Evidence for imidazoline binding sites in basolateral membranes from rabbit kidney

An Imidazoline 2 Receptor Ligand Relaxes Mouse Aorta via Off-Target Mechanisms Resistant to Aging

Imidazoline receptors (IR) are classified into three receptor subtypes (I1R, I2R, and I3R) and previous studies showed that regulation of I2R signaling has neuroprotective potential. In order to know if I2R has a role in modulating vascular tone in health and disease, we evaluated the putative vasoactive effects of two recently synthesized I2R ligands, diethyl (1RS,3aSR,6aSR)-5-(3-chloro-4-fluorophenyl)-4,6-dioxo-1-phenyl-1,3a,4,5,6,6a-hexahydropyrrolo[3,4-c]pyrrole -1-phosphonate (B06) and diethyl [(1-(3-chloro-4-fluorobenzyl)-5,5-dimethyl-4-phenyl-4,5-dihydro-1H-imidazol-4-yl]phosphonate] (MCR5). Thoracic aortas from Oncins France 1 (3- to 4-months-old) and C57BL/6 (3- to 4- and 16- to 17-months-old mice) were mounted in tissue baths to measure isometric tension. In young mice of both strains, MCR5 induced greater relaxations than either B06 or the high-affinity I2R selective ligand 2-(2-benzofuranyl)-2-imidazoline (2-BFI), which evoked marginal responses. MCR5 relaxations were independent of I2R, as IR ligands did not significantly affect them, involved activation of smooth muscle KATP channels and inhibition of L-type voltage-gated Ca2+ channels, and were only slightly modulated by endothelium-derived nitric oxide (negatively) and prostacyclin (positively). Notably, despite the presence of endothelial dysfunction in old mice, MCR5 relaxations were preserved. In conclusion, the present study provides evidence against a functional contribution of I2R in the modulation of vascular tone in the mouse aorta. Moreover, the I2R ligand MCR5 is an endothelium-independent vasodilator that acts largely via I2R-independent pathways and is resistant to aging. We propose MCR5 as a candidate drug for the management of vascular disease in the elderly.

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.

Imidazoline I2 receptors: An update

Since first introduced more than two decades ago, the research in imidazoline I₂ receptors has been steadily increasing. This review provides an update on the current status of I₂ receptor pharmacology. Imidazoline I₂ receptors or I₂ binding sites refer to several (at least four) different proteins that bind to [³H]-idazoxan and [³H]-2-BFI with high affinity. The molecular identities of the proteins remain elusive. One of the proteins (45kD) seems to be consistent with the identity of brain creatine kinase. The biological functions of I₂ receptors have been primarily unveiled by the studies of selective I₂ receptor ligands. Accumulating evidence suggests that I₂ receptor ligands are effective analgesics for persistent and chronic painful conditions such as inflammatory, neuropathic and postoperative pain. One selective I₂ receptor ligand, CR4056, has been advanced to phase II clinical trial with the therapeutic indication of chronic inflammatory pain (osteoarthritis). The expansion to the treatment of other chronic pain conditions should be expected if CR4056 could eventually be approved as a new drug. I₂ receptor ligands also demonstrate robust discriminative stimulus activity and induce a characteristic discriminative cue in animals. Biochemical and preclinical in vivo investigations also suggest that I₂ receptor ligands have neuroprotective activity and modulate body temperature. The emerging discrepancies of a range of purported selective I₂ receptor ligands suggest different pharmacological effects mediated by discrete I₂ receptor components which likely attribute to the I₂ receptor-related proteins. It is proposed that the I₂ receptors represent an emerging drug target for the treatment of neurological disorders such as pain and stroke, and deserve more research attention to translate preclinical findings to pharmacotherapies.

EFFECTS OF RILMENIDINE ON PROXIMAL TUBULAR FLUID ABSORPTION IN RATS

The antihypertensive agent rilmenidine has threefold higher affinity for I(1) imidazoline receptors compared with alpha(2)-adrenoceptors and acts on the central nervous system by reducing sympathetic activity and in the kidney by inhibiting Na(+)/H(+) exchange activity. In the present study, we examined: (i) the effects of luminal and peritubular administration of rilmenidine on fluid absorption in superficial proximal tubules; and (ii) the nature of the receptors involved in mediating the action of this drug in the presence of specific antagonists (efaroxan, idazoxan and 2-methoxy-idazoxan). Studies were performed in anaesthetized Sprague-Dawley rats using shrinking split-drop micropuncture. Luminal administration of rilmenidine (10(-5) and 10(-13) mol/L) inhibited proximal tubular fluid absorption. Peritubular rilmenidine at 10(-12) and 10(-13) mol/L also inhibited fluid uptake, whereas rilmenidine at 10(-11) mol/L had a significant stimulatory action. In the presence of the I(2) > I(1)/alpha(2)-adrenoceptor antagonist idazoxan (10(-5) mol/L), luminal rilmenidine (10(-5) mol/L) stimulated fluid absorption. Stimulation of fluid uptake was also observed when rilmenidine (10(-5) mol/L) and the I(1) imidazoline receptor antagonist efaroxan (10(-5) mol/L) were added together in the luminal fluid. Luminal administration of the selective alpha(2)-adrenoceptor antagonist 2-methoxy-idazoxan (10(-5) mol/L) resulted in significant attenuation of the inhibitory action of luminal rilmenidine (10(-5) mol/L). This indicates that both I(1) imidazoline receptors and alpha(2)-adrenoceptors are involved in the luminal actions of rilmenidine. The effects of luminal and peritubular administration of alpha-methylnoradrenaline (an alpha(2)-adrenoceptor agonist) were compared with those of rilmenidine. Luminal alpha-methylnoradrenaline, at higher concentrations (10(-7) and 10(-5) mol/L), inhibited fluid absorption, as was seen with peritubular rilmenidine, but, in contrast with rilmenidine, no stimulatory action was observed. Peritubular alpha-methylnoradrenaline inhibited fluid uptake at higher concentrations (10(-5) and 10(-7) mol/L), whereas rilmenidine at these concentrations had no effect. The differences in the concentration-dependent responses for rilmenidine and alpha-methylnoradrenaline indicate that both imidazoline receptors and alpha(2)-adrenoceptors are involved in the actions of these compounds on proximal fluid uptake.

