The effect of age on binding of MK-801 in the cat visual cortex

Factors that are critical for plasticity in the visual cortex

Factors that may be critical for plasticity in the visual cortex are evaluated according to three criteria. (1) Do antagonists to the factor abolish plasticity? (2) Does the concentration or activity of the factor peak with the critical period for plasticity? (3) Does rearing in the dark, which postpones the critical period, affect the factor in a similar fashion? N-methyl-D-aspartate receptors fulfil all three criteria. Metabotropic glutamate receptors fulfil two of them. Most other putative factors do not fulfil more than one.

Experience-dependent changes in NMDAR1 expression in the visual cortex of an animal model for amblyopia

When normal binocular visual experience is disrupted during postnatal development, it affects the maturation of cortical circuits and often results in the development of poor visual acuity known as amblyopia. Two main factors contribute to the development of amblyopia: visual deprivation and reduced binocular competition. We investigated the affect of these two amblyogenic factors on the expression of the NMDAR1 subunit in the visual cortex because activation of the NMDA receptor is a key mechanism of developmental neural plasticity. We found that disruption of binocular correlations by monocular deprivation promoted a topographic loss of NMDAR1 expression within the cortical representations of the central visual field and the vertical and horizontal meridians. In contrast, binocular deprivation, which primarily affects visual deprivation, promoted an increase in NMDAR1 expression throughout the visual cortex. These different changes in NMDAR1 expression can be described as topographic and homeostatic plasticity of NMDA expression, respectively. In addition, the changes in NMDA expression in the visual cortex provide a greater understanding of the neural mechanisms that underlie the development of amblyopia and the potential for visual recovery.

Synaptic and Molecular Mechanisms Regulating Plasticity during Early Learning

Many behaviors are learned most easily during a discrete developmental period, and it is generally agreed that these "sensitive periods" for learning reflect the developmental regulation of molecular or synaptic properties that underlie experience-dependent changes in neural organization and function. Avian song learning provides one example of such temporally restricted learning, and several features of this behavior and its underlying neural circuitry make it a powerful model for studying how early experience sculpts neural and behavioral organization. Here we describe evidence that within the basal ganglia-thalamocortical loop implicated in vocal learning, song acquisition engages N-methyl-d-aspartate receptors (NMDARs), as well as signal transduction cascades strongly implicated in other instances of learning. Furthermore, NMDAR phenotype changes in parallel with developmental and seasonal periods for vocal plasticity. We also review recent studies in the avian song system that challenge the popular notion that sensitive periods for learning reflect developmental changes in the NMDAR that alter thresholds for synaptic plasticity.

Postnatal phencyclidine-induced deficit in adult water maze performance is associated with N-methyl-D-aspartate receptor upregulation

The N-methyl-D-aspartate (NMDA) receptor plays an important role in developmental plasticity. Earlier, we have shown that blocking the NMDA receptor with the non-competitive antagonist phencyclidine (PCP), during a brief postnatal period, disrupts the water maze performance in young juvenile rats (starting at 25 days of age). We now show the long-term effects of postnatal phencyclidine exposure on spatial learning and memory. Male and female rats were exposed to PCP (1 and 5mg/kg) or saline, from postnatal days 5-15, and their performance in the Morris water maze (MWM) was tested both as adolescents (starting on postnatal day (PD) 35) and as adults (starting on postnatal day 60). Separate groups of adult male and female postnatal PCP-treated and saline-treated rats were sacrificed and saturation [3H]MK-801 binding experiments were carried out in their hippocampi and frontal cortices; hippocampus and frontal cortex have high densities of NMDA receptors and both regions are important in spatial learning and memory. Postnatal PCP administration disrupted the water maze performance both in adolescent and adult rats of both sexes. Adult male and female rats treated postnatally with PCP had increased maximal [3H]MK-801 binding in the hippocampus and frontal cortex compared to same-sex saline-treated controls. Taken together, repeated postnatal PCP (RPP) administration impaired the acquisition of spatial learning in adolescent and adult male and female rats, and this cognitive deficit was associated with increased [3H]MK-801 labeled NMDA receptor in the hippocampus and frontal cortex. These findings are consistent with the hypothesis that PCP treatment during the postnatal period produces deficits in the water maze performance by disrupting the developing glutamatergic system.

