Immunocytochemical localization of gamma-aminobutyric acid transaminase at cellular and ultrastructural levels

Localization of AMPA, kainate, and NMDA receptor mRNAs in the pigeon cerebellum

In the present study, we investigated the location of mRNAs for three types of ionotropic glutamate receptors (iGluRs) in the pigeon cerebellum and then compared the results with those of mammals. The following nine iGluRs subunits were analyzed by in situ hybridization: AMPA receptors (GluA1, GluA2, GluA3, and GluA4), kainate receptors (GluK1, GluK2, and GluK4), and NMDA receptors (GluN1 and GluN2A). Subunit hybridization revealed expression in different cell types of the cerebellar cortex: Purkinje cells expressed most subunits, including AMPA receptors (GluA1, GluA2, and GluA3), kainate receptors (GluK1 and GluK4), and NMDA receptors (GluN1); granule cells expressed four subunits of kainate (GluK1 and GluK2) and NMDA receptors (GluN1 and GluN2A); stellate and basket cells expressed GluK1, GluK2, and GluN1; Golgi cells expressed GluA1, GluA3, and GluN1; and Bergmann glial cells expressed only AMPA receptors (GluA2 and GluA4). Cerebellar nuclei showed no AMPA subunit signals, whereas kainate and NMDA receptors were observed in the five cerebellar nuclei divisions (CbL, CbMic, CbMim, CbMin, and CbMvm). The five divisions showed weak expression of GluK1, GluK2, and GluN2A; moderate to intense expression of GluK4; and intense expression of GluN1. These results demonstrate that in pigeons the cerebellar cortex expresses AMPA, kainate, and NMDA receptors, while the cerebellar nuclei express kainate and NMDA receptors. Taken together, these findings provide anatomical data for further analysis of the functions of iGluR-expressing neurons in glutamatergic circuits of the avian cerebellum.

Cannabinoid 1 receptor signaling on GABAergic neurons influences astrocytes in the ageing brain

Astrocytes, key regulators of brain homeostasis, interact with neighboring glial cells, neurons and the vasculature through complex processes involving different signaling pathways. It is not entirely clear how these interactions change in the ageing brain and which factors influence astrocyte ageing. Here, we investigate the role of endocannabinoid signaling, because it is an important modulator of neuron and astrocyte functions, as well as brain ageing. We demonstrate that mice with a specific deletion of CB1 receptors on GABAergic neurons (GABA-Cnr1-/- mice), which show a phenotype of accelerated brain ageing, affects age-related changes in the morphology of astrocytes in the hippocampus. Thus, GABA-Cnr1-/- mice showed a much more pronounced age-related and layer-specific increase in GFAP-positive areas in the hippocampus compared to wild-type animals. The number of astrocytes, in contrast, was similar between the two genotypes. Astrocytes in the hippocampus of old GABA-Cnr1-/- mice also showed a different morphology with enhanced GFAP-positive process branching and a less polarized intrahippocampal distribution. Furthermore, astrocytic TNFα levels were higher in GABA-Cnr1-/- mice, indicating that these morphological changes were accompanied by a more pro-inflammatory function. These findings demonstrate that the disruption of endocannabinoid signaling on GABAergic neurons is accompanied by functional changes in astrocyte activity, which are relevant to brain ageing.

Gamma-vinyl GABA inhibits cocaine-triggered reinstatement of drug-seeking behavior in rats by a non-dopaminergic mechanism

Relapse to drug use is a core feature of addiction. Previous studies demonstrate that gamma-vinyl GABA (GVG), an irreversible GABA transaminase inhibitor, attenuates the acute rewarding effects of cocaine and other addictive drugs. We here report that systemic administration of GVG (25-300 mg/kg) dose-dependently inhibits cocaine- or sucrose-induced reinstatement of reward-seeking behavior in rats. In vivo microdialysis data indicated that the same doses of GVG dose-dependently elevate extracellular GABA levels in the nucleus accumbens (NAc). However, GVG, when administered systemically or locally into the NAc, failed to inhibit either basal or cocaine-priming enhanced NAc dopamine in either naïve rats or cocaine extinction rats. These data suggest that: (1) GVG significantly inhibits cocaine- or sucrose-triggered reinstatement of reward-seeking behavior; and (2) a GABAergic-, but not dopaminergic-, dependent mechanism may underlie the antagonism by GVG of cocaine-triggered reinstatement of drug-seeking behavior, at least with respect to GVG's action on the NAc.

