Lithium administration modulates platelet Gi in humans

Genetic association of cyclic AMP signaling genes with bipolar disorder

The genetic basis for bipolar disorder (BPD) is complex with the involvement of multiple genes. As it is well established that cyclic adenosine monophosphate (cAMP) signaling regulates behavior, we tested variants in 29 genes that encode components of this signaling pathway for associations with BPD type I (BPD I) and BPD type II (BPD II). A total of 1172 individuals with BPD I, 516 individuals with BPD II and 1728 controls were analyzed. Single SNP (single-nucleotide polymorphism), haplotype and SNP × SNP interactions were examined for association with BPD. Several statistically significant single-SNP associations were observed between BPD I and variants in the PDE10A gene and between BPD II and variants in the DISC1 and GNAS genes. Haplotype analysis supported the conclusion that variation in these genes is associated with BPD. We followed-up PDE10A's association with BPD I by sequencing a 23-kb region in 30 subjects homozygous for seven minor allele risk SNPs and discovered eight additional rare variants (minor allele frequency < 1%). These single-nucleotide variants were genotyped in 999 BPD cases and 801 controls. We obtained a significant association for these variants in the combined sample using multiple methods for rare variant analysis. After using newly developed methods to account for potential bias from sequencing BPD cases only, the results remained significant. In addition, SNP × SNP interaction studies suggested that variants in several cAMP signaling pathway genes interact to increase the risk of BPD. This report is among the first to use multiple rare variant analysis methods following common tagSNPs associations with BPD.

Chronic lithium administration enhances noradrenergic responses to intravenous administration of the alpha2 antagonist idazoxan in healthy volunteers

The acute and chronic effects of lithium carbonate administration at therapeutic blood levels on peripheral noradrenergic activity and sympathetic responses to alpha2 adrenoceptor blockade were examined in 10 medically and psychiatrically healthy volunteers. Supine resting levels of plasma norepinephrine and the increases in norepinephrine following intravenous infusion of 200 microg/kg of idazoxan, a selective alpha2 adrenoceptor antagonist, were determined before lithium (Li+) administration and after 5 days and after 4 weeks of daily Li+ treatment. Chronic Li+ treatment significantly increased mean resting plasma norepinephrine levels by 53.6%. The noradrenergic responses to infusions of idazoxan were slightly enhanced after 5 days of Li+ administration and significantly increased following 4 weeks of Li+ treatment. The possibility that Li+ produces functional alpha2 subsensitivity causing enhanced peripheral noradrenergic activity in humans is supported by the findings of increased mean resting plasma norepinephrine and increased response to idazoxan following chronic Li+ administration. Alteration of regulatory mechanisms in the noradrenergic system may be relevant to understanding the clinical effects of Li+ in manic-depressive illness.

Effect of alpha 2-adrenoceptor blockade on lithium action in the rat brain

The inhibitory effect of different concentrations of lithium (0.15-10 x 10(-3) M) on cAMP production induced by isoprenaline (1 x 10(-4) M) after the blockade of alpha(2)-adrenoceptors in the rat cerebral cortex was investigated. Low lithium concentrations (0.3-0.6 x 10(-3) M) exerted a significant inhibitory effect after yohimbine (1 x 10(-5) M) addition, but had no effect when isoprenaline alone or prazosin (1 x 10(-7) M) was added. The recovery of [3H]yohimbine binding after irreversible inactivation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) was evaluated in cortical membranes to study how alpha(2)-adrenoceptor repopulation affects the action of lithium on the adenylyl cyclase-cAMP system. When the density of alpha(2)-adrenoceptors was lower than 21%, lithium showed a significant inhibitory effect at all concentrations tested. However, at higher densities, increased concentrations of lithium were required to inhibit cAMP production. Our results suggest that the inhibitory effect of lithium on cAMP levels in the rat brain is conditioned by alpha(2D)-adrenoceptors.

