Lesions of the Basal forebrain cholinergic system impair task acquisition and abolish cortical plasticity associated with motor skill learning

α4* Nicotinic acetylcholine receptors modulate experience-based cortical depression in the adult mouse somatosensory cortex

The molecular mechanisms that mediate experience-based changes in the function of the cerebral cortex, particularly in the adult animal, are poorly understood. Here we show using in vivo voltage-sensitive dye imaging, that whisker trimming leads to depression of whisker-evoked sensory responses in primary, secondary and associative somatosensory cortical regions. Given the importance of cholinergic neurotransmission in cognitive and sensory functions, we examined whether α4-containing (α4*) nicotinic acetylcholine receptors (nAChRs) mediate cortical depression. Using knock-in mice that express YFP-tagged α4 nAChRs subunits, we show that whisker trimming selectively increased the number α4*-YFP nAChRs in layer 4 of deprived barrel columns within 24 h, which persisted until whiskers regrew. Confocal and electron microscopy revealed that these receptors were preferentially increased on the cell bodies of GABAergic neurons. To directly link these receptors with functional cortical depression, we show that depression could be induced in normal mice by topical application or micro-injection of α4* nAChR agonist in the somatosensory cortex. Furthermore, cortical depression could be blocked after whisker trimming with chronic infusions of an α4* nAChR antagonist. Collectively, these results uncover a new role for α4* nAChRs in regulating rapid changes in the functional responsiveness of the adult somatosensory cortex.

Use it and improve it or lose it: interactions between arm function and use in humans post-stroke

"Use it and improve it, or lose it" is one of the axioms of motor therapy after stroke. There is, however, little understanding of the interactions between arm function and use in humans post-stroke. Here, we explored putative non-linear interactions between upper extremity function and use by developing a first-order dynamical model of stroke recovery with longitudinal data from participants receiving constraint induced movement therapy (CIMT) in the EXCITE clinical trial. Using a Bayesian regression framework, we systematically compared this model with competitive models that included, or not, interactions between function and use. Model comparisons showed that the model with the predicted interactions between arm function and use was the best fitting model. Furthermore, by comparing the model parameters before and after CIMT intervention in participants receiving the intervention one year after randomization, we found that therapy increased the parameter that controls the effect of arm function on arm use. Increase in this parameter, which can be thought of as the confidence to use the arm for a given level of function, lead to increase in spontaneous use after therapy compared to before therapy.

Selective optogenetic stimulation of cholinergic axons in neocortex

Acetylcholine profoundly affects neocortical function, being involved in arousal, attention, learning, memory, sensory and motor function, and plasticity. The majority of cholinergic afferents to neocortex are from neurons in nucleus basalis. Nucleus basalis also contains projecting neurons that release other transmitters, including GABA and possibly glutamate. Hence, electrical stimulation of nucleus basalis evokes the release of a mixture of neurotransmitters in neocortex, and this lack of selectivity has impeded research on cholinergic signaling in neocortex. We describe a method for the selective stimulation of cholinergic axons in neocortex. We used the Cre-lox system and a viral vector to express the light-activated protein channelrhodopsin-2 in cholinergic neurons in nucleus basalis and their axons in neocortex. Labeled neurons depolarized on illumination with blue light but were otherwise unchanged. In anesthetized mice, illumination of neocortex desynchronized the local field potential, indicating that light evoked release of ACh. This novel technique will enable many new studies of the cellular, network, and behavioral physiology of ACh in neocortex.

Cognitive consilience: primate non-primary neuroanatomical circuits underlying cognition

Interactions between the cerebral cortex, thalamus, and basal ganglia form the basis of cognitive information processing in the mammalian brain. Understanding the principles of neuroanatomical organization in these structures is critical to understanding the functions they perform and ultimately how the human brain works. We have manually distilled and synthesized hundreds of primate neuroanatomy facts into a single interactive visualization. The resulting picture represents the fundamental neuroanatomical blueprint upon which cognitive functions must be implemented. Within this framework we hypothesize and detail 7 functional circuits corresponding to psychological perspectives on the brain: consolidated long-term declarative memory, short-term declarative memory, working memory/information processing, behavioral memory selection, behavioral memory output, cognitive control, and cortical information flow regulation. Each circuit is described in terms of distinguishable neuronal groups including the cerebral isocortex (9 pyramidal neuronal groups), parahippocampal gyrus and hippocampus, thalamus (4 neuronal groups), basal ganglia (7 neuronal groups), metencephalon, basal forebrain, and other subcortical nuclei. We focus on neuroanatomy related to primate non-primary cortical systems to elucidate the basis underlying the distinct homotypical cognitive architecture. To display the breadth of this review, we introduce a novel method of integrating and presenting data in multiple independent visualizations: an interactive website (http://www.frontiersin.org/files/cognitiveconsilience/index.html) and standalone iPhone and iPad applications. With these tools we present a unique, annotated view of neuroanatomical consilience (integration of knowledge).

Pairing tone trains with vagus nerve stimulation induces temporal plasticity in auditory cortex

The selectivity of neurons in sensory cortex can be modified by pairing neuromodulator release with sensory stimulation. Repeated pairing of electrical stimulation of the cholinergic nucleus basalis, for example, induces input specific plasticity in primary auditory cortex (A1). Pairing nucleus basalis stimulation (NBS) with a tone increases the number of A1 neurons that respond to the paired tone frequency. Pairing NBS with fast or slow tone trains can respectively increase or decrease the ability of A1 neurons to respond to rapidly presented tones. Pairing vagus nerve stimulation (VNS) with a single tone alters spectral tuning in the same way as NBS-tone pairing without the need for brain surgery. In this study, we tested whether pairing VNS with tone trains can change the temporal response properties of A1 neurons. In naïve rats, A1 neurons respond strongly to tones repeated at rates up to 10 pulses per second (pps). Repeatedly pairing VNS with 15 pps tone trains increased the temporal following capacity of A1 neurons and repeatedly pairing VNS with 5 pps tone trains decreased the temporal following capacity of A1 neurons. Pairing VNS with tone trains did not alter the frequency selectivity or tonotopic organization of auditory cortex neurons. Since VNS is well tolerated by patients, VNS-tone train pairing represents a viable method to direct temporal plasticity in a variety of human conditions associated with temporal processing deficits.

Cortical plasticity during motor learning and recovery after ischemic stroke

The motor system has the ability to adapt to environmental constraints and injury to itself. This adaptation is often referred to as a form of plasticity allowing for livelong acquisition of new movements and for recovery after stroke. We are not sure whether learning and recovery work via same or similar neural mechanisms. But, all these processes require widespread changes within the matrix of the brain. Here, basic mechanisms of these adaptations on the level of cortical circuitry and networks are reviewed. We focus on the motor cortices because their role in learning and recovery has been investigated more thoroughly than other brain regions.

