The mechanical nociceptive thresholds determined with the von Frey test before minimum alveolar concentration (MAC) determination and the administration of 4 µg·kg⁻¹·min⁻¹ remifentanil with 0.17 ng·kg⁻¹·min⁻¹ naloxone, 4 µg·kg⁻¹·min⁻¹ remifentanil alone, 0.17 ng·kg⁻¹·min⁻¹ naloxone alone, or saline on days 0, 2, and 4. Data are expressed as the mean ± SD; n = 6–7 animals per group. * Significantly different from the control group (saline), P < 0.05. † Significantly different from baseline (day 0), P < 0.05.