GABAA receptor stimulation promotes survival of embryonic rat striatal neurons in culture

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.

Microsomal prostaglandin E synthase-1 is a critical factor in dopaminergic neurodegeneration in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder of uncertain pathogenesis characterized by the loss of nigrostriatal dopaminergic neurons. Although increased production of prostaglandin E₂ (PGE₂) has been implicated in tissue damage in several pathological settings, the role of microsomal prostaglandin E synthase-1 (mPGES-1), an inducible terminal enzyme for PGE₂ synthesis, in dopaminergic neurodegeneration remains unclear. Here we show that mPGES-1 is up-regulated in the dopaminergic neurons of the substantia nigra of postmortem brain tissue from PD patients and in neurotoxin 6-hydroxydopamine (6-OHDA)-induced PD mice. The expression of mPGES-1 was also up-regulated in cultured dopaminergic neurons stimulated with 6-OHDA. The genetic deletion of mPGES-1 not only abolished 6-OHDA-induced PGE₂ production but also inhibited 6-OHDA-induced dopaminergic neurodegeneration both in vitro and in vivo. Nigrostriatal projections, striatal dopamine content, and neurological functions were significantly impaired by 6-OHDA administration in wild-type (WT) mice, but not in mPGES-1 knockout (KO) mice. Furthermore, in cultured primary mesencephalic neurons, addition of PGE₂ to compensate for the deficiency of 6-OHDA-induced PGE₂ production in mPGES-1 KO neurons recovered 6-OHDA toxicity to almost the same extent as that seen in WT neurons. These results suggest that induction of mPGES-1 enhances 6-OHDA-induced dopaminergic neuronal death through excessive PGE₂ production. Thus, mPGES-1 may be a valuable therapeutic target for treatment of PD.

Human breast cancer metastases to the brain display GABAergic properties in the neural niche

Dispersion of tumors throughout the body is a neoplastic process responsible for the vast majority of deaths from cancer. Despite disseminating to distant organs as malignant scouts, most tumor cells fail to remain viable after their arrival. The physiologic microenvironment of the brain must become a tumor-favorable microenvironment for successful metastatic colonization by circulating breast cancer cells. Bidirectional interplay of breast cancer cells and native brain cells in metastasis is poorly understood and rarely studied. We had the rare opportunity to investigate uncommonly available specimens of matched fresh breast-to-brain metastases tissue and derived cells from patients undergoing neurosurgical resection. We hypothesized that, to metastasize, breast cancers may escape their normative genetic constraints by accommodating and coinhabiting the neural niche. This acquisition or expression of brain-like properties by breast cancer cells could be a malignant adaptation required for brain colonization. Indeed, we found breast-to-brain metastatic tissue and cells displayed a GABAergic phenotype similar to that of neuronal cells. The GABAA receptor, GABA transporter, GABA transaminase, parvalbumin, and reelin were all highly expressed in breast cancer metastases to the brain. Proliferative advantage was conferred by the ability of breast-to-brain metastases to take up and catabolize GABA into succinate with the resultant formation of NADH as a biosynthetic source through the GABA shunt. The results suggest that breast cancers exhibit neural characteristics when occupying the brain microenvironment and co-opt GABA as an oncometabolite.

γ-Aminobutyric acid regulates both the survival and replication of human β-cells

γ-Aminobutyric acid (GABA) has been shown to inhibit apoptosis of rodent β-cells in vitro. In this study, we show that activation of GABAA receptors (GABAA-Rs) or GABAB-Rs significantly inhibits oxidative stress-related β-cell apoptosis and preserves pancreatic β-cells in streptozotocin-rendered hyperglycemic mice. Moreover, treatment with GABA, or a GABAA-R- or GABAB-R-specific agonist, inhibited human β-cell apoptosis following islet transplantation into NOD/scid mice. Accordingly, activation of GABAA-Rs and/or GABAB-Rs may be a useful adjunct therapy for human islet transplantation. GABA-R agonists also promoted β-cell replication in hyperglycemic mice. While a number of agents can promote rodent β-cell replication, most fail to provide similar activities with human β-cells. In this study, we show that GABA administration promotes β-cell replication and functional recovery in human islets following implantation into NOD/scid mice. Human β-cell replication was induced by both GABAA-R and GABAB-R activation. Hence, GABA regulates both the survival and replication of human β-cells. These actions, together with the anti-inflammatory properties of GABA, suggest that modulation of peripheral GABA-Rs may represent a promising new therapeutic strategy for improving β-cell survival following human islet transplantation and increasing β-cells in patients with diabetes.

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.

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.

Dynamic regulation of glutamic acid decarboxylase 65 gene expression in rat testis

Glutamate decarboxylase 65 (GAD65) produces gamma-aminobutyric acid, the main inhibitory neurotransmitter in adult mammalian brain. Previous experiments, performed in brain, showed that GAD65 gene possesses two TATA-less promoters, although the significance is unknown. Here, by rapid amplification of cDNA ends method, two distinct GAD65 mRNA isoforms transcribed from two independent clusters of transcription start sites were identified in post-natal rat testis. RT-PCR results revealed that the two mRNA isoforms had distinct expression patterns during post-natal testis maturation, suggesting that GAD65 gene expression was regulated by alternative promoters at the transcription level. By using GAD65-specific antibodies, western blotting analysis showed that the 58-kDa GAD65, N-terminal 69 amino acids truncated form of full-length GAD65 protein, was developmentally expressed during post-natal testis maturation, suggesting that GAD65 gene expression in testis may also be regulated by post-translational processing. Confocal immunofluorescence microscopy revealed that GAD65 protein was presented in Leydig cells of Day 1 testis, primary spermatocytes and spermatids of postnatal of Day 90 testis. The above results suggested that GAD65 gene expression is dynamically regulated at multiple levels during post-natal testis maturation.

Allopregnanolone-induced rise in intracellular calcium in embryonic hippocampal neurons parallels their proliferative potential

Background

Factors that regulate intracellular calcium concentration are known to play a critical role in brain function and neural development, including neural plasticity and neurogenesis. We previously demonstrated that the neurosteroid allopregnanolone (APα; 5α-pregnan-3α-ol-20-one) promotes neural progenitor proliferation in vitro in cultures of rodent hippocampal and human cortical neural progenitors, and in vivo in triple transgenic Alzheimer's disease mice dentate gyrus. We also found that APα-induced proliferation of neural progenitors is abolished by a calcium channel blocker, nifedipine, indicating a calcium dependent mechanism for the proliferation.

Methods

In the present study, we investigated the effect of APα on the regulation of intracellular calcium concentration in E18 rat hippocampal neurons using ratiometric Fura2-AM imaging.

