Effects of selective doses of x-irradiation on the levels of several amino acids in the cerebellum of the rat

Transient Sensitivity of Rat Cerebellar Purkinje Cells to N-methyl-D-aspartate during Development. A Voltage Clamp Study in in vitro Slices

In vitro sagittal slices of immature rat cerebellum were used to study the development of the sensitivity of Purkinje cells (PC) to L-glutamate (Glu) and N-methyl-D-aspartate (NMDA). In 8-day-old animals, all PCs recorded in magnesium-free medium responded to iontophoretic applications of both agonists by transient and dose dependent inward currents which, in both cases, were heavily contaminated by a Glu and NMDA-induced synaptic noise. When 5 x 10-6 M tetrodotoxin (TTX) was added to the perfusing medium, this evoked synaptic noise was completely abolished in most cells whereas clear-cut inward currents induced in PCs by Glu and NMDA applications on their dendrites were still visible. These responses were selectively antagonized by the non-NMDA glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and by the NMDA receptor antagonist D-2-aminophosphono-5-valeric acid (2APV) respectively. Excitatory responses induced by aspartate in 8 - 10-day-old PCs were also markedly antagonized by CNQX. At this stage, the sensitivity of PCs to NMDA was about one order of magnitude less than that to Glu. In 15 - 20-day-old animals, all PCs were still responsive to Glu whereas only 70% of them were still excited by NMDA in the presence of TTX in the bath. Furthermore, the sensitivity of PCs to Glu was higher than at 8 days of age, whereas that to NMDA was significantly lower, even when considering only those cells which still responded to this agonist. This trend was still accentuated later on since at 2 months of age, only 25% of PCs were excited by NMDA whereas their sensitivity to Glu was similar to that observed in 15 - 20-day-old animals. Therefore, the present results are fully consistent with the view that PCs have a transient expression of NMDA receptors during development.

Hippocampal lesions induced by ionizing radiation: a parametric study

The selective lesion of granule cell populations in the dentate gyrus induced by ionizing radiation has been proposed as a useful method for evaluating the effects of hippocampal lesions on behavioral tasks. In the first part of the present study we confirmed the induction of the selective lesion of hippocampal dentate gyrus by ionizing radiation in infant Wistar rats, reported previously, but to a smaller extent with less cell loss. A parametric study was thus performed to assess the effect of modification of the parameters previously tested, comprising three further steps: an increase in the total dose of X-rays and modification of the fractionating schedule; use of three radiation types, X-ray, gamma-ray, and electrons (at two energy levels, 3 and 7 mev); use of three X-ray energy levels, 180, 200 and 250 kVp; and assessment of the effect of five total X-ray doses, at 200 kVp, 10, 14, 16, 18 and 20 gy (grays). The data suggests that X-ray radiation, in a total dose of 14 gy, at the 200 kVp energy level, fractionated into seven consecutive exposures of 2 gy each and produces a lesion of about 85% of the dentate gyrus granule cells.

Differences in transmitter release, morphology, and ischemia-induced cell injury between cerebellar granule cell cultures developing in the presence and in the absence of a depolarizing potassium concentration

Release of glutamate and aspartate was measured in mouse cerebellar granule cells in primary cultures grown for 4-16 days in serum-containing tissue culture medium with either a partially depolarizing (25 mM) or a physiological concentration of potassium (5.4 mM). The cells migrated to form aggregates connected by a network of processes during the first week in culture and both groups of cultures survived for at least 2 weeks. In cultures grown in the presence of 25 mM potassium for at least 8 days there was a large (approximately 10 nmol/min/mg protein), calcium-dependent glutamate release and a smaller aspartate release during superfusion with 50 mM potassium. This response was not present in cultures grown in the physiological medium. Nevertheless, exposure to an elevated potassium concentration caused a normal, or even enhanced calcium entry into the cells. Phase contrast microscopy showed a similar appearance of the cellular aggregates under each of the two conditions. Electron microscopy revealed that the aggregates consisted of a centrally located neuropil and peripherally located granule cell bodies. The morphology of the cell bodies and the neuropil in the cells grown at the high potassium concentration closely resembled that of cerebellar granule cells in vivo. In the cells grown at the low potassium concentration, cell bodies, axons and synaptic vesicles looked normal, but the remainder of the neuropil, especially dendrites, showed massive degeneration. Immunochemical measurements demonstrated similar amounts of synaptophysin under each of the two culturing conditions, thus confirming our impression that there were similar numbers of synaptic vesicles and hence presynaptic elements in the two types of cultures. Fluorescence microscopy, using fluorescein diacetate to stain living cells and propidium iodide to stain dead cells, indicated a much greater resistance to ischemic cell injury in the cells cultured at the low potassium concentration. Possible reasons for this difference are discussed.

Development of monoamine oxidase activity and monoamine effects on glutamate release in cerebellar neurons and astrocytes

Activities of monoamine oxidase (MAO) A and B were measured during the first month of postnatal development in mouse cerebellum and in primary cultures of either cerebellar granule cells or cerebellar astrocytes, derived from 7-day-old cerebella. In addition, effects of the two monoamines, serotonin (a MAO A substrate) and phenylethylamine (a MAO B substrate) on the release of glutamate under resting conditions and in a transmitter related fashion (i.e., potassium-induced, calcium-dependent glutamate release) were studied during the same period. Both MAO A and MAO B activities increased during in vivo development (beginning around postnatal day 14) and in cultured astrocytes (during a comparable time period and to a similar extent), but remained constant at a low level in granule cells. In 4-day-old cerebellar granule cell cultures there was no potassium-induced glutamate release but serotonin as well as phenylethylamine reduced the release in both the presence and absence of excess potassium. In 8- and 12-day-old granule cell cultures and in 8- and 18-day old astrocyte cultures there was a pronounced glutamate release during superfusion with 50 mM K+. In both neurons and astrocytes this response was inhibited by 1 nM of either serotonin or phenylethylamine. In the astrocytes the inhibition was followed by an increased release of glutamate in both the presence and absence of the high potassium concentration, whereas the 8-day-old neurons showed only a slight increase in glutamate release after the withdrawal of the monoamine and only in the absence of excess potassium. The response was almost identical in 8- and 18-day-old astrocytes in spite of the marked difference in MAO activities.

