Climbing fiber multi-innervation of mouse Purkinje dendrites with arborization common to human

Implementing feature binding through dendritic networks of a single neuron

A single neuron receives an extensive array of synaptic inputs through its dendrites, raising the fundamental question of how these inputs undergo integration and summation, culminating in the initiation of spikes in the soma. Experimental and computational investigations have revealed various modes of integration operations that include linear, superlinear, and sublinear summation. Interestingly, different types of neurons exhibit diverse patterns of dendritic integration depending on the spatial distribution of dendrites. The functional implications of these specific integration modalities remain largely unexplored. In this study, we employ the Purkinje cell (PC) as a model system to investigate these complex questions. Our findings reveal that PCs generally exhibit sublinear summation across their expansive dendrites. Both spatial and temporal input dynamically modulates the degree of sublinearity. Strong sublinearity necessitates the synaptic distribution in PCs to be globally scattered sensitive, whereas weak sublinearity facilitates the generation of complex firing patterns in PCs. Using dendritic branches characterized by strong sublinearity as computational units, we demonstrate that a neuron can successfully address the feature binding problem. Taken together, these results offer a systematic perspective on the functional role of dendritic sublinearity, inspiring a broader understanding of dendritic integration in various neuronal types.

Mapping the developmental profile of ventricular zone-derived neurons in the human cerebellum

The cerebellar ventricular zone (VZ) is the primary source of progenitors that generate cerebellar GABAergic neurons, including Purkinje cells (PCs) and interneurons (INs). This study provides detailed characterization of human cerebellar GABAergic neurogenesis using transcriptomic and histopathological analyses and reveals conserved and unique features compared to rodents. We show that the sequential progression of neurogenesis is conserved and occurs before 8 postconception weeks. Notably, PC differentiation occurs in the outer subventricular zone (SVZ), a region absent in the mouse cerebellum. Human PCs are generated during a compact two-week period before the onset of cerebral cortex histogenesis. A subset of human PCs retain proliferative marker expression weeks after leaving the VZ, another feature not observed in rodents. Human PC maturation is protracted with an extensive migration and reorganization throughout development with dendritic arborization developing in late gestation. We define a continuous transcriptional cascade of PC development from neuroepithelial cells to mature PCs. In contrast, while human interneuronal progenitors are born beginning in early fetal development, they exhibit an even more protracted differentiation across late gestation and into postnatal ages. These findings show dynamic developmental process for human cerebellar GABAergic neurons and underscore the importance of the embryonic environment, with early disruptions having potentially significant impacts.

Non-allometric expansion and enhanced compartmentalization of Purkinje cell dendrites in the human cerebellum

Purkinje cell (PC) dendrites are optimized to integrate the vast cerebellar input array and drive the sole cortical output. PCs are classically seen as stereotypical computational units, yet mouse PCs are morphologically diverse and those with multi-branched structure can receive non-canonical climbing fiber (CF) multi-innervation that confers independent compartment-specific signaling. While otherwise uncharacterized, human PCs are universally multi-branched. Do they exceed allometry to achieve enhanced integrative capacities relative to mouse PCs? To answer this, we used several comparative histology techniques in adult human and mouse to analyze cellular morphology, parallel fiber (PF) and CF input arrangement, and regional PC demographics. Human PCs are substantially larger than previously described; they exceed allometric constraint by cortical thickness and are the largest neuron in the brain with 6-7 cm total dendritic length. Unlike mouse, human PC dendrites ramify horizontally to form a multi-compartment motif that we show can receive multiple CFs. Human spines are denser (6.9 vs 4.9 spines/μm), larger (~0.36 vs 0.29 μm), and include an unreported 'spine cluster' structure-features that may be congruent with enhanced PF association and amplification as human-specific adaptations. By extrapolation, human PCs may receive 500,000 to 1 million synaptic inputs compared with 30-40,000 in mouse. Collectively, human PC morphology and input arrangement is quantitatively and qualitatively distinct from rodent. Multi-branched PCs are more prevalent in posterior and lateral cerebellum, co-varying with functional boundaries, supporting the hypothesis that this morphological motif permits expanded input multiplexing and may subserve task-dependent needs for input association.

