Chronic exposure of human cells in culture to the tricyclic antidepressant desipramine reduces the number of beta-adrenoceptors

Is depression a disorder of a receptor superfamily? A critical review of the receptor theory of depression and the appraisal of a new heuristic model

The monoamine hypothesis of depression and its direct derivation, the receptor theory, have constituted for several years a frame of reference for researchers working in the field of biological psychiatry. Although most of the data are derived from animal findings and must be considered inconclusive in view of various controversies, some guidelines may be identified: these would suggest that changes in postsynaptic beta-adrenoreceptors, presynaptic alpha 2-adrenoreceptors, as well as in type 2 serotonin receptors and dopaminergic autoreceptors may be involved in the mode of action of antidepressant drugs and, consequently, in the pathophysiology of depression. Nowadays, any attempt to correlate depression with the dysfunction of a single neurotransmitter or receptor is no longer tenable, since it is clear that depression is a heterogeneous disorder which involves abnormalities in the interactive relationships between neurotransmitters and receptors. If, on the one hand, this new model has opened up new fields of research and has led to the investigation of new systems,egthe GABAergic and GABA B receptors, on the other hand, it has been strongly limited by the lack of research tools and reliable peripheral CNS models forin vivostudies. A possible approach to this unresolved dilemma may be provided by molecular biology techniques, which have permitted the identification of the genes and sequencing of the primary structure of several membrane receptors. It is now established that receptors may be grouped into four superfamilies; in depression, there exists compelling evidence of alterations mainly in receptors belonging to the G-protein-coupled family: it is plausible that depression may be related to a disorder of the G-protein-coupled receptor superfamily. Such an hypothesis would represent an attempt to unify the different receptor abnormalities found in depression or following antidepressant treatments, and to shift from the monoamine paradigm to a new heuristic model. In addition, it would accommodate the various dysfunctions likely to be encountered and would open up new theoretical perspectives in the treatment of depression.

Human Dermal Fibroblast: A Promising Cellular Model to Study Biological Mechanisms of Major Depression and Antidepressant Drug Response

BACKGROUND

Human dermal fibroblasts (HDF) can be used as a cellular model relatively easily and without genetic engineering. Therefore, HDF represent an interesting tool to study several human diseases including psychiatric disorders. Despite major depressive disorder (MDD) being the second cause of disability in the world, the efficacy of antidepressant drug (AD) treatment is not sufficient and the underlying mechanisms of MDD and the mechanisms of action of AD are poorly understood.

OBJECTIVE

The aim of this review is to highlight the potential of HDF in the study of cellular mechanisms involved in MDD pathophysiology and in the action of AD response.

METHODS

The first part is a systematic review following PRISMA guidelines on the use of HDF in MDD research. The second part reports the mechanisms and molecules both present in HDF and relevant regarding MDD pathophysiology and AD mechanisms of action.

RESULTS

HDFs from MDD patients have been investigated in a relatively small number of works and most of them focused on the adrenergic pathway and metabolism-related gene expression as compared to HDF from healthy controls. The second part listed an important number of papers demonstrating the presence of many molecular processes in HDF, involved in MDD and AD mechanisms of action.

CONCLUSION

The imbalance in the number of papers between the two parts highlights the great and still underused potential of HDF, which stands out as a very promising tool in our understanding of MDD and AD mechanisms of action.

Concomitant inhibition of primary equine bronchial fibroblast proliferation and differentiation by selective β2-adrenoceptor agonists and dexamethasone

Altered airway cell proliferation plays an important role in the pathogenesis of human bronchial asthma and chronic obstructive pulmonary disease (COPD) as well as the equine recurrent airway obstruction (RAO) with consistent changes, i.e. narrowing the airway wall, explained by proliferation and differentiation of fibroblasts. In permanent cell lines, it has been suggested that β2-adrenoceptor agonists and glucocorticoids regulate cell proliferation via the β2-adrenoceptor pathway; indeed, no study was carried out in fresh isolated primary equine bronchial fibroblasts (EBF). We characterized the β-adrenoceptors in EBF, and compared effects of long-acting (clenbuterol) and short-acting (salbutamol and isoproterenol) β2-agonists and dexamethasone on proliferation, differentiation and collagen synthesis. High density (Bmax; 5037±494 sites/cell) of β2-adrenoceptor subtype was expressed in EBF. β2-agonists inhibited concentration-dependently EBF proliferation with potency of clenbuterol>salbutamol »isoproterenol which was inhibited by ICI 118.551 and propranolol but not by CGP 20712A. In contrast, dexamethasone alone inhibited less EBF proliferation, but the effect was high when dexamethasone was combined with β2-agonists. Transforming growth factor-β1 (TGF-β1) increased transformation of fibroblasts into myofibroblasts, which was inhibited by clenbuterol and dexamethasone alone and drug combination resulted in high inhibition rate. Collagen synthesis in EBF was rather hampered by dexamethasone than by β-agonists. Collectively, the expression of β2-adrenoceptor subtype in EBF and the anti-proliferative effect of clenbuterol suggest that β2-adrenoceptors are growth inhibitory and anti-fibrotic in EBF. These β2-agonist effects in EBF were synergistically enhanced by dexamethasone, providing the additive effects of glucocorticoids to counteract airway remodelling and morbidity of asthma and RAO.

