Opioid receptor agonists enhance the phosphorylation state of Fas-associated death domain (FADD) protein in the rat brain: Functional interactions with casein kinase Iα, Gαi proteins, and ERK1/2 signaling

Opioid drugs have been proposed to promote anti-apoptotic signals in brain through inhibition of FADD protein [García-Fuster et al., 2007. Effects of opiate drugs on Fas-associated protein with death domain (FADD) and effector caspases in the rat brain: Regulation by the ERK1/2 MAP kinase pathway. Neuropsychopharmacology 32, 399–411]. FADD phosphorylation by casein kinase Iα (CKIα) appears to regulate its non-apoptotic activity. This study investigated the effects of opioids on p-FADD in rat brain, as well as various mechanisms that could link opioid receptors with p-FADD, including the modulation of CKIα, Gαi proteins and ERK1/2 signaling. In rat, mouse and human brains, various anti-p-FADD antibodies immunodetected the monomeric and oligomeric forms of this protein, irrespective of the antibody origin and specific Ser191 or Ser194 phosphorylation site. Acute μ- and δ-agonists increased, through specific opioid receptor mechanisms, the content of oligomeric and monomeric p-FADD forms in rat cortical homogenates (25–61%) and subcellular compartments, with most relevant effects for sufentanil in membrane (239%) and nucleus (136%). p-FADD induction vanished with repeated (5 days) morphine but not SNC-80, and opioid withdrawal induced a new (morphine) or sustained (SNC-80) stimulatory effect (32–33%). The κ-agonist (−)-U-50488H failed to stimulate p-FADD. Sufentanil reduced CKI protein and kinase activity in the cytosol (30–37%). Morphine, but not SNC-80, augmented CKIα in cytosol, membrane and nucleus (36–104%). In contrast to FADD, the ability of SNC-80 to stimulate p-FADD was not sensitive to ERK1/2 blockade. Pertussis toxin did not prevent the opposite effects of SNC-80 on p-FADD and FADD because the toxin by itself markedly altered their basal contents, indicating that FADD could be a novel toxin target. The upregulation of p-FADD induced by μ/δ-agonists could play a relevant role in the anti-apoptotic and/or neuroplastic effects of opioids.