Thermally potentiated responses to algesic substances of visceral nociceptors.

Using spermatic nerve preparations from testis superior, the effects of temperature rise on chemical responses were studied in vitro, within both subthreshold and suprathreshold ranges for testicular polymodal receptors, one type of visceral nociceptor. In the range of temperature subthreshold for polymodal receptors, the responses to algesic substances tested were greater at higher temperatures. The mean discharge rates induced by various concentrations of bradykinin (BK, 9 X 10(-9)-9 X 10(-6) M), were significantly greater at 36 degrees C than at 30 degrees C. Responses to hypertonic saline, tested in the temperature range, 34-43 degrees C, showed similar temperature-dependent increases, and Q10 values between 2.3 and 4.2. In the units exhibiting 'heat sensitization' with repeated testing of suprathreshold temperature rises, the response to hypertonic saline (616 mM) at 34 degrees C also increased. Although calor in the inflamed tissues is in itself not high enough to cause excitation of 'pain receptors,' temperature-dependent augmentation of chemical responses of the polymodal receptor might partly explain peripheral hyperalgesic behaviour observable in inflamed tissue on the basis of sensory receptor activity.