Embryonic hypothalamic expression of functional glutamate receptors

iPSC-Derived Striatal Medium Spiny Neurons from Patients with Multiple System Atrophy Show Hypoexcitability and Elevated α-Synuclein Release

Multiple system atrophy of the parkinsonian type (MSA-P) is a rare, fatal neurodegenerative disease with sporadic onset. It is still unknown if MSA-P is a primary oligodendropathy or caused by neuronal pathophysiology leading to severe, α-synuclein-associated neurodegeneration, mainly in the striatum. In this study, we generated and differentiated induced pluripotent stem cells (iPSCs) from patients with the clinical diagnosis of probable MSA-P (n = 3) and from three matched healthy controls into GABAergic striatal medium spiny neurons (MSNs). We found a significantly elevated release and neuronal distribution for α-synuclein, as well as hypoexcitability in the MSNs derived from the MSA-P patients compared to the healthy controls. These data suggest that the striatal hypoexcitable neurons of MSA-P patients contribute to a pathological α-synuclein burden which is likely to spread to neighboring cells and projection targets, facilitating disease progression.

Efficacy of probiotics on the modulation of gut microbiota in the treatment of diabetic nephropathy

Diabetic nephropathy (DN) is a major cause of end-stage renal disease, and therapeutic options for preventing its progression are insufficient. The number of patients with DN has been increasing in Asian countries because of westernization of dietary lifestyle, which may be associated with the following changes in gut microbiota. Alterations in the gut microbiota composition can lead to an imbalanced gastrointestinal environment that promotes abnormal production of metabolites and/or inflammatory status. Functional microenvironments of the gut could be changed in the different stages of DN. In particular, altered levels of short chain fatty acids, D-amino acids, and reactive oxygen species biosynthesis in the gut have been shown to be relevant to the pathogenesis of the DN. So far, evidence suggests that the gut microbiota may play a key role in determining networks in the development of DN. Interventions directing the gut microbiota deserve further investigation as a new protective therapy in DN. In this review, we discuss the potential roles of the gut microbiota and future perspectives in the protection and/or treatment of kidneys.

A novel approach for testing the teratogenic potential of chemicals on the platform of metabolomics: studies employing HR-MAS nuclear magnetic resonance spectroscopy

The objective is to develop a quick, reliable method for testing the teratogenic potential of a new chemical entity (NCE) on the platform of metabonomics, as an alternative to conventional procedures.

Glutamate carboxypeptidase II gene polymorphisms and neural tube defects in a high-risk Chinese population

Glutamate carboxypeptidase II (GCPII) catalyzes the hydrolysis of N-acetylaspartylglutamate into N-acetylaspartate and glutamate in the brain. Animal experiments suggested that GCPII plays an essential role in early embryonic development. Previous studies provided conflicting results on the effect of the GCPII rs61886492 C>T (or 1561C>T) polymorphism on NTDs. In the Lvliang area of Shanxi province, where the incidence of NTDs is the highest in China, a case-control study was conducted to investigate possible association between the GCPII rs61886492 and rs202676 polymorphisms and NTD risk. Results indicated all the case and control samples displayed the rs61886492 GG genotype. Although no significant differences in rs202676 genotype or allele frequencies were found between the NTD and control groups, the combined AG+GG genotype group was significantly associated with anencephaly (p = 0.03, OR = 2.11, 95% CI, 1.11-4.01), but not with spina bifida or encephalocele. Overall, the rs202676 A>G polymorphism is a potential risk factor for anencephaly. The results of this study suggest that phenotypic heterogeneity may exist among NTDs in this Chinese population.

Chapter 2: hypothalamic neural systems controlling the female reproductive life cycle gonadotropin-releasing hormone, glutamate, and GABA

The hypothalamic-pituitary-gonadal (HPG) axis undergoes a number of changes throughout the reproductive life cycle that are responsible for the development, puberty, adulthood, and senescence of reproductive systems. This natural progression is dictated by the neural network controlling the hypothalamus including the cells that synthesize and release gonadotropin-releasing hormone (GnRH) and their regulatory neurotransmitters. Glutamate and GABA are the primary excitatory and inhibitory neurotransmitters in the central nervous system, and as such contribute a great deal to modulating this axis throughout the lifetime via their actions on receptors in the hypothalamus, both directly on GnRH neurons as well as indirectly through other hypothalamic neural networks. Interactions among GnRH neurons, glutamate, and GABA, including the regulation of GnRH gene and protein expression, hormone release, and modulation by estrogen, are critical to age-appropriate changes in reproductive function. Here, we present evidence for the modulation of GnRH neurosecretory cells by the balance of glutamate and GABA in the hypothalamus, and the functional consequences of these interactions on reproductive physiology across the life cycle.

Activity-dependent neuroprotection and cAMP response element-binding protein (CREB): kinase coupling, stimulus intensity, and temporal regulation of CREB phosphorylation at serine 133

The dual nature of the NMDA receptor as a mediator of excitotoxic cell death and activity-dependent cell survival likely results from divergent patterns of kinase activation, transcription factor activation, and gene expression. To begin to address this divergence, we examined cellular and molecular signaling events that couple excitotoxic and nontoxic levels of NMDA receptor stimulation to activation of the cAMP response element-binding protein (CREB)/cAMP response element (CRE) pathway in cultured cortical neurons. Pulses (10 min) of NMDA receptor-mediated synaptic activity (nontoxic) triggered sustained (up to 3 h) CREB phosphorylation (pCREB) at serine 133. In contrast, brief stimulation with an excitotoxic concentration of NMDA (50 microm) triggered transient pCREB. The duration of pCREB was dependent on calcineurin activity. Excitotoxic levels of NMDA stimulated calcineurin activity, whereas synaptic activity did not. Calcineurin inhibition reduced NMDA toxicity and converted the transient increase in pCREB into a sustained increase. In accordance with these observations, sustained pCREB (up to 3 h) did not require persistent kinase pathway activity. The sequence of stimulation with excitotoxic levels of NMDA and neuroprotective synaptic activity determined which stimulus exerted control over pCREB duration. Constitutively active and dominant-negative CREB constructs were used to implicate CREB in synaptic activity-dependent neuroprotection against NMDA-induced excitotoxicity. Together these data provide a framework to begin to understand how the neuroprotective and excitotoxic effects of NMDA receptor activity function in an antagonistic manner at the level of the CREB/CRE transcriptional pathway.

Age-related changes in the expression of axonal and glial group I metabotropic glutamate receptor in the rat substantia nigra pars reticulata

Neuronal systems undergo many significant changes during the course of brain development. To characterize the developmental changes in the substantia nigra pars reticulata (SNr) associated with the expression of group I metabotropic glutamate receptors (mGluRs), we used the immunoperoxidase and immunogold methods at the electron microscope level to determine whether the subcellular and subsynaptic patterns of distribution of mGluR1a and mGluR5 differ between young (P14-P18) and adult (>2 months) rats. The SNr of young rats contained a significantly higher density of labeled unmyelinated axons for both receptor subtypes. In addition, mGluR5-immunoreactive glial processes were very abundant in young rats but absent in the adults. On the other hand, the relative proportion of immunoreactive dendrites was the same for both age groups. Analysis of immunogold-labeled rat SNr revealed similar proportions of plasma membrane-bound mGluR1a and mGluR5 in adult (59.8 and 19.4%, respectively) and young (60.6 and 18.4%, respectively) rats. The pattern of subsynaptic localization of mGluR1a also remained the same between young and adults. However, the proportion of extrasynaptic mGluR5 decreased, whereas proportions of gold particles associated with symmetric synapses increased in adults. The results of this study demonstrate significant differences in the expression of group I mGluRs in the SNr of young and adult rats. These findings support a role for group I mGluRs during development and emphasize the importance of using brain tissue from age-matched subjects when attempting to correlate functional data from young rat brain slices with immunocytochemical localization of group I mGluRs.

