Postnatal development of NMDA receptor-mediated synaptic transmission in cat visual cortex

New insights into cortical development and plasticity: from molecules to behavior

The human brain contains 100 billion neurons, and each neuron can have up to 200,000 connections to other neurons. Recent advancements in neuroscience-ranging from molecular studies in animal models to behavioral studies in humans-have given us deeper insights into the development of this extraordinarily intricate system. Studies show a complex interaction between biological predispositions and environment; while the gross neuroanatomy and low-level functions develop early prior to receiving environmental inputs, functional selectivity is shaped through experience, governed by the maturation of local excitatory and inhibitory circuits and synaptic plasticity during sensitive periods early in development. Plasticity does not end with the closing of the early sensitive period - the environment continues to play an important role in learning throughout the lifespan. Recent work delineating the cascade of events that initiates, controls and ends sensitive periods, offers new hope of eventually being able to remediate various clinical conditions by selectively reopening plasticity.

Fast recruitment of recurrent inhibition in the cat visual cortex

Neurons of the same column in L4 of the cat visual cortex are likely to share the same sensory input from the same region of the visual field. Using visually-guided patch clamp recordings we investigated the biophysical properties of the synapses of neighboring layer 4 neurons. We recorded synaptic connections between all types of excitatory and inhibitory neurons in L4. The E–E, E–I, and I–E connections had moderate CVs and failure rates. However, E–I connections had larger amplitudes, faster rise-times, and shorter latencies. Identification of the sites of putative synaptic contacts together with compartmental simulations on 3D reconstructed cells, suggested that E–I synapses tended to be located on proximal dendritic branches, which would explain their larger EPSP amplitudes and faster kinetics. Excitatory and inhibitory synapses were located at the same distance on distal dendrites of excitatory neurons. We hypothesize that this co-localization and the fast recruitment of local inhibition provides an efficient means of modulating excitation in a precisely timed way.

Role of pre- and postsynaptic activity in thalamocortical axon branching

Axonal branching is thought to be regulated not only by genetically defined programs but also by neural activity in the developing nervous system. Here we investigated the role of pre- and postsynaptic activity in axon branching in the thalamocortical (TC) projection using organotypic coculture preparations of the thalamus and cortex. Individual TC axons were labeled with enhanced yellow fluorescent protein by transfection into thalamic neurons. To manipulate firing activity, a vector encoding an inward rectifying potassium channel (Kir2.1) was introduced into either thalamic or cortical cells. Firing activity was monitored with multielectrode dishes during culturing. We found that axon branching was markedly suppressed in Kir2.1-overexpressing thalamic cells, in which neural activity was silenced. Similar suppression of TC axon branching was also found when cortical cell activity was reduced by expressing Kir2.1. These results indicate that both pre- and postsynaptic activity is required for TC axon branching during development.

Postnatal development of excitatory postsynaptic currents in nucleus accumbens medium spiny neurons

We have recorded excitatory postsynaptic currents (EPSCs) evoked by local electrical stimulation in 243 nucleus accumbens (nAcb) neurons in vitro during postnatal development from the day of birth (postnatal day 0; P0) to P27 and in young adults rats (P59-P71). An EPSC sensitive to glutamatergic antagonists was found in all neurons. In the majority of cases (189/243), the EPSC had two distinct components: an early one sensitive to 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and a late one that was sensitive to D-2-amino-5-phosphonovaleric acid (APV) showing that early and late components of the EPSC were mediated by AMPA/kainate (KA) and N-methyl-D-aspartate (NMDA) receptors respectively. During the first four postnatal days, the amplitudes of both the AMPA/KA and NMDA components of the EPSC were relatively small and then began to increase until the end of the second postnatal week. Whereas the amplitude of the early component appeared to stabilize from that point on, the late component began to decrease and became virtually undetectable in preparations from animals older than 3 weeks unless the AMPA/KA response was blocked with CNQX. In addition, the ratio between the amplitude of the NMDA and AMPA/KA receptor-mediated components of the EPSC followed a developmental pattern parallel to that of the NMDA receptor component showing an increase during the first two postnatal weeks followed by a decrease. Together, these results show that, during postnatal development, there is a period when NMDA receptor-mediated EPSC are preeminent and that time frame might represent a period during which the development of the nAcb might be sensitive to environmental manipulation.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Development of NMDA and non-NMDA receptor-mediated excitatory synaptic transmission in geniculocortical and corticocortical connections in organotypic coculture preparations

