Protection of focal ischemic infarction by rilmenidine in the animal: evidence that interactions with central imidazoline receptors may be neuroprotective

Imidazoline receptors as a new therapeutic target in Huntington's disease: A preclinical overview

An autosomal dominant neurodegenerative disease called Huntington's disease (HD) is characterized by motor dysfunction, cognitive decline, and a variety of psychiatric symptoms due to the expansion of polyglutamine in the Huntingtin gene. The disease primarily affects the striatal neurons within the basal ganglia, leading to significant neuronal loss and associated symptoms such as chorea and dystonia. Current therapeutic approaches focus on symptom management without altering the disease's progression, highlighting a pressing need for novel treatment strategies. Recent studies have identified imidazoline receptors (IRs) as promising targets for neuroprotective and disease-modifying interventions in HD. IRs, particularly the I1 and I2 subtypes, are involved in critical physiological processes such as neurotransmission, neuronal excitability, and cell survival. Activation of these receptors has been shown to modulate neurotransmitter release and provide neuroprotective effects in preclinical models of neurodegeneration. This review discusses the potential of IR-targeted therapies to not only alleviate multiple symptoms of HD but also possibly slow the progression of the disease. We emphasize the necessity for ongoing research to further elucidate the role of IRs in HD and develop selective ligands that could lead to effective and safe treatments, thereby significantly improving patient outcomes and quality of life.

Dexmedetomidine protects against oxygen–glucose deprivation-induced injury through the I2 imidazoline receptor-PI3K/AKT pathway in rat C6 glioma cells

Objectives

To explore the protection and the mechanism of dexmedetomidine on the oxygen–glucose deprivation (OGD) insults in rat C6 glioma cells.

Methods

Cells were subjected to OGD then assessed by viability studies. After dexmedetomidine treatment, p-AKT, hypoxia-inducible factor (HIF)-1α, vascular endothelial growth factor (VEGF) and RTP801 expression were measured.

Key findings

Three hours of OGD decreased cell viability to 48.8%, which was reversed to 67.4% by 1 µm dexmedetomidine. Hoechst 33342 and propidium iodide double stains showed that the protection of dexmedetomidine was mainly by an anti-apoptosis effect, which was also strengthened by decreasing caspase-3 expression. Dexmedetomidine protection was mainly blocked by the I2 imidazoline receptor antagonist idazoxan and BU 224, but not by the α1-adrenoceptor antagonist prazosin, the α2-adrenoceptor antagonist yohimbine and RX 821002, or the I1 imidazoline receptor antagonist efaroxan. On the other hand, dexmedetomidine enhanced AKT phosphorylation. Furthermore, the protection of dexmedetomidine was blocked by the PI3K/AKT inhibitor wortmannin. The proteins of HIF-1α, VEGF and RTP801 were significantly increased by dexmedetomidine treatment.

Conclusions

Dexmedetomidine activated the I2 imidazoline receptor-PI3K/AKT pathway, and up-regulated HIF-1α, VEGF and RTP801 expression to protect against OGD-induced injury in rat C6 cells.

Review Article: Dexmedetomidine: Does it Have Potential in Palliative Medicine?

Dexmedetomidine, is a α 2 adrenergic agonist approved by the Food and Drug administration for sedation and analgesia. A highly potent α2 adrenergic agonist, it has quick onset of action, with peak effects within 1 hour of administration. It is metabolized in the liver and eliminated in the urine as a glucuronide. Dexmedetomidine is a substrate and inhibitor of cytochrome oxidase 2D6, but clinical evidence of significant drug interactions is lacking. Clinical trials suggest efficacy for the treatment of delirium in the intensive care unit setting with efficacy comparable to haloperidol and benzodiazepines. Dexmedetomidine also has an opioid-sparing action and can act to enhance analgesia. The purpose of this article is to review the pharmacodynamics and pharmacology of dexmedetomidine, and examine its potential use in the palliative care population, especially with regard to the management of delirium.

