Differential dendritic Ca2+ signalling in young and mature hippocampal granule cells

T-Type Ca2+ Channels Mediate a Critical Period of Plasticity in Adult-Born Granule Cells

Adult-born granule cells (abGCs) exhibit a transient period of elevated synaptic plasticity that plays an important role in hippocampal function. Various mechanisms have been implicated in this critical period for enhanced plasticity, including minimal GABAergic inhibition and high intrinsic excitability conferred by T-type Ca2+ channels. Here we assess the contribution of synaptic inhibition and intrinsic excitability to long-term potentiation (LTP) in abGCs of adult male and female mice using perforated patch recordings. We show that the timing of critical period plasticity is unaffected by intact GABAergic inhibition such that 4-6-week-old abGCs exhibit LTP that is absent by 8 weeks. Blocking GABAA receptors, or partial blockade of GABA release from PV and nNos-expressing interneurons by a µ-opioid receptor agonist, strongly enhances LTP in 4-week-old GCs, suggesting that minimal inhibition does not underlie critical period plasticity. Instead, the closure of the critical period coincides with a reduction in the contribution of T-type Ca2+ channels to intrinsic excitability, and a selective T-type Ca2+ channel antagonist prevents LTP in 4-week-old but not mature GCs. Interestingly, whole-cell recordings that facilitate T-type Ca2+ channel activity in mature GCs unmasks LTP (with inhibition intact) that is also sensitive to a T-type Ca2+ channel antagonist, suggesting T-type channel activity in mature GCs is suppressed by native intracellular signaling. Together these results show that abGCs use T-type Ca2+ channels to overcome inhibition, providing new insight into how high intrinsic excitability provides young abGCs a competitive advantage for experience-dependent synaptic plasticity.

Excitatory selective LTP of supramammillary glutamatergic/GABAergic cotransmission potentiates dentate granule cell firing

It is now established that many neurons can release multiple transmitters. Recent studies revealed that fast-acting neurotransmitters, glutamate and GABA, are coreleased from the same presynaptic terminals in some adult brain regions. The dentate gyrus (DG) granule cells (GCs) are innervated by the hypothalamic supramammillary nucleus (SuM) afferents that corelease glutamate and GABA. However, how these functionally opposing neurotransmitters contribute to DG information processing remains unclear. We show that glutamatergic, but not GABAergic, cotransmission exhibits long-term potentiation (LTP) at SuM-GC synapses. By the excitatory selective LTP, the excitation/inhibition balance of SuM inputs increases, and GC firing is enhanced. This study provides evidence that glutamatergic/GABAergic cotransmission balance is rapidly changed in an activity-dependent manner, and such plasticity may modulate DG activity.

Emerging evidence indicates that the functionally opposing neurotransmitters, glutamate and GABA, are coreleased from the same presynaptic terminals in some adult brain regions. The supramammillary nucleus (SuM) is one region that coreleases glutamate and GABA in the dentate gyrus (DG) through its afferents. Although the SuM-DG pathway has been implicated in various brain functions, little is known about the functional roles of the peculiar features of glutamate/GABA corelease. Here, we show that depolarization of granule cells (GCs) triggers postsynaptic long-term potentiation (LTP) of glutamatergic, but not GABAergic, cotransmission at SuM-GC synapses. Moreover, the burst activity of perforant-path inputs heterosynaptically induces LTP at excitatory SuM-GC synapses. This non-Hebbian LTP requires postsynaptic Ca²⁺ influx, Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) activity, and exocytosis of AMPA receptors. Glutamatergic transmission-selective expression of LTP increases the excitatory drive such that SuM inputs become sufficient to discharge GCs. Our results highlight a form of LTP, which dynamically and rapidly changes the glutamatergic/GABAergic cotransmission balance and contributes to DG network activity.

Corticohippocampal circuit dysfunction in a mouse model of Dravet syndrome

Dravet syndrome (DS) is a neurodevelopmental disorder due to pathogenic variants in SCN1A encoding the Nav1.1 sodium channel subunit, characterized by treatment-resistant epilepsy, temperature-sensitive seizures, developmental delay/intellectual disability with features of autism spectrum disorder, and increased risk of sudden death. Convergent data suggest hippocampal dentate gyrus (DG) pathology in DS (Scn1a+/-) mice. We performed two-photon calcium imaging in brain slice to uncover a profound dysfunction of filtering of perforant path input by DG in young adult Scn1a+/- mice. This was not due to dysfunction of DG parvalbumin inhibitory interneurons (PV-INs), which were only mildly impaired at this timepoint; however, we identified enhanced excitatory input to granule cells, suggesting that circuit dysfunction is due to excessive excitation rather than impaired inhibition. We confirmed that both optogenetic stimulation of entorhinal cortex and selective chemogenetic inhibition of DG PV-INs lowered seizure threshold in vivo in young adult Scn1a+/- mice. Optogenetic activation of PV-INs, on the other hand, normalized evoked responses in granule cells in vitro. These results establish the corticohippocampal circuit as a key locus of pathology in Scn1a+/- mice and suggest that PV-INs retain powerful inhibitory function and may be harnessed as a potential therapeutic approach toward seizure modulation.

