Immunochemical studies of brain glutamate decarboxylase and GABA-transaminase of six inbred strains of mice

Glucose, Lactate, β-Hydroxybutyrate, Acetate, GABA, and Succinate as Substrates for Synthesis of Glutamate and GABA in the Glutamine-Glutamate/GABA Cycle

The glutamine-glutamate/GABA cycle is an astrocytic-neuronal pathway transferring precursors for transmitter glutamate and GABA from astrocytes to neurons. In addition, the cycle carries released transmitter back to astrocytes, where a minor fraction (~25 %) is degraded (requiring a similar amount of resynthesis) and the remainder returned to the neurons for reuse. The flux in the cycle is intense, amounting to the same value as neuronal glucose utilization rate or 75-80 % of total cortical glucose consumption. This glucose:glutamate ratio is reduced when high amounts of β-hydroxybutyrate are present, but β-hydroxybutyrate can at most replace 60 % of glucose during awake brain function. The cycle is initiated by α-ketoglutarate production in astrocytes and its conversion via glutamate to glutamine which is released. A crucial reaction in the cycle is metabolism of glutamine after its accumulation in neurons. In glutamatergic neurons all generated glutamate enters the mitochondria and its exit to the cytosol occurs in a process resembling the malate-aspartate shuttle and therefore requiring concomitant pyruvate metabolism. In GABAergic neurons one half enters the mitochondria, whereas the other one half is released directly from the cytosol. A revised concept is proposed for the synthesis and metabolism of vesicular and nonvesicular GABA. It includes the well-established neuronal GABA reuptake, its metabolism, and use for resynthesis of vesicular GABA. In contrast, mitochondrial glutamate is by transamination to α-ketoglutarate and subsequent retransamination to releasable glutamate essential for the transaminations occurring during metabolism of accumulated GABA and subsequent resynthesis of vesicular GABA.

Why are astrocytes important?

Astrocytes, which populate the grey and white mater of the brain and the spinal cord are highly heterogeneous in their morphology and function. These cells are primarily responsible for homeostasis of the central nervous system (CNS). Most central synapses are surrounded by exceedingly thin astroglial perisynaptic processes, which act as "astroglial cradle" critical for genesis, maturation and maintenance of synaptic connectivity. The perisynaptic glial processes are densely packed with numerous transporters, which provide for homeostasis of ions and neurotransmitters in the synaptic cleft, for local metabolic support and for release of astroglial derived scavengers of reactive oxygen species. Through perivascular processes astrocytes contribute to blood-brain barrier and form "glymphatic" drainage system of the CNS. Furthermore astrocytes are indispensible for glutamatergic and γ-aminobutyrate-ergic synaptic transmission being the supplier of neurotransmitters precursor glutamine via an astrocytic/neuronal cycle. Pathogenesis of many neurological disorders, including neuropsychiatric and neurodegenerative diseases is defined by loss of homeostatic function (astroglial asthenia) or remodelling of astroglial homoeostatic capabilities. Astroglial cells further contribute to neuropathologies through mounting complex defensive programme generally known as reactive astrogliosis.

Evaluation of the glutamate decarboxylase genes Gad1 and Gad2 as candidate genes for acute ethanol withdrawal severity in mice