Acute changes in the neuronal expression of GABA and glutamate decarboxylase isoforms in the rat piriform cortex following status epilepticus

The piriform cortex (PC) is the largest region of the mammalian olfactory cortex with strong connections to other limbic structures, including the amygdala, hippocampus, and entorhinal cortex. In addition to its functional importance in the classification of olfactory stimuli, the PC has been implicated in the study of memory processing, spread of excitatory information, and the facilitation and propagation of seizures within the limbic system. Previous data from the kindling model of epilepsy indicated that alterations in GABAergic inhibition in the transition zone between the anterior and posterior PC, termed here central PC, are particularly involved in the processes underlying seizure propagation. In the present study we studied alterations in GABAergic neurons in different parts of the PC following seizures induced by kainate or pilocarpine in rats. GABA neurons were labeled either immunohistochemically for GABA or its synthesizing enzyme glutamate decarboxylase (GAD) or by in situ hybridization using antisense probes for GAD65 and GAD67 mRNAs. For comparison with the PC, labeled neurons were examined in the basolateral amygdala, substantia nigra pars reticulata, and the hippocampal formation. In the PC of controls, immunohistochemical labeling for GABA and GAD yielded consistently higher neuronal densities in most cell layers than labeling for GAD65 or GAD67 mRNAs, indicating a low basal activity of these neurons. Eight hours following kainate- or pilocarpine-induced seizures, severe neuronal damage was observed in the PC. Counting of GABA neurons in the PC demonstrated significant decreases in densities of neurons labeled for GABA or GAD proteins. However, a significantly increased density of neurons labeled for GAD65 and GAD67 mRNAs was determined in layer II of the central PC, indicating that a subpopulation of remaining neurons up-regulated the mRNAs for the GAD isoenzymes. One likely explanation for this finding is that remaining GABA neurons in layer II of the central PC maintain high levels of activity to control the increased excitability of the region. In line with previous studies, an up-regulation of GAD67 mRNA, but not GAD65 mRNA, was observed in dentate granule cells following seizures, whereas no indication of such up-regulation was determined for the other brain regions examined. The data substantiate the particular susceptibility of the central PC to seizure-induced plasticity and indicate that this brain region provides an interesting tool to study the regulation of GAD isoenzymes.

Age-related changes of γ-aminobutyric acid transaminase immunoreactivity in the hippocampus and dentate gyrus of the Mongolian gerbil

We investigated the age-related changes of gamma-aminobutyric acid transaminase (GABA-T, a GABA degradation enzyme) in the hippocampus and dentate gyrus of the gerbil at postnatal month 1 (PM 1), PM 3, PM 6, PM 12, and PM 24. Age-related changes of GABA-T immunoreactivity were distinct in the hippocampal CA1 region and in the dentate gyrus. GABA-T immunoreactivity was weak at PM 1, but at PM 3, it had increased significantly, and then increased further. Between PM 6 and PM 12, strong GABA-T immunoreactivity was found in nonpyramidal cells (GABAergic) in the stratum pyramidale of the CA1 region, and at PM 6, strong GABA-T immunoreactivity was found in neurons of the dentate gyrus subgranular zone. At PM 24, CA1 pyramidal cells showed strong GABA-T immunoreactivity. Western blot analysis showed a pattern of GABA-T expression similar to that shown by immunohistochemistry at various ages. In conclusion, our results suggest that the age-related changes of GABA-T provide important information about the aged brain with GABA dysfunction.

In vivo nuclear magnetic resonance studies of glutamate-gamma-aminobutyric acid-glutamine cycling in rodent and human cortex: the central role of glutamine

It has been recognized for many years that the metabolism of brain glutamate and gamma-aminobutyric acid (GABA), the major excitatory and inhibitory neurotransmitters, is linked to a substrate cycle between neurons and astrocytes involving glutamine. However, the quantitative significance of these fluxes in vivo was not known. Recent in vivo 13C and 15N NMR studies in rodents and 13C NMR in humans indicate that glutamine synthesis is substantial and that the total glutamate-GABA-glutamine cycling flux, necessary to replenish neurotransmitter glutamate and GABA, accounts for >80% of net glutamine synthesis. In studies of the rodent cortex, a linear relationship exists between the rate of glucose oxidation and total glutamate-GABA-glutamine cycling flux over a large range of cortical electrical activity. The molar stoichiometric relationship (approximately 1:1) found between these fluxes suggests that they share a common mechanism and that the glutamate-GABA-glutamine cycle is coupled to a major fraction of cortical glucose utilization. Thus, glutamine appears to play a central role in the normal functional energetics of the cerebral cortex.

Reciprocal communication systems between astrocytes and neurones

Over the past decade, a growing body of evidence has emerged on the existence in the brain of a close bidirectional communication system between neurones and astrocytes. This article reviews recent advances in understanding the rules governing these interactions and describes putative, novel functions attributable to astrocytes in neuronal transmission. Astrocytes can respond to the neurotransmitter released from active synaptic terminals, with cytosolic Ca(2+) oscillations whose frequency is under the dynamic control of neuronal activity. In response to these neuronal signals, astrocytes can signal back to neurones by releasing various neurone active compounds, such as the excitatory neurotransmitter glutamate. Interestingly, there is accumulating evidence that glutamate is released via a Ca(2+)-dependent mechanism which may share common properties with neurotransmitter exocytosis in neurones. This bidirectional communication system between neurones and astrocytes may lead to profound changes in neuronal excitability and synaptic transmission. While there clearly is an enormous amount of experimental and theoretical work yet to figure out, a coherent view is now emerging which incorporates the astrocyte, with the presynaptic terminal and the postsynaptic target neurone, as a possible third functional element of the synapse.

