The role of excitatory amino acid transmission in development and plasticity of SI barrel cortex

Astrocyte and Neuronal Plasticity in the Somatosensory System

Changing the whisker complement on a rodent's snout can lead to two forms of experience-dependent plasticity (EDP) in the neurons of the barrel cortex, where whiskers are somatotopically represented. One form, termed coding plasticity, concerns changes in synaptic transmission and connectivity between neurons. This is thought to underlie learning and memory processes and so adaptation to a changing environment. The second, called homeostatic plasticity, serves to maintain a restricted dynamic range of neuronal activity thus preventing its saturation or total downregulation. Current explanatory models of cortical EDP are almost exclusively neurocentric. However, in recent years, increasing evidence has emerged on the role of astrocytes in brain function, including plasticity. Indeed, astrocytes appear as necessary partners of neurons at the core of the mechanisms of coding and homeostatic plasticity recorded in neurons. In addition to neuronal plasticity, several different forms of astrocytic plasticity have recently been discovered. They extend from changes in receptor expression and dynamic changes in morphology to alteration in gliotransmitter release. It is however unclear how astrocytic plasticity contributes to the neuronal EDP. Here, we review the known and possible roles for astrocytes in the barrel cortex, including its plasticity.

Developmental decline in modulation of glutamatergic synapses in layer IV of the barrel cortex by group II metabotropic glutamate receptors

Previously, we demonstrated that group II metabotropic glutamate receptors (mGluRs) reduce glutamate release from thalamocortical synapses during early postnatal development (P7-11). To further examine the role of group II mGluRs in the modulation of somatosensory circuitry, we determined whether group II mGluRs continue to modulate thalamocortical synapses until adulthood and whether these receptors also modulate intra-cortical synapses in the barrel cortex. To address these issues, we examined the effect of the group II mGluR agonists on thalamocortical excitatory postsynaptic currents (EPSCs) and intra-barrel EPSCs in slices from animals of different ages (P7-53). We found that the depression of thalamocortical EPSCs by group II mGluRs rapidly declined after the second postnatal week. In contrast, adenosine continued to depress thalamocortical EPSCs via a presynaptic mechanism in young adult mice (P30-50). Activation of group II mGluRs also reduced intra-barrel EPSCs through a postsynaptic mechanism in young mice (P7-11). Similar to the thalamocortical synapses, the group II mGluR modulation of intra-barrel excitatory synapses declined with development. In young adult animals (P30-50), group II mGluR stimulation had little effect on intra-barrel EPSCs but did hyperpolarize the neurons. Together our results demonstrate that group II mGluRs modulate barrel cortex circuitry by presynaptic and postsynaptic mechanisms depending on the source of the synapse and that this modulation declines with development.

Prenatal and Lactational Exposure to Bisphenol A in Mice Alters Expression of Genes Involved in Cortical Barrel Development without Morphological Changes

It has been reported that premature infants in neonatal intensive care units are exposed to a high rate of bisphenol A (BPA), an endocrine disrupting chemical. Our previous studies demonstrated that corticothalamic projection was disrupted by prenatal exposure to BPA, which persisted even in adult mice. We therefore analyzed whether prenatal and lactational exposure to low doses of BPA affected the formation of the cortical barrel, the barreloid of the thalamus, and the barrelette of the brainstem in terms of the histology and the expression of genes involved in the barrel development. Pregnant mice were injected subcutaneously with 20 µg/kg of BPA daily from embryonic day 0 (E0) to postnatal 3 weeks (P3W), while the control mice received a vehicle alone. The barrel, barreloid and barrelette of the adult mice were examined by cytochrome C oxidase (COX) staining. There were no significant differences in the total and septal areas and the patterning of the posterior medial barrel subfield (PMBSF), barreloid and barrelette, between the BPA-exposure and control groups in the adult mice. The developmental study at postnatal day 1 (PD1), PD4 and PD8 revealed that the cortical barrel vaguely appeared at PD4 and completely formed at PD8 in both groups. The expression pattern of some genes was spatiotemporally altered depending on the sex and the treatment. These results suggest that the trigeminal projection and the thalamic relay to the cortical barrel were spared after prenatal and lactational exposure to low doses of BPA, although prenatal exposure to BPA was previously shown to disrupt the corticothalamic projection.