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.

Glutamate-like immunoreactivity revealed in rat olfactory bulb, hippocampus and cerebellum by monoclonal antibody and sensitive staining method

Although there is good evidence favoring L-glutamate as a major excitatory amino acid transmitter, relatively little is known about the distribution of nerve terminals using this substance. A method visualizing glutamate-like immunoreactivity at the light microscopic level by means of a monoclonal antibody, mAb 2D7, is described. --The antigen used for immunization was a glutaraldehyde-linked glutamate-BSA conjugate, and hybridomas were differentially screened by ELISA for production of antibodies recognizing glutamate- but not aspartate-BSA. The crossreactivity of 'anti-glutamate' mAb 2D7 as estimated in absorption tests was low even with conjugates closely related to glutamate-BSA.--Semithin sections from rapidly perfusion-fixed, plastic-embedded rat brain tissues were etched and stained by a combination of the peroxidase-antiperoxidase method and silver enhancement of the diaminobenzidine reaction product. Only this amongst several other immunohistochemical methods tried produced labeling patterns which showed terminal-like elements in brain regions such as olfactory bulb, hippocampus and cerebellum, and which were mostly consistent with already available information on systems using glutamate as neurotransmitter. Particularly striking was the staining of elements reminiscent of mossy fiber terminals in hippocampus and cerebellum as well as of cerebellar parallel fiber terminals.

ATP-dependent glutamate uptake into synaptic vesicles from cerebellar mutant mice

The ATP-dependent glutamate uptake system in synaptic vesicles prepared from mouse cerebellum was characterized, and the levels of glutamate uptake were investigated in the cerebellar mutant mice, staggerer and weaver, whose main defect is the loss of cerebellar granule cells, and the nervous mutant, whose main defect is the loss of Purkinje cells. The ATP-dependent glutamate uptake is stimulated by low concentrations of chloride, is insensitive to aspartate, and is inhibited by agents known to dissipate the electrochemical proton gradient. These properties are similar to those of the glutamate uptake system observed in the highly purified synaptic vesicles prepared from bovine cortex. The ATP-dependent glutamate uptake system is reduced by 68% in the staggerer and 57-67% in the weaver mutant; these reductions parallel the substantial loss of granule cells in those mutants. In contrast, the cerebellar levels of glutamate uptake are not altered significantly in the nervous mutant, which has lost Purkinje cells, but not granule cells. In view of evidence that granule cells are glutamatergic neurons and Purkinje cells are GABAergic neurons, these observations support the notion that the ATP-dependent glutamate uptake system is present in synaptic vesicles of glutamatergic neurons.

Activation of protein kinase C induces a long-term depression of glutamate sensitivity of cerebellar Purkinje cells. An in vitro study

In immature rat cerebellar slices in vitro, a long term depression (LTD) of the responses of Purkinje cells (PCs) to L-glutamate (Glu) was achieved in 30% of the recorded cells by simultaneous stimulation of the neurones by Glu and by climbing fibres (CFs). This effect was not observed for L-aspartate (Asp)-induced responses. Similarly, selective LTD of Glu-induced responses was obtained in 22% of the cells by pairing Glu applications with direct stimulation of the cells which elicited calcium spikes in these neurones. Finally, bath application of phorbol esters also induced a selective LTD of Glu-induced responses in all cells tested. These results suggest that protein kinase C is involved in cerebellar synaptic plasticity.

Quantitative assessment of taurine-like immunoreactivity in different cell types and processes in rat cerebellum: an electronmicroscopic study based on a postembedding immunogold labelling procedure

Ultrathin sections of plastic-embedded rat cerebella were incubated with an antiserum against conjugated taurine and subsequently treated with a secondary antibody coupled to colloidal gold. The density of gold particles in various cellular profiles was calculated with the assistance of a computer. In the cerebellar cortex the highest density was found in the somata, dendrites, and dendritic spines of the Purkinje cells, supporting parallel light-microscopical observations in postembedding stained semithin sections from the same tissue blocks. The remaining profiles could be divided into three groups according to their immunolabelling intensity, in descending order: 1) somata and processes of granule and Golgi cells; 2) somata and processes of stellate, basket, and glial cells, and 3) mossy fiber terminals. In a representative experiment, the structures in the first and second groups showed gold particle densities in the range of 19-25%, and 4-11%, respectively, of that in the Purkinje cell somata (values corrected for background) whereas the particle density in the mossy fiber terminals was not significantly above background level. In the cerebellar nuclei, taurine-like immunoreactivity was concentrated in terminals that typically established symmetric or intermediate type contacts with weakly labelled dendrites and cell bodies. These terminals, which shared the ultrastructural features of Purkinje cell terminals, showed an average gold particle density that was about 60% higher than that of the Purkinje somata. For specificity control, ultrathin sections containing a series of different amino acid conjugates were incubated in the same drops of sera as the tissue sections. The highly selective labelling of the taurine conjugate indicated that the distribution of gold particles in the tissue was not confounded by crossreactivity with GABA, glutamate or other common amino acids but adequately reflected the distribution of fixed taurine. For additional control of specificity, the taurine antiserum was applied to the soluble fraction of a rat brain extract separated by thin layer chromatography. In this system the taurine antiserum stained a single spot that comigrated with free taurine. The present results suggest that all cell types and processes in the rat cerebellum (with the exception of the mossy fiber terminals) contain taurine. However, the concentration of taurine appears to vary considerably among the different cell types and may also differ between different parts of the same neuron.

