Glycogen phosphorylase activity in the cerebral cortex of rats during development: effect of homocysteine-induced seizures

Glycogen serves as an energy source that maintains astrocyte cell proliferation in the neonatal telencephalon

Large amounts of energy are required when cells undergo cell proliferation and differentiation for mammalian neuronal development. Early neonatal mice face transient starvation and use stored energy for survival or to support development. Glycogen is a branched polysaccharide that is formed by glucose, and serves as an astrocytic energy store for rapid energy requirements. Although it is present in radial glial cells and astrocytes, the role of glycogen during development remains unclear. In the present study, we demonstrated that glycogen accumulated in glutamate aspartate transporter (GLAST)+ astrocytes in the subventricular zone and rostral migratory stream. Glycogen levels markedly decreased after birth due to the increase of glycogen phosphorylase, an essential enzyme for glycogen metabolism. In primary cultures and in vivo, the inhibition of glycogen phosphorylase decreased the proliferation of astrocytic cells. The number of cells in the G1 phase increased in combination with the up-regulation of cyclin-dependent kinase inhibitors or down-regulation of the phosphorylation of retinoblastoma protein (pRB), a determinant for cell cycle progression. These results suggest that glycogen accumulates in astrocytes located in specific areas during the prenatal stage and is used as an energy source to maintain normal development in the early postnatal stage.

The Glutamate-Glutamine (GABA) Cycle: Importance of Late Postnatal Development and Potential Reciprocal Interactions between Biosynthesis and Degradation

The gold standard for studies of glutamate-glutamine (GABA) cycling and its connections to brain biosynthesis from glucose of glutamate and GABA and their subsequent metabolism are the elegant in vivo studies by (13)C magnetic resonance spectroscopy (NMR), showing the large fluxes in the cycle. However, simpler experiments in intact brain tissue (e.g., immunohistochemistry), brain slices, cultured brain cells, and mitochondria have also made important contributions to the understanding of details, mechanisms, and functional consequences of glutamate/GABA biosynthesis and degradation. The purpose of this review is to attempt to integrate evidence from different sources regarding (i) the enzyme(s) responsible for the initial conversion of α-ketoglutarate to glutamate; (ii) the possibility that especially glutamate oxidation is essentially confined to astrocytes; and (iii) the ontogenetically very late onset and maturation of glutamine-glutamate (GABA) cycle function. Pathway models based on the functional importance of aspartate for glutamate synthesis suggest the possibility of interacting pathways for biosynthesis and degradation of glutamate and GABA and the use of transamination as the default mechanism for initiation of glutamate oxidation. The late development and maturation are related to the late cortical gliogenesis and convert brain cortical function from being purely neuronal to becoming neuronal-astrocytic. This conversion is associated with huge increases in energy demand and production, and the character of potentially incurred gains of function are discussed. These may include alterations in learning mechanisms, in mice indicated by lack of pairing of odor learning with aversive stimuli in newborn animals but the development of such an association 10-12 days later. The possibility is suggested that analogous maturational changes may contribute to differences in the way learning is accomplished in the newborn human brain and during later development.

Analysis of age dependent effects of heat stress on EEG frequency components in rats

OBJECTIVE

To demonstrate changes in different frequencies of cerebral electrical activity or electroencephalogram (EEG) following exposure to high environmental heat in three different age groups of freely moving' rats.

METHODS

Rats were divided into three groups (i) acute heat stress--subjected to a single exposure for four hours at 38 degrees C; (ii) chronic heat stress--exposed for 21 days daily for one hour at 38 degrees C, and (iii) handling control groups. The digital polygraphic sleep-EEG recordings were performed just after the heat exposure from acute stressed rats and on 22nd day from chronic stressed rats by simultaneous recording of cortical EEG, EOG (electrooculogram), and EMG (electromyogram). Further, power spectrum analyses were performed to analyze the effects of heat stress.

RESULTS

The frequency analysis of EEG signals following exposure to high environmental heat revealed that in all three age groups of rats, changes in higher frequency components (beta 2) were significant in all sleep-wake states following both acute and chronic heat stress conditions. After exposure to acute heat, significant changes in EEG frequencies with respect to their control groups were observed, which were reversed partly or fully in four hours of EEG recording. On the other hand, due to repetitive chronic exposure to hot environment, adaptive and long-term changes in EEG frequency patterns were observed.

CONCLUSION

The present study has exhibited that the cortical EEG is sensitive to environmental heat and alterations in EEG frequencies in different sleep-wake states due to heat stress can be differentiated efficiently by EEG power spectrum analysis.

