Cooperative changes in GABA, glutamate and activity levels: the missing link in cortical plasticity

Induction of excitatory brain state governs plastic functional changes in visual cortical topology

Adult visual plasticity underlying local remodeling of the cortical circuitry in vivo appears to be associated with a spatiotemporal pattern of strongly increased spontaneous and evoked activity of populations of cells. Here we review and discuss pioneering work by us and others about principles of plasticity in the adult visual cortex, starting with our study which showed that a confined lesion in the cat retina causes increased excitability in the affected region in the primary visual cortex accompanied by fine-tuned restructuring of neuronal function. The underlying remodeling processes was further visualized with voltage-sensitive dye (VSD) imaging that allowed a direct tracking of retinal lesion-induced reorganization across horizontal cortical circuitries. Nowadays, application of noninvasive stimulation methods pursues the idea further of increased cortical excitability along with decreased inhibition as key factors for the induction of adult cortical plasticity. We used high-frequency transcranial magnetic stimulation (TMS), for the first time in combination with VSD optical imaging, and provided evidence that TMS-amplified excitability across large pools of neurons forms the basis for noninvasively targeting reorganization of orientation maps in the visual cortex. Our review has been compiled on the basis of these four own studies, which we discuss in the context of historical developments in the field of visual cortical plasticity and the current state of the literature. Overall, we suggest markers of LTP-like cortical changes at mesoscopic population level as a main driving force for the induction of visual plasticity in the adult. Elevations in excitability that predispose towards cortical plasticity are most likely a common property of all cortical modalities. Thus, interventions that increase cortical excitability are a promising starting point to drive perceptual and potentially motor learning in therapeutic applications.

Postoperative speech impairment and cranial nerve deficits after secondary surgery of posterior fossa tumours in childhood: a prospective European multicentre study

PURPOSE

Brain tumours constitute 25% of childhood neoplasms, and half of them are in the posterior fossa. Surgery is a fundamental component of therapy, because gross total resection is associated with a higher progression-free survival. Patients with residual tumour, progression of residual tumour or disease recurrence commonly require secondary surgery. We prospectively investigated the risk of postoperative speech impairment (POSI) and cranial nerve dysfunction (CND) following primary and secondary resection for posterior cranial fossa tumours.

METHODS

In the Nordic-European study of the cerebellar mutism syndrome, we prospectively included children undergoing posterior fossa tumour resection or open biopsy in one of the 26 participating European centres. Neurological status was assessed preoperatively, and surgical details were noted post-operatively. Patients were followed up 2 weeks, 2 months and 1 year postoperatively. Here, we analyse the risk of postoperative speech impairment (POSI), defined as either mutism or reduced speech, and cranial nerve dysfunction (CND) following secondary, as compared to primary, surgery.

RESULTS

We analysed 426 children undergoing primary and 78 undergoing secondary surgery between 2014 and 2020. The incidence of POSI was significantly lower after secondary (12%) compared with primary (28%, p = 0.0084) surgery. In a multivariate analysis adjusting for tumour histology, the odds ratio for developing POSI after secondary surgery was 0.23, compared with primary surgery (95% confidence interval: 0.08-0.65, p = 0.006). The frequency of postoperative CND did not differ significantly after primary vs. secondary surgery (p = 0.21).

CONCLUSION

Children have a lower risk of POSI after secondary than after primary surgery for posterior fossa tumours but remain at significant risk of both POSI and CND. The present findings should be taken in account when weighing risks and benefits of secondary surgery for posterior fossa tumours.

NGF Eye Administration Recovers the TrkB and Glutamate/GABA Marker Deficit in the Adult Visual Cortex Following Optic Nerve Crush

Eye-drop recombinant human nerve growth factor (ed-rhNGF) has proved to recover the retina and optic nerve damage in animal models, including the unilateral optic nerve crush (ONC), and to improve visual acuity in humans. These data, associated with evidence that ed-rhNGF stimulates the brain derived neurotrophic factor (BDNF) in retina and cortex, suggests that NGF might exert retino-fugal effects by affecting BDNF and its receptor TrkB. To address these questions, their expression and relationship with the GABAergic and glutamatergic transmission markers, GAD65 and GAD67, vesicular inhibitory amino acid transporter (VGAT), and vesicular glutamate transporters 1 and 2 (VGLUT-1 and VGLUT-2) were investigated in adult ONC rats contralateral and ipsilateral visual cortex (VCx). Ed-rhNGF recovers the ONC-induced alteration of GABAergic and glutamatergic markers in contralateral VCx, induces an upregulation of TrkB, which is positively correlated with BDNF precursor (proBDNF) decrease in both VCx sides, and strongly enhances TrkB+ cell soma and neuronal endings surrounded by GAD65 immuno-reactive afferents. These findings contribute to enlarging the knowledge on the mechanism of actions and cellular targets of exogenously administrated NGF, and suggest that ed-rhNGF might act by potentiating the activity-dependent TrkB expression in GAD+ cells in VCx following retina damage and/or ONC.

Effects of Paired Associative Stimulation on Metabolites in Ischemia Stroke Rats Model as Studied by Nuclear Magnetic Resonance Spectrum

Paired associated stimulation (PAS) has been confirmed to play a role in motor recovery after stroke, but the underlying mechanism has not been fully elucidated. In this study, we employed a comprehensive battery of measurements, including behavioral test, electrophysiology and 1H-NMR approaches, to investigate the therapeutic effects of PAS in rat model of cerebral ischemia and its underlying mechanism. Rats were randomly divided into a transient middle cerebral artery occlusion group (tMCAO group), a tMCAO + PAS group (PAS group), and a sham group. PAS was applied over 7 consecutive days in PAS group. The behavioral function of rats was evaluated by modified Garcia Scores and Rota-rod test. Electrophysiological changes were measured by motor evoked potentials (MEP). Metabolic changes of ischemic penumbra were detected by 1H-NMR. After PAS intervention, the performances on Rota-rod test and Garcia test improved and the amplitude of MEP increased significantly. The gamma-aminobutyric acid (GABA) in penumbra cortex was decreased significantly, whereas the glutamate showed the opposite changes. The results suggested that post-stroke recovery promoted by PAS may be related to the metabolites alteration in ischemic penumbra and also regulate the excitability of motor cortex.

Disinhibition of Human Primary Somatosensory Cortex After Median Nerve Transection and Reinnervation

Despite state-of-the-art surgical and postoperative treatment, median nerve transection causes lasting impaired hand function due to limitations in the nerve’s reinnervation ability. The defective innervation and thus controllability of the affected hand can shape the brain’s control of manual behaviors. Earlier studies of changes in the processing of tactile stimuli have focused mainly on stimulation of the reinnervated hand and lack sufficient control over the brain’s use of the tactile input in perceptual terms. Here we used fMRI to measure brain activity (BOLD-signal) in 11 people with median nerve injury and healthy controls (N = 11) when performing demanding tactile tasks using the tip of either the index or little finger of either hand. For the nerve-injured group, the left median nerve had been traumatically transected in the distal forearm and surgically repaired on average 8 years before the study. The hand representation of their contralesional (right) primary somatosensory cortex (S1) showed greater activity compared to controls when the left reinnervated index finger was used, but also when the left-hand little finger and the fingers of the right hand innervated by uninjured nerves were used. We argue that the overall increase in activity reflects a general disinhibition of contralesional S1 consistent with an augmented functional reorganizational plasticity being an ongoing feature of chronic recovery from nerve injury. Also, the nerve-injured showed increased activity within three prefrontal cortical areas implicated in higher-level behavioral processing (dorsal anterior cingulate cortex, left ventrolateral prefrontal and right dorsolateral prefrontal cortex), suggesting that processes supporting decision-making and response-selection were computationally more demanding due to the compromised tactile sensibility.

