Body weight and food intake at early estrus of rats on a high-fat diet.

Body weight, food intake, and age at vaginal opening and estrus were studied for two groups of weanling rats (age 21 days), fed on high-fat (24.6% by weight) and low-fat (5.0%) diets. Fat was substituted isocalorically for carbohydrate in the high-fat diet. The high-fat rats had estrus at 33.3 +/- 0.8 days, significantly earlier (P less than 0.001) than the age at estrus, 37.4 +/- 0.7 days, of the low-fat rats. Estrus was simultaneous with vaginal opening in 81% of the high-fat rats, in comparison to 48% of the low-fat rats. The caloric intake per 100 g of body weight of the high-fat and low-fat rats did not differ at vaginal opening or at estrus, whereas the two groups differed significantly at both events in age, body weight, absolute food intake (g/day), and relative food intake (g/100 g of body weight per day) and absolute caloric intake (calories/day). Caloric intake/100 g of body weight as a function of chronological age first increased and then decreased steadily before estrus for both high-fat and low-fat rats. The findings support Kennedy's hypothesis that a food intake signal, now further defined as caloric intake/100 g of body weight, is a signal for puberty, and are in accord with the hypothesis that a critical body composition of fatness is essential for estrus in the rat, as in the human female.     

慢性情绪应激对高脂饮食大鼠炎症反应和主动脉Toll样受体4表达的影响

目的通过观察慢性情绪应激对高脂饮食大鼠脂代谢、炎症反应和主动脉内皮细胞TLR4表达的影响,探讨慢性情绪应激在动脉粥样硬化病变形成中的作用、机制。方法雄性Wistar大鼠40只,随机分为正常对照组(NC)、无应激组(NS)、生理应激组(PS)和情绪应激组(ES),每组各10只,并制作慢性情绪应激模型。于末次应激结束后采集血标本,全自动生化分析仪检测血清TC、TG、HDL-C、LDL-C和hs-CRP,ELISA法测定血清ox-LDL、放免法检测TNF-α水平;HE染色观察大鼠主动脉形态学变化;免疫组化法(S-P)测定主动脉内皮细胞TLR4表达。结果 (1)ES组大鼠较其他3组出现了明显的脂代谢紊乱和炎症反应。与PS组、NS组、NC组比较,ES组TC、LDL-C、ox-LDL水平明显升高[TC:(5.30±0.69)mmol/L比(3.94±0.42)mmol/L、(3.82±0.48)mmol/L、(2.07±0.26)mmol/L;LDL-C:(1.57±0.22)mmol/L比(1.18±0.13)mmol/L、(1.11±0.11)mmol/L、(0.75±0.11)mmol/L;ox-LDL:(65.18±6.51)μg/dl比(45.65±2.70)μg/dl、(38.35±2.27)μg/dl、(14.99±1.46)μg/dl,均为P<0.01];ES组HDL-C[(0.94±0.14)mmol/L]低于NS组[(1.09±0.14)mmol/L,P<0.05],低于NC组[(1.16±0.18)mmol/L,P<0.01];与PS组、NS组、NC组比较,ES组hs-CRP、TNF-α水平升高[hs-CRP:(1.748±0.082)mg/L比(1.485±0.067)mg/L、(1.381±0.067)mg/L、(0.757±0.069)mg/L;TNF-α:(2.447±0.083)μg/L比(2.189±0.099)μg/L、(2.181±0.085)μg/L、(1.772±0.075)μg/L,均为P<0.01];(2)ES组大鼠主动脉出现早期动脉粥样硬化性改变;(3)ES组大鼠主动脉内皮细胞TLR4表达明显上调,其阳性细胞单位面积平均吸光度值(A)高于PS组、NS组和NC组[(0.334±0.010)比(0.250±0.012)、(0.238±0.015)、(0.082±0.008),均为P<0.01]。结论慢性情绪应激可能在动脉粥样硬化形成的早期,部分通过激活TLR4释放炎性细胞因子引起机体的炎症反应,进而导致动脉粥样硬化病变形成。     

Skeletal muscle oxidative capacity in rats fed high-fat diet.

