Reciprocal trophic interactions between climbing fibres and Purkinje cells in the rat cerebellum

Deletion of Calsyntenin-3, an atypical cadherin, suppresses inhibitory synapses but increases excitatory parallel-fiber synapses in cerebellum

Cadherins contribute to the organization of nearly all tissues, but the functions of several evolutionarily conserved cadherins, including those of calsyntenins, remain enigmatic. Puzzlingly, two distinct, non-overlapping functions for calsyntenins were proposed: As postsynaptic neurexin ligands in synapse formation, or as presynaptic kinesin adaptors in vesicular transport. Here, we show that, surprisingly, acute CRISPR-mediated deletion of calsyntenin-3 in mouse cerebellum in vivo causes a large decrease in inhibitory synapse, but a robust increase in excitatory parallel-fiber synapses in Purkinje cells. As a result, inhibitory synaptic transmission was suppressed, whereas parallel-fiber synaptic transmission was enhanced in Purkinje cells by the calsyntenin-3 deletion. No changes in the dendritic architecture of Purkinje cells or in climbing-fiber synapses were detected. Sparse selective deletion of calsyntenin-3 only in Purkinje cells recapitulated the synaptic phenotype, indicating that calsyntenin-3 acts by a cell-autonomous postsynaptic mechanism in cerebellum. Thus, by inhibiting formation of excitatory parallel-fiber synapses and promoting formation of inhibitory synapses in the same neuron, calsyntenin-3 functions as a postsynaptic adhesion molecule that regulates the excitatory/inhibitory balance in Purkinje cells.

Increased Climbing Fiber Lateral Crossings on Purkinje Cell Dendrites in the Cerebellar Hemisphere in Essential Tremor

BACKGROUND

Climbing fibers (CFs) innervate Purkinje cells (PCs) with 1:1 relationship to ensure proper cerebellar function. Although CFs abnormally extend into the parallel fiber domain of PC dendrites in essential tremor (ET), the architecture of CFs in relation to PCs has yet to be investigated in detail.

OBJECTIVE

The aim of this work was to study the architecture of CFs in relation to PCs in ET.

METHODS

The number of PC somas and PC dendrites that a single CF crossed was quantified in the postmortem cerebellum of 15 ET cases and 15 control cases.

RESULTS

In ET, CFs crossed a greater number of PC somas and PC dendrites than in control cases, raising the possibility that there is abnormal CF wiring onto the PCs. Interestingly, the increase in CF-PC crossings positively correlated with tremor severity.

CONCLUSIONS

Patients with ET have increased CF crossings on PC dendrites. This abnormal architectural arrangement may contribute to synchronous brain activity and tremor. © 2021 International Parkinson and Movement Disorder Society.

Severe Cerebellar Degeneration and Chiari I Malformation - Speculative pathophysiology based on a systematic review

BACKGROUND

Symptomatic Chiari Type I Malformation (CM) is treated with posterior fossa decompression with or without duroplasty. We have noticed some cases with concomitant severe cerebellar ataxia due to cerebellar atrophy. The aim of this study is to review the literature of CM associated with severe cerebellar atrophy and discuss its potential physiopathology.

METHODS

A systematic literature review in the Pubmed Database was performed using the following key-terms: "cerebellar atrophy Chiari", and "cerebellar degeneration Chiari". Articles reporting the presence of cerebellar degeneration/atrophy associated with CM were included.

RESULTS

We found only six studies directly discussing the association of cerebellar atrophy with CM, with a total of seven cases. We added one case of our own practice for additional discussion. Only speculative causes were described to justify cerebellar atrophy. The potential causes of cerebellar atrophy were diffuse cerebellar ischemia from chronic compression of small vessels (the most mentioned speculative cause), chronic raised intracranial pressure due to CSF block, chronic venous hypertension, and association with platybasia with ventral compression of the brainstem resulting in injury of the inferior olivary nuclei leading to mutual trophic effects in the cerebellum. Additionally, it is not impossible to rule out a degenerative cause for cerebellar atrophy without a causative reason.

CONCLUSIONS

Severe cerebellar atrophy is found in some patients with CM. Although chronic ischemia due to compression is the most presumed cause, other etiologies were proposed. The real reasons for cerebellar degeneration are not known. Further studies are necessary.

