Role of inhibition in cortical reorganization of the adult raccoon revealed by microiontophoretic blockade of GABA(A) receptors

Age-Related Reductions in Tactile and Motor Inhibitory Function Start Early but Are Independent

Aging is associated with declines in motor and somatosensory function. Some of these motor declines have been linked to age-related reductions in inhibitory function. Here we examined whether tactile surround inhibition also changes with age and whether these changes are associated with those in the motor domain. We tested a group of 56 participants spanning a wide age range (18-76 years old), allowing us to examine when age differences emerge across the lifespan. Participants performed tactile and motor tasks that have previously been linked to inter- and intra-hemispheric inhibition in the somatosensory and motor systems. The results showed that aging is associated with reductions in inhibitory function in both the tactile and motor systems starting around 40 years of age; however, age effects in the two systems were not correlated. The independent effects of age on tactile and motor inhibitory function suggest that distinct mechanisms may underlie age-related reductions in inhibition in the somatosensory and motor systems.

Serotonin rebalances cortical tuning and behavior linked to autism symptoms in 15q11-13 CNV mice

Serotonin is a critical modulator of cortical function, and its metabolism is defective in autism spectrum disorder (ASD) brain. How serotonin metabolism regulates cortical physiology and contributes to the pathological and behavioral symptoms of ASD remains unknown. We show that normal serotonin levels are essential for the maintenance of neocortical excitation/inhibition balance, correct sensory stimulus tuning, and social behavior. Conversely, low serotonin levels in 15q dup mice (a model for ASD with the human 15q11-13 duplication) result in impairment of the same phenotypes. Restoration of normal serotonin levels in 15q dup mice revealed the reversibility of a subset of ASD-related symptoms in the adult. These findings suggest that serotonin may have therapeutic potential for discrete ASD symptoms.

[Maladaptive plasticity in chronic and neuropathic pain]

Chronic pain results in structural and functional changes of the brain. However, most of the neurophysiologic and imaging studies have been conducted with small sample sizes, some have been reproduced, but studies on larger populations are lacking. Larger epidemiologic studies are currently being performed to show specific structural changes due to chronic pain. Longitudinal studies using neurophysiologic or imaging methods are very rare and often not feasible. Most methods are very complex, which hampers their application in daily practice. But it is not only the complexity of methods, but also a lack of interaction between researchers and practitioners to formulate joint research topics and targets. This article tries to fill the gap between the practicing pain therapist and the researcher in summarizing neurophysiological and imaging results on neuropathic and chronic pain in a clear and simple manner.

Rapid homeostasis by disinhibition during whisker map plasticity

How homeostatic processes contribute to map plasticity and stability in sensory cortex is not well-understood. Classically, sensory deprivation first drives rapid Hebbian weakening of spiking responses to deprived inputs, which is followed days later by a slow homeostatic increase in spiking responses mediated by excitatory synaptic scaling. Recently, more rapid homeostasis by inhibitory circuit plasticity has been discovered in visual cortex, but whether this process occurs in other brain areas is not known. We tested for rapid homeostasis in layer 2/3 (L2/3) of rodent somatosensory cortex, where D-row whisker deprivation drives Hebbian weakening of whisker-evoked spiking responses after an unexplained initial delay, but no homeostasis of deprived whisker responses is known. We hypothesized that the delay reflects rapid homeostasis through disinhibition, which masks the onset of Hebbian weakening of L2/3 excitatory input. We found that deprivation (3 d) transiently increased whisker-evoked spiking responses in L2/3 single units before classical Hebbian weakening (≥5 d), whereas whisker-evoked synaptic input was reduced during both periods. This finding suggests a transient homeostatic increase in L2/3 excitability. In whole-cell recordings from L2/3 neurons in vivo, brief deprivation decreased whisker-evoked inhibition more than excitation and increased the excitation-inhibition ratio. In contrast, synaptic scaling and increased intrinsic excitability were absent. Thus, disinhibition is a rapid homeostatic plasticity mechanism in rodent somatosensory cortex that transiently maintains whisker-evoked spiking in L2/3, despite the onset of Hebbian weakening of excitatory input.

Spike-timing-dependent potentiation of sensory surround in the somatosensory cortex is facilitated by deprivation-mediated disinhibition

Functional maps in the cerebral cortex reorganize in response to changes in experience, but the synaptic underpinnings remain uncertain. Here, we demonstrate that layer (L) 2/3 pyramidal cell synapses in mouse barrel cortex can be potentiated upon pairing of whisker-evoked postsynaptic potentials (PSPs) with action potentials (APs). This spike-timing-dependent long-term potentiation (STD-LTP) was only effective for PSPs evoked by deflections of a whisker in the neuron's receptive field center, and not its surround. Trimming of all except two whiskers rapidly opened the possibility to drive STD-LTP by the spared surround whisker. This facilitated STD-LTP was associated with a strong decrease in the surrounding whisker-evoked inhibitory conductance and partially occluded picrotoxin-mediated LTP facilitation. Taken together, our data demonstrate that sensory deprivation-mediated disinhibition facilitates STD-LTP from the sensory surround, which may promote correlation- and experience-dependent expansion of receptive fields.

