Study on the peripheral mechanism of hyperalgesia to mechanical and cold stimulations

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The purpose of this study is to clarify the peripheral mechanisms of mechanical and cold hyperalgesia. We found :1 ) Inflammatory mediators such as bradykinin, histamine and prostaglandin E2 as well as substances that were supposed to be second messengers of these mediators, sensitized the mechanical response of canine testicular polymodal receptors in vitro.2 ) Bradykinin response of Of C-fiber nociceptors recorded in rats with adjuvant-induced polyarthritis showed more than 1000 times higher sensitivity to bradykinin. Based on the assumption that this increased sensitivity might be resulted from the increased expression of B2 bradykinin receptor in sensory neurons, we measured B2 receptor mRNA in dorsal root ganglia (DRG_s) with RT-PCR method. We found 2.5 times increase in B2 receptor in DRG_s 1 week after inoculation of adjuvant.3 ) Animals with adjuvant-induced monoarthritis showed hyperalgesia to mechanical stimulation with von Frey hairs and to mild cold stimulation with aceton drops 10-14 days after injection.4 ) Corresponding to the existence of cold hyperalgesia in monoarthritic rat the percentage of C-fiber nociceptors responding to cold stimulation up to 2℃ increased, and C-low threshold mechanoreceptors showed augmented responses to mild cold stimulation. These results suggest that increased cold sensitivity of not only nociceptors but also low threshold mechanoreceptors, which are commonly believed not to involved in nociception, play some role in hyperalgesia to cold.