Electrophysiological experiments have been carried out on rats to examine the relationship between plasma extravasation into skin and antidromic activation of individual unmyelinated afferent fibres supplying cutaneous receptors, particularly polymodal nociceptors. Normal animals and ones in which the skin had been reinnervated after peripheral nerve transection 26–52 weeks before were studied. In the normal animals 12 of 18 (66%) polymodal nociceptor afferents showed evidence of plasma extravasation after antidromic stimulation of the nerve filament from which they were initially recorded. Antidromic stimulation of other types of unmyelinated unit gave no detectable signs of plasma extravasation. The results from the animals with regenerated nerves were similar with 15 of 21 (71%) of the polymodal nociceptor afferents showing clear signs of plasma extravasation after antidromic activation and the other types of unit giving no signs of such a response.

These results show that while antidromic stimulation of a cutaneous nerve after intravascular administration of Evans Blue dye can be used to get a general view of the extent of innervation by polymodal nociceptors, it does not necessarily show the entire distribution of these receptors. It is likely that the ability or lack of ability of a polymodal nociceptor afferent to enhance vascular permeability, and so bring about plasma extravasation, is related to its neuropeptide content, and thus it follows that this might be a useful criterion for separating this broad class of cutaneous receptor into sub-groups.