Loss of HCN channel mediated Ih current following seizures accounts for movement dysfunction

Abstract

Hyperpolarisation-activated, cyclic nucleotide-gated (HCN) channel mutations are linked to disorders characterized by the occurrence of recurrent seizures (epilepsies) and HCN channel dysfunction has also been observed following evoked seizures in otherwise typical brains. Whether HCN channels contribute to the behavioral co-morbidities associated with epilepsy has received limited attention, despite extensive work to show that they serve as key regulators of neuronal excitability. A recent article, co-authored by us, provides evidence that experimental seizures disrupt HCN channel function and that this type of disruption leads to long-term impairments in skilled motor behavior. Given that interictal motor impairments are observed following experimental seizures and clinical epilepsy, this new study suggests an intriguing possibility that HCN channels represent a novel therapeutic target to treat co-morbidities of this disorder. HCN channels provide a diverse set of contributions to brain excitability. At the level of individual neurons, the current mediated by HCN channels, referred to by many names including Ih, affects integration of synaptic input as well as patterns of action potential firing. It has previously been shown that HCN channels serve as a mechanism to restrict spatial firing fields within entorhinal cortex. HCN channels also restrict hippocampal-dependent spatial memory. In our recent work, we sought to examine the role of HCN channels for networks located in motor cortex. The study manipulated HCN channels using 3 separate approaches. Repeated experimental seizures were used as they reduce Ih in layer 5 pyramidal cells that make up the cortical spinal tract. The pharmacological blocker ZD7288 was locally applied within motor cortex and global HCN1 knockout (HCN1) mice were used as a genetic strategy. In order to examine network function of motor cortex, in vivo studies were performed using standard intracortical microstimulation (ICMS) to systematically measure evoked forelimb movement responses across sites within neocortex. With short-train ICMS parameters, stimulation at individual sites within motor cortex predominantly results in responses on the contralateral side of the body that are characterized by simple flexion or extension across a single joint. With repeated seizures there was a substantial increase in the number of stimulation sites that exhibited complex forelimb movement responses rather than the simple flexion or extension movements. These new complex forelimb responses occurred as combinations of simple movement responses and were present contralateral to stimulation and often bilaterally. This result was also seen after direct application of ZD7228 and in HCN1 mice. Additionally, no further change in ICMS responses occurred when the effects of ZD7228 were tested in HCN1 mice. Since experimental seizures had previously been shown to impair skilled motor behavior, additional experiments tested whether genetic or pharmacological manipulation of HCN channels also affected skilled motor behavior in awake behaving rodents. HCN1 mice exhibited decreased performance and atypical movements on a skilled forelimb-reaching task. Na€ıve rats given local