The mGlu5 metabotropic glutamate receptor is expressed in zones of active neurogenesis of the embryonic and postnatal brain

Metabotropic glutamate (mGlu) receptors have been implicated in the regulation of developmental plasticity. Here, we examined the expression of mGlu1a-b, -2, -3, -4a-b, and -5a receptor subtypes from embryonic day 12 (E12) to the early and late postnatal life. While all transcripts (with the exception of mGlu4 mRNA) were detected prenatally, only the mGlu5 receptor protein was found in detectable amounts in the embryonic brain. Immunohistochemical analysis showed that the mGlu5 receptor was mainly expressed by cells surrounding the ventricles at E15, whereas it was more diffusely expressed at E18. In the postnatal life, besides its classical expression sites, the mGlu5 receptor was found in zones of active neurogenesis such as the external granular layer (EGL) of the cerebellar cortex and the subventricular zone. In these regions, the presence of actively proliferating progenitor cells was detected by BrdU staining. No other subtype (among those we have examined) was found to be expressed in regions enriched of BrdU(+) cells. These data suggest a role for mGlu5 receptors in the early brain development and in basic cellular processes such as proliferation and/or differentiation.

Active surface transport of metabotropic glutamate receptors through binding to microtubules and actin flow

Receptors for neurotransmitters are concentrated and stabilized at given sites such as synapses through interactions with scaffolding proteins and cytoskeletal elements. The transport of receptors first involves directed vesicular trafficking of intracellularly stored receptors followed by their targeting to the plasma membrane. Once expressed at the cell surface, receptors are thought to reach their final location by random Brownian diffusion in the plasma membrane plane. Here, we investigate whether the metabotropic glutamate receptor mGluR5 can also be transported actively on the cell surface. We used single particle tracking to follow mGluR5 movement in real time at the surface of neuronal growth cones or fibroblast lamellipodia, both of which bear a particularly active cytoskeleton. We found that after a certain lag time mGluR5 undergoes directed rearward transport, which depends on actin flow. On actin depolymerization, directed movement was suppressed, but receptors still bound to a rigid structure. By contrast, receptor transport and immobilization was fully suppressed by microtubule depolymerization but favored by microtubule stabilization. Furthermore, mGluR5 could be immunoprecipitated with tubulin from rat brains, confirming the ability of mGluR5 to bind to microtubules. We propose that mGluR5 can be transported on the cell surface through actin-mediated retrograde transport of microtubules. This process may play a role in receptor targeting and organization during synapse formation or during glutamate-mediated growth cone chemotaxis.

Excitatory actions of GABA increase BDNF expression via a MAPK-CREB-dependent mechanism--a positive feedback circuit in developing neurons

During early neuronal development, GABA functions as an excitatory neurotransmitter, triggering membrane depolarization, action potentials, and the opening of plasma membrane Ca(2+) channels. These excitatory actions of GABA lead to a number of changes in neuronal structure and function. Although the effects of GABA on membrane biophysics during early development have been well documented, little work has been done to examine the possible mechanisms underlying GABA-regulated plastic changes in the developing brain. This study focuses on GABA-regulated kinase activity and transcriptional control. We utilized a combination of Western blotting and immunocytochemical techniques to examine two potential downstream pathways regulated by GABA excitation: the p42/44 mitogen-activated protein kinase (MAPK) cascade and the transcription factor cyclic AMP response element binding protein (CREB). During early development of cultured hypothalamic neurons (5 days in vitro), stimulation with GABA triggered activation of the MAPK cascade and phosphorylation of CREB at Ser 133. These effects were mediated by the GABA(A) receptor, since administration of the GABA(A) receptor-specific agonist muscimol (50 microM) triggered pathway activation, and pretreatment with the GABA(A)-receptor specific antagonist bicuculline (20 microM) blocked pathway activation. Immunocytochemistry revealed a spatial and temporal correlation between activation of the MAPK cascade and CREB phosphorylation. Pretreatment with the MAPK/ERK kinase (MEK) inhibitor U0126 (10 microM) attenuated CREB phosphorylation, indicating that the MAPK pathway regulates that activation state of CREB. In contrast to the excitatory effects observed during early development, in more mature neurons, GABA functions as an inhibitory transmitter. Consistent with this observation, GABA(A) receptor activation did not stimulate MAPK cascade activation or CREB phosphorylation in mature cultures (18 days in vitro). To determine whether GABA(A) receptor activation during early development stimulates gene expression, we examined the inducible expression of the neurotrophin brain-derived neurotrophic factor (BDNF). Both GABA and muscimol stimulated BDNF expression, and pretreatment with U0126 attenuated GABA-induced BDNF expression. Whole cell electrophysiological recording was used to assess the effects of BDNF on GABA release. BDNF (100 ng/ml) dramatically increased the frequency of excitatory GABAergic spontaneous postsynaptic currents. Together, these data suggest a positive excitatory feedback loop between GABA and BDNF expression during early development, where GABA facilitates BDNF expression, and BDNF facilitates the synaptic release of GABA. Signaling via the MAPK cascade and the transcription factor CREB appear to play a substantial role in this process.

Expression and plasticity of glutamate receptors in the supraoptic nucleus of the hypothalamus

Magnocellular neuroendocrine cells (MNCs) of the supraoptic nucleus of the hypothalamus (SON) produce and release the hormones vasopressin (VP) and oxytocin (OT) in response to a variety of stimuli to regulate body water and salt, parturition and lactation. Hormone release is influenced by the pattern of neuronal firing of these MNCs, which, in turn, is governed by intrinsic conductances and synaptic inputs, including those mediated by the neurotransmitter glutamate. Functional and molecular evidence has confirmed the expression of AMPA-, NMDA-, and metabotropic-type glutamate receptors in the SON, that together may orchestrate the effects of glutamatergic transmission on neuroendocrine function. However, the specific roles of the different subtypes of glutamate receptors is not yet clear. As with other central neurons, the subunit composition of glutamate receptors on MNCs will likely determine their properties and may potentially help define the differential properties of VP- and OT-producing MNCs. Possible functions of glutamate receptors on SON MNCs include altering excitatory synaptic transmission of osmotic information, neuronal firing, hormone production and release, and calcium signaling. Of interest are the anatomical, molecular, and functional changes at glutamatergic synapses in the SON that occur in response to pertinent physiological stimuli or development. These types of plasticity may include changes in glutamatergic synaptic density, glutamate receptor levels, or glutamate receptor subunit expression, all of which can affect the efficiency of synaptic transmission.

