Stimulation of serotonin transport by the cyclic GMP phosphodiesterase-5 inhibitor sildenafil

Functional and pathological consequences of being fast on the uptake: Protein kinase G and p38α MAPK regulation of serotonin transporters

Serotonin (5-hydroxytryptamine, 5-HT) signaling plays an important role in dynamic control of peripheral and central nervous system physiology, with altered 5-HT homeostasis implicated in a significant number of disorders, ranging from pulmonary, bowel, and metabolic disease to depression, obsessive-compulsive disorder, and autism spectrum disorder (ASD). The presynaptic, 5-HT transporter (SERT) has a well-established role in regulating 5-HT signaling and is a target of widely prescribed psychotherapeutics, the 5-HT selective reuptake inhibitors (SSRIs). Although SSRI therapy provides symptom relief for many suffering from mood and anxiety disorders, response to these medications is slow (weeks), and too many receive modest or no benefit. At present, all prescribed SSRIs act as competitive SERT antagonists. Although non-serotonergic therapeutics for mood disorders deserve aggressive investigation, the development of agents that target SERT regulatory pathways have yet to be considered for their possible utility and may possibly offer improved efficacy and more rapid onset. Here, we focus attention on a significant body of evidence that SERT transport activity can be rapidly elevated by protein kinase G (PKG) and p38α mitogen activated protein kinase (MAPK) linked pathways, mechanisms that are impacted by disease-associated genetic variation. Here, we provide a brief overview of kinase-linked, posttranslational regulation of SERT, with a particular focus on evidence from pharmacological and genetic studies that the transporter's regulation by PKG/p38α MAPK associated pathways offers an opportunity to more subtly adjust, rather than eliminate, SERT function as a therapeutic strategy.

Kinase-dependent Regulation of Monoamine Neurotransmitter Transporters

Modulation of neurotransmission by the monoamines dopamine (DA), norepinephrine (NE), and serotonin (5-HT) is critical for normal nervous system function. Precise temporal and spatial control of this signaling in mediated in large part by the actions of monoamine transporters (DAT, NET, and SERT, respectively). These transporters act to recapture their respective neurotransmitters after release, and disruption of clearance and reuptake has significant effects on physiology and behavior and has been linked to a number of neuropsychiatric disorders. To ensure adequate and dynamic control of these transporters, multiple modes of control have evolved to regulate their activity and trafficking. Central to many of these modes of control are the actions of protein kinases, whose actions can be direct or indirectly mediated by kinase-modulated protein interactions. Here, we summarize the current state of our understanding of how protein kinases regulate monoamine transporters through changes in activity, trafficking, phosphorylation state, and interacting partners. We highlight genetic, biochemical, and pharmacological evidence for kinase-linked control of DAT, NET, and SERT and, where applicable, provide evidence for endogenous activators of these pathways. We hope our discussion can lead to a more nuanced and integrated understanding of how neurotransmitter transporters are controlled and may contribute to disorders that feature perturbed monoamine signaling, with an ultimate goal of developing better therapeutic strategies.

Mice genetically depleted of brain serotonin do not display a depression-like behavioral phenotype

Reductions in function within the serotonin (5HT) neuronal system have long been proposed as etiological factors in depression. Selective serotonin reuptake inhibitors (SSRIs) are the most common treatment for depression, and their therapeutic effect is generally attributed to their ability to increase the synaptic levels of 5HT. Tryptophan hydroxylase 2 (TPH2) is the initial and rate-limiting enzyme in the biosynthetic pathway of 5HT in the CNS, and losses in its catalytic activity lead to reductions in 5HT production and release. The time differential between the onset of 5HT reuptake inhibition by SSRIs (minutes) and onset of their antidepressant efficacy (weeks to months), when considered with their overall poor therapeutic effectiveness, has cast some doubt on the role of 5HT in depression. Mice lacking the gene for TPH2 are genetically depleted of brain 5HT and were tested for a depression-like behavioral phenotype using a battery of valid tests for affective-like disorders in animals. The behavior of TPH2(-/-) mice on the sucrose preference test, tail suspension test, and forced swim test and their responses in the unpredictable chronic mild stress and learned helplessness paradigms was the same as wild-type controls. While TPH2(-/-) mice as a group were not responsive to SSRIs, a subset responded to treatment with SSRIs in the same manner as wild-type controls with significant reductions in immobility time on the tail suspension test, indicative of antidepressant drug effects. The behavioral phenotype of the TPH2(-/-) mouse questions the role of 5HT in depression. Furthermore, the TPH2(-/-) mouse may serve as a useful model in the search for new medications that have therapeutic targets for depression that are outside of the 5HT neuronal system.

