Circadian rhythm of corticosterone in diabetic rats

Circadian clocks and metabolism

Circadian clocks maintain periodicity in internal cycles of behavior, physiology, and metabolism, enabling organisms to anticipate the 24-h rotation of the Earth. In mammals, circadian integration of metabolic systems optimizes energy harvesting and utilization across the light/dark cycle. Disruption of clock genes has recently been linked to sleep disorders and to the development of cardiometabolic disease. Conversely, aberrant nutrient signaling affects circadian rhythms of behavior. This chapter reviews the emerging relationship between the molecular clock and metabolic systems and examines evidence that circadian disruption exerts deleterious consequences on human health.

Circadian rhythms and obesity in mammals

Obesity has become a serious public health problem and a major risk factor for the development of illnesses, such as insulin resistance and hypertension. Attempts to understand the causes of obesity and develop new therapeutic strategies have mostly focused on caloric intake and energy expenditure. Recent studies have shown that the circadian clock controls energy homeostasis by regulating the circadian expression and/or activity of enzymes, hormones, and transport systems involved in metabolism. Moreover, disruption of circadian rhythms leads to obesity and metabolic disorders. Therefore, it is plausible that resetting of the circadian clock can be used as a new approach to attenuate obesity. Feeding regimens, such as restricted feeding (RF), calorie restriction (CR), and intermittent fasting (IF), provide a time cue and reset the circadian clock and lead to better health. In contrast, high-fat (HF) diet leads to disrupted circadian expression of metabolic factors and obesity. This paper focuses on circadian rhythms and their link to obesity.

Diabetes as a chronic metabolic stressor: causes, consequences and clinical complications

Diabetes mellitus is an endocrine disorder resulting from inadequate insulin release and/or reduced insulin sensitivity. The complications of diabetes are well characterized in peripheral tissues, but there is a growing appreciation that the complications of diabetes extend to the central nervous system (CNS). One of the potential neurological complications of diabetes is cognitive deficits. Interestingly, the structural, electrophysiological, neurochemical and anatomical underpinnings responsible for cognitive deficits in diabetes are strikingly similar to those observed in animals subjected to chronic stress, as well as in patients with stress-related psychiatric illnesses such as major depressive disorder. Since diabetes is a chronic metabolic stressor, this has led to the suggestion that common mechanistic mediators are responsible for neuroplasticity deficits in both diabetes and depression. Moreover, these common mechanistic mediators may be responsible for the increase in the risk of depressive illness in diabetes patients. In view of these observations, the aims of this review are (1) to describe the neuroplasticity deficits observed in diabetic rodents and patients; (2) to summarize the similarities in the clinical and preclinical studies of depression and diabetes; and (3) to highlight the diabetes-induced neuroplasticity deficits in those brain regions that have been implicated as important pathological centers in depressive illness, namely, the hippocampus, the amygdala and the prefrontal cortex.

The circadian clock and metabolism

Mammals have developed an endogenous circadian clock located in the SCN (suprachiasmatic nuclei) of the anterior hypothalamus that responds to the environmental light–dark cycle. Human homoeostatic systems have adapted to daily changes in a way that the body anticipates the sleep and activity periods. Similar clocks have been found in peripheral tissues, such as the liver, intestine and adipose tissue. Recently it has been found that the circadian clock regulates cellular and physiological functions in addition to the expression and/or activity of enzymes and hormones involved in metabolism. In turn, key metabolic enzymes and transcription activators interact with and affect the core clock mechanism. Animals with mutations in clock genes that disrupt cellular rhythmicity have provided evidence to the relationship between the circadian clock and metabolic homoeostasis. The present review will summarize recent findings concerning the relationship between metabolism and circadian rhythms.

Metabolism and circadian rhythms--implications for obesity

Obesity has become a serious public health problem and a major risk factor for the development of illnesses, such as insulin resistance and hypertension. Human homeostatic systems have adapted to daily changes in light and dark in a way that the body anticipates the sleep and activity periods. Mammals have developed an endogenous circadian clock located in the suprachiasmatic nuclei of the anterior hypothalamus that responds to the environmental light-dark cycle. Similar clocks have been found in peripheral tissues, such as the liver, intestine, and adipose tissue, regulating cellular and physiological functions. The circadian clock has been reported to regulate metabolism and energy homeostasis in the liver and other peripheral tissues. This is achieved by mediating the expression and/or activity of certain metabolic enzymes and transport systems. In return, key metabolic enzymes and transcription activators interact with and affect the core clock mechanism. In addition, the core clock mechanism has been shown to be linked with lipogenic and adipogenic pathways. Animals with mutations in clock genes that disrupt cellular rhythmicity have provided evidence for the relationship between the circadian clock and metabolic homeostasis. In addition, clinical studies in shift workers and obese patients accentuate the link between the circadian clock and metabolism. This review will focus on the interconnection between the circadian clock and metabolism, with implications for obesity and how the circadian clock is influenced by hormones, nutrients, and timed meals.

An association between stress-induced disruption of the hypothalamic-pituitary-adrenal axis and disordered glucose metabolism in an animal model of post-traumatic stress disorder