Large brains: Big unknowns in cellular neuroscience

Contemporary cellular neuroscience is strong on small but weak on large brains. Large brains have lower neuronal densities than smaller brains. We outline opposing functional interpretations of this result. Analysis of human brains supports the idea that dendritic complexity matters and might even correlate with intellectual ability. Cortical connectomics revealed an elaboration of disinhibitory motifs in human brains. There is disagreement as to whether glia-to-neuron ratios differ between small and large brains. The elaborate myeloarchitecture of the human brain has long been recognized and novel evidence indicates myelin might play nonconventional structural functions in larger brains. Three-dimensional body-part models in the cortex of tactile specialists point to the significance of the three-dimensional structure of cortical networks. The comparative assessment of brain performance remains one of the biggest challenges in neurobiology. Understanding cellular differences between small and large brains is a neglected, yet fundamental issue for neuroscience and translation.

Ignoring the cerebellum is hindering progress in neuroscience

Traditionally considered a motor structure, the cerebellum has been shown to play a key role in several cognitive functions. However, for decades, the cerebellum has been largely overlooked and even deliberately excluded from 'whole-brain' neuroimaging studies. Here, we propose that the continued exclusion of the cerebellum has limited our understanding of whole-brain function. We describe reasons - both warranted and unwarranted - behind its historical exclusion from the neuroimaging literature, review literature describing the importance of the cerebellum and its unique role in brain function, and outline the potential unintended negative consequences of exclusion of the cerebellum for our comprehensive understanding of brain function and clinical disorders.

Cerebellar control of targeted tongue movements

The cerebellum is critical for coordinating movements related to eating, drinking and swallowing, all of which require proper control of the tongue. Cerebellar Purkinje cells can encode tongue movements, but it is unclear how their simple spikes and complex spikes induce changes in the shape of the tongue that contribute to goal-directed movements. To study these relations, we recorded and stimulated Purkinje cells in the vermis and hemispheres of mice during spontaneous licking from a stationary or moving water spout. We found that Purkinje cells can encode rhythmic licking with both their simple spikes and complex spikes. Increased simple spike firing during tongue protrusion induces ipsiversive bending of the tongue. Unexpected changes in the target location trigger complex spikes that alter simple spike firing during subsequent licks, adjusting the tongue trajectory. Furthermore, we observed increased complex spike firing during behavioural state changes at both the start and the end of licking bouts. Using machine learning, we confirmed that alterations in Purkinje cell activity accompany licking, with different Purkinje cells often exerting heterogeneous encoding schemes. Our data highlight that directional movement control is paramount in cerebellar function and that modulation of the complex spikes and that of the simple spikes are complementary during acquisition and execution of sensorimotor coordination. These results bring us closer to understanding the clinical implications of cerebellar disorders during eating, drinking and swallowing. KEY POINTS: When drinking, mice make rhythmic tongue movements directed towards the water source. Cerebellar Purkinje cells can fire rhythmically in tune with the tongue movements. Purkinje cells encode changes in the position of the water source with complex spikes. Purkinje cell simple spike firing affects the direction of tongue movements. Purkinje cells that report changes in the position of the target can also adjust movements in the right direction.

The endocytic adaptor AP-2 maintains Purkinje cell function by balancing cerebellar parallel and climbing fiber synapses

The loss of cerebellar Purkinje cells is a hallmark of neurodegenerative movement disorders, but the mechanisms remain enigmatic. We show that endocytic adaptor protein complex 2 (AP-2) is crucial for Purkinje cell survival. Using mouse genetics, viral tracing, calcium imaging, and kinematic analysis, we demonstrate that loss of the AP-2 μ subunit in Purkinje cells leads to early-onset ataxia and progressive degeneration. Synaptic dysfunction, marked by an overrepresentation of parallel fibers (PFs) over climbing fibers (CFs), precedes Purkinje cell loss. Mechanistically, AP-2 interacts with the PF-enriched protein GRID2IP, and its loss triggers GRID2IP degradation and glutamate δ2 receptor (GLURδ2) accumulation, leading to an excess of PFs while CFs are reduced. The overrepresentation of PFs increases Purkinje cell network activity, which is mitigated by enhancing glutamate clearance with ceftriaxone. These findings highlight the role of AP-2 in regulating GRID2IP levels in Purkinje cells to maintain PF-CF synaptic balance and prevent motor dysfunction.

