced as an anti-idiotypic antiserum, from purified IgG selective for imidazolines. Despite such diverse origins, both antisera co-identified an 85 kDa band on western blots from a variety of tissues. The integrity of the 85 kDa band was dependent on protection by eight different protease inhibitors. Other proteolytic breakdown products (obtained after homogenization with only one protease inhibitor) were comparable in size to previously reported smaller immunoreactive bands. The full-size 85 kDa band was also enriched in plasma membrane fractions and abundant in rat PC12 cells and brain regions known to be abundant in I1 binding sites. Furthermore, the immunodensity of the 85 kDa band, against anti-idiotypic antiserum, was linearly correlated with reported I1 site radioligand Bmax values (r2 = 0.8736, P = 0.0002) across nine rat tissues. Therefore, a possible candidate for the full-length imidazoline receptor(s) appears to be an 85 kDa protein.

Imidazoline receptors of the paraventricular nucleus on the pressor response induced by stimulation of the subfornical organ

In the present experiments we investigated a possible involvement of imidazoline receptors of the paraventricular nucleus (PVN) of the hypothalamus on the pressor effects of the angiotensin II (ANG II) injected into the subfornical organ (SFO), in male Holtzman rats (250-300 g) with a cannula implanted into the third ventricle (3rdV), PVN and SFO. At first we tested the participation of alpha 2 and imidazoline agonist and antagonist compounds on the pressor effect of ANG II injected into the 3rdV. Based on the results we may conclude that clonidine associated with rilmenidine was able to block the hypertensive response to ANG II. The ANG II (20 pmol) injected into SFO induced a robust increase in blood pressure (37 +/- 2 mmHg). Isotonic saline (0.15 M) NaCl did not produce any change in blood pressure (5 +/- 2 mmHg). The injection of rilmenidine (30 micrograms/kg/1 microL), an imidazoline agonist agent injected into PVN before ANG II injection into SFO, blocked the pressor effect of ANG II (5 +/- 2 mmHg). Also, the injection of idazoxan (60 micrograms/kg/microL) before rilmenidine blocked the inhibitory effect of rilmenidine on blood pressure (39 +/- 4 mmHg). The injection of clonidine (20 nmol/microL) prior to ANG II into the 3rdV produced a decreased in arterial blood pressure (37 +/- 2 mmHg) to (15 +/- 4 mmHg). The injection of yohimbine (80 nmol/microL) prior to clonidine blocked the effect of clonidine on the effect of ANG II (27 +/- 2 mmHg). The injection of rilmenidine prior to ANG II also induced a decrease in arterial blood pressure (10 +/- 3 mmHg). The injection of idazoxan prior to rilmenidine also blocked the inhibitory effect of rilmenidine (24 +/- 3 mmHg). In summary, the present study demonstrated that rilmenidine decreases the hypertensive effect of ANG II, with more potency than clonidine, even when injected into 3rdV or PVN. This study established that the PVN interacts with SFO by imidazoline receptors in order to control the arterial blood pressure.

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.

Clonidine and ST-91 may activate imidazoline binding sites in the heart to release atrial natriuretic peptide

