Redefining the boundaries of the hippocampal CA2 subfield in the mouse using gene expression and 3-dimensional reconstruction

RGS14 is a natural suppressor of both synaptic plasticity in CA2 neurons and hippocampal-based learning and memory

Learning and memory have been closely linked to strengthening of synaptic connections between neurons (i.e., synaptic plasticity) within the dentate gyrus (DG)-CA3-CA1 trisynaptic circuit of the hippocampus. Conspicuously absent from this circuit is area CA2, an intervening hippocampal region that is poorly understood. Schaffer collateral synapses on CA2 neurons are distinct from those on other hippocampal neurons in that they exhibit a perplexing lack of synaptic long-term potentiation (LTP). Here we demonstrate that the signaling protein RGS14 is highly enriched in CA2 pyramidal neurons and plays a role in suppression of both synaptic plasticity at these synapses and hippocampal-based learning and memory. RGS14 is a scaffolding protein that integrates G protein and H-Ras/ERK/MAP kinase signaling pathways, thereby making it well positioned to suppress plasticity in CA2 neurons. Supporting this idea, deletion of exons 2-7 of the RGS14 gene yields mice that lack RGS14 (RGS14-KO) and now express robust LTP at glutamatergic synapses in CA2 neurons with no impact on synaptic plasticity in CA1 neurons. Treatment of RGS14-deficient CA2 neurons with a specific MEK inhibitor blocked this LTP, suggesting a role for ERK/MAP kinase signaling pathways in this process. When tested behaviorally, RGS14-KO mice exhibited marked enhancement in spatial learning and in object recognition memory compared with their wild-type littermates, but showed no differences in their performance on tests of nonhippocampal-dependent behaviors. These results demonstrate that RGS14 is a key regulator of signaling pathways linking synaptic plasticity in CA2 pyramidal neurons to hippocampal-based learning and memory but distinct from the canonical DG-CA3-CA1 circuit.

Distinctive hippocampal CA2 subfield of the Amazon rodent Proechimys

Previous data of our laboratory have shown that the Amazonian rodents Proechimys do not present spontaneous seizures in different models of epilepsy, suggesting endogenous inhibitory mechanisms. Here, we describe a remarkably different Proechimy's cytoarchitecture organization of the hippocampal cornu Ammonis 2 (CA2) subfield. We identified a very distinctive Proechimy's CA2 sector exhibiting disorganized cell presentation of the pyramidal layer and atypical dispersion of the pyramidal-like cells to the stratum oriens, strongly contrasting to the densely packed CA2 cells in the Wistar rats. Studies showed that CA2 is the only cornu ammonis (CA) subfield resistant to the extensive pyramidal neural loss in mesial temporal lobe epilepsy (MTLE) associated to hippocampal sclerosis. Thus, in order to investigate this region, we used Nissl and Timm staining, stereological approach to count neurons and immunohistochemistry to neuronal nuclei (NeuN), parvalbumin (PV), calbindin (CB) and calretinin (CR). We did not notice statistically significant differences in the total number of neurons of the CA2 region between Proechimys and Wistar. However, Proechimys rodents presented higher CA2 volume than Wistar rats. Furthermore, no significant difference in the optical density of parvalbumin-immunoreactivity was found between subject groups. On the other hand, Proechimys presented significant higher density of calbindin and calretinin-immunoreactivity when compared to Wistar rats. In this context, this unique CA2 subfield seen in Proechimys opens up a new set of possibilities to explore the contribution of CA2 neurons in normal and pathological brain circuits.

