The structural and functional heterogeneity of glutamic acid decarboxylase: a review

GABAergic neuronal activity and mRNA levels for both forms of glutamic acid decarboxylase (GAD65 and GAD67) are reduced in the diagonal band of Broca during the afternoon of proestrus

There is considerable evidence that GABAergic neurons play an important role in the regulation of gonadotropin-releasing hormone (GnRH) secretion, and that these neurons may mediate the feedback actions of gonadal steroids on GnRH neurons. The aim of the present study was to investigate whether endogenous changes in ovarian steroid secretion during the estrous cycle influenced GABAergic neuronal activity in the preoptic region of the hypothalamus, and in other steroid-sensitive brain regions. Intact, adult female rats were sacrificed at various times during the days of metestrus or proestrus. GABAergic neuronal activity was estimated by measuring the rate of accumulation of GABA in microdissected brain regions after pharmacological inhibition of GABA degradation. Concentrations of mRNA for both forms of glutamic acid decarboxylase (GAD65 and GAD67) were quantified in microdissected brain regions by a microlysate ribonuclease protection assay. In the diagonal band of Broca at the level of the organum vasculosum of the lamina terminalis (DBB(ovlt)), GABAergic neuronal activity was significantly reduced during the afternoon of proestrus compared with the morning of either proestrus or metestrus. In the lateral septal nucleus, GABAergic neuronal activity was significantly increased in the afternoon of proestrus compared with the morning. There were no significant effects of time of day or day of estrous cycle in the medial preoptic nucleus, median eminence, ventromedial nucleus, suprachiasmatic nucleus, medial septal nucleus, hippocampus (CA1 region), or cingulate cortex. In the DBB(ovlt), mRNA levels for both GAD65 and GAD67 were significantly reduced in the afternoon of proestrus compared with the afternoon of metestrus. By contrast, there was no change in GAD65 and GAD67 mRNA levels in the cingulate cortex at any of the times examined. These results demonstrate that GABAergic neuronal activity, and mRNA levels for both GAD65 and GAD67, are reduced in the DBB(ovlt) during the afternoon of proestrus. These results support the hypothesis that decreased GABAergic neuronal activity in this region plays a major permissive role in the generation and maintenance of the estrogen-induced LH surge.

Presence of mRNA for glutamic acid decarboxylase in both excitatory and inhibitory neurons

Neurons in very low density hippocampal cultures that are physiologically identified as either GABAergic inhibitory or glutamatergic excitatory all contain mRNA for the gamma-aminobutyric acid (GABA) synthetic enzyme, glutamic acid decarboxylase (GAD), as detected by single cell mRNA amplification and PCR. However, consistent with the physiology, immunocytochemistry revealed that only a subset of the neurons stain for either GAD protein or GABA. A similar fraction hybridize with RNA probes for GAD65 and GAD67. Hippocampal CA1 pyramidal neurons in slice preparations, which are traditionally thought to be excitatory, also contain mRNA for GAD65 and GAD67. Hippocampal neurons in culture did not contain mRNA for two other neurotransmitter synthesizing enzymes, tyrosine hydroxylase, and choline acetyl transferase. These data suggest that in some neurons, presumably the excitatory neurons, GAD mRNA is selectively regulated at the level of translation. We propose that neurotransmitter phenotype may be posttranscriptionally regulated and neurons may exhibit transient phenotypic plasticity in response to environmental influences.

Expression of AMPA-glutamate receptor B subunit in rat hippocampal GABAergic neurons

The physiological properties of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptor (GluR) channels are determined by their subunit composition. In particular, the expression of the GluR-B subunit determines the divalent ion selectivity of the channel. We studied the distribution of GluR-B mRNA and protein in adult rat hippocampal GABAergic neurons combining non-radioactive in situ hybridization with immunocytochemistry. The majority of GABAergic hippocampal neurons were GluR-B mRNA-positive, but overall the levels of GluR-B transcript were lower than in pyramidal and granule cells, which showed the highest hybridization signal. The different GluR-B mRNA expression levels in GABAergic versus principal neurons were also observed at the protein level. There was a paucity of GluR-B/C immunoreactivity in the vast majority of somata of the GABAergic neurons studied, which contrasted with the strong expression of GluR-B/C proteins in the hippocampal principal neurons. Our results demonstrate the general low expression of the GluR-B subunit mRNA and GluR-B/C proteins in GABAergic hippocampal neurons. Considering the dominant role of the GluR-B subunit in determining the divalent ion permeability of the receptor, it is likely that most GABAergic neurons express AMPA receptor channels with different calcium permeabilities.

Immunological characterization of antibodies against synthetic peptides of glutamic acid decarboxylase

We characterized antibodies against synthetic N-terminal peptides (glutamic acid decarboxylase; GAD65N and GAD67N) and a C-terminal peptide (GAD67C) of human GAD isoforms. On Western blots, the GAD65N antibody specifically stained the 65 kDa isoform and the GAD67N antibody the 67 kDa one in various mammalian brain tissues, whereas the GAD67C antibody stained both. The immunotrapped GAD enzyme activity increased in a dose-dependent manner with increasing concentration of the N-terminal peptide antibodies, but the activity was completely inhibited by the C-terminal peptide antibody. By an enzyme-linked immunosorbent assay using rat brain GAD purified on a GAD67C antibody-affinity column, we detected GAD antibodies in 40% (24/60) of the patients with long-standing insulin-dependent diabetes mellitus (IDDM). These antipeptide antibodies are a useful tool not only for identifying the GAD isoforms, but also for purifying GAD.

Unilateral nigrostriatal lesions induce a bilateral increase in glutamate decarboxylase messenger RNA in the reticular thalamic nucleus

The reticular thalamic nucleus consists of densely packed neurons containing the neurotransmitter GABA. It surrounds the lateral border of the thalamus, has extensive reciprocal connections with thalamocortical neurons, and is thought to be involved in attentional processes. The reticular thalamic nucleus also receives direct and indirect inputs from the basal ganglia, suggesting that it may be involved in relaying motor information to the thalamus and cortex. We examined the possibility that decreased dopaminergic transmission in the basal ganglia indirectly affects the reticular thalamic nucleus. Rats received unilateral 6-hydroxydopamine lesions of the substantia nigra pars compacta and were killed two or three weeks after the lesion. Sections of the reticular thalamic nucleus were processed for in situ hybridization histochemistry at the single cell level with RNA probes for both isoforms of glutamate decarboxylase (M(r) 65,000: glutamate decarboxylase 65 and M(r) 67,000: glutamate decarboxylase 67), the rate limiting enzyme of GABA synthesis. Unilateral nigrostriatal dopaminergic lesions induced a topographically specific, bilateral increase in glutamate decarboxylase 67 messenger RNA in neurons of the lateral and ventral reticular thalamic nucleus. A much smaller increase in glutamate decarboxylase 65 messenger RNA was observed which was significant only ipsilateral to the lesion. Short- (seven day) and long-term (eight month) treatments with the antipsychotic drug haloperidol, in regimens that preferentially block D2 dopamine receptors, induced catalepsy and orofacial dyskinesia, respectively, but did not alter glutamate decarboxylase 67 messenger RNA levels in the reticular thalamic nucleus. Thus, loss of dopaminergic terminals, but not blockade of D2 dopamine receptors, induced the effects observed in the reticular thalamic nucleus. The results reveal a novel bilateral effect of unilateral dopamine depletion. In view of the role of the reticular thalamic nucleus in tremor and attentional processes, which are altered in Parkinson's disease, this effect may contribute to the clinical manifestations of nigrostriatal dopamine depletion.

Activation of a recombinant petunia glutamate decarboxylase by calcium/calmodulin or by a monoclonal antibody which recognizes the calmodulin binding domain

To date, only plants have been shown to possess a form of glutamate decarboxylase (GAD) that binds calmodulin. In the present study, a recombinant calmodulin-binding 58-kDa petunia GAD produced in Escherichia coli was purified to homogeneity using calmodulin-affinity chromatography, and its responsiveness to calcium and calmodulin was examined in vitro. At pH 7.0-7.5, the purified recombinant enzyme was essentially inactive in the absence of calcium and calmodulin, but it could be stimulated to high levels of activity (Vmax = 30 micromol of CO2 min-1 mg of protein-1) by the addition of exogenous calmodulin (K0.5 = 15 nM) in the presence of calcium (K0.5 = 0.8 microM). Neither calcium nor calmodulin alone had any effect on GAD activity. Recombinant GAD displayed hyperbolic kinetics at pH 7.3 (Km = 8.2 mM). A monoclonal antibody directed against the carboxyl-terminal region, which contains the calmodulin-binding domain of GAD, was able to fully activate GAD in a dose-dependent manner in the absence of calcium and calmodulin, whereas an antibody recognizing an epitope outside of this region was unable to activate GAD. This study provides the first evidence that the activity of the purified 58-kDa GAD polypeptide is essentially calcium/calmodulin-dependent at physiological pH. Furthermore, activation of GAD by two different proteins that interact with the calmodulin-binding domain, a monoclonal antibody or calcium/calmodulin, suggests that this domain plays a major role in the regulation of plant GAD activity.