Results

Results indicate that APα rapidly increased intracellular calcium concentration in a dose-dependent and developmentally regulated manner, with an EC₅₀ of 110 ± 15 nM and a maximal response occurring at three days in vitro. The stereoisomers 3β-hydroxy-5α-hydroxy-pregnan-20-one, and 3β-hydroxy-5β-hydroxy-pregnan-20-one, as well as progesterone, were without significant effect. APα-induced intracellular calcium concentration increase was not observed in calcium depleted medium and was blocked in the presence of the broad spectrum calcium channel blocker La³⁺, or the L-type calcium channel blocker nifedipine. Furthermore, the GABA_A receptor blockers bicuculline and picrotoxin abolished APα-induced intracellular calcium concentration rise.

Conclusion

Collectively, these data indicate that APα promotes a rapid, dose-dependent, stereo-specific, and developmentally regulated increase of intracellular calcium concentration in rat embryonic hippocampal neurons via a mechanism that requires both the GABA_A receptor and L-type calcium channel. These data suggest that APα-induced intracellular calcium concentration increase serves as the initiation mechanism whereby APα promotes neurogenesis.

Posttraumatic GABA(A)-mediated [Ca2+]i increase is essential for the induction of brain-derived neurotrophic factor-dependent survival of mature central neurons

A shift of GABA(A)-mediated responses from hyperpolarizing to depolarizing after neuronal injury leads to GABA(A)-mediated increase in [Ca2+](i). In addition, central neurons become dependent on BDNF for survival. Whether these two mechanisms are causally interrelated is an open question. Here, we show in lesioned CA3 hippocampal neurons in vitro and in axotomized corticospinal neurons in vivo that posttraumatic downregulation of the neuron-specific K-Cl cotransporter KCC2 leads to intracellular chloride accumulation by the Na-K-2Cl cotransporter NKCC1, resulting in GABA-induced [Ca2+](i) transients. This mechanism is required by a population of neurons to survive in a BDNF-dependent manner after injury, because blocking GABA(A)-depolarization with the NKCC1 inhibitor bumetanide prevents the loss of neurons on BDNF withdrawal. The resurgence of KCC2 expression during recovery coincides with loss of BDNF dependency for survival. This is likely mediated through BDNF itself, because injured neurons reverse their response to this neurotrophin by switching the BDNF-induced downregulation of KCC2 to upregulation.

GABAA receptors, anesthetics and anticonvulsants in brain development

GABA, acting via GABA(A) receptors, is well-accepted as the main inhibitory neurotransmitter of the mature brain, where it dampens neuronal excitability. The receptor's properties have been studied extensively, yielding important information about its structure, pharmacology, and regulation that are summarized in this review. Several GABAergic drugs have been commonly used as anesthetics, sedatives, and anticonvulsants for decades. However, findings that GABA has critical functions in brain development, in particular during the late embryonic and neonatal period, raise worthwhile questions regarding the side effects of GABAergic drugs that may lead to long-term cognitive deficits. Here, we will review some of these drugs in parallel with the control of CNS development that GABA exerts via activation of GABA(A) receptors. This review aims to provide a basic science and clinical perspective on the function of GABA and related pharmaceuticals acting at GABA(A) receptors.

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.

Role of maternal corticosterone in the development and maturation of the aminoacidergic systems of the rat brain

Previous studies have suggested an important role for maternal glucocorticoids in the development of the aminoacidergic systems of the rat brain. This study examines the effect of metyrapone (2-methyl-1,2-di-3-pyridyl-1-propanone), i.p.-administered to gestating mothers, on the maturation of the aminoacidergic systems of their offsprings' brains. gamma-Aminobutyric acid (GABA) and glutamate concentrations were determined in male and female offspring at postnatal days (PN) 23 and 90 in four brain areas: the hippocampus, hypothalamus, striatum and cortex. The activity of glutamic acid decarboxylase (GAD), the most important enzyme in the synthesis of GABA, was also analysed. The results show that a reduction in maternal corticosterone during gestation leads to a reduced GABAergic content in all brain areas studied at PN23; permanent organizational changes occurred in the cortex, striatum and hypothalamus. Maternal metyrapone treatment also affected the development of the glutamatergic systems, females being more affected than males at both PN23 and PN90 particularly in the hypothalamus and cortex. The metyrapone treatment produced no changes in GAD activity at PN23, but induced an important increase in this activity at PN90.

Sex differences in the chloride cotransporters, NKCC1 and KCC2, in the developing hypothalamus

In immature neurones, high basal [Cl(-)](i) results in membrane depolarisation following GABA(A) receptor activation, which is critical for various developmental processes including steroid-mediated sexual differentiation of the hypothalamus. Previously, we demonstrated that oestradiol enhances GABA-mediated Ca(2+) influx in neonate hypothalamus and that Ca(2+) induced activation of the transcription factor, cyclicAMP response element binding protein (CREB), was higher in male (high oestradiol) relative to female neonate hypothalamus. Based on these results, we hypothesised that expression of developmentally regulated chloride cotransporters may be sexually dimorphic. Here, we investigate the expression of the chloride cotransporters, NKCC1 (Na-K-2Cl(-)) and KCC2 (K-Cl(-)) in neonate mediobasal hypothalamus of male and female rats. The NKCC1 transporter moves Cl(-) into cells and helps maintain depolarising GABA action while the KCC2 transporter has the opposite effect by moving Cl(-) out of cells. NKCC1 mRNA levels were higher in males than females on the day of birth (postnatal day 0; PND 0) and total NKCC1 protein levels were significantly higher in males than females on embryonic day (ED) 20 and PND0. Levels of activated phosphorylated NKCC1 (pNKCC1) were not sexually dimorphic. Females were treated with a masculinising dose of oestradiol benzoate (EB; 100 microg; EB-females) on PND0. Total NKCC1 protein levels in tissue processed on PND1 and PND2 were similar in EB-females and oil-treated PND0 males and females. However, pNKCC1 protein levels measured on PND2 (but not PND1) were significantly higher in EB-treated females relative to oil-treated males and females. By contrast, KCC2 mRNA levels were significantly lower in males relative to females on PND0. KCC2 protein was not detectable on ED20 or PND0 but was significantly lower in males relative to females on PND5. These results suggest a complex relationship between KCC2 and NKCC1 mRNA and protein in developing brain that is not easily linked to regulation by oestradiol.

Regulation of pancreatic islet cell survival and replication by gamma-aminobutyric acid

AIMS/HYPOTHESIS

Pancreatic islets have evolved remarkable, though poorly understood mechanisms to modify beta cell mass when nutrient intake fluctuates or cells are damaged. We hypothesised that appropriate and timely adjustments in cell number occur because beta cells release proliferative signals to surrounding cells when stimulated by nutrients and 'bleed' these growth factors upon injury.

