Sensitivity of Purkinje cell dendrites to glutamic acid

Enkephalin Expression in Spinal Cord Neurons is Modulated by Drugs Related to Classical and Peptidergic Transmitters

The effects of various neurotransmitter agonists and antagonists on the synthesis and release of methionine enkephalin (mENK) in neuronal cultures of mouse spinal cord and dorsal root ganglia have been measured. Blockade of electrical activity with tetrodotoxin between days 9 and 13 in culture caused a > 95% decrease in the number of mENK-immunoreactive neurons. This effect was also seen upon the blockade of glycine and beta-adrenergic receptors with strychnine and propranolol, respectively, and stimulation of GABA receptors with muscimol. Stimulation of beta-adrenergic receptors with isoproterenol, or blockade of glutamate and GABA receptors with 2-aminophosphonovalerate and strychnine, respectively, had a qualitatively opposite action on both the number of mENK-immunoreactive neurons and enkephalin peptide levels measured by radioimmunoassay. Application of substance P also enhanced the mENK cell number. These data suggest that, at least in the spinal cord, characteristics other than the average level of impulse activity in the afferent input may be critical to the regulation of expression of mENK.

Distribution of NMDA and AMPA receptors in the cerebellar cortex of rhesus macaques

The distribution of glutamate receptors in the cerebellar cortex of the rhesus macaque was examined by light microscopic immunocytochemistry using an antibody specific to the N-methyl-D-aspartate (NMDA) R1 receptor subunit (i.e. NMDAR1) as well as antibodies specific to alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunits (i.e. GluR1, GluR2/3, and GluR4). NMDAR1 immunolabeling was most prevalent in the Purkinje cell perikarya and dendrities, but was also significant in the stellate and basket cells of the granular layer and Golgi cells of the molecular layer. On the other hand, GluRl and GluR4 immunolabeling was concentrated principally in the processes of the Bergmann glia located in the vicinity of the Purkinje cell perikarya. Although GluR2/3 immunolabeling also occurred in these Bergmann glia processes as well as in the Bergmann fibers, it was more pronounced in the Purkinje cell perikarya and dendrites; additionally, significant GluR2/3 labeling was evident in the stellate and basket cells of the molecular layer and medium-size soma of the granular layer (most likely Golgi cells). In situ hybridization histochemistry (ISHH), using cRNA probes to NMDAR1. GluR1.GluR2, and GluR3, showed glutamate receptor mRNA distribution patterns consistent with those disclosed in the immunocytochemical study. Furthermore, the ISHH findings suggest that the positive immunocytochemical labeling of Purkinje cells with the GluR2/3 antibody is most likely due to the gene expression of both GluR2 and GluR3 AMPA receptor subtypes. Taken together, the results are potentially important for the elucidation of mechanisms that control aspects of cerebellar function, such as long-term depression.

NMDA receptor expression in the mouse cerebellar cortex

A detailed, light microscopic study on the distribution of the N-methyl- D-aspartate receptor subunit 1 (NMDAR1) was carried out with immunohistochemistry and in situ hybridization on the cerebellar cortex of the mouse. With a monoclonal antibody, labeling of Purkinje cell bodies varied from intense to negative, while heavy dendritic staining was limited to the proximal dendrites (unlike the rat, which also had heavily stained distal dendrites). In the granular layer, the cell bodies and and the dendritic shafts of Golgi II cells were only moderately stained, but very intense labeling was associated with granule cell bodies, and with their dendrites and dendritic endings in the glomeruli. The mossy and climbing fibers were negative. In situ hybridization with a cRNA probe showed levels and spatial distributions of NMDAR1 mRNA consistent with the immunolabeling pattern, in that signals were strongest in the granular and Purkinje cell layers and relatively low or absent in the molecular layer and white matter. The findings are consistent with the hypothesis that NMDAR1 may be especially well concentrated at the synaptic target sites of the mossy and climbing fibers. In the mouse, NMDAR1 at the parallel fiber sites associated with Purkinje cell spiny branchlets may differ from the rat in its level of expression or in its molecular configuration.

