Ongoing studies from our group support the existence and biological importance of a distinct cellular signaling pathway involving endogenously synthesized, chemically authentic, l-morphine, its cognate mu(3) opiate receptor subtype, and constitutive NO synthase. Based on prior studies indicating evolutionary conservation and adaptation of morphinergic/NO-coupled signaling to mediate autocrine/paracrine control of cellular functions, our goal was to determine whether a functionally competent mu(3) opiate receptor/NO-coupled regulatory pathway exists in human multilineage progenitor cells (MLPC) prepared from umbilical cord blood. Real-time PCR analysis indicated significant expression of mu(3) opiate receptor-encoding RNA by undifferentiated human MLPC, in the absence of traditional mu(1) opioid receptor-encoding RNA expression. Unpredictably, confirmatory RT-PCR analyses indicated cellular expression of a splice variant of the previously characterized mu(3) opiate receptor-encoding mRNA. Pharmacological analyses provided critical validating evidence of functional mu(3)-like opiate receptor/NO-coupled signaling within primary cultures of undifferentiated human MLPC via morphine-evoke real-time release of NO. Control analyses indicated that morphine-stimulated NO release was markedly inhibited by prior treatment with the opiate antagonist l-naloxone or the constitutive NO synthase inhibitor N(G)-nitro-l-arginine methyl ester and unresponsive to stimulation by the opioid peptide methionine enkephalin. Complementary microarray analysis demonstrated that traditional mu(1), delta, and kappa opioid receptor gene expression is not detected in both undifferentiated and differentiated MLPC. Chemical differentiation of MLPC into neuronal progenitor cells effected significant phenotypic expression of a variety of neurally-associated genes. Our data provide compelling evidence in support of both the evolutionary primacy and primordial regulatory role of mu(3)-like opiate receptor/NO signaling in embryogenesis.