Effect of sensory deafferentation on immunoreactivity of GABAergic cells and on GABA receptors in the adult cat visual cortex

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.

Growth rules for the repair of Asynchronous Irregular neuronal networks after peripheral lesions

Several homeostatic mechanisms enable the brain to maintain desired levels of neuronal activity. One of these, homeostatic structural plasticity, has been reported to restore activity in networks disrupted by peripheral lesions by altering their neuronal connectivity. While multiple lesion experiments have studied the changes in neurite morphology that underlie modifications of synapses in these networks, the underlying mechanisms that drive these changes are yet to be explained. Evidence suggests that neuronal activity modulates neurite morphology and may stimulate neurites to selective sprout or retract to restore network activity levels. We developed a new spiking network model of peripheral lesioning and accurately reproduced the characteristics of network repair after deafferentation that are reported in experiments to study the activity dependent growth regimes of neurites. To ensure that our simulations closely resemble the behaviour of networks in the brain, we model deafferentation in a biologically realistic balanced network model that exhibits low frequency Asynchronous Irregular (AI) activity as observed in cerebral cortex. Our simulation results indicate that the re-establishment of activity in neurons both within and outside the deprived region, the Lesion Projection Zone (LPZ), requires opposite activity dependent growth rules for excitatory and inhibitory post-synaptic elements. Analysis of these growth regimes indicates that they also contribute to the maintenance of activity levels in individual neurons. Furthermore, in our model, the directional formation of synapses that is observed in experiments requires that pre-synaptic excitatory and inhibitory elements also follow opposite growth rules. Lastly, we observe that our proposed structural plasticity growth rules and the inhibitory synaptic plasticity mechanism that also balances our AI network both contribute to the restoration of the network to pre-deafferentation stable activity levels.

An accumulating body of evidence suggests that our brain can compensate for peripheral lesions by adaptive rewiring of its neuronal circuitry. The underlying process, structural plasticity, can modify the connectivity of neuronal networks in the brain, thus affecting their function. To better understand the mechanisms of structural plasticity in the brain, we have developed a novel model of peripheral lesions and the resulting activity-dependent rewiring in a simplified balanced cortical network model that exhibits biologically realistic Asynchronous Irregular (AI) activity. In order to accurately reproduce the directionality and course of network rewiring after injury that is observed in peripheral lesion experiments, we derive activity dependent growth rules for different synaptic elements: dendritic and axonal contacts. Our simulation results suggest that excitatory and inhibitory synaptic elements have to react to changes in neuronal activity in opposite ways. We show that these rules result in a homeostatic stabilisation of activity in individual neurons. In our simulations, both synaptic and structural plasticity mechanisms contribute to network repair. Furthermore, our simulations indicate that while activity is restored in neurons deprived by the peripheral lesion, the temporal firing characteristics of the network may not be retained by the rewiring process.

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.

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.

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.

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.

Large-scale axonal reorganization of inhibitory neurons following retinal lesions

The functional properties of adult cortical neurons are subject to alterations in sensory experience. Retinal lesions lead to remapping of cortical topography in the region of primary visual cortex representing the lesioned part of the retina, the lesion projection zone (LPZ), with receptive fields shifting to the intact parts of the retina. Neurons within the LPZ receive strengthened input from the surrounding region by growth of the plexus of excitatory long-range horizontal connections. Here, by combining cell type-specific labeling with a genetically engineered recombinant adeno-associated virus and in vivo two-photon microscopy in adult macaques, we showed that the remapping was also associated with alterations in the axonal arbors of inhibitory neurons, which underwent a parallel process of pruning and growth. The axons of inhibitory neurons located within the LPZ extended across the LPZ border, suggesting a mechanism by which new excitatory input arising from the peri-LPZ is balanced by reciprocal inhibition arising from the LPZ.

Olfactory Hallucinations without Clinical Motor Activity: A Comparison of Unirhinal with Birhinal Phantosmia

Olfactory hallucinations without subsequent myoclonic activity have not been well characterized or understood. Herein we describe, in a retrospective study, two major forms of olfactory hallucinations labeled phantosmias: one, unirhinal, the other, birhinal. To describe these disorders we performed several procedures to elucidate similarities and differences between these processes. From 1272, patients evaluated for taste and smell dysfunction at The Taste and Smell Clinic, Washington, DC with clinical history, neurological and otolaryngological examinations, evaluations of taste and smell function, EEG and neuroradiological studies 40 exhibited cyclic unirhinal phantosmia (CUP) usually without hyposmia whereas 88 exhibited non-cyclic birhinal phantosmia with associated symptomology (BPAS) with hyposmia. Patients with CUP developed phantosmia spontaneously or after laughing, coughing or shouting initially with spontaneous inhibition and subsequently with Valsalva maneuvers, sleep or nasal water inhalation; they had frequent EEG changes usually ipsilateral sharp waves. Patients with BPAS developed phantosmia secondary to several clinical events usually after hyposmia onset with few EEG changes; their phantosmia could not be initiated or inhibited by any physiological maneuver. CUP is uncommonly encountered and represents a newly defined clinical syndrome. BPAS is commonly encountered, has been observed previously but has not been clearly defined. Mechanisms responsible for phantosmia in each group were related to decreased gamma-aminobutyric acid (GABA) activity in specific brain regions. Treatment which activated brain GABA inhibited phantosmia in both groups.

Visual cortex organisation in a macaque monkey with macular degeneration

The visual field is retinotopically represented in early visual areas. It has been suggested that when adult primary visual cortex (V1) is deprived of normal retinal input it is capable of large-scale reorganisation, with neurons inside the lesion projection zone (LPZ) being visually driven by inputs from intact retinal regions. Early functional magnetic resonance imaging (fMRI) studies in humans with macular degeneration (MD) report > 1 cm spread of activity inside the LPZ border, whereas recent results report no shift of the LPZ border. Here, we used fMRI population receptive field measurements to study, for the first time, the visual cortex organisation of one macaque monkey with MD and to compare it with normal controls. Our results showed that the border of the V1 LPZ remained stable, suggesting that the deafferented area V1 zone of the MD animal has limited capacity for reorganisation. Interestingly, the pRF size of non-deafferented V1 voxels increased slightly (~20% on average), although this effect appears weaker than that in previous single-unit recording reports. Area V2 also showed limited reorganisation. Remarkably, area V5/MT of the MD animal showed extensive activation compared to controls stimulated over the part of the visual field that was spared in the MD animal. Furthermore, population receptive field size distributions differed markedly in area V5/MT of the MD animal. Taken together, these results suggest that V5/MT has a higher potential for reorganisation after MD than earlier visual cortex.