Characterization of intracellular regions in the human serotonin transporter for phosphorylation sites

In the central nervous system, synaptic levels of the monoamine neurotransmitter serotonin are mainly controlled by the serotonin transporter (SERT), and drugs used in the treatment of various psychiatric diseases have SERT as primary target. SERT is a phosphoprotein that undergoes phosphorylation/dephosphorylation during transporter regulation by multiple pathways. In particular, activation and/or inhibition of kinases including PKC, PKG, p38MAPK, and CaMKII modulate SERT function and trafficking. The molecular mechanisms by which kinase activity is linked to SERT regulation are poorly understood, including the identity of specific phosphorylated residues. To elucidate SERT phosphorylation sites, we have generated peptides corresponding to the entire intracellular region of human SERT and performed in vitro phosphorylation assays with a panel of kinases suggested to be involved in SERT regulation or for which canonical phosphorylation sites are predicted. Peptide analysis by liquid chromatography-tandem mass spectrometry (LC-MS/MS) was used to identify and quantify site-specific phosphorylation. Five residues located in the N- and C-termini and in intracellular loop 1 and 2 were identified as phosphorylation sites; Ser149, Ser277, and Thr603 for PKC, Ser13 for CaMKII, and Thr616 for p38MAPK. Possible regulatory roles of these potential phosphoacceptors for SERT function and surface expression were investigated using phospho-mimicking and phosphodeficient mutations, coexpression of constitutively active kinases and pharmacological kinase induction in a heterologous expression system. Our results suggest that Ser277 is involved in an initial phase of PKC-mediated down-regulation of SERT. The five identified sites can guide future studies of direct links between SERT phosphorylation and regulatory processes.

A dialogue between the immune system and brain, spoken in the language of serotonin

Neuropsychiatric disorders have long been linked to both immune system activation and alterations in serotonin (5-HT) signaling. In the CNS, the contributions of 5-HT modulate a broad range of targets, most notably, hypothalamic, limbic and cortical circuits linked to the control of mood and mood disorders. In the periphery, many are aware of the production and actions of 5-HT in the gut but are unaware that the molecule and its receptors are also present in the immune system where evidence suggests they contribute to the both innate and adaptive responses. In addition, there is clear evidence that the immune system communicates to the brain via both humoral and neuronal mechanisms, and that CNS 5-HT neurons are a direct or indirect target for these actions. Following a brief primer on the immune system, we describe our current understanding of the synthesis, release, and actions of 5-HT in modulating immune function, including the expression of 5-HT biosynthetic enzymes, receptors, and transporters that are typically studied with respect to the roles in the CNS. We then orient our presentation to recent findings that pro-inflammatory cytokines can modulate CNS 5-HT signaling, leading to a conceptualization that among the many roles of 5-HT in the body is an integrated physiological and behavioral response to inflammatory events and pathogens. From this perspective, altered 5-HT/immune conversations are likely to contribute to risk for neurobehavioral disorders historically linked to compromised 5-HT function or ameliorated by 5-HT targeted medications, including depression and anxiety disorders, obsessive-compulsive disorder (OCD), and autism. Our review raises the question as to whether genetic variation impacting 5-HT signaling genes may contribute to maladaptive behavior as much through perturbed immune system modulation as through altered brain mechanisms. Conversely, targeting the immune system for therapeutic development may provide an important opportunity to treat mental illness.

