Expression patterns of GABAA receptor subtypes in developing hippocampal neurons

Reduction in hippocampal GABAergic transmission in a low birth weight rat model of depression

Prenatal stress is believed to increase the risk of developing neuropsychiatric disorders, including major depression. Adverse genetic and environmental impacts during early development, such as glucocorticoid hyper-exposure, can lead to changes in the foetal brain, linked to mental illnesses developed in later life. Dysfunction in the GABAergic inhibitory system is associated with depressive disorders. However, the pathophysiology of GABAergic signalling is poorly understood in mood disorders. Here, we investigated GABAergic neurotransmission in the low birth weight (LBW) rat model of depression. Pregnant rats, exposed to dexamethasone, a synthetic glucocorticoid, during the last week of gestation, yielded LBW offspring showing anxiety- and depressive-like behaviour in adulthood. Patch-clamp recordings from dentate gyrus granule cells in brain slices were used to examine phasic and tonic GABAA receptor-mediated currents. The transcriptional levels of selected genes associated with synaptic vesicle proteins and GABAergic neurotransmission were investigated. The frequency of spontaneous inhibitory postsynaptic currents (sIPSC) was similar in control and LBW rats. Using a paired-pulse protocol to stimulate GABAergic fibres impinging onto granule cells, we found indications of decreased probability of GABA release in LBW rats. However, tonic GABAergic currents and miniature IPSCs, reflecting quantal vesicle release, appeared normal. Additionally, we found elevated expression levels of two presynaptic proteins, Snap-25 and Scamp2, components of the vesicle release machinery. The results suggest that altered GABA release may be an essential feature in the depressive-like phenotype of LBW rats.

Third trimester-equivalent ethanol exposure is characterized by an acute cellular stress response and an ontogenetic disruption of genes critical for synaptic establishment and function in mice

The developing brain is remarkably sensitive to alcohol exposure, resulting in the wide range of cognitive and neurobehavioral characteristics categorized under the term fetal alcohol spectrum disorders (FASD). The brain is particularly susceptible to alcohol during synaptogenesis, a process that occurs heavily during the third trimester and is characterized by the establishment and pruning of neural circuitry; however, the molecular response of the brain to ethanol during synaptogenesis has not been documented. To model a binge-like exposure during the third-trimester neurodevelopmental equivalent, neonate mice were given a high (5 g/kg over 2 h) dose of ethanol at postnatal day 7. Acute transcript changes within the brain were assessed using expression arrays and analyzed for associations with gene ontology functional categories, canonical pathways, and gene network interactions. The short-term effect of ethanol was characterized by an acute stress response and a downregulation of energetically costly cellular processes. Further, alterations to a number of genes with roles in synaptic transmission and hormonal signaling, particularly those associated with the neuroendocrine development and function, were evident. Ethanol exposure during synaptogenesis was also associated with altered histone deacetylase and microRNA transcript levels, suggesting that abnormal epigenetic patterning may maintain some of the persistent molecular consequences of developmental ethanol exposure. The results shed insight into the sensitivity of the brain to ethanol during the third-trimester equivalent and outline how ethanol-induced alterations to genes associated with neural connectivity may contribute to FASD phenotypes.

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

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

Selective modulation of different GABAA receptor isoforms by diazepam and etomidate in hippocampal neurons

Diazepam modulation of native γ2-containing GABA(A) (γGABA(A)) receptors increases channel conductance by facilitating protein interactions involving the γ2-subunit amphipathic (MA) region, which is found in the cytoplasmic loop between transmembrane domains 3 and 4 (Everitt et al., 2009). However, many drugs, predicted to act on different GABA(A) receptor subtypes, increase channel conductance leading us to hypothesize that conductance variation in GABA(A) receptors may be a general property, mediated by protein interactions involving the cytoplasmic MA stretch of amino acids. In this study we have tested this hypothesis by potentiating extrasynaptic GABA(A) currents with etomidate and examining the ability of peptides mimicking either the γ2- or δ-subunit MA region to affect conductance. In inside-out hippocampal patches from newborn rats the general anesthetic etomidate potentiated GABA currents, producing either an increase in open probability and single-channel conductance or an increase in open probability, as described previously (Seymour et al., 2009). In patches displaying high conductance channels application of a δ((392-422)) MA peptide, but not a scrambled version or the equivalent γ2((381-403)) MA peptide, reduced the potentiating effects of etomidate, significantly reducing single-channel conductance. In contrast, when GABA currents were potentiated by the γ2-specific drug diazepam the δ MA peptide had no effect. These data reveal that diazepam and etomidate potentiate different extrasynaptic GABA(A) receptor subtypes but both drugs modulate conductance similarly. One interpretation of the data is that these drugs elicit potentiation through protein interactions and that the MA peptides compete with these interactions to disrupt this process.

Effects of temperature elevation on neuronal inhibition in hippocampal neurons of immature and mature rats

Febrile seizures are the most common seizure type in children, and hyperthermia may contribute to seizure generation during fever. We have previously demonstrated that hyperthermia suppressed gamma-aminobutyric acid (GABA)-ergic synaptic transmission in CA1 neurons of immature rats. However, whether this suppression is age-dependent is unknown. Moreover, it is unclear whether hyperthermia has differential effects on neuronal inhibition in CA1 pyramidal cells (PCs) and dentate gyrus granule cells (GCs). In this study, we investigated the effects of hyperthermia on GABA(A) and GABA(B) receptor-mediated inhibitory postsynaptic currents (IPSCs) in CA1 and DG neurons from immature (11-17 days old) and mature (6-8 weeks old) rats using whole-cell recordings in vitro. In immature rats, hyperthermia decreased the peak amplitude of GABA(A) receptor-mediated IPSCs (GABA(A) IPSCs) in PCs but not in GCs. However, hyperthermia decreased the decay time constant of GABA(A) IPSCs to a similar extent in both PCs and GCs. In mature rats, hyperthermia decreased the peak amplitude but not the decay time constant of GABA(A) IPSCs in both PCs and GCs. Hyperthermia decreased charge transfer (area) of the GABA(A) IPSC of PCs more in immature than in mature rats. In contrast, hyperthermia decreased the GABA(B) receptor-mediated IPSCs to the same degree in immature and mature rats, for either CA1 or DG neurons. Because the hippocampus has been found to be involved in hyperthermia-induced behavioral seizures in immature rats, we suggest that the higher sensitivity of CA1 inhibitory synaptic function to hyperthermia in immature compared with mature rats might partially explain the higher susceptibility for febrile seizures in immature animals.

Effects of sodium valproate on synaptic transmission and neuronal excitability in rat hippocampus

1. Valproate (VPA) has long been used in the treatment of both generalized and partial seizures. However, its cellular mechanisms of action remain unclear. 2. In the present study, the effects of VPA on synaptic transmission and neuronal excitability were examined in the hippocampal CA1 region using whole-cell patch clamp recordings. 3. Perfusion with VPA, at therapeutically attainable concentrations (i.e. 0.3 and 0.6 mmol/L), significantly increased the frequency (112 +/- 2 and 133 +/- 2% of control, respectively; n = 5; both P < 0.05), but not the average amplitude, of miniature inhibitory post-synaptic currents (mIPSCs). Perfusion with VPA had no effect on either the amplitude or the frequency of miniature excitatory post-synaptic currents (mEPSCs). 4. In acutely dissociated CA1 pyramidal neurons, VPA had no effect on 10 micromol/L GABA-induced currents. Furthermore, following the administration of 0.3 and 0.6 mmol/L VPA, the frequency of action potential firing was significantly reduced from 18.0 +/- 1.1 to 15.3 +/- 0.9 and from 18.6 +/- 0.9 to 12.6 +/- 0.6, respectively (n = 8; both P < 0.05). In contrast, 0.3 and 0.6 mmol/L VPA significantly increased spike frequency adaptation from 4.02 +/- 0.47 to 4.72 +/- 0.55 and from 3.47 +/- 0.41 to 4.48 +/- 0.58, respectively (n = 8; P < 0.05). 5. The results of the present study suggest that VPA presynaptically increases inhibitory synaptic activity without modifying excitatory synaptic transmission and reduces neuronal excitability. Any or all of these effects may contribute to its anticonvulsant action.