The clinical significance of an autoimmune response against glutamic acid decarboxylase

Glutamic acid decarboxylase is attracting much interest because of its putative involvement in two clinical disorders: stiffman syndrome and insulin-dependent diabetes. Here we discuss the clinical significance of an autoimmune response against GAD and consider how such information may help identify the disease mechanisms of these disorders.

The effect of gabapentin on brain gamma-aminobutyric acid in patients with epilepsy

Gabapentin has come into clinical use as adjunctive therapy in the treatment of epilepsy. Designed to mimic gamma-aminobutyric acid (GABA), its mechanism of action remains elusive. In vivo measurements of GABA in human brain were made using 1H magnetic resonance spectroscopy. We used a 2.1-T magnetic resonance imager-spectrometer and an 8-cm surface coil to measure a 13.5-cm3 volume in the occipital cortex. GABA levels were measured in 14 patients enrolled in an open-lbel trial of gabapentin. GABA was elevated in patients taking gabapentin compared with 14 complex partial epilepsy patients, matched for antiepileptic drug treatment. Brain GABA levels appeared to be higher in patients taking high-dose gabapentin (3,300-3,600 mg/day) than in those taking standard doses (1,200-2,400 mg/day). Gabapentin appears to increase human brain GABA levels.

Displaced starburst amacrine cells of the rabbit retina contain the 67-kDa isoform, but not the 65-kDa isoform, of glutamate decarboxylase

The cholinergic identity of retinal starburst amacrine neurons is well established, but recent evidence suggests that these cells are GABAergic as well. Confirmation of this dual transmitter function requires the demonstration of glutamate decarboxylase (GAD), the biosynthetic enzyme for GABA, within starburst cells. The current work was undertaken to determine whether rabbit retinal starburst amacrine neurons contain either of the two known isoforms of GAD. To do this, we have examined the localization of the following: (1) the 65-kDa isoform of GAD; (2) the 67-kDa isoform of GAD; (3) choline acetyltransferase; and (4) the fluorescent dye DAPI, a marker for cholinergic amacrine cells. In addition, we labeled displaced starburst neurons directly, by injecting them with Lucifer Yellow in vitro. Four strata within the inner plexiform layer contained immunoreactive GAD65. A non-GAD65-immunoreactive zone separated the two innermost strata (G3 and G4); this zone contained (1) the dendrites of individual Lucifer Yellow-injected, displaced starburst amacrine cells; (2) dendrites immunoreactive for choline acetyltransferase; and (3) processes of DAPI-labeled amacrine cells. Immunoreactive GAD67 appeared in the same strata that contained GAD65, and in at least two additional strata, one of which lay at precisely the same depth as the proximal cholinergic stratum. In addition, the somas of displaced starburst cells were strongly immunoreactive for GAD67, but not for GAD65. These results demonstrate (1) that displaced starburst amacrine cells contain the 67-kDa isoform of GAD, but not the 65-kDa isoform; and (2) that the dendrites of starburst (67-kDa GAD) amacrines, and the dendrites of 65-kDa-GAD-containing amacrines, occupy different strata within the inner plexiform layer. Thus, displaced starburst cells do contain GAD, and can, presumably, manufacture GABA. The reasons for their preferential use of the 67-kDa GAD isoform remain to be elucidated.

Transcription and translation of two glutamate decarboxylase genes in the ileum of rat, mouse and guinea pig

gamma-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter, synthesised from glutamate by glutamate decarboxylase (GAD), in the central nervous system. Two forms of GAD, designated GAD 65 and GAD 67, are encoded by distinct genes and have been demonstrated in the mammalian brain. GABA has been postulated to be synthesised in neurons of the enteric nervous system (ENS), but evidence for its role as an enteric neurotransmitter is equivocal. We therefore aimed to determine whether GAD 65 and GAD 67 messenger RNAs (mRNAs) and proteins were expressed in the ileum of mice, rats and guinea pigs. Using an RNase protection assay, both GAD 65 and GAD 67 mRNAs were detected in the rodent small intestine. Antisera specific for GAD 65 or GAD 67, used in immunoblot analyses, revealed GAD 65-like and GAD 67-like immunoreactivity in rat and guinea pig ileum. Anti-GAD 65 antisera detected a major band of 65 kDa. Anti-GAD 67 antisera detected a major band of 55 kDa, which probably represented a breakdown product, and a minor band of 67 kDa. Analysis of immunoblot extracts of rat and guinea pig ileum revealed more GAD 67-like than GAD 65-like immunoreactivity. GAD enzymatic activity was high in the rat and guinea-pig brain, and low in the whole and dissected ileum. These results demonstrate that both GAD 65 and GAD 67 genes are transcribed and translated in the ileum of three rodent species and lend indirect support to the postulate that GABA is synthesised by neurons of the ENS and intestinal endocrine cells.

Estrogen modulation of mRNA levels for the two forms of glutamic acid decarboxylase (GAD) in female rat brain

Two separate forms of glutamic acid decarboxylase, now termed GAD65 and GAD67, are the rate limiting enzymes for synthesis of gamma-aminobutyric acid (GABA). Because of the significance of GABA to neuroendocrine processes, numerous attempts have been made to determine the impact of gonadal steroids on enzyme functioning with inconclusive results. Therefore, we attempted to determine the impact of estradiol on mRNA levels for each form of GAD by quantitative in situ hybridization histochemistry in various brain regions. Ovariectomized rats were treated with estradiol benzoate or oil vehicle on 2 consecutive days and the brains collected on the third day. DNA probes specific for GAD65 and GAD67 were radiolabeled with CTP32 using asymmetric polymerase chain reaction. Results of in situ hybridizations for each probe on alternate sections from the same animals were analyzed for magnocellular preoptic area (McPOA), dorsal medial nucleus of the hypothalamus (DMN), zona incerta (ZI), and midbrain central gray (MCG). In the McPOA, estradiol exerted opposite effects on the frequency distribution of pixels per cell for two GAD mRNA probes, significantly increasing GAD65 (P < .05) and decreasing GAD67 (P < .01; Kolmogorov-Smirnov). In the DMN, estradiol treatment significantly increased GAD67 by 60% (P < .05; two-way ANOVA) but decreased GAD65 mRNA by 73% (P < .01). Note the direction of effects are opposite between McPOA and DMN. In MCG, analysis showed no estradiol effect on GAD mRNA levels/cells, but the proportion of cells expressing detectable levels of GAD65 or GAD67 increased by 33-40% in estradiol-treated rats (chi 2, P < .001).

