α6 integrin subunit regulates cerebellar development

Mutations in genes encoding several basal lamina components as well as their cellular receptors disrupt normal deposition and remodeling of the cortical basement membrane resulting in a disorganized cerebral and cerebellar cortex. The α6 integrin was the first α subunit associated with cortical lamination defects and formation of neural ectopias. In order to understand the precise role of α6 integrin in the central nervous system (CNS), we have generated mutant mice carrying specific deletion of α6 integrin in neuronal and glia precursors by crossing α6 conditional knockout mice with Nestin-Cre line. Cerebral cortex development occurred properly in the resulting α6^{fl/fl;nestin-Cre} mutant animals. Interestingly, however, cerebellum displayed foliation pattern defects although granule cell (GC) proliferation and migration were not affected. Intriguingly, analysis of Bergmann glial (BG) scaffold revealed abnormalities in fibers morphology associated with reduced processes outgrowth and altered actin cytoskeleton. Overall, these data show that α6 integrin receptors are required in BG cells to provide a proper fissure formation during cerebellum morphogenesis.     

Nicotinic mechanisms in the autonomic control of organ systems

Most visceral organs are under the control of the autonomic nervous system (ANS). Information on the state and function of these organs is constantly relayed to the central nervous system (CNS) by sensory afferent fibers. The CNS integrates the sensory inputs and sends neural commands back to the organ through the ANS. The autonomic ganglia are the final site for the integration of the message traveling from the CNS. Nicotinic acetylcholine receptors (nAChRs) are the main mediators of fast synaptic transmission in ganglia, and therefore, are key molecules for the processing of neural information in the ANS. This review focuses on the role of nAChRs in the control of organ systems such as heart, gut, and bladder. The autonomic control of these organ systems is discussed in the light of the results obtained from the analysis of mice carrying mutations targeted to nAChR subunits expressed in the ANS.     

Tenascin-C and its functions in neuronal plasticity

The extracellular matrix glycoprotein tenascin-C (TN-C), a molecule highly conserved in vertebrates, is widely expressed in neural and non-neural tissue during development, repair processes in the adult organism, and tumorigenesis. In the developing central nervous system (CNS), in different brain regions TN-C is expressed in specific spatial and temporal patterns. In the adult CNS, its expression remains in areas of active neurogenesis and areas that exhibit neuronal plasticity. Understanding of the contribution of this extracellular matrix constituent to the major developmental processes such as cell proliferation and migration, axonal guidance, as well as synaptic plasticity, is derived from studies on TN-C deficient mice. Studies on these mice demonstrated that TN-C plays an important role in neuronal plasticity in the cerebral cortex, hippocampus and cerebellum, possibly by modulating the activity of L-type voltage-dependent Ca(2+) channels.     

The central and peripheral renin-angiotensin and noradrenergic systems in the spontaneous hypertensive rats (SHR)

The aim of this study was to evaluate the components of the renin-angiotensin system in the periphery and in the central nervous system (CNS) of the spontaneous hypertensive rats (SHR). On the other hand, the norepinephrine (NE) content of the different areas and of the mesenteric artery were also measured. Sixteen SHR and 9 Wistar Kyoto (WKY) control animals were used at about 6 months of age. Blood and cerebrospinal fluid (CSF) samples were collected. The brain was dissected into several areas and the mesenteric artery was excised. Plasma renin activity (PRA), plasma angiotensinogen concentration (P1AoC), brain renin (RC) and angiotensinogen concentrations (AoC) were evaluated by radioimmunoassay. NE was determined in all the tissues by a fluorimetric technique. PRA, P1AoC and NE concentration in the mesenteric artery were similar in both groups. An increase in the NE content of the cerebellum was detected in the SHR without changes in the other areas of the CNS. AoC was decreased in the CSF and in the brain stem of the SHR animals. RC was evaluated in the hypothalamus, brain stem, cerebral cortex and cerebellum of the same strain of rats. These results seem to indicate the some alteration of the peptidergic system in the CNS is present in the hypertensive animals.     