Androgens predispose males to GABAA-mediated excitotoxicity in the developing hippocampus

Clinical evidence and animal models indicate greater brain damage in newborn males following injury. In adults, glutamate is the primary source of excitotoxic cell death and the steroid, estradiol, is neuroprotective. In neonatal brain, membrane depolarization following activation of GABAA receptors is the major source of excitation. Consequent influx of calcium via L-type channels is normally trophic, but becomes excitotoxic during periods of excessive activation of GABAA receptors, such as hypoxia-ischemia, alcohol exposure and seizures. The use of sex-specific hippocampal cultures revealed greater cell death induced by the GABAA agonist, muscimol, in male- versus female-derived cultures. Pretreatment with the androgen, dihydrotestosterone (DHT) increased muscimol-induced death in both sexes. Exploration of calcium dynamics indicated that, counter to expectation, female neurons achieved higher [Ca2+]i than male, but the calcium transient duration was shorter due to faster rise and decay. However, a second exposure to muscimol within minutes of the first, caused significant attenuation of [Ca2+]i in female neurons. In contrast, while male neurons exposed to muscimol for the first time exhibited lower maximal [Ca2+]i, when exposed to muscimol again there was no attenuation in [Ca2+]i. The latter effect was induced in females by DHT, and inversely correlated with the amount of gamma2 subunit of the GABAA receptor. This novel effect of androgen on GABA-mediated excitotoxicty suggests a unique opportunity for a sex-specific therapeutic approach involving antagonism of the androgen receptor in neonatal males at risk for brain injury.

Development of gamma-aminobutyric acidergic synapses in cultured hippocampal neurons

The formation and maturation of gamma-aminobutyric acid (GABA)-ergic synapses was studied in cultured hippocampal pyramidal neurons by both performing immunocytochemistry for GABAergic markers and recording miniature inhibitory postsynaptic currents (mIPSCs). Nascent GABAergic synapses appeared between 3 and 8 days in vitro (DIV), with GABAA receptor subunit clusters appearing first, followed by GAD-65 puncta, then functional synapses. The number of GABAergic synapses increased from 7 to 14 DIV, with a corresponding increase in frequency of mIPSCs. Moreover, these new GABAergic synapses formed on neuronal processes farther from the soma, contributing to decreased mIPSC amplitude and slowed mIPSC 19-90% rise time. The mIPSC decay quickened from 7 to 14 DIV, with a parallel change in the distribution of the alpha5 subunit from diffuse expression at 7 DIV to clustered expression at 14 DIV. These alpha5 clusters were mostly extrasynaptic. The alpha1 subunit was expressed as clusters in none of the neurons at 7 DIV, in 20% at 14 DIV, and in 80% at 21 DIV. Most of these alpha1 clusters were expressed at GABAergic synapses. In addition, puncta of GABA transporter 1 (GAT-1) were localized to GABAergic synapses at 14 DIV but were not expressed at 7 DIV. These studies demonstrate that mIPSCs appear after pre- and postsynaptic elements are in place. Furthermore, the process of maturation of GABAergic synapses involves increased synapse formation at distal processes, expression of new GABAA receptor subunits, and GAT-1 expression at synapses; these changes are reflected in altered frequency, kinetics, and drug sensitivity of mIPSCs.

Age-related alterations in GABAA receptor subunits in the nonhuman primate hippocampus

Pharmacological studies have documented that altered drug responses, particularly to benzodiazepines, are common in elderly populations. While numerous factors may contribute to changes in drug response, age-related alterations in the molecular composition of GABA(A) receptors may be a key factor in regulating these responses. We employed quantitative densitometry to examine the cytological features and density of highly prevalent hippocampal GABA(A) receptor subunits (alpha1 and beta2/3) in young and aged rhesus monkeys. alpha1 and beta2/3 subunit immunostaining was differentially distributed throughout the hippocampus. In addition, beta2/3 immunolabeling in aged monkeys was characterized by marked intersubject variability in labeling intensity, with dramatic reductions present in 3 of 5 samples. alpha1 immunolabeling in aged monkeys was significantly reduced in the CA2 and CA3 subregions, and in hilus/polymorphic layer of the dentate gyrus. Collectively, our findings demonstrate that not only are GABA(A) receptor subunits differentially distributed throughout the hippocampus, but they are also differentially altered with increased age--changes that may have an important impact on the binding properties of GABA(A) receptor pharmacological agents.

Clustered and non-clustered GABAA receptors in cultured hippocampal neurons

In cultured hippocampal neurons, gamma2 subunit-containing GABA(A) Rs form large postsynaptic clusters at GABAergic synapses and small clusters outside GABAergic synapses. We now show that a pool of non-clustered gamma2 subunit-containing GABA(A) Rs are also present at the cell surface. We also demonstrate that myc- or EGFP-tagged gamma2, alpha2, beta3 or alpha1 subunits expressed in these neurons assemble with endogenous subunits, forming GABA(A) Rs that target large postsynaptic clusters, small clusters outside GABAergic synapses or a pool of non-clustered surface GABA(A) Rs. In contrast, myc- or EGFP-tagged delta subunits only form non-clustered GABA(A) Rs, which can be induced to form clusters by antibody capping. A myc-tagged chimeric gamma2 subunit possessing the large intracellular loop (IL) of the delta-subunit IL (myc gamma2S/delta-IL) assembled into GABA(A) Rs, but it did not form clusters, therefore behaving like the delta subunit. Thus, the large intracellular loops of gamma2 and delta play an important role in determining the synaptic clustering/non-clustering capacity of the GABA(A) Rs.

Dynamic mobility of functional GABAA receptors at inhibitory synapses

Importing functional GABAA receptors into synapses is fundamental for establishing and maintaining inhibitory transmission and for controlling neuronal excitability. By introducing a binding site for an irreversible inhibitor into the GABAA receptor alpha1 subunit channel lining region that can be accessed only when the receptor is activated, we have determined the dynamics of receptor mobility between synaptic and extrasynaptic locations in hippocampal pyramidal neurons. We demonstrate that the cell surface GABAA receptor population shows no fast recovery after irreversible inhibition. In contrast, after selective inhibition, the synaptic receptor population rapidly recovers by the import of new functional entities within minutes. The trafficking pathways that promote rapid importation of synaptic receptors do not involve insertion from intracellular pools, but reflect receptor diffusion within the plane of the membrane. This process offers the synapse a rapid mechanism to replenish functional GABAA receptors at inhibitory synapses and a means to control synaptic efficacy.

Glutamate and GABA receptor signalling in the developing brain

Our understanding of the role played by neurotransmitter receptors in the developing brain has advanced in recent years. The major excitatory and inhibitory neurotransmitters in the brain, glutamate and GABA, activate both ionotropic (ligand-gated ion channels) and metabotropic (G protein-coupled) receptors, and are generally associated with neuronal communication in the mature brain. However, before the emergence of their role in neurotransmission in adulthood, they also act to influence earlier developmental events, some of which occur prior to synapse formation: such as proliferation, migration, differentiation or survival processes during neural development. To fulfill these actions in the constructing of the nervous system, different types of glutamate and GABA receptors need to be expressed both at the right time and at the right place. The identification by molecular cloning of 16 ionotropic glutamate receptor subunits, eight metabotropic glutamate receptor subtypes, 21 ionotropic and two metabotropic GABA receptor subunits, some of which exist in alternatively splice variants, has enriched our appreciation of how molecular diversity leads to functional diversity in the brain. It now appears that many different types of glutamate and GABA receptor subunits have prominent expression in the embryonic and/or postnatal brain, whereas others are mainly present in the adult brain. Although the significance of this differential expression of subunits is not fully understood, it appears that the change in subunit composition is essential for normal development in particular brain regions. This review focuses on emerging information relating to the expression and role of glutamatergic and GABAergic neurotransmitter receptors during prenatal and postnatal development.

