I'd puzzled about placebos in rats earlier. I asked Dr. Coe for information, and he was very helpful.
If one broadens the definition a bit to include 'conditioned effects' and anticipatory effects, then there is even more in the animal literature.
For example, one can condition an allergic person to have an allergy and histamine response to a color or odor by associating it just once with the allergen.
An interesting observation in the Viagra studies is that it is hard to see a drug effect over the placebo effects. 30% of men report enhancement of sexual performance when given sugar pills.
In rats I wouldn't necessarily talk about it as placebo, but any time you do something repetitively in animals, they both will anticipate it and show a conditioned response.
Doesn't necessarily even require the brain. Expose a rat repeatedly to bacterial endotoxin, and there is the development of tolerance by the second exposure. So even the immune system has elements of learning and memory.
Habituation and sensitization are important features of many processes, and are probably related to the placebo effect.
Certainly with pain, which is in a real sense a created experience both out in the periphery and in the CNS, there is plenty of opportunity for modulation to occur. We have been conducting studies on fibromyalgia, where there is real pain, but also a big influence of psychological factors on pain symptoms.