Síndrome metabólico y melatonina : estudio de dos modelos experimentales en ratas

Resumen: El presente Trabajo de Tesis persiguió analizar y comparar algunas de las secuelas del síndrome metabólico (SM) en dos modelos experimentales en ratas, la ingesta de una dieta hipergrasa y la administración de fructosa en el agua de bebida. La comparación de distintos parámetros del SM en ambos modelos experimentales indicó que comparten secuelas somáticas y bioquímicas relevantes como el aumento de peso corporal y de la PA sistólica, la intolerancia a una sobrecarga de glucosa indicativa de una anormalmente alta resistencia a la insulina y cambios en analitos usados en clínica para el diagnóstico de SM tales como la hipertrigliceridemia, hipercolesterolemia, hiperuricemia y el aumento en colesterol-LDL. Pudo verificarse que tanto en el SM por ingesta de una dieta rica en grasa como en el producido por la administración de fructosa se detecta una inhibición del eje hipófiso-gonadal de origen testicular, indicado por la inhibición de la secreción de testosterona en presencia de niveles anormalmente elevados de LH circulante. Se analizó también la evolución del SM mediante el estudio de las etapas incipiente y establecida del SM por administración de fructosa, verificándose que existe una etapa inicial de mayor tolerancia a una sobrecarga de glucosa (indicativa de menor resistencia a la insulina), que coexiste con un aumento de PA sistólica y un desarrollo parcial de secuelas dislipémicas (hipercolesterolemia).En el SM por dieta rica en grasas se producen alteraciones en los ritmos diarios de adiponectina, leptina, insulina, glucosa, triglicéridos y colesterol plasmáticos, compatibles con un relevante efecto de la dieta en la sincronización del sistema circadiano. En vista de que la melatonina combina propiedades cronobióticas y citoprotectoras que pueden ser de relevancia en la prevención y el tratamiento del SM estudiamos distintos aspectos de la actividad de la melatonina en los dos modelos experimentales citados. La administración concomitante de melatonina en el agua de bebida fue eficaz para revertir los aumentos de peso y de PA sistólica, la anormal resistencia a la insulina, la dislipemia y la hiperuricemia que se producen tanto en el SM por ingesta de una dieta rica en grasa como en el producido por la administración de fructosa. Este efecto correctivo de la melatonina es ya evidente en la etapa inicial de sensibilidad aumentada a la insulina que se observa en el SM incipiente por administración de 5% de fructosa. La melatonina no corrigió la inhibición del eje hipófiso-gonadal de origen testicular en el SM y mostró una actividad inhibitoria de la síntesis de testosterona cuando se administró a ratas con dieta normal. La melatonina fue eficaz para normalizar las alteraciones en los ritmos diarios de adipocitoquinas y señales metabólicas circulantes que se observan en el SM. En conclusión, debido a sus efectos sobre el sistema circadiano y a sus potentes propiedades citoprotectoras la melatonina puede ser de utilidad terapéutica en el SM.Abstract: The objective of this Doctoral Thesis work was to analyze and to compare some of the consequences of metabolic syndrome (MS) in two experimental models, i.e. rats eating a high fat diet and rats drinking a high fructose solution. The comparison of various parameters of the MS in both experimental models indicated that they share relevant biochemical sequelae such as increased body weight and abnormally high systolic blood pressure, impaired glucose overload (indicative of an abnormally high insulin resistance) and changes in analytes used clinically for the diagnosis of MS like hypertriglyceridemia, hypercholesterolemia, hyperuricemia and increased cholesterol-LDL. In both types of MS an inhibition of the pituitary-gonadal at the testicular was detectable, as indicated by the inhibition of the secretion of testosterone in the presence of abnormally high circulating LH. We also analyzed the development of MS by studying the early stage and the established stage of MS brought about by fructose administration, verifying that initially there is a higher tolerance to a glucose load (indicative of lower insulin resistance), which coexists with a increased systolic and partial development of dislipemic sequelae (hypercholesterolemia). In the established SM following a high fat diet, alterations of daily rhythms of circulating adiponectin, leptin, insulin, glucose, triglycerides and cholesterol levels were found, consistent with a significant effect of diet on the circadian timing system. In view that melatonin combines chronobiotic and cytoprotective properties which may be relevant in the prevention and treatment of MS, the activity of melatonin in the two experimental models above cited was examined. The concomitant administration of melatonin in the drinking water was effective in reversing the increase in weight and systolic BP, abnormal insulin resistance, dyslipidemia and hyperuricemia occurring both in MS. The corrective effect of melatonin was already evident at the initial stage of increased insulin sensitivity after the administration of 5% fructose. Melatonin did not correct the inhibition of the pituitary-gonadal axis in MS; rather it showed an inhibitory activity per se on testosterone synthesis in rats fed with a normal diet. Melatonin was effective to normalize the disrupted daily rhythms of circulating adipocytokines and metabolic signals found in MS. In conclusion, the results demonstrate that due to its effects on the circadian system and its potent cytoprotective properties, melatonin could be therapeutically useful in the MS

