Eric Rinvik

Origin and distribution of glutamate decar☐ylase in substantia nigra of the cat

The topographical distribution of glutamate decar☐ylase (GAD) in substantia nigra in unoperated and operated cats was studied in samples microdissected from freeze-dried tissue sections. The concentration of GAD, the enzyme synthesizing γ-aminobutyric acid (GABA), was highest in the medial part of pars reticulata, and decreased in the mediolateral direction. In pars compacta, on the other hand, the highest enzyme activity was found in the lateral part which merges with pars reticulata, and it decreased gradually in the latero-medial direction. The activity of GAD was always lower in the medial part of pars compacta, which contains the highest concentration of cell bodies. GAD in substantia nigra decreased after lesions in putamen, nucleus caudatus, globus pallidus and nucleus entopeduncularis. The loss of enzyme activity was strictly localized and was related to the site of termination of the degenerating striato-nigral fibers. The reduction of GAD in substantia nigra following lesions of globus pallidus or nucleus entopeduncularis may be ascribed to the interruption of striato-nigral fibers passing through these regions. The results thus indicate that the fibers of the GAD-containing axon terminals in substantia nigra of the cat originate in putamen and nucleus caudatus. Subcellular fractionation showed that about 85% of GAD and about 25% of lactate dehydrogenase were present in particles (probably synaptosomes) from substantia nigra in unoperated animals. Electron microscopic examination revealed that 11.5% of the tissue volume of pars reticulata was occupied by boutons compared to 5.9% for pars compacta. The concentration of GABA in pars reticulata was found to be 9 mM. From these data the intraterminal concentration of GABA was estimated to be at least 60 mM, probably over 100 mM. DOPA decar☐ylase was mainly found in pars compacta. Acetylcholin-esterase showed a very high activity in substantia nigra, the highest concentration being found in the medial part of pars reticulata. In contrast, the concentration of choline acetyltransferase was very low. The ratio of acetylcholinesterase activity to choline acetyltransferase activity was 1000. DOPA decar☐ylase and the cholinergic enzymes were little affected by the above described lesions.

Mesencephalic and diencephalic afferents to the superior colliculus and periaqueductal gray substance demonstrated by retrograde axonal transport of horseradish peroxidase in the cat.

The mesencephalic and diencephalic afferent connections to the superior colliculus and the central gray substance in the cat were examined by means of the retrograde transport of horseradish peroxidase (HRP). After deep collicular injections numerous labeled cells were consistently found in the parabigeminal nucleus, the mesencephalic reticular formation, substantia nigra pars reticulata, the nucleus of posterior commissure, the pretectal area, zona incerta, and the ventral nucleus of the lateral geniculate body. A smaller number of cells was found in the inferior colluculus, the nucleus of the lateral lemniscus, the central gray substance, nucleus reticularis thalami, the anterior hypothalamic area, and, in some cases, in the contralateral superior colliculus, Forels field, and the ventromedial hypothalamic nucleus. Only the parabigeminal nucleus and the pretectal area showed labeled cells following injections in the superficial layers of the superior colliculus. In the cats submitted to injections in the central gray substance, labeled cells were consistently found in the contralateral superior colliculus, the mesencephalic reticular formation, substantia nigra parts reticulata, zona incerta and various hypothalamic areas, especially the ventromedial nucleus. In some cases, HRP-positive cells were seen in the nucleus of posterior commissure, the pretectal area, Forels field, and nucleus reticularis thalami. A large injection in the mediodorsal part of the caudal mesencephalic reticular formation, which included the superior colliculus and the central gray substance, resulted in numerous labeled cells in nucleus reticularis thalami. The findings are discussed with respect to the suggested functional division of the superior colliculus into deep and superficial layers. Furthermore, the possible implications of labeled cells in zona incerta and the reticular thalamic nucleus are briefly discussed.

An experimental electron microscopic study on the striatonigral projection in the cat.

