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Dentate gyrus network dysfunctions precede the symptomatic phase in a genetic mouse model of seizures.

Toader O, Forte N, Orlando M, Ferrea E, Raimondi A, Baldelli P, Benfenati F, Medrihan L - Front Cell Neurosci (2013)

Bottom Line: Neuronal circuit disturbances that lead to hyperexcitability in the cortico-hippocampal network are one of the landmarks of temporal lobe epilepsy.We made use of a high-resolution microelectrode array (4096 electrodes) and patch-clamp recordings, and found that in acute hippocampal slices of young pre-symptomatic (3-6 week-old) Syn II(-/-) mice excitatory synaptic output of the mossy fibers is reduced.Moreover, we showed that the main excitatory neurons present in the polymorphic layer of the DG, hilar mossy cells, display a reduced excitability.

View Article: PubMed Central - PubMed

Affiliation: Department of Neuroscience and Brain Technologies, Fondazione Istituto Italiano di Tecnologia Genoa, Italy ; International Max-Planck Research School for Neurosciences Göttingen, Germany.

ABSTRACT
Neuronal circuit disturbances that lead to hyperexcitability in the cortico-hippocampal network are one of the landmarks of temporal lobe epilepsy. The dentate gyrus (DG) network plays an important role in regulating the excitability of the entire hippocampus by filtering and integrating information received via the perforant path. Here, we investigated possible epileptogenic abnormalities in the function of the DG neuronal network in the Synapsin II (Syn II) knockout mouse (Syn II(-/-)), a genetic mouse model of epilepsy. Syn II is a presynaptic protein whose deletion in mice reproducibly leads to generalized seizures starting at the age of 2 months. We made use of a high-resolution microelectrode array (4096 electrodes) and patch-clamp recordings, and found that in acute hippocampal slices of young pre-symptomatic (3-6 week-old) Syn II(-/-) mice excitatory synaptic output of the mossy fibers is reduced. Moreover, we showed that the main excitatory neurons present in the polymorphic layer of the DG, hilar mossy cells, display a reduced excitability. We also provide evidence of a predominantly inhibitory regulatory output from mossy cells to granule cells, through feed-forward inhibition, and show that the excitatory-inhibitory ratio is increased in both pre-symptomatic and symptomatic Syn II(-/-) mice. These results support the key role of the hilar mossy neurons in maintaining the normal excitability of the hippocampal network and show that the late epileptic phenotype of the Syn II(-/-) mice is preceded by neuronal circuitry dysfunctions. Our data provide new insights into the mechanisms of epileptogenesis in the Syn II(-/-) mice and open the possibility for early diagnosis and therapeutic interventions.

No MeSH data available.


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The hilar mossy cell phenotype of pre-symptomatic Syn II−/− mice is maintained in adult symptomatic Syn II−/− mice. (A) Representative mEPSC traces (A) and cumulative distributions (B) of their amplitude and frequency from 4 to 6 months old WT (black) and Syn II−/− (red) mossy cells; ***p < 0.001, Kolmogorov–Smirnov test. (C) Mean (±s.e.m.) rise-time (10–90%) and mono-exponential τ of decay of mEPSCs from 4–6 months old WT (black bars) and Syn II−/− (red bars) neurons. (D) Representative traces of current-clamp recordings from hilar mossy neurons in acute slices of WT (black) and symptomatic Syn II−/− (red) mice. (E) Frequency of APs plotted as a function of the injected current for both genotypes. (F) Mean (±s.e.m.) rheobase and input resistance; *p < 0.05, two-tailed unpaired Student's t-test.
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Figure 6: The hilar mossy cell phenotype of pre-symptomatic Syn II−/− mice is maintained in adult symptomatic Syn II−/− mice. (A) Representative mEPSC traces (A) and cumulative distributions (B) of their amplitude and frequency from 4 to 6 months old WT (black) and Syn II−/− (red) mossy cells; ***p < 0.001, Kolmogorov–Smirnov test. (C) Mean (±s.e.m.) rise-time (10–90%) and mono-exponential τ of decay of mEPSCs from 4–6 months old WT (black bars) and Syn II−/− (red bars) neurons. (D) Representative traces of current-clamp recordings from hilar mossy neurons in acute slices of WT (black) and symptomatic Syn II−/− (red) mice. (E) Frequency of APs plotted as a function of the injected current for both genotypes. (F) Mean (±s.e.m.) rheobase and input resistance; *p < 0.05, two-tailed unpaired Student's t-test.

