Immature Neocortex Research Articles

Glutamate released by Cajal-Retzius cells impacts specific hippocampal circuits and behaviors.

SUMMARYThe impact of Cajal-Retzius cells on the regulation of hippocampal circuits and related behaviors is unresolved. Here, we directly address this issue by impairing the glutamatergic output of Cajal-Retzius cells with the conditional ablation of vGluT2, which is their main vesicular glutamate transporter. Although two distinct conditional knockout lines do not reveal major alterations in hippocampal-layer organization and dendritic length of principal neurons or GABAergic cells, we find parallel deficits in specific hippocampal-dependent behaviors and in their putative underlying microcircuits. First, conditional knockout animals show increased innate anxiety and decreased feedforward GABAergic inhibition on dentate gyrus granule cells. Second, we observe impaired spatial memory processing, which is associated with decreased spine density and reduced AMPA/NMDA ratio of postsynaptic responses at the perforant- and entorhino-hippocampal pathways. We conclude that glutamate synaptically released by Cajal-Retzius cells is critical for the regulation of hippocampal microcircuits and specific types of behaviors.

COUP-TFI/Nr2f1 Orchestrates Intrinsic Neuronal Activity during Development of the Somatosensory Cortex.

The formation of functional cortical maps in the cerebral cortex results from a timely regulated interaction between intrinsic genetic mechanisms and electrical activity. To understand how transcriptional regulation influences network activity and neuronal excitability within the neocortex, we used mice deficient for Nr2f1 (also known as COUP-TFI), a key determinant of primary somatosensory (S1) area specification during development. We found that the cortical loss of Nr2f1 impacts on spontaneous network activity and synchronization of S1 cortex at perinatal stages. In addition, we observed alterations in the intrinsic excitability and morphological features of layer V pyramidal neurons. Accordingly, we identified distinct voltage-gated ion channels regulated by Nr2f1 that might directly influence intrinsic bioelectrical properties during critical time windows of S1 cortex specification. Altogether, our data suggest a tight link between Nr2f1 and neuronal excitability in the developmental sequence that ultimately sculpts the emergence of cortical network activity within the immature neocortex.

Putative Role of Taurine as Neurotransmitter During Perinatal Cortical Development.

Neurotransmitters and neuronal activity affect neurodevelopmental events like neurogenesis, neuronal migration, apoptosis and differentiation. Beside glutamate and gamma-amino butyric acid, the aminosulfonic acid taurine has been considered as possible neurotransmitter that influences early neuronal development. In this article I review recent studies of our group which demonstrate that taurine can affect a variety of identified neuronal populations in the immature neocortex and directly modulates neuronal activity. These experiments revealed that taurine evoke dose-dependent membrane responses in a variety of neocortical neuron populations, including Cajal-Retzius cells, subplate neurons and GABAergic interneurons. Taurine responses persist in the presence of GABA(A) receptor antagonists and are reduced by the addition of strychnine, suggesting that glycine receptors are involved in taurine-mediated membrane responses. Gramicidin-perforated patch-clamp and cell-attached recordings demonstrated that taurine evokes depolarizing and mainly excitatory membrane responses, in accordance with the high intracellular Cl- concentration in immature neurons. In addition, taurine increases the frequency of postsynaptic GABAergic currents (PSCs) in a considerable fraction of immature pyramidal neurons, indicating a specific activation of presynaptic GABAergic networks projecting toward and exciting pyramidal neurons. In summary, these results suggest that taurine may be critically involved in the regulation of network excitability in the immature neocortex and hippocampus via interactions with glycine receptors.

