Effects of increased gamma-aminobutyric acid levels on GAD67 protein and mRNA levels in rat cerebral cortex

Pentylenetetrazole kindling induces dynamic changes in GAD65 expression in hippocampal somatostatin interneurons

INTRODUCTION

One of the mechanisms of epileptgenesis is impairment of inhibitory neural circuits. Several studies have compared neural changes among subtypes of gamma-aminobutyric acid-related (GABAergic) neurons after acquired epileptic seizure. However, it is unclear that GABAergic neural modifications that occur during acquisition process of epileptic seizure.

METHODS

Male rats were injected with pentylenetetrazole (PTZ kindling: n = 30) or saline (control: n = 15) every other day to observe the development of epileptic seizure stages. Two time points were identified: the point at which seizures were most difficult to induce, and the point at which seizures were most easy to induce. The expression of GABAergic neuron-related proteins in the hippocampus was immunohistochemically compared among GABAergic subtypes at each of these time points.

RESULTS

Bimodal changes in seizure stages were observed in response to PTZ kindling. The increase of seizure stage was transiently suppressed after 8 or 10 injections, and then progressed again by the 16th injection. Based on these results, we defined 10 injections as a short-term injection period during which seizures are less likely to occur, and 20 injections as a long-term injection period during which continuous seizures are likely to occur. The immunohistochemical analysis showed that hippocampal glutamic acid decarboxylase 65 (GAD65) expression was increased after short-term kindling but unchanged after long-term kindling. Increased GAD65 expression was limited to somatostatin-positive (SOM+) cells among several GABAergic subtypes. By contrast, GAD, GABA, GABAAR α1, GABABR1, and VGAT cells showed no change following short- or long-term PTZ kindling.

CONCLUSION

PTZ kindling induces bimodal changes in the epileptic seizure stage. Seizure stage is transiently suppressed after short-term PTZ injection with GAD65 upregulation in SOM+ cells. The seizure stage is progressed again after long-term PTZ injection with GAD65 reduction to baseline level.

Neonatal Oxidative Stress Impairs Cortical Synapse Formation and GABA Homeostasis in Parvalbumin-Expressing Interneurons

Neonatal brain injury is often caused by preterm birth. Brain development is vulnerable to increased environmental stress, including oxidative stress challenges. Due to a premature change of the fetal living environment from low oxygen in utero into postnatal high-oxygen room air conditions ex utero, the immature preterm brain is exposed to a relative hyperoxia, which can induce oxidative stress and impair neuronal cell development. To simulate the drastic increase of oxygen exposure in the immature brain, 5-day-old C57BL/6 mice were exposed to hyperoxia (80% oxygen) for 48 hours or kept in room air (normoxia, 21% oxygen) and mice were analyzed for maturational alterations of cortical GABAergic interneurons. As a result, oxidative stress was indicated by elevated tyrosine nitration of proteins. We found perturbation of perineuronal net formation in line with decreased density of parvalbumin-expressing (PVALB) cortical interneurons in hyperoxic mice. Moreover, maturational deficits of cortical PVALB+ interneurons were obtained by decreased glutamate decarboxylase 67 (GAD67) protein expression in Western blot analysis and lower gamma-aminobutyric acid (GABA) fluorescence intensity in immunostaining. Hyperoxia-induced oxidative stress affected cortical synaptogenesis by decreasing synapsin 1, synapsin 2, and synaptophysin expression. Developmental delay of synaptic marker expression was demonstrated together with decreased PI3K-signaling as a pathway being involved in synaptogenesis. These results elucidate that neonatal oxidative stress caused by increased oxygen exposure can lead to GABAergic interneuron damage which may serve as an explanation for the high incidence of psychiatric and behavioral alterations found in preterm infants.

Heterogeneous GAD65 Expression in Subtypes of GABAergic Neurons Across Layers of the Cerebral Cortex and Hippocampus

Gamma-aminobutyric acid (GABA), a major inhibitory transmitter in the central nervous system, is synthesized via either of two enzyme isoforms, GAD65 or GAD67. GAD65 is synthesized in the soma but functions at synaptic terminals in an activity-dependent manner, playing a distinct role in excitatory-inhibitory balance. However, the extent to which each GABAergic subtype expresses GAD65 in the resting state remains unclear. In this study, we compared GAD65 expression among six GABAergic subtypes: NPY⁺, nNOS⁺, PV⁺, SOM⁺, CR⁺, and CCK⁺. According to the results, the GABAergic subtypes were classified into two groups per region based on GAD65 expression levels: high-expression (NPY⁺ and nNOS⁺) and low-expression groups (PV⁺, SOM⁺, CR⁺, and CCK⁺) in the cerebral cortex and high-expression (NPY⁺, nNOS⁺, and CCK⁺) and low-expression groups (PV⁺, SOM⁺, and CR⁺) in the hippocampus. Moreover, these expression patterns revealed a distinct laminar distribution in the cerebral cortex and hippocampus. To investigate the extent of GAD65 transport from the soma to synaptic terminals, we examined GAD65 expression in colchicine-treated rats in which GAD65 was synthesized in the soma but not transported to terminals. We found a significant positive correlation in GAD65 expression across subtypes between colchicine-treated and control rats. In summary, each GABAergic subtype exhibits a distinct GAD65 expression pattern across layers of the cerebral cortex and hippocampus. In addition, the level of GAD65 expression in the soma can be used as a proxy for the amount of GAD65 in the cytoplasm. These findings suggest that exploration of the distinct profiles of GAD65 expression among GABAergic subtypes could clarify the roles that GABAergic subtypes play in maintaining the excitatory-inhibitory balance.

Targeted neurotransmitter metabolomics profiling of oleanolic acid in the treatment of spontaneously hypertensive rats

Essential hypertension (EH) is a prevalent chronic medical condition and a major risk factor for cardiovascular morbidity and mortality. Neurotransmitters are involved in the physiological process of blood pressure regulation in the body. Studies have shown that oleanolic acid (OA) can effectively regulate neurotransmitter-related metabolic disorders caused by EH, but the mechanism is still unclear. Here, we studied the neurotransmitter metabolic profiles in five brain regions by targeted metabolomics approaches in spontaneously hypertensive rats (SHRs) treated with OA and vehicle. Samples from five brain regions (hippocampus, striatum, hypothalamus, temporal lobe, and frontal lobe) were collected from the control group, the spontaneously hypertensive rat (SHR) group, and the OA group. Targeted metabolomics based on UPLC-Q-Exactive-MS was employed to characterize the dramatically changed neurotransmitters in the brain regions of SHRs treated with OA and vehicle. The expressions of the key enzymes involved in the neurotransmitter metabolism were detected by the reverse transcription-polymerase chain reaction (RT-PCR). The metabolomic profiles of SHRs pre-protected by OA were significantly different from those of unprotected SHRs. A total of 18 neurotransmitters could be confirmed as significantly altered metabolites, which were involved in tyrosine and glutamate metabolism as well as other pathways. The results showing seven key enzymes in neurotransmitter metabolism further validated the changes in the metabolic pathways. OA could effectively restore tyrosine metabolism in the striatum and hypothalamus, glutamate metabolism in the hippocampus, striatum and temporal lobe, cholinergic metabolism in the striatum, and histidine metabolism in the hypothalamus due to its inhibition of inflammatory reactions, structural damage of the neuronal cells, and increase in sedative activity. This study indicated that brain region-targeted metabolomics can provide a powerful tool to further investigate the possible mechanism of OA in EH.

Adenosine A2A Receptor Gene Knockout Prevents l-3,4-Dihydroxyphenylalanine-Induced Dyskinesia by Downregulation of Striatal GAD67 in 6-OHDA-Lesioned Parkinson's Mice

l-3,4-Dihydroxyphenylalanine (l-DOPA) remains the primary pharmacological agent for the symptomatic treatment of Parkinson’s disease (PD). However, the development of l-DOPA-induced dyskinesia (LID) limits the long-term use of l-DOPA for PD patients. Some data have reported that adenosine A_2A receptor (A_2AR) antagonists prevented LID in animal model of PD. However, the mechanism in which adenosine A_2AR blockade alleviates the symptoms of LID has not been fully clarified. Here, we determined to knock out (KO) the gene of A_2AR and explored the possible underlying mechanisms implicated in development of LID in a mouse model of PD. A_2AR gene KO mice were unilaterally injected into the striatum with 6-hydroxydopamine (6-OHDA) in order to damage dopamine neurons on one side of the brain. 6-OHDA-lesioned mice were then injected once daily for 21 days with l-DOPA. Abnormal involuntary movements (AIMs) were evaluated on days 3, 8, 13, and 18 after l-DOPA administration, and real-time polymerase chain reaction and immunohistochemistry for glutamic acid decarboxylase (GAD) 65 and GAD67 were performed. We found that A_2AR gene KO was effective in reducing AIM scores and accompanied with decrease of striatal GAD67, rather than GAD65. These results demonstrated that the possible mechanism involved in alleviation of AIM symptoms by A_2AR gene KO might be through reducing the expression of striatal GAD67.

