A conditional transgenic reporter of presynaptic terminals reveals novel features of the mouse corticospinal tract

The alpha2 nicotinic acetylcholine receptor, a subunit with unique and selective expression in inhibitory interneurons associated with principal cells

Nicotinic acetylcholine receptors (nAChRs) play crucial roles in various human disorders, with the α7, α4, α6, and α3-containing nAChR subtypes extensively studied in relation to conditions such as Alzheimer's disease, Parkinson's disease, nicotine dependence, mood disorders, and stress disorders. In contrast, the α2-nAChR subunit has received less attention due to its more restricted expression and the scarcity of specific agonists and antagonists for studying its function. Nevertheless, recent research has shed light on the unique expression pattern of the Chrna2 gene, which encodes the α2-nAChR subunit, and its involvement in distinct populations of inhibitory interneurons. This review highlights the structure, pharmacology, localization, function, and disease associations of α2-containing nAChRs and points to the unique expression pattern of the Chrna2 gene and its role in different inhibitory interneuron populations. These populations, including the oriens lacunosum moleculare (OLM) cells in the hippocampus, Martinotti cells in the neocortex, and Renshaw cells in the spinal cord, share common features and contribute to recurrent inhibitory microcircuits. Thus, the α2-nAChR subunit's unique expression pattern in specific interneuron populations and its role in recurrent inhibitory microcircuits highlight its importance in various physiological processes. Further research is necessary to uncover the comprehensive functionality of α2-containing nAChRs, delineate their specific contributions to neuronal circuits, and investigate their potential as therapeutic targets for related disorders.

Exploiting the molecular diversity of the synapse to investigate neuronal communication: A guide through the current toolkit

Chemical synapses are tiny and overcrowded environments, deeply embedded inside brain tissue and enriched with thousands of protein species. Many efforts have been devoted to developing custom approaches for evaluating and modifying synaptic activity. Most of these methods are based on the engineering of one or more synaptic protein scaffolds used to target active moieties to the synaptic compartment or to manipulate synaptic functioning. In this review, we summarize the most recent methodological advances and provide a description of the involved proteins as well as the operation principle. Furthermore, we highlight their advantages and limitations in relation to studies of synaptic transmission in vitro and in vivo. Concerning the labelling methods, the most important challenge is how to extend the available approaches to the in vivo setting. On the other hand, for those methods that allow manipulation of synaptic function, this limit has been overcome using optogenetic approaches that can be more easily applied to the living brain. Finally, future applications of these methods to neuroscience, as well as new potential routes for development, are discussed.

Corticospinal populations broadcast complex motor signals to coordinated spinal and striatal circuits

Many models of motor control emphasize the role of sensorimotor cortex in movement, principally through the projections that corticospinal neurons (CSNs) make to the spinal cord. Additionally, CSNs possess expansive supraspinal axon collaterals, the functional organization of which is largely unknown. Using anatomical and electrophysiological circuit-mapping techniques in the mouse, we reveal dorsolateral striatum as the preeminent target of CSN collateral innervation. We found that this innervation is biased so that CSNs targeting different striatal pathways show biased targeting of spinal cord circuits. Contrary to more conventional perspectives, CSNs encode not only individual movements, but also information related to the onset and offset of motor sequences. Furthermore, similar activity patterns are broadcast by CSN populations targeting different striatal circuits. Our results reveal a logic of coordinated connectivity between forebrain and spinal circuits, where separate CSN modules broadcast similarly complex information to downstream circuits, suggesting that differences in postsynaptic connectivity dictate motor specificity.

Activity-based anorexia animal model: a review of the main neurobiological findings

BACKGROUND

The genesis of anorexia nervosa (AN), a severe eating disorder with a pervasive effect on many brain functions such as attention, emotions, reward processing, cognition and motor control, has not yet been understood. Since our current knowledge of the genetic aspects of AN is limited, we are left with a large and diversified number of biological, psychological and environmental risk factors, called into question as potential triggers of this chronic condition with a high relapse rate. One of the most valid and used animal models for AN is the activity-based anorexia (ABA), which recapitulates important features of the human condition. This model is generated from naïve rodents by a self-motivated caloric restriction, where a fixed schedule food delivery induces spontaneous increased physical activity.