Retrospective clinical reports suggesting that traumatic stress populations display an increased propensity for glucose metabolism disorders were examined in a controlled prospective animal model. Stress-induced behavioural and hypothalamic-pituitary-adrenal (HPA) axis response patterns were correlated to central and peripheral parameters of glucose metabolism and signalling, and to body measurements in Sprague-Dawley rats exposed to predator scent stress. Forty days post-exposure, fasting blood glucose and insulin levels, oral glucose tolerance test, body weight and white adipose tissue mass, systemic corticosterone levels and brain expression of insulin receptor (IR) and insulin-sensitive glucose transporter 4 (GLUT4) protein levels were evaluated. In a second experiment inbred strains with hyper- (Fischer) and hypo- (Lewis) reactive HPA axes were employed to assess the association of metabolic data with behavioural phenomenology versus HPA axis response profile. For data analysis, animals were classified according to their individual behavioural response patterns (assessed at day 7) into extreme, partial and minimal response groups. The exposed Sprague-Dawley rats fulfilling criteria for extreme behavioural response (EBR) (20.55%) also exhibited significant increases in body weight, abdominal circumference and abdominal white adipose tissue mass; a hyperglycaemic oral glucose tolerance test; and fasting hyperglycaemia, hyperinsulinaemia and hypercorticosteronemia, whereas minimal responders (MBR) and control animals displayed no such disturbances. Hippocampal and hypothalamic expression of IR and GLUT4 protein were significantly lower in EBR than in MBR and control rats. The inbred strains showed no metabolic differences at baseline. Exposed Fischer rats displayed hyperglycaemia and hyperinsulinaemia, whereas Lewis rats did not. A significant protracted disorder of glucose metabolism was induced by exposure to a stress paradigm. This metabolic response was associated with the characteristic pattern of HPA axis (corticosterone) response, which underlies the behavioural response to stress.

Clock genes and metabolic disease

The circadian system is a key integrator of behavior and metabolism that synchronizes physiological processes with the rotation of the Earth on its axis. In mammals, the clock is present not only within the central pacemaker neurons of the hypothalamus, but also within extra-suprachiasmatic nucleus (SCN) regions of brain and nearly all peripheral tissues. Recent evidence suggests that the complex feedback networks that encompass both the circadian and metabolic systems are intimately intertwined and that disruption of either system leads to reciprocal disturbances in the other. We anticipate that improved understanding of the interconnections between the circadian and metabolic networks will open new windows on the treatment of sleep and metabolic disorders, including diabetes mellitus and obesity.

Treadmill running improves long-term potentiation (LTP) defects in streptozotocin-induced diabetes at dentate gyrus in rats

OBJECTIVES

It has been demonstrated that exercise has neuroprotective effects in the central nervous system (CNS), especially in hippocampus. Previous studies have indicated that diabetes mellitus affects synaptic plasticity in the hippocampus leading to impairments in learning and memory. The aim of this study was to evaluate the effects of treadmill running on synaptic plasticity at dentate gyrus (DG) of streptozotocin-induced diabetic rats.

STUDY DESIGN

Experimental groups were the control, the diabetes and the diabetes-exercise groups. Long-term potentiation (LTP) in perforant path-DG synapses was assessed (by 400Hz tetanization) in order to investigate the effect of exercise on synaptic plasticity. Field excitatory post-synaptic potential (fEPSP) slope and population spike (PS) amplitude were measured.

RESULTS

With respect to the control group, fEPSP were significantly decreased in the diabetes group. However, there were no differences between responses of the diabetes-exercise group and the control.

CONCLUSION

The present results suggest that LTP induction in the dentate gyrus is affected under diabetic conditions and that treadmill running prevents these effects. The data suggest that treadmill running protect against diabetes-induced decrease of learning ability and memory function of the hippocampus.

The As and Ds of stress: metabolic, morphological and behavioral consequences

Unlike responses to acute stressful events that are protective and adaptive in nature, chronic stress elicits neurochemical, neuroanatomical and cellular changes that may have deleterious consequences upon higher brain functioning. For example, while exposure to acute stress facilitates memory formation and consolidation, chronic stress or chronic exposure to stress levels of glucocorticoids impairs cognitive performance. Chronic stress or glucocorticoid exposure, as well as impairments in hypothalamic-pituitary-adrenal (HPA) axis function are proposed to participate in the etiology and progression of neurological disorders such as depressive illness, anxiety disorders and post-traumatic stress disorder (PTSD). HPA axis dysfunction, impaired stress responses and elevated basal levels of glucocorticoids are also hallmark features of experimental models of type 1 and type 2 diabetes, as well as diabetic subjects in poor glycemic control. Such results suggest that stress and glucocorticoids contribute to the neurological complications observed in diabetes patients. Interestingly, many of the hyperglycemia mediated changes in the brain are similar to those observed in depressive illness patients and in experimental models of chronic stress. Such results suggest that common mechanisms may be involved in the development of the neurological complications associated with Anxiety, Depressive illness and Diabetes: the As and Ds of stress. The aim of the current review will be to discuss the mechanisms through which limbic structures such as the hippocampus and amygdala respond and adapt to the deleterious consequences of chronic stress and hyperglycemia.

The clockwork of metabolism

The observation that cycles of sleep and wakefulness occur with a periodicity fixed in time to match the rotation of the Earth on its axis provided a key to unlock the first genetic code for a neurobehavioral pathway in flies and ultimately in mice. As a remarkable outcome of this discovery, we have gained an unprecedented view of the conserved genetic program that encodes a sense of time across all kingdoms of life. The tools are now in hand to begin to understand how important processes such as energy homeostasis and fuel utilization are coordinated to anticipate daily changes in environment caused by the rising and setting of the sun. A better understanding of the impact of circadian gene networks on nutrient balance at the molecular, cellular, and system levels promises to shed light on the emerging association between disorders of diabetes, obesity, sleep, and circadian timing.

Insulin signaling effects on memory and mood

The escalating obesity/diabetes epidemic is an important health-care issue that has crucial socio-economic ramifications. The complications of diabetes/obesity phenotypes extend to the central nervous system (CNS), including the hippocampus, a brain region that is particularly vulnerable to hyperglycemia and insulin resistance. Deficits in hippocampal synaptic plasticity observed in diabetes ultimately have deleterious consequences upon cognitive function. For example, recent studies using brain imaging technologies have identified cerebral atrophy in diabetic patients, suggesting that the neuroanatomical changes observed in experimental models of diabetes may accurately reflect what is occurring in the clinical setting. Deficits in insulin receptor (IR) signaling and impairments in hypothalamic-pituitary-adrenal (HPA) axis function also contribute to the neurological complications of diabetes phenotypes. The pathophysiological similarities between diabetes and stress-related mood disorders suggest that common mechanistic mediators may be involved in the etiology and progression of the neurological complications of these disorders. When combined with the accumulating evidence from pre-clinical models, these data support the hypothesis that a long-term consequence of diabetes/obesity phenotypes is accelerated brain aging that results in neuropsychological deficits and increased vulnerability to co-morbidities such as depressive illness.

