Differential laminar expression of AMPA receptor genes in the developing rat visual cortex using in situ hybridization histochemistry. Effect of visual deprivation

The role of nitric oxide and GluR1 in presynaptic and postsynaptic components of neocortical potentiation

In this study, we investigated the mechanisms underlying synaptic plasticity at the layer IV to II/III pathway in barrel cortex of mice aged 6-13 weeks. This pathway is one of the likely candidates for expression of experience-dependent plasticity in the barrel cortex and may serve as a model for other IV to II/III synapses in the neocortex. We found that postsynaptic autocamtide-2-inhibitory peptide is sufficient to block long-term potentiation (LTP) (IC50 of 500 nm), implicating postsynaptic calcium/calmodulin-dependent kinase II in LTP induction. AMPA receptor subunit 1 (GluR1) knock-out mice also showed LTP in this pathway, but potentiation was predominantly presynaptic in origin as determined by paired-pulse analysis, coefficient of variation analysis, and quantal analysis, whereas wild types showed a mixed presynaptic and postsynaptic locus. Quantal analysis at this synapse was validated by measuring uniquantal events in the presence of strontium. The predominantly presynaptic LTP in the GluR1 knock-outs was blocked by postsynaptic antagonism of nitric oxide synthase (NOS), either with intracellular N-omega-nitro-L-arginine methyl ester or N-nitro-L-arginine, providing the first evidence for a retrograde transmitter role for NO at this synapse. Antagonism of NOS in wild types significantly reduced but did not eliminate LTP (group average reduction of 50%). The residual LTP formed a variable proportion of the total LTP in each cell and was found to be postsynaptic in origin. We found no evidence for silent synapses in this pathway at this age. Finally, application of NO via a donor induced potentiation in layer II/III cells and caused an increase in frequency but not amplitude of miniature EPSPs, again implicating NO in presynaptic plasticity.

Postnatal expression and distribution of Refsum disease gene associated protein in the rat retina and visual cortex: effect of binocular visual deprivation

Previously, phytanoyl-CoA alpha-hydroxylase-associated protein 1 (PAHX-AP1) was isolated as a novel neuron-specific protein to interact with Refsum disease (RfD) gene PAHX. Its expression in the brain increased after eyelid opening, and the elevated level was maintained through adulthood. In this report, to verify the hypothesis that light could trigger this increase, we have examined the developmental distribution pattern of PAHX-AP1 in rat retina and visual cortex, and changes of its expression by binocular deprivation. Northern blot analyses demonstrated PAHX-AP1 expression reached its highest level in the visual cortex and eyeball at 4 weeks after birth, and these levels were maintained through adult life. Two weeks after visual deprivation, its expression in the eyeball and visual cortex decreased compared with the control. In situ hybridization analyses of the retina showed that PAHX-AP1 expression was limited to the ganglionic cell layer at 10 days after birth, but expressed in the inner nuclear cell layer and extended to the outer nuclear cell layer at 2 and 3 weeks after birth, respectively. Two weeks after visual deprivation, however, it decreased in the ganglionic and inner nuclear cell layer, and disappeared in the rod and cone cell layers. In the visual cortex, strong signals of PAHX-AP1 were detected in layers IV and VI, and II-VI at 10 days and 2 weeks after birth, respectively. Its expression decreased after 2 weeks of visual deprivation. These results indicate that visual stimulation is essential for the maintenance of PAHX-AP1 expressions in the retina, especially in the rod and cone cell layers, and visual cortex, and suggest that PAHX-AP1 may be involved in the developmental regulation of the photoreceptor's function.