Distinct PKA and Epac compartmentalization in airway function and plasticity

Asthma and chronic obstructive pulmonary disease (COPD) are obstructive lung diseases characterized by airway obstruction, airway inflammation and airway remodelling. Next to inflammatory cells and airway epithelial cells, airway mesenchymal cells, including airway smooth muscle cells and (myo)fibroblasts, substantially contribute to disease features by the release of inflammatory mediators, smooth muscle contraction, extracellular matrix deposition and structural changes in the airways. Current pharmacological treatment of both diseases intends to target the dynamic features of the endogenous intracellular suppressor cyclic AMP (cAMP). This review will summarize our current knowledge on cAMP and will emphasize on key discoveries and paradigm shifts reflecting the complex spatio-temporal nature of compartmentalized cAMP signalling networks in health and disease. As airway fibroblasts and airway smooth muscle cells are recognized as central players in the development and progression of asthma and COPD, we will focus on the role of cAMP signalling in their function in relation to airway function and plasticity. We will recapture on the recent identification of cAMP-sensing multi-protein complexes maintained by cAMP effectors, including A-kinase anchoring proteins (AKAPs), proteins kinase A (PKA), exchange protein directly activated by cAMP (Epac), cAMP-elevating seven-transmembrane (7TM) receptors and phosphodiesterases (PDEs) and we will report on findings indicating that the pertubation of compartmentalized cAMP signalling correlates with the pathopysiology of obstructive lung diseases. Future challenges include studies on cAMP dynamics and compartmentalization in the lung and the development of novel drugs targeting these systems for therapeutic interventions in chronic obstructive inflammatory diseases.

Antidepressants increase β-arrestin 2 ubiquitinylation and degradation by the proteasomal pathway in C6 rat glioma cells

beta-Arrestins, regulators of G protein-coupled receptor-G protein coupling and receptor desensitization and internalization, function also as scaffolding proteins mediating cellular signaling events. beta-Arrestin1 was previously implicated by us in the pathophysiology of depression and in the mechanism of action of antidepressants (ADs). The ubiquitously expressed beta-arrestins1 and 2 are structurally highly homologous. There has been extensive investigation of these two proteins to determine whether they serve different roles in receptor signaling. In this study, we show that treatment of C(6) rat glioma cells with ADs of various types for 3 days resulted in decreased beta-arrestin2 levels. In contrast, beta-arrestin2 mRNA expression was found to be up-regulated by ADs. To unravel the mechanism for these opposite effects several possible beta-arrestin2 post-transcriptional events and modifications were examined. C(6) rat glioma cells transfected with beta-arrestin1-targeted short hairpin RNA showed similar effects of ADs on beta-arrestin2 levels. AD-induced decreases in beta-arrestin2 protein levels were not due to cytosolic membrane translocation. Immunoprecipitation experiments showed that ADs were able to increase coimmunoprecipitation of ubiquitin with beta-arrestin2. AD-induced increases in beta-arrestin2 ubiquitinylation led to its degradation by the proteasomal pathway, as the proteasome inhibitor N-[(phenylmethoxy)carbonyl]-l-leucyl-N-[(1S)-1-formyl-3-methylbutyl]-l-leucinamide (MG-132) prevented antidepressant-induced decreases in beta-arrestin2 protein levels.

Pulmonary fibroblasts, an emerging target for anti-obstructive drugs

Fibrotic alterations are part of the airway re-modelling processes observed in asthma and chronic obstructive pulmonary disease. There is increasing evidence that in addition to acute bronchodilatory effects, classical anti-obstructive drugs such as muscarinic antagonists and beta-adrenoceptor agonists may also modulate long-term re-modelling processes. The present review aims to summarise muscarinic and beta-adrenergic effects on pulmonary fibroblasts. Recent experimental evidence demonstrated muscarinic stimulatory effects on pulmonary fibroblasts, and long-term blockade of these pro-fibrotic effects may contribute to the beneficial effects of muscarinic antagonists, as observed particularly for the long-acting muscarinic antagonist tiotropium. On the other hand, beta-adrenoceptor agonists, via activation of adenylyl cyclase, can also exert various inhibitory effects on pulmonary fibroblasts, and these anti-fibrotic effects are mimicked by other agents that cause an increase in intracellular cyclic adenosine monophosphate (cAMP), such as phosphodiesterase inhibitors or EP2 prostanoid receptor agonists. In addition, the role of the extracellular signal-regulated kinase-mitogen-activated protein kinase pathway, protein kinase A and exchange protein activated by cAMP (Epac) and potential interactions between these cellular signalling pathways are discussed.

G protein-coupled receptors in major psychiatric disorders

Although the molecular mechanisms underlying psychiatric illnesses such as depression, bipolar disorder and schizophrenia remain incompletely understood, there is increasing clinical, pharmacologic, and genetic evidence that G protein-coupled receptors (GPCRs) play critical roles in these disorders and their treatments. This perspectives paper reviews and synthesizes the available data. Dysfunction of multiple neurotransmitter and neuropeptide GPCRs in frontal cortex and limbic-related regions, such as the hippocampus, hypothalamus and brainstem, likely underlies the complex clinical picture that includes cognitive, perceptual, affective and motoric symptoms. The future development of novel agents targeting GPCR signaling cascades remains an exciting prospect for patients refractory to existing therapeutics.

Regulators of G-protein-coupled receptor-G-protein coupling: antidepressants mechanism of action

There is a significant gap between advances in medication for mental disorders and the present static situation of biological diagnosis and monitoring treatment. The system of neural transmission and signal transduction is a complicated, highly regulated cascade of biochemical events. Growing evidence suggests that receptor-G-protein coupling may be involved in both the pathogenesis and treatment of mood disorders. Our knowledge concerning the basic mechanisms underlying the phenomenon of desensitization, internalization, downregulation and resensitization of the G-protein-coupled receptor has been advanced during the last decade. The present review discusses the possible involvement of regulators of G-protein-coupled receptor-G-protein coupling: beta-arrestins, G-protein-coupled receptor kinases and phosducin-like proteins, as well as beta-arrestins alternative signaling events, in the pathophysiology, diagnosis and treatment monitoring of mood disorders and in the mechanism of action of antidepressant medications.