Membrane properties underlying patterns of GABA-dependent action potentials in developing mouse hypothalamic neurons

Spikes may play an important role in modulating a number of aspects of brain development. In early hypothalamic development, GABA can either evoke action potentials, or it can shunt other excitatory activity. In both slices and cultures of the mouse hypothalamus, we observed a heterogeneity of spike patterns and frequency in response to GABA. To examine the mechanisms underlying patterns and frequency of GABA-evoked spikes, we used conventional whole cell and gramicidin perforation recordings of neurons (n = 282) in slices and cultures of developing mouse hypothalamus. Recorded with gramicidin pipettes, GABA application evoked action potentials in hypothalamic neurons in brain slices of postnatal day 2-9 (P2-9) mice. With conventional patch pipettes (containing 29 mM Cl-), action potentials were also elicited by GABA from neurons of 2-13 days in vitro (2-13 DIV) embryonic hypothalamic cultures. Depolarizing responses to GABA could be generally classified into three types: depolarization with no spike, a single spike, or complex patterns of multiple spikes. In parallel experiments in slices, electrical stimulation of GABAergic mediobasal hypothalamic neurons in the presence of glutamate receptor antagonists [10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 100 microM 2-amino-5-phosphonopentanoic acid (AP5)] resulted in the occurrence of spikes that were blocked by bicuculline (20 microM). Blocking ionotropic glutamate receptors with AP5 and CNQX did not block GABA-mediated multiple spikes. Similarly, when synaptic transmission was blocked with Cd(2+) (200 microM) and Ni(2+) (300 microM), GABA still induced multiple spikes, suggesting that the multiple spikes can be an intrinsic membrane property of GABA excitation and were not based on local interneurons. When the pipette [Cl-] was 29 or 45 mM, GABA evoked multiple spikes. In contrast, spikes were not detected with 2 or 10 mM intracellular [Cl-]. With gramicidin pipettes, we found that the mean reversal potential of GABA-evoked current (E(GABA)) was positive to the resting membrane potential, suggesting a high intracellular [Cl-] in developing mouse neurons. Varying the holding potential from -80 to 0 mV revealed an inverted U-shaped effect on spike probability. Blocking voltage-dependent Na+ channels with tetrodotoxin eliminated GABA-evoked spikes, but not the GABA-evoked depolarization. Removing Ca(2+) from the extracellular solution did not block spikes, indicating GABA-evoked Na+ -based spikes. Although E(GABA) was more positive within 2-5 days in culture, the probability of GABA-evoked spikes was greater in 6- to 9-day cells. Mechanistically, this appears to be due to a greater Na+ current found in the older cells during a period when the E(GABA) is still positive to the resting membrane potential. GABA evoked similar spike patterns in HEPES and bicarbonate buffers, suggesting that Cl-, not bicarbonate, was primarily responsible for generating multiple spikes. GABA evoked either single or multiple spikes; neurons with multiple spikes had a greater Na+ current, a lower conductance, a more negative spike threshold, and a greater difference between the peak of depolarization and the spike threshold. Taken together, the present results indicate that the patterns of multiple action potentials evoked by GABA are an inherent property of the developing hypothalamic neuron.

Differing, spatially restricted roles of ionotropic glutamate receptors in regulating the migration of gnrh neurons during embryogenesis

We have examined here the role of glutamate in regulating the process of tangential neuronal migration during embryogenesis by investigating the roles of AMPA and NMDA receptors in the migration of the gonadotropin-releasing hormone (GnRH) neurons from the nose to the hypothalamus. We first determined that GluR1-4 subunit mRNAs were present from embryonic day (E) 12.5 along the complete nose-brain migratory pathway of the GnRH neurons, whereas that of the obligatory NMDAR1 transcript was present only in brain regions of GnRH migration. In vivo studies revealed that AMPA receptor antagonism between E12.5 and E16.5 resulted in a significant (p < 0.05) accumulation of GnRH neurons in the nose adjacent to the cribiform plate. In contrast, NMDA receptor antagonism over E12.5-E16.5 or E13.5-E16.5 caused a selective increase (p < 0.05) in the number of GnRH neurons located in their final resting place within the diagonal band of Broca and preoptic area. Dual-labeling studies using GnRH promoter-LacZ transgenic mice, which facilitate the identification of receptors in GnRH neurons, identified the presence of NMDAR1 receptors in approximately 6% of embryonic GnRH neurons located throughout the migratory pathway. Postnatally, the percentage of GnRH neurons expressing NMDAR1 increased to 50%. These results indicate that tonic AMPA receptor activation enhances the migration of GnRH neurons from the nose into the brain, whereas that of NMDA receptor activation slows the final phase of GnRH migration within the forebrain. These in vivo observations demonstrate differing, spatially restricted roles for AMPA and NMDA receptor activation in the process of tangential neuronal migration.

GABA, not glutamate, a primary transmitter driving action potentials in developing hypothalamic neurons

Neuronal activity is critical for many aspects of brain development. It has often been assumed that the primary excitatory transmitter driving this activity is glutamate. In contrast, we report that during early development, synaptic release of GABA, the primary inhibitory neurotransmitter in the mature brain, is not only excitatory but in addition plays a more robust role than glutamate in generating spike activity in mouse hypothalamic neurons. Based on gramicidin perforated whole cell and extracellular recording, which leave intracellular Cl(-) unperturbed in brain slices and cultures, the GABA(A) receptor antagonist bicuculline induced a dramatic decrease in spike frequency (83% decrease) in developing neurons, three times greater than that generated by glutamate receptor antagonists 2-amino-5-phosphono-pentanoic acid and 6-cyano-7-nitroquinoxalene-2,3-dione. Thus a number of factors related to spike-dependent stabilization of neuronal connections, including Hebbian mechanisms, that are generally applied to glutamate transmission may also participate in stabilization of GABA circuits.

Late embryonic expression of AMPA receptor function in the CA1 region of the intact hippocampus in vitro

Studies in slices suggest that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated synaptic currents are not present in CA1 (Cornu ammonis) pyramidal neurons at birth (P0). We have re-examined this issue in the rat intact hippocampal formation (IHF) in vitro. Injections of biocytin or carbocyanine show that the temporo-ammonic, commissural and Schaffer collateral pathways are present at birth in the marginal zone of CA1. Electrical stimulation of these pathways evoked field excitatory postsynaptic potentials (fEPSPs) in the marginal zone of CA1 from embryonic day 19 (E19) to postnatal day 9 (P9). These fEPSPs are mediated by synaptic AMPA receptors as they are reduced or completely blocked by: (i) tetrodotoxin; (ii) high divalent cation concentrations; (iii) the adenosine A1 receptor agonist CPA; (iv) anoxic episodes; (v) the selective AMPA receptor antagonist 1-(4-aminophenyl)-3-methylcarbamyl-4-methyl-7, 8-methylenedioxy-3,4-dihydro-5H-2,3-benzodiazepine (GYKI-53655) or the mixed AMPA-kainate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione (NBQX). The amplitude of the fEPSPs is also reduced by D(-)-2-amino-5-phosphonopentanoic acid (D-APV) and its duration is increased by bicuculline suggesting the participation of N-methyl-D-aspartate (NMDA) and GABAA (gamma-aminobutyric acid) receptors. Finally, AMPA receptor-mediated fEPSPs are also recorded in P0 slices, but they are smaller and more labile than in the IHF. Our results suggest that in embryonic CA1 neurons, glutamate acting on AMPA receptors already provides a substantial part of the excitatory drive and may play an important role in the activity-dependent development of the hippocampus. Furthermore, the IHF may be a convenient preparation to investigate the properties of the developing hippocampus.

Ca(2+)-permeable AMPA receptors and spontaneous presynaptic transmitter release at developing excitatory spinal synapses

At many mature vertebrate glutamatergic synapses, excitatory transmission strength and plasticity are regulated by AMPA and NMDA receptor (AMPA-R and NMDA-R) activation and by patterns of presynaptic transmitter release. Both receptors potentially direct neuronal differentiation by mediating postsynaptic Ca(2+) influx during early development. However, the development of synaptic receptor expression and colocalization has been examined developmentally in only a few systems, and changes in release properties at neuronal synapses have not been characterized extensively. We recorded miniature EPSCs (mEPSCs) from spinal interneurons in Xenopus embryos and larvae. In mature 5-8 d larvae, approximately 70% of mEPSCs in Mg(2+)-free saline are composed of both a fast AMPA-R-mediated component and a slower NMDA-R-mediated decay, indicating receptor colocalization at most synapses. By contrast, in 39-40 hr embryos approximately 65% of mEPSCs are exclusively fast, suggesting that these synapses initially express predominantly AMPA-R. In a physiological Mg(2+) concentration (1 mM), mEPSCs throughout development are mainly AMPA-R-mediated at negative potentials. Embryonic synaptic AMPA-R are highly Ca(2+)-permeable, mEPSC amplitude is over twofold larger than at mature synapses, and mEPSCs frequently occur in bursts consistent with asynchronous multiquantal release. AMPA-R function in this motor pathway thus appears to be independent of previous NMDA-R activation, unlike other regions of the developing nervous system, ensuring a greater reliability for embryonic excitatory transmission. Early spontaneous excitatory activity is specialized to promote AMPA-R-mediated synaptic Ca(2+) influx, which likely has significant roles in neuronal development.