Early Loss of Vision Results in Extensive Reorganization of Plasticity-Related Receptors and Alterations in Hippocampal Function That Extend Through Adulthood

Although by adulthood cortical structures and their capacity for processing sensory information have become established and stabilized, under conditions of cortical injury, or sensory deprivation, rapid reorganization occurs. Little is known as to the impact of this kind of adaptation on cellular processes related to memory encoding. However, imaging studies in humans suggest that following loss or impairment of a sensory modality, not only cortical but also subcortical structures begin to reorganize. It is likely that these processes are supported by neurotransmitter receptors that enable synaptic and cortical plasticity. Here, we explored to what extent the expression of plasticity-related proteins (GABA-A, GABA-B, GluN1, GluN2A, GluN2B) is altered following early vision loss, and whether this impacts on hippocampal function. We observed that in the period of 2-4 months postnatally in CBA/J-mice that experience hereditary postnatal retinal degeneration, systematic changes of GABA-receptor and NMDA-receptor subunit expression occurred that emerged first in the hippocampus and developed later in the cortex, compared to control mice that had normal vision. Changes were accompanied by significant impairments in hippocampal long-term potentiation and hippocampus-dependent learning. These data indicate that during cortical adaptation to early loss of vision, hippocampal information processing is compromised, and this status impacts on the acquisition of spatial representations.

MEGA-PRESS and PRESS measure oxidation of glutathione in a phantom

PURPOSE

Investigate the possibility of measuring changes in glutathione (GSH) concentration using the MRS PRESS and MEGA-PRESS sequences by tracking the natural oxidation of GSH, and to examine the accuracy of the two methods.

METHODS

122 GSH edited MEGA-PRESS and PRESS acquisitions were acquired on a "braino" based phantom +3.0 mM GSH during a period of 11 days. All spectra were analyzed in LCModel. (The MEGA-PRESS data were first preprocessed in Matlab). Degradation curves were modeled. A one year follow-up on the same phantom and measurements from a similar phantom without GSH and one pure GSH phantom were also included.

RESULTS

Both MEGA-PRESS and PRESS showed degradation of the measured GSH signal. Modeling the exponential decay of the GSH signal in MEGA-PRESS and PRESS gave for t = 0; 2.9 i.u. for MEGA-PRESS and 2.3 i.u. for PRESS. As t increased, the GSH concentration converged to zero for MEGA-PRESS but not for PRESS (0.7 i.u.). GSH for the one year follow up were 0.0 i.u. for MEGA-PRESS and 0.6 i.u. for PRESS. Similar phantom without GSH yielded 0.0 i.u. for both MEGA-PRESS and PRESS.

CONCLUSION

It is possible to measure changes in GSH concentration in a phantom using both PRESS and MEGA-PRESS techniques, however the PRESS spectrum appears to include oxidized GSH (GSSG). In addition, GSH edited MEGA-PRESS measurement gives more precise values at lower GSH concentrations.

Time course of cytochrome oxidase blob plasticity in the primary visual cortex of adult monkeys after retinal laser lesions

We studied the time course of changes of cytochrome oxidase (CytOx) blob spatial density and blob cross-sectional area of deprived (D) and nondeprived (ND) portions of V1 in four capuchin monkeys after massive and restricted retinal laser lesions. Laser shots at the border of the optic disc produced massive retinal lesions, while low power laser shots in the retina produced restricted retinal lesions. These massive and restricted retinal lesions were intended to simulate glaucoma and diabetic retinopathy, respectively. We used a Neodymium-YAG dual frequency laser to make the lesions. We measured Layer III blobs in CytOx-reacted tangential sections of flat-mounted preparations of V1. The plasticity of the blob system and that of the ocular dominance columns (ODC) varied with the degree of retinal lesions. We found that changes in the blob system were different from that of the ODC. Blob sizes changed drastically in the region corresponding to the retinal lesion. Blobs were larger and subjectively darker above and below the non deprived ODC than in the deprived columns. With restricted lesions, blobs corresponding to the ND columns had sizes similar to those from non-lesioned areas. In contrast, blobs corresponding to the deprived columns were smaller than those from nonlesioned areas. With massive lesions, ND blobs were larger than the deprived blobs. Plastic changes in blobs described here occur much earlier than previously described.

GABA and GABA receptors alterations in the primary visual cortex of concave lens-induced myopic model

Until recently most researches on myopia mechanisms have mainly been focused on the eye ball and few investigations were explored on the upper visual pathway, such as the visual cortex. The roles of gamma-aminobutyric acid (GABA) in the retinal and in the upper visual pathway are inter-correlated. As the retinal glutamate decarboxylase (GAD), GABA, and the mRNA levels of GABA receptors increased during the concave lens induced myopia formation, however, whether GABA alterations also occurred in the visual cortex during the concave lens induction is still unknown. In the present study, using HPLC, Enzyme-Linked Immunosorbent Assay (ELISA) and Real-Time Quantitative-PCR (RT-PCR) methods, we observed the changing trends of GABA, glutamate decarboxylase (GAD), and GABA receptors in the visual cortex of concave lens-induced myopic guinea pigs. Similar to the changing patterns of retinal GABA, the concentrations of GAD, GABA and the mRNA levels of GABA receptors in the visual cortex also increased. These results indicate that the exploration on myopia mechanisms should possibly be investigated on the whole visual pathway and the detailed significance of cortical GABA alterations needs further investigation.

Glutamine and Glutamate Complex, as Measured by Functional Magnetic Resonance Spectroscopy, Alters During Face-Name Association Task in Patients with Mild Cognitive Impairment and Alzheimer's Disease

BACKGROUND

The metabolite response during a memory task in Alzheimer's disease (AD) patients is unknown.

OBJECTIVE

To investigate the metabolite changes in subjects with AD, amnestic mild cognitive impairment (aMCI), and cognitively normal (CN) elderly during a memory task using functional magnetic resonance spectroscopy (fMRS).

METHODS

This study involved 23 young normal controls (YC), 24 CN elderly, 24 aMCI, and 24 mild and probable AD individuals. fMRS data were acquired at the precuneus and posterior cingulate brain regions during a face-name association task. Statistical analyses of quantified metabolites were performed to evaluate differences of the metabolite values between the stimulation conditions and among the four subject groups. Receiver operating curve analysis was performed to evaluate whether the metabolic changes after functional activations can differentiate the subject groups.

RESULT

Glutamine and glutamate complex (Glx) was statistically significantly different between the fixation and repeat conditions in aMCI (p = 0.0492) as well as between the fixation and the novel conditions in the AD (p = 0.0412) group. The total N-acetylaspartate (tNAA) was statistically significantly different among the four subject groups in the fixation condition (DF = 3, F = 7.673, p <  0.001), the novel condition (DF = 3, F = 6.945, p <  0.001), and the repeat condition (DF = 3, F = 7.127, p <  0.001). tNAA, tCr, and mIns could be used to differentiate CN from aMCI. Furthermore, tNAA, tCr, Glx, and Glu could also differentiate CN from AD, and aMCI from AD.

CONCLUSION

Glx was altered during a stimulation that may be used to evaluate neuronal dysfunction in a demented patient. tNAA and tCr were reduced in patients with AD.

A Tool for Brain-Wide Quantitative Analysis of Molecular Data upon Projection into a Planar View of Choice

Several techniques, allowing the reconstruction and visualization of functional, anatomical or molecular information from tissue and organ slices, have been developed over the years. Yet none allow direct comparison without reprocessing the same slices. Alternative methods using publicly available reference maps like the Allen Brain Atlas lack flexibility with respect to age and species. We propose a new approach to reconstruct a segmented region of interest from serial slices by projecting the optical density values representing a given molecular signal to a plane of view of choice, and to generalize the results into a reference map, which is built from the individual maps of all animals under study. Furthermore, to allow quantitative comparison between experimental conditions, a non-parametric pseudo t-test has been implemented. This new mapping tool was applied, optimized and validated making use of an in situ hybridization dataset that represents the spatiotemporal expression changes for the neuronal activity reporter gene zif268, in relation to cortical plasticity induced by monocular enucleation, covering the entire mouse visual cortex. The created top view maps of the mouse brain allow precisely delineating and interpreting 11 extrastriate areas surrounding mouse V1. As such, and because of the opportunity to create a planar projection of choice, these molecular maps can in the future easily be compared with functional or physiological imaging maps created with other techniques such as Ca2+, flavoprotein and optical imaging.

