Effects of Three Consecutive Days of Morphine or Methadone Administration on Analgesia and Open-Field Activity in Mice with Ehrlich Carcinoma

Abstract

This study assessed the exploratory behavioral responses in BALB/c mice inoculated with Ehrlich ascitic carcinoma after 3 consecutive days of treatment with morphine or methadone. Fifty-three female mice, 60 ± 10 d old, were used. Seven days after intraperitoneal tumor inoculation (2 × 10⁶ cells), the animals were randomized into 7 groups: morphine 5 mg/kg (MO₅), morphine 7.5 mg/kg (MO_7.5), morphine 10 mg/kg (MO₁₀), methadone 2.85 mg/kg (ME_2.85), methadone 4.3 mg/kg (ME_4.3), methadone 5.7 mg/kg (ME_5.7), and 0.9% NaCl (Saline) (n = 7). Drug treatments were administered subcutaneously every 6 h for 3 d. The animals were evaluated for analgesia using the mouse grimace scale (MGS) and for general activity using the open field test. The MGS was performed before tumor inoculation (day 0), on day 7 at 40, 90, 150, 240, and 360 min after drug injection, and on days 8 and 9 at 40, 150, 240, and 360 min after drug injection. The open field test was performed before tumor inoculation (day 0), on day 7 after inoculation at 40, 90, 150, 240, and 360 min after drug injection, and on days 8 and 9 after inoculation at 40, 150, and 360 min after drug injection. MGS results indicated that administration of morphine promoted analgesia for up to 240 min. Conversely, methadone reduced MGS scores only at 40 min. All tested doses promoted a significant dose-dependent increase in the total distance traveled and the average speed, and increase that was markedly pronounced on days 8 and 9 as compared with day 7. The frequencies of rearing and self-grooming decreased significantly after morphine or methadone administration. Despite the difference in analgesia, both drugs increased locomotion and reduced the frequency of rearing and self-grooming as compared with the untreated control animals.

Figure 1.

Schematic timeline of the experimental period. Mice were inoculated intraperitoneally with 2 × 10⁶ Ehrlich tumor cells with > 95% viability. On day 7, the animals were sorted into 7 groups, and given repeated administration (every 6 h) of morphine (MO), methadone (ME) or Saline, for 3 d (day 7, 8 and 9 after tumor inoculation).

Figure 2.

(A) MGS scores over time after repeated administration (every 6 h) of morphine (Mo) or (B) methadone (Me) in different dosages, for 3 d (day 7, 8 and 9 after tumor inoculation). Values are expressed as median (interquartile range) (n = 7). Black symbols express difference to the baseline values and * express differences to the Saline group (Kruskal–Wallis and Dunn posthoc test P < 0.05).

Figure 3.

Locomotor activity (distance traveled and average speed) over time after repeated administration (every 6 h) of morphine (MO) or methadone (ME) in different dosages, for 3 d (day 7, 8 and 9 after tumor inoculation). Values are expressed as mean ± error deviation (n = 7). Black symbols express difference to the baseline values and * express differences to the Saline group (Two-way ANOVA and Dunnet posthoc test, P < 0.05). (A) Morphine distance traveled, (B) methadone distance traveled, (C) average speed morphine, (D) average speed methadone.