Peter Somogyi

Neuronal Diversity and Temporal Dynamics: The Unity of Hippocampal Circuit Operations

In the cerebral cortex, diverse types of neurons form intricate circuits and cooperate in time for the processing and storage of information. Recent advances reveal a spatiotemporal division of labor in cortical circuits, as exemplified in the CA1 hippocampal area. In particular, distinct GABAergic (γ-aminobutyric acid–releasing) cell types subdivide the surface of pyramidal cells and act in discrete time windows, either on the same or on different subcellular compartments. They also interact with glutamatergic pyramidal cell inputs in a domain-specific manner and support synaptic temporal dynamics, network oscillations, selection of cell assemblies, and the implementation of brain states. The spatiotemporal specializations in cortical circuits reveal that cellular diversity and temporal dynamics coemerged during evolution, providing a basis for cognitive behavior.

Brain-state- and cell-type-specific firing of hippocampal interneurons in vivo

Neural-network oscillations at distinct frequencies have been implicated in the encoding, consolidation and retrieval of information in the hippocampus. Some GABA (γ-aminobutyric acid)-containing interneurons fire phase-locked to theta oscillations (4–8 Hz) or to sharp-wave-associated ripple oscillations (120–200 Hz), which represent different behavioural states. Interneurons also entrain pyramidal cells in vitro. The large diversity of interneurons poses the question of whether they have specific roles in shaping distinct network activities in vivo. Here we report that three distinct interneuron types—basket, axo-axonic and oriens–lacunosum-moleculare cells—visualized and defined by synaptic connectivity as well as by neurochemical markers, contribute differentially to theta and ripple oscillations in anaesthetized rats. The firing patterns of individual cells of the same class are remarkably stereotyped and provide unique signatures for each class. We conclude that the diversity of interneurons, innervating distinct domains of pyramidal cells, emerged to coordinate the activity of pyramidal cells in a temporally distinct and brain-state-dependent manner.

The metabotropic glutamate receptor (mGluRlα) is concentrated at perisynaptic membrane of neuronal subpopulations as detected by immunogold reaction

An antiserum to mGluR1 alpha labeled a 160 kd protein in immunoblots of membranes derived from rat brain or cells transfected with mGluR1 alpha. Immunoreactivity for mGluR1 alpha was present in discrete subpopulations of neurons. The GABAergic neurons of the cerebellar cortex were strongly immunoreactive; only some Golgi cells were immunonegative. Somatostatin/GABA-immunopositive cells in the neocortex and hippocampus were enriched in mGluR1 alpha. The hippocampal cells had spiny dendrites that were precisely codistributed with the local axon collaterals of pyramidal and granule cells. Electron microscopic immunometal detection of mGluR1 alpha showed a preferential localization at the periphery of the extensive postsynaptic densities of type 1 synapses in both the cerebellum and the hippocampus. The receptor was also present at sites in the dendritic and somatic membrane where synapses were not located.

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.

Segregation of Different GABAA Receptors to Synaptic and Extrasynaptic Membranes of Cerebellar Granule Cells

Two types of GABAA receptor-mediated inhibition (phasic and tonic) have been described in cerebellar granule cells, although these cells receive GABAergic input only from a single cell type, the Golgi cell. In adult rats, granule cells express six GABAAreceptor subunits abundantly (α1, α6, β2, β3, γ2, and δ), which are coassembled into at least four to six distinct GABAA receptor subtypes. We tested whether a differential distribution of GABAA receptors on the surface of granule cells could play a role in the different forms of inhibition, assuming that phasic inhibition originates from the activation of synaptic receptors, whereas tonic inhibition is provided mainly by extrasynaptic receptors. The α1, α6, β2/3, and γ2 subunits have been found by immunogold localizations to be concentrated in GABAergic Golgi synapses and also are present in the extrasynaptic membrane at a lower concentration. In contrast, immunoparticles for the δ subunit could not be detected in synaptic junctions, although they were abundantly present in the extrasynaptic dendritic and somatic membranes. Gold particles for the α6, γ2, and β2/3, but not the α1 and δ, subunits also were concentrated in some glutamatergic mossy fiber synapses, where their colocalization with AMPA-type glutamate receptors was demonstrated. The exclusive extrasynaptic presence of the δ subunit-containing receptors, together with their kinetic properties, suggests that tonic inhibition could be mediated mainly byextrasynaptic α6β2/3δ receptors, whereas phasic inhibition is attributable to the activation of synapticα1β2/3γ2, α6β2/3γ2, and α1α6β2/3γ2receptors.

