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A Beaudet

Publications and source records attributed to A Beaudet.

At least 163 records · Page 9Linked to original sources
PubMed1981

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.

Animals↗
PubMed1981

Synaptic remodeling of serotonin axon terminals in rat agranular cerebellum.

In order to assess the influence of the target zone on the synaptic modeling of central serotonin (5-HT) axons, the 5-HT innervation of the posterior vermal cortex was studied by high resolution radioautography in both normal and X-ray-induced agranular rat cerebella, following topical application of [3H]5-HT. Two major systems of 5-HT afferents were identified in normal cerebellar cortex: (1) typical mossy fibers confined to the granular layer and (2) fine beaded axons diffusely distributed through all layers. The density of this innervation was estimated to be approximately 240,000 varicosities/cu.mm of cortex. The labeled mossy terminals all established synaptic contacts with the dendrites of granule cells. In contrast, only 3% of the varicosities belonging to the 'diffuse system' exhibited active zones in single thin sections, implying that less than 9% were actually engaged in junctional synaptic relationships. In the agranular cerebellar cortex, all 5-HT terminals belonging to the so-called 'diffuse system'. Their density was more than 8 times higher than in normal rat (2 million/cu.mm of cortex), an increase accounted for by the smaller volume of the experimental cerebellum. Thirty-five per cent of these 5-HT varicosities were seen in synaptic contact, indicating that all established at least one junctional complex. Most of these synapses were made on the branchlet spines of Purkinje cell dendrites, but some were also observed on the dendritic shafts of Golgi cells. Thus, in the absence of granule cells, the 5-HT innervation of rat cerebellar cortex evolves from a mostly 'non-junctional' into an entirely 'junctional' input. This finding indicates that the territory of innervation can exert a determinant influence on the synaptic modeling of incoming 5-HT afferents.

Afferent Pathways↗
PubMed1981

The fine structure of central serotonin neurons.

1. Most of the current knowledge concerning the fine structure of serotonin (5-HT) neurons in mammalian CNS is derived from radioautographic studies following uptake of tritiated 5-HT in vivo. 2. (3H)5-HT-labelled perikaria and proximal dendrites have thus far been visualized ultrastructurally in nuclei raphe dorsalis, paragigantocellularis lateralis and dorsomedialis hypothalami. In none of these regions were they found to possess cytological features which would constitute a hallmark of their serotoninergic nature. 3. Most central 5-HT neurons give rise to thin and unmyelinated, albeit long and ramified axons. These innervate the entire neuraxis, issuing billions of varicose terminals along their paths. Such varicosities usually consist of small axonal enlargements (0.5-1 micrometers in diameter) containing clustered vesicular organelles. These organelles include small (15-50 nm), pleomorphic agranular vesicles and larger (50-130 nm), round or flattened dense-cored vesicles. 4. A salient feature of central 5-HT varicosities is that they often lack the junctional complexes classically considered as the morphological substrate for chemical transmission in CNS. The occurrence of synaptic specializations varies from one brain region to another. When present, synaptic contacts are generally found on dendritic branches or spines of nonserotoninergic neurons, but axo-somatic or axo-axonic synapses, as well as contacts with 5-HT elements, have also been reported. The frequency with which 5-HT varicosities establish junctional relationships may be more dependent on the territory of innervation than characteristic of any subset of 5-HT fibers or nerve cell bodies. 5. It is currently assumed that both non-junctional and junctional 5-HT varicosities can release their endogenous amine upon axonal depolarization. This mode of action, which evokes that of a local neurohormone, might subtend the modulatory effects of 5-HT in the CNS.

Afferent Pathways↗
PubMed1981

Selective retrograde labeling of cholinergic neurons with [3H]choline.

