Regulation of Perineuronal Nets in the Adult Cortex by the Activity of the Cortical Network

Perineuronal net (PNN) accumulation around parvalbumin-expressing (PV) inhibitory interneurons marks the closure of critical periods of high plasticity, whereas PNN removal reinstates juvenile plasticity in the adult cortex. Using targeted chemogenetic in vivo approaches in the adult mouse visual cortex, we found that transient inhibition of PV interneurons, through metabotropic or ionotropic chemogenetic tools, induced PNN regression. EEG recordings indicated that inhibition of PV interneurons did not elicit unbalanced network excitation. Likewise, inhibition of local excitatory neurons also induced PNN regression, whereas chemogenetic excitation of either PV or excitatory neurons did not reduce the PNN. We also observed that chemogenetically inhibited PV interneurons exhibited reduced PNN compared with their untransduced neighbors, and confirmed that single PV interneurons express multiple genes enabling individual regulation of their own PNN density. Our results indicate that PNN density is regulated in the adult cortex by local changes of network activity that can be triggered by modulation of PV interneurons. PNN regulation may provide adult cortical circuits with an activity-dependent mechanism to control their local remodeling.SIGNIFICANCE STATEMENT The perineuronal net is an extracellular matrix, which accumulates around individual parvalbumin-expressing inhibitory neurons during postnatal development, and is seen as a barrier that prevents plasticity of neuronal circuits in the adult cerebral cortex. We found that transiently inhibiting parvalbumin-expressing or excitatory cortical neurons triggers a local decrease of perineuronal net density. Our results indicate that perineuronal nets are regulated in the adult cortex depending on the activity of local microcircuits. These findings uncover an activity-dependent mechanism by which adult cortical circuits may locally control their plasticity.     

Neonatal capsaicin treatment modulates experience‐dependent plasticity in the rat barrel cortex

Previous studies have reported that capsaicin‐induced C‐fiber depletion results in expansion of low threshold somatosensory mechanoreceptive fields. Here we used this paradigm to investigate its effect on experience‐dependent plasticity in the barrel cortex of rats. All but the D2 vibrissa were first plucked on postnatal day 0 (P0), P5, or P8, and kept plucked for a period of 30 days before being allowed to regrow for 7–9 days prior to the recording session. To assess receptive field characteristics the spared D2 principal whisker (PW) and the deprived D1 adjacent whisker (AW) were moved either singly or in concert, neuronal responses being recorded in layers IV and V of the D2 barrel. In vehicle‐treated rats, PW‐evoked ON responses (layer IV) were increased only in those animals that first had their vibrissae plucked on P0, whereas AW‐evoked ON responses (layers IV and V) were decreased in the P0, P5, and P8 groups. In the capsaicin‐treated animals, PW‐evoked ON responses (layer IV) were increased in all three groups, but no decrease was recorded in the AW‐evoked ON (layers IV and V) responses. In the vehicle‐ and capsaicin‐treated animals, the greatest decrease in inhibitory interactions was observed in the P5 and P0 groups, respectively. These findings indicate that, following the induction of experience‐dependent plasticity, the resultant changes in excitatory and integrative circuits can be further influenced by C‐fiber depletion. J. Comp. Neurol. 518:3427–3438, 2010. © 2010 Wiley‐Liss, Inc.     

The effects of peripheral deafferentation on spontaneously bursting neurons in the somatosensory cortex of waking cats

Single neurons (n=356) were studied in the forelimb representation of awake, quietly resting cats. Thirty-five spontaneously bursting neurons in a sample of 206 cells recorded before forelimb deafferentation were compared to 39 spontaneously bursting neurons in a sample of 127 neurons studied 1–3 weeks after deafferentation. The probability of encountering bursting neurons increased significantly following deafferentation from 17% to 31% of the sample (P<0.005). The same 5 classes of bursting cells were observed after deafferentation but there were significant changes in the duration of interspike intervals in some classes, in the probability of observing certain classes, and in the proportion of spikes found in bursts. The probability of encountering class III cells, a class thought to consist primarily of non-inactivating pyramidal burst neurons, nearly doubled and the average interspike interval length within the burst increased from 1.9 to 3.0 ms. The burst structure in the other classes did not change but they were found less frequently. These other classes may include inhibitory interneurons which receive less excitatory drive after deafferentation and therefore provide less inhibition to class III cells. The differential behavior of the different classes of bursting cells may be one reason why the overall level of spontaneous activity does not change after deafferentation and it suggests that there are homeostatic mechanisms in primary somatosensory cortex that maintain a certain level of neural activity.     