Zbtb20 is essential for the specification of CA1 field identity in the developing hippocampus

The development of hippocampal circuitry depends on the proper assembly of correctly specified and fully differentiated hippocampal neurons. Little is known about factors that control the hippocampal specification. Here, we show that zinc finger protein Zbtb20 is essential for the specification of hippocampal CA1 field identity. We found that Zbtb20 expression was initially activated in the hippocampal anlage at the onset of corticogenesis, and persisted in immature hippocampal neurons. Targeted deletion of Zbtb20 in mice did not compromise the progenitor proliferation in the hippocampal and adjacent transitional ventricular zone, but led to the transformation of the hippocampal CA1 field into a transitional neocortex-like structure, as evidenced by cytoarchitectural, neuronal migration, and gene expression phenotypes. Correspondingly, the subiculum was ectopically located adjacent to the CA3 in mutant. Although the field identities of the mutant CA3 and dentate gyrus (DG) were largely maintained, their projections were severely impaired. The hippocampus of Zbtb20 null mice was reduced in size, and exhibited increased apoptotic cell death during postnatal development. Our data establish an essential role of Zbtb20 in the specification of CA1 field identity by repressing adjacent transitional neocortex-specific fate determination.

Insulin/PI3K signaling protects dentate neurons from oxygen-glucose deprivation in organotypic slice cultures

It is known that ischemia/reperfusion induces neurodegeneration in the hippocampus in a subregion-dependent manner. This study investigated the mechanism of selective resistance/vulnerability to oxygen-glucose deprivation (OGD) using mouse organotypic hippocampal cultures. Analysis of propidium iodide uptake showed that OGD-induced duration- and subregion-dependent neuronal injury. When compared with the CA1-3 subregions, dentate neuronal survival was more sensitive to inhibition of phosphatidylinositol 3-kinase (PI3K)/Akt signaling under basal conditions. Dentate neuronal sensitivity to PI3K/Akt signaling activation was inversely related to its vulnerability to OGD-induced injury; insulin/insulin-like growth factor 1 pre-treatment conferred neuroprotection to dentate neurons via activation of PI3K/Akt signaling. In contrast, CA1 and CA3 neurons were less sensitive to disruptions of endogenous PI3K/Akt signaling and protective effects of insulin/insulin-like growth factor 1, but more vulnerable to OGD. OGD-induced injury in CA1 was reduced by inhibition of NMDA receptor or mitogen-activated protein kinase signaling, and was prevented by blocking NMDA receptor in the presence of insulin. The CA2 subregion was distinctive in its response to glutamate, OGD, and insulin, compared with other CA subregions. CA2 neurons were sensitive to the protective effects of insulin against OGD-induced injury, but more resistant to glutamate. Distinctive distribution of insulin receptor beta and basal phospho-Akt was detected in our slice cultures. Our results suggest a role for insulin signaling in subregional resistance/vulnerability to cerebral ischemia.

Genomic-anatomic evidence for distinct functional domains in hippocampal field CA1

Functional heterogeneity has been investigated for decades in the hippocampal region of the mammalian cerebral cortex, and evidence for vaguely defined "dorsal" and "ventral" regions is emerging. Direct evidence that hippocampal field CA1 displays clear regional, laminar, and pyramidal neuron differentiation is presented here, based on a systematic high-resolution analysis of a publicly accessible, genome-wide expression digital library (Allen Brain Atlas) [Lein et al. (2007) Genome-wide atlas of gene expression in the adult mouse brain. Nature 445:168-176]. First, genetic markers reveal distinct spatial expression domains and subdomains along the longitudinal (dorsal/septal/posterior to ventral/temporal/anterior) axis of field CA1. Second, genetic markers divide field CA1 pyramidal neurons into multiple subtypes with characteristic laminar distributions. And third, subcortical brain regions receiving axonal projections from molecularly distinct spatial domains of field CA1 display distinct global gene expression patterns, suggesting that field CA1 spatial domains may be genetically wired independently to form distinct functional networks related to cognition and emotion. Insights emerging from this genomic-anatomic approach provide a starting point for a detailed analysis of differential hippocampal structure-function organization.

Distribution of neuromedin U binding sites in the rat CNS revealed by in vitro receptor autoradiography

Neuromedin U (NMU), a neuropeptide implicated in feeding, inflammation, pain control and anxiety-related behaviours, is widely distributed in peripheral organs and the CNS. These effects are thought to be mediated by its receptors NMU(1) and NMU(2). Since its precise sites of interaction in the CNS were to date unknown, we studied the distribution of in vitro binding sites for (125)I-NMU-23 in the rat CNS by receptor autoradiography. High-density specific binding was found in discrete areas of the brain and spinal cord, namely in the limbic system (hippocampal formation, septohippocampal nucleus, indusium griseum, hypothalamus, amygdaloid nuclei), superior colliculus, dorsal raphé, and substantia gelatinosa of the spinal cord. Our findings provide further supportive evidence for a multifunctional role for the peptide in the brain and spinal cord.