Androgen-induced norepinephrine release mediating guinea pig seminal vesicle smooth muscle proliferation: potential role of pre-synaptic alpha2-adrenoceptors

BACKGROUND

Recent studies from our laboratory have demonstrated that androgen-induced basal norepinephrine (NE) release is responsible for the onset of proliferation in seminal vesicle smooth muscle (SVM) cells during early puberty. With subsequent sexual maturation, SVM was irreversibly differentiated to an androgen-resistant-amitotic state in which basal NE release remained elevated and resistant to androgen withdrawal or repletion. Based on these findings, we hypothesized that this irreversible elevation of basal NE release during pubertal development is caused, at least in part, by the down-regulation of pre-synaptic NE feedback inhibition, secondary to irreversible reduction in the expression of neuronal (pre-synaptic) alpha(2)-adrenoceptors. Functional alpha(2)-adrenoceptors are selectively localized to pre-synaptic sites in SVM.

METHODS

To test this hypothesis, we employed ligand binding techniques with [(3)H]RX821002, an antagonist which labeled all alpha(2)-adrenoceptor sub-types. Initial experiments focused on analysis of competitor specificity to identify the predominant alpha(2)-adrenoceptor sub-type in SVM. Subsequently, we quantified the changes in the receptor concentration (B(max)) for [(3)H]RX821002 at the point of maximal dihydrotestosterone (DHT)-induced change in basal NE release.

RESULTS

Based on competitor specificity for [(3)H]RX821002, the alpha(2D)-adrenoceptor sub-type predominated in SVM. We treated pre-pubertal castrate animals with DHT for 7 days, which was previously demonstrated to maximally induce basal NE release. This treatment reduced the pre-synaptic alpha(2)-adrenoceptor B(max) 4-fold. In animals which had been castrated as adults, the B(max) for [(3)H]RX821002 remained irreversibly suppressed.

CONCLUSIONS

The DHT-dependent reduction in the alpha(2)-adrenoceptor concentration is consistent with the developmental pattern of increased basal NE release. These findings support the hypothesis that the down-regulation of pre-synaptic NE feedback is mechanistically involved in the irreversible elevation of basal NE release. NE mediates proliferation in SVM in early pubertal development. Thus, the androgen-dependent pubertal growth of smooth muscle cells may be indirectly controlled at the level of neurotransmission.

Pharmacological characterization of unique prazosin-binding sites in human kidney

In human kidney, we found unique prazosin-binding sites that were insensitive to phentolamine and were thus unlikely to be alpha(1)-adrenoceptors. As the binding of [(3)H]prazosin to phentolamine-insensitive sites was prevented by 100 microM guanabenz, the insensitive sites were evaluated by subtracting [(3)H]prazosin binding in the presence of 100 microM guanabenz from that in the presence of 10 microM phentolamine. [(3)H]Prazosin bound to the phentolamine-insensitive sites monophasically with a high affinity (pK(d); 9.1+/-0.08, n=8), and the B(max) value (814+/-204 fmol mg(-1) protein, n=8) was more than ten times that of the phentolamine-sensitive alpha(1)-adrenoceptor (pK(d)=9.9+/-0.13, B(max)=66+/-23 fmol mg(-1) protein, n=7). The phentolamine-insensitive sites in human kidney were highly sensitive to other quinazoline derivatives such as terazosin and doxazosin. However, other alpha(1)-adrenoceptor antagonists (tamsulosin, WB4101 and corynanthine) did not inhibit the binding at a range of concentrations that generally exhibit alpha(1)-adrenoceptor antagonism, and noradrenaline, rauwolscine and propranolol were without effect on the [(3)H]prazosin binding. On the other hand, ligands for the renal Na(+)-transporter (amiloride and triamterene) and for imidazoline recognition sites (guanabenz, guanfacine and agmatine) displaced the binding of [(3)H]prazosin to phentolamine-insensitive sites at micromolar concentrations. Photoaffinity labeling with [(125)I]iodoarylazidoprazosin showed phentolamine-insensitive labeling at around 100 kDa, a molecular size larger than that of human alpha(1a)- and alpha(1b)-adrenoceptors expressed in 293 cells (50-60 and 70-80 kDa, respectively) on electrophoresis. In contrast, there was no detectable phentolamine-insensitive binding site but were phentolamine-sensitive alpha(1)-adrenoceptors in human liver (pK(d)=10.0+/-0.06, B(max)=44+/-6 fmol mg(-1) protein, n=3). Phentolamine-insensitive prazosin binding sites were also detected in rabbit kidney (approximately 50% of specific binding sites) but were minor in rat kidney (less than 20%). In conclusion, there are unique prazosin-binding sites in human kidney, the pharmacological profiles of which were distinct from those of known adrenoceptors.

Agmatine inhibits arginine vasopressin-stimulated urea transport in the rat inner medullary collecting duct

BACKGROUND

Agmatine, a putative endogenous ligand for imidazoline receptors, induces numerous biological effects. The agonist clonidine binds to alpha-2 (alpha2) adrenoceptors and imidazoline receptors, and inhibits arginine vasopressin (AVP)-stimulated urea permeability (Pu) in the rat inner medullary collecting duct (IMCD). Dexmedetomidine, a selective alpha2 agonist, does not inhibit AVP-stimulated Pu. This study was conducted to determine if agmatine affects Pu in the rat IMCD and to investigate the possibility of an imidazoline-mediated mechanism.

METHODS

The isolated-perfused tubule technique was used to measure Pu in IMCDs from Wistar rats. AVP at 220 pmol/L or 8-chlorophenylthio cyclic adenosine monophosphate (8CPT cAMP) was used to stimulate Pu. Agmatine and other agents were added to the bath.

RESULTS

Agmatine at 1 micromol/L inhibited AVP-stimulated Pu by 50%. Agmatine-induced inhibition could not be separated completely from inhibition produced by the non-imidazoline, catecholamine epinephrine. Of three antagonists selective for alpha2 adrenoceptors (rauwolscine, yohimbine, and RX821002), only rauwolscine reversed inhibition, whereas each of the three imidazoline-selective antagonists tested (atipamezole, idazoxan, and BU239) produced a significant reversal. Agmatine did not affect basal Pu or inhibit 8CPTcAMP-stimulated Pu.

CONCLUSION

Our results indicate that agmatine inhibits AVP stimulated Pu by a cAMP-dependent mechanism. Imidazoline receptors are probably not involved. The possibility exists of an unknown agmatine-selective receptor modulating urea transport in the rat IMCD.

