Studies on convulsants in the isolated frog spinal cord. II. Effects on root potentials.

1. In the isolated frog spinal cord picrotoxin, bicuculline, and strychnine were evaluated for their effects on synaptically induced root potentials recorded by the sucrose gap technique. 2. Picrotoxin (greater than 10- minus 4 M) completely blocked the dorsal root potential (DRP) elicited by stimulating the ventral root of the same segment (VR-DRP). Although picrotoxin antagonized the DRP elicited by stimulation of either an adjacent dorsal root (DR-DRP) or the lateral column (LC-DRP), a slower component to these potentials appeared and increased in size as the concentration of picrotoxin was increased. Thus picrotoxin brings out a later, picrotoxin resistant component to the DR-DRP and LC-DRP. 3. Strychnine (10- minus 8-10- minus 5 M) reduced and abolished the VR-DRP without prolongation and progressively increased and prolonged the DR-DRP (and LC-DRP) and the DR-VRP. Strychnine in higher concentrations (greater than 10- minus 4 M) also reduced the amplitude and prolonged the duration of the compound action potential of afferent fibres. 4. These results combined with those presented in the preceding paper (Barker, Nicoll & Padjen, 1975) suggest that (1) a GABA-like transmitter mediates the final step in the DR-DRP and LC-DRP pathways and that (2) either taurine or beta-alanine may mediate the last step in the VR-DRP pathway.