Signalling routes and developmental regulation of group I metabotropic glutamate receptors in rat auditory midbrain neurons

Group I metabotropic glutamate receptors (mGluRs) are linked to intracellular Ca(2+) signalling and play important roles related to synaptic plasticity and development. In neurons from the central nucleus of the inferior colliculus (CIC), the activation of these receptors evokes large [Ca(2+) ](i) responses. By using optical imaging of the fluorescent Ca(2+) -sensitive dye Fura-2, we have explored which [Ca(2+) ](i) routes are triggered by group I mGluR activation in young CIC neurons and whether mGluR-induced [Ca(2+) ](i) responses are regulated during postnatal development. In addition, real-time quantitative RT-PCR was used to study the developmental expression of both group I mGluR subtypes, mGluR1 and mGluR5. Application of DHPG, a specific agonist of group I mGluRs, was used on CIC slices from young rats to elicit [Ca(2+) ](i) responses. A majority of responses consisted of an initial thapsigargin-sensitive Ca(2+) peak, related to store depletion, followed by a plateau phase, sensitive to the store-operated Ca(2+) entry blocker 2-APB. During postnatal development, from P6 to P17, DHPG-induced [Ca(2+) ](i) responses changed. The largest Ca(2+) responses were reached at P6, whereas lower peak and plateau responses were found after hearing onset, at P13-P14 and P17. qRT-PCR analysis also revealed important differences in the expression of both mGluR1 and mGluR5 subtypes during development, with the highest levels of both subtypes at P7 and a developmental decrease of both transcripts. Our results suggest both intra- and extracellular routes for [Ca(2+) ](i) increases linked to group I mGluRs in CIC neurons and a regulation of group I mGluR activity and expression during auditory development.

Temporal and regional alterations in NMDA receptor expression in Mecp2-null mice

Our previous postmortem study of girls with Rett Syndrome (RTT), a development disorder caused by MECP2 mutations, found increases in the density of N-Methyl-D-aspartate (NMDA) receptors in the prefrontal cortex of 2-8-year-old girls, whereas girls older than 10 years had reductions in NMDA receptors compared with age-matched controls (Blue et al., Ann Neurol 1999b;45:541-545). Using [(3)H]-CGP to label NMDA-type glutamate receptors in 2- and 7-week old wild-type (WT), Mecp2-null, and Mecp2-heterozygous (HET) mice (Bird model), we found that frontal areas of the brain also exhibited a bimodal pattern in NMDA expression, with increased densities of NMDA receptors in Mecp2-null mice at 2 weeks of age but decreased densities at 7 weeks of age. Visual cortex showed a similar pattern, while other cortical regions only exhibited changes in NMDA receptor densities at 2 weeks (retrosplenial granular) or 7 weeks (somatosensory). In thalamus of null mice, NMDA receptors were increased at 2 and 7 weeks. No significant differences in density were found between HET and WT mice at both ages. Western blots for NMDAR1 expression in frontal brain showed higher levels of expression in Mecp2-null mice at 2 weeks of age but not at 1 or 7 weeks of age. Our mouse data support the notion that deficient MeCP2 function is the primary cause of the NMDA receptor changes we observed in RTT. Furthermore, the findings of regional and temporal differences in NMDA expression illustrate the importance of age and brain region in evaluating different genotypes of mice.

Roles of mGluR5 in synaptic function and plasticity of the mouse thalamocortical pathway

The group I metabotropic glutamate receptor 5 (mGluR5) has been implicated in the development of cortical sensory maps. However, its precise roles in the synaptic function and plasticity of thalamocortical (TC) connections remain unknown. Here we first show that in mGluR5 knockout (KO) mice bred onto a C57BL6 background cytoarchitectonic differentiation into barrels is missing, but the representations for large whiskers are identifiable as clusters of TC afferents. The altered dendritic morphology of cortical layer IV spiny stellate neurons in mGluR5 KO mice implicates a role for mGluR5 in the dendritic morphogenesis of excitatory neurons. Next, in vivo single-unit recordings of whisker-evoked activity in mGluR5 KO adults demonstrated a preserved topographical organization of the whisker representation, but a significantly diminished temporal discrimination of center to surround whiskers in the responses of individual neurons. To evaluate synaptic function at TC synapses in mGluR5 KO mice, whole-cell voltage-clamp recording was conducted in acute TC brain slices prepared from postnatal day 4-11 mice. At mGluR5 KO TC synapses, N-methyl-D-aspartate (NMDA) currents decayed faster and synaptic strength was more easily reduced, but more difficult to strengthen by Hebbian-type pairing protocols, despite a normal developmental increase in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated currents and presynaptic function. We have therefore demonstrated that mGluR5 is required for synaptic function/plasticity at TC synapses as barrels are forming, and we propose that these functional alterations at the TC synapse are the basis of the abnormal anatomical and functional development of the somatosensory cortex in the mGluR5 KO mouse.