Voltage-activated K+ channels and membrane depolarization regulate accumulation of the cyclin-dependent kinase inhibitors p27(Kip1) and p21(CIP1) in glial progenitor cells

Neural cell development is regulated by membrane ion channel activity. We have previously demonstrated that cell membrane depolarization with veratridine or blockage of K+ channels with tetraethylammonium (TEA) inhibit oligodendrocyte progenitor (OP) proliferation and differentiation (); however the molecular events involved are largely unknown. Here we show that forskolin (FSK) and its derivative dideoxyforskolin (DFSK) block K+ channels in OPs and inhibit cell proliferation. The antiproliferative effects of TEA, FSK, DFSK, and veratridine were attributable to OP cell cycle arrest in G1 phase. In fact, (1) cyclin D accumulation in synchronized OP cells was not affected by K+ channel blockers or veratridine; (2) these agents prevented OP cell proliferation only if present during G1 phase; and (3) G1 blockers, such as rapamycin and deferoxamine, mimicked the anti-proliferative effects of K+ channel blockers. DFSK also prevented OP differentiation, whereas FSK had no effect. Blockage of K+ channels and membrane depolarization also caused accumulation of the cyclin-dependent kinase inhibitors p27(Kip1) and p21(CIP1) in OP cells. The antiproliferative effects of K+ channel blockers and veratridine were still present in OP cells isolated from INK4a-/- mice, lacking the cyclin-dependent kinase inhibitors p16(INK4a) and p19(ARF). Our results demonstrate that blockage of K+ channels and cell depolarization induce G1 arrest in the OP cell cycle through a mechanism that may involve p27(Kip1) and p21(CIP1) and further support the conclusion that OP cell cycle arrest and differentiation are two uncoupled events.

GABAB receptor-mediated regulation of glutamate-activated calcium transients in hypothalamic and cortical neuron development

In the mature nervous system excitatory neurotransmission mediated by glutamate is balanced by the inhibitory actions of GABA. However, during early development, GABA acting at the ligand-gated GABAA Cl- channel also exerts excitatory actions. This raises a question as to whether GABA can exert inhibitory activity during early development, possibly by a mechanism that involves activation of the G protein-coupled GABAB receptor. To address this question we used Ca2+ digital imaging to assess the modulatory role of GABAB receptor signaling in relation to the excitatory effects of glutamate during hypothalamic and cortical neuron development. Ca2+ transients mediated by synaptic glutamate release in neurons cultured from embryonic rat were dramatically depressed by the administration of the GABAB receptor agonist baclofen in a dose-dependent manner. The inhibitory effects of GABAB receptor activation persisted for the duration of baclofen administration (>10 min). Preincubation with the Gi protein inhibitor pertussis toxin resulted in a substantial decrease in the inhibitory actions of baclofen, confirming that a Gi-dependent mechanism mediated the effects of the GABAB receptor. Co-administration of the GABAB receptor antagonist 2-hydroxy-saclofen eliminated the inhibitory action of baclofen. Alone, GABAB antagonist application elicited a marked potentiation of Ca2+ transients mediated by glutamatergic neurotransmission, suggesting that tonic synaptic GABA release exerts an inhibitory tone on glutamate receptor-mediated Ca2+ transients via GABAB receptor activation. In the presence of TTX to block action potential-mediated neurotransmitter release, stimulation with exogenously applied glutamate triggered a robust postsynaptic Ca2+ rise that was dramatically depressed (>70% in cortical neurons, >40% in hypothalamic neurons) by baclofen. Together these data suggest both a pre- and postsynaptic component for the modulatory actions of the GABAB receptor. These results indicate a potentially important role for the GABAB receptor as a modulator of the excitatory actions of glutamate in developing neurons.

Functional activation of punch-cultured magnocellular neuroendocrine cells by glutamate receptor subtypes

To provide a simple means to isolate and study the cellular functions of small groups of neurons, we developed a modified 'punch' culture procedure that facilitates acute and long-term in vitro physiological studies. Primary 'punch' cultures of magnocellular neuroendocrine cells (MNCs) from the supraoptic nucleus (SON) were established and the basic physiological effects of subtype-specific glutamate receptor agonists were characterized. MNCs from the punch cultures established a mature morphology in culture with extensive outgrowth of axons and varicosities. After 8 days, a single cultured SON punch produced an average of 10.0 +/- 2.1 pg AVP and contained an average of 222 +/- 53 vasopressin-neurophysin immunoreactive cells. Patch clamp recordings from MNCs demonstrated the presence of N-methyl-D-aspartate (NMDA)-sensitive and DL, alpha-amino-3-hydroxy-5-methylisoxazole propionic acid (AMPA)-receptors. Stimulation of metabotropic receptors with 1S,3R ACPD induced acute or gradual increases in intracellular calcium. NMDA, AMPA and metabotropic receptors all contributed to the secretion of vasopressin from the punch cultures with an agonist rank order potency of: NMDA (10 microM) > AMPA (1 microM) = 1S,3R ACPD (100 microM) > kainate (10 microM). This culture preparation should be useful for cellular studies of small groups of neuroendocrine and other cells.

Glutamatergic induction of CREB phosphorylation and Fos expression in primary cultures of the suprachiasmatic hypothalamus in vitro is mediated by co-ordinate activity of NMDA and non-NMDA receptors

Exposure of Syrian hamsters to light 1 h after lights-off rapidly (10 min) induced nuclear immunoreactivity (-ir) to the phospho-Ser133 form of the Ca2+/cAMP response element (CRE) binding protein (pCREB) in the retinorecipient zone of the suprachiasmatic nuclei (SCN). Light also induced nuclear Fos-ir in the same region of the SCN after 1 h. The glutamatergic N-methyl-D-aspartate (NMDA) receptor blocker MK801 attenuated the photic induction of both factors. To investigate glutamatergic regulation of pCREB and Fos further, tissue blocks and primary cultures of neonatal hamster SCN were examined by Western blotting and immunocytochemistry in vitro. On Western blots of SCN tissue, the pCREB-ir signal at 45 kDa was enhanced by glutamate or a mixture of glutamatergic agonists (NMDA, amino-methyl proprionic acid (AMPA), and Kainate (KA)), whereas total CREB did not change. Glutamate or the mixture of agonists also induced a 56 kDa band identified as Fos protein in SCN tissue. In dissociated cultures of SCN, glutamate caused a rapid (15 min) induction of nuclear pCREB-ir and Fos-ir (after 60 min) exclusively in neurones, both GABA-ir and others. Treatment with NMDA alone had no effect on pCREB-ir. AMPA alone caused a slight increase in pCREB-ir. However, kainate alone or in combination with NMDA and AMPA induced nuclear pCREB-ir equal to that induced by glutamate. The effects of glutamate on pCREB-ir and Fos-ir were blocked by antagonists of both NMDA (MK801) and AMPA/KA (NBQX) receptors. In the absence of extracellular Mg2+, MK801 blocked glutamatergic induction of Fos-ir. However, the AMPA/KA receptor antagonist was no longer effective at blocking glutamatergic induction of either Fos-ir or pCREB-ir, consistent with the model that glutamate regulates gene expression in the SCN by a co-ordinate action through both NMDA and AMPA/KA receptors. Glutamatergic induction of nuclear pCREB-ir in GABA-ir neurones was blocked by KN-62 an inhibitor of Ca2+/Calmodulin (CaM)-dependent kinases, implicating Ca2+-dependent signalling pathways in the glutamatergic regulation of gene expression in the SCN.

