Characterization of voltage - gated K + currents contributing to subthreshold 1 membrane potential oscillations in hippocampal CA 1 interneurons 2 3 4

31 32 CA1 inhibitory interneurons at the stratum lacunosum-moleculare and radiatum 33 junction (LM/RAD-INs) display subthreshold membrane potential oscillations (MPOs) 34 involving voltage-dependent Na and A-type K currents. LM/RAD-INs also express 35 other voltage-gated K currents, however their properties and role in MPOs remain 36 unclear. Here, we characterized these voltage-gated K currents and investigated their 37 role in MPOs. Using outside-out patch recordings from LM/RAD-IN somata, we 38 distinguished four voltage-gated K currents based on their pharmacology and 39 activation/inactivation properties: a fast delayed rectifier current (IKfast), a slow delayed 40 rectifier current (IKslow), a rapidly inactivating A-type current (IA), and a slowly inactivating 41 current (ID). Their relative contribution to the total K current was IA > IKfast > IKslow = ID. 42 The presence of ID and the relative contributions of K currents in LM/RAD-INs are 43 different from that of other CA1 interneurons, suggesting the presence of differential 44 complement of K currents in subgroups of interneurons. We next determined if these K 45 currents were sufficient for MPO generation using a single-compartment model of 46 LM/RAD-INs. The model captured the subthreshold voltage dependence of MPOs. 47 Moreover, all K currents were active at subthreshold potentials but ID, IA and the 48 persistent sodium current (INaP) were most active near threshold. Using impedance 49 analysis, we found that IA and INaP contribute to MPO generation by modulating peak 50 spectral frequency during MPOs and governing the voltage range over which MPOs 51 occur. Our findings uncover a differential expression of a complement of K channels 52 that underlies intrinsic rhythmic activity in inhibitory interneurons. 53