Group II metabotropic glutamate receptors reduce excitatory but not inhibitory neurotransmission in rat barrel cortex in vivo

Group II metabotropic (mGlu) receptors are known to play an important role in regulating the release of excitatory transmitter in a number of brain areas. Previous experiments demonstrated that (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) depressed excitatory transmission in the adult rat barrel cortex. Here we show, using in vivo extracellular single unit recordings and iontophoretic application of drugs, that selective activation of Group II mGlu receptors depresses excitatory but not inhibitory transmission. The selective Group II receptor agonist (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC) had similar depressant effects to 1S,3R-ACPD on tactile evoked responses of rapidly adapting neurons. The depressant effects were seen on shorter latency (<12 ms) responses, were most pronounced in layers 3-4 (and 5b for 2R,4R-APDC only), and were reversibly antagonized by the Group II receptor antagonist (2S)-alpha-ethylglutamic acid (EGLU) relative to depressions produced by iontophoretic GABA. Where 1S,3R-ACPD and 2R,4R-APDC depressed excitatory transmission, there was little or no effect on postsynaptic excitations produced by iontophoretic AMPA--a result that supports a presynaptic location of Group II receptors on excitatory terminals. To assess the possible involvement of Group II mGlu receptors in the modulation of inhibition, we studied the effect of iontophoretic 1S,3R-ACPD in a condition-test protocol. The results contrasted markedly from those previously observed using the Group III agonist L(+)-2-amino-4-phosphonobutyric acid in that activation of Group II receptors using 1S,3R-ACPD did not modulate inhibition. Therefore our results show that Group II mGlu receptors play an important role in modulating excitatory, but not inhibitory, transmission. We propose that the Group II mGlu receptors are located on excitatory terminals, and act as autoreceptors. Their role appears to be important in the early stages of cortical processing, by keeping excitatory inputs within specified physiological limits, and possibly by mediating depression evidenced during synaptic plasticity.

Early Postnatal Plasticity in Neocortex of Fmr1 Knockout Mice

Fragile X syndrome is produced by a defect in a single X-linked gene, called Fmr1, and is characterized by abnormal dendritic spine morphologies with spines that are longer and thinner in neocortex than those from age-matched controls. Studies using Fmr1 knockout mice indicate that spine abnormalities are especially pronounced in the first month of life, suggesting that altered developmental plasticity underlies some of the behavioral phenotypes associated with the syndrome. To address this issue, we used intracellular recordings in neocortical slices from early postnatal mice to examine the effects of Fmr1 disruption on two forms of plasticity active during development. One of these, long-term potentiation of intrinsic excitability, is intrinsic in expression and requires mGluR5 activation. The other, spike timing-dependent plasticity, is synaptic in expression and requires N-methyl-d-aspartate receptor activation. While intrinsic plasticity was normal in the knockout mice, synaptic plasticity was altered in an unusual and striking way: long-term depression was robust but long-term potentiation was entirely absent. These findings underscore the ideas that Fmr1 has highly selective effects on plasticity and that abnormal postnatal development is an important component of the disorder.

Metabotropic glutamate receptors

Metabotropic glutamate receptors (mGlus) are a family of G-protein-coupled receptors activated by the neurotransmitter glutamate. Molecular cloning has revealed eight different subtypes (mGlu1-8) with distinct molecular and pharmacological properties. Multiplicity in this receptor family is further generated through alternative splicing. mGlus activate a multitude of signalling pathways important for modulating neuronal excitability, synaptic plasticity and feedback regulation of neurotransmitter release. In this review, we summarize anatomical findings (from our work and that of other laboratories) describing their distribution in the central nervous system. Recent evidence regarding the localization of these receptors in peripheral tissues will also be examined. The distinct regional, cellular and subcellular distribution of mGlus in the brain will be discussed in view of their relationship to neurotransmitter release sites and of possible functional implications.

