Key morphological features of human pyramidal neurons.

The basic building block of the cerebral cortex, the pyramidal cell, has been shown to be characterized by a markedly different dendritic structure among layers, cortical areas, and species. Functionally, differences in the structure of their dendrites and axons are critical in determining how neurons integrate information. However, within the human cortex, these neurons have not been quantified in detail. In the present work, we performed intracellular injections of Lucifer Yellow and 3D reconstructed over 200 pyramidal neurons, including apical and basal dendritic and local axonal arbors and dendritic spines, from human occipital primary visual area and associative temporal cortex. We found that human pyramidal neurons from temporal cortex were larger, displayed more complex apical and basal structural organization, and had more spines compared to those in primary sensory cortex. Moreover, these human neocortical neurons displayed specific shared and distinct characteristics in comparison to previously published human hippocampal pyramidal neurons. Additionally, we identified distinct morphological features in human neurons that set them apart from mouse neurons. Lastly, we observed certain consistent organizational patterns shared across species. This study emphasizes the existing diversity within pyramidal cell structures across different cortical areas and species, suggesting substantial species-specific variations in their computational properties.

Alternative Brain Slice-on-a-Chip for Organotypic Culture and Effective Fluorescence Injection Testing.

Mouse brain slices are one of the most common models to study brain development and functioning, increasing the number of study models that integrate microfluidic systems for hippocampal slice cultures. This report presents an alternative brain slice-on-a-chip, integrating an injection system inside the chip to dispense a fluorescent dye for long-term monitoring. Hippocampal slices have been cultured inside these chips, observing fluorescence signals from living cells, maintaining the cytoarchitecture of the slices. Having fluorescence images of biological samples inside the chip demonstrates the effectiveness of the staining process using the injection method avoiding leaks or biological contamination. The technology developed in this study presents a significant improvement in the local administration of reagents within a brain slice-on-a-chip system, which could be a suitable option for organotypic cultures in a microfluidic chip acting as a highly effective bioreactor.

Differential Structure of Hippocampal CA1 Pyramidal Neurons in the Human and Mouse.

Pyramidal neurons are the most common cell type and are considered the main output neuron in most mammalian forebrain structures. In terms of function, differences in the structure of the dendrites of these neurons appear to be crucial in determining how neurons integrate information. To further shed light on the structure of the human pyramidal neurons we investigated the geometry of pyramidal cells in the human and mouse CA1 region-one of the most evolutionary conserved archicortical regions, which is critically involved in the formation, consolidation, and retrieval of memory. We aimed to assess to what extent neurons corresponding to a homologous region in different species have parallel morphologies. Over 100 intracellularly injected and 3D-reconstructed cells across both species revealed that dendritic and axonal morphologies of human cells are not only larger but also have structural differences, when compared to mouse. The results show that human CA1 pyramidal cells are not a stretched version of mouse CA1 cells. These results indicate that there are some morphological parameters of the pyramidal cells that are conserved, whereas others are species-specific.

Slow-Wave Activity in the S1HL Cortex Is Contributed by Different Layer-Specific Field Potential Sources during Development.

Spontaneous correlated activity in cortical columns is critical for postnatal circuit refinement. We used spatial discrimination techniques to explore the late maturation of synaptic pathways through the laminar distribution of the field potential (FP) generators underlying spontaneous and evoked activities of the S1HL cortex in juvenile (P14-P16) and adult anesthetized rats. Juveniles exhibit an intermittent FP pattern resembling Up/Down states in adults, but with much reduced power and different laminar distribution. Whereas FPs in active periods are dominated by a layer VI generator in juveniles, in adults a developing multipart generator takes over, displaying current sinks in middle layers (III-V). The blockade of excitatory transmission in upper and middle layers of adults recovered the juvenile-like FP profiles. In addition to the layer VI generator, a gamma-specific generator in supragranular layers was the same in both age groups. While searching for dynamical coupling among generators in juveniles we found significant cross-correlation in ∼one-half of the tested pairs, whereas excessive coherence hindered their efficient separation in adults. Also, potentials evoked by tactile and electrical stimuli showed different short-latency dipoles between the two age groups, and the juveniles lacked the characteristic long latency UP state currents in middle layers. In addition, the mean firing rate of neurons was lower in juveniles. Thus, cortical FPs originate from different intra-columnar segments as they become active postnatally. We suggest that although some cortical segments are active early postnatally, a functional sensory-motor control relies on a delayed maturation and network integration of synaptic connections in middle layers.SIGNIFICANCE STATEMENT Early postnatal activity in the rodent cortex is mostly endogenous, whereas it becomes driven by peripheral input at later stages. The precise schedule for the maturation of synaptic pathways is largely unknown. We explored this in the somatosensory hindlimb cortex at an age when animals begin to use their limbs by uncovering the laminar distribution of the field potential generators underlying the dominant delta waves in juveniles and adults. Our results suggest that field potentials are mostly generated by a pathway in deep layers, whereas other pathways mature later in middle layers and take over in adults. We suggest that a functional sensory-motor control relies on a delayed maturation and network integration of synaptic connections in middle layers.

