Motor cortical HCN channels: a contributor to coordinated forelimb movements in rodents?

Our ability to generate movements is what allows us to interact with the world in purposeful and goal‐directed ways. The movement repertoire of the mammalian central nervous system is immense and ranges from simple rhythmic locomotion, such as swimming and walking, to more complex skilled forelimb movements, such as reaching and grasping for objects.

One particular brain region, long known for playing a central role in the generation of voluntary movements, is the motor cortex (MCtx). Observational studies, using a variety of motor tasks, have shown that MCtx neurones robustly change their discharge rate during preparation, execution and adaptation of movements. In addition, direct excitation of MCtx layer V projection neurones, using electrical stimulation, evoke limb movement in experimental animals (Asanuma & Rosén, 1972). The MCtx is functionally organised and an early concept was that neuronal assemblies in a particular area encode a specific movement, giving rise to a MCtx motor map representation (Asanuma & Rosén, 1972). However, it is now firmly established that extensive overlapping of boundaries between these representations exist in the MCtx, and a single area can encode different movements, involving various body parts, depending on specific circumstances such as neuronal discharge rate (Graziano & Aflalo, 2007).

The detailed cellular mechanisms underlying the encoding of separate movement responses executed upon MCtx stimulation are poorly understood. Although a great deal is known about the composition of ion channels in MCtx neurones, the mechanisms by which specific conductances participate in the separation, generation and representation of distinct movements in MCtx remain largely unknown.

Given their ability to affect synaptic integration and modulate discharge probability (Lüthi & McCormick, 1998), hyperpolarisation‐activated non‐selective cation (HCN) channels are a potential molecular candidate for orchestrating the segregation of movements in MCtx.

The present study by Boychuk et al. (2017), recently published in The Journal of Physiology, investigated the role of HCN channels for the execution of movement behaviours elicited by electrical MCtx stimulation, in addition to their role in the fidelity and accuracy of skilled reaching. The authors utilised pharmacological techniques, induction of repeated seizures, and a genetic knockout model. In this Journal Club article we focus on the experiments employing pharmacology and genetic knockout.

Boychuk et al. (2017) used the classical technique of intra‐cortical micro‐stimulation (ICMS) to elicit forelimb movements in anaesthetised rodents. Following delivery of incremental amounts of current through a stimulation electrode at a single site in MCtx layer V, primary and multiple movements were observed and quantified. The authors determined segregation thresholds when ICMS applied to a given area triggered the primary movement or multiple complex movements, enabling them to investigate the causal role of HCN channels in the organisation of the motor map.

ICMS experiments in anaesthetised rats showed that local suppression of HCN channels in MCtx, using the antagonist ZD7288, increased the proportion of stimulation sites where multiple complex movements could be elicited. These multiple movements consisted of some primary, lower‐threshold and some additional, higher‐threshold movements. This set of experiments thus compellingly revealed that multiple movement representations exist within a single site of rat MCtx and that these representations can be exposed by local suppression of HCN channels. Intriguingly, the stimulus threshold required to evoke primary movements was sensitive to HCN suppression only in rats anaesthetised with α‐chloralose, but not with ketamine–xylazine. This suggests that ZD7288‐mediated HCN channel modulation may depend on the anaesthetic used.

Next, Boychuk et al. evaluated the effect of ZD7288 on MCtx motor map representations in mice. Similar to rats, ZD7288 increased the proportion of sites where multiple movements could be triggered. This also enabled the authors to use a transgenic knockout line lacking the HCN1 gene (HCN^KO) that encodes a type of HCN channel. The proportion of the motor map area that evoked multiple movements was higher in the HCN1^KO mice than in wild‐type, and ZD7288 did not increase this proportion further. This indicates that HCN channels play a causal role in the observed effect of ZD7288. Somewhat counter‐intuitively, and opposite to that observed in rats, the stimulus threshold for evoking multiple movements was significantly increased in wild‐type mice following ZD7288 application. As expected, this manipulation had no effect on the HCN1^KO mice. In addition, these experiments revealed that the stimulus threshold required to evoke primary movements was significantly increased after ZD7288 application in wild‐type mice, while HCN1^KO mice were insensitive to this effect. Altogether, these experiments led investigators to suggest that MCtx HCN channels are causally involved in the separation of discrete primary movements, but also, that the ZD7288 sensitivity of primary forelimb movement threshold differs between mice and rats.

