Nischarin Is Not the Functional I1 Imidazoline Receptor Involved in Blood Pressure Regulation.

ABSTRACT: Imidazoline receptor antisera selected/Nischarin was proposed several years ago as the functional entity for the I1 medullary receptors (I1Rs) targeted, together with α2-adrenoceptors, by the centrally acting antihypertensive drugs, such as clonidine. The objective of this study was to test this assumption using a pyrroline analog of clonidine, LNP599, which, unlike clonidine and related compounds, displays high selectivity toward I1Rs. Cardiovascular effects of LNP599 (3 mg/kg intravenous) were evaluated in anesthetized, artificially ventilated nischarin mutant rats expressing a truncated form of nischarin lacking the putative imidazoline binding site. LNP599 induced a rapid and pronounced fall in arterial blood pressure in wild-type animals (-42.7% ± 11.0% after 15 minutes), associated with a ≈30% heart rate reduction. Similar effects were obtained in homozygous and heterozygous nischarin mutant rats. The observation that the hypotensive response to I1R activation is not affected by the absence of the putative imidazoline binding site on nischarin strongly suggests that nischarin cannot be regarded as the functional I1R. Carbohydrate regulation was improved in nischarin mutant rats, further supporting the conclusion that nischarin and I1R are 2 distinct molecular entities.

Protective effects of the imidazoline-like drug lnp599 in a marmoset model of obesity-induced metabolic disorders.

BACKGROUND/OBJECTIVES: Overweight and obesity are undoubtable risk factors for type 2 diabetes and cardiovascular diseases and significantly contribute to the global morbi-mortality. We previoulsy reported that LNP599, a pharmacological imidazoline-like activator of hepatic AMPK/adiponectin signaling, protects against the development of adiposity and obesity and the associated cardio-metabolic disorders, suggesting that it may be a suitable drug candidate for a therapeutic approach targeting the development of obesity at very early stages. The objective of the present study was to evaluate the metabolic effects of LNP599 in a model of diet-induced overweight and metabolic disorders in a nonhuman primate, the common marmoset (Callithrix jacchus), and more particularly to establish the impact of the compound on cholesterol homeostasis, i.e., HDL and LDL/VLDL lipoproteins. METHODS: Marmosets were fed normal (NC) or hypercaloric (HC) chow during 16 weeks. Diet-induced changes in body weight and metabolism were assessed. Effects of LNP599 were evaluated in a subset of HC animals (HC-LNP) receiving the compound at a daily dose of 10 mg/kg over the 16 weeks. RESULTS: HC-feeding induced significant overweight associated with a marked dyslipidemia (hypertriglyceridemia, hypercholesterolemia, and reduced HDL over LDL/VLDL cholesterol ratio). LNP599 blunted the diet-induced body weight gain and largely protected against the development of hypertriglyceridemia. Total cholesterol was unchanged but the ratio of HDL over LDL/VLDL cholesterol was more than doubled. CONCLUSIONS: The profile of metabolic troubles obtained upon enriched diet mimicked the disorders associated with spontaneous obesity in marmosets. HC marmosets represent an experimental model of high clinical relevance to study the pathophysiology of obesity and related dyslipidemia and to evaluate the effects of emerging therapies targeting these disorders. Our data confirm the preventing effects of LNP599 in a nonhuman primate model and demonstrate for the first time the high potency of this drug in promoting HDL-cholesterol.

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 ß-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.

Increased expression of blood muscarinic receptors in patients with reflex syncope.