The glutamate receptor subtype mediating parallel fibre-Purkinje cell transmission in rabbit cerebellar cortex

Responses of Purkinje cells to parallel fibre stimulation and to ionophoretically applied glutamate agonists, L-glutamate, L-aspartate, quisqualate, kainate and N-methyl-D-aspartate (NMDA), were extracellularly recorded in a superficial folium of the dorsal paraflocculus of high-decerebrate rabbits. NMDA caused an inhibition of simple spike discharge from Purkinje cells. 2-Amino-5-phosphonovalerate (APV), a selective NMDA receptor antagonist, effectively antagonized this inhibition. However, APV did not antagonize the excitation of Purkinje cells caused by quisqualate or parallel fibre stimulation. By contrast, both kynurenate and gamma-D-glutamylglycine (gamma-DGG) effectively antagonized the excitation of Purkinje cells by parallel fibre stimulation in a dose-dependent manner. Both kynurenate and gamma-DGG also antagonized the excitation of Purkinje cells caused by quisqualate, L-glutamate, L-aspartate or kainate. However, the antagonism was more prominent to L-aspartate and kainate than to quisqualate and L-glutamate. Quisqualate response was antagonized to an extent comparable with the response to parallel fibre stimulation. These results indicate that the receptor for the neurotransmitter at parallel fibre-Purkinje cell synapses is of quisqualate-specific type. The present data are consistent with the evidence that L-glutamate is the endogenous transmitter at these synapses.

Cerebellar benzodiazepine receptors: cellular localization and consequences of neurological mutations in mice

The distribution of cerebellar [3H]flunitrazepam binding sites was studied autoradiographically in Purkinje cell degeneration (pcd/pcd), weaver (wv/wv), staggerer (sg/sg) and reeler (rl/rl) mutant mice. In the normal 78-day-old C57BL/6J mouse cerebellum, the highest concentration of [3H]flunitrazepam binding sites was observed over the molecular layer. Intermediate grain density was present over the Purkinje cell layer and intermediate to high density over the deep cerebellar nuclei. Low labeling was observed over the granule cell layer. Negligible concentrations of binding sites were seen in the white matter. In 45-49-day-old Purkinje cell degeneration mutants, where essentially all Purkinje cells have disappeared by day 45, there was a small decrease in grain density over the cerebellar cortex. Concomitantly, a substantial increase in grain density was observed over the deep cerebellar nuclei of the pcd/pcd mutants when compared to littermate controls. A significant increase in [3H]flunitrazepam labeling was observed over the cerebellar cortex of 81-86-day-old wv/wv mutants; this was most pronounced in the vermis where the granule cell loss was greatest. Over the hemispheres, where fewer granule cells degenerate, a lower density of binding sites was seen. Grain density over the wv/wv deep cerebellar nuclei was comparable to that of littermates. Substantially lower [3H]flunitrazepam labeling was detected over the cerebellar cortex of 25-27-day-old sg/sg mutants in which the number of granule, Purkinje and Golgi cells is greatly reduced; the labeling over the deep nuclei, however, was significantly increased. In 27-29-day-old rl/rl mutant cerebella, where all classes of cells are malpositioned, labeling density over all areas of the cerebellar cortex, including the Purkinje cell masses, was increased. Our autoradiographic data suggest that a proportion of cerebellar cortical benzodiazepine receptors are associated with Purkinje cells; we propose that the remainder of the receptors are localized on Golgi cells, while granule cells are devoid of receptors. In the deep cerebellar nuclei, the observed increase in benzodiazepine receptors in the pcd/pcd and sg/sg mutants may be a manifestation of denervation supersensitivity subsequent to the loss of innervation by Purkinje cell axon terminals. The finding of a high receptor density in the Purkinje cell masses of the rl/rl mutant, where Purkinje cells are devoid of afferent basket cell input, suggests that benzodiazepine receptors are expressed and maintained in the absence of a full complement of GABAergic afferents.

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.

The laminar distribution of amino acids in the caudal cerebellum and electrosensory lateral line lobe of weakly electric fish (Gymnotidae)

We have studied the distribution of the putative amino acid neurotransmitters glutamate, aspartate, gamma-aminobutyric acid (GABA), glycine, taurine and beta-alanine in the caudal cerebellar lobe and electrosensory lateral line lobe (ELL) of weakly electric gymnotid fish. In the caudal lobe of the cerebellum, the levels of the various amino acids in the granular and molecular layers are comparable to the levels in the rat cerebellum, with the exception of taurine which is present in greater amounts in the gymnotid. In the ELL, these amino acids are differentially distributed in the various layers of this structure. Glutamate and taurine are enriched in the molecular layer, whereas GABA, aspartate, and beta-alanine are enriched in the deep neuropil + granular layers. Glycine is slightly enriched in the pyramidal cell layer.