Hypobaric Hypoxia Induces Deficits in Adult Neurogenesis and Social Interaction via Cyclooxygenase-1/ EP1 Receptor Pathway Activating NLRP3 Inflammasome

Low oxygen environments, like hypobaric hypoxia (HH), are common nodes in a number of diseases characterized by neuroinflammation, which is detrimental to the structural and functional aspects of hippocampal circuitry. Hypoxic conditions lead to elevation of inflammasome-mediated inflammation that may contribute to cognitive deficits. However, a systematic investigation of the impact of inflammasome-mediated neuroinflammation on the components of neurogenic niche during HH remains to be elusive. Cerebral hypoxia was induced in adult male Sprague Dawley rats via decreasing partial pressure of oxygen. The effect of HH (1, 3, and 7 days at 25,000 ft) on social memory, anxiety, adult neurogenesis, and NLRP3- (NLR family pyrin domain containing 3) mediated neuroinflammation in the dentate gyrus (DG) was explored in detail. Furthermore, we explored the therapeutic efficacy of cyclooxygenase-1 inhibitor (valeryl salicylate, 5 mg/kg/day, i.p.) and EP1 receptor (EP1R) antagonist (SC19220, 1 mg/kg/day, i.p.) on HH-induced deficits. Seven days of HH exposure induced alteration in social and anxiety-like behavior along with perturbation in adult neurogenesis. Elevation in NLRP3, caspase-1, and IL-1β levels was observed during HH from day 1. A notable increase in the COX-1/EP1R pathway in activated glial cells in DG was evident during HH. COX-1 inhibitor and EP1R antagonist mitigated the detrimental effects of HH on social memory, adult neurogenesis via blunting NLRP3-mediated inflammation. Our data showed induction of the COX-1/EP1R pathway in the glial cells, which is detrimental to neurogenesis and social memory, opening up the possibility that the COX-1/EP1R pathway is a plausible target for inflammasome-related neurogenesis impairments.

Leucine-rich repeats and immunoglobulin-like domains 1 deficiency affects hippocampal dendrite complexity and impairs cognitive function

Leucine-rich repeat (LRR) transmembrane proteins have been directly linked to neurodevelopmental and cognitive disorders. We have previously shown that the LRR transmembrane protein, leucine-rich repeats and immunoglobulin-like domains 1 (Lrig1), is a physiological regulator of dendrite complexity of hippocampal pyramidal neurons and social behavior. In this study, we performed a battery of behavioral tests to evaluate spatial memory and cognitive capabilities in Lrig1 mutant mice. The cognitive assessment demonstrated deficits in recognition and spatial memory, evaluated by novel object recognition and object location tests. Moreover, we found that Lrig1-deficient mice present specific impairments in the processing of similar but not dissimilar locations in a spatial pattern separation task, which was correlated with an enhanced dendritic growth and branching of Doublecortin-positive immature granule cells of the dentate gyrus. Altogether, these findings indicate that Lrig1 plays an essential role in controlling morphological and functional plasticity in the hippocampus.

Ion-channel regulation of response decorrelation in a heterogeneous multi-scale model of the dentate gyrus

Heterogeneities in biological neural circuits manifest in afferent connectivity as well as in local-circuit components such as neuronal excitability, neural structure and local synaptic strengths. The expression of adult neurogenesis in the dentate gyrus (DG) amplifies local-circuit heterogeneities and guides heterogeneities in afferent connectivity. How do neurons and their networks endowed with these distinct forms of heterogeneities respond to perturbations to individual ion channels, which are known to change under several physiological and pathophysiological conditions? We sequentially traversed the ion channels-neurons-network scales and assessed the impact of eliminating individual ion channels on conductance-based neuronal and network models endowed with disparate local-circuit and afferent heterogeneities. We found that many ion channels differentially contributed to specific neuronal or network measurements, and the elimination of any given ion channel altered several functional measurements. We then quantified the impact of ion-channel elimination on response decorrelation, a well-established metric to assess the ability of neurons in a network to convey complementary information, in DG networks endowed with different forms of heterogeneities. Notably, we found that networks constructed with structurally immature neurons exhibited functional robustness, manifesting as minimal changes in response decorrelation in the face of ion-channel elimination. Importantly, the average change in output correlation was dependent on the eliminated ion channel but invariant to input correlation. Our analyses suggest that neurogenesis-driven structural heterogeneities could assist the DG network in providing functional resilience to molecular perturbations.

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Perturbations at one scale result in a cascading impact on physiology across scales.

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Heterogeneous multi-scale models used to assess the impact of ion-channel deletion.

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Mapping of structural components to functional outcomes is many-to-many.

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Differential & variable impact of ion channel deletion on response decorrelation.

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Neurogenesis-induced structural heterogeneity confers resilience to perturbations.

GABAergic Input Affects Intracellular Calcium Levels in Developing Granule Cells of Adult Rat Hippocampus

In the dentate gyrus (DG) of the mammalian hippocampus, granule neurons are generated from neural stem cells (NSCs) throughout the life span and are integrated into the hippocampal network. Adult DG neurogenesis is regulated by multiple intrinsic and extrinsic factors that control NSC proliferation, maintenance, and differentiation into mature neurons. γ-Aminobutyric acid (GABA), released by local interneurons, regulates the development of neurons born in adulthood by activating extrasynaptic and synaptic GABA_A receptors. In the present work, patch-clamp and calcium imaging techniques were used to record very immature granule cells of adult rat dentate gyrus for investigating the actual role of GABA_A receptor activation in intracellular calcium level regulation at an early stage of maturation. Our findings highlight a novel molecular and electrophysiological mechanism, involving calcium-activated potassium channels (BK) and T-type voltage-dependent calcium channels, through which GABA fine-tunes intracellular calcium homeostasis in rat adult-born granule neurons early during their maturation. This mechanism might be instrumental in promoting newborn cell survival.

Amyloid pathology-produced unexpected modifications of calcium homeostasis in hippocampal subicular dendrites

INTRODUCTION

Alzheimer's disease (AD) is linked to neuronal calcium dyshomeostasis, which is associated with network hyperexcitability. Decreased expression of the calcium-binding protein cal- bindin-D28K (CB) might be a susceptibility factor for AD. The subiculum is affected early in AD, for unknown reasons.

METHODS

In AD, CB knock-out and control mice fluorescence Ca2+ imaging combined with patch clamp were used to characterize Ca2+ dynamics, resting Ca2+ , and Ca2+ -buffering capacity in subicular neurons. CB expression levels in wild-type and AD mice were also analyzed.