AIMS: Pathophysiology of reflex syncope is not fully understood but a vagal overactivity might be involved in this syncope. Previously, overexpression of muscarinic M2 receptors and acetylcholinesterase was found in particular in the heart and in lymphocytes of rabbits with vagal overactivity as well as in hearts of Sudden Infant Death Syndromes. The aim of this present study was to look at M2 receptor expression in blood of patients with reflex syncope. The second objective was to measure acetylcholinesterase expression in these patients. METHODS AND RESULTS: 136 subjects were enrolled. This monocenter study pooled 45 adults exhibiting recurrent reflex syncope compared with 32 healthy adult volunteers (18-50 years) and 38 children exhibiting reflex syncope requiring hospitalization compared with 21 controls (1-17 years). One blood sample was taken from each subject and blood mRNA expression of M2 receptors was assessed by qRT-PCR. Taking into account the non-symmetric distributions of values in both groups, statistical interferences were assessed using bayesian techniques. A M2 receptor overexpression was observed in adult and pediatric patients compared to controls. The medians [q1;q3] were 0.9 [0.3;1.9] in patients versus 0.2 [0.1;1.0] in controls; the probability that M2 receptor expression was higher in patients than in controls (Pr[patients>controls]) was estimated at 0.99. Acetylcholinesterase expression was also increased 0.7 [0.4;1.6] in patients versus 0.4 [0.2;1.1] in controls; the probability that acetylcholinesterase expression was higher in patients than in controls (Pr[patients>controls]) was estimated at 0.97. Both in adults and children, the expression ratio of M2 receptors over acetylcholinesterase was greater in the patient group compared with the control group. CONCLUSION: M2 receptor overexpression has been detected in the blood of both, adults and children, exhibiting reflex syncope. As in our experimental model, i.e. rabbits with vagal overactivity, acetylcholinesterase overexpression was associated with M2 receptor overexpression. For the first time, biological abnormalities are identified in vagal syncope in which only clinical signs are, so far, taken into account for differential diagnosis and therapeutic management. Further work will be needed to validate potential biomarkers of risk or severity associated with the cholinergic system.

Effects of imidazoline-like drugs on liver and adipose tissues, and their role in preventing obesity and associated cardio-metabolic disorders.

BACKGROUND/OBJECTIVES: We previously observed that selective agonists of the sympatho-inhibitory I1 imidazoline receptors (LNP ligands) have favorable effects on several cardiovascular and metabolic disorders defining the metabolic syndrome, including body weight. The objectives of this study were to explore the effects of LNPs on adiposity and the mechanisms involved, and to evaluate their impact on metabolic homeostasis. METHODS: Young Zucker fa/fa rats were treated with LNP599 (10 mg/kg/day) for 12 weeks. Effects on body weight, adiposity (regional re-distribution, morphology, and function of adipose tissues), cardiovascular and metabolic homeostasis, and liver function were evaluated. Direct effects on insulin and AMP-activated protein kinase (AMPK) signaling were studied in human hepatoma HepG2 cells. RESULTS: LNP599 treatment limited the age-dependent remodeling and inflammation of subcutaneous, epididymal, and visceral adipose tissues, and prevented total fat deposits and the development of obesity. Body-weight stabilization was not related to reduced food intake but rather to enhanced energy expenditure and thermogenesis. Cardiovascular and metabolic parameters were also improved and were significantly correlated with body weight but not with plasma norepinephrine. Insulin and AMPK signaling were enhanced in hepatic tissues of treated animals, whereas blood markers of hepatic disease and pro-inflammatory cytokine levels were reduced. In cultured HepG2 cells, LNP ligands phosphorylated AMPK and the downstream acetyl-CoA carboxylase and prevented oleic acid-induced intracellular lipid accumulation. They also significantly potentiated insulin-mediated AKT activation and this was independent from AMPK. CONCLUSIONS: Selective I1 imidazoline receptor agonists protect against the development of adiposity and obesity, and the associated cardio-metabolic disorders. Activation of I1 receptors in the liver, leading to stimulation of the cellular energy sensor AMPK and insulin sensitization, and in adipose tissues, leading to improvement of morphology and function, are identified as peripheral mechanisms involved in the beneficial actions of these ligands.

I1-imidazoline receptor-mediated cardiovascular and metabolic effects in high-fat diet-induced metabolic syndrome in rats.

OBJECTIVES: The objective of this study was to investigate the effects of a new I1-imidazoline receptor-selective pyrroline compound on the hemodynamic, metabolic and microvascular alterations in a high-fat diet (HFD)-induced model of metabolic syndrome in rats. METHODS: In total, twenty adult male Wistar rats were fed a high-fat diet (HFD, n = 20) for 20 weeks. Thereafter, the rats received a new pyrroline compound selective for I1-imidazoline receptors (LNP599; 10 mg/kg/day) or vehicle (n = 10/group) orally by gavage for 4 weeks. Functional microcirculation was assessed using intravital video microscopy, and structural microcirculation was evaluated using histochemical analysis. RESULTS: LNP599 induced concomitant reductions in the SBP, HR and plasma catecholamine levels. The animals treated with this new antihypertensive compound also presented an improvement in body weight and the metabolic parameters related to metabolic syndrome, such as the glucose and lipid profiles. These effects were accompanied by a reversal of the functional and structural capillary rarefaction in the skeletal muscle. CONCLUSIONS: The modulation of the sympathetic nervous system by a selective agonist for I1-imidazoline receptors improves the hemodynamic and metabolic parameters in an experimental model of metabolic syndrome. LNP599 can also contribute to the restoration of microcirculatory parameters.