Non-allometric expansion and enhanced compartmentalization of Purkinje cell dendrites in the human cerebellum

Purkinje cell (PC) dendrites are optimized to integrate the vast cerebellar input array and drive the sole cortical output. PCs are classically seen as stereotypical computational units, yet mouse PCs are morphologically diverse and those with multi-branched structure can receive non-canonical climbing fiber (CF) multi-innervation that confers independent compartment-specific signaling. While otherwise uncharacterized, human PCs are universally multi-branched. Do they exceed allometry to achieve enhanced integrative capacities relative to mouse PCs? To answer this, we used several comparative histology techniques in adult human and mouse to analyze cellular morphology, parallel fiber (PF) and CF input arrangement, and regional PC demographics. Human PCs are substantially larger than previously described; they exceed allometric constraint by cortical thickness and are the largest neuron in the brain with 6-7cm total dendritic length. Unlike mouse, human PC dendrites ramify horizontally to form a multi-compartment motif that we show can receive multiple CFs. Human spines are denser (6.9 vs 4.9 spines/μm), larger (~0.36 vs 0.29μm), and include an unreported 'spine cluster' structure-features that may be congruent with enhanced PF association and amplification as human-specific adaptations. By extrapolation, human PCs may receive 500,000 to 1 million synaptic inputs compared with 30-40,000 in mouse. Collectively, human PC morphology and input arrangement is quantitatively and qualitatively distinct from rodent. Multi-branched PCs are more prevalent in posterior and lateral cerebellum, co-varying with functional boundaries, supporting the hypothesis that this morphological motif permits expanded input multiplexing and may subserve task-dependent needs for input association.

Cerebellar asymmetries

The cerebellum is a subcortical structure tucked underneath the cerebrum that contains the majority of neurons in the brain, despite its small size. While it has received less attention in the study of brain asymmetries than the cerebrum, structural asymmetries in the cerebellum have been found in cerebellar volume that mirror cerebral asymmetries. Larger cerebellar structures have been reported on the right compared to the left, either for the whole cerebellar hemisphere or the anterior part of the cerebellum, with the latter accompanied by a left increase in the posterior cerebellum. Cerebellar asymmetries are considered evolutionary recent and have been observed prenatally and in early development. Both asymmetries in anterior-posterior divisions and specific lobules have been linked to handedness and cognitive abilities, in particular language. Functional lateralization in the cerebellum varies across motor and cognitive functions, with language activation predominantly localized in the right hemisphere, contralateral to cerebral activation. Meanwhile, working memory and executive functions are not lateralized to one hemisphere. New neuroimaging methods and resources, including a symmetric functional atlas of the cerebellum that enables precision mapping, open novel avenues for exploring cerebellar asymmetries and answering questions about the developmental timeline, relationships to behavior, and clinical relevance.

If you please, draw me a neuron - linking evolutionary tinkering with human neuron evolution

Animal speciation often involves novel behavioral features that rely on nervous system evolution. Human-specific brain features have been proposed to underlie specialized cognitive functions and to be linked, at least in part, to the evolution of synapses, neurons, and circuits of the cerebral cortex. Here, we review recent results showing that, while the human cortex is composed of a repertoire of cells that appears to be largely similar to the one found in other mammals, human cortical neurons do display specialized features at many levels, from gene expression to intrinsic physiological properties. The molecular mechanisms underlying human species-specific neuronal features remain largely unknown but implicate hominid-specific gene duplicates that encode novel molecular modifiers of neuronal function. The identification of human-specific genetic modifiers of neuronal function brings novel insights on brain evolution and function and, could also provide new insights on human species-specific vulnerabilities to brain disorders.