Development of N-methyl-D-aspartate (NMDA) and non-NMDA receptor-mediated excitatory synaptic transmission was studied in the visual cortex using organotypic slice cocultures. A slice of visual cortex (VC) dissected from newborn rats was cocultured with either a chunk of embryonic lateral geniculate nucleus (LGN) or another VC. During 7-38 days in vitro (DIV), geniculocortical monosynaptic excitatory postsynaptic potentials (EPSPs) were recorded from layer IV neurons in response to stimulation of the LGN in cocultures of the VC with the LGN. Similarly, corticocortical monosynaptic EPSPs were recorded from layers II/III and V/VI neurons in cocultures of two VCs when stimulating the partner VC. The initial slopes of the non-NMDA and NMDA receptor-mediated components of the EPSPs, which were dissociated pharmacologically, were assessed and compared among three different culture stages, early (7-11 DIV), middle (12-15 DIV) and late (17-38 DIV). With progression of the culture stage, the non-NMDA component tended to increase in both the geniculocortical and corticocortical connections. In contrast, the NMDA component exhibited distinct developmental changes. The NMDA component in layer IV neurons, which receive geniculate inputs, showed a transient increase in the middle stage. In the corticocortical connection, the magnitude of the NMDA component was large in the early stage and maintained through all culture stages in layer V/VI cells, whereas in layer II/III cells it decreased sharply by the late stage. Our results suggest that glutamatergic transmission in the visual cortex develops differently in the geniculocortical and corticocortical connections.

Confocal imaging of N-methyl-D-aspartate receptors in living cortical neurons

The fluorescence-conjugated N-methyl-D-aspartate receptor-selective antagonist, BODIPY-conantokin-G, was employed to label N-methyl-D-aspartate receptors in living neurons derived from the visual cortex of embryonic rats. The fluorescent labeling was visualized and analysed using confocal microscopy and digital imaging techniques. BODIPY-conantokin-G binding sites were homogeneously distributed across somata four days after neurons (E17-20) were placed in culture. In five-day-old cultures, BODIPY-conantokin-G binding sites became clusters of fluorescently labeled spots which were arranged irregularly on somata and proximal neurites. Distal neurites displayed fluorescent labeling after 10-15 days in culture. Displacement experiments showed that spermine and unlabeled conantokin-G compete with BODIPY-conantokin-G labeling at the N-methyl-D-aspartate receptor-associated polyamine site. The N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovaleric acid also depressed the labeling but with a weaker effect, probably due to interactions occurring between the N-methyl-D-aspartate receptor agonist binding site and the polyamine modulatory site. The fluorescent dyes FM 1-43 and FM 4-64 were used in double-labeling studies to compare the distribution of nerve terminals with that of BODIPY-conantokin-G binding sites. BODIPY-conantokin-G binding clusters were associated with presynaptic nerve terminals while isolated BODIPY-conantokin-G binding sites were not always opposed to terminals. The aggregation of receptors to form clusters may lead to the functional formation of excitatory synapses. To investigate whether modulation of membrane potentials affected the formation of N-methyl-D-aspartate receptor clusters, cultured neurons were chronically treated for a week with either tetrodotoxin (to block membrane action potentials) or a high concentration of potassium to depolarize the membrane. While neurons in the tetrodotoxin-treated group showed a similar number of fluorescently labeled clusters compared with the control group, neurons in the high potassium group exhibited a higher number of fluorescently labeled receptor clusters. These results suggest that more active neurons may tend to form more N-methyl-D-aspartate synapses during early development.

Synaptogenesis and ultrastructural localization of the polysialylated neural cell adhesion molecule in the developing striatum

The polysialylated neural cell adhesion molecule (PSA-NCAM) plays a role in axonal development and synaptic plasticity. Its pattern of expression is regulated temporally and topographically in the brain during development. However, it is unclear whether or not its subcellular location also changes. We have examined PSA-NCAM expression in relation to synapse formation in the developing rat striatum with immunohistochemistry and electron microscopy. Early in development, PSA-NCAM was present along the cytoplasmic membranes of neurons and in growth cones. PSA-NCAM expression became progressively confined to pre- and postsynaptic elements as neurons matured morphologically. Confirming previous results, a marked increase in the density of asymmetric synapses determined by using the physical dissector method was observed in the dorsolateral striatum between postnatal day 14 (P14) and P18. It was followed by a reduction between P18 and P25, when asymmetric synapse density reached adult levels. In contrast, the density of symmetric synapses had surpassed adult levels by P14. In the dorsomedial striatum, the density of asymmetric and symmetric synapses was similar at P18, at P25, and in adults. PSA-NCAM was associated with most asymmetric and symmetric synapses at P14 and P18 and was expressed in both pre- and postsynaptic elements of a majority (P14) or approximately half (P18) of the synapses. Most synapses lost PSA-NCAM expression between P18 and P25 in the dorsolateral striatum and between P25 and adult in the dorsomedial striatum. The data indicate that PSA-NCAM expression becomes restricted topographically during neuronal maturation but remains strategically associated with developing synapses during late postnatal development in the striatum.

