Antidromic vasodilation in frog: identification of the nerve fiber types involved

In anesthetized, immobilized frogs arteriolar vasodilation in the submaxillaris muscle in response to electrical stimulation of the submaxillar nerve (peripheral end) was observed directly and vasodilation in the hind leg in response to stimulation of the sciatic nerve (peripheral end) measured by plethysmography. With pulses of 0.1 ms duration at 20 Hz, the threshold for arteriolar vasodilation in the submaxillaris muscle was close to 3 T, where T was the activation threshold of the most excitable fraction of motor fibers of the submaxillar nerve. Atropine had no effect on the arteriolar vasodilation. When the sciatic nerve was stimulated with pulses of 0.1 ms duration, the threshold for vasodilation in the hind leg was 3.6±1.2 T (mean ± SEM). The thresholds for excitation of the A_αβ, A_δ and C-afferent fibers in the sciatic nerve and the range of stimulus intensities for recruiting each of these fiber groups were evaluated by recording compound action potentials in the VIII-X dorsal roots. Excitation of A_δ-afferent fibers was found to occur in the same intensity range as that which evoked vasodilation in the hind leg. It is concluded that, in the frog, these myelinated afferent fibers are capable of dilating the blood vessels by antidromic action in both submaxillaris muscle and hind leg. This finding is in accordance with recent reports of an antidromic vasodilator action of A_δ-afferent fibers in rabbit and rat skin.