Imidazoline I2 receptors: target for new analgesics?

Multitarget ligands that comprise opioid/nonopioid pharmacophores for pain management: Current state of the science

Chronic pain, which affects more than one-third of the world's population, represents one of the greatest medical challenges of the 21st century, yet its effective management remains sub-optimal. The 'gold standard' for the treatment of moderate to severe pain consists of opioid ligands, such as morphine and fentanyl, that target the µ-opioid receptor (MOP). Paradoxically, these opioids also cause serious side effects, including constipation, respiratory depression, tolerance, and addiction. In addition, the development of opioid-use disorders, such as opioid diversion, misuse, and abuse, has led to the current opioid crisis, with dramatic increases in addiction, overdoses, and ultimately deaths. As pain is a complex, multidimensional experience involving a variety of pathways and mediators, dual or multitarget ligands that can bind to more than one receptor and exert complementary analgesic effects, represent a promising avenue for pain relief. Indeed, unlike monomodal therapeutic approaches, the modulation of several endogenous nociceptive systems can often result in an additive or even synergistic effect, thereby improving the analgesic-to-side-effect ratio. Here, we provide a comprehensive overview of research efforts towards the development of dual- or multi-targeting opioid/nonopioid hybrid ligands for effective and safer pain management. We reflect on the underpinning discovery rationale by discussing the design, medicinal chemistry, and in vivo pharmacological effects of multitarget antinociceptive compounds.

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.

Recent developments and challenges in positron emission tomography imaging of gliosis in chronic neuropathic pain

ABSTRACT

Understanding the mechanisms that underpin the transition from acute to chronic pain is critical for the development of more effective and targeted treatments. There is growing interest in the contribution of glial cells to this process, with cross-sectional preclinical studies demonstrating specific changes in these cell types capturing targeted timepoints from the acute phase and the chronic phase. In vivo longitudinal assessment of the development and evolution of these changes in experimental animals and humans has presented a significant challenge. Recent technological advances in preclinical and clinical positron emission tomography, including the development of specific radiotracers for gliosis, offer great promise for the field. These advances now permit tracking of glial changes over time and provide the ability to relate these changes to pain-relevant symptomology, comorbid psychiatric conditions, and treatment outcomes at both a group and an individual level. In this article, we summarize evidence for gliosis in the transition from acute to chronic pain and provide an overview of the specific radiotracers available to measure this process, highlighting their potential, particularly when combined with ex vivo / in vitro techniques, to understand the pathophysiology of chronic neuropathic pain. These complementary investigations can be used to bridge the existing gap in the field concerning the contribution of gliosis to neuropathic pain and identify potential targets for interventions.

Molecular Modeling, Synthesis, and Antihyperglycemic Activity of the New Benzimidazole Derivatives - Imidazoline Receptor Agonists

Introduction

The paper presents the results of a study on the first synthesized benzimidazole derivatives obtained from labile nature carboxylic acids. The synthesis conditions of these substances were studied, their structure was proved, and some components were found to have sugar-reducing activity on the model of alloxan diabetes in rats.

Methods

The study used molecular modeling methods such as docking based on the evolutionary model (igemdock), RP_HPLC method to monitor the synthesis reaction, and 1H NMR and 13C NMR, and other methods of organic chemistry to confirm the structures of synthesized substances.

Results & Discussion

The docking showed that the ursodeoxycholic acid benzimidazole derivatives have high tropics to all imidazoline receptor carriers (PDB ID: 2XCG, 2bk3, 3p0c, 1QH4). The ursodeoxycholic acid benzimidazole derivative and arginine and histidine benzimidazole derivatives showed the highest sugar-lowering activity in the experiment on alloxan-diabetic rats. For these derivatives, the difference in glucose levels of treated rats was significant against untreated control. Therefore, the new derivatives of benzimidazole and labile natural organic acids can be used to create new classes of imidazoline receptor inhibitors for the treatment of diabetes mellitus and hypertension.

Exploring the reactivity of bicyclic α-iminophosphonates to access new imidazoline I2 receptor ligands

Recent studies pointed out the modulation of imidazoline I2 receptors (I2-IR) by selective ligands as a putative strategy to face neurodegenerative diseases. Foregoing the classical 2-imidazoline/imidazole-containing I2-IR ligands, we report a family of bicyclic α-iminophosphonates endowed with high affinity and selectivity upon I2-IR and we advanced a representative compound B06 in preclinical phases. In this paper, we describe the synthetic possibilities of bicyclic α-iminophosphonates by exploring its ambivalent reactivity, leading to unprecedented molecules that showed promising activities as I2-IR ligands in human brain tissues and good BBB permeation capabilities. After in silico ADME prediction studies, we assessed the neuroprotective properties of selected compounds and beneficial effect in an in vitro model of Alzheimeŕs and Parkinson's disease. Along with their neuroprotective effect, compounds showed a potent anti-inflammatory response when evaluated in a neuroinflammation cellular model. Moreover, this is the first time that the neuroprotective effects of imidazoline I2-IR ligands in a transgenic Alzheimer's disease Caenorhabditis elegans strain are investigated. Using a thrashing assay, we found a significant cognition improvement in this in vivo model after treatment with the new bicyclic α-phosphoprolines. Therefore, our results confirmed the need of exploring structurally new I2-IR ligands and their potential for therapeutic strategies in neurodegeneration.

Insights into the Pharmacokinetics and In Vitro Cell-Based Studies of the Imidazoline I2 Receptor Ligand B06

The impact of neurodegenerative diseases (ND) is becoming unbearable for humankind due to their vast prevalence and the lack of efficacious treatments. In this scenario, we focused on imidazoline I₂ receptors (I₂-IR) that are widely distributed in the brain and are altered in patients with brain disorders. We took the challenge of modulating I₂-IR by developing structurally new molecules, in particular, a family of bicyclic α-iminophosphonates, endowed with high affinity and selectivity to these receptors. Treatment of two murine models, one for age-related cognitive decline and the other for Alzheimer's disease (AD), with representative compound B06 ameliorated their cognitive impairment and improved their behavioural condition. Furthermore, B06 revealed beneficial in vitro ADME-Tox properties. The pharmacokinetics (PK) and metabolic profile are reported to de-risk B06 for progressing in the preclinical development. To further characterize the pharmacological properties of B06, we assessed its neuroprotective properties and beneficial effect in an in vitro model of Parkinson's disease (PD). B06 rescued the human dopaminergic cell line SH-SY5Y from death after treatment with 6-hydroxydopamine (6-OHDA) and showed a crucial anti-inflammatory effect in a cellular model of neuroinflammation. This research reveals B06 as a putative candidate for advancing in the difficult path of drug discovery and supports the modulation of I₂-IR as a fresh approach for the therapy of ND.

Fentanyl Structure as a Scaffold for Opioid/Non-Opioid Multitarget Analgesics

One of the strategies in the search for safe and effective analgesic drugs is the design of multitarget analgesics. Such compounds are intended to have high affinity and activity at more than one molecular target involved in pain modulation. In the present contribution we summarize the attempts in which fentanyl or its substructures were used as a μ-opioid receptor pharmacophoric fragment and a scaffold to which fragments related to non-opioid receptors were attached. The non-opioid ‘second’ targets included proteins as diverse as imidazoline I₂ binding sites, CB₁ cannabinoid receptor, NK₁ tachykinin receptor, D₂ dopamine receptor, cyclooxygenases, fatty acid amide hydrolase and monoacylglycerol lipase and σ₁ receptor. Reviewing the individual attempts, we outline the chemistry, the obtained pharmacological properties and structure-activity relationships. Finally, we discuss the possible directions for future work.

