Moderate recurrent hypoglycemia during early development leads to persistent changes in affective behavior in the rat

Hypoglycaemia aggravates impaired endothelial-dependent vasodilation in diabetes by suppressing endothelial nitric oxide synthase activity and stimulating inducible nitric oxide synthase expression

BACKGROUND

Diabetes exacerbates vascular injury by triggering endothelial dysfunction. Endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS) both play major roles in endothelial dysfunction. However, effects of hypoglycaemia, the main complication of the insulin therapy to the glycemic control in diabetes, on eNOS activity and iNOS expression, and underlying mechanisms in diabetes remain unknown. Hence, we aimed to determine the effects of hypoglycaemia on eNOS activity and iNOS expression in different arterial beds of diabetic rats.

METHODS

Sprague-Dawley rats were subjected to Streptozotocin (STZ) combined with high fat diet (HFD) to induce diabetes and then received insulin injection to attain acute and recurrent hypoglycaemia. Immunoblotting was used to analyse the phosphorylation and O-glycosylation status of eNOS and iNOS level from thoracic aorta and mesenteric artery tissue. Indicators of oxidative stress from plasm were determined, and endothelial-dependent vasodilation was detected via wire myograph system.

RESULTS

Hypoglycaemia was associated with a marked increase in eNOS O-GlcNAcylation and decrease in Serine (Ser)-1177 phosphorylation from thoracic aortas and mesenteric arteries. Moreover, hypoglycaemia resulted in elevated phosphorylation of eNOS at Threonine (Thr)-495 site in mesenteric arteries. Besides, changes in these post-translational modifications were associated with increased O-GlcNAc transferase (OGT), decreased phosphorylation of Akt at Ser-473, and increased protein kinase C α subunit (PKCα). iNOS expression was induced in hypoglycaemia. Furthermore, endothelial-dependent vasodilation was impaired under insulin-induced hypoglycaemia, and further in recurrent hypoglycaemia.

CONCLUSIONS

Conclusively, these findings strongly indicate that hypoglycaemia-dependent vascular dysfunction in diabetes is mediated through altered eNOS activity and iNOS expression. Therefore, this implies that therapeutic modulation of eNOS activity and iNOS expression in diabetics under intensive glucose control may prevent and treat adverse cardiovascular events.

Cerebral Effects of Neonatal Dysglycemia

This article summarizes the available evidence reporting the relationship between perinatal dysglycemia and long-term neurodevelopment. We review the physiology of perinatal glucose metabolism and discuss the controversies surrounding definitions of perinatal dysglycemia. We briefly review the epidemiology of hypoglycemia and hyperglycemia in fetal, preterm, and term infants. We discuss potential pathophysiologic mechanisms contributing to dysglycemia and its effect on neurodevelopment. We highlight current strategies to prevent and treat dysglycemia in the context of neurodevelopmental outcomes. Finally, we discuss areas of future research and the potential role of continuous glucose monitoring.

Hypoglycemia increases endothelial-dependent vasodilation through suppressing phosphorylation at Threonine 495/497 site of endothelial nitric oxide synthase

OBJECTIVE

Phosphorylation plays an essential role in the regulation of endothelial nitric oxide synthase (eNOS) activity. However, the phosphorylation of eNOS under hypoglycemia and whether hypoglycemia changes eNOS activity is unknown. This paper aims to clarify the regulation of eNOS phosphorylation and its activity change under hypoglycemia.

METHODS

Bovine aortic endothelial cells (BAECs) and Sprague-Dawley rats were treated with hypoglycemia, and the phosphorylation of eNOS was subjected to western blot. Blood nitric oxide (NO) concentration was determined by NO kit and endothelial-dependent vasodilation was detected by multi-wire myograph.

RESULTS

In both BAECs and rats' thoracic aorta, hypoglycemia induced eNOS phosphorylation decrease specifically on Threonine (Thr) 497. Inhibition of ubiquitination of protein kinase C α subunit (PKCα) reverses the decrease of eNOS phosphorylation in hypoglycemia. Ubiquitinated PKCα can be reversed by AMPK knockdown. In rats, insulin induced hypoglycemia increased the concentration of NO in arterial blood, and progressively enhanced the endothelium-dependent vasodilation of the thoracic and mesenteric aorta.

CONCLUSIONS

In vitro, the activation of AMPK may lead to the expression of PKCα by regulating ubiquitination, resulting in a decrease in the level of P-eNOS Thr497 phosphorylation under hypoglycemia. In vivo, insulin-induced hypoglycemia produces a beneficial cardiovascular effect on rats.

