The in vivo extracellular single unit recordings in the CN were performed according to a previously published method41. In brief, rats were deeply anesthetized with an intraperitoneal (i.p.) injection of sodium pentobarbital (60 mg/kg) for surgery, were mounted on the stereotaxic frame, and were supplemented with sodium pentobarbital (5 mg/kg/h) infused intravenously through a jugular vein catheter during the recording process to maintain anesthesia. The adequacy of anesthesia was monitored by the lack of withdrawal reflexes to noxious stimuli and the absence of corneal blink reflexes. Body temperature was maintained at 37 °C with a thermostatically controlled heating blanket with a rectal probe. Extracellular potentials of single neurons in the CN were recorded using a carbon-filament glass microelectrode (Carbostar-1, Kation Scientific, USA) at a depth of 0.0–0.8 mm ventral from the dorsal surface of the medulla, 1 mm caudal to the obex and 1–2 mm lateral from the midline40. Neurons were classified as low-threshold (LT) or wide-dynamic range (WDR) neurons based on the response to mechanical somatic stimuli. LT neurons were excited by only innocuous stimuli (light pressure), and WDR neurons responded to both innocuous (light pressure) and noxious stimuli (blunt clip). The signals were amplified (Iso80, World Precision Instruments, USA) and recorded using a micro1401 and Spike2 software (Cambridge Electronic Design, UK).