Sildenafil, a phosphodiesterase type 5 inhibitor, enhances the activity of two atypical antidepressant drugs, mianserin and tianeptine, in the forced swim test in mice

Sildenafil, a selective phosphodiesterase type 5 inhibitor, has recently been reported to abolish anti-immobility action of antidepressant drugs, i.e., bupropion, venlafaxine and S-citalopram, in the forced swim test in mice. The present study was designed to investigate the influence of sildenafil on the potential of two atypical antidepressants, namely mianserin and tianeptine. Swim sessions were conducted by placing mice in glass cylinders filled with water for 6 min and the duration of the behavioral immobility during the last 4 min of the test was evaluated. Locomotor activity was measured with photoresistor actimeters. To evaluate the potential pharmocokinetic interaction, total brain concentrations of the studied antidepressants were determined by HPLC method. Sildenafil at a dose of 2.5 mg/kg did not affect the activity of mianserin (20 mg/kg) in the forced swim test. Interestingly, at higher doses (5 and 10 mg/kg), sildenafil significantly enhanced the anti-immobility action of mianserin. Likewise, sildenafil (5, 10 and 20 mg/kg) robustly augmented the antidepressant activity of tianeptine (30 mg/kg). Mianserin alone, as well as in a combination with sildenafil at the highest dose, caused a potent reduction in locomotor activity. However, the changes in motor activity did not interfere with the data obtained in the forced swim test. Sildenafil significantly increased the total brain tianeptine concentration. No alteration in mianserin level in the brain after sildenafil co-administration was observed. The present study suggests that sildenafil enhances the activity of mianserin and tianeptine in the forced swim test in mice. The changes in the antidepressant activity of mianserin evoked by sildenafil co-administration were related to pharmacodynamic interaction while the interaction between tianeptine and sildenafil was, at least in part, pharmacokinetic in nature.

SLC6 neurotransmitter transporters: structure, function, and regulation

The neurotransmitter transporters (NTTs) belonging to the solute carrier 6 (SLC6) gene family (also referred to as the neurotransmitter-sodium-symporter family or Na(+)/Cl(-)-dependent transporters) comprise a group of nine sodium- and chloride-dependent plasma membrane transporters for the monoamine neurotransmitters serotonin (5-hydroxytryptamine), dopamine, and norepinephrine, and the amino acid neurotransmitters GABA and glycine. The SLC6 NTTs are widely expressed in the mammalian brain and play an essential role in regulating neurotransmitter signaling and homeostasis by mediating uptake of released neurotransmitters from the extracellular space into neurons and glial cells. The transporters are targets for a wide range of therapeutic drugs used in treatment of psychiatric diseases, including major depression, anxiety disorders, attention deficit hyperactivity disorder and epilepsy. Furthermore, psychostimulants such as cocaine and amphetamines have the SLC6 NTTs as primary targets. Beginning with the determination of a high-resolution structure of a prokaryotic homolog of the mammalian SLC6 transporters in 2005, the understanding of the molecular structure, function, and pharmacology of these proteins has advanced rapidly. Furthermore, intensive efforts have been directed toward understanding the molecular and cellular mechanisms involved in regulation of the activity of this important class of transporters, leading to new methodological developments and important insights. This review provides an update of these advances and their implications for the current understanding of the SLC6 NTTs.

Transgenic elimination of high-affinity antidepressant and cocaine sensitivity in the presynaptic serotonin transporter

Serotonin [i.e., 5-hydroxytryptamine (5-HT)]-targeted antidepressants are in wide use for the treatment of mood disorders, although many patients do not show a response or experience unpleasant side effects. Psychostimulants, such as cocaine and 3,4-methylenedioxymethamphetamine (i.e., "ecstasy"), also impact 5-HT signaling. To help dissect the contribution of 5-HT signaling to the actions of these and other agents, we developed transgenic mice in which high-affinity recognition of multiple antidepressants and cocaine is eliminated. Our animals possess a modified copy of the 5-HT transporter (i.e., SERT, slc6a4) that bears a single amino acid substitution, I172M, proximal to the 5-HT binding site. Although the M172 substitution does not impact the recognition of 5-HT, this mutation disrupts high-affinity binding of many competitive antagonists in transfected cells. Here, we demonstrate that, in M172 knock-in mice, basal SERT protein levels, 5-HT transport rates, and 5-HT levels are normal. However, SERT M172 mice display a substantial loss of sensitivity to the selective 5-HT reuptake inhibitors fluoxetine and citalopram, as well as to cocaine. Through a series of biochemical, electrophysiological, and behavioral assays, we demonstrate the unique properties of this model and establish directly that SERT is the sole protein responsible for selective 5-HT reuptake inhibitor-mediated alterations in 5-HT clearance, in 5-HT1A autoreceptor modulation of raphe neuron firing, and in behaviors used to predict the utility of antidepressants.