Postnatal development of the chemosensitivity of rat cerebellar Purkinje cells to excitatory amino acids. An in vitro study

In vitro sagittal slices of immature rat cerebellum were used to study the development of the sensitivity of Purkinje cells (PCs) to L-aspartate (L-Asp), L-glutamate (L-Glu) and related derivatives. As early as postnatal day 0 all PCs already displayed clear excitatory responses to short iontophoretic applications of L-Asp, L-Glu and quisqualate while in the same conditions no effect of N-methyl-D,L-aspartate (NMDLA) was detected. By postnatal day 5, i.e. after the onset of the synaptogenesis, the sensitivity of PCs to L-Asp, L-Glu and quisqualate significantly increased up to values similar to those recorded in adult rat cerebellum and surprisingly nearly all (87%) the recorded cells now also displayed excitatory responses to NMDLA. Although this sensitivity of PCs to NMDLA was significantly lower than that observed with the other drugs, it persisted until the end of the first postnatal month when the adult type of connectivity is already well established but at this stage only 30 per cent of the tested cells were still sensitive to the agonist. During this period, excitatory responses elicited by NMDLA were selectively antagonized by 2-amino-5-phosphonovalerate (2-APV), suggesting that during postnatal development, NMDA receptor types are transiently expressed on PCs membranes since in the adult, NMDLA no longer had an excitatory effect. Instead, this drug now exerted a preferential antagonistic action on the excitatory response elicited by L-Asp. Also in the adult, no major changes occurred in the sensitivity of PCs to L-Asp, L-Glu and quisqualate when these drugs were ejected at a dendritic site whereas, when ejected at the somatic level, the sensitivity of the cell appeared 2-3 times lower.

Quantification of immunogold labelling reveals enrichment of glutamate in mossy and parallel fibre terminals in cat cerebellum

The glutamate immunoreactivity of different cell populations was compared quantitatively in the cerebellar cortex of cat, using an antiserum raised against glutamate coupled to bovine serum albumin by glutaraldehyde. Neuronal and glial processes were identified on serial electron microscopic sections which were processed by a postembedding immunogold procedure. The surface density of colloidal gold particles was used for statistical comparison of the relative levels of glutamate in cell populations, or in different parts of the same population. The terminals of mossy and parallel fibres had significantly higher levels of glutamate immunoreactivity than Golgi cell terminals, granule cell dendritic digits, Purkinje cell dendrites or dendritic spines. Golgi cell terminals were identified by their position and GABA immunoreactivity as revealed by immunogold in serial sections. The dendritic digits of the putative glutamatergic granule cells had significantly higher glutamate immunoreactivity than did Purkinje cell dendrites and dendritic spines. Glial cell processes in the molecular layer had lower level of glutamate immunoreactivity than any of the neuronal processes. The results demonstrate that the highest levels of glutamate immunoreactivity occur in mossy and parallel fibre presynaptic terminals that are known to have an excitatory effect. This supports previous suggestions that glutamate may be a transmitter at these synapses. The measurement of the levels of putative amino acid transmitters in identified neuronal populations, or in different parts of the same population, could have wide applications in studies on the chemical neuroanatomy of the nervous system.

Amino acid levels in the guinea pig spinal gray matter after axotomy of primary sensory and descending tracts

This study attempts to determine if the axonal endings of dorsal root sensory fibers and of descending axons to the spinal gray matter in the guinea pig store glutamate and/or aspartate. Bilateral dorsal rhizotomy (spinal segments C5-T1) and partial cordotomy (segment C5, right side) were used to interrupt primary sensory and descending tracts, respectively. At 1 and 2 days after surgery, amino acid levels were determined in regions microdissected from areas of the gray matter of spinal segment C7 that receive heavy projections from the primary sensory and the descending tracts. These regions were identified by visualizing the degeneration of axons and their terminal fields in silver-impregnated light microscopic preparations of the spinal cord. After dorsal rhizotomy, the heaviest degeneration in the spinal gray appeared centrally in laminae II-IV and medially in laminae IV-VI. The levels of aspartate, glutamate, and gamma-aminobutyrate were reduced by 34, 21, and 26% in laminae II-IV and 28, 33, and 23% in medial laminae IV-VI. The levels of glycine, alanine, and threonine-serine-glutamine (unseparated) were increased. After partial cordotomy, the heaviest degeneration in the spinal gray appeared laterally in laminae IV-VI, dorsolaterally in lamina VII, and in lamina IX. The levels of aspartate and glutamate were reduced by 22 and 28% in lateral laminae IV-VI and by 26 and 28% in dorsolateral laminae VII and IX. Glycine levels were reduced by 9% in dorsolateral laminae VII and IX. The levels of gamma-aminobutyrate, alanine, and threonine-serine-glutamine were either unchanged or raised. These findings suggest that the axonal endings of the primary sensory and of one or more of the descending tracts probably contain relatively high levels of glutamate and aspartate, and that they may use these amino acids as transmitters. The partial deafferentation of spinal interneurons and the destruction of some propriospinal fibers probably caused the losses of gamma-aminobutyrate and glycine, and contributed modestly to those of aspartate.

Regional distribution and extracellular levels of amino acids in rat central nervous system

The extracellular levels of aspartate, glutamate, serine, glutamine, glycine, alanine and GABA were studied in vivo with the microdialysis technique in 15 different regions of the rat brain. The effect of high K+ on the overflow of these amino acids was also studied. These results were compared with those from a regional dissection of 17 brain regions in which the tissue content of the same amino acids was determined. The in vivo data showed an unevenly distributed KCl response of aspartate, glutamate, taurine and GABA, all of which are putative neurotransmitters. It was not possible to predict the response to high K+ from the magnitude of the unstimulated overflow. Glutamine overflow was inversely related to that of glutamate during the high K+ stimulus, which is consistent with glutamine being the main precursor of glutamate. Only for GABA and alanine was overflow proportional to the tissue level in the different regions studied.

Serotonin inhibits the depolarization-evoked release of endogenous glutamate from rat cerebellar nerve endings

Glutamic acid (Glu) has been proposed as the neurotransmitter of cerebellar granule cells. 5-Hydroxytryptamine (5-HT) afferents project to the cerebellar cortex. The possible interaction between 5-HT and Glu was investigated by studying the effect of 5-HT on Glu release. The Ca2+-dependent depolarization-evoked release of endogenous Glu from superfused rat cerebellar synaptosomes was potently inhibited by 5-HT. Methiothepin, but not ketanserin, cinanserin or methysergide, antagonized 5-HT. It is concluded that the release of Glu can be modulated by 5-HT through receptors sited on Glu terminals. These receptors belong to the 5-HT1-type.