Bral2 is indispensable for the proper localization of brevican and the structural integrity of the perineuronal net in the brainstem and cerebellum

Perineuronal nets (PNNs) are pericellular coats of condensed matrix that enwrap the cell bodies and dendrites of many adult central nervous system (CNS) neurons. These extracellular matrices (ECMs) play a structural role as well as instructive roles in the control of CNS plasticity and the termination of critical periods. The cartilage link protein Crtl1/Hapln1 was reported to be a trigger for the formation of PNNs in the visual cortex. Bral2/Hapln4 is another link protein that is expressed in PNNs, mainly in the brainstem and cerebellum. To assess the role of Bral2 in PNN formation, we examined the expression of PNN components in targeted mouse mutants lacking Bral2. We show here that Bral2-deficient mice have attenuated PNNs, but the overall levels of chondroitin sulfate proteoglycans, lecticans, are unchanged with the exception of neurocan. Bral2 deficiency markedly affected the localization of brevican in all of the nuclei tested, and neurocan concomitant with Crtl1 in some of the nuclei, whereas no effect was seen on aggrecan even with the attenuation of Crtl1. Bral2 may have a role in the organization of the PNN, in association with brevican, that is independent of aggrecan binding. There was a heterogenous attenuation of PNN components, including glycosaminoglycans, indicating the elaborate molecular organization of the PNN components. Strikingly, a slight decrease in the number of synapses in deep cerebellar nuclei neurons was found. Taken together, these results imply that Bral2-brevican interaction may play a key role in synaptic stabilization and the structural integrity of the PNN.     

Spatiotemporal distribution of tenascin-R in the developing human cerebral cortex parallels neuronal migration

Tenascin-R is an extracellular matrix glycoprotein that is restricted to the central nervous system, where it acts as a multifunctional and versatile molecule. We report spatial and temporal distribution of tenascin-R in the developing human cerebral cortex for the first time. At 7.5 gestational weeks (GW), tenascin-R was expressed in a restricted area of the basal telencephalon. At 9.5 and 11 GW, it showed a unique double band expression pattern that delineated the boundaries of the future cortical plate. From 14 to 30 GW, tenascin-R labeling extended to the whole cortex from the deep layers toward the marginal zone with an inside-to-outside progression pattern reminiscent of neuronal migration. Moreover, tenascin-R labeling initially appeared in the form of thin, straight, or slightly tortuous intercellular processes directed toward the surface in parallel with the axis of neuronal migration. At the end of pregnancy and at adulthood, diffuse and homogeneous immunolabeling of the whole cortex thickness was observed. The striatum and thalamus were faintly positive for TNR as early as 14 GW, and this positivity intensified with brain maturation. At all developmental stages, the germinative zone, the corpus callosum, the anterior commissure, and the internal capsule appeared clearly negative for tenascin-R immunostaining whereas the adjacent parenchyma was immunopositive. Our results show that tenascin-R expression is tightly regulated in a spatiotemporal manner during brain development, especially cortical plate formation. Its pattern of expression suggests a role for tenascin-R in corticogenesis.     

Functional connectivity between somatosensory and visual cortex in early blind humans

Crossmodal plasticity occurs when loss of input in one sensory modality leads to reorganization in brain representations of other sensory modalities. In congenital blindness the visual cortex becomes responsive to somatosensory input such as occurs during Braille reading. The route by which somatosensory information reaches the visual cortex is not known. Here, we used repetitive transcranial magnetic stimulation (rTMS) to probe the connection between primary somatosensory cortex (S1) and early visual cortex (V1 and neighboring areas), combining rTMS with positron emission tomography (PET). We applied stimulation over S1 in sighted, early blind and late blind individuals. Baseline regional cerebral blood flow in occipital cortex was highest in early blind and lowest in late blind individuals. Only the early blind group showed significant activation of early visual areas when rTMS was delivered over S1. This activation was significantly higher in early than in late blind, but not relative to sighted controls. These results are consistent with the hypothesis that tactile information may reach early visual areas in early blind humans through cortico-cortical pathways, possibly supporting enhanced tactile information processing.     