Intrathalamic striatal grafts survive and affect circling behaviour in adult rats with excitotoxically lesioned striatum

Current models of basal ganglia disorders suggest that choreoathetosis is the end result of reduced GABAergic inhibition of the motor thalamus. Graft-derived release of GABA from intrastriatal striatal grafts has also been reported. In the present work, cell suspension grafts from embryonic day 14-15 rat striatal primordia were implanted close to the ventromedial thalamic nucleus to investigate whether they can develop and survive in this ectopic location, and whether they induce changes in the circling behaviour of the host. The grafts were implanted either in normal rats or in rats whose striatum had been lesioned with ibotenic acid. These grafts were implanted either ipsilateral or contralateral to the lesioned striatum. Additionally, some rats received intrastriatal grafts, and lesioned but non-grafted rats and lesioned rats that had received injections of saline or of cell suspensions from fetal spinal cord in the thalamus were used as control. Four to eight months after transplantation, circling behaviour after amphetamine or apomorphine injection was evaluated. Serial sections were stained with Cresyl Violet and studied immunohistochemically with antibodies against DARPP-32 (dopamine- and adenosine 3',5'-monophosphate-regulated phosphoprotein, as striatal marker), Fos protein, glutamate decarboxylase (67,000 mol. wt), glutamate decarboxylase (65,000 mol. wt) and GABA. Cresyl Violet sections showed that the intrathalamic striatal grafts developed into tissue masses resembling those observed in intrastriatal striatal grafts. DARPP-32 immunohistochemistry revealed that the grafts were composed of DARPP-32 immunoreactive (striatum-like) and DARPP-32-negative patches. The intrathalamic grafts of rats which had received a low dose of apomorphine (0.25 mg/kg) 2 h before perfusion showed clusters of intensely Fos-immunoreactive nuclei throughout the transplant, indicating that these cells had developed dopamine receptors and supersensitivity to dopamine agonists. Double Fos and DARPP-32 immunohistochemistry revealed that the Fos-positive nuclei were located in the striatum-like areas. Finally, the intrathalamic grafts also contained neurons immunoreactive to GABA and glutamate decarboxylase (65,000 and 67,000 mol. wt). Rats that had received intrathalamic grafts contralateral to the lesioned striatum (i.e. contralateral to the lesion-induced turning direction) showed a significant reduction of circling both after amphetamine (78% reduction) or apomorphine (77% reduction) injection. Rats that had received grafts ipsilateral to the lesioned striatum showed a 75% decrease in amphetamine-induced circling, but no significant change in apomorphine-induced circling. No significant drug-induced circling was observed in non-lesioned and grafted rats. Sham grafting (saline) or grafting of weakly GABAergic tissue (fetal spinal cord) had no significant effects on lesion-induced circling behaviour.(ABSTRACT TRUNCATED AT 400 WORDS)

gamma-Aminobutyric acid (GABA) metabolism in mammalian neural and nonneural tissues

4-Aminobutyric acid (GABA), a major inhibitory neurotransmitter of mammalian central nervous system, is found in a wide range of organisms, from prokaryotes to vertebrates. GABA is widely distributed in nonneural tissue including peripheral nervous and endocrine systems. GABA acts on GABAA and GABAB receptors. GABAA receptors are ligand-gated chloride channels modulated by a variety of drugs. GABAB receptors are essentially presynaptic, usually coupled to potassium or calcium channels, and they function via a GTP binding protein. In neural and nonneural tissues, GABA is metabolized by three enzymes--glutamic acid decarboxylase (GAD), which produces GABA from glutamic acid, and the catabolic enzymes GABA-transaminase (GABA-T) and succinic semialdehyde dehydrogenase (SSADH). Production of succinic acid by SSADH allows entry of the GABA carbon skeleton into the tricarboxylic acid cycle. Alternate sources of GABA include putrescine, spermine, spermidine and ornithine, which produce GABA via deamination and decarboxylation reactions, while L-glutamine is an additional source of glutamic acid via deamination. GAD from mammalian brain occurs in two molecular forms, GAD65 and GAD67 (referring to subunit relative molecular weight (Mr) in kilodaltons). These different forms of GAD are the product of different genes, differing in nucleotide sequence, immunoreactivity and subcellular localization. The presence and characteristics of GAD have been investigated in a wide variety of nonneural tissues including liver, kidney, pancreas, testis, ova, oviduct, adrenal, sympathetic ganglia, gastrointestinal tract and circulating erythrocytes. In some tissues, one form (GAD65 or GAD67) predominates. GABA-T has been located in most of the same tissues, primarily through histochemical and/or immunochemical methods; GABA-T is also present in a variety of circulating cells, including platelets and lymphocytes. SSADH, the final enzyme GABA catabolism, has been detected in some of the tissues in which GAD and GABA-T have been identified, although the presence of this enzyme has not been in mammalian pancreas, ova, oviduct, testis or sympathetic ganglia.

NGF mRNA is expressed by GABAergic but not cholinergic neurons in rat basal forebrain

Nerve growth factor (NGF) supports the survival and biosynthetic activities of basal forebrain cholinergic neurons and is expressed by neurons within lateral aspects of this system including the horizontal limb of the diagonal bands and magnocellular preoptic areas. In the present study, colormetric and isotopic in situ hybridization techniques were combined to identify the neurotransmitter phenotype of the NGF-producing cells in these two areas. Adult rat forebrain tissue was processed for the colocalization of mRNA for NGF with mRNA for either choline acetyltransferase, a cholinergic cell marker, or glutamic acid decarboxylase, a GABAergic cell marker. In both regions, many neurons were single-labeled for choline acetyltransferase mRNA, but cells containing both choline acetyltransferase and NGF mRNA were not detected. In these fields, virtually all NGF mRNA-positive neurons contained glutamic acid decarboxylase mRNA. The double-labeled cells comprised a subpopulation of GABAergic neurons; numerous cells labeled with glutamic acid decarboxylase cRNA alone were codistributed with the double-labeled neurons. These data demonstrate that in basal forebrain GABAergic neurons are the principal source of locally produced NGF.

Islet cell antibodies: variable immunostaining of pancreatic islet cells and carcinoid tissue

OBJECTIVES

Islet cell antibodies (ICA) in sera of patients with autoimmune diabetes mellitus generally stain not only the insulin-producing beta cells but also the non-beta cells of the islets of Langerhans. The antibodies have been reported to react also with the chromaffin cells of carcinoid tissue. In the present study, we examined in detail the reactivity of 10 ICA-positive sera of patients with new onset insulin-dependent diabetes mellitus (IDDM) and two sera of patients with stiff-man syndrome.

DESIGN

The sera were analysed by immunofluorescence and by immunoperoxidase staining of human islets as well as by immunoprecipitations using 35S-methionine labelled rat islet lysates. In addition, immunofluorescence analyses of carcinoid tissues were carried out.

RESULTS

Eight of the 10 IDDM-positive sera reacted with all islet endocrine cells, whereas two sera showed staining restricted to the beta cells, as did the two sera of the patients with stiff-man syndrome. All beta cell 'selective' sera, but only 6 of 8 'whole' islet positive sera, immunoprecipitated the 64.kDa glutamic acid decarboxylase (GAD) antigen. The staining of carcinoid tissue was variable and did not correlate with the 'whole' or 'selective' staining pattern of islets.

CONCLUSION

The data underline a heterogeneity of ICA and indicate the presence of a separate, non-GAD antigen in islet cells. It is possible that in future studies, a resolution of ICA titres with respect to different types of islet cellular reactivity might provide insights into the pathogenesis of IDDM and improve the prognostic implications of antibody determinations.

Effect of sensory deafferentation on immunoreactivity of GABAergic cells and on GABA receptors in the adult cat visual cortex

To investigate the effects of sensory deafferentation on the cortical GABAergic circuitry in adult cats, glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) immunoreactivity and GABA receptor binding were studied in the visual cortex of normal cats and compared with cats that had received restricted binocular central lesions of the retina and had survived for 2 weeks postlesion in a normal visual environment. In the visual cortex of lesioned cats, two changes were observed in the number of GAD-immunoreactive elements in the regions affected by the retinal lesions: the number of GAD-positive puncta decreased, whereas that of GAD-immunoreactive somata increased. In contrast, no detectable changes were measured in the number of GABA-immunopositive somata or puncta. At the receptor level, we observed no differences in either the laminar distribution or the affinity of cortical GABAA and GABAB receptors labeled with [3H]-muscimol and [3H]-baclofen, respectively, in the lesioned versus normal cats. We present the hypothesis that sensory deafferentation in these adult cats (1) leads to a reduction of cortical GABAergic inhibition in the deafferented region, and (2) that this decreased inhibition may permit changes in efficiency of synapses and (3) that these changes may represent a first stage of events underlying the retinotopic reorganization preceeding the structural changes.

Transient expression of glutamate decarboxylase and gamma-amino butyric acid in embryonic lens fibers of the rat

We have determined the localization and developmental expression of glutamate decarboxylase (GAD67) in the rat lens. Immunofluorescence experiments showed that GAD67 was transiently expressed in the nuclear fiber cells of the lens between embryonic days (E) 15 and 20, with maximal immunostaining occurring on E17 and E18. gamma-amino butyric acid (GABA) co-localized with GAD67 in the embryonic nuclear fiber cells. Reverse transcription-polymerase chain reaction (RT-PCR) tests showed that at least three alternatively spliced forms of GAD67 mRNA, including mRNAs with and without the I80 and the I86 insert, were transiently co-expressed with GAD67 in the embryonic lens. The major GAD67 protein in the lens was 67 kDa. We conclude that enzymatically active GAD67 is transiently expressed in the lens nuclear fiber cells of the embryonic rat. The transient expression is regulated by transcriptional and/or posttranscriptional processes. We speculate on the basis of possible common gene regulatory elements for glutamate and ornithine decarboxylases and the involvement of these enzymes with polyamine synthesis, that the transient expression of GAD67 may be connected to nuclear and/or DNA breakdown during lens fiber cell differentiation.