The controlled generation of functional basal forebrain cholinergic neurons from human embryonic stem cells

An early substantial loss of basal forebrain cholinergic neurons (BFCN) is a constant feature of Alzheimer's disease and is associated with deficits in spatial learning and memory. The ability to selectively control the differentiation of human embryonic stem cells (hESCs) into BFCN would be a significant step toward a cell replacement therapy. We demonstrate here a method for the derivation of a predominantly pure population of BFCN from hESC cells using diffusible ligands present in the forebrain at developmentally relevant time periods. Overexpression of two relevant human transcription factors in hESC-derived neural progenitors also generates BFCN. These neurons express only those markers characteristic of BFCN, generate action potentials, and form functional cholinergic synapses in murine hippocampal slice cultures. siRNA-mediated knockdown of the transcription factors blocks BFCN generation by the diffusible ligands, clearly demonstrating the factors both necessary and sufficient for the controlled derivation of this neuronal population. The ability to selectively control the differentiation of hESCs into BFCN is a significant step both for understanding mechanisms regulating BFCN lineage commitment and for the development of both cell transplant-mediated therapeutic interventions for Alzheimer's disease and high-throughput screening for agents that promote BFCN survival.

Postlesion estradiol treatment increases cortical cholinergic innervations via estrogen receptor-α dependent nonclassical estrogen signaling in vivo

17β-Estradiol (E2) treatment exerts rapid, nonclassical actions via intracellular signal transduction system in basal forebrain cholinergic (BFC) neurons in vivo. Here we examined the effect of E2 treatment on lesioned BFC neurons in ovariectomized mice and the role of E2-induced nonclassical action in this treatment. Mice given an N-methyl-d-aspartic acid (NMDA) injection into the substantia innominata-nucleus basalis magnocellularis complex (SI-NBM) exhibited cholinergic cell loss in the SI-NBM and ipsilateral cholinergic fiber loss in the cortex. A single injection of E2 after NMDA lesion did not have an effect on cholinergic cell loss in the SI-NBM, but it restored the ipsilateral cholinergic fiber density in the cortex in a time- and dose-dependent manner. The most effective cholinergic fiber restoration was observed with 33 ng/g E2 treatment at 1 h after NMDA lesion. The E2-induced cholinergic fiber restoration was absent in neuron-specific estrogen receptor-α knockout mice in vivo. Selective activation of nonclassical estrogen signaling in vivo by estren induced E2-like restorative actions. Selective blockade of the MAPK or protein kinase A pathway in vivo prevented E2's ability to restore cholinergic fiber loss. Finally, studies in intact female mice revealed an E2-induced restorative effect that was similar to that of E2-treated ovariectomized mice. These observations demonstrate that a single E2 treatment restores the BFC fiber loss in the cortex, regardless of endogenous E2 levels. They also reveal the critical role of nonclassical estrogen signaling via estrogen receptor-α and protein kinase A-MAPK pathways in E2-induced restorative action in the cholinergic system in vivo.

Feasibility of the adaptive and automatic presentation of tasks (ADAPT) system for rehabilitation of upper extremity function post-stroke

BACKGROUND

Current guidelines for rehabilitation of arm and hand function after stroke recommend that motor training focus on realistic tasks that require reaching and manipulation and engage the patient intensively, actively, and adaptively. Here, we investigated the feasibility of a novel robotic task-practice system, ADAPT, designed in accordance with such guidelines. At each trial, ADAPT selects a functional task according to a training schedule and with difficulty based on previous performance. Once the task is selected, the robot picks up and presents the corresponding tool, simulates the dynamics of the tasks, and the patient interacts with the tool to perform the task.

METHODS

Five participants with chronic stroke with mild to moderate impairments (> 9 months post-stroke; Fugl-Meyer arm score 49.2 ± 5.6) practiced four functional tasks (selected out of six in a pre-test) with ADAPT for about one and half hour and 144 trials in a pseudo-random schedule of 3-trial blocks per task.

RESULTS

No adverse events occurred and ADAPT successfully presented the six functional tasks without human intervention for a total of 900 trials. Qualitative analysis of trajectories showed that ADAPT simulated the desired task dynamics adequately, and participants reported good, although not excellent, task fidelity. During training, the adaptive difficulty algorithm progressively increased task difficulty leading towards an optimal challenge point based on performance; difficulty was then continuously adjusted to keep performance around the challenge point. Furthermore, the time to complete all trained tasks decreased significantly from pretest to one-hour post-test. Finally, post-training questionnaires demonstrated positive patient acceptance of ADAPT.

CONCLUSIONS

ADAPT successfully provided adaptive progressive training for multiple functional tasks based on participant's performance. Our encouraging results establish the feasibility of ADAPT; its efficacy will next be tested in a clinical trial.

Reflections of experience-expectant development in repair of the adult damaged brain

Behavioral experience has long been known to influence functional outcome after brain injury, but only recently has its pervasive role in the reorganization of the adult brain after damage become appreciated. We briefly review findings from animal models on the role of experience in shaping neuronal events after stroke-like injury. Experience-dependent neural plasticity can be enhanced or impaired by brain damage, depending upon injury parameters and timing. The neuronal growth response to some experiences is heightened due to interactions with denervation-induced plasticity. This includes compensatory behavioral strategies developed in response to functional impairments. Early behavioral experiences can constrain later experience-dependent plasticity, leading to suboptimal functional outcome. Time dependencies and facets of neural growth patterns are reminiscent of experience-expectant processes that shape brain development. As with sensitive periods in brain development, this process may establish behavioral patterns early after brain injury which are relatively resistant to later change.

Cholinergic modulation of cognition: insights from human pharmacological functional neuroimaging

Evidence from lesion and cortical-slice studies implicate the neocortical cholinergic system in the modulation of sensory, attentional and memory processing. In this review we consider findings from sixty-three healthy human cholinergic functional neuroimaging studies that probe interactions of cholinergic drugs with brain activation profiles, and relate these to contemporary neurobiological models. Consistent patterns that emerge are: (1) the direction of cholinergic modulation of sensory cortex activations depends upon top-down influences; (2) cholinergic hyperstimulation reduces top-down selective modulation of sensory cortices; (3) cholinergic hyperstimulation interacts with task-specific frontoparietal activations according to one of several patterns, including: suppression of parietal-mediated reorienting; decreasing ‘effort’-associated activations in prefrontal regions; and deactivation of a ‘resting-state network’ in medial cortex, with reciprocal recruitment of dorsolateral frontoparietal regions during performance-challenging conditions; (4) encoding-related activations in both neocortical and hippocampal regions are disrupted by cholinergic blockade, or enhanced with cholinergic stimulation, while the opposite profile is observed during retrieval; (5) many examples exist of an ‘inverted-U shaped’ pattern of cholinergic influences by which the direction of functional neural activation (and performance) depends upon both task (e.g. relative difficulty) and subject (e.g. age) factors. Overall, human cholinergic functional neuroimaging studies both corroborate and extend physiological accounts of cholinergic function arising from other experimental contexts, while providing mechanistic insights into cholinergic-acting drugs and their potential clinical applications.