Anticonvulsant action of an antagonist of metabotropic glutamate receptors mGluR5 MPEP in immature rats

Antagonists of type I of metabotropic glutamate receptors exhibit anticonvulsant action in adult as well as immature rodents. To know the anticonvulsant profile of a specific mGluR5 antagonist MPEP in developing rats, two models of epileptic seizures were used. MPEP (10, 20 or 40 mg/kg i.p.) suppressed in a dose-dependent manner epileptic afterdischarges induced by electrical stimulation of sensorimotor cortical area in three age groups (12, 18 and 25 days old). The anticonvulsant action was more expressed in the youngest group than in older animals so that in 25-day-old rats an additional dose of 80 mg/kg was used. In contrast to this marked anticonvulsant action, MPEP at a dose of 40 mg/kg i.p. in 18-day-old rat pups and at doses of 40 and 80 mg/kg in 25-day-old rat pups did not affect episodes of spike-and-wave rhythm elicited by low doses of pentetrazol. Our results delineate the profile of the anticonvulsant action of MPEP and confirm the higher efficacy of this antagonist at early developmental stages in comparison with prepubertal animals.

Activity-dependent expression of Egr1 mRNA in somatosensory cortex of developing rats

The rat barrel field in somatosensory cortex is a well-characterized model of neocortical development, with activity-dependent and activity-independent components. Egr1 encodes an inducible transcription factor that is required for certain forms activity-dependent plasticity. This study examines Egr1 mRNA expression in the developing barrel field under basal conditions and after short-term deprivation or stimulation of whiskers. Egr1 mRNA was measured with in situ hybridization at postnatal Day (P) 6, P9, P12, P15, and P21. For short-term deprivation, whiskers were trimmed close to the skin and Egr1 mRNA was examined 3 hr later. For controlled stimulation of a single whisker, surrounding whiskers were trimmed, a wire was glued to the designated whisker, and animals were placed in an AC magnetic field pulsed at 2 Hz, 10 mT rms for 15 min. Egr1 mRNA was examined 30 min later. At P6, basal Egr1 mRNA in the barrel field was very low and was increased only slightly by stimulation (P < 0.05). At each of the later ages, there was a large increase in Egr1 mRNA in stimulated versus deprived barrels (P < 0.001). Egr1 mRNA expression after whisker stimulation increased exponentially with age through P15 (P < 0.001) and then declined between P15 and P21. The onset of Egr1 responses to whisker stimulation at P9 and the striking increase in activity-dependent Egr1 mRNA expression in the second postnatal week suggest that this transcription factor may play a role in activity-dependent processes that occur in this developmental period, such as maturation of barrel cortex circuitry.

Molecular biology and ontogeny of glutamate receptors in the mammalian central nervous system

Glutamate is the principal excitatory neurotransmitter in the mammalian central nervous system. After release from presynaptic terminals, glutamate binds to both ionotropic and metabotropic receptors to mediate fast, slow, and persistent effects on synaptic transmission and integrity. There are three types of ionotropic glutamate receptors. N-Methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA), and kainate receptors are principally activated by the agonist bearing its name and are permeable to cationic flux; hence, their activation results in membrane depolarization. All ionotropic glutamate receptors are believed to be composed of four distinct subunits, each of which is topologically arranged with three transmembrane-spanning and one pore-lining (hairpin loop) domain. In contrast, metabotropic glutamate receptors are G protein (guanine nucleotide-binding protein) -coupled receptors linked to second-messenger systems. Group I metabotropic glutamate receptors are linked to phospholipase C, which results in phosphoinositide hydrolysis and release of calcium from intracellular stores. Group II and group III metabotropic glutamate receptors are negatively linked to adenylate cyclase, which catalyzes the production of cyclic adenosine monophosphate. Each metabotropic glutamate receptor is composed of seven transmembrane-spanning domains, similar to other members of the superfamily of metabotropic receptors, which includes noradrenergic, muscarinic acetylcholinergic, dopaminergic, serotonergic (except type 3 receptors), and gamma-aminobutyric acid (GABA) type B receptors. This review summarizes the relevant molecular biology and ontogeny of glutamate receptors in the central nervous system and highlights some of the roles that they can play during brain development and in certain disease states.

