Jean-Marie Maloteaux

Dopamine receptors—physiological understanding to therapeutic intervention potential

There are two families of dopamine (DA) receptors, called D1 and D2, respectively. The D1 family consists of D1- and D5-receptor subtypes and the D2 family consists of D2-, D3-, and D4-receptor subtypes. The amino acid sequences of these receptors show that they all belong to a large superfamily of receptors with seven transmembrane domains, which are coupled to their intracellular signal transduction systems by G-proteins. The implications of DA receptors in neuropsychiatry and cardiovascular and renal diseases are discussed. Neuropsychiatry indications include Parkinsons disease, schizophrenia, migraine, drug dependence, mania and depression, and Gilles de la Tourette syndrome. The underlying dysfunction of dopaminergic systems and the potential benefits of dopaminergic therapy in these different indications are critically examined. With respect to the pharmacological treatment of Parkinsons disease, a range of DA agonists are in various stages of preclinical and clinical development. D2-receptor agonist activity is predominant in most effective antiparkinsonian DA agonists. However, in practice, it is difficult to treat patients for several years with DA agonists alone; therapeutic benefit is not sustained. Rather, the use of a combination of DA agonists and levodopa is considered preferable. Reports of the efficacy of DA partial agonists await confirmation, and recent clinical investigations also suggest the potential of D1 receptor agonists as antiparkinson drugs. Regarding migraine pathogenesis, clinical and pharmacological evidence suggests that DA is involved in this disorder. Most prodromal and accompanying symptoms may be related to dopaminergic activation. Several drugs acting on DA receptors are effective in migraine treatment. Furthermore, migraine patients show a higher incidence of dopaminergic symptoms following acute DA agonist administration, when compared with normal controls. In cardiology, the therapeutic benefits of DA agonists are noted in the treatment of heart failure. Low doses of DA are widely used for its specific dopaminergic effects on renal function, which are suggested to be beneficial, and for its alpha- and beta-adrenergic-mediated responses that occur with higher doses. However, studies have been unable to demonstrate that DA can prevent acute renal failure or reduce mortality. It appears that the significant progress that is being made in the molecular understanding of DA receptors will continue to have a tremendous impact in the pharmacological treatment of neuropsychiatric, cardiovascular, and renal diseases.

Regional and subcellular localization in human brain of [3H]paroxetine binding, a marker of serotonin uptake sites.

The characteristics of the binding of [3H]paroxetine, a selective serotonin (5-HT) uptake blocker, were investigated in human brain. The Kd value was 0.23 +/- 0.07 nM, and the Bmax value was 190 +/- 39 fmol/mg protein in the putamen. The capacity of various antidepressive drugs to inhibit [3H]paroxetine-specific binding in human brain was well correlated with their capacity to inhibit [3H]5-HT uptake in rat brain. The highest concentrations of [3H]paroxetine-specific binding sites were found in the substantia nigra, hypothalamus, and hippocampus. Lower values were obtained in the basal ganglia and the thalamus. The specific binding was very low in cerebral and cerebellar cortices. The regional distribution of [3H]paroxetine binding sites differs from that of [3H]ketanserin binding to S2 serotonin receptors. The subcellular distribution of the [3H]paroxetine-specific binding sites obtained by differential centrifugation revealed a synaptosomal enrichment in the frontal cortex and striatum, whereas an enrichment in the microsomal fraction was found in striatum. The results show that [3H]paroxetine is a ligand of choice to label the 5-HT uptake molecular complex in human brain.

Interleukin-1 beta induces long-term increase of axonally transported opiate receptors and substance P.

Interleukin-1 is known to exert pleiotropic effects in host defence mechanisms and in inflammation. Chronic pain, inflammation and interleukin-1 beta enhance the production of substance P. Recently, axonal transport of opiate receptors was found to increase in rat sciatic nerves in the model of Freunds adjuvant-induced arthritis. Here we show that a single intraplantar injection of interleukin-1 beta is able to enhance the axonal transport of mu and kappa opiate receptors and substance P. Indeed, their accumulation was markedly increased in the proximal part of ligated sciatic nerves, but only in the paw injected with interleukin-1. The time course revealed a delayed onset and, more importantly, a long-term increase lasting at least six days, which is in contrast with the short-term pyrogenic effect of interleukin-1. Pretreatment of rats with capsaicin or administration of dexamethasone completely prevented the interleukin-1 beta effect. The present results suggest that interleukin-1 beta may serve as a mediator to sensitize nociceptors in chronic inflammation and possibly in hyperalgesia through long-term changes in neuronal plasticity.

Acute up-regulation of glutamate uptake mediated by mGluR5a in reactive astrocytes.