3-MA Inhibits Autophagy and Favors Long-Term Integration of Grafted Gad67–GFP GABAergic Precursors in the Developing Neocortex by Preventing Apoptosis

In human neonates, immature GABAergic interneurons are markedly affected by an excitotoxic insult. While in adults the interest of cell transplantation has been demonstrated in several neurological disorders, few data are available regarding the immature brain. The low survival rate constitutes a strong limitation in the capacity of transplanted neurons to integrate the host tissue. Because i) autophagy is an adaptive process to energetic/nutrient deprivation essential for cell survival and ii) literature describes cross-talks between autophagy and apoptosis, we hypothesized that regulation of autophagy would represent an original strategy to favor long-term survival of GABAergic precursors grafted in the immature neocortex. Morphological, neurochemical, and functional data showed that in control conditions, few grafted Gad67-GFP precursors survived. The first hours following transplantation were a critical period with intense apoptosis. Experiments performed on E15.5 ganglionic eminences revealed that Gad67-GFP precursors were highly sensitive to autophagy. Rapamycin and 3-MA impacted on LC3 cleavage, LC3II translocation, and autophagosome formation. Quantification of Bax, mitochondrial integrity, caspase-3 cleavage, and caspase-3 immunolocalization and activity showed that 3-MA induced a significant decrease of Gad67-GFP precursor apoptosis. In vivo, 3-MA induced, within the first 24 h, a diffuse LC3 pattern of grafted Gad67-GFP precursors, an increase of precursors with neurites, a reduction of the density of caspase-3 immunoreactive cells. A twofold increase in the survival rate occurred 15 days after the graft. Surviving neurons were localized in the cortical layers II-IV, which were still immature when the transplantation was done. Altogether, these data indicate that inhibition of autophagy represents an original strategy to allow GABAergic interneurons to overpass the first critical hours following transplantation and to increase their long-term survival in mice neonates.

Resonance properties of GABAergic interneurons in immature GAD67-GFP mouse neocortex

Subthreshold resonance is a characteristic membrane property of different neuronal classes, is critically involved in the generation of network oscillations, and tunes the integration of synaptic inputs to particular frequency ranges. In order to investigate whether neocortical GABAergic interneurons show resonant behavior already during early postnatal development, we performed whole-cell patch-clamp recordings from visually identified interneurons in supragranular layers of parietal regions in coronal neocortical slices from postnatal day (P) P6–P13 GAD67-GFP knock-in mice. Subthreshold resonance was analyzed by injection of sinusoidal current with varying frequency. About 50% of the investigated GABAergic interneurons showed subthreshold resonance with an average frequency of 2.0±0.2Hz (n=38). Membrane hyperpolarization to –86mV attenuated the frequency and strength of subthreshold resonance. In the presence of 1mM Ni2+ subthreshold resonance was virtually abolished, suggesting that T-type Ca2+ currents are critically involved in the generation of resonance. In contrast, subthreshold resonance was not affected by ZD7288, a blocker of HCN channels. Application of TTX suppressed subthreshold resonance at depolarized, but not hyperpolarized membrane potential, suggesting that persistent Na+ current contribute to the amplification of membrane resonance. In summary, these results demonstrate that GABAergic interneurons express subthreshold resonance at low frequencies, with T-type Ca2+ and persistent Na+ currents underlying the generation of membrane resonance. The membrane resonance of immature interneurons may contribute to the generation of slow oscillatory activity pattern in the immature neocortex and enhance the temporal precision of synaptic integration in developing cortical neurons.