Imidazoline receptors in cardiovascular and metabolic diseases

The site of the hypotensive action of imidazoline compounds, such as clonidine, was first identified within the rostroventrolateral part of the brainstem. Afterwards, it was shown that imidazolines reduced blood pressure when applied in this area, whereas no catecholamine was capable of such an effect. These data led us to suggest the existence of receptors specific for imidazolines different from the alpha-adrenergic receptors. Soon after, the existence of imidazoline binding sites (IBS) was reported in the brain and in a variety of peripheral tissues including pancreatic gland and kidney. As expected, these specific binding sites do not bind the catecholamines. The IBS are classified in two groups: the I1 type, sensitive to clonidine and idazoxan; and the I2 type, sensitive to idazoxan and largely insensitive to clonidine. Imidazoline receptors were shown to be involved in several physiological regulations and pathological processes such as hypertension, diabetes mellitus and some mood disorders. Evidence for their implication in the nervous regulation of blood pressure and in the insulin secretion control will be presented. The hypotensive effects of clonidine-like drugs involve imidazoline receptors (I1Rs), while their most frequent side-effects only involve alpha2-adrenergic receptors. A new class of centrally acting antihypertensive drugs selective for I1Rs is now available. At hypotensive doses, these drugs are devoid of significant side effects. It was shown that the good acceptability of these drugs is likely due to their selectivity for I1Rs.

Synthesis and biological evaluation of pyrrolinic isosteres of rilmenidine. Discovery of cis-/trans-dicyclopropylmethyl-(4,5-dimethyl-4,5-dihydro-3H-pyrrol-2-yl)-amine (LNP 509), an I1 imidazoline receptor selective ligand with hypotensive activity

To find new compounds selective for purported I1 imidazoline receptors (I1Rs) over I2 imidazoline binding sites (I2BS) and alpha2-adrenoceptors (alpha2ARs), a series of pyrrolinic isosteres of rilmenidine has been prepared and their biological activity at I1Rs, I2BS, and alpha2ARs evaluated. This isosteric replacement provided us with compounds which still bound to I1Rs but not to I2BS nor to alpha2ARs. A limited structure-affinity relationship was generated around the heterocyclic moiety of these ligands. One compound in this series, LNP 509 (1e) [cis-/trans-dicyclopropylmethyl-(4,5-dimethyl-4,5-dihydro-3H-pyrrol-2-yl)-amine], had no detectable affinity at alpha2ARs yet was capable of lowering blood pressure after central administration. These pyrrolinic analogues constitute a new chemical class of imidazoline related compounds with high selectivity for the I1Rs. They could be used as new tools in the study of I1Rs and in the conception of new centrally acting hypotensive drugs.

Synthesis and pharmacological evaluation of imidazoline sites I1 and I2 selective ligands

Several series of 2-aryl or heterocyclic-imidazoline compounds have been prepared and evaluated in vitro as imidazoline sites (I1 and I2) and alpha-adrenergic (alpha1 and alpha2) receptor ligands. Their pKi values indicate that linkage of the imidazoline moiety at the 2-position with an aromatic substituent dramatically decreases alpha-adrenergic affinity. I1 sites are more accessible by phenyl imidazolines substituted by a methyl or a methoxy group at the ortho or meta position. Indeed, 2-(2'-methoxyphenyl)-imidazoline (17) is one of the best I1 ligands ever reported (pKi = 8.53 and I1/I2 > 3388). On the other hand, I2 selectivity increases in the presence of a methyl group in the para position. The original compound, 2-(3'-fluoro-4'-tolyl)-imidazoline (31) is a new potent ligand for the I2 sites with high selectivity (pKi = 8.53 and I2/I1 > 3388).

Analysis of the pharmacological and molecular heterogeneity of I(2)-imidazoline-binding proteins using monoamine oxidase-deficient mouse models

The I(2) subgroup of imidazoline-binding sites was identified as monoamine oxidases (MAOs), but it is unclear whether there are I(2)-binding sites located on proteins distinct from MAOs. To address this issue, we characterized I(2)-binding proteins in liver and brain of wild-type and MAO A- and MAO B-deficient mice. I(2)-binding sites were identified using [(3)H]idazoxan and the photoaffinity adduct 2-[3-azido-4-[(125)I]iodophenoxyl]methylimidazoline ([(125)I]AZIPI). [(3)H]Idazoxan labeled binding sites with ligand recognition properties typical of I(2) sites in both brain and liver of wild-type mice. High-affinity, specific [(3)H]idazoxan binding were not altered in MAO A knockout (KO) mice. In contrast, [(3)H]idazoxan binding was completely abolished in both liver and brain of MAO B KO mice. In wild-type mice, [(125)I]AZIPI photolabeled three proteins with apparent molecular masses of approximately 28 (liver), approximately 61 (brain), and approximately 55 kDa (liver and brain). The photolabeling of each protein was blocked by the imidazoline cirazoline (10 microM). Photolabeling of the approximately 61- and approximately 55-kDa proteins was not observed in MAO A and B KO mice, respectively. In contrast, photolabeling of the liver approximately 28-kDa protein was still observed in MAO-deficient mice, indicating that this protein is unrelated to MAOs. These data indicate that I(2) imidazoline-binding sites identified by [(3)H]idazoxan reside solely on MAO B. The binding sites on MAO A and the liver approximately 28-kDa protein may represent additional subtypes of the family of the imidazoline-binding sites.

Imidazoline receptors: a challenge

The hypotensive effect of imidazoline-like drugs (IMs) directly injected into the rostroventrolateral part of the brainstem (NRL/RVLM) was shown to involve non-adrenergic imidazoline specific receptors (IRs). Some IMs caused hypotension when injected there, irrespective of their affinity and selectivity for any alpha-adrenoceptor subtype. Compounds, such as LNP 509, S 23515, S 23757 or benazoline with very high selectivities for IRs over alpha 2-adrenoceptors (A2Rs), became available recently. Some of these compounds (LNP 509, S 23515) caused hypotension when injected alone into the NRL/RVLM region. Nevertheless, high selectivity for IRs will not predict by its own the capability of IMs to elicit hypotension as some of these substances behaved as antagonists towards the hypotensive effects of the latter. As far as hybrid drugs, i.e., with mixed binding profiles (I1/alpha 2), were concerned, a significant correlation has been reported between their central hypotensive effect and their affinity for IRs. Imidazoline antagonists, such as idazoxan, were repeatedly shown to competitively prevent and reverse the centrally induced hypotensive effect of IMs. The sole stimulation of A2Rs within the NRL/RVLM region was not sufficient to decrease blood pressure as much as IMs did, as shown by the lack of significant blood pressure lowering effect of alpha-methylnoradrenaline (alpha-MNA). No correlation was observed between affinity of IMs for A2Rs and their central hypotensive effects. It is also noticeable that yohimbine, an A2Rs antagonist, was repeatedly shown to abolish the hypotensive effect of hybrids but usually in a non-competitive manner. Mutation of A2Rs was shown to prevent the hypotensive effects of centrally acting drugs. It is concluded that (i) drugs highly selective for I1Rs over A2Rs can reduce blood pressure by their own; (ii) the central hypotensive effect of IMs needs implication of IRs and appears to be facilitated by additional activation of A2Rs; and (iii) this effect requires intact A2Rs along the sympathetic pathways.