Glutamic acid decarboxylase isoform distribution in transgenic mouse septum: an anti-GFP immunofluorescence study

The septum is a basal forebrain region located between the lateral ventricles in rodents. It consists of lateral and medial divisions. Medial septal projections regulate hippocampal theta rhythm whereas lateral septal projections are involved in processes such as affective functions, memory formation, and behavioral responses. Gamma-aminobutyric acidergic neurons of the septal region possess the 65 and 67 isoforms of the enzyme glutamic acid decarboxylase. Although data on the glutamic acid decarboxylase isoform distribution in the septal region generally appears to indicate glutamic acid decarboxylase 67 dominance, different studies have given inconsistent results in this regard. The aim of this study was therefore to obtain information on the distributions of both of these glutamic acid decarboxylase isoforms in the septal region in transgenic mice. Two animal groups of glutamic acid decarboxylase-green fluorescent protein knock-in transgenic mice were utilized in the experiment. Brain sections from the region were taken for anti-green fluorescent protein immunohistochemistry in order to obtain estimated quantitative data on the number of gamma-aminobutyric acidergic neurons. Following the immunohistochemical procedures, the mean numbers of labeled cells in the lateral and medial septal nuclei were obtained for the two isoform groups. Statistical analysis yielded significant results which indicated that the 65 isoform of glutamic acid decarboxylase predominates in both lateral and medial septal nuclei (unpaired two-tailed t-test p < 0.0001 for LS, p < 0.01 for MS). This study is the first to reveal the dominance of glutamic acid decarboxylase isoform 65 in the septal region in glutamic acid decarboxylase-green fluorescent protein transgenic mice.

Immunomodulatory Effect of Toll-Like Receptor-3 Ligand Poly I:C on Cortical Spreading Depression

The release of inflammatory mediators following cortical spreading depression (CSD) is suggested to play a role in pathophysiology of CSD-related neurological disorders. Toll-like receptors (TLR) are master regulators of innate immune function and involved in the activation of inflammatory responses in the brain. TLR3 agonist poly I:C exerts anti-inflammatory effect and prevents cell injury in the brain. The aim of the present study was to examine the effect of systemic administration of poly I:C on the release of cytokines (TNF-α, IFN-γ, IL-4, TGF-β1, and GM-CSF) in the brain and spleen, splenic lymphocyte proliferation, expression of GAD65, GABAAα, GABAAβ as well as Hsp70, and production of dark neurons after induction of repetitive CSD in juvenile rats. Poly I:C significantly attenuated CSD-induced production of TNF-α and IFN-γ in the brain as well as TNF-α and IL-4 in the spleen. Poly I:C did not affect enhancement of splenic lymphocyte proliferation after CSD. Administration of poly I:C increased expression of GABAAα, GABAAβ as well as Hsp70 and decreased expression of GAD65 in the entorhinal cortex compared to CSD-treated tissues. In addition, poly I:C significantly prevented production of CSD-induced dark neurons. The data indicate neuroprotective and anti-inflammatory effects of TLR3 activation on CSD-induced neuroinflammation. Targeting TLR3 may provide a novel strategy for developing new treatments for CSD-related neurological disorders.

Immunofluorescently labeling glutamic acid decarboxylase 65 coupled with confocal imaging for identifying GABAergic somata in the rat dentate gyrus-A comparison with labeling glutamic acid decarboxylase 67

As γ-aminobutyric acid (GABA) is synthesized by two isoforms of glutamic acid decarboxylase (GAD), namely, GAD65 and GAD67, immunohistochemically targeting either isoform of GAD is theoretically useful for identifying GABAergic cell bodies. In practice, targeting GAD67 remains to be a popular choice. However, identifying GABAergic cell bodies with GAD67 immunoreactivity in the hippocampal dentate gyrus, especially in the hilus, is not without pitfalls. In the present study, we compared the characteristics of GAD65 immunoreactivity to GAD67 immunoreactivity in the rat dentate gyrus and examined perikaryal expression of GAD65 in four neurochemically prevalent subgroups of interneurons in the hilus. Experiments were done in normal adult Sprague-Dawley rats and GAD67-GFP knock-in mice. Horizontal hippocampal slices cut from the ventral portion of hippocampi were immunofluorescently stained and scanned using a confocal microscope. Immunoreactivity for both GAD67 and GAD65 was visible throughout the dentate gyrus. Perikaryal GAD67 immunoreactivity was denser but variable in terms of distribution pattern and intensity among cells whereas perikaryal GAD65 immunoreactivity displayed similar distribution pattern and staining intensity. Among different layers of the dentate gyrus, GAD67 immunoreactivity was densest in the hilus despite GAD65 immunoreactivity being more intense in the granule cell layer. Co-localization experiments showed that GAD65, but not GAD67, was expressed in all hilar calretinin (CR)-, neuronal nitric oxide synthase (nNOS)-, parvalbumin (PV)- or somatostatin (SOM)-positive somata. Labeling CR, nNOS, PV, and SOM in sections obtained from GAD67-GFP knock-in mice revealed that a large portion of SOM-positive cells had weak GFP expression. In addition, double labeling of GAD65/GABA and GAD67/GABA showed that nearly all of GABA-immunoreactive cells had perikaryal GAD65 expression whereas more than one-tenth of GABA-immunoreactive cells lacked perikaryal GAD67 immunoreactivity. Inhibition of axonal transport with colchicine dramatically improved perikaryal GAD65 immunoreactivity in GABAergic cells without significant augmentation to be seen in granule cells. Double labeling GAD65 and GAD67 in the sections obtained from colchicine-pretreated animals confirmed that a portion of GAD65-immunoreactive cells had weak or even no GAD67 immunoreactivity. We conclude that for confocal imaging, immunofluorescently labeling GAD65 for identifying GABAergic somata in the hilus of the dentate gyrus has advantages over labeling GAD67 in terms of easier recognition of perikaryal labeling and more consistent expression in GABAergic somata. Inhibition of axonal transport with colchicine further improves perikaryal GAD65 labeling, making GABAergic cells more distinguishable.

Potential for Cell-Transplant Therapy with Human Neuronal Precursors to Treat Neuropathic Pain in Models of PNS and CNS Injury: Comparison of hNT2.17 and hNT2.19 Cell Lines

Effective treatment of sensory neuropathies in peripheral neuropathies and spinal cord injury (SCI) is one of the most difficult problems in modern clinical practice. Cell therapy to release antinociceptive agents near the injured spinal cord is a logical next step in the development of treatment modalities. But few clinical trials, especially for chronic pain, have tested the potential of transplant of cells to treat chronic pain. Cell lines derived from the human neuronal NT2 cell line parentage, the hNT2.17 and hNT2.19 lines, which synthesize and release the neurotransmitters gamma-aminobutyric acid (GABA) and serotonin (5HT), respectively, have been used to evaluate the potential of cell-based release of antinociceptive agents near the lumbar dorsal (horn) spinal sensory cell centers to relieve neuropathic pain after PNS (partial nerve and diabetes-related injury) and CNS (spinal cord injury) damage in rat models. Both cell lines transplants potently and permanently reverse behavioral hypersensitivity without inducing tumors or other complications after grafting. Functioning as cellular minipumps for antinociception, human neuronal precursors, like these NT2-derived cell lines, would likely provide a useful adjuvant or replacement for current pharmacological treatments for neuropathic pain.

Cortical deficits of glutamic acid decarboxylase 67 expression in schizophrenia: clinical, protein, and cell type-specific features

OBJECTIVE

Cognitive deficits in schizophrenia are associated with altered activity of the dorsolateral prefrontal cortex, which has been attributed to lower expression of the 67 kDa isoform of glutamic acid decarboxylase (GAD67), the major γ-aminobutyric acid (GABA)-synthesizing enzyme. However, little is known about the relationship of prefrontal GAD67 mRNA levels and illness severity, translation of the transcript into protein, and protein levels in axon terminals, the key site of GABA production and function.

METHOD

Quantitative polymerase chain reaction was used to measure GAD67 mRNA levels in postmortem specimens of dorsolateral prefrontal cortex from subjects with schizophrenia and matched comparison subjects with no known history of psychiatric or neurological disorders (N=42 pairs). In a subset of this cohort in which potential confounds of protein measures were controlled (N=19 pairs), Western blotting was used to quantify tissue levels of GAD67 protein in tissue. In five of these pairs, multilabel confocal immunofluorescence was used to quantify GAD67 protein levels in the axon terminals of parvalbumin-containing GABA neurons, which are known to have low levels of GAD67 mRNA in schizophrenia.

RESULTS

GAD67 mRNA levels were significantly lower in schizophrenia subjects (by 15%), but transcript levels were not associated with predictors or measures of illness severity or chronicity. In schizophrenia subjects, GAD67 protein levels were significantly lower in total gray matter (by 10%) and in parvalbumin axon terminals (by 49%).