Taurine activates GABAergic networks in the neocortex of immature mice

Although it has been suggested that taurine is the main endogenous neurotransmitter acting on glycine receptors, the implications of glycine receptor-mediated taurine actions on immature neocortical networks have not been addressed yet. To investigate the influence of taurine on the excitability of neuronal networks in the immature neocortex, we performed whole-cell patch-clamp recordings from visually identified pyramidal neurons and interneurons in coronal slices from C57Bl/6 and GAD67-green fluorescent protein (GFP) transgenic mice (postnatal days 2–4). In 46% of the pyramidal neurons bath-application of taurine at concentrations ≥ 300 μM significantly enhanced the frequency of postsynaptic currents (PSCs) by 744.3 ± 93.8% (n = 120 cells). This taurine-induced increase of PSC frequency was abolished by 0.2 μM tetrodotoxin (TTX), 1 μM strychnine or 3 μM gabazine, but was unaffected by the glutamatergic antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and (±) R(-)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid (CPP), suggesting that taurine specifically activates GABAergic network activity projecting to pyramidal neurons. Cell-attached recordings revealed that taurine enhanced the frequency of action potentials (APs) in pyramidal neurons, indicating an excitatory action of the GABAergic PSCs. In order to identify the presynaptic targets of taurine we demonstrate that bath application of taurine induced in GAD67-GFP labeled interneurons an inward current that is mainly mediated by glycine receptors and can generate APs in these cells. We conclude from these results that taurine can enhance network excitability in the immature neocortex by selectively activating GABAergic interneurons via interactions with glycine receptors.

Kainate receptors in epilepsy

AbstractKainate receptors (KARs) belong to the glutamatergic receptor family and they are assembled from various combinations of GluK1–GluK5 subunits. In the central nervous system (CNS), these receptors are involved in fundamental neuronal operation‐like synaptic transmission. Thus, excitatory postsynaptic events with a KAR‐mediated component have been identified in different structures of the CNS including the hippocampus, amygdala, cerebellum, spinal cord, retina, and immature neocortex. A remarkable feature of KAR‐mediated synaptic events is their small amplitude and slow kinetics. These receptors also play a major role in the control of neuronal excitability via the modulation of transmitter release at both excitatory and inhibitory connections, and through metabotropic signaling. More recently, it has been discovered that KARs are also key players in CNS pathology as in temporal lobe epilepsy. Indeed, in this pathology abnormal synaptic connections in dentate granule cells (DGCs) operate via KARs not present in naïve conditions. These aberrant KAR‐operated synapses drastically alter the computational properties of DGCs. These latest discoveries open a promising and new vista for the development of novel antiepileptic strategies. WIREs Membr Transp Signal 2013, 2:75–83. doi: 10.1002/wmts.80For further resources related to this article, please visit the WIREs website.

Resonance properties of different neuronal populations in the immature mouse neocortex

In vivo recordings in the immature neocortex revealed spontaneous and sensory-driven oscillatory activity from delta (0.5-4 Hz) to gamma (30-100 Hz) frequencies. In order to investigate whether the resonance properties of distinct neuronal populations in the immature neocortex contribute to these network oscillations, we performed whole-cell patch-clamp recordings from visually identified neurons in tangential and coronal neocortical slices from postnatal day (P)0-P7 C57Bl/6 mice. Subthreshold resonance was analysed by sinusoidal current injection of varying frequency. All Cajal-Retzius cells showed subthreshold resonance, with an average frequency of 2.6 ± 0.1 Hz (n = 60), which was massively reduced by ZD7288, a blocker of hyperpolarization-activated cation currents. Approximately 65.6% (n = 61) of the supragranular pyramidal neurons showed subthreshold resonance, with an average frequency of 1.4 ± 0.1 Hz (n = 40). Application of Ni(2+) suppressed subthreshold resonance, suggesting that low-threshold calcium currents contribute to resonance in these neurons. Approximately 63.6% (n = 77) of the layer V pyramidal neurons showed subthreshold resonance, with an average frequency of 1.4 ± 0.2 Hz (n = 49), which was abolished by ZD7288. Only 44.1% (n = 59) of the subplate neurons showed subthreshold resonance, with an average frequency of 1.3 ± 0.2 Hz (n = 26) and a small resonance strength. In summary, these results demonstrate that neurons in all investigated layers show resonance behavior, with either hyperpolarization-activated cation or low-threshold calcium currents contributing to the subthreshold resonance. The observed resonance frequencies are in the range of slow activity patterns observed in the immature neocortex, suggesting that subthreshold resonance may support the generation of this activity.