Cultured Hippocampal Pyramidal Neurons Express Two Kinds of GABAA Receptors

We combined a study of the subcellular distribution of the alpha1, alpha2, alpha4, beta1, beta2/3, gamma2, and delta subunits of the GABAA receptor with an electrophysiological analysis of GABAA receptor currents determine the to types of receptors expressed on cultured hippocampal pyramidal neurons. The immunocytochemistry study demonstrated that alpha1, alpha2, beta2/3, and gamma2 subunits formed distinct clusters of various sizes, which were colocalized with clusters of glutamate decarboxylase (GAD) immunoreactivity at rates ranging from 22 to 58%. In contrast, alpha4, beta1, and delta subunits were distributed diffusely over the cell soma and neuronal processes of cultured neurons and did not colocalize with the synaptic marker GAD. Whole-cell GABA receptor currents were moderately sensitive to GABAA and were modulated by diazepam. The whole-cell currents were also enhanced by the neurosteroid allopregnanolone (10 nM). Tonic currents, measured as changes in baseline current and noise, were sensitive to Zn2+, furosemide, and loreclezole; they were insensitive to diazepam. These studies suggest that two kinds of GABAA receptors are expressed on cultured hippocampal neurons. One kind of receptor formed clusters, which were present at GABAergic synapses and in the extrasynaptic membrane. The alpha1, alpha2, beta2/3, and gamma2 subunits were contained in clustered receptors. The second kind was distributed diffusely in the extrasynaptic membrane. The alpha4, beta1, and delta subunits were contained in these diffusely distributed receptors. The properties of tonic currents recorded from these neurons were similar to those from recombinant receptors containing alpha4, beta1, and delta subunits.

Maturation of respiratory reflex responses in the fetus and neonate

Respiratory control in the fetus and neonate is quite immature when compared to that of adults. This immaturity involves all facets of respiration including respiratory responses to hypoxia, hypercapnia, an exaggerated apnoeic response to laryngeal stimulation and immature responses to activation of pulmonary afferents. The net result of this immaturity of breathing responses is the vulnerability of neonates and especially preterm infants to apnoea and respiratory pauses. The mechanisms behind immature control of breathing are not fully understood, but seem to originate from a predominance of inhibitory input early in life on respiratory centres. The relative contribution of up-regulation of inhibitory pathways versus down-regulation of excitatory ones is not clear. Multiple neurotransmitters have been implicated in the regulation of breathing in mammals and some of them are discussed in this chapter.

Axonal sorting of Kir3.3 defines a GABA-containing neuron in the CA3 region of rodent hippocampus

Hippocampal interneurons comprise a heterogeneous group of locally acting GABAergic neurons. In addition to their variability in cotransmitter content and receptor profile, they express a variety of potassium channels that specify their individual properties. Here we describe a new type of large GABA-containing neuron in rodent hippocampus that is characterized by an axonal sorting of the potassium channel Kir3.3. The parent cell bodies of the Kir3.3-positive axons are located in CA3, as assessed by primary cultures derived from hippocampal subareas. At postnatal day 14 these neurons appear at the border between stratum oriens and stratum pyramidale of CA3, from where their axons pass through stratum pyramidale to join the mossy fiber tract. In adult hippocampus, high levels of Kir3.3 channel protein exist in axons that run with the mossy fiber tract. Kir3.3 and the vesicular GABA transporter could be identified in subpopulations of large synaptic terminals that probably derive from Kir3.3 neurons. Axonal sorting of Kir3.3 appears to be typical of a group of large inhibitory neurons, including Purkinje cells and a novel type of interneuron in CA3. Kir3.3 neurons might modulate the activity of CA3 circuitries and consequently memory processing in the hippocampus.

Ethanol withdrawal influences survival and morphology of developing rat hippocampal neurons in vitro

BACKGROUND

Previous studies in this laboratory have shown that, like their counterparts in vivo, fetal rat hippocampal pyramidal neurons in culture develop abnormally small dendritic arbors when exposed to ethanol. This study asked whether ethanol's inhibitory effects on dendritic development differ when the duration of ethanol exposure and timing of withdrawal are varied to correspond with early versus later stages of development and whether ethanol withdrawal influences survival of these neurons.

METHODS

We compared neurons exposed continuously for 6 or 14 days to ethanol (70 mM) with neurons transferred from ethanol-containing medium to control medium either 1 day after adding ethanol (before dendrites elongated) or 6 days after adding ethanol (after dendrites began elongating). We then performed morphometric and cell density analyses at 6 and 14 days using digital images of neurons immunostained with microtubule-associated protein 2 (MAP2) to visualize dendrites.

RESULTS

Continuous exposure to ethanol decreased the length and number of dendrites formed but had no effect on neuron survival compared with controls without ethanol. Dendritic length was less inhibited when ethanol was withdrawn after 1 day, but the number of dendrites per cell was unchanged compared with neurons continuously exposed to ethanol. Withdrawal from ethanol at 1 day slightly enhanced the survival of neurons assessed at 14 days compared with neurons in control medium and with neurons exposed continuously to ethanol. In contrast, withdrawal from ethanol at 6 days severely decreased the number of neurons at 14 days.

CONCLUSIONS

These results suggest that dendrites can achieve normal length when ethanol exposure is limited to only 1 day and withdrawal occurs before dendrites begin elongating. However, a persistent reduction in dendrite number results in smaller overall dendritic arbor size. Although continuous exposure to ethanol has little effect on neuron survival in these cultures, and exposure limited to 1 day followed by withdrawal can be neuroprotective against cell death associated with increased time in culture, longer exposure before withdrawal can trigger cell death.

Early establishment of multiple release site connectivity between interneurons and pyramidal neurons in the developing hippocampus

The strength of the synaptic transmission between two neurons critically depends on the number of release sites connecting the neurons. Here we examine the development of connectivity between gamma-aminobutyric acid (GABA)ergic interneurons and CA1 pyramidal neurons in the hippocampus. GABAergic postsynaptic currents (PSCs) were recorded in whole-cell voltage-clamped CA1 pyramidal neurons. By comparing spontaneous and miniature (action potential-independent) GABAergic PSCs, we found that multiple release site connectivity is established already at the first postnatal day and that the degree of connectivity remains unaltered into adulthood. During the same time there is a dramatic increase in the number of GABAergic synapses on each pyramidal neuron as indicated by the increase in frequency of miniature GABAergic PSCs. These results indicate that during development a given interneuron contacts an increasing number of target pyramidal neurons but with the same multiple release site connectivity. It has been shown previously that the connectivity between CA3 and CA1 pyramidal neurons is initially restricted to one release site, and develops gradually. The present result thus suggests different mechanisms to govern the maturation of excitatory and inhibitory synaptic transmissions.

Membrane and cytoskeleton dynamics during axonal elongation and stabilization

Proper nervous activities are gradually developing events. Reflecting this, embryonic neurons start differentiation by sprouting multiple extensions, neurites, which do not bear clear axonal or dendritic structural and molecular characteristics. Later in development one of these multiple neurites elongates further, generating a morphologically polarized neuron with a single long axon and many short dendrites. Still, despite such morphological differences these processes can switch destiny, further reflecting their immaturity. Final and irreversible axonal and dendritic commitment occurs after both axons and dendrites have elongated considerably. Recent evidence suggests that the transition from axonal immaturity to maturity reflects changes in the mechanisms used by neurons to control the precise membrane and cytoskeleton polarization. This chapter provides an overview of how these mechanisms contribute to the formation of an axon.

