Developmental profile of GABAA-receptors in the marmoset monkey: expression of distinct subtypes in pre- and postnatal brain

GABA system as the cause and effect in early development

GABA is the primary inhibitory neurotransmitter in the adult brain and through its actions on GABAARs, it protects against excitotoxicity and seizure activity, ensures temporal fidelity of neurotransmission, and regulates concerted rhythmic activity of neuronal populations. In the developing brain, the development of GABAergic neurons precedes that of glutamatergic neurons and the GABA system serves as a guide and framework for the development of other brain systems. Despite this early start, the maturation of the GABA system also continues well into the early postnatal period. In this review, we organize evidence around two scenarios based on the essential and protracted nature of GABA system development: 1) disruptions in the development of the GABA system can lead to large scale disruptions in other developmental processes (i.e., GABA as the cause), 2) protracted maturation of this system makes it vulnerable to the effects of developmental insults (i.e., GABA as the effect). While ample evidence supports the importance of GABA/GABAAR system in both scenarios, large gaps in existing knowledge prevent strong mechanistic conclusions.

A Primer on Constructing Plasticity Phenotypes to Classify Experience-Dependent Development of the Visual Cortex

Many neural mechanisms regulate experience-dependent plasticity in the visual cortex (V1), and new techniques for quantifying large numbers of proteins or genes are transforming how plasticity is studied into the era of big data. With those large data sets comes the challenge of extracting biologically meaningful results about visual plasticity from data-driven analytical methods designed for high-dimensional data. In other areas of neuroscience, high-information content methodologies are revealing more subtle aspects of neural development and individual variations that give rise to a richer picture of brain disorders. We have developed an approach for studying V1 plasticity that takes advantage of the known functions of many synaptic proteins for regulating visual plasticity. We use that knowledge to rebrand protein measurements into plasticity features and combine those into a plasticity phenotype. Here, we provide a primer for analyzing experience-dependent plasticity in V1 using example R code to identify high-dimensional changes in a group of proteins. We describe using PCA to classify high-dimensional plasticity features and use them to construct a plasticity phenotype. In the examples, we show how to use this analytical framework to study and compare experience-dependent development and plasticity of V1 and apply the plasticity phenotype to translational research questions. We include an R package “PlasticityPhenotypes” that aggregates the coding packages and custom code written in RStudio to construct and analyze plasticity phenotypes.

Neonatal Clonazepam Administration Induced Long-Lasting Changes in GABAA and GABAB Receptors

Benzodiazepines (BZDs) are widely used in patients of all ages. Unlike adults, neonatal animals treated with BZDs exhibit a variety of behavioral deficits later in life; however, the mechanisms underlying these deficits are poorly understood. This study aims to examine whether administration of clonazepam (CZP; 1 mg/kg/day) in 7-11-day-old rats affects Gama aminobutyric acid (GABA)ergic receptors in both the short and long terms. Using RT-PCR and quantitative autoradiography, we examined the expression of the selected GABA_A receptor subunits (α1, α2, α4, γ2, and δ) and the GABA_B B2 subunit, and GABA_A, benzodiazepine, and GABA_B receptor binding 48 h, 1 week, and 2 months after treatment discontinuation. Within one week after CZP cessation, the expression of the α2 subunit was upregulated, whereas that of the δ subunit was downregulated in both the hippocampus and cortex. In the hippocampus, the α4 subunit was downregulated after the 2-month interval. Changes in receptor binding were highly dependent on the receptor type, the interval after treatment cessation, and the brain structure. GABA_A receptor binding was increased in almost all of the brain structures after the 48-h interval. BZD-binding was decreased in many brain structures involved in the neuronal networks associated with emotional behavior, anxiety, and cognitive functions after the 2-month interval. Binding of the GABA_B receptors changed depending on the interval and brain structure. Overall, the described changes may affect both synaptic development and functioning and may potentially cause behavioral impairment.

GABAARα2 is Decreased in the Axon Initial Segment of Pyramidal Cells in Specific Areas of the Prefrontal Cortex in Autism

Some forms of Autism Spectrum Disorder, a neurodevelopmental syndrome characterized by impaired communication and social skills as well as repetitive behaviors, are purportedly associated with dysregulation of the excitation/inhibition balance in the cerebral cortex. Through human postmortem tissue analysis, we previously found a significant decrease in the number of a gamma-aminobutyric acid (GABA)ergic interneuron subtype, the chandelier (Ch) cell, in the prefrontal cortex of subjects with autism. Ch cells exclusively target the axon initial segment (AIS) of excitatory pyramidal (Pyr) neurons, and a single Ch cell forms synapses on hundreds of Pyr cells, indicating a possible role in maintaining electrical balance. Thus, we herein investigated this crucial link between Ch and Pyr cells in the anatomy of autism neuropathology by examining GABA receptor protein expression in the Pyr cell AIS in subjects with autism. We collected tissue from the prefrontal cortex (Brodmann Areas (BA) 9, 46, and 47) of 20 subjects with autism and 20 age- and sex-matched control subjects. Immunohistochemical staining with antibodies against the GABA_A receptor subunit α2 (GABA_ARα2) - the subunit most prevalent in the Pyr cell AIS - revealed a significantly decreased GABA_ARα2 protein in the Pyr cell AIS in supragranular layers of prefrontal cortical areas BA9 and BA47 in autism. Downregulated GABA_ARα2 protein in the Pyr cell AIS may result from decreased GABA synthesis in the prefrontal cortex of subjects with autism, and thereby contribute to an excitation/inhibition imbalance. Our findings support the potential for GABA receptor agonists asa therapeutic tool for autism.

Functional characterization of GABAA receptor-mediated modulation of cortical neuron network activity in microelectrode array recordings

The numerous γ-aminobutyric acid type A receptor (GABA_AR) subtypes are differentially expressed and mediate distinct functions at neuronal level. In this study we have investigated GABA_AR-mediated modulation of the spontaneous activity patterns of primary neuronal networks from murine frontal cortex by characterizing the effects induced by a wide selection of pharmacological tools at a plethora of activity parameters in microelectrode array (MEA) recordings. The basic characteristics of the primary cortical neurons used in the recordings were studied in some detail, and the expression levels of various GABA_AR subunits were investigated by western blotting and RT-qPCR. In the MEA recordings, the pan-GABA_AR agonist muscimol and the GABA_BR agonist baclofen were observed to mediate phenotypically distinct changes in cortical network activity. Selective augmentation of αβγ GABA_AR signaling by diazepam and of δ-containing GABA_AR (δ-GABA_AR) signaling by DS1 produced pronounced changes in the majority of the activity parameters, both drugs mediating similar patterns of activity changes as muscimol. The apparent importance of δ-GABA_AR signaling for network activity was largely corroborated by the effects induced by the functionally selective δ-GABA_AR agonists THIP and Thio-THIP, whereas the δ-GABA_AR selective potentiator DS2 only mediated modest effects on network activity, even when co-applied with low THIP concentrations. Interestingly, diazepam exhibited dramatically right-shifted concentration-response relationships at many of the activity parameters when co-applied with a trace concentration of DS1 compared to when applied alone. In contrast, the potencies and efficacies displayed by DS1 at the networks were not substantially altered by the concomitant presence of diazepam. In conclusion, the holistic nature of the information extractable from the MEA recordings offers interesting insights into the contributions of various GABA_AR subtypes/subgroups to cortical network activity and the putative functional interplay between these receptors in these neurons.

GABAA receptor expression and white matter disruption in intrauterine growth restricted piglets

Intrauterine growth restriction (IUGR) is one of the most common causes of perinatal mortality and morbidity. White matter and neuronal injury are major pathophysiological features of the IUGR neonatal brain. GABA_A (γ-aminobutyric acid type A) receptors have been shown to play a role in oligodendrocyte differentiation and proliferation in the neonatal brain and may be a key factor in white matter injury and myelination in IUGR neonates. Whether there are impairments to the GABAergic system and neuronal cytoskeleton in IUGR brain has yet to be elucidated. This study aims to examine GABA_A receptor α₁ and α₃ subunit protein expression and distribution in parietal cortex and hippocampus of the IUGR piglet at four different ages (term=115d - days gestational age), 100d, 104d, birth (postnatal day 0-P0) and P7 and to examine neuronal and myelination patterns. Significant alterations to GABA_A receptor α₁ and α₃ protein expression levels were observed in the IUGR piglet brain of P7 IUGR piglets with significantly greater α₃ expression compared to α₁ expression in the hippocampus while there was virtually no difference between the two subunits in the parietal cortex. However a significantly lower α₁/α₃ ratio was evident in P7 IUGR cortex when compared with P7 NG cortex. Neuronal somatodendrites studied using MAP2 immunohistochemistry showed reduced and disrupted somatodendrites while MBP immunolabelling showed loss of axonal fibres from gestational day 104d through to P7. These findings provide insights into the effects of IUGR on the development of the GABA system, altered developmental maturation of GABA_A receptor subunit expression in the IUGR brain may influence myelination and may partly explain the cognitive disabilities observed in IUGR. Understanding the mechanisms behind grey and white matter injury in the IUGR infant is essential to identifying targets for treatments to improve long-term outcomes for IUGR infants.

