Figure 9.

GABA_A activation by optical stimulation of juvenile VGAT slices causes pyramidal cell excitation in 4-AP conditions. (A) Pyramidal cell response to optical stimuli before bath application of 4-AP showing membrane hyperpolarization after 10 ms, 20 ms, and 30 ms optical pulses. Simultaneous field potential recordings in the same location (bottom trace) show no detectable population response to the optical pulse. (B) In the presence of 100μM 4-AP, optical stimulus pulses produced membrane depolarization and bursts of action potential firing that correlated with an optically-evoked population response detected in a nearby extracellular recording electrode. (C) Patch-clam mode injected current pulse protocol (top) with 10 ms optical pulse (arrow) during 4-AP application. Optical stimulus produced a very large depolarization response at low membrane potential, but still produced hyperpolarization when the membrane was clamped above −60 mV. This is highlighted in (D) which shows a magnified view of the marked region in (C) to illustrate how optical stimuli produced an initial membrane response with a reversal potential of approximately −63 mV (horizontal dotted line), as before 4-AP application. Cells showed two phases of depolarization when the membrane voltage started at very hyperpolarized potentials (D, i and D, ii), and action potential bursting in response to optical stimuli occurred during the second depolarization phase. Field potential recordings (bottom traces) confirmed that pyramidal cell bursting correlated with optically-evoked population bursting. (E) Summary of optical response reversal potentials in normal and 4-AP conditions. Results are mean ± SEM, P=0.394, Student’s t-test.