Chronic restraint stress decreases the expression of glutathione S-transferase pi2 in the mouse hippocampus

Chronic restraint stress in mice affects hippocampal structure and function. Mice were subjected to daily restraint for 3 weeks, and gene expression in hippocampus was compared to controls using large-scale cDNA microarrays. We found that 444 genes were differentially expressed, and further analysis of 6 genes by real-time reverse transcription PCR confirmed that 3 of them were downregulated by stress. These 3 genes, growth factor receptor-bound protein 2 (Grb2), phosphatidylinositol-4-phosphate 5-kinase, type 1 beta (Pip5k1b), and glutathione S-transferase, pi2 (Gstp2), were also analyzed by in situ hybridization. The downregulation of Gstp2 may induce an increase of oxidative damage in the pyramidal cells of the CA1 and CA3 regions and granular layer of the dentate gyrus, leading to structural and functional damage. Those regions are affected by stress, and our results could help understand further the mechanisms involved in the occurrence of stress-related disorders.

Neuronal insulin signal transduction mechanisms in diabetes phenotypes

The hippocampus is an important integration center for learning and memory in the mammalian central nervous system (CNS) and is particularly sensitive and responsive to changes in insulin and glucose concentrations. Insulin administration improves cognitive performance in a variety of physiological and pathophysiological settings, including diabetes phenotypes. Our previous studies demonstrated that hyperglycemia produces behavioral, neuroanatomical and neurochemical changes in the adult rat hippocampus that are indicative of accelerated brain aging. In addition, the trafficking of insulin-sensitive glucose transporters (GLUTs) is impaired in experimental models of diabetes. Such results suggest that insulin receptor (IR) signaling may be disrupted in diabetes phenotypes, although the signaling mechanisms utilized by neurons are not clearly defined. To this end, we have employed in vivo and in vitro approaches to determine the insulin signaling pathways utilized by neurons. These methodologies provide insight into the signaling mechanisms utilized by neuronal IRs and ultimately will allow for determination of the IR signaling deficits that may contribute to accelerated brain aging in the hippocampus of diabetic subjects.

Immunocytochemical analysis of synaptic proteins provides new insights into diabetes-mediated plasticity in the rat hippocampus

The hippocampus, an important integration center for learning and memory in the mammalian brain, undergoes neurological changes in response to a variety of stimuli that are suggestive of ongoing synaptic reorganization. Accordingly, the aim of this study was to identify markers of synaptic plasticity using rapid and reliable techniques such as radioimmunocytochemistry and confocal microscopy, thereby providing a "birds-eye view" of the whole hippocampus under hypercorticosteronemic conditions. The regulation of microtubule-associated protein 2, synaptophysin and postsynaptic density-95 was examined in two different animal models of hypercorticosteronemia: corticosterone administration and streptozotocin-induced diabetes using both a short-term (1 week) and long-term (5 weeks) treatment. Glucocorticoids and/or hyperglycemia increased synaptophysin expression in CA1, CA3 and the dentate gyrus, regions that exhibit synaptic plasticity in response to glucocorticoid exposure. In these models, postsynaptic density-95 expression increased in the CA3 region, particularly in the diabetic rats, while microtubule-associated protein 2 exhibited more selective changes. Fluoro-Jade histochemistry did not detect neuronal damage, suggesting that glucocorticoids and/or hyperglycemia induce plastic and not irreversible neuronal changes at these time points. Collectively, these results demonstrate that changes in the expression and distribution of synaptic proteins provide another measure of synaptic plasticity in the rat hippocampus in response to glucocorticoid exposure, changes that may accompany or contribute to neuroanatomical, neurochemical, and behavioral changes observed in experimental models of type 1 diabetes.

Partial leptin restoration increases hypothalamic-pituitary-adrenal activity while diminishing weight loss and hyperphagia in streptozotocin diabetic rats

Chronic leptin administration at pharmacologic doses normalizes food intake and body weight in streptozotocin (STZ)-diabetic rats. We examined the metabolic effects of acute partial physiological leptin restoration in STZ-diabetic rats by using subcutaneous osmotic mini pumps. Groups: (1) Rats infused with vehicle (DV); (2) rats infused with recombinant murine methionine leptin (DL) at 4.5 microg . (kg body weight . d)(-1); (3)pair-fed rats (DP) given a food ration matching that consumed by the DL group. A fourth group of nondiabetic, normal (N) rats was also studied to assess normal metabolic efficiency, hypothalamic-pituitary-adrenal (HPA) activity and sympathoadrenal activity. Following leptin infusion, food consumption by DL rats was significantly lower than in DV rats. Paradoxically, despite a similar food intake to that of the DP group, which demonstrated a 40% reduction in body mass, DL rats increased their initial body weight by approximately 20% (P < .05). Plasma corticosterone and ACTH concentrations were elevated by 2-fold to 3-fold in DL versus N, DP, and DV rats. In the pars distalis, glucocorticoid receptor (GR) mRNA levels were significantly higher in DL and DP rats compared with N and DV rats. Our results suggest that partial restoration of physiologic leptin: (1) successfully reduces hyperphagia while allowing body weight gain in STZ-diabetic rats; (2) increases corticosterone levels in STZ-diabetic rats, which may in turn counteract the anorexic effects of diabetes; and (3) is associated with increased pituitary GR mRNA levels, despite elevated corticosterone levels, suggesting that leptin may interfere with the negative feedback regulation of the HPA axis.