AIM

In this review, we sought to provide a summary of the experimental research conducted using the ABA model in the pursuit of potential neurobiological mechanism(s) underlying AN.

METHOD

The experimental work presented here includes evidence for neuroanatomical and neurophysiological changes in several brain regions as well as for the dysregulation of specific neurochemical synaptic and neurohormonal pathways.

RESULTS

The most likely hypothesis for the mechanism behind the development of the ABA phenotype relates to an imbalance of the neural circuitry that mediates reward processing. Evidence collected here suggests that ABA animals show a large set of alterations, involving regions whose functions extend way beyond the control of reward mechanisms and eating habits. Hence, we cannot exclude a primary role of these alterations from a mechanistic theory of ABA induction.

CONCLUSIONS

These findings are not sufficient to solve such a major enigma in neuroscience, still they could be used to design ad hoc further experimental investigation. The prospect is that, since treatment of AN is still challenging, the ABA model could be more effectively used to shed light on the complex AN neurobiological framework, thus supporting the future development of therapeutic strategies but also the identification of biomarkers and diagnostic tools. Anorexia Nervosa (AN) is a severe eating disorder with a dramatic effect on many functions of our brain, such as attention, emotions, cognition and motion control. Since our current knowledge of the genetic aspects behind the development of AN is still limited, many biological, psychological and environmental factors must be taken into account as potential triggers of this condition. One of the most valid animal models for studying AN is the activity-based anorexia (ABA). In this model, rodents spontaneously limit food intake and start performing increased physical activity on a running wheel, a result of the imposition of a fixed time schedule for food delivery. In this review, we provide a detailed summary of the experimental research conducted using the ABA model, which includes extended evidence for changes in the anatomy and function of the brain of ABA rodents. The hope is that such integrated view will support the design of future experiments that will shed light on the complex brain mechanisms behind AN. Such advanced knowledge is crucial to find new, effective strategies for both the early diagnosis of AN and for its treatment.

Converging integration between ascending proprioceptive inputs and the corticospinal tract motor circuit underlying skilled movement control

Converging interactions between ascending proprioceptive afferents and descending corticospinal tract projections are critical in the modulation and coordination of skilled motor behaviors. Fundamental to these processes are the functional inputs and the mechanisms of integration in the brain and spinal cord between proprioceptive and corticospinal tract information. In this review, we first highlight key connections between corticospinal tract motor circuit and spinal interneurons that receive proprioceptive inputs. We will also address corticospinal tract access to the presynaptic inhibitory system in the spinal cord and its role in modulating proprioceptive stimuli. Lastly, we will focus on the corticospinal neuron influences on the dorsal column nuclei complex, an integration hub for processing ascending somatosensory information.

Absence of Subcerebral Projection Neurons Is Beneficial in a Mouse Model of Amyotrophic Lateral Sclerosis

OBJECTIVE

Recent studies carried out on amyotrophic lateral sclerosis patients suggest that the disease might initiate in the motor cortex and spread to its targets along the corticofugal tracts. In this study, we aimed to test the corticofugal hypothesis of amyotrophic lateral sclerosis experimentally.

METHODS

Sod1G86R and Fezf2 knockout mouse lines were crossed to generate a model that expresses a mutant of the murine Sod1 gene ubiquitously, a condition sufficient to induce progressive motor symptoms and premature death, but genetically lacks corticospinal neurons and other subcerebral projection neurons, one of the main populations of corticofugal neurons. Disease onset and survival were recorded, and weight and motor behavior were followed longitudinally. Hyper-reflexia and spasticity were monitored using electromyographic recordings. Neurodegeneration and gliosis were assessed by histological techniques.

RESULTS

Absence of subcerebral projection neurons delayed disease onset, reduced weight loss and motor impairment, and increased survival without modifying disease duration. Absence of corticospinal neurons also limited presymptomatic hyper-reflexia, a typical component of the upper motoneuron syndrome.

INTERPRETATION

Major corticofugal tracts are crucial to the onset and progression of amyotrophic lateral sclerosis. In the context of the disease, subcerebral projection neurons might carry detrimental signals to their downstream targets. In its entirety, this study provides the first experimental arguments in favor of the corticofugal hypothesis of amyotrophic lateral sclerosis. ANN NEUROL 2020;88:688-702.