Role of excitatory aminoacids in neonatal hypoglycemia

Neonatal Hypoglycemia and Brain Vulnerability

Neonatal hypoglycemia is a common condition. A transient reduction in blood glucose values is part of a transitional metabolic adaptation following birth, which resolves within the first 48 to 72 h of life. In addition, several factors may interfere with glucose homeostasis, especially in case of limited metabolic stores or increased energy expenditure. Although the effect of mild transient asymptomatic hypoglycemia on brain development remains unclear, a correlation between severe and prolonged hypoglycemia and cerebral damage has been proven. A selective vulnerability of some brain regions to hypoglycemia including the second and the third superficial layers of the cerebral cortex, the dentate gyrus, the subiculum, the CA1 regions in the hippocampus, and the caudate-putamen nuclei has been observed. Several mechanisms contribute to neuronal damage during hypoglycemia. Neuronal depolarization induced by hypoglycemia leads to an elevated release of glutamate and aspartate, thus promoting excitotoxicity, and to an increased release of zinc to the extracellular space, causing the extensive activation of poly ADP-ribose polymerase-1 which promotes neuronal death. In this review we discuss the cerebral glucose homeostasis, the mechanisms of brain injury following neonatal hypoglycemia and the possible treatment strategies to reduce its occurrence.

Neuroprotective potential of Bacopa monnieri and Bacoside A against dopamine receptor dysfunction in the cerebral cortex of neonatal hypoglycaemic rats

Neonatal hypoglycaemia initiates a series of events leading to neuronal death, even if glucose and glycogen stores return to normal. Disturbances in the cortical dopaminergic function affect memory and cognition. We recommend Bacopa monnieri extract or Bacoside A to treat neonatal hypoglycaemia. We investigated the alterations in dopaminergic functions by studying the Dopamine D1 and D2 receptor subtypes. Receptor-binding studies revealed a significant decrease (p < 0.001) in dopamine D1 receptor number in the hypoglycaemic condition, suggesting cognitive dysfunction. cAMP content was significantly (p < 0.001) downregulated in hypoglycaemic neonatal rats indicating the reduction in cell signalling of the dopamine D1 receptors. It is attributed to the deficits in spatial learning and memory. Hypoglycaemic neonatal rats treated with Bacopa extract alone and Bacoside A ameliorated the dopaminergic and cAMP imbalance as effectively as the glucose therapy. The upregulated Bax expression in the present study indicates the high cell death in hypoglycaemic neonatal rats. Enzyme assay of SOD confirmed cortical cell death due to free radical accumulation. The gene expression of SOD in the cortex was significantly downregulated (p < 0.001). Bacopa treatment showed a significant reversal in the altered gene expression parameters (p < 0.001) of Bax and SOD. Our results suggest that in the rat experimental model of neonatal hypoglycaemia, Bacopa extract improved alterations in D1, D2 receptor expression, cAMP signalling and cell death resulting from oxidative stress. This is an important area of study given the significant motor and cognitive impairment that may arise from neonatal hypoglycaemia if proper treatment is not implemented.

Decreased cholinergic receptor expression in the striatum: motor function deficit in hypoglycemic and diabetic rats

Hypoglycemic brain injury is a common and serious complication of insulin therapy associated with diabetes. This study evaluated the effect of insulin-induced hypoglycemia and STZ-induced diabetes on striatal cholinergic receptors and enzyme expression and on motor function. Cholinergic enzymes: AChE and ChAT gene expression, radioreceptor binding assay and immunohistochemistry of muscarinic M1, M3 receptors and α7nAChR were carried out. Motor performance on grid walk test was analysed. AChE and ChAT expression significantly downregulated in hypoglycemic and diabetic rats. Total muscarinic and Muscarinic M3 receptor binding decreased in hypoglycemic rats compared to diabetic rats whereas muscarinic M1 receptor binding increased in hypoglycemic rats compared to diabetic rats. Real-time PCR analysis and confocal imaging of muscarinic M1, M3 receptors confirmed the changes in muscarinic receptor binding in hypoglycemic and diabetic rats. In hypoglycemic rats, α7nAChR expression significantly up regulated compared to diabetic rats. Grid walk test demonstrated the impairment in motor function and coordination in hypoglycemic and hyperglycemic rats. Neurochemical changes along with the behavioral data implicate a role for impaired striatal cholinergic receptor function inducing motor function deficit induced by hypo and hyperglycemia. Hypoglycemia exacerbated the neurobehavioral deficit in diabetes which has clinical significance in the treatment of diabetes.