Smad3 deficiency inhibits dentate gyrus LTP by enhancing GABAA neurotransmission.

Transforming growth factor-ß signaling through intracellular Smad3 has been implicated in Parkinson's disease (PD) and it fulfills an important role in the neurogenesis and synaptic plasticity that occurs in the adult dentate gyrus (DG). The long-term potentiation (LTP) induced in the DG by high-frequency stimulation of the medial perforant pathway is abolished in the DG of Smad3-deficient mice, but not in the CA1 hippocampal region. Here, we show that NMDA- and AMPA-type glutamate receptors do not participate in the inhibition of LTP associated with Smad3 deficiency. Moreover, there is no difference in the hippocampal GAD65 and GAD67 content, suggesting that GABA biosynthesis remains unaffected. Increased conductance and higher action potential firing thresholds were evident in intracellular recordings of granule cells from Smad3 deficient mice. Interestingly, phasic and tonic GABAA receptor (GABAA R)-mediated neurotransmission is enhanced in the DG of Smad3-deficient mice, and LTP induction can be rescued by inhibiting GABAA R with picrotoxin. Hence, Smad3 signaling in the DG appears to be necessary to induce LTP by regulating GABAA neurotransmission, suggesting a central role of this intracellular signaling pathway in the hippocampal brain plasticity related to learning and memory. Smad3 deficient mice represent a new and interesting model of Parkinson's disease, displaying hippocampal dysfunctions that include decreased neurogenesis and the failure to induce LTP in the dentate gyrus. Here we show that Smad3 deficiency inhibits LTP induction by enhancing phasic and tonic GABAA receptor-mediated neurotransmission, while LTP induction can be rescued with a GABAA receptor antagonist. Alteration of GABA neurotransmission is thought to produce hippocampal cognitive dysfunction in Down's syndrome or Alzheimer's disease, and here we provide new insights into the hippocampal changes in an animal model of Parkinson's disease.

Smad3 is required for the survival of proliferative intermediate progenitor cells in the dentate gyrus of adult mice.

BACKGROUND: New neurons are continuously being generated in the adult hippocampus, a phenomenon that is regulated by external stimuli, such as learning, memory, exercise, environment or stress. However, the molecular mechanisms underlying neuron production and how they are integrated into existing circuits under such physiological conditions remain unclear. Indeed, the intracellular modulators that transduce the extracellular signals are not yet fully understood. RESULTS: We show that Smad3, an intracellular molecule involved in the transforming growth factor (TGF)-ß signaling cascade, is strongly expressed by granule cells in the dentate gyrus (DG) of adult mice, although the loss of Smad3 in null mutant mice does not affect their survival. Smad3 is also expressed by adult progenitor cells in the subgranular zone (SGZ) and more specifically, it is first expressed by Type 2 cells (intermediate progenitor cells). Its expression persists through the distinct cell stages towards that of the mature neuron. Interestingly, proliferative intermediate progenitor cells die in Smad3 deficiency, which is associated with a large decrease in the production of newborn neurons in Smad3 deficient mice. Smad3 signaling appears to influence adult neurogenesis fulfilling distinct roles in the rostral and mid-caudal regions of the DG. In rostral areas, Smad3 deficiency increases proliferation and promotes the cell cycle exit of undifferentiated progenitor cells. By contrast, Smad3 deficiency impairs the survival of newborn neurons in the mid-caudal region of the DG at early proliferative stages, activating apoptosis of intermediate progenitor cells. Furthermore, long-term potentiation (LTP) after high frequency stimulation (HFS) to the medial perforant path (MPP) was abolished in the DG of Smad3-deficient mice. CONCLUSIONS: These data show that endogenous Smad3 signaling is central to neurogenesis and LTP induction in the adult DG, these being two forms of hippocampal brain plasticity related to learning and memory that decline with aging and as a result of neurological disorders.

Secretagogin as a marker to distinguish between different neuron types in human frontal and temporal cortex.