Chronophysiology of melatonin : therapeutical implications

Normal circadian rhythms are synchronized to a regular 24 hr environmental light/dark cycle. Both the suprachiasmatic nucleus (SCN) and melatonin are essential for this adaptation. Melatonin exerts its chronophysiological action in part by acting through specific receptors (MT1, MT2) which have been identified in the plasma membrane of SCN as well as in several neural and non-neural tissues. Both receptors have been cloned and share general features with other G protein linked receptors. Melatonin also exerts direct effects on intracellular proteins such as calmodulin or tubulin and has strong free radical scavenger properties which are non-receptor mediated. Within the SCN, melatonin reduces neuronal activity in a time-dependent manner. SCN MT1 and MT2 receptors appear to be insensitive during the day, but sensitive at dusk and dawn (MT2; causes phase shifts) or during early night period (MT1; decreases neuronal firing rate). Melatonin secreted during nighttime provides enough inertia to resist minor perturbations of the circadian timing system. The disruption of these circadian mechanisms cause a number of sleep disorders named according to the International Classification of Sleep Disorders as circadian rhythm sleep disorders (CRSDs). CRSDs include delayed or advanced sleep phase syndromes, non 24 hr sleep/wake rhythm disorder, time zone change syndrome (“jet lag”) and shift work sleep disorder. Disturbances in the circadian phase position of plasma melatonin levels have been found in all these disorders. In addition, co-morbility of severe circadian alterations with neurodegenerative diseases like Alzheimer´disease (AD) has been documented. Although further research involving larger number of patients suffering CRSDs is required, currently there is sufficient evidence to implicate endogenous melatonin as an important mediator in CRSD pathophysiology. Melatonin and its analogs can constitute useful therapeutic tools to treat disturbed sleep-wake rhythms in CRSDs. Melatonin secretion decreases in AD patients and its administration improves sleep efficiency, sundowning and cognitive function. This effect can be particularly important in mild cognitive impairment, an etiologically heterogeneous syndrome characterized by cognitive impairment preceding dementia

Melatonin and diet-induced metabolic syndrome in rats. Impact on the hypophysial-testicular axis

Abstract: Combinations of fructose- and fat-rich diets in experimental animals can model the human metabolic syndrome (MS). In rats the increase in blood pressure (BP) after diet manipulation is sex-related and highly dependent on testosterone secretion. However, the extent of diet impact on rodent hypophysial-testicular axis remains undefined. In the present study rats drinking a 10% fructose solution or fed a high fat (35%) diet for 10 weeks had higher plasma levels of luteinizing hormone (LH) and lower plasma levels of testosterone, with absence of significant changes in circulating follicle-stimulating hormone (FSH) or in weight of most reproductive organs. Diet manipulation brought about a significant increase in body weight, systolic BP, area under the curve (AUC) of glycemia after an i.p. glucose tolerance test (IPGTT) and plasma lowdensity lipoprotein-cholesterol, cholesterol, triglycerides and uric acid levels. The concomitant administration of melatonin (25 μg/mL of drinking water) normalized the abnormally high LH levels but did not affect the inhibited testosterone secretion found in fructose- or high fat-fed rats. Rather melatonin per se inhibited testosterone secretion. Melatonin significantly blunted the body weight and systolic BP increase, the increase in the AUC of glycemia after an IPGTT and the changes in circulating lipid profile and uric acid found in both MS models. The results are compatible with a primary inhibition of testicular function in the diet-induced MS in rats and with the partial effectiveness of melatonin to counteract the metabolic but not the testicular sequels of rodent M