Development of the spinal-medullary projection from the mouse barrel field

Neurons in layer V of the murine posteromedial barrel subfield (PMBSF) project to structures at or caudal to the spinal-medullary junction. During postnatal development a reduction occurs in the density of the neurons which form this projection. In principle, three processes might be expected to contribute to this reduction: cell death, tissue growth, and axon pruning. Three different paradigms in which cells of origin of the projection are labeled retrogradely with True Blue, injected into the spinal-medullary junction, taken together with an estimate of the relative growth of layer V, provide separate estimates of the magnitude and rate of reduction consequent to these different processes during the first 3 postnatal weeks. The density of neurons in an index sector of layer V of the PMBSF which contribute to the projection at varied ages is estimated by injections made at a range of ages from postnatal day 1 (P1) to P16, with a survival of 4 days in each instance. Overall reduction in density is 80%. The component due primarily to axon pruning is estimated to be 50% by injections delivered at graded ages from P1 to P16 with survival to P20 in each instance. The component of the reduction attributable to increase in volume is estimated at 30% by a series of injections delivered at P1 with graded survival times from P5 through P20. A reduction due to cell death is not identified. The reduction in density due to tissue growth is essentially linear through the interval P5-P11. At all ages, neuronal somata of origin of the spinal-medullary projection are located within layer V. Subsequent to P15 they are confined to sublayer Vb; at earlier ages somata in Va and Vc also contribute axons to the projection. Although volume increase due to growth of the neuropil reduces the density of the population contributing to the projection equally in all three sublayers, final elimination of all contributions from Va and Vc depends upon axon pruning.     

Chondroitin sulfate proteoglycans in the developing cerebral cortex: The distribution of neurocan distinguishes forming afferent and efferent axonal pathways

The pharmacological and physiological properties of ligand-gated ion channels are dependent on their subunit composition; spontaneously occurring changes in subunit composition during neuronal development may result in dramatic functional differences between embryonic and adult forms of the receptor complex. In the present study, in situ hybridization with antisense cRNA probes was used to examine the subunit composition of the γ-aminobutyric acid_A/benzodiazepine (GABA_A/BZ) receptor in the developing inferior olivary complex. This receptor is thought to be a pentameric chloride channel comprised of selected α, β, γ, δ, and ρ subunits, the majority of which have several isoforms: α_1?6, β_1?4, γ_1?4 and ρ_{1, 2}. Among the 13 subunit variants present in the mammalian central nervous system, α_2?5 β₃, and γ_{1, 2} mRNAs are expressed at significant levels in the inferior olivary complex. Two clearly different temporal patterns of GABA_A/BZ receptor subunit mRNA expression were observed: The expression of α₃, α₅, β₃, and γ₂ mRNAs was at a peak during embryonic and early postnatal development followed by rapid down-regulation thereafter. Conversely, α₂, α₄, and γ₁ mRNA expression was very low or absent during early development, and a pronounced increase was observed at the end of postnatal week 1. These studies suggest that there are developmental changes in the subunit composition of the GABA_A/BZ receptor in inferior olivary neurons. These changes in subunit expression, which occur during a period of major alterations in afferent and efferent synaptic connections, may subserve a change in the role of GABA from its function as a neurotrophic factor to that of an inhibitory neurotransmitter. © 1995 Wiley-Liss, Inc.     