Mentions: To verify if the reduced excitability of hilar mossy cells in pre-symptomatic Syn II−/− slices persists after the initiation of epileptic seizures in these mice, we repeated the experiments from Figures 4, 5 on adult (4–6 months old) Syn II−/− mouse slices. As in pre-symptomatic mice, both the amplitude and the frequency distributions of mEPSCs were significantly shifted toward lower values in Syn II−/− hilar mossy neurons (n = 4 neurons/3 mice for WT and 6 neurons/3 mice for Syn II−/−; Kolmogorov–Smirnov test, p < 0.001) (Figures 6A,B). The smaller amplitude distribution of Syn II−/− cells was not accompanied by any change in the kinetic parameters of the response with respect to the WT (rise-time 10–90%: 1.36 ± 0.2 vs. 1.08 ± 0.1 ms, p = 0.296; decay τ: 5.58 ± 1.1 vs. 5.63 ± 0.6 ms, p = 0.967; two-tailed unpaired Student's t-test) (Figure 6C). Moreover, the firing rate of mossy cells was lower in adult Syn II−/− (Figures 6D,E), with a significant increase in rheobase (60.0 ± 5.7, n = 3 neurons/3 mice for WT vs. 85.0 ± 8.6 pA for Syn II−/−, n = 8 neurons/3 mice; two-tailed unpaired Student's t-test, p = 0.043) (Figure 6F, left) and a decrease in input resistance (422.0 ± 12.5 MΩ, n = 3 neurons/3 mice for WT vs. 269.0 ± 38.0 for Syn II−/−, n = 8 neurons/3 mice; two-tailed unpaired Student's t-test, p = 0.042) (Figure 6F, right). These results show that the cellular phenotype of Syn II−/− mice appears long before the appearance of an overt epileptic phenotype.


Dentate gyrus network dysfunctions precede the symptomatic phase in a genetic mouse model of seizures.

Toader O, Forte N, Orlando M, Ferrea E, Raimondi A, Baldelli P, Benfenati F, Medrihan L - Front Cell Neurosci (2013)

The hilar mossy cell phenotype of pre-symptomatic Syn II−/− mice is maintained in adult symptomatic Syn II−/− mice. (A) Representative mEPSC traces (A) and cumulative distributions (B) of their amplitude and frequency from 4 to 6 months old WT (black) and Syn II−/− (red) mossy cells; ***p < 0.001, Kolmogorov–Smirnov test. (C) Mean (±s.e.m.) rise-time (10–90%) and mono-exponential τ of decay of mEPSCs from 4–6 months old WT (black bars) and Syn II−/− (red bars) neurons. (D) Representative traces of current-clamp recordings from hilar mossy neurons in acute slices of WT (black) and symptomatic Syn II−/− (red) mice. (E) Frequency of APs plotted as a function of the injected current for both genotypes. (F) Mean (±s.e.m.) rheobase and input resistance; *p < 0.05, two-tailed unpaired Student's t-test.
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Figure 6: The hilar mossy cell phenotype of pre-symptomatic Syn II−/− mice is maintained in adult symptomatic Syn II−/− mice. (A) Representative mEPSC traces (A) and cumulative distributions (B) of their amplitude and frequency from 4 to 6 months old WT (black) and Syn II−/− (red) mossy cells; ***p < 0.001, Kolmogorov–Smirnov test. (C) Mean (±s.e.m.) rise-time (10–90%) and mono-exponential τ of decay of mEPSCs from 4–6 months old WT (black bars) and Syn II−/− (red bars) neurons. (D) Representative traces of current-clamp recordings from hilar mossy neurons in acute slices of WT (black) and symptomatic Syn II−/− (red) mice. (E) Frequency of APs plotted as a function of the injected current for both genotypes. (F) Mean (±s.e.m.) rheobase and input resistance; *p < 0.05, two-tailed unpaired Student's t-test.
Mentions: To verify if the reduced excitability of hilar mossy cells in pre-symptomatic Syn II−/− slices persists after the initiation of epileptic seizures in these mice, we repeated the experiments from Figures 4, 5 on adult (4–6 months old) Syn II−/− mouse slices. As in pre-symptomatic mice, both the amplitude and the frequency distributions of mEPSCs were significantly shifted toward lower values in Syn II−/− hilar mossy neurons (n = 4 neurons/3 mice for WT and 6 neurons/3 mice for Syn II−/−; Kolmogorov–Smirnov test, p < 0.001) (Figures 6A,B). The smaller amplitude distribution of Syn II−/− cells was not accompanied by any change in the kinetic parameters of the response with respect to the WT (rise-time 10–90%: 1.36 ± 0.2 vs. 1.08 ± 0.1 ms, p = 0.296; decay τ: 5.58 ± 1.1 vs. 5.63 ± 0.6 ms, p = 0.967; two-tailed unpaired Student's t-test) (Figure 6C). Moreover, the firing rate of mossy cells was lower in adult Syn II−/− (Figures 6D,E), with a significant increase in rheobase (60.0 ± 5.7, n = 3 neurons/3 mice for WT vs. 85.0 ± 8.6 pA for Syn II−/−, n = 8 neurons/3 mice; two-tailed unpaired Student's t-test, p = 0.043) (Figure 6F, left) and a decrease in input resistance (422.0 ± 12.5 MΩ, n = 3 neurons/3 mice for WT vs. 269.0 ± 38.0 for Syn II−/−, n = 8 neurons/3 mice; two-tailed unpaired Student's t-test, p = 0.042) (Figure 6F, right). These results show that the cellular phenotype of Syn II−/− mice appears long before the appearance of an overt epileptic phenotype.

