Evidence that Drug Binding to Non-Adrenergic [3H]-Idazoxan Binding Sites (I-Receptors) Occurs to Interacting or Interconvertible Affinity Forms of the Receptor

Co-operative regulation of ligand binding to melanocortin receptor subtypes: Evidence for interacting binding sites

This study evaluates the binding the melanocyte stimulating hormone peptide analogue [125I]NDP-MSH to melanocortin receptors MC1, MC3, MC4 and MC5 in insect cell membranes produced by baculovirus expression systems. The presence of Ca2+ was found to be mandatory to achieve specific [125I]NDP-MSH binding to the melanocortin receptors. Although association kinetics of [125I]NDP-MSH followed the regularities of simple bimolecular reactions, the dissociation of [125I]NDP-MSH from the melanocortin receptors was heterogeneous. Eleven linear and cyclic MSH peptides studied displaced the [125I]NDP-MSH binding to the studied melanocortin receptors, with the shapes of their competition curves varying from biphasic or shallow to super-steep (Hill coefficients ranging from 0.4 to 1.5). Notably the same peptide often gave highly different patterns on different melanocortin receptor subtypes; e.g. the MC4 receptor selective antagonist HS131 gave a Hill coefficient of 1.5 on the MC1 receptor but 0.5-0.7 on the MC(3-5) receptors. Adding a mask of one of the peptides to block its high affinity binding did not prevent other competing peptides to yield biphasic competition curves. The data indicate that the binding of MSH peptides to melanocortin receptors are governed by a complex dynamic homotropic co-operative regulations.

Characterization of the discriminable stimulus produced by 2-BFI: effects of imidazoline I(2)-site ligands, MAOIs, beta-carbolines, agmatine and ibogaine

1. The molecular nature and functions of the I(2) subtype of imidazoline binding sites are unknown but evidence suggests an association with monoamine oxidase (MAO). Rats can distinguish the selective imidazoline I(2)-site ligand 2-BFI from vehicle in drug discrimination, indicating functional consequences of occupation of these sites. We have used drug discrimination to investigate the nature of the discriminable stimulus, especially in relation to MAO inhibition. 2. Following training to distinguish 2-BFI 7 mg kg(-1) i.p. from saline vehicle in two-lever operant-chambers, male Hooded Lister rats underwent sessions where test substances were given instead and the proportion of lever presses on the 2-BFI-associated lever (substitution) recorded. 3. 2-BFI; its cogeners BU216, BU224, BU226 and LSL60101; the reversible MAO-A inhibitors moclobemide and RO41-1049; the beta-carbolines harmane, norharmane and harmaline which also reversibly inhibit MAO-A, and the anti-addictive substance ibogaine exhibited potent, dose-dependent substitution for 2-BFI. 4. Agmatine, and LSL60125 substituted at one dose only. The reversible MAO-B inhibitors lazabemide and RO16-1649; the sigma(2)-site ligand SKF10,047 and the I(2A)-site ligand, amiloride, failed to substitute. The irreversible inhibitor of MAO, deprenyl, substituted for 2-BFI while clorgyline did not. 5. These results suggest imidazoline I(2) site ligands produce a common discriminable stimulus that appears associated with reversible inhibition of MAO-A rather than MAO-B, possibly through increases in extracellular concentration of one or more monoamines. Ibogaine exhibits a commonality in its subjective effects with those of I(2)-site ligands.

Differential [(3)H]idazoxan and [(3)H]2-(2-benzofuranyl)-2-imidazoline (2-BFI) binding to imidazoline I(2) receptors in human postmortem frontal cortex

[(3)H]2-(2-benzofuranyl)-2-imidazoline (2-BFI) and [(3)H]idazoxan are the most used tools to characterise imidazoline I(2) receptors. We evaluated the binding of both radioligands to human postmortem frontal cortex membranes. Saturation binding analyses revealed that [(3)H]idazoxan (in the presence of 2 microM efaroxan to avoid radioligand binding to alpha(2)-adrenoceptors and imidazoline I(1) receptors) and [(3)H]2-BFI bound with high affinity to an apparent single population of sites. However, in competition studies whereas [(3)H]idazoxan (10 nM) binding was displaced monophasically by idazoxan and 2-BFI, both drugs displayed biphasic curves for [(3)H]2-BFI (1 nM). The proportion of the low-affinity binding site increased from 17% to 25% when 10 nM [(3)H]2-BFI was displaced by idazoxan. Amiloride inhibited [(3)H]2-BFI (10 nM) binding with low affinity and in a monophasic way. These data indicate that [(3)H]2-BFI recognises in human postmortem brain membranes a second binding site different from the imidazoline I(2) receptors labelled by [(3)H]idazoxan.

Non-adrenergic binding of [3H]atipamezole in rat kidney--regional distribution and comparison to alpha2-adrenoceptors

1 Atipamezole (4-(2-ethyl-2,3-dihydro-1H-inden-2-yl)-1H-imidazole) was first introduced as a potent and specific alpha2-adrenoceptor antagonist, but in some tissues [3H]atipamezole identifies an additional population of binding sites, distinct from both classical alpha2-adrenoceptors and I1- and I2-imidazoline receptors identified with [3H]para-aminoclonidine or [3H]idazoxan. 2 In the present study we have characterized [3H]atipamezole binding sites in rat kidney by receptor autoradiography and membrane binding assays and determined whether they are pharmacologically identical with the previously described binding sites for [3H]para-aminoclonidine and [3H]idazoxan. [3H]RX821002 and [3H]rauwolscine were used to compare the regional distribution of alpha2-adrenoceptors to that of non-adrenergic binding sites of [3H]atipamezole. 3 Comparative autoradiographic experiments demonstrated the differential localisation of [3H]atipamezole, [3H]RX821002 and [3H]rauwolscine binding sites in rat kidney. The pattern of distribution of non-adrenergic [3H]atipamezole binding sites is clearly distinct from that of alpha2-adrenoceptors. 4 The non-adrenergic binding of [3H]atipamezole in rat kidney does not fall into any of the previously identified three classes of imidazoline receptors studied with [3H]para-aminoclonidine, [3H]idazoxan and [3H]RX821002. 5 Atipamezole had no inhibitory effect on MAO-A or MAO-B activity in renal membranes, which speaks against the involvement of MAOs in the observed radioligand binding.