MATERIALS AND METHODS

In rat pancreatic islets, we measured DNA content, insulin content, insulin secretion after treatment, immunoblots of apoptotic proteins and the uptake of nucleoside analogues to assess the ability of gamma-aminobutyric acid (GABA), which is highly concentrated in beta cells, to act as a growth and survival factor. This focus is supported by work from others demonstrating that GABA increases cell proliferation in the developing nervous system, acts as a survival factor for differentiated neurons and, interestingly, protects plants under stress.

RESULTS

Our results show that DNA, insulin content and insulin secretion are higher in freshly isolated islets treated with GABA or GABA B receptor agonists. Exposure to GABA upregulated the anti-apoptotic protein B-cell chronic lymphocytic leukaemia XL and limited activation of caspase 3 in islets. The cellular proliferation rate in GABA-treated islets was twice that of untreated controls.

CONCLUSIONS/INTERPRETATION

We conclude that GABA serves diverse purposes in the islet, meeting a number of functional criteria to act as an endogenous co-regulator of beta cell mass.

Spatial and temporal patterns of proliferation and differentiation in the developing turtle eye

Here we show for the first time different aspects of the pattern of neurogenesis in the developing turtle retina by using different morphological and molecular clues. We show the chronotopographical fashion of occurrence of three major aspects of retinal development: (1) morphogenesis of the optic primordia and emergence of the different retinal layers, (2) the temporal progression of neurogenesis by the cessation of proliferative activity, and (3) the apparition and cellular localization of different antigens and neuroactive substances. Retinal cells were generated in a conserved temporal order with ganglion cells born first, followed by amacrine, photoreceptor, horizontal and bipolar/Müller cells. While eventually expressed in many types of retinal neurons, Islet1 was permanently expressed in differentiating and mature ganglion cells. Calbindin-immunoreactive elements were found in the ganglion cell layer and the inner nuclear layer. Interestingly, at later stages the amount of expressing cells in these layers was reduced dramatically. On the contrary, the number of calbindin-immunoreactive photoreceptors increased as development proceeded. In addition, calretinin expressing cells were prominent in the horizontal cell bodies, and their processes extending into the outer plexiform layer were also strongly labeled. Finally, the synthesis of gamma-aminobutyric acid (GABA) was detected in developing and matured horizontal and amacrine cells. All these maturational features began in the dorso-central area, in a region slightly displaced towards the temporal retina.

Long-term effects of diazepam treatment of epileptic GABAA receptor beta3 subunit knockout mouse in early life

The knockout mouse for the beta3 subunit of the GABAA receptor exhibits spontaneous epilepsy and hyperactivity, and has been proposed as a model for the severe developmental disorder, Angelman's syndrome, which is known to be of genetic origin. We have used this mutant to test an approach of therapeutic intervention prior to seizure onset by daily injection with diazepam during either the first or second postnatal week. Results showed differences between postnatal week 1 and week 2 injections both acutely, with respect to sedative effects, and in long-term outcome, with respect to EEG and behavioral tests measured at 12-14 weeks of age. The EEG of control mice remained unaffected under all conditions, but the EEG of beta3 (-/-) injected with diazepam in week 1 was worsened, showing increased oscillatory activity at 5-6Hz, and more myoclonic jerks, particularly among males. For beta3 (-/-) injected with diazepam in week 2, the EEG was normalized in half the mice but worsened similarly to week 1 in the other half. Neonatal diazepam injection had a long-term normalizing effect on behavior of beta3 (-/-) mice injected in week 1, but diazepam treatment in week 2 did not affect the hyperactive and circling behavior characteristic of the beta3 knockout mouse. Diazepam treatment in postnatal week 2 significantly decreased anxiety in the adult beta3 group. Diazepam treatment in both postnatal weeks 1 and 2 improved the motor coordination of beta3 (-/-) on the rotarod, although performance of control mice injected with diazepam in postnatal week 2 was significantly impaired. The observed long-term outcome of neonatal diazepam injections may result from interference with developmental processes, and shows that enhancing GABAergic activity with diazepam during the period where GABA can be excitatory can produce narrow stage-related effects on brain development.

Microglia-specific expression of microsomal prostaglandin E2 synthase-1 contributes to lipopolysaccharide-induced prostaglandin E2 production

Microsomal prostaglandin E2 synthase (mPGES)-1 is an inducible protein recently shown to be an important enzyme in inflammatory prostaglandin E2 (PGE2) production in some peripheral inflammatory lesions. However, in inflammatory sites in the brain, the induction of mPGES-1 is poorly understood. In this study, we demonstrated the expression of mPGES-1 in the brain parenchyma in a lipopolysaccharide (LPS)-induced inflammation model. A local injection of LPS into the rat substantia nigra led to the induction of mPGES-1 in activated microglia. In neuron-glial mixed cultures, mPGES-1 was co-induced with cyclooxygenase-2 (COX-2) specifically in microglia, but not in astrocytes, oligodendrocytes or neurons. In microglia-enriched cultures, the induction of mPGES-1, the activity of PGES and the production of PGE2 were preceded by the induction of mPGES-1 mRNA and almost completely inhibited by the synthetic glucocorticoid dexamethasone. The induction of mPGES-1 and production of PGE2 were also either attenuated or absent in microglia treated with mPGES-1 antisense oligonucleotide or microglia from mPGES-1 knockout (KO) mice, respectively, suggesting the necessity of mPGES-1 for microglial PGE2 production. These results suggest that the activation of microglia contributes to PGE2 production through the concerted de novo synthesis of mPGES-1 and COX-2 at sites of inflammation of the brain parenchyma.

Reversal of prenatal diazepam-induced deficit in a spatial-object learning task by brief, periodic maternal separation in adult rats

In the rat, prenatal exposure to diazepam (DZ) induces a permanent reduction in GABA/BZ receptor (R) function and behavioural abnormalities. Environmental modifications during early stages of life can influence brain development and induce neurobiological and behavioural changes throughout adulthood. Indeed, a subtle, periodic, postnatal manipulation increases GABA/BZ R activity and produces facilitatory effects on neuroendocrine and behavioural responses. We here investigated the impact of prenatal treatment with DZ on learning performance in adult 3- and 8-month-old male rats and the influence of a brief, periodic maternal separation on the effects exerted by prenatal DZ exposure. Learning performance was examined employing a non-aversive spatial, visual and/or tactile task, the "Can test". Behavioural reactivity, emotional state and fear/anxiety-driven behaviour were also examined using open field (OF), acoustic startle reflex (ASR) and elevated plus-maze (EPM) tests. A single daily injection of DZ (1.5mg/kg, s.c.), over gestational days (GD) 14-20, induced, in an age-independent manner, a severe deficit in learning performance, a decrease in locomotor and explorative activity and an increase in peak amplitude in the ASR. Furthermore, anxiety-driven behaviour in EPM was disrupted. Daily maternal separation for 15 min over postnatal days 2-21 exerted opposite effects in all the paradigms examined. Prenatally DZ-exposed maternal separated rats, in contrast to respective non-separated rats, showed an improvement in learning performance, a decrease in emotionality and a normalization of the exploratory behaviour in EPM. These results suggest that a greater maternal care, induced by separation, can serve as a source for the developing brain to enhance neuronal plasticity and to prevent the behavioural abnormalities induced by prenatal DZ exposure.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Role of calcium and kinases on the neurotrophic effect induced by gamma-aminobutyric acid