Deafferentation induced changes in GAD67 and GluR2 mRNA expression in mouse somatosensory cortex

Partial vibrissectomy in adult mice induces body map plasticity in SI barrel cortex. To examine if the disturbed balance of cortical activation affects the excitatory and inhibitory neurotransmitter systems, we studied glutamic acid decarboxylase (GAD 67) and AMPA receptor subunit GluR2 mRNA expression in the barrel cortex. At varying times post-vibrissectomy, sparing row C of whiskers on one side of the snout, the brains were processed for in situ hybridization using specific [(35)S]oligonucleotides to detect the laminar localization of GAD67 and GluR2 mRNAs. Three and seven days after vibrissectomy, the expression of GAD67 was decreased in the deafferented cortex, while 30 days post-lesion, no effects were observed. At 3 days post-lesion, an ipsilateral decrease in GAD67 mRNA expression was also observed. No decreases in GluR2 transcripts were found in the deafferented cortex, but an increased expression was observed in the representation of the spared row C of whiskers 3 days after vibrissectomy. Seven and 30 days post lesion no changes in GluR2 expression were found. These data indicate that in the barrel cortex, peripheral deafferentation transiently regulates GAD67 and GluR2 expression at the transcriptional level. We suggest that this may be a manifestation of adaptive processes.

Laminar and cellular distribution of AMPA, kainate, and NMDA receptor subunits in monkey sensory-motor cortex

In situ hybridization histochemistry and immunocytochemistry were used to examine lamina- and cell-specific expression of glutamate receptor (GluR) mRNAs and polypeptide subunits in motor and somatosensory cortex of macaque monkeys. Radioactive complementary RNA (cRNA) probes were prepared from cDNAs specific for alpha-amino-3-hydroxy-5-methylisoxozolepropionate (AMPA)/kainate (GluR1-GluR4), kainate (GluR5-GluR7), and N-methyl-D-aspartate (NMDA; NR1, NR2A-NR2D) receptor subunits. AMPA/kainate and NR1, NR2A, and NR2B receptor transcripts show higher expression than other transcripts. All transcripts show lamina-specific patterns of distribution. GluR2 and GluR4 mRNAs show higher expression than do GluR1 and GluR3 mRNAs. GluR6 transcript expression is higher than that of GluR5 and GluR7. NR1 mRNA expression is much higher than that of NR2 mRNAs. NR2C subunit expression is very low except for a very distinct band of high expression in layer IV of area 3b. Immunocytochemistry, using subunit-specific antisera and double labeling for calbindin, parvalbumin, or alpha type II Ca2+/calmodulin-dependent protein kinase (CaMKII-alpha), allowed identification of cell types expressing different subunit genes. GluR1 and GluR5/6/7 immunoreactivity is found in both pyramidal cells and gamma-amino butyric acid (GABA) cells; GluR2/3 immunoreactivity is preferentially found in pyramidal cells, whereas GluR4 immunoreactivity is largely restricted to GABA cells; NMDA receptor subunit immunoreactivity is far greater in excitatory cells than in GABA cells. The density of expression of AMPA/kainate, kainate, and NMDA receptor subunit mRNAs differed within and across the architectonic fields of sensory-motor cortex. This finding and the lamina- and cell-specific patterns of expression suggest assembly of functional receptors from different arrangements of available subunits in specific neuronal populations.

Stable properties of spontaneous EPSCs and miniature retinal EPSCs during the development of ON/OFF sublamination in the ferret lateral geniculate nucleus

Retinal projections to the lateral geniculate nucleus (LGN) in ferrets progressively segregate into eye-specific laminae and subsequently into sublaminae that receive inputs from either ON-center or OFF-center afferents. To study the development of synaptic efficacy during a period of activity-dependent growth and reorganization in the CNS, we recorded spontaneous EPSCs (sEPSCs) from cells of the LGN during ON/OFF sublamination. We also examined retinal inputs specifically by stimulating the optic tract in the presence of strontium and recording evoked miniature EPSCs (emEPSCs). The rise times, areas, half-widths, and decay times of sEPSCs and emEPSCs and interevent intervals of sEPSCs recorded at the beginning of ON/OFF sublamination were not different from those recorded after its completion. Typically EPSC areas were small (10-20 fC) but varied greatly both within and between neurons. The frequency of sEPSCs was also quite variable, ranging from 0.2 to 5 Hz. sEPSCs were equivalent to miniature EPSCs recorded in the presence of tetrodotoxin, and both sEPSCs and emEPSCs were CNQX-sensitive. No difference was observed between sEPSCs recorded at room temperature and those recorded at 34 degreesC, and strontium could be substituted for calcium with no effect on sEPSC shape. These data argue for a remarkable stability in the components of at least AMPA-mediated synaptic transmission during a period of major synaptic rearrangement in the LGN.