Pharmacologically distinct glutamate receptors on cerebellar granule cells

Cultured cerebellar granule cells were found to exhibit calcium-dependent release of 3H-D-aspartate when stimulated with excitatory amino acids. L-glutamate and L-aspartate were found to be potent stimulators of 3H-D-aspartate release, D-aspartate was weaker and only minor effects were seen with D-glutamate, quisqualate, kainate, N-methyl-D-aspartate (NMDA) and L-alpha-aminoadipate (L-alpha AA). It was also found that only L-glutamate and L-aspartate showed high affinity for the 3H-L-glutamate binding sites on granule cell membranes. Stimulation by L-glutamate of 3H-D-aspartate release could be blocked by various excitatory amino acid antagonists. From the relative potencies of agonists and antagonists on D-aspartate release it is suggested that cerebellar granule cells express functionally active glutamate receptors with pharmacological characteristics different from all known excitatory amino acid receptors.

Assessment of some transmitter actions in rat cerebellar slices

The effects of 100 microM norepinephrine (NE), GABA, aspartate, glutamate, and carbachol on the release of endogenous NE, GABA, aspartate, and glutamate from slices of rat cerebellum were examined. The 35 mM K+-stimulated release of NE was potentiated by GABA (136% of control), glutamate (123%), and carbachol (123%); aspartate had no effect. Glutamate increased the release of GABA to 250% of control levels, while neither NE nor carbachol exerted any effect. Glutamate and GABA increased aspartate release to 260% and 300% of control values, respectively. NE decreased the release of aspartate to 86% of control levels while carbachol had no effect. The stimulated release of glutamate was increased by GABA (166% of control) but was unaffected by NE and carbachol. All of these effects were observed only under depolarizing conditions and in the presence of extracellular Ca2+. These data suggest a cholinergic, GABAergic and glutamatergic control of the noradrenergic system in the cerebellum; the presence of a specific aspartergic system in the cerebellum; and a net excitatory action of GABA may be present within the cerebellum.

Studies on neurotransmitter markers and neuronal cell density in the cerebellar system in olivopontocerebellar atrophy and cortical cerebellar atrophy

Glutamate, aspartate and gamma-aminobutyrate (GABA) concentrations and choline acetyltransferase (ChAT) activity were measured in postmortem cerebellar cortical areas and brainstem nuclei of 10 normal controls, 5 patients of olivopontocerebellar atrophy (OPCA) with multiple system atrophy (MSA) and 2 patients of cortical cerebellar atrophy (CCA). In addition, the neuronal cell density in the cerebellar cortex and the brainstem nuclei was determined, and the correlation between neurotransmitter markers and the neuronal cell densities were investigated. Glutamate and aspartate concentrations in the cerebellar cortical tissues were markedly varied from case to case of MSA (OPCA) and CCA patients. However, glutamate concentration in the anterior vermis showed a positive correlation coefficient with the density of granule cells (r = 0.554, 0.05 less than P less than 0.10) and, those in the posterior vermis and in the cerebellar hemisphere were positively correlated with cells in the inferior olive (r = 0.707 and 0.607, P less than 0.05, respectively). Aspartate concentration in the anterior vermis also has a positive correlation coefficient (r = 0.571, 0.05 less than P less than 0.10) with the density of cells in the inferior olive. GABA concentrations in the dentate nucleus were decreased in all cases of MSA (OPCA) and CCA, and were positively correlated with the degree of loss of Purkinje cells (r = 0.765, P less than 0.01). ChAT activities were decreased in certain cases of MSA (OPCA), but conversely, increased in CCA patients. ChAT activity in the posterior vermis has a positive correlation coefficient (r = 0.613, 0.05 less than P0.10) with the cell density in the pontine nucleus. A possibility of a compensatory increase of ChAT activity in CCA patients was discussed.

Immunocytochemical visualization of taurine: neuronal localization in the rat cerebellum

A novel technique for immunocytochemical demonstration of taurine (Tau) is presented. Antisera raised against Tau conjugated to protein by glutaraldehyde (GA) react selectively with similar conjugates in model systems and in tissue fixed with GA. In rat cerebellum, Tau-like immunoreactivity is high in the Purkinje cells but low in other cell types, including the stellate cells for which Tau has been proposed as transmitter.

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.

Excitatory and inhibitory responses of Purkinje cells, in the rat cerebellum in vivo, induced by excitatory amino acids

Responses of rat cerebellar Purkinje cells to iontophoretically administered excitatory amino acids have been studied in vivo. Responses to N-methyl-D-aspartate (NMDA) were either biphasic (excitation followed by inhibition) or purely inhibitory and were antagonized by the selective NMDA-receptor antagonist, 2-amino-5-phosphonovalerate. Quisqualate and kainate either excited or induced biphasic responses, in these neurones, which were only reduced by amino acid antagonists that acted at non-NMDA receptors. The excitatory amino acid-induced inhibitions were also antagonized by the selective gamma-aminobutyric acid (GABA) antagonist, picrotoxin, suggesting that they were indirectly mediated via GABAergic inhibitory interneurones, which could be excited via NMDA and non-NMDA receptors.

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.

Evidence against neurotransmitter mediation of sprouting in granuloprival cerebellar cultures

Cerebellar cultures derived from neonatal mice undergo a remarkable sprouting of Purkinje cell recurrent axon collaterals after exposure for the 1st 5 days in vitro to cytosine arabinoside to destroy granule cells. Such cultures were simultaneously exposed to large concentrations of the putative neurotransmitters glutamate and GABA, with subsequent continued exposure to the amino acids until the time of fixation at 15 or 16 days in vitro. Axon collateral sprouting was not prevented by glutamate or GABA, suggesting the sprouting is not mediated by the relevant neurotransmitters, but is more likely due to an alteration or lack of development of some trophic interaction between granule cells and their target Purkinje cells.

