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Enhanced GABAergic Inputs Contribute to Functional Alterations of Cholinergic Interneurons in the R6/2 Mouse Model of Huntington's Disease.

Holley SM, Joshi PR, Parievsky A, Galvan L, Chen JY, Fisher YE, Huynh MN, Cepeda C, Levine MS - eNeuro (2015 Jan-Feb)

Bottom Line: In Huntington's disease (HD), a hereditary neurodegenerative disorder, striatal medium-sized spiny neurons undergo degenerative changes.They also displayed a higher frequency of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) and larger amplitude of electrically evoked IPSCs.In contrast, glutamatergic spontaneous or evoked postsynaptic currents were not affected.

View Article: PubMed Central - HTML - PubMed

Affiliation: Intellectual and Developmental Disabilities Research Center, Semel Institute for Neuroscience and Human Behavior, University of California at Los Angeles, Los Angeles, CA 90095.

ABSTRACT

In Huntington's disease (HD), a hereditary neurodegenerative disorder, striatal medium-sized spiny neurons undergo degenerative changes. In contrast, large cholinergic interneurons (LCIs) are relatively spared. However, their ability to release acetylcholine (ACh) is impaired. The present experiments examined morphological and electrophysiological properties of LCIs in the R6/2 mouse model of HD. R6/2 mice show a severe, rapidly progressing phenotype. Immunocytochemical analysis of choline acetyltransferase-positive striatal neurons showed that, although the total number of cells was not changed, somatic areas were significantly smaller in symptomatic R6/2 mice compared to wildtype (WT) littermates, For electrophysiology, brain slices were obtained from presymptomatic (3-4 weeks) and symptomatic (>8 weeks) R6/2 mice and their WT littermates. Striatal LCIs were identified by somatic size and spontaneous action potential firing in the cell-attached mode. Passive and active membrane properties of LCIs were similar in presymptomatic R6/2 and WT mice. In contrast, LCIs from symptomatic R6/2 animals displayed smaller membrane capacitance and higher input resistance, consistent with reduced somatic size. In addition, more LCIs from symptomatic mice displayed irregular firing patterns and bursts of action potentials. They also displayed a higher frequency of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) and larger amplitude of electrically evoked IPSCs. Selective optogenetic stimulation of somatostatin- but not parvalbumin-containing interneurons also evoked larger amplitude IPSCs in LCIs from R6/2 mice. In contrast, glutamatergic spontaneous or evoked postsynaptic currents were not affected. Morphological and electrophysiological alterations, in conjunction with the presence of mutant huntingtin in LCIs, could explain impaired ACh release in HD mouse models.

No MeSH data available.


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A, Confocal image (z-stack) of a LCI recorded and filled with biocytin (red). The LCI is surrounded by ChR2-EYFP terminals from SOM interneurons (green). Arrows indicate SOM-expressing interneurons. B, IPSCs evoked in LCIs by optogenetic stimulation of SOM-expressing interneurons. Responses were significantly larger in LCIs from R6/2 mice. These responses were completely blocked by BIC. C, Graphs show significant increases in amplitude and charge in LCIs from R6/2 mice compared to WTs. D, LCI surrounded by ChR2-EYFP terminals from PV interneurons. Arrow indicates a PV-expressing interneuron. E, IPSCs evoked in LCIs by optogenetic stimulation of PV-expressing interneurons were smaller than those evoked by SOM terminal stimulation and were not different between WT and R6/2 animals. Responses were completely blocked by BIC. F, Graphs show the lack of significant differences in peak amplitude and charge in LCIs from WT and R6/2 mice. *p < 0.05.
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Figure 5: A, Confocal image (z-stack) of a LCI recorded and filled with biocytin (red). The LCI is surrounded by ChR2-EYFP terminals from SOM interneurons (green). Arrows indicate SOM-expressing interneurons. B, IPSCs evoked in LCIs by optogenetic stimulation of SOM-expressing interneurons. Responses were significantly larger in LCIs from R6/2 mice. These responses were completely blocked by BIC. C, Graphs show significant increases in amplitude and charge in LCIs from R6/2 mice compared to WTs. D, LCI surrounded by ChR2-EYFP terminals from PV interneurons. Arrow indicates a PV-expressing interneuron. E, IPSCs evoked in LCIs by optogenetic stimulation of PV-expressing interneurons were smaller than those evoked by SOM terminal stimulation and were not different between WT and R6/2 animals. Responses were completely blocked by BIC. F, Graphs show the lack of significant differences in peak amplitude and charge in LCIs from WT and R6/2 mice. *p < 0.05.

