Amino acid antagonists do not block the depolarizing effects of potassium ions on frog primary afferents

Abstract

The effects of picrotoxin, bicuculline and strychnine on the dorsal root potential evoked by dorsal root stimulation and on the responses of primary afferents to elevated [K+] and to γ-aminobutyric acid (and in some cases to β-alanine) were studied by sucrose gap recording from dorsal roots of the isolated hemisected frog spinal cord. Bicuculline and picrotoxin when applied in concentrations sufficient to block depolarizations of dorsal roots evoked by γ-aminobutyrate and the early phase (initial 150-200 ms) of the dorsal root potential did not alter the sensitivity of primary afferents to the depolarizing effects of K+. Strychnine produced inconsistent effects on the early component of the dorsal root potential, but in low or moderate concentrations (<10-3 M) which antagonized β-alanine-induced depolarizations, did not affect K+-induced depolarizations. In high concentrations (10-3M), strychnine decreased the responses of dorsal roots to K+, β-alanine and γ-aminobutyrate. All the convulsants tested prolonged and augmented a late phase of the dorsal root potential. These observations indicate that primary afferent depolarization (as measured by the dorsal root potential) consists of two components which appear to be pharmacologically distinct. The results also indicate that the reduction of the early component of primary afferent depolarization by picrotoxin and bicuculline result from a blockage of the effects of γ-aminobutyrate on primary afferents rather than from a reduced sensitivity of these afferents to elevations in the concentration of extracellular K+. The ability of picrotoxin, bicuculline and strychnine to facilitate the late phase of the dorsal root potential, may be related to the ability of K+ to contribute to the enhancement of the late component of primary afferent depolarization produced by convulsants. © 1980.