An operant assay of thermal pain in conscious, unrestrained rats

Methods are described which provide quantification of learned operant and innate reflex responses to a thermal stimulus (heat or cold) and provide matched motor controls. The apparati and procedures consist of (1) an 'Escapetest' which measures latencies and durations of escape from a compartment where the floor is heated or cooled to a platform at neutral temperature in an adjacent compartment; (2) a motor and motivational control for the Escapetest, the 'Darkboxtest', which measures escape latency from bright light in a shuttle box; and (3) assessment of latencies and durations of licking or guarding responses to thermal stimulation in the absence of the escape option. Avoidance responses in the Escapetest (retreating to the escape platform in the absence of an experience of pain) are discouraged by bright illumination of the compartment containing the escape platform (brightly lit areas are aversive to rodents). Stimulus-response functions for escape from heat and cold are compared to stimulus response functions for innate lick/guard responses to the same temperatures. Substantial differences in the relationships between learned or innate responses and temperature attest to a need for methods which evaluate operant responses to nociceptive stimulation.