Dual effects of DAMGO [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin and CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2) on adenylyl cyclase activity: implications for mu-opioid receptor Gs coupling.
The mu-opioid receptor (MOR) couples to multiple G proteins, of which coupling to Gs has long been debated. As expected, in opioid naive Chinese hamster ovary cells expressing recombinant MOR, the predominant action of [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) is inhibitory. However, inactivation of Gi/Go proteins via pertussis toxin (PTX) unmasks its ability to facilitate forskolin activation of adenylyl cyclase (AC) activity. Tolerance develops to this effect of DAMGO, which can also be attenuated by cholera toxin (CTX). The latter suggests G mediation. D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP), previously considered to be a neutral MOR antagonist, also produces a facilitation of forskolin (FSK) activation of AC that is augmented by chronic morphine. Facilitative effects of CTAP in naive as well as its augmentation in tolerant membranes are both substantially reduced by CTX. This suggests that not only Gs mediation but also G(salpha)-linked signaling is critical to the chronic morphine-induced enhanced facilitative action of CTAP. Interestingly, the (augmented) CTAP facilitation of FSK-stimulated AC activity that is observed in opioid tolerant (but not in naive) membranes is also sensitive to PTX. This can best be explained by postulating the involvement of Gi-derived G(betagamma), which would stimulate type 2 ACs, conditional on the presence of activated G(salpha). The emergence of a G(betagamma) dimension of AC stimulation by CTAP after chronic morphine could explain its ability to augment the stimulatory action of CTAP on AC. These results support putative MOR coupling to Gs and underscore the multifaceted nature and plasticity of MOR G protein coupling.
Szücs
Mária
M
Department of Biochemistry, State University of New York, Downstate Medical Center, Brooklyn, New York 11203, USA.
Boda
Krisztina
K
Gintzler
Alan R
AR
D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D016466:CHO Cells; D005576:Colforsin / Q000494:pharmacology; D006224:Cricetinae; D004361:Drug Tolerance; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology*; D004745:Enkephalins / Q000737:chemistry / Q000494:pharmacology; D005260:Female; D019205:GTP-Binding Protein alpha Subunits, Gs / Q000378:metabolism*; D009294:Narcotics / Q000009:adverse effects; D010446:Peptide Fragments; D010455:Peptides / Q000494:pharmacology*; D037342:Pertussis Toxin / Q000494:pharmacology; D017450:Receptors, Opioid, mu / Q000378:metabolism*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D013004:Somatostatin
C529311:CTAP octapeptide; D004745:Enkephalins; D009294:Narcotics; D010446:Peptide Fragments; D010455:Peptides; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D005576:Colforsin; D013004:Somatostatin; D037342:Pertussis Toxin; D019205:GTP-Binding Protein alpha Subunits, Gs; D000262:Adenylyl Cyclases