Emodin down-regulates expression of TRPV1 mRNA and its function in DRG neurons in vitro

Emodin is a principle ingredient isolated from rhubarb rhizome, which is commonly used for constipation or pain-related diseases in traditional Chinese medicine (TCM) practice. The transient receptor potential vanilloid 1 ion channel proteins (TRPV1) are abundantly expressed in the peripheral sensory neurons and are assumed to act as a kind of nociceptor involved in the perception of pain and development of hyperalgesia. The aim of this study was to further unravel the analgesic mechanisms of rhubarb through investigating the effects of its main constitutive ingredient emodin on the expression of TRPV1 mRNA as well as on its calcium- mediating functions in vitro. The primary DRG neurons with a high purity and viability were obtained, and the TRPV1 mRNA expression levels were examined by using real-time RT-PCR and the elevated amplitudes of intracellular [ Ca2+ ] i in the DRG neurons evoked by TRPV1 agonist capsaicin were examined by confocal microscopy. The results showed that emodin could significantly down-regulate both the mRNA expression of TRPV1 and the capsaicin-evoked intracellular fluorescent intensity in the DRG neurons under both 37°C and 39°C in vitro. Concomitantly, all of the changes induced by emodin could not be blocked by pretreatment of the primary neurons with capsazepine, an antagonist of TRPV1. In conclusion, we established that the mRNA expression level of TRPV1 and its calcium-mediating function in naive DRG neurons could be down-regulated by emodin through perhaps the non-TRPV1 channel pathways, and this might be the molecular mechanisms for rhubarb to inhibit hyperalgesia induced by inflammatory stimuli.