Removal of GABAergic inhibition alters subthreshold input in neurons in forepaw barrel subfield (FBS) in rat first somatosensory cortex (SI) after digit stimulation

Lower jaw-to-forepaw rapid and delayed reorganization in the rat forepaw barrel subfield in primary somatosensory cortex

We used the forepaw barrel subfield (FBS), that normally receives input from the forepaw skin surface, in rat primary somatosensory cortex as a model system to study rapid and delayed lower jaw-to-forepaw cortical reorganization. Single and multi-unit recording from FBS neurons was used to examine the FBS for the presence of "new" lower jaw input following deafferentations that include forelimb amputation, brachial plexus nerve cut, and brachial plexus anesthesia. The major findings are as follows: (1) immediately following forelimb deafferentations, new input from the lower jaw becomes expressed in the anterior FBS; (2) 7-27 weeks after forelimb amputation, new input from the lower jaw is expressed in both anterior and posterior FBS; (3) evoked response latencies recorded in the deafferented FBS following electrical stimulation of the lower jaw skin surface are significantly longer in both rapid and delayed deafferents compared to control latencies for input from the forepaw to reach the FBS or for input from lower jaw to reach the LJBSF; (4) the longer latencies suggest that an additional relay site is imposed along the somatosensory pathway for lower jaw input to access the deafferented FBS. We conclude that different sources of input and different mechanisms underlie rapid and delayed reorganization in the FBS and suggest that these findings are relevant, as an initial step, for developing a rodent animal model to investigate phantom limb phenomena.

Effects of Repetitive Peripheral Sensory Stimulation in the Subacute and Chronic Phases After Stroke: Study Protocol for a Pilot Randomized Trial

Background

Repetitive peripheral nerve sensory stimulation (RPSS) is a potential add-on intervention to motor training for rehabilitation of upper limb paresis after stroke. Benefits of RPSS were reported in subjects in the chronic phase after stroke, but there is limited information about the effects of this intervention within the 1st weeks or months. The primary goal of this study is to compare, in a head-to-head proof-of-principle study, the impact of a single session of suprasensory vs. subsensory RPSS on the upper limb motor performance and learning in subjects at different phases after stroke subacute and chronic phases and mild upper limb motor impairments after stroke. In addition, we examine the effects of RPSS on brain perfusion, functional imaging activation, and γ-aminobutyric acid (GABA) levels. Subjects with mild upper limb motor impairments will be tested with MRI and clinical assessment either at an early (7 days to 3 months post-stroke) or at a chronic (>6 months) stage after stroke.

Methods

In this multicenter, randomized, parallel-group, proof-of-principle clinical trial with blinded assessment of outcomes, we compare the effects of one session of suprasensory or subsensory RPSS in patients with ischemic or hemorrhagic stroke and upper limb paresis. Clinical assessment and MRI will be performed only once in each subject (either at an early or at a chronic stage). The primary outcome is the change in performance in the Jebsen–Taylor test. Secondary outcomes: hand strength, cerebral blood flow assessed with arterial spin labeling, changes in the blood oxygenation level-dependent (BOLD) effect in ipsilesional and contralesional primary motor cortex (M1) on the left and the right hemispheres assessed with functional MRI (fMRI) during a finger-tapping task performed with the paretic hand, and changes in GABA levels in ipsilesional and contralesional M1 evaluated with spectroscopy. The changes in outcomes will be compared in four groups: suprasensory, early; subsensory, early; suprasensory, chronic; and subsensory, chronic.

Discussion

The results of this study are relevant to inform future clinical trials to tailor RPSS to patients more likely to benefit from this intervention.

Trial Registration

NCT03956407.

Repetitive microstimulation in rat primary somatosensory cortex (SI) strengthens the connection between homotopic sites in the opposite SI and leads to expression of previously ineffective input from the ipsilateral forelimb

