Mature Purkinje cells in cerebellar tissue cultures: an ultrastructural study

Activity-Dependent Inhibitory Synaptogenesis in Cerebellar Cultures

Inhibitory synapses on Purkinje cell somata in organotypic cerebellar cultures derived from newborn mice were increased after chronic exposure post explantation to agents that enhance neuronal activity. Inhibitory synaptogenesis was reduced in similar cultures after continuous blockade of spontaneous neuronal discharges. By contrast, excitatory synapses developed fully in the absence of neuronal activity. The reduction of inhibitory synaptogenesis was prevented by the simultaneous application of activity blocking agents and neurotrophins BDNF or NT-4, which are TrkB receptor ligands, but not with NT-3, a TrkC receptor ligand. The effect of endogenous neurotrophins was evaluated by continuously exposing cerebellar cultures to antibodies to BDNF and NT-4, which caused a significant reduction in the development of inhibitory Purkinje cell axosomatic synapses. These combined results indicated a role for TrkB receptors in activity-dependent inhibitory synaptogenesis. This concept was supported by the promotion of inhibitory synaptogenesis by specific antibody activation of TrkB receptors.

The changeable nervous system: studies on neuroplasticity in cerebellar cultures

Circuit reorganization after injury was studied in a cerebellar culture model. When cerebellar cultures derived from newborn mice were exposed at explantation to a preparation of cytosine arabinoside that destroyed granule cells and oligodendrocytes and compromised astrocytes, Purkinje cells surviving in greater than usual numbers were unensheathed by astrocytic processes and received twice the control number of inhibitory axosomatic synapses. Purkinje cell axon collaterals sprouted and many of their terminals formed heterotypical synapses with other Purkinje cell dendritic spines. The resulting circuit reorganization preserved inhibition in the cerebellar cortex. Following this reorganization, replacement of the missing granule cells and glia was followed by a restitution of the normal circuitry. Most of these developmental and reconstructive changes were not dependent on neuronal activity, the major exception being inhibitory synaptogenesis. The full complement of inhibitory synapses did not develop in the absence of neuronal activity, which could be mitigated by application of exogenous TrkB receptor ligands. Inhibitory synaptogenesis could also be promoted by activity-induced release of endogenous TrkB receptor ligands or by antibody activation of the TrkB receptor.

Morphological study of organotypic cerebellar cultures

Organotypic cerebellar cultures from 8-days-old (P8) mouse pups were studied following 11 days of in vitro (I IDIV) culturing. The cerebellar cytoarchitectonic structure was maintained in most parasagittal cerebellar cortical slice cultures (also containing the deep cerebellar nuclei). The two main extrinsic excitatory inputs (the climbing and the mossy fibers) seem to be replaced by other axonal types: in the molecular layer mostly by parallel fibers (for climbing fibers) and in the granular layer by intrinsic mossy fiber collaterals of local excitatory interneurons, the unipolar brush cells. However, in a few organotypic cultures, which (although preserving the trilaminar cerebellar cortical structure) were "granuloprival" but also contained some of the deep cerebellar nuclei, the participation of extracortical axons from the deep cerebellar nuclei in the replacement of the missing afferents is suggested.

Altered dendritic development of cerebellar Purkinje cells in slice cultures from protein kinase Cgamma-deficient mice

Protein kinase C (PKC) is a key molecule for the expression of long-term depression at the parallel fiber-Purkinje cell synapse in the cerebellum, a well known model for synaptic plasticity. We have recently shown that activity of PKC also profoundly affects the dendritic morphology of Purkinje cells in rat cerebellar slice cultures suggesting that synaptic efficacy and dendritic development may be controlled by similar intracellular signalling pathways. Here we have analyzed the role of the gamma-isoform of protein kinase C (PKCgamma), which is strongly and specifically expressed in Purkinje cells, during dendritic development. After pharmacological treatment with PKC modulators, phosphorylation of PKCgamma at serine 660 was altered in cerebellar slices suggesting that a change of PKCgamma activity by these treatments was taking place within the Purkinje cells. In PKCgamma-deficient mice, Purkinje cell dendritic trees were enlarged and had an increased number of branching points compared to wild-type mice indicating a role for the PKCgamma isoform as a negative regulator of dendritic growth and branching. Furthermore, the branching-stimulating effects of the PKC inhibitors 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl)maleimide and Gö6976 found in wild-type cultures were abolished in the absence of PKCgamma. In contrast, the strong inhibitory effect on dendritic growth by the PKC activator phorbol-12-myristate-13-acetate (PMA) did not require the presence of the PKCgamma isoform since it was still present in the cultures of PKCgamma-deficient mice. Our results clearly demonstrate an involvement of PKCgamma in Purkinje cell dendritic differentiation in cerebellar slice cultures.