Hypoglycemia induced changes in cholinergic receptor expression in the cerebellum of diabetic rats

Glucose homeostasis in humans is an important factor for the functioning of nervous system. Hypoglycemia and hyperglycemia is found to be associated with central and peripheral nerve system dysfunction. Changes in acetylcholine receptors have been implicated in the pathophysiology of many major diseases of the central nervous system (CNS). In the present study we showed the effects of insulin induced hypoglycemia and streptozotocin induced diabetes on the cerebellar cholinergic receptors, GLUT3 and muscle cholinergic activity. Results showed enhanced binding parameters and gene expression of Muscarinic M1, M3 receptor subtypes in cerebellum of diabetic (D) and hypoglycemic group (D + IIH and C + IIH). α7nAchR gene expression showed a significant upregulation in diabetic group and showed further upregulated expression in both D + IIH and C + IIH group. AchE expression significantly upregulated in hypoglycemic and diabetic group. ChAT showed downregulation and GLUT3 expression showed a significant upregulation in D + IIH and C + IIH and diabetic group. AchE activity enhanced in the muscle of hypoglycemic and diabetic rats. Our studies demonstrated a functional disturbance in the neuronal glucose transporter GLUT3 in the cerebellum during insulin induced hypoglycemia in diabetic rats. Altered expression of muscarinic M1, M3 and α7nAchR and increased muscle AchE activity in hypoglycemic rats in cerebellum is suggested to cause cognitive and motor dysfunction. Hypoglycemia induced changes in ChAT and AchE gene expression is suggested to cause impaired acetycholine metabolism in the cerebellum. Cerebellar dysfunction is associated with seizure generation, motor deficits and memory impairment. The results shows that cerebellar cholinergic neurotransmission is impaired during hyperglycemia and hypoglycemia and the hypoglycemia is causing more prominent imbalance in cholinergic neurotransmission which is suggested to be a cause of cerebellar dysfunction associated with hypoglycemia.

Enhanced [3H] Glutamate Binding in the Cerebellum of Insulin-Induced Hypoglycaemic and Streptozotocin-Induced Diabetic Rats

AIM

Energy deprivation causes neuronal death affecting the cognitive and memory ability of an individual. The kinetic parameters of glutamate dehydrogenase (GDH), the enzyme involved in the production of glutamate, was studied in the cerebellum and liver and the binding parameters of glutamate receptors in the cerebellum of insulin-induced hypoglycaemic and streptozotocin-induced diabetic rats were studied to reveal the role of glutamate excitotoxicity.

METHODS

A single intrafemoral dose of streptozotocin was administered to induce diabetes. Hypoglycaemia was induced by appropriate doses of insulin subcutaneously in control and diabetic rats. The kinetic parameters V (max) and K (m) of GDH were studied spectrophotometrically at different substrate concentrations of alpha-ketoglutarate. Glutamate receptor binding assay was done with different concentrations of [3H] Glutamate.

RESULTS

The GDH enzyme assay showed a significant increase (P < 0.001) in the V (max) of the enzyme in the cerebellum of hypoglycaemic and diabetic rat groups when compared to control. The V (max) of hypoglycaemic groups was significantly increased (P < 0.001) when compared to diabetic group. In the liver, the V (max) of GDH was significantly increased (P < 0.001) in the diabetic and diabetic hypoglycaemia group when compared to control. The V (max) of GDH increased significantly (P < 0.001) in the diabetic hypoglycaemic rats compared to diabetic group, whereas the control hypoglycaemic rats showed a significant decrease in V (max) (P < 0.001) when compared to diabetic and diabetic hypoglycaemic rats. The K (m) showed no significant change amongst the groups in cerebellum and liver. Scatchard analysis showed a significant increase (P < 0.001) in B (max) in the cerebellum of hypoglycaemic and diabetic rats when compared to control. The B (max) of hypoglycaemic rats significantly increased (P < 0.001) when compared to diabetic group. In hypoglycaemic groups, B (max) of the control hypoglycaemic rats showed a significant increase (P < 0.001) compared to diabetic hypoglycaemic rats. The K (d) of the diabetic group decreased significantly (P < 0.01) when compared to control and control hypoglycaemic rats. There was a significant decrease (P < 0.05) in the K (d) of diabetic hypoglycaemic group when compared to the control hypoglycaemic rats.

CONCLUSION

Our studies demonstrated the increased enzyme activity in the hypoglycaemic rats with increased production of extracellular glutamate. The present study also revealed increased binding parameters of glutamate receptors reflecting an increased receptor number with increase in the affinity. This increased number of receptors and the increased glutamate production will lead to glutamate excitotoxicity and neuronal degeneration which has an impact on the cognitive and memory ability. This has immense clinical significance in the management of diabetes and insulin therapy.

Cellular mechanisms of brain hypoglycemia

Data on intracellular processes induced by a low glucose level in nerve tissue are presented. The involvement of glutamate and adenosine receptors, mitochondria, reactive oxygen species (ROS), and calcium ions in the development of hypoglycemia-induced damage of neurons is considered. Hypoglycemia-induced calcium overload of neuronal mitochondria is suggested to be responsible for the increased ROS production by mitochondria.

