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Presynaptic calcium signalling in cerebellar mossy fibres.

Thomsen LB, Jörntell H, Midtgaard J - Front Neural Circuits (2010)

Bottom Line: A paired-pulse depression of the calcium signal lasting more than 1 s affected burst firing in mossy fibres; this paired-pulse depression was reduced by GABA B antagonists.While our results indicated that a presynaptic rosette electrophysiologically functioned as a unit, topical GABA application showed that calcium signals in the branches of complex rosettes could be modulated locally, suggesting that cerebellar glomeruli may be dynamically sub-compartmentalized due to ongoing inhibition mediated by Golgi cells.This could provide a fine-grained control of mossy fibre-granule cell information transfer and synaptic plasticity within a mossy fibre rosette.

View Article: PubMed Central - PubMed

Affiliation: Department of Neuroscience and Pharmacology, University of Copenhagen Copenhagen, Denmark.

ABSTRACT
Whole-cell recordings were obtained from mossy fibre terminals in adult turtles in order to characterize the basic membrane properties. Calcium imaging of presynaptic calcium signals was carried out in order to analyse calcium dynamics and presynaptic GABA B inhibition. A tetrodotoxin (TTX)-sensitive fast Na(+) spike faithfully followed repetitive depolarizing pulses with little change in spike duration or amplitude, while a strong outward rectification dominated responses to long-lasting depolarizations. High-threshold calcium spikes were uncovered following addition of potassium channel blockers. Calcium imaging using Calcium-Green dextran revealed a stimulus-evoked all-or-none TTX-sensitive calcium signal in simple and complex rosettes. All compartments of a complex rosette were activated during electrical activation of the mossy fibre, while individual simple and complex rosettes along an axon appeared to be isolated from one another in terms of calcium signalling. CGP55845 application showed that GABA B receptors mediated presynaptic inhibition of the calcium signal over the entire firing frequency range of mossy fibres. A paired-pulse depression of the calcium signal lasting more than 1 s affected burst firing in mossy fibres; this paired-pulse depression was reduced by GABA B antagonists. While our results indicated that a presynaptic rosette electrophysiologically functioned as a unit, topical GABA application showed that calcium signals in the branches of complex rosettes could be modulated locally, suggesting that cerebellar glomeruli may be dynamically sub-compartmentalized due to ongoing inhibition mediated by Golgi cells. This could provide a fine-grained control of mossy fibre-granule cell information transfer and synaptic plasticity within a mossy fibre rosette.

No MeSH data available.


Related in: MedlinePlus

Electrophysiological membrane properties in mossy fibre rosettes.  Responses to sub- and suprathreshold depolarization in normal medium (A) and in 10 μM TTX (B). Inset to right in (A) shows suprathreshold responses in normal medium and in TTX (arrow). Inset to right in (B) shows local response in normal medium (arrow) and the effect of TTX (arrow) on the local response. (C) Control sweeps in 10 μM TTX. (D) Addition of 4-AP (1 mM) results in local responses (arrowheads) and a large long lasting spike, which was blocked by Co2+ (E); 3 mM. (A), (B) and (C–E) from two different axons.
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Figure 2: Electrophysiological membrane properties in mossy fibre rosettes. Responses to sub- and suprathreshold depolarization in normal medium (A) and in 10 μM TTX (B). Inset to right in (A) shows suprathreshold responses in normal medium and in TTX (arrow). Inset to right in (B) shows local response in normal medium (arrow) and the effect of TTX (arrow) on the local response. (C) Control sweeps in 10 μM TTX. (D) Addition of 4-AP (1 mM) results in local responses (arrowheads) and a large long lasting spike, which was blocked by Co2+ (E); 3 mM. (A), (B) and (C–E) from two different axons.