Impact of Intrauterine Insults on Fetal and Postnatal Cerebellar Development in Humans and Rodents

Many children suffer from neurodevelopmental aberrations that have long-term effects. To understand the consequences of pathological processes during particular periods in neurodevelopment, one has to understand the differences in the developmental timelines of brain regions. The cerebellum is one of the first brain structures to differentiate during development but one of the last to achieve maturity. This relatively long period of development underscores its vulnerability to detrimental environmental exposures throughout gestation. Moreover, as postnatal functionality of the cerebellum is multifaceted, enveloping sensorimotor, cognitive, and emotional domains, prenatal disruptions in cerebellar development can result in a large variety of neurological and mental health disorders. Here, we review major intrauterine insults that affect cerebellar development in both humans and rodents, ranging from abuse of toxic chemical agents, such as alcohol, nicotine, cannabis, and opioids, to stress, malnutrition, and infections. Understanding these pathological mechanisms in the context of the different stages of cerebellar development in humans and rodents can help us to identify critical and vulnerable periods and thereby prevent the risk of associated prenatal and early postnatal damage that can lead to lifelong neurological and cognitive disabilities. The aim of the review is to raise awareness and to provide information for obstetricians and other healthcare professionals to eventually design strategies for preventing or rescuing related neurodevelopmental disorders.

Increased understanding of complex neuronal circuits in the cerebellar cortex

The prevailing belief has been that the fundamental structures of cerebellar neuronal circuits, consisting of a few major neuron types, are simple and well understood. Given that the cerebellum has long been known to be crucial for motor behaviors, these simple yet organized circuit structures seemed beneficial for theoretical studies proposing neural mechanisms underlying cerebellar motor functions and learning. On the other hand, experimental studies using advanced techniques have revealed numerous structural properties that were not traditionally defined. These include subdivided neuronal types and their circuit structures, feedback pathways from output Purkinje cells, and the multidimensional organization of neuronal interactions. With the recent recognition of the cerebellar involvement in non-motor functions, it is possible that these newly identified structural properties, which are potentially capable of generating greater complexity than previously recognized, are associated with increased information capacity. This, in turn, could contribute to the wide range of cerebellar functions. However, it remains largely unknown how such structural properties contribute to cerebellar neural computations through the regulation of neuronal activity or synaptic transmissions. To promote further research into cerebellar circuit structures and their functional significance, we aim to summarize the newly identified structural properties of the cerebellar cortex and discuss future research directions concerning cerebellar circuit structures and their potential functions.

Branch-specific clustered parallel fiber input controls dendritic computation in Purkinje cells

Most central neurons have intricately branched dendritic trees that integrate massive numbers of synaptic inputs. Intrinsic active mechanisms in dendrites can be heterogeneous and be modulated in a branch-specific way. However, it remains poorly understood how heterogeneous intrinsic properties contribute to processing of synaptic input. We propose the first computational model of the cerebellar Purkinje cell with dendritic heterogeneity, in which each branch is an individual unit and is characterized by its own set of ion channel conductance densities. When simultaneously activating a cluster of parallel fiber synapses, we measure the peak amplitude of a response and observe how changes in P-type calcium channel conductance density shift the dendritic responses from a linear one to a bimodal one including dendritic calcium spikes and vice-versa. These changes relate to the morphology of each branch. We show how dendritic calcium spikes propagate and how Kv4.3 channels block spreading depolarization to nearby branches.

Purkinje cell models: past, present and future

The investigation of the dynamics of Purkinje cell (PC) activity is crucial to unravel the role of the cerebellum in motor control, learning and cognitive processes. Within the cerebellar cortex (CC), these neurons receive all the incoming sensory and motor information, transform it and generate the entire cerebellar output. The relatively homogenous and repetitive structure of the CC, common to all vertebrate species, suggests a single computation mechanism shared across all PCs. While PC models have been developed since the 70's, a comprehensive review of contemporary models is currently lacking. Here, we provide an overview of PC models, ranging from the ones focused on single cell intracellular PC dynamics, through complex models which include synaptic and extrasynaptic inputs. We review how PC models can reproduce physiological activity of the neuron, including firing patterns, current and multistable dynamics, plateau potentials, calcium signaling, intrinsic and synaptic plasticity and input/output computations. We consider models focusing both on somatic and on dendritic computations. Our review provides a critical performance analysis of PC models with respect to known physiological data. We expect our synthesis to be useful in guiding future development of computational models that capture real-life PC dynamics in the context of cerebellar computations.

