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

1. The effect of temperature on the kinetics of the activation and inactivation phases of gamma-aminobutyric acid (GABA)-induced Cl- current (ICl) was examined in frog isolated sensory neurones. 2. The peak ICl 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 degrees C. 3. At both room temperature (20 degrees C) and 10 degrees C, the GABA dose-response curve was sigmoidal with a Hill coefficient of 2 and half-maximal responses to GABA, Kd, of 1.3 x 10(-5)M and 1.1 x 10(-5)M, 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 ICl consisted of double exponentials, fast and slow components respectively, in the temperature range of 10 to 30 degrees C. 5. The fast (tau af) and slow (tau 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 ICl was due to a reduction in the slow time constant with no significant change in the fast time constant. 6. Both the fast (tau if) and slow (tau is) inactivation time constants were also increased by cooling to 10 degrees C; heating to 30 degrees C had little effect. 7. The concentration-dependence (10(-5) to 10(-3)M) of the slow activation (tau as) and inactivation (tau is) time constants was unaltered by the change in temperature. Similarly, the lack of concentration-dependence shown by the fast activation (tau af) and inactivation (tau 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 degrees C compared to room temperature (20 degrees C), with no change in channel conductance. 9. The change in the GABA-induced ICl at different temperatures is, therefore, not due to changes in binding but to subsequent channel activation. Possible mechanisms whereby this occurs are discussed.