Heterogeneity of Age-Related Neural Hyperactivity along the CA3 Transverse Axis.

Age-related memory deficits are correlated with neural hyperactivity in the CA3 region of the hippocampus. Abnormal CA3 hyperactivity in aged rats has been proposed to contribute to an imbalance between pattern separation and pattern completion, resulting in overly rigid representations. Recent evidence of functional heterogeneity along the CA3 transverse axis suggests that proximal CA3 supports pattern separation while distal CA3 supports pattern completion. It is not known whether age-related CA3 hyperactivity is uniformly represented along the CA3 transverse axis. We examined the firing rates of CA3 neurons from young and aged, male, Long-Evans rats along the CA3 transverse axis. Consistent with prior studies, young CA3 cells showed an increasing gradient in mean firing rate from proximal to distal CA3. However, aged CA3 cells showed an opposite, decreasing trend, in that CA3 cells in aged rats were hyperactive in proximal CA3, but possibly hypoactive in distal CA3, compared with young (Y) rats. We suggest that, in combination with altered inputs from the entorhinal cortex and dentate gyrus (DG), the proximal CA3 region of aged rats may switch from its normal function that reflects the pattern separation output of the DG and instead performs a computation that reflects an abnormal bias toward pattern completion. In parallel, distal CA3 of aged rats may create weaker attractor basins that promote abnormal, bistable representations under certain conditions.SIGNIFICANCE STATEMENT Prior work suggested that age-related CA3 hyperactivity enhances pattern completion, resulting in rigid representations. Implicit in prior studies is the notion that hyperactivity is present throughout a functionally homogeneous CA3 network. However, more recent work has demonstrated functional heterogeneity along the CA3 transverse axis, in that proximal CA3 is involved in pattern separation and distal CA3 is involved in pattern completion. Here, we show that age-related hyperactivity is present only in proximal CA3, with potential hypoactivity in distal CA3. This result provides new insight in the role of CA3 in age-related memory impairments, suggesting that the rigid representations in aging result primarily from dysfunction of computational circuits involving the dentate gyrus (DG) and proximal CA3.

Na+-K+-ATPase functions in the developing hippocampus: regional differences in CA1 and CA3 neuronal excitability and role in epileptiform network bursting.

The Na+-K+-ATPase (Na+-K+ pump) is essential for setting resting membrane potential and restoring transmembrane Na+ and K+ gradients after neuronal firing, yet its roles in developing neurons are not well understood. This study examined the contribution of the Na+-K+ pump to resting membrane potential and membrane excitability of developing CA1 and CA3 neurons and its role in maintaining synchronous network bursting. Experiments were conducted in postnatal day (P)9 to P13 rat hippocampal slices using whole cell patch-clamp and extracellular field-potential recordings. Blockade of the Na+-K+ pump with strophanthidin caused marked depolarization (23.1 mV) in CA3 neurons but only a modest depolarization (3.3 mV) in CA1 neurons. Regarding other membrane properties, strophanthidin differentially altered the voltage-current responses, input resistance, action-potential threshold and amplitude, rheobase, and input-output relationship in CA3 vs. CA1 neurons. At the network level, strophanthidin stopped synchronous epileptiform bursting in CA3 induced by 0 Mg2+ and 4-aminopyridine. Furthermore, dual whole cell recordings revealed that strophanthidin disrupted the synchrony of CA3 neuronal firing. Finally, strophanthidin reduced spontaneous excitatory postsynaptic current (sEPSC) bursts (i.e., synchronous transmitter release) and transformed them into individual sEPSC events (i.e., nonsynchronous transmitter release). These data suggest that the Na+-K+ pump plays a more profound role in membrane excitability in developing CA3 neurons than in CA1 neurons and that the pump is essential for the maintenance of synchronous network bursting in CA3. Compromised Na+-K+ pump function leads to cessation of ongoing synchronous network activity, by desynchronizing neuronal firing and neurotransmitter release in the CA3 synaptic network. These findings have implications for the regulation of network excitability and seizure generation in the developing brain.NEW & NOTEWORTHY Despite the extensive literature showing the importance of the Na+-K+ pump in various neuronal functions, its roles in the developing brain are not well understood. This study reveals that the Na+-K+ pump differentially regulates the excitability of CA3 and CA1 neurons in the developing hippocampus, and the pump activity is crucial for maintaining network activity. Compromised Na+-K+ pump activity desynchronizes neuronal firing and transmitter release, leading to cessation of ongoing epileptiform network bursting.