Excitatory transmission in the CNS necessitates the existence of dynamic controls of the glutamate uptake achieved by astrocytes, both in physiological conditions and under pathological circumstances characterized by gliosis. In this context, this study was aimed at evaluating the involvement of group I metabotropic glutamate receptors (mGluR) in the regulation of glutamate transport in a model of rat astrocytes undergoing in vitro activation using a cocktail of growth factors (G5 supplement). The vast majority of the cells were found to take up aspartate, mainly through the glutamate/aspartate transporter (GLAST), and at least 60% expressed functional mGluR5a. When exposed for 15 s to the selective group I mGluR agonist (S)‐3,5‐dihydroxyphenylglycine, reactive astrocytes showed a significant increase in their capacity to take up aspartate. This effect was confirmed at the single‐cell level, since activation of mGluRs significantly increased the initial slope of aspartate‐dependent Na+ entry associated with the activity of glutamate transporters. This up‐regulation was inhibited by an antagonist of mGluR5 and, more importantly, was sensitive to a specific glutamate transporter 1 (GLT‐1) blocker. The acute influence of mGluR5 on aspartate uptake was phospholipase C‐ and protein kinase C‐dependent, and was mimicked by phorbol esters. We conclude that mGluR5a contributes to a dynamic control of GLT‐1 function in activated astrocytes, acting as a glial sensor of the extracellular glutamate concentration in order to acutely regulate the excitatory transmission.

Mechanisms of regulation of neurotensin receptors.

Since its discovery in 1973, the neuropeptide neurotensin has been demonstrated to be involved in the control of a broad variety of physiological activities in both the central nervous system and in the periphery. Pharmacological studies have shown that the biological effects elicited by neurotensin result from its specific binding to cell membrane neurotensin receptors that have been characterized in various tissue and in cell preparations. In addition, it is now well documented that most of these responses are subject to rapid desensitization. Such desensitization results in transient responses to sustained peptide applications, or to tachyphylaxis during successive stimulations in the same conditions. More recently, desensitization of neurotensin signalling was investigated at the cellular and molecular levels. In cultured cells, regulation at the second messenger level, receptor internalization, and receptor down-regulation processes have been reported. These are proposed to play a critical role in the control of cell responsiveness to neurotensin. This review aims to compile recent data on the different biochemical processes involved in the regulation of the neurotensin receptor and to discuss the physiological consequences of this regulation in vivo.

Modulation of the neuronal dopamine transporter activity by the metabotropic glutamate receptor mGluR5 in rat striatal synaptosomes through phosphorylation mediated processes

There is considerable evidence that the activity of the neuronal dopamine transporter (DAT) is dynamically regulated and a putative implication of its phosphorylation in this process has been proposed. However, there is little information available regarding the nature of physiological stimuli that contribute to the endogenous control of the DAT function. Based on the close relationship between glutamatergic and dopaminergic systems in the striatum, we investigated the modulation of the DAT activity by metabotropic glutamate receptors (mGluRs). Short‐term incubations of rat striatal synaptosomes with micromolar concentrations of the group I mGluR selective agonist (S)‐3,5‐dihydroxyphenylglycine were found to significantly decrease the DAT capacity and efficiency. This alteration was completely prevented by a highly selective mGluR5 antagonist, 2‐methyl‐6‐(phenylethynyl)pyridine hydrochloride (MPEP). The effect of (S)‐3,5‐dihydroxyphenylglycine was also inhibited by staurosporine and by selective inhibitors of protein kinase C and calcium calmodulin‐dependent protein kinase II. Co‐application of okadaic acid prolonged the transient effect of the agonist, supporting a critical role for phosphorylation in the modulation of the DAT activity by mGluRs. In conclusion, we propose that striatal mGluR5 contribute to the control of the DAT activity through concomitant activation of both protein kinase C and calcium calmodulin‐dependent protein kinase II.

Chimerization of astroglial population in the lumbar spinal cord after mesenchymal stem cell transplantation prolongs survival in a rat model of amyotrophic lateral sclerosis

Adult mesenchymal stem cells (MSCs) exhibit neuroprotective properties when introduced into the degenerating central nervous system through different putative mechanisms including secretion of growth factors and transdifferentiation. In the present study, we injected MSCs into the cerebrospinal fluid of symptomatic hSOD1G93A rats, a transgenic animal model of familial amyotrophic lateral sclerosis (ALS) expressing a mutated form of the human superoxide dismutase. MSCs were found to infiltrate the nervous parenchyma and migrate substantially into the ventral gray matter, where motor neurons degenerate. Even though overall astrogliosis was not modified, MSCs differentiated massively into astrocytes at the site of degeneration. The intrathecal delivery of MSCs and the subsequent generation of healthy astrocytes at symptomatic stage decreased motor neuron loss in the lumbar spinal cord, preserving motor functions and extending the survival of hSOD1G93A rats. This neuroprotection was correlated with decreased inflammation, as shown by the lower proliferation of microglial cells and the reduced expressiontion of COX‐2 and NOX‐2. Together, these data highlight the protective capacity of adult MSC‐derived astrocytes when grafted into the central nervous system and illustrate an attractive strategy to target excessive inflammation in ALS.