A neural field model of the somatosensory cortex: formation, maintenance and reorganization of ordered topographic maps

We investigate the formation and maintenance of ordered topographic maps in the primary somatosensory cortex as well as the reorganization of representations after sensory deprivation or cortical lesion. We consider both the critical period (postnatal) where representations are shaped and the post-critical period where representations are maintained and possibly reorganized. We hypothesize that feed-forward thalamocortical connections are an adequate site of plasticity while cortico-cortical connections are believed to drive a competitive mechanism that is critical for learning. We model a small skin patch located on the distal phalangeal surface of a digit as a set of 256 Merkel ending complexes (MEC) that feed a computational model of the primary somatosensory cortex (area 3b). This model is a two-dimensional neural field where spatially localized solutions (a.k.a. bumps) drive cortical plasticity through a Hebbian-like learning rule. Simulations explain the initial formation of ordered representations following repetitive and random stimulations of the skin patch. Skin lesions as well as cortical lesions are also studied and results confirm the possibility to reorganize representations using the same learning rule and depending on the type of the lesion. For severe lesions, the model suggests that cortico-cortical connections may play an important role in complete recovery.

Expression and rapid experience-dependent regulation of type-A GABAergic receptors in the songbird auditory forebrain

GABAergic transmission influences sensory processing and experience-dependent plasticity in the adult brain. Little is known about the functional organization of inhibitory circuits in the auditory forebrain of songbirds, a robust model extensively used in the study of central auditory processing of behaviorally relevant communication signals. In particular, no information is currently available on the expression and organization of GABAA receptor-expressing neurons. Here, we studied the distribution and regulation of GABAA receptors in the songbird auditory forebrain, with a specific focus on α5, a subunit implicated in tonic inhibition and sensory learning. We obtained a zebra finch cDNA that encodes the α5-subunit (GABRA5) and carried out a detailed analysis of its expression via in situ hybridization. GABRA5 was highly expressed in the caudomedial nidopallium (NCM), caudomedial mesopallium, and field L2. Using double fluorescence in situ hybridization, we demonstrate that a large fraction of GABRA5-expressing neurons is engaged by auditory experience, as revealed by the song-induced expression of the activity-dependent gene zenk. Remarkably, we also found that α5 expression is rapidly regulated by sensory stimulation: 30 min of conspecific song playbacks significantly increase the number of GABRA5-expressing neurons in NCM, but not in other auditory areas. This effect is selective for α5, but not γ2 transcripts. Our results suggest that α5-containing GABAA receptors likely play a key role in central auditory processing and may contribute to the experience-dependent plasticity underlying auditory learning.

Learning-stage-dependent, field-specific, map plasticity in the rat auditory cortex during appetitive operant conditioning

Cortical reorganizations during acquisition of motor skills and experience-dependent recovery after deafferentation consist of several distinct phases, in which expansion of receptive fields is followed by the shrinkage and use-dependent refinement. In perceptual learning, however, such non-monotonic, stage-dependent plasticity remains elusive in the sensory cortex. In the present study, microelectrode mapping characterized plasticity in the rat auditory cortex, including primary, anterior, and ventral/suprarhinal auditory fields (A1, AAF, and VAF/SRAF), at the early and late stages of appetitive operant conditioning. We first demonstrate that most plasticity at the early stage was tentative, and that long-lasting plasticity after extended training was able to be categorized into either early- or late-stage-dominant plasticity. Second, training-induced plasticity occurred both locally and globally with a specific temporal order. Conditioned-stimulus (CS) frequency used in the task tended to be locally over-represented in AAF at the early stage and in VAF/SRAF at the late stage. The behavioral relevance of neural responses suggests that the local plasticity also occurred in A1 at the early stage. In parallel, the tone-responsive area globally shrank at the late stage independently of CS frequency, and this shrinkage was also correlated with the behavioral improvements. Thus, the stage-dependent plasticity may commonly underlie cortical reorganization in the perceptual learning, yet the interactions of local and global plasticity have led to more complicated reorganization than previously thought. Field-specific plasticity has important implications for how each field subserves in the learning; for example, consistent with recent notions, A1 should construct filters to better identify auditory objects at the early stage, while VAF/SRAF contribute to hierarchical computation and storage at the late stage.