Salient features of synaptic organisation in the cerebral cortex

The neuronal and synaptic organisation of the cerebral cortex appears exceedingly complex, and the definition of a basic cortical circuit in terms of defined classes of cells and connections is necessary to facilitate progress of its analysis. During the last two decades quantitative studies of the synaptic connectivity of identified cortical neurones and their molecular dissection revealed a number of general rules that apply to all areas of cortex. In this review, first the precise location of postsynaptic GABA and glutamate receptors is examined at cortical synapses, in order to define the site of synaptic interactions. It is argued that, due to the exclusion of G protein-coupled receptors from the postsynaptic density, the presence of extrasynaptic receptors and the molecular compartmentalisation of the postsynaptic membrane, the synapse should include membrane areas beyond the membrane specialisation. Subsequently, the following organisational principles are examined: 1. The cerebral cortex consists of: (i) a large population of principal neurones reciprocally connected to the thalamus and to each other via axon collaterals releasing excitatory amino acids, and, (ii) a smaller population of mainly local circuit GABAergic neurones. 2. Differential reciprocal connections are also formed amongst GABAergic neurones. 3. All extrinsic and intracortical glutamatergic pathways terminate on both the principal and the GABAergic neurones, differentially weighted according to the pathway. 4. Synapses of multiple sets of glutamatergic and GABAergic afferents subdivide the surface of cortical neurones and are often co-aligned on the dendritic domain. 5. A unique feature of the cortex is the GABAergic axo-axonic cell, influencing principal cells through GABAA receptors at synapses located exclusively on the axon initial segment. The analysis of these salient features of connectivity has revealed a remarkably selective array of connections, yet a highly adaptable design of the basic circuit emerges when comparisons are made between cortical areas or layers. The basic circuit is most obvious in the hippocampus where a relatively homogeneous set of spatially aligned principal cells allows an easy visualization of the organisational rules. Those principles which have been examined in the isocortex proved to be identical or very similar. In the isocortex, the basic circuit, scaled to specific requirements, is repeated in each layer. As multiple sets of output neurones evolved, requiring subtly different needs for their inputs, the basic circuit may be superimposed several times in the same layer. Tangential intralaminar connections in both the hippocampus and isocortex also connect output neurones with similar properties, as best seen in the patchy connections in the isocortex. The additional radial superposition of several laminae of distinct sets of output neurones, each representing and supported by its basic circuit, requires a co-ordination of their activity that is mediated by highly selective interlaminar connections, involving both the GABAergic and the excitatory amino acid releasing neurones. The remarkable specificity in the geometry of cells and the selectivity in placement of neurotransmitter receptors and synapses on their surface, strongly suggest a predominant role for time in the coding of information, but this does not exclude an important role also for the rate of action potential discharge in cortical representation of information.

Perisynaptic location of metabotropic glutamate receptors mGluR1 and mGluR5 on dendrites and dendritic spines in the rat hippocampus

Ionotropic and metabotropic (mGluR1a) glutamate receptors were reported to be segregated from each other within the postsynaptic membrane at individual synapses. In order to establish whether this pattern of distribution applies to the hippocampal principal cells and to other postsynaptic metabotropic glutamate receptors, the mGluR1a/b/c and mGluR5 subtypes were localized by immunocytochemistry. Principal cells in all hippocampal fields were reactive for mGluR5, the strata oriens and radiatum of the CA1 area being most strongly immunolabelled. Labelling for mGluR1b/c was strongest on some pyramids in the CA3 area, weaker on granule cells and absent on CA1 pyramids. Subpopulations of non‐principal cells showed strong mGluR1 or mGluR5 immunoreactivity. Electron microscopic pre‐embedding immunoperoxidase and both pre‐ and postembedding immunogold methods consistently revealed the extrasynaptic location of both mGluRs in the somatic and dendritic membrane of pyramidal and granule cells. The density of immunolabelling was highest on dendritic spines. At synapses, immunoparticles for both mGluR1 and mGluR5 were found always outside the postsynaptic membrane specializations. Receptors were particularly concentrated in a perisynaptic annulus around type I synaptic junctions, including the invaginations at ‘perforated’synapses. Measurements of immunolabelling on dendritic spines showed decreasing levels of receptor as a function of distance from the edge of the synaptic specialization. We propose that glutamatergic synapses with an irregular edge develop in order to increase the circumference of synaptic junctions leading to an increase in the metabotropic to ionotropic glutamate receptor ratio at glutamate release sites. The perisynaptic position of postsynaptic metabotropic glutamate receptors appears to be a general feature of glutamatergic synaptic organization and may apply to other G‐protein‐coupled receptors.