Differential maturation of vesicular glutamate and GABA transporter expression in the mouse auditory forebrain during the first weeks of hearing

Vesicular transporter proteins are an essential component of the presynaptic machinery that regulates neurotransmitter storage and release. They also provide a key point of control for homeostatic signaling pathways that maintain balanced excitation and inhibition following changes in activity levels, including the onset of sensory experience. To advance understanding of their roles in the developing auditory forebrain, we tracked the expression of the vesicular transporters of glutamate (VGluT1, VGluT2) and GABA (VGAT) in primary auditory cortex (A1) and medial geniculate body (MGB) of developing mice (P7, P11, P14, P21, adult) before and after ear canal opening (~P11-P13). RNA sequencing, in situ hybridization, and immunohistochemistry were combined to track changes in transporter expression and document regional patterns of transcript and protein localization. Overall, vesicular transporter expression changed the most between P7 and P21. The expression patterns and maturational trajectories of each marker varied by brain region, cortical layer, and MGB subdivision. VGluT1 expression was highest in A1, moderate in MGB, and increased with age in both regions. VGluT2 mRNA levels were low in A1 at all ages, but high in MGB, where adult levels were reached by P14. VGluT2 immunoreactivity was prominent in both regions. VGluT1 (+) and VGluT2 (+) transcripts were co-expressed in MGB and A1 somata, but co-localization of immunoreactive puncta was not detected. In A1, VGAT mRNA levels were relatively stable from P7 to adult, while immunoreactivity increased steadily. VGAT (+) transcripts were rare in MGB neurons, whereas VGAT immunoreactivity was robust at all ages. Morphological changes in immunoreactive puncta were found in two regions after ear canal opening. In the ventral MGB, a decrease in VGluT2 puncta density was accompanied by an increase in puncta size. In A1, perisomatic VGAT and VGluT1 terminals became prominent around the neuronal somata. Overall, the observed changes in gene and protein expression, regional architecture, and morphology relate to-and to some extent may enable-the emergence of mature sound-evoked activity patterns. In that regard, the findings of this study expand our understanding of the presynaptic mechanisms that regulate critical period formation associated with experience-dependent refinement of sound processing in auditory forebrain circuits.

Characteristics of the cation cotransporter NKCC1 in human brain: alternate transcripts, expression in development, and potential relationships to brain function and schizophrenia

Early in development, GABA, an inhibitory neurotransmitter in adults, is excitatory. NKCC1 (SLC12A2) encodes one of two cation chloride cotransporters mediating the conversion of GABA from excitatory to inhibitory. Using 3' and 5' RACE and PCR, we verified previously characterized alternative transcripts of NKCC1a (1-27) and NKCC1b (1-27(Δ21)), identified new NKCC1 transcripts, and explored their expression patterns during human prefrontal cortical development. A novel ultra-short transcript (1-2a) was expressed preferentially in the fetus. Expression of NKCC1b and 1-2a were decreased in schizophrenia compared with controls (NKCC1b: 0.8-fold decrease, p = 0.013; 1-2a: 0.8-fold decrease, p = 0.006). Furthermore, the expression of NKCC1b was associated with NKCC1 polymorphism rs3087889. The minor allele at rs3087889, associated with reduced NKCC1b expression (homozygous for major allele: N = 37; homozygous for minor allele: N = 15; 1.5-fold decrease; p < 0.01), was also associated with a modest increase in schizophrenia risk in a case-control sample (controls: N = 435; cases: N = 397, OR = 1.5). This same allele was then found associated with cognitive (n = 369) and fMRI (n = 313) intermediate phenotypes associated with schizophrenia-working memory (Cohen's d = 0.35), global cognition or g (d = 0.18), and prefrontal inefficiency (d = 0.36) as measured by BOLD fMRI during a working memory task. Together, these preclinical and clinical results suggest that variation in NKCC1 may increase risk for schizophrenia via alterations of mRNA expression at the molecular level and impairment of optimal prefrontal function at the macro or systems level.

Bumetanide, an inhibitor of cation-chloride cotransporter isoform 1, inhibits γ-aminobutyric acidergic excitatory actions and enhances sedative actions of midazolam in neonatal rats

BACKGROUND

It has been shown that γ-aminobutyric acid exerts excitatory actions on the immature brain due to the increased expression of Na(+)-K(+)-2Cl(-) cotransporter isoform 1. The authors sought to clarify whether midazolam, a γ-aminobutyric acid-mimetic hypnotic agent, causes neuronal excitation that can be blocked by bumetanide, a selective inhibitor of Na(+)-K(+)-2Cl(-) cotransporter isoform 1. Furthermore, the authors examined whether bumetanide potentiates the sedative effects of midazolam in neonatal rats.

METHODS

The authors measured the effects of midazolam with or without bumetanide on the cytosolic Ca(2+) concentration ([Ca](2+)(i)) in hippocampal slices (n=3 in each condition) from rats at postnatal days 4, 7, and 28 (P4, P7, and P28) using fura-2 microfluorometry. Neuronal activity in the hippocampus and thalamus after intraperitoneal administration of midazolam with or without bumetanide was estimated by immunostaining of phosphorylated cyclic adenosine monophosphate-response element-binding protein (n=12 in each condition). Furthermore, the authors assessed effects of bumetanide on the sedative effect of midazolam by measuring righting reflex latency (n=6 in each condition).

RESULTS

Midazolam significantly increased [Ca](2+)(i) in the CA3 area at P4 and P7 but not at P28. Bumetanide inhibited midazolam-induced increase in [Ca](2+)(i). Midazolam significantly up-regulated phosphorylated cyclic adenosine monophosphate-response element-binding protein expression in a bumetanide-sensitive manner in the hippocampus at P7 but not P28. Bumetanide enhanced the sedative effects of midazolam in P4 and P7 but not P28 rats.

CONCLUSION

These results suggest that γ-aminobutyric acid A receptor-mediated excitation plays an important role in attenuated sedative effects of midazolam in immature rats.

Role of tonic GABAergic currents during pre- and early postnatal rodent development

In the last three decades it became evident that the GABAergic system plays an essential role for the development of the central nervous system, by influencing the proliferation of neuronal precursors, neuronal migration and differentiation, as well as by controlling early activity patterns and thus formation of neuronal networks. GABA controls neuronal development via depolarizing membrane responses upon activation of ionotropic GABA receptors. However, many of these effects occur before the onset of synaptic GABAergic activity and thus require the presence of extrasynaptic tonic currents in neuronal precursors and immature neurons. This review summarizes our current knowledge about the role of tonic GABAergic currents during early brain development. In this review we compare the temporal sequence of the expression and functional relevance of different GABA receptor subunits, GABA synthesizing enzymes and GABA transporters. We also refer to other possible endogenous agonists of GABAA receptors. In addition, we describe functional consequences mediated by the GABAergic system during early developmental periods and discuss current models about the origin of extrasynaptic GABA and/or other endogenous GABAergic agonists during early developmental states. Finally, we present evidence that tonic GABAergic activity is also critically involved in the generation of physiological as well as pathophysiological activity patterns before and after the establishment of functional GABAergic synaptic connections.

Postnatal maturation of GABAergic transmission in the rat basolateral amygdala

Many psychiatric disorders, including anxiety and autism spectrum disorders, have early ages of onset and high incidence in juveniles. To better treat and prevent these disorders, it is important to first understand normal development of brain circuits that process emotion. Healthy and maladaptive emotional processing involve the basolateral amygdala (BLA), dysfunction of which has been implicated in numerous psychiatric disorders. Normal function of the adult BLA relies on a fine balance of glutamatergic excitation and GABAergic inhibition. Elsewhere in the brain GABAergic transmission changes throughout development, but little is known about the maturation of GABAergic transmission in the BLA. Here we used whole cell patch-clamp recording and single-cell RT-PCR to study GABAergic transmission in rat BLA principal neurons at postnatal day (P)7, P14, P21, P28, and P35. GABAA currents exhibited a significant twofold reduction in rise time and nearly 25% reduction in decay time constant between P7 and P28. This corresponded with a shift in expression of GABAA receptor subunit mRNA from the α2- to the α1-subunit. The reversal potential for GABAA receptors transitioned from depolarizing to hyperpolarizing with age, from around -55 mV at P7 to -70 mV by P21. There was a corresponding shift in expression of opposing chloride pumps that influence the reversal, from NKCC1 to KCC2. Finally, we observed short-term depression of GABAA postsynaptic currents in immature neurons that was significantly and gradually abolished by P28. These findings reveal that in the developing BLA GABAergic transmission is highly dynamic, reaching maturity at the end of the first postnatal month.

