The rats had undergone surgery, and they were about to traverse some paper in the name of science.
Tim LaBranche and his colleagues at the drug maker Pfizer had operated on a group of lab rodents to replicate the effects of osteoarthritic joint damage. They had also given some of them a new pain drug, called an NGF inhibitor, while others got a placebo.
The researchers wanted to know if the drug would not only blunt the rats’ chronic pain, but also some useful pain, the kind that reminds humans to steer clear of a hot stove — or to stop walking on a damaged knee. The idea was to see whether the inky footprints of rats that had been treated were thicker than those of the control group, evidence that they were overburdening an injured leg.