Fig 1.
Components of the TailTimer device for use in the tail immersion assay.
[A] The Raspberry Pi 3 computer. [B] The TailTimer software in operation. [C] Thermal probe to detect water temperature. [D] Electrical ground wire that remains immersed in the water with the thermal probe (C). [E] Scannable RFID implanted subcutaneously in the rat for identification. [F] Electrical latency wire to be dipped into and withdrawn from the water simultaneously with the rat’s tail to start and stop the timer, respectively. [G] USB RFID scanner. [H] Scannable RFID command fobs used to navigate the TailTimer program in place of a keyboard and mouse. The six necessary fobs are used to enter the user ID, set the target temperature, start a new rat, test the same rat again, delete the last latency, and exit the program.
Fig 2.
Latency varies across water conditions.
[A] Tail withdrawal latency measured at different water temperatures. Each data point (+) represents one of the 2–3 latency measures obtained for each individual rat at each temperature tested. As water temperature is decreased, latency is lengthened. Linear regression indicated that temperature explains 63% of the variance in latency when tested between 47–50°C with the water mixing at a constant, low speed. [B] Tail withdrawal latency measured at a constant temperature (48°C ± 0.25) and different water mixing speeds. Lengthening of latency occurs as the water mixing speed is decreased with the longest latencies occurring when the water is still (i.e., not being mixed by the stir bar). Data are expressed as mean ± SEM; **p = 0.001, ***p < 0.001.
Fig 3.
TailTimer detects acute changes in tail withdrawal latency following oral oxycodone exposure.
Pain thresholds of SD males (n = 8) were evaluated at 15 min, 1 h, and 4 h post-gavage of 3 mg/kg oxycodone or distilled water at a controlled water temperature of 48°C ± 0.25 and a fixed, low water mixing speed (setting 1). Latencies were significantly longer at 15 min and 1 h, but not 4 h, post-gavage of oxycodone versus distilled water. Following gavage of the distilled water control, latencies were not significantly different from baseline at any time point. Mean latencies reflect the average of the (two–four) measurements per individual rat averaged across timepoints. Data are expressed as mean ± SEM; ***p < 0.001.
Fig 4.
TailTimer detects strain differences in baseline pain thresholds.
The tail immersion assay was performed to determine baseline tail withdrawal latencies for males of the outbred SD strain (n = 8) and of the WLI (n = 7) and WMI (n = 6) inbred strains. Baseline measurements were collected for each rat on two separate, consecutive days. Mean latencies reflect the average of the (four–eight) tests per individual rat, averaged across strain. Data are expressed as mean ± SEM; **p = 0.006, ***p < 0.001.