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)

Free amino acids in synaptic vesicles isolated from the cerebellum and cerebral hemispheres of control and neonatally X-irradiated rats

X-irradiation of the rat brain (1000R, at two days of age), suppresses the normal age-related increase in the weight of the cerebellum and cerebral hemispheres and influences amino acid levels. The decrease in glutamic acid concentration, particularly in the cerebellum, supports the previously advanced proposition that this amino acid may be associated with or may be the transmitter of, the rat cerebellar granule cells. Subfractionation of the cerebellar tissue reveals that the decrease in the glutamic acid level consequent to the loss of granule cells, is reflected in the cytoplasmic fraction but not in the synaptic vesicle subfraction, where glutamic acid was increased. The reduced weight gain in the cerebral hemispheres after irradiation, is accompanied by a significant decrease of aspartate in the cytoplasmic fraction, changes which suggest that a specific cell type, with aspartic acid as its neurotransmitter (possibly in the hippocampus), may also be radiosensitive in the early postnatal period. In contrast, in the synaptic vesicle fraction from cerebral hemispheres, all free amino acids, with the exception of glutamine, increased significantly. Overall, the changes in free amino acid concentration induced by X-irradiation in the cytoplasmic fraction in both brain regions studied are opposite to those found in the synaptic vesicle fraction and although they may indicate changes in specific cell populations, as proposed above, they could also reflect changes in cellular compartmentalization and metabolism or changes in the relative axonal arborization of the affected regions.

Neurotransmitter amino acids in the CNS. II. Some changes in amino acid levels in rat brain synaptosomes during and after in vitro anoxia and simulated ischemia

The effects of in vitro anoxia and membrane depolarization by veratridine on the uptake and release of amino acids were investigated in suspensions of synaptosomes isolated from the forebrains of rats. It was observed that GABA, aspartate and glutamate were released from synaptosomes in anaerobic conditions and upon addition of veratridine in a time-dependent manner. The release of the two latter amino acids was faster and more pronounced than that of GABA. The other amino acids were not affected in any systematic way by either condition. Re-introduction of oxygen or addition of tetrodotoxin to veratridine-treated synaptosomes resulted in the re-uptake of GABA, aspartate and glutamate, which was much faster and more complete for GABA than for the acidic amino acids, especially at acid pH values. The amounts of aspartate and glutamate in the incubation mixture remained constant during all the manipulations whereas that of GABA increased by about 30% during anaerobiosis, in agreement with the results obtained during in vivo ischemia. It is postulated that synaptosomes which utilize glutamate and aspartate as neurotransmitters are more damaged by anoxia and depolarization with veratridine than the population which utilizes GABA. These observations may explain reports that those neurons which are thought to receive major glutamatergic input are particularly sensitive to the lack of oxygen.

Neurotransmitter amino acids in the CNS. I. Regional changes in amino acid levels in rat brain during ischemia and reperfusion

The levels of amino acids in 6 regions of the brain (cortex, hippocampus, striatum, diencephalon, stem and cerebellum) were determined during an ischemic insult of 30 min and after recovery periods of up to 10 h. The results were analyzed in two groups: putative neurotransmitters (GABA, aspartate, glutamate, taurine, glycine and alanine) and non-neurotransmitters. In the neurotransmitter group, it was found that at the end of 30 min ischemia the levels of aspartate and glutamate slightly decreased whereas those of GABA and alanine rose substantially. The amounts of glycine and taurine remained unchanged. In 30 min after the ischemic insult, there were much larger decreases in aspartate and glutamate and increases in GABA and alanine with no change in glycine and taurine. At 2 h recovery the levels of the neurotransmitter amino acids had almost returned to control values and were fully recovered by 10 h after ischemia. It is postulated that glutamate and aspartate are released during ischemia into the extracellular space and subsequently 'washed-out' into the blood during the reperfusion. Release of GABA, if it occurs, is however, compensated by increase in its synthesis and decrease in its degradation under anaerobic conditions, both of which contribute to the rise in its steady-state level. In the non-transmitter category, increases were seen in amino acids present normally in very small concentrations; tyrosine, lysine, leucine and 3 hydrophobic amino acids: valine, methionine and phenylalanine, which were most pronounced at 2 h after ischemia. It is suggested that the rise in the levels of these molecules is the consequence of stimulation of protein breakdown caused by activation of intracellular proteases by calcium and H+ during the ischemic episode. Regional variations in the patterns of changes were small although in the ischemic models used the brainstem seemed to be least affected.

Effect of excitatory amino acids on Purkinje cell dendrites in cerebellar slices from normal and staggerer mice

The sensitivity of Purkinje cells to short pulse applications of L-aspartate, L-glutamate and related derivatives in their dendritic fields was tested in normal and staggerer mutant mice using cerebellar slices maintained in vitro. In normal mice, the response of Purkinje cells to L-aspartate and L-glutamate consisted of a transient and dose-dependent increase of their firing of simple spikes. The potency of L-aspartate in exciting Purkinje cells was lower than that of L-glutamate when the two drugs were released from adjacent barrels of the same iontophoretic electrode. Quisqualate was an even more potent excitant of these cells than L-aspartate and L-glutamate, whereas N-methyl-DL-aspartate had little or no effect. In staggerer mutant mice, the sensitivity of Purkinje cells to L-aspartate, L-glutamate and quisqualate was not significantly altered. On the contrary, N-methyl-DL-aspartate had a much stronger potency than normal in exciting Purkinje cells although this was still smaller than that of the other agonists tested. These results suggest that the sensitivity of Purkinje cells to L-aspartate and L-glutamate, i.e. the putative neurotransmitters of the climbing and parallel fibers respectively, remains largely normal in staggerer mice. In contrast, in the mutant, N-methyl-D-aspartate receptors are likely to be much more developed than normal.

