Our team, led by graduate student Sydney Lee, published a paper in Experimental Neurology addressing how spinal cord injury (SCI) affects the relative salience of anxiety vs. heat sensitivity (access pdf here also).
To explore the effects of SCI on mouse behaviors, we used our newly developed Thermal Increments Dark-Light (TIDAL) conflict test (Fig. 1) (for more details, see our related preprint). This test places mice in a two-chambered apparatus: one chamber remains dark but gradually heats to aversive temperatures, whereas the other chamber is illuminated and remains isothermic. If SCI caused strongly increased heat sensitivity, then mice would leave the dark, heating plate more quickly than control sham mice. Conversely, if SCI increased the salience of anxiety more than heat, then mice would persist on the dark plate longer despite increasing temperatures.
Figure 1. The Thermal Increments Dark-Light (TIDAL) conflict test is used in mice to assess anxiety- vs. heat sensitivity-related behaviors after spinal cord injury. A: The TIDAL conflict test places an anxiety-inducing stimulus (light vs. dark) in conflict with aversive heat. B: A control condition is thermal place preference, with both plates illuminated. C: Experimental conditions: for the dark plate, 5 minutes are spent at each temperature noted (i.e., 2 x 5 min at 31°C, then 5 min at each degree between 39-44°C), whereas the isothermic plate is maintained at 31°C throughout. D: TIDAL interpretations for possible behavioral shifts induced by SCI.
Our previous work showed that female mice exhibit increased preference for the dark, heating plate, suggesting that females exhibit increased salience of anxiety-related stimuli compared to males. Here, we again showed that females display increased anxiety-like behavior.
Interestingly, SCI in both females and males increased preference for the dark, heating plate compared to sham control mice (Fig. 2 below; Fig. 3 and 4 in the paper). This was not simply increased preference for warmth - SCI mice with both plates illuminated (thermal place preference, TPP) had reduced preference for the heated plate compared to mice on TIDAL. SCI increased dark-heated plate preference at all heated temperatures, from 39-44°C. Therefore, female and male mice at 7 d post-SCI displayed increased salience of anxiety.
Figure 2. After SCI, female and male mice in the TIDAL conflict test exhibit increased anxiety-like behavior. A: In the initial dark-light test with both plates at 31°C, female and male SCI mice exhibit only minor preference for the dark plate. B,C: Control condition - in the thermal place preference with both plates illuminated, uninjured and SCI mice show no significant differences in preference for the heating plate. D,E: In the TIDAL conflict test, both female and male mice with SCI show increased preference for the dark, heating plate compared to uninjured control mice.
A subset of mice were tested at 7 d post-operative (dpo) and re-tested at 21 dpo to establish whether anxiety-related differences persist, or whether differences are dampened and/or affected by learning (Fig. 5 and 6 in the paper). Our data showed differential effects in females and males. Females with SCI or sham surgery at 21 dpo both showed high initial dark plate preference, but decreased preference more quickly at higher temperatures (42-44°C). Males with SCI showed increased preference for the dark plate at 21 dpo compared to sham mice (a difference that was not apparent at 7 dpo), suggesting that male mice with SCI maintain higher anxiety-like behaviors than sham males.
Overall, we use our new TIDAL conflict test to reveal that SCI in mice drives previously underappreciated increases in anxiety-like behavior. Previous studies had suggested mice display minor anxiety-like behavior. Our data imply that placing an anxiety-driving stimulus in conflict with another stimulus - here, heat - can unmask underestimated anxiety-like behaviors. Testing using TIDAL revealed that both female and male mice with SCI exhibit increased anxiety-like behaviors at 7 dpo. Future studies could use TIDAL or other conflict tests after SCI to explore behaviors related to mood and anxiety. Ultimately, studying behaviors related to mood and anxiety after SCI could enhance our understanding of mechanisms and targets for improving mood-related therapies for humans afflicted with SCI or other neurologic conditions.