The involvement of G proteins and regulators of receptor-G protein coupling in the pathophysiology, diagnosis and treatment of mood disorders

Biochemical research in mood disorders has focused, along the cascade of events involved in signal transduction, from studies at the level of the monoamine neurotransmitter to the level of the neurotransmitter receptors, and lately to information transduction mechanisms beyond receptors, involving the coupling of receptors with signal transducers. We review findings concerning (a) the involvement of G proteins, in the pathophysiology, diagnosis and treatment of mood disorders; (b) the importance of regulation of receptor-G protein coupling, G protein-coupled receptor kinases (GRKs), beta-arrestins, to the pathophysiology of mood disorders and the mechanism of action of antidepressants. We relate to the special complexity of mental disorders with regards to etiology and pathophysiological diagnosis as well as to the strength and limitations of the 'pharmacological bridge' approach governing studies to unravel the etiology of mental disorders. There are presently no established and reliable, sensitive and specific objective biological diagnostic markers in psychiatry that can serve as 'gold standards'. The future achievement of an objective biochemical differential diagnostic system for major mental disorders that will also enable an objective biological treatment monitoring is expected to be revolutionary for psychiatry with a magnitude similar to the impact of the discovery of psychopharmacological treatments for mental disorders more than 50 years ago.

G protein signaling and the molecular basis of antidepressant action

Over the past four decades, a variety of interventions have been used for the treatment of clinical depression and other affective disorders. Several distinct pharmacological compounds show therapeutic efficacy. There are three major classes of antidepressant drugs: monoamine oxidase inhibitors (MAOIs), selective serotonin reuptake inhibitors (SSRIs), and tricyclic compounds. There are also a variety of atypical antidepressant drugs, which defy ready classification. Finally, there is electroconvulsive therapy, ECT. All require chronic (2-3 weeks) treatment to achieve a clinical response. To date, no truly inclusive hypothesis concerning a mechanism of action for these diverse therapies has been formed. This review is intended to give an overview of research concerning G protein signaling and the molecular basis of antidepressant action. In it, the authors attempt to discuss progress that has been made in this arena as well as the possibility that some point (or points) along a G protein signaling cascade represent a molecular target for antidepressant therapy that might lead toward a unifying hypothesis for depression. This review is not designed to address the clinical studies. Furthermore, as it is a relatively short paper, citations to the literature are necessarily selective. The authors apologize in advance to authors whose work we have failed to cite.

Antidepressant-induced switch of beta 1-adrenoceptor trafficking as a mechanism for drug action

Reduction in surface beta(1)-adrenoceptor (beta1AR) density is thought to play a critical role in mediating the therapeutic long term effects of antidepressants. Since antidepressants are neither agonists nor antagonists for G protein-coupled receptors, receptor density must be regulated through processes independent of direct receptor activation. Endocytosis and recycling of the beta1AR fused to green fluorescent protein at its carboxyl-terminus (beta1AR-GFP) were analyzed by confocal fluorescence microscopy of live cells and complementary ligand binding studies. In stably transfected C6 glioblastoma cells, beta1AR-GFP displayed identical ligand-binding isotherms and adenylyl cyclase activation as native beta1AR. Upon exposure to isoproterenol, a fraction of beta1AR-GFP (10-15%) internalized rapidly and colocalized with endocytosed transferrin receptors in an early endosomal compartment in the perinuclear region. Chronic treatment with the tricyclic antidepressant desipramine (DMI) did not affect internalization characteristics of beta1AR-GFP when challenged with isoproterenol. However, internalized receptors were not able to recycle back to the cell surface in DMI-treated cells, whereas recycling of transferrin receptors was not affected. Endocytosed receptors were absent from structures that stained with fluorescently labeled dextran, and inhibition of lysosomal protease activity did not restore receptor recycling, indicating that beta1AR-GFP did not immediately enter the lysosomal compartment. The data suggest a new mode of drug action causing a "switch" of receptor fate from a fast recycling pathway to a slowly exchanging perinuclear compartment. Antidepressant-induced reduction of receptor surface expression may thus be caused by modulation of receptor trafficking routes.

Plasmalogen content and beta-adrenoceptor signalling in fibroblasts from patients with Zellweger syndrome. Effects of hexadecylglycerol

In Zellweger or cerebro-hepato-renal syndrome (CHRS), the assembly of peroxisomes is defective, resulting in deficient plasmalogen formation. Plasmalogens are part of the membrane lipid composition. In fibroblasts of CHRS patients, the plasmalogen fraction of phosphatidylethanolamine (PPE) was about half of that in control cells while total phospholipid (PL) content, individual PL and plasma membrane fluidity were normal. CHRS cell strains had higher beta-adrenoceptor numbers and isoproterenol-stimulated cAMP responses. Receptors were more efficiently coupled to adenylate cyclase than in control cells. Stimulations of cAMP with NaF or forskolin were the same as in control cells. Restoring synthesis of plasmalogens with hexadecylglycerol (HDG), a plasmalogen precursor, resulted in a proportionate increase in PPE of about 40% in both control and CHRS fibroblasts. Exposure to HDG reduced surface beta-adrenoceptor sites and cAMP-responses to isoproterenol in CHRS cells only, while post-receptor stimulations of cAMP were reduced in both cell types. Plasmalogen contents inversely correlated with isoproterenol-stimulated cAMP levels. The increased numbers of functional beta-adrenoceptors in CHRS fibroblasts may be the result of a higher expression and/or of a prolonged functional half-life of the receptor protein. In vivo, this may contribute to the clinical manifestations of the disease.