Application of microarray outlier detection methodology to psychiatric research

BACKGROUND

Most microarray data processing methods negate extreme expression values or alter them so that they do not lie outside the mean level of variation of the system. While microarrays generate a substantial amount of false positive and spurious results, some of the extreme expression values may be valid and could represent true biological findings.

METHODS

We propose a simple method to screen brain microarray data to detect individual differences across a psychiatric sample set. We demonstrate in two different samples how this method can be applied.

RESULTS

This method targets high-throughput technology to psychiatric research on a subject-specific basis.

CONCLUSION

Assessing microarray data for both mean group effects and individual effects can lead to more robust findings in psychiatric genetics.

Synaptic plasticity (and the lack thereof) in hippocampal CA2 neurons

The hippocampus is critical for some forms of memory and spatial navigation, but previous research has mostly neglected the CA2, a unique region situated between CA3 and CA1. Here, we show that CA2 pyramidal neurons have distinctive physiological characteristics that include an unprecedented synaptic stability. Although basal synaptic currents in CA1 and CA2 are quite similar, synaptic plasticity including long-term potentiation and long-term depression is absent or less likely to be induced with conventional methods of stimulation in CA2. We also find that CA2 neurons have larger leak currents and more negative resting membrane potentials than CA1 neurons, and consequently, more current is required for action potential generation in CA2 neurons. These data suggest that the molecular "conspiracy against plasticity" in CA2 makes it functionally distinct from the other hippocampal CA regions. This work provides critical insight into hippocampal function and may lead to an understanding of the resistance of CA2 to damage from disease, trauma, and hypoxia.

Characterization of neurons in the CA2 subfield of the adult rat hippocampus

The hippocampal cornu ammonis 2 (CA2) region is unique in being the only CA region receiving inputs from the hypothalamic supramammillary nucleus, of importance in modulating hippocampal theta rhythm, and is seizure resistant in temporal lobe epilepsy. CA2 has, however, been little studied, possibly because of its small size and difficulty encountered in defining its borders. To investigate the properties of CA2 interneurons, intracellular recordings with biocytin filling were made in adult hippocampal slices. Two types of basket cells were identified. A minority resembled those in CA1, with fast spiking behavior, vertically oriented dendrites, and axons confined to the region of origin. In contrast, the majority of parvalbumin-immunopositive CA2 basket and bistratified cells had long, horizontally oriented, sparsely spiny dendrites extending into all CA subfields in stratum oriens, adapting firing patterns and a pronounced "sag" in voltage responses to hyperpolarizing current, indicative of I(h). Broad CA2 basket cells innervated all three CA subfields and could thus provide CA1 and CA2 with feedforward and CA3 with feedback inhibition. In contrast, CA2 bistratified cell axons displayed striking subfield preference, innervating stratum oriens and stratum radiatum of CA2 and CA1 but stopping abruptly at the CA2/CA3 border, implying feedforward inhibition of CA2 and CA1. These unique features suggest that CA2 is more than a transitional region between CA1 and CA3. The pronounced slow sag current of many CA2 interneurons may contribute to coordination of pyramidal cell firing during theta, whereas the fast spiking behavior of a smaller population of interneurons supports more localized gamma.