Developmental plasticity of NR1 and NR2B subunit expression in the supraoptic nucleus of the rat hypothalamus

Vasopressin and oxytocin neuroendocrine cells within the supraoptic nucleus of the adult hypothalamus (SON) display mRNA expression for the NMDA receptor subunits, NR1 and NR2B, NR2C and NR2D. The NR2B subunit confers slow decay kinetics (relative to NR1/NR2A receptors) and high magnesium sensitivity to NMDA receptor responses--properties which may contribute to the NMDA receptor-mediated bursting manifested by these cells. Therefore, we examined NR2B protein expression and its developmental profile in the SON and compared it to that in the cortex and cerebellum--areas which have been studied previously. We performed Western blot analysis on SON homogenates from embryonic, postnatal (PN7, 14, 21), and adult rats using an NR2B-specific antibody. Adult NR2B levels in the SON and PVN were similar but low relative to those of cortex. SON NR2B protein levels rose in the first postnatal week, remained high through PN21, and later declined to significantly lower levels in the adult. A similar profile was observed in cerebellum, where NR2B expression displayed a sharp peak at PN14 and later declined to minimal or undetectable levels in the adult. In contrast, NR2B continued to be overexpressed through adulthood in the cortex. The ontogenic pattern for NR1 expression, which included unregulation during early postnatal life and adulthood, was similar in the SON and cortex. A different pattern was observed for the cerebellum, where NR1 levels increased gradually after ED17 to reach significantly greater adult levels. Of all three areas studied, the SON displayed the earliest developmental rise in NR1 levels. SON explant cultures proved to be a useful preparation, since they contained neurons which synthesized NR1 and NR2B subunits in quantities similar to those of ED17 SON. Our findings suggest that NMDA receptors on SON neuroendocrine cells are assembled using NR1 and NR2B subunits, and that their plastic expression in early postnatal life may play a role during development.

AMPA and NMDA receptors expressed by differentiating Xenopus spinal neurons

N-methyl--aspartate (NMDA) receptors are often the first ionotropic glutamate receptors expressed at early stages of development and appear to influence neuronal differentiation by mediating Ca2+ influx. Although less well studied, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors also can generate Ca2+ elevations and may have developmental roles. We document the presence of AMPA and NMDA class receptors and the absence of kainate class receptors with whole cell voltage-clamp recordings from Xenopus embryonic spinal neurons differentiated in vitro. Reversal potential measurements indicate that AMPA receptors are permeable to Ca2+ both in differentiated neurons and at the time they first are expressed. The PCa/Pmonocation of 1.9 is close to that of cloned Ca2+-permeable AMPA receptors expressed in heterologous systems. Ca2+ imaging reveals that Ca2+ elevations are elicited by AMPA or NMDA in the absence of Mg2+. The amplitudes and durations of these agonist-induced Ca2+ elevations are similar to those of spontaneous Ca2+ transients known to act as differentiation signals in these cells. Two sources of Ca2+ amplify AMPA- and NMDA-induced Ca2+ elevations. Activation of voltage-gated Ca2+ channels by AMPA- or NMDA-mediated depolarization contributes approximately 15 or 30% of cytosolic Ca2+ elevations, respectively. Activation of either class of receptor produces elevations of Ca2+ that elicit further release of Ca2+ from thapsigargin-sensitive but ryanodine-insensitive stores, contributing an additional approximately 30% of Ca2+ elevations. Voltage-clamp recordings and Ca2+ imaging both show that these spinal neurons express functional AMPA receptors soon after neurite initiation and before expression of NMDA receptors. The Ca2+ permeability of AMPA receptors, their ability to generate significant elevations of [Ca2+]i, and their appearance before synapse formation position them to play roles in neural development. Spontaneous release of agonists from growth cones is detected with glutamate receptors in outside-out patches, suggesting that spinal neurons are early, nonsynaptic sources of glutamate that can influence neuronal differentiation in vivo.

Channel properties of NMDA receptors on magnocellular neuroendocrine cells cultured from the rat supraoptic nucleus

Application of N-methyl-D-aspartate (NMDA) to the supraoptic nucleus of the hypothalamus (SON) generates clustered firing that may be important in hormone release. However, synaptically evoked EPSPs recorded from SON neurons exhibit varying contributions from NMDA receptors. We used the high resolution of single-channel recording to examine the receptor and ion channel properties of NMDA receptors expressed by SON neurons in 'punch' culture. Biocytin introduced into individual neurons during patch clamp recording revealed large (32.1+/-3.3 micron), oblong somas and bipolar extensions typical of magnocellular neuroendocrine cells (MNCs). Rapid application of NMDA (100-300 microM) in the presence of 10 microM glycine to outside-out macropatches resulted in openings with an average conductance of 46. 9 pS and reversal potential of +3.9 mV. Increasing glycine from 0.03 to 30 microM increased the apparent frequency, duration and occurrence of overlapping NMDA-elicited openings. NMDA responses were inhibited by Mg2+ in a voltage-dependent manner and by the NMDA-site antagonist, D-(-)-2-amino-5-phosphonovaleric acid (D-APV). Application of saturating NMDA or glycine alone with the glycine-site antagonist, 5,7-dichlorokynurenate (DCK) or with D-APV, respectively, did not result in agonist-induced openings. NR1 immunoreactivity was observed in large neurons (>25 micron) with MNC-like morphology. These single-channel and immunocytochemical data confirm the presence of functional NR1-containing NMDA receptors in MNCs.

GABAB receptor-mediated inhibition of GABAA receptor calcium elevations in developing hypothalamic neurons

In the CNS, gamma-aminobutyric acid (GABA) affects neuronal activity through both the ligand-gated GABAA receptor channel and the G protein-coupled GABAB receptor. In the mature nervous system, both receptor subtypes decrease neural excitability, whereas in most neurons during development, the GABAA receptor increases neural excitability and raises cytosolic Ca2+ levels. We used Ca2+ digital imaging to test the hypothesis that GABAA receptor-mediated Ca2+ rises were regulated by GABAB receptor activation. In young, embryonic day 18, hypothalamic neurons cultured for 5 +/- 2 days in vitro, we found that cytosolic Ca2+ rises triggered by synaptically activated GABAA receptors were dramatically depressed (>80%) in a dose-dependent manner by application of the GABAB receptor agonist baclofen (100 nM-100 microM). Coadministration of the GABAB receptor antagonist 2-hydroxy-saclofen or CGP 35348 reduced the inhibitory action of baclofen. Administration of the GABAB antagonist alone elicited a reproducible Ca2+ rise in >25% of all synaptically active neurons, suggesting that synaptic GABA release exerts a tonic inhibitory tone on GABAA receptor-mediated Ca2+ rises via GABAB receptor activation. In the presence of tetrodotoxin the GABAA receptor agonist muscimol elicited robust postsynaptic Ca2+ rises that were depressed by baclofen coadministration. Baclofen-mediated depression of muscimol-evoked Ca2+ rises were observed in both the cell bodies and neurites of hypothalamic neurons taken at embryonic day 15 and cultured for three days, suggesting that GABAB receptors are functionally active at an early stage of neuronal development. Ca2+ rises elicited by electrically induced synaptic release of GABA were largely inhibited (>86%) by baclofen. These results indicate that GABAB receptor activation depresses GABAA receptor-mediated Ca2+ rises by both reducing the synaptic release of GABA and decreasing the postsynaptic Ca2+ responsiveness. Collectively, these data suggest that GABAB receptors play an important inhibitory role regulating Ca2+ rises elicited by GABAA receptor activation. Changes in cytosolic Ca2+ during early neural development would, in turn, profoundly affect a wide array of physiological processes, such as gene expression, neurite outgrowth, transmitter release, and synaptogenesis.