Bottom Line: Neuronal circuit disturbances that lead to hyperexcitability in the cortico-hippocampal network are one of the landmarks of temporal lobe epilepsy.We made use of a high-resolution microelectrode array (4096 electrodes) and patch-clamp recordings, and found that in acute hippocampal slices of young pre-symptomatic (3-6 week-old) Syn II(-/-) mice excitatory synaptic output of the mossy fibers is reduced.Moreover, we showed that the main excitatory neurons present in the polymorphic layer of the DG, hilar mossy cells, display a reduced excitability.

View Article: PubMed Central - PubMed

Affiliation: Department of Neuroscience and Brain Technologies, Fondazione Istituto Italiano di Tecnologia Genoa, Italy ; International Max-Planck Research School for Neurosciences Göttingen, Germany.

ABSTRACT
Neuronal circuit disturbances that lead to hyperexcitability in the cortico-hippocampal network are one of the landmarks of temporal lobe epilepsy. The dentate gyrus (DG) network plays an important role in regulating the excitability of the entire hippocampus by filtering and integrating information received via the perforant path. Here, we investigated possible epileptogenic abnormalities in the function of the DG neuronal network in the Synapsin II (Syn II) knockout mouse (Syn II(-/-)), a genetic mouse model of epilepsy. Syn II is a presynaptic protein whose deletion in mice reproducibly leads to generalized seizures starting at the age of 2 months. We made use of a high-resolution microelectrode array (4096 electrodes) and patch-clamp recordings, and found that in acute hippocampal slices of young pre-symptomatic (3-6 week-old) Syn II(-/-) mice excitatory synaptic output of the mossy fibers is reduced. Moreover, we showed that the main excitatory neurons present in the polymorphic layer of the DG, hilar mossy cells, display a reduced excitability. We also provide evidence of a predominantly inhibitory regulatory output from mossy cells to granule cells, through feed-forward inhibition, and show that the excitatory-inhibitory ratio is increased in both pre-symptomatic and symptomatic Syn II(-/-) mice. These results support the key role of the hilar mossy neurons in maintaining the normal excitability of the hippocampal network and show that the late epileptic phenotype of the Syn II(-/-) mice is preceded by neuronal circuitry dysfunctions. Our data provide new insights into the mechanisms of epileptogenesis in the Syn II(-/-) mice and open the possibility for early diagnosis and therapeutic interventions.

No MeSH data available.


Related in: MedlinePlus