Characterisation of hyperpolarization-activated currents (Ih) in the medial septum/diagonal band complex in the mouse

Hyperpolarization-activated cyclic nucleotide gated (HCN) channel subunits are distributed widely, but selectively, in the central nervous system, and underlie hyperpolarization-activated currents (I-h) that contribute to rhythmicity in a variety of neurons. This study investigates, using current and voltage-clamp techniques in brain slices from young mice, the properties of I-h currents in medial septum/diagonal band (MS/ 1313) neurons. Subsets of neurons in this complex, including GABAergic and cholinergic neurons, innervate the hippocampal formation, and play a role in modulating hippocampal theta rhythm. In support of a potential role for I-h in regulating MS/DB firing properties and consequently hippocampal neuron rhythmicity, I-h currents were present in around 60% of midline MS/DB complex neurons. The I-h currents were sensitive to the selective blocker ZD7288 (10 muM). The Ih current had a time constant of activation of around 220 ms (at - 130 mV), and tail current analysis revealed a half-activation voltage of - 98 mV. Notably, the amplitude and kinetics of I-h currents in MS/DB neurons were insensitive to the cAMP membrane permeable analogue 8-bromo-cAMP (1 mM), and application of muscarine (100 muM). Immunofluoresence using antibodies against HCN1, 2 and 4 channel subunits revealed that all three HCN subunits are expressed in neurons in the MS/DB, including neurons that express the calcium binding protein parvalbumin (marker of fast spiking GABAergic septo-hippocampal projection neurons). The results demonstrate, for the first time, that specific HCN channel subunits are likely to be coexpressed in subsets of MS/DB neurons, and that the resultant I-h currents show both similarities, and differences, to previously described I-h currents in other CNS neurons. (C) 2004 Elsevier B.V. All rights reserved.