Differential localization of metabotropic glutamate receptors during postnatal development

The localization of metabotropic glutamate receptors (mGluRs) during development has been associated with brain maturation and plasticity. The developmental immunohistochemical analysis of mGluR1alpha, mGluR2/3 and mGluR4a expression was performed in the cerebral cortex, hippocampus and basal ganglia at postnatal days (P) 4, 8, 12, 35 and 60. In early stages (P4 and P8) mGluR1alpha-like immunoreactivity (mGluR1alpha-LI) was detected in cell bodies and fibers of the frontal cortex, hippocampus and globus pallidus. At P35 and P60, the staining was observed in pyramidal cells and fibers in the deepest layers of the cortex and in stratum oriens of the hippocampus, while a lower labeling was observed in fibers of the globus pallidus. No immunostaining was observed in substantia nigra pars reticulata until P12, when a dense network of fiber staining was detected through the adult stages (P35, P60). mGluR2/3-LI was present from the second week of development in fibers and cell bodies of the stratum lacunosum moleculare of the CA1-CA3 and striatum; this staining pattern persisted until adult stages. mGluR4a-LI was observed at P12 in neuronal bodies of the cortex, in pyramidal cells of the hippocampus and in neuronal cells of the striatum. At P35 and P60, a strong signal was observed in a reduced number of labeled cells of the cerebral cortex, in fibers of the stratum oriens of CA1 and in long processes of substantia nigra pars reticulata. Our results indicate that there are significant changes in the protein expression of mGluR subunits through postnatal development. These differences may play a significant role in the establishment of proper synaptic circuitry in early postnatal life, as well as contributing to the maintenance, stabilization, and plasticity of the rat forebrain, particularly through the participation of mGluR1alpha and mGluR4a.     

Localization of urokinase-type plasminogen activator, its receptor, and inhibitors in mouse forebrain during postnatal development

Proteolytic enzymes are postulated to play a role in cell migration and synapse organization during brain development. Among these, urokinase-type plasminogen activator (uPA) has been studied in neoplastic and cultured brain cells extensively. We hypothesized that uPA, its receptor, and its inhibitors would be expressed in immature glial and neuronal cells in postnatal mouse forebrain. Immature cortical neurons were immunoreactive for uPA, its receptor, and its substrate plasminogen peaking at the end of postnatal week two, consistent with the postulated role in synaptogenesis. Immunoreactivity for uPA receptor was also observed on astroglial cells in vitro. Neither it nor uPA were convincingly detected in subventricular zone precursor cells, immature white matter or pre-labeled immature cells that had been transplanted into brain. Plasminogen activator inhibitor type 1 immunoreactivity was observed on endothelia up to 12 days age, and type 2 was observed to surround immature cells. We conclude, based on the spatial and temporal distribution of immunoreactivity, that uPA and its receptor may be relatively more important for synaptogenesis, remodeling, and reactive processes than for cell migration in developing mouse brain.     

Changes in D-serine levels and localization during postnatal development of the rat vestibular nuclei

The patterns of development of the vestibular nuclei (VN) and their main connections involving glutamate neurotransmission offer a good model for studying the function of the glial-derived neuromodulator D-serine in synaptic plasticity. In this study we show that D-serine is present in the VN and we analyzed its distribution and the levels of expression of serine racemase and D-amino acid oxidase (D-AAO) at different stages of postnatal (P) development. From birth to P21, high levels of D-serine were detected in glial cells and processes in all parts of the VN. This period corresponded to high expression of serine racemase and low expression of D-AAO. On the other hand, in the mature VN D-serine displayed very low levels and was mainly localized in neuronal cell bodies and dendrites. This drop of D-serine in adult stages corresponded to an increasing expression of D-AAO at mature stages. High levels of glial D-serine during the first 3 weeks of postnatal development correspond to an intense period of plasticity and synaptogenesis and maturation of VN afferents, suggesting that D-serine could be involved in these phenomena. These results demonstrate for the first time that changes in D-serine levels and distribution occur during postnatal development in the central nervous system. The strong decrease of D-serine levels and the glial-to-neuronal switch suggests that D-serine may have distinct functional roles depending on the developmental stage of the vestibular network.     

