Neuroprotective effects of the alpha2-adrenoceptor antagonists, (+)-efaroxan and (+/-)-idazoxan, against quinolinic acid-induced lesions of the rat striatum

Effects of Manipulation of Noradrenergic Activities on the Expression of Dopaminergic Phenotypes in Aged Rat Brains

This study investigated the effects of the pharmacological manipulation of noradrenergic activities on dopaminergic phenotypes in aged rats. Results showed that the administration of L-threo-3,4-dihydroxyphenylserine (L-DOPS) for 21 days significantly increased the expression of tyrosine hydroxylase (TH) and dopamine transporter (DAT) in the striatum and substantia nigra (SN) of 23-month-old rats. Furthermore, this treatment significantly increased norepinephrine/DA concentrations in the striatum and caused a deficit of sensorimotor gating as measured by prepulse inhibition (PPI). Next, old rats were injected with the α2-adrenoceptor antagonist 2-methoxy idazoxan or β2-adrenoceptor agonist salmeterol for 21 days. Both drugs produced similar changes of TH and DAT in the striatum and SN. Moreover, treatments with L-DOPS, 2-methoxy idazoxan, or salmeterol significantly increased the protein levels of phosphorylated Akt in rat striatum and SN. However, although a combination of 2-methoxy idazoxan and salmeterol resulted in a deficit of PPI in these rats, the administration of 2-methoxy idazoxan alone showed an opposite behavioral change. The in vitro experiments revealed that treatments with norepinephrine markedly increased mRNAs and proteins of ATF2 and CBP/p300 and reduced mRNA and proteins of HDAC2 and HDAC5 in MN9D cells. A ChIP assay showed that norepinephrine significantly increased CBP/p300 binding or reduced HDAC2 and HDAC5 binding on the TH promoter. The present results indicate that facilitating noradrenergic activity in the brain can improve the functions of dopaminergic neurons in aged animals. While this improvement may have biochemically therapeutic indication for the status involving the degeneration of dopaminergic neurons, it may not definitely include behavioral improvements, as indicated by using 2-methoxy idazoxan only.

The Novel Alpha-2 Adrenoceptor Inhibitor Beditin Reduces Cytotoxicity and Huntingtin Aggregates in Cell Models of Huntington's Disease

Huntington's disease (HD) is a monogenetic neurodegenerative disorder characterized by the accumulation of polyglutamine-expanded huntingtin (mHTT). There is currently no cure, and therefore disease-slowing remedies are sought to alleviate symptoms of the multifaceted disorder. Encouraging findings in Alzheimer's and Parkinson's disease on alpha-2 adrenoceptor (α2-AR) inhibition have shown neuroprotective and aggregation-reducing effects in cell and animal models. Here, we analyzed the effect of beditin, a novel α2- adrenoceptor (AR) antagonist, on cell viability and mHTT protein levels in cell models of HD using Western blot, time-resolved Foerster resonance energy transfer (TR-FRET), lactate dehydrogenase (LDH) and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) cytotoxicity assays. Beditin decreases cytotoxicity, as measured by TUNEL staining and LDH release, in a neuronal progenitor cell model (STHdh cells) of HD and decreases the aggregation propensity of HTT exon 1 fragments in an overexpression model using human embryonic kidney (HEK) 293T cells. α2-AR is a promising therapeutic target for further characterization in HD models. Our data allow us to suggest beditin as a valuable candidate for the pharmaceutical manipulation of α2-AR, as it is capable of modulating neuronal cell survival and the level of mHTT.

Norepinephrine upregulates the expression of tyrosine hydroxylase and protects dopaminegic neurons against 6-hydrodopamine toxicity

As a classic neurotransmitter in the brain, norepinephrine (NE) also is an important modulator to other neuronal systems. Using primary cultures from rat ventral mesencephalon (VM) and dopaminergic cell line MN9D, the present study examined the neuroprotective effects of NE and its effects on the expression of tyrosine hydroxylase (TH). The results showed that NE protected both VM cultures and MN9D cells against 6-hydroxydopamine-caused apoptosis, with possible involvement of adrenal receptors. In addition, treatment with NE upregulated TH protein levels in dose- and time-dependent manner. Further experiments to investigate the potential mechanisms underlying this NE-induced upregulation of TH demonstrated a marked increase in protein levels of the brain-derived neurotrophic factor (BDNF) and the phosphorylated extracellular signal-regulated protein kinase 1 and 2 (pERK1/2) in VM cultures treated with NE. In MN9D cells, a significantly increase of TH and pERK1/2 protein levels were observed after their transfection with BDNF cDNA or exposure to BDNF peptides. Treatment of VM cultures with K252a, an antagonist of the tropomyosin-related kinase B, blocked the upregulatory effects of NE on TH, BDNF and pERK1/2. Administration of MEK1 & MEK2 inhibitors also reversed NE-induced upregulation of TH and pERK1/2. Moreover, ChIP assay showed that treatment with NE or BDNF increased H4 acetylation in the TH promoter. These results suggest that the neuroprotection and modulation of NE on dopaminergic neurons are mediated via BDNF and MAPK/ERK pathways, as well as through epigenetic histone modification, which may have implications for the improvement of therapeutic strategies for Parkinson's disease.