The role of N-methyl-D-aspartate receptors in synaptic plasticity of rat visual cortex in vitro: effect of sensory experience

We examined the role of N-methyl-D-aspartate (NMDA) receptors in synaptic plasticity of visual cortex of light (LR) and dark (DR) reared adult rats in vitro. Layer IV stimulation resulted in field potentials in layer II/III, consisting of two excitatory postsynaptic potentials (EPSP) called EPSP1 and EPSP2. Tetanic stimulation induced long-term potentiation (LTP) in EPSP2 of both LR and DR visual cortices. NMDA receptor antagonist D, L-2-amino-5-phosphono-valeric acid (AP5) completely blocked the LTP of EPSP2 in DR visual cortex while it reduced slightly the extent of LTP of EPSP2 in LR ones. Another NMDA receptor antagonist ketamine blocked potentiation of EPSP1 as well as EPSP2 in both groups. Our findings demonstrate that dependency of LTP on NMDA receptors and/or sensitivity of these receptors to the antagonists are different in LR and DR animals.

Cyclic AMP-dependent protein kinase mediates ocular dominance shifts in cat visual cortex

Visual experience during a critical period early in postnatal development can change connections within mammalian visual cortex. In a kitten at the peak of the critical period (approximately P28-42), brief monocular deprivation can lead to complete dominance by the open eye, an ocular dominance shift. This process is driven by activity from the eyes, and depends on N-methyl-D-aspartate (NMDA) receptor activation. The components of the intracellular signaling cascade underlying these changes have not all been identified. Here we show that inhibition of protein kinase A (PKA) by Rp-8-Cl-cAMPS blocks ocular dominance shifts that occur following monocular deprivation early in the critical period. Inhibition of protein kinase G by Rp-8-Br-PET-cGMPS had no effect, indicating a specificity for the PKA pathway. Enhancement of PKA activity late in the critical period with Sp-8-Cl-cAMPS did not increase plasticity. PKA is a necessary component of the pathway leading to cortical plasticity during the critical period.

Early sensory and hormonal experience modulate age-related changes in NR2B mRNA within a forebrain region controlling avian vocal learning

Male zebra finches are most apt to mimic songs heard between posthatch days (PHD) 35 and 65, and this vocal learning depends, in part, on the activation of N-methyl-D-aspartate receptors (NMDAR) within a discrete forebrain circuit that includes the lateral magnocellular nucleus of the anterior neostriatum (lMAN) and area X. Using in situ hybridization, we show that transcripts for both the constitutive NMDAR subunit NR1 and the modulatory subunit NR2B decrease abruptly in the lMAN between PHD20 and 40. This downregulation corresponds to the onset of song learning and a transition from slow to faster NMDAR currents in lMAN neurons. In area X, NR1 mRNA increases as NR2B mRNA decreases during song development. To understand how these changes in NMDAR mRNA might regulate song learning, we next investigated how manipulations that influence song development affect NMDAR mRNA expression. Early isolation from conspecific song (which delays closure of the sensitive period for song learning) selectively increases NR2B, but not NR1 mRNA, within lMAN at PHD60. In contrast, exposure to testosterone beginning at PHD20 (which impairs song development and hastens the developmental transition to faster NMDAR current kinetics within lMAN) accelerates the decline in NR2B mRNA in lMAN, again without affecting NR1 transcript levels. Neither manipulation significantly effects NR1 or NR2B mRNA levels in area X. Our data suggest that developmental changes in the expression of specific NMDAR subunits may regulate periods of neural and behavioral plasticity and that flexibility in the timing of these sensitive periods may be achieved through experience and/or hormone-dependent modulation of NMDAR gene expression.

