Imidazoline Binding Sites Research Articles

Inhibition of bovine plasma semicarbazide‐sensitive amine oxidase by caffeine

Semicarbazide-sensitive amine oxidase (SSAO) is a copper-containing enzyme that catalyzes the oxidative deamination of endogenous and exogenous primary amines. SSAO exists in mammals both as a plasma-soluble and as a membrane-bound form, and its active site is able to come into contact with numerous xenobiotic, amine-containing compounds. The kinetic studies performed in this work showed that caffeine inhibition of bovine serum amine oxidase was noncompetitive when benzylamine was used as substrate and mixed when the substrate used was methylamine. Since caffeine contains an imidazole ring, it cannot be excluded that it might bind to an inhibitory imidazoline-binding site on SSAO.

Effects of imidazoline compounds on cytoplasmic Ca2+ concentration and ATP-sensitive K+ channels in pancreatic B-cells.

Phentolamine, an alpha-adrenoceptor-blocking agent with an imidazoline structure, induces an increase in the cytoplasmic Ca2+ concentration of pancreatic B-cells. This effect occurs at a concentration (32 microM) at which phentolamine is able to enhance glucose-induced insulin secretion. The increase in cytoplasmic Ca2+ concentration caused by phentolamine is additive to the one elicited by a maximally effective concentration of tolbutamide (100 microM). Imidazoline-binding sites in insulin-secreting HIT cells can also be occupied by the guanidinium compound guanabenz, which was found to be a potent and reversible blocker of ATP-dependent K(+)-channels in B-cells. In contrast to phentolamine, guanabenz blocks the ATP-dependent K(+)-channels only in the inside-out mode, but not in the cell-attached mode of the patch-clamp technique. In conclusion, imidazolines and structurally related compounds block ATP-dependent K(+)-channels by binding to the cytoplasmic face of the plasma membrane, and may have effects on other ion channels which contribute to the elevation of cytoplasmic Ca2+ concentration and, consequently, to insulin release.

Central and peripheral mechanisms involved in the agmatine-induced gastroprotection in the rat

Introduction: In 1994 the biosynthesis of agmatine in mammalian brain has been reported. Agmatine was described as an endogenous ligand of the imidazoline binding sites, however, several data indicates that it can also interact with numerous other receptors, like α2-adrenoceptors, NMDA- or serotonin 5-HT3 receptors. It is widely distributed in the CNS and exerts several central (e.g. antidepressant or anxiolytic) effects. Moreover, agmatine and imidazoline receptors have also been identified in the gastrointestinal tract. Aims: We analysed the effect of centrally injected agmatine on gastric mucosal injury induced by a necrotizing agent. Methods: The ethanol-ulcer model was used. After 24h starvation, male Wistar rats were given 0.5ml acidified ethanol orally. The mucosal lesions were evaluated 1h later. Agmatine was given intracerebroventricularly (i.c.v.) 10min before the ethanol challenge. Antagonists were injected peripherally (s.c., i.v. or per os) or centrally (i.c.v.) before the administration of agmatine. Results: 1. Agmatine induced dose-dependent gastroprotective effect after central (0.044–1.76 nmol/rat i.c.v.) administration. 2. The imidazoline I1 receptor antagonist efaroxan (4 nmol/rat i.c.v.) as well as the non-selective α2-adrenoceptor antagonist yohimbine (5µmol/kg s.c.) antagonized the agmatine-induced mucosal protection. 3. Similarly, the effect of agmatine was abolished after bilateral cervical vagotomy. 4. The nitric oxide synthase inhibitor N(G)-nitro-L-arginine (L-NNA, 13.7µmol/kg i.v.) and the cyclooxigenase inhibitor indomethacin (56 mmol/kg p.os) also reduced the protective effect of agmatine. Conclusions: Agmatine via activation of central imidazoline I1 receptors and α2-adrenoceptors may initiate a chain of events which result in the gastric mucosal protection. The effect is conveyed by the vagal nerve to the periphery, where nitric oxide and prostaglandins may mediate it.

