Parallel Fiber Activity Research Articles

Brief dendritic calcium signals initiate long-lasting synaptic depression in cerebellar Purkinje cells.

We have performed experiments designed to test the hypothesis that long-term depression (LTD) of excitatory synaptic transmission in the cerebellar cortex is caused by a rise in postsynaptic Ca concentration. These experiments combined measurements of synaptic efficacy, performed with the thin slice patch clamp technique, with fura-2 measurements of intracellular Ca concentration ([Ca]i) in single cerebellar Purkinje cells. Simultaneous activation of the climbing fiber and parallel fibers innervating single Purkinje cells caused a LTD of transmission of the parallel fiber-Purkinje cell excitatory synapse. This LTD was associated with large and transient rises in [Ca]i in the Purkinje cell and apparently was due to Ca entry through voltage-gated Ca channels in the Purkinje cell dendrites. The rise in [Ca]i produced by climbing fiber activity was necessary for LTD, because addition of the Ca chelator bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA) to the interior of the Purkinje cell blocked LTD. Further, elevation of [Ca]i, produced by depolarizing pulses delivered in conjunction with parallel fiber activation, induced a depression of synaptic activity that closely resembled LTD in both time course and magnitude. Thus, a rise in [Ca]i appears to be sufficient to initiate LTD. From these results, we conclude that LTD of the parallel fiber-Purkinje cell synapse is initiated by a brief, climbing fiber-mediated rise in postsynaptic [Ca]i and that LTD is maintained by other, longer-lived processes that are triggered by the rise in postsynaptic [Ca]i.

Coactivation of D 1 and D 2 dopamine receptors is required for long-term synaptic depression in the striatum

Long-term changes of synaptic transmission following brief trains of high-frequency stimulation of excitatory pathways in the brain have attracted attention as a possible correlate of memory. In the cerebellum, concurrent activation of parallel fibers and climbing fibers leads to a long-term depression (LTD) of synaptic transmission, which may be the cellular substrate of motor learning in this structure. We report here for the first time that high-frequency stimulation of corticostriatal glutamatergic fibers in the striatum, another brain structure strongly involved in motor control, also induces LTD of synaptic transmission. Induction of striatal LTD is blocked either by SCH 23390, a D 1 dopamine (DA) receptor antagonist or by l-sulpiride, a D 2 DA receptor antagonist. The lesion of the nigrostriatal DAergic pathway abolishes LTD. After DA depletion, LTD can be restored by the application of exogenous DA. LTD can also be restored by coadministration of D 1 and D 2 DA receptor agonists, but not by the application of a single class of DA agonists alone. Our data show that coactivation of D 1 and D 2 DA receptors is required for LTD in the striatum. D 1/D 2 receptor cooperation in the induction of LTD may play a crucial role in the behavioural function of DA and in the therapeutic effects of DA agonists in Parkinson's disease.

Why run parallel fibers parallel? Teleostean purkinje cells as possible coincidence detectors, in a timing device subserving spatial coding of temporal differences

