Response properties of polymodal receptors studied using in vitro testis superior spermatic nerve preparations of dogs

Abstract

Characteristics of the polymodal receptor were studied using in vitro testis superior spermatic nerve preparations excised from anesthetized dogs. They were in most aspects similar to those reported previously using in vivo preparations. The majority (90%) of the tested polymodal units had small myelinated nerve fibers; the rest had nonmyelinated fibers. The mean mechanical threshold as determined by von Frey hairs was 17.5 g/mm2 (n = 476). There was a tendency for a unit with a higher conduction velocity to have a lower mechanical threshold. Bradykinin and hypertonic saline consistently caused a dose-dependent increase in discharge rate of these units; high K+ solution was also found to be a consistent stimulant. The responses of C-fiber receptors were not significantly different from those of A-delta-fibers. Heat stimulation up to 50 degrees C evoked discharges in 99 out of the total 103 units tested. The mean threshold temperature was 44.4 degrees C for the first trial. In 19 units in which the same heat stimulation was tested after an interval of 10 min, 10 units showed sensitization; 3 units were deactivated; and no clear difference was observed in the rest. No unit responded with a substantial increase in discharge rate to cold stimuli of 20 degrees C or less. A small temperature rise of 2 degrees C from the normal surface temperature of the testis (34 degrees C) significantly increased the response to hypertonic saline (616 mM) (2.41 +/- 0.22 impulses/s at 34 degrees C to 3.23 +/- 0.44 impulses/s at 36 degrees C) and to bradykinin (9 X 10(-8) M) (1.95 +/- 0.35 impulses/s at 34 degrees C to 2.85 +/- 0.19 impulses/s at 36 degrees C). The majority of the units recorded from the superior spermatic nerve in this experiment were most probably of polymodal receptor type, although the heat response was tested in a limited number of units. A very small number of a different type of receptor was discovered: rapidly adapting mechano-receptors, which responded almost exclusively to mechanical stimulation and were especially sensitive to a light mechanical stimulus moving across the receptive fields. The response properties of receptors studied in vitro remained practically unchanged during the experiments of several hours. The present experiments have shown that this preparation is suitable for systematic investigations, especially of the effects of chemical agents, on the polymodal receptor, which plays important roles in nociceptive functions.