Release of labelled taurine from the rat dorsal medulla and cerebellum in vivo

Stimulus-induced release of labelled taurine has been studied in the superfused rat cerebellar cortex and dorsal medulla in vivo. In the cerebellum both elevated potassium and electrically induced depolarization consistently produced marked increases in the efflux of exogenously applied taurine in a calcium-dependent fashion. Veratridine-stimulation evoked a large Ca2+-independent taurine efflux which was, however, prevented by tetrodotoxin. In the dorsal medulla, both high K+ and veratridine induced a clear Ca2+-independent increase in taurine efflux. Electrical stimulation was always ineffective in changing taurine efflux from the dorsal medulla. These data strongly support a possible neurohumoral role for taurine in the cerebellum but not in the dorsal medulla.

High affinity uptake and Ca2+-dependent release of glutamic acid in the developing cerebellum

This study was undertaken in order to evaluate the postulated role of glutamic acid as the neurotransmitter for the parallel fibers of the cerebellar cortex. We studied the Ca2+-dependent release and the high affinity uptake of glutamic acid in the developing cerebellum. The Ca2+-dependent release of glutamic acid from cerebellar molecular layer during development closely follows the time course of parallel fibers synaptogenesis. Little glutamic acid release was observed at 15 days, then it increased to the adult values at the 21st postnatal day. In the rat the bulk of synapses of the parallel fibers appear between the 15th and the 21st postnatal days, the time at which the nerve terminals of the climbing fibers, the other excitatory input to the Purkinje cells, are already developed. An enhanced Na+-dependent, high affinity uptake of glutamic acid was observed in the developing cerebellum relative to the adult rat. That this higher accumulation of glutamic acid is not related to a releasable pool is suggested by the fact that an enhanced glutamic acid, Ca2+-dependent release relative to the adult was not observed. These results support the view that glutamic acid is the transmitter for the cerebellar parallel fibers.

Release of endogenous and accumulated exogenous amino acids from slices of normal and climbing fibre-deprived rat cerebellar slices

Efflux of various amino acids from slices of rat cerebellar hemispheres was determined under resting or depolarizing conditions. It was increased under high K+(50 mM) as compared to low K+ (5 mM) conditions by 1258 pmol/mg protein for aspartate, 478 for gamma-aminobutyric acid (GABA), 44,693 for glutamate, and 615 for glycine. These were significantly higher than the corresponding values obtained under low-Ca2+ (0.1 mM), high-Mg2+ (12 mM) conditions, whereas for 11 other amino acids the K+-induced efflux was similar under normal and low-Ca2+ concentrations. The K+-induced efflux of exogenously accumulated L-[3H]aspartate, D-[3H]aspartate, and L-[3H]glutamate was higher by factors of 2, 5.8, and 6.3, respectively, under normal Ca2+ conditions, as compared with low-Ca2+, high-Mg2+ conditions. After climbing fibre degeneration induced by destruction of the inferior olive with 3-acetylpyridine, release of endogenous aspartate and exogenous L-[3H]glutamate and D-[3H]aspartate was significantly reduced, by 26%, 38%, and 27%, respectively. These results support the hypothesis that climbing fibres may use aspartate or a related compound as a neurotransmitter. In rat cerebellar tissue, L-[3H]glutamate and L-[3H]aspartate differ in several aspects: (1) L-[3H]glutamate uptake was 4 times higher than that of L-[3H]aspartate; (2) fractional rate constant of K+- evoked release of L-[3H]aspartate was 7% X 2.5 min-1, and of L-[3H]glutamate 36% X 2.5 min-1; and (3) specific activity of L-[3H]glutamate in the eluate collected during K+ stimulation was 3.5 times the value in the tissue, whereas for L-[3H]aspartate, specific activities in the eluate and tissue were similar.

Voltage clamp analysis of the effect of excitatory amino acids and derivatives on Purkinje cell dendrites in rat cerebellar slices maintained in vitro

A voltage clamp analysis of the effects of L-aspartate, L-glutamate and related derivatives on Purkinje cell dendrites was performed in rat cerebellar slices maintained in vitro. Short iontophoretic pulse applications of L-aspartate and L-glutamate in the dendritic field of Purkinje cells induced dose-dependent inward currents with fast onset and recovery. Quisqualate application also gave rise to well developed inward currents with fast onset and slow recovery, whereas N-methyl-D,L-aspartate had no or little effect on Purkinje cell membranes unless prolonged (several seconds) applications were used. Steady applications of low doses of N-methyl-D,L-aspartate much more severely depressed L-aspartate than L-glutamate mediated responses, whereas inward currents due to quisqualate were unaffected. Inward currents due to quisqualate were often more reduced than those due to L-aspartate by steady applications of 2-amino-5-phosphonovalerate, and the antagonistic action of this drug on responses due to L-glutamate was very weak. These results suggest that receptors of Purkinje cells for glutamate and aspartate are different, and are also different from N-methyl-D-aspartate and quisqualate receptors.

