Neuropeptide

Neuropeptides are chemical messengers made up of small chains of amino acids. When they bind to receptors, they cause nerve activity. They also act on other tissues like the gut, muscles, and heart. There are over a hundred different types in our bodies.
Peptides are ancient signalling systems that are in almost all animals on Earth (sponges are the exception). 
Neuropeptide signalling may predate the development of nervous tissues or neurotransmitters.