Primed-bursts induced long-term potentiation in rat visual cortex: effects of dark-rearing

Theta burst stimulation (TBS) and primed bursts (PBs) stimulation are among the effective tetanic stimulations for induction of long-term potentiation (LTP) in the hippocampus. Recent studies have indicated that TBS is effective in LTP induction of layer III synapses of neocortex, only if applied to layer IV. However, the possibility of neocortical LTP induction using PBs has not been investigated yet. Sensory deprivation greatly influences the development of neocortex. According to the effect of sensory deprivation on synaptic plasticity of developing neocortex, we studied the induction of LTP by PBs in visual cortical slices of control and dark-reared rats. The results showed that application of PBs to layer IV could effectively induce LTP of layer II/III field potentials. These potentials are consisted of two components: pEPSP1, (population excitatory postsynaptic potential 1) and pEPSP2. In control slices PBs led to selective potentiation of pEPSP2. Visual deprivation increased the incidence of LTP of pEPSP1 and decreased the amount of LTP of pEPSP2. These findings showed that PBs could be used as an effective tetanic stimulation to study the synaptic plasticity in neocortex. The effects of visual deprivation on PBs-induced LTP are consistent with its role in the development of excitatory system in neocortex.

Visual deprivation increases capability of layer II/III for epileptiform activity in the rat visual cortical slices

Effects of visual deprivation on the induction of epileptiform activity were studied in layer II/III of 29-39-day-old rat primary visual cortex. Field potentials were evoked by stimulation of layer IV in slices from control (CON) and dark-reared (DR) rats. Picrotoxin (PTX)-induced epileptiform activity was characterized by spontaneous and evoked epileptic field potentials (EFPs). The results showed that DR slices demonstrate greater susceptibility for induction of spontaneous EFP. PTX-induced changes in the characteristics of evoked field potentials also showed higher tendency of DR animals to generate epileptiform activity. In both groups, field potentials consisted of pEPSP(1) (population excitatory postsynaptic potential 1, i.e., first negativity) and pEPSP(2) (second negativity), respectively. There was no significant difference between the characteristics of field potentials in CON and DR slices. PTX significantly increased amplitude and duration of pEPSP(2), but it had no significant effect on pEPSP(1). Effects of PTX on pEPSP(2) were significantly higher in DR slices. It is concluded that visual deprivation results in a heightened potential in layer II/III of the rat visual cortex to generate PTX-induced epileptiform activity.

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.

Patchy distribution of NMDAR1 subunit immunoreactivity in developing visual cortex

Development of ocular dominance columns is dependent on patterned retinal activity, and yet patterned activity alone cannot explain all aspects of cortical column development. Features intrinsic to the cortex have been proposed to interact with activity to guide the patterning of cortical columns (), and the NMDA receptor, because of its role in experience-dependent plasticity, is an obvious candidate. Using immunohistochemical techniques, we found a transiently patchy distribution of the NMDA receptor 1 (NMDAR1) subunit in kitten visual cortex. Regularly spaced patches of NMDAR1-immunoreactive neurons were found at the top of the cortical plate in the developing visual cortex at 2 weeks of age. At 4-5 weeks of age, the radial extent of the NMDAR1 patches spanned the supragranular layers, and by 12 weeks of age, this nonuniform pattern of NMDAR1 immunostaining was no longer apparent. Monocular visual experience prevented the expression of the NMDAR1 patches, but just 4 d of subsequent binocular visual experience was sufficient to promote expression of the patches. Furthermore, the NMDAR1 patches tended to be associated with the borders of ocular dominance columns. These results suggest that the degree of plasticity associated with NMDA-mediated mechanisms is elevated in local regions across the tangential extent of the visual cortex and that the NMDAR1 patches may participate in sculpting the overall arrangement of visual cortical columns.