The Current Status of Somatostatin-Interneurons in Inhibitory Control of Brain Function and Plasticity

The mammalian neocortex contains many distinct inhibitory neuronal populations to balance excitatory neurotransmission. A correct excitation/inhibition equilibrium is crucial for normal brain development, functioning, and controlling lifelong cortical plasticity. Knowledge about how the inhibitory network contributes to brain plasticity however remains incomplete. Somatostatin- (SST-) interneurons constitute a large neocortical subpopulation of interneurons, next to parvalbumin- (PV-) and vasoactive intestinal peptide- (VIP-) interneurons. Unlike the extensively studied PV-interneurons, acknowledged as key components in guiding ocular dominance plasticity, the contribution of SST-interneurons is less understood. Nevertheless, SST-interneurons are ideally situated within cortical networks to integrate unimodal or cross-modal sensory information processing and therefore likely to be important mediators of experience-dependent plasticity. The lack of knowledge on SST-interneurons partially relates to the wide variety of distinct subpopulations present in the sensory neocortex. This review informs on those SST-subpopulations hitherto described based on anatomical, molecular, or electrophysiological characteristics and whose functional roles can be attributed based on specific cortical wiring patterns. A possible role for these subpopulations in experience-dependent plasticity will be discussed, emphasizing on learning-induced plasticity and on unimodal and cross-modal plasticity upon sensory loss. This knowledge will ultimately contribute to guide brain plasticity into well-defined directions to restore sensory function and promote lifelong learning.

Obstructive sleep apnea is associated with low GABA and high glutamate in the insular cortex

The insular cortex is injured in obstructive sleep apnea (OSA) and responds inappropriately to autonomic challenges, suggesting neural reorganization. The objective of this study was to assess whether the neural changes might result from γ-aminobutyric acid (GABA) and glutamate alterations. We studied 14 OSA patients [mean age ± standard deviation (SD): 47.5 ± 10.5 years; nine male; apnea-hypopnea index (AHI): 29.5 ± 15.6 events h(-1) ] and 22 healthy participants (47.5 ± 10.1 years; 11 male), using magnetic resonance spectroscopy to detect GABA and glutamate levels in insular cortices. We localized the cortices with anatomical scans, and measured neurochemical levels from anterior to mid-regions. Left and right anterior insular cortices showed lower GABA and higher glutamate in OSA versus healthy subjects [GABA left: OSA n = 6: 0.36 ± 0.10 (mean ± SD), healthy n = 5: 0.62 ± 0.18; P < 0.05), right: OSA n = 11: 0.27 ± 0.09, healthy n = 14: 0.45 ± 0.16; P < 0.05; glutamate left: OSA n = 6: 1.61 ± 0.32, healthy n = 8: 0.94 ± 0.34; P < 0.05, right: OSA n = 14: 1.26 ± 0.28, healthy n = 19: 1.02 ± 0.28; P < 0.05]. GABA and glutamate levels were correlated only within the healthy group in the left insula (r: -0.9, P < 0.05). The altered anterior insular levels of GABA and glutamate may modify integration and projections to autonomic areas, contributing to the impaired cardiovascular regulation in OSA.

Retinal lesions induce fast intrinsic cortical plasticity in adult mouse visual system

Neuronal activity plays an important role in the development and structural-functional maintenance of the brain as well as in its life-long plastic response to changes in sensory stimulation. We characterized the impact of unilateral 15° laser lesions in the temporal lower visual field of the retina, on visually driven neuronal activity in the afferent visual pathway of adult mice using in situ hybridization for the activity reporter gene zif268. In the first days post-lesion, we detected a discrete zone of reduced zif268 expression in the contralateral hemisphere, spanning the border between the monocular segment of the primary visual cortex (V1) with extrastriate visual area V2M. We could not detect a clear lesion projection zone (LPZ) in areas lateral to V1 whereas medial to V2M, agranular and granular retrosplenial cortex showed decreased zif268 levels over their full extent. All affected areas displayed a return to normal zif268 levels, and this was faster in higher order visual areas than in V1. The lesion did, however, induce a permanent LPZ in the retinorecipient layers of the superior colliculus. We identified a retinotopy-based intrinsic capacity of adult mouse visual cortex to recover from restricted vision loss, with recovery speed reflecting the areal cortical magnification factor. Our observations predict incomplete visual field representations for areas lateral to V1 vs. lack of retinotopic organization for areas medial to V2M. The validation of this mouse model paves the way for future interrogations of cortical region- and cell-type-specific contributions to functional recovery, up to microcircuit level.

Regional Specificity of GABAergic Regulation of Cross-Modal Plasticity in Mouse Visual Cortex after Unilateral Enucleation

In adult mice, monocular enucleation (ME) results in an immediate deactivation of the contralateral medial monocular visual cortex. An early restricted reactivation by open eye potentiation is followed by a late overt cross-modal reactivation by whiskers (Van Brussel et al., 2011). In adolescence (P45), extensive recovery of cortical activity after ME fails as a result of suppression or functional immaturity of the cross-modal mechanisms (Nys et al., 2014). Here, we show that dark exposure before ME in adulthood also prevents the late cross-modal reactivation component, thereby converting the outcome of long-term ME into a more P45-like response. Because dark exposure affects GABAergic synaptic transmission in binocular V1 and the plastic immunity observed at P45 is reminiscent of the refractory period for inhibitory plasticity reported by Huang et al. (2010), we molecularly examined whether GABAergic inhibition also regulates ME-induced cross-modal plasticity. Comparison of the adaptation of the medial monocular and binocular cortices to long-term ME or dark exposure or a combinatorial deprivation revealed striking differences. In the medial monocular cortex, cortical inhibition via the GABAA receptor α1 subunit restricts cross-modal plasticity in P45 mice but is relaxed in adults to allow the whisker-mediated reactivation. In line, in vivo pharmacological activation of α1 subunit-containing GABAA receptors in adult ME mice specifically reduces the cross-modal aspect of reactivation. Together with region-specific changes in glutamate acid decarboxylase (GAD) and vesicular GABA transporter expression, these findings put intracortical inhibition forward as an important regulator of the age-, experience-, and cortical region-dependent cross-modal response to unilateral visual deprivation.

SIGNIFICANCE STATEMENT

In adult mice, vision loss through one eye instantly reduces neuronal activity in the visual cortex. Strengthening of remaining eye inputs in the binocular cortex is followed by cross-modal adaptations in the monocular cortex, in which whiskers become a dominant nonvisual input source to attain extensive cortical reactivation. We show that the cross-modal component does not occur in adolescence because of increased intracortical inhibition, a phenotype that was mimicked in adult enucleated mice when treated with indiplon, a GABAA receptor α1 agonist. The cross-modal versus unimodal responses of the adult monocular and binocular cortices also mirror regional specificity in inhibitory alterations after visual deprivation. Understanding cross-modal plasticity in response to sensory loss is essential to maximize patient susceptibility to sensory prosthetics.

Adult plasticity and cortical reorganization after peripheral lesions

Following loss of input due to peripheral lesions, functional reorganization occurs in the deprived cortical region in adults. Over a period of hours to months, cells in the lesion projection zone (LPZ) begin to respond to novel stimuli. This reorganization is mediated by two processes: a reduction of inhibition in a gradient throughout the cortex and input remapping via sprouting of axonal arbors from cortical regions spatially adjacent to the LPZ, and strengthening of pre-existing subthreshold inputs. Together these inputs facilitate receptive field remapping of cells in the LPZ. Recent experiments have revealed time courses and potential interactions of the mechanisms associated with functional reorganization, suggesting that large scale reorganization in the adult may utilize plasticity mechanisms prominent during development.

Evaluation of disease progression in INCL by MR spectroscopy

OBJECTIVE

Infantile neuronal ceroid lipofuscinosis (INCL) is a devastating neurodegenerative storage disease caused by palmitoyl-protein thioesterase-1 deficiency, which impairs degradation of palmitoylated proteins (constituents of ceroid) by lysosomal hydrolases. Consequent lysosomal ceroid accumulation leads to neuronal injury. As part of a pilot study to evaluate treatment benefits of cysteamine bitartrate and N-acetylcysteine, we quantitatively measured brain metabolite levels using magnetic resonance spectroscopy (MRS).

METHODS

A subset of two patients from a larger treatment and follow-up study underwent serial quantitative single-voxel MRS examinations of five anatomical sites. Three echo times were acquired in order to estimate metabolite T2. Measured metabolite levels included correction for partial volume of cerebrospinal fluid. Comparison of INCL patients was made to a reference group composed of asymptomatic and minimally symptomatic Niemann-Pick disease type C patients.

RESULTS

In INCL patients, N-acetylaspartate (NAA) was abnormally low at all locations upon initial measurement, and further declined throughout the follow-up period. In the cerebrum (affected early in the disease course), choline and myo-inositol were initially elevated and fell during the follow-up period, whereas in the cerebellum and brainstem (affected later), choline and myo-inositol were initially normal and rose subsequently.