The principal aim of the present work was to chemically characterize the population of neurons labeled for the calcium binding protein secretagogin (SCGN) in the human frontal and temporal cortices (Brodmann's area 10 and 21, respectively). Both cortical regions are involved in many high cognitive functions that are especially well developed (or unique) in humans, but with different functional roles. The pattern of SCGN immunostaining was rather similar in BA10 and BA21, with all the labeled neurons displaying a non-pyramidal morphology (interneurons). Although SCGN cells were present throughout all layers, they were more frequently observed in layers II, III and IV, whereas in layer I they were found only occasionally. We examined the degree of colocalization of SCGN with parvalbumin (PV) and calretinin (CR), as well as with nitric oxide synthase (nNOS; the enzyme responsible for the synthesis of nitric oxide by neurons) by triple immunostaining. We looked for possible similarities or differences in the coexpression patterns of SCGN with PV, CR and nNOS between BA10 and BA21 throughout the different cortical layers (I-VI). The percentage of colocalization was estimated by counting the number of all labeled cells through columns (1,100-1,400 µm wide) across the entire thickness of the cortex (from the pial surface to the white matter) in 50 µm-thick sections. Several hundred neurons were examined in both cortical regions. We found that SCGN cells include multiple neurochemical subtypes, whose abundance varies according to the cortical area and layer. The present results further highlight the regional specialization of cortical neurons and underline the importance of performing additional experiments to characterize the subpopulation of SCGN cells in the human cerebral cortex in greater detail.

Ex vivo, in situ perfusion protocol for human brain fixation compatible with microscopy, MRI techniques, and anatomical studies.

We present a method for human brain fixation based on simultaneous perfusion of 4% paraformaldehyde through carotids after a flush with saline. The left carotid cannula is used to perfuse the body with 10% formalin, to allow further use of the body for anatomical research or teaching. The aim of our method is to develop a vascular fixation protocol for the human brain, by adapting protocols that are commonly used in experimental animal studies. We show that a variety of histological procedures can be carried out (cyto- and myeloarchitectonics, histochemistry, immunohistochemistry, intracellular cell injection, and electron microscopy). In addition, ex vivo, ex situ high-resolution MRI (9.4T) can be obtained in the same specimens. This procedure resulted in similar morphological features to those obtained by intravascular perfusion in experimental animals, provided that the postmortem interval was under 10 h for several of the techniques used and under 4 h in the case of intracellular injections and electron microscopy. The use of intravascular fixation of the brain inside the skull provides a fixed whole human brain, perfectly fitted to the skull, with negligible deformation compared to conventional techniques. Given this characteristic of ex vivo, in situ fixation, this procedure can probably be considered the most suitable one available for ex vivo MRI scans of the brain. We describe the compatibility of the method proposed for intravascular fixation of the human brain and fixation of the donor's body for anatomical purposes. Thus, body donor programs can provide human brain tissue, while the remainder of the body can also be fixed for anatomical studies. Therefore, this method of human brain fixation through the carotid system optimizes the procurement of human brain tissue, allowing a greater understanding of human neurological diseases, while benefiting anatomy departments by making the remainder of the body available for teaching purposes.

Metabolic Changes in Brain Slices over Time: a Multiplatform Metabolomics Approach.

Brain slice preparations are widely used for research in neuroscience. However, a high-quality preparation is essential and there is no consensus regarding stable parameters that can be used to define the status of the brain slice preparation after its collection at different time points. Thus, it is critical to fully characterize the experimental conditions for ex vivo studies using brain slices for electrophysiological recording. In this study, we used a multiplatform (LC-MS and GC-MS) untargeted metabolomics-based approach to shed light on the metabolome and lipidome changes taking place at different time intervals during the brain slice preparation process. We have found significant modifications in the levels of 300 compounds, including several lipid classes and their derivatives, as well as metabolites involved in the GABAergic pathway and the TCA cycle. All these preparation-dependent changes in the brain biochemistry related to the time interval should be taken into consideration for future studies to facilitate non-biased interpretations of the experimental results.

Calbindin immunostaining in the CA1 hippocampal pyramidal cell layer of the human and mouse: A comparative study.

Immunostaining for calbindin (CB) is commonly used to label particular populations of neurons. Recently, it has been shown that the CA1 pyramidal cells in the mouse can be subdivided along the radial axis into superficial and deep pyramidal cells and that this segregation in the radial axis may represent a general principle of structural and functional organization of the hippocampus. One of the most widely used markers of the superficial pyramidal cells is CB. However, this laminar segregation of pyramidal cells has not been reported in the human CA1 using CB immunostaining. The problem is that the different pattern of CB immunostaining observed in the mouse compared to the human could be explained by technical features, of which one of the most important is the postmortem time (PT) delay typical of the brain tissue obtained from humans. In the present study, we have studied the influences of PT delays and fixation procedures and we found that the clear differences found between the CA1 of the human and mouse do not depend on the fixation, but represent actual species-specific differences. These remarkable differences between species should be taken into consideration when making interpretations in translational studies from mouse to human brains.