Mentions: Feedforward inhibition, in particular from SOM-expressing persistent low-threshold spiking (PLTS) interneurons, plays a crucial role in the increased GABA synaptic activity observed in MSNs from R6/2 mice (Cepeda et al., 2013). To determine if SOM-expressing PLTS interneurons serve as a source for increased GABA synaptic activity in LCIs, we used an optogenetic approach to isolate the contribution of these interneurons (Fig. 5A). LCIs were voltage clamped at +10 mV and IPSCs were evoked by optically activating ChR2 expressed in SOM interneurons (Fig. 5B, top). In LCIs from WT mice (n = 7, age 71 ± 2 d), 62% of cells responded to SOM interneuron activation (8/13) and in LCIs from R6/2 mice (n = 6, age 70 ± 1 d), 82% of cells responded (9/11 cells) (p = 0.27). The eIPSCs in LCIs from R6/2 mice had significantly larger peak amplitudes (p < 0.05), greater charge (p < 0.05 pA) (Fig. 5C), and longer decay times (125 ± 8 ms vs 98 ± 10 ms, p < 0.05, for R6/2 and WT, respectively) when compared to responses recorded in WT LCIs. All eIPSCs were identified as GABAergic, as all recordings were performed in the presence of NBQX and APV (10 μM and 50 μM, respectively), and responses were completely eliminated after addition of BIC (10 μM) (Fig. 5B, bottom).


Enhanced GABAergic Inputs Contribute to Functional Alterations of Cholinergic Interneurons in the R6/2 Mouse Model of Huntington's Disease.

Holley SM, Joshi PR, Parievsky A, Galvan L, Chen JY, Fisher YE, Huynh MN, Cepeda C, Levine MS - eNeuro (2015 Jan-Feb)