Bilateral multielectrode neurophysiological recordings coupled to local pharmacology in awake songbirds

Here we describe a protocol for bilateral multielectrode neurophysiological recordings during intracerebral pharmacological manipulations in awake songbirds. This protocol encompasses fitting adult animals with head-posts and recording chambers, and acclimating them to periods of restraint. The adaptation period is followed by bilateral penetrations of multiple electrodes to obtain acute, sensory-driven neurophysiological responses before versus during the application of pharmacological agents of interest. These local manipulations are achieved by simultaneous and restricted drug infusions carried out independently for each hemisphere. We have used this protocol to elucidate how neurotransmitter and neuroendocrine systems shape the auditory and perceptual processing of natural, learned communication signals. However, this protocol can be used to explore the neurochemical basis of sensory processing in other small vertebrates. Representative results and troubleshooting of key steps of this protocol are presented. Following the animal's recovery from head-post and recording chamber implantation surgery, the length of the procedure is 2 d.

Anatomical and Functional Organization of Inhibitory Circuits in the Songbird Auditory Forebrain

Recent studies on the anatomical and functional organization of GABAergic networks in central auditory circuits of the zebra finch have highlighted the strong impact of inhibitory mechanisms on both the central encoding and processing of acoustic information in a vocal learning species. Most of this work has focused on the caudomedial nidopallium (NCM), a forebrain area postulated to be the songbird analogue of the mammalian auditory association cortex. NCM houses neurons with selective responses to conspecific songs and is a site thought to house auditory memories required for vocal learning and, likely, individual identification. Here we review our recent work on the anatomical distribution of GABAergic cells in NCM, their engagement in response to song and the roles for inhibitory transmission in the physiology of NCM at rest and during the processing of natural communication signals. GABAergic cells are highly abundant in the songbird auditory forebrain and account for nearly half of the overall neuronal population in NCM with a large fraction of these neurons activated by song in freely-behaving animals. GABAergic synapses provide considerable local, tonic inhibition to NCM neurons at rest and, during sound processing, may contain the spread of excitation away from un-activated or quiescent parts of the network. Finally, we review our work showing that GABAA-mediated inhibition directly regulates the temporal organization of song-driven responses in awake songbirds, and appears to enhance the reliability of auditory encoding in NCM.

Synapses of horizontal connections in adult rat somatosensory cortex have different properties depending on the source of their axons

In somatosensory cortex (S1) tactile stimulation activates specific regions. The borders between representations of different body parts constrain the spread of excitation and inhibition: connections that cross from one representation to another (cross-border, CB) are weaker than those remaining within the representation (noncross border, NCB). Thus, physiological properties of CB and NCB synapses onto layer 2/3 pyramidal neurons were compared using whole-cell recordings in layer 2/3 neurons close to the border between the forepaw and lower jaw representations. Electrical stimulation of CB and NCB connections was used to activate synaptic potentials. Properties of excitatory (EPSPs) and inhibitory (IPSPs) postsynaptic potentials (PSP) were determined using 3 methods: 1) minimal stimulation to elicit single-fiber responses; 2) stimulation in the presence of extracellular Sr(2+) to elicit asynchronous quantal responses; 3) short trains of stimulation at various frequencies to examine postsynaptic potential (PSP) dynamics. Both minimal and asynchronous quantal EPSPs were smaller when evoked by CB than NCB stimulation. However, the dynamics of EPSP and IPSP trains were not different between CB and NCB stimulation. These data suggest that individual excitatory synapses from connections that cross a border (CB) have smaller amplitudes than those that come from within a representation (NCB), and suggest a postsynaptic locus for the difference.

Reversal of the expression pattern of Aldolase C mRNA in Purkinje cells and Ube 1x mRNA in Golgi cells by a dopamine D1 receptor agonist injections in the methamphetamine sensitized-rat cerebellum

The cerebellum has a parasagittal modular structure, in which Zebrin (Aldolase) positive and negative bands expressed in Purkinje cell layers alternate, and is involved in amphetamine psychosis. Administration of SKF38393, a D1 receptor agonist, reversed the behavioral sensitization of methamphetamine. In the vermis, there were the binding sites of SKF38393. In methamphetamine-sensitized rats the expression of the Aldolase mRNA positive bands move laterally in the rat vermis. We provide here the evidence that the D1 agonist injections also reversed the expression pattern of both the Aldolase mRNA in Purkinje cells and Ube (ubiquitin activating enzyme) 1x mRNA in Golgi interneurons of the sensitized rats. Thus the reverse changes in gene expression pattern in the vermis may be involved in the mechanisms of the behavioral plasticity and suggests the new treatment of drug abuse.