Protection in glutamate-induced neurotoxicity by imidazoline receptor agonist moxonidine

In the present study we investigated the effects of mixed imidazoline-1 and alpha(2)-adrenoceptor agonist, moxonidine, in glutamate-induced neurotoxicity in frontal cortical cell cultures of rat pups by dye exclusion test. Also, phosphorylated p38 mitogen activated protein kinases (p-p38 MAPK) levels were determined from rat frontal cortical tissue homogenates by two dimensional gel electrophoresis and semidry western blotting. Glutamate at a concentration of 10(-6) M was found neurotoxic when applied for 16 hr in cell cultures. Dead cell mean scores were 12.8 +/- 0.5 for control and 52.3 +/- 4.8 for glutamate (p < .001). On the other hand, p-p38 MAPK levels start to increase at a glutamate concentration of 10(-7) M for 20 min application. Moxonidine was found to have an U-shape neuroprotective effect in glutamate-induced neurotoxicity in neuronal cell culture experiments. Even though moxonidine did not induce neurotoxicity alone between the doses of 10(-8) to 10(-4) M concentrations in cell culture series, it caused the reduction of glutamate-induced dead cell population 23.07 +/- 3.6% in 10(-6) M and 26.7 +/- 2.1% in 10(-5) M concentrations (p <.001 for both, in respect to control values). The protective effect of moxonidine was confirmed in 10(-8) and 10(-7) M, but not in higher concentrations in glutamate neurotoxicity in gel electrophoresis and western blotting of p-p38 MAPK levels. In addition to other studies that revealed an antihypertensive feature of moxonidine, we demonstrated a possible partial neuroprotective role in lower doses for it in glutamate-mediated neurotoxicity model.

Endogenous alpha2-adrenergic receptor-mediated neuroprotection after severe hypoxia in preterm fetal sheep

Central alpha-adrenergic receptor activity is important for fetal adaptation to hypoxia before birth. It is unclear whether it is also important during recovery. We therefore tested the hypothesis that an infusion of the specific alpha(2)-adrenergic receptor antagonist idazoxan (1 mg/kg/h i.v.) from 15 min to 4 h after profound hypoxia induced by 25 min umbilical cord occlusion in fetal sheep at 70% of gestation (equivalent to the 28-32 weeks in humans) would increase neural injury. After 3 days' recovery, idazoxan infusion was associated with a significant increase in neuronal loss in the hippocampus (P<0.05), expression of cleaved caspase-3 (P<0.05), and numbers of activated microglia (P<0.05). There was no significant effect on other neuronal regions or on loss of O4-positive premyelinating oligodendrocytes in the subcortical white matter. Idazoxan was associated with an increase in evolving epileptiform electroencephalographic (EEG) transient activity after occlusion (difference at peak 2.5+/-1.0 vs. 11.7+/-4.7 counts/min, P<0.05) and significantly reduced average spectral edge frequency, but not EEG intensity, from 54 until 72 h after occlusion (P<0.05). Hippocampal neuronal loss was correlated with total numbers of epileptiform transients during idazoxan infusion (P<0.01; r(2)=0.7). In conclusion, endogenous inhibitory alpha(2)-adrenergic receptor activation after severe hypoxia appears to significantly limit evolving hippocampal damage in the immature brain.

Neuroprotection by alpha 2-adrenergic agonists in cerebral ischemia

Ischemic brain injury is implicated in the pathophysiology of stroke and brain trauma, which are among the top killers worldwide, and intensive studies have been performed to reduce neural cell death after cerebral ischemia. Alpha 2-adrenergic agonists have been shown to improve the histomorphological and neurological outcome after cerebral ischemic injury when administered during ischemia, and recent studies have provided considerable evidence that alpha 2-adrenergic agonists can protect the brain from ischemia/reperfusion injury. Thus, alpha 2-adrenergic agonists are promising potential drugs in preventing cerebral ischemic injury, but the mechanisms by which alpha 2-adrenergic agonists exert their neuroprotective effect are unclear. Activation of both the alpha 2-adrenergic receptor and imidazoline receptor may be involved. This mini review examines the recent progress in alpha 2-adrenergic agonists - induced neuroprotection and its proposed mechanisms in cerebral ischemic injury.

Noradrenergic agonists and antagonists influence migration of cortical spreading depression in rat-a possible mechanism of migraine prophylaxis and prevention of postischemic neuronal damage

Cortical spreading depression (CSD) is thought to be a neuronal mechanism that expands the penumbra zone after focal brain ischemia and that causes migraine aura. Both adrenergic agonists and antagonists significantly influence the size of the penumbra zone and decline the frequency of migraine. To study whether these compounds act by influencing CSD, we applied different drugs topically to an area of the exposed cortex of anesthetized adult rats and observed the migration of CSD-related DC potential deflections across the treated area. The adrenergic agonist norepinephrine (1 mmol/L) and the alpha(2)-agonist clonidine (0.56 mmol/L) blocked reversibly the migration of CSD. The beta-blocker propranolol (250 micromol/L to 1 mmol/L) dose-dependently diminished migration velocity or even blocked migration of CSD. The CSD blockade by the alpha(2)-antagonist yohimbine (1.75 mmol/L) was because of its action on inhibitory 5-HT(1A) receptors. None of the substances in the concentrations used had influence on regional cerebral blood flow or on systemic arterial blood pressure. The data suggest that the interference of these compounds with CSD may contribute to their beneficial therapeutic effect. The effect of beta-receptor antagonists in human migraine needs further exploration, since these drugs also work in migraine without aura.