The effects of synthetic and endogenous imidazoline binding site ligands on neuronal activity in discrete brain regions of naive and restraint-stressed rats

We have mapped Fos expression to investigate brain regions activated by synthetic selective I 2 (BU224) and endogenous (harmane) imidazoline binding site ligands in naive and restraint-stressed rats. Systemic administration of BU224 or harmane to naive rats increased Fos-like immunoreactivity (FLI) in the hypothalamic paraventricular nucleus (PVN), hippocampal dentate gyrus (DG), central and medial nuclei of the amygdala (CeA, MeA) and the locus coeruleus (LC). FLI in restraint-stressed rats was increased in all 5 regions by harmane, and in the CeA, MeA and LC by BU224. Dual-labelling of FLI cells in the PVN of naive rats showed an increase in the number of corticotrophin-releasing-factor-containing cells (CRF) activated by BU224 and harmane. Several CRF-containing neurons in the PVN expressed α 1-adrenoceptors and were densely surrounded by catecholaminergic axons and terminals. Our results provide a functional neuroanatomical framework which may explain the stimulatory effects of imidazoline ligands on basal and stress-induced neuronal and neuroendocrine activity.

Modulation of stress by imidazoline binding sites: Implications for psychiatric disorders

In this review, we present evidence for the involvement of imidazoline binding sites (IBS) in modulating responses to stress, through central control of monoaminergic and hypothalamo–pituitary–adrenal (HPA) axis activity. Pharmacological and physiological evidence is presented for differential effects of different IBS subtypes on serotoninergic and catecholaminergic pathways involved in control of basal and stress-stimulated HPA axis activity. IBS ligands can modulate behavioural and neuroendocrine responses in animal models of stress, depression and anxiety, and a body of evidence exists for alterations in central IBS expression in psychiatric patients, which can be normalised partially or fully by treatment with antidepressants. Dysfunction in monoaminergic systems and the HPA axis under basal and stress-induced activation has been extensively reported in psychiatric illnesses. On the basis of the literature, we suggest a potential therapeutic role for selective IBS ligands in the treatment of depression and anxiety disorders.

Novel Ligands Rationally Designed for Characterizing I2−Imidazoline Binding Sites Nature and Functions

The study of two series of 2-aryl-ethylen-imidazolines 3-7 and 8-12 inspired by I2-IBS ligands phenyzoline (1) and diphenyzoline (2), respectively, confirmed the interesting "positive" or "negative" morphine analgesia modulation displayed by their corresponding leads and demonstrated that these effects might be correlated with morphine tolerance and dependence, respectively. By comparative examination of rationally designed compounds, some analogies between binding site cavity of I2-IBS proteins and alpha 2C-adrenoreceptor emerged.

Clonidine accumulation in human neuronal cells

After transport across several epithelial barriers including the blood–brain barrier, clonidine interacts with α 2-adrenergic receptors and imidazoline binding sites in the brain. We hypothesized that neuronal cells take up clonidine thereby removing the drug from the extracellular fluid compartment. Uptake of [ 3H]clonidine into SH-SY5Y neuroblastoma cells was linear for up to 1 min, unaffected by inside directed Na + or Cl − gradients but strongly inhibited by an outside pH of 6.0. The cells accumulated [ 3H]clonidine 50–70-fold uphill against a concentration gradient. Unlabeled clonidine, guanabenz, imipramine, diphenhydramine, maprotiline, quinine and the endogenous monoamine phenylethylamine (2 mM) strongly inhibited the [ 3H]clonidine uptake by 60–95%. Tetraethylammonium, choline and N-methyl-4-phenylpyridinium had no effect. The accumulation at pH 7.5 was saturable with an apparent Michaelis–Menten constant ( K t) of 0.7 mM. We conclude that SH-SY5Y cells not only bind clonidine to extracellular receptors but also take up the drug rapidly by a specific and concentrative mechanism.