Prolidase enzyme is required for extracellular matrix integrity and impacts on postnatal cerebellar cortex development

The extracellular matrix is essential for brain development, lamination, and synaptogenesis. In particular, the basement membrane below the pial meninx (pBM) is required for correct cortical development. The last step in the catabolism of the most abundant protein in pBM, collagen Type IV, requires prolidase, an exopeptidase cleaving the imidodipeptides containing pro or hyp at the C-terminal end. Mutations impairing prolidase activity lead in humans to the rare disease prolidase deficiency characterized by severe skin ulcers and mental impairment. Thus, the dark-like (dal) mouse, in which the prolidase is knocked-out, was used to investigate whether the deficiency of prolidase affects the neuronal maturation during development of a brain cortex area. Focusing on the cerebellar cortex, thinner collagen fibers and disorganized pBM were found. Aberrant cortical granule cell proliferation and migration occurred, associated to defects in brain lamination, and in particular in maturation of Purkinje neurons and formation of synaptic contacts. This study deeply elucidates a link between prolidase activity and neuronal maturation shedding new light on the molecular basis of functional aspects in the prolidase deficiency.

Principal cell activity induces spine relocation of adult-born interneurons in the olfactory bulb

Adult-born neurons adjust olfactory bulb (OB) network functioning in response to changing environmental conditions by the formation, retraction and/or stabilization of new synaptic contacts. While some changes in the odour environment are rapid, the synaptogenesis of adult-born neurons occurs over a longer time scale. It remains unknown how the bulbar network functions when rapid and persistent changes in environmental conditions occur but when new synapses have not been formed. Here we reveal a new form of structural remodelling where mature spines of adult-born but not early-born neurons relocate in an activity-dependent manner. Principal cell activity induces directional growth of spine head filopodia (SHF) followed by spine relocation. Principal cell-derived glutamate and BDNF regulate SHF motility and directional spine relocation, respectively; and spines with SHF are selectively preserved following sensory deprivation. Our three-dimensional model suggests that spine relocation allows fast reorganization of OB network with functional consequences for odour information processing.

The mechanism by which adult-born neurons quickly adjust olfactory bulb network functioning is not understood. Here the authors describe a novel form of structural plasticity in which mature spines relocate toward active mitral cell dendrite along spine head filopodia via AMPA and BDNF mediated signalling.

Laser nanosurgery of cerebellar axons in vivo

Only a few neuronal populations in the central nervous system (CNS) of adult mammals show local regrowth upon dissection of their axon. In order to understand the mechanism that promotes neuronal regeneration, an in-depth analysis of the neuronal types that can remodel after injury is needed. Several studies showed that damaged climbing fibers are capable of regrowing also in adult animals. The investigation of the time-lapse dynamics of degeneration and regeneration of these axons within their complex environment can be performed by time-lapse two-photon fluorescence (TPF) imaging in vivo. This technique is here combined with laser surgery, which proved to be a highly selective tool to disrupt fluorescent structures in the intact mouse cortex. This protocol describes how to perform TPF time-lapse imaging and laser nanosurgery of single axonal branches in the cerebellum in vivo. Olivocerebellar neurons are labeled by anterograde tracing with a dextran-conjugated dye and then monitored by TPF imaging through a cranial window. The terminal portion of their axons are then dissected by irradiation with a Ti:Sapphire laser at high power. The degeneration and potential regrowth of the damaged neuron are monitored by TPF in vivo imaging during the days following the injury.

Guided motor training induces dendritic spine plastic changes in adult rat cerebellar purkinje cells

The simple cerebellar lobule is involved in several neuromotor processes and it is activated during guided exercise. Although guided exercises are essential for motor rehabilitation, the plastic events that occur in the simple cerebellar lobule during motor training remain unknown. In this study, normal adult rats were intensely trained on a motorized treadmill during a period of four weeks (IT group) varying both the velocity and the slope of the moving belt, and they were compared to a mildly trained (MC) group and an intact control group (IC). Dendritic spine density and proportions of the different spine types on Purkinje cells was assessed in the cerebellar simple lobule, as was drebrin A expression. Both dendritic spine density and drebrin expression increased in the MC and IT groups. Stubby spines were more abundant in the MC animals, while there was an increase in both stubby and wide spines in IT rats. In addition, mushroom spines were more numerous in the IT group. Increases in stubby and wide spines could be related to regulation of the excitability in Purkinje cells due to the motor training regime experienced by the MC and IT rats. Moreover, the observed increase in mushroom spines in the IT group could be related with the motor adjustments imposed by training.

