Peripheral versus central mechanisms for secondary hyperalgesia Is the Controversy Resolved?

Responses of primate spinomesencephalic tract cells to intradermal capsaicin

The responses of 32 spinomesencephalic tract cells to intradermal capsaicin were examined in anesthetized monkeys. Wide dynamic range (n = 20) and nociceptive specific (n = 6) cells showed two types of excitatory responses to intradermal injection of capsaicin. The first excitatory response shown by the majority of wide dynamic range (n = 13) and nociceptive specific (n = 4) cells was consistent with their sensitization by capsaicin. The cells showed an acute and prolonged increase in ongoing activity with capsaicin injection. Responses to mechanical stimuli were substantially increased after capsaicin and an expansion of receptive field was often observed. The responses of the same cells to excitatory amino acid agonists applied locally by iontophoresis also increased. All cells showing sensitization were antidromically activated from periaqueductal gray regions dorsal to the sulcus limitans. Electrical stimulation at these sites did not affect the ongoing or evoked discharges of these cells. The second excitatory response of wide dynamic range (n = 5) and nociceptive specific (n = 1) cells was a novel pattern not consistent with sensitization. These cells nevertheless showed an acute and prolonged increase in background activity after capsaicin injection. Yet, there was no change or a decrease in responses to cutaneous stimuli, no evidence for change in receptive field size and no increase in responses to locally released excitatory amino acids. These cells projected to regions in the periaqueductal gray ventral to the sulcus limitans. Electrical stimulation at these sites produced a decrease in spontaneous activity of the same cell. Low threshold mesencephalic-projecting neurons (n = 6) showed a single inhibitory pattern (n = 4) of responses to capsaicin. The injection produced an acute decrease in spontaneous activity that was sustained for at least 30 min after injection. The responses to cutaneous stimuli and to excitatory amino acids were also substantially reduced. Low threshold cells were found that projected to both dorsal-lateral and ventral-lateral regions of the periaqueductal gray. In summary, three patterns of responses shown by primate spinomesencephalic tract cells to intradermal capsaicin appear dependent on the functional regions of the periaqueductal gray to which they project. These results suggest that inputs of spinomesencephalic tract neurons to the periaqueductal gray may evoke important components of the systemic response to the neurogenic hyperalgesia produced by intradermal capsaicin.

Secondary hyperalgesia and perceptual wind-up following intradermal injection of capsaicin in humans

Wind-up and secondary hyperalgesia both are related to central sensitization, but whereas the former is explained by homosynaptic facilitation, the latter is due to heterosynaptic facilitation. To investigate possible interactions between both types of facilitation, we tested for alterations of perceptual wind-up in the secondary hyperalgesic skin zone adjacent to a capsaicin injection with light touch (by a cotton wisp) and punctate stimuli (calibrated von Frey hairs and pin pricks). Temporal summation of pain sensation (perceptual wind-up) was only observed with a clearly noxious stimulus (pin prick) presented at a repetition frequency of 0.6 s(-1), but not 0.2 s(-1). Pain ratings to trains of pin pricks reached a plateau after 3-4 repetitions, which was 1.65 times the initial rating ('wind-up ratio'). Injection of capsaicin induced a tenderness to mechanical stimuli in adjacent uninjured skin (secondary hyperalgesia), including hyperalgesia to light touch (allodynia) and hyperalgesia to punctate stimuli. Hyperalgesia to punctate stimuli was characterized by a leftward shift of the stimulus response function, corresponding to a decrease in pain threshold and an increase of painfulness of suprathreshold stimuli by a factor of 3-4. After capsaicin, the difference between the ratings of the first and last stimuli of trains of pin pricks was increased, but the ratio was unchanged. This behavior is equivalent to an increase in effective stimulus intensity, and could be mimicked by increasing the pin prick force from 20 mN to 40 and 80 mN in normal skin. Thus, the leftward shift of the stimulus response function fully accounts for all alterations of pain sensitivity to punctate stimuli in the zone of secondary hyperalgesia. We conclude that when the gain of spinal transmission was changed in secondary hyperalgesia, the gain of wind-up remained unchanged. These findings indicate that secondary hyperalgesia (heterotopic facilitation) and wind-up of pain sensation (homotopic facilitation) are independent phenomena.