Displaying the autonomic processing network in humans - a global tractography approach.

Regulation of the internal homeostasis is modulated by the central autonomic system. So far, the view of this system is determined by animal and human research focusing on cortical and subcortical grey substance regions. To provide an overview based on white matter architecture, we used a global tractography approach to reconstruct a network of tracts interconnecting brain regions that are known to be involved in autonomic processing. Diffusion weighted imaging data were obtained from subjects of the human connectome project (HCP) database. Resulting tracts are in good agreement with previous studies assuming a division of the central autonomic system into a cortical (CAN) and a subcortical network (SAN): the CAN consist of three subsystems that encompass all cerebral lobes and overlap within the insular cortex: a parieto-anterior-temporal pathway (PATP), an occipito-posterior-temporo-frontal pathway (OPTFP) and a limbic pathway. The SAN on the other hand connects the hypothalamus to the periaqueductal grey and locus coeruleus, before it branches into a dorsal and a lateral part that target autonomic nuclei in the rostral medulla oblongata. Our approach furthermore reveals how the CAN and SAN are interconnected: the hypothalamus can be considered as the interface-structure of the SAN, whereas the insula is the central hub of the CAN. The hypothalamus receives input from prefrontal cortical fields but is also connected to the ventral apex of the insular cortex. Thus, a holistic view of the central autonomic system could be created that may promote the understanding of autonomic signaling under physiological and pathophysiological conditions.

Motor Learning Induces Profound but Delayed Dendritic Plasticity in M1 Layer II/III Pyramidal Neurons.

Motor learning depends on plastic reorganization of neural networks within the primary motor cortex (M1). In the circuitry of M1, integration and processing of afferent inputs is executed by pyramidal neurons of layer II/III. Thus, an involvement of these layer II/III pyramids in learning-induced changes is highly plausible. We therefore analyzed dendritic plasticity in layer II/III pyramidal cells on Golgi-Cox silver-impregnated sections after training of a forelimb reaching task. Based on their location within layer II/III, neurons were assigned to either a superficial or a deep population. After training, morphological changes occurred in both superficial and deep layer II/III pyramids. Overall, a decrease in dendritic length could be observed. In detail, superficial cells showed a significant reduction in the length of the apical dendrite after training ended in contrast to deep layer II/III pyramids, where dendritic length initially remained stable. Both types of neurons showed a transient increment in complexity of the distal apical dendrite 30 days after training. Findings were different in basal dendrites: length and complexity continuously decreased in superficial and deep layer II/III pyramids. Spine density increased in apical and basal dendrites of both superficial and deep layer II/III neurons, likely an effect of ageing that occurred independently from motor learning. This increase in spine density was accompanied with a morphological change towards stubby- and mushroom-like spines. Thus, profound but delayed changes occurred within the dendritic compartment of layer II/III pyramidal cells.

Progressive secondary exo-focal dopaminergic neurodegeneration occurs in not directly connected midbrain nuclei after pure motor-cortical stroke.

Transsynaptic anterograde and retrograde degeneration of neurons and neural fibers are assumed to trigger local excitotoxicity and inflammatory processes. These processes in turn are thought to drive exo-focal neurodegeneration in remote areas connected to the infarcted tissue after ischemic stroke. In the case of middle cerebral artery occlusion (MCAO), in which striato-nigral connections are affected, the hypothesis of inflammation-induced remote neurodegeneration is based on the temporal dynamics of an early appearance of inflammatory markers in midbrain followed by dopaminergic neuronal loss. To test the hypothesis of a direct transsynaptic mediation of secondary exo-focal post-ischemic neurodegeneration, we used a photochemical induction of a stroke (PTS) in Sprague-Dawley rats restricted to motor cortex (MC), thereby sparing the striatal connections to dopaminergic midbrain nuclei. To dissect the temporal dynamics of post-ischemic neurodegeneration, we analyzed brain sections harvested at day 7 and 14 post stroke. Here, an unexpectedly pronounced and widespread loss of dopaminergic neurons occurred 14 days after stroke also affecting dopaminergic nuclei that are not directly coupled to MC. Since the pattern of neurodegeneration in case of a pure motor stroke is similar to a major stroke including the striatum, it is unlikely that direct synaptic coupling is a prerequisite for delayed secondary exo-focal post ischemic neurodegeneration. Furthermore, dopaminergic neurodegeneration was already detected by Fluoro-Jade C staining at day 7, coinciding with a solely slight inflammatory response. Thus, inflammation cannot be assumed to be the primary driver of exo-focal post-ischemic cell death. Moreover, nigral substance P (SP) expression indicated intact striato-nigral innervation after PTS, whereas opposing effects on SP expression after striatal infarcts argue against a critical role of SP in neurodegenerative or inflammatory processes during exo-focal neurodegeneration.