OBJECTIVE: To investigate whether young rats respond to high-fat feeding through changes in energy efficiency and fuel partitioning at the level of skeletal muscle, to avoid obesity development. In addition, to establish whether the two mitochondrial subpopulations, which exist in skeletal muscle, ie subsarcolemmal and intermyofibrillar, are differently affected by high-fat feeding. DESIGN: Weaning rats were fed a low-fat or a high-fat diet for 15 days. MEASUREMENTS: Energy balance and lipid partitioning in the whole animal. State 3 and state 4 oxygen consumption rates in whole skeletal muscle homogenate. State 3 and state 4 oxygen consumption rates, membrane potential and uncoupling effect of palmitate in subsarcolemmal and intermyofibrillar mitochondria from skeletal muscle. RESULTS: Rats fed a high-fat diet showed an increased whole body lipid utilization. Skeletal muscle NAD-linked and lipid oxidative capacity significantly increased at the whole-tissue level, due to an increase in lipid oxidative capacity in subsarcolemmal and intermyofibrillar mitochondria and in NAD-linked activity only in intermyofibrillar ones. In addition, rats fed a high-fat diet showed an increase in the uncoupling effect of palmitate in both the mitochondrial populations. CONCLUSIONS: In young rats fed a high-fat diet, skeletal muscle contributes to enhanced whole body lipid oxidation through an increased mitochondrial capacity to use lipids as metabolic fuels, associated with a decrease in energy coupling.     

罗格列酮和二甲双胍对高脂饲养造成的胰岛素抵抗的影响

目的　观察高脂饲养正常大鼠造成胰岛素抵抗 (IR)及不同药物干预的影响。方法将 8周龄雄性SD大鼠随机分为 4组 ,每组各 11只 :正常饲养组 (NC)、高脂饲养组 (HF)、高脂 二甲双胍组 (HF Met)、高脂 罗格列酮组 (HF Ros)。罗格列酮 3mg·kg-1·d-1或二甲双胍 30 0mg·kg-1·d-1剂量进行灌胃。饲养 8周时取空腹血测游离脂肪酸 (FFA)、甘油三酯 (TG)、脂联素等。胰岛素敏感性用正常血糖高胰岛素钳夹技术稳态时的葡萄糖输注率 (GIR)来评价。结果　高脂饲养 8周后 ,与NC组相比 ,HF组呈现肥胖 ,血浆脂联素水平低 4 3 7% (P <0 0 1) ,GIR低 5 1 3% (P <0 0 1)。HF Met组体重和TG均低于HF组 ,分别低 8 4 %和 4 0 5 % ,P值均 <0 0 1,GIR高5 8 9% (P <0 0 1)。HF Ros组FFA和TG均显著低于HF组 ,分别低 2 5 3% (P <0 0 5 )和 5 4 0 %(P <0 0 1) ,血浆脂联素水平高 6 0 % (P <0 0 1) ,GIR高 14 9 6 % (P <0 0 1)。结论　 (1)正常热量高脂饲养正常SD大鼠 8周可引起肥胖 ,血浆脂联素水平下降和胰岛素抵抗。 (2 )罗格列酮和二甲双胍均可显著改善高脂饲养引发的胰岛素抵抗。前者伴有显著血FFA、TG降低和脂联素升高 ,后者伴有体重及血TG降低。     

Effects of four Bifidobacteria on obesity in high-fat diet induced rats

AIM:To compare the effects of four Bifidobacteria strains(Bifidobacteria L66-5,L75-4,M13-4 and FS31-12,originated from normal human intestines) on weight gain,lipid metabolism,glucose metabolism in an obese murine model induced by high-fat diet.METHODS:Forty-eight Sprague-Dawley rats were randomly divided into six groups.Control group received standard chow,model group received high-fat diet,and intervention groups received high-fat diet added with different Bifidobacteria strains isolated from healthy volu...     