Developmental regulation and lateralisation of the α7 and α4 subunits of nicotinic acetylcholine receptors in developing rat hippocampus.

The purpose of this study was to describe the distinct regional distribution patterns of expression of the α7 and α4 subunits of nicotinic acetylcholine receptors (nAChRs) and their left-right lateralisation in the rat hippocampus during the first 2 weeks of postnatal (P) development. Eighteen male pups were randomly divided into three groups: P0, P7, and P14. After removing the newborn brains, real-time polymerase chain reaction, western blot, and immunohistochemistry techniques were used to evaluate expression of the receptors. Results indicated that the expression profile of these receptors were time- and spatially dependent. A significant increase was observed in the distribution of α7 and α4 nAChR subunits in the developing rat hippocampus from P0 to P7 (p < .001); however, there was a significant decrease from P7 to P14 (p < .05). As a spatial effect, the highest optical density (OD) was observed in the CA3 and CA2 regions of the hippocampus, while the lowest OD was in the dentate gyrus. Moreover, the distribution of α7 and α4 nAChR subunits in the left hippocampus was significantly higher than their counterparts in the right (p < .05). From these data, the expression patterns of α7 and α4 nAChR subunits exhibited left-right asymmetry in the developing rat hippocampus.

Impaired regulation of KCC2 phosphorylation leads to neuronal network dysfunction and neurodevelopmental pathology.

KCC2 is a vital neuronal K+/Cl- cotransporter that is implicated in the etiology of numerous neurological diseases. In normal cells, KCC2 undergoes developmental dephosphorylation at Thr906 and Thr1007 We engineered mice with heterozygous phosphomimetic mutations T906E and T1007E (KCC2E/+ ) to prevent the normal developmental dephosphorylation of these sites. Immature (postnatal day 15) but not juvenile (postnatal day 30) KCC2E/+ mice exhibited altered GABAergic inhibition, an increased glutamate/GABA synaptic ratio, and greater susceptibility to seizure. KCC2E/+ mice also had abnormal ultrasonic vocalizations at postnatal days 10 to 12 and impaired social behavior at postnatal day 60. Postnatal bumetanide treatment restored network activity by postnatal day 15 but failed to restore social behavior by postnatal day 60. Our data indicate that posttranslational KCC2 regulation controls the GABAergic developmental sequence in vivo, indicating that deregulation of KCC2 could be a risk factor for the emergence of neurological pathology.

Functions of adult-born neurons in hippocampal memory interference and indexing.

The dentate gyrus-CA3 circuit of the hippocampus is continuously modified by the integration of adult-born dentate granule cells (abDGCs). All abDGCs undergo a prolonged period of maturation, during which they exhibit heightened synaptic plasticity and refinement of electrophysiological properties and connectivity. Consistent with theoretical models and the known functions of the dentate gyrus-CA3 circuit, acute or chronic manipulations of abDGCs support a role for abDGCs in the regulation of memory interference. In this Review, we integrate insights from studies that examine the maturation of abDGCs and their integration into the circuit with network mechanisms that support memory discrimination, consolidation and clearance. We propose that adult hippocampal neurogenesis enables the generation of a library of experiences, each registered in mature abDGC physiology and connectivity. Mature abDGCs recruit inhibitory microcircuits to support pattern separation and memory indexing.

Developmental regulation and lateralization of GABA receptors in the rat hippocampus.

