NOTE: All procedures for animal breeding and cares were conducted according to the Institutional Animal Care and Use Committee (IACUC) regulation of Icahn School of Medicine at Mount Sinai. Be sure to use sterile gloves throughout the procedure. 

1. Drug and Virus Preparation

1. Drug Preparation
   1. Prepare 15.0 ml ketamine/xylazine anesthetic by dissolving 1.35 ml of 100 mg/ml Ketamine and 0.75 ml of 20 mg/ml xylazine in 12.9 ml of 0.9% Saline solution. Thoroughly mix solution.
2. Virus Preparation
   1. Clone the mGlu2 and mGlu2ΔTM4N constructs into a bicistronic herpes simplex virus (HSV) vector following standard protocols previously described⁶. Package the viral particles as previously described^6,13,14. Substitution of residues Ala-677^4.40, Ala-681^4.44 and Ala685^4.48 in mGlu2 for Ser686^4.40, Phe690^4.44 and Gly-694^4.48 in mGlu3 (HA-mGlu2ΔTM4N) have been described previously⁶.
      NOTE: It was previously demonstrated that the chimeric construct HA-mGlu2ΔTM4N is expressed at the plasma membrane with intact G protein-dependent signaling⁶.
   2. Store viral vectors in -80 °C when not in use. Thaw viral vector on ice, and then aliquot into 10 μl aliquots. For the surgical procedure, keep on ice.

2. Surgery

1. Surgery Preparation
   1. Weigh mouse and inject mouse with appropriate dose of ketamine/xylazine cocktail (for details, see 1.1.1).
   2. Check mouse to see if properly anesthetized, squeeze foot and tail for pain response, if unable to elicit a response, the mouse is properly anesthetized.
   3. Shave mouse head from the base of the skull to the tip of the nose using clippers. Apply ophthalmic gel to the mouse afterward to prevent blindness of the mouse.
   4. Load each syringe onto the stereotaxic frame. Then tilt perpendicular portion of each arm of the stereotaxic frame so that they are 10 degrees away from the normal. Ensure that the arms are tilted, such that the needles are facing each other.
   5. Clean each syringe by filling the needle with 70% ethanol. Fill the needle at least three times to ensure that the syringe is clean.
   6. Once the needle has been cleaned, flush the needle by filling the needle with double distilled H₂O. Once flushed, fill each needle with 1.3 µl of double distilled H₂O. Twist the plunger of the syringe to release 0.3 µl of double distilled H₂O. If water beads at the tip of the needle, carefully wipe away water. If nothing comes out of the syringe, push the plunger completely down and then repeat cleaning of syringe.
   7. After filling with water, then pull up the syringe filling the syringe with 0.5 µl of air.
   8. Once the air and water are in the syringe, carefully fill the syringe with 1.3 µl of virus solution. At this point ensure that the total volume in the syringe is 2.8 µl. Again twist the tip of the syringe to release 0.3 µl of virus. If liquid beads at the tip of the needle, carefully wipe away liquid. If nothing comes out of the syringe, push the plunger completely down and then repeat cleaning of syringe.
2. Surgery
   1. Attach the mouse to the stereotaxic frame, making sure to adjust the stereotaxic frame so that the skull is level and flat. Apply povodine-iodine to the exposed scalp. Using a scalpel, make a sagittal incision along the midline of the skull within the exposed shaved area. Then attach the buret clamps to the skin at the incision site to make sure that the skull remains exposed.
   2. Use H₂O₂ to dissolve away the periosteum to expose the sutures of the skull. Now that the bregma and sutures are visible, be sure to adjust the stereotaxic frame to make sure that the skull is level.
   3. Align the needle tips of the syringes with the bregma and record the coordinates of the bregma. Calculate the coordinates of the where the needles are going to be inserted.
      1. For the Rostral-Cauldal (R-C) plane, add 1.6 mm to the recorded R-C bregma coordinates (+1.6 from Bregma). For the Dorsal-Ventral (D-V) plane, subtract 2.4 mm from the recorded D-V bregma coordinates (-2.4 from Bregma).
      2. Finally, for the Medial Lateral (M-L) plane, add 2.6 mm to the recorded M-L bregma coordinates (+2.6 from Bregma). For all coordinates be sure to record both left and right coordinates, as this is a bilateral injection.
   4. Bring the needles to the desired coordinates. Mark the places of where the needles are going to be inserted and with a drill, drill the marked areas.
   5. With a cotton tip applicator wipe away any excess blood or bone fragment.
   6. Bring the needles to the skull where the tips of the needles are touching the surface of the brain. Then lower the needles to the desired coordinates slowly lowering them.
   7. Once the needles are at the desired coordinates, slowly inject the contents of the syringe by twisting the plunger of the needle 0.1 µl per minute over the course of 5 min (in total 0.5 µl).
   8. Once the injection has been made leave the syringe in cortex for another 5 min.
3. Closing Up/Care
   1. Remove the needles from the mouse cortex steadily and slowly. Then remove the mouse from the stereotaxic frame.
   2. Apply cyanoacrylate (dermal adhesive) to the base flaps of skin from the incision and then with forceps grab the flaps of skin and place them together.
   3. Allow the cyanoacrylate to dry. Place the mouse in cage over a heating pad (heating pad is optional if the surgical suite is kept under RT of 37 °C – otherwise not necessary). Be sure to place the mouse on a paper towel to make sure that bedding does not adhere to surgical site.
   4. Depending on the length of the surgical procedure, ensure that the mice is out of anesthesia within 30 – 60 min after procedure. Following surgery, the animal is placed in its own cage and monitored until it becomes conscious before returning to its room to recover. No analgesics are used post-operatively, because they may alter the results of our experiments by modifying some of the brain biochemical pathways. However, animals are monitored until they recover from anesthesia on the day of surgery, and daily post-operation for signs of infection and evaluation of pain/distress.