CONCLUSIONS

The findings that lower GAD67 mRNA expression is common in schizophrenia, that it is not a consequence of having the illness, and that it leads to less translation of the protein, especially in the axon terminals of parvalbumin-containing neurons, support the hypothesis that lower GABA synthesis in parvalbumin neurons contributes to dorsolateral prefrontal cortex dysfunction and impaired cognition in schizophrenia.

A sensitive period of mice inhibitory system to neonatal GABA enhancement by vigabatrin is brain region dependent

Neurodevelopmental disorders, such as schizophrenia and autism, have been associated with disturbances of the GABAergic system in the brain. We examined immediate and long-lasting influences of exposure to the GABA-potentiating drug vigabatrin (GVG) on the GABAergic system in the hippocampus and cerebral cortex, before and during the developmental switch in GABA function (postnatal days P1-7 and P4-14). GVG induced a transient elevation of GABA levels. A feedback response to GABA enhancement was evident by a short-term decrease in glutamate decarboxylase (GAD) 65 and 67 levels. However, the number of GAD65/67-immunoreactive (IR) cells was greater in 2-week-old GVG-treated mice. A long-term increase in GAD65 and GAD67 levels was dependent on brain region and treatment period. Vesicular GABA transporter was insensitive to GVG. The overall effect of GVG on the Cl(-) co-transporters NKCC1 and KCC2 was an enhancement of their synthesis, which was dependent on the treatment period and brain region studied. In addition, a short-term increase was followed by a long-term decrease in KCC2 oligomerization in the cell membrane of P4-14 hippocampi and cerebral cortices. Analysis of the Ca(2+) binding proteins expressed in subpopulations of GABAergic cells, parvalbumin and calbindin, showed region-specific effects of GVG during P4-14 on parvalbumin-IR cell density. Moreover, calbindin levels were elevated in GVG mice compared to controls during this period. Cumulatively, these results suggest a particular susceptibility of the hippocampus to GVG when exposed during days P4-14. In conclusion, our studies have identified modifications of key components in the inhibitory system during a critical developmental period. These findings provide novel insights into the deleterious consequences observed in children following prenatal and neonatal exposure to GABA-potentiating drugs.

Facilitatory actions of serotonin type 3 receptors on GABAergic inhibitory synaptic transmission in the spinal superficial dorsal horn

Analgesic effects of serotonin (5-hydroxytryptamine [5-HT]) type 3 (5-HT3) receptors may involve the release of gamma-aminobutyric acid (GABA) in the spinal dorsal horn. However, the precise synaptic mechanisms for 5-HT3 receptor-mediated spinal analgesia are not clear. In this study, we investigated whether GABAergic neurons in the superficial dorsal horn (SDH) express functional 5-HT3 receptors and how these 5-HT3 receptors affect GABAergic inhibitory synaptic transmission in the SDH, by using slice preparations from adult glutamate decarboxylase 67-green fluorescent protein (GAD67-GFP) knock-in mice. Tight-seal whole cell recordings from GFP-positive and -negative neurons showed that 5-HT3 receptor-specific agonist 2-methyl-serotonin (2-Me-5-HT) induced inward currents in a substantial population of both GFP-positive and -negative neurons. Additionally, we confirmed expression of 5-HT3 receptors in both types of neurons by single-cell reverse transcription-polymerase chain reaction (RT-PCR) analysis. Further, GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs)-both those evoked by electrical stimulation and those occurring spontaneously in tetrodotoxin (i.e., miniature IPSCs [mIPSCs])-were recorded from GFP-negative neurons. 2-Me-5-HT increased the amplitude of the evoked IPSCs and the frequency of mIPSCs. The amplitude of mIPSCs was not affected by 2-Me-5-HT, suggesting that 5-HT augments GABAergic synaptic transmission via presynaptic mechanisms. The present observations indicate that 5-HT3 receptors are expressed on both somadendritic regions and presynaptic terminals of GABAergic neurons and regulate GABAA receptor-mediated inhibitory synaptic transmission in the SDH. Taken together, these results provide clues for the underlying mechanisms of the antinociceptive actions of 5-HT3 receptors in the spinal dorsal horn.

The neuroprotective properties of calorie restriction, the ketogenic diet, and ketone bodies

Both calorie restriction and the ketogenic diet possess broad therapeutic potential in various clinical settings and in various animal models of neurological disease. Following calorie restriction or consumption of a ketogenic diet, there is notable improvement in mitochondrial function, a decrease in the expression of apoptotic and inflammatory mediators and an increase in the activity of neurotrophic factors. However, despite these intriguing observations, it is not yet clear which of these mechanisms account for the observed neuroprotective effects. Furthermore, limited compliance and concern for adverse effects hamper efforts at broader clinical application. Recent research aimed at identifying compounds that can reproduce, at least partially, the neuroprotective effects of the diets with less demanding changes to food intake suggests that ketone bodies might represent an appropriate candidate. Ketone bodies protect neurons against multiple types of neuronal injury and are associated with mitochondrial effects similar to those described during calorie restriction or ketogenic diet treatment. The present review summarizes the neuroprotective effects of calorie restriction, of the ketogenic diet and of ketone bodies, and compares their putative mechanisms of action.

Allelic variation in GAD1 (GAD67) is associated with schizophrenia and influences cortical function and gene expression

Cortical GABAergic dysfunction has been implicated as a key component of the pathophysiology of schizophrenia and decreased expression of the gamma-aminobutyric acid (GABA) synthetic enzyme glutamic acid decarboxylase 67 (GAD(67)), encoded by GAD1, is found in schizophrenic post-mortem brain. We report evidence of distorted transmission of single-nucleotide polymorphism (SNP) alleles in two independent schizophrenia family-based samples. In both samples, allelic association was dependent on the gender of the affected offspring, and in the Clinical Brain Disorders Branch/National Institute of Mental Health (CBDB/NIMH) sample it was also dependent on catechol-O-methyltransferase (COMT) Val158Met genotype. Quantitative transmission disequilibrium test analyses revealed that variation in GAD1 influenced multiple domains of cognition, including declarative memory, attention and working memory. A 5' flanking SNP affecting cognition in the families was also associated in unrelated healthy individuals with inefficient BOLD functional magnetic resonance imaging activation of dorsal prefrontal cortex (PFC) during a working memory task, a physiologic phenotype associated with schizophrenia and altered cortical inhibition. In addition, a SNP in the 5' untranslated (and predicted promoter) region that also influenced cognition was associated with decreased expression of GAD1 mRNA in the PFC of schizophrenic brain. Finally, we observed evidence of statistical epistasis between two SNPs in COMT and SNPs in GAD1, suggesting a potential biological synergism leading to increased risk. These coincident results implicate GAD1 in the etiology of schizophrenia and suggest that the mechanism involves altered cortical GABA inhibitory activity, perhaps modulated by dopaminergic function.

The effects of GABA agonists on glutamic acid decarboxylase, GABA-transaminase, activin, salmon gonadotrophin-releasing hormone and tyrosine hydroxylase mRNA in the goldfish (Carassius auratus) neuroendocrine brain

GABA plays a pivotal role in reproduction by regulating luteinising hormone (LH) release from the anterior pituitary. Current evidence indicates that there is a prominent stimulatory effect of GABA on LH release in teleost fish which results from enhanced gonadotrophin-releasing hormone (GnRH) release and decreased dopamine turnover in the brain and pituitary. We hypothesised that there may be additional mechanisms underlying LH release in goldfish and investigated the relative mRNA levels of GABA synthesising enzymes (GAD65 and GAD67), degrading enzyme (GABA-T), activin betaa and betab, salmon GnRH (sGnRH), and tyrosine hydroxylase (TH) with the real-time reverse transcriptase-polymerase chain reaction after GABA agonist treatment. Sexually regressed female goldfish were i.p. injected with either the GABA(A) agonist muscimol (1 microg/g body weight) or the GABA(B) agonist baclofen (10 microg/g body weight). Both agonists significantly increased serum LH after 6 h. Muscimol decreased GAD65 (approximately ten-fold), GABA-T (approximately 15-fold) and TH (approximately three-fold) mRNA in the telencephalon. Baclofen significantly reduced GAD67 (approximately two-fold) and GABA-T (approximately two-fold) mRNA levels in the hypothalamus. Activin betaa, but not activin betab, steady-state mRNA was increased approximately three- to four-fold in both the hypothalamus and telencephalon after baclofen treatment. There was no change in sGnRH mRNA levels in either tissue after GABA agonist treatment. We show that the GABA(A) and GABA(B) receptor agonists have differing and rapid effects on gene transcription in the goldfish neuroendocrine brain and, by affecting specific targets, we identify putative genomic mechanisms underlying GABA-stimulated LH release in fish.