The role of CREB and other transcription factors in the pharmacotherapy and etiology of depression

Psychiatric diseases are genetically complex and consequently, altered programs of gene expression have been hypothesized as the molecular basis of psychopathology. Since transcription factors represent the final communicative link between receptor activation and the orchestration of programs of gene expression, they are prime targets for studies on both the pharmacotherapy and the etiology of depression. The cyclic AMP response element binding protein (CREB) and the glucocorticoid receptor (GR) are altered by chronic treatment with antidepressants. Since it is phosphorylated CREB (pCREB) that determines its transcriptional activity, it is pertinent that some antidepressants have been shown to reduce pCREB in brain in vivo and in tissue culture in vitro. Moreover, pCREB is down-regulated in human fibroblasts from patients with major depression and in postmortem brain of suicide victims with a history of depression. With regard to GR, its mRNA, immunoreactivity, density and cytoplasmic-nuclear translocation are increased by antidepressants. While transcription factor mediated programs of gene expression relevant to either the pharmacotherapy or the etiology of depression are still largely elusive, studies utilizing modern technologies such as differential display and cDNA microarrays promise to lead eventually to the identification of structure and function of psychopathologically relevant target genes.

Cellular mechanisms in the vulnerability to depression and response to antidepressants

As a testable heuristic, the concept of stress response and adaptation is highly appealing, and the support for the concept is strong. This explanatory model of depression may account for hitherto apparently discordant facts--contradictory symptoms, antidepressant drugs that act on differing systems, facilitation of antidepressant response by augmentation, and response to psychotherapy and pharmacotherapy. This article has focused narrowly on specific cellular elements of the stress-adaptational mechanisms, including the AC-PKA and PLC-PKC transductional cascades, together with specific response elements, such as the HPA axis, BDNF, and NMDA receptors; however, other important mechanisms, including specific receptor subtypes (e.g., 5-HT1A and NE alpha 2), transmitter systems (e.g., acetylcholine and depamine), and hormones (e.g., thyroid and growth hormones and prolactin), which may be important, have not been discussed. As the complex interactions of these systems gradually yield to investigation, not only will new treatments be developed, but better matching of treatment to patient may become an achievable goal.

Neutrophil beta(2)-adrenoceptor function in major depression: G(s) coupling, effects of imipramine and relationship to treatment outcome

Abnormal beta(2)-adrenoceptor density and beta(2)-adrenoceptor-mediated cyclic adenosine monophosphate (cAMP) responses were inconsistently reported in major depressive disorder. Tricyclic antidepressants downregulate beta-adrenoceptor density and decrease coupling to G(s) protein. Abnormal beta-adrenoceptor coupling may exist in major depressive disorder and may relate to treatment response. We investigated beta(2)-adrenoceptor coupling to G(s) protein in 25 controls, 23 major depressive disorder drug-free patients and 16 major depressive disorder patients after chronic imipramine treatment using agonist displacement experiments. Pretreatment beta(2)-adrenoceptor coupling and density were normal in patients as a whole. Chronic imipramine induced beta(2)-adrenoceptor uncoupling. This effect was observed in treatment responders who had increased beta(2)-adrenoceptor density in the high-conformational state and supercoupling prior to treatment. Beta(2)-adrenoceptor density decreased after imipramine treatment. Treatment non-responders had seemingly normal pretreatment beta(2)-adrenoceptor function, which was not changed by imipramine. Differences in beta(2)-adrenoceptor regulation in major depressive disorder may underlie treatment response. The results indirectly implicate abnormal agonist-mediated beta(2)-adrenoceptor gene expression, protein kinase A, and protein kinase C in major depressive disorder.

The combined effects of chronic ethanol/desipramine treatment on beta-adrenoceptor density and coupling efficiency in rat brain

Both ethanol and desipramine influence beta-adrenoceptor regulation. We reported previously that ethanol partially counteracted desipramine's effects on beta-adrenoceptor. Previous studies utilized beta-adrenoceptor radioligands that also bind to 5-HT1B receptors, thus, changes in 5-HT1B receptors could have confounded the results. The effects of chronic ethanol, desipramine and ethanol/desipramine treatment on beta-adrenoceptor coupling efficiency to Gs protein in rat brain were examined using 125I-iodocyanopindolol after blocking binding to 5-HT1B receptors. In the frontal cortex, ethanol uncoupled beta-adrenoceptor from GS. Desipramine decreased beta-adrenoceptor density, particularly in the high-conformational state, with no effect on coupling. In combined treatment, desipramine prevented ethanol-induced uncoupling. In the hippocampus, desipramine enhanced beta-adrenoceptor coupling, but ethanol had no effect. In combination with desipramine, ethanol enhanced desipramine-induced decrease in beta-adrenoceptor density in the high-conformational state, but uncoupled beta-adrenoceptors, an effect not observed with ethanol alone. These results suggest a complex interplay between ethanol and antidepressants in modulating beta-adrenoceptor function.

