Endomorphin-induced motivational effect: differential mechanism of endomorphin-1 and endomorphin-2
by
Narita M, Ozaki S, Suzuki T.
Department of Toxicology,
School of Pharmacy, Hoshi University, Tokyo, Japan.
Jpn J Pharmacol 2002 Jul;89(3):224-8

ABSTRACT

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.

Pain
LAAM
Tolerance
Sugar junkies
Knockout mice
Opium timeline
Opioid receptors
Endomorphins 1 and 2
The Pleasures of Opium
Endomorphins and the mouse
Endomorphins and rodent brains
Adenylyl cyclase superactivation
The degradation of endomorphins
Endomorphins 1 and 2 as antidepressants
Endomorphins and the mu-opioid receptor
Endomorphins and related opioid peptides
Morphine: a mood-brightening smart-drug?
Opioids, depression and learned helplessness
Endomorphin-1, accumbal dopamine and the mu-opioid receptor

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