Cerebral Microvascular Dysfunction and Inflammation Are Improved by Centrally Acting Antihypertensive Drugs in Metabolic Syndrome.

BACKGROUND: We aimed to investigate the effects of chronic oral treatment with centrally acting antihypertensive drugs, such as clonidine (CLO), an α2-adrenoceptor agonist, or LNP599, a selective I1 imidazoline receptor agonist, on brain microvascular function in rats with high-fat diet (HFD)-induced metabolic syndrome. METHODS: Male Wistar Kyoto rats were maintained on a normal diet (CON) or a HFD for 20 weeks. After this period, the HFD group received oral CLO (0.1 mg/kg), LNP599 (20 mg/kg), or vehicle daily for 4 weeks. Systolic blood pressure and heart rate (HR) were evaluated by photoplethysmography. Functional capillary density, endothelial function, and endothelial-leukocyte interactions in the brain were investigated by intravital video microscopy. Cerebral microcirculatory flow was evaluated by laser speckle contrast imaging. Brain tissue endothelial nitric oxide synthase, oxidative enzyme, and inflammatory marker expression levels were analyzed. RESULTS: Metabolic syndrome decreased brain functional capillary density and microvascular blood perfusion, changes accompanied by deficient brain microcirculation vasodilatory responses to acetylcholine. Significant numbers of rolling and adherent leukocytes were also observed in the brain venules. Chronic sympathetic inhibition with clonidine and LNP599 reduced blood pressure and HR. These effects were accompanied by reversals of cerebral capillary rarefaction, improvements in cerebral microvascular blood flow and endothelial function, and decreases in endothelial-leukocyte interactions in the cerebral venules. CONCLUSIONS: Our results suggest that central sympathetic inhibition exerts beneficial effects by increasing perfusion and reducing inflammatory marker expression and oxidative stress in the brains of rats with metabolic syndrome. Centrally acting antihypertensive drugs may be helpful in regulating cerebral microcirculatory function and vascular inflammation in metabolic syndrome.

Central Sympathetic Modulation Reverses Microvascular Alterations in a Rat Model of High-Fat Diet-Induced Metabolic Syndrome.

OBJECTIVES: The objective of this study was to investigate the role of the SNS on hemodynamic, metabolic, and microvascular alterations in a rat model of HFD-induced MS with salt supplementation. METHODS: In total, 40 adult male Wistar rats were fed normal chow (n = 10) or a HFD (n = 30) for 20 weeks. Thereafter, the HFD group received the centrally acting sympatho-modulatory drugs clonidine (0.1 mg/kg) or rilmenidine (1 mg/kg) or vehicle (n = 10/group) orally by gavage. FCD was evaluated using intravital video microscopy, and the SCD was evaluated using histochemical analysis. RESULTS: The pharmacological modulation of the SNS induced concomitant reductions in SBP, HR and plasma catecholamine levels. These effects were accompanied by a reversal of functional and structural capillary rarefaction in the skeletal muscle in both treated groups and an increase in SCD in the left ventricle only in the rilmenidine group. Improvement of the lipid profile and of glucose intolerance was also obtained only with rilmenidine treatment. CONCLUSIONS: Modulation of sympathetic overactivity results in the reversal of microvascular rarefaction in the skeletal muscle and left ventricle and improves metabolic parameters in an experimental model of MS in rats.

Imidazoline-like drugs improve insulin sensitivity through peripheral stimulation of adiponectin and AMPK pathways in a rat model of glucose intolerance.

