Activated human platelets in plasma excite nociceptors in rat skin, in vitro.

Extravascular activation of thrombocytes may contribute to nociceptor excitation and pain, since platelets store and, upon stimulation, release potential algogenic substances such as serotonin, histamine and precursor molecules of bradykinin. To test this hypothesis, a skin-nerve preparation of rat hairy skin, in vitro, was used that allows to record and characterize single afferent nerve fibers. In a first protocol, receptive fields of nociceptive C-fibers, at the corium side of the skin patch, were exposed to adenosine diphosphate (ADP), to heparinized human platelet-rich plasma (PRP) and to PRP activated by ADP. Such activated platelets excited 9/11 units characterized as mechano-heat responsive C-nociceptors (CMH); peak discharges of more than 10 spikes/s were observed. After application of activated PRP, 4/5 high threshold mechanosensitive C-units and 4/5 mechano-cold sensitive C-units became responsive to heat stimulation but only few of these fibers were excited (1/5 in each group). In a second series of experiments the exposure to native PRP was prolonged to test for the effect of spontaneous platelet activation resulting from cutaneous collagen. Prolonged exposure did, but not significantly, enhance fiber discharge. During subsequent exposure to activated PRP, the discharge commenced, on average, after a significant delay of about three minutes. With this protocol 5/7 CMH units were driven by activated platelets. Following both protocols, mechanical (v.Frey) and thermal thresholds of the CMH units were not significantly altered. The findings demonstrate that nociceptors can indeed be driven and sensitized by activated platelets. This pain inducing mechanism may be relevant to certain clinical conditions, and it appears promising to scrutinize the chemical factors involved.