Mentions: Mossy fibres are myelinated, but myelination is interrupted at the presynaptic rosettes (Mugnaini et al., 1974; Palay and Chan-Palay, 1974). Extracellular recordings have previously been made from elements in the turtle cerebellar granule cell layer, including mossy fibres (Walsh et al., 1972, 1974). We obtained whole cell recordings from mossy fibres by blind patch in the intact cerebellum (Blanton et al., 1989) in 15 cases and by visually guided patch in thin slices in two cases. The fibres were identified by the inclusion of biocytin or a fluorescent dye in the recording pipette and their distinct electrophysiological properties (see Materials and Methods; Figure 1A: whole mount fibre; Figure 1C: visually patched fibre; Movie in Supplementary Material). The recordings from visually identified rosettes revealed similar firing patterns as blind recordings. For a subset of randomly selected fibres, the resting potential was −53.4 mV (SD ± 5.3; n = 7), the input resistance for hyperpolarizing current pulses at rest was 1064 MOhm (SD ± 285; n = 7) and the membrane time constant was 22.3 ms (SD ± 16.3; n = 7). A strong outward rectification was present just above the resting potential and subthreshold for spike initiation, while hyperpolarizing current steps revealed a much larger steady state input resistance and a pronounced inward rectification, followed by a repolarizing overshoot (Figures 1D,F). The inward and outward rectifying properties are similar to the responses seen in other types of axons (Bostock, 1995; Geiger and Jonas, 2000), including cerebellar mossy fibres in mammals (Rancz et al., 2007). In addition, at identified presynaptic terminals, a local response subthreshold for spike initiation was observed (Figure 1E). Spike-amplitude was 66.9 mV (SD ± 14.6, n = 7) and duration 1.22 ms (half-width; SD ± 0.38; n = 7). The basic electrophysiological responses of mossy fibres have been described by Rancz et al. (2007), and extracellular recordings have been used to characterize membrane conductances in mammalian mossy fibre terminals (Maffei et al., 2002). In order to characterize the underlying conductances using whole-cell recordings, we analysed the pharmacology of some fundamental electrophysiological properties found in mossy fibres. TTX (10 μM) abolished the local potential and the fast spike (n = 6; Figures 2A,B).


Presynaptic calcium signalling in cerebellar mossy fibres.

Thomsen LB, Jörntell H, Midtgaard J - Front Neural Circuits (2010)

Electrophysiological membrane properties in mossy fibre rosettes.  Responses to sub- and suprathreshold depolarization in normal medium (A) and in 10 μM TTX (B). Inset to right in (A) shows suprathreshold responses in normal medium and in TTX (arrow). Inset to right in (B) shows local response in normal medium (arrow) and the effect of TTX (arrow) on the local response. (C) Control sweeps in 10 μM TTX. (D) Addition of 4-AP (1 mM) results in local responses (arrowheads) and a large long lasting spike, which was blocked by Co2+ (E); 3 mM. (A), (B) and (C–E) from two different axons.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
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Figure 2: Electrophysiological membrane properties in mossy fibre rosettes. Responses to sub- and suprathreshold depolarization in normal medium (A) and in 10 μM TTX (B). Inset to right in (A) shows suprathreshold responses in normal medium and in TTX (arrow). Inset to right in (B) shows local response in normal medium (arrow) and the effect of TTX (arrow) on the local response. (C) Control sweeps in 10 μM TTX. (D) Addition of 4-AP (1 mM) results in local responses (arrowheads) and a large long lasting spike, which was blocked by Co2+ (E); 3 mM. (A), (B) and (C–E) from two different axons.
Mentions: Mossy fibres are myelinated, but myelination is interrupted at the presynaptic rosettes (Mugnaini et al., 1974; Palay and Chan-Palay, 1974). Extracellular recordings have previously been made from elements in the turtle cerebellar granule cell layer, including mossy fibres (Walsh et al., 1972, 1974). We obtained whole cell recordings from mossy fibres by blind patch in the intact cerebellum (Blanton et al., 1989) in 15 cases and by visually guided patch in thin slices in two cases. The fibres were identified by the inclusion of biocytin or a fluorescent dye in the recording pipette and their distinct electrophysiological properties (see Materials and Methods; Figure 1A: whole mount fibre; Figure 1C: visually patched fibre; Movie in Supplementary Material). The recordings from visually identified rosettes revealed similar firing patterns as blind recordings. For a subset of randomly selected fibres, the resting potential was −53.4 mV (SD ± 5.3; n = 7), the input resistance for hyperpolarizing current pulses at rest was 1064 MOhm (SD ± 285; n = 7) and the membrane time constant was 22.3 ms (SD ± 16.3; n = 7). A strong outward rectification was present just above the resting potential and subthreshold for spike initiation, while hyperpolarizing current steps revealed a much larger steady state input resistance and a pronounced inward rectification, followed by a repolarizing overshoot (Figures 1D,F). The inward and outward rectifying properties are similar to the responses seen in other types of axons (Bostock, 1995; Geiger and Jonas, 2000), including cerebellar mossy fibres in mammals (Rancz et al., 2007). In addition, at identified presynaptic terminals, a local response subthreshold for spike initiation was observed (Figure 1E). Spike-amplitude was 66.9 mV (SD ± 14.6, n = 7) and duration 1.22 ms (half-width; SD ± 0.38; n = 7). The basic electrophysiological responses of mossy fibres have been described by Rancz et al. (2007), and extracellular recordings have been used to characterize membrane conductances in mammalian mossy fibre terminals (Maffei et al., 2002). In order to characterize the underlying conductances using whole-cell recordings, we analysed the pharmacology of some fundamental electrophysiological properties found in mossy fibres. TTX (10 μM) abolished the local potential and the fast spike (n = 6; Figures 2A,B).