Modulatory actions of gamma aminobutyric acid (GABA) on GABA type A receptor subunit expression and function

Gamma aminobutyric acid (GABA) is present in the central nervous system (CNS) during very early embryogenesis. It is therefore likely to play a role not only as a neurotransmitter but also as a signal molecule for neuronal differentiation, growth, and development. It has been firmly established that formation of synapses is strengthened by GABA, and the expression of certain subunits of the GABA type A (GABAA) receptor complex is clearly promoted by GABA. This latter effect of GABA may have profound implications for the functional activity of GABAergic synapses since the pharmacological properties of GABAA receptors are governed by the subunit composition of the receptor complex. Dynamic changes in GABAA receptor expression and diversity during development and differentiation may therefore play important roles for the inhibitory potential of the CNS during mature stages.

A convulsant, 3-mercaptopropionic acid, decreases the level of GABA and GAD in rat pancreatic islets and brain

To investigate the properties of the gamma-aminobutyric acid (GABA) synthesizing enzyme, glutamate decarboxylase (GAD), in the brain and the pancreatic islets of the rat, GABA concentration in the brain and the pancreatic islets was measured after intraperitoneal administration of 3-mercaptopropionic acid (3-MP) at 25 mg/kg. 60 min after the administration of 3-MP, GABA concentration in the hypothalamus, the superior colliculus and the hippocampus of the brain decreased by 20-30% and in the pancreatic islets by 35%. The concentration in the pancreatic acini did not change. Western blotting showed that GAD activity in the pancreatic islets decreased after administration of 3-MP compared to the control. The activity of GAD in the pancreatic islets as well as brain can be modified by a convulsant, in this case 3-MP. These results suggest the properties of GAD may be similar in the pancreatic islets and brain.

Differences between immature and adult rats in brain glutamate decarboxylase inhibition by 3-mercaptopropionic acid

Glutamate decarboxylase (EC 4.1.1.15, GAD) activity was studied in the brain of 12-day-old and adult rats treated with 3-mercaptopropionic acid (3-MPA), an inhibitor of GAD competitive with glutamate. Control GAD activity in the brains of immature animals (91.8 +/- 18.2 nmol/h/mg of protein) was lower than that of the adult rats (228 +/- 37.5 nmol/h/mg of protein). Brain GAD inhibition in adult rats was 58% at the onset of seizures (9 min on the average after administration of 70 mg 3-MPA/kg). At the same time, 3-MPA-treated young rats exhibited 76% inhibition of GAD despite the fact that at 9 min these animals were not yet having seizures. At the onset of seizures (19 min after 3-MPA on the average) their GAD activity remained at the same level. The difference between the groups was not related to the presence of the coenzyme pyridoxal-5'-phosphate in the enzyme assay. The inhibition of GAD by 3-MPA in vitro in the immature and adult brains was similar (Ki at 5.1 microM and 4.8 microM concentrations of 3-MPA, respectively). Identical values were found for Km of GAD (at 4.5 mM concentration of L-glutamate). Calculations based on the results suggest that 3-MPA enters the immature brain more easily than the brain of the adult animals. While GAD inhibition by 3-MPA is the primary cause of seizures, their onset is influenced by other factors, in which the immature brain differs from the adult one and which may include less sensitivity to GABA decrease due to relative overactivity of the GABA system.

A role for glutamate decarboxylase during tomato ripening: the characterisation of a cDNA encoding a putative glutamate decarboxylase with a calmodulin-binding site

A tomato fruit cDNA library was differentially screened to identify mRNAs present at higher levels in fruit of the tomato ripening mutant rin (ripening inhibitor). Complete sequencing of a unique clone ERT D1 revealed an open reading frame with homology to several glutamate decarboxylases. The deduced polypeptide sequence has 80% overall amino acid sequence similarity to a Petunia hybrida glutamate decarboxylase (petGAD) which carries a calmodulin-binding site at its carboxyl terminus and ERT D1 appears to have a similar domain. ERT D1 mRNA levels peaked at the first visible sign of fruit colour change during normal tomato ripening and then declined, whereas in fruit of the ripening impaired mutant, rin, accumulation of this mRNA continued until at least 14 days after the onset of ripening. This mRNA was present at much lower levels in other tissues, such as leaves, roots and stem, and was not increased by wounding. Possible roles for GAD, and its product gamma-aminobutyric acid (GABA) in fruit, are discussed.

Targeting of the 67-kDa isoform of glutamic acid decarboxylase to intracellular organelles is mediated by its interaction with the NH2-terminal region of the 65-kDa isoform of glutamic acid decarboxylase

The two isoforms of glutamic acid decarboxylase (GAD), GAD67 and GAD65, synthesize the neurotransmitter gamma-aminobutyric acid in neurons and pancreatic beta-cells. Previous studies suggest that GAD67 is a soluble cytosolic protein, whereas GAD65 is membrane-associated. Here, we study the intracellular distribution of GAD67 in neurons, pancreatic beta-cells, and fibroblasts transfected either with GAD65 and GAD67 together or with GAD67 alone. Neuronal GAD67 is partially recovered with GAD65 in membrane-containing pellet fractions and Triton X-114 detergent phases. The two proteins co-immunoprecipitate from extracts of brain and GAD65-GAD67 co-transfected fibroblasts, but not when extracts of GAD65 and GAD67 transfected fibroblasts were mixed and used as a starting material for immunoprecipitation. GAD67 is concentrated in the Golgi complex region in GAD65-GAD67 co-transfected fibroblasts, but not in fibroblasts transfected with GAD67 alone. A pool of neuronal GAD67 co-localizes with GAD65 in the Golgi complex region and in many synapses. The two proteins also co-localize in the perinuclear region of some pancreatic beta-cells. GAD67 interacts with the NH2-terminal region of GAD65, even in the absence of palmitoylation of this region of GAD65. Taken together, our results indicate that GAD65-GAD67 association occurs in vivo and is required for the targeting of GAD67 to membranes.

Glutamate decarboxylase solubilized from the rat cerebral cortex by two different concentrations of Triton X-100: effects of glutamate analogues and analysis by SDS-PAGE/western blotting using GAD6 and K2 antibodies

Analysis of two preparations (containing 0.1% and 0.5% Triton X-100) of glutamate decarboxylase (GAD) by Western blotting using GAD6 and K2 antibodies specifically recognizing two GAD isoenzymes, GAD65 and GAD67, respectively, indicated that the higher concentration of Triton X-100 at best only moderately favoured solubilization of GAD67. Several glutamate analogues were found to be either equally potent or equally inactive as inhibitors of glutamate decarboxylase activities in the two preparations. Among typical ligands for glutamate receptors and transporters, only quinolinic and L-cysteine sulphinic acids were weak inhibitors of GAD. Kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA), 3-((RS)-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), L-threo-3-hydroxy-aspartate, L-trans-pyrrolidine-2,4-dicarboxylate, dihydrokainate, kynurenic acid and N-methyl-D-aspartate were inactive. Even though the activity of glutamate decarboxylase in homogenates of rat cerebral cortex is higher at 0.5% than at 0.1% Triton X-100, structural requirements of the enzyme active site appear to be independent of Triton X-100 concentration. Furthermore, since the less soluble component of the enzyme activity contains about the same ratio of GAD65 to GAD67 as the more soluble one, it does not seem that the fractionation with Triton X-100 can be easily used to separate the two isoenzymes from each other.

Synaptic vesicle-associated glutamate decarboxylase: identification and relationship to insulin-dependent diabetes mellitus

Glutamic acid decarboxylase (GAD) catalyzes the biosynthesis of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). GAD has been suggested as an autoantigen in insulin-dependent diabetes mellitus and stiff-man syndrome. Recently, three forms of membrane-associated GAD (MGAD) have been characterized in porcine brain, but the subcellular localization and function of these proteins are unknown. We present evidence that GAD activity is associated with synaptic vesicles from porcine brain. These vesicles contain a 60 kDa protein recognized by serum from patients with insulin-dependent diabetes mellitus, probably MGADII, as shown by subcellular fractionation and immunoblotting. These results raise the possibility that the association of MGADII with synaptic vesicles may be crucial for its role as an autoantigen in insulin-dependent diabetes mellitus.