Cortical map plasticity improves learning but is not necessary for improved performance

Cortical map plasticity is believed to be a key substrate of perceptual and skill learning. In the current study, we quantified changes in perceptual ability after pairing tones with stimulation of the cholinergic nucleus basalis to induce auditory cortex map plasticity outside of a behavioral context. Our results provide evidence that cortical map plasticity can enhance perceptual learning. However, auditory cortex map plasticity fades over weeks even though tone discrimination performance remains stable. This observation is consistent with recent reports that cortical map expansions associated with perceptual and motor learning are followed by a period of map renormalization without a decrement in performance. Our results indicate that cortical map plasticity enhances perceptual learning, but is not necessary to maintain improved discriminative ability.

Breeder and batch-dependent variability in the acquisition and performance of a motor skill in adult Long-Evans rats

Reaching tasks are popular tools for investigating the neural mechanisms of motor skill learning and recovery from brain damage in rodents, but there is considerable unexplained variability across studies using these tasks. We investigated whether breeder, batch effects, experimenter, time of year, weight and other factors contribute to differences in the acquisition and performance of a skilled reaching task, the single pellet retrieval task, in adult male Long-Evans hooded rats. First, we retrospectively analyzed task acquisition and performance in rats from different breeding colonies that were used in several studies spanning a 3 year period in our laboratory. Second, we compared reaching variables in age-matched rats from different breeders that were trained together as a batch by the same experimenters. All rats had received daily training on the reaching task until they reached a criterion of successful reaches per attempt. We found significant breeder-dependent differences in learning rate and final performance level. This was found even when age-matched rats from different breeders were trained together by the same experimenters. There was also significant batch-to-batch variability within rats from the same breeder trained by the same experimenter. Other factors, including weight, paw preference and the experimenter, were not as strong or consistent in their contributions to differences across studies. The breeder and batch effects found within the same rat strain may reflect genetic and environmental influences on the neural substrates of motor skill learning. This is an important consideration when comparing baseline performance across studies and for controlling variability within studies.

Revisiting the cholinergic hypothesis in the development of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia affecting the elderly population today; however, there is currently no accurate description of the etiology of this devastating disorder. No single factor has been demonstrated as being causative; however, an alternative co-factors theory suggests that the interaction of multiple risk factors is responsible for AD. We have used this model, in combination with the original cholinergic hypothesis of AD to propose a "new" cholinergic hypothesis that we present in this review. This new version takes into account recent findings from the literature and our reports of removal of medial septum cholinergic projections to the hippocampus reduces both behavioural and anatomical plasticity, resulting in greater cognitive impairment in response to secondary insults (stress, injury, disease, etc.). We will first summarize the experimental results and discuss some potential mechanisms that could explain our results. We will then present our 'new' version of the cholinergic hypothesis and how it relates to the field of AD research today. Finally we will discuss some of the implications for treatment that arise from this model and present directions for future study.

Structural plasticity within highly specific neuronal populations identifies a unique parcellation of motor learning in the adult brain

Cortical networks undergo adaptations during learning, including increases in dendritic complexity and spines. We hypothesized that structural elaborations during learning are restricted to discrete subsets of cells preferentially activated by, and relevant to, novel experience. Accordingly, we examined corticospinal motor neurons segregated on the basis of their distinct descending projection patterns, and their contribution to specific aspects of motor control during a forelimb skilled grasping task in adult rats. Learning-mediated structural adaptations, including extensive expansions of spine density and dendritic complexity, were restricted solely to neurons associated with control of distal forelimb musculature required for skilled grasping; neurons associated with control of proximal musculature were unchanged by the experience. We further found that distal forelimb-projecting and proximal forelimb-projecting neurons are intermingled within motor cortex, and that this distribution does not change as a function of skill acquisition. These findings indicate that representations of novel experience in the adult motor cortex are associated with selective structural expansion in networks of functionally related, active neurons that are distributed across a single cortical domain. These results identify a distinct parcellation of cortical resources in support of learning.

ProNGF induces PTEN via p75NTR to suppress Trk-mediated survival signaling in brain neurons

Proneurotrophins and mature neurotrophins activate different signaling pathways with distinct effects on their target cells: proneurotrophins can induce apoptotic signaling via p75(NTR), whereas mature neurotrophins activate Trk receptors to influence survival and differentiation. Here, we demonstrate that the PTEN (phosphatase and tensin homolog deleted on chromosome 10) phosphatase represents a novel switch between the survival and apoptotic signaling pathways in rat CNS neurons. Simultaneous activation of p75(NTR) by proNGF and TrkB signaling by BDNF elicited apoptosis despite TrkB phosphorylation. Apoptosis induced by p75(NTR) required suppression of TrkB-induced phosphoinositide-3 kinase signaling, mediated by induction of PTEN, for apoptosis to proceed. Inhibition of PTEN restored the ability of BDNF to phosphorylate Akt and protect cultured basal forebrain neurons from proNGF-induced death. In vivo, inhibition or knockdown of PTEN after pilocarpine-induced seizures protected CNS neurons from p75(NTR)-mediated death, demonstrating that PTEN is a crucial factor mediating the balance between p75(NTR)-induced apoptotic signaling and Trk-mediated survival signaling in brain neurons.

Sleep, off-line processing, and vocal learning

The study of song learning and the neural song system has provided an important comparative model system for the study of speech and language acquisition. We describe some recent advances in the bird song system, focusing on the role of off-line processing including sleep in processing sensory information and in guiding developmental song learning. These observations motivate a new model of the organization and role of the sensory memories in vocal learning.

Reorganization in motor cortex after brachial plexus avulsion injury and repair with the contralateral C7 root transfer in rats

The purpose of our study was to establish the profile of cortical reorganization in whole BPAI on rats and evaluate changes of cortical reorganization after repair of the median nerve with the contralateral C7 root transfer. Forty adult SD rats underwent whole roots avulsion of left brachial plexus, among them 20 received contralateral C7 root transfer to the injured median nerve. Intracortical microstimulation was performed in primary motor cortex (M1) at intervals of 3, 5, 7, and 10 months, postoperatively. The maps of motor cortical responses were constructed. Five normal rats were used as the control. Results showed that stimulating right M1 elicited motion of left vibrissae, submaxilla, neck, back, and left hindlimb after left BPAI, among them neck representation area replaced the forelimb area throughout the reorganization process. The left forelimb representation area was found in the left motor cortex 5 months after the contralateral C7 root transfer and existed in both motor cortexes at 7th postoperative month. The left forelimb representation area was detected only in right motor cortex at 10th month, postoperatively. In conclusions, after the contralateral C7 root transfer for repair of the median nerve in BPAI, the cortical reorganization occurred in a time-dependent reorganization. The findings from this study demonstrate that brain involves in the functional recovery after BPAI and repair with nerve transfer and suggest that efforts to improve the results from nerve repair should address the peripheral nerve as well as the brain.