An excitatory lateral hypothalamic circuit orchestrating pain behaviors in mice

Understanding how neuronal circuits control nociceptive processing will advance the search for novel analgesics. We use functional imaging to demonstrate that lateral hypothalamic parvalbumin-positive (LHPV) glutamatergic neurons respond to acute thermal stimuli and a persistent inflammatory irritant. Moreover, their chemogenetic modulation alters both pain-related behavioral adaptations and the unpleasantness of a noxious stimulus. In two models of persistent pain, optogenetic activation of LHPV neurons or their ventrolateral periaqueductal gray area (vlPAG) axonal projections attenuates nociception, and neuroanatomical tracing reveals that LHPV neurons preferentially target glutamatergic over GABAergic neurons in the vlPAG. By contrast, LHPV projections to the lateral habenula regulate aversion but not nociception. Finally, we find that LHPV activation evokes additive to synergistic antinociceptive interactions with morphine and restores morphine antinociception following the development of morphine tolerance. Our findings identify LHPV neurons as a lateral hypothalamic cell type involved in nociception and demonstrate their potential as a target for analgesia.

Benzofuranyl-2-imidazoles as imidazoline I2 receptor ligands for Alzheimer's disease

Recent findings unveil the pharmacological modulation of imidazoline I₂ receptors (I₂-IR) as a novel strategy to face unmet medical neurodegenerative diseases. In this work, we report the chemical characterization, three-dimensional quantitative structure-activity relationship (3D-QSAR) and ADMET in silico of a family of benzofuranyl-2-imidazoles that exhibit affinity against human brain I₂-IR and most of them have been predicted to be brain permeable. Acute treatment in mice with 2-(2-benzofuranyl)-2-imidazole, known as LSL60101 (garsevil), showed non-warning properties in the ADMET studies and an optimal pharmacokinetic profile. Moreover, LSL60101 induced hypothermia in mice while decreased pro-apoptotic FADD protein in the hippocampus. In vivo studies in the familial Alzheimer's disease 5xFAD murine model with the representative compound, revealed significant decreases in the protein expression levels of antioxidant enzymes superoxide dismutase and glutathione peroxidase in hippocampus. Overall, LSL60101 plays a neuroprotective role by reducing apoptosis and modulating oxidative stress.

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.

Amelioration of BPSD-Like Phenotype and Cognitive Decline in SAMP8 Mice Model Accompanied by Molecular Changes after Treatment with I2-Imidazoline Receptor Ligand MCR5

Behavioural and psychological symptoms of dementia (BPSD), including fear-anxiety- and depressive-like behaviour, are present in Alzheimer's disease (AD), together with memory decline. I₂-imidazoline receptors (I₂-IRs) have been associated with neuropsychiatric and neurodegenerative disorders, further, I₂-IR ligands have demonstrated a neuroprotective role in the central nervous system (CNS). In this study, we assessed the effect of the I₂-IR ligand MCR5 on both cognitive and non-cognitive symptoms in the Senescence accelerated mice prone 8 (SAMP8) mouse model. Oral administration of I₂-IR ligand MCR5 (5 mg/kg/day for four weeks) in 10-month SAMP8 mice ameliorated both BPSD-like phenotype and cognitive decline by attenuating depressive-like behaviour, reducing fear-anxiety-like behaviour and improving cognitive performance using different tasks. Interaction of I₂-IR ligand MCR5 with serotoninergic system did not account for behavioural or cognitive improvement, although changes in molecular pathways underlying depression and anxiety phenotype were observed. MCR5 increased levels of p-AKT, phosphorylated glycogen synthase kinase 3 β (GSK3β) at Ser9 and phosphorylated mammalian target of rapamycin complex 1 (mTORC1) levels in SAMP8 treated mice compared to SAMP8 control. Moreover, MCR5 treatment altered N-methyl-d-aspartate receptor (NMDA) 2B phosphorylation, and decreased the protein levels of phosphorylated cyclin-dependent kinase 5 (p-CDK5) and dopamine- and cyclic adenosine monophosphate (cAMP)-regulated phosphoprotein of Mr 32 kDa phosphorylated at Thr75 (p-DARPP32), with a parallel increase in protein kinase A (PKA) and p-cAMP response element-binding (pCREB) levels. Consistent with these changes MCR5 attenuated neuroinflammation by decreasing expression of pro-inflammatory markers such as Tumor necrosis factor-alpha (Tnf-α), Interleukin 1β (Il-1β), Interleukin 6 (Il-6), and promoted synaptic plasticity by increasing levels of postsynaptic density protein 95 (PSD95) as well as ameliorating tropomyosin-related kinase B (TrkB) and nerve growth factor receptor (NGFR) signalling. Collectively, these results increase the potential of highly selective I₂-IR ligands as therapeutic agents in age-related BPSD and cognitive alterations.

CR4056, a powerful analgesic imidazoline-2 receptor ligand, inhibits the inflammation-induced PKCε phosphorylation and membrane translocation in sensory neurons

BACKGROUND AND PURPOSE

CR4056 is a first-in-class imidazoline-2 (I₂ ) receptor ligand characterized by potent analgesic activity in different experimental animal models of pain. In a recent phase II clinical trial, CR4056 effectively reduced pain in patients with knee osteoarthritis. In the present study, we investigated the effects of CR4056 on PKCε translocation in vitro and on PKCε activation in vivo in dorsal root ganglia (DRG) neurons.

EXPERIMENTAL APPROACH

Effects of CR4056 on bradykinin-induced PKCε translocation were studied in rat sensory neurons by immunocytochemistry. PKCε activation was investigated by immunohistochemistry analysis of DRG from complete Freund's adjuvant-treated animals developing local hyperalgesia. The analgesic activity of CR4056 was tested on the same animals.

KEY RESULTS

CR4056 inhibited PKCε translocation with very rapid and long-lasting activity. CR4056 decreased hyperalgesia and phospho-PKCε immunoreactivity in the DRG neurons innervating the inflamed paw. The effect of CR4056 on PKCε translocation was blocked by pertussis toxin, implying that the intracellular pathways involved G_i proteins. The inhibition of PKCε translocation by CR4056 was independent of the α₂ -adrenoeceptor and, surprisingly, was also independent of idazoxan-sensitive I₂ binding sites. The I₂ agonist 2BFI had no effect alone but potentiated the activity of low concentrations of CR4056.

CONCLUSIONS AND IMPLICATIONS

Our results demonstrate that CR4056 shares the ability to inhibit PKCε translocation with other analgesics. Whether the inhibition of PKCε involves binding to specific subtype(s) of I₂ receptors should be further investigated. If so, this would be a new mode of action of a highly specific I₂ receptor ligand.

