Regional and synaptosomal levels of amino acid neurotransmitters in the 3-acetylpyridine deafferentated rat cerebellum

3-acetylpyridine induced behavioral dysfunction and neuronal loss in the striatum and hippocampus of adult male rats

3-acetylpyridine (3-AP) is a neurotoxin that is known to mainly affect the inferior olivary nucleus (ION) in the brain stem. Although several studies have explored the effect of this neurotoxin, still further investigation is required to understand the impact of this toxin on different parts of the brain. In this research, two groups of rats were studied, the 3-AP-treated and the control groups. Behavioral, stereological, and immunohistochemical analyses were performed. The locomotor activity of the 3-AP-treated rats decreased whereas their anxiety levels were higher than in normal controls. Also, memory performance was impaired in animals in the 3-AP group. Microscopic observations showed a decline in the numerical density of neurons in the hippocampus and striatum along with gliosis. Although this toxin is used to affect the ION, it exerts a neurotoxic effect on different brain regions.

Differential distribution of vesicular glutamate transporters in the rat cerebellar cortex

The chemical organization of excitatory axon terminals in the rat cerebellar cortex was examined by immunocytochemistry and in situ hybridization histochemistry of vesicular glutamate transporters 1 and 2 (VGluT1 and VGluT2). Chemical depletion of the inferior olivary complex neurons by 3-acetylpyridine treatment almost completely removed VGluT2 immunoreactivity from the molecular layer, leaving VGluT1 immunoreactivity apparently intact. On the other hand, neuronal deprivation of the cerebellar cortex by kainic acid injection induced a large loss of VGluT1 immunoreactivity in the molecular layer. In the cerebellar granular layer, both VGluT1 and VGluT2 immunoreactivities were found in mossy fiber terminals, and the two immunoreactivities were mostly colocalized in single-axon terminals. Signals for mRNA encoding VGluT2 were found in the inferior olivary complex, and those for VGluT1 and VGluT2 mRNAs were observed in most brainstem precerebellar nuclei sending mossy fibers, such as the pontine, pontine tegmental reticular, lateral reticular and external cuneate nuclei. These results indicate that climbing and parallel fibers selectively use VGluT2 and VGluT1, respectively, whereas mossy fibers apply both VGluT1 and VGluT2 together to accumulate glutamate into synaptic vesicles. Since climbing-fiber and parallel-fiber terminals are known to make depressing and facilitating synapses, respectively, VGluT1 and VGluT2 might have distinct properties associated with those synaptic characteristics. Thus, it would be the next interesting issue to determine whether mossy-fiber terminals co-expressing VGluT1 and VGluT2 show synaptic facilitation or depression.

Effect of Climbing Fibre Deprivation on the K+-evoked Release of Endogenous Adenosine from Rat Cerebellar Slices

We report the identification of a compound whose K+-induced Ca2+-dependent release in rat cerebellar slices was reduced following climbing fibre deprivation by 3-acetylpyridine (3-AP) treatment. Based on HPLC retention time, UV absorption spectrum, and mass spectrometry, this compound was identified as adenosine. The K+-induced, Ca2+-dependent release of adenosine was subsequently quantified in control and 3-AP-treated rats. It decreased by 60 - 70% in both the cerebellar vermis and hemispheres following climbing fibre deprivation, while 3-AP treatment had no effect on adenosine release in the cerebral cortex. Inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP and GMP decreased basal and stimulated efflux of adenosine in the cerebellum by 50 - 60%, indicating that a significant proportion of adenosine was derived from the extracellular metabolism of released nucleotides. Taken with the reports of other groups on adenosine in cerebellum, these results suggest that climbing fibre activity increases the extracellular level of adenosine, probably through the metabolism of released nucleotides. This adenosine could then cause presynaptic inhibition of the release of the parallel fibre transmitter, which is presumably glutamate. This may account for the climbing fibre-evoked depression of Purkinje cell sensitivity to parallel fibre input.

GABA-ergic transmission in deep cerebellar nuclei

gamma-Aminobutyric acid (GABA) is the inhibitory transmitter released at Purkinje cell axon terminals in deep cerebellar nuclei (DCN). Neurons in DCN also receive excitatory glutamatergic inputs from the inferior olive. The output of DCN neurons, which depends on the balance between excitation and inhibition on these cells, is involved in cerebellar control of motor coordination. Plasticity of synaptic transmission observed in other areas of the mammalian central nervous system (CNS) has received wide attention. If GABA-ergic and/or glutamatergic synapses in DCN also undergo plasticity, it would have major implications for cerebellar function. In this review, literature evidence for GABA-ergic synaptic transmission in DCN as well as its plasticity are discussed. Studies indicate that fast inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) in neurons of DCN are mediated by GABAA receptors. While GABAB receptors are present in DCN, they do not appear to be activated by Purkinje cell axons. The IPSPs undergo paired-pulse, as well as frequency-dependent, depressions. In addition, tetanic stimulation of inputs can induce a long-term depression (LTD) of the IPSPs and IPSCs. Excitatory synapses do not appear to undergo long-term potentiation or LTD. The LTD of the IPSP is not input-specific, as it can be induced heterosynaptically and is associated with a reduced response of DCN neurons to a GABAA receptor agonist. Postsynaptic Ca2+ and protein phosphatases appear to contribute to the LTD. The N-methyl-D-aspartate receptor-gated, as well as the voltage-gated Ca2+ channels are proposed to be sources of the Ca2+. It is suggested that LTD of GABA-ergic transmission, by regulating DCN output, can modulate cerebellar function.

Glutamate-immunoreactive climbing fibres in the cerebellar cortex of the rat

The climbing fibre system, one of the two main excitatory inputs to the cerebellar cortex, is anatomically and physiologically well characterized, while the nature of its neurotransmitter is still a matter of debate. We wished to determine whether glutamate-immunoreactive profiles with the morphological characteristics of climbing fibres could be found in the rat cerebellar cortex. For this purpose, a monoclonal 'anti-glutamate' antibody has been used in combination with a sensitive postembedding immunoperoxidase method on semi-thin sections or in combination with a postembedding immunogold method on ultrathin sections. At the light microscopic level, climbing fibres appeared as strongly stained fibrous profiles, chains of interconnected varicosities or heavily labelled dots of various sizes, often in close apposition to principal Purkinje cell dendrites. At the electron microscopic level, certain labelled varicosities or more elongated profiles resembling climbing fibre terminals were in synaptic contact with dendritic spines of Purkinje cells. Quantitative analysis of gold particle densities showed that such elements were about three to four times more heavily labelled than their postsynaptic partners. The results obtained in this study demonstrate that at least a subset of climbing fibres and their terminals contain relatively high levels of glutamate-like immunoreactivity and provide additional evidence for a role of glutamate as transmitter in these cerebellar afferents.

