Is spinal cord dorsolateral funiculus involved in hypoalgesia induced by counter-irritation?

Several previous studies have demonstrated that, depending upon the behavioral test used, counter-irritation (i.e. the pain-relieving effects of pain elicited from heterotopic body areas) can produce hypoalgesia. In the present study, behavioral responses were elicited in the rat by increasing calibrated pressure applied to a hindpaw (Randall-Selitto test; 'passive' stimulus) and were studied before and after a subcutaneous formalin injection ('active' stimulus). The vocalization threshold to the pressure was clearly increased after injection of the algogenic solution either in the forepaw or in the cheek. Using this vocalization threshold, the counter-irritation-produced hypoalgesia was generally unchanged by unilateral dorsolateral funiculus (DLF) lesions. Following bilateral DLF lesions, hypoalgesia was decreased when formalin was injected in the forepaw, but was unaffected when the algogen was injected in the cheek. The present results partly contrast with previous papers from our group, where it has been assumed that the DLF is mainly involved in the neural circuitry subserving diffuse noxious inhibitory controls (DNIC), which have been considered as one possible neurophysiological basis for counter-irritation phenomena. They are discussed with reference to various hypotheses, including DNIC, as explanations for counter-irritation.