Decreased GAD67 mRNA levels in cerebellar Purkinje cells in autism: pathophysiological implications

The recent identification of decreased protein levels of glutamate decarboxylase (GAD) 65 and 67 isoforms in the autistic cerebellar tissue raises the possibility that abnormal regulation of GABA production in individual neurons may contribute to the clinical features of autism. Reductions in Purkinje cell number have been widely reported in autism. It is not known whether the GAD changes also occur in Purkinje cells at the level of transcription. Using a novel approach, the present study quantified GAD67 mRNA, the most abundant isoform in Purkinje cells, using in situ hybridization in adult autistic and control cases. The results indicate that GAD67 mRNA level was reduced by 40% in the autistic group (P < 0.0001; two-tailed t test), suggesting that reduced Purkinje cell GABA input to the cerebellar nuclei potentially disrupts cerebellar output to higher association cortices affecting motor and/or cognitive function. These findings may also contribute to the understanding of previous reports of alterations in the GABAergic system in limbic and cerebro-cortical areas contributing to a more widespread pathophysiology in autistic brains.

Expression of GAD65 and GAD67 immunoreactivity in MPTP-treated monkeys with or without l-DOPA administration

This study investigated the consequences of levodopa treatment on the expression of the 65- and 67-kDa isoforms of glutamate decarboxylase (GAD65 and GAD67) immunoreactivity in the basal ganglia and cortex of monkeys rendered Parkinsonian by systemic MPTP administration. All MPTP-treated monkeys showed Parkinsonian impairment and selective loss of tyrosine hydroxylase (TH) with sparing of GAD immunoreactive (-ir) fibers and terminals in basal ganglia. The distribution of GAD65- and GAD67-ir in the cortex, caudate, and putamen was not significantly different in MPTP vs. naïve monkeys nor as a function of L-DOPA treatment. In comparison, the expression of GAD67- but not GAD65-ir was augmented in the globus pallidus in MPTP-treated monkeys. Quantification revealed significant increases in number of GAD67-ir neurons in the external and internal segments of the globus pallidus while no significant difference in the number of GAD65-ir neurons was observed. L-DOPA treatment did not significantly change the number of GAD65- or GAD67-ir pallidal neurons following MPTP. These results support and extend the findings that transcriptional elevation of GAD67 occurs in the globus pallidus and demonstrate that GAD65 and GAD67 are differentially altered following lesion. The finding of elevated GAD67 expression in the pallidum is consistent with alterations in inhibitory neurocircuitry playing a key role in the pathophysiology of motor disturbances in Parkinson's disease.

Glutamate Decarboxylase: Loss of N-terminal Segment Does Not Affect Homodimerization and Determination of the Oxidation State of Cysteine Residues

Glutamate decarboxylase (GAD) produces GABA, the main inhibitory neurotransmitter in adult mammalian brain. The physical characteristics of GAD were studied using mass spectrometry and partial protein digests. The N-termini of the two main isoforms, GAD65 and GAD67, were processed by removal of the initial methionine residues and acetylation of the penultimate alanines. Native recombinant GAD65 and GAD67 exist as homodimers that can be dissociated with non-reducing methods, indicating that homodimerization does not involve intermolecular disulfide bonds. Truncation of the N-terminal segment with trypsin digestion did not affect homodimerization but increased activity by decreasing the Km of GAD67 and increasing the Vmax of both isoforms. Of the 15 cysteines in GAD65, the six found in the N-terminal segment can form disulfide bonds and of the 13 cysteines in GAD67, cysteines 32 and 38 can form a disulfide bond. The in vitro formation of disulfide bonds in the N-termini, and the removal of the termini with relatively low amounts of trypsin, indicate that the N-terminal segments of GAD65 and GAD67 are exposed and flexible. The formation of a disulfide bridge between cysteines 30 and 45 of GAD65 suggests that alteration of normal redox conditions could affect GAD targeting.

Effects of creatine treatment on survival and differentiation of GABA-ergic neurons in cultured striatal tissue

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder, characterized by a prominent loss of GABA-ergic medium-sized spiny neurons in the caudate putamen. There is evidence that impaired energy metabolism contributes to neuronal death in HD. Creatine is an endogenous substrate for creatine kinases and thereby supports cellular ATP levels. This study investigated the effects of creatine supplementation (5 mm) on cell survival and neuronal differentiation in striatal cultures. Chronic creatine treatment resulted in significant increased densities of GABA-immunoreactive (-ir) neurons, although total neuronal cell number and general viability were not affected. Similar effects were seen after short-term treatment, suggesting that creatine acted as a differentiation factor. Inhibitors of transcription or translation did not abolish the creatine-mediated effects, nor did omission of extracellular calcium, whereas inhibition of mitogen-activated protein kinase and phosphatidylinositol-3-kinase significantly attenuated the creatine induced increase in GABA-ir cell densities. Creatine exhibited significant neuroprotection against toxicity instigated either by glucose- and serum deprivation or addition of 3-nitropropionic acid. In sum, the neuroprotective properties in combination with promotion of neuronal differentiation suggest that creatine has potential as a therapeutic drug in the treatment of neurodegenerative diseases, like HD.

GABAergic modulation of the expression of genes involved in GABA synaptic transmission and stress in the hypothalamus and telencephalon of the female goldfish (Carassius auratus)

GABA is one of the most abundant neurotransmitters in the vertebrate central nervous system and is involved in neuroendocrine processes such as development, reproduction, feeding and stress. To examine the effect of GABA on gene expression in the brain, we used a cDNA macroarray containing 26 genes involved in GABA synaptic transmission (GABA receptor subunits, GABA transporters), reproduction (gonadotrophin-releasing hormone isoforms and oestrogen receptor alpha), feeding (neuropeptide Y and cholecystokinin), and stress [corticotrophin-releasing factor (CRF)]. To elevate GABA levels in the brain, we injected female goldfish with gamma-vinyl GABA (300 microg/g of body weight) (24 h), an irreversible inhibitor of the enzyme GABA transaminase (GABA-T). We found that increased levels of GABA in the hypothalamus resulted in a 2.2-fold down-regulation of GABA(A) receptor beta4 subunit mRNA. In the telencephalon, we found that increased GABA levels resulted in a 1.5-fold increase of CRF mRNA and a 1.8-fold decrease of GABA(A) receptor beta2 subunit mRNA. Increasing GABA in the hypothalamus and telencephalon of the goldfish did not significantly affect the mRNA abundance of genes involved in GABA synthesis (glutamic acid decarboxylase isoforms) and degradation (GABA-T), feeding, or reproduction. Our preliminary study suggests that the regulation of GABA receptor subunit mRNA expression by GABA may be a conserved evolutionary mechanism in vertebrates to modulate GABAergic synaptic transmission.

Kinetic differences between the isoforms of glutamate decarboxylase: implications for the regulation of GABA synthesis

Glutamate decarboxylase (GAD) exists as two isoforms, GAD65 and GAD67. GAD activity is regulated by a cycle of activation and inactivation determined by the binding and release of its co-factor, pyridoxal 5'-phosphate. Holoenzyme (GAD with bound co-factor) decarboxylates glutamate to form GABA, but it also catalyzes a slower transamination reaction that produces inactive apoGAD (without bound co-factor). Apoenzyme can reassociate with pyridoxal phosphate to form holoGAD, thus completing the cycle. Within cells, GAD65 is largely apoenzyme (approximately 93%) while GAD67 is mainly holoenzyme (approximately 72%). We found striking kinetic differences between the GAD isoforms that appear to account for this difference in co-factor saturation. The glutamate dependent conversion of holoGAD65 to apoGAD was about 15 times faster than that of holoGAD67 at saturating glutamate. Aspartate and GABA also converted holoGAD65 to apoGAD at higher rates than they did holoGAD67. Nucleoside triphosphates (such as ATP) are known to affect the activation reactions of the cycle. ATP slowed the activation of GAD65 and markedly reduced its steady-state activity, but had little affect on the activation of GAD67 or its steady-state activity. Inorganic phosphate opposed the effect of ATP; it increased the rate of apoGAD65 activation but had little effect on apoGAD67 activation. We conclude that the apo-/holoenzyme cycle of inactivation and reactivation is more important in regulating the activity of GAD65 than of GAD67.

Expression of the brain-specific membrane adapter protein p42IP4/centaurin alpha, a Ins(1,3,4,5)P4/PtdIns(3,4,5)P3 binding protein, in developing rat brain

Inositolphosphates and phosphatidylinositides are important second messengers. Previously p42(IP4), a protein with high affinity for both Ins(1,3,4,5)P(4) and PtdIns(3,4,5)P(3) has been characterized in our laboratory. In the present study mRNA levels of p42(IP4) were quantified during development (ages: 7, 14, 21 days and adult) by means of ribonuclease protection assay in various rat brain regions (cerebellum, cortex, striatum, thalamus, hypothalamus, olfactory bulb, hippocampus and tectum (superior and inferior colliculus)). A high level of p42(IP4) mRNA was detected in the cortex (ca. 1 pg specific RNA per microg of total RNA) which stayed highly independent of the age of the animals. In hippocampus and in the thalamus, p42(IP4) mRNA levels were comparable to those in the cortex in the first and second week postnatally, but decreased to lower levels in the adult brain. In striatum, the mRNA increased, albeit less intensely than in hippocampus and thalamus, until day 21 postnatally, and then decreased in the adult rat brain. Cerebellar p42(IP4) mRNA showed a slow increase within the first 3 weeks postnatally, and remained rather high in the adult brain. The protein expression of p42(IP4), tested within the same samples by Western blot staining, was consistent with mRNA values. For comparison, glutamic acid decarboxylase (isoforms GAD65/GAD67), an enzyme, for which some regional brain specific distribution is already known, was also examined. The mRNA levels of GAD and its developmental regulation clearly differed from that of p42(IP4). In summary, p42(IP4) expressed in several neuronal cell types, did not seem to be restricted to specific developmental stages, but the high absolute expression levels at all developmental stages indicated that p42(IP4) is a protein fundamental for neuronal functioning.

