Effects of age on the axon reflex response to noxious chemical stimulation

The axon reflex flare response to noxious stimulation of the skin is mediated by polymodal nociceptors of "C" fibre primary afferent nerves. Topical application of capsaicin initiates such a flare. The mediator of the response is presumed to be substance P. In this study we examined the flare response to topical capsaicin (1g/L in 70% alcohol for 30 minutes; n = 220) and the substance P content of autopsied skin (3mm punch biopsy; n = 14). The area of the flare response was measured by tracing a Leitz ASM probe onto traced flare outlines. Skin was extracted in acetic acid and acetone and substance P measured by radio-immunoassay. Skin thickness was also measured in parallel biopsy specimens. The flare response decreased with increasing age at sites examined on the trunk and proximal extremities (for the trapezoid ridge r = -0.41; p less than 0.001). The substance P content of skin also decreased with age at those sites examined (for the cubital fossa r = -0.57; p less than 0.05). There was no relationship between age and skin thickness. The results demonstrate decreased activity of the axon reflex mechanism with increasing age. This may be due to changes in substance P containing nerve terminal density. The importance of a reduced neurogenic inflammatory response in the pathology of ageing is unknown.