Structure-activity relationship at alpha-adrenergic receptors within a series of imidazoline analogues of cirazoline

A Concise and Useful Guide to Understand How Alpha1 Adrenoceptor Antagonists Work

Adrenoceptors are the receptors for the catecholamines, adrenaline and noradrenaline. They are divided in α (α1 and α2) and β (β1, β2 and β3). α1-Adrenoceptors are subdivided in α1A, α1B and α1D. Most tissues express mixtures of α1-adrenoceptors subtypes, which appear to coexist in different densities and ratios, and in most cases their responses are probably due to the activation of more than one type. The three subtypes of α1-adrenoceptors are G-protein-coupled receptors (GPCR), specifically coupled to Gq/11. Additionally, the activation of these receptors may activate other signaling pathways or different components of these pathways, which leads to a great variety of possible cellular effects. The first clinically used α1 antagonist was Prazosin, for Systemic Arterial Hypertension (SAH). It was followed by its congeners, Terazosin and Doxazosin. Nowadays, there are many classes of α-adrenergic antagonists with different selectivity profiles. In addition to SAH, the α1-adrenoceptors are used for the treatment of Benign Prostatic Hyperplasia (BPH) and urolithiasis. This antagonism may be part of the mechanism of action of tricyclic antidepressants. Moreover, the activation of these receptors may lead to adverse effects such as orthostatic hypotension, similar to what happens with the antidepressants and with some antipsychotic. Structure-activity relationships can explain, in part, how antagonists work and how selective they can be for each one of the subtypes. However, it is necessary to develop new molecules which antagonize the α1-adrenoceptors or make chemical modifications in these molecules to improve the selectivity, pharmacokinetic profile and/or reduce the adverse effects of known drugs.

Corticotropin-releasing factor in the basolateral amygdala enhances memory consolidation via an interaction with the beta-adrenoceptor-cAMP pathway: dependence on glucocorticoid receptor activation

Extensive evidence indicates that stress hormone effects on the consolidation of emotionally influenced memory involve noradrenergic activation of the basolateral complex of the amygdala (BLA). The present experiments examined whether corticotropin-releasing factor (CRF) modulates memory consolidation via an interaction with the beta-adrenoceptor-cAMP system in the BLA. In a first experiment, male Sprague Dawley rats received bilateral infusions of the CRF-binding protein ligand inhibitor CRF(6-33) into the BLA either alone or together with the CRF receptor antagonist alpha-helical CRF(9-41) immediately after inhibitory avoidance training. CRF(6-33) induced dose-dependent enhancement of 48 h retention latencies, which was blocked by coadministration of alpha-helical CRF(9-41), suggesting that CRF(6-33) enhances memory consolidation by displacing CRF from its binding protein, thereby increasing "free" endogenous CRF concentrations. In a second experiment, intra-BLA infusions of atenolol (beta-adrenoceptor antagonist) and Rp-cAMPS (cAMP inhibitor), but not prazosin (alpha(1)-adrenoceptor antagonist), blocked CRF(6-33)-induced retention enhancement. In a third experiment, the CRF receptor antagonist alpha-helical CRF(9-41) administered into the BLA immediately after training attenuated the dose-response effects of concurrent intra-BLA infusions of clenbuterol (beta-adrenoceptor agonist). In contrast, alpha-helical CRF(9-41) did not alter retention enhancement induced by posttraining intra-BLA infusions of either cirazoline (alpha(1)-adrenoceptor agonist) or 8-br-cAMP (cAMP analog). These findings suggest that CRF facilitates the memory-modulatory effects of noradrenergic stimulation in the BLA via an interaction with the beta-adrenoceptor-cAMP cascade, at a locus between the membrane-bound beta-adrenoceptor and the intracellular cAMP formation site. Moreover, consistent with evidence that glucocorticoids enhance memory consolidation via a similar interaction with the beta-adrenoceptor-cAMP cascade, a last experiment found that the CRF and glucocorticoid systems within the BLA interact in influencing beta-adrenoceptor-cAMP effects on memory consolidation.

