Calcium-binding proteins in primate cerebellum

Molecular probes of the vestibular nerve. II. Characterization of neurons in Scarpa's ganglion to determine separate populations within the nerve

An unambiguous delineation of the exact numbers and/or proportions of calyx-only, dimorph, and bouton-only vestibular afferents is needed to continue studies concerning vestibular integration in the nervous system. Here, we take advantage of immunocytochemical properties of three groups of vestibular afferents. We utilize calretinin to delineate the calyx-only population, and peripherin to stain the bouton-only afferents. An additional subgroup of afferents that stain with calbindin, but not calretinin is also introduced. The size of the cells that stain with these markers was determined. Cells that are calbindin-positive overlap the sizes of Nissl-stained somata. Cells that stain with peripherin or calretinin are non-overlapping with calretinin cells being the largest and peripherin-positive cells the smallest. Twenty percent of the ganglion cells were peripherin positive, another 20% stained with calretinin antibodies, 30% stained with calbindin, and all cells in Scarpa's ganglion stained with parvalbumin. Most of the calretinin-positive cells also stained with calbindin. One-third of the calbindin-positive population stained only with calbindin. These studies indicate that the calyx- and bouton-only populations of vestibular afferents in gerbil comprise at least 40% of the nerve. In addition, at least 10% of the nerve also stains with calbindin and neither calretinin nor peripherin. Based on indirect evidence, we hypothesize that these are a subpopulation of dimorph afferents. This study has provided an anatomical instrument (in addition to intracellular physiological methods) to study separate populations of vestibular afferents.

Localization of ataxin-2 within the cerebellar cortex of the rat

Spinocerebellar ataxia type 2 is caused by a polyglutamine stretch in the protein ataxin-2 that is due to an expansion of a CAG repeat in the spinocerebellar ataxia-2 gene. The function of wild-type ataxin-2 has not been clarified. A widespread distribution of this protein throughout the brain has been reported. We examined the expression of ataxin-2 in cortical cerebellar cells of the adult rat. We performed a single label immunohistochemical study of ataxin-2 and a single label immunofluorescence study of ataxin-2 and zebrin on adjacent sections, to compare the distribution of the observed parasagittal band pattern. We also performed a double label immunofluorescence study of ataxin-2 and one of each parvalbumin, calbindin, and calretinin. Single label studies revealed that between 50% and 70% of the Purkinje cells express ataxin-2. The abundance of ataxin-2 was different between hemisphere and vermis, with a clear prevalence for the former. Furthermore, the distribution of ataxin-2-positive Purkinje cells showed a peculiar alternating parasagittal band pattern. Among the other cortical cerebellar cells only basket and granule cells showed ataxin-2 staining. Our dual label studies showed that about 50% of calbindin and more than 70% of parvalbumin-immunoreactive Purkinje cells were also labeled for ataxin-2. The uneven distribution of ataxin-2 expression in the Purkinje cell layer does not support the hypothesized link between ataxin-2 content and cell vulnerability. The differences in ataxin-2 expression among the cell types of cerebellar cortex, on the other hand, suggest a possible correlation between ataxin-2 content and cell function.

Cerebellar long-term depression: characterization, signal transduction, and functional roles

Cerebellar Purkinje cells exhibit a unique type of synaptic plasticity, namely, long-term depression (LTD). When two inputs to a Purkinje cell, one from a climbing fiber and the other from a set of granule cell axons, are repeatedly associated, the input efficacy of the granule cell axons in exciting the Purkinje cell is persistently depressed. Section I of this review briefly describes the history of research around LTD, and section II specifies physiological characteristics of LTD. Sections III and IV then review the massive data accumulated during the past two decades, which have revealed complex networks of signal transduction underlying LTD. Section III deals with a variety of first messengers, receptors, ion channels, transporters, G proteins, and phospholipases. Section IV covers second messengers, protein kinases, phosphatases and other elements, eventually leading to inactivation of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolone-propionate-selective glutamate receptors that mediate granule cell-to-Purkinje cell transmission. Section V defines roles of LTD in the light of the microcomplex concept of the cerebellum as functionally eliminating those synaptic connections associated with errors during repeated exercises, while preserving other connections leading to the successful execution of movements. Section VI examines the validity of this microcomplex concept based on the data collected from recent numerous studies of various forms of motor learning in ocular reflexes, eye-blink conditioning, posture, locomotion, and hand/arm movements. Section VII emphasizes the importance of integrating studies on LTD and learning and raises future possibilities of extending cerebellar research to reveal memory mechanisms of implicit learning in general.

