Are synaptic boutons always located on axons? I am learning about the protein Synaptophysin and I have read that it is an integral membrane protein localised to synaptic vesicles. I have also read that it is a specific and sensitive marker for synaptic terminals. On this site, I have read that synaptic terminals are the presynaptic endings and on Wikipedia that the presynaptic axon terminal is also known as the synaptic bouton. From what I have read online it seems to me that synaptic boutons are located only on axons. However, I have found out about dendrodendritic synapses which are connections between the dendrites of two different neurons. Can the presynaptic ending of a dendrodendritic synapse be referred to as a synaptic bouton or are synaptic boutons only located on axons? Any insights are appreciated.

Axonal boutons are named as such due to their shape. Dendro-dendritic synapses do not resemble this shape and are never referred to as boutons. It does not appear that synaptophysin is expressed on dendrites (Fletcher et al. 1991), but I am unaware of a study that targeted dendro-dendritic synapses specifically.

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**References**

Fletcher, TL, P Cameron, P De Camilli, and G Banker. ‘The Distribution of Synapsin I and Synaptophysin in Hippocampal Neurons Developing in Culture’. The Journal of Neuroscience 11, no. 6 (1 June 1991): 1617. <