Tools for Cre-Mediated Conditional Deletion of Floxed Alleles from Developing Cerebellar Purkinje Cells

The Cre-lox system is an indispensable tool in neuroscience research for targeting gene deletions to specific cellular populations. Here we assess the utility of several transgenic Cre lines, along with a viral approach, for targeting cerebellar Purkinje cells (PCs) in mice. Using a combination of a fluorescent reporter line (Ai14) to indicate Cre-mediated recombination and a floxed Dystroglycan line (Dag1flox ), we show that reporter expression does not always align precisely with loss of protein. The commonly used Pcp2Cre line exhibits a gradual mosaic pattern of Cre recombination in PCs from Postnatal Day 7 (P7) to P14, while loss of Dag1 protein is not complete until P30. Ptf1aCre drives recombination in precursor cells that give rise to GABAergic neurons in the embryonic cerebellum, including PCs and molecular layer interneurons. However, due to its transient expression in precursors, Ptf1aCre results in stochastic loss of Dag1 protein in these neurons. NestinCre , which is often described as a "pan-neuronal" Cre line for the central nervous system, does not drive Cre-mediated recombination in PCs. We identify a Calb1Cre line that drives efficient and complete recombination in embryonic PCs, resulting in loss of Dag1 protein before the period of synaptogenesis. AAV8-mediated delivery of Cre at P0 results in gradual transduction of PCs during the second postnatal week, with loss of Dag1 protein not reaching appreciable levels until P35. These results characterize several tools for targeting conditional deletions in cerebellar PCs at different developmental stages and illustrate the importance of validating the loss of protein following recombination.

Intrinsic and synaptic determinants of receptive field plasticity in Purkinje cells of the mouse cerebellum

Non-synaptic (intrinsic) plasticity of membrane excitability contributes to aspects of memory formation, but it remains unclear whether it merely facilitates synaptic long-term potentiation or plays a permissive role in determining the impact of synaptic weight increase. We use tactile stimulation and electrical activation of parallel fibers to probe intrinsic and synaptic contributions to receptive field plasticity in awake mice during two-photon calcium imaging of cerebellar Purkinje cells. Repetitive activation of both stimuli induced response potentiation that is impaired in mice with selective deficits in either synaptic or intrinsic plasticity. Spatial analysis of calcium signals demonstrated that intrinsic, but not synaptic plasticity, enhances the spread of dendritic parallel fiber response potentiation. Simultaneous dendrite and axon initial segment recordings confirm these dendritic events affect axonal output. Our findings support the hypothesis that intrinsic plasticity provides an amplification mechanism that exerts a permissive control over the impact of long-term potentiation on neuronal responsiveness.

The olivary input to the cerebellum dissociates sensory events from movement plans

Neurons in the inferior olive are thought to anatomically organize the Purkinje cells (P-cells) of the cerebellum into computational modules, but what is computed by each module? Here, we designed a saccade task in marmosets that dissociated sensory events from motor events and then recorded the complex and simple spikes of hundreds of P-cells. We found that when a visual target was presented at a random location, the olive reported the direction of that sensory event to one group of P-cells, but not to a second group. However, just before movement onset, it reported the direction of the planned movement to both groups, even if that movement was not toward the target. At the end of the movement if the subject experienced an error but chose to withhold the corrective movement, only the first group received information about the sensory prediction error. We organized the P-cells based on the information content of their olivary input and found that in the group that received sensory information, the simple spikes were suppressed during fixation, then produced a burst before saccade onset in a direction consistent with assisting the movement. In the second group, the simple spikes were not suppressed during fixation but burst near saccade deceleration in a direction consistent with stopping the movement. Thus, the olive differentiated the P-cells based on whether they would receive sensory or motor information, and this defined their contributions to control of movements as well as holding still.