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.

Imidazolines and the pancreatic B-cell. Actions and binding sites

Stimulation of insulin secretion by imidazoline compounds displays variable characteristics. Phentolamine (10-100 microM) increased secretion of perifused mouse islets at nonstimulatory glucose concentrations (5 mM) and even in the absence of glucose. Idazoxan (20-100 microM) elicited a moderate increase in insulin secretion, which required the presence of a stimulatory glucose concentration (10 mM). Phentolamine is therefore a stimulator of secretion in its own right, whereas idazoxan may be termed an enhancer of secretion. Both compounds inhibited the activity of ATP-dependent K+ channels in inside-out patches from B-cells; however, idazoxan achieved only an incomplete block. Both compounds depolarized the B-cell plasma membrane to an extent that permitted the opening of voltage-dependent Ca2+ channels (-40 to -30 mV). An increase in cytoplasmic Ca2+ concentration was induced by phentolamine and much less so by idazoxan. Activation of protein kinase C, a possible mechanism to amplify Ca(2+)-induced secretion, could not be verified for phentolamine. It thus appears that stimulation of insulin secretion by phentolamine is due to its blocking effect on KATP channels, which may be the correlate of non-adrenergic imidazoline binding sites which were characterized in insulin-secreting HIT cells. Whether incomplete closure of KATP channels by idazoxan or additional effects are responsible for the requirement of high glucose to stimulate secretion remains to be clarified.

Pharmacologic and molecular discrimination of I2-imidazoline receptor subtypes

I2-imidazoline receptors (I2-IR) are characterized by their high affinity for imidazolines and guanidines and medium affinity for imidazolidines. The differential recognition of I2-IR by amiloride led to subtype these sites as amiloride-sensitive (I2A-IR) and amiloride-insensitive (I2B-IR). I2-IR labeled with [3H]idazoxan or [3H]2-BFI in the rabbit cerebral cortex (I2A-IR) displayed higher affinities for amiloride and amiloride analogs than in the rat cerebral cortex (I2B-IR). Other drugs tested displayed biphasic curves in competition experiments, indicating the existence of high and low affinity sites for both I2-IR subtypes. The drugs (+)- and (-)-medetomidine, bromoxidine, moxonidine, and clorgyline were more potent on the high and/or low affinity sites of I2B-IR than on I2A-IR. Preincubation (30 min at 25 degrees C) with 10(-6) M isothiocyanatobenzyl imidazoline (IBI) or with 10(-6) M clorgyline reduced by 40% and 26%, respectively, the binding of [3H]2-BFI to I2B-IR, but it did not alter the binding of the radioligand to I2A-IR. These results indicated that the I2-IR subtypes differ in their pharmacologic profiles and in the nature of the imidazoline binding site involved in clorgyline and IBI alkylation. In rat cortical membranes, western blot detection of immunoreactive imidazoline receptor proteins revealed a double band of approximately 29/30 kD and three less intense bands of approximately 45, approximately 66, and approximately 85 kD. In rabbit cortical membranes the antibody detected proteins of approximately 30, approximately 57, approximately 66, and approximately 85 kD. It is suggested that I2-IR may be related to more than one receptor protein and that I2-IR subtypes differ in the nature of the proteins implicated.

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.

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.

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.

Presynaptic imidazoline receptors and non-adrenoceptor [3H]-idazoxan binding sites in human cardiovascular tissues

1 In segments of human right atrial appendages and pulmonary arteries preincubated with [3H]-noradrenaline and superfused with physiological salt solution containing desipramine and corticosterone, the involvement of imidazoline receptors in the modulation of [3H]-noradrenaline release was investigated. 2 In human atrial appendages, the guanidines aganodine and DTG (1,3-di(2-tolyl)guanidine) which activate presynaptic imidazoline receptors, inhibited electrically-evoked [3H]-noradrenaline release. The inhibition was not affected by blockade of alpha 2-adrenoceptors with 1 microM rauwolscine, but antagonized by extremely high concentrations of this drug (10 and/or 30 microM; apparent pA2 against aganodine and DTG: 5.55 and 5.21, respectively). 3 In the presence of 1 microM rauwolscine, [3H]-noradrenaline release in human atrial appendages was also inhibited by the imidazolines idazoxan and cirazoline, but not by agmatine and noradrenaline. The inhibitory effects of 100 microM idazoxan and 30 microM cirazoline were abolished by 30 microM rauwolscine. 4 In the atrial appendages, the rank order of potency of all guidelines and imidazolines for their inhibitory effect on electrically-evoked [3H]-noradrenaline release in the presence of 1 microM rauwolscine was: aganodine > or = BDF 6143 [4-chloro-2-(2-imidazolin-2-yl-amino)-isoindoline] > DTG > or = clonidine > cirazoline > idazoxan (BDF 6143 and clonidine were previously studied under identical conditions). This potency order corresponded to that previously determined at the presynaptic imidazoline receptors in the rabbit aorta. 5 When, in the experiments in the human pulmonary artery, rauwolscine was absent from the superfusion fluid, the concentration-response curve for BDF 6143 (a mixed alpha 2-adrenoceptor antagonist/imidazoline receptor agonist) for its facilitatory effect on electrically-evoked [3H]-noradrenaline release was bell-shaped. In the presence of 1 microM rauwolscine, BDF 6143 and cirazoline concentration-dependently inhibited the evoked [3H]-noradrenaline release. 6 In human atrial appendages, non-adrenoceptor [3H]-idazoxan binding sites were identified and characterized. The binding of [3H]-idazoxan was specific, reversible, saturable and of high affinity (KD: 25.4 nM). The specific binding of [3H]-idazoxan (defined by cirazoline 0.1 mM) to membranes of human atrial appendages was concentration-dependently inhibited by several imidazolines and guanidines, but not by rauwolscine and agmatine. In most cases, the competition curves were best fitted to a two-site model. 7 The rank order of affinity for the high affinity site (in a few cases for the only detectable site; cirazoline = idazoxan > BDF 6143>DTG> or = clonidine) is compatible with the pharmacological properties of I2-imidazoline binding sites, but is clearly different from the rank order of potency for inhibiting evoked noradrenaline release from sympathetic nerves in the same tissue. 8 It is concluded that noradrenaline release in the human atrium and, less well established, in the pulmonary artery is inhibited via presynaptic imidazoline receptors. These presynaptic imidazoline receptors appear to be related to those previously characterized in rabbit aorta and pulmonary artery, but differ clearly from I1 and I2 imidazoline binding sites.

