Neuroepithelial cells in the rat spinal cord express glutamate decarboxylase immunoreactivity in vivo and in vitro

GABAergic neurons regulate lateral ventricular development via transcription factor Pax5

Postmortem studies have revealed a downregulation of the transcription factor Pax5 in GABAergic neurons in bipolar disorder, a neurodevelopmental disorder, raising the question whether Pax5 in GABAergic neurons has a role in normal brain development. In a genetic approach to study functions of Pax5 in GABAergic neurons, Pax5 was specifically deleted in GABAergic neurons from Pax5 floxed mice using a novel Gad1-Cre transgenic mouse line expressing Cre recombinase in Gad1-positive, that is, GABAergic neurons. Surprisingly, these mice developed a marked enlargement of the lateral ventricles at approximately 7 weeks of age, which was lethal within 1-2 weeks of its appearance. This hydrocephalus phenotype was observed in mice homozygous or heterozygous for the Pax5 conditional knockout, with a gene dosage-dependent penetrance. By QTL (quantitative trait loci) mapping, a 3.5 Mb segment on mouse chromosome 4 flanked by markers D4Mit237 and D4Mit214 containing approximately 92 genes including Pax5 has previously been linked to differences in lateral ventricular size. Our findings are consistent with Pax5 being a relevant gene underlying this QTL phenotype and demonstrate that Pax5 in GABAergic neurons is essential for normal ventricular development.

Neuroendocrine alterations in the fragile X mouse

The expression of GABA(A) receptors in the fragile X mouse brain is significantly downregulated. We additionally found that the expression of somatostatin and voltage-sensitive calcium channels (VSCCs) is also reduced. GABA(A) and the VSCCs, through a synergistic interaction, perform a critical role in mediating activity-dependent developmental processes. In the developing brain, GABA is excitatory and its actions are mediated through GABA(A) receptors. Subsequent to GABA-mediated depolarization, the VSCCs are activated and intracellular calcium is increased, which mediates gene transcription and other cellular events. GABAergic excitation mediated through GABA(A) receptors and the subsequent activation of the VSCCs are critically important for the establishment of neuronal connectivity within immature neuronal networks. Data from our laboratories suggest that there is a dysregulation of axonal pathfinding during development in the fragile X mouse brain and that this is likely due to a dysregulation of the synergistic interactions of GABA and VSCC. Thus, we hypothesize that the altered expression of these critical channels in the early stages of brain development leads to altered activity-dependent gene expression that may potentially lead to the developmental delay characteristic of the fragile X syndrome.

Segmental, synaptic actions of commissural interneurons in the mouse spinal cord

Left-right alternation depends on activity in commissural interneurons (CINs) that have axons crossing in the midline. In this study, we investigate the CIN connectivity to local motor neurons using a newly developed preparation of the in vitro neonatal mouse spinal cord that allows us to identify all classes of CINs. Nineteen of 29 short-range CINs with axonal projections <1.5 segments (sCINs) directly excited, directly inhibited, or indirectly inhibited contralateral motor neurons in the quiescent spinal cord. Excitation was glutamatergic and inhibition was mixed glycinergic and/or GABAergic. Long-range CINs were also found to have input to local, contralateral motor neurons. Thirteen of 29 descending CINs had similar synaptic connectivity to contralateral motor neurons as the sCINs, including direct excitation and direct and indirect inhibition. Some (9 of 23) rostrally projecting ascending CINs, and a few (2 of 10) CINs with bifurcating axons that both ascend and descend, indirectly inhibited local, contralateral motor neurons. Rhythmic firing during locomotor-like activity was observed in a number of CINs with segmental synaptic effects on contralateral motor neurons. This study outlines the basic connectivity pattern of CINs in the mouse spinal cord on a segmental level. Our study suggests that, based on observed synaptic connectivity, both short- and long-range CINs are likely involved in segmental left-right coordination and that the CIN system is organized into a dual-inhibitory and single-excitatory system. These systems are organized in a way that they could provide appropriate coordination during locomotion.

GABA signalling: therapeutic targets for epilepsy, Parkinson's disease and Huntington's disease

Temporal lobe epilepsy (TLE), Parkinson's disease (PD) and Huntington's disease (HD) are neurodegenerative disorders that involve disruptions in gamma-amino butyric acid (GABA) signalling. GABA is the major inhibitory neurotransmitter in the central nervous system (CNS). TLE seizures reflect excess excitation, which may result from local inhibitory circuit dysfunction. PD devastates the input to striatal GABAergic neurones and HD destroys striatal GABAergic neurones. Controlling GABA delivery to specific brain areas should benefit each of these diseases. The molecules responsible for GABA release and signalling are ideal targets for new therapies. In this paper, we discuss the role of GABA in the circuitry affected by each of these diseases and suggest potential sites for intervention. GABA is unique among neurotransmitters because it can be synthesised by either of two related enzymes. Intracellular GABA is found throughout the cytosol and in synaptic vesicles. GABA can be released either through exocytosis, or via the plasma membrane transporter. The synthesising enzyme probably determines the intracellular location and hence the mechanism for GABA release. Directing GABA synthesis, degradation, transport or receptors can control GABA signalling. We propose that new drugs and devices aimed at GABA synthesis, release and binding will offer novel and highly effective treatments for neurodegenerative diseases.