The primary somatosensory cortex (SI) receives input from the contralateral forelimb and projects to homotopic sites in the opposite SI. Since homotopic sites in SI are linked by a callosal pathway, we proposed that repetitive intracortical microstimulation (ICMSr) of neurons in layer V of SI forelimb cortex would increase spike firing in the opposite SI cortex thereby strengthening the callosal pathway sufficiently to allow normally ineffective stimuli from the ipsilateral forelimb to excite cells in the ipsilateral SI. The forelimb representation in SI in one hemisphere was mapped using mechanical and electrical stimulation of the contralateral forelimb, a homotopic site was similarly identified in the opposite SI, the presence of ipsilateral peripheral input was tested in both homotopic sites, and ICMS was used to establish an interhemispheric connection between the two homotopic recording sites. The major findings are: (1) each homotopic forelimb site in SI initially received short latency input only from the contralateral forelimb; (2) homotopic sites in layer V in each SI were interconnected by a callosal pathway; (3) ICMSr delivered to layer V of the homotopic SI in one hemisphere generally increased evoked response spike firing in layer V in the opposite homotopic site; (4) increased spike firing was often followed by the expression of a longer latency normally ineffective input from the ipsilateral forelimb; (5) these longer latency ipsilateral responses are consistent with a delay time sufficient to account for travel across the callosal pathway; (6) increased spike firing and the resulting ipsilateral peripheral input were also corroborated using in-vivo intracellular recording; and (7) inactivation of the stimulating site in SI by lidocaine injection or local surface cooling abolished the ipsilateral response, suggesting that the ipsilateral response was very likely relayed across the callosal pathway. These results suggest that repetitive microstimulation can do more than expand receptive fields in the territory adjacent to the stimulating electrode but in addition can also alter receptive fields in homotopic sites in the opposite SI to bring about the expression of previously ineffective input from the ipsilateral forelimb.

Lateral Orbitofrontal Cortical Modulation on the Medial Prefrontal Cortex-Amygdala Pathway: Differential Regulation of Intra-Amygdala GABAA and GABAB Receptors

BACKGROUND

The basolateral complex of the amygdala receives inputs from neocortical areas, including the medial prefrontal cortex and lateral orbitofrontal cortex. Earlier studies have shown that lateral orbitofrontal cortex activation exerts an inhibitory gating on medial prefrontal cortex-amygdala information flow. Here we examined the individual role of GABAA and GABAB receptors in this process.

METHODS

In vivo extracellular single-unit recordings were done in anesthetized rats. We searched amygdala neurons that fire in response to medial prefrontal cortex activation, tested lateral orbitofrontal cortex gating at different delays (lateral orbitofrontal cortex-medial prefrontal cortex delays: 25, 50, 100, 250, 500, and 1000 milliseconds), and examined differential contribution of GABAA and GABAB receptors with iontophoresis.

RESULTS

Relative to baseline, lateral orbitofrontal cortex stimulation exerted an inhibitory modulatory gating on the medial prefrontal cortex-amygdala pathway and was effective up to a long delay of 500 ms (long-delay latencies at 100, 250, and 500 milliseconds). Moreover, blockade of intra-amygdala GABAA receptors with bicuculline abolished the lateral orbitofrontal cortex inhibitory gating at both short- (25 milliseconds) and long-delay (100 milliseconds) intervals, while blockade of GABAB receptors with saclofen reversed the inhibitory gating at long delay (100 milliseconds) only. Among the majority of the neurons examined (8 of 9), inactivation of either GABAA or GABAB receptors during baseline did not change evoked probability per se, suggesting that local feed-forward inhibitory mechanism is pathway specific.

CONCLUSIONS

Our results suggest that the effect of lateral orbitofrontal cortex inhibitory modulatory gating was effective up to 500 milliseconds and that intra-amygdala GABAA and GABAB receptors differentially modulate the short- and long-delay lateral orbitofrontal cortex inhibitory gating on the medial prefrontal cortex-amygdala pathway.

Neuromagnetic correlates of adaptive plasticity across the hand-face border in human primary somatosensory cortex

It is well established that permanent or transient reduction of somatosensory inputs, following hand deafferentation or anesthesia, induces plastic changes across the hand-face border, supposedly responsible for some altered perceptual phenomena such as tactile sensations being referred from the face to the phantom hand. It is also known that transient increase of hand somatosensory inputs, via repetitive somatosensory stimulation (RSS) at a fingertip, induces local somatosensory discriminative improvement accompanied by cortical representational changes in the primary somatosensory cortex (SI). We recently demonstrated that RSS at the tip of the right index finger induces similar training-independent perceptual learning across the hand-face border, improving somatosensory perception at the lips (Muret D, Dinse HR, Macchione S, Urquizar C, Farnè A, Reilly KT.Curr Biol24: R736-R737, 2014). Whether neural plastic changes across the hand-face border accompany such remote and adaptive perceptual plasticity remains unknown. Here we used magnetoencephalography to investigate the electrophysiological correlates underlying RSS-induced behavioral changes across the hand-face border. The results highlight significant changes in dipole location after RSS both for the stimulated finger and for the lips. These findings reveal plastic changes that cross the hand-face border after an increase, instead of a decrease, in somatosensory inputs.