Reductions in N-acetylaspartylglutamate and the 67 kDa form of glutamic acid decarboxylase immunoreactivities in the visual system of albino and pigmented rats after optic nerve transections

This study compares the immunohistochemical distributions of N-acetylaspartylglutamate (NAAG) and the large isoform of the gamma-aminobutyric acid (GABA)-synthesizing enzyme glutamic acid decarboxylase (GAD(67)) in the visual system of albino and pigmented rats. Most retinal ganglion cells and their axons were strongly immunoreactive for NAAG, whereas GAD(67) immunoreactivity was very sparse in these cells and projections. In retinorecipient zones, NAAG and GAD(67) immunoreactivities occurred in distinct populations of neurons and in dense networks of strongly immunoreactive fibers and synapses. Dual-labeling immunohistochemistry indicated that principal neurons were stained for NAAG, whereas local interneurons were stained for GAD(67). In contrast to the distribution observed in retinorecipient zones, most or all neurons were doubly stained for NAAG and GAD(67) in the thalamic reticular nucleus. Ten days after unilateral optic nerve transection, NAAG-immunoreactive fibers and synapses were substantially reduced in all contralateral retinal terminal zones. The posttransection pattern of NAAG-immunoreactive synaptic loss demarcated the contralateral and ipsilateral divisions of the retinal projections. In addition, an apparent transynaptic reduction in GAD(67) immunoreactivity was observed in some deafferented areas, such as the lateral geniculate. These findings suggest a complicated picture in which NAAG and GABA are segregated in distinct neuronal populations in primary visual targets, yet they are colocalized in neurons of the thalamic reticular nucleus. This is consistent with NAAG acting as a neurotransmitter release modulator that is coreleased with a variety of classical transmitters in specific neural pathways.

Gastrodin decreases immunoreactivities of gamma-aminobutyric acid shunt enzymes in the hippocampus of seizure-sensitive gerbils

Gastrodin is one of the natural compound isolated from Gastrodia elata and has known anticonvulsant effects, although the exact pharmacological principles of this natural compound and its effects on other aspects of gamma-aminobutyric acid (GABA) metabolism in vivo have not been explored. Therefore, in the present study, the effects of gastrodin on GABA metabolism in the gerbil hippocampus were examined, in an effort to identify the antiepileptic characteristics of this substance. Gastrodin reduced the seizure score in the treated group, although the immunoreactivities of GABA synthetic enzymes and GABA transporters were unaltered in gastrodin-treated animals. Interestingly, in the gastrodin-treated group, GABA transaminase (GABA-T) immunoreactivity in the hippocampus, particularly in neurons, was significantly decreased. In the gastrodin-treated group, both succinic semialdehyde dehydrogenase (SSADH) and succinic semialdehyde reductase (SSAR) immunoreactivities in the hippocampus was also decreased significantly, which stood in contrast to the nontreated group, in which strong SSADH and SSAR immunoreactivities were detected. From the neuroanatomical viewpoint, these findings suggest that gastrodin may cause the elevation of GABA concentration by inhibiting the GABA shunt.

In vivo monitoring of rat brain metabolites during vigabatrin treatment using localized 2D-COSY

A two-dimensional COSY-based localization sequence was designed to allow the in vivo monitoring of proton metabolites in rat brain [particularly gamma-aminobutyric acid (GABA), glutamine, taurine and myo-inositol]. The sequence incorporated OSIRIS signal localization, B1-insensitive water suppression and phase-sensitive COSY acquisition. The method was used to study the effects of the GABA-transaminase inhibitor vigabatrin on rat brain metabolite concentrations. Wistar rats were treated daily for 3 days with an oral dose of vigabatrin (200 mg/kg, n = 4). Localized COSY spectra were obtained during a 120 min acquisition from a 270 microl central brain voxel and compared with nine untreated control animals. Significant elevations were observed in GABA (267% of control, p < 0.005, Mann-Witney test), glutamine (130% of control, p < 0.005) and taurine (113% of control, p < 0.05). Changes in GABA and taurine were consistent with previous data on the action of Vigabatrin, and support a previously hypothesized link between these compounds. The increase in glutamine was more surprising and may reflect the balance between the level and/or site of GABA-transaminase inhibition and downregulation of GABA synthesis.

Effects of vigabatrin on brain GABA+/Cr signals in focus-distant and focus-near brain regions monitored by 1H-NMR spectroscopy

The new antiepileptic drug vigabatrin (VGB) increases gamma-aminobutyric acid (GABA) in the brain. We compared GABA+/Cr signals measured focus-near and focus-distant and correlated it with the degree of response to VGB. Brain GABA+/Cr signals were measured in 17 epileptic patients in structurally normal appearing tissue by nuclear proton magnetic resonance (1H-NMR) spectroscopy using a special editing sequence for GABA. In 11 patients the measurements were done in brain areas distant to focus and in six near to focus. Full-responders (seizure reduction of >or=50% at the end of the treatment phase) and partial-responders (seizure reduction of >or=50% at the end of the first month of treatment but <or=50% at end of treatment) had lower GABA+/Cr signals in the hemisphere with the epileptogenic focus and increases of the GABA+/Cr signals with VGB. Non-responders (seizure reduction of <or=50%) had no side difference in the GABA+/Cr signals before treatment and no increase during treatment. These observations were made in structurally normal appearing tissue near to the focus and distant to the focus. A side difference in brain GABA+/Cr signal between the epileptogenic and non-epileptogenic hemisphere before VGB treatment correlates with an improved seizure control under VGB treatment regardless whether the measurement is done focus-near or focus-distant.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

Opposite roles of GABA and excitatory amino acids on the control of GAD expression in cultured retina cells

The mechanism of control of GAD expression by GABA and excitatory amino acids (EAAs) was studied in chick and rat retina cultures using immunohistochemical and PAGE-immunoblot detection of the enzyme, as well as by measuring enzyme activity. Aggregate cultures were prepared with retina cells obtained from chick embryos at embryonic days 8-9 (E8-E9). Organotypical cultures were also prepared with retinas from E14 chick embryos, post-hatched chicken and P21 rats. GABA (1-20 mM) fully prevented GAD expression in aggregate and organotypical cultures from chick embryo retinas. A substantial, but not complete, reduction of GAD was also observed in organotypical cultures of post-hatched chicken and P21 rats, in which both forms of the enzyme (GAD65 and 67) were affected. The GABA effect was not mimicked by THIP (100 microM), baclofen (100 microM) or CACA (300 microM), agonists of GABAa, b and c receptors, respectively. NNC-711, a potent inhibitor of GABA transporters, reduced by 50% the inhibition of GAD activity promoted by GABA. Aggregates exposed to GABA and treated with glutamate (5 mM) or kainate (100 microM) displayed an intense GAD-like immunoreactivity in many cell bodies, but not in neurite regions. Immunoblot analysis revealed that the increase in GAD-like immunoreactivity by EAA corresponded to a 67-kDa protein. However, GAD activity was not detected. Treatment of aggregates or retina homogenates with SNAP, a NO producing agent (but not its oxidized form), reduced GAD activity by more than 60% indicating that the lack of enzyme activity in GAD-like immunoreactive cells, could be due to NO production by EAA stimulation.

Influence of pyridoxal 5'-phosphate alone and in combination with vigabatrin on brain GABA measured by 1H-NMR-spectroscopy

Both iso-forms of the gamma-aminobutyric acid (GABA) synthesising enzyme and also the GABA degrading enzyme need pyridoxal 5'-phosphate (PP) as co-enzyme. The aim of the study was to investigate the influence of PP alone and in combination with various doses of vigabatrin (VGB) on brain GABA levels. In eight healthy subjects 300 mg/d PP and various doses of VGB (range, 1000 mg/d to 4000 mg/d) were given alone or in combination. The GABA+/creatine (Cr) signals in both occipital lobes were measured before treatment, during monotherapy with PP or VGB, and during combination of both using 1H-NMR-spectroscopy (1H-NMRS). PP alone did not change the GABA+/Cr signals. VGB alone increased the GABA+/Cr signals in both hemispheres. The combination PP and low-medium dosed VGB (1000-2000 mg/d) did not increase the GABA+/Cr signals. The effects of the combination of PP and high dosed (3000-4000 mg/d) VGB on the GABA+/Cr signals varied depending on the sequence of the drugs and dose of VGB. PP alone has no effect on the GABA+/Cr signals in healthy volunteers. The combination of PP and low-high dosed VGB had inconsistent effects on the GABA+/Cr signals compared to a VGB monotherapy because PP activates also the GABA-degrading enzyme GABA-transaminase.