Circadian regulation of islet genes involved in insulin production and secretion

Both transcription factors albumin site d-binding protein (DBP) and thyrotroph embryonic factor (TEF) are elements of the "cell-clock". Their circadian accumulation in suprachiasmatic nucleus (SCN) and peripheral tissues such as liver, kidney and lung is thought to participate in controlling circadian regulation of downstream genes. TEF and DBP control elements have never been investigated in the insulin-secreting cells, but impairment of the circadian rhythm of the beta-cells might be involved in the development of diabetic state as type 2 diabetics have lost daily temporal variations of insulin secretion. We investigated the expression pattern of TEF and DBP in insulin-secreting cells. TEF and DBP transcripts are expressed at extremely high levels in human pancreatic islets compared to other tissues, suggesting a potentially important circadian regulation of these cells. Both TEF and DPB accumulate in a circadian way in insulin-secreting cells after a serum shock known to restore circadian rhythms in cultured cells. In addition, the expression of islet-specific genes involved in glucose sensing (glucose transporter 2 (Glut2), glucokinase), insulin production (insulin) and secretion (migration inhibitory factor (MIF), somatostatin and syntaxin 1A) were modulated in the same daily rhythm as well. The circadian deregulation of these genes could therefore participate in the diabetic state development.

Biphasic effects of stress upon GLUT8 glucose transporter expression and trafficking in the diabetic rat hippocampus

Disease states such as diabetes mellitus are known to impair hippocampal glucoregulatory activities, which may contribute to cognitive deficits observed in diabetic subjects. Stress or exposure to stress levels of glucocorticoids (GCs) are also intimately involved in hippocampal glucoregulatory activities and the actions of GCs are often most evident in hyperglycemic states. Glucose transporter (GLUT) expression, activity and translocation represent components of the glucoregulatory activities of the hippocampus that may be disrupted by diabetes and stress. Accordingly, the current study examined the effects of stress, streptozotocin (STZ)-induced diabetes and the combined actions of stress and hyperglycemia upon GLUT8 mRNA expression, protein levels and intracellular trafficking in the rat hippocampus. Short-term stress in euglycemic rats had no effect upon GLUT8 mRNA, while restraint stress normalized diabetes mediated increases in GLUT8 mRNA expression in STZ treated rats. Radioimmunocytochemical analysis revealed that total GLUT8 protein levels were not altered by diabetes, short-term stress or the combined actions of hyperglycemia and stress. However, subcellular compartmentalization of GLUT8 was modulated by stress in that hippocampal GLUT8 protein levels were increased in high-density microsomal (HDM) fractions isolated from rats subjected to stress. In contrast, STZ-diabetes decreased GLUT8 protein levels in the HDM fraction, an effect that was potentiated by stress. Collectively, these results demonstrate that the actions of GCs may be dramatically different in euglycemic and hyperglycemic/insulinopenic states, suggesting that stress may increase hippocampal neuronal responsiveness under normal physiological conditions while increasing hippocampal neuronal vulnerability in pathophysiological settings such as in type 1 diabetes.

Role of 5-ht1b receptors in entrainment disorder of otsuka long evans tokushima fatty (oletf) rats

The role of 5-HT1A and 5-HT1B receptors in entrainment function was studied in Otsuka Long Evans Tokushima fatty (OLETF) rats and control Long Evans Tokushima Otsuka (LETO) rats. Light-induced (100 lux, 30 min) Fos expression in the suprachiasmatic nucleus was studied. Light-induced Fos expression was significantly decreased in OLETF rats compared to that in LETO rats. The decrease of light-induced Fos expression in OLETF rats was significantly reversed by pretreatment with the 5-HT1B receptor antagonist, isamoltan (3 mg/kg, i.p.). Simultaneous administration of CGS12066B (5 mg/kg, i.p.), a 5-HT1B agonist, blocked the reversal effect of isamoltan on Fos expression. Fos expression was not changed in LETO rats by pretreatment with isamoltan (3 mg/kg, i.p.). The Fos expression in LETO and OLETF rats was significantly decreased by pretreatment with the 5-HT1A antagonist, WAY-100,635. Phase shifts in locomotor activity paralleled the Fos expression. Light-induced phase shifts of locomotor activity in OLETF rats were significantly smaller than those in LETO rats. The phase shifts were significantly increased by isamoltan (3 mg/kg, i.p.) in OLETF rats. These results suggest that 5-HT1B receptors are involved in the reduced entrainment function of OLETF rats.

Region specific increases in oxidative stress and superoxide dismutase in the hippocampus of diabetic rats subjected to stress

Oxidative stress and modulation of anti-oxidant enzymes may contribute to the deleterious consequences of diabetes mellitus and to the effects of chronic (i.e. 21 day) stress in the CNS. We therefore compared the effects of short- and long-term exposure to diabetes-induced hyperglycemia, restraint stress and the combined effects of restraint stress and diabetes upon parameters of oxidative stress in the rat hippocampus. Whereas 7 days of restraint stress or hyperglycemia, or the combination, produced similar increases in oxidative stress markers 4-hydroxy-2-nonenal (HNE) and malondialdehyde (MDA) throughout the hippocampus, 21 days of stress or hyperglycemia did not increase these markers in the dentate gyrus. In contrast, Ammon's horn still showed elevated levels of these lipid peroxidation products, especially in diabetic rats subjected to 21 days of restraint stress. The expression of two anti-oxidant enzymes, copper/zinc superoxide dismutase (Cu/Zn-SOD) and manganese SOD, was also differentially regulated by stress and hyperglycemia in a time- and region-specific manner in the rat hippocampus. Although long-term stress decreased both SOD isoforms, diabetes increased Cu/Zn-SOD expression in DG with or without 21 days of repeated stress. These increases may account for the finding that protein-conjugated HNE and MDA levels returned to control levels between 7 days and 21 days of hyperglycemia or the combination of diabetes and stress. These results suggest that while other anti-oxidant pathways may account for decreases in oxidative stress in the long-term stress paradigm, increases in Cu/Zn-SOD expression may contribute to the region-specific attenuation of oxidative stress in the diabetic rat hippocampus.