GABA is the chief inhibitory neurotransmitter in the adult brain. However, in the developing brain it acts as an excitatory transmitter causing depolarization. Thereby, activates calcium-dependent processes that are crucial for brain development. Accordingly, GABA receptors have the great role in the brain development, especially in the area with persisting neurogenesis such as hippocampus. The present study investigated the development and lateralization of two important subunits of GABA receptors, GABAAα1 and GABAB1, in the developing rat hippocampus during the neurogenesis-active period, at the first two postnatal weeks. Real-time PCR, western blot and immunohistochemistry were used. We found that the mRNA and protein of these GABA receptor subunits have already been expressed at birth and significantly increased at postnatal day (P) 7, and also at P14. Also, regarding the optical densities of GABAAα1 and GABAB1 expressing hippocampal cells, we found a significant increase in the distribution pattern of these subunits in the all hippocampal subregions on day 14 after birth. The highest optical density of GABAAα1 was observed in the CA3, and GABAB1 in the CA2. Nevertheless, our results did not show a significant laterality differences in the expression of these subunits. Regarding the crucial role of GABA receptors in the hippocampus development; they probably have the same effects on development of the rat hippocampus on both sides.

Insufficient glutamine synthetase activity during synaptogenesis causes spatial memory impairment in adult mice.

Glutamatergic synapses constitute a major excitatory neurotransmission system and are regulated by glutamate/glutamine (Gln) cycling between neurons and astrocytes. Gln synthetase (GS) produced by astrocytes plays an important role in maintaining the cycle. However, the significance of GS during synaptogenesis has not been clarified. GS activity and expression significantly increase from postnatal day (PD) 7 to 21, and GS is expressed prior to glial fibrillary acidic protein (GFAP) and is more abundant than GFAP throughout synaptogenesis. These observations suggest that GS plays an important role in synaptogenesis. We investigated this by inhibiting GS activity in neonatal mice and assessed the consequences in adult animals. Lower expression levels of GS and GFAP were found in the CA3 region of the hippocampus but not in the CA1 region. Moreover, synaptic puncta and glutamatergic neurotransmission were also decreased in CA3. Behaviorally, mice with inhibited GS during synaptogenesis showed spatial memory-related impairment as adults. These results suggest that postnatal GS activity is important for glutamatergic synapse development in CA3.

The GABA Developmental Shift Is Abolished by Maternal Immune Activation Already at Birth.

Epidemiological and experimental studies suggest that maternal immune activation (MIA) leads to developmental brain disorders, but whether the pathogenic mechanism impacts neurons already at birth is not known. We now report that MIA abolishes in mice the oxytocin-mediated delivery γ-aminobutyric acid (GABA) shift from depolarizing to hyperpolarizing in CA3 pyramidal neurons, and this is restored by the NKCC1 chloride importer antagonist bumetanide. Furthermore, MIA hippocampal pyramidal neurons at birth have a more exuberant apical arbor organization and increased apical dendritic length than age-matched controls. The frequency of spontaneous glutamatergic postsynaptic currents is also increased in MIA offspring, as well as the pairwise correlation of the synchronized firing of active cells in CA3. These alterations produced by MIA persist, since at P14-15 GABA action remains depolarizing, produces excitatory action, and network activity remains elevated with a higher frequency of spontaneous glutamatergic postsynaptic currents. Therefore, the pathogenic actions of MIA lead to important morphophysiological and network alterations in the hippocampus already at birth.

α4ßδ GABAA Receptors Trigger Synaptic Pruning and Reduce Dendritic Length of Female Mouse CA3 Hippocampal Pyramidal Cells at Puberty.