Increased neurosteroid sensitivity of hippocampal GABAA receptors during postnatal development

To investigate developmental changes in neurosteroid modulation of GABA(A) receptors, whole-cell currents were elicited by applying GABA with allopregnanolone or pregnenolone sulfate (PS) to dentate granule cells (DGCs), acutely isolated from 7-14-day-old and adult rats. GABA evoked larger currents from dentate granule cells acutely isolated from adult rats (adult DGCs) than from neonatal DGCs, due to increased efficacy (1662+/-267 pA in adult DGCs versus 1094+/-198 pA in neonatal DGCs, P=0.004), and current density (0.072+/-0.01 pA/microm(2) in neonatal rat DGCs to 0.178+/-0.02 pA/microm(2) in adult DGCs), but unchanged potency (EC(50) was 18.5+/-2 microm in adult DGCs, and 26.6+/-7.9 microm in neonatal DGCs, P=0.21). Allopregnanolone sensitivity of GABA(A) receptor currents increased during development due to an increased potency (21.1+/-4.7 nM in adult DGCs versus 94.6+/-9 nM in neonatal DGCs, P=0.0002). The potency and efficacy of PS inhibition of GABA(A) receptor currents were remained unchanged during development (13+/-6 microm and 13.2+/-5.9 microm, P=0.71 and 85.5%+/-3.5% and 83.6%+/-0.8%, P=0.29, respectively). To investigate possible mechanism of developmental changes in GABA(A) receptor properties, in situ hybridization for alpha1, alpha4 and gamma2 subunit mRNAs was performed in dentate gyrus of the two age groups. Qualitatively, alpha1 subunit mRNA was expressed at low levels in neonatal rats while it was well expressed in adult rats. The alpha4 and gamma2 subunits were well expressed in the dentate gyrus of adult and neonatal rats. Immunohistochemical staining for alpha1 subunit in hippocampal slices from neonatal and adult rats was examined under confocal laser scanning microscope. This demonstrated that cell bodies and dendrites of granule cells are moderately positive for the alpha1 staining in adult rats but weakly so in neonatal rats. Higher-magnification images demonstrate large number of clusters of alpha1-subunit in the cell bodies of dentate granule cells of adult rat but rare clusters in granule cells of neonatal rats. Maturation of GABA(A) receptors in DGCs is characterized by increased number of GABA(A) receptors that are more sensitive to endogenous neurosteroid allopregnanolone, which might be related to increased expression of alpha1 subunit.

Molecular biology and ontogeny of gamma-aminobutyric acid (GABA) receptors in the mammalian central nervous system

gamma-Aminobutyric acid (GABA) is the predominant inhibitory neurotransmitter in the mammalian central nervous system. After release from nerve terminals, GABA binds to at least two classes of postsynaptic receptors (ie, GABAA and GABAB), which are nearly ubiquitous in the brain. GABAA receptors are postsynaptic heteropentameric complexes that display unique physiologic and pharmacologic properties based on subunit composition. Activation of GABAA receptors in mature neurons results in membrane hyperpolarization, which is mediated principally by inward chloride flux, whereas in early stages of brain development, GABAA receptor activation causes depolarization of the postsynaptic membrane. GABA, receptors reside both presynaptically and postsynaptically, exist as heterodimers and are coupled to voltage-dependent ion channels through interactions with heterotrimeric G proteins. This review summarizes the molecular biology and ontogeny of GABAA and GABAB receptors, highlighting some of their putative roles during normal brain development as well as in disease states such as epilepsy.

Development of GABA(A) receptor-mediated inhibitory postsynaptic currents in hippocampus

Hippocampal CA1 pyramidal cells receive two kinetic classes of GABA(A) receptor-mediated inhibition: slow dendritic inhibitory postsynaptic currents (GABA(A,slow) IPSCs) and fast perisomatic (GABA(A,fast)) IPSCs. These two classes of IPSCs are likely generated by two distinct groups of interneurons, and we have previously shown that the kinetics of the IPSCs have important functional consequences for generating synchronous firing patterns. Here, we studied developmental changes in the properties of GABA(A,fast) and GABA(A,slow) spontaneous, miniature, and evoked IPSCs (sIPSCs, mIPSCs, and eIPSCs, respectively) using whole cell voltage-clamp recordings in brain slices from animals aged P10-P35. We found that the rate of GABA(A,slow) sIPSCs increased by over 70-fold between P11 and P35 (from 0.0017 to 0.12 s(-1)). Over this same age range, we observed a >3.5-fold increase in the maximal amplitude of GABA(A,slow) eIPSCs evoked by stratum lacunosum-moleculare (SL-M) stimuli. However, the rate and amplitude of GABA(A,slow) mIPSCs remained unchanged between P10 and P30, suggesting that the properties of GABA(A,slow) synapses remained stable over this age range, and that the increase in sIPSC rate and in eIPSC amplitude was due to increased excitability or excitation of GABA(A,slow) interneurons. This hypothesis was tested using bath application of norepinephrine (NE), which we found at low concentrations (1 microM) selectively increased the rate of GABA(A,slow) sIPSCs while leaving GABA(A,fast) sIPSCs unchanged. This effect was observed in animals as young as P13 and was blocked by coapplication of tetrodotoxin, suggesting that NE was acting to increase the spontaneous firing rate of GABA(A,slow) interneurons and consistent with our hypothesis that developmental changes in GABA(A,slow) IPSCs are due to changes in presynaptic excitability. In contrast to the changes we observed in GABA(A,slow) IPSCs, the properties of GABA(A,fast) sIPSCs remained largely constant between P11 and P35, whereas the rate, amplitude, and kinetics of GABA(A,fast) mIPSCs showed significant changes between P10 and P30, suggesting counterbalancing changes in action potential-dependent GABA(A,fast) sIPSCs. These observations suggest differential developmental regulation of the firing properties of GABA(A,fast) and GABA(A,slow) interneurons in CA1 between P10 and P35.

Morphologic and neurotoxic effects of ethanol vary with timing of exposure in vitro

Results of investigations with animal models of fetal alcohol syndrome (FAS) seem to indicate that neuronal vulnerability to ethanol-induced cell death may be correlated with specific developmental events. In the present study, we sought to test this observation in a cell culture model of neuronal development in which morphogenesis as well as survival could be assessed. Using embryonic rat hippocampal pyramidal neurons in primary cultures, we compared the sensitivity of neurons to ethanol added, at 400 mg/dl, to the medium at different times relative to the development of axons and dendrites. Quantitative morphometric analysis was performed by using phase contrast at 12 h (0.5 day) and 24 h (1 day), or fluorescence microscopy after microtubule-associated protein-2 (MAP2) immunostaining at 6 and 14 days. Survival was assessed by counting the number of neurons per unit area of the substrate at 14 days. Addition of ethanol 1 day after plating, when most neurons had developed an axon, had no effect on survival up to 14 days in vitro, but resulted in significantly shorter, less branched dendrites than observed when ethanol was added 2 h after plating. Despite the shorter duration of ethanol exposure, the addition of ethanol on day 6, after rapid growth of dendrites and synapses had begun, resulted in loss of all but about one third of the neurons by 14 days. This supports the suggestion that increased neuronal vulnerability to the morphoregulatory effects of ethanol is correlated with the establishment of polarity, but that the sensitivity of neurons to the cytotoxic effects of ethanol occurs later, when dendrites and synapses are rapidly forming.

Axon specification in hippocampal neurons

Neurons are the most highly polarized cells, comprised of two structurally and functionally distinct parts, axons and dendrites. This asymmetry enables a vectorial flow of signaling within neurons. One of the most fundamental questions still to be answered in neuroscience is how these two specialized processes initially develop. The first manifestation of polarization occurs when one of the immature neurites acquires axonal characteristics. We review recent advances that have highlighted the involvement of several cellular events in the initial formation of the axon, including membrane traffic and cytoskeletal rearrangement. We then discuss the molecular mechanisms underlying axon formation, focusing on the Rho family small GTPases and an axon-inducing neuronal protein, CRMP-2.

GABAergic innervation organizes synaptic and extrasynaptic GABAA receptor clustering in cultured hippocampal neurons

We have studied the effects of GABAergic innervation on the clustering of GABA(A) receptors (GABA(A)Rs) in cultured hippocampal neurons. In the absence of GABAergic innervation, pyramidal cells form small (0.36 +/- 0.01 micrometer diameter) GABA(A)R clusters at their surface in the dendrites and soma. When receiving GABAergic innervation from glutamic acid decarboxylase-containing interneurons, pyramidal cells form large (1.62 +/- 0.08 micrometer breadth) GABA(A)R clusters at GABAergic synapses. This is accompanied by a disappearance of the small GABA(A)R clusters in the local area surrounding each GABAergic synapse. Although the large synaptic GABA(A)R clusters of any neuron contained all GABA(A)R subunits and isoforms expressed by that neuron, the small clusters not localized at GABAergic synapses showed significant heterogeneity in subunit and isoform composition. Another difference between large GABAergic and small non-GABAergic GABA(A)R clusters was that a significant proportion of the latter was juxtaposed to postsynaptic markers of glutamatergic synapses such as PSD-95 and AMPA receptor GluR1 subunit. The densities of both the glutamate receptor-associated and non-associated small GABA(A)R clusters were decreased in areas surrounding GABAergic synapses. However, no effect on the density or distribution of glutamate receptor clusters was observed. The results suggest that there are local signals generated at GABAergic synapses that induce both assembly of large synaptic GABA(A)R clusters at the synapse and disappearance of the small GABA(A)R clusters in the surrounding area. In the absence of GABAergic innervation, weaker GABA(A)R-clustering signals, generated at glutamatergic synapses, induce the formation of small postsynaptic GABA(A)R clusters that remain juxtaposed to glutamate receptors at glutamatergic synapses.