Inhibition of nigral dopamine neurons by systemic and local apomorphine: Possible contribution of dendritic autoreceptors

Peripheral administration of low doses of dopamine agonist apomorphine induces a strong and short-latency inhibition of dopamine neurons in the substantia nigra, presumably via the activation of somatodendritic autoreceptors. We studied the site of action of apomorphine in anesthetized rats using volume-controlled pressure microejection combined with single unit recordings. Microapplication of apomorphine in the immediate vicinity of nigral dopamine neurons did not mimic the effect of intravenous administration of apomorphine (50 micrograms/kg), regardless of the concentration or volume used (10(-10)-10(-2) M, 10-100 nl). In contrast, the inhibition produced by systemic apomorphine was mimicked by drug application at a site 300 microns lateral and 600 microns ventral from the recording site in the zona reticulata of the substantia nigra, a region rich in dendrites of dopamine neurons. The inhibition induced by such a distant application of apomorphine could be reversed by systemic injection of D2, but not D1, receptor antagonists. Non-dopaminergic substances such as GABA, bicuculline or lidocaine were more effective when ejected close to rather than distant from the recording site, in a manner opposite to that of apomorphine. Similar to apomorphine, dopamine and D2 receptor agonists were more potent when intranigral applications were made at sites distant from, rather than close to, the recorded dopamine cells. Ejection of D2 antagonists in the substantia nigra zona reticulata attenuated the inhibitory effect of subsequent systemic apomorphine. Our results, together with other previous studies on the location of D2 receptors on dopamine neurons, suggest that peripheral administration of low doses of apomorphine inhibits nigral dopamine neurons by acting at D2 receptors located on the dendrites of these neurons.

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.

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.

The effects of calcium channel agonists and antagonists on the release of endogenous glutamate from cerebellar slices

The effects of compounds acting at the calcium channel on neurotransmitter release are equivocal. We report here the effects of the antagonists, verapamil, diltiazem and nifedipine; the agonists, bay K8644 and the calcium ionophore, A23187 on the release of endogenous glutamate from rat cerebellar slices. Of these compounds, only verapamil and diltiazem modified glutamate release and these were effective at relatively high concentrations (greater than 1 x 10(-5) M). It is suggested that the high-affinity binding sites found in neuronal tissue for the dihydropyridine-like compounds are not involved in neurotransmitter release.

Effects of noradrenaline on the responsiveness of cultured cerebellar neurons to excitatory amino acids

The effects of noradrenaline (NA) on the responsiveness of cultured cerebellar neurons to excitatory amino acids were intracellularly investigated. NA applied to external medium to a final concentration of 10 microM or lower slightly decreased the firing frequency of spontaneous spikes, induced a small hyperpolarization or slightly increased the input resistance of Purkinje cells. In addition, bath-applied NA was found to enhance the depolarizations induced by iontophoretically applied glutamate and aspartate but to a smaller extent for the latter. These direct and modulating effects of NA were also observed when NA was applied by iontophoresis. The sites sensitive to iontophoresed NA were found to be not uniformly distributed but localized in restricted regions on individual Purkinje cells. The enhancement by NA of the glutamate or aspartate response was blocked by beta-adrenergic antagonists, propranolol or pindolol, and extracellularly applied cAMP mimicked the NA action. These results suggest the possibility that NA physiologically modulates excitatory amino acid-mediating synaptic transmission in the cerebellum probably by acting on beta-rather than alpha-adrenergic receptors.