Methylprednisolone treatment delays remote cell death after focal brain lesion

Glucocorticoids have a prominent role in the treatment of CNS injuries. However, the cellular consequences of glucocorticoid treatment on remote degenerative responses after focal brain lesions have been poorly investigated. Here we examine the effectiveness of a high dose (50 mg/kg) of methylprednisolone sodium succinate (MPSS) in reducing neuronal loss, glial response and glial-derived inflammatory mediators in inferior olive and pontine nuclei after lesion of the contralateral cerebellar hemisphere using immunohistochemistry and Western blot techniques. Quantitative analysis demonstrated that MPSS treatment significantly improved the survival of neurons in remote precerebellar stations. This survival was accompanied by reduction in the postlesional activation of microglia, astrocytes and interleukin-1 beta (IL-1beta). Cell death resumed after suspension of MPSS treatment and this delayed wave of cell loss was paralleled by reactivation of the inflammatory markers analyzed. The present study confirms the importance of inflammatory events in inducing remote cell death and that this type of degeneration can be delayed by MPSS treatment. Furthermore, the sustained effect of MPSS treatment, up to 28 days postlesion, and the reactivation of the degenerative phenomena after its suspension, support the hypothesis that glucocorticoid treatment, although capable of delaying cell death mechanisms, is not effective in blocking the cascade of remote degenerative events started by the primary lesion.

Diminished climbing fiber innervation of Purkinje cells in the cerebellum of myosin Va mutant mice and rats

Myosin Va is an actin-based molecular motor that is involved in organelle transport and membrane trafficking. Here, we explored the role of myosin Va in the formation of synaptic circuitry by examining climbing fiber (CF) innervation of Purkinje cells (PCs) in the cerebella of dilute-neurological (d-n) mice and dilute-opisthotonus (dop) rats that have mutations in dilute-encoded myosin Va. Anterograde labeling of CFs with biotinylated dextran amine (BDA) revealed that they arborized poorly and that their tips extended only half way through the thickness of the molecular layer (ML) in adult d-n mice. Using immunohistochemistry specific for vesicular glutamate transporter 2 (VGluT2) to visualize CF synaptic terminals, we found that during development and in adulthood, these terminals did not ascend as far along the proximal shaft dendrites of PCs in d-n mice and dop rats as they did in normal animals. An irregular distribution of BDA-labeled bulbous varicosities and VGluT2 spots along CF branches were also noted in these animals. Finally, VGluT2-positive CF terminals were occasionally localized on the PC somata of adult d-n cerebella. These phenotypes are consistent with our electrophysiological findings that CF-mediated excitatory postsynaptic currents (EPSCs) were significantly smaller in amplitude and faster in decay in adult d-n mice, and that the regression of multiple CFs was slightly delayed in developing d-n mice. Taken together, our results suggest that myosin Va is essential for terminal CF extension and for the establishment of CF synapses within the proper dendritic territories of PCs.

Afferent-target interactions during olivocerebellar development: transcommissural reinnervation indicates interdependence of Purkinje cell maturation and climbing fibre synapse elimination

We have used a model of postlesional reinnervation to observe the interactions between synaptic partners during neosynaptogenesis to determine how the developmental states of the pre- and postsynaptic cells influence circuit maturation. After unilateral transection of the neonatal rat olivocerebellar pathway (pedunculotomy), axons from the remaining ipsilateral inferior olive grow into the denervated hemicerebellum and develop climbing fibre (CF) terminal arbors on Purkinje cells (PCs) at a later stage of development than normal. However, the significance of delayed CF-PC interactions on subsequent circuit maturation remains poorly defined. To examine this question, we recorded CF-induced currents in PCs and analysed PC morphology during the first two postnatal weeks in control animals and following left unilateral inferior cerebellar pedunculotomy on postnatal day (P)3. Our results show that transcommissural olivary axons multiply-reinnervate PCs in the denervated hemisphere over 4 days following pedunculotomy. Each PC received fewer CFs than did age-matched controls and the maximal multi-reinnervation was reached on P7, 2 days later than in controls. Consequently, the onset of CF synapse elimination in reinnervated PCs was delayed, but then proceeded in parallel with controls so that all PCs were monoinnervated by P15. Furthermore, reinnervated PCs had delayed dendritic maturation and subsequent dendritic abnormalities consistent with the role of CF innervation in PC dendritic growth. Thus, within the olivocerebellar system, our data suggest that target neurons depend upon sufficient afferent investment arriving at the correct time for their normal development, and maturation of the target neuron regulates afferent selection and therefore circuit maturation.