Distribution of metabotropic GABA receptor subunits GABAB1a/b and GABAB2 in the rat hippocampus during prenatal and postnatal development

Metabotropic gamma-aminobutyric acid receptors (GABAB) play modulatory roles in central synaptic transmission and are involved in controlling neuronal migration during development. We used immunohistochemical methods to elucidate the expression pattern as well as the cellular and the precise subcellular localization of the GABA(B1a/b) and GABAB2 subunits in the rat hippocampus during prenatal and postnatal development. At the light microscopic level, both GABA(B1a/b) and GABAB2 were expressed in the hippocampal primordium from embryonic day E14. During postnatal development, immunoreactivity for GABA(B1a/b) and GABAB2 was distributed mainly in pyramidal cells, with discrete GABA(B1a/b)-immunopositive cell bodies of interneurons present throughout the hippocampus. Using double immunofluorescence, we demonstrated that during the second week of postnatal development, GABA(B1a/b) but not GABAB2 was expressed in glial cells throughout the hippocampal formation. At the electron microscopic level, GABA(B1a/b) and GABAB2 showed a similar distribution pattern during postnatal development. Thus, at all ages the two receptor subunits were located postsynaptically in dendritic spines and shafts at extrasynaptic and perisynaptic sites in both pyramidal and nonpyramidal cells. We further demonstrated that the two subunits were localized presynaptically along the extrasynaptic plasma membrane of axon terminals and along the presynaptic active zone in both asymmetrical and, to a lesser extent, symmetrical synapses. These results suggest that GABAB receptors are widely expressed in the hippocampus throughout development and that GABA(B1a/b) and GABAB2 form both pre- and postsynaptic receptors.     

Expression of group I and II metabotropic glutamate receptors in trigeminal neurons during postnatal development

Trigeminal motoneurons (Mo5), mesencephalic trigeminal neurons (Me5), supratrigeminal neurons (Su5), and intertrigeminal neurons (I5) are important constituents of the neural circuitry responsible for jaw movements. Glutamate neurotransmission, mediated by ionotropic and metabotropic glutamate receptors (mGluRs), is critical for the production of jaw movements. To better understand the role of mGluR-mediated modulation of these neurons during early postnatal development, we conducted a series of experiments to illustrate the ontogeny of mGluRs 1, 5 (group I) and mGluRs 2, 3 (group II) in Mo5, Me5, Su5, and I5 neurons using receptor immunohistochemistry. Results demonstrate that the temporal expression of mGluRs is differentially regulated between, and within these trigeminal nuclei. The localization of group I and II mGluRs in these nuclei suggests a role for these receptors in mediating glutamatergic neurotransmission in brainstem circuits responsible for oral-motor behaviors.     

Synaptic patterns of thalamocortical afferents in mouse barrels at postnatal day 11.

This study focuses on the synaptic output patterns of thalamocortical axons in mouse barrel cortex at postnatal day (P) 11. Axons were labeled by biotinylated dextran amine transported anterogradely following injection in vivo into the ventrobasal thalamus. Labeled axons in the posteromedial barrel subfield were examined by light and electron microscopy and then reconstructed in three dimensions to assess the spatial distribution of their synapses. Thalamocortical axons form asymmetrical synapses, both at varicosities and along cylindrical portions of the axons; usually, only one synapse occurs per site, contrasting with the case in the adult, in which multiple synapses are typical. At P11, varicosities without synapses are common. As in adult barrels, approximately 80% of synapses formed by thalamocortical axons are with dendritic spines; 20% are with dendritic shafts. The similarity in the distribution of thalamocortical synapses onto spines vs. dendrites in developing and mature barrels indicates that adult synaptic patterns already are specified at a very early stage of thalamocortical synaptogenesis.     

Novel metabotropic glutamate receptor negatively coupled to adenylyl cyclase in cultured rat cerebellar astrocytes