Binding of [3H]2-(2-benzofuranyl)-2-imidazoline (BFI) to human brain: potentiation by tranylcypromine

Recently, a new imidazoline (I2) ligand, [3H]2-(2-benzofuranyl)-2-imidazoline (BFI) was shown to be more selective for I2 vs alpha 2 binding in rodent brain. We characterized [3H]BFI binding in human brain cortex and lateral reticular nucleus (NRL). Membranes were incubated with [3H]BFI at 22 degrees C in 50 mM Tris, 1.5 mM EDTA at pH 7.5. Saturation experiments with [3H]BFI (0.5-80 nM) were analyzed using non-linear curve fitting. The NRL had 4X more binding sites than cortex with similar affinity (Bmax = 2085 +/- 732 and 471 +/- 41 fmol/mg protein; KD = 9.3 +/- 3.5 and 11.9 +/- 2.7 nM, respectively). In competition studies, cortical [3H]BFI binding was displaced in order of decreasing potency by clorgyline > BFI > or = cirazoline > idazoxan > or = guanabenz > clonidine > RX821002. The monoamine oxidase (MAO) inhibitor tranylcypromine (TCP) (1 nM-10 microM), markedly enhanced [3H]BFI binding in both NRL and cortex. Enhanced binding was maximal at 300 nM (12 X control) and returned to baseline at 30 microM. Potentiation was not seen with pargyline or clorgyline. TCP did not effect [3H]BFI binding in rat cortex, or [3H]idazoxan binding in human cortex and NRL. In human cortex, inhibition of MAO by preincubation with pargyline (10 micro M) abolished the TCP effect. Upon preincubation with TCP, the stimulation of [3H]BFI binding was dose-dependently related to a simultaneous inhibition of MAO. Thus, [3H]BFI labels a site in human NRL and cortex that appears similar to the previously described I2 site labeled by [3H]idazoxan. However, [3H]BFI binding is dramatically stimulated by TCP in human brain via a mechanism dependent on endogenous MAO activity.

A series of novel imidazoline I2-receptor selective Schiff bases of 1-(benzylidenamino)-3,3-dimethylguanidine

A series of 12 Schiff bases of 1-(benyzylidenamino)-3,3-dimethylguanidine were evaluated for their affinities at imidazoline I2-receptors and alpha 2-adrenoceptors in the guinea pig cerebral cortex by using a 2-curve assay with [3H]idazoxan as ligand and the simultaneous computer analysis of (-)adrenaline masked and non-masked competition curves. All of the dimethylguanidines were selective for I2-receptors with the selectivity ranging 6-110-fold, and the most selective being WAP8 (2-bromobenzylidenamino-3,3-dimethylguanidine) closely followed by FLA151 (2-chlorobenzylidenamino-3, 3-dimethylguanidine). Some of the dimethylguanidines were also tested for affinity at bovine kidney cortex I2-receptors and guinea pig alpha 1-adrenoceptors. These substances were found to have similar affinities for bovine kidney I2-receptors as for guinea pig cerebral cortex I2-receptors, but their affinities for the alpha 1-adrenoceptors were considerably lower.

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.

Evaluation in rat of RS-79948-197 as a potential PET ligand for central alpha 2-adrenoceptors

Tritium-labelled RS-79948-197 {(8aR,12aS,13aS)-5, 8,8a,9,10,11,12,12a,13,13a-decahydro-3-methoxy-12-(ethylsulphon yl)-6H-iso- quino[2,1-g][1,6]naphthyridine} was evaluated in rat brain as an in vivo ligand for central alpha 2-adrenoceptors, as a preliminary step in the development of a radioligand for positron-emission tomography (PET) studies. The maximal receptor-specific signal was achieved within 90-120 min after i.v. injection of [ethyl-3H]RS-79948-197 and was selective for the alpha 2- compared with the alpha 1-adrenoceptor, with no detectable binding to the imidazoline-I2 site. Estimates for binding potential (approximating to Bmax/Kd) ranged between 3.4 in entorhinal cortex and 0.5 in medulla oblongata. The results, which indicate a similarly localised but 2-fold increase in specific binding compared with that previously demonstrated using [3H]RX 821002 (2-methoxy-idazoxan), are sufficiently encouraging as to support further investment in the development of 11C-labelled RS-79948-197, or a close structural analogue, as a ligand for clinical PET.

Pharmacological and molecular discrimination of brain I2-imidazoline receptor subtypes

I2-imidazoline receptors labelled with [3H]-idazoxan in the rabbit and rat brains displayed high and low affinity, respectively, for the guanidide amiloride; reinforcing the previous definition of I2A-imidazoline receptors expressed in the rabbit brain and I2B-imidazoline receptors expressed in the rat brain. Other drugs tested displayed biphasic curves in competition experiments, indicating the existence of high and low affinity sites for both subtypes of I2-imidazoline receptors. Among the drugs studied, bromoxidine, moxonidine, (+)- and (-)-medetomidine and clorgyline were more potent on the high and/or low affinity sites of I2B-than on their corresponding of I2A-imidazoline receptors (KiH ratios 20 to 65). No correlation was found for the potencies of the drugs tested at the low affinity sites of both I2-imidazoline receptor subtypes. Preincubation (30 min at 25 degrees C) with 10(-6) M clorgyline reduced by 60% the Bmax of [3H]-idazoxan binding to I2B-imidazoline receptors in the rat brain, but it did not affect the binding parameters of the radioligand saturation curves to I2A-imidazoline receptors in the rabbit brain. These results indicated that I2A- and I2B-imidazoline receptor subtypes differ in the pharmacological profiles of their high and low affinity sites and in the ability to irreversibly bind clorgyline. In rat cortical membranes western blot detection of immunoreactive imidazoline receptors proteins revealed a double band of approximately 29/30 kDa and two less intense bands of approximately 45 and approximately 66 kDa. In rabbit cortical membranes the antibody used detected proteins of approximately 30, approximately 57 and approximately 66 kDa. It is suggested that different imidazoline receptor proteins (approximately 45 vs approximately 57 kDa) may account for the different pharmacological profiles of I2-imidazoline receptor subtypes.

