Identification of dorsal-ventral hippocampal differentiation in neonatal rats

Dissecting gene expression networks in the developing hippocampus through the lens of NEIL3 depletion

Gene regulation in the hippocampus is fundamental for its development, synaptic plasticity, memory formation, and adaptability. Comparisons of gene expression among different developmental stages, distinct cell types, and specific experimental conditions have identified differentially expressed genes contributing to the organization and functionality of hippocampal circuits. The NEIL3 DNA glycosylase, one of the DNA repair enzymes, plays an important role in hippocampal maturation and neuron functionality by shaping transcription. While differential gene expression (DGE) analysis has identified key genes involved, broader gene expression patterns crucial for high-order hippocampal functions remain uncharted. By utilizing the weighted gene co-expression network analysis (WGCNA), we mapped gene expression networks in immature (p8-neonatal) and mature (3 m-adult) hippocampal circuits in wild-type and NEIL3-deficient mice. Our study unveiled intricate gene network structures underlying hippocampal maturation, delineated modules of co-expressed genes, and pinpointed highly interconnected hub genes specific to the maturity of hippocampal subregions. We investigated variations within distinct gene network modules following NEIL3 depletion, uncovering NEIL3-targeted hub genes that influence module connectivity and specificity. By integrating WGCNA with DGE, we delve deeper into the NEIL3-dependent molecular intricacies of hippocampal maturation. This study provides a comprehensive systems-level analysis for assessing the potential correlation between gene connectivity and functional connectivity within the hippocampal network, thus shaping hippocampal function throughout development.

Grid cells in rats deprived of geometric experience during development

The medial entorhinal cortex (MEC) is part of the brain's network for dynamic representation of location. The most abundant class of neurons in this circuit is the grid cell, characterized by its periodic, hexagonally patterned firing fields. While in developing animals some MEC cell types express adult-like firing patterns already on the first exposure to an open spatial environment, only days after eye opening, grid cells mature more slowly, over a 1-to-2-wk period after the animals leave their nest. Whether the later emergence of a periodic grid pattern reflects a need for experience with spatial environments has not been determined. We here show that grid-like firing patterns continue to appear during exploration of open square environments in rats that are raised for the first months of their life in opaque spherical environments, in the absence of stable reference boundaries to guide spatial orientation. While strictly periodic firing fields were initially absent in these animals, clear grid patterns developed after only a few trials of training. In rats that were tested in the same open environment but raised for the first months of life in opaque cubes, with sharp vertical boundaries, grid-like firing was from the beginning indistinguishable from that of nondeprived control animals growing up in large enriched cages. Thus, although a minimum of experience with peripheral geometric boundaries is required for expression of regular grid patterns in a new environment, the effect of restricted spatial experience is overcome with short training, consistent with a preconfigured experience-independent basis for the grid pattern.

Transcriptional development of the hippocampus and the dorsal-intermediate-ventral axis in rats

Risk and resilience for neuropsychiatric illnesses are established during brain development, and transcriptional markers of risk may be identifiable in early development. The dorsal-ventral axis of the hippocampus has behavioral, electrophysiological, anatomical, and transcriptional gradients and abnormal hippocampus development is associated with autism, schizophrenia, epilepsy, and mood disorders. We previously showed that differential gene expression along the dorsoventral hippocampus in rats was present at birth (postnatal day 0, P0), and that a subset of differentially expressed genes (DEGs) was present at all postnatal ages examined (P0, P9, P18, and P60). Here, we extend the analysis of that gene expression data to understand the development of the hippocampus as a whole by examining DEGs that change with age. We additionally examine development of the dorsoventral axis by looking at DEGs along the axis at each age. Using both unsupervised and supervised analyses, we find that the majority of DEGs are present from P0 to P18, with many expression profiles presenting peaks or dips at P9/18. During development of the hippocampus, enriched pathways associated with learning, memory, and cognition increase with age, as do pathways associated with neurotransmission and synaptic function. Development of the dorsoventral axis is greatest at P9 and P18 and is marked by DEGs associated with metabolic functions. Our data indicate that neurodevelopmental disorders like epilepsy, schizophrenia and affective disorders are enriched with developmental DEGs in the hippocampus, regardless of dorsoventral location, with the greatest enrichment of these clinical disorders seen in genes whose expression changes from P0-9. When comparing DEGs from the ventral and dorsal poles, the greatest number of neurodevelopmental disorders is enriched with DEGs found at P18. Taken together, the developing hippocampus undergoes substantial transcriptional maturation during early postnatal development, with expression of genes involved in neurodevelopmental disorders also showing maximal expression changes within this developmental period.