Temporal course of gene expression during motor memory formation in primary motor cortex of rats.

Motor learning is associated with plastic reorganization of neural networks in primary motor cortex (M1) that depends on changes in gene expression. Here, we investigate the temporal profile of these changes during motor memory formation in response to a skilled reaching task in rats. mRNA-levels were measured 1h, 7h and 24h after the end of a training session using microarray technique. To assure learning specificity, trained animals were compared to a control group. In response to motor learning, genes are sequentially regulated with high time-point specificity and a shift from initial suppression to later activation. The majority of regulated genes can be linked to learning-related plasticity. In the gene-expression cascade following motor learning, three different steps can be defined: (1) an initial suppression of genes influencing gene transcription. (2) Expression of genes that support translation of mRNA in defined compartments. (3) Expression of genes that immediately mediates plastic changes. Gene expression peaks after 24h - this is a much slower time-course when compared to hippocampus-dependent learning, where peaks of gene-expression can be observed 6-12h after training ended.

The effect of surgery and intracerebral injections on motor skill learning in rats: results from a database analysis.

Male Long-Evans rats are often used to investigate neural mechanisms of learning in the motor system. Successful acquisition of a skilled motor task is influenced by various variables such as animal supplier and batch membership. In this retrospective analysis of our laboratory database, we investigate how head and brain surgery as well as intracerebral injections that were performed to address particular scientific questions affect motor learning. Overall, invasive interventions (n=90) slow the acquisition of a skilled-reaching task when compared to naïve animals (n=184; P=0.01). With respect to subgroups, this detrimental effect widely differs between particular procedures: whereas epidural implantations of thin-film electrode arrays and punctual injection through pre-implanted cannulas into primary motor cortex (M1) do not interfere with learning, skill acquisition is slowed after chronic infusion using osmotic minipumps into M1 and skill acquisition is lastingly impaired after bilateral cannula implantation within the dorsal striatum. In line with previous reports, breeder-specific differences could be observed in the analysis of the overall population. In summary, interventions may impair learning-behavior in an unpredictable fashion. Thus, a comparison of behavioral data to a naïve population is recommended to be aware of these drawbacks.

Impaired implicit learning and feedback processing after stroke.

The ability to learn is assumed to support successful recovery and rehabilitation therapy after stroke. Hence, learning impairments may reduce the recovery potential. Here, the hypothesis is tested that stroke survivors have deficits in feedback-driven implicit learning. Stroke survivors (n=30) and healthy age-matched control subjects (n=21) learned a probabilistic classification task with brain activation measured using functional magnetic resonance imaging in a subset of these individuals (17 stroke and 10 controls). Stroke subjects learned slower than controls to classify cues. After being rewarded with a smiley face, they were less likely to give the same response when the cue was repeated. Stroke subjects showed reduced brain activation in putamen, pallidum, thalamus, frontal and prefrontal cortices and cerebellum when compared with controls. Lesion analysis identified those stroke survivors as learning-impaired who had lesions in frontal areas, putamen, thalamus, caudate and insula. Lesion laterality had no effect on learning efficacy or brain activation. These findings suggest that stroke survivors have deficits in reinforcement learning that may be related to dysfunctional processing of feedback-based decision-making, reward signals and working memory.

Biphasic plasticity of dendritic fields in layer V motor neurons in response to motor learning.