Regulation of O-GlcNAcylation on endothelial nitric oxide synthase by glucose deprivation and identification of its O-GlcNAcylation sites

As an energy-sensitive post-translational modification, O-GlcNAcylation plays a major role in endothelial nitric oxide synthase (eNOS) activity regulation. However, effects of glucose deprivation on eNOS O-GlcNAcylation and the presence of novel O-GlcNAcylation sites of eNOS under glucose deprivation remain unknown. Hence, we aim to determine the effects of glucose deprivation on O-GlcNAcylation and novel O-GlcNAcylation sites of eNOS. Bovine aortic endothelial cells (BAECs) and Sprague-Dawley rats were induced by glucose deprivation and their eNOS O-GlcNAcylation was subjected to immunoblotting. eNOS and transfected eNOS were purified by pull-down assay and immunoprecipitation respectively. Novel O-GlcNAcylation sites of eNOS were predicted by HPLC-MS and MS/MS Ion and determined by immunoblotting. eNOS activity was detected by Elisa and isotope labeling method. In BAECs and rat thoracic aorta, low glucose-associated activation of eNOS was accompanied by elevated O-GlcNAcylation, which did not affect O-linked serine phosphorylation at 1179/1177 residues. Changes in this post-translational modification were associated with increased O-GlcNAc transferase (OGT) expression and were reversed by AMPK knockdown. Immunoblot analysis of cells expressing His-tagged wild-type human eNOS and human eNOS carrying a mutation at the Ser1177 phosphorylation site confirmed an increase in O-GlcNAcylation by glucose deprivation. A marked increase in O-GlcNAcylation indicated that eNOS contained novel O-GlcNAcylation sites that were activated by glucose deprivation. Immunoblot analysis of cells expressing His-tagged human eNOS carrying a mutation at Ser738 and Ser867 confirmed an increase in O-GlcNAcylation by glucose deprivation. Conversely, in His-tagged human eNOS carrying a mutation at Thr866, O-GlcNAcylation was unaffected by glucose deprivation. Differences in culture conditions were identified using two-way analysis of variance (ANOVA), one-way ANOVA, and unpaired Student's t-test. Glucose deprivation increases O-GlcNAcylation and activity of eNOS, potentially by the AMPK-OGT pathway, suggesting that Thr866 is a novel O-GlcNAcylation site involved in glucose-deprivation mediated eNOS activation.

Recurrent nonsevere hypoglycemia exacerbates imbalance of mitochondrial homeostasis leading to synapse injury and cognitive deficit in diabetes

Recurrent nonsevere hypoglycemia (RH) can lead to cognitive dysfunction in patients with diabetes, although the involved mechanisms remain unclear. Here, we aimed to investigate the mechanism underlying RH-induced cognitive deficits with a focus on mitochondrial homeostasis. To establish a model that mimicked RH in patients with type 1 diabetes (T1DM) receiving insulin therapy, streptozotocin-induced mice with T1DM were subjected to recurrent, twice-weekly insulin injections over 4 wk. We found that RH disrupted the mitochondrial fine structure, reduced the number of mitochondria, and upregulated the expression of mitochondrial dynamics and mitophagy markers, including dynamin-related protein 1 (Drp1), Bcl-2/adenovirus E1B 19-kDa-interacting protein-3 (BNIP3), and microtubule-associated protein 1 light-chain 3 (LC3) in the hippocampus of T1DM mice. Moreover, RH and chronic hyperglycemia synergistically promoted the production of reactive oxygen species, impaired mitochondrial membrane potential, and suppressed mitochondrial energy metabolism. Under diabetic conditions, RH also altered the synaptic morphology and reduced the expression of synaptic marker proteins. Long-term recognition memory and spatial memory, assessed with the Morris water maze test, were also impaired. However, these effects were largely prevented by mitochondrial division inhibitor 1, a potent and selective Drp1 inhibitor. Thus, it appears that RH exacerbates the imbalance of mitochondrial homeostasis, leading to synapse injury and cognitive deficits in diabetes. The adjustment of mitochondrial homeostasis could serve as an effective neuroprotective approach when addressing low blood sugar conditions.