INTERPRETATION

Choline and myo-inositol levels in our patients are consistent with patterns of neuroinflammation observed in two INCL mouse models. Low, persistently declining NAA was expected based on the progressive, irreversible nature of the disease. Progression of metabolite levels in INCL has not been previously quantified; therefore the results of this study serve as a reference for quantitative evaluation of future therapeutic interventions.

Distribution and effects of the muscarinic receptor subtypes in the primary visual cortex

Muscarinic cholinergic receptors modulate the activity and plasticity of the visual cortex. Muscarinic receptors are divided into five subtypes that are not homogeneously distributed throughout the cortical layers and cells types. This distribution results in complex action of the muscarinic receptors in the integration of visual stimuli. Selective activation of the different subtypes can either strengthen or weaken cortical connectivity (e.g., thalamocortical vs. corticocortical), i.e., it can influence the processing of certain stimuli over others. Moreover, muscarinic receptors differentially modulate some functional properties of neurons during experience-dependent activity and cognitive processes and they contribute to the fine-tuning of visual processing. These functions are involved in the mechanisms of attention, maturation and learning in the visual cortex. This minireview describes the anatomo-functional aspects of muscarinic modulation of the primary visual cortex's (V1) microcircuitry.

fMRI and MRS measures of neuroplasticity in the pharyngeal motor cortex

INTRODUCTION

Paired associative stimulation (PAS), is a novel non-invasive technique where two neural substrates are employed in a temporally coordinated manner in order to modulate cortico-motor excitability within the motor cortex (M1). In swallowing, combined pharyngeal electrical and transcranial-magnetic-stimulation induced beneficial neurophysiological and behavioural effects in healthy subjects and dysphagic stroke patients. Here, we aimed to investigate the whole-brain changes in neural activation during swallowing using functional magnetic resonance imaging (fMRI) following PAS application and in parallel assess associated GABA changes with magnetic resonance spectroscopy (MRS).

METHODS

Healthy adults (n=11, 38±9years old) were randomised to receive real and sham PAS to the 'stronger' motor cortex pharyngeal representation, on 2 separate visits. Following PAS, event-related fMRI was performed to assess changes in brain activation in response to water and saliva swallowing and during rest. Data were analysed (SPM8) at P<.001. MRS data were acquired using MEGA-PRESS before and after the fMRI acquisitions on both visits and GABA concentrations were measured (AMARES, jMRUI).

RESULTS

Following real PAS, BOLD signal changes (group analyses) increased at the site of stimulation during water and saliva swallowing, compared to sham PAS. It is also evident that PAS induced significant increases in BOLD signal to contralateral (to stimulation) hemispheric areas that are of importance to the swallowing neural network. Following real PAS, GABA:creatine ratio showed a trend to increase contralateral to PAS.

CONCLUSION

Targeted PAS applied to the human pharyngeal motor cortex induces local and remote changes in both primary and non-primary areas for water and saliva tasks. There is a possibility that changes of the inhibitory neurotransmitter, GABA, may play a role in the changes in BOLD signal. These findings provide evidence for the mechanisms underlying the beneficial effects of PAS on the brain swallowing network.

Regulation of Local Ambient GABA Levels via Transporter-Mediated GABA Import and Export for Subliminal Learning

Perception of supraliminal stimuli might in general be reflected in bursts of action potentials (spikes), and their memory traces could be formed through spike-timing-dependent plasticity (STDP). Memory traces for subliminal stimuli might be formed in a different manner, because subliminal stimulation evokes a fraction (but not a burst) of spikes. Simulations of a cortical neural network model showed that a subliminal stimulus that was too brief (10 msec) to perceive transiently (more than about 500 msec) depolarized stimulus-relevant principal cells and hyperpolarized stimulus-irrelevant principal cells in a subthreshold manner. This led to a small increase or decrease in ongoing-spontaneous spiking activity frequency (less than 1 Hz). Synaptic modification based on STDP during this period effectively enhanced relevant synaptic weights, by which subliminal learning was improved. GABA transporters on GABAergic interneurons modulated local levels of ambient GABA. Ambient GABA molecules acted on extrasynaptic receptors, provided principal cells with tonic inhibitory currents, and contributed to achieving the subthreshold neuronal state. We suggest that ongoing-spontaneous synaptic alteration through STDP following subliminal stimulation may be a possible neuronal mechanism for leaving its memory trace in cortical circuitry. Regulation of local ambient GABA levels by transporter-mediated GABA import and export may be crucial for subliminal learning.

Glutamate dysfunction associated with developmental cerebellar damage: relevance to autism spectrum disorders

Neural abnormalities commonly associated with autism spectrum disorders include prefrontal cortex (PFC) dysfunction and cerebellar pathology in the form of Purkinje cell loss and cerebellar hypoplasia. It has been reported that loss of cerebellar Purkinje cells results in aberrant dopamine neurotransmission in the PFC which occurs via dysregulation of multisynaptic efferents from the cerebellum to the PFC. Using a mouse model, we investigated the possibility that developmental cerebellar Purkinje cell loss could disrupt glutamatergic cerebellar projections to the PFC that ultimately modulate DA release. We measured glutamate release evoked by local electrical stimulation using fixed-potential amperometry in combination with glutamate selective enzyme-based recording probes in urethane-anesthetized Lurcher mutant and wildtype mice. Target sites included the mediodorsal and ventrolateral thalamic nuclei, reticulotegmental nuclei, pedunculopontine nuclei, and ventral tegmental area. With the exception of the ventral tegmental area, the results indicated that in comparison to wildtype mice, evoked glutamate release was reduced in Lurcher mutants by between 9 and 72% at all stimulated sites. These results are consistent with the notion that developmental loss of cerebellar Purkinje cells drives reductions in evoked glutamate release in cerebellar efferent pathways that ultimately influence PFC dopamine release. Possible mechanisms whereby reductions in glutamate release could occur are discussed.

Molecular correlates of cortical network modulation by long-term sensory experience in the adult rat barrel cortex

Modulation of cortical network connectivity is crucial for an adaptive response to experience. In the rat barrel cortex, long-term sensory stimulation induces cortical network modifications and neuronal response changes of which the molecular basis is unknown. Here, we show that long-term somatosensory stimulation by enriched environment up-regulates cortical expression of neuropeptide mRNAs and down-regulates immediate-early gene (IEG) mRNAs specifically in the barrel cortex, and not in other brain regions. The present data suggest a central role of neuropeptides in the fine-tuning of sensory cortical circuits by long-term experience.

Highly specific structural plasticity of inhibitory circuits in the adult neocortex

Inhibitory neurons are known to play a vital role in defining the window for critical period plasticity during development, and it is increasingly apparent that they continue to exert powerful control over experience-dependent cortical plasticity in adulthood. Recent in vivo imaging studies demonstrate that long-term plasticity of inhibitory circuits is manifested at an anatomical level. Changes in sensory experience drive structural remodeling in inhibitory interneurons in a cell-type and circuit-specific manner. Inhibitory synapse formation and elimination can occur with a great deal of spatial and temporal precision and are locally coordinated with excitatory synaptic changes on the same neuron. We suggest that the specificity of inhibitory synapse dynamics may serve to differentially modulate activity across the dendritic arbor, to selectively tune parts of a local circuit, or potentially discriminate between activities in distinct local circuits. We further review evidence suggesting that inhibitory circuit structural changes instruct excitatory/inhibitory balance while enabling functional reorganization to occur through Hebbian forms of plasticity.

Quantification of early stages of cortical reorganization of the topographic map of V1 following retinal lesions in monkeys

We quantified the capacity for reorganization of the topographic representation of area V1 in adult monkeys. Bias-free automated mapping methods were used to delineate receptive fields (RFs) of an array of neuronal clusters prior to, and up to 6 h following retinal lesions. Monocular lesions caused a significant reorganization of the topographic map in this area, both inside and outside the cortical lesion projection zone (LPZ). Small flashed stimuli revealed responses up to 0.85 mm inside the boundaries of the LPZ, with RFs representing regions of undamaged retina immediately surrounding the lesion. In contrast, long moving bars that spanned the scotoma resulting from the lesion revealed responsive units up to 1.87 mm inside the LPZ, with RFs representing interpolated responses in this region. This reorganization is present immediately after monocular retinal lesioning. Both stimuli showed a similar and significant (5-fold) increase of the RF scatter in the LPZ, 0.56 mm (median), compared with the undamaged retina, 0.12 mm. Our results reveal an array of preexisting subthreshold functional connections of up to 2 mm in V1, which can be rapidly mobilized independently from the differential qualitative reorganization elicited by each stimulus.