Evidence for lysosomotropism of memantine in cultured human cells: cellular kinetics and effects of memantine on phospholipid content and composition, membrane fluidity and beta-adrenergic transmission

Memantine, an amantadine derivative, is therapeutically used for the treatment of various neurological and psychiatric disorders such as Parkinson's disease, spasticity, and dementia. Pharmacokinetics of memantine and its effects on phospholipid content and composition, on membrane properties and functions such as fluidity and beta-adrenergic transmission were studied in cultured human fibroblasts and macrophages. The kinetic behaviour of memantine was characteristic for a lysosomotropic drug. Fibroblasts exposed to 14C-memantine in the microM range accumulated the drug up to 200 fold above initial medium concentrations. Lysosomal drug storage was proven by indirect evidence and by analyses of subcellular fractions. Repetitive exposure to memantine resulted in a cumulative uptake. While memantine uptake after single exposure was fully reversible, the rate and extent of release of chronically accumulated drug was reduced but could be enhanced by the addition of unlabelled memantine or ammonium chloride to the medium. Chronic, but not single, exposure to memantine above 10 microM resulted in a concentration dependent phospholipid accumulation and in a shift in the phospholipid composition. There was an overproportionate increase in phosphatidylinositol at the expense of phosphatidylserine and sphingomyelin. Chronic exposure of cultured cells to memantine increased fluidity in the superficial layers of the plasma membrane and reduced the isoproterenol-stimulated cAMP-response without affecting beta-adrenoceptor density. All these findings were compatible with the kinetic behaviour and the effectiveness expected of a weak lysosomotropic drug.

Cellular accumulation of amiodarone and desethylamiodarone in cultured human cells. Consequences of drug accumulation on cellular lipid metabolism and plasma membrane properties of chronically exposed cells

Amiodarone (AMIO), a potent antiarrhythmic drug, is clinically widely used despite its frequent side effects after chronic administration. These side effects coincide with an intralysosomal accumulation of AMIO and its main metabolite desethylamiodarone (DEA) and may be causally related to the drug-induced intracellular storage of phospholipids (PL). Kinetics of cellular uptake and release of radiolabelled AMIO and DEA were studied following single and multiple exposures of cultured human skin fibroblasts to 5 and 10 microM drug concentrations. AMIO and DEA were efficiently taken up into cultured cells. The rate of uptake was slower than that of other cationic amphiphilic drugs. The intracellular steady state concentrations were in the millimolar range suggesting a lysosomal trapping. Repetitive exposures of cultures resulted in a cumulative and partly saturable drug uptake. The accumulation of DEA was higher than that of AMIO throughout. AMIO and DEA previously taken up into the cells during a 2 hr exposure were completely released into the washing media, suggesting an exchangeable form of the accumulated drugs. Following repetitive exposures only part of the drugs was released. Under chasing conditions using washing media containing non-labelled AMIO and DEA respectively or ammonium chloride the release of the chronically accumulated 14C-labelled drugs was increased. This suggested a drug storage in the form of complexes in acidic compartments. Phospholipid (PL) content as well as individual PL fractions were changed in whole cells and in isolated plasma membranes. PL accumulation is assumed to occur by inhibition of PL degradation due to formation of non-degradable drug-PL complexes or by inhibition of phospholipase activities. Cellular PL accumulation seemed to interfere with PL recycling. Changes in PL composition of purified plasma membranes were in part complementary to the ones in whole cells. The alterations in membrane PL composition may explain the changes in membrane fluidity and the decrease in beta-adrenoceptor density and in isoproterenol-stimulated cAMP formation. The results obtained provide an explanation for the pharmacokinetic, and possibly for the pharmacodynamic and also toxicological behaviour of AMIO and DEA in vivo.

Idazoxan down-regulates β-adrenoceptors on C6 glioma cells in vitro

Incubation of the C6 cells with 10 microM idazoxan (an alpha 2-adrenoceptor antagonist and putative antidepressant) for 5 days in vitro resulted in a 23% reduction of beta-adrenoceptor number and a 37% decrease in isoproterenol-induced cyclic AMP accumulation. In contrast, post-receptor stimulated cyclic AMP accumulation (by the use of forskolin or cholera toxin) was unaffected. The desensitization of the beta-adrenoceptor was accompanied by an increase in the KL/KH ratio for this receptor. Chronic in vitro treatment of C6 glioma cells with idazoxan did not significantly affect cholera or pertussis toxin catalyzed ribosylation of Gs and Gi/Go in these cells. Similarly, idazoxan did not alter either the basal levels of protein kinase C (PKC) alpha, or its cytoplasm to membrane translocation. These results suggest that idazoxan may have direct postsynaptic effects, the site of which may be at the level of receptor/G protein interaction.

Signal-transducing G proteins and antidepressant drugs: evidence for modulation of alpha subunit gene expression in rat brain

Signal-transducing G proteins, heterotrimers formed of alpha, beta, and gamma subunits, are central to the coordination of receptor-effector communication. They are derived from a large gene family, and recent cloning and sequencing of cDNAs encoding the alpha subunits, which confer receptor and effector specificity on the heterotrimer, have defined four major classes, Gs, Gi, Gq, and G12, with at least 16 isotypes. The G proteins that coordinate receptor-effector activity are especially important in the central nervous system (CNS), where they serve widespread, critical roles in the regulation of neuronal function, maintain the functional balance between neurotransmitter systems, and, as such, represent attractive potential targets for antidepressant drugs. We describe an integrated series of animal and cell culture studies aimed at testing the hypothesis that alterations in G protein function may contribute the complex neuroadaptive mechanisms involved in the clinical actions of antidepressants, and demonstrate that long-term administration of a wide spectrum of antidepressant drugs regulate G alpha s, G alpha i1, G alpha i2, G alpha o, G alpha q, and G alpha 12 mRNA and protein expression in various areas of the rat brain. Additionally, we present the polymerase chain reaction-(PCR) mediated cross-species partial cDNA cloning and sequencing of rat and human G alpha o and rat G alpha 12, illustrate the regional distribution of G alpha mRNA and protein in rat brain, and provide evidence that different classes of antidepressants alter expression and/or stability of the recently identified G alpha 12 mRNA. We conclude that long-term treatment with antidepressant drugs exerts differential effects on G alpha mRNA and protein expression in rat brain, thus modifying signal transduction as an integral part of complex neuroadaptive mechanisms that may underlie their therapeutic efficacy. The development of novel drugs with G proteins as primary targets remains an attractive prospect for the future.