Functional adenosine A1 receptors in goldfish brain: regional distribution and inhibition of K(+)-evoked glutamate release from cerebellar slices

In goldfish brain, [3H]cyclohexyladenosine binding sites are ubiquitously distributed with a maximum in the hypothalamus and a minimum in the spinal cord. The binding parameters measured in cerebellar membranes (Kd = 0.88 +/- 0.08 nM; Bmax = 59.65 +/- 2.62 fmol/mg protein) are not significantly different from those of the whole brain. In perfused goldfish cerebellar slices, stimulation of cyclic AMP accumulation by 10(-5) M forskolin was markedly reduced (58.7%) by treatment with 10(-4) M cyclohexyladenosine, an adenosine A1 receptor agonist, and the reduction was reversed in the presence of 10(-4) M 8-cyclopentyltheophylline, a selective A1 receptor antagonist. In the same brain preparation, 30 mM K+ stimulated the release of glutamate, glutamine, glycine and GABA in a Ca(2+)-dependent manner, whereas the aspartate and taurine release was Ca(2+)-independent. Cyclohexyladenosine inhibited the 30 mM K(+)-evoked release of glutamate in a dose-related manner. This effect was reversed by 8-cyclopentyltheophylline. These results support the hypothesis that adenosine A1 receptors present in goldfish cerebellum are involved in the modulation of glutamate transmitter release.

Immunocytochemical localization of the amino acid neurotransmitters in the chicken retina

Postembedding immunocytochemistry was used to determine the cellular localization of the amino acid neurotransmitters glutamate, aspartate, gamma-aminobutyric acid (GABA), and glycine in the avian retina. The through retinal pathway was glutamatergic, with all photoreceptors, bipolar cells, and ganglion cells being immunoreactive for glutamate. Bipolar cells displayed the highest level of glutamate immunoreactivity, with the cell bodies terminating just below the middle of the inner nuclear layer. All lateral elements, horizontal cells, amacrine cells, and interplexiform cells were immunoreactive for glycine or GABA. The GABAergic neurons consisted of two classes of horizontal cells and amacrine cells located in the lower part of the inner nuclear layer. GABA was also localized in displaced amacrine cells in the ganglion cell layer, and a population of ganglion cells that co-localize glutamate and GABA. Both the horizontal cells and GABAergic amacrine cells had high levels of glutamate immunoreactivity, which probably reflects a metabolic pool. At least two types of horizontal cells in the avian retina could be discriminated on the basis of the presence of aspartate immunoreactivity in the H2 horizontal cells. Glycine was contained in a subclass of amacrine cells, with their cell bodies located between the bipolar cells and GABAergic amacrine cells, two subclasses of bipolar cells, displaced amacrine cells in the ganglion cell layer, and ganglion cells that colocalize glutamate and glycine. Glycinergic amacrine cells had low levels of glutamate. We have also identified a new class of glycinergic interplexiform cell, with its stellate cell body located in the middle of the inner nuclear layer among the cell bodies of bipolar cells. Neurochemical signatures obtained by analyzing data from serial sections allowed the classification of subclasses of horizontal cells, bipolar cells, amacrine cells, and ganglion cells.

Glutamate-like immunoreactivity in chick cerebellum and optic tectum

Glutamate was coupled via glutaraldehyde to bovine serum albumin. The conjugate was used for raising specific anti-glutamate antibodies. The purified antibody was used for immunostaining of chick cerebellum and optic tectum. Staining was intense in the molecular layer and in cell bodies of the granule cell layer. In the optic tectum a diffuse staining was detected in the superficial layers of stratum griseum fibrosum superficiale and in cell bodies especially in the layers a and e. Large cell bodies located in the stratum griseum centrale were also stained.

GABA levels and GAD immunoreactivity in the deep cerebellar nuclei of rats with altered olivo-cerebellar function

Immunocytochemistry was used to examine the distribution, size, and density of glutamic acid decarboxylase immunoreactive (GAD+) puncta in two animal models with movement disorders, the genetically dystonic (dt) rat and rats with 3-acetylpyridine (3AP) lesions of the inferior olive. In both models, GAD activity is increased in the deep cerebellar nuclei (DCN) where the enzyme is localized primarily in the terminals of Purkinje cells. GABA levels were also measured in the DCN. The general distribution of GAD+ puncta in the DCN was similar in all groups. Immediately after the 3AP lesions, however, GABA levels were elevated in 3AP rats in comparison with both normal rats and age-matched dt rats. GAD+ puncta were also larger than normal in the 3AP group at this time, although the magnitude of this effect declined over a 2-week recovery period. Puncta density was decreased in the medial nucleus only in 25-day-old dt rats in comparisons with normal littermates. These findings are discussed in the context of previously reported differences in the firing rate of Purkinje cells in dt and 3AP-treated rats.

Excitatory and inhibitory amino acid neurotransmitter binding sites in the cerebellar cortex of the pigeon (Columba livia)

We used receptor autoradiography to determine the distribution of excitatory and inhibitory amino acid neurotransmitter binding sites in the cerebellar cortex of the pigeon (Columba livia). alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid, kainate and metabotropic binding sites had highest levels in the molecular layer. N-methyl-D-aspartate binding sites, assayed with both [3H]glutamate under selective conditions and with [3H]glycine binding to the associated strychnine-insensitive glycine site, had highest levels in the granule cell layer. There was little specific binding of the non-competitive N-methyl-D-aspartate antagonist, [3H]MK-801. The level of gamma-aminobutyric acid (GABA)-A binding sites was higher than GABA-B binding sites in both molecular and granule cell layers with the highest level of GABA-A sites in the granule cell layer. The highest level of GABA-B binding sites was in the molecular layer. [3H]Flunitrazepam binding levels were approximately the same in both molecular and granule cell layers. With the exception of kainate binding sites, the distribution of binding sites was identical to that seen in the cerebellar cortex of mammals. Our results support the concept that the chemoarchitecture of the cerebellar cortex has been conserved in the course of vertebrate evolution.

Receptor sub-types involved in responses of Purkinje cell to exogenous excitatory amino acids and local electrical stimulation in cerebellar slices in the rat

The effects of the NMDA receptor antagonist, 2-amino-5-phosphonovalerate (APV) and non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) on responses of Purkinje cells to exogenous excitatory amino acids and to electrical stimulation of the parallel fibres, were investigated in slices of the cerebellum of the rat. Glutamate, aspartate, kainate and quisqualate all induced excitation of Purkinje cells. Responses to kainate and quisqualate were blocked by CNQX (10 microM) but not by APV (10 microM). N-Methyl-D-aspartate induced biphasic excitatory-inhibitory responses, both components of which were blocked by APV but not by CNQX. The inhibitory component was less sensitive to blockade by APV but was totally blocked by bicuculline, the GABAA receptor antagonist. Parallel fibre stimulation most commonly induced inhibition of Purkinje cells, with or without preceding excitation. This inhibition was blocked by APV and excitatory responses were often revealed. A less commonly-observed predominantly excitatory response was blocked by CNQX but not by APV and inhibition tended to be revealed. These data suggest that parallel fibre-Purkinje cell synapses possess non-NMDA postsynaptic receptors, while the parallel fibre-inhibitory interneuron synapses possess functional NMDA receptors.

