Environmental and genetic determinants of connectivity in the central nervous system--an approach through dendritic field analysis

Dendritic differentiation of cerebellar Purkinje cells is promoted by ryanodine receptors expressed by Purkinje and granule cells

Cerebellar Purkinje cells have the most elaborate dendritic trees among neurons in the brain. We examined the roles of ryanodine receptor (RyR), an intracellular Ca(2+) release channel, in the dendrite formation of Purkinje cells using cerebellar cell cultures. In the cerebellum, Purkinje cells express RyR1 and RyR2, whereas granule cells express RyR2. When ryanodine (10 µM), a blocker of RyR, was added to the culture medium, the elongation and branching of Purkinje cell dendrites were markedly inhibited. When we transferred small interfering RNA (siRNA) against RyR1 into Purkinje cells using single-cell electroporation, dendritic branching but not elongation of the electroporated Purkinje cells was inhibited. On the other hand, transfection of RyR2 siRNA into granule cells also inhibited dendritic branching of Purkinje cells. Furthermore, ryanodine reduced the levels of brain-derived neurotrophic factor (BDNF) in the culture medium. The ryanodine-induced inhibition of dendritic differentiation was partially rescued when BDNF was exogenously added to the culture medium in addition to ryanodine. Overall, these results suggest that RyRs expressed by both Purkinje and granule cells play important roles in promoting the dendritic differentiation of Purkinje cells and that RyR2 expressed by granule cells is involved in the secretion of BDNF from granule cells.

Disruption of metabotropic glutamate receptor signalling is a major defect at cerebellar parallel fibre-Purkinje cell synapses in staggerer mutant mice

Staggerer mutant mice have functional loss of a transcription factor, retinoid-related orphan receptor α (RORα), which is abundantly expressed in Purkinje cells (PCs) of the cerebellum.Homozygous staggerer (sg/sg)mice show cerebellar hypoplasia and congenital ataxia. Sg/sg mice serve as an important extreme mouse model of the hereditary spinocerebellar ataxia type 1 (SCA1), since it has been shown that RORα dysfunction is strongly correlated with SCA1 pathogenesis. However, synaptic abnormalities, especially at parallel fibre (PF)-PC synapses, in SCA1-related sg/sg mice have not been examined in detail electrophysiologically. In this study, we report that PFs can still establish functional synapses onto PCs in sg/sg mice in spite of reduction in the number of PF-PC synapses. Compared with PF-evoked EPSCs in the wild-type or heterozygotes, the success rate of the EPSC recordings in sg/sg was quite low (∼40%) and the EPSCs showed faster kinetics and slightly decreased paired pulse facilitation at short intervals. The prominent synaptic dysfunction is that sg/sg mice lack metabotropic glutamate receptor (mGluR)-mediated slow EPSCs completely. Neither intense PF stimulation nor an exogenously applied mGluR agonist, DHPG, could elicit mGluR-mediated responses.Western blot analysis in the sg/sg cerebellum revealed low-level expression of mGluR1 and TRPC3, both of which underlie mGluR-mediated slow currents in PCs. Immunohistochemical data demonstrated marked mislocalization of mGluR1 on sg/sg PCs.We found that mGluR-mediated retrograde suppression of PF-PC EPSCs by endocannabinoid is also impaired completely in sg/sg mice. These results suggest that disruption of mGluR signalling at PF-PC synapses is one of the major synaptic defects in sg/sg mice and may manifest itself in SCA1 pathology.

Dendrite formation of cerebellar Purkinje cells

During postnatal cerebellar development, Purkinje cells form the most elaborate dendritic trees among neurons in the brain, which have been of great interest to many investigators. This article overviews various examples of cellular and molecular mechanisms of formation of Purkinje cell dendrites as well as the methodological aspects of investigating those mechanisms.

