Stefano, G.B.: The Beginnings of the Stress Response: Opioid Modulation in Invertebrates

It is evident that chemical signal molecules mediate intercellular communication. During evolution, organisms using this form of communication have increased their chances of survival, passing this trait on to their descendents. Chemical communication exhibits a greater level of sophistication, i.e., synaptic, hormonal. The end result of such chemical-communication mechanisms would be a higher degree of sophistication and detailed information transfer, which allows for a greater number of behavioral characteristics (including afferent hormonal influences), enhancing an organism’s chance for survival in a changing environment. Periodically, environmental changes may represent an immediate adverse event, regardless of where an organism is on the evolutionary tree. In this regard, concerning any organism, a correct response to ensure survival is required: thus, the presence of morphine and proenkephalin, prodynorphin and proopiomelanocortin, and their derived peptides. In simple animals, the presence of such derived peptides is not surprising since they must respond to similar challenges from the environment, i.e., bacterial. Given the success of these signal molecules in modulating the internal stress response in invertebrates, they must have been conserved during evolution, presumably because the signaling process was so difficult to achieve in the first place. Recent findings not only document the invertebrate presence of signal molecules, exhibiting high sequence identity associated with the “mammalian” hypothalamic-pituitary –adrenal axis but, their receptors as well. All in all, it appears that the mammalian “stress-associated” processes first existed in invertebrates, if not earlier. In this regard, evolution simply placed cognition on this basic foundation in man.