Altered adiponectin signaling and chronic sympathetic hyperactivity have both been proposed as key factors in the pathogenesis of metabolic syndrome. We recently reported that activation of I1 imidazoline receptors (I1R) improves several symptoms of the metabolic syndrome through sympathoinhibition and increases adiponectin plasma levels in a rat model of metabolic syndrome (Fellmann L, Regnault V, Greney H, et al. J Pharmacol Exp Ther 346: 370-380, 2013). The present study was designed to explore the peripheral component of the beneficial actions of I1R ligands (i.e., sympathoinhibitory independent effects). Aged rats displaying insulin resistance and glucose intolerance were treated with LNP509, a peripherally acting I1R agonist. Glucose tolerance, insulin sensitivity, and adiponectin signaling were assessed at the end of the treatment. Direct actions of the ligand on hepatocyte and adipocyte signaling were also studied. LNP509 reduced the area under the curve of the intravenous glucose tolerance test and enhanced insulin hypoglycemic action and intracellular signaling (Akt phosphorylation), indicating improved glucose tolerance and insulin sensitivity. LNP509 stimulated adiponectin secretion acting at I1R on adipocytes, resulting in increased plasma levels of adiponectin; it also enhanced AMPK phosphorylation in hepatic tissues. Additionally, I1R activation on hepatocytes directly enhanced AMPK phosphorylation. To conclude, I1R ligands can improve insulin sensitivity acting peripherally, independently of sympathoinhibition; stimulation of adiponectin and AMPK pathways at insulin target tissues may account for this effect. This may open a promising new way for the treatment of the metabolic syndrome.

Synthesis and biological evaluation of 2-aryliminopyrrolidines as selective ligands for I1 imidazoline receptors: discovery of new sympatho-inhibitory hypotensive agents with potential beneficial effects in metabolic syndrome.

New 2-aryliminopyrrolidines (1-18) were synthesized and tested for their binding properties on I1 imidazoline receptors vs α2-adrenergic receptors and their blood pressure effects after both systemic and intracerebral administrations. The purposes of this study were: (i) to analyze structure-activity and affinity relationships on I1 imdazoline receptors and (ii) to propose some leader compounds for the development of new sympatho-inhibitory drugs with potential applications in hypertension and/or metabolic syndrome, i.e., a cluster of cardiovascular (hypertension) and metabolic disorders. Our study highlights decisive arguments of SAR concerning both the affinity for I1Rs and the hypotensive activity of 2-aryliminopyrrolidines. Binding assays showed high affinity and selectivity of some compounds for I1 imidazoline receptors over α2-adreergic receptors. Compound 13 (laboratory reference LNP599; Ki = 3.2 nM on I1imidazoline receptors) is the prototype for the development of new centrally acting agents targeting specifically I1imidazoline receptors to be used in the management of hypertension and/or metabolic syndrome.

A new pyrroline compound selective for I1-imidazoline receptors improves metabolic syndrome in rats.

Symptoms of the metabolic syndrome (MetS), such as insulin resistance, obesity, and hypertension, have been associated with sympathetic hyperactivity. In addition, the adiponectin pathway has interesting therapeutic potentials in MetS. Our purpose was to investigate how targeting both the sympathetic nervous system and the adipose tissue (adiponectin secretion) with a drug selective for nonadrenergic I1-imidazoline receptors (I1Rs) may represent a new concept in MetS pharmacotherapy. LNP599 [3-chloro-2-methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride], a new pyrroline derivative, displaced the specific [(125)I]para-iodoclonidine binding to I1R with nanomolar affinity and had no significant affinity for a large set of receptors, transporters, and enzymes. In addition, it can cross the blood-brain barrier and has good intestinal absorption, permitting oral as well as intravenous delivery. The presence of I1Rs was demonstrated in 3T3-L1 adipocytes; LNP599 had a specific stimulatory action on adiponectin secretion in adipocytes. Short-term administration of LNP599 (10 mg/kg i.v.) in anesthetized Sprague-Dawley rats markedly decreased sympathetic activity, causing hypotension and bradycardia. Long-term treatment of spontaneously hypertensive heart failure rats with LNP599 (20 mg/kg PO) had favorable effects on blood pressure, body weight, insulin resistance, glucose tolerance, and lipid profile, and it increased plasma adiponectin. The pyrroline derivative, which inhibits sympathetic activity and stimulates adiponectin secretion, has beneficial effects on all the MetS abnormalities. The use of one single drug with both actions may constitute an innovative strategy for the management of MetS.

Murine models for pharmacological studies of the metabolic syndrome.