Treatment with the noradrenaline re-uptake inhibitor atomoxetine alone and in combination with the α2-adrenoceptor antagonist idazoxan attenuates loss of dopamine and associated motor deficits in the LPS inflammatory rat model of Parkinson's disease

The impact of treatment with the noradrenaline (NA) re-uptake inhibitor atomoxetine and the α₂-adrenoceptor (AR) antagonist idazoxan in an animal model of Parkinson's disease (PD) was assessed. Concurrent systemic treatment with atomoxetine and idazoxan, a combination which serves to enhance the extra-synaptic availability of NA, exerts anti-inflammatory and neuroprotective effects following delivery of an inflammatory stimulus, the bacterial endotoxin, lipopolysaccharide (LPS) into the substantia nigra. Lesion-induced deficits in motor function (akinesia, forelimb-use asymmetry) and striatal dopamine (DA) loss were rescued to varying degrees depending on the treatment. Treatment with atomoxetine following LPS-induced lesion to the substantia nigra, yielded a robust anti-inflammatory effect, suppressing microglial activation and expression of the pro-inflammatory cytokine TNF-α whilst increasing the expression of neurotrophic factors. Furthermore atomoxetine treatment prevented loss of tyrosine hydroxylase (TH) positive nigral dopaminergic neurons and resulted in functional improvements in motor behaviours. Atomoxetine alone was sufficient to achieve most of the observed effects. In combination with idazoxan, an additional improvement in the impairment of contralateral limb use 7 days post lesion and a reduction in amphetamine-mediated rotational asymmetry 14 days post-lesion was observed, compared to atomoxetine or idazoxan treatments alone. The results indicate that increases in central NA tone has the propensity to regulate the neuroinflammatory phenotype in vivo and may act as an endogenous neuroprotective mechanism where inflammation contributes to the progression of DA loss. In accordance with this, the clinical use of agents such as NA re-uptake inhibitors and α₂-AR antagonists may prove useful in enhancing the endogenous neuroimmunomodulatory potential of NA in conditions associated with brain inflammation.

Role of catalpol in ameliorating the pathogenesis of experimental autoimmune encephalomyelitis by increasing the level of noradrenaline in the locus coeruleus

The endogenous neurotransmitter, noradrenaline, exerts anti-inflammatory and neuroprotective effects in vivo and in vitro. Reduced noradrenaline levels results in increased inflammation and neuronal damage. The primary source of noradrenaline in the central nervous system is tyrosine hydroxylase (TH)-positive neurons, located in the locus coeruleus (LC). TH is the rate-limiting enzyme for noradrenaline synthesis; therefore, regulation of TH protein expression and intrinsic enzyme activity represents the central means for controlling the synthesis of noradrenaline. Catalpol is an iridoid glycoside purified from Rehmannia glutinosa Libosch, which exerts a neuroprotective effect in multiple sclerosis (MS). The present study used an experimental mouse model of autoimmune encephalomyelitis to verify the neuroprotective effects of catalpol. Significant improvements in the clinical scores were observed in catalpol-treated mice. Furthermore, catalpol increased TH expression and increased noradrenaline levels in the spinal cord. In primary cultures, catalpol exerted a neuroprotective effect in rat LC neurons by increasing the noradrenaline output. These results suggested that drugs targeting LC survival and function, including catalpol, may be able to benefit patients with MS.

Causes, consequences, and cures for neuroinflammation mediated via the locus coeruleus: noradrenergic signaling system

Aside from its roles in as a classical neurotransmitter involved in regulation of behavior, noradrenaline (NA) has other functions in the CNS. This includes restricting the development of neuroinflammatory activation, providing neurotrophic support to neurons, and providing neuroprotection against oxidative stress. In recent years, it has become evident that disruption of physiological NA levels or signaling is a contributing factor to a variety of neurological diseases and conditions including Alzheimer's disease (AD) and Multiple Sclerosis. The basis for dysregulation in these diseases is, in many cases, due to damage occurring to noradrenergic neurons present in the locus coeruleus (LC), the major source of NA in the CNS. LC damage is present in AD, multiple sclerosis, and a large number of other diseases and conditions. Studies using animal models have shown that experimentally induced lesion of LC neurons exacerbates neuropathology while treatments to compensate for NA depletion, or to reduce LC neuronal damage, provide benefit. In this review, we will summarize the anti-inflammatory and neuroprotective actions of NA, summarize examples of how LC damage worsens disease, and discuss several approaches taken to treat or prevent reductions in NA levels and LC neuronal damage. Further understanding of these events will be of value for the development of treatments for AD, multiple sclerosis, and other diseases and conditions having a neuroinflammatory component. The classical neurotransmitter noradrenaline (NA) has critical roles in modulating behaviors including those involved in sleep, anxiety, and depression. However, NA can also elicit anti-inflammatory responses in glial cells, can increase neuronal viability by inducing neurotrophic factor expression, and can reduce neuronal damage due to oxidative stress by scavenging free radicals. NA is primarily produced by tyrosine hydroxylase (TH) expressing neurons in the locus coeruleus (LC), a relatively small brainstem nucleus near the IVth ventricle which sends projections throughout the brain and spinal cord. It has been known for close to 50 years that LC neurons are lost during normal aging, and that loss is exacerbated in neurological diseases including Parkinson's disease and Alzheimer's disease. LC neuronal damage and glial activation has now been documented in a variety of other neurological conditions and diseases, however, the causes of LC damage and cell loss remain largely unknown. A number of approaches have been developed to address the loss of NA and increased inflammation associated with LC damage, and several methods are being explored to directly minimize the extent of LC neuronal cell loss or function. In this review, we will summarize some of the consequences of LC loss, consider several factors that likely contribute to that loss, and discuss various ways that have been used to increase NA or to reduce LC damage. This article is part of the 60th Anniversary special issue.

Neuroprotective properties of cannabigerol in Huntington's disease: studies in R6/2 mice and 3-nitropropionate-lesioned mice

Different plant-derived and synthetic cannabinoids have shown to be neuroprotective in experimental models of Huntington's disease (HD) through cannabinoid receptor-dependent and/or independent mechanisms. Herein, we studied the effects of cannabigerol (CBG), a nonpsychotropic phytocannabinoid, in 2 different in vivo models of HD. CBG was extremely active as neuroprotectant in mice intoxicated with 3-nitropropionate (3NP), improving motor deficits and preserving striatal neurons against 3NP toxicity. In addition, CBG attenuated the reactive microgliosis and the upregulation of proinflammatory markers induced by 3NP, and improved the levels of antioxidant defenses that were also significantly reduced by 3NP. We also investigated the neuroprotective properties of CBG in R6/2 mice. Treatment with this phytocannabinoid produced a much lower, but significant, recovery in the deteriorated rotarod performance typical of R6/2 mice. Using HD array analysis, we were able to identify a series of genes linked to this disease (e.g., symplekin, Sin3a, Rcor1, histone deacetylase 2, huntingtin-associated protein 1, δ subunit of the gamma-aminobutyric acid-A receptor (GABA-A), and hippocalcin), whose expression was altered in R6/2 mice but partially normalized by CBG treatment. We also observed a modest improvement in the gene expression for brain-derived neurotrophic factor (BDNF), insulin-like growth factor-1 (IGF-1), and peroxisome proliferator-activated receptor-γ (PPARγ), which is altered in these mice, as well as a small, but significant, reduction in the aggregation of mutant huntingtin in the striatal parenchyma in CBG-treated animals. In conclusion, our results open new research avenues for the use of CBG, alone or in combination with other phytocannabinoids or therapies, for the treatment of neurodegenerative diseases such as HD.