Diabetes, but not stress, reduces neuronal nitric oxide synthase expression in rat hippocampus: implications for hippocampal synaptic plasticity

Neuronal nitric oxide synthase (nNOS) plays an important role in synaptic plasticity and learning and memory. Since deficits in long-term potentiation (LTP) and learning are observed in diabetic rats and following stress, we examined the expression of nNOS mRNA and protein in the hippocampus of streptozotocin (STZ) diabetic rats and rats subjected to restraint stress. Stress did not modulate nNOS expression, while nNOS mRNA and protein levels were significantly decreased in the hippocampus of STZ diabetic rats. These results suggest that: (1) decreased expression of nNOS mRNA and protein may contribute to deficits in hippocampal dependent learning and LTP in diabetic rats; and (2) other mechanisms may be involved in stress mediated decreases in hippocampal synaptic plasticity.

Experimental diabetes in rats causes hippocampal dendritic and synaptic reorganization and increased glucocorticoid reactivity to stress

We report that 9 d of uncontrolled experimental diabetes induced by streptozotocin (STZ) in rats is an endogenous chronic stressor that produces retraction and simplification of apical dendrites of hippocampal CA3 pyramidal neurons, an effect also observed in nondiabetic rats after 21 d of repeated restraint stress or chronic corticosterone (Cort) treatment. Diabetes also induces morphological changes in the presynaptic mossy fiber terminals (MFT) that form excitatory synaptic contacts with the proximal CA3 apical dendrites. One effect, synaptic vesicle depletion, occurs in diabetes as well as after repeated stress and Cort treatment. However, diabetes produced other MFT structural changes that differ qualitatively and quantitatively from other treatments. Furthermore, whereas 7 d of repeated stress was insufficient to produce dendritic or synaptic remodeling in nondiabetic rats, it potentiated both dendritic atrophy and MFT synaptic vesicle depletion in STZ rats. These changes occurred in concert with adrenal hypertrophy and elevated basal Cort release as well as hypersensitivity and defective shutoff of Cort secretion after stress. Thus, as an endogenous stressor, STZ diabetes not only accelerates the effects of exogenous stress to alter hippocampal morphology; it also produces structural changes that overlap only partially with those produced by stress and Cort in the nondiabetic state.

Induction of rat hepatic aryl sulfotransferase (SULT1A1) gene expression by triamcinolone acetonide: impact on minoxidil-mediated hypotension

The hypotensive agent minoxidil (6-imino-1, 2-dihydro-1-hydroxy-2-imino-4-piperidinopyrimidine) depends upon aryl sulfotransferase (SULT1)-catalyzed sulfation for its bioactivation. Previous reports suggest that glucocorticoids induce class-specific SULT1 and isoform-specific SULT1A1 gene expression in rat liver. In the present study, rats were treated with the glucocorticoid triamcinolone acetonide (TA, 5 mg/kg/day i.p. x 3 days) or its vehicle, 2% Tween-20, prior to minoxidil, and subsequent effects on mean arterial pressure (MAP), heart rate (HR), and hepatic SULT1 gene expression were characterized. Minoxidil treatment (1.5 mg/kg) resulted in a steady decline in MAP values of 16.3 to 18.6% relative to basal control levels at 35 to 60 min following minoxidil injection. Pentachlorophenol (PCP, 40 micromol/kg i.p.), an inhibitor of SULT1 enzyme activity, effectively ablated the hypotensive effects of minoxidil. By contrast, pretreatment with TA significantly enhanced minoxidil-induced hypotension. Relative to vehicle-treated controls, TA-treated rats displayed a steeper rate of decline in MAP and more profound levels of hypotension with decreases in MAP following minoxidil administration of 27.8%. TA also produced significant increases in hepatic SULT1 mRNA expression (of 271%) and SULT1A1 immunoreactive protein levels (of 273%), relative to vehicle-treated controls. These results provide physiological evidence to support the biological relevance of SULT1A1 induction by glucocorticoids. The data indicate that steroid treatment induces SULT1A1 gene expression and, as a consequence, accentuates the hypotensive effects of minoxidil.

Oxidative stress and HNE conjugation of GLUT3 are increased in the hippocampus of diabetic rats subjected to stress

Recent studies demonstrate that cellular, molecular and morphological changes induced by stress in rats are accelerated when there is a pre-existing strain upon their already compromised adaptive responses to internal or external stimuli, such as may occur with uncontrolled diabetes mellitus. The deleterious actions of diabetes and stress may increase oxidative stress in the brain, leading to increases in neuronal vulnerability. In an attempt to determine if stress, diabetes or stress+diabetes increases oxidative stress in the hippocampus, radioimmunocytochemistry was performed using polyclonal antisera that recognize proteins conjugated by the lipid peroxidation product 4-hydroxy-2-nonenal (HNE). Radioimmunocytochemistry revealed that HNE protein conjugation is increased in all subregions of the hippocampus of streptozotocin (STZ) diabetic rats, rats subjected to restraint stress and STZ diabetic rats subjected to stress. Such increases were not significant in the cortex. Because increases in oxidative stress may contribute to stress- and diabetes-mediated decreases in hippocampal neuronal glucose utilization, we examined the stress/diabetes mediated HNE protein conjugation of the neuron specific glucose transporter, GLUT3. GLUT3 immunoprecipitated from hippocampal membranes of diabetic rats subjected to stress exhibited significant increases in HNE immunolabeling compared to control rats, suggesting that HNE protein conjugation of GLUT3 contributes to decreases in neuronal glucose utilization observed during diabetes and exposure to stress. Collectively, these results demonstrate that the hippocampus is vulnerable to increases in oxidative stress produced by diabetes and stress. In addition, increases in HNE protein conjugation of GLUT3 provide a potential mechanism for stress- and diabetes-mediated decreases in hippocampal neuronal glucose utilization.

