C. I. De Zeeuw

Deletion of FMR1 in Purkinje cells enhances parallel fiber LTD, enlarges spines, and attenuates cerebellar eyelid conditioning in Fragile X syndrome.

Absence of functional FMRP causes Fragile X syndrome. Abnormalities in synaptic processes in the cerebral cortex and hippocampus contribute to cognitive deficits in Fragile X patients. So far, the potential roles of cerebellar deficits have not been investigated. Here, we demonstrate that both global and Purkinje cell-specific knockouts of Fmr1 show deficits in classical delay eye-blink conditioning in that the percentage of conditioned responses as well as their peak amplitude and peak velocity are reduced. Purkinje cells of these mice show elongated spines and enhanced LTD induction at the parallel fiber synapses that innervate these spines. Moreover, Fragile X patients display the same cerebellar deficits in eye-blink conditioning as the mutant mice. These data indicate that a lack of FMRP leads to cerebellar deficits at both the cellular and behavioral levels and raise the possibility that cerebellar dysfunctions can contribute to motor learning deficits in Fragile X patients.

More on climbing fiber signals and their consequence(s)

The persistence of many contrasting notions of climbing fiber function after years of investigation testifies that the issue of climbing fiber contributions to cerebellar transactions is still unresolved. The proposed capabilities of the climbing fibers cover an impressive spectrum. For many researchers, the climbing fibers signal errors in motor performance, either in the conventional manner of frequency modulation or as a single announcement of an “unexpected event”. More controversial is the effect of these signals on the simple spike modulation of Purkinje cells. In some hands, they lead to a long-term depression of the strength of parallel fiber synapses, while, in other hands, they lead to a short-lasting enhancement of the responsiveness of Purkinje cells to mossy fiber inputs or contribute to the often-seen reciprocal relation between complex and simple spike modulation. For still other investigators, the climbing fibers serve internal timing functions through their capacity for synchronous and rhythmic firing. The above viewpoints are presented in the spirit of trying to reach some consensus about climbing fiber function. Each point of view is introduced by summarizing first the key observations made by the respective proponents; then the issues of short-lasting enhancement, reciprocity between complex and simple spikes, and synchrony and rhythmicity are addressed in the context of the visual climbing fiber system of the vestibulocerebellum.

A comparison of video and magnetic search coil recordings of mouse eye movements.

Interest in connecting molecular biology and behavior is motivating research into the eye movements of mice. Unfortunately, recording eye movements in this diminutive animal is technically difficult. We present the first method for obtaining calibrated video oculography, and contrast the results with simultaneously obtained scleral search coil recordings in C57BL/6 pigmented mice. We determined the distance of the pupil from the center of corneal curvature, based upon relative motions of the pupil and corneal reflections during camera movements, and used the distance to convert subsequent video measurements of pupil motion to eye rotation. We recorded responses during sinusoidal rotation (0.1-1.6 Hz) in the light, by video prior to search coil implantation, and by video and search coil simultaneously following implantation. Pre-implantation, video-derived gains ranged from 0.86+/-0.03 (mean+/-SD) at 0.1 Hz to 0.95+/-0.03 at 0.8 Hz. Phase progressed monotonically from -3.1+/-2. 6 degrees (eye leads head) at 0.1 Hz to +5.9+/-1.1 degrees at 1.6 Hz. Coil implantation reduced the range of video-derived gains to 0. 64-0.79. This reduction reflects disruption of normal behavior by the coil. Coil data confirmed the video results. Video and search coil techniques each have advantages. Specific precautions are required when designing and interpreting experiments using the coil technique.

A new combination of WGA-HRP anterograde tracing and GABA immunocytochemistry applied to afferents of the cat inferior olive at the ultrastructural level ☆

In order to identify cerebellar terminals in the cat inferior olive which contain gamma-aminobutyric acid (GABA), a technique was developed combining anterograde transport of wheatgerm agglutinine-conjugated horseradish peroxidase (WGA-HRP) with gold-immunocytochemistry. With this technique both the HRP reaction product and the immunogold labelling can be visualized in a single ultrathin section. Our results suggest that most, if not all of the WGA-HRP-labelled cerebellar terminals in the rostral medial accessory olive (MAO) and the rostral principal olive (PO) are GABAergic. In an additional experiment the GABAergic innervation of the rostral MAO was studied in combination with WGA-HRP anterograde tracing from the rostral mesencephalon. In this case the WGA-HRP-labelled terminals were never found to be GABA-positive.

Increased noise level of purkinje cell activities minimizes impact of their modulation during sensorimotor control

While firing rate is well established as a relevant parameter for encoding information exchanged between neurons, the significance of other parameters is more conjectural. Here, we show that regularity of neuronal spike activities affects sensorimotor processing in tottering mutants, which suffer from a mutation in P/Q-type voltage-gated calcium channels. While the modulation amplitude of the simple spike firing rate of their floccular Purkinje cells during optokinetic stimulation is indistinguishable from that of wild-types, the regularity of their firing is markedly disrupted. The gain and phase values of totterings compensatory eye movements are indistinguishable from those of flocculectomized wild-types or from totterings with the flocculus treated with P/Q-type calcium channel blockers. Moreover, normal eye movements can be evoked in tottering when the flocculus is electrically stimulated with regular spike trains mimicking the firing pattern of normal simple spikes. This study demonstrates the importance of regularity of firing in Purkinje cells for neuronal information processing.

