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Agrin mediates a rapid switch from electrical coupling to chemical neurotransmission during synaptogenesis.

Martin AO, Alonso G, Guérineau NC - J. Cell Biol. (2005)

Bottom Line: When applied at the developing splanchnic nerve-chromaffin cell cholinergic synapse in rat adrenal acute slices, agrin rapidly modified cell-to-cell communication mechanisms.This developmental switch from predominantly electrical to chemical communication was fully operational within one hour and depended on the activation of Src family-related tyrosine kinases.Hence, agrin may play a pivotal role in synaptogenesis in promoting a rapid switch between electrical coupling and synaptic neurotransmission.

View Article: PubMed Central - PubMed

Affiliation: CNRS UMR5203, INSERM U661, Université Montpellier I, Département d'Endocrinologie, Institut de Génomique Fonctionnelle, 34094 Montpellier Cedex 5, France.

ABSTRACT
In contrast to its well-established actions as an organizer of synaptic differentiation at the neuromuscular junction, the proteoglycan agrin is still in search of a function in the nervous system. Here, we report an entirely unanticipated role for agrin in the dual modulation of electrical and chemical intercellular communication that occurs during the critical period of synapse formation. When applied at the developing splanchnic nerve-chromaffin cell cholinergic synapse in rat adrenal acute slices, agrin rapidly modified cell-to-cell communication mechanisms. Specifically, it led to decreased gap junction-mediated electrical coupling that preceded an increase in nicotinic synaptic transmission. This developmental switch from predominantly electrical to chemical communication was fully operational within one hour and depended on the activation of Src family-related tyrosine kinases. Hence, agrin may play a pivotal role in synaptogenesis in promoting a rapid switch between electrical coupling and synaptic neurotransmission.

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Agrin-induced changes in sEPSC and capacitive current kinetics. (A) Effect of agrin on the sEPSC decay phase. Typical examples of a double-exponential sEPSC recorded in neonate and a single-exponential sEPSC recorded in adult and in neonate after agrin treatment. Pooled data in the histogram show the percentage of sEPSCs that could be fitted by a single exponential. *, P < 0.01 when compared with control slices in neonates. (B) Similar effect of agrin on the decay phase of capacitive currents evoked by square voltage pulses from −80 to −90 mV (100-ms duration).
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fig4: Agrin-induced changes in sEPSC and capacitive current kinetics. (A) Effect of agrin on the sEPSC decay phase. Typical examples of a double-exponential sEPSC recorded in neonate and a single-exponential sEPSC recorded in adult and in neonate after agrin treatment. Pooled data in the histogram show the percentage of sEPSCs that could be fitted by a single exponential. *, P < 0.01 when compared with control slices in neonates. (B) Similar effect of agrin on the decay phase of capacitive currents evoked by square voltage pulses from −80 to −90 mV (100-ms duration).

