Paramembranous specialized formations of the synaptic cytoskeleton -dense projections and postsynaptic condensation of axospinous synapses of the molecular layer of white rat sensorimotor and cerebellar cortex -in health and acute total ischemia are studied by selective contrast staining with phosphotungstic acid. A direct relationship is revealed between the pattern and degree of deformation of the contact plane, on the one hand, and the postsynaptic condensation and ratio of the volumes of pre-and postsynaptic accumulations of paramembranous fdaments, on the other. Key Words: neocortex; cerebellum; interneuronal synapse; ischemiaSynaptic deformation is regarded as the basis of morphological characterization of synapse shape [3]. Experimental findings attest to a relationship between the pattern of deformation of the plane of a synaptic contact and its functional state [2,3,7]. The factors determining the type (positive or negative) and degree of deformation include various external and internal factors (state of the synaptic cytoskeleton, type of transmitter metabolism, size of the synapse, complexity of arrangement of the contact, its localization on the neuron, and its appurtenance to a particular brain compartment) [2,3,5,6]. Discrepancies in the data, however, leave unsolved the problem of the basic structural mechanisms of synapse formation in health and disease.In this study we assessed the effect of the spatial organization of the system of paramembranous fdamentous formations (dense projections, postsynaptic condensation) on the degree of changes in the deformation of the synaptic contact plane in health and ischemia. MATERIALS AND METHODSThe objects of this study were the axospinous synapses of the molecular layer of the sensorimotor cortex (SMC) and the cerebellar cortex, which differ appreciably in biochemical composition and in the spatial arrangement of postsynaptic condensation [4]. Experiments were carried out with 6 male white rats weighing 190 to 210 g under ether narcosis. Acute total ischemia of the brain was induced by 10-rain clamping of the cardiac vascular bundle [1]. The brain was fixed by perfusion of a mixture of 1% glutaraldehyde, 4% paraformaldehyde, and 5% sucrose in phosphate buffer (pH 7.4) through the left ventricle of the heart for 15 min, and then postfixed for 2 h in the same solution at 4"C. Oriented pyramidal fragments of SMC and cerebellar cortex were contrast-stained in ethyl solution of phosphotungstic acid as described elsewhere [7]. The fragments were embedded in an epon-araldite mixture. Ultrathin slices were prepared in a tangential plane at the level of the molecular layer of the SMC and cerebeUar cortex. The slices were examined under an EVM 100AK electron microscope; 30 random visual fields of the neuropil were photographed at the standard 15,000
Studies on white mongrel rats addressed the reorganization of the synaptoarchitectonics of layer I of the cerebral cortex in diffuse-focal injuries. The experiments used models of acute arrest of the systemic circulation (clinical death) resulting from exposure to mechanical asphyxia for 6 min, clamping of the common carotid arteries for 20 min (ischemia), and imposition of sublethal rotatory trauma by the Noble-Collip method (craniocerebral trauma). Electron microscopy and morphometric analysis showed that reductions in the total number density of synapses were accompanied by changes in the relative and absolute contents of the major types of synaptic apparatuses. There were increases in the numbers of large simple and perforated contacts and synapses with invaginated synaptic membranes, mitochondria, and spine apparatuses. These changes were interpreted as the structural basis for the mechanisms of synaptic plasticity in diffuse-focal brain injuries.
Perforated synapses (PS) are a variety of synapse with a presynaptic grid (PG) of complex organization, a post-synaptic thickening (PT), and a higher nerve impulse transmission efficiency. The center of this type of synaptic contact generally has one or several regions of PG and PT lacking filamentous components. Some authors believe PS are independent stable synapses, while others regard them as intermediate and unstable forms reflecting the process of reorganization of functionally mature synapses. Nearly all investigators emphasize the significant role of PS formation for producing synaptic plasticity in conditions of normal and pathological brain function [6, 7, 9]. A number of reports have been published describing changes in the PS content in the brain during the post-ischemic period [5].The aim of the present work was to make a quantitative assessment of the synapse pool in the molecular layer of the neocortex and to determine the role of perforated synapses in reorganizing interneuron interactions in the sensorimotor cortex during the post-ischemic period. MATERIALS AND METHODSExperiments were carried out using 23 white male rats (190-210 g) under ether anesthesia. Total ischemia (cessation of systemic circulation) was modeled by mechanical asphyxiation produced by clamping the intubation tube for 6 min. Recovery of heart function and the systemic circulation was aided by indirect cardiac massage and artificial pulmonary ventilation. In similar conditions, the latent period of anoxic depolarization of cerebrocortical neurons was 75 + 3 sec, and the total duration of cortical electrical silence was 21 + 1 min [2]. After 6 h, 1, 3, 7, 14, and 30 days (three rats at each time point) of the post-ischemic period, brains were fixed by perfusion with a mix of solutions containing 4% paraformaldehyde, 1% glutaraldehyde, 5% sucrose, and 0.1 M phosphate buffer (pH 7.4) administered via the ascending aorta for 15 min, at a pressure of 100-110 mmHg. Controls (five rats) underwent intubation under ether anesthesia. After f'Lxation, the sensorimotor cortex zone was cut into oriented blocks of pyramidal shape. Half of the blocks were contrasted with an ethanolic solution of phosphotungstic acid, and the other half with osmium tetroxide [6]. Material was embedded in a mix of Epon and Araldite. Ultrathin tangential sections were cut at the level of layer I of the sensorimotor cortex. Osmium-stained material was additionally contrasted with lead citrate and uranyl acetate. Photographs were made of 30 randomly selected fields of the sensorimotor cortex layer I neuropile for each animal, at a magnification of • 15,000. Using a final magnification of • photographs were used to determine the number densities of desmosome-like contacts, perforated and non-perforated synapses, and small (up to 400 mm) and large (greater than 400 mm) synaptic contacts, as well as the proportions of simple and complex synapses [6, 7]. Number densities were calculated per I00 ~m 2 of area of neuropile sections. Results were evaluated statistically ...
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