Expression of cytosolic branched chain aminotransferase (BCATc) mRNA in the developing mouse brain

Branched-chain aminotransferase (BCAT) catalyzes the transamination of essential branched-chain amino acids (BCAAs: leucine, isoleucine and valine) with alpha-ketoglutarate. Through this reaction, BCAAs provide nitrogen for the synthesis of glutamate, the predominant excitatory neurotransmitter. Two BCAT isoforms have been identified: one cytosolic (BCATc) and one mitochondrial (BCATm). In adult rodents, BCATc is expressed in a wide variety of structures of the central nervous system (CNS), in neurons. So far, no data were available about the expression of BCATc in the developing CNS. Here, we analyse the expression profile of BCATc mRNA in the mouse brain from embryonic day 12.5 to adult age. BCATc mRNA gradually appears in different brain regions starting from early stages of neural development, and is maintained until adulthood. BCATc mRNA is predominantly present in the cerebral cortex, hippocampus, thalamus, ventral midbrain, raphe, cerebellum and precerebellar system. This study represents the first detailed analysis of BCATc mRNA expression in the developing mouse brain.     

Developmental Switch in Neurovascular Coupling in the Immature Rodent Barrel Cortex

Neurovascular coupling (NVC) in the adult central nervous system (CNS) is a mechanism that provides regions of the brain with more oxygen and glucose upon increased levels of neural activation. Hemodynamic changes that go along with neural activation evoke a blood oxygen level-dependent (BOLD) signal in functional magnetic resonance imaging (fMRI) that can be used to study brain activity non-invasively. A correct correlation of the BOLD signal to neural activity is pivotal to understand this signal in neuronal development, health and disease. However, the function of NVC during development is largely unknown. The rodent whisker-to-barrel cortex is an experimentally well established model to study neurovascular interdependences. Using extracellular multi-electrode recordings and laser-Doppler-flowmetry (LDF) we show in the murine barrel cortex of postnatal day 7 (P7) and P30 mice in vivo that NVC undergoes a physiological shift during the first month of life. In the mature CNS it is well accepted that cortical sensory processing results in a rise in regional cerebral blood flow (rCBF). We show in P7 animals that rCBF decreases during prolonged multi-whisker stimulation and goes along with multi unit activity (MUA) fatigue. In contrast at P30, MUA remains stable during repetitive stimulation and is associated with an increase in rCBF. Further we characterize in both age groups the responses in NVC to single sensory stimuli. We suggest that the observed shift in NVC is an important process in cortical development that may be of high relevance for the correct interpretation of brain activity e.g. in fMRI studies of the immature central nervous system (CNS).     

Developmental expression of sorting nexin 3 in the mouse central nervous system

We previously reported that sorting nexin 3 (SNX3), a protein belonging to the sorting nexin family, regulates neurite outgrowth in mouse N1E-115 neuroblastoma cells. The snx3 gene is disrupted in patients with microcephaly, microphthalmia, ectrodactyly, and prognathism (MMEP) and mental retardation, demonstrating that SNX3 plays an important role in the genesis of these organs during development. The present study was designed to determine the expression pattern of snx3 mRNA, particularly in the mouse central nervous system (CNS), from the embryonic stage to adulthood. Whole mount in situ hybridization of embryonic day (E) 9.5 and 10.5 mouse embryos revealed strong positive signals for snx3 mRNA in the forebrain, pharyngeal arches, eyes, and limb buds. In situ hybridization analyses of embryonic and neonatal brain sections revealed that snx3 mRNA is mainly expressed in the cerebral cortex, hippocampus, piriform cortex, cerebellum, and spinal cord. In adulthood, the expression of snx3 mRNA is observed in the cerebral cortex, hippocampus, piriform cortex, and cerebellar neurons. Thus, snx3 mRNA is expressed during neural development and in adult neural tissues, suggesting that SNX3 may play an important role in the development and function of the CNS.     

Receptor sites for atrial natriuretic factors in brain and associated structures: an overview

1. Recent data have clearly shown the existence of specific receptor binding sites for atrial natriuretic factors (ANF) or polypeptides in mammalian brain tissues. 2. Ligand selectivity pattern and coupling to cGMP production suggest that brain ANF sites are similar to high-affinity/low-capacity sites found in various peripheral tissues (kidney, adrenal gland, blood vessels). These brain ANF sites possibly are of the B-ANP subtype. 3. High densities of ANF binding sites are found especially in areas of the central nervous system associated with the control of various cardiovascular parameters (such as the subfornical organ and area postrema). However, high densities of sites are also present in other regions such as the hippocampus, cerebellum, and thalamus in the brain of certain mammalian species, suggesting that brain ANF could act as a neuromodulator of noncardiovascular functions. 4. The density of brain ANF binding sites is modified in certain animal models of cardiovascular disorders and during postnatal ontogeny, demonstrating the plasticity of these sites in the central nervous system (CNS). 5. Specific ANF binding sites are also found in various other CNS-associated tissues such as the eye, pituitary gland, and adrenal medulla. In these tissues ANF appears to act as a modulator of fluid production and hormone release. 6. Thus, ANF-like peptides and ANF receptor sites are present in brain and various peripheral tissues, demonstrating the existence of a family of brain/heart peptides.     