Peripheral glucose metabolism is altered by epileptic seizures

The aim of the present study was to investigate the status of jejunal absorption and peripheral metabolism of glucose in Wistar Audiogenic Rats (WAR), a genetic model of epilepsy, after seizures induced by intensive sound exposure. The jejunal loop of rats was isolated and infused (0.5 mL min(-1)) with Tyrode solution containing twice the normal concentrations of glucose, sodium, and potassium. Samples were taken at 5 or 10-min intervals over a 40-min period. At the end of the experiment, samples of liver and gastrocnemius muscle were taken to measure the levels of glycogen, glucose-6-phosphate, fructose-6-phosphate and glucose transporter-4 (GLUT4). Hepatic glucose-6-phosphate increased in WAR submitted to audiogenic seizure (21.90 +/- 3.08) as compared to non-susceptible Wistar rats (8.12 +/- 0.87) and to WAR not submitted to audiogenic stimulation (5.17 +/- 0.97). In addition, an increase in hepatic fructose-6-phosphate, an intermediate metabolite of the glycolytic pathway, was observed in WAR submitted to audiogenic seizure (5.98 +/- 0.99) compared to non-susceptible Wistar rats (2.38 +/- 0.53). According to the present results, jejunal absorption of glucose was not changed by seizures. However, generalized tonic-clonic seizures produced by sound stimulation resulted in a decrease in muscle glycogen content. In addition, our results demonstrated that the concentration of GLUT4 in the gastrocnemius muscle of WAR was 1.6-fold higher than that observed in resistant rats and that the audiogenic stimulus led to decreased concentration of this receptor in the muscle of WAR animals.

Immunocytochemical analysis of glycogen phosphorylase isozymes in the developing and adult retina of the domestic chicken (Gallus domesticus)

Glycogen is the major energy reserve in neural tissues including the retina. A key-enzyme in glycogen metabolism is glycogen phosphorylase (GP) which exists in three differentially regulated isoforms. By applying isozyme-specific antibodies it could be demonstrated that the GP BB (brain), but not the GP MM (muscle) isoform is expressed in the chicken retina in neuronal and glial (Müller) cells. In the embryonic chicken retina, GP showed a development-dependent expression pattern. Double-labeling experiments with cell type-specific antibodies revealed that GP is expressed in various layers of the retina some of which, e.g., the photoreceptor inner segments, are known to be sites of high energy consumption. This suggests important roles of GP BB, and therefore glycogen, in early differentiation, spontaneous wave generation and in formation and stabilization of synapses.

Glycogen phosphorylase isozyme pattern in mammalian retinal Müller (glial) cells and in astrocytes of retina and optic nerve

Müller cells, the radially oriented dominant macroglial cells of the retina, are known to contain abundant glycogen as well as the key enzyme for its degradation, glycogen phosphorylase (GP), but the expressed isozyme pattern is unknown. To elucidate the isoform expression pattern, specific antisera directed against the brain (BB) and muscle (MM) isoforms of GP were applied to retinal sections, isolated Müller cells, and sections of the optic nerve. We show that Müller cells of rat, rabbit, guinea pig, and mouse retina exclusively express the BB isoform. Astrocytes of rat and rabbit optic nerve, as well as retina express only the BB isoform. In contrast, astrocytes in the brain and spinal cord as well as the epithelial cells of the pars caeca and of the ciliary body express both the BB and MM isoform. This result may indicate some differences in the role of glycogen in retinal macroglia and brain astrocytes, reflecting a local specialization of macroglia in the retina proper.

Histochemical mapping of the substrate for brain-stimulation reward with glycogen phosphorylase

Glycogen phosphorylase is the enzyme that regulates glycogenolysis and it appears that there is a relationship between central levels of glycogen and neuronal activity, which is influenced by a variety of neurotransmitters. In the present study, glycogen phosphorylase histochemistry was used to correlate changes in metabolic activity in response to rewarding lateral hypothalamic stimulation. Rats were allowed to self-stimulate for 1 h per day for ten consecutive days following which postmortem phosphorylase a activity was examined. Significant differences in optical density between the stimulated and contralateral hemispheres were found in three of the eight analyzed structures, two of which, the diagonal band of Broca and the caudate nucleus, showed a greater density of glycogen phosphorylase a on the stimulated side and the third, the habenula, had greater contralateral activity. In conclusion, our data suggest that glycogen phosphorylase activity is a viable but not weighty marker of energy alterations induced by chronic exposure to intracranial self-stimulation, and that it is generally consistent with the patterns revealed by other metabolic indices such as cytochrome oxidase and 2-deoxyglucose autoradiography.

Effects of sleep deprivation in neonatal rats

This investigation represents the first systematic study of sleep homeostasis in developing mammals that spans the preweaning and postweaning periods. Neonatal rats from 12 to 24 days of postnatal life (P12-P24) were anesthetized with Metofane (methoxyflurane) and implanted with miniaturized electroencephalographic (EEG) and electromyographic electrodes. After 48 h of recovery, neonatal rats were sleep deprived for 3 h by either gentle handling or forced locomotion. We find that 3-h sleep deprivation produces dramatically different compensatory responses at different stages of postnatal development. In striking contrast to adult rats, sleep deprivation does not increase slow-wave sleep EEG delta (0.5-4.0 Hz) activity in rats younger than P24. However, P12-P20 rats do show evidence of sleep regulation because they show compensatory increases in sleep time and sleep continuity during recovery. In P12 rats, approximately 90% of total slow wave sleep time lost during the sleep-deprivation period was recovered during subsequent sleep. A similar recovery of active sleep time was observed in P20-P24 rats. These findings suggest not only that sleep is regulated in neonatal rats but that the accumulation and/or discharge of sleep need changes dramatically between the third and fourth postnatal weeks.