Question 1

What is the tail suspension test (TST) and what does it measure?

Accepted Answer

The tail suspension test (TST) is a behavioral despair paradigm used to assess depression-like behavior in mice. Developed by Steru et al. (1985), the mouse is suspended by its tail from a horizontal bar approximately 50 cm above the floor for a standard 6-minute session. The animal initially engages in vigorous escape-oriented movements (struggling, body torsion, running movements) but progressively adopts an immobile posture. Immobility is interpreted as a state of behavioral despair — the animal has ceased active attempts to escape the inescapable stressor. The primary measure is total immobility time or percent immobility, with antidepressant drugs characteristically reducing immobility.

Question 2

How is immobility defined in the TST?

Accepted Answer

Immobility in the TST is defined as the absence of any initiated movement — the animal hangs passively and completely motionless. Small movements caused by momentum (swinging) from prior struggling are not counted as active movement. Trained observers or automated systems (e.g., force transducers, video tracking) typically require a minimum continuous immobility duration of 1-2 seconds to avoid counting brief pauses between active movements. When using force transducers, immobility is detected when the variance in the force signal drops below a calibrated threshold, indicating the animal has stopped moving.

Question 3

What is the standard TST session duration and why 6 minutes?

Accepted Answer

The standard TST session is 6 minutes (360 seconds), established by Steru et al. (1985) in the original description of the test. This duration was chosen because immobility typically develops within the first 1-2 minutes and stabilizes by 3-4 minutes, providing a reliable window for measuring drug effects. Some protocols analyze only the last 4 minutes (minutes 3-6) to allow for an initial habituation period, though total session immobility is the most common primary endpoint. Shorter (4-minute) or longer (10-minute) sessions have been used, but 6 minutes remains the standard and is recommended for comparability across studies.

Question 4

How does the TST differ from the forced swim test (FST)?

Accepted Answer

Both the TST and FST are behavioral despair paradigms that measure immobility as an index of depression-like behavior, but they differ in important ways. In the TST, the mouse is suspended by its tail (dry, no thermoregulation confound), while in the FST, the animal is placed in a cylinder of water (swimming, with hypothermia risk). The TST is predominantly used in mice (rats tend to curl up and climb their tails), while the FST works in both species. The TST avoids the confound of water temperature affecting immobility and does not require drying/warming animals afterward. The TST may be more sensitive to serotonergic antidepressants, while the FST may better detect noradrenergic compounds. Many labs run both tests in the same cohort for complementary data.

Question 5

What is latency to first immobility and why is it important?

Accepted Answer

Latency to first immobility is the time (in seconds) from the start of the session until the animal first becomes immobile for the minimum bout duration. It reflects how quickly the animal transitions from active coping (struggling) to passive coping (immobility). Antidepressant treatments typically increase the latency to first immobility — treated animals persist in active escape attempts for longer before giving up. Latency is a useful secondary outcome because it is less sensitive to ceiling/floor effects than total immobility time and can detect drug effects even when total immobility differences are marginal.

Question 6

What is tail climbing and how does it affect TST scoring?

Accepted Answer

Tail climbing occurs when a mouse curls upward and grabs or climbs its own tail during the TST, which is particularly common in certain strains (e.g., C57BL/6) and in rats (which is why the TST is primarily a mouse test). Tail climbing is an active behavior but represents a qualitatively different response from normal struggling — the animal is not attempting to escape the suspension per se. Most protocols exclude tail-climbing episodes from both the immobility and the active movement categories, or apply adhesive tape to the tail to prevent climbing. Automated force-transducer systems can detect climbing as a distinctive force signature. If tail climbing is frequent, it should be reported and its handling described in the methods.

Question 7

Can I use time-bin analysis to examine temporal patterns in the TST?

Accepted Answer

Yes. Dividing the 6-minute session into 1-minute bins (or other intervals) reveals the temporal dynamics of immobility development. Normal mice show increasing immobility across bins as active escape attempts wane. Drug-treated animals may show a delayed onset of immobility (flat or low immobility in early bins) or reduced immobility in later bins compared to vehicle controls. Time-bin analysis can reveal drug effects that are masked by total immobility scores — for example, a drug that delays onset but does not reduce terminal immobility. This calculator computes per-bin immobility automatically.

Tail Suspension Test Immobility Calculator.

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What Is the Tail Suspension Test?

Scoring Immobility in the TST

TST vs. FST: Choosing the Right Despair Test

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