The present paper explores the possible functional significance of the parallel orientation of parallel fibers in teleostean cerebellar and cerebelloid molecular layers, taking advantage of the restricted width of these molecular layers compared with mammalian ones and several specific configurations of granule cells. These configurations include: (i) a unilateral location, i.e. at only one (lateral) side of the molecular layer, giving rise to parallel fibers without bifurcation in a unidirectional molecular layer, where all parallel fibers conduct signals in the same direction; (ii) a bilateral location at both sides of the molecular layer giving rise to a bidirectional molecular layer where parallel fibers conduct signals in two opposite directions originating from two discrete sources; and (iii) a basal (or sometimes apical) location underneath (or opposite to) the layer of Purkinje cells, giving rise to a bidirectional molecular layer where parallel fibers conduct signals in two opposite directions originating from a continuous range of sources. It is argued that molecular layers with a bilateral location of granule cells, exemplified by the mormyrid lobus transitorius, represent an optimal configuration for the analysis of small temporal differences (up to 4 ms) between inputs to the right and left granule cell mass, by means of detection of the site of coincidence of parallel fiber activity running from left to right and vice versa. Morphological aspects that probably optimize such a function include not only the parallel course and bilateral origin of parallel fibers, but also their small diameter, large number and co-extensive location, as well as the sagittal orientation and the presence of many spines of Purkinje cell dendrites and the presence of stellate and other inhibitory interneurons. The only assumption underlying the present coincidence detection hypothesis is that Purkinje cells are supposed to be maximally stimulated by parallel fiber input when all spines are activated in such a way that their excitatory postsynaptic potentials reach the axon hillock simultaneously. For molecular layers with a unilateral location of granule cells, exemplified by the teleostean torus longitudinalis-tectal marginal parallel fiber system, a similar coincidence detecting mechanism is proposed on the basis of the presence of two populations of parallel fibers with slightly different conduction velocities. Such a system might be suitable to adapt the location of coincidence peaks to topographic maps present in deeper layers of nervous tissue. Molecular layers with basally (or apically) located granule cells as encountered in the teleostean corpus cerebelli, are probably involved in the analysis of specific spatio-temporal input waves directed centripetally towards different Purkinje cells. Compared with monoor bilateral locations of granule cells, a basal location of granule cells adds specificity at the expense of sensitivity to the coincidence detecting properties of the molecular layer, and requires a more precise topographic organization of mossy fiber input. The possible functional significance of the configurations analysed is discussed on the basis of the literature available, while their specific advantages and constraints are evaluated using a model of the teleostean lateral line system. This system presents a nice example of different configurations of similar sensors at the same sensory interface working within the same space-time domain. Comparison of the teleostean cerebellar configurations analysed with the mammalian one suggests that the dramatically larger width of the latter does not only allow for the detection of a larger range of temporal differences and spatiotemporal input wave patterns, but might also reduce artefacts by juxtaposition of distinct zones with a similar input. The presence of basket cells and the discongruent extension of parallel fibers in the mammalian cerebellum might also optimize coincidence detection. Comparison with the acoustic and electrosensory communication system, where similar coincidence detection mechanisms have been demonstrated, suggests that these subserve fast analysis of phase differences of relatively simple, repetitive input of high frequency (an acoustic or electric tone), whereas cerebellar coincidence detection is probably involved in slower analysis of temporal differences of single or low-frequency repetitive inputs with a more complex and noisy shape and pattern. Finally, it is suggested that the specific palisade orientation of the dendrites of mormyrid Purkinje cells might well increase the tuning of Purkinje cells for specific input waves, not only in the transverse direction, but also in the apico-basal and rostro-caudal direction, and thus probably represents an ultimate optimization for cerebellar coincidence detection. In contrast, the mammalian Purkinje cell configuration seems optimal for integration of parallel fiber input with climbing fiber input, possibly involved in gain control of coincidence detection.

Palisade pattern of mormyrid Purkinje cells: A correlated light and electron microscopic study

The present study is devoted to a detailed analysis of the structural and synaptic organization of mormyrid Purkinje cells in order to evaluate the possible functional significance of their dendritic palisade pattern. For this purpose, the properties of Golgi-impregnated as well as unimpregnated Purkinje cells in lobe C1 and C3 of the cerebellum of Gnathonemus petersii were light and electron microscopically analyzed, quantified, reconstructed, and mutually compared. Special attention was paid to the degree of regularity of their dendritic trees, their relations with Bergmann glia, and the distribution and numerical properties of their synaptic connections with parallel fibers, stellate cells, "climbing" fibers, and Purkinje axonal boutons. The highest degree of palisade specialization was encountered in lobe C1, where Purkinje cells have on average 50 palisade dendrites with a very regular distribution in a sagittal plane. Their spine density decreases from superficial to deep (from 14 to 6 per micron dendritic length), a gradient correlated with a decreasing parallel fiber density but an increasing parallel fiber diameter. Each Purkinje cell makes on average 75,000 synaptic contacts with parallel fibers, some of which are rather coarse (0.45 microns), and provided with numerous short collaterals. Climbing fibers do not climb, since their synaptic contacts are restricted to the ganglionic layer (i.e., the layer of Purkinje and eurydendroid projection cells), where they make about 130 synaptic contacts per cell with 2 or 3 clusters of thorns on the proximal dendrites. These clusters contain also a type of "shunting" elements that make desmosome-like junctions with both the climbing fiber boutons and the necks of the thorns. The axons of Purkinje cells in lobe C1 make small terminal arborizations, with about 20 boutons, that may be substantially (up to 500 microns) displaced rostrally or caudally with respect to the soma. Purkinje axonal boutons were observed to make synaptic contacts with eurydendroid projection cells and with the proximal dendritic and somatic receptive surface of Purkinje cells, where about 15 randomly distributed boutons per neuron occur. The organization of Purkinje cells in lobe C3 differs markedly from that in C1 and seems to be less regular and specialized, although the overall palisade pattern is even more regular than in lobe C1 because of the absence of large eurydendroid neurons. However, individual neurons have a less regular dendritic tree, there is no apical-basal gradient in spine density or parallel fiber density and diameter, and there are no "shunting" elements in the climbing fiber glomeruli.(ABSTRACT TRUNCATED AT 400 WORDS)