Effects of genetic depletion of monoamines on somatosensory cortical development

Raised levels of serotonin cause alterations in the development of the barrelfield of the primary somatosensory cortex (S1) in rodents. We examined the development of S1 in genetic mouse models in which the levels of serotonin and/or dopamine and noradrenaline are drastically reduced. Mice lacking the vesicular monoamine transporter type 2 (VMAT2 KO) are hypomorphic with rare pups surviving until postnatal day (P) 6. Serotonin, dopamine and noradrenaline are almost undetectable in the brain. In S1 we find that the segregation of thalamocortical axons into whisker patterns is delayed by 1 day and that layer IV granular neurons fail to form normal barrels. Moreover, the growth of cortical layers II-IV is reduced. Despite severe malnutrition, we show that these alterations are not caused by increased cell death in the thalamus or S1. Moreover, the maturation of cortical neurons is not altered as reflected by calcium-binding protein immunolabeling. Mice lacking both VMAT2 and monoamine oxidase type A (MAOA) were generated. VMAT2-MAOA DKO mice are hypomorphic but survive until P13. Increased levels of serotonin but profoundly reduced dopamine and noradrenaline levels are found in the brains. In S1, alterations are similar to those observed in MAOA KO mice: thalamocortical axons and granular neurons failed to form barrels. In addition there is a severe reduction in the thickness of the upper cortical layers as in the VMAT2 KO mice. These results show that monoamines have no instructive effect per se on the formation of thalamocortical patterning in S1. However, monoamines appear to be essential for the normal cytoarchitectonic maturation of the granular (IV) and supragranular cortical layers (II-III). Since developmental cell death and chemoarchitectonic differentiation of these neurons are not modified, it is possible that these alterations result from migration defects and/or from altered synaptic maturation.

Differential localization of metabotropic glutamate receptors during postnatal development

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

Neonatal electrolytic lesions of the basal forebrain stunt plasticity in mouse barrel field cortex

Previous studies have shown that neonatal electrolytic lesions of basal forebrain cholinergic projections in mice lead to a transient cholinergic depletion of neocortex and to permanent alterations in cortical cytoarchitecture and in cognitive performance. The present study examines whether neonatal electrolytic lesions of the basal forebrain modify neocortical plasticity. Using cytochrome oxidase histochemistry, we compared cross-sectional areas of individual barrels in the barrel field of four groups of postnatal day 8 (P8) old mice that on P1 received either (1) right electrolytic lesions of the basal forebrain, (2) left C row 1-4 whisker follicle ablations, (3) combined lesion treatments or (4) ice anesthesia only. The size of barrels in basal forebrain lesioned animals was not significantly different from controls. However, the plastic response to whisker removal was compromised in basal forebrain lesioned animals. An index of plasticity, the ratio of row D/row C areas, was reduced significantly in the combined nBM lesioned/follicle ablation group. Compared to whisker-lesioned mice, the expansion in rows B and D and the shrinkage in the lesioned row C area were diminished in the combined treatment group. The present findings correspond to those from a study of rats injected with a cholinergic immunotoxin [Cereb. Cortex 8 (1998) 63]. These results suggest that cholinergic inputs play a role in regulating plasticity as well as in the morphogenesis of mouse sensory-motor cortex.

New concepts in neonatal seizures

The immature brain is more prone to seizures than the older brain as a result of an imbalance between excitatory and inhibitory input. The depolarizing, rather than hyperpolarizing effect of GABA(A) during the first week of life in the rodent, and the delay in postsynaptic GABA(B) inhibition coupled with the over-expression of glutamatergic synapses contribute to this increased propensity toward seizures. It is now clear that seizures can be injurious to the immature brain, although the pattern of seizure-induced injury is age-related. While the immature brain is resistant to acute seizure-induced cell loss, there are functional abnormalities following seizures with impairment of visual-spatial memory and reduced seizure threshold. Neonatal seizures are also associated with a number of activity-dependent changes in brain development including altered synaptogenesis and reduction in neurogenesis. These results argue that neonatal seizures should no longer be considered as benign events.