Altered circadian responses to light in streptozotocin-induced diabetic mice

Diabetes mellitus affects the daily expression of many behavioral and metabolic processes. Recent studies indicate that changes in brain glucose metabolism alter the entraining effects of light of the circadian pacemaker. To test whether diabetes-associated diurnal changes are related to alterations in the responses of the circadian pacemaker to light, photic phase resetting of the circadian rhythm of locomotor activity was analyzed in diabetic mice housed in constant darkness. Multiple low doses of streptozotocin, which damages pancreatic beta-insulin-producing cells, were used to render C57BL/6J mice mildly diabetic. In those mice treated with streptozotocin, serum glucose was increased by 25% and circadian responses to light either were increased by 40% for phase delays or were close to those observed in control animals for phase advances. Furthermore, insulin-induced hypoglycemia normalized light-induced phase delays in diabetic animals, without altering those in nondiabetic mice. These results show that abnormalities of daily temporal organization associated with diabetes can result from altered circadian responses to the daily variation in ambient light. Such alterations could be normalized with appropriate insulin therapy.

Lowered entrainment function in the suprachiasmatic nucleus of Otsuka Long Evans Tokushima Fatty (OLETF) rats

The entrainment function in the suprachiasmatic nucleus (SCN) of young non-diabetic Otsuka Long Evans Tokushima Fatty (OLETF) rats was studied. OLETF rats significantly needed more days for re-entrainment to a new light-dark cycle than control Long Evans Tokushima Otsuka (LETO) rats. We also assessed Fos expression in the SCN induced by dim light exposure. The number of Fos-immunoreactive cells was significantly decreased in 5- to 13-week-old OLETF rats compared with LETO rats. Moreover, the effect of glutamate on neuronal activity in the SCN of OLETF rats were investigated. In young non-diabetic OLETF rats, the phase delay in the SCN neuronal firing rhythm induced by 1 microM glutamate was significantly less than that in LETO rats. These results suggested that the entrainment function is reduced in OLETF rats before the onset of diabetes.

Regulation of GLUT-3 glucose transporter in the hippocampus of diabetic rats subjected to stress

Previous studies from our laboratory have demonstrated that chronic stress produces molecular, morphological, and ultrastructural changes in the rat hippocampus that are accompanied by cognitive deficits. Glucocorticoid attenuation of glucose utilization is proposed to be one of the causative factors involved in stress-induced changes in the hippocampus, producing an energy-compromised environment that may make hippocampal neuronal populations more vulnerable to neurotoxic insults. Similarly, diabetes potentiates neuronal damage in acute neurotoxic events, such as ischemia and stroke. Accordingly, the current study examined the regulation of the neuron-specific glucose transporter, GLUT-3, in the hippocampus of streptozotocin-induced diabetic rats subjected to restraint stress. Diabetes leads to significant increases in GLUT-3 mRNA and protein expression in the hippocampus, increases that are not affected by stress. Collectively, these results suggest that streptozotocin-induced increases in GLUT-3 mRNA and protein expression in the hippocampus may represent a compensatory mechanism to increase glucose utilization during diabetes and also suggest that modulation of GLUT-3 expression is not responsible for glucocorticoid impairment of glucose utilization.

Decreased level of light-induced Fos expression in the suprachiasmatic nucleus of diabetic rats

We assessed light-induced Fos-immunoreactive cells in the suprachiasmatic nucleus of diabetic rats. The number of Fos-immunoreactive cells significantly decreased in diabetic Otsuka Long-Evans Tokushima Fatty (OLETF) rats as compared with control Long-Evans Tokushima Otsuka (LETO) rats. In contrast there was no decrease in the number of Fos-immunoreactive cells in young OLETF rats which have not yet developed diabetes. Two months after the administration of streptozotocin (STZ) to Wistar rats, the number of Fos-immunoreactive cells significantly decreased, although 1 week after the administration of STZ, the number had not yet changed in these STZ-induced diabetic rats. These results suggest that chronic diabetic (hyperglycemic) conditions may affect the light entraining responses in the suprachiasmatic nucleus (SCN).

Alteration of antioxidant status in diabetic rats by chronic exposure to psychological stressors

Antioxidant status was measured in heart, liver, kidney, lung, and erythrocytes of 2-week streptozotocin-diabetic male Wistar rats exposed to chronic intermittent psychological stress consisting of 1 h of restraint twice daily for 14 days. Diabetes reduced erythrocyte and heart and liver susceptibility to hydrogen peroxide-induced glutathione depletion. Susceptibility to peroxide-induced thiobarbituric acid reactive substance (TBARS) formation increased in erythrocytes, liver, kidney, and lung but decreased in heart. Significant changes also occurred in glutathione levels (increased in heart and decreased in liver) and in the activities of catalase (reduced in liver and kidney), glutathione reductase (elevated in heart and liver), and glutathione peroxidase (decreased in liver and lung), but not Cu,Zn-superoxide dismutase. Stress potentiated diabetes-associated hyperglycemia and attenuated diabetes-induced hyperlipidemia. In addition, the reduction in peroxide-induced glutathione depletion in heart and liver and the increased TBARS formation in kidney and lung were reversed. Similarly, the diabetes-induced induced increase in liver glutathione reductase and decreases in liver and lung glutathione peroxidase activities were abolished by stress. Thus, the relative resistance of antioxidant systems to stress can be modified under pathologic conditions in which antioxidant alterations are present.