Mesoscale simulations predict the role of synergistic cerebellar plasticity during classical eyeblink conditioning

According to the motor learning theory by Albus and Ito, synaptic depression at the parallel fibre to Purkinje cells synapse (pf-PC) is the main substrate responsible for learning sensorimotor contingencies under climbing fibre control. However, recent experimental evidence challenges this relatively monopolistic view of cerebellar learning. Bidirectional plasticity appears crucial for learning, in which different microzones can undergo opposite changes of synaptic strength (e.g. downbound microzones-more likely depression, upbound microzones-more likely potentiation), and multiple forms of plasticity have been identified, distributed over different cerebellar circuit synapses. Here, we have simulated classical eyeblink conditioning (CEBC) using an advanced spiking cerebellar model embedding downbound and upbound modules that are subject to multiple plasticity rules. Simulations indicate that synaptic plasticity regulates the cascade of precise spiking patterns spreading throughout the cerebellar cortex and cerebellar nuclei. CEBC was supported by plasticity at the pf-PC synapses as well as at the synapses of the molecular layer interneurons (MLIs), but only the combined switch-off of both sites of plasticity compromised learning significantly. By differentially engaging climbing fibre information and related forms of synaptic plasticity, both microzones contributed to generate a well-timed conditioned response, but it was the downbound module that played the major role in this process. The outcomes of our simulations closely align with the behavioural and electrophysiological phenotypes of mutant mice suffering from cell-specific mutations that affect processing of their PC and/or MLI synapses. Our data highlight that a synergy of bidirectional plasticity rules distributed across the cerebellum can facilitate finetuning of adaptive associative behaviours at a high spatiotemporal resolution.

Tools for Cre-mediated conditional deletion of floxed alleles from developing cerebellar Purkinje cells

The Cre-lox system is an indispensable tool in neuroscience research for targeting gene deletions to specific cellular populations. Here we assess the utility of several transgenic Cre lines, along with a viral approach, for targeting cerebellar Purkinje cells. Using a combination of a fluorescent reporter line (Ai14) to indicate Cre-mediated recombination and a floxed Dystroglycan line (Dag1flox) we show that reporter expression does not always align precisely with loss of protein. The commonly used Pcp2Cre line exhibits a gradual mosaic pattern of Cre recombination in Purkinje cells from P7-P14, while loss of Dag1 protein is not complete until P30. Ptf1aCre drives recombination in precursor cells that give rise to GABAergic neurons in the embryonic cerebellum, including Purkinje cells and molecular layer interneurons. However, due to its transient expression in precursors, Ptf1aCre results in stochastic loss of Dag1 protein in these neurons. NestinCre, which is often described as a "pan-neuronal" Cre line for the central nervous system, does not drive Cre-mediated recombination in Purkinje cells. We identify a Calb1Cre line that drives efficient and complete recombination in embryonic Purkinje cells, resulting in loss of Dag1 protein before the period of synaptogenesis. AAV8-mediated delivery of Cre at P0 results in gradual transduction of Purkinje cells during the second postnatal week, with loss of Dag1 protein not reaching appreciable levels until P35. These results characterize several tools for targeting conditional deletions in cerebellar Purkinje cells at different developmental stages and illustrate the importance of validating the loss of protein following recombination.

Structure, biophysics, and circuit function of a "giant" cortical presynaptic terminal

The hippocampal mossy fiber synapse, formed between axons of dentate gyrus granule cells and dendrites of CA3 pyramidal neurons, is a key synapse in the trisynaptic circuitry of the hippocampus. Because of its comparatively large size, this synapse is accessible to direct presynaptic recording, allowing a rigorous investigation of the biophysical mechanisms of synaptic transmission and plasticity. Furthermore, because of its placement in the very center of the hippocampal memory circuit, this synapse seems to be critically involved in several higher network functions, such as learning, memory, pattern separation, and pattern completion. Recent work based on new technologies in both nanoanatomy and nanophysiology, including presynaptic patch-clamp recording, paired recording, super-resolution light microscopy, and freeze-fracture and "flash-and-freeze" electron microscopy, has provided new insights into the structure, biophysics, and network function of this intriguing synapse. This brings us one step closer to answering a fundamental question in neuroscience: how basic synaptic properties shape higher network computations.