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.

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.

Design and synthesis of imidazoline derivatives active on glucose homeostasis in a rat model of type II diabetes. 2. Syntheses and biological activities of 1,4-dialkyl-, 1,4-dibenzyl, and 1-benzyl-4-alkyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)piperazines and isosteric analogues of imidazoline

Piperazine derivatives have been identified as new antidiabetic compounds. Structure-activity relationship studies in a series of 1-benzyl-4-alkyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)piperazines resulted in the identification of 1-methyl-4-(2', 4'-dichlorobenzyl)-2-(4',5'-dihydro-1'H-imidazol-2'-yl)piperazine, PMS 812 (S-21663), as a highly potent antidiabetic agent on a rat model of diabetes, mediated by an important increase of insulin secretion independently of alpha2 adrenoceptor blockage. These studies were extended to find additional compounds in these series with improved properties. In such a way, substitution of both piperazine N atoms was first optimized by using various alkyl, branched or not, and benzyl groups. Second, some modifications of the imidazoline ring and its replacement by isosteric heterocycles were carried out, proceeding from PMS 812, to evaluate their influence on the antidiabetic activity. The importance of the distance between the imidazoline ring and the piperazine skeleton was studied third. Finally, the influence of the N-benzyl moiety was also analyzed compared to a direct N-phenyl substitution. The pharmacological evaluation was performed in vivo using glucose tolerance tests on a rat model of type II diabetes. The most active compounds were 1,4-diisopropyl-2-(4', 5'-dihydro-1'H-imidazol-2'-yl)piperazine (41a), PMS 847 (S-22068), and 1,4-diisobutyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)piperazine (41b), PMS 889 (S-22575), which strongly improved glucose tolerance without any side event or hypoglycemic effect. More particularly, PMS 847 proved to be as potent after po (100 micromol/kg) as after ip administration and appears as a good candidate for clinical investigations.

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

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

Characterization of [3H]idazoxan binding proteins in solubilized membranes from rabbit and human liver

Several studies have shown that I2 imidazoline binding sites are localized on monoamine oxidases. Recent results obtained after solubilization of rat brain membranes and analysis by size-exclusion chromatography suggested the existence of additional I2 imidazoline binding sites located on proteins distinct from monoamine oxidases. In order to define whether such binding sites are expressed in human and rabbit liver, we solubilized I2 imidazoline binding sites and monoamine oxidases and compared their elution profile by size-exclusion chromatography. I2 binding sites were labeled using [3H]idazoxan. Monoamine oxidases were identified by the measure of [14C]tyramine oxidation and Western blot analysis using an anti-MAO-A/MAO-B polyclonal antiserum. After solubilization of rabbit or human liver using 1% digitonin, 90% of [3H]idazoxan binding eluted in a major peak corresponding to a Mr of approximately 175000 Da. A minor peak, (Mr approximately equal to 100000 Da) representing 10% of the recovered [3H]idazoxan binding, was also observed. [14C]tyramine oxidation as well as immunoreactive bands corresponding to MAOs were exclusively detected in fractions containing [3H]idazoxan binding. These results show that solubilized I2 imidazoline binding sites distinct from monoamine oxidases are not detectable in rabbit and human liver.

Does a second generation of centrally acting antihypertensive drugs really exist?

The site of the hypotensive action of imidazoline compounds, such as clonidine, was first identified within the rostroventrolateral part of the brainstem: the nucleus reticularis lateralis. After that, it was shown that imidazolines and related substances reduced blood pressure when applied in this area whereas catecholamines were not capable of producing such an effect. These data led us to suggest the existence of receptors specific for imidazoline-like compounds different from the alpha2-adrenoceptors. Soon after, the existence of imidazoline binding sites was reported in the brain and in a variety of peripheral tissues including the human kidney. As expected, these specific binding sites do not bind the catecholamines. The imidazoline binding sites are already subclassified in two groups: the I1-subtype sensitive to clonidine and idazoxan, and the I2-subtype, sensitive to idazoxan and nearly insensitive to clonidine. Functional studies confirmed that the hypotensive effects of clonidine-like drugs involved imidazoline receptors while their most frequent side effects only involved alpha2-adrenoceptors. However, recent functional evidence suggests that a cross talk between imidazoline receptors and alpha2-adrenoceptors is necessary to trigger a hypotensive effect within the ventral brainstem. Rilmenidine and Moxonidine are the leader compounds of a new class of antihypertensive drugs selective for imidazoline receptors. At hypotensive doses, these drugs are devoid of significant sedative effect. Rilmenidine evoked hypotension when injected within the nucleus reticularis lateralis region; it competed for [3H]-clonidine bound to specific imidazoline binding sites in human medullary membrane preparations but proved more selective for cerebral imidazoline receptors than clonidine. It is suggested that this selectivity might explain the low incidence of their side effects. Additional potentially beneficial actions on cardiac arrhythmias or congestive heart failure enlarge the therapeutic interest of imidazoline-related drugs. Recent binding and functional data throw a new light on the optimal pharmacological profile of this second generation of centrally acting antihypertensive drugs.

1,5-Bis(4-amidinophenoxy)pentane (pentamidine) is a potent inhibitor of [3H]idazoxan binding to imidazoline I2 binding sites

The aromatic diamidine 1,5-bis(4-amidinophenoxy)pentane (pentamidine) is used for treatment and prophylaxis of Pneumocystis carinii pneumonia in patients with Acquired Immune Deficiency Syndrome. Clinical use of pentamidine has been restricted by significant toxicity, that includes hypotension, and hypoglycemia. Although clinical toxicity is well described, the mechanisms are still poorly understood. Competitive binding analyses using [3H]idazoxan as the radioligand, and cirazoline to define non-specific binding, demonstrate that pentamidine binds to an imidazoline I2 binding site on rat liver membranes with a Ki of 1.4+/-0.22 nM. The Ki indicates that pentamidine inhibits radioligand binding at imidazoline I2 sites with an affinity approximating the most potent known ligands and may be related to pentamidine toxicity. Moreover, pentamidine analogs inhibit radioligand binding with a range of affinities that vary according to their structure. Two candidate drugs, Compounds 5 and 6, are more active than pentamidine in the corticosteroid-suppressed rat model of P. carinii pneumonia, yet have different affinities for the imidazoline I2 site (Ki 5 = 50.1+/-1.06 nM and Ki 6 = approximately 3500 nM). Affinity for this site does not correlate with antimicrobial activity (r = 0.60; p = 0.09) or the calculated log of the octanol:water partition coefficient (ClogP) (r = -0.38; p = 0.22).