Expression of MAP1B protein and its phosphorylated form MAP1B-P in the CNS of a continuously growing fish, the rainbow trout

Microtubule-associated protein-1B (MAP1B), and particularly its phosphorylated isoform MAP1B-P, play an important role in axonal outgrowth during development of the mammalian nervous system and have also been shown to be associated with axonal plasticity in the adult. Here, we used antibodies and mRNA probes directed against mammalian MAP1B to extend our analysis to fish species, trout (Oncorhynchus mykiss), at different stages of development. The specificity of the cross-reaction of our anti-total-MAP1B/MAP1B-P antibodies was confirmed by Western blotting. Trout MAP1B-like proteins exhibited about the same apparent molecular weight (320 kDa) as rat-MAP1B. Immunohistochemistry and in situ hybridization analysis performed on hindbrain and spinal cord revealed the presence of MAP1B in neurons and some glial subpopulations. Primary sensory neurons and motoneurons maintain high levels of MAP1B expression from early stages throughout adulthood, as has been shown for mammals. Unlike mammals, however, MAP1B and axon-specific MAP1B-P continue to be strongly expressed by hindbrain neurons projecting into spinal cord, with the important exception of Mauthner cells. MAP1B/MAP1B-P immunostaining were also detected elsewhere within the brain, including axons of the retino-tectal projection. This obvious difference between adult fish and mammals is likely to reflect the capacity of fish for continued growth and regeneration. Our results suggest that MAP1B/MAP1B-P expression is generally maintained in neurons known to regenerate after axotomy. The regenerative potential of the adult nervous system may in fact depend on continued expression of neuron-intrinsic growth related proteins, a feature of MAP1B that appears phylogenetically conserved.

Retrograde activation of presynaptic NMDA receptors enhances GABA release at cerebellar interneuron–Purkinje cell synapses

Synaptic inhibition is a vital component in the control of cell excitability within the brain. Here we report a newly identified form of inhibitory synaptic plasticity, termed depolarization-induced potentiation of inhibition, in rodents. This mechanism strongly potentiated synaptic transmission from interneurons to Purkinje cells after the termination of depolarization-induced suppression of inhibition. It was triggered by an elevation of Ca(2+) in Purkinje cells and the subsequent retrograde activation of presynaptic NMDA receptors. These glutamate receptors promoted the spontaneous release of Ca(2+) from presynaptic ryanodine-sensitive Ca(2+) stores. Thus, NMDA receptor-mediated facilitation of transmission at this synapse provides a regulatory mechanism that can dynamically alter the synaptic efficacy at inhibitory synapses.

Post-lesion transcommissural growth of olivary climbing fibres creates functional synaptic microzones

In the adult mammalian central nervous system, reinnervation and recovery from trauma is limited. During development, however, postlesion plasticity may generate alternate paths, providing models to investigate reinnervating axon-target interactions. After unilateral transection of the neonatal rat olivocerebellar path, axons from the ipsilateral inferior olive grow into the denervated hemicerebellum and develop climbing fibre (CF)-like arbors on Purkinje cells (PCs). However, the synaptic function and extent of PC reinnervation remain unknown. In adult rats pedunculotomized on postnatal day 3 the morphological and electrophysiological properties of reinnervating olivocerebellar axons were studied, using axonal reconstruction and patch-clamp PC recording of CF-induced synaptic currents. Reinnervated PCs displayed normal CF currents, and the frequency of PC reinnervation decreased with increasing laterality. Reinnervating CF arbors were predominantly normal but 6% branched within the molecular layer forming smaller secondary arbors. CFs arose from transcommissural olivary axons, which branched extensively near their target PCs to produce on average 36 CFs, which is six times more than normal. Axons terminating in the hemisphere developed more CFs than those terminating in the vermis. However, the precise parasagittal microzone organization was preserved. Transcommissural axons also branched, although to a lesser extent, to the deep cerebellar nuclei and terminated in a distribution indicative of the olivo-cortico-nuclear circuit. These results show that reinnervating olivocerebellar axons are highly plastic in the cerebellum, compensating anatomically and functionally for early postnatal denervation, and that this reparation obeys precise topographic constraints although axonal plasticity is modified by target (PC or deep nuclear neurons) interactions.

