Surgical Procedure for Intracerebral Electrode Implantation

Eight mice were anesthetized with 3% isoflurane in 2 L/min oxygen until the loss of pinch reflex and maintained at 2% isoflurane during the surgery. The head was shaved and disinfected with Videne. Before surgery, animals were administered with a prophylactic antibiotic, Baytril® (Bayer Health group, Germany, 5 mg/kg, s.c.), and an analgesic, buprenorphine (Vetergesic®, Reckitt Benckiser Healthcare, UK, 0.3 mg/kg, i.m.). An incision was made through the skin, muscle, and connective tissue over the skull, as described previously (31). Bilateral burr holes of 1.8 mm diameter were made through the skull corresponding to the cerebral hemispheres and underlying hippocampi (5 mm cranial to lambda and 2 mm lateral to midline over each hemisphere). We used sterile DSI electrode wires (Data Scientific International, USA). The electrodes used were twisted (coiled bifilar) and bipolar, i.e., they measure the voltage between the two leads between the electrode coil in the (+) red insulated lead and the electrode coil in the negative (−) clear insulated lead. Each of the two electrode leads consists of a high-performance nickel cobalt alloy as used in medical implants. The electrode is a coil consisting of two wires (bifilar), but the two wires are electrically common to one another. The diameter of the coil was 0.356 mm. The coil insulation was a medical grade silicone with an outer diameter of 0.94 mm. Following removal of the electrodes' insulation, a V-shaped bend was made in the wire with an extended flat terminal end (~2 mm). The length/depth of the V-shaped electrode was 2.8 mm, which was enough to penetrate the cortex. The width of the V bend varied from bottom to the top (0.5–2 mm), which was sufficient to pass through the 1.8 mm burr hole. The V-shaped electrode was manually inserted through the burr hole. The flat free terminal end and an insulated wire at the other end of the hole were touching the skull. Electrodes were secured to the skull with dental cement constituted from Alphacryl Rapid Repair and methyl acrylate (National Dental Supplies, Southport, U.K). The incision was sutured using Polysorb 4.0 and reinforced with Vetbond™ Tissue adhesive (3M Animal Care Products, USA). All animals were given subcutaneous injections of saline for rehydration and soft food to facilitate recovery of their body weight, post-surgery.