Synaptic pruning during adolescence is critical for optimal cognition. The CA3 hippocampus contains unique spine types and plays a pivotal role in pattern separation and seizure generation, where sex differences exist, but adolescent pruning has only been studied in the male. Thus, for the present study we assessed pruning of specific spine types in the CA3 hippocampus during adolescence and investigated a possible mechanism in the female mouse. To this end, we used Golgi-impregnated brains from pubertal (∼PND 35, assessed by vaginal opening) and post-pubertal (PND 56) mice. Spine density was assessed from z-stack (0.1-µm steps) images taken using a Nikon DS-U3 camera through a Nikon Eclipse Ci-L microscope and analyzed with NIS Elements. Spine density decreased significantly (P < 0.05) during adolescence, with 50-60% decreases in mushroom and stubby spine-types (P < 0.05, ∼PND35 vs. PND56) in non-proestrous mice. This was associated with decreases in kalirin-7, a spine protein which stabilizes the cytoskeleton and is required for spine maintenance. Because our previous findings suggest that pubertal increases in α4ßδ GABAA receptors (GABARs) trigger pruning in CA1, we investigated their role in CA3. α4 expression in CA3 hippocampus increased 4-fold at puberty (P < 0.05), assessed by immunostaining and verified electrophysiologically by an increased response to gaboxadol (100 nM), which is selective for α4ßδ. Knock-out of α4 prevented the pubertal decrease in kalirin-7 and synaptic pruning and also increased the dendritic length, demonstrating a functional link. These data suggest that pubertal α4ßδ GABARs alter dendritic morphology and trigger pruning in female CA3 hippocampus.

Adenosine Receptor-Mediated Developmental Loss of Spike Timing-Dependent Depression in the Hippocampus.

Critical periods of synaptic plasticity facilitate the reordering and refining of neural connections during development, allowing the definitive synaptic circuits responsible for correct adult physiology to be established. Presynaptic spike timing-dependent long-term depression (t-LTD) exists in the hippocampus, which depends on the activation of NMDARs and that probably fulfills a role in synaptic refinement. This t-LTD is present until the third postnatal week in mice, disappearing in the fourth week of postnatal development. We were interested in the mechanisms underlying this maturation related loss of t-LTD and we found that at CA3-CA1 synapses, presynaptic NMDA receptors (pre-NMDARs) are tonically active between P13 and P21, mediating an increase in glutamate release during this critical period of plasticity. Conversely, at the end of this critical period (P22-P30) and coinciding with the loss of t-LTD, these pre-NMDARs are no longer tonically active. Using immunogold electron microscopy, we demonstrated the existence of pre-NMDARs at Schaffer collateral synaptic boutons, where a decrease in the number of pre-NMDARs during development coincides with the loss of both tonic pre-NMDAR activation and t-LTD. Interestingly, this t-LTD can be completely recovered by antagonizing adenosine type 1 receptors (A1R), which also recovers the tonic activation of pre-NMDARs at P22-P30. By contrast, the induction of t-LTD was prevented at P13-P21 by an agonist of A1R, as was tonic pre-NMDAR activation. Furthermore, we found that the adenosine that mediated the loss of t-LTD during the fourth week of development is supplied by astrocytes. These results provide direct evidence for the mechanism that closes the window of plasticity associated with t-LTD, revealing novel events probably involved in synaptic remodeling during development.

It's All in the Details: Relations Between Young Children's Developing Pattern Separation Abilities and Hippocampal Subfield Volumes.

The ability to keep similar experiences separate in memory is critical for forming unique and lasting memories, as many events share overlapping features (e.g., birthday parties, holidays). Research on memory in young children suggests their memories often lack high-resolution details, i.e., show impoverished pattern separation (PS). Recently developed assessments of PS suitable for children allow us to relate the formation of distinct, detailed memories for the development of the hippocampus, a neural structure critical for this ability in adults. The hippocampus displays a protracted developmental profile and underlies the ability to form detailed memories. This study examined age-related differences in hippocampal subfield volumes in 4- to 8-year-old children and relations with performance on a mnemonic similarity task (MST) designed to index memory specificity. Results revealed age-moderated associations between MST performance and cornu ammonis 2-4/dentate gyrus subfields. Specifically, age-related differences in the ability to form detailed memories tracked with normative patterns of volume increases followed by reductions over this age range. That is, greater volume correlated with better performance in younger children, whereas smaller volume correlated with better performance in older children. These findings support the hypothesis that developmental differences in hippocampal circuitry contribute to age-related improvements in detailed memory formation during this period.

Asc-1 transporter activation: an alternative to rescue age-related alterations in functional plasticity at rat hippocampal CA3/CA1 synapses.