Design and synthesis of imidazoline derivatives active on glucose homeostasis in a rat model of type II diabetes. 1. Synthesis and biological activities of N-benzyl-N'-(arylalkyl)-2-(4',5'-dihydro-1'H-imidazol-2'-yl)piperazines

The physiopathology of non-insulin-dependent diabetes mellitus is associated with a dysfunction in the regulation of insulin secretion. The alpha 2-adrenoceptors have been reported to be involved in this alteration, although alpha 2-antagonists containing an imidazoline ring may stimulate insulin secretion independently of alpha 2-adrenoceptor blockage. Recently, a new "imidazoline-binding site" involved in the control of K(+)-ATP channels in the B cell has been proposed. In the course of searching for new antidiabetic agents, 1-alkyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)-4-benzylpiperazines, 1-benzyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)-4-alkylpiperazines, and 1-benzyl-2-(4',5'-dihydro-1'H-imidazol-2'-yl)-4-benzylpiperazines have been designed and evaluated as potential adrenoceptor antagonists. Pharmacological evaluation was performed in vivo using glucose tolerance tests performed on a rat model of type II diabetes obtained by injection of a low dose (35 mg/kg) of streptozotocin (STZ). For some compounds, binding experiments were performed on alpha 2 adrenoceptors and I1 and I2 imidazoline-binding sites. The biological and physicochemical data have been combined with molecular modeling studies to establish structure-activity relationships. The most active compound was 1-(2',4'-dichlorobenzyl)-2-(4',5'-dihydro-1'H-imidazol-2'-yl)- 4-methylpiperazine (7f); intraperitoneal administration (100 mumol/kg) of 7f strongly improved glucose tolerance in STZ diabetic rats. This effect seemed at least partly mediated by a significant increase of insulin secretion. Other compounds of the same family (7b, 16f, 23b) have also shown potent activity. We found no correlation between in vivo antihyperglycemic properties and in vitro affinities for alpha 2-adrenoceptors or I1, and I2 binding sites. These compounds can be considered as antihyperglycemic agents potentially useful for treatment of type II diabetes and are currently under complementary investigation.

Binding of new cirazoline derivative to imidazoline receptors from human brain

Imidazoline compounds are known to interact with alpha 2-adrenoceptors as well as with specific non-adrenergic binding sites. Such binding sites are present in the brain and in peripheral tissues. Hypotensive effects of imidazolines were shown to be related to specific interaction with imidazoline binding sites within the brainstem. Heterogeneity of these sites based on differences in selectivities was reported. In order to facilitate the characterization of human brain imidazoline receptors, we synthetized new ligands by substitutions on the cirazoline phenyl ring. Affinities of these cirazoline derivatives were determined in two imidazoline binding site models, namely the human brain and the rabbit kidney. Interaction of these compounds with imidazoline binding sites from the human brain appeared more sensitive to structural variations of the imidazoline than those with rabbit kidney sites. Moreover, no correlation was found between affinities for imidazoline binding sites and those for alpha 2-adrenoceptors of the rat brain. Arylazide derivative of 2-(5-amino-2-methyl-phenoxymethyl)-imidazoline exhibited a higher affinity for human brain imidazoline binding sites than for human brain alpha 2-adrenoceptors. Photoincorporation of this azido-compound in human brain imidazoline binding sites was achieved and blockade of [3H]idazoxan imidazoline specific binding observed. These new tools may allow fine characterization of the different subtypes of imidazoline binding proteins.

Relationship between imidazoline/guanidinium receptive sites and monoamine oxidase A and B

Imidazoline binding sites or imidazoline/guanidinium receptive sites (IGRS) recognize bioactive endogenous substances and a variety of pharmacologically active compounds containing imidazoline or guanidinium moieties. The family of imidazoline binding proteins consists of multiple membrane-associated proteins that differ in their tissue/subcellular localization, M(r) and ligand recognition properties. Two of the imidazoline binding proteins are identical to the mitochondrial enzyme monoamine oxidase (MAO) A and B isoforms, which contain imidazoline binding domains distinct from the enzyme active site. The relationship between the imidazoline binding proteins and monoamine oxidases was further characterized in the present report using a covalent probe (2-[3-azido-4[125I]iodophenoxy] methyl imidazoline, [125I]-AZIPI) to label the imidazoline binding proteins in different species and following transient expression of MAO- A and -B in COS 7 cells. Species homologues of MAO-A and -B in rat and human differ in their apparent molecular weight by approximately 2000 Da. In rat and human liver [125I]-AZIPI identified peptides with apparent molecular weights similar to those of the species homologues of MAO. Peptides of M(r) approximately 63,000 (MAO-A) and approximately 59,000 (MAO-B) were also photolabeled in membranes prepared from COS-7 cells transfected with human cDNA clones encoding MAO-A or -B. Additional experiments indicate that the imidazoline binding domains on MAO-A and -B exhibit different ligand recognition properties. The covalent labeling of human liver MAO-B was more sensitive than that of placenta MAO-A to inhibition by the imidazoline 2-(4,5-dihydroimidaz-2-yl)-quinoline (BU224). These data indicate that the A and B isoforms of MAO possess imidazoline binding domains that differ in their ligand recognition properties. Allosteric regulation of the activity of MAO via the imidazoline binding domains may be of significance in various disease states associated with elevated enzyme expression or in which the enzyme is a therapeutic target.