RESULTS

The subiculum and dentate gyrus of wild-type mice showed age-related decline in CB expression not observed in AD mice. Resting Ca2+ and Ca2+ -buffering capacity was increased in aged AD mice subicular dendrites. Modeling suggests that AD calcium changes can be explained by alterations of Ca2+ extrusion pumps rather than by buffers.

DISCUSSION

Overall, abnormal Ca2+ homeostasis in AD has an age dependency that comprises multiple mechanisms, including compensatory processes.

Mechanisms of Supralinear Calcium Integration in Dendrites of Hippocampal CA1 Fast-Spiking Cells

In fast-spiking (FS), parvalbumin-expressing interneurons of the CA1 hippocampus, activation of the GluA2-lacking Ca²⁺-permeable AMPA receptors (CP-AMPARs) in basal dendrites is coupled to Ca²⁺-induced Ca²⁺-release (CICR), and can result in a supralinear summation of postsynaptic Ca²⁺-transients (post-CaTs). While this mechanism is important in controlling the direction of long-term plasticity, it is still unknown whether it can operate at all excitatory synapses converging onto FS cells or at a set of synapses receiving a particular input. Using a combination of patch-clamp recordings and two-photon Ca²⁺ imaging in acute mouse hippocampal slices with computational simulations, here we compared the generation of supralinear post-CaTs between apical and basal dendrites of FS cells. We found that, similar to basal dendrites, apical post-CaTs summated supralinearly and relied mainly on the activation of the CP-AMPARs, with a variable contribution of other Ca²⁺ sources, such as NMDA receptors, L-type voltage-gated Ca²⁺-channels and Ca²⁺ release. In addition, supralinear post-CaTs generated in apical dendrites had a slower decay time and a larger cumulative charge than those in basal, and were associated with a stronger level of somatic depolarization. The model predicted that modulation of ryanodine receptors and of the Ca²⁺ extrusion mechanisms, such as the Na⁺/Ca²⁺-exchanger and SERCA pump, had a major impact on the magnitude of supralinear post-CaTs. These data reveal that supralinear Ca²⁺ summation is a common mechanism of Ca²⁺ signaling at CP-AMPAR-containing synapses. Shaped in a location-specific manner through modulation of ryanodine receptors and Ca²⁺ extrusion mechanisms, CP-AMPAR/CICR signaling is suitable for synapse-specific bidirectional modification of incoming inputs in the absence of active dendritic conductances.

Cell-Biological Requirements for the Generation of Dentate Gyrus Granule Neurons

The dentate gyrus (DG) receives highly processed information from the associative cortices functionally integrated in the trisynaptic hippocampal circuit, which contributes to the formation of new episodic memories and the spontaneous exploration of novel environments. Remarkably, the DG is the only brain region currently known to have high rates of neurogenesis in adults (Andersen et al., 1966, 1971). The DG is involved in several neurodegenerative disorders, including clinical dementia, schizophrenia, depression, bipolar disorder and temporal lobe epilepsy. The principal neurons of the DG are the granule cells. DG granule cells generated in culture would be an ideal model to investigate their normal development and the causes of the pathologies in which they are involved and as well as possible therapies. Essential to establish such in vitro models is the precise definition of the most important cell-biological requirements for the differentiation of DG granule cells. This requires a deeper understanding of the precise molecular and functional attributes of the DG granule cells in vivo as well as the DG cells derived in vitro. In this review we outline the neuroanatomical, molecular and cell-biological components of the granule cell differentiation pathway, including some growth- and transcription factors essential for their development. We summarize the functional characteristics of DG granule neurons, including the electrophysiological features of immature and mature granule cells and the axonal pathfinding characteristics of DG neurons. Additionally, we discuss landmark studies on the generation of dorsal telencephalic precursors from pluripotent stem cells (PSCs) as well as DG neuron differentiation in culture. Finally, we provide an outlook and comment critical aspects.

T2N as a new tool for robust electrophysiological modeling demonstrated for mature and adult-born dentate granule cells

Compartmental models are the theoretical tool of choice for understanding single neuron computations. However, many models are incomplete, built ad hoc and require tuning for each novel condition rendering them of limited usability. Here, we present T2N, a powerful interface to control NEURON with Matlab and TREES toolbox, which supports generating models stable over a broad range of reconstructed and synthetic morphologies. We illustrate this for a novel, highly detailed active model of dentate granule cells (GCs) replicating a wide palette of experiments from various labs. By implementing known differences in ion channel composition and morphology, our model reproduces data from mouse or rat, mature or adult-born GCs as well as pharmacological interventions and epileptic conditions. This work sets a new benchmark for detailed compartmental modeling. T2N is suitable for creating robust models useful for large-scale networks that could lead to novel predictions. We discuss possible T2N application in degeneracy studies.

Synaptic integrative mechanisms for spatial cognition

Synaptic integrative mechanisms have profound effects on electrical signaling in the brain that, although largely hidden from recording methods that observe the spiking activity of neurons, may be critical for the encoding, storage and retrieval of information. Here we review roles for synaptic integrative mechanisms in the selection, generation and plasticity of place and grid fields, and in related temporal codes for the representation of space. We outline outstanding questions and challenges in the testing of hypothesized models for spatial computation and memory.

Neural mechanisms underlying GABAergic regulation of adult hippocampal neurogenesis

Within the dentate gyrus of the adult hippocampus is the subgranular zone, which contains a neurogenic niche for radial-glia like cells, the most primitive neural stem cells in the adult brain. The quiescence of neural stem cells is maintained by tonic gamma-aminobutyric acid (GABA) released from local interneurons. Once these cells differentiate into neural progenitor cells, GABA continues to regulate their development into mature granule cells, the principal cell type of the dentate gyrus. Here, we review the role of GABA circuits, signaling, and receptors in regulating development of adult-born cells, as well as the molecular players that modulate GABA signaling. Furthermore, we review recent findings linking dysregulation of adult hippocampal neurogenesis to the altered GABAergic circuitry and signaling under various pathological conditions.