Unipolar brush cells in marmoset cerebellum and cochlear nuclei express calbindin

Unipolar brush cells (UBCs) are excitatory neurons in the mammalian cerebellum and cochlear nuclei (CN), including the CN of primates, as shown only recently. UBCs are readily identified by their expression of the calcium-binding protein calretinin (CR), except for those of the primate CN that hardly immunostain for CR. The present findings corroborate the existence of UBCs in the CN of a primate, Callithrix. Furthermore, evidence is presented for UBCs, in the cerebellum and a small subpopulation of UBCs in the CN of Callithrix to express the calcium-binding protein calbindin (CB). This may be unique for Callithrix as CB-expressing UBCs have not been recognized in any other mammal. Presence of CB but not CR in UBCs of the Callithrix CN may indicate a certain interchangeability between these two calcium-binding proteins.

Calbindin immunoreactivity in the developing and adult human cerebellum

Calbindin (CALB), a calcium-binding protein, is known to be expressed in the embryonic nervous system. In this study, we have examined its distribution in the cerebellum of human fetuses (11-25 weeks of gestation) and adult by immunohistochemistry. At the gestational age of 11-12 weeks. CALB immunoreactivity was present in granule and Purkinje cells throughout the cerebellum. By 16-21 weeks of gestation, immunoreactive Purkinje cells were well-differentiated in the vermis and flocculus, and their axons ran towards the deep cerebellar nuclei area, while the axon collaterals were seen to be distributed into adjacent folia. At the gestational period of 24-25 weeks, most Purkinje cells of the flocculus and vermis were arranged in one to two rows, while those of the hemispheres were still undifferentiated. A few Golgi cells of the vermis showed immunoreactivity. The neurons of the deep nuclei were immunonegative right from the gestational age of 11 weeks although a fine stippled staining of fibers was present throughout the body of all nuclei. The fibers lying close to the hilum of the dentate nucleus were strongly CALB-positive. The vestibulocerebellar fibers, being traced at the level of lower pons and upper medulla oblongata were stained as early as 11 weeks of gestation, whereas the olivocerebellar fibers were stained from 16 weeks onward. In the adult cerebellum, Purkinje cells were moderately immunopositive while granule cells were faintly stained; no other cells, including those of the deep nuclei were stained. In the medulla oblongata, the inferior olivary nucleus and olivocerebellar fibers were strongly CALB-positive. Our results indicate that CALB is expressed in early migratory Purkinje cells, and their maturation occurs in a vermal-to-hemisphere gradient. It is likely that CALB plays a significant role in the regulation of Ca2+-dependent activities in the developing cerebellum.

Unipolar brush cells in the cochlear nuclei of a primate (Callithrix jacchus)

Unipolar brush cells (UBCs) have been recognized in the cerebella of many species including primates. They have also been identified in the cochlear nuclei (CN) of many mammals, however, not in any primate. Our immunohistochemical study in the marmoset demonstrates the presence of abundant calretinin immunoreactive (CR-ir) UBCs in the cerebellum, as in other mammals. In the marmoset CN, in contrast, CR-ir UBCs could hardly be identified. The reverse pattern was evident in sections immunostained for a metabotropic glutamate receptor (mGluR2/3-IR): beautifully stained UBCs were present in the CN, but the stained cerebellar structures were difficult to identify as UBCs. The present findings indicate that UBCs are present in the CN of primates, as in other mammals, but that cerebellar and CN UBCs might differ in their molecular equipment in primates.