Unipolar brush cells form a glutamatergic projection system within the mouse cerebellar cortex

Unipolar brush cells (UBCs) of the mammalian vestibulocerebellum receive mossy fiber projections primarily from the vestibular ganglion and vestibular nuclei. Recently, the axons of UBCs have been shown to generate an extensive system of cortex-intrinsic mossy fibers, which resemble traditional cerebellar mossy fiber afferents and synapse with granule cell dendrites and other UBCs. However, the neurotransmitter used by the UBC axon is still unknown. In this study, we used long-term organotypic slice cultures of the isolated nodulus (lobule X) from postnatal day 8 mouse cerebella to identify the neurotransmitter and receptors at synapses of the UBC axon terminals, relying on the notion that, in these cultures, all of the cortex-extrinsic fibers had degenerated during the first few days in vitro. Quantification of glutamate immunogold labeling showed that the UBC axon terminals have the same high gold-particle density as the glutamatergic parallel fiber varicosities. Furthermore, UBCs identified by calretinin immunoreactivity expressed the glutamate receptor subunits GluR2/3, NMDAR1, and mGluR2/3, like they do in the mature mouse cerebellum in situ. Evoked excitatory postsynaptic currents (EPSCs), spontaneous EPSCs, and burst discharges were demonstrated in UBCs and granule cells by patch-clamp recording. Both the evoked and spontaneous EPSCs were blocked by ionotropic glutamate receptor antagonists CNQX and D-AP5. We conclude that neurotransmission at the UBC axon terminals is glutamatergic. Thus, UBCs provide a powerful network of feedforward excitation within the granular layer, which may amplify vestibular signals and synchronize activity in clusters of functionally related granule cells which project vertically to patches of Purkinje cells.

Protein kinase C activity modulates dendritic differentiation of rat Purkinje cells in cerebellar slice cultures

The molecular mechanisms underlying dendritic differentiation in neurons are currently poorly understood. We used slice cultures from rat cerebellum of postnatal day 8 to investigate the effect of protein kinase C (PKC) activity on dendritic development of Purkinje cells. After 12 days in culture under control conditions, Purkinje cells had developed a typical dendritic tree consisting of a few long primary dendrites with shorter side branches. Following treatment with the PKC agonist, phorbol-12-myristate-13-acetate (PMA), the dendritic tree area was strongly reduced to 32% of control and primary dendrites were short with only a few side branches. Delayed addition of PMA after 6 days resulted in a retraction of existing dendrites, whereas discontinuation of PMA treatment after 6 days resulted in a recovery of the dendritic tree to almost control values. In the presence of the PKC inhibitor, 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl)maleimide (GF109203X), the dendritic tree area was increased to 158% of control with much more ramified branches after 12 days. The overall morphology of the cultures and the survival of Purkinje cells were unaffected by PKC modulators. Our data show that increased activity of PKC inhibits, and reduced activity of PKC promotes dendritic growth. This suggests that PKC activity is a critical regulator of dendritic growth and differentiation in cerebellar Purkinje cells.

Influence of functional glia on the electrophysiology of Purkinje cells in organotypic cerebellar cultures

Previous studies have shown that exposure of organotypic cerebellar explants to cytosine arabinoside (Sigma) for the first five days in vitro drastically reduced the granule cell population and severely affected glial function. Myelination was absent and astrocytes failed to ensheath Purkinje cells. In the absence of astrocytic ensheathment, Purkinje cell somata became hyperinnervated by Purkinje cell recurrent axon collaterals. Recurrent axon collaterals also projected to Purkinje cell dendritic spines. In later studies, exposure of cerebellar cultures to a different formulation of cytosine arabinoside (Pfanstiehl) also affected granule cells and oligodendrocytes but did not compromise astrocyte function. The different susceptibility of astrocytes to the two preparations of cytosine arabinoside (Sigma and Pfanstiehl) has provided the opportunity to examine the electrophysiological properties of Purkinje cells in the presence and absence of functional glia. Ensheathed Purkinje cells in granuloprival cultures exhibit within two weeks in vitro similar passive membrane properties as Purkinje cells in control cultures. Their input resistance is significantly higher and their spontaneous single-unit discharge is significantly lower than that of unensheathed Purkinje cells. This effect suggests that ensheathed Purkinje cells in cytosine arabinoside (Pfanstiehl)-treated cultures are more responsive to the profuse Purkinje cell recurrent axon collateral inhibitory projection to dendritic spines. These studies also show that the presence of functional glia and/or astrocytic ensheathment can be correlated with the development of complex spike activity by Purkinje cells in vitro. Purkinje cells in cultures treated with cytosine arabinoside (Pfanstiehl), which does not compromise astrocytic ensheathment, display membrane conductances and spike activity similar to mature Purkinje cells in control cultures. By contrast, Purkinje cells in cultures treated with cytosine arabinoside (Sigma), and devoid of astrocytic ensheathment, display mainly simple spike activity reminiscent of the type of activity seen in less mature neurons.