It is well established that the antihypertensive drug clonidine acts through specific imidazoline receptors in the brain and kidney to increase diuresis, natriuresis, and kaliuresis. We have previously shown that the effects of clonidine are associated with elevated plasma atrial natriuretic peptide (ANP). Similar to clonidine, ST-91, a clonidine analogue that does not cross the blood-brain barrier, evokes renal responses that are also associated with elevated plasma ANP. The mechanisms of ANP increase elicited by these imidazoline drugs are unclear. Since ANP is primarily released from the cardiac atria, we investigated the direct effect of the imidazoline drugs on ANP release by incubating left and right atrial sections with 10(-6) mol/L ST-91 in the presence and absence of efaroxan, a selective imidazoline I1 receptor antagonist, for 30 minutes at 37 degrees C. ST-91 significantly stimulated ANP release, and the effect was inhibited by 10(-6) mol/L efaroxan. Further studies using heart perfusion with the imidazoline drugs with and without antagonists over 30 minutes revealed that both clonidine and ST-91 gradually stimulated ANP release. Also, perfusion with these compounds resulted in a gradual decrease in heart rate, but bradycardia was significant only with clonidine. The effects of ST-91 were inhibited by 10(-6) mol/L efaroxan and to a lesser extent by 10(-6) mol/L yohimbine, implying that the actions of ST-91 were mainly mediated by I1 receptors. On the other hand, the actions of clonidine were inhibited by 10(-5) mol/L efaroxan and by 10(-6) mol/L yohimbine, an alpha2-adrenoceptor antagonist, which may suggest that the actions of clonidine were preferentially mediated by alpha2-adrenoceptors in the heart. These results indicate that the peripheral actions of clonidine are probably mediated by alpha2 and imidazoline receptors and may involve direct stimulation of ANP release by the cardiac atria--an effect that may account for the increase in plasma ANP levels and diuresis and natriuresis observed in vivo after administration of clonidine and its analogues.

The involvement of pertussis toxin-sensitive G proteins in the post receptor mechanism of central I1-imidazoline receptors

1. To elucidate the possible involvement of pertussis toxin (PTX)-sensitive G proteins in the post receptor mechanism of alpha 2-adrenoceptors and imidazoline receptors, we examined the effect of pretreatment of the central nervous system with PTX on the antidysrhythmic effect of dexmedetomidine, a selective alpha 2-adrenoceptor agonist, and rilmenidine, a selective I1-imidazoline receptor agonist on halothane-adrenaline dysrhythmias in rats. 2. Dexmedetomidine (0, 1.0, 2.0, 5.0 micrograms kg-1 min-1.i.v.) and rilmenidine (0, 1.0, 3.0, 10, 20 micrograms kg-1, i.v.) prevented the genesis of halothane-adrenaline dysrhythmias in a dose-dependent fashion. Both idazoxan (10, 20 micrograms kg-1, intracerebroventricularly (i.c.v.)), an alpha 2-adrenoceptor antagonist with high affinity for imidazoline receptors, and rauwolscine, (40 micrograms kg-1, i.c.v.), an alpha 2-adrenoceptor antagonist with low affinity for imidazoline receptors inhibited the action of dexmedetomidine (5.0 micrograms kg-1, min-1, i.v.), but the inhibitory potency of idazoxan was much greater than that of rauwolscine. While the pretreatment with PTX (0.1, 0.5, 1.0 micrograms kg-1, i.c.v.) did not change the dysrhythmogenecity of adrenaline, this treatment completely blocked the antidysrhythmic property of rilmenidine (20 micrograms kg-1, i.v.) as well as dexmedetomidine (5.0 micrograms kg-1 min-1, i.v.). 3. It is suggested that central I1-imidazoline receptors as well as alpha 2-adrenoceptors may be functionally coupled to PTX-sensitive G proteins.

Activation of phosphatidylcholine-selective phospholipase C by I1-imidazoline receptors in PC12 cells and rostral ventrolateral medulla

The I1-imidazoline receptor is expressed in the rostral ventrolateral medulla (RVLM) where it mediates vasodepression, and in PC12 pheochromocytoma cells where it elicits generation of diacylglycerol independent of phosphatidylinositol turnover or activation of phospholipase D. We hypothesized that the I1-imidazoline receptor couples to a phosphatidylcholine-selective phospholipase C (PC-PLC). The I1-agonist moxonidine elicited diacyglyceride accumulation and release of [3H]phosphocholine from PC12 cells prelabeled with [3H]choline. The PC-PLC inhibitor D609 abolished both responses. Microinjection of D609 into the RVLM of hypertensive rats blocked the vasodepressor response to intravenous moxonidine. These data implicate PC-PLC in cellular and organismic responses to I1-receptor stimulation.

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.

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.