Effect of agmatine on heteromeric N-methyl-d-aspartate receptor channels

Endogenous polyamines like spermine are known to have four distinct effects on recombinant N-methyl-d-aspartate (NMDA) receptor channels: voltage-dependent inhibition, glycine-dependent stimulation, glycine-independent stimulation and decreased affinity to the agonist (l-glutamate). These effects are highly dependent on the constituting epsilon subunits (epsilon1-epsilon4) of the recombinant NMDA receptor channels. Agmatine reportedly inhibits native NMDA receptor channels in cultured hippocampal neurons. In the present investigation, the effects of agmatine on the epsilon/zeta heteromeric NMDA receptor channels expressed in Xenopus laevis oocytes were examined using the two-electrode voltage clamp method. Agmatine inhibited the four epsilon/zeta (epsilon1/zeta1, epsilon2/zeta1, epsilon3/zeta1 and epsilon4/zeta1) channels with similar sensitivity (an IC50 value of about 300microM at -70mV). This effect was dependent on the membrane potential and was more pronounced at hyperpolarized membrane potentials (voltage-dependent inhibition). Agmatine did not exhibit other stimulatory (glycine-dependent and -independent effects) or inhibitory (decreased affinity to l-glutamate) effects. These properties are similar to the pharmacological profile of well-characterized NMDA receptor channel blockers like phencyclidine and ketamine. Thus, regarding the effects on the NMDA receptor channels, agmatine is not like other endogenous polyamines rather it acts as a channel blocker.

Protective effects of rilmenidine and AGN 192403 on oxidative cytotoxicity and mitochondrial inhibitor-induced cytotoxicity in astrocytes

Oxidative stress and mitochondrial dysfunction are important aspects of pathogenesis, particularly in the brain, which is highly dependent on oxygen, and the protection of astrocytes is essential for neuroprotection. In this context, imidazoline drugs have been reported to be neuroprotective. Our recent study showed that imidazoline drugs, including guanabenz, inhibit the naphthazarin-induced oxidative cytotoxicity associated with lysosomal destabilization. We now report on a study into the protective effects of rilmenidine and AGN 192403, which have affinity for imidazoline-1 receptors, on the cytotoxicity induced by naphthazarin and inhibitors of mitochondrial respiration in astrocytes. Cytotoxicity was measured grossly by LDH release and by measuring changes in lysosomal membrane stability and features of mitochondrial membrane permeabilization. Naphthazarin-induced cytotoxicity was evidenced by the ordered development of lysosomal acridine orange relocation, decrease in mitochondrial potential, cytochrome c release, and caspase-9 activation, and was inhibited by guanabenz, rilmenidine, and AGN 192403. Antimycin A and rotenone induced mitochondrial dysfunction primarily, and their cytotoxicities were inhibited only by AGN 192403. Rilmenidine and guanabenz may have a lysosomal stabilizing effect, which underlies their protective effects. AGN 192403 might affect the mitochondrial cell death cascades, and had a novel protective effect on the cytotoxicity associated with mitochondrial dysfunction.

Imidazoline drugs stabilize lysosomes and inhibit oxidative cytotoxicity in astrocytes

Oxidative stress is a primary pathogenesis in the brain, which is particularly vulnerable to oxidative stress. Maintenance of astrocyte functions under oxidative stress is essential to prevent neuronal injuries and to recover neuronal functions in various pathologic conditions. Imidazoline drugs have affinities for imidazoline receptors, which are highly distributed in the brain, and have been shown to be neuroprotective. This study presented the protective effects of several imidazoline drugs against oxidative cytotoxicity, in primary cultures of astrocytes. Imidazoline drugs, such as idazoxan, guanabenz, guanfacine, BU224, and RS-45041-190, showed protective effects against naphthazarin-induced oxidative cytotoxicity, as evidenced by LDH release and Hoechst 33342/propidium iodide staining. The imidazoline drugs stabilized lysosomes and inhibited naphthazarin-induced lysosomal destabilization, as evidenced by acridine orange relocation. Guanabenz inhibited, the leakage of lysosomal cathepsin D to cytosol, the decreased mitochondrial potential, and the release of mitochondrial cytochrome c, which were induced by naphthazarin. The lysosomal destabilization by oxidative stress and other apoptotic signals and subsequent cathepsin D leakage to the cytosol can induce apoptotic changes of mitochondria and eventually cell death. Therefore, lysosomal stabilization by imidazoline drugs may be ascribed to their protective effects against oxidative cytotoxicity.

