The ontogeny of glutamate receptors in rat barrel field cortex

Hypoxic-ischemic encephalopathy in the term infant

Hypoxia-ischemia in the perinatal period is an important cause of cerebral palsy and associated disabilities in children. There has been significant research progress in hypoxic-ischemic encephalopathy over the last 2 decades, and many new molecular mechanisms have been identified. Despite all these advances, therapeutic interventions are still limited. In this article the authors discuss several molecular pathways involved in hypoxia-ischemia, and potential therapeutic targets.

Postsynaptic deregulation in GAP-43 heterozygous mouse barrel cortex

Formation of whisker-related barrels in primary somatosensory cortex (S1) requires communication between presynaptic thalamocortical afferents (TCAs) and postsynaptic cortical neurons. GAP-43 is crucially involved in targeting TCAs to postsynaptic S1 neurons but its influence on the interactions between these 2 elements has not been explored. Here, we tested the hypothesis that reduced early expression of presynaptic GAP-43 (GAP-43 heterozygous [HZ] mice) alters postsynaptic differentiation of barrel cells. We found a transient increase in cytochrome oxidase staining between P6 and P14 in HZ animals, indicative of increased metabolic activity in barrel cortex during this time. Golgi impregnation and microtubule-associated protein 2 immunohistochemistry showed anomalous dendritic patterning in GAP-43 HZ cortex at P5, with altered dendritic length and branching and abnormal retention of dendrites that extend into developing septa. This deficiency was no longer apparent at P7, suggesting partial recovery of dendritic pruning processes. Finally, we showed early defects in synaptogenesis from P4 to P5 with increased colocalization of NR1 and GluR1 staining in HZ mice. By P7, this colocalization had normalized to wild type levels. Taken together, our findings suggest abnormal postsynaptic differentiation in GAP-43 HZ cortex during early barrel development, followed by adaptive compensation and partial phenotypic rescue.

The minicolumnopathy of autism: A link between migraine and gastrointestinal symptoms

Gastrointestinal symptoms are common medical problems among autistic patients. A leaky gut and viruses have been proposed as possible culprits but evidence for these etiological agents remains elusive. In this article, we put forward an alternate etiology: abdominal migraines. Recent postmortem studies in autism indicate the presence of a minicolumnopathy and its relationship to both serotonergic abnormalities and a hyperexcitable cortex. These features of phenomenology are also observed in miganeurs. A putative relationship between autism and migraine is further suggested by similarities in clinical histories and laboratory evidence. Some commonalities include the presence of neuroinflammation, sensory overstimulation (e.g., flickering of fluorescent lights), "food allergies", benefits from similar diets, and the role of nitric oxide. Abdominal migraine therefore stands as a falsifiable hypothesis with added importance accrued to potential therapeutic interventions.

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.

A morphogenetic role for acetylcholine in mouse cerebral neocortex

Recently, cholinergic afferents to cerebral cortex have met renewed attention regarding the regulation of plasticity as well as cognitive processing. My laboratory has developed a mouse neonatal basal forebrain lesion paradigm that has contributed considerably to the understanding of cholinergic mechanisms in cortical development. We have shown that transient cholinergic deafferentation, beginning at birth, precipitates alterations in neuronal differentiation and synaptic connectivity that persist into maturity, and contribute to altered cognitive behavior. These data are in general agreement with studies in rats in which the cholinergic basal forebrain is lesioned very early in development but contrast with effects of later developmental lesions. Moreover, in mouse, both morphological and behavioral consequences of the lesion are sex dependent. Studies of receptors and secondary messengers that are instrumental in morphogenesis and plasticity suggest that sex dependent molecular alterations occur within days if not hours following cortical cholinergic deafferentation.

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.

MK801 decreases glutamate release and oxidative metabolism during hypoglycemic coma in piglets

