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Neuronal network activity in the developing brain is generated in a discontinuous manner. In the visual cortex during the period of physiological blindness of immaturity, this activity mainly comprises retinally triggered spindle bursts or Ca2 + clusters thought to contribute to the activity-dependent construction of cortical circuits. In spite of potentially important developmental functions, the spatial structure of these activity patterns remains largely unclear. In order to address this issue, we here used three-dimensional two-photon Ca2 + imaging in the visual cortex of neonatal mice at postnatal days (P) 3–4 in vivo. Large-scale voxel imaging covering a cortical depth of 200 μm revealed that Ca2 + clusters, identified as spindle bursts in simultaneous extracellular recordings, recruit cortical glutamatergic neurons of the upper cortical plate (CP) in a column-like manner. Specifically, the majority of Ca2 + clusters exhibit prominent horizontal confinement and high intra-cluster density of activation involving the entire depth of the upper CP. Moreover, using simultaneous Ca2 + imaging from hundreds of neurons at single-cellular resolution, we demonstrate that the degree of neuronal co-activation within Ca2 + clusters displays substantial heterogeneity. We further provide evidence that co-activated cells within Ca2 + clusters are spatially distributed in a non-stochastic manner. In summary, our data support the conclusion that dense coding in the form of column-like Ca2 + clusters is a characteristic property of network activity in the developing visual neocortex. Such knowledge is expected to be relevant for a refined understanding of how specific spatiotemporal characteristics of early network activity instruct the development of cortical circuits.

In recent years, considerable progress has been achieved in deciphering the cellular and network functions of GABAergic transmission in the intact developing brain. First, in vivo studies in non-mammalian and mammalian species confirmed the long-held assumption that GABA acts as a mainly depolarizing neurotransmitter at early developmental stages. At the same time, GABAergic transmission was shown to spatiotemporally constrain spontaneous cortical activity, whereas firm evidence for GABAergic excitation in vivo is currently missing. Second, there is a growing body of evidence indicating that depolarizing GABA may contribute to the activity-dependent refinement of neural circuits. Third, alterations in GABA actions have been causally linked to developmental brain disorders and identified as potential targets of timed prophylactic interventions. In this article, we review these major recent findings and argue that both depolarizing and inhibitory GABA actions may be crucial for physiological brain maturation.

Summary AMPA receptors are essential for fast excitatory transmission in the CNS. Autoantibodies to AMPA receptors have been identified in humans with autoimmune encephalitis and severe defects of hippocampal function. Here, combining electrophysiology and high-resolution imaging with neuronal culture preparations and passive-transfer models in wild-type and GluA1-knockout mice, we analyze how specific human autoantibodies against the AMPA receptor subunit GluA2 affect receptor function and composition, synaptic transmission, and plasticity. Anti-GluA2 antibodies induce receptor internalization and a reduction of synaptic GluA2-containing AMPARs followed by compensatory ryanodine receptor-dependent incorporation of synaptic non-GluA2 AMPARs. Furthermore, application of human pathogenic anti-GluA2 antibodies to mice impairs long-term synaptic plasticity in vitro and affects learning and memory in vivo. Our results identify a specific immune-neuronal rearrangement of AMPA receptor subunits, providing a framework to explain disease symptoms.

γ-aminobutyric acid (GABA) mediates synaptic inhibition in the adult brain, but acts as a predominantly depolarizing and partially excitatory neurotransmitter in immature neurons. Recent

Zusammenfassung:Während γ-Aminobuttersäure (GABA) im adulten Gehirn synaptische Hemmung vermittelt, wirkt es auf unreife Nervenzellen zumeist depolarisierend und teilweise erregend.