Differential Pattern of Interhemispheric Connections Between Homotopic Layer V Regions in the Forelimb Representation in Rat Barrel Field Cortex

Layer V neurons in forelimb and shoulder representations in rat first somatosensory cortex (SI) project to the contralateral SI. However, few studies have addressed whether projections from specific subregions of the forelimb representation, namely forepaw, wrist, or forearm, terminate at homotopic sites in the contralateral SI. Neuroanatomical retrograde (cholera toxin B subunit [CT-B]) or anterograde (biodextran amine [BDA]) tracers were injected into physiologically identified sites in layer V in specific forelimb and/or shoulder representations in SI to examine the projection to contralateral SI in young adult rats (N = 17). Injection and target sites were flattened and cut in a tangential plane to relate labeling to the body map or cut along a coronal plane to relate labeling to cortical layers. Results indicate that layer V neurons project to cortical laminae II-VI in contralateral SI, with the densest labeling in layer V followed by layer III. In contrast, layer V neurons send sparse projections to layer IV. Furthermore, layer V neurons in wrist, forearm, and shoulder project to homotopic sites in contralateral layer V, while neurons in the forepaw representation project largely to sites in perigranular and dysgranular cortex adjacent to their homotopic territory. Our results provide evidence for a differential pattern of interhemispheric projections from forelimb and shoulder representations to the opposite SI and a detailed description of areal and laminar projection patterns of layer V neurons in the SI forelimb and shoulder cortices.

Forelimb amputation-induced reorganization in the ventral posterior lateral nucleus (VPL) provides a substrate for large-scale cortical reorganization in rat forepaw barrel subfield (FBS)

In this study, we examined the role of the ventral posterior lateral nucleus (VPL) as a possible substrate for large-scale cortical reorganization in the forepaw barrel subfield (FBS) of primary somatosensory cortex (SI) that follows forelimb amputation. Previously, we reported that, 6 weeks after forelimb amputation in young adult rats, new input from the shoulder becomes expressed throughout the FBS that quite likely has a subcortical origin. Subsequent examination of the cuneate nucleus (CN) 1 to 30 weeks following forelimb amputation showed that CN played an insignificant role in cortical reorganization and led to the present investigation of VPL. As a first step, we used electrophysiological recordings in forelimb intact adult rats (n=8) to map the body representation in VPL with particular emphasis on the forepaw and shoulder representations and showed that VPL was somatotopically organized. We next used stimulation and recording techniques in forelimb intact rats (n=5) and examined the pattern of projection (a) from the forelimb and shoulder to SI, (b) from the forepaw and shoulder to VPL, and (c) from sites in the forepaw and shoulder representation in VPL to forelimb and shoulder sites in SI. The results showed that the projections were narrowly focused and homotopic. Electrophysiological recordings were then used to map the former forepaw representation in forelimb amputated young adult rats (n=5) at 7 to 24 weeks after amputation. At each time period, new input from the shoulder was observed in the deafferented forepaw region in VPL. To determine whether the new shoulder input in the deafferented forepaw VPL projected to a new shoulder site in the deafferented FBS, we examined the thalamocortical pathway in 2 forelimb-amputated rats. Stimulation of a new shoulder site in deafferented FBS antidromically-activated a cell in the former forepaw territory in VPL; however, similar stimulation from a site in the original shoulder representation, outside the deafferented region, in SI did not activate cells in the former forepaw VPL. These results suggest that the new shoulder input in deafferented FBS is relayed from cells in the former forepaw region in VPL. In the last step, we used anatomical tracing and stimulation and recording techniques in forelimb intact rats (n=9) to examine the cuneothalamic pathway from shoulder and forepaw receptive field zones in CN to determine whether projections from the shoulder zone might provide a possible source of shoulder input to forepaw VPL. Injection of biotinylated dextran amine (BDA) into physiologically identified shoulder responsive sites in CN densely labeled axon terminals in the shoulder representation in VPL, but also gave off small collateral branches into forepaw VPL. In addition, microstimulation delivered to forepaw VPL antidromically-activated cells in shoulder receptive field sites in CN. These results suggest that forepaw VPL also receives input from shoulder receptive sites in CN that are latent or subthreshold in forelimb intact rats. However, we speculate that following amputation these latent shoulder inputs become expressed, possibly as a down-regulation of GABA inhibition from the reticular nucleus (RTN). These results, taken together, suggest that VPL provides a substrate for large-scale cortical reorganization that follows forelimb amputation.