Mature granule cells of the dentate gyrus--Passive bystanders or principal performers in hippocampal function?

The dentate gyrus is the main entrance of highly processed information to the hippocampus which derives from associative cortices and it is one of the few privileged areas in the brain where adult neurogenesis occurs. This creates the unique situation that neurons of diverse maturation stages are part of one neuronal network at any given point in life. While recently adult-born cells have a low induction threshold for long-term potentiation several studies suggest that following maturation granule cells are poorly excitable and they exhibit reduced Hebbian synaptic plasticity to an extent that it was even suggested that they functionally retire. Here, we review the functional properties of mature granule cells and discuss how plasticity of intrinsic excitability and alterations in excitation-inhibition balance might impact on their role in hippocampal information processing.

Priming Spatial Activity by Single-Cell Stimulation in the Dentate Gyrus of Freely Moving Rats

An essential requirement for hippocampal circuits to function in episodic memory is the ability to rapidly disambiguate and store incoming sensory information. This "pattern separation" function has been classically associated to the dentate gyrus, where spatial learning is accompanied by rapid and persistent modifications of place-cell representation. How these rapid modifications are implemented at the cellular level has remained largely unresolved. Here, we tested whether plasticity-inducing stimuli--spike trains--evoked in postsynaptic neurons are sufficient for the rapid induction of place-field activity in the dentate gyrus. We juxtacellularly stimulated 67 silent granule cells while rats explored a maze for the first time. Spike trains with different characteristics (e.g., number of spikes, frequency, and theta-rhythmicity) were evoked at randomly selected spatial locations. We found that, under novelty, ∼30% (10/33) of the stimulated neurons fired selectively at the "primed" spatial location on subsequent laps. Induced place fields were either transient or persisted for multiple laps. The "priming" effect was experience dependent, as it was less frequently observed in habituated animals (3/34 neurons), and it correlated with the number of spikes and theta-rhythmicity of the stimulus trains. These data indicate that, albeit with low efficiency, evoked theta-rhythmic spike trains can be sufficient for priming spatial activity in the dentate gyrus and thus recruiting silent granule cells into the coding population.

Bidirectional GABAergic control of action potential firing in newborn hippocampal granule cells

Newly generated young neurons in the adult hippocampus receive GABAergic synaptic inputs, which are crucial for activity-dependent survival and functional maturation between 1-3 weeks after mitosis. We found synaptically driven action potential (AP) firing in these newborn young cells in adult mice. Although glutamatergic synaptic inputs remained subthreshold, activation of GABAergic synaptic inputs depolarized young neurons and reliably evoked APs. Furthermore, pairing of subthreshold excitatory postsynaptic potentials or somatic current injection with brief bursts of GABAergic inputs revealed efficient GABAergic excitation at conductances of ∼ 1.5 nS, corresponding to the activity of only three or four interneurons. Stronger GABAergic inputs (>4 nS) effectively blocked AP firing via shunting inhibition, which might be important to dynamically control spiking output in both directions. Taken together, GABAergic interneurons differentially recruit newborn young granule cells by supporting either AP generation or shunting inhibition dependent on hippocampal network activity.

Maturation and Functional Integration of New Granule Cells into the Adult Hippocampus

The adult hippocampus generates functional dentate granule cells (GCs) that release glutamate onto target cells in the hilus and cornus ammonis (CA)3 region, and receive glutamatergic and γ-aminobutyric acid (GABA)ergic inputs that tightly control their spiking activity. The slow and sequential development of their excitatory and inhibitory inputs makes them particularly relevant for information processing. Although they are still immature, new neurons are recruited by afferent activity and display increased excitability, enhanced activity-dependent plasticity of their input and output connections, and a high rate of synaptogenesis. Once fully mature, new GCs show all the hallmarks of neurons generated during development. In this review, we focus on how developing neurons remodel the adult dentate gyrus and discuss key aspects that illustrate the potential of neurogenesis as a mechanism for circuit plasticity and function.

Buffer mobility and the regulation of neuronal calcium domains

The diffusion of calcium inside neurons is determined in part by the intracellular calcium binding species that rapidly bind to free calcium ions upon entry. It has long been known that some portion of a neuron's intracellular calcium binding capacity must be fixed or poorly mobile, as calcium diffusion is strongly slowed in the intracellular environment relative to diffusion in cytosolic extract. The working assumption was that these immobile calcium binding sites are provided by structural proteins bound to the cytoskeleton or intracellular membranes and may thereby be relatively similar in composition and capacity across different cell types. However, recent evidence suggests that the immobile buffering capacity can vary greatly between cell types and that some mobile calcium binding proteins may alter their mobility upon binding calcium, thus blurring the line between mobile and immobile. The ways in which immobile buffering capacity might be relevant to different calcium domains within neurons has been explored primarily through modeling. In certain regimes, the presence of immobile buffers and the interaction between mobile and immobile buffers have been shown to result in complex spatiotemporal patterns of free calcium. In total, these experimental and modeling findings call for a more nuanced consideration of the local intracellular calcium microenvironment. In this review we focus on the different amounts, affinities, and mobilities of immobile calcium binding species; propose a new conceptual category of physically diffusible but functionally immobile buffers; and discuss how these buffers might interact with mobile calcium binding partners to generate characteristic calcium domains.