Single-cell study of motor cortex projections to the barrel field in rats

In freely moving rats, whisking is associated with a slow modulation of neuronal excitability in the primary somatosensory cortex. Because it persists after the blockade of vibrissa input, it was suggested that the slow modulation might be mediated by motor-sensory corticocortical connections and perhaps result from the corollary discharges of corticofugal cells. In the present study, we identified motor cortical cells that project to the barrel field and reconstructed their axonal projections after juxtacellularly staining single cells with a biotinylated tracer. On the basis of the final destination of main axons, two groups of neurons contribute to motor-sensory projections: callosal cells (87.5%) and corticofugal cells (12.5%). Axon collaterals of callosal cells arborize in layers five to six of the granular and dysgranular zones and give off several branches that ascend between the barrels to ramify in the molecular layer. In contrast, the axon collaterals of corticofugal cells do not ramify in the infragranular layers but in layer 1. The origin of the majority of motor sensory projections from callosally projecting cells does not support the notion that the slow modulation results from the corollary discharges of corticofugal axons. It would rather originate from a separate population of cells, which could output the slow signal to the barrel field in parallel with the corticofugal commands to a brainstem pattern generator. As free whisking is characterized by bilateral concerted movements of the vibrissae, the transcallosal contribution of motor-sensory axons represents a substrate for synchronizing the slow modulation across both hemispheres.     

Effects of whisker trimming on GABAA receptor binding in the barrel cortex of developing and adult rats

Both sensory deprivation and blockade of γ-aminobutyric acid _A (GABA_A) receptors result in signs of cortical disinhibition. To investigate whether down-regulation of GABA_A receptors could underlie effects of sensory deprivation, [³H]muscimol binding was assessed in rat whisker barrels after chronic whisker trimming. Vibrissae in row C or rows A,B,D, and E were trimmed during certain developmental periods. When whiskers were trimmed for the first 6 postnatal weeks, [³H]muscimol binding was 8.3% lower in deprived barrel rows than in adjacent nondeprived rows (P < 0.001). The effect may be somewhat selective for GABA_A receptors because there was no evident change in N-methyl-D-aspartate (NMDA) receptors as indicated by [³H]MK-801 binding. Ten weeks after whiskers were allowed to regrow, the decrease in [³H]muscimol binding was partly reversed (P < 0.002), leaving a 3.3% decrease (P < 0.001). These declines in GABA_A receptors could contribute to persisting electrophysiological signs of reduced inhibition in similarly deprived barrel neurons (Simons and Land [1987] Nature 326:694–697). A 6-week deprivation beginning in adulthood resulted in a 7.7% decrease (P < 0.001), indicating that the effect is not restricted to an early critical period. In rats trimmed for the first 10 postnatal days, [³H]muscimol binding declined 2.3% (P < 0.05), which is a small change compared with the magnitude of the developmental peak; thus, normal whisker input apparently is not required for the developmental increase in GABA_A receptors. The present study suggests that sensory input can regulate cortical GABA_A receptors in adulthood and during ontogeny. Down-regulation of cortical GABA_A receptors may be a compensatory mechanism that serves to disinhibit the reduced sensory input. J. Comp. Neurol. 395:209–216, 1998. © 1998 Wiley-Liss, Inc.     

Reversal of the physiological effects of monocular deprivation in adult dark-reared cats

If kittens are dark-reared for 4 months and subsequently monocularly sutured, cells in area 17 become dominated by the experienced eye. We now find that the effects of monocular deprivation in adult dark-reared cats can be reversed by suturing the experienced eye and allowing the cat to use the deprived eye, an effect that has previously been shown only in young kittens. The presence of continuous or nearly continuous visual experience during infancy is required for the critical period to exhaust itself--brief periods of visual experience will not suffice.     

Metabolic barrel representations with various patterns of neonatal whisker deafferentation in rats

With various patterns of whisker deafferentation, C3 whisker stimulation produced divergently shaped metabolic barrel representations in layer IV of the primary somatosensory cortex. Whisker deafferentation results in functional and structural reorganization of the barrels in the primary somatosensory cortex. The present study examines the alteration of the metabolic barrel representations in layer IV with various configurations of selective whisker deafferentation in neonates, using [14C]2-deoxyglucose autoradiography. The deafferentation was produced by unilateral ablation of whiskers, leaving certain follicles intact. Configurations of intact follicles included: (I) row C follicles; (II) B3, C3, and D3 follicles; (III) B3, B4, C3, and C4 follicles; (IV) C2, C3, D2, and D3 follicles. The metabolic C3 barrel representations in layer IV after the deafferentations were found to have expanded only toward the barrel sites in which the corresponding whiskers were ablated, with no expansion toward the neighboring barrels. Expansion toward row D was significantly more pronounced than expansion toward row B, and expansion toward the C2 barrel was significantly more pronounced than expansion toward the C4 barrel. From these results, it can be inferred that asymmetric intrinsic structural connections are reflected in the functional metabolic barrel representation under the condition of neural plasticity in the barrel cortex following whisker deafferentation.     