Noradrenergic regulation of glial activation: molecular mechanisms and therapeutic implications

It has been known for many years that the endogenous neurotransmitter noradrenaline (NA) exerts anti-inflammatory and neuroprotective effects both in vitro and in vivo. In many cases the site of action of NA are beta-adrenergic receptors (βARs), causing an increase in intracellular levels of cAMP which initiates a broad cascade of events including suppression of inflammatory transcription factor activities, alterations in nuclear localization of proteins, and induction of patterns of gene expression mediated through activity of the CREB transcription factor. These changes lead not only to reduced inflammatory events, but also contribute to neuroprotective actions of NA by increasing expression of neurotrophic substances including BDNF, GDNF, and NGF. These properties have prompted studies to determine if treatments with drugs to raise CNS NA levels could provide benefit in various neurological conditions and diseases having an inflammatory component. Moreover, increasing evidence shows that disruptions in endogenous NA levels occurs in several diseases and conditions including Alzheimer's disease (AD), Parkinson's disease (PD), Down's syndrome, posttraumatic stress disorder (PTSD), and multiple sclerosis (MS), suggesting that damage to NA producing neurons is a common factor that contributes to the initiation or progression of neuropathology. Methods to increase NA levels, or to reduce damage to noradrenergic neurons, therefore represent potential preventative as well as therapeutic approaches to disease.

The therapeutic potential of G-protein coupled receptors in Huntington's disease

Huntington's disease is a late-onset autosomal dominant inherited neurodegenerative disease characterised by increased symptom severity over time and ultimately premature death. An expanded CAG repeat sequence in the huntingtin gene leads to a polyglutamine expansion in the expressed protein, resulting in complex dysfunctions including cellular excitotoxicity and transcriptional dysregulation. Symptoms include cognitive deficits, psychiatric changes and a movement disorder often referred to as Huntington's chorea, which involves characteristic involuntary dance-like writhing movements. Neuropathologically Huntington's disease is characterised by neuronal dysfunction and death in the striatum and cortex with an overall decrease in cerebral volume (Ho et al., 2001). Neuronal dysfunction begins prior to symptom presentation, and cells of particular vulnerability include the striatal medium spiny neurons. Huntington's is a devastating disease for patients and their families and there is currently no cure, or even an effective therapy for disease symptoms. G-protein coupled receptors are the most abundant receptor type in the central nervous system and are linked to complex downstream pathways, manipulation of which may have therapeutic application in many neurological diseases. This review will highlight the potential of G-protein coupled receptor drug targets as emerging therapies for Huntington's disease.

From the cell to the clinic: a comparative review of the partial D₂/D₃receptor agonist and α2-adrenoceptor antagonist, piribedil, in the treatment of Parkinson's disease

Though L-3,4-dihydroxyphenylalanine (L-DOPA) is universally employed for alleviation of motor dysfunction in Parkinson's disease (PD), it is poorly-effective against co-morbid symptoms like cognitive impairment and depression. Further, it elicits dyskinesia, its pharmacokinetics are highly variable, and efficacy wanes upon long-term administration. Accordingly, "dopaminergic agonists" are increasingly employed both as adjuncts to L-DOPA and as monotherapy. While all recognize dopamine D(2) receptors, they display contrasting patterns of interaction with other classes of monoaminergic receptor. For example, pramipexole and ropinirole are high efficacy agonists at D(2) and D(3) receptors, while pergolide recognizes D(1), D(2) and D(3) receptors and a broad suite of serotonergic receptors. Interestingly, several antiparkinson drugs display modest efficacy at D(2) receptors. Of these, piribedil displays the unique cellular signature of: 1), signal-specific partial agonist actions at dopamine D(2)and D(3) receptors; 2), antagonist properties at α(2)-adrenoceptors and 3), minimal interaction with serotonergic receptors. Dopamine-deprived striatal D(2) receptors are supersensitive in PD, so partial agonism is sufficient for relief of motor dysfunction while limiting undesirable effects due to "over-dosage" of "normosensitive" D(2) receptors elsewhere. Further, α(2)-adrenoceptor antagonism reinforces adrenergic, dopaminergic and cholinergic transmission to favourably influence motor function, cognition, mood and the integrity of dopaminergic neurones. In reviewing the above issues, the present paper focuses on the distinctive cellular, preclinical and therapeutic profile of piribedil, comparisons to pramipexole, ropinirole and pergolide, and the core triad of symptoms that characterises PD-motor dysfunction, depressed mood and cognitive impairment. The article concludes by highlighting perspectives for clarifying the mechanisms of action of piribedil and other antiparkinson agents, and for optimizing their clinical exploitation.

