Ultrastructure of neuron-glia interaction in the norm and experimental pathology

Abstract

Long-term experience was gained in analyzing synapses and their glial surroundings in normal, natural, and experimental models of functional plasticity and brain pathology development. However, most studies in this area rely on electrophysiological techniques combined with fluorescent imaging. Notably, fine synapse structure and astrocytic processes cannot be resolved using light or some electron microscopy techniques. Studies on experimental brain pathology using volume electron microscopy methods [1] were previously restricted due to their high labor intensity. However, automated methods for sample preparation and analysis, using machine vision and artificial intelligence, significantly simplified this task. Using transmission electron microscopy methods and 3D reconstructions, this study examined the Str. radiatum CA1 hippocampal region of rat brains in a chronic lithium-pilocarpine model of epilepsy. The results indicate a decrease in synaptic number along with an increase in their size and a reduction in astrocytic isolation of the active zones. A decrease in glial ensheathment of enlarged active zones and facilitation of neurotransmitter diffusion to active synapses may have a multiplicative effect on epileptiform activity growth and excitotoxicity. The simplification of the astrocytes meshwork in the somatosensory cortex’s layer 2/3 is comparable to that of layer 1. This decrease in layer 1’s inhibitory action enables pyramidal neurons in layer 2/3 to potentially exhibit epileptiform activity. Thus, the superficial cortical layers’ structural-functional aspects can be used as a natural cellular model in epilepsy development studies. Reduction of Ca2+ events in astrocyte processes in lithium-pilocarpine induced epilepsy may result from the low buffer capacity of Ca2+ ions in the smooth endoplasmic reticulum (sER) and/or impaired Ca2+ wave transmission through the gap junctions between astrocytic processes. High resolution is necessary to analyze the gap junctions, and special methods are required for sER visualization in perisynaptic astrocytic processes. We developed original sER staining methods [2] to quantitatively evaluate gap junctions and sER cisternae within astrocytic meshworks in layers 1 and 2/3 of the somatosensory cortex. In layer 1, the area of gap junctions in relation to the volume of an astrocyte was twice as high as in layer 2/3. The proportion of sER volume differed between layer 1 and layer 2/3 tissues. Specifically, the total sER cisternae volume in layer 2/3 was twice as high as in layer 1, relative to the volume of astrocytic processes [3]. Additionally, a doubling of single astrocytic gap junction area concomitant with decreased calcium events was observed. The results suggest a normal balance between Ca2+ stores (sER) and gap junctions, whose disruption may contribute to the development of seizures.