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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.


Related in: MedlinePlus

Hilar mossy cells of pre-symptomatic Syn II−/− mice display decreased excitability. (A) Representative traces of whole cell current-clamp recordings from hilar mossy neurons in acute slices of WT (black) and pre-symptomatic Syn II−/− (red) mice. (B) Action potential (AP) frequency plotted as a function of the injected current for both genotypes. (C) Mean (±s.e.m.) rheobase and input resistance; *p < 0.05, **p < 0.01, two-tailed unpaired Student's t-test. (D,E) Representative traces of whole cell current-clamp recordings from granule neurons (D) in acute slices of WT (black) and pre-symptomatic Syn II−/− (red) mice and AP frequency (E) plotted as a function of the injected current for both genotypes.
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Figure 5: Hilar mossy cells of pre-symptomatic Syn II−/− mice display decreased excitability. (A) Representative traces of whole cell current-clamp recordings from hilar mossy neurons in acute slices of WT (black) and pre-symptomatic Syn II−/− (red) mice. (B) Action potential (AP) frequency plotted as a function of the injected current for both genotypes. (C) Mean (±s.e.m.) rheobase and input resistance; *p < 0.05, **p < 0.01, two-tailed unpaired Student's t-test. (D,E) Representative traces of whole cell current-clamp recordings from granule neurons (D) in acute slices of WT (black) and pre-symptomatic Syn II−/− (red) mice and AP frequency (E) plotted as a function of the injected current for both genotypes.

Mentions: Extracellular fPSPs are the summation of a series of events, notably synaptic activity and synchronous firing of APs by groups of neurons (Buzsaki et al., 2012). Thus, in the next experiment, we evaluated the firing rate of hilar mossy neurons from pre-symptomatic Syn II−/− in the current clamp configuration. In the presence of specific antagonists that fully block synaptic activity, mossy cells were injected with 30 current steps, lasting 1 s and ranging from −100 to +200 pA, in 10 pA increments (Figures 5A,B). The firing rate of mossy neurons was lower in Syn II−/− slices with respect to WT recordings (Figure 5B) and was accompanied by a significant increase in the rheobase (45.0 ± 6.7, n = 11 neurons/5 mice for WT vs. 83.3 ± 8.8 for Syn II−/−, n = 6 neurons/4 mice; two-tailed unpaired Student's t-test, p = 0.003) (Figure 5C, left). Input resistance, a parameter correlated with the firing rate, was also significantly reduced in Syn II−/− (396.0 ± 45.2 MΩ, n = 11 neurons/5 mice for WT vs. 248.7 ± 23.2 for Syn II−/−, n = 6 neurons/4 mice; two-tailed unpaired Student's t-test, p = 0.031) (Figure 5C, right). On the contrary, recording from granule cells revealed no differences in the firing rates of WT and Syn II−/− neurons (Figures 5D,E), with no genotype-dependent difference in either input resistance or rheobase (data not shown). Other intrinsic membrane properties (resting and threshold potential, AP amplitude, half-amplitude width, after-hyperpolarization current) were similar for the two genotypes in both mossy and granule neurons (data not shown). These results indicate that the reduced fPSPs in the hilar region of pre-symptomatic Syn II−/− mice (Figure 1) are associated with reduced excitability of Syn II−/− mossy cells, but not granule cells.


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)

Hilar mossy cells of pre-symptomatic Syn II−/− mice display decreased excitability. (A) Representative traces of whole cell current-clamp recordings from hilar mossy neurons in acute slices of WT (black) and pre-symptomatic Syn II−/− (red) mice. (B) Action potential (AP) frequency plotted as a function of the injected current for both genotypes. (C) Mean (±s.e.m.) rheobase and input resistance; *p < 0.05, **p < 0.01, two-tailed unpaired Student's t-test. (D,E) Representative traces of whole cell current-clamp recordings from granule neurons (D) in acute slices of WT (black) and pre-symptomatic Syn II−/− (red) mice and AP frequency (E) plotted as a function of the injected current for both genotypes.
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Figure 5: Hilar mossy cells of pre-symptomatic Syn II−/− mice display decreased excitability. (A) Representative traces of whole cell current-clamp recordings from hilar mossy neurons in acute slices of WT (black) and pre-symptomatic Syn II−/− (red) mice. (B) Action potential (AP) frequency plotted as a function of the injected current for both genotypes. (C) Mean (±s.e.m.) rheobase and input resistance; *p < 0.05, **p < 0.01, two-tailed unpaired Student's t-test. (D,E) Representative traces of whole cell current-clamp recordings from granule neurons (D) in acute slices of WT (black) and pre-symptomatic Syn II−/− (red) mice and AP frequency (E) plotted as a function of the injected current for both genotypes.
Mentions: Extracellular fPSPs are the summation of a series of events, notably synaptic activity and synchronous firing of APs by groups of neurons (Buzsaki et al., 2012). Thus, in the next experiment, we evaluated the firing rate of hilar mossy neurons from pre-symptomatic Syn II−/− in the current clamp configuration. In the presence of specific antagonists that fully block synaptic activity, mossy cells were injected with 30 current steps, lasting 1 s and ranging from −100 to +200 pA, in 10 pA increments (Figures 5A,B). The firing rate of mossy neurons was lower in Syn II−/− slices with respect to WT recordings (Figure 5B) and was accompanied by a significant increase in the rheobase (45.0 ± 6.7, n = 11 neurons/5 mice for WT vs. 83.3 ± 8.8 for Syn II−/−, n = 6 neurons/4 mice; two-tailed unpaired Student's t-test, p = 0.003) (Figure 5C, left). Input resistance, a parameter correlated with the firing rate, was also significantly reduced in Syn II−/− (396.0 ± 45.2 MΩ, n = 11 neurons/5 mice for WT vs. 248.7 ± 23.2 for Syn II−/−, n = 6 neurons/4 mice; two-tailed unpaired Student's t-test, p = 0.031) (Figure 5C, right). On the contrary, recording from granule cells revealed no differences in the firing rates of WT and Syn II−/− neurons (Figures 5D,E), with no genotype-dependent difference in either input resistance or rheobase (data not shown). Other intrinsic membrane properties (resting and threshold potential, AP amplitude, half-amplitude width, after-hyperpolarization current) were similar for the two genotypes in both mossy and granule neurons (data not shown). These results indicate that the reduced fPSPs in the hilar region of pre-symptomatic Syn II−/− mice (Figure 1) are associated with reduced excitability of Syn II−/− mossy cells, but not granule cells.

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