Anatomical and synaptic substrates for avian song learning

In songbirds, vocal learning occurs during periods of major cellular and synaptic change. This neural reorganization includes massive synaptogenesis associated with the addition of new neurons into the vocal motor pathway, as well as pruning of connections between song regions. These observations, coupled with behavioral evidence that song development requires NMDA receptor activation in specific song nuclei, suggest that experiences associated with vocal learning are encoded by activity driven, Hebbianlike processes of synaptic change akin to those implicated in many other forms of developmental plasticity and learning. In this review we discuss the hypothesis that develpmental and/or seasonal changes in NMDA receptor function and the availability of new synapses may modulate thresholds for plasticity and thereby define sensitive periods for vocal learning.

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.

Development of MK-801, kainate, AMPA, and muscimol binding sites and the effect of dark rearing in rat visual cortex

We used quantitative autoradiography to determine whether the development of glutamate receptors correlates with the plastic period for monocular deprivation in rat visual cortex. To study glutamate receptors, we incubated sections of rat 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. To compare glutamate binding sites with a nonglutamate binding site, we studied [3H]muscimol, which binds to gamma-aminobutyric acid (GABA)A receptors. [3H]MK-801 binding was maximal at postnatal day 26 (P26) and decreased in adulthood. [3H]AMPA binding was maximal at P18. [3H]kainate binding and [3H]muscimol binding were not age dependent. Dark rearing partially prevented the age-dependent decrease in [3H]MK-801 binding but had no effect on [3H]kainate or [3H]AMPA binding. Dark rearing decreased muscimol binding in adult animals. These results suggest that NMDA receptors, but not other glutamate receptors or GABAA receptors, are likely to be critical for developmental plasticity in rat visual cortex.

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.

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.

Glutamate receptors and development of the visual cortex: effect of metabotropic agonists on cAMP

Glutamate receptors are more active in several respects in young animals than in adults. Here we examine the effect of metabotropic glutamate agonists on rat cortical cAMP during and after the critical period for visual cortex plasticity. Quisqualate produced a substantial increase in cAMP, which was larger during the critical period than in the adult. The increase was not affected by CNQX or APV, showing that it was not due to the action of quisqualate on ionotropic glutamate receptors. Both Type I mGluRs (mGluRs 1 and/or 5) and Type II mGluRs (mGluRs 2 and/or 3) probably contributed to the cAMP increase because (i) ACPD and L-CCG-I, which are more active on Type II mGluRs, were more effective than DHPG, which is more active on Type I mGluRs; and (ii) there was a significant difference in the effect of ACPD on the increase in cAMP, comparing mGluR1 knockout mice with control mice. Agonists which produce large stimulation of cAMP production (ACPD, L-CCG-I), as well as L-AP4, also produced small attenuations of forskolin-stimulated cAMP, but only at high concentrations. Thus, we conclude that it is the stimulation and/or potentiation of cAMP production that is significant, rather than the attenuation of forskolin-stimulated cAMP. Since this stimulation and/or potentiation is higher during the critical period than in the adult, and the cAMP second messenger system has been implicated in hippocampal plasticity, it may also play a role in visual cortex plasticity.

Treatment with NMDA receptor antagonists does not affect developmental changes in NMDA receptor properties in vivo