Role of dopamine in the plasticity of glutamic acid decarboxylase messenger RNA in the rat frontal cortex and the nucleus accumbens

The modulatory role of dopamine (DA) on the expression of mRNA encoding the large isoform of glutamic acid decarboxylase (GAD67), the biosynthesis enzyme of gamma aminobutyric acid (GABA), was examined in GABA neurons of two structures innervated by DA neurons originating from the ventral tegmental area (VTA): the medial frontal cortex (MFC) and the nucleus accumbens (NAcc). A bilateral electrolytic lesion of VTA was performed in rats to produce a DA denervation of both the MFC and NAcc. The efficacy of VTA lesions was verified by measurement of locomotor activity and by immunohistochemical detection of tyrosine hydroxylase in the mesencephalon. GAD67 mRNA was detected by in situ hybridization histochemistry using a 35S-labelled cDNA probe. Densitometric analysis of GAD67 mRNA hybridization signals revealed in VTA-lesioned rats a significant decrease (-24%) in GAD67 mRNA levels in the prelimbic area of the MFC and no significant effect in the anterior cingulate area or the frontoparietal cortex. Single cell analyses by computer-assisted grain counting showed that the decrease in GAD67 mRNA levels in prelimbic MFC was due to a change in GAD67 mRNA expression in a subpopulation of GABA interneurons located in the deep cortical layers (V-VI). By contrast, in the NAcc of VTA-lesioned rats, GAD67 mRNA levels were significantly increased in the anterior part and in the core but were unchanged in the shell part. These results suggest that in two target structures of VTA DA neurons, GAD67 mRNA expression is, in normal conditions, under a tonic stimulatory and a tonic inhibitory DA control in the MFC and the NAcc respectively. A schematic diagram is proposed for functional interactions between these structures.

Distinct protein forms are produced from alternatively spliced bicistronic glutamic acid decarboxylase mRNAs during development

It has been shown that the enzyme glutamic acid decarboxylase (GAD; EC 4.1.1.15), which catalyzes the conversion of L-glutamate to gamma-aminobutyric acid in the central nervous system of vertebrates, can be first detected in rodents at late embryonic stages. In contrast, we have found that the gene coding for the 67-kDa form of GAD is already transcriptionally active at embryonic day E10.5 in the mouse. In addition to the 3.5-kb adult-type mRNA, we have detected two 2-kb embryonic messages that contain alternatively spliced exons of 80 (I-80) and 86 (I-86) bp, respectively. The overlapping stop-start codon TGATG, found in the embryonic exons, converts the monocistronic adult-type transcript into a bicistronic one, coding for a 25-kDa leader peptide and a 44-kDa enzymatically active truncated GAD. A second stop codon at the 3' end of the 86-bp exon abolishes the expression of truncated GAD. The products of the two embryonic mRNAs were identified in a rabbit reticulocyte in vitro translation system, COS cells, and mouse embryos. The two GAD embryonic forms represent distinct functional domains and display characteristic developmental patterns, consistent with a possible role in the formation of the gamma-aminobutyric acid-ergic inhibitory synapses.

Distinct protein forms are produced from alternatively spliced bicistronic glutamic acid decarboxylase mRNAs during development

It has been shown that the enzyme glutamic acid decarboxylase (GAD; EC 4.1.1.15), which catalyzes the conversion of L-glutamate to gamma-aminobutyric acid in the central nervous system of vertebrates, can be first detected in rodents at late embryonic stages. In contrast, we have found that the gene coding for the 67-kDa form of GAD is already transcriptionally active at embryonic day E10.5 in the mouse. In addition to the 3.5-kb adult-type mRNA, we have detected two 2-kb embryonic messages that contain alternatively spliced exons of 80 (I-80) and 86 (I-86) bp, respectively. The overlapping stop-start codon TGATG, found in the embryonic exons, converts the monocistronic adult-type transcript into a bicistronic one, coding for a 25-kDa leader peptide and a 44-kDa enzymatically active truncated GAD. A second stop codon at the 3' end of the 86-bp exon abolishes the expression of truncated GAD. The products of the two embryonic mRNAs were identified in a rabbit reticulocyte in vitro translation system, COS cells, and mouse embryos. The two GAD embryonic forms represent distinct functional domains and display characteristic developmental patterns, consistent with a possible role in the formation of the gamma-aminobutyric acid-ergic inhibitory synapses.

Role of Protein Phosphorylation in Regulation of Brain L-Glutamate Decarboxylase Activity

In the brain, the gamma-aminobutyric acid (GABA) level is primarily controlled by the activity of its synthesizing enzyme, L-glutamate decarboxylase (GAD). At present, mechanisms responsible for regulation of GAD activity remain largely unknown. Here we report that GAD activity is inhibited by conditions favoring protein phosphorylation, and this inhibition can be reversed by phosphatase treatment. Furthermore, this inhibition appears to result from the suppression of a Ca(2+)-dependent phosphatase. Phosphorylation of GAD is demonstrated by direct incorporation of (32)P into the GAD protein. These results suggest that GAD activity in the brain is inhibited by phosphorylation and activated by dephosphorylation. A model for regulation of GABA synthesis related to neuronal excitation is discussed. Copyright 1994 S. Karger AG, Basel

Localization of mRNAs encoding two forms of glutamic acid decarboxylase in the rat hippocampal formation

The mRNAs for two forms of glutamic acid decarboxylase (GAD65 and GAD67) were localized in the rat hippocampal formation by nonradioactive in situ hybridization methods with digoxigenin-labeled cRNA probes. Some neurons in all layers of the hippocampus and dentate gyrus were readily labeled for each GAD mRNA, and the patterns of labeling for GAD65 and GAD67 mRNAs were very similar. All major groups of previously described GAD- and GABA-containing neurons appeared to be labeled for each GAD mRNA. Such findings suggest that most GABA neurons in the hippocampal formation contain both GAD mRNAs. When the labeling of neurons in the hippocampal formation and cerebral cortex was compared in the same sections, the intensity of neuronal labeling for GAD67 mRNA was generally similar in the two regions. However, the intensity of labeling for GAD65 mRNA was generally stronger for many neurons in the hippocampal formation than for most neurons in the cerebral cortex. Neurons in the hilus of the dentate gyrus were particularly well labeled for GAD65. The nonradioactive labeling for the GAD mRNAs was confined to the cytoplasm of neuronal cell bodies, and this allowed a clear visualization of the relative number and location of labeled neurons. Several distinct patterns of GAD mRNA-containing neurons were observed among different regions of the hippocampal formation. In the hilus of the dentate gyrus, GAD mRNA-containing neurons were numerous in the regions deep to the granule cell layer as well as in more central parts of the hilus. Within CA3, the densities (quantities) of labeled neurons varied among the regions. In the inner or hilar segment of CA3, the density of labeled neurons was often lower than that in the outer part of CA3 where numerous labeled neurons were distributed throughout all layers. In CA1, GAD mRNA-labeled neurons were distributed in a relatively laminar pattern with the highest density in stratum pyramidale and moderate densities in stratum oriens and at the interface between strata radiatum and lacunosum-moleculare. Lower densities were found within the latter two layers. The prominent localization of the two GAD mRNAs in the hippocampal formation suggests that a dual system for GABA synthesis is necessary for normal GABAergic function in this brain region. Most putative GABA neurons contain relatively high levels of GAD67 mRNA as might be expected if this GAD form is responsible for the synthesis of GABA for metabolic and baseline synaptic function.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of GAD mRNA in spinal cord neurons of normal and monoarthritic rats

This study was carried out to investigate whether the increase of GABA levels in spinal cord dorsal horn in response to chronic inflammatory lesions results from an enhanced expression of the gene that governs the production of glutamate decarboxylase (GAD), the enzyme responsible for GABA synthesis. In situ hybridization was used to visualize neurons expressing GAD mRNA within the spinal cord, in both intact rats and in animals bearing chronic monoarthritis induced by intraarticular injection of complete Freund's adjuvant. In control normal animals, neuronal labeling by an antisense oligonucleotide probe occurred throughout the spinal gray matter, except in the motoneuronal pool of Rexed's lamina IX. In treated animals 4 days after the induction of monoarthritis, a significant increase in the number of labeled cells occurred in the superficial laminae (25.3%) and the neck (17.2%) of the ipsilateral dorsal horn at segments L4-L5 which contain the projection domain of the ankle joint. At 2 weeks, values were, respectively, 20.2% and 13.9% over contralateral values, and an increase of 12.4% was found in the ventral horn. At 3 weeks, the ipsilateral increase of labeled cells was restricted to the superficial dorsal horn (15.2%). These findings emphasize the role played by the spinal GABAergic system in the modulation of chronic nociceptive input. It is suggested that the response of the spinal GABAergic system depends on the activation of GAD gene transcription in spinal neurons.

