Densities of I2-imidazoline receptors, alpha 2-adrenoceptors and monoamine oxidase B in brains of suicide victims

Imidazoline ligand BU224 reverses cognitive deficits, reduces microgliosis and enhances synaptic connectivity in a mouse model of Alzheimer's disease

BACKGROUND AND PURPOSE

Activation of type 2 imidazoline receptors has been shown to exhibit neuroprotective properties including anti-apoptotic and anti-inflammatory effects, suggesting a potential therapeutic value in Alzheimer's disease (AD). Here, we explored the effects of the imidazoline-2 ligand BU224 in a model of amyloidosis.

EXPERIMENTAL APPROACH

Six-month-old female transgenic 5XFAD and wild-type (WT) mice were treated intraperitoneally with 5-mg·kg^-1 BU224 or vehicle twice a day for 10 days. Behavioural tests were performed for cognitive functions and neuropathological changes were investigated by immunohistochemistry, Western blot, elisa and qPCR. Effects of BU224 on amyloid precursor protein (APP) processing, spine density and calcium imaging were analysed in brain organotypic cultures and N2a cells.

KEY RESULTS

BU224 treatment attenuated spatial and perirhinal cortex-dependent recognition memory deficits in 5XFAD mice. Fear-conditioning testing revealed that BU224 also improved both associative learning and hippocampal- and amygdala-dependent memory in transgenic but not in WT mice. In the brain, BU224 reduced levels of the microglial marker Iba1 and pro-inflammatory cytokines IL-1β and TNF-α and increased the expression of astrocytic marker GFAP in 5XFAD mice. These beneficial effects were not associated with changes in amyloid pathology, neuronal apoptosis, mitochondrial density, oxidative stress or autophagy markers. Interestingly, ex vivo and in vitro studies suggested that BU224 treatment increased the size of dendritic spines and induced a threefold reduction in amyloid-β (Aβ)-induced functional changes in NMDA receptors.

CONCLUSION AND IMPLICATIONS

Sub-chronic treatment with BU224 restores memory and reduces inflammation in transgenic AD mice, at stages when animals display severe pathology.

Advances in CNS PET: the state-of-the-art for new imaging targets for pathophysiology and drug development

PURPOSE

A limit on developing new treatments for a number of central nervous system (CNS) disorders has been the inadequate understanding of the in vivo pathophysiology underlying neurological and psychiatric disorders and the lack of in vivo tools to determine brain penetrance, target engagement, and relevant molecular activity of novel drugs. Molecular neuroimaging provides the tools to address this. This article aims to provide a state-of-the-art review of new PET tracers for CNS targets, focusing on developments in the last 5 years for targets recently available for in-human imaging.

METHODS

We provide an overview of the criteria used to evaluate PET tracers. We then used the National Institute of Mental Health Research Priorities list to identify the key CNS targets. We conducted a PubMed search (search period 1st of January 2013 to 31st of December 2018), which yielded 40 new PET tracers across 16 CNS targets which met our selectivity criteria. For each tracer, we summarised the evidence of its properties and potential for use in studies of CNS pathophysiology and drug evaluation, including its target selectivity and affinity, inter and intra-subject variability, and pharmacokinetic parameters. We also consider its potential limitations and missing characterisation data, but not specific applications in drug development. Where multiple tracers were present for a target, we provide a comparison of their properties.

RESULTS AND CONCLUSIONS

Our review shows that multiple new tracers have been developed for proteinopathy targets, particularly tau, as well as the purinoceptor P2X7, phosphodiesterase enzyme PDE10A, and synaptic vesicle glycoprotein 2A (SV2A), amongst others. Some of the most promising of these include 18F-MK-6240 for tau imaging, 11C-UCB-J for imaging SV2A, 11C-CURB and 11C-MK-3168 for characterisation of fatty acid amide hydrolase, 18F-FIMX for metabotropic glutamate receptor 1, and 18F-MNI-444 for imaging adenosine 2A. Our review also identifies recurrent issues within the field. Many of the tracers discussed lack in vivo blocking data, reducing confidence in selectivity. Additionally, late-stage identification of substantial off-target sites for multiple tracers highlights incomplete pre-clinical characterisation prior to translation, as well as human disease state studies carried out without confirmation of test-retest reproducibility.