Pharmacological characterization of I1 and I2 imidazoline receptors in human striatum

[3H]RX821002, [3H]clonidine and [3H]idazoxan have previously been shown to selectively label alpha 2-adrenergic receptors, I1 and I2 imidazoline receptors in the human central nervous system, respectively. Idazoxan shows relatively high affinity for all three receptors. We investigated the possible selectivity of several compounds towards one of those receptors in human striatum. Addition of an alkoxy group at the 2-position of the benzodioxan moiety of idazoxan (ethoxy-idazoxan, methoxy-idazoxan) increases the alpha 2-selectivity in human brain. Efaroxan is also alpha 2-selective. On the contrary, BU224, BU239, cirazoline and RX801077 display imidazoline receptor selectivity. Our results indicate that for all molecules tested, idazoxan and 'flat' analogs possess I1/I2 receptor selectivity. A 'bulky' substituent at the 2-position of the benzodioxan ring gives rise to alpha 2-adrenergic receptor selectivity. Until now, we found no more than 3-fold difference in IC50 between both imidazoline receptors. Both receptors also display similar stereoselectivity, suggesting that they might be 'interconnected' in the human striatum.

[3H]dexmedetomidine, an alpha 2-adrenoceptor agonist, detects a novel imidazole binding site in adult rat spinal cord

Binding properties of [3H]dexmedetomidine [(+)-(S)-4-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole] as an agonist-type radioligand for alpha 2-adrenoceptors were characterised for the first time in tissues relevant to its analgesic (spinal cord from neonatal or adult rats) and behavioural (rat cerebral cortex) actions. In membranes of rat cerebral cortex (KdHigh 0.2 +/- 0.03 nM, KdLow 8.8 +/- 1.4 nM with Bmax High 130 +/- 11 fmol/mg protein, RHigh 16%) and neonatal spinal cord (KdHigh 0.3 +/- 0.04 nM, KdLow 14 +/- 3.7 nM with Bmax High 290 +/- 40 fmol/mg protein, RHigh 25%) Gpp(NH)p modifies the biphasic binding to monophasic and binding is competed with specifically by alpha 2-adrenoceptor compounds. Binding to rat cerebral cortex is not modified by pretreatment with the noradrenergic neurotoxin, DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine). In contrast, [3H]dexmedetomidine binding to adult rat spinal cord membranes is more complex and both saturation analysis and competition experiments indicate the presence of a non-adrenergic component of binding (about 40% of total binding) which is sensitive to imidazole-type compounds. This non-adrenergic component of [3H]dexmedetomidine binding can be defined as a novel type of imidazole binding site such that, of the imidazoline I1 or I2 receptor ligands, only cimetidine has relatively high affinity. In conclusion, [3H]dexmedetomidine shows very complex binding characteristics that limit its use as an agonist-type radioligand for alpha 2-adrenoceptors but it may be a useful tool for imidazoline receptor characterisation.

Modulation of opioid analgesia by agmatine

Administered alone, agmatine at doses of 0.1 or 10 mg/kg is without effect in the mouse tailflick assay. However, agmatine enhances morphine analgesia in a dose-dependent manner, shifting morphine's ED50 over 5-fold. A far greater effect is observed when morphine is given intrathecally (9-fold shift) than after intracerebroventricular administration (2-fold). In contrast to the potentiation of morphine analgesia, agmatine (10 mg/kg) has no effect on morphine's inhibition of gastrointestinal transit. delta-Opioid receptor-mediated analgesia also is potentiated by agmatine, but kappa1-receptor-mediated (U50,488H; trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] benzeneacetemide) and kappa3-opioid receptor-mediated (naloxone benzoylhydrazone) analgesia is not significantly enhanced by any dose of agmatine tested in this acute model. In chronic studies, agmatine at a low dose (0.1 mg/kg) which does not affect morphine analgesia acutely prevents tolerance following chronic morphine dosing for 10 days. A higher agmatine dose (10 mg/kg) has a similar effect. Agmatine also blocks tolerance to the delta-opioid receptor ligand [D-Pen2,D-Pen5]enkephalin given intrathecally, but not to the kappa3-opioid receptor agonist naloxone benzoylhydrazone. Despite its inactivity on kappa1-opioid analgesia in the acute model, agmatine prevents kappa1-opioid receptor-mediated tolerance. These studies demonstrate the dramatic interactions between agmatine and opioid analgesia and tolerance.

Antagonism by idazoxan at low dose but not high dose, of the natriuretic action of moxonidine

1. Recent studies concerning the imidazoline receptor have utilized idazoxan as a specific imidazoline receptor antagonist. The aim of the present study was to describe the in vivo effects of various doses of idazoxan on renal function, in the presence and absence of moxonidine, an I1 imidazoline receptor agonist. 2. In anaesthetized, unilaterally nephrectomized (7 to 10 days) Sprague Dawley rats, an intrarenal infusion of moxonidine (3 nmol kg-1 min-1) increased urine flow rate, sodium excretion and osmolar clearance without altering free water clearance. Pretreatment with intravenous idazoxan at 0.1 and 0.3 mg kg-1 produced a dose-related decrease in the renal actions of moxonidine. However, a higher dose of idazoxan (1 mg kg-1) was not as effective as the 0.3 mg kg-1 dose in blocking the effects of moxonidine. 3. In a separate series of experiments, the direct renal actions of idazoxan alone were investigated. Idazoxan at 0.3 mg kg-1 failed to alter urine flow rate and sodium excretion. However, idazoxan at 1 mg kg-1 produced a significant increase in urine flow rate and sodium excretion in association with an increase in osmolar clearance. 4. These results do not prove but are consistent with low doses of idazoxan antagonizing the sites stimulated by moxonidine (renal imidazoline receptors). However, at higher doses, idazoxan may function as a partial agonist and/or interact with other receptors to increase urine flow rate, independent of imidazoline receptor blockade. These studies underscore the importance of the dose of idazoxan administered when this antagonist is used as a tool to investigate imidazoline receptors.