Peripubertal stress increases play fighting at adolescence and modulates nucleus accumbens CB1 receptor expression and mitochondrial function in the amygdala

Play fighting is a highly rewarding behavior that helps individuals to develop social skills. Early-life stress has been shown to alter play fighting in rats and hamsters as well as to increase aggressive behaviors at adulthood. However, it is not known whether individual differences in stress-induced play fighting are related to differential developmental trajectories towards adult aggression. To address this question, we used a rat model of peripubertal stress (PPS)-induced psychopathology that involves increased aggression at adulthood. We report that, indeed, PPS leads to enhanced play fighting at adolescence. Using a stratification approach, we identify individuals with heightened levels of play fighting as the ones that show abnormal forms of aggression at adulthood. These animals showed as well a rapid habituation of their corticosterone responsiveness to repeated stressor exposure at peripuberty. They also showed a striking increase in mitochondrial function in the amygdala-but not nucleus accumbens-when tested ex vivo. Conversely, low, but not high players, displayed increased expression of the CB1 cannabinoid receptor in the nucleus accumbens shell. Our results highlight adolescence as a potential critical period in which aberrant play fighting is linked to the emergence of adult aggression. They also point at brain energy metabolism during adolescence as a possible target to prevent adult aggression.

Reduction in N-methyl-D-aspartate Receptor-mediated Cell Death in Hippocampal Neurons by Glucose Reduction Preconditioning

BACKGROUND

Repeated episodes of reduced glucose availability can precondition the brain against damage caused by severe hypoglycemia. Because N-methyl-D-aspartate (NMDA) receptor activation may contribute to neuronal loss in the hippocampus following glucose deprivation, we tested the hypothesis that preconditioning with reduced glucose decreased NMDA receptor-mediated cell death in hippocampal neurons.

METHODS

Hippocampal slice cultures from 7-day old rats were used to study glucose reduction preconditioning and N-methyl-D-aspartate receptor (NMDAR)-mediated cell death. Preconditioning involved reductions in glucose to the following levels: 0.1 mM, 0.5, or 1.0 mM for 30 minutes, 60 minutes, or 90 minutes on 3 consecutive days. Cell death following 1-hour total glucose deprivation was measured with a vital dye technique (SYTOX fluorescence). As an index of NMDAR activity, cell death following application of 1 mM NMDA, was also measured.

RESULTS

A preconditioning protocol of 30 minutes of 0.1 mM glucose per day for 3 days reduced cell death following 1-hour total glucose by 65% to 70%, depending on cellular region. No reduction in NMDAR-mediated cell death was seen following any of the preconditioning treatments. However, when NMDAR-mediated cell death was assessed following preconditioning combined with subsequent total glucose deprivation, cell death was reduced in the cultures that had been preconditioned with 0.1 mM glucose for 30 minutes×3 days.

CONCLUSIONS

We found that that glucose reduction preconditioning protects hippocampal neurons against severe glucose deprivation-induced neuronal damage. This preconditioning was not associated with reductions in NMDAR-mediated cell death except when the preconditioning was combined with an additional exposure to a period of total glucose deprivation.

Recurrent Moderate Hypoglycemia Suppresses Brain-Derived Neurotrophic Factor Expression in the Prefrontal Cortex and Impairs Sensorimotor Gating in the Posthypoglycemic Period in Young Rats

Recurrent hypoglycemia is common in infants and children. In developing rat models, recurrent moderate hypoglycemia leads to neuronal injury in the medial prefrontal cortex. To understand the effects beyond neuronal injury, 3-week-old male rats were subjected to 5 episodes of moderate hypoglycemia (blood glucose concentration, approx. 30 mg/dl for 90 min) once daily from postnatal day 24 to 28. Neuronal injury was determined using Fluoro-Jade B histochemistry on postnatal day 29. The effects on brain-derived neurotrophic factor (BDNF) and its cognate receptor, tyrosine kinase receptor B (TrkB) expression, which is critical for prefrontal cortex development, were determined on postnatal day 29 and at adulthood. The effects on prefrontal cortex-mediated function were determined by assessing the prepulse inhibition of the acoustic startle reflex on postnatal day 29 and 2 weeks later, and by testing for fear-potentiated startle at adulthood. Recurrent hypoglycemia led to neuronal injury confined primarily to the medial prefrontal cortex. BDNF/TrkB expression in the prefrontal cortex was suppressed on postnatal day 29 and was accompanied by lower prepulse inhibition, suggesting impaired sensorimotor gating. Following the cessation of recurrent hypoglycemia, the prepulse inhibition had recovered at 2 weeks. BDNF/TrkB expression in the prefrontal cortex had normalized and fear-potentiated startle was intact at adulthood. Recurrent moderate hypoglycemia during development has significant adverse effects on the prefrontal cortex in the posthypoglycemic period.

