Fibre photometry
Recombinant AAV vectors (Viral Vector Facility, Zurich Neuroscience Center, ETHZ/UZH) were used for pathway specific expression. For calcium-dependent fluorescent GCaMP6 expression in BA-NAc neurons, a vector encoding GCaMP6 incorporated into a construct with FLEX, ssAAV-9/2-hSyn1-chI-dlox-GCaMP6m-dlox-WPRE-SV40p(A) (“rAAV-FLEX-GCaMP6”; 4.2 x 1012 vg/ml, 300 nl) , was injected unilaterally in the BA (see Stereotactic surgery for retrograde tracing and fibre photometry). For Ca2+-independent EGFP expression, as a control to determine whether certain social behaviours generated movement-related artifacts in the fibre photometry signal, a vector encoding EGFP incorporated into a construct with FLEX, ssAAV-9/2-hSyn1-dlox-EGFP(rev)-dlox-WPRE-hGHp(A) (“rAAV-FLEX-EGFP”; 8.2 x 1012 vg/ml, 300 nl) was injected unilaterally in the BA. To induce GCaMP6 or EGFPexpression in BA-NAc neurons specifically, a retrograde vector encodingCre-recombinase , rAAV-2-retro-hsyn1-Cre-mCherry (“rAAV-retro-Cre”; 8.4.1012 vg/ml, 300 nl) was injected in the ipsilateral NAc. After BA vector injection, a fibre-optic probe (Ø = 200 nm) was implanted directly above the injection site (bregma AP -1.8 mm, ML ±3.2 mm, DV -4.9 mm). Stable adhesion of the fibre-optic probe onto the cranium was achieved as described previously . Mice were distributed equally in terms of whether the left or right hemisphere was studied.
Fibre photometry for optical recording of neural activity in freely moving mice was performed largely as described previously . Briefly, a laser as excitation light source, a high-sensitivity photoreceiver (model 2151, New Focus), and customized software for signal processing, were used. To provide excitation, a 488 nm laser light was focused into a fibre patch cord and delivered at the optic fibre tip in the BA. The patch cord was connected via a ceramic sheath to the optic fibre ferrule on the mouse cranium. Back-propagated GCaMP6 or EGFP fluorescence was focused on a photoreceiver, and custom-written software code was used for data acquisition (LABView, 2020). Testing was conducted in an attenuation chamber equipped with a loudspeaker emitting low-level white noise. A camera (model C920, Logitech) was fixed to the underside of the ceiling of the attenuation chamber and allowed for video recording of the tests on the control PC running LabView. An opening in the ceiling of the attenuation chamber allowed for the passage of the patch cord. A modified cage lid was prepared out of transparent Plexiglas and contained an opening along its length to allow for passage of the patch cord and free movement of the mouse in the test cage during testing. Before the testing of each mouse, the cage lid and the divider used to divide the cage into two compartments were cleaned with 70% ethanol.