Melatonin and mitochondrial dysfunction in the central nervous system

Abstract: Cell death and survival are critical events for neurodegeneration, mitochondria being increasingly seen as important determinants of both. Mitochondrial dysfunction is considered a major causative factor in Alzheimer’s disease (AD), Parkinson’s disease (PD) and Huntington’s disease (HD). Increased free radical generation, enhanced mitochondrial inducible nitric oxide (NO) synthase activity and NO production, and disrupted electron transport system and mitochondrial permeability transition, have all been involved in impaired mitochondrial function. Melatonin, the major secretory product of the pineal gland, is an antioxidant and an effective protector of mitochondrial bioenergetic function. Both in vitro and in vivo, melatonin was effective to prevent oxidative stress/nitrosative stress-induced mitochondrial dysfunction seen in experimental models of AD, PD and HD. These effects are seen at doses 2-3 orders of magnitude higher than those required to affect sleep and circadian rhythms, both conspicuous targets of melatonin action. Melatonin is selectively taken up by mitochondria, a function not shared by other antioxidants. A limited number of clinical studies indicate that melatonin can improve sleep and circadian rhythm disruption in PD and AD patients. More recently, attention has been focused on the development of potent melatonin analogs with prolonged effects which were employed in clinical trials in sleep-disturbed or depressed patients in doses considerably higher than those employed for melatonin. In view that the relative potencies of the analogs are higher than that of the natural compound, clinical trials employing melatonin in the range of 50-100 mg/day are needed to assess its therapeutic validity in neurodegenerative disorders

Disrupted chronobiology of sleep and cytoprotection in obesity : possible therapeutic value of melatonin

Abstract: From a physiological perspective the sleep-wake cycle can be envisioned as a sequence of three physiological states (wakefulness, non-rapid eye movement, NREM, sleep and REM sleep) which are defined by a particular neuroendocrine-immune profile regulating the metabolic balance, body weight and inflammatory responses. Sleep deprivation and circadian disruption in contemporary “24/7 Society” lead to the predominance of pro-orexic and proinflammatory mechanisms that contribute to a pandemic metabolic syndrome (MS) including obesity, diabetes and atherosclerotic disease. Thus, a successful management of MS may require a drug that besides antagonizing the trigger factors of MS could also correct a disturbed sleep-wake rhythm. This review deals with the analysis of the therapeutic validity of melatonin in MS. Melatonin is an effective chronobiotic agent changing the phase and amplitude of the sleep/wake rhythm and having cytoprotective and immunomodulatory properties useful to prevent a number of MS sequels. Several studies support that melatonin can prevent hyperadiposity in animal models of obesity. Melatonin at a low dose (2‐5 mg/day) has been used for improving sleep in patients with insomnia and circadian rhythm sleep disorders. More recently, attention has been focused on the development of potent melatonin analogs with prolonged effects (ramelteon, agomelatine, tasimelteon, TK 301). In clinical trials these analogs were employed in doses considerably higher than those usually employed for melatonin. In view that the relative potencies of the analogs are higher than that of the natural compound, clinical trials employing melatonin doses in the range of 50-100 mg/day are needed to assess its therapeutic value in MS

Melatonin may curtail the metabolic syndrome: studies on initial and fully established fructose-induced metabolic syndrome in rats

Abstract: To examine the effect of melatonin given to rats simultaneously with fructose on initial and fully developed metabolic syndrome, male Wistar rats had free access to chow and 5% or 10% fructose drinking solution for 8 weeks. As compared to controls, systolic blood pressure augmented significantly under both treatments whereas excessive body weight was seen in rats receiving the 10% fructose only. Rats drinking 5% fructose showed a greater tolerance to a glucose load while rats having access to a 10% fructose drinking solution exhibited the expected impaired glucose tolerance found in the metabolic syndrome. Circulating triglyceride and low density lipoproteins-cholesterol (LDL-c) concentration augmented significantly in rats showing a fully developed metabolic syndrome only, while high blood cholesterol levels were found at both stages examined. Melatonin (25 μg/mL drinking solution) counteracted the changes in body weight and systolic blood pressure found in rats administered with fructose. Melatonin decreased the abnormal hyperglycemia seen after a glucose load in 10% fructose-treated rats but it did not modify the greater tolerance to glucose observed in animals drinking 5% fructose. Melatonin also counteracted the changes in plasma LDL-c, triglyceride and cholesterol levels and decreased plasma uric acid levels. The results underline a possible therapeutical role of melatonin in the metabolic syndrome, both at initial and established phase

