Behavioural consequences of p-glycoprotein deficiency in mice, with special focus on stress-related mechanisms.

P-glycoprotein (P-gp), an efflux transporter localised in the blood-brain barrier, limits the access of multiple xenobiotics to the central nervous system. Whether it is also implemented in the transport of the endogenous glucocorticoid corticosterone is a matter of debate. The P-gp knockout mouse model [abcb1a/b (-/-)] has been shown to differ in the functioning of the hypothalamic-pituitary adrenal (HPA) axis. In the present study, we investigated the behaviour of abcb1a/b (-/-) and wild-type mice with respect to stress-related tests and the effects of corticosterone. Behavioural activities were assessed in the open field (OF) test for 4 days, and in the forced swimming test (FST) and tail suspension test (TST) under naïve and stressed conditions. The FST was also conducted after exogenous corticosterone injection (0.25 and 2.5 mg/kg). Moreover, the elevated plus maze test and the RotaRod test (RotaRod Advanced; TSE Systems, Bad Homburg, Germany) were assessed. Brain corticosterone levels were determined by an immunoassay and expression of glucocorticoid receptors by western blot analysis. Abcb1a/1b (-/-) mice showed significantly decreased brain corticosterone levels and elevated glucocorticoid receptor expression. Behavioural analysis revealed a significantly decreased activity in the OF test on the first 2 days in abcb1a/1b (-/-) mice compared to wild-type mice, although the differences disappeared under habituation. Immobility time in the FST was significantly decreased in abcb1a/1b (-/-) mice under basal and under stressed conditions, whereas immobility in the TST was significantly elevated in these mice under all conditions. Injection of exogenous corticosterone resulted in significant reductions of immobility in the FST in abcb1a/1b (-/-) mice, whereas wild-type mice did not respond to the same doses. There were no differences in the elevated plus maze test and RotaRod test. The results obtained in the present study demonstrate that a P-gp deficiency has an impact on the stress-related behaviour, possibly as a result of differences in HPA axis-feedback regulation.