Endomorphin-induced motivational effect: differential mechanism of endomorphin-1 and endomorphin-2.
The newly discovered endogenous mu-opioid receptor (MOP-R) ligands endomorphin-1 (EM-1) and -2 (EM-2) exhibit the highest specificity and affinity for the MOP-R of any endogenous substance so far described in the mammalian nervous system. This review focuses on differential mechanism of the motivational effects induced by EM-1 and EM-2. In the [35S]GTPgammaS binding assay, either EM-1 or EM-2 causes a concentration-dependent G-protein activation in brain membrane of normal mice, whereas neither EM-1 nor EM-2 produces any activation of G-protein in membranes obtained from the MOP-R knockout mice. These results provide direct evidence at the molecular level that both EMs act on the MOP-R as the endogenous MOP-R agonists. Based on the conditioned place preference paradigm in mice, EM-1 given intracerebroventriculally produced a dose-related place preference. This effect was abolished by pretreatment with the MOP-R antagonist beta-funaltrexamine (FNA) but not the delta-opioid receptor (DOP-R) antagonist naltrindole and the kappa-opioid receptor (KOP-R) antagonist nor-bialtorphimine (BNI). Unlike EM-1, EM-2 exhibited a place aversion. The aversive effect was inhibited by not only beta-FNA but also nor-BNI. Place aversion produced by EM-2 was also attenuated by pretreatment with an antiserum against an endogenous KOP-R ligand dynorphin A(1-17). These findings indicate that EM-1 may produce its rewarding effect via MOP-Rs. Furthermore, the aversive effect induced by EM-2 may be associated with the stimulation of the EM-1-insensitive MOP-R subtype and necessarily activate an endogenous KOPergic system in the mouse brain.
Narita
Minoru
M
Department of Toxicology, School of Pharmacy, Hoshi University, Tokyo, Japan.
Ozaki
Satoru
S
Suzuki
Tsutomu
T
D000818:Animals; D001921:Brain / Q000187:drug effects / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D006801:Humans; D009042:Motivation; D009842:Oligopeptides / Q000378:metabolism / Q000494:pharmacology*; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000378:metabolism
D009842:Oligopeptides; D017450:Receptors, Opioid, mu; C105293:endomorphin 1; C105294:endomorphin 2