Hyperpolarization of the presynaptic olfactory mitral cells decreases the strength of mitral–mitral cell connections. A, Pairs of olfactory mitral cells (M₁, presynaptic; M₂, postsynaptic) projecting to the same glomerulus were found by electroporation with Alexa-594. Dashed circles in glomerular layer indicate glomeruli and projecting pairs of mitral cells. ML, Mitral cell layer; EPL, external plexiform layer; GL, glomerular layer. See the schematic representation of experimental design on the right. Pairs of somatic APs (20 Hz) were evoked by current injection (1.8–2.4 nA, 2 ms) in M₁ either at V_rest (black) or V_hyper (gray). B, Representative experiment shows a pair of APs in M₁ delivered either at V_rest (black) or V_hyper (gray) and the average of evoked EPSPs in M₂ recorded at V_rest. Colors sign corresponding recordings in M₁ and M₂. C₁, EPSPs were recorded in cohorts of five sweeps by alternating presynaptic membrane potential between V_rest (black) and V_hyper (gray). C₂, EPSP amplitudes (left) and PPRs (right) are shown at presynaptic V_rest (rest) and V_hyper (hyper). Black circles connected with thick black lines represent the mean of eight individual experiments. ***p < 0.001. D, Recovery of the mitral–mitral synapse from depression was fitted with a single exponential (inter-stimulus interval [ISI]). E₁, The amplitude of EPSP1 (filled triangles) and EPSP2 (open triangles) plotted as a function of coupling coefficient (CC). E₂, PPR_rest evoked at V_rest plotted as a function of coupling coefficient.