Neural activity: sculptor of 'barrels' in the neocortex

A major portion of the primary somatosensory cortex of rodents is characterized by the discrete and patterned distribution of thalamocortical axons and layer IV granule cells ('barrels'), which correspond to the spatial distribution of whiskers and sinus hairs on the snout. In recent years several mutant mouse models began unveiling the cellular and molecular mechanisms by which these patterns emerge presynaptically and are reflected postsynaptically. Neural activity plays a crucial role in conferring presynaptic patterns to postsynaptic cells via neurotransmitter receptor-mediated intracellular signals. Here we review recent evidence that is finally opening the doors to understanding the cellular and molecular mechanisms of pattern formation in the neocortex.

PLC-beta1, activated via mGluRs, mediates activity-dependent differentiation in cerebral cortex

During development of the cerebral cortex, the invasion of thalamic axons and subsequent differentiation of cortical neurons are tightly coordinated. Here we provide evidence that glutamate neurotransmission triggers a critical signaling mechanism involving the activation of phospholipase C-beta1 (PLC-beta1) by metabotropic glutamate receptors (mGluRs). Homozygous null mutation of either PLC-beta1 or mGluR5 dramatically disrupts the cytoarchitectural differentiation of 'barrels' in the mouse somatosensory cortex, despite segregation in the pattern of thalamic innervation. Furthermore, group 1 mGluR-stimulated phosphoinositide hydrolysis is dramatically reduced in PLC-beta1-/- mice during barrel development. Our data indicate that PLC-beta1 activation via mGluR5 is critical for the coordinated development of the neocortex, and that presynaptic and postsynaptic components of cortical differentiation can be genetically dissociated.

Neonatal lead exposure impairs development of rodent barrel field cortex

Childhood exposure to low-level lead can permanently reduce intelligence, but the neurobiologic mechanism for this effect is unknown. We examined the impact of lead exposure on the development of cortical columns, using the rodent barrel field as a model. In all areas of mammalian neocortex, cortical columns constitute a fundamental structural unit subserving information processing. Barrel field cortex contains columnar processing units with distinct clusters of layer IV neurons that receive sensory input from individual whiskers. In this study, rat pups were exposed to 0, 0.2, 1, 1.5, or 2 g/liter lead acetate in their dam's drinking water from birth through postnatal day 10. This treatment, which coincides with the development of segregated columns in the barrel field, produced blood lead concentrations from 1 to 31 microg/dl. On postnatal day 10, the area of the barrel field and of individual barrels was measured. A dose-related reduction in barrel field area was observed (Pearson correlation = -0.740; P < 0.001); mean barrel field area in the highest exposure group was decreased 12% versus controls. Individual barrels in the physiologically more active caudoventral group were affected preferentially. Total cortical area measured in the same sections was not altered significantly by lead exposure. These data support the hypothesis that lead exposure may impair the development of columnar processing units in immature neocortex. We demonstrate that low levels of blood lead, in the range seen in many impoverished inner-city children, cause structural alterations in a neocortical somatosensory map.

Dopamine D(3) receptor is selectively and transiently expressed in the developing whisker barrel cortex of the rat

The rodent primary somatosensory cortex (SI) contains a map of the body surface, the most conspicuous part of which are "barrels," neuronal aggregates in layer IV that receive somatotopic projections from whiskers on the rodent's snout. We report that the D(3) dopamine receptor (D(3)R) is selectively and transiently expressed in SI during the first 2 weeks of postnatal development. D(3)R binding sites and mRNA overlap completely and are limited to layer IV of SI. D(3)R/mRNA are organized in a pattern corresponding to somatotopic representations of the body (e.g., whiskers, jaws, paws, etc.) with the highest expression in the barrel field. D(3) mRNA is first detected at postnatal day (P)4, increases rapidly until P7-10, and sharply decreases after P14. D(3)R binding sites are detectable at P6, peak at P14, and decline afterwards. D(1), D(2), D(4), or D(5) mRNAs display dissimilar expression pattern. D(1) mRNA is mostly confined to infragranular layers throughout the cortex. D(4) mRNA expression in layer IV rises by 4 weeks postnatal, when D(3)R expression is virtually undetectable. Quantitative analysis of D(3) mRNA expression demonstrates that the proportion of D(3) mRNA-positive cells decreases between P7 and P14, whereas mRNA concentration per cell remains stable. Moreover, D(3)R number continues to rise, whereas mRNA levels begin to decline. Thus, a process limiting D(3)R expression to fewer cells may occur that also induces changes in post-transcriptional regulation of D(3)R expression in remaining cells. These findings indicate that dopamine acting via D(3)R may play an important role in the development or function of the SI.