Development of functional organization of the pallid bat auditory cortex

The primary auditory cortex is characterized by a tonotopic map and a clustered organization of binaural properties. The factors involved in the development of overlain representation of these two properties are unclear. We addressed this issue in the auditory cortex of the pallid bat. The adult pallid bat cortex contains a systematic relationship between best frequency (BF) and binaural properties. Most neurons with BF<30 kHz are binaurally inhibited (EO/I), while most neurons with BF>30 kHz are monaural (EO). As in other species, binaural properties are clustered. The EO/I cluster contains a systematic map of interaural intensity difference (IID) sensitivity. We asked if these properties are present at the time the bat acquires its full audible range (postnatal day [P] 15). Tonotopy, relationship between BF and binaural properties, and the map of IID sensitivity are adult-like at P15. However, binaural facilitation is only observed in pups older than P25. Frequency selectivity shows a BF-dependent sharpening during development. Thus, overlain representation of binaural properties and tonotopy in the pallid bat cortex is remarkably adult-like at an age when the full audible range is first present, suggesting an experience-independent development of overlapping feature maps.

Anatomical pathways and molecular mechanisms for plasticity in the barrel cortex

The barrel cortex has yielded a wealth of information about cortical plasticity in recent years. Barrel cortex is one of the few cortical areas studied so far where plasticity can be examined from birth through to adulthood. This review looks at plasticity mechanisms in three periods of life: early post-natal development, adolescence and adulthood. Separate consideration is given to depression and potentiation mechanisms. Plasticity can be induced in barrel cortex by whisker deprivation. Single whisker experience leads to expansion of the area of cortex responding to the spared whisker. In early post-natal life, plasticity occurs in thalamocortical pathways, while later in adolescence, intracortical pathways become more important. Ablation of the spared whisker's barrel prevents expression of plasticity in the cortex. A row of lesions between the spared and an adjacent barrel prevents expression of plasticity in the adjacent barrel. This evidence, together with latency of response data and an analysis of pathways capable of inducing long-term potentiation (LTP) within barrel cortex, leads to the view that horizontal and/or diagonal pathways between barrels are responsible for plasticity expression. The mouse has become the most commonly mutated mammalian species and has a well-developed barrel cortex. Therefore, mutations can be used to study the role of particular molecules in experience-dependent plasticity of barrel cortex. Through this work, it has become clear that the major post-synaptic density protein, alpha-CaMKII, and its T286 autophosphorylation site are essential for experience-dependent plasticity. This points to a major role for excitatory transmission in cortical plasticity and raises the possibility that LTP like mechanisms are involved. Furthermore, transgenic mice carrying a reporter gene for CRE have provided evidence that CRE-mediated gene expression is also involved in barrel cortex plasticity. This view is supported by studies on alpha/delta CREB knockouts, and provides a starting point for studying the role of gene expression in experience-dependent cortical plasticity.

Normotopic and heterotopic cortical representations of mystacial vibrissae in rats with subcortical band heterotopia

The tish rat is a neurological mutant exhibiting bilateral cortical heterotopia similar to those found in certain epileptic patients. Previous work has shown that thalamocortical fibers originating in the ventroposteromedial nucleus, which in normal animals segregate as 'barrel' representations for individual whiskers, terminate in both normotopic and heterotopic areas of the tish cortex (Schottler et al., 1998). Thalamocortical innervation terminates as barrels in layer IV and diffusely in layer VI of the normotopic area. Discrete patches of terminals are also observed in the underlying heterotopic area suggesting that representations of individual vibrissa may be present in the heterotopic somatosensory areas. The present study examines this issue by investigating the organization of the vibrissal somatosensory system in the tish cortex. Staining for cytochrome oxidase or Nissl substance reveals a normal complement of vibrissal barrels in the normotopic area of the tish cortex. Dense patches of cytochrome oxidase staining are also found in the underlying lateral portions of the heterotopic area (i.e. the same area that is innervated by the ventroposteromedial nucleus). Injections of retrograde tracers into vibrissal areas of either the normotopic or heterotopic area produce topographically organized labeling of neurons restricted to one or a small number of barreloids within the ventroposteromedial nucleus of the thalamus. Physical stimulation of a single whisker (D3 or E3) elicits enhanced uptake of [(14)C]2-deoxyglucose in restricted zones of both the normotopic and heterotopic areas, demonstrating that single whisker stimulation can increase functional activity in both normotopic and heterotopic neurons. These findings indicate that the barrels are intact in the normotopic area and are most consistent with the hypothesis that at least some of the individual vibrissae are 'dually' represented in normotopic and heterotopic positions in the primary somatosensory areas of the tish cortex.

