gamma‐Aminobutyric‐acid‐ and pentobarbitone‐gated chloride currents in internally perfused frog sensory neurones.

Abstract

SUMMARY1. y-Aminobutyric-acid-(GABA) and pentobarbitone-induced C1-currents (IC1) were studied in isolated frog sensory neurones after suppression of Na+, K+ and Ca2+ currents using a suction-pipette technique combining internal perfusion with voltage clamp.2. All GABA-sensitive neurones responded to pentobarbitone. Both GABA-and pentobarbitone-induced IC, reversed at the Cl-equilibrium potential (EC1).3. The dose-response curve for maxima of GABA-induced IC, was sigmoidal with a mean concentration producing a half-maximum response, Ka of 2 x 1O-5 M at a Hill coefficient of 1-8. In the presence of pentobarbitone, the GABA dose-response curve shifted to the left without affecting the saturating maximum current. 4. At high concentrations, both GABA and pentobarbitone could also potentiate the pentobarbitone-and GABA-induced IC, respectively, while pre-treatment with one of the two markedly attenuated currents induced by the other, indicating a 'cross-desensitization '. 5. In the presence of pentobarbitone, the augmented response was voltage dependent and this augmentation was much greater in the inward-current direction than outward.6. In producing IC,, pentobarbitone and its stereoisomers were potent in the order of (-) isomer > (± ) racemic mixture > (+ ) isomer. A stereospecific facilitatory action of pentobarbitone on GABA responses was also found in the same order.7. Responses to GABA, homotaurine, taurine, ,-alanine, 5-aminovaleric acid, (+ )-and (-)-y-amino-/3-hydroxybutyric acid and muscimol were equally enhanced by pentobarbitone, though its action on glycine-induced IC, was less effective.8. Picrotoxin inhibited the GABA-and pentobarbitone-induced IC, from either side of membrane, while internal application of GABA and pentobarbitone did not exert any effect. 9. It was concluded that pentobarbitone binds to the 'barbiturate receptors' located close to the GABA receptor-Cl-channel complex, and directly affects the GABA-GABA receptor interactions rather than the ionic channels.

Sodium‐dependent suppression of gamma‐aminobutyric‐acid‐gated chloride currents in internally perfused frog sensory neurones.

“…The reversal potentials of 10-5 M-GABA-induced Ici in the Tris+ and Na+ external solutions were assessed from the I-V relationship obtained by passing a depolarizing voltage ramp of 250 mV/s steepness immediately followed by a mirror-image hyperpolarizing ramp (Akaike et al 1985a). The VH was kept at -10 mV, and the voltagedependent Na+ current was completely inactivated.…”

“…Further experiments were undertaken with this working hypothesis in mind. If the GABA concentration is low enough (3 x 10-6 M) to prevent desensitization Hattori et al 1984;Akaike et al 1985a), then the GABA-gated Ic, amplitude under voltage clamp will depend on two factors: (1) equilibration of GABA with its receptors and (2) the uptake mechanism that limits access of GABA to its receptors. Thus, we examined the effect of the flow rate of the 3 x 10-6 M-GABA-containing solution (i.e.…”

“…The isolated nerve cell body was then internally perfused, the amount of solution pefusing the interior of the suction pipette being 1 ml/min. At this flow rate, complete change of the ionic environment of the cell interior occurs within 10-15 min (Akaike et al 1985a).…”

“…Recently, 'pure' chloride currents induced by GABA in frog d.r.g. cells have been isolated by applying the internal perfusion technique Hattori, Akaike, Oomura & Kuraoka, 1984;Ishizuka, Hattori & Akaike, 1984;Akaike, Hattori, Inomata & Oomura, 1985a;Akaike, Hattori, Oomura & Carpenter, 1985b;Akaike, Inoue & Krishtal, 1986; Inoue, Oomura, Yakushiji & Hattori, Oomura & Akaike, 1986). Using both internal perfusion and single-electrode voltage-clamp techniques, we studied electrophysiologically the GABA uptake process in frog d.r.g.…”

“…Current-voltage (I-V) relations were measured by applying a voltage ramp of 250 mV/s. These procedural details were described elsewhere (Akaike et al 1985a …”