Further observations on the role of -aminobutyric acid in insulin-induced hypoglycaemic convulsions

Insulin effects on core neurotransmitter pathways involved in schizophrenia neurobiology: a meta-analysis of preclinical studies. Implications for the treatment

Impairment of insulin action and metabolic dysregulation have traditionally been associated with schizophrenia, although the molecular basis of such association remains still elusive. The present meta-analysis aims to assess the impact of insulin action manipulations (i.e., hyperinsulinemia, hypoinsulinemia, systemic or brain insulin resistance) on glutamatergic, dopaminergic, γ-aminobutyric acid (GABA)ergic, and serotonergic pathways in the central nervous system. More than one hundred outcomes, including transcript or protein levels, kinetic parameters, and other components of the neurotransmitter pathways, were collected from cultured cells, animals, or humans, and meta-analyzed by applying a random-effects model and adopting Hedges'g to compare means. Two hundred fifteen studies met the inclusion criteria, of which 180 entered the quantitative synthesis. Significant impairments in key regulators of synaptic plasticity processes were detected as the result of insulin handlings. Specifically, protein levels of N-methyl-D-aspartate receptor (NMDAR) subunits including type 2A (NR2A) (Hedges' g = -0.95, 95%C.I. = -1.50, -0.39; p = 0.001; I2 = 47.46%) and 2B (NR2B) (Hedges'g = -0.69, 95%C.I. = -1.35, -0.02; p = 0.043; I2 = 62.09%), and Postsynaptic density protein 95 (PSD-95) (Hedges'g = -0.91, 95%C.I. = -1.51, -0.32; p = 0.003; I2 = 77.81%) were found reduced in insulin-resistant animal models. Moreover, insulin-resistant animals showed significantly impaired dopamine transporter activity, whereas the dopamine D2 receptor mRNA expression (Hedges'g = 3.259; 95%C.I. = 0.497, 6.020; p = 0.021; I2 = 90.61%) increased under insulin deficiency conditions. Insulin action modulated glutamate and GABA release, as well as several enzymes involved in GABA and serotonin synthesis. These results suggest that brain neurotransmitter systems are susceptible to insulin signaling abnormalities, resembling the discrete psychotic disorders' neurobiology and possibly contributing to the development of neurobiological hallmarks of treatment-resistant schizophrenia.

The effect of postischemic blood glucose levels on ischemic brain damage in the rat

The effect of insulin-induced hypoglycemia following 10.5 minutes of forebrain ischemia was studied in the rat. All groups received preischemic glucose loading (2 gm/kg) to promote brain infarction. Following completion of ischemia, rats received either 2 to 3 IU/kg (low-dose group) or 8 to 20 IU/kg (high-dose group) insulin. During the survival period, blood glucose concentrations were maintained in the ranges of 1.2 to 2.9 mM and 2.9 to 4.9 mM, respectively, for the low-dose and high-dose insulin groups. Control rats were given 2 gm/kg glucose immediately following ischemia. During the recovery period, until perfusion at 7 days, they were given glucose, 2 gm/kg, twice daily by intraperitoneal injection, and their drinking water was supplemented with 25% glucose. Mortality (p less than 0.05) and postischemic seizure incidence (p less than 0.01) were significantly reduced in the low-dose insulin group compared to the control group. Mortality was increased in the high-dose insulin group compared to the control group and was associated with an increased incidence of postischemic seizures. Neuropathological examination revealed no cortical infarction in the low-dose or high-dose insulin-treated rats compared to a 60% incidence of cortical infarction in the control group. In addition, the high-dose insulin-treated group showed a significant reduction in striatal and hippocampal CA1 selective neuronal necrosis compared to control rats with comparable survivals (p less than 0.05). The findings suggest that postischemic blood glucose concentrations play an important role in modulating both ischemic infarction and selective neuronal necrosis.

Progress review: hypoglycemic brain damage

The central question to be addressed in this review can be stated as "How does hypoglycemia kill neurons?" Initial research on hypoglycemic brain damage in the 1930s was aimed at demonstrating the existence of any brain damage whatsoever due to insulin. Recent results indicate that uncomplicated hypoglycemia is capable of killing neurons in the brain. However, the mechanism does not appear to be simply glucose starvation of the neuron resulting in neuronal breakdown. Rather than such an "internal catabolic death" current evidence suggests that in hypoglycemia, neurons are killed from without, i.e. from the extracellular space. Around the time the EEG becomes isoelectric, an endogenous neurotoxin is produced, and is released by the brain into tissue and cerebrospinal fluid. The distribution of necrotic neurons is unlike that in ischemia, being related to white matter and cerebrospinal fluid pathways. The toxin acts by first disrupting dendritic trees, sparing intermediate axons, indicating it to be an excitotoxin. Exact mechanisms of excitotoxic neuronal necrosis are not yet clear, but neuronal death involves hyperexcitation, and culminates in cell membrane rupture. Endogenous excitotoxins produced during hypoglycemia may explain the tendency toward seizure activity often seen clinically. The recent research results on which these findings are based are reviewed, and clinical implications are discussed.

Regional amino acid distribution in relation to function in insulin hypoglycaemia

The amino acids glutamate, aspartate, gamma-aminobutyric acid (GABA), and glutamine were measured as their dansyl derivatives in whole brain and specific brain regions by a sensitive double-labelling technique at various times during the development of hypoglycaemic encephalopathy. Hypoglycaemia was induced by administration of insulin (100 i.u./kg) to 24-h fasted rats. No significant changes in glutamate, GABA, or glutamine were detected in whole brain at any time up to and including the onset of hypoglycaemic convulsions. In cerebral cortex, however, GABA levels were reduced to 65% or normal prior to the appearance of neurological symptoms of hypoglycaemia. Onset of symptoms (severe catalepsy and loss of righting reflex, but before the onset of convulsions) was accompanied by marked decreases of glutamate and glutamine in striatum and hippocampus. These regions, in addition to cerebral cortex, show the greatest vulnerability to hypoglycaemic insult, according to previous anatomical studies. Aspartate levels were significantly increased (p less than 0.01) in the cerebral cortex of convulsing animals, confirming a previous report. No changes were detectable in any of the amino acids studied in medulla-pons at any time during the progression of hypoglycaemia. Cerebral cortex and striatum showed a selective net loss of amino acids (2.2 and 3.5 mumol/g. respectively) prior to the onset of insulin-hypoglycaemic convulsions.