Figure 1. Opsin expression in cultured neurons does not alter the spontaneous network electrical activity.

Transparent glass microelectrode arrays (MEAs), used as substrates for long-term primary cortical cultures, were employed to record light-evoked (A) and spontaneous (B) neuronal electrical activity, 28 days after plating. Brief light pulses were delivered wide field by a computer-controlled LED, focused on the entire inner area of the MEA (A), and found to elicit neuronal spiking activity analyzed from extracellular raw voltage signals. Transduction of ChR2 and mCherry by AAV vectors, with a CaMKIIα promotor, altered neither the spontaneous network bursting (B), nor induced significant changes in neuronal firing rate, burst rate and duration, or in the number of active microelectrodes (C, quantified over 11 control and 46 transduced MEAs). NeuN immunostaining (green) and fluorescent mCherry (red) imaging highlighted (D) the ChR2 expression in putative excitatory neurons, with spatially unrestricted presence over the cell surface.