Imaging of cerebellar surface activation In vivo using voltage sensitive dyes

The understanding of the information processing performed by complex neuronal networks in the central nervous system will require techniques permitting the simultaneous monitoring of the electrical activity of neuronal ensembles. Voltage sensitive dyes offer the potential for non-invasive optical monitoring of the activity in large populations of neurons. In this report we describe the use of voltage sensitive dyes and image processing techniques to monitor in vivo the activation of parallel fibers and associated neuronal events produced by stimulation of the cerebellar cortex in the rat. Despite the temporal limitations of video processing a relatively brief set of neuronal events was successfully imaged. Using this methodology we demonstrate that the detected fluorescent light changes were highly correlated with the evoked extracellular field potentials. Graded surface stimulation produced graded spatial patterns consistent with known parallel fiber anatomy and physiology. The optical signals were dependent on the presence of the voltage sensitive dyes and were abolished by topical application of a local anesthetic agent. In essence, activation of a parallel fiber beam and associated activity were imaged at relatively high resolution.

Behavior of extracellular K + and pH in skate ( Raja erinacea) cerebellum

Ion-selective microelectrodes were used to measure extracellular K + concentrations ([K +] o) and extracellular pH (pH o) in skate cerebellum under resting and stimulated conditions. Consistent with earlier ion analysis of elasmobranch cerebrospinal fluid (CSF), [K +] o was3.6 ± 0.1mM. During parallel fiber activation, [K +] o increased to an upper limit of 12–14 mM with an linear approximately linear dependence on stimulation frequency (1–20 Hz). Post-stimulus undershoots 0.1–0.6 mM were seen throughout an animal temperature range of 13–18 °C. When stimulation produced spreading depression (SD), [K +] o first increased to about 10 mM, then rose more rapidly to about 30 mM. These observations indicate a K + ceiling of 10–12 mM in elasmobranchs. This ceiling is the same as that seen in mammals, despite marked differences in CSF composition and osmolality between mammalian and elasmobranch species. Extracellular acidification were characteristic of the response. These pH o transients were similar to those reported in other preparations, although the alkaline shift was enhanced. This may be attributed to the relatively low buffering capacity of elasmobranch CSF and to summation with a generally smaller acid shift.

Electrophysiological demonstration of a synaptic integration of transplanted purkinje cells into the cerebellum of the adult purkinje cell degeneration mutant mouse

After implantation of solid pieces of cerebellar primordia from 12-day-old C57BL embryos into the cerebellar parenchyma of 3- to 4-month-old “Purkinje cell degeneration” mutant mice, Purkinje cells from the donor leave the implant and differentiate while migrating into the host molecular layer. Electrophysiological studies were performed using in vitro cerebellar slice preparations from “Purkinje cell degeneration” mutants 1–2 months after grafting, when grafted Purkinje cells have reached their final location in the host molecular layer and have completed their morphological differentiation. Intracellular recordings obtained from 45 Purkinje cells in mutant mice demonstrated that such grafted neurons have normal bioelectrical properties including sodium and calcium conductances and inward rectification. Moreover, all grafted Purkinje cells responded to electrical white matter stimulation by a typical all-or-none climbing fiber response. Responses mediated through the activation of mossy and parallel fibers, as well as inhibitory postsynaptic potentials, were also recorded in a significant number of grafted Purkinje cells. On the whole, all these excitatory and inhibitory responses in grafted “Purkinje cell degeneration” mutant mice have characteristics comparable to those in control mice. After electrophysiological studies, Purkinje cells were further characterized by their positive staining by calbindin antibody. Neurons of this class were dispersed throughout the molecular layer of the host folia in which the electrophysiological recordings had been performed. The ectopic location of their perikarya, the presence of dendritic trees spanning most of the molecular layer (without entering the granular layer), and the occasional presence of axons emerging from the ectopic neurons and forming loose bundles at the white matter axis of the folia, corroborate the grafted nature of the Purkinje cells studied. Therefore, these experiments demonstrate that embryonic Purkinje cells from the graft can complete differentiation in the adult host cerebellum, and establish specific synaptic contacts with the presynaptic elements previously impinging on the missing neurons of “Purkinje cell degeneration” mutants. This process leads to a qualitative functional synaptic restoration of the cortical cerebellar network.