Hyperthermia and fatigue.

The present review addresses mechanisms of importance for hyperthermia-induced fatigue during short intense activities and prolonged exercise in the heat. Inferior performance during physical activities with intensities that elicit maximal oxygen uptake is to a large extent related to perturbation of the cardiovascular function, which eventually reduces arterial oxygen delivery to the exercising muscles. Accordingly, aerobic energy turnover is impaired and anaerobic metabolism provokes peripheral fatigue. In contrast, metabolic disturbances of muscle homeostasis are less important during prolonged exercise in the heat, because increased oxygen extraction compensates for the reduction in systemic blood flow. The decrease in endurance seems to involve changes in the function of the central nervous system (CNS) that lead to fatigue. The CNS fatigue appears to be influenced by neurotransmitter activity of the dopaminergic system, but may primarily relate to inhibitory signals from the hypothalamus arising secondary to an increase in brain temperature. Fatigue is an integrated phenomenon, and psychological factors, including the anticipation of fatigue, should not be neglected and the interaction between central and peripheral physiological factors also needs to be considered.     

Plasticity of central autonomic neural circuits in diabetes

Regulation of energy metabolism is controlled by the brain, in which key central neuronal circuits process a variety of information reflecting nutritional state. Special sensory and gastrointestinal afferent neural signals, along with blood-borne metabolic signals, impinge on parallel central autonomic circuits located in the brainstem and hypothalamus to signal changes in metabolic balance. Specifically, neural and humoral signals converge on the brainstem vagal system and similar signals concentrate in the hypothalamus, with significant overlap between both sensory and motor components of each system and extensive cross-talk between the systems. This ultimately results in production of coordinated regulatory autonomic and neuroendocrine cues to maintain energy homeostasis. Therapeutic metabolic adjustments can be accomplished by modulating viscerosensory input or autonomic motor output, including altering parasympathetic circuitry related to GI, pancreas, and liver regulation. These alterations can include pharmacological manipulation, but surgical modification of neural signaling should also be considered. In addition, central control of visceral function is often compromised by diabetes mellitus, indicating that circuit modification should be studied in the context of its effect on neurons in the diabetic state. Diabetes has traditionally been handled as a peripheral metabolic disease, but the central nervous system plays a crucial role in regulating glucose homeostasis. This review focuses on key autonomic brain areas associated with management of energy homeostasis and functional changes in these areas associated with the development of diabetes.     

Genome-Wide Divergence of DNA Methylation Marks in Cerebral and Cerebellar Cortices

Background

Emerging evidence suggests that DNA methylation plays an expansive role in the central nervous system (CNS). Large-scale whole genome DNA methylation profiling of the normal human brain offers tremendous potential in understanding the role of DNA methylation in brain development and function.

Methodology/Significant Findings

Using methylation-sensitive SNP chip analysis (MSNP), we performed whole genome DNA methylation profiling of the prefrontal, occipital, and temporal regions of cerebral cortex, as well as cerebellum. These data provide an unbiased representation of CpG sites comprising 377,509 CpG dinucleotides within both the genic and intergenic euchromatic region of the genome. Our large-scale genome DNA methylation profiling reveals that the prefrontal, occipital, and temporal regions of the cerebral cortex compared to cerebellum have markedly different DNA methylation signatures, with the cerebral cortex being hypermethylated and cerebellum being hypomethylated. Such differences were observed in distinct genomic regions, including genes involved in CNS function. The MSNP data were validated for a subset of these genes, by performing bisulfite cloning and sequencing and confirming that prefrontal, occipital, and temporal cortices are significantly more methylated as compared to the cerebellum.