Summary Epileptic seizures rank among the most frequent neurologic symptoms during the neonatal period. Accumulating data from experimental animal studies and clinical trials in humans suggest that neonatal seizures could adversely affect normal brain development and result in long-term neurologic sequelae. Unfortunately, currently used anticonvulsive drugs are often ineffective in the neonatal period. One particularity of the immature neuronal network during neonatal development is that the neurotransmitter γ-aminobutyric acid (GABA) is mainly depolarizing, rather than hyperpolarizing as commonly observed in adults. This might, in part, explain not only the higher seizure propensity of the immature neuronal network, but also the limited anticonvulsive efficacy of GABA-enhancing drugs during early postnatal life. Accordingly, pharmacologic attenuation of GABAergic depolarization has been proposed as a strategy for neonatal seizure control. However, the underlying conjecture of a depolarizing mode of GABA action has been seriously challenged recently. In the present review, we will summarize the state of knowledge regarding GABAergic depolarization in early life and discuss how these data might impact a currently tested anticonvulsive strategy.

A large body of evidence from in vitro studies suggests that GABA is depolarizing during early postnatal development. However, the mode of GABA action in the intact developing brain is unknown. Here we examine the in vivo effects of GABA in cells of the upper cortical plate using a combination of electrophysiological and Ca2+-imaging techniques. We report that at postnatal days (P) 3–4, GABA depolarizes the majority of immature neurons in the occipital cortex of anaesthetized mice. At the same time, GABA does not efficiently activate voltage-gated Ca2+ channels and fails to induce action potential firing. Blocking GABAA receptors disinhibits spontaneous network activity, whereas allosteric activation of GABAA receptors has the opposite effect. In summary, our data provide evidence that in vivo GABA acts as a depolarizing neurotransmitter imposing an inhibitory control on network activity in the neonatal (P3–4) neocortex. GABA depolarizes immature neurons in the central nervous system, yet the mode of GABA action in the developing brain is unknown. Here the authors demonstrate thatin vivoGABA acts as a depolarizing neurotransmitter imposing inhibitory control on network activity in the mouse postnatal day 3–4 neocortex.

The sulphydryl alkylating agent N-ethylmaleimide (NEM) has been often used as an uncoupler of pertussis toxin-sensitive G-proteins. However, the effects of NEM on gamma-aminobutyric acid (GABA)ergic synaptic transmission remain controversial. Using the whole-cell patch-clamp technique, GABA(A) receptor-mediated postsynaptic currents (IPSCs) have been recorded from Cajal-Retzius (CR) cells in layer I of the neonatal mouse visual cortex. NEM increased the frequencies of both spontaneous and miniature IPSCs (mIPSCs) without an effect on the median mIPSC amplitudes or mIPSC kinetics. The NEM actions on mIPSCs did not depend on the extracellular Ca(2+), Ca(2+) release from intracellular stores, adenylyl cyclase and protein kinase A activities. NEM increased the mean amplitudes of evoked IPSCs and strongly decreased the paired-pulse ratio. The size of the readily releasable pool of presynaptic vesicles (RRP) was estimated using a high-frequency stimulation protocol. The RRP size was not affected by NEM. In addition, NEM significantly decreased the latency between the stimulus and the onset of GABA release. These results suggest that NEM selectively increases GABA release probability. At postnatal day 2, mIPSCs were observed only in about 30% of CR cells. NEM application revealed, however, that more than 90% of CR cells receive GABAergic inputs. Therefore, NEM seems to be a useful tool to verify the existence of 'silent' GABAergic synapses.

Background Delayed cerebral vasospasm is the most common cause of mortality and severe neurological impairment in patients who survive subarachnoid hemorrhage. Despite improvements in the field of diagnostic imaging, options for prevention and medical treatment—primarily with the calcium channel antagonist nimodipine or hemodynamic manipulations—are insufficient. Previous studies have suggested that heme and bilirubin oxidation end products, originating from degraded hemoglobin around ruptured blood vessels, are involved in the development of vasospasm by inhibiting large conductance BK C a potassium channels in vascular smooth muscle cells. In this study, we identify individual heme degradation products regulating arteriolar diameter in dependence of BK C a channel activity. Methods and Results Using differential interference contrast video microscopy in acute brain slices, we determined diameter changes of intracerebral arterioles in mouse visual cortex. In preconstricted vessels, the specific BK C a channel blockers paxilline and iberiotoxin as well as iron‐containing hemin caused vasoconstriction. In addition, the bilirubin oxidation end product Z ‐ BOX A showed a stronger vasoconstrictive potency than its regio‐isomer Z ‐ BOX B. Importantly, Z ‐ BOX A had the same vasoconstrictive effect, independent of its origin from oxidative degradation or chemical synthesis. Finally, in slices of Slo1‐deficient knockout mice, paxilline and Z ‐ BOX A remained ineffective in changing arteriole diameter. Conclusions We identified individual components of the oxidative bilirubin degradation that led to vasoconstriction of cerebral arterioles. The vasoconstrictive effect of Z ‐ BOX A and Z ‐ BOX B was mediated by BK C a channel activity that might represent a signaling pathway in the occurrence of delayed cerebral vasospasm in subarachnoid hemorrhage patients.