Lateral mobility and anchoring of recombinant GABAA receptors depend on subunit composition

The clustering of type A gamma-aminobutyric acid receptors (GABA(A)R) at discrete and functionally significant domains on the nerve cell surface is an important determinant in the integration of synaptic inputs. To discern the role that the subunits of the GABA(A)R play in determining the receptor's cell surface topography and mobility, the alpha1, beta1, beta3, and gamma2s subunits were transfected into COS7, HEK293, and PC12 cells and the distribution and cell surface mobility of these recombinant receptors were examined. Our results show that alpha1 subunits are retained in the endoplasmic reticulum while beta1 and beta3 subunits are sorted to the plasma membrane where they form clusters. Co-expression and co-assembly of alpha1 and beta3 subunits result in the rescue of intracellular alpha1 subunits, which are transported as alphabeta subunit complexes to the cell surface where they formed clusters. Fluorescence photobleach recovery and single particle tracking of recombinant receptors show that, despite clustering, beta3 subunit homooligomers are mobile within a cell surface domain. Inclusion of alpha1 in beta3 or beta3gamma2s complexes, however, dramatically reduces the receptor's lateral mobility in COS 7 and PC12 cells and anchors GABA(A)Rs on the cell surface, suggesting the formation of a direct link to a component of the cytoskeleton. The mobility of recombinant receptors that include the alpha1 subunit mirrors the mobility of GABA(A)Rs on cell bodies and dendrites of cortical and spinal cord neurons. The results suggest that incorporation of alpha1 subunits give rise to a population of GABA(A)Rs that are immobilized on the cell surface.

Redistribution and increased specificity of GABA(B) receptors during development of the rostral nucleus of the solitary tract

Recent results show that there is an abundance of gamma-aminobutyric acid (GABA) before GABAergic synapses have formed in the gustatory zone of the nucleus of the solitary tract. These results suggest that a non-synaptic, developmental function may exist for GABA prior to synaptogenesis. However, GABA exerts its physiological effect via its receptors, the development of which is a largely unknown process. The developmental expression of one of the GABA receptors in the young nucleus of the solitary tract is the focus of this study. The development of GABA(B) receptors was investigated by light and electron microscopy. The results suggest that before the development of GABAergic synapses, GABA(B) receptors are diffusely distributed. When GABAergic synapses form, the receptors become clustered. Quantitative postembedding immunohistochemical studies at the electron microscopic level show that extrasynaptic labeling for GABA(B) receptors decreases during development, but synaptic labeling increases. Increased specificity of neurotransmitter receptors at synapses has been shown in other systems during development, including other central nervous system structures, but this may be the first demonstration of the phenomenon using quantitative electron microscopy.

Visualizing synapse formation and remodeling: recent advances in real-time imaging of CNS synapses

The formation and maintenance of synaptic connections are critical in the development and plasticity of the central nervous system (CNS). Until recently, there have been few studies that followed the molecular sequences of the CNS synapse formation and maintenance. This situation changed dramatically after the introduction of green fluorescent protein (GFP)-based fluorescent probes and the development of lipophilic tracers of endocytotic membranes. These techniques enabled us to visualize presynaptic and postsynaptic structures in living neurons and illustrated active transport and remodeling of synaptic components. Furthermore, recent attempts to identify correlation between presynaptic and postsynaptic morphogenesis suggested very rapid time course of synapse formation at the individual axo-dendritic contact sites. These recent works clearly demonstrated the power of real-time imaging studies. Development of a wide variety of fluorescent probes and advances in the imaging techniques in future will further extend our knowledge on the molecular events that take place in the process of the development and maturation of synaptic junctions.

Sorting and directed transport of membrane proteins during development of hippocampal neurons in culture

Hippocampal neurons in culture develop morphological polarity in a sequential pattern; axons form before dendrites. Molecular differences, particularly those of membrane proteins, underlie the functional polarity of these domains, yet little is known about the temporal relationship between membrane protein polarization and morphological polarization. We took advantage of viral expression systems to determine when during development the polarization of membrane proteins arises. All markers were unpolarized in neurons before axonogenesis. In neurons with a morphologically distinguishable axon, even on the first day in culture, both axonal and dendritic proteins were polarized. The degree of polarization at these early stages was somewhat less than in mature cells and varied from cell to cell. The cellular mechanism responsible for the polarization of the dendritic marker protein transferrin receptor (TfR) in mature cells centers on directed transport to the dendritic domain. To examine the relationship between cell surface polarization and transport, we assessed the selectivity of transport by live cell imaging. TfR-green fluorescent protein-containing vesicles were already preferentially transported into dendrites at 2 days, the earliest time point we could measure. The selectivity of transport also varied somewhat among cells, and the amount of TfR-green fluorescent protein fluorescence on intracellular structures within the axon correlated with the amount of cell surface expression. This observation implies that selective microtubule-based transport is the primary mechanism that underlies the polarization of TfR on the cell surface. By 5 days in culture, the extent of polarization on the cell surface and the selectivity of transport reached mature levels.

Central GABAergic mechanisms are involved in apnea induced by SLN stimulation in piglets

Stimulation of the superior laryngeal nerve (SLN) results in apnea in animals of different species, the mechanism of which is not known. We studied the effect of the GABA(A) receptor blocker bicuculline, given intravenously and intracisternally, on apnea induced by SLN stimulation. Eighteen 5- to 10-day-old piglets were studied: bicuculline was administered intravenously to nine animals and intracisternally to nine animals. The animals were anesthetized and then decerebrated, vagotomized, ventilated, and paralyzed. The phrenic nerve responses to four levels of electrical SLN stimulation were measured before and after bicuculline. SLN stimulation caused a significant decrease in phrenic nerve amplitude, phrenic nerve frequency, minute phrenic activity, and inspiratory time (P < 0.01) that was proportional to the level of electrical stimulation. Increased levels of stimulation were more likely to induce apnea during stimulation that often persisted beyond cessation of the stimulus. Bicuculline, administered intravenously or intracisternally, decreased the SLN stimulation-induced decrease in phrenic nerve amplitude, minute phrenic activity, and phrenic nerve frequency (P < 0.05). Bicuculline also reduced SLN-induced apnea and duration of poststimulation apnea (P < 0.05). We conclude that centrally mediated GABAergic pathways are involved in laryngeal stimulation-induced apnea.

Synaptic and extrasynaptic gamma -aminobutyric acid type A receptor clusters in rat hippocampal cultures during development

We have simultaneously measured the expression of postsynaptic gamma-aminobutyric acid type A (GABA(A)) receptor clusters and of presynaptic boutons in neonatal rat hippocampal cultures between days 1 and 30. GABA(A) receptors were labeled with antibodies recognizing the extracellular domains of beta2/3 and gamma2 subunits. Boutons were visualized by activity-dependent uptake of the styryl dye FM4-64, or by antibodies against the presynaptic vesicular protein SV2 or the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD). GABA(A) receptor clusters could be seen in living neurons already 6 h after culturing, much before presynaptic markers could be identified in nerve terminals. The densities of receptor clusters that contained the beta2/3 subunits were constant between days 10 and 30 in culture, whereas gamma2 subunit-containing clusters fluctuated and reached a maximum on day 20. SV2 and GAD staining could be measured from day 2 onwards. Clustering of GAD in presynaptic terminals and FM4-64 uptake were observed only at day 5 and afterward. SV2 staining and FM4-64 uptake increased in parallel between days 5 and 20 and remained constant thereafter. GAD-stained boutons were fewer than those labeled with other, less specific, presynaptic stains. They reached a maximum on day 20 and fell again toward day 30. Double labeling of GABA(A) receptors and of presynaptic boutons in neurons during differentiation showed that, even after 30 days in culture, large fractions of GABA(A) receptor clusters containing beta2/3 and/or gamma2 subunits remained extrasynaptic.

