Regional differences in cofactor saturation of glutamate decarboxylase (GAD) in discrete brain nuclei of the rat. Effect of repeated administration of haloperidol on GAD activity in the substantia nigra

Neurochemical effects of motor cortex stimulation in the periaqueductal gray during neuropathic pain

OBJECTIVE

Motor cortex stimulation (MCS) is a neurosurgical technique used to treat patients with refractory neuropathic pain syndromes. MCS activates the periaqueductal gray (PAG) matter, which is one of the major centers of the descending pain inhibitory system. However, the neurochemical mechanisms in the PAG that underlie the analgesic effect of MCS have not yet been described. The main goal of this study was to investigate the neurochemical mechanisms involved in the analgesic effect induced by MCS in neuropathic pain. Specifically, we investigated the release of γ-aminobutyric acid (GABA), glycine, and glutamate in the PAG and performed pharmacological antagonism experiments to validate of our findings.

METHODS

Male Wistar rats with surgically induced chronic constriction of the sciatic nerve, along with sham-operated rats and naive rats, were implanted with both unilateral transdural electrodes in the motor cortex and a microdialysis guide cannula in the PAG and subjected to MCS. The MCS was delivered in single 15-minute sessions. Neurotransmitter release was evaluated in the PAG before, during, and after MCS. Quantification of the neurotransmitters GABA, glycine, and glutamate was performed using a high-performance liquid chromatography system. The mechanical nociceptive threshold was evaluated initially, on the 14th day following the surgery, and during the MCS. In another group of neuropathic rats, once the analgesic effect after MCS was confirmed by the mechanical nociceptive test, rats were microinjected with saline or a glycine antagonist (strychnine), a GABA antagonist (bicuculline), or a combination of glycine and GABA antagonists (strychnine+bicuculline) and reevaluated for the mechanical nociceptive threshold during MCS.

RESULTS

MCS reversed the hyperalgesia induced by peripheral neuropathy in the rats with chronic sciatic nerve constriction and induced a significant increase in the glycine and GABA levels in the PAG in comparison with the naive and sham-treated rats. The glutamate levels remained stable under all conditions. The antagonism of glycine, GABA, and the combination of glycine and GABA reversed the MCS-induced analgesia.

CONCLUSIONS

These results suggest that the neurotransmitters glycine and GABA released in the PAG may be involved in the analgesia induced by cortical stimulation in animals with neuropathic pain. Further investigation of the mechanisms involved in MCS-induced analgesia may contribute to clinical improvements for the treatment of persistent neuropathic pain syndromes.

Altered GABAergic and glutamatergic activity within the rat hippocampus and amygdala in rats subjected to repeated corticosterone administration but not restraint stress

We investigated the effect of two well characterized preclinical animal models of depression - repeated injections of corticosterone (CORT) and repeated restraint stress - on markers of GABAergic and glutamatergic activity in the hippocampus and amygdala. Stress is an identified risk factor for the onset of major depression, but the neurobiological mechanisms by which stress may produce depressogenic effects are not clear. Rats received one of the following four treatments for 21 consecutive days: daily single CORT injections (40mg/kg), daily single vehicle injections, daily 6h of restraint stress, or daily handling. After the 21-day stress period, all rats were sacrificed and hippocampal and amygdalar tissue was collected and prepared for Western blot analyses. We examined the effect of CORT and restraint stress on glutamate decarboxylase (GAD)-65 and GAD67, as well as the α1, α2, α3, and β2-3 GABA(A) receptor subunits, and the vesicular glutamate transporter (VGLUT)-2. We found that CORT significantly decreased GAD65 and the α2 receptor subunit and increased VGLUT2 within the hippocampus. We also found that CORT decreased GAD67 and the α2 receptor subunit in the amygdala. However, restraint stress had no significant effect on protein expression in either the hippocampus or the amygdala. These findings parallel our previous results showing that repeated CORT injections, but not restraint stress, increase depression-like behavior in rats, and suggest that the depressogenic effects of CORT may be related to alterations in GABAergic and glutamatergic neurotransmission in stress-sensitive regions of the brain.

Evidence that GAD65mediates increased GABA synthesis during intense neuronal activityin vivo

In this study we tested the hypothesis that the 65-kDa isoform of glutamate decarboxylase (GAD(65)) mediates activity-dependent GABA synthesis as invoked by seizures in anesthetized rats. GABA synthesis was measured following acute GABA-transaminase inhibition by gabaculine using spatially localized (1)H NMR spectroscopy before and after bicuculline-induced seizures. Experiments were conducted with animals pre-treated with vigabatrin 24 h earlier in order to reduce GAD(67) protein and also with non-treated controls. GAD isoform content was quantified by immunoblotting. GABA was higher in vigabatrin-treated rats compared to non-treated controls. In vigabatrin-treated animals, GABA synthesis was 28% lower compared to controls [p < 0.05; vigabatrin-treated, 0.043 +/- 0.011 micromol/(g min); non-treated, 0.060 +/- 0.014 micromol/(g min)] and GAD(67) was 60% lower. No difference between groups was observed for GAD(65). Seizures increased GABA synthesis in both control [174%; control, 0.060 +/- 0.014 micromol/(g min) vs. seizures, 0.105 +/- 0.043 micromol/(g min)] and vigabatrin-treated rats [214%; control, 0.043 +/- 0.011 micromol/(g min); seizures, 0.092 +/- 0.018 micromol/(g min)]. GAD(67) could account for at least half of basal GABA synthesis but only 20% of the two-fold increase observed in vigabatrin-treated rats during seizures. The seizure-induced activation of GAD(65) in control cortex occurs concomitantly with a 2.3-fold increase in inorganic phosphate, known to be a potent activator of apoGAD(65)in vitro. Our results are consistent with a major role for GAD(65) in activity-dependent GABA synthesis.