Place cells are nonrandomly clustered by field location in CA1 hippocampus

A challenge in both modern and historic neuroscience has been achieving an understanding of neuron circuits, and determining the computational and organizational principles that underlie these circuits. Deeper understanding of the organization of brain circuits and cell types, including in the hippocampus, is required for advances in behavioral and cognitive neuroscience, as well as for understanding principles governing brain development and evolution. In this manuscript, we pioneer a new method to analyze the spatial clustering of active neurons in the hippocampus. We use calcium imaging and a rewarded navigation task to record from 100 s of place cells in the CA1 of freely moving rats. We then use statistical techniques developed for and in widespread use in geographic mapping studies, global Moran's I, and local Moran's I to demonstrate that cells that code for similar spatial locations tend to form small spatial clusters. We present evidence that this clustering is not the result of artifacts from calcium imaging, and show that these clusters are primarily formed by cells that have place fields around previously rewarded locations. We go on to show that, although cells with similar place fields tend to form clusters, there is no obvious topographic mapping of environmental location onto the hippocampus, such as seen in the visual cortex. Insights into hippocampal organization, as in this study, can elucidate mechanisms underlying motivational behaviors, spatial navigation, and memory formation.

Postnatal development of projections of the postrhinal cortex to the entorhinal cortex in the rat

The ability to encode and retrieve contextual information is an inherent feature of episodic memory that starts to develop during childhood. The postrhinal cortex, an area of the parahippocampal region, has a crucial role in encoding object-space information and translating egocentric to allocentric representation of local space. The strong connectivity of POR with the adjacent entorhinal cortex, and consequently the hippocampus, suggests that the development of these connections could support the postnatal development of contextual memory. Here, we report that postrhinal cortex projections of the rat develop progressively from the first to the third postnatal week starting in the medial entorhinal cortex before spreading to the lateral entorhinal cortex. The increased spread and complexity of postrhinal axonal distributions is accompanied by an increased complexity of entorhinal dendritic trees and an increase of postrhinal - entorhinal synapses, which supports a gradual maturation in functional activity.SIGNIFICANCE STATEMENTPostrhinal-entorhinal cortical interplay mediates important aspects of encoding and retrieval of contextual information that is important for episodic memory. To better understand the function of the postrhinal interactions with the entorhinal cortex we studied the postnatal development of the connection between the two cortical areas. Our study describes the postnatal development of the postrhinal-to-entorhinal projections as established with neuroanatomical and electrophysiological methods. The projections gradually reach functionally different areas of the entorhinal cortex, reaching the area involved in spatial functions first, followed by the part involved in representing information about objects and sequences of events.

Single Cell Transcriptomic and Chromatin Profiles Suggest Layer Vb Is the Only Layer With Shared Excitatory Cell Types in the Medial and Lateral Entorhinal Cortex

All brain functionality arises from the activity in neural circuits in different anatomical regions. These regions contain different circuits comprising unique cell types. An integral part to understanding neural circuits is a full census of the constituent parts, i.e., the neural cell types. This census can be based on different characteristics. Previously combinations of morphology and physiology, gene expression, and chromatin accessibility have been used in various cortical and subcortical regions. This has given an extensive yet incomplete overview of neural cell types. However, these techniques have not been applied to all brain regions. Here we apply single cell analysis of accessible chromatin on two similar but different cortical regions, the medial and the lateral entorhinal cortices. Even though these two regions are anatomically similar, their intrinsic and extrinsic connectivity are different. In 4,136 cells we identify 20 different clusters representing different cell types. As expected, excitatory cells show regionally specific clusters, whereas inhibitory neurons are shared between regions. We find that several deep layer excitatory neuronal cell types as defined by chromatin profile are also shared between the two different regions. Integration with a larger scRNA-seq dataset maintains this shared characteristic for cells in Layer Vb. Interestingly, this layer contains three clusters, two specific to either subregion and one shared between the two. These clusters can be putatively associated with particular functional and anatomical cell types found in this layer. This information is a step forwards into elucidating the cell types within the entorhinal circuit and by extension its functional underpinnings.

