Characterization of imidazoline binding protein(s) solubilized from human brainstem: studies with [3H]idazoxan and [3H]clonidine

Imidazoline Receptor System: The Past, the Present, and the Future

Imidazoline receptors historically referred to a family of nonadrenergic binding sites that recognize compounds with an imidazoline moiety, although this has proven to be an oversimplification. For example, none of the proposed endogenous ligands for imidazoline receptors contain an imidazoline moiety but they are diverse in their chemical structure. Three receptor subtypes (I₁, I₂, and I₃) have been proposed and the understanding of each has seen differing progress over the decades. I₁ receptors partially mediate the central hypotensive effects of clonidine-like drugs. Moxonidine and rilmenidine have better therapeutic profiles (fewer side effects) than clonidine as antihypertensive drugs, thought to be due to their higher I₁/α ₂-adrenoceptor selectivity. Newer I₁ receptor agonists such as LNP599 [3-chloro-2-methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride] have little to no activity on α ₂-adrenoceptors and demonstrate promising therapeutic potential for hypertension and metabolic syndrome. I₂ receptors associate with several distinct proteins, but the identities of these proteins remain elusive. I₂ receptor agonists have demonstrated various centrally mediated effects including antinociception and neuroprotection. A new I₂ receptor agonist, CR4056 [2-phenyl-6-(1H-imidazol-1yl) quinazoline], demonstrated clear analgesic activity in a recently completed phase II clinical trial and holds great promise as a novel I₂ receptor-based first-in-class nonopioid analgesic. The understanding of I₃ receptors is relatively limited. Existing data suggest that I₃ receptors may represent a binding site at the Kir6.2-subtype ATP-sensitive potassium channels in pancreatic β-cells and may be involved in insulin secretion. Despite the elusive nature of their molecular identities, recent progress on drug discovery targeting imidazoline receptors (I₁ and I₂) demonstrates the exciting potential of these compounds to elicit neuroprotection and to treat various disorders such as hypertension, metabolic syndrome, and chronic pain.

S1P-receptors in PC12 and transfected HEK293 cells: molecular targets of hypotensive imidazoline I(1) receptor ligands

The present study aimed at elucidating the molecular identity of the proposed "I(1)-imidazoline receptors", i.e. non-adrenoceptor recognition sites via which the centrally acting imidazolines clonidine and moxonidine mediate a major part of their effects. In radioligand binding experiments with [(3)H]clonidine and [(3)H]lysophosphatidic acid on intact, alpha(2)-adrenoceptor-deficient PC12 cells, moxonidine, clonidine, lysophosphatidic acid and sphingosine-1-phosphate (S1P) competed for the specific binding sites of both radioligands with similar affinities. RNA interference with the rat S1P(1)-, S1P(2)- or S1P(3)-receptor abolished specific [(3)H]lysophosphatidic acid binding. [(3)H]Clonidine binding was markedly decreased by siRNA targeting S1P(1)- and S1P(3)-receptors but not by siRNA against S1P(2)-receptors. Finally, in HEK293 cells transiently expressing human S1P(3)-receptors, sphingosine-1-phosphate, clonidine and moxonidine induced increases in intracellular calcium concentration, moxonidine being more potent than clonidine; this is in agreement with the known properties of the "I(1)-imidazoline receptors". The present results indicate that the "I(1)-imidazoline receptors" mediating effects of clonidine and moxonidine in PC12 and the transfected HEK293 cells belong to the S1P-receptor family; in particular, the data obtained in PC12 cells suggest that the I(1) imidazoline receptors represent a mixture of S1P(1)- and S1P(3)-receptors and/or hetero-dimers of both.

Imidazoline receptors associated with noradrenergic terminals in the rostral ventrolateral medulla mediate the hypotensive responses of moxonidine but not clonidine