Differential expression of secretagogin immunostaining in the hippocampal formation and the entorhinal and perirhinal cortices of humans, rats, and mice.

Secretagogin (SCGN) is a recently discovered calcium-binding protein belonging to the group of EF-hand calcium-binding proteins. SCGN immunostaining has been described in various regions of the human, rat and mouse brain. In these studies, it has been reported that, in general, the patterns of SCGN staining differ between rodents and human brains. These differences have been interpreted as uncovering phylogenetic differences in SCGN expression. Nevertheless, an important aspect that is not usually taken into account is that different methods are used for obtaining and processing brain tissue coming from humans and experimental animals. This is a critical issue since it has been shown that post-mortem time delay and the method of fixation (i.e., perfused vs. nonperfused brains) may influence the results of the immunostaining. Thus, it is not clear whether differences found in comparative studies with the human brain are simply due to technical factors or species-specific differences. In the present study, we analyzed the pattern of SCGN immunostaining in the adult human hippocampal formation (DG, CA1, CA2, CA3, subiculum, presubiculum, and parasubiculum) as well as in the entorhinal and perirhinal cortices. This pattern of immunostaining was compared with rat and mouse that were fixed either by perfusion or immersion and with different post-mortem time delays (up to 5 hr) to mimic the way the human brain tissue is usually processed. We found a number of clear similarities and differences in the pattern of labeling among the human, rat, and mouse in these brain regions as well as between the different brain regions examined within each species. These differences were not due to the fixation.

Modifications of the axon initial segment during the hibernation of the Syrian hamster.

Mammalian hibernation is a natural process in which the brain undergoes profound adaptive changes that appear to protect the brain from extreme hypoxia and hypothermia. In addition to a virtual cessation of neural and metabolic activity, these changes include a decrease in adult neurogenesis; the retraction of neuronal dendritic trees; changes in dendritic spines and synaptic connections; fragmentation of the Golgi apparatus; and the phosphorylation of the microtubule-associated protein tau. Furthermore, alterations of microglial cells also occur in torpor. Importantly, all of these changes are rapidly and fully reversed when the animals arouse from torpor state, with no apparent brain damage occurring. Thus, hibernating animals are excellent natural models to study different aspects of brain plasticity. The axon initial segment (AIS) is critical for the initiation of action potentials in neurons and is an efficient site for the regulation of neural activity. This specialized structure-characterized by the expression of different types of ion channels and adhesion, scaffolding and cytoskeleton proteins-is subjected to morpho-functional plastic changes upon variations in neural activity or in pathological conditions. Here, we used immunocytochemistry and 3D confocal microscopy reconstruction techniques to measure the possible morphological differences in the AIS of neocortical (layers II-III and V) and hippocampal (CA1) neurons during the hibernation of the Syrian hamster. Our results indicate that the general integrity of the AIS is resistant to the ischemia/hypoxia conditions that are characteristic of the torpor phase of hibernation. In addition, the length of the AIS significantly increased in all the regions studied-by about 16-20% in torpor animals compared to controls, suggesting the existence of compensatory mechanisms in response to a decrease in neuronal activity during the torpor phase of hibernation. Furthermore, in double-labeling experiment, we found that the AIS in layer V of torpid animals was longer in neurons expressing phospho-tau than in those not labeled for phospho-tau. This suggests that AIS plastic changes were more marked in phospho-tau accumulating neurons. Overall, the results further emphasize that mammalian hibernation is a good physiological model to study AIS plasticity mechanisms in non-pathological conditions.

Regional Diversity in the Postsynaptic Proteome of the Mouse Brain.

The proteome of the postsynaptic terminal of excitatory synapses comprises over one thousand proteins in vertebrate species and plays a central role in behavior and brain disease. The brain is organized into anatomically distinct regions and whether the synapse proteome differs across these regions is poorly understood. Postsynaptic proteomes were isolated from seven forebrain and hindbrain regions in mice and their composition determined using proteomic mass spectrometry. Seventy-four percent of proteins showed differential expression and each region displayed a unique compositional signature. These signatures correlated with the anatomical divisions of the brain and their embryological origins. Biochemical pathways controlling plasticity and disease, protein interaction networks and individual proteins involved with cognition all showed differential regional expression. Combining proteomic and connectomic data shows that interconnected regions have specific proteome signatures. Diversity in synapse proteome composition is key feature of mouse and human brain structure.

