RATIONALE

Marble burying and forced swimming behavior are widely used and sensitive tests for identifying clinically effective antidepressant drugs, although the underlying neurobiology of these behaviors is not fully elucidated.

OBJECTIVES

The objective of this study was to determine the relationship between the behavioral effects of antidepressant drugs and their ability to modulate extracellular neurotransmitter levels in the prefrontal cortex.

MATERIALS AND METHODS

The effects of fluoxetine, fluvoxamine, citalopram, imipramine, and desipramine (0 to 60 mg/kg by oral gavage, except fluoxetine at 0 to 40 mg/kg) were studied independently in CD-1 mice in the marble-burying task, forced swim task and on extracellular concentrations of serotonin, norepinephrine, and dopamine in the prefrontal cortex by freely moving microdialysis.

RESULTS

Fluvoxamine, fluoxetine, and citalopram all suppressed marble-burying behavior, but produced no change in immobility time in the forced swim test. In contrast, imipramine and desipramine suppressed both marble-burying behavior and increased swimming time in the forced swim test, although desipramine mildly suppressed locomotor activity at the maximal dose. Fluvoxamine, fluoxetine, and citalopram all increased extracellular levels of cortical serotonin. Desipramine and imipramine increased extracellular dopamine levels. Fluoxetine, desipramine, and imipramine increased extracellular norepinephrine levels. Correlational analysis revealed a positive correlation between efficacy of drugs in the forced swim test and cortical extracellular dopamine levels, whereas a positive correlation was found between efficacy in the marble-burying test and extracellular serotonin levels.

CONCLUSIONS

Although marble burying and forced swimming behavior have strong predictive validity in tests of antidepressant action, each assay appears to be underpinned by entirely different neurochemical systems.