Tail Immersion Test

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The tail immersion assay is a thermal test for evaluating the analgesic potential of compounds. 

A number of clinically approved pharmacological agents have been demonstrated to delay the onset of heat sensitivity upon tail exposure to heat including opioids such as morphine, and alpha adrenergic compounds. 

The tail immersion test has been reputed to measure spinally driven aspects of pain and has the advantage of measurements not being affected by sedation as they would be in other assays such as the hot plate test.

The study summarized below evaluated the time course of the analgesic effects of morphine and oxycodone against vehicle.

Tail withdraw latency of mice treated with morphine or oxycodone during the tail immersion test.

Tail Immersion Assay. The distal half of each mice tail was placed in a water bath. The tail withdraw latency was recorded with a 10 second time-out. An increased sensitivity to thermal stimuli was displayed in mice that received vehicle treatment, shown by a shorter latency to pain response.

Mice were tested twice for baseline response at 30 minutes prior to drug administration. After baseline, mice were administered vehicle, morphine or oxycodone and tested at 10, 20, 30, 45, 60, 90 and 120 minutes after drug administration. A maximum score was assigned (100%) to animals not responding within 10 seconds to avoid tissue damage.

Oxycodone and morphine both significantly increased the latency to pain response, with a majority of the animals in these two groups timing out at all time points. The oxycodone and morphine treated mice had significantly higher pain thresholds up to two hours post dose. The vehicle treated animals showed a significantly lower pain threshold over the duration of the study. Data are mean ± SEM; ****p<0.0001 compared to vehicle.

The mouse tail immersion model shows a good response in measuring pain reactions to thermal stimuli. Using various time points up to two hours post-dose, mice in the vehicle group displayed an increased sensitivity throughout the duration of the study. The Morphine and Oxycodone groups both exhibited a significant decrease in sensitivity to thermal stimuli, with the majority of the mice in both groups reaching the maximum latency for tail withdraw