GABA immunoreactivity in auditory and song control brain areas of zebra finches

Inhibitory transmission is critical to sensory and motor processing and is believed to play a role in experience-dependent plasticity. The main inhibitory neurotransmitter in vertebrates, GABA, has been implicated in both sensory and motor aspects of vocalizations in songbirds. To understand the role of GABAergic mechanisms in vocal communication, GABAergic elements must be characterized fully. Hence, we investigated GABA immunohistochemistry in the zebra finch brain, emphasizing auditory areas and song control nuclei. Several nuclei of the ascending auditory pathway showed a moderate to high density of GABAergic neurons including the cochlear nuclei, nucleus laminaris, superior olivary nucleus, mesencephalic nucleus lateralis pars dorsalis, and nucleus ovoidalis. Telencephalic auditory areas, including field L subfields L1, L2a and L3, as well as the caudomedial nidopallium (NCM) and mesopallium (CMM), contained GABAergic cells at particularly high densities. Considerable GABA labeling was also seen in the shelf area of caudodorsal nidopallium, and the cup area in the arcopallium, as well as in area X, the lateral magnocellular nucleus of the anterior nidopallium, the robust nucleus of the arcopallium and nidopallial nucleus HVC. GABAergic cells were typically small, most likely local inhibitory interneurons, although large GABA-positive cells that were sparsely distributed were also identified. GABA-positive neurites and puncta were identified in most nuclei of the ascending auditory pathway and in song control nuclei. Our data are in accordance with a prominent role of GABAergic mechanisms in regulating the neural circuits involved in song perceptual processing, motor production, and vocal learning in songbirds.

Role of GABA A inhibition in modulation of pyramidal tract neuron activity during postural corrections

In a previous study we demonstrated that the activity of pyramidal tract neurons (PTNs) of the motor cortex is modulated in relation to postural corrections evoked by periodical tilts of the animal. The modulation included an increase in activity in one phase of the tilt cycle and a decrease in the other phase. It is known that the motor cortex contains a large population of inhibitory GABAergic neurons. How do these neurons participate in periodic modulation of PTNs? The goal of this study was to investigate the role of GABA(A) inhibitory neurons of the motor cortex in the modulation of postural-related PTN activity. Using extracellular electrodes with attached micropipettes, we recorded the activity of PTNs in cats maintaining balance on a tilting platform both before and after iontophoretic application of the GABA(A) receptor antagonists gabazine or bicuculline. The tilt-related activity of 93% of PTNs was affected by GABA(A) receptor antagonists. In 88% of cells, peak activity increased by 75 +/- 50% (mean +/- SD). In contrast, the trough activity changed by a much smaller value and almost as many neurons showed a decrease as showed an increase. In 73% of the neurons, the phase position of the peak activity did not change or changed by no more than 0.1 of a cycle. We conclude that the GABAergic system of the motor cortex reduces the posture-related responses of PTNs but has little role in determining their response timing.

Cholinergic modulation of sensory interference in rat primary somatosensory cortical neurons

Sensory interaction was studied using extracellular recordings from 275 neurons in the primary somatosensory (SI) cortex of pentobarbital-anesthetized rats. Tactile stimulation was applied to the receptive field using a 1 mm diameter probe that indented the skin for 20 ms, at 0.5 Hz, (test stimulus). Tactile test responses of SI neurons decreased during simultaneous application of a gentle tickling (distracter stimuli) continuously for 60 s on a separate receptive field located in the same or the contralateral hindlimb (ipsi- or contralateral distraction). This decrease in neural response produced by distracter stimuli was interpreted as "sensory interference". Sensory interference was observed in 66% and 61% of recorded SI neurons when ipsi- or contralateral distracters were applied, respectively and was blocked by a novel stimulus obtained by increasing the stimulation frequency of the test tactile stimuli from 0.5 to 2 Hz. The number of neurons showing sensory interference in response to a contralateral distracter was not modified after corpus callosum transection, suggesting that interhemispheric connections are not crucial for sensory interference. In contrast, the number of neurons showing sensory interference decreased in animals with 192 IgG-saporin basal forebrain lesions that decreased the number of cortical cholinergic fibers. This finding indicates that cholinergic afferents from the basal forebrain are fundamental to sensory interference and suggests that the associative cortices - basal forebrain - sensory cortices network may be implicated in sensory interference.

