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

The present study examines the biophysical properties and functional implications of Ih 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), shal- low voltage dependence with a hyperpolarized half- maximal activation (V1/2=−91 mV), and kinetics character- ized by double-exponential functions. The functional consequences of Ih were examined with regard to temporal summation and impedance measurements. For temporal summation experiments, 5-pulse mossy fiber input trains were activated. Blocking Ih with 50 μM ZD7288 resulted in an increase in temporal summation, suggesting that Ih supports sensitivity of response amplitude to relative input timing. Impedance was assessed by applying sinusoidal current commands. From impedance measurements, we found that Ih did not confer theta-band resonance, but flattened the impedance-frequency relations instead. Dou- ble 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 electrophysio- logical studies. Finally, a model of Ih was employed in computational analyses to confirm and elaborate upon the contributions of Ih to impedance and temporal summation.