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The Journal of Neuroscience, August 15, 1999, 19(16):6897-6906
Three-Dimensional Relationships between Hippocampal Synapses
and Astrocytes
Rachel
Ventura1 and
Kristen M.
Harris2
1 Harvard College and 2 Program in
Neuroscience, Harvard Medical School, Division of Neuroscience in the
Department of Neurology, Children's Hospital, Boston, Massachusetts
02115
Recent studies show that glutamate transporter-mediated currents
occur in astrocytes when glutamate is released from hippocampal synapses. These transporters remove excess glutamate from the extracellular space, thereby facilitating synaptic input specificity and preventing neurotoxicity. Little is known about the position of
astrocytic processes at hippocampal synapses. Serial electron microscopy and three-dimensional analyses were used to investigate structural relationships between astrocytes and synapses in stratum radiatum of hippocampal area CA1 in the mature rat in
vivo and in slices. Only 57 ± 11% of the synapses had
astrocytic processes apposed to them. Of these, the astrocytic
processes surrounded less than half (0.43 ± 22) of the synaptic
interface. Other studies suggest that astrocytes extend processes
toward higher concentrations of glutamate; thus the presence of
astrocytic processes at particular hippocampal synapses might signal
which ones are releasing glutamate. The distance between nearest
neighboring synapses was usually (~95%) <1 µm. Astrocytic
processes occurred along the extracellular path between 33% of the
neighboring synapses, neuronal processes occurred along the path
between another 66% of the neighboring synapses, and only 1% of the
synapses were close enough such that neither astrocytic nor neuronal
processes occurred between them. These morphological arrangements
suggest that the glutamate released at approximately two-thirds of
hippocampal synapses might diffuse to other synapses, unless neuronal
glutamate transporters are more effective than previously reported. The
findings also suggest that physiological recordings made from
hippocampal astrocytes do not uniformly sample the glutamate released
from all hippocampal synapses.
Key words:
Key words: astrocytes; serial electron microscopy; glutamate spillover; transporters; long-term potentiation; multiple
synapse boutons
Copyright © 1999 Society for Neuroscience 0270-6474/99/19166897-10$05.00/0
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