Effect of apocalmodulin on recombinant human brain glutamic acid decarboxylase

In this work, we report that the recombinant glutathione S-transferase (GST)-human L-glutamic acid decarboxylase (HGAD) isoforms, 65-kDa L-glutamic acid decarboxylase (GAD) (GST-HGAD65) fusion protein or free truncated HGAD65, were activated by apocalmodulin (ApoCaM) to an extent of 60%. Both truncated forms of GAD67 (tGAD67), HGAD67(Delta1-70) and HGAD67(Delta1-90), were markedly activated by ApoCaM to an extent of 141 and 85%, respectively, while GST-HGAD67 was not significantly affected. The activation appears to be due to an increase of GAD affinity for its cofactor, pyridoxal phosphate (PLP). This conclusion is based on the following observations. Firstly, the V(max) of GAD was increased when ApoCaM was present whereas the affinity for the substrate, glutamate, was not affected. Secondly, the affinity of GAD for PLP was increased in the presence of ApoCaM. Thirdly, results from calmodulin-agarose affinity column chromatography studies indicated a direct interaction or binding between ApoCaM and GAD. Fourthly, ApoCaM was found to be copurified with GAD65/GAD67 by anti-GAD65/67 immunoaffinity column using rat brain extract. Hence, it is proposed that a conformational change is induced when ApoCaM interacts with GAD65 or tGAD67, resulting in an increase of GAD affinity for PLP and the activation of GAD. The physiological significance of the interaction between GAD and ApoCaM is discussed.

Molecular Identification of the 62 kd Form of Glutamic Acid Decarboxylase from the Mouse

A series of overlapping clones coding for L-glutamic acid decarboxylase was purified from a mouse brain cDNA library, the longest of which contains a 1869 bp open reading frame and 913 bp of non-coding sequence. By comparison with the corresponding sequences from the mouse genome, it was determined that the first methionine in the longest cDNA represents the initiation codon. Expression of this cDNA in eukaryotic cells produces a 62 kd protein that is recognized by antiserum against rat GAD and which displays GAD activity commensurate with the amount of protein produced. Antibodies raised against the purified product of this cDNA recognize a 62 kd protein from mouse brain on immunoblots, specifically stain GABA-ergic neurons in brain sections, and are capable of immunoprecipitating most GAD activity from mouse brain extracts. These results provide the first definitive identification of a cDNA coding for the larger of two forms of GAD in mouse brain, and suggest that the two forms are closely related.

Immunohistochemical studies of brain pyridoxine-5'-phosphate oxidase

A total of six hybridoma cell lines, which produce monoclonal antibodies (mAbs) against the sheep brain pyridoxine-5'-phosphate oxidase (PNP oxidase), were established. Isotype analysis revealed that all antibodies corresponded to the IgG 2B kappa subclass. Immunoblotting with various tissue homogenates indicated that all the mAbs specifically recognize a single protein band of 30 kDa. They also appear to be extensively cross-reactive among different mammalian and avian sources. These results demonstrated that only one type of immunologically similar PNP oxidase is present in all of the mammalian tissues tested. When the purified PNP oxidase was incubated with the mAbs, the enzyme activity was inhibited up to a maximum of 81%. Furthermore, these antibodies were successfully applied in immunohistochemistry in order to detect PNP oxidase in various regions of rat brain tissues. The immunoreactive neurons in PNP oxidase were found in cerebellar cortex, hippocampus, amygdala, paraventricular nucleus, cerebral cortex and ependyma. This result suggests that PNP oxidase may play an important role in the neuronal metabolism.

Increased expression of GAD65 and GABA in pancreatic beta-cells impairs first-phase insulin secretion

The functional role of glutamate decarboxylase (GAD) and its product GABA in pancreatic islets has remained elusive. Mouse beta-cells express the larger isoform GAD67, whereas human islets express only the smaller isoform GAD65. We have generated two lines of transgenic mice expressing human GAD65 in pancreatic beta-cells (RIP7-hGAD65, Lines 1 and 2) to study the effect that GABA generated by this isoform has on islet cell function. The ascending order of hGAD65 expression and/or activity in beta-cells was Line 1 heterozygotes < Line 2 heterozygotes < Line 1 homozygotes. Line 1 heterozygotes have normal glucose tolerance, whereas Line 1 homozygotes and Line 2 heterozygotes exhibit impaired glucose tolerance and inhibition of insulin secretion in vivo in response to glucose. In addition, fasting levels of blood glucose are elevated and insulin is decreased in Line 1 homozygotes. Pancreas perfusion experiments suggest that GABA generated by GAD65 may function as a negative regulator of first-phase insulin secretion in response to glucose by affecting a step proximal to or at the K(ATP)(+) channel.

The hydrophilic isoform of glutamate decarboxylase, GAD67, is targeted to membranes and nerve terminals independent of dimerization with the hydrophobic membrane-anchored isoform, GAD65

GAD67, the larger isoform of the gamma-aminobutyric acid-synthesizing enzyme glutamic acid decarboxylase, is a hydrophilic soluble molecule, postulated to localize at nerve terminals and membrane compartments by heterodimerization with the smaller membrane-anchored isoform GAD65. We here show that the dimerization region in GAD65 is distinct from the NH(2)-terminal membrane-anchoring region and that a membrane anchoring GAD65 subunit can indeed target a soluble subunit to membrane compartments by dimerization. However, only a fraction of membrane-bound GAD67 is engaged in a heterodimer with GAD65 in rat brain. Furthermore, in GAD65-/- mouse brain, GAD67, which no longer partitions into the Triton X-114 detergent phase, still anchors to membranes at similar levels as in wild-type mice. Similarly, in primary cultures of neurons derived from GAD65-/- mice, GAD67 is targeted to nerve terminals, where it co-localizes with the synaptic vesicle marker SV2. Thus, axonal targeting and membrane anchoring is an intrinsic property of GAD67 and does not require GAD65. The results suggest that three distinct moieties of glutamate decarboxylase localize to membrane compartments, an amphiphilic GAD65 homodimer, an amphiphilic GAD65/67 heterodimer, tethered to membranes via the GAD65 subunit, and a hydrophilic GAD67 homodimer, which associates with membranes by a distinct mechanism.