Effects of the noradrenergic system in rat white matter exposed to oxygen-glucose deprivation in vitro

Norepinephrine (NE) is released in excess into the extracellular space during oxygen-glucose deprivation (OGD) in brain, increasing neuronal metabolism and aggravating glutamate excitoxicity. We used isolated rat optic nerve and spinal cord dorsal columns to determine whether the noradrenergic system influences axonal damage in white matter. Tissue was studied electrophysiologically by recording the compound action potential (CAP) before and after exposure to 60 min of OGD at 36 degrees C. Depleting catecholamine stores with reserpine was protective and improved CAP recovery after 1 h of reperfusion from 17% (control) to 35%. Adding NE during OGD decreased CAP recovery to 8%, and adding NE to reserpine during OGD eliminated the protective effect of the latter. Selective inhibitors of Na(+)-dependent norepinephrine transport desipramine and nisoxetine improved recovery to 58% and 44%, respectively. alpha2 adrenergic receptor agonists UK14,304 and medetomidine improved CAP recovery to 41% and 46% after 1 h of OGD. Curiously, alpha2 antagonists alone were also highly protective (e.g., atipamezole: 86% CAP recovery), at concentrations that did not affect baseline excitability. The protective effect of alpha2 receptor modulation was corroborated by imaging fluorescent Ca(2+) and Na(+) indicators within axons during OGD. Both agonists and antagonists significantly reduced axonal Ca(2+) and Na(+) accumulation in injured axons. These data suggest that the noradrenergic system plays an active role in the pathophysiology of axonal ischemia and that alpha2 receptor modulation may be useful against white matter injury.

Novelty determines the effects of olfactory enrichment on memory and neurogenesis through noradrenergic mechanisms

Commonly used experimental paradigms of environmental enrichment combine increased social interactions and sensory inputs and renewal of the objects present in the environment. However, the specific contribution of novelty to the effects of enrichment is unclear. Here, we show that repeated daily exposure to single novel odorants and not to an enriched but stable olfactory environment improves short-term olfactory memory and neurogenesis in the mouse olfactory bulb. In addition, these positive effects are mediated by noradrenalin as they are blocked by a noradrenergic receptor antagonist. These data suggest that novelty recognition and noradrenergic mechanisms are crucial in mediating neural plasticity induced by olfactory enrichment.

Delivery of peptide and protein drugs over the blood-brain barrier

Peptide and protein (P/P) drugs have been identified as showing great promises for the treatment of various neurodegenerative diseases. A major challenge in this regard, however, is the delivery of P/P drugs over the blood-brain barrier (BBB). Intense research over the last 25 years has enabled a better understanding of the cellular and molecular transport mechanisms at the BBB, and several strategies for enhanced P/P drug delivery over the BBB have been developed and tested in preclinical and clinical-experimental research. Among them, technology-based approaches (comprising functionalized nanocarriers and liposomes) and pharmacological strategies (such as the use of carrier systems and chimeric peptide technology) appear to be the most promising ones. This review combines a comprehensive overview on the current understanding of the transport mechanisms at the BBB with promising selected strategies published so far that can be applied to facilitate enhanced P/P drug delivery over the BBB.

Negative regulation of caspase-3 expression in the neonatal cerebral cortex by alpha2A-adrenoceptors

Antisense oligonucleotide to alpha2A-adrenoceptors increased the levels of mRNA (reverse polymerase chain reaction) and protein for a key executioner protease of apoptosis caspase-3 (immunoblotting assay) in the cerebral cortex of newborn rats. The relationship between the observed effect and low expression of alpha2-adrenoceptors was confirmed by the possibility of correcting this phenomenon by clonidine (stimulator of alpha2-adrenoceptors).

Evidence for imidazoline receptors involvement in the agmatine antidepressant-like effect in the forced swimming test

This study investigated the involvement of the imidazoline receptors in the antidepressant-like effect of agmatine in the forced swimming test. The antidepressant-like effects of agmatine (10 mg/kg, i.p.) in the forced swimming test was blocked by pretreatment of mice with efaroxan (1 mg/kg, i.p., an imidazoline I1/alpha2-adrenoceptor antagonist), idazoxan (0.06 mg/kg, i.p., an imidazoline I2/alpha2-adrenoceptor antagonist) and antazoline (5 mg/kg, i.p., a ligand with high affinity for the I2 receptor). A subeffective dose of agmatine (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with clonidine (0.06 mg/kg, i.p, an imidazoline I1/alpha2-adrenoceptor agonist), moxonidine (0.5 mg/kg, i.p., an imidazoline I1/alpha2-adrenoceptor agonist), antazoline (1 mg/kg, i.p.) and MK-801 (0.001 mg/kg, i.p., a non-competitive NMDA receptor antagonist), but not with efaroxan (1 mg/kg, i.p.) and idazoxan (0.06 mg/kg, i.p.). Pretreatment of mice with yohimbine (1 mg/kg, i.p., an alpha2-adrenoceptor antagonist) blocked the synergistic antidepressant-like effect of agmatine (0.001 mg/kg, i.p.) with clonidine (0.06 mg/kg, i.p). A subeffective dose of MK-801 (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with antazoline (5 mg/kg, i.p.), but not with efaroxan (1 mg/kg, i.p.) or idazoxan (0.06 mg/kg, i.p.). In conclusion, this study suggests that the anti-immobility effect of agmatine in the forced swimming test is dependent on its interaction with imidazoline I1 and I2 receptors.

Beta-amyloid-dependent expression of NOS2 in neurons: prevention by an alpha2-adrenergic antagonist

The neurotransmitter noradrenaline (NA) exerts important antiinflammatory effects on glial cells including suppression of the inducible form of nitric oxide synthase (NOS2). The authors examined the consequences of manipulating NA in vivo by treating adult rats with the neurotoxin DSP4, which selectively lesions noradrenergic neurons of the locus ceruleus (LC), and reduces cortical NA levels. Following LC lesion, intracortical injection of aggregated amyloid beta 1-42 (Abeta1-42) caused appearance of NOS2 within neurons, and increased neuronal damage assessed by staining for nonphosphorylated neurofilament proteins with antibody SMI-32. Co-treatment with a selective alpha2-adrenergic antagonist reduced neuronal NOS2 staining as well as SMI-32 staining. Neuronal damage was dependent on NOS2 expression since injection of Abeta1-42 into DSP4-treated NOS2-deficient mice did not result in neuronal damage. These results demonstrate that decrease of NA levels in vivo can exacerbate inflammatory responses and neuronal damage due to inflammatory stimuli such as Abeta. These findings suggest that alpha2-adrenergic antagonists could provide therapeutic benefit in neurological diseases such as AD or PD where LC loss is known to occur.