The use of moxonidine in the treatment of hypertension

BACKGROUND

Imidazoline I1-receptor agonism represents a new mode of antihypertensive action to inhibit peripheral alpha-adrenergic tone by a central mechanism. Adrenaline, noradrenaline and renin levels are reduced, a finding consistent with central inhibition of sympathetic tone. Acute haemodynamic studies indicate that moxonidine results in an acute decrease in blood pressure due to a fall in systemic vascular resistance, whereas the heart rate, cardiac output, stroke volume and pulmonary artery pressures are not affected. Left ventricular end systolic and diastolic volumes are reduced. Left ventricular hypertrophy has been found to regress after 6 months of treatment.

PHARMACOKINETICS

Following oral administration, maximum concentration is reached at about 1 h, and bioavailability approaches 90%. Moxonidine is mostly excreted unchanged, and biotransformation is unimportant. The half-life of moxonidine is 2.5 h, which is prolonged by renal insufficiency. However, the antihypertensive effect lasts longer than would be expected from the half-life, suggesting possible retention in the central nervous system.

DRUG EFFECTS

Decreases of about 20-30 mmHg systolic and 10-20 mmHg diastolic blood pressure have been found in open studies with moxonidine. The dosage of 0.2-0.4 mg moxonidine daily controls hypertension in most patients. Moxonidine has been compared with representatives from each important class of antihypertensive drugs, with clonidine, diuretics, both alpha- and beta-blocking drugs, calcium antagonists and angiotensin converting enzyme inhibitors. Blood pressure control has been observed to be similar with moxonidine and these other agents. Generally, the overall incidence of side-effects has been found to be similar, although the incidence of side-effects with clonidine is greater than that seen with moxonidine.

CONCLUSIONS

A meta-analysis of controlled studies with moxonidine found that moxonidine gave similar reductions in blood pressure in both men and women, in those aged below 50, 50-60 and over 60 years, and regardless of body weight. As often seen with some other drugs, higher systolic blood pressures are associated with larger reductions in systolic blood pressure and the same appears to be the case with diastolic blood pressure.

Membrane localization and guanine nucleotide sensitivity of medullary I1-imidazoline binding sites

Imidazoline binding sites are labeled by [3H]clonidine (I1) or by [3H]idazoxan (I2). I2-sites are mitochondrial. The subcellular localization of I1-sites in brain is unknown. Crude membranes from bovine rostral ventrolateral medulla (RVLM) were further purified by discontinuous sucrose density gradient. Fractions were assayed for I1-site density (Bmax) with [125I]p-iodoclonidine. Nonspecific binding was defined by 10 microM BDF-6143, and alpha 2-adrenergic binding was defined by 10 microM epinephrine. The proportions of I1 and alpha 2 in mitochondrial fractions were similar (28 +/- 3 and 24 +/- 4%, respectively), and both I1 and alpha 2 showed the greatest enrichment within the membrane-enriched fraction (58 +/- 13 and 38 +/- 4%). The myelin fraction contained a higher proportion of alpha 2 than I1 (38 +/- 4 and 15 +/- 2%), consistent with expression of alpha 2, but not I1, by glia. The enrichment of I1 and alpha 2 in cellular membranes and alpha 2 in myelin was confirmed by further purification of these fractions over a second discontinuous gradient. Following irreversible inactivation of alpha 2, the remaining I1 sites in RVLM crude membranes were inhibited by Gpp(NH)p but not by ATP. We conclude that I1-imidazoline sites are non-mitochondrial membrane proteins sensitive to guanine nucleotide and may be functional receptors.

Identification and characterization of imidazoline-binding sites from calf striatum

"Non-adrenoceptor'-binding sites for [3H]clonidine (I1-sites) and [3H]idazoxan (I2-sites) are identified in calf striatum membranes. The pharmacological profile of both subtypes was investigated by competition binding with the imidazolines idazoxan, cirazoline, Bu 224 (2-(4,5-dihydroimidaz-2-yl)-quinoline) and Bu 239 (2-(4,5-dihydroimidaz-2-yl)-quinoxaline); the guanidino derivatives clonidine, moxonidine, guanabenz, amiloride and agmatine; the oxazoline rilmenidine and the imidazole histamine. The competition experiments indicate that both populations of imidazoline-binding sites in calf striatum consist of a high- (H) and a low-affinity (L) compartment. The monoamine oxidase (MAO) inhibitors pargyline (non-selective) and deprenyl (MAO-B-selective) have micromolar affinity for the I1-sites and much lower affinity for the I2-sites. The venom of the marine snail Conus geographus is the most potent of the 13 tested Conus venom preparations. None of the tested venoms is able to discriminate between both sites.