SummaryThe ultrastructure of the cats substantia nigra was investigated from 2–21 days following large lesions of the caudate nucleus and the putamen. From 4 days on a large number of degenerating boutons and degenerating unmyelinated fibers are seen in the substantia nigra, in the pars compacta as well as the pars reticulata. Both parts, mainly the latter, receive striatal afferents. The degeneration in the substantia nigra following striatal lesions is of the dark type. Most of the degenerating boutons apparently are of the type I (see Rinvik and Grofová, 1970) and end on all parts of the nigral cell surface, including the dendritic spines. One instance of a degenerating presynaptic bouton in an axo-axonic synapse has been found. Some degenerating boutons also probably belong to the type II bouton, while degenerating boutons of type III were never seen following the striatal lesions. The electron microscopic identification of early axonal degeneration in the central nervous system, is discussed with reference to the paper of Cohen and Pappas (1969). Problems concerning the pars compacta versus the pars reticulata of the substantia nigra are taken up. The possible sources of origin of the different types of boutons in the cats substantia nigra, is discussed.

Observations on the fine structure of the substantia nigra in the cat

SummaryA light and electron microscopical investigation has been undertaken of the substantia nigra in the normal cat. The pars reticulata partly contains the arborization of dendrites whose cell bodies are located in the so-called pars compacta. There is a considerable overlap of the dendritic fields in the rostrocaudal direction, while the dendritic fields are very restricted in the mediolateral extension of the substantia nigra. The secondary and all subsequent branches of the dendrites of nigral cells are for considerable distances completely covered by boutons. Only few boutons contact the cell bodies. Three types of boutons are distinguished in the substantia nigra in the cat. Type I, about 90 % of the total, is of the terminal type, contains pleomorphic vesicles and establishes symmetrical synapses with nigral cell soma, dendritic trunks and spines. The type II bouton (about 10 % of the total number) is most commonly of the terminal type, contains spherical vesicles and establishes asymmetrical synapses with cell bodies and dendritic trunks of nigral cells. The type III bouton (about 2 % of the boutons) is always of the en passage type, contains pleomorphic vesicles and establishes symmetrical contacts with dendrites. All boutons in the cats substantia nigra contain several large (700–1200 Å) dense core vesicles. Occasional axo-axonic contacts between type I and type III boutons are observed. Type I bouton is invariably presynaptic to the other.The findings are discussed in relation to some relevant problems.

GABA, glycine, aspartate, glutamate and taurine in the vestibular nuclei: an immunocytochemical investigation in the cat.

SummaryThe distributions of five amino acids with well-established neuroexcitatory or neuroinhibitory properties were investigated in the feline vestibular complex. Consecutive semithin sections of plastic-embedded tissue were incubated with antisera raised against protein-glutaraldehyde conjugates of GABA, glycine, aspartate, glutamate and taurine. This approach allowed us to study the relative densities of the different immunoreactivities at the level of individual cell profiles. The results indicate that in the vestibular nuclei, neuronal colocalization of two or more neuroactive amino acids is the rule rather than an exception. Colocalization was found of immunoreactivities for GABA and glycine; glycine, aspartate and glutamate; glycine and aspartate, and glutamate and aspartate. GABA immunoreactive neurons were generally small and were found scattered throughout the vestibular complex. Glycine immunoreactive neurons were similarly distributed, except in the superior nucleus where the latter type of neuron could not be detected. Neuronal profiles colocalizing immunoreactivities for GABA and glycine occurred in all nuclei, but were most numerous in the lateral nucleus. The vast majority of the neurons showed noteworthy staining for glutamate and aspartate, although the level of immunoreactivities varied (e.g., the large neurons in the lateral and descending nuclei were more intensely aspartate immunoreactive than the smaller ones). Taurine-like immunoreactivity did not occur in neuronal cell bodies but appeared in Purkinje cell axons and in glial cell profiles. The functional significance of the complex pattern of amino acid colocalization remains to be clarified. In particular it needs to be distinguished between metabolic and transmitter pools of the different amino acids. The present results call for caution when attempts are made to conclude about transmitter identity on the basis of amino acid contents alone.

