The cellular distribution of certain enzymes associated with the metabolic compartmentation of glutamate

Glial fibrillary acidic protein and the mesolimbic dopamine system: regulation by chronic morphine and Lewis-Fischer strain differences in the rat ventral tegmental area

In this study we demonstrate that a 51-kDa phosphoprotein, previously identified as morphine regulated and showing different basal levels among rat strains, is glial fibrillary acidic protein (GFAP). Chronic morphine increased levels of GFAP immunoreactivity by > 70% in the ventral tegmental area (VTA) of outbred Sprague-Dawley rats. This increase in GFAP content was not observed in rats that were treated concomitantly with morphine and naltrexone, an opiate receptor antagonist, and did not occur in response to a single acute injection with morphine. No alterations in GFAP levels were observed in response to chronic morphine in several other regions of the CNS studied, including the substantia nigra, locus coeruleus, cerebral cortex, and spinal cord. There were also inherent differences in levels of GFAP immunoreactivity in the VTA of drug-naive Fischer 344 and Lewis rats, two inbred rat strains that differ in their relative preference for morphine and other drugs of abuse. The VTA of drug-naive Lewis rats contained more than twofold higher levels of GFAP compared with drug-naive Fischer rats. This strain difference was also apparent in the locus coeruleus but not in several other brain regions or in spinal cord. Because the mesolimbic dopamine system is thought to play a critical role in mediating the reinforcing properties of opiates and other drugs of abuse, it is possible that the opiate induction of GFAP and inherent Lewis versus Fischer strain differences in GFAP levels in the VTA may be related to the reinforcing and/or addictive properties of opiates mediated by this brain region, as well as to genetic differences in drug preference.

Alteration of striatal glutamate release after glutamine synthetase inhibition

The effect of the glutamine synthetase (GS) inhibitor, methionine sulfoximine (MSO), on glutamate levels in, and glutamate release from, rat striatal tissue was examined. Tissue levels of glutamate were unchanged 24 h after an intraventricular injection of MSO, but tissue glutamine levels were decreased 50%. Calcium-dependent, potassium-stimulated glutamate release was diminished in tissue prisms from animals pretreated with MSO compared to controls. The decreased release of glutamate correlated over time with the inhibition of GS following an intraventricular injection of MSO. The maximum diminution of calcium-dependent, potassium-stimulated glutamate release (50%) and the maximum inhibition of GS activity (51%) were observed 24 h after MSO. The addition of 0.5 mM glutamine to the perfusion medium completely reversed the effects of MSO pretreatment on calcium-dependent, potassium-stimulated glutamate release. Since GS is localized in glial cells and the measured glutamate release is presumed to occur from neurons, the data support the contention that astroglial glutamine synthesis is an important contributor to normal neuronal neurotransmitter release.

[Biosynthesis of amino acids from glucose in the central nervous system in the Parkinson syndrome]

The incorporation of labelled carbon from glucose U-14C into CSF amino acids was investigated in three patients with Parkinson's disease and in three control persons with comparable age and physical stature. Comparing the specific radioactivities of serum and CSF one can postulate that the labelled amino acids found in the CSF are synthesized mainly by brain tissue. The resorption of glucose into the CNS and therefore the synthesis of amino acids from glucose was more rapid in controls; labelled alanine and glutamine appeared later in the CSF of the patients. As expected, in the controls the specific radioactivity of glutamic acid was found to be higher than that of glutamine, in patients the labelling of glutamine was higher as was that of serine, glycine, aspartic acid and asparagine. From our knowledge concerning the compartmentation of the metabolism of glutamate, we assume that in Parkinsonism the metabolic activity of neurons is reduced but that of astroglia is enhanced.

Putative acidic amino acid transmitters in the cerebellum. II. Electron microscopic localization of transport sites

In structurally preserved cerebellar slices, the sites of high affinity uptake of acidic amino acids were analyzed using the nonmetabolizable analogue, D-[3H]aspartate. Electron microscopic autoradiography showed the greatest accumulation of grains to be over glial structures. The labelling of the perikarya, dendrites and axons of the putatively glutamatergic granule cells was very low. However, "hypothetical grain" analysis indicated that the terminals of these cells are probable sources of radioactivity even though they contained less than 9% of the total grains in the molecular layer. The resolution of the autoradiographic technique did not permit definitive conclusion, as the parallel fibre terminals are too small and are ensheathed by thin glial processes. Nevertheless, further supporting evidence for some D-[3H]aspartate uptake into parallel fibre terminals was obtained using mechanically chopped cerebellar slices in which compared with glia presynaptic structures are selectively preserved. It is concluded that in line with hypotheses relating to the compartmentation of glutamate metabolism, the principal sites of uptake of acidic amino acids in the cerebellum are the glial cells. The results have clear implications regarding the use of high affinity uptake as a marker for glutamatergic nerve terminals.

Putative acidic amino acid transmitters in the cerebellum. I. Depolarization-induced release

In the present investigation we studied the autoradiographic localization and the characteristics of the depolarization-induced release of acidic amino acids in in vitro rat cerebellar preparations. Light microscopy autoradiography of cerebellar slices preincubated in the presence of the non-metabolized glutamate analogue D-[3H]aspartate showed a large accumulation of radioactivity over glial cells, and very little labelling of the granule cells, whose putative neurotransmitter may be glutamate. In spite of its predominant localization in glia, D-[3H]aspartate (and [14C]glutamate) was released from cerebellar slices depolarized with high [K+] in a Ca2+-dependent way, and the release elicited by veratrine was prevented by TTX. These findings, together with the observation that freshly isolated or cultured glial cells did not show any Ca2+-dependent, depolarization-induced release of D-[3H]aspartate, suggest that the radioactive amino acid released from slices has a neuronal origin. The high [K+]-induced release of exogenous radioactive acidic amino acids from superfused cerebellar synaptosomal preparations exhibited, as best, a modest Ca2+-dependence, a result probably due to the existence of a substantial non-Ca2+-dependent release of the amino acid from glial fragments contaminating the preparation. However, both the K+-evoked release of endogenous glutamate, and that of [14C]glutamate previously synthesized from [14C]glutamine were largely Ca2+-dependent, suggesting that nerve endings are the main sites involved in the stimulus-coupled secretion. In the experiments in which synaptosomes had been prelabelled with [14C]glutamine, a study of the specific radioactivity of the glutamate released and of that present in synaptosomes at the beginning and at the end of superfusion period provided evidence in favour of a preferential release of the newly synthesized [14C]glutamate. In contrast to glutamate, endogenous aspartate was not released in a Ca2+-dependent manner, and the efflux of newly formed [14C]aspartate was only slightly potentiated by Ca2+, which suggests that glutamate and aspartate are not released from the same sites. Studies on preparations (slices and synaptosomes) from immature, 8-day-old cerebella showed that neither the K+-evoked release of D-[3H]aspartate, nor that of endogenous glutamate was Ca2+-dependent. In conclusion, the data presented are consistent with the proposition that glutamate has a neurotransmitter role in the cerebellum.U