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

Diagnostic Criteria for Metabolic Syndrome in Diet-Induced Rodent Models: A Systematic Review

BACKGROUND

Thousands of publications in recent years have addressed the induction of metabolic syndrome (MetS) in rodents. However, the criteria and the reference values for diagnosing this disease have not been defined.

OBJECTIVE

Our main objective was to carry out a systematic review to gather evidence about the criteria for biochemical and anthropometric parameters in which scientific studies have relied on to report that rats developed MetS from a previous dietary manipulation.

METHODS

We compiled characteristics and findings of diet-induced MetS with high-fat, high-carbohydrate, high-fat/high-carbohydrates, and cafeteria diet from PubMed and Science Direct databases published in the last 5 years.

RESULTS

The results on the principal determinants for the syndrome, published in the reviewed articles, were chosen to propose reference values in the rat models of food induction.

CONCLUSION

The values obtained will serve as reference cut-of points in the development of the disease; in addition, the compilation of data will be useful in planning and executing research protocols in animal models.

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.

Cardiovascular Effects Mediated by Imidazoline Drugs: An Update

OBJECTIVE

Clonidine is a centrally acting antihypertensive drug. Hypotensive effect of clonidine is mediated mainly by central α2-adrenoceptors and/or imidazoline receptors located in a complex network of the brainstem. Unfortunately, clonidine produces side effects such as sedation, mouth dry, and depression. Moxonidine and rilmenidine, compounds of the second generation of imidazoline drugs, with fewer side effects, display a higher affinity for the imidazoline receptors compared with α2-adrenoceptors. The antihypertensive action of these drugs is due to inhibition of the sympathetic outflow primarily through central I1-imidazoline receptors in the RVLM, although others anatomical sites and mechanisms/receptors are involved. Agmatine is regarded as the endogenous ligand for imidazoline receptors. This amine modulates the cardiovascular function. Indeed, when administered in the RVLM mimics the hypotension of clonidine.

RESULTS

Recent findings have shown that imidazoline drugs also exert biological response directly on the cardiovascular tissues, which can contribute to their antihypertensive response. Currently, new imidazoline receptors ligands are in development.

CONCLUSION

In the present review, we provide a brief update on the cardiovascular effects of clonidine, moxonidine, rilmenidine, and the novel imidazoline agents since representing an important therapeutic target for some cardiovascular diseases.

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.