Molecular targets of lithium action

Lithium is an effective drug for both the treatment and prophylaxis of bipolar disorder. However, the precise mechanism of lithium action is not yet well understood. Extensive research aiming to elucidate the molecular mechanisms underlying the therapeutic effects of lithium has revealed several possible targets. The behavioral and physiological manifestations of the illness are complex and are mediated by a network of interconnected neurotransmitter pathways. Thus, lithium's ability to modulate the release of serotonin at presynaptic sites and modulate receptor-mediated supersensitivity in the brain remains a relevant line of investigation. However, it is at the molecular level that some of the most exciting advances in the understanding of the long-term therapeutic action of lithium will continue in the coming years. The lithium cation possesses the selective ability, at clinically relevant concentrations, to alter the PI second-messenger system, potentially altering the activity and dynamic regulation of receptors that are coupled to this intracellular response. Subtypes of muscarinic receptors in the limbic system may represent particularly sensitive targets in this regard. Likewise, preclinical data have shown that lithium regulates arachidonic acid and the protein kinase C signaling cascades. It also indirectly regulates a number of factors involved in cell survival pathways, including cAMP response element binding protein, brain-derived neurotrophic factor, bcl-2 and mitogen-activated protein kinases, and may thus bring about delayed long-term beneficial effects via under-appreciated neurotrophic effects. Identification of the molecular targets for lithium in the brain could lead to the elucidation of the pathophysiology of bipolar disorder and the discovery of a new generation of mood stabilizers, which in turn may lead to improvements in the long-term outcome of this devastating illness (1).

Mood stabilizer psychopharmacology

Mood stabilizers represent a class of drugs that are efficacious in the treatment of bipolar disorder. The most established medications in this class are lithium, valproic acid, and carbamazepine. In addition to their therapeutic effects for treatment of acute manic episodes, these medications often are useful as prophylaxis against future episodes and as adjunctive antidepressant medications. While important extracellular effects have not been excluded, most available evidence suggests that the therapeutically relevant targets of this class of medications are in the interior of cells. Herein we give a prospective of a rapidly evolving field, discussing common effects of mood stabilizers as well as effects that are unique to individual medications. Mood stabilizers have been shown to modulate the activity of enzymes, ion channels, arachidonic acid turnover, G protein coupled receptors and intracellular pathways involved in synaptic plasticity and neuroprotection. Understanding the therapeutic targets of mood stabilizers will undoubtedly lead to a better understanding of the pathophysiology of bipolar disorder and to the development of improved therapeutics for the treatment of this disease. Furthermore, the involvement of mood stabilizers in pathways operative in neuroprotection suggests that they may have utility in the treatment of classical neurodegenerative disorders.

Signaling networks in the pathophysiology and treatment of mood disorders

Over the past decade, the focus of research into the pathophysiology of mood disorders (bipolar disorder and unipolar depression in particular) has shifted from an interest in the biogenic amines to an emphasis on second messenger systems within cells. Second messenger systems rely on cell membrane receptors to relay information from the extracellular environment to the interior of the cell. Within the cell, this information is processed and altered, eventually to the point where gene and protein expression patterns are changed. There is a preponderance of evidence implicating second messenger systems and their primary contact with the extracellular environment, G proteins, in the pathophysiology of mood disorders. After an introduction to G proteins and second messenger pathways, this review focuses on the evidence implicating G proteins and two second messenger systems-the adenylate cyclase (cyclic adenosine monophosphate, cAMP) and phosphoinositide (protein kinase C, PKC) intracellular signaling cascades-in the pathophysiology and treatment of bipolar disorder and unipolar depression. Emerging evidence implicates changes in cellular resiliency, neuroplasticity and additional cellular pathways in the pathophysiology of mood disorders. The systems discussed within this review have been implicated in neuroplastic processes and in modulation of many other cellular pathways, making them likely candidates for mediators of these findings.

