THE PLACEBO RESPONSE FOR PAIN TREATMENT: ANIMALS
People tend to assume that the placebo phenomenon must be the result of some complex, sophisticated, cognitive process and must therefore be unique to humans. However, we are moving to the conclusion that the phenomenon has to do with expectations, and animals have expectations and learned predictions. Rodents have a highly developed one, learning through trials to avoid any food which makes them sick. That is why rats and mice are so difficult to control by poisoned bait. If a rat is given an injection of a small dose of apomorphine in a space with which he is familiar, he salivates, his hair stands on end and he curls up looking miserable for half an hour. Months later, if the same rat is put in the same space and injected with a dose of saline, he puts on an identical performance as though he was again injected with apomorphine. This is a learned nocebo response.If a rabbit has experienced a series of small insulin injections that decrease the blood glucose and is then given a saline injection in the same conditions, the animal reacts by raising its blood glucose. The animal has learned to counteract the effects of the drug by raising its blood sugar. With a saline injection, it reacts as though it has received the insulin. This is neither nocebo or placebo but shows that even an animal learns to counteract the expected effect. The discovery of this type of preemptive reaction in animals led to the whole phenomenon being classed as simple Pavlovian conditioning, and it was suggested that the same explanation must apply to the human placebo response. As I will explain below, I think that label is misleading.*68\219\2*