Imidazoline 2 binding sites reflecting astroglia pathology in Parkinson's disease: an in vivo11C-BU99008 PET study

Astroglia are multifunctional cells that regulate neuroinflammation and maintain homeostasis within the brain. Astroglial α-synuclein-positive cytoplasmic accumulations have been shown post-mortem in patients with Parkinson's disease and therefore astroglia may play an important role in the initiation and progression of Parkinson's disease. Imidazoline 2 binding sites are expressed on activated astroglia in the cortex, hippocampus, basal ganglia and brainstem; therefore, by measuring imidazoline 2 binding site levels we can indirectly evaluate astrogliosis in patients with Parkinson's disease. Here, we aimed to evaluate the role of astroglia activation in vivo in patients with Parkinson's disease using 11C-BU99008 PET, a novel radioligand with high specificity and selectivity for imidazoline 2 binding sites. Twenty-two patients with Parkinson's disease and 14 healthy control subjects underwent 3 T MRI and a 120-min 11C-BU99008 PET scan with volume of distribution (VT) estimated using a two-tissue compartmental model with a metabolite corrected arterial plasma input function. Parkinson's disease patients were stratified into early (n = 8) and moderate/advanced (n = 14) groups according to disease stage. In early Parkinson's disease, increased 11C-BU99008 VT uptake was observed in frontal (P = 0.022), temporal (P = 0.02), parietal (P = 0.026) and occipital (P = 0.047) cortical regions compared with healthy controls. The greatest 11C-BU99008 VT increase in patients with early Parkinson's disease was observed in the brainstem (52%; P = 0.018). In patients with moderate/advanced Parkinson's disease, loss of 11C-BU99008 VT was observed across frontal (P = 0.002), temporal (P < 0.001), parietal (P = 0.039), occipital (P = 0.024), and insula (P < 0.001) cortices; and in the subcortical regions of caudate (P < 0.001), putamen (P < 0.001) and thalamus (P < 0.001); and in the brainstem (P = 0.018) compared with healthy controls. In patients with Parkinson's disease, loss of 11C-BU99008 VT in cortical regions, striatum, thalamus and brainstem correlated with longer disease duration (P < 0.05) and higher disease burden scores, measured with Movement Disorder Society Unified Parkinson's Disease Rating Scale (P < 0.05). In the subgroup of patients with moderate/advanced Parkinson's disease, loss of 11C-BU99008 VT in the frontal (r = 0.79; P = 0.001), temporal (r = 0.74; P = 0.002) and parietal (r = 0.89; P < 0.001) cortex correlated with global cognitive impairment. This study demonstrates in vivo the role of astroglia in the initiation and progression of Parkinson's disease. Reactive astroglia observed early in Parkinson's disease could reflect a neuroprotective compensatory mechanisms and pro-inflammatory upregulation in response to α-synuclein accumulation. However, as the disease progresses and significant neurodegeneration occurs, astroglia lose their reactive function and such loss in the cortex has clinical relevance in the development of cognitive impairment.

Alternatives to Opioid Analgesia in Small Animal Anesthesia: Alpha-2 Agonists

Alpha-2 agonists have potent analgesic effects, in addition to their sedative actions. Alpha-2 agonists provide analgesia through any of several routes of administration, including parenteral, oral, epidural or intrathecal and intraarticular, because of spinal and supraspinal actions. Systemic doses are short acting, whereas local administration at the site of action result in longer analgesic effects. The potent cardiovascular and respiratory effects of alpha-2 agonists should be considered when used as analgesics.

Behavioral and Cognitive Improvement Induced by Novel Imidazoline I2 Receptor Ligands in Female SAMP8 Mice

As populations increase their life expectancy, age-related neurodegenerative disorders such as Alzheimer's disease have become more common. I2-Imidazoline receptors (I2-IR) are widely distributed in the central nervous system, and dysregulation of I2-IR in patients with neurodegenerative diseases has been reported, suggesting their implication in cognitive impairment. This evidence indicates that high-affinity selective I2-IR ligands potentially contribute to the delay of neurodegeneration. In vivo studies in the female senescence accelerated mouse-prone 8 mice have shown that treatment with I2-IR ligands, MCR5 and MCR9, produce beneficial effects in behavior and cognition. Changes in molecular pathways implicated in oxidative stress, inflammation, synaptic plasticity, and apoptotic cell death were also studied. Furthermore, treatments with these I2-IR ligands diminished the amyloid precursor protein processing pathway and increased Aβ degrading enzymes in the hippocampus of SAMP8 mice. These results collectively demonstrate the neuroprotective role of these new I2-IR ligands in a mouse model of brain aging through specific pathways and suggest their potential as therapeutic agents in brain disorders and age-related neurodegenerative diseases.

Therapeutic Effect of Agmatine on Neurological Disease: Focus on Ion Channels and Receptors

The central nervous system (CNS) is the most injury-prone part of the mammalian body. Any acute or chronic, central or peripheral neurological disorder is related to abnormal biochemical and electrical signals in the brain cells. As a result, ion channels and receptors that are abundant in the nervous system and control the electrical and biochemical environment of the CNS play a vital role in neurological disease. The N-methyl-D-aspartate receptor, 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl) propanoic acid receptor, kainate receptor, acetylcholine receptor, serotonin receptor, α2-adrenoreceptor, and acid-sensing ion channels are among the major channels and receptors known to be key components of pathophysiological events in the CNS. The primary amine agmatine, a neuromodulator synthesized in the brain by decarboxylation of L-arginine, can regulate ion channel cascades and receptors that are related to the major CNS disorders. In our previous studies, we established that agmatine was related to the regulation of cell differentiation, nitric oxide synthesis, and murine brain endothelial cell migration, relief of chronic pain, cerebral edema, and apoptotic cell death in experimental CNS disorders. In this review, we will focus on the pathophysiological aspects of the neurological disorders regulated by these ion channels and receptors, and their interaction with agmatine in CNS injury.

The α2A -adrenoceptor subtype plays a key role in the analgesic and sedative effects of xylazine

Xylazine, the classical α₂ -adrenoceptor (α₂ -AR) agonist, is still used as an analgesic and sedative in veterinary medicine, despite its low potency and affinity for α₂ -ARs. Previous pharmacological studies suggested that the α_2A -AR subtype plays a role in mediating the clinical effects of xylazine; however, these studies were hampered by the poor subtype-selectivity of the antagonists used and a lack of knowledge of their bioavailability in vivo. Here, we attempted to elucidate the role of the α_2A -AR subtype in mediating the clinical effects of xylazine by comparing the analgesic and sedative effects of this drug in wild-type mice with those in α_2A -AR functional knockout mice using the hot-plate and open field tests, respectively. Hippocampal noradrenaline turnover in both mice was also measured to evaluate the contribution of α_2A -AR subtype to the inhibitory effect of xylazine on presynaptic noradrenaline release. In wild-type mice, xylazine (10 or 30 mg/kg) increased the hot-plate latency. Furthermore, xylazine (3 or 10 mg/kg) inhibited the open field locomotor activity and decreased hippocampal noradrenaline turnover. By contrast, all of these effects were abolished in α_2A -AR functional knockout mice. These results indicate that the α_2A -AR subtype is mainly responsible for the clinical effects of xylazine.

Tolerance and cross-tolerance to the antinociceptive effects of oxycodone and the imidazoline I2 receptor agonist phenyzoline in adult male rats

RATIONALE

Emerging evidence suggests the potential utility of combining opioids with imidazoline I₂ receptor agonists for chronic pain. However, chronic pain management requires prolonged pharmacotherapy, and the consequence of such combination therapy remains unclear.