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.

Expression of I2-imidazoline sites in rat prostate. Effect of castration and aging

Clonidine, idazoxan, and related imidazoline adrenergic drugs bind to non-adrenergic sites in brain and several peripheral tissues. These sites, termed imidazoline receptors, appear to exist in two major subclasses, I1 sites labeled by clonidine and I2 sites labeled by idazoxan. In this study, we investigated whether rat prostate expresses imidazoline receptors and, if so, whether their expression can be regulated by circulating testosterone. Studies in rat ventral prostate membrane revealed that [3H]idazoxan, but not [3H]p-aminoclonidine, bound to non-adrenergic sites. The binding of [3H]idazoxan was saturable (Bmax: 941 +/- 105 fmol/mg protein) and high affinity (KD: 16.4 +/- 2.3 nM). The rank order of the inhibition of binding by imidazoline ligands was cirazoline > clonidine > UK 14,304 > guanabenz, indicating an I2 subclass of imidazoline receptors. Bilateral orchiectomy increased the number of binding sites (Bmax) for [3H]idazoxan without changing the affinity (KD). Testosterone replacement, while completely restoring the plasma testosterone levels, only partially reversed the increase in Bmax. In contrast, the binding of [3H]idazoxan to prostate membranes of rats in different age groups (4, 7, and 16 months) revealed a progressive decrease in the Bmax without any change in KD. We conclude that the rat prostate expresses the I2 subclass of imidazoline receptors and that the expression is regulated by circulating testosterone.

Renal effects of infusion of rilmenidine and guanabenz in conscious dogs: contribution of peripheral and central nervous system alpha 2-adrenoceptors

1. We tested the renal effects of the alpha 2-adrenoceptor agonists, rilmenidine and guanabenz and the antagonists, 2-methoxyidazoxan and idazoxan, in conscious dogs. Our aim was to test the hypothesis that putative imidazoline (I) receptors influence renal function. We reasoned that since rilmenidine and guanabenz are selective for I1- and I2-binding sites respectively, an influence of one of these receptive sites on renal function would be reflected in qualitative differences between the effects of these agents. Moreover, effects mediated by putative I-receptors should be relatively resistant to antagonism by the selective alpha 2-adrenoceptor antagonist, 2-methoxyidazoxan. Since the effects of these drugs on renal function could be mediated in the central nervous system or periphery, the dogs were studied under both normal and ganglion-blocked conditions. 2. In dogs with intact autonomic reflexes, 2-methoxyidazoxan (15 micrograms kg-1 plus 0.6 micrograms kg-1 min-1) produced effects consistent with a generalized increase in sympathetic drive, including increases in mean arterial pressure and plasma renin activity, and a reduction in sodium excretion. In ganglion-blocked dogs, 2-methoxyidazoxan reduced sodium excretion but had no discernible effect on systemic or renal haemodynamics. We conclude that an alpha 2-adrenoceptor-mediated mechanism in the central nervous system tonically inhibits sympathetic drive in the conscious dog. 3. In ganglion-blocked dogs idazoxan (3-300 micrograms kg-1) dose-dependently increased arterial pressure. This was not abolished by concomitant administration of 2-methoxyidazoxan (0.3-30 micrograms kg-1). The pressor effect of idazoxan is therefore probably mediated by an agonist action at alpha 1-adrenoceptors. 4. The effects of infusions of rilmenidine (0.1-1.0 mg kg-1) and guanabenz (10-100 micrograms kg-1) were indistinguishable. They comprised dose-dependent increases in mean arterial pressure, urine excretion, and glomerular filtration rate (the latter in ganglion blocked dogs only), and dose-dependent reductions in heart rate, renal blood flow and sodium excretion (only in dogs with intact autonomic reflexes). All of these effects were antagonized by 2-methoxyidazoxan. 5. We conclude that the renal effects of rilmenidine and guanabenz infusions in conscious dogs are predominantly, if not completely, attributable to activation of alpha 2-adrenoceptors. Our results do not support the hypothesis that putative I-receptors contribute towards the renal effects of these agents.

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)

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

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

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.

Dynamic properties of I2-receptors

Evidence is presented that drug binding to I2-sites in the guinea pig kidney and liver is subjected to dynamic homeotropic allosteric regulation. Drugs have been found that are capable of inducing both positive and negative cooperativity on both kidney and liver I2-sites when studied in competition with [3H]idazoxan. Moreover the kinetics of [3H]idazoxan association with guinea pig kidney I2-sites was complex and occurred with three different rate constants. However, on dissociation of [3H]idazoxan from its binding sites in the kidney a substantial number of the slower sites had been converted to faster sites, thus supporting the idea of a dynamic interconversion of I2 sites. The guinea pig liver and kidney I2-sites appear to be similar in their pharmacological properties. They also bear some resemblance to the I2-sites of the smooth muscle of the guinea pig ileum, but they appear to be more distant to the guinea pig cerebral cortex I2-sites. Nevertheless, data indicate that considerable overall heterogeneity exists between the guinea pig kidney and liver I2-sites and the guinea pig ileum and cerebral cortex I2-sites.