Activation of the N-methyl-D-aspartate subtype of glutamate receptor (NMDA-R) represents a key functional process for memory formation. A decreased synthesis of the NMDA-R co-agonist d-serine was recently proposed to contribute to alterations of hippocampus-dependent memory mechanisms with ageing. Nevertheless, other pathways could also be involved and thus considered to be targets of interest to prevent cognitive ageing. Herein, we demonstrate that the Asc-1 subtype of neutral amino acid (nAA) transporters that regulates d-serine and glycine release from neurons could be viewed as one of these targets. At CA3/CA1 hippocampal synapses, Asc-1 activation did not modify basal glutamate neurotransmission either in adult or aged rats. In contrast, Asc-1 activation significantly increased NMDA-R-dependent long-term potentiation (LTP) in both groups of animals and fully rescued the age-related LTP deficits. This rescue in aged animals was observed only when Asc-1 activation was selectively managed by d-Isoleucine (d-Ile), but not when less specifically driven by a mixture of nAA. Similarly, while any activation of Asc-1 improved the isolated NMDA-R-induced synaptic potentials in adult rats, only d-Ile was efficient in aged animals. Taken together, these results strengthen the interest in specifically targeting Asc-1 transporters to better cure age-associated memory decline. OPEN PRACTICES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Development-related aberrations in Kv1.1 α-subunit exert disruptive effects on bioelectrical activities of neurons in a mouse model of fragile X syndrome.

Kv1.1, a Shaker homologue potassium channel, plays a critical role in homeostatic regulation of neuronal excitability. Aberrations in the functional properties of Kv1.1 have been implicated in several neurological disorders featured by neuronal hyperexcitability. Fragile X syndrome (FXS), the most common form of inherited mental retardation, is characterized by hyperexcitability in neural network and intrinsic membrane properties. The Kv1.1 channel provides an intriguing mechanistic candidate for FXS. We investigated the development-related expression pattern of the Kv1.1 α-subunit by using a Fmr1 knockout (KO) mouse model of FXS. Markedly decreased protein expression of Kv1.1 was found in neonatal and adult stages when compared to age-matched wild-type (WT) mice. Immunohistochemical investigations supported the delayed development-related increases in Kv1.1 expression, especially in CA3 pyramidal neurons. By applying a Kv1.1-specific blocker, dendrotoxin-κ (DTX-κ), we isolated the Kv1.1-mediated currents in the CA3 pyramidal neurons. The isolated DTX-κ-sensitive current of neurons from KO mice exhibited decreased amplitude, lower threshold of activation, and faster recovery from inactivation. The equivalent reduction in potassium current in the WT neurons following application of the appropriate amount of DTX-κ reproduced the enhanced firing abilities of KO neurons, suggesting the Kv1.1 channel as a critical contributor to the hyperexcitability of KO neurons. The role of Kv1.1 in controlling neuronal discharges was further supported by the parallel developmental trajectories of Kv1.1 expression, current amplitude, and discharge impacts, with a significant correlation between the amplitude of Kv1.1-mediated currents and Kv1.1-blocking-induced firing enhancement. These data suggest that the expression of the Kv1.1 α-subunit has a profound pathological relevance to hyperexcitability in FXS, as well as implications for normal development, maintenance, and control of neuronal activities.

Critical periods regulating the circuit integration of adult-born hippocampal neurons.

The dentate gyrus (DG) in the adult brain maintains the capability to generate new granule neurons throughout life. Neural stem cell-derived new-born neurons emerge to play key functions in the way information is processed in the DG and then conveyed to the CA3 hippocampal area, yet accumulating evidence indicates that both the maturation process and the connectivity pattern of new granule neurons are not prefigured but can be modulated by the activity of local microcircuits and, on a network level, by experience. Although most of the activity- and experience-dependent changes described so far appear to be restricted to critical periods during the development of new granule neurons, it is becoming increasingly clear that the surrounding circuits may play equally key roles in accommodating and perhaps fostering, these changes. Here, we review some of the most recent insights into this almost unique form of plasticity in the adult brain by focusing on those critical periods marked by pronounced changes in structure and function of the new granule neurons and discuss how the activity of putative synaptic partners may contribute to shape the circuit module in which new neurons become finally integrated.