Dysregulation of IRAS/nischarin and other potential I1-imidazoline receptors in major depression postmortem brain: Downregulation of basal contents by antidepressant drug treatments

BACKGROUND

Major depressive disorder (MDD) has been associated with altered brain densities of imidazoline receptors (I₁-IR and I₂-IR types).

METHODS

The contents of potential I₁-IR IRAS/nischarin (167kDa) and, for comparison, those of I₁- (85kDa) and I₂- (45kDa and 30kDa) IR proteins were quantified by western blotting in postmortem prefrontal cortex (PFC/BA9) of antidepressant-free ([MDD(-)], n=9) and antidepressant-treated ([MDD(+)], n=12) subjects and matched controls (n=19).

RESULTS

In MDD, regardless of antidepressant treatment (n=21), IRAS/nischarin was not altered in PFC/BA9. However, the content of IRAS/nischarin was found modestly and not significantly increased (+19%, p=0.075) in MDD(-) and significantly decreased (-24%, p=0.001) in MDD(+), revealing that basal I₁-IR content was downregulated by antidepressants. Putative 85kDa I₁-IR was upregulated (+35%, p=0.035) in MDD(-) but it was not reduced (-14%, p=0.37) in MDD(+). There was a positive correlation (r=0.33, p=0.037, n=40) between the contents of IRAS/nischarin and 85kDa IR proteins in PFC/BA9 (control and MDD subjects). In MDD and regardless of antidepressants, the content of cortical 45kDa I₂-IR was increased (+31%, p=0.006) and that of 30kDa I₂-IR decreased (-14%, p=0.002), indicating basal dysregulations of these potential IRs.

LIMITATIONS

MDD(+) subjects had been treated with a variety of antidepressant drugs. The results must be understood in the context of suicide victims with MDD.

CONCLUSIONS

The dysregulation of IRAS/nischarin in depressed brains is a major novel finding that supports a role of this potential I₁-IR in the neurobiology of MDD and in the molecular mechanisms of antidepressant drugs.

A useful PET probe [11C]BU99008 with ultra-high specific radioactivity for small animal PET imaging of I2-imidazoline receptors in the hypothalamus

INTRODUCTION

A positron emission tomography (PET) probe with ultra-high specific radioactivity (SA) enables measuring high receptor specific binding in brain regions by avoiding mass effect of the PET probe itself. It has been reported that PET probe with ultra-high SA can detect small change caused by endogenous or exogenous ligand. Recently, Kealey et al. developed [¹¹C]BU99008, a more potent PET probe for I₂-imidazoline receptors (I₂Rs) imaging, with a conventional SA (mean 76GBq/μmol) showed higher specific binding in the brain. Here, to detect small change of specific binding for I₂Rs caused by endogenous or exogenous ligand in an extremely small region, such as hypothalamus in the brain, we synthesized and evaluated [¹¹C]BU99008 with ultra-high SA as a useful PET probe for small-animal PET imaging of I₂Rs.

METHODS

[¹¹C]BU99008 was prepared by [¹¹C]methylation of N-desmethyl precursor with [¹¹C]methyl iodide. Biodistribution, metabolite analysis, and brain PET studies were conducted in rats.

RESULTS

[¹¹C]BU99008 with ultra-high SA in the range of 5400-16,600GBq/μmol were successfully synthesized (n=7), and had appropriate radioactivity for in vivo study. In the biodistribution study, the mean radioactivity levels in all investigated tissues except for the kidney did not show significant difference between [¹¹C]BU99008 with ultra-high SA and that with conventional SA. In the metabolite analysis, the percentage of unchanged [¹¹C]BU99008 at 30min after the injection of probes with ultra-high and conventional SA was similar in rat brain and plasma. In the PET study of rats' brain, radioactivity level (AUC_{30-60 min}) in the hypothalamus of rats injected with [¹¹C]BU99008 with ultra-high SA (64 [SUV ∙ min]) was significantly higher than that observed for that with conventional SA (50 [SUV ∙ min]). The specific binding of [¹¹C]BU99008 with ultra-high SA (86% of total binding) for I₂R was higher than that of conventional SA (76% of total binding).