GABA(A) receptor imaging with positron emission tomography in the human newborn: a unique binding pattern

Flumazenil is a specific, reversibly bound antagonist at benzodiazepine binding sites of gamma-aminobutyric acid A receptors; these sites can be imaged using positron emission tomography with 11C-flumazenil. We reported an exponential decline of flumazenil volume of distribution (proportional to receptor binding) of gamma-aminobutyric acid A receptors in children 2 to 17 years. Six newborns (33.3-46.7 weeks' postconception) were studied. All had experienced epileptic seizures and undergone 60-minute dynamic 11C-flumazenil-positron emission tomography imaging after injection of 0.4 mCi/kg of 11C-flumazenil. All newborns were scanned during their natural sleep. Binding potential (indicating flumazenil receptor binding) was calculated using Logan-plot analysis. Visual and quantitative analyses showed highest receptor binding in the amygdala-hippocampus region, sensory-motor cortex, thalamus, brainstem and basal ganglia, in that order. Cerebellum and most of the cerebral cortex showed relatively low binding. This is the first demonstration of gamma-aminobutyric acid A receptor binding in human neonates and is strikingly different from that in older children/adults, showing a programmed pattern of expression. The ontogeny data of flumazenil receptor binding from children may contribute to understanding regional differences in synaptic plasticity and improve rational therapeutic use of drugs acting at the gamma-aminobutyric acid A receptor in the pediatric population.

Short- and long-term effects of juvenile stressor exposure on the expression of GABAA receptor subunits in rats

During the juvenile period rodents are particularly sensitive to stressors. Aversive events encountered during this period may have enduring effects that are not evident among animals initially stressed as adults. Interestingly, experiencing stressor during juvenile period was found to elicit a biphasic behavioral pattern over the course of development. During the juvenile period, the expression of several GABAA receptor subunits is subject to elevated plasticity, rendering the GABAergic system sensitive to stressors. In the present investigation, animals were exposed to a juvenile variable stressor regimen (JUV-S) at 27-29 postnatal days (PND): 27 PND-acute swim stress (10 min), 28 PND-elevated platform stress (3 sessions×30 min each), and 29 PND-restraint (2 h). One hour following the last exposure to stressor or in adulthood (60 PND), anxiety-related behaviors were assessed in a 5-min elevated plus maze test. The western blotting technique was used to evaluate whether the juvenile stress induced behavioral pattern will be accompanied by respective changes in GABAA α1, α2, and α3 protein expression in male rats. Our findings further established that juvenile stressor elicits hyper-reactivity when rats were tested as juveniles, whereas rats exhibited reduced activity and increased anxiety when tested as adults. Additionally, the effects of juvenile stressor on α1, α2, and α3 were more pronounced among juvenile stressed rats that were challenged as adults compared with rats that were only challenged as juveniles. Interestingly, the stress-induced modulation of the subunits was particularly evident in the amygdala, a brain region closely associated with anxiety. Thus, age- and region-specific alterations of the α subunits may contribute to the age-specific behavioral alterations observed following juvenile stress exposure.

Decreased GABAA receptor binding in the medullary serotonergic system in the sudden infant death syndrome

γ-Aminobutyric acid (GABA) neurons in the medulla oblongata help regulate homeostasis, in part through interactions with the medullary serotonergic (5-HT) system. Previously, we reported abnormalities in multiple 5-HT markers in the medullary 5-HT system of infants dying from sudden infant death syndrome (SIDS), suggesting that 5-HT dysfunction is involved in its pathogenesis. Here, we tested the hypothesis that markers of GABAA receptors are decreased in the medullary 5-HT system in SIDS cases compared with controls. Using tissue receptor autoradiography with the radioligand H-GABA, we found 25% to 52% reductions in GABAA receptor binding density in 7 of 10 key nuclei sampled of the medullary 5-HT system in the SIDS cases (postconceptional age [PCA] = 51.7 ± 8.3, n = 28) versus age-adjusted controls (PCA = 55.3 ± 13.5, n = 8) (p ≤ 0.04). By Western blotting, there was 46.2% reduction in GABAAα3 subunit levels in the gigantocellularis (component of the medullary 5-HT system) of SIDS cases (PCA = 53.9 ± 8.4, n = 24) versus controls (PCA = 55.3 ± 8.3, n = 8) (56.8% standard in SIDS cases vs 99.35% in controls; p = 0.026). These data suggest that medullary GABAA receptors are abnormal in SIDS infants and that SIDS is a complex disorder of a homeostatic network in the medulla that involves deficits of the GABAergic and 5-HT systems.

Distinct α GABA(A)R subunits influence structural and transcriptional properties of CA1 hippocampal neurons

The hippocampus is recognized as a major telencephalic area modulating learning and episodic memory through the activation of its different subregions. The various functional properties of Ammon's horn 1 (Cornu Amonis 1; CA1) area have been shown to rely on GABAergic and Glutamat- (Glu)-ergic neuronal signals during both postnatal and adult stages. For this purpose, it was the aim of the present study to establish whether certain alpha GABA(A)R subunits (alpha(2,5)) were capable of modifying CA1 structural and functional features via their interaction with specific NMDA receptor subunits such as NR1 during early development stages of the hibernating hamster (Mesocricetus auratus). Indeed, in vitro addition of the selective alpha(2,5) GABA(A)R agonist diazepam (DZP; alpha(2,5)) accounted for early neuronal formations that were blocked by its antagonist flumazenil (FLM). In particular, the former drug caused very great (p<0.001) increases of dendritic sprouting and branching processes mainly at day in vitro (DIV) 3, while its effects still continued to be responsible for moderate (p<0.05) increases of axonal length during the entire culture period. Contextually, DZP was also responsible for a very great up-regulated expression of neuritic NR1 and MAP2 together with a great (p<0.01) increase of synaptophysin at DIV7. Overall, this first study suggests a specifically tight cross-talking relationship of GABAergic/Gluergic mechanisms operating during CA1 neuronal development, which may bring us closer to the identification of more selective therapeutic targets for hippocampal-linked neurological disorders.

Developmental expression and distribution of GABA(A) receptor α1-, α3- and β2-subunits in pig brain

The principal function of the γ-aminobutyric acid (GABA) system in the adult brain is inhibition; however, in the neonatal brain, GABA provides much of the excitatory drive. As the brain develops, transmembrane chloride gradients change and the inhibitory role of GABA is initiated and continues throughout juvenile and adult life. Previous studies have shown that GABA(A) receptor subunit expression is developmentally regulated, and it is thought that the change in GABA function from excitation to inhibition corresponds to the changeover in expression of 'immature' to 'mature' subunit isoforms. We examined the protein expression pattern and distribution of GABA type A (GABA(A)) receptor α1-, α3- and β2-subunits in the parietal cortex and hippocampus of the developing piglet brain. Four perinatal ages were studied; 14 days preterm (P-14), 10 days preterm (P-10), day of birth (P0) and at postnatal day 7 (P7). Animals were obtained by either caesarean section or spontaneous birth. Protein expression levels and subunit localization were analysed by Western blotting and immunohistochemistry, respectively. In the cortex and hippocampus, GABA(A) receptor α1-subunit showed greatest expression at P7 when compared to all other age groups (p < 0.05). In contrast, α3 expression in the cortex was elevated in preterm brain, peaking at P0, followed by a significant reduction by P7 (p < 0.05); a similar trend was observed in the hippocampus. GABA(A) receptor β2-subunit protein expression appeared relatively constant across all time points studied in both the cortex and hippocampus. Immunolabelling of the α1-, α3- and β2-subunits was observed throughout all cortical layers at every age. GABA(A) receptor α3-subunit appeared to show specific localization to layers V and VI whilst labelling for the β2-subunit was observed in layer IV. In the hippocampus of all animals, the α1- and β2-subunits were shown to immunolabel various cells and processes in the dentate gyrus (DG), CA1 and CA3; the α3-subunit was barely observed except at the stratum moleculare of the DG. We report for the first time the ontogenesis of GABA(A) receptor subunits α1, α3 and β2 in the perinatal pig brain.