Acoustic experience alters the aged auditory system

OBJECTIVES

Presbyacusis, one of the most common ailments of the elderly, is often treated with hearing aids, which serve to reintroduce some or all of those sounds lost to peripheral hearing loss. However, little is known about the underlying changes to the ear and brain as a result of such experience with sound late in life. The present study attempts to model this process by rearing aged CBA mice in an augmented acoustic environment (AAE).

DESIGN

Aged (22-23 months) male (n = 12) and female (n = 9) CBA/CaJ mice were reared in either 6 weeks of low-level (70 dB SPL) broadband noise stimulation (AAE) or normal vivarium conditions. Changes as a function of the treatment were measured for behavior, auditory brainstem response thresholds, hair cell cochleograms, and gamma aminobutyric acid neurochemistry in the key central auditory structures of the inferior colliculus and primary auditory cortex.

RESULTS

The AAE-exposed group was associated with sex-specific changes in cochlear pathology, auditory brainstem response thresholds, and gamma aminobutyric acid neurochemistry. Males exhibited significantly better thresholds and reduced hair cell loss (relative to controls) whereas females exhibited the opposite effect. AAE was associated with increased glutamic acid decarboxylase (GAD67) levels in the inferior colliculus of both male and female mice. However, in primary auditory cortex AAE exposure was associated with increased GAD67 labeling in females and decreased GAD67 in males.

CONCLUSIONS

These findings suggest that exposing aged mice to a low-level AAE alters both peripheral and central properties of the auditory system and these changes partially interact with sex or the degree of hearing loss before AAE. Although direct application of these findings to hearing aid use or auditory training in aged humans would be premature, the results do begin to provide direct evidence for the underlying changes that might be occurring as a result of hearing aid use late in life. These results suggest the aged brain retains significantly anatomical, electrophysiological, and neurochemical plasticity.

Structural plasticity of GABAergic axons is regulated by network activity and GABAA receptor activation

Coordinated changes at excitatory and inhibitory synapses are essential for normal brain development and function. It is well established that excitatory neurons undergo structural changes, but our knowledge about inhibitory structural plasticity is rather scarce. Here we present a quantitative analysis of the dynamics of GABAergic boutons in the dendritic region of the hippocampal CA1 area using time-lapse two-photon imaging in organotypic hippocampal cultures from GAD65-GFP mice. We show that ~20% of inhibitory boutons are not stable. They are appearing, disappearing and reappearing at specific locations along the inhibitory axon and reflect immature or incomplete synapses. Furthermore, we observed that persistent boutons show large volume fluctuations over several hours, suggesting that presynaptic content of inhibitory synapses is not constant. Our data show that inhibitory boutons are highly dynamic structures and suggest that inhibitory axons are continuously probing potential locations for inhibitory synapse formation by redistributing presynaptic material along the axon. In addition, we found that neuronal activity affects the exploratory dynamics of inhibitory axons. Blocking network activity rapidly reduces the number of transient boutons, whereas enhancing activity reduces the number of persistent inhibitory boutons, possibly reflecting enhanced competition between boutons along the axon. The latter effect requires signaling through GABA_A receptors. We propose that activity-dependent regulation of bouton dynamics contributes to inhibitory synaptic plasticity.

Presynaptic GABA(B) receptors decrease neurotransmitter release in vestibular nuclei neurons during vestibular compensation

Unilateral damage to the peripheral vestibular receptors precipitates a debilitating syndrome of oculomotor and balance deficits at rest, which extensively normalize during the first week after the lesion due to vestibular compensation. In vivo studies suggest that GABA(B) receptor activation facilitates recovery. However, the presynaptic or postsynaptic sites of action of GABA(B) receptors in vestibular nuclei neurons after lesions have not been determined. Accordingly, here presynaptic and postsynaptic GABA(B) receptor activity in principal cells of the tangential nucleus, a major avian vestibular nucleus, was investigated using patch-clamp recordings correlated with immunolabeling and confocal imaging of the GABA(B) receptor subunit-2 (GABA(B)R2) in controls and operated chickens shortly after unilateral vestibular ganglionectomy (UVG). Baclofen, a GABA(B) agonist, generated no postsynaptic currents in principal cells in controls, which correlated with weak GABA(B)R2 immunolabeling on principal cell surfaces. However, baclofen decreased miniature excitatory postsynaptic current (mEPSC) and GABAergic miniature inhibitory postsynaptic current (mIPSC) events in principal cells in controls, compensating and uncompensated chickens three days after UVG, indicating the presence of functional GABA(B) receptors on presynaptic terminals. Baclofen decreased GABAergic mIPSC frequency to the greatest extent in principal cells on the intact side of compensating chickens, with concurrent increases in GABA(B)R2 pixel brightness and percentage overlap in synaptotagmin 2-labeled terminals. In uncompensated chickens, baclofen decreased mEPSC frequency to the greatest extent in principal cells on the intact side, with concurrent increases in GABA(B)R2 pixel brightness and percentage overlap in synaptotagmin 1-labeled terminals. Altogether, these results revealed changes in presynaptic GABA(B) receptor function and expression which differed in compensating and uncompensated chickens shortly after UVG. This work supports an important role for GABA(B) autoreceptor-mediated inhibition in vestibular nuclei neurons on the intact side during early stages of vestibular compensation, and a role for GABA(B) heteroreceptor-mediated inhibition of glutamatergic terminals on the intact side in the failure to recover function.

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.