Structural and Functional Aspects of the Neurodevelopmental Gene NR2F1: From Animal Models to Human Pathology

The assembly and maturation of the mammalian brain result from an intricate cascade of highly coordinated developmental events, such as cell proliferation, migration, and differentiation. Any impairment of this delicate multi-factorial process can lead to complex neurodevelopmental diseases, sharing common pathogenic mechanisms and molecular pathways resulting in multiple clinical signs. A recently described monogenic neurodevelopmental syndrome named Bosch-Boonstra-Schaaf Optic Atrophy Syndrome (BBSOAS) is caused by NR2F1 haploinsufficiency. The NR2F1 gene, coding for a transcriptional regulator belonging to the steroid/thyroid hormone receptor superfamily, is known to play key roles in several brain developmental processes, from proliferation and differentiation of neural progenitors to migration and identity acquisition of neocortical neurons. In a clinical context, the disruption of these cellular processes could underlie the pathogenesis of several symptoms affecting BBSOAS patients, such as intellectual disability, visual impairment, epilepsy, and autistic traits. In this review, we will introduce NR2F1 protein structure, molecular functioning, and expression profile in the developing mouse brain. Then, we will focus on Nr2f1 several functions during cortical development, from neocortical area and cell-type specification to maturation of network activity, hippocampal development governing learning behaviors, assembly of the visual system, and finally establishment of cortico-spinal descending tracts regulating motor execution. Whenever possible, we will link experimental findings in animal or cellular models to corresponding features of the human pathology. Finally, we will highlight some of the unresolved questions on the diverse functions played by Nr2f1 during brain development, in order to propose future research directions. All in all, we believe that understanding BBSOAS mechanisms will contribute to further unveiling pathophysiological mechanisms shared by several neurodevelopmental disorders and eventually lead to effective treatments.

Topiramate-chitosan nanoparticles prevent morphine reinstatement with no memory impairment: Dopaminergic and glutamatergic molecular aspects in rats

Besides their clinical application, chronic misuse of opioids has often been associated to drug addiction due to their addictive properties, underlying neuroadaptations of AMPA glutamate-receptor-dependent synaptic plasticity. Topiramate (TPM), an AMPAR antagonist, has been used to treat psychostimulants addiction, despite its harmful effects on memory. This study aimed to evaluate the effects of a novel topiramate nanosystem on molecular changes related to morphine reinstatement. Rats were previously exposed to morphine in conditioned place preference (CPP) paradigm and treated with topiramate-chitosan nanoparticles (TPM-CS-NP) or non-encapsulated topiramate in solution (S-TPM) during CPP extinction; following memory performance evaluation, they were re-exposed to morphine reinstatement. While morphine-CPP extinction was comparable among all experimental groups, TPM-CS-NP treatment prevented morphine reinstatement, preserving memory performance, which was impaired by both morphine-conditioning and S-TPM treatment. In the NAc, morphine increased D1R, D2R, D3R, DAT, GluA1 and MOR immunoreactivity. It also increased D1R, DAT, GluA1 and MOR in the dorsal hippocampus. TPM-CS-NP treatment decreased D1R, D3R and GluA1 and increased DAT in the NAc, decreasing GluA1 and increasing D2 and DAT in the dorsal hippocampus. Taken together, we may infer that TPM-CS-NP treatment was able to prevent the morphine reinstatement without memory impairment. Therefore, TPM-CS-NP may be considered an innovative therapeutic tool due to its property to prevent opioid reinstatement because it acts modifying both dopaminergic and glutamatergic neurotransmission, which are commonly related to morphine addiction.

A molecular gradient along the longitudinal axis of the human hippocampus informs large-scale behavioral systems

The functional organization of the hippocampus is distributed as a gradient along its longitudinal axis that explains its differential interaction with diverse brain systems. We show that the location of human tissue samples extracted along the longitudinal axis of the adult human hippocampus can be predicted within 2mm using the expression pattern of less than 100 genes. Futhermore, this model generalizes to an external set of tissue samples from prenatal human hippocampi. We examine variation in this specific gene expression pattern across the whole brain, finding a distinct anterioventral-posteriodorsal gradient. We find frontal and anterior temporal regions involved in social and motivational behaviors, and more functionally connected to the anterior hippocampus, to be clearly differentiated from posterior parieto-occipital regions involved in visuospatial cognition and more functionally connected to the posterior hippocampus. These findings place the human hippocampus at the interface of two major brain systems defined by a single molecular gradient.

