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Morphogenetic Plasticity of Neuronal Elements in Cerebellar Glomeruli During Deafferentation-Induced Synaptic Reorganization J6zsefhhmori, Robert L

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Morphogenetic Plasticity of Neuronal Elements in Cerebellar Glomeruli During Deafferentation-Induced Synaptic Reorganization J6zsefhhmori, Robert L (C)Freund Publishing House Ltd., 1997 Morphogenetic Plasticity of Neuronal Elements in Cerebellar Glomeruli during Deafferentation-Induced Synaptic Reorganization J6zsefHhmori, Robert L. Jakab and J6zsefTakb.cs Department ofAnatomy, Laboratory ofNeurobiology, Semmelweis University, Medical School, Budapest, Hungary, H-1094; Section ofNeurobiology, Yale University School ofMedicine, New Haven, CT, USA SUMMARY represented only one-fifth of all synaptic junctions. The quantitative data of the Reorganization of the cerebellar glomerulus, reorganized cerebellar glomerulus demonstrate the main synaptic complex within the granule both a remarkable constancy and a plasticity of cell layer, was investigated using quantitative the excitatory granule cells and inhibitory Golgi morphological techniques. All afferents to the neurons building up this synaptic complex. cerebellar cortex, including mossy-fibers, were Constancy (the preservation of certain specific surgically destroyed by undercutting the structural features) is represented by an cerebellar vermis. Fifteen days after the eventually unchanged number of dendrites and operation, which resulted in the removal of the synaptic junctions within the deafferented main excitatory afferent to the glomerulus, a glomerulus. Such constancy was made possible, significant reorganization of the whole synaptic however, by the morphogenetic plasticity of complex was observed, whereas the structural both nerve-cell types to produce new, dendro- integrity of the glomerulus was remarkably well dendritic and axo-dendritic synapses to preserved. This was indicated by the compensate for the loss of mossy-fiber synapses. observation that the number of granule cell dendrites per glomerulus), as well as the (50 KEY number of dendritic digits (210 per WORDS glomerulus) bearing most of the 230 synaptic eerebellar cortex, deafferentation, synaptic junctions per glomerulus, did not change EM significantly after mossy-fiber degeneration. The plasticity, quantitative total number of synapses in the reorganized glomerulus did not change either, despite the INTRODUCTION disappearance of two-thirds of (excitatory) synaptic junctions caused by mossy-fiber Morphological evidence for the plasticity of degeneration. In the reorganized glomeruli, neuronal networks or nerve cells in the adult brain is however, the inhibitory, GABA-containing provided by the observation that certain neurons in Golgi axonal varicosities became the dominant the central nervous system (CNS) maintain their synaptic type--about four-fifths (200) of all potential to form new synaptic junctions in response synapses within the glomerulus--whereas the to deafferentations or lesions. The resulting dendritic synapses between the granule cells neosynaptogenesis is called induced or reactive synaptogenesis/19/. Two morphologically distinct types of reactive synaptogenesis have been described in the adult Reprint address: CNS. The axonal type of synaptogenesis has been Prof. J. Hfimori demonstrated in many regions; for example, aRer Department of Anatomy, Semmelweis Medical University denervation of nuclei Tiizolt6 58, H-1094 Budapest, Hungary septal /23,24/, hippocampus Fax: +36-1-217-6937 /4,15,30,31/, red nucleus/1,20,24/, among others. VOLUME 6, NO. 1, 1997 11 12 J. HAMORI ET AL. In axonal reactive synaptogenesis, the emphasis is elements, the excitatory mossy terminals and the on the active role of axonal processes to find and to inhibitory Golgi axon varicosities, establish multiple reoccupy the deafferentation-caused, vacant, post- synaptic contacts with many dendritic processes of synaptic sites. The second, dendritic type of granule cells. Quantitative morphological and reactive synaptogenesis implies the active stereological studies/16,17/have provided the most participation of postsynaptic dendritic processes in important data about the number of participating forming new synapses/7, 8,32/. neuronal processes and the number of synaptic The most spectacular example of dendritic junctions in simple type /17/ glomeruli. This reactive synaptogenesis was seen in the lateral information makes it possible to compare synaptic geniculate nucleus/10/and in cerebellar glomeruli numbers between control and mossy fiber /11/ where, after deafferentation, the otherwise deafferented cerebellar glomeruli, that is, to express exclusively postsynaptic dendrites of relay cells/10/ neosynaptogenetic plasticity in quantitative terms. or granule neurons/11/, both dendritic types were In the present study, we used quantitative shown to develop axonal, presynaptic sites to form