Current status of putative imidazoline (I1) receptors and renal mechanisms in relation to their antihypertensive therapeutic potential

1. A 'second generation' of centrally acting antihypertensive agents has recently been developed. Unlike the 'first generation' of these agents (e.g. alpha-methyldopa, clonidine, guanabenz), which act predominantly by an agonist action at a alpha 2-adrenoceptors, these agents (e.g. rilmenidine, moxonidine) are believed to exert their antihypertensive effects chiefly by an interaction at putative imidazoline (I) receptors of the I1-type, and so have a reduced profile of alpha 2-adrenoceptor-mediated side effects. There is also evidence from studies in experimental animals that activation of I1-receptors mediates a natriuretic effect. This review evaluates the evidence that they mediate renal effects different from those of alpha 2-adrenoceptors that could contribute to their long-term efficacy. 2. Data from binding studies suggest that I1-binding sites are heterogeneous. There is conflicting evidence concerning whether any of these binding sites are truly receptors. Indeed, the best evidence for the existence of I1-receptors comes from in vivo experiments indicating that imidazoline compounds act at non-adrenoceptor receptive sites in the central nervous system to reduce sympathetic drive and blood pressure. 3. There are a wide range of potential sites and mechanisms through which centrally acting antihypertensive agents can affect renal function, including actions mediated within the central nervous system, heart, systemic circulation and within the kidneys themselves. 'First generation' centrally acting antihypertensive agents cause diuresis and natriuresis in rats, while in dogs and humans a diuresis is often seen with variable effects on sodium excretion. 4. Evidence from studies in anaesthetized rats indicates that rilmenidine and moxonidine can promote sodium excretion by interacting with both central nervous system and renal putative I1-receptors. This does not appear to necessarily be the case in other species. At this time there are few or no published data from clinical studies to suggest that 'second generation' centrally acting antihypertensive agents affect salt and water balance differently from 'first generation' agents.

Sympathetic nervous system in salt-sensitive and obese hypertension: amelioration of multiple abnormalities by a central sympatholytic agent

Excess activity of the sympathetic nervous system (SNS) is linked to human obese hypertension and to salt-sensitive hypertension. Paradoxically, reduced SNS activity has been implicated as a contributor to obesity, particularly in animal models, and salt loading usually inhibits SNS activity. We have investigated the relationship between SNS activity, diet, and hypertension in the obese spontaneously hypertensive rat (SHROB), a model with a recessive obesity trait superimposed on a hypertensive background with multiple metabolic abnormalities resembling human syndrome X. We examined the role of SNS overactivity in the adverse impact of excess dietary salt and the possible beneficial effects of sympatholytic therapy. Mean blood pressure (MBP) was increased in SHROB and SHR fed a 4% NaCl diet. The pressor effect of dietary salt was abolished by ganglionic blockade, suggesting that increased SNS activity contributed to the pressor effect of the high-salt diet. Moxonidine, a second-generation central antihypertensive, controlled hypertension in both SHROB and SHR. Kidney damage in SHROB was accelerated by dietary salt and was reduced by moxonidine. Moxonidine elicited progressive weight loss in SHROB but not in SHR. Food intake in SHROB was reduced to the level of lean SHR. SHROB and SHR treated with moxonidine showed improved glucose tolerance. Additionally, SHROB showed reduced levels of triglycerides, cholesterol, and insulin following moxonidine therapy. Inhibition of the SNS, as with moxonidine therapy, may ameliorate multiple abnormalities and have therapeutic advantages in obese hypertensive syndromes.