Hypoglycemic coma increases extracellular excitatory amino acids, which mediate hypoglycemic neuronal degeneration. Cerebral oxygen consumption increases during hypoglycemic coma in piglets. We tested the hypothesis that the NMDA-receptor antagonist dizocilpine (MK801) attenuates the increase in cerebral oxygen consumption during hypoglycemia. We measured EEG, cerebral blood flow (CBF), cerebral oxygen consumption (CMRO(2)) and cortical microdialysate levels of glutamate, aspartate and glycine in pentobarbital-anesthetized piglets during 60 min of insulin-induced hypoglycemic coma. NMDA-receptor distribution was measured by autoradiography. MK801 (0.75 mg/kg i.v.) was given within 5 min after onset of isoelectric EEG. Saline- and MK801-treated normoglycemic control animals were also studied. Brain temperature was maintained at 38.5+/-0.5 degrees C. MK801 prevented the 5--10-fold increase in glutamate and aspartate occurring in saline-treated hypoglycemic animals, and attenuated the increase in CMRO(2). Increases in CBF of 200--400% during hypoglycemic coma were not affected by MK801. MK801 did not alter CBF, CMRO(2) or microdialysate amino acid levels in normoglycemic control animals. Parietal cortex corresponding to microdialysis sites was highly enriched in NMDA receptors, and the density and distribution overall of NMDA receptor binding sites were comparable to that reported in other species. We conclude that NMDA receptor activation plays a central role in hypoglycemia-induced glutamate release, and contributes to increased cerebral oxygen consumption. Neuroprotective effects of MK801 during hypoglycemia in piglets may involve inhibitory effects on glutamate release and oxidative metabolism.

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.

Phosphorylated MAP-1B isoforms in the developing mouse barrel cortex

Developmental expression of two phosphorylation modes of microtubule-associated protein 1B (MAP-1B) has been studied in the barrel cortex of mice at postnatal days (P)5, P12, P21 and P90 using immunocytochemistry with antibodies 125 and 150 that recognize phosphorylation modes II and I, respectively. The antibody 125 immunoreactive processes, identified as dendrites, are not yet detectable at P5; they are already present at P12 and become more evident at P21. In the barrel cortex of P90 animals the antibody 125 immunopositive dendrites are still present, although they are much less pronounced. The antibody 150 punctate immunostaining seen at P5 is not detectable at P12. At P21, however, thin immunopositive fibres appear, implicating a re-expression of the microtubule-associated protein 1B phosphorylation mode I in a portion of axons. The antibody 150 immunopositive axons are no longer present in the P90 barrel cortex. The re-expression of the MAP-1B phosphorylation mode I, which is a juvenile isoform characteristic for growing axons, may imply induction of mechanisms providing mouse barrel cortex neurons with the potency for plastic changes at a terminal stage of synaptogenesis.

Ionotropic glutamate receptor expression in human spinal cord during first trimester development

Quantitative receptor autoradiography and immunoblotting were used to study the expression and distribution of AMPA, kainate and NMDA receptors in first trimester human spinal cord obtained from elective abortions ranging from 4 to 11.5 weeks of gestational age. Spinal cord tissue sections were processed for receptor autoradiography with the ligands [3H]AMPA, [3H]kainate and [3H]MK-801 and the optical density was measured separately in a dorsal region (alar plate) and ventral region (basal plate) of the autoradiographs. Binding sites for all three ligands were demonstrated already at 4-5.5 weeks of gestation and increased continuously during the first trimester both in the dorsal and ventral regions. [3H]AMPA binding to both high- and low-affinity sites increased from undetectable levels to about 35 and 400 fmol/mg tissue, respectively, during this period. A temporal difference in the distribution of [3H]AMPA binding sites was observed. The early homogeneous pattern of [3H]AMPA binding in both alar and basal plates had changed to a heterogeneous pattern at 11 weeks of gestation with the highest density of [3H]AMPA binding sites in the superficial layers of the immature dorsal horn. [3H]kainate and [3H]MK-801 binding sites were densely and homogeneously distributed already at 4 weeks, and steadily increased six- and two-fold, respectively, to about 100 fmol/mg tissue at 11.5 weeks of gestation. Immunoreactive bands corresponding to the NMDA receptor subunits NR1, NR2A, NR2B, NR2C and NR2D were demonstrated by immunoblotting at the earliest between 4.5 and 7 weeks and increasing concentrations were seen up to 11 weeks of gestation. These results suggest that AMPA, kainate and NMDA receptors are expressed in the human spinal cord early in embryogenesis.

