Figure 2. Depolarizing versus hyperpolarizing nature of perisomatic inhibition in the DG and CA1. A, Left, Single action potentials evoked in a DG PII generate negative uFields in the PCL. Right, Single action potentials evoked in a PII located in CA1 give rise to positive uFields. Boxes show the PII axon in the PCL at higher magnification. B, Gabazine (5 μm) blocks uFields, confirming their GABAA receptor-mediated nature. C, When the recording pipette (rec) is moved to the ML, uFields reverse their polarity. D, Biexponential fit (black) to the decay phase of an average uField recorded in the DG reveals its fast time course. E, Summary plot of the 20–80% rise time (RT; left scale) and the decay time constant (decay τ; right scale) of average uFields. F, Multiple-pulse depression of uFields in response to a presynaptic train of five action potentials at 50 Hz. Bottom, The first and the fifth uField of the train are superimposed. G, uFields in the DG of mature rats (P57–P61) are negative in amplitude and can be blocked by gabazine.