Evidence is presented which is consistent with a specific retrograde labeling of cholinergic neurons following [3H]choline application in their zone of termination. [3H]Choline injection in the rat hippocampus leads to perikaryal retrograde labeling in the ipsilateral medial septal nuclease and nucleus of the diagonal band, thus delineating an established cholinergic pathway, while only diffuse presumably anterograde labeling was observed in the lateral septum, the entorhinal cortex, and the opposite hippocampus. After [3H]choline injection in the pigeon visual Wulst, only the ipsilateral thalamic relay, of all inputs, showed similar perikaryal retrograde labeling, an observation supporting the suggestion that at least some thalamo-Wulst neurons are cholinergic.

Animals↗
PubMed1979

Influence of experimentally induced agranularity on the synaptogenesis of serotonin nerve terminals in rat cerebellar cortex.

The serotonin (5-HT) innervation of the posterior vermis was studied by high resolution radioautography in both normal and X-ray-induced agranular rat cerebella, following 3 h topical superfusion with 10(-4) M 3H-5-HT. In the normal cerebellar cortex, 5-HT axonal varicosities are scarce and only rarely exhibit the membrane differentiations characterizing synaptic contacts. In the agranular cerebellum, 5-HT terminals appear to have a much higher density than in normal controls, although their absolute number may not be significantly different when the important reduction in volume of this experimental cerebellum is taken into account. These terminals frequently show typical synaptic contracts. Most of them are established on the branchlet spines of Purkinje cell dendrites, but some are also observed on the shafts of Golgi cell dendrites. The 5-HT innervation of the cerebellar cortex thus undergoes important changes in the absence of granule cells. It is suggested that these modifications may be part of the general reorganization process of the cerebellar circuitry consequent on the early destruction of the external granular layer. This new example of synaptic remodelling could imply that the formation of cerebellar connectivity is modulated, to a certain extent, by the local cellular environment.

Animals↗
PubMed1979

[Visualization of opiate receptors in the locus coeruleus of rats: high resolution radioautography after administration of a tritiated met-enkephalin analog].

Opiate receptor sites were visualized by high resolution radioautography in the locus coeruleus of the Rat following intraventricular injection of the met-enkephalin analogue FK 33-824 tritiated with a high specific activity (3H-FK). After administration of a tracer dose of 3H-FK, more than 75% of the radioactivity detected in the locus coeruleus is specifically bound to opiate receptor sites. These are distributed both between and inside neuronal perikarya. After administration of 3H-FK at high concentration, electron microscope radioautography shows the presence of specific opiate binding sites at the level of axo-somatic and axo-dendritic synaptic junctions. These synaptic binding sites could correspond to receptor sites for endogenous enkephalins.

Animals↗
PubMed1979

Radioautographic characterization of a serotonin-accumulating nerve cell group in adult rat hypothalamus.

Intensely labeled nerve cell bodies were identified by radioautography within the pars ventralis of nucleus dorsomedialis hypothalami (hdv), following intraventricular perfusion with 10(-5) or 10(-4) M tritiated serotonin [3H]5-HT in adult rats pretreated with a monoamine oxidase inhibitor. This selective reaction, which involved approximately 1000 neurons on each side of the third ventricle, was unaltered by concomitant administration of 10(-3) M non-radioactive norepinephrine, and was absent after intraventricular injection of 10(-5) or 10(-4) M tritiated norepinephrine. The 3H-labeled 5-HT nerve cell bodies were loosely grouped within the inner and caudal half of the hdv, and appeared morphologically similar to the unreactive neurons among which they were interspersed. Within the same region, numerous labeled axonal varicosities were also detected, which were never found in synaptic contact with the reactive cells. If the 3H-labeled 5-HT neurons contain endogenous 5-HT, they might constitute an intrinsic source of 5-HT innervation in the adult rat hypothalamus.

Animals↗
PubMed1976

Quantitative data on serotonin nerve terminals in adult rat neocortex.