Experimental models of epilepsy in young animals

Seizures occur more frequently early in life. Some of these early seizures may eventually become epilepsy. Others are reactive seizures due to excessive environmental stimuli that, in any other age group, might not have elicited a similar response. To understand the developmental aspects of seizures and epilepsy in humans, it is important to study these processes in animals of equivalent ages. In this paper, we describe several animal models of developmental seizures, including their electroclinical manifestations and their validity in respect to human epileptic syndromes. There are several factors that may account for the increased seizure susceptibility of the immature brain, including the delayed development of effective systems or synaptic networks that are involved in the suppression of seizures. A better insight of the basic pathophysiology of seizures as a function of age in animal models will lead to the development of new therapeutic approaches for the treatment of age-specific epileptic disorders in humans.

Quantitative immunofluorescence data suggest a permanently enhanced GAD67/GAD65 ratio in nerve endings in rat cerebral cortex damaged by early postnatal hypoxia-ischemia: a comparison between two computer-assisted procedures for quantification of confocal laser scanning microscopic immunofluorescence images

The aim of the present study was 2-fold: (1) to determine the ratio between the amount of GAD67 and GAD65 (two isoforms of the GABA synthetizing enzyme glutamic acid decarboxylase) in nerve endings in the mature rat cerebral cortex damaged by hypoxia-ischemia during early postnatal life; and (2) to compare two different computer-assisted procedures developed for quantitative analysis of immunofluorescence images obtained with a confocal laser scanning microscope (CLSM). One procedure was based on a program present in the standard Leica CLSM software packet for full-field analysis, the other on a specially written program for object-oriented analysis run on a Kontron IBAS-KAT image analysis system. To this end, rat pups were unilaterally exposed to hypoxic-ischemic conditions and, after a survival period of 6.5 months, sacrificed by perfusion fixation. After dissection of the brain and vibratome sectioning, three animals with substantial damage on one cortical side were selected. Sections of these animals were double-stained with primary antibodies against GAD67 and GAD65 and fluorophore-conjugated secondary antibodies and subsequently sampled with a CLSM. Analysis of the CLSM images with both computer-assisted procedures showed for all three animals a clear tendency to higher GAD67/GAD65 ratios in cortical GABAergic nerve endings on the hypoxia-damaged side than in matched areas on the contralateral side. This outcome led to the following conclusions. (1) The correspondence between the outcome of both analysis procedures indicates that both procedures are valid for quantification of immunofluorescence images of nerve endings obtained with a CLSM. (2) The outcome lends further support to our view that hypoxic-ischemic encephalopathy, sustained during early postnatal life, may result in an unstable cortical network generating abnormal synchronizations and oscillations which can be amplified and propagated as true epileptic discharges. In such a network both excitatory and inhibitory processes are tonically enhanced, the latter probably as a homeostatic reaction tending to keep abnormal excitation within physiological limits.

A signal located within amino acids 1-27 of GAD65 is required for its targeting to the Golgi complex region

The mechanisms involved in the targeting of proteins to different cytosolic compartments are still largely unknown. In this study we have investigated the targeting signal of the 65-kD isoform of glutamic acid decarboxylase (GAD65), a major autoantigen in two autoimmune diseases: Stiff-Man syndrome and insulin-dependent diabetes mellitus. GAD65 is expressed in neurons and in pancreatic beta-cells, where it is concentrated in the Golgi complex region and in proximity to GABA-containing vesicles. GAD65, but not the similar isoform GAD67 which has a more diffuse cytosolic distribution, is palmitoylated within its first 100 amino acids (a.a.). We have previously demonstrated that the domain corresponding to a.a. 1-83 of GAD65 is required for the targeting of GAD65 to the Golgi complex region. Here we show that this domain is sufficient to target an unrelated protein, beta-galactosidase, to the same region. Site-directed mutagenesis of all the putative acceptor sites for thiopalmitoylation within this domain did not abolish targeting of GAD65 to the Golgi complex region. The replacement of a.a. 1-29 of GAD67 with the corresponding a.a. 1-27 of GAD65 was sufficient to target the otherwise soluble GAD67 to the Golgi complex region. Conversely, the replacement of a.a. 1-27 of GAD65 with a.a. 1-29 of GAD67 resulted in a GAD65 protein that had a diffuse cytosolic distribution and was primarily hydrophilic, suggesting that targeting to the Golgi complex region is required for palmitoylation of GAD65. We propose that the domain corresponding to a.a. 1-27 of GAD65, contains a signal required for the targeting of GAD65 to the Golgi complex region.

Distributions of GABAA, GABAB, and benzodiazepine receptors in the forebrain and midbrain of pigeons

Autoradiographic and immunohistochemical methods were used to study the distributions of GABAA, GABAB and benzodiazepine (BDZ) receptors in the pigeon fore- and midbrain. GABAA, GABAB and BDZ binding sites were found to be abundant although heterogeneously distributed in the telencephalon. The primary sensory areas of the pallium of the avian telencephalon (Wulst and dorsal ventricular ridge) tended to be low in all three binding sites, while the surrounding second order belt regions of the pallium were typically high in all three. Finally, the outermost rind of the pallium (termed the pallium externum by us), which surrounds the belt regions and projects to the striatum of the basal ganglia, was intermediate in all three GABAergic receptors types. Although both GABAA and benzodiazepine receptors were abundant within the basal ganglia, GABAA binding sites were densest in the striatum and BDZ binding sites were densest in the pallidum. Among the brainstem regions receiving GABAergic basal ganglia input, the anterior and posterior nuclei of the ansa lenticularis showed very low levels of all three receptors, while the lateral spiriform nucleus and the ventral tegmental area/substantia nigra complex contained moderate abundance of the three binding sites. The dorsalmost part of the dorsal thalamus (containing nonspecific nuclei) was rich in all three binding sites, while the more ventral part of the dorsal thalamus (containing specific sensory nuclei), the ventral thalamus and the hypothalamus were poor in all three binding sites. The pretectum was also generally poor in all three, although some nuclei displayed higher levels of one or more binding sites. The optic tectum, inferior colliculus, and central gray were rich in all three sites, while among the isthmic nuclei, the parvicellular isthmic nucleus was conspicuously rich in BDZ sites. The results show a strong correlation of the regional abundance of GABA binding sites with previously described distributions of GABAergic fibers and terminals in the avian forebrain and midbrain. The regional distribution of these binding sites is also remarkably similar to that in mammals, indicating a conservative evolution of forebrain and midbrain GABA systems among amniotes.

Drosophila GABAergic systems. II. Mutational analysis of chromosomal segment 64AB, a region containing the glutamic acid decarboxylase gene

The Drosophila melanogaster Gad gene maps to region 64A3-5 of chromosome 3L and encodes glutamic acid decarboxylase (GAD), the rate-limiting enzyme for the synthesis of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Because this neurotransmitter has been implicated in developmental functions, we have begun to study the role of GABA synthesis during Drosophila embryogenesis. We show that Gad mRNA is expressed in a widespread pattern within the embryonic nervous system. Similarly, GAD-immunoreactive protein is present during embryogenesis. These results prompted us to screen for embryonic lethal mutations that affect GAD activity. The chromosomal region to which Gad maps, however, has not been subjected to an extensive mutational analysis, even though it contains several genes encoding important neurobiological, developmental, or cellular functions. Therefore, we have initially generated both chromosomal rearrangements and point mutations that map to the Drosophila 64AB interval. Altogether, a total of 33 rearrangements and putative point mutations were identified within region 64A3-5 to 64B12. Genetic complementation analysis suggests that this cytogenetic interval contains a minimum of 19 essential genes. Within our collection of lethal mutations are several chromosomal rearrangements, two of which are in the vicinity of the Gad locus. One of these rearrangements, Df(3L)C175, is a small deletion that removes the Gad locus and at least two essential genes; the second, T(2;3)F10, is a reciprocal translocation involving the second and third chromosomes with a break within region 64A3-5. Both of these rearrangements are associated with embryonic lethality and decreased GAD enzymatic activity.