Serotonin-glutamate interaction in rat cerebellum: involvement of 5-HT1 and 5-HT2 receptors

The effects of serotonin (5-HT) on the release of endogenous glutamate (GLU) in rat cerebellum were investigated in slices depolarized with 35 mM K+. The Ca2+-dependent release of GLU was potently inhibited by 5-HT in a concentration-dependent way. Release was also inhibited by the 5-HT1 receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and by the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI). The inhibition by 10 nM 5-HT was partly (35-40%) counteracted by the 5-HT2 receptor antagonist ketanserin but was fully blocked by the mixed 5-HT1/5-HT2 receptor antagonist methiothepin. The effect of 8-OH-DPAT was not affected by ketanserin but was totally antagonized by methiothepin, while the effect of DOI was entirely suppressed by ketanserin. Ketanserin or methiothepin themselves increased (by 23 and 55%, respectively, at 10 nM) the K+-evoked release of GLU. In conclusion the release of endogenous GLU in rat cerebellum can be inhibited by 5-HT through receptors of the 5-HT1 and 5-HT2 type. The enhancement of GLU release by ketanserin or methiothepin could suggest a tonic inhibition. The possible localization of the 5-HT receptors involved in the interaction with the GLU systems is discussed.

Development of spontaneous and glutamate-evoked activity is altered by chronic ethanol in cultured cerebellar Purkinje neurons

The effects of continuous exposure to ethanol on the cytological and physiological development of a central nervous system (CNS) neuron were studied using the cultured Purkinje neuron of the rat cerebellar cortex. Purkinje neurons in fetal rat brain cultures which are established at one day before birth show development comparable to that described in vivo in other studies. In culture, Purkinje neurons progress from immature rounded cells with fine neurites to mature neurons with a branched dendritic structure. These structural changes are accompanied by an increase in the duration and complexity of the excitatory response to glutamate, by transitions in the patterns of spontaneous activity, and by an increase in mean firing rate. Our results demonstrate that chronic exposure to a low concentration of ethanol (90 mg%; 19.5 mM) during development selectively alters the electrophysiological but not the morphological properties of Purkinje neurons. Specifically, ethanol treatment reduces the responsiveness of these neurons to glutamate, delays the expected developmental transitions in patterns of spontaneous activity, and induces increased spontaneous bursting activity, particularly at the stage of dendritic formation. Impairment of responsiveness to glutamate is significant in that it may reflect the compromise by ethanol of a major excitatory pathway in the cerebellar cortex, resulting from the decreased efficacy of glutamatergic input from parallel fibers. In contrast to the results of other studies using adult neurons as a model for the effects of ethanol, our work suggests that the developing CNS neuron does not become tolerant; that is, in the continuing presence of ethanol, it does not express physiological function equivalent to that of the control.

L-aspartate and L-glutamate binding sites in developing normal and 'nervous' mutant mouse cerebellum

This study concerns the ontogeny and the cellular localization of L-aspartate and L-glutamate binding sites in normal and 'nervous' mutant mouse cerebellar membranes. The binding kinetics revealed for L-aspartate a single binding system (Kd = 750 nM) and for L-glutamate also a single binding component of higher affinity (Kd = 344 nM). The pharmacological study, using various amino acid analogues, revealed a differential specificity for the binding sites of the two amino acids. The developmental study showed that the binding sites of both amino acids appear mainly during the second and third week of life, a period when parallel and climbing fiber synaptogenesis occurs, but they follow a slightly different developmental pattern. The study using 'nervous', mutant mouse cerebellum showed an age-dependent decrease of L-aspartate and L-glutamate binding, which coincides in time with the Purkinje cell degeneration in this mutant, indicating a cellular localization of these binding sites on the Purkinje cell membranes. These results suggest that L-aspartate and L-glutamate binding sites may be respectively associated with the postsynaptic target of climbing and parallel fibers on the Purkinje cell dendrites. However, the decrease of specific binding in 'nervous' mutant mouse cerebellum was about 50% for L-aspartate and 60% for L-glutamate, implying that a significant number of L-aspartate and L-glutamate binding sites are located on cerebellar elements other than the Purkinje cell membranes.

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.

Binding sites for [3H]glutamate and [3H]aspartate in human cerebellum

The binding of [3H]aspartate and [3H]glutamate to membranes prepared from frozen human cerebellar cortex was studied. The binding sites differed in their relative proportions, their inhibition by amino acids and analogues, and by the effects of cations. A proportion (about 30%) of [3H]glutamate binding was to sites similar to those labelled by [3H]aspartate. An additional component of [3H]glutamate binding (about 50%) was displaced by quisqualate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, and may represent a "quisqualate-preferring" receptor. Neither N-methyl-D-aspartic acid-sensitive nor DL-2-amino-4-phosphonobutyric acid-sensitive [3H]glutamate binding was detected.