Development and topographic organization of subicular projections to lateral septum in the rat brain

One of the main subcortical targets of hippocampal formation efferents is the lateral septum. Previous studies on the subicular projections, as a main output structure of the hippocampus, have shown a clear topographic organization of septal innervation, related to the origin of the fibres along the dorsoventral axis of the subiculum in the adult brain. In contrast, studies on the developing brain depict an extensive rearrangement of subicular projections during the prenatal period, shifting from the medial septum to the lateral septum. Our study aimed to describe the postnatal development of subicular projections to the septum. We injected anterograde tracers into the subiculum of 57 pups of different postnatal ages. Injections covered the proximodistal and dorsoventral axis of the subiculum. The age of the pups at day of tracer injection ranged from the day of birth to postnatal day 30. Analyses revealed that from the first postnatal day projections from subiculum preferentially target the lateral septum. Sparse innervation in the lateral septum was already present in the first few postnatal days, and during the following 3 weeks, the axonal distribution gradually expanded. Subicular projections to the lateral septum are topographically organized depending on the origin along the dorsoventral axis of the subiculum, in line with the adult innervation pattern. Different origins along the proximodistal axis of the subiculum are reflected in changes in the strength of septal innervation. The findings demonstrate that in case of the development of subicular projections, axonal expansion is more prominent than axonal pruning.

Postnatal Development of Functional Projections from Parasubiculum and Presubiculum to Medial Entorhinal Cortex in the Rat

Neurons in parasubiculum (PaS), presubiculum (PrS), and medial entorhinal cortex (MEC) code for place (grid cells) and head direction. Directional input has been shown to be important for stable grid cell properties in MEC, and PaS and PrS have been postulated to provide this information to MEC. In line with this, head direction cells in those brain areas are present at postnatal day 11 (P11), having directional tuning that stabilizes shortly after eye opening, which is before premature grid cells emerge in MEC at P16. Whether functional connectivity between these structures exists at those early postnatal stages is unclear. Using anatomical tracing, voltage-sensitive dye imaging and single-cell patch recordings in female and male rat brain slices between P2 and P61, we determined when the pathways from PaS and PrS to MEC emerge, become functional, and how they develop. Anatomical connections from PaS and PrS to superficial MEC emerge between P4 and P6. Monosynaptic connectivity from PaS and PrS to superficial MEC was measurable from P9 to P10 onward, whereas connectivity with deep MEC was measurable from P11 to P12. From P14/P15 on, reactivity of MEC neurons to parasubicular and presubicular inputs becomes adult-like and continues to develop until P28-P30. The maturation of the efficacy of both inputs between P9 and P21 is paralleled by maturation of morphological properties, changes in intrinsic properties of MEC principal neurons, and changes in the GABAergic network of MEC. In conclusion, synaptic projections from PaS and PrS to MEC become functional and adult-like before the emergence of grid cells in MEC.SIGNIFICANCE STATEMENT Head direction information, crucial for grid cells in medial entorhinal cortex (MEC), is thought to enter MEC via parasubiculum (PaS) and presubiculum (PrS). Unraveling the development of functional connections between PaS, PrS, and MEC is key to understanding how spatial navigation, an important cognitive function, may evolve. To gain insight into the development, we used anatomical tracing techniques, voltage-sensitive dye imaging, and single-cell recordings. The combined data led us to conclude that synaptic projections from PaS and PrS to MEC become functional and adult-like before eye opening, allowing crucial head direction information to influence place encoding before the emergence of grid cells in rat MEC.

Neurons and networks in the entorhinal cortex: A reappraisal of the lateral and medial entorhinal subdivisions mediating parallel cortical pathways

In this review, we aim to reappraise the organization of intrinsic and extrinsic networks of the entorhinal cortex with a focus on the concept of parallel cortical connectivity streams. The concept of two entorhinal areas, the lateral and medial entorhinal cortex, belonging to two parallel input-output streams mediating the encoding and storage of respectively what and where information hinges on the claim that a major component of their cortical connections is with the perirhinal cortex and postrhinal or parahippocampal cortex in, respectively, rodents or primates. In this scenario, the lateral entorhinal cortex and the perirhinal cortex are connectionally associated and likewise the postrhinal/parahippocampal cortex and the medial entorhinal cortex are partners. In contrast, here we argue that the connectivity matrix emphasizes the potential of substantial integration of cortical information through interactions between the two entorhinal subdivisions and between the perirhinal and postrhinal/parahippocampal cortices, but most importantly through a new observation that the postrhinal/parahippocampal cortex projects to both lateral and medial entorhinal cortex. We suggest that entorhinal inputs provide the hippocampus with high-order complex representations of the external environment, its stability, as well as apparent changes either as an inherent feature of a biological environment or as the result of navigating the environment. This thus indicates that the current connectional model of the parahippocampal region as part of the medial temporal lobe memory system needs to be revised.

