Acute versus repeated administration of desipramine in rats and mice: relationships between brain concentrations and reduction of immobility in the swimming test.

Immobility scores in the swimming test and brain concentrations of desipramine were determined in rats and mice following repeated injection of the antidepressant versus acute administration of either a behaviorally effective or ineffective dose of the drug. Five injections (IP) of desipramine (each injection being performed at the measured T1/2 of the drug in the brain) reduced immobility scores by 30%, whereas this regimen resulted in brain drug concentrations not different from those obtained after a single, behaviorally ineffective dose of desipramine. It is suggested that the enhanced "antidepressant" response such as that frequently observed in animals after repeated injection of imipramine-like drugs does not involve accumulation of the drug in the brain.