GABA-dependent firing of glutamate-evoked action potentials at AMPA/kainate receptors in developing hypothalamic neurons

Although it plays a major inhibitory role in the adult mammalian CNS, gamma-aminobutyric acid (GABA) may have an excitatory function in developing neurons. The present study focuses on the dependence of glutamate on GABA to generate action potentials in developing hypothalamic neurons. Under conditions where glutamate by itself could not evoke an action potential, GABA facilitated glutamate-mediated depolarization to fire action potentials. This facilitation had a broad time window during the decaying phase of the GABA-mediated depolarization in developing neurons in culture. The glutamate-mediated depolarization was shunted only during the peak of GABA-mediated depolarization, but was facilitated after that. Similar results were obtained in the presence of 2-amino-5-phosphonopentanoic acid (AP5), indicating that GABA can facilitate glutamate responses independent of relief of the Mg2+ block of the N-methyl-D-aspartate (NMDA) receptor. This novel interaction between GABA- and glutamate-mediated excitation could play a role in strengthening neuronal circuits during early development and would exert a maximal effect if GABA and glutamate receptors were activated after a slight temporal delay.

Somatostatin receptor subtypes sst1 and sst2 elicit opposite effects on the response to glutamate of mouse hypothalamic neurones: an electrophysiological and single cell RT-PCR study

We have previously shown that somatostatin can either enhance or decrease AMPA/kainate receptor-mediated responses to glutamate in mouse-dissociated hypothalamic neurones grown in vitro. To investigate whether this effect is due to differential activation of somatostatin (SRIF) receptor subtypes, we compared modulation of the response to glutamate by SRIF with that induced by CH-275 and octreotide, two selective agonists of sst1 and sst2/sst5 receptors, respectively. Somatostatin either significantly decreased (49%) or increased (30%) peak currents induced by glutamate, and was ineffective in the remaining cells. Only the decreased response was obtained with octreotide, whereas only increased responses were elicited by CH-275 (47 and 35% of the tested cells, respectively). Mean amplitude variations under somatostatin or octreotide on the one hand, and under somatostatin or CH-275 on the other hand, were equivalent. Pertussis toxin pretreatment significantly decreased the number of cells inhibited by somatostatin or octreotide, but had no effect on the frequency of neurones showing increased sensitivity to glutamate during somatostatin or CH-275 application. About half of the neurones tested by single cell reverse transcriptase polymerase chain reaction (RT-PCR) expressed only one sst receptor (sst1 in 26% and sst2 in 22% of studied cells). Out of the remaining neurones, 34% displayed neither sst1 nor sst2 mRNAs, whereas 18% showed a simultaneous expression of both mRNA subtypes. Expression of sst1 or sst2 mRNA subtypes matched totally with the effects of somatostatin on sensitivity to glutamate in 79% of the neurones processed for PCR after recordings. These data show that pertussis toxin-insensitive activation of the sst1 receptor subtype mediates somatostatin-induced increase in sensitivity to glutamate, whereas decrease in the response to glutamate is linked to pertussis toxin-sensitive activation of the sst2 receptor subtype.

AMPA/kainate receptors modulate the survival in vitro of embryonic brainstem cells

This study aimed at analyzing the involvement of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate (AMPA/kainate) receptors in the survival of cultured rat embryonic brainstem cells, dissociated on embryonic day 14. The cell number was estimated after pharmacological manipulation of the receptors by exposure to agonists or antagonists. The developmental stage at the moment of drug application was critical for cell survival. We observed after 8 days in vitro a much stronger decrease in the number of gamma-enolase-positive cells when the cultures were treated for 3 days with the antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) starting on the day of plating than when DNQX was added after 5 days in vitro. Conversely, exposure to the agonists (RS)-2-amino-3-(3-hydroxy-5-tri-fluoromethyl-4-isoxazolyl)-propion ic acid (T-AMPA) or kainate for 3 days significantly reduced cell survival only when the treatment was initiated after 5 days in vitro. Survival of S-100-positive cells was not affected after exposure to either agonists or antagonists. Neither agonist nor antagonist treatment modified cell proliferation, as assessed by 5-bromo-2'-deoxyuridine (BrdU) staining, suggesting that the decrease in the number of gamma-enolase-positive cells is essentially due to cell death. If some of the processes we observed in vitro correspond to analogous events in vivo, then exposure to excitatory amino acid receptor agonists or antagonists at critical stages of embryogenesis may alter the development of the central nervous system.

Role of glutamate in the regulation of the outgrowth and motility of neurites from mouse spinal cord neurons in culture

The excitatory amino acid glutamate has been shown to be toxic to a number of neuronal cell types both in vitro and in vivo. It has also been shown to be capable of controlling the development of neurons grown in vitro. Using time-lapse video microscopy techniques the effects of glutamate on the rate of neurite outgrowth and growth cone motility were examined on cultured mouse spinal cord neurons. Concentrations in the range of 1 to 100 microM caused a significant inhibition of neurite outgrowth and concentrations of 10 and 100 microM significantly inhibited growth cone activity. In addition it was shown that the kainate/AMPA receptor antagonist (+/-)3-(2-carbvoxypiperazin-4-yl)-propyl-l-phosphonic acid, but not the NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione, was capable of blocking the inhibitory actions of glutamate on both outgrowth and motility. These results show that, at least in the culture system employed, glutamate might have a role in regulating neuronal development and function.

GABA activity mediating cytosolic Ca2+ rises in developing neurons is modulated by cAMP-dependent signal transduction

In the majority of developing neurons, GABA can exert depolarizing actions, thereby raising neuronal Ca2+. Ca2+ elevations can have broad consequences during development, inducing gene expression, altering neurite outgrowth and growth cone turning, activating enzyme pathways, and influencing neuronal survival. We used fura-2 and fluo-3 Ca2+ digital imaging to assess the effects of inhibiting or activating the cAMP signal transduction pathway on GABA activity mediating Ca2+ rises during the early stages of in vitro hypothalamic neural development. Our experiments stemmed from the finding that stimulation of transmitter receptors shown to either activate or inhibit adenylyl cyclase activity caused a rapid decrease in Ca2+ rises mediated by synaptically released GABA. Both the adenylyl cyclase activator forskolin and the inhibitor SQ-22,536 reduced the Ca2+ rise elicited by the synaptic release of GABA. Bath application of the membrane-permeable cAMP analogs 8-bromo-cAMP (8-Br-cAMP) or 8-(4-chlorophenylthio)-cAMP (0.2-5 mM) produced a rapid, reversible, dose-dependent inhibition of Ca2+ rises triggered by synaptic GABA release. Potentiation of GABAergic activity mediating Ca2+ rises was observed in some neurons at relatively low concentrations of the membrane-permeable cAMP analogs (20-50 microM). In the presence of tetrodotoxin (TTX), postsynaptic Ca2+ rises triggered by the bath application of GABA were only moderately depressed (13%) by 8-Br-cAMP (1 mM), suggesting that the inhibitory effects of 8-Br-cAMP were largely the result of a presynaptic mechanism. The protein kinase A (PKA) inhibitors H89 and Rp-3', 5'-cyclic monophosphothioate triethylamine also caused a large reduction (>70%) in Ca2+ rises triggered by synaptic GABA release. Unlike the short-term depression elicited by activation of the cAMP signal transduction pathway, Ca2+ depression elicited by PKA inhibition persisted for an extended period (>30 min) after PKA inhibitor washout. Postsynaptic depression of GABA-evoked Ca2+ rises triggered by H89 (in the presence of TTX) recovered rapidly, suggesting that the extended depression observed during synaptic GABA release was largely through a presynaptic mechanism. Long-term Ca2+ modulation by cAMP-regulating hypothalamic peptides may be mediated through a parallel mechanism. Together, these results suggest that GABAergic activity mediating Ca2+ rises is dependent on ongoing PKA activity that is maintained within a narrow zone for GABA to elicit a maximal Ca2+ elevation. Thus, neuromodulator-mediated changes in the cAMP-dependent signal transduction pathway (activation or inhibition) could lead to a substantial decrease in GABA-mediated Ca2+ rises during early development.

