The effect of temperature on the GABA‐induced chloride current in isolated sensory neurones of the frog

Abstract

1 The effect of temperature on the kinetics of the activation and inactivation phases of γ-aminobutyric acid (GABA)-induced Cl⁻ current (I_Cl) was examined in frog isolated sensory neurones. 2 The peak I_Cl was reversibly reduced on changing the temperature and temperature-dependent coefficients were shown to exist, with the highest Q₁₀ (1.58) occurring between 5–15°C. 3 At both room temperature (20°C) and 10°C, the GABA dose-response curve was sigmoidal with a Hill coefficient of 2 and half-maximal responses to GABA, K_d, of 1.3 × 10⁻⁵m and 1.1 × 10⁻⁵m, respectively. Thus, indicating no change in the binding affinity of GABA when the temperature was decreased. 4 At GABA concentrations greater than 10⁻⁵ m, both the activation and inactivation phases of the GABA-induced I_Cl consisted of double exponentials, fast and slow components respectively, in the temperature range of 10 to 30°C. 5 The fast (ẗ_af) and slow (ẗ_as) activation time constants decreased with an increase in temperature and increased with a reduction in temperature. With an increased temperature, the reduction in peak I_Cl was due to a reduction in the slow time constant with no significant change in the fast time constant. 6 Both the fast (T̈_IF) and slow (ẗ_is) inactivation time constants were also increased by cooling to 10°C; heating to 30°C had little effect. 7 The concentration-dependence (10⁻⁵ to 10⁻³ m) of the slow activation (ẗ_as) and inactivation (ẗ_is) time constants was unaltered by the change in temperature. Similarly, the lack of concentration-dependence shown by the fast activation (ẗ_af) and inactivation (T̈_IF) time constants was unaltered by the temperature change. 8 From recordings made with ‘inside-out’ patches, the probability of opening of the GABA-induced Cl⁻ channels showed a marked increase with cooling to 10°C compared to room temperature (20°C), with no change in channel conductance. 9 The change in the GABA-induced I_Cl at different temperatures is, therefore, not due to changes in binding but to subsequent channel activation. Possible mechanisms whereby this occurs are discussed.