Dendritic morphogenesis of cerebellar Purkinje cells through extension and retraction revealed by long-term tracking of living cells in vitro

Cerebellar Purkinje cells have the most elaborate dendritic trees among the neurons in the CNS. To investigate the dynamic aspects of dendritic morphogenesis of Purkinje cells, we performed a long-term analysis of living cells in cerebellar cell cultures derived from glutamate decarboxylase 67-green fluorescent protein mice. Most Purkinje cells had several primary dendrites during the 25-day culture period. Repeated observation of green fluorescent protein-expressing Purkinje cells over a period of 10-25 days in vitro demonstrated that not only extension, but also retraction of primary dendrites occurred during this culture period. Interestingly, both extension and retraction of primary dendrites were active between 10 and 15 days in vitro, and retraction of a primary dendrite occurred concomitantly with elongation of other primary dendrites in the same cell. Analysis of the morphological characteristics of the retracted primary dendrites demonstrated that shorter and less branched primary dendrites tended to retract. Furthermore, treatment with an inhibitor of calcium/calmodulin-dependent protein kinase II reduced the number of primary dendrites specifically during 5-15 days in vitro, the culture period when the extension and retraction of primary dendrites occurred actively. Blockade of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate-type glutamate receptors also reduced the number of primary dendrites during the same culture period, while inhibition of glutamate transporters increased the number. These findings suggest that the final morphology of Purkinje cells is achieved not only through extension, but also through retraction of their dendrites, and that calcium/calmodulin-dependent protein kinase II and neuronal activity are involved in this dendritic morphogenesis.

A chondroitin sulfate proteoglycan PTPzeta /RPTPbeta regulates the morphogenesis of Purkinje cell dendrites in the developing cerebellum

PTPzeta/RPTPbeta, a receptor-type protein tyrosine phosphatase synthesized as a chondroitin sulfate (CS) proteoglycan, uses a heparin-binding growth factor pleiotrophin (PTN) as a ligand, in which the CS portion plays an essential role in ligand binding. Using an organotypic slice culture system, we tested the hypothesis that PTN-PTPzeta signaling is involved in the morphogenesis of Purkinje cell dendrites. An aberrant morphology of Purkinje cell dendrites such as multiple and disoriented primary dendrites was induced in slice cultures by (1) addition of a polyclonal antibody against the extracellular domain of PTPzeta, (2) inhibition of protein tyrosine phosphatase activity, (3) enzymatic removal of the CS chains, (4) addition of exogenous CS chains, and (5) addition of exogenous PTN, all of which disturb PTN-PTPzeta signaling. These treatments also reduced the immunoreactivity to GLAST, a glial glutamate transporter, on Bergmann glial processes. Furthermore, a glutamate transporter inhibitor also induced the abnormal morphogenesis of Purkinje cell dendrites. Altogether, these findings suggest that PTN-PTPzeta signaling regulates the morphogenesis of Purkinje cell dendrites and that the mechanisms underlying that regulation involve the GLAST activity in Bergmann glial processes.

Cell death of motoneurons in the chick embryo spinal cord. X. Synapse formation on motoneurons following the reduction of cell death by neuromuscular blockade

Chronic treatment of chick embryos with neuromuscular blocking agents, such as curare, rescues motoneurons from naturally occurring cell death. In the present study, embryos treated with curare from E6 to E9 had 35% more motoneurons than controls on E10 and 42% more than controls on E16. Previous studies have shown that several aspects of motoneuron differentiation occur normally in curare-treated embryos. We report here that dendrite growth and arborization is also unaltered on E10 and E16 following curare treatment. A quantitative analysis of afferent synapses on motoneurons shows that the packing density of both axosomatic and axodendritic synapses is also normal on E10 in curare-treated embryos, despite the greater number of motoneurons present. This indicates that the interneurons that provide presynaptic input to motoneurons are able to compensate for the increased number of synaptic sites made available by curare treatment. However, by E16 the packing density of synapses is reduced by about half. Because motoneurons and their dendrites continue to grow between E10 and E16, the further increase in synaptic sites made available in curare-treated embryos apparently exceeds the compensatory capacity of presynaptic interneurons on E16. One can conclude from these results that the increased survival of motoneurons in curare-treated embryos is not owing to an increase in afferent synapses. Motoneurons in these embryos continue to survive in the face of either no change (E10) or a reduction (E16) in the number of axodendritic and axosomatic synapses. Therefore, increased motoneuron survival in this situation is very likely regulated primarily by motoneuron-target interactions.