Effects of acute and long-term treatment of neonatal rats with N-methyl-D-aspartate (NMDA) receptor antagonists on changes in NMDA receptor properties were examined. Animals received either on postnatal day 6 a single dose of the antagonists MK-801 (1 mg/kg), or D-CPPene (2 mg/kg) or during the period from postnatal day 5 to 14, two daily injections of MK-801 (0.25 mg/kg) or D-CPPene (0.75 mg/kg). Control littermates received saline injections. In both cases animals were sacrificed one day after the last injection. NMDA receptor properties were examined in membrane preparations derived from the cerebral cortex by studying the modulation of [3H]MK-801 binding by glutamate, Mg2+ and D-CPPene. The density of agonist- and antagonist-binding sites in the CA1 region of the hippocampus were determined by autoradiography, using [3H]CGP39653 or [3H]glutamate as ligands. A significant developmental increase in NMDA receptor binding sites was detected both in preparations of cerebral cortical membranes and in the CA1 area of the hippocampus. In addition, the Mg2+ sensitivity of [3H]MK-801 binding was significantly higher in membrane preparations from the cerebral cortex of postnatal day 15 compared to postnatal day 7 animals. Neither the single nor the subchronic treatment with NMDA receptor antagonists exerted a significant influence on the density of antagonist binding sites or on the modulation of [3H]MK-801 binding by glutamate, Mg2+ or D-CPPene. We conclude therefore that neonatal treatment with NMDA receptor antagonists in vivo does not involve significant alterations in the properties and the densities of NMDA receptors in the brain regions studies, i.e., during the period when expression of these receptors is subject to pronounced developmental regulation.

NMDA receptors in mouse barrel cortex during normal development and following vibrissectomy

The development of N-methyl-D-aspartate (NMDA) receptors and the effects of vibrissectomy upon [3H]MK-801 binding were examined in the barrel cortex of mice. Autoradiographic studies showed that initially very low binding of [3H]MK-801 sharply increased during the second postnatal week reaching the adult level by the end of the third week. Scatchard analysis performed on cortical membrane preparations indicated that this rise of [3H]MK-801 labelling was due to an increase in the number of binding sites and a decrease of Kd at postnatal day 15 and 28. The interlaminar differences of labelling were registered from postnatal day 8. Changes of interlaminar distribution were found during the second and third postnatal weeks. In adult barrel cortex the highest binding was found in supragranular layers. In layer IV of the cortex, the pattern of binding resembled the pattern of barrels. Unilateral denervation of vibrissae performed in neonatal and adult mice did not alter the intensity of [3H]MK-801 labelling or the laminar distribution of binding sites. These results suggest that NMDA receptor binding does not reflect the plastic changes occurring in the barrel cortex.

The intensity of a fetal taste aversion is modulated by the anesthesia used during conditioning

Rat fetuses (E18) can learn a taste aversion in utero if experience with a sweet flavor (saccharin = Sac) is followed by a malaise-producing injection of lithium chloride (LiCl). Here we report that this phenomenon can be significantly modulated by the type of anesthesia administered to the pregnant dam before the conditioning procedure. Dams were anesthetized with one of the following drugs or drug combinations: (1) sodium pentobarbital; (2) ketamine hydrochloride and xylazine; or (3) sodium pentobarbital and ketamine hydrochloride. While under the influence of these anesthetics, rat fetuses received pairings of Sac + LiCl or one of the following sets of oral and systemic (i.p.) control injections: Sac + Saline, H2O + LiCl; H2O + Saline. At age 15 days neonatal rats were given a taste preference test by allowing them to select nipples painted with either saccharin or vehicle (H2O). After weaning, rats were given an additional taste preference test where they were allowed to drink from bottles filled with either 0.30% saccharin or water. Neonates that received Sac + LiCl injections avoided saccharin-painted nipples while neonates that received control injections in utero preferred saccharin-painted nipples. Rats that acquired the taste aversion under the influence of ketamine showed a significantly stronger conditioned taste aversion on the nipple preference test than did those from dams injected with sodium pentobarbital. The conditioned taste aversion was not detectable later during the bottle preference test. Since ketamine blocks N-methyl-D-aspartate (NMDA) glutamate receptors, and these receptors have been implicated in neural plasticity during development, our data suggest that NMDA antagonism can potentiate fetal learning. Ketamine has been used as an obstetrical and pediatric anesthetic.(ABSTRACT TRUNCATED AT 250 WORDS)

The development of MK-801, kainate, AMPA, and muscimol binding sites in cat visual cortex