Ontogeny of non-NMDA glutamate receptors in rat barrel field cortex: II. Alpha-AMPA and kainate receptors

The ontogeny of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate (KA) glutamate receptors in rat barrel field cortex was characterized by using receptor autoradiography and immunocytochemistry. A somatotopic pattern of AMPA receptors with fewer [3H]AMPA sites in barrel centers than in surrounding cortex did not emerge until postnatal day 10 (P10). After reaching a peak density at P14, the density of [3H]AMPA receptors declined in both barrel centers and surrounding cortex. Compared with AMPA receptors, the density of [3H]KA sites at all ages was low, a somatotopic expression of [3H]KA sites was missing, and the developmental curve for [3H]KA sites was more shallow than that for [3H]AMPA binding sites. A differential ontogeny of AMPA and KA receptors in barrel field cortex was also demonstrated in immunocytochemical studies with antibodies to the AMPA receptor subunits GluR1 and GluR2,3 and the KA receptor subunits GluR6,7. GluR1 and GluR2,3 staining was more dense in barrel septa than in barrel centers; this pattern persisted into adulthood. GluR1 and GluR2,3 receptors were localized to cell bodies and dendrites as well as the neuropil, but different populations of cortical neurons expressed these receptors. At P10, KA receptor subunits GluR6,7 exhibited a contrasting pattern to that of AMPA receptor subunits, with slightly more neuropil staining in barrel centers than in surrounding cortex. After that point, the somatotopic pattern of GluR6,7 subunit expression was lost. The contrasting developmental patterns of expression of the AMPA and KA receptors in the barrel field suggest that they may play different roles in the whisker-to-barrel pathway.

Differential distribution of AMPA receptors and glutamate during pre- and postnatal development in the visual cortex of ferrets

Immunohistochemical methods were used to study the distribution and time-course of appearance of cells expressing glutamate and alpha-amino-3-hydroxy-5-methyl-4-isoaxazole propionic acid (AMPA)-type glutamate receptors (GluR1 and GluR2/3) during development of the ferret visual cortex. Glutamate is present in many neurons in the ventricular zone, intermediate zone, developing cortical plate, and marginal zone as early as embryonic day (E) 34 (birth is at E41 in ferrets). Glutamate attains its adult distribution coincident with the completion of cellular migration. By contrast, GluR1 immunoreactivity emerges more slowly. By birth, GluR1 immunoreactivity is present only in a few neurons in the marginal zone and ventricular zone but is abundant in the marginal zone and subplate, where synaptogenesis commences. The number and staining intensity of GluR1-positive cells increases dramatically during the first two postnatal weeks and is maximal between the second and third week, before slowly declining to adult levels. Cortical cells immunopositive for GluR2/3 follow a similar pattern, although their distribution differs: GluR2/3-positive cells are mainly pyramidal cells. During the first postnatal week, GluR2/3 is also transiently present in fibers in the intermediate zone, which at this stage contains many thalamocortical and callosal and corticofugal axons. The abundance of glutamate at fetal stages, especially in the ventricular zone, is consistent with the previously proposed role of glutamate in mediating trophic effects in vivo, as previously demonstrated in vitro. The expression of AMPA receptors, as well as their transient overexpression, confirms the results of in situ hybridization studies and may imply a developmental role in neuronal differentiation for these receptors, in addition to their mature role in mediating cortical transmission.

Laminar distribution of MK-801, kainate, AMPA, and muscimol binding sites in cat visual cortex: a developmental study

We used quantitative autoradiography to determine whether the development of glutamate receptors correlates with the sensitive period for monocular deprivation in the visual cortex. To study glutamate receptors, we incubated sections of cat visual cortex with tritiated (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10imin e-maleate (MK-801), tritiated kainate, and tritiated amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA). [3H]MK-801 is a noncompetitive ligand for the N-methyl-D-aspartate (NMDA) receptor. [3H]kainate and [3H]AMPA are competitive ligands for non-NMDA receptors. We used [3H]muscimol, which binds to GABAA receptors, so that we would have one control ligand that binds to a nonglutamate receptor. When all layers were combined, the results confirmed our previous studies with homogenate binding. [3H]MK-801 and [3H]kainate binding were significantly greater at 42 days than at earlier or later times. [3H]AMPA and [3H]muscimol binding did not show such a peak. This suggests that MK-801 and kainate binding sites are more likely to be involved in plasticity than are AMPA and muscimol binding sites. In layers 2/3, MK-801 had the greatest age-dependent changes; in layers 5 and 6, kainate binding changed most with age. This suggests that the mechanisms of plasticity may vary with cortical layer.