Unique developmental patterns of GABAergic neurons in rat spinal cord

Gamma-aminobutyric acid (GABA)ergic neurons have been postulated to compose an important component of local circuits in the adult spinal cord, yet their identity and axonal projections have not been well defined. We have found that, during early embryonic ages (E12-E16), both glutamic acid decarboxylase 65 (GAD65) and GABA were expressed in cell bodies and growing axons, whereas at older ages (E17-P28), they were localized primarily in terminal-like structures. To determine whether these developmental changes in GAD65 and GABA were due to an intracellular shift in the distribution pattern of GAD proteins, we used a spinal cord slice model. Initial experiments demonstrated that the pattern of GABAergic neurons within organotypic cultures mimicked the expression pattern seen in embryos. Sixteen-day-old embryonic slices grown 1 day in vitro contained many GAD65- and GAD67-labeled somata, whereas those grown 4 days in vitro contained primarily terminal-like varicosities. When isolated E14-E16 slices were grown for 4 days in vitro, the width of the GAD65-labeled ventral marginal zone decreased by 40-50%, a finding that suggests these GABAergic axons originated from sources both intrinsic and extrinsic to the slices. Finally, when axonal transport was blocked in vitro, the developmental subcellular localization of GAD65 and GAD67 was reversed, so that GABAergic cell bodies were detected at all ages examined. These data indicate that an intracellular redistribution of both forms of GAD underlie the developmental changes observed in GABAergic spinal cord neurons. Taken together, our findings suggest a rapid translocation of GAD proteins from cell bodies to synaptic terminals following axonal outgrowth and synaptogenesis.

Activity-dependent modulation of GABAergic synapses in developing rat spinal networks in vitro

The role of activity-dependent plasticity in modulating inhibitory synapses was investigated in embryonic rat spinal cord slice cultures, by chronic exposure to non-NMDA receptor blockers. GABAergic synaptic efficacy in control and chronic-treated cultures was investigated by patch-recordings from visually identified spinal interneurons. In both culture groups proximal stimulation induced the appearance of postsynaptic currents (PSCs), which were fully antagonized by 20 microM bicuculline application and reverse polarity at potential values close to those reported for spontaneous GABAergic PSCs. In chronically treated cells GABAergic evoked PSCs displayed a larger failure rate and a smaller coefficient of variation of mean PSC amplitude, when compared to controls. As opposed to controls, chronic GABAergic evoked PSCs did not facilitate upon paired-pulse stimulation. Facilitation at chronic synapses was observed when extracellular calcium levels were decreased below physiological values (< 2 mM). Kainate was used to disclose any functional differences between control and treated slices. In accordance with the presynaptic action of kainate, the application of this drug along with GYKI, an AMPA receptor selective antagonist, changed, with analogous potency, short-term plasticity of GABAergic synapses from control and treated cultures. Nevertheless, in chronic cultures, the downstream effects of such activation unmasked short-term depression. Ultrastructural analysis of synapses in chronically treated cultures showed a reduction both in symmetric synapses and in the number of vesicles at symmetric terminals. Thus, based on electrophysiological and ultrastructural data, it could be suggested that during the development of spinal circuits, GABAergic synapses are modulated by glutamatergic transmission, and thus implying that excitatory transmission regulates the strength of GABAergic synapses.

Acetylcholine stimulates cortical precursor cell proliferation in vitro via muscarinic receptor activation and MAP kinase phosphorylation

Increasing evidence has shown that some neurotransmitters act as growth-regulatory signals during brain development. Here we report a role for the classical neurotransmitter acetylcholine (ACh) to stimulate proliferation of neural stem cells and stem cell-derived progenitor cells during neural cell lineage progression in vitro. Neuroepithelial cells in the ventricular zone of the embryonic rat cortex were found to express the m2 subtype of the muscarinic receptor. Neural precursor cells dissociated from the embryonic rat cortical neuroepithelium were expanded in culture with basic fibroblast growth factor (bFGF). reverse transcriptase-polymerase chain reaction (RT-PCR) revealed the presence of m2, m3 and m4 muscarinic receptor subtype transcripts, while immunocytochemistry demonstrated m2 protein. ACh and carbachol induced an increase in cytosolic Ca2+ and membrane currents in proliferating (BrdU+) cells, both of which were abolished by atropine. Exposure of bFGF-deprived precursor cells to muscarinic agonists not only increased both cell number and DNA synthesis, but also enhanced differentiation of neurons. These effects were blocked by atropine, indicating the involvement of muscarinic ACh receptors. The growth-stimulating effects were also antagonized by a panel of inhibitors of second messengers, including 1,2-bis-(O-aminophenoxy)-ethane-N,N,N', N'-tetraacetic acid (BAPTA-AM) to chelate cytosolic Ca2+, EGTA to complex extracellular Ca2+, pertussis toxin, which uncouples certain G-proteins, the protein kinase C inhibitor H7 and the mitogen-activated protein kinase (MAPK) inhibitor PD98059. Muscarinic agonists activated MAPK, which was significantly inhibited by atropine and the same panel of inhibitors. Thus, muscarinic receptors expressed by neural precursors transduce a growth-regulatory signal during neurogenesis via pathways involving pertussis toxin-sensitive G-proteins, Ca2+ signalling, protein kinase C activation, MAPK phosphorylation and DNA synthesis.

Kir 4.1 channel expression in neuroblastomaxglioma hybrid NG108-15 cell line

To study a possible involvement of inwardly rectifying K+ 4.1 (Kir 4. 1) channels in neural cell development, RT-PCR, immunocytochemistry and whole-cell patch-clamp techniques were used to assess expression of Kir 4.1 channels in proliferating and differentiated NG108-15 cells. RT-PCR revealed co-expression of Kir 4.1 and rat ether-a-go-go-related gene (R-ERG) mRNAs in both proliferating and differentiated cells. The relative Kir 4.1 mRNA concentration increased markedly as cells progressed from undifferentiated to differentiated cells. Kir 4.1-immunoreactivity was barely detectable in undifferentiated cells, but clearly detected in differentiated cells, indicating that Kir 4.1 gene and protein expressions are developmentally regulated. However, corresponding Kir 4.1 current could not be detected in differentiated cells using whole-cell patch-clamp recording. The 'silent' channel/receptor, often found in tumor cells, may carry genetic defects, which prevent functional expression of the channel. NG108-15 may serve as unique model for studying the relationship between the expression of an ion channel gene and the electrophysiological phenotype it encodes.