We determined whether the cardiovascular actions of central anti-hypertensive agents clonidine and moxonidine are dependent on noradrenergic or serotonergic innervation of the rostral ventrolateral medulla (RVLM) in conscious rabbits. 6-Hydroxydopamine (6-OHDA) or 5,6-dihydroxytriptamine (5,6-DHT) was injected into the RVLM to deplete noradrenergic and serotonergic terminals respectively. One, 2 and 4 weeks later, responses to fourth ventricular (4V) clonidine (0.65 microg/kg) and moxonidine (0.44 microg/kg) were examined. Destruction of noradrenergic pathways in the RVLM by 6-OHDA reduced the hypotensive response to 4V moxonidine to 62%, 47% and 60% of that observed in vehicle treated rabbits at weeks 1, 2 and 4 respectively. The moxonidine induced bradycardia was similarly attenuated (to 46% of vehicle). Conversely, 6-OHDA had no effect on the hypotensive or bradycardic effects of 4V clonidine. Efaroxan (I(1)-imidazoline receptor/alpha(2)-adrenoceptor antagonist; 3.5, 11, 35 microg/kg) and 2-methoxyidazoxan (alpha(2)-adrenoceptor antagonist; 0.3, 0.9, 3 microg/kg) equally reversed the hypotension to 4V clonidine, suggesting a mainly alpha(2)-adrenoceptor mechanism. Efaroxan preferentially reversed responses to moxonidine in both vehicle and 5,6-DHT groups and in the 1st week after 6-OHDA, suggesting a mechanism involving mainly I(1)-imidazoline receptors. This selectivity was subsequently lost in the 2nd and 4th weeks when the remaining hypotension was mainly mediated by alpha(2)-adrenoceptors. Depletion of serotonergic terminals did not alter the responses to either agonist nor did it change the relative effectiveness of the antagonists. Western blots of RVLM tissues probed with imidazoline and alpha(2)-adrenoceptor antisera showed a pattern of bands close to that reported in other species. The main effect of 6-OHDA was an 18% lower level of the 42 kDa imidazoline protein (P<0.05). We conclude that the hypotensive and bradycardic actions of moxonidine but not clonidine are mediated through imidazoline receptors and are dependent on intact noradrenergic pathways within the RVLM. Furthermore, the noradrenergic innervation may be associated with a 42 kDa imidazoline receptor protein.

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.

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.

Identification of human I1 receptors and their relationship to alpha 2-adrenoceptors

I1 imidazoline receptors (I1R) were defined as receptors insensitive to catecholamines and highly sensitive to [3H]clonidine and analogs. By contrast, the I2R subtype is more sensitive to [3H]idazoxan. [3H]clonidine and [3H]idazoxan imidazoline specific binding sites (IBS) have been detected in crude human membranes. Pharmacologic characterization by binding assays clearly differentiates IBS from alpha 2-adrenoceptors, whereas differences between [3H]clonidine and [3H]idazoxan IBS are less clear in crude preparations. In fact, only moderate affinity for [3H]clonidine was detectable in such preparations. However, purification procedures allowed detection of high affinity [3H]clonidine IBS in the human brain, corresponding to the I1R. Difficulties in the characterization of the I1R in crude membranes are due to multiple factors including heterogeneity of IBS, their low Bmax value, the existence of allosteric modulation, and possibly the presence of natural binding inhibitors. Immunologic studies with specific anti-idiotypic antibodies revealed a 43-kD protein as the best candidate for I1R as binding activity coincides with immunodetection. No cross-reaction was found with anti-monoamine oxidase (MAO) A/B antibodies and the 43-kD protein, ruling out the possibility of this protein being an MAO-associated I2R. Neither anti-alpha 2A- nor anti-alpha 2B-specific antibodies were able to immunodetect the 43-kD protein in crude membrane preparations or in purified fractions. These results and further biochemical characterization (pHi, N-glycosylation) of the 43-kD protein definitely assessed that human brain I1R and alpha 2-adrenoceptors clearly differ physically. However, coexpression of I1R and alpha 2-adrenoceptors in synaptic plasma membranes of the bovine brainstem reinforce the possibility of a functional relationship between the two types of receptor.