Motor cortex reorganization across the lifespan

The brain is a highly dynamic structure with the capacity for profound structural and functional change. Such neural plasticity has been well characterized within motor cortex and is believed to represent one of the neural mechanisms for acquiring and modifying motor behaviors. A number of behavioral and neural signals have been identified that modulate motor cortex plasticity throughout the lifespan in both the intact and damaged brain. Specific signals discussed in this review include: motor learning in the intact brain, motor relearning in the damaged brain, cortical stimulation, stage of development and genotype. Clinicians are encouraged to harness these signals in the development and implementation of treatment so as to maximally drive neural plasticity and functional improvements in speech, language and swallowing.

LEARNING OUTCOMES

Readers will be able to: (1) describe a set of behavioral and neural signals that modulate motor cortex plasticity in the intact and damaged brain; (2) describe the influence of stage of development on plasticity and functional outcomes; and (3) identify a known genotype that alters the capacity for motor learning and brain plasticity.

Unique contributions of distinct cholinergic projections to motor cortical plasticity and learning

The cholinergic basal forebrain projects throughout the neocortex, exerting a critical role in modulating plasticity associated with normal learning. Cholinergic modulation of cortical plasticity could arise from 3 distinct mechanisms by 1) "direct" modulation via cholinergic inputs to regions undergoing plasticity, 2) "indirect" modulation via cholinergic projections to anterior, prefrontal attentional systems, or 3) modulating more global aspects of processing via distributed inputs throughout the cortex. To segregate these potential mechanisms, we investigated cholinergic-dependent reorganization of cortical motor representations in rats undergoing skilled motor learning. Behavioral and electrophysiological consequences of depleting cholinergic inputs to either motor cortex, prefrontal cortex, or globally, were compared. We find that local depletion of cholinergic afferents to motor cortex significantly disrupts map plasticity and skilled motor behavior, whereas prefrontal cholinergic depletion has no effect on these measures. Global cholinergic depletion perturbs map plasticity comparable with motor cortex depletions but results in significantly greater impairments in skilled motor acquisition. These findings indicate that local cholinergic activation within motor cortex, as opposed to indirect regulation of prefrontal systems, modulate cortical map plasticity and motor learning. More globally acting cholinergic mechanisms provide additional support for the acquisition of skilled motor behaviors, beyond those associated with cortical map reorganization.

Axotomy-induced neurotrophic withdrawal causes the loss of phenotypic differentiation and downregulation of NGF signalling, but not death of septal cholinergic neurons

BACKGROUND

Septal cholinergic neurons account for most of the cholinergic innervations of the hippocampus, playing a key role in the regulation of hippocampal synaptic activity. Disruption of the septo-hippocampal pathway by an experimental transection of the fimbria-fornix drastically reduces the target-derived trophic support received by cholinergic septal neurons, mainly nerve growth factor (NGF) from the hippocampus. Axotomy of cholinergic neurons induces a reduction in the number of neurons positive for cholinergic markers in the medial septum. In several studies, the reduction of cholinergic markers has been interpreted as analogous to the neurodegeneration of cholinergic cells, ruling out the possibility that neurons lose their cholinergic phenotype without dying. Understanding the mechanism of cholinergic neurodegeneration after axotomy is relevant, since this paradigm has been extensively explored as an animal model of the cholinergic impairment observed in neuropathologies such as Alzheimer's disease.The principal aim of this study was to evaluate, using modern quantitative confocal microscopy, neurodegenerative changes in septal cholinergic neurons after axotomy and to assess their response to delayed infusion of NGF in rats.

RESULTS

We found that there is a slow reduction of cholinergic cells labeled by ChAT and p75 after axotomy. However, this phenomenon is not accompanied by neurodegenerative changes or by a decrease in total neuronal number in the medial septum. Although the remaining axotomized-neurons appear healthy, they are unable to respond to delayed NGF infusion.

CONCLUSIONS

Our results demonstrate that at 3 weeks, axotomized cholinergic neurons lose their cholinergic phenotype without dying and down-regulate their NGF-receptors, precluding the possibility of a response to NGF. Therefore, the physiological role of NGF in the adult septal cholinergic system is to support phenotypic differentiation and not survival of neurons. This evidence raises questions about the relationship between transcriptional regulation of the cholinergic phenotype by retrograde-derived trophic signaling and the transcriptional changes experienced when retrograde transport is impaired due to neuropathological conditions.

Enhanced spatial memory and hippocampal long-term potentiation in p75 neurotrophin receptor knockout mice

Previous reports have described increases in the size and number of cholinergic neurons in the basal forebrain in p75 neurotrophin receptor (p75(NTR)) knockout mice. In an earlier study, we also found improved spatial memory in these mice, raising the possibility that p75(NTR) regulates hippocampal function by its effects on the cholinergic basal forebrain. We therefore investigated hippocampal long-term potentiation in p75(NTR) knockout mice that shared the same genetic background as control 129/Sv mice. We also investigated heterozygous mice, carrying just one functional p75(NTR) allele. The p75(NTR) knockout mice had enhanced long-term potentiation in the Schafer collateral fiber synapses of the hippocampus. Heterozygous mice had an intermediate level, greater than controls but less than knockout mice. Hippocampal choline acetyltransferase activity was also markedly elevated in p75(NTR) knockout mice, with a smaller increase in heterozygous mice. In the Barnes maze, p75(NTR) knockout mice displayed markedly superior learning to controls, and this was evident over the three age brackets tested. At each age, the performance of heterozygous mice was intermediate to the other groups. In the open field test, p75(NTR) knockout mice exhibited greater stress-related behavioral responses, including freezing, than did control animals. There were no differences between the three groups in a test of olfactory function. The dose-dependent effects of p75(NTR) gene copy number on hippocampal plasticity and spatial memory indicate that p75(NTR) has profound effects on hippocampal function. Bearing in mind that p75(NTR) is very sparsely expressed in the adult hippocampus and has a potent effect on hippocampal choline acetyltransferase activity, the effects of p75(NTR) on hippocampal function are likely to be mediated indirectly, by its actions on basal forebrain cholinergic neurons.

A functional threshold for long-term use of hand and arm function can be determined: predictions from a computational model and supporting data from the Extremity Constraint-Induced Therapy Evaluation (EXCITE) Trial

BACKGROUND

Although spontaneous use of the more-affected arm and hand after stroke is an important determinant of participation and quality of life, a number of patients exhibit decreases in use following rehabilitative therapy. A previous neurocomputational model predicted that if the dose of therapy is sufficient to bring performance above a certain threshold, training can be stopped.

