Effects of the NMDA receptor channel blockers memantine and MRZ 2/579 on morphine withdrawal-facilitated aggression in mice

Rationale: Opioid withdrawal is known to facilitate aggressive behavior in laboratory rodents. Aggression develops as the somatic signs disappear and thus may reflect protracted withdrawal-related behavioral alterations. Antagonists acting at the NMDA receptor are known to attenuate the expression of morphine withdrawal syndrome in laboratory animals. Objective: The present study aimed to evaluate the effects of low-affinity NMDA receptor channel blockers (memantine and MRZ 2/579) on aggression facilitated by morphine withdrawal in mice. Methods: Significant increases in aggressive behavior were observed 48 h after repeated morphine administration (8 days, b.i.d., 10–80 mg/kg, SC) was discontinued. Separate groups of mice were treated intraperitoneally with vehicles or different doses of memantine (1, 3, 10 or 30 mg/kg) or MRZ 2/579 (1, 3 or 10 mg/kg) 48 h after the last morphine injection. Results: Both compounds dose-dependently reduced the expression of aggressive behavior while having no significant effect upon the intensity of non-aggressive social contacts. Memantine significantly diminished the occurrence of all recorded components of aggressive behavior (attacks/bites, threats, tail rattling) while MRZ 2/579 affected mainly the appetitive events of aggressive bursts (threats, tail rattling). For both compounds, anti-aggressive effects occurred at dose levels that did not produce motor impairment in the Rotarod test. Conclusions: Taken together with the evidence on the lack of selective anti-aggressive effects of these drugs in morphine-naive mice, attenuation of the aggression observed in the present study may be due to specific interaction with morphine withdrawal-triggered processes. Received: 2 May 1999 / Final version: 17 November 1999