Fig. 1.

Slow GABAergic inhibition of mitral cells of the rat olfactory bulb. (A) The spontaneous activity of a mitral cell monitored under current-clamp conditions is sporadically inhibited by slow hyperpolarizing events (asterisks, Upper; n = 73 slow hyperpolarizations for 20 cells) that are detected as SOCs of variable kinetics in voltage-clamp recordings (E_Cl⁻ = −78 mV; Lower; same cell). (B) SOCs reverse at the Cl⁻ equilibrium potential E_Cl⁻ = −50 mV (n = 7 cells). (C Top and Middle) Bicuculline (40 μM) completely blocks fast synaptic outward currents but only partially blocks SOCs (asterisks). The frequency of SOCs in the presence of bicuculline is decreased by 85.8 ± 4.7% (n = 5 cells; P = 0.03). (C Bottom) Picrotoxin (100 μM; n = 7 cells) blocks all SOCs and a tonic inhibitory current (V_H = −10 mV). (D) Time-to-peak distribution of all outward currents recorded at V_H = −10 mV (n = 5 cells). In control condition (Upper), most events correspond to fast, synaptic GABAergic currents. Enlargement of the histogram shows the rare SOCs (double white arrowhead). In the presence of 40 μM bicuculline (Lower), synaptic currents are abolished, whereas some SOCs persist.