Excitatory and inhibitory amino acid binding sites in human dentate nucleus

Autoradiography of excitatory and inhibitory amino acid binding sites in human dentate nuclei indicated virtually no binding to N-methyl-D-aspartate (NMDA) or gamma-aminobutyric acidB (GABAB) binding sites, and a low density of kainate binding sites. alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid, metabotropic-quisqualate, benzodiazepine, and gamma-aminobutyric acidA (GABAA) binding sites were present in moderate abundance. Our NMDA results differ from those found previously in rodents. GABAA receptors are probably the primary mediators of inhibitory neurotransmission and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and metabotropic-quisqualate receptors are probably the primary mediators of excitatory neurotransmission within the human deep cerebellar nuclei.

Effect of 3-acetylpyridine on serotonin uptake and release from rat cerebellar slices

The cerebellum receives indolaminergic fibers influencing Purkinje cell discharges. Data from our laboratories have demonstrated an endogenous release of serotonin (5-HT) and a Na(+)-dependent uptake and Ca(2+)-dependent release of [3H]5-HT from slices, homogenates and synaptosomal fractions of the rat cerebellar molecular layer. While the neurotransmitter produced by climbing fibers has been sought for in several studies and some of the classical transmitters have been ruled out, as yet this neurotransmitter is unknown. The aim of this work was to measure the 5-HT uptake and release from rat cerebellar slices, 6 h and 15 days after intraperitoneal injection of 3-acetylpyridine (3-AP) (75 mg/kg), harmaline (15 mg/kg) and nicotinamide (300 mg/kg). A histological study of medulla and cerebellar cortex in these animals showed destruction of neurons in the inferior olivary nuclei and changes in the granulation of the cortical molecular layer in the cerebellum. A significant reduction of the 5-HT content (100%), 5-HT uptake (60%) and its Vmax (60%) was seen on the 5th day, in cerebellar preparations obtained from rats injected with 3-AP. The Ca(2+)-dependent release of 5-HT from these preparations was found to be similar to the basal values, in spite of depolarizing stimuli with 53 mM KCl or veratrine (60 micrograms/ml). The results suggest that 5-HT could play an important role as neurotransmitter produced by some climbing fibers.

Quantitative autoradiographic study of L-glutamate binding sites in normal and atrophic human cerebellum

In the present work the distribution of L-glutamate binding sites in the different layers of human cerebellum of normal individuals and of seven patients who died with olivopontocerebellar atrophy (OPCA) was examined with the technique of quantitative autoradiography. Specific L-[3H]glutamate binding was higher in the molecular than in the granule cell layer of normal cerebellar tissue. A significant decrease of L-[3H]glutamate specific binding was observed in the molecular layer of all OPCA tissues. In the granule cell layer L-[3H]glutamate binding was decreased only in two patients who suffered from late-onset sporadic OPCA and in one patient who suffered from a form of OPCA inherited in a dominant manner. Quisqualate-sensitive binding sites were the most abundant binding sites in the molecular layer of normal cerebella, whereas N-methyl-D-aspartic acid (NMDA)-sensitive binding sites were the most abundant type in the granule cell layer. A significant decrease of quisqualate-sensitive and an increase in NMDA-sensitive binding sites were observed in the molecular layer of OPCA cerebellar tissues. No significant changes were observed in the granule cell layer of these tissues.

Decreased sensitivity of cerebellar nuclei neurons to GABA and taurine: effects of long-term inferior olive destruction in the rat

The effects of iontophoretically applying the presumed Purkinje cell inhibitory neurotransmitters, GABA and taurine, were tested on neurons of the cerebellar nuclei in normal and in climbing-fiber-deafferented cerebella. Rats treated with 3-acetylpyridine to totally destroy the inferior olive were used for acute experiments 105-185 days after treatment. In controls, nearly all neuronal firing was dose-dependently depressed by both inhibitory amino acids. The depression in firing for both were antagonized by bicuculline and picrotoxin but not by strychnine while TAG specifically antagonized only responses to taurine. At sufficient doses, bicuculline and TAG induced disinhibitory responses (significant release of neuron discharge) in the absence of applied antagonist. In deafferented animals, the inhibitory efficacy of GABA and taurine were drastically reduced; most of the neurons failed to respond to these amino acids at the same iontophoretic parameters as for the control rats. Moreover, high doses of bicuculline and TAG did not induce any disinhibitory response (no significant increase in discharge rate) in most of the neurons tested. These results clearly demonstrate that climbing fiber deafferentation reduces postsynaptic sensitivity of the cerebellar nuclei neurons for the presumed Purkinje cell inhibitory neurotransmitters.

Cerebellar excitatory amino acid binding sites in normal, granuloprival, and Purkinje cell-deficient mice

Using quantitative autoradiography, the cellular localization and characterization of cerebellar excitatory amino acid binding sites in normal, Purkinje cell-deficient and granuloprival (granule cell-deficient) mouse cerebella were investigated. In the molecular layer of normal mouse cerebellum, the quisqualate subtype of excitatory amino acid receptor (assayed by [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, quisqualate-sensitive L-[3H]glutamate, and [3H]6-cyano-7-nitroquinoxaline-2,3-dione binding) predominated. In the granule cell layer of the cerebellum, N-methyl-D-aspartate-sensitive L-[3H]glutamate and [3H]glycine binding sites were predominant. In the molecular layer of Purkinje cell-deficient mutant mice, [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding sites and [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding were reduced to 24% (P less than 0.01) and 36% (P less than 0.001) of control, respectively, while quisqualate-sensitive [3H]glutamate binding sites were reduced to 54% of control (P less than 0.01). N-Methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were unchanged. In the granule cell layer of these mouse cerebella, there was no change in excitatory amino acid receptor binding. In the molecular layer of granuloprival mouse cerebella, [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding was increased to 205% of control (P less than 0.01), [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding was increased to 136% of control (P less than 0.02), and quisqualate-sensitive [3H]glutamate binding was increased to 152% of control (P less than 0.01). N-Methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were unchanged. In areas of granule cell depletion N-methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were reduced to 68% (P less than 0.01) and 59% (P less than 0.01) of control, respectively. In the granule cell layer, binding to quisqualate receptors was not significantly different from binding in controls with any of the ligands tested. These results suggest that three different receptor assays: [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, quisqualate-sensitive L-[3H]glutamate, and [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding can be used to demonstrate that quisqualate receptor specific binding sites are located on Purkinje cell dendrites in the molecular layer of cerebellum, and that these binding sites apparently up-regulate in response to granule cell ablation and Purkinje cell deafferentation.

