Evy Grini Iversen

LOCALIZATION OF GABAERGIC, CHOLINERGIC AND AMINERGIC STRUCTURES IN THE MESOLIMBIC SYSTEM

Abstract— The localization of cholinergic, GABAergic and aminergic structures in the ‘mesolimbic’ system has been discussed from studies on the topographical distribution of choline acetyltransferase, glutamate decarboxylase and aromatic amino acid decarboxylase in normal rat brain and in brains hemitransected at the level of globus pallidus. The structures analysed included nucleus accumbens, olfactory tubercle, septum, medial forebrain bundle, striatum, substantia nigra, ventral tegmental area and nucleus interpeduncularis.

Up-regulation of hippocampal glutamate transport during chronic treatment with sodium valproate

Excessive glutamatergic neurotransmission has been implicated in some neurodegenerative disorders. It would be of value to know whether glutamate transport, which terminates the glutamate signal, can be up‐regulated pharmacologically. Here we show that chronic treatment of rats with the anti‐epileptic drug sodium valproate (200 mg or 400 mg/kg bodyweight, twice per day for 90 days) leads to a dose‐dependent increase in hippocampal glutamate uptake capacity as measured by uptake of [3H]glutamate into proteoliposomes. The level of glutamate transporters EAAT1 and EAAT2 in hippocampus also increased dose‐dependently. No effect of sodium valproate on glutamate transport was seen in frontal or parietal cortices or in cerebellum. The hippocampal levels of glial fibrillary acidic protein and glutamine synthetase were unaffected by valproate treatment, whereas the levels of synapsin I and phosphate‐activated glutaminase were reduced by valproate treatment, suggesting that the increase in glutamate transporters was not caused by astrocytosis or increased synaptogenesis. A direct effect of sodium valproate on the glutamate transporters could be excluded. The results show that hippocampal glutamate transport is an accessible target for pharmacological intervention and that sodium valproate may have a role in the treatment of excitotoxic states in the hippocampus.

Inhibition of l-glutamate uptake into synaptic vesicles

The effects of different agents similar in structure to glutamate were tested for inhibition of the vesicular uptake of L-glutamate. Kainate and L-homocysteate turned out to be non-competitive inhibitors of the L-glutamate uptake. Kainate was not taken up by the vesicle fraction. The vesicular uptake of gamma-aminobutyric acid (GABA) was also inhibited by kainate and L-homocysteate. Kynurenate, on the other hand, strongly inhibited the uptake of L-glutamate, whereas the uptake of GABA was hardly affected. L-alpha-Aminoadipate and D-glutamate inhibited the uptake of L-glutamate, whereas L- and D-aspartate and L-cysteate only weakly inhibited the uptake of L-glutamate. GABA, glycine, L-serine and taurine did not inhibit the uptake of L-glutamate.

Effects of area 17 ablation on neurotransmitter parameters in efferents to area 18, the lateral geniculate body, pulvinar and superior colliculus in the cat

The result of unilateral ablation of visual cortical area 17 in adult cats was consistent with glutamate-aspartate being the neurotransmitter in efferents to the lateral geniculate body, the pulvinar and the visual part of superior colliculus but not in efferents to area 18 and the non-visual strata of superior colliculus. Furthermore, the distribution of glutamatergic, GABAergic and cholinergic markers within the various subdivisions of the cat visual system complied well with observations made previously with biochemical, neurophysiological, histochemical and immunohistochemical methods, in this and other mammalian species.

Valproate is neuroprotective against malonate toxicity in rat striatum: an association with augmentation of high-affinity glutamate uptake.

The antiepileptic drug valproate (VPA) may be neuroprotective. We treated rats with VPA for 14 days (300 mg/kg twice daily) before intrastriatal injection of 1.5 μmol (1 M) of the succinate dehydrogenase inhibitor malonate. VPA-treated animals developed smaller lesions than control animals: 10 ± 2 mm3 versus 26 ± 8 mm3 (means ± SD; P = 10−4). Injection of NaCl that was equiosmolar with 1 M malonate caused lesions of only 1.2 ± 0.4 mm3 in control animals, whereas physiologic saline produced no lesion. VPA pretreatment reduced the malonate-induced extracellular accumulation of glutamate. This effect paralleled an increase in the striatal level of the glutamate transporter GLT, which augmented high-affinity glutamate uptake by 25%, as determined from the uptake of [3H] glutamate into striatal proteoliposomes. Malonate caused a 76% reduction in striatal adenosine triphosphate (ATP) content, but the glial, ATP-dependent formation of glutamine from radiolabeled glucose or glutamate was intact, indicating that glial ATP production supported uptake of glutamate. Striatal levels of HSP-70 and fos were reduced, and the levels of bcl-2 and phosphorylated extracellular signal-regulated kinase remained unaffected, but histone acetylation was increased by VPA treatment. The results suggest that augmentation of glutamate uptake may contribute importantly to VPA-mediated neuroprotection in striatum.

