A rapid procedure for obtaining a preparation of large fragments of the cerebellar glomeruli in high purity

Mechanism of action of rab3A in mossy fiber LTP

In mossy fiber synapses of the hippocampal CA3 region, LTP is induced by cAMP and requires the synaptic vesicle protein rab3A. In contrast, CA1-region synapses do not exhibit this type of LTP. We now show that cAMP enhances glutamate release from CA3 but not CA1 synaptosomes by (1) increasing the readily releasable pool as tested by hypertonic sucrose; (2) potentiating release evoked by KCl depolarization, which opens voltage-gated Ca2+ channels; and (3) by enhancing Ca2+ action on the secretory apparatus as monitored by the Ca2+-ionophore ionomycin. In rab3A-deficient synaptosomes, forskolin still enhances KCl- and sucrose-induced glutamate release but not ionomycin-induced release. Our results show that cAMP has multiple actions in mossy fiber synapses, of which only the direct activation of the secretory apparatus requires rab3A and functions in mfLTP.

Increase in syntaxin 1B and glutamate release in mossy fibre terminals following induction of LTP in the dentate gyrus: a candidate molecular mechanism underlying transsynaptic plasticity

A growing body of evidence suggests that modulation of certain proteins of the exocytotic machinery is, in part, involved in the biochemical changes that underlie long-term synaptic plasticity. We have previously shown that the induction of long-term potentiation (LTP) at perforant path to dentate granule cell synapses in the rat hippocampus induces changes in the mRNA levels of syntaxin 1B and synapsin I, known to be involved in neurotransmitter release. Immunohistochemical staining suggested that concomitant changes in these proteins occurred at mossy fibre synapses, downstream of those synapses at which LTP was induced, leading us to postulate that such a mechanism might underlie a form of transsynaptic plasticity. Here we have used a specific mossy-fibre synaptosome preparation to quantify levels of proteins and measure, using a chemiluminescent glutamate assay, depolarization-induced glutamate release from these synaptosomes after induction of LTP in the dentate gyrus in vivo. We show that 5 h after the induction of LTP, there is an increase in the protein levels of syntaxin 1B and, although to a lesser extent, the synapsins I and II, associated with an increase in depolarization-induced release of glutamate within these terminals. Increases in both the protein levels and glutamate release were not observed when dentate gyrus LTP was blocked by an NMDA receptor antagonist. From these results we propose a molecular mechanism for the propagation of synaptic plasticity through hippocampal circuits.

Region-specific phosphorylation of rabphilin in mossy fiber nerve terminals of the hippocampus

In mossy fiber synapses of the CA3 region of the hippocampus, long-term potentiation (LTP) is induced presynaptically by activation of cAMP-dependent protein kinase A (PKA). Rab3A is a synaptic vesicle protein that regulates vesicle fusion and is essential for mossy fiber LTP. Rab3A probably acts via two effector proteins, rabphilin and RIM, of which rabphilin is an in vitro substrate for PKA. To test if rabphilin is phosphorylated in nerve terminals and if its PKA-dependent phosphorylation correlates with the PKA-dependent induction of LTP in mossy fiber terminals, we have studied the phosphorylation of rabphilin in synaptosomes isolated from the CA1 and CA3 regions of the hippocampus. Rabphilin was phosphorylated in both CA1 and CA3 synaptosomes. However, when we treated the CA1 and CA3 synaptosomes with forskolin (an agent that enhances PKA activity) or induced Ca2+ influx into synaptosomes with high K+, rabphilin phosphorylation was increased selectively in mossy fiber CA3 synaptosomes, but not in CA1 synaptosomes. In contrast, the phosphorylation of synapsin, studied as a control for the specificity of the region-specific phosphorylation of rabphilin, was augmented similarly by both treatments in CA1 and CA3 synaptosomes. These results reveal that the phosphorylation states of two synaptic substrates for PKA and CaM KII, rabphilin and synapsin, are regulated differentially in a region-specific manner, an unexpected finding because rabphilin and synapsin are similarly present in CA1 and CA3 synaptosomes and are colocalized on the same synaptic vesicles. The region-specific phosphorylation of rabphilin agrees well with the restricted induction of LTP by presynaptic PKA activation in mossy fiber, but not CA1, nerve terminals.

Nitric oxide inhibition of the depolarization-evoked glutamate release from synaptosomes of rat cerebellum

Effects of nitric oxide (NO) on release of amino acid transmitter were investigated by superfusion of synaptosomes prepared from rat cerebellum. After constant basal levels of amino acid release were obtained, exposure to a depolarizing concentration of KCl (30 mM) evoked 4.05-, 2.18- and 3.00-fold increases in release of glutamic acid (Glu), aspartic acid (Asp) and gamma-aminobutyric acid (GABA) from synaptosomes. The perfusion with NO-donors inhibited the evoked increases in release of Glu and Asp in a concentration-dependent manner, but not that in GABA release. A membrane-permeable analog of cyclic GMP, but not that of cyclic AMP, caused a similar reduction in the evoked release. The concentration of nitroprusside to increase cyclic GMP levels corresponded to that of nitroprusside to reduce the evoked release. These data suggest that NO may directly act upon the nerve terminals to inhibit release of excitatory amino acid transmitters.