Several excitatory amino acid ligands were found potently to inhibit forskolin-stimulated cAMP accumulation in rat cultured cerebellar astrocytes: L-cysteine sulfinic acid (L-CSA) = L-aspartate > L-glutamate >/= the glutamate uptake inhibitor, L-PDC. This property did not reflect activation of conventional glutamate receptors, since the selective ionotropic glutamate receptor agonists NMDA, AMPA, and kainate, as well as several mGlu receptor agonists [(1S,3R)-ACPD, (S)-DHPG, DCG-IV, L-AP4, L-quisqualate, and L-CCG-I], were without activity. In addition, the mGlu receptor antagonists, L-AP3, (S)-4CPG, Eglu, LY341495, (RS)-CPPG, and (S)-MCPG failed to reverse 30 microM glutamate-mediated inhibitory responses. L-PDC-mediated inhibition was abolished by the addition of the enzyme glutamate-pyruvate transaminase. This finding suggests that the effect of L-PDC is indirect and that it is mediated through endogenously released L-glutamate. Interestingly, L-glutamate-mediated inhibitory responses were resistant to pertussis toxin, suggesting that G(i)/G(o) type G proteins were not involved. However, inhibition of protein kinase C (PKC, either via the selective PKC inhibitor GF109203X or chronic PMA treatment) augmented glutamate-mediated inhibitory responses. Although mGlu3 receptors (which are negatively coupled to adenylyl cyclase) are expressed in astrocyte populations, in our study Western blot analysis indicated that this receptor type was not expressed in cerebellar astrocytes. We therefore suggest that cerebellar astrocytes express a novel mGlu receptor, which is negatively coupled to adenylyl cyclase, and possesses an atypical pharmacological profile.     

Fine structural localization of connexin-36 immunoreactivity in mouse cerebral cortex and thalamus

The mounting physiological evidence for low-resistance electrical coupling between thalamic and cortical neurons contrasts with a lack of morphological data on gap junctions in thalamus and cortex. Connexin-36 is a neuronally specific protein associated with low-resistance gap junctions in the central nervous system. Connexin-36 localization was studied in the mouse somatosensory cortex and thalamus by using immunocytochemistry and immunoelectron microscopy. Expression of connexin-36 immunoreactivity is widespread in the forebrain and significantly enhanced in the barrel cortex and thalamic reticular nucleus during the second postnatal week, but it extends to other thalamic nuclei as well. At the electron microscopic level, pre- and postembedding immunogold labeling revealed that 70-76% of connexin-36-immunolabeled particles were localized at focal sites on apposed plasma membranes of cortical and thalamic dendrites; approximately 5% of the particles were associated with parasynaptic membranes; but on no occasion could overt, morphologically identifiable gap junctions be demonstrated in association with connexin-36 immunoreactivity. The widespread distribution of focal concentrations of connexin-36 subunits could provide a basis for the electrical coupling that exists between cortical and reticular thalamic neurons, but morphologically definable gap junctions may be too small to be adequately visualized by conventional immunoelectron microscopy.     

Cajal-Retzius cells in early postnatal mouse cortex selectively express functional metabotropic glutamate receptors

Glutamate receptors have been linked to the regulation of several developmental events in the CNS. By using cortical slices of early postnatal mice, we show that in layer I cells, glutamate produces intracellular calcium ([Ca(2+)](i)) elevations mediated by ionotropic and metabotropic glutamate receptors (mGluRs). The contribution of mGluRs to these responses was demonstrated by application of tACPD, an agonist to groups I and II mGluRs, which evoked [Ca(2+)](i) increases that could be reversibly blocked by MCPG, an antagonist to groups I and II mGluRs. In the absence of extracellular Ca(2+), repetitive applications of tACPD or quisqualate, an agonist to group I mGluRs, elicited decreasing [Ca(2+)](i) responses that were restored by refilling a thapsigargin-sensitive Ca(2+) store. The use of specific group I mGluR agonists CHPG and DHPG indicated that the functional mGluR in layer I was of the mGluR1 subtype. Subtype specific antibodies confirmed the presence of mGlur1 alpha, but not mGluR5, in Cajal-Retzius (Reelin-immunoreactive) neurons.     

Selective alterations in glutamate receptor subtypes after unilateral orbital enucleation