GABAA receptor subunit profiles of tangentially migrating neurons derived from the medial ganglionic eminence

During rodent corticogenesis, a sizeable subpopulation of γ-aminobutyric acid (GABA)ergic interneurons arises extracortically from the medial ganglionic eminence (MGE). These neurons progressively acquire responsiveness to GABA in the course of corticopetal tangential migration, a process regulated by ambient GABA and mediated by GABA(A) receptors. Here, we combined patch clamp electrophysiology and single-cell reverse transcription-polymerase chain reaction to examine GABA(A) receptor expression in green fluorescent MGE-derived (eGFP+) cells in telencephalic slices from gestational day 14.5 BAC-Lhx6 embryos. GABA concentration-response curves revealed lower apparent affinity and efficacy in eGFP+ cells in and around the MGE than their counterparts in the cortex. Pharmacological tests revealed subunit-selective response profiles in the MGE and cortex consistent with differential expression of GABA(A) receptor isoforms. Profiling of GABA(A) receptor subunit transcripts (α1-5, β1-3, and γ1-3, δ) uncovered increased expression of the α1-, α2-, α5-, γ2-, and γ3-subunit messenger RNAs in the cortex. We propose that the dynamic expression of certain GABA(A) receptor subunits contributes to assembling receptor isoforms that confer functional attributes important in regulating the migration and maturation of primordial GABAergic cortical interneurons.

Prefrontal GABA(A) receptor alpha-subunit expression in normal postnatal human development and schizophrenia

Cortical GABA deficits that are consistently reported in schizophrenia may reflect an etiology of failed normal postnatal neurotransmitter maturation. Previous studies have found prefrontal cortical GABA(A) receptor alpha subunit alterations in schizophrenia, yet their relationship to normal developmental expression profiles in the human cortex has not been determined. The aim of this study was to quantify GABA(A) receptor alpha-subunit mRNA expression patterns in human dorsolateral prefrontal cortex (DLPFC) during normal postnatal development and in schizophrenia cases compared to controls. Transcript levels of GABA(A) receptor alpha subunits were measured using microarray and qPCR analysis of 60 normal individuals aged 6weeks to 49years and in 37 patients with schizophrenia/schizoaffective disorder and 37 matched controls. We detected robust opposing changes in cortical GABA(A) receptor alpha1 and alpha5 subunits during the first few years of postnatal development, with a 60% decrease in alpha5 mRNA expression and a doubling of alpha1 mRNA expression with increasing age. In our Australian schizophrenia cohort we detected decreased GAD67 mRNA expression (p=0.0012) and decreased alpha5 mRNA expression (p=0.038) in the DLPFC with no significant change of other alpha subunits. Our findings confirm that GABA deficits (reduced GAD67) are a consistent feature of schizophrenia postmortem brain studies. Our study does not confirm alterations in cortical alpha1 or alpha2 mRNA levels in the schizophrenic DLPFC, as seen in previous studies, but instead we report a novel down-regulation of alpha5 subunit mRNA suggesting that post-synaptic alterations of inhibitory receptors are an important feature of schizophrenia but may vary between cohorts.

Developmental changes in GABAergic mechanisms in human visual cortex across the lifespan

Functional maturation of visual cortex is linked with dynamic changes in synaptic expression of GABAergic mechanisms. These include setting the excitation-inhibition balance required for experience-dependent plasticity, as well as, intracortical inhibition underlying development and aging of receptive field properties. Animal studies have shown that there is developmental regulation of GABAergic mechanisms in visual cortex. In this study, we show for the first time how these mechanisms develop in the human visual cortex across the lifespan. We used Western blot analysis of postmortem tissue from human primary visual cortex (n = 30, range: 20 days to 80 years) to quantify expression of eight pre- and post-synaptic GABAergic markers. We quantified the inhibitory modulating cannabinoid receptor (CB1), GABA vesicular transporter (VGAT), GABA synthesizing enzymes (GAD65/GAD67), GABA(A) receptor anchoring protein (Gephyrin), and GABA(A) receptor subunits (GABA(A)alpha1, GABA(A)alpha2, GABA(A)alpha3). We found a complex pattern of different developmental trajectories, many of which were prolonged and continued well into the teen, young adult, and even older adult years. These included a monotonic increase or decrease (GABA(A)alpha1, GABA(A)alpha2), a biphasic increase then decrease (GAD65, Gephyrin), or multiple increases and decreases (VGAT, CB1) across the lifespan. Comparing the balances between the pre- and post-synaptic markers we found three main transition stages (early childhood, early teen years, aging) when there were rapid switches in the composition of the GABAergic signaling system, indicating that functioning of the GABAergic system must change as the visual cortex develops and ages. Furthermore, these results provide key information for translating therapies developed in animal models into effective treatments for amblyopia in humans.

Angelman syndrome: need for further illumination in the theater of the happy puppet

Analysis of the Characteristics of Epilepsy in 37 Patients with the Molecular Diagnosis of Angelman Syndrome

Galvan-Manso M, Campistol J, Conill J, Sanmarti FX

Epileptic Disord 2005;7:19–25

Angelman syndrome is a genetic disorder caused by defects in the maternally inherited imprinted domain located on chromosome 15q11-q13. Most patients with Angelman syndrome have severe mental retardation, characteristic physical appearance, behavioral traits, and severe, early-onset epilepsy. We retrospectively reviewed the medical histories of 37 patients, all with the molecular diagnosis of Angelman syndrome and at least 3 years of follow-up in our neurology department, for further information about their epilepsy: age at onset, type of seizures initially and during follow-up, EEG recordings, treatments, and response. The molecular studies showed 87% deletions de novo; 8% uniparental, paternal disomy; and 5% imprinting defects. The median age at diagnosis was 6.5 years, with 20% having begun to manifest febrile seizures at an average age of 1.9 years. Nearly all (95%) had epilepsy, the majority younger than 3 years (76%). The most frequent seizure types were myoclonic, atonic, generalized tonic–clonic, and atypical absences. At onset, two patients exhibited West syndrome. EEG recordings typical of Angelman syndrome were found in 68%. Normalization of EEG appeared in 12 patients after 9 years. Control of epileptic seizures improved after the age of 8.5 years. The most effective treatments were valproic acid and clonazepam. We conclude that epilepsy was present in nearly all of our cases with Angelman syndrome and that the EEG can be a useful diagnostic tool. On comparing the severity of epilepsy with the type of genetic alteration, we did not find any statistically significant correlations.

Neuroanatomic relationships between the GABAergic and serotonergic systems in the developing human medulla

gamma-Amino butyric (GABA) critically influences serotonergic (5-HT) neurons in the raphé and extra-raphé of the medulla oblongata. In this study we hypothesize that there are marked changes in the developmental profile of markers of the human medullary GABAergic system relative to the 5-HT system in early life. We used single- and double-label immunocytochemistry and tissue receptor autoradiography in 15 human medullae from fetal and infant cases ranging from 15 gestational weeks to 10 postnatal months, and compared our findings with an extensive 5-HT-related database in our laboratory. In the raphé obscurus, we identified two subsets of GABAergic neurons using glutamic acid decarboxylase (GAD65/67) immunostaining: one comprised of small, round neurons; the other, medium, spindle-shaped neurons. In three term medullae cases, positive immunofluorescent neurons for both tryptophan hydroxylase and GAD65/67 were counted within the raphé obscurus. This revealed that approximately 6% of the total neurons counted in this nucleus expressed both GAD65/67 and TPOH suggesting co-production of GABA by a subset of 5-HT neurons. The distribution of GABA(A) binding was ubiquitous across medullary nuclei, with highest binding in the raphé obscurus. GABA(A) receptor subtypes alpha1 and alpha3 were expressed by 5-HT neurons, indicating the site of interaction of GABA with 5-HT neurons. These receptor subtypes and KCC2, a major chloride transporter, were differentially expressed across early development, from midgestation (20 weeks) and thereafter. The developmental profile of GABAergic markers changed dramatically relative to the 5-HT markers. These data provide baseline information for medullary studies of human pediatric disorders, such as sudden infant death syndrome.

GABAergic control of neurite outgrowth and remodeling during development and adult neurogenesis: general rules and differences in diverse systems

During development, Gamma-aminobutyric acidergic (GABAergic) neurons mature at early stages, long before excitatory neurons. Conversely, GABA reuptake transporters become operative later than glutamate transporters. GABA is therefore not removed efficiently from the extracellular domain and it can exert significant paracrine effects. Hence, GABA-mediated activity is a prominent source of overall neural activity in developing CNS networks, while neurons extend dendrites and axons, and establish synaptic connections. One of the unique features of GABAergic functional plasticity is that in early development, activation of GABA_A receptors results in depolarizing (mainly excitatory) responses and Ca²⁺ influx. Although there is strong evidence from several areas of the CNS that GABA plays a significant role in neurite growth not only during development but also during adult neurogenesis, surprisingly little effort has been made into putting all these observations into a common framework in an attempt to understand the general rules that regulate these basic and evolutionary well-conserved processes. In this review, we discuss the current knowledge in this important field. In order to decipher common, universal features and highlight differences between systems throughout development, we compare findings about dendritic proliferation and remodeling in different areas of the nervous system and species, and we also review recent evidence for a role in axonal elongation. In addition to early developmental aspects, we also consider the GABAergic role in dendritic growth during adult neurogenesis, extending our discussion to the roles played by GABA during dendritic proliferation in early developing networks versus adult, well established networks.