Spatiotemporal patterning of glutamate receptors in developing ferret striate cortex

We have studied glutamate receptor levels during very early phases of cortical formation by using quantitative in vitro autoradiography to map the expression of NMDA, AMPA and kainate receptors in the developing primary visual cortex of the ferret. NMDA and non-NMDA receptors exhibit very different developmental profiles in primary visual cortex. NMDA receptor density is low at birth and increases throughout the first 2 postnatal months, rising between threefold (layers II/III) and ninefold (layer VI). In contrast, AMPA receptors are abundant at birth and their density remains constant for the first postnatal month, before rising by a maximum of 1.7-fold (layer I) at around the time of eye-opening (postnatal day 32). Kainate receptors are also present in high levels at birth and their expression levels rise in the early postnatal period by between 1. 5-fold (layer I) and threefold (layers V/VI) to a peak just after eye-opening. The proportion of the total ionotropic glutamate receptor binding contributed by NMDA receptors thus rises from 5% at birth to a maximum of 22% at 2 months of age, while the AMPA receptor contribution falls from 87% to 72% over the same period. Below cortex, all three glutamate receptor subtypes are expressed in the subplate region for the first 3 postnatal weeks. These developmental patterns, combined with the fact that AMPA receptors are densely expressed in the proliferative zones underlying presumptive area 17, indicate that non-NMDA receptor expression levels in primary visual cortex are mostly specified much earlier than those of NMDA receptors.

Involvement of NMDA in a plasticity phenomenon observed in the adult frog monocular optokinetic nystagmus

The frog horizontal monocular optokinetic nystagmus (H-OKN) is asymmetrical, the reflex being evoked by a temporal-nasal (T-N) component, but not by a nasal-temporal (N-T) component. Coil recordings showed that, in adult animals, 8 days of monocular deprivation (by unilateral eyelid suture) provoked the appearance of a N-T component, the H-OKN becoming symmetrical, reacting for both directions of stimulation. This delay was shortened to 2 days following two successive unilateral pretectal administrations of NMDA or of LY 285 265, an NMDA agonist, the first 2 days of eyelid suture. The same results were obtained when chronic microinjections of NMDA or LY 285 265 were achieved, the frogs being maintained in total darkness during the week of eyelid suture. These data indicate that the plasticity phenomenon evidenced in the monocular frog H-OKN depends on the activation of the NMDA receptors of one pretectum. This activation was obtained either by a monocular light stimulation of 8 days duration, or by unilateral administration of drugs activating the NMDA glutamatergic pretectal system. In this last case, the light stimulation was no longer necessary.

Effect of chronic administration of NMDA antagonists on synaptic development

The current research assessed the role of the N-methyl-D-aspartate (NMDA) receptor in developmental synaptic plasticity. This was accomplished by quantitative analysis of synaptic number and morphology following pharmacological manipulation of NMDA receptor activity using either the competitive antagonist 2-amino-5-phosphonovaleric acid (APV) or the noncompetitive antagonist phencyclidine (PCP). In the first group, 15-day-old male Long-Evans rats were implanted with osmotic minipumps, which administered 50 mM APV or vehicle at a rate of 0.5 microliter per h into the subjects' occipital cortex for 14 days. At age 30 days (P30), the rats were sacrificed and their occipital neocortices were examined. A second group of rats was given subcutaneous injections of 10 mg/kg PCP or vehicle once daily beginning on P5 for a period of 15 days, and was sacrificed on P20. To determine the effects following withdrawal from long-term NMDA antagonism, a third group of animals was given the same PCP injection routine until P20, but was sacrificed on P21, P26, P36, and P56. Developmental administration of APV was associated with a decreased molecular layer depth and estimated total number of synapses. Similarly, PCP induced a reduction in brain weight, molecular layer depth, and estimated total number of synapses. Withdrawal from NMDA antagonism was initially associated with similar results, i.e., reduced brain weight, cortex depth, synaptic density, and estimated total number of synapses, along with an increase in synaptic length. By P36, however, there was a transitory rebound associated with increased molecular layer depth and estimated total number of synapses. These results support the suggestion that NMDA receptor activation is integral to naturally occurring developmental synaptogenesis, and underscore the importance of NMDA receptor involvement in the process of synaptic plasticity.

