Properties and functional implications of I h in hippocampal area CA3 interneurons

The present study examines the biophysical properties and functional implications of I h in hippocampal area CA3 interneurons with somata in strata radiatum and lacunosum-moleculare . Characterization studies showed a small maximum h-conductance (2.6 ± 0.3 nS, n = 11), shallow voltage dependence with a hyperpolarized half-maximal activation ( V 1/2 = −91 mV), and kinetics characterized by double-exponential functions. The functional consequences of I h were examined with regard to temporal summation and impedance measurements. For temporal summation experiments, 5-pulse mossy fiber input trains were activated. Blocking I h with 50 μM ZD7288 resulted in an increase in temporal summation, suggesting that I h supports sensitivity of response amplitude to relative input timing. Impedance was assessed by applying sinusoidal current commands. From impedance measurements, we found that I h did not confer theta-band resonance, but flattened the impedance–frequency relations instead. Double immunolabeling for hyperpolarization-activated cyclic nucleotide-gated proteins and glutamate decarboxylase 67 suggests that all four subunits are present in GABAergic interneurons from the strata considered for electrophysiological studies. Finally, a model of I h was employed in computational analyses to confirm and elaborate upon the contributions of I h to impedance and temporal summation.