Melatonin ameliorates nonalcoholic fatty liver induced by high-fat diet in rats

Nonalcoholic fatty liver disease (NAFLD) is an increasingly recognized condition that may progress to end-stage liver disease, which ranges from simple steatosis to steatohepatitis, advanced fibrosis, and cirrhosis. Oxidative stress and lipid peroxidation are key pathophysiological mechanisms in NAFLD. We investigate the preventive effects of intraperitoneal administration of melatonin (2.5, 5, 10 mg/kg, daily, respectively) in NAFLD rats induced by high-fat diets for 12 wk. Liver damage was evaluated by serological analysis, serum and hepatic lipid assay as well as hematoxylin-eosin staining in liver sections. Oxidative stress and lipid peroxidation were assessed by measuring malondialdehyde (MDA) levels and superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) activities in liver. The results showed that high-fat diet induced oxidative stress with extensive liver steatosis in rats. Melatonin (5 or 10 mg/kg) was effective in reducing hepatic steatosis and inflammation with lowering serum alanine aminotransferase, aspartate aminotransferase, and levels liver total cholesterol and triglycerides in high-fat diet rats. Moreover, melatonin (2.5, 5, 10 mg/kg) increased SOD and GSH-Px activities and the 10 mg/kg dose of melatonin reduced MDA levels in liver. This study shows that melatonin exerts protective effects against fatty liver in rats induced by high-fat diet possibly through its antioxidant actions.     

Sex-specific effects of ketogenic diet after pre-exposure to a high-fat,high-sugar diet in rats

Background and aimsThe objectives were to evaluate the relationship between ketogenic diets, the ketone body beta-hydroxybutyrate (BHB), parameters known to increase risk for cardiovascular and metabolic diseases in both sexes, using a pre-clinical model of obesity.Methods and resultsRats had access to a diet high in fat and sugar (HFS) for 12 weeks. After HFS, they switched to chow (HFS–CH) or ketogenic diet (HFS-KD) for 3 weeks to model a dietary intervention. Body weight, adiposity, and food intake were measured. Glucose tolerance and corticosterone response to stress were measured after HFS, then again after the intervention. Both sexes increased body weight, food intake, and adiposity compared to control (CTL) while on HFS. HFS females showed impaired glucose tolerance. HFS males developed a dampened corticosterone to stress, whereas HFS females developed an exacerbated response. The effects of HFS on adiposity and corticosterone were reversed in HFS–CH males. These same improvements were observed in HFS–CH females, although they still had impaired glucose tolerance. HFS-KD males showed some improvements, however, they still had higher body weight and adiposity than CTL. The same pattern was observed in females. These beneficial effects of KD correlated with plasma BHB levels in females but not in males.ConclusionsThese data model effects reported in clinical literature and serve as a valuable translational tool to further test causal mechanisms that lead to desirable outcomes of KD. These sex-specific relationships are important, as KD could potentially affect endocrine mechanisms differently in males and females.     

非酒精性脂肪性肝病大鼠内生性乙醇和肝组织乙醇代谢相关酶的变化*

目的 观察高脂饮食和高果糖饮食对大鼠肝脏病理、内生性乙醇、乙醇代谢酶的影响。方法 将SD大鼠18只随机分为正常组（n=6）,予以普通饲料喂养,高果糖组（n=6）接受普通饲料和含20%果糖水喂养,高脂组（n=6）接受高脂饲料喂养。造模16 w后,取肝组织进行非酒精性脂肪性肝病活动性评分（NAS）;采用Biovison试剂盒检测门静脉血乙醇水平;采用实时荧光定量PCR法检测大鼠肝组织乙醇脱氢酶（ADH1）和乙醛脱氢酶（ALDH2）mRNA水平;采用Western blot法测定肝组织ADH1和ALDH2蛋白表达水平;使用乙醇脱氢酶活性试剂盒测定肝组织匀浆乙醇脱氢酶活性。结果 在16 w末,高果糖组大鼠肝组织表现为脂肪变性,高脂组大鼠肝组织主要表现为NASH;高脂组大鼠NAS评分为5.40±0.32,显著高于对照组【（1.10±0.25）,P<0.05】和高果糖组【（2.94±0.40）,P<0.05】 ;高脂组大鼠血清内生性乙醇水平【（1.30±0.15）nmol/μL】显著高于正常组【（1.00±0.10）nmol/μL,P<0.05）】和高果糖组【（1.04±0.23）nmol/μL,P<0.05）】;高脂组大鼠ADH1 mRNA水平显著高于正常组（（1.30倍,P<0.05）和高果糖组（1.36倍,P<0.05）;高脂组大鼠ALDH2mRNA水平显著高于正常组（1.55倍,P<0.05）和高果糖组（1.44倍,P<0.05）;高脂组大鼠ADH1蛋白表达显著高于正常组（2.56倍,P<0.05）和高果糖组（2.52倍,P<0.05）;高脂组大鼠ALDH2蛋白表达显著高于正常组（1.41倍,P<0.05）和高果糖组（1.57倍,P<0.05）;高脂组大鼠肝脏乙醇脱氢酶活性为（175±28）μ/L,显著高于正常组【（72±13）μ/L,P<0.05）】和高果糖组【( 78±9)μ/L,P<0.05）】。结论 高脂饮食造模大鼠内生性乙醇浓度显著升高,乙醇脱氢酶和乙醛脱氢酶蛋白水平升高,乙醇脱氢酶活性显著升高。     

