Self-inhibiting action of nortriptyline's anti-immobility effect at high plasma and brain levels in mice.

Animals were treated acutely with 0, 2.5, 5, 10, 20 and 40 mg/kg nortriptyline (NT) 30 min before the tail suspension test (TST). They were sacrificed after test for evaluation of plasma and brain levels of NT. The anti-immobility effect increased with increasing doses and concentrations of the drug, reaching statistical significance (P less than 0.01, Dunnett test) at a dose of 20 mg/kg, 865 ng/ml in plasma and 11 micrograms/g in brain tissue. The anti-immobility effect was, however, blocked with the highest, non-toxic, concentrations. Results seem to indicate a biphasic curvilinear relationship between plasma and brain levels of NT and behaviour in mice.