Localization of AMPA-selective glutamate receptor subunits in the adult cat visual cortex

We have studied the presence and distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-selective glutamate receptor subunits (GluR1, 2, 3, and 4) in the adult cat visual cortical areas 17, 18, 19, and the lateral suprasylvian areas (LSA). Reverse transcription-polymerase chain reaction (RT-PCR) amplification indicated that the genes encoding GluR1, 2, 3, and 4 are expressed in these areas and Western blot analysis revealed that the size of the corresponding peptides is similar to those described in the rat brain. In situ hybridization (ISH) using digoxigenin-labeled riboprobes showed that mRNAs coding for GluR1 and GluR3 were located in cells in all layers of the areas examined and also in the underlying white matter. GluR1 mRNA was relatively abundant throughout layers II-VI while GluR3 mRNA revealed a more laminated pattern of expression, preferentially labeling cells in layers II, III, V, and VI. The distribution of AMPA-selective receptor subunit peptides was studied by immunohistochemistry using subunit specific antibodies and found to be consistent with ISH results. In addition, we observed that most of the cells strongly labeled by the anti-GluR1 antibody were non-pyramidal neurons and that intense GluR2/3 immunoreactivity was seen preferentially in pyramidal neurons. Interestingly, double-labeling experiments indicated that neurons expressing gamma-aminobutyric acid (GABA) as well as the GluR1 subunit were particularly abundant in deeper layers. The GluR4 peptide was predominantly found in a relatively low number of layer III and layer V neurons with either pyramidal or non-pyramidal morphology. Finally, the distribution of neurons expressing the various receptor subunits was similar in all the visual cortical areas studied. These findings indicate a high expression of GluR1-3 subunits in the cat visual cortex and that GluR1 and GluR2/3 subunits are particularly abundant in non-pyramidal and pyramidal neurons, respectively. In addition, the results described here provide a reference for future studies dealing with the effect of visual deprivation on the expression of this receptor type.

Differential effect of whisker trimming on excitatory and inhibitory transmission in primary somatosensory cortex of the adult rat in vivo

The effects of sensory deprivation on excitatory and inhibitory activity in the primary somatosensory cortex were studied in the adult rat. Excitatory and inhibitory transmission generated by whisker stimulation, and neuronal responsiveness to iontophoretically applied excitatory amino acids were recorded. Whisker input deprivation, through whisker trimming for a median of 24 days, resulted in a significant decrease in excitatory transmission to surround whisker stimulation. In contrast, the response magnitude to principal whisker stimulation remained unchanged. However, the response latencies to principal whisker and surround whisker stimulation were significantly reduced, which led to altered temporal response distributions in deprived cells. Neurons deprived of sensory input were significantly less responsive to glutamate, N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionate and kainate. Following deprivation, no change was observed in cortical inhibitory transmission measured 30-200 ms post-stimulus. These results show that excitatory transmission (including excitatory amino acid receptor function) is altered by adult whisker deprivation.