The alpha7 nicotinic acetylcholine receptor in neuronal plasticity

A growing body of evidence indicates that neuronal nicotinic acetylcholine receptors (nAChRs), in addition to promoting fast cholinergic transmission, may modulate other neuronal activities within the central nervous system (CNS). In particular, the alpha7 nAChR is highly permeable to Ca2+ and may serve a distinct role in regulating neuronal plasticity. By elevating intracellular Ca2+ levels in discrete neuronal locations, these ligand-gated ion channels may influence numerous physiological processes in developing and adult CNS. In this article, we review evidence that both pre- and postsynaptic alpha7 nAChRs modulate transmitter release in the brain and periphery through Ca2+-dependent mechanisms. The possible role of alpha7 nAChRs in regulating neuronal growth and differentiation in developing CNS is also evaluated. We consider an interaction between cholinergic and glutamatergic transmission and propose a hypothesis on the possible coregulation of intracellular Ca2+ by N-methyl-D-aspartate (NMDA) receptors and alpha7 nAChRs. Finally, the clinical significance of alterations in the normal function of alpha7 nAChRs is discussed as it pertains to prenatal nicotine exposure, schizophrenia, and epilepsy.

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.

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.

The effects of neonatal basal forebrain lesions on cognition: towards understanding the developmental role of the cholinergic basal forebrain

Abnormal development of the cholinergic basal forebrain has been implicated in numerous developmental disabilities such as Rett Syndrome and Down Syndrome. This review summarizes recent data using two rodent animal models that involve interrupting cholinergic basal forebrain projections on postnatal day 1 and postnatal day 7 when basal forebrain fibers are beginning to innervate their neocortical and hippocampal targets, respectively. In one model, electrolytic lesions in mice aimed at the basal forebrain on postnatal day 1 transiently reduce cholinergic markers in neocortex which induce permanent alterations in neocortical anatomy that correlate with impairments on cognitive tasks. Furthermore, the lesion effects are sex dependent. In another model, 192 IgG saporin lesions in rats on postnatal day 7 permanently reduce cholinergic markers in neocortex and hippocampus, and result in mild impairments in spatial processing, acquisition and exploratory activities. These data suggest that during the first postnatal week of development the cholinergic basal forebrain system is critical for normal neocortical differentiation and, possibly synaptogenesis in neural circuits that will be important for spatial memory and acquisition of spatial data. During the second postnatal week of development, the cholinergic basal forebrain system appears to take on a role largely similar to its adult role in selective attention and processing of new information. These studies also suggest strongly that interrupting cholinergic basal forebrain innervation of neocortex and hippocampus leads to anatomical and neurochemical abnormalities that may serve as neural substrates for some of the cognitive deficits seen in disorders such as Rett Syndrome and Down Syndrome.

Modular distribution of vasoactive intestinal polypeptide in the rat barrel cortex: changes induced by neonatal removal of vibrissae

The distribution of vasoactive intestinal polypeptide-immunoreactive neuronal structures in the barrel cortex (posteromedial barrel subfield) of adult rats was analysed after unilateral removal of the vibrissal follicles of row C in neonatal rats. The hypothesis was tested whether the distribution of vasoactive intestinal polypeptide-immunoreactive structures depends on the normal anatomical organization of the specific sensory input. After three months survival the distribution of the vasoactive intestinal polypeptide-immunoreactive structures was morphometrically evaluated. This approach revealed alterations in the contralateral posteromedial barrel subfield, where the disappearance of barrel row C and a substantial increase in size mainly of barrel row D, but also of other rows could be detected. Increase in row D included both barrels and the interspace (septal segments between barrels in one row). As vasoactive intestinal polypeptide immunoreactivity of the barrel field was found previously to be localized in synaptic boutons involved in symmetric synapses, our present findings suggest that (i) the interspace is enriched in inhibitory vasoactive intestinal polypeptide-immunoreactive synapses as opposed to the excitatory thalamocortical input reaching the barrel hollow, (ii) the spatial distribution of the vasoactive intestinal polypeptide system in the barrel cortex is closely associated with the neuronal organization of the sensory input and reacts with a considerable plasticity to lesion-induced changes of the input, and (iii) the compensatory barrel hypertrophy in a row neighbouring the deafferented row involves an increasing number of vasoactive intestinal polypeptide-immunoreactive synapses per barrel.