Stress and GABA receptors

GABA(A) receptors are sensitive to subtle changes in the environment in both early-life and adulthood. These neurochemical responses to stress in adulthood are sex-dependent. Acute stress induces rapid changes in GABA(A) receptors in experimental animals, with the direction of the changes varying according to the sex of the animals and the stress-paradigm studied. These rapid alterations are of particular interest as they provide an example of fast neurotransmitter system plasticity that may be mediated by stress-induced increases in neurosteroids, perhaps via effects on phosphorylation and/or receptor trafficking. Interestingly, some studies have also provided evidence for long-lasting changes in GABA(A) receptors as a result of exposure to stressors in early-life. The short- and long-term stress sensitivity of the GABAergic system implicates GABA(A) receptors in the non-genetic etiology of psychiatric illnesses such as depression and schizophrenia in which stress may be an important factor.

Neurodevelopmental mechanisms of schizophrenia: understanding disturbed postnatal brain maturation through neuregulin-1-ErbB4 and DISC1

Schizophrenia (SZ) is primarily an adult psychiatric disorder in which disturbances caused by susceptibility genes and environmental insults during early neurodevelopment initiate neurophysiological changes over a long time course, culminating in the onset of full-blown disease nearly two decades later. Aberrant postnatal brain maturation is an essential mechanism underlying the disease. Currently, symptoms of SZ are treated with anti-psychotic medications that have variable efficacy and severe side effects. There has been much interest in the prodromal phase and the possibility of preventing SZ by interfering with the aberrant postnatal brain maturation associated with this disorder. Thus, it is crucial to understand the mechanisms that underlie the long-term progression to full disease manifestation to identify the best targets and approaches towards this goal. We believe that studies of certain SZ genetic susceptibility factors with neurodevelopmental implications will be key tools in this task. Accumulating evidence suggests that neuregulin-1 (NRG1) and disrupted-in-schizophrenia-1 (DISC1) are probably functionally convergent and play key roles in brain development. We provide an update on the role of these emerging concepts in understanding the complex time course of SZ from early neurodevelopmental disturbances to later onset and suggest ways of testing these in the future.

PET in the Assessment of Pediatric Brain Development and Developmental Disorders

This article discusses and reviews the role and contribution of PET in understanding the structural and functional changes that occur during brain development, and how these changes relate to behavioral and cognitive development in the infant and child. Data regarding various aspects of brain development, such as glucose metabolism, protein synthesis, and maturation and development of neurotransmitter systems will help in understanding the pathogenesis and neurologic basis of various developmental and neurologic disorders. This may help in following disease evolution and progression, planning and development of various therapeutic interventions, timing these interventions and monitoring their responses, and rendering long-term prognostication.

Protracted developmental trajectories of GABAA receptor alpha1 and alpha2 subunit expression in primate prefrontal cortex

BACKGROUND

In schizophrenia, working memory dysfunction is associated with altered expression of gamma-aminobutyric acid (GABA)(A) receptor alpha1 and alpha2 subunits in the dorsolateral prefrontal cortex (DLPFC). In rodents, cortical alpha subunit expression shifts from low alpha1 and high alpha2 to high alpha1 and low alpha2 during early postnatal development. Because these two alpha subunits confer different functional properties to the GABA(A) receptors containing them, we determined whether this shift in alpha1 and alpha2 subunit expression continues through adolescence in the primate DLPFC, potentially contributing to the maturation of working memory during this developmental period.

METHODS

Levels of GABA(A) receptor alpha1 and alpha2 subunit mRNAs were determined in the DLPFC of monkeys aged 1 week, 4 weeks, 3 months, 15-17 months (prepubertal), and 43-47 months (postpubertal) and in adult monkeys using in situ hybridization, followed by the quantification of alpha1 subunit protein by western blotting. We also performed whole-cell patch clamp recording of miniature inhibitory postsynaptic potentials (mIPSPs) in DLPFC slices prepared from pre- and postpubertal monkeys.

RESULTS

The mRNA and protein levels of alpha1 and alpha2 subunits progressively increased and decreased, respectively, throughout postnatal development including adolescence. Furthermore, as predicted by the different functional properties of alpha1-containing versus alpha2-containing GABA(A) receptors, the mIPSP duration was significantly shorter in postpubertal than in prepubertal animals.

CONCLUSIONS

In contrast to rodents, the developmental shift in GABA(A) receptor alpha subunit expression continues through adolescence in primate DLPFC, inducing a marked change in the kinetics of GABA neurotransmission. Disturbances in this shift might underlie impaired working memory in schizophrenia.

Neurochemical and physiological correlates of a critical period of respiratory development in the rat

Despite its vital importance to life, respiration is not mature at birth in mammals, but rather, it undergoes a great deal of growth, refinement, and adjustments postnatally. Many adjustments do not follow smooth paths, but assume abrupt changes during certain postnatal periods that may render the animal less capable of responding to respiratory stressors. The present review focuses on neurochemical and physiological correlates of a critical period of respiratory development in the rat. In addition to an imbalanced expression of reduced excitatory and enhanced inhibitory neurotransmitters, a switch in the expressions of gamma-aminobutyric acid (GABA)A receptor subunits from alpha3 to alpha1 occurs around postnatal day (P)12 in the pre-Bötzinger nucleus and the ventrolateral subnucleus of the solitary tract nucleus. Possible subunit switches in a number of other neurotransmitter receptors are discussed. These neurochemical changes are paralleled by ventilatory adjustments at the end of the second postnatal week. At P13 and under normoxia, respiratory frequency reaches its peak before assuming a gradual fall, and both tidal volume and minute ventilation exhibit a significant rise prior to a plateau or a gradual decline until P21. The response to acute hypoxia is markedly reduced between P12 and P16, being lowest at P13. Thus, the end of the second postnatal week can be considered as a critical period of respiratory development, during which multiple neurochemical and physiological adjustments and switches are orchestrated at the same time, rendering the system extremely dynamic but, at the same time, vulnerable to externally imposed perturbations and insults. The critical period embodies a time of multi-system, multifaceted growth and adjustments. It is a plastic, transitional period that is also a part of the normal development of the respiratory system.

Variations in excitatory and inhibitory postsynaptic protein content in rat cerebral cortex with respect to aging and cognitive status

Age-related cognitive impairments are associated with structural and functional changes in the cerebral cortex. We have previously demonstrated in the rat that excitatory and inhibitory pre- and postsynaptic changes occur with respect to age and cognitive status; however, in aged cognitively impaired animals, we have shown a significant imbalance in postsynaptic markers of excitatory versus inhibitory synapses, using markers of excitatory versus inhibitory neurotransmitter-related scaffolding proteins [postsynaptic density-95 (PSD95)/synapse associated protein-90 (SAP90) and gephyrin, respectively]. The present study focuses on whether the expression of various excitatory and inhibitory postsynaptic proteins is affected by ageing and cognitive status. Thus, aged animals were segregated into aged cognitively impaired (AI) and aged cognitively unimpaired (AU) groups using the Morris water maze. We applied Western immunoblotting to reveal the expression patterns of a number of relevant excitatory and inhibitory receptors in the prefrontal and parietal cortices of young (Y), AU and AI animals, and performed semi-quantitative analyses to statistically tabulate changes among the three animal groups. A significant increase in the inhibitory postsynaptic scaffold protein, gephyrin, was observed in the parietal cortex of AI animals. Similarly, an increase in GABA(A) receptor subunit alpha1 was observed in the parietal cortex of AI animals. An increase in the excitatory N-methyl-d-aspartate receptor subunit N-methyl-d-aspartate receptor 1 expression was observed in the parietal cortex of AI animals, whereas a significant decrease in AMPA receptor subunit glutamate receptor 2 expression was found in the prefrontal cortex of AI animals. Finally, the excitatory, postsynaptic neuronal cell-adhesion receptor, neuroligin-1, was found to be significantly increased in both the prefrontal and parietal cortical areas of AI animals.

Safety pharmacology assessment of central nervous system function in juvenile and adult rats: effects of pharmacological reference compounds

INTRODUCTION

Recent EU/US pediatric legislation and FDA/EMEA guidelines recognize the potential differences in safety profiles of drugs in adults versus young patients. Hence safety studies are recommended to investigate key functional domains of e.g. the developing CNS.

METHODS

Selected psychoactive stimulants (caffeine, d-amphetamine, scopolamine) and depressants (baclofen, diazepam, haloperidol, chlorpromazine, imipramine, morphine) were characterized upon single administration with regard to behavioural parameters, locomotor activity, body temperature, pro-/anti-convulsive activity (pentylenetetrazole, PTZ), and nocifensive responses (hotplate) in neonatal (2 weeks), juvenile (4 weeks) and adult rats (8-9 weeks).

