The voltage-dependent nature of the GABA-induced conductance change recorded from the ganglion cell of Aplysia.

gamma-Aminobutyric acid (GABA) produces 2 types of hyperpolarizing responses in Aplysia ganglion cells: One is fast and Cl(-)-dependent whereas the other is slow and K+-dependent. This experiment was performed only on the cells which showed the fast, Cl(-)-dependent receptor activity. GABA-induced increase in membrane conductance (delta G) was evaluated under the voltage clamp at different potential levels of the resting membrane. The delta G was found to decrease when the membrane was hyperpolarized. The more the membrane was hyperpolarized, the greater was the decrease. The dose-response curve shifted to the right when the resting membrane was hyperpolarized, a fact suggesting an increase in apparent dissociation constant (Kapparent) of the receptor-GABA complex. In fact, Kapparent increased e-fold for every 11-mV hyperpolarization of the resting membrane. In contrast, neither parameter of other receptor activities, such as the rate of desensitization, the intrinsic activity, and the Hill coefficient, was altered by hyperpolarization of the receptor membrane. It was concluded that the voltage dependent nature observed in this type of GABA receptor might be due to the change in stability of the Cl(-)-channel in an open state at the receptor membrane.