A single-compartment model of calcium dynamics in nerve terminals and dendrites

This introduction describes a single-compartment model of calcium dynamics that has been applied to fluorescence measurements of intracellular free calcium concentration ([Ca(2+)]i) changes in neurons. The model describes intracellular calcium handling under simplified conditions, for which analytical expressions for the amplitude and the time constants of [Ca(2+)]i changes can be explicitly derived. In particular, it reveals the dependence of the measured [Ca(2+)]i changes on the calcium indicator concentration. Applied to experimental data from small cells or subcellular compartments, the model equations have been extremely useful for obtaining quantitative information about essential parameters of Ca(2+) influx, buffering, and clearance. We illustrate also several changes that occur when the basic assumptions do not hold (e.g., when calcium diffusion, dye saturation, or kinetic effects become significant). Finally, we discuss how the changes in calcium dynamics, which are explained by the model, have been exploited for measuring properties of calcium-driven reactions, such as those regulating short-term synaptic enhancement, vesicle recycling, and adaptation.

Supralinear dendritic Ca(2+) signalling in young developing CA1 pyramidal cells

Although Ca(2+) is critically important in activity-dependent neuronal development, not much is known about the regulation of dendritic Ca(2+) signals in developing neurons. Here, we used ratiometric Ca(2+) imaging to investigate dendritic Ca(2+) signalling in rat hippocampal pyramidal cells during the first 1-4 weeks of postnatal development. We show that active dendritic backpropagation of Nav channel-dependent action potentials (APs) evoked already large dendritic Ca(2+) transients in animals aged 1 week with amplitudes of ∼150 nm, similar to the amplitudes of ∼160 nM seen in animals aged 4 weeks. Although the AP-evoked dendritic Ca(2+) load increased about four times during the first 4 weeks, the peak amplitude of free Ca(2+) concentration was balanced by a four-fold increase in Ca(2+) buffer capacity κs (∼70 vs. ∼280). Furthermore, Ca(2+) extrusion rates increased with postnatal development, leading to a slower decay time course (∼0.2 s vs. ∼0.1 s) and more effective temporal summation of Ca(2+) signals in young cells. Most importantly, during prolonged theta-burst stimulation dendritic Ca(2+) signals were up to three times larger in cells at 1 week than at 4 weeks of age and much larger than predicted by linear summation, which is attributable to an activity-dependent slow-down of Ca(2+) extrusion. As Ca(2+) influx is four-fold smaller in young cells, the larger Ca(2+) signals are generated using four times less ATP consumption. Taken together, the data suggest that active backpropagations regulate dendritic Ca(2+) signals during early postnatal development. Remarkably, during prolonged AP firing, Ca(2+) signals are several times larger in young than in mature cells as a result of activity-dependent regulation of Ca(2+) extrusion rates.

Tuning local calcium availability: cell-type-specific immobile calcium buffer capacity in hippocampal neurons

It has remained difficult to ascribe a specific functional role to immobile or fixed intracellular calcium buffers in central neurons because the amount of these buffers is unknown. Here, we explicitly isolated the fixed buffer fraction by prolonged whole-cell patch-clamp dialysis and quantified its buffering capacity in murine hippocampal slices using confocal calcium imaging and the "added-buffer" approach. In dentate granule cells, the calcium binding ratio (κ) after complete washout of calbindin D28k (Cb), κfixed, displayed a substantial value of ∼100. In contrast, in CA1 oriens lacunosum moleculare (OLM) interneurons, which do not contain any known calcium-binding protein(s), κfixed amounted to only ∼30. Based on these values, a theoretical analysis of dendritic spread of calcium after local entry showed that fixed buffers, in the absence of mobile species, decrease intracellular calcium mobility 100- and 30-fold in granule cells and OLM cells, respectively, and thereby strongly slow calcium signals. Although the large κfixed alone strongly delays the spread of calcium in granule cells, this value optimizes the benefits of additionally expressing the mobile calcium binding protein Cb. With such high κfixed, Cb effectively increases the propagation velocity to levels seen in OLM cells and, contrary to expectation, does not affect the peak calcium concentration close to the source but sharpens the spatial and temporal calcium gradients. The data suggest that the amount of fixed buffers determines the temporal availability of calcium for calcium-binding partners and plays a pivotal role in setting the repertoire of cellular calcium signaling regimens.

Functional properties of extrasynaptic AMPA and NMDA receptors during postnatal hippocampal neurogenesis

In the mammalian hippocampus, new granule cells are continuously generated throughout life. Although it is well known that they rapidly form several thousand new glutamatergic synapses, the underlying mechanisms are not well understood. As extrasynaptic NMDA receptors are believed to support the generation of new spines, we have studied the functional properties of extrasynaptic ionotropic glutamate receptors in newborn granule cells in juvenile rats during and after synaptic integration. Using the fast application of glutamate to outside-out membrane patches, we show that all immature granule cells express functional AMPA and NMDA receptors. The density of AMPA receptors was small in cells starting to receive excitatory synaptic input (∼30 pS μm(-2)) but substantially increased during synaptic integration to finally reach ∼120 pS μm(-2) in fully mature cells. Interestingly, AMPA receptors showed a biphasic change in desensitization time constant which was slowest during synaptic integration and substantially faster before and afterwards. This was paralleled by a change in the non-desensitizing current component which was maximal during synaptic integration and about 50% smaller afterwards. Surprisingly, the NMDA receptor kinetics and density in young cells was already comparable to mature cells (∼10 pS μm(-2)), leading to an enhanced NMDA/AMPA receptor density ratio. Similar to somatic outside-out patches, iontophoretic application of glutamate onto dendrites also revealed an enhanced dendritic NMDA/AMPA ratio in young cells. These data indicate that prolonged AMPA receptor currents in newly generated young granule cells might support the effective activation of extrasynaptic NMDA receptors and therefore constitute a competitive advantage over mature cells for new synapse formation.

cAMP promotes the differentiation of neural progenitor cells in vitro via modulation of voltage-gated calcium channels

