EFFECTS OF MORPHINE HYDROCHLORIDE ON SOCIAL ENCOUNTERS BETWEEN MALE-MICE

The effects of a single injection of morphine hydrochloride (0.3, 0.6, or 1.25 mg/kg) or physiological saline (0.9% NaCl) on the agonistic behaviour elicited by isolation in male mice were examined. Individually housed mice were exposed to anosmic ''standard opponents'' 30 minutes after the drug administration, and the encounters were videotaped and evaluated using an ethologically based analysis. Morphine (at 0.6 and 1.25 mg/kg) significantly and dose-dependently decreased time spent in offensive (''threat'' and ''attack'') and ''digging'' behaviours but markedly increased ''non-social exploration'' without a significant increase of ''immobility.'' The lowest dose was completely ineffective in producing changes in any of the behaviours studied. It is concluded that these results present a specific ethopharmacological profile characterized by suppression of aggressive behaviour, increase in non-social exploration, and no evident impairment of motor activity. (C) 1993 Wiley-Liss, Inc.