Transient inhibition of responses to thermal stimuli of spinal sensory tract neurons in monkeys during sensitization by intradermal capsaicin.

A secondary zone of cutaneous hyperalgesia in humans is characterized by increased pain to mechanical stimuli. In contrast, the perception of heat stimuli delivered to a secondary zone of hyperalgesia in humans often shows a paradoxical decrease in magnitude. A number of studies have shown that the responses of spinal cord neurons to cutaneous mechanical stimuli after peripheral injury model well the mechanical hyperalgesia. However, the responses of dorsal horn neurons to thermal stimuli after peripheral injury have not been as carefully studied. The present study examined the effects of intradermal capsaicin on the responses of spinal sensory projection neurons to cutaneous mechanical and thermal stimuli. Our observations indicate that the responses of identified sensory projection neurons to heat are transiently reduced at the same time that responses of these cells to mechanical stimuli are increased. These results confirm a role for sensory projection cells in signaling the sensory discriminative aspects of pain in humans and underscore the emerging complexity of dorsal horn circuitry and sensory processing.