Small animal stereotax

Five- to six-month-old mice were anesthetized with isoflurane and placed in a small animal stereotax (David Kopf Instruments, St. Tujunga, CA, USA). They were implanted with a manually-constructed optic fiber [200 um core, NA = 0.22 (FG200UCC, Thorlabs, Newton, NJ, USA)] held in a ceramic ferrule (CF270-10, Thorlabs, Newton, NJ, USA). Measurements were made relative to Bregma, coordinates: −3.1, 0.35–0.37, −4.3. The fiber was held in place to the skull with adhesive cement (C&B Metabond; Parkell Inc., Brentwood, NY, USA). The incision was closed with vetbond and mice were recovered on a heat pad in a clean cage. Mice were group housed pre and post surgery. Fiber placement was confirmed after behavior with serial coronal sections, anti-TH staining to identify the VTA and epifluorescent imaging (Supplementary Figure S1).

For all subjects, surgery was performed under aseptic conditions using a small animal stereotax (David Kopf Instruments, Tujunga, CA, USA) and body temperature was maintained using a heating pad. Anesthesia was induced using a 5% mixture of isoflurane and oxygen. Following induction, this mixture was reduced to 2–2.5% and was maintained throughout the duration of the procedure (0.5 L/min oxygen flow rate). Once subjects were adequately anesthetized, ophthalmic ointment was applied to the subject’s eyes, hair was removed from the incision site using hair clippers, and the area was scrubbed using 70% alcohol and betadine 3 times each in alternation, with an injection of 2% lidocaine under the skin for local anesthetic. All measurements for viral injections, electrode implants, or optrode implants were made with Bregma as the origin. Following surgery, subjects were placed in clean cages with water-softened mouse chow to recover. Cages were placed either under a heating lamp or on a heating pad to aid in recovery.

Once animals obtained the necessary behavioral requirements for discrimination among magnitudes, they were given 3 d ad libitum access to food and water in preparation for surgery. Anesthesia was administered using isoflurane initially in an induction chamber (5%) and then using a nosecone (1–3%) with a precision vaporizer (EZ Anesthesia). Buprenorphine (Buprenex, Reckitt Benckiser Healthcare) was administered 15–20 min prior to general anesthesia. The scalp was shaved, and the animal was placed into a small animal stereotax (Kopf Instruments) using blunt 45^° earbars to stabilize the head position and to prevent any damage to the ear. Sterile eye ointment (Webster) was used to prevent desiccation. A povidone-iodine solution (10% Betadine, Henry Schein, Inc.) was used as an antiseptic treatment for the skin, and lidocaine (0. 2 ml) was injected as a topical analgesic. A longitudinal incision was made, and skin was retracted over the skull. To ensure precise electrode implantation, the bone landmarks (lambda and bregma) were leveled, and skull screws were inserted (two anterior and two posterior) following bilateral craniotomies for left and right hemisphere electrode implantation.