Role of synaptic vesicle proton gradient and protein phosphorylation on ATP-mediated activation of membrane-associated brain glutamate decarboxylase

Previously, we have shown that the soluble form of brain glutamic acid decarboxylase (GAD) is inhibited by ATP through protein phosphorylation and is activated by calcineurin-mediated protein dephosphorylation (Bao, J., Cheung, W. Y., and Wu, J. Y. (1995) J. Biol. Chem. 270, 6464-6467). Here we report that the membrane-associated form of GAD (MGAD) is greatly activated by ATP, whereas adenosine 5'-[beta,gamma-imido]triphosphate (AMP-PNP), a non-hydrolyzable ATP analog, has no effect on MGAD activity. ATP activation of MGAD is abolished by conditions that disrupt the proton gradient of synaptic vesicles, e.g. the presence of vesicular proton pump inhibitor, bafilomycin A1, the protonophore carbonyl cyanide m-chorophenylhydrazone or the ionophore gramicidin, indicating that the synaptic vesicle proton gradient is essential in ATP activation of MGAD. Furthermore, direct incorporation of (32)P from [gamma-(32)P]ATP into MGAD has been demonstrated. In addition, MGAD (presumably GAD65, since it is recognized by specific monoclonal antibody, GAD6, as well as specific anti-GAD65) has been reported to be associated with synaptic vesicles. Based on these results, a model linking gamma-aminobutyric acid (GABA) synthesis by MGAD to GABA packaging into synaptic vesicles by proton gradient-mediated GABA transport is presented. Activation of MGAD by phosphorylation appears to be mediated by a vesicular protein kinase that is controlled by the vesicular proton gradient.

Regional distribution and relative amounts of glutamate decarboxylase isoforms in rat and mouse brain

The levels of the two isoforms of glutamate decarboxylase (GAD) were measured in 12 regions of adult rat brain and three regions of mouse brain by sodium dodecylsulfate-polyacrylamide gel electrophoresis and immunoblotting with an antiserum that recognizes the identical C-terminal sequence in both isoforms from both species. In rat brain the amount of smaller isoform, GAD65, was greater than that of the larger isoform, GAD67, in all twelve regions. GAD65 ranged from 77-89% of total GAD in frontal cortex, hippocampus, hypothalamus, midbrain, olfactory bulb, periaqueductal gray matter, substantia nigra, striatum, thalamus and the ventral tegmental area. The proportion of GAD65 was lower in amygdala and cerebellum but still greater than half of the total. There was a strong correlation between total GAD protein and GAD activity. In the three mouse brain regions analysed (cerebellum, cerebral cortex and hippocampus) the proportion of GAD65 (35,47, and 51% of total GAD) was significantly lower than in the corresponding rat-brain regions. The amount of GAD67 was greater than the amount of GAD65 in mouse cerebellum and was approximately equal to the amount of GAD65 in mouse cerebral cortex and hippocampus.

Increased anxiety and altered responses to anxiolytics in mice deficient in the 65-kDa isoform of glutamic acid decarboxylase

The larger isoform of the enzyme glutamate decarboxylase, GAD67, synthesizes >90% of basal levels of gamma-aminobutyric acid (GABA) in the brain. In contrast, the smaller isoform, GAD65, has been implicated in the fine-tuning of inhibitory neurotransmission. Mice deficient in GAD65 exhibit increased anxiety-like responses in both the open field and elevated zero maze assays. Additionally, GAD65-deficient mice have a diminished response to the anxiolytics diazepam and pentobarbital, both of which interact with GABA-A receptors in a GABA-dependent fashion to facilitate GABAergic neurotransmission. Loss of GAD65-generated GABA does not appear to result in compensatory postsynaptic GABA-A receptor changes based on radioligand receptor binding studies, which revealed no change in the postsynaptic GABA-A receptor density. Furthermore, mutant and wild-type animals do not differ in their behavioral response to muscimol, which acts independently of the presence of GABA. We propose that stress-induced GABA release is impaired in GAD65-deficient mice, resulting in increased anxiety-like responses and a diminished response to the acute effects of drugs that facilitate the actions of released GABA.

Structural characteristics of brain glutamate decarboxylase in relation to its interaction and activation

The conformation, stability, cofactor interactions, and activation of a recombinant 65-kDa form of rat brain glutamate decarboxylase (GAD65) were investigated by using UV/visible spectrophotometry, fluorescence spectroscopy, circular dichroism, and differential scanning microcalorimetry. The enzyme was prepared from Sf9 insect cells infected with a recombinant baculovirus containing the entire GAD65 coding region. The UV/visible absorption spectrum of purified holoenzyme (holoGAD) exhibits two peaks in the range of 300-450 nm, which are due to the formation of a Schiff base when pyridoxal phosphate (pyridoxal-P) binds to GAD. Fluorescence emission intensity (excited at 295 or 280 nm) was substantially enhanced when pyridoxal-P was removed from holoGAD and quenched when pyridoxal-P was added to the apoenzyme (apoGAD). These observations implied that a significant enzyme conformational change occurs during the formation of holoGAD. Circular dichroism provided additional evidence for a conformational change, as the ellipticity of both negative (202-242 nm) and positive (188-202 nm) bands decreased when pyridoxal-P was removed from holoGAD. Secondary structure determination estimated that holoGAD contains a higher content of alpha-helix (34% versus 24%) and a lower content of beta-sheet (18% versus 30%) than apo-GAD. Differential scanning microcalorimetry indicated that holoGAD exhibits a much larger enthalpy and a 3 degrees C higher temperature of thermal unfolding than apoGAD, suggesting that holoGAD has a much tighter conformation and greater stability than apoGAD. A model describing the interaction of pyridoxal-P with GAD is presented, which proposes that an intermediate complex involving ionic interaction between the phosphate group of pyridoxal-P and the positive, charged residues in the active site of GAD maintains the pyridoxal-P molecule in an appropriate position in the active center. Simultaneously, this complex formation is accompanied by a moderate enzyme conformational change, providing a favorable configuration that enables the epsilon-amino of the active-site lysine to react with the aldehyde group of pyridoxal-P. The formation of active holoGAD involves a large enzyme conformational change, which leads to increased stability.

