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

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.

Ehrlich carcinoma is a well-known transplantable tumor in which cells from mammary adenocarcinoma are inoculated subcutaneously or intraperitoneally, and grow into either solid or ascitic tumors, respectively.^6,8 This tumor is considered to cause pain yet is widely used to determine the influence of drugs and other therapeutic substances on the inhibition of tumor growth.^23,27,33 The ascitic form of Ehrlich carcinoma is characterized by a proinflammatory response induced by tumor cells in the peritoneum and increased vascular permeability.⁴¹ Tumor cells promote a progressive increase in the secretion of interleukin-1β (IL1β),¹⁶ monocyte chemoattractant protein-1 (MCP-1)⁵⁴ and prostaglandin E2 (PGE-2),²⁸ substances all related to the phenomenon of hyperalgesia.¹⁵Recognition and management of pain are an important component of international standards designed to ensure the welfare of research animals. These factors are closely related to the survival and quality of life.^34,55 Although a test to directly measure pain in animals is currently unavailable, changes in behavioral patterns can indicate pain (for example, agitation, reduced ambulation, and changes in the sequence and frequency of self-grooming and vocalization).⁹ Opioid analgesics, such as morphine and methadone, although frequently regarded as the most effective treatment of cancer pain,^12,38,53 tend to alter locomotor activity and exploratory behavior in mice.^22,43,47 Although morphine alters behavioral patterns, it does not interfere with facial expression in the absence of pain.³⁰Morphine is a potent opioid that acts mainly through the occupation of pre- and postsynaptic µ-opioid receptors, which modulate the perception of pain.⁵² Methadone has affinity for µ-receptors, is also an antagonist of N-methyl-D-Aspartate receptors (NMDA), and is considered an ideal treatment choice in cases of tolerance to morphine.²⁰ The condition of cancer pain requires long-term analgesic treatment. However, some disagreement remains regarding the optimal doses and frequency of administration of morphine and methadone in mice, and few studies have evaluated the effects of these drugs on cancer pain in mice or the effect of long-term administration.^35,39,43,46The current study aimed to evaluate the analgesic effect of morphine and methadone in BALB/c mice with Ehrlich ascitic carcinoma by observing the influence of these opioids on behavior. The hypothesis was that morphine and methadone would provide analgesia and mitigate pain-related behavioral changes in mice with Ehrlich ascitic carcinoma in a dose-dependent manner.