Brain development and generation of brain pathologies

The timing of brain development is genetically programmed and can be profoundly altered by a number of abnormal genes that control proliferation, differentiation, migration, synapse formation, and elimination of cells and synapses. With advances in genetic techniques both the gene and gene product responsible for brain malformation are being identified which will permit a better understanding of the pathophysiology of brain malformations. In addition to genetic abnormalities, brain development can be influenced by a variety of acquired disorders. The type of brain abnormality is highly dependent on the stage of brain development during which the insult occurs. While our understanding of experience- or activity-dependent processes on brain development is limited, it is now clear that this activity can significantly alter the timing of a number of maturational events including receptor maturation and neurogenesis. The future challenge will be both to understand the biological basis of these processes and to use this information to enhance brain development.

Protracted postnatal development of inhibitory synaptic transmission in rat hippocampal area CA1 neurons

In the CNS, inhibitory synaptic function undergoes profound transformation during early postnatal development. This is due to variations in the subunit composition of subsynaptic GABA(A) receptors (GABA(A)Rs) at differing developmental stages as well as other factors. These include changes in the driving force for chloride-mediated conductances as well as the quantity and/or cleft lifetime of released neurotransmitter. The present study was undertaken to investigate the nature and time course of developmental maturation of GABAergic synaptic function in hippocampal CA1 pyramidal neurons. In neonatal [postnatal day (P) 1-7] and immature (P8-14) CA1 neurons, miniature inhibitory postsynaptic currents (mIPSCs) were significantly larger, were less frequent, and had slower kinetics compared with mIPSCs recorded in more mature neurons. Adult mIPSC kinetics were achieved by the third postnatal week in CA1 neurons. However, despite this apparent maturation of mIPSC kinetics, significant differences in modulation of mIPSCs by allosteric agonists in adolescent (P15-21) neurons were still evident. Diazepam (1-300 nM) and zolpidem (200 nM) increased the amplitude of mIPSCs in adolescent but not adult neurons. Both drugs increased mIPSC decay times equally at both ages. These differential agonist effects on mIPSC amplitude suggest that in adolescent CA1 neurons, inhibitory synapses operate differently than adult synapses and function as if subsynaptic receptors are not fully occupied by quantal release of GABA. Rapid agonist application experiments on perisomatic patches pulled from adolescent neurons provided additional support for this hypothesis. In GABA(A)R currents recorded in these patches, benzodiazepine amplitude augmentation effects were evident only when nonsaturating GABA concentrations were applied. Furthermore nonstationary noise analysis of mIPSCs in P15-21 neurons revealed that zolpidem-induced mIPSC augmentation was not due to an increase in single-channel conductance of subsynaptic GABA(A)Rs but rather to an increase in the number of open channels responding to a single GABA quantum, further supporting the hypothesis that synaptic receptors may not be saturated during synaptic function in adolescent neurons. These data demonstrate that inhibitory synaptic transmission undergoes a markedly protracted postnatal maturation in rat CA1 pyramidal neurons. In the first two postnatal weeks, mIPSCs are large in amplitude, are slow, and occur infrequently. By the third postnatal week, mIPSCs have matured kinetically but retain distinct responses to modulatory drugs, possibly reflecting continued immaturity in synaptic structure and function persisting through adolescence.

Neonatal 6-hydroxydopamine treatment affects GABA(A) receptor subunit expression during postnatal development of the rat cerebellum

Neurotoxic elimination of noradrenergic terminals by 6-hydroxydopamine (6-OHDA) leads to alteration of the granule cell layer formation. We have studied the developmental expression of GABA(A) receptor subunits in rat cerebellum after neonatal administration of 6-OHDA during the first postnatal month of life. 6-OHDA was injected subcutaneously. The expression of GABA(A) receptor subunits was studied by in situ hybridization and immunohistochemistry. The alterations were observed in the neocerebellum - the part of the cerebellum which starts development postnatally. The migration of granule cells was delayed, and the total area of the granule cell layer in the neocerebellum from 6-OHDA-treated rats was reduced to 22.6+/-5% of the corresponding area from control rats. In situ hybridization with subunit-specific antisense oligonucleotide probes was performed for alpha1, alpha2, alpha3, alpha5, alpha6, beta1, beta2, gamma1 and gamma2 subunits of the GABA(A) receptor. In neocerebellum, 6-OHDA treatment caused a significant reduction in the alpha1, alpha6 and gamma2 subunit mRNA levels. The expression of the other subunits was not changed. It has been shown that in the postnatal cerebellum alpha1 and alpha6 subunits can be detected in granule cells only when the cells had migrated to their final destination. Our findings indicate that a noradrenergic influence may be necessary for the normal maturation and migration of cerebellar granule cells.

Heterogeneous conductance levels of native AMPA receptors

The single-channel properties of AMPA receptors can affect information processing in neurons by influencing the amplitude and kinetics of synaptic currents, yet little is known about the unitary properties of native AMPA receptors in situ. Using whole-cell and outside-out patch-clamp recordings from granule cells in acute cerebellar slices, we found that migrating granule cells begin to express AMPA receptors before they arrive in the internal granule cell layer and receive synaptic input. At saturating agonist concentrations, the open probability of channels in outside-out patches from migrating cells was very high, allowing us to identify patches that contained only one or two active channels. Analysis of the single-channel activity in these patches showed that individual AMPA receptors exhibit as many as four distinguishable conductance levels. The conductance levels observed varied substantially for different channels, although on average the values fell within the range of unitary conductances estimated previously for synaptic AMPA receptors. In contrast to patches from migrating granule cells, we rarely observed directly resolvable single-channel currents in patches excised from the somata of granule cells in the internal granular layer, even though these cells gave large AMPA receptor whole-cell currents. We did, however, detect AMPA receptors with apparent unitary conductances of <1 pS in patches from both migrating and mature granule cells. Our results suggest that granule cells express a heterogeneous population of AMPA receptors, a subset of which are segregated to postsynaptic sites after synaptogenesis.

Expression of Kv1 potassium channels in mouse hippocampal primary cultures: development and activity-dependent regulation

Excitability and discharge behavior of neurons depends on the highly variable expression pattern of voltage-dependent potassium (Kv) channels throughout the nervous system. To learn more about distribution, development, and activity-dependent regulation of Kv channel subunit expression in the rodent hippocampus, we studied the protein expression of members of the Kv1 subfamily in mouse hippocampus in situ and in primary cultures. In adult hippocampus, Kv1 (1-6) channel alpha-subunits were present, whereas at postnatal day 2, none of these proteins could be detected in CA1-CA3 and dentate gyrus. Kv1.1 was the first channel to be observed at postnatal day 6. The delayed postnatal expression and most of the subcellular distribution observed in hippocampal sections were mimicked by cultured hippocampal neurons in which Kv channels appeared only after 10 days in vitro. This developmental upregulation was paralleled by a dramatic increase in total K(+) current, as well as an elevated GABA release in the presence of 4-aminopyridine. Thus, the developmental profile, subcellular localization, and functionality of Kv1 channels in primary culture of hippocampus closely resembles the in situ situation. Impairing secretion by clostridial neurotoxins or blocking activity by tetrodotoxin inhibited the expression of Kv1.1, Kv1.2, and Kv1.4, whereas the other Kv1 channels still appeared. This activity-dependent depression was only observed before the initial appearance of the respective channels and lost after they had been expressed. Our data show that hippocampal neurons in culture are a convenient model to study the developmental expression and regulation of Kv1 channels. The ontogenetic regulation and the activity-dependent expression of Kv1.1, Kv1.2, and Kv1.4 indicate that neuronal activity plays a crucial role for the development of the mature Kv channel pattern in hippocampal neurons.

