Chronic NMDA receptor blockade or muscimol inhibition of cerebellar cortical neuronal activity alters the development of spinocerebellar afferent topography

Purkinje cell neurotransmission patterns cerebellar basket cells into zonal modules defined by distinct pinceau sizes

Ramón y Cajal proclaimed the neuron doctrine based on circuit features he exemplified using cerebellar basket cell projections. Basket cells form dense inhibitory plexuses that wrap Purkinje cell somata and terminate as pinceaux at the initial segment of axons. Here, we demonstrate that HCN1, Kv1.1, PSD95 and GAD67 unexpectedly mark patterns of basket cell pinceaux that map onto Purkinje cell functional zones. Using cell-specific genetic tracing with an Ascl1^CreERT2 mouse conditional allele, we reveal that basket cell zones comprise different sizes of pinceaux. We tested whether Purkinje cells instruct the assembly of inhibitory projections into zones, as they do for excitatory afferents. Genetically silencing Purkinje cell neurotransmission blocks the formation of sharp Purkinje cell zones and disrupts excitatory axon patterning. The distribution of pinceaux into size-specific zones is eliminated without Purkinje cell GABAergic output. Our data uncover the cellular and molecular diversity of a foundational synapse that revolutionized neuroscience.

Insights into cerebellar development and connectivity

The cerebellum has a well-established role in controlling motor functions such coordination, balance, posture, and skilled learning. There is mounting evidence that it might also play a critical role in non-motor functions such as cognition and emotion. It is therefore not surprising that cerebellar defects are associated with a wide array of diseases including ataxia, dystonia, tremor, schizophrenia, dyslexia, and autism spectrum disorder. What is intriguing is that a seemingly uniform circuit that is often described as being "simple" should carry out all of these behaviors. Analyses of how cerebellar circuits develop have revealed that such descriptions massively underestimate the complexity of the cerebellum. The cerebellum is in fact highly patterned and organized around a series of parasagittal stripes and transverse zones. This topographic architecture partitions all cerebellar circuits into functional modules that are thought to enhance processing power during cerebellar dependent behaviors. What are arguably the most remarkable features of cerebellar topography are the developmental processes that produce them. This review is concerned with the genetic and cellular mechanisms that orchestrate cerebellar patterning. We place a major focus on how Purkinje cells control multiple aspects of cerebellar circuit assembly. Using this model, we discuss evidence for how "zebra-like" patterns in Purkinje cells sculpt the cerebellum, how specific genetic cues mediate the process, and how activity refines the patterns into an adult map that is capable of executing various functions. We also discuss how defective Purkinje cell patterning might impact the pathogenesis of neurological conditions.

Mossy Fibers Terminate Directly Within Purkinje Cell Zones During Mouse Development

The cerebellum is organized into a map of zones that is manifested in various ways according to gene expression, anatomical connectivity, neuronal firing properties, behavioral specificity, and susceptibility to disease. At the center of every zone is the Purkinje cell, the principal cell type of the cerebellum and sole output of the cerebellar cortex. During development, Purkinje cells are thought to coordinate the zonal patterning of all other cell types. However, the morphogenetic mechanism that mediates the interaction between Purkinje cells and afferent fibers remains unclear. To address this problem in vivo, I took advantage of a rapid fluorescent-based transynaptic tracing approach to determine the nature of mossy fiber to Purkinje cell connectivity during early postnatal development, a period when the afferent map is assembling into clear-cut zonal circuits. By injecting WGA-Alexa 555 into the lower thoracic-upper lumber spinal cord, I found that spinocerebellar mossy fibers transynaptically transfer tracer into zones of Purkinje cells that are directly adjacent to the fibers. The traced Purkinje cell zones formed a zebrin-like pattern that was defined by the expression of neurofilament heavy chain (NFH), a marker of zones in the postnatal developing cerebellum. These results suggest that Purkinje cells generate the zonal circuit map by using molecular cues, neuronal activity, and synaptic contact.

Embryonic stages in cerebellar afferent development

The cerebellum is important for motor control, cognition, and language processing. Afferent and efferent fibers are major components of cerebellar circuitry and impairment of these circuits causes severe cerebellar malfunction, such as ataxia. The cerebellum receives information from two major afferent types – climbing fibers and mossy fibers. In addition, a third set of afferents project to the cerebellum as neuromodulatory fibers. The spatiotemporal pattern of early cerebellar afferents that enter the developing embryonic cerebellum is not fully understood. In this review, we will discuss the cerebellar architecture and connectivity specifically related to afferents during development in different species. We will also consider the order of afferent fiber arrival into the developing cerebellum to establish neural connectivity.