Metabolic syndrome has been described as the association of insulin resistance, hypertension, hyperlipidemia and obesity. Its prevalence increased dramatically, mainly in developed countries. Animal models are essential to understand the pathophysiology of this syndrome. This review presents the murine models of metabolic syndrome the most often used in pharmacological studies. The most common metabolic syndrome models exhibit a non-functional leptin pathway, or metabolic disorders induced by high fat diets. In a first part, and after a short introduction on leptin, its receptor and mechanism of action, we provide a detailed description of each model: SHROB, SHHF, JCR:LA-cp, Zucker, ZDF, Wistar Ottawa Karlsburg W, and Otsuka Long-Evans Tokushima Fatty rats, ob/ob, db/db, agouti yellow and Mc4R KO mice. The second part of this review is dedicated to metabolic syndrome models obtained by high fat feeding.

Methylation of imidazoline related compounds leads to loss of α2-adrenoceptor affinity. Synthesis and biological evaluation of selective I1 imidazoline receptor ligands.

Methylated analogues of imidazoline related compounds (IRC) were prepared; their abilities to bind I(1) imidazoline receptors (I(1)Rs), I(2) imidazoline binding sites (I(2)BS) and α(2)-adrenoceptor subtypes (α(2)ARs) were assessed. Methylation of the heterocyclic moiety of IRC resulted in a significant loss of α(2)AR affinity. Amongst the selective ligands obtained, LNP 630 (4) constitutes the first highly selective I(1)R agent showing hypotensive activity after intravenous administration.

Polarization state studies in second harmonic generation signals to trace atherosclerosis lesions.

We have performed multi-photon image reconstructions as well as polarization state analyses inside an artery wall affected by atherosclerosis to investigate the changes in collagen structure. Mice, either healthy or affected by spontaneous atherosclerosis, have been used for this purpose. A two-photon imaging system has been used to investigate atherosclerotic lesions in the ascending aorta of mice. Second harmonic imaging has been performed alternatively on healthy samples and on affected region. The reconstructed images show that the spatial distribution of the collagen network seems disorganized by the disease. The polarization state studies reveal however that the apparent disorganization of the collagen is related to its spatially diffuse distribution and that the internal structure of the collagen fibers is not affected by the disease. In addition, a theoretical simulation of the second harmonic polarization states shows that they are consistent with the known 3D structure of the collagen network.

Cardiac muscarinic receptor overexpression in sudden infant death syndrome.

BACKGROUND: Sudden infant death syndrome (SIDS) remains the leading cause of death among infants less than 1 year of age. Disturbed expression of some neurotransmitters and their receptors has been shown in the central nervous system of SIDS victims but no biological abnormality of the peripheral vago-cardiac system has been demonstrated to date. The present study aimed to seek vago-cardiac abnormalities in SIDS victims. The cardiac level of expression of muscarinic receptors, as well as acetylcholinesterase enzyme activity were investigated. METHODOLOGY/PRINCIPAL FINDINGS: Left ventricular samples and blood samples were obtained from autopsies of SIDS and children deceased from non cardiac causes. Binding experiments performed with [(3)H]NMS, a selective muscarinic ligand, in cardiac membrane preparations showed that the density of cardiac muscarinic receptors was increased as shown by a more than doubled B(max) value in SIDS (n = 9 SIDS versus 8 controls). On average, the erythrocyte acetylcholinesterase enzyme activity was also significantly increased (n = 9 SIDS versus 11 controls). CONCLUSIONS: In the present study, it has been shown for the first time that cardiac muscarinic receptor overexpression is associated with SIDS. The increase of acetylcholinesterase enzyme activity appears as a possible regulatory mechanism.

The purinergic P2Y1 receptor supports leptin secretion in adipose tissue.

Extracellular nucleotides have been shown to trigger intracellular calcium release and influence leptin secretion in differentiated white and brown adipocytes through activation of various but not clearly identified P2 receptors. In the present study, we wished to assess whether or not the P2Y1 ADP receptor is functional in white adipocytes and whether it could affect the secretion of adipocyte-derived hormones. Stromal cells and mature adipocytes were isolated from epididymal adipose tissue from wild-type and P2Y1 knockout (KO) C57-black/six male mice. The expression of the P2Y1 receptor in adipocytes was confirmed by RT-PCR and intracellular calcium measurements with fura 2-AM. KO of P2Y1 receptors did not affect the cell size and lipid content of mature adipocytes or the differentiation of the stromal cell fraction, but the leptin production of mature adipocytes was decreased under basal and insulin-stimulated conditions. A selective P2Y1 antagonist, MRS2500, reduced leptin release in isolated adipocytes. The plasma and adipose tissue mRNA levels of leptin were also lower in P2Y1 KO mice as compared with wild-type animals. However, in mice fed a high-fat diet, the plasma leptin levels were greatly enhanced and the inhibitory effect of P2Y1 KO was not observed. These results show that the P2Y1 receptor supports leptin production in isolated white adipocytes through a transcriptional mechanism. This function of the receptor may regulate plasma leptin in lean mice but is overcome in obese animals.