OBJECTIVE

This study examined the anti-hyperalgesic effect of the opioid oxycodone, the selective I₂ receptor agonist phenyzoline, alone and in combination, during prolonged treatment.

METHODS

Von Frey filament test was used to examine the anti-hyperalgesic effect of drugs in complete Freund's adjuvant (CFA)-induced inflammatory pain or chronic constriction injury (CCI)-induced neuropathic pain in rats. Twice-daily treatment with oxycodone and phenyzoline, alone or in combination, was continued until the development of significant tolerance (oxycodone) or as long as 19 days passed (phenyzoline).

RESULTS

In rats receiving CFA or CCI manipulation, mechanical hyperalgesia was dose-dependently reversed by oxycodone and phenyzoline. Twice-daily treatment with 2 × ED₅₀ dose of oxycodone for 7 days led to significant antinociceptive tolerance to oxycodone but not cross-tolerance to phenyzoline. Similarly, twice-daily treatment with 2 × ED₅₀ dose of phenyzoline for 19 days led to significant antinociceptive tolerance to phenyzoline but not cross-tolerance to oxycodone. Twice-daily treatment with the combined oxycodone and phenyzoline using different ratios (1:3, 1:1 and 3: 1) at the doses that were functionally equivalent to the treatment doses of oxycodone and phenyzoline for 13-19 days generally led to delayed antinociceptive tolerance.

CONCLUSIONS

Combination therapy with oxycodone and I₂ receptor agonists maintains prolonged antinociceptive effectiveness with reduced propensity to develop tolerance.

Efficacy of CR4056, a first-in-class imidazoline-2 analgesic drug, in comparison with naproxen in two rat models of osteoarthritis

Purpose

CR4056, (2-phenyl-6-(1H-imidazol-1yl) quinazoline), an imidazoline-2 (I2) receptor ligand, is a promising analgesic drug that has been reported to be effective in several animal models of pain. The aim of this study was to evaluate the effects of CR4056 in two well-established rat models of osteoarthritis (OA), mimicking the painful and structural components of human OA.

Methods

Knee OA was induced either by single intra-articular injection of monoiodoacetate (MIA) or by medial meniscal tear (MMT) in the right knee of male rats. In the MIA model, allodynia and hyperalgesia were measured as paw withdrawal threshold to mechanical stimulation. In the MMT model, pain behavior was analyzed as weight-bearing asymmetry (i.e. difference in hind paw weight distribution, HPWD) between the injured and the contralateral limbs.

Results

Acute oral administration of CR4056, 14 days after MIA injection, significantly and dose-dependently reduced allodynia and hyperalgesia 90 minutes after treatment, whereas acute naproxen administration significantly reduced allodynia but not hyperalgesia. After 7 days of repeated treatment, both CR4056 and naproxen showed significant anti-allodynic and anti-hyperalgesic effects in the MIA model. Rats undergoing MMT surgery developed a significant and progressive asymmetry in HPWD compared with sham-operated animals. Repeated treatment with CR4056 significantly reduced the progression of the pain behavior, whereas naproxen had no effects.

Conclusion

The data presented here show that the I2 ligand CR4056 could be a new effective treatment for OA pain. The compound is currently under Phase II clinical evaluation for this indication.

Upregulation of IRAS/nischarin (I1-imidazoline receptor), a regulatory protein of μ-opioid receptor trafficking, in postmortem prefrontal cortex of long-term opiate and mixed opiate/cocaine abusers

Imidazoline receptor antisera-selected (IRAS)/nischarin, a putative I₁-imidazoline receptor, has recently been shown to regulate μ-opioid receptor (OR) trafficking and resensitisation. To study a possible involvement of this μ-OR regulator in opiate dependence, the present study assessed by Western blot analysis the contents of IRAS/nischarin and μ-OR in total homogenates and subcellular preparations of postmortem human prefrontal cortex (PFC/BA9) of long-term opiate and mixed opiate/cocaine abusers as well as of matched healthy control subjects. In the PFC/BA9 of long-term opiate/cocaine abusers (all subjects together) IRAS/nischarin content was increased (+67%, p < 0.01, n = 11) when compared with matched controls (n = 10). Similar increases were found for the subgroups of opiate (+72%, n = 6) and mixed opiate/cocaine (+61%, n = 5) abusers. IRAS/nischarin immunocontents were also found increased in subcellular membrane preparations (+61%, p < 0.05, n = 10) of PFC/BA9 from opiate addicts. In the same brain samples, the levels of μ-OR were not different to those in control subjects. Based on the increased contents in brains of opiate abusers and the reported function as μ-OR regulator, IRAS/nischarin could represent a new promising target for treatment of opiate use disorder.

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.

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.

Analysing the effect of I1 imidazoline receptor ligands on DSS-induced acute colitis in mice

Imidazoline receptors (IRs) have been recognized as promising targets in the treatment of numerous diseases; and moxonidine and rilmenidine, agonists of I₁-IRs, are widely used as antihypertensive agents. Some evidence suggests that IR ligands may induce anti-inflammatory effects acting on I₁-IRs or other molecular targets, which could be beneficial in patients with inflammatory bowel disease (IBD). On the other hand, several IR ligands may stimulate also alpha₂-adrenoceptors, which were earlier shown to inhibit, but in more recent studies to rather aggravate colitis. Hence, this study aimed to analyse for the first time the effect of various I₁-IR ligands on intestinal inflammation. Colitis was induced in C57BL/6 mice by adding dextran sulphate sodium (DSS) to the drinking water for 7 days. Mice were treated daily with different IR ligands: moxonidine and rilmenidine (I₁-IR agonists), AGN 192403 (highly selective I₁-IR ligand, putative antagonist), efaroxan (I₁-IR antagonist), as well as with the endogenous IR agonists agmatine and harmane. It was found that moxonidine and rilmenidine at clinically relevant doses, similarly to the other IR ligands, do not have a significant impact on the macroscopic and histological signs of DSS-evoked inflammation. Likewise, colonic myeloperoxidase and serum interleukin-6 levels remained unchanged in response to these agents. Thus, our study demonstrates that imidazoline ligands do not influence significantly the severity of DSS-colitis in mice and suggest that they probably neither affect the course of IBD in humans. However, the translational value of these findings needs to be verified with other experimental colitis models and human studies.

Investigation of the role of alpha-2 adrenergic receptors on prepulse inhibition of acoustic startle reflex in rats

OBJECTIVES

Alpha-2 adrenergic receptors target several behavioral functions. These receptors may connect with the brain pathways mediating sensorimotor gating system that associate with psychoses, and the literature that investigate the relationship between alpha-2 receptors and sensorimotor gating system is very limited and some results are controversial. Thus, we aimed to investigate the role of alpha-2 receptors on prepulse inhibition (PPI) of acoustic startle reflex which is a measure of sensorimotor gating.

EXPERIMENTAL DESIGN

Adult male Wistar rats were subjects. PPI was measured as the per cent inhibition of the startle reflex produced by a startling pulse stimulus. The average PPI levels were used in the further analyses. Clonidine (0.03-1 mg/kg), an agonist of alpha-2 receptors, idazoxan (10 mg/kg), an antagonist alpha-2 receptors, and saline were injected to rats intraperitoneally. PPI was evaluated at two different startle intensity levels (78 and 86 dB, respectively).