The last cohort of experiments performed in the present study evaluated the accuracy of a skilled forelimb motor task before and after acute MCtx HCN channel suppression, using local ZD7288 infusion. For this purpose, the authors employed a behavioural protocol in which rats were trained to successfully reach and grasp for a single sugar pellet. Supporting and extending their electrophysiological results, Boychuk et al. (2017) showed, that acute MCtx HCN channel suppression significantly decreased the success rate in the reaching task, and that specific movement components of this motor behaviour were especially affected, i.e. advancing and grasping. Taken together, this experiment demonstrates a functional impairment of skilled forelimb motor function following MCtx HCN channel suppression, suggesting an important role of HCN channels for accurate and effortless goal‐directed forelimb movement execution.

This study by Boychuk et al. brings novel and important insights into what cellular mechanisms form the basis for the functional organisation of MCtx sub‐regions and the division of representational motor maps. The proposed involvement of HCN channels in MCtx and their role in motor control micro‐circuits has previously been suggested (Sheets et al. 2011). However, the work by Boychuk et al. (2017) translates and extends previous in vitro studies and hypotheses into the living, behaving rodent thereby providing unique experimental evidence for the involvement of HCN channels in MCtx movement representation segregation.

An intriguing and less obvious finding from the study by Boychuk et al. is the observation that the consequence of MCtx HCN channel suppression on forelimb movement threshold is sensitive to the choice of anaesthesia. Suppressing HCN channels in the presence of ketamine–xylazine, that inhibits glutamatergic NMDA receptors and lowers global noradrenaline levels, either increased or did not change primary forelimb threshold. In contrast, the same experiment decreased primary forelimb threshold when GABAergic transmission was increased with α‐chloralose. This difference in the effect of ZD7288 could be due to a direct effect on HCN channels mediated by the anaesthetic used. For example, the concentrations of ketamine used for anaesthesia also modulate HCN1 channels (Chen et al. 2009).

An alternative interpretation might be that the MCtx network state, and composition and levels of neuromodulators, are important determinants for the HCN channel‐mediated effects on MCtx neuronal excitability and synaptic integration. As described, the ICMS experiments conducted in the present study were performed in anaesthetised rodents, albeit movement and locomotion are behaviours usually associated with states of wakefulness and higher levels of arousal. It is now well established that during anaesthesia slow‐wave sleep and quiet wakefulness, cortical networks, including MCtx, remain in a synchronised state and then during active behaviour the cortical networks enter an activated asynchronous state. This transformation in cortical state profoundly alters membrane potential dynamics and discharge patterns of cortical pyramidal neurones. Since the HCN channels are non‐selective cation channels and are highly sensitive to membrane potential fluctuations, their overall effect on synaptic integration and discharge probability can vary with membrane potential changes (Lüthi & McCormick, 1998). For future experiments, it should be investigated if the effects of HCN suppression correlate with MCtx network state.

In conclusion, the present work by Boychuk et al. (2017) shows that MCtx HCN channel suppression increases the representation of ICMS‐evoked complex multiple forelimb movements in rodents. Furthermore, acute local MCtx HCN channel suppression resulted in impaired reaching abilities in a skilled goal‐directed forelimb motor task. Altogether, these suggest that MCtx HCN channels act as important gatekeepers for the execution of movement representations. The findings made by Boychuk et al. (2017) will thus unquestionably pave the way for future studies interrogating the mechanistic basis for how movements are represented in MCtx and how the activity within these neuronal circuits accomplishes coordinated movement generation.

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Competing interests

The authors declare that their opinion was provided in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.