LSL 60101, a selective ligand for imidazoline I2 receptors, on glial fibrillary acidic protein concentration

The concentration of the astrocytic marker, glial fibrillary acidic protein (GFAP) was quantitated by immunoblotting (Western blotting) in the rat brain after treatment with the novel ligand for imidazoline I2 receptors LSL 60101 [2-(2-benzofuranyl)imidazole] and its 6-methoxy derivative LSL 60125. Chronic (7-21 days), but not acute (1 day) or short-term (3 days), treatment with LSL 60101 (10 mg/kg i.p.) markedly increased (44-49%) GFAP immunoreactivity in the rat cerebral cortex. In contrast, chronic (7 days) treatment with LSL 60125 (10 mg/kg i.p.) did not significantly modify GFAP concentrations. In vitro, both drugs displayed moderate high affinity and high selectivity for imidazoline I2 receptors versus alpha 2-adrenoceptors; however, only chronic treatment with LSL 60101 (10 mg/kg i.p.) but not with LSL 60125 (10 mg/kg i.p.) was associated with an up-regulation of imidazoline I2 receptors. These data indicate that glial imidazoline I2 receptors may have a direct physiological function related to GFAP expression and that LSL 60101 could be a good tool for the study of the implication of these receptors on astrocyte activation and neuronal regeneration.

Comparison of the binding activities of some drugs on alpha 2A, alpha 2B and alpha 2C-adrenoceptors and non-adrenergic imidazoline sites in the guinea pig

Simultaneous computer modelling of control and guanfacine-masked [3H]-MK 912 saturation curves as well as guanfacine competition curves revealed that both alpha 2A- and alpha 2C-adrenoceptor subtypes were present in the guinea pig cerebral cortex. The Kd value of [3H]-MK 912 determined for the alpha 2A-subtype was 403 pM and for the alpha 2C-subtype 79.8 pM; the receptor sites showing capacities 172 and 19.5 fmol/mg protein, respectively. The Kds of guanfacine were 20 and 880 nM for the alpha 2A- and alpha 2C-adrenoceptor, respectively. In the guinea pig kidney [3H]-MK 912 bound to a single saturable site with Kd 8.34 nM and capacity 285 fmol/mg protein, the site showing pharmacological properties like an alpha 2B-adrenoceptor. Binding constants of 22 compounds for the three guinea pig alpha 2-adrenoceptor subtypes were determined by computer modelling competition curves using for the cerebral cortex a "3-curve assay", for the kidney an "1-curve assay", and using [3H]-MK 912 as labelled ligand. Of the tested drugs guanfacine and BRL 44408 were found to be clearly alpha 2A-selective, Spiroxatrine, yohimbine, rauwolscine and WB 4101, as well as [3H]-MK 912 itself, were found to be alpha 2C-selective. The most selective compounds for alpha 2B-adrenoceptors, when compared to alpha 2A-adrenoceptors, were ARC 239 and prazosin. In the guinea pig kidney [3H]-p-aminoclonidine bound to alpha as well as to non-adrenergic imidazoline sites. The alpha 2-adrenoceptors could be completely blocked using 10 microM (-)-adrenaline without the non-adrenergic sites being affected. During these conditions the analysis of combined saturation and competition studies using labelled and unlabelled p-aminoclonidine with computer modelling revealed that the ligand labelled two different sites with Kds of 310 and 47,000 nM, respectively. Competition curves of 16 compounds for the non-adrenergic [3H]-p-aminoclonidine sites were shallow and resolved into two-site fits. For the high affinity [3H]-p-aminoclonidine site the highest affinities were shown by 1-medetomidine, UK-14,304, guanabenz and detomidine; the Kds of these drugs ranging 26-72 nM. All drugs tested showed low but varying affinities for the low affinity [3H]-p-aminoclonidine site. These data indicated that the [3H]-p-aminoclonidine binding sites of the guinea pig kidney are grossly different from the [3H]-idazoxan binding I2-receptors previously demonstrated also to be present in the guinea pig kidney.

Characterization of non-adrenergic [3H]clonidine binding sites in rat stomach: high affinity of imidazolines, guanidines and sigma ligands

We have identified and characterized non-adrenergic [3H]clonidine binding sites in rat stomach. The binding of [3H]clonidine was rapid, reversible, partly specific (as defined by cirazoline 0.1 nmol/l), saturable and of high affinity. The specific binding of [3H]clonidine to rat stomach membranes was concentration-dependently inhibited by various imidazolines and guanidines including the sigma site ligand 1,2-di-(2-tolyl)guanidine (DTG), by the butyrophenone derivative (+)-3-PPP[(R)-3-(3-hydroxyphenyl)-N-propylpiperidine]; the latter two compounds are also known to exhibit affinity for sigma sites. In contrast, rauwolscine, histamine, ranitidine and the non-hydrolysable GTP-analogue Gpp(NH)p (5' guanylylimidodiphosphate) did not, or with negligible affinity, inhibit [3H]clonidine binding. In most cases, the competition curves were best fitted to a two-site model. The rank order of affinity for the high affinity site (in a few cases for a single detectable site) was as follows: cirazoline > idazoxan > or = DTG > (+)-3-PPP > chlonidine > guanabenz > haloperidol. This rank order is not compatible with the pharmacological properties of either I1- or I2-imidazoline binding sites. However, the ability of haloperidol, (+)-3-PPP and DTG to displace [3H]clonidine (the latter two with high affinity) suggests that the [3H] clonidine binding sites in rat stomach may be related to sigma-like sites.

Metabolism of agmatine (clonidine-displacing substance) by diamine oxidase and the possible implications for studies of imidazoline receptors

Clonidine-displacing substance, thought to be the endogenous ligand for imidazoline receptors, has been identified recently as agmatine (1-amino-4-guanidinobutane). The similarity of this compound's structure to that of the diamine oxidase (DAO) inhibitor, aminoguanidine, led us to investigate the possibility that agmatine might be a substrate for this enzyme. The metabolism of agmatine by purified porcine kidney DAO was measured by a peroxidase-linked colorimetric assay. Agmatine was a substrate for this enzyme and, under the experimental conditions used here, was metabolised at a rate of 0.8 mumol agmatine h-1 (unit DAO activity)-1. In contrast, agmatine was a substrate neither for rat brain monoamine oxidase (MAO) -A or -B, nor for rat brown adipose tissue semicarbazide-sensitive amine oxidase (SSAO). The metabolism of agmatine by DAO was inhibited by aminoguanidine (IC50 14.9 nM) and by the antidepressant, phenelzine (IC50 1.95 microM). These results suggest that administration of DAO inhibitors may increase endogenous agmatine levels and thus alter imidazoline receptor densities. A review of the literature documenting ligand affinities for idazoxan-preferring (I2) imidazoline binding site subtypes and drug affinities for DAO enzymes indicates that some of the I2 sites described elsewhere may correspond to DAO and not to an imidazoline receptor.