Development and topographical organization of projections from the hippocampus and parahippocampus to the retrosplenial cortex

The rat hippocampal formation (HF), parahippocampal region (PHR), and retrosplenial cortex (RSC) play critical roles in spatial processing. These regions are interconnected, and functionally dependent. The neuronal networks mediating this reciprocal dependency are largely unknown. Establishing the developmental timing of network formation will help to understand the emergence of this dependency. We questioned whether the long‐range outputs from HF‐PHR to RSC in Long Evans rats develop during the same time periods as previously reported for the intrinsic HF‐PHR connectivity and the projections from RSC to HF‐PHR. The results of a series of retrograde and anterograde tracing experiments in rats of different postnatal ages show that the postnatal projections from HF‐PHR to RSC display low densities around birth, but develop during the first postnatal week, reaching adult‐like densities around the time of eye‐opening. Developing projections display a topographical organization similar to adult projections. We conclude that the long‐range projections from HF‐PHR to RSC develop in parallel with the intrinsic circuitry of HF‐PHR and the projections of RSC to HF‐PHR.

Cajal-Retzius neurons are required for the development of the human hippocampal fissure

Cajal-Retzius neurons (CRN) are the main source of Reelin in the marginal zone of the developing neocortex and hippocampus (HC). They also express the transcription factor p73 and are complemented by later-appearing GABAergic Reelin⁺ interneurons. The human dorsal HC forms at gestational week 10 (GW10), when it develops a rudimentary Ammonic plate and incipient dentate migration, although the dorsal hippocampal fissure (HF) remains shallow and contains few CRN. The dorsal HC transforms into the indusium griseum (IG), concurrently with the rostro-caudal appearance of the corpus callosum, by GW14-17. Dorsal and ventral HC merge at the site of the former caudal hem, which is located at the level of the future atrium of the lateral ventricle and closely connected with the choroid plexus. The ventral HC forms at GW11 in the temporal lobe. The ventral HF is wide open at GW14-16 and densely populated by large numbers of CRNs. These are in intimate contact with the meninges and meningeal blood vessels, suggesting signalling through diverse pathways. At GW17, the fissure deepens and begins to fuse, although it is still marked by p73/Reelin⁺ CRNs. The p73KO mouse illustrates the importance of p73 in CRN for HF formation. In the mutant, Tbr1/Reelin⁺ CRNs are born in the hem but do not leave it and subsequently disappear, so that the mutant cortex and HC lack CRN from the onset of corticogenesis. The HF is absent, which leads to profound architectonic alterations of the HC. To determine which p73 isoform is important for HF formation, isoform-specific TAp73- and DeltaNp73-deficient embryonic and early postnatal mice were examined. In both mutants, the number of CRNs was reduced, but each of their phenotypes was much milder than in the global p73KO mutant missing both isoforms. In the TAp73KO mice, the HF of the dorsal HC failed to form, but was present in the ventral HC. In the DeltaNp73KO mice, the HC had a mild patterning defect along with a shorter HF. Complex interactions between both isoforms in CRNs may contribute to their crucial activity in the developing brain.

Marked Diversity of Unique Cortical Enhancers Enables Neuron-Specific Tools by Enhancer-Driven Gene Expression

Understanding neural circuit function requires individually addressing their component parts: specific neuronal cell types. However, not only do the precise genetic mechanisms specifying neuronal cell types remain obscure, access to these neuronal cell types by transgenic techniques also remains elusive. Whereas most genes are expressed in the brain, the vast majority are expressed in many different kinds of neurons, suggesting that promoters alone are not sufficiently specific to distinguish cell types. However, there are orders of magnitude more distal genetic cis-regulatory elements controlling transcription (i.e., enhancers), so we screened for enhancer activity in microdissected samples of mouse cortical subregions. This identified thousands of novel putative enhancers, many unique to particular cortical subregions. Pronuclear injection of expression constructs containing such region-specific enhancers resulted in transgenic lines driving expression in distinct sets of cells specifically in the targeted cortical subregions, even though the parent gene's promoter was relatively non-specific. These data showcase the promise of utilizing the genetic mechanisms underlying the specification of diverse neuronal cell types for the development of genetic tools potentially capable of targeting any neuronal circuit of interest, an approach we call enhancer-driven gene expression (EDGE).