GABA immunoreactivity in mouse barrel field after aversive and appetitive classical conditioning training involving facial vibrissae

We have previously reported that a classical conditioning paradigm involving stimulation of a row of facial vibrissae produced an expansion of the cortical representation of the "trained row", labeled with 2-deoxyglucose (2DG), in layer IV of the barrel field. The present study has examined the pattern of GABA immunoreactivity (GABA-IR) in the cortical representation of row B of the facial vibrissae after (i) 3 days of aversive training, and (ii) 2 months of appetitive training, where stimulation of row B of vibrissae on one side of the snout was used as a conditioned stimulus. The most notable observation was a greater density of GABA-IR cells concentrated in the hollows of the "trained row" B barrels compared to the hollows in the barrel field of the opposite hemisphere in the same mouse. After aversive training, we noted a 2-fold increase in the density of GABA-IR neurons in the hollows of row B; after reward training, the increase amounted to 49%. In contrast, GABA-IR was unchanged in the control groups, which received only stimulation of vibrissae without the unconditioned stimulus. The classification of labeled neurons according to size revealed that the increase in density of GABA-IR neurons was confined to the small (12-15 microm) diameter group. We concluded that the GABAergic system undergoes up-regulation, after both associative learning paradigms, and that the population of small, GABAergic neurons plays an active role in use-dependent plasticity.     

Rapid modulation of cortical proprioceptive activity induced by transient cutaneous deafferentation: neurophysiological evidence of short-term plasticity across different somatosensory modalities in humans

Single cell recording in non-human primates shows plastic changes of cortical somatic representations across different types of somatic inputs originating from the same peripheral territory. In humans, muscle afferents from first dorsal interosseus are supplied by the ulnar nerve while the cutaneous territory overlying this muscle is supplied by the radial nerve. This peculiar anatomical nervous distribution allowed us to devise an experimental model which provided a unique opportunity to assess, in humans with a non-invasive technique, the functional relationships between cutaneous and muscle afferent inputs originating from the same peripheral territory. We recorded spinal, brainstem and cortical somatosensory potentials evoked by stimulation of muscle afferents of the right first dorsal interosseus before, during and after anaesthetic block of the sensitive branch of the ipsilateral radial nerve. Amplitude of parietal N20 and P27 and frontal N30 somatosensory evoked potential components showed an increase of amplitudes with more profound anaesthesia. Amplitudes returned to pre-anaesthetic values several minutes after anaesthesia. By contrast, spinal N13 and brainstem P14 potentials did not change throughout the experiment. Results show, for the first time in humans, that a transient cutaneous deafferentation may induce rapid modulation of cortical activity evoked by stimulation of muscle afferents originating in the anaesthetic territory.     

Neonatal facial nerve extirpations fail to produce alterations in the barrel field in the primary somatosensory cortex of mice

The possible excitatory effect of N-acetyl-alpha- aspartylglutamate ( NAAG ) was studied in 3 different systems. First on the increase in 45Ca2+ influx into rat brain cortex slices in vitro, a process that is enhanced by excitatory substances. In this system 1.25 mM NAAG was entirely inactive, nor did it potentiate the excitatory effect of 0.5 mM L-glutamate. NAAG (1 mM) was able to inhibit the specific binding of [3H]kainic acid to its receptors in rat brain cortex membranes by 57.2%, but such inhibition could be accounted by the release of L-glutamate because of hydrolysis of NAAG during the incubation. In vivo infusion of NAAG (10 or 100 micrograms) through permanently implanted cannulas into the cat dorsal hippocampus, or into the pulvinar nucleus of the thalamus, was also without effect. NAAG was also unable to potentiate or to antagonize the excitatory effects of glutamate in this preparation.     

