Fig. 2

Experimental procedures. a Schematic representation of the fiber photometry system with two channels. b Target sites in this study: B9, LC and VTA. c The fluorescence signal intensity was abruptly increased when the position of the optical fiber tip was placed just above target site. d Schematic representation of the procedure of the recording. Fiber implantation was performed under isoflurane anesthesia. We set 3 h before the beginning of experimental sessions so that anesthesia would not affect experimental sessions. Total four stimuli (two kinds of acute nociceptive stimuli and two kinds of noninvasive control stimuli) were set in order from weaker stimulus to stronger stimulus as follows; the first is low temperature heat stimulus at 25 °C, the second is a gentle touch, the third is acute heat stimulus at 55 °C, and the last is acute mechanical tail pinch stimulus at the force of 400 g. Inter-stimulus interval was set as 30 min to reduce possible carryover effect from the previous stimulus