Agmatine, an endogenous
ligand for the I1-imidazoline receptor, has previously been shown to prevent
morphine dependence in rats and mice. To investigate the role of
imidazoline receptor antisera-selected
protein (IRAS), a strong candidate for I1R, in
morphine dependence, two CHO cell lines were created, in which
mu opioid receptor (MOR) was stably expressed alone (CHO-mu) or MOR and IRAS were stably co-expressed (CHO-mu/IRAS). After 48 h administration of
morphine (10 microM),
naloxone induced a cAMP overshoot in both cell lines, suggesting cellular
morphine dependence had been produced.
Agmatine (0.1-2.5 microM) concentration-dependently inhibited the
naloxone-precipitated cAMP overshoot when co-pretreated with
morphine in CHO-mu/IRAS, but not in CHO-mu.
Agmatine at 5-100 microM also inhibited the cAMP overshoot in CHO/mu and CHO-mu/IRAS.
Efaroxan, an I1R-preferential antagonist, completely blocked the effect of
agmatine on the cAMP overshoot at 0.1-2.5 microM in CHO-mu/IRAS, while partially reversing the effects of
agmatine at 5-100 microM.
L-type calcium channel blocker
nifedipine entirely mimicked the effects of
agmatine at high concentrations on
forskolin-stimulated cAMP formation in CHO-mu and
naloxone-precipitated cAMP overshoot in
morphine-pretreated CHO-mu. Therefore, IRAS, in the co-transfected CHO-mu/IRAS cell line, appears necessary for low concentrations of
agmatine to cause attenuation of cellular
morphine dependence. An additional effect of
agmatine at higher concentrations seems to relate to both transfected IRAS and some naive elements in CHO cells, and L-type voltage-gated
calcium channels are not ruled out. This study suggests that IRAS mediates
agmatine's high affinity effects on cellular
morphine dependence and may play a role in
opioid dependence.