Destruction of meningeal cells over the newborn hamster cerebellum with 6-hydroxydopamine prevents foliation and lamination in the rostral cerebellum

Intracisternal injection of 30 micrograms 6-hydroxydopamine was used to destroy meningeal cells in the newborn hamster. After 20 or 30 days the cerebella of treated animals showed severe morphological alterations including: an absence of distinct folia anterior to the primary fissure; a disruption of lamination in the same region by the displacement of both Purkinje cells and cerebellar interneurons; a reduction in size and frequency of branching of the medullary tree with anomalous anterobasal branches and splaying; reductions in the area of the molecular layer, the total area occupied by granule cells, the length of the pial surface and the length of the Purkinje cell layer of 29, 21, 57 and 27%, respectively; disorganization of the radially organized glial scaffold by outgrowth of Bergmann glial fibers and displacement of their cell bodies, the Golgi epithelial cells, and anomalous orientation, polarity, size and branching frequency of Purkinje cell dendritic trees. These findings support our earlier hypothesis that the initial destruction of meningeal cells destabilizes the cerebellar surface (basal lamina and glia limitans superficialis) and disorganizes the glial scaffold, while the neuronal cerebellar malformations are secondary to this glial defect.

A quantitative Golgi analysis of the postnatal maturation of dendrites in the central nucleus of the inferior colliculus of the rat

In this work, we used Golgi impregnation and computer assistance to analyze the quantitative development of the dendrites of neurons in the central nucleus of the inferior colliculus in the rat from 8 (P8) to 20 (P20) days of age, i.e., during the period of functional maturation of the auditory system. The number and length of the dendritic segments and the number of spines of a total of 516 impregnated cells were studied. The variations of both the mean number of peripheral dendritic segments and the mean dendritic domain area suggest that the dendritic tree of these cells is orientated preferentially in the sagittal plane before the onset of functional maturation, but undergoes a reorientation in the frontal plane during this critical period. The length of dendritic segments varies as a function of both the stage of the period of functional maturation and the order of magnitude of these segments. During the first stage, from P8 to P16, only proximal and intermediate segments lengthen, the distal segments increasing their length later from P16 to P20. The lengthening of the dendritic segments is associated with dramatic reabsorption of the dendritic spines on all dendritic segments and seems closely related to the functional maturation of the auditory pathways.

Vertex analysis of Purkinje cell dendritic trees in the cerebellum of the rat

All networks are made up of vertices (points interconnected by segments), which include terminals interconnected by terminal segments, nodes interconnected by link segments and the root point connected to the tree by the root segment. All nodes may be classified into unique types according to the number of terminal and link segments they drain. For example, there are three distinct dichotomous nodes, a 'primary' node draining two terminal segments, a 'secondary' node draining one terminal segment and a link segment, and a 'tertiary' node draining two link segments. The numbers of primary and tertiary nodes approximate to equality in large networks and thus the ratio of primary to secondary nodes defines topology. All higher order nodes ( trichotomous and beyond) may be resolved into dichotomous forms and incorporated into the analysis. Different forms of growth may thus be analysed by comparing the frequency distributions of nodes with those generated by computer simulated growth models. Moreover, all vertices can be ordered so that metrical parameters are easily incorporated and the hierarchical arrangements of vertices of different order discerned. The dendritic trees of 48 Purkinje cells, taken from folia along the primary fissure, were analysed using vertex analysis. The mean number of segments in Purkinje cell trees was 881 +/- 23 (s.e.) and mean total dendritic length 7959 +/- 233 (s.e.) micrometers. Segment lengths were longest over proximal segments but over most of the tree segment lengths were constant at 10 +/- 0.2 (s.e.) micrometers. Vertex, segment and terminal frequency distributions of equivalent orders were all normal with a slight positive skew. Peak frequencies were recorded at the 12th equivalent order. The mean primary/secondary nodal vertex ratio was 0.93 and the proportion of trichotomous branch points in the tree was 5%. Comparison of the frequency distribution of all vertices with computer generated models showed that growth of the Purkinje cell was most closely simulated by a random terminal growth model, incorporating 5% trichotomy , in which the branching of high order terminals was more likely than low order terminals. It was concluded that growth of the Purkinje cell tree could proceed by random terminal branching with growth occurring preferentially over a front composed of terminals that are ascending through a corridor in the molecular layer whose margins are defined by neighbouring trees.

