J. Hámori

Immunohistochemical visualization of a metabotropic glutamate receptor.

The immunocytochemical localization of the recently cloned metabotropic glutamate receptor 1 alpha (mGluR1 alpha) was demonstrated with a C-terminus specific antibody in rat cerebellar cortex. This antibody detects a 138-140 kDa major, and a 46 kDa minor band in membrane preparations of rat cortex and cerebellum. mGluR1 alpha immunoreactivity (mGRi) was present in Purkinje and basket cells. Purkinje cell dendritic spines and their postsynaptic membranes showed selective labelling. Presynaptic membranes, parallel fibres and glial processes were devoid of mGRi. It is suggested that the selective postsynaptic localization of this receptor at the dendritic spines of Purkinje cells serves as the morphological basis for long term depression processes in the molecular layer of the cerebellar cortex.

Cellular and subcellular localization of the mGluR5a metabotropic glutamate receptor in rat spinal cord

THE cellular, and subcellular distribution of the mGluR5a metabotropic glutamate receptor was studied in the spinal cord of the rat using an antibody raised against a mGluR5a-specific carboxy-terminal peptide. Strong mGluR5a-immunoreactivity (mGluR5a-ir) was found in the laminae I-II of the dorsal horn, which gradually decreased towards the deeper layers. At the electron microscopical level, mGluR5a-ir was present exclusively in neuronal somata, and dendrites. Immunometal labelling revealed that mGluR5a-ir is concentrated at the periphery of postsynaptic densities of asymmetrical synapses or localized extrasynaptically at dendritic, and somatic membranes. The mGluR5a-immunoreactive dendritic profiles were often targeted by synaptic boutons with the morphological characteristics of C-fibre terminals. These observations provide evidence for mGluR5a being involved in the nociceptive transmission at the dorsal horn.

The use of CNN models in the subcortical visual pathway

The equivalent notions of neuroanatomy and the cellular neural network (CNN) model are discussed with a view toward studying the visual system. Various mainly subcortical phenomena are studied and simple effects like directional sensitivity and length tuning are modeled. A more accurate retina model has been developed, taking into account some effects of amacrine cells. It is shown that the standard errors occurring in simple models of retinal illusions can be eliminated by using the more accurate models including delays. Lateral geniculate nucleus (LGN) effects with and without cortical feedback are modeled as well; their CNN models are simple. Simple texture detection effects and motion illusions are explained by neuromorphic CNN models. The goal is to translate known effects into CNN models and to provide a framework for further studies. >

Immunogold electron microscopic demonstration of glutamate and GABA in normal and deafferented cerebellar cortex: Correlation between transmitter content and synaptic vesicle size

Selective labeling of mossy fiber terminals and parallel fibers was obtained in rat cerebellar cortex by a glutamate antibody produced and characterized by Hepler et al. The high-resolution electron microscopic immunogold demonstration of this amino acid offered the possibility of determining the size and shape of synaptic vesicles in glutamate-positive mossy endings. Mossy terminals that stained with the glutamate antibody formed two distinct populations, one with spherical synaptic vesicles with an average diameter of 34.0 nm (more than 90% of all mossy fiber endings) and one with pleomorphic and smaller synaptic vesicles which had an average diameter of 28.5 nm. We present experimental evidence that the mossy terminals with large round vesicles are of extracerebellar origin, whereas those with small pleomorphic synaptic vesicles are endings of nucleocortical fibers. The presence of two distinct classes of gamma-aminobutyric acid (GABA)-containing axon terminals within cerebellar glomeruli has also been demonstrated; those originating from the cerebellar nuclei contain large (36.2 nm) synaptic vesicles, whereas the majority of GABA-stained axon terminals that are of local (cortical) origin contain small (29.1 nm) synaptic vesicles. It therefore appears that, at least in the case of glutamate and GABA, morphological characterization of the axon terminals based on the size and shape of synaptic vesicles is not a reliable indicator of their functional nature (i.e., whether they are excitatory or inhibitory); convincing evidence for the identity of the transmitter can be obtained only by electron microscopic immunostaining procedures. Our results also suggest the existence of both inhibitory and excitatory feedback from cerebellar nuclei to cerebellar cortex.

CNN model of the feature-linked synchronized activities in the visual thalamo-cortical system

In order to investigate whether thalamic synchronization may contribute to the binding of different cortical representations in visual information processing, we made a neuromorphic model of the thalamo-cortical loop, using the cellular neural network (CNN) model and simulator. It is demonstrated with the aid of this model that the cortico-thalamic feedback by synchronizing dLGN relay cell responses, provides a temporal code and this may induce correlated activities in the target cortical neuron population. Our results support the notion that using synchronization of temporally structured activities as general integration mechanism, the processing of visual information occurs simultaneously in the highly interconnected thalamo-cortical system.