Conclusions

These findings are consistent with known developmental differences in nucleosome repeat lengths in cerebral and cerebellar cortices, with cerebrum exhibiting shorter repeat lengths than cerebellum. Our observed differences in DNA methylation profiles in these regions underscores the potential role of DNA methylation in chromatin structure and organization in CNS, reflecting functional specialization within cortical regions.     

Distribution of the thy-1 antigen in cellular and subcellular fractions of adult mouse brain.

By using a cytotoxicity inhibition assay employing AKR anti-C3H thymocyte antiserum, we have determined the degree of expression of the thy-1 antigen in fractions of adult mouse brain. As expressed as cytotoxicity inhibitory capacity per mg protein with C3H whole brain arbitrarily assigned a value of 1.0, the following values were found: C3H cerebral cortex, 5.8; C3H cerebral cortex synaptosomes, 2.5: C3H whole brain myelin, 0.65; C3H cerebral cortex neurons, 0.16; and C3H cerebral cortex mitochondria 0.10. Neither C1300 neuroblastoma cells nor any AKR neural fraction had detectable levels of thy-1. The findings indicate that the thy-1 antigen is found mainly in mouse cerebral cortex and in synaptosomal fractions, whereas myelin fractions contain lower but perhaps significant amounts of thy-1. Cerebral cortex neurons, isolated by a method requiring a 90-min mild trypsinization at 37 degrees C, did not display significant amounts of the thy-1 antigen. These results lend themselves to further study in the area of differentiation and development of central nervous system components and in the area of central nervous system immunopathology.     

The role of the insulin-like growth factors in the central nervous system

Increasing evidence strongly supports a role for insulin-like growth factor-I (IGF-I) in central nervous system (CNS) development. IGF-I, IGF-II, the type IIGF receptor (the cell surface tyrosine kinase receptor that mediates IGF signals), and some IGF binding proteins (IGFBPs; secreted proteins that modulate IGF actions) are expressed in many regions of the CNS beginningin utero. The expression pattern of IGF system proteins during brain growth suggests highly regulated and developmentally timed IGF actions on specific neural cell populations. IGF-I expression is predominantly in neurons and, in many brain regions, peaks in a fashion temporally coincident with periods in development when neuron progenitor proliferation and/or neuritic outgrowth occurs. In contrast, IGF-II expression is confined mainly to cells of mesenchymal and neural crest origin. While expression of type I IGF receptors appears ubiquitous, that of IGFBPs is characterized by regional and developmental specificity, and often occurs coordinately with peaks of IGF expression. In vitro IGF-I has been shown to stimulate the proliferation of neuron progenitors and/or the survival of neurons and oligodendrocytes, and in some cultured neurons, to stimulate function. Transgenic (Tg) mice that overexpress IGF-I in the brain exhibit postnatal brain overgrowth without anatomic abnormality (20–85% increases in weight, depending on the magnitude of expression). In contrast, Tg mice that exhibit ectopic brain expression of IGFBP-1, an inhibitor of IGF action when present in molar excess, manifest postnatal brain growth retardation, and mice with ablated IGF-I gene expression, accomplished by homologous recombination, have brains that are 60% of normal size as adults. Taken together, these in vivo studies indicate that IGF-I can influence the development of most, if not all, brain regions, and suggest that the cerebral cortex and cerebellum are especially sensitive to IGF-I actions. IGF-I’s growth-promoting in vivo actions result from its capacity to increase neuron number, at least in certain populations, and from its potent stimulation of myelination. These IGF-I actions, taken together with its neuroprotective effects following CNS and peripheral nerve injury, suggest that it may be of therapeutic benefit in a wide variety of disorders affecting the nervous system.     

In Situ Fluorescence Imaging of Myelination

We describe a novel fluorescent dye, 3-(4-aminophenyl)-2H-chromen-2-one (termed case myelin compound or CMC), that can be used for in situ fluorescent imaging of myelin in the vertebrate nervous system. When administered via intravenous injection into the tail vein, CMC selectively stained large bundles of myelinated fibers in both the central nervous system (CNS) and the peripheral nervous system (PNS). In the CNS, CMC readily entered the brain and selectively localized in myelinated regions such as the corpus callosum and cerebellum. CMC also selectively stained myelinated nerves in the PNS. The staining patterns of CMC in a hypermyelinated mouse model were consistent with immunohistochemical staining. Similar to immunohistochemical staining, CMC selectively bound to myelin sheaths present in the white matter tracts. Unlike CMC, conventional antibody staining for myelin basic protein also stained oligodendrocyte cytoplasm in the striatum as well as granule layers in the cerebellum. In vivo application of CMC was also demonstrated by fluorescence imaging of myelinated nerves in the PNS. (J Histochem Cytochem 58:611–621, 2010)     

