Abstract
Hyperpolarisation-activated (Ih) currents are considered important for dendritic integration, synaptic transmission, setting membrane potential and rhythmic action potential (AP) discharge in neurons of the central nervous system. Hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels underlie these currents and are composed of homo- and hetero-tetramers of HCN channel subunits (HCN1–4), which confer distinct biophysical properties on the channel. Despite understanding the structure–function relationships of HCN channels with different subunit stoichiometry, our knowledge of their expression in defined neuronal populations remains limited. Recently, we have shown that HCN subunit expression is a feature of a specific population of dorsal horn interneurons that exhibit high-frequency AP discharge. Here we expand on this observation and use neuroanatomical markers to first identify well-characterised neuronal populations in the lumbar spinal cord and hippocampus and subsequently determine whether HCN4 expression correlates with high-frequency AP discharge in these populations. In the spinal cord, HCN4 is expressed in several putative inhibitory interneuron populations including parvalbumin (PV)-expressing islet cells (84.1%; SD: ±2.87), in addition to all putative Renshaw cells and Ia inhibitory interneurons. Similarly, virtually all PV-expressing cells in the hippocampal CA1 subfield (93.5%; ±3.40) and the dentate gyrus (90.9%; ±6.38) also express HCN4. This HCN4 expression profile in inhibitory interneurons mirrors both the prevalence of Ih sub-threshold currents and high-frequency AP discharge. Our findings indicate that HCN4 subunits are expressed in several populations of spinal and hippocampal interneurons, which are known to express both Ih sub-threshold currents and exhibit high-frequency AP discharge. As HCN channel function plays a critical role in pain perception, learning and memory, and sleep as well as the pathogenesis of several neurological diseases, these findings provide important insights into the identity and neurochemical status of cells that could underlie such conditions.
Highlights
Neurons in the central nervous system (CNS) exhibit diverse physiological, morphological and neurochemical properties and these features have been used as defining criteria to help identify functionally distinct populations
As Hyperpolarisationactivated cyclic nucleotide-gated (HCN) channelopathies are being implicated in a number of neurological diseases including epilepsy and chronic pain (Chaplan et al, 2003; Lewis and Chetkovich, 2011), we focus on HCN4 expression in defined subpopulations of neurons in the spinal cord and hippocampus
This study details the pattern of HCN4-expression in neurochemically defined populations of interneurons in the lumbar spinal cord and hippocampus
Summary
Neurons in the central nervous system (CNS) exhibit diverse physiological, morphological and neurochemical properties and these features have been used as defining criteria to help identify functionally distinct populations. Four genes are known to encode for HCN channel proteins and these homologous HCN channel subunits (HCN1–4) can assemble as homomeric or heteromeric tetramer complexes to form channels with differing kinetics and voltage activation profiles (Wahl-Schott and Biel, 2009) These properties allow HCN channels to play a critical role in setting resting membrane potential, regulating repetitive AP discharge, and shaping dendritic processing in neuron populations. HCN channel subunit expression is likely to be an important determinant of membrane properties in CNS neurons and we propose that HCN4 expression in these selected inhibitory interneuron populations is linked to high-frequency AP discharge and the presence of Ih currents (Southan et al, 2000; Dudman and Nolan, 2009). We have used the neurochemical profiles of spinal and hippocampal neurons to first identify populations reported to exhibit either high-frequency AP discharge (e.g. islet cells, Renshaw cells (RCs), Ia inhibitory interneurons, basket cells and oriens-lacunosum moleculare cells) or transient/regular-spiking discharge patterns (motor neurons, pyramidal cells, granule cells) and determine whether they express immunolabelling for the HCN4 channel subunit
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