Serotonin (5-HT) nerve terminals, specifically labeled with [3H]5-HT have been counted in light microscope radioautographs from the fronto-parietal neocortex of adult rats, following prolonged superfusions with relatively high concentrations of tracer (10(-5) M or 10(-4) M), and after 15 or 30 days of radioautographic exposure. Comparative analysis of the results indicated that all 5-HT varicosities did not possess the same capacity to accumulate the exogenous amine. Nevertheless, superfusions with 10(-4) M[3H]5-HT provided a sufficient loading of 5-HT boutons to allow their complete detection within any given layer of the neocortex. The labeled varicosities were found to be present within all cortical layers, except layer VI. Their total number was extrapolated to be approximately 1 X 10(6)/cu.mm of cortex. This figure represented a mean incidence of one 5-HT nerve terminal/850-1400 cortical synapses (0.07-0.12%). The intralaminar density of 5-HT innervation increased progressively from layer V to layer I, in a distribution pattern suggestive of unspecific afferents. Based on endogenous 5-HT levels of 346 +/- 32 ng/g for the fronto-parietal neocortex, the mean 5-HT content per varicosity could be estimated to be 0.35 X 10(-3) pg, and the concentration 1900 ng/g wet weight of terminal or 0.2%. In view of their particular ultrastructural features7, ubiquitous repartition and endogenous amine content, 5-HT nerve endings may be considered as capable of exerting a rather widespread influence in the neocortex.

Animals↗
PubMed1975

Serotonin nerve terminals in adult rat neocortex.

Axonal processes which take up and retain exogenous tritiated serotonin ([3H]5-HT) have been demonstrated in the fronto-parietal cortex of adult rats, by means of high resolution radioautography. Prolonged topical superfusion with relatively high concentrations of ([3H]5-HT allowed detection of a maximal number of axonal reactions in the upper 3 layers of cortex. Comparison of results obtained from normal rats and animals pretreated with 6-hydroxydopamine or bearing midbrain raphe lesions established the specificity of this labeling. All reactive axons seemed to arise from the serotonin nerve cell bodies in groups B7 and B8 of Dahlström and Fuxe15. In electron microscope radioautographs, the serotonin fibers appeared as tenuous, naked axonal processes (0.1-0.5 mum in diameter) exhibiting small enlargements (0.7 mum in mean diameter) spaced at frequent intervals (1-3 mum). These boutons contained occasional mitochondria, small, round, agranular 'synaptic' vesicles and large granular vesicles. With axons, [3H]5-HT was concentrated in the boutons, and to a much lesser extent in connecting segments. This reactive pattern resembled that revealed by the fluorescence technique for endogenous serotonin. Preferential accumulations of the tracer by mitochondria and vesicular organelles indicated that these elements could sequester exogenous serotonin. Large granular vesicles were not necessarily visible in random thin sections of the labeled varicosities, and thus could not serve as the unique criterion for electron microscopic identification of 5-HT terminals. Moreover, these organelles are known to be present in other types of nerve endings. Topometric analysis of serial thin sections nevertheless demonstrated that large granular vesicles were potentially detectable in every 5-HT containing bouton, and also enabled extrapolation of their average number at 7 per varicosity. This low number makes it unlikely that large granular vesicles primarily represent storage sites. They could rather serve as a carrier for particle-bound enzymes essential to the local metabolism of serotonin or its precursors. A very small fraction of the serotonin varicosities exhibited the membrane differentiations of typical synaptic terminals. Extensive sampling in serial thin sections revealed junctional complexes in only 5% of labeled boutons, as opposed to 50% of unlabeled nerve endings in the surrounding neuropil. The data do not preclude the possibility that other monoaminergic neurons also share similar characteristics. It is probable that endogenous serotonin can be liberated from all axonal varicosities including those lacking strictu senso synaptic relationships. The overall configuration and ultrastructural features of cortical serotonin fibers suggest intrinsic dynamic properties which could assume particular significance in terms of function, plasticity and regrowth.

Animals↗