Functional Cortical Connectomics through Co-Expression of Genetically Expressed Voltage and Calcium Indicators

Abstract

The flow of information through networks of cortical neurons is thought to provide the processing power that underpins cognition. Traditional structural methods to map connectivity are blind to the functional dynamics of cortical circuits. Emerging approaches use genetically encoded activity indicators such as GCaMP. However they are limited to reporting spiking activity and are blind to subthreshold excitatory and inhibitory events. The latter are, however, essential for the functional characterization of cortical circuits. This problem could be overcome by a strategy based on the co-expression of a genetically expressed, voltage sensitive fluorescent protein (VSFP) and calcium indicator (GECI) in the same neuron. Co-expression of VSFPs and GECIs in specific cell-types facilitates direct visualization of sub and supra-threshold activity, illuminating functional cortical circuits. This approach is possible in a new quadruple transgenic mouse: Rasgrf2-2A-dCre;CamK2a-tTA;Chi-VSFPBlfy1.2(TITL-GCaMP6f). This mouse line uses intersectional genetic targeting with (i) a GECI (GCaMP6f) targeted to the TIGRE locus (TITL) with expression driven in L2/3 excitatory neurons via Cre and tTA activities (Rasgrf2 and CamK2A) and (ii) a VSFP (Chimeric-VSFPBlfy1.2) under a tetracycline response element (pTRE) with expression driven by CamK2A to excitatory neurons. The probability of GCaMP6f expression can be controlled by titrated trimethoprim application while VSFPs are broadly expressed in all excitatory cells. The co-expression of the two indicators combined with 2-P microscopy gives simultaneous monitoring of both signals in the same neuron and across networks of neurons in vivo. We defined the spatio-temporal profile of VSFP responses that co-occur with GCaMP transients in single neurons expressing both indicators in vitro and in vivo. We then create event-triggered maps of co-active neural assemblies during visual stimulation. Thus we provide an approach to dissect functional cortical microcircuits in the intact cortex.