Nursing-induced somatosensory cortex plasticity: temporally decoupled changes in neuronal receptive field properties are accompanied by modifications in activity-dependent protein expression

This study is an attempt to gain insight into the malleability of representational maps in the primary somatosensory cortex in relation to the expression of proteins involved in inhibitory and excitatory neurotransmitter systems that contribute to maintain these maps in a dynamic state. Malleability of somatosensory maps is characterized by changes in the sizes of neuron receptive fields (RFs) affecting the representational grain and in the locations and submodalities of these RFs modifying the map extent. The concomitance of these alterations remains so far hypothetical. We used nursing as an evolving source of ethologically significant cutaneous stimulation. This cyclic behavior is particularly suited to investigating the time course of experience-dependent cortical changes. Electrophysiological maps of the ventrum skin were recorded twice in the same lactating rats between nursing initiation and several weeks after nursing. We found that reduction in RF size occurred earlier than map expansion. As nursing time declined, the map expansion was maintained longer than the RF sharpening. Based on this difference in time course, we compared the expression patterns of several activity-dependent proteins in relation to the RF plasticity. Western blot analysis showed an increase in glutamic acid decarboxylase expression that was concomitant with RF contraction. In contrast, NR2A subunit of NMDA and alpha calcium/calmodulin kinase type II were upregulated at times when map expansion was observed. We propose that inhibitory and excitatory plasticity mechanisms operating with different time courses may contribute to the temporal dissociation of nursing-induced RF reshaping and map expansion.

A homeostatic rule for inhibitory synapses promotes temporal sharpening and cortical reorganization

Experience with transient stimuli leads to stronger neural responses that also rise and fall more sharply in time. This sharpening enhances the processing of transients and may be especially relevant for speech perception. We consider a learning rule for inhibitory connections that promotes this sharpening effect by adjusting these connections to maintain a target homeostatic level of activity in excitatory neurons. We analyze this rule in a recurrent network model of excitatory and inhibitory units. Strengthening inhibitory-->excitatory connections along with excitatory-->excitatory connections is required to obtain a sharpening effect. Using the homeostatic rule, we show that repeated presentations of a transient signal will "teach" the network to respond to the signal with both higher amplitude and shorter duration. The model also captures reorganization of receptive fields in the sensory hand area after amputation or peripheral nerve resection.

The surround inhibition determines therapy-induced cortical reorganization

Rehabilitation can induce cortical reorganization in chronic stroke patients. In this study we investigated the mechanisms underlying treatment-associated plasticity. Eight patients with a stroke >6 months earlier participated in a 4-week period of physiotherapy based on a forced use concept. Before and after treatment, focal transcranial magnetic stimulation over the affected hemisphere was used to assess the motor output map of the paretic first dorsal interosseous muscle. Using a paired pulse paradigm, intracortical inhibition was investigated at the center of the cortical output map (CoG) and one cm anterior, posterior, lateral and medial of that position. Motor function was evaluated with the Motor Activity Log and the Wolf Motor Function Test. After therapy, the cortical representation size of the affected hand muscle was increased. In each patient, the CoG moved in the direction where intracortical inhibition had been lowest prior to therapy. Significant correlations were found between motor function tests and changes of output map size and CoG shifts, respectively. We conclude that treatment-associated cortical reorganization is influenced by the distribution of inhibitory properties within the representation area prior to therapy, since the CoG moved in the direction of lowest inhibition. The correlations between motor functions and electrophysiological results indicate a functional relevance of the observed reorganization pattern.

Disparity for disinhibitory and excitatory effects during cortical reorganization

Inhibitory and excitatory transmission have been shown to be two dominant forces in the process of injury-induced cortical reorganization. However, the contributions of each individual neurotransmitter system have been difficult to characterize, mainly because they often affect overlapping domains of the reorganizing cell's receptive field (RF). The present work differentiates the effects of either neurotransmitter system by studying reorganizing neurons where pharmacological intervention directed at glutamatergic or GABAergic networks affected spatially non-overlapping subfields within a cell's RF. Microiontophoresis was used to apply glutamate and bicuculline, a selective GABA-A receptor antagonist, to cells in the deafferented cortex at various post-amputation times, that ranged from 2 to 37 weeks. It was found that at all post-amputation times studied, glutamate increased the neural activity that could be evoked from mechanical stimulation over the wound area, in addition to decreasing the threshold for activation. Disinhibition with bicuculline application often lowered the response threshold for subfields within the reorganized RF, usually located on the digits, and could also expand RF area. Interestingly, the effects of GABAergic transmission to the process of reorganization appeared later than those mediated through glutamatergic networks. These findings suggest that recruitment of inhibition in reorganizing cells relies on activity-dependent mechanisms, with reinstatement of GABAergic transmission being secondary to the formation of novel glutamatergic drives.