Myoclonic Twitching and Sleep-Dependent Plasticity in the Developing Sensorimotor System

As bodies grow and change throughout early development and across the lifespan, animals must develop, refine, and maintain accurate sensorimotor maps. Here we review evidence that myoclonic twitches-brief and discrete contractions of the muscles, occurring exclusively during REM (or active) sleep, that result in jerks of the limbs-help animals map their ever-changing bodies by activating skeletal muscles to produce corresponding sensory feedback, or reafference. First, we highlight the spatiotemporal characteristics of twitches. Second, we review findings in infant rats regarding the multitude of brain areas that are activated by twitches during sleep. Third, we discuss evidence demonstrating that the sensorimotor processing of twitches is different from that of wake movements; this state-related difference in sensorimotor processing provides perhaps the strongest evidence yet that twitches are uniquely suited to drive certain aspects of sensorimotor development. Finally, we suggest that twitching may help inform our understanding of neurodevelopmental disorders, perhaps even providing opportunities for their early detection and treatment.

Cerebellar zonal patterning relies on Purkinje cell neurotransmission

Cerebellar circuits are patterned into an array of topographic parasagittal domains called zones. The proper connectivity of zones is critical for motor coordination and motor learning, and in several neurological diseases cerebellar circuits degenerate in zonal patterns. Despite recent advances in understanding zone function, we still have a limited understanding of how zones are formed. Here, we focused our attention on Purkinje cells to gain a better understanding of their specific role in establishing zonal circuits. We used conditional mouse genetics to test the hypothesis that Purkinje cell neurotransmission is essential for refining prefunctional developmental zones into sharp functional zones. Our results show that inhibitory synaptic transmission in Purkinje cells is necessary for the precise patterning of Purkinje cell zones and the topographic targeting of mossy fiber afferents. As expected, blocking Purkinje cell neurotransmission caused ataxia. Using in vivo electrophysiology, we demonstrate that loss of Purkinje cell communication altered the firing rate and pattern of their target cerebellar nuclear neurons. Analysis of Purkinje cell complex spike firing revealed that feedback in the cerebellar nuclei to inferior olive to Purkinje cell loop is obstructed. Loss of Purkinje neurotransmission also caused ectopic zonal expression of tyrosine hydroxylase, which is only expressed in adult Purkinje cells when calcium is dysregulated and if excitability is altered. Our results suggest that Purkinje cell inhibitory neurotransmission establishes the functional circuitry of the cerebellum by patterning the molecular zones, fine-tuning afferent circuitry, and shaping neuronal activity.

REM sleep twitches rouse nascent cerebellar circuits: Implications for sensorimotor development

The cerebellum is critical for sensorimotor integration and undergoes extensive postnatal development. During the first postnatal week in rats, climbing fibers polyinnervate Purkinje cells and, before granule cell migration, mossy fibers make transient, direct connections with Purkinje cells. Activity-dependent processes are assumed to play a critical role in the development and refinement of these and other aspects of cerebellar circuitry. However, the sources and patterning of activity have not been described. We hypothesize that sensory feedback (i.e., reafference) from myoclonic twitches in sleeping newborn rats is a prominent driver of activity for the developing cerebellum. Here, in 6-day-old rats, we show that Purkinje cells exhibit substantial state-dependent changes in complex and simple spike activity-primarily during active sleep. In addition, this activity increases significantly during bouts of twitching. Moreover, the surprising observation of twitch-dependent increases in simple spike activity at this age suggests a functional engagement of mossy fibers before the parallel fiber system has developed. Based on these and other results, we propose that twitching comprises a unique class of self-produced movement that drives critical aspects of activity-dependent development in the cerebellum and other sensorimotor systems.

Mechanisms underlying reorganization of fractured tactile cerebellar maps after deafferentation in developing and adult rats

Our previous studies showed that fractured tactile cerebellar maps in rats reorganize after deafferentation during development and in adulthood while maintaining a fractured somatotopy. Several months after deafferentation of the infraorbital branch of the trigeminal nerve, the missing upper lip innervation is replaced in the tactile maps in the granule cell layer of crus IIa. The predominant input into the denervated area is always the upper incisor representation. This study examined whether this reorganization was caused by mechanisms intrinsic to the cerebellum or extrinsic, i.e., occurring in somatosensory structures afferent to the cerebellum. We first compared normal and deafferented maps and found that the expansion of the upper incisor is not caused by a preexisting bias in the strength or abundance of upper incisor input in normal animals. We then mapped tactile representations before and immediately after denervation. We found that the pattern of reorganization observed in the cerebellum several months later is not caused by unmasking of a silent or weaker upper incisor representation. Both results indicate that the reorganization is not a result of subsequent growth or sprouting mechanism within the cerebellum itself. Finally, we compared postlesion maps in the cerebellum and the somatosensory cortex. We found that the upper incisor representation significantly expands in both regions and that this expansion is correlated, suggesting that reorganization in the cerebellum is a passive consequence of reorganization in afferent cerebellar pathways. This result has important developmental and functional implications.