Regional and subtype selective changes of neurotransmitter receptor density in a rat transgenic for the Huntington's disease mutation

Huntington's disease (HD) is an autosomal dominantly inherited progressive neurodegenerative disorder caused by a CAG/polyglutamine repeat expansion in the gene encoding the huntingtin protein. We have recently generated a rat model transgenic for HD, which displays a slowly progressive phenotype resembling the human adult-onset type of disease. In this study we systematically assessed the distribution and density of 17 transmitter receptors in the brains of 2-year-old rats using quantitative multi-tracer autoradiography and high-resolution positron emission tomography. Heterozygous animals expressed increased densities of M(2) acetylcholine (increase of 148 +/- 16% of controls; p > 0.001; n = 7), nicotine (increase of 149 +/- 16% of controls; p > 0.01; n = 6), and alpha(2) noradrenergic receptors (increase of 141 +/- 15% of controls; p > 0.001; n = 6), respectively. Densities of these receptors were decreased in homozygous animals. Decreases of receptor density in both hetero- and homozygous animals were found for M1 acetylcholine, 5-HT 2A serotonin, A 2A adenosine, D1 and D2 dopamine, and GABA(A) receptors, respectively. Other investigated receptor systems showed small changes or were not affected. The present data suggest that the moderate increase of CAG/polyglutamine repeat expansions in the present rat model of Huntington's disease is characterized by subtype-selective and region-specific changes of neuroreceptor densities. In particular, there is evidence for a contribution of predominantly presynaptically localized cholinergic and noradrenergic receptors in the response to Huntington's disease pathology.

Activation of noradrenergic transmission by alpha2-adrenoceptor antagonists counteracts deafferentation-induced neuronal death and cell proliferation in the adult mouse olfactory bulb

The olfactory bulb is the target of neural progenitor cells that are generated in the subventricular zone of the lateral ventricle in the adult brain. This permanent neurogenesis is likely influenced by olfactory input to the bulb since previous studies have shown that cell proliferation and/or apoptotic death are stimulated by naris closure or surgical transection of the olfactory nerve. Since the olfactory bulb is densely innervated by noradrenergic afferents originating in the locus coeruleus, we have studied the impact of pharmacologically activating this noradrenergic system on cell death and proliferation following unilateral olfactory axotomy in the adult mouse olfactory bulb. We found that noradrenaline release in the olfactory bulb was significantly increased by intraperitoneal injections of the selective alpha(2)-adrenoceptor antagonists, dexefaroxan (0.63 mg/kg) and 5-fluoro-methoxyidazoxan (F 14413; 0.16 mg/kg). A chronic treatment with either compound for 7 days following olfactory axotomy significantly reduced neuronal death, glial activation and cell proliferation in the deafferented olfactory bulb. These data (1) confirm that alpha(2)-adrenoceptor antagonists, presumably by facilitating central noradrenergic transmission, afford neuroprotection in vivo, as previously shown in models of cerebral ischemia, excitotoxicity and devascularization-induced neurodegeneration, and (2) support a role of the locus coeruleus noradrenergic system in promoting survival of neurons in areas of the brain where neurogenesis persists in the adult.

The paradigm of Huntington's disease: therapeutic opportunities in neurodegeneration

Despite a relatively small number of affected patients, Huntington's disease (HD) has been a historically important disease, embodying many of the major themes in modern neuroscience, including molecular genetics, selective neuronal vulnerability, excitotoxicity, mitochondrial dysfunction, apoptosis, and transcriptional dysregulation. The discovery of the HD gene in 1993 opened the door to the mechanisms of HD pathogenesis. Multiple pathologic mechanisms have been discovered, each one serving as a potential therapeutic target. HD thus continues to serve as a paradigmatic disorder, with basic bench research generating clinically relevant insights and stimulating the development of therapeutic human trials.

Reduced MPTP toxicity in noradrenaline transporter knockout mice

The noradrenergic neurons of the locus coeruleus (LC) are damaged in Parkinson's disease (PD). Neurotoxin ablation of the LC noradrenergic neurons has been shown to exacerbate the dopaminergic toxicity of MPTP, suggesting that the noradrenergic system protects dopamine neurons. We utilized mice that exhibit elevated synaptic noradrenaline (NA) by genetically deleting the noradrenaline transporter (NET), a key regulator of the noradrenergic system (NET KO mice). NET KO and wild-type littermates were administered MPTP and striatal dopamine terminal integrity was assessed by HPLC of monoamines, immmunoblotting for dopaminergic markers and tyrosine hydroxylase (TH) immunohistochemistry. MPTP significantly reduced striatal dopamine in wild-type mice, but not in the NET KO mice. To confirm that the protection observed in the NET KO mice was due to the lack of NET, we treated wild-type mice with the specific NET inhibitor, nisoxetine, and then challenged them with MPTP. Nisoxetine conferred protection to the dopaminergic system. These data indicate that NA can modulate MPTP toxicity and suggest that manipulation of the noradrenergic system may have therapeutic value in PD.

In vivo upregulation of endogenous NGF in the rat brain by the alpha2-adrenoreceptor antagonist dexefaroxan: potential role in the protection of the basalocortical cholinergic system during neurodegeneration

We have previously reported that the alpha2-adrenoceptor antagonist dexefaroxan protects against the degeneration of nucleus basalis magnocellularis (NbM) cholinergic neurons following cortical devascularization in the adult rat. Since nerve growth factor (NGF) is critical to the survival of NbM cholinergic neurons in the adult brain and its synthesis is known to be regulated by noradrenergic mechanisms, we examined whether the protective effect of dexefaroxan in the devascularization model was associated with regional induction of NGF biosynthesis. Dexefaroxan or vehicle was administered to rats via subcutaneous minipumps for 28 days following devascularization or sham operation procedures. In vehicle-treated devascularized rats, NGF protein levels in the cortex were increased at 5 days but had normalized by 2 weeks postoperation; NGF levels in NbM remained unchanged during this time. In dexefaroxan-treated devascularized rats, increases in NGF protein levels (2-fold) and immunoreactivity were maintained in both the cortex and NbM over the entire 28-day postoperation period; these increases were coincident with changes in functional markers characteristic of NGF's actions, including increases in choline acetyltransferase (ChAT), p75 and TrkA immunoreactivities, and a preservation of NbM cholinergic cell numbers. Dexefaroxan also increased NGF protein levels in sham-operated rats, but without any significant consequence to the otherwise normal NbM cholinergic phenotype in these animals. Results indicate that activation of endogenous NGF systems could contribute to the cholinergic protective effect of dexefaroxan in the cortical devascularization model, and provide further support for a potential therapeutic utility of dexefaroxan in neurodegenerative diseases where central cholinergic function is progressively compromised.