GABAergic synapses: their plasticity and role in sensory cortex

The mammalian neocortex is composed of a variety of cell types organized in a highly interconnected circuit. GABAergic neurons account for only about 20% of cortical neurons. However, they show widespread connectivity and a high degree of diversity in morphology, location, electrophysiological properties and gene expression. In addition, distinct populations of inhibitory neurons have different sensory response properties, capacities for plasticity and sensitivities to changes in sensory experience. In this review we summarize experimental evidence regarding the properties of GABAergic neurons in primary sensory cortex. We will discuss how distinct GABAergic neurons and different forms of GABAergic inhibitory plasticity may contribute to shaping sensory cortical circuit activity and function.

Forelimb amputation-induced reorganization in the cuneate nucleus (CN) is not reflected in large-scale reorganization in rat forepaw barrel subfield cortex (FBS)

We examined reorganization in cuneate nucleus (CN) in juvenile rat following forelimb amputation (n=34) and in intact controls (n=5) to determine whether CN forms a substrate for large-scale reorganization in forepaw barrel subfield (FBS) cortex. New input from the shoulder first appears in the FBS 4 weeks after amputation, and by 6 weeks, the new shoulder input comes to occupy most of the FBS. Electrophysiological recording was used to map CN in controls and in forelimb amputees during the first 12 weeks following deafferentation and at 26 and 30 weeks post-amputation. Mapping was confined to a location 300 μm anterior to the obex where a medial-to-lateral row of electrode penetrations traversed through a complete complement of cytochrome-oxidase stained clusters (called barrelettes) that are associated with the representation of the glabrous forepaw digits and pads and adjacent non-cluster zones that are associated with the representation of the wrist, arm, and shoulder. Following amputation, non-cluster zones became occupied with new input from the body/chest and head/neck, while the cluster zone remained largely devoid of new input except at the border. A regression analysis comparing controls and amputees over the first 12 weeks post-amputation found significant differences for the total area of new input from the body/chest and head/neck in the non-cluster zones, while no significant differences were found for any new input into the cluster zone. When the averaged areas of a body-part representation were re-examined as a percentage of the averaged zonal area, a non-significant increase in new input from the body was observed within the cluster zone during post-amputation weeks 2-3 that returned to baseline in the subsequent weeks. In contrast, significant differences in averaged area of body-part representations for body/chest and head/neck were found in non-cluster zones over the first 12 weeks post-amputation. The present findings suggest that reorganization occurs only within the non-cluster zones whereby new input from the body/chest and head/neck moves in and occupies the deafferented territory immediately after amputation. Additionally, the lack of significant differences in new shoulder input in either cluster or non-cluster zones over the first 12 weeks after amputation suggests that CN provides an unlikely substrate for large-scale reorganization in the FBS.

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.

Functional and structural organization of the forelimb representation in cuneate nucleus in rat

We examined the physiological representation of the forelimb in the cuneate nucleus (CN) of forelimb-intact young adult rats (n=38) as the first part in a series of studies aimed at understanding the possible role that CN plays in delayed cortical reorganization that follows forelimb amputation. Metabolic labeling with cytochrome oxidase (CO) and electrophysiological mapping were used to examine the relationship between the structural and functional organization of CN. CN is a cylinder-shaped structure that lies bilaterally in the brainstem and extends nearly 4mm in the rostrocaudal direction. The forelimb is represented along the rostrocaudal extent. CN contains three zones; the rostral and caudal zones receive input largely from deep muscle and joint receptors and a middle zone, in the vicinity of the obex, receives input primarily from cutaneous receptors in the skin. The middle zone is somatotopically organized with the glabrous digits represented centrally, bordered on the medial side by ulnar wrist, ulnar forearm, and posterior upper arm representations; on the lateral side by radial wrist, radial forearm, and anterior upper arm representations; and on dorsal side by the dorsal digits and dorsal hand. The middle zone also contains well-defined CO-filled glomerular structures, called barrelettes, which are located within a homogenously stained field. The barrelettes are associated with the representation of the glabrous digits, with D5 represented most dorsal followed sequentially in a ventral-to-lateral direction by the representation of D4, D3, D2, and D1. The digit representations are topographically organized with the distal digit surface represented laterally with respect to the more medially lying proximal digit surface. The digit and palmar pads are also represented by barrelettes located on the medial side of CN. In contrast, the dorsal digit surfaces are represented dorsally and the dorsal hand is represented directly beneath the cuneate fasciculus, in a region devoid of barrelettes. The representations of the ulnar and radial wrist, forearm, and upper arm also lie within the homogeneously stained field in CN. The forelimb representation is bordered on the medial side by representation of trunk and hindlimb, and on the lateral side by representation of shoulder, ear, and head. While the present findings support and extend previous electrophysiological and anatomical studies of CN in the rat, they also provide a detailed physiological description of the functional organization of CN that is necessary for subsequent understanding of the functional reorganization of CN that may result following forelimb amputation.