Developmental and comparative aspects of posterior medial thalamocortical innervation of the barrel cortex in mice and rats

The thalamocortical projection to the rodent barrel cortex consists of inputs from the ventral posterior medial (VPM) and posterior medial (POm) nuclei that terminate in largely nonoverlapping territories in and outside of layer IV. This projection in both rats and mice has been used extensively to study development and plasticity of highly organized synaptic circuits. Whereas the VPM pathway has been well characterized in both rats and mice, organization of the POm pathway has only been described in rats, and no studies have focused exclusively on the development of the POm projection. Here, using transport of Phaseolus vulgaris leucoagglutinin(PHA-L) or carbocyanine dyes, we characterize the POm thalamocortical innervation of adult mouse barrel cortex and describe its early postnatal development in both mice and rats. In adult mice, POm inputs form a dense plexus in layer Va that extends uniformly underneath layer IV barrels and septa. Innervation of layer IV is very sparse; a clear septal innervation pattern is evident only at the layer IV/Va border. This pattern differs subtly from that described previously in rats. Developmentally, in both species, POm axons are present in barrel cortex at birth. In mice, they occupy layer IV as it differentiates, whereas in rats, POm axons do not enter layer IV until 1-2 days after its emergence from the cortical plate. In both species, arbors undergo progressive and directed growth. However, no layer IV septal innervation pattern emerges until several days after the cytoarchitectonic appearance of barrels and well after the emergence of whisker-related clusters of VPM thalamocortical axons. The mature pattern resolves earlier in rats than in mice. Taken together, these data reveal anatomical differences between mice and rats in the development and organization of POm inputs to barrel cortex, with implications for species differences in the nature and plasticity of lemniscal and paralemniscal information processing.     

Chondroitinase ABC promotes compensatory sprouting of the intact corticospinal tract and recovery of forelimb function following unilateral pyramidotomy in adult mice

Chondroitin sulphate proteoglycans (CSPGs) are extracellular matrix molecules whose inhibitory activity is attenuated by the enzyme chondroitinase ABC (ChABC). Here we assess whether CSPG degradation can promote compensatory sprouting of the intact corticospinal tract (CST) following unilateral injury and restore function to the denervated forelimb. Adult C57BL/6 mice underwent unilateral pyramidotomy and treatment with either ChABC or a vehicle control. Significant impairments in forepaw symmetry were observed following pyramidotomy, with injured mice preferentially using their intact paw during spontaneous vertical exploration of a cylinder. No recovery on this task was observed in vehicle‐treated mice. However, ChABC‐treated mice showed a marked recovery of function, with forelimb symmetry fully restored by 5 weeks post‐injury. Functional recovery was associated with robust sprouting of the uninjured CST, with numerous axons observed crossing the midline in the brainstem and spinal cord and terminating in denervated grey matter. CST fibres in the denervated side of the spinal cord following ChABC treatment were closely associated with the synaptic marker vGlut1. Immunohistochemical assessment of chondroitin‐4‐sulphate revealed that CSPGs were heavily digested around lamina X, alongside midline crossing axons and in grey matter regions where sprouting axons and reduced peri‐neuronal net staining was observed. Thus, we demonstrate that CSPG degradation promotes midline crossing and reinnervation of denervated target regions by intact CST axons and leads to restored function in the denervated forepaw. Enhancing compensatory sprouting using ChABC provides a route to restore function that could be applied to disorders such as spinal cord injury and stroke.     

Temporal and spatial regulation of chondroitin sulfate, radial glial cells, growing commissural axons, and other hippocampal efferents in developing hamsters