The extracellular matrix molecule, laminin, induces purkinje cell dendritic spine proliferation in granule cell depleted cerebellar cultures

Granule cells and glia were eliminated or reduced in organotypic cerebellar cultures exposed to cytosine arabinoside. Transplantation of such granuloprival cultures with glia or exposure to astrocyte conditioned medium in the absence of parallel fibers (granule cell axons) resulted in proliferation of Purkinje cell dendritic spines. The aim of the present study was to identify specific astrocyte secreted factors that induced dendritic spine proliferation. Known astrocyte secreted, neurite promoting factors were screened by application to granuloprival cultures and assayed for dendritic spine proliferation by electron microscopy. An extracellular matrix molecule, laminin, evoked sprouting of Purkinje cell dendritic spines. Dendritic spine proliferation was not associated with known neurite promoting parts of the laminin molecule, as two laminin-derived peptides with identified neurite promoting domains did not induce dendritic spine sprouting. The purpose of laminin-induced dendritic spine proliferation may be to elaborate postsynaptic membrane, thereby increasing the target area for arriving axon terminals during development or regeneration, both of which have been associated with the presence of laminin secreting astrocytes.

Serial changes in granuloprival cerebellar cultures after transplantation with granule cells and glia: a timed ultrastructural study

Granuloprival cerebellar cultures derived from neonatal mice were transplanted at nine days in vitro with granule cells and glia, and the changes induced in the host explants were examined daily with the electron microscope from one to nine days post-transplantation. Granule cells and astrocytes had migrated into the host cultures within 24 h, and astrocytic processes began to ensheath Purkinje cells and to interpose themselves between axon terminals and Purkinje cell somata, reducing the number of axosomatic synapses. Occasional degenerating Purkinje cells were present. At two days post-transplantation, synapse formation between parallel fibre terminals and Purkinje cell dendritic spines was initially evident, and Purkinje cells began to proliferate dendritic spines near astrocytic processes. Degenerating Purkinje cells were more frequently encountered. Myelin was first observed in host cultures at three days after transplantation, and astrocytes continued to ensheath Purkinje cells and reduce the population of axosomatic synapses, a process that began to stabilize at four days post-transplantation. At this time astrocytic ensheathment had extended to Purkinje cell dendrites and dendritic spine synapses. Proliferation of Purkinje cell dendritic spines accelerated, and occasional synapses with presumptive parallel fibre terminals were present among clusters of proliferated spines. At five days after transplantation, contours of Purkinje cells were rounded, and there was a decrease of somatic spines and of synapses with somatic spines. Purkinje cells were fully ensheathed by astrocytic processes by six days post-transplantation and had assumed a mature appearance. Homotypical parallel fibre-Purkinje cell dendritic spine synapses were predominant in more developed areas of cortical neuropil as heterotypical recurrent axon collateral-Purkinje cell dendritic spine synapses were reduced. Increasing synapse formation was evident among clusters of proliferated spines, which continued at seven days post-transplantation, as the spine clusters became less frequent. At eight days after transplantation, space between Purkinje cells had increased and the cortical neuropil resembled that of comparably aged control cultures. Occasional degenerating Purkinje cells were still evident at nine days post-transplantation, at which time residual clusters of proliferated unattached dendritic spines were scarce. The sequence of changes after transplantation was consistent with the specific roles of the transplanted elements. Astrocytes were involved with the regulation of synapse density, including reduction of some heterotypical synapses, and induced proliferation of Purkinje cell dendritic spines. Granule cell axons synapsed with Purkinje cell dendritic spines, further reducing heterotypical synapses and restoring cortical circuitry to a near control state. The loss of heterotypical synapses was associated with programmed cell death of excess Purkinje cells, reducing the Purkinje cell population to control levels.