Differential localization of two glutamic acid decarboxylases (GAD65 and GAD67) in adult monkey visual cortex

Adult monkey primary visual cortex contains a diverse population of stellate neurons that utilize the neurotransmitter gamma aminobutyric acid (GABA). Two glutamic acid decarboxylase (GAD) enzymes that synthesize GABA, GAD65 and GAD67, were localized within these stellate neurons by in situ hybridization of 35S or digoxigenin (DIG) labeled riboprobes. Double labels were done by using 35S GAD67 riboprobe and GABA immunocytochemistry on the same section to verify that the neuronal population identified by immunocytochemistry was the same one studied in the in situ hybridization experiments. We find that GAD65 mRNA and GAD67 mRNA are widely distributed in the cortex, with four bands of heavily labeled neurons in upper layer 2, lower 3, 4C, and 6. GAD67 labeled neurons were more obvious in layer 4C beta, while GAD65 containing neurons were common in layer 1 and white matter. Northern blots and in situ hybridization on sections with both 35S and DIG riboprobes indicate that cortical neurons typically contain more GAD67 mRNA. Cell counts show that 18% of all cortical neurons contain GAD67 mRNA and 13% contain GAD65 mRNA, suggesting that a small population of GABA neurons might lack GAD65. Cell bodies that contain high amounts of GAD65 mRNA are prominent in layers deep 3, 4B, 4C alpha, and 6 and often are the largest cells in their respective layers. Double labels demonstrate that 96% of all GABA+ neurons contain GAD67 mRNA. Neurons heavily labeled for GABA tend to have smaller cell bodies and contain less GAD67 mRNA, while lightly labeled GABA neurons are larger and contain more GAD67 mRNA. These data indicate that most GABA neurons in monkey striate cortex contain both GAD enzymes. Although the differences in GABA content, cell size, laminar distribution, and GAD mRNA concentration suggest different requirements for GAD67 and GAD65 in cortical circuits, our experiments do not reveal what different roles these two enzymes subserve within GABAergic stellate neurons.

An integral membrane protein form of brain L-glutamate decarboxylase: purification, characterization and its relationship to insulin-dependent diabetes mellitus

A new and novel form of L-glutamate decarboxylase (GAD; EC 4.1.1.15) was purified from whole porcine brain to apparent homogeneity by a combination of column chromatographies on DE-52, ultragel AcA 34, hydroxylapatite and Sephadex G-200, and native gel electrophoresis. The purified GAD was established as an integral membrane protein based on hydrophobic interaction chromatography and membrane extraction studies. This membrane GAD (MGAD) has a native molecular weight of 120 +/- 5 kDa and is a homodimer of 60 +/- 2 kDa. Immunoprecipitation and immunoblotting tests using the sera from insulin-dependent diabetes mellitus (IDDM) patients revealed the presence of antibodies against this newly identified MGAD in IDDM. The role of MGAD in the pathogenesis of IDDM and related autoimmune disorders is also discussed.

Intracerebral administration of antisense oligodeoxynucleotides to GAD65 and GAD67 mRNAs modulate reproductive behavior in the female rat

Increased GABA activity in the medial hypothalamus (HYP) and midbrain central gray (MCG), but not the preoptic area (POA), facilitates sexual receptivity in the female rat [40]. In the current experiments, ovariectomized females were chronically treated with estrogen (via silastic capsules) to maintain a continuously high level of lordosis response. Administration of crystalline antisense oligodeoxynucleotide to the GABA synthetic enzyme, GAD67, into the HYP and MCG significantly and reversibly reduced lordosis response for 1-2 days, but did not inhibit lordosis when administered into the POA. Administration of a control oligonucleotide, consisting of the same nucleotide bases but in a scrambled sequence, did not significantly modulate behavior when infused into any brain areas. When oligodeoxynucleotide antisense to GAD67 was suspended in oil and then infused into the HYP or MCG it was more effective and resulted in less inter-animal variability. Subsequent experiments involving infusions into the MCG compared the effectiveness of antisense oligonucleotides to the two different forms of GAD, known as GAD65 and GAD67. Oligodeoxynucleotides antisense to the mRNA for either gene were effective at reducing lordosis behavior but with a different time course. Oligonucleotide antisense to GAD67 significantly reduced behavior within 24 h of infusion and there was full recovery by 4 days post-infusion. GAD65 antisense oligonucleotide did not significantly reduce behavior until 48 h post infusion and animals did not fully recover to pretest levels of lordosis until 5 days post-infusion. When antisense oligonucleotide for the two genes was administered simultaneously, the inhibition of lordosis was maximal at 24 h and stayed depressed for 4 days. There did not appear to be an additive effect of the two different antisense oligonucleotides when administered together. Tissue GABA levels in HYP and MCG of individual rats assayed by HPLC were no longer correlated with lordosis score after antisense oligonucleotide infusion but were after infusions of scrambled control oligos. Immunoblotting for the two forms of GAD revealed that GAD67 antisense oligonucleotide infusion led to significant decreases in both GAD67 and GAD65 protein levels as compared to infusions of scrambled control oligo. In addition, the levels of a neuronal marker, neuron-specific enolase, also decreased (although nonsignificantly) suggesting either a temporary shutdown of protein synthesis or a degeneration of GABAergic neurons after GAD67 antisense oligonucleotide infusion.

Convulsions and inhibition of glutamate decarboxylase by pyridoxal phosphate-gamma-glutamyl hydrazone in the developing rat

We have previously shown that in the adult rat the inhibition of brain glutamate decarboxylase (GAD) activity by pyridoxal phosphate-gamma-glutamyl hydrazone (PLPGH) administration does not result in convulsions, whereas in the adult mouse intense convulsions invariably occur. In the present study we report that, surprisingly, immature rats from 2 to 20 days of age treated with PLPGH (80 mg/kg) showed generalized tonic-clonic convulsions, whereas no convulsions at all were present in 30 days-old or older rats. GAD activity, measured by enzymic determination of GABA formed in forebrain homogenates, was inhibited by about 60% at the time of convulsions in 15 days-old and younger rats, whereas the inhibition was between 40 and 50% in older animals. The addition of the coenzyme pyridoxal 5'-phosphate to the incubation medium completely reversed this inhibition. In all treated animals GABA levels were lower compared to controls. The results indicate that the susceptibility of GAD in vivo to a diminished cofactor concentration decreases with age. It seems possible that changes in the expression of enzyme forms are reflected in developmental variations in the susceptibility to seizures induced by vitamin B6 depletion, but alterations of other B6-dependent biochemical pathways cannot be discarded.

The distribution of GABA-containing perikarya, fibers, and terminals in the forebrain and midbrain of pigeons, with particular reference to the basal ganglia and its projection targets

Immunohistochemical techniques were used to study the distributions of glutamic acid decarboxylase (GAD) and gamma-aminobutyric acid (GABA) in pigeon forebrain and midbrain to determine the organization of GABAergic systems in these brain areas in birds. In the basal ganglia, numerous medium-sized neurons throughout the striatum were labeled for GABA, while pallidal neurons, as well as a small population of large, aspiny striatal neurons, labeled for GAD and GABA. GAD+ and GABA+ fibers and terminals were abundant throughout the basal ganglia, and GABAergic fibers were found in all extratelencephalic targets of the basal ganglia. Most of these targets also contained numerous GABAergic neurons. In pallial regions, approximately 10-12% of the neurons were GABAergic. The outer rind of the pallium was more intensely labeled for GABAergic fibers than the core. The olfactory tubercle region, the ventral pallidum, and the hypothalamus were extremely densely labeled for GABAergic fibers, while GABAergic neurons were unevenly distributed in the hypothalamus. GABAergic neurons and fibers were abundant in the dorsalmost part of thalamus and the dorsal geniculate region, while GABAergic neurons and fibers were sparse (or lightly labeled) in the thalamic nuclei rotundus, triangularis, and ovoidalis. Further, GABAergic neurons were abundant in the superficial tectal layers, the magnocellular isthmic nucleus, the inferior colliculus, the intercollicular region, the central gray, and the reticular formation. GABAergic fibers were particularly abundant in the superficial tectal layers, the parvocellular isthmic nucleus, the inferior colliculus, the intercollicular region, the central gray, and the interpeduncular nucleus. These results suggest that GABA plays a role as a neurotransmitter in nearly all fore- and midbrain regions of birds, and in many instances the observed distributions of GABAergic neurons and fibers closely resemble the patterns seen in mammals, as well as in other vertebrates.