Loss of sensory input causes rapid structural changes of inhibitory neurons in adult mouse visual cortex

A fundamental property of neuronal circuits is the ability to adapt to altered sensory inputs. It is well established that the functional synaptic changes underlying this adaptation are reflected by structural modifications in excitatory neurons. In contrast, the degree to which structural plasticity in inhibitory neurons accompanies functional changes is less clear. Here, we use two-photon imaging to monitor the fine structure of inhibitory neurons in mouse visual cortex after deprivation induced by retinal lesions. We find that a subset of inhibitory neurons carry dendritic spines, which form glutamatergic synapses. Removal of visual input correlates with a rapid and lasting reduction in the number of inhibitory cell spines. Similar to the effects seen for dendritic spines, the number of inhibitory neuron boutons dropped sharply after retinal lesions. Together, these data suggest that structural changes in inhibitory neurons may precede structural changes in excitatory circuitry, which ultimately result in functional adaptation following sensory deprivation.

Difference in binocularity and ocular dominance plasticity between GABAergic and excitatory cortical neurons

Neuronal circuits in the cerebral cortex consist mainly of glutamatergic/excitatory and GABAergic/inhibitory neurons. In the visual cortex, the binocular responsiveness of neurons is modified by monocular visual deprivation during the critical period of postnatal development. Although GABAergic neurons are considered to play a key role in the expression of the critical period, it is not known whether their binocular responsiveness and ocular dominance plasticity are different from those of excitatory neurons. Recently, the end of the critical period was found to be not strict so that cortical neurons in the adult still have some ocular dominance plasticity. It is not known, however, which type of neurons or both maintain such plasticity in adulthood. To address these issues, we applied in vivo two-photon functional Ca(2+) imaging to transgenic mice whose GABAergic neurons express a yellow fluorescent protein called Venus. We found that GABAergic neurons are more binocular than excitatory neurons in the normal visual cortex, and both types of neurons show the same degree of modifiability to monocular visual deprivation during the critical period, but the modifiability of GABAergic neurons is stronger than that of excitatory neurons after the end of the critical period.

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.

Cortical inhibition and habituation to evoked potentials: relevance for pathophysiology of migraine

Dysfunction of neuronal cortical excitability has been supposed to play an important role in etiopathogenesis of migraine. Neurophysiological techniques like evoked potentials (EP) and in the last years non-invasive brain stimulation techniques like transcranial magnetic stimulation (TMS) and transcranial direct current stimulation gave important contribution to understanding of such issue highlighting possible mechanisms of cortical dysfunctions in migraine. EP studies showed impaired habituation to repeated sensorial stimulation and this abnormality was confirmed across all sensorial modalities, making defective habituation a neurophysiological hallmark of the disease. TMS was employed to test more directly cortical excitability in visual cortex and then also in motor cortex. Contradictory results have been reported pointing towards hyperexcitability or on the contrary to reduced preactivation of sensory cortex in migraine. Other experimental evidence speaks in favour of impairment of inhibitory circuits and analogies have been proposed between migraine and conditions of sensory deafferentation in which down-regulation of GABA circuits is considered the more relevant pathophysiological mechanism. Whatever the mechanism involved, it has been found that repeated sessions of high-frequency rTMS trains that have been shown to up-regulate inhibitory circuits could persistently normalize habituation in migraine. This could give interesting insight into pathophysiology establishing a link between cortical inhibition and habituation and opening also new treatment strategies in migraine.

Microstimulation of visual cortex to restore vision

This review argues that one reason why a functional visuo-cortical prosthetic device has not been developed to restore even minimal vision to blind individuals is because there is no animal model to guide the design and development of such a device. Over the past 8 years we have been conducting electrical microstimulation experiments on alert behaving monkeys with the aim of better understanding how electrical stimulation of the striate cortex (area V1) affects oculo- and skeleto-motor behaviors. Based on this work and upon review of the literature, we arrive at several conclusions: (1) As with the development of the cochlear implant, the development of a visuo-cortical prosthesis can be accelerated by using animals to test the perceptual effects of microstimulating V1 in intact and blind monkeys. (2) Although a saccade-based paradigm is very convenient for studying the effectiveness of delivering stimulation to V1 to elicit saccadic eye movements, it is less ideal for probing the volitional state of monkeys, as they perceive electrically induced phosphenes. (3) Electrical stimulation of V1 can delay visually guided saccades generated to a punctate target positioned in the receptive field of the stimulated neurons. We call the region of visual space affected by the stimulation a delay field. The study of delay fields has proven to be an efficient way to study the size and shape of phosphenes generated by stimulation of macaque V1. (4) An alternative approach to ascertain what monkeys see during electrical stimulation of V1 is to have them signal the detection of current with a lever press. Monkeys can readily detect currents of 1-2 microA delivered to V1. In order to evoke featured phosphenes currents of under 5 microA will be necessary. (5) Partially lesioning the retinae of monkeys is superior to completely lesioning the retinae when determining how blindness affects phosphene induction. We finish by proposing a future experimental paradigm designed to determine what monkeys see when stimulation is delivered to V1, by assessing how electrical fields generated through multiple electrodes interact for the production of phosphenes, and by depicting a V1 circuit that could mediate electrically induced phosphenes.