Glutamate is the major excitatory neurotransmitter in the rat visual system. Using quantitative autoradiography the effect of unilateral orbital enucleation on [³H]kainate, [³H]-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid ([³H]AMPA) and [³H]glutamate binding to kainate, quisqualate and NMDA receptors respectively has been examined within anatomical components of the visual pathway at 4 time points up to 20 days post-lesion. The time course for the degeneration of retinal projection fibres was assessed in a separate group of animals by quantifying [³H]cyclohexyladenosine ([³H]CHA) binding to presynaptic adenosine A₁ receptors. Over the first 5 days after orbital enucleation, there were no significant alterations in glutamate or adenosine A₁ receptor binding in visual structures of the visually deprived hemisphere. However, at 10 days post-lesion [³H]AMPA binding was significantly reduced (30%) in the visually deprived superior colliculus but unaltered in other visual structures. At this time point there was also a significant reduction (50%) in [³H]CHA binding in the visually deprived superior colliculus but not in other retino-recipient nuclei. There were similar changes in [³H]AMPA and [³H]CHA binding at 20 days post-enucleation. [³H]Kainate binding was significantly increased in the visually deprived superior colliculus only at 20 days post-enucleation. Saturation analysis of [³H]kainate and [³H]AMPA binding at this time point indicated a selective increase in the b_max value for the high affinity [³H]kainate binding site and a concomitant decrease in the b_max value for the high affinity [³H]AMPA binding site in the visually deprived superior colliculus. There were, however, no significant alterations in [³H]AMPA or [³H]kainate binding in other primary projection areas or in secondary visual areas (e.g. visual cortex) at any time point. NMDA sensitive [³H]glutamate binding was unaltered in the visually deprived hemisphere up to 20 days post-enucleation. These results suggest an upregulation of kainate receptors in the visually deprived superior colliculus after orbital enucleation and a loss of presynaptic quisqualate receptors on degenerating retinal fibres. The plastic alterations in kainate receptors in the superior colliculus are supportive of electrophysiological data suggesting a physiological role for these sites in mediating excitatory postsynaptic potentials in tectal neurons.     

Changes in metabotropic glutamate receptor expression following spinal cord injury

Spinal cord injury (SCI) initiates biochemical events that lead to an increase in extracellular excitatory amino acid concentrations, resulting in glutamate receptor-mediated excitotoxic events. These receptors include the three groups of metabotropic glutamate receptors (mGluRs). Group I mGluR activation can initiate a number of intracellular pathways that increase neuronal excitability. Group II and III mGluRs may function as autoreceptors to modulate neurotransmission. Thus, all three groups may contribute to the mechanisms of central sensitization and chronic central pain. To begin evaluating mGluRs in SCI, we quantified the changes in mGluR expression after SCI in control (naive), sham, and impact injured adult male Sprague-Dawley rats (200-250 g). SCI was produced at spinal segment T10 with a New York University impactor (12.5-mm drop, 10-g rod of 2-mm diameter). Expression levels were determined by Western blot and immunohistochemistry analyses at the epicenter of injury, as well as segments rostral and caudal. The group I subtype mGluR1 was increased over control levels in segments rostral and caudal by postsurgical day (PSD) 7 and remained elevated through PSD 60. The group I subtype mGluR5 was unchanged in all segments rostral and caudal to the injury at every time point measured. Group II mGluRs were decreased compared to control levels from PSD 7 through PSD 60 in all segments. These results suggest that different subtypes of mGluRs have different spatial and temporal expression patterns following SCI. The expression changes in mGluRs parallel the development of mechanical allodynia and thermal hyperalgesia following SCI; therefore, understanding the expression of mGluRs after SCI may give insight into mechanisms underlying the development of chronic central pain.     

Metabotropic glutamate receptor mGluR1 distribution and ultrastructural localization in hypothalamus

The metabotropic glutamate receptor mGluR1 is a G-protein-coupled glutamate receptor whose activation induces phosphotidylinositol hydrolysis and increases diacylglycerol and cytoplasmic calcium. By using affinity-purified antisera against a partial amino acid sequence of mGluR1α, deduced from the nucleotide sequence of the cloned gene, the heterogeneous expression of this glutamate receptor was studied immunocytochemically with light and electron microscopy in the rat hypothalamus. Immunoreactivity was restricted to cell bodies and dendrites throughout many regions of the adult hypothalamus, including the preoptic area, anterior hypothalamus, suprachiasmatic nucleus, dorsomedial hypothalamus, and periventricular region. Strong immunolabeling was found in the lateral hypothalamus where immunoreactivity could be detected as early as embryonic day 18. Intense immunoreactivity was also found in the medial mammillary nuclei. In contrast to the strong labeling in many other regions, the neuroendocrine neurons of the arcuate, supraoptic, and paraventricular nuclei showed relatively little staining in adults. With light microscopy, immunoperoxidase labeling was found distributed in patches on the cytoplasmic side of the plasma membrane of immunoreactive neurons. When the same tissue was examined ultrastructurally, the patches were not restricted to synaptic specializations but were also found distributed on perikaryal and dendritic membranes sometimes associated with synapses and sometimes not. Some immunoreactive membranes showed no immunolabeling at the synaptic junction. When the tissue was strongly stained, labeling could be found in the cytoplasm of immunoreactive cells. No immunostaining was found on axons or presynaptic boutons. Together with other evidence showing a widespread expression of many different subtypes of both ionotropic and metabotropic receptors, these data support the hypothesis that glutamate may regulate hypothalamic cellular activity with a number of physiologycally different mechanisms, and these mechanisms include second-messenger systems activated by G proteins. © 1994 Wiley-Liss, Inc.     

