Sensitization to mechanical stimulation by inflammatory mediators and by mild burn in canine visceral nociceptors in vitro

Hyperalgesia to mechanical stimulation and heat is commonly observed in inflamed conditions. Although sensitization to heat is well documented and its mechanism has also been well studied, it remains unclear whether and how nociceptors are sensitized to mechanical stimulation. Therefore we conducted in vitro investigation of which inflammatory mediators (bradykinin, histamine, prostaglandin E2, and protons) sensitize nociceptors to supra-threshold mechanical stimulation and at what concentrations. In addition, we studied the effects of possible second messengers for these mediators downstream of the receptors and also the effects of mild burn. Single polymodal receptor activities were recorded in canine testis-spermatic nerve preparations excised from deeply anesthetized dogs. Mechanical stimulation was applied to the identified receptive field for 10 s with a servo-controlled mechanical stimulator. Bradykinin at 0.001 muM induced neither excitation nor facilitation of the mechanical response; however, it facilitated the mechanical response at 0.01 muM and higher, levels at which significant excitation was also induced by bradykinin alone. Histamine excited the nociceptor and sensitized it to mechanical stimulation at 10 muM and higher. PG E-2 also sensitized the mechanical response, but starting at 1 muM, without inducing excitation by itself. The effects of two possible intracellular messengers for these mediators were studied using forskolin (10 muM), which increases intracellular cAMP, and a protein-kinase-C-stimulating phorbol ester, phorbol 12,13-dibutyrate (0.1 muM). Both substances reversibly facilitated the mechanical response of testicular polymodal receptors. In contrast, low-pH solution (pH: 6.6-4.5) seldom induced excitation and failed to facilitate the mechanical response. After 55degreesC, 30-s heat stimulation, testicular polymodal receptors were sensitized to mechanical stimulation. These results demonstrated that inflammatory mediators and burn sensitized nociceptor responses to mechanical stimulation and provide support for the idea that peripheral nociceptor sensitization is a mechanism involved in hyperalgesia to mechanical stimulation in inflamed tissues.