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.

Effect of calcium ions on the sensitivities of cultured cerebellar neurons to glutamate and aspartate

Iontophoretically applied glutamate and aspartate induced depolarizations in immature (6-13 days in culture) and mature (25-45 days) cultured chick cerebellar neurons, immature neurons being less sensitive. The input resistances of the neurons were variously changed by these amino acids. Reversal potentials of the depolarizations induced by both amino acids were similar in either immature or mature neurons. The population of amino acid-sensitive neurons increased with maturation. In mature neurons, the amplitude of glutamate- or aspartate-induced depolarization was decreased by addition of 10 mM Ca2+ to normal Tyrode's solution, aspartate responses being decreased more greatly. In low-Na+ solution (2.7 mM), however, high Ca2+ significantly enhanced amino acid-induced depolarizations. In immature neurons, on the other hand, the amplitude of glutamate- or aspartate-induced depolarization was drastically and consistently increased when 10 mM Ca2+ was added either to normal solution or to the low-Na+ solution. These enhancing actions of Ca2+ were abolished by Mn2+, but only partially by 10 mM glutamic acid diethylester or 1 mM D-alpha-aminoadipate, though responses to both amino acids in normal solution were blocked by these antagonists at the same concentrations. These results suggest that calcium ions enhance the effect of glutamate and aspartate in immature neurons, possibly by interacting with the ionophores involved in amino acid responses.

Modulatory action of serotonin on glutamate-induced excitation of cerebellar Purkinje cells

The effects of serotonin (5-hydroxytryptamine, 5-HT) on glutamate-induced excitation of Purkinje cells were examined. Pulsatile iontophoretic applications of glutamate (1-25 nA) induced consistent increases in the spontaneous activity of Purkinje cells. When serotonin was applied continuously with currents that elicited minimum changes in the spontaneous rate, it inhibited or blocked glutamate-induced excitations significantly in most Purkinje cells. We also examined the effects of high currents of serotonin on spontaneous activity of Purkinje cells. High currents of serotonin induced 3 different effects: inhibitions, biphasic effects comprising transient inhibition followed by excitation, and excitations. Nonetheless, whether it inhibited or excited the activity of Purkinje cells, serotonin inhibited glutamate-induced excitations consistently. The effect of a putative 5-HT antagonist methysergide (UML) was also examined. Methysergide consistently attenuated or antagonized the inhibitory effects of serotonin on glutamate-induced excitations. This finding suggests strongly that inhibitory effects of 5-HT on glutamate excitations observed in the present study is the specific action of serotonin.

Electrophysiological studies on the postnatal development of intracerebellar nuclei neurons in rat cerebellar slices maintained in vitro. I. Postsynaptic potentials

The development of the synaptic responses of intracerebellar nuclei neurons was studied in the rat by the use of thick sagittal cerebellar slices maintained in vitro. It has been shown that functional excitatory synapses are present on these neurons from birth, probably due to climbing and/or mossy fiber collaterals; functional inhibitory synapses, due to monosynaptic projections of Purkinje cell axons onto intracerebellar nuclei, are present as early as postnatal day 2; and a more complex pattern of synaptic responses, including short latency excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs), longer latency IPSPs, and late depolarizing responses, can be elicited in nuclear neurons as early as postnatal day 3, indicating an early development of some complete functional cerebellar circuits involving the intracerebellar nuclei.

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.

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.

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.