RESULTS

In vehicle-treated rats, behavioural patterns matured with age, locomotor activity and handling-induced rise in body temperature were enhanced, whereas PTZ convulsion threshold dose and nocifensive response latency decreased. Single test compound treatment elicited behavioural effects characteristic for psychoactive drugs with stimulating and depressing properties regardless of age. However, incidence of certain behaviours, and magnitude of effects on locomotor activity and body temperature varied with age and became generally more pronounced in adult rats. Pro-/anti-convulsive effects and delayed nocifensive responses did not differ between juvenile and adult rats.

CONCLUSION

CNS effects of selected psychoactive reference compounds were qualitatively similar, but quantitatively different in neonatal, juvenile and adult rats.

GABA(A) receptors in normal development and seizures: friends or foes?

GABA(A) receptors have an age-adapted function in the brain. During early development, they mediate excitatory effects resulting in activation of calcium sensitive signaling processes that are important for the differentiation of the brain. In more mature stages of development and in adults, GABA(A) receptors transmit inhibitory signals. The maturation of GABA(A) signaling follows sex-specific patterns, which appear to also be important for the sexual differentiation of the brain. The inhibitory effects of GABA(A) receptor activation have been widely exploited in the treatment of conditions where neuronal silencing is necessary. For instance, drugs that target GABA(A) receptors are the mainstay of treatment of seizures. Recent evidence suggests however that the physiology and function of GABA(A) receptors changes in the brain of a subject that has epilepsy or status epilepticus.This review will summarize the physiology of and the developmental factors regulating the signaling and function of GABA(A) receptors; how these may change in the brain that has experienced prior seizures; what are the implications for the age and sex specific treatment of seizures and status epilepticus. Finally, the implications of these changes for the treatment of certain forms of medically refractory epilepsies and status epilepticus will be discussed.

Taurine acts as a glycine receptor agonist in slices of rat inferior colliculus

Taurine is an important endogenous amino acid for neural development and for many physiological functions, but little is known about its functional role in the central auditory system. We investigated in young rats (P10-P14) the effects of taurine on the neuronal responses and synaptic transmissions in the central nucleus of the inferior colliculus (ICC) with a brain slice preparation and with whole-cell patch-clamp recordings. Perfusion of taurine at 1mM reliably evoked a current across the membrane and decreased the input resistance in neurons of the ICC. Taurine also depressed the spontaneous and current-evoked firing of ICC neurons. All these effects were reversible after washout and could be blocked by 3 microM strychnine, an antagonist of glycine receptors, but not by 10 microM bicuculline, an antagonist of GABA(A) receptors. When the inhibitory receptors were not pharmacologically blocked, taurine reversibly reduced the postsynaptic currents/potentials evoked by electrically stimulating the commissure of the inferior colliculus or the ipsilateral lateral lemniscus. The results demonstrate that taurine reduces the neuronal excitability and depresses the synaptic transmission in the ICC by activating glycine-gated chloride channels. Our findings suggest that taurine acts as a ligand of glycine receptors in the ICC and can be involved in the information processing of the central auditory system similarly like the neurotransmitter glycine.

Developmental changes in the expression of GABAA receptor subunits alpha1, alpha2, and alpha3 in brain stem nuclei of rats

Gamma-aminobutyric acid (GABA)(A) receptor subunit switching is a suggested postnatal mechanism for changes in GABA transmission from depolarization to hyperpolarization. Previously, we found an apparent switch between GABA(A) alpha3 and alpha1 subunit expression in the rat pre-Bötzinger complex (PBC) on postnatal day (P) 12, a presumed peak critical period of respiratory nuclei development. The present study aimed at determining if GABA(A) subunit switching occurred in another respiratory nucleus, the ventrolateral subnucleus of the solitary tract nucleus (NTS(VL)), and in a non-respiratory cuneate nucleus (CN) of P0 to P21 rats. In both nuclei: (1) the expression of GABA(A) alpha1 subunit was relatively low at birth but increased with development; (2) that of GABA(A) alpha3 was relatively high at birth but declined with age; and (3) GABA(A) alpha2 remained relatively low and constant throughout development. However, the specific patterns differed between the two nuclei, but were similar between the NTS(VL) and the PBC. In the NTS(VL), GABA(A) alpha1 expression gradually increased from birth and peaked at P12, whereas that in the CN sharply rose from P7 and peaked at P10. GABA(A) alpha3 expression had a prominent decrease from P11 to P12 in the NTS(VL), whereas that in the CN only gradually declined from P10 to P12. The developmental trends of alpha1 and alpha3 in the NTS(VL) intersected close to P12, whereas those in the CN intersected at P10. Despite differences in timing, GABA(A) alpha subunit switching may be a common theme in the brain stem that may mediate different functional properties of GABA transmission.

Transitory uptake of serotonin in the developing sensory pathways of the common marmoset

Serotonin (5-HT) affects brain development during sensitive developmental periods. In rodents, transient sites of high affinity capture of 5-HT were demonstrated in the primary sensory neurons and in the sensory thalamocortical afferents. This uptake is required to adjust 5-HT receptor stimulation during the formation of sensory maps. To determine whether similar mechanisms exist in primates, we analyzed staged embryos and postnatal pups in the common marmoset (total gestation time, 142 days). Immunocytochemical analyses were performed using antisera to 5-HT, to the serotonin transporter (SERT), and to the vesicular monoamine transporter (VMAT2). 5-HT, SERT, and VMAT2 labeled the raphe neurons and their terminal network from embryonic day (E)70 to adulthood. In addition, from E70-130 VMAT2 and SERT were observed in all the sensory cranial nerves, the olfactory nerve, the gustatory, the trigeminal, the auditory fibers, in the retinal ganglion cells, and the optic tract up to the lateral geniculate nucleus and the superior colliculus. All the spinal sensory ganglia and their peripheral sensory branches were labeled. Accumulation of 5-HT was observed in all the sensory neurons expressing SERT and the corresponding axon tracts. Since these neurons were missing tryptophan hydroxylase (TPH), the synthesizing enzyme for 5-HT, they most likely accumulated 5-HT through the action of the amine transporters, as has been shown in rodents. No transient expression of 5-HT markers was detectable in the sensory thalamocortical axons at any of the ages examined. Thus, the existence of 5-HT uptake in nonserotoninergic neurons appears to be a conserved feature in primates, although the topographic extent of this transient expression is more restricted than that previously demonstrated in rodents.

Neurochemical development of brain stem nuclei involved in the control of respiration

The first two postnatal weeks are the most dynamic in the development of brain stem respiratory nuclei in the rat, the primary model for this review. Several neurochemicals (glutamate, glycine receptors, choline acetyltransferase, serotonin, norepinephrine, and thyrotropin-releasing hormone) increase expression with age, while others (GABA, serotonin receptor 1A, substance P, neurokinin 1 receptor, and somatostatin) decrease their expression. Surprisingly, a dramatic shift occurs at postnatal day (P) 12 in the rat. Excitatory neurotransmitter glutamate and its NMDA receptors fall precipitously, whereas inhibitory neurotransmitter GABA, GABA(B), and glycine receptors rise sharply. A concomitant drop in cytochrome oxidase activity occurs in respiratory neurons. Several receptor types undergo subunit switches during development. Notably, GABA(A) receptors switch prevalence from alpha3- to an alpha1-dominant form at P12 in the pre-Bötzinger complex of the rat. The transient dominance of inhibitory over excitatory neurotransmission around P12 may render the respiratory system sensitive to failure when stressed. Relating these neurochemical changes to physiological responses in animals and to sudden infant death syndrome in humans will be a challenge for future research.

Developmental changes in GABA receptor subunit composition within the gonadotrophin-releasing hormone-1 neuronal system

It is becoming increasingly evident that GABA plays an important role in the regulation of gonadotrophin-releasing hormone (GnRH)-1 neurones via the GABAA receptor. The aim of the present study was to characterise expression of the GABAA receptor within the GnRH-1 system across development. The expression pattern of five GABAAalpha subunits and one GABAAbeta subunit was first examined within individual GnRH-1 neurones by the polymerase chain reaction. A significant increase in the expression of GABAAalpha2 and a significant decrease in the expression of GABAAalpha6 over time were found. Of the other subunits examined, two (alpha1 and alpha3) showed no differences in expression and two (alpha4 and beta3) showed variable low incidence of expression. Given the reciprocal relationship of alpha2 and alpha6 expression, we hypothesised that there is a developmental switch in the expression of these subunits in GnRH-1 neurones. To investigate this hypothesis, single- and double-label immunocytochemistry for GABAAalpha2 and alpha6 and GnRH-1 was performed in tissue from ages E12.5 to adulthood, as well as in nasal explants. We show that GABAAalpha2 and alpha6 are present in the GnRH-1 neuronal system both in vivo and in vitro and that the levels of expression are altered as a function of age.