morphological and stereological methods to new, dendro-dendritic synapses, probably to compare the structures of control and mossy-fiber compensate for the lost axo-dendritic synaptic deafferented, synaptically reorganized glomeruli. junctions. Such dendritic synaptogenesis occurred in brain regions (for example, the cerebellar cortex, some thalamic nuclei) where the neuronal basis of MATERIALS AND METHODS axonal-type reactive synaptogenesis (sprouting of local or extrinsic axons) is either negligible or Tissue preparation lacking/5/. Previous studies in our laboratory have In two adult male rats the upper cerebellar demonstrated the presence of both dendritic types vermis, including lobe 4, was undercut by a surgical /11/ and also a moderate axonal /12/ type of knife, resulting in a complete isolation of the vermal synaptogenesis in the granular layer of cerebellar cortex. This procedure secures a complete cortex. In normal, intact cerebellar cortex, all nerve transection of all afferents (climbing fibers, mossy cells are classical neurons/2,22/, the dendrites and fibers, and catecholamine fibers), as well as efferent somata of which exhibit only postsynaptic sites. (Purkinje) axons without disturbing the normal Under abnormal conditions, however, as in blood supply from the pial vessels to the isolated organotypic cerebellar cultures /18/, in cerebellar vermis. Fitteen days after the operation, the animals mutant mice/29/, or in undercut, isolated cerebellar were perfused with a fixative solutions, consisting cortex /11/, the granule cells develop presynaptic of 1% paraformaldehyde and 1% glutaraldehyde in sites on the somata and dendritic processes. 0.1 M phosphate buffer (pH 7.4). As a control, the Similarly, the other neuron type of the granular normal cortices of two intact animals/17/from the layer, the Golgi neurons, accumulate synaptic same brood were used atter perfusion with the same vesicles in their dendritic processes. The aldehyde fixative. observation that the newly formed presynaptic Small blocks of the perfused vermal cortices dendrites and somata were engaged in various were excised and (a) immersed in buffered 2% synaptic arrangements indicates the potential of osmic acid (pH 7.4), (b) dehydrated in ethanol, both nerve-cell types for deafferentation-caused (c) stained for 60 min in 1% uranyl acetate compensatory reactions. dissolved in 70% ethanol, and then (d) embedded in The cerebellar glomerulus is the main structural Durkupan. locus in the cerebellar cortex where reactive Serial ultrathin sections were cut using an LKB synaptogenetic processes have been observed, both V Ultrotom. Ribbons of 120-250 consecutive during development and in adults. The structural sections were mounted on formvar-coated, single- organization of this complex synaptic structure has slot grids and then stained with lead citrate. Four been described by several authors /2,9,16,22/. complete glomeruli from the control cerebellar Accordingly, in the glomerulus two presynaptic cortex and six glomeruli from the deafferented JOAL OF NEURAL TRANSPLANTATION & PLASTICITY REORGANIZTION OF MOSSY-FIBER DEAFFERENTED GLOMERULUS 13 cerebellar cortex were photographed at 12 000 x TABLE 1 and analyzed for quantitation at 27 000 x. The Total granule cell dendrites and dendritic digits in control number of successive sections covering complete and mossy-fiber-deafferented cerebellar glomeruli glomeruli were 131, 123, 159, 170 in the control Glomemli Granule cell Dendritic Number glomeruli; and in the deafferented cortex: 110, 131, dendrites (Grd) digits digits/Grd 194, 135, 193, 133. Control 53:!:2. I 208+30.8 3.9:k-0.5 Before the lead a few sections were staining, b also immunostained for GABA, using the Deafferented 52+10 217+36.8 4.3:k-0.1 postembedding method of Somogyi and Hodgson lean of four glomeruli in each group; bMean of six glomeruli in /28/. The GABA antiserum (Amel Co., New York) each group; Vlean 5: S.E.M. was used at a dilution of 1:3000. For the EM immunoreaction, 15 nm gold-labeled Immunogold where the Golgi axons were exclusively located at goat anti-rabbit antibody was used. the periphery of the synaptic complex, the GABA- containing Golgi axons in the deafferented glomerulus were also seen to occur in the interior of RESULTS the glomerulus, and the number of Golgi axonal varicosities appeared to be increased compared with Structure of normal cerebellar glomerulus that of the control. (b) As previously observed/11/, new heterologous synaptic contacts between An electron micrograph of the normal, intact dendritic digits of granule cells were found in the cerebellar glomerulus is presented in Fig.1. The mossy-fiber-deprived glomeruli. The newly clawlike, postsynaptic dendriti digits of the granule developed dendro-dendritic synapses were the cell dendrites surround the large, bulbous, mossy asymmetric type, and the synaptic vesicles were terminal, which is usually located in the center
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