Mineralocorticoid receptor blockade inhibits aggressive behaviour in male rats

Previous experiments have demonstrated that aggressive behaviour of male rats in a territorial setting is facilitated by corticosterone. Moreover, the inhibition of the endogenous corticosterone response prevents agonistic behaviour. The aim of the present paper was to investigate the effect of mineralocorticoid receptor (MR) blockade on the expression of aggressive behaviour at the beginning of the dark phase, when MRs are mostly occupied due to the diurnal peak of corticosterone secretion. High levels of aggressive behaviour were induced in male Wistar rats cohabiting with females by exposing them 3 times to an intruder male rat of smaller size. Intruder males were introduced at the beginning of the active period on every second day for 20 minutes, while the female was temporarily removed. A gradual increase in the number of biting attacks was noticed, the rats performing 6.7 +/- 2.0 attacks per 20 min on the last day (n=8). One hour before the fourth encounter rats were injected with the MR blocker spironolactone (10 mg/kg). Attacking behaviour was almost totally abolished (0.87 +/- .35 attacks per 20 min; n=8). Vehicle injections were ineffective (9.3 +/- 2.1 attacks per 20 min; n=8). Offensive threats underwent similar changes while other behaviours showed non-significant variation, with the exception of resting which increased towards the end of the observation period. The time course of these effects showed that the primary action was on offensive aggressive behaviour. This report is the first to show that the almost total MR occupancy at the beginning of the dark (active) period of the day is a prerequisite for the expression of aggressiveness in response to a social challenge.