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Three-dimensional relationships between hippocampal synapses and astrocytes.
1Harvard College, Harvard Medical School, Division of Neuroscience in the Department of Neurology, Children's Hospital, Boston, Massachusetts 02115, USA.
Summary
Astrocytes contact only 57% of hippocampal synapses, suggesting glutamate may diffuse between synapses. This impacts understanding astrocyte signaling and glutamate transporter function in the hippocampus.
Area of Science:
- Neuroscience
- Cell Biology
- Synaptic Plasticity
Background:
- Astrocytes play a crucial role in regulating extracellular glutamate levels at synapses.
- Glutamate transporter-mediated currents in astrocytes are essential for synaptic function and preventing excitotoxicity.
- The precise structural relationship between astrocytes and hippocampal synapses remains largely uncharacterized.
Purpose of the Study:
- To investigate the morphological relationship between astrocytic processes and synapses in the CA1 region of the rat hippocampus.
- To determine the extent of astrocytic coverage at hippocampal synapses and the extracellular space between them.
Main Methods:
- Serial electron microscopy and 3D reconstruction techniques were employed.
- Analysis focused on mature rat hippocampus in vivo and in slice preparations.
- Quantified the apposition of astrocytic processes to synapses and their coverage of the synaptic interface.
Main Results:
- Astrocytic processes were apposed to only 57% of synapses, covering less than half of the synaptic interface.
- Neighboring synapses were frequently separated by neuronal processes (66%) or astrocytic processes (33%).
- A significant portion of synapses (approximately two-thirds) may experience glutamate diffusion to neighboring synapses.
Conclusions:
- The limited astrocytic coverage suggests that glutamate released from hippocampal synapses may diffuse to extrasynaptic sites or neighboring synapses.
- This diffusion could impact synaptic specificity and neuronal excitability.
- Physiological recordings from astrocytes may not capture glutamate signaling from all synapses uniformly.