ELEVATED PLUS MAZE

ELEVATED PLUS MAZE

Primary Disciplinary Field(s): Behavioral Neuroscience, Psychopharmacology, Experimental Psychology

1. Core Definition and Function

The Elevated Plus Maze (EPM) is a widely utilized scientific apparatus and behavioral assay developed primarily for testing anxiety-like behaviors in small laboratory animals, most commonly rats and mice. This standardized test exploits the natural conflict inherent in rodents between their innate tendency to explore novel environments and their aversion to open, elevated spaces. The EPM is regarded as one of the most reliable and efficient measures for screening the effects of pharmacological agents, genetic manipulations, or environmental stressors on anxiety levels, thereby serving as a crucial tool in the development of anxiolytic (anxiety-reducing) and anxiogenic (anxiety-inducing) drugs.

In essence, the EPM functions on the principle that animals experiencing higher levels of fear or anxiety will actively avoid exposure to potential danger, which, in the context of the maze, is represented by the open arms. Conversely, animals exhibiting lower anxiety, or those treated with an effective anxiolytic compound (such as a benzodiazepine), will demonstrate increased exploration of the risky, open areas. This behavioral output provides quantitative data regarding emotional state, making the EPM a cornerstone methodology in preclinical neuropsychiatric research.

The procedure is relatively simple and non-invasive, relying on ethologically relevant behaviors rather than requiring extensive prior training or conditioning. Its ability to discriminate between generalized locomotor activity and specific anxiety-related behavior is paramount, allowing researchers to accurately attribute observed changes to alterations in emotional processing rather than mere changes in movement or motivation.

2. Apparatus Design and Specifications

The physical design of the Elevated Plus Maze is central to its effectiveness and standardized application. The apparatus consists of four arms extending from a central platform, forming a shape similar to a plus sign (+). Crucially, the maze is raised high above the floor, typically 50 to 100 centimeters, which contributes significantly to the animal’s fear response.

The four arms are structurally differentiated into two pairs: two opposing arms are designated as open arms, lacking surrounding walls or barriers, and the other two opposing arms are designated as closed arms, enclosed by high walls (usually 40 cm tall). The floor of all arms and the central platform are typically narrow (around 10 cm wide). This design creates an explicit conflict zone: the closed arms offer shelter and safety, satisfying the rodent’s thigmotactic tendency (the preference for movement along vertical surfaces), while the open arms present a potential threat (exposure and elevation) but also represent the novel environment they are driven to explore.

Standardization of size, color (often black or gray to minimize visual distraction), lighting conditions, and elevation is critical for ensuring experimental reproducibility across different laboratories. Variations in these parameters can dramatically affect baseline anxiety levels observed in control animals. For instance, extremely bright lighting conditions tend to increase the aversion to the open arms, thus magnifying anxiety-like behavior.

3. Underlying Behavioral Principles and Ethology

The validity of the EPM as an anxiety assay rests upon specific, evolutionarily conserved behavioral principles. Rodents, as prey animals, possess an inherent fear of high, exposed places, which is intensified by the absence of vertical walls—a phenomenon known as the fear of heights/open spaces. This anxiety is juxtaposed against the natural drive for exploration, which is necessary for locating food, mates, and new shelter.

Anxiety, in this context, is measured as the result of a conflict experienced by the animal. When placed on the maze, the animal must choose between remaining in the dark, safe, closed arms (reducing anxiety) or entering the open arms (increasing anxiety but satisfying the exploratory drive). The proportion of time spent in the open arms is therefore inversely proportional to the animal’s perceived anxiety level. A highly anxious animal will prioritize safety, remaining sequestered in the closed arms for the majority of the testing period.

Furthermore, the EPM differentiates anxiety from general depression or sedation. If an animal is merely sedated, its overall movement (locomotor activity, measured by total entries into all arms) will decrease, but the ratio of open arm time to total time might remain similar to the control group. Conversely, a true anxiolytic drug will specifically increase the relative time and entries into the open arms without necessarily affecting overall mobility, indicating a reduction in fear rather than just motor function impairment.

4. Methodology and Experimental Procedures

The protocol for the Elevated Plus Maze test involves careful handling and precise observation to ensure accurate data collection. Typically, the experimental session lasts for a fixed duration, most commonly five minutes.

The procedure begins by gently placing the test animal (rat or mouse) onto the central platform, facing one of the open arms. This placement minimizes initial bias. For the duration of the test, the experimenter, often observing from a distance or using automated video tracking software, records several key dependent variables. It is imperative that the testing room is quiet and the environment is consistent to avoid introducing extraneous stressors that could confound the results. Following each trial, the maze must be thoroughly cleaned with a non-residual cleaning solution (e.g., alcohol solution) to remove scent markers that could influence the behavior of the subsequent animal.

