Choanoflagellates
3
Communication via:
sodium channel,
calcium channel
and another for calcium signalling and cytoskeleton regulation that is used for cell-to-cell communication in multicellular organisms but not in the unicellular ones.
From 2011
“Choanoflagellates are aquatic organisms found in oceans and rivers around the globe. Being a single cell, they do not have nerves, yet the team found both proteins in the choanoflagellate Monosiga brevicollis, and the interaction between the two was the same as in neurons (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1106189108). These proteins are found in every nerve cell and control the release of the chemicals which neurons use to talk to each other, called neurotransmitters.”
https://www.newscientist.com/article/mg21128283-800-your-brain-chemistry-existed-before-animals-did/