Asymmetric distribution of acetylcholine receptors and M channels on prepyriform neurons

The responses of pyramidal neurons of rat prepyriform cortex to ionophoretic application of acetylcholine (ACh) were studied in a submerged, perfused brain slice. ACh excited some neurons but only if applied to an area near to the cut surface of the slice. This area contained the basal dendrites of the pyramidal cells and some cell bodies. No excitation was seen if ACh was applied at depths of 250 microns or more from the cut surface, an area which contained only apical dendrites, although the apical dendrites were very sensitive to excitatory amino acids such as aspartate (Asp) and glutamate (Glu). On all neurons which did not discharge to ionophoretic application of ACh, ACh potentiated the response to Glu and Asp. No potentiation of amino acid responses was obtained on apical dendrites. The potentiation had a time course similar to that of the discharge of neurons which fired to ACh. This observation suggests that pyramidal neurons have receptors for ACh on basal but not apical dendrites. The ACh response in the basal dendrite-soma region was elicted by pilocarpine and blocked by atropine but not curare. This was true whether the response studied was direct excitation or potentiation of the response to an amino acid. The ACh response was associated with a voltage-dependent increase in membrane resistance which had a slow time course and appeared to be due to a turning off of an M current, as described by Brown and Adams (1980) in sympathetic ganglion cells. The effects of ACh were minimal at the resting potential but increased with depolartization. ACh had no effect on the current-voltage relation of the cell, except at depolarized potentials of less than -60 mV. Ionophoretic application of Ba2+ to the basal dendritic region resulted in potentiation of the amino acid responses and sometimes induced a discharge similar to that of ACh. Since Ba2+ mimics the ACh response, presumably by a direct blockade of the M channel, the effects of Ba2+ on apical dendrites were tested to determine whether these dendrites contain M channels associated with a transmitter receptor other than ACh. However, Ba2+ did not induce potentiation in apical dendrites, suggesting that M channels are also restricted to the basal dendrites or cell bodies.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Modulatory action of taurine on the release of GABA in cerebellar slices of the guinea pig

For the purpose of demonstrating the action of taurine as a neuromodulator in addition to its suggested neurotransmitter function, the effects of taurine and muscimol on the depolarization-induced Ca-dependent release of [3H] gamma-aminobutyric acid ([3H]GABA) and L-[3H]glutamate in cerebellar slices from guinea pigs were investigated. The release of [3H]GABA was found to be greatly decreased by a GABA agonist, muscimol, and by taurine, but not by glycine. The release of L-[3H]glutamate was little affected by taurine. The release of [3H]GABA, was enhanced by bicuculline and strychnine, but not by picrotoxin, and the suppressive action of muscimol on the GABA release was antagonized by bicuculline, picrotoxin, and strychnine, suggesting the possible existence of presynaptic autoreceptors for GABA in the cerebellum. The suppressive action of taurine on the release of [3H]GABA, on the other hand, was blocked only by bicuculline. These results suggest that taurine reduced the release of [3H]GABA from cerebellar slices by acting on the GABA autoreceptors or, more likely, on other types of receptors that are sensitive to bicuculline. As a possible mechanism for this modulatory action of taurine, the blockade by this amino acid of the influx of Ca2+ into cerebellar tissues was tentatively suggested.

Noradrenergic modulation of glutamate release in the cerebellum

The effect of alpha- and beta-adrenergic agonists has been studied on the release of newly synthesized [3H]glutamate and [3H]GABA from slices of rat cerebellum. The beta 2-adrenergic agonist salbutamol, and also noradrenaline in the presence of the alpha-adrenergic antagonist phentolamine, both potentiated the K+-evoked release of [3H]glutamate. This potentiation appears to be mediated by adenylate cyclase activation. No effects of beta-adrenergic stimulation were observed on [3H]GABA release. The alpha-adrenergic agonist clonidine inhibited both the [3H]glutamate and the [3H]GABA release evoked by K+. The results suggest that noradrenergic modulation of cerebellar activity may have a presynaptic as well as postsynaptic origin.

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.

