additional supporting information for the article Neil Smalheiser, University of Illinois at Chicago 3 April 2008 In my article (section 5d, Cargo proteins within postsynaptic exosomes, subsection b, AMPA receptors), I made the statement that “to my knowledge, no physiologic role for presynaptic AMPA receptors has been described.” Thus, I surmised that AMPA receptor proteins and/or mRNAs carried from postsynaptic to presynaptic terminals would have no physiological significance, but might instead provide a way for the postsynaptic terminal to shed AMPA receptors. However, more recently, I have become aware that there is an extensive literature documenting the presence and physiological relevance of presynaptic AMPA receptors in hippocampal and sensory ganglion neurons, among other neuronal cell types. (See Matsuda et al (2008)  for discussion and a partial list of references.) This provides further biological rationale in favor of the hypothesis that exosomes may provide a means of signaling across synapses, particularly from postsynaptic to presynaptic terminals. 1. Matsuda S, Miura E, Matsuda K, Kakegawa W, Kohda K, Watanabe M, Yuzaki M: Accumulation of AMPA receptors in autophagosomes in neuronal axons lacking adaptor protein AP-4. Neuron 2008, 57:730-745. Competing interests the author declares that there are no competing interests.