Effects of normothermic versus mild hyperthermic forebrain ischemia in rats

We compared the neuropathological consequences of global forebrain ischemia under normothermia versus mild hyperthermia. Twenty-one rats underwent 20 minutes of four-vessel occlusion during which brain temperature was maintained at either 37 degrees C (normothermia, n = 9) or 39 degrees C (hyperthermia, n = 12). Quantitative neuropathological assessment was conducted 1 or 3 days later. At 1 day following the ischemic insult, normothermic rats demonstrated neuronal injury mainly confined to the most dorsolateral striatum. By 3 days, ischemic cells were present throughout the striatum and CA1 hippocampus in normothermic animals. Compared with normothermic rats, intraischemic hyperthermia significantly increased the extent and severity of brain damage at 1 day after the ischemic insult. Areas of severe neuronal necrosis and frank infarction included the cerebral cortex, CA1 hippocampus, striatum, and thalamus. Morphologic damage was also detected in the cerebellum and pars reticulata of the substantia nigra. An overall mortality rate of 83% was demonstrated at 3 days in the hyperthermic ischemic group. We conclude that intraischemic hyperthermia 1) markedly augments ischemic brain damage and mortality compared with normothermia, 2) transforms ischemic cell injury into frank infarction, and 3) accelerates the morphological appearance of ischemic brain injury in regions usually demonstrating delayed neuronal necrosis. These observations on mild hyperthermia may have important implications for patients undergoing cardiac or cerebrovascular surgery as well as patients following cardiac arrest or those with stroke-in-evolution.

Autoradiographic localization of inhibitory and excitatory amino acid neurotransmitter receptors in human normal and olivopontocerebellar atrophy cerebellar cortex

We used standard techniques of receptor autoradiography to study the distribution of inhibitory and excitatory amino acid neurotransmitter receptors in human normal cerebellar cortex. Benzodiazepine (BDZ) receptor density was relatively high in both granule cell and molecular layers. GABAA receptor density was highest in granule cell layer with lower receptor density in molecular layer. There was a lower density of GABAB receptors than GABAA receptors in both molecular and granule cell layers with a relatively higher density of GABAB receptors in molecular layer than in granule cell layer. In granule cell layer, the density of the N-methyl-D-aspartate (NMDA) subtype of excitatory amino acid receptors was greatest whereas in molecular layer the quisqualate (QA) receptor subtype density was greatest. With [3H]N-(1-[2-thienyl]cyclohexyl)3-4-piperidine as a ligand, there was no specific binding to the phencyclidine receptor. Molecular layer was also characterized by relatively high density of a non-NMDA/non-QA displaceable glutamate binding site. We studied also the cerebellar cortex of 4 cases of olivopontocerebellar atrophy (OPCA), a syndrome in which Purkinje and granule cells degenerate. In these specimens, there was significant decrement of BDZ and GABAA receptors in both molecular and granule cell layers, with loss of GABAB receptors in molecular layer. NMDA receptors were depleted in granule cell layer while QA receptors and the non-NMDA/non-QA glutamate binding site were significantly depleted in molecular layer. Our normal human and OPCA data are largely consistent with animal data about the cellular localization of cerebellar cortical amino acid neurotransmitter receptors.

Immunolocalization of cytosolic aspartate aminotransferase (cAAT) in axon terminals that form synapses in the rat cerebellar cortex. A study at the electron microscopic level

This study was conducted to determine the ultrastructural localization of cytosolic aspartate aminotransferase (cAAT)-like immunoreactivity in the cerebellar cortex in the rat. The isoenzyme was found both in excitatory and inhibitory axon terminals, but not in the climbing fibers of the molecular layer. These findings suggest that cAAT may have a different role in the excitatory and inhibitory synapses, and that climbing fibers of the molecular layer do not appear to use aspartate as neurotransmitter.

Effect of climbing fiber deprivation on release of endogenous aspartate, glutamate, and homocysteate in slices of rat cerebellar hemispheres and vermis

Aspartate (Asp) and/or glutamate (Glu) have been proposed as putative excitatory transmitters released from synaptic terminals of the olivo-cerebellar climbing fiber afferents to the Purkinje cells. Investigations of the climbing fiber transmitter(s) separately for hemispheres and vermis were performed to examine whether the current controversy over the role of Asp as a neurotransmitter in the climbing fibers may be due to topographic differences. K(+)-induced Ca2(+)-dependent release of endogenous substances was investigated in slices of cerebellar hemisphere and vermis of control rats and those deprived of climbing fibers by 3-acetylpyridine (3-AP) treatment. A release of Asp and Glu, as well as a small but significant release of homocysteic acid (HCA) was confirmed in control rats. Climbing fiber deprivation by 3-AP treatment reduced the stimulated release of Asp by 48% in slices of cerebellar hemispheres, but not in vermis. Climbing fiber deprivation completely abolished the release of HCA in both hemispheres and vermis. The release of HCA, Asp, and Glu from slices of control and climbing fiber-deprived rats evoked by 50 mM K+ was greater than 90% Ca2(+)-dependent. These results support the hypothesis that Asp is a transmitter candidate of the climbing fibers projecting to the cerebellar hemispheres, but not to the vermis, and provide the first evidence that HCA can be linked to a specific pathway.

Quantitative analysis of gamma-aminobutyric acid (GABA) receptors of Purkinje cell layer from rat cerebellar slices

1. Extracellular recordings were made from Purkinje cell layer of rat cerebellar slices. Cell activity was sensitive to both magnesium and manganese ions. 2. Glutamic and aspartic acids both excited cell activity while gamma-aminobutyric acid (GABA), muscimol, taurine, beta-alanine and delta-aminolaevulinic acid all inhibited activity. 3. The sensitivity to GABA varied with depth while no such effect was observed with muscimol. 4. Bicuculline methiodide, picrotoxin and pitrazepin blocked the action of muscimol with pA2 values of 5.92, 5.97 and 5.71 respectively. 5. The benzodiazepines flurazepam and RU 32007 both potentiated the GABA inhibition and this potentiation was blocked by Ro 15-1788, a benzodiazepine antagonist.