Neural mechanisms of short-term plasticity in the human visual system

Following circumscribed retinal damage, extensive reorganization of topographically organized visual cortical areas has been demonstrated in several species of mammals (including humans). Although reorganization is often studied over extended time scales, neural response properties change within seconds of retinal deafferentation. Understanding the mechanisms underlying these short-term effects is essential for developing a complete picture of representational plasticity. One approach to the study of short-term plasticity has been to use an artificial scotoma, a stimulus-induced analog of a retinal scotoma, as a model. Here, we use event-related potentials in an artificial scotoma paradigm to examine 2 aspects of short-term plasticity in the human visual system. First, we investigated the changes within visual representations temporarily deprived of patterned visual input by probing the inner boundaries of an artificial scotoma. We found an enhanced early sensory P1, consistent with a reduction in inhibition (disinhibition), a proposed mechanism of short-term visual plasticity. Second, we investigated mechanisms through which representations of surrounding space invade a visually deprived area by probing the outer boundaries of an artificial scotoma. In this case, a later visual component, the N1, was enhanced, suggesting that feedback may provide a source of unmasked, or invading, activity to visually deprived representations.

MR spectroscopic studies of the brain in psychiatric disorders

The measurement of brain metabolites with magnetic resonance spectroscopy (MRS) provides a unique perspective on the brain bases of neuropsychiatric disorders. As a context for interpreting MRS studies of neuropsychiatric disorders, we review the characteristic MRS signals, the metabolic dynamics,and the neurobiological significance of the major brain metabolites that can be measured using clinical MRS systems. These metabolites include N-acetylaspartate(NAA), creatine, choline-containing compounds, myo-inositol, glutamate and glutamine, lactate, and gamma-amino butyric acid (GABA). For the major adult neuropsychiatric disorders (schizophrenia, bipolar disorder, major depression, and the anxiety disorders), we highlight the most consistent MRS findings, with an emphasis on those with potential clinical or translational significance. Reduced NAA in specific brain regions in schizophrenia, bipolar disorder, post-traumatic stress disorder, and obsessive–compulsive disorder corroborate findings of reduced brain volumes in the same regions. Future MRS studies may help determine the extent to which the neuronal dysfunction suggested by these findings is reversible in these disorders. Elevated glutamate and glutamine (Glx) in patients with bipolar disorder and reduced Glx in patients with unipolar major depression support models of increased and decreased glutamatergic function, respectively, in those conditions. Reduced phosphomonoesters and intracellular pH in bipolar disorder and elevated dynamic lactate responses in panic disorder are consistent with metabolic models of pathogenesis in those disorders. Preliminary findings of an increased glutamine/glutamate ratio and decreased GABA in patients with schizophrenia are consistent with a model of NMDA hypofunction in that disorder. As MRS methods continue to improve, future studies may further advance our understanding of the natural history of psychiatric illnesses, improve our ability to test translational models of pathogenesis, clarify therapeutic mechanisms of action,and allow clinical monitoring of the effects of interventions on brain metabolicmarkers

Subthreshold Membrane Depolarization as Memory Trace for Perceptual Learning

Experience-dependent synaptic plasticity characterizes the adaptable brain and is believed to be the cellular substrate for perceptual learning. A chemical agent such as gamma-aminobutyric acid (GABA) is known to affect synaptic alteration, perhaps gating perceptual learning. We examined whether and how ambient (extrasynaptic) GABA affects experience-dependent synaptic alteration. A cortical neural network model was simulated. Transporters on GABAergic interneurons regulate ambient GABA levels around their axonal target neurons by removing GABA from (forward transport) or releasing it into (reverse transport) the extracellular space. The ambient GABA provides neurons with tonic inhibitory currents by activating extrasynaptic GABAa receptors. During repeated exposures to the same stimulus, we modified the synaptic connection strength between principal cells in a spike-timing-dependent manner. This modulated the activity of GABAergic interneurons, and reduced or augmented ambient GABA concentration. Reduction in ambient GABA concentration led to slight depolarization (less than several millivolts) in ongoing-spontaneous membrane potential. This was a subthreshold neuronal behavior because ongoing-spontaneous spiking activity remained almost unchanged. The ongoing-spontaneous subthreshold depolarization improved a suprathreshold neuronal response. If the stimulus was long absent for perceptual learning, augmentation of ambient GABA concentration took place and the ongoing-spontaneous subthreshold depolarization was depressed. We suggest that a perceptual memory trace could be left in neuronal circuitry as an ongoing-spontaneous subthreshold membrane depolarization, which would allow that memory to be accessed easily afterward, whereas a trace of a memory that has not recently been retrieved fades away when the ongoing-spontaneous subthreshold membrane depolarization built by previous perceptual learning is depressed. This would lead that memory to be accessed with some difficulty. In the brain, ambient GABA, whose level could be regulated by transporter may have an important role in leaving memory trace for perceptual learning.

Vigorous exercise increases brain lactate and Glx (glutamate+glutamine): a dynamic 1H-MRS study

Vigorous exercise increases lactate and glucose uptake by the brain in excess of the increase in brain oxygen uptake. The metabolic fate of this non-oxidized carbohydrate entering the brain is poorly understood, but accumulation of lactate in the brain and/or increased net synthesis of amino acid neurotransmitters are possible explanations. Previous proton magnetic resonance spectroscopy (1H-MRS) studies using conventional pulse sequences have not detected changes in brain lactate following exercise. This contrasts with 1H-MRS studies showing increased brain lactate when blood lactate levels are raised by an intravenous infusion of sodium lactate. Using a J-editing 1H-MRS technique for measuring lactate, we demonstrated a significant 19% increase in lactate in the visual cortex following graded exercise to approximately 85% of predicted maximum heart rate. However, the magnitude of the increase was insufficient to account for more than a small fraction of the non-oxidized carbohydrate entering the brain with exercise. We also report a significant 18% increase in Glx (combined signal from glutamate and glutamine) in visual cortex following exercise, which may represent an activity-dependent increase in glutamate. Future studies will be necessary to test the hypothesis that non-oxidized carbohydrate entering the brain during vigorous exercise is directed, in part, toward increased net synthesis of amino acid neurotransmitters. The possible relevance of these findings to panic disorder and major depression is discussed.

Dilantin therapy in an experimental model of traumatic brain injury: effects of limited versus daily treatment on neurological and behavioral recovery

The mechanisms by which Dilantin confers anticonvulsant benefits may also be neuroprotective by attenuating the acute excitatory insult in cortical and subcortical structures when the drug is given in the acute phase after traumatic brain injury (TBI). However, when Dilantin is used for prolonged periods, we hypothesized that it may impede recovery, synaptic plasticity may be impaired, and neuroprotective benefits may be lost. As such, we assessed the effect of daily chronic administration (75 mg/kg day 0 followed by 50 mg/kg daily i.p.) and acute administration (75 mg/kg day 0 followed by 50 mg/kg i.p. day 1) of Dilantin in young adult male rats on motor performance, y-maze exploration, Morris Water Maze (MWM), hippocampal (HC) cell survival, contusion size, and regional expression of neuroplasticity markers after controlled cortical impact (CCI) injury. Chronic daily Dilantin administration resulted in beam walking impairments on day 6, whereas acute Dilantin administration resulted in beam walking impairments on days 3 and 4. Chronic Dilantin administration also resulted in worse MWM performance, more HC cell loss and no increases in neuroplasticity markers compared to rats with CCI receiving chronic vehicle. Conversely, rats receiving acute Dilantin administration exhibited more novel arm exploration in the y-maze, greater HC cell sparing, and greater growth-associated protein 43 (GAP-43) expression in the HC ipsilateral to the CCI, compared to injured rats receiving vehicle. MWM was not influenced by acute Dilantin administration. These results suggest that there are beneficial effects of limited acute Dilantin therapy after TBI, and that extended daily Dilantin therapy has deleterious effects on neural recovery. These findings support clinical guidelines for limited use of Dilantin in seizure prophylaxis after TBI.