Persistent and specific influences of early acoustic environments on primary auditory cortex

This study demonstrates that the adult form of 'tonotopic maps' of sound frequency in the rat primary auditory cortex (A1) arises from parallel developmental processes involving two cortical zones: the progressive differentiation and refinement of selectively tone-responsive receptive fields within an initially broadly-tuned posterior zone, and the progressive loss of tone-evoked, short-latency response over an initially large, very broadly tuned anterior zone. The formation of tonotopic maps in A1 was specifically influenced by a rat pup's early acoustic environments. Exposure to pulsed tones resulted in accelerated emergence and an expansion of A1 representations of those specific tone frequencies, as well as a deteriorated tonotopicity and broader-than-normal receptive fields. Thus, auditory experiences during early postnatal development are important in shaping the functional development of auditory cortical representations of specific acoustic environments.

Neuronal activity influences the growth of barrels in developing rat primary somatosensory cortex without affecting the expression pattern of four major GABAA receptor alpha subunits

Thalamic innervation plays a major role in parcellation of neocortex and maturation of cortical circuits. While the underlying mechanisms are unknown, lesion studies have identified GABAA receptors in neocortex as molecular targets of thalamic regulation [J. Paysan, A. Kossel, J. Bolz, J.M. Fritschy, Area-specific regulation of gamma-aminobutyric acid A receptor subtypes by thalamic afferents in developing rat neocortex, Proc. Natl. Acad. Sci. USA 94 (1997) 6995-7000]. To determine the factors regulating the expression of GABAA receptors, the overall level of neuronal activity was chronically modulated in neonatal rat cortex. Slices of Elvax polymer loaded with the N-methyl-D-asparate (NMDA) receptor antagonist MK-801 or with brain derived neurotrophic factor (BDNF) were placed unilaterally over the left parietal cortex in newborn animals. Unlike thalamic lesions (Paysan et al., 1997), these chronic drug treatments did not alter the laminar distribution or the expression level of the four major GABAA receptor alpha subunit isoforms (alpha 1, alpha 2, alpha 3, alpha 5) in primary somatosensory cortex (S1), as assessed immunohistochemically after one week. In particular, the staining of the barrel field in layers III-IV, which is very prominent with the alpha 1-subunit, was preserved in the drug-treated hemisphere. Even systemic administration of MK-801 at birth, which resulted in pronounced retardation of cortical development, had no effect on the laminar distribution and staining intensity of the four GABAA receptor alpha subunit variants. However, the size of barrels in S1, as measured in tangential sections stained for the GABAA receptor alpha 1 subunit, was enlarged upon chronic, topical blockade of NMDA receptors with MK-801 and was reduced to the same extent upon chronic exposure to BDNF. Thus, these pharmacological treatments modulated cortical growth, possibly by exerting opposite effects on neuronal activity in S1. The results suggest that the parcellation of somatosensory cortex and the laminar distribution of GABAA receptor subtypes are governed primarily by factors independent of thalamocortical activity.

Experience-dependent depression of vibrissae responses in adolescent rat barrel cortex

A short period of vibrissae deprivation in an adolescent (approximately 1 month old) rat can lead to depression of the cortical response to stimulation of the regrown vibrissae. In a barrel column representing the deprived vibrissa, depression is greater for neurons located close to the barrel column representing the spared vibrissa. One possible explanation is that the spared vibrissa produces heterosynaptic depression of the principal vibrissa response (Glazewski & Fox, 1996). To test this idea further, we compared the effect of depriving all vibrissae (no heterosynaptic influence at all) with depriving a single vibrissa (maximal heterosynaptic influence expected). In addition we tested the origin of the depression by recording from subcortical structures. After 7 days' deprivation and 6-8 days' regrowth, we tested the responses of barrel cortex cells, thalamic VPm neurons and trigeminal ganglion cells to stimulation of the regrown vibrissae. We found that depression was greater in cortex if a single vibrissa had been deprived than if all vibrissae had been deprived. (Average principal vibrissae responses in single vibrissae deprived animals were 36% of those in all vibrissae deprived animals for layer II/III and 41% for layer IV.) This implicates the spared vibrissae in actively down-regulating responses to the deprived vibrissae. However, some depression could also be produced in animals deprived of all vibrissae (layers II/III were 39% and layer IV 74% of control levels). These results indicate that simple withdrawal of activation has a depressive effect on responses but that depression is far greater if some active inputs remain. Neither form of deprivation had an effect on responses to principal vibrissa stimulation in the thalamus or trigeminal ganglion however, suggesting that depression originates in the cortex. Within the cortex, intracortical connections seem most affected as the greatest depression was found in layers II/III and in layer IV among cells responding at intermediate latencies (9-14 ms).