An increasing body of evidence supports a trophic action of gamma-aminobutyric acid (GABA) during nervous system development. The purported mediator of these trophic effects is a depolarizing response triggered by GABA, which elicits a calcium influx in immature CNS cells. This Mini-Review focuses on the neurotrophic role of neural activity and GABA and some of the most common intracellular cascades activated by depolarization and trophic factors. Several biological effects induced by GABA in the developing nervous system are reviewed, with particular emphasis on what is known about calcium-dependent neurotrophic effects induced by GABA and its intracellular mechanisms.

Ontogeny of gamma-aminobutyric acid-immunoreactive neurons in the rhombencephalon and spinal cord of the sea lamprey

The development of neurons expressing gamma-aminobutyric acid (GABA) in the rhombencephalon and spinal cord of the sea lamprey (Petromyzon marinus) was studied for the first time with an anti-GABA antibody. The earliest GABA-immunoreactive (GABAir) neurons appear in late embryos in the basal plate of the isthmus, caudal rhombencephalon, and rostral spinal cord. In prolarvae, the GABAir neurons of the rhombencephalon appear to be distributed in spatially restricted cellular domains that, at the end of the prolarval period, form four longitudinal GABAir bands (alar dorsal, alar ventral, dorsal basal, and ventral basal). In the spinal cord, we observed only three GABAir longitudinal bands (dorsal, intermediate, and ventral). The larval pattern of GABAir neuronal populations was established by the 30-mm stage, and the same populations were observed in premetamorphic and adult lampreys. The ontogeny of GABAergic populations in the lamprey rhombencephalon and spinal cord is, in general, similar to that previously described in mouse and Xenopus.

Autocrine/paracrine activation of the GABA(A) receptor inhibits the proliferation of neurogenic polysialylated neural cell adhesion molecule-positive (PSA-NCAM+) precursor cells from postnatal striatum

GABA and its type A receptor (GABA(A)R) are present in the immature CNS and may function as growth-regulatory signals during the development of embryonic neural precursor cells. In the present study, on the basis of their isopycnic properties in a buoyant density gradient, we developed an isolation procedure that allowed us to purify proliferative neural precursor cells from early postnatal rat striatum, which expressed the polysialylated form of the neural cell adhesion molecule (PSA-NCAM). These postnatal striatal PSA-NCAM+ cells were shown to proliferate in the presence of epidermal growth factor (EGF) and formed spheres that preferentially generated neurons in vitro. We demonstrated that PSA-NCAM+ neuronal precursors from postnatal striatum expressed GABA(A)R subunits in vitro and in situ. GABA elicited chloride currents in PSA-NCAM+ cells by activation of functional GABA(A)R that displayed a typical pharmacological profile. GABA(A)R activation in PSA-NCAM+ cells triggered a complex intracellular signaling combining a tonic inhibition of the mitogen-activated protein kinase cascade and an increase of intracellular calcium concentration by opening of voltage-gated calcium channels. We observed that the activation of GABA(A)R in PSA-NCAM+ neuronal precursors from postnatal striatum inhibited cell cycle progression both in neurospheres and in organotypic slices. Furthermore, postnatal PSA-NCAM+ striatal cells synthesized and released GABA, thus creating an autocrine/paracrine mechanism that controls their proliferation. We showed that EGF modulated this autocrine/paracrine loop by decreasing GABA production in PSA-NCAM+ cells. This demonstration of GABA synthesis and GABA(A)R function in striatal PSA-NCAM+ cells may shed new light on the understanding of key extrinsic cues that regulate the developmental potential of postnatal neuronal precursors in the CNS.

Excitatory actions of GABA in developing brain are mediated by l-type Ca2+ channels and dependent on age, sex, and brain region

Although GABA is the major inhibitory neurotransmitter in adult brain, it exerts depolarizing actions in developing neurons that include activation of voltage-gated calcium channels. The depolarizing actions of GABA serve an obvious trophic function, but the specific physiological significance of excitatory versus inhibitory GABA action has been largely ignored. We previously demonstrated that estradiol enhances the magnitude and duration of calcium influx through L-type voltage-gated calcium channels following GABA(A) receptor activation in neonatal hypothalamic neurons. This has led us to propose that GABA action represents a major divergence point in steroid-mediated sexual differentiation of rat brain. Presently, we examined sex differences in phosphorylation of the calcium-regulated transcription factor, cyclic AMP response element binding protein, following activation of the GABA(A) receptor with muscimol, in vivo. Muscimol given 30 min before killing significantly increased the number of neurons exhibiting phosphorylated cyclic AMP response element binding protein in newborn male hypothalamus and CA1 hippocampus but decreased phosphorylated cyclic AMP response element binding protein in most brain regions in females. Muscimol-induced increases in phosphorylated cyclic AMP response element binding protein in hypothalamus and hippocampus of newborn males were attenuated by pretreatment with the L-type voltage-gated calcium channel blocker, nimodipine, suggesting that calcium influx is involved in phosphorylation of cyclic AMP response element binding protein in neonate brain. Muscimol treatment had no effect on hypothalamic or hippocampal phosphorylated cyclic AMP response element binding protein levels in juvenile males and females. These results are consistent with a divergence in male and female rat brain in the calcium-mediated cellular response to muscimol that is restricted to the early neonatal period, a time critical for estradiol-mediated sexual differentiation.

Glutamate promotes proliferation of striatal neuronal progenitors by an NMDA receptor-mediated mechanism

Increasing evidence suggests that classical neurotransmitters play important roles in the development of the mammalian CNS. We used in vivo and in vitro models to identify a novel role for glutamate in striatal neurogenesis mediated by NMDA receptors. In utero exposure to NMDA receptor antagonists during striatal neurogenesis caused a dramatic reduction in the total number of adult striatal neurons. In contrast, embryos exposed to NMDA receptor antagonists immediately after the main period of neurogenesis showed no significant change in neuronal number in the adult striatum. In addition, examination of embryos shortly after NMDA receptor blockade revealed reduced proliferation in the lateral ganglionic eminence (LGE). In culture, dividing neuronal progenitors derived from the embryonic LGE showed marked reduction in 5'-bromodeoxyuridine (BrdU) uptake when exposed to NMDA receptor antagonists, indicating reduced DNA synthesis. Low concentrations of NMDA significantly increased proliferation, whereas high concentrations were toxic. AMPA-KA receptor antagonists had no significant effect on striatal neuroblast proliferation either in vivo or in vitro. These results support the hypothesis that glutamate plays a novel role during early development of the ventral telencephalon, promoting proliferation of striatal neuronal progenitors by an NMDA receptor-dependent mechanism. In contrast, previous findings suggest that proliferation of cortical progenitors derived from the dorsal telencephalon is regulated by activation of AMPA-KA but not NMDA receptors. Heterogeneous responses to glutamate in different germinal zones of the telencephalon may be an important mechanism contributing to generating neuronal diversity in the forebrain.