Bottom Line: A paired-pulse depression of the calcium signal lasting more than 1 s affected burst firing in mossy fibres; this paired-pulse depression was reduced by GABA B antagonists.While our results indicated that a presynaptic rosette electrophysiologically functioned as a unit, topical GABA application showed that calcium signals in the branches of complex rosettes could be modulated locally, suggesting that cerebellar glomeruli may be dynamically sub-compartmentalized due to ongoing inhibition mediated by Golgi cells.This could provide a fine-grained control of mossy fibre-granule cell information transfer and synaptic plasticity within a mossy fibre rosette.

View Article: PubMed Central - PubMed

Affiliation: Department of Neuroscience and Pharmacology, University of Copenhagen Copenhagen, Denmark.

ABSTRACT
Whole-cell recordings were obtained from mossy fibre terminals in adult turtles in order to characterize the basic membrane properties. Calcium imaging of presynaptic calcium signals was carried out in order to analyse calcium dynamics and presynaptic GABA B inhibition. A tetrodotoxin (TTX)-sensitive fast Na(+) spike faithfully followed repetitive depolarizing pulses with little change in spike duration or amplitude, while a strong outward rectification dominated responses to long-lasting depolarizations. High-threshold calcium spikes were uncovered following addition of potassium channel blockers. Calcium imaging using Calcium-Green dextran revealed a stimulus-evoked all-or-none TTX-sensitive calcium signal in simple and complex rosettes. All compartments of a complex rosette were activated during electrical activation of the mossy fibre, while individual simple and complex rosettes along an axon appeared to be isolated from one another in terms of calcium signalling. CGP55845 application showed that GABA B receptors mediated presynaptic inhibition of the calcium signal over the entire firing frequency range of mossy fibres. A paired-pulse depression of the calcium signal lasting more than 1 s affected burst firing in mossy fibres; this paired-pulse depression was reduced by GABA B antagonists. While our results indicated that a presynaptic rosette electrophysiologically functioned as a unit, topical GABA application showed that calcium signals in the branches of complex rosettes could be modulated locally, suggesting that cerebellar glomeruli may be dynamically sub-compartmentalized due to ongoing inhibition mediated by Golgi cells. This could provide a fine-grained control of mossy fibre-granule cell information transfer and synaptic plasticity within a mossy fibre rosette.

No MeSH data available.


Related in: MedlinePlus