Presynaptic Markers of Synapses

There are a number of presynaptic molecular markers of synapse formation. GAP43 is a marker of the growth cone and has been shown to increase in the VMN after estrogen treatment (151); however, no studies of this type have been done on the hippocampus. SNAP-25 is a marker of synaptic vesicles (152,153), as are the synaptotagmins (154-156), synaptoporin (157,158), synaptophysin (159), and the synapsins (160-162). Overexpression of synapsins in cultured cells increases synapse formation, suggesting that they may be rate-limiting and that enhanced expression could drive new synapse production (160,163), although synapsins are not essential for synapse formation, because knock-outs lacking two forms of synapsin are viable and form synaptic connections (161).

It is possible to use antibodies to visualize markers of synapses, such as synaptoporin, and of growth cones, such as GAP43, to study reactive synaptogenesis by means of conventional immunocytochemistry and confocal imaging (164). However, to look at the mRNAs for presynaptic proteins, it is necessary to study the cell bodies of afferent projections. Principal afferents to hippocampal CA1 neurons include the Schaffer collateral from CA3 and efferents from the entorhinal cortex (78).

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