Thenar Muscle Motor Imagery Increases Spinal Motor Neuron Excitability of the Abductor Digiti Minimi Muscle

When a person attempts intended finger movements, unintended finger movement also occur, a phenomenon called “enslaving”. Given that motor imagery (MI) and motor execution (ME) share a common neural foundation, we hypothesized that the enslaving effect on the spinal motor neuron excitability occurs during MI. To investigate this hypothesis, electromyography (EMG) and F-wave analysis were conducted in 11 healthy male volunteers. Initially, the EMG activity of the left abductor digiti minimi (ADM) muscle during isometric opposition pinch movement by the left thumb and index finger at 50% maximal effort was compared with EMG activity during the Rest condition. Next, the F-wave and background EMG recordings were performed under the Rest condition, followed by the MI condition. Specifically, in the Rest condition, subjects maintained relaxation. In the MI condition, they imagined isometric left thenar muscle activity at 50% maximal voluntary contraction (MVC). During ME, ADM muscle activity was confirmed. During the MI condition, both F-wave persistence and the F-wave/M-wave amplitude ratio obtained from the ADM muscle were significantly increased compared with that obtained during the Rest condition. No difference was observed in the background EMG between the Rest and MI conditions. These results suggest that MI of isometric intended finger muscle activity at 50% MVC facilitates spinal motor neuron excitability corresponding to unintended finger muscle. Furthermore, MI may induce similar modulation of spinal motor neuron excitability as actual movement.

CX3CL1 action on microglia protects from diet-induced obesity by restoring POMC neuronal excitability and melanocortin system activity impaired by high-fat diet feeding

Objective: Diet-induced obesity (DIO) is associated with hypothalamic microglial activation and dysfunction of the melanocortin pathway, but the molecular mechanisms linking the two remain unclear. Previous studies have hypothesized that microglial inflammatory signaling is linked with impaired pro-opiomelanocortin (POMC) neuron function, but this mechanism has never been directly tested in vivo. We addressed this hypothesis using the specific microglial silencing molecule, CX3CL1 (fractalkine), to determine whether reducing hypothalamic microglial activation can restore POMC/melanocortin signaling in the brain to protect against DIO. Methods: We performed metabolic analyses in mice with targeted viral overexpression of CX3CL1 in the hypothalamus exposed to high fat diet (HFD). Electrophysiologic recording in hypothalamic slices from POMC-MAPT-GFP mice was used to determine the effects of HFD feeding and microglial silencing via minocycline or CX3CL1 on GFP-labeled POMC neurons. Finally, mice with hypothalamic overexpression of CX3CL1 received central treatment with the melanocortin receptor antagonist SHU-9119 to determine whether melanocortin signaling is required for the metabolic benefits of CX3CL1. Results: We found that targeted expression of both soluble and membrane-bound forms of CX3CL1 in the mediobasal hypothalamus potently reduced weight gain and increased leptin sensitivity in animals exposed to high fat diet. The protective effect of CX3CL1 rescued diet-induced changes in POMC neuron excitability and required intact melanocortin receptor signaling in vivo. Conclusion: Our results provide the first evidence that HFD-induced POMC neuron dysfunction involves microglial activation. Furthermore, our study suggests that the anti-obesity action of CX3CL1 is mediated through the restoration of POMC neuron excitability and melanocortin signaling.

TRPC1/5-CaV3 Complex Mediates Leptin-Induced Excitability in Hypothalamic Neurons

