The influence of non-nociceptive factors on hot-plate latency in rats

Abstract

The hot plate is a widely used test to assess nociception. The effect of non-nociceptive factors (weight, sex, activity, habituation, and repeated testing) on hot-plate latency was examined. Comparison of body weight and hot-plate latency revealed a small but significant inverse correlation (light rats had longer latencies). Habituating rats to the test room for 1 hour prior to testing did not decrease hot-plate latency except for female rats tested on days 2 to 4. Hot-plate latency decreased with repeated daily testing, but this was not caused by a decrease in locomotor activity or learning to respond. Activity on the hot plate was consistent across all 4 trials, and prior exposure to a room-temperature plate caused a similar decrease in latency as rats tested repeatedly on the hot plate. Despite this decrease in baseline hot-plate latency, there was no difference in morphine antinociceptive potency. The present study shows that weight, habituation to the test room, and repeated testing can alter baseline hot-plate latency, but these effects are small and have relatively little impact on morphine antinociception.

Perspective: This manuscript shows that non-nociceptive factors such as body weight, habituation, and repeated testing can alter hot-plate latency, but these factors do not alter morphine potency. In sum, the hot-plate test is an easy to use and reliable method to assess supraspinally organized nociceptive responses.

Figure 1

Inverse correlation between body weight and hot plate (HP) latency. There is a slight but significant decrease in hot plate latency as body weight increases (r = -0.26). This correlation breaks down for rats over 300 g.

Figure 2

Effect of sex and habituation to the test room on hot plate (HP) latency. There was a significant decrease in hot plate latency from Trial 1 to 4 (F(3,174) = 15.613, p < .05). There was no difference in hot plate latency between male and female rats on Trial 1. Habituation to the test room for 1 hr had no effect on hot plate latency on Trial 1 in female rats, but caused a shorter hot plate latency on Trials 2 – 4 compared to male rats or non-habituated (Naïve) female rats. Habituation to the test room had no effect on hot plate latency for male rats. Sample size varied from 14 – 16 rats in each group.

Figure 3

No change in locomotor activity on the hot plate in male or female rats across trials. Locomotion on the hot plate was consistent across trials whether measured as the average time to cross to a new quadrant (Figure A; hot plate latency/crosses) or the number of crosses during the first 6 s of the test period (Figure B). These data indicate that changes in activity are not responsible for the decrease in hot plate latency with repeated testing.

Figure 4

Decrease in hot plate latency following pre-exposure to a room temperature plate. The decrease in hot plate (HP) latency with repeated testing occurs whether the hot plate is heated (52.5 °C) or not (Normal Plate) compared to rats that are naïve to the hot plate apparatus (Home Cage; *p < .05). Data are compared from Trial 5. N = 8 or 9/group.

Figure 5

No effect of pre-exposure treatment on morphine potency. Dose response curves for morphine antinociception were the same despite differences in baseline hot plate latencies caused by pre-exposure to the hot plate apparatus. Data were collected on Trial 5 after twice daily exposure to the hot plate (52.5 °C) or a room temperature plate (Normal Plate) for 2 days. Control rats (Home Cage) were handled for 50 s on each trial and then returned to their home cage. N = 8 or 9/group.