Heterogeneity of imidazoline binding sites revealed by a cirazoline derivative

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.

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.

Further biochemical characterization of imidazoline binding sites from the human brainstem

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

Identification and characterization of imidazoline-binding sites from calf striatum

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

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

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