Regulation of flagellar length in Chlamydomonas

The length of eukaryotic cilia and flagella depends on the cell cycle-regulated assembly and disassembly of at least 9 doublet and 2 central microtubules, their associated proteins, and the surrounding membrane. In light-synchronized Chlamydomonas cells, flagella assembled to 10-14 microm in length near the beginning of the light period and they disassembled prior to cell division, during the dark period. Flagella on light-synchronized pf18 Chlamydomonas mutants grew to 10-12 microm near the beginning of the light period but shortened by 50% or more by the end of the light period. Flagellar length was cell-cycle regulated: when flagella were amputated at various times during the light period, new flagella regenerated to the lengths of control cells at that time of the light cycle. The later in the cycle pf18 cells were deflagellated, the shorter were the regenerated flagella. Flagellar shortening was not affected, in either pf18 or wild-type (wt) cells, by inhibitors of protein synthesis or of microtubule assembly, so flagellar length cannot depend on protein turnover. Shortening in pf18 was attenuated by Li+, which stimulated flagellar growth in wt cells, by red light, by protein kinase inhibitors, and by the Ca2+ channel blockers La3+ and Cd2+. Shortening was increased by cAMP, Na+, K+, and EGTA. Ca2+-CAM blockers did not affect pf18 shortening but they increased shortening in wt and fa1 cells. We propose that flagellar length is regulated by a signal transduction pathway that is sensitive to Ca2+ levels and red light.

Signalling pathways in the brain: cellular transduction of mood stabilisation in the treatment of manic-depressive illness

The long-term treatment of manic-depressive illness (MDI) likely involves the strategic regulation of signalling pathways and gene expression in critical neuronal circuits. Accumulated evidence has identified signalling pathways, in particular the family of protein kinase C (PKC) isozymes, as targets for the long-term action of lithium. Chronic lithium administration produces a reduction in the expression of PKC alpha and epsilon, as well as a major PKC substrate, MARCKS, which has been implicated in long-term neuroplastic events in the developing and adult brain. More recently, studies have demonstrated robust effects of lithium on another kinase system, GSK-3beta, and on neuroprotective/neurotrophic proteins in the brain. Given the key roles of these signalling cascades in the amplification and integration of signals in the central nervous system, these findings have clear implications not only for research into the neurobiology of MDI, but also for the future development of novel and innovative treatment strategies.

Regulation of signal transduction pathways and gene expression by mood stabilizers and antidepressants

OBJECTIVE

To determine whether the currently available evidence supports the hypothesis that antidepressants and mood stabilizers may bring about some of their long-term therapeutic effects by regulating signal transduction pathways and gene expression in the central nervous system.

METHODS

To address this question, we reviewed the evidence showing that chronic administration of antidepressants and mood stabilizers involves alterations in signaling pathways and gene expression in the central nervous system.

RESULTS

A large body of data has shown that lithium and valproate exert effects on the protein kinase C signaling pathway and the activator protein 1 family of transcription factors; in contrast, antidepressants affect the cyclic adenosine monophosphate pathway and may bring about their therapeutic effects by modulating cyclic adenosine monophosphate-regulated gene expression in the central nervous system.

CONCLUSIONS

Given the key roles of these signaling cascades in the amplification and integration of signals in the central nervous system, the findings have clear implications not only for research into the etiology and pathophysiology of the severe mood disorders but also for the development of novel and innovative treatment strategies.

Effects of lithium on receptor-mediated activation of G proteins in rat brain cortical membranes

The underlying molecular mechanism of action of lithium in the treatment of manic-depressive illness is not clear. The effect of chronic lithium on GTP-binding and toxin-mediated ADP-ribosylation of specific G proteins in brain cortical membranes was examined. Incubation of cortical membranes with 5-HT increased [35S]GTPgammaS binding to Galphas, Galphai, Galphao and Galphaq proteins. Six weeks but not 1 week of lithium treatment reduced the increases in [35S]GTPgammaS binding to Galphas, Galphai and Galphao which are produced by 5-HT by 75-85%, whereas 5-HT stimulated [35S]GTPgammaS binding to Galphaq was reduced by 38%. No changes in membrane levels of Galpha and Gbeta proteins were noted in lithium-treated rats. Pertussis toxin (PTX)-mediated ADP-ribosylation of Galphai/o was increased by 60% in cortical membranes of chronically treated rats. Lithium treatment did not affect cholera toxin-mediated ribosylation of Galphas. Increases in [35S]GTPgammaS binding to Galpha proteins evoked by 5-HT were also inhibited by 0.5-2.0 mM lithium chloride added in vitro to the assay mixture. Rubidium and cesium did not change 5-HT-stimulated G protein activation. ADP-ribosylation of Galphai/o catalyzed by PTX was not changed by in vitro LiCl. The inhibitions of 5-HT-stimulated increases in [35S]GTPgammaS-binding to Galphas and Galphaq were completely suppressed by 2.4 mM MgCl2 this concentration of MgCl2 inhibited the effect of lithium on Galphai and Galphao by 50%. Similar findings were also noted when [alpha-32P]GTP was used in the binding assay. The results suggest that lithium interferes with receptor-G protein coupling via a Mg2+-sensitive mechanism. This action of the drug is more effective for Gs, Gi and Go than for Gq and may result from its interference with the recycling of trimeric G proteins.