Differential effects of childhood neglect and abuse during sensitive exposure periods on male and female hippocampus.

The hippocampus is a highly stress susceptible structure and hippocampal abnormalities have been reported in a host of psychiatric disorders including major depression and post-traumatic stress disorder (PTSD). The hippocampus appears to be particularly susceptible to early life stress with a graded reduction in volume based on number of types (multiplicity) or severity of maltreatment. We assessed whether the most important predictors of adult hippocampal volume were multiplicity, severity or duration of exposure or timing of maltreatment during developmental sensitive periods. 3T MRIs were collected on 336 unmedicated, right-handed subjects (132M/204F, 18-25 years). Exposure to broad categories of abuse and neglect during each year of childhood were assessed using the Maltreatment and Abuse Chronology of Exposure scale and evaluated using artificial intelligence and predictive analytics. Male hippocampal volume was predicted by neglect, but not abuse, up through 7 years of age. Female hippocampal volume was predicted by abuse, but not neglect, at 10, 11, 15 and 16 years. Exposure at peak age had greater predictive importance than multiplicity, severity or duration. There were also marked gender differences in subfields and portions (head, body or tail) affected by exposure. History and symptoms of major depression, PTSD or anxiety disorders were not predictive of hippocampal volume once maltreatment was accounted for. Neglect appears to foster inadequate hippocampal development in males while abuse appears to produce a stress-related deficit in females. Studies assessing hippocampal volume in psychiatric disorders need to control for the gender-specific effects of abuse and neglect.

Immature Status Epilepticus: In Vitro Models Reveal Differences in Cholinergic Control and HFO Properties of Adult CA3 Interictal Discharges in Temporal vs Septal Hippocampus.

We have earlier demonstrated that a Status Epilepticus (SE) during CNS development has long-lasting effects on cholinergic neurotransmission, detectable in vitro and in vivo. In this work, we aimed to localize changes in temporal (T) vs septal (S) hippocampus and to correlate adult CA3 interictal epileptiform discharge (IED) frequency changes to those of Ripples (R) and Fast Ripples (FR) of the High-Frequency Oscillations (HFOs). Spontaneous IEDs were induced by bathing slices in Mg2+-free ACSF or in 4-Aminopyridine (4-AP, 50 µM) and data were analyzed separately for each model. IED frequencies were similar in same origin normal (N) slices across models, but differed in SE slices, being lower in Mg2+-free ACSF than in 4-AP, suggesting a post-SE long-term increase in a K+ conductance. Rs and FRs detected within IEDs had generally higher power in 4-AP than in Mg2+-free ACSF; FR/R ratio was the highest in T-SE slices in 4-AP and similar in all other slice groups. Carbachol or eserine increased IED rates universally, but had region- and conditioning-specific effects on HFOs, suggesting that IED frequency and HFOs represent possibly independent indices of excitability. The muscarinic antagonist atropine depressed IED rates with increasing effectiveness in S slices post-SE in both models. In conclusion, the long-term effects of an immature SE are region-specific within the hippocampus, affect differently synchronizing components like the IED frequency and HFOs and may shape neurotransmitter effects (ACh) on neuronal networks, thus affecting seizure threshold and information processing, especially in behavioral conditions of rising extracellular ACh levels.

Afferent specific role of NMDA receptors for the circuit integration of hippocampal neurogliaform cells.

Appropriate integration of GABAergic interneurons into nascent cortical circuits is critical for ensuring normal information processing within the brain. Network and cognitive deficits associated with neurological disorders, such as schizophrenia, that result from NMDA receptor-hypofunction have been mainly attributed to dysfunction of parvalbumin-expressing interneurons that paradoxically express low levels of synaptic NMDA receptors. Here, we reveal that throughout postnatal development, thalamic, and entorhinal cortical inputs onto hippocampal neurogliaform cells are characterized by a large NMDA receptor-mediated component. This NMDA receptor-signaling is prerequisite for developmental programs ultimately responsible for the appropriate long-range AMPAR-mediated recruitment of neurogliaform cells. In contrast, AMPAR-mediated input at local Schaffer-collateral synapses on neurogliaform cells remains normal following NMDA receptor-ablation. These afferent specific deficits potentially impact neurogliaform cell mediated inhibition within the hippocampus and our findings reveal circuit loci implicating this relatively understudied interneuron subtype in the etiology of neurodevelopmental disorders characterized by NMDA receptor-hypofunction.Proper brain function depends on the correct assembly of excitatory and inhibitory neurons into neural circuits. Here the authors show that during early postnatal development in mice, NMDAR signaling via activity of long-range synaptic inputs onto neurogliaform cells is required for their appropriate integration into the hippocampal circuitry.