Early induction of Talpha1 alpha-tubulin transcription in neurons of the developing nervous system

In this report, we have examined the relationship between the onset of neuronal gene transcription and neuronal development by characterizing expression of the early panneuronal Talpha1 alpha-tubulin promoter in developing neurons. In the peripheral nervous system, detectable expression of a beta-galactosidase transgene driven by the Talpha1 promoter (Talpha1:nlacZ) was coincident with neuronal birth dates, with the exception of sympathetic neuroblasts, which expressed the transgene prior to terminal mitosis. Similarly, in the central nervous system, the onset of beta-galactosidase expression was coincident with neuronal birth dates in most identifiable populations of central neurons. A small subpopulation of transgene-positive cells localized to ventricular zones, but the vast majority was observed in locations consistent with their identification as migrating and/or differentiating neurons. To determine more precisely the temporal relationship between transgene expression and terminal mitosis, we analyzed cultures of cortical progenitors that become postmitotic neurons in vitro. When initially plated, the vast majority of cells consisted of dividing, nestin-positive progenitors. Neurons differentiated from these progenitors as early as 1 day in vitro, as indicated by immunostaining for betaIII-tubulin, a neuron-specific tubulin isotype that is turned on shortly after terminal mitosis. Double-labeling studies showed that Talpha1:nlacZ expression was detectable in the same cells and at approximately the same time as was betaIII-tubulin, indicating that detectable transcription of the Talpha1 alpha-tubulin promoter commences at the time of terminal mitosis, at least in culture. This promoter, therefore, provides a valuable tool for genetic manipulation of early developing neurons in transgenic mice.

Basic FGF-responsive telencephalic precursor cells express functional GABA(A) receptor/Cl-channels in vitro

We have previously described the expression of specific gamma-aminobutyric acid (GABA)A receptor subunits and their transcripts in the cortical neuroepithelium (Ma and Barker, 1995, 1998). However, it is not clear whether neural precursor cells exposed to basic fibroblast growth factor (bFGF) in vitro reproduce the biological properties of neuroepithelial cells in vivo within the embryonic ventricular zone. In the present study, neural precursor cells were isolated from the telencephalic neuroepithelium of embryonic day 13-13.5 rats and exposed to bFGF in serum-free medium. Basic FGF-responsive cells were capable of dividing and differentiating into neurons and astrocytes. The rapidly dividing cells formed multicellular spheres and then a rosette-like formation in which a majority of cells expressed GABA(A) receptor alpha4, beta1, or gamma1 subunit proteins. We found in perforated patch-clamp recordings that GABA depolarized bromodeoxyundine (BrdU)+ precursor cells, and under voltage-clamp induced a bicuculline-sensitive current that reversed at the Cl- equilibrium potential. GABA also increased cytoplasmic Ca2+ in a significant number of BrdU+ cells that was blocked by bicuculline. The bicuculline sensitivity of these pharmacological effects implicates GABA(A) receptor/Cl- channels, since bicuculline is a competitive GABA(A) antagonist at these channels in well-differentiated cells. It is possible that the three GABA(A) receptor subunits (alpha4, beta1, and gamma1) found in precursor cells form the Cl- channels detected electrophysiologically. The functional GABA(A) receptor/Cl- channels and associated regulation of their cytoplasmic Ca2+ levels via bicuculline-sensitive mechanisms may play significant roles in the regulation of neural cell proliferation in this model neuroepithelium.

Initially expressed early rat embryonic GABA(A) receptor Cl- ion channels exhibit heterogeneous channel properties

We have studied the earliest expression of GABA-induced CI- channels in the rat embryonic dorsal spinal cord (DSC) using in situ hybridization, immunocytochemistry, flow cytometry and electrophysiology. At embryonic day 13 (E13) cells in the dorsal region are still proliferating. In situ hybridization consistently showed transcripts encoding only three GABAA receptor subunits (alpha4, beta1 and gammal); immunocytochemistry both in tissue sections and in acutely isolated cells in suspension demonstrated the expression of the corresponding proteins and also revealed staining for other subunits (alpha2, alpha3, beta3, gamma2). In patch-recordings performed in cells acutely isolated from the dorsal cord, responses to GABA were detected in 356 out of 889 cells. GABA-evoked responses, which often displayed the opening of a few channels, were mediated by CI- ions, were inhibited by bicuculline and picrotoxin, and potentiated by benzodiazepines. Taken together, these observations indicate that CI- channels likely involve GABAA type receptors. Fluctuation analysis revealed channel kinetics consisting of three exponential components (Ts: approximately 1,9 and 90 ms) and a wide variety of inferred unitary conductance values, ranging between 4 and 40 pS. A comparison of these results with observations in other, later embryonic cell types and recombinant receptors suggests that most of the earliest E13 DSC GABAA receptors may include alpha3 subunit. These GABAA receptor Cl- channels may be activated physiologically as both GABA synthesizing enzymes and GABA are present in the E13 dorsal cord.