Mentions: In addition to the effect on the sEPSC variance amplitude, we found that agrin also altered the sEPSC kinetics by modifying the decay phase (Fig. 4 A). No effect was observed on the activation phase (unpublished data). In adults, most of sEPSCs were well fitted by a single exponential curve (71% of sEPSCs, n = 156), as previously reported (Barbara and Takeda, 1996). Conversely, in neonates the decay phase preferentially displayed bi-exponential kinetics (80% of sEPSCs, n = 39). Nevertheless, after agrin treatment, 69% sEPSCs (n = 97) in neonates exhibited a single exponential decay phase, equivalent to the adult curve (P > 0.01). In adults, agrin treatment remained without significant effect on sEPSC kinetics (71% monoexponential sEPSCs in control vs. 78% in agrin-treated slices, P > 0.01). Similar findings were obtained for capacitive currents. In neonates, 63.6% capacitive currents generated by square voltage pulses at −90 mV (100-ms duration) displayed a bi-exponential decay phase (n = 22). After prolonged agrin exposure, the percentage dropped to 37.8% (Fig. 4 B; n = 45, P < 0.01). Because gap junctional coupling modifies decay kinetics of capacitive currents, i.e., uncoupled cells exhibit single-exponential capacitive current decays while the decays are fitted with a double-exponential in coupled cells (Moser, 1998; Postma et al., 1998), we hypothesized that agrin decreased gap junctional coupling between neonate chromaffin cells. sEPSCs were then recorded in the presence of the decoupling agent carbenoxolone (Ishimatsu and Williams, 1996). Carbenoxolone (100 μM, 10 min before recordings) mimicked, although to a lesser extent, the effect of agrin on sEPSC amplitude (unpublished data). The average amplitude significantly increased (70.9 ± 4.5 pA, n = 89 sEPSCs in carbenoxolone-treated slices vs. 51.8 ± 2.6 pA, n = 47 sEPSCs in control slices, P < 0.01). Similarly, the mean current amplitude evoked by a brief application of nicotine (100 μM, 100 ms) significantly increased in the presence of the gap junction blocker (116.9 ± 33.4 pA, n = 6 in carbenoxolone-treated slices vs. 68.4 ± 19.1 pA, n = 18 in control slices, P < 0.01; unpublished data). In addition, in the presence of carbenoxolone the percentage of sESPCs in which the decay phase could be fitted by a single exponential significantly increased (unpublished data), approaching values found in agrin-treated slices. In adults, 86.3% sEPSC decays were fitted by a single exponential in the presence of carbenoxolone (63 out of 73 sEPSCs, 8 cells) versus 71.2% in untreated slices, consistent with the uncoupling effect of carbenoxolone on gap junctional coupling in adult chromaffin cells (Martin et al., 2001). Together, these findings confirmed that a double-exponential fit reflects the presence of electrical coupling between the recorded cell and nearby cells. In this case the slow time constant τslow likely corresponded to the electrotonic spread of current from coupled cells.


Agrin mediates a rapid switch from electrical coupling to chemical neurotransmission during synaptogenesis.

Martin AO, Alonso G, Guérineau NC - J. Cell Biol. (2005)

Agrin-induced changes in sEPSC and capacitive current kinetics. (A) Effect of agrin on the sEPSC decay phase. Typical examples of a double-exponential sEPSC recorded in neonate and a single-exponential sEPSC recorded in adult and in neonate after agrin treatment. Pooled data in the histogram show the percentage of sEPSCs that could be fitted by a single exponential. *, P < 0.01 when compared with control slices in neonates. (B) Similar effect of agrin on the decay phase of capacitive currents evoked by square voltage pulses from −80 to −90 mV (100-ms duration).
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Related In: Results  -  Collection