Initiation and slow propagation of epileptiform activity from ventral to dorsal medial entorhinal cortex is constrained by an inhibitory gradient

KEY POINTS

The medial entorhinal cortex (mEC) has an important role in initiation and propagation of seizure activity. Several anatomical relationships exist in neurophysiological properties of mEC neurons; however, in the context of hyperexcitability, previous studies often considered it as a homogeneous structure. Using multi-site extracellular recording techniques, ictal-like activity was observed along the dorso-ventral axis of the mEC in vitro in response to various ictogenic stimuli. This originated predominantly from ventral areas, spreading to dorsal mEC with a surprisingly slow velocity. Modulation of inhibitory tone was capable of changing the slope of ictal initiation, suggesting seizure propagation behaviours are highly dependent on levels of GABAergic function in this region. A distinct disinhibition model also showed, in the absence of inhibition, a prevalence for interictal-like initiation in ventral mEC, reflecting the intrinsic differences in mEC neurons. These findings suggest the ventral mEC is more prone to hyperexcitable discharge than the dorsal mEC, which may be relevant under pathological conditions.

ABSTRACT

The medial entorhinal cortex (mEC) has an important role in the generation and propagation of seizure activity. The organization of the mEC is such that a number of dorso-ventral relationships exist in neurophysiological properties of neurons. These range from intrinsic and synaptic properties to density of inhibitory connectivity. We examined the influence of these gradients on generation and propagation of epileptiform activity in the mEC. Using a 16-shank silicon probe array to record along the dorso-ventral axis of the mEC in vitro, we found 4-aminopyridine application produces ictal-like activity originating predominantly in ventral areas. This activity spreads to dorsal mEC at a surprisingly slow velocity (138 μm s^-1 ), while cross-site interictal-like activity appeared relatively synchronous. We propose that ictal propagation is constrained by differential levels of GABAergic control since increasing (diazepam) or decreasing (Ro19-4603) GABA_A receptor activation, respectively, reduced or increased the slope of ictal initiation. The observation that ictal activity is predominately generated in ventral mEC was replicated using a separate 0-Mg²⁺ model of epileptiform activity in vitro. By using a distinct disinhibition model (co-application of kainate and picrotoxin) we show that additional physiological features (for example intrinsic properties of mEC neurons) still produce a prevalence for interictal-like initiation in ventral mEC. These findings suggest that the ventral mEC is more likely to initiate hyperexcitable discharges than the dorsal mEC, and that seizure propagation is highly dependent on levels of GABAergic expression across the mEC.

Dorsal and Ventral Hippocampus Differentiate in Functional Pathways and Differentially Associate with Neurological Disease-Related Genes during Postnatal Development

The dorsal and ventral regions of the hippocampus are important in cognitive and emotional processing, respectively. Various approaches have revealed the differential molecular and structural characteristics, and functional roles of the hippocampus. Recent RNA sequencing (RNA-seq) technology has enriched our understanding of the hippocampus by elucidating more detailed information on gene expression patterns. However, no RNA-seq–based study on gene profiles in the developing hippocampus has been reported. Using RNA-seq–based bioinformatic analysis in conjunction with quantitative real-time polymerase chain reaction analysis and a comparison of in situ hybridization data obtained from the Allen Brain Atlas, we provide a thorough analysis of differentially expressed genes in the dorsal and ventral hippocampus at specific developmental ages representing the postnatally maturing hippocampus. Genes associated with particular functional pathways and marker genes for particular neurological diseases were found to be distinctively segregated within either the dorsal or ventral hippocampus at specific or at all developmental ages examined. We also report novel molecular markers enriched in the dorsal or ventral hippocampus. Taken together, this study provides insights into the molecular mechanisms underlying physiological functions linked to the dorsal or ventral hippocampus. The information provided in the study also contributes to a better understanding of brain functions and serves as a resource for future studies on the pathophysiology of dorsal and ventral hippocampal functions.

