Interactions between chemical and thermal cutaneous stimuli: inhibition (counterirritation) and integration

Methyl salicylate, a commonly used chemical counterirritant, was applied topically to the forearm to determine whether a nonpainful chemical irritation could inhibit the perception of another (weaker) chemical irritation. In the first experiment, sensations of irritation (burning and stinging) produced by a 10% solution of methyl salicylate were significantly attenuated when a 15% solution of the same chemical was applied to the opposite forearm. In the second experiment, neither the perception of warmth nor the heat pain threshold was affected by application of 10% or 15% methyl salicylate to a site 10 cm from the thermal stimulus. Inhibition did, however, occur in the opposite direction: Chemical irritation was reduced after the thermal stimulus reached a painful level. In the third experiment, a 15% solution of methyl salicylate was applied immediately adjacent to the thermal stimulus, with the result that ratings of warmth intensity increased rather than decreased, and perceived irritation was again attenuated following a painful heat stimulus. Overall, the results indicate that (1) chemical counterirritation can occur at nonpainful levels; (2) the resulting inhibition is confined to the nociceptive system; and (3) when the nociceptive and warmth system are activated together, the tendency is toward integration rather than inhibition.