Hypothalamic-Pituitary-Adrenal Axis Programming after Recurrent Hypoglycemia during Development

Permanent brain injury is a complication of recurrent hypoglycemia during development. Recurrent hypoglycemia also has adverse consequences on the neuroendocrine system. Hypoglycemia-associated autonomic failure, characterized by ineffective glucose counterregulation during hypoglycemia, is well described in children and adults on insulin therapy for diabetes mellitus. Whether recurrent hypoglycemia also has a programming effect on the hypothalamus-pituitary-adrenal cortex (HPA) axis has not been well studied. Hypoglycemia is a potent stress that leads to increased glucocorticoid secretion in all age groups, including the perinatal period. Other conditions associated with exposure to excess glucocorticoid in the perinatal period have a programming effect on the HPA axis activity. Limited animal data suggest the possibility of similar programming effect after recurrent hypoglycemia in the postnatal period. The age at exposure to hypoglycemia likely determines the HPA axis response in adulthood. Recurrent hypoglycemia in the early postnatal period likely leads to a hyperresponsive HPA axis, whereas recurrent hypoglycemia in the late postnatal period lead to a hyporesponsive HPA axis in adulthood. The age-specific programming effects may determine the neuroendocrine response during hypoglycemia and other stressful events in individuals with history of recurrent hypoglycemia during development.

Acute hypoglycemia results in reduced cortical neuronal injury in the developing IUGR rat

BACKGROUND

Hypoglycemia (HG) is common in intrauterine growth restricted (IUGR) neonates. In normally grown (NG) neonatal rats, acute HG causes neuronal injury in the brain; the cerebral cortex is more vulnerable than the hippocampus (HPC). We hypothesized that the IUGR brain is less vulnerable to HG-induced injury while preserving regional variation in vulnerability.

METHODS

We induced IUGR via bilateral uterine artery ligation on gestational day 19 (term 22 d) rats. On postnatal day 14, insulin-induced HG of equivalent severity and duration (blood glucose < 40 mg/dl for 240 min) was produced in IUGR and NG (IUGR/HG and NG/HG). Neuronal injury in the cortex and HPC was quantified 6-72 h later using Fluoro-Jade B (FJB) histochemistry. The mRNA expression of monocarboxylate transporters, MCT1 and MCT2, and glucose transporters, GLUT1 and GLUT3, was determined using quantitative PCR.

RESULTS

There were fewer FJB-positive (FJB+) cells in the cortex of IUGR/HG; no difference was observed in FJB+ cells in HPC. Core body temperature was lower in IUGR/HG compared with NG/HG. MCT2 expression was increased in the IUGR cortex.

CONCLUSION

HG-induced neuronal injury is decreased in the cortex of the developing IUGR brain. Adaptations including systemic hypothermia and enhanced delivery of alternative substrates via MCT2 might protect against HG-induced neuronal injury in IUGR.

Hyperglycemia accentuates and ketonemia attenuates hypoglycemia-induced neuronal injury in the developing rat brain

BACKGROUND

Prolonged hypoglycemia leads to brain injury, despite treatment with 10% dextrose. Whether induction of hyperglycemia or ketonemia achieves better neuroprotection is unknown. Hyperglycemia is neuroprotective in other brain injuries during development; however, it worsens hypoglycemia-induced injury in the adult brain via poly(ADP-ribose)polymerase-1 (PARP-1) overactivation.

METHODS

Three-week-old rats were subjected to insulin-induced hypoglycemia and treated with 10% dextrose or 50% dextrose. Neuronal injury, PARP-1, and brain-derived neurotrophic factor (BDNF) III/TrkB/p75(NTR) expressions were determined. In the second experiment, ketonemia was induced by administering β-hydroxybutyrate during hypoglycemia and its effect on neuronal injury was compared with those conventionally treated using 10% dextrose.

RESULTS

Both 10 and 50% dextrose administration led to hyperglycemia (50% dextrose > 10% dextrose). Compared with the 10% dextrose group, neuronal injury was greater in the 50% dextrose group and was accompanied by PARP-1 overactivation. BDNF III and p75(NTR), but not TrkBFL, mRNA expressions were upregulated. Neuronal injury was less severe in the rats subjected to ketonemia, compared with those conventionally treated using 10% dextrose.

CONCLUSION

Hyperglycemia accentuated hypoglycemia-induced neuronal injury, likely via PARP-1 overactivation. Although BDNF was upregulated, it was not neuroprotective and potentially exaggerated injury by binding to p75(NTR) receptor. Conversely, ketonemia during hypoglycemia attenuated neuronal injury.