[3H]cirazoline as a tool for the characterization of imidazoline sites

Recent studies have shown that cirazoline, an alpha 1-adrenoceptor agonist, has greater affinity than do other imidazoline or guanidinium compounds at imidazoline recognition sites. In this report we used [3H]cirazoline as a probe to characterize imidazoline recognition sites present in membrane homogenates of rat brain and kidney as well as pancreatic beta HIT T15 cells. Specific binding of [3H]cirazoline to these various homogenates was saturable and reversible and was resolved into two classes of high affinity binding sites. Competition inhibition studies of [3H]cirazoline binding to these different membrane preparations were performed with alkaloid, phenylethylamine, imidazoline, and guanidinium compounds. Catecholamines and non-imidazoline adrenoceptor ligands such as epinephrine, benextramine, prazosin, propranolol, rauwolscine, or adrenoceptor ligands such as epinephrine, benextramine, prazosin, propranolol, rauwolscine, or yohimbine did not compete with [3H]cirazoline (Ki > 10 microM). Under our experimental conditions, only guanidinium and imidazoline derivatives had high affinities for [3H]cirazoline binding sites. Unlabeled cirazoline, clonidine, bromoxidine, idazoxan, and amiloride had the highest affinities with this respective rank order. These results suggest that [3H]cirazoline is a novel high affinity radioligand that specifically labels nonadrenergic imidazoline-guanidinium sites in the brain, kidney, and beta cells. Furthermore, the obtained rank order of inhibition suggests that [3H]cirazoline binding does not distinguish between I1 and I2 sites. In addition, we compared the specific binding of [3H]cirazoline with that of the alpha 2-adrenoceptor antagonist [3H]rauwolscine in chinese hamster ovary (CHO) cell lines stably expressing human alpha 2C2-, alpha 2C4-, and alpha 2C10-adrenoceptor subtypes. Using [3H]rauwolscine as a probe, each of these transfected cell lines expressed high levels for the three different alpha 2-adrenoceptor subtypes (Bmax values were between 2 and 7 pmol.mg-1 protein). In contrast, none of these cell lines displayed measurable imidazoline recognition sites. In summary, [3H]cirazoline is a novel high affinity radioligand that specifically labels imidazoline recognition sites without significant alpha- or beta-adrenoceptor binding. Furthermore, our results using alpha 2-adrenoceptor transfected cells confirm that the imidazoline recognition sites and each of the cloned alpha 2-adrenoceptor subtypes represent distinct macromolecular entities.

Binding of [3H]cirazoline to an imidazoline site in rat brain and kidney membranes

Two classes of high-affinity sites for [3H]cirazoline were characterized in rat brain and kidney membranes. In both tissues, the binding parameters for the high- and low-affinity sites are similar with Bmax values of approximately 50 fmol/mg protein, Kd approximately 0.6 nM and Bmax approximately 470 fmol/mg protein, Kd approximately 11 nM respectively. Inhibition studies of [3H]cirazoline binding to the lower affinity site revealed that only guanidinium or imidazoline derivatives compete with the specific binding of this radioligand. Our results suggest that [3H]cirazoline could be used as a novel ligand to label the non-adrenergic imidazoline-preferring sites.

Localization of I2-imidazoline binding sites on monoamine oxidases

Imidazoline binding sites (IBS) were proposed to be responsible for some of the pharmacological and therapeutic activities of imidazoline and related compounds and have been classified into two subtypes, I1BS and I2BS. Convergent studies attribute a role in central blood pressure regulation to the I1BS. In contrast, the function of I2BS remains unknown. In the present study, by combining biochemical and molecular biology approaches, we show that 1) microsequencing of I2BS purified from rabbit kidney mitochondria allowed the recovery of four peptide sequence stretches displaying up to 85.7% similarity with human, rat, and bovine monoamine oxidases (MAO)-A and -B; 2) I2BS and MAO displayed identical biophysical characteristics as their activities, measured by [3H]idazoxan binding and [14C]tyramine oxidation, respectively, could not be separated using various chromatographic procedures; and 3) heterologous expression of human placenta MAO-A and human liver MAO-B in yeast, inherently devoid of I2BS and MAO activities, led to the coexpression of [3H]idazoxan binding sites displaying ligand-recognition properties typical of I2BS. These results show definitely that I2BS is located on both MAO-A and -B. The fact that I2BS ligands inhibited MAO activity independently of the interaction with the catalytic region suggests that I2BS might be a previously unknown MAO regulatory site.

Role of imidazoline receptors in cardiovascular regulation

The involvement of nonadrenergic imidazoline specific receptors in the central control of the vasomotor tone and in the mechanism of action of drugs bearing an imidazoline structure, or analogs, is now well documented. Imidazoline-specific binding sites were found in many tissues and species. Moreover, until now, it is only in the brainstem that such binding sites are associated with a function: the hypotensive effect of imidazoline-like drugs. Rilmenidine, which is an oxazoline structurally related to the reference imidazolines, exerts a central hypotensive effect of central origin involving imidazoline receptors. The selectivity of rilmenidine for the imidazoline receptors compared to alpha 2-adrenergic receptors could explain the low incidence of sedative side effects observed with this antihypertensive drug. A specific anti-imidazoline radioimmunoassay allowed us to detect the presence of an immunoreactive imidazoline-like substance in human sera. High levels of this immunoreactive substance are associated with high blood pressure in 20-30% of the hypertensive patients. This observation indicates that high levels of this immunoreactive substance in the serum can be associated with some kinds of primary hypertension. The cause-and-effect relation between these 2 phenomena has not yet been determined. This substance is in process of purification; it could be a candidate to be an endogenous ligand of the imidazoline receptors.

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.

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.

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

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

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.

Ocular alpha 2-receptor subclasses and antiglaucoma efficacy

An overview of the ocular hypotensive actions of some alpha 2-agonists with imidazoline structures is presented. These agents inhibit isoproterenol-stimulated adenylate cyclase (AC) activity in ciliary process membrane through a Na+ and GTP-dependent mechanism. Receptor binding studies with the alpha 2-agonist radioligand [125I] p-iodoclonidine ([125I]PIC) in rabbit ciliary body membranes indicate that the alpha 2-receptor subtype is alpha 2A. Gpp(NH)p and NaCl dose-dependently decreased the number of [125I]PIC binding sites by shifting the receptor-G protein complexes from the high affinity state to the low affinity state for agonist binding. This is consistent with the observations that inhibition of AC was Na+ and GTP dependent. The NaCl and Gpp(NH)p effects on binding appeared to be through different mechanisms. The alpha 2-receptor in ciliary process thus appears to be an alpha 2A-receptor that is negatively coupled to the AC-cAMP generating system.

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.

Relevance of the use of [3H]-clonidine to identify imidazoline receptors in the rabbit brainstem

1. [3H]-clonidine binding was investigated in membranes isolated from the ventral medulla oblongata of the rabbit, where clonidine produced a hypotensive effect which was not mediated by adrenoceptors. [3H]-clonidine specific binding, as defined by the difference between the binding of [3H]-clonidine in the presence and in the absence of 10 microM cirazoline, occurred at two sites: a high affinity site with a KD = 2.9 +/- 0.7 nM and a Bmax of 40 +/- 8 fmol mg-1 protein and a low affinity site with a KD = 18.2 +/- 0.4 nM and a Bmax of 66 +/- 14 fmol mg-1 protein. 2. The high affinity sites being catecholamine-sensitive were identified as alpha 2-adrenoceptors. The low affinity binding of [3H]-clonidine was insensitive to catecholamines, as well as to other alpha 2-adrenoceptor specific probes, and could be inhibited with high affinity only by compounds which lowered blood pressure when directly injected in the nucleus reticularis lateralis of the ventral brainstem, or by antagonists. 3. It was concluded that in the ventral medulla of the rabbit, [3H]-clonidine labelled alpha 2-adrenoceptors and imidazoline receptors (IRs). Only the latter were related to the hypotensive effects of clonidine and rilmenidine directly injected into the rostroventrolateral medulla oblongata (RVLM) of the rabbit. The methodological problems regarding the study of IRs with [3H]-clonidine are discussed.