Pharmacology and subcellular distribution of [3H]rilmenidine binding sites in rat brain

We have previously reported that in rat brain membranes, [3H]rilmenidine, in addition to labelling alpha2-adrenoceptors and the I2B-subtype of imidazoline receptor binding site (I2B-RBS), may label an additional I-RBS population, distinct from previously classified I1-RBS and I2-RBS. In this study, using crude or fractionated rat brain membranes we examined the possible association of [3H]rilmenidine-labelled I-RBS with the A- and B-isoforms of monoamine oxidase (MAO) by studying the inhibition of [3H]rilmenidine binding by a number of MAO inhibitors; and comparing the maximal binding density (Bmax) and subcellular distribution of [3H]rilmenidine binding sites with that of MAO-A and MAO-B catalytic sites labelled by [3H]RO41-1049 and [3H]RO19-6327 and 12-RBS labelled by [3H]2-BFI. Inhibition of [3H]rilmenidine binding by all MAO inhibitors tested produced very shallow curves (slope 0.29-0.56). Clorgyline and moclobemide (selective MAO-A inhibitors) displayed moderate affinities (60-140 nM), while pargyline (non-selective MAO-inhibitor), RO41-1049 (selective MAO-A inhibitor) and RO19-6327 (selective MAO-B inhibitor) exhibited very low affinities (> 2 microM) for 50-75% of [3H]rilmenidine-labelled I-RBS in crude brain membranes and even lower affinity for the remaining binding. Under identical buffer conditions, the Bmax of [3H]rilmenidine-labelled I-RBS (1.45+/-0.14 pmol/mg protein) was considerably lower than those of MAO-A (13.10+/-0.15 pmol/mg) and MAO-B (10.35+/-0.50 pmol/mg) sites. These results suggest that [3H]rilmenidine does not interact directly with the active catalytic site of either MAO enzyme and could at best only associate with a subpopulation of MAO molecules. Binding studies on five fractions of rat cortex homogenates-nuclear (N), heavy (M) and light (L) mitochondrial, microsomal non-mitochondrial (P), and soluble cytosolic (S) fractions-revealed that 45% of total [3H]rilmenidine binding was present in the P fraction cf. 20 and 23% in the M and L fractions, in contrast to [3H]RO19-6327 and [3H]2-BFI which bound 11-13% in the P fraction and 36-38% and 35-44% in the M and L fractions, respectively. Binding of all ligands in the N fraction was 6-15% of total. These studies reveal that [3H]rilmenidine-labelled I-RBS, unlike the I2-RBS, are not predominantly associated with mitochondrial fractions containing the MAO enzymes (and cytochrome oxidase activity), but appear to be distributed in both the mitochondrial and plasma membrane fractions in rat cerebral cortex.

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.

Imidazoline receptor proteins in brains of patients with Alzheimer's disease

Imidazoline receptors (29/30- and 45-kDa proteins) were quantitated in postmortem brains of patients with Alzheimer's disease (AD) by using immunoblotting techniques and a specific antiserum. Increased levels of the 29/30-kDa protein (30%), 45-kDa protein (36%) and glial fibrillary acidic protein (88%) were found in the frontal cortex of AD patients. These findings are in line with the reported higher density of imidazoline receptors labelled by [3H]idazoxan in AD brains, suggesting that these imidazoline receptor proteins are related to the I2-imidazoline receptor located in mitochondria of glial (astrocyte) cells.

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.

Densities of I2-imidazoline receptors, alpha 2-adrenoceptors and monoamine oxidase B in brains of suicide victims

In the frontal cortex of suicide victims, the density of I2-imidazoline receptors labeled by [3H]idazoxan was lower (40%) than that in healthy subjects. In the same brains, the immunoreactivity of a 29/30 kDa imidazoline receptor protein was also found to be significantly decreased (19%) and it showed a positive correlation with the density of I2-imidazoline receptors. Also in the same brains, the density of alpha 2-adrenoceptor antagonist binding sites labeled by [3H]RX821002 (2-methoxy idazoxan) was found to be unchanged. The density of brain I2-imidazoline receptors, correlated with aging in control subjects but not in suicides. The density of brain MAO-B sites labeled by [3H]Ro 19-6327 (lazabemide) in suicides was no different to that in age-matched controls. As expected the density of brain MAO-B sites in suicides showed a positive correlation with age, but it did not correlate with the density of I2-imidazoline receptors in the same brains. The data indicate that the I2-imidazoline receptors labeled by [3H]idazoxan in the brain of suicides is related to a 29/30 kDa imidazoline receptor protein identified by immunoblot analysis. The data also indicate that the brain I2-imidazoline receptor cannot be identified with the MAO-B isoenzyme. The decreased density of I2-imidazoline receptors in the brain of suicide victims might play a role in the pathogenesis of major depression.

