Electrode implantation and recordings

Each rat was anesthetized with sodium pentobarbital (50 mg/kg, i.p.), of which supplemental doses (16 mg/kg, i.p.) were administered when necessary. Craniotomies were performed to expose the brain surface vertical to the recording sites. The coordinates for implantation were as follows: (1) mediodorsal thalamic nucleus, medial part (MDM): right or left: 0.4 mm, posterior: 3 mm, depth: 4.5–5.5 mm; (2) rostral anterior insular cortex (RAIC): right or left: 3–5 mm, anterior: 1–4 mm, depth: 5–6 mm; (3) mediodorsal thalamic nucleus, lateral part (MDL): right: 1.2 mm, posterior: 2.5 mm, depth: 4.5–5.5 mm; (4) anterior cingulate cortex (ACC): right: 1.2 mm, anterior: 1–4 mm, depth: 2.5–3 mm; (5) ventral posterolateral thalamic nucleus (VP): right: 2.8–3.3 mm, posterior: 2.8 mm, depth: 5.5–6.5 mm; (6) S1: right: 1–3 mm, posterior: 1 mm, depth: 600–800 μm. For thalamus recording, we used a bundled microarray electrode consisting of seven tungsten microwires with diameters of 35 μm bare and 50 μm insulated (#100211; California Fine Wire) in a 29-G guide tube [32]. The electrode set for the cortex recording consisted of eight stainless steel microwires arranged in a 2–3-mm-wide array. Each rat was implanted with a matching thalamic and cortical target set (MDM-RAIC, MDL-ACC and VP-SI) of electrodes. Only those rats with good evoked cortical responses under low intensity (below 10 μA) thalamic stimulation were used to ensure the accuracy of the cortical implantations. Four stainless steel screws were set in each rat’s skull to serve as anchors for the electrode sets. To ground the array electrodes, a copper wire was fixed around the anchoring screw positioned at the occipital bone. When all the electrodes were in place, the surface of the skull was covered with dental cement and the wound was sutured. Lincomycin hydrochloride (30 mg/kg, i.m.) was administrated to prevent infection.

After recovering for 1 week, single unit activities were recorded while SNI surgery was conducted. Multiple-channel cortical unit activities were transmitted to a multichannel acquisition processor system (MAP, Plexon, Dallas, USA) through a connecting cable. To record single unit activities while performing SNI surgery, the anesthesia procedure and surgery preparation were identical to those of the fMRI experiment. Single unit recording was initiated 5 min before sciatic nerve transection and continued for 25 min after transection, resulting in 30 min of continuous recording. Spike signals were amplified 7000–32,000-fold, bandpass-filtered at 250 Hz–13 kHz, and digitized at 40 kHz. Well isolated single unit activities in one area were linearly added for a quantitative estimation of the activity change in that cortical area.