Placebo effect

Does the placebo effect work on lab rats, or just on people?

I read of an experiment in which doses of morphine were administered to patients in pain, and the last dose was saline--but had the same effect as morphine. (Unless the saline included naloxone, which blocks morphine's effect.)

You could train lab rats to come for their pain reliever dose at regular times. If you had some way of telling if the rat was in pain you could tell if a saline dose at the end of a string of morphine doses (just as was done with the humans) relieved pain or not.

It sounds straightforward, and would say something about what kind of expectation helps with placebos.

Update post with some info from a researcher.