Chronic cocaine enhances defensive behaviour in the laboratory mouse: involvement of D2 dopamine receptors

C57BL/6 male mice injected with a challenge dose (20 mg/kg) of cocaine 72 h after the end of chronic intermittent treatment with the psychostimulant (two daily injections of 20 mg/kg for 10 days) exhibited a clear-cut increase in defensive upright and sideways postures and escape when confronted with a non-drugged conspecific. Treated mice spent 40% of time showing defensive acts over the 5-min testing session. Administration of the selective D2 receptor antagonist (-)-sulpiride (25 mg/kg) before the challenge dose of cocaine completely antagonized the increase in defensive behaviour, while the selective D1 receptor antagonist SCH 23390 (0.25-0.50 mg/kg) did not significantly affect defensive behavioural patterns. These results suggest the involvement of D2 receptors in cocaine-induced hyperdefensiveness. The hypothesis that alteration in D2 receptor functioning produced by chronic cocaine administration may produce hyperdefensiveness possibly due to altered perceptive processes is discussed.