High density of beta 2-adrenoceptors in a human keratinocyte cell line with complete epidermal differentiation capacity (HaCaT)

A non-tumorigenic keratinocyte cell line with complete epidermal differentiation capacity (HaCaT) was used in radioligand binding experiments to determine the number of beta-adrenoceptors. Intact cells were saturated with 3H-labelled (-)CGP-12177 (CGP), a hydrophilic non-selective beta-adrenergic antagonist as radioligand. In order to investigate the beta-adrenergic subtype selectivity, displacement experiments were performed with different antagonists and agonists. Binding of CGP to keratinocytes has been shown to be reversible and saturable and to have high affinity (Bmax = 114.0 +/- 8.8 fmol/10(7) cells with 6866 receptors/cell, KD = 0.095 +/- 0.017 nmol/l; n = 11). Beta-adrenergic antagonists inhibited binding yielding monophasic displacement curves. IC50-values (nmol/l) were: propranolol (non-selective) 1.68; CGP-12177 (non-selective) 1.08; ICI 118,551 (beta 2-selective) 2.92; bisoprolol (beta 1-selective) 1230; and CGP-20712 (beta 1-selective) 24,980. Agonists displaced CGP in the order isoprenaline greater than adrenaline greater than noradrenaline. We conclude that HaCaT cells express a high density of beta2-adrenoceptors providing a good model system to study adrenergic receptor mechanisms under reproducible experimental conditions in keratinocytes.

Relationship between membrane fluidity and adrenoceptor binding in depression

Membrane fluidity and adrenergic receptor binding were studied in platelets of depressed patients before and during treatment with desmethylimipramine to investigate the relationship between the alpha 2-adrenergic receptor and its membrane environment in depression. Most samples came from a previous study in which we observed higher 3H-para-aminoclonidine (3H-PAC) binding in platelets from depressed patients compared to healthy subjects. Fluidity was measured by steady state diphenylhexatriene (DPH) anisotropy in both purified plasma membranes and in intracellular membrane preparations from platelets. No differences were observed in DPH membrane fluidity, per se, indicating that fluidity changes probably do not underlie either the increased alpha 2-adrenergic receptor binding in depression or the normalization of binding during treatment. However, lower intracellular membrane fluidity was correlated with higher binding to 3H-PAC site-1 in healthy subjects, but not in depressed patients. Thus, during depression there may be a disruption in the normal relationship between the adrenergic receptor and its membrane environment.

Chronic exposure of C6 glioma cells to desipramine desensitizes beta-adrenoceptors, but increases KL/KH ratio

Incubation of rat glioma C6 cells with 10 microM desipramine for five days in vitro resulted in a 31% reduction of beta-adrenoceptors and a 38% reduction in isoproterenol-stimulated cyclic AMP accumulation. In contrast, forskolin or cholera toxin-stimulated cyclic AMP was unaffected by desipramine. Surprisingly, the beta-adrenoceptor desensitization was accompanied by an increase in the ratio of dissociation constants (KL/KH) for the low and high affinity states of the beta-adrenoceptor respectively and supports the concept of a complex interaction between the receptor and Gs protein.

Characterization and regulation of beta 1-adrenergic receptors in a human neuroepithelioma cell line

Intact human neuroepithelioma SK-N-MC cells bound the beta-adrenergic antagonist (-)-[3H]-CGP 12177 with a KD of 0.13 nM and a Bmax of 17,500 sites/cell. When the cells were exposed to beta-adrenergic agonists, they accumulated cyclic AMP in the following order of potency: isoproterenol much greater than norepinephrine greater than epinephrine, which is indicative of a beta 1-subtype receptor. Membranes prepared from the cells bound (-)-3-[125I]iodocyanopindolol with a KD of 11.5 pM. Inhibition of agonist-stimulated cyclic AMP production and competition binding experiments indicated that the beta 1-selective antagonists CGP 20712A and ICI 89,406 were much more potent than the beta 2-selective antagonist ICI 118,551. Analysis of the displacement curves indicated that the cells contained only beta 1-adrenergic receptors. Northern blot analysis of SK-N-MC mRNA using cDNA probes for the beta 1- and beta 2-adrenergic receptors revealed the presence of a very strong beta 1-adrenergic receptor mRNA signal, while under the same conditions no beta 2-adrenergic receptor mRNA was observed. Thus, SK-N-MC cells appear to express a pure population of beta 1-adrenergic receptors. When the cells were exposed to isoproterenol, there was no observable desensitization during the first hour. After longer exposure, desensitization slowly occurred and the receptors slowly down-regulated to 50% of control levels by 24 h. Other agents that elevate cyclic AMP levels, such as forskolin, cholera toxin, and cyclic AMP analogues, caused no or little substantial receptor loss.

Radioligand binding characteristics of beta 2-adrenoceptors of cultured melanoma cells

The existence of beta-adrenoceptors on intact cells of the human malignant melanoma cell line A-375 was investigated using the binding properties of the tritiated radioligand (-)-[3H]CGP-12177, a hydrophilic non-selective beta-adrenoceptor antagonist. Displacement experiments of the radioligand from its binding site were performed with antagonists and agonists to determine the beta-adrenoceptor subtype selectivity. The binding of (-)-[3H]CGP-12177 was saturable, of high affinity (KD = 0.025 nmol/l, n = 12) and was rapid and readily reversible. The maximal number of binding sites (Bmax) was 33.5 +/- 1.9 fmol/10(7) cells or 2018 +/- 114 receptors per cell. beta-adrenoceptor antagonists inhibited binding of the radioligand with monophasic displacement curves. IC50 values were (nmol/l): propranolol (non-selective) 2.82, alprenolol (non-selective) 2.0, ICI 118,551 (beta 2-selective) 3.5 and bisoprolol (beta 1-selective) 2200. Agonists inhibited binding in the order of potency of isoprenaline greater than adrenaline greater than noradrenaline. It is concluded that cells of the melanoma cell line A-375 contain a homogeneous population of beta 2-adrenoceptors.

