SUMO1 modulates the excitability of hippocampal neurons. Rat hippocampal neurons exposed via the pipette to SUMO1₁₀₁, SUMO1₉₅, heat-denatured SUMO1₁₀₁, SENP1, or SENP1-C603S and assessed for Vm, R_IN, and Ff (with 2-s current injections of 10 pA) in whole cell mode. Recording conditions (Plant et al., 2006) and biophysical parameters are in Table I. Bars in A are 50 mV and 1 s, and also apply to B–D. (A) Photomicrograph of a neuron (left); bar, 20 µm. Evoked action potentials under baseline conditions (right). (B) Action potential firing increases in frequency with 7 pM SUMO1₁₀₁ in the pipette (left) and is increased further with 75 pM SUMO1₁₀₁ (right). (C) Ff is insensitive to linkage-incompetent SUMO1₉₅ (left) and heat-denatured SUMO1₁₀₁ (right). (D) Excitability is dampened by 250 pM SENP1 (left), but not by inactive SENP1-C603S (right). (E) Mean ± SEM Vm (left), R_IN (middle), and Ff (right) for 8–20 cells studied with the indicated proteins (Table I).