Inhibitory neurotransmission, plasticity and aging in the mammalian central auditory system

Aging and acoustic trauma may result in partial peripheral deafferentation in the central auditory pathway of the mammalian brain. In accord with homeostatic plasticity, loss of sensory input results in a change in pre- and postsynaptic GABAergic and glycinergic inhibitory neurotransmission. As seen in development, age-related changes may be activity dependent. Age-related presynaptic changes in the cochlear nucleus include reduced glycine levels, while in the auditory midbrain and cortex, GABA synthesis and release are altered. Presumably, in response to age-related decreases in presynaptic release of inhibitory neurotransmitters, there are age-related postsynaptic subunit changes in the composition of the glycine (GlyR) and GABA(A) (GABA(A)R) receptors. Age-related changes in the subunit makeup of inhibitory pentameric receptor constructs result in altered pharmacological and physiological responses consistent with a net down-regulation of functional inhibition. Age-related functional changes associated with glycine neurotransmission in dorsal cochlear nucleus (DCN) include altered intensity and temporal coding by DCN projection neurons. Loss of synaptic inhibition in the superior olivary complex (SOC) and the inferior colliculus (IC) likely affect the ability of aged animals to localize sounds in their natural environment. Age-related postsynaptic GABA(A)R changes in IC and primary auditory cortex (A1) involve changes in the subunit makeup of GABA(A)Rs. In turn, these changes cause age-related changes in the pharmacology and response properties of neurons in IC and A1 circuits, which collectively may affect temporal processing and response reliability. Findings of age-related inhibitory changes within mammalian auditory circuits are similar to age and deafferentation plasticity changes observed in other sensory systems. Although few studies have examined sensory aging in the wild, these age-related changes would likely compromise an animal's ability to avoid predation or to be a successful predator in their natural environment.

Does reorganization in the cuneate nucleus following neonatal forelimb amputation influence development of anomalous circuits within the somatosensory cortex?

Neonatal forelimb amputation in rats produces sprouting of sciatic nerve afferent fibers into the cuneate nucleus (CN) and results in 40% of individual CN neurons expressing both forelimb-stump and hindlimb receptive fields. The forelimb-stump region of primary somatosensory cortex (S-I) of these rats contains neurons in layer IV that express both stump and hindlimb receptive fields. However, the source of the aberrant input is the S-I hindlimb region conveyed to the S-I forelimb-stump region via intracortical projections. Although the reorganization in S-I reflects changes in cortical circuitry, it is possible that these in turn are dependent on the CN reorganization. The present study was designed to directly test whether the sprouting of sciatic afferents into the CN is required for expression of the hindlimb inputs in the S-I forelimb-stump field. To inhibit sprouting, neurotrophin-3 (NT-3) was applied to the cut nerves following amputation. At P60 or older, NT-3-treated rats showed minimal sciatic nerve fibers in the CN. Multiunit electrophysiological recordings in the CN of NT-3-treated, amputated rats revealed 6.3% of sites were both stump/hindlimb responsive, compared with 30.5% in saline-treated amputated animals. Evaluation of the S-I following GABA receptor blockade, revealed that the percentage of hindlimb responsive sites in the stump representation of the NT-3-treated rats (34.2%) was not significantly different from that in saline-treated rats (31.5%). These results indicate that brain stem reorganization in the form of sprouting of sciatic afferents into the CN is not necessary for development of anomalous hindlimb receptive fields within the S-I forelimb/stump region.

Laminar distribution and co-distribution of neurotransmitter receptors in early human visual cortex

The laminar distributions of 16 neurotransmitter receptor binding sites were analysed in visual cortical areas V1-V3 by quantitative in vitro receptor autoradiography. For each receptor (glutamatergic: AMPA, kainate, NMDA; cholinergic: M1, M2, M3, nicotinic; GABAergic: GABAA, GABAB, benzodiazepine binding-sites; adrenergic: alpha1, alpha2; serotoninergic: 5-HT1A, 5-HT2; dopaminergic: D1; Adenosine: A1), density profiles extracted perpendicular to the cortical surface were compared to cyto- and myeloarchitectonic profiles sampled at corresponding cortical sites. When testing for differences in laminar distribution patterns, all receptor-density profiles differed significantly from the cyto- and myeloarchitectonic ones. These results indicate that receptor distribution is an independent feature of the cortical architecture not predictable by densities of cell bodies or myelinated fibres. Receptor co-distribution was studied by cluster analyses, revealing several groups of receptors, which showed similar laminar distribution patterns across all analysed areas (V1-V3). Other receptors were co-distributed in extrastriate but not primary visual cortex. Finally, some receptors were not co-distributed with any of the analysed other ones. A comparison of the laminar patterns of receptor binding sites in the human visual cortex with those reported for non-human primates and other mammals showed that the laminar distributions of cholinergic and glutamatergic receptors seem largely preserved, while serotoninergic and adrenergic receptors appear to be more variable between different species.

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.

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.

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.

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.

Lack of long-term cortical reorganization after macaque retinal lesions

Several aspects of cortical organization are thought to remain plastic into adulthood, allowing cortical sensorimotor maps to be modified continuously by experience. This dynamic nature of cortical circuitry is important for learning, as well as for repair after injury to the nervous system. Electrophysiology studies suggest that adult macaque primary visual cortex (V1) undergoes large-scale reorganization within a few months after retinal lesioning, but this issue has not been conclusively settled. Here we applied the technique of functional magnetic resonance imaging (fMRI) to detect changes in the cortical topography of macaque area V1 after binocular retinal lesions. fMRI allows non-invasive, in vivo, long-term monitoring of cortical activity with a wide field of view, sampling signals from multiple neurons per unit cortical area. We show that, in contrast with previous studies, adult macaque V1 does not approach normal responsivity during 7.5 months of follow-up after retinal lesions, and its topography does not change. Electrophysiology experiments corroborated the fMRI results. This indicates that adult macaque V1 has limited potential for reorganization in the months following retinal injury.

New GABAergic interneurons in the adult neocortex and striatum are generated from different precursors

Ongoing neurogenesis in the adult mammalian dentate gyrus and olfactory bulb is generally accepted, but its existence in other adult brain regions is highly controversial. We labeled newly born cells in adult rats with the S-phase marker bromodeoxyuridine (BrdU) and used neuronal markers to characterize new cells at different time points after cell division. In the neocortex and striatum, we found BrdU-labeled cells that expressed each of the eight neuronal markers. Their size as well as staining for γ-aminobutyric acid (GABA), glutamic acid decarboxylase 67, calretinin and/or calbindin, suggest that new neurons in both regions are GABAergic interneurons. BrdU and doublecortin-immunoreactive (BrdU+/DCX+) cells were seen within the striatum, suggesting migration of immature neurons from the subventricular zone. Surprisingly, no DCX+ cells were found within the neocortex. NG2 immunoreactivity in some new neocortical neurons suggested that they may instead be generated from the NG2+ precursors that reside within the cortex itself.