Further biochemical characterization of imidazoline binding sites from the human brainstem

Biochemical characteristics of imidazoline specific binding sites from the human brainstem were further investigated using [3H]idazoxan as radiolabeled ligand. The study of the interaction of [3H]idazoxan binding sites with heparin and lectins (soybean and lentil lectin) confirm the heterogeneity of these sites in the human brain. In fact, about 10-15% of [3H]idazoxan binding sites were retained by each of the three supports used, leading to the hypothesis that two populations of sites, with different biochemical characteristics, coexist in this tissue. A small proportion of [3H]idazoxan binding sites was retained on an affinity chromatography support consisting of a clonidine-derived Pharmalink column. The binding activity of these clonidine-eluted sites was markedly and dose-dependently improved by the addition of 'treated fall-through' fraction from the same column. On the other hand, this 'treated fall-through' fraction inhibited the binding activity detected in the solubilized human brainstem membranes. These results also suggest the existence of heterogeneous imidazoline specific binding sites in the human brainstem and the existence of endogenous factors able to discriminate between them.

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.

Imidazoline binding sites and signal transduction pathways

1. Discrete, non-adrenergic binding sites for imidazolines have been characterized in the brain and periphery. The I1 clonidine-preferring site is mainly distributed in the brain and brain stem, while the I2 idazoxan-preferring site is more widely distributed. 2. The I1 site appears to be associated with modulation of blood pressure. Imidazolines act within the rostral ventrolateral medulla to produce hypotension. The underlying signal transduction mechanism is poorly understood. 3. The imidazolines clonidine and cirazoline inhibited nicotine-stimulated calcium entry into rat phaeochromocytoma (PC-12) cells by a non-adrenergic mechanism. This effect was not attributable to the stimulation of protein kinases. 4. Similarly, clonidine and cirazoline inhibited nicotine-stimulated inward currents into PC-12 cells. This inhibitory action was not altered by inhibitors of signal transducing G-proteins. 5. Clonidine and cirazoline displaced the ion channel ligand [3H]-phencyclidine from nicotinic acetylcholine receptors, suggesting that these drugs act by direct blockade of the intrinsic ion channel of the nicotinic acetylcholine receptor. 6. This ion channel-blocking activity represents a novel action of these imidazolines and may underlie some of the proposed physiological actions of I1 sites.

Polyclonal anti-idiotypic antibodies to idazoxan and their interaction with human brain imidazoline binding sites

Polyclonal antibodies were raised in rabbits against purified polyclonal anti-idazoxan antibodies. The anti-idiotypic antibodies thus obtained, proved able to inhibit [3H]idazoxan specific binding to anti-idazoxan antibodies. Applied to human nucleus reticularis lateralis membrane preparations, these antibodies (20 micrograms) inhibited about 50 and 70% of the imidazoline specific binding of [3H]idazoxan and [3H]clonidine, respectively. Furthermore, they specifically immunoprecipitated 50% of [3H]idazoxan binding activity of imidazoline binding sites solubilized from the same tissue. [3H]Rauwolscine binding to alpha 2-adrenoceptors in rat cortex was not significantly affected by these antibodies. The antibodies labeled a 43 kDa protein in Western blots of partially purified imidazoline binding sites from human brain. In conclusion, these anti-idiotypic antibodies recognize imidazoline binding sites from human brain and allow the detection of a 43 kDa binding protein associated with or representing the imidazoline receptor expressed in human brain.

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)

Heterogeneity of imidazoline binding sites revealed by a cirazoline derivative

The affinity of AMPI (2-[3-aminophenoxy]methyl imidazoline) for [3H]clonidine and [3H]idazoxan imidazoline binding sites was determined in various rabbit and human tissues. Although cirazoline showed a high affinity (nM range) in all the tested tissues, its derivative, AMPI, had a high affinity (nM range) in rabbit brain and kidney but a low affinity (microM range) in the human brain. These differences in affinities were very similar to those obtained with amiloride. The same results were obtained when considering [3H]clonidine or [3H]idazoxan specific imidazoline binding sites.

Isolation of a human cerebral imidazoline-specific binding protein

The first isolation of a human brain specific imidazoline binding protein is described. This protein was obtained using affinity chromatography and was revealed with the aid of an anti-idiotypic antibody specific for imidazoline binding sites. The protein (43 kDa) differs from other imidazoline binding proteins previously isolated from peripheral tissues, in particular by being also sensitive to clonidine.