The biological presence and regulatory function of the plant alkaloid morphine in relatively simple and complex integrated animal systems has previously been shown. The pivotal role of dopamine as a chemical intermediate in the morphine biosynthetic pathway in plants establishes a functional basis for its expansion into an essential role as the progenitor catecholamine signaling molecule. In invertebrate neural systems, dopamine serves as the preeminent catecholamine signaling molecule, with the emergence and limited utilization of norepinephrine and its biosynthetic enzyme dopamine β-hydroxylase in newly defined adaptational chemical circuits required by a rapidly expanding set of physiological demands. In vertebrates, epinephrine emerges as the major end of the catecholamine synthetic pathway consistent with a newly incorporated regulatory modification, i.e. N-methylation of norepinephrine. Given the striking similarities between the enzymatic steps in the morphine biosynthetic pathway and those driving the evolutionary adaptation of catecholamine chemical species to accommodate an expansion of interactive but distinct signaling systems, we surmise that the evolutionary emergence of catecholamine systems required conservation and selective ‘retrofit’ of specific enzyme activities, i.e. catechol O-methyl transferase and phenylethanol-amine N-methyl transferase, drawn from cellular morphine expression. This hypothesis is further supported by the critical recruitment of enzymatically synthesized tetrahydrobiopterin (BH4) both as an essential cofactor for tyrosine hydroxylase-mediated dopamine production and as a secondary electron donor for nitric oxide synthase-mediated nitric oxide (NO) production. The establishment of a reciprocal regulatory linkage between NO and catecholaminergic processes, as mediated by BH4, subserves a pivotal capacity to promote autocrine and paracrine regulation of signaling molecules. In summary, ongoing development and adaptation of catecholamine signaling pathways in animals appear to be related to their mobile lifestyle associated with complex feeding, sexual and protective processes, which also generate free radicals, thus requiring morphinergic signaling coupling to NO release.

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