Dietary magnesium intake influences exercise capacity and hematologic parameters in rats

The effects of feeding varying concentrations of dietary Mg (50, 100, 200, or 400 ppm) for 22 days on exercise capacity and hematologic parameters were investigated in male rats. Five-week-old male Osborne-Mendel rats fed diets containing 50 or 100 ppm Mg developed transitory priapism and hyperemia, signs of Mg deficiency. Based on a treadmill test, these rats showed a markedly lower exercise endurance capacity (four hours) than rats fed the higher levels of dietary Mg (six hours). They also showed a mild macrocytic anemia. Consumption of a mineral water containing 85 ppm Mg prevented these signs of Mg deficiency. These results show that a reduction in exercise capacity can be an early effect of Mg deficiency. Hematologic changes that occurred with Mg deficiency such as macrocytic anemia may be responsible, at least in part, for the depression of exercise observed in these rats. These data are important because they illustrate the potential significance of even a marginal deprivation of dietary magnesium.     

Decreased carbohydrate intake is more important than increased fat intake in the glucose intolerance by a low-carbohydrate/high-fat diet.

A low-carbohydrate/high-fat evening meal impaired the glucose tolerance of healthy subjects to almost the same extent as the impairment caused by overnight fasting. This suggests that the decreased component (carbohydrate) is more relevant than the increased component (fat) to the impairment of glucose tolerance by a low-carbohydrate/high-fat diet.     

Zinc deficiency and a high-fat diet during growth: Metabolic and adipocyte alterations in rats

Background and aimsTo evaluate the effects of a high-fat diet during post-weaning growth on intermediate metabolism and retroperitoneal adipose tissue, in adult male rats exposed to adequate or deficient zinc intake during prenatal and postnatal life.Methods and resultsFemale Wistar rats were fed low- or control-zinc diets from pregnancy to offspring weaning. Male offspring born from control mothers were fed either control or high-fat, control-zinc diets for 60 days. Male offspring born from zinc deficient mothers were fed either low-zinc or high-fat, low-zinc diets for 60 days. At 74 days of life, oral glucose tolerance test was performed. In 81-day-old offspring, blood pressure, lipid profile, plasmatic lipid peroxidation and serum adiponectin level were determined. In retroperitoneal adipose tissue, we evaluated oxidative stress, morphology and adipocytokines mRNA expression. Low-zinc diet induced adipocytes hypertrophy, increased oxidative stress, and decreased adiponectin mRNA expression in adipose tissue. Low-zinc diet increased systolic blood pressure, triglyceridemia, plasmatic lipid peroxidation and glycemia at 3 h after glucose overload. Animals fed high-fat or high-fat, low-zinc diets showed adipocytes hypertrophy, decreased adiponectin mRNA expression, and increased leptin mRNA expression and oxidative stress in adipose tissue. They also exhibited decreased serum adiponectin levels, increased triglyceridemia, plasmatic lipid peroxidation and area under the oral glucose tolerance curve. High-fat, low-zinc diet induced greater alterations in adipocyte hypertrophy, leptin mRNA expression and glucose tolerance test than high-fat diet.ConclusionZinc deficiency since early stages of intrauterine life could increase susceptibility to metabolic alterations induced by high-fat diets during postnatal life.     

