198 Brain Research, 55 (1973) 198- 202 ~:) Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherla~lds Two negative dorsal root potentials evoke a positive dorsal root potential LORNE MENDELL Department of Physiology and Pharmacology, Duke Medical Center, Durham, N.C. 27710 (U.S.A.) (Accepted February 27th, 1973) The dorsal root potential (DRP) reflects changes in polarization of presynaptic terminals of primary afferent fibers. There is general agreement that primary afferent depolarization (PAD -- negative DRP)2,s,14, 21 and primary afferent hyperpolariza- tion (PAH -- positive DRP) 1,7,11,13,17-2°,22 can be evoked by single shock stimuli to peripheral nerves. If the positive DRP is elicited during a steady negative DRP produced by repetitive stimulation of another nerve, its amplitude is increased 7,19. This observation supports the view that the positive DRP results from inhibition of a tonic depolarizing mechanismS,15,16, 22 whose level of activity can be varied by periph- eral stimulation. It is assumed implicitly that the positive DRPs evoked under these two conditions (i.e., (a) single shock; (b) single shock during steady negative DRP) have the same neuronal mechanism. However, it will be shown in this report that the positive DRPs evoked under these two conditions have different properties, suggesting two mechanisms for the positive DRP. In the absence of any other stimulation, a positive DRP can be elicited by single shocks to muscle nerves17,18. These positive DRPs have the following properties: (a) a central delay of about 25 msec ~7 which is much longer than the 2-5 msec central delay for the negative DRp~4; (b) extreme sensitivity to barbiturate anesthesia unlike the negative DRP 17; and (c) a requirement for shocks of sufficient strength to stimu- late group III afferent fibers 17,18, unlike the negative DRP which requires only shocks of sufficient strength to stimulate the group I afferent fibers. These experiments have been carried out on unanesthetized cats made spinal at C1 under temporary anesthesia and paralyzed with gaUamine (Flaxedil). DRPs have been recorded from small dorsal root filaments in L6 and L7 using DC amplification. The signals often have been averaged 16 or 32 times to improve the signal to noise ratio. In Fig. 1A1 a single shock of group III intensity delivered to the gastrocnemius- soleus (G-S) nerve evokes a large negative DRP followed by a small positive DRP (top trace). A 200 Hz train of shocks applied to the tibial nerve produces a steady negative DRP in the same filament (middle trace). When the single G-S shock is timed to evoke its DRP during the steady negative DRP, the resultant DRP has a smaller negative and a larger positive component than the control, where these potentials are measured with respect to the level of the steady negative DRP (bottom trace). In Fig. 1A2, the