OBJECTIVE

The aim of this study was to test the hypothesis that there exists a threshold for function of the paretic arm and hand after therapy. If function is above this threshold, spontaneous use will increase in the months following therapy. In contrast, if function is below this threshold, spontaneous use will decrease.

METHODS

New computer simulations are presented showing that changes in arm use following therapy depend on a performance threshold. This prediction was tested by reanalyzing the data from the Extremity Constraint-Induced Therapy Evaluation (EXCITE) trial, a phase III randomized controlled trial in which participants received constraint-induced movement therapy for 2 weeks and were tested both 1 week and 1 year after therapy.

RESULTS

The results demonstrate that arm and hand function measured immediately after therapy predicts, on average, the long-term change of arm use. Above a functional threshold, use improves. Below this threshold, use decreases.

LIMITATIONS

The reanalysis of the EXCITE trial data provides a "group" threshold above which a majority of patients, but not all, improve spontaneously. A goal of future research is to provide the means to assess when patients reach their individual threshold.

CONCLUSION

Understanding of the causal and nonlinear relationship between limb function and daily use is important for the future development of cost-effective interventions and prevention of "rehabilitation in vain."

NGF is essential for hippocampal plasticity and learning

Nerve growth factor (NGF) is produced in the hippocampus throughout life and is retrogradely trafficked to septal cholinergic neurons, providing a potential mechanism for modulating cholinergic inputs and, thereby, hippocampal plasticity. To explore NGF modulation of hippocampal plasticity and function, NGF levels were augmented or blocked in intact adult rats, and subsequent in vivo effects on cholinergic neurons, hippocampal long-term potentiation (LTP), and learning were examined. NGF augmentation significantly enhanced cholinergic neuronal markers and facilitated induction of hippocampal LTP. Blockade of endogenous NGF significantly reduced hippocampal LTP and impaired retention of spatial memory. These findings reveal an essential role for NGF in regulating biological mechanisms related to plasticity and memory in the intact adult brain.

Dopamine in motor cortex is necessary for skill learning and synaptic plasticity

Preliminary evidence indicates that dopamine given by mouth facilitates the learning of motor skills and improves the recovery of movement after stroke. The mechanism of these phenomena is unknown. Here, we describe a mechanism by demonstrating in rat that dopaminergic terminals and receptors in primary motor cortex (M1) enable motor skill learning and enhance M1 synaptic plasticity. Elimination of dopaminergic terminals in M1 specifically impaired motor skill acquisition, which was restored upon DA substitution. Execution of a previously acquired skill was unaffected. Reversible blockade of M1 D1 and D2 receptors temporarily impaired skill acquisition but not execution, and reduced long-term potentiation (LTP) within M1, a form of synaptic plasticity critically involved in skill learning. These findings identify a behavioral and functional role of dopaminergic signaling in M1. DA in M1 optimizes the learning of a novel motor skill.

The basal forebrain cholinergic system is required specifically for behaviorally mediated cortical map plasticity

The basal forebrain cholinergic system has been implicated in the reorganization of adult cortical sensory and motor representations under many, but not all, experimental conditions. It is still not fully understood which types of plasticity require the cholinergic system and which do not. In this study, we examine the hypothesis that the basal forebrain cholinergic system is required for eliciting plasticity associated with complex cognitive processing (e.g., behavioral experiences that drive cortical reorganization) but is not required for plasticity mediated under behaviorally independent conditions. We used established experimental manipulations to elicit two distinct forms of plasticity within the motor cortex: facial nerve transections evoke reorganization of cortical motor representations independent of behavioral experience, and skilled forelimb training induces behaviorally dependent expansion of forelimb motor representations. In animals that underwent skilled forelimb training in conjunction with a facial nerve lesion, cholinergic mechanisms were required for mediating the behaviorally dependent plasticity associated with the skilled motor training but were not necessary for mediating plasticity associated with the facial nerve transection. These results dissociate the contribution of cholinergic mechanisms to distinct forms of cortical plasticity and support the hypothesis that the forebrain cholinergic system is selectively required for modulating complex forms of cortical plasticity driven by behavioral experience.

Severe scene learning impairment, but intact recognition memory, after cholinergic depletion of inferotemporal cortex followed by fornix transection

To examine the generality of cholinergic involvement in visual memory in primates, we trained macaque monkeys either on an object-in-place scene learning task or in delayed nonmatching-to-sample (DNMS). Each monkey received either selective cholinergic depletion of inferotemporal cortex (including the entorhinal cortex and perirhinal cortex) with injections of the immunotoxin ME20.4-saporin or saline injections as a control and was postoperatively retested. Cholinergic depletion of inferotemporal cortex was without effect on either task. Each monkey then received fornix transection because previous studies have shown that multiple disconnections of temporal cortex can produce synergistic impairments in memory. Fornix transection mildly impaired scene learning in monkeys that had received saline injections but severely impaired scene learning in monkeys that had received cholinergic lesions of inferotemporal cortex. This synergistic effect was not seen in monkeys performing DNMS. These findings confirm a synergistic interaction in a macaque monkey model of episodic memory between connections carried by the fornix and cholinergic input to the inferotemporal cortex. They support the notion that the mnemonic functions tapped by scene learning and DNMS have dissociable neural substrates. Finally, cholinergic depletion of inferotemporal cortex, in this study, appears insufficient to impair memory functions dependent on an intact inferotemporal cortex.

Sensorimotor behavioral effects of endothelin-1 induced small cortical infarcts in C57BL/6 mice

Mouse models have not paralleled rat models of stroke in advances in sensitive, species appropriate measures of neurological and behavioral recovery. Most available tests of mouse sensorimotor function are adaptations of those originally developed in rats and may not be as sensitive in detecting behavioral deficits after small cortical lesions in mice. Our purpose was to test the use of a vasoconstricting peptide, endothelin-1 (ET-1), to produce focal infarcts of the mouse sensorimotor cortex and to establish a behavioral test battery sensitive to resulting sensorimotor deficits. Young adult (3-5-month-old) male C57BL/6 mice received intracortical infusions of ET-1 that produced unilateral lesions of the forelimb region of the sensorimotor cortex, intracortical infusions of sterile saline, or sham surgeries. Pre-operatively and at various time points over 3 weeks post-surgery, they were administered a test battery that included measures of sensorimotor asymmetry (Corner and Bilateral Tactile Stimulation Tests), coordinated forepaw use (Cylinder and Ladder Rung Tests), and dexterous forepaw function (Pasta Matrix Reaching Test). ET-1 infusions resulted in consistently placed, focal cortical infarcts and forelimb impairments as measured with the Ladder Rung, Bilateral Tactile Stimulation, and Pasta Matrix Reaching Tests. On the Bilateral Tactile Stimulation and Pasta Matrix Reaching Tests, impairments persisted throughout the time span of observation (26 days). These results support ET-1 as a viable option for creating small, reproducible lesions of anatomical subregions in the mouse neocortex that result in lasting functional impairments in the forelimb, as observed with sufficiently sensitive measures.