Climbing fibre induced depression of both mossy fibre responsiveness and glutamate sensitivity of cerebellar Purkinje cells

1. In high decerebrate rabbits, cells were sampled extracellularly from the rostral flocculus. Purkinje cells were identified by their characteristic responses to stimulation of the contralateral inferior olive. Identification of basket cells was based on the absence of olivary responses and also on their location in the molecular layer adjacent to identified Purkinje cells. Mass field potentials in the flocculus were also studied.2. Single pulse stimulation of a vestibular nerve, either ipsilateral or contralateral, at a rate of 2/sec excited Purkinje cells with a latency of 3-6 msec. This early excitation represents activation through vestibular mossy fibres, granule cells and their axons (parallel fibres). Similar early excitation also occurred in putative basket cells.3. Conjunctive stimulation of a vestibular nerve at 20/sec and the inferior olive at 4/sec, for 25 sec per trial, effectively depressed the early excitation of Purkinje cells by that nerve, without an associated change in spontaneous discharge. The depression recovered in about ten minutes. This recovery was followed by the onset of a slow depression lasting for an hour.4. Conjunctive vestibular-olivary stimulation produced no such depression in the following responses: early excitation in Purkinje cells induced from the vestibular nerve not involved in the conjunctive stimulation; early excitation in putative basket cells from either vestibular nerve; inhibition or rebound facilitation in Purkinje cells following the early excitation; vestibular-evoked field potentials in the granular layer and white matter of the flocculus. These observations lead to the conclusion that the depression occurs specifically at parallel fibre-Purkinje cell synapses involved in conjunctive stimulation.5. Ionophoretic application of glutamate to Purkinje cells in conjunction with 4/sec olivary stimulation depressed the glutamate sensitivity of Purkinje cells; aspartate sensitivity was depressed to a much lesser degree. The depression diminished in about 10 min, but this recovery was succeeded by a slow depression lasting for an hour. The depression was seen only when glutamate sensitivity was relatively high, suggesting that the micro-electrode was impinging onto Purkinje cell dendrites. These observations suggest that subsynaptic chemosensitivity of Purkinje cells to the putative neurotransmitter of parallel fibres is involved in the depression observed after conjunctive stimulation of a vestibular nerve and the inferior olive.6. The present results are consistent with the Marr-Albus assumption concerning plasticity of cerebellar neuronal networks.

Effect of glutamate, aspartate and related derivatives on cerebellar purkinje cell dendrites in the rat: an in vitro study

1. The responses of Purkinje cells to short duration (pulse) ionophoretic applications of L-aspartate (L-asp), L-glutamate (L-glu), N-methyl DL-aspartate (NMDLA) and quisqualic acid in their dendritic fields were studied in vitro on sagittal slices of lobules IX and X of the adult rat cerebellum.2. Pulse application of L-asp or L-glu evoked transient and dose-dependent increases in the firing rate of the simple spikes recorded extracellularly as single units. When the ionophoretic electrode was positioned in the dendritic field of the Purkinje cells, the lowest thresholds for L-glu and L-asp mediated excitations of the cells were as low as 25 and 35 pC respectively, with a latency for maximal responses as brief as 7 ms.3. In intracellular recordings these excitatory responses consisted of depolarizations of up to 18 mV in amplitude and with depolarizing slopes up to 0.52 mV/ms. They were generally unaccompanied by changes in cell input resistance in contrast to the marked decrease which occurred in response to steady applications of large doses of L-asp and L-glu.4. The spatial distribution of the excitatory sites confirmed that the dendritic sensitivity to L-glu was greater than that of the soma and showed that the same was true for L-asp. In 34% of cells the sensitivity for L-asp declined markedly in the upper region of the molecular layer, whereas it remained high for L-glu; no such differential sensitivity was detected in the remaining 66% of cells.5. Inhibitory responses, antagonized by 10(-5) M-bicuculline in the bath, were also induced in Purkinje cells by L-glu and L-asp when the ionophoretic electrode was withdrawn from the excitatory sites by as little as 8 mum and up to 40 mum upward or downward along the track of parallel fibres or positioned as far as 250 mum laterally.6. Whenever it was applied in the molecular layer, the pulse application of NMDLA elicited no excitatory response in Purkinje cells recorded extra or intracellularly. However, slow depolarizations accompanied by a slight increase in cell input resistance were obtained with steady applications of 20-50 nA of the drug for 20-30 s.7. In contrast, pulse application of quisqualic acid appeared to have the same type of fast excitatory effect on Purkinje cells as L-asp and L-glu, but its potency was greater and its action more prolonged. Furthermore, its steady application led to an abrupt and marked decrease in cell membrane resistance.8. The excitatory effects of L-asp, L-glu and quisqualic acid were antagonized by L-glutamic acid diethyl ester more consistently than by D-alpha-aminoadipate, suggesting together with previous observations that L-asp and L-glu act on Purkinje cells via quisqualic acid rather than via NMDLA receptors.