Concentration of amino acid neurotransmitters in the somatosensory cortex of the rat after surgical or functional deafferentation

Hindlimb unloading is considered as a model of functional deafferentation, since in this situation the tactile information from the paw and the proprioceptive input from the limb are dramatically reduced. Unloading induces a shrinkage of the cortical representation of the affected body part associated to a reorganization of topographic maps and to an expansion of receptive fields. Previous studies have suggested that cortical plasticity was the result of a change in the balance of excitation and inhibition in the cortex. The aim of the present study was thus to determine whether deafferentation of the hindlimb representation in the somatosensory cortex, by 14 days of unloading or by surgical means (selective dorsal rhizotomy during 17 days), can change the concentration in various amino acid neurotransmitters in the deprived cortex. The present findings indicate that both types of deafferentation result in a decrease in inhibitory amino acids (GABA, taurine) without significant changes in the main excitatory amino acid (glutamate). In conclusion, the present results support the idea that cortical changes are more likely due to a release from inhibition than to an increased excitation.

Incongruent restoration of inhibitory transmission and general metabolic activity during reorganization of somatosensory cortex

Activity markers cytochrome oxidase (CO) and glutamic acid decarboxylase (GAD) were analyzed in the primary somatosensory cortex of raccoons that underwent digit amputation. Subjects recovered for 2, 15, and 23 weeks following amputation of the fourth forepaw digit. Histochemistry was used to assess relative activity levels of both enzymes. We found a pronounced decrease in the numbers of CO intense patches in the cortical gyrus that had lost its original sensory input from the fourth digit. This decrease in CO activity was still apparent 15 weeks post-amputation. Conversely, no clear decrease in GAD levels could be identified in connection with the amputation procedure. Our findings present evidence that a significant decrease in metabolic activity results from the loss of the primary afferent sensory drive. The remaining GAD activity suggests that the absence of electrical activity, characteristic of reorganizing cortex, is likely to depend in part on lateral inhibitory cortical connections.

Inhibitory sharpening of receptive fields contributes to whisker map plasticity in rat somatosensory cortex

The role of inhibition in sensory cortical map plasticity is not well understood. Here we tested whether inhibition contributes to expression of receptive field plasticity in developing rat somatosensory (S1) cortex. In normal rats, microiontophoresis of gabazine (SR 95531), a competitive gamma-aminobutyric acid (GABA)-A receptor antagonist, preferentially disinhibited surround whisker responses relative to principal whisker responses, indicating that GABA(A) inhibition normally acts to sharpen whisker tuning. Plasticity was induced by transiently depriving adolescent rats of all but one whisker; this causes layer 2/3 (L2/3) receptive fields to shift away from the deprived principal whisker and toward the spared surround whisker. In units with shifted receptive fields, gabazine preferentially disinhibited responses to the deprived principal whisker, unlike in controls, suggesting that GABA(A) inhibition was acting to preferentially suppress these responses relative to spared whisker responses. This effect was not observed for L2/3 units that did not express receptive field plasticity or in layer 4, where receptive field plasticity did not occur. Thus GABA(A) inhibition promoted expression of sensory map plasticity by helping to sharpen receptive fields around the spared input.

Rapid functional plasticity in the primary somatomotor cortex and perceptual changes after nerve block

The mature human primary somatosensory cortex displays a striking plastic capacity to reorganize itself in response to changes in sensory input. Following the elimination of afferent return, produced by either amputation, deafferentation by dorsal rhizotomy, or nerve block, there is a well-known but little-understood 'invasion' of the deafferented region of the brain by the cortical representation zones of still-intact portions of the brain adjacent to it. We report here that within an hour of abolishing sensation from the radial and medial three-quarters of the hand by pharmacological blockade of the radial and median nerves, magnetic source imaging showed that the cortical representation of the little finger and the skin beneath the lower lip, whose intact cortical representation zones are adjacent to the deafferented region, had moved closer together, presumably because of their expansion across the deafferented area. A paired-pulse transcranial magnetic stimulation procedure revealed a motor cortex disinhibition for two muscles supplied by the unaffected ulnar nerve. In addition, two notable perceptual changes were observed: increased two-point discrimination ability near the lip and mislocalization of touch of the intact ulnar portion of the fourth finger to the neighbouring third finger whose nerve supply was blocked. We suggest that disinhibition within the somatosensory system as a functional correlate for the known enlargement of cortical representation zones might account for not only the 'invasion' phenomenon, but also for the observed behavioural correlates of the nerve block.

Morphological correlates of injury-induced reorganization in primate somatosensory cortex

Background

Topographic reorganization of central maps following peripheral nerve injury has been well characterized. Despite extensive documentation of these physiological changes, the underlying anatomical correlates have yet to be fully explored. In this study, we used Golgi impregnation and light microscopy to assess dendritic morphology following denervation of the glabrous hand surface in adult primates.