Cellular and subcellular distribution of spinophilin, a PP1 regulatory protein that bundles F-actin in dendritic spines

Spinophilin is an actin binding protein that positions protein phosphatase 1 next to its substrates in dendritic spines. It contains a single PDZ domain and has the biochemical characteristics of a cytoskeletal scaffolding protein. Previous studies suggest that spinophilin is present in most spines, but the concentration of spinophilin varies from brain region to region in a manner that does not simply reflect differences in spine density. Here, we show that spinophilin is enriched in the great majority of dendritic spines in cerebral cortex, caudatoputamen, hippocampal formation, and cerebellum, irrespective of regional differences in spinophilin concentration. In addition, spinophilin is present postsynaptic to asymmetrical contacts on interneuronal dendritic shafts. We further show that, in hippocampus and ventral pallidum, spinophilin is occasionally present in dendritic shafts adjacent to gamma-aminobutyric acid-containing contacts. Thus, the functional role of spinophilin may not be exclusively restricted to excitatory synapses and may be significant at a small fraction of inhibitory contacts. These data also suggest that the concentration of spinophilin per spine is variable and is likely regulated by local physiological factors and/or regional influences.     

Glial coverage of the small cell somata in the rat nucleus of tractus solitarius during postnatal development

Astrocytes are thought to be active participants in synaptic plasticity in the developing nervous system. Previous studies suggested that axosomatic synapses decreased in number on the small cells of the rat caudal nucleus of tractus solitarius (cNTS) toward the end of the first postnatal week. Astrocytes might be involved in this phenomenon. We examined the morphological development of astrocytic processes around the small cell soma in the rat cNTS using light and electron microscopy. Glial fibrillary acidic protein (GFAP), glutamate-aspartate transporter (GLAST), and glutamate transporter-1 (GLT-1)-positive structures within the cNTS became more intensely stained as development proceeded. GLAST-positive structures encompassed calbindin-positive small cell somata after postnatal day 10. Electron microscopic observations indicated that astrocytic processes encompass the small cell soma, while the number of axosomatic synapses decreases as development proceeds. The timing for glial coverage of the small cell soma appears to be consistent with the decrease in axosomatic synapses on the small cells. These observations imply that astrocytes may participate actively in regulating the decrease of axosomatic synapses on small cells in the cNTS during postnatal development.     

Developmental course of EphA4 cellular and subcellular localization in the postnatal rat hippocampus

From embryonic development to adulthood, the EphA4 receptor and several of its ephrin‐A or ‐B ligands are expressed in the hippocampus, where they presumably play distinct roles at different developmental stages. To help clarify these diverse roles in the assembly and function of the hippocampus, we examined the cellular and subcellular localization of EphA4 in postnatal rat hippocampus by light and electron microscopic immunocytochemistry. On postnatal day (P) 1, the EphA4 immunostaining was robust in most layers of CA1, CA3, and dentate gyrus and then decreased gradually, until P21, especially in the cell body layers. At the ultrastructural level, focal spots of EphA4 immunoreactivity were detected all over the plasma membrane of pyramidal and granule cells, between P1 and P14, from the perikarya to the dendritic and axonal extremities, including growth cones and filopodia. This cell surface immunoreactivity then became restricted to the synapse‐associated dendritic spines and axon terminals by P21. In astrocytes, the EphA4 immunolabeling showed a similar cell surface redistribution, from the perikarya and large processes at P1–P7, to small perisynaptic processes at P14–P21. In both cell types, spots of EphA4 immunoreactivity were also detected, with an incidence decreasing with maturation, on the endoplasmic reticulum, Golgi apparatus, and vesicles, organelles involved in protein synthesis, posttranslational modifications, and transport. The cell surface evolution of EphA4 localization in neuronal and glial cells is consistent with successive involvements in the developmental movements of cell bodies first, followed by process outgrowth and guidance, synaptogenesis, and finally synaptic maintenance and plasticity. J. Comp. Neurol. 512:798–813, 2009. © 2008 Wiley‐Liss, Inc.     