CONCLUSION

A PET study using [¹¹C]BU99008 with ultra-high SA would thus contribute to the detection of small changes in or small regions with I₂R expression and hence may be useful in elucidating new functions of I₂R.

Imaging imidazoline-I2 binding sites in porcine brain using 11C-BU99008

Changes in the density of imidazoline-I(2) binding sites have been observed in a range of neurologic disorders including Alzheimer's disease, Huntington's chorea, and glial tumor; however, the precise function of these sites remains unclear. A PET probe for I(2) binding sites would further our understanding of the target and may find application as a biomarker for early disease diagnosis. Compound BU99008 has previously been identified as a promising I(2) ligand from autoradiography studies, displaying high affinity and good selectivity toward the target. In this study, BU99008 was radiolabeled with (11)C in order to image the I(2) binding sites in vivo using PET.

METHODS

(11)C-BU99008 was radiolabeled by N-alkylation of the desmethyl precursor using (11)C-methyl iodide. A series of PET experiments was performed to investigate the binding of (11)C-BU99008 in porcine brains, in the presence or absence of a nonradiolabeled, competing I(2) ligand, BU224.

RESULTS

(11)C-BU99008 was obtained in good yield and specific activity. In vivo, (11)C-BU99008 displayed good brain penetration and gave a heterogeneous distribution with high uptake in the thalamus and low uptake in the cortex and cerebellum. (11)C-BU99008 brain kinetics were well described by the 1-tissue-compartment model, which was used to provide estimates for the total volume of distribution (V(T)) across brain regions of interest. Baseline V(T) values were ranked in the following order: thalamus > striatum > hippocampus > frontal cortex ≥ cerebellum, consistent with the known distribution and concentration of I(2) binding sites. Administration of a selective I(2) binding site ligand, BU224, reduced the V(T) to near-homogeneous levels in all brain regions.

CONCLUSION

(11)C-BU99008 appears to be a suitable PET radioligand for imaging the I(2) binding sites in vivo.

α2 adrenergic receptor dysregulation in depressive disorders: implications for the neurobiology of depression and antidepressant therapy

Dysfunction in noradrenergic neurotransmission has long been theorized to occur in depressive disorders. The α2 adrenergic receptor (AR) family, as a group of key players in regulating the noradrenergic system, has been investigated for involvement in the neurobiology of depression and mechanisms of antidepressant therapies. However, a clear picture of the α2ARs in depressive disorders has not been established due to the existence of apparently conflicting findings in the literature. In this article, we report that a careful accounting of methodological differences within the literature can resolve the present lack of consensus on involvement of α2ARs in depression. In particular, the pharmacological properties of the radioligand (e.g. agonist versus antagonist) utilized for determining receptor density are crucial in determining study outcome. Upregulation of α2AR density detected by radiolabeled agonists but not by antagonists in patients with depressive disorders suggests a selective increase in the density of high-affinity conformational state α2ARs, which is indicative of enhanced G protein coupling to the receptor. Importantly, this high-affinity state α2AR upregulation can be normalized with antidepressant treatments. Thus, depressive disorders appear to be associated with increased α2AR sensitivity and responsiveness, which may represent a physiological basis for the putative noradrenergic dysfunction in depressive disorders. In addition, we review changes in some key α2AR accessory proteins in depressive disorders and discuss their potential contribution to α2AR dysfunction.