Intrinsic activation of GABAA receptors suppresses epileptiform activity in the cerebral cortex of immature mice

Activation of ionotropic γ-aminobutyric acid type A (GABA(A) ) receptors induces in immature neocortical neurons a membrane depolarization that may contribute to the higher epilepsy susceptibility in newborns. To elucidate whether depolarizing GABAergic responses enhance or attenuate epileptiform activity in the immature neocortex, we investigated the effect of agonists, antagonists, and positive modulators of GABA(A) receptors on epileptiform activity. We performed in vitro field potential recordings on isolated whole neocortex preparations and whole cell recordings of identified pyramidal neurons in 400-μm slices of immature (postnatal day 1-7) mice. Epileptiform activity was induced by low Mg²(+) solutions with or without 50-100 μm 4-aminopyridine. Bath application of GABA (3-100 μm, in the presence of tiagabine) attenuated epileptiform activity. The GABA transporter isoform 1 (GAT-1) inhibitor tiagabine (30 μm) and the GAT-2/3 specific inhibitor SNAP 5114 (40 μm) reduced the frequency of epileptiform activity. The benzodiazepines midazolam (0.2 μm) and zolpidem (0.5 μm) as well as the barbiturate phenobarbital (30 μm) slightly attenuated epileptiform activity. Continuous bath application of the GABAergic antagonist gabazine (SR-95531, 2-3 μm) or picrotoxin (15 μm) induced epileptiform discharges. These results demonstrate, that (1) the activation or positive modulation of GABA(A) receptors attenuates epileptiform activity, (2) GABA(A) antagonists mediate a disinhibition, and (3) GABA uptake contributes to the regulation of extracellular GABA in immature neocortex. We conclude from these findings that a constant inhibition via GABA(A) receptors is required to suppress epileptiform activity already in the immature neocortex.

Early NMDA receptor‐driven waves of activity in the developing neocortex: physiological or pathological network oscillations?

Several patterns of coherent activity have been described in developing cortical structures, thus providing a general framework for network maturation. A detailed timely description of network patterns at circuit and cell levels is essential for the understanding of pathogenic processes occurring during brain development. Disturbances in the expression timetable of this pattern sequence are very likely to affect network maturation. This review focuses on the maturation of coherent activity patterns in developing neocortical structures. It emphasizes the intrinsic and synaptic cellular properties that are unique to the immature neocortex and, in particular, the critical role played by extracellular glutamate in controlling network excitability and triggering synchronous network waves of activity.

Excitatory GABA in Rodent Developing Neocortex In Vitro

GABA depolarizes immature cortical neurons. However, whether GABA excites immature neocortical neurons and drives network oscillations as in other brain structures remains controversial. Excitatory actions of GABA depend on three fundamental parameters: the resting membrane potential (Em), reversal potential of GABA (E(GABA)), and threshold of action potential generation (Vthr). We have shown recently that conventional invasive recording techniques provide an erroneous estimation of these parameters in immature neurons. In this study, we used noninvasive single N-methyl-d-aspartate and GABA channel recordings in rodent brain slices to measure both Em and E(GABA) in the same neuron. We show that GABA strongly depolarizes pyramidal neurons and interneurons in both deep and superficial layers of the immature neocortex (P2-P10). However, GABA generates action potentials in layer 5/6 (L5/6) but not L2/3 pyramidal cells, since L5/6 pyramidal cells have more depolarized resting potentials and more hyperpolarized Vthr. The excitatory GABA transiently drives oscillations generated by L5/6 pyramidal cells and interneurons during development (P5-P12). The NKCC1 co-transporter antagonist bumetanide strongly reduces [Cl(-)]i, GABA-induced depolarization, and network oscillations, confirming the importance of GABA signaling. Thus a strong GABA excitatory drive coupled with high intrinsic excitability of L5/6 pyramidal neurons and interneurons provide a powerful mechanism of synapse-driven oscillatory activity in the rodent neocortex in vitro. In the companion paper, we show that the excitatory GABA drives layer-specific seizures in the immature neocortex.