GABAergic deafferentation hypothesis of brain aging and Alzheimer's disease revisited

Considering the mechanisms responsible for age- and Alzheimer's disease (AD)-related neuronal degeneration, little attention was paid to the opposing relationships between the energy-rich phosphates, mainly the availability of the adenosine triphosphate (ATP), and the activity of the glutamic acid decarboxylase (GAD), the rate-limiting enzyme synthesizing the gamma-amino butyric acid (GABA). Here, it is postulated that in all neuronal phenotypes the declining ATP-mediated negative control of GABA synthesis gradually declines and results in age- and AD-related increases of GABA synthesis. The Ca2+-independent carrier-mediated GABA release interferes with Ca2+-dependent exocytotic release of all transmitter-modulators, because the interstitial (ambient) GABA acts on axonal preterminal and terminal varicosities endowed with depolarizing GABA(A)-benzodiazepine receptors; this makes GABA the "executor" of virtually all age- and AD-related neurodegenerative processes. Such a role of GABA is diametrically opposite to that in the perinatal phase, when the carrier-mediated GABA release, acting on GABA(A)/chloride ionophore receptors, positively controls chemotactic migration of neuronal precursor cells, has trophic actions and initiates synaptogenesis, thereby enabling retrograde axonal transport of target produced factors that trigger differentiation of neuronal phenotypes. However, with advancing age, and prematurely in AD, the declining mitochondrial ATP synthesis unleashes GABA synthesis, and its carrier-mediated release blocks Ca2+-dependent exocytotic release of all transmitter-modulators, leading to dystrophy of chronically depolarized axon terminals and block of retrograde transport of target-produced trophins, causing "starvation" and death of neuronal somata. The above scenario is consistent with the following observations: 1) a 10-month daily administration to aging rats of the GABA-chloride ionophore antagonist, pentylenetetrazol, or of the BDZ antagonist, flumazenil (FL), each forestalls the age-related decline in cognitive functions and losses of hippocampal neurons; 2) the brains of aging rats, relative to young animals, and the postmortem brains of AD patients, relative to age-matched controls, show up to two-fold increases in GABA synthesis; 3) the aging humans and those showing symptoms of AD, as well as the aging nonhuman primates and rodents--all show in the forebrain dystrophic axonal varicosities, losses of transmitter vesicles, and swollen mitochondria. These markers, currently regarded as the earliest signs of aging and AD, can be reproduced in vitro cell cultures by 1 microM GABA; the development of these markers can be prevented by substituting Cl- with SO4(2-); 4) the extrasynaptic GABA suppresses the membrane Na+, K+-ATPase and ion pumping, while the resulting depolarization of soma-dendrites relieves the "protective" voltage-dependent Mg2+ control of the N-methyl-D-aspartate (NMDA) channels, thereby enabling Ca2+-dependent persistent toxic actions of the excitatory amino acids (EAA); and 5) in whole-cell patch-clamp recording from neurons of aging rats, relative to young rats, the application of 3 microM GABA, causes twofold increases in the whole-cell membrane Cl- conductances and a loss of the physiologically important neuronal ability to desensitize to repeated GABA applications. These age-related alterations in neuronal membrane functions are amplified by 150% in the presence of agonists of BDZ recognition sites located on GABA receptor. The GABA deafferentation hypothesis also accounts for the age- and AD-related degeneration in the forebrain ascending cholinergic, glutamatergic, and the ascending mesencephalic monoaminergic system, despite that the latter, to foster the distribution-utilization of locally produced trophins, evolved syncytium-like connectivities among neuronal somata, axon collaterals, and dendrites, to bidirectionally transport trophins. (ABSTRACT TRUNCATED)

Neuronal and glial epitopes and transmitter-synthesizing enzymes appear in parallel with membrane excitability during neuroblastoma x glioma hybrid differentiation

The membrane excitability and the presence of neural proteins, including neuronal and glial markers and neurotransmitter-synthesizing enzymes, were examined in parallel while the NG108-15 cell line was maintained in a serum-free medium. Whole-cell recordings in voltage-clamp or current-clamp configurations were used to evaluate the membrane excitability, and immunostaining was done with a panel of well-characterized antibodies against NSE, NF150, S-100 beta, GFAP, ChAT and TH. Culture for 4 to 10 days led to a striking rise in neurite outgrowth, electrical excitability and expression of neural proteins in type I neuron-like cells, which were of both neuronal and glial character, and expressed both cholinergic and adrenergic traits. After about 2 weeks, type II cells which lack neurite processes began to emerge. The type II cells proliferated, as revealed by BrdU uptake, and gradually overgrew differentiated cell types. They exhibited little or no membrane excitability and absence of immunoreactivity for the neuronal and glial specific proteins tested. These measurements indicate that the presence of these neural proteins at crucial stages of membrane excitability development is an important characteristics of NG108-15 cell differentiation, providing insights into the neural development and the reversible nature of neoplasia in the nervous system.

GABA, GAD, and GABA(A) receptor alpha4, beta1, and gamma1 subunits are expressed in the late embryonic and early postnatal neocortical germinal matrix and coincide with gliogenesis

Increasing evidence indicates that the classical, fast-acting neurotransmitter gamma-amino butyric acid (GABA) may initially act as morphogen in cell proliferation and differentiation via specific receptors. In view of the potential roles for GABA in central nervous system development, we examined the expression of GABA, GABA(A) receptor beta1 and gamma1 subunits by immunocytochemistry and the expression of transcripts for two GABA-synthesizing enzymes, glutamate decarboxylase (GAD65, GAD67 mRNAs), and for alpha4, beta1, and gamma1 subunits of GABA(A) receptor by in situ hybridization in the developing neocortex. Tissue sections were taken from embryonic days (E) 17 and E20 embryos and newborn rats (P0). The embryos' mothers and newborn rats had been injected with 5-bromo-2'-deoxyuridine (BrdU) and had survived for 2 hours. At E17, BrdU-positive cells were largely restricted in the synthetic zone at the ventricular margin when cortical neurogenesis was still active. GAD mRNAs and GABA immunoreactivity were detected in the subventricular zone, while alpha4, beta1, and gamma1 subunits were abundant in the ventricular zone. At E20 and P0, when neurogenesis had largely ceased and gliogenesis had commenced, BrdU-positive cells were found throughout the ventricular zone with GABA, GAD mRNAs, and alpha4, beta1, and gamma1 subunits. GABA, GAD mRNAs and alpha4, beta1, and gamma1 subunit signals intensified in the ventricular zone from E17 to P0 as gliogenesis proceeded. Thus, specific components of a putative GABAergic circuit are expressed in cells of the ventricular zone during the late embryonic/early postnatal period coincident with gliogenesis, suggesting a role for GABA in glial cell proliferation.