Effects of moxonidine on corticocerebral blood flow under normal and ischemic conditions in conscious rabbits

Hypertension associated with excessive liberation of circulating and tissue catecholamines is an independent risk factor for further cardiovascular complications and an important predictor of stroke. Moxonidine is a centrally acting anti-hypertensive drug with potent action on I1-imidazoline receptors. It inhibits catecholamine release and is therefore expected to exert an antiadrenergic effect at various levels in the regulation of the cardiovascular system. The aim of this study was to investigate the effect of moxonidine (0.025-0.1 mg/kg, i.v.) on the normal and unilateral carotid occlusion-induced impaired corticocerebral blood flow (cCBF) determined by hydrogen polarography, on mean arterial blood pressure (MABP) and heart rate (HR) in conscious rabbits. Moxonidine produced a reduction of MABP and HR. On the other hand, after administration of the drug, a significant increase in the normal and impaired cCBF was observed. Because the improvement in cCBF was conspicuous in both normal and ischemic conditions, moxonidine might be beneficial not only in the treatment of hypertension but also in the management of cerebral ischemia.

Idazoxan does not prevent but worsens focal hypoxic-ischemic brain damage in neonatal Wistar rats

We examined the neuroprotective efficacy of a post-treatment with idazoxan (Idaz): an alpha2-adrenoceptor antagonist with activity at the I1- and I2-subtypes of the imidazoline receptor (I-receptor), in an experimental model of perinatal hypoxic-ischemic (HI) brain damage. Seventy-two, 7-day-old Wistar rats were subjected to permanent unilateral ligation of the common carotid artery and transient (2 hr) hypoxia (8% O(2)). The surviving animals were sub-divided into 3 groups: one "control" group received intraperitoneal (i.p.) injection of saline (Sigma; n = 21) and two "treated" groups received, 10 min post-HI, i.p. treatments with Idaz (I3: 3 mg/kg; n = 19) or (I8: 8 mg/kg; n = 20). Idaz effects were assessed by TTC-staining 72 hr post-HI for Sigma (n = 13), I3 (n = 11), and I8 (n = 12) groups and by MRI-examination 5 weeks post-HI for Sigma (n = 8), I3 (n = 8), and I8 (n = 6) groups. Total ratio of brain infarct areas were significantly (P < 0.01) different between Sigma and Idaz-treated rats: 20.9 +/- 4.0%, 35.6 +/- 5.9 % and 36.8 +/- 5.8% for Sigma, I3 and I8, respectively, when determined with TTC-staining and; 23.3 +/- 3.7%, 39.8 +/- 4.2%, and 43.2 +/- 10.1%, for Sigma, I3, and I8, respectively, when assessed by MRI. Our results suggest that Idaz, given as a post-HI treatment, does not exert neuroprotective effects but enhances the brain injury induced by focal neonatal cerebral HI. The deleterious mechanism may result from an overactivity of sympathetic tone and/or the immaturity of central I-receptors in newborn rats.

Recent advances in imidazoline receptor research: ligands--localization and isolation--signaling--functional and clinical studies

In this article we outline the highlights of this special issue of the journal containing a series of articles covering many aspects of current interest in the field of imidazoline receptor research. This volume is the result of an international symposium held in September 1997 in Melbourne as an official satellite of the inaugural meeting of the International Society of Autonomic Neurosciences held in Cairns, Australia. A wide range of topics relating to imidazoline receptors were canvassed, including endogenous and synthetic ligands, identification and localisation of binding sites, putative transduction mechanisms and experimental and clinical functional studies.

Influence of flupirtine on a G-protein coupled inwardly rectifying potassium current in hippocampal neurones

