Figure 3. Mean curves illustrate the differential effects of intravenous morphine on the noxious heat-evoked responses of lumbar convergent neurons for stimulated areas of 1.9 (open symbols) and 18 cm²(filled symbols). Ordinate = percentages of the control predrug values (mean +/− SD). *Significant difference compared with predrug values (repeated-measurement analysis of variance). (A) In the control group (n = 10), the firing frequencies elicited by the 1.9- and the 18-cm²stimuli, expressed as percentages of the control predrug responses (mean +/− SD), did not vary significantly at any time during the experiments after either saline or naloxone (P = 0.84 and 0.78, respectively). (B) In the M1 group (n = 14), the responses of the convergent neurons, elicited by noxious stimulation limited to the area (1.9 cm²) of their excitatory receptive fields, were not modified after either 1 mg/kg intravenous morphine or naloxone. In contrast, responses elicited by stimulating a much larger area (18 cm²) increased significantly to reach 159 +/− 13% of mean predrug values (P = 0.0006). This effect was fully reversed by 0.4 mg/kg intravenous naloxone. (C) Responses elicited by the 1.9-cm²stimulus were decreased to 11 +/− 1% of predrug values 20 min after 10 mg/kg intravenous morphine (P < 0.0001). A similar effect, but with a smaller magnitude, was observed for responses elicited by the 18-cm²stimuli: This decreased to 42 +/− 9% of the predrug values 20 min after injection (P < 0.0001). In both cases, the responses returned to the predrug values after naloxone.