Functional effectiveness of the blood-brain barrier to small water-soluble molecules in developing and adult opossum (Monodelphis domestica)

We have evaluated a small water-soluble molecule, biotin ethylenediamine (BED, 286 Da), as a permeability tracer across the blood-brain barrier. This molecule was found to have suitable characteristics in that it is stable in plasma, has low plasma protein binding, and appears to behave in a similar manner across brain barriers as established by permeability markers such as sucrose. BED, together with a 3000-Da biotin-dextran (BDA3000), was used to investigate the effectiveness of tight junctions in cortical vessels during development and adulthood of a marsupial opossum (Monodelphis domestica). Marsupial species are born at an early stage of brain development when cortical vessels are just beginning to appear. The tracers were administered systemically to opossums at various ages and localized in brains with light and electron microscopy. In adults, the tight junctions restricted the movement of both tracers. In neonates, as soon as vessels grow into the neocortex, their tight junctions are functionally restrictive, a finding supported by the presence of claudin-5 in endothelial cells. However, both tracers are also found within brain extracellular space soon after intraperitoneal administration. The main route of entry for the tracers into immature neocortex appears to be via the cerebrospinal fluid over the outer (subarachnoid) and inner (ventricular) surfaces of the brain. These experiments demonstrate that the previously described higher permeability of barriers to small molecules in the developing brain does not seem to be due to leakiness of cerebral endothelial tight junctions, but to a route of entry probably via the choroid plexuses and cerebrospinal fluid.

Glutamate transporters prevent the generation of seizures in the developing rat neocortex.

Glutamate transporters are operative at an early developmental stage well before synapse formation, but their functional significance has not been determined. We now report that blockade of glutamate transporters in the immature neocortex generates recurrent NMDA receptor-mediated currents associated with synchronous oscillations of [Ca2+]i in the entire neuronal population. Intracerebroventricular injections of the blocker to pups generate seizures that are prevented by coinjections of NMDA receptor blockers. Therefore, the early expression of glutamate transporters plays a central role to prevent the activation by local glutamate concentrations of NMDA receptors and the generation of seizures that may alter the construction of cortical networks. A dysfunction of glutamate transporters may be a central event in early infancy epilepsy syndromes.

Maturational Aspects of Epilepsy Mechanisms and Consequences for the Immature Brain

Maturational Aspects of Epilepsy Mechanisms and Consequences for the Immature Brain

Endogenous acetylcholine facilitates epileptogenesis in immature rat neocortex

In the presence of the γ-amino butyric acid-A (GABA A) antagonist bicuculline methiodide (50 μM), synchronous spontaneous and evoked potentials were recorded extracellularly from the deep layers of immature neocortex (postnatal days 10–31, P10–P31) in vitro. Addition of the anticholinesterase eserine (10 μM) depressed the amplitude (by 29.5±6.6%, n=13) and duration (by 26.3±4.7%, n=11) of the evoked field potentials in 13/19 slices (68%), and increased significantly the rates of occurrence of spontaneous epileptiform discharges or induced them in 9/19 slices (47%). All these effects were blocked by the muscarinic antagonist atropine (2.5 μM, n=3), suggesting that they were mediated by the activation of muscarinic receptors by endogenous acetylcholine. The cholinergic inhibitory effect is unlikely to terminate seizures, while the excitatory effect, could conceivably promote or aggravate their manifestation. In conclusion, these findings demonstrate that endogenous acetylcholine may contribute to epileptogenesis in immature neocortex.

Modulation of intrinsic circuits in developing neocortex.

