Figure 9.

VSFP reporter kinetics and signal/noise of action-potential-induced fluorescence responses. (A) Spike pattern evoked by injection of a constant current pulse into the cell body of the Purkinje neuron (holding current: −20 pA; pulse: 0 pA; 200 ms) and of the layer 5 pyramidal neuron (holding current: 0 pA; pulse: +200 pA). Start of current pulses indicated by arrows. (B1) Activation state (n_R+ in %; somatic compartment) of the VSFP reporter in the Purkinje (left) and L5 neuron models including VSFP2.3 (Model III) at 500 VSDs/μm² using the same stimulus as in A. (B2) Optical response signal ΔF/F₀ corresponding to B1 with (green) and without (black) photon shot noise in the case of a single trail (upper trace) and a 20 trial average (lower trace). Shot noise was simulated for a single spherical cell body (25 μm diameter) at 1.5 kHz sampling rate. (C1) Activation state (n_R+; in %) of the VSFP reporter in the Purkinje and L5 neuron models including VSFP3.1 (Model III) at 500 VSDs/μm² applying the same stimulus as in A. (C2) Optical response signal ΔF/F₀ corresponding to C1 with (green) and without (black) photon shot noise in the case of a single trail (upper trace) and a 80 trial average (lower trace). Shot noise simulated as in B2. (D) S/N ratio for the optical detection of the first action potential in spike trains as in A by a VSFP probe with kinetics identical to VSFP3.1 as function of indicator sensitivity at half activation S_1/2 and VSFP membrane density on a 40 × 50 parameter grid. S/N values above 2 are coded according to the color scale to the right. The dashed line indicates the voltage sensitivity of VSFP3.1 for comparison.