Motor learning is associated with plastic reorganization of neural networks in primary motor cortex (M1) that advances through stages. An initial increment in spine formation is followed by pruning and maturation one week after training ended. A similar biphasic course was described for the size of the forelimb representation in M1. This study investigates the evolution of the dendritic architecture in response to motor skill training using Golgy-Cox silver impregnation in rat M1. After learning of a unilateral forelimb-reaching task to plateau performance, an increase in dendritic length of layer V pyramidal neurons (i.e. motor neurons) was observed that peaked one month after training ended. This increment in dendritic length reflected an expansion of the distal dendritic compartment. After one month dendritic arborization shrinks even though animals retain task performance. This pattern of evolution was observed for apical and basal dendrites alike - although the increase in dendritic length occurs faster in basal than in apical dendrites. Dendritic plasticity in response to motor training follows a biphasic course with initial expansion and subsequent shrinkage. This evolution takes fourth as long as the biphasic reorganization of spines or motor representations.

Sub-processes of motor learning revealed by a robotic manipulandum for rodents.

Rodent models are widely used to investigate neural changes in response to motor learning. Usually, the behavioral readout of motor learning tasks used for this purpose is restricted to a binary measure of performance (i.e. "successful" movement vs. "failure"). Thus, the assignability of research in rodents to concepts gained in human research - implying diverse internal models that constitute motor learning - is still limited. To solve this problem, we recently introduced a three-degree-of-freedom robotic platform designed for rats (the ETH-Pattus) that combines an accurate behavioral readout (in the form of kinematics) with the possibility to invasively assess learning related changes within the brain (e.g. by performing immunohistochemistry or electrophysiology in acute slice preparations). Here, we validate this platform as a tool to study motor learning by establishing two forelimb-reaching paradigms that differ in degree of skill. Both conditions can be precisely differentiated in terms of their temporal pattern and performance levels. Based on behavioral data, we hypothesize the presence of several sub-processes contributing to motor learning. These share close similarities with concepts gained in humans or primates.

Region and task-specific activation of Arc in primary motor cortex of rats following motor skill learning.

Motor learning requires protein synthesis within the primary motor cortex (M1). Here, we show that the immediate early gene Arc/Arg3.1 is specifically induced in M1 by learning a motor skill. Arc mRNA was quantified using a fluorescent in situ hybridization assay in adult Long-Evans rats learning a skilled reaching task (SRT), in rats performing reaching-like forelimb movement without learning (ACT) and in rats that were trained in the operant but not the motor elements of the task (controls). Apart from M1, Arc expression was assessed within the rostral motor area (RMA), primary somatosensory cortex (S1), striatum (ST) and cerebellum. In SRT animals, Arc mRNA levels in M1 contralateral to the trained limb were 31% higher than ipsilateral (p<0.001), 31% higher than in the contralateral M1 of ACT animals (p<0.001) and 48% higher than in controls (p<0.001). Arc mRNA expression in SRT was positively correlated with learning success between two sessions (r=0.52; p=0.026). For RMA, S1, ST or cerebellum no significant differences in Arc mRNA expression were found between hemispheres or across behaviors. As Arc expression has been related to different forms of cellular plasticity, these findings suggest a link between M1 Arc expression and motor skill learning in rats.

Dopaminergic modulation of motor maps in rat motor cortex: an in vivo study.

While the primary motor cortex (M1) is know to receive dopaminergic projections, the functional role of these projections is poorly characterized. Here, it is hypothesized that dopaminergic signals modulate M1 excitability and somatotopy, two features of the M1 network relevant for movement execution and learning. To test this hypothesis, movement responses evoked by electrical stimulation using an electrode grid implanted epidurally over the caudal motor cortex (M1) were assessed before and after an intracortical injection of D1- (R-(+),8-chloro,7-hydroxy,2,3,4,5,-tetra-hydro,3-methyl,5-phenyl,1-H,3-benzazepine maleate, SCH 23390) or D2-receptor (raclopride) antagonists into the M1 forelimb area of rats. Stimulation mapping of M1 was repeated after 24 h. D2-inhibition reduced the size of the forelimb representation by 68.5% (P<0.001). Movements thresholds, i.e., minimal currents required to induce movement responses increased by 37.5% (P<0.001), and latencies increased by 35.9% (P<0.01). Twenty-4 h after the injections these effects were reversed. No changes were observed with D1-antagonist or vehicle. By enhancing intracortical excitability and signal transduction, D2-mediated dopaminergic signaling may affect movement execution, e.g. by enabling task-related muscle activation synergies, and learning.