Regulatory cell interactions between retinal ganglion cells and radial glia during axonal and dendritic outgrowth

Neuronal differentiation and the formation of cell polarity are crucial events during the development of the nervous system. Cell polarity is a prerequisite for directed information flux within neuronal networks. In this article, we focus on neuro-glial cell interactions that influence the establishment of neural cell polarity and the directed outgrowth of axons versus dendrites. The cellular model discussed in detail is the retinal ganglion cell (RGC) of the chick retina, which is investigated by a comprehensive set of in vitro assays. The experiments demonstrate that retinal microenvironment determines axon vs. dendrite formation of RGCs. The instructive differences in different retinal microenvironments are substantially influenced by radial glia. Different glial domains support or inhibit axon vs. dendrite outgrowth. The data support the notion that neuro-glial interactions are crucial for directed neurite outgrowth.

Changes in membrane trafficking and actin dynamics during axon formation in cultured hippocampal neurons

Neurons begin to polarize when one of the neurites becomes the axon. Hippocampal neurons in cell culture have a sharp transition between their unpolarized and polarized stage revealed by the rapid growth of the future axon. Recent progress shows that both a cytoplasmic membrane flow and actin dynamics govern axon formation, and thereby initial neuronal polarization. We here review these mechanisms, evaluate their physiological role, and show similarities to the transient polarization of migrating fibroblasts. Finally, we present a model how actin dynamics and vectorial membrane flow may interact to achieve axon formation.

The establishment of GABAergic and glutamatergic synapses on CA1 pyramidal neurons is sequential and correlates with the development of the apical dendrite

We have performed a morphofunctional analysis of CA1 pyramidal neurons at birth to examine the sequence of formation of GABAergic and glutamatergic postsynaptic currents (PSCs) and to determine their relation to the dendritic arborization of pyramidal neurons. We report that at birth pyramidal neurons are heterogeneous. Three stages of development can be identified: (1) the majority of the neurons (80%) have small somata, an anlage of apical dendrite, and neither spontaneous nor evoked PSCs; (2) 10% of the neurons have a small apical dendrite restricted to the stratum radiatum and PSCs mediated only by GABA(A) receptors; and (3) 10% of the neurons have an apical dendrite that reaches the stratum lacunosum moleculare and PSCs mediated both by GABA(A) and glutamate receptors. These three groups of pyramidal neurons can be differentiated by their capacitance (C(m) = 17.9 +/- 0.8; 30.2 +/- 1.6; 43.2 +/- 3.0 pF, respectively). At birth, the synaptic markers synapsin-1 and synaptophysin labeling are present in dendritic layers but not in the stratum pyramidale, suggesting that GABAergic peridendritic synapses are established before perisomatic ones. The present observations demonstrate that GABAergic and glutamatergic synapses are established sequentially with GABAergic synapses being established first most likely on the apical dendrites of the principal neurons. We propose that different sets of conditions are required for the establishment of functional GABA and glutamate synapses, the latter necessitating more developed neurons that have apical dendrites that reach the lacunosum moleculare region.

Subcellular localization of full-length and truncated Trk receptor isoforms in polarized neurons and epithelial cells

Neurotrophins affect neuronal development and plasticity via spatially localized effects, yet little is known about the subcellular distribution of the Trk neurotrophin receptors and the impact of this distribution on neurotrophin action. To address this, we examined the subcellular location of full-length TrkB and TrkC tyrosine kinase receptors and truncated TrkB isoforms after transfection of Madin-Darby canine kidney (MDCK) cells, dissociated primary hippocampal neurons, and cortical neurons within intact brain slices. Myc-, herpes virus glycoprotein (HVG)-, or FLAG-derived epitope-tagged receptor isoforms were created to allow their unambiguous identification and localization after transfection. All tagged receptors were appropriately synthesized, and full-length myc-TrkB and myc-TrkC mediated appropriate neurotrophin-signaling events. We found that full-length TrkB receptors were excluded from the apical domain of MDCK cells but that TrkC receptors were present in both apical and basolateral domains. Full-length TrkB and TrkC were found throughout transfected primary cultured hippocampal neurons and transfected neurons in neocortical brain slices and showed no evidence of vectorial sorting. Truncated forms of TrkB were also homogeneously distributed in MDCK cells, dissociated hippocampal neurons, and cortical neurons within slice preparations. Levels of full-length and truncated TrkB were examined in postsynaptic densities; both receptor isoforms were present but only moderately enriched in these structures. Together, these findings suggest that Trk receptors are uniformly distributed in both axonal and dendritic compartments and that local neurotrophin responses are controlled by other mechanisms.

Changes in respiratory timing induced by hypercapnia in maturing rats

Premature infants respond to hypercapnia by an attenuated ventilatory response that is characterized by a decrease in respiratory frequency. We hypothesized that this impaired hypercapnic ventilatory response is of central origin and is mediated via gamma-aminobutyric acid-ergic (GABAergic) pathways. We therefore studied two groups of maturing Sprague-Dawley rats: unrestrained rats in a whole body plethysmograph at four postnatal ages (5, 16-17, 22-23, and 41-42 days); and ventilated, decerebrate, vagotomized, paralyzed rats in which phrenic nerve responses to hypercapnia were measured at 4-6 and 37-39 days of age. In the unrestrained group, the increase in minute ventilation induced by hypercapnia was significantly lower at 5 days vs. beyond 16 days. Although there was an increase in tidal volume at all ages, frequency decreased significantly from baseline at 5 days, whereas it increased significantly at 16-17, 22-23, and 41-42 days. The decrease in frequency at 5 days of age was mainly due to a significant prolongation in expiratory duration (TE). In the ventilated group, hypercapnia also caused prolongation in TE at 4-6 days but not at 37-39 days of age. Intravenous administration of bicuculline (GABA(A)-receptor blocker) abolished the prolongation of TE in response to hypercapnia in the newborn rats. We conclude that newborn rat pups exhibit a characteristic ventilatory response to CO(2) expressed as a centrally mediated prolongation of TE that appears to be mediated by GABAergic mechanisms.

Expression of functional alpha7 nicotinic acetylcholine receptor during mammalian muscle development and denervation

We have studied, on the transcriptional, protein and functional level, the expression of alpha7 nicotinic acetylcholine receptors (nAChR) in the course of rat muscle development, denervation and renervation. At foetal day 13, alpha7 nAChR expression was observed in somites and developing muscles of the back, but not yet in migrating myoblasts. Two days later, concomitant with myoblast aggregation, the alpha7 isoform began to be expressed in isolated myoblasts, with the highest level of expression in the frontal zone of the migrating wave. On foetal day 18, a time when the myoblasts in the upper hindleg have fused, alpha7 nAChR expression was most prominent in the outer layer of muscle tissue. The highest level of expression was observed in the first postnatal week, when practically all muscle cells stained positively for alpha7 protein. During the following weeks, alpha7 nAChR expression slowly decreased and practically disappeared in adult hindleg muscle. Following chronic denervation of adult Soleus muscle fibres, expression of alpha7 nAChR returned within 2-4 weeks. Electrophysiological measurements showed that the alpha7 nAChR of chronically denervated soleus muscle fibres were functional and, in particular, that they could be activated by choline. The presence of the alpha7 nAChR in developing and denervated muscle suggests a role for this nicotinic receptor in neuronal pathfinding and/or endplate stabilization.

Developmental study of miniature IPSCs of CA3 hippocampal cells: modulation by midazolam

Maturation of GABAA/benzodiazepine receptors is associated with changes in their subunit composition. We have investigated whether these changes are accompanied by a developmental modification in the kinetic properties of miniature IPSCs (mIPSCs) and sensitivity to midazolam, a benzodiazepine agonist. In the presence of TTX (10 microM) and excitatory amino acid antagonists, AP5 (50 microM) and CNQX (50 microM), we whole-cell recorded mIPSCs in CA3 cells of hippocampal slices of adult and young (4-8 days) rats. mIPSCs were mediated by GABAA receptors as they were suppressed by bicuculline (10 microM). In both the adult and young rats, mIPSCs were similar in amplitude and kinetic properties. However, the mIPSCs frequency markedly increased with age from 4+/-3 Hz in the young rats to 20+/-9 Hz in the adult rats. In both age groups, midazolam (0.01 microM(-10) microM) and pentobarbital (30 microM) did not affect the amplitude, frequency and rise time of the mIPSCs but they increased to a similar extent their decay time constant. The current responses to isoguvacine, a GABAA agonist, were potentiated by 0.1 microM midazolam in both age groups. It is concluded that in immature and adult rats, synaptic GABAA receptors of CA3 were not different in their kinetic properties and sensitivity to midazolam.

