Animals were anesthetized as above and positioned in a stereotaxic frame (Kopf Instruments, Tujunga, CA). Unless otherwise noted, the incisor bar was set to the `flat-skull' position. To test the efficacy of the re-expressing and knockdown viruses in vivo, bilateral injections were made into the hippocampus of mice or rats, respectively. This area was chosen based on the constitutive expression of α5 nAChR subunit mRNA in wildtype animals. In mice, six bilateral injections (1 μl each at a flow rate of 1 μl per min) were made at the following coordinates: anterior-posterior (AP): −1.7 mm from bregma; medial-lateral (ML): ±0.75 mm from midline; dorsal-ventral (DV): −2.05 mm, −1.80 mm and −1.3 5mm from brain surface2. In rats, the six hippocampal injections (three 2 μl injections per side at a flow rate of 1 μl per min) were made at the following coordinates: AP: −3.3 mm from bregma; ML: ±1.1 mm from midline; DV: −3.6 mm, −3.0 mm and −2.4 mm from brain surface3. For habenular injections in mice, the needle was angled 20° toward midline, and bilateral injections (0.375 μl