Immunocytochemical mapping of NPY and VIP neuronal elements in the cat subcortical visual nuclei, with special reference to the pretectum and accessory optic system

The aim of this study was to describe the distribution patterns of neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP)-immunoreactive (ir) neuronal elements in subcortical visual centers of the cat. Numerous NPY-ir neurons were present in the feline nucleus of the optic tract and in the anterior pretectal nucleus. Only a few NPY-ir neurons were found in the posterior, medial and olivary pretectal nuclei and in the accessory optic nuclei. Diffuse and heavily beaded NPY-ir fiber plexuses were observed throughout the superior colliculus, pretectum, and accessory optic system. Extensively arborising NPY-ir fibers were present in the mesencephalon and ventral lateral geniculate nucleus, while the dorsal visual thalamic nuclei contained only a few NPY-ir fibers. VIP-ir cells were present mainly in the accessory optic nuclei, and they were absent in the dorsal visual thalamus. Both NPY- and VIP-ir neurons were multipolar and fusiform in shape in the regions studied. Enucleation did not alter the appearance of NPY- and VIP-containing neuronal elements in the superior colliculus and pretectum while in the thalamus a subset of NPY-ir fiber population disappeared, indicating their retinal origin. Although there is a partial overlap in the topographical localization of the NPY- and VIP-ergic neurons in the pretectum, the colocalization of the two peptides could not be demonstrated. The present observations demonstrate the existence of two different and separate peptidergic (NPY and VIP) neuronal populations in the pretectum.

Hyperacuity in time: a CNN model of a time-coding pathway of sound localization

This paper discusses a new multilayer one-dimensional (1-D) cellular neural network model of the time-coding pathway of sound localization. The key feature of the model is lateral inhibition, which is supposed to play a crucial role in sound localization. The possible role of this inhibition is examined on the basis of our model and several conclusions are drawn concerning the expected nature of inhibition. It is also shown that by use of inhibition, a group of neurons may be much more sensitive to interaural time difference than one individual neuron. Thus, our model of the first stage of the sound localization system solves a hyperacuity in time problem. The second part of the paper introduces a CNN model of that part of the sound localization system which is characterized by a massive convergence of different frequency channels to resolve the so-called phase ambiguity problem. We show that with inhibition good results can be achieved here too. Quantitative studies show the robustness of the model.

Some cortical spiking neuron models using CNN

In this paper we show cellular neural network (CNN) models of some basic types of cells characterised by diverse spiking patterns. After showing some preliminary models (ion channels, neurons), CNN models of the action potential generation are given followed by an analysis of the rate coding capabilities of the models. Furthermore, CNN models of neurons with diverse intrinsic firing patterns are presented followed by some conclusions.<<ETX>>

Innervation of a single vibrissa in the whisker-pad of rats

Innervation of the γ straddler vibrissa and distribution of individual γ vibrissal sensory nerve fibers within infraorbital nerve branches were investigated in adult rats. In the close vicinity (2 mm) of the whisker-pad, one-fifth of the nerve fibers innervating the γ vibrissa were still located in nerves to neighboring (β, δ, C1 and D1) whiskers, while one-tenth of the innervating fibers were placed in nerves of more distant sinus hairs. This convergence of γ vibrissal sensory nerve fibers to single vibrissa from neighboring nerves is suggested to be a consequence of overlapping vibrissal representation in the Gasserian ganglion.

Morphogenetic Plasticity of Neuronal Elements in Cerebellar Glomeruli during Deafferentation-Induced Synaptic Reorganization

Reorganization of the cerebellar glomerulus, the main synaptic complex within the granule cell layer, was investigated using quantitative morphological techniques. All afferents to the cerebellar cortex, including mossy-fibers, were surgically destroyed by undercutting the cerebellar vermis. Fifteen days after the operation, which resulted in the removal of the main excitatory afferent to the glomerulus, a significant reorganization of the whole synaptic complex was observed, whereas the structural integrity of the glomerulus was remarkably well preserved. This was indicated by the observation that the number of granule cell dendrites (≈50 per glomerulus), as well as the number of dendritic digits (≈210 per glomerulus) bearing most of the ≈230 synaptic junctions per glomerulus, did not change significantly after mossy-fiber degeneration. The total number of synapses in the reorganized glomerulus did not change either, despite the disappearance of two-thirds of (excitatory) synaptic junctions caused by mossy-fiber degeneration. In the reorganized glomeruli, however, the inhibitory, GABA-containing Golgi axonal varicosities became the dominant synaptic type—about four-fifths (≈200) of all synapses within the glomerulus—whereas the dendritic synapses between the granule cells represented only one-fifth of all synaptic junctions. The quantitative data of the reorganized cerebellar glomerulus demonstrate both a remarkable constancy and a plasticity of he excitatory granule cells and inhibitory Golgi neurons building up this synaptic complex. constancy (the preservation of certain specific structural features) is represented by an eventually unchanged number of dendrites and synaptic junctions within the deafferented lomerulus. Such constancy was made possible, however, by the morphogenetic plasticity of both nerve-cell types to produce new, dendrodendritic and axo-dendritic synapses to compensate for the loss of mossy-fiber synapses.