Neuromodulatory effects of corticotropin releasing factor on cerebellar Purkinje cells: an in vivo study in the cat

Corticotropin releasing factor, a 41 amino acid peptide, has been localized in climbing fibers and mossy fibers in the cat's cerebellar cortex. In the present study, corticotropin releasing factor was iontophoretically applied to Purkinje cells, isolated extracellularly, to assess the effect of this peptide on the firing rate of the neuron. By itself corticotropin releasing factor had little or no effect on cellular activity. However, this peptide potentiated the excitatory effects of aspartate and glutamate, the putative neurotransmitters of the climbing fiber and mossy fiber-parallel fiber systems, respectively. In addition, corticotropin releasing factor blocked the suppressive effects induced by the iontophoretic application of GABA. Finally, it shortened or eliminated the period of suppression produced by activation of climbing fibers in the cerebellar cortex. These data suggest that corticotropin releasing factor functions as a neuromodulator rather than as a neurotransmitter in cerebellar circuitry.

The effects of two anticonvulsants on amino acid levels in the developing rat cerebellum

Two anticonvulsants were administered pre- and postnatally to determine their effects on putative amino acid neurotransmitter levels in the rat cerebellum. The amino acids were quantitated using precolumn fluorescence derivatization and reverse-phase high performance liquid chromatography at various postnatal intervals. Treatment with clonazepam produced an initial depression in levels of most of the amino acids analyised. By three weeks postnatal all the amino acids, with the exception of GABA, had returned to control levels. GABA levels were still depressed five weeks after the cessation of treatment. Phenobarbital treatment produced an initial elevation in the level of GABA. At three weeks postnatal, both GABA and glutamate levels were elevated and remained so at eight weeks postnatal. In conclusion, the data demonstrated that each anticonvulsant produced unique, acute and chronic alterations in the levels of the cerebellar amino acids.

Glutaminase-like immunoreactivity in the lower brainstem and cerebellum of the adult rat

Distribution of putative glutamatergic neurons in the lower brainstem and cerebellum of the rat was examined immunocytochemically by using a monoclonal antibody against phosphate-activated glutaminase, which has been proposed to be a major synthetic enzyme of transmitter glutamate and so may serve as a marker for glutamatergic neurons in the central nervous system. Intensely-immunolabeled neuronal cell bodies were densely distributed in the main precerebellar nuclei sending mossy fibers to the cerebellum; in the pontine nuclei, pontine tegmental reticular nucleus of Bechterew, external cuneate nucleus, and lateral reticular nucleus of the medulla oblongata. Phosphate-activated glutaminase-immunoreactive granular deposits were densely seen in the brachium pontis and restiform body, suggesting the immunolabeling of mossy fibers of passage. In the cerebellum, neuropil within the granule cell layer of the cerebellar cortex displayed intense phosphate-activated glutaminase-immunoreactivity, and that within the deep cerebellar nuclei showed moderate immunoreactivity. These results indicate that many mossy fiber terminals originate from phosphate-activated glutaminase-containing neurons and utilize phosphate-activated glutaminase for the synthesis of transmitter glutamate. Intensely-immunostained neuronal cell bodies were further observed in other regions which have been reported to contain neurons sending mossy fibers to the cerebellum; in the dorsal part of the principal sensory trigeminal nucleus, dorsomedial part of the oral subnucleus of the spinal trigeminal nucleus, interpolar subnucleus of the spinal trigeminal nucleus, paratrigeminal nucleus, supragenual nucleus, regions dorsal to the abducens nucleus and genu of the facial nerve, superior and medial vestibular nuclei, cell groups f, x and y, hypoglossal prepositus nucleus, intercalated nucleus, nucleus of Roller, reticular regions intercalated between the motor trigeminal and principal sensory trigeminal nuclei, linear nucleus, and gigantocellular and paramedian reticular formation. Neuronal cell bodies with intense phosphate-activated glutaminase-immunoreactivity were also found in other brainstem regions, such as the paracochlear glial substance, posterior ventral cochlear nucleus, and cell group e. Although it is still controversial whether all glutamatergic neurons use phosphate-activated glutaminase in a transmitter-related process and whether phosphate-activated glutaminase is involved in other metabolism-related processes, the neurons showing intense phosphate-activated glutaminase-immunoreactivity in the present study were suggested to be putative glutamatergic neurons.

Changes in the content of glutamate and GABA in the cerebellar vermis and hemispheres of the Purkinje cell degeneration (pcd) mutant

The contents of glutamate and GABA, as well as aspartate, glycine, and alanine, were examined in the cerebellar vermis and hemispheres of normal and Purkinje cell degeneration (pcd) mutant mice at 6, 9, and 12 months of age. Relative to normal values, the content of glutamate was approximately 50% lower in the vermis for the 3 age groups. In the hemispheres, the content of glutamate was also lower than control values and showed a progressive loss from 30 to 47% with age. On the other hand, in the case of GABA in the vermis, the level was 39% lower in the pcd mutant at 6 months of age but no different from control values at 12 months. However, relative to data for normal mice, the content of GABA in the hemispheres was consistently lower (20%) for all age groups. The level of aspartate was approximately 60% lower in the cerebellar vermis and 45 to 55% lower in the hemispheres of the mutant with respect to control data for all three age groups. Likewise, alanine showed a reduced content in the hemispheres (36-46%) and vermis (24%) in the mutant relative to normal values at 6, 9, and 12 months of age. On the other hand, the level of glycine was 43-64% higher in the vermis and 77-100% greater in the hemispheres of the mutant than in the control group. The higher values for glycine were observed at the two oldest ages. In conclusions, the data are consistent with the idea that glutamate and GABA are present in high concentrations in granule and Purkinje cells, respectively, and provide additional support for a transmitter function for both amino acids in the cerebellum.

Temporal sequence of motor disturbances and increased cerebellar glutamic acid decarboxylase activity following 3-acetylpyridine lesions in adult rats

Adult male rats were administered 75 mg/kg of the neurotoxin 3-acetylpyridine to produce lesions of the inferior olive-climbing fiber projection to the cerebellum. At selected times ranging from 6 h to 43 days postlesion, rats were evaluated for motor dysfunction, and glutamic acid decarboxylase (GAD) activity was determined in the deep cerebellar nuclei and cerebellar vermis. In the deep nuclei non-monotonic changes in GAD activity were found following climbing fiber destruction. Initially, there was a steady increase in GAD activity which peaked at 38% above control values 14 days postlesion. GAD activity then slowly declined, although it remained significantly above control levels at 43 days postlesion, the latest time point examined. In the vermis, GAD activity was significantly increased at 4 days postlesion (+8%) and remained at approximately this level throughout the experiment. The initial behavioral effects of climbing fiber loss included hypotonia and ataxia with severely reduced mobility. With time, the ataxia and hypotonia decreased and movements such as mud-walking and pivoting developed. As these behaviors diminished, other novel conditions such as movement-associated tremor and hopping appeared. These results are discussed in the context of the previously reported effects of climbing fiber lesions on the firing rates of Purkinje cells and deep nuclei cells.

