Amino acids in 'light' and 'heavy' synaptosome fractions from rat olfactory lobes and their release by electrical stimulation

Localization of the clustering protein gephyrin at GABAergic synapses in the main olfactory bulb of the rat

The tubulin-binding protein gephyrin is essential for the formation of postsynaptic glycine-receptor clusters in cultured spinal neurons. In addition, there is increasing evidence that gephyrin can also be present at nonglycinergic synapses. Here we analyzed immunocytochemically the subcellular localization of gephyrin in the main olfactory bulb of the rat and compared its distribution with that of gamma-aminobutyric acid (GABA) and of two major GABA(A)-receptor subunits. Gephyrin was selectively localized to the postsynaptic side of symmetric synaptic junctions, where the presynaptic terminals contained GABA. Moreover, gephyrin colocalized extensively with the alpha1 and gamma2 subunits of the GABA(A) receptor. In contrast, gephyrin was not detected at presumed glutamatergic synapses. These results indicate that gephyrin is not uniquely associated with glycine receptors, but can also be found at distinct GABAergic synapses. Thus, they raise the possibility that gephyrin is involved in anchoring certain GABA(A)-receptor subtypes in the postsynaptic membrane.

A fraction enriched in dendrodendritic synaptosomes isolated from rat olfactory bulb: morphology and transmitter release

A fraction enriched in dendro-dendritic synaptosomes was isolated from rat olfactory bulb by a rapid method. Synaptosomes preserved their ultrastructure and showed configurational changes in relation to incubation in physiological ion medium as described earlier in the case of cortical synaptosomes. Dendro-dendritic synaptosomes were larger and contained more mitochondria than cortical synaptosomes. Doublets of terminals synapsing with each other were frequently seen and each terminal contained synaptic vesicles. Oxygen consumption of dendro-dendritic synaptosomes was decreased by ouabain and increased by 2,4-dinitrophenol. High-potassium medium evoked a considerable release of GABA and dopamine but not of noradrenaline or serotonin in accordance with histochemical published data.

An analysis of the effects of excitatory amino acid receptor antagonists on evoked field potentials in the olfactory bulb

The effects of excitatory amino acid antagonists on extracellular field potentials in the olfactory bulb produced by lateral olfactory tract stimulation were analysed in vivo. The compounds tested D-2-amino-5-phosphonovalerate, L-(+)2-amino-4-phosphonobutyrate, gamma-D-glutamylglycine, L-glutamic acid diethylester and cis-2,3-piperidine dicarboxylic acid, were administered by brain dialysis. Of the compounds tested, only cis-2,3 piperidine-dicarboxylic acid and gamma-D-glutamylglycine were able to suppress the synaptic excitation of granule cells. This pharmacological profile suggests the involvement of non-N-methyl-D-aspartate receptors. However, the suppression was accompanied by a reduction in the amplitude of the presynaptic volley. A second finding was that D-2-amino-5-phosphono-valerate and gamma-D-glutamyl glycine attenuated granule cell mediated inhibition of mitral cells, suggesting the involvement of N-methyl-D-aspartate-sensitive receptors. The possibility that mitral cells and that either centrifugal fibres, or an intrinsic olfactory bulb feedback loop might use an excitatory amino acid as its neurotransmitter is therefore discussed.

Kainic acid-induced changes of extracellular amino acid levels, evoked potentials and EEG activity in the rabbit olfactory bulb

The effect of kainic acid (KA) on the extracellular content of amino acids in the rabbit olfactory bulb was investigated, both in vivo with the brain dialysis technique and in vitro with a superfused tissue slice preparation. Olfactory bulb EEG activity and lateral olfactory tract (LOT)-evoked field potentials were monitored simultaneously during dialysis experiments. KA induced a rapid (within 5 min) increase of extracellular aspartate, glutamate, GABA, phosphoethanolamine and taurine in vivo. LOT-evoked potentials were altered concomitantly in a concentration- and time-dependent manner. The antidromic invasion of mitral/tufted cells was depressed and the synaptic activation of granule cells was abolished in the presence of KA. Olfactory bulb EEG activity was also affected. Oscillatory bursts in olfactory bulb EEG were abolished by 10 mM in most experiments, whereas sustained oscillations were induced by 1 mM KA. The effects of KA may partly be due to a blockade of amino acid reuptake since dihydrokainate (DKA) perfusion was found to increase extracellular aspartate and glutamate. However, DKA had no significant effect on EEG or evoked potentials. In vitro, aspartate and glutamate were selectively increased during KA perfusion.