Neuroimmunomodulatory actions of hypothalamic interferon-alpha

Recent studies have revealed that the brain produces interferon-alpha (IFN-alpha) in response to noninflammatory as well as inflammatory stress and that it might have a role in normal physiology. When administered intracerebrally, IFN-alpha causes diverse effects including fever, anorexia, analgesia and changes in the central neuronal activities. These responses are inhibited by the opioid receptor antagonist naloxone. This is consistent with the reports suggesting that recombinant human (rh) IFN-alpha binds to opioid receptors in rodent brain membrane. We revealed that rhIFN-alpha altered the activity of thermosensitive neurons in the medial preoptic area (MPO) and glucose-responsive neurons in the ventromedial hypothalamus in an opioid-receptor-dependent way. As a stress which produces opioid-dependent analgesia is known to suppress the cytotoxicity of splenic natural killer cells, we investigated whether the administration of beta-endorphin and rhIFN-alpha may induce a similar immunosuppression. We found that central, but not peripheral, injection of both compounds inhibited natural killer (NK) cytotoxicity. Further studies revealed that rhIFN-alpha decreased the activity of MPO neurons via opioid receptors and the altered activity of MPO neurons in turn resulted in the activation of corticotropin-releasing factor neurons, thereby suppressing NK cytotoxicity predominantly through activation of the splenic sympathetic nerve and beta-receptor mechanisms in splenocytes. Thus, IFN-alpha may alter the brain activity to exert a feedback effect on the immune system. Further detailed whole-cell clamping analyses on neuronal mechanisms in rat brain tissue slices showed that the inhibitory effect of rhIFN-alpha on N-methyl-D-aspartate-induced membrane current responses of MPO neurons was mediated not only by opioid receptors but also by the local production of reactive oxygen intermediates, nitric oxide and prostanoids, possibly due to neuron-glial cell interaction.

Signal transduction by platelet adenylate cyclase: alterations in depressed patients may reflect impairment in the coordinated integration of cellular signals (coincidence detection)

BACKGROUND

Adenylate cyclase (AC) responds to distinct but coincident signals from the agonist-stimulated G-protein Gs and the inhibitory G-protein Gi by generating a greater output signal-to-noise ratio--i.e., agonist-stimulated to basal ratio (fold-stimulation)--through coincidence detection than that generated by a single input (Gs) alone. Such coincidence detection by murine brain AC was found to be enhanced during chronic antidepressant treatment with imipramine.

METHODS

We examined and compared the basal, agonist-stimulated, and guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) or AlF4 ion postreceptor-stimulated AC activities in mononuclear leukocytes and platelets from the same blood specimens obtained from depressed patients (n = 27) and control subjects (n = 19).

RESULTS

In all subjects, the differences (delta GTP gamma S or delta AlF4) between postreceptor measures of AC in mononuclear leukocytes (where AC is regulated by Gs but not by Gi) and platelets (where AC is regulated by both Gs and Gi) were highly significant. In controls, the relationships between delta GTP gamma S or delta AlF4 and basal, agonist-stimulated, and the fold-stimulation of agonist-stimulated platelet AC resembled the regulation of AC by Gi in model-membrane systems. Comparable relationships between delta GTP gamma S or delta AlF4 and basal, agonist-stimulated, and the fold-stimulation of agonist-stimulated platelet AC activities were not observed in depressed patients.

