Mechanisms of allodynia: interactions between sensitive mechanoreceptors and nociceptors

We examined whether stimulation of sensitive mechanoreceptors from an area of allodynia evokes nociceptor activity expressed as axon reflexes. Experiments were conducted on human volunteers. Cutaneous blood flow was measured with a laser Doppler flowmeter. Allodynia was induced with mustard oil (25-100%) or by intradermal injections of capsaicin (25-50 micrograms) in the skin of the forearm or the hand. Tactile stimulation of normal skin or outside zones of allodynia did not evoke axon reflexes. The same stimulation in areas of allodynia evoked pain as well as axon reflexes. Cooling the area of primary hyperalgesia or blocking the A fibres in the nerve that innervated the allodynia area abolished the allodynia and the axon reflex. These results demonstrate central interactions between sensitive mechanoreceptors and nociceptors concomitant with the development of allodynia.