GABA immunoreactivity in hypothalamic neurons and growth cones in early development in vitro before synapse formation

GABA (gamma-aminobutyrate) is the most prevalent inhibitory transmitter in the mature hypothalamus. In contrast, in the developing hypothalamus, GABA may exert depolarizing actions leading to neuronal excitation. To determine whether GABA is present in hypothalamic neurons early in development, and whether there is a preferential expression in axonal growth cones, immunogold and peroxidase studies were used with light and whole mount transmission electron microscopy. At embryonic day 15, a stage of development at the beginning of hypothalamic neurogenesis, histological sections showed GABA immunoreactivity in fibers and weakly stained perikarya. Hypothalamic neurons (13%) cultured at embryonic day 15 were immunoreactive after 1 day in vitro. The percentage of neurons stained, and the intensity of staining increased during the next few days to 39% at 4 days in vitro. Neuritic growth cones, including lamellipodia and long filopodia, showed strong immunoreactivity before synaptogenesis. By using neuronal whole mounts studied with transmission electron microscopy and GABA silver-enhanced immunogold staining, a quantitative comparison of growth cones after a day and a half in culture revealed that the growth cone of the longest process, the putative axon, had a greater level of immunogold labeling than that of the shorter processes, the putative dendrites. This finding is one of the earliest biochemical differences between putative axons and dendrites. Astrocytes in the same cultures showed no immunolabeling. These results indicate that GABA is present very early in the development of hypothalamic neurons and is in a position to be released.

Local synaptic release of glutamate from neurons in the rat hypothalamic arcuate nucleus

1. The hypothalamic arcuate nucleus (ARC) contains neuroendocrine neurons that regulate endocrine secretions by releasing substances which control anterior pituitary hormonal release into the portal blood stream. Many neuroactive substances have been identified in the ARC, but the existence of excitatory neurons in the ARC and the identity of an excitatory transmitter have not been investigated physiologically. 2. In the present experiments using whole-cell current- and voltage-clamp recording of neurons from cultures and slices of the ARC, we demonstrate for the first time that some of the neurons in the ARC secrete glutamate as their transmitter. 3. Using microdrop stimulation of presynaptic neurons in ARC slices, we found that local axons from these glutamatergic neurons make local synaptic contact with other neurons in the ARC and that all evoked excitatory postsynaptic potentials could be blocked by the selective ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 10 microM) and D,L-2-amino-5-phosphonovalerate (AP5; 100 microM). To determine the identity of ARC neurons postsynaptic to local glutamatergic neurons, we used antidromic stimulation to reveal that many of these cells were neuroendocrine neurons by virtue of their maintaining axon terminals in the median eminence. 4. In ARC cultures, postsynaptic potentials, both excitatory and inhibitory, were virtually eliminated by the glutamate receptor antagonists AP5 and CNQX, underlining the functional importance of glutamate within this part of the neuroendocrine brain. 5. GABA was secreted by a subset of ARC neurons from local axons. The GABAA receptor antagonist bicuculline released glutamatergic neurons from chronic inhibition mediated by synaptically released GABA, resulting in further depolarization and an increase in the amplitude and frequency of glutamate-mediated excitatory postsynaptic potentials.

Excitatory actions of GABA after neuronal trauma

GABA is the dominant inhibitory neurotransmitter in the CNS. By opening Cl- channels, GABA generally hyperpolarizes the membrane potential, decreases neuronal activity, and reduces intracellular Ca2+ of mature neurons. In the present experiment, we show that after neuronal trauma, GABA, both synaptically released and exogenously applied, exerted a novel and opposite effect, depolarizing neurons and increasing intracellular Ca2+. Different types of trauma that were effective included neurite transection, replating, osmotic imbalance, and excess heat. The depolarizing actions of GABA after trauma increased Ca2+ levels up to fourfold in some neurons, occurred in more than half of the severely injured neurons, and was long lasting (>1 week). The mechanism for the reversed action of GABA appears to be a depolarized Cl- reversal potential that results in outward rather than inward movement of Cl-, as revealed by gramicidin-perforated whole-cell patch-clamp recording. The consequent depolarization and resultant activation of the nimodipine sensitive L- and conotoxin-sensitive N-type voltage-activated Ca2+ channel allows extracellular Ca2+ to enter the neuron. The long-lasting capacity to raise Ca2+ may give GABA a greater role during recovery from trauma in modulating gene expression, and directing and enhancing outgrowth of regenerating neurites. On the negative side, by its depolarizing actions, GABA could increase neuronal damage by raising cytosolic Ca2+ levels in injured cells. Furthermore, the excitatory actions of GABA after neuronal injury may contribute to maladaptive signal transmission in affected GABAergic brain circuits.

Neuropeptide Y-mediated long-term depression of excitatory activity in suprachiasmatic nucleus neurons

A brief exposure to light can shift the phase of mammalian circadian rhythms by 1 hr or more. Neuropeptide Y (NPY) administration to the hypothalamic suprachiasmatic nucleus, the circadian clock in the brain, also causes a phase shift in circadian rhythms. After a phase shift, the neural clock responds differently to light, suggesting that learning has occurred in neural circuits related to clock function. Thus, certain stimuli can produce effects that last for an extended period, but possible mechanisms of this long-term effect have not been previously examined at the cellular level. Here, we report that NPY caused a long-term depression in both electrical activity and intracellular calcium levels of neurons, as studied with whole-cell patch-clamp recording and Fura-2 digital imaging. In contrast to the immediate (1 sec) recovery after relief from glutamate receptor blockade, a brief single application of NPY (100 nM) depressed cytosolic Ca2+ for > 1 hr. The mechanism of this long-term calcium depression, a form of cellular learning, is dependent on the simultaneous release of glutamate and activation of NPY receptors, because both the extended response to NPY and any aftereffect were blocked by coapplication of glutamate receptor antagonists. Postsynaptic actions of NPY, mediated by both Y1- and Y2-like receptors, were short term and recovered rapidly. The primary site of long-term NPY actions may be on presynaptic glutamatergic axons, because the frequency of miniature excitatory postsynaptic currents in the presence of tetrodotoxin was reduced by transient exposure to NPY in both cultures and slices.

