What Is the Tail Suspension Test?
The tail suspension test (TST) was introduced by Steru, Chermat, Thierry, and Simon in 1985 as a mouse model of behavioral despair. A mouse is suspended by its tail from a horizontal bar, typically using adhesive tape, approximately 50 cm above the surface below. The test is based on the observation that rodents placed in an inescapable stressful situation will, after an initial period of vigorous activity, adopt an immobile posture. This immobility is interpreted as a failure to persist in escape-directed behavior, i.e., behavioral despair. The TST has become one of the two most widely used screening tests for antidepressant activity in mice, alongside the forced swim test (FST). The test is valued for its simplicity, reliability, sensitivity to all major classes of antidepressants (SSRIs, SNRIs, tricyclics, MAOIs, atypicals), and its suitability for high-throughput screening. Unlike the FST, the TST does not involve water exposure, eliminating confounds related to thermoregulation, swimming ability, and post-test drying. Standard parameters include total immobility time, percent immobility, and latency to first immobility episode.
Scoring Immobility in the TST
Immobility can be scored manually by trained observers or automatically using force-transducer systems or video-based tracking. Manual scoring involves continuous or time-sampled observation during the 6-minute session, with the observer pressing a key or timer whenever the mouse is immobile. Inter-rater reliability should be established (Cohen kappa > 0.80) before collecting experimental data. Automated force-transducer systems attach a strain gauge to the suspension bar and record the force fluctuations caused by the animal's movements. When the animal is immobile, force variance drops to near zero; thresholds are calibrated against manual scoring. Video-based systems (e.g., ConductVision) use computer vision to detect movement from video frames, applying pixel-change or pose-estimation algorithms. Regardless of method, a minimum immobility bout duration of 1-2 seconds is applied to filter brief pauses. The key metrics are: total immobility time (seconds), percent immobility (immobility time / session duration x 100), latency to first immobility (seconds from session start), and number of immobility bouts. Time-bin analysis (typically 1-minute bins) reveals the temporal pattern of immobility onset and can detect drug effects not apparent in total scores.
TST vs. FST: Choosing the Right Despair Test
The tail suspension test (TST) and forced swim test (FST) are the two canonical behavioral despair paradigms, and choosing between them depends on species, strain, and experimental goals. The TST is exclusively a mouse test — rats and heavier mice tend to climb their tails, making scoring unreliable. The FST works in both rats and mice. The TST is a dry test with no water immersion, eliminating hypothermia and swimming-ability confounds. The FST requires precise water temperature control (typically 23-25 C) and post-swim warming. In terms of pharmacological sensitivity, the TST is broadly sensitive to antidepressants and has been validated for SSRIs (fluoxetine, paroxetine), SNRIs (venlafaxine), tricyclics (imipramine, desipramine), MAOIs (tranylcypromine), and atypical antidepressants. Some studies suggest the TST may have higher sensitivity to serotonergic compounds, while the FST (particularly the modified Cryan scoring with swimming, climbing, and immobility) provides additional behavioral categorization that can distinguish serotonergic from noradrenergic mechanisms. For comprehensive phenotyping, many labs run both the TST and FST in the same cohort (with appropriate spacing between tests) to provide converging evidence. Strain considerations are critical: C57BL/6J mice show moderate baseline immobility in both tests, while BALB/cJ mice show high baseline immobility, and some strains (e.g., Swiss Webster) may show excessive tail climbing in the TST.