Selective changes in the shapes of parasagittal bands of Aldoc (Zebrin) mRNA in the rat vermis of the cerebellum after repeated methamphetamine injections

In the cerebellum the mossy and climbing projections, which excite Purkinje cells, display a parasagittal and striped organization. These projections also excite Zebrin (aldolase C: Aldoc) parasagittally. To evaluate the possibility that external stimuli can change the organization of the bands of Aldoc mRNA, we compared the effects of repeated methamphetamine administration on the Aldoc mRNA stripes in the four transverse (anterior, central, posterior and nodular) regions of the vermis with the effects on the glutamate transporter EAAT4 (SCL1A 6) mRNA stripes. In the posterior region the injections four times daily increased the fragmentation of the Aldoc mRNA stripes. The presence of a large amount of fragmentation (forty/cerebellum slice), was accompanied with large lateral dislocations of the Aldoc mRNA stripes. In the central and nodular regions, where the size of the stripe areas decreased significantly the stripes were dislocated laterally. The dislocations of the Aldoc mRNA bands did not occur after a single methamphetamine injection and thus repeated injections were necessary to change the distributions of the lateral bands. In contrast, the distributions of the SCL1A 6 mRNA stripes did not change, even though there was mild fragmentation (six/slice) of the SLC1A 6 mRNA stripes in the anterior region and decreases in the numbers (twelve/slice) in the nodular region. We concluded that excess dopamine selectively changes the location of the Aldoc mRNA compartments in the vermis while the SLC1A 6 mRNA stripes could be changed by other inputs and thus the specific transmitter system might change the specific compartment of the cerebellum.

Axotomy dependent purinergic and nitrergic co-expression

Different lines of evidence indicate that ATP and nitric oxide (NO) play key roles in mediating neuronal responses after cell damage. Purinergic and nitrergic interactions have been proposed in non neural tissues physiological functions and, in different experimental models of brain injury, both purinergic and nitrergic activations have been reported. The present study was planned to ascertain possible relations of these two systems after brain damage. Variations in the expression of the nitric oxide synthase neuronal isoform (nNOS) enzyme, and of two subunits of purinergic ionotrophic receptors (P2X) namely P2X(1) and P2X(2) in precerebellar stations after cerebellar lesion in rats were analyzed and compared. After the lesion nNOS positive cells presented a clear increment followed by a decrement. Conversely, nNOS negative cells presented a rapid decrement in the first postlesional weeks that continued less pronounced afterward. Postlesional nNOS activation was related with time course of P2X(1) and P2X(2) activations. The capacity of the same cells to express both nNOS and P2X markers was investigated immunocytochemically. Confocal microscopy of double immunofluorescence showed a high percentage of co-localization among P2X(1)/nNOS, P2X(2)/nNOS and P2X(1)/P2X(2) in olivary and pontine neurons. In addition, NeuN/P2X(1) and NeuN/P2X(2) double immunofluorescence showed P2X(1) expressed only in neurons while P2X(2) expressed by both neurons and glia. Present data demonstrate that after cerebellar lesion nitrergic and purinergic systems are activated with similar time courses in precerebellar stations. Further, time differences in the relation between nNOS expression and cell survival suggest a multifarious role of NO in mediating cell reaction to axotomy. The tight cellular co-localization and temporal co-activation of purinergic and nitrergic markers indicate possible interactions between these two systems also in the CNS.

