The Gaudet lab published an exciting new paper in Journal of Neurotrauma titled, “Sex differences in pain: Spinal cord injury in female and male mice elicits behaviors related to neuropathic pain.”
Chronic pain afflicts 65% of individuals with spinal cord injury (SCI). Women are more susceptible to pain symptoms than men. Although previous preclinical studies have explored sex differences in SCI-elicited pain-like behaviors, it remains understudied how SCI-driven pain behaviors develop over time and differ between the sexes. Here, we hypothesized that moderate force T9 contusion SCI in mice would cause neuropathic pain-like behaviors, and that these would be exacerbated in females compared to males.
First, we explored the effects of two different SCI forces on pain-like behaviors in female mice (Fig. 1). Both 60 and 75 kDyn SCI elicited mechanical allodynia (von Frey test) and heat hyperalgesia (Hargreaves test). The more severe 75 kDyn SCI caused excessive sensorimotor deficits at 7 d post-operative (dpo) that confounded sensory testing, so we used the less-severe 60 kDyn force for subsequent studies.
Figure 1. In female mice, spinal cord injury (SCI) of different forces causes graded locomotor deficits (a) and pain-like behaviors (b,c) compared to sham controls.
Next, we compared female and male mice with SCI regarding locomotor recovery and pain-like behavior (Fig. 2). Using the Basso Mouse Scale for locomotor recovery, females recovered slightly slower than males. For mechanical sensitivity, SCI caused increased hypersensitivity in mice of both sexes across all times tested (weekly from 6-27 dpo), and there was no substantial difference between females and males (Fig. 2 below; and Fig. 2-4 in the paper). However, females did show increased hypersensitivity at 27 dpo (Fig. 3 below; and Fig. 4 in the paper). For heat sensitivity, SCI again increased heat hypersensitivity at all times tested. Females with SCI exhibited increased hypersensitivity compared to males (Fig. 2 and Fig. 4 below; and Fig. 2, 3, and 5 in the paper).
Figure 2. SCI causes locomotor deficits and pain-like behaviors over 28 days post-operative (dpo) in female and male mice. (a-c) SCI in female and male mice caused locomotor deficits, which were slightly worse at acute-to-subacute times in females. (d-f) SCI elicited mechanical hypersensitivity compared to sham mice, as measured using the von Frey test. Females and males exhibited a similar extent of hypersensitivity. (g-i) SCI evoked heat pain-related behaviors in mice of both sexes; heat hypersensitivity was amplified in females.
Figure 3. A 27 dpo, SCI causes mechanical hypersensitivity. SCI-driven hypersensitivity is increased in female compared to male mice.
One strength of our study was the uniquely large size of our dataset. We have completed several experiments using age-matched mice of both sexes, so here we compiled a rigorous and comprehensive analysis of pain-like behavior in female and male mice. One limitation of our study is we assessed pain-like behaviors using only evoked tests, which may have limited relevance for chronic pain related to dull, ongoing sensations or spontaneous pain. Separately, we also created a test that attempted to unveil affective-related pain-like behaviors, though instead this revealed increased anxiety-like symptoms. Thus, it is challenging to develop effective tests for spontaneous or affective pain-like behavior in mice with SCI, particularly given the confounding combination of motor and sensory deficits. Others are also working on this challenge; e.g., Angelo Lepore’s group has used the grimace scale in mice to measure spontaneous pain-like behavior after SCI.
Figure 4. SCI elicits acute-to-chronic heat hypersensitivity, which is exacerbated in females compared to males.
Overall, our data suggest that moderate T9 contusion SCI in mice reliably causes mechanical and heat hypersensitivity, and that females with SCI exhibit increased heat hypersensitivity compared to males. Sex differences in pain-like behavior may relate to sex-specific underlying mechanisms. Future research should continue integrating sensory testing and both sexes. Ultimately, these studies will help unravel the relationship between sex/gender and neuropathic pain and to develop new therapies to benefit individuals with SCI.