Metabolomics and neuroanatomical evaluation of post-mortem changes in the hippocampus.

Understanding the human brain is the ultimate goal in neuroscience, but this is extremely challenging in part due to the fact that brain tissue obtained from autopsy is practically the only source of normal brain tissue and also since changes at different levels of biological organization (genetic, molecular, biochemical, anatomical) occur after death due to multiple mechanisms. Here we used metabolomic and anatomical techniques to study the possible relationship between post-mortem time (PT)-induced changes that may occur at both the metabolomics and anatomical levels in the same brains. Our experiments have mainly focused on the hippocampus of the mouse. We found significant metabolomic changes at 2 h PT, whereas the integrity of neurons and glia, at the anatomical/ neurochemical level, was not significantly altered during the first 5 h PT for the majority of histological markers.

Dopamine and α-synuclein dysfunction in Smad3 null mice.

BACKGROUND: Parkinson's disease (PD) is characterized by dopaminergic neurodegeneration in the substantia nigra (SN). Transforming growth factor-ß1 (TGF-ß1) levels increase in patients with PD, although the effects of this increment remain unclear. We have examined the mesostriatal system in adult mice deficient in Smad3, a molecule involved in the intracellular TGF-ß1 signalling cascade. RESULTS: Striatal monoamine oxidase (MAO)-mediated dopamine (DA) catabolism to 3,4-dihydroxyphenylacetic acid (DOPAC) is strongly increased, promoting oxidative stress that is reflected by an increase in glutathione levels. Fewer astrocytes are detected in the ventral midbrain (VM) and striatal matrix, suggesting decreased trophic support to dopaminergic neurons. The SN of these mice has dopaminergic neuronal degeneration in its rostral portion, and the pro-survival Erk1/2 signalling is diminished in nigra dopaminergic neurons, not associated with alterations to p-JNK or p-p38. Furthermore, inclusions of α-synuclein are evident in selected brain areas, both in the perikaryon (SN and paralemniscal nucleus) or neurites (motor and cingulate cortices, striatum and spinal cord). Interestingly, these α-synuclein deposits are detected with ubiquitin and P(S129)-α-synuclein in a core/halo cellular distribution, which resemble those observed in human Lewy bodies (LB). CONCLUSIONS: Smad3 deficiency promotes strong catabolism of DA in the striatum (ST), decrease trophic and astrocytic support to dopaminergic neurons and may induce α-synuclein aggregation, which may be related to early parkinsonism. These data underline a role for Smad3 in α-synuclein and DA homeostasis, and suggest that modulatory molecules of this signalling pathway should be evaluated as possible neuroprotective agents.

Selective oestrogen receptor (ER) modulators reduce microglia reactivity in vivo after peripheral inflammation: potential role of microglial ERs.

It has been previously reported that the neuroprotective hormone oestradiol reduces microglia inflammatory activity. The objective of this study was to test whether two selective oestrogen receptor modulators, tamoxifen and raloxifene, modulate in vivo the activation of microglia induced by the peripheral administration of lipopolysaccharide (LPS). Activation of microglia was assessed in the white matter of the cerebellum using immunoreactivity for major histocompatability complex-II. Oestradiol, tamoxifen and raloxifene decreased microglia activation induced by LPS in male and ovariectomized female rats, although the doses of oestradiol that were effective in decreasing microglia reactivity were not the same in both sexes. Tamoxifen reduced microglia activation in all experimental groups at all doses tested (0.5-2 mg/kg b.w.) while raloxifene lost its anti-inflammatory activity at the higher dose tested (2 mg/kg b.w). In addition, raloxifene had per se a moderate pro-inflammatory activity in the brain of control female rats and its anti-inflammatory activity was partially impaired in female animals after 1 month of deprivation of ovarian hormones. Spots of oestrogen receptor (ER)-alpha immunoreactivity were detected in the soma and cell processes of microglia. Treatment with LPS, oestradiol or tamoxifen induced an increase of ER-alpha immunoreactive spots in the perikaryon of microglia, while oestradiol antagonized the effect of LPS. The results indicate that some oestrogenic compounds decrease brain inflammation by a mechanism that may involve ERs expressed by microglia. The findings support the potential therapeutic role of oestrogenic compounds as protective anti-inflammatory agents for the central nervous system.