Excitatory effect of L-aspartate and L-glutamate on Purkinje cells in rat cerebellum

L-aspartate and L-glutamate were microiontophoretically applied onto Purkinje cells and unidentified cerebellar neurons of urethane-anesthetized rats. Both amino acids produced a dose-dependent increase in the spontaneous firing rate of all cells tested. Fifty-three percent (8 of 15 cells) of the dose-response relationships for L-aspartate as compared to those for L-glutamate on Purkinje cells were not parallel, implying different mechanisms of action (suggesting different receptors). On these 8 Purkinje cells, L-glutamate was three times more potent than L-aspartate. Only thirty-three percent of the dose-response relationships (8 of 24 cells) for the two agents on the unidentified cerebellar cells were not parallel. There was no statistical difference in the potency of L-aspartate as compared to L-glutamate on these particular cells. Tests for antagonism on Purkinje cells revealed L-glutamic acid diethyl ester (GDEE) to be a more effective blocker of L-aspartate than of L-glutamate while DL-alpha-aminoadipic acid (DL alpha AA)) was not selective in antagonizing the action of either amino acid. These data are discussed in terms for L-aspartate functioning as a neurotransmitter in the cerebellum of rat and possessing receptor sites distinct from those for L-glutamate.

Autoradiographic localization of specific kainic acid binding sites in pigeon and rat cerebellum

The pigeons cerebellum contains a unique kainic acid binding site characterized by a relatively low binding affinity and cooperative binding properties. Dry-mount autoradiographs of incubated tissue sections demonstrate that this type of binding site is exclusively localized in the molecular and Purkinje cell layers of the cerebellar cortex, suggesting that it is related to the Purkinje cell dendritic tree. The affinity, cooperativity and pharmacological properties of the binding were not modified by prior perfusion of the brain with a low concentration of formaldehyde. Moreover, the maximal binding was higher in tissue sections than in membrane preparations. The selective localization and specific kinetic properties of this binding site suggest that it may play a physiological role. The high affinity type of binding site, which is present in both rat and pigeon cerebellum, is localized in the molecular, and to a lesser extent, the granular layer. This may imply that this second binding site is related to other types of cellular elements, such as the granule cells.

Morphological and electrophysiological characteristics of rat cerebellar slices maintained in vitro

1. The morphological and electrophysiological characteristics of sagittal cerebellar slices of adult rat cerebellum maintained in vitro were studied. 2. The ultrastructural preservation of the different neuronal cell types in many areas of these slices after 2-3 h incubation was very similar to that observed in material fixed in situ. A limited degree of glial swelling was observed in some regions. 3. The conduction velocity of parallel fibres was within the normal in vivo range and the fibres retained their ability to activate Purkinje cells and inhibitory interneurones. 4. Purkinje cells, recorded intrasomatically, responded to white matter stimulation with characteristic antidromic activation and climbing fibre responses, and typical parallel fibre responses were evoked following parallel fibre stimulation. 5. Climbing fibre excitatory post-synaptic potentials (e.p.s.p.s) were very similar whether recorded in the dendrites or somata of Purkinje cells. By contrast, marked differences in the associated spike potentials were evident, the initial fast, low-threshold somatic spike appearing in the dendrites as a slow, high-threshold spike. The secondary spikes, both in the soma and dendrites, were of the latter type. 6. The initial somatic spike was readily inactivated by cell depolarization but resisted moderate hyperpolarization, whereas the converse was true for the slow, high-threshold spikes recorded in the dendrites. These differences suggest that these responses are generated in the soma and in the dendrites respectively. 7. Climbing fibre and parallel fibre e.p.s.p.s recorded in Purkinje cell somata were reversed under depolarizing current injected through the recording micro-electrode. As in vivo, the parallel fibre e.p.s.p.s was more sensitive to injected current than the climbing fibre e.p.s.p. in several instances, despite the more proximal location of the synapses involved.