Interactions of imidazoline ligands with the active site of purified monoamine oxidase A

The two forms of monoamine oxidase, monoamine oxidase A and monoamine oxidase B, have been associated with imidazoline-binding sites (type 2). Imidazoline ligands saturate the imidazoline-binding sites at nanomolar concentrations, but inhibit monoamine oxidase activity only at micromolar concentrations, suggesting two different binding sites [Ozaita A, Olmos G, Boronat MA, Lizcano JM, Unzeta M & García-Sevilla JA (1997) Br J Pharmacol121, 901-912]. When purified human monoamine oxidase A was used to examine the interaction with the active site, inhibition by guanabenz, 2-(2-benzofuranyl)-2-imidazoline and idazoxan was competitive with kynuramine as substrate, giving K(i) values of 3 microM, 26 microM and 125 microM, respectively. Titration of monoamine oxidase A with imidazoline ligands induced spectral changes that were used to measure the binding affinities for guanabenz (19.3 +/- 3.9 microM) and 2-(2-benzofuranyl)-2-imidazoline (49 +/- 8 microM). Only one type of binding site was detected. Agmatine, a putative endogenous ligand for some imidazoline sites, reduced monoamine oxidase A under anaerobic conditions, indicating that it binds close to the flavin in the active site. Flexible docking studies revealed multiple orientations within the large active site, including orientations close to the flavin that would allow oxidation of agmatine.

Synthesis and pharmacological studies of new hybrid derivatives of fentanyl active at the μ-opioid receptor and I2–imidazoline binding sites

Two series of fentanyl-derived hybrid molecules bearing potent I2–imidazoline binding site (IBS) ligands (i.e., guanidine and BU224 moieties) linked with an aliphatic (m=2, 3, 4, 6, 7, 8, 9 and 12 methylene units) or aromatic spacer were prepared. Their affinities for the μ-opioid receptors and for the I2–IBS were determined through competition binding studies on human postmortem brain membranes. Whereas the BU224 hybrid molecules bound to the μ-opioid receptor and the I2–IBS in the micromolar to low micromolar range, the alkaneguanidine series exhibited remarkable affinities in the nanomolar range for both receptors. [35S]GTPγS functional assays were performed on human postmortem brain membranes with selected ligands from each series (4f and 8g) showing the highest dual affinity for the μ-opioid receptor and I2–IBS affinities. Both compounds displayed agonist properties: at the μ-opioid receptor for the alkaneguanidine derivative 4f (spacer: six methylene units) and at a G-protein coupled receptor (GPCR) which remains to be determined for 8g. The lack of analgesic properties of 4f in vivo (i.e., hot plate and writhing tests in mice), discordant with the good in vitro binding data (Ki μ=1.04±0.28nM, Ki I2=409±238nM), may possibly be due to the low intrinsic efficacy of the compound. Alternatively, a low access to the central nervous system for this kind of hybrid molecules cannot be ruled out. Two new compounds reported here (9f and 13), which were not dual acting, are worth mentioning for their outstanding binding affinities; 9f bound to the μ-opioid receptor with a picomolar affinity (Ki=0.0098±0.0033nM), whereas 13 presented an I2–IBS affinity (Ki=18±11nM) similar to the reference compound BU224.

Autoradiographical distribution of imidazoline binding sites in monoamine oxidase A deficient mice

This study has used receptor autoradiography to characterize imidazoline binding sites (I-BS) in monoamine oxidase (MAO) A knockout and wild-type mice. A comparison between MAO-A and MAO-B, binding of the endogenous beta-carboline [(3)H]harmane, and I-BS, has been made using sections from brain and kidney. The loss of binding to MAO-A in the knockout animals was confirmed using the selective radioligand [(3)H]Ro41-1049, with labelling reduced to background levels. The binding of [(3)H]Ro19-6327 to MAO-B was unaffected, indicating no change in this isoform in response to the loss of MAO-A. A reduction in binding to the I(2)-BS, as labelled by both [(3)H]idazoxan and [(3)H]2-BFI (2-(2-benzofuranyl)-2-imidazoline), was seen in the MAO-A knockout animals in both brain and kidney sections, whereas binding to the I(1)-BS in kidney sections remained unchanged. The loss of I(2) binding was found to be regionally dependent and was positively correlated with the relative expression of MAO-A in specific regions in the wild-type animals. Using the MAO-A knockout mice it was also possible to demonstrate a non-MAO-A population of binding sites labelled by the putative I-BS endogenous ligand, harmane.