Defined types of cortical interneurone structure space and spike timing in the hippocampus

The cerebral cortex encodes, stores and combines information about the internal and external environment in rhythmic activity of multiple frequency ranges. Neurones of the cortex can be defined, recognized and compared on the comprehensive application of the following measures: (i) brain area‐ and cell domain‐specific distribution of input and output synapses, (ii) expression of molecules involved in cell signalling, (iii) membrane and synaptic properties reflecting the expression of membrane proteins, (iv) temporal structure of firing in vivo, resulting from (i)–(iii). Spatial and temporal measures of neurones in the network reflect an indivisible unity of evolutionary design, i.e. neurones do not have separate structure or function. The blueprint of this design is most easily accessible in the CA1 area of the hippocampus, where a relatively uniform population of pyramidal cells and their inputs follow an instantly recognizable laminated pattern and act within stereotyped network activity patterns. Reviewing the cell types and their spatio‐temporal interactions, we suggest that CA1 pyramidal cells are supported by at least 16 distinct types of GABAergic neurone. During a given behaviour‐contingent network oscillation, interneurones of a given type exhibit similar firing patterns. During different network oscillations representing two distinct brain states, interneurones of the same class show different firing patterns modulating their postsynaptic target‐domain in a brain‐state‐dependent manner. These results suggest roles for specific interneurone types in structuring the activity of pyramidal cells via their respective target domains, and accurately timing and synchronizing pyramidal cell discharge, rather than providing generalized inhibition. Finally, interneurones belonging to different classes may fire preferentially at distinct time points during a given oscillation. As different interneurones innervate distinct domains of the pyramidal cells, the different compartments will receive GABAergic input differentiated in time. Such a dynamic, spatio‐temporal, GABAergic control, which evolves distinct patterns during different brain states, is ideally suited to regulating the input integration of individual pyramidal cells contributing to the formation of cell assemblies and representations in the hippocampus and, probably, throughout the cerebral cortex.

Cell type and pathway dependence of synaptic AMPA receptor number and variability in the hippocampus.

It has been suggested that some glutamatergic synapses lack functional AMPA receptors. We used quantitative immunogold localization to determine the number and variability of synaptic AMPA receptors in the rat hippocampus. Three classes of synapses show distinct patterns of AMPA receptor content. Mossy fiber synapses on CA3 pyramidal spines and synapses on GABAergic interneurons are all immunopositive, have less variability, and contain 4 times as many AMPA receptors as synapses made by Schaffer collaterals on CA1 pyramidal spines and by commissural/ associational (C/A) terminals on CA3 pyramidal spines. Up to 17% of synapses in the latter two connections are immunonegative. After calibrating the immunosignal (1 gold = 2.3 functional receptors) at mossy synapses of a 17-day-old rat, we estimate that the AMPA receptor content of C/A synapses on CA3 pyramidal spines ranges from <3 to 140. A similar range is found in adult Schaffer collateral and C/A synapses.

Glutamatergic synapses on oligodendrocyte precursor cells in the hippocampus.

Fast excitatory neurotransmission in the central nervous system occurs at specialized synaptic junctions between neurons, where a high concentration of glutamate directly activates receptor channels. Low-affinity AMPA (α-amino-3-hydroxy-5-methyl isoxazole propionic acid) and kainate glutamate receptors are also expressed by some glial cells, including oligodendrocyte precursor cells (OPCs). However, the conditions that result in activation of glutamate receptors on these non-neuronal cells are not known. Here we report that stimulation of excitatory axons in the hippocampus elicits inward currents in OPCs that are mediated by AMPA receptors. The quantal nature of these responses and their rapid kinetics indicate that they are produced by the exocytosis of vesicles filled with glutamate directly opposite these receptors. Some of these AMPA receptors are permeable to calcium ions, providing a link between axonal activity and internal calcium levels in OPCs. Electron microscopic analysis revealed that vesicle-filled axon terminals make synaptic junctions with the processes of OPCs in both the young and adult hippocampus. These results demonstrate the existence of a rapid signalling pathway from pyramidal neurons to OPCs in the mammalian hippocampus that is mediated by excitatory, glutamatergic synapses.