Errorless practice as a possible adjuvant to donepezil in Alzheimer's disease

Six individuals with probable Alzheimer's disease (AD) participated in a phase 1 study employing a repeated measures, parallel baseline design testing the hypothesis that error-free experience during word production practice combined with an acetyl cholinesterase inhibitor would improve confrontation naming ability. While acetyl cholinesterase inhibitors are safe and delay cognition decline associated with AD, improvement over baseline cognition is less evident; clinically significant cognitive deficits persist and progress. Both animal and clinical research strongly implicate acetylcholine in learning, a form of neuroplasticity. In clinical practice, however, people with AD are given cholinergic medications without concomitant systematic/targeted retraining. In this study six participants with probable AD and taking donepezil participated in targeted word production practice using an errorless learning strategy. Results showed that combining behavioral enrichment training and an acetyl cholinesterase inhibitor resulted in significant improvements in verbal confrontation naming of trained items for three of six participants. Differences in baseline dementia severity, living conditions, and medications may have influenced the training response. Detection of substantial treatment effects in 50% of subjects suggests further language treatment studies in AD in combination with an acetyl cholinesterase inhibitor are warranted and provide useful information on inclusion/exclusion criteria for use in subsequent studies.

Reduced expression of the vesicular acetylcholine transporter causes learning deficits in mice

Storage of acetylcholine in synaptic vesicles plays a key role in maintaining cholinergic function. Here we used mice with a targeted mutation in the vesicular acetylcholine transporter (VAChT) gene that reduces transporter expression by 40% to investigate cognitive processing under conditions of VAChT deficiency. Motor skill learning in the rotarod revealed that VAChT mutant mice were slower to learn this task, but once they reached maximum performance they were indistinguishable from wild-type mice. Interestingly, motor skill performance maintenance after 10 days was unaffected in these mutant mice. We also tested whether reduced VAChT levels affected learning in an object recognition memory task. We found that VAChT mutant mice presented a deficit in memory encoding necessary for the temporal order version of the object recognition memory, but showed no alteration in spatial working memory, or spatial memory in general when tested in the Morris water maze test. The memory deficit in object recognition memory observed in VAChT mutant mice could be reversed by cholinesterase inhibitors, suggesting that learning deficits caused by reduced VAChT expression can be ameliorated by restoring ACh levels in the synapse. These data indicate an important role for cholinergic tone in motor learning and object recognition memory.

Stroke rehabilitation reaches a threshold

Motor training with the upper limb affected by stroke partially reverses the loss of cortical representation after lesion and has been proposed to increase spontaneous arm use. Moreover, repeated attempts to use the affected hand in daily activities create a form of practice that can potentially lead to further improvement in motor performance. We thus hypothesized that if motor retraining after stroke increases spontaneous arm use sufficiently, then the patient will enter a virtuous circle in which spontaneous arm use and motor performance reinforce each other. In contrast, if the dose of therapy is not sufficient to bring spontaneous use above threshold, then performance will not increase and the patient will further develop compensatory strategies with the less affected hand. To refine this hypothesis, we developed a computational model of bilateral hand use in arm reaching to study the interactions between adaptive decision making and motor relearning after motor cortex lesion. The model contains a left and a right motor cortex, each controlling the opposite arm, and a single action choice module. The action choice module learns, via reinforcement learning, the value of using each arm for reaching in specific directions. Each motor cortex uses a neural population code to specify the initial direction along which the contralateral hand moves towards a target. The motor cortex learns to minimize directional errors and to maximize neuronal activity for each movement. The derived learning rule accounts for the reversal of the loss of cortical representation after rehabilitation and the increase of this loss after stroke with insufficient rehabilitation. Further, our model exhibits nonlinear and bistable behavior: if natural recovery, motor training, or both, brings performance above a certain threshold, then training can be stopped, as the repeated spontaneous arm use provides a form of motor learning that further bootstraps performance and spontaneous use. Below this threshold, motor training is “in vain”: there is little spontaneous arm use after training, the model exhibits learned nonuse, and compensatory movements with the less affected hand are reinforced. By exploring the nonlinear dynamics of stroke recovery using a biologically plausible neural model that accounts for reversal of the loss of motor cortex representation following rehabilitation or the lack thereof, respectively, we can explain previously hard to reconcile data on spontaneous arm use in stroke recovery. Further, our threshold prediction could be tested with an adaptive train–wait–train paradigm: if spontaneous arm use has increased in the “wait” period, then the threshold has been reached, and rehabilitation can be stopped. If spontaneous arm use is still low or has decreased, then another bout of rehabilitation is to be provided.

Stroke often leaves patients with predominantly unilateral functional limitations of the arm and hand. Although recovery of function after stroke is often achieved by compensatory use of the less affected limb, improving use of the more affected limb has been associated with increased quality of life. Here, we developed a biologically plausible model of bilateral reaching movements to investigate the mechanisms and conditions leading to effective rehabilitation. Our motor cortex model accounts for the experimental observation that motor training can reverse the loss of cortical representation due to lesion. Further, our model predicts that if spontaneous arm use is above a certain threshold, then training can be stopped, as the repeated spontaneous use provides a form of motor learning that further improves performance and spontaneous use. Below this threshold, training is “in vain,” and compensatory movements with the less affected hand are reinforced. Our model is a first step in the development of adaptive and cost-effective rehabilitation methods tailored to individuals poststroke.

Functional convergence of dopaminergic and cholinergic input is critical for hippocampus-dependent working memory

Although Parkinson's disease is a movement disorder, in many patients cognitive dysfunction is an important clinical sign. It is not yet clear whether this is attributable solely to a decrease in dopamine levels, or whether other neurotransmitter systems might be involved as well. In the present study, the importance of the mesocorticolimbic dopamine pathway and a possible convergence with forebrain cholinergic projections to neocortex and hippocampus in the regulation of learning and memory abilities were investigated by using specific lesion paradigms in one or both systems. Lesioning of dopaminergic neurons in the ventral tegmental area resulted in an impaired performance in the reference memory task, whereas the execution of the working memory tasks appeared to be unaffected in the Morris water maze. Analysis of the swim paths revealed that the dopamine-depleted animals were capable of adapting a search strategy on a given testing day but failed to transfer this information to the next day, suggesting a deficit in information storage and/or recall. In contrast, cholinergic lesions alone were without effect in all test paradigms. However, when both dopamine and acetylcholine were depleted, animals were also impaired in the working memory task, indicating that a functional convergence of the inputs from these systems was critical for acquisition of spatial memory. Interestingly, such an additional acquisition deficit appeared only after hippocampal cholinergic depletion regardless of a concurrent disruption of basalo cortical cholinergic afferents. Thus, further analyses of cholinergic alterations may prove useful in better understanding the cognitive symptoms in Parkinson's disease.

