In humans, neuropathic pain is associated with significant co-morbidity, including anxiety and depression, which impact considerably on the overall patient experience and quality of life.1
To date, reports of investigations of the presence of such co-morbid behaviour in rodent models of neuropathic pain are limited and somewhat conflicting.21
Therefore, we have used three well-characterized behavioural paradigms to determine whether traumatic peripheral nerve injury-induced neuropathic pain behaviour in male C57BL/6J mice is associated with increased fear-avoidance- and depression-related behaviour
Firstly, we chose to utilize two well-validated measures of fear-avoidance behaviour: the novel open-field test and the EPM. Both tests are based upon conflict between an innate aversion to exposed, well-lit spaces and a tendency to explore novel environments.10
As these tests may therefore detect subtle differences in the behaviour, we felt it necessary to investigate behavioural responses in both paradigms. Results showed that although hypersensitivity to punctate mechanical and cool stimulation was sustained for 28 days after partial sciatic nerve injury, there was no evidence of increased fear-avoidance behaviour after PSNL in either test. In fact, mice that demonstrated an increased sensitivity in stimulus-evoked hind limb reflex tests after PSNL tended to demonstrate evidence of reduced fear-avoidance behaviour, in that they showed more approach behaviour into potentially aversive environments. Specifically, PSNL-injured animals spent a greater amount of time in the centre of the open-field arena compared with sham-operated mice. No clear differences were seen on measures of fear-avoidance behaviour in the EPM. Thus, these findings therefore do not support our hypothesis that traumatic peripheral neuropathy is associated with increased fear-avoidance behaviour, and that this effect is more pronounced as the chronicity of pain is increased.
Secondly, we used a pharmacologically validated test for depression-related behaviour; the TST. The TST23
is frequently used to assess depression-related behaviour, validated by its sensitivity to clinically effective antidepressants that cause mice to actively and persistently engage in escape-directed behaviours compared with non-treated controls.10
Depression may be a major symptom associated with chronic neuropathic pain.2
However, there was no evidence of depression-related behavioural abnormalities in nerve-injured animals in this test. Again, these findings fail to support our original hypothesis that mice displaying significant pain behaviour will also show signs of co-morbidity depression-related behaviours. Taken together, these data may suggest that conventional murine models of pain do not fully recapitulate the range of symptoms displayed by humans suffering from neuropathic pain, at least to the extent emotional symptoms can be measured in mice.
In contrast to our findings, a recent study in C57BL/6J mice showed increased fear-avoidance behaviours on the light–dark exploration test and EPM 4 weeks after intraplantar injection of complete Freund's adjuvant or PSNL.14
The reason for these discrepancies is not clear, but may reflect differences in experimental protocols and paradigm design (e.g. it is not clear whether separate animals were used at each of the four time points examined or whether the same animals were re-tested at different time points, which is likely to reduce validity of behavioural testing).25
Another potentially salient factor in the study of fear-avoidance and pain in mice is the presence of marked strain differences. Data from strain comparison studies in tests of fear-avoidance demonstrate a significant strain effect on baseline activity,26
and a similar strain difference exists for various tests of nociceptive sensitivity.27
For example, the 129S1 strain tends to be the most sensitive to von Frey hair stimulation at baseline and develops the greatest magnitude of sensitivity after peripheral nerve injury. However, on threshold withdrawal from a heat stimulus (Hargreave's test), 129S1 mice are less sensitive than C57BL/6J mice. Interestingly, after peripheral nerve injury, 129S1 mice develop a significantly greater hypersensitivity compared with C57BL/6J mice.27
In contrast, in an alternative test of ongoing pain, the formalin test, 129S1 mice show reduced pain behaviour compared with C57BL/6J mice.27
This suggests that the strain used must be taken into consideration when modelling human conditions, including pain and fear-avoidance behaviour. This raises the possibility that genetic factors influence the effects of neuropathic pain on measures of fear-avoidance- and depression-related behaviour, and it would be of interest to compare effects of our pain model across different strains.
Another factor to consider is potential species differences between rats and mice. A number of rat models of pain have been shown to be associated with increased fear-avoidance behaviour in the open-field paradigm.15
Therefore, when assessing the suitability of integrated behavioural paradigms for the detection of pain behaviour, we must consider the suitability of the tests with regard to species as it appears that there is not only a strain-specific, but also a species-specific effect on fear-avoidance behaviour. Our findings suggest that mice, though widely used in paradigms of fear-avoidance- and depression-related behaviour, may not be a suitable species for the investigation of neuropathic pain co-morbidity. A better species for this purpose may be rats15
in which pharmacologically sensitive fear-avoidance behaviour, characterized by increased thigmotaxsis (‘wall-hugging’) and reduced entry into the centre of an open-field arena, is correlated with mechanical hypersensitivity.
In conclusion, we have shown that in contrast to previous findings,14
C57BL/6J mice displaying nerve injury-induced pain-like behaviour, as determined by standard hind paw withdrawal reflex tests, do not display increased fear-avoidance or depression-related behaviour for up to 4 weeks after injury. In contrast, neuropathic mice tended towards less fear-avoidance or depressed-related state in all paradigms assessed. This behaviour may be a correlation of altered endogenous systems known to be associated with fear-avoidance and stress states such as the opioid or serotonergic systems. However, the involvement of such mechanisms in the behaviour we have tested is yet to be determined and merits further investigation. This suggests, however, that these tests could be utilized to assess the involvement of fear-avoidance mechanisms in the endogenous control of pain and co-morbid behaviour.