I have heard that pain and pleasure cause the changes in the bodies
chemistry or is it brain waves. Is it true? Where can I read about
this research?

You may be referring to endorphins, chemicals produced in response to
both pain and pleasure.

http://en.wikipedia.org/wiki/Endorphin

Well pain and pleasure can change your bodies chemistry and brain waves.
Brain waves can actually be explained via biochemistry.
But what I think you are talking about is, chemistry ... as a feedback loops.
Biological constructs within biological pathways.

I would suggest the best place to look for information concerning this
would be any depression drug documentation (but I could be wrong).

Best regards, Sean.