This work was supported by the National Natural Science Foundation of China (Grant Nos. 39770848, 39830160), and a grant from NIH (DA 03983), USA. It is now well established that nerve growth factor (NGF) plays a key role in inammation-induced hyperalgesia. It was also reported that brain derived neurotrophic factor (BDNF), another member of neurotrophins, contributed to the pain pathway as a neurotransmitter in the CNS. The present work demonstrated a down-regulation of glial cell line-derived neurotrophic factor (GDNF) mRNA expression in dorsal spinal cord in complete Freund's adjuvant-induced unilateral arthritic rats serving as a chronic pain model. The fast occurring and long lasting down-regulations suggest that GDNF might contribute to pain pathway in a way different from neurotrophins and might play a role in the maintenance of chronic pain status. NeuroReport 11:737-741