Increased thermal and mechanical nociceptive thresholds in rats with depressive-like behaviors

Clinical observations suggest that depressed patients were less sensitive to experimental pain than healthy subjects. However, few animal studies are reported concerning the association of depression and pain. The purpose of this study was to investigate the effects of unpredictable chronic mild stress (UCMS) induced depression on the perceived intensity of painful stimulation in rats. We measured the thermal and mechanical paw withdrawal thresholds (PWT) of normal and spinal nerve ligated (SNL) rats using hot plate test and von Frey test, respectively. The results showed that rats exposed to UCMS exhibited significantly higher thermal and mechanical pain thresholds in comparison to the non-depressed controls. In particular, the PWT of the SNL group was restored to nearly normal level after three weeks of UCMS, and even comparable to that of the control group. These results strongly suggest that the depressed subjects have decreased sensitivity to externally applied noxious stimulation, which is consistent with our previous findings.

Research highlights
► Unpredictable chronic mild stress (UCMS) induces depressive behaviors in rats.
► UCMS elevates contact heat paw withdrawal threshold in normal rats.
► UCMS elevates mechanical paw withdrawal threshold in normal rats.
► UCMS elevates mechanical paw withdrawal threshold in SNL rats.