Molecular and cellular mechanisms of the age-dependency of opioid analgesia and tolerance. Mechanisms of opioid analgesia and tolerance at the molecular and cellular levels are complex, and many of them require a modification of the expression and functions of signaling molecules. Aging has a significant impact on almost every aspect of the opioid receptor signaling systems that underlie opioid analgesia and tolerance. ① Endogenous opioid peptides and opioid receptors are differentially expressed in different developmental stages, and aging is associated with changes in the number and/or affinity of opioid receptors and opioid receptor-like 1 (ORL1). ② The expression of β-arrestin, which plays a prominent part in opioid receptor desensitization, is determined by neural differentiation and aging. The increased expression of β-arrestin is accompanied by a parallel increase in G protein-coupled receptor kinase (GRK) expression during prenatal development. ③ The phosphorylation of opioid receptors by GRK and the binding of β-arrestin initiate the internalization of the ligand-bound receptors. The internalization of epidermal growth factor (EGF) receptors and interleukin 2 (IL2) receptors and clathrin-associated endocytosis are age-dependent, which implies that the same might be also true for opioid receptor systems. ④ Aging affects the expression and function of the N-methyl-D-aspartic acid (NMDA) receptor and its subunits—calmodulin (CaM) and protein kinase C (PKC) and its various isoforms—as well as other neuropeptides known to have anti-opioid effects. ⑤ The expression, regulation, and function of specific G protein signaling (RGS) members are affected by age during embryonic development and neuronal differentiation. ⑥ Development and aging differentially regulate G protein-mediated adenylate cyclase (AC) signaling. The activities of AC, guanylate cyclase (GC), cyclic AMP (cAMP), phosphodiesterase, and cyclic GMP (cGMP) phosphodiesterase in the frontal cortex and cerebellum show age-related changes.