Myristoylation of cGMP-dependent protein kinase dictates isoform specificity for serotonin transporter regulation

By transporting serotonin (5-HT) into neurons and other cells, serotonin transporter (SERT) modulates the action of 5-HT at cell surface receptors. SERT itself is modulated by several processes, including the cGMP signaling pathway. Activation of SERT by cGMP requires the cGMP-dependent protein kinase (PKG). Here we show that in HeLa cells lacking endogenous PKG, expression of PKGIα or PKGIβ was required for 8-bromoguanosine-3',5'-cyclic monophosphate (8-Br-cGMP) to stimulate SERT phosphorylation and 5-HT influx. Catalytically inactive PKG mutants and wild-type PKGII did not support this stimulation. However, a mutant PKGII (G2A) that was not myristoylated substituted for functional PKGI, suggesting that myristoylation and subsequent membrane association blocked productive interaction with SERT. PKG also influenced SERT expression and localization. PKGI isoforms increased total and cell surface SERT levels, and PKGII decreased cell surface SERT without altering total expression. Remarkably, these changes did not require 8-Br-cGMP or functional kinase activity and were also observed with a SERT mutant resistant to activation by PKG. Both PKGIα and PKGIβ formed detergent-stable complexes with SERT, and this association did not require catalytic activity. The nonmyristoylated PKGII G2A mutant stimulated SERT expression similar to PKGI isoforms. These results suggest multiple mechanisms by which PKG can modulate SERT and demonstrate that the functional difference between PKG isoforms results from myristoylation of PKGII.

Regulation of monoamine transporters: Role of transporter phosphorylation

Presynaptic biogenic amine transporters mediate reuptake of released amines from the synapse, thus regulating serotonin, dopamine and norepinephrine neurotransmission. Medications utilized in the treatment of depression, attention deficit-hyperactivity disorder and other psychiatric disorders possess high affinity for amine transporters. In addition, amine transporters are targets for psychostimulants. Altered expression of biogenic amine transporters has long been implicated in several psychiatric and degenerative disorders. Therefore, appropriate regulation and maintenance of biogenic amine transporter activity is critical for the maintenance of normal amine homoeostasis. Accumulating evidence suggests that cellular protein kinases and phosphatases regulate amine transporter expression, activity, trafficking and degradation. Amine transporters are phosphoproteins that undergo dynamic control under the influence of various kinase and phosphatase activities. This review presents a brief overview of the role of amine transporter phosphorylation in the regulation of amine transport in the normal and diseased brain. Understanding the molecular mechanisms by which phosphorylation events affect amine transporter activity is essential for understanding the contribution of transporter phosphorylation to the regulation of monoamine neurotransmission and for identifying potential new targets for the treatment of various brain diseases.

Real-time, spatially resolved analysis of serotonin transporter activity and regulation using the fluorescent substrate, ASP+