Expression of glutamic acid decarboxylase during human neuronal differentiation: studies using the NTera-2 culture system

Human NTera-2N neurons, but not the parental NTera-2 teratocarcinoma line, decarboxylate [2-(15)N]glutamine to form gamma-[15N]aminobutyric acid (GABA). The reverse transcriptase-polymerase chain reaction (RT-PCR) followed by Southern blotting showed that NTera-2N neurons transcribe the glutamic acid decarboxylase p67 (GAD67) gene, and also demonstrated that there is developmentally regulated alternative splicing of GAD67 mRNA in NTera-2N neurons. As in rat central nervous system (CNS), this mRNA processing generates two RNA transcripts, owing to the inclusion or exclusion of an approximately 80 bp coding region insert. In embryonic day 16 (E16) rat brain, the larger of the two GAD67 mRNAs, which encodes a truncated, inactive apoenzyme, reaches a concentration almost equal to that of the smaller transcript, which encodes functional GAD67. In developing NTera-2N neurons, however, the larger transcript is barely detectable by RT-PCR. RT-PCR also revealed that rat CNS of all ages examined contains GAD65 mRNA, and that GAD65 mRNA is below the detectable range in NTera-2N neurons.

Anatomical gradients in proliferation and differentiation of embryonic rat CNS accessed by buoyant density fractionation: alpha 3, beta 3 and gamma 2 GABAA receptor subunit co-expression by post-mitotic neocortical neurons correlates directly with cell buoyancy

Development of the CNS occurs as a complex cascade of pre-programmed events involving distinct phases of cell proliferation and differentiation. Here we show these phases correlate with cells of specific buoyant densities which can be readily accessed by density gradient fractionation. Sprague-Dawley dams were pulse-labelled with bromodeoxyuridine (BrdU) and selected regions of embryonic (E) CNS tissues at E11-22 dissociated with papain into single-cell suspensions. Proliferative cell populations were assessed by anti-BrdU and propidium iodide staining using flow cytometry. Cell differentiation was evaluated using molecular and immunocytochemical probes against mRNAs and antigens differentiating the neuroepithelial, neuronal and glial cell lineages. The results show the emergence of distinctive spatiotemporal changes in BrdU+ populations throughout the CNS during embryonic development, which were followed by corresponding changes in the cellular distributions of antigens distinguishing specific cell types. Fractionation of neocortical cells using discontinuous Percoll gradients revealed that an increasing number of cells increase their buoyancy during corticogenesis. Immunocytochemical and molecular characterization showed that the proliferative and progenitor cell populations are for the most part associated with lower buoyancy or higher specific buoyant densities (> 1.056 g/ml) whereas the post-mitotic, differentiated neurons generally separated into fractions of higher buoyancy or lower specific buoyant densities (< 1.043 g/ml). Immunostaining with antibodies against several GABAA receptor subunits (alpha 3, beta 3, gamma 2) revealed that the highest percent (70-90%) of immunopositive cells could be identified in the most buoyant, differentiating neurons found in the cortical plate/subplate regions, with the lowest percent of the immunopositive cells found in the least buoyant, proliferative and progenitor cell populations originating from the ventricular/subventricular zones. Taken together, these results indicate that buoyant density is a distinguishing characteristic of embryonic CNS cells transforming from primarily proliferative to mainly differentiating, and that fractionation of these cells according to their buoyant densities provides rapid access to the properties of specific cell lineages during the prenatal period of CNS development.

Prominent expression of two forms of glutamate decarboxylase in the embryonic and early postnatal rat hippocampal formation

Immunohistochemical methods were used to determine the earliest times of detection for two forms of glutamate decarboxylase (GAD67 and GAD65) in the embryonic and early postnatal rat hippocampal formation and to determine whether their distribution patterns differed from each other and from those of the adult. Both GAD67- and GAD65-containing neurons were observed as early as embryonic day 17 (E17)-E18 in the hippocampus and E19 in the dentate gyrus, and this was substantially earlier than GAD had been detected previously in the hippocampal formation. The two GAD isoforms displayed very similar distribution patterns, but these patterns were distinctly different from those of the adult. From E17 to E20, GAD67 and GAD65 were expressed in neuronal cell bodies throughout the hippocampal and dentate marginal zones (future dendritic layers), and relatively few existed within the principal cell body layers, where GAD-positive neurons are frequently concentrated in the adult. At E21 to postnatal day 1 (P1), there was a sudden shift from a predominance of GAD-containing cell bodies within the developing dendritic regions to a meshwork of GAD-positive processes with terminal-like varicosities in these same regions. This pattern also contrasted with that of the adult, in which GAD-labeled terminals are highly concentrated in the principal cell layers. Electron microscopic observations of the GAD-labeled processes at P1 confirmed their axon-like appearance and demonstrated that the immunoreactivity was consistently localized in vesicle-filled regions that were often closely apposed to and, in some instances, established synaptic contacts with dendritic profiles. The present identification of an early abundance of GAD-containing structures in the hippocampal formation and the marked change in their distribution during development complement recent observations of developmental changes in the functioning of the GABA system and provide additional support for the early involvement of this neurotransmitter system in hippocampal development.

