One-Time Morphine Use Could Raise Addiction Risk, Study Says / Join Together, 27 April 2007
http://digbig.com/4sstj
"A single dose of morphine blocks nerve-cell connections in the brain and causes a spike in dopamine levels, possibly raising the risk of addiction, according to Brown University researchers."

Opioids block long-term potentiation of inhibitory synapses
Nature
http://www.nature.com/nature/journal/v446/n7139/full/nature05726.html
Fereshteh S. Nugent, Esther C. Penick, Julie A. Kauer Nature 446, 1086 - 1090 (26 Apr 2007) Letters to Editor [Sub required]