Age-related loss of the GABA synthetic enzyme glutamic acid decarboxylase in rat primary auditory cortex

Age-related changes within the auditory brainstem typically include alterations in inhibitory neurotransmission and coding mediated by GABA and glycinergic circuits. As part of an effort to evaluate the impact of aging on neurotransmission in the higher auditory centers, the present study examined age-related changes in the GABA synthetic enzyme, glutamic acid decarboxylase (GAD), in rat primary auditory cortex (AI), which contains a vast network of intrinsic and extrinsic GABAergic circuits throughout its layers. Message levels of the two GAD isoforms found in brain, GAD(65) and GAD(67), and GAD(67) protein levels were compared in young adult, middle-aged and aged rats using in situ hybridization and quantitative immunocytochemistry, respectively. For comparison, age-related GAD changes were also assessed in the parietal cortex and hippocampus. Significant age-related decreases in GAD(65&67) messages were observed in AI layers II-VI of aged rats relative to their young adult cohorts. The largest changes were identified in layer II (GAD(65): -26.6% and GAD(67): -40.1%). GAD(67) protein expression decreased significantly in parallel with mRNA decreases in all layers of AI. Adjacent regions of parietal cortex showed no significant GAD(67) protein changes among the age groups, except in layer IV. As previously described, GAD(67) message and protein levels in selected hippocampal regions were significantly reduced in aged rats. Age-related GAD reductions likely reflect decreases in both metabolic and pre-synaptic GABA levels suggesting a plastic down-regulation of normal adult inhibitory GABA neurotransmission. Consistent with the present findings, functional studies in primate visual cortex and preliminary studies in AI find coding changes suggestive of altered inhibitory processing in aged animals. An age-related loss of normal adult GABA neurotransmission in AI would likely alter temporal coding properties and could contribute to the loss in speech understanding observed in the elderly.

Local GABA Receptor Blockade Reveals Hindlimb Responses in the SI Forelimb-Stump Representation of Neonatally Amputated Rats

In adult rats that sustained forelimb amputation on the day of birth, there are numerous multi-unit recording sites in the forelimb-stump representation of primary somatosensory cortex (SI) that also respond to cutaneous stimulation of the hindlimb when cortical receptors for GABA are blocked. These normally suppressed hindlimb inputs originate in the SI hindlimb representation and synapse in the dysgranular cortex before exciting SI forelimb-stump neurons. In our previous studies, GABA (A + B) receptor blockade was achieved by topically applying a bicuculline methiodide/saclofen solution (BMI/SAC) to the cortical surface. This treatment blocks receptors throughout SI and does not allow determination of where along the above circuit the GABA-mediated suppression of hindlimb information occurs. In this study, focal injections of BMI/SAC were delivered to three distinct cortical regions that are involved in the hindlimb-to-forelimb-stump pathway. Blocking GABA receptors in the SI hindlimb representation and in the dysgranular cortex was largely ineffective in revealing hindlimb inputs (∼10% of hindlimb inputs were revealed in both cases). In contrast, when the blockade was targeted at forelimb-stump recording sites, >80% of hindlimb inputs were revealed. Thus GABAergic interneurons within the forelimb-stump representation suppress the expression of reorganized hindlimb inputs to the region. A circuit model incorporating these and previous observations is presented and discussed.

Retinotopic map plasticity in adult cat visual cortex is accompanied by changes in Ca2+/calmodulin-dependent protein kinase II alpha autophosphorylation

In adult cats, the induction of homonymous binocular central retinal lesions causes a dramatic reorganization of the topographic map in the sensory-deprived region of the primary visual cortex. To investigate the possible involvement of the alpha-subunit of the calcium/calmodulin dependent protein kinase type II (alphaCaMKII) in this form of brain plasticity, we performed in situ hybridization and Western blotting experiments to analyze mRNA, protein and autophosphorylation levels of this multifunctional kinase. No differences in the mRNA or protein levels were observed between the central, sensory-deprived and the peripheral, non-deprived regions of area 17 of retinal lesion animals or between corresponding cortical regions of normal control animals. Western blotting with an alphaCaMKII threonine-286 phosphorylation-state specific antiserum consistently showed a small, albeit not significant, increase of alphaCaMKII autophosphorylation in the central versus the peripheral region of cortical area 17, and this both in normal subjects as well as in retinal lesion animals with a 3-day post-lesion survival time. In contrast, a post-lesion survival time of 14 days resulted in a alphaCaMKII autophosphorylation level that was four times higher in visually-deprived area 17 than in the non-deprived cortical region. This increased phosphorylation state is not a direct consequence of the decrease in visual activity in these neurons, because we would have expected to see a similar change at shorter or longer post-lesion survival times or in the visually deprived visual cortex of animals in which the left optic tract and the corpus callosum were surgically cut. No such changes were observed, leading to the conclusion that the phosphorylation changes observed at 14 days are related to a delayed reorganization of the retinotopic map of the striate 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.

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.

Retinal lesions affect extracellular glutamate levels in sensory-deprived and remote non-deprived regions of cat area 17 as revealed by in vivo microdialysis

This study aimed at gaining insight into the role of the excitatory neurotransmitter glutamate in topographic map reorganization in the sensory systems of adult mammals after restricted deafferentations. Hereto, in vivo microdialysis was used to sample extracellular glutamate from sensory-deprived and non-deprived visual cortex of adult awake cats 18 to 53 days after the induction of restricted binocular retinal lesions, and in topographically corresponding cortical regions of control animals. A microbore HPLC-ED method was applied for the analysis of the microdialysates. In normal subjects, the visual cortex subserving central and peripheral vision showed similar extracellular fluid glutamate concentrations. In contrast, in animals with homonymous central retinal lesions, the extracellular glutamate concentration was significantly lower in central, sensory-deprived cortex compared to peripheral, non-deprived cortex. Compared to control regions in normal subjects, glutamate decreased in the extracellular fluid of deprived cortex but increased significantly in remote non-deprived visual cortex. These results not only suggest an activity-dependent regulation of the glutamate levels in visual cortex but also imply a role for perilesional cortical regions in topographic map reorganization following sensory deafferentation.