Sexual behavior and some of its physiological consequences in persistently estrous aged female rats

Sexual behavior and some of its consequences were assessed in female rats which, with advancing age, had come to exhibit persistent vaginal estrus (PVE). Sexual receptivity, defined by the animal’s readiness to exhibit lordosis when mounted, was comparable across all ages evaluated. However, sexual proceptivity, defined by measures indicative of the female’s seeking mating opportunities, was determined to be much suppressed in 18 to 20 month old PVE females compared to either younger cyclic females or 12 to 15 month old PVE females. Mating disrupted the PVE pattern. The length of time animals remained out of estrus was positively correlated with the number of intromissions achieved by the male. However, pregnancy was not observed in any of the PVE females and examination of their ovaries and the responses of their uteri to traumatization provided no evidence of progestational responses in these animals. The behavioral data indicate that with continued exposure to the hormonal conditions of PVE there is a suppression of systems mediating the proceptive components of sexual behavior in female rats. It is suggested that changes in the concentrations of pituitary and/or ovarian hormones reported to occur as PVE rats continue to age may account for these changes in behavior.     

高脂喂食对自发性高血压大鼠动脉内皮功能的影响及鱼油的干预作用

目的观察高饱和脂肪酸对自发性高血压大鼠(SHR)血压和内皮功能的影响及鱼油的干预作用。方法选择8周龄雄性SHR 30只,适应性喂养至14周龄,按完全随机分组法分为普食组、高脂组、高脂加鱼油组共3组,每组10只,分别给予普通喂养、高脂喂养、高脂加鱼油喂养,持续喂养至36周龄,共干预22周。同龄WKY大鼠30只作为对照,给予同样分组干预。干预后每2周测定鼠尾血压,喂养结束后测定血浆一氧化氮含量,应用组织多普勒超声法测定中心动脉脉搏波传导速度(APWV)。大鼠处死后,测定动脉环内皮功能,测定一氧化氮含量和内皮型一氧化氮合酶(eNOS)活性,反转录聚合酶链反应法测量eNOS的mRNA表达。结果开始干预后,SHR普食组血压略微上升,高脂组明显上升,高脂加鱼油组呈现下降趋势。至36周龄时,SHR高脂组与SHR普食组比较,APWV明显升高[(16.7±2.0)mm/ms比(10.4±3.1) mm/ms,P<0.05],血浆一氧化氮含量、动脉内皮细胞eNOS活性降低[(32.1±3.3)μmol/g pro比(35.0±2.7)μmol/g pro,(0.672±0.029)U/ml比(0.727±0.030)U/ml,均为P<0.05],最大内皮依赖性血管舒张功能降低(51.61%±4.18%比55.75%±3.56%,P<0.05)。SHR高脂加鱼油组与SHR高脂组比较,APWV降低[(9.4±2.1) mm/ms比(16.7±2.0) mm/ms,P<0.05],血浆一氧化氮含量、动脉内皮细胞eNOS活性升高[(35.2±2.4)μmol/g pro比(32.1±3.3)μmol/g pro,(0.735±0.043)U/ml比(0.672±0.029)U/ml,均为P<0.05],最大内皮依赖性血管舒张功能升高(59.34%±3.75%比51.61%±10.18%,P<0.05)。结论高脂喂食能升高SHR血压、抑制大动脉血管内皮细胞eNOS的活性、降低内皮依赖性血管舒张功能及增加动脉僵硬度,而鱼油可逆转这些作用。     

Oleate metabolism and endogenous triacylglycerol hydrolysis in isolated hepatocytes from rats fed a high-fat diet

In isolated hepatocytes of fat-fed rats, as compared to control fed animals, the cellular uptake of [1-14C] oleate and its oxidation to CO2 were similar but the incorporation of the label into water-soluble products (mainly ketone bodies) was increased by 36.6% whereas its esterification to triacylglycerols and phospholipids decreased by 36%. While endogenous ketogenesis was slightly but not significantly increased, ketone body synthesis from both 2 mM octanoate and 0.7 mM oleate was stimulated two fold. Thus, in the fatfed rats the oxidative pathway is clearly activated whereas long chain fatty acids are preferentially channelled into the oxidation pathway at the expense of esterification. Yet, hepatocyte triacylglycerol content was 3-fold higher after fat-feeding. In this regard, lysosomal triacylglycerol lipase (EC 3.1.1.3) activity, in homogenates of hepatocytes was decreased by 32% (p less than 0.01). This findings suggest a lower breakdown of endogenous triacylglycerols, which, taken together with decreased secretion of VLDL lipoprotein triacylglycerol (Kalopissis et al. Biochem. J. 198: 373, 1981) and an in vivo increased fatty acid influx to the liver may contribute to the accumulation of lipids in the livers of fat-fed rats.     