GABA expression dominates neuronal lineage progression in the embryonic rat neocortex and facilitates neurite outgrowth via GABA(A) autoreceptor/Cl- channels

GABA emerges as a trophic signal during rat neocortical development in which it modulates proliferation of neuronal progenitors in the ventricular/subventricular zone (VZ/SVZ) and mediates radial migration of neurons from the VZ/SVZ to the cortical plate/subplate (CP/SP) region. In this study we investigated the role of GABA in the earliest phases of neuronal differentiation in the CP/SP. GABAergic-signaling components emerging during neuronal lineage progression were comprehensively characterized using flow cytometry and immunophenotyping together with physiological indicator dyes. During migration from the VZ/SVZ to the CP/SP, differentiating cortical neurons became predominantly GABAergic, and their dominant GABA(A) receptor subunit expression pattern changed from alpha4beta1gamma1 to alpha3beta3gamma2gamma3 coincident with an increasing potency of GABA on GABA(A) receptor-mediated depolarization. GABA(A) autoreceptor/Cl(-) channel activity in cultured CP/SP neurons dominated their baseline potential and indirectly their cytosolic Ca(2+) (Ca(2+)c) levels via Ca(2+) entry through L-type Ca(2+) channels. Block of this autocrine circuit at the level of GABA synthesis, GABA(A) receptor activation, intracellular Cl(-) ion homeostasis, or L-type Ca(2+) channels attenuated neurite outgrowth in most GABAergic CP/SP neurons. In the absence of autocrine GABAergic signaling, neuritogenesis could be preserved by depolarizing cells and elevating Ca(2+)c. These results reveal a morphogenic role for GABA during embryonic neocortical neuron development that involves GABA(A) autoreceptors and L-type Ca(2+) channels.

Dynamic expression of a glutamate decarboxylase gene in multiple non-neural tissues during mouse development

BACKGROUND

Glutamate decarboxylase (GAD) is the biosynthetic enzyme for the neurotransmitter gamma-aminobutyric acid (GABA). Mouse embryos lacking the 67-kDa isoform of GAD (encoded by the Gad1 gene) develop a complete cleft of the secondary palate. This phenotype suggests that this gene may be involved in the normal development of tissues outside of the CNS. Although Gad1 expression in adult non-CNS tissues has been noted previously, no systematic analysis of its embryonic expression outside of the nervous system has been performed. The objective of this study was to define additional structures outside of the central nervous system that express Gad1, indicating those structures that may require its function for normal development.

RESULTS

Our analysis detected the localized expression of Gad1 transcripts in several developing tissues in the mouse embryo from E9.0-E14.5. Tissues expressing Gad1 included the tail bud mesenchyme, the pharyngeal pouches and arches, the ectodermal placodes of the developing vibrissae, and the apical ectodermal ridge (AER), mesenchyme and ectoderm of the limb buds.

CONCLUSIONS

Some of the sites of Gad1 expression are tissues that emit signals required for patterning and differentiation (AER, vibrissal placodes). Other sites correspond to proliferating stem cell populations that give rise to multiple differentiated tissues (tail bud mesenchyme, pharyngeal endoderm and mesenchyme). The dynamic expression of Gad1 in such tissues suggests a wider role for GABA signaling in development than was previously appreciated.

Role of glutamatergic and GABAergic systems in alcoholism

The pharmacological effects of ethanol are complex and widespread without a well-defined target. Since glutamatergic and GABAergic innervation are both dense and diffuse and account for more than 80% of the neuronal circuitry in the human brain, alterations in glutamatergic and GABAergic function could affect the function of all neurotransmitter systems. Here, we review recent progress in glutamatergic and GABAergic systems with a special focus on their roles in alcohol dependence and alcohol withdrawal-induced seizures. In particular, NMDA-receptors appear to play a central role in alcohol dependence and alcohol-induced neurological disorders. Hence, NMDA receptor antagonists may have multiple functions in treating alcoholism and other addictions and they may become important therapeutics for numerous disorders including epilepsy, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's chorea, anxiety, neurotoxicity, ischemic stroke, and chronic pain. One of the new family of NMDA receptor antagonists, such as DETC-MESO, which regulate the redox site of NMDA receptors, may prove to be the drug of choice for treating alcoholism as well as many neurological diseases.

Effects of vigabatrin on brain GABA+/CR signals in patients with epilepsy monitored by 1H-NMR-spectroscopy: responder characteristics

PURPOSE

Vigabatrin (VGB) is a new antiepileptic drug that increases the human brain gamma-aminobutyric acid (GABA) level by irreversibly inhibiting GABA transaminase. Although some patients respond to VGB with a significant seizure reduction, others do not. The aim of this study was to identify possible responders before or in an early phase of VGB treatment by measuring the GABA and homocarnosine contaminated with macromolecules/creatine and phosphocreatine ratio (GABA+/Cr) signal by means of proton-nuclear magnetic resonance (1H NMR) spectroscopy.

METHODS

Measurements were performed immediately before and after a titration period of 1 month (2 g/day during the past 2 weeks). A third measurement followed a maintenance period of 3 months (2 or 3 g/day). In 14 patients with drug-resistant temporal lobe epilepsy and 3 patients with occipital lobe epilepsy, GABA+/Cr was measured in the ipsilateral (i.e., epileptogenic) hemisphere and contralateral (i.e., nonepileptogenic) hemisphere in a volume of 8 cm3.

RESULTS

Depending on the therapeutic efficacy of VGB, we defined three groups: (a) full responders (n = 7), (b) nonresponders (n = 7), and (c) partial responders (n = 3). The nonresponders had no significant change in the GABA+/Cr signal during the treatment compared with baseline. The full responders had a significant increase of the GABA+/Cr signal during the whole treatment phase and a lower ipsilateral level at baseline. The partial responders had also a lowered ipsilateral GABA+/Cr signal at baseline and an increase during treatment but a decrease when the seizures started again.

CONCLUSIONS

Responders to VGB could be identified by a lower ipsilateral baseline GABA+/Cr signal and a steeper increase during VGB treatment. However, it was not possible to predict the duration of the response (full versus partial responder) with these criteria.

GAD levels and muscimol binding in rat inferior colliculus following acoustic trauma

Pharmacological studies of the inferior colliculus (IC) suggest that the inhibitory amino acid neurotransmitter gamma-aminobutyric acid (GABA) plays an important role in shaping responses to simple and complex acoustic stimuli. Several models of auditory dysfunction, including age-related hearing loss, tinnitus, and peripheral deafferentation, suggest an alteration of normal GABA neurotransmission in central auditory pathways. The present study attempts to further characterize noise-induced changes in GABA markers in the IC. Four groups (unexposed control, 0 h post-exposure, 42 h post-exposure, and 30 days post-exposure) of 3-month-old male Fischer 344 rats were exposed to a high intensity sound (12 kHz, 106 dB) for 10 h. Observed hair cell damage was primarily confined to the basal half of the cochlea. There was a significant decrease in glutamic acid decarboxylase (GAD(65)) immunoreactivity in the IC membrane fraction compared to controls (P<0.05) at 0 h (-41%) and 42 h (-28%) post-exposure, with complete recovery by 30 days post-exposure (P>0.98). Observed decreases in cytosolic levels of GAD(65) were not significant. Quantitative muscimol receptor binding revealed a significant increase (+20%) in IC 30 days after sound exposure (P<0.05). These data suggest that changes in GABA neurotransmission occur in the IC of animals exposed to intense sound. Additional studies are needed to determine whether these changes are a result of protective/compensatory mechanisms or merely peripheral differentiation, as well as whether these changes preserve or diminish central auditory system function.

Transplantation of embryonic Raphe cells regulates the modifications of the gabaergic phenotype occurring in the injured spinal cord

Transection of the spinal cord yields a permanent deficit due to the interruption of descending and ascending tracts which subserve the supraspinal control of spinal cord functions. We have shown previously that transplantation below the level of the section of embryonic monoaminergic neurons can promote the recovery of some segmental functions via a local serotonergic and noradrenergic reinnervation. Moreover, the up-regulation of the corresponding receptors resulting from the section was corrected by the transplants. The aim of the present work was to determine whether such a graft could also influence non-monoaminergic local neurons, the GABAergic interneurons of the spinal cord. Following spinal cord transection, the number of cells which express glutamate decarboxylase (mol. wt 67,000) messenger RNA--a marker of GABA synthesis--increased significantly below the lesion compared with the intact animal. In contrast, in lesioned animals which had been grafted one week later with raphe neuroblasts, this number was close to control level. These post-grafting modifications were further associated with increased GABA immunoreactivity in the host tissue. These data suggest that the graft of embryonic raphe cells which compensates the deficit of serotonin in the distal segment also regulates the expression of the GABAergic phenotype in the host spinal cord. This regulation could be mediated by the re-establishment of a local functional innervation by both serotonin and GABAergic transplanted neurons and/or by trophic factors released from the embryonic cells. It appears then that grafted cells influence the host tissue in a complex manner, through the release and/or regulation of several neurotransmitter systems.