Localization of GABA transaminase immunoreactivity in the rat substantia nigra pars reticulata

Precise cellular localization of gamma-aminobutyric acid transaminase (GABA(T)), a degrading enzyme for the neurotransmitter GABA, was determined in the rat substantia nigra (SN) by immunocytochemical experiments using a recently developed monoclonal antibody. In order to characterize the GABA(T)-immunoreactive neurons, double immunocytochemistry was also performed using tyrosine hydroxylase (TH) as a neurochemical marker for dopaminergic neurons in the substantia nigra pars compacta (SNc). Immunoreactivity for GABA(T) was primarily localized in perikarya of the SN. There were only a few GABA(T)-immunoreactive neurons found to display TH immunoreactivity. Most of the GABA(T)-immunoreactive neurons were then identified as reticulata neurons. These results indicate that reticulata neurons are the major nigral neurons that express GABA(T) immunoreactivity and there may be functional compartmentalization of the GABA metabolism in the rat substantia nigra pars reticulata (SNr).

The gamma-hydroxybutyrate signalling system in brain: organization and functional implications

gamma-Hydroxybutyrate is a metabolite of GABA which is synthesized and accumulated by neurons in brain. This substance is present in micromolar quantities in all brain regions investigated as well as in several peripheral organs. Neuronal depolarization releases gamma-hydroxybutyrate into the extracellular space in a Ca(2+)-dependent manner. Gamma-hydroxybutyrate high-affinity receptors are present only in neurons, with a restricted specific distribution in the hippocampus, cortex and dopaminergic structures of rat brain (the striatum in general, olfactory bulbs and tubercles, frontal cortex, dopaminergic nuclei A9, A10 and A12). Stimulation of these receptors with low amounts of gamma-hydroxybutyrate induces in general hyperpolarizations in dopaminergic structures with a reduction of dopamine release. However, in the hippocampus and the frontal cortex, it seems that gamma-hydroxybutyrate induces depolarization with an accumulation of cGMP and an increase in inositol phosphate turnover. Some of the electrophysiological effects of GHB are blocked by NCS-382, a gamma-hydroxybutyrate receptor antagonist while some others are strongly attenuated by GABAB receptors antagonists. Gamma-hydroxybutyrate penetrates freely into the brain when administered intravenously or intraperitoneally. This is a unique situation for a molecule with signalling properties in the brain. Thus, the gamma-hydroxybutyrate concentration in brain easily can be increased more than 100 times. Under these conditions, gamma-hydroxybutyrate receptors are saturated and probably desensitized and down-regulated. It is unlikely that GABAB receptors could be stimulated directly by GHB. Most probably, GABA is released in part under the control of GHB receptors in specific pathways expressing GABAB receptors. Alternatively, GABAB receptors might be specifically stimulated by the GABA formed via the metabolism of gamma-hydroxybutyrate in brain. In animals and man, these GHBergic and GABAergic potentiations induce dopaminergic hyperactivity (which follows the first phase of dopaminergic terminal hyperpolarization), a strong sedation with anaesthesia and some EEG changes with epileptic spikes. It is presumed that, under pathological conditions (hepatic failure, alcoholic intoxication, succinic semialdehyde dehydrogenase defects), the rate of GHB synthesis or degradation in the peripheral organ is modified and induces increased GHB levels which could interfere with the normal brain mechanisms. This pathological status could benefit from treatments with gamma-hydroxybutyric and/or GABAB receptors antagonists. Nevertheless, the regulating properties of the endogenous gamma-hydroxybutyrate system on the dopaminergic pathways are a cause for the recent interest in synthetic ligands acting specifically at gamma-hydroxybutyrate receptors and devoid of any role as metabolic precursor of GABA in brain.

A rat brain cDNA encodes enzymatically active GABA transaminase and provides a molecular probe for GABA-catabolizing cells

cDNAs encoding gamma-aminobutyric acid aminotransferase (GABA-T) were isolated from a lambda ZAP rat hippocampal cDNA expression library by two independent cloning methods, immunological screening with an antimouse GABA-T antibody and plaque hybridization with a GABA-T cDNA probe derived by polymerase chain reaction. We have produced enzymatically active GABA-T from a rat brain cDNA containing the full-length GABA-T coding region. Our rat brain GABA-T cDNAs hybridize to mRNAs in brain and peripheral tissues, including liver, kidney, and testis. We have also detected GABA-T mRNA in GABAergic cells of rat cerebellar cortex by in situ hybridization. Our rat brain GABA-T probe hybridizes to Purkinje, basket, stellate, and Golgi II cells, the same GABAergic neurons previously shown to contain glutamate decarboxylase GAD65 and GAD67.