The molecular mechanisms underlying the differentiation of neural progenitor cells (NPCs) remain poorly understood. In this study we investigated the role of Ca²⁺ and cAMP (cyclic adenosine monophosphate) in the differentiation of NPCs extracted from the subventricular zone of E14.5 rat embryos. Patch clamp recordings revealed that increasing cAMP-signaling with Forskolin or IBMX (3-isobutyl-1-methylxantine) significantly facilitated neuronal functional maturation. A continuous application of IBMX to the differentiation medium substantially increased the functional expression of voltage-gated Na⁺ and K⁺ channels, as well as neuronal firing frequency. Furthermore, we observed an increase in the frequency of spontaneous synaptic currents and in the amplitude of evoked glutamatergic and GABAergic synaptic currents. The most prominent acute effect of applying IBMX was an increase in L-type Ca²⁺currents. Conversely, blocking L-type channels strongly inhibited dendritic outgrowth and synapse formation even in the presence of IBMX, indicating that voltage-gated Ca²⁺ influx plays a major role in neuronal differentiation. Finally, we found that nifedipine completely blocks IBMX-induced CREB phosphorylation (cAMP-response-element-binding protein), indicating that the activity of this important transcription factor equally depends on both enhanced cAMP and voltage-gated Ca²⁺-signaling. Taken together, these data indicate that the up-regulation of voltage-gated L-type Ca²⁺-channels and early electrical excitability are critical steps in the cAMP-dependent differentiation of SVZ-derived NPCs into functional neurons. To our knowledge, this is the first demonstration of the acute effects of cAMP on voltage-gated Ca⁺²channels in NPC-derived developing neurons.

GABA depolarization is required for experience-dependent synapse unsilencing in adult-born neurons

Neural activity enhances adult neurogenesis, enabling experience to influence the construction of new circuits. GABAA receptor-mediated depolarization of newborn neurons in the adult and developing brain promotes glutamatergic synaptic integration since chronic reduction of GABA depolarization impairs morphological maturation and formation of glutamatergic synapses. Here we demonstrate an acute role of GABA depolarization in glutamatergic synaptic integration. Using proopiomelanocortin enhanced-green fluorescent protein reporter mice, we identify a developmental stage when adult-generated neurons have glutamatergic synaptic transmission mediated solely by NMDA receptors (NMDARs), representing the initial silent synapses before AMPA receptor (AMPAR)-mediated functional transmission. We show that pairing synaptic stimulation with postsynaptic depolarization results in synapse unsilencing that requires NMDAR activation. GABA synaptic depolarization enables activation of NMDARs in the absence of AMPAR-mediated transmission and is required for synapse unsilencing induced by synaptic activity in vitro as well as a brief exposure to an enriched environment in vivo. The rapid appearance of AMPAR-mediated EPSCs and the lack of maturational changes show that GABA depolarization acutely allows NMDAR activation required for initial synapse unsilencing. Together, these results also reveal that adult-generated neurons in a critical period for survival use GABA signaling to rapidly initiate functional glutamate-mediated transmission in response to experience.

Neurogenesis in the dentate gyrus: carrying the message or dictating the tone

The dentate gyrus (DG) is a region in the mammalian brain critical for memory encoding with a neuronal architecture and function that deviates considerably from other cortical areas. One of the major differences of the DG compared to other brain regions is the finding that the dentate gyrus generates new principal neurons that are continuously integrated into a fully functional neural circuit throughout life. Another distinguishing characteristic of the dentate network is that the majority of principal neurons are held under strong inhibition and rarely fire action potentials. These two findings raise the question why a predominantly silent network would need to continually incorporate more functional units. The sparse nature of the neural code in the DG is thought to be fundamental to dentate network function, yet the relationship between neurogenesis and low activity levels in the network remains largely unknown. Clues to the functional role of new neurons come from inquiries at the cellular as well as the behavioral level. Few studies have bridged the gap between these levels of inquiry by considering the role of young neurons within the complex dentate network during distinct stages of memory processing. We will review and discuss from a network perspective, the functional role of immature neurons and how their unique cellular properties can modulate the dentate network in memory guided behaviors.

Properties of doublecortin expressing neurons in the adult mouse dentate gyrus

The dentate gyrus is a neurogenic zone where neurons continue to be born throughout life, mature and integrate into the local circuitry. In adults, this generation of new neurons is thought to contribute to learning and memory formation. As newborn neurons mature, they undergo a developmental sequence in which different stages of development are marked by expression of different proteins. Doublecortin (DCX) is an early marker that is expressed in immature granule cells that are beginning migration and dendritic growth but is turned off before neurons reach maturity. In the present study, we use a mouse strain in which enhanced green fluorescent protein (EGFP) is expressed under the control of the DCX promoter. We show that these neurons have high input resistances and some cells can discharge trains of action potentials. In mature granule cells, action potentials are followed by a slow afterhyperpolarization that is absent in EGFP-positive neurons. EGFP-positive neurons had a lower spine density than mature neurons and stimulation of either the medial or lateral perforant pathway activated dual component glutamatergic synapses that had both AMPA and NMDA receptors. NMDA receptors present at these synapses had slow kinetics and were blocked by ifenprodil, indicative of high GluN2B subunit content. These results show that EGFP-positive neurons in the DCX-EGFP mice are functionally immature both in their firing properties and excitatory synapses.

Myenteric neurons of the mouse small intestine undergo significant electrophysiological and morphological changes during postnatal development