CONCLUSIONS

Our results suggest that in controls, platelet AC enzyme activity is determined (in part) by the coordinated integration of signals from Gs and Gi through coincidence detection, while such coincidence detection by platelet AC may be impaired in patients with depressive disorders.

Guanine nucleotide binding proteins in opioid-dependent patients

Guanine nucleotide binding (G) protein levels and functioning in the platelets of 19 methadone-maintained patients were compared to age and sex matched, normal controls. We found that in the methadone patients, G alpha s-levels were significantly higher, while the levels of G alpha i 1/2 and pertussis toxin catalyzed [32P]ADP ribosylation were significantly lower compared to control subjects in platelet membranes. We have further found that when all three of these biochemical indicators were combined in a discriminant function analysis, 79% of the methadone patients were correctly classified and 83% of the controls were correctly classified.

Platelet pertussis toxin-sensitive G proteins in affective disorders

Pertussis toxin (islet-activating protein, IAP) sensitive guanine nucleotide-binding regulatory (G) proteins were quantitatively determined using [32P]ADP-ribosylating response in the platelet membranes prepared from patients with affective disorders (3 bipolar, 10 major depression) and sex- and age-matched controls. IAP-catalyzed [32P]ADP-ribosylation was not significantly different between patients and controls, suggesting that the quantity of IAP-sensitive G proteins is unaltered in affective disorder patients. The implication of this result was discussed with special reference to the previous reports dealing with the role of G proteins in affective disorders.

Actions of lithium on the cyclic AMP signalling system in various regions of the brain--possible relations to its psychotropic actions. A study on the adenylate cyclase in rat cerebral cortex, corpus striatum and hippocampus

It has been estimated that in most industrialized countries 1 person out of every 1000 in the population is undergoing lithium treatment to stabilize their episodic mood disturbances due to manic-depressive illness. Lithium may stabilize mood swings by altering the action of certain neurotransmitters at the synaptic level in the brain. Recent research suggests that lithium alters neurotransmission by affecting neurotransmitter-coupled second messenger systems. A major second messenger system is the adenylate cyclase, which generates intracellular cAMP from ATP. The adenylate cyclases (type I-IV) are regulated by stimulatory and inhibitory receptors, which either stimulate or inhibit the adenylate cyclase activity. The stimulatory and inhibitory neurotransmitter-receptor signals are transferred to the catalytic unit of the adenylate cyclase by Gs and Gi, respectively. The activated receptor induces GTP stimulation of the heterotrimeric G protein, leading to a dissociation of the protein into the active alpha*GTP and the beta gamma complex. The former stimulates the catalytic unit of adenylate cyclase. The stimulation is terminated by a GTPase located on the alpha subunit that converts GTP to inactive GDP. At present, G proteins are known to play a central role in coupling receptors to effector proteins. In addition to extracellular regulation due to neurotransmitters, some adenylate cyclases (type I, III) are regulated by CaM as a consequence of enhanced intracellular concentrations of free Ca2+. The Ca(2+)-dependent stimulation of adenylate cyclase by CaM is assumed to occur by a direct effect on the catalytic unit. The catalytic units sensitive to Ca(2+)-CaM are also subjected to regulation by stimulatory and inhibitory neurotransmitter stimuli. Magnesium is essential for adenylate cyclase activity, since MgATP2- is the enzyme substrate. Furthermore, one Mg2+ site located on the G protein regulates both the receptor agonist affinity and the dissociation of the G protein during the activation cycle. A second Mg2+ site on the catalytic unit is responsible for Mg2+ regulation of the catalytic activity. The present work aimed at investigating the mechanisms by which lithium in vitro and after chronic treatment (ex vivo) affects adenylate cyclase activities in various regions of the rat brain. Lithium in vitro and ex vivo inhibited the selective stimulation of adenylate cyclase by Ca(2+)-CaM in the cerebral cortex. Furthermore, lithium in vitro interacted directly with the catalytic unit of adenylate cyclase.(ABSTRACT TRUNCATED AT 400 WORDS)

CNS signal transduction in the pathophysiology and pharmacotherapy of affective disorders and schizophrenia