Numerous experimental studies have indicated that the developing rat brain is very susceptible to seizures. Immature rats have a low threshold for flurothyland kainic acid-induced seizures. Additionally, prolonged epileptiform activity is observed in slices of immature rat brain exposed to convulsant drugs in vitro. The mechanisms responsible for this seizure-prone period in development are poorly understood. Developmental changes in expression of metabotropic glutamate receptors (mGluRs) and local circuit abnormalities may contribute to the increase in seizure susceptibility of the immature brain. We have hypothesized that because other neuromodulators display marked developmental changes in receptor expression, these systems also could contribute to epileptogenesis. The intricate neural circuits of the mammalian neocortex develop as a result of a series of complex, precisely timed events. These events include cell differentiation, migration, synapse formation, and stabilization. The cerebral cortex is innervated by monoaminergic afferents at early stages of development, suggesting that these systems play a prominent role in synaptogenesis and circuit formation. In particular, disruptions of the serotonin (5-HT) system during embryonic or early postnatal development are known to produce, in adults, profound disturbances of sleep patterns, learning, and behavior. 5-HT has been implicated in drug-induced psychoses and a number of psychiatric disorders, major depression and schizophrenia in particular. Increasing 5-HT levels in neonatal rats cause impairments in performance on passive avoidance learning paradigms when these rats are tested 1-2 months later. Decreasing neonatal 5-HT levels have been associated with adult deficits in attention, spatial memory, and learning. Although these manipulations are not physiologic, environmental enrichment, a treatment known to augment neuronal plasticity, also has been shown to alter 5-HT mRNA expression and binding in rat brain. Abnormal 5-HT levels during development also have been implicated in a number of human neurologic disorders such as schizophrenia, Rett syndrome, and autism. In addition to involvement with mechanisms of brain development, serotonin has been recently shown to be an endogenous modulator of epileptiform activity in the rat hippocampus (1). The 5-HT reuptake inhibitor fluoxetine has anticonvulsant effects as an add-on therapy in patients (2). Multiple 5-HT-receptor subtypes are expressed in the cerebral cortex ( 3 ) . 5-HT, receptors are principally expressed in inhibitory neurons (4), whereas 5-HT2, receptors are heavily expressed in pyramidal cells and to a lesser extent in inhibitory neurons. Based on experiments using in vivo microiontophoretic methods, the predominant effect of 5-HT on cerebral cortical pyramidal neurons was believed to be inhibition of spontaneous spiking, but the underlying mechanisms was not clear (see 5 for review). Intracellular studies in vitro have reported varying postsynaptic effects of 5-HT. In neocortical neurons, 5-HT has been reported to have no effect on input resistance and membrane potential (6) or to both hyperpolarize and to depolarize neurons (6,7). These multiple effects likely result from the diversity of ion channels that 5-HT modulates and the multiplicity of 5-HT receptors. Because there is a transient expression of a dense innervation of 5-HT receptors in sensory areas in the immature neocortex (8,9), we compared the effects of 5-HT on immature and mature neocortex. To examine the effects of 5-HT on activation of local circuits in the neocortex, optical imaging of voltagesensitive dye signals was used. This technique allows measurement of the spatial temporal pattern of excitation of a large area of cortex, Because we have shown that 5-HT enhances the activity of inhibitory interneurons and increases GABA-mediated synaptic activity in mature neocortex (Zhou and Hablitz, unpublished observations), it was reasoned that 5-HT-receptor activation should decrease voltage-sensitive dye signals. Stimulation was applied in deeper cortical layers. As shown in Fig. 1A (left), dye signal responses were initially recorded in the region of cortex overlying the stimulation site. This activity then spread laterally over a distance of 5 1,000 Fm. The area of cortex excited and the spread of activity was reduced by application of 5-HT (Fig. lA, right). These effects were reversible on washing. These

Neonatal lead exposure impairs development of rodent barrel field cortex.

