Progesterone does not inhibit aggression induced by testosterone metabolites in castrated male mice

Sufficient data exist indicating that testosterone (T) or metabolites of T, i.e., dihydrotestosterone (DHT) and estrogen (E), induce aggressive behavior. The effects of T have also been shown to be suppressed by concurrent administration of progesterone (P). It is unknown if the effects of P are exerted on T directly or upon the T metabolites. The objective of this experiment was to evaluate the inhibitory effects of P on aggression induced by T and T metabolites. Seven male mice were randomly allocated to one of the following treatment groups: castration (C), C + T, C + 5 alpha-dihydrotestosterone (DHT), C + estrogen (E), C + DHT + E (DE), T + P, and DE + P. The males were then placed with androgenized females in three 10-min trials to determine the effects of each treatment on hormone induced aggression. The degree of aggressive behavior displayed was measured by number of trials where attacks occurred (N), mean latency to attack (L), frequency of attacks (AF), duration of attacks (D), and a composite index score (INDEX). All androgen treated groups showed significantly higher degrees of aggression compared to C and T + P animals. P inhibited aggression in T-treated males but did not inhibit aggression in the DE treated group. The data suggest that P inhibits the aggressive effects of T by acting on receptors of the metabolite-producing enzymes or by direct competitive binding for 5 alpha-reductase.