Amputation with median nerve redirection (targeted reinnervation) reactivates forepaw barrel subfield in rats

Prosthetic limbs are difficult to control and do not provide sensory feedback. Targeted reinnervation was developed as a neural-machine interface for amputees to address these issues. In targeted reinnervation, amputated nerves are redirected to proximal muscles and skin, creating nerve interfaces for prosthesis control and sensory feedback. Touching the reinnervated skin causes sensation to be projected to the missing limb. Here we use electrophysiological brain recording in the Sprague Dawley rat to investigate the changes to somatosensory cortex (S1) following amputation and nerve redirection with the intent to provide insight into the sensory phenomena observed in human targeted reinnervation amputees. Recordings revealed that redirected nerves established an expanded representation in S1, which may help to explain the projected sensations that encompass large areas of the hand in targeted reinnervation amputees. These results also provide evidence that the reinnervated target skin could serve as a line of communication from a prosthesis to cortical hand processing regions. S1 border regions were simultaneously responsive to reinnervated input and also vibrissae, lower lip, and hindfoot, suggesting competition for deactivated cortical territory. Electrically evoked potential latencies from reinnervated skin to cortex suggest direct connection of the redirected afferents to the forepaw processing region of S1. Latencies also provide evidence that the widespread reactivation of S1 cortex may arise from central anatomical interconnectivity. Targeted reinnervation offers the opportunity to examine the cortical plasticity effects when behaviorally important sensory afferents are redirected from their original location to a new skin surface on a different part of the body.

The environmental pollutant endosulfan disrupts cerebral cortical function at low doses

Endosulfan can induce convulsions that could lead to brain damage. The variability and lack of specificity of neurological signs and symptoms in the pre-convulsive stages makes early diagnosis difficult. We sought to determine if electrophysiological exploration of the cerebral cortex could yield objective signs of endosulfan intoxication at levels that do not elicit convulsions. Endosulfan was administered intravenously to Sprague-Dawley adult rats under urethane anesthesia at doses from 0.5 to 4mg/kg. EEG power and the evoked potentials (EP) to forepaw electrical stimulation were studied over the contralateral (S1CL) and homolateral (S1HL) cortical somatosensory areas and the contralateral visual area (V1CL). At each area, five EP waves were measured. Arterial blood pressure, heart rate and body temperature were also recorded. Endosulfan induced a dose-related increase in EPs at all sites. At S1CL, EP peak amplitude was greater than baseline at 1, 2 and 4mg/kg for the first negative, second positive and third negative waves, and at 2 and 4mg/kg for the first and third positive waves. Similar but less marked trends were observed at S1HL and V1CL. A shift of EEG power to higher frequencies (alpha and beta EEG bands) was only present at 4mg/kg. In conclusion, endosulfan induced a large increase of cortical evoked potentials amplitudes at doses that did not elicit convulsions. These responses could be used as a non-invasive diagnostic tool to detect low-level endosulfan intoxication in humans and to help establish the NOAEL and LOAEL levels of this pollutant.

A comparative perspective on minicolumns and inhibitory GABAergic interneurons in the neocortex

Neocortical columns are functional and morphological units whose architecture may have been under selective evolutionary pressure in different mammalian lineages in response to encephalization and specializations of cognitive abilities. Inhibitory interneurons make a substantial contribution to the morphology and distribution of minicolumns within the cortex. In this context, we review differences in minicolumns and GABAergic interneurons among species and discuss possible implications for signaling among and within minicolumns. Furthermore, we discuss how abnormalities of both minicolumn disposition and inhibitory interneurons might be associated with neuropathological processes, such as Alzheimer's disease, autism, and schizophrenia. Specifically, we explore the possibility that phylogenetic variability in calcium-binding protein-expressing interneuron subtypes is directly related to differences in minicolumn morphology among species and might contribute to neuropathological susceptibility in humans.

Neuroanatomical identification of crossmodal auditory inputs to interneurons in somatosensory cortex

Multisensory convergence is the first, requisite step in the process that generates neural responses to events involving more than one sensory modality. Although anatomical studies have documented the merging of afferents from different sensory modalities within a given area, they do not provide insight into the architecture of connectivity at the neuronal level that underlies multisensory processing. In fact, few anatomical studies of multisensory convergence at the neuronal level have been conducted. The present study used a combination of tract-tracing, immunocytochemistry, and confocal microscopic techniques to examine the connections related to crossmodal auditory cortical inputs to somatosensory area SIV. Axons labeled from auditory cortex were found in contact with immunolabeled interneurons in SIV, some of which also colocalized vesicular glutamate transporter 1, indicating the presence of an active, glutamatergic synapse. No specific subtype of inhibitory interneuron appeared to be targeted by the crossmodal contacts. These results provide insight into the structural basis for multisensory processing at the neuronal level and offer anatomical evidence for the direct involvement of inhibitory interneurons in multisensory processing.

