Pacemaker channel function and regulation in the healthy and epileptic thalamus

The control of electrical activity in the brain is important for all brain-related behavior, including attention, arousal, action, but also drowsiness and sleep. Voltage-gated ion channels are central to all these aspects of neuronal excitability. Accordingly, malfunction of ion channels, be it inherited or acquired, tremendously compromises brain function, and leads to states of epilepsy, movement disorders, sensory deficits and neuropsychiatric disease. Voltage-gated ion channels form a large class of pore-forming, transmembrane proteins, and include channels selective for the major ions Na+, Ca2+, K+, Cl-, and HCO3 -. This family also includes ion channels permeable for several ions. Amongst these, the hyperpolarizationactivated cyclic nucleotide-gated cation-nonselective (HCN) channels occupy a unique position. First, being hyperpolarization-, not depolarization-activated, these channels possess the capacity to function as pacemakers. Second, in addition to the voltage-gating, HCN channels are directly modulated by intracellular cAMP levels. Third, the channels show the greatest sensitivity to brief periods of abnormal neuronal activity documented so far that manifests as a change in expression and function after periods of hours to days following abnormal electrical activity. This unique sensitivity has prompted an interest into how HCN channels may underlie the transformation of well-balanced neuronal circuits into hyperexcitable networks typically observed after an epileptic insult or after injury. The wealth of novel information about the molecular and regulatory properties of HCN channels accumulated over the past years raised a series of questions related to the function of this unique ion channel in neuronal cells and networks, including those in the intact animal. 1. At the level of the neuronal network: How does abnormal HCN channel expression and function causally relate to the emergence of pathological neuronal activity? 2. At the level of the neuron: Are there, and if yes, which are the cell-type specific modes of cAMP-dependent regulation of the channels? In my thesis, I have addressed these questions by combining electrophysiological, imaging, and molecular biological techniques in healthy animals and a rat model of epilepsy. 1. We have used the GAERS model to investigate the properties of HCN channel regulation in both pre-epileptic and chronically epileptic stages. This approach has allowed us to address the temporal relation between abnormal HCN channel function and the emergence of epilepsy. The findings imply that pacemaker currents undergo an abnormal regulation in the cause of epileptogenesis, but remain unaffected in chronic epilepsy. Interestingly, thalamic cells overcome these deficits by developing compensatory changes that stabilize HCN channel function. 2. Neurotransmitter-mediated cAMP synthesis and subsequent enhancement of HCNcurrents is a well-established mechanism that controls thalamic relay functions. The maintenance of arousal and wakefulness is connected with tonic activity of the noradrenergic locus coeruleus in the thalamocortical system. How and whether prolonged noradrenergic input modulates HCN channels in thalamic nuclei is subject of the second part of my thesis. Furthermore, a differential β-adrenergic subtype expression pattern in functionally distinct thalamic nuclei suggests that there could be a nucleus-specific component in the control of waking and sleep homeostasis. The results of my study indeed reveal a distinct β-adrenergic regulation of HCN channels within the thalamus. A strong β-adrenergic regulation of HCNcurrents appears to be pronounced in those portions being involved in sensory relay, while they may not be associated with general arousal functions

Regulation of recombinant and native hyperpolarization-activated cation channels

Ionic currents generated by hyperpolarization-activated cation-nonselective (HCN) channels have been principally known as pacemaker h-currents (Ih), because they allow cardiac and neuronal cells to be rhythmically active over precise intervals of time. Presently, these currents are implicated in numerous additional cellular functions, including neuronal integration, synaptic transmission, and sensory reception. These roles are accomplished by virtue of the regulation of Ih by both voltage and ligands. The article summarizes recent developments on the properties and allosteric interactions of these two regulatory pathways in cloned and native channels. Additionally, it discusses how the expression and properties of native channels may be controlled via regulation of the transcription of the HCN channel gene family and the assembly of channel subunits. Recently, several cardiac and neurological diseases were found to be intimately associated with a dysregulation of HCN gene transcription, suggesting that HCN-mediated currents may be involved in the pathophysiology of excitable systems. As a starting point, we briefly review the general characteristics of Ih and the regulatory mechanisms identified in heterologously expressed HCN channel