A minimum-stress procedure for repeated measurements of nociceptive threshold and analgesia

A procedure was developed which allows repeated measurement of threshold responses to a thermal pain stimulus administered to an unrestrained animal. The apparatus consists of a metal plate whose temperature can be rapidly incremented and decremented. The device was tested with rats using two experimental procedures. In a titration procedure, temperature was raised gradually until the subject licked its paws. At this time, the temperature was gradually decreased, with the amount of decrement determined by the duration of the paw licking. In a discrete trials procedure, temperature was increased rapidly until paw licking occurred, and then returned rapidly to room temperature. Morphine reliably reduced the responses to heat. The effect was dose-dependent and diminished with repeated doses.