Mature HE synapses were thought to contain both acetylcholine (ACh) and a peptide, FMRFamide. Using FMRFamide markers, synapses did not appear until very late.

 

Surprisingly, we found that appearance of the synapses required rhythmic activity and that it was rhythmic activity of the postsynaptic muscle that was needed.

 

However, since that very activity was imposed by the HE, the question arose - Where does the heart muscle rhythmic activity come from before HE synapses form?

 

This led Dan Kueh in my lab to a startling discovery. There evidently were some HE synapses on the heart that did not contain both neurotransmitters. These synapses evidently do contain ACh, innervate the junctions between lateral vessels and the heart and serve a role in establishing rhythmic activity early in development.