Have you already searched for Papers in ScienceDirect?

http://www.sciencedirect.com/science?_ob=ArticleListURL&_method=list&_ArticleListID=-1041965669&_sort=r&_st=13&view=c&md5=8a86c399f6a570b918a4fd8968de8335&searchtype=a

Dear Chhuon,

I wrote a review article on this field and this may give you a good starting point. There are long list of research articles that you can choose from.

best wishes,

Refik

Article Opioids and pain

I suggest to search Pubmed for the following authors:

Bowsher D    -   Boivie J   -  Baron R  -  Rowbotham M

Thanks so much everyone for telling me and giving me hints. I'll look into everything. Hopefully everything will go well.

Article Central Neuropathic Pain in Spinal Cord Injury