Microvascular effects of centrally acting antihypertensive drugs in spontaneously hypertensive rats.

We investigated the effects of oral long-term antihypertensive treatment using centrally acting sympathoinhibitory drugs on capillary density in the skin, skeletal muscle, and heart in the spontaneously hypertensive rat (SHR). Wistar Kyoto rats (WKY) were used as normotensive control groups. Functional capillary density was assessed using intravital fluorescence videomicroscopy and structural capillary density with histochemical analysis. Groups of 10 SHRs were orally treated over 28 days with clonidine (0.1 mg x kg x d), rilmenidine (1 mg x kg x d), or moxonidine (10 mg x kg x d). A group of WKY was also treated with clonidine (0.1 mg x kg x d). Treatment with all antihypertensive drugs induced a normalization of arterial pressure accompanied by a reversion of functional capillary rarefaction in the skeletal muscle and skin of SHR. Clonidine treatment also reduced arterial pressure and increased functional capillary density in the skin and skeletal muscle of WKY. Histochemical analysis showed that SHR had a lower capillary to fiber ratio in the skeletal muscle (P < 0.0001), which was normalized by all treatments. The capillary volume density to fiber volume density ratio in the left ventricle of SHR was also significantly reduced (P < 0.0001). However, myocardial capillary rarefaction was not altered by the different treatments. In conclusion, the results showed that long-term antihypertensive treatment with centrally acting drugs enhanced tissue perfusion and reversed capillary rarefaction in the skeletal muscle of SHRs.

Constitutive overexpression of muscarinic receptors leads to vagal hyperreactivity.

BACKGROUND: Alterations in muscarinic receptor expression and acetylcholinesterase (AchE) activity have been observed in tissues from Sudden Infant Death Syndrome (SIDS). Vagal overactivity has been proposed as a possible cause of SIDS as well as of vasovagal syncopes. The aim of the present study was to seek whether muscarinic receptor overexpression may be the underlying mechanism of vagal hyperreactivity. Rabbits with marked vagal pauses following injection of phenylephrine were selected and crossed to obtain a vagal hyperreactive strain. The density of cardiac muscarinic receptors and acetylcholinesterase (AchE) gene expression were assessed. Blood markers of the observed cardiac abnormalities were also sought. METHODOLOGY/PRINCIPAL FINDINGS: Cardiac muscarinic M(2) and M(3) receptors were overexpressed in hyperreactive rabbits compared to control animals (2.3-fold and 2.5-fold, respectively) and the severity of the phenylephrine-induced bradycardia was correlated with their densities. A similar overexpression of M(2) receptors was observed in peripheral mononuclear white blood cells, suggesting that cardiac M(2) receptor expression can be inferred with high confidence from measurements in blood cells. Sequencing of the coding fragment of the M(2) receptor gene revealed a single nucleotide mutation in 83% of hyperreactive animals, possibly contributing for the transcript overexpression. Significant increases in AchE expression and activity were also assessed (AchE mRNA amplification ratio of 3.6 versus normal rabbits). This phenomenon might represent a compensatory consequence of muscarinic receptors overexpression. Alterations in M(2) receptor and AchE expression occurred between the 5th and the 7th week of age, a critical period also characterized by a higher mortality rate of hyperreactive rabbits (52% in H rabbits versus 13% in normal rabbits) and preceeded the appearance of functional disorders. CONCLUSIONS/SIGNIFICANCE: The results suggest that cardiac muscarinic receptor overexpression plays a critical role in the development of vagal hyperreactivity, whereas AchE hyperactivity appears as a compensatory consequence of it. Since similar vagal disorders were observed recently by us in SIDS, muscarinic receptor overexpression could become a marker of risk of vasovagal syncopes and SIDS.