Alterations in geniculate ganglion proteins following fungiform receptor damage

Previous anatomical studies in rat have shown that damage produced to fungiform receptors of the anterior tongue at postnatal age 2 (P2) alters the growth and ramification of primary gustatory axons in the rostral nucleus of the solitary tract (NST). Studies employing artificial rearing (AR) procedures, which functionally deprive rat pups of orochemical stimulation during critical periods of postnatal life, produce similar alterations in the development of primary gustatory axons in the NST. Therefore, orochemical stimulation during rat's early postnatal life is necessary for normal development of primary gustatory axons in the rostral NST. One hypothesis concerning receptor-damage effects and AR effects is that receptor damage during critical periods of development may alter the regulation (i.e. transcription/translation) and/or distribution (i.e. transport) of proteins in geniculate ganglion neurons, thereby affecting growth of primary gustatory axons in the rostral NST. Specific aims of the present experiments were to comprehensively examine electrophoretic profiles of geniculate ganglion proteins following P2 receptor damage and late (> P40) receptor damage. Results show that concentrations of particular geniculate ganglion proteins are differentially altered following P2 receptor damage and late receptor damage, and that early receptor damage and late receptor damage produces distinct effects on the electrophoretic profiles of particular classes of proteins. Between the ages of P7-P38, P2 receptor damage lowers ganglion concentration of an acidic membrane glycoprotein designated as A1, with an apparent M(r) of 64-67 kDa and a pI of 4.8-5.2 P2 receptor damage also lowers ganglion concentrations of GAP-43. P2 receptor damage produces transient decreases in ganglion concentrations of NF-160, NF-200, and 8 additional acidic proteins. Three of these proteins may correspond to peripheral nerve sheath proteins analyzed in previous studies of the sciatic nerve, and one of these proteins may correspond to a 24 kDa growth-associated protein characterized in regenerating optic nerve. The time-course for changes observed in ganglion proteins following P2 damage was consistent with that observed for normal anatomical development of primary gustatory axons in both the lingual epithelium and NST. Receptor damage produced at P40 and later yielded different patterns of changes in geniculate ganglion proteins. Late receptor damage produced a transient increase in ganglion concentrations of NF-160, NF-200, GAP-43 and four additional acidic proteins within the 29-57 kDa M(r) range. Late receptor damage also produced a transient decrease in the concentrations of protein A1 and a 30 kDa protein that was not affected by P2 damage. Therefore, proteins that were preferentially affected by P2 damage may be involved in the regulation of initial axonal growth within the lingual epithelium and NST, as opposed to the structural repair or maintenance of extant axons. Relationships between normal anatomical development in peripheral and central components of primary gustatory axons are discussed in relation to availability of particular cytoskeletal and growth-associated proteins.

Embryonic hypothalamic expression of functional glutamate receptors

Glutamate can play a number of roles in the developing brain, including modulation of gene expression, cell motility, neurite growth and neuronal survival, all critical for the final organization and function of the mature brain. These functions are dependent on the early expression of glutamate receptors and on glutamate release in developing neurons. This subject has received little attention in the hypothalamus, despite glutamate's critical role as an excitatory transmitter in hypothalamic control of circadian rhythms, endocrine secretion, temperature regulation, and autonomic control. A total of 10,922 rat hypothalamic neurons were studied with digital Ca2+ imaging with the ratiometric dye fura-2 to examine their responses to glutamate receptor agonists and antagonists during embryonic development and maturation in vitro. Functional glutamate receptors were found very early in development (embryonic day 15-E15) with both Ca2+ imaging and with patch clamp recording. This is a time when the hypothalamus is beginning to undergo neurogenesis. Ca2+ responses from N-methyl-D-aspartate receptors developed later than those from non-N-methyl-D-aspartate ionotropic receptors that responded to kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate. The responses of immature E15 cells after one day in vitro were compared with more mature cells after six days in vitro to examine the response to repeated 3 min applications of 100 microM kainate (n = 108). Immature cells showed similar Ca2+ rises (+232nM Ca2+) with each kainate stimulation. In contrast, more mature cells showed an initial Ca2+ rise of 307 nM, with the second rise only to 147 nM above the initial baseline. Immature cells more quickly returned to their pre-kainate baseline than did older cells. The expression of metabotropic glutamate receptors was studied with the selective agonist trans-1-amino-cyclopentyl-1,3-dicarboxylic acid and with glutamate stimulation in the absence of extracellular Ca2+ and presence of 1 mM EGTA. After five days in vitro. E16 astrocytes showed a greater response than did neurons to conditions that would activate the metabotropic glutamate receptor. A dramatic increase in the percentage of cells that responded to N-methyl-D-aspartate was found after only a few days in culture. Only a small number of E15 cells studied on the day of culture (4% of 694 cells) showed a response to 100 microM N-methyl-D-aspartate. Thirty-eight percent of 120 E18 cells cultured for one day in vitro showed an N-methyl-D-aspartate response.(ABSTRACT TRUNCATED AT 400 WORDS)