Neuropeptide Y, the hypothalamus, and diabetes: insights into the central control of metabolism

Neuropeptide Y (NPY), a major brain neurotransmitter, is expressed in neurons of the hypothalamic arcuate nucleus (ARC) that project mainly to the paraventricular nucleus (PVN), an important site of NPY release. NPY synthesis in the ARC is thought to be regulated by several factors, notably insulin, which may exert an inhibitory action. The effects of NPY injected into the PVN and other sites include hyperphagia, reduced energy expenditure and enhanced weight gain, insulin secretion, and stimulation of corticotropin and corticosterone release. The ARC-PVN projection appears to be overactive in insulin-deficient diabetic rats, and could contribute to the compensatory hyperphagia and reduced energy expenditure, and pituitary dysfunction found in these animals; overactivity of these NPY neurons may be due to reduction of insulin's normal inhibitory effect. The ARC-PVN projection is also stimulated in rat models of obesity +/- non-insulin diabetes, possibly because the hypothalamus is resistant to inhibition by insulin; in these animals, enhanced activity of ARC NPY neurons could cause hyperphagia, reduced energy expenditure, and obesity, and perhaps contribute to hyperinsulinemia and altered pituitary secretion. Overall, these findings suggest that NPY released in the hypothalamuss, especially from the ARC-PVN projection, plays a key role in the hypothalamic regulation of energy balance and metabolism. NPY is also found in the human hypothalamus. Its roles (if any) in human homeostasis and glucoregulation remain enigmatic, but the animal studies have identified it as a potential target for new drugs to treat obesity and perhaps NIDDM.

Biphasic regulation of cytochrome P450 2B1/2 mRNA expression by dexamethasone in primary cultures of adult rat hepatocytes maintained on matrigel

We have demonstrated recently that although rat hepatocytes rapidly lose their cytochrome P450 mRNA content following their introduction into primary culture, hepatocytes cultured on Matrigel, a reconstituted basement membrane, subsequently spontaneously "reexpress" the mRNAs of some constitutive P450 forms (Kocarek et al., Mol Pharmacol 43: 328-334, 1993). In the present study, we used the Matrigel cell culture system to examine the dose-dependent effects of dexamethasone (DEX) treatments on the mRNAs for two of the P450 forms that are reexpressed spontaneously between days 3 and 5 in culture, 2B1/2 and 2C6. Treatment of cultured hepatocytes with low doses of DEX (10(-9) to 10(-8) M) that induced the mRNA for tyrosine aminotransferase, a model glucocorticoid-inducible gene, suppressed the spontaneous appearance of 2B1/2 mRNA while having little or no effect on the level of 2C6 mRNA or on beta-actin mRNA. However, treatment of the hepatocyte cultures with high doses of DEX (10(-6) to 10(-5) M) that induced P450 3A1 mRNA increased the amounts of the 2B1/2 and 2C6 mRNAs (4.1- and 2.4-fold, respectively, at 10(-5) M DEX). In contrast to the suppressive effects on the spontaneous increases in 2B1/2 mRNA, low doses of DEX (10(-8) to 10(-7) M) enhanced the induction of 2B1/2 mRNA by phenobarbital (2.5-fold at 10(-7) M DEX). Treatment of the hepatocyte cultures with triamcinolone acetonide, another potent glucocorticoid, suppressed spontaneous 2B1/2 mRNA expression at low doses, but did not induce 2B1/2 mRNA at high doses. Treatments with steroids of other classes, including dihydrotestosterone, 17 alpha-ethinylestradiol, fludrocortisone or R-5020, failed to suppress 2B1/2 mRNA levels at low doses. Additionally, treatment with RU-486, a glucocorticoid/progestin receptor antagonist, induced 2B1/2 mRNA at high doses (10(-6) to 10(-5) M). The suppressive effects of DEX on spontaneous 2B1/2 mRNA expression observed at low doses are consistent with a classical glucocorticoid-mediated mechanism, while the high-dose inductive effects of DEX appear to be exerted through a nonclassical mechanism, perhaps akin to that for induction of 3A1.

The effects of chronic cold exposure on diurnal corticosterone and aldosterone rhythms in Sprague-Dawley rats

Plasma corticosterone (CORT) and aldosterone (ALDO) exhibit diurnal rhythmicity. Cold exposure increases basal plasma CORT and ALDO levels. This study investigated the effects of cold exposure on CORT and ALDO diurnal rhythms. Twelve male Sprague-Dawley rats were housed at 23 degrees C, adapted to a 12:12 day:night cycle (lights on 0900 h), and provided chow and water ad lib. On day 7, 100 microliters tail blood samples were obtained every 4 h for 24 h beginning at 0900 h. The temperature of the animal room was lowered to 4 degrees C. On days 7 and 14 of cold exposure, blood samples were obtained every 4 h as above. After 7 days at 4 degrees C, CORT levels were elevated at 0900 h, 1300 h, and 0500 h (p < 0.05) as compared to levels observed at 23 degrees C. Peak CORT levels were observed at 2100 h in each 24-h sampling session with mean levels of 181.3 +/- 23.3 ng/ml at 23 degrees C, and 200.5 +/- 16.4 ng/ml and 188.1 +/- 19.9 ng/ml after 7 and 14 days at 4 degrees C. In contrast, ALDO levels were elevated at all time points across the day:night cycle after 7 days at 4 degrees C. After 14 days, ALDO levels were elevated at 0900 h, 1300 h, and 2100 h (p < 0.05) as compared to levels observed at 23 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Aldosterone diurnal rhythm in the rat: a question of cross-reactivity?