Apoptosis and necrosis occur in separate neuronal populations in hippocampus and cerebellum after ischemia and are associated with differential alterations in metabotropic glutamate receptor signaling pathways

It was evaluated whether postischemic neurodegeneration is apoptosis and occurs with alterations in phosphoinositide-linked metabotropic glutamate receptors (mGluRs) and their associated signaling pathways. A dog model of transient global incomplete cerebral ischemia was used. The CA1 pyramidal cells and cerebellar Purkinje cells underwent progressive delayed degeneration. By in situ end-labeling of DNA, death of CA1 and Purkinje cells was greater at 7 days than 1 day after ischemia, whereas death of granule neurons in dentate gyrus and cerebellar cortex was greater at 1 than at 7 days. Ultrastructurally, degenerating CA1 pyramidal neurons and cerebellar Purkinje cells were necrotic; in contrast, degenerating granule neurons were apoptotic. In agarose gels of regional DNA extracts, random DNA fragmentation coexisted with internucleosomal fragmentation. By immunoblotting of regional homogenates, mGluR1alpha, mGluR5, phospholipase Cbeta (PLCbeta), and Galphaq/11 protein levels in hippocampus at 1 and 7 days after ischemia were similar to control levels, but in cerebellar cortex, mGluR1alpha and mGluR5 were decreased but PLCbeta was increased. By immunocytochemistry, mGluR and PLCbeta immunoreactivity dissipated in CA1 and cerebellar Purkinje cell/ molecular layers, whereas immunoreactivities for these proteins were enhanced in granule neurons. It was concluded that neuronal death after global ischemia exists as two distinct, temporally overlapping forms in hippocampus and cerebellum: necrosis of pyramidal neurons and Purkinje cells and apoptosis of granule neurons. Neuronal necrosis is associated with a loss of phosphoinositide-linked mGluR transduction proteins, whereas neuronal apoptosis occurs with increased mGluR signaling.

Altered development of glutamate and GABA receptors in the basal ganglia of girls with Rett syndrome

Rett syndrome (RS), a genetic disorder found almost exclusively in females, is associated with psychomotor regression and stereotyped hand movements. To determine whether a defect in basal ganglia amino acid neurotransmission plays a role in RS, NMDA-, AMPA-, kainate (KA)-, and metabotropic (mGluR)-type glutamate receptors (GluRs) and GABA receptors were labeled autoradiographically in the caudate, putamen, and globus pallidus of postmortem brain slices from 9 RS girls and 10 age-related controls. The cases were divided into younger (8 years or younger) and older age groups to study age-related changes in receptor binding density. We found significant reductions in AMPA and NMDA receptor density in the putamen and in KA receptor density in the caudate of older RS cases compared to controls. In contrast, mGluR density in the basal ganglia of RS patients was not altered significantly. The density of GluRs in control subjects generally showed more limited changes with age than in RS cases. In contrast to ionotropic GluRs, GABA receptor density was significantly increased in the caudate of young RS patients. The effects on GluR density in the putamen, which serves a primary motor function, were consistent with the motor deficits observed in RS, while those on amino acid transmitter receptors in the caudate may account for some cognitive features. Our studies demonstrate regional, receptor-subtype, and age-specific alterations in amino acid neurotransmitter receptors in the basal ganglia of RS girls. These changes may correlate with age-related clinical stages observed in RS.

Development of amino acid receptors in frontal cortex from girls with Rett syndrome

To determine whether a disorder of excitatory neurotransmission plays a role in the pathophysiology of Rett syndrome (RS), N-methyl-D-aspartate (NMDA), adenosine monophosphate acid (AMPA), kainate, and metabotropic types of glutamate receptors were labeled autoradiographically in the superior frontal gyrus (SFG) from 9 RS patients and 10 female controls. The results showed a trend for the densities of NMDA, AMPA, gamma-aminobutyric acid, and metabotropic glutamate receptors to be higher in younger patients than in controls and for densities in older patients to fall below those of controls. The age-related changes in SFG NMDA receptor density may be correlated with the shift from psychomotor regression and seizures in younger stage II/III RS girls to the less epileptic plateau stage in older girls.