Comparative study between 4-aminobutyrate-2-oxoglutarate aminotransferase (GABA-T) from rat forebrain and cerebellum

In this study differences in the biochemical properties of 4-aminobutyric acid aminotransferase (GABA-T) from forebrain and cerebellum were detected. These differences may be related to: a) the characteristics of the catalytic site, b) the substrate affinities and c) their pyridoxal-phosphate requirements which suggests that PLP could be a physiological regulator of these forms of brain GABA-T.

Cerebellar nuclei and the nucleocortical projections in the rat: Retrograde tracing coupled to GABA and glutamate immunohistochemistry

The amino acids GABA and glutamate (Glu) are thought to be the principal substances in the central nervous system responsible for neuronal inhibition and excitation. Their distributions among the different neurons in a defined pathway may thus be indicative of the contributions of the cells to pathway function. Examples of such neurons are those of the cerebellar nuclei which, while regulating output from the Purkinje cells of the cerebellar cortex, are also found to project back to the cerebellar cortex. Immunohistochemical experiments were done to identify GABA and glutamate (Glu) containing cells in the adult rat cerebellar nuclei. Consecutive semithin and serial vibratome sections were incubated with antisera raised in rabbit against GABA and Glu. In semithin sections, only small neurons were intensely GABA immunoreactive (GABA-IR) (31.7%), and the majority (80.5%) were Glu immunoreactive (Glu-IR) of different sizes. Consistent with Glu being a metabolic precursor for GABA, 75.4% of the GABA-IR population colocalized Glu. In vibratome sections GABA-IR neurons showed some local differences in number, whereas the Glu-IR were uniformly distributed in the three nuclei studied. Measured mean diameters for these neurons showed a distinct size difference for the GABA- and Glu-IR with little overlap. Cerebellar nuclei neurons projecting to the cortex (nucleocortical neurons, NCN) were identified by locally preinjecting the retrograde transported WGA-apoHRP-colloidal gold complex in the cerebellar cortex. Vibratome sections of these cerebellar were silver intensified for the retrograde tracer and double labeled for GABA and Glu. Of the total number of identified NCN, 8.7% were GABA-IR (10 animals) and 47.7% Glu-IR (5 animals). Many retrograde labeled NCN in the core of the thick sections were immunonegative for both amino acids due to poor antibody penetration, thus underestimating the proportions of cells containing GABA and Glu. The size distributions for the GABA-IR and Glu-IR NCN were similar to those measured in non-retrograde labeled nuclei in thick sections. The conclusions reached are that GABA-IR neurons of the cerebellar nuclei, including the NCN, use GABA as the presumed inhibitory neurotransmitter and that Glu-IR neurons may use Glu or another excitatory neurotransmitter.

Immunocytochemical identification of GABA in astrocytes located in white matter after inhibition of GABA-transaminase with gamma-acetylenic GABA

Several lines of evidence suggest that astrocytes contribute to the uptake and degradation of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Recent immunohistochemical studies have shown that GABA-like immunoreactivity can be demonstrated in astrocytes in the grey matter of the rat's brainstem. The present study investigates whether GABA is also present in astrocytes located in the white matter. Adult rats were given gamma-acetylenic GABA (GAG), which inhibits the GABA-degratory enzyme GABA-alpha-ketoglutaric acid aminotransferase, and tissue sections from the cerebral cortex and brainstem were processed for GABA immunohistochemistry using an antiserum to GABA. Light microscopic examination of the sections showed numerous small GABA-immunoreactive cells in fibre tracts as well as in nuclear regions. Electron microscopic examination of the immunoreactive cells showed that they were fibrous astrocytes. The results provide evidence that the large increase in GABA in fibre tracts found in biochemical studies of rats injected with GAG is due to an increase in astrocytic GABA and suggest that fibrous astrocytes regulate GABA levels in the extracellular space of white matter.

GABAergic innervation in cerebral blood vessels: an immunohistochemical demonstration of L-glutamic acid decarboxylase and GABA transaminase

The presence of GABAergic innervation in cerebral arteries of several species was investigated by an immunohistochemical method using antibodies against glutamic acid decarboxylase (GAD) and GABA transaminase (GABA-T). Both GAD and GABA-T immunoreactivities were found to be associated with large bundles and single fibers in the adventitial layer of arteries examined. The density and distribution pattern of both GAD- and GABA-T-immunoreactive fibers were found to be comparable at most regions examined. Both fibers were found to be most dense in the anterior cerebral artery and its adjacent part of the circle of Willis. Several peripheral arteries were found to receive very sparse or no GAD- and GABA-T-immunoreactive fibers. Superior cervical ganglionectomy did not appreciably affect the distribution of both fibers. Cold-storage denervation, however, resulted in a drastic decrease in both fibers. At ultrastructural levels, both GAD- and GABA-T-immunoreactive nerve profiles were found to be very close to the smooth muscle cells. These results demonstrate the presence of a potentially functional GABAergic innervation in cerebral circulation. On few occasions, GAD immunoreactivities were also found in some endothelial cells, suggesting that a nonneuronal GABA system may also be present in cerebral arteries.

