Effects of an alpha 2 antagonist in a 20-year-old Java monkey with MPTP-induced parkinsonian signs

Binding of dopamine and 3-methoxytyramine as l-DOPA metabolites to human alpha(2)-adrenergic and dopaminergic receptors

The ability of l-3,4-dihydroxyphenylalanine (l-DOPA), l-DOPA-methyl ester and their major metabolites, dopamine, dihydroxyphenylacetic acid (DOPAC), homovanillic (HVA), 3-O-methyldopa and 3-methoxytyramine (3-MT) to bind to alpha(2) adrenergic and D1 and D2 dopamine receptors was assessed by radioligand binding to cloned human receptors expressed in cell lines. As anticipated, dopamine bound with high affinity to D1 (IC(50) 1.1 + or - 0.16 microM) and D2 (IC(50) 0.7 + or - 0.3 microM) dopamine receptors. However, dopamine also bound with high affinity to alpha(2A) (IC(50) was 2.6 + or - 0.5 microM), alpha(2C) (IC(50) 3.2 + or - 0.7 microM). 3-MT bound to alpha(2A) with high affinity (IC(50), 3.6 + or - 0.2 microM) though moderate affinity to alpha(2)c, D1 and D2 receptors (values of IC(50) were 55 + or - 14, 121 + or - 43, 36 + or - 14 microM, respectively). l-DOPA-methyl ester bound with high affinity to alpha(2) (IC(50) 17-36 microM) but not dopamine receptors (IC(50) 0.9-2.5 mM). l-DOPA, 3-O-methyldopa and DOPAC had no observable effect on binding to any of the receptors tested. These data suggest that the effects of l-DOPA in Parkinson's disease may result from actions of its metabolites dopamine and 3-MT on both dopaminergic and non-dopaminergic receptors. These findings may provide explanations for the differences between l-DOPA and dopamine receptor agonists in mediating anti-parkinsonian effects and propensity to be associated with dyskinesia and motor complications such as wearing-off and on-off.

Noradrenergic modulation of subthalamic nucleus activity: behavioral and electrophysiological evidence in intact and 6-hydroxydopamine-lesioned rats

The subthalamic nucleus (STN) plays a key role in the pathophysiology of Parkinson's disease. The modulation of the STN by norepinephrine, however, is unknown. The present study aims at characterizing the effects of systemic administration of noradrenergic agents on locomotor activity and on in vivo extracellularly recorded STN neuronal activity in intact and 6-hydroxydopamine (6-OHDA)-lesioned rats. Using selective agonists and antagonists of alpha1 and alpha2 adrenergic receptors (ARs), we show that STN neurons have functional alpha1- and alpha2-AR controlling STN firing with an impact on locomotor activity. We further demonstrate that those systemic effects are supported, at least in part, by a direct modulation of STN neuronal activity, using patch-clamp recordings of STN neurons in brain slices. These findings support the premise that hypokinesia is associated with an increased STN neuronal activity, and that improvements of parkinsonian motor abnormalities are associated with a decrease in STN activity. Our data challenge assumptions about the role of alpha1-AR and alpha2-AR in the regulation of STN neurons in both intact and 6-OHDA-lesioned rats and further ground the rationale for using alpha2-AR noradrenergic antagonists in Parkinson's disease, albeit via an unexpected mechanism.

Novel 4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)methylbenzofuran derivatives as selective α2C-adrenergic receptor antagonists

The synthesis of a series of 4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)methyl-2-arylbenzofuran and 4-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinolin-2-yl)methylbenzofuran-2-carboxamide derivatives as novel alpha(2C)-adrenergic receptor antagonists are described. Their affinity at three different human alpha(2)-adrenergic receptors is reported, and some of these compounds exhibited high affinity for the alpha(2C)-adrenergic receptor with high subtype selectivity. Among them, compound 10e has been found to show the anti-L-dopa-induced dyskinetic activity in marmosets. The structure-activity relationship of these compounds is also discussed.

Present and future drug treatment for Parkinson's disease

Considerable advances made in defining the aetiology, pathogenesis, and pathology of Parkinson's disease (PD) have resulted in the development and rapid expansion of the pharmacopoeia available for treatment. Anticholinergics were used before the introduction of levodopa which is now the drug most commonly used. Dopamine agonists are effective when used alone or as an adjunct to levodopa, while monoamine oxidase B inhibitors improve motor function in early and advanced PD. However, treatment mainly addresses the dopaminergic features of the disease and leaves its progressive course unaffected; the drug treatment available for the management of non-motor symptoms is limited. This article seeks to set current treatment options in context, review emerging and novel drug treatments for PD, and assess the prospects for disease modification. Surgical therapies are not considered.

