*2.6. Injection of Adeno-Associated Viruses (AAV) to the Hippocampus*

The adeno-associated virus (AAV) injection method used to knock down the dopamine D1 receptor and the following behavioral evaluation were performed as described in our previous study [9,10]. The AAV construct that expresses artificial microRNA (miRNA) targeting the dopamine D1 receptor with an emerald green fluorescent protein (EmGFP) under the control of the elongation factor (EF)1α promoter only in the presence of Cre recombinase (AAV10-EF1α-double-floxed inverted (DIO)-EmGFP-D1miRNA), an AAV construct that expresses control miRNA in the same arrangement (AAV10-EF1α-DIO-EmGFP-control), and an AAV construct that expresses Cre recombinase under the CMV promoter (AAV10-CMV-Cre) were produced as previously described [9]. After anesthetized with sodium pentobarbital, 8-week-old Crl:CD1 male mice were injected with 0.5 <sup>μ</sup>L of the AAV solution (1.0 × 1012 genomics copies/mL) per site, applied at two sites in the hippocampal regions of both hemispheres (four sites per mouse; from bregma: posterior, −3.5 mm; lateral, ± 3 mm; ventral, −3.8 mm and −1.8 mm) according to the atlas of Paxinos and Franklin [11] and using a PV-830 Pneumatic PicoPump (World Precision Instruments, Sarasota, FL, USA). Mice were allowed to recover for 4 weeks and then used for the Y-maze test.