GABAergic neural elements in the rat basilar pons: electron microscopic immunochemistry

Previous light microscopic immunoperoxidase studies of glutamic acid decarboxylase (GAD)-immunoreactive neural elements in the rat basilar pontine nuclei revealed immunocytochemical reaction product in neuronal somata and axon terminals. In the present study, pre-embedding immunoperoxidase labeling of GAD or gamma-aminobutyric acid (GABA) and postembedding immunogold labeling of GABA allowed the ultrastructural visualization of these neural elements in the basilar pontine nuclei of colchicine-treated animals. At the electron microscopic level, immunolabeled neuronal somata exhibited smoothly contoured nuclei, whereas some dendrites also contained reaction product after immunocytochemical treatment and were postsynaptic to both immunoreactive and nonimmunoreactive axon terminals. Synaptic boutons immunoreactive for GAD or GABA exhibited cross-sectional areas that ranged from 0.1 to 3.8 microns 2 and generally appeared round or elongated in most sections. The majority (95%) of immunolabeled boutons contained pleomorphic synaptic vesicles and formed symmetric synapses at their postsynaptic loci; however, boutons exhibiting round vesicles and boutons forming asymmetric synapses (5%) were also immunopositive. Small (less than 1.5 microns 2) GAD- or GABA-labeled axon terminals formed synaptic contact mainly with small dendritic profiles, dendritic spines, and neuronal somata, whereas large labeled boutons (greater than 1.5 microns 2) formed synapses with all sizes of dendritic profiles. Occasionally, a single immunolabeled bouton formed synaptic contact with two separate postsynaptic dendrites. It is suggested that the immunolabeled neuronal somata and dendrites observed in the rat basilar pontine nuclei represent a population of pontine local circuit neurons; however, it is known that GABAergic cell groups extrinsic to the pontine gray provide afferent projections to the basilar pons, and therefore at least some immunoreactive axon terminals present in the pontine nuclei are derived from these extrinsic sources. The ultrastructural observation of GABAergic neural elements in the rat basilar pontine nuclei confirms previous light microscopic findings and provides an anatomical substrate through which GABAergic neurons, whether arising from an intrinsic or extrinsic source, might exert an inhibitory influence on target cells within the pontine nuclei.

Two types of GABA-containing axon terminals in cerebellar glomeruli of cat: an immunogold-EM study

Immunogold demonstration of GABA was used in ultrathin sections of cerebellar cortex of cat to identify GABA(+) profiles in cerebellar glomeruli. In addition to small, GABA-containing axonal varicosities found at the periphery of all glomeruli, a few complex glomeruli were found to contain a second type of GABA(+) terminal, with a mossy ending appearance. GABA(+) type "I" axon terminals, which were identified as the axonal arborization of local Golgi cells, contained ovoid, small synaptic vesicles. GABA(+) type "II" terminals, however, exhibited large, spheroid synaptic vesicles. Experimental evidence is provided that type "II" GABA(+) mossy like terminals are the endings of nucleocortical fibers.

Abundant GABAergic innervation of rat posterior pituitary revealed by inhibition of GABA-transaminase

An antibody against gamma-aminobutyric acid (GABA) was used to identify GABAergic elements immunocytochemically in the rat posterior pituitary. In order to increase the intracellular concentration of GABA, rats were treated with the GABA-transaminase inhibitor gamma-vinyl-GABA (GVG). Light-microscopic observations of Vibratome and semithin sections revealed the presence of numerous immunoreactive nerve fibers throughout the neural lobe; the mean number and length of these fibers increased by 90% after GVG treatment. Electron microscopy demonstrated the immunostained axons to be of small diameter. The reaction product was confined to small vesicles. No immunostaining occurred in pituicytes. The richness of the GABAergic innervation of the neural lobe contrasts with previous reports using antibodies against glutamate decarboxylase and supports the idea that GABA participates in the presynaptic control of neurosecretion.

Localization of L-glutamate decarboxylase immunoreactivity in the major pelvic ganglion and in the coeliac-superior mesenteric ganglion complex of the rat

The localization of L-glutamate decarboxylase (GAD), the GABA-synthesizing enzyme, was studied in the rat major pelvic ganglion and in the coeliac-superior mesenteric ganglion complex by indirect immunofluorescence technique with a specific antiserum raised in rabbits. GAD immunoreactivity was demonstrated in small cells of these ganglia. The GAD-immunoreactive small cells were 10-20 microns in diameter and formed clusters or occurred as solitary cells. The principal neurons were non-reactive but they were surrounded by immunoreactive processes. Studies on colocalization of GAD with tyrosine hydroxylase (TH), the rate-limiting enzyme of the catecholamine synthesis, in the major pelvic ganglion and in the coeliac-superior mesenteric ganglion complex indicated that all GAD-immunoreactive small cells were also labelled with TH. In the major pelvic ganglion all TH-immunoreactive SIF cells were also immunoreactive for GAD. However, in the coeliac-superior mesenteric ganglion complex there occurred TH-immunoreactive small cells which showed no immunoreactivity to GAD. It is suggested that the small GAD-immunoreactive cells represent small intensely fluorescent (SIF) cells.