Cortico-cortical relations of cat somatosensory areas SIV and SV

Sensory cortex is characterized by multiple representations of a given modality which are generally highly interconnected and hierarchically arranged. The cat cerebral cortex contains at least five major areas dedicated to somatosensory processing, yet aside from areas SI and SII, little is known regarding the interconnectivity of the other, higher-level regions, such as SIV and SV. Therefore, this investigation examined the anatomical relationship of somatosensory areas SIV and SV to each other. In adult cats, wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) injected into SIV produced retrogradely labeled neurons in SV in a bilaminar pattern. When biotinylated dextran amine (BDA) was injected into SV, orthogradely labeled axon terminals were found in SIV across all laminae but predominated in supragranular locations. In the reciprocal direction, neurons located in both the supra- and infragranular layers of SIV projected across all laminae of SV, but also in a manner that favored the supragranular layers. Because local inhibitory circuits are critical for specific somatosensory response properties, the distribution of GABA-ergic neurons and their co-localized markers calbindin (CB), calretinin (CR) and parvalbumin (PV) was also compared for SIV and SV using immunocytochemical techniques. Although fundamental differences in laminar arrangement were observed between the different GABA-ergic subtypes, the distribution for each subtype was essentially the same in both SIV and SV. Collectively, these connectional, cytoarchitectonic and organizational similarities indicate that SIV and SV are reciprocally connected and share many somatosensory processing and connectional features.

Neocortical neuron types in Xenarthra and Afrotheria: implications for brain evolution in mammals

Interpreting the evolution of neuronal types in the cerebral cortex of mammals requires information from a diversity of species. However, there is currently a paucity of data from the Xenarthra and Afrotheria, two major phylogenetic groups that diverged close to the base of the eutherian mammal adaptive radiation. In this study, we used immunohistochemistry to examine the distribution and morphology of neocortical neurons stained for nonphosphorylated neurofilament protein, calbindin, calretinin, parvalbumin, and neuropeptide Y in three xenarthran species-the giant anteater (Myrmecophaga tridactyla), the lesser anteater (Tamandua tetradactyla), and the two-toed sloth (Choloepus didactylus)-and two afrotherian species-the rock hyrax (Procavia capensis) and the black and rufous giant elephant shrew (Rhynchocyon petersi). We also studied the distribution and morphology of astrocytes using glial fibrillary acidic protein as a marker. In all of these species, nonphosphorylated neurofilament protein-immunoreactive neurons predominated in layer V. These neurons exhibited diverse morphologies with regional variation. Specifically, high proportions of atypical neurofilament-enriched neuron classes were observed, including extraverted neurons, inverted pyramidal neurons, fusiform neurons, and other multipolar types. In addition, many projection neurons in layers II-III were found to contain calbindin. Among interneurons, parvalbumin- and calbindin-expressing cells were generally denser compared to calretinin-immunoreactive cells. We traced the evolution of certain cortical architectural traits using phylogenetic analysis. Based on our reconstruction of character evolution, we found that the living xenarthrans and afrotherians show many similarities to the stem eutherian mammal, whereas other eutherian lineages display a greater number of derived traits.

Experience-dependent changes in spatiotemporal properties of cutaneous inputs remodel somatosensory cortical maps following skin flap rotation

Contiguous skin surfaces that tend to be synchronously stimulated are represented in neighbouring sectors of primary somatosensory maps. Moreover, neuronal receptive fields (RFs) are reshaped through ongoing competitive/cooperative interactions that segregate/desegregate inputs converging onto cortical neuronal targets. The present study was designed to evaluate the influence of spatio-temporal constraints on somatotopic map organization. A vascularized and innervated pedicle flap of the ventrum skin bearing nipples was rotated by 180 degrees . Electrophysiological maps of ventrum skin were elaborated in the same rats at 24 h after surgery and 2 weeks after parturition. Neurones with split RFs resulting from the surgical separation of formerly adjoining skin surfaces were more numerous in non-nursing than nursing rats. RFs that included newly adjacent skin surfaces on both sides of the scar line emerged in nursing rats, suggesting that the spatial contiguity of formerly separated skin surfaces induced a fusion of their cortical representations through nursing-induced stimulation. In addition, nursing-dependent inputs were found to reincorporate the rotated skin flap representation in an updated topographical organization of the cortical map. A skin territory including recipient and translocated skin areas was costimulated for 7 h, using a brushing device. Neural responses evoked by a piezoelectric-induced skin indentation before and after skin brushing confirmed the emergence of RFs crossing the scar line and contraction of non-brushed components of split RFs. Our findings provide further evidence that the spatiotemporal structure of sensory inputs changing rapidly or evolving in a natural context is critical for experience-dependent reorganization of cortical map topography.