We investigated the time and space relationship between growth of hippocampal efferents, particularly those forming the hippocampal commissure, and expression of extracellular matrix components related to radial glial cells. Developing hamster brains from embryonic day (E) 13 to postnatal day (P) 7 had 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) crystals implanted into the hippocampus or were processed for fluorescent immunohistochemistry against chondroitin sulfate (CS) glycosaminoglycans and glial fibrillary acidic protein (GFAP). The first, pioneer fibers from the hippocampus were seen crossing the midline at E15 and arriving at the contralateral hippocampus 24-48 hours later (P1), followed closely by a thick front of growing fibers. Before E15, CS expression was preceded by septal fusion and was concomitant with formation of the commissural tract. On E15, CS expression formed a U-shaped border below the fimbria. From E15 to P3, CS became expressed between the hippocampal commissure and the third ventricle and at the caudal borders of the fornix columns. As the hippocampal commissure expanded, CS expression became gradually lighter to virtually disappear by P7. On E15 and P1, GFAP-positive radial glial cells were present caudal (but not rostral) to the commissure at the midline, partially overlapping CS expression. Similar cells were present dorsal to the fimbria, extending their processes perpendicularly over the growing axons. The data reveal that CS and radial glial cells form a tunnel surrounding the developing fimbria and a border at the midline caudal to the hippocampal commissure. It is suggested that these cellular and molecular borders play a role in guidance of hippocampal efferents.     

Ontogeny of non-NMDA glutamate receptors in rat barrel field cortex: I. metabotropic receptors

The ontogeny of metabotropic excitatory amino acid receptors (mGluRs) in rat barrel field cortex was characterized by using receptor autoradiography and immunocytochemistry to test the hypothesis that changes in mGluR expression coincide with the emergence of somatotopic patterns in this region. On postnatal days 1 (P1) and 3, [³H]glutamate binding to mGluRs was not distributed in a somatotopic pattern. By P5, mGluRs exhibited a whisker-related pattern, with higher densities of mGluRs in barrel centers than in surrounding cortex. Between P5 and P14 and at P60, the overall binding density remained higher in barrels than in surrounding cortex. At P60, a somatotopic pattern of binding was not apparent. The majority of mGluR sites in the barrel field were blocked by the metabotropic agonist trans-1-aminocyclopentane-1,3-dicarboxylic acid but were not significantly displaced by quisqualate. Immunocytochemical studies of phosphoinositide-linked mGluRs, mGluR5 and mGluR1α, showed that the developmental expression of mGluR5 mirrored that of the pattern of autoradiographically labeled mGluRs. The immature barrel field (ages P5–P14) was enriched in mGluR5, with greater concentrations of mGluR5 immunoreactivity in barrels than in surrounding cortex. Within barrel centers, mGluR5 was localized within the neuropil, on the surfaces of cell bodies and dendrites in layer IV. A somatotopic pattern of mGluR5 immunoreactivity persisted into adulthood, although the pattern was less pronounced after P14. In contrast, mGluR1α was never localized in a somatotopic pattern in barrel field cortex. We conclude from the developmental localization of mGluRs that the spatiotemporal regulated expression of these receptors may influence barrel maturation and plasticity. J. Comp. Neurol. 386:16–28, 1997. © 1997 Wiley-Liss, Inc.     

Distribution and synthesis of extracellular matrix proteoglycans, hyaluronan, link proteins and tenascin-R in the rat spinal cord

Perineuronal nets (PNNs) are dense extracellular matrix (ECM) structures that form around many neuronal cell bodies and dendrites late in development. They contain several chondroitin sulphate proteoglycans (CSPGs), hyaluronan, link proteins and tenascin-R. Their time of appearance correlates with the ending of the critical period for plasticity, and they have been implicated in this process. The distribution of PNNs in the spinal cord was examined using Wisteria floribunda agglutinin lectin and staining for chondroitin sulphate stubs after chondroitinase digestion. Double labelling with the neuronal marker, NeuN, showed that PNNs were present surrounding ∼ 30% of motoneurons in the ventral horn, 50% of large interneurons in the intermediate grey and 20% of neurons in the dorsal horn. These PNNs formed in the second week of postnatal development. Immunohistochemical staining demonstrated that the PNNs contain a mixture of CSPGs, hyaluronan, link proteins and tenascin-R. Of the CSPGs, aggrecan was present in all PNNs while neurocan, versican and phosphacan/RPTPβ were present in some but not all PNNs. In situ hybridization showed that aggrecan and cartilage link protein (CRTL 1) and brain link protein-2 (BRAL 2) are produced by neurons. PNN-bearing neurons express hyaluronan synthase, and this enzyme and phosphacan/RPTPβ may attach PNNs to the cell surface. During postnatal development the expression of link protein and aggrecan mRNA is up-regulated at the time of PNN formation, and these molecules may therefore trigger their formation.