IL-4 increases GABAergic phenotype in rat retinal cell cultures: involvement of muscarinic receptors and protein kinase C

Interleukin-4 (IL-4) is an anti-inflammatory cytokine. During injuries, infections and neurodegenerative diseases, high levels of this molecule are expressed in the brain. In the present work, we investigated the effect of IL-4 on GABAergic differentiation of retinal cells kept in vitro. We analyzed either the uptake of [3H]-gamma-aminobutyric acid (GABA) or the expression of glutamic acid decarboxylase (GAD-67) following IL-4 treatment. We have also investigated the pharmacological modulation of the [3H]-GABA uptake by cholinergic activation. Our results demonstrate that IL-4 increases the uptake of [3H]-GABA after 48 h in culture in a dose-dependent manner (0.5-100 U/ml). The maximal effect was obtained with 5 U/ml (75% increase). This effect was blocked by 1 mM of nipecotic acid, demonstrating the involvement of the GAT-1 subtype of GABA transporter. The IL-4 effect depends on M1 muscarinic activity, an increase in intracellular calcium levels, tyrosine kinase activity and protein kinase C (PKC) activity. Treatment with IL-4 for 48 h induced an increase of 90% in the number of GAD- and GABA-immunoreactive cells when compared with control cultures. Our results indicate that IL-4 modulates the GABAergic phenotype of retinal cells in culture. This result can suggest an important role for this cytokine either during the normal development of retinal circuitry or during neuroprotection after injuries.

Is there more to GABA than synaptic inhibition?

In the mature brain, GABA (gamma-aminobutyric acid) functions primarily as an inhibitory neurotransmitter. But it can also act as a trophic factor during nervous system development to influence events such as proliferation, migration, differentiation, synapse maturation and cell death. GABA mediates these processes by the activation of traditional ionotropic and metabotropic receptors, and probably by both synaptic and non-synaptic mechanisms. However, the functional properties of GABA receptor signalling in the immature brain are significantly different from, and in some ways opposite to, those found in the adult brain. The unique features of the early-appearing GABA signalling systems might help to explain how GABA acts as a developmental signal.

Functional glycine receptors are expressed by postnatal nestin-positive neural stem/progenitor cells

Multipotent neural stem and progenitor cells (NS/PCs) are well-established cell subpopulations occurring in the developing, and also in the mature mammalian nervous systems. Trophic and transcription factors are currently the main signals known to influence the development and the commitment of NS/PCs and their progeny. However, recent studies suggest that neurotransmitters could also contribute to neural development. In that respect, rodent-cultured embryonic NS/PCs have been reported to express functional neurotransmitter receptors. No similar investigation has, however, been made in postnatal and/or in adult rodent brain stem cells. In this study, using RT-PCR and immunocytochemical methods, we show that alpha(1)-, alpha(2)- and beta-subunit mRNAs and alpha-subunit proteins of the glycine ionotropic receptor are expressed by 80.5 +/- 0.9% of postnatal rat striatum-derived, nestin-positive cells within cultured neurospheres. Whole-cell patch-clamp experiments further demonstrated that glycine triggers in 33.5% of these cells currents that can be reversibly blocked by strychnine and picrotoxin. This demonstrates that NS/PCs express functional glycine receptors, the consequence(s) of their activation remaining unknown.

The effect of GABA receptor ligands in experimental spina bifida occulta

BACKGROUND

The pathophysiology behind spina bifida and other neural tube defects (NTDs) is unclear. Folic acid is one variable, but other factors remain. Studies suggest that substances active at the GABA receptor may produce NTDs. To test this hypothesis pregnant rats were exposed to either the GABA a agonist muscimol (1, 2 or 4 mg/kg), the GABA a antagonist bicuculline (.5, 1, or 2 mg/kg), the GABA b agonist baclofen (15, 30, 60 mg/kg), or the GABA b antagonist hydroxysaclofen (1, 3, or 5 mg/kg) during neural tube formation. Normal saline was used as a control and valproic acid (600 mg/kg) as a positive control. The embryos were analyzed for the presence of a spina bifida like NTD.

RESULTS

After drug administration the pregnancies were allowed to proceed to the 21st day of gestation. Then embryos were removed and skeletons staining and cleared. Vertebral arch closure was measured. Results indicate that the GABAa receptor agonist muscimol, the GABAa receptor antagonist bicuculline, and the GABAb agonist baclofen produced NTDs characterized by widening of the vertebral arch. Oppositely the GABAb antagonist hydroxysaclofen produced narrowing of the vertebral arches.

CONCLUSIONS

The findings indicate that GABA a or b ligands are capable of altering neural formation. GABA may play a greater than appreciated role in neural tube formation and may be important in NTDs. The narrowing of the vertebral arch produced by the GABA b antagonist hydroxysalcofen suggests that GABA b receptor may play an undefined role in neural tube closure that differs from the GABA a receptor.

GABA promotes survival but not proliferation of parvalbumin-immunoreactive interneurons in rodent neostriatum: an in vivo study with stereology

Amino-acid neurotransmitters regulate a wide variety of developmental processes in the mammalian CNS including neurogenesis, cell migration, and apoptosis. In order to investigate the role of GABA in early development of forebrain interneurons, we determined the survival of parvalbumin-immunoreactive GABAergic interneurons in the adult rat striatum following prenatal exposure to either GABA(A) receptor agonist or antagonist. Unbiased stereology was used to quantify parvalbumin-immunoreactive neuron number in the neostriatum of adult rats exposed to the drugs in utero, and the results were compared to pair-fed or vehicle controls. Embryos were exposed to the GABA(A) antagonist (bicuculline) or agonist (muscimol) during previously defined proliferative or post-proliferative periods for parvalbumin-immunoreactive interneurons. Unbiased stereology using the optical fractionator was used to estimate the total number of parvalbumin-immunoreactive neurons in neostriatum of experimental and control rats. No significant alteration in parvalbumin-immunoreactive neuron number was observed in rats treated with either bicuculline (1 or 2mg/kg/day) or muscimol (1mg/kg/day) during the proliferative phase. Administration of bicuculline during the post-proliferative phase significantly reduced parvalbumin-immunoreactive neuron number in the neostriatum. A concomitant decrease in neostriatal volume was also observed, suggesting that the effect is not restricted to parvalbumin-immunoreactive interneurons. Positional analysis revealed loss of normal regional distribution gradients for parvalbumin-immunoreactive neurons in neostriatum of rats exposed to bicuculline in the embryonic post-proliferative phase. This data collectively suggests that GABA promotes survival but not proliferation of parvalbumin-immunoreactive progenitors. GABA may also promote migration of subpopulations of interneurons that ultimately populate the ventral telencephalon.