Glutamate hyperexcitability and seizure-like activity throughout the brain and spinal cord upon relief from chronic glutamate receptor blockade in culture

Cortical structures such as the hippocampus and cerebral cortex are considered to be particularly susceptible to seizure and epileptiform electrical activity and, as such, are the focus of intense investigation relative to hyperexcitability. To determine whether parallel glutamate-mediated hyperexcitability and seizure-like activity in the rat can be generated by neurons irrespective of their origin within the CNS, we maintained cells from the spinal cord,hippocampus, olfactory bulb, striatum, hypothalamus, and cortex in the long-term presence of glutamate receptor antagonists 2-amino-5-phosphonovalerate and 6-cyano-7-nitroquinoxaline-2-3-dione. After removal of chronic (three to 11 weeks) glutamate receptor block, whole-cell patch-clamp recordings from current-clamped neurons (n = 94) revealed an immediate increase in large excitatory postsynaptic potentials and a depolarization of 20-35 mV that was often sustained for recording periods lasting 5 min (54% of 66 neurons from all six areas). The intense activity was not seen in age-matched control neurons not subjected to chronic glutamate receptor block. Selective blockade of ionotropic glutamate receptors showed that the hyperexcitability was due to an enhanced response through both AMPA/kainate and N-methyl-D-aspartate receptors. Relief from chronic glutamate receptor block also increased inhibitory activity, as revealed by an increase in inhibitory postsynaptic currents while neurons were voltage-clamped at -25 mV. These inhibitory postsynaptic currents could be blocked with bicuculline, indicating that they were mediated by an enhanced GABA release. This enhanced GABA activity reduced, but did not eliminate, the glutamate-mediated hyperactivity, shown by an increase in both intracellular Ca2+ and excitatory electrical activity when bicuculline was added. When the glutamate receptor block was removed, cells (n > 1000) from all six regions showed exaggerated Ca2+ activity, characterized by abnormally high increases in intracellular Ca2+, rising from basal levels of 50-100 nM up to 150-1600 nM. Cd2+ eliminated the hyperexcitability by blocking Ca2+ channels, and reducing excitatory transmitter release and response. Fura-2 digital imaging revealed Ca2+ oscillations with periods ranging from 4 to 60 s. Ca2+ peaks in oscillations in oscillations were synchronized among most neurons recorded simultaneously. That synchronization was dependent on a mechanism involving voltage-dependent Na+ channels was demonstrated with experiments with tetrodotoxin that blocked Ca2+ rises and synchronous cellular behavior. Removal of the glutamate receptor antagonists resulted in the glutamate-mediated death of 44% of the cells after 23 days of chronic block and 82% cell death after 40 days of chronic block. Nimodipine substantially reduced cell death, indicating that one mechanism responsible for the enhanced cell death after relief from chronic glutamate receptor block was increased intracellular Ca2+ entry through L-type voltage-gated calcium channels. These data indicate that glutamate is released by neurons from all areas studied, including the spinal cord. Sufficient amounts of glutamate can be released from axon terminals from all areas to cause cell hippocampal and cortical neurons, but also by neurons from any of the brain regions tested after chronic deprivation of glutamate receptor stimulation during development. This hyperexcitability is mediated by glutamatergic mechanisms independent of the specific excitatory connections existing in vivo. The epileptiform activity of neurons from one region is indistinguishable from that of another in culture, underlining the importance of synaptic connections in vivo that define the responses characteristic of neurons from different brain regions.

Growth cone calcium elevation by GABA

Cytoplasmic calcium plays a key role in neurite growth. In contrast to previous work suggesting that gamma aminobutyrate's (GABA) role in regulating growth cone calcium is primarily to antagonize the effects of glutamate, we report that GABA can act in an excitatory manner on developing hypothalamic neurites, independently raising calcium in growing neurites and their growth cones. Time-lapse digital video and confocal laser microscopy with the calcium-sensitive dyes fluo-3 and fura-2 were used to study the influence of GABA on neurite calcium levels. GABA (10 microM) evoked a calcium rise in both bicarbonate- and Hepes-based buffers. The calcium rise was greatly reduced after chloride transport was blocked. GABA raised calcium by stimulating the cell body, resulting in an increase in calcium throughout the neuronal cell body and dendritic arbor. GABA also acted locally, stimulating a neuritic calcium rise only in a single dendrite or growth cone. In some neurites and growth cones during early development, GABA generated a greater calcium rise than did glutamate. Bicuculline, a GABAA receptor antagonist, reduced calcium levels in neurites of young synaptically coupled neurons, indicating that ongoing synaptic release of GABA raised neuritic calcium. These data suggest that during early development, GABA may play a significant role in regulating process growth and modulating the formation of early connections in the hypothalamus. Our data support the hypothesis that GABA receptors are functionally active and may play a calcium regulating role similar to that of glutamate in neuronal development. This is particularly true in early development, as later in development GABA's role becomes more inhibitory, and glutamate plays the primary excitatory role.

Neuropeptide Y depresses GABA-mediated calcium transients in developing suprachiasmatic nucleus neurons: a novel form of calcium long-term depression

In contrast to its inhibitory role in mature neurons, GABA can exert excitatory actions in developing neurons, including mediation of increases in cytosolic Ca2+. Modulation of this excitatory activity has not been studied previously. We used Ca2+ digital imaging with Fura-2 to test the hypothesis that neuropeptide Y (NPY) would depress GABA-mediated Ca2+ rises in neurons cultured from the developing suprachiasmatic nucleus (SCN). SCN neurons were chosen as a model system for this study because SCN neurons are primarily GABAergic, they express high levels of NPY and GABA receptors, and functionally, NPY causes profound phase-shifts in SCN-generated circadian rhythms. Vigorous GABA-mediated Ca2+ activity was found in young SCN neurons that were maintained in vitro for 4-14 d. NPY showed a dose-dependent rapid depression of the amplitude of Ca2+ rises generated by GABA released from presynaptic SCN axons. NPY exerted a long-term depression of cytosolic CA2+ in the majority of neurons tested, which lasted more than 1 hr after NPY washout. The magnitude of the NPY depression was dose-dependent. NPY did not affect Ca2+ levels when GABAA receptor activity was blocked by bicuculline; however, when bicuculline and NPY were withdrawn from the perfusion solution, the subsequent CA2+ rise was either significantly reduced or completely absent, suggesting that the NPY receptor was activated in the absence of elevated intracellular Ca2+ and GABAA receptor activity, and that the latent effect of NPY was revealed only after depolarizing GABA stimulation was renewed. Pretreating neurons with pertussis toxin greatly reduced the ability of NPY to depress GABAergic Ca2+ rises, suggesting that the NPY modulation of the GABA activity was based largely on a mechanism involving pertussis toxin-sensitive Gi/Go proteins. NPY receptor stimulation depressed (< 30%) postsynaptic Ca2+ rises evoked by GABA (20 microM) application in the presence of tetrodotoxin (TTX). The effects of NPY were mimicked by the NPY Y1 receptor agonist [Pro34,Leu31] NPY and the Y2 receptor agonist NPY 13-36 and by peptide YY (PYY). Together, our data suggest that the Y1 and Y2 type NPY receptors act both presynaptically and postsynaptically to depress GABA-mediated Ca2+ rises. If related mechanisms exist in peptide modulation of inhibitory GABA activity in mature neurons, this could underlie long-term changes in the behavior of neurons of the SCN necessary for phase-shifting the circadian clock by NPY, NPY also modulated GABA responses in neuroendocrine neurons from the hypothalamic arcuate nucleus. NPY thus can play an important role in evoking long-term depression of GABA-mediated Ca2+ activity in these developing neurons, allowing NPY-secreting cells to modulate the effects of GABA on neurite outgrowth, gene expression, and physiological stimulation. This is the first example of such a cellular memory: that is, long-term Ca2+ depression based on modulation of depolarizing GABA activity.