A membrane form of brain L-glutamate decarboxylase: identification, isolation, and its relation to insulin-dependent mellitus

A membrane form of L-glutamate decarboxylase (GAD) was identified and purified to apparent homogeneity from hog brain. The purified GAD was established as an integral membrane protein by phase-partitioning assay, charge-shift electrophoresis, and chromatography on a hydrophobic interaction column. This membrane GAD has a native molecular mass of 96 +/- 5 kDa and is a homodimer of 48 +/- 3-kDa subunits. Immunoprecipitation and immunoblotting tests revealed the presence of antibodies against this membrane GAD in sera from patients with insulin-dependent diabetes mellitus. Since this form of GAD appears to be an integral membrane protein and is presumed to have extracellular domains exposed, it seems reasonable to suggest that membrane GAD is more likely than soluble GAD to be involved in the pathogenesis of insulin-dependent diabetes and related autoimmune disorders such as stiff-man syndrome.

A convulsant, 3-mercaptopropionic acid, decreases the level of GABA in pancreatic islets of rat as well as that of brain

To investigate the properties of the gamma-aminobutyric acid (GABA) synthesizing enzyme, glutamate decarboxylase (GAD), in the brain and the pancreatic islets of the rat, GABA concentration in the brain and the pancreatic islets was measured after administration of 3-mercaptopropionic acid (3-MP) at 25 mg/kg intraperitoneally. Sixty minutes after the administration of 3-MP, GABA concentration in the hypothalamus, the superior colliculus and the hippocampus of the brain decreased by 20-30% and in the pancreatic islets by 35%. The activities of GAD in the pancreatic islets and brain can be modified by a convulsant, in this case 3-MP. These results suggest the properties of GAD may be similar in the pancreatic islets and brain.

Permanent increase of the GAD67/synaptophysin ratio in rat cerebral cortex nerve endings as a result of hypoxic ischemic encephalopathy sustained in early postnatal life: a confocal laser scanning microscopic study

The aim of this study was to investigate whether perinatal hypoxia-ischemia preferentially destroys GABAergic nerve endings in rat cerebral cortex tissue which, in its turn, could then account for the reported higher risk of developing epilepsy later in life. To that end rat pups, with an age of 12-13 days postnatally, were unilaterally exposed to hypoxic-ischemic conditions. After a survival period of 2 to 6 months, the animals were sacrificed by perfusion fixation and their brains were used for cutting transversal vibratome and frozen sections. These sections were double-stained with primary antibodies against one of the two GABA synthesizing enzymes, glutamic acid decarboxylase with a mol. wt. of 66,600 (GAD67) and one of the intrinsic membrane proteins of small synaptic vesicles, synaptophysin, followed by fluorophore-conjugated second antibodies. By using the confocal laser scanning microscope, we determined the ratio between the amount of GAD67/synaptophysin immunofluorescence in nerve endings per unit volume of tissue in the hypoxia-damaged neocortex. It turned out that this ratio, contrary to expectations, was significantly higher in the hypoxia-damaged cortical areas than in matched areas on the contralateral side. It appeared, moreover, that this effect was directly proportional to the severity of the incurred damage. The conclusion was drawn that these observations do not support the hypothesis that perinatal hypoxia-ischemia ultimately leads to a preferential loss of GABAergic nerve endings in the damaged neocortex and, as such, to a shortage of inhibition.

Identification of a dominant epitope of glutamic acid decarboxylase (GAD-65) recognized by autoantibodies in stiff-man syndrome

Glutamic acid decarboxylase (GAD) is the enzyme that synthesizes the neurotransmitter gamma-aminobutyric acid (GABA) in neurons and in pancreatic beta cells. It is a major target of autoimmunity in Stiff-Man syndrome (SMS), a rare neurological disease, and in insulin-dependent diabetes mellitus. The two GAD isoforms, GAD-65 and GAD-67, are the products of two different genes. GAD-67 and GAD-65 are very similar to each other in amino acid sequence and differ substantially only at their NH2-terminal region. We have investigated the reactivity of autoantibodies of 30 Stiff-Man syndrome patients to GAD. All patient sera contained antibodies that recognize strongly GAD-65, but also GAD-67, when tested by immunoprecipitation on brain extracts and by immunoprecipitation or immunocytochemistry on cells transfected with either the GAD-65 or the GAD-67 gene. When tested by Western blotting, all patient sera selectively recognized GAD-65. Western blot analysis of deletion mutants of GAD-65 demonstrated that autoantibodies are directed predominantly against two regions of the GAD-65 molecule. All SMS sera strongly recognized a fragment contained between amino acid 475 and the COOH terminus (amino acid 585). Within this region, amino acids 475-484 and 571-585 were required for reactivity. The requirement of these two discontinuous segments implies that the epitope is influenced by conformation. This reactivity is similar to that displayed by the monoclonal antibody GAD 6, suggesting the presence of a single immunodominant epitope (SMS-E1) in this region of GAD-65. In addition, most SMS sera recognized at least one epitope (SMS-E2) in the NH2-terminal domain of GAD-65 (amino acids 1-95). The demonstration in SMS patients of a strikingly homogeneous humoral autoimmune response against GAD and the identification of dominant autoreactive target regions may help to elucidate the molecular mechanisms of GAD processing and presentation involved in GAD autoimmunity. Moreover, the reactivity reported here of GAD autoantibodies in SMS partially differs from the reactivity of GAD autoantibodies in insulin-dependent diabetes mellitus, suggesting a link between the pattern of humoral autoimmunity and the clinical condition.

Glutamic acid decarboxylase gene expression in thalamic reticular neurons transplanted as a cell suspension in the adult thalamus

The goal of the present study was to determine whether alterations in neuronal morphology and connections in thalamic grafts were accompanied by changes in the expression of mRNA encoding glutamic acid decarboxylase (GAD), the key enzyme in the synthesis of GABA, the normal neurotransmitter of neurons of the thalamic reticular nucleus. Cell suspensions of rat fetal tissue containing both thalamic reticular nucleus and ventrobasal primordia were transplanted into the excitotoxically lesioned somatosensory thalamus of adult rats. Levels of messenger RNA (mRNA) encoding GAD (Mr 67,000; GAD67) were measured 7 days to 4 months following transplantation via quantitative in situ hybridization with 35S-radiolabeled antisense RNAs. Expression of GAD67 mRNA in the thalamic reticular nucleus was analyzed in parallel in rat pups between 0 and 30 days postnatally, and in adult animals. As already observed with immunohistochemistry, transplanted neurons of the thalamic reticular nucleus did not group in specific clusters but rather mingled with unlabeled (putatively ventrobasal) neurons. Levels of labelling for GAD67 mRNA per neuron increased over time and reached adult levels during the third week post-grafting, i.e. 2 weeks after the theoretical birthdate of the neurons (grafted at embryonic days 15-16). Similar values were observed and a plateau was reached at similar time points during normal ontogeny. The results suggest that, in contrast to morphology and size of the neuronal cell bodies, gene expression of GAD67 develops normally despite the ectopic location of neurons of the thalamic reticular nucleus in the somatosensory thalamus, the abnormal connectivity and the lack of segregation from non-GABAergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Transient presence of GABA in astrocytes of the developing optic nerve

Immunostaining and high-pressure liquid chromatography (HPLC) were used to study the developmental time course of astrocytic gamma-aminobutyric acid (GABA) expression in rat optic nerve. GABA immunostaining was carried out on cultured astrocytes, and on whole optic nerve. Confocal scanning laser microscopy was used to obtain optical sections in excised whole tissue in order to localize the cellular origins of GABA within the relatively intact optic nerve. GABA immunoreactivity was localized in astrocytes identified by GFAP staining; GABA staining was most intense in early neonatal optic nerve and attenuated over 3 weeks of postnatal development. The staining was pronounced in the astrocyte cell bodies and processes but not in the nucleus. There was a paucity of GABA immunoreactivity by postnatal day 20, both in culture and in whole optic nerve. A biochemical assay for optic nerve GABA using HPLC indicated a relatively high concentration of GABA in the neonate, which rapidly attenuated over the first 3 postnatal weeks. Immunoreactivity for the GABA synthesis enzyme glutamic acid decarboxylase (GAD) was pronounced in neonates but also attenuated with development. These results indicate that GABA and the GABA synthesis enzyme GAD are localized in astrocytes of optic nerve, and that their expression is transient during postnatal development.