The Transcription Factors COUP-TFI and COUP-TFII have Distinct Roles in Arealisation and GABAergic Interneuron Specification in the Early Human Fetal Telencephalon

In human telencephalon at 8-12 postconceptional weeks, ribonucleic acid quantitative sequencing and immunohistochemistry revealed cortical chicken ovalbumin upstream promotor-transcription factor 1 (COUP-TFI) expression in a high ventro-posterior to low anterior gradient except for raised immunoreactivity in the anterior ventral pallium. Unlike in mouse, COUP-TFI and SP8 were extensively co-expressed in dorsal sensory neocortex and dorsal hippocampus whereas COUPTFI/COUPTFII co-expression defined ventral temporal cortex and ventral hippocampus. In the ganglionic eminences (GEs) COUP-TFI immunoreactivity demarcated the proliferative zones of caudal GE (CGE), dorsal medial GE (MGE), MGE/lateral GE (LGE) boundary, and ventral LGE whereas COUP-TFII was limited to ventral CGE and the MGE/LGE boundary. Co-labeling with gamma amino butyric acidergic interneuron markers revealed that COUP-TFI was expressed in subpopulations of either MGE-derived (SOX6+) or CGE-derived (calretinin+/SP8+) interneurons. COUP-TFII was mainly confined to CGE-derived interneurons. Twice as many GAD67+ cortical cells co-labeled for COUP-TFI than for COUP-TFII. A fifth of COUP-TFI cells also co-expressed COUP-TFII, and cells expressing either transcription factor followed posterior or anterio-lateral pathways into the cortex, therefore, a segregation of migration pathways according to COUP-TF expression as proposed in mouse was not observed. In cultures differentiated from isolated human cortical progenitors, many cells expressed either COUP-TF and 30% also co-expressed GABA, however no cells expressed NKX2.1. This suggests interneurons could be generated intracortically from progenitors expressing either COUP-TF.

Sex-dependent effects of maternal corticosterone and SSRI treatment on hippocampal neurogenesis across development

Background

Postpartum depression affects approximately 15% of mothers and represents a form of early life adversity for developing offspring. Postpartum depression can be treated with prescription antidepressants like fluoxetine (FLX). However, FLX can remain active in breast milk, raising concerns about the consequences of neonatal FLX exposure. The hippocampus is highly sensitive to developmental stress, and males and females respond differently to stress at many endpoints, including hippocampal plasticity. However, it is unclear how developmental exposure to FLX alters the trajectory of hippocampal development. The goal of this study was to examine the long-term effects of maternal postpartum corticosterone (CORT, a model of postpartum depression) and concurrent FLX on hippocampal neurogenesis in male and female offspring.

Methods

Female Sprague-Dawley rat dams were treated daily with either CORT or oil and FLX or saline from postpartum days 2–23. Offspring were perfused on postnatal day 31 (pre-adolescent), postnatal day 42 (adolescent), and postnatal day 69 (adult). Tissue was processed for doublecortin (DCX), an endogenous marker of immature neurons, in the dorsal and ventral hippocampus.

Results

Maternal postpartum CORT reduced density of DCX-expressing cells in the dorsal hippocampus of pre-adolescent males and increased it in adolescent males, suggesting that postpartum CORT exposure disrupted the typical progression of the density of DCX-expressing cells. Further, among offspring of oil-treated dams, pre-adolescent males had greater density of DCX-expressing cells than pre-adolescent females, and maternal postpartum CORT prevented this sex difference. In pre-adolescent females, maternal postpartum FLX decreased the density of DCX-expressing cells in the dorsal hippocampus compared to saline. As expected, maternal CORT reduced the density of DCX-expressing cells in adult female, but not male, offspring. The combination of maternal postpartum CORT/FLX diminished density of DCX-expressing cells in dorsal hippocampus regardless of sex or age.

Conclusions

These findings reveal how modeling treatment of postpartum depression with FLX alters hippocampal neurogenesis in developing offspring differently depending on sex, predominantly in the dorsal dentate gyrus and earlier in life.

MicroRNAs contribute to postnatal development of laminar differences and neuronal subtypes in the rat medial entorhinal cortex

The medial entorhinal cortex (MEC) is important in spatial navigation and memory formation and its layers have distinct neuronal subtypes, connectivity, spatial properties, and disease susceptibility. As little is known about the molecular basis for the development of these laminar differences, we analyzed microRNA (miRNA) and messenger RNA (mRNA) expression differences between rat MEC layer II and layers III–VI during postnatal development. We identified layer and age-specific regulation of gene expression by miRNAs, which included processes related to neuron specialization and locomotor behavior. Further analyses by retrograde labeling and expression profiling of layer II stellate neurons and in situ hybridization revealed that the miRNA most up-regulated in layer II, miR-143, was enriched in stellate neurons, whereas the miRNA most up-regulated in deep layers, miR-219-5p, was expressed in ependymal cells, oligodendrocytes and glia. Bioinformatics analyses of predicted mRNA targets with negatively correlated expression patterns to miR-143 found that miR-143 likely regulates the Lmo4 gene, which is known to influence hippocampal-based spatial learning.