Theoretical study of structure, p Ka, lipophilicity, solubility, absorption, and polar surface area of some centrally acting antihypertensives

The methods of theoretical chemistry have been used to elucidate the molecular properties of the substituted imidazoline and oxazoline structures, a class of potent agonists and antagonists of imidazoline receptors. The geometries of various tautomers and isomers of 2-[2,6-dichlorophenylimino]imidazolidine (clonidine), 1-( N-dicyclopropylmethyl)amino-2-oxazoline (rilmenidine), 4-chloro- N-(4,5-dihydro-1 H-imidazol-2yl)-6-methoxy-2-methyl-5-pyrimidinamine (moxonidine), N-(dicyclopropylmethyl)-4,5-dihydro-1 H-pyrrol-2-amine (aminopyrroline), N-dicyclopropylmethyl-4,5-dihydrothiazol-2-amine (aminothiazoline), 4,5-dihydro-2-(2-methoxyphenyl)-1 H-imidazole (compound_6), 4,5-dihydro-2-(3-methylthiophen-2-yl)-1 H-imidazole (compound_7), N-(2-chloro-4-iodophenyl)-4,5-dihydro-5-methyl-3 H-pyrrol-2-amine (LNP_911), N-amidino-3,5-diamino-6-chloropyrazine-carboxamide (amiloride), 2-(1,4-benzodioxan-2-yl)-2-imidazoline (idazoxan), (±)-2-(2-ethyl-2,3-dihydro-2-benzofuranyl)-2-imidazoline (efaroxan), (4-aminobutyl)guaninine (agmatine), and 1-methyl-9 H-pyrido[3,4- b]indole (harmane) have been studied using Becke3LYP/6–31+G(d,p) and BP86/TZ2P DFT methods. The optimized geometries indicate that these molecules show a distinctly nonplanar configuration of the imidazoline and oxazoline moieties. In the gas-phase, rilmenidine and aminothiazoline exist in two forms (amino and imino), the amino tautomers being more stable by about 6 kJ/mol. The calculations showed, in agreement with experiments, that clonidine, moxonidine, and LNP_911 exist in a more stable imino tautomer. The tautomer containing the amino group is by about 30 kJ/mol less stable. Computations that include the effect of solvation indicated that also in water the relative stability order of individual tautomers (amino and imino forms) is preserved. The computed p K a values varied between 6.7 and 9.0, and correlate well with the available experimental p K a’s found in the literature. Among the clinically useful antihypertensives moxonidine exhibits the lowest basicity in water. At pH = 7.4 only about 50% of this drug exists in ionized form. The available experimental partition coefficients of compounds investigated are best reproduced by the CLOGP method. The computed partition coefficients varied between −1.80 (agmatine) and 5.35 (LNP_911) (CLOGP). Clonidine, moxonidine, and rilmenidine are moderately lipophilic compounds with lipophilicities between these two extreme values. The computed solubilities (about 0.1–4 g/L) show that the imidazoline and oxazoline derivatives studied have very low water solubility. The analysis of molecular descriptors defined by Lipinski has shown that most of the compounds studied obey ‘rule of five’. Amiloride and agmatine ‘outlets’ exhibit also the lowest absorption. Therefore, in the early stages of the design of ligands acting on imidazoline binding sites, it is becoming more important to determine the p K a, lipophilicity, water solubility, polar surface area, absorption, and other physicochemical properties associated with a drug, before synthetic work is undertaken, with the aim of avoiding the synthesis of compounds that are predicted to have poor biopharmaceutical characteristics.

In vitro and ex vivo distribution of [ 3H]harmane, an endogenous β-carboline, in rat brain

The endogenous β-carboline, harmane, has been shown to bind to monoamine oxidase A (MAO-A) and a separate, high affinity, non-MAO site. Research in our laboratory has shown that harmane is an active component of clonidine-displacing substance (CDS), the proposed endogenous ligand for imidazoline binding sites (IBS). In the present study we have investigated the distribution of [ 3H]harmane in rat brain, and related the binding profile to the distribution of the MAO-A selective ligand [ 3H]Ro41-1049 and the I 2BS ligand [ 3H]2-BFI. The in vivo distribution of [ 3H]harmane following intravenous administration was also investigated. Receptor autoradiography revealed a highly significant correlation for the distribution of [ 3H]harmane and [ 3H]Ro41-1049, and a significant correlation for [ 3H]harmane and the I 2BS ligand [ 3H]2-BFI. The in vivo distribution of [ 3H]harmane suggests that the ligand accumulates in the adrenal gland and throughout the brain with the primary route of excretion occurring via the duodenum. In conclusion, these studies have shown that [ 3H]harmane labels a population of binding sites that reflect the distribution of MAO-A. Further evidence for a non-MAO, IBS [ 3H]harmane population has not been shown but the high level of expression of the MAO-A site is likely to have masked the much smaller population of I 2BS.

