Signaling mechanisms in protozoa and invertebrates

Comparative endocrinology explores the origins of homeostatic regulation across organisms. Early experiments revealed hormonal regulation in invertebrates, showcasing mechanisms that share similarities with, yet differ from, those in vertebrates. The advent of advanced receptor studies in mammalian endocrinology spurred interest in invertebrate hormone receptors, leading to the application of sophisticated methods for hormone determination. These studies confirmed the existence and functional similarities of these structures and signaling molecules. However, information regarding hormones and receptors in the simplest metazoans and the highest protozoans was scarce. Around two decades ago, initial findings indicated the presence of hormone receptors in protozoa that respond to vertebrate hormones. Since the early 1980s, it has been established that hormone-like molecules akin to those in higher vertebrates exist in unicellular organisms. Research has identified second messengers in Tetrahymena, and many structures once thought exclusive to vertebrates, such as opiate receptors, have been discovered in unicellular life forms. These findings support the notion of a primitive endocrine system in protozoa, challenging the assumption that such systems were unnecessary in unicellular organisms. Ultimately, it appears that the foundations of endocrine communication extend back at least 2 billion years.