Modern implementations often utilize sophisticated computer tracking systems, such as video-based analysis software, to automatically quantify the animal’s position and movement patterns. This minimizes human error, ensures precise timing, and allows for the calculation of more complex metrics, such as speed and specific paths taken, in addition to the traditional time and entry counts.

5. Interpretation of Results and Key Metrics

Accurate interpretation of EPM data relies on analyzing specific behavioral metrics that reflect anxiety-like behavior versus general activity. The primary indicators of reduced anxiety (anxiolytic effect) are:

• Percentage of Time Spent in Open Arms: This is the ratio of time spent in the open arms relative to the total time spent in all arms (excluding the center platform time, in some protocols). An increase in this percentage is the strongest indicator of anxiolysis.
• Percentage of Entries into Open Arms: This metric measures the frequency of exploring the risky areas. An increase suggests that the animal is less inhibited by fear when choosing new paths.
• Total Arm Entries: The sum of entries into all four arms (open and closed). This variable serves as an index of overall locomotor activity or general exploration. Changes in this metric, without corresponding changes in the open arm percentages, may indicate sedation or hyperactivity rather than a specific anxiety effect.

Researchers look for a dissociation between these measures. For example, effective anxiolytic agents like Diazepam (Valium) typically increase the percentage of time and entries into open arms, often without significantly altering the total number of entries. Conversely, anxiogenic compounds, such as certain stress hormones or inverse agonists of benzodiazepine receptors, cause a marked decrease in open arm exploration relative to controls.

6. Validation, Reliability, and Historical Development

The Elevated Plus Maze was originally developed by S.P. Handley and colleagues in 1985 as a simplification and improvement upon earlier conflict tests, such as the open-field test. Its immediate acceptance stemmed from its ability to rapidly and reliably screen compounds previously validated using more complex and time-consuming tests like the Geller-Seifter procedure.

The EPM’s predictive validity is high because its results correlate strongly with clinically established anxiolytic drugs. Compounds known to reduce human anxiety consistently increase open arm exploration in the EPM. Furthermore, the EPM demonstrates good construct validity, as stress manipulations (e.g., chronic mild stress or social defeat) reliably decrease open arm exploration, mirroring increased anxiety-like states.

However, reliability is highly dependent on strict standardization of the environment. Variables such as the time of day, intensity of ambient light, noise levels, and even the gender and experience of the experimenter can introduce variability. Consequently, rigorous adherence to published protocols and the use of appropriate control groups are essential prerequisites for generating reliable and reproducible scientific data.

7. Variations and Related Methodologies

Due to subtle limitations of the standard EPM, several variations have been developed to refine the measurement of anxiety and mitigate potential confounding factors:

• The Elevated Zero Maze (EZM): This variation is a continuous, ring-shaped track elevated off the ground, where two opposing quadrants are open and two are enclosed. This design eliminates the central platform found in the EPM, addressing the critique that the central platform acts as a third, ambiguous zone (neither safe nor dangerous), potentially skewing results as animals tend to spend disproportionate time there. The EZM forces animals into immediate proximity to a decision point regarding safety or exposure.
• The Elevated T-Maze: Used primarily to study inhibitory avoidance and memory, rather than immediate anxiety. It tests the animal’s ability to remember and avoid a previously punished location.
• The Elevated Plus-Maze with Additional Stimuli: Some protocols introduce supplementary aversive stimuli, such as a bright light or an air puff, specifically targeting the open arms to increase the sensitivity of the assay, particularly when testing subtle genetic manipulations.

These modifications allow researchers to tailor the conflict intensity and complexity of the task based on the specific pharmacological or behavioral questions being investigated, providing a broader methodological toolkit for anxiety research.

8. Debates and Limitations

Despite its widespread use, the Elevated Plus Maze is not without methodological criticisms and limitations, primarily centered on the interpretation of observed behavior:

One major limitation is the inherent difficulty in distinguishing true anxiety reduction from other behavioral effects, such as increased motivation, impulsivity, or impaired motor coordination at high drug doses. While the total entries metric attempts to address locomotor changes, subtle impairments can still affect performance ratios. Furthermore, the test primarily measures acute, unconditioned fear responses rather than learned fear or chronic anxiety states, which may require different assays (like the conditioned fear paradigm).

Another significant debate concerns the impact of previous handling and habituation. Highly stressed or inexperienced animals may exhibit ceiling effects (maximal avoidance of open arms), masking potential anxiolytic effects of tested compounds. Conversely, overly habituated animals may show floor effects (minimal anxiety), making it difficult to detect anxiogenic effects. Researchers must carefully control pre-test procedures to ensure the baseline anxiety level is appropriate for the experimental question. Finally, the observed behaviors, while consistent across rodents, represent anxiety-like behaviors in an artificial environment and must be cautiously extrapolated to human clinical anxiety disorders.

Further Reading

• Elevated plus maze (Wikipedia)
• The elevated plus-maze: pharmacologic validation and practical guidelines.
• Ethological Approach to Elevated Plus Maze