Neural plasticity in cerebellar cultures

Cerebellar granule cells and oligodendrocytes are destroyed and astrocytes are functionally compromised by exposure of organotypic cerebellar cultures derived from newborn mice to cytosine arabinoside for the first 5 days in vitro. Consequently, myelin does not form and Purkinje cells survive in increased numbers, but without astrocytic ensheathment. In the absence of glial sheaths, Purkinje cells have altered membrane properties and reduced input resistance. Their inhibitory recurrent axon collaterals sprout enormously and hyperinnervate the unensheathed somata of other Purkinje cells and form heterotypical synapses with Purkinje cell dendritic spines normally occupied by homotypical excitatory parallel fiber (granule cell axon) terminals. This reorganization of the cortical circuitry, in which recurrent axon collaterals are the dominant inhibitory elements, allows retention of some inhibition in the absence of parallel fiber excitation of the inhibitory interneurons. In the absence of neuronal activity, the full complement of inhibitory synapses is not developed and the cultures exhibit sustained cortical hyperactivity after recovery from the blockade. If granule cells and glia are replaced, a second round of reorganization ensues, in the direction of restoration of the normal cortical circuitry. The cultures are myelinated and the number of recurrent axon collaterals is reduced. Astrocytes ensheath Purkinje cell somata and strip excess axosomatic synapses, as well as eliminate some of the heterotypical synapses in the cortical neuropil. Parallel fibers synapse with already present Purkinje cell dendritic spines and with newly proliferated spines, the latter induced by an astrocyte secreted factor. As homotypical synapses develop and heterotypical synapses decline, Purkinje cells undergo apoptosis and their population is reduced to control levels. With the restoration of parallel fiber excitation, recurrent axon collaterals are no longer the dominant cortical inhibitory elements. If neuronal activity is blocked as the granule cells and glia are replaced, there is incomplete formation of inhibitory synapses, and cortical discharges are hyperactive after recovery from activity blockade.

Electrophysiological differences between Purkinje cells in organotypic and granuloprival cerebellar cultures

Organotypic cerebellar cultures derived from newborn mice were exposed to cytosine arabinoside for the first five days in vitro to destroy granule cells and functionally compromise glia. Such granuloprival cultures undergo a circuit reorganization featured by Purkinje cells sprouting recurrent axon collaterals that hyperinnervate other Purkinje cells. Intracellular recordings were used to compare the electrophysiological properties of Purkinje cells in granuloprival cultures to those of Purkinje cells in standard cultures. Purkinje cells in granuloprival cultures have similar membrane potentials to those of Purkinje cells in standard cultures, but have a lower input resistance. A reduced input resistance could affect the effectiveness of inhibitory synaptic input. Intracellular recordings from Purkinje cells of standard cerebellar cultures between 13 and 21 days in vitro exhibit spike activity consisting of a mixture of complex and simple spikes. The complex spikes contain a fast rising action potential followed by a depolarizing potential on which a plateau and several spike-like components are superimposed. This type of activity has been observed in mature Purkinje cells in vivo and in vitro. By contrast, at resting membrane potential Purkinje cells in granuloprival cultures have simple spike activity reminiscent of the type of activity seen in immature Purkinje cells, while at hyperpolarized potentials they generate complex spikes. These observations indicate differences in the expression of intrinsic electrophysiological properties underlying complex spike generation between Purkinje cells of organotypic and granuloprival cerebellar cultures. Our results illustrate the considerable plasticity of Purkinje cells in the presence of altered neuronal circuitry. In the absence of normal excitatory input, their spontaneous activity is regulated by intrinsic membrane properties.

Cerebellar Purkinje cells from the lurcher mutant and wild-type mouse grown in vitro: a light and electron microscope study

Lurcher is an autosomal semidominant murine mutation. Lurcher heterozygotes (+/Lc) lose all their cerebellar Purkinje cells by adulthood. Explants from 2 days postnatal (P2) wild-type (+/+) and +/Lc cerebellar cortex were grown in vitro to investigate the role of local neuronal environment and afferent input on the degenerating +/Lc Purkinje cell. In Lurcher explants, Purkinje cells were maintained for up to 25 days in vitro. No significant difference was observed between +/+ and +/Lc Purkinje cell numbers from 10 to 20 days in vitro, as revealed by calbindin-D immunoreactivity. Growing +/Lc explants in association with +/+ explants resulted in no significant difference in Purkinje cell survival (10-20 days in vitro). Image analysis of the gross morphology of calbindin-D-immunostained Purkinje cells from +/+ and +/Lc explants grown in vitro revealed a significant decrease in the total area and dendritic lengths of +/Lc Purkinje cells (15 and 20 days in vitro). The fine structure of +/Lc and +/+ Purkinje cells was examined under the electron microscope (10-25 days in vitro). No difference in ultrastructure was observed between +/Lc and +/+ Purkinje cells grown in vitro, and many features similar to normal Purkinje cell development in vivo were present. These included monosynaptic parallel fibre synapses with Purkinje cell dendritic spines, other interneuron synapses with Purkinje cell dendrites and soma, astroglial investment, and minimal extracellular space in the neuropil. Unusual features observed included a persistence of the perisomatic spines in some Purkinje cells, an absence of Nissl bodies in the Purkinje cell perikaryon, naked Purkinje cell dendritic spines, and occasional heterologous synapses. The results are discussed in the light of previous chimeric analysis of the Lurcher mutation, and a hypothesis is put forward to explain the survival of +/Lc Purkinje cells in vitro.