1. Previous studies have shown that flupirtine, a centrally acting, non-opioid analgesic agent, also exhibits neuroprotective activity in focal cerebral ischaemia in mice and reduces apoptosis induced by NMDA, gp 120 of HIV, prior protein fragment or lead acetate as well as necrosis induced by glutamate or NMDA in cell culture. To study the potential mechanism of the neuroprotective action of flupirtine, we investigated whether flupirtine is able to modulate potassium or NMDA-induced currents in rat cultured hippocampal neurones by use of the whole-cell configuration of the patch-clamp technique. 2. We demonstrated that 1 microM flupirtine activated an inwardly rectifying potassium current (K(ir)) in hippocampal neurones (deltaI=-39+/-18 pA at -130 mV; n=10). This effect was dose-dependent (EC50=0.6 microM). The reversal potential for K(ir) was in agreement with the potassium equilibrium potential predicted from the Nernst equation showing that K(ir) was predominantly carried by K+. Furthermore, the induced current was blocked completely by Ba2+ (1 mM), an effect typical for K(ir). 3. The activation of K(ir) by flupirtine was largely prevented by pretreatment of the cells with pertussis toxin (PTX) indicating the involvement of a PTX-sensitive G-protein in the transduction mechanism (deltaI=-3+/-6 pA at -130 mV; n=8). Inclusion of cyclic AMP in the intracellular solution completely abolished the activation of K(ir) (n=7). 4. The selective alpha2-adrenoceptor antagonist SKF-86466 (10 microM), the selective 5-HT1A antagonist NAN 190 as well as the selective GABA(B) antagonist 2-hydroxysaclofen (10 microM) failed to block the flupirtine effect on the inward rectifier. 5. Flupirtine (1 microM) could not change the current induced by 50 microM NMDA. 6. These results show that in cultured hippocampal neurones flupirtine activates an inwardly rectifying potassium current and that a PTX-sensitive G-protein is involved in the transduction mechanism.

Imidazoline receptor agonist drugs: a new approach to the treatment of systemic hypertension

The imidazoline receptors have recently been discovered to be involved in central nervous system control of blood pressure (I-1 receptor) and in neuroprotection for cerebral ischemia (I-2 receptor). A new class of central-acting antihypertensive agents has been developed, the imidazoline receptor agonists (rilmenidine and moxonidine), which control blood pressure effectively without the adverse effects of sedation and mental depression that are usually associated with central-acting antihypertensives. This new generation of central-acting antihypertensive agents are highly selective for the imidazoline receptor, while having a low affinity for alpha 2-adrenergic receptors.

Endogenous ligands of imidazoline receptors: classic and immunoreactive clonidine-displacing substance and agmatine

1. There are several endogenous ligands that bind to I-receptors of both the I1 and I2 subclass. These include: (a) classic CDS, a partially purified entity isolated by the criteria that it displaces binding ligands to alpha 2- and I-receptors; (b) immunoreactive (ir)-CDS, a moiety that binds to antibodies raised against clonidine, para-amino-clonidine, or idazoxan; and (c) agmatine. 2. Classic-CDS, not yet defined structurally, binds to I1, I2, and alpha 2-adrenergic receptors, is neither a peptide nor a catecholamine, and has purportedly a molecular weight of 588 Da. By ligand binding assays, it was found in brain, serum, CSF, and placenta and in a neural-glial cell line. Partially purified classic CDS is bioactive. Like clonidine, it contracts aorta and vas deferens and inhibits platelet aggregation, effects largely attributable to agonism at alpha 2-adrenergic receptors. Unlike clonidine, it contracts rat gastric fundus and releases catecholamines from chromaffin cells, effects attributable to actions at I-receptors. Injected into the RVL, classic CDS alters arterial pressure, but the direction of change of pressure has differed between groups of investigators. However, in the absence of structure, it is possible that ligand binding and bioactivity may be attributable to different molecules. 3. Ir-CDS, also of unknown structure, is a material(s) that binds to antibodies raised against clonidine, PAC, or idazoxan. Ir-CDS, measured by radioimmunoassay, is unevenly distributed in brain with highest concentrations in the hypothalamus, midbrain, and dorsal medulla. It is contained in the gastric fundus, adrenal gland, heart, kidney, and serum in amounts substantially higher than found in brain. Ir-CDS may be elevated in the serum of some patients with hypertension and in the CSF of patients with structural brain disease. The concentration of ir-CDS and bioactivity on gastric fundus directly correlates, suggesting that it may share similarities with classic-CDS. However, until the structure of classic and ir-CDS is determined, the possibility that ligand binding and antibody recognition are properties of different molecules must be considered. 4. Agmatine (decarboxylated arginine) is the only endogenous molecule that, like CDS, binds to alpha 2- and I-receptors of both classes. It and its biosynthetic enzyme arginine decarboxylase are present in brain, and agmatine is widely distributed throughout the body. However, the distribution of agmatine and ir-CDS differs, whereas the biological actions of agmatine do not mimic those of classic CDS. Its presence raises the possibility of an alternative pathway for polyamine biosynthesis.(ABSTRACT TRUNCATED AT 400 WORDS)