5-HT2 presynaptic receptors mediate inhibition of glutamate release from cerebellar mossy fibre terminals

'Giant' synaptosomes originating from mossy fibre terminals and having sedimentation properties different from those of standard synaptosomes were obtained from rat cerebellum. Exposure of superfused giant synaptosomes to 15 mM KCl caused the release of endogenous glutamate in a largely (about 80%) calcium-dependent manner. The K(+)-evoked overflow of glutamate was inhibited in a concentration-dependent manner by 5-hydroxytryptamine (5-HT) and by the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI), but not by the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The effects of 5-HT and DOI were quite potent, already reaching significant inhibition (about 25%) at 10 nM. The 5-HT2 receptor antagonist ketanserin counteracted the inhibitory effect of 5-HT. In cerebellar slices, ketanserin increased on its own the calcium-dependent K(+)-evoked release of glutamate and this effect was not prevented by tetrodotoxin (TTX). The results support the idea that cerebellar mossy fibres use glutamate as a transmitter and show that the release of glutamate can be inhibited via presynaptic heteroreceptors of the 5-HT2 type probably localized on the mossy fibre terminals.

An improved method for the preparation of rat cerebellar glomeruli

Cerebellar glomeruli consist of large portions of the mossy fiber giant terminal, granule cell dendrites and Golgi neuron terminals. By modifying previously reported procedures we have developed a new method for bulk preparation of this polysynaptic complex from rat cerebellum. We obtained well preserved isolated glomeruli of satisfactory purity and homogeneity as indicated by electron microscopy and by determination of appropriate biochemical markers. The method is fast and simple, and it provides a glomerular fraction suitable for investigation of neurotransmitter receptors.

Non-NMDA excitatory amino acid receptors in a subcellular fraction enriched in cerebellar glomeruli

In the internal granular layer of the cerebellar cortex the polysynaptic complexes called glomeruli consist mainly of homogeneous populations of glutamatergic and GABAergic synapses, both located on granule cell dendrites. A subcellular fraction enriched in glomeruli was prepared from rat cerebellum, and the distribution of the different types of NMDA and non-NMDA glutamate binding sites was studied in the membranes derived from this fraction (fraction G) as compared to that in the membranes prepared from a total cerebellar homogenate (fraction T). Cl-/Ca2+ independent [3H]glutamate binding sites were not abundant and could be reliably measured only in fraction G. Cl- dependent/Ca2+ activated [3H]glutamate binding sites were more abundant and exhibited a single KD in both fractions G and T. Quisqualate, NMDA, kainate, L-AP4 and trans-ACPD inhibited [3H]glutamate binding to different extents in the two membrane fractions. Quisqualate sensitive sites were predominant in all cases but more abundant in fraction T than in fraction G. An opposite distribution was observed for the NMDA sensitive binding sites while kainate sensitive binding sites were scarce everywhere. Trans-ACPD, a ligand presumed selective for metabotropic glutamate binding sites, displaced [3H]glutamate from fraction T but nor from fraction G, suggesting the absence of these sites from glomeruli. Similarly, no L-AP4 sensitive sites were present in fraction G while they were abundant in fraction T. Binding sites associated with ionotropic receptors of the quisqualate type were determined by measuring [3H]AMPA binding. The density of the high affinity [3H]AMPA binding sites in fraction T was twice as high as in fraction G, indicating that these sites are abundant in structures other than glomeruli.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparative study and partial characterization of multi-uptake systems for gamma-aminobutyric acid

Previous work by the authors had indicated that synaptosome-enriched preparations from the cerebral cortex of the rat contained a high-, a medium-, and a low-affinity uptake system for gamma-aminobutyric acid (GABA). The present study demonstrated that this phenomenon also prevailed in synaptosomes from rat diencephalon, mesencephalon, and cerebellum, although the Vmax values for the high- and medium-affinity systems in the cerebellum were very low relative to those of the other regions. When a different type of preparation containing nerve endings (glomeruli) was obtained from the cerebellum, it possessed a Vmax value for the high-affinity system that was more similar to that for the corresponding system in synaptosomes from the other brain regions. In contrast to the above situation, synaptosomes from rat olfactory bulb lacked the low-affinity uptake system, as did synaptosomes from dog olfactory bulb. The aspartate/glutamate uptake systems, as measured with D-aspartate, provided a regional pattern quite different from those of GABA uptake. Only two uptake systems, a high- and low-affinity system, were observed in all regions tested. All three GABA uptake systems were present in cortical synaptosomes from the mouse, hamster, and guinea pig, and all three systems were sodium dependent, energy dependent, temperature sensitive, and totally inhibited by nipecotic acid.