Mechanisms of ischemic cerebral injury

Normal compensatory mechanisms protect the central nervous system (CNS) from moderate hypoxia and ischemia; however, after more severe ischemia progressive brain hypoperfusion ensues and irreversible damage occurs. Ischemic brain injury remains greatly significant clinically and elucidating the determinants of ischemic neuronal injury and death continues to challenge researchers. Although altered perfusion and decreased energy charge may contribute to the production of irreversible damage, the distribution of lesions seen after insult does not correspond with the degree of ischemic blood flow impairment, nor can neuronal energy deprivation explain the cell damage. Other factors, such as derangements in astrocyte function, calcium homeostasis, free radical metabolism, acid-base regulation and excitatory neurotransmitters also probably mediate ischemic neuronal death. Continued investigation to establish the cellular pathophysiology of cerebral ischemia can guide rational research and therapeutic strategies.

Autoradiographic localization of cerebellar excitatory amino acid binding sites in the mouse

We have investigated the cellular localization of cerebellar excitatory amino acid binding sites in normal mice, in mice deficient in granule cells and, perhaps, stellate, basket and Golgi cells (granuloprival mice) and in mice lacking Purkinje cells. In the molecular layer of normal mouse cerebellum, the quisqualate-sensitive binding sites were the predominant type of excitatory amino acid receptor and there were relatively few N-methyl-D-aspartate or kainate-sensitive binding sites. The granule cell layer of normal mice contained a mixture of all 3 types, the N-methyl-D-aspartate-sensitive binding sites being predominant. In the molecular layer of granuloprival mice, the number of quisqualate-sensitive binding sites was increased to 214% of control (P less than 0.01), whereas N-methyl-D-aspartate-sensitive binding sites were decreased to 62% of control (P less than 0.001) and kainate-sensitive binding sites were unchanged. In the granule cell layer of these mice, quisqualate-sensitive binding sites were increased to 200% (P less than 0.01), N-methyl-D-aspartate-sensitive binding sites were decreased to 47% (P less than 0.001) and kainate-sensitive binding sites were decreased to 49% (P less than 0.01 of their respective control values. In the molecular layer of mice lacking Purkinje cells, quisqualate-sensitive binding sites were reduced to 29% (P less than 0.001) of control and N-methyl-D-aspartate-sensitive binding sites were unchanged. In the granule cell layer of these mice, neither quisqualate nor N-methyl-D-aspartate-sensitive binding sites were changed. These results suggest that (1) quisqualate-sensitive binding sites are located principally on dendrites of Purkinje cells and that they up-regulate after deafferentation; (2) N-methyl-D-aspartate-sensitive binding sites are located on granule cells and, perhaps, stellate, basket and Golgi cells, and (3) kainate binding sites are located on cell bodies of granule and, perhaps, Golgi cells.

Autoradiographic analysis of [3H]kainic acid binding in primate brain

The distribution of [3H]kainic acid binding sites was studied in the primate brain using semiquantitative autoradiography. The highest levels of binding were observed in the hippocampal area CA3 and the dentate gyrus. The deep layers of pyriform, cingulate and insular cortex, the central nucleus of the amygdala and the caudate nucleus also displayed high levels of [3H]kainic acid binding. Although these areas receive putative excitatory amino acid-containing afferents, other regions containing a similar input displayed low levels of binding. Some similarities were apparent between the distribution of binding sites and pathological changes in human neurodegenerative disorders such as temporal lobe epilepsy.

Modification of drug-induced tremor by systemic administration of kainic acid and quisqualic acid in mice

The effects of excitatory amino acids, kainic acid and quisqualic acid, on the tremorine- and harmaline-induced tremor were quantitatively examined in mice using the power spectral analyzing method. The severity of the tremor was determined quantitatively in terms of the cumulative sum of the mean square value of the data. Kainic acid enhanced the tremor induced by tremorine but depressed the tremor induced by harmaline. Quisqualic acid depressed the tremor induced by both tremorine and harmaline in a dose-dependent manner. Kainic acid shifted the frequency of each component of the tremor induced by tremorine to the high frequency side, but quisqualic acid did not affect the frequency of tremor of the tremor induced by tremorine. The frequency of tremor of the tremor induced by harmaline was shifted by both excitatory amino acids to the low frequency side, and another component of tremor in the power spectral densities developed, of which the mean square values were very small. The present results suggest that, at least in part, the glutamatergic system can take a role on the modification of drug-induced tremor.

Effect of thyroid deficiency on the regional development of glutaminase, a glutamatergic neuron marker, in the rat brain

The effect of thyroid deficiency on the activity of phosphate-activated glutaminase (the marker for glutamatergic neurons) was studied in different parts of the rat brain at ages 5, 10, 15 and 25 days, and at day 130 following 102 days of rehabilitation. The brain regions investigated were the cerebral cortex, basal forebrain, hippocampus and cerebellum. During normal development, the activity of glutaminase increased relatively earlier in the cerebral cortex and hippocampus than in the cerebellum, while the absolute value reached a much higher level in the hippocampus than in other brain regions. In the basal forebrain, the developmental pattern of glutaminase was bimodal, and the rise in enzyme activity after 15 days coincided with the decrease in the cerebral cortex. These regional developmental changes in glutaminase activity correlated well with known information on the formation of glutamatergic cells and pathways in the brain. Neonatal thyroid deficiency had little effect on the developmental patterns of enzyme activity, the exception being a transient decrease in 10-day-old hypothyroid hippocampus. The present results, together with previous findings, indicate that the effect of thyroid hormone on neural maturation is cell-type specific and the glutamatergic neurons are not the main targets of thyroid hormone action.

Release of endogenous aspartate and glutamate induced by electrical stimulation in guinea pig cerebellar slices

Whether endogenous aspartate and glutamate, candidates for the excitatory neurotransmitter of cerebellar climbing and parallel fibers, are actually released from guinea pig cerebellar slices by electrical stimulation of the cerebellar white matter, was examined by means of mass fragmentography using gas chromatograph-mass spectrometer and thin layer chromatography. Both endogenous aspartate and glutamate were found to be significantly released in a Ca- and stimulus-frequency-dependent manner. Although the origin of each amino acid could not be specified in spite of pharmacological attempt to selectively block the mossy fiber-granule cell (parallel fiber) system, these results were at least in favor of the electrophysiologically and pharmacologically suggested candidacy of these amino acids for the transmitters of cerebellar climbing and parallel fibers.

Immunocytochemical localization of aspartate aminotransferase and glutaminase immunoreactivities in the cerebellum

The distributions of glutaminase and aspartate aminotransferase were studied immunocytochemically in the cerebellum of the guinea pig and the rat. In the granule cell layer, both antibodies gave a similar staining pattern. Granule cell bodies were labeled, but staining was also found to lie outside the cell body, associated with what appear to be synaptic processes. In the molecular and Purkinje cell layers, aspartate aminotransferase was concentrated in stellate and basket cell bodies and in terminal baskets beneath Purkinje cells. Glutaminase, however, was not concentrated in these structures.