Two glutamate decarboxylase forms corresponding to the mammalian GAD65 and GAD67 are expressed during development of the chick telencephalon

The gamma-aminobutyric acid (GABA)-synthesizing enzyme glutamate decarboxylase (GAD) was studied during development of the chick telencephalon. By means of reverse-phase HPLC analysis, we showed that GABA indeed accumulates during embryogenesis, whereas the levels of glutamate, the substrate for GAD, are more or less unchanged up to later developmental stages. The enzyme activity increased approximately 25-fold from embryonic day 3 to embryonic day 17. Immunoblotting data revealed that two GAD proteins, of approximately 65 and 67 kDa, were present during the period investigated. Furthermore, Northern blot analysis with probes obtained from rat cDNA sequences, as well as a chicken-specific probe for GAD65 generated by means of reverse transcriptase-polymerase chain reaction (RT-PCR), strengthened the interpretation that the chick embryo expresses genes corresponding to GAD65 and GAD67. The rat probes recognized transcript sizes of 3.9 kb (GAD65) and 5.6 kb (GAD67), sizes which are different from those of the rat brain (Erlander et al., Neuron, 7, 91-100, 1991). Sequencing of the RT-PCR products revealed a high level of homology (82% at the nucleotide level) between the mammalian and chick GAD65 genes. Taken together, these findings suggest that the chick embryo expresses two GAD genes during embryogenesis. The functional properties of each gene product remain to be investigated.

Bromodeoxyuridine: a diagnostic tool in biology and medicine, Part III. Proliferation in normal, injured and diseased tissue, growth factors, differentiation, DNA replication sites and in situ hybridization

This paper is a continuation of parts I (history, methods and cell kinetics) and II (clinical applications and carcinogenesis) published previously (Dolbeare, 1995 Histochem. J. 27, 339, 923). Incorporation of bromodeoxyuridine (BrdUrd) into DNA is used to measure proliferation in normal, diseased and injured tissue and to follow the effect of growth factors. Immunochemical detection of BrdUrd can be used to determine proliferative characteristics of differentiating tissues and to obtain birth dates for actual differentiation events. Studies are also described in which BrdUrd is used to follow the order of DNA replication in specific chromosomes, DNA replication sites in the nucleus and to monitor DNA repair. BrdUrd incorporation has been used as a tool for in situ hybridization experiments.

Dividing neuron precursors express neuron-specific tubulin

Neuronal differentiation involves specific molecular and morphological changes in precursors and results in mature, postmitotic neurons. The expression of neuron-specific beta tubulin, as detected by the monoclonal antibody TuJ1, begins during the period of neurogenesis. Indeed, TuJ1 expression precedes that of the 160 kD neurofilament protein in both the central and peripheral nervous systems. In the embryonic rat spinal cord, bipolar cells and some mitotic cells in the ventricular zone were TuJ1 immunoreactive (IR). Sensory ganglia also contained cells with TuJ1-IR mitotic spindles in situ. In embryonic rat sensory and sympathetic ganglion cell cultures pulsed with the thymidine analog bromodeoxyuridine (BrdU), TuJ1 label was detected in the spindle of mitotic cells and in the midbody of cells joined at cytokinesis, indicating that neuron-specific tubulin expression was initiated during or before the final mitosis of neuronal progenitors. Dorsal root ganglion cultures included TuJ1-IR cells with several shapes that may reflect morphological transitions, from flattened stellate neural crest-like cells to differentiated bipolar neurons. Indeed, the presence of flattened TuJ1-IR cells was correlated with neurogenesis. Some sympathetic neuron precursors possessed long TuJ1-IR neurites, as well as TuJ1-IR spindle microtubules and BrdU-labeled chromosomes, indicating that these precursors can possess long processes during metaphase. These results support the hypothesis that neuron-specific tubulin expression represents an early molecular event in neuronal differentiation exhibited by a wide range of neuronal precursors. The cessation of proliferation can occur at different points during neuronal differentiation, as TuJ1-IR was detected in cells undergoing mitosis. Future studies directed toward understanding the molecules that initiate neuron-specific tubulin expression may lead to the factors that control the initial phases of neuronal differentiation.

Developmental patterns of somatostatin-receptors and somatostatin-immunoreactivity during early neurogenesis in the rat

The temporal pattern of distribution of somatostatin receptor was investigated using the somatostatin analogue [125I]Tyr0-DTrp8-somatostatin14 as a ligand and compared with that of somatostatin immunoreactivity during early developmental stages in the spinal cord and the sensory derivatives in rat fetuses. Qualitative and quantitative analysis showed that somatostatin receptors were detected in a transient manner. In the neural tube, they were clearly associated with immature premigratory cells and with the developing white matter. During the time-period examined (from day 10.5 to 16.5), the disappearance of somatostatin receptors followed a ventro to dorsal gradient probably linked to the regression of the ventricular zone. In sensory derivatives, they were expressed in the forming ganglia and their central and peripheral nerves from embryonic day 12.5 to 16.5 inclusive, with a peak around day 14.5 and low levels observed at day 16.5. Competition experiments performed at embryonic day 14.5 demonstrated that somatostatin1-14, somatostatin1-28, and Octreotide displaced specific binding with nanomolar affinities while CGP 23996 was only active at micromalar doses. Such displacements are compatible with the SSTR2 and/or SSTR4 pharmacology. During the time period examined, some transient somatostatin immunoreactive cell bodies and fibers were detected in the neural tube and in the sensory derivatives. These results demonstrate the existence, in neuronal derivatives, of a complex temporal and anatomical pattern of expression of somatostatin receptors, from the SSTR2/SSTR4 subtype(s), and somatostatin immunoreactivity. It appears that the transient expression of somatostatin receptors and/or somatostatin immunoreactivity characterizes critical episodes in the development of a cohort of neurons; a fact that unequivocally reinforces the notion that somatostatin plays a fundamental role during neurogenesis in vertebrates.