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

Imidazoline binding sites from the human brainstem were solubilized with 3-[(3-cholamido-propyl)-dimethylammonio]-1-propane-sulfonate (CHAPS). [3H]idazoxan and [3H]clonidine were used as ligands to characterize the solubilized binding sites. In both the soluble and membrane fractions, [3H]idazoxan binding was saturable, stereoselective, sensitive to imidazolines and insensitive to (-)norepinephrine and to amiloride. The affinities of [3H]idazoxan for the soluble and membrane sites were similar (KD = 25 +/- 11 nM and 20 +/- 3 nM). In both soluble and membrane fractions, the alpha 2-adrenoceptor binding being masked with (-)norepinephrine, [3H]clonidine bound to a low affinity site which was insensitive to (-)norepinephrine and which exhibited the same selectivity for various drugs as the [3H]idazoxan binding site. alpha 2-adrenoceptor binding was present in the membrane and the soluble fractions although it was difficult to detect in the soluble fraction because of inhibition of [3H]rauwolscine binding by the CHAPS detergent.

Characterization of binding sites for [3H]-idazoxan, [3H]-P-aminoclonidine and [3H]-rauwolscine in the kidney of the dog

1. We characterized the binding of [3H]-rauwolscine, [3H]-p-aminoclonidine and [3H]-idazoxan in a dog kidney membrane preparation. Our aim was to determine the pharmacological nature of the alpha 2-adrenoceptor- and imidazoline-preferring binding sites in this organ. 2. [3H]-Rauwolscine bound to an apparent single site with an affinity (KD) of 2.2 nmol/L and a maximum density (Bmax) of 58.5 fmol/mg protein, when 10 mumol/L idazoxan defined non-specific binding. However displacement studies demonstrated that a number of compounds, including prazosin, inhibited [3H]-rauwolscine binding in a complex manner consistent with displacement from two distinct binding sites. The majority (69%) of the [3H]-rauwolscine binding sites had a relatively low affinity for prazosin (KI = 398 nmol/L), while the remainder had a relatively high affinity for prazosin (KI = 7.9 nmol/L). 3. [3H]-p-Aminoclonidine bound to an apparent single site (KD = 5.2 nmol/L; Bmax = 72.4 fmol/mg protein), when 10 mumol/L phentolamine defined non-specific binding. When 1 mumol/L of the potent and selective alpha 2-adrenoceptor antagonist 2-methoxyidazoxan was included in the incubate, no specific binding was detected. We therefore conclude that under the conditions of this experiment [3H]-p-aminoclonidine binds only to alpha 2-adrenoceptors in the dog kidney. 4. [3H]-Idazoxan bound to two sites, with a higher (KD = 0.95 nmol/L; Bmax = 43.9 fmol/mg protein) and lower (KD = 9.1 nmol/L; Bmax = 93.8 fmol/mg protein) affinity, respectively, when 1 mmol/L phentolamine defined non-specific binding. When 10 mumol/L GTP gamma S was included in the incubate, the low affinity site was unaffected but the maximum binding at the higher affinity site was reduced by 79%. 2-Methoxyidazoxan displaced [3H]-idazoxan in a monophasic manner and with low potency (IC50 = 11.5 mumol/L). Yohimbine, efaroxan, clonidine, rilmenidine, guanabenz and idazoxan itself displaced [3H]-idazoxan in a complex manner; the slope of the displacement curves being less than unity. 5. We conclude that the dog kidney contains a heterogeneous population of alpha 2-adrenoceptors that can be labelled either with [3H]-rauwolscine or [3H]-p-aminoclonidine. The dog kidney also contains a heterogeneous population of non-adrenoceptor imidazoline-preferring binding sites of the I2-subtype, that can be labelled with [3H]-idazoxan. The binding site for which [3H]-idazoxan has the highest affinity appears to be coupled to a guanine nucleotide binding regulatory protein.

Imidazoline receptors, non-adrenergic idazoxan binding sites and alpha 2-adrenoceptors in the human central nervous system

Both [3H]clonidine and [3H]idazoxan bind to alpha 2 adrenoceptors. The former also labels imidazoline receptors, and the latter non-adrenergic idazoxan binding sites. In order to investigate whether the imidazoline receptors and non-adrenergic idazoxan binding sites are identical, we compared the binding characteristics of [3H]clonidine and [3H]idazoxan to these sites by radioligand binding experiments on ultra-thin slices and homogenates of human striatum. A good correlation was found between the effect of different ions on the binding characteristics of [3H]clonidine and [3H]idazoxan, and the affinities of most competing drugs. However, clonidine and rilmenidine displayed a 100- and 10-fold lower affinity, respectively, for the idazoxan binding sites than for the imidazoline receptors. Autoradiography with [3H]clonidine showed that high densities of imidazoline receptors were present in the striatum, pallidum, gyrus dentatus of the hippocampus, amygdala, and substantia nigra. Moderate densities were found throughout the cerebral cortex, thalamus and several brainstem nuclei including the nucleus olivarius inferior. Low densities were seen in the cerebellum, spinal cord and pituitary gland. As for the non-adrenergic sites labelled by [3H]idazoxan, the imidazoline receptors can be found in all major brain areas examined. However, there are some striking differences between the concentrations of imidazoline receptors and non-adrenergic idazoxan binding sites in certain brain regions. To reconcile distribution and pharmacologic data, we propose that imidazoline receptors and non-adrenergic idazoxan binding sites represent different proteins or protein complexes and that at least in the nucleus reticularis lateralis and the striatum, imidazoline receptors and non-adrenergic idazoxan binding sites may be physically associated. The regional distribution of alpha 2 adrenoceptors within the human CNS was determined by quantitative autoradiography with [3H]RX821002. The highest densities of alpha 2 adrenoceptors were found in the cerebral and cerebellar cortex, and certain regions in the medulla oblongata (floor of the IV ventricle, reticular formation, hypoglossal nucleus and nucleus olivarius inferior). No alpha 2 adrenoceptors were detected in the pituitary gland. There exists no relationship between the distribution pattern of imidazoline receptors and alpha 2 adrenoceptors, indicating that these binding sites are independent from each other.