Epilepsy in mice deficient in the 65-kDa isoform of glutamic acid decarboxylase

gamma-Aminobutyric acid (GABA), the major inhibitory neurotransmitter in the mammalian brain, is synthesized by two glutamate decarboxylase isoforms, GAD65 and GAD67. The separate role of the two isoforms is unknown, but differences in saturation with cofactor and subcellular localization suggest that GAD65 may provide reserve pools of GABA for regulation of inhibitory neurotransmission. We have disrupted the gene encoding GAD65 and backcrossed the mutation into the C57BL/6 strain of mice. In contrast to GAD67-/- animals, which are born with developmental abnormalities and die shortly after birth, GAD65-/- mice appear normal at birth. Basal GABA levels and holo-GAD activity are normal, but the pyridoxal 5' phosphate-inducible apo-enzyme reservoir is significantly decreased. GAD65-/- mice develop spontaneous seizures that result in increased mortality. Seizures can be precipitated by fear or mild stress. Seizure susceptibility is dramatically increased in GAD65-/- mice backcrossed into a second genetic background, the nonobese diabetic (NOD/LtJ) strain of mice enabling electroencephalogram analysis of the seizures. The generally higher basal brain GABA levels in this backcross are significantly decreased by the GAD65-/- mutation, suggesting that the relative contribution of GABA synthesized by GAD65 to total brain GABA levels is genetically determined. Seizure-associated c-fos-like immunoreactivity reveals the involvement of limbic regions of the brain. These data suggest that GABA synthesized by GAD65 is important in the dynamic regulation of neural network excitability, implicate at least one modifier locus in the NOD/LtJ strain, and present GAD65-/- animals as a model of epilepsy involving GABA-ergic pathways.

Phosphorylation of serine residues 3, 6, 10, and 13 distinguishes membrane anchored from soluble glutamic acid decarboxylase 65 and is restricted to glutamic acid decarboxylase 65alpha

GAD65, the smaller isoform of the gamma-aminobutyric acid-synthesizing enzyme glutamic acid decarboxylase is detected as an alpha/beta doublet of distinct mobility on SDS-polyacrylamide gel electrophoresis. Glutamic acid decarboxylase (GAD) 65 is reversibly anchored to the membrane of synaptic vesicles in neurons and synaptic-like microvesicles in pancreatic beta-cells. Here we demonstrate that GAD65alpha but not beta is phosphorylated in vivo and in vitro in several cell types. Phosphorylation is not the cause of the alpha/beta heterogeneity but represents a unique post-translational modification of GAD65alpha. Two-dimensional protein analyses identified five phosphorylated species of three different charges, which are likely to represent mono-, di-, and triphosphorylated GAD65alpha in different combinations of phosphorylated serines. Phosphorylation of GAD65alpha was located at serine residues 3, 6, 10, and 13, shown to be mediated by a membrane bound kinase, and distinguish the membrane anchored, and soluble forms of the enzyme. Phosphorylation status does not affect membrane anchoring of GAD65, nor its Km or Vmax for glutamate. The results are consistent with a model in which GAD65alpha and -beta constitute the two subunits of the native GAD65 dimer, only one of which, alpha, undergoes phosphorylation following membrane anchoring, perhaps to regulate specific aspects of GAD65 function in the synaptic vesicle membrane.

Comparative distribution of GAD65 and GAD67 mRNAs and proteins in the rat spinal cord supports a differential regulation of these two glutamate decarboxylases in vivo

Gamma-aminobutyric acid (GABA) synthesis can result from the action of at least two glutamic acid decarboxylase (GAD) isoforms, GAD65 and GAD67, possibly involved in distinct mechanisms. We have made the hypothesis that GAD65 may respond to short-term changes and is present in neurons with a phasic activity, while GAD67 may rather provide GABA for the metabolic pool and for supporting tonic levels of synaptic transmission (Erlander et al.: Neuron 7:91-100, 1991; Feldblum et al.: J Neurosci Res 34:689-706, 1993). In the present work we have tested this hypothesis in the rat spinal cord where both types of activities have been identified. The correlation of GABA immunodetection with the distribution of GAD65 and GAD67 mRNAs and proteins has evinced in the dorsal horn a differential regulation of the two isoforms. In situ hybridization has revealed, in the dorsal horn, relatively higher levels of GAD67 mRNA than of GAD65, while immunodetection of the proteins demonstrated numerous punctate profiles with both GAD antisera. Reverse transcription-polymerase chain reaction (RT-PCR) data confirmed the abundance of the GAD67 transcripts compared to GAD65 in the rat spinal cord. In contrast, within the ventral horn, there was a greter number of GAD67-immunoreactive (IR) profiles mostly located around motoneurons. The paucity of GAD65 immunoreactivity in the ventral horn cannot be related to a different accessibility of the antigens to the epitopes since on the same section a dense GAD65 staining was detected in the dorsal horn. Hence, a number of biochemical and electrophysiological data support the concept of the involvement of glycine as the major inhibitory system within the ventral horn which may explain the low levels of GAD transcription in this region. The paucity of GAD65 in the ventral horn may also reflect a functional difference, suggesting a predominance of GAD67 in neurons under tonic activity. In the dorsal horn, where neurons with phasic and tonic firing patterns have been disclosed, GAD65 may, in addition, provide GABA for responses to short-term changes.

Different distributions of GAD65 and GAD67 mRNAs suggest that the two glutamate decarboxylases play distinctive functional roles

Two genes encode two forms of glutamate decarboxylase, GAD65 and GAD67. Because the two GADs differ in subcellular distribution and interactions with the cofactor pyridoxal phosphate, the two enzymes may play different roles in gamma-aminobutyric acid (GABA) production. In this study we have used in situ hybridization to compare the regional and cellular distributions of the two GAD mRNAs in rat brain. Both GAD mRNAs are abundant in olfactory bulb, olfactory tubercle, zona incerta, reticular nucleus of the thalamus, oculomotor nuclei, and pontine tegmental area. GAD65 mRNA is more abundant in several structures of the visual system, including the lateral geniculate nuclei, superior colliculi, and olivary pretectal nucleus, as well as in several hypothalamic and pontine nuclei. In contrast, GAD67 mRNA is more abundant in neocortex, the granular layer of olfactory bulb, lateral and medial septum, globus pallidus, inferior colliculi, and cerebellar cortex. Both GAD mRNAs are present in interneurons as well as in projection neurons, and both are present in neurons with different types of synapses, including dendrodendiritic, axosomatic, and axodendritic synapses. GAD65 mRNA predominates in the visual and the neuroendocrine systems, which are more subject to phasic changes, while GAD67 is present at relatively higher concentrations in many tonically active neurons. GAD65 and GAD67 together may provide more flexibility in the regulation of GABA synthesis than either could alone.