Distribution of synaptic zinc in the developing mouse somatosensory barrel cortex

Histochemical localization of synaptic zinc was examined in the somatosensory (SI) barrel cortex of mouse. The laminar distribution and distribution within the barrel field were described. At postnatal day 3 (P3) and 5 (P5), very faint and uniform zinc staining was present in the lower part of the subplate. At P6, subtle laminar variations emerged. At P8, these variations were clearly observed. Intense zinc staining was found in layers I, II, III, and V. Layers IV and VI showed a weaker staining. From this postnatal age to adult, uneven patchy distribution of synaptic zinc in layer IV could be distinguished in coronal sections. In tangential sections through layer IV, zinc staining showed a barrel-like pattern due to a higher zinc concentration in septa and the surrounding cortex. Barrel sides revealed a lower zinc concentration compared with the barrel hollow. With brain maturation, the zinc staining increased more intensely outside the barrel field, thus producing a progressively higher contrast between the barrel field and adjacent cortical regions. The differences in zinc staining between the barrel side and barrel hollow diminished with age but were still visible at P70. The changes in synaptic zinc distribution probably reflect the process of synaptic maturation of glutamatergic terminals projecting to the SI cortex. The time course of postnatal changes in terminal zinc distribution suggests that synaptic zinc is not involved in the mechanisms of barrel formation.

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.

Postnatal ontogeny of glutamate receptors in the rat nucleus tractus solitarii and ventrolateral medulla

The nucleus tractus solitarii and the ventrolateral medulla are two brainstem regions involved in regulation of autonomic functions. Glutamate (Glu) receptors localized within these two regions play a key role in neural control of swallowing and breathing and in blood pressure regulation. In the present study, postnatal changes in global [3H]Glu binding and in [3H]Glu binding to N-methyl-D-aspartate (NMDA) receptors were analyzed in the nucleus tractus solitarii and the ventrolateral medulla using in vitro receptor autoradiography. Similar results were obtained in both regions. When expressed as density values (fmol/mg tissue), both global and NMDA-sensitive Glu binding increased by approximately 50-70% between birth and postnatal day 9 (P9) and then decreased until P30. When expressed as binding per nucleus (i.e., after correction for tissue growth), global Glu binding still increased between birth and P9 and decreased between P9 and P30 whereas NMDA-sensitive binding increased until P9 and remained stable thereafter. Saturation studies showed a postnatal increase in Glu receptor number per nucleus, which occurred mainly between birth and P9, and a decrease in Glu receptor affinity between P9 and adulthood. These results indicate that dramatic changes in glutamatergic neurotransmission occur in the nucleus tractus solitarii and the ventrolateral medulla during the first month of postnatal life. They suggest that both neonates and young animals may not be fully mature as regard to central regulation of autonomic functions.

Implication of NMDA type glutamate receptors in neural regeneration and neoformation of synapses after excitotoxic injury in the guinea pig cochlea

In the adult mammalian cochlea, the ability of nerve fibres to regenerate has been observed following disruption of the organ of Corti by various means, or transsection of the cochlear nerve in the internal auditory meatus. Based upon the implication of glutamate as a neurotransmitter at synapses between sensory hair cells and terminal dendrites of the auditory nerve in the mammalian cochlea, we have developed, in a previous study, an in vivo model of neural regeneration and formation of synapses after the destruction of the afferent nerve endings by local application of the glutamate agonist alpha-amino-3-hydroxy-5-methyl-isoxazol-propionic acid (AMPA). In situ hybridization experiments performed during the re-innervation process revealed an overexpression of mRNA coding for NR1 subunit of N-methyl-D-aspartate (NMDA) receptors in the spiral ganglion neurons, suggesting that these receptors are implicated in neural regenerative processes. The present study has been designed to study the functional implication of NMDA receptors in the regrowth and synaptic repair of auditory dendrites in the guinea pig cochlea, by blocking the NMDA receptors during the period of normal functional recovery. In a first set of experiments, we recorded compound action potential after acute perilymphatic perfusion of cumulative doses (0.03-10mM) of DL 2-amino-5-phosphonovalerate (D-AP5), a NMDA antagonist, to determine the efficiency of the drug. In a second set of experiments, the auditory dendrites were destroyed by local application of the glutamate agonist AMPA. The blockage of NMDA by the antagonist D-AP5 applied with an osmotic micropump delayed the functional recovery and the regrowth of auditory dendrites. The findings of our study support the hypothesis that, in addition to acting as a fast transmitter, glutamate has a neurotrophic role via the activation of NMDA receptors.