Rapid modulation of GABA in sensorimotor cortex induced by acute deafferentation

Recovery of function after acute injury to the central nervous system may be controlled by the availability of gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the cerebral cortex. Acute lesions as well as manipulation of sensory inputs can lead to rapid reorganization of the cerebral cortex, occurring within minutes to hours. Reduction of cortical inhibitory tone through a decrease in the availability of GABA has been suggested as a possible mechanism; however, the degree and temporal course of the changes in brain GABA are not known. A novel method using two-dimensional J-resolved magnetic resonance spectroscopy showed that GABA levels in the human sensorimotor cortex are quickly reduced within minutes of deafferentation. This finding strongly supports the view that the release of latent corticocortical projections from tonic inhibition through decreased GABA availability is a mechanism of rapid cortical plasticity. Reduction of brain GABA can play a pivotal role in regulating the extent of rapid cortical reorganization after lesions or changes in sensory input.

Differential expression of c-fos in subtypes of GABAergic cells following sensory stimulation in the cat primary visual cortex

Recent immunocytochemical stainings on cat visual cortex, visually stimulated for 1 h, showed a strong induction of Fos expression in cortical neurons. We initiated immunocytochemical double staining experiments with different cytochemical markers to investigate the neurochemical and morphological character of these activated neurons showing Fos induction after sensory stimulation. Double staining with Fos and glutamic acid decarboxylase (GAD) demonstrated the presence of Fos in the nuclei of GABAergic neurons of the primary visual cortex. To further subdivide this Fos/GABAergic cell population we investigated whether Fos colocalized with parvalbumin, calbindin or calretinin. Colocalization of Fos with these calcium-binding proteins delineated distinct neuronal subclasses of Fos-immunoreactive neurons in supra- and infragranular layers of cat area 17. Quantitative analysis of the proportion of immunoreactive local circuit neurons revealed that 35% of the GABAergic neurons showed Fos induction in supragranular layers, whereas in infragranular layers a mere 10% of the GABAergic cells revealed Fos expression. Fos coexisted in about 24% of the calbindin-immunopositive cells within supra- and infragranular layers, but only a minority of the parvalbumin and the calretinin neuronal subgroups were immunopositive for Fos in the corresponding layers of area 17. These findings suggest that visual stimulation induces Fos expression in distinct subsets of inhibitory neurons in cat primary visual cortex.

A method to measure the effective spread of focally injected muscimol into the central nervous system with electrophysiology and light microscopy

A method was developed to quantitate the volume of brain inactivated by muscimol focally injected. Tritiated muscimol was injected into the cerebellum and closely spaced sequential microelectrode recordings made at different depths by penetrations in an X-Y pattern centered at the injection site to evaluate changes in spontaneous activity in the tissue volume. Animals were euthanized after survivals from 40 min to 6 h, the cerebellum sectioned in the sagittal plane, and the sections dried onto glass slides. The slides were dipped in photographic emulsion, exposed in the dark and developed. Silver grain densities were quantitated by light microscopy from measured standards. The extent and concentration of bound, labeled muscimol co-varied with the observed reduction in recorded spontaneous activity. For future studies, the distribution and density of silver grains alone can serve as an accurate spatial indicator of the area of muscimol inactivation at high spatial resolution.

Gamma-aminobutyric acid circuits shape response properties of auditory cortex neurons

Neurons containing gamma aminobutyric acid (GABA) are widely distributed throughout the primary auditory cortex (AI). We investigated the effects of endogenous GABA by comparing response properties of 110 neurons in chinchilla AI before and after iontophoresis of bicuculline, a GABA(A) receptor antagonist, and/or CGP35348, a GABA(B) receptor antagonist. GABA(A) receptor blockade significantly increased spontaneous and driven discharge rates, dramatically decreased the thresholds of many neurons, and constricted the range of thresholds across the neural population. Some neurons with 'non-onset' temporal discharge patterns developed an onset pattern that was followed by a long pause. Interestingly, the excitatory response area typically expanded on both sides of the characteristic frequency; this expansion exceeded one octave in a third of the sample. Although GABA(B) receptor blockade had little effect alone, the combination of CGP35348 and bicuculline produced greater increases in driven rate and expansion of the frequency response area than GABA(A) receptor blockade alone, suggesting a modulatory role of local GABA(B) receptors. The results suggest that local GABA inhibition contributes significantly to intensity and frequency coding by controlling the range of intensities over which cortical neurons operate and the range of frequencies to which they respond. The inhibitory circuits that generate nonmonotonic rate-level functions are separate from those that influence other response properties of AI neurons.

Functional MRI of congenital hyposmia: brain activation to odors and imagination of odors and tastes

PURPOSE

Our goal was to use functional MRI (fMRI) to define brain activation in response to odors and imagination ("memory") of odors and tastes in patients who never recognized odors (congenital hyposmia).

METHOD

Functional MR brain scans were obtained in nine patients with congenital hyposmia using multislice echo planar imaging (EPI) in response to odors of amyl acetate, menthone, and pyridine and to imagination ("memory") of banana and peppermint odors and to salt and sweet tastes. Functional MR brain scans were compared with those in normal subjects and patients with acquired hyposmia. Activation images were derived using correlation analysis, and ratios of areas of brain activated to total and hemispheric brain areas were calculated. Total and hemispheric activated pixel counts were used to quantitate regional brain activation.

RESULTS

Brain activation in response to odors was present in patients with congenital hyposmia. Activation was significantly lower than in normal subjects and patients with acquired hyposmia and did not demonstrate differential vapor pressure-dependent detection responsiveness or odor response lateralization. Regional activation localization was in anterior frontal and temporal cortex similar to that in normal subjects and patients with acquired hyposmia. Activation in response to presented odors was diverse, with a larger group exhibiting little or no activation with localization only in anterior frontal and temporal cortex and a smaller group exhibiting greater activation with localization extending to more complex olfactory integration sites. "Memory" of odors and tastes elicited activation in the same central nervous system (CNS) regions in which activation in response to presented odors occurred, but responses were significantly lower than in normal subjects and patients with acquired hyposmia and did not lateralize.