The serotonin transporter (SERT) mediates clearance of serotonin from the synapse, thereby, regulating extracellular serotonin concentrations. Radioligand uptake techniques are typically used to assess SERT function in tissue and heterologous expression systems. The need for sufficient protein in samples, however, requires use of homogenate preparations, potentially masking effects limited to specific cell populations. 4-(4-(dimethylamino)-styryl)-N-methylpyridinium (ASP(+)) is a fluorescent monoamine transporter substrate that has been used for real-time monitoring of dopamine and norepinephrine transporter function in single cells. The present live cell imaging studies examine the utility of ASP(+) for quantifying human SERT function in HEK293 and neuroblastoma cells. We show rapid membrane binding and intracellular ASP(+) accumulation in human SERT-expressing cells. Accumulation is saturable; dependent on temperature and the presence of sodium and chloride in the media, and attenuated by serotonin. Acute or prolonged exposure of cells to serotonin re-uptake inhibitors produces a concentration-dependent decrease in accumulation. Similar effects are produced by protein kinase C activation whereas p38 MAPK activation increases ASP(+) accumulation. These data demonstrate the validity of ASP(+) as a probe for monitoring SERT function in living cells. Alterations in SERT binding and uptake can be quantified in the same cell and use of a within-cell design permits analysis of time-related alterations in SERT function.

The locust foraging gene

Our knowledge of how genes act on the nervous system in response to the environment to generate behavioral plasticity is limited. A number of recent advancements in this area concern food-related behaviors and a specific gene family called foraging (for), which encodes a cGMP-dependent protein kinase (PKG). The desert locust (Schistocerca gregaria) is notorious for its destructive feeding and long-term migratory behavior. Locust phase polyphenism is an extreme example of environmentally induced behavioral plasticity. In response to changes in population density, locusts dramatically alter their behavior, from solitary and relatively sedentary behavior to active aggregation and swarming. Very little is known about the molecular and genetic basis of this striking behavioral phenomenon. Here we initiated studies into the locust for gene by identifying, cloning, and studying expression of the gene in the locust brain. We determined the phylogenetic relationships between the locust PKG and other known PKG proteins in insects. FOR expression was found to be confined to neurons of the anterior midline of the brain, the pars intercerebralis. Our results suggest that differences in PKG enzyme activity are correlated to well-established phase-related behavioral differences. These results lay the groundwork for functional studies of the locust for gene and its possible relations to locust phase polyphenism.

Effect of sildenafil on the anticonvulsant action of classical and second-generation antiepileptic drugs in maximal electroshock-induced seizures in mice

PURPOSE

The goal of the present study was to evaluate the effects of sildenafil on the threshold for electrically induced seizures in mice. In addition, interactions between sildenafil and classical and second-generation antiepileptic drugs (AEDs), that is, carbamazepine (CBZ), phenobarbital (PB), phenytoin (PHT), valproate (VPA), lamotrigine (LTG), topiramate (TPM), and oxcarbazepine (OXC) were evaluated.

METHODS

Two electroconvulsive tests were used: maximal electroshock seizure threshold (MEST) and maximal electroshock seizure (MES) tests in mice. Acute adverse effects of the studied combinations were investigated in the chimney test, step-through passive avoidance task, and grip-strength test. Total brain and free plasma concentrations of AEDs were also determined.

RESULTS

Sildenafil raised the threshold for electroconvulsions in a dose-dependent manner. It also increased the anticonvulsant activity of CBZ, VPA, and TPM in the MES test, whereas the activity of the remaining AEDs was not significantly changed. Sildenafil increased total brain and free (protein unbound) plasma CBZ concentrations and total brain VPA concentration. Neither sildenafil nor its coadministration with the studied AEDs affected motor coordination and long-term memory in mice. Interestingly, sildenafil dose-dependently enhanced the skeletal muscle strength in mice, although combinations of sildenafil with AEDs were ineffective in this respect.

CONCLUSIONS

Sildenafil significantly raised the threshold for electroconvulsions in mice without any impairment of motor performance and long-term memory, but it enhanced muscle strength. Treatment of patients on CBZ or VPA with sildenafil may not be recommended due to pharmacokinetic interactions. Coadministration of sildenafil with other AEDs, especially with TPM, seems to be a reasonable choice.

cGMP-dependent protein kinase Ialpha associates with the antidepressant-sensitive serotonin transporter and dictates rapid modulation of serotonin uptake

Background

The Na⁺/Cl^--dependent serotonin (5-hydroxytryptamine, 5-HT) transporter (SERT) is a critical element in neuronal 5-HT signaling, being responsible for the efficient elimination of 5-HT after release. SERTs are not only targets for exogenous addictive and therapeutic agents but also can be modulated by endogenous, receptor-linked signaling pathways. We have shown that neuronal A3 adenosine receptor activation leads to enhanced presynaptic 5-HT transport in vitro and an increased rate of SERT-mediated 5-HT clearance in vivo. SERT stimulation by A3 adenosine receptors derives from an elevation of cGMP and subsequent activation of both cGMP-dependent protein kinase (PKG) and p38 mitogen-activated protein kinase. PKG activators such as 8-Br-cGMP are known to lead to transporter phosphorylation, though how this modification supports SERT regulation is unclear.