Contiguity in perception: origins in cellular associative computations

Our brains are good at detecting and learning associative structures; according to some linguistic theories, this capacity even constitutes a prerequisite for the development of syntax and compositionality in language and verbalized thought. I will argue that the search for associative motifs in input patterns is an evolutionary old brain function that enables contiguity in sensory perception and orientation in time and space. It has its origins in an elementary material property of cells that is particularly evident at chemical synapses: input-assigned calcium influx that activates calcium sensor proteins involved in memory storage. This machinery for the detection and learning of associative motifs generates knowledge about input relationships and integrates this knowledge into existing networks through updates in connectivity patterns.

Neuropeptides and Their Roles in the Cerebellum

Although more than 30 different types of neuropeptides have been identified in various cell types and circuits of the cerebellum, their unique functions in the cerebellum remain poorly understood. Given the nature of their diffuse distribution, peptidergic systems are generally assumed to exert a modulatory effect on the cerebellum via adaptively tuning neuronal excitability, synaptic transmission, and synaptic plasticity within cerebellar circuits. Moreover, cerebellar neuropeptides have also been revealed to be involved in the neurogenetic and developmental regulation of the developing cerebellum, including survival, migration, differentiation, and maturation of the Purkinje cells and granule cells in the cerebellar cortex. On the other hand, cerebellar neuropeptides hold a critical position in the pathophysiology and pathogenesis of many cerebellar-related motor and psychiatric disorders, such as cerebellar ataxias and autism. Over the past two decades, a growing body of evidence has indicated neuropeptides as potential therapeutic targets to ameliorate these diseases effectively. Therefore, this review focuses on eight cerebellar neuropeptides that have attracted more attention in recent years and have significant potential for clinical application associated with neurodegenerative and/or neuropsychiatric disorders, including brain-derived neurotrophic factor, corticotropin-releasing factor, angiotensin II, neuropeptide Y, orexin, thyrotropin-releasing hormone, oxytocin, and secretin, which may provide novel insights and a framework for our understanding of cerebellar-related disorders and have implications for novel treatments targeting neuropeptide systems.

Human Purkinje cells outperform mouse Purkinje cells in dendritic complexity and computational capacity

Purkinje cells in the cerebellum are among the largest neurons in the brain and have been extensively investigated in rodents. However, their morphological and physiological properties remain poorly understood in humans. In this study, we utilized high-resolution morphological reconstructions and unique electrophysiological recordings of human Purkinje cells ex vivo to generate computational models and estimate computational capacity. An inter-species comparison showed that human Purkinje cell had similar fractal structures but were larger than those of mouse Purkinje cells. Consequently, given a similar spine density (2/μm), human Purkinje cell hosted approximately 7.5 times more dendritic spines than those of mice. Moreover, human Purkinje cells had a higher dendritic complexity than mouse Purkinje cells and usually emitted 2-3 main dendritic trunks instead of one. Intrinsic electro-responsiveness was similar between the two species, but model simulations revealed that the dendrites could process ~6.5 times (n = 51 vs. n = 8) more input patterns in human Purkinje cells than in mouse Purkinje cells. Thus, while human Purkinje cells maintained spike discharge properties similar to those of rodents during evolution, they developed more complex dendrites, enhancing computational capacity.

Molecular and cellular mechanisms of developmental synapse elimination in the cerebellum: Involvement of autism spectrum disorder-related genes

Neural circuits are initially created with excessive synapse formation until around birth and undergo massive reorganization until they mature. During postnatal development, necessary synapses strengthen and remain, whereas unnecessary ones are weakened and eventually eliminated. These events, collectively called "synapse elimination" or "synapse pruning", are thought to be fundamental for creating functionally mature neural circuits in adult animals. In the cerebellum of neonatal rodents, Purkinje cells (PCs) receive synaptic inputs from multiple climbing fibers (CFs). Then, inputs from a single CF are strengthened and those from the other CFs are eliminated, and most PCs become innervated by single CFs by the end of the third postnatal week. These events are regarded as a representative model of synapse elimination. This review examines the molecular and cellular mechanisms of CF synapse elimination in the developing cerebellum and argues how autism spectrum disorder (ASD)-related genes are involved in CF synapse development. We introduce recent studies to update our knowledge, incorporate new data into the known scheme, and discuss the remaining issues and future directions.