Reduction in beta-adrenoceptor density in cultured rat glioma C6 cells after incubation with antidepressants is dependent upon the culturing conditions used

The hydrophilic beta-adrenoceptor ligand (-)-[3H]CGP-12177 binds to intact C6 cells with a high affinity (KD approximately 0.1 nM) and with a high degree of specificity. The binding was inhibited by DL-propranolol (Ki approximately 1 nM). Treatment of cells cultured in Dulbecco's modified Eagle medium (DMEM) without fetal calf serum for 4 days with desipramine reduced the (-)-[3H]CGP-12177 specific binding in a concentration-dependent manner, a reduction from 127 to 102 fmol/mg of protein being found at a ligand concentration of 1 nM after treatment with 10 microM desipramine. Lesser effects were seen after treatment for 1 day. A similar result was found with maprotiline, and reductions in specific binding were seen after 4 days of treatment with amitriptyline, iprindole, and citalopram. The reduction in binding-site density (measured per milligram of protein to compensate for variability in cell density per well), however, was paralleled in all cases by a reduction in the rate of cell proliferation. When C6 glioma cells were cultured in Ham's medium without fetal calf serum during the antidepressant treatment period, a higher specific binding was observed than for the DMEM-cultured cells, and 10 microM desipramine was without effect on either the (-)-[3H]CGP-12177 specific binding or cell proliferation. It is concluded that the effects of the antidepressants tested upon the density of (-)-[3H]CGP-12177 specific binding sites in intact C6 cells may be secondary to the toxicity of the compounds under the conditions used.

Single and multiple desipramine exposures of cultured cells. Changes in cellular anisotropy and in lipid composition of whole cells and of plasma membranes

Effects of the antidepressant drug desipramine (DMI) on fluorescence anisotropy were studied in living cultured human fibroblasts, rat brain astrocytes and rat ROC-1 hybridoma cells (oligodendrocytes x C6). Fluorescence anisotropy, a measure for fluidity, was measured by means of a fluorescence polarization technique using a set of n-(9-anthroyloxy) fatty acids as markers. Apparent fluorescence anisotropies were determined in cells following single or multiple dose exposures to 5 microM DMI at 37 degrees and compared to control cells. In all three cell types single doses of DMI led to significant decreases in anisotropies of the deeper layers (12-AS) of the membranes only, suggesting increases in fluidity. Repeated exposures to 5 microM DMI led to cell specific, significant changes in anisotropies of the superficial membrane layers, as determined by 2-AP, 6-, 7- and 9-AS. The resulting anisotropy values of the three different cell types became more alike than prior to DMI exposure. Alterations in anisotropies were accompanied with changes in the phospholipid patterns of whole cells and isolated plasma membrane vesicles. The changes of PC/PE ratios were consistent with changes observed in fluorescence anisotropies. Such alterations may be individual regulatory responses of the cells to the chronic presence of the drug within the membranes.

Effect of prolonged 5-hydroxytryptamine uptake inhibition by paroxetine on cortical beta 1 and beta 2-adrenoceptors in rat brain

The effects of prolonged (21 day) oral administration of the antidepressants paroxetine (0.9 to 8.9 mg/kg/day) and amitriptyline (2.7 to 27 mg/kg/day), on rat brain cortical beta 1- and beta 2-adrenoceptor numbers and affinities were investigated using [3H]-CGP 12177. Although amitriptyline, 27 mg/kg, caused a significant (p less than 0.05) 20% reduction in the number of beta 1-adrenoceptors, paroxetine, at doses up to 8.9 mg/kg p.o., did not influence binding of [3H]-CGP 12177 to cortical beta 1- or beta 2-adrenoceptors. This study with paroxetine provides further evidence that the down-regulation of central beta 1-adrenoceptors in rat brain after repeated administration is not a property of all antidepressant drugs.

Effects of long-term administration of antidepressants and neuroleptics on receptors in the central nervous system

1. A review of the effects of long-term administration of antidepressants and neuroleptics on receptors in the central nervous system is presented. 2. The effects of antidepressants on adenylate cyclase activity and on receptor binding in brain tissue are discussed. Effects on a variety of receptor types are considered. 3. The utilization of electrophysiological, behavioral, and neurochemical studies to assess receptor function after chronic antidepressant administration is discussed, as is the use of peripheral receptor estimations in clinical studies. 4. Animal studies on the actions of chronic administration of neuroleptics on pre- and postsynaptic dopamine receptors are reviewed. Effects of these drugs on dopamine receptors in humans are considered from the following perspectives: postmortem and in vivo binding studies in schizophrenia, tardive dyskinesia, and central versus peripheral receptor estimation.