Results

In this report, we explore the kinase isoform specificity underlying the rapid stimulation of SERT activity by PKG activators. Using immortalized, rat serotonergic raphe neurons (RN46A) previously shown to support 8-Br-cGMP stimulation of SERT surface trafficking, we document expression of PKGI, and to a lower extent, PKGII. Quantitative analysis of staining profiles using permeabilized or nonpermeabilized conditions reveals that SERT colocalizes with PKGI in both intracellular and cell surface domains of RN46A cell bodies, and exhibits a more restricted, intracellular pattern of colocalization in neuritic processes. In the same cells, SERT demonstrates a lack of colocalization with PKGII in either intracellular or surface membranes. In keeping with the ability of the membrane permeant kinase inhibitor DT-2 to block 8-Br-cGMP stimulation of SERT, we found that DT-2 treatment eliminated cGMP-dependent kinase activity in PKGI-immunoreactive extracts resolved by liquid chromatography. Similarly, treatment of SERT-transfected HeLa cells with small interfering RNAs targeting endogenous PKGI eliminated 8-Br-cGMP-induced regulation of SERT activity. Co-immunoprecipitation studies show that, in transporter/kinase co-transfected cells, PKGIα specifically associates with hSERT.

Conclusion

Our findings provide evidence of a physical and compartmentalized association between SERT and PKGIα that supports rapid, 8-Br-cGMP-induced regulation of SERT. We discuss a model wherein SERT-associated PKGIα supports sequentially the mobilization of intracellular transporter-containing vesicles, leading to enhanced surface expression, and the production of catalytic-modulatory SERT phosphorylation, leading to a maximal enhancement of 5-HT clearance capacity.

Phosphodiesterase 5 inhibitors prevent 3,4-methylenedioxymethamphetamine-induced 5-HT deficits in the rat

Phosphodiesterase 5 (PDE5) inhibitors are often used in combination with club drugs such as 3,4-methylenedioxymethamphetamine (MDMA or ecstasy). We investigated the consequences of such combination in the serotonergic system of the rat. Oral administration of sildenafil citrate (1.5 or 8 mg/kg) increased brain cGMP levels and protected in a dose-dependent manner against 5-hydroxytryptamine depletions caused by MDMA (3 x 5 mg/kg, i.p., every 2 h) in the striatum, frontal cortex and hippocampus without altering the acute hyperthermic response to MDMA. Intrastriatal administration of the protein kinase G (PKG) inhibitor, KT5823 [(9S, 10R, 12R)-2,3,9,10,11,12-Hexahydro-10-methoxy-2,9-dimethyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid, methyl ester)], suppressed sildenafil-mediated protection. By contrast, the cell permeable cGMP analogue, 8-bromoguanosine cyclic 3',5'-monophosphate, mimicked sildenafil effects further suggesting the involvement of the PKG pathway in mediating sildenafil protection. Because mitochondrial ATP-sensitive K(+) channels are a target for PKG, we next administered the specific mitochondrial ATP-sensitive K(+) channel blocker, 5-hydroxydecanoic acid, 30 min before sildenafil. 5-hydroxydecanoic acid completely reversed the protection afforded by sildenafil, thereby implicating the involvement of mitochondrial ATP-sensitive K(+) channels. Sildenafil also increased Akt phosphorylation, and so the possible involvement of the Akt/endothelial nitric oxide synthase (eNOS)/sGC signalling pathway was analysed. Neither the phosphatidylinositol 3-kinase inhibitor, wortmannin, nor the selective eNOS inhibitor, L-N5-(1-iminoethyl)-L-