Neurotransmitters of the cerebellar glomeruli: uptake and release of labeled gamma-aminobutyric acid, glycine, serotonin and choline in a purified glomerulus fraction and in granular layer slices

We have studied some properties of the uptake and release of labeled gamma-aminobutyric acid (GABA), glycine, serotonin and choline in a purified fraction of glomeruli and in slices of the granular layer of the rat cerebellum. The uptake of both GABA and glycine into the glomerulus particles was dependent on the presence of Na+ in the medium. In contrast, the uptake of both serotonin and choline was Na+-independent. In slices of the granular layer also a slight Na+-dependence was observed for both serotonin and choline uptake; imipramine and hemicholinium partially inhibited the uptake of serotonin and choline, respectively. Choline uptake into the glomerulus particles showed two components, with apparent Km values of 16.8 and 102 microM. GABA release was stimulated by K+-depolarization about 100% (peak stimulation) and this value was reduced to 50% when Ca2+ was omitted. The release of glycine was stimulated more rapidly and notably than GABA (200%) and this stimulation was completely abolished in the absence of Ca2+. Serotonin release from the glomerulus particles was only slightly stimulated by depolarization, but this stimulation was strictly Ca2+-dependent. In slices of the granular layer, this stimulation was considerably larger (about 40%) and it was also almost totally dependent on Ca2+. In contrast, after loading with labeled choline the release of radioactivity from both the glomerulus particles and the cerebellar slices was not stimulated at all by K+-depolarization, either in the presence or in the absence of Ca2+. Most of the radioactivity released spontaneously corresponded to choline, and only a small proportion (8-14%) to acetylcholine. From the results of the release experiments and taking into account the pertinent data from the literature, it is concluded that GABA and glycine are probably the transmitters of different populations of Golgi axon terminals, whereas serotonin might be the transmitter of at least a certain population of the mossy fiber giant terminals, in the rat cerebellar glomeruli. In contrast, acetylcholine does not seem to have any transmitter role in the synaptic structures of the glomeruli.

Uptake, exchange, and release of GABA by cerebellar glomeruli

Glomerular particles were isolated from the bovine cerebellar vermis and studied in vitro to further assess the possibility that gamma-aminobutyric acid (GABA) is utilized as a neurotransmitter in this synaptic complex. Cerebellar glomeruli accumulated [3H]GABA at two different high affinity sites, with affinities (KT) of 2.2 X 10(-6) M and 3.0 X 10(-5) M. These uptake sites could not be distinguished on the basis of their temperature sensitivities, sodium dependence, substrate specificities or responses to metabolic inhibitors. Although an exchange process contributed to the uptake measured in these experiments, a considerable amount of the [3H]GABA accumulated by glomerular particles was stored in an osmotically-sensitive, nonexchangeable pool. Glomerular particles preloaded with [3H]GABA exhibited a Ca2+-independent release of this amino acid in response to membrane depolarization. However, when preloaded glomerular particles were exposed to unlabeled GABA, which presumably displaced [3H]GABA from the exchangeable pool, a K+-evoked and Ca2+-dependent release of the remaining [3H]GABA occurred. The observed net uptake, together with the depolarization-induced and Ca2+-dependent release, of [3H]GABA from glomerular particles supports the suggestion that functionally active GABAergic synapses are present in these structures.

Isolation of glomeruli from areas of bovine cerebellum and comparison of [3H]serotonin uptake

A method for bulk preparation of glomerular particles from subdivisions of the bovine cerebellum is presented. This method represents a modification of that previously reported by Hajos et al. [8], which increases the yield of glomerular protein by five-fold (5.5 mg/g wet wt) without compromising structural integrity or homogeneity. In addition, it offers the advantage of allowing one to study intraregional variations in the metabolic properties of cerebellar glomeruli. [3H]Serotonin (5-HT) uptake was measured in this preparation and it was demonstrated that glomeruli possess an active high affinity mechanism for this substrate. Comparison of [3H]5-HT uptake by glomeruli isolated from the cerebellar cortices of the lateral hemispheres and vermis revealed no differences in their kinetic properties.

Distribution of GABA receptors in the rat cerebellum

The distribution of GABA receptors in the cerebellum is not homogeneous. In comparison with detergent-treated membranes from the whole tissue the number of [3H]muscimol binding sites per mg protein (Bmax) is about doubled in preparations enriched in large fragments of the cerebellar glomeruli, and it is about one-third in the dissected deep nuclei. On the other hand, the apparent affinity (Kd) is similar in the different preparations. Comparison of the results with earlier studies suggests a heterogeneity in cerebellar GABA receptors and/or their control.