Veratridine-induced release in vivo and in vitro of amino acids in the rabbit olfactory bulb

Free amino acids were studied in the olfactory bulb of the rabbit during basal conditions and veratridine-induced depolarization, in vitro with a tissue slice preparation and in vivo with a perfusion-dialysis technique. In vivo, basal extracellular concentrations of GABA, beta-alanine and aspartate were low, while glutamine showed the highest level. The basal steady-state concentration ratio between the total tissue pool of free amino acids and amino acids in the extracellular fluid was high for GABA, aspartate and glutamate, while low for glutamine and other 'non-transmitter' amino acids. Veratridine induced a marked TTX-sensitive release of GABA (40-50 times the control) both in vivo and in vitro. In vivo, the GABA release showed a peak during the first minutes of veratridine perfusion. The TTX-sensitive release of aspartate and glutamate, on the other hand, was approximately 5 times higher in vitro than in vivo. Furthermore, a prolonged response to veratridine was seen for glutamate and aspartate in vivo consisting of an early peak, followed by a sustained release. Taurine showed a time-delayed veratridine response, both in vivo and in vitro, whereas glutamine displayed a slow, TTX-sensitive decrease. No effect of veratridine was seen on beta-alanine or carnosine-threonine levels.

Selective retrograde transport of tritiated D-aspartate from the olfactory bulb to the anterior olfactory nucleus, pyriform cortex and nucleus of the lateral olfactory tract in the rat

After an injection of [3H]D-aspartate into the olfactory bulb of the rat, retrogradely labeled cells were detected bilaterally in the anterior olfactory nucleus (AON), and ipsilaterally in the pyriform cortex (PC) and nucleus of the lateral olfactory tract (NLOT). These results suggest a certain selective retrograde transport of this amino acid, and are discussed in relation to transmitter candidates in the olfactory bulb.

Evidence for compartmentalization of glutamate in rat brain synaptosomes using the glutamate sensitivity of phosphate-activated glutaminase as a functional test

The glutamate content of rat brain synaptosomes was measured by high performance liquid chromatography to be 39 micromol/g protein. If uncompartmentalized this glutamate (4 mM) would inhibit phosphate-activated glutaminase considerably. Since the action of any endogenous effector on the enzyme is assumed to be negligible following disruption of the synaptosomes, due to dilution with the incubation medium, the inhibition by glumate and activation by phosphate were compared in intact and disrupted synaptosomes. The inhibition by endogenous glutamate in intact synaptosomes was found to correspond to less than that of 0.5 mM of added glutamate to disrupted synaptosomes, indicating that the major fraction of synaptosomal glutamate is compartmentalized.

Aspartate and not glutamate is the likely transmitter of the rat lateral olfactory tract fibres

The protoveratrine A-evoked release of endogenous amino acid neurotransmitter candidates from olfactory cortex slices taken from control and bulbectomized rats has been monitored. Bulbectomy is accompanied by a statistically significant and specific attenuation of drug-evoked aspartate release suggesting that the excitatory transmitter of the lateral olfactory tract fibres is aspartate rather than glutamate.

Subcellular distribution of glutamate decarboxylase in rat olfactory bulb: high content in dendrodendritic synaptosomes

The olfactory bulbs in the CNS contain reciprocal dendrodendritic synapses between the granule cells and the secondary dendrites of mitral cells. Based on pharmacologic and electrophysiologic evidence, these synapses are believed to utilize GABA as an inhibitory neurotransmitter. A dendrodendritic synaptosomal fraction has been isolated from rat olfactory bulbs. The upper portion (PB) of the crude nuclear pellet contains 30-40% of the GAD (glutamate decarboxylase) activity of the olfactory bulb homogenate. When PB is purified on a discontinuous sucrose density gradient, 78-85% of the GAD activity is localized to the region containing the dendrodendritic synaptosomes, which were identified by transmission electron microscopy. The presence of a substantial proportion of GAD, the enzyme that catalyzes synthesis of GABA, in the DDS provides neurochemical support for the hypothesis that GABA functions at the reciprocal dendrodendritic synapses in the olfactory bulb.

