• Tianjiao Xia, Yin Cui, Yue Qian, Shuaishuai Chu, Jia Song, Xiaoping Gu, and Zhengliang Ma.
• From the Department of Anesthesiology, Affiliated Drum Tower Hospital of Medical Department, Nanjing University, Nanjing, Jiangsu, China.
• Anesth. Analg. 2016 Feb 1; 122 (2): 542-52.

BackgroundNumerous clinical investigations have revealed the circadian rhythm changes in the perception of chronic pain, and most clinical chronic pain types peak in the night. However, it is still undiscovered whether circadian rhythm of pain exists in rodents and the specific mechanism that may underlie it. Our study was conducted to investigate the rhythmic changes of hyperalgesia behavior in a chronic constrictive injury (CCI) model of rodents and to explore the role of the N-methyl-d-aspartate receptor 2B (NR2B)-cAMP response element binding protein (CREB)-CREB-regulated transcription coactivator 1 (CRTC1) signaling pathway in this pain rhythm.MethodsA CCI operation was performed to mimic clinical chronic pain. Paw mechanical withdrawal threshold and paw withdrawal thermal latency were used to test pain behavior in rats; a von Frey cilia test was used to test mechanical hyperalgesia in mice at Zeitgeber time (ZT) 4, ZT10, ZT16, and ZT22 for 14 contiguous days. The relative mRNA and protein expression of NR2B, CREB and CRTC1 in the suprachiasmatic nuclei and the dorsal horn were measured by real-time polymerase chain reaction and Western blot. CRTC1 and CREB interference adenovirus vectors were injected intrathecally at 2 time points, respectively (ZT12 and ZT0), to further explore the proper time point for pain treatment.ResultsDuring the period of chronic pain state, the pain behavior of CCI rodents showed a circadian rhythm with the peak at ZT4 or ZT10 daily. The pain thresholds were significantly different between the activity period and the rest period. The expressions of NR2B, CRTC1, and CREB at the spinal level were consistent with the pain rhythm. The intrathecal treatment with CRTC1 or CREB interference adenovirus from day 7 to day 9 after CCI surgery markedly improved pain behaviors. Nevertheless, when given at ZT0, they were both more effective at relieving peak pain than drugs given at ZT12.ConclusionsPain behavior in the chronic pain of CCI displayed circadian rhythm and was associated with circadian secretion of pain-related receptors. The NR2B-CREB-CRTC1 signaling pathway may play a crucial role in this rhythm. Moreover, our results suggest that measures to relieve pain should be taken before pain reaches its peak.

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