Neuroprotection of cultured foetal rat hippocampal cells against glucose deprivation: are GABAergic neurons less vulnerable or more sensitive to TCP protection?

In the rat brain, hippocampal neurons are particularly sensitive to secondary excitotoxic injury induced by ischaemia or hypoglycaemia. To determine some distinctive features of vulnerability among neuronal phenotypes in the hippocampus following a metabolic insult, we used an in vitro model of mild glucose deprivation. Primary cultures from the rat hippocampus (21 days in vitro) were deprived of glucose for 4 h and then were returned to the standard medium for 24 or 48 h. Survival of the GABAergic neuronal population was evaluated both by measuring [3H]GABA uptake and by counting GAD65-immunostained cells. This was compared with the survival of the total neuronal population evaluated by counting the neurofilament-200-immunostained cells. Glucose deprivation for 4 h followed by a recovery period of 48 h induced a decrease of 59% and 40% in the number of GAD65- and neurofilament-200-immunostained cells, respectively. Thus, GABAergic neurons were slightly more vulnerable to glucose deprivation than the other neurons in the hippocampal cell cultures. When the excitotoxic component of cellular death was blocked in the presence of TCP, an NMDA-antagonist, the survival of GABAergic neurons was almost complete after 48 h of recovery. In contrast, measurements of the release of lactate dehydrogenase in the medium indicated that TCP largely protected hippocampal cells after 24 h but was ineffective after 48 h. This observation was confirmed by immunostaining data which showed that after 48 h TCP did not significantly increase the survival of neurofilament-200-immunostained cells. These results indicate that after glucose deprivation and a recovery period of 48 h, GABAergic neurons in hippocampal cell cultures are not more resistant than other neurons but they are more sensitive to TCP protection.

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.

GABAergic system in the developing mammalian retina: dual sources of GABA at early stages of postnatal development

In the present work, we have characterized the maturation of the GABAergic system in mammalian retina. Immunoreactivity for GABA, GAD (glutamic acid decarboxylase, EC 4.1.1.15) -65 and -67 in the adult rat retina was localized in cells in the inner nuclear and ganglion cell layers. This pattern was established around postnatal day 8 and included transient GABA and GAD-67 expression in horizontal cells. GAD activity was very low at P1 and P4, increasing after P8, reaching maximal activity by P21 and decreasing to attain adult values by P30. GABA content was approximately constant from P1 to P13, increasing thereafter to reach adult levels. GAD protein content increased progressively with postnatal development and the two isoforms could be distinguished at P8. The disparity between retinal GABA content vs. presence and activity of the synthesizing enzyme, led us to investigate the alternative pathway for GABA synthesis that utilizes putrescine as a substrate. Highest levels of ornithine decarboxylase activity (the limiting step for putrescine synthesis) were found between P1 and P4, decreasing to very low levels after P13. The same pattern was observed for putrescine content in the retina. Highest amounts were found at P1, that decreased and remained constant after P13. Additionally, approximately 40% of tritiated putrescine incorporated by P1, P4 and adult retinas was converted into GABA. Our results suggest the existence of two different sources of GABA in mammalian retina, one that uses glutamate as a precursor and predominates in the mature nervous system and another that utilizes putrescine and is present transiently at early developmental stages.

Changes in GAD- and GABA- immunoreactivity in the spinal dorsal horn after peripheral nerve injury and promotion of recovery by lumbar transplant of immortalized serotonergic precursors

We have utilized RN46A cells, an immortalized neuronal cell line derived from E13 brainstem raphe, as a model for transplant of bioengineered serotonergic cells. RN46A cells require brain-derived neurotrophic factor (BDNF) for increased survival and serotonin (5HT) synthesis in vitro and in vivo. RN46A cells were transfected with the rat BDNF gene, and the 46A-B14 cell line was subcloned. These cells survive longer than 7 weeks after transplantation into the subarachnoid space of the lumbar spinal cord and synthesize 5HT and BDNF. Chronic constriction injury (CCI) of the sciatic nerve was used to induce chronic neuropathic pain in the affected hindpaw in rats. Transplants of 46A-B14 cells placed 1 week after CCI alleviated chronic neuropathic pain, while transplants of 46A-V1 control cells, negative for 5HT and without the BDNF gene, had no effect on the induction of thermal and tactile nociception. When endogenous cells of the dorsal horn which contain the neurotransmitter gamma-aminobutyric acid (GABA) and its synthetic enzyme glutamate decarboxylase (GAD) were immunohistochemically quantified in the lumbar spinal cord 3 days and 1-8 weeks after CCI, the number of GABA- and GAD-immunoreactive (ir) cells decreased bilateral to the nerve injury as soon as 3 days after CCI. At 1 week after CCI, the number of GABA-ir cells continued to significantly decline bilaterally, returning to near normal numbers on the side contralateral to the nerve injury by 8 weeks after the nerve injury. The number of GAD-ir cells began to increase bilaterally to the nerve injury at 1 week after CCI and continued to significantly increase in numbers over normal values by 8 weeks after the nerve injury. When examined 2 and 8 weeks after CCI plus cell transplants, the transplants of 46A-B14 cells reversed the increase in GAD-ir cell numbers and the decrease in GABA-ir cells by 1 week after transplantation, while 46A-V1 control cell transplants after CCI had no effect on the changes in numbers of GAD-ir or GABA-ir cells. Collectively, these data suggest that altered 5HT levels, and perhaps BDNF secretion, related to the transplants ameliorate chronic pain and reverse the induction and maintenance of an endogenous pain mechanism in the dorsal horn. This induction mechanism is likely dependent on altered GAD regulation and GABA synthesis, initiated by CCI.

Two isoforms of glutamate decarboxylase: why?

Adults express two isoforms of glutamate decarboxylase (GAD), GAD67 and GAD65, which are encoded by different independently regulated genes, a situation that differs from that of other neurotransmitters. In this article, J-J. Soghomonian and David Martin review current knowledge on the differences between these two isoforms. Both isoforms are present in most GABA-containing neurones in the CNS, but GAD65 appears to be targeted to membranes and nerve endings, whereas GAD67 is more widely distributed in cells. Both forms can synthesize transmitter GABA, but GAD67 might preferentially synthesize cytoplasmic GABA and GAD65 might preferentially synthesize GABA for vesicular release. Several lines of evidence suggest that the two forms have different roles in the coding of information by GABA-containing neurones.

Glutamate-modulated production of GABA in immortalized astrocytes transduced by a glutamic acid decarboxylase-expressing retrovirus

Replication-defective Moloney murine leukemia virus expressing the GAD67 gene under the control of the GFAP promoter was produced using selected clones of a fibroblast-packaging cell line. A spontaneously immortalized astrocyte cell line was infected with this virus and cellular clones expressing GAD67 selected. Astrocyte and fibroblast clones expressed functional GAD (detected by glutamic acid decarboxylation), but only fibroblasts were able to also produce GABA in the extracellular medium. When exposed to 200 microM glutamate, despite an observed difference in the rates of glutamate accumulation in control and GAD67-expressing astrocytes, similar proportions of glutamate taken up were detected. In GAD67-expressing astrocytes, the glutamate was mainly converted into GABA, suggesting GAD transgene activity to be dominant over other glutamate metabolic pathways, such as glutamine synthetase and glutamate dehydrogenase. Moreover, rapid GABA release into the cell medium was also observed, suggesting the involvement of reverse GABA transporters. The use of the GFAP promoter might be able to take advantage of its activation in response to factors inducing reactive gliosis observed in pathological insults. GAD67-expressing astrocytes might therefore be used for future grafting in pathological situations in which an excess of glutamate results in neuronal dysfunction or cell death.

Retinal ganglion cell depletion alters the phenotypic expression of GABA and GAD in the rat retina

We have looked at the phenotypic expression of gamma-aminobutyric acid (GABA) and the two isoforms of its synthetic enzyme [glutamic acid decarboxylase (GAD)-65 and -67] in adult rat retinas that had the superior colliculus, pretectum and optic tract lesioned unilaterally at birth. It has been shown previously that this type of manipulation induces retrograde degeneration of retinal ganglion cells presumably without affecting other intraretinal neurons. We present evidence that GABAergic amacrine cells are affected by such manipulation. The number of cells immunoreactive for GABA, GAD-65 and GAD-67 decreased in the inner nuclear layer. In the retinal ganglion cell layer, however, the number of GABA- and GAD-65-labelled cells increased, while the number of GAD-67-labelled cells did not change. Biochemical assay showed that overall GAD activity was not altered in retinas of lesioned animals. Our results support the notion that, while neonatal lesion reorganizes the expression of GABA and GAD in the retina, enzyme activity is maintained within normal levels.