Circuit reorganization in granuloprival cerebellar cultures in the absence of neuronal activity

Neonatal mouse cerebellar cultures exposed to cytosine arabinoside for the first 5 days in vitro to destroy granule cells and compromise glia undergo a circuit reorganization featured by profuse sprouting of Purkinje cell recurrent axon collaterals, which hyperinnervate the somata of other Purkinje cells and project to Purkinje cell dendritic spines. Such granuloprival cultures were exposed continuously from explanation to tetrodotoxin and elevated levels of magnesium to block neuronal activity. A similar circuit reorganization occurred, except that there was a reduction in the number of axospinous synapses and Purkinje cell axosomatic synapses, which in this case were all inhibitory. Functionally, after recovery from the blockade, granuloprival cultures developed sustained cortical hyperactivity, which was consistent with the reduction of inhibitory synapses. While the absence of neuronal activity did not prevent reorganizational changes following granule cell loss, the full development of the inhibitory circuitry was not attained. These results further support the concept that spontaneous neuronal activity is necessary for the complete development of inhibitory synapses.

Persistence of heterotypical synapses in transplanted cerebellar cultures in the absence of functional glia

Neonatal mouse cerebellar cultures depleted of granule cells and functional glia by exposure to cytosine arabinoside were transplanted with either granule cells and glia or with granule cells in the absence of functional glia. Myelination was evident in cultures transplanted with granule cells and glia, excess sprouted cortical neurites were reduced, Purkinje cells acquired astrocytic sheaths and had a near normal complement of axosomatic synapses, and homotypical parallel fiber-Purkinje cell dendritic spine synapses were present in a 2.4:1 ratio to heterotypical recurrent axon collateral-Purkinje cell dendritic spine synapses. Cultures transplanted with granule cells were not myelinated, sprouted cortical neurites were not reduced. Purkinje cells lacked astrocytic sheaths and their somata remained hyperinnervated, and the ratio of homotypical to heterotypical dendritic spine synapses was 1.4:1. In the absence of functional glia there was a greater persistence of heterotypical recurrent axon collateral-Purkinje cell dendritic spine synapses. These results are consistent with a previously described astrocytic role in the regulation of axosomatic synapse density on glially ensheathed neurons, and suggest astrocytic participation in the reduction of heterotypical axospinous synapses. Astrocyte-mediated synapse reduction may be an important mechanism for circuit reorganization after transplantation or during development.

Observation of the highly organized development of granule cells in rat cerebellar organotypic cultures

Cerebellar slices of 9-day-old rats were cultured for a week at the interface between air and a culture medium containing horse serum and hormone cocktail, and the development of granule cells was characterized morphologically. The typical layered structure of the cerebellar cortex was well preserved during the cultivation. Many granule cells in the external granular layer (EGL) proliferated actively within the early culture period. They were migrating downward at 3 days in vitro (DIV) and almost completed the migration to the internal granular layer (IGL) after 6 DIV. In the middle and upper molecular layer (ML), parallel fibers were elongated horizontally, which is the orientation 90 degrees rotated compared to that in vivo. They formed synapses with Purkinje cell dendrites. Regional differences in synapse density and maturity existed which might reflect a gradient in progressive synapse formation comparable to that in vivo. Thus, a serial process of highly organized development of granule cells was realized for the first time in vitro, although some spatial or temporal modifications existed. Such a culture system could be a useful experimental model for the study of cellular and molecular mechanisms of spatiotemporally organized neuronal development.

Morphological correlates of altered neuronal activity in organotypic cerebellar cultures chronically exposed to anti-GABA agents

Organotypic cerebellar cultures derived from newborn mice were chronically exposed to medium containing picrotoxin or bicuculline from explanation until they were recorded from extracellularly or fixed for morphological studies. Cultures exposed to anti-GABA agents for 13-18 days in vitro had decreased spontaneous cortical discharge rates when compared with sister control cultures and prolonged inhibitory responses to cortical stimulation. Electron microscopic examination of exposed cultures after 14-16 days in vitro revealed a hyperinnervation of Purkinje cell somata by inhibitory terminals predominantly of basket cell origin. The sprouted terminals penetrated otherwise intact Purkinje cell astrocytic sheaths. These changes represent a departure from the usual developmental pattern, a departure induced by exposure to anti-GABA agents that increased neuronal activity early in the development of the cerebellar circuitry in vitro. The precise signals that initiated the changes are unknown, but the altered development is most likely in response to increased Purkinje cell activity.

