I just stumbled on some research that is worth investigating. There is a chemical called sialorphin that appears to play an important role in sexual behavior. The research below is especially interesting because it was conducted on aging rats.

More about sialophin at these links:

http://www.sciencedaily.com/releases/2009/09/090918181456.htm

http://www3.interscience.wiley.com/cgi-bin/fulltext/118508324/PDFSTART