Excitation and inhibition jointly regulate cortical reorganization in adult rats

The primary somatosensory cortex (SI) retains its capability for cortical reorganization after injury or differential use into adulthood. The plastic response of SI cells to peripheral stimulation is characterized by extension of cortical representations accompanied by changes of the receptive field size of neurons. We used intracortical microstimulation that is known to enforce local, intracortical synchronous activity, to induce cortical reorganization and applied immunohistochemical methods in the same individual animals to investigate how plasticity in the cortical topographic maps is linked to changes in the spatial layout of the inhibitory and excitatory neurotransmitter systems. The results reveal a differential spatiotemporal pattern of upregulation and downregulation of specific factors for an excitatory (glutamatergic) and an inhibitory (GABAergic) system, associated with changes of receptive field size and reorganization of the somatotopic map in the rat SI. Predominantly local mechanisms are the specific reduction of the calcium-binding protein parvalbumin in inhibitory neurons and the low expression of the activity marker c-Fos. Reorganization in the hindpaw representation and in the adjacent SI cortical areas (motor cortex and parietal cortex) is accompanied by a major increase of the excitatory transmitter glutamate and c-Fos. The spatial extent of the reorganization appears to be limited by an increase of glutamic acid decarboxylase and the inhibitory transmitter GABA. The local and medium-range net effects are excitatory and can facilitate receptive field enlargements and cortical map expansion. The longer-range increase of inhibition appears suited to limit these effects and to prevent neurons from pathological hyperexcitability.

Strengthening of lateral activation in adult rat visual cortex after retinal lesions captured with voltage-sensitive dye imaging in vivo

Sensory deprivation caused by peripheral injury can trigger functional cortical reorganization across the initially silenced cortical area. It is proposed that intracortical connectivity enables recovery of function within such a lesion projection zone (LPZ), thus substituting lost subcortical input. Here, we investigated retinal lesion-induced changes in the function of lateral connections in the primary visual cortex of the adult rat. Using voltage-sensitive dye recordings, we visualized in millisecond-time resolution spreading synaptic activity across the LPZ. Shortly after lesion, the majority of neurons within the LPZ were subthresholdly activated by delayed propagation of activity that originated from unaffected cortical regions. With longer recovery time, latencies within the LPZ gradually decreased, and activation reached suprathreshold levels. Targeted electrode recordings confirmed that receptive fields of intra-LPZ neurons were displaced to the retinal lesion border while displaying normal orientation and direction selectivity. These results corroborate the view that cortical horizontal connections have a central role in functional reorganization, as revealed here by progressive facilitation of synaptic activity and the traveling wave of excitation that propagates horizontally into the deprived cortical region.

Analysis of c-fos and zif268 expression reveals time-dependent changes in activity inside and outside the lesion projection zone in adult cat area 17 after retinal lesions

Retinal lesions induce a topographic reorganization in the corresponding lesion projection zone (LPZ) in the visual cortex of adult cats. To gain a better insight into the reactivation dynamics, we investigated the alterations in cortical activity throughout area 17. We implemented in situ hybridization and real-time polymerase chain reaction to analyze the spatiotemporal expression patterns of the activity marker genes zif268 and c-fos. The immediate early gene (IEG) data confirmed a strong and permanent activity decrease in the center of the LPZ as previously described by electrophysiology. A recovery of IEG expression was clearly measured in the border of the LPZ. We were able to register reorganization over 2.5-6 mm. We also present evidence that the central retinal lesions concomitantly influence the activity in far peripheral parts of area 17. Its IEG expression levels appeared dependent of time and distance from the LPZ. We therefore propose that coupled changes in activity occur inside and outside the LPZ. In conclusion, alterations in activity reporter gene expression throughout area 17 contribute to the lesion-induced functional reorganization.

Perceptual plasticity in damaged adult visual systems

Plasticity appears to be a ubiquitous property of nervous systems, regardless of developmental stage or complexity. In the visual system of higher mammals, perceptual plasticity has been intensively studied, both during development and in adulthood. However, the last few years have seen some significant controversies arise about the existence and properties of visual plasticity after permanent damage to the adult visual system. The study of perceptual plasticity in damaged, adult visual systems is of interest for several reasons. First, it is an important means of unmasking the relative contribution of individual visual areas to visual learning, adaptation and priming, among other plastic phenomena. Second, it can provide knowledge that is essential for the development of effective therapies to rehabilitate the increasing number of people who suffer the functional consequences of damage at different levels of their visual hierarchy. This review summarizes the available evidence on the subject and proposes that visual plasticity may be just as ubiquitous after damage as it is in the intact visual system. However, damage may alter visual plasticity in ways that are still being defined.

Pharmacological modulation of cortical plasticity following kainic acid lesion in rat barrel cortex

OBJECT

The brain shows remarkable capacity for plasticity in response to injury. To maximize the benefits of current neurological treatment and to minimize the impact of injury, the authors examined the ability of commonly administered drugs, dextroamphetamine (D-amphetamine) and phenytoin, to positively or negatively affect the functional recovery of the cerebral cortex following excitotoxic injury.

METHODS

Previous work from the same laboratory has demonstrated reorganization of whisker functional responses (WFRs) in the rat barrel cortex after excitotoxic lesions were created with kainic acid (KA). In the present study, WFRs were mapped using intrinsic optical signal imaging before and 9 days after creation of the KA lesions. During the post-lesion survival period, animals were either treated with intraperitoneal D-amphetamine, phenytoin, or saline or received no treatment. Following the survival period, WFRs were again measured and compared with prelesion data.

RESULTS

The findings suggest that KA lesions cause increases in WFR areas when compared with controls. Treatment with D-amphetamine further increased the WFR area (p < 0.05) while phenytoin-treated rats showed decreases in WFR areas. There was also a statistically significant difference (p < 0.05) between the D-amphetamine and phenytoin groups.

CONCLUSIONS

These results show that 2 commonly used drugs, D-amphetamine and phenytoin, have opposite effects in the functional recovery/plasticity of injured cerebral cortex. The authors' findings emphasize the complex nature of the cortical response to injury and have implications for understanding the biology of the effects of different medications on eventual functional brain recovery.

Cortical reorganization consistent with spike timing–but not correlation-dependent plasticity

The receptive fields of neurons in primary visual cortex that are inactivated by retinal damage are known to 'shift' to nondamaged retinal locations, seemingly due to the plasticity of intracortical connections. We have observed in cats that these shifts occur in a pattern that is highly convergent, even among receptive fields that are separated by large distances before inactivation. Here we show, using a computational model of primary visual cortex, that the observed convergent shifts are inconsistent with the common assumption that the underlying intracortical connection plasticity is dependent on the temporal correlation of pre- and postsynaptic action potentials. The shifts are, however, consistent with the hypothesis that this plasticity is dependent on the temporal order of pre- and postsynaptic action potentials. This convergent reorganization seems to require increased neuronal gain, revealing a mechanism that networks may use to selectively facilitate the didactic transfer of neuronal response properties.

Effect of binocular retinal lesions on CRMP2 and CRMP4 but not Dyn I and Syt I expression in adult cat area 17

Removal of retinal input from a restricted region of adult cat visual cortex leads to a substantial reorganization of the retinotopy within the sensory-deprived cortical lesion projection zone (LPZ). Still little is known about the molecular mechanisms underlying this cortical map reorganization. We chose two members of the collapsin response mediator protein (CRMP) family, CRMP2 and CRMP4, because of their involvement in neurite growth, and compared gene and protein expression levels between normal control and reorganizing visual cortex upon induction of central retinal lesions. Parallel analysis of Dynamin I (Dyn I) and Synaptotagmin I (Syt I), two molecules implicated in the exocytosis-endocytosis cycle, was performed because changes in neurotransmitter release have been implicated in cortical plasticity. Western blotting and real-time polymerase chain reaction revealed a clear time-dependent effect of retinal lesioning on CRMP2 and CRMP4 expression, with maximal impact 2 weeks post-lesion. Altered CRMP levels were not a direct consequence of decreased visual activity in the LPZ as complete surgical removal of retinal input to one hemisphere had no effect on CRMP2 or CRMP4 expression. Thus, CRMP expression is correlated to cortical reorganization following partial deafferentation of adult visual cortex. In contrast, Dyn I and Syt I were not influenced and thereby do not promote exocytosis-endocytosis cycle modifications in adult cat cortical plasticity.

