This short review on silent synapses is clearly written and well focused. I would be more cautious about the interpretation of pair recording experiments. I would stress the possibility that previous experience or the frequency of stimulation may convert active synapses into silent ones (see Gasparini et al. 2000; Xiao et al. 2004). This may imply a presynaptic mechanism related to vesicle recycling.
This is a nicely written and concise article. My only suggestion is to define the term “silent synapse” at the beginning of the article taking both pre- and post- synaptic components into account. At the present the presynaptic mechanisms are confined to the “basement” of the article as an afterthought. This should be changed since the presynaptically silent synapses that do not release neurotransmitter are no less silent than synapses with ineffective postsynaptic receptors.
In addition to the changes the author made in the revision I suggest to add at least one current reference on presynaptically silent synapses to bring the topic to the 21 century. One possibility is the aricle by Peled and Isacoff (2011) 0. E. S. Peled and E. Y. Isacoff. Optical qunatal analysis of synaptic transmission in wild-type and rab3-mutant Drosophila motor axons. Nature Neurosci. 14:519-526, 2011. This work confirms earlier studies by Atwood and colleagues showing heterogeneity of synaptic release sites with some being "silent" but recruitable. The work is doubly important in showing what happens to transmitter release during synaptic facilitation and depression.