Signals from the cerebellum guide the pathfinding of inferior olivary axons

During development, inferior olivary axons cross the floor plate and project from the caudal to the rostral hindbrain, whence they grow into the cerebellar plate. We have investigated the axon guidance signals involved in the formation of this projection in vitro. When the cerebellar plate was grafted ectopically along the margin of the hindbrain in organotypic cultures, inferior olivary axons could pathfind to the ectopic cerebellum, establishing a topographically normal projection. Following rostrocaudal reversal of a region of tissue in the axon pathway between the inferior olive and the cerebellum, olivary axons still navigated towards the cerebellum. Moreover, olivary axons could cross a bridging tissue explant (spinal cord) to reach a cerebellar explant. In collagen gel cultures of inferior olive explants, olivary axon outgrowth increased significantly in the presence of cerebellar explants and axons deflected towards the cerebellar tissue. These results show that the cerebellum is a source of diffusible axon guidance signals for olivary axons. We also found that, in organotypic cultures, olivary axons which had crossed the floor plate showed an increased tendency to respond to cerebellar cues. Taken together, these results indicate that the cerebellum is the source of cues that are chemoattractant and growth-promoting for inferior olivary axons; prior exposure to the floor plate increases responsiveness to these cues.

Distinctive cerebellar immunoreactivity for the prion protein in familial (E200K) Creutzfeldt-Jakob disease

We have compared the immunomorphological spectrum of the deposition of the disease-associated prion protein (PrP(Sc)) in the cerebral and cerebellar cortex of 32 Creutzfeldt-Jakob disease (CJD) patients with the PrP gene (PRNP) E200K mutation to 45 sporadic CJD and 14 other genetic prion disease cases. PrP deposits correlate with the genotype at the methionine/valine (MV) polymorphic codon 129. While the diffuse/synaptic and patchy/perivacuolar PrP deposits and PrP plaques have a similar distribution and correlation with the genotype at codon 129 as in sporadic CJD, an additional peculiar PrP immunostaining pattern occurs in the cerebellum in 81% E200K mutation brains including 93% of M129M, 71% of M129V, but not in the single V129V case. It is localized to the molecular layer and consists of coarse granular PrP deposits arranged in a stripe-like manner predominantly perpendicular to the surface, closely resembling the parasagittal arborization of climbing fibers. Our results suggest that (1) the type of PrP deposits in the cerebellum may suggest genetic disease and the need for genetic testing; and (2) the peculiar stripes of PrP deposits might reflect selective vulnerability of cerebellar structures.

Cerebellar lesion up-regulates P2X1 and P2X2 purinergic receptors in precerebellar nuclei

ATP released in the extracellular space by neuronal injury can influence neighboring neurons via activation of purinergic receptors. In vitro data suggest the involvement of ATP and purinergic receptors as trophic agents in different biological events such as neuritogenesis and cell survival. Recently, in vivo studies have demonstrated modifications in the glial expression of ionotropic purinergic receptors after CNS lesions. In the present study, we investigated the effects of CNS lesion on the neuronal expression of P2X(1) and P2X(2) receptor subunits by immunohistochemistry and western blotting techniques. In the precerebellar structures of normal animals the expression of P2X(1) and P2X(2) was lower than previously reported. P2X(1) immunostaining was confined only to fibers, while P2X(2) immunostaining demonstrated a neuronal expression. After unilateral cerebellar lesion (hemicerebellectomy) axotomized precerebellar neurons underwent marked cell loss; however, some precerebellar neurons did not degenerate. Seven to 35 days after hemicerebellectomy, a transient, time-dependent, marked increase in the number of immunopositive P2X(1) and P2X(2) neurons was observed in the precerebellar nuclei of the experimental side. An even distribution of immunopositive neurons was present in almost all precerebellar nuclei examined, except for the inferior olive. In this latter structure, differences in the distribution of immunopositive neurons were evident among the subnuclei. Up-regulation of immunoreactivity over relatively long time periods, distribution selectivity and absence of degenerating morphological features in immunopositive neurons suggest that purinergic receptors may have a role in mediating the survival of neuronal responses to axotomy. The present findings are the first report in the CNS of P2X(1) and P2X(2) receptor subunit involvement in neuronal reaction to axotomy. They provide in vivo evidence of a correlation between purinergic receptor subunit up-regulation and survival of injured neurons.