Treatment with alpha2-adrenoceptor antagonist, 2-methoxy idazoxan, protects 6-hydroxydopamine-induced Parkinsonian symptoms in rats: neurochemical and behavioral evidence

Noradrenaline, not only functions as a synaptic transmitter, but also promotes neural differentiation and regenerative processes. In Parkinson's disease, besides the dopaminergic degeneration, noradrenergic neurons of locus coeruleus origin degenerate as well. Drugs enhancing noradrenergic transmission in the locus coeruleus (e.g. alpha2-adrenoceptor antagonists) have been shown to be neuroprotective against Huntington's and ischemic animal models. However, in Parkinsonian animal models, most of the studies evaluated the worsening of experimental nigral neurodegeneration after locus coeruleus lesions. Here, it has been tested, whether treatment with the selective alpha2-adrenoceptor antagonist, 2-methoxy idazoxan (2.5 mg/kg i.p., twice daily for 5 days), before an experimental lesion to nigra, protects dopaminergic neurodegeneration. Dopaminergic degeneration was produced by 6-hydroxydopamine lesion in the median forebrain bundle. The concentrations of dopamine, 5-hydroxytryptamine and its metabolites were analysed in the various regions of the basal ganglia. The concentrations of noradrenaline and dopamine were measured in the regions innervated by locus coeruleus neurons and in the basal ganglia respectively, after 2-methoxy idazoxan treatment. The Parkinsonian behavior was assessed by catalepsy and activity test. 2-Methoxy idazoxan specifically increased the concentration of noradrenaline in the brain regions, innervated by locus coeruleus neurons. 6-OHDA lesion strongly depleted the concentration of dopamine and its metabolites in the striatum and SN, producing catalepsy and hypoactivity. Multiple treatments with 2-methoxy idazoxan reduced some of the observed neurochemical and behavioral indices of 6-hydroxydopamine-induced Parkinsonism, indicating neuroprotection. Although the mechanism underlying the neuroprotective property remains elusive, the therapeutic usage of alpha2-antagonists might be helpful in slowing the neuronal death and progression of Parkinson's disease.

Clonidine increases caspase-3 mRNA level and DNA fragmentation in the developing rat brainstem

The densities of alpha2-adrenergic receptors, labeled by 3H-clonidine or 3H-RX821002, reach a peak in the rat brainstem during the first week of its life. This enables the agonist of alpha2-adrenergic receptor clonidine, which is used as a component of anaesthetic solution in infants and children, to have specific effects in this structure of the developing brain. Clonidine was injected into the fetal brain (5 microg in 5 microl of saline) or subcutaneously to the pups (1, 10 microg in 50 microl of saline) 3 days before investigation. Clonidine increased the level of apoptotic enzyme caspase-3 mRNA expression, as measured by RT-PCR and enhanced the DNA fragmentation, as determined by gel electrophoresis, in the brainstem of the 21-day-old fetuses and 8-day-old rats. In the cortex of 8-day-old rat, the alpha2-adrenergic receptors are at a much lower level than the brainstem. Clonidine treatment had no evident effects on caspase-3 mRNA level and DNA fragmentation in the cortex of an 8-day-old rat. The data suggest that clonidine facilitates cell death in the developing brainstem. This drug effect provides a potential mechanism whereby clonidine during early life could induce long-lasting alterations in brain neurochemistry, autonomic functions and behavior.

Noradrenergic mechanisms in neurodegenerative diseases: a theory

A deficiency in the noradrenergic system of the brain, originating largely from cells in the locus coeruleus (LC), is theorized to play a critical role in the progression of a family of neurodegenerative disorders that includes Parkinson's disease (PD) and Alzheimer's disease (AD). Consideration is given here to evidence that several neurodegenerative diseases and syndromes share common elements, including profound LC cell loss, and may in fact be different manifestations of a common pathophysiological process. Findings in animal models of PD indicate that the modification of LC-noradrenergic activity alters electrophysiological, neurochemical and behavioral indices of neurotransmission in the nigrostriatal dopaminergic system, and influences the response of this system to experimental lesions. In models related to AD, noradrenergic mechanisms appear to play important roles in modulating the activity of the basalocortical cholinergic system and its response to injury, and to modify cognitive functions including memory and attention. Mechanisms by which noradrenaline may protect or promote recovery from neural damage are reviewed, including effects on neuroplasticity, neurotrophic factors, neurogenesis, inflammation, cellular energy metabolism and excitotoxicity, and oxidative stress. Based on evidence for facilitatory effects on transmitter release, motor function, memory, neuroprotection and recovery of function after brain injury, a rationale for the potential of noradrenergic-based approaches, specifically alpha2-adrenoceptor antagonists, in the treatment of central neurodegenerative diseases is presented.

Protective effect of the α2-adrenoceptor antagonist, dexefaroxan, against spatial memory deficit induced by cortical devascularization in the adult rat

The alpha2-adrenoceptor antagonist, dexefaroxan, has been shown in the rat to have neuroprotective and plastic effects against degenerative structural changes in elements of the basalocortical cholinergic system that result from cortical devascularization [Neuroscience 115 (2002) 41]. The present study, using the same experimental protocol, examined the functional consequences of cortical devascularization and dexefaroxan treatment in the Morris water maze memory test. Rats were first trained to find the hidden platform in the test, and then subjected to the devascularization procedure. Thirty-one days later, lesioned rats exhibited a significant deficit in recalling the platform location, compared with sham control animals. A 28-day subcutaneous infusion with dexefaroxan (0.63, 2.5, and 10 mg rat(-1) day(-1)), starting from the moment of the devascularization, protected against this spatial memory deficit.