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.

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.

Potentiation of parkinsonian symptoms by depletion of locus coeruleus noradrenaline in 6-hydroxydopamine-induced partial degeneration of substantia nigra in rats

Parkinson's disease is characterized not only by a progressive loss of dopaminergic neurons in the substantia nigra but also by a degeneration of locus coeruleus noradrenergic neurons. The present study addresses the question of whether a partial neurodegeneration of dopaminergic neurons using 6-hydroxydopamine in rat, not sufficient to produce motor disturbances, is potentiated by prior selective denervation of locus coeruleus noradrenergic terminal fields using N-ethyl-2-bromobenzylamine. Two types of denervations, one causing dopamine deficiency alone and the other causing noradrenaline and dopamine deficiency, were performed. Noradrenaline, 5-hydroxytryptamine, 5-hydroxyindole acetic acid, dopamine and its metabolites were analysed in various brain regions. Behaviour was evaluated by catalepsy tests and activity box. N-ethyl-2-bromobenzylamine selectively depleted noradrenaline from neurons of locus coeruleus origin. Decreased dopamine content in the striatum, substantia nigra and pre-frontal cortex was observed after dopaminergic lesion with 6-hydroxydopamine (42.9%). Additional locus coeruleus noradrenaline depletion with N-ethyl-2-bromobenzylamine aggravated the dopamine depletion (61.2%). The lesion in the noradrenergic and dopaminergic neurodegenerated group was not sufficient to induce consistent catalepsy and akinesia. However, after a subthreshold dose of haloperidol (0.1 mg/kg), the expression of catalepsy and akinesia was strong in the dual-lesioned group and less in the 6-hydroxydopamine-lesioned group. These results indicate that denervation of locus coeruleus noradrenergic terminals with N-ethyl-2-bromobenzylamine potentiates the 6-hydroxydopamine-induced partial dopaminergic neurodegeneration and parkinsonian symptoms. Based on the present findings and existing reports, it can be concluded that noradrenergic neurons of locus coeruleus have neuromodulatory and neuroprotective properties on the dopaminergic neurons of basal ganglia and that noradrenergic degeneration may contribute to the aetiology and pathophysiology of Parkinson's disease.

Pathophysiology of levodopa-induced dyskinesia: potential for new therapies

Involuntary movements--or dyskinesias--are a debilitating complication of levodopa therapy for Parkinson's disease, and is experienced in most patients. Despite the importance of this problem, little was known about the cause of dyskinesia until recently; however, this situation has changed significantly in the past few years. Our increased understanding of levodopa-induced dyskinesia is not only valuable for improving patient care, but also in providing us with new insights into the functional organization of the basal ganglia and motor systems.

Neural mechanisms underlying peak-dose dyskinesia induced by levodopa and apomorphine are distinct: evidence from the effects of the alpha(2) adrenoceptor antagonist idazoxan

Dyskinesia, secondary to dopamine replacement therapy, is the major complication of currently available therapies for Parkinson's disease. Alpha(2) adrenoceptor antagonists, such as idazoxan, can significantly reduce levodopa-induced dyskinesia in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned, nonhuman primate model of Parkinson's disease and in human. This action of adrenoceptor antagonists may involve blockade of the actions of noradrenaline synthesised from levodopa. We hypothesise that, because dopamine receptor agonists, such as apomorphine, cannot be metabolised to produce noradrenaline, activation of adrenoceptors may not be involved in dyskinesia produced by such agents. If this were the case, idazoxan would not be expected to reduce apomorphine-induced dyskinesia. MPTP-lesioned marmosets with stable dyskinesia induced by prolonged levodopa therapy were given an acute challenge with apomorphine (0.3 mg/kg subcutaneously) or levodopa (8.0 mg/kg orally), these doses produced equivalent peak-dose dyskinesia. Idazoxan (2.5 mg/kg p.o.), or vehicle, was then administered with either apomorphine or levodopa. Idazoxan abolished levodopa-induced dyskinesia but did not affect apomorphine-induced dyskinesia (P < 0.05 and P > 0.05, respectively, Wilcoxon matched pairs test). Idazoxan also extended the anti-parkinsonian actions of levodopa but did not affect those of apomorphine. The pharmacological characteristics of the neural mechanisms underlying levodopa-induced dyskinesia and apomorphine-induced dyskinesia in parkinsonism thus appear to be distinct, at least with respect to the involvement of alpha(2) adrenoceptors. Specifically, levodopa, but not apomorphine-induced dyskinesia, involves activation of adrenoceptors. This finding may have major implications for understanding dyskinesia and should be borne in mind when designing clinical studies in which levodopa or dopamine receptor agonist challenges are employed to assess potential anti-dyskinetic properties of drugs.