CONCLUSION

Odors induced CNS activation in patients with congenital hyposmia, which distinguishes olfaction from vision and audition since neither light nor acoustic stimuli induce CNS activation. Odor activation localized to anterior frontal and temporal cortex, consistent with the hypothesis that olfactory pathways are hard-wired into the CNS and that further pathways are undeveloped with primary olfactory system CNS connections but lack of secondary connections. However, some patients exhibited greater odor activation with response localization extending to cingulate and opercular cortex, indicating some olfactory signals impinge on and maintain secondary connections consistent with similar functions in vision and audition. Activation localization of taste "memory" to anterior frontal and temporal cortex is consistent with CNS plasticity and cross-modal CNS reorganization as described for vision and audition. Thus, there are differences and similarities between olfaction, vision, and audition, the differences dependent on unique qualities of olfaction, perhaps due to its diffuse, primitive, fundamental role in survival. Response heterogeneity to odors may reflect heterogeneous genetic abnormalities, independent of anatomic or hormonal changes but dependent on molecular abnormalities in growth factor function interfering with growth factor/stem cell interactions. Patients with congenital hyposmia offer an unique model system not previously explored in which congenital smell lack as measured by fMRI is reflective of congenital dysfunction of a major sensory system.

In vivo microdialysis in the visual cortex of awake cat. III: histological verification

In vivo microdialysis sampling extracellular excitatory and inhibitory amino acids from the striate cortex of mammals will provide important information for visual system research. To facilitate the interpretation of microdialysis results, this protocol critically examines: (1) the location of probe implantation in the visual cortex using Nissl staining; (2) the morphological changes after probe implantation by visualization of neurons containing glutamate; (3) the morphological changes after probe implantation by visualization of gliosis using glial fibrillary acidic protein (GFAP) immunocytochemistry; (4) the implantation of the probe in sensory-deprived versus non-deprived cortical regions by visualization of neurons containing c-Fos protein after limited retinal lesion. The histochemical and immunocytochemical methods of Glu, GFAP and c-Fos used are described.

Effect of partial sensory deprivation on monoaminergic neuromodulators in striate cortex of adult cat

The role of monoaminergic neuromodulators in the reorganization of cortical topography following limited sensory deprivation in the adult cat was investigated. The total concentrations of dopamine, noradrenaline, serotonin and their major metabolites were measured in the visual cortex of both normal control and experimental animals using microbore high-performance liquid chromatography coupled with electrochemical detection. The experimental animals were subjected to a binocular retinal lesion corresponding to the central 10 degrees of vision and killed two weeks post-lesion. The sensory deprivation was confirmed in area 17 by measuring immediate-early gene zif-268 messenger RNA expression. Following the retinal lesion, the total concentrations of noradrenaline and dopamine were significantly higher in the non-deprived cortex of retinal lesion cats than in the deprived cortex of retinal lesion cats and the cortex of normal animals. This pattern follows the release of the excitatory neurotransmitter glutamate under the same conditions. Serotonin levels were significantly lower in the deprived cortex, and its metabolite 5-hydroxyindole-3-acetic acid was significantly higher in the non-deprived cortex than in deprived cortex and normal cortex. From these results, we suggest that the modulation of noradrenaline, dopamine and serotonin is regulated by visual afferent activity.

Analysis of extracellular gamma-aminobutyric acid, glutamate and aspartate in cat visual cortex by in vivo microdialysis and capillary electrophoresis-laser induced fluorescence detection

To investigate the influence of a partial sensory deprivation on the extracellular concentration of amino acid neurotransmitters in cat visual cortex, a capillary electrophoresis method was developed for the quantification of gamma-aminobutyric acid (GABA), glutamate (Glu) and aspartate (Asp) in in vivo microdialysis samples of cat brain. Microdialysis samples from different regions of area 17 were obtained every 15-min using CMA 12 2-mm probes perfused with synthetic cerebrospinal fluid and derivatized using fluorescein isothiocyanate (FITC). Laser-induced fluorescence (LIF) detection was employed. Good selectivity was obtained with a borate buffer (20 mM, pH 10.25). The whole procedure, including the washing step takes only 15 min. The conditions for derivatization and separation were optimized. The parameters for validation such as linearity, precision and detection limit are also reported. The results are consistent with those of HPLC but, as the sample volumes needed are only 1--5 nl, a much better time resolution can be obtained.

Hyperexcitatory activity in visual cortex in homonymous hemianopia after stroke

OBJECTIVES

Damage to and destruction of neural afferents result in a disruption of sensory input, which causes reduced activity in the corresponding cortical areas. Conversely, there is also evidence that lesions in the sensory pathway induce changes in the intracortical connectivity resulting in augmented cortical activity due to disinhibition. As disinhibition is assumed to be involved in the reconfiguration of neural networks, its appearance after brain lesions might be relevant for the restitution of impaired brain functions.

METHODS

The effects of lesions in the visual pathway on the activity in visual cortex were studied using magnetoencephalography. In order to compare the neural activity affected by the lesion with the activity associated with intact visual processing, only patients with unilateral, post-chiasmatic lesions resulting in homonymous hemianopia were examined.

RESULTS

Stimulation within the scotoma resulted in reduced magnetic activity compared to the stimulation of the intact hemifield. Increased activity was observed when the border region of the scotoma was stimulated.

CONCLUSIONS

It is concluded that the magnetic hyperactivity reflects cortical disinhibition induced by lesions in the visual system. Furthermore, the possible role of cortical disinhibition as a basis for cortical reorganization and as a precondition for the recovery of impaired visual functions is discussed.