Previous work using homogenate binding has shown that the development of (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10imin e maleate (MK-801) binding in cat visual cortex increases from 21 days to 42 days, the height of the plastic period, and decreases in adulthood. We have studied the generality of this finding by examining the development of NMDA binding sites in several brain regions and by examining the development of other binding sites in the visual cortex. After confirming the original finding, we extended it by showing that the sensitivity of MK-801 binding sites to glutamate and glycine decreases when the cat becomes an adult. We then examined the regional specificity of MK-801 binding. Retinal binding did not change significantly with age. Binding in both visual cortex and hippocampus increased significantly from 7 days to 42 days regardless of whether binding was measured per milligram wet weight or per milligram protein. The decline from 42 days to adulthood was less dramatic in the hippocampus than in the visual cortex and was statistically significant only when binding was measured per milligram protein. Saturation analyses also showed a difference in the two structures. Bmax in the visual cortex, but not in the hippocampus, decreased from 42 days to adulthood. To determine whether these developmental changes were specific to MK-801 binding sites, we compared the age-dependent binding of MK-801, kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and muscimol. Like MK-801, kainate binding increased from 7 days to 42 days and decreased from 42 days to adulthood. AMPA and muscimol binding showed a similar increase in binding from 7 days to 42 days but did not decrease significantly from 42 days to adulthood. Displacement experiments suggest that AMPA and kainate bind to separate sites. The 42-day peak in NMDA and kainate binding suggests that their associated receptors may have a role in determining the plastic period of visual cortex.

The ontogeny of glutamate receptors in rat barrel field cortex

The ontogeny of N-methyl-D-aspartate (NMDA) and non-NMDA excitatory amino acid receptors in rat barrel field cortex were characterized using receptor autoradiography. NMDA receptors showed a different pattern of development than that of non-NMDA receptors recognizing quisqualate (QUIS sites). During the first 14 days, high densities of QUIS sites were localized in barrel centers forming a sensory map of the rat whisker pad. After that time, the density of QUIS sites in barrel centers decreased so that the pattern was no longer apparent by postnatal day 21. In contrast to QUIS sites, NMDA sites did not exhibit a somatotopic pattern until postnatal day 21, when the lower density of sites in barrel septa formed an outline of barrel centers. At all ages examined, the density of NMDA sites did not differ significantly between barrel centers and surrounding cortex. Of the non-NMDA receptors examined in the postnatal day 10 old rat, both metabotropic sites and the NNKQ sites, which are [3H]glutamate binding sites that are not displaceable by NMDA, kainate or QUIS, showed a pattern of higher densities in barrel centers than surrounding tissue, whereas AMPA sites exhibited a complementary pattern. [3H]Glutamate binding to metabotropic sites was not significantly displaced by QUIS, whereas both NNKQ sites and metabotropic sites were potently blocked by the metabotropic agonist trans-ACPD. These results suggest that the NNKQ sites are low affinity QUIS metabotropic receptors, which, due to their high density in the immature barrel field, are in a position to influence barrel formation.

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.

Regulation of N-methyl-D-aspartate (NMDA) receptor function during the rearrangement of developing neuronal connections

There is evidence from a number of studies that the molecular and biophysical properties of NMDA receptors are altered during normal development. A temporal correlation with changes in NMDA receptor efficacy and periods of synaptic plasticity has been demonstrated in several systems, suggesting that NMDA receptors have a critical function in determining periods of synaptic plasticity. Data from our laboratory demonstrate reduced NMDA sensitivity of the tectal evoked potential following chronic application of NMDA to the tadpole tectum, a treatment that may mimic a naturally occurring mechanism for limiting neuronal plasticity to certain stages of development. Our analysis of the expression pattern of mRNA coding for various glutamate receptor subunits in the rat retinocollicular system establishes that differential regulation of NMDA receptor subunits at the mRNA level could be a molecular basis for changes in biophysical and pharmacological properties of the NMDA receptor complex. However, even though the NMDA receptor is the best studied candidate to function as a 'plasticity switch', there are large gaps in our understanding of the complete set of factors that control the ability of synapses to rearrange during development.