New substituted 1-(2,3-dihydrobenzo[1, 4]dioxin-2-ylmethyl)piperidin-4-yl derivatives with alpha(2)-adrenoceptor antagonist activity

The emergence of a novel theory concerning the role of noradrenaline in the progression and the treatment of neurodegenerative diseases such as Parkinson's and Alzheimer's diseases has provided a new impetus toward the discovery of novel compounds acting at alpha(2)-adrenoceptors. A series of substituted 1-(2, 3-dihydrobenzo[1,4]dioxin-2-ylmethyl)piperidin-4-yl derivatives bearing an amide, urea, or imidazolidinone moiety was studied. Some members of this series of compounds proved to be potent alpha(2)-adrenoceptor antagonists with good selectivity versus alpha(1)-adrenergic and D(2)-dopamine receptors. Particular emphasis is given to compound 33g which displays potent alpha(2)-adrenoceptor binding affinity in vitro and central effects in vivo following oral administration.

Effect of the alpha 2 adrenoreceptor antagonist, idazoxan, on motor disabilities in MPTP-treated monkey

1. The motor effect of the alpha 2 adrenoreceptor antagonist, idazoxan, was compared to that of L-dopa in MPTP-treated monkeys. 2. Idazoxan 2.0 mg/kg improved parkinsonian motor abnormalities which was comparable to the effects of a minimal effective dose of L-dopa. 3. At 2.0 and 5.0 mg/kg, the parkinsonian rigidity was the item most frequently alleviated by idazoxan (respectively 63.6% and 68.2%). 4. These findings provide support for the therapeutic utility of alpha 2 antagonists in the treatment of Parkinson's disease.

The alpha2-adrenergic receptor antagonist idazoxan reduces dyskinesia and enhances anti-parkinsonian actions of L-dopa in the MPTP-lesioned primate model of Parkinson's disease

Dopamine replacement therapy in patients with Parkinson's disease is plagued by the emergence of abnormal involuntary movements known as L-dopa-induced dyskinesias. It has been demonstrated that yohimbine can reduce L-dopa-induced dyskinesia in the MPTP-lesioned primate model of Parkinson's disease. Yohimbine is, among other things, an alpha-adrenergic receptor antagonist. In this study, we demonstrate that the selective and potent alpha2-adrenergic receptor antagonist idazoxan reduces L-dopa-induced dyskinesia in the MPTP-lesioned marmoset model of Parkinson's disease. The alpha2-adrenergic receptor antagonists rauwolscine and yohimbine also reduce L-dopa-induced dyskinesia. Furthermore, we demonstrate that coadministration of idazoxan with L-dopa can provide an anti-parkinsonian action more than twice the length of that seen with L-dopa alone. However, idazoxan as a monotherapy displayed no anti-parkinsonian actions. We propose that idazoxan in combination with L-dopa may provide a novel approach to the treatment of Parkinson's disease that will not only reduce the dyskinetic side effects, but extend the anti-parkinsonian actions of L-dopa. Idazoxan, as an adjunct to dopamine replacement, may prove useful in the treatment of parkinsonian patients at all stages of disease progression.

Animal models of tremor

Animal models of tremor have been widely used in experimental neurology, because they are an indispensable requirement for understanding the pathophysiology of human tremor disorders and the development of new therapeutic agents. This review focuses on three approaches to produce tremor in animals (application of tremorgenic drugs, experimental central nervous system lesions, study of genetic mutants) and their use in simulating tremor syndromes of humans. Whereas harmaline induces a postural/kinetic tremor in animals that shares some features with human essential tremor/enhanced physiological tremor, MPTP tremor is the best model available for rest tremor in people. The tremor following experimental lesion of the ventromedial tegmentum in primates closely resembles Holmes tremor in humans, whereas cerebellar intention tremor is mimicked by cooling of the lateral cerebellar nuclei. The "campus syndrome," discovered in a breed of Pietrain pigs, might be a useful model of human orthostatic tremor. However, no animal model has yet been generated that exactly recreates all features of any of the known tremor disorders in humans. Problems encountered when comparing tremor in animals and humans include differing tremor frequencies and the uncertainty, if specific transmitter abnormalities/central nervous system lesions seen in animal tremor models are characteristic for their human counterparts. The search for adequate tremor models continues.

