Sara Fernández-García, Mercè Masana, Javier G. Orlandi, Gerardo García-Díaz Barriga, Jordi Soriano and Jordi Alberch

SFN Meeting, 2017

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Striatum, the main hub of the basal ganglia circuitry, is the most affected region in Huntington’s disease (HD). In HD, mutant-huntingtin (mHtt) causes an excitatory-inhibitory imbalance of the basal ganglia output pathways and induces motor symptoms. Although alterations of striatal Medium-sized Spiny Neurons (MSN) occur at early stages of the disease, little is known about how this translates into functional changes in the network dynamics. Using high-speed, high-resolution calcium imaging, we have recorded simultaneously hundreds of cells from striatal primary cultures in WT and the R6/1 mouse model of HD and characterized the spontaneous activity patterns of every cell as well as their collective activity. Although most of the striatum is composed by inhibitory neurons, we showed that isolated striatal cultures are functionally active, without the need of excitatory inputs. The percentage of active population is ~10% whilst in cortex ~50% of the population showed spontaneous active in basal conditions. At the single cell level, we identified three populations based on their fluorescence activity traces in both WT and R6/1. Type 1 (fast increase/exponential decay) characteristic of neurons; Type 2 (slow calcium transients) characteristic of astrocytic calcium waves; and Type 3 (non-detectable fluorescence changes). Heterogeneity in cell populations was confirmed by immunocytochemistry against neuronal and glial (MAP2/GFAP) markers. At the population level, both WT and R6/1 include a subset of neurons that display highly coherent activity, indicating the presence of a functional network. Bursting properties of this group are similar between genotypes. Blockade of GABA_A receptors by bicuculline increased number of active neurons while decreased astrocytic signal. Also, the previously reported silent population became active, showing a characteristic neuronal activity profile. The large silent population was inhibited in basal conditions, but the underlying network is revealed through system-wide disinhibition. Indeed, bicuculline boosts coherent activity throughout the culture and increases burst duration and amplitude, but not frequency, in both genotypes. These results indicate that striatal network dysfunction in HD may not arise from local disinhibition but probably from aberrant afferent activity. Therefore, further analysis is required to characterize the contribution of different neurotransmitter systems to the striatal network dynamics in healthy and HD cultures. Understanding functional network alterations mediated by mHtt is fundamental to decipher initial key mechanisms to finally target early symptoms in HD.