Voltage imaging reveals the CA1 region at the CA2 border as a focus for epileptiform discharges and long-term potentiation in hippocampal slices

Voltage-sensitive-dye imaging was used to study the initiation and propagation of epileptiform activity in transverse hippocampal slices. A portion of the slices tested generated epileptiform discharges in response to electrical shocks under normal physiological conditions. The fraction of slices showing epileptiform responses increased from 44 to 86% when bathing [K+] increased from 3.2 to 4 mM. Regardless of stimulation site in the dentate gyrus and hippocampus, discharges generally initiated in the CA3 region. After onset, discharges abruptly appeared in the CA1 region, right at the CA2 border. This spread from the CA3 region to the CA1 region was saltatory, occurring before detectable activity in the intervening CA2 and CA3 regions. Discharges did eventually propagate smoothly through the intervening CA3 region into the CA2 region, but on a slower timescale. The surge in the CA1 region did not spread back into the CA2 region, but spread through the CA1 region toward the subiculum. Tetanic stimulation, theta bursts, and GABA(A) receptor antagonists failed to alter this characteristic pattern, but did reduce the latency of discharge onset. The part of the CA1 region at the CA2 border, where epileptic responses emerged with relatively short latency, also expressed stronger long-term potentiation (LTP) than the rest of the CA1 region. The CA2 region, where discharges had long latencies and low amplitudes, expressed weaker LTP. Thus the CA1 region at the CA2 border has unique properties, which make this part of the hippocampus an important locus for both epileptiform activity and plasticity.

Nucleus- and cell-specific gene expression in monkey thalamus

Nuclei of the mammalian thalamus are aggregations of neurons with unique architectures and input-output connections, yet the molecular determinants of their organizational specificity remain unknown. By comparing expression profiles of thalamus and cerebral cortex in adult rhesus monkeys, we identified transcripts that are unique to dorsal thalamus or to individual nuclei within it. Real-time quantitative PCR and in situ hybridization analyses confirmed the findings. Expression profiling of individual nuclei microdissected from the dorsal thalamus revealed additional subsets of nucleus-specific genes. Functional annotation using Gene Ontology (GO) vocabulary and Ingenuity Pathways Analysis revealed overrepresentation of GO categories related to development, morphogenesis, cell-cell interactions, and extracellular matrix within the thalamus- and nucleus-specific genes, many involved in the Wnt signaling pathway. Examples included the transcription factor TCF7L2, localized exclusively to excitatory neurons; a calmodulin-binding protein PCP4; the bone extracellular matrix molecules SPP1 and SPARC; and other genes involved in axon outgrowth and cell matrix interactions. Other nucleus-specific genes such as CBLN1 are involved in synaptogenesis. The genes identified likely underlie nuclear specification, cell phenotype, and connectivity during development and their maintenance in the adult thalamus.

The dentate mossy fibers: structural organization, development and plasticity

Hippocampal mossy fibers are the axons of the dentate granule cells and project to hippocampal CA3 pyramidal cells and mossy cells of the dentate hilus (CA4) as well as a number of interneurons in the two areas. Besides their role in hippocampal function, studies of which are still evolving and taking interesting turns, the mossy fibers display a number of unique features with regard to axonal projections, terminal structures and synaptic contacts, development and variations among species and strains, as well as to normal occurring and lesion-induced plasticity and neural transplantation. These features are the topic of this review, which will use the mossy fiber system of the rat as basis and reference in its aim to provide an up-to-date, yet historically based guide to students in the field.

Genome-wide atlas of gene expression in the adult mouse brain

Molecular approaches to understanding the functional circuitry of the nervous system promise new insights into the relationship between genes, brain and behaviour. The cellular diversity of the brain necessitates a cellular resolution approach towards understanding the functional genomics of the nervous system. We describe here an anatomically comprehensive digital atlas containing the expression patterns of approximately 20,000 genes in the adult mouse brain. Data were generated using automated high-throughput procedures for in situ hybridization and data acquisition, and are publicly accessible online. Newly developed image-based informatics tools allow global genome-scale structural analysis and cross-correlation, as well as identification of regionally enriched genes. Unbiased fine-resolution analysis has identified highly specific cellular markers as well as extensive evidence of cellular heterogeneity not evident in classical neuroanatomical atlases. This highly standardized atlas provides an open, primary data resource for a wide variety of further studies concerning brain organization and function.

Juvenile administration of methylphenidate attenuates adult hippocampal neurogenesis

BACKGROUND

The neural consequences of early-life exposure to methylphenidate (MPH; Ritalin) are of great interest given the widespread, and sometimes inappropriate, use in children. Here we examine the impact of juvenile MPH exposure on adult hippocampal neurogenesis.