Specific binding of [3H]Ro 19-6327 (lazabemide) to monoamine oxidase B is increased in frontal cortex of suicide victims after controlling for age at death

Previous studies have reported negative findings for the association among brain monoamine oxidase B (MAO-B) and suicidal behaviour. However those studies did not adequately control their main results for the influence of confounding variables such as age at death. We have evaluated the association of MAO-B density (assessed by [3H]Ro 19-6327 - lazabemide - binding) with type of death (suicide victims vs non-suicide controls) after controlling for age at death. Frontal cortex samples from 43 subjects (21 suicides, 22 controls) were assayed for MAO-B density at a single concentration of lazabemide (8 nM). A linear regression modelling approach comparing nested models resulted with both type of death (p<0.05) and age of death (p<0.01) as main explanatory variables for the variability of MAO-B density. Suicide victims had >30% more binding sites for lazabemide than controls. Contrary to previous reports, MAO-B density seems to increase in suicide victims.

Interactions of imidazoline ligands with the active site of purified monoamine oxidase A

The two forms of monoamine oxidase, monoamine oxidase A and monoamine oxidase B, have been associated with imidazoline-binding sites (type 2). Imidazoline ligands saturate the imidazoline-binding sites at nanomolar concentrations, but inhibit monoamine oxidase activity only at micromolar concentrations, suggesting two different binding sites [Ozaita A, Olmos G, Boronat MA, Lizcano JM, Unzeta M & García-Sevilla JA (1997) Br J Pharmacol121, 901-912]. When purified human monoamine oxidase A was used to examine the interaction with the active site, inhibition by guanabenz, 2-(2-benzofuranyl)-2-imidazoline and idazoxan was competitive with kynuramine as substrate, giving K(i) values of 3 microM, 26 microM and 125 microM, respectively. Titration of monoamine oxidase A with imidazoline ligands induced spectral changes that were used to measure the binding affinities for guanabenz (19.3 +/- 3.9 microM) and 2-(2-benzofuranyl)-2-imidazoline (49 +/- 8 microM). Only one type of binding site was detected. Agmatine, a putative endogenous ligand for some imidazoline sites, reduced monoamine oxidase A under anaerobic conditions, indicating that it binds close to the flavin in the active site. Flexible docking studies revealed multiple orientations within the large active site, including orientations close to the flavin that would allow oxidation of agmatine.

Antidepressant-like effect of harmane and other beta-carbolines in the mouse forced swim test

The purpose of the present study was to determine the effects of harmane, norharmane and harmine on the immobility time in the mouse forced swim test (FST) - an animal model of depression. After 30 min of the beta-carbolines injections, mice were placed individually in a vertical glass cylinder (height, 25 cm; diameter, 12 cm) containing water about 15 cm deep at 22+/-1 degrees C and forced to swim. Treatment of animals with harmane (5-15 mg/kg, i.p.), norharmane (2.5-10 mg/kg, i.p.) and harmine (5-15 mg/kg, i.p.) reduced dose-dependently the time of immobility. Their antidepressant-like effects were not affected by pretreatment with reserpine at the dose of 5 mg/kg, i.p., 18 h before the test, which did not modify the immobility time. Conversely, when flumazenil (5 mg/kg, i.p.) was administered 30 min before the test, it was able to antagonize completely the antidepressant-like effects of harmane, norharmane and harmine. It was concluded that harmane, norharmane and harmine reduce the immobility time in this test, suggesting an antidepressant-like effect, via an inverse-agonistic mechanism located in the benzodiazepine receptors.

Invited review: the evolution of antidepressant mechanisms

Present antidepressants are all descendents of the serendipitous findings in the 1950s that the monoamine oxidase inhibitor iproniazid and the tricyclic antidepressant imipramine were effective antidepressants. The identification of their mechanism of action, and those of reserpine and amphetamine, in the 1960s, led to the monoamine theories of depression being postulated; first, with noradrenaline then 5-hydroxytryptamine being considered the more important amine. These monoamine theories of depression predominated both industrial and academic research for four decades. Recently, in attempts to design new drugs with faster onsets of action and more universal therapeutic action, downstream alterations common to current antidepressants are being examined as potential antidepressants. Additionally, the use of animal models has identified a number of novel targets some of which have been subjected to clinical trials in humans. However, monoamine antidepressants remain the best current medications and it may be some time before they are dislodged as the market leaders.