Postnatal changes in NMDAR1 subunit expression in the rat trigeminal pathway to barrel field cortex

N-methyl-D-aspartate (NMDA) type glutamate receptors are constituted of one obligatory subunit (NR1), expressed as eight splice variants, combined with one or more of four NMDAR2 subunits. Polyclonal antibodies were produced to an N-terminal domain of the NR1 subunit that recognize all eight splice variants. The antibody was used to localize NR1 in the trigeminal pathway to barrel field cortex in rats. The distribution and density of NR1 changes between birth (postnatal day 0 = P-0) and P-360. The trigeminal nuclei already contain a high level of NR1 immunoreactivity on the day of birth. The ventral posterior lateral, ventral posterior medial, and posterior nucleus, medial division, thalamic nuclei show fluctuations in NR1 immunoreactivity levels, starting at birth with moderate densities in neuropil which decrease at P-7, and peak again in neuronal cell bodies as well as the neuropil at P-21. In the cortex, the density of NR1 in layer VI fluctuates with low points at P-7 and P-40. Superficial cortical layers I, II, and III reach adult levels at P-14 and remain high. NR1 levels decrease sharply in layer IV just prior to P-40 and then slowly recover over the next 3 months to stabilize at moderate levels in the adult. In addition to neuronal expression there is a transient high level of labeling in glial cells with a peak density of staining at P-21. The results emphasize that NR1 subunit expression is finely regulated in rat somatic sensory pathways for periods as long as 7-8 weeks after birth in the barrel field cortex.

Ontogeny of NMDA R1 subunit protein expression in five regions of rat brain

A polyclonal antiserum to a fusion protein corresponding to a region of the NMDA R1 (NR1) subunit (amino acids 656-811) was produced and affinity purified. A quantitative immunoblotting technique was developed using the fusion protein as a standard. By employing this method, ontogenic studies (day 2-42) of the density of NR1 protein were carried out in several regions of rat brain. The results showed that in all five of the brain regions examined [olfactory bulb (Ob), cortex (Cx), hippocampus (Hp), midbrain (Mb) and cerebellum (Cb)], levels of NR1 protein are low at birth and increase with similar patterns having a sharp rise within the first 3 weeks after birth. Levels increased 2.0 to 4.5-fold from postnatal day 2 to postnatal day 42. Although the general patterns of developmental expression are similar, large differences in the absolute amounts of NR1 protein among the five brain regions were observed. The maximal levels (pmol of fusion protein equivalent/mg +/- S.E.) of NR1 subunit attained during development in the five regions are: Hp 2.0 +/- 0.37 > Cx 1.4 +/- 0.11 > Ob 1.3 +/- 0.2 > Mb 1.0 +/- 0.10 > Cb 0.57 +/- 0.13. The temporal patterns of expression of NR1 protein are similar to results from studies examining the expression of NR1 mRNA. Furthermore, the absolute numbers obtained from our studies are close to those found using [(3)H]MK-801 binding suggesting that many of the NR1 subunits expressed in the brain exist in an active form.

Transient co-localization of calretinin, parvalbumin, and calbindin-D28K in developing visual cortex of monkey

This paper reports a double-labelling immunocytochemical study of the three calcium-binding proteins calretinin, parvalbumin, and calbindin-D28k in developing and adult Macaca primary visual cortex. In adult visual cortex, each protein marks a subset of GABAergic neurons with a characteristic laminar distribution and virtually no co-localization was found between these three proteins, suggesting that each calcium-binding protein may serve as a marker for one or more cortical subcircuits. The immature visual cortex, immunostained using identical techniques was then analysed to determine if each calcium-binding protein could serve as a developmental marker for these circuits. The Cajal-Retzius cells of layer 1 contained all three proteins during development. Calbindin-D28k and calretinin were co-localized starting at Fd (foetal day) 45 and after Fd125, parvalbumin also was present in the same Cajal-Retzius cells. All three proteins continued to be expressed until the Cajal-Retzius disappeared postnatally. In layers 2-6 calbindin-D28k and calretinin were never co-localized. In contrast, parvalbumin and calretinin were found in neurons of deep layer 3 from Fd 155 to postnatal (P6) weeks with a few persisting even later. Before birth almost all PV+ neurons in layers 4-6 were CaB+, but by P3 weeks only a few PV+/CaB+ neurons remained in layer 4C and these completely disappeared by P6 weeks. Co-localization in layer 4 neurons overlaps the period of ocular dominance segregation, suggesting that the onset of cortical maturity coincides with segregation of calcium-binding proteins within the GABA interneurons.

Plasticity of excitatory synaptic transmission in kitten visual cortex depends on voltage-dependent Ca2+ channels but not on NMDA receptors

Long-term potentiation (LTP) of excitatory synaptic transmission occurs in in vitro slices of cat and rat visual cortex. Earlier studies suggested that activation of N-methyl-D-aspartate (NMDA)-selective glutamate receptors is essential for the induction of LTP. However, our studies on kitten visual cortex demonstrate that LTP induction requires the activation of low-threshold Ca2+ channels in postsynaptic cells but not of NMDA receptors.