Background: The daily fluctuations of many physiologic and behavioral parameters are differentially influenced by either central or peripheral clocks in mammals. Since substance P (SP) oscillates in some brain tissues and plays an indispensable role in modulating inflammatory pain at the spinal level, we speculated that SP mediates circadian nociception transmission at the spinal level.
Methods: In the present study behavioral observation, real-time polymerase chain reaction, luciferase assay, chromatin immunoprecipitation, and immunohistochemistry stain methods were used to investigate the role of SP in the spinal circadian nociception transmission and its regulation mechanism.
Results: Our results showed that under transcriptional regulation of BMAL1:CLOCK heterodimers, SP's coding gene Tac1 expression oscillates in dorsal root ganglion (n = 36), but not in the spinal dorsal horn. Further, the expression of SP cycled in the spinal dorsal horn, and this rhythmicity was potentially determined by circadian expression of Tac1 in dorsal root ganglion. Furthermore, the variation of SP expression induced by formalin was fluctuated in a similar rhythm to behavioral nociceptive response induced by formalin (n = 48); and the nociceptive behavioral circadian rhythm could be abolished through blockade of the SP-Neurokinin 1 receptor pathway (n = 70). Lastly, the variations of spinal SP expression and behavioral nociceptive response were in step, and both were changed by the deletion mutation of clock gene.
Conclusions: We conclude that spinal SP probably plays a pivotal role in modulating circadian inflammatory pain and suggest that peripheral circadian-regulated signaling is potentially an essential pathway for circadian nociceptive transmission.