Previous studies in crosses between the C57BL/6J (B6) and the DBA/2J (D2) mice have implicated a role of the genes encoding for the 67- and 65-kDa isoforms of the glutamate decarboxylase (Gad1 and Gad2) in the manifestation and severity of multiple ethanol-related traits such as acute ethanol withdrawal severity [Buck, K.J., Metten, P., Belknap, J.K., Crabbe, J.C., 1997. Quantitative trait loci involved in genetic predisposition to acute alcohol withdrawal in mice. J. Neurosci. 17, 3946-3955], ethanol preference [Phillips, T.J., Belknap, J.K., Buck, K.J., Cunningham, C.L., 1998. Genes on mouse chromosomes 2 and 9 determine variation in ethanol consumption. Mamm. Genome 9, 936-941] and ethanol-induced locomotion [Demarest, K., McCaughran Jr., J., Mahjubi, E., Cipp, L., Hitzemann, R., 1999. Identification of an acute ethanol response quantitative trait locus on mouse chromosome 2. J. Neurosci. 19, 549-561]. Strain-specific sequencing experiments as well as gene expression studies in drug-naive and ethanol-treated D2 and B6 mice were carried out. The Gad1 sequence was similar, the Gad2 cDNA carried only a silent polymorphism (1017 G>C) between both strains. In addition, no significant GAD65 or GAD67 expression differences were detected in either drug-nai;ve or acute ethanol withdrawn animals by Western blot experiments. Therefore, these results do not support the hypothesis of an involvement of Gad1 or Gad2 in the pathophysiology of acute ethanol withdrawal severity and the other ethanol related traits.

Gamma-aminobutyric acidB receptors and the development of the ventromedial nucleus of the hypothalamus

Gamma-aminobutyric acid (GABA) is a highly abundant neurotransmitter in the brain and the ligand for GABA(A), GABA(B), and GABA(C) receptors. Unlike GABA(A) and GABA(C) receptors, which are chloride channels, GABA(B) receptors are G-protein linked and alter cell-signaling pathways. Electrophysiological studies have found GABA(B) receptors in cultured embryonic hypothalamus, but the distribution of these receptors remains unknown. In the present study, we examined the expression of GABA(B) receptors in the ventromedial nucleus of the hypothalamus (VMH) during embryonic mouse development. GABA(B) receptors were present in the VMH at all ages examined, from embryonic day 13 to postnatal day 6. Using a brain slice preparation, we examined the effect of GABA(B) receptor activation on cell movement in the embryonic VMH as the nucleus forms in vitro. The GABA(B) receptor agonist baclofen decreased the rate of cell movement in a dose-dependent manner. Baclofen reduced cell movement by up to 56% compared with vehicle-treated controls. The percentage of cells moving per field and the angles of cell movement were not affected. With our previous findings of GABA(A) receptor activation, it is likely that GABA influences VMH development via multiple mechanisms.

Synaptic control of glycine and GABA(A) receptors and gephyrin expression in cultured motoneurons

We have evaluated the influence of the secretory phenotype of presynaptic boutons on the accumulation of postsynaptic glycine receptors (GlyRs), type A GABA receptors (GABA(A)Rs), and gephyrin clusters. The cellular distribution of these components was analyzed on motoneurons cultured either alone or with glycinergic and/or GABAergic neurons. In motoneurons cultured alone, we observed gephyrin clusters at nonsynaptic sites and in front of cholinergic boutons, whereas glycine and GABA(A) receptors formed nonsynaptic clusters. These receptors are functionally and pharmacologically similar to those found in cultures of all spinal neurons. Motoneurons receiving GABAergic innervation from dorsal root ganglia neurons displayed postsynaptic clusters of gephyrin and GABA(A)Rbeta but not of GlyRalpha/beta subunits. In motoneurons receiving glycinergic and GABAergic innervation from spinal interneurons, gephyrin, GlyRalpha/beta, and GABA(A)Rbeta formed mosaics at synaptic loci. These results indicate that (1) the transmitter phenotype of the presynaptic element determines the postsynaptic accumulation of specific receptors but not of gephyrin and (2) the postsynaptic accumulation of gephyrin alone cannot account for the formation of GlyR-rich microdomains.