Development of human visual cortex: a balance between excitatory and inhibitory plasticity mechanisms

Formation of neural circuitry in the developing visual cortex is shaped by experience during the critical period. A number of mechanisms, including N-methyl-D-aspartate (NMDA) receptor activation and gamma-aminobutyric acid (GABA)-mediated inhibition, are crucial in determining onset and closure of the critical period for visual plasticity. Animal models have shown that a threshold level of tonic inhibition must be reached for critical period plasticity to occur and that NMDA receptors contribute to Hebbian synaptic plasticity in the developing visual cortex. There are a number of developmental changes in these glutamatergic and GABAergic mechanisms that have been linked to plasticity; however, those changes have been shown only in animal models, and their development in the human visual cortex is not known. We have addressed this question by studying the expression of the major glutamatergic receptors, GABA(A) receptors, and glutamic acid decarboxylase (GAD) isoforms during the first 6 years of postnatal development of human visual cortex. There are significant changes in the expression of these proteins during postnatal development of human visual cortex. The time course of the changes is quite prolonged and suggests that it may set the pace for the prolonged critical period in human visual development. The changes also affect the nature of spatial and temporal integration in visual cortical neurons and thereby contribute to the maturation of visual functions.

Perinatal maternal exposure to picrotoxin: Effects on sexual behavior in female rat offspring

A previous study in our laboratory showed that perinatal maternal picrotoxin exposure (0.75 mg/kg) in rats improved heterosexual behavior in male offspring. In the present study, we examined the effects of this maternal treatment on sexual behavior in the female offspring. The dams received 0.75 mg/kg picrotoxin treatment (PT) once a day on the 18th and 21st day of pregnancy, 2 h after parturition and once a day during the first 4 days of lactation. The results showed that (1) at birth, the body weight and anogenital distance were not modified by treatment; (2) female sexual behavior was improved in experimental animals. These results demonstrate that perinatal picrotoxin exposure improves adult sexual behavior in female rat offspring as suggested by increase in the lordosis quotient.

GABAergic signaling in the developing cerebellum

In the adult central nervous system (CNS), GABA is a predominant inhibitory neurotransmitter that regulates glutamatergic activity. Recent studies have revealed that GABA serves as an excitatory transmitter in the immature CNS and acts as a trophic factor for brain development. Furthermore, synaptic transmission by GABA is also involved in the expression of higher brain functions, such as memory, learning and anxiety. These results indicate that GABA plays various roles in the expression of brain functions and GABAergic roles change developmentally in accordance with alterations in GABAergic transmission and signaling. We have investigated morphologically the developmental changes in the GABAergic transmission system and the key factors important for the formation of GABAergic synapses and networks using the mouse cerebellum, which provides an ideal system for the investigation of brain development. Here, we focus on GABA and GABA(A) receptors in the developing cerebellum and address the processes of how GABA exerts its effect on developing neurons and the mechanisms underlying the formation of functional GABAergic synapses.

Organization of GABA receptor alpha-subunit clustering in the developing rat neocortex and hippocampus

We compared the expression and co-expression of alpha1, alpha2, alpha3, and alpha5-subunit protein clusters of the gamma-aminobutyric acid (GABA)(A) receptor in the neocortex and hippocampus of rat at postnatal days (PND) 5-10 and 30-40 in order to understand how inhibitory receptors reorganize during brain maturation. The size, intensity, density and pattern of co-localization of fluorescently tagged subunit clusters were determined in deconvolved digital images using a novel 2D cross-correlational analysis. The cross-correlation analysis allowed an unbiased identification of GABA(A) receptor subunit clusters based on staining intensity. Cluster size increased through development; only the alpha2 clusters in dentate gyrus (DG) decreased in size. alpha5-subunit cluster density either increased or decreased with maturation depending on the brain region. For the other subunits, the cluster density remained rather constant, with noted exceptions (increase in alpha2 clusters in cortical layer 5 but a decrease of alpha3 clusters in hilus). The co-localization of alpha1-subunit with the others was unique and not correlated to overall changes in subunit abundance between developmental époques. So, although alpha2-subunit expression went up in the DG, the clusters became less co-localized with alpha1. In contrast, alpha5-subunit clusters became more co-localized with alpha1 as the alpha5-subunit expression declined in cortex and CA1. The co-localization of alpha3 with alpha1 also became greater in layer 6. In the adult brain not all clustering was associated with synapses, as many alpha-subunit clusters did not co-localize with synaptophysin. Overall, these data indicate that the regulation of GABA(A) receptor clustering is both synaptic and extrasynaptic, presumably reflecting complex cellular trafficking mechanisms.

Developmental changes in the expression of GABAA receptor subunits alpha1, alpha2, and alpha3 in the rat pre-Botzinger complex

Previously, we reported that the pre-Bötzinger complex (PBC) exhibited a dramatic reduction in cytochrome oxidase activity at postnatal day (P) 12. This coincided in time with decreases in glutamate and NMDA receptor subunit 1 and increases in GABA, GABAB, glycine receptor, and glutamate receptor GluR2. To test our hypothesis that various alpha-subunits of GABAA receptors also undergo changes in their expression during postnatal development, as they do in other brain regions, we undertook an in-depth immunohistochemical study of GABAA receptor subunits alpha1, alpha2, and alpha3 in the PBC of P0 to P21 rats. We found that 1) GABAA alpha3-subunit was expressed at relatively high levels at P0, which then declined with age; 2) GABAA alpha1-subunit was expressed at relatively low levels at P0 but increased with age; 3) the developmental trends of subunits alpha1 and alpha3 intersected at P12; and 4) GABAA alpha2-subunit expression was moderate to light at P0 and remained quite constant during development, being lowest at P21. These findings suggest that the apparent switch in relative expressions of subunits alpha3 and alpha1 during development and the intersection of slopes around P12 may be associated with possible changes in GABAA receptor subtypes that would mediate different functional properties of GABA transmission, such as primarily a less efficient inhibitory transmission before P12 and a more mature inhibitory effect at P12 and thereafter, as suggested by the kinetics of distinct postsynaptic potentials. This mechanism may contribute partially to the dramatic reduction in cytochrome oxidase activity within the PBC at P12, as shown previously.

Postnatal changes in cytochrome oxidase expressions in brain stem nuclei of rats: implications for sensitive periods

Previously, we reported that cytochrome oxidase (CO) activity in the rat pre-Bötzinger complex (PBC) exhibited a plateau on postnatal days (P) 3-4 and a prominent decrease on P12 (Liu and Wong-Riley, J Appl Physiol 92: 923-934, 2002). These changes were correlated with a concomitant reduction in the expression of glutamate and N-methyl-d-aspartate receptor subunit 1 and an increase in GABA, GABAB, glycine receptor, and glutamate receptor 2. To determine whether changes were limited to the PBC, the present study aimed at examining the expression of CO in a number of brain stem nuclei, with or without known respiratory functions from P0 to P21 in rats: the ventrolateral subnucleus of the solitary tract nucleus, nucleus ambiguus, hypoglossal nucleus, nucleus raphe obscurus, dorsal motor nucleus of the vagus nerve, medial accessory olivary nucleus, spinal nucleus of the trigeminal nerve, and medial vestibular nucleus (MVe). Results indicated that, in all of the brain stem nuclei examined, CO activity exhibited a general increase with age from P0 to P21, with MVe having the slowest rise. Notably, in all of the nuclei examined except for MVe, there was a plateau or decrease at P3-P4 and a prominent rise-fall-rise pattern at P11-P13, similar to that observed in the PBC. In addition, there was a fall-rise-fall pattern at P15-P17 in these nuclei, instead of a plateau pattern in the PBC. Our data suggest that the two postnatal periods with reduced CO activity, P3-P4 and especially P12, may represent common sensitive periods for most of the brain stem nuclei with known or suspected respiratory control functions.

