Fig. 2. C fiber–evoked firing responses (mean ± SEM in percent of baseline). The responses were significantly increased for 1 h (P < 0.001) after conditioning compared with baseline and those in control animals. The values shown are based on the average of four consecutive recorded responses (100%= 7.8 and 11.5 action potentials in groups 2 and 3, respectively). Morphine was applied 1 h after tetanization (group 2) or a waiting period (group 3). The time from morphine was applied until the C fiber–evoked response was zero varied from cell to cell (2–4 h). Naloxone, however, reversed the C fiber–mediated response to the control level in group 3 and to the LTP level in group 2.