Hippocampal CA3-dentate gyrus volume uniquely linked to improvement in associative memory from childhood to adulthood.

Associative memory develops into adulthood and critically depends on the hippocampus. The hippocampus is a complex structure composed of subfields that are functionally-distinct, and anterior-posterior divisions along the length of the hippocampal horizontal axis that may also differ by cognitive correlates. Although each of these aspects has been considered independently, here we evaluate their relative contributions as correlates of age-related improvement in memory. Volumes of hippocampal subfields (subiculum, CA1-2, CA3-dentate gyrus) and anterior-posterior divisions (hippocampal head, body, tail) were manually segmented from high-resolution images in a sample of healthy participants (age 8-25 years). Adults had smaller CA3-dentate gyrus volume as compared to children, which accounted for 67% of the indirect effect of age predicting better associative memory via hippocampal volumes. Whereas hippocampal body volume demonstrated non-linear age differences, larger hippocampal body volume was weakly related to better associative memory only when accounting for the mutual correlation with subfields measured within that region. Thus, typical development of associative memory was largely explained by age-related differences in CA3-dentate gyrus.

Roles of afadin in the formation of the cellular architecture of the mouse hippocampus and dentate gyrus.

The hippocampal formation with tightly packed neurons, mainly at the dentate gyrus, CA3, CA2, and CA1 regions, constitutes a one-way neural circuit, which is associated with learning and memory. We previously showed that the cell adhesion molecules nectins and its binding protein afadin play roles in the formation of the mossy fiber synapses which are formed between the mossy fibers of the dentate gyrus granule cells and the dendrites of the CA3 pyramidal cells. We showed here that in the afadin-deficient hippocampal formation, the dentate gyrus granules cells and the CA3, CA2, and CA1 pyramidal cells were abnormally located; the mossy fiber trajectory was abnormally elongated; the CA3 pyramidal cells were abnormally differentiated; and the densities of the presynaptic boutons on the mossy fibers and the apical dendrites of the CA3 pyramidal cells were decreased. These results indicate that afadin plays roles not only in the formation of the mossy fiber synapses but also in the formation of the cellular architecture of the hippocampus and the dentate gyrus.

Gap junctions between CA3 pyramidal cells contribute to network synchronization in neonatal hippocampus.

Direct electrical coupling between neurons through gap junctions is prominent during development, when synaptic connectivity is scarce, providing the additional intercellular connectivity. However, functional studies of gap junctions are hampered by the unspecificity of pharmacological tools available. Here we have investigated gap-junctional coupling between CA3 pyramidal cells in neonatal hippocampus and its contribution to early network activity. Four different gap junction inhibitors, including the general blocker carbenoxolone, decreased the frequency of network activity bursts in CA3 area of hippocampus of P3-6 rats, suggesting the involvement of electrical connections in the generation of spontaneous network activity. In CA3 pyramidal cells, spikelets evoked by local stimulation of stratum oriens, were inhibited by carbenoxolone, but not by inhibitors of glutamatergic and GABAergic synaptic transmission, signifying the presence of electrical connectivity through axo-axonic gap junctions. Carbenoxolone also decreased the success rate of firing antidromic action potentials in response to stimulation, and changed the pattern of spontaneous action potential firing of CA3 pyramidal cells. Altogether, these data suggest that electrical coupling of CA3 pyramidal cells contribute to the generation of the early network events in neonatal hippocampus by modulating their firing pattern and synchronization.