SOME COMMENTS ON THE PYRAMIDAL TRACT, WITH SPECIAL REFERENCE TO ITS INDIVIDUAL VARIATIONS IN MAN

It has for a long time been generally accepted that a lesion of the pyramidal tract usually results in a spastic hemiplegia, characterized by impairment of skilled voluntary movements, increased myotatic (deep or tendon) rcflcxes, and weakened or abolished cutaneous reflexes with an extensor plantar response. However, after sectioning the middle third of the ccrebral peduncle in man containing the pyramidal tract fibres scvcral authors have reported that there is no, or only a slight and transient, spasticity and that the capacity to perform skilled voluntary moverncnts is only nioderately impaired ( WaIker 1849, W h i f e 1050, Bzicy 1857, Bucy & Keplinger 1961). These observations lctl Rucy to ask “Is there a pyramidal tract?”, and to suggest that cortical impulscs responsible for the production of skilled voluntary movements might take an “extrapyramidal” routc. Although symptomatological variations which may be seen following lesions supposed to involve the pyramidal tract infantile hemiplegia included may be due to concomitant damage to “extrapyramidal” structures, a problem to which we will briefly return in the final section of this paper, the question has often been raised whether such variations are due to a greater or lesser extent to individual morphological variations of the pyramidal tract. The chief purpose of the present paper is to review what is known of individual variations of this fibre system in man with regard to the following items : decussation of fibres, aberrant fibres, number and size of fibres and myelination of fibres. For a proper discussion of this, certain relevant data on the anatomy of the pyramidal tract have to be considered as well. Only few systematical studies on the individual variations of this fibre system in man have, however, been made. The data on which this review is based are, therefore, largely collcctcd from clinical case reports in the literature. Where it is relevant, data from other species will be considered.

A re-evaluation of the cytoarchitecture of the ventral nuclear complex of the cat's thalamus on the basis of corticothalamic connections

Summary A short description has been given of the thalamic nuclei which receive afferent connections from the frontal cerebral cortex and the first and second somatic sensory cortical areas in the cat. The findings reported correspond largely to previous observations on the topography of the thalamus. However, studies on corticothalamic connections have proved to be of value in delimiting some thalamic structures more accurately. The areas of termination of corticothalamic fibres in VPM and VPL from face and limb areas, respectively, of the cerebral cortex are separated by a thick myelinated fibre bundle. Furthermore, a thin fibre bundle separates the 2 parts of VPL which receive afferent fibres from hind- and forelimb areas of the sensorimotor cortical areas. Marked differences in cytoarchitecture help to separate VPMpc and VPI from VPM. However, these nuclei appear to receive afferent connections from largely identical cortical areas. The posterior group of nuclei (PO) is distinct from the ventral group by differences in cytoarchitecture and particulars in the corticothalamic connections. In PO are included the caudal end of VPL1, rostral MGmc and ventral part of caudal LP. It is emphasized that a profound knowledge of fibre connections combined with the use of other methods such as the Golgi technique, histochemistry and electron microscopy, is needed in order to obtain a complete and rational comprehension of thalamic topography.

The corticothalamic projection from the pericruciate and coronal gyri in the cat. An experimental study with silver-impregnation methods