Effect of a high-fat diet on 24-hour pattern in expression of prolactin and redox pathway enzymes in the rat adenohypophysis

The effect of a high-fat diet (35% fat) on 24-h changes in expression of prolactin and redox pathway enzyme genes was examined in the anterior pituitary of male rats. When body weight of high-fat fed rats attained values about 20- 25 % higher than controls (after 66 days of treatment) animals were sacrificed at 6 different time intervals throughout a 24-h cycle. Anterior pituitary mRNA levels were measured by real-time PCR analysis. In control rats expression of the prolactin gene peaked in anterior pituitary at early scotophase, 3 h in advance to the peak in plasma prolactin levels. In high-fat fed rats this correlation was lost. A disruption of 24-h rhythmicity of pituitary gene expression of heme oxygenase (HO)-2, Cu/Zn- superoxide dismutases (SOD), Mn-SOD, catalase, glutathione peroxidase (GPx) and glutathione reductase (GR) was apparent in high-fat fed rats. Anterior pituitary mRNA levels for nitric oxide synthase (NOS)-2 and HO-2, Cu/Zn- and Mn- SOD and catalase augmented in high-fat fed rats, whereas those of GPx and GR decreased. The results indicate a disrupted coordination between prolactin gene expression and prolactin release in highfat fed rats that comes along with a disturbed expression of redox enzyme genes in the anterior pituitary

Effect of cadmium on 24-hour pattern in expression of redox enzime and clock genes in rat medial basal hypothalamus

The effect of cadmium (Cd) in the brain has been attributed to an increase in reactive oxygen species in cells, particularly when high amounts of the metal are given. In this study we examined the effect of a low dose of Cd (7.5 μg/day) on 24-h changes in expression of redox pathway enzyme and circadian genes in rat medial basal hypothalamus (MBH). Rats receiving CdCl2 (5 ppm in drinking water) or tap water for one month were killed at 6 different time intervals throughout a 24 h cycle. MBH mRNA levels were measured by real-time PCR analysis. In CdCl2 treated rats a disruption of 24-h pattern of hypothalamic gene expression of nitric oxide synthase (NOS)-1 and -2, heme oxygenase (HO)-1 and -2, Mn- superoxide dismutase (SOD), catalase, glutathione peroxidase and glutathione reductase was detectable. Mean levels of MBH mRNA for HO-2, Mn-SOD and catalase augmented after Cd intake, whereas those of NOS-2 decreased. After CdCl2 intake rats the 24-h pattern of clock gene expression in MBH seen in controls was significantly suppressed (Bmal1) or changed in phase (Per1, Per2, Cry2) while in the case of Clock significant 24-h variations were induced. The results are compatible with the view that a low amount of Cd given in tap water brought about significant changes in circadian expression of redox enzyme and clock genes in rat MBH

Melatonin effect on plasma adiponectin, leptin, insulin, glucose, triglycerides and cholesterol in normal and high-fat fed rats

Melatonin effect on body weight progression, mean levels and 24‐hour pattern of circulating adiponectin, leptin, insulin, glucose, triglycerides and cholesterol were examined in rats fed a normal or a high‐fat diet. In experiment 1, rats fed a normal diet were divided into two groups, receiving melatonin (25 μg/mL drinking water) or vehicle for 9 weeks. In experiment 2, animals were divided into 3 groups, two fed with a high‐fat diet (35% fat) and melatonin (25 μg/mL) or vehicle in drinking water for 11 weeks, while a third group was given a normal diet (4% fat). At the end of experiments groups of 8 rats were killed at 6 different time intervals throughout a 24 hour period. Melatonin administration for 9 weeks decreased body weight gain from the 3rd week on without affecting food intake. A significant reduction of circulating insulin, glucose and triglyceride mean levels and disrupted daily patterns of plasma adiponectin, leptin and insulin were observed after melatonin. In high‐fat fed rats, melatonin attenuated body weight increase, hyperglycemia and hyperinsulinemia, as well as the increase in mean plasma adiponectin, leptin, triglycerides and cholesterol levels. The high‐fat diet disrupted normal 24 h patterns of circulating adiponectin, insulin and cholesterol, the effects on insulin and cholesterol being counteracted by melatonin. Nocturnal plasma melatonin concentration in control and obese rats receiving melatonin for 11 weeks attained values 21‐24‐fold greater than controls. The results indicate that melatonin counteracts some of the disrupting effects of diet‐induced obesity in rats