酒精联合高脂饮食对大鼠胰岛素敏感性的影响及其分子机制

研究酒精和高脂对大鼠胰岛素敏感性的影响.测大鼠血糖、血胰岛素和葡萄糖转运蛋白4(GLUT-4)mRNA及蛋白表达水平.结果 示各剂量组血糖、胰岛素抵抗指数升高(均P<0.05),GLUT-4mRNA及蛋白表达显著降低(均P<0.05).     

替米沙坦对高脂饮食大鼠肥胖和血脂水平的影响

目的 探讨运用替米沙坦阻断肾索-血管紧张素系统(RAS)对大鼠肥胖和血脂水平的影响.方法 40只雄性SD大鼠随机分为对照组、对照治疗组、高脂组、高脂治疗组,分别用正常饮食、正常饮食联合替米沙坦、高脂饮食、高脂饮食联合替米沙坦处理.各组大鼠体质量每周测量一次,喂养10周后取大鼠静脉血检测血脂,测量内脏脂肪质量,计算Lee's指数.结果 高脂饮食组大鼠平均体质鼍随时间变化的趋势显著高于对照组和高脂治疗组(P=0.046,P=0.035).高脂治疗组大鼠平均体质量的增加和平均内脏脂肪质量均显著低于高脂组(P=0.012,P=0.024).高脂治疗组大鼠平均甘油三酯(TG)水平显著低于高脂组(P=0.002),但平均高密度脂蛋白胆固醇(HDL-c)水平显著高于高脂组(P=0.023).结论 替米沙坦可以抑制大鼠高脂饮食引起的肥胖,同时改善高脂饮食引起的血脂紊乱.     

高脂饮食对老年大鼠肝脏脂肪酸代谢及胰岛素敏感性的影响

目的 探讨增龄和高脂饮食对大鼠肝脏脂肪酸代谢及胰岛素敏感性的影响,了解老年大鼠胰岛素抵抗(IR)的发病机制. 方法 将22～24月龄雄性Wistar大鼠随机分为老年对照组和高脂组;4～5月龄大鼠作为青年对照组.老年对照组和青年对照组给予基础饲料,高脂组给予高脂饲料,喂养8周.用高胰岛素-正葡萄糖钳夹实验评价各组大鼠胰岛素敏感性;肝脏三酰甘油经氯仿/甲醇抽提后用全自动生化分析仪测定. 结果 (1)老年对照组空腹血糖、胰岛素和游离脂肪酸均高于青年对照组,高脂组进一步升高,且血清三酰甘油和总胆同醇水平增高;(2)葡萄糖输注率老年对照组[(23.80±2.79)mU·kg-1·min-1]较青年对照组[(30.08±3.89)mU·kg-1·min-1]低,高脂组((18.83±2.18)mU·kg-1·min-1]最低,差异有统计学意义(P<0.01);高脂组8周末较4周末低;(3)肝脏三酰甘油老年对照组较青年对照组升高,分别为(16.6±4.8)μmol/g和(9.6±2.2)μmol/g,高脂组(24.7±6.6)μmol/g较老年对照组进一步升高(P<0.01);在老年组中,肝脏三酰甘油与葡萄糖输注率呈负相关.与空腹血糖呈正相关. 结论 与青年对照组比较,老年对照组更易出现脂肪酸代谢异常及IR;高脂饮食导致老年大鼠肝脏脂质积聚更加严重,进一步加重IR;肝脏脂质堆积可能参与了与增龄和高脂饮食相关IR的发生.     

Pineal melatonin synthesis and release are not altered throughout the estrous cycle in female rats

Melatonin times reproduction with seasons in many photoperiodic mammalian species. Whether sexual hormones reflect on melatonin synthesis is still debated. The aim of this work was to study, using a large panel of technical approaches, whether the daily profile of pineal melatonin synthesis and release varies with the estrous cycle in the female rat. The mRNA levels and enzyme activities of the melatonin synthesizing enzymes, arylalkylamine N-acetyltransferase and hydroxyindole-O-methyltransferase were similar at the four stages of the rat estrous cycle. The endogenous release of melatonin, followed by transpineal microdialysis during six consecutive days in cycling female rats, displayed no significant variation during this interval. Taken together, the present results demonstrate that there is no regular fluctuation in the pineal metabolism leading to melatonin synthesis and release throughout the estrous cycle in female rats.