Regulatory role of cysteine dioxygenase in cerebral biosynthesis of taurine. Analysis using cerebellum from 3-acetylpyridine-treated rat

The effect of 3-acetylpyridine (3-AP) administration on the biosynthesis of taurine in the rat brain has been studied. Treatment with 3-AP induced a significant decrease in the cerebellar contents of taurine and its metabolic precursors, cysteine sulfinic acid (CSA) and cysteic acid (CA), as well as a selective degeneration of climbing fibers in the molecular layer of the cerebellum. It was found that the activity of cerebral cysteine dioxygenase, the enzyme catalyzing the formation of CSA from cysteine, consisted of two systems with low and high Km values. The 3-AP-induced attenuation of cysteine dioxygenase activity with a low Km value was noted only in the cerebellum, while that with a high Km value was detected not only in the cerebellum but also in other brain areas such as the medulla oblongata, striatum and cerebral cortex. In contrast, no alteration in the activity of cysteine sulfinic acid decarboxylase (CSD) was observed in any brain areas examined following the administration of 3-AP. Furthermore, it was found that essentially no cystamine as well as a very low activity of cysteamine dioxygenase is present in the brain. The present results suggest that taurine in the brain is synthesized from cysteine, mainly by the CSA and CA pathways, and the observed decline of cerebellar taurine in 3-AP-treated rats may be due to an attenuation of the biosynthesis, possibly at the step of cysteine dioxygenase. A possible regulatory role of cysteine dioxygenase with a low Km value in the biosynthesis of cerebral taurine is also suggested.

Sites of D-[3H]aspartate accumulation in mouse cerebellar slices

Slices of mouse cerebellar vermis, cut in the parasagittal plane, were incubated for various times (up to 3 h) in the presence of 1 microM D-[3H]aspartate, a non-metabolized substrate for the glutamate/aspartate carrier in brain tissue. Light microscopic autoradiography indicated that in regions away from the cut edges of the slices the amino acid accumulated in glia and granule cells. Relatively few grains were seen over Purkinje, Golgi, stellate and basket cells or over white matter. Grain counts over the granule cell layers in the middle parts of the slices indicated that after short (15 min) exposures to the labelled substrate, non-granule cell areas (which included glia) contained, on average, slightly more grains than granule cells but with longer exposures (1.5 and 3 h) the relative grain density over granule cells became much higher, possibly because glial uptake prevents D-[3H]aspartate gaining access to neuronal sites in adequate amounts during short incubations and/or because the longer incubations allow time for retrograde migration of the label from parallel fibre terminals to occur. The demonstration of selective uptake of D-[3H]aspartate into granule cells contrasts with previous autoradiographic results (possible reasons for which are discussed) and supports the notion that L-glutamate is the transmitter of granule cells. The results also have a bearing on the importance of the metabolic compartmentation of glutamate in relation to its proposed transmitter role.

Selective retrograde labelling of the rat olivocerebellar climbing fiber system with D-[3H]aspartate

Selective retrograde labelling of the olivocerebellar climbing fiber system with D-[3H]aspartate has been observed in the rat, and the results have implications for the identification of a transmitter candidate as well as the neuroanatomical understanding of these cerebellar afferents. Microinjections of D-[3H]aspartate (50 nl, ca 10-2 M) were made into various parts of cerebellar cortex. Survival times were 6, 12 or 24 h. Pronounced diffusion of the tracer resulted in large injection sites. Within the zone of injection, glial elements were labelled over background. Most granule cells exposed to the tracer were unlabelled; the small numbers demonstrating labelling were believed to have been injured by the micropipette penetration. Beneath injection sites, large numbers of well-labelled nerve fibers appeared in the white matter and could be followed through the brainstem to the contralateral inferior olive, where labelled perikarya were found. After the inferior olivary neurons had been effectively destroyed with 3-acetylpyridine, evidence of cerebellar afferent labelling with D-[3H]aspartate was missing. Retrograde labelling of the olivocerebellar system was also observed after superfusion of the vermis with D-[3H]aspartate at concentrations in the range of Km for high affinity uptake (10(-5) or 10(-4) M, for 2 h). Mossy fiber or monoaminergic afferents to the cerebellum were never labelled with D-[3H]aspartate. The distribution of labelled cells in the olivary subnuclei after injections in different cerebellar areas was in line with the olivocerebellar organization previously described in the cat. Moreover, it was demonstrated that fibers from the different subnuclei follow different routes through the brainstem towards the cerebellum. Labelling of climbing fiber collaterals in uninjected parts of cerebellum indicated that some of the retrogradely migrating D-[3H]aspartate was directed in anterograde direction at axonal branching points. Collaterals were demonstrated in all deep cerebellar and Deiters' nuclei, and the results of intranuclear injections suggested that virtually every olivary neuron sends collaterals to these nuclei. Intracortical collaterals were organized in sagittal zones. Midline injections into the anterior lobe and VI lobule labelled collaterals in several zones of the posterior lobe spinal area and uninjected parts of the anterior lobe vermis. Hemispheral injection into copula pyramidis labelled collaterals in two prominent bundles in the anterior lobe.(ABSTRACT TRUNCATED AT 400 WORDS)

Excitatory amino acid receptors and guanosine 3',5'-cyclic monophosphate in incubated slices of immature and adult rat cerebellum

Addition of the excitatory amino acids l-glutamate, l-aspartate and their analogues kainate and N-methyl-d-aspartate to incubated slices of adult rat cerebellum led to large increases in cyclic GMP levels. The order of apparent potencies was kainate greater than N-methyl-d-aspartate greater than glutamate and aspartate. D-alpha-aminoadipate and Mg2-+ inhibited responses to N-methyl-d-aspartate while glutamic acid diethyl ester was most effective against those to glutamate; responses to kainate were least affected by the antagonists. The exicitant amino acids also elicited large elevations of cyclic GMP levels in slices of immature (8 day) cerebellum. Kainate was less effective than in adult but induced two responses distinguishable by their different time courses, concentration dependencies and sensitivity to antagonists. N-methyl-d-aspartate, glutamate and aspartate were 5 to 10-fold more potent than in the adult. Responses to N-methyl-d-aspartate were similarly inhibited by d-alpha-aminoadipate and Mg2+ but those to glutamate were more resistant to glutamic acid diethyl ester than in the adult. It is concluded that the accumulation of cyclic GMP in response to excitant amino acids in the adult cerebellum is mediated via the operation of receptor types showing pharmacological characteristics expected of excitatory amino acid receptors. The actions of kainate in the immature cerebellum appear to be mediated by receptors different from those on which it acts primarily in the adult.