Radioimmunoassay (RIA) of plasma aldosterone (ALDO) can be hampered by cross-reactivity with plasma corticosterone (CORT). The purpose of this study was to determine and adjust for CORT crossreactivity when assaying for ALDO in blood samples taken throughout a 24-h period. Plasma was obtained from 10 adrenalectomized male Sprague-Dawley rats. Corticosterone-free plasma was spiked with 0, 50, 75, 100, 150, 200, and 300 ng/ml CORT. Aldosterone determination was performed by RIA. An equation for ALDO concentration adjusted for plasma CORT concentration was calculated (ALDO = measurable ALDO - 0.377 x CORT - 5.324). Twenty male Sprague-Dawley rats were adapted to a 12/12 light/dark cycle (lights on at 0900 h). They were divided into two groups based on body weight. Blood samples were obtained every 4 h beginning at 0900 h from rats in group 1 and beginning at 1100 h from rats in group 2. Distinct diurnal CORT and ALDO rhythms were observed. Corticosterone levels were highest at 2100 h (131.8 +/- 17.8 ng/ml) and lowest at 1300 h (04.6 +/- 1.8 ng/ml). Aldosterone levels were highest at 1900 h (276.50 +/- 53.60 pg/ml) and lowest at 1500 h (27.53 +/- 6.84 pg/ml). Corticosterone and ALDO levels were often significantly correlated (r > 0.60), but not at times when CORT and ALDO levels were at their highest. These results suggest that ALDO and CORT may be regulated by different mechanisms or may have a regulatory influence upon each other throughout the day/night cycle.

Glucocorticoids in the nonobese diabetic (NOD) mouse: basal serum levels, effect of endocrine manipulation and immobilization stress

The NOD mouse is a recognized model for studying immunologically mediated insulin-dependent diabetes mellitus (IDDM). In most colonies, the disease appears with a greater preponderance in females than in males and castration alters the expression of the disease. The prevalence of diabetes may also vary depending upon environmental factors such as stress. Therefore, we measured in the NOD mouse serum glucocorticoid concentrations in basal and stress conditions. We observed in NOD as well as in C57BL/6 mice, taken as controls, a circadian rhythm of corticosterone, with females having higher values than males. After a single restraint stress, female and male NOD mice exhibit a comparable response, whereas after repeated stress, males respond significantly less than females, suggesting an adaptation phenomenon. In contrast, there is no difference in the pattern of corticosterone response of C57BL/6 females and males to both types of stress, but females always respond better than males. Moreover, whatever the stress considered, NOD mice generally exhibit a higher corticosterone response than C57BL/6 mice. The sexual dimorphism in diabetes expression in NOD mice may be related to the levels of corticosterone, a hyperglycemic hormone, in both basal and stress conditions. However, the understanding of corticosteroid effects in this model of type I IDDM is rather complex given their well known anti-inflammatory and immunosuppressive effects in other models of autoimmune diseases.

The functional significance of biochemical alterations in streptozotocin-induced diabetes

These experiments examined the effects of restraint stress on dopamine (DA) and 5-hydroxytryptamine (5-HT) and their principal metabolites dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA), respectively, in 4 brain regions, as well as on plasma corticosterone concentration (CORT) and behavior in streptozotocin-induced diabetic rats and nondiabetic controls. Diabetic rats had widespread reductions in DA and 5-HT turnover (DOPAC/DA and 5-HIAA/5-HT ratios). Restraint led to equivalent increases in DA turnover in diabetics and nondiabetics, but attenuated increases in 5-HT turnover in diabetic rats. CORT concentration of diabetics and nondiabetics measured in complete quiet did not differ. Relative to these measures, only diabetics had elevated CORT when either restrained or kept in the same room with restrained rats with food and water removed. Open-field exploration was suppressed by restraint in diabetics only. All diabetic rats showed decreased locomotion in a novel environment which was normalized during a second exposure to the apparatus. Together, these results suggest that diabetes-induced disruptions in open-field activity are related to anxiety rather than to motor or energy deficits, and may be related to impaired 5-HT and CORT systems.

Effect of restraint stress on prolactin and corticosterone levels in streptozotocin-induced diabetic rats

Changes in neuroendocrine function have been shown to occur in diabetic animals. The aim of the present study was to examine both the prolactin (PRL) and corticosterone (CORT) responses to a short period of restraint stress after the animals had been made diabetic for six weeks. The streptozotocin - induced diabetic rats had resting CORT levels which were significantly higher than the control animals. Acute restraint significantly increased CORT levels in both the control and diabetic rats. The CORT levels after stress were higher in the diabetic rats. However, the magnitude of the response (percent increase) was less in these animals. The resting PRL levels were not significantly different in the diabetic and control animals. The PRL levels significantly increased in both the control and diabetic rats when they were exposed to the restraint stress. The PRL levels after stress were significantly less in the diabetic rats, indicating a blunted PRL stress response. These results indicate that the diabetic state can affect an animals PRL and CORT response to a new acute stress.

Sex steroids, glucocorticoids, stress and autoimmunity

Interest in the field of neuroimmunoendocrinology is in full expansion. With regard to this, steroid influence on the immune system, in particular sex steroids and glucocorticoids, has been known for a long time. Sex steroids are part of the mechanism underlying the immune sexual dimorphism, as particularly emphasized in autoimmune diseases. Immunosuppressive and anti-inflammatory effects of glucocorticoids are now considered a physiological negative feedback loop to cytokines produced during an immune and/or inflammatory response. Psychosocial factors may play a role in the development of immunologically-mediated diseases, e.g. autoimmune diseases. The nonobese diabetic (NOD) mouse, that develops an immunologically-mediated insulin-dependent diabetes mellitus (IDDM) is an interesting model to study the role of endogenous steroids. Insulitis is present in both sexes, but diabetes has a strong preponderance in females. Hormonal alteration, such as castration, modulates the incidence of diabetes, whereas environmental factors, such as stress, accelerate the disease. In the present paper, we have reviewed the role of gender, sex steroid hormones, stress and glucocorticoids in autoimmunity as well as analyzed their different levels of actions and interrelationships, focusing particular attention on the immunologically-mediated IDDM of the NOD mouse.