Kindled seizures increase metabotropic glutamate receptor expression and function in the rat supraoptic nucleus

The spread of experimentally kindled seizures in rats results in sustained increases in plasma vasopressin (VP) and VP mRNA in the supraoptic nucleus (SON). These increases provide an excellent example of the pathological plasticity that can develop in normal cells exposed to recurrent seizure activity. To test whether this plasticity might be due in part to changes in metabotropic glutamate receptors (mGluRs), we examined mGluR mRNA expression in the SON 1 month after stage 5 amygdala kindling. Three mGluR subtypes were detected by in situ hybridization in the SON in the following relative levels: mGluR3 > mGluR1 > mGluR7. Both mGluR1 and mGluR3 mRNAs were significantly increased in the SON (+28–61%) and cortex (+27–42%) after kindling. Immunoreactivity for mGluR1 but not mGluR2/ 3 was significantly increased in vivo in the SON. Receptor protein expression and intracellular calcium accumulation in response to the mGluR agonist, 1S,3R ACPD, were evaluated after in vitro “kindling” of neuroendocrine cells by Mg²⁺ deprivation. Increased immunoreactivity for mGluR1 and mGluR2/ 3 was seen in all cultures 3 days after a brief exposure to Mg²⁺-free medium. 1S,3R 1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) induced rapid peak responses and gradual accumulations of intracellular Ca²⁺ in neurons. Both responses were increased in the “kindled” cells. Increases in the expression of functional mGluR1 and perhaps mGluR3 receptors may contribute to the development of long-lasting plastic changes associated with seizure activity. J. Neurosci. Res. 54:412–423, 1998. © 1998 Wiley-Liss, Inc.     

Expression of the metabotropic glutamate receptor mGluR1α and the ionotropic glutamate receptor GluR1 in the brain during the postnatal development of normal mouse and in the cerebellum from mutant mice

Expression of the metabotropic glutamate receptor type 1α (mGluR1α) and the non-N-methyl-D-aspartate (NMDA) ionotropic glutamate receptor type 1 (GluR1) in mouse brain was investigated using the antibodies raised against the synthetic peptides corresponding to their C-terminal amino acid sequences. Both receptor proteins are glycosylated predominantly in an asparagine-linked manner, and are abundant in post-synaptic membranes. We showed that mGluR1α and GluR1 expression within the first 3 postnatal weeks undergoes dramatic changes in time and space, i.e., in the hippocampus and cerebellum. These spatio-temporal expression patterns appear to be correlated with the postnatal ontogenesis and establishment of the glutamatergic neurotransmission system in the hippocampus and cerebellum, cell migration, dendritic and axonal growth, spine formation, and synaptogenesis. In the adult cerebellum, mGluR1α is intensely expressed in Purkinje neurons and GluR1 in Bergmann glial cells. Both receptors are expressed to a fair degree in weaver mutant cerebellum despite granule cell degeneration. However, the intrinsic expression levels of both mGluR1α and GluR1 are markedly reduced in the cerebellum of the Purkinje cell-deficient and underdeveloped mutant mice, Purkinje-cell-degeneration, Lurcher, and staggerer, suggesting that GluR1 expression in Bergmann glia cells may be correlated with the sustained interaction with adjacent Purkinje neurons. © 1993 Wiley-Liss, Inc.     

Social isolation perturbs experience‐driven synaptic glutamate receptor subunit 4 delivery in the developing rat barrel cortex

In neonates, the stress of social isolation can alter developing neural circuits and cause mental illness. However, the molecular and cellular bases for these effects are poorly understood. Experience‐driven synaptic AMPA receptor delivery is crucial for circuit organisation during development. In the rat, whisker experience drives the delivery of glutamate receptor subunit 4 (GluA4) but not glutamate receptor subunit 1 (GluA1) to layer 4–2/3 pyramidal synapses in the barrel cortex during postnatal day (P)8–10, whereas GluA1 but not GluA4 is delivered to these synapses during P12–14. We recently reported that early social isolation disrupts experience‐driven GluA1 delivery to layer 4–2/3 pyramidal synapses during P12–14. Here, we report that neonatal isolation affects even earlier stages of development by preventing experience‐dependent synaptic GluA4 delivery. Thus, social isolation severely affects synaptic maturation throughout early postnatal development.