Excitatory versus inhibitory GABA as a divergence point in steroid-mediated sexual differentiation of the brain

Whereas adult sex differences in brain morphology and behavior result from developmental exposure to steroid hormones, the mechanism by which steroids differentiate the brain is unknown. Studies to date have described subtle sex differences in levels of proteins and neurotransmitters during brain development, but these have lacked explanatory power for the profound sex differences induced by steroids. We report here a major divergence in the response to injection of the gamma-aminobutyric acid type A (GABA(A)) agonist, muscimol, in newborn male and female rats. In females, muscimol treatment primarily decreased the phosphorylation of cAMP response element binding protein (CREB) within the hypothalamus and the CA1 region of the hippocampus. In contrast, muscimol increased the phosphorylation of CREB in males within these same brain regions. Within the arcuate nucleus, muscimol treatment increased the phosphorylation of CREB in both females and males. Thus, the response to GABA can be excitatory or inhibitory on signal-transduction pathways that alter CREB phosphorylation depending on the sex and the region in developing brain. This divergence in response to GABA allows for a previously unknown form of steroid-mediated neuronal plasticity and may be an initial step in establishing sexually dimorphic signal-transduction pathways in developing brain.

Effect of embryonic knock-down of GABAA receptors on the levels of monoamines and their metabolites in the CNS of the mouse

In vitro evidence indicates that gamma-aminobutyric acid (GABA), acting at GABA(A) receptors, exerts a positive trophic effect on monoaminergic neurons during embryogenesis. To determine whether in vivo antagonism of GABA(A) receptors during embryogenesis interferes with the development of monoaminergic neurons, we used mice in which the number of GABA(A) receptors was decreased by 50% by targeted deletion of the beta(3) subunit gene of the GABA(A) receptor. Levels of serotonin, dopamine, norepinephrine, and the metabolites 3,4-deoxyphenylacetic acid, homovanillic acid, and 5-hydroxyindoleacetic acid were measured in the brainstem, cortex, striatum and spinal cord of female adult homozygous null (beta3-/-) and wild-type (beta3+/+) mice, as well as progenitor C57BL/6J and Strain 129/SvJ mice. The level of norepinephrine in the spinal cord of beta3-/- mice was 44% less than that of beta3+/+ mice, and did not differ in the brainstem, cortex or striatum. This finding suggests that beta3 subunit-containing GABA(A) receptors mediate the trophic effects of GABA on a subpopulation of spinally-projecting noradrenergic neurons. In contrast, the levels of serotonin, dopamine or their metabolites were unaffected, suggesting that the development of serotonergic and dopaminergic neurons may require activation of only a small fraction of GABA(A) receptors or may not be dependent on beta3 subunit-containing GABA(A) receptors. Finally, Strain 129/SvJ and C57BL/6J mice differed with respect to the levels of dopamine and its metabolites in the brainstem, spinal cord and cortex. These differences may need to be considered when assessing the phenotype of gene-targeted mice for which these mice serve as progenitor strains.

Estradiol enhances excitatory gamma-aminobutyric [corrected] acid-mediated calcium signaling in neonatal hypothalamic neurons

Contrary to the situation in adulthood, gamma-aminobutyric [corrected] acid (GABA)(A) receptor activation during early brain development depolarizes neurons sufficiently to open L-type voltage-gated Ca(2+) channels. Because GABA is excitatory during the sensitive period of steroid-mediated brain sexual differentiation, we investigated whether estradiol modulates excitatory GABA during this period, by examining two parameters: 1) magnitude of GABA-induced calcium transients; and 2) developmental duration of excitatory GABA. Dissociated hypothalamic neurons from embryonic-day-15 rat embryos were loaded with the Ca(2+) indicator, fura-2, and transient rises in [Ca(2+)](i) (Ca(2+) transient) were measured after application of 10 microM muscimol, a GABA(A) receptor agonist. Cells were treated with 10(-10) M estradiol or vehicle from 0-3 days in vitro (DIV) and imaged on 4 DIV, whereas others were treated from 3-6 DIV and imaged on 7 DIV. The mean amplitude of Ca(2+) transients after muscimol administration were 68% and 61% higher in estradiol-treated neurons on 4 DIV and 7 DIV, respectively, relative to controls. Consistent with GABA becoming inhibitory in mature neurons, 50% fewer control neurons responded on DIV 7, relative to DIV 4. However, estradiol treatment maintained excitatory GABA on DIV 7 (72% in estradiol-treated vs. 35% in control). This is the first report of hormonal modulation of excitatory GABA, and it suggests that estradiol may mediate sexual differentiation by enhancing GABA-induced increases in intracellular Ca(2+).

GABA itself promotes the developmental switch of neuronal GABAergic responses from excitation to inhibition

GABA is the main inhibitory neurotransmitter in the adult brain. Early in development, however, GABAergic synaptic transmission is excitatory and can exert widespread trophic effects. During the postnatal period, GABAergic responses undergo a switch from being excitatory to inhibitory. Here, we show that the switch is delayed by chronic blockade of GABA(A) receptors, and accelerated by increased GABA(A) receptor activation. In contrast, blockade of glutamatergic transmission or action potentials has no effect. Furthermore, GABAergic activity modulated the mRNA levels of KCC2, a K(+)-Cl(-) cotransporter whose expression correlates with the switch. Finally, we report that GABA can alter the properties of depolarization-induced Ca(2+) influx. Thus, GABA acts as a self-limiting trophic factor during neural development.

Dynamic expression of a glutamate decarboxylase gene in multiple non-neural tissues during mouse development

BACKGROUND

Glutamate decarboxylase (GAD) is the biosynthetic enzyme for the neurotransmitter gamma-aminobutyric acid (GABA). Mouse embryos lacking the 67-kDa isoform of GAD (encoded by the Gad1 gene) develop a complete cleft of the secondary palate. This phenotype suggests that this gene may be involved in the normal development of tissues outside of the CNS. Although Gad1 expression in adult non-CNS tissues has been noted previously, no systematic analysis of its embryonic expression outside of the nervous system has been performed. The objective of this study was to define additional structures outside of the central nervous system that express Gad1, indicating those structures that may require its function for normal development.