PRINCIPAL OBSERVATIONS

Treatments produced some significant changes on PPI of startle reflex at all two levels of startle intensity. While clonidine (0.06, 0.25, 0.5, and 1 mg/kg) disrupted significantly PPI, idazoxan (10 mg/kg) did not produce any significant effect on PPI. However, pretreatment with idazoxan reversed significantly clonidine-induced disruption of PPI. Neither idazoxan (10 mg/kg) nor clonidine (1 mg/kg) produces any significant change on locomotor activity in naive rats.

CONCLUSION

Because idazoxan and clonidine also act through imidazoline receptors, our results suggest that alpha-2 and/or imidazoline receptors are associated with PPI of acoustic startle reflex in rats. Stimulation of these receptors may cause sensorimotor gating disturbances.

A Comparative Study of Chromatographic Behavior and Lipophilicity of Selected Imidazoline Derivatives

Chromatographic behavior and lipophilicity of 20 selected imidazoline derivatives were examined by thin-layer chromatography using CN, RP-2, RP-8 and RP-18 as the stationary phases and a mixture of methanol, water and ammonia as the mobile phase. In all examined chromatographic systems, linear relationships were established between retention parameters and the volume fraction of methanol in the mobile phase (r > 0.985, 0.978, 0.981, 0.988 for the CN, RP-2, RP-8 and RP-18, respectively). The highest correlation between the obtained [Formula: see text] values was observed for RP-2 and RP-8 stationary phases. The experimental lipophilicity indices ([Formula: see text], m and C0) obtained from the retention data were used in correlation study with the calculated logP values. Experimentally determined [Formula: see text] values for all investigated chromatographic systems exhibited the highest correlation with the calculated ClogP values (r: 0.880, 0.872, 0.897 and 0.889 for the CN, RP-2, RP-8 and RP-18 stationary phases, respectively). In addition, principal component analysis enables new information about similarity and differences between tested compounds as well as experimental lipophilicity indices and calculated logP values. Performed QSRR analysis showed that the frequency of C-C at topological distance 1 and CATS2D Lipophilic-Lipophilic at lag 01 were important descriptors with influence on the [Formula: see text] values in all the examined chromatographic systems, while the differences in the retention behavior of compounds on the examined stationary phases can be distinguished based on their specific geometrical, electronic and constitutional properties.

Modulation of imidazoline I2 binding sites by CR4056 relieves postoperative hyperalgesia in male and female rats

BACKGROUND AND PURPOSE

CR4056 is a novel imidazoline-2 (I2 ) ligand exhibiting potent analgesic activity in animal models of pain. In this study, we investigated the effects of CR4056 in a well-established model of postoperative pain where rats develop hyperalgesia in the injured hind paw.

EXPERIMENTAL APPROACH

By measuring paw withdrawal threshold to mechanical pressure, we studied the pharmacology of CR4056, potential sex differences in pain perception and response to treatment, and the pharmacodynamic interaction of CR4056 with morphine.

KEY RESULTS

Oral CR4056 and subcutaneous morphine dose-dependently reversed the hyperalgesic response. Analgesic effects of CR4056 were completely suppressed by the non-selective imidazoline I2 /α2 -adrenoceptor antagonist idazoxan, were partially reduced (~30%; P < 0.05) by the selective α2 -adrenoceptor antagonist yohimbine, but were not influenced by the non-selective I1 /α2 -adrenoceptor antagonist efaroxan or by the μ opioid receptor antagonist naloxone. We found no differences in responses to CR4056 or morphine between male and female rats. However, females had a lower pain threshold than males, and needed lower doses of drugs to reach a significant analgesia. When CR4056 and morphine were combined, their median effective doses were lower than expected for additive effects, both in males and in females. Isobolographic analysis confirmed a synergism between CR4056 and morphine.

CONCLUSIONS AND IMPLICATIONS

CR4056 is a novel pharmacological agent under development for postoperative pain both as stand-alone treatment and in association with morphine. CR4056 has successfully completed Phase I studies for tolerability and pharmacokinetics in healthy volunteers, and is currently entering the first proof-of-concept study in patients.

Interactions between imidazoline I2 receptor ligands and acetaminophen in adult male rats: antinociception and schedule-controlled responding

RATIONALE

Recent evidence suggests that imidazoline I2 receptor ligands are suitable for combination therapy with opioids. Quantitative analysis of I2 receptor ligands combined with non-opioid drugs is necessary for the justification of alternative pain therapies.

OBJECTIVE

This study systematically examined the antihyperalgesic and response rate-suppressing effects of selective I2 receptor ligands (2-BFI and phenyzoline) alone and in combination with acetaminophen.

METHODS

Von Frey and Hargreaves tests were used to examine the antihyperalgesic effects of drugs in complete Freund's adjuvant (CFA)-induced inflammatory pain in rats. Food-reinforced schedule-controlled responding was used to assess the rate-suppressing effects of study drugs. Dose-addition and isobolographic analyses were used to assess drug-drug interactions for all assays.

RESULTS

2-BFI (3.2-17.8 mg/kg, i.p.), phenyzoline (17.8-100 mg/kg, i.p.), and acetaminophen (56-178 mg/kg, i.p.) all dose-dependently produced significant antinociceptive effects. When studied as combinations, 2-BFI and acetaminophen produced infra-additive to additive interactions while phenyzoline and acetaminophen produced additive to supra-additive interactions. The same drug combinations suppressed response rate in a supra-additive manner.

CONCLUSIONS

Quantitative analysis of the antihyperalgesic and response rate-suppressing effects suggests that I2 receptor ligands are not well suited to combination therapy with acetaminophen.

Opioid adjuvant strategy: improving opioid effectiveness

Opioid analgesics continue to be the mainstay of pharmacologic treatment of moderate to severe pain. Many patients, particularly those suffering from chronic pain, require chronic high-dose analgesic therapy. Achieving clinical efficacy and tolerability of such treatment regimens is hampered by the appearance of opioid-induced side effects such as tolerance, hyperalgesia and withdrawal syndrome. Among the therapeutic options to improve the opioid effectiveness, this current review focuses on strategies combining opioids to other drugs that can modulate opioid-mediated effects. We will discuss about experimental evidences reported for several potential opioid adjuvants, including N-methyl-d-aspartate receptor antagonists, 5-HT7 agonists, sigma-1 antagonists, I2-R ligands, cholecystokinin antagonists, neuropeptide FF-R antagonists and toll-like receptor 4 antagonists.

Effects of the imidazoline I2 receptor agonist 2-BFI on the development of tolerance to and behavioural/physical dependence on morphine in rats

BACKGROUND AND PURPOSE

This study examined the effects of imidazoline I2 receptor agonists on the development of tolerance to and physical dependence on repeated morphine treatment in rats.

