Studies on convulsants in the isolated frog spinal cord. I. Antagonism of amino acid responses.

Abstract

SUMMARY1. The isolated frog spinal cord was used to study the effects of picrotoxin, bicuculline, and strychnine on the responses of primary afferents to amino acids. Recording was by sucrose gap technique.2. A series of neutral amino acids was found to depolarize primary afferents. Optimal activity was obtained by an amino acid whose carboxyl and amino groups were separated by a three-carbon chain length (i.e. GABA). Amino acids with shorter (i.e. f-alanine, glycine) or longer (i.e. a-aminovaleric acid, e-aminocaproic acid) distances between the charged groups were less potent. Imidazoleacetic acid was the most potent depolarizing agent tested.3. Picrotoxin and bicuculline antagonized the primary afferent depolarizations of a number of amino acids tested with equal specificity.Depolarizing responses to standard (10-3 M) concentrations of fl-alanine and taurine were completely blocked by these convulsants, while depolarizations to 103 M y-aminobutyric acid (GABA) were only partially antagonized. Glycine responses were unaffected by these agents.4. Strychnine completely blocked ,8-alanine and taurine depolarizations and incompletely antagonized several other neutral amino acids. GABA, glutamate, and glycine depolarizations were not affected.5. These results suggest that there are at least three distinct populations of neutral amino acid receptors on primary afferent terminals: a GABAlike receptor, a taurine/f,-alanine receptor, and a glycine-like receptor. The

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

“…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.…”

“…The isolated, hemisected frog spinal cord represents a preparation of choice for such a study. In the preceding paper (Barker et al 1975) the antagonistic action of convulsants on amino acid responses of primary afferent fibres was described. In this paper, quantitative data are presented on the interaction between the same convulsants and the synaptically elicited root potentials.…”

THE ACTION OF ACETYLCHOLINE ANTAGONISTS ON AMINO ACID RESPONSES IN THE FROG SPINAL CORD in vitro

“…However, glycine depolarizations were also antagonized and hyperpolarizing components were observed with Atropine behaved in a manner identical to strychnine Barker et al, 1975a;Nicoll et al, 1975), specifically antagonizing the ,B-alanine responses on primary afferents and the hyperpolarizing motoneurone responses to glycine and ,B-alanine. This action of atropine was approximately 50 times weaker than that of strychnine.…”

“…GABA, glutamate and glycine responses were resistant to atropine, even in concentrations of 2.5 x 10-3 M. Figure 4b mately 2.5 x 1-3 M. Atropine also completely antagonized the response to taurine (not illustrated). This selective reduction of ,B-alanine (and taurine) responses is identical to the action of strychnine on responses of primary afferents to amino acids Barker et al, 1975a (Figure 5a The effect of atropine on amino acid hyperpolarizations was examined in four preparations.…”

“…The dissection and sucrose gap recording method used for recording drug responses from frog (Rana pipiens) spinal roots has recently been described (Barker, Nicoll & Padjen, 1975a Figure 1) (Barker et al, 1975b Since the sensitivity of primary afferents to glycine is considerably less than to these other amino acids it was usually not possible to match this response without approaching the peak of the dose-response curve and using very high concentrations of glycine (>5 x 10-2 M). In such cases a compromise was reached whereby a glycine response of approximately 80% of the maximal response was used.…”