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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

Local inhibition of calcium signalling in complex rosettes by focal activation of GABA B receptors.  (A) Complex rosette branching off a mossy fibre axon (cf. Figure 1). The position of the GABA pipette is indicated diagrammatically. For each region of interest indicated by the square boxes 1–4, the fluorescence signals to five electrical stimuli at 50 Hz (black arrowhead below dF/F traces) are shown as control and 300 ms after a GABA puff (open arrowhead). Each trace is the average of at least three trials. In location 4 (C), GABA had no effect, whereas in location 1–3 (B), (D, E) prior GABA application depressed the fluorescence response to subsequent electrical activation. The results suggest that proximal regions affected by GABA (1, 2) did not prevent the action potentials from propagating to more distal regions (4, (C)) unaffected by GABA. All regions showed a CGP55845-sensitive paired-pulse depression at inter-burst intervals of 500 ms (F), 1–4; panel numbers correspond to regions in (A). The rosette was confined to the side branches: the axon showed no fluorescence signal increase, while the whole extent of the side branches gave a fluorescence response to electrical stimulation, similar to other complex rosettes shown. The size of this rosette was comparable to the lateral extent of rosettes in fixed and dehydrated tissue (Figure 1). In other experiments (not shown) similar results were obtained in complex rosettes along the axon: local GABA application at one end of long rosette (cf. Mugnaini et al., 1974) resulted in local reduction in electrically-evoked fluorescence signals near the GABA-containing pipette. GABA pipette puff duration: 25 ms. Frame rate 10 Hz. CGP55845 concentration: 50 μM. All experiments in 30 μM picrotoxin. Double-headed arrow in (A) indicates the cerebellar sagittal direction. (G) Summary diagram of widespread calcium signal in control situation (no GABA application prior to electrical stimulation of mossy fibre), cf. panels (B–E). (H) Diagram of local GABA application prior to electrical activation (shaded grey; cf. panels (B–E)) reducing the calcium signal from some branches of the complex rosette. The depolarization, e.g. an action potential (AP), spreads (arrows) from the parent mossy fibre (MF) through rosette branches with calcium signals inhibited by local GABA B receptor activation, to branches showing no GABA B effect.
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Figure 7: Local inhibition of calcium signalling in complex rosettes by focal activation of GABA B receptors. (A) Complex rosette branching off a mossy fibre axon (cf. Figure 1). The position of the GABA pipette is indicated diagrammatically. For each region of interest indicated by the square boxes 1–4, the fluorescence signals to five electrical stimuli at 50 Hz (black arrowhead below dF/F traces) are shown as control and 300 ms after a GABA puff (open arrowhead). Each trace is the average of at least three trials. In location 4 (C), GABA had no effect, whereas in location 1–3 (B), (D, E) prior GABA application depressed the fluorescence response to subsequent electrical activation. The results suggest that proximal regions affected by GABA (1, 2) did not prevent the action potentials from propagating to more distal regions (4, (C)) unaffected by GABA. All regions showed a CGP55845-sensitive paired-pulse depression at inter-burst intervals of 500 ms (F), 1–4; panel numbers correspond to regions in (A). The rosette was confined to the side branches: the axon showed no fluorescence signal increase, while the whole extent of the side branches gave a fluorescence response to electrical stimulation, similar to other complex rosettes shown. The size of this rosette was comparable to the lateral extent of rosettes in fixed and dehydrated tissue (Figure 1). In other experiments (not shown) similar results were obtained in complex rosettes along the axon: local GABA application at one end of long rosette (cf. Mugnaini et al., 1974) resulted in local reduction in electrically-evoked fluorescence signals near the GABA-containing pipette. GABA pipette puff duration: 25 ms. Frame rate 10 Hz. CGP55845 concentration: 50 μM. All experiments in 30 μM picrotoxin. Double-headed arrow in (A) indicates the cerebellar sagittal direction. (G) Summary diagram of widespread calcium signal in control situation (no GABA application prior to electrical stimulation of mossy fibre), cf. panels (B–E). (H) Diagram of local GABA application prior to electrical activation (shaded grey; cf. panels (B–E)) reducing the calcium signal from some branches of the complex rosette. The depolarization, e.g. an action potential (AP), spreads (arrows) from the parent mossy fibre (MF) through rosette branches with calcium signals inhibited by local GABA B receptor activation, to branches showing no GABA B effect.

Mentions: The large size and often structure of complex rosettes suggest that local modulation of calcium dependent processes such as transmitter release could take place, since calcium diffusion is restricted in axons (Collin et al., 2005). Localised modulation of calcium entry in a rosette could in principle occur due to localised presynaptic plasticity or local modulatory effects due to presynaptic transmitter effects. For instance, differential activity of Golgi cells innervating the same glomerulus (Fox et al., 1967; Mugnaini, 1972) could result in a non-uniform extracellular GABA signal in a glomerulus. This could lead to differential calcium influx in the rosette innervating that glomerulus. In order to test this possibility, we pressure-applied GABA to one part of large complex rosettes before electrical stimulation of the axon (Figure 7), using a stimulus interval of 300 ms, similar to the time frame for a robust paired-pulse depression (cf. Figures 6G,H). Picrotoxin was present throughout in order to block any potential presynaptic GABA A receptors (Engelman and MacDermott, 2004; Alle and Geiger, 2007).