GABA-immunoreactive neurons in the rat cerebellum: a light and electron microscope study

An antibody raised against gamma - amino-butyric acid (GABA) coupled to bovine serum albumin with glutaraldehyde (Hodgson et al: J. Histochem. Cytochem. 33:229-239, '85) was used to localise immunocytochemically the presumptive GABAergic neuronal elements in the cerebellar cortex of the adult rat. employing the unlabelled antibody enzyme method with pre- and post-embedding immunocytochemical procedures, the following cellular structures were observed to be GABA-immunopositive in both the light and electron microscopes: the somata, dendrites, and axonal processes (including axon terminals) of stellate, basket, and Golgi neurons. In immunopositive neuronal somata and dendrites, the reaction product was found to be associated with all intracellular organelles and with the postsynaptic densities of synaptic junctions. Specific GABA-like immunoreactivity was also seen around outer mitochondrial membranes, microtubules, and neurofilaments, and coating synaptic vesicles in presynaptic axon terminals. In the pre-embedding procedure with dilutions of the antiserum between 1:1,000 and 1:2,000, the perikarya and dendrites of Purkinje cells were GABA-immunonegative, whereas at an antiserum dilution of 1:500 the somata of Purkinje cells were mildly GABA-immunoreactive. Purkinje cell axon terminals in the infra- and supraganglionic plexuses and in the deep cerebellar nuclei were always strongly immunopositive. Neuroglia were invariably GABA-immunonegative, as were the dendrites, axons (parallel fibres), and somata of granule cells. Mossy fibre and climbing fibre afferents were also immunonegative. The pattern of immunoreactivity obtained with this antiserum directed against the inhibitory neurotransmitter GABA was found to resemble closely the immunocytochemical distribution of GABA and of the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD) as reported previously in other immunocytochemical investigations (Oertel et al. and Wu et al: Cytochemical Methods in Neuroanatomy. New York: A. R. Liss, '82; Seguela et al: Neuroscience 16:865-874, '85; Mugnaini and Oertel: GABA and Neuropeptides in the CNS. Handbook of Chemical Neuroanatomy, Vol. 4, Part I. Amsterdam: Elsevier, '85.

A monoclonal antibody to rabbit brain GABA transaminase

A monoclonal antibody of class IgG (subclass IgG1) has been prepared to rabbit brain GABA transaminase (GABA-T). This antibody reveals a single band of molecular weight 52,000 on a nitrocellulose filter blotted with purified GABA-T. On a filter blotted with unfractionated rabbit brain supernatant a major band of molecular weight 58,000 is revealed. An immunoaffinity column was prepared by coupling proteins from ascites fluid containing anti-rabbit GABA-T antibody to Bio-Rad Affi-Gel 15. This column bound purified GABA-T and extracted from unfractionated rabbit brain supernatant a protein of molecular weight 58,000, which was almost homogeneous and which had GABA-T enzyme activity. Using immunoaffinity chromatography, therefore, a high degree of purification of GABA-T may be achieved in a single step. Further, this technique may preserve an authentic form of the enzyme that is lost during the conventional purification procedure. The antibody inhibits GABA-T enzyme activity, up to a maximum of 35%.

Intrinsic GABAergic system of adrenal chromaffin cells

Histochemical and biochemical studies demonstrate that gamma-aminobutyric acid (GABA), glutamic acid decarboxylase (EC 4.1.1.15), and GABA aminotransferase (EC 2.6.1.19) are present in bovine adrenal chromaffin cells. Moreover, [3H]GABA can be taken up and stored by primary cultures of adrenal chromaffin cells. Nicotinic receptor stimulation or KCl depolarization releases the [3H]GABA taken up by these cell cultures. GABA and benzodiazepine recognition sites located in chromaffin cells interact with each other with modalities similar to those described for GABA and benzodiazepine recognition sites located in synaptic membranes prepared from brain tissue. Bicuculline facilitates the release of catecholamine from chromaffin cells induced by nicotinic receptor stimulation but it fails to influence the release of catecholamine evoked by K+ depolarization. Since the GABA-benzodiazepine receptor system appears to modulate nicotinic receptor function, it is suggested that GABA transmission might participate in modulating responsiveness of chromaffin cells to incoming cholinergic stimuli.