Isolation and characterization of postsynaptic densities from various brain regions: enrichment of different types of postsynaptic densities

Postsynaptic densities (PSDs) have been isolated from cerebral cortex, midbrain, cerebellum, and brain stem by the Triton X-100 method previously used in the isolation of cerebral PSDs (Cohen et al., 1977, J. Cell Biol. 74:181). These PSDs have been compared in protein composition, protein phosphorylation, and morphology. Thin-section electron microscopy revealed that cerebral cortex and midbrain PSDs were identical, being approximately 57 nm thick and composed of apparent aggregates 20-30 nm in diameter. Isolated cerebellar PSDs appeared thinner (33 nm) than cerebral cortex PSDs and lacked the apparent 20- to 30-nm aggregates, but had a latticelike structure. In unidirectional and rotary-shadowed replicas, the cerebrum and midbrain PSDs were circular in shape with a large central perforation or hole in the center of them. Cerebellum PSDs did not have a large perforation, but did have numerous smaller perforations in a lattice like structure. Filaments (6-9 nm) were observed connecting possible 20- to 30-nm aggregates in cerebrum PSDs and were also observed radiating from one side of the PSD. Both cerebral cortex and midbrain PSDs exhibited identical protein patterns on SDS gel electrophoresis. In comparison, cerebellar PSDs (a) lacked the major 51,000 Mr protein, (b) contained two times less calmodulin, and (c) contained a unique protein at 73,000 Mr. Calcium plus calmodulin stimulated the phosphorylation of the 51,000 and 62,000 Mr bands in both cerebral cortex and midbrain PSDs. In cerebellar PSDs, only the 58,000 and 62,000 Mr bands were phosphorylated. In the PSDs from all brain regions, cAMP stimulated the phosphorylation of Protein Ia (73,000 Mr), Protein Ib (68.000 Mr), and a 60,000 Mr protein, although cerebrum and midbrain PSDs contained very much higher levels of phosphorylated protein than did the cerebellum. On the basis of the morphological criteria, it is possible that PSDs isolated from cerebrum and midbrain were derived from the Gray type I, or asymmetric, synapses, whereas cerebellum PSDs were derived from the Gray type II, or symmetric, synapses. Since there is some evidence that the type I synapses are involved in excitatory mechanisms while the type II are involved in inhibitory mechanisms, the role of the PSD and of some of its proteins in these synaptic responses is discussed.

Localization of acetylcholinesterase in the main and accessory olfactory bulbs of the mouse by light and electron microscopic histochemistry

Experiments were conducted to examine by light and electron microscopy the localization of acetylcholinesterase (AChE) in the main (MOB) and accessory (AOB) olfactory bulbs of the normal mouse. Evidence from the literature for cholinergic innervation of the mammalian olfactory bulb was then assessed in light of possible correlation between reported sites of termination of centrifugal fibers to the olfactory bulb and the localization of AChE. AChE-positive nerve fibers were concentrated in the periglomerular region and internal plexiform layer of the MOB. Stained fibers were also present in the granule cell, mitral cell, and external plexiform layers as well as within glomeruli. A few neurons in all layers of the MOB contained AChE reaction product. Unlike the MOB, AChE-positive fibers were not present in the glomerular layer of the AOB. AChE-positive fibers were concentrated in the inner plexiform layer, whereas fewer stained fibers were observed in the external plexiform and mitral cell layer and granule cell layer. Lightly stained neurons were found in the deeper portions of the external plexiform and mitral cell layer and granule cell layer. Ultrastructurally, AChE reaction product in the MOB and AOB was predominantly associated with small unmyelinated axons. Reaction product was also observed adjacent to axon terminals and dendrites. Occasionally within the MOB, AChE activity was found within periglomerular, tufted, short-axon, mitral, and granule cells. In the AOB, however, intracellular AChE activity was observed within some mitral/tufted cells and only a few granule cells. In conclusion, the AChE reaction product was mainly associated with axons in regions of the MOB where centrifugal fibers have been reported. Accessory olfactory bulb AChE localization was different from that of the MOB, suggesting a different pattern of cholinergic input to the AOB. The small amounts and sites of intraneuronal AChE reaction product in cells of the olfactory bulb indicate cholinoceptive rather than cholinergic function.