Neurotoxicity of albumin in vivo

The neurotoxicity of albumin was studied in the rat. Solutions of rat albumin (3, 10 and 30 mg/ml) essentially free of fatty acids and globulins were injected into one neostriatum, physiological saline into the other. Injections were also performed with sodium glutamate (10 and 30 mM). Both albumin and glutamate produced lesions in a concentration-dependent manner. Thus 3, 10 and 30 mg/ml albumin produced lesions in excess of saline of 22 +/- 24 microns3, 67 +/- 25 microns3 and 170 +/- 44 microns3, (P = 0.82, 0.03 and 0.0005, respectively). 10 and 30 mM sodium glutamate caused lesions of 45 +/- 14 microns3 and 315 +/- 56 microns3 in excess of saline (P = 0.04 and 0.0004, respectively). Injection of 10 mg/ml albumin together with 10 mM sodium glutamate caused lesions of 70 +/- 11 microns3 in excess of saline (P = 0.005). This was not significantly different from the lesions caused by any of the two substances alone. Thus no potentiating effect of one substance on the toxicity of the other was seen in this study. The neurotoxicity of albumin could be of importance in disease states which are accompanied by leakiness of the blood-brain barrier.

Glutamate transport, glutamine synthetase and phosphate-activated glutaminase in rat CNS white matter. A quantitative study.

Glutamatergic signal transduction occurs in CNS white matter, but quantitative data on glutamate uptake and metabolism are lacking. We report that the level of the astrocytic glutamate transporter GLT in rat fimbria and corpus callosum was ∼ 35% of that in parietal cortex; uptake of [3H]glutamate was 24 and 43%, respectively, of the cortical value. In fimbria and corpus callosum levels of synaptic proteins, synapsin I and synaptophysin were 15–20% of those in cortex; the activities of glutamine synthetase and phosphate‐activated glutaminase, enzymes involved in metabolism of transmitter glutamate, were 11–25% of cortical values, and activities of aspartate and alanine aminotransferases were 50–70% of cortical values. The glutamate level in fimbria and corpus callosum was 5–6 nmol/mg tissue, half the cortical value. These data suggest a certain capacity for glutamatergic neurotransmission. In optic and trigeminal nerves, [3H]glutamate uptake was < 10% of the cortical uptake. Formation of [14C]glutamate from [U‐14C]glucose in fimbria and corpus callosum of awake rats was 30% of cortical values, in optic nerve it was 13%, illustrating extensive glutamate metabolism in white matter in vivo. Glutamate transporters in brain white matter may be important both physiologically and during energy failure when reversal of glutamate uptake may contribute to excitotoxicity.

Impaired reference memory and reduced glutamergic activity in rats with temporo-entorhinal connections disrupted

SummaryThe purpose of the present study was to investigate whether effects of temporo-entorhinal disconnections on acquisition and retention of a visual discrimination task might be associated with neurochemical dysfunctions. The results revealed that the present lesions impaired both acquisition and retention of the discrimination task. This impairment was accompanied by decreased glutamergic activity in both temporal and entorhinal cortices. No changes were seen in levels of acetylcholine or GABA. Further, the distribution of glutamate/ aspartate was related to both regional and hemispheric differences. The results are discussed in terms of a highly integrative role of the lateral entorhinal cortex and in terms of other putative neurotransmitter systems involved in the function of memory.

Differential rearing conditions in rats: Effects on neurochemistry in neocortical areas and cognitive behaviors

Previous studies have shown that environmental factors can influence several aspects of the central nervous system that are associated with behavioral changes. In the present study, an attempt was made to investigate how cholinergic and glutamergic transmission systems in neocortical areas might respond to differential rearing conditions and how potential neurochemical changes might be accompanied by alterations in behavior. The results show that only glutamergic levels in the lateral entorhinal cortex (LEC) responded to differential environmental stimulation. The levels of glutamergic activity in LEC correlated significantly with learning and retention of a visual discrimination task and total time exploring objects in a novelty test. A comparatively complex pattern of neurochemical relations was seen in terms of differences across brain structures and hemispheres for both glutamergic and cholinergic activity. The results are interpreted as supporting the glutamergic hypothesis of Alzheimers disease.

Evidence for a higher glycolytic than oxidative metabolic activity in white matter of rat brain

Different values exist for glucose metabolism in white matter; it appears higher when measured as accumulation of 2-deoxyglucose than when measured as formation of glutamate from isotopically labeled glucose, possibly because the two methods reflect glycolytic and tricarboxylic acid (TCA) cycle activities, respectively. We compared glycolytic and TCA cycle activity in rat white structures (corpus callosum, fimbria, and optic nerve) to activities in parietal cortex, which has a tight glycolytic-oxidative coupling. White structures had an uptake of [(3)H]2-deoxyglucose in vivo and activities of hexokinase, glucose-6-phosphate isomerase, and lactate dehydrogenase that were 40-50% of values in parietal cortex. In contrast, formation of aspartate from [U-(14)C]glucose in awake rats (which reflects the passage of (14)C through the whole TCA cycle) and activities of pyruvate dehydrogenase, citrate synthase, alpha-ketoglutarate dehydrogenase, and fumarase in white structures were 10-23% of cortical values, optic nerve showing the lowest values. The data suggest a higher glycolytic than oxidative metabolism in white matter, possibly leading to surplus formation of pyruvate or lactate. Phosphoglucomutase activity, which interconverts glucose-6-phosphate and glucose-1-phosphate, was similar in white structures and parietal cortex ( approximately 3 nmol/mg tissue/min), in spite of the lower glucose uptake in the former, suggesting that a larger fraction of glucose is converted into glucose-1-phosphate in white than in gray matter. However, the white matter glycogen synthase level was only 20-40% of that in cortex, suggesting that not all glucose-1-phosphate is destined for glycogen formation.