Biphasic response of spinal GABAergic neurons after a lumbar rhizotomy in the adult rat

The expression of gamma-aminobutyric acid (GABA) and of the isoforms of the enzyme involved in its synthesis, glutamic acid decarboxylase (GAD), is modified in several rat brain structures in different injury models. The aim of the present work was to determine whether such plasticity of the GABAergic system also occurred in the deafferented adult rat spinal cord, a model where a major reorganization of neural circuits takes place. GABAergic expression following unilateral dorsal rhizotomy was studied by means of non-radioactive in situ hybridization to detect GAD67 mRNA and by immunohistochemistry to detect GAD67 protein and GABA. Three days following rhizotomy the number of GAD67 mRNA-expressing neurons was decreased in the superficial layers of the deafferented horn, while GABA immunostaining of axonal fibres located in this region was highly increased. Seven days after lesion, on the other hand, many GAD67 mRNA-expression neurons were bilaterally detected in deep dorsal and ventral layers, this expression being correlated with the increased detection of GAD67 immunostained somata and with the reduction of GABA immunostaining of axons. GABA immunostaining was frequently found to be associated with reactive astrocytes that exhibited intense immunostaining for glial fibrillary acidic protein (GFAP) but remained GAD67 negative. These results indicate that degeneration of afferent terminals induces a biphasic response of GABAergic spinal neurons located in the dorsal horn and show that many spinal neurons located in deeper regions re-express GAD67, suggesting a possible participation of the local GABAergic system in the reorganization of disturbed spinal networks.

Basal expression and induction of glutamate decarboxylase and GABA in excitatory granule cells of the rat and monkey hippocampal dentate gyrus

The excitatory, glutamatergic granule cells of the hippocampal dentate gyrus are presumed to play central roles in normal learning and memory, and in the genesis of spontaneous seizure discharges that originate within the temporal lobe. In localizing the two GABA-producing forms of glutamate decarboxylase (GAD65 and GAD67) in the normal hippocampus as a prelude to experimental epilepsy studies, we unexpectedly discovered that, in addition to its presence in hippocampal nonprincipal cells, GAD67-like immunoreactivity (LI) was present in the excitatory axons (the mossy fibers) of normal dentate granule cells of rats, mice, and the monkey Macaca nemestrina. Using improved immunocytochemical methods, we were also able to detect GABA-LI in normal granule cell somata and processes. Conversely, GAD65-LI was undetectable in normal granule cells. Perforant pathway stimulation for 24 hours, which evoked population spikes and epileptiform discharges in both dentate granule cells and hippocampal pyramidal neurons, induced GAD65-, GAD67-, and GABA-LI only in granule cells. Despite prolonged excitation, normally GAD- and GABA-negative dentate hilar neurons and hippocampal pyramidal cells remained immunonegative. Induced granule cell GAD65-, GAD67-, and GABA-LI remained elevated above control immunoreactivity for at least 4 days after the end of stimulation. Pre-embedding immunocytochemical electron microscopy confirmed that GAD67- and GABA-LI were induced selectively within granule cells; granule cell layer glia and endothelial cells were GAD- and GABA-immunonegative. In situ hybridization after stimulation revealed a similarly selective induction of GAD65 and GAD67 mRNA in dentate granule cells. Neurochemical analysis of the microdissected dentate gyrus and area CA1 determined whether changes in GAD- and GABA-LI reflect changes in the concentrations of chemically identified GAD and GABA. Stimulation for 24 hours increased GAD67 and GABA concentrations sixfold in the dentate gyrus, and decreased the concentrations of the GABA precursors glutamate and glutamine. No significant change in GAD65 concentration was detected in the microdissected dentate gyrus despite the induction of GAD65-LI. The concentrations of GAD65, GAD67, GABA, glutamate and glutamine in area CA1 were not significantly different from control concentrations. These results indicate that dentate granule cells normally contain two "fast-acting" amino acid neurotransmitters, one excitatory and one inhibitory, and may therefore produce both excitatory and inhibitory effects. Although the physiological role of granule cell GABA is unknown, the discovery of both basal and activity-dependent GAD and GABA expression in glutamatergic dentate granule cells may have fundamental implications for physiological plasticity presumed to underlie normal learning and memory. Furthermore, the induction of granule cell GAD and GABA by afferent excitation may constitute a mechanism by which epileptic seizures trigger compensatory interictal network inhibition or GABA-mediated neurotrophic effects.

The rate of turnover of cortical GABA from [1-13C]glucose is reduced in rats treated with the GABA-transaminase inhibitor vigabatrin (gamma-vinyl GABA)

Brain GABA levels rise and plateau following prolonged administration of the irreversible GABA-transaminase inhibitor vigabatrin (gamma-vinylGABA). Recently it has been shown that increased GABA levels reduces GAD67 protein, one of two major isoforms of glutamic acid decarboxylase (GAD). The effects of GABA elevation on GABA synthesis were assessed in vivo using 1H and 13C-edited NMR spectroscopy. Rates of turnover of cortical glutamate and GABA from intravenously administered [1-13C]glucose were measured in alpha-chloralose anesthetized rats 24 hours after receiving vigabatrin (500 mg/kg, i.p.) and in non-treated controls. GABA concentration was increased 2-fold at 24 hours (from 1.3 +/- 0.4 to 2.7 +/- 0.9 mumol/g) and GABA-T activity was inhibited by 60%. Tricarboxylic acid cycle flux was not affected by vigabatrin treatment compared to non-treated rats (0.47 +/- 0.19 versus 0.52 +/- 0.18 mumol/g, respectively). GABA-C2 fractional enrichment (FE) measured in acid extracts rose more slowly in vigabatrin-treated compared to non-treated rats, reaching > 90% of the glutamate FE after 3 hours. In contrast, GABA FE > or = glutamate FE in non-treated rats. A metabolic model consisting of a single glutamate pool failed to account for the rapid labeling of GABA from glutamate. Metabolic modelling analysis based on two (non-communicating) glutamate pools revealed a approximately 70% decrease in the rate of GABA synthesis following vigabatrin-treatment, from 0.14 (non-treated) to 0.04 mumol/g/min (vigabatrin-treated). These findings, in conjunction with the previously reported differential effects of elevated GABA on the GAD isoforms, suggests that GAD67 may account for a major fraction of cortical GABA synthesis in the alpha-chloralose anesthetized rat brain in vivo.

In vivo regulation of specific GABAA receptor subunit messenger RNAs by increased GABA concentrations in rat brain

This study has examined whether changes in endogenous GABA concentrations influence GABAA receptor subunit mRNA expression in vivo. Increased GABA concentrations were achieved by treating female rats with gamma-vinyl-GABA (15 mg/100 g), an irreversible inhibitor of the GABA transaminase, daily for three days. High performance liquid chromatography analysis of brain punches from specific brain regions showed that gamma-vinyl-GABA treatment resulted in approximately two-fold increases in brain GABA content. Using in situ hybridization techniques with specific 35S-labelled oligonucleotides, the mRNA expression of the alpha 1, alpha 2, beta 2, beta 3, gamma 1 and/or gamma 2 subunits of the GABAA receptor was quantified in various brain regions including the medial preoptic nucleus, bed nucleus of the stria terminalis, bed nucleus of the anterior commissure, supraoptic and paraventricular nuclei of the hypothalamus, globus pallidus and cingulate cortex. Silver grain density analysis showed that gamma-vinyl-GABA treatment induced a significant 35 and 49% decrease in gamma 1 mRNA expression in the medial preoptic nucleus and the principle encapsulated nucleus of the bed nucleus of the stria terminalis respectively, and a significant 20% decrease in alpha 2 mRNA expression in the cingulate cortex. Expression of alpha 2 and beta 3 in the former areas was unchanged as was alpha 1, beta 2, beta 3 and gamma 2 subunit expression in the cingulate cortex. Elevation of brain GABA levels also resulted in a specific and significant 17% increase in gamma 2 mRNA expression in the supraoptic nucleus. In the globus pallidus, gamma-vinyl-GABA treatment induced a significant 29% increase in alpha 1 mRNA expression combined with 19 and 30% decreases in beta 2 and gamma 2 mRNA expression, respectively. Levels of GABAA receptor subunits expressed in the bed nucleus of the anterior commissure (alpha 2, beta 3, gamma 1) and paraventricular nucleus (alpha 1, alpha 2, beta 2, gamma 2) were not changed by gamma-vinyl-GABA treatment. These results provide in vivo evidence for a region- and subunit-specific regulation of GABAA receptor subunit mRNA levels following the elevation of brain GABA concentrations and suggest that endogenous GABA levels influence GABAA receptor subunit mRNA expression.

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.

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).