Cerebellar macroneurons in microexplant cell culture. Postsynaptic amino acid pharmacology

Cerebellar neurons derived from 17- to 19-day-old fetal rats have been grown in a monolayer in microexplant cell culture, and intracellular recording coupled with iontophoresis of amino acid neurotransmitters has been employed to characterize their amino acid chemosensitivity. Although these cultures contain at least 3 different neuronal cell types, intracellular recordings were obtained from large neurons (diameter greater than 15 microns) with 1-5 dendritic shafts and fine dendritic arborizations and which could, on morphological grounds, be identified as Purkinje cells. All neurons with resting membrane potentials greater than 25 mV and with action potentials evoked by intracellular stimulation, responded to iontophoretically applied glutamate and GABA. There was essentially no chemosensitivity to glycine, beta-alanine or taurine. Aspartate application evoked only small responses at high iontophoretic currents. GABA reversibly increased membrane conductance and produced hyperpolarization at resting membrane potential with reversal potentials between -50 and -40 mV (5-10 mV more negative than resting membrane potential). Glutamate reversibly increased membrane conductance and produced depolarizing responses with extrapolated reversal potentials between 0 and -10 mV. Aspartate augmented glutamate responses at low iontophoretic currents which did not directly alter membrane potential or conductance. Thus Purkinje cells grown in the absence of parallel fiber and climbing fiber input develop autonomous neuropharmacologic specificity similar to that of Purkinje cells in vivo.

Putative acidic amino acid transmitters in the cerebellum. II. Electron microscopic localization of transport sites

In structurally preserved cerebellar slices, the sites of high affinity uptake of acidic amino acids were analyzed using the nonmetabolizable analogue, D-[3H]aspartate. Electron microscopic autoradiography showed the greatest accumulation of grains to be over glial structures. The labelling of the perikarya, dendrites and axons of the putatively glutamatergic granule cells was very low. However, "hypothetical grain" analysis indicated that the terminals of these cells are probable sources of radioactivity even though they contained less than 9% of the total grains in the molecular layer. The resolution of the autoradiographic technique did not permit definitive conclusion, as the parallel fibre terminals are too small and are ensheathed by thin glial processes. Nevertheless, further supporting evidence for some D-[3H]aspartate uptake into parallel fibre terminals was obtained using mechanically chopped cerebellar slices in which compared with glia presynaptic structures are selectively preserved. It is concluded that in line with hypotheses relating to the compartmentation of glutamate metabolism, the principal sites of uptake of acidic amino acids in the cerebellum are the glial cells. The results have clear implications regarding the use of high affinity uptake as a marker for glutamatergic nerve terminals.

Putative acidic amino acid transmitters in the cerebellum. I. Depolarization-induced release

In the present investigation we studied the autoradiographic localization and the characteristics of the depolarization-induced release of acidic amino acids in in vitro rat cerebellar preparations. Light microscopy autoradiography of cerebellar slices preincubated in the presence of the non-metabolized glutamate analogue D-[3H]aspartate showed a large accumulation of radioactivity over glial cells, and very little labelling of the granule cells, whose putative neurotransmitter may be glutamate. In spite of its predominant localization in glia, D-[3H]aspartate (and [14C]glutamate) was released from cerebellar slices depolarized with high [K+] in a Ca2+-dependent way, and the release elicited by veratrine was prevented by TTX. These findings, together with the observation that freshly isolated or cultured glial cells did not show any Ca2+-dependent, depolarization-induced release of D-[3H]aspartate, suggest that the radioactive amino acid released from slices has a neuronal origin. The high [K+]-induced release of exogenous radioactive acidic amino acids from superfused cerebellar synaptosomal preparations exhibited, as best, a modest Ca2+-dependence, a result probably due to the existence of a substantial non-Ca2+-dependent release of the amino acid from glial fragments contaminating the preparation. However, both the K+-evoked release of endogenous glutamate, and that of [14C]glutamate previously synthesized from [14C]glutamine were largely Ca2+-dependent, suggesting that nerve endings are the main sites involved in the stimulus-coupled secretion. In the experiments in which synaptosomes had been prelabelled with [14C]glutamine, a study of the specific radioactivity of the glutamate released and of that present in synaptosomes at the beginning and at the end of superfusion period provided evidence in favour of a preferential release of the newly synthesized [14C]glutamate. In contrast to glutamate, endogenous aspartate was not released in a Ca2+-dependent manner, and the efflux of newly formed [14C]aspartate was only slightly potentiated by Ca2+, which suggests that glutamate and aspartate are not released from the same sites. Studies on preparations (slices and synaptosomes) from immature, 8-day-old cerebella showed that neither the K+-evoked release of D-[3H]aspartate, nor that of endogenous glutamate was Ca2+-dependent. In conclusion, the data presented are consistent with the proposition that glutamate has a neurotransmitter role in the cerebellum.U

Specific high-affinity binding of L-[3H]aspartate to rat brain membranes

The binding of L-[3H]aspartate was investigated in washed membranes prepared from whole rat brain. We were able to differentiate two separate binding sites differing in their Na dependence. The Na-independent binding was saturable, reversible, and optimal at 20 degrees C and at pHs in the neutral range. The dissociation constant (Kd) at 20 degrees C was about 200 nM. This binding site seemed to be modulated by magnesium and calcium at physiological concentrations. None of the amino acids tested was a potent competitor for Na-independent L-[3H]aspartate binding. This binding site was unevenly distributed in the rat central nervous system: cerebellum = cerebral cortex greater than pons-medulla greater than spinal cord. Destruction of the intrinsic neurons of the cerebellum by injecting kainic acid 30 days before sacrifice resulted in a 53% reduction in Na-independent binding in this region. The Na-dependent binding of L-[3H]-aspartate (Kd = 4894 nM) was strongly inhibited by D-aspartate, L-glutamate, D,L-aspartate beta-hydroxamate; was unaffected by calcium and magnesium; and showed a different pattern of distribution: cerebral cortex greater than cerebellum = pons-medulla = spinal cord. This binding in cerebellum was unaffected by injections of kainic acid.

Aspartate: possible neurotransmitter in cerebellar climbing fibers

Autoradiography demonstrated prominent retrograde labeling of olivocerebellar climbing fiber neurons after injection of tritiated D-aspartate into the rat cerebellar cortex or deep nuclei. Mossy fiber systems originating in the brainstem and spinal cord remained unlabeled. Potassium ion-induced depolarization of cerebellar slices resulted in calcium ion-dependent release of endogenous L-aspartate, L-glutamate, gamma-aminobutyric acid, and glycine. A 26 percent decrease in aspartate release was observed after 3-acetylpyridine-induced destruction of the inferior olive, supporting the hypothesis that aspartate is a neurotransmitter in climbing fibers.