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fig4: Agrin-induced changes in sEPSC and capacitive current kinetics. (A) Effect of agrin on the sEPSC decay phase. Typical examples of a double-exponential sEPSC recorded in neonate and a single-exponential sEPSC recorded in adult and in neonate after agrin treatment. Pooled data in the histogram show the percentage of sEPSCs that could be fitted by a single exponential. *, P < 0.01 when compared with control slices in neonates. (B) Similar effect of agrin on the decay phase of capacitive currents evoked by square voltage pulses from −80 to −90 mV (100-ms duration).
Mentions: In addition to the effect on the sEPSC variance amplitude, we found that agrin also altered the sEPSC kinetics by modifying the decay phase (Fig. 4 A). No effect was observed on the activation phase (unpublished data). In adults, most of sEPSCs were well fitted by a single exponential curve (71% of sEPSCs, n = 156), as previously reported (Barbara and Takeda, 1996). Conversely, in neonates the decay phase preferentially displayed bi-exponential kinetics (80% of sEPSCs, n = 39). Nevertheless, after agrin treatment, 69% sEPSCs (n = 97) in neonates exhibited a single exponential decay phase, equivalent to the adult curve (P > 0.01). In adults, agrin treatment remained without significant effect on sEPSC kinetics (71% monoexponential sEPSCs in control vs. 78% in agrin-treated slices, P > 0.01). Similar findings were obtained for capacitive currents. In neonates, 63.6% capacitive currents generated by square voltage pulses at −90 mV (100-ms duration) displayed a bi-exponential decay phase (n = 22). After prolonged agrin exposure, the percentage dropped to 37.8% (Fig. 4 B; n = 45, P < 0.01). Because gap junctional coupling modifies decay kinetics of capacitive currents, i.e., uncoupled cells exhibit single-exponential capacitive current decays while the decays are fitted with a double-exponential in coupled cells (Moser, 1998; Postma et al., 1998), we hypothesized that agrin decreased gap junctional coupling between neonate chromaffin cells. sEPSCs were then recorded in the presence of the decoupling agent carbenoxolone (Ishimatsu and Williams, 1996). Carbenoxolone (100 μM, 10 min before recordings) mimicked, although to a lesser extent, the effect of agrin on sEPSC amplitude (unpublished data). The average amplitude significantly increased (70.9 ± 4.5 pA, n = 89 sEPSCs in carbenoxolone-treated slices vs. 51.8 ± 2.6 pA, n = 47 sEPSCs in control slices, P < 0.01). Similarly, the mean current amplitude evoked by a brief application of nicotine (100 μM, 100 ms) significantly increased in the presence of the gap junction blocker (116.9 ± 33.4 pA, n = 6 in carbenoxolone-treated slices vs. 68.4 ± 19.1 pA, n = 18 in control slices, P < 0.01; unpublished data). In addition, in the presence of carbenoxolone the percentage of sESPCs in which the decay phase could be fitted by a single exponential significantly increased (unpublished data), approaching values found in agrin-treated slices. In adults, 86.3% sEPSC decays were fitted by a single exponential in the presence of carbenoxolone (63 out of 73 sEPSCs, 8 cells) versus 71.2% in untreated slices, consistent with the uncoupling effect of carbenoxolone on gap junctional coupling in adult chromaffin cells (Martin et al., 2001). Together, these findings confirmed that a double-exponential fit reflects the presence of electrical coupling between the recorded cell and nearby cells. In this case the slow time constant τslow likely corresponded to the electrotonic spread of current from coupled cells.

Bottom Line: When applied at the developing splanchnic nerve-chromaffin cell cholinergic synapse in rat adrenal acute slices, agrin rapidly modified cell-to-cell communication mechanisms.This developmental switch from predominantly electrical to chemical communication was fully operational within one hour and depended on the activation of Src family-related tyrosine kinases.Hence, agrin may play a pivotal role in synaptogenesis in promoting a rapid switch between electrical coupling and synaptic neurotransmission.

View Article: PubMed Central - PubMed

Affiliation: CNRS UMR5203, INSERM U661, Université Montpellier I, Département d'Endocrinologie, Institut de Génomique Fonctionnelle, 34094 Montpellier Cedex 5, France.

ABSTRACT
In contrast to its well-established actions as an organizer of synaptic differentiation at the neuromuscular junction, the proteoglycan agrin is still in search of a function in the nervous system. Here, we report an entirely unanticipated role for agrin in the dual modulation of electrical and chemical intercellular communication that occurs during the critical period of synapse formation. When applied at the developing splanchnic nerve-chromaffin cell cholinergic synapse in rat adrenal acute slices, agrin rapidly modified cell-to-cell communication mechanisms. Specifically, it led to decreased gap junction-mediated electrical coupling that preceded an increase in nicotinic synaptic transmission. This developmental switch from predominantly electrical to chemical communication was fully operational within one hour and depended on the activation of Src family-related tyrosine kinases. Hence, agrin may play a pivotal role in synaptogenesis in promoting a rapid switch between electrical coupling and synaptic neurotransmission.

Show MeSH
Related in: MedlinePlus