The localisation of urocortin in the adult rat cerebellum: a light and electron microscopic study

Light and electron microscopic immunocytochemistry was used to identify the cellular and subcellular localisation of urocortin in the adult rat cerebellum. Urocortin immunoreactivity (UCN-ir) was visualised throughout the cerebellum, yet predominated in the posterior vermal lobules, especially lobules IX and X, the flocculus, paraflocculus and deep cerebellar nuclei. Cortical immunoreactivity was most evident in the Purkinje cell layer and molecular layer. Reaction product, though sparse, was found in the somata of Purkinje cells, primarily in the region of the Golgi apparatus. Purkinje cell dendritic UCN-ir was compartmentalised, with it being prevalent in proximal regions especially where climbing fibres synapsed, yet absent in distal regions where parallel fibres synapsed. In the Purkinje cell layer, the labelling was also contained in axonal terminals, synapsing directly on Purkinje cell somata. These were identified as axon terminals of basket cells based on their morphology. Terminals of stellate cells in the upper molecular layer also expressed the peptide. Whilst somata of inferior olivary neurones showed intense immunoreactivity, axonal labelling was indistinct, with only the terminals of climbing fibres containing reaction product. UCN-ir in the mossy fibre-parallel fibre system was restricted to mossy fibre rosettes of mainly posterior lobules and the varicose terminals of parallel fibres. Furthermore, labelling also was prevalent in glial perikarya and their sheaths. The current study shows, firstly, that urocortin enjoys a close ligand-receptor symmetry in the cerebellum, probably to a greater degree than corticotropin-releasing factor since corticotropin-releasing factor itself is found exclusively in the two major cerebellar afferent systems. Its congregation in excitatory and inhibitory axonal terminals suggests a significant degree of participation in the synaptic milieu, perhaps in the capacity as a neurotransmitter or effecting the release of co-localised neurotransmitters. Finally, its unique distribution in the Purkinje cell dendrite might serve as an anatomical marker of discrete populations of dendritic spines.

Cerebellar synaptic protein expression in schizophrenia

A cortico-subcortico-cerebellar neural circuit has been postulated to be important in the pathophysiology of schizophrenia. This study investigated whether there are synaptic changes in the cerebellum to accompany its putative involvement in the disorder. We measured the expression of three synaptic proteins (synaptophysin, complexin I and complexin II) in the cerebellar cortex of 16 subjects with schizophrenia and 16 controls using in situ hybridisation histochemistry and immunoautoradiography. Complexin I and II are expressed predominantly by inhibitory and excitatory neurones respectively. In schizophrenia, synaptophysin mRNA was decreased, as was complexin II and its mRNA. Complexin I mRNA and protein levels were unaltered. Expression of the mRNAs in the rat cerebellum was unaffected by 2 weeks administration of antipsychotic drugs (haloperidol, chlorpromazine, risperidone, olanzapine or clozapine). We conclude that there is synaptic pathology in the cerebellum in schizophrenia. By disrupting neural circuits, the alterations may contribute to the cerebellar dysfunction thought to occur in the disorder.

Three-dimensional morphology of cerebellar climbing fibers. A study by means of confocal laser scanning microscopy and scanning electron microscopy

The intracortical pathway of cerebellar climbing fibers have been traced by means of scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) to study the degree of lateral collateralization of these fibers in the granular Purkinje cell and molecular layers. Samples of teleost fish were processed for conventional and freeze-fracture SEM. Samples of hamster cerebellum were examined by means of CLSM using FM4-64 as an intracellular stain. High resolution in lens SEM of primate cerebellar cortex was carried out using chromium coating. At scanning electron and confocal laser microscopy levels, the climbing fibers appeared at the white matter and granular layer as fine fibers with a typical arborescence or crossing-over branching pattern, whereas the mossy fibers exhibited a characteristic dichotomous bifurcation. At the granular layer, the parent climbing fibers and their tendrils collaterals appeared to be surrounding granule and Golgi cells. At the interface between granule and Purkinje cell layers, the climbing fibers were observed giving off three types of collateral processes: those remaining in the granular layer, others approaching the Purkinje cell bodies, and a third type ascending directly to the molecular layer. At this layer, retrograde collaterals were seen descending to the granular layer. By field emission high-resolution SEM of primate cerebellar cortex, the climbing fiber terminal collaterals were appreciated ending by means of round synaptic knobs upon the spines of secondary and tertiary Purkinje cell dendrites.

Axonal atrophy: the retraction reaction

Recent studies indicate that morphological alterations of axon branches that are removed during normal development are similar to those that occur following ablation of postsynaptic cells in adult animals. In both situations, axons retract (rather than degenerate), the calibers of withdrawing axon branches are markedly reduced, and spherical swellings near (or at) the axon terminations appear. The similarity between naturally occurring and target-deprived axon withdrawal suggests that both developing and adult axons withdraw from target cells that no longer provide support.