Remodelling during development of the Purkinje cell dendritic tree in the mouse

The development of the Purkinje cells in normal C57 mice was studied from 7-100 d post natum . The growth of the dendritic trees was analysed both metrically and topologically using the method of vertex analysis (Berry & Flinn 1983 a). Granule and PUrkinje cell counts were made so that Purkinje cell segment production could be correlated with the number of parallel fibres deposited. Both topological and metrical results indicate that from 7 to 30 d post natum the Purkinje cell dendritic trees expand massively; accounting for 87% of total segment elaboration, reaching their lateral boundaries by 12-15 d post natum and then advancing towards the pial surface. Continued lateral expansion is constrained by the proximity of dendrites from neighbouring trees. Growth proceeds upwards through the neuropil as a front of prolific random terminal branching with inhibitory forces acting at the edges of the growth corridor and behind the growth front to prevent overlapping of dendrites. By 30 d post natum all boundaries are reached and the size of the dendritic field is fixed. Trees averaged 711.2 segments +/- 21.45 with a mean distance from root to terminal segment of 133.5 +/- 2.9 micrometers. The Va/Vb vertex ratios and the levels of trichotomy during this period indicate that branching patterns deviate from pure random terminal additions in a dichotomous tree. There is opportunity for non-random growth at the areas of inhibitory action. Beyond 30 d post natum remodelling occurs within the arbor which involves segment loss in the subpial region (orders above 16) and segment elaboration within the tree (orders 8-16) causing increased density of dendrites and overlapping of segments. The frequencies of segments and terminals are restored to symmetrical distributions through the orders of the trees from the skewed distributions associated with the frontal advance in earlier growth. During remodelling the Va/Vb vertex ratios and percentage of trichotomous nodes are consistent with growth through dichotomous random terminal branching. Path lengths of 8 micrometers between each order are seen as regular increments throughout entire trees at 100 d post natum . The final tree produced is indistinguishable from a network grown entirely by random terminal dichotomous branching with some 6% trichotomy and a Va/Vb vertex ratios of 0.92. Granule cell number within the granular layer increases rapidly up to 15 d post natum after which cell death causes a decrease to stable levels beyond 30 d post natum .(ABSTRACT TRUNCATED AT 400 WORDS)

Gradients of cellular maturation and synaptogenesis in the superior colliculus of the fetal rhesus monkey

Light (LM) and electron microscopic (EM) qualitative and quantitative analyses were employed to determine the tempo and spatial gradients of synaptogenesis and cellular differentiation in the superficial superior colliculus (SC) of the rhesus monkey between embryonic (E) days E47 and E84. By E47, a majority of the neurons of the prospective superficial gray layer (SGS) have arrived at their final positions and contribute to a uniform band of small, darkly Nissl-stained neurons at the outer surface of the SC. By E54, cells in the middle of the rostral pole of the superficial SC become considerably larger, paler staining, and less densely packed than the more medially or laterally located cells. These regional differences, which extend posteriorly through about the middle of the SC at this age, are evident on both the LM and EM levels and were confirmed by a quantitative EM analysis of the cytodifferentiation and synaptogenesis in the SGS. Several overlapping EM probes made across the medial, middle, and lateral regions of the SGS at each of three coronal levels reveal consistently more developed neuropil and smaller amounts of extracellular space in the middle region than in the medial and lateral portions of the more anterior SC. Further, the densities of synapses, both in terms of synapses/micron2 of total cross-sectional area and synapses/micron2 of neuropil alone, are also higher in the middle than the peripheral regions. Most of the middle-peripheral differences found in the mid-E50s are still evident by the early E60s, but have disappeared by midgestation (E80s). The present results are interpreted to indicate that the middle region of the SGS at a given transverse level begins to mature significantly earlier than the medial or lateral areas. Since our previous 3H-thymidine analysis (Cooper and Rakic, '81a) failed to reveal significant regional variation in the time of neuron origin in the superficial SC, the observed spatiotemporal gradients of neuronal maturation in the primate SGS probably do not arise from underlying gradients of cellular proliferation.

