Depression shows divergent effects on evoked and spontaneous pain behaviors in rats

Although it has been accepted that depression and pain are common comorbidities, their interaction is not fully understood. The present study was aimed to investigate the effects of depression on both evoked pain behavior (thermal-induced nociception and hyperalgesia) and spontaneous pain behavior (formalin pain) in rats. An unpredictable chronic mild stress (UCMS) paradigm was employed to develop a classical depression. The emotional behaviors were assessed by sucrose preference test, open field test, and elevated plus-maze test. The results showed that the depressed rats always exhibited stronger tolerance to noxious thermal stimulation under both normal and complete Freund's adjuvant (CFA)-induced chronic pain conditions, when compared to nondepressed animals. Interestingly, the spontaneous nociceptive behaviors induced by formalin injection were significantly enhanced in rats exposed to UCMS in comparison to those without UCMS. Systemic administration of antidepressant fluoxetine significantly restored the nociceptive behaviors to normal level in depressed animals. An additional finding was that the inflammatory rats tended to display depressive-like behaviors without being exposed to UCMS. These results demonstrated that depression can have different effects on stimulus-evoked pain and spontaneous pain, with alleviation in the former while aggravation in the latter.

The present study provides evidence that depression can have divergent effects on stimulus-evoked and spontaneous pain by confirming that rats exposed to chronic mild stress tend to exhibit decreased pain sensitivity to experimental stimuli but increased intensity of ongoing pain. This may contribute to further understanding of the perplexing relationship between clinical depression and chronic pain.