Novel 1-oxyl-2-substitutedphenyl-4,4,5,5-tetramethylimidazolines: Synthesis, selectively analgesic action, and QSAR analysis

Based on the knowledge that imidazoline can result in analgesic action due to its selective binding with the prostacyclin receptor, 20 1-oxyl-2-substitutedphenyl-4,4,5,5-tetramethylimidazolines (3a-t) were prepared in moderate yields. At 0.13 mmol/kg dose, their in vivo analgesic activities were evaluated after the mice were administered at 30, 60, 90, and 150 min. Compared with the pain threshold (12.27+/-9.56-17.71+/-7.00%) of normal saline (NS) receiving mice, the pain threshold (23.42+/-8.14% to 102.58+/-10.66%) of 3a-t receiving mice increases significantly. Considering a prostacyclin receptor targeting analgesic agent usually had bleeding action and to appraise the bleeding risk, the in vivo tail bleeding time of 1.30 mmol/kg 3a-t receiving mice was found to be ranged from 116.3+/-8.2s to 120.3+/-9.2s, which was substantially equal to that (117.8+/-8.4s to 119.0+/-8.6s) of NS receiving mice. Based on the possibility of imidazoline acting as vasodilator, the in vitro vasorelaxations of 3a-t were tested using the rat aortic strip model. When the aortic strip contracted by noradrenaline (NE, final concentration 10(-7)mol/l) was treated with 3a-t (final concentration 5 x 10(-4)mol/l), only lower percentage inhibitions (6.55+/-5.70-37.40+/-4.07%) were recorded, implying that the vasorelaxation of 3a-t was neglectable. By selecting appropriate molecular descriptors generated from e-dragon server, the QSAR model of the analgesic activities of 3a-t was constructed using the multiple linear regression method. The established QSAR model showed reasonable accuracy and thus it is promising to be used for screening new 1-oxyl-2-substitutedphenyl-4,4,5,5-tetramethylimidazoline derivatives as analgesic agents.

Synthesis and biological studies of yohimbine derivatives on human α2C-adrenergic receptors

A series of yohimbine derivatives was synthesized and evaluated for binding affinity at the human alpha(2C)-adrenergic receptors expressed in Chinese hamster ovary cells. It has been found that compound 5 shows a higher affinity for alpha(2C)-AR than the parent compound yohimbine 1, thereby illustrating that the nature of the linkers affect binding potencies on these receptors.

α2-Adrenoreceptors Profile Modulation. 2. Biphenyline Analogues as Tools for Selective Activation of the α2C-Subtype

A series of derivatives structurally related to biphenyline (3) was designed with the aim to modulate selectivity toward the alpha(2)-AR subtypes. The results obtained demonstrated that the presence of a correctly oriented function with positive electronic effect (+sigma) in portion X of the ligands is an important factor for significant alpha(2C)-subtype selectivity (imidazolines 5, 13, 16, and 19). Homology modeling and docking studies support experimental data and highlight the crucial role for the hydrogen bond between the pyridine nitrogen in position 3 of 5 and the NH-indole ring of Trp6.48, which is favorably oriented in the alpha(2C)-subtype, only.

Synthesis and biological evaluation of new 2-(4,5-dihydro-1H-imidazol-2-yl)-3,4-dihydro-2H-1,4-benzoxazine derivatives

2-(4,5-Dihydro-1H-imidazol-2-yl)-3,4-dihydro-2H-1,4-benzoxazine derivatives and tricyclic analogues with a fused additional ring on the nitrogen atom of the benzoxazine moiety have been prepared and evaluated for their cardiovascular effects as potential antihypertensive agents. The imidazoline ring was generated by reaction of the corresponding ethyl ester with ethylenediamine. Affinities for imidazoline binding sites (IBS) I(1) and I(2) and alpha(1) and alpha(2) adrenergic receptors were evaluated as well as the effects on mean arterial blood pressure (MAP) and heart rate (HR) of spontaneously hypertensive rats. With few exceptions the most active compounds on MAP were those with high affinities for IBS and alpha(2) receptor. Among these, compound 4h was the most interesting and is now, together with its enantiomers, under complementary pharmacological evaluation.

Alpha2-adrenoreceptors profile modulation and high antinociceptive activity of (S)-(-)-2-[1-(biphenyl-2-yloxy)ethyl]-4,5-dihydro-1H-imidazole

A number of derivatives structurally related to cirazoline (1) were synthesized and studied with the purpose of modulating alpha2-adrenoreceptors selectivity versus both alpha1-adrenoreceptors and I2 imidazoline binding sites. The most potent alpha2-agonist was 2-[1-(biphenyl-2-yloxy)ethyl]-4,5-dihydro-1H-imidazole (7), whose key pharmacophoric features closely matched those found in the alpha2-agonist 2-(3-exo-(3-phenylprop-1-yl)-2-exo-norbornyl)amino-2-oxazoline (15). (S)-(-)-7 was the most potent of the two enantiomers, confirming the stereospecificity of the interaction with alpha2-adrenoreceptors. This eutomer was tested on two algesiometric paradigms and, because of the interaction with alpha2-adrenoreceptors, showed a potent and long-lasting antinociceptive activity, since it was abolished by the selective alpha2-antagonist RX821002.