GABA induces down-regulation of the benzodiazepine-GABA receptor complex in the rat cultured neurons.

Cultured neurons from embryonic rat brain display central type benzodiazepine receptors characterized by high-affinity binding of [3H]flunitrazepam which is allosterically enhanced in the presence of gamma-aminobutyric acid (GABA). A 48 h treatment of the cultured neurons with 1 microM diazepam, 0.1 microM clonazepam or 0.1 microM beta-carboline ester derivatives did not change either Bmax or KD values of the [3H]flunitrazepam specific binding. A 48 h incubation in the presence of GABA (1 mM) or muscimol (0.1 mM) induced a 30% decrease of the Bmax value of [3H]flunitrazepam specific binding without change of the KD value. The down-regulation was dependent on GABA concentrations and temperature, and was partially inhibited by bicuculline but not by the benzodiazepine antagonist Ro 15-1788. The other subunits of the benzodiazepine-GABA-chloride channel receptor complex also seemed to be down-regulated by GABA since there was a decrease of the specific binding of [3H]muscimol and [35S]t-butylbicyclophosphorothionate (TBPS) to the GABAA and chloride channel sites respectively. The GABA-induced down-regulation of the GABA-benzodiazepine receptor seems to be selective since the specific binding of ligands to other receptors was not affected. Our results suggests that activation of the low-affinity GABA subunit which is involved in cellular electrophysiological responses, induced the receptor down-regulation.

Current therapeutic uses and potential of beta-adrenoceptor agonists and antagonists

Abstractβ-Adrenoceptors are members of a large family of hormone and neurotransmitter receptors that initiate their biological function by coupling to GTP-binding regulatory proteins. β-Adrenoceptors can be subdivided into two main subgroups, designated β1 and β2. Atypical β-adrenoceptors or β3-adrenoceptors, which are present on adipocytes, have been demonstrated pharmacologically. Their function in adipose tissue is currently being investigated.β2-Adrenoceptor agonists have played a key role in the treatment of asthma for some 30 years, being used for the relief and prophylaxis of symptoms. There is, however, no evidence that tolerance to the bronchodilator or anti-bronchoconstrictor effects of these drugs is responsible for the deleterious effects reported with the regular use of bronchodilators.In neuropsychiatry, β-adrenoceptor antagonists have been used for the treatment of acute stress reactions and generalised anxiety, essential tremor and prophylaxis of migraine. In general, they are effective in anxiety disorders if the somatic symptoms are not extreme. For prophylactic treatment of migraine, β-adrenoceptor antagonists such as propranolol, metoprolol, nadolol and atenolol are the drugs of first choice.In cardiology, β-adrenoceptor antagonists are an important class for the treatment of high blood pressure, arrhythmias and angina pectoris, and for prevention of myocardial infarction. With chronic treatment, they reduce mortality in hypertension and prolong survival in patients with coronary heart disease.

Involvement of the sigma1 receptor in the modulation of dopaminergic transmission by amantadine

Pharmacological effects of amantadine on dopaminergic transmission are proposed to result from an uncompetitive antagonism at glutamate N‐methyl‐d‐aspartate (NMDA) receptors. However, our previous studies examining amantadine‐mediated dopamine receptor regulation in the rat striatum revealed a discrepancy from a direct interference with glutamate transmission. Preliminary in vitro binding data from the literature suggested the interaction of amantadine with the sigma1 receptor. Therefore, we have now further characterized the pharmacological properties of amantadine and memantine at this receptor and investigated its involvement in the modulation of striatal dopaminergic transmission. Our binding studies using [3H]‐(+)SKF‐10,047 indicated that amantadine and memantine behave as ligands of the sigma1 receptor in rat forebrain homogenates (Ki values of 7.44 ± 0.82 and 2.60 ± 0.62 µm, respectively). In NG108‐15 neuroblastoma cells, both drugs (amantadine (100 µm) and memantine (10 µm)) potentiated the bradykinin‐induced mobilization of intracellular Ca2+, mimicking the effect of the sigma1 receptor agonist PRE‐084 (1 µm). Finally, we previously showed that in striatal membranes from amantadine‐treated rats, the functional coupling of dopamine receptors with G‐proteins was enhanced. Similarly, PRE‐084 dose‐dependently increased the [35S]GTPγS binding induced by dopamine (Emax 28 and 26% of basal, 0.3 and 1 mg/kg PRE‐084, respectively). By contrast, BD1047, which is without effect on its own, antagonized the effects of amantadine and PRE‐084. Together, these data demonstrate that aminoadamantanes behave as sigma1 receptor agonists, and confirm an involvement of this receptor in modulating dopamine receptors exerted by therapeutically relevant concentrations of amantadine.