Neural plasticity after peripheral nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory and autonomic functions conveyed by the lesioned nerves to the denervated segments of the body, due to the interruption of axons continuity, degeneration of nerve fibers distal to the lesion and eventual death of axotomized neurons. Injuries to the peripheral nervous system may thus result in considerable disability. After axotomy, neuronal phenotype switches from a transmitter to a regenerative state, inducing the down- and up-regulation of numerous cellular components as well as the synthesis de novo of some molecules normally not expressed in adult neurons. These changes in gene expression activate and regulate the pathways responsible for neuronal survival and axonal regeneration. Functional deficits caused by nerve injuries can be compensated by three neural mechanisms: the reinnervation of denervated targets by regeneration of injured axons, the reinnervation by collateral branching of undamaged axons, and the remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of specificity in target reinnervation; plasticity in human has, however, limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain, hyperreflexia and dystonia. Recent research has uncovered that peripheral nerve injuries induce a concurrent cascade of events, at the systemic, cellular and molecular levels, initiated by the nerve injury and progressing throughout plastic changes at the spinal cord, brainstem relay nuclei, thalamus and brain cortex. Mechanisms for these changes are ubiquitous in central substrates and include neurochemical changes, functional alterations of excitatory and inhibitory connections, atrophy and degeneration of normal substrates, sprouting of new connections, and reorganization of somatosensory and motor maps. An important direction for ongoing research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, but are also able to modulate central nervous system reorganization, amplifying those positive adaptive changes that help to improve functional recovery but also diminishing undesirable consequences.

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.

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.

Scaling of inhibitory interneurons in areas v1 and v2 of anthropoid primates as revealed by calcium-binding protein immunohistochemistry

Inhibitory GABAergic interneurons are important for shaping patterns of activity in neocortical networks. We examined the distributions of inhibitory interneuron subtypes in layer II/III of areas V1 and V2 in 18 genera of anthropoid primates including New World monkeys, Old World monkeys, and hominoids (apes and humans). Interneuron subtypes were identified by immunohistochemical staining for calbindin, calretinin, and parvalbumin and densities were quantified using the optical disector method. In both V1 and V2, calbindin-immunoreactive neuron density decreased disproportionately with decreasing total neuronal density. Thus, V1 and V2 of hominoids were occupied by a smaller percentage of calbindin-immunoreactive interneurons compared to monkeys who have greater overall neuronal densities. At the transition from V1 to V2 across all individuals, we found a tendency for increased percentages of calbindin-immunoreactive multipolar cells and calretinin-immunoreactive interneurons. In addition, parvalbumin-immunoreactive cell soma volumes increased from V1 to V2. These findings suggest that modifications of specific aspects of inhibition might be critical to establishing the receptive field properties that distinguish visual areas. Furthermore, these results show that phylogenetic variation exists in the microcircuitry of visual cortex that could have general implications for sensory processing.

Cortical influences on rapid brainstem plasticity

Cortical contributions to brainstem plasticity in the somatosensory system are poorly understood. Tactile receptive fields (RFs) of brainstem dorsal column nuclei (DCN) neurons rapidly enlarge when peripheral inputs are disrupted by local anesthetic blocks with lidocaine (LID). Cortical inputs appear to influence this plasticity because enlargements have been shown to be greater when cortical inputs are disrupted. Like disruptions of peripheral inputs, disruptions of DCN inhibition by DCN administration of the GABAA receptor antagonist bicuculline methiodide (BMI) also cause rapid enlargements of DCN RFs when cortical inputs are intact. These findings leave questions about interactions between cortical inputs, DCN inhibition, and DCN RF plasticity. To study potential interactions, the present experiments evaluated RF sizes of DCN tactilely responsive neurons in anesthetized rats following DCN microinjection of BMI when cortical inputs were acutely disrupted or intact. These tests were also supplemented by subsequent LID tests to directly compare post-BMI and post-LID effects on the same RF. BMI caused DCN RF enlargements when cortical inputs were disrupted or intact; however, enlargements after cortical input disruption were greater than when cortical inputs were intact. Following RF enlargement and retraction after BMI, LID often caused a second enlargement of the same RF, across skin that partially matched skin involved in the enlargement after BMI. This occurred when cortical inputs were disrupted or intact. We hypothesize that cortical inputs are not required for BMI and LID to initiate partially matching enlargements in individual DCN tactile RFs, however, cortical inputs constrain magnitudes of these enlargements.