Cholinergic mediation of attention: contributions of phasic and tonic increases in prefrontal cholinergic activity

Contrary to the classic description of acetylcholine (ACh) as a slowly acting neuromodulator that influences arousal states, results from experiments that employed enzyme-selective microelectrodes for the real-time monitoring of ACh release in the cortex of attentional task-performing rats indicate that cholinergic signals manifesting on multiple timescales (seconds, tens of seconds, and minutes) support, and are necessary for, the mediation of defined cognitive operations. Specifically, in the prefrontal cortex, second-based cholinergic signals support the detection of behaviorally significant cues. In contrast to these prefrontal cholinergic transients, performance-associated cholinergic activity that manifested at lower temporal resolution also was observed elsewhere in the cortex. Although tonic cholinergic signal levels were correlated with the amplitudes of cue-evoked cholinergic transients, and the latter with response latencies, the interrelationships and interactions between the multiple cholinergic signaling modes remains unclear. Hypotheses concerning the afferent circuitry contributing to the regulation of second- versus minute-based cholinergic signals are discussed. The discovery of cholinergic transients and their crucial role in cue detection and attentional performance form the basis for new hypotheses about the nature of cholinergic dysfunction in cognitive disorders and offer new targets for the development of treatments for the cognitive symptoms of neuropsychiatric and neurodegenerative disorders.

c-Fos Expression After Chronic Electrical Stimulation of Sensorimotor Cortex in Rats

Objectives.  Motor cortex stimulation has been used as a treatment for intractable pain. However, the mechanisms underlying its effects remain unclear. In this study, neuroplasticity induced by chronic sensorimotor cortex stimulation was investigated experimentally on the basis of c-Fos expression. Materials and Methods.  The experimental animals employed were adult male Wistar rats. A quadripolar stimulation electrode was positioned over the sensorimotor cortex. We examined the neural activation in response to chronic stimulation using c-Fos immunopositivity. Results.  The results are as follows: 1) c-Fos was significantly expressed immediately after the stimulation compared with that in the control; 2) c-Fos expression became extensive over the various regions with an increase in stimulation duration; and 3) after two months of stimulation, c-Fos was expressed not only on the stimulation side, but also within the contralateral cerebral hemisphere. Conclusions.  Changes in c-Fos expression induced by long-term stimulation indicate the existence of a time-dependent neural plasticity.

Cortical modulation by nucleus basalis magnocellularis corticopetal cholinergic neurons during anxiety-like states is reflected by decreases in delta

Previous research has demonstrated that nucleus basalis magnocellularis (nbm) corticopetal cholinergic neurons modulate anxiety-like states, but cortical modulation by these neurons during anxiety-like states has not been characterized. In order to address this, we documented the effect of nbm corticopetal cholinergic lesions on cortical activity in direct (prefrontal cortex) and indirect (retrosplenial cortex) targets of nbm corticopetal cholinergic neurons during footshock induced operant suppression. The gamma/delta ratio and theta were used as indices of cortical activity, because these components of the electroencephalogram (EEG) are sensitive to basal forebrain corticopetal cholinergic modulation. During operant suppression, increases in the gamma/delta ratio and augmented theta were observed in both cortical EEGs. Lesions attenuated operant suppression and the gamma/delta ratio, but had no effect on increased theta. The effect of nbm corticopetal cholinergic lesions on the gamma/delta ratio was driven by the effect of the lesions on the delta band. The results of the study demonstrate that during anxiety-like states 1) decreases in delta reflect the action of nbm corticopetal cholinergic neurons, 2) nbm corticopetal cholinergic neurons alter neural processes in direct and indirect cortical targets, and 3) cortical theta is not dependent on the integrity of nbm corticopetal cholinergic neurons.

Learning by structural remodeling in a class of single cell models

Changes in neural connectivity are thought to underlie the most permanent forms of memory in the brain. We consider two models, derived from the clusteron (Mel, Adv Neural Inf Process Syst 4:35-42, 1992), to study this method of learning. The models show a direct relationship between the speed of memory acquisition and the probability of forming appropriate synaptic connections. Moreover, the strength of learned associations grows with the number of fibers that have taken part in the learning process. We provide simple and intuitive explanations of these two results by analyzing the distribution of synaptic activations. The obtained insights are then used to extend the model to perform novel tasks: feature detection, and learning spatio-temporal patterns. We also provide an analytically tractable approximation to the model to put these observations on a firm basis. The behavior of both the numerical and analytical models correlate well with experimental results of learning tasks which are thought to require a reorganization of neuronal networks.

The vermicelli handling test: a simple quantitative measure of dexterous forepaw function in rats

Loss of function in the hands occurs with many brain disorders, but there are few measures of skillful forepaw use in rats available to model these impairments that are both sensitive and simple to administer. Whishaw and Coles previously described the dexterous manner in which rats manipulate food items with their paws, including thin pieces of pasta [Whishaw IQ, Coles BL. Varieties of paw and digit movement during spontaneous food handling in rats: postures, bimanual coordination, preferences, and the effect of forelimb cortex lesions. Behav Brain Res 1996;77:135-48]. We set out to develop a measure of this food handling behavior that would be quantitative, easy to administer, sensitive to the effects of damage to sensory and motor systems of the CNS and useful for identifying the side of lateralized impairments. When rats handle 7 cm lengths of vermicelli, they manipulate the pasta by repeatedly adjusting the forepaw hold on the pasta piece. As operationally defined, these adjustments can be easily identified and counted by an experimenter without specialized equipment. After unilateral sensorimotor cortex (SMC) lesions, transient middle cerebral artery occlusion (MCAO) and striatal dopamine depleting (6-hydroxydopamine, 6-OHDA) lesions in adult rats, there were enduring reductions in adjustments made with the contralateral forepaw. Additional pasta handling characteristics distinguished between the lesion types. MCAO and 6-OHDA lesions increased the frequency of several identified atypical handling patterns. Severe dopamine depletion increased eating time and adjustments made with the ipsilateral forepaw. However, contralateral forepaw adjustment number most sensitively detected enduring impairments across lesion types. Because of its ease of administration and sensitivity to lateralized impairments in skilled forepaw use, this measure may be useful in rat models of upper extremity impairment.

Principles of experience-dependent neural plasticity: implications for rehabilitation after brain damage

PURPOSE

This paper reviews 10 principles of experience-dependent neural plasticity and considerations in applying them to the damaged brain.

METHOD

Neuroscience research using a variety of models of learning, neurological disease, and trauma are reviewed from the perspective of basic neuroscientists but in a manner intended to be useful for the development of more effective clinical rehabilitation interventions.

