TOXIN-I, BUT NOT 4-AMINOPYRIDINE, BLOCKS THE LATE INHIBITORY COMPONENT OF THE DORSAL-ROOT REFLEX IN AN ISOLATED PREPARATION OF RAT SPINAL-CORD

Bursts of spontaneous antidromic dorsal root action potentials, and ev oked dorsal root reflexes (DRR), have been recorded from lumbar roots of isolated spinal cord preparations of rats weighing 70-90 g. The pat tern of dorsal root activity was similar to that reported for isolated cord preparations from hamsters, but the frequency of spontaneous dor sal root activity was approximately 10 times slower in the rat. Toxin I and 4-aminopyridine (4-AP) both increased the frequency of spontaneo us dorsal root activity. The threshold concentration of 4-AP was 1 mu M, with EC50 at 20 mu M. Insufficient Toxin I was available to reach a maximal response, but the threshold concentration for producing an in crease in spontaneous activity was 0.1 mu M, and the curve appeared to be parallel to that of 4-AP. The patterns of spontaneous dorsal root activity in the presence of 4-AP and Toxin I differed. In 4-AP bursts of large amplitude action potentials were followed by periods of depre ssed activity lasting up to 500 ms, whereas in Toxin I bursts of large amplitude action potentials caused no change in the continuously firi ng small amplitude action potentials. DRR evoked by stimulation of adj acent dorsal roots also showed differences in the presence of 4-AP and Toxin I. In 4-AP the excitatory phase of the reflex was followed by a period of depressed activity lasting up to 500 ms. This was was reduc ed or absent in the presence of Toxin I. Paired pulse stimulation conf irmed the presence of inhibition in 4-AP, and its reduction in Toxin I . Examination of the pattern of spontaneous dorsal root activity follo wing dorsal root stimulation showed strong oscillatory activity in the presence of 4-AP, but little such activity in the presence of Toxin I . It was concluded that the actions of 4-AP and Toxin I on the isolate d preparation of rat spinal cord are similar in that both cause an inc rease in the spontaneous dorsal root firing rate, but that Toxin I als o blocks the period of inhibition which follows bursts of large amplit ude action potentials. (C) 1997 Elsevier Science B.V.