Characterization of a hyperpolarization-activated inward current in Cajal-Retzius cells in rat neonatal neocortex

Cajal-Retzius cells are among the first neurons appearing during corticogenesis and play an important role in the establishment of cortical lamination. To characterize the hyperpolarization-activated inward current (I-h) and to investigate whether I-h contributes to the relatively positive resting membrane potential (RMP) of these cells, we analyzed the properties of I-h in visually identified Cajal-Retzius cells in cortical slices from neonatal rats using the whole cell patch-clamp technique. Membrane hyperpolarization to -90 mV activated a prominent inward current that was inhibited by 1 mM Cs+ and was insensitive to 1 mM Ba2+. The activation time constant for I-h was strongly voltage dependent. In Na+-free solution, I-h was reduced, indicating a contribution of Na+. An analysis of the tail currents revealed a reversal potential of -45.2 mV, corresponding to a permeability coefficient (pNa(+)/pK(+)) of 0.13. While an increase in the extracellular K+ concentration ([K+](e)) enhances I-h, it was reduced by a [K+](e) decrease. This [K+](e) dependence could not be explained by an effect on the electromotive force on K+ but suggested an additional extracellular binding site for K+ with an apparent dissociation constant of 7.2 mM. Complete Cl- substitution by Br-, I-,or NO3- had no significant effect on I-h, whereas a complete Cl- substitution by the organic compounds methylsulfate, isethionate, or gluconate reduced I-h by similar to 40%. The I-h reduction observed in gluconate could be abolished by the addition of Cl-. The analysis of the [Cl-](e) dependence of I-h revealed a dissociation constant of 9.8 mM and a Hill-coefficient of 2.5, while the assumption of a gluconate-dependent I-h reduction required an unreasonably high Hill-coefficient >20. An internal perfusion with the lidocaine derivative lidocaine N-ethyl bromide blocks I-h within 1 min after establishment of the whole cell configuration. An inhibition of I-h by 1 mM Cs+ was without an effect on RMP, action potential amplitude, threshold, width, or afterhyperpolarization. We conclude from these results that Cajal-Retzius cells express a prominent I-h with characteristic properties that does not contribute to the RMP.