Neuronal damage and cognitive impairment associated with hypoglycemia: An integrated view

The aim of the present review is to offer a current perspective about the consequences of hypoglycemia and its impact on the diabetic disorder due to the increasing incidence of diabetes around the world. The main consequence of insulin treatment in type 1 diabetic patients is the occurrence of repetitive periods of hypoglycemia and even episodes of severe hypoglycemia leading to coma. In the latter, selective neuronal death is observed in brain vulnerable regions both in humans and animal models, such as the cortex and the hippocampus. Cognitive damage subsequent to hypoglycemic coma has been associated with neuronal death in the hippocampus. The mechanisms implicated in selective damage are not completely understood but many factors have been identified including excitotoxicity, oxidative stress, zinc release, PARP-1 activation and mitochondrial dysfunction. Importantly, the diabetic condition aggravates neuronal damage and cognitive failure induced by hypoglycemia. In the absence of coma prolonged and severe hypoglycemia leads to increased oxidative stress and discrete neuronal death mainly in the cerebral cortex. The mechanisms responsible for cell damage in this condition are still unknown. Recurrent moderate hypoglycemia is far more common in diabetic patients than severe hypoglycemia and currently important efforts are being done in order to elucidate the relationship between cognitive deficits and recurrent hypoglycemia in diabetics. Human studies suggest impaired performance mainly in memory and attention tasks in healthy and diabetic individuals under the hypoglycemic condition. Only scarce neuronal death has been observed under moderate repetitive hypoglycemia but studies suggest that impaired hippocampal synaptic function might be one of the causes of cognitive failure. Recent studies have also implicated altered mitochondrial function and mitochondrial oxidative stress.

Correction of long-lasting negative effects of neonatal isolation in white rats using semax

Adverse experience during the early postnatal period induces negative alterations in physiological and neurobiological functions, resulting in long-term disorder in animal behavior. The aim of the present work was to study the long-lasting effects of chronic neonatal stress in white rats and to estimate the possibility of their correction using Semax, an analogue of ACTH fragment (4-10). Early neonatal isolation was used as a model of early-life stress. Rat pups were separated from their mothers and littermates for 5 h daily during postnatal days 1-14. The pups of the control group were left undisturbed with the dams. Half of the rats subjected to neonatal isolation received an intranasal injection of Semax at a dose of 50 µg/kg daily, from postnatal day 15 until day 28. The other animals received intranasal vehicle injections daily at the same time points. It was shown that neonatal isolation leads to a delay in physical development, metabolic disturbances, and a decrease in the corticosterone stress response in white rats. These changes were observed during the first two months of life. Semax administration weakened the influence of neonatal isolation on the animals, body weight , reduced metabolic dysfunction, and led to an increase in stress-induced corticosterone release to the control values. So the chronic intranasal administration of Semax after termination of the neonatal isolation procedure diminishes the negative effects of neonatal stress.

Evidence for a hyporesponsive limbic-hypothalamic-pituitary-adrenal axis following early-life repetitive hypoglycemia in adult male rats

The developing limbic-hypothalamic-pituitary-adrenal (LHPA) axis is highly vulnerable to programming by early-life environmental factors, including exposure to synthetic glucocorticoids and nutrient deficiencies. Early-life repetitive hypoglycemia (RHG) is a common complication of insulin therapy for type-1 diabetes that may have long-term consequences in adulthood. Recent observations in a rat model of early RHG suggest persistent changes in LHPA axis function, including changes in relevant hormones and affective behaviors, which support a hyperresponsive LHPA axis. Thus, we hypothesized that early RHG would alter the expression of key genes regulating LHPA axis function in adulthood. The present study employed a rat model of insulin-induced RHG spanning postnatal days (P)24-28, a neurodevelopmental equivalent of early childhood in humans, to assess the long-term effects on mRNA levels for proteins relevant to the LHPA function and the corticosterone responses to ACTH stimulation of dispersed adrenocortical cells in vitro and restraint stress in vivo at adulthood. This early RHG model resulted in a hyporesponsive LHPA axis characterized by impaired corticosterone response, increased hippocampal glucocorticoid and mineralocorticoid receptor (GR and MR), decreased hypothalamic corticotropin-releasing hormone, increased adrenal steroidogenic-acute-regulatory protein and GR, and decreased adrenal MR, melanocortin-type-2 receptor and low-density lipoprotein receptor expression. Our findings highlight a complex environmental-gene interaction between RHG and LHPA axis during development that influences regulation of this axis in adulthood. The findings are consistent with the developmental origins of disease and underscore the influences of early-life events on the programming of a major regulatory system.