An ultrastructural study into the effect of global transient cerebral ischaemia on the synaptic population of the cerebellar cortex in rats

Abstract

The density of synapses, the shape and size of presynaptic dense projections (PDP), and the curvature of synaptic appositions in the molecular layer of the cerebellum cortex of rat at 10 min ischaemia and after 90 min, 1, 3, 7, 30 days of re-circulation were examined using quantitative ultra-structural techniques. The numerical density of mature junctions decreased significantly (44.0%) after 1 and 3 days of re-circulation, and was increased to 149.8% of the value in the control animals after 7 days of re-circulation. The restoration of the population of mature synaptic junctions was accompanied by a considerable increase of the number of immature junctions. We found a close association between the synaptic curvature and the size of PDP. The curvature of the larger junctions was consistently associated with a reduced height of PDP and a rounder shape. Synaptic curvature increased from 0.0885 (control) to 0.2041 (3 days of re-circulation) and to 0.2128 (7 days re-circulation). The maximum reduction in synaptic numerical density and larger junction curvature was found in zones of irreversibly damaged Purkinje cells. Our results revealed that the synaptic curvature and the height of the pre-synaptic dense projections undergo reciprocal changes after global transient cerebral ischemia. It is tempting to hypothesize that the positive synaptic curvature occurs as a result of changes in morphological conditions for the PDP filaments and in the shape and size of PDP and depends on the level of Ca2+ in synaptic appositions.