A, Confocal image (z-stack) of a LCI recorded and filled with biocytin (red). The LCI is surrounded by ChR2-EYFP terminals from SOM interneurons (green). Arrows indicate SOM-expressing interneurons. B, IPSCs evoked in LCIs by optogenetic stimulation of SOM-expressing interneurons. Responses were significantly larger in LCIs from R6/2 mice. These responses were completely blocked by BIC. C, Graphs show significant increases in amplitude and charge in LCIs from R6/2 mice compared to WTs. D, LCI surrounded by ChR2-EYFP terminals from PV interneurons. Arrow indicates a PV-expressing interneuron. E, IPSCs evoked in LCIs by optogenetic stimulation of PV-expressing interneurons were smaller than those evoked by SOM terminal stimulation and were not different between WT and R6/2 animals. Responses were completely blocked by BIC. F, Graphs show the lack of significant differences in peak amplitude and charge in LCIs from WT and R6/2 mice. *p < 0.05.
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Figure 5: A, Confocal image (z-stack) of a LCI recorded and filled with biocytin (red). The LCI is surrounded by ChR2-EYFP terminals from SOM interneurons (green). Arrows indicate SOM-expressing interneurons. B, IPSCs evoked in LCIs by optogenetic stimulation of SOM-expressing interneurons. Responses were significantly larger in LCIs from R6/2 mice. These responses were completely blocked by BIC. C, Graphs show significant increases in amplitude and charge in LCIs from R6/2 mice compared to WTs. D, LCI surrounded by ChR2-EYFP terminals from PV interneurons. Arrow indicates a PV-expressing interneuron. E, IPSCs evoked in LCIs by optogenetic stimulation of PV-expressing interneurons were smaller than those evoked by SOM terminal stimulation and were not different between WT and R6/2 animals. Responses were completely blocked by BIC. F, Graphs show the lack of significant differences in peak amplitude and charge in LCIs from WT and R6/2 mice. *p < 0.05.
Mentions: Feedforward inhibition, in particular from SOM-expressing persistent low-threshold spiking (PLTS) interneurons, plays a crucial role in the increased GABA synaptic activity observed in MSNs from R6/2 mice (Cepeda et al., 2013). To determine if SOM-expressing PLTS interneurons serve as a source for increased GABA synaptic activity in LCIs, we used an optogenetic approach to isolate the contribution of these interneurons (Fig. 5A). LCIs were voltage clamped at +10 mV and IPSCs were evoked by optically activating ChR2 expressed in SOM interneurons (Fig. 5B, top). In LCIs from WT mice (n = 7, age 71 ± 2 d), 62% of cells responded to SOM interneuron activation (8/13) and in LCIs from R6/2 mice (n = 6, age 70 ± 1 d), 82% of cells responded (9/11 cells) (p = 0.27). The eIPSCs in LCIs from R6/2 mice had significantly larger peak amplitudes (p < 0.05), greater charge (p < 0.05 pA) (Fig. 5C), and longer decay times (125 ± 8 ms vs 98 ± 10 ms, p < 0.05, for R6/2 and WT, respectively) when compared to responses recorded in WT LCIs. All eIPSCs were identified as GABAergic, as all recordings were performed in the presence of NBQX and APV (10 μM and 50 μM, respectively), and responses were completely eliminated after addition of BIC (10 μM) (Fig. 5B, bottom).

Bottom Line: In Huntington's disease (HD), a hereditary neurodegenerative disorder, striatal medium-sized spiny neurons undergo degenerative changes.They also displayed a higher frequency of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) and larger amplitude of electrically evoked IPSCs.In contrast, glutamatergic spontaneous or evoked postsynaptic currents were not affected.

View Article: PubMed Central - HTML - PubMed

Affiliation: Intellectual and Developmental Disabilities Research Center, Semel Institute for Neuroscience and Human Behavior, University of California at Los Angeles, Los Angeles, CA 90095.

ABSTRACT

In Huntington's disease (HD), a hereditary neurodegenerative disorder, striatal medium-sized spiny neurons undergo degenerative changes. In contrast, large cholinergic interneurons (LCIs) are relatively spared. However, their ability to release acetylcholine (ACh) is impaired. The present experiments examined morphological and electrophysiological properties of LCIs in the R6/2 mouse model of HD. R6/2 mice show a severe, rapidly progressing phenotype. Immunocytochemical analysis of choline acetyltransferase-positive striatal neurons showed that, although the total number of cells was not changed, somatic areas were significantly smaller in symptomatic R6/2 mice compared to wildtype (WT) littermates, For electrophysiology, brain slices were obtained from presymptomatic (3-4 weeks) and symptomatic (>8 weeks) R6/2 mice and their WT littermates. Striatal LCIs were identified by somatic size and spontaneous action potential firing in the cell-attached mode. Passive and active membrane properties of LCIs were similar in presymptomatic R6/2 and WT mice. In contrast, LCIs from symptomatic R6/2 animals displayed smaller membrane capacitance and higher input resistance, consistent with reduced somatic size. In addition, more LCIs from symptomatic mice displayed irregular firing patterns and bursts of action potentials. They also displayed a higher frequency of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) and larger amplitude of electrically evoked IPSCs. Selective optogenetic stimulation of somatostatin- but not parvalbumin-containing interneurons also evoked larger amplitude IPSCs in LCIs from R6/2 mice. In contrast, glutamatergic spontaneous or evoked postsynaptic currents were not affected. Morphological and electrophysiological alterations, in conjunction with the presence of mutant huntingtin in LCIs, could explain impaired ACh release in HD mouse models.

No MeSH data available.


Related in: MedlinePlus