Differential effect of a chemical algogen on two nociceptive thresholds.

Several previous studies have demonstrated that, depending upon the behavioral test used, counterirritation can elicit hypoalgesia at body sites distant from the second painful stimulus. Behavioral responses were elicited in the rat by increasing calibrated pressure on a hindpaw (Randall-Selitto test) and were studied before and after a subcutaneous formalin injection. The vocalization threshold to the pressure was clearly increased after injection of the algogenic solution either in the forepaw or in the cheek, whereas the struggle threshold was unchanged. These results are discussed in terms of the ability of an "active" localized noxious stimulation to induce heterotopic hypoalgesic effects dependent on the level of integration of the "passive" pain behavior tested.