Figure 5.

GABA can be endocytosed in the absence of Ca²⁺, provided Ca²⁺-dependent exocytosis has occurred previously. In this figure, the black traces are bicuculline-sensitive autaptic currents recorded in CNQX plus d-APV, and the gray traces are whole-cell currents elicited by bath application of the indicated solutions. A, After superfusion of a high-K⁺ GABA loading solution that lacks added Ca²⁺, there is little change in the autaptic current (middle black trace). Next, the same cell was superfused briefly (5 s) with a solution containing 2 mm Ca²⁺ and 40 mm K⁺ but no GABA to trigger exocytosis. The bath solution was then immediately switched to one containing 100 mm GABA and normal K⁺ but no added Ca²⁺. After endocytosis for 60 s in this Ca²⁺-free loading solution, a large postload IPSC emerged (right black trace). B, Exocytosis was triggered by a train of 300 autaptic stimuli at 20 Hz in normal Ca²⁺-containing bath solution. The inset (gray trace) shows the beginning and end of this train. Starting 1.5 s after the end of the train, the cell was superfused for 60 s with nominally Ca²⁺-free GABA loading solution. After this, a large postload IPSC emerged (right black trace).