Postnatal development of pre- and postsynaptic GABA markers at chandelier cell connections with pyramidal neurons in monkey prefrontal cortex

The protracted postnatal maturation of the primate prefrontal cortex (PFC) is associated with substantial changes in the number of excitatory synapses on pyramidal neurons, whereas the total number of inhibitory synapses appears to remain constant. In this study, we sought to determine whether the developmental changes in excitatory input to pyramidal cells are paralleled by changes in functional markers of inhibitory inputs to pyramidal neurons. The chandelier subclass of gamma-aminobutyric acid (GABA) neurons provides potent inhibitory control over pyramidal neurons by virtue of their axon terminals, which form distinct vertical structures (termed cartridges) that synapse at the axon initial segment (AIS) of pyramidal neurons. Thus, we examined the relative densities, laminar distributions, and lengths of presynaptic chandelier axon cartridges immunoreactive for the GABA membrane transporter 1 (GAT1) or the calcium-binding protein parvalbumin (PV) and of postsynaptic pyramidal neuron AIS immunoreactive for the GABA(A) receptor alpha(2) subunit (GABA(A) alpha(2)) in PFC area 46 of 38 rhesus monkeys (Macaca mulatta). From birth through 2 years of age, the relative densities and laminar distributions of these three markers exhibited different trajectories, suggesting developmental shifts in the weighting of at least some factors that determine inhibition at the AIS. In contrast, from 2 to 4 years of age, all three markers exhibited similar declines in density and length that paralleled the periadolescent pruning of excitatory synapses to pyramidal neurons. Across development, the predominant laminar location of PV-labeled cartridges and GABA(A) alpha(2)-immunoreactive AIS shifted from the middle to superficial layers, whereas the laminar distribution of GAT1-positive cartridges did not change. Together, these findings suggest that the maturation of inhibitory inputs to the AIS of PFC pyramidal neurons is a complex process that may differentially affect the firing patterns of subpopulations of pyramidal neurons at specific postnatal time points.

Structural and functional maturation of the developing primate brain

Descriptive studies have established that the developmental events responsible for the assembly of neural systems and circuitry are conserved across mammalian species. However, primates are unique regarding the time during which histogenesis occurs and the extended postnatal period during which myelination of pathways and circuitry formation occur and are then subsequently modified, particularly in the cerebral cortex. As in lower mammals, the framework for subcortical-cortical connectivity in primates is established before midgestation and already begins to remodel before birth. Association systems, responsible for modulating intracortical circuits that integrate information across functional domains, also form before birth, but their growth and reorganization extend into puberty. There are substantial differences across species in the patterns of development of specific neurochemical systems. The complexity is even greater when considering that the development of any particular cellular component may differ among cortical areas in the same primate species. Developmental and behavioral neurobiologists, psychologists, and pediatricians are challenged with understanding how functional maturation relates to the evolving anatomical organization of the human brain during childhood, and moreover, how genetic and environmental perturbations affect the adaptive changes exhibited by neural circuits in response to developmental disruption.

Carotid body denervation effect on cytochrome oxidase activity in pre-Botzinger complex of developing rats

Previously, we found that the rat pre-Bötzinger complex (PBC) exhibited reduced cytochrome oxidase (CO) activity on postnatal days (P) 3-4 and especially on P12, with a concomitant decrease in glutamate and N-methyl-d-aspartate receptor subunit 1, and an increase in GABA, GABA(B), glycine receptor, and glutamate subunit 2. We hypothesized that the PBC would be more affected by carotid body denervation (CBD) during the two critical windows than at other times. Pairs of CBD and sham animals at each postnatal day from P2 to P14 and at P21 were operated on and survived for 3 days. Brain stems were processed for CO and neurokinin-1 receptor for the identification of PBC. Results indicate that CBD caused a significant loss in body weight in all animals and a reduction in PBC somal size when the surgery was between P2 and P7. CBD also induced a significant decrease in CO activity of the PBC in most animals and a distinct delay, as well as prolongation of the maturational process, especially when induced close to P3 and P11-P13.

GABA(A) receptor beta3 subunit deletion decreases alpha2/3 subunits and IPSC duration

Deletion of the beta3 subunit of the GABA(A) receptor produces severe behavioral deficits and epilepsy. GABA(A) receptor-mediated miniature inhibitory postsynaptic currents (mIPSCs) in cortical neurons in cultures from beta3 -/- mice were significantly faster than those in beta3 +/+ mice and were more prolonged by zolpidem. Surface staining revealed that the number of beta2/3, alpha2, and alpha3 (but not of alpha1) subunit-expressing neurons and the intensity of subunit clusters were significantly reduced in beta3 -/- mice. Transfection of beta3 -/- neurons with beta3 cDNA restored beta2/3, alpha2, and alpha3 subunits immunostaining and slowed mIPSCs decay. We show that the deletion of the beta3 subunit causes the loss of a subset of GABA(A) receptors with alpha2 and alpha3 subunits while leaving a receptor population containing predominantly alpha1 subunit with fast spontaneous IPSC decay and increased zolpidem sensitivity.

Hypotheses from functional neuroimaging studies

Functional neuroimaging, especially positron emission tomography (PET) using various tracers, provided new insights into the pathophysiology of West syndrome in the past decade. Glucose PET studies revealed a unique corticosubcortical circuitry assumed to be involved in the age-dependent generalization of seizure activity leading to symmetric spasms. The findings strongly suggested that cortical abnormalities, mostly consistent with dysplastic lesions or diffuse cortical dysfunction due to an underlying systemic disorder, trigger brain stem nuclei and activate basal ganglia bilaterally. PET is also able to investigate developmental abnormalities of serotonergic and GABAergic neurotransmitter systems in vivo. Involvement of these systems in the pathophysiology of infantile spasms is strongly supported by animal data and can be further elucidated by future PET studies. In addition, the development of new PET tracers (such as neurotracers for imaging NMDA receptors) could help further clarify the role of altered neurotransmission in generation of spasms. This review of the most important functional neuroimaging findings illustrates how human PET and single photon emission computed tomography data help answer basic questions regarding the pathomechanisms involved in this often devastating condition and how these findings might facilitate development of a useful animal model of West syndrome.

Expression of the alpha4 isoform of the nicotinic acetylcholine receptor in the fetal human cerebral cortex

Nicotinic acetylcholine receptors are likely to play an important role in neuronal migration during development. Furthermore, the alpha4 receptor subunit gene is related to a hereditary juvenile form of epilepsy. Only little information is available, however, on the expression of cerebrocortical nicotinic acetylcholine receptors during human fetal development. Using non-isotopic in situ hybridization and immunohistochemistry, we have studied the distribution of the alpha4 subunit of the nicotinic acetylcholine receptor mRNA and protein in the human frontal cortex at middle (17-24 weeks of gestation) and late (34-42 weeks of gestation) fetal stages. Both, alpha4 receptor mRNA and alpha4 receptor protein were observed beginning during week 17-18 of gestation. At this time of development, a few weakly labeled mRNA-containing cells were present mainly in the ventricular zone, the subplate and the cortical plate. A similar distribution pattern was found for the receptor protein. Around week 38 of gestation, the distribution in the cerebral cortex of alpha4 subunit-containing cells was similar to that of adult human cortices with the highest densities of labeled neurons found in layers II/III, followed by layers V and VI. Nicotinic acetylcholine receptor-containing neurons appear rather early in human fetal development. Given functional maturity, they may interact during cortical development with acetylcholine released from corticopetal fibers or other yet unknown sources subserving the process of neuronal migration and pathfinding.

Neuroradiological assessment of brain structure and function and its implication in the pathogenesis of West syndrome

Neuroimaging studies with magnetic resonance imaging (MRI) and positron emission tomography (PET) scanning have contributed significantly to our understanding of West syndrome. Cortical dysplastic lesions are the most common abnormalities seen with MRI in infants with spasms, but other structural lesions are also detected occasionally. An underlying cortical dysplasia may not be apparent until myelination has advanced in the brain and poor gray-white matter differentiation becomes observable. Many cortical dysplastic lesions can only be detected using PET scanning of glucose metabolism or gamma-aminobutyric acid(A) (GABA(A)) receptor binding. The MRI and PET findings, together with neurophysiological observations, strongly suggest that infantile spasms are initiated as cortical epileptic discharges that, during a 'critical' developmental period, may undergo secondary generalization in an age-dependent mechanism to emerge as spasms. The onset of spasms often coincides with the functional maturation of cerebral cortex. Based on data from glucose metabolism PET scanning as well as electrophysiological and neurochemical findings on infants with spasms, we have postulated that the offending lesion is a focal or diffuse cortical abnormality which, at a critical stage of maturation, causes abnormal functional interactions with brainstem raphe nuclei which project widely throughout the brain. Raphe-cortical projections could mediate the hypsarrhythmic changes seen on EEG. The prominent serotonergic raphe-striatal pathway and descending spinal pathways may be responsible for secondary generalization of the cortical discharges to result in the relatively symmetric spasms. It is likely that additional factors (e.g. genetic) play a role in the manifestation of the age-specific electroclinical features of West syndrome. Recently developed PET tracers can be used to detect epileptogenic brain regions and also to investigate developmental abnormalities of serotonergic (using the tracer alpha[(11)C]methyl-L-tryptophan) and GABAergic (using [(11)C]flumazenil) neurotransmitter systems. These systems are implicated in epileptogenesis, and their involvement in the pathophysiology of West syndrome can be further addressed by future functional neuroimaging studies.