2.2. Medial septum viral injections

Three weeks before the chemogenetic experiments, ChAT-Cre:: Tg2576 mice were anesthetized with 3% isoflurane and then transferred to a stereotaxic apparatus where anesthesia was maintained with 1–2% isoflurane. Mice were placed on a heating pad (#50–7220F, Harvard Apparatus) with a rectal probe for control of body temperature. Next, buprenorphine (0.05 mg/kg) was injected subcutaneously (s.c.) to reduce discomfort. One burr hole was drilled above the MS (+0.7 mm Anterior-Posterior; A-P, 0 mm Medio-Lateral; M-L) and 500 nL of AAV containing a modified designer receptor (hM4D) carrying an inhibitory signaling cascade (Gi; AAV5-hSyn-DIO-hM4D(Gi)-mCherry, Addgene) was stereotaxically injected over 10 min into the MS (−3.5 mm Dorso-Ventral; D-V, measured from the surface of the dura as before Lisgaras and Scharfman, 2022b). The needle was then left in place for another 10 min and it was slowly retracted to avoid backflow of the injected virus. During the same surgery, one depth electrode was implanted in the left DG following procedures described below. Chemogenetic experiments were conducted 3 weeks after EEG surgery to allow for sufficient expression of the injected virus and recovery from EEG surgery.