Compartmentation of the reeler cerebellum: segregation and overlap of spinocerebellar and secondary vestibulocerebellar fibers and their target cells

The cerebellum of the reeler mutant mouse has an abnormal organization; its single lobule is composed of a severely hypogranular cortex and a central cerebellar mass (CCM) consisting of Purkinje cell clusters intermixing with the cerebellar nuclei. As such the reeler represents an excellent model in which to examine the effect of the abnormal distribution of cerebellar cells on afferent-target relationships. To this effect we studied the organization of the spinocerebellar and secondary vestibulocerebellar afferent projections in homozygous reeler mice (rl/rl) using anterograde tracing techniques. Spinal cord injections resulted in labeled spinocerebellar mossy fiber rosettes in specific anterior and posterior regions of the cerebellar cortex. Some vestiges of parasagittal organization may be present in the anterior projection area. Within the CCM, labeled fibers appeared to terminate on distinct groups of Purkinje cells. Thus, the spinocerebellar mossy fibers seem to form both normal and heterologous synapses in the reeler cerebellum. Secondary vestibular injections resulted in both retrograde and anterograde labeling. Retrograde labeling was seen in clusters of Purkinje cells and cerebellar nuclear cells; anterograde labeling was distributed in the white matter and in specific regions of the anterior and posterior cortex of the cerebellum. The labeled spinocerebellar and secondary vestibulocerebellar afferents overlapped in the anterior region but in the posterior region the vestibulocerebellar termination area was ventral to the spinocerebellar area. An area devoid of labeled terminals was also observed ventral to the posterior secondary vestibulocerebellar termination field. Using calretinin immunostaining it was determined that this area contains unipolar brush cells, a cell type found primarily in the vestibulocerebellum of normal mice. Our data indicate that despite of the lack of known landmarks (fissures, lobules) the spinocerebellar and vestibulocerebellar afferent projections in the reeler cerebellum do not distribute randomly but have specific target regions, and the position of these regions, relative to each other, appears to be conserved. Two caveats to this were the finding of overlapping terminal fields of these afferents in the anterior region, and a posteroventral region that contains unipolar brush cells yet is devoid of secondary vestibulocerebellar afferents. The distribution of Purkinje cells and cerebellar nuclear cells is not random either; those that give rise to cerebellovestibular efferents form distinct groups within the central cerebellar mass.

Evidence of spinocerebellar mossy fiber segregation in the juvenile staggerer cerebellum

Developmental and experimental studies of climbing fiber and mossy fiber connectivity in the cerebellum have suggested that Purkinje cells are the critical organizing elements for connectivity patterns. This hypothesis is supported by evidence that spinocerebellar mossy fiber projections are abnormally diffuse in P25 sg/sg mutant mice in which the differentiation of a reduced number of sg/sg Purkinje cells is blocked due to a cell autonomous defect. However, mossy fiber distribution may be disrupted in sg/sg mutants not because of the Purkinje cell deficits, but because of the death of virtually all granule cells following the 4th postnatal week. To test this hypothesis, we have analyzed the distribution of wheat germ agglutinin-horseradish peroxidase (WGA-HRP)-labeled spinocerebellar mossy fiber terminals in sg/sg mutants at the end of the period of granule cell genesis (postnatal day [P] 12-P13) and before massive granule cell death (P16). Two percent WGA-HRP was injected into the lower thoracic/upper lumbar region of the spinal cord of eight homozygous sg/sg mutants (P12-P16) and five controls (+/sg and +/+). We have found that spinocerebellar mossy fibers segregate into distinct terminal fields in the anterior cerebellar lobules of P12 to P16 sg/sg mutants, although the medial-lateral distribution of spinocerebellar mossy fiber projections is different from controls. The results from this study and previous analysis of sg/sg mutants support the hypothesis that topographic cues are expressed in the early postnatal staggerer mutant, but mossy fiber terminals become disorganized or retract as granule cells die in the older staggerer mutant. J. Comp. Neurol. 378:354-362, 1997.

Spinocerebellar mossy fiber terminal topography in the NR2C/PKC gamma double mutant cerebellum

The spatiotemporal expression patterns of the NR2C subunit of the NMDA receptor and PKC gamma isoform during cerebellar development suggests that both proteins are involved in the molecular mechanisms of synaptogenesis. However, the topographic distribution of WGA-HRP labeled spinocerebellar mossy fiber terminals in NR2C/PKC gamma double mutants (n = 4) appears similar to controls (n = 3). While the results do not rule out a role for NR2C receptor subunits and the PKC gamma isoform in cerebellar synaptogenesis, they indicate that neither is necessary for the formation or maintenance of normal spinocerebellar mossy fiber afferent maps.

