Endogenous indoleamines demonstrate moderate affinity for I2 binding sites

Imidazoline Receptor System: The Past, the Present, and the Future

Imidazoline receptors historically referred to a family of nonadrenergic binding sites that recognize compounds with an imidazoline moiety, although this has proven to be an oversimplification. For example, none of the proposed endogenous ligands for imidazoline receptors contain an imidazoline moiety but they are diverse in their chemical structure. Three receptor subtypes (I₁, I₂, and I₃) have been proposed and the understanding of each has seen differing progress over the decades. I₁ receptors partially mediate the central hypotensive effects of clonidine-like drugs. Moxonidine and rilmenidine have better therapeutic profiles (fewer side effects) than clonidine as antihypertensive drugs, thought to be due to their higher I₁/α ₂-adrenoceptor selectivity. Newer I₁ receptor agonists such as LNP599 [3-chloro-2-methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride] have little to no activity on α ₂-adrenoceptors and demonstrate promising therapeutic potential for hypertension and metabolic syndrome. I₂ receptors associate with several distinct proteins, but the identities of these proteins remain elusive. I₂ receptor agonists have demonstrated various centrally mediated effects including antinociception and neuroprotection. A new I₂ receptor agonist, CR4056 [2-phenyl-6-(1H-imidazol-1yl) quinazoline], demonstrated clear analgesic activity in a recently completed phase II clinical trial and holds great promise as a novel I₂ receptor-based first-in-class nonopioid analgesic. The understanding of I₃ receptors is relatively limited. Existing data suggest that I₃ receptors may represent a binding site at the Kir6.2-subtype ATP-sensitive potassium channels in pancreatic β-cells and may be involved in insulin secretion. Despite the elusive nature of their molecular identities, recent progress on drug discovery targeting imidazoline receptors (I₁ and I₂) demonstrates the exciting potential of these compounds to elicit neuroprotection and to treat various disorders such as hypertension, metabolic syndrome, and chronic pain.

Imidazoline binding sites and their ligands: an overview of the different chemical structures

Since Bousquet et al. discovered the imidazoline binding sites (IBS) two decades ago, when they realized that the antihypertensive drug clonidine interacts not only with the alpha2-adrenenoceptors (alpha2-AR) but also with a distinct imidazoline preferring binding site, these receptors have been paid a great deal of attention. At least two subtypes, I1 and I2, have been characterised based on their binding affinity for different radioligands, but their structures still remain unknown. The pharmacological profile of these IBSs has been the objective of several and very thorough reviews. However, a medicinal chemistry overview of the different IBS ligands prepared to date has never been attempted. In this study, we attempt to compile all the different chemical structures reported to date as IBS ligands and classify them in function of their chemical structure and binding affinity for the different IBS subtypes. Thus, we comment on the different endogenous IBS ligands known as well as the drugs described to interact with the I1-IBS which have found application as antihypertensive drugs. Then, we review those compounds described in the literature to interact with the I2-IBS, classifying them by their chemical families (imidazolines, guanidines, 2-aminoimidazolines, beta-carbolines). Finally, some conclusions are drawn.

Behavioral, neuroendocrine and neurochemical effects of the imidazoline I2 receptor selective ligand BU224 in naive rats and rats exposed to the stress of the forced swim test

RATIONALE

There is evidence for alterations in imidazoline(2) (I(2)) receptor density in depressed patients. Selective I(2) receptor ligands modulate central monoamine levels and activate the hypothalamo-pituitary-adrenal (HPA) axis and may have potential as antidepressants.

OBJECTIVES

To study the behavioral effects of the selective I(2) receptor ligand BU224 in the rat forced swim test (FST) and its effects on the HPA axis and central monoaminergic responses.

METHODS

Rats received saline or BU224 (10 mg/kg IP) 24, 18 and 1 h prior to 15 min exposure to the FST. Saline- and BU224-treated non-stressed groups were included. Time spent immobile, struggling and swimming calmly was measured. Plasma adrenocorticotrophic hormone (ACTH) and corticosterone levels 90 min post-BU224 were measured in addition to tissue levels of monoamines and metabolites in the frontal cortex, hippocampus and hypothalamus.

RESULTS

Administration of BU224 significantly reduced immobility and increased mild swimming without affecting struggling. Exposure to the FST significantly increased plasma ACTH and corticosterone levels. BU224 administration also increased ACTH and potentiated the ACTH response to FST with no effect on corticosterone. BU224 administration significantly increased frontal cortex 5-hydroxytryptamine (5-HT) levels and decreased 5-HT turnover in the frontal cortex and hypothalamus of rats exposed to FST. In non-stressed rats, BU224 decreased 5-HT turnover in the hippocampus and hypothalamus and decreased norepinephrine turnover in the frontal cortex.

CONCLUSIONS

The selective I(2) receptor ligand BU224 reduces immobility of rats in the FST, indicative of antidepressant-like activity. This effect is accompanied by alterations in HPA axis and central monoaminergic activity.