Hand posture modulates cortical finger representation in SII

Somatosensory magnetic fields evoked by electrical stimuli of the thumb or the index finger were recorded using a whole head magnetoencephalography (MEG) system in 10 subjects performing different finger postures, open hand posture and close hand posture for picking up a small object. The mean Euclidean distances between the ECD (equivalent current dipole) locations for the thumb and index finger in the secondary somatosensory cortex (SII) across the subjects were 8.5 +/- 2.1 mm in the close hand posture and 11.2 +/- 2.6 mm in the open hand posture. The distance was significantly shorter in the close hand posture (paired t test, P = 0.002, n = 8). However, the distances of the P38m and P60m components in the primary somatosensory cortex (SI) were not significantly different between the two hand postures (P38m: 13.4 +/- 5.6 mm in the open and 13.5 +/- 3.9 mm in the close; P60m: 12.4 +/- 2.6 mm in the open and 16.2 +/- 5.3 mm in the close). This shortening of the spatial distance between the cortical finger representations suggests a similarity in humans of the rapid changes in the dynamics of cortical circuits reported in animal studies. In addition, the overlap of the cortical finger representations, which might be suggested by the shortening of the distance between the ECDs in SII, is likely to play a role in information integration between sensory inputs from the thumb and index finger.

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.

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.

Role of development in reorganization of the SI forelimb-stump representation in fetally, neonatally, and adult amputated rats

Rats that sustain forelimb removal on postnatal day (P) 0 exhibit numerous multi-unit recording sites in the forelimb-stump representation of primary somatosensory cortex (SI) that also respond to hindlimb stimulation when cortical GABAA+B receptors are blocked. Most of these hindlimb inputs originate in the medial SI hindlimb representation. Although many forelimb-stump sites in these animals respond to hindlimb stimulation, very few respond to stimulation of the face (vibrissae or lower jaw), which is represented in SI just lateral to the forelimb. The lateral to medial development of SI may influence the capacity of hindlimb (but not face) inputs to "invade" the forelimb-stump region in neonatal amputees. The SI forelimb-stump was mapped in adult (>60 days) rats that had sustained amputation on embryonic day (E) 16, on P0, or during adulthood. GABA receptors were blocked and subsequent mapping revealed increases in nonstump inputs in E16 and P0 amputees: fetal amputees exhibited forelimb-stump sites responsive to face (34%), hindlimb (10%), and both (22%); neonatal amputees exhibited 10% face, 39% hindlimb, and 5% both; adult amputees exhibited 10% face, 5% hindlimb, and 0% both, with approximately 80% stump-only sites. These results indicate age-dependent differences in receptive-field reorganization of the forelimb-stump representation, which may reflect the spatiotemporal development of SI. Results from cobalt chloride inactivation of the SI vibrissae region and electrolesioning of the dysgranular cortex suggest that normally suppressed vibrissae inputs to the SI forelimb-stump area originate in the SI vibrissae region and synapse in the dysgranular cortex.

A comparison of the distribution of GABA-ergic neurons in cortices representing different sensory modalities

It is well known that sensory receptive field properties are shaped by inhibitory processes. Given the physiological and perceptual distinctions among the different sensory modalities, it might be expected that the contribution of GABA-ergic inhibition to the process would vary from area to area, depending on the sensory modality represented. Furthermore, as receptive field properties become progressively more complex at higher cortical levels, differences in the inhibitory contributions to these computations would be reflected in differences in GABA-ergic neuronal distribution. These possibilities were examined in the cortices surrounding the cat Anterior Ectosylvian Sulcus (AES) which contains higher order visual (AEV), somatosensory (SIV) and auditory (Field AES) representations, and is located between the lower-level primary (AI) and secondary auditory (AII) and somatosensory (SII) areas. Using standard immunocytochemical and light-microscopic techniques, the distribution of GABA-ergic neurons (and their co-localized calcium-binding proteins: calbindin (CB), calretinin (CR) and parvalbumin (PV)) was determined for each area. When normalized for differences in cortical thickness, the depth distribution of each of the immunopositive types was plotted. These data confirmed that there were striking differences in the distribution of GABA-, CB-, CR- and PV-positive neurons. However, the laminar organization for a given marker was remarkably similar for the different subregions, irrespective of modality or hierarchical level. These data indicate that, instead of underlying processing differences among different sensory and hierarchical representations, the distribution of GABA-ergic inhibitory neurons reveals common organizational features across sensory cortex.