A quantitative method for assessing of the affective component of the pain: conditioned response associated with CO 2 laser-induced nocifensive reaction

Abstract

Sensory and affective components are included in the overall behavioral manifestation in a nocifensive reaction. We have developed a behavioral model using classical conditioning to differentiate the affective component from the sensory responses following a thermal noxious stimulus. In laser-pain conditioning, free moving rats were trained to associate a tone (conditioned stimulus, CS) and short CO 2 laser pulsation (unconditioned stimulus, US). A monotonous tone (800 Hz, 0.6 s) was delivered through a loudspeaker as the CS. CO 2 laser pulses (5 W at 100 ms in duration) applied to the hind paw were adopted as the US. The CS–US interval was 0.5 s. The conditioned responses as quantitatively measured by their body movement were developed over a period of 40 CS–US pairings. These conditioned responses were found retained when the rats were tested by presenting CS alone, immediate to and 24 h subsequent to training. The conditioned responses however diminished significantly following both morphine and buspirone treatment. This method demonstrated that neutral auditory stimuli could form association with unlearned nocifensive responses evoked by noxious CO 2 laser pulses stimuli. Thus, the assessment of conditioned response may be a valuable tool for the measurement of the affective component of nociception.