Enhancement of EFS-induced contractions, by agmatine, in guinea pig gallbladder smooth muscle strips

Electrical field stimulation of gallbladder muscle strips produces frequency-dependent contractions by activating cholinergic nerves. The cholinergic motor function of the gallbladder and enteric system is also modulated by other mediators. The aim of this study was to investigate the role of agmatine, a ligand for alpha 2-adrenoceptors and imidazoline binding sites, in the cholinergic motor activity of guinea pig gallbladder smooth muscle. Gallbladder muscle strips obtained from guinea pigs were subjected to electrical field stimulation (1-64 Hz, 100 V, 1-ms pulse width, and 10-s train duration). Frequency-response contractions of gallbladder muscle strips were traced before and after the addition of cumulative concentrations of agmatine (10(-5)-10(-3) M) to the tissue bath. The same set of experiments was repeated in the presence of different antagonists. Agmatine by itself did not produce any contractions in guinea pig gallbladder muscle strips, but significantly enhanced the contractile response produced by electrical field stimulation. Yohimbine (10(-6) M), a selective alpha 2-adrenergic blocker, neither decreased nor increased the enhancement induced by agmatine. However, idazoxan (10(-4) M), an alpha-receptor blocker and imidazoline receptor antagonist, abolished this enhanced contractile response. Pretreatment with N(W)-nitro L-arginine methyl ester (L-NAME; 30 microM), and indomethacin (10 microM) did not inhibit the effect of agmatine. Our findings indicate that agmatine has a modulator role in the electrical field stimulation-induced cholinergic contractions of guinea pig gallbladder smooth muscle strips, and this role could be mediated by imidazoline receptors. Receptor binding studies should be done to determine the presence of endogenous agmatine and imidazoline receptors in gallbladder smooth muscle.

The imidazoline-like drug S23515 affects lipid metabolism in hepatocyte by inhibiting the oxidosqualene:lanosterol cyclase activity

Imidazoline-like drugs are centrally-acting antihypertensive agents that inhibit the activity of the sympathetic nervous system by interacting with the α2-adrenoreceptor and also with a non-adrenergic imidazoline binding site called the imidazoline 1 receptor. Recently, these molecules were proposed to play an additional role in cardiovascular diseases by acting on glucose and lipid metabolism. We used S23515, a potent imidazoline-like molecule acting selectively on blood pressure through the imidazoline 1 receptor, to decipher the effects of these drugs on lipid metabolism. We found that S23515 inhibited specifically and dose-dependently cholesterol synthesis in cultured rodent and primate hepatocytes. This hypocholesterolemic effect was likely due to the inhibition of the oxido:lanosterol cyclase (OSC), a rate-limiting enzyme in the cholesterol biosynthetic pathway. Partial OSC inhibition induced by S23515 led to the generation of 24( S),25-epoxycholesterol, a potent ligand for the liver X receptor (LXR). Furthermore, S23515 increased in human macrophages the expression of both ABCA1 and G1, the 2 ATP binding cassette transporters, which play a pivotal role in the reverse cholesterol transport. Thus, these results suggest that S23515, and potentially other imidazoline-like drugs, could exert hypolipidemic effects in addition to their hypotensive activities.

Evidence for different pharmacological targets for imidazoline compounds inhibiting settlement of the barnacleBalanus improvisus