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.

Differential [(3)H]idazoxan and [(3)H]2-(2-benzofuranyl)-2-imidazoline (2-BFI) binding to imidazoline I(2) receptors in human postmortem frontal cortex

[(3)H]2-(2-benzofuranyl)-2-imidazoline (2-BFI) and [(3)H]idazoxan are the most used tools to characterise imidazoline I(2) receptors. We evaluated the binding of both radioligands to human postmortem frontal cortex membranes. Saturation binding analyses revealed that [(3)H]idazoxan (in the presence of 2 microM efaroxan to avoid radioligand binding to alpha(2)-adrenoceptors and imidazoline I(1) receptors) and [(3)H]2-BFI bound with high affinity to an apparent single population of sites. However, in competition studies whereas [(3)H]idazoxan (10 nM) binding was displaced monophasically by idazoxan and 2-BFI, both drugs displayed biphasic curves for [(3)H]2-BFI (1 nM). The proportion of the low-affinity binding site increased from 17% to 25% when 10 nM [(3)H]2-BFI was displaced by idazoxan. Amiloride inhibited [(3)H]2-BFI (10 nM) binding with low affinity and in a monophasic way. These data indicate that [(3)H]2-BFI recognises in human postmortem brain membranes a second binding site different from the imidazoline I(2) receptors labelled by [(3)H]idazoxan.

Relationships between beta- and alpha2-adrenoceptors and G coupling proteins in the human brain: effects of age and suicide

Interactions between brain alpha2- and beta-adrenoceptors are of interest in physiological (aging) and pathological (major depression) processes involving both receptors. In this study, total beta-adrenoceptors and beta1/2-subtypes were quantitated in postmortem human brains to investigate their relationships with alpha2A-adrenoceptors and specific G proteins during the process of aging and in brains of suicide victims. Analysis of [3H]CGP12177 binding, in the presence of CGP20712A (beta1-antagonist), indicated that the predominant beta-adrenoceptor in the frontal cortex is the beta1-subtype (65-75%). The density of total beta- (r=-0.60, n=44) or beta1-adrenoceptors (r=-0.78, n=22), but not the beta2-subtype, declined with aging (3-80 years). The density of total beta- or beta1-adrenoceptors, but not the beta2-subtype, correlated with the number of alpha2-adrenoceptors quantitated in the same brains with the agonist [3H]UK14304 (r=0.71-0.81) or the antagonist [3H]RX821002 (r=0.61-0.66). Interestingly, the ratios alpha2/beta- or alpha2/beta1-adrenoceptors did not correlate with the age of the subject at death, indicating that the proportion of alpha2/beta-adrenoceptors in brain remains rather constant during the process of aging. The density of beta-adrenoceptors correlated with the immunodensity of G(alpha)s (r=0.55) and Gbeta (r=0.61) proteins, and that of alpha2-adrenoceptors with those of G(alpha)i1/2 (r=0.88) and Gbeta (r=0.65). In brains of suicides, compared to controls, the ratio between alpha2- and beta- or beta1-adrenoceptors (alpha2-full agonist sites/beta-sites) was greater (1.3- to 2.0-fold; P<0.05). The results demonstrate a close interdependence between brain alpha2- and beta-adrenoceptors during aging, and in brains of suicides. The quantitation of the alpha2A/beta-adrenoceptor ratio could represent a relevant neurochemical index in the study of brain pathologies in which both receptors are involved.

Imidazoline receptor proteins are decreased in the hippocampus of individuals with major depression

BACKGROUND

A downregulation of I(2)-imidazoline binding sites has been reported in frontal cortices of depressed suicide victims, according to I(2)-radioligand binding and confirmed by Western blotting. We now report Western blots of imidazoline receptor proteins in hippocampi of subjects with and without depression at the time of death.