Presynaptic fibres of spiral neurons and reciprocal synapses in the organ of Corti in culture

Isolated segments of the newborn mouse organ of Corti were explanted together with the spiral ganglion components. Within the innervation provided by the spiral neurons, we observed presynaptic vesiculated nerve endings that form reciprocal ribbon-afferent/efferent synapses with inner hair cells. These intracochlear presynaptic fibres are characteristically located between adjoining inner hair cells, on the modiolar side, low and close to the supporting cells. The presynaptic fibres display different modes of synaptic connectivity, forming repetitive reciprocal synapses on single inner hair cells or on adjoining hair cells, or connecting adjoining inner hair cells through simultaneous efferent synapses. Many presynaptic fibres exhibit a distinctive ultrastructure: defined clusters of synaptic vesicles, dense core vesicles, coated vesicles, and mitochondria. These organelles occur focally at the synaptic sites; beyond the efferent synaptic specializations, the endings appear quite nondescript and afferent-like. We believe that the reciprocal synapses, although observed in cultures of the organ of Corti, represent real intracochlear synaptic arrangements providing a feedback mechanism between the primary sensory receptors and a special class of spiral ganglion cells that have yet to be recognized in the organ in situ.

Cytosine arabinoside effects in mouse cerebellar cultures in the presence of astrocytes

Organotypic cerebellar cultures derived from neonatal mice were exposed to recent preparations of cytosine arabinoside that destroyed oligodendrocytes and drastically reduced granule cells, but did not reduce the astrocyte population. The cultures were analysed by light and electron microscopy, and by extracellular electrophysiological recording. Purkinje cells survived in greater numbers than in untreated explants and sprouted excess recurrent axon collaterals that formed heterotypical synapses with Purkinje cell dendritic spines. These changes were similar to those found in earlier studies with a cytosine arabinoside preparation that did reduce the astrocyte population, in addition to destroying oligodendrocytes and granule cells. Results with recent cytosine arabinoside preparations that differed from those obtained previously included astrocytic ensheathment of Purkinje cells and apposition of many unattached dendritic spines, encasement of heterotypical synapses by astroglial processes, a loss of Purkinje cell somatic spines, and a lack of somatic hyperinnervation of Purkinje cells by sprouted recurrent axon collateral terminals. All of these differences were attributed to the presence of adequate numbers of competent astrocytes. Heterotypical synapses formed by sprouted recurrent axon collateral terminals and Purkinje cell dendritic spines were functional, as indicated by cortical inhibition in response to antidromic Purkinje cell activation in the absence of somatic hyperinnervation. These results give further definition to the role of astrocytes in cerebellar development and plasticity.

Tyrosine hydroxylase expression in non-catecholaminergic cells in cerebellar cultures

Organotypic cerebellar cultures, some with incorporated portions of brainstem, were immunostained after 12-19 days in vitro with three different antibodies to tyrosine hydroxylase. Similar cultures were reacted with glyoxylic acid and examined for catecholamine histofluorescence. Locus coeruleus and other subcortical neurons were positive for tyrosine hydroxylase, as were Purkinje cells and outgrowth zone astrocytes. By contrast, only locus coeruleus neurons and their axons exhibited catecholamine histofluorescence after reaction with glyoxylic acid. These results confirm previously reported in vivo developmental studies indicating that tyrosine hydroxylase can be expressed in the absence of its normal biosynthetic products, and suggest that tyrosine hydroxylase cannot be considered to be a specific marker for catecholaminergic neurons in vitro, as well as during development in vivo.

