Comparison of agmatine with moxonidine and rilmenidine in morphine dependence in vitro: Role of imidazoline I1 receptors

Moxonidine and rilmenidine are classical imidazoline I1 receptor agonists, and used as anti-hypertension drugs in clinical practice. Agmatine is an imidazoline I1 receptor endogenous ligand as well as its agonist, but more and more evidences suggest it has no influence on blood pressure. In the present study we compared the effects of moxonidine, rilmenidine and agmatine in the development of morphine dependence, and investigated the role of imidazoline I1 receptor in the effects of these agents. Chinese hamster ovary cells co-expressing μ opioid receptor and imidazoline receptor antisera-selected protein (IRAS), the strong candidate for imidazoline I1 receptor, were used as the cell line. cAMP overshoot, which represents an opioid dependent state in vitro, was measured to study the effects on morphine dependence. siRNA against IRAS was carried out to investigate the role of imidazoline I1 receptor. Moxonidine and rilmenidine (0.01–10 µM) were ineffective on cAMP level in the cells when given alone, and failed to inhibit chronic morphine exposure, naloxone-precipitated cAMP overshoot when co-pretreated with morphine. Agmatine (0.01–10 µM) by itself was ineffective but co-pretreated with morphine concentration-dependently inhibited chronic morphine exposure, naloxone-precipitated cAMP overshoot in the cells. Furthermore, we found that the inhibitory effect of agmatine (100 nM and 1 µM) on cAMP overshoot was significantly reduced by siRNA against IRAS. This study indicates that agmatine can inhibit the development of morphine dependence in vitro, whereas moxonidine and rilmenidine have no the effect. Imidazoline I1 receptor plays an important role in agmatine inhibiting morphine dependence.