METHODS

Postmortem diagnoses were obtained from 17 cases of Axis I major depressive disorder and 17 cases without Axis I psychopathology. No psychotropic compounds were found in body fluids. Hippocampi were removed, sectioned, and assessed histologically. Throughout the analysis, each major depressive disorder sample was paired with a sample from a psychiatrically healthy subject based on equivalent life spans and postmortem delays. The antiserum was identical to that used in previous studies that reported a downregulation of cortical 29/30-kd imidazoline receptor-binding proteins in depression.

RESULTS

A triad of imidazoline receptor-binding protein bands (40-50 kd) was detected in the human hippocampus. Subjects with major depressive disorder had significantly less intensity in each imidazoline receptor-binding proteins band compared with control subjects (p =. 01 for overall bands).

CONCLUSIONS

The present results can be aligned with previous reports of downregulation of I(2)-radioligand binding sites in both cortices and platelets of depressed patients.

Unaltered alpha(2)-noradrenergic/imidazoline receptors in suicide victims: a postmortem brain autoradiographic analysis

In vitro quantitative autoradiography of alpha(2)-adrenergic/imidazoline receptors, using [(125)I]iodoclonidine as a ligand, was performed on 24 human brains postmortem. Twelve brains were obtained from suicide victims and 12 from matched controls. We found no significant, region-dependent alterations in the density of alpha(2)-adrenergic receptors in brains of suicide victims as compared to matched controls. We also report age-dependent reductions in binding in the prefrontal cortex and hippocampus, as well as significant recent alcohol ingestion-dependent reductions in binding in the prefrontal cortex. Sex and time from death to autopsy did not affect iodoclonidine binding in our sample.

Imidazoline receptors: from discovery to antihypertensive therapy (facts and doubts)

The hypothesis and indirect evidence of imidazoline receptors has been promoted since some 15 years ago and it gave a substantial impetus for research in this field, resulting in a better understanding of neuronal and cardiovascular regulatory processes. The nomenclature of the imidazoline receptors has been accepted by international forums but no direct proof for the existence of these receptors has been published. Authors summarise the most important available data, including facts and doubts as far as the discovery, characterisation, and function of imidazoline receptors and their subtypes, the differences between imidazoline receptors and alpha-2 adrenoceptors, and also on their participation in regulatory processes.

Imidazoline receptors and human brain disorders

Major depression, opioid addiction, neurodegenerative diseases, and glial tumors are associated with disturbances of imidazoline receptors (IR) in the human brain. In depression, the level of a 45-kD IR protein (putative I1-IR) is increased in the brain of suicide victims (51%) and in platelets of depressed patients (40%). The density of platelet I1-IR ([125I]-p-iodoclonidine binding) is also increased in depression (135%). The 29/30-kD IR protein (putative I2B-IR) is downregulated (19%) in suicide victims in parallel with a reduction (40%) in the density of I2B-IR ([3H]idazoxan binding). Antidepressant drugs induce downregulation of 45-kD IR protein and I1-sites in platelets of depressed patients and upregulation of I2-sites in rat brain. The densities of I2B-IR and the related 29/30-kD IR protein are decreased (39% and 28%) in the brain of heroin addicts. The density of I2B-IR is increased in Alzheimer's disease (63%) and decreased in Huntington's disease (56%). Brain I2B-IR is not altered in Parkinson's disease. The level of I2-IR in glial tumors is increased (two-fivefold) in parallel with the abundance of the related 29/30-kD IR protein (39%), whereas the level of 45-kD IR protein is decreased (39%). The possible functional relevance of these findings in the context of the pathogenesis of these disorders remains to be elucidated.