Summary The corticothalamic projections from the pericruciate and coronal gyri in adult cats have been studied with the silver-impregnation method of Nauta. The efferent fibers from these cortical areas follow three different routes before they reach their area of termination in the ipsilateral thalamus. The majority of corticothalamic fibers course in nucleus reticularis thalami (R) and the external medullary lamina and enter different thalamic nuclei at successively caudal levels. Other corticofugal fibers leave the internal capsule at rostral levels, pierce R and course through various rostral thalamic nuclei before they terminate at more caudal diencephalic levels. By a third and more circuitous route, fibers leave the cerebral peduncle at the meso-diencephalic junction and course in a dorsorostral direction towards posteroinferior and medial thalamic structures. The postcruciate gyrus sends fibers to the following thalamic nuclei: reticularis (R), ventralis lateralis (VL), ventralis posterolateralis (VPL), posterior group (PO), centrum medianum (CM), parafascicularis (Pf) and paracentralis (Pc). In addition to the above-mentioned nuclei, the precruciate gyrus projects to ventralis anterior (VA), ventralis medialis (VM) and the lateral part of caudal medialis dorsalis (MD). The coronal gyrus projects upon R, VA, VL, PO, CM, Pf, Pc, lateral part of caudal MD, centralis lateralis (CL), ventralis posteromedialis (VPM), the parvocellular part of VPM (VPMpc) and ventralis posteroinferior (VPI). Fibers from medial and lateral parts of the pericruciate gyri end in lateral and medial regions, respectively, of VPL. Fibers from the coronal gyrus end in VPM. This topographical organization of the corticothalamic fibers is also seen to a minor extent in R, VL and PO. Most efferent fibers to VL from the precruciate gyrus, however, project diffusely upon a zone which extends throughout the rostrocaudal extent of VL medial to the VB complex. Many corticofugal fibers to VL and the VB complex from the pericruciate and coronal gyri are of thick caliber, those to the former nucleus being on an average somewhat thinner than those to the latter. The fibers to CM, Pf, Pc and MD are quite thin. The problem of retrograde fiber degeneration in thalamocortical axons following cortical lesions is discussed. The advantages and limitations of the Nauta method have been discussed in light of recent electron microscope studies on degeneration processes in the nervous system. The importance of varying the survival time in experimental degeneration studies is emphasized. The findings are finally discussed in the context of earlier reports with respect to the anatomy and physiology of corticothalamic connections.

Terminals of subthalamonigral fibres are enriched with glutamate-like immunoreactivity: An electron microscopic, immunogold analysis in the cat

Wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) histochemistry was combined with post-embedding immunogold cytochemistry in order to establish whether the subthalamic nucleus (STN) gives origin to glutamate (Glu)-enriched nerve terminals in substantia nigra, pars reticulata (SNr). Two adult cats served as normal controls and in two other animals crystalline WGA-HRP had been implanted bilaterally in STN. In all four animals ultrathin sections from SN were subjected to an immunogold procedure using antiserum raised against either Glu or gamma-aminobutyric acid (GABA). In some experiments the sections were subjected to consecutive incubations with both GABA and Glu antisera. These two antisera label two morphologically distinct types of boutons in SNr. The GABA antiserum labels boutons with pleomorphic vesicles, and they establish symmetrical synaptic contacts, mainly with dendritic shafts and spines, and occasionally with cell bodies. The Glu antiserum labels boutons with vesicles which are smaller and more uniform with regard to size and shape than those seen in the GABA-labelled boutons. The Glu-labelled boutons are engaged in asymmetrical synaptic contacts mainly with dendritic shafts and more rarely with cell bodies. The number of GABA-labelled boutons in SNr greatly exceeds the number of Glu-labelled ones. In the experimental material a considerable number of boutons in SNr are labelled with WGA-HRP reaction product. Several of these boutons are enriched in Glu-like immunoreactivity (Glu-LI), but not in GABA-LI. It is concluded that the subthalamonigral projection in the cat is likely to use Glu as a transmitter. The findings are briefly discussed with respect to the role played by STN in movement disorders and the involvement of excitatory amino acids in SN for the propagation of epileptic seizures and development of neurotoxicity.

The arrangement of glutamate receptors in excitatory synapses.

ABSTRACT: Electron microscopic immunogold analyses have revealed a highly differentiated arrangement of glutamate receptors at excitatory synapses in the central nervous system. Studies focused on the hippocampus and cerebellum have shown that the postsynaptic specialization is the preferential site of NMDA and AMPA receptor expression, and that the δ2 receptor is similarly concentrated at this site. In cases of colocalization (AMPA and NMDA, or AMPA and δ2) the two receptor types appear to be intermingled rather than segregated to separate parts of the membrane. The different groups of metabotropic receptor exhibit distinct distributions at the synapse: group I receptors occur in membrane domains lateral to the postsynaptic specialization; group II receptors are expressed in preterminal membranes or extra‐synaptically; whereas group III receptors are found in, or close to, the presynaptic active zone consistent with their roles as autoreceptors. The differentiated distribution of glutamate receptors reflects their functional heterogeneity and explains why some receptors are activated only at high firing frequencies.