Distribution of GABA-T-intensive neurons in the rat forebrain and midbrain

The neuronal distribution of gamma-aminobutyric acid (GABA) transaminase (GABA-T), the enzyme which metabolizes GABA, has been mapped in rat brain. The method involves staining for newly synthesized GABA-T by the previously established nitro blue tetrazolium technique in animals killed 8-48 hours after administration of gabaculine, an irreversible inhibitor of GABA-T. Neuronal staining is obscured by staining of other elements if initial suppression is inadequate or survival times postgabaculine are too long. With appropriate conditions, GABA-T-positive neuronal somata can be widely detected. The stained cells include neuronal groups previously reported to be GABAergic on the basis of glutamate decarboxylase (GAD)-colchicine immunocytochemistry and other methods, i.e.: Purkinje, basket, Golgi, and stellate neurons of the cerebellum; basket and stellate neurons of the hippocampus; granule and periglomerular cells of the olfactory bulb; magnocellular neurons of the hypothalamus; and neurons of the striatum, pallidum, entopeduncular nucleus, cortex, medial septal area, diagonal band, substantia innominata, reticular nucleus of the thalamus, substantia nigra, and dorsal raphe. Other cells that stain intensely for GABA-T and may be GABAergic include neurons in the midlateral septal area, accumbens, the central medial and basal nuclei of the amygdala, zona incerta, the brainstem reticular formation, central gray, interstitial nucleus of Cajal, and various thalamic nuclei including the periventricular, intralaminar, rhomboid, and subparafascicular. Known non-GABA neuronal groups are negative for GABA-T staining under these conditions, reinforcing the hypothesis that GABA neurons are far more GABA-T intensive than other neurons.

Synthesizing enzymes for four neuroactive substances in motor neurons and neuromuscular junctions: light and electron microscopic immunocytochemistry

Immunocytochemical evidence is presented for the existence of choline acetyltransferase (ChoAcTase), cysteine sulfinic acid decarboxylase (CSADCase), tyrosine hydroxylase (TyrOHase), and glutamic acid decarboxylase (GluDCase) in large motor neurons of the hypoglossal nucleus and the spinal cord and in nerve terminals of motor end plates in tongue and skeletal muscle of five mammalian species, including man. These enzymes, which are responsible for the synthesis of acetylcholine (AcCho), taurine, dopamine, and gamma-aminobutyrate (GABA), respectively, were detected by immunocytochemical studies with monoclonal or polyclonal antibodies raised against the enzymes. Electron microscopy of the neuromuscular junctions showed that the immunoreactivity in each case was confined to the cytoplasmic matrix of presynaptic nerve terminals. Immunoreactivity obtained for each enzyme antibody varied with the species. It was highest in fresh, unfixed muscle and lowest in aldehyde-fixed specimens. Negative controls were obtained with preimmune sera and antisera preabsorbed with pure ChoAcTase, CSADCase, or GluDCase antigen. Double-labeling studies with ChoAcTase antibodies and acetylcholinesterase (AcChoEase) antibodies, AcChoEase enzyme activity, or alpha-bungarotoxin binding indicated that ChoAcTase, AcChoEase, and AcCho receptors were colocalized at the same end plates.

Taurine in the mammalian cerebellum: demonstration by autoradiography with [3H]taurine and immunocytochemistry with antibodies against the taurine-synthesizing enzyme, cysteine-sulfinic acid decarboxylase

Taurine neurons and their dendrites and axons were visualized in the mammalian cerebellum by autoradiography, after in vivo injections of [(3)H]taurine directly into the cerebellar cortex or deep cerebellar nuclei, and by immunocytochemistry at the light- and electron-microscope levels with antibodies against cysteine-sulfinic acid decarboxylase (CSADCase; L-cysteine-sulfinate carboxylyase, EC 4.1.1.29). Uptake and sequestration of [(3)H]taurine labeled numerous Purkinje cell somata, primary dendrites, and axons; many granule cell somata, dendrites, and parallel fibers; stellate, basket, and Golgi cells; the larger neurons in all deep cerebellar nuclei; the largest neurons in the lateral vestibular nucleus; and, more rarely, Purkinje cell axonal terminals in the neuropil. The label at all sites was diminished by preinjection into the cerebellum of hypotaurine, p-chloromercuriphenylsulfonic acid, or beta-alanine, and was virtually eliminated by strychnine. Immunocytochemical labeling with polyclonal antibodies directed against CSADCase, the enzyme responsible for the synthesis of hypotaurine from cysteine sulfinic acid and taurine from cysteic acid, had a similar distribution. In electron micrographs, immunoreactivity within Purkinje cell somata and dendrites was localized to the Golgi apparatus, the inner plasma membrane, and condensed nonmembranous foci (120 nm in diameter) marked by clumps of peroxidase reaction product. Large Nissl bodies were usually not CSADCase immunoreactive. Numerous immunoreactive granule cells, dendrites, and parallel fibers were recognized. Pretreatment of the animals with colchicine increased the intensity of CSADCase immunoreactivity but did not change the number or distribution of labeled cells. These experiments indicate that taurine is synthesized and involved in a specific uptake process by cerebellar neurons. Neuroglial cells do not synthesize taurine but some neuroglia take up [(3)H]taurine. These findings call for a reexamination of the physiological function of taurine in the cerebellum. A hypothesis is proposed that taurine may be involved in the regulation of calcium, in dendritic spike generation, and in the inhibition of impulse propagation in major Purkinje cell dendrites.