Regulation of gamma-aminobutyric acid synthesis in the brain

gamma-Aminobutyric acid (GABA) is synthesized in brain in at least two compartments, commonly called the transmitter and metabolic compartments, and because regulatory processes must serve the physiologic function of each compartment, the regulation of GABA synthesis presents a complex problem. Brain contains at least two molecular forms of glutamate decarboxylase (GAD), the principal synthetic enzyme for GABA. Two forms, termed GAD65 and GAD67, are the products of two genes and differ in sequence, molecular weight, interaction with the cofactor, pyridoxal 5'-phosphate (pyridoxal-P), and level of expression among brain regions. GAD65 appears to be localized in nerve terminals to a greater degree than GAD67, which appears to be more uniformly distributed throughout the cell. The interaction of GAD with pyridoxal-P is a major factor in the short-term regulation of GAD activity. At least 50% of GAD is present in brain as apoenzyme (GAD without bound cofactor; apoGAD), which serves as a reservoir of inactive GAD that can be drawn on when additional GABA synthesis is needed. A substantial majority of apoGAD in brain is accounted for by GAD65, but GAD67 also contributes to the pool of apoGAD. The apparent localization of GAD65 in nerve terminals and the large reserve of apoGAD65 suggest that GAD65 is specialized to respond to short-term changes in demand for transmitter GABA.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative distribution of messenger RNAs encoding glutamic acid decarboxylases (Mr 65,000 and Mr 67,000) in the basal ganglia of the rat

Glutamic acid decarboxylase, the enzyme required for GABA synthesis, exists as distinct isoforms, which have recently been found to be encoded by different genes. The relative expression of messenger RNAs encoding two isoforms of glutamic acid decarboxylase (Mr 67,000 and Mr 65,000) was measured at the single-cell level in neurons of the rat basal ganglia with in situ hybridization histochemistry. Both messenger RNAs were expressed in neurons of the striatum, pallidum, and substantia nigra pars reticulata, but marked differences in the relative level of labelling were observed with the two probes. In striatum, efferent neurons were more densely labelled for the messenger RNA encoding glutamic acid decarboxylase (Mr 65,000) than for the messenger RNA encoding glutamic acid decarboxylase (Mr 67,000), whereas the reverse was observed for GABA-ergic interneurons. Neurons of the entopeduncular nucleus were much more densely labelled for messenger RNA encoding glutamic acid decarboxylase (Mr 65,000) than for messenger RNA encoding glutamic acid decarboxylase (Mr 67,000). In addition, labelling for messenger RNA encoding glutamic acid decarboxylase (Mr 65,000) was higher in the entopeduncular nucleus (internal pallidum) than in the globus pallidus (external pallidum), a structure which expressed similar levels of both mRNAs. In contrast to neurons of the internal pallidum, efferent neurons of the substantia nigra pars reticulata expressed slightly more messenger RNA encoding glutamic acid decarboxylase (Mr 67,000) than that encoding the other isoform of the enzyme. The results suggest a differential expression of the messenger RNAs encoding the two isoforms of glutamic acid decarboxylase in subpopulations of basal ganglia neurons in rats.

GAD and GABA in an enriched population of cultured GABAergic neurons from rat cerebral cortex

To study various aspects of GABAergic metabolism in an easily accessible system, dissociated cells from postnatal rat cerebral cortex were cultured in a serum-based medium and characterized morphologically and biochemically. The majority (70-96%) of the neurons were GABAergic as determined by three double-labeling procedures. The specific activity of glutamine synthetase in the cultures was 4-5% of the levels in rat astrocyte cultures and intact rat brain, indicating that glia were a minor component. The developmental increase of GABA levels preceded the increase of GAD activity in both immunocytochemical and biochemical experiments. GABA turnover rates also increased with culture age and were 20-30% of GAD activity. Four anti-GAD antibodies, which recognize GAD subunits with differing molecular masses to varying degrees, were used to stain cultured neurons and make immunoblots. Immunoblots showed that the neurons contained two major subunits of GAD which differed in mass by 2 kDa. All four antibodies immunostained both neuronal perikarya and neurites but one antibody, which on the immunoblots predominantly labeled the GAD protein with the lower molecular weight, showed a somewhat more pronounced punctate staining, possibly indicating a principal localization to neurites.

Two genes encode distinct glutamate decarboxylases

gamma-Aminobutyric acid (GABA) is the most widely distributed known inhibitory neurotransmitter in the vertebrate brain. GABA also serves regulatory and trophic roles in several other organs, including the pancreas. The brain contains two forms of the GABA synthetic enzyme glutamate decarboxylase (GAD), which differ in molecular size, amino acid sequence, antigenicity, cellular and subcellular location, and interaction with the GAD cofactor pyridoxal phosphate. These forms, GAD65 and GAD67, derive from two genes. The distinctive properties of the two GADs provide a substrate for understanding not only the multiple roles of GABA in the nervous system, but also the autoimmune response to GAD in insulin-dependent diabetes mellitus.

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

Studies of the GABA-synthetic enzyme glutamate decarboxylase (glutamic acid decarboxylase; GAD; E.C.4.1.1.15) began in 1951 with the work of Roberts and his colleagues. Since then, many investigators have demonstrated the structural and functional heterogeneity of brain GAD. At least part of this heterogeneity derives from the existence of two GAD genes.