We describe the effect of eight different imidazoline/guanidinium compounds on the settlement and metamorphosis of larvae of the barnacle Balanus improvisus. These agents were chosen on the basis of their similar pharmacological classification in vertebrates and their chemical similarity to medetomidine and clonidine, previously described as highly potent settlement inhibitors (nanomolar range). Seven of the tested compounds were found to inhibit settlement in a dose-dependent manner in concentrations ranging from 100 nM to 10 microM without any significant lethal effects. In vertebrate systems these substances have overlapping functions and interact with both alpha-adrenoceptors as well as imidazoline binding sites. Antagonizing experiments using the highly specific alpha(2)-antagonist methoxy-idazoxan or agmatine (the putative endogenous ligand at imidazoline receptors) were performed to discriminate between putative pharmacological mechanisms involved in the inhibition of cyprid settlement. Agmatine was not able to reverse the effect of any of the tested compounds. However, methoxy-idazoxan almost completely abolished the settlement inhibition mediated by guanabenz (alpha(2)-agonist, I(2) ligand), moxonidine (alpha(2)-agonist, I(1) ligand) and tetrahydrozoline (alpha-agonist, I(2) ligand). The actions of cirazoline (alpha(1)-agonist, I(2) ligand) BU 224 (I(2) ligand) and metrazoline (I(2) ligand) were not reversed by treatment with methoxy-idazoxan. These results suggest that the settlement inhibition evoked by the I(2) ligands and alpha(2)-agonists used in this study of the neurologically simple but well-organized barnacle larva is mediated through different physiological targets important in the overall settlement process.

Harmane and Harmalan Are Bioactive Components of Classical Clonidine-Displacing Substance

Elucidation of the structure of the endogenous ligand(s) for imidazoline binding sites, clonidine-displacing substance (CDS), has been a major goal for many years. Crude CDS from bovine lung was purified by reverse-phase high-pressure liquid chromatography. Electrospray mass spectrometry (ESMS) and nuclear magnetic resonance ((1)H NMR) analysis revealed the presence of L-tryptophan and 1-carboxy-1-methyltetrahydro-beta-carboline in the active CDS extract. Competition radioligand binding studies, however, failed to show displacement of specific [(3)H]clonidine binding to rat brain membranes for either compound. Further purification of the bovine lung extract allowed the isolation of the beta-carbolines harmane and harmalan as confirmed by ESMS, (1)H NMR, and comparison with synthetic standards. Both compounds exhibited a high (nanomolar) affinity for both type 1 and type 2 imidazoline binding sites, and the synthetic standards were shown to coelute with the active classical CDS extracts. We therefore propose that the beta-carbolines harmane and harmalan represent active components of classical CDS. The identification of these compounds will allow us to establish clear physiological roles for CDS.

Imidazoline Receptors and their Ligands as Potentiators of Nutrient-Induced Insulin Secretion

It is now well established that many tissues express binding sites that are defined pharmacologically as “imidazoline” receptors. These are still undergoing full molecular characterisation but candidate clones have been identified for both the I 1 and I2 subtypes. The physiological roles of these receptors remain to be the subject of continuing investigation although there is strong evidence that I1 sites play a role in the central regulation of blood pressure while I2 sites may control feeding and other aspects of behaviour. In molecular terms, the third imidazoline receptor subtype (I3) is the least well characterised although progress in understanding its functional pharmacology is continuing. This receptor plays a role in the regulation of insulin secretion and mediates glucose-dependent increases in secretion in response to synthetic ligands such as efaroxan and antazoline. These agents were originally developed as selective α2-adrenoceptor antagonists but it is now clear that they also act as I3 agonists. Molecular characterisation of the functional activity of I3 agonists has been complicated by the fact that most of the ligands also interact with other cellular proteins, leading to potential difficulties in the interpretation of experimental data. In particular, it was established more than a decade ago that many I3-agonists induce the closure of ATP-sensitive potassium (KATP) channels and it is now clear that an imidazoline binding site is located within the pore-forming subunit (Kir6.2) of the KATP channel. However, increasing evidence implies that closure of this channel is not primarily responsible for the ability of imidazoline drugs to enhance insulin secretion. In support of this, imidazoline compounds have recently been described that do not close KATP channels but which potentiate insulin secretion very effectively. These probably interact with intracellular targets within pancreatic β- cells and the results suggest that the I3 receptor may be distinct from the imidazoline binding site associated with Kir6.2. Rather, the I3-site appears to control a distal step in the exocytotic pathway. The present review examines recent progress in identifying the I 3 receptor and describes the pharmacology of I3-agonists that regulate insulin secretion. The possibility that these agonists might then serve as prototypes for the development of novel ligands having therapeutic potential is also considered. Keywords: type 2 diabetes, imidazoline receptors, efaroxan, clonidine-displacing substance, islets of langerhans, insulin secretion, glucagon secretion