Specific lysis of GABAergic synaptosomes by an antiserum to glutamate decarboxylase

An antiserum to pure glutamate decarboxylase (GAD) when incubated with rat cortical synaptosomes in the presence of complement caused release of 33-53% of lactate dehydrogenase (LDH) and 22-41% of total GAD. In addition most of the gamma-aminobutyrate (GABA) present was released. Anti-GAD antiserum alone, or complement alone, were without action. The antiserum plus complement had no effect on noradrenaline or choline uptake, and did not release choline acetylase (ChAT). Anti-ChAT serum plus complement released 30-37% of ChAT and 10-13% of LDH. It prevented choline uptake. This serum did not produce GAD release or prevent GABA, choline or noradrenaline uptake. When cortical synaptosomes were exposed to both antisera plus complement, their actions were strictly additive. The data indicate specific lysis of GABAergic and cholinergic synaptosomal sub-populations.

Gamma-aminobutyric acid interneurons in the rat hippocampal region studied by retrograde transport of glutamic acid decarboxylase antibody after in vivo injections

The retrograde axonal labeling of hippocampal GABA-ergic neurons was studied after in vivo injections of a characterized antibody against glutamic acid decarboxylase (GAD) into different parts of the hippocampal region. Small injections (50 nl) of undiluted GAD antibody into the area dentata (AD) labelled fusiform and dentate pyramidal basket cells within the AD and fusiform and multipolar cells in subfields CA2/CA3 (a and b) of Ammon's horn. The labeled cells were characterized by intense immunoreactivity of the soma and proximal parts of the dendrites, while the nucleus contained little or none. The morphological appearance and laminar positions of these cells corresponded to hippocampal GAD-positive neurons, as shown previously (Ribak et al. 1978) with immunocytochemistry. Injections of anti-GAD into the medial entorhinal area, subiculum, and CA1 region labeled cells in strata oriens, pyramidale and radiatum of CA2 and CA3a, but injections of the antibody into these latter areas failed to label cells in the medial CA1 and subiculum, thus suggesting a preferential organization of hippocampal GABA neuronal projections in a lateral to medial direction. Injections of preimmune sera or antiserum preabsorbed with the pure enzyme antigen GAD failed to label cells in a manner similar to that described for the anti-GAD injections. These observations, taken together with the finding that injections of anti-GAD into the terminal field of non-GABA-ergic pathways never resulted in retrograde axonal transport of the antibody-antigen complex, suggest that the in vivo injection of GAD antibody is a useful method to study the organization of hippocampal GABA-ergic neurons and their projections.

Ultrastructure of GABA-neurons in cultures of rat neostriatum

The subcellular distribution of glutamate decarboxylase (GAD) was revealed by electron microscopic immunocytochemistry in primary cultures of the rat neostriatum. Immunoreactivity was restricted to a small nerve cell type with rich amounts of free ribosomes and an indented nucleus. Glial cells were non-reactive. The results indicate that GABA-synthesizing neurons are viable in culture conditions and suggest that neostriatal GABA-neurons are small cells with simple ultrastructure since larger cells did not possess immunoreactivity.

Immunocytochemical localization of glutamic acid decarboxylase in monkey striate cortex

Neuronal cell bodies and synaptic terminals positive for glutamic acid decarboxylase, the enzyme responsible for synthesizing gamma-amino butyric acid, have been located by immunocytochemical staining in all layers of the macaque monkey cortex. In layers II and III the staining pattern of periodic dots is identical with that seen in sections stained for cytochrome oxidase. The rows of dots run parallel with the ocular dominance columns, suggesting that the labelled neurones are preferentially related to each eye.

Demonstration of enkephalin-, substance P- and glutamate decarboxylase-like immunoreactivity in cultured cells derived from newborn rat neostriatum

The presence of cells exhibiting leucine-enkephalin-, substance P- and glutamate decarboxylase-like immunoreactivity was demonstrated in dissociated cultures from newborn rat neostriatum. The size and shape of the enkephalin-immunoreactive cells varied, but they were generally larger than substance P- and glutamate decarboxylase-immunoreactive cells, which formed relatively uniform cell populations. Cells of apparently non-neuronal origin did not show any immunoreactivity. It is unlikely that enkephalin is present in the same cells that contain substance P or glutamate decarboxylase because of morphological differences between these cells. The possible coexistence of substance P and glutamate decarboxylase in the same cells however, could not be excluded. The results of this study confirm that the cell bodies of neurons containing three possible neurotransmitters are located in the neostriatum.