3. Head Twitch Response Experiment

1. Set-up
   1. Carry out all behavioral testing between 10:00 AM and 2:00 PM, 2 – 3 days after stereotactic injection of viral particles.
   2. Dissolve (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI) into a 0.9% saline solution to 2.0 mg/kg. Also prepare a 0.9% saline solution.
   3. Prepare a home cage (28 x 18 x 15 cm) without any bedding and using a tri-pod, adjust a camera so that the view of the video camera is directly above the home cage.
   4. Habituate the mice to the room for at least 4 hr prior to the beginning of the experiment.
   5. Set up a camcorder to record the head twitch.
2. Experiment
   1. Position the camera so that it is directly over a home cage. Calibrate the camera so that the entire home cage is in the field of view.
   2. Weigh mouse and inject mouse intraperitoneally with appropriate dose of either 0.9% saline or DOI (0.01 ml/g). NOTE: If a mouse weighs 25 grams, administer dose to a total volume of 0.25 ml.
   3. Place each mouse back in their home cage for 10 min. After 10 min, place mouse into the center of the empty home cage and validate that there are no blind spots in the field of view of the camera. Press record on the camcorder. Leave the room.
      NOTE: Mouse movements and various behavioral responses therein (head twitch, ear scratch, etc. Please refer to supplemental data table 1 of Gonzalez-Maeso et al. 2007 for full list of behavioral responses induced by DOI.)³, will be recorded for 30 min. Therefore, it is important there are no blind spots in the field of view recorded.
   4. After 30 min stop recording on the camcorder and place mouse back in original home cage. Repeat this process for each mouse.
3. Review
   1. Have each referee review the tapes blind to the experimental conditions of mouse (i.e., drugs used during head twitch experiment or virus used during intracranial injection)). Manually record every head twitch throughout the video.
      NOTE: Head-twitch is defined as a rapid shaking head movement conducted by a mouse (supplemental video).
   2. For each mouse, average the final HTR response from the three totals of the blind referees. Then group these values by experimental condition and carry out statistical analysis (i.e., t-test or ANOVA).