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.

Human brain imidazoline receptors: further characterization with [3H]clonidine

The aim of the present study was to further characterize [3H]clonidine binding in the ventrolateral medulla of the human brainstem, the region involved in the vasodepressor effect of imidazoline drugs of the clonidine type. Under basal conditions, [3H]clonidine can bind both to the imidazoline receptors and to the alpha-adrenoceptors. The latter represent only a small part of the total [3H]clonidine binding with a Bmax of 61 +/- 13 fmol/mg proteins and a KD of 4.9 +/- 2.2 nM. Most of the binding was associated with imidazoline receptors with a KD of 67 +/- 13 nM and a Bmax of 677 +/- 136 fmol/mg protein. alpha-Adrenoceptor binding of [3H]clonidine could be completely prevented when membranes were either treated during preparation with the aIkylating agent phenoxybenzamine or incubated in the presence of 30 microM (-)-noradrenaline or in the presence of the non-hydrolysable analogue of GTP, guanylyl imidodiphosphate (Gpp(NH)p). When the alpha-adrenoceptors binding was prevented, we demonstrated the insensitivity of [3H]clonidine binding to Gpp(NH)p and showed that the competition between clonidine and idazoxan for imidazoline receptors was insensitive to Gpp(NH)p suggesting that imidazoline receptors are not G protein coupled receptors. The specificity of [3H]cloniding binding to imidazoline receptors in the human ventrolateral medulla indicates that these receptors are different from imidazole receptors as defined with p-aminoclonidine in the bovine brainstem.

Differential effects of the alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline on brain alpha 2-adrenoceptors and I2-imidazoline sites in vitro and in vivo

The alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) is a peptide-coupling agent that is being used to inactivate irreversibly alpha 2-adrenoceptors and other receptors. The aim of the present study was to assess the in vitro and in vivo effects of EEDQ on the newly discovered brain I2-imidazoline sites, located mainly in mitochondria. Preincubation of rat cortical membranes with EEDQ (10(-8)-10(-5) M) markedly decreased (20-90%) the specific binding of the selective antagonist [3H]RX821002 to alpha 2-adrenoceptors without affecting that of [3H]idazoxan (in the presence of adrenaline) to I2-imidazoline sites. In EEDQ-pretreated membranes (10(-5) M, 30 min at 25 degrees C), the density of I2-imidazoline sites (Bmax = 80 +/- 4 fmol/mg of protein) was not different from that determined in untreated membranes in the presence of (10(-6) M (-)-adrenaline (Bmax = 83 +/- 4 fmol/mg of protein), and both densities were lower (24%, p < 0.05) than the total native density of [3H]idazoxan binding sites (Bmax = 107 +/- 6 fmol/mg of protein) (I2-imidazoline sites plus alpha 2-adrenoceptors). Treatment of rats with an optimal dose of EEDQ (1.6 mg/kg, i.p., for 2 h to 30 days) reduced maximally at 6 h (by 95 +/- 1%) the specific binding of [3H]-RX821002 to alpha 2-adrenoceptors, but also the binding of [3H]idazoxan to I2-imidazoline sites (by 44 +/- 5%). Pretreatment with yohimbine (10 mg/kg, i.p.) fully protected against EEDQ-induced alpha 2-adrenoceptor inactivation.(ABSTRACT TRUNCATED AT 250 WORDS)

Detomidine binding to guinea pig liver imidazoline receptors (I-receptors) shows marked positive cooperativity

The binding of [3H]-idazoxan to guinea pig liver membranes was measured in the presence of 3 microM rauwolscine, which prevented the binding [3H]-idazoxan to alpha 2-adrenergic receptors. Under these conditions the radioligand bound to saturable imidazoline receptors (I-receptors) with a Kd of 18 nM and a Bmax of 665 fmol mg-1 protein. Six drugs which were used to compete for [3H]-idazoxan in the liver caused competition curves of widely varying steepness. Fitting the competition curves to the standard four parameter logistic function showed that the Hill coefficients (nH) varied from 2.02 (detomidine) to 0.43 (UK-14,304), The nH's obtained in liver for the six compounds correlated strongly (r = 0.99; P < 0.001) with the corresponding nH's obtained in a previous study on the guinea pig kidney where the drugs were also tested in competition with [3H]-idazoxan (Wikberg et al. 1992). Good correlation was also found for the Log(Ki) values of drugs determined in the two tissues (r = 0.96; P < 0.005). Whereas the standard logistic function accurately described the competition curves of the 5 drugs tested in the liver for which the competition curve Hill coefficients varied between 0.43 to 1.41 (UK-14,304, rilmenidine, histamine and d- and l-medetomidine), it did not accurately fit the detomidine competition curves. Instead the detomidine competition curves could be more accurately described by a model composed of the sum of two independent logistic functions.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of [3H]clonidine to I1-imidazoline sites in bovine adrenal medullary membranes