RESULTS

Our analysis detected the localized expression of Gad1 transcripts in several developing tissues in the mouse embryo from E9.0-E14.5. Tissues expressing Gad1 included the tail bud mesenchyme, the pharyngeal pouches and arches, the ectodermal placodes of the developing vibrissae, and the apical ectodermal ridge (AER), mesenchyme and ectoderm of the limb buds.

CONCLUSIONS

Some of the sites of Gad1 expression are tissues that emit signals required for patterning and differentiation (AER, vibrissal placodes). Other sites correspond to proliferating stem cell populations that give rise to multiple differentiated tissues (tail bud mesenchyme, pharyngeal endoderm and mesenchyme). The dynamic expression of Gad1 in such tissues suggests a wider role for GABA signaling in development than was previously appreciated.

Neurogenesis of GABAergic cells in the chick retina

Two classes of retinal neurons in the chick retina, the horizontal and the amacrine cells, are GABAergic. This study evaluates the neurogenesis of glutamic acid decarboxylase immunoreactive cells in the chick retina. Twenty-five microCi [3H]thymidine was injected into eggs of 2-10 days and the embryos were sacrificed at embryonic day 18 (E18). Glutamic acid decarboxylase immunohistochemistry was revealed by avidin-biotin complex method followed by autoradiography of thymidine. We used the cumulative method for counting autoradiographic grains. At E3, 10% of the amacrine cells were thymidine negative/glutamic acid decarboxylase positive and this rate remained constant until E6. From E6 to E8 about 80% of the amacrine cells were thymidine negative/glutamic acid decarboxylase positive. At E9, 100% of these neurons had been generated. On the other hand, at E3 only 1.5% of the horizontal cells had been generated (thymidine negative/glutamic acid decarboxylase positive) while at E6 this number increased to 10%. From E6 to E9 the neurogenesis pattern was similar to that found for amacrine cells. Our data show that the great majority (80%) of glutamic acid decarboxylase positive amacrine and horizontal cells proliferate between E6 and E9, i.e. the last 3 days of the neurogenesis period. From E3 to E6 only 20% of the glutamic acid decarboxylase positive amacrine and horizontal cells are generated, which suggests that glutamic acid decarboxylase positive cells may require a specific signal at about E6, which triggers their withdrawal from the cell cycle.

Sex-specific effects of in utero manipulation of GABA(A) receptors on pre- and postnatal expression of BDNF in rats

Exposure to diazepam (DZ) during the last week of in utero development in rats induces neurobehavioral effects that do not become apparent in exposed animals until young adult ages. Some of the effects are sex specific. This study evaluated the hypothesis that late gestational exposure to DZ, a positive modulator of GABA(A) receptors, affects the developmental appearance of brain-derived neurotrophic factor (BDNF), an effect that could be linked to the later consequences of the exposure. Pregnant Long-Evans rats were injected with DZ (2.5 mg/kg) over gestation days 14-20, and their male and female offspring were evaluated for levels of BDNF mRNA and protein in the cerebral cortex and hypothalamus at fetal day 20 and at postnatal ages spanning birth to young adulthood. The effects of the exposure were sex and region specific. At fetal day 20 the expression of BDNF was reduced by about 20% in the hypothalamus of males only. The early exposure affected postnatal expression of BDNF in the hypothalamus only modestly, influencing the age-related profile in both sexes. Postnatal development of BDNF in the cerebral cortex was significantly affected by the in utero exposure in males only with mRNA levels lower in the exposed group and protein levels higher during juvenile ages. At adulthood, both levels were lower in DZ-exposed males. GABA serves a role as a trophic factor during early development, and these results suggest that manipulation of GABA(A) receptors during early development could interact with the developmental action of other trophic factors thereby leading to altered neural organization and later neurobehavioral dysfunction.

Embryonic striatal neurons from niemann-pick type C mice exhibit defects in cholesterol metabolism and neurotrophin responsiveness

Niemann-Pick type C (NP-C) disease is a progressive and fatal neuropathological disorder previously characterized by abnormal cholesterol metabolism in peripheral tissues. Although a defective gene has been identified in both humans and the npc(nih) mouse model of NP-C disease, how this leads to abnormal neuronal function is unclear. Here we show that whereas embryonic striatal neurons from npc(nih) mice can take up low density lipoprotein-derived cholesterol, its subsequent hydrolysis and esterification are significantly reduced. Given the importance of cholesterol to a variety of signal transduction mechanisms, we assessed the effect of this abnormality on the ability of these neurons to respond to brain-derived neurotrophic factor (BDNF). In contrast to its effects on wild type neurons, BDNF failed to induce autophosphorylation of the TrkB receptor and to increase neurite outgrowth in npc(nih) neurons, despite expression of TrkB on the cell surface. The results suggest that abnormal cholesterol metabolism occurs in neurons in the brain during NP-C disease, even at embryonic stages of development prior to the onset of phenotypic symptoms. Moreover, this defect is associated with a lack of TrkB function and BDNF responsiveness, which may contribute to the loss of neuronal function observed in NP-C disease.

Changing properties of GABA(A) receptor-mediated signaling during early neocortical development

Evidence from several brain regions suggests gamma-aminobutyric acid (GABA) can exert a trophic influence during development, expanding the role of this amino acid beyond its function as an inhibitory neurotransmitter. Proliferating precursor cells in the neocortical ventricular zone (VZ) express functional GABA(A) receptors as do immature postmigratory neurons in the developing cortical plate (CP); however, GABA(A) receptor properties in these distinct cell populations have not been compared. Using electrophysiological techniques in embryonic and early postnatal neocortex, we find that GABA(A) receptors expressed by VZ cells have a higher apparent affinity for GABA and are relatively insensitive to receptor desensitization compared with neurons in the CP. GABA-induced current magnitude increases with maturation with the smallest responses found in recordings from precursor cells in the VZ. No evidence was found that GABA(A) receptors on VZ cells are activated synaptically, consistent with previous data suggesting that these receptors are activated in a paracrine fashion by nonsynaptically released ligand. After neurons are born and migrate to the CP, they begin to demonstrate spontaneous synaptic activity, the majority of which is GABA(A) mediated. These spontaneous GABA(A) postsynaptic currents (sPSCs) first were detected at embryonic day 18 (E18). At birth, approximately 50% of recordings from cortical neurons demonstrated GABA(A)-mediated sPSCs, and this value increased with age. GABA(A)-mediated sPSCs were action potential dependent and arose from local GABAergic interneurons. GABA application could evoke action potential-dependent PSCs in neonatal cortical neurons, suggesting that during the first few postnatal days, GABA can act as an excitatory neurotransmitter. Finally, N-methyl-D-aspartate (NMDA)- but not non-NMDA-mediated sPSCs were also present in early postnatal neurons. These events were not observed in cells voltage clamped at negative holding potentials (-60 to -70 mV) but were evident when the holding potential was set at positive values (+30 to +60 mV). Together these results provide evidence for the early maturation of GABAergic communication in the neocortex and a functional change in GABA(A)-receptor properties between precursor cells and early postmitotic neurons. The change in GABA(A)-receptor properties may reflect the shift from paracrine to synaptic receptor activation.