Childhood exposure to low-level lead can permanently reduce intelligence, but the neurobiologic mechanism for this effect is unknown. We examined the impact of lead exposure on the development of cortical columns, using the rodent barrel field as a model. In all areas of mammalian neocortex, cortical columns constitute a fundamental structural unit subserving information processing. Barrel field cortex contains columnar processing units with distinct clusters of layer IV neurons that receive sensory input from individual whiskers. In this study, rat pups were exposed to 0, 0.2, 1, 1.5, or 2 g/liter lead acetate in their dam's drinking water from birth through postnatal day 10. This treatment, which coincides with the development of segregated columns in the barrel field, produced blood lead concentrations from 1 to 31 microg/dl. On postnatal day 10, the area of the barrel field and of individual barrels was measured. A dose-related reduction in barrel field area was observed (Pearson correlation = -0.740; P < 0.001); mean barrel field area in the highest exposure group was decreased 12% versus controls. Individual barrels in the physiologically more active caudoventral group were affected preferentially. Total cortical area measured in the same sections was not altered significantly by lead exposure. These data support the hypothesis that lead exposure may impair the development of columnar processing units in immature neocortex. We demonstrate that low levels of blood lead, in the range seen in many impoverished inner-city children, cause structural alterations in a neocortical somatosensory map.

Formation of Cortical Fields on a Reduced Cortical Sheet

Theories of both cortical field development and cortical evolution propose that thalamocortical projections play a critical role in the differentiation of cortical fields (; ). In the present study, we examined how changing the size of the immature neocortex before the establishment of thalamocortical connections affects the subsequent development and organization of the adult neocortex. This alteration in cortex is consistent with one of the most profound changes made to the mammalian neocortex throughout evolution: cortical size. Removing the caudal one-third to three-fourths of the cortical neuroepithelial sheet unilaterally at an early stage of development in marsupials resulted in normal spatial relationships between visual, somatosensory, and auditory cortical fields on the remaining cortical sheet. Injections of neuroanatomical tracers into the reduced cortex revealed in an altered distribution of thalamocortical axons; this alteration allowed the maintenance of their original anteroposterior distribution. These results demonstrate the capacity of the cortical neuroepithelium to accommodate different cortical fields at early stages of development, although the anteroposterior and mediolateral relationships between cortical fields appear to be invariant. The shifting of afferents and efferents with cortical reduction or expansion at very early stages of development may have occurred naturally in different lineages over time and may be sufficient to explain much of the phenotypic variation in cortical field number and organization in different mammals.

Developmental regulation of action potential-induced Ca2+ entry in neocortical neurons.

Intracellular calcium plays a critical role in the regulation of membrane excitability, synaptic integration and synaptic plasticity. Using whole-cell recording and calcium imaging, we investigated intracellular calcium dynamics in apical dendrites of layer V pyramidal cells in the immature rat neocortex. Dendritic calcium increases induced by action potentials were only small on postnatal days 4-6 (P4-6), then underwent a gradual enhancement during the second post-natal week and were 2- to 3-fold larger on P16-18 than on P4-6. At P15-18 there were no significant differences in the calcium increases among the three subclasses of layer V pyramidal neurons: adapting regular spiking (RS), non-adapting RS and intrinsically bursting (IB) neurons. Developmental regulation of dendritic calcium dynamics may be crucial for functional maturation of the neocortex.

Nonsynaptic Glycine Receptor Activation during Early Neocortical Development

Glycine receptors (GlyRs) contribute to fast inhibitory synaptic transmission in the brain stem and spinal cord. GlyR subunits are expressed in the developing neocortex, but a neurotransmitter system involving cortical GlyRs has yet to be demonstrated. Here, we show that GlyRs in immature neocortex are excitatory and activated by a nonsynaptically released endogenous ligand. Of the potential ligands for cortical GlyRs, taurine is by far the most abundant in the developing neocortex. We found that taurine is stored in immature cortical neurons and that manipulations known to elevate extracellular taurine cause GlyR activation. These data indicate that nonsynaptically released taurine activates GlyRs during neocortical development. As fetal taurine deprivation can cause cortical dysgenesis, it is possible that taurine influences neocortical development by activating GlyRs.