Reorganization of organotypic cultures of mouse cerebellum exposed to cytosine arabinoside: a timed ultrastructural study

This study was designed to examine the sequential changes in the developing granuloprival cerebellar culture. In this model of anomalous cerebellar development, organotypic cultures derived from newborn Swiss-Webster mice were exposed to the DNA synthesis inhibitor, cytosine arabinoside, at explantation and were fixed for electron microscopic examination on successive days in vitro. Similar developmental stages were compared in control explants. Granule cell destruction began early, and was widespread by 2 days in vitro, when oligodendrocyte destruction also began in treated cultures. A few granule cells survived, but no recognizable oligodendrocytes remained by 7 days in vitro, at a time when myelin was initially evident in control explants. Purkinje cell recurrent axon collateral sprouting began at 3 days in vitro in cultures exposed to cytosine arabinoside, and the sprouted terminals initially synapsed with Purkinje cell somata, somatic spines and dendritic shafts. Synapses with Purkinje cell dendritic spines developed later, at approximately the same time as parallel fiber-Purkinje cell dendritic spine synapses formed in control cultures. Astrocytic ensheathment of control Purkinje cells was well underway by 6 days in vitro and Purkinje cell somata were relatively rounded and almost completely ensheathed by 9 days in vitro. Glial ensheathment did not occur in cytosine arabinoside treated cultures, and Purkinje cell somata were scalloped at 7 days in vitro by excess impinging recurrent axon collateral terminals, and never developed the smooth contours characteristic of control Purkinje cells. Purkinje cell somatic spines persisted in treated explants, and reduction of excess extracellular space was delayed until 12 days in vitro, when most of the developmental changes had been completed. The earlier development of synapses by excess recurrent axon collateral terminals with Purkinje cell somata, somatic spines and dendritic shafts, followed by the later development of heterotypical synapses with dendritic spines, in parallel with synapse formation by normal presynaptic elements, suggests that the sequence of development of synapses is a function of the maturational state of the postsynaptic components.

Computer-assisted analysis of the developing Purkinje neuron. I. Effects of the age of the animal at the moment of explantation on the subsequent dendritic development in organotypic cultures

Purkinje cell dendritic arborization were studied in intracellularly horseradish peroxidase-stained mature neurons grown during 30-40 days in organotypic cerebellar cultures from kittens of various ages. The effects of the age of the animal at the moment of explantation upon the subsequent dendritic and axonal development were studied in kitten cerebella of 1, 12 and 21 postnatal days old. These effects were investigated by computer-assisted methods. Qualitative data were found to be different in these 3 in vitro models explanted at 3 different ages except for the spine development which remained the same. Quantitative data, obtained from 15 measurable parameters, showed that the degree of dendritic development was higher for the Purkinje cells explanted at the latest postnatal age: the older the animal, the larger the dendritic tree in culture. A discrimination analysis permitted a recognition of the 3 differently aged populations (94% well classified cells) based on 3 selected variables, number of primary dendritic trunks, number of rows and dendritic field areas. These results lend further support to the major role played in the final dendritic arrangement by extrinsic (discriminant factors, their pre-existence in vivo being responsible for the subsequent degree of the in vitro development of the Purkinje cell dendritic arbor.

Distribution of carbonic anhydrase activity in the rat central nervous system, as revealed by a new semipermeable technique

Results obtained with a new method provided evidence for the extraneuronal localization of CA and supports the idea that the enzyme is very widespread in non-neuronal cell types of the CNS. Most of these cells were considered to be oligodendroglia, but probably the astrocytes also contributed to the reactivity of the neuropil. The perineuronal CA activity observed in the spinal cord, brainstem, cerebellum, and hippocampus could be astrocytic in origin. Our observations concerning the widespread CA staining of the CNS vessels pointed to the possible functional significance of CA in the vessel wall. This activity could be due not only to pericytes but also to astrocytic processes. We have not found stained myelin sheats although biochemically the myelin contained the enzyme. Might be that our histochemical reaction was not sensitive to the membrane bound form of the CA. The fact that the reaction of the nucleoli did not disappear after acetazolamide treatment could be explained on the basis of binding of the cobalt-phosphate complex to the proteins of the nucleolus.

Astrocytes play a role in regulation of synaptic density

Exposure of neonatal cerebellar explants to cytosine arabinoside destroys granule cells and arrests surviving glia in an early stage of maturation. Purkinje cells lack astroglial ensheathment and are hyperinnervated by sprouted Purkinje cell recurrent axon collateral terminals. Such granuloprival cultures were transplanted with optic nerve in order to supply mature glial cells. It was observed that not only were Purkinje cells almost completely ensheathed by astroglia, but there was a greater than 60% reduction in the number of somatic synapses compared to the non-transplanted granuloprival cultures. This astroglial ensheathment, which may be neuronally directed, could be the physical element provoking the reduction in the number of synapses.