The enigma of deep-body thermosensory specificity

Information provided by the analysis of peripheral cold and warm receptors may be considered a useful guide for assessing the specificity of thermal information originating in deep-body tissues. A wealth of data concerning the location of deep-body thermosensors and their neuronal correlates and modes of transduction permits the following theses to be proposed. 1. Unlike the peripheral warm and cold receptors, deep-body thermosensors are only in part represented by afferent fibers, mostly warm sensitive ones that are not character- ized in detail, as the source of thermal information outside the central nervous system (CNS). The more important thermal information generated in the CNS originates mainly from warm-sensitive neurons but contributions of cold-sensitive neurons are not definitely excluded. 2. Unlike the peripheral thermoreceptors, monomodality with respect to natural physical stimuli does not seem to be an essential property of deep-body thermosensors. By contrast, multimodality may underlie at least some of the multitude of interactions between thermoregulatory and other homeostatic control systems. 3. Temperature transduction seems to utilize molecular mechanisms that are also found in neurons that lack any thermosensory functions, and so the transduction mechanisms identified in warm-sensitive CNS neurons do not seem to be specific per se. 4. The observation of a multitude of temperature/response characteristics for thermosensitive CNS neurons has been helpful for categorizing these neurons, but there is no clear information that any one might be particularly relevant. 5. Originating from the peripheral cold and warm receptors two separate but interacting cold- and warm-signal pathways ascend multisynaptically to the hypothalamus as the highest level of thermoregulatory control, and to some extent go further to the sensory cortex. The signal contributed by a deep-body thermosensitive neuron, irrespective of its location, attains specificity by being fed properly into one of the two ascending thermosensory pathways. Received: 25 January 2000 / Accepted: 10 April 2000     

Ethanol exposure alters neurotrophin receptor expression in the rat central nervous system: Effects of neonatal exposure

The detrimental effects of ethanol exposure during nervous system development have been well established. The cellular mechanisms of ethanol neurotoxicity, however, have not been clearly defined. Recent studies suggest that neurotrophin signaling pathways may be involved in ethanol-mediated neuronal death. The present investigation, therefore, was designed to examine ethanol-induced alterations in neurotrophin receptor protein levels in the developing central nervous system (CNS) following chronic ethanol treatment administered during the early neonatal period. For this study, rats were exposed to ethanol via vapor inhalation from postnatal day 4 (P4) to P10. Brains were then dissected on P10 or P21, and Western blots used to quantify expression of neurotrophin receptors TrkA, TrkB, TrkC, and p75. This early postnatal ethanol treatment produced significant alterations in receptor levels in hippocampus, septum, cerebral cortex, and cerebellum. The alterations seen were variable, with decreases generally found in hippocampus and cerebellum, increases noted in septum, and changes in both directions occurring in cortex. These alterations were generally more prevalent in males than in females. While most of the receptor changes observed were transient, sustained or delayed alterations were occasionally seen in hippocampus, cortex, and cerebellum. These results suggest that developmental ethanol exposure modulates expression of these neurotrophin receptors throughout the CNS, alterations which could have wide-ranging effects on functional CNS development. The possible linkage between such changes and abnormalities encountered in the fetal alcohol syndrome are considered.     

Ethanol exposure alters neurotrophin receptor expression in the rat central nervous system: Effects of prenatal exposure

Developmental ethanol exposure produces significant central nervous system (CNS) abnormalities. The cellular mechanisms of ethanol neurotoxicity, however, remain elusive. Recent data implicate altered neurotrophin signaling pathways in ethanol-mediated neuronal death. The present study investigated ethanol-induced alterations in neurotrophin receptor proteins in the rat CNS following chronic ethanol treatment during gestation, via liquid diet to pregnant dams. Brains were dissected on P1 and P10, and Western blots for the neurotrophin receptors TrkA, TrkB, TrkC, and p75 were quantified. Such ethanol treatment produced significant changes in neurotrophin receptor levels in the hippocampus, septum, cerebral cortex, and cerebellum. Receptor levels in hippocampus, septum, and cerebellum, tended to be decreased, while levels in cortex were consistently increased. Males were generally more affected than females. While most of these alterations were transient, sustained or delayed changes were present in P10 septum, cortex, and cerebellum. These results indicate that developmental ethanol exposure produces major changes in the normal physiological levels of the neurotrophin receptors throughout the CNS. These changes in the receptor complement during critical prenatal stages could relate to the anomalous development of the CNS seen in the fetal alcohol syndrome. This relationship is discussed, together with the potential biological effects of such dramatic changes in neurotrophin receptor expression.     