Levels of glutamate, aspartate, GABA, and taurine in different regions of the cerebellum after x-irradiation-induced neuronal loss

The levels of glutamate (Glu), aspartate (Asp), gamma-amino-n-butyric acid (GABA), and taurine (Tau) were determined in the cortex, molecular layer, and deep nuclei of cerebella of adult rats exposed to X-irradiation at 12-15 days following birth (to prevent the acquisition of late-forming granule cells; 12-15x group) and 8-15 days following birth (to prevent the acquisition of granule and stellate cells; 8-15x group). Also, the levels of the four amino acids were measured in the crude synaptosomal fraction (P2) isolated from the whole cerebella of the control, 12-15x, and 8-15x groups. The level of Glu was significantly decreased by (1) 6-20% in the cerebellar cortex; (2) 15-20% in the molecular layer; and (3) 25-50% in the P2 fraction of the X-irradiated groups relative to control values. The content of Glu in the deep nuclei was not changed by X-irradiation treatment. Regional levels of Asp were unchanged by X-irradiation, while its level in P2 decreased by 15-30% after treatment. The levels of GABA and Tau in the molecular layer, deep nuclei, or P2 were not changed in the experimental groups. However, there was a 15% increase in the levels of GABA and Tau in the cerebellar cortex of the 8-15x group relative to control values. The data support the proposed role of glutamate as the excitatory transmitter released from the cerebellar granule cells but are inconclusive regarding a transmitter role for either Tau or GABA from cerebellar stellate cells.

On the postsynaptic action of glutamate in frog spinal motoneurons

In the isolated frog spinal cord depolarization of motoneurons (MNs) induced by glutamate (GLUT) was not accompanied by measurable changes of neuronal input resistance when chemical synaptic transmission was blocked by Mn2+ or Mg2+. The GLUT depolarization was, however, paralleled by a considerable increase in K+ in the extracellular space. To clarify, whether the GLUT depolarization was exclusively due to a reduction of the transmembrane K+ gradient or whether ion conductances not detectable by measurements of neuronal input resistance were involved, membrane potential (MP) was plotted semilogarithmically versus extracellular K+ activity (aKe+). During experimental elevation of aKe+ the function delta MP/dec. delta aKe+ was found to agree fairly with the Nernst equation. The slope of this function was much steeper during GLUT superfusion, indicating an influx of positive ions. The elevation of aKe+ during the GLUT action can mimic postsynaptic effects by release of transmitter from presynaptic terminals synapsing with the recorded cell. In vivo preparations do not allow blockade of chemical synaptic transmission. Therefore, it is impossible to decide, whether the recorded cell is depolarized either postsynaptically by GLUT or by K+ release from surrounding GLUT sensitive cells. As an experimental proof of the postsynaptic GLUT action is not feasible in such preparations, the ubiquitous action of GLUT in the CNS may have been overestimated.

Glutamate as the neurotransmitter of cerebellar granule cells in the rat: electrophysiological evidence

1 Glutamate and the excitatory aminoacid antagonist, alpha-aminoadipic acid (alphaAA), have been applied by microiontophoresis to Purkinje cells in the rat cerebellum. 2 Glutemate produced excitation of Purkinje cells and alpha AA selectively reduced that excitation without affecting responses to acetylcholine or hydrogen ions. 3 Monosynaptic spikes were evoked in Purkinje cells by stimulating the parallel fibres. alpha AA had little effect on these spikes when applied alone. 4 When the Purkinje cell excitability was reduced by the iontophoresis of gamma-aminobutyric acid, alpha AA then produced railure of the monosynaptic spike on 10 of 13 Purkinje cells, in doses shown to be selectively antagonistic towards aminoacids. 5 These results support neurochemical evidence that glutamic acid may be the neurotransmitter released by granule cell parallel fibres.