INCREASED RESPONSIVENESS OF SENSORY NEURONS IN THE SAPHENOUS NERVE OF THE STREPTOZOTOCIN-DIABETIC RAT

1. This study examined sensory neurons in the saphenous nerve of rats treated with streptozotocin to induce diabetes (STZ-D). Several physiological properties of sensory neurons were not significantly different in STZ-D compared with control (CON) rats, including percentage and rate of spontaneous activity seen in the whole nerve and mechanical and thermal thresholds of individual C-fibers. 2. The response of STZ-D and CON C-fibers to a sustained (1 min) mechanical stimulus of threshold force was similar. However, during the 5 min immediately after removal of this stimulus, there was a much greater afterdischarge in STZ-D rats (STZ-D: n = 35; 14.6 +/- 5.1 action potentials/ 5 min, mean +/- SE; CON: n = 34; 3.9 +/- 0.7 action potentials/5 min). The number of action potentials during a sustained (1 min) suprathreshold mechanical (445 g) stimulus was also significantly greater in the C-fibers from STZ-D rats (STZ-D: n = 44; 149.7 +/- 18.4 action potentials; CON: n = 45; 84.7 +/- 12.2 action potentials). The afterdischarge during the 5 min immediately after removal of the sustained suprathreshold stimulus was also greater in C-fibers from STZ-D rats (STZ-D: 38.7 +/- 13.1 action potentials/5 min; CON: 9.3 +/- 2.3 action potentials/5 min). 3. There was a significant difference between C-fibers from STZ-D and CON rats with respect to the distribution among certain sensory classes. Thus the proportion of mechanically and thermally insensitive C-fibers was 12% in CON rats but only 5% in STZ-D rats; the proportion responding to cold (C-cold and C-mechanocold) increased from 9% in CON rats to 19% in STZ-D rats. The proportion responding to mechanical stimuli did not change. 4. The mechanical threshold of Adelta fibers was decreased in STZ-D rats (STZ-D: n = 26, 8.1 +/- 3.5 g; CON: n = 28, 29.2 +/- 10.2 g). There was no change in the distribution of the Adelta fiber population with respect to sensory class. 5. In conclusion, the present data suggest that an increased and prolonged responsiveness of C-fiber nociceptors may contribute to the lowered mechanical nociceptive threshold reported in behavioral studies of diabetic rats. A decrease in Adelta mechanical threshold may also contribute. We found no evidence for a contribution of changes in Abeta fibers to altered nociception in the diabetic rat.