METHODS

Rats received MPH (2.0 mg/kg, intraperitoneal, twice daily) or saline (SAL) during preadolescence (postnatal days 20-35). Hippocampal cell proliferation (Experiment 1), neurogenesis (Experiment 2), and stress-induced changes in cell proliferation (Experiment 3) were assessed at several developmental stages including adulthood.

RESULTS

Juvenile exposure to MPH did not alter proliferation at any developmental time point relative to control rats; however, exposure to MPH significantly decreased the long-term survival of newborn cells in adult rats, particularly in the temporal hippocampus. Although MPH-treated rats had higher levels of corticosterone after restraint stress, they did not show the expected greater decrease in hippocampal cell proliferation relative to control animals.

CONCLUSIONS

Early-life exposure to MPH inhibits the survival of adult-generated neurons in the temporal hippocampus and may reduce progenitor sensitivity to corticosterone-induced decreases in proliferation. These findings suggest that decreased adult neurogenesis is an enduring consequence of early-life exposure to MPH and are discussed for their relevance to humans.

The vasopressin 1b receptor is prominent in the hippocampal area CA2 where it is unaffected by restraint stress or adrenalectomy

The vasopressin 1b receptor (Avpr1b) is one of two principal receptors mediating the behavioral effects of vasopressin (Avp) in the brain. Avpr1b has recently been shown to strongly influence social forms of aggression in mice and hamsters. This receptor appears to play a role in social recognition and motivation as well as in regulating the hypothalamic-pituitary-adrenal axis. Most of these studies have been performed in knockout mice, a species in which the localization of the Avpr1b has not been described, thus precluding correlations with the behaviors. We performed in situ hybridization histochemistry (ISHH) with specific probes and found especially prominent expression within the CA2 pyramidal neurons of the hippocampus, with much lower expression in the hypothalamic paraventricular nucleus and amygdala. Reverse transcriptase-polymerase chain reaction (RT-PCR) confirmed expression in those as well other areas in which the ISHH was not sensitive enough to detect labeled cells (e.g. piriform cortex, septum, caudate-putamen and lower brainstem areas). Mouse Avpr1b transcript levels were not altered in the CA2 field by restraint stress or adrenalectomy. Finally, ISHH and RT-PCR showed expression of the Avpr1b gene in the rat and human hippocampi as well. We suggest that the CA2 field may form or retrieve associations (memories) between olfactory cues and social encounters.

Opiates, psychostimulants, and adult hippocampal neurogenesis: Insights for addiction and stem cell biology

Once thought to produce global, nonspecific brain injury, drugs of abuse are now known to produce selective neuro-adaptations in particular brain regions. These neuro-adaptations are being closely examined for clues to the development, maintenance, and treatment of addiction. The hippocampus is an area of particular interest, as it is central to many aspects of the addictive process, including relapse to drug taking. A recently appreciated hippocampal neuro-adaptation produced by drugs as diverse as opiates and psychostimulants is decreased neurogenesis in the sub-granular zone (SGZ). While the role of adult-generated neurons is not clear, their functional integration into hippocampal circuitry raises the possibility that decreased adult SGZ neurogenesis may alter hippocampal function in such a way as to maintain addictive behavior or contribute to relapse. Here, we review the impact of opiates and psychostimulants on the different stages of cell development in the adult brain, as well as the different stages of the addictive process. We discuss how examination of drug-induced alterations of adult neurogenesis advances our understanding of the complex mechanisms by which opiates and psychostimulants affect brain function while also opening avenues for novel ways of assessing the functional role of adult-generated neurons. In addition, we highlight key discrepancies in the field and underscore the necessity to move "beyond BrdU"--beyond merely counting new hippocampal cells labeled with the S phase marker bromodeoxyuridine--so as to probe mechanistic questions about how drug-induced alterations in adult hippocampal neurogenesis occur and what the functional ramifications of alterations in neurogenesis are for addiction.