Monocular visual deprivation at the critical period modulates photic evoked responses

Photic evoked responses were recorded from the striate cortex of Long-Evans hooded normal (control) rats and from monocular visual deprivation (MD) rats. The averaged visual evoked responses (AVER) were obtained from both hemispheres and provide comparison between the contralateral and the ipsilateral striate cortex with relation to the monocular deprived eye. The AVER recorded following binocular photic stimulation after 1 month of monocular deprivation demonstrated that the two visual cortexes responded differently. In the contralateral hemisphere of the visual cortex (related to the MD eye), all three components (P2, N2 and P3) of the AVER of the MD rats had significant increases in their peak amplitude as compared to the control recordings. In the ipsilateral cortex, the amplitude of component P2 and N2 was significantly reduced as a result of 1 month of MD. Comparing the AVER amplitudes of the two homotopic sites of the visual cortex obtained from the control group reveals no differences between the two hemispheres but markedly significant differences in P2, N2 and P3 components for the MD group. Based on the literature, the possibility that the monocular visual deprivation at the critical period in early developmental stage modulates the AVER as a result from the neurocytological alteration from altering of GABA and ACh within the striate cortex was discussed. In conclusion, the AVER is a reliable and practical method for studying the effects of monocular deprivation and neuroplasticity in the rat visual cortex.

Effects of orochemical stimulation on postnatal development of gustatory recipient zones within the nucleus of the solitary tract

Previous receptor damage studies and artificial rearing (AR) studies in rat have demonstrated that orochemical stimulation between the postnatal ages of P2 and P14 is necessary for development of primary gustatory axons and terminal endings in the rostral nucleus of the solitary tract (NST). Objectives of the present experiment were to evaluate the qualitative nature of orochemical stimulation and amount of orochemical stimulation that is necessary to produce normal axonal and terminal development in gustatory recipient zones of the rostral and intermediate NST. To this end, ultra-pure water, 30, 150, and 500 mM NaCl, 80 and 340 mM lactose, whole rat milk, and rat milk that had been subject to extensive dialysis (12-14 kD MWCO) was delivered to independent groups of rat pups during AR procedures. Unstimulated AR animals and matched mother-reared (MR) animals were used as controls. AR animals received experimental orochemical stimulation between the ages of P4 and P10, and were thereafter returned to lactating dams until the time of weaning; MR animals received experimental orochemical stimulation during the course of normal nursing. Following maturation, anterograde fluorescent dual-labeling experiments were conducted to map the course and distribution of primary gustatory axons within the NST. Results show that experimental stimualtion with water during AR procedures is not sufficient to produce normal development of primary gustatory axons and terminal endings in the gustatory NST. Stimulation with 30, 150, and 500 mM NaCl produced normal axonal development in the NST, as did 80 and 340 mM lactose, whole rat milk, and dialyzed rat milk.(ABSTRACT TRUNCATED AT 250 WORDS)

Postnatal development of NMDA, AMPA, and kainate receptors in individual layers of rat visual cortex and the effect of monocular deprivation

The postnatal development of the ligand binding to N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and kainate receptor sites was studied in individual layers of rat visual cortex with [3H]MK-801, [3H]CNQX and [3H]kainate, respectively. The studies were performed by quantitative receptor autoradiography in the different visual cortical layers from normally raised rats and from monocularly deprived animals. In normally raised rats, in all visual cortical layers, [3H]MK-801 and [3H]CNQX binding increased significantly from birth to around postnatal day 20, at which age already the adult values are reached. In contrast, the increase in [3H]kainate binding from day 1 to day 15 is followed by a significant decrease in binding sites up to day 25; at this age the adult binding level is nearly attained. Monocular deprivation, by unilateral eyelid suture at the age of 11 days onwards, leads to a transient decrease in [3H]CNQX binding in all visual cortical layers, whereas [3H]kainate binding results to a permanent decrease in AMPA receptor sites in cortical layers II to VI in both sides of the cortex. In contrast, only long-term monocular deprivation until the age of 90 days resulted in decreased [3H]MK-801 binding levels as compared to age-matched controls. The data suggest that the laminar development of glutamate receptor subtypes is differentially affected by visual experience during early brain maturation.