Nineteen Drd1a-tdTomato/Drd2-EGFP transgenic mice received an intracerebral injection of 6-OHDA (catalog no. H4381; Sigma-Aldrich, Saint-Louis, MO, USA) in the right medial forebrain bundle (mfb). Approximately 30 minutes before 6-OHDA administration, mice received intraperitoneal injection of desipramine (25 mg/kg) diluted in saline (0.9%) at a concentration of 2 mg/mL. Mice were then anaesthetized using 2% isoflurane and their head were fixed in a stereotaxic apparatus. A hole was drilled and the following stereotaxic coordinates were aimed: anteroposterior (bregma) = −1.2 mm; mediolateral = 1.1 mm; dorsoventral = −5.0 mm, corresponding to the mfb, according to the mouse brain atlas of Franklin and Paxinos70. A glass micropipette of 35 μm diameter at the tip containing a freshly prepared 6-OHDA solution was introduced in the mfb. The 6-OHDA was then pressure-injected and the micropipette was left in place for 2 min both prior and following the injection. The 6-OHDA was diluted in ascorbic acid (catalog no. A5960; Sigma-Aldrich) at a concentration of 6 μg/μL. A total volume of 0.25 μL of 6-OHDA, corresponding to a dose of 1.5 μg of the neurotoxin, was