EXPERIMENTAL APPROACH

Two groups of rats (n = 9 per group) were trained to lever press for sucrose (10%) presentation under a fixed-ratio 10 schedule. The rate-suppressing effects of the opioid receptor ligands morphine and naltrexone and the I2 receptor agonist 2-BFI were examined weekly in rats treated with either daily morphine (20 mg·kg(-1) , s.c.), alone or in combination with 2-BFI (10 mg·kg(-1) ) for 3 weeks. Changes in body weight were measured following naltrexone tests in both groups of rats. In separate experiments, the antinociceptive effects of morphine were assessed using a warm-water tail-withdrawal procedure in rats before and after daily treatments (7 days) with morphine (32 mg·kg(-1) , i.p.) alone or in combination with various doses of the I2 receptor agonists 2-BFI, BU224 and CR4056.

KEY RESULTS

Daily treatment for 3 weeks, with morphine in combination with 2-BFI produced significantly less tolerance to the rate-suppressing effects of morphine and produced a decreased sensitivity to the rate-suppressing effects of naltrexone as well as decreased naltrexone-induced weight loss, compared with morphine-alone group. Repeated treatment for 7 days with morphine produced antinociceptive tolerance, which was attenuated by co-administration with 2-BFI, BU224 or CR4056.

CONCLUSIONS AND IMPLICATIONS

Imidazoline I2 receptor agonists attenuated the development of tolerance to and physical dependence on morphine, further supporting the therapeutic potential of combining I2 receptor agonists and opioids for pain treatment.

Design, synthesis and biological evaluation of novel analgesic agents targeting both cyclooxygenase and TRPV1

Multitarget-directed ligands might offer certain advantages over traditional single-target drugs and/or drug combinations. In the present study, a series of novel analgesic agents targeting both cyclooxygenase and TRPV1 were prepared and evaluated in an effort to optimize properties of previously described lead compounds from piperazine, ethanediamine cores. These compounds were evaluated for antagonism of hTRPV1 activation by capsaicin and the ability to inhibit Ovine COX-1 and human recombinant COX-2 in vitro. The favorable potentials of these test compounds were further characterized in preliminary analgesic and side-effects tests in vivo. On the basis of comprehensive evaluations, compound 8d which showed strong TRPV1 antagonistic activity, middle COX-2 inhibition, weak ulcerogenic action and had no hyperthermia side-effect was considered as a safe candidate for the further development of analgesic drugs.

Immunodetection and subcellular distribution of imidazoline receptor proteins with three antibodies in mouse and human brains: Effects of treatments with I1- and I2-imidazoline drugs

Various imidazoline receptor (IR) proteins have been proposed to mediate the effects of selective I1- and I2-IR drugs. However, the association of these IR-binding proteins with classic I1- and I2-radioligand binding sites remains somewhat controversial. In this study, three IR antibodies (anti-NISCH and anti-nischarin for I1-IRs; and anti-IRBP for I1/I2-IRs) were used to immunodetect, characterize and compare IR protein patterns in brain (mouse and human; total homogenate, subcellular fractionation, grey and white matter) and some cell systems (neurones, astrocytes, human platelets). Various immunoreactive IRs (specific molecular weight bands coincidently detected with the different antibodies) were related to I1-IR (167 kDa, 105/115 kDa and 85 kDa proteins) or I2-IR (66 kDa, 45 kDa and 30 kDa proteins) types. The biochemical characterization of cortical 167 kDa protein, localized in the membrane/cytosol but not in the nucleus, indicated that this I1-IR also forms part of higher order nischarin-related complexes. The contents of I1-IR (167 kDa, 105/115 kDa, and 85 kDa) proteins in mouse brain cortex were upregulated by treatment with I1-drugs (moxonidine, efaroxan) but not with I2-drugs (BU-224, LSL 61122). Conversely, the contents of I2-IR (66 kDa, 45 kDa and 30 kDa) proteins in mouse brain cortex were modulated by treatment with I2-drugs (decreases after BU-224 and LSL 61122, and increases after idazoxan) but not with I1-drugs (with the exception of moxonidine). These findings further indicate that brain immunoreactive IR proteins exist in multiple forms that can be grouped in the already known I1- and I2-IR types, which are expressed both in neurones and astrocytes.

Anti-hyperalgesic effects of imidazoline I2 receptor ligands in a rat model of inflammatory pain: interactions with oxycodone

RATIONALE

Emerging preclinical evidence suggests that imidazoline I2 receptor ligands may be effective analgesics. Quantitative analysis of the combined I2 receptor ligands and opioids is needed for the justification of combination therapy.

OBJECTIVE

This study systematically examined the anti-hyperalgesic and response rate-suppressing effects of selective I2 receptor ligands (2-BFI and phenyzoline) alone and in combination with oxycodone in rats.

METHODS

Von Frey filament test was used to examine the anti-hyperalgesic effects of drugs in a rat model of complete Freund's adjuvant (CFA)-induced inflammatory pain. Schedule-controlled responding was used to assess the rate-altering effects of study drugs. Duration of actions of individual drugs (2-BFI, phenyzoline, and oxycodone) alone or in combination was studied. Dose-addition analysis was employed to assess the anti-hyperalgesic interactions between drugs.

RESULTS

Oxycodone (0.1-3.2 mg/kg, i.p.), 2-BFI (1-17.8 mg/kg, i.p.), and phenyzoline (17.8-56 mg/kg, i.p.) all dose-dependently produced significant antinociceptive effects. When studied as combinations, 2-BFI and oxycodone produced additive interactions while phenyzoline and oxycodone produced supra-additive interactions under all fixed ratios. The same drug combinations did not alter or significantly reduced the operant responding depending on the ratios of the drug combinations.

CONCLUSIONS

Quantitative analysis of the anti-hyperalgesic effects of I2 receptor ligands strongly supports the therapeutic potential of I2 receptor ligands against inflammatory pain. In addition, the data reveal that phenyzoline is superior to the prototypic I2 receptor ligand 2-BFI for the management of pain and warrants further consideration as a novel analgesic.

The imidazoline receptors and ligands in pain modulation

Pain is an unpleasant experience and effects daily routine negatively. Although there are various drugs, many of them are not entirely successful in relieving pain, since pain modulation is a complex process involving numerous mediators and receptors. Therefore, it is a rational approach to identify the factors involved in the complex process and develop new agents that act on these pain producing mechanisms. In this respect, the involvement of the imidazoline receptors in pain modulation has drawn attention in recent years. In this review, it is aimed to focus on the imidazoline receptors and their ligands which contribute to the pain modulation. It is demonstrated that imidazoline-2 (I2) receptors are steady new drug targets for analgesics. Even if the mechanism of I2 receptor is not well known in the modulation of pain, it is known that it plays a role in tonic and chronic pain but not in acute phasic pain. Moreover, the I2 receptor ligands increase the analgesic effects of opioids in both acute and chronic pain and prevent the development of opioid tolerance. So, they are valuable for the chronic pain treatment and also therapeutic coadjuvants in the management of chronic pain with opiate drugs due to the attenuation of opioid tolerance and addiction. Thus, the use of the ligands which bind to the imidazoline receptors is an effective strategy for relieving pain. This educational forum exhibits the role of imidazoline receptors and ligands in pain process by utilizing experimental studies.

Allantoin ameliorates chemically-induced pancreatic β-cell damage through activation of the imidazoline I3 receptors

Objective. Allantoin is the primary active compound in yams (Dioscorea spp.). Recently, allantoin has been demonstrated to activate imidazoline 3 (I3) receptors located in pancreatic tissues. Thus, the present study aimed to investigate the role of allantoin in the effect to improve damage induced in pancreatic β-cells by streptozotocin (STZ) via the I3 receptors.