Developmental sex differences in amino acid neurotransmitter levels in hypothalamic and limbic areas of rat brain

GABA, glutamate and aspartate are the predominant amino acid neurotransmitters in the mammalian brain. We have previously reported a developmental sex difference in messenger RNA levels of glutamate decarboxylase, the rate-limiting enzyme in GABA synthesis [Davis A. M. et al. (1996) Horm. Behav. 30, 538-552]. Males were found to have significantly higher levels of messenger RNA in many steroid-concentrating regions of the hypothalamus and limbic system on day 1 of life. Therefore, in this study, we have examined levels of amino acid neurotransmitters during early postnatal development in many of the same or related brain areas. We found that levels of all three transmitters change as animals age. While both GABA and aspartate concentrations increase, glutamate levels decrease. In addition, there are sex differences in neurotransmitter levels in several areas examined, including the ventromedial and arcuate nuclei of the hypothalamus, and the CA1 region of the hippocampus. Sex differences for GABA occur only on postnatal days 1 and 5. However, sex differences in aspartate occur later in development (postnatal day 20). The CA1 region of males has a significantly greater concentration of GABA, glutamate and aspartate than females on postnatal day 1. In addition, treatment of females with testosterone propionate on the day of birth results in increased GABA levels, suggesting that these sex differences may be the result of hormone exposure during development. We hypothesize that these hormonally mediated sex differences in amino acid transmitters early in development contribute to the establishment of sexually dimorphic neuronal architecture in the adult.

Regulation of intracellular chloride by cotransporters in developing lateral superior olive neurons

The regulatory mechanisms of intracellular Cl- concentration ([Cl-]i) were investigated in the lateral superior olive (LSO) neurons of various developmental stages by taking advantage of gramicidin perforated patch recording mode, which enables neuronal [Cl-]i measurement. Responses to glycine changed from depolarization to hyperpolarization during the second week after birth, resulting from [Cl-]i decrease. Furosemide equally altered the [Cl-]i of both immature and mature LSO neurons, indicating substantial contributions of furosemide-sensitive intracellular Cl- regulators; i.e., K+-Cl- cotransporter (KCC) and Na+-K+-Cl- cotransporter (NKCC), throughout this early development. Increase of extracellular K+ concentration and replacement of intracellular K+ with Cs+ resulted in [Cl-]i elevation at postnatal days 13-15 (P13-P15), but not at P0-P2, indicating that the mechanism of neuronal Cl- extrusion is sensitive to both furosemide and K+-gradient and poorly developed in immature LSO neurons. In addition, removal of extracellular Na+ decreased [Cl-]i at P0-P2, suggesting the existence of extracellular Na+-dependent and furosemide-sensitive Cl- accumulation in immature LSO neurons. These data show clearly that developmental changes of Cl- cotransporters alter [Cl-]i and are responsible for the switch from the neonatal Cl- efflux to the mature Cl- influx in LSO neurons. Such maturational changes in Cl- cotransporters might have the important functional roles for glycinergic and GABAergic synaptic transmission and the broader implications for LSO and auditory development.

GABA and histogenesis in fetal and neonatal mouse brain lacking both the isoforms of glutamic acid decarboxylase

Recent in vitro investigations have suggested that GABA is involved in the development of the mammalian central nervous system. To evaluate the roles of GABA in neurogenesis in vivo, we generated mice lacking both the isoforms of glutamic acid decarboxylase (GAD), GAD65 and GAD67, by mating GAD65- and GAD67-mutant mice generated by homologous recombination in this laboratory. Similar to GAD67-deficient mice, the GAD65/67-deficient mice did not survive after birth because of cleft palate. We thus analyzed these mice at the fetal and newborn stages. GABA was scarcely detectable in the GAD65/67-deficient brains, indicating that the GAD-independent GABA synthetic pathway was not active. The activity of ornithine decarboxylase, which is possibly involved in such a pathway, did not increase with the GAD deficiency. Histological and immunohistochemical studies of the GAD65/67-deficient brain did not reveal any discernible disorders of histogenesis. The discrepancy between the results of previous in vitro investigations, performed mostly on rat tissue, and those of the present analysis on mutant mice may be attributed to the different species used or to the possibility that other mediators can compensate for GABA functions in vivo.

Regulation of GABA(A) receptor subunit mRNA expression by the pesticide dieldrin in embryonic brainstem cultures: a quantitative, competitive reverse transcription-polymerase chain reaction study

Cyclodiene organochlorine pesticides, such as dieldrin, inhibit gamma-aminobutyric acid (GABA)ergic neurotransmission by blocking the Cl- channel of GABA(A) receptors. This action may make the developing nervous system especially vulnerable to these neurotoxins, which could interfere with the trophic actions of GABA on developing neurons and alter expression of GABA(A) receptors. We have used an in vitro model to determine whether exposure to dieldrin alters developmental expression of GABA(A) receptor subunit mRNA transcripts. Dissociated cell cultures were prepared from embryonic day 14 (E14) brainstem and cultured in serum-containing medium for 1 day in vitro (DIV), then treated for 2 DIV with 10 microM dieldrin in serum-free medium. This dose was based on preliminary experiments and previous studies (Nagata et al.: Brain Res 645:19-26, 1994; Pomes et al.: J Pharmacol Exp Ther 271:1616-1623, 1994). Absolute amounts of alpha1, beta3, gamma1, gamma2S and gamma2L mRNA transcripts were quantified in these cultures by quantitative, competitive reverse transcription-polymerase chain reaction (RT-PCR) using subunit-selective internal standards. The most abundant GABA(A) subunit transcript was beta3, which was much more highly expressed than gamma2S, gamma1, gamma2L, or alpha1 subunit mRNAs. Dieldrin differentially regulated expression of these transcripts. Levels of beta3 subunit transcripts were significantly increased (by 300%) by dieldrin, whereas expression of gamma2S and gamma2L transcripts were decreased (by 50% and 40%, respectively). However, dieldrin did not alter the ratio of gamma2S to gamma2L transcripts, indicating that it did not affect alternative splicing of gamma2 transcripts. Dieldrin appeared to increase expression of alpha1 subunit transcripts, but this effect was not statistically significant. Dieldrin did not significantly alter expression of gamma1 subunit transcripts. These results support the hypothesis that in utero exposure to cyclodiene pesticides could pose a risk to the developing brain by virtue of their ability to alter gene expression of GABA(A) receptor subunits, which could produce GABA(A) receptors with altered functional properties.