A regional study of 4-aminobutyrate metabolism and amino acid levels in rat brain following chronic oral administration of ethanolamine O-sulphate

Ethanolamine O-sulphate (EOS) dissolved in the drinking water (5 mg . ml(-1) was administered ad libitum to rats for 26 days. At the end of this period, glutamate decarboxylase (GAD) and GAA-transaminase (GABA-T) activities, 4-aminobutyrate (GABA) concentration, and the levels of six other amino acids were measured in various brain regions. Significant inhibition of GABA-T accompanied by significant increases in GABA content were observed throughout the brain, although the magnitudes of these effects varied according to region. GAD activity was significantly reduced in most brain regions, although this effect was apparently not related to cofactor availability or the direct actions of EOS or increased GABA concentration. Glutamine levels were significantly reduced to approximately 72% of control values in all brain regions. Aspartate levels were significantly reduced to approximately 84% of control values in all regions except the striatum and cerebellum. Minor changes in other amino acid levels were also detected. These neurochemical changes which accompanied the primary effect of EOS on GABA-T are discussed in terms of indirect secondary metabolic changes rather than nonspecific enzyme inhibition by EOS.

Chemical heterogeneity in cerebellar Purkinje cells: existence and coexistence of glutamic acid decarboxylase-like and motilin-like immunoreactivities

Purkinje neurons of the cerebellar cortex from a chemically and morphologically heterogeneous population containing some members that have gamma-aminobutyric acid (GABA), others that have immunoreactivity for motilin, and a small number that have both. The remaining 30-40% of all Purkinje cells have neither of these two neuroactive substances, leaving possibilities for other transmitter candidates. The evidence was compiled from double-staining immunocytochemical procedures performed on single sections of the cerebellum and brain stem in rat, mouse, and monkey. Two polyclonal antibodies were applied in succession, one directed against the midregion and COOH terminus of the 22-amino acid polypeptide motilin and the other against glutamic acid decarboxylase (glutamate decarboxylase; L-glutamate 1-carboxy-lyase, EC 4.1.1.15), the rate-limiting enzyme in the synthesis of the neurotransmitter GABA. The staining combinations employed the immunoperoxidase method, with different chromogens for distinguishing the motilin-like immunoreactivity from glutamic acid decarboxylase immunoreactivity by different colors, or the immunoperoxidase method for one antiserum and immunofluorescence for the other. The locations of both motilin and GABA cell types were mapped. The recognition of motilin in Purkinje cells calls for experimental definition of the role of this substance in the cerebellum and for reevaluation of the roles of Purkinje cells and of GABA in cerebellar function. The significant motilin representation in the flocculus, paraflocculus, and vermis suggests that it may be the Purkinje cell mediative chemical in the vestibular parts of the cerebellum. However, the presence of GABA as well in the same regions indicates that the chemical preference may be at least bimodal.

An ultrastructural immunocytochemical study of nerve-specific protein in rat cerebellum

The ultrastructural localization of the neuron-specific enolase (14-3-2 protein) has been investigated in the cerebellum of the adult rat using the indirect antibody immunohistochemical method. The protein was found exclusively in neurons: perikaryal cytoplasm, axons and dendrites were labelled while nuclei were not. Reaction product was found to be attached to intracytoplasmic membranes, the surface membranes of mitochondria and microtubules in addition to its dispersion as a flocculent material throughout the cytoplasm. All classes of cerebellar neurons were found to be labelled though large variations in the level of labelling between different types of neuron were noted. Purkinje cells appeared to have a much lower cytoplasmic concentration of this protein than other neurons.

Abnormalities of neurotransmitter enzymes in Huntington's chorea

The activities of L-glutamate decarboxylase (GAD), GABA-transaminase (GABA-T), choline acetyltransferase (CAT), and cysteic and cysteinesulfinic acids decarboxylase (CAD/CSAD) in putamen and frontal cortex in both Huntington's chorea and normal tissues were measured. The greatest difference between Huntington's and normal tissues occurred in putamen, in which the apparent CSAD activity was reduced by 85%, while no difference was observed in frontal cortex. GAD, CAD, and CAT activities were also reduced in putamen by 65%, 63%, and 42%, respectively (P less than 0.05). Slight reduction in the enzyme activities was also observed in frontal cortex. However, these reductions appeared to be statistically insignificant (P greater than 0.05 in all cases). GABA-T showed little difference in both putamen and frontal cortex in Huntington's chorea and normal tissues. GAD and GABA-T from Huntington's tissues were indistinguishable from those obtained from normal tissues by double diffusion test and by microcomplement fixation test, which is capable of distinguishing proteins with a single amino acid substitution. Furthermore, the similarity of the complement fixation curves for GAD from Huntington's and normal tissues suggests that the decrease in GAD activity is probably due to the reduction in the number of GAD molecules, presumably through the loss of neurons, and not due to the inhibition or inactivation of GAD activity by toxic substances which might be present in Huntington's chorea.

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

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