Organized motility patterns in the gut depend on circuitry within the enteric nervous system (ENS), but little is known about the development of electrophysiological properties and synapses within the ENS. We examined the electrophysiology and morphology of myenteric neurons in the mouse duodenum at three developmental stages: postnatal day (P)0, P10–11, and adult. Like adults, two main classes of neurons could be identified at P0 and P10–11 based on morphology: neurons with multiple long processes that projected circumferentially (Dogiel type II morphology) and neurons with a single long process. However, postnatal Dogiel type II neurons differed in several electrophysiological properties from adult Dogiel type II neurons. P0 and P10–11 Dogiel type II neurons exhibited very prominent Ca(2+)-mediated after depolarizing potentials (ADPs) following action potentials compared to adult neurons. Adult Dogiel type II neurons are characterized by the presence of a prolonged after hyperpolarizing potential (AHP), but AHPs were very rarely observed at P0. The projection lengths of the long processes of Dogiel type II neurons were mature by P10–11. Uniaxonal neurons in adults typically have fast excitatory postsynaptic potentials (fEPSPs, ‘S-type' electrophysiology) mainly mediated by nicotinic receptors. Nicotinic-fEPSPs were also recorded from neurons with a single long process at P0 and P10–11. However, these neurons underwent major developmental changes in morphology, from predominantly filamentous neurites at birth to lamellar dendrites in mature mice. Unlike Dogiel type II neurons, the projection lengths of neurons with a single long process matured after P10–11. Slow EPSPs were rarely observed in P0/P10–11 neurons. This work shows that, although functional synapses are present and two classes of neurons can be distinguished electrophysiologically and morphologically at P0, major changes in electrophysiological properties and morphology occur during the postnatal development of the ENS.

Adult neurogenesis and the plasticity of the dentate gyrus network

The granule cell layer (GCL) of the dentate gyrus contains neurons generated during embryonic, early postnatal and adult life. During adulthood there is a continuous production of neuronal cohorts that develop and functionally integrate in the preexisting circuits. This morphogenic process generates a stratified GCL, with the outermost layers containing dentate granule cells (DGCs) generated during perinatal life, and the innermost layers containing adult-born DGCs. In this review we analyse the functional profile of the different neuronal populations of the GCL, with an emphasis on adult-born neurons as they develop, mature and integrate in the dentate gyrus network. We focus on the contribution of adult-born neurons to activity-dependent synaptic modification in the dentate gyrus and, in turn, discuss how network activity modulates integration and survival of new neurons.

M(1)-like muscarinic acetylcholine receptors regulate fast-spiking interneuron excitability in rat dentate gyrus

Cholinergic transmission through muscarinic acetylcholine receptors (mAChRs) plays a key role in cortical oscillations. Although fast-spiking (FS), parvalbumin-expressing basket cells (BCs) are proposed to be the cellular substrates of gamma oscillations, previous studies reported that FS nonpyramidal cells in neocortical areas are unresponsive to cholinergic modulation. Dentate gyrus (DG) is an independent gamma oscillator in the hippocampal formation. However, in contrast to other cortical regions, the direct impact of mAChR activation on FS BC excitability in this area has not been investigated. Here, we show that bath-applied muscarine or carbachol, two mAChR agonists, depolarize DG BCs in the acute brain slices, leading to action potential firing in the theta-gamma bands in the presence of blockers of ionotropic glutamate and gamma-aminobutyric acid type A receptors at physiological temperatures. The depolarizing action persists in the presence of tetrodotoxin, a voltage-gated Na(+) channel blocker. In voltage-clamp recordings, muscarine markedly reduces background K(+) currents. These effects are mimicked by oxotremorine methiodide, an mAChR-specific agonist, and largely reversed by atropine, a non-selective mAChR antagonist, or pirenzepine, an M(1) receptor antagonist, but not by gallamine, an M(2/4) receptor antagonist. Interestingly, in contrast to M(1)-receptor-mediated depolarization, M(2) receptor activation by the specific agonist arecaidine but-2-ynyl ester tosylate down-regulates GABA release at BC axons-the effect is occluded by gallamine, an M(2) receptor antagonist. Overall, muscarinic activation results in a net increase in phasic inhibitory output to the target cells. Thus, cholinergic activation through M(1)-like receptor enhances BC activity and promotes the generation of nested theta and gamma rhythms, thereby enhancing hippocampal function and associated performance.

Full system bifurcation analysis of endocrine bursting models

Plateau bursting is typical of many electrically excitable cells, such as endocrine cells that secrete hormones and some types of neurons that secrete neurotransmitters. Although in many of these cell types the bursting patterns are regulated by the interplay between voltage-gated calcium channels and calcium-sensitive potassium channels, they can be very different. We investigate so-called square-wave and pseudo-plateau bursting patterns found in endocrine cell models that are characterized by a super- or subcritical Hopf bifurcation in the fast subsystem, respectively. By using the polynomial model of Hindmarsh and Rose (Proceedings of the Royal Society of London B 221 (1222) 87-102), which preserves the main properties of the biophysical class of models that we consider, we perform a detailed bifurcation analysis of the full fast-slow system for both bursting patterns. We find that both cases lead to the same possibility of two routes to bursting, that is, the criticality of the Hopf bifurcation is not relevant for characterizing the route to bursting. The actual route depends on the relative location of the full-system's fixed point with respect to a homoclinic bifurcation of the fast subsystem. Our full-system bifurcation analysis reveals properties of endocrine bursting that are not captured by the standard fast-slow analysis.

Estimating intracellular Ca2+ concentrations and buffering in a dendritic inhibitory hippocampal interneuron