The Engrailed-2 homeobox gene and patterning of spinocerebellar mossy fiber afferents

The mouse Engrailed-2 gene, En-2, appears to be involved in cerebellar pattern formation. Homozygous null mutants for En-2 have abnormal foliation patterns in the posterior half of the cerebellum and there are changes in Purkinje and granule cell gene expression in some posterior folia, possibly reflecting changes in cell identity. We have examined the distribution of spinocerebellar mossy fiber terminals in homozygous En-2hd null mutants to determine if En-2 is involved in regulating the pattern of afferent connectivity in the cerebellum. Spinocerebellar mossy fiber terminals were labeled following WGA-HRP injections in the lumbar region of 5 homozygous En-2hd mutants and 4 heterozygous controls. The distribution of spinocerebellar mossy fiber terminals was consistently altered in lobules VIII and IX of the En-2hd mutants. The principal changes were a reduction in the number of mossy fiber terminal fields in the dorsal aspect of lobule VIII and the dorsal midline field in lobule IX was fused into a single compartment. The results suggest that the deletion of En-2 expression does not transform lobule identity, at least with respect to afferent fiber positional information cues. However, the changes in foliation and afferent connectivity in the En-2 mutant support a broad role for the En-2 gene in cerebellar patterning.

Partial ablation of the neonatal external granular layer disrupts mossy fiber topography in the adult rat cerebellum

The spinocerebellar projection in the rat is compartmentalized in an array of parasagittal bands of mossy fiber terminals. These bands align reproducibly with bands of Purkinje cells that differentially express zebrin II. To investigate whether this alignment is obligatory, Purkinje cell and mossy fiber compartmentation has been compared in the rat cerebellum where the cytoarchitecture was contorted by neonatal administration of methylazoxymethanol. Methylazoxymethanol ablates many granule cell precursors, leaving a much reduced external granular layer, and adult rats that received a single methylazoxymethanol injection at birth showed varying degrees of abnormal cerebellar foliation. Zebrin II immunocytochemistry nevertheless revealed no fundamental abnormality in the Purkinje cell compartments. However, despite the normal Purkinje cell compartmentation being retained, the spinocerebellar mossy fiber-Purkinje cell topography is disrupted by methylazoxymethanol treatment. The normal precise array of parasagittal mossy fiber terminal fields becomes blurred across the lobule, and the normal clear banding is difficult to follow. These data suggest that, despite the early topography being dependent on the Purkinje cells, the granule cell-mossy fiber interactions also regulate the topography of the spinocerebellar projection.

CGP 39653 binding in the chick CNS after NMDA receptor antagonist treatment

To analyze the affect of blocking neuronal activity on NMDA receptor levels during development, we have injected chick embryos with the competitive NMDA receptor antagonist NPC 12626 from E17 to E19. Brains from drug-treated (n = 7) and control (n = 6) embryos (E20) were processed for receptor autoradiography using the NMDA competitive antagonist [3H]CGP 39653. NPC 12626 treatment caused a significant 24 to 46% increase in [3H]CGP 39653 binding in both the forebrain and cerebellar cortex. The results support the hypothesis that NMDA receptor levels are regulated by activity-dependent mechanisms.

Na+ channel beta 1 subunit mRNA expression in developing rat central nervous system

The sodium channel beta 1 subunit (Na beta 1) is a component of the rat brain voltage-dependent sodium channel. We have used nonradioactive in situ hybridization cytochemical techniques to demonstrate that transcript levels of Na beta 1 are differentially upregulated during postnatal development of several CNS regions, with selective labeling of specific neuronal populations. In the hippocampus, labeling of the pyramidal cell layer (particularly in the CA3 region) and dentate granule cells was initially observed at postnatal day 2 (P2) and P10, respectively, and became progressively more intense with maturation. Labeled cells were first observed in the hilus at P10. In the developing cerebellum, transient labeling was observed in the external granule cell layer beginning at P1 while label increased in the internal granule cell layer up to P21. Purkinje cells showed significant label beginning at P4 and increasing up to P21. Weak signal was seen in neurons of deep nuclei at P1 and increased up to P21. Na beta 1 labeling in the spinal cord was first observed in the ventral horn at P2, and the intensity of labeling in these large motoneurons gradually increased. In addition, there was a ventral-dorsal gradient in this region, with label appearing subsequently in neurons of Rexed laminae IX, VII and VIII, and in the dorsal horn (Rexed laminae I-VI). In these regions, the labeling reached a plateau within the first 2-3 weeks after birth and persisted into the adult rat. The time course and regional heterogeneity of Na beta 1 expression are consistent with the hypothesis that the expression of mature Na+ channels, including Na beta 1, contributes to the development of circuitry that supports complex patterns of electrogenesis.