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

Abstract

SUMMARY1. 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 ( > 104 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 (10-10-5 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 (> 104 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 ,/-alanine may mediate the last step in the VR-DRP pathway.

Dorsal root potentials and changes in extracellular potassium in the spinal cord of the frog.

“…Kuffler et al 1966) which is similar to that observed in other systems (cf. Orkand et al 1966;Ransom & Goldring, 1973b) Two observations support the hypothesis that during high frequency neuronal activity induced by stimulating one dorsal root, the primary afferents entering through an adjacent dorsal root do see the resulting A[K]e. First, the addition of the GABA antagonist picrotoxin, which reduces the early, presumably GABA mediated component of the d.r.-d.r.p., results in the appearance of a much slower depolarization (Barker et al 1975b;Lothman & Somjen, 1976). Simultaneous measurements of [K]e indicate that the time of appearance and the magnitude of this picrotoxin resistant depolarization correlate well with the observed A[K]e. The large change in [K]e seen in the presence of picrotoxin is presumably due to the marked increase in neuronal excitability resulting in intense high frequency neuronal discharges.…”

“…stimulation. As reported previously (Barker et al 1975b) picrotoxin antagonizes the d.r.-d.r.p., but a much slower, picrotoxin resistant depolarization appears in the presence of picrotoxin. Simultaneous recording from the d.r.…”

Ventral Root Responses of the Hemisected Amphibian Spinal Cord to Perfused Amino Acids in the Presence of Procaine

“…Effects of strychnine and picrotoxin on ventral root potentials of the unblocked amphibian spinal cord have been reported previously (Tebecis & Phillis, 1969;Barker & Nicoll, 1973;Barker, Nicoll & Padjen, 1975 Picrotoxin antagonized the responses produced by GABA but not those produced by L-glutamate; glycine, f-alanine or taurine (Figure 2c responses produced by taurine and ,B-alanine were blocked by strychnine but those produced by L-glutamate, glycine and GABA were not ( Figure 2d). It should be noted here that strychnine, even in a 1 mM concentration, had no depressant effect on responses to glycine; indeed a potentiation was observed.…”

“…Barker et al (1975) found that picrotoxin was equally effective as an antagonist of dorsal root depolarizations produced by taurine, ,B-alanine or GABA. Thus it would seem that receptors for taurine, ,B-alanine and GABA on dorsal root fibres and primary afferent terminals are less specific than those of motoneurones.…”

Action of 5-Hydroxytryptamine on Isolated Spinal Cord of Bullfrogs

“…Thus it appears that 5-HT stimulates interneurons, and unknown transmitters released from these interneurons depolarize the dorsal root nerve terminals in the amphibian spinal cord. In fact, the dorsal root potential (DR-DRP) contained a component which was not sensitive to picrotoxin (16,17).…”