Calcium is known to regulate several phenomena like neuronal excitability and plasticity. Interestingly, the spatiotemporal profile of dendritic calcium depends on several processes, specific to each neuronal type. In this study, we investigated Ca(2+) buffering and action potential (AP)-evoked Ca(2+) signaling in the dendrites of anatomically identified oriens lacunosum-moleculare (O-LM) cells, a major type of dendrite-targeting interneurons in the hippocampal CA1 region, using a combination of whole-cell patch-clamp recording and fast Ca(2+) imaging in acute rat brain slices. Cells were loaded with fluorescent Ca(2+) indicators fura-2 or Oregon Green BAPTA-1 (OGB-1) via patch-clamping electrode, and the effect of fura-2 on AP-evoked dendritic Ca(2+) transients was determined by ratiometric Ca(2+) imaging. To estimate intracellular Ca(2+) concentrations ([Ca(2+)](i)) and endogenous Ca(2+)-binding ratio (kappa(s)) in the proximal dendrite, fluorescence signals were converted into [Ca(2+)](i) using the ratioing method and were analyzed on the basis of the "single compartment model." Resting [Ca(2+)](i) was 22+/-5 nM and the build-up of [Ca(2+)](i) during a single AP was up to 656+/-226 nM. Analysis of Ca(2+) transients revealed that O-LM cells have a relatively low endogenous Ca(2+)-binding ratio (kappa(s)): the kappa(s) was 20+/-8 estimated during fura-2 loading and 27 estimated under steady-state fura-2 concentrations, respectively. To further examine the spatial profile of dendritic Ca(2+) transients, we measured somatic AP-evoked Ca(2+) transients beyond proximal dendrites using OGB-1. Dendritic Ca(2+) transients evoked by single APs or AP trains are not limited to regions close to the soma. The amplitude and decay of [Ca(2+)](i) associated with backpropagating APs are relatively independent of the distance from the soma. In sum, O-LM cells exhibit low endogenous Ca(2+)-binding ratios and relatively distance-independent Ca(2+) dynamics in the dendrites. These special features of Ca(2+) signaling in O-LM cells may have important functional implications for both normal and pathological conditions.

Endogenous calcium buffering capacity of substantia nigral dopamine neurons

Dopamine (DA)-containing cells from the substantia nigra pars compacta (SNc) play a major role in the initiation of movement. Loss of these cells results in Parkinson's disease (PD). Changes in intracellular calcium ion concentration ([Ca(2+)](i)) elicit several events in DA cells, including spike afterhyperpolarizations (AHPs) and subthreshold oscillations underlying autonomous firing. Continuous Ca(2+) load due to Ca(2+)-dependent rhythmicity has been proposed to cause the death of DA cells in PD and normal aging. Because of the physiological and pathophysiological importance of [Ca(2+)](i) in DA cells, we characterized their intrinsic Ca(2+)-buffering capacity (K(S)) in brain slices. We introduced a fluorescent Ca(2+)-sensitive exogenous buffer (200 microM fura-2) and cells were tracked from break-in until steady state by stimulating with a single action potential (AP) every 30 s and measuring the Ca(2+) transient from the proximal dendrite. DA neurons filled exponentially with a tau of about 5-6 min. [Ca(2+)](i) was assumed to equilibrate between the endogenous Ca(2+) buffer and the exogenous Ca(2+) indicator buffer. Intrinsic buffering was estimated by extrapolating from the linear relationships between the amplitude or time constant of the Ca(2+) transients versus [fura-2]. Extrapolated Ca(2+)-transients in the absence of fura-2 had mean peak amplitudes of 293.7 +/- 65.3 nM and tau = 124 +/- 13 ms (postnatal day 13 [P13] to P17 animals). Intrinsic buffering increased with age in DA neurons. For cells from animals P13-P17, K(S) was estimated to be about 110 (n = 20). In older animals (P25-P32), the estimate was about 179 (n = 10). These relatively low values may reflect the need for rapid Ca(2+) signaling, e.g., to allow activation of sK channels, which shape autonomous oscillations and burst firing. Low intrinsic buffering may also make DA cells vulnerable to Ca(2+)-dependent pathology.

Calcium buffering in rodent olfactory bulb granule cells and mitral cells

In the mammalian olfactory bulb, axonless granule cells (GCs) mediate self- and lateral inhibitory interactions between mitral cells (MCs) via reciprocal dendrodendritic synapses. Calcium signals in the GC dendrites and reciprocal spines appear to decay unusually slowly, hence GC calcium handling might contribute to the known asynchronous release at this synapse. By recording fluorescence transients of different Ca(2+)-sensitive dyes at variable concentrations evoked by backpropagating action potentials (APs) and saturating AP trains we extrapolated Ca(2+) dynamics to conditions of zero added buffer for juvenile rat GC apical dendrites and spines and MC lateral dendrites. Resting [Ca(2+)] was at approximately 50 nM in both GC dendrites and spines. The average endogenous GC buffer capacities (kappa(E)) were within a range of 80-90 in the dendrites and 110-140 in the spines. The extrusion rate (gamma) was estimated as 570 s(-1) for dendrites and 870 s(-1) for spines and the decay time constant as approximately 200 ms for both. Single-current-evoked APs resulted in a [Ca(2+)] elevation of approximately 250 nM. Calcium handling in juvenile and adult mouse GCs appeared mostly similar. In MC lateral dendrites, we found AP-mediated [Ca(2+)] elevations of approximately 130 nM with a similar decay to that in GC dendrites, while kappa(E) and gamma were roughly 4-fold higher. In conclusion, the slow GC Ca(2+) dynamics are due mostly to sluggish Ca(2+) extrusion. Under physiological conditions this slow removal may well contribute to delayed release and also feed into other Ca(2+)-dependent mechanisms that foster asynchronous output from the reciprocal spine.

Characterization of somatic Ca2+ clearance mechanisms in young and mature hippocampal granule cells

Calcium is a key regulator for expression of genes relevant to survival and maturation of newborn neurons. Mammalian hippocampal dentate gyrus generates new granule cells (GCs) throughout adult life. We identified young and mature GCs in hippocampi of young adult mice according to their electrical properties, and investigated contributions of Na/Ca exchanger (NCX), sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA), plasma membrane Ca(2+)-ATPase (PMCA) and mitochondria to Ca(2+) clearance in somata of GCs. Somatic Ca(2+) clearance was increased by about 50% as GCs matured. NCX activity increased proportionally during maturation with its relative contribution kept about 40% both in young and mature GCs. On the other hand, the developmental increases in activities of mitochondria and SERCA resulted in higher contributions to Ca(2+) clearance in mature GCs than in young GCs. Especially mitochondrial function was most highly enhanced during maturation. PMCA activity, however, did not increase during maturation. Low Ca(2+) clearance in immature GCs might facilitate higher Ca(2+) accumulation during network activity, which in turn help survival of young GCs.