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Expression of a Functional Hyperpolarization-Activated Current (I_h) in the Mouse Nucleus Reticularis Thalami

Laboratoire de Neurophysiologie et Nouvelles Microscopies, Institut National de la Santé et de la Recherche Médicale U603; Centre Nationale de la Recherche Scientifique FRE2500; Ecole Supérieure de Physique et de Chimie Industrielles; and Université Paris Descartes, Paris, France

Submitted 2 September 2005; accepted in final form 13 January 2006

The GABAergic neurons of the nucleus reticularis thalami (nRT) express the type 2 hyperpolarization-activated cAMP-sensitive (HCN2) subunit mRNA, but surprisingly, they were reported to lack the hyperpolarization-activated (I_h) current carried by this subunit. Using the voltage-clamp recordings in the thalamocortical slice preparation of the newborn and juvenile mice (P6–P23), we demonstrate that, in the presence of 1 mM barium (Ba²⁺), the nRT neurons express a slow hyperpolarization-activated inward current, suggesting that the I_h is present but masked in control conditions by K⁺ leak currents. We investigate the identity of the hyperpolarization-activated current in the nRT by studying its physiological and pharmacological profile in presence of Ba²⁺. We show that it has voltage- and time-dependent properties typical of the I_h, that it is blocked by cesium and ZD7288, two blockers of the I_h, and that it is carried both by the K⁺ and Na⁺ ions. We could also alter the gating characteristics of the hyperpolarization-activated current in the nRT by adding a nonhydrolysable analogue of cAMP to the pipette solution. Finally, using the current-clamp recording, we showed that blocking the hyperpolarization-activated current induced an hyperpolarization correlated with an increase of the R_in of the nRT neurons. In conclusion, our results demonstrate that the nRT neurons express the I_h with slow kinetics similar to those described for the homomeric HCN2 channels, and we show that the I_h of the nRT contributes to the excitability of the nRT neurons in normal conditions.

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