Free amino acids in the synaptosome and synaptic vesicle fractions of different bovine brain areas

Free amino acids were quantitatively estimated in intact tissues and isolated synaptosome and synaptic vesicle fractions of the bovine brain regions with the aid of a sensitive amino acid analyzer. The brain areas studied were frontal, parietal and occipital cerebral cortices, cerebellar cortex, caudate and lenticular nuclei, superior colliculi, thalamus, pons and medulla. The most abundant amino acid in tissue samples and synaptosome fractions was glutamic acid, followed by glutamine, aspartic acid, GABA and taurine. The dominatating amino acid in all isolated synaptic vesicle fractions was taurine. The concentrations of glutamic acid, glutamine, GABA and aspartic acid were generally much lower. The 5 transmitter candidates, viz. GABA, glycine, glutamic acid, as particaid and taurine, comprised about one-half of the total amino acids in all samples. Taurine was the only amino acid highly enriched in the vesicle fractions. This enrichment was discernible in all brain areas. It is suggested therefore that taurine is rather a ubiquitous associate of synaptic membrane structures than a specific inhibitory transmitter.

The effect of depolarizing potassium concentrations on the efflux of GABA from rat dorsal medulla in vivo and from slices and synaptosomes

The efflux of [3H]GABA from the dorsal surface of adult rat medulla overlying the dorsal column nuclei (DCN) was found not be influenced by increasing the concentration of potassium in the superfusing solution to 40 mequiv. Similarly, raised potassium was found not to influence the efflux from slices of the dorsal region of the caudal medulla containing the dorsal column nuclei. This lack of effect of raised potassium is not thought to be due to lack of GABAergic terminals in this region because there is good pharmacological evidence for their presence and bacause both electrical stimulation and 100 microM veratridine increased the efflux of [3H]GABA from such slices. Also, 40 mequiv potassium was found to increase the efflux of both endogenous and [3H]GABA from crude synaptosome preparations of this region without influencing the efflux of [14C]sucrose or [3H]leucine. This release of [3H]GABA was calcium-dependent, and was similar whether produced by 20, 40 of 60 mequiv potassium and occurred whether eos or AOAA was used to inhibit GABA metabolism. Release from synaptosomes could also be induced with 100 microM veratridine. Raised potassium was additionally found to prevent the increased efflux from slices produced by electrical stimulation and to increase the efflux from slices prepared from the brains of rats 14 days old. It is suggested that the astrocytic swelling produced by raised potassium concentration restricts the diffusion of GABA away from depolarized terminals.

Amino acids in the synaptic vesicle fraction from calf brain: content, uptake and metabolism

A synaptic vesicle fraction was prepared from calf brain cortex, containing 10 identified amino acids and two unidentified ninhydrin-positive compounds, one of which is apparently a peptide. The most plentiful amino acids were taurine (1.8 nmol/g original tissue), glutamic acid (1.8), serine (0.9), aspartic acid (0.8) and GABA (0.8); the others identified were cysteic acid (or cysteinesulphinic acid), glutamine, alanine, glycine and lysine. The unknown peptide occurred in a high concentration (about 16 alanine equivalents/g), and contained mainly aspartic acid and serine. Cysteic acid (or cysteinesulphinic acid) also occurred in relatively high amounts, but its peak contained acid-labile impurities. The influx of [14C]glutamate into the vesicles took place by means of non-saturable migration, while two saturable systems having very similar properties were dominant only at low glutamate concentrations. Influx constants for these quantitatively low uptake systems were Km, 34 and 92 micrometer, and Vmax, 33 and 49 nmol/min/g obtained by v versus v/S plot. Almost the same values were also obtained by a 1/v versus 1/S plot. GAD and GABA-T activities in the vesicles were only 1/200th of those in the synaptosomes.