Dual roles of the retinoblastoma protein in cell cycle regulation and neuron differentiation

To assess the functions of the retinoblastoma protein (RB) during normal development, we have analyzed mouse embryos that lack a functional copy of the retinoblastoma gene (genotype: Rb-1 delta 20/Rb-1 delta 20). Our findings demonstrate that RB plays an important role in the regulation of the neuronal cell cycle. In mutant embryos, dividing cells are found well outside of the normal neurogenic regions in both the central and peripheral nervous systems. In addition to abnormal cell cycle regulation, however, the mutant embryos show two less expected phenotypes. First, many of the ectopically dividing cells die by apoptosis shortly after their entrance into S phase. In sensory ganglia, most nerve cells die by this process, beginning at about the same time as normal target-related neuronal death. Second, although the expression of certain differentiation markers such as N-CAM and Brn-3.0 appears to be near normal, nerve cells, especially in sensory ganglia, do not mature properly. Their morphology is stunted and expression of neuronal beta II tubulin is greatly reduced. Preferential reduction in the expression of TrkA, TrkB, and the low-affinity neurotrophin receptor p75LNGFR may be relevant to neuronal cell death and lack of neuronal differentiation seen in the mutant embryos. Primary cultures of dorsal root and trigeminal ganglion cells from later stage mutant embryos reveal a decrease in neuronal cell survival and in neurite outgrowth even in the presence of the appropriate neurotrophins. Taken together, these results suggest that the p110RB protein not only regulates progression through the cell cycle but is also important for cell survival and differentiation.

Transient increase in expression of GAD65 and GAD67 mRNAs during postnatal development of rat spinal cord

Gamma-aminobutyric acid (GABA) is thought to be one of the classic neurotransmitters acting as a developmental signal. To understand the role for GABA in development, we investigated the expression of transcripts encoding two forms of the GABA-synthesizing enzyme glutamate decarboxylase (GAD65 and GAD67) in the cervical enlargement of the rat spinal cord at successive postnatal days--P0, P7, P14, P21, and P90 (adult)--by using in situ hybridization histochemistry. Cells hybridized with two oligonucleotide probes designed to detect GAD65 and GAD67 mRNAs were widely distributed in all laminae, except in motoneurons of the spinal cord. The integrated densities of hybridization signals were measured across all layers of the gray matter. The relative number of GAD mRNA-labeled cells was determined within each of four regions: laminae I-III, laminae IV-VI, laminae VII and VIII, and lamina X. There was a transient increase in both the integrated density and the relative number of hybridized cells between P7 and P14, after which there was a marked decline to adult levels (lowest). An overall decrease in the number of GAD mRNA-labeled cells was evident in all layers, but a dramatic drop occurred in a subpopulation of cells within ventral portions of the spinal cord. The distribution patterns and postnatal changes in expression of the mRNAs encoding GAD65 and GAD67 were similar and closely paralleled reported changes in the abundance of GAD65 and GAD67 proteins and their product, GABA. Transient increases in GAD mRNA expression during the early postnatal period coincide with, and may be linked to, synapse formation and synapse elimination of the developing spinal cord.

Developmental changes in the distribution of gamma-aminobutyric acid-immunoreactive neurons in the embryonic chick lumbosacral spinal cord

The development of gamma-aminobutyric acid (GABA)-immunoreactive neurons was investigated in the embryonic and posthatch chick lumbosacral spinal cord by using pre- and postembedding immunostaining with an anti-GABA antiserum. The first GABA-immunoreactive cells were detected in the ventral one-half of the spinal cord dorsal to the lateral motor column at E4. GABAergic neurons in this location sharply increased in number and, with the exception of the lateral motor column, appeared throughout the entire extent of the ventral one-half of the spinal gray matter by E6. Thereafter, GABA-immunoreactive neurons extended from ventral to dorsal regions. Stained perikarya first appeared at E8 and then progressively accumulated in the dorsal horn, while immunoreactive neurons gradually declined in the ventral horn. The general pattern of GABA immunoreactivity characteristic of mature animals had been achieved by E12 and was only slightly altered afterwards. In the dorsal horn, most of the stained neurons were observed in laminae I-III, both at the upper (LS 1-3) and at the lower (LS 5-7) segments of the lumbosacral spinal cord. In the ventral horn, the upper and lower lumbosacral segments showed marked differences in the distribution of stained perikarya. GABAergic neurons were scattered in a relatively large region dorsomedial to the lateral motor column at the level of the upper lumbosacral segments, whereas they were confined to the dorsalmost region of lamina VII at the lower segments. The early expression of GABA immunoreactivity may indicate a trophic and synaptogenetic role for GABA in early phases of spinal cord development.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of the anatomical distribution of GAD67 mRNA encoding truncated glutamic acid decarboxylase proteins in the embryonic rat brain

During development of the central nervous system (CNS) the gene that encodes the 67 kDa form of glutamic acid decarboxylase (GAD) undergoes alternative splicing. The alternatively spliced variants include an exon (referred to as ES, for embryonic stop) that contains a premature stop codon. The detection of mRNA containing the ES exon in embryonic rat brain has been previously reported (Proc. Natl. Acad. Sci., 87 (1990) 8771-8775). We have used in situ hybridization to identify the anatomical distribution of ES mRNA in the embryonic rat brain during two stages of development, embryonic day 17 (E17) and E20. At E17, GAD67 mRNA was expressed in several CNS regions that were destined to contain GABAergic neurons when mature. ES transcripts were predominantly localized to ventricular zones and other regions associated with populations of proliferative cells at E17 and E20. At both ages, however, the alternatively spliced variants were also detected in regions of brain associated with migratory or post-mitotic neurons. GAD67 transcripts that did not include the ES exon were localized to anatomical areas that contained post-mitotic, and often post-migratory neurons. The temporal and spatial disappearance of mRNA containing the ES exon generally followed a caudal-to-rostral gradient which paralleled neuronal terminal mitosis and differentiation.

