Fibre photometry target validation
After completion of the experiment, mice were deeply anaesthetised and underwent brain perfusion-fixation for histological assessment in terms of BA probe placement and BA GCaMP6/EGFP and NAc mCherry expression. As described in detail elsewhere , the optic fibre implant was removed, and the brain sectioned coronally at 100 μm using a vibratome (Leica). Sections underwent Nissl staining (NeuroTrace 640/660 Deep-Red Fluorescent Nissl Stain, Thermo Fisher), followed by washing in PBS, mounting on microscope slides, addition of Dako/DAPI fluorescence mounting medium (Sigma Aldrich), and cover-slipping. Using an epifluorescence microscope (Axio Observer.Z.1, Zeiss), mounting medium allowed for localization of GCaMP6/EGFP expression, and Nissl staining allowed for localization of the probe placement and tracing of the BA-NAc pathway using mCherry reporter. Using a mouse brain atlas (Franklin and Paxinos, 2019), the bregma levels of the BA section that included the most ventral position of the fibre tip in the BA and of the NAc section with the highest mCherry expression, were identified. Two GCaMP6 mice were excluded due to a misplaced BA AAV vector injection and optic fibre placement, resulting in final sample sizes of n=10 for GCaMP6 and n=4 for EGFP.