Dynamics and specificity of cortical map reorganization after retinal lesions

Neurons in the mature visual cortex deprived of their normal retinotopic inputs by matched binocular retinal lesions are initially silenced but become reactivated with time when the "blind" cortical lesion projection zone (LPZ) is filled in by new suprathreshold visual responses. In an attempt to gain further insight into the dynamics of this process, we investigated in detail the spatiotemporal pattern of single-cell properties and recording probability during cortical reorganization up to 12 months after retinal lesions. In the early phases of filling in, a transient peak of hyperactivity moves from the border of the normal cortex into the LPZ and forms the leading edge of a functional reconnection process. In the course of this process hyperactive cells inside the LPZ develop ectopic receptive fields that are initially enlarged and regain orientation specificity. During the proceeding recovery, hyperactivity and receptive field size normalize, while the quality of orientation tuning remains reduced at longer distances inside the LPZ at all stages of recovery up to 1 year. Within the adult anatomical framework of cortical connectivity, the maximal lateral distance of reconnection is limited, and the probability to encounter spiking cells decreases with increasing distance inside the LPZ. However, this recording probability was significantly increased after 1 year.

Axons and synaptic boutons are highly dynamic in adult visual cortex

While recent studies of synaptic stability in adult cerebral cortex have focused on dendrites, how much axons change is unknown. We have used advances in axon labeling by viruses and in vivo two-photon microscopy to investigate axon branching and bouton dynamics in primary visual cortex (V1) of adult Macaque monkeys. A nonreplicative adeno-associated virus bearing the gene for enhanced green fluorescent protein (AAV.EGFP) provided persistent labeling of axons, and a custom-designed two-photon microscope enabled repeated imaging of the intact brain over several weeks. We found that large-scale branching patterns were stable but that a subset of small branches associated with terminaux boutons, as well as a subset of en passant boutons, appeared and disappeared every week. Bouton losses and gains were both approximately 7% of the total population per week, with no net change in the overall density. These results suggest ongoing processes of synaptogenesis and elimination in adult V1.

Age-dependent alterations in CRMP2 and CRMP4 protein expression profiles in cat visual cortex

We monitored the protein expression profiles of collapsin response mediator protein 2 and 4 (CRMP2 and CRMP4) throughout cat primary visual area 17 at different postnatal ages. Single immunocytochemical stainings revealed a clear effect of cortical maturation on the spatial and laminar distribution profile of CRMP2 and CRMP4. In kittens of postnatal day 10 (P10) and 30 (P30), CRMP2 and CRMP4 immunoreactivity was exclusively present in fibers running perpendicular to the cortical surface and crossing all cortical layers, but was never found in neuronal cell bodies. The immunoreactive fibers were embedded in an intensely and homogeneously stained neuropil. In contrast, mature visual cortex immunocytochemistry located CRMP2 and CRMP4 in the somatodendritic compartment of neurons with a clear CRMP-specific lamination pattern. Similar to kitten, neuropil staining was clearly observed but showed a decreasing gradient from layer I to VI in adult area 17. Detailed analysis of cellular morphology and size classified the CRMP2- and CRMP4-immunopositive cells in distinct neuronal populations. Double labeling of CRMP2 or CRMP4 with the typical interneuron marker parvalbumin (PV) showed many double-labeled cells immunoreactive for CRMP4 and PV, but not for CRMP2 and PV, corroborating the cell type-specific character of each CRMP. Our present results clearly illustrate that CRMP2 and CRMP4 may play an important role in visual cortex, possibly providing different classes of neurons with the potential to form a functionally meaningful network, not only during development, but also in adulthood, coincident with the belief that CRMPs are involved in neurite growth and guidance.

Critical period plasticity in local cortical circuits

Neuronal circuits in the brain are shaped by experience during 'critical periods' in early postnatal life. In the primary visual cortex, this activity-dependent development is triggered by the functional maturation of local inhibitory connections and driven by a specific, late-developing subset of interneurons. Ultimately, the structural consolidation of competing sensory inputs is mediated by a proteolytic reorganization of the extracellular matrix that occurs only during the critical period. The reactivation of this process, and subsequent recovery of function in conditions such as amblyopia, can now be studied with realistic circuit models that might generalize across systems.

Neuroscience: rewiring the adult brain

Any analysis of plastic reorganization at a neuronal locus needs a veridical measure of changes in the functional output--that is, spiking responses of the neurons in question. In a study of the effect of retinal lesions on adult primary visual cortex (V1), Smirnakis et al. propose that there is no cortical reorganization. Their results are based, however, on BOLD (blood-oxygen-level-dependent) fMRI (functional magnetic resonance imaging), which provides an unreliable gauge of spiking activity. We therefore question their criterion for lack of plasticity, particularly in the light of the large body of earlier work that demonstrates cortical plasticity.

Critical Period Mechanisms in Developing Visual Cortex

Binocular vision is shaped by experience during a critical period of early postnatal life. Loss of visual acuity following monocular deprivation is mediated by a shift of spiking output from the primary visual cortex. Both synaptic and network explanations have been offered for this heightened brain plasticity. Direct experimental control over its timing, duration, and closure has now been achieved through a consideration of balanced local circuit excitation-inhibition. Notably, canonical models of homosynaptic plasticity at excitatory synapses alone (LTP/LTD) fail to produce predictable manipulations of the critical period in vivo. Instead, a late functional maturation of intracortical inhibition is the driving force, with one subtype in particular standing out. Parvalbumin-positive large basket cells that innervate target cell bodies with synapses containing the alpha1-subunit of GABA(A) receptors appear to be critical. With age, these cells are preferentially enwrapped in peri-neuronal nets of extracellular matrix molecules, whose disruption by chondroitinase treatment reactivates ocular dominance plasticity in adulthood. In fact, critical period plasticity is best viewed as a continuum of local circuit computations ending in structural consolidation of inputs. Monocular deprivation induces an increase of endogenous proteolytic (tPA-plasmin) activity and consequently motility of spines followed by their pruning, then re-growth. These early morphological events faithfully reflect competition only during the critical period and lie downstream of excitatory-inhibitory balance on a timescale (of days) consistent with the physiological loss of deprived-eye responses in vivo. Ultimately, thalamic afferents retract or expand accordingly to hardwire the rapid functional changes in connectivity. Competition detected by local inhibitory circuits then implemented at an extracellular locus by proteases represents a novel, cellular understanding of the critical period mechanism. It is hoped that this paradigm shift will lead to novel therapies and training strategies for rehabilitation, recovery from injury, and lifelong learning in adulthood.

Enhancement of synaptic strength in the somatosensory cortex following nerve injury does not parallel behavioural alterations

Following infraorbital nerve transection, underlying mechanisms of the altered synaptic strength were studied in rat barrel cortex slice experiments. In addition to the in vitro electrophysiological studies, open-field tests were run to detect possible behavioural changes associated with cortical oversensitization. Enhanced NMDA receptor-mediated component of the evoked field response appeared in the barrel cortex after nerve injury. The alteration was transient, very distinct on the first day following injury, and almost returned to normal level by the end of the second week. Behavioural changes had not followed this time-course since long-lasting alterations were detected in the open-field test. These observations are in agreement with findings that showed biphasic regenerative processes following nerve injuries in other cortical areas.

P300-amplitudes in upper limb amputees with and without phantom limb pain in a visual oddball paradigm

The aim of the study was to investigate to what extent cortical hyper-reactivity to visual stimuli is present in upper limb amputees. Five amputees with phantom limb pain (PLP), five amputees without PLP (Non-PLP) and 10 healthy controls (HC) were investigated using a visual oddball paradigm. Two hundred visual stimuli were presented with target stimuli occurring at a probability of 25% and standard stimuli at a probability of 75%. Event-related potentials were recorded from nine scalp positions (F3, F4, Fz, C3, C4, Cz, P3, P4, Pz). The PLP-patients had significantly higher P300-amplitudes to both types of stimuli compared to the non-PLP-patients. The HC were not significantly different from both amputee groups. P300-amplitude to targets at frontal sites in the hemisphere contralateral to the amputation was higher in the PLP patients. P300-latencies to target stimuli differed only at frontal sites with PLP-patients showing significantly longer latencies than non-PLP-patients. To standard stimuli, however, they showed significantly shorter latencies at central and parietal scalp positions. The HC had significantly shorter latencies than both amputee groups. The size of the P300-amplitude was positively correlated with the intensity of PLP. These findings suggest a higher magnitude of non-specific cortical excitability in amputees with PLP and a reduced excitability in amputees without PLP. This extends previous findings of differences in cortical excitability in PLP and non-PLP patients in the sensorimotor domain.