Ontogeny of GABAA receptor subunit mRNAs in rat spinal cord and dorsal root ganglia

Relatively little is known about the development of GABAA receptor subunits and their gene expression in mammalian spinal cord. The expression of mRNAs encoding 13 GABAA receptor subunits (alpha 1-6, beta 1-3, gamma 1-3, and delta) in embryonic, postnatal, and adult rat spinal cord and dorsal root ganglia (DRG) cells were studied by in situ hybridization and reverse transcription-polymerase chain reaction (RT-PCR) analysis. Both techniques revealed the presence of all subunit mRNAs originally found in the rat brain, except for alpha 6, which was not detectable, and delta, which was weakly detected only by RT-PCR. Two anatomically distinctive sets of subunit mRNAs were found by in situ hybridization within the ventricular zone (VZ) and mantle zone (MZ). The trio of alpha 4, beta 1, and gamma 1 subunit mRNAs emerged exclusively in neuroepithelial cells at embryonic day 13 (E13) and remained detectable in the VZ until E17. In the MZ, beta 3 subunit mRNA was first detected at E12, while alpha 2, alpha 3, alpha 5, beta 2, gamma 2, and gamma 3 transcripts appeared at E13. Expressions of the subunit mRNAs in the MZ rapidly increased and expanded in a ventrodorsal sequence from motoneurons to dorsal horn neurons before reaching a peak in the late embryonic/early postnatal period. The mRNA expressions declined during postnatal development, by region-selective depletion, with alpha 4, alpha 5, beta 1, beta 2, gamma 1, and gamma 3 subunit mRNAs becoming barely detectable. In contrast, alpha 2, alpha 3, beta 3, and gamma 2 transcripts persisted into adulthood with distinct anatomical distributions. RT-PCR analysis revealed unique developmental patterns in the intensities of PCR products, most of which were in good agreement with developmental changes in the densities of hybridized mRNA signals. However, RT-PCR amplified minute amounts of mRNAs for alpha 1, alpha 4, alpha 5, beta 1, beta 2, gamma 1, gamma 3, and delta subunits in adults, which were not found in film autoradiograms, but could be detected in a few grain-positive cells in emulsion-dipped sections. DRG cells expressed alpha 2, alpha 3, alpha 5, beta 2, beta 3, and gamma 2 subunit mRNAs during embryogenesis but only alpha 2, beta 3, and gamma 2 subunit mRNAs were reliably detected in the adult.(ABSTRACT TRUNCATED AT 400 WORDS)

GABA alters GABAA receptor mRNAs and increases ligand binding

In addition to its role as an inhibitory neurotransmitter, gamma-aminobutyric acid (GABA) influences the cytodifferentiation of developing neurons both in culture and in vivo. Here, we report some of the targets of GABA action and the mechanism through which GABA acts. In primary cultures of cerebellar granule cells, GABA specifically stimulates an increase in the levels of mRNAs for alpha 1 and beta 2 GABAA receptor subunits. The GABAA agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) mimics this effect, and the GABAA antagonist bicuculline prevents it. In addition, GABA and THIP trigger an increase in the number of GABA binding sites. This increase parallels that seen in vivo, where the total number of GABAA receptor sites increases during postnatal cerebellar development. It is interesting that the period of the greatest increase in the number of receptor sites coincides with the development of the granule cells. Taken together, our data suggest that GABA may play an important role during maturation of cerebellar granule cells by influencing the number and composition of its own receptors.

Transient expression of GABA immunoreactivity in the developing rat spinal cord

The development of GABAergic neurons in the spinal cord of the rat has been investigated by immunocytochemical staining of frozen sections with anti-gamma-aminobutyric acid (GABA) antiserum. In the cervical cord, GABA-immunoreactive fibers first appeared at embryonic day (E) 13 in the presumptive white matter within the ventral commissure, ventral funiculus, and dorsal root entrance zone, and in the ventral roots. There were no GABA-immunoreactive cell bodies detected at this age. By E14, motoneurons, the earliest generated spinal cells, were the first cell population to become GABA-immunoreactive at the cell body level. Thereafter, GABA-immunoreactive neurons increased progressively in number and extended from ventral to dorsal regions. GABA-immunoreactive relay neurons within lamina I of the dorsal horn were initially detected at E17. Interneurons in the substantia gelatinosa, the latest generated cells in the spinal cord, were also the last to express the GABA immunoreactivity at E18. Immunoreactive neurons peaked in intensity and extent at E18 and 19. GABA immunoreactivity was only detectable in neurons within the intermediate and marginal zones 1-3 days after they withdrew from the cell cycle. This contrasts to glutamate decarboxylase immunoreactivity, which is detected in precursor cells in the ventricular zone prior to, or during, withdrawal from the cell cycle. Toward the end of gestation, GABA immunoreactivity declined in intensity and extent. This regression began in the ventral horn of the cervical region and ended in the dorsal horn of the lumbosacral region. During the first week after birth, immunoreactivity in motoneurons and in many other neurons within the ventral horn, intermediate gray, and deeper layers of the dorsal horn disappeared, and only in those neurons predominantly within the superficial layers of the dorsal horn did it persist into adulthood. Thus, the expression and regression of GABA immunoreactivity in the spinal cord followed ventral-to-dorsal, rostral-to-caudal, and medial-to-lateral gradients. These observations indicate that the majority of embryonic spinal neurons pass through a stage of transient expression of GABA immunoreactivity. The functional significance of this transient expression is unknown, but it coincides with the period of intense neurite growth of motoneurons, sensory neurons, and interneurons, and of neuromuscular junction formation, suggesting that the transient presence of GABA may play an important role in the differentiation of sensorimotor neuronal circuits.