Clusters of GABAA receptors on cultured hippocampal cells correlate only partially with functional synapses

We describe a method to label gamma-aminobutyric acid (GABA)A receptors on the surface of living hippocampal neurons in primary culture, and we compare the distribution of receptors with that of active synapses. To visualize GABAA receptors, the affinity-purified antibody beta3(1-13), recognizing the extracellular N-termini of the GABAA receptor beta2- and beta3-subunits, was used in combination with fluorescent secondary antibodies. The beta2- and beta3-subunits belong to the predominant GABAA receptor subunits in the hippocampus. As expected for aggregates of GABAA receptors in the somato-dendritic plasma membrane, a patchy staining pattern similar to that seen by labelling neurons after fixation was obtained. An antiserum recognizing an intracellular epitope of GABAA receptor beta3-subunits did not label the receptors in living neurons. Whole-cell recordings of GABA-evoked Cl - currents were not affected after decorating GABAA receptors with antibody beta3(1-13). Combining the staining of GABAA receptors with the labelling of active presynaptic terminals with the fluorescent dyes FM1-43 or FM4-64, consistently resulted in the detection of GABAA receptor clusters that were not located at active synapses. These amounted to approximately 50% of all labelled GABAA receptor clusters. GABAA receptor clusters that were not associated with active presynaptic terminals partially colocalized with the synaptic vesicle marker protein sv2, while another fraction had no presynaptic counterpart at all. These findings suggest the presence of presynaptically silent GABAergic synapses in cultured hippocampal neurons. They also indicate that for the maintenance of GABAA receptor aggregates, the release of GABA from an opposing active terminal is not essential.

Maturation of the axonal plasma membrane requires upregulation of sphingomyelin synthesis and formation of protein-lipid complexes

Neuronal maturation is a gradual process; first axons and dendrites are established as distinct morphological entities; next the different intracellular organization of these processes occurs; and finally the specialized plasma membrane domains of these two compartments are formed. Only when this has been accomplished does proper neuronal function take place. In this work we present evidence that the correct distribution of a class of axonal membrane proteins requires a mechanism which involves formation of protein-lipid (sphingomyelin/cholesterol) detergent-insoluble complexes (DIGs). Using biochemistry and immunofluorescence microscopy we now show that in developing neurons the randomly distributed Thy-1 does not interact with lipids into DIGs (in fully developed neurons the formation of such complexes is essential for the correct axonal targeting of this protein). Using lipid mass spectrometry and thin layer chromatography we show that the DIG lipid missing in the developing neurons is sphingomyelin, but not cholesterol or glucosylceramide. Finally, by increasing the intracellular levels of sphingomyelin in the young neurons the formation of Thy-1/DIGs was induced and, consistent with a role in sorting, proper axonal distribution was facilitated. These results emphasize the role of sphingomyelin in axonal, and therefore, neuronal maturation.

Localization of the brain-specific high-affinity l-proline transporter in cultured hippocampal neurons: molecular heterogeneity of synaptic terminals

The expression of a brain-specific, high-affinity Na+-(and Cl--)dependent l-proline transporter in subpopulations of putative glutamatergic pathways in mammalian brain suggests a physiological role for this carrier in excitatory neurotransmission (Fremeau et al. , Neuron 8: 915-926, 1992). To assess further the cell-type and subcellular localization of PROT, we examined its distribution in low-density cultures of embryonic rat hippocampus. PROT immunoreactivity was detected beginning at 8 days in culture in a highly punctate pattern localizing to a subset of synaptic terminals. PROT was not detected at GABAergic terminals but was specifically localized to a subset of excitatory nerve terminals. PROT-labeled terminals showed partial apposition to AMPA-type and NMDA-type glutamate receptor clusters. Immunolabeling of isolated neurons grown in microisland cultures revealed that PROT was expressed by 60% of cultured hippocampal neurons. Individual microisland cultures were immunopositive for either PROT or glutamic acid decarboxylase, but never both. In the expressing pyramidal neurons, PROT was targeted to all presynaptic terminals. These findings indicate that PROT contributes to the molecular heterogeneity of glutamatergic terminals and suggest a novel presynaptic regulatory role for PROT in excitatory transmission at specific glutamatergic synapses.

Temporal patterns and depolarizing actions of spontaneous GABAA receptor activation in granule cells of the early postnatal dentate gyrus

Whole cell patch-clamp recordings were used to investigate the properties of the gamma-aminobutyric acid type A (GABAA) receptor-mediated spontaneous synaptic events in immature granule cells of the developing, early postnatal day (P0-P6) rat dentate gyrus. With Cs-gluconate-filled whole cell patch pipettes at 0 mV in control medium, spontaneous inhibitory postsynaptic currents (sIPSCs) occurred in prominent bursts (peak amplitude of the bursts 406.9 +/- 58.4 pA; intraburst IPSC frequency 71.0 +/- 12.4 Hz) at 0.05 +/- 0.02 Hz in every immature granule cell younger than P7. Between the bursts of IPSCs, lower frequency (1.7 +/- 0.7 Hz), interburst IPSCs could be observed. Bicuculline and picrotoxin as well as the intracellularly applied chloride-channel blockers CsF- and 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid (DIDS) abolished the intraburst as well as the interburst IPSCs, indicating that the IPSCs were mediated by GABAA receptor channels. The bursts of IPSCs, but not the interburst IPSCs, were blocked by the simultaneous application of the glutamate receptor antagonists 2-amino-5-phosphovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione, indicating the importance of the glutamatergic excitatory drive onto the interneurons in the early postnatal dentate gyrus. The spontaneously occurring excitatory postsynaptic currents in immature granule cells, observable after the intracellular blockade of GABAA receptor channels with CsF- and DIDS, appeared exclusively as single events at low frequencies, i.e., they did not occur in prominent bursts. Gramicidin-based perforated patch-clamp recordings determined that the reversal potential for the burst of IPSCs (-46.6 +/- 3.1 mV) was more depolarized than the resting membrane potential (-54.2 +/- 4.2 mV) but more hyperpolarized than the action potential threshold (-41. 8 +/- 1.7 mV). The depolarizing action of the bursts of synaptic events most often evoked only a single action potential per burst. Simultaneous whole cell patch recordings, with KCl-filled patch pipettes at -60 mV in current clamp from pairs of immature granule cells of the developing dentate gyrus, determined that the bursts of IPSPs took place in a similar temporal pattern but with imperfect synchrony in neighboring granule cells (average lag between the onsets of the bursts between granule cell pairs 77.7 +/- 8.6 ms). These results show that the spontaneous activation of GABAA receptors in immature dentate granule cells displays unique properties that are distinct from the temporal patterns and biophysical features of spontaneous GABAA receptor activation taking place in the developing Ammon's horn and in the adult dentate gyrus.

Selective changes in single cell GABA(A) receptor subunit expression and function in temporal lobe epilepsy

Temporal lobe epilepsy is the most prevalent seizure disorder in adults. Compromised inhibitory neurotransmitter function in the hippocampus contributes to the hyperexcitability generating this condition, but the underlying molecular mechanisms are unknown. Combining patch-clamp recording and single-cell mRNA amplification (aRNA) techniques in single dentate granule cells, we demonstrate that expression of GABA(A) receptor subunit mRNAs is substantially altered in neurons from epileptic rats. These changes in gene expression precede epilepsy onset by weeks and correlate with profound alterations in receptor function, indicating that aberrant GABA(A) receptor expression and function has an essential role in the process of epileptogenesis.