Behavioural differences between artificially selected aggressive and non-aggressive mice: response to apomorphine

The present study reports a first attempt to unravel the neurochemical background that underlies the difference in behavioural profiles between aggressive and non-aggressive male mice. For this purpose two bidirectionally selected lines for attack latency (SAL and LAL) were used. In pursuit of Cools'9 approach, the susceptibility of individuals of both selection lines to the dopamine agonist apomorphine was measured. The apomorphine was injected subcutaneously at dose levels of 2.5 and 5.0 mg/kg. Apomorphine is considered to stimulate the dopamine receptors in the telencephalon and induces stereotyped behaviour. The responsivity to apomorphine can be rated as a total stereotypy-score. SAL (aggressive) mice showed a significantly greater enhancement of stereotyped behaviour in response to apomorphine than LAL (non-aggressive) mice. In addition, it was demonstrated that this difference is of a quantitative rather than qualitative character. Pharmacokinetic variation between the two lines could be ruled out as cause of the difference. Hence, it was concluded that SAL mice are more sensitive to apomorphine than LAL males, which provides evidence for a difference in the dopaminergic system between the two selection lines. It was suggested that this difference underlies the difference in flexibility in behaviour between aggressive and non-aggressive male mice.