Postnatal expression of glutamate decarboxylases in developing rat cerebellum

The recent identification of two genes encoding distinct forms of the GABA synthetic enzyme, glutamate decarboxylase (GAD), raises the possibility that varying expression of the two genes may contribute to the regulation of GABA production in individual neurons. We investigated the postnatal development the two forms of GAD in the rat cerebellum. The mRNA for GAD67, the form which is less dependent on the presence of the cofactor, pyridoxal phosphate (PLP), is present at birth in presumptive Purkinje cells and increases during postnatal development. GAD67 mRNA predominates in the cerebellum. The mRNA for GAD65, which displays marked PLP-dependence for enzyme activity, cannot be detected in cerebellar cortex by in situ hybridization until P7 in Purkinje cells, and later in other GABA neurons. In deep cerebellar nuclei, which mature prenatally, both forms of GAD mRNA can be detected at birth. The amounts of immunoreactice GAD and GAD enzyme activity parallel changes in mRNA levels. We suggest that the delayed appearance of GAD65 is coincident with synapse formation between GABA neurons and their targets during the second postnatal week. GAD67 mRNA may be present prior to synaptogenesis to produce GABA for trophic and metabolic functions.

Cofactor interactions and the regulation of glutamate decarboxylase activity

More than 50% of glutamate decarboxylase (GAD) in brain is present as apoenzyme. Recent work has opened the possibility that apoGAD can be studied in brain by labeling with radioactive cofactor. Such studies would be aided by a compound that inhibits specific binding. One possibility is 4-deoxy-pyridoxine 5'-phosphate, a close structural analog of the cofactor pyridoxal 5'-phosphate. The effects of deoxypyridoxine-P on the cyclic series of reactions that interconverts apo- and holoGAD was investigated and found to be consistent with simple competitive inhibition of the activation of apoGAD by pyridoxal-P. As expected from the cycle GAD was inactivated when incubated with glutamate and deoxypyridoxine-P even though cofactor was present, but no inactivation was observed with deoxypyridoxine-P in the absence of glutamate. Deoxypyridoxine-P also stabilized apoGAD against heat denaturation. These effects were quantitatively accounted for by a kinetic model of the apo-holoGAD cycle. Deoxypyridoxine-P inhibited the labeling by [32P]pyridoxal-P of GAD isolated from rat brain. Hippocampal extracts were labeled with [32P]pyridoxal-P and analyzed by SDS-polyacrylamide gel electrophoresis. Remarkably few bands were strongly labeled. The major labeled band (at 63 kDa) corresponded to one of the forms of GAD. Other strongly-labeled bands were observed at 65 kDa (corresponding to the higher molecular weight form of GAD) and at 69--72 kDa. Labeling of the 63- and 65-kDa bands was inhibited by deoxypyridoxine-P, but the 69-72 kDa bands were unaffected, suggesting that the latter were non-specifically labeled. The results suggest that the 63-kDa form of GAD makes up the majority of apoGAD in hippocampus.

Glutamate-dependent active-site labeling of brain glutamate decarboxylase

A major regulatory feature of brain glutamate decarboxylase (GAD) is a cyclic reaction that controls the relative amounts of holoenzyme and apoenzyme [active and inactive GAD with and without bound pyridoxal 5'-phosphate (pyridoxal-P, the cofactor), respectively]. Previous studies have indicated that progression of the enzyme around the cycle should be stimulated strongly by the substrate, glutamate. To test this prediction, the effect of glutamate on the incorporation of pyridoxal-P into rat-brain GAD was studied by incubating GAD with [32P]pyridoxal-P, followed by reduction with NaBH4 to link irreversibly the cofactor to the enzyme. Adding glutamate to the reaction mixture strongly stimulated labeling of GAD, as expected. 4-Deoxypyridoxine 5'-phosphate (deoxypyridoxine-P), a close structural analogue of pyridoxal-P, was a competitive inhibitor of the activation of glutamate apodecarboxylase by pyridoxal-P (Ki = 0.27 microM) and strongly inhibited glutamate-dependent labeling of GAD. Analysis of labeled GAD by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis showed two labeled proteins with apparent molecular masses of 59 and 63 kDa. Both proteins could be purified by immunoaffinity chromatography on a column prepared with a monoclonal antibody to GAD, and both were labeled in a glutamate-dependent, deoxypyridoxine-P-sensitive manner, indicating that both were GAD. Three peaks of GAD activity (termed peaks I, II, and III) were separated by chromatography on phenyl-Sepharose, labeled with [32P]pyridoxal-P, purified by immunoaffinity chromatography, and analyzed by SDS-polyacrylamide gel electrophoresis. Peak I contained only the 59-kDa labeled protein. Peaks II and III contained the both the 59- and 63-kDa proteins, but in differing proportions.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of acute barbiturate administration, tolerance and dependence on brain GABA system: comparison to alcohol and benzodiazepines

Central nervous system depressants, e.g., barbiturates, alcohol and benzodiazepines, have a wide spectrum of activity in humans and animals. Evidence accumulated suggests that some of the pharmacological actions exerted by these agents may be mediated through GABA system by mimicking GABAergic transmission. This review attempts to summarize the evidence available as to how the GABA system plays a part in the barbiturate actions and the development of tolerance to and physical dependence on barbiturates. The comparisons of the effects of alcohol, barbiturates and benzodiazepines at different steps of GABA synapse are also presented. Furthermore, the results which have been reported in the literature are inconsistent. This may be due to differences in: (a) animal models used; (b) brain regions used; (c) protocols (dose, duration, form and route of administration, etc.) used in treating animals and/or (d) techniques (pharmacological, biochemical, physiological, etc.) used.

Stimulation of synaptosomal gamma-aminobutyric acid synthesis by glutamate and glutamine

gamma-Aminobutyric acid (GABA) synthesis was studied in rat brain synaptosomes by measuring the increase of GABA level in the presence of the GABA-transaminase inhibitor gabaculine. The basal rate of synaptosomal GABA synthesis in glucose-containing medium (25.9 nmol/h/mg of protein) was only 3% of the maximal activity of glutamate decarboxylase (GAD; 804 +/- 83 nmol/h/mg of protein), a result indicating that synaptosomal GAD operates at only a small fraction of its catalytic capacity. Synaptosomal GABA synthesis was stimulated more than threefold by adding 500 microM glutamine. Glutamate also stimulated GABA synthesis, but the effect was smaller (1.5-fold). These results indicate that synaptosomal GAD is not saturated by endogenous levels of its substrate, glutamate, and account for part of the unused catalytic capacity. The greater stimulation of GABA synthesis by glutamine indicates that the GAD-containing compartment is more accessible to extrasynaptosomal glutamine than glutamate. The strong stimulation by glutamine also shows that the rates of uptake of glutamine and its conversion to glutamate can be sufficiently rapid to support GABA synthesis in nerve terminals. Synaptosomes carried out a slow net synthesis of aspartate in glucose-containing medium (7.7 nmol/h/mg of protein). Aspartate synthesis was strongly stimulated by glutamate and glutamine, but in this case the stimulation by glutamate was greater. Thus, the larger part of synaptosomal aspartate synthesis occurs in a different compartment than does GABA synthesis.