Changes in amino acids and nitric oxide concentration in cerebrospinal fluid during labor pain

This study analyzes the relationship between amino acids and pain perception during active labor. Cerebrospinal fluid (CSF) levels of the excitatory amino acids (EAAs)-glutamate, aspartate and their amide forms, inhibitory amino acids (IAAs)-glycine, gamma-amino butyric acid (GABA) and taurine and nitric oxide (NO) related compounds-arginine and citrulline (by-product of NO synthesis) were compared between pregnant women at term pregnancy with labor pain (n = 38) and without labor pain (Caesarian section; n = 30). The levels of aspartate, glycine, GABA and citrulline were significantly higher; whilst taurine was significantly lower in the labor pain group. These findings suggest that aspartate and NO are associated with labor pain. An inhibitory role for the IAA taurine and a pronociceptive role for glycine in labor pain are proposed.

Susceptibility of the developing brain to acute hypoglycemia involving A1 adenosine receptor activation

It has been suggested that the developing brain is less vulnerable to the adverse effects of hypoglycemia than the mature brain; however, this issue remains controversial. We also do not know the magnitude or duration of hypoglycemia needed to trigger hypoglycemic brain injury during development. To address this issue a series of in vivo and in vitro studies were performed. First, we established an acute model of insulin-induced hypoglycemia in mice by administering 3 U/kg of neutral-protamine Hagadorn insulin subcutaneously. When we examined degenerating neurons in hippocampus and striatum by TUNEL labeling, injury was observed after 4 h of hypoglycemia in postnatal day (P)7 mice, and we observed more cell injury in animals rendered hypoglycemic at P7 than at P21. Studies of hippocampal slice cultures revealed that reduction in glucose concentration induced more neuronal injury in slices prepared from P3 and P7 than from P14 and P21 mice. Treatment of slices with an adenosine A(1) receptor (A(1)AR) antagonist reduced the hypoglycemic damage, whereas agonists increased damage, particularly in slices prepared from very young pups. This suggests a critically important role for A(1)ARs, which was further demonstrated by the reduction of hypoglycemic damage in hippocampal slices prepared from A(1)AR(-/-) mice. Furthermore, insulin-induced hypoglycemia in P7 A(1)AR(-/-) mice did not increase TUNEL-positive cells, but a major increase was seen in A(1)AR(+/-) mice. These observations show that the developing nervous system is indeed sensitive to acute hypoglycemic injury and that A(1)AR activation contributes to damage induced by hypoglycemia, particularly in immature mouse brain.

Hypoglycemic injury to the immature brain

Despite the fact that hypoglycemia is an extremely common disorder of the newborn, consensus has been difficult to reach regarding definition, diagnosis, outcome, and treatment. With improved neuroradiologic techniques, such as MRI and PET scanning becoming increasingly available, studies to determine the correlation between hypoglycemia and outcome will help to clarify issues surrounding the effects of hypoglycemia on brain pathology. Long-term epidemiologic studies correlating the severity and duration of hypoglycemia with neurologic consequences are required, and can be complemented by appropriate parallel investigations in animal models of neonatal hypoglycemia.

Hypoglycemia and the breastfed neonate

Healthy, full-term infants are functionally and metabolically programmed to make the transition from their intrauterine dependent environment to their extrauterine existence without the need for metabolic monitoring or interference with the natural breastfeeding process. Full-term infants are equipped with homeostatic mechanisms that preserve adequate energy substrate to the brain and other vital organs. Thermal stability and early, properly guided, frequent, exclusive breastfeeding are the keys to success. Thus, routine screening for blood glucose concentrations or feeding sugar water is not necessary and potentially counterproductive to the establishment of a healthy mother-infant dyad.

Excitatory amino acids and taurine levels in cerebrospinal fluid of hypoxic ischemic encephalopathy in newborn

The recent studies indicating the transiently enhanced expression of excitatory amino acid receptors in hypoxia vulnerable brain regions and the elevated concentration of aspartate and glutamate in cerebrospinal fluid of asphyxiated newborns strongly suggest the role of excitatory amino acids in hypoxic ischemic brain damage in the developing human brain. In this study, we compared the concentrations of glutamate, aspartate, taurine and glycine in the cerebrospinal fluid of asphyxiated infants with values of a healthy control group. The concentrations of aspartate (5.82 +/- 3.36), glutamate (1.76 +/- 1.0) and taurine (9.32 +/- 9.1) were significantly elevated in cerebrospinal fluid of asphyxiated infants (P < 0.05). When compared to the control group, the high levels of aspartate was correlated with the degrees of hypoxic-ischemic encephalopathy (HIE) and the varying outcome. The high levels of aspartate and glutamate in the asphyxiated patients adds further evidence to the role of excitotoxicity in hypoxic ischemic encephalopathy. The mental and motor development of the patients in asphyxiated group was followed for 3 years.