Dissimilar pharmacological responses by a new series of imidazoline derivatives at precoupled and ligand-activated alpha 2A-adrenoceptor states: evidence for effector pathway-dependent differential antagonism

Whereas agonist-directed differential signaling at a single receptor subtype has become an accepted pharmacological concept, distinct behaviors by ligands that are assumed to be antagonists is less documented. The intrinsic activity and capacity of antagonism for a new series of imidazoline-derived adrenergic ligands analogous to dexefaroxan were investigated by measuring two distinct signaling pathways at the recombinant human alpha 2A-adrenoceptor (alpha 2A AR): 1) pertussis toxin-resistant guanosine 5'-O-(3-[35S]thio)triphosphate ([35S]GTP gamma S) binding responses mediated by either a recombinant G alpha oCys351Ile or G alpha i2Cys352Ile protein in CHO-K1 cells, and 2) inhibition of cAMP formation in a stably transfected C6-glial cell line. Ligands could be differentiated as inverse agonists [i.e., 2-(4-methoxy-2-ethyl-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 851062], neutral antagonists [i.e., 2-(4-hydroxy-2-ethyl-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 851057], partial [i.e., 2-(4-chloro-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 821008], and high-efficacy [i.e., 2-(6,7-dichloro-2,3-dihydrobenzofuran-2-yl)-4,5-dihydro-1H-imidazole; RX 821010] agonists at a precoupled alpha 2A AR state in the copresence of a G alpha oCys351Ile protein but not G alpha i2Cys352Ile protein by monitoring [35S]GTP gamma S binding responses. Neither positive nor negative efficacy was observed for these compounds by monitoring the adenylate cyclase pathway at a presumably low-affinity alpha 2A AR state. The capacity of the dexefaroxan analogs to antagonize the (-)-epinephrine-mediated [35S]GTP gamma S binding response at a G alpha oCys351Ile protein was inversely correlated with their magnitude of intrinsic activity and unrelated to their ligand binding affinity for the alpha 2A AR. On the other hand, their capacity to antagonize either (-)-epinephrine or 5-bromo-6-(2-imidazolin-2-ylamino)quinoxaline tartrate (UK 14304)-mediated inhibition of forskolin-stimulated cAMP formation was not related with the rank order of antagonist capacity for the (-)-epinephrine-mediated [35S]GTP gamma S binding response. In conclusion, these data demonstrate that certain alpha2 AR ligands that are assumed to be antagonists, may yield dissimilar pharmacological responses, dependent on the investigated agonist-stimulated effector pathway.

Effects of the alpha 2-adrenoreceptor antagonist dexefaroxan on neurogenesis in the olfactory bulb of the adult rat in vivo: selective protection against neuronal death

A dysfunction of noradrenergic mechanisms originating in the locus coeruleus has been hypothesised to be the critical factor underlying the evolution of central neurodegenerative diseases [Colpaert FC (1994) Noradrenergic mechanism Parkinson's disease: a theory. In: Noradrenergic mechanisms in Parkinson's disease (Briley M, Marien M, eds) pp 225-254. Boca Raton, FL, USA: CRC Press Inc.]. alpha(2)-Adrenoceptor antagonists, presumably in part by facilitating central noradrenergic transmission, afford neuroprotection in vivo in models of cerebral ischaemia, excitotoxicity and devascularization-induced neurodegeneration. The present study utilised the rat olfactory bulb as a model system for examining the effects of the selective alpha(2)-adrenoceptor antagonist dexefaroxan upon determinants of neurogenesis (proliferation, survival and death) in the adult brain in vivo. Cell proliferation (5-bromo-2'-deoxyuridine labelling) and cell death associated with DNA fragmentation (terminal dideoxynucleotidyl transferase-catalysed 2'-deoxyuridine-5'-triphosphate nick end-labelling assay) were quantified following a 7-day treatment with either vehicle or dexefaroxan (0.63 mg/kg i.p., three times daily), followed by a 3-day washout period. The number of terminal dideoxynucleotidyl transferase-catalysed 2'-deoxyuridine-5'-triphosphate nick end-labelling-positive nuclei in the olfactory bulb was lower in dexefaroxan-treated rats, this difference being greatest and significant in the subependymal layer (-52%). In contrast, 5-bromo-2'-deoxyuridine-immunoreactive nuclei were more numerous (+68%) in the bulbs of dexefaroxan-treated rats whilst no differences were detected in the proliferating region of the subventricular zone. Terminal dideoxynucleotidyl transferase-catalysed 2'-deoxyuridine-5'-triphosphate nick end-labelling combination with glial fibrillary acidic protein or neuronal-specific antigen immunohistochemistry revealed that terminal dideoxynucleotidyl transferase-catalysed 2'-deoxyuridine-5'-triphosphate nick end-labelling-positive nuclei were associated primarily with a neuronal cell phenotype. These findings suggest that dexefaroxan increases neuron survival in the olfactory bulb of the adult rat in vivo, putatively as a result of reducing the apoptotic fate of telencephalic stem cell progenies.