Down-regulation of imidazoline sites in rabbit kidney

The effects of 6 days treatment with guanabenz, clonidine, rilmenidine and idazoxan on [3H]idazoxan and [3H]clonidine binding to imidazoline sites on rabbit kidney membranes were compared. Guanabenz and rilmenidine treatment resulted in a decrease in Bmax for [3H]clonidine binding and guanabenz treatment for [3H]idazoxan binding consistent with the compounds in vitro affinities for the imidazoline binding sites. [3H]Idazoxan binding was also decreased by idazoxan treatment suggesting that idazoxan may act as an agonist at this site. These results show that imidazoline sites in kidney may be modified during chronic drug treatment and provide further evidence for differences between the sites labelled by the two ligands.

Identification of alpha 2-adrenoceptors and of non-adrenergic idazoxan binding sites in pancreatic islets from young and adult hamsters

1. The current study was undertaken to investigate the characteristics of alpha 2-adrenoceptors and to search for the presence of NAIBS in hamster pancreatic islets. 2. Pancreatic islets were isolated from young (6-7 weeks) and adult (14-15 weeks) animals. 3. The identification of alpha 2-adrenoceptors using [3H]RX821002 indicated that adults exhibited higher number of alpha 2-adrenoceptors than the young animals (194 +/- 20 vs 105 +/- 16 fmol/mg protein) while the Kd value was unchanged. 4. Glucose-evoked insulin release was completely inhibited by the alpha 2-agonist clonidine (0.1 microM) whatever the age of the animals. Agonist inhibition curves showed the following rank order of potency: clonidine > UK14304 > adrenaline. 5. Blockade of UK14304-elicited inhibition by various antagonists indicated that yohimbine has a low affinity for the receptor supporting the conclusion that the receptor is of the alpha 2-D subtype. 6. Binding experiments with [3H]idazoxan under conditions allowing to discriminate between alpha 2-adrenoceptors and NAIBS showed that hamster pancreatic islets express a high number of NAIBS. The density of NAIBS was similar in young and adult hamsters (1550 +/- 245 and 1342 +/- 332 fmol/mg protein).

[3H]-idazoxan binding to rabbit cerebral cortex recognises multiple imidazoline I2-type receptors: pharmacological characterization and relationship to monoamine oxidase

1. In rabbit cerebral cortical homogenates, saturation analysis of [3H]-idazoxan, an alpha 2-adrenoceptor antagonist, revealed high affinity binding to a single site with high density. Competition experiments demonstrated that the [3H]-idazoxan recognition site was insensitive to the catecholamines, adrenaline and noradrenaline and possessed a low affinity for the alpha 2- and alpha 1-adrenoceptor antagonists, rauwolscine, yohimbine and prazosin, suggesting that the site was not an adrenoceptor. Mapping [3H]-idazoxan binding sites in the forebrain of rabbits by autoradiography, showed high densities of I2 sites in the medial preoptic area and in the stria terminalis. Moderate binding was found in caudate nucleus, putamen, cerebral cortex and hippocampus. 2. The imidazolines cirazoline, naphazoline, guanabenz and BRL44408 along with amiloride, which is structurally related to the imidazolines, all had high affinity for the [3H]-idazoxan site, suggesting that the site was related to the I2 imidazoline-recognition site described by other groups. However, the imidazolines, clonidine and UK-14,304 and the structurally related rilmenidine all had a low affinity for the binding site, showing that [3H]-idazoxan was not binding to the I1 imidazoline-recognition site found in rat, bovine and human medulla oblongata. 3. Naphazoline, guanabenz, clonidine and amiloride competition studies had Hill slopes which were significantly different from unity (P < 0.01) and computer analysis showed that the [3H]-idazoxan binding data could be best fitted to a model which considers binding to two sites (P < 0.01). One site has a high affinity for idazoxan, cirazoline, naphazoline, guanabenz and amiloride and a moderate affinity for BRL44408 and clonidine (70% of binding) and the second site (30% of binding) has a high affinity for idazoxan and cirazoline, but a lower affinity for naphazoline, guanabenz, amiloride,BRL44408 and clonidine.4. Experiments using [3H]-RX821002, in contrast to [3H]-idazoxan, clearly demonstrated the presence ofa single type of alpha2-adrenoceptor in rabbit cortex with a pharmacological profile which is similar to the alpha2A-adrenoceptor possessing a high affinity for yohimbine, rauwolscine, BRL44408 and oxymetazoline,but a lower affinity for prazosin.5. The monoamine oxidase inhibitors, clorgyline, pargyline and deprenyl had at least a ten fold lower affinity at the rabbirt cortex I2 site as compared to their known affinity at monoamine oxidase suggesting that the I2 site is not related to the active site of the enzyme, monoamine oxidase. In addition, the peripheral benzodiazepine ligands, PK-11195 or Ro 5-4864 both had very low affinities at the I2 site in rabbit cortex suggesting that the [3H]-idazoxan binding was not to the peripheral benzodiazepine binding site.

Polyclonal anti-idazoxan antibodies: characterization and purification

Amino-idazoxan coupled to hemocyanine was used to raise anti-idazoxan antibodies in the rabbit. The antibodies were affinity purified with an amino-idazoxan affinity column. Binding studies with [3H]idazoxan showed a dissociation constant of 2.2 +/- 1.4 nM. The specificity spectrum of these antibodies indicates that the imidazoline part of idazoxan is more important for recognition than the benzodioxan ring as imidazoline substances (clonidine, cirazoline) are powerful competitors of [3H]idazoxan binding on the antibodies. Catecholamines or imidazoles were unable to displace [3H]idazoxan from the antibodies. These anti-idazoxan antibodies present specificity similarities with the imidazoline receptor as did our previously obtained anti-clonidine antibodies. Affinity-purified antibodies represent useful tools for studying the imidazoline receptors particularly with an anti-idiotypic approach.