Results

After survival durations that permit complete physiologically-defined reorganization, we find a systematic change in the dendritic arborization pattern of both layer II/III pyramidal and layer IV spiny stellate cells in the contralateral hand region of area 3b, compared to unaffected cortical areas. In general, our analyses indicate a progressive expansion of distal regions of the dendritic arbor with no appreciable changes proximally. This pattern of distal dendritic elaboration occurs for both basilar and apical dendrites.

Conclusions

These observations are consistent with the notion that latent inputs gain expression in reorganized cortex after nerve injury via their influence through contacts with more distally located termination sites.

GABAergic neurons participate in the brain's response to birdsong auditory stimulation

Birdsong is a learned vocal behaviour that requires intact hearing for its development in juveniles and for its maintenance during adulthood. However, the functional organization of the brain circuits involved in the perceptual processing of song has remained obscure. Here we provide evidence that GABAergic mechanisms are an important component of these circuits and participate in the auditory processing of birdsong. We first cloned a zebra finch homologue of the gene encoding the 65-kDa isoform of glutamic acid decarboxylase (zGAD-65), a specific GABAergic marker, and conducted an expression analysis by in situ hybridization to identify GABAergic cells and to map their distribution throughout auditory telencephalic areas. The results showed that field L2, the caudomedial nidopallium (NCM) and the caudomedial mesopallium (CMM) contain a high number of GABAergic cells. Using patch-clamp brain slice recordings, we found abundant GABAergic mIPSCs in NCM. Pharmacological antagonism of mIPSCs induced large EPSC bursts, suggesting that tonic inhibition helps to stabilize NCM against runaway excitation via activation of GABA-A receptors. Next, using double fluorescence in situ hybridization and double immunocytochemical labelling, we demonstrated that large numbers of GABAergic cells in NCM and CMM show inducible expression of the transcriptional regulator ZENK in response to song auditory stimulation. These data provide direct evidence that GABAergic neurons in auditory brain regions are activated by song stimulation. Altogether, our results suggest that GABAergic mechanisms participate in auditory processing and perception, and might contribute to the memorization of birdsong.

The integrated nature of motor cortical function

Recent studies on the functional organization and operational principles of motor cortical function, taken together, strongly support the notion that the motor cortex controls the muscle activities subserving movements in an integrated manner. For example, during pointing the shoulder, elbow and wrist muscles appear to be controlled as a coupled functional system, rather than individually and separately. The pattern of intrinsic connections between motor cortical points is likely part of the explanation of this operational principle. So too is the manner in which muscles and muscle synergies are represented in the motor cortex. However, selection of movement-related muscle synergies is likely a dynamic process involving the functional linking of a variety of motor cortical points, rather than the selection of fixed patterns embedded in the motor cortical circuitry. One of the mechanisms that may be involved in the functional linking of motor cortical points is disinhibition. Thus, motor cortical points are recruited into action by selected excitation as well as by selected release from inhibition. The incoordination of limb movements in patients after a stroke may be understood, at least in part, as a disruption of the connections between motor cortical points and of the neural mechanisms involved in their functional linking.

Changes in corticothalamic modulation of receptive fields during peripheral injury-induced reorganization

The influence of corticothalamic projections on the thalamus during different stages of reorganization was determined in anesthetized raccoons that had undergone previous removal of a single forepaw digit. Single-unit recordings were made from 522 sites in the somatosensory nucleus of the thalamus (ventroposterior lateral nucleus) before and after lesioning parts of primary somatosensory cortex. In those parts of ventroposterior lateral nucleus that had intact input from the periphery, the cortical lesion resulted in an immediate 85% increase in receptive field (RF) size. In animals studied 2-6 weeks after digit amputation, peripherally denervated thalamic neurons had unique RFs that were larger than normal, and these were not further enlarged by cortical lesion. However, at longer periods of reorganization (>4 mo), when the new RFs of denervated neurons had decreased in size, cortical lesion again produced expansion of RF size. These data demonstrate that corticothalamic fibers modulate the spatial extent of thalamic RFs in intact animals, probably by controlling intrathalamic inhibition. This corticothalamic modulation is ineffective during the early stages of injury-induced reorganization when new RFs are being formed, but is reinstated after the new RFs have become stabilized. The fact that neurons in the denervated thalamic region retained their unique RFs after cortical lesion indicates that their new inputs are not being relayed from a reorganized cortex and support the view that some plasticity occurs in or below the thalamus.

Synaptic basis for developmental plasticity in somatosensory cortex

Sensory experience drives plasticity of the body map in developing and adult somatosensory cortex, but the synaptic mechanisms underlying such plasticity are not well understood. Recently, several mechanisms that are likely to contribute to map plasticity have been directly observed in response to altered experience in vivo. These mechanisms include long-term potentiation and long-term depression at specific excitatory synapses, competition between lemniscal (barrel) and non-lemniscal (septal) processing streams, and regulation of the number of inhibitory synapses.