The alleged dopamine D1 receptor agonist SKF 83959 is a dopamine D1 receptor antagonist in primate cells and interacts with other receptors

So far, no clear correlation has been found between the effects of dopamine D1 receptor agonists on motor behavior in primate models of Parkinson's disease and their ability to stimulate adenylate cyclase in rats, the benzazepine SKF 83959 (3-methyl-6-chloro-7,8-hydroxy-1-[3-methylphenyl]-2,3,4,5-tetrahydro-]H- 3-benzazepine) being the most striking example. Since this discrepancy might be attributed to: (A) the different species used to study these effects or (B) the interaction of SKF 83959 with other catecholamine receptors, the aims of this study were: (1) to study the ability of SKF 83959 to stimulate adenylate cyclase in cultured human and monkey glial cells equipped with dopamine D1 receptors and (2) to evaluate the affinity for and the functional interaction of SKF 83959 with other catecholamine receptors. Binding studies revealed that SKF 83959 displayed the highest affinity for the dopamine D1 receptor (pKi=6.72) and the alpha2-adrenoceptor (pKi=6.41) and moderate affinity for the dopamine D2 receptor and the noradrenaline transporter. In monkey and human cells, SKF 83959 did not stimulate cyclic adenosine monophosphate (cAMP) formation to a significant extent, but antagonized very potently the dopamine-induced stimulation of cAMP formation in both cell types. The compound stimulated basal dopamine outflow and inhibited depolarization-induced acetylcholine release only at concentrations > 10 microM. Finally, SKF 83959 concentration dependently increased electrically evoked noradrenaline release, indicating that it had alpha2-adrenoceptor blocking activity and interfered with the noradrenaline transporter. In conclusion, SKF 83959 is a potent dopamine D1 receptor and alpha2-adrenoceptor antagonist. Thus, the anti-parkinsonian effects of SKF 83959 in primates are not mediated by striatal dopamine D1 receptors coupled to adenylate cyclase in a stimulatory way.

6-Hydroxydopamine lesion of locus coeruleus and the antiparkinsonian potential of NMDA-receptor antagonists in rats

Behavioral and neurochemical effects after bilateral 6-hydroxy-dopamine locus coeruleus- (LC) lesion were examined in rats and compared to sham-lesioned controls. Behavior after treatment with the antiakinetic drugs dizocilpine, amantadine, memantine or L-DOPA as well as joint treatment of these drugs with haloperidol were tested in an open field with holeboard and in an experimental chamber. Under saline spontaneous activity (open field with holeboard) and sniffing (experimental chamber) were reduced after lesion. Injection of the proparkinsonian drug haloperidol decreased sniffing in all rats but to a greater extent in LC-lesioned rats. In combination with haloperidol none of the tested drugs could completely compensate for the motor deficits induced by the lesion. Neurochemical data revealed a reduced content of noradrenaline in the prefrontal cortex and in the posterior striatum of LC-lesioned rats. These results indicate that loss of LC neurons intensifies parkinsonian symptoms induced by blockade of dopamine D2-receptors, and lowers the antiakinetic potential of dizocilpine, amantadine, memantine or L-DOPA.

Blink rate, neurocognitive impairments, and symptoms in schizophrenia

This study addresses the relationship between blink rate (as a measure of central dopaminergic activity), symptoms, neuropsychological performance, as well as neurological signs in schizophrenia. Blink rate between schizophrenic patients and a matched control group was compared in both "relaxed" and "attentive" conditions. Blink rate was increased in schizophrenic patients in the "relaxed" condition but not in the "attentive" condition. The relationship between blink rate and clinical variables was examined in a larger sample of 204 patients. In both "relaxed" and "attentive" conditions, blink rate was related to "hallucinations," "anxiety," negative symptoms, and neuroleptic dosage. Attention was a significant predictor only in the "relaxed" condition. No other significant correlation was found between blink rate, neuropsychological impairments, and neurological signs. These findings suggest that blink rate is not related to neurocognitive impairments in schizophrenia.