Turnover rates of amino acid neurotransmitters in regions of rat cerebellum

The turnover rates of aspartate, gamma-aminobutyric acid (GABA), glutamate, glutamine, alanine, serine, and glycine were measured in five regions of rat cerebellum. Turnover rates of the putative neurotransmitters (aspartate, glutamate, and GABA) were 2-20-fold higher than those of alanine and serine, and generally consistent with the proposed neurotransmitter functions for these amino acids. However, glutamate turnover was high and similar in magnitude in the deep nuclei and granule layer, suggesting possible release, not only from parallel fibers, but from mossy fibers as well. The differential distribution of turnover rates for GABA supports its neuronal release by Purkinje, stellate, basket, and Golgi cells, whereas aspartate may be released by both climbing and mossy fibers. The distribution of glycine turnover rates is consistent with release from Golgi cells, whereas alanine may be released from granule cell parallel fibers. Turnover rates measured in two other motor areas, the striatum and motor cortex, indicated that utilization of these amino acid neurotransmitters is differentially distributed in brain motor regions. The data indicate that turnover rate measurements may be useful in identifying neurotransmitter function where content measurements alone are insufficient.

Evidence for taurine as an inhibitory neurotransmitter in cerebellar stellate interneurons: selective antagonism by TAG (6-aminomethyl-3-methyl-4H,1,2,4-benzothiadiazine-1,1-dioxide)

Using a novel amino acid antagonist, TAG (6-aminomethyl-3-methyl-4H,1,2,4-benzothiadiazine-1,1-dioxide), intradendritic electrophysiological investigations were carried out to obtain evidence for taurine as a neurotransmitter in the cerebellum. The hyperpolarizing action of taurine on Purkinje cell dendrites in guinea pig cerebellar slices was selectively antagonized by TAG (200 microM), while the actions of GABA, glycine and beta-alanine were virtually unaffected. TAG shifted the log dose-response curve of the taurine action to the right in parallel, indicating a competitive antagonism. A hyperpolarizing synaptic potential which was evoked by electrical stimulation of the upper region of the cerebellar molecular layer and recorded from a Purkinje cell dendrite, was reversed to a depolarizing one at a membrane potential of -70 mV. The hyperpolarization induced by exogenously applied taurine was also reversed at the same potential. Moreover, TAG (200 microM) completely and reversibly blocked the synaptic potential. These results suggest that taurine may be an inhibitory neurotransmitter in stellate neuronal synapses on Purkinje cell dendrites.

Cultured cerebellar neurons: endogenous and exogenous components of Purkinje cell activity and membrane response to putative transmitters

Modified explant cultures of fetal rat cerebellum were developed for electrophysiological and pharmacological studies, at the membrane level, of Purkinje neurons. The goals of the present series of experiments were to identify possible endogenous and exogenous components to the electrical activity of Purkinje neurons, to assess the sensitivity of these neurons to putative excitatory and inhibitory neurotransmitters, and to characterize the membrane response to the transmitters. Intracellular recordings were made from Purkinje neurons, identified on a morphological basis, using conventional electrophysiological techniques. Virtually all Purkinje neurons displayed spontaneous activity. A contribution of both endogenous and exogenous components to the spontaneous activity was indicated by alterations in the pattern and amount of activity when the membrane potential was varied and by the characteristics of the individual potentials themselves. Several types of activity were considered to be endogenous: the most common type consisted of pacemaker-like potentials which generated a pattern of firing similar to that characterized as simple spike activity in previous in vivo studies; another type of endogenous activity consisted of large membrane depolarizations that evoked one or two spikes. These depolarizing responses were similar to the membrane response generated by climbing fiber input to Purkinje cells in vivo. The exogenous components to the spontaneous activity consisted of synaptic potentials including excitatory (EPSPs) and inhibitory (IPSPs) synaptic potentials and biphasic EPSP/IPSPs. Several putative transmitters thought to mediate these synaptic potentials were tested by focal micropressure application to determine if they could mimic the action of the endogenous transmitters. The putative transmitter glutamate depolarized the cultured Purkinje neurons and evoked action potentials, characteristics which were displayed by the excitatory synaptic potentials. The putative inhibitory transmitter GABA hyperpolarized the cultured Purkinje neurons and depressed activity, characteristics which were displayed by the inhibitory synaptic potentials. The putative inhibitory transmitters glycine and taurine were ineffective. Norepinephrine, the transmitter mediating the inhibitory input from the locus coeruleus to Purkinje neurons, was also tested. When applied in the microM range, NE effects were variable. When applied in the mM range, NE depressed the spontaneous activity in a manner suggestive of a presynaptic action.

The content of amino acids in the developing cerebellar cortex and deep cerebellar nuclei of granule cell deficient mutant mice

Glutamic acid is the only free amino acid to be clearly reduced in mature granule cell deficient cerebellum. The correlation between concentration of glutamic acid and extent of granule cell loss suggests that it may serve as a neurotransmitter. Curiously, in two neurological mouse mutants, glutamic acid is also decreased in the deep cerebellar nuclei where there are no granule cells. We have now examined the amino acid content of cerebellar cortex and deep cerebellar nuclei of the granule cell deficient mutants, weaver and staggerer, during the postnatal period in which granule cell development takes place. We have found: (1) an early and transient deficit in taurine in weaver cerebellar cortex during the period of granule cell migration, (2) deficits during the second postnatal week in taurine, aspartic and glutamic acids in both weaver and staggerer cerebellar cortex, (3) that aspartic and glutamic acid deficits result from failure to increase concentrations at the normal rate after birth rather than from a fall from normal levels, (4) decreased concentrations of glutamic acid but not of taurine and aspartic acids apparent in the deep nuclei of both weaver and staggerer at about the same time as in cerebellar cortex, (5) amino acid changes in weaver heterozygote cerebellum which result in values intermediate in magnitude between normal and homozygous weaver animals and (6) an early and persistent reduction in staggerer deep nuclei of gamma-aminobutyric acid (GABA), the Purkinje cell transmitter, indicating early denervation or lack of full innervation of deep nuclei by Purkinje cells.

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. 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