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There are several promising techniques for connectomics based on iterative sectioning and imaging of tissue with scanning electron microscopes (e.g. FIB-SEM and ATUM)

By looking at such micrographs, one can identify synapses:

    enter image description here

Is there any way to measure synaptic strength using these types of techniques?

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i guess you cant determine synaptic strength by just detecting synapses.. perhaps by labeling the receptors you might be able to say something using confocal.. there are some studies like that – WYSIWYG Sep 27 '13 at 16:47
    
Thank you @WYSIWYG. Would you mind pointing me to some of those papers? – Josh Sep 27 '13 at 17:38
    
I am sorry.. I don't know of papers. I just heard them in some talk.. i shall look up and get back.. – WYSIWYG Sep 28 '13 at 5:01
    
@WYSIWYG: I would be extremely wary of using such approach... you should really combine it with some ephys. – nico Sep 28 '13 at 7:38
    
@nico: totally agree on that.. i just said that there are some studies which check protein activity at the synapses using labelled proteins. It is not absolute.. Btw i still need to find reference for it. – WYSIWYG Sep 28 '13 at 10:48
up vote 1 down vote accepted

First of all, definitions. By synaptic strength let's understand ability of post-synapse to elicit current flow/depolarization in response to neurotransmitter appearance in the synapse. How can NT appear? By release from pre-synapse or through uncaging (optical activation of "caged", or inactive NT). Second, post-synapse contain all sort of stuff, but main sources of current/depolarization are ionotropic receptors of NTs, e.g. NMDA-R or AMPA-R. This molecules are not just floating ion membrane but attached to extensive scaffolding just below of the membrane. Scaffolding includes actin, and more importantly PSD95 (in case of excitatory synapses).

I work in vivo and don't do any EM. But in vivo issue of observing molecules is that you can't label sensitive proteins without disturbing function. Some things are just too delicate to be labeled with fusing GFP.

PSD95 has been used for in vivo labeling with GFP. One paper I will point you to, is this knock-in work in mice: Live Imaging of Endogenous PSD-95 Using ENABLED: A Conditional Strategy to Fluorescently Label Endogenous Proteins, J Neurosc. 2014

This is the figure that shows strong correlation between brightness of synapse (aka number of PSD95 molecules) and current that occurs in cell body when authors uncaged glutamate at different synapses.

An image of a representative CA1 oblique dendrite (top), a family of uEPSCs elicited from the spines indicated in the top (middle), and uEPSC amplitudes as a function of PSD-95mVenus fluorescence (bottom). Colored dots indicate sites of two-photon glutamate uncaging

Figure description

Now, how does that relates to your image? PSD stands for "post-synaptic density" protein, because in EM synapse is very dark. I don't know links, but I would assume some correlation between brightness of synapse in EM and its strength.

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