Area-specific regulation of gamma-aminobutyric acid type A receptor subtypes by thalamic afferents in developing rat neocortex

Targeting and innervation of the cerebral cortex by thalamic afferents is a key event in the specification of cortical areas. The molecular targets of thalamic regulation, however, have remained elusive. We now demonstrate that thalamic afferents regulate the expression of gamma-aminobutyric acid type A (GABAA) receptors in developing rat neocortex, leading to the area-specific expression of receptor subtypes in the primary visual (V1) and somatosensory (S1) areas. Most strikingly, the alpha1- and alpha5-GABAA receptors exhibited a reciprocal expression pattern, which precisely reflected the distribution of thalamocortical afferents at postnatal day 7. Following unilateral lesions at the birth of the thalamic nuclei innervating V1 and S1 (lateral geniculate nucleus and ventrobasal complex, respectively), profound changes in subunit expression were detected 1 week later in the deprived cortical territories (layers III-IV of V1 and S1). The expression of the alpha1 subunit was strongly down-regulated in these layers to a level comparable to that in neighboring areas. Conversely, the alpha5 subunit was up-regulated and areal boundaries were no longer discernible in the lesioned hemisphere. Changes similar to the alpha5 subunit were also seen for the alpha2 and alpha3 subunits. These results indicate that the differential expression of GABAA receptor subtypes in developing neocortex is dependent on thalamic innervation, contributing to the emergence of functionally distinct areas.

Transient uptake and storage of serotonin in developing thalamic neurons

Serotonin (5-HT) has been shown to affect the development and patterning of the mouse barrelfield. We show that the dense transient 5-HT innervation of the somatosensory, visual, and auditory cortices originates in the thalamus rather than in the raphe: 5-HT is detected in thalamocortical fibers and most 5-HT cortical labeling disappears after thalamic lesions. Thalamic neurons do not synthesize 5-HT but take up exogenous 5-HT through 5-HT high affinity uptake sites located on thalamocortical axons and terminals. 3H-5-HT injected into the cortex is retrogradely transported to thalamic neurons. In situ hybridization shows a transient expression of the genes encoding the serotonin transporter and the vesicular monoamine transporter in thalamic sensory neurons. In these glutamatergic neurons, internalized 5-HT might thus be stored and used as a "borrowed transmitter" for extraneuronal signaling or could exert an intraneuronal control on thalamic maturation.

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.

Developmental aspects of epileptogenesis

Several factors may contribute to the propensity for the developing brain to have seizures and develop epilepsy. Hypersynchrony of neuronal circuits contributes to the seizure potential and several neurobiological features of the immature brain may support synchronized neuronal firing. The immature cerebral cortex and hippocampus have an increased density of synapses compared to adults and also a higher density of gap junctions and of excitatory amino acid receptors. Enhanced regenerative responses to injury in the developing brain may also contribute to the formation of abnormal hippocampal connections that support epilepsy. Molecular mechanisms that contribute to enhanced synaptic plasticity in the child's brain can also contribute to epileptogenesis in certain circumstances. The phenomenon of kindling, where repeated electrical stimulation of neuronal circuits leads to the development of epileptic seizures, is easily elicited in young animals. Long-term potentiation (LTP), where repeated synaptic stimulation leads to a reduced threshold for activation of that pathway and enhanced postsynaptic potentials, is much more robust in the immature cerebral cortex and may contribute to kindling and epileptogenesis. Age related enhancement of N-methyl-D-aspartate-type glutamate receptors, which are important for the activity dependent plasticity in the developing brain, appears to participate in LTP. This information suggests that normal developmental features of synaptic development make the immature brain more excitable than the adult brain and may contribute to epileptogenesis.

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.

Neurotransmitters and vulnerability of the developing brain

The immature human brain undergoes remarkable organizational changes during intrauterine and postnatal life. These changes create potential temporal 'windows' of selective vulnerability to damage. For example, the temporary germinal matrix is vulnerable to hemorrhage in the third trimester fetus and premature infant. The immature oligodendroglia present in developing white matter of the fetus are also vulnerable to injury producing periventricular leukomalacia. Similar changes take place in the synapses that make up the infant's neuronal circuitry. In human cerebral cortex, synapses are produced in greater than adult numbers by postnatal age 2 years and then reduced over the next decade. Over the same period receptors for glutamate, the most important excitatory neurotransmitter, change their characteristics to allow them to participate in activity dependent synaptic plasticity. For example, the immature N-methyl-D-aspartate (NMDA) type glutamate receptor/channel complex, which plays important roles in long term potentiation (LTP), neuronal migration and synaptic pruning, contains subunits that allow the channel to be opened more easily for a longer period than adult channels. These developmental changes make the immature brain selectively vulnerable to NMDA receptor overstimulation that can occur during hypoxia-ischemia and other insults. Several types of neuropathology in the developing brain can be understood on the basis of these organizational principles.