Cell surface exposure of phosphatidylserine during apoptosis is phylogenetically conserved

Exposure of the aminophospholipid phosphatidylserine at the outer leaflet of the plasma membrane by apoptotic cells can trigger phagocytic removal of these dying cells. This functionality of phosphatidylserine exposure in the process of phagocytosis is indicated by in vitro studies of mammalian and insect phagocytes. We have studied the in vivo distribution of cell-surface exposed phosphatidylserine by injecting biotinylated Annexin V, a Ca 2+ -dependent phosphatidyl-serine binding protein, into viable mouse and chick embryos and Drosophila pupae. The apparent binding of Annexin V to cells with a morphology which is characteristicof apoptosis and which was present in regions of developmental cell death indicates that phosphatidylserine exposure by apoptotic cells is a phylogenetically conserved mechanism.

Mesodiencephalic and cerebellar terminals terminate upon the same dendritic spines in the glomeruli of the cat and rat inferior olive: An ultrastructural study using a combination of [3H]-leucine and wheat germ agglutinin coupled horseradish peroxidase anterograde tracing

The mesodiencephalic and cerebellar afferents in the rostral medial accessory and principal olive of the cat and rat were studied following anterograde transport of tritiated leucine combined with anterograde transport of wheat germ agglutinin coupled horseradish peroxidase in the same animals. In all studied areas at least one-third of the labelled glomeruli appeared to contain both mesodiencephalic and cerebellar terminals. In many of these cases it was found that the terminals from both afferent systems contacted the same dendritic spines. Therefore, these olivary spines may be, as will be discussed, well suited for being involved in a timing process.

Single Purkinje Cell Can Innervate Multiple Classes of Projection Neurons in the Cerebellar Nuclei of the Rat: A Light Microscopic and Ultrastructural Triple-Tracer Study in the Rat

Two different populations of projection neurons are intermingled in the cerebellar nuclei. One group consists of small, γ‐aminobutyric acid‐containing (GABAergic) neurons that project to the inferior olive, and the other group consists of larger, non‐GABAergic neurons that provide an input to one or more, usually premotor, centers in the brainstem, such as the red nucleus, the thalamus, and the superior colliculus. All cerebellar nuclear neurons are innervated by GABAergic Purkinje cells. In this study, we investigated whether individual Purkinje cells of the C1 zone of the paramedian lobe of the rat innervate both groups of projection neurons in the anterior interposed nucleus. Two different, retrogradely transported tracers, either cholera toxin β subunit (CTb) or wheat germ agglutinin coupled to horseradish peroxidase (WGA‐HRP) and a gold lectin tracer were injected into the red nucleus and the inferior olive, respectively, whereas Purkinje cell axons were anterogradely labeled with biotinylated dextran amine (BDA) injected into the paramedian lobule.

Temporal relations of the complex spike activity of Purkinje cell pairs in the vestibulocerebellum of rabbits

Parasagittal zones in the vestibulocerebellum contain Purkinje cells whose complex spike (CS) activity is modulated in response to rotational optokinetic stimulation (OKS) about either the vertical axis (VA) or a horizontal axis (HA) that is approximately perpendicular to the ipsilateral anterior canal. In rabbits, there are two VA zones in both the ventral nodulus and flocculus, two HA zones in the flocculus, and one HA zone in the ventral nodulus. We investigated the temporal relationship of the CS activity of Purkinje cell pairs in the same or different zones of the vestibulocerebellum in ketamine-anesthetized pigmented rabbits. A synchronous temporal relationship was defined as the tendency of the CS of each Purkinje cell to fire within, at most, 2 msec of one another. Generally, neurons in the same zone showed a tendency to exhibit CS synchrony. Of 82 pairs consisting of two Purkinje cells in the same zone (e.g., two nodulus HA cells), 33 were synchronous. In contrast, none of 26 pairs consisting of two neurons in functionally different zones (e.g., a VA cell paired with an HA cell), showed CS synchrony. Pairs consisting of neurons in spatially separated VA zones in the ventral nodulus also showed a tendency to be synchronously related (6/16), as did pairs consisting of a nodulus VA cell and a flocculus VA cell (3/14). The CS synchrony was higher during OKS in the preferred direction than during spontaneous activity. This is the first demonstration that CS synchrony in the vestibulocerebellum can be manipulated with a natural sensory stimulus.

The dynamic characteristics of the mouse horizontal vestibulo-ocular and optokinetic response.

In the present study the optokinetic reflex, vestibulo-ocular reflex and their interaction were investigated in the mouse, using a modified subconjunctival search coil technique. Gain of the ocular response to sinusoidal optokinetic stimulation was relatively constant for peak velocities lower than 8 degrees /s, ranging from 0.7 to 0.8. Gain decreased proportionally to velocity for faster stimuli. The vestibulo-ocular reflex acted to produce a sinusoidal compensatory eye movement in response to sinusoidal stimuli. The phase of the eye movement with respect to head movement advanced as stimulus frequency decreased, the familiar signature of the torsion pendulum behavior of the semicircular canals. The first-order time constant of the vestibulo-ocular reflex, as measured from the eye velocity decay after a vestibular velocity step, was 660 ms. The response of the vestibulo-ocular reflex changed with stimulus amplitude, having a higher gain and smaller phase lead when stimulus amplitude was increased. As a result of this nonlinear behavior, reflex gain correlated strongly with stimulus acceleration over the 0.1-1.6 Hz frequency range. When whole body rotation was performed in the light the optokinetic and vestibular system combined to generate nearly constant response gain (approximately 0.8) and phase (approximately 0 degrees ) over the tested frequency range of 0.1-1.6 Hz. We conclude that the compensatory eye movements of the mouse are similar to those found in other afoveate mammals, but there are also significant differences, namely shorter apparent time constants of the angular VOR and stronger nonlinearities.