Research Design and Methods. The effect of allantoin on STZ-induced apoptosis in pancreatic β-cells was examined using the ApoTox-Glo triplex assay, live/dead cell double staining assay, flow cytometric analysis, and Western blottings. The potential mechanism was investigated using KU14R: an I3 receptor antagonist, and U73122: a phospholipase C (PLC) inhibitor. The effects of allantoin on serum glucose and insulin secretion were measured in STZ-treated rats.

Results. Allantoin attenuated apoptosis and cytotoxicity and increased the viability of STZ-induced β-cells in a dose-dependent manner; this effect was suppressed by KU14R and U73112. Allantoin decreased the level of caspase-3 and increased the level of phosphorylated B-cell lymphoma 2 (Bcl-2) expression detected by Western blotting. The improvement in β-cells viability was confirmed using flow cytometry analysis. Daily injection of allantoin for 8 days in STZ-treated rats significantly lowered plasma glucose and increased plasma insulin levels. This action was inhibited by treatment with KU14R.

Conclusion. Allantoin ameliorates the damage of β-cells induced by STZ. The blockade by pharmacological inhibitors indicated that allantoin can activate the I3 receptors through a PLC-related pathway to decrease this damage. Therefore, allantoin and related analogs may be effective in the therapy for β-cell damage.

Panel docking of small-molecule libraries - Prospects to improve efficiency of lead compound discovery

Computational docking as a means to prioritise small molecules in drug discovery projects remains a highly popular in silico screening approach. Contemporary docking approaches without experimental parametrisation can reliably differentiate active and inactive chemotypes in a protein binding site, but the absence of a correlation between the score of a predicted binding pose and the biological activity of the molecule presents a clear limitation. Several novel or improved computational approaches have been developed in the recent past to aid in screening and profiling of small-molecule ligands for drug discovery, but also more broadly in developing conceptual relationships between different protein targets by chemical probing. Among those new methodologies is a strategy known as inverse virtual screening, which involves the docking of a compound into different protein structures. In the present article, we review the different computational screening methodologies that employ docking of atomic models, and, by means of a case study, present an approach that expands the inverse virtual screening concept. By computationally screening a reasonably sized library of 1235 compounds against a panel of 48 mostly human kinases, we have been able to identify five groups of putative lead compounds with substantial diversity when compared to each other. One representative of each of the five groups was synthesised, and tested in kinase inhibition assays, yielding two compounds with micro-molar inhibition in five human kinases. This highly economic and cost-effective methodology holds great promise for drug discovery projects, especially in cases where a group of target proteins share high structural similarity in their binding sites.

Discriminative stimulus effects of the imidazoline I2 receptor ligands BU224 and phenyzoline in rats

Although imidazoline I2 receptor ligands have been used as discriminative stimuli, the role of efficacy of I2 receptor ligands as a critical determinant in drug discrimination has not been explored. This study characterized the discriminative stimulus effects of selective imidazoline I2 receptor ligands BU224 (a low-efficacy I2 receptor ligand) and phenyzoline (a higher efficacy I2 receptor ligand) in rats. Two groups of male Sprague-Dawley rats were trained to discriminate 5.6mg/kg BU224 or 32mg/kg phenyzoline (i.p.) from their vehicle in a two-lever food-reinforced drug discrimination procedure, respectively. All rats acquired the discriminations after an average of 18 (BU224) and 56 (phenyzoline) training sessions, respectively. BU224 and phenyzoline completely substituted for one another symmetrically. Several I2 receptor ligands (tracizoline, CR4056, RS45041, and idazoxan) all occasioned>80% drug-associated lever responding in both discriminations. The I2 receptor ligand 2-BFI and a monoamine oxidase inhibitor harmane occasioned>80% drug-associated lever responding in rats discriminating BU224. Other drugs that occasioned partial or less substitution to BU224 cue included clonidine, methamphetamine, ketamine, morphine, methadone and agmatine. Clonidine, methamphetamine and morphine also only produced partial substitution to phenyzoline cue. Naltrexone, dopamine D2 receptor antagonist haloperidol and serotonin (5-HT)2A receptor antagonist MDL100907 failed to alter the discriminative stimulus effects of BU224 or phenyzoline. Combined, these results are the first to demonstrate that BU224 and phenyzoline can serve as discriminative stimuli and that the low-efficacy I2 receptor ligand BU224 shares similar discriminative stimulus effects with higher-efficacy I2 receptor ligands such as phenyzoline and 2-BFI.

Behavioral effects of the imidazoline I(2) receptor ligand BU99006 in rats

The imidazoline I2 receptor ligand BU99006 binds to and attenuates effects mediated by I2 receptors in vitro, although its effects in vivo have not been studied previously. This study examined the effects of BU99006 in two behavioral assays in rats: hypothermia and 2-BFI discrimination. BU99006 (3.2-15 mg/kg, intraperitoneally) produced a dose-dependent hypothermic effect (rectal temperature), which was antagonized by the I2 receptor antagonist idazoxan. BU99006 (3.2 or 10 mg/kg administered 10 min or 2 h before the session, respectively) did not significantly alter hypothermia produced by the I2 receptor agonist 2-BFI (10 mg/kg). In rats discriminating 5.6 mg/kg 2-BFI, BU99006 (1.78-17.8 mg/kg, intraperitoneally) produced 40 and 82% responding on the 2-BFI-associated lever when it was administered immediately or 2 h before the test sessions, respectively. BU99006 enhanced the discriminative stimulus and rate-suppressing effects of 2-BFI. Collectively, these data suggest that BU99006 is an imidazoline I2 receptor agonist with no evidence of I2 receptor antagonism in rats.

Antihyperalgesic effects of imidazoline I(2) receptor ligands in rat models of inflammatory and neuropathic pain

BACKGROUND AND PURPOSE

A new imidazoline I2 receptor ligand, CR4056, is effective for chronic inflammatory pain and diabetic neuropathy. However, it is unclear whether other I2 receptor ligands have similar effects and whether antinociceptive tolerance develops with repeated treatment.

EXPERIMENTAL APPROACH

The Von Frey filament test was used to measure mechanical hyperalgesia and the plantar test to measure thermal hyperalgesia in rats injected with complete Freund's adjuvant (CFA) treatment or had undergone surgery to induce chronic constriction injury (CCI), models of inflammatory pain and peripheral neuropathic pain respectively. The effects of morphine and I2 receptor ligands, 2-BFI, BU224, tracizoline and CR4056, 3.2-32 mg·kg(-1) , i.p., on hyperalgesia or affective pain (as measured by a place escape/avoidance paradigm) were studied in separate experiments.

KEY RESULTS

Morphine and the I2 receptor ligands (2-BFI, BU224 and tracizoline) all dose-dependently attenuated mechanical and thermal hyperalgesia in CFA-treated rats. The anti-hyperalgesic effects of 2-BFI in CFA-treated and CCI rats were attenuated by the I2 receptor antagonist idazoxan. The combination of 2-BFI and morphine produced additive effects against mechanical hyperalgesia in CFA-treated rats. Repeated treatment (daily for 7-9 days) with 2-BFI or CR4056 did not produce antinociceptive tolerance in CFA-treated or CCI rats. Morphine and the I2 receptor ligands (2-BFI, BU224 and CR4056) were all effective at attenuating place escape/avoidance behaviour in CFA-treated rats.