A large body of evidence suggests that the neurotransmitter GABA undergoes a developmental switch from being predominantly depolarizing–excitatory to predominantly hyperpolarizing–inhibitory. Recently published data, however, point to the possibility that the presumed depolarizing mode of GABA action during early development might represent an artifact due to an insufficient energy supply of thein vitropreparations used. Specifically, addition of the ketone bodydl-β-hydroxybutyrate (βHB) to the extracellular medium was shown to prevent GABA from exerting excitatory effects. Applying a complementary set of minimally invasive optical and electrophysiological techniques in brain slices from neonatal mice, we investigated the effects of βHB on GABA actions in immature cells of the upper cortical plate. Fluorescence imaging revealed that GABA-mediated somatic [Ca2+] transients, that required activation of GABAAreceptors and voltage-gated Ca2+channels, remained unaffected by βHB. Cell-attached current-clamp recordings showed that, in the presence of βHB, GABA still induced a membrane potential depolarization. To estimate membrane potential changes quantitatively, we used cell-attached recordings of voltage-gated potassium currents and demonstrated that the GABA-mediated depolarization was independent of supplementation of the extracellular solution with βHB. We conclude that,in vitro, GABA depolarizes immature cells of the upper cortical plate in the presence of the ketone body βHB. Our data thereby support the general concept of an excitatory-to-inhibitory switch of GABA action during early development.

At early stages of brain development, GABA plays a dual role. It fulfills important trophic functions and provides a major excitatory drive for the immature neuronal network. Here, we investigated whether GABA itself can limit the strength of excitatory GABAergic synapses on Cajal-Retzius (CR) cells in sagittal slices from the mouse visual cortex. (2S)-3-[[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl](phenylmethyl)phosphinic acid (CGP55845), a specific GABABreceptor (GABABR) blocker, increased the frequency of spontaneous Ca2+transients and spontaneous and miniature IPSCs (mIPSCs) but did not affect mIPSC amplitudes or kinetics.CGP55845significantly increased evoked IPSC (eIPSC) amplitudes and decreased the paired-pulse ratio (PPR). Baclofen, a specific GABABR agonist, produced opposite effects. The size of the readily releasable pool was not affected by these GABABR modulators. The sameCGP55845actions were observed at physiological temperatures, but they were abolished after glutamate decarboxylase block with 3-mercaptopropionic acid (3-MP). These results indicate that presynaptic GABABRs dynamically regulate GABA release probability. SNAP-5114, a specific GABA transporter-2/3 (GAT-2/3) blocker, enhanced mIPSC frequencies, decreased PPR, and increased eIPSC amplitudes without changing eIPSC kinetics. These effects were blocked byCGP55845and 3-MP. NO-711, a specific GAT-1 blocker, prolonged eIPSC decay and decreased eIPSC/mIPSC amplitudes. These NO-711-mediated effects were not sensitive toCGP55845and 3-MP. We conclude that the strength of GABAergic inputs to CR cells is constrained by GABABRs that are persistently activated by ambient GABA. The latter is also provided by GAT-2/3 operating in the reversed mode. Presynaptic GAT-1 functions in the uptake mode and possibly provides GABA for presynaptic vesicle filling.