Comparison of the expression of two forms of glutamic acid decarboxylase (GAD67 and GAD65) in the visual cortex of normal and dark-reared cats

In normal development, there are dramatic changes in both the level and the laminar pattern of expression of the two forms of glutamate decarboxylase (GAD67, GAD65), the synthetic enzyme for gamma-aminobutyric acid (GABA). We have used antibodies to determine whether these normal postnatal changes in the expression of the two GADs depend on visual input by comparing normal and dark-reared cat visual cortex. Western blot analysis showed no significant differences in the levels of expression of the two enzymes between rearing conditions at either 5 or 20 weeks. Immunohistochemistry was used to compare the laminar distribution of the GADs in the two rearing conditions. At 1 week of age, both GAD67 and GAD65 immunoreactivity is concentrated in deep layers of visual cortex. At 5 and 20 weeks in both rearing conditions, GAD67-stained cells bodies were distributed rather uniformly across all cortical layers. GAD65 primarily labeled puncta (synaptic terminals) and these were also distributed rather uniformly across all visual cortical layers in both rearing conditions. Counts of GAD67-positive cell bodies and GAD65-positive puncta also revealed no differences between the rearing conditions. Thus, both GAD67, which produces the basal pool of GABA, and GAD65, which is specialized to respond to short-term increases in demand in synaptic terminals, developed normal levels of expression and normal intracellular and laminar distributions in the absence of visual input. Physiological studies suggest immaturity in the GABA system of dark-reared visual cortex. The present results indicate that such abnormalities are not due to presynaptic alterations in GABA synthetic enzymes.

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

Different intracortical mechanisms have been reported to contribute to the substantial topographic reorganization of the mammalian primary visual cortex in response to matching lesions in the two retinas: an immediate expansion of receptive fields followed by a gradual shift of excitability into the deprived area and finally axonal sprouting of laterally projecting neurons months after the lesion. To gain insight into the molecular mechanisms of this adult plasticity, we used immunocytochemical and bioanalytical methods to measure the glutamate and GABA neurotransmitter levels in the visual cortex of adult cats with binocular central retinal lesions. Two to four weeks after the lesions, glutamate immunoreactivity was decreased in sensory-deprived cortex as confirmed by HPLC analysis of the glutamate concentration. Within three months normal glutamate immunoreactivity was restored. In addition, the edge of the unresponsive cortex was characterized by markedly increased glutamate immunoreactivity 2-12 weeks postlesion. This glutamate immunoreactivity peak moved into the deprived area over time. These glutamate changes corresponded to decreased spontaneous and visually driven activity in unresponsive cortex and to strikingly increased neuronal activity at the border of this cortical zone. Furthermore, the previously reported decrease in glutamic acid decarboxylase immunoreactivity was found to reflect decreased GABA levels in sensory-deprived cortex. Increased glutamate concentrations and neuronal activity, and decreased GABA concentrations, may be related to changes in synaptic efficiency and could represent a mechanism underlying the retinotopic reorganization that occurs well after the immediate receptive field expansion but long before the late axonal sprouting.

Investigation of cortical reorganization in area 17 and nine extrastriate visual areas through the detection of changes in immediate early gene expression as induced by retinal lesions

The effect of binocular central retinal lesions on the expression of the immediate early genes c-fos and zif268 in the dorsal lateral geniculate nucleus (dLGN) and the visual cortex of adult cats was investigated by in situ hybridization and immunocytochemistry. In the deafferented region of the dLGN, the c-fos mRNA level was decreased within 3 days. The dimensions of the geniculate region showing decreased amounts of c-fos mRNA matched the predictions based on the lesion size and the retinotopic maps of Sanderson ([1971] J. Comp. Neurol. 143:101-118). We did not detect zif268 mRNA in the dLGN. At the cortical level, both c-fos and zif268 mRNA expression decreased in the sensory-deprived region of area 17. In addition, the portions of areas 18, 19, 21a, 21b, and 7, as well as the posterior medial lateral suprasylvian area, the posterior lateral lateral suprasylvian area, the ventral lateral suprasylvian area, and the dorsal lateral suprasylvian area corresponding to the retinal lesions also displayed decreased c-fos and zif268 mRNA levels. Immunocytochemistry revealed similar changes for Zif268 and Fos protein. Three days post lesion, the dimensions of the lesion-affected cortical loci exceeded the predictions in relation to the size of the retinal lesions and the available retinotopic maps. Longer postlesion survival times clearly resulted in a time-dependent restoration of immediate early gene expression from the border to the center of the lesion-affected cortical portions. Our findings represent a new approach for investigating the capacity of adult sensory systems to undergo plastic changes following sensory deprivation and for defining the topographic nature of sensory subcortical and cortical structures.

Sampling extracellular aspartate, glutamate and gamma-aminobutyric acid in striate cortex of awake cat by in vivo microdialysis: surgical and methodological aspects

A method which permits repeated microdialysis in the cortical layers of area 17 of the awake cat is described. Under visual control through a surgical microscope and using a stereotactic instrument, four probe guides are permanently implanted in area 17 of one hemisphere of the anesthetized animal and two fixation bars are mounted on the skull to allow fixation of the cat in a stereotactic frame. The implantation of four probe guides in the same hemisphere allows simultaneous sampling from different cortical regions serving different parts of the visual field. A removable transparent cover protects the probe guides. After recovery from surgery the awake cats are trained to adapt to a fixation of 5 h in a stereotaxic apparatus. Once adapted to that situation, the cats are ready for microdialysis experiments without anesthesia. The day of the experiment, the awake animal was fixed in the stereotactic frame and the probes inserted into the guides. To test the validity of the method, the basal efflux and the depolarization efflux, triggered by the addition of 65 mM K(+) to the artificial cerebrospinal fluid, of the amino acids aspartate, glutamate and gamma-aminobutyric acid are measured by two HPLC-electrochemical detection methods. The exact localization of the probes and the reaction of the surrounding tissue is studied using immunocytochemistry for glutamate and glial fibrilary acidic protein. Our neurochemical and morphological results suggest the feasibility of multiple and repeated probe insertions for microdialysis experiments in the cerebral cortex of awake and behaving cat. This method provides a new tool to investigate the cortical plasticity.