Sympatho-modulatory therapies in perioperative medicine

With increasing life expectancy and improved surgical technology an ever-larger number of elderly patients with cardiovascular disease, or significant cardiovascular risk factors will undergo major surgery. More than 5% of an unselected surgical population undergoing non-cardiac surgery will suffer from perioperative cardiovascular complications including myocardial infarction and cardiac death. The incidence of adverse cardiac events may even reach 30% in high-risk patients undergoing vascular surgery causing a substantial financial burden of perioperative health care costs. 59 Thus, all therapeutic measures should be undertaken to reach the challenging goal of a lower incidence of perioperative cardiovascular complications. Gaining control over sympathetic nervous system activity, that is blunting the adrenergic response to the surgical trauma, traditionally represents an important aspect of anaesthetic practice. 21 Anaesthesiology has been regarded as the ‘practice of autonomic nervous system medicine’. While variable and moderate changes in sympathetic nervous system activity function as a servo-control mechanism and are even required to maintain and optimize cardiac performance, undue liberation of excitotoxic substances such as catecholamines and inflammatory cytokines, particularly during emergence from anaesthesia and the painful postoperative period, facilitates the occurrence of cardiovascular complications. 83 84 At this point, the life-supporting adrenergic drive (‘fight-or-flight-response’) turns into a potentially hazardous life-threatening maladaptation. In support of this concept, the beneficial effects of antiadrenergic treatment regimens in perioperative medicine have been confirmed, in observational studies, metaanalyses 55 1 66 and randomized controlled clinical trials. 43 54 59 63 80 84 However, the seemingly established concept of ‘sympatholysis’ as an effective cardioprotective treatment modality needs considerable refinement in the light of the many new experimental and clinical findings. The parlance of ‘sympatholytic’ protection erroneously equates annihilation of any type of adrenergic stimulation with cardioprotection and should be replaced by ‘sympathomodulatory’protection. The present review summarizes findings from large-scale heart failure trials and discusses basic and clinical aspects of individual sympatho-modulatory therapies, as currently used in perioperative medicine, including b-adrenergic antagonism, a2-agonism, and regional anaesthetic techniques. For limitation of space, reviews will often be cited where further references to the primary literature may be found.

Imidazoline binding sites and their ligands: an overview of the different chemical structures.

Since Bousquet et al. discovered the imidazoline binding sites (IBS) two decades ago, when they realized that the antihypertensive drug clonidine interacts not only with the alpha2-adrenenoceptors (alpha2-AR) but also with a distinct imidazoline preferring binding site, these receptors have been paid a great deal of attention. At least two subtypes, I1 and I2, have been characterised based on their binding affinity for different radioligands, but their structures still remain unknown. The pharmacological profile of these IBSs has been the objective of several and very thorough reviews. However, a medicinal chemistry overview of the different IBS ligands prepared to date has never been attempted. In this study, we attempt to compile all the different chemical structures reported to date as IBS ligands and classify them in function of their chemical structure and binding affinity for the different IBS subtypes. Thus, we comment on the different endogenous IBS ligands known as well as the drugs described to interact with the I1-IBS which have found application as antihypertensive drugs. Then, we review those compounds described in the literature to interact with the I2-IBS, classifying them by their chemical families (imidazolines, guanidines, 2-aminoimidazolines, beta-carbolines). Finally, some conclusions are drawn.

Pharmacophore Development and 3D-QSAR Study of I1 Imidazoline Binding Site Ligands

Binding affinities at I 1 and I 2 Imidazoline Binding Sites (IBS) of a number of properly substituted imidazoline, oxazoline and pyrroline ligands were studied through an integrated modelling approach based on a coordinated application of 2D- (MGZ) and 3D-QSAR (CoMFA-and GRID-GOLPE) analyses and on the development of a quantitative pharmacophore (HypoGen). Congruent and significant cross-validated 2D and 3D correlations, having promising predictive ability, indicated that electrostatic and steric interactions differently modulate the binding affinities at I 1 /I 2 IBS. Stereoselective interactions of chiral a or β substituted imidazolines, leading to diverse I 1 /I 2 binding affinities and selectivities, were correctly predicted by the models. Slightly different pharmacophore features were detected by means of Hypogen for I 1 and I 2 -IBS ligands.