EXCITATION OF CENTRAL AND PERIPHERAL TERMINALS OF PRIMARY AFFERENT NEURONS BY CAPSAICIN IN-VIVO

In three groups of rats discharge activity was recorded (i) from the p eripheral stump of the cut saphenous nerve (saphenous-receptor prepara tion); (ii) from the central stump of the cut L(4) or L(5) dorsal root (dorsal root preparation); (iii) from the peripheral stump of the sap henous nerve segment cut at both ends (axon preparation) during slow i ntraarterial infusion of capsaicin (30-300 mu g/kg/min for 5 min) into the carotid artery. Capsaicin produced excitation, i.e. an increase i n frequency of action potentials in the same dose range (100-300 mu g/ kg/min) in both the saphenous-receptor and dorsal root preparations, w hile the axon preparations remained unresponsive. In the cat, close ar terial injection of capsaicin (up to 20 mu g) into a collateral branch of the saphenous artery failed to evoke discharges in the saphenous a xon preparation, although similar injection of 4-aminopyridine (60 mu g), a K+ channel blocking agent was readily effective. These results i ndicate that after systemic application of capsaicin the peripheral an d central endings of primary afferent neurons are equally important si tes for activation and are much more sensitive to capsaicin than the a xons of the nerve trunk.