Imidazolines bind with high affinity not only to alpha-adrenoceptors but also to specific imidazoline binding sites (IBS) labelled by either [3H]clonidine or [3H]idazoxan and termed I1- and I2-IBS, respectively. Since bovine adrenal chromaffin cells lack alpha 2-adrenoceptors, we investigated the pharmacological characteristics of [3H]clonidine binding sites in the bovine adrenal medulla. The binding of [3H]clonidine was rapid, reversible, partly specific (as defined by naphazoline 0.1 mmol/l; 55% specific binding at [3H]clonidine 10 nmol/l), saturable and of high affinity. The specific binding of [3H]clonidine to bovine adrenal medullary membranes was concentration-dependently inhibited by various imidazolines, guanidines and an oxazoline derivative but not, or with negligible affinity, by rauwolscine and (-)-adrenaline. In most cases, the competition curves were best fitted to a two-site model. The rank order of affinity for the high affinity site (in a few cases the single detectable site) was as follows: naphazoline > or = BDF 7579 (4-chloro-2-isoindolinyl guanidine) > or = clonidine > or = cirazoline > or = BDF 6143 (4-chloro-2-(2-imidazoline-2-ylamino)-isoindoline hydrochloride) > BDF 7572 (4,7-chloro-2-(2-imidazolin-2-ylamino)-isoindoline) > moxonidine = rilmenidine > BDF 6100 (2-(2-imidazoline-2-ylamino)-isoindoline) = idazoxan > phentolamine > aganodine = guanabenz > amiloride > histamine. This rank order is compatible with the pharmacological properties of the I1-IBS. The non-hydrolysable GTP-analogue Gpp(NH)p (5'guanylylimidodiphosphate; 100 mumol/l) inhibited specific [3H]clonidine binding by about 50%. Equilibrium [3H]clonidine binding was also significantly reduced by K+ and Mg2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of non-adrenergic imidazoline sites in chromaffin cells and mitochondrial membranes of bovine adrenal medulla

We sought to characterize the non-adrenergic binding site for imidazolines, the imidazoline receptor in whole membranes and subcellular compartments of chromaffin cells of bovine adrenal medulla. [3H]Idazoxan exhibited saturable and high affinity (KD = 5 nM) binding to chromaffin cell membranes fully displaceable by idazoxan and cirazoline but not by epinephrine or rauwolscine. Binding sites were highly enriched in mitochondrial but not plasma membranes and absent from nuclear fractions. The rank order of potency for displacement of [3H]idazoxan from mitochondrial membranes was: cirazoline > idazoxan > naphazoline > amiloride > detomedine > clonidine >> phentolamine > cimetidine = imidazole 4-acetic acid > p-iodoclonidine = epinephrine = norepinephrine = rauwolscine. Binding was also inhibited with high affinity by the purported endogenous ligand clonidine-displacing substance and by K+ and the K(+)-channel antagonists 4-aminopyridine and tetraethylammonium bromide but not Na+. We conclude that: (a) adrenal chromaffin cells express imidazoline receptors but not alpha 2-adrenergic receptors; (b) these sites are predominantly localized to adrenal medullary mitochondria; and (c) imidazoline receptors conform to an idazoxan preferring (I-2) rather than the clonidine preferring (I-1) subclass and are amiloride sensitive. The data support the view that alpha 2-adrenergic and imidazoline receptors are distinct receptor species and that adrenal chromaffin cells would be a useful cultured cell system, expressing only imidazoline receptors, for further molecular and functional studies of the receptors.

Chronic treatment with the monoamine oxidase inhibitors clorgyline and pargyline down-regulates non-adrenoceptor [3H]-idazoxan binding sites in the rat brain

1. The binding of [3H]-idazoxan in the presence of 10(-6) M (-)-adrenaline was used to quantitate non-adrenoceptor idazoxan binding sites (NAIBS) in the rat brain after treatment with various psychotropic drugs. 2. Chronic treatment (14 days) with the monoamine oxidase (MAO) inhibitors clorgyline (0.3-10 mg kg-1, i.p.) and pargyline (10 mg kg-1, i.p.), but not with Ro 41-1049 (1 mg kg-1, i.p.), markedly decreased (30-50%) the density of NAIBS in the cerebral cortex without any apparent change in the affinity of the radioligand. 3. Acute (1 day) and/or chronic treatments (14 days) with other psychotropic drugs such as desipramine (3 mg kg-1, i.p.), cocaine (10 mg kg-1, i.p.), reserpine (0.12 mg kg-1, s.c.), haloperidol (1 mg kg-1, i.p.) and diazepam (10 mg kg-1, i.p.) did not alter the density of NAIBS in the cerebral cortex. 4. In vitro, the propargylamines clorgyline, pargyline and deprenyl displaced the binding of [3H]-idazoxan to NAIBS from two distinct sites, but only clorgyline displayed an apparent very high affinity for a relevant population of NAIBS (KiH = 40 pM; KiL = 10.6 microM). The structurally diverse MAO inhibitors Ro 16-6491 (selective for MAO-B) and Ro 41-1049 (selective for MAO-A), as well as the other psychotropic drugs (desipramine, cocaine, reserpine and haloperidol) displaced the binding of [3H]-idazoxan to NAIBS monophasically and with very low potencies. As expected, the MAO inhibitors clorgyline and Ro 41-1049 displaced the binding of [3H]-Ro 41-1049 to MAO-A monophasically and with high potencies (Ki values: 0.18 nM and 22 nM, respectively). In contrast, idazoxan displayed very low affinity (Ki =40 microM) against the binding of pH]-Ro 41-1049 to MAO-A. These results disprove a direct interaction between [3H]-idazoxan and the enzyme MAO.5. Preincubation of cortical membranes with clorgyline (10-9M or 10-6 M for 30 min) or pargyline(10-6 M or 10-5M for 30 min), reduced by 30-50% and by 17-30%, respectively, the total density of NAIBS without any apparent change in the affinity of the radioligand. Preincubation with 10-6M clorgyline did not alter the affinity of cirazoline for the two populations of NAIBS, but reduced by 60%the binding of [3H]-idazoxan to the high affinity site without affecting the binding of the radioligand to the low affinity site. These results indicate that the two MAO inhibitors irreversibly block the binding of[3H]-idazoxan to NAIBS.6. In vivo, however, various acute treatments with clorgyline (1-20 mg kg-1, i.p.) for different time intervals (6-48 h) did not alter the density of NAIBS. In vivo, only very high doses of clorgyline (40 and 80 mg kg-1, i.p.) induced modest decreases (21-28%) in the density of NAIBS in the cerebral cortex.7. Together the results indicate that the irreversible binding of clorgyline and pargyline to NAIBSfound in vitro does not fully explain the marked decreases in the density of NAIBS found in vivo after the chronic treatments. It is suggested that the down-regulation of NAIBS induced in vivo by clorgyline and pargyline, through a direct or indirect mechanism, may have functional implications.