Stability and activation of glutamate apodecarboxylase from pig brain

The stability and activation of glutamate apodecarboxylase was studied with three forms of the enzyme from pig brain (referred to as the alpha, beta, and gamma forms). Apoenzyme was prepared by incubating the holoenzyme with aspartate followed by chromatography on Sephadex G-25. Apoenzyme was much less stable than holoenzyme to inactivation by heat (for beta-glutamate decarboxylase (beta-GAD) at 30 degrees C, t1/2 values of apo- and holoenzyme were 17 and greater than 100 min). ATP protected holoenzyme and apoenzyme against heat inactivation. The kinetics of reactivation of apoenzyme by pyridoxal-P was consistent with a two-step mechanism comprised of a rapid, reversible association of the cofactor with apoenzyme followed by a slow conversion of the complex to active holoenzyme. The reactivation rate constant (kr) and apparent dissociation constant (KD) for the binding of pyridoxal-P to apoenzyme differed substantially among the forms (for alpha-, beta-, and gamma-GAD, kr = 0.032, 0.17, and 0.27 min-1, and KD = 0.014, 0.018, and 0.04 microM). ATP was a strong competitive inhibitor of activation (Ki = 0.45, 0.18, and 0.39 microM for alpha-, beta-, and gamma-GAD). In contrast, Pi stimulated activation at 1-5 mM but inhibited at much higher concentrations. The results suggest that ATP is important in stabilizing the apoenzyme in brain and that ATP, Pi, and other compounds regulate its activation.

Regulatory properties of brain glutamate decarboxylase

1. Glutamate decarboxylase is a focal point for controlling gamma-aminobutyric acid (GABA) synthesis in brain. Several factors that appear to be important in the regulation of GABA synthesis have been identified by relating studies of purified glutamate decarboxylase to conditions in vivo. 2. The interaction of glutamate decarboxylase with its cofactor, pyridoxal 5'-phosphate, is a regulated process and appears to be one of the major means of controlling enzyme activity. The enzyme is present in brain predominantly as apoenzyme (inactive enzyme without bound cofactor). Studies with purified enzyme indicate that the relative amounts of apo- and holoenzyme are determined by the balance in a cycle that continuously interconverts the two. 3. The cycle that interconverts apo- and holoenzyme is part of the normal catalytic mechanism of the enzyme and is strongly affected by several probable regulatory compounds including pyridoxal 5'-phosphate, ATP, inorganic phosphate, and the amino acids glutamate, GABA, and aspartate. ATP and the amino acids promote apoenzyme formation and pyridoxal 5'-phosphate and inorganic phosphate promote holoenzyme formation. 4. Numerous studies indicate that brain contains multiple molecular forms of glutamate decarboxylase. Multiple forms that differ markedly in kinetic properties including their interactions with the cofactor have been isolated and characterized. The kinetic differences among the forms suggest that they play a significant role in the regulation of GABA synthesis.

Mass fragmentographic determination of gamma-aminobutyric acid and glutamic acid in discrete amygdaloid nuclei of rat brain

A mass fragmentographic method for the simultaneous quantification of gamma-aminobutyric acid (GABA) and glutamic acid is described. In a convenient one-step reaction, the two amino acids were derivatized with pentafluoropropionic anhydride and pentafluoropropanol. The derivatization products were stable for several days. The technique has been applied to the assay of GABA and Glu in five amygdaloid nuclei of the rat brain. The GABA level was high in the central and medial nuclei, whereas the Glu level was high in the lateral and basal nuclei. The regional distribution of GABA was different from that of Glu within the amygdaloid nuclei.

Rapid inactivation of brain glutamate decarboxylase by aspartate

In the absence of its cofactor, pyridoxal 5'-phosphate (pyridoxal-P), glutamate decarboxylase is rapidly inactivated by aspartate. Inactivation is a first-order process and the apparent rate constant is a simple saturation function of the concentration of aspartate. For the beta-form of the enzyme, the concentration of aspartate giving the half-maximal rate of inactivation is 6.1 +/- 1.3 mM and the maximal apparent rate constant is 1.02 +/- 0.09 min-1, which corresponds to a half-time of inactivation of 41 s. The rate of inactivation by aspartate is about 25 times faster than inactivation by glutamate or gamma-aminobutyric acid (GABA). Inactivation is accompanied by a rapid conversion of holoenzyme to apoenzyme and is opposed by pyridoxal-P, suggesting that inactivation results from an alternative transamination of aspartate catalyzed by the enzyme, as previously observed with glutamate and GABA. Consistent with this mechanism pyridoxamine 5'-phosphate, an expected transamination product, was formed when the enzyme was incubated with aspartate and pyridoxal-P. The rate of transamination relative to the rate of decarboxylation was much greater for aspartate than for glutamate. Apoenzyme formed by transamination of aspartate was reactivated with pyridoxal-P. In view of the high rate of inactivation, aspartate may affect the level of apoenzyme in brain.

