Effect of type of opponent on aggression in male mice with particular reference to studies with antihormones

The potential influence of the type of opponent used in intermale aggression encounters to assess the actions of drugs was examined. Two experiments were carried out, one with the antiandrogen Cyproterone Acetate and the other with the antioestrogen CI-680 (both administered every three days over 25 days). In both experiments the antihormone-treated subjects encountered different opponents, namely: a) an antihormone-treated male, b) a non aggressive anosmic male or c) a vehicle-treated male. Vehicle-treated subjects also confronted a vehicle-treated or an anosmic opponent. The behaviour displayed by antihormone-treated subjects varied according to the characteristics of the partner, suggesting that the effects of drugs might be interpreted differently depending on the type of animal employed as an adversary. In fact, some of the apparently contradictory results reported in the literature seem to be consequences of the utilisation of different kinds of opponents. It is concluded that the choice of the opponent is of paramount importance in the study of drug actions on intermale aggression tests and that using more than one type of opponent can provide more complete information about the actions of particular drugs.