Javier DeFelipe

Petilla terminology: nomenclature of features of GABAergic interneurons of the cerebral cortex.

Neuroscience produces a vast amount of data from an enormous diversity of neurons. A neuronal classification system is essential to organize such data and the knowledge that is derived from them. Classification depends on the unequivocal identification of the features that distinguish one type of neuron from another. The problems inherent in this are particularly acute when studying cortical interneurons. To tackle this, we convened a representative group of researchers to agree on a set of terms to describe the anatomical, physiological and molecular features of GABAergic interneurons of the cerebral cortex. The resulting terminology might provide a stepping stone towards a future classification of these complex and heterogeneous cells. Consistent adoption will be important for the success of such an initiative, and we also encourage the active involvement of the broader scientific community in the dynamic evolution of this project.

Types of neurons, synaptic connections and chemical characteristics of cells immunoreactive for calbindin-D28K, parvalbumin and calretinin in the neocortex

This article provides a general account of types of neurons, synaptic connections and chemical characteristics (colocalization studies) of cells immunoreactive for the three main calcium-binding proteins found in the neocortex, namely, calbindin-D28K, parvalbumin and calretinin. The main conclusion is two-fold. First, all, or the majority, of calbindin-, parvalbumin- and calretinin-immunoreactive cells are smooth nonpyramidal neurons (interneurons) which participate in a variety of complex cortical circuits that may differ depending on the species, cortical area or layer where they are located. Second, in general, different types of nonpyramidal neurons are stained for each of these calcium-binding proteins and display different chemical characteristics regarding a variety of neurotransmitters (or related compounds), cell surface markers and receptors. However, a certain overlap exits, which also shows regional and species differences.

Microstructure of the neocortex: Comparative aspects

The appearance of the neocortex, its expansion, and its differentiation in mammals, represents one of the principal episodes in the evolution of the vertebrate brain. One of the fundamental questions in neuroscience is what is special about the neocortex of humans and how does it differ from that of other species? It is clear that distinct cortical areas show important differences within both the same and different species, and this has led to some researchers emphasizing the similarities whereas others focus on the differences. In general, despite of the large number of different elements that contribute to neocortical circuits, it is thought that neocortical neurons are organized into multiple, small repeating microcircuits, based around pyramidal cells and their input-output connections. These inputs originate from extrinsic afferent systems, excitatory glutamatergic spiny cells (which include other pyramidal cells and spiny stellate cells), and inhibitory GABAergic interneurons. The problem is that the neuronal elements that make up the basic microcircuit are differentiated into subtypes, some of which are lacking or highly modified in different cortical areas or species. Furthermore, the number of neurons contained in a discrete vertical cylinder of cortical tissue varies across species. Additionally, it has been shown that the neuropil in different cortical areas of the human, rat and mouse has a characteristic layer specific synaptology. These variations most likely reflect functional differences in the specific cortical circuits. The laminar specific similarities between cortical areas and between species, with respect to the percentage, length and density of excitatory and inhibitory synapses, and to the number of synapses per neuron, might be considered as the basic cortical building bricks. In turn, the differences probably indicate the evolutionary adaptation of excitatory and inhibitory circuits to particular functions.

Reconstruction and Simulation of Neocortical Microcircuitry

UNLABELLED We present a first-draft digital reconstruction of the microcircuitry of somatosensory cortex of juvenile rat. The reconstruction uses cellular and synaptic organizing principles to algorithmically reconstruct detailed anatomy and physiology from sparse experimental data. An objective anatomical method defines a neocortical volume of 0.29 ± 0.01 mm(3) containing ~31,000 neurons, and patch-clamp studies identify 55 layer-specific morphological and 207 morpho-electrical neuron subtypes. When digitally reconstructed neurons are positioned in the volume and synapse formation is restricted to biological bouton densities and numbers of synapses per connection, their overlapping arbors form ~8 million connections with ~37 million synapses. Simulations reproduce an array of in vitro and in vivo experiments without parameter tuning. Additionally, we find a spectrum of network states with a sharp transition from synchronous to asynchronous activity, modulated by physiological mechanisms. The spectrum of network states, dynamically reconfigured around this transition, supports diverse information processing strategies. PAPERCLIP VIDEO ABSTRACT.

Silencing microRNA-134 produces neuroprotective and prolonged seizure-suppressive effects

Temporal lobe epilepsy is a common, chronic neurological disorder characterized by recurrent spontaneous seizures. MicroRNAs (miRNAs) are small, noncoding RNAs that regulate post-transcriptional expression of protein-coding mRNAs, which may have key roles in the pathogenesis of neurological disorders. In experimental models of prolonged, injurious seizures (status epilepticus) and in human epilepsy, we found upregulation of miR-134, a brain-specific, activity-regulated miRNA that has been implicated in the control of dendritic spine morphology. Silencing of miR-134 expression in vivo using antagomirs reduced hippocampal CA3 pyramidal neuron dendrite spine density by 21% and rendered mice refractory to seizures and hippocampal injury caused by status epilepticus. Depletion of miR-134 after status epilepticus in mice reduced the later occurrence of spontaneous seizures by over 90% and mitigated the attendant pathological features of temporal lobe epilepsy. Thus, silencing miR-134 exerts prolonged seizure-suppressant and neuroprotective actions; determining whether these are anticonvulsant effects or are truly antiepileptogenic effects requires additional experimentation.

