Not every touch is a kiss. Synapses and appositions between dendrites and axons: a correlated light and electron microscopy study
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1
UZH/ETHZ, Institute of Neuroinformatics, Switzerland
The use of light microscopy (LM) to identify synaptic connections between different labeled neurons could be of great help in the reconstruction of cortical circuitry. However it remains to be demonstrated that a contact between two neurons seen at LM reflects synaptic connectivity. Theoretically this is an unlikely scenario, given that the resolution of light microscopy does not go beyond 200nm, and so it is expected that at the LM level it will not be possible to know if two labeled structures are in physical contact or not, even less forming a synapse.
We used correlated light and serial section electron microscopy (EM) to investigate 122 contacts seen at LM, between labeled dendrites and labeled thalamic axons in the primary visual cortex of the cat.
Electron microscopic observations showed that 70% of the contacts seen at LM did not form synapses. Contacts seen at LM could be divided into those that were a physical apposition and those that were not, i.e. had other unlabeled structures between labeled profiles. When we consider only those contacts seen at LM that were truly physical appositions seen at EM, then 72% form synapses. These results are consistent with the electron microscopy observation that on average a spine is in contact with 1.6 boutons, synapting usually with only one. The fact that more than one potential target exist, alone, makes it impossible to judge at the LM level if a labeled dendrite and a labeled axon form a synapse with each other.
Our results show that the LM observation of a contact between an axon and a dendrite is not sufficient to infer synaptic connectivity. Of the contacts seen at LM that did not reflect real physical apposition, the mean distance between labeled structures was 400nm. While this proximity creates the opportunity for local selection and rearrangement of connections, offering the potential for structural plasticity, it is unclear how many of the numerous potential structural circuits actually lead to different circuit function.
Conference:
11th Meeting of the Portuguese Society for Neuroscience, Braga, Portugal, 4 Jun - 6 Jun, 2009.
Presentation Type:
Oral Presentation
Topic:
Symposium 1– Neuronal Communication
Citation:
Macarico-da-Costa
N and
Martin
KA
(2009). Not every touch is a kiss. Synapses and appositions between dendrites and axons: a correlated light and electron microscopy study.
Front. Neurosci.
Conference Abstract:
11th Meeting of the Portuguese Society for Neuroscience.
doi: 10.3389/conf.neuro.01.2009.11.005
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Received:
05 Aug 2009;
Published Online:
05 Aug 2009.
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Correspondence:
Nuno Macarico-da-Costa, UZH/ETHZ, Institute of Neuroinformatics, Zurich, Switzerland, nunod@alleninstitute.org