Visual deprivation decreases long-term potentiation in rat visual cortical slices

A major finding in the visual plasticity literature is that visual deprivation is effective only during an early 'sensitive' period, which is lengthened by dark rearing. Unresolved is whether the visual cortex is in a normally plastic state prior to light stimulation. This cannot be addressed using paradigms employing light exposure to assess plasticity. Several developmental studies have investigated a plastic phenomenon termed long-term potentiation (LTP) in slices from cat (J. Neurophysiol., 59 (1988) 124-141) and rat (Brain Res., 439 (1988) 222-229) visual cortex. Susceptibility to the induction of LTP parallels the period of sensitivity to visual deprivation. This suggests that slices can be used to assay visual cortical plasticity, avoiding light exposure. In the present study, field potentials were recorded from slices of the primary visual cortices of dark-reared (DR) and control (CONT) Long Evans hooded rats (17 to 21 days). Field potential profiles recorded before and 90 min following tetanic electrical stimulation were subjected to current source density analysis, yielding extracellular current sink amplitudes. Tetanus resulted in LTP in both CONT and DR slices, but DR slices were significantly less potentiated. These results indicate that the primary visual cortex of DR animals is not fully plastic, indicating a role for light stimulation in inducing visual cortical plasticity.

Laminar distribution of NMDA receptors in cat and monkey visual cortex visualized by [3H]-MK-801 binding

Glutamate is the major excitatory neurotransmitter of the mammalian central nervous system. Two major classes of glutamate receptors have been reported. The actions of glutamate on its N-methyl-D-aspartate (NMDA)-type receptor may underlie developmental and adult plasticity as well as neurotoxicity. The NMDA-type of glutamate receptor in cat and monkey visual cortex was visualized by means of in vitro receptor autoradiography with the noncompetitive NMDA-receptor antagonist [3H]-MK-801. The kinetics, performed on tissue sections, revealed an apparently single, saturable site with an approximate dissociation constant (KD) of 18.5 nM in cat and 15.9 nM in monkey visual cortex. Autoradiography, performed on frontal sections of cat and monkey visual cortex, revealed a heterogeneous laminar distribution of NMDA receptors. Cat areas 17, 18, 19, and the lateral suprasylvian areas exhibited a similar NMDA-receptor distribution. In these areas, NMDA receptors were most prominent in layer II and the upper part of layer III. In monkey striate cortex, NMDA receptors were primarily concentrated in layers II, upper III, IVc, V, and VI. In monkey secondary visual cortex, [3H]-MK-801 labeling was most prominent in layers II, V, and VI; whereas in the temporal visual areas included in this study layer II displayed the heaviest receptor labeling. In neither cat nor monkey could we observe significant differences in NMDA-receptor distribution between different retinotopic subdivisions within a single visual area. Neither did we detect any periodic changes in NMDA-receptor distribution that would correspond to the compartments defined by cytochrome-oxidase in monkey V1 and V2.

Do NMDA receptors have a critical function in visual cortical plasticity?

The theoretical framework, by which we understand the function of NMDA receptors, is derived, in large part, from work conducted on the hippocampal slice preparation, where NMDA receptors are crucial for a form of synaptic plasticity known as long-term potentiation (LTP). Establishing their role in plasticity mechanisms in the neocortex is proving to be far more difficult than originally envisaged, in part due to the fact that the operation of NMDA receptors is different in the intact animal than in vitro. For example, NMDA receptors are activated at low levels of sensory input in intact animals but only by high levels of input in slice preparations. Recent results suggest that a re-evaluation of the role of NMDA receptors in neocortical plasticity is required. Here we discuss some of the issues and introduce four criteria by which any factor supposedly involved in plasticity can be judged. NMDA receptors fulfill more of these criteria than any of the other factors so far investigated in the visual cortex, but maybe this is because they have been studied more intensively.