Inhibition of monoamine oxidase A and B activities by imidazol(ine)/guanidine drugs, nature of the interaction and distinction from I(2)-imidazoline receptors in rat liver

1. I(2)-Imidazoline sites ([(3)H]-idazoxan binding) have been identified on monoamine oxidase (MAO) and proposed to modulate the activity of the enzyme through an allosteric inhibitory mechanism (Tesson et al., 1995). The main aim of this study was to assess the inhibitory effects and nature of the inhibition of imidazol(ine)/guanidine drugs on rat liver MAO-A and MAO-B isoforms and to compare their inhibitory potencies with their affinities for the sites labelled by [(3)H]-clonidine in the same tissue. 2. Competition for [(3)H]-clonidine binding in rat liver mitochondrial fractions by imidazol(ine)/guanidine compounds revealed that the pharmacological profile of the interaction (2 - styryl - 2 - imidazoline, LSL 61112>idazoxan>2 - benzofuranyl - 2 - imidazoline, 2-BFI=cirazoline>guanabenz>oxymetazoline>>clonidine) was typical of that for I(2)-sites. 3. Clonidine inhibited rat liver MAO-A and MAO-B activities with very low potency (IC(50)s: 700 μM and 6 mM, respectively) and displayed the typical pattern of competitive enzyme inhibition (Lineweaver-Burk plots: increased K(m) and unchanged V(max) values). Other imidazol(ine)/guanidine drugs also were weak MAO inhibitors with the exception of guanabenz, 2-BFI and cirazoline on MAO-A (IC(50)s: 4–11 μM) and 2-benzofuranyl-2-imidazol (LSL 60101) on MAO-B (IC(50): 16 μM). Idazoxan was a full inhibitor, although with rather low potency, on both MAO-A and MAO-B isoenzymes (IC(50)s: 280 μM and 624 μM, respectively). Kinetic analyses of MAO-A inhibition by these drugs revealed that the interactions were competitive. For the same drugs acting on MAO-B the interactions were of the mixed type inhibition (increased K(m) and decreased V(max) values), although the greater inhibitory effects on the apparent value of V(max)/K(m) than on the V(max) value indicated that the competitive element of the MAO-B inhibition predominated. 4. Competition for [(3)H]-Ro 41-1049 binding to MAO-A or [(3)H]-Ro 19-6327 binding to MAO-B in rat liver mitochondrial fractions by imidazol(ine)/guanidine compounds revealed that the drug inhibition constants (K(i) values) were similar to the IC(50) values displayed for the inhibition of MAO-A or MAO-B activities. In fact, very good correlations were obtained when the affinities of drugs at MAO-A or MAO-B catalytic sites were correlated with their potencies in inhibiting MAO-A (r=0.92) or MAO-B (r=0.99) activity. This further suggested a direct drug interaction with the catalytic sites of MAO-A and MAO-B isoforms. 5. No significant correlations were found when the potencies of imidazol(ine)/guanidine drugs at the high affinity site (pK(iH), nanomolar range) or the low-affinity site (pK(iL), micromolar range) of I(2)-imidazoline receptors labelled with [(3)H]-clonidine were correlated with the pIC(50) values of the same drugs for inhibition of MAO-A or MAO-B activity. These discrepancies indicated that I(2)-imidazoline receptors are not directly related to the site of action of these drugs on MAO activity in rat liver mitochondrial fractions. 6. Although these studies cannot exclude the presence of additional binding sites on MAO that do not affect the activity of the enzyme, they would suggest that I(2)-imidazoline receptors represent molecular species that are distinct from MAO

Activation of I(2)-imidazoline receptors enhances supraspinal morphine analgesia in mice: a model to detect agonist and antagonist activities at these receptors

1. This work investigates the receptor acted upon by imidazoline compounds in the modulation of morphine analgesia. The effects of highly selective imidazoline ligands on the supraspinal antinociception induced by morphine in mice were determined. 2. Intracerebroventricular (i.c.v.) or subcutaneous (s.c.) administration of ligands selective for the I(2)-imidazoline receptor, 2-BFI, LSL 60101, LSL 61122 and aganodine, and the non selective ligand agmatine, increased morphine antinociception in a dose-dependent manner. Neither moxonidine, a mixed I(1)-imidazoline and α(2)-adrenoceptor agonist, RX821002, a potent α(2)-adrenoceptor antagonist that displays low affinity at I(2)-imidazoline receptors, nor the selective non-imidazoline α(2)-adrenoceptor antagonist RS-15385-197, modified the analgesic responses to morphine. 3. Administration of pertussis toxin (0.25 μg per mouse, i.c.v.) 6 days before the analgesic test blocked the ability of the I(2)-imidazoline ligands to potentiate morphine antinociception. 4. The increased effect of morphine induced by I(2)-imidazoline ligands (agonists) was completely reversed by idazoxan and BU 224. Identical results were obtained with IBI, which alkylates I(2)-imidazoline binding sites. Thus, both agonist and antagonist properties of imidazoline ligands at the I(2)-imidazoline receptors were observed. 5. Pre-treatment (30 min) with deprenyl, an irreversible inhibitor of monoamine oxidase B (IMAO-B), produced an increase of morphine antinociception. Clorgyline, an irreversible IMAO-A, given 30 min before morphine did not alter the effect of the opioid. At longer intervals (24 h) a single dose of either clorgyline or deprenyl reduced the density of I(2)-imidazoline receptors and prevented the I(2)-mediated potentiation of morphine analgesia. 6. These results demonstrate functional interaction between I(2)-imidazoline and opioid receptors. The involvement of G(i)-G(o) transducer proteins in this modulatory effect is also suggested

Computational comparison of imidazoline association with the 12 binding site in human monoamine oxidases

Imidazoline ligands in I2-type binding sites in the brain alter monoamine turnover and release. One example of an 12 binding site characterized by binding studies, kinetics, and crystal structure has been described in monoamine oxidase B (MAO B). MAO A also binds imidazolines but has a different active site structure. Docking and molecular dynamics were used to explore how 2-(2-benzofuranyl)-2-imidazoline hydrochloride (2-BFI) binds to MAO A and to explain why tranylcypromine increases tight binding to MAO B. The energy for 2-BFI binding to MAO A was comparable to that for tranylcypromine-modified MAO B, but the location of 2-BFI in the MAO A could be anywhere in the monopartite substrate cavity. Binding to the tranylc-ypromine-modified MAO B was with high affinity and in the entrance cavity as in the crystal structure, but the energies of interaction with the native MAO B were less favorable. Molecular dynamics revealed that the entrance cavity of MAO B after tranylcypromine modification is both smaller and less flexible. This change in the presence of tranylcypromine may be responsible for the greater affinity of tranylcypromine-modified MAO B for imidazoline ligands.</p