Maturation of NADPH-d activity in the rat's barrel-field cortex and its relationship to cytochrome oxidase activity

Histochemical detection of NADPH-d activity in rat barrel-field cortex reveals four types of distributions. (i) A transient, diffuse neuropil staining is visible in the cortical plate and in deeper layers until postnatal day (P) 4. Thereafter, until P15, it is segregated in whisker-specific patches in layer IV, then the pattern gradually disappears, becoming virtually indistinct by P21. This transient patterning of diffuse NADPH-d activity in layer IV disappears after cortical injections of kainic acid and is affected by neonatal damage to the contralateral snout. An intense labeling (ii) of scattered cells and (iii) of a plexus of fibers is present. With maturation, the cells become localized mostly in layers II/III, in the lower part of layer V, and in layer VI. They are sparse in layer I, in upper layer V, and in layer IV where their somata are located primarily in the interbarrel septa. (iv) Light staining of cortical neurons is detected mostly in layers II-IV but occasionally also in layers V-VI. Cytochrome c oxidase (CO)-positive patches associated with barrels are first detected in layer IV around P4-P5; their staining density increases with development, then stays high. In the adult, CO activity is moderate in supragranular layers, highest in the barrels in layer IV, low in upper layer V, medium dense in the deeper half of layer V, and low in lamina VI. Thus, NADPH-d and CO activities are not necessarily colocalized in the rodent barrel-field cortex. The varied (transient and long-lasting) distributions of NADPH-d activity indicate that the enzyme and its associated production of NO serve multiple roles in developing and adult barrel-field cortex.

Selective vulnerability of the developing brain to lead

Environmental lead exposure in young children who ingest household paint dust or other sources impairs their potential intelligence in a linear, dose-dependent fashion in contrast to its far more subtle effects on other neurologic functions. Basic investigations have identified three interrelated steps in synaptic neurotransmission at which low levels of lead can disrupt signal processing. Lead enhances background transmitter release, but impairs stimulated release, inhibits function at the N-methyl-D-aspartate-type glutamate receptor and stimulates background levels of the intracellular messenger protein kinase C. Taken together these effects have the effect of diminishing the synaptic signal to noise ratio. The ability of lead to enhance 'synaptic noise' during a critical early period of postnatal development may permanently disrupt the architecture of cortical processing units by depriving them of high resolution environmental signals needed to refine synaptic connections.

Effects of neonatal cholinergic basal forebrain lesions on excitatory amino acid receptors in neocortex

The role of cholinergic basal forebrain projections in the modulation of cortical plasticity and associated functional changes is currently the subject of renewed attention. Excitatory amino acid receptors have been identified as mediators of cortical topographic efferent and afferent information. In addition some of these receptors, notably the NMDA and metabotropic [mGluR] type, participate in cortical plasticity. Growing evidence suggests that interactions between cholinergic and glutamatergic systems contribute to cognitive cortical functions and their anatomical and physiological substrates. Though cholinergic and glutamatergic mechanisms have both been shown to be involved in cortical morphogenesis, few studies have attempted to study their interactions in development. The present study investigates the effect of neonatal lesions to the cholinergic basal forebrain on NMDA, AMPA and mGluR receptors in BALB/CByJ mice, at two different developmental ages. We demonstrated previously that nBM lesions at birth result in transient cholinergic depletion for the first two postnatal weeks, substantial morphogenetic alterations in neocortex and cognitive deficits by adulthood. We show here that unilateral neonatal lesions result in decreases in NMDA and AMPA receptors but increases in mGluRs during the second postnatal week (PND 14). At 30 days postnatal, lesion mediated changes were attenuated, compared with PND 14, but significant sex differences in control and nBM lesioned mice were apparent. These data support the notion that cholinergic/glutamatergic interactions are important during early cortical morphogenesis. Moreover, our results highlight the fact that cholinergic as well glutamatergic developmental mechanisms are sexually dimorphic.

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.