Figure 6. Prolonged depolarizations restore GABA responses in immature neurons, but reduce GABA responses in mature neurons only transiently.

A, averaged (7 cells) GABA-induced Ca²⁺ responses are shown for cells at DIV 10. During repetitive applications of GABA (G, 50 μm, 15 s pulses) [Ca²⁺]_i responses decreased. After an elevation of [K⁺]_o (horizontal bar, K20, 20 mm, 8 min) the GABA-induced [Ca²⁺]_i responses re-increased and declined again with further GABA applications. The decrement was reversed for a second time by a second elevation of [K⁺]_o (5 min). B, reduction of [Ca²⁺]_i signals induced by repetitive GABA applications can be reversed by intermediate elevation of [K⁺]_o (K20, 8 min, hatched bar) but not by a 15 min wash period (15′, open bar). The amplitude of the second response elicited by two subsequent GABA applications was on average reduced to 60 % of the first response under control conditions (Con, filled bar, n = 6). A washout period of 15 min between the two applications did not restore the response (15′, open bar, n = 4). However, elevation of [K⁺]_o to 20 mm (K20, 8 min, n = 9) between GABA applications led to an increase of the following responses. C, averaged (13 cells) glycine-induced [Ca²⁺]_i responses are displayed for cells at DIV 29. Repetitive applications of glycine (Gly) induced transient decreases in [Ca²⁺]_i. Transient elevation of [K⁺]_o (K20, 3–5 min) led to a reduction of the glycine-induced Ca²⁺ signals that recovered despite repetitive glycine applications. In this experiment, GABA_A receptors were blocked by bicuculline (20 μm).