Ultrastructure of Dendritic Spines: Correlation Between Synaptic and Spine Morphologies

Dendritic spines are critical elements of cortical circuits, since they establish most excitatory synapses. Recent studies have reported correlations between morphological and functional parameters of spines. Specifically, the spine head volume is correlated with the area of the postsynaptic density (PSD), the number of postsynaptic receptors and the ready-releasable pool of transmitter, whereas the length of the spine neck is proportional to the degree of biochemical and electrical isolation of the spine from its parent dendrite. Therefore, the morphology of a spine could determine its synaptic strength and learning rules. To better understand the natural variability of neocortical spine morphologies, we used a combination of gold-toned Golgi impregnations and serial thin-section electron microscopy and performed three-dimensional reconstructions of spines from layer 2/3 pyramidal cells from mouse visual cortex. We characterized the structure and synaptic features of 144 completed reconstructed spines, and analyzed their morphologies according to their positions. For all morphological parameters analyzed, spines exhibited a continuum of variability, without clearly distinguishable subtypes of spines or clear dependence of their morphologies on their distance to the soma. On average, the spine head volume was correlated strongly with PSD area and weakly with neck diameter, but not with neck length. The large morphological diversity suggests an equally large variability of synaptic strength and learning rules.

Synapses of double bouquet cells in monkey cerebral cortex visualized by calbindin immunoreactivity

In the monkey neocortex, immunoreactivity for the 28-kDa vitamin D-dependent calcium binding protein (Calbindin) is contained in a set of GABAergic intrinsic neurons whose small size and laminar locations render them very distinct from a second set of GABAergic intrinsic neurons that show immunoreactivity for another calcium-binding protein, parvalbumin. A conspicuous feature of many calbindin-immunoreactive cells is their possession of long, vertically oriented bundles of immunoreactive processes that descend or ascend vertically through several cortical layers. These are components of the radial fasciculi of the cortex and are here shown by correlative electron microscopic immunocytochemistry to consist of both immunoreactive dendrites and unmyelinated axons. The morphology of the bundles and the parent cells indicates that the cells are classical double bouquet cells. The calbindin-positive axons in the radial fasciculi in the present study formed symmetric synapses on unlabeled dendritic shafts (62%) and spines (38%). Despite the close-packed nature of the immunoreactive axons, relatively few terminals of the same axon converged on a single postsynaptic profile. The postsynaptic profiles were identified in certain cases as side branches of pyramidal cell apical and basal dendrites. Mainstem apical dendrites generally did not receive synapses derived from the calbindin-positive axons. These results indicate that double bouquet cells can be distinguished both by their GABAergic character and by their possession of calbindin immunoreactivity. They are probably major contributors to the vertical flow of inhibitory influences across laminae of the cerebral cortex.

The Evolution of the Brain, the Human Nature of Cortical Circuits, and Intellectual Creativity

The tremendous expansion and the differentiation of the neocortex constitute two major events in the evolution of the mammalian brain. The increase in size and complexity of our brains opened the way to a spectacular development of cognitive and mental skills. This expansion during evolution facilitated the addition of microcircuits with a similar basic structure, which increased the complexity of the human brain and contributed to its uniqueness. However, fundamental differences even exist between distinct mammalian species. Here, we shall discuss the issue of our humanity from a neurobiological and historical perspective.

Distribution and patterns of connectivity of interneurons containing calbindin, calretinin, and parvalbumin in visual areas of the occipital and temporal lobes of the macaque monkey

Immunocytochemical techniques were used to examine the distribution of double‐bouquet cells and chandelier cells that were immunoreactive (‐ir) for the calcium‐binding proteins calbindin (CB), calretinin (CR), and parvalbumin (PV) in the primary visual area (V1), the second visual area (V2), and cytoarchitectonic area TE in the macaque monkey. Furthermore, the connections between CB‐, CR‐, and PV‐ir neurons in these visual areas were investigated at the light microscope level by using a dual‐immunocytochemical staining procedure. The most significant findings were three‐fold. First, the number and distribution of CB‐ir and CR‐ir double‐bouquet cells and PV‐ir chandelier cells differed considerably between different visual areas. In particular, the different distribution of double‐bouquet cells was illustrated dramatically at the V1/V2 border, where CB‐ir double‐bouquet axons were very few or lacking in V1 but were very numerous in V2. Furthermore, PV‐ir chandelier cell terminals were relatively sparse in V1, more frequent in V2, and most frequent in area TE. Second, the percentage of CB‐, CR‐, and PV‐ir neurons receiving multiple contacts on their somata and proximal dendrites from other calcium‐binding protein neurons varied between 22% and 85%. The highest percentage of contacts found between immunolabelled cells and multiterminals were for the combinations CR/CB (76–85%; percent of cells immunoreactive for CB that were innervated by multiterminals immunoreactive for CR), followed by the combination PV/CR (42–48%), and then by the other combinations that had similar percentages (22–32% for CR/PV; 26–37% for CB/CR; 29–42% for CR/PV). Third, differences in the relative proportions of CB, CR, and PV terminals in contact with CB‐, CR‐, and PV‐ir neurons were consistent between the different cortical areas studied. Thus, certain characteristics of intraareal circuits differ, whereas others remain similar, in different areas of the occipitotemporal visual pathway. The differences may represent regional specializations related to the different processing of visual stimuli, whereas the similarities may be attributed to general functional requisites for interneuronal circuitry. J. Comp. Neurol. 412:515–526, 1999.

Non-synaptic dendritic spines in neocortex.

A long-held assumption states that each dendritic spine in the cerebral cortex forms a synapse, although this issue has not been systematically investigated. We performed complete ultrastructural reconstructions of a large (n=144) population of identified spines in adult mouse neocortex finding that only 3.6% of the spines clearly lacked synapses. Nonsynaptic spines were small and had no clear head, resembling dendritic filopodia, and could represent a source of new synaptic connections in the adult cerebral cortex.