Nature Reviews Neuroscience - Highlights
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June 2005 Vol 6 No 6 HIGHLIGHTS
Nature Reviews Neuroscience 6, 416 (2005); doi:10.1038/nrn1690
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SYNAPTIC PHYSIOLOGY
Please release me
Rebecca Craven
The number of 'readily releasable' synaptic vesicles shapes the postsynaptic
response to presynaptic action potentials. However, new data indicate that
vesicles in the readily releasable pool (RRP) are not always ready and willing
to be released. Reporting in The Journal of Neuroscience, Moulder and
Mennerick describe a population of 'reluctant' vesicles that might increase
the range of potential outputs at some synapses.
The RRP contains the subset of synaptic vesicles
that are on standby for release, and modulation
of RRP size is a fundamental means by which
synaptic strength can be altered. To estimate the
size of the RRP, a stimulus is typically used to
deplete the pool and the resulting postsynaptic
response is measured. However, different
depletion techniques have resulted in
discrepancies in the RRP size estimates obtained
at some synapses. Do these techniques probe
the same pool of vesicles?
To address this question, Moulder and Mennerick looked at excitatory
(glutamate-releasing) and inhibitory (GABA ( -aminobutyric acid)-releasing)
neurons in hippocampal cultures. They made electrophysiological recordings
from single cells that had formed autaptic contacts synapses between a
neuron and its own dendrites -- and compared the postsynaptic response to
stimulation with hypertonic sucrose or a high-frequency train of action
potentials. In glutamatergic neurons, the sucrose-depleted RRP was
estimated to contain 12,681 vesicles, whereas train-evoked responses gave
a maximum estimate of just 1,945 vesicles.
In the case of GABA-releasing neurons, the authors found no difference
between sucrose and train estimates of RRP size, which indicates that
specific presynaptic or postsynaptic factors contribute to the discrepancy
seen in glutamatergic cells. Postsynaptic factors that could confound
estimates of RRP size, such as glutamate receptor saturation, accounted for
only a small part of the difference in excitatory neurons. The authors
therefore looked for presynaptic factors that could explain the discrepancy.
http://www.nature.com/nrn/journal/v6/n6/full/nrn1690_fs.html (1 z 2) [2.6.2005 10:19:14]
Nature Reviews Neuroscience - Highlights
They found that longer trains of action potentials gave RRP estimates closer
to those obtained using sucrose, and that by increasing Ca2+ influx during
trains, they could eliminate the discrepancy. RRP size estimates made with a
strong depolarizing stimulus high extracellular potassium -- matched those
obtained with sucrose.
These findings point to a heterogeneous population of releasable vesicles
that can be differentially released with changing Ca2+ concentrations. The
authors comment that the presence of 'reluctant' vesicles in the RRP might
allow greater plasticity at hippocampal glutamatergic synapses. An
interesting question that remains is whether modulators of synaptic
transmission can make readily releasable vesicles more or less reluctant.
References and links
ORIGINAL RESEARCH PAPER
Moulder, K. L. & Mennerick, S. Reluctant vesicles contribute to the total
readily releasable pool in glutamatergic hippocampal neurons. J. Neurosci.
25, 3842­3850 (2005) | Article | PubMed | ChemPort |
FURTHER READING
Rizzoli, S. O. & Betz, W. J. Synaptic vesicle pools. Nature Rev. Neurosci. 6,
57­69 (2005) | Article | PubMed | ChemPort |
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 2005 Nature Publishing Group
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