CONCLUSIONS AND IMPLICATIONS

Imidazoline I2 receptor ligands have antihyperalgesic effects in rat models of inflammatory and neuropathic pain and may represent a new class of pharmacotherapeutics for the management of chronic pain.

Discriminative stimulus effects of the novel imidazoline I₂ receptor ligand CR4056 in rats

This study examined whether a novel imidazoline I₂ receptor ligand CR4056 could serve as a discriminative stimulus and whether it shares similar discriminative stimulus effects with other reported I₂ receptor ligands. Eight male Sprague-Dawley rats were trained to discriminate 10.0 mg/kg CR4056 (i.p.) from vehicle in a two-lever food-reinforced drug discrimination procedure. Once rats acquired the discrimination, substitution and combination studies were conducted to elucidate the underlying receptor mechanisms. All rats acquired CR4056 discrimination after an average of 26 training sessions. Several I₂ receptor ligands (phenyzoline, tracizoline, RS45041, and idazoxan, 3.2–75 mg/kg, i.p.) all occasioned > 80% CR4056-associated lever responding. Other drugs that occasioned partial or no CR4056-associated lever responding included methamphetamine, ketamine, the endogenous imidazoline ligand agmatine, the monoamine oxidase (MAO) inhibitor harmane, the α₂-adrenoceptor agonist clonidine, the μ-opioid receptor agonists morphine and methadone, and the selective I₂ receptor ligands BU224 and 2-BFI. The α₁ adrenoceptor antagonist WB4101, α₂ adrenoceptor antagonist yohimbine and μ-opioid receptor antagonist naltrexone failed to alter the stimulus effects of CR4056. Together, these results show that CR4056 can serve as a discriminative stimulus in rats, which demonstrates high pharmacological specificity and appears to be mediated by imidazoline I₂ receptors.

Gender difference in epileptogenic effects of 2-BFI and BU224 in mice

Imidazoline I2 receptors are involved in pain modulation and psychiatric disorders and its ligands may represent a new therapeutic strategy against pain and depression. In particular, 2-BFI and BU224 are the two most widely studied I2 receptor ligands and have antinociceptive and antidepressant-like activities in rodents. However, little is known of the toxicological effects and potential gender differences of these I2 receptor ligands. This study examined the epileptogenic activities of 2-BFI and BU224 in male and female mice and also examined their underlying receptor mechanisms. 2-BFI (10-40 mg/kg, i.p.) and BU224 (10-40 mg/kg) produced epileptic seizures in a dose-related manner, as did the epileptogenic agent, pentylenetetrazole (PTZ, 15-60 mg/kg). However, female mice were significantly more sensitive than male mice in all the measures. The commonly used I2 receptor antagonist, idazoxan (10mg/kg), did not block the onset and magnitude of the epileptic seizures or lethality induced by 2-BFI and BU224. When studied in combination, PTZ potentiated the epileptogenic effect of 2-BFI and BU224. The lack of antagonism by idazoxan of the epileptogenic activities of 2-BFI and BU224 suggests that the epileptogenic effects of 2-BFI and BU224 are mediated by non-imidazoline I2 receptors and that I2 receptors remain a viable therapeutic target for neurological disorders such as pain.

Participation of central imidazoline binding sites in antinociceptive effect of ethanol and nicotine in rats

Despite synergistic morbidity and mortality, concomitant consumption of alcohol and tobacco is increasing, and their antinociceptive effect has been linked with co-abuse. Present study was designed to investigate the role of imidazoline binding sites in the antinociceptive effect of nicotine, ethanol, and their combination. Separate group of male Sprague-Dawley rats (200-250 g) were treated with different doses of alcohol (0.50-2 g/kg, i.p.) or nicotine (0.25-1 mg/kg, i.p.), and their combination evaluated in tail flick test. Influence of endogenous imidazoline binding site ligands, agonist, and antagonists were determined by their prior treatment with effective or subeffective doses of either ethanol or nicotine. Ethanol, nicotine, or their subeffective dose combination exhibited significant antinociceptive effects in dose-dependent manner. Antinociceptive effect of ethanol and nicotine was significantly augmented by intracerebroventricular (i.c.v.) administration of endogenous imidazoline receptor ligands, harmane (25 μg/rat, i.c.v.) and agmatine (10 μg/rat, i.c.v.), as well as imidazoline I1 /α2 adrenergic receptor agonist, clonidine (2 μg/rat, i.c.v.), I1 agonist moxonidine (25 μg/rat, i.c.v.), and imidazoline I2 agonist, 2-BFI (10 μg/rat, i.c.v.). Conversely, antinociception elicited by ethanol or nicotine or their subeffective dose combination was antagonized by pretreatment with imidazoline I1 antagonist, efaroxan (10 μg/rat, i.c.v.), and I2 antagonist, idazoxan (4 μg/rat, i.c.v.), at their per se ineffective doses. These findings project imidazoline binding ligands as important therapeutic molecules for central antinociceptive activity as well as may reduce the co-abuse potential of alcohol and nicotine.

Chronic treatment with selective I2-imidazoline receptor ligands decreases the content of pro-apoptotic markers in rat brain

Selective I(2)-imidazoline receptor ligands induce neuroprotection through various molecular mechanisms including blockade of N-methyl-D-aspartate (NMDA) receptors. To investigate new neuroprotective mechanisms associated with I(2)-imidazoline receptors, the effects of selective (2-styryl-2-imidazoline (LSL 61122), 2-(2-benzofuranyl)-2-imidazoline (2-BFI), 2-(4,5-dihydroimidazol-2-yl) quinoline hydrochloride (BU-224)) and non-selective (idazoxan) I(2)-drugs on canonical apoptotic pathways were assessed in rat brain cortex. The acute treatment with LSL 61122 (10 mg/kg) reduced the content of mitochondrial (pro-apoptotic) Bax (-33%) and cytochrome c (-31%), which was prevented by idazoxan, an I(2)-receptor antagonist. The sustained stimulation of I(2)-imidazoline receptors with selective drugs (10 mg/kg, every 12 h for seven days) was associated with down-regulation of key components of the extrinsic (Fas receptor: -20%; Fas associated protein with death domain (FADD) adaptor: -47-54%) and/or intrinsic (Bax: -20-23%; cytochrome c: -22-28%) apoptotic signalling and/or up-regulation of survival anti-apoptotic factors (p-Ser194 FADD/FADD ratio: +1.6-2.5-fold; and/or Bcl-2/Bax ratio: +1.5-fold), which in the long-term could dampen cell death in the brain. Similar chronic treatments with LSL 60101 (the imidazole analogue of 2-BFI) and idazoxan (a mixed I(2)/α(2)-ligand) did not induce significant alterations of pro- or anti-apoptotic proteins. The disclosed anti-apoptotic mechanisms of selective I(2)-imidazoline drugs may work in concert with other molecular mechanisms of neuroprotection (e.g. blockade of NMDA receptors) that are engaged by I(2)-ligands.