Leptin regulates hypothalamic POMC+ (pro-opiomelanocortin) neurons by inducing TRPC (Transient Receptor Potential Cation) channel-mediate membrane depolarization. The role of TRPC channels in POMC neuron excitability is clearly established; however, it remains unknown whether their activity alone is sufficient to trigger excitability. Here we show that the right-shift voltage induced by the leptin-induced TRPC channel-mediated depolarization of the resting membrane potential brings T-type channels into the active window current range, resulting in an increase of the steady state T-type calcium current from 40 to 70% resulting in increased intrinsic excitability of POMC neurons. We assessed the role and timing of T-type channels on excitability and leptin-induced depolarization in vitro in cultured mouse POMC neurons. The involvement of TRPC channels in the leptin-induced excitability of POMC neurons was corroborated by using the TRPC channel inhibitor 2APB, which precluded the effect of leptin. We demonstrate T-type currents are indispensable for both processes, as treatment with NNC-55-0396 prevented the membrane depolarization and rheobase changes induced by leptin. Furthermore, co-immunoprecipitation experiments suggest that TRPC1/5 channels and CaV3.1 and CaV3.2 channels co-exist in complex. The functional relevance of this complex was corroborated using intracellular Ca2+ chelators; intracellular BAPTA (but not EGTA) application was sufficient to preclude POMC neuron excitability. However, leptin-induced depolarization still occurred in the presence of either BAPTA or EGTA suggesting that the calcium entry necessary to self-activate the TRPC1/5 complex is not blocked by the presence of BAPTA in hypothalamic neurons. Our study establishes T-type channels as integral part of the signaling cascade induced by leptin, modulating POMC neuron excitability. Leptin activation of TRPC channels existing in a macromolecular complex with T-type channels recruits the latter by locally induced membrane depolarization, further depolarizing POMC neurons, triggering action potentials and excitability.

Analysis of Age-Dependent Alterations in Excitability Properties of CA1 Pyramidal Neurons in an APPPS1 Model of Alzheimer’s Disease

Age-dependent accumulation of amyloid-β, provoking increasing brain amyloidopathy, triggers abnormal patterns of neuron activity and circuit synchronization in Alzheimer’s disease (AD) as observed in human AD patients and AD mouse models. Recent studies on AD mouse models, mimicking this age-dependent amyloidopathy, identified alterations in CA1 neuron excitability. However, these models generally also overexpress mutated amyloid precursor protein (APP) and presenilin 1 (PS1) and there is a lack of a clear correlation of neuronal excitability alterations with progressive amyloidopathy. The active development of computational models of AD points out the need of collecting such experimental data to build a reliable disease model exhibiting AD-like disease progression. We therefore used the feature extraction tool of the Human Brain Project (HBP) Brain Simulation Platform to systematically analyze the excitability profile of CA1 pyramidal neuron in the APPPS1 mouse model. We identified specific features of neuron excitability that best correlate either with over-expression of mutated APP and PS1 or increasing Aβ amyloidopathy. Notably, we report strong alterations in membrane time constant and action potential width and weak alterations in firing behavior. Also, using a CA1 pyramidal neuron model, we evidence amyloidopathy-dependent alterations in Ih. Finally, cluster analysis of these recordings showed that we could reliably assign a trace to its correct group, opening the door to a more refined, less variable analysis of AD-affected neurons. This inter-disciplinary analysis, bringing together experimentalists and modelers, helps to further unravel the neuronal mechanisms most affected by AD and to build a biologically plausible computational model of the AD brain.

DNA Methyltransferase 1 and 3a Expression in the Frontal Cortex Regulates Palatable Food Consumption

DNA methylation is an important regulatory mechanism in the control of neuronal function. Both during development and following exposure to salient stimuli, plasticity in the methylation of cytosine residues leads to a change in neuron excitability that subsequently sculpts animal behavior. However, although the response of DNA methyltransferase enzymes in adult neurons to stimuli such as drugs of abuse have been described, less is known about how these enzymes regulate methylation at specific loci to change the drive to ingest natural rewards. Specifically, we do not understand how changes in methylation within important brain areas known to regulate palatable food intake can affect ingestion, while a detailed investigation of the neurophysiological and genomic effects of perturbing methyltransferase function has not been pursued. By deleting DNA methyltransferase 1 and 3a in the mouse prefrontal cortex, we observed the requirement for these enzymes in the regulation of nutrient rich food consumption in the absence of any effect on the intake of low fat and low sugar chow. We also determined that the deletion profoundly affected neuron excitability within pyramidal cells resident in superficial layers II/III of the cortex but had little effect in deep layer V neurons. Finally, reduced representation bisulfite sequencing revealed both hypo and hypermethylation in response to methyltransferase deletion, an effect that was observed in binding sites for retinoic acid receptor beta (RARβ) located within regulatory regions of genes known to affect neuronal function. Together, our data suggest that alterations in the actions of RARβ could shift neuronal activity to reduce palatable food intake.