Pyridoxal phosphate-unrelated inhibition of hippocampal glutamic acid decarboxylase by convulsant pyridoxal sulphate

Previous studies from this laboratory have shown that pyridoxal-5'-sulphate, the synthetic analogue of pyridoxal phosphate, causes epileptic seizures including tonic-clonic convulsions. These seizure activities are prevented or reversed by GABA or muscimol. In an attempt to delineate the biochemical basis of these seizure processes further, we have studied and shown that pyridoxal sulphate is a competitive inhibitor of glutamic acid decarboxylase. In addition, the chronic administration of pyridoxal sulphate was shown to reduce the concentration of pyridoxal phosphate in the cerebellum, the cerebrum, and basal ganglion, but not in the hippocampus. The activity of hippocampal glutamic acid decarboxylase was reduced after 1, 3, and 5 days of chronic application of pyridoxal sulphate. The inhibition was demonstrated, whether glutamic acid decarboxylase was assayed in the presence or absence of its coenzyme pyridoxal phosphate. Unlike findings in the hippocampus, the activity of glutamic acid decarboxylase in other brain regions was unaffected following chronic application of pyridoxal sulphate. The selective toxic effects of pyridoxal sulfate to the hippocampus, a brain area well known for its high susceptibility to seizure discharges, deserve additional indepth investigation.

Comparative characterization of glutamate decarboxylase in crude homogenates of oviduct, ovary, and hypothalamus

Some biochemical characteristics of L-glutamate decarboxylase (GAD) were compared using crude homogenates of the rat oviduct, ovary, and hypothalamus. As estimated by the measurement of CO2 production, the specific activities of oviductal and ovarian GAD were about 10 and 3% of the hypothalamic value, respectively. Stoichiometric formation of gamma-aminobutyric acid (GABA) and CO2 from L-glutamate could be observed in oviduct and hypothalamus, while in ovarian homogenates the production of CO2 was more than nine times that of GABA. Hypothalamic and tubal GAD showed similar time course, temperature dependence, and pH dependence. The dependence on protein concentration and on exogenous cofactor supply was also similar in these two tissues. The kinetic constant for L-glutamate as a substrate was nearly the same in oviduct (1.30 mM) and hypothalamus (1.64 mM). The responsiveness of tubal and hypothalamic GAD to various inhibitors was also similar. The present findings indicate that the oviductal and the hypothalamic GAD may be identical, and they suggest a possible GABAergic innervation of the Fallopian tube.

Evidence for feedback regulation of glutamate decarboxylase by gamma-aminobutyric acid

Although feedback control mechanisms for regulating the synthesis of various neurotransmitters have been demonstrated no such mechanism has been described for gamma-aminobutyric acid (GABA) in mammalian brain. Physiological concentrations of GABA inactivated glutamate decarboxylase, the enzyme responsible for GABA synthesis, by converting it to apoenzyme. This inactivation was opposed by the cofactor, pyridoxal 5'-phosphate (pyridoxal-P), and was promoted by ATP. GABA also competitively inhibited the enzyme, and the Ki for inhibition was essentially the same as the concentration of GABA giving the half-maximal rate of inactivation (16 mM). These results provide a mechanism for direct feedback control of presynaptic GABA synthesis and provide further support for the regulation of glutamate decarboxylase in vivo by a cycle of inactivation and reactivation.

Effect of L-dopa on glutamate decarboxylase activity in the hypothalamic and amygdaloid nuclei

Repeated administration of L-dopa methylester produced a significant increase in glutamate decarboxylase (GAD) activity without pyridoxal-5'-phosphate in the lateral hypothalamic area and medial amygdaloid nucleus. The effect of L-dopa on GAD activity was opposite to that of haloperidol in the lateral hypothalamic area.

Effect of haloperidol on glutamate decarboxylase activity in discrete brain areas of the rat

Using freeze-dried samples of rat brain, the effect of haloperidol on glutamate decarboxylase (GAD) activity without exogenously added pyridoxal-5'-phosphate (PLP) was studied in discrete brain nuclei and areas. Repeated injections of haloperidol produced significant changes in GAD activity in the dorsal part of the caudate nucleus, entopeduncular nucleus, pars reticulata of the substantia nigra, lateral hypothalamic area, and dorsomedial hypothalamic nucleus. A reduction of GAD activity after haloperidol was observed in the entopeduncular nucleus and pars reticulata of the substantia nigra. This finding demonstrates biochemically that haloperidol-induced extrapyramidal behavior may be involved in the reduction of GABAergic transmission in the entopeduncular nucleus and pars reticulata of the substantia nigra. A decrease in GAD activity in the lateral hypothalamic area indicates that interaction between GABAergic neurons as well as dopaminergic neurons may be involved in the haloperidol-induced behavioral changes. In addition, close interaction between GABAergic and dopaminergic systems in the dorsomedial hypothalamic nucleus and dorsal part to the caudate nucleus was demonstrated.

Reduction of nigral glutamic acid decarboxylase in rats with neuroleptic-induced oral dyskinesia

Following eight monthly haloperidol decanoate injections rats showed an increased rate of vacuous chewing movements (VCM's), which gradually disappeared within 4 drug-free months. Another single dose of non-decanoate haloperidol reinstated a second increase in VCM rate which was still significant after 2 months. The glutamic acid decarboxylase (GAD) activity in the substantia nigra of these chronically haloperidol-treated rats was lower than untreated controls. Furthermore, there was a significant negative correlation between individual VCM rates and nigral GAD activity. No corresponding changes occurred in other brain regions. The depression of nigral GAD may reflect a reduced tissue density of GABA-ergic axon terminals within the descending striato-nigral pathway.

The selective inhibition of hippocampal glutamic acid decarboxylase in zinc-induced epileptic seizures

The intracerebroventricular administration of Zn2+ (0.3 mumol/10 microliters) causes epileptic seizures characterized by running fits, jumping, vocalization, fasiculation of facial muscles, myoclonic movements of the limbs and tonic-clonic convulsions. These episodes are blocked or reversed by gamma-aminobutyric acid (0.4 mumol/10 microliters). When assayed under conditions where pyridoxal phosphate was not added, the activity of glutamic acid decarboxylase decreased significantly in hippocampus from 18.9 to 15.3 and 9.7 mumols 14CO2 formed/gram proteins/20 min, 15 and 30 min following administration of Zn2+. The inhibition of glutamic acid decarboxylase by Zn2+ was selective occurring only in hippocampus and not in the hypothalamus, amygdala, caudate or thalamus. The inhibition of glutamic acid decarboxylase was not due to a reduction in the concentration of endogenous pyridoxal phosphate which remained unaltered in hippocampus following Zn2+ administration.