Properties of a hyperpolarization-activated cation current in interneuronsin the rat lateral geniculate nucleus

A hyperpolarization-activated cation conductance contributes to the membran e properties of a variety of cell types. In the thalamus, a prominent hyper polarization-activated cation conductance exists in thalamocortical cells, and this current is implicated in the neuromodulation of complex firing beh aviors. In contrast, the GABAergic cells in the reticular nucleus in the th alamus appear to lack this conductance. The presence and role of this catio n conductance in the other type of thalamic GABAergic cells, local interneu rons, is still unclear. To resolve this issue, we studied 54 physiologicall y and morphologically identified local interneurons in the rat dorsal later al geniculate nucleus using an in vitro whole-cell patch recording techniqu e. We found that hyperpolarizing current injections induced depolarizing vo ltage sags in these geniculate interneurons. The I-V relationship revealed an inward rectification. Voltage-clamp study indicated that a slow, hyperpo larization-activated cation conductance was responsible for the inward rect ification. We then confirmed that this slow conductance had properties of t he hyperpolarization-activated cation conductance described in other cell t ypes. The slow conductance was insensitive to 10 mM tetraethylammonium and 0.5 mM 4-aminopyridine, but was largely blocked by 1-1.5 mM Cs+. It was per meable to both K+ and Na+ ions and had a reversal potential of -44 mV. The voltage dependence of the hyperpolarization-activated cation conductance in interneurons was also studied: the activation threshold was about -55 mV, half-activation potential was about -80 mV and maximal conductance was abou t 1 nS. The activation and deactivation time constants of the conductance r anged from 100 to 1000 ms, depending on membrane potential. The depolarizin g voltage sags and I-V relationship were further simulated in a model inter neuron, using the parameters of the hyperpolarization-activated cation cond uctance obtained from the voltage-clamp study. The time-course and voltage dependence of the depolarizing voltage sags and I-V relationship in the mod el cell were very similar to those found in geniculate interneurons in curr ent clamp. Taken together, the results of the present study suggest that thalamic loca l interneurons possess a prominent hyperpolarization-activated cation condu ctance, which may play important roles in determining basic membrane proper ties and in modulating firing patterns. (C) 1999 IBRO. Published by Elsevie r Science Ltd.