Structural development and dorsoventral maturation of the medial entorhinal cortex

We investigated the structural development of superficial-layers of medial entorhinal cortex and parasubiculum in rats. The grid-layout and cholinergic-innervation of calbindin-positive pyramidal-cells in layer-2 emerged around birth while reelin-positive stellate-cells were scattered throughout development. Layer-3 and parasubiculum neurons had a transient calbindin-expression, which declined with age. Early postnatally, layer-2 pyramidal but not stellate-cells co-localized with doublecortin - a marker of immature neurons - suggesting delayed functional-maturation of pyramidal-cells. Three observations indicated a dorsal-to-ventral maturation of entorhinal cortex and parasubiculum: (i) calbindin-expression in layer-3 neurons decreased progressively from dorsal-to-ventral, (ii) doublecortin in layer-2 calbindin-positive-patches disappeared dorsally before ventrally, and (iii) wolframin-expression emerged earlier in dorsal than ventral parasubiculum. The early appearance of calbindin-pyramidal-grid-organization in layer-2 suggests that this pattern is instructed by genetic information rather than experience. Superficial-layer-microcircuits mature earlier in dorsal entorhinal cortex, where small spatial-scales are represented. Maturation of ventral-entorhinal-microcircuits - representing larger spatial-scales - follows later around the onset of exploratory behavior.

Postnatal development of retrosplenial projections to the parahippocampal region of the rat

The rat parahippocampal region (PHR) and retrosplenial cortex (RSC) are cortical areas important for spatial cognition. In PHR, head-direction cells are present before eye-opening, earliest detected in postnatal day (P)11 animals. Border cells have been recorded around eye-opening (P16), while grid cells do not obtain adult-like features until the fourth postnatal week. In view of these developmental time-lines, we aimed to explore when afferents originating in RSC arrive in PHR. To this end, we injected rats aged P0-P28 with anterograde tracers into RSC. First, we characterized the organization of RSC-PHR projections in postnatal rats and compared these results with data obtained in the adult. Second, we described the morphological development of axonal plexus in PHR. We conclude that the first arriving RSC-axons in PHR, present from P1 onwards, already show a topographical organization similar to that seen in adults, although the labeled plexus does not obtain adult-like densities until P12.

Mapping the electrophysiological and morphological properties of CA1 pyramidal neurons along the longitudinal hippocampal axis

Differences in behavioral roles, anatomical connectivity, and gene expression patterns in the dorsal, intermediate, and ventral regions of the hippocampus are well characterized. Relatively fewer studies have, however, focused on comparing the physiological properties of neurons located at different dorsoventral extents of the hippocampus. Recently, we reported that dorsal CA1 neurons are less excitable than ventral neurons. There is little or no information for how neurons in the intermediate hippocampus compare to those from the dorsal and ventral ends. Also, it is not known whether the transition of properties along the dorsoventral axis is gradual or segmented. In this study, we developed a statistical model to predict the dorsoventral position of transverse hippocampal slices. Using current clamp recordings combined with this model, we found that CA1 neurons in dorsal, intermediate, and ventral hippocampus have distinct electrophysiological and morphological properties and that the transition in most (but not all) of these properties from the ventral to dorsal end is gradual. Using linear and segmented regression analyses, we found that input resistance and resting membrane potential changed linearly along the V-D axis. Interestingly, the transition in resonance frequency, rebound slope, dendritic branching in stratum radiatum, and action potential properties was segmented along the V-D axis. Together, the findings from this study highlight the heterogeneity in CA1 neuronal properties along the entire longitudinal axis of hippocampus.

A ventral view on antidepressant action: roles for adult hippocampal neurogenesis along the dorsoventral axis

Adult hippocampal neurogenesis is implicated in antidepressant action, stress responses, and cognitive functioning. The hippocampus is functionally segregated along its longitudinal axis into dorsal (dHi) and ventral (vHi) regions in rodents, and analogous posterior and anterior regions in primates, whereby the vHi preferentially regulates stress and anxiety, while the dHi preferentially regulates spatial learning and memory. Given the role of neurogenesis in functions preferentially regulated by the dHi or vHi, it is plausible that neurogenesis is preferentially regulated in either the dHi or vHi depending upon the stimulus. We appraise here the literature on the effects of stress and antidepressants on neurogenesis along the hippocampal longitudinal axis and explore whether preferential regulation of neurogenesis in the vHi/anterior hippocampus contributes to stress resilience and antidepressant action.