Stimulation of locus coeruleus neurons by non-I1/I2-type imidazoline receptors: an in vivo and in vitro electrophysiological study

1. Imidazoline binding sites have been reported to be present in the locus coeruleus (LC). To investigate the role of these sites in the control of LC neuron activity, we studied the effect of imidazolines using in vivo and in vitro single-unit extracellular recording techniques. 2. In anaesthetized rats, local (27 pmoles) and systemic (1 mg kg(-1), i.v.) administrations of 2-(2-benzofuranyl)-2-imidazoline (2-BFI), a selective I-imidazoline receptor ligand, increased the firing rate of LC cells (maximal increase: 22+/-5%, P<0.001 and 16+/-7%, P<0.001 respectively). Chronic pretreatment with the irreversible monoamine oxidase inhibitor clorgyline (3 mg kg(-1), i.p., every 12 h for 14 days) abolished this effect. 3. In rat midpontine brain slices containing the LC, bath application (1 mM) of the imidazolines 2-BFI, 2-(4,5-dihydroimidaz-2-yl)-quinoline (BU224), idazoxan, efaroxan, phentolamine and (2-2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline (RX821002) reversibly stimulated LC cells. The maximal effect was approximately 90% except for RX821002 and efaroxan which induced smaller maximal effects (approximately 58% and approximately 35% respectively). Simultaneous application of idazoxan and 2BFI did not lead to additive effects. 4. Bath application of the alpha2-adrenoceptor antagonists, yohimbine (1 - 10 microM) and N-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ) (10 microM), failed to modify LC activity. The irreversible blockade of alpha2-adrenoceptors with EEDQ (10 microM) did not alter the effect of idazoxan or that of efaroxan. Previous application of clorgyline (10 microM) did not modify the excitatory effect of 2-BFI or efaroxan. 5. Changes in the pH of the bathing solution (6.84-7.84) did not influence the effect caused by idazoxan. Bath application of 2-BFI (1 mM) reversed the inhibition induced by diazoxide (300 microM), an ATP-sensitive K+ channel opener, whereas application of glibenclamide (3 microM), an ATP-sensitive K+ channel blocker, partially blocked the effect of 2-BFI. 6. This study shows that imidazoline compounds stimulate the firing rate of LC neurons. This effect is not mediated by alpha2-adrenoceptors nor by I1 or I2-imidazoline receptors but involves a different subtype of imidazoline receptor. Our results indicate that this receptor is located extracellularly and modulates ATP-sensitive K+ channels.

Inhibition of monoamine oxidase A and B activities by imidazol(ine)/guanidine drugs, nature of the interaction and distinction from I2-imidazoline receptors in rat liver

1. I2-Imidazoline sites ([3H]-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 [3H]-clonidine in the same tissue. 2. Competition for [3H]-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 I2-sites. 3. Clonidine inhibited rat liver MAO-A and MAO-B activities with very low potency (IC50S: 700 microM and 6 mM, respectively) and displayed the typical pattern of competitive enzyme inhibition (lineweaver-Burk plots: increased K(m) and unchanged Vmax values). Other imidazol(ine)/guanidine drugs also were weak MAO inhibitors with the exception of guanabenz, 2-BFI and cirazoline on MAO-A (IC50S: 4-11 microM) and 2-benzofuranyl-2-imidazol (LSL 60101) on MAO-B (IC50: 16 microM). Idazoxan was a full inhibitor although with rather low potency, on both MAO-A and MAO-B isoenzymes (IC50S: 280 microM and 624 microM, 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 Vmax values), although the greater inhibitory effects on the apparent value of Vmax/K(m) than on the Vmax value indicated that the competitive element of the MAO-B inhibition predominated. 4. Competition for [3H]-Ro 41-1049 binding to MAO-A or [3H]-Ro 19-6327 binding to MAO-B in rat liver mitochondrial fractions by imidazol(ine)/guanidine compounds revealed that the drug inhibition constants (Ki values) were similar to the IC50 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 (pKiH, nanomolar range) or the low-affinity site (pKiL, micromolar range) of I2-imidazoline receptors labelled with [3H]-clonidine were correlated with the pIC50 values of the same drugs for inhibition of MAO-A or MAO-B activity. These discrepancies indicated that I2-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 I2-imidazoline receptors represent molecular species that are distinct from MAO.