RESULTS

Neural plasticity is believed to be the basis for both learning in the intact brain and relearning in the damaged brain that occurs through physical rehabilitation. Neuroscience research has made significant advances in understanding experience-dependent neural plasticity, and these findings are beginning to be integrated with research on the degenerative and regenerative effects of brain damage. The qualities and constraints of experience-dependent neural plasticity are likely to be of major relevance to rehabilitation efforts in humans with brain damage. However, some research topics need much more attention in order to enhance the translation of this area of neuroscience to clinical research and practice.

CONCLUSION

The growing understanding of the nature of brain plasticity raises optimism that this knowledge can be capitalized upon to improve rehabilitation efforts and to optimize functional outcome.

Prefrontal acetylcholine release controls cue detection on multiple timescales

Cholinergic neurons originating from the basal forebrain innervate the entire cortical mantle. Choline-sensitive microelectrodes were used to measure the synaptic release of cortical acetylcholine (ACh) at a subsecond resolution in rats performing a task involving the detection of cues. Cues that were detected, defined behaviorally, evoked transient increases in cholinergic activity (at the scale of seconds) in the medial prefrontal cortex (mPFC), but not in a nonassociational control region (motor cortex). In trials involving missed cues, cholinergic transients were not observed. Cholinergic deafferentation of the mPFC, but not motor cortex, impaired cue detection. Furthermore, decreases and increases in precue cholinergic activity predicted subsequent cue detection or misses, respectively. Finally, cue-evoked cholinergic transients were superimposed over slower (at the timescale of minutes) changes in cholinergic activity. Cortical cholinergic neurotransmission is regulated on multiple timescales to mediate the detection of behaviorally significant cues and to support cognitive performance.

The stroke rehabilitation paradigm

The matrix of stroke rehabilitation is evolving as we look outside the box of traditional therapy type, timing, and intensity of rehabilitation techniques. For inpatient wards, the goal of medical stability and prompt resolution of complications to maximize participation in therapy remains paramount. In the current medical model, we focus on teaching compensatory strategies and rarely on restorative approaches because of time and financial limitations. Researchers aim to identify new technologic and molecular approaches to improve functional outcomes and more accurately predict disability. This article examines different concepts surrounding the comprehensive rehabilitation paradigm of stroke survivors.

The role of the nucleus basalis magnocellularis in fear conditioning consolidation in the rat

The nucleus basalis magnocellularis (NBM) is known to be involved in the memorization of several conditioned responses. To investigate the role of the NBM in fear conditioning memorization, this neural site was subjected to fully reversible tetrodotoxin (TTX) inactivation during consolidation in adult male Wistar rats that had undergone fear training to acoustic conditioned stimulus (CS) and context. TTX was stereotaxically administered to different groups of rats at increasing intervals after the acquisition session. Memory was assessed as the conditioned freezing duration measured during retention testing, always performed 72 and 96 h after TTX administration. In this way, there was no interference with normal NBM function during either acquisition or retrieval phases, allowing any amnesic effect to be due only to consolidation disruption. The results show that for contextual fear response memory consolidation, NBM functional integrity is necessary up to 24 h post-acquisition. On the other hand, NBM functional integrity was shown to be necessary for memory consolidation of the acoustic CS fear response only immediately after acquisition and not 24-h post-acquisition. The present findings help to elucidate the role of the NBM in memory consolidation and better define the neural circuits involved in fear memories.

The application of positron-emitting molecular imaging tracers in Alzheimer's disease

The symptomatology and known pathology of Alzheimer's disease are restricted to the central nervous system. This review details studies of PET tracers aimed at interrogating cholinergic, serotonergic, opiate, benzodiazepine, and inflammatory pathways as well as PET tracers that illuminate amyloid plaques and neurofibrillary tangles in AD. Progress has been remarkable. Together with studies of brain structure with MRI and of functional regional brain activity, e.g., through measures of blood flow and glucose metabolic rate, molecular imaging promises to dramatically alter our understanding of the structural and physiological abnormalities underlying AD symptomatology. A more immediate impact on the diagnosis and treatment evaluation of AD patients in clinical trials is predicted while the possibility of personalized treatment or prevention of AD may not be that far away.

Nerve growth factor gene delivery: animal models to clinical trials

Cell death is the final common pathway of cognitive decline in Alzheimer's disease (AD). Nervous system growth factors, or neurotrophic factors, are substances naturally produced in the nervous system that support neuronal survival during development and influence neuronal function throughout adulthood. Notably, in animal models, including primates, neurotrophic factors prevent neuronal death after injury and can reverse spontaneous neuronal atrophy in aging. Thus, neurotrophic factor therapy has the potential to prevent or reduce ongoing cell loss in disorders such as AD. The main challenge in clinical testing of neurotrophic factors has been their delivery to the brain in sufficient doses to impact cell function, while restricting their delivery to specific sites to prevent adverse effects from broad distribution. This article reviews progress in evaluating the therapeutic potential of growth factors, from early animal models to human clinical trials currently underway in AD.

Reorganization of functional brain maps after exercise training: Importance of cerebellar-thalamic-cortical pathway

Exercise training (ET) causes functional and morphologic changes in normal and injured brain. While studies have examined effects of short-term (same day) training on functional brain activation, less work has evaluated effects of long-term training, in particular treadmill running. An improved understanding is relevant as changes in neural reorganization typically require days to weeks, and treadmill training is a component of many neurorehabilitation programs. Adult, male rats (n=10) trained to run for 40 min/day, 5 days/week on a Rotarod treadmill at 11.5 cm/s, while control animals (n=10) walked for 1 min/day at 1.2 cm/s. Six weeks later, [(14)C]-iodoantipyrine was injected intravenously during treadmill walking. Regional cerebral blood flow-related tissue radioactivity was quantified by autoradiography and analyzed in the three-dimensionally reconstructed brain by statistical parametric mapping. Exercised compared to nonexercised rats demonstrated increased influence of the cerebellar-thalamic-cortical (CbTC) circuit, with relative increases in perfusion in deep cerebellar nuclei (medial, interposed, lateral), thalamus (ventrolateral, midline, intralaminar), and paravermis, but with decreases in the vermis. In the basal ganglia-thalamic-cortical circuit, significant decreases were noted in sensorimotor cortex and striatum, with associated increases in the globus pallidus. Additional significant changes were noted in the ventral pallidum, superior colliculus, dentate gyrus (increases), and red nucleus (decreases). Following ET, the new dynamic equilibrium of the brain is characterized by increases in the efficiency of neural processing (sensorimotor cortex, striatum, vermis) and an increased influence of the CbTC circuit. Cerebral regions demonstrating changes in neural activation may point to alternate circuits, which may be mobilized during neurorehabilitation.