Intrinsic and extrinsic determinants of neuronal development: relation to infantile autism

This paper attempts to view the autistic syndrome in the context of a disorder of brain development. The authors review some of the known or suspected causes of the autistic syndrome: maternal rubella, metabolic diseases, and heredity. Some basic principles of cellular neuroanatomy and chemical neurotransmission are sketched. The stages of human brain development from neurulation through histogenesis, cell migration, and elaboration of dendritic trees and axonal projections are described. The authors conclude that there are a limited number of developmental loci that could be disrupted and lead to the autistic syndrome, and that these most probably occur in the end stages of neuronal development, after the migrating neurons have reached their final place in the brain and have begun to elaborate communicative processes. Finally, the authors speculate on how neurochemical disturbances might alter end stage neuronal differentiation leading to the pathology of infantile autism.

Transneuronal vestibular afferent influence on the nodular molecular layer synaptogenesis

The effect of vestibular afferent deprivation on the synaptogenesis of the nodular molecular layer has been studied quantitatively. No detectable effect on the time sequence of the development of the molecular layer and the external granular layer was found. Only around hatching a significantly reduced synaptic profile density was found in otocyst-deprived chickens on both halves of the nodulus. This effect can most easily be explained by the assumption of an anterograde transient transneuronal influence of vestibular afferents on the ability of parallel fibers to form synapses.

Malformation of Purkinje cell dendrites induced by graft-versus-host disease

Impairment of cerebral Purkinje cell growth was assessed in Golgi-Cox stained tissue in 14 day old (Fischer X DA)F1 hybrid rats subjected to graft versus host disease (GVHD). GVHD was induced by grafting parental strain lymph node cells (PSLNC) into the anterior facial vein on the day of birth. We have previously described GVHD induced changes in nucleotide and protein content [12] and RNA function [14] as well as alterations in cell numbers and areas of the external granular and molecular layers in diseased animals [13]. In this report some effects of GVHD on the gross morphology of Purkinje cells in 14 day old animals are enumerated. Most Purkinje cells (62%) from GVHD animals had a height to width ratio greater than 1, whereas most from control animals (71%) had ratios of less than 1. The majority of Purkinje cells (67%) from diseased animals had elongated main dendrites which were devoid of branches and often (13%) these elongated main dendrites were S-shaped. In addition, comparison of Purkinje cells from GVHD and control animals revealed a greater tendency (15%) toward retention of extrasomal process in GVHD animals. These findings suggest that GVHD affects nondividing, differentiating cell populations as well as those which are proliferating and migrating. Our evidence that, as a result of GVHD, the protein synthesizing capacity of the cerebellum is altered [14] and that Purkinje cells are more closely spaced at day 11 suggests that both intrinsic and extrinsic factors are involved in producing the changes in dendritic shape reported here.

Contextual memory in the molecular domain

Contextually rich recall of past events and actions indicates the formation of complex memory traces in which many items of information are integrated. The speed of this process and the inference that large numbers of cortical neurons are involved argue against synaptic transmission of all of the information required. The intercellular electromagnetic field giving rise to the EEG may function as an additional carrier of information essential to contextual processing. Recent experiments have led to models of the neural membrane that show very great sensitivities to the intercellular field. Changes of arrayed molecular conformations in this membrane due to cooperative effects in the intercellular field may provide a contextual memory located within the dense dendritic network of the cortex. Integrative effects within the volume structure of complex electromagnetic fields may thus provide a means of high-speed contextual processing and discrimination.