Inhibitory effects of imidazoline receptor ligands on basal and kainic acid-induced neurotoxic signalling in mice

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.

Assessment of drug-drug interactions between moxonidine and antiepileptic drugs in the maximal electroshock seizure test in mice

Hypertension is a common comorbid condition with epilepsy, and drug interactions between antihypertensive and antiepileptic drugs (AEDs) are likely in patients. Experimental studies showed that centrally active imidazoline compounds belonging to antihypertensive drugs can affect seizure susceptibility. The purpose of this study was to assess the effect of moxonidine, an I₁ -imidazoline receptor agonist, on the anticonvulsant efficacy of numerous AEDs (carbamazepine, phenobarbital, valproate, phenytoin, oxcarbazepine, topiramate and lamotrigine) in the mouse model of maximal electroshock. Besides, the combinations of moxonidine and AEDs were investigated for adverse effects in the passive avoidance task and the chimney test. Drugs were administered intraperitoneally (ip). Moxonidine at doses of 1 and 2 mg/kg ip did not affect the convulsive threshold. Among tested AEDs, moxonidine (2 mg/kg) potentiated the protective effect of valproate against maximal electroshock. This interaction could be pharmacodynamic because the brain concentration of valproate was not significantly changed by moxonidine. The antihypertensive drug did not cause adverse effects when combined with AEDs. This study shows that moxonidine may have a neutral or positive effect on the anticonvulsant activity of AEDs in patients with epilepsy. The enhancement of the anticonvulsant action of valproate by moxonidine needs further investigations to elucidate potential mechanisms involved.

The 2-(2-benzofuranyl)-2-imidazoline provides neuroprotection against focal cerebral ischemia-reperfusion injury in diabetic rats: Influence of microglia and possible mechanisms of action

Increased microglial NADPH oxidase (NOX₂) production may make an important contribution to the increased incidence and severity of ischemic stroke associated with diabetes. Imidazoline receptors are closely associated with neuroprotection, but the neuroprotective effects of the selective I₂-imidazoline receptor ligand 2-(2-benzofuranyl)-2-imidazoline (2BFI) in diabetes has not been established. The effect of 2BFI on microglial NOX₂ production was investigated using a co-culture of neurons and microglia, and the effect on cerebral ischemia-reperfusion (IR) injury was determined in diabetic rats. Garcia neurological scores, brain infarct volumes, brain water content, TUNEL staining, blood-brain barrier, and immunofluorescent labeling for microglia were evaluated. Western blots were used to determine gp91^phox and Tyr1472 expression. Anti-inflammatory cytokine (IL-10) and inflammatory cytokine secretion was determined using ELISA kits. The brain infarct volumes, TUNEL-positive neurons, expression of microglia, brain water content, blood-brain barrier structure damage, and gp91^phox and Tyr1472 expression were increased, the Garcia neurological scores were significantly decreased in the IR group, and 2BFI relieved these alterations. The IL-10 concentration was increased in the IR group; 2BFI significantly improved this increase. The neuron apoptosis and necrosis rates, and production of reactive oxygen species (ROS) and inflammatory cytokines, including IL-6, IL-8, TNF-α, and 8-iso-PGF2α, were significantly increased by high glucose stimulation combined with oxygen-glucose deprivation treatment, which were inhibited by 2BFI. The 2BFI ameliorated cerebral ischemia-reperfusion injury in diabetes and decreased neuron death in an in vitro model. The mechanism underlying these findings may be related to the decreased production of inflammatory factors and reactive oxygen species from microglia.

Restoration of Rostral Ventrolateral Medulla Cystathionine-γ Lyase Activity Underlies Moxonidine-Evoked Neuroprotection and Sympathoinhibition in Diabetic Rats