The contribution of metabolites to the rapid and potent down-regulation of rat cortical beta-adrenoceptors by the putative antidepressant sibutramine hydrochloride

Sibutramine HCl is an inhibitor of the reuptake of monoamines with a pharmacological profile in rodents indicative of antidepressant activity. The secondary (BTS 54 354) and primary (BTS 54 505) amine metabolites of the tertiary amine sibutramine HCl exhibit similar in vivo pharmacological activity to the parent compound. Thus, each compound displays potent activity in acute behavioural models predictive of antidepressant effects and a comparable ability to inhibit the uptake of monoamines in vivo. In addition, BTS 54 354 and BTS 54 505 induce an equally rapid and potent down-regulation of cortical beta-adrenoceptors in the rat as sibutramine HCl. The secondary and primary amines are, however, considerably more active than sibutramine HCl as inhibitors of the uptake of noradrenaline, dopamine and 5-hydroxytryptamine in vitro. The potent inhibition of the reuptake of noradrenaline by the secondary and primary amine metabolites probably contributes to the rapid and potent down-regulation of beta-adrenoceptors in the rat, induced by the putative antidepressant sibutramine HCl.

Organ-specific, qualitative changes in the phospholipid composition of rats after chronic administration of the antidepressant drug desipramine

Rats were chronically treated with daily i.p. injections of 10 mg/kg desipramine for 21 days. A 30% decrease in the number of beta-adrenoceptors was observed in brain. A receptor desensitization of similar extent was noted in submaxillary glands and lung. No change in beta-adrenoceptor number was present in heart. Total phospholipid contents were not altered in these organs after chronic drug treatment. However, organ-specific changes were found in the phospholipid composition of submaxillary glands, lung and liver but not in whole brain and heart. The changes were variable but an increase in phosphatidylinositol and decreases in phosphatidylethanolamine and sphingomyelin were consistent. Possible alterations in the phospholipid composition of the brain might have been masked by the large and stable pool of myelin phospholipids. A casual relationship between changes in the phospholipid composition and beta-adrenoceptor desensitization is discussed.

Independent regulation of beta 1- and beta 2-adrenoceptors

1 The down-regulation of beta-adrenoceptors has been postulated as a biochemical marker of antidepressant efficacy. Here we demonstrate that chronic treatment with desipramine down-regulates beta 1-adrenoceptors in rat cerebral cortex and that beta-adrenoceptor subtypes can be independently regulated by treatment with different beta-adrenoceptor agonists. 2 Desipramine, (+/-)-clenbuterol, prenalterol, corwin (20 mg kg-1 daily) and corwin (10 mg kg-1 daily) were administered to male, Sprague-Dawley rats, over eight days, by means of osmotic Alzet pumps placed subcutaneously and removed 24 h before analysis. Control rats received vehicle only. The beta 1- and beta 2-adrenoceptor populations were measured in cerebral cortex by a modified (-)-[125I]-pindolol receptor binding assay. 3 The conventional antidepressant, desipramine, preferentially down-regulated beta 1-adrenoceptors whereas the non-selective beta-adrenoceptor agonist (+/-)-clenbuterol preferentially down-regulated beta 2-adrenoceptors. The beta 1-selective partial agonist, prenalterol, up-regulated beta 1-adrenoceptors perhaps acting more as an antagonist than as an agonist. Finally, neither dose of corwin had any significant effect on beta-adrenoceptor number.

Effect of the tricyclic antidepressant desipramine on beta-adrenergic receptors in cultured rat glioma C6 cells

Rat glioma C6 cells, cultured in the presence of the tricyclic antidepressant desipramine, lost a significant number of beta-adrenergic receptors in a time- and dose-dependent manner. A similar loss was observed whether binding was determined on intact cells with the hydrophilic beta-adrenergic antagonist (+/-)-[3H]4-(3-tert-butylamino-2-hydroxypropoxyl)benzimidazole-2-o n HCl ([3H]CGP-12177) or on cell lysates with the more hydrophobic antagonists [125I]iodocyanopindolol or [3H]dihydroalprenolol. When stimulated with the agonist isoproterenol, desipramine-treated cells accumulated less cyclic AMP than control cells. The affinity of the beta-adrenergic receptors for either antagonist or agonist was unchanged after desipramine treatment. Desipramine interacted only weakly with the receptors and competed for [125I]iodocyanopindolol binding with a Ki of 30 microM. The presence in the culture medium of alprenolol or propranolol, potent beta-adrenergic antagonists, however, did not prevent the reduction in receptors by desipramine. Desipramine also caused a loss of beta-adrenergic receptors from cells maintained in serum-free medium and the cells themselves did not contain or secrete endogenous catecholamines. Although desipramine is a potent inhibitor of catecholamine uptake, it appears unlikely that the observed loss of beta-adrenergic receptors in rat glioma C6 cells exposed to the drug is due to an increase in extracellular catecholamine levels or to a direct interaction with the receptors.

Subcellular distribution of the antidepressant drug desipramine in cultured human fibroblasts after chronic administration. Drug-effect on the subcellular distribution of accumulated phospholipids

Desipramine (DMI) is an important antidepressant drug and a lysosomotropic substance. In cultured fibroblasts it interferes with lysosomal functions, e.g. phospholipid degradation. Chronic exposure of cells with DMI induces storage of phospholipids. Subcellular fractionations of cultured human fibroblasts that had been exposed to a short pulse of 3H-DMI showed accumulation of DMI in two acidic compartments, one of high density represented the lysosomes and one of much lower density may contain pinosomes. In chronically exposed cells DMI accumulated in the subcellular fractions of lower density only. DMI induced an important shift of lysosomal enzymes from vesicles of high density to the ones of lower density. Phospholipids were accumulating in those vesicles of lower density as well as in the fractions that contained plasma membranes. DMI also accumulated in one part of the Golgi vesicles of acute and chronically exposed cells. In the latter phospholipids and arylsulfatase A activity were also accumulating. DMI possibly interferes with membrane recycling. This eventually could induce changes in phospholipid content and composition in the plasma membrane which may have important implications for membrane functions.