Until recently, research on the neurochemical basis of affective disorders (AD) and schizophrenia (SCZ) focused on detecting postulated disturbances in presynaptic neurotransmitter release and metabolism, or postsynaptic receptor function. New insights into the molecular mechanisms involved in the propagation of neurotransmitter signals across biological membranes and in the regulation of neuronal responses have allowed the development of novel hypotheses, which may explain the altered postsynaptic neuroreceptor responsivity thought to be integral to the pathophysiology of these disorders. In this review we evaluate evidence from both basic science and clinical research implicating disturbances in postreceptor signal transduction in the pathophysiology and pharmacotherapy of AD and SCZ. Specific findings regarding potential postreceptor sites of pathophysiology are highlighted in each of these disorders, together with the growing body of data on the possible postreceptor loci of psychotropic drug action, especially lithium and antidepressants.

5-HT-1A receptor-mediated effects of antidepressants

1. Antidepressant (AD) drugs in general induce subsensitivity of behavioural functions associated with activation of 5-HT-1a receptors in animals. 2. Electrophysiological studies in animals in general indicate increased serotonergic transmission after AD administration, mediated partly by increased functioning of post-synaptic 5-HT-1a receptors in the hippocampus. 3. Binding studies have in general shown no change in 5-HT-1a receptor number either pre-or post-synaptically, while results of second messenger studies (inhibition of adenylate cyclase) indicate subsensitivity after AD administration. 4. Human studies also indicate subsensitivity of 5-HT-1a receptors after ADs.

In vivo evidence that lithium inactivates Gi modulation of adenylate cyclase in brain

In vivo microdialysis of cyclic AMP from prefrontal cortex complemented by ex vivo measures was used to investigate the possibility that lithium produces functional changes in G proteins that could account for its effects on adenylate cyclase activity. Four weeks of lithium administration (serum lithium concentration of 0.85 +/- 0.05 mM; n = 11) significantly increased the basal cyclic AMP content in dialysate from prefrontal cortex of anesthetized rats. Forskolin infused through the probe increased dialysate cyclic AMP, but the magnitude of this increase was unaffected by chronic lithium administration. Inactivation of the inhibitory guanine nucleotide binding protein Gi with pertussis toxin increased dialysate cyclic AMP in control rats, as did stimulation with cholera toxin (which activates the stimulatory guanine nucleotide binding protein Gs). The effect of pertussis toxin was abolished following chronic lithium, whereas the increase in cyclic AMP after cholera toxin was enhanced. In vitro pertussis toxin-catalyzed ADP ribosylation of alpha i (and alpha o) was increased by 20% in prefrontal cortex from lithium-treated rats, but the alpha i and alpha s contents (as determined by immunoblot) as well as the cholera toxin-catalyzed ADP ribosylation of alpha s were unchanged. Taken together, these results suggest that chronic lithium administration may interfere with the dissociation of Gi into its active components and thereby remove a tonic inhibitory influence on adenylate cyclase, with resultant enhanced basal and cholera toxin-stimulated adenylate cyclase activity.

Quantitative determination of pertussis toxin-sensitive G proteins using [32P]ADP-ribosylation in human platelet membranes: negative correlation with ages

The optimum condition to quantitate the [32P]ADP-ribosylation catalyzed by pertussis toxin (islet-activating protein, IAP) in human platelet membranes was investigated. Autoradiography indicated the incorporation of 32P into the band corresponding to the molecular weight of 40-41 kDa, which was augmented by the addition of GTP in the presence of 10 mM MgCl2. On the other hand, non-hydrolyzable GTP analogue, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) enhanced the IAP-catalyzed [32P]ADP-ribosylation only in the absence of MgCl2. The amounts of IAP-catalyzed [32P]ADP-ribosylation in the presence of 100 microM GTP and 10 mM MgCl2 were linear in proportion to the protein concentrations within the limited range of protein concentrations, indicating that this simple quantitative method could be adequately used to evaluate the IAP-sensitive G proteins. Data from fifteen healthy volunteers (7 males and 8 females ranging 24 to 60 years old) indicate that the amounts of IAP-sensitive G proteins in platelet membranes are significantly negatively correlated with ages.