Detection of cold pain, cold allodynia and cold hyperalgesia in freely behaving rats

Abstract

Background: Pain is elicited by cold, and a major feature of many neuropathic pain states is that normally innocuous cool stimuli begin to produce pain (cold allodynia). To expand our understanding of cold induced pain states we have studied cold pain behaviors over a range of temperatures in several animal models of chronic pain.

Results: We demonstrate that a Peltier-cooled cold plate with +/- 1 degrees C sensitivity enables quantitative measurement of a detection withdrawal response to cold stimuli in unrestrained rats. In naïve rats the threshold for eliciting cold pain behavior is 5 degrees C. The withdrawal threshold for cold allodynia is 15 degrees C in both the spared nerve injury and spinal nerve ligation models of neuropathic pain. Cold hyperalgesia is present in the spared nerve injury model animals, manifesting as a reduced latency of withdrawal response threshold at temperatures that elicit cold pain in naïve rats. We also show that following the peripheral inflammation produced by intraplantar injection of complete Freund's adjuvant, a hypersensitivity to cold occurs.

Conclusion: The peltier-cooled provides an effective means of assaying cold sensitivity in unrestrained rats. Behavioral testing of cold allodynia, hyperalgesia and pain will greatly facilitate the study of the neurobiological mechanisms involved in cold/cool sensations and enable measurement of the efficacy of pharmacological treatments to reduce these symptoms.

Figure 1

Cold pain threshold in naïve rats (n = 6). The mean latency for withdrawal ± S.E.M became significantly different from the baseline at surface temperatures of 5°C and below. Temperatures of 10°C and above can therefore be considered innocuous.

Figure 2

Cold allodynia and cold hyperalgesia in the Spared Nerve Injury and Spinal Nerve Ligation models of neuropathic pain. At 15°C both the SNI and SNL have significantly reduced latencies to withdrawal (A and B) compared to both naïve controls (n = 6). At 10°C the SNI model shows a significantly reduced latency (C), but the SNL model does not (D). The SNI shows cold allodynia (a significantly reduced response to a non-noxious temperature) at both 10°C and 15°C. The SNL model only displays this at 15°C. At -5°C, a temperature known to be painful in the naïve rat, the SNI model displays a further reduction in withdrawal latency (E); the SNL does not show any further reduction (F). Data are mean ± S.E.M.

Figure 3

Temporal development of cold allodynia (cold plate surface temperature 10°C) in the Spared Nerve Injury model of neuropathic pain. Following SNI (n = 6), the latency to withdrawal is consistently lower than sham controls (n = 6). It is significantly lower from 2 days to 5 weeks post surgery. Data are mean ± S.E.M.

Figure 4

Cold sensitivity following CFA inflammation of the hindpaw. The latency to withdrawal following intraplantar injection of CFA (n = 6 to 12) is significantly lower than naïve controls (n = 6 to 12) at 15°C, 10°C and 5°C. From this we can conclude that CFA induced inflammation of the hindpaw has both cold allodynic and cold hyperalgesic components. Data are mean ± S.E.M.

Figure 5

The relationship between cold plate set temperature (y-axis) and actual surface temperature (x-axis). Surface temperatures were measured at 4 different positions on the cold plate and mean ± S.E.M. temperature obtained. The discrepancy between set and actual temperature is due to the cold plate temperature probe being positioned in the core of the plate rather than on the surface. All temperatures used in this study are surface temperature.