Protective effects of the alpha 2-adrenoceptor antagonist, dexefaroxan, against degeneration of the basalocortical cholinergic system induced by cortical devascularization in the adult rat

It has been hypothesized [Colpaert, F.C., 1994. In: Briley, M., Marien, M. (Eds.), Noradrenergic Mechanisms in Parkinson's Disease. CRC Press, Boca Raton, FL, pp. 225-254] that a deficiency in the noradrenergic system originating from the locus coeruleus is a decisive factor in the progression of central neurodegenerative disorders including Alzheimer's disease, and that treatments which boost noradrenergic transmission (e.g. via blockade of alpha(2)-adrenoceptors) could provide both symptomatic and trophic benefits against the disease. Studies in the rat in vivo demonstrating that the selective alpha(2)-adrenoceptor antagonist dexefaroxan increases acetylcholine release in the cortex, improves measures of cognitive performance and protects against excitotoxin lesions, support this concept. As a further test of the hypothesis, we investigated the effect of dexefaroxan in a rat model of unilateral cortical devascularization that induces a loss of the cortical cholinergic terminal network and a retrograde degeneration of the cholinergic projections that originate in the nucleus basalis magnocellularis. Lesioned and sham-operated rats received a 28-day subcutaneous infusion of dexefaroxan (0.63 mg/rat/day) or vehicle, delivered by osmotic minipumps implanted on the day of the cortical devascularization procedure. In lesioned rats, the dexefaroxan treatment was associated with a significantly higher number and size of vesicular acetylcholine transporter-immunoreactive boutons in comparison to the vehicle treatment; this effect was most marked within cortical layer V. Dexefaroxan also significantly reduced the atrophy of cholinergic neurons within the nucleus basalis magnocellularis. Dexefaroxan had no observable effect on any of these parameters in sham-operated cohorts. These results show that systemically administered dexefaroxan mitigates cholinergic neuronal degeneration in vivo, and provide further evidence for a therapeutic potential of the drug in neurodegenerative diseases such as Alzheimer's disease, where central cholinergic function is progressively compromised.

Activation of the mitogen-activated protein kinase (ERK(1/2)) signaling pathway by cyclic AMP potentiates the neuroprotective effect of the neurotransmitter noradrenaline on dopaminergic neurons

We have shown previously that low concentrations of noradrenaline (NA) confer long-term but partial protection to tyrosine hydroxylase (TH(+)) dopaminergic neurons by reducing spontaneously occurring oxidative stress. We demonstrate here that the effect of NA is strongly enhanced by cAMP-elevating agents, in particular forskolin (FK), through a mechanism that does not involve activation of adrenoceptors. FK also enhanced the neuroprotective action of antioxidants that mimic the trophic effects of NA, such as trolox and pyrocatechol, but was totally ineffective by itself, suggesting that inhibition of oxidative stress was a required step to reveal the cAMP-dependent mechanism. Neuroprotection afforded by FK was rapidly reversible, optimal when the treatment was initiated in the early phase of the culture and exquisitely specific to dopaminergic neurons. FK stimulated the phosphorylation of extracellular signal-activated kinases (ERK)(1/2) in a subpopulation of dopaminergic neurons, suggesting that the mitogen-activated protein kinase (MAPK) pathway was involved in the effects of cAMP-elevating agents. Accordingly, inhibition of the upstream kinases of ERK(1/2) by 2'-amino-3'-methoxyflavone (PD98059) not only suppressed MAPK activation caused by FK but also abolished the survival promoting activity that this compound exerts on TH(+) neurons. PD98059 did not reduce, however, the trophic effects provided by NA alone. Surprisingly, the archetypal cAMP-dependent protein kinase was apparently not responsible for ERK(1/2) activation. The data suggest that the MAPK signaling pathway plays a key role in the trophic effects that cAMP elevating agents and NA cooperatively exert on TH(+) neurons.

Effects of acute and subchronic administration of dexefaroxan, an alpha(2)-adrenoceptor antagonist, on memory performance in young adult and aged rodents

The present study examined the influence of dexefaroxan, a potent and selective alpha(2)-adrenoceptor antagonist, on cognitive performance in rodents. In young adult rats, dexefaroxan reversed the deficits induced by UK 14304 [5-bromo-N-(4,5-dihydro-1-H-imidazol-2-yl)-6-quinoxalinamine], scopolamine, and diazepam in a passive avoidance task. In this test, dexefaroxan also attenuated the spontaneous forgetting induced by a 15-week training-testing interval. Moreover, dexefaroxan, given immediately after training, increased the memory performance of rats trained with a weak electric footshock in the passive avoidance test, facilitated spatial memory processes in the Morris water maze task in rats, and increased the performance of mice in an object recognition test. Thus, dexefaroxan appears to have a promnesic effect in these tests by facilitating the processes of memory retention, rather than acquisition or other noncognitive influences. The facilitatory effects of dexefaroxan in young adult rats persisted even after a 21- to 25-day constant subcutaneous infusion by using osmotic minipumps, indicating that tolerance to the promnesic effect of the drug did not occur during this prolonged treatment interval. Furthermore, in the passive avoidance and Morris water maze tests, dexefaroxan ameliorated the age-related memory deficits of 24-month-old rats to a level that was comparable to that of young adult animals, and reversed the memory deficits induced by excitotoxin lesions of the nucleus basalis magnocellularis region. Together, these findings support a potential utility of dexefaroxan in the treatment of cognitive deficits occurring in Alzheimer's disease.

Noradrenaline provides long-term protection to dopaminergic neurons by reducing oxidative stress

To better understand the neurotrophic function of the neurotransmitter noradrenaline, we have developed a model of mesencephalic cultures in which we find low concentrations (0.3-10 microM) of noradrenaline to be remarkably effective in promoting long-term survival and function of dopaminergic neurons. This protective action reproduced the effect of caspase inhibition. It was atypical in that it occurred independently of adrenoceptor activation and was mimicked by some antioxidants, redox metal chelators and the hydroxyl radical detoxifying enzyme catalase. Interestingly, intracellular reactive oxygen species (ROS) were drastically reduced by treatment with noradrenaline, indicating that the neurotransmitter itself acted as an antioxidant. Prevention of oxidative stress was, however, independent of the glutathione antioxidant defense system. Chemical analogues of noradrenaline bearing two free hydroxyl groups in the ortho position of the aromatic ring (o-catechols), as well as o-catechol itself, mimicked the survival promoting effects of the neurotransmitter, suggesting that this diphenolic structure was critical for both neuroprotection and reduction of ROS production. Paradoxically, the autoxidation of noradrenaline and the ensuing production of quinone metabolites may be required for both effects, as the neurotransmitter was spontaneously and rapidly degraded over time in the culture medium. These results support the concept that central noradrenergic mechanisms have a neuroprotective role, perhaps in part by reducing oxidative stress.