We recently demonstrated a fundamental role for cystathionine-γ lyase (CSE)-derived hydrogen sulfide (H₂S) in the cardioprotective effect of the centrally acting drug moxonidine in diabetic rats. Whether a downregulated CSE/H₂S system in the rostral ventrolateral medulla (RVLM) underlies neuronal oxidative stress and sympathoexcitation in diabetes has not been investigated. Along with addressing this question, we tested the hypothesis that moxonidine prevents the diabetes-evoked neurochemical effects by restoring CSE/H₂S function within its major site of action, the RVLM. Ex vivo studies were performed on RVLM tissues of streptozotocin (55 mg/kg, i.p.) diabetic rats treated daily for 3 weeks with moxonidine (2 or 6 mg/kg; gavage), H₂S donor sodium hydrosulfide (NaHS) (3.4 mg/kg, i.p.), CSE inhibitor DL-propargylglycine (DLP) (37.5 mg/kg, i.p.), a combination of DLP with moxonidine, or their vehicle. Moxonidine alleviated RVLM oxidative stress, neuronal injury, and increased tyrosine hydroxylase immunoreactivity (sympathoexcitation) by restoring CSE expression/activity as well as heme oxygenase-1 (HO-1) expression. A pivotal role for H₂S in moxonidine-evoked neuroprotection is supported by the following: 1) NaHS replicated the moxonidine-evoked neuroprotection, and the restoration of RVLM HO-1 expression in diabetic rats; and 2) DLP abolished moxonidine-evoked neuroprotection in diabetic rats, and caused RVLM neurotoxicity, reminiscent of a diabetes-evoked neuronal phenotype, in healthy rats. These findings suggest a novel role for RVLM CSE/H₂S/HO-1 in moxonidine-evoked neuroprotection and sympathoinhibition, and as a therapeutic target for developing new drugs for alleviating diabetes-evoked RVLM neurotoxicity and cardiovascular anomalies.

Neuroprotective Effects of a Structurally New Family of High Affinity Imidazoline I2 Receptor Ligands

The imidazoline I₂ receptors (I₂-IRs) are widely distributed in the brain, and I₂-IR ligands may have therapeutic potential as neuroprotective agents. Since structural data for I₂-IR remains unknown, the discovery of selective I₂-IR ligands devoid of α₂-adrenoceptor (α₂-AR) affinity is likely to provide valuable tools in defining the pharmacological characterization of these receptors. We report the pharmacological characterization of a new family of (2-imidazolin-4-yl)phosphonates. Radioligand binding studies showed that they displayed a higher affinity for I₂-IRs than idazoxan, and high I₂/α₂ selectivity. In vivo studies in mice showed that acute treatments with 1b and 2c significantly increased p-FADD/FADD ratio (an index of cell survival) in the hippocampus when compared with vehicle-treated controls. Additionally, acute and repeated treatments with 2c, but not with 1b, markedly reduced hippocampal p35 cleavage into neurotoxic p25. The present results indicate a neuroprotective potential of (2-imidazolin-4-yl)phosphonates acting at I₂-IRs.

Imidazoline I2 receptors: An update

Since first introduced more than two decades ago, the research in imidazoline I₂ receptors has been steadily increasing. This review provides an update on the current status of I₂ receptor pharmacology. Imidazoline I₂ receptors or I₂ binding sites refer to several (at least four) different proteins that bind to [³H]-idazoxan and [³H]-2-BFI with high affinity. The molecular identities of the proteins remain elusive. One of the proteins (45kD) seems to be consistent with the identity of brain creatine kinase. The biological functions of I₂ receptors have been primarily unveiled by the studies of selective I₂ receptor ligands. Accumulating evidence suggests that I₂ receptor ligands are effective analgesics for persistent and chronic painful conditions such as inflammatory, neuropathic and postoperative pain. One selective I₂ receptor ligand, CR4056, has been advanced to phase II clinical trial with the therapeutic indication of chronic inflammatory pain (osteoarthritis). The expansion to the treatment of other chronic pain conditions should be expected if CR4056 could eventually be approved as a new drug. I₂ receptor ligands also demonstrate robust discriminative stimulus activity and induce a characteristic discriminative cue in animals. Biochemical and preclinical in vivo investigations also suggest that I₂ receptor ligands have neuroprotective activity and modulate body temperature. The emerging discrepancies of a range of purported selective I₂ receptor ligands suggest different pharmacological effects mediated by discrete I₂ receptor components which likely attribute to the I₂ receptor-related proteins. It is proposed that the I₂ receptors represent an emerging drug target for the treatment of neurological disorders such as pain and stroke, and deserve more research attention to translate preclinical findings to pharmacotherapies.