The effect of acclimatization and ambient temperature on heat withdrawal threshold in rats

Skin temperature contribution to the decrease in withdrawal latency following chronic constriction injury

BACKGROUND

Chronic constriction injury (CCI) is widely used as an animal neuropathic pain model. Neuropathic pain is considered to exist when withdrawal latency to thermal stimulation is decreased after inducing a CCI to the sciatic nerve. However, it is known that CCI leads to changes in skin temperature and that skin temperature can affect withdrawal latency. Aim of this study was to compare withdrawal latencies of constricted and contralateral hind limbs, to thermal stimulation, at the same artificially-induced skin temperatures.

METHODS

Neuropathic pain was induced by four ligatures on the left sciatic nerve in adult male Wistar rats. Withdrawal latencies were measured from the 11th to 14th day after ligation, in different ambient temperatures, using the plantar test (Hargreaves method). By changing ambient we produced different hind limb skin temperatures.

RESULTS

Our results show that (1) CCI cause an increase in skin temperature; (2) the withdrawal latency was inversely related to ambient and skin temperature in the same manner for both the ligated and contralateral hind limbs; and (3) withdrawal latencies did not differ significantly for the ligated and contralateral hind limbs when the temperature of the hind limbs was artificially made the same (i.e., by changing the ambient temperature).

CONCLUSIONS

Withdrawal latencies to thermal stimulation did not differ on ligated and contralateral hind limb after CCI to the sciatic nerve if the temperature of the hind limbs was artificially or mathematically made the same. This finding may have significant impact on the interpretation results of neuropathic pain research.

Confounding Factors in the Interpretation of Preclinical Studies

A number of issues may arise during the conduct of a study which can complicate interpretation of in vitro and in vivo datasets. Speakers discussed the implications of differing interpretations and how to avoid complicating factors during study planning and execution. Consideration needs to be given to study design factors including defining objectives, consideration of expected pharmacological effects, dose selection and drug kinetics, species used, and vehicle selection. In addition, the effects of vivarium temperature effects on various endpoints, how to control variables affecting clinical pathology, and how early death animals, common background findings, and artifacts can affect histopathology interpretation all play into the final interpretation of study data.

Laboratory environmental factors and pain behavior: the relevance of unknown unknowns to reproducibility and translation

The poor record of basic-to-clinical translation in recent decades has led to speculation that preclinical research is "irreproducible", and this irreproducibility in turn has largely been attributed to deficiencies in reporting and statistical practices. There are, however, a number of other reasonable explanations of both poor translation and difficulties in one laboratory replicating the results of another. This article examines these explanations as they pertain to preclinical pain research. I submit that many instances of apparent irreproducibility are actually attributable to interactions between the phenomena and interventions under study and "latent" environmental factors affecting the rodent subjects. These environmental variables-often causing stress, and related to both animal husbandry and the specific testing context-differ greatly between labs, and continue to be identified, suggesting that our knowledge of their existence is far from complete. In pain research in particular, laboratory stressors can produce great variability of unpredictable direction, as stress is known to produce increases (stress-induced hyperalgesia) or decreases (stress-induced analgesia) in pain depending on its parameters. Much greater attention needs to be paid to the study of the laboratory environment if replication and translation are to be improved.

Baclofen reversed thermal place preference in rats with chronic constriction injury

Chronic constriction injury to the sciatic nerve was used as an animal model of neuropathic pain. Instead of frequently used reflex-based tests we used an operant thermal place preference test to evaluate signs of neuropathic pain and the effect of baclofen administration in rats with neuropathy. Chronic constriction injury was induced by four loose ligations of the sciatic nerve. Thermal place preference (45 °C vs. 22 °C and 45 °C vs. 11 °C) was measured after the ligation and after the administration of baclofen in sham and experimental rats. Rats with the chronic constriction injury spent significantly less time on the colder plate compared to sham operated animals at the combination 45 °C vs. 11 °C. After administration of baclofen (10 mg/kg s.c.), the aversion to the colder plate in rats with chronic constriction injury disappeared. At the combination 45 °C vs. 22 °C, no difference in time spent on colder and/or warmer plate was found between sham and experimental animals. These findings show the importance of cold allodynia evaluation in rats with chronic constriction injury and the effectiveness of baclofen in this neuropathic pain model.