Shuttle maze test: A novel operant method to repeatedly assess presumed nociception in freely behaving rats after nerve injury

Operant methods that allow animals to avoid painful stimuli are interpreted to assess the aversive quality of pain, however, such measurements involve stimulation delivery by the experimenter and can induce fear, anxiety, or stress in animals, all of which can affect nociceptive threshold or mask pain behaviors. Here we developed a new operant method, the shuttle maze test, to assess the aversive quality of mecha nical stimuli in rats after L5–L6 spinal nerve ligation (SNL) without externally applied stimuli by the experimenter. The shuttle maze test is based on the motivation for chocolate flavor cereal as a treat, and animals ambulates back and forth between two treat feeders by taking either a short route with a prickly–surfaced arch or a longer route with a smooth floor. The preference for taking the short route is a primary outcome measure of the test. Importantly, the animals do not have any painful consequences of not performing the task in the shuttle maze. SNL reduced the preference for the short route with the arch, correlated with hypers ensitivity in the hindpaw. Oral gabapentin (30 and 100 mg/kg), pregabalin (100 mg/kg), and duloxetine (50 mg/kg) restored the short route preference and reduced hypersensitivity in SNL rats. These results suggest that SNL injury alters behavior in the shuttle maze test and that shuttle maze test shows comparable results to reflexive hypersensitivity after SNL in response to analgesics.