Field potential analysis in elasmobranch cerebellum

Stimulation of cerebellar white matter (WM) in the skate, an elasmobranch fish, evokes a distinctive set of cortical field potentials characterized by 3 negativities and a positivity. The first negativity (N1) has 1 msec latency and is largest in the lateral regions of the corpus cerebelli where Purkinje cells and white matter are most densely congregated. The second negativity (N2) occurs at 2--4 msec latency and is localized to the granular layer. The third negativity (N3) follows with a latency of 4--6 msec and is prominent in the molecular layer. The positivity (P) correlates with N2 in time course and dominates in the midline where granule cell axons ascend en masse to form parallel fibers in the molecular layer. A preceding WM stimulus blocks the N1 potential for 20 msec and the N2 potential for 60 msec and the N2 potential for 20 msec. A conditioning stimulus, applied to the parallel fibers in the dorsal midline (LOC), suppresses the N1 potential for 20 msec and the N2 potential for 40 msec. Intracellular recordings from the Purkinje cell layer reveals short latency action potentials correlating in time with N1. These findings suggest that N1 derives from antidromic Purkinje cell activation and mossy fiber excitation, that N2 represents orthodromic granule cell excitation by white matter fibers, and that N3 stems from parallel fiber activity. Because of its close association with parallel fibers and the similarity of its time course to N2, the positivity is attributed to current sources in parallel fibers generated by granule cell current sinks. Differences between the cerebellar field potentials found in elasmobranchs and mammals can be explained by the unique anatomic arrangement of granule cell axons in the former.

Cerebellar Purkinje cell responses to afferent inputs. II. Mossy fiber activation

(1) The mossy fiber pathway to cerebellar Purkinje cells was activated in decerebrate and lightly thiopental anesthetized cats by electrical stimulation of the cerebellar cortex surface, the lateral reticular nucleus (LRN), and various hind- and forelimb nerves. (2) On-beam activation of parallel fibers could be accomplished by any of the 3 types of stimulations used. Such on-beam activation rarely evoked more than a single spike in Purkinje cells, and nerver more than three, in either preparation. The average number of spikes evoked per stimulus graded in a delicate manner when the strength of the stimulus at any of the 3 sites was increased. (3) Similarly, inhibition of Purkinje cells firing could be produced by either on-or off-beam activation of parallel fiber by any of the 3 stimulations. This inhibition was of a highly variable duration, at times greater than 200 msec, and also graded with stimulus strength. (4) Surface stimulation could be arranged at times to produce sudden, ungraded inhibition, presumably by direct excitation of Golgi cells. Direct excitation of mossy fibers with surface stimulation was also at times observed. (5) The latency of the LRN evoked response in Purkinje cells was of the order of 3.0–6.0 msec. A slow negative field potential was sometimes observed following the N 2 potential. (6) A hypothesis is put forward that the mossy fiber pathway carries rapidly changing proprioceptive information from peripheral receptors which enables Purkinje cells to compute appropriate motor responses.

Parallel fiber and white matter activation of Purkinje cells in a reptilian cerebellum ( Lacerta viridis)

Parallel fiber and white matter activation of Purkinje cells in a reptilian cerebellum ( Lacerta viridis)

Inhibition in the frog cerebellar cortex following parallel fiber activation

Inhibition in the frog cerebellar cortex following parallel fiber activation