Presynaptic calcium signalling in cerebellar mossy fibres.

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

Local inhibition of calcium signalling in complex rosettes by focal activation of GABA B receptors.  (A) Complex rosette branching off a mossy fibre axon (cf. Figure 1). The position of the GABA pipette is indicated diagrammatically. For each region of interest indicated by the square boxes 1–4, the fluorescence signals to five electrical stimuli at 50 Hz (black arrowhead below dF/F traces) are shown as control and 300 ms after a GABA puff (open arrowhead). Each trace is the average of at least three trials. In location 4 (C), GABA had no effect, whereas in location 1–3 (B), (D, E) prior GABA application depressed the fluorescence response to subsequent electrical activation. The results suggest that proximal regions affected by GABA (1, 2) did not prevent the action potentials from propagating to more distal regions (4, (C)) unaffected by GABA. All regions showed a CGP55845-sensitive paired-pulse depression at inter-burst intervals of 500 ms (F), 1–4; panel numbers correspond to regions in (A). The rosette was confined to the side branches: the axon showed no fluorescence signal increase, while the whole extent of the side branches gave a fluorescence response to electrical stimulation, similar to other complex rosettes shown. The size of this rosette was comparable to the lateral extent of rosettes in fixed and dehydrated tissue (Figure 1). In other experiments (not shown) similar results were obtained in complex rosettes along the axon: local GABA application at one end of long rosette (cf. Mugnaini et al., 1974) resulted in local reduction in electrically-evoked fluorescence signals near the GABA-containing pipette. GABA pipette puff duration: 25 ms. Frame rate 10 Hz. CGP55845 concentration: 50 μM. All experiments in 30 μM picrotoxin. Double-headed arrow in (A) indicates the cerebellar sagittal direction. (G) Summary diagram of widespread calcium signal in control situation (no GABA application prior to electrical stimulation of mossy fibre), cf. panels (B–E). (H) Diagram of local GABA application prior to electrical activation (shaded grey; cf. panels (B–E)) reducing the calcium signal from some branches of the complex rosette. The depolarization, e.g. an action potential (AP), spreads (arrows) from the parent mossy fibre (MF) through rosette branches with calcium signals inhibited by local GABA B receptor activation, to branches showing no GABA B effect.
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Figure 7: Local inhibition of calcium signalling in complex rosettes by focal activation of GABA B receptors. (A) Complex rosette branching off a mossy fibre axon (cf. Figure 1). The position of the GABA pipette is indicated diagrammatically. For each region of interest indicated by the square boxes 1–4, the fluorescence signals to five electrical stimuli at 50 Hz (black arrowhead below dF/F traces) are shown as control and 300 ms after a GABA puff (open arrowhead). Each trace is the average of at least three trials. In location 4 (C), GABA had no effect, whereas in location 1–3 (B), (D, E) prior GABA application depressed the fluorescence response to subsequent electrical activation. The results suggest that proximal regions affected by GABA (1, 2) did not prevent the action potentials from propagating to more distal regions (4, (C)) unaffected by GABA. All regions showed a CGP55845-sensitive paired-pulse depression at inter-burst intervals of 500 ms (F), 1–4; panel numbers correspond to regions in (A). The rosette was confined to the side branches: the axon showed no fluorescence signal increase, while the whole extent of the side branches gave a fluorescence response to electrical stimulation, similar to other complex rosettes shown. The size of this rosette was comparable to the lateral extent of rosettes in fixed and dehydrated tissue (Figure 1). In other experiments (not shown) similar results were obtained in complex rosettes along the axon: local GABA application at one end of long rosette (cf. Mugnaini et al., 1974) resulted in local reduction in electrically-evoked fluorescence signals near the GABA-containing pipette. GABA pipette puff duration: 25 ms. Frame rate 10 Hz. CGP55845 concentration: 50 μM. All experiments in 30 μM picrotoxin. Double-headed arrow in (A) indicates the cerebellar sagittal direction. (G) Summary diagram of widespread calcium signal in control situation (no GABA application prior to electrical stimulation of mossy fibre), cf. panels (B–E). (H) Diagram of local GABA application prior to electrical activation (shaded grey; cf. panels (B–E)) reducing the calcium signal from some branches of the complex rosette. The depolarization, e.g. an action potential (AP), spreads (arrows) from the parent mossy fibre (MF) through rosette branches with calcium signals inhibited by local GABA B receptor activation, to branches showing no GABA B effect.
Mentions: The large size and often structure of complex rosettes suggest that local modulation of calcium dependent processes such as transmitter release could take place, since calcium diffusion is restricted in axons (Collin et al., 2005). Localised modulation of calcium entry in a rosette could in principle occur due to localised presynaptic plasticity or local modulatory effects due to presynaptic transmitter effects. For instance, differential activity of Golgi cells innervating the same glomerulus (Fox et al., 1967; Mugnaini, 1972) could result in a non-uniform extracellular GABA signal in a glomerulus. This could lead to differential calcium influx in the rosette innervating that glomerulus. In order to test this possibility, we pressure-applied GABA to one part of large complex rosettes before electrical stimulation of the axon (Figure 7), using a stimulus interval of 300 ms, similar to the time frame for a robust paired-pulse depression (cf. Figures 6G,H). Picrotoxin was present throughout in order to block any potential presynaptic GABA A receptors (Engelman and MacDermott, 2004; Alle and Geiger, 2007).

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