Maintenance of immunocytologically identified Purkinje cells from mouse cerebellum in monolayer culture

Purkinje cells were identified in monolayer cultures obtained from trypsin-dissociated cerebella of embryonic and early postnatal mice by the Purkinje cell-specific monoclonal antibodies PC1, PC2, PC3 and UCHT1. These cells also expressed the neuronal marker L1 antigen but not the glial markers, glial fibrillary acidic protein or 04 antigen. They also expressed tetanus toxin receptors, PC4, M1 and Thy-1 antigens. Survival of Purkinje cells was best: (a) when cerebella were taken from mice not older than one day of age: (b) when cells were seeded at higher plating densities; and (c) cultured in chemically defined medium which facilitates the survival of neurons. No Purkinje cells could be detected in cultures from mice older than 6 days. PC1 antigen expression developed in vitro on the same time scale as in vivo, i.e. it was first detectable at the equivalent of postnatal days 3-4. At this stage cell bodies had a size of 13-14 micron in diameter and few processes. Dendrite-like arborizations, with more than one primary dendrite, extension of usually only one thin and long (0.5-1.6 mm) axon-like process and collaterals directed preferentially towards other Purkinje cells, developed with time in culture until the final form was reached by the equivalent of approximately day 16. Cell body size was 18-19 micron in diameter at this stage. Cell shapes were reminiscent of those described in certain cerebellar mouse mutants and in experimentally produced agranular cerebella. Many ultrastructural features of these cells correlated with those described for the in vivo counterpart. However, there was a lack of spiny branchlets and abnormally long persisting somatic spines. Synaptic contacts of the 'en passant' type could be seen at the Purkinje cell soma. Gray type I synapses were seen on Purkinje cell dendrites and spines.

Slice cultures of cerebellar, hippocampal and hypothalamic tissue

Cerebellar, hippocampal and hypothalamic slices prepared from newborn and 7-day-old rats were cultured by means of the roller-tube technique. Identification of cells was made easier by the fact that at least part of the characteristic cytoarchitecture of the tissue was preserved in vitro. Cerebellar Purkinje cells and neurones of the deep cerebellar nuclei were recognized on the basis of their size, their location within the culture and their dendritic arborization. Pyramidal cells of all hippocampal subfields displayed their characteristic basal and apical dendritic trees with numerous spinous processes. Hippocampal granule cells gave rise to a monopolar dendritic arbor; their axons terminated in the dentate hilus and CA3 region. Golgi-like immuniperoxidase staining allowed localization of groups of neurophysin-positive neurones in slices prepared from the anterior hypothalamus. These neurones, bilaterally bordering the third ventricle, usually displayed a simple dendritic arborization and fine beaded axons. - Cultivation of brain slices prepared from young rats offers particular advantages in that the cultured cells are organized in an organotypic monolayer and individual living neurones may be directly visualized.

The dendritic trees of Purkinje cells: a computer assisted analysis of HRP labeled neurons in organotypic cultures of kitten cerebellum

Purkinje cell dendritic arborizations were studied in HRP intracellularly stained mature neurons grown during 30-50 days in organotypic cerebellar cultures from newborn kittens. The effects of afferent fiber depletions on the final dendritic topological parameters were investigated by computer assisted methods. Three differently deafferented models lacking both parallel and climbing fibers (PF and CF), only PF or only CF were thus studied. Qualitative data were found to be common to these 3 in vitro models and similar to those following other in vivo deafferenting procedures. Quantitative data obtained from 16 measurable parameters showed that all dendritic trees were markedly reduced in size as indicated by a decrease in total dendritic length and total number of segments although in all models individual segment lengths remained largely unaltered. A discriminant analysis permitted the recognition of 3 populations (100% well classified cells) based on 3 selected variables: dendritic field area, mean path length and total number of segments (with the lowest values for the model lacking only PF). These results are shown to lend further support to the role played in the final dendritic arrangement by both intrinsic (non-discriminant) and extrinsic (discriminant) factors and more precisely by the two distinct specific afferent systems (PF and CF).

Reorganization in granuloprival cerebellar cultures after transplantation of granule cells and glia. II. Ultrastructural studies

Cytosine arabinoside-induced granuloprival cerebellar cultures lack both granule cells and differentiated glia and demonstrate marked synaptic reorganization. After kainic acid-exposed cerebellar explants, which contain granule cells and mature glia, were transplanted to the granuloprival cultures, the following ultrastructural features were noted: (1) parallel fibers formed normal synapses with Purkinje cell dendritic spines as well as with basket/stellate cell somata; (2) sprouted Purkinje cell recurrent axon collateral terminals were markedly reduced in number; (3) Purkinje cells matured and lost perisomatic spines; (4) astroglia formed sheaths around Purkinje cell somata and dendrites; and (5) axonal myelination occurred. The transplanted cultures demonstrated ultrastructural restitution toward normal after addition of missing elements.