Functional reorganization of visual cortex maps after ischemic lesions is accompanied by changes in expression of cytoskeletal proteins and NMDA and GABA(A) receptor subunits

Reorganization of cortical representations after focal visual cortex lesions has been documented. It has been suggested that functional reorganization may rely on cellular mechanisms involving modifications in the excitatory/inhibitory neurotransmission balance and on morphological changes of neurons peripheral to the lesion. We explored functional reorganization of cortical retinotopic maps after a focal ischemic lesion in primary visual cortex of kittens using optical imaging of intrinsic signals. After 1, 2, and 5 weeks postlesion (wPL), we addressed whether functional reorganization correlated in time with changes in the expression of MAP-2, GAP-43, GFAP, GABA(A) receptor subunit alpha1 (GABA(A)alpha1), subunit 1 of the NMDA receptor (NMDAR1), and in neurotransmitter levels at the border of the lesion. Our results show that: (1) retinotopic maps reorganize with time after an ischemic lesion; (2) MAP-2 levels increase gradually from 1wPL to 5wPL; (3) MAP-2 upregulation is associated with an increase in dendritic-like structures surrounding the lesion and a decrease in GFAP-positive cells; (4) GAP-43 levels reach the highest point at 2wPL; (5) NMDAR1 and glutamate contents increase in parallel from 1wPL to 5wPL; (6) GABA(A)alpha1 levels increase from 1wPL to 2wPL but do not change after this time point; and (7) GABA contents remain low from 1wPL to 5wPL. This is a comprehensive study showing for the first time that functional reorganization correlates in time with dendritic sprouting and with changes in the excitatory/inhibitory neurotransmission systems previously proposed to participate in cortical remodeling and suggests mechanisms by which plasticity of cortical representations may occur.

Distribution and morphological characterization of phosphate-activated glutaminase-immunoreactive neurons in cat visual cortex

Phosphate-activated glutaminase (PAG) is the major enzyme involved in the synthesis of the excitatory neurotransmitter glutamate in cortical neurons of the mammalian cerebral cortex. In this study, the distribution and morphology of glutamatergic neurons in cat visual cortex was monitored through immunocytochemistry for PAG. We first determined the specificity of the anti-rat brain PAG polyclonal antibody for cat brain PAG. We then examined the laminar expression profile and the phenotype of PAG-immunopositive neurons in area 17 and 18 of cat visual cortex. Neuronal cell bodies with moderate to intense PAG immunoreactivity were distributed throughout cortical layers II-VI and near the border with the white matter of both visual areas. The largest and most intensely labeled cells were mainly restricted to cortical layers III and V. Careful examination of the typology of PAG-immunoreactive cells based on the size and shape of the cell body together with the dendritic pattern indicated that the vast majority of these cells were pyramidal neurons. However, PAG immunoreactivity was also observed in a paucity of non-pyramidal neurons in cortical layers IV and VI of both visual areas. To further characterize the PAG-immunopositive neuronal population we performed double-stainings between PAG and three calcium-binding proteins, parvalbumin, calbindin and calretinin, to determine whether GABAergic non-pyramidal cells can express PAG, and neurofilament protein, a marker for a subset of pyramidal neurons in mammalian neocortex. We here present PAG as a neurochemical marker to map excitatory cortical neurons that use the amino acid glutamate as their neurotransmitter in cat visual cortex.

Differential display implicates cyclophilin A in adult cortical plasticity

Removal of retinal input from a restricted region of adult cat visual cortex leads to a substantial reorganization of the retinotopy within the sensory-deprived cortical zone. Little is known about the molecular mechanisms underlying this reorganization. We used differential mRNA display (DDRT-PCR) to compare gene expression patterns between normal control and reorganizing visual cortex (area 17-18), 3 days after induction of central retinal lesions. Systematic screening revealed a decrease in the mRNA encoding cyclophilin A in lesion-affected cortex. In situ hybridization and competitive PCR confirmed the decreased cyclophilin A mRNA levels in reorganizing cortex and extended this finding to longer postlesion survival times as well. Western blotting and immunocytochemistry extended these data to the protein level. In situ hybridization and immunocytochemistry further demonstrated that cyclophilin A mRNA and protein are present in neurons. To exclude the possibility that differences in neuronal activity per se can induce alterations in cyclophilin A mRNA and protein expression, we analyzed cyclophilin A expression in the dorsal lateral geniculate nucleus (dLGN) of retinally lesioned cats and in area 17 and the dLGN of isolated hemisphere cats. In these control experiments cyclophilin A mRNA and protein were distributed as in normal control subjects indicating that the decreased cyclophilin A levels, as observed in sensory-deprived area 17 of retinal lesion cats, are not merely a reflection of changes in neuronal activity. Instead our findings identify cyclophilin A, classically considered a housekeeping gene, as a gene with a brain plasticity-related expression in the central nervous system.

Neuronal activity and neurotrophic factors regulate GAD-65/67 mRNA and protein expression in organotypic cultures of rat visual cortex

Environmental factors are known to regulate the molecular differentiation of neocortical interneurons. Their class-defining transmitter synthetic enzymes are the glutamic acid decarboxylases (GAD); yet, fairly little is known about the developmental regulation of transcription and translation of the GAD-65/67 isoforms. We have characterized the role of neuronal activity, neurotrophins and afferent systems for GAD-65/67 expression in visual cortex in organotypic cultures (OTC) compared with in vivo in order to identify cortex-intrinsic regulatory mechanisms. Spontaneously active OTC prepared at postnatal day 0 displayed from 10 days in vitro (DIV) onwards 12-14% GAD-65/GAD-67 neurons similar to in vivo. However, GAD-65 mRNA was higher, whereas GAD-67 protein was lower, than in vivo. During the first week neurotrophins increased whereas the Trk receptor inhibitor K252a and MEK inhibitors decreased both GAD mRNAs and proteins. After 10 DIV GAD expression no longer depended on neurotrophin signalling. Activity-deprived OTC revealed only 6% GAD-67 neurons and mRNA and protein were reduced by 50%. GAD-65 mRNA was less reduced, but protein was reduced by half, suggesting translational regulation. Upon recovery of activity GAD mRNAs, cell numbers, and both proteins quickly returned to normal and these 'adult' levels were resistant to late-onset deprivation. In 20 DIV activity-deprived OTC, only neurotrophin 4 increased GAD-65/67 mRNAs, rescued the percentage of GAD-67 neurons and increased both proteins in a TrkB-dependent manner. Activity deprivation had thus shifted the period of neurotrophin sensitivity to older ages. The results suggested neuronal activity as a major regulator differentially affecting transcription and translation of the GAD isoforms. The early presence of neuronal activity promoted the GAD expression in OTC to a neurotrophin-independent state suggesting that neurotrophins play a context-dependent role.

Extracellular GABA concentrations in area 17 of cat visual cortex during topographic map reorganization following binocular central retinal lesioning

Gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system of mammals, plays an important role in cortical reorganization following sensory deprivation, by regulating the level of cortical inhibition and gating changes in receptive field size and synaptic efficacy. In cats it has been shown that 2 weeks after the induction of binocular retinal lesions, GABAergic inhibition, as determined by immunocytochemistry, is decreased in the deafferented region of area 17, whereas 3 months post-lesion, normal GABAergic control is restored within the cortical scotoma. In this study we used in vivo microdialysis to investigate the extracellular GABA concentrations 1-2 months post-lesion, in the sensory-deprived and remote, non-deprived region of area 17. Data were collected at those sample times and sites for which the extracellular glutamate concentrations had been determined in a previous investigation to elucidate the role of this excitatory neurotransmitter in cortical reorganization. As for glutamate, we observed significantly increased extracellular GABA concentrations in non-deprived area 17, whereas in deafferented area 17, extracellular GABA concentrations were comparable to those observed in normal, control subjects. These data suggest that 1-2 months post-lesion the deafferented cortex behaves like normal visual cortex, in contrast to remote, non-deprived cortex. Notwithstanding the increase in extracellular GABA concentration of 134%, the parallel increase in glutamate concentration of 269% could give rise to a net increase in excitability in remote area 17. We therefore suggest that LTP-like mechanisms, and thereby cortical reorganization, might still be facilitated, while possible excessive hyperexcitability is balanced by the moderately increased GABAergic control.