Comparison of reorganization of the somatosensory system in rats that sustained forelimb removal as neonates and as adults

Studies of sensory pathways in several species indicate that the extent and form of reorganization resulting from deafferentation early in life vs. adulthood are not the same. The reasons for such differences are not well understood. To gain further insight into age-dependent mechanisms of reorganization, this study compared the consequences of neonatal vs. adult forelimb amputation in rats at multiple levels of the sensory pathway, including primary somatosensory cortex, brainstem, and dorsal root ganglia. At the cortical level, the average area of the functional forelimb-stump representation from rats amputated as adults was significantly smaller (P < 0.05) than that of neonatally amputated rats (4.3 +/- 1.3 mm(2) vs. 6.6 +/- 1.5 mm(2), respectively). At the brainstem level, neonatally amputated rat cuneate neurons possessed the following responsivities: 20% stump responsive, 40% responsive to both stump and hindlimb, 30% responsive to another body region, and 10% unresponsive. In contrast, cuneate neurons of adult amputated rats were 70% stump responsive, 2% responsive to both stump and hindlimb, and 30% unresponsive. A significantly (P < 0.001) greater percentage of the C(6)-C(8) dorsal root ganglia neurons of adult amputated rats were unresponsive to peripheral stimulation vs. neurons from neonatally amputated rats (48% vs. 16%, respectively). These results indicate that the reorganization that occurs in response to forelimb amputation at birth vs. adulthood is distinctly different at each of these levels of the dorsal column-medial lemniscal pathway. Possible mechanisms to account for these differences are considered.

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.

Induction of bilateral plasticity in sensory cortical maps by small unilateral cortical infarcts in rats

Behavioural impairments caused by brain lesions show a considerable, though often incomplete, recovery. It is hypothesized that cortical and subcortical plasticity of sensory representations contribute to this recovery. In the hindpaw representation of somatosensory cortex of adult rats we investigated the effects of focal unilateral cortical lesions on remote areas. Cortical lesions with a diameter of approximately 2 mm were induced in the parietal cortex by photothrombosis with the photosensitive dye Rose Bengal. Subsequently, animals were kept in standard cages for 7 days. On day seven, animals were anaesthetized and cutaneous receptive fields in the cortical hindpaw representations ipsi- and contralateral to the lesion were constructed from extracellular recordings of neurons in layer IV using glass microelectrodes. Receptive fields in the lesioned animals were compared to receptive fields measured in nonlesioned animals serving as controls. Quantitative analysis of receptive fields revealed a significant increase in size in the lesioned animals. This doubling in receptive field size was observed equally in the hemispheres ipsi- and contralateral to the lesion. The results indicate that the functional consequences of restricted cortical lesions are not limited to the area surrounding the lesion, but affect the cortical maps on the contralateral, nonlesioned hemisphere.

Complexity of sensory environment drives the expression of candidate-plasticity gene, nerve growth factor induced-A

Exposure of animals to an enriched environment triggers widespread modifications in brain circuitry and function. While this paradigm leads to marked plasticity in animals chronically or acutely exposed to the enriched environment, the molecular mechanisms that enable or regulate such modifications require further characterization. To this end, we have investigated the expression profiles of both mRNA and protein products of a candidate-plasticity gene, nerve growth factor induced-A (NGFI-A), in the brains of rats exposed to increased environmental complexity. We found that NGFI-A mRNA is markedly up-regulated throughout the brains of animals exposed to the enriched environment, but not in the brains of either handled-only or undisturbed control groups. The most pronounced effects were observed in the somatosensory and visual cortices, in layers III and V, while more modest increases were observed in all other cortical layers, with the exception of layer I. A striking NGFI-A mRNA up-regulation was also observed in the striatum and hippocampal formation, notably in the CA1 subfield, of animals exposed to the enriched environment paradigm. Immunocytochemistry was also used to investigate the distribution of NGFI-A protein in response to the environmental enrichment protocol. A marked increase in the number of NGFI-A positive nuclei was identified in the enriched environment condition, as compared to undisturbed and handled-only controls, throughout the rat brain. While the greatest number of NGFI-A immunolabeled neurons was found in cortical layers III and V, up-regulation of NGFI-A protein was also detectable in layers II, IV and VI, in both the somatosensory and visual cortices. NGFI-A immunopositive neurons were also more numerous in the CA1 subfield of the hippocampal formation of animals exposed to the enriched environment, but remained at basal levels in both control groups. Our results implicate NGFI-A as one of the possible early genetic signals that ultimately lead to plastic changes in the CNS.