Expression of Bcl-2, Bax and Caspase-3 in Nerve Tissues of Rats Chronically Exposed to 2,5-hexanedione

Occupational exposure and experimental intoxication with n-hexane or its metabolite 2,5-hexanedione (HD) produce a central-peripheral neuropathy. However, the mechanism remains unknown. We hypothesized that HD affected the expression of Bcl-2, Bax and Caspase-3 in the central nervous system (CNS) and the peripheral nervous system (PNS). Male adult Wistar rats were administered by intraperitoneal injection at a dosage of 200 or 400 mg/kg HD, five days per week for 8 weeks. Samples of the cerebral cortex, cerebellum, spinal cord and sciatic nerves were collected and examined for Bcl-2, Bax and Caspase-3 expression using Western blotting. Subchronic exposure to HD resulted in significantly increased expression of both anti-apoptotic protein Bcl-2 and pro-apoptotic protein Bax and Caspase-3 in cerebral cortex and cerebellum, which exhibited a dose-dependent pattern. Though little change was detected in spinal cord, our results showed that the expression of Bcl-2, Bax and Caspase-3 was markedly enhanced in the sciatic nerves. These findings suggested that the changes of apoptosis-related protein level in rat nerve tissues were associated with the intoxication of HD, which might be involved in early molecular regulatory mechanism of apoptosis in the HD-induced neuropathy.     

Increased monoamine oxidase activity in cardiovascular and central nervous systems of DOCA-NaCl hypertensive rabbits

Monoamine oxidase (MAO) activity and sodium and potassium concentrations were determined in the cardiovascular and central nervous systems of rabbits made hypertensive with desoxycorticosterone acetate (DOCA) and sodium chloride. MAO was increased materially above control values in heart, arterial tissue and hypothalamus of hypertensive rabbits. The sodium concentraion of arterial tissue and of the hypothalamus but not of the cerebral cortex, was also elevated significantly in hypertension. It is concluded that both peripheral and central components may be involved in DOCA-NaCl hypertension in the rabbit.     

Neurotransmitters, neuropeptides and calcium binding proteins in developing human cerebellum: a review

Many endogenous neurochemicals that are known to have important functions in the mature central nervous system have also been found in the developing human cerebellum. Cholinergic neurons, as revealed by immunoreactivities towards choline acetyltransferase or acetylcholinesterase, appear early at 23 weeks of gestation in the cerebellar cortex and deep nuclei. Immunoreactivities gradually increase until the first postnatal month. Enkephalin is localized in the developing cerebellum, initially in the fibers of the cortex and deep nuclei at 16–20 weeks and then also in the Purkinje cells, granule cells, basket cells and Golgi cells at 23 weeks onward. Another neuropeptide, substance P, is localized mainly in the fibers of the dentate nucleus from 9 to 24 weeks but substance P immunoreactivity declines thereafter. GABA, an inhibitory neurotransmitter of the central nervous system, starts to appear at 16 weeks in the Purkinje cells, stellate cells, basket cells, mossy fibers and neurons of deep nuclei. GABA expression is gradually upregulated toward term forming networks of GABA-positive fibers and neurons. Catecholaminergic fibers and neurons are also detected in the cortex and deep nuclei at as early as 16 weeks. Calcium binding proteins, calbindin D28K and parvalbumin, make their first appearance in the cortex and deep nuclei at 14 weeks and then their expression decreases toward term, while calretinin appears later at 21 weeks but its expression increases with fetal age. The above findings suggest that many neurotransmitters, neuropeptides and calcium binding proteins (1) appear early during development of the cerebellum; (2) have specific temporal and spatial expression patterns; (3) may have functions other than those found in the mature neural systems; and (4) may be able to interact with each other during early development.