Gamma-aminobutyric acid pathways in the cerebellum studied by retrograde and anterograde transport of glutamic acid decarboxylase antibody after in vivo injections

Injections of characterized antibody against glutamic acid decarboxylase (GAD), the enzyme responsible for the synthesis of gamma-aminobutyric acid (GABA), were made into the cerebellum. Small cortical injections of anti-GAD antibody produced labeled stellate, basket, Purkinje, and Golgi cells and their processes at the injection site. Anterograde transport of GAD antigen-antibody complexes in Purkinje cell axons caused intense labelling of terminals in deep cerebellar and several vestibular nuclei. Small groups of mossy fiber rosettes labeled and produced retrograde labeling and GAD immunoreactivity in a small number of pleomorphic neurons in the deep cerebellar nuclei. Injections into the dentate nucleus produced retrograde labeling in Purkinje cell bodies and anterograde label in a small number of mossy fiber rosettes. All projections conformed to previously reported topographic distributions of corticonuclear and nucleocortical cerebellar pathways. These findings confirm the GABA content of most Purkinje cell-deep nuclei connections and provide new evidence for a GABA component in part of the nucleocortical pathway in the cerebellum. Immunocytochemical controls for specificity were conducted by injections of preimmune rabbit serum as a substitute for GAD antibody. Only nonspecific labeling was obtained in these cases. Colchicine caused a cumulative enhancement of GAD immunoreactivity in all cases. The present studies indicate that the method of in vivo antibody injections can be utilized to study chemically specific connections in nervous tissue.

L-Glutamate decarboxylase

Much progress has been made in recent years regarding enzymological aspects of mammalian brain GAD, such as its purification and characterization, but some uncertainty still remains concerning its molecular weight and forms, and its subunit structure. The availability of antibodies to this enzyme has allowed immunocytochemical studies which have provided important information on the intrinsic organization of GABA-ergic neurones in the CNS, particularly in the cerebellum and nigrostriatal pathway. With the increased understanding of the enzymology of GAD and the distribution of central GABA-ergic neurones, it is becoming feasible to study the regulatory biochemistry of GAD in terms of control and adaptive mechanisms at the cellular level. In our own laboratory, as well as in others, initial approaches have already begun. Obviously, cellular regulation of this phenotypic enzyme is an important issue for the understanding of GABA-ergic neurones and their functions.

Non-synaptic chemical neurotransmission

Images with apparently gemmulofugal polarity in the EPL of the olfactory bulb are the result of sectioning, along misleading planes, gemmulopetal synapses containing postsynaptic vesicles. Unless one accepts a bidirectional conduction for chemical synapses, the internal granule cells lack actual gemmulofugal synapses and the neurotransmitter contained on their vesicles must act at non-synaptic membranes.

Genetic variation in brain L-glutamate decarboxylase activity from two inbred strains of mice

Two inbred strains of mice, C57BL/6Bg and DBA/1Bg, were compared for genetic varaition in brain L-glutamate decarboxylase (GAD) activity. Although no large difference was found between the strains in whole brain GAD activity at adult age (27--45 days postnatally), regional examination revealed a difference in GAD activity in the cerebral cortex (15% higher in DBA); subcellular examination revealed a difference in synaptosomal fraction (21% higher in DBA). When GAD activity was measured in synaptosomal fractions prepared from dissected cerebral cortex, DBA was 34% higher than C57BL. In addition, differences in GAD activity between the two strains could be observed even in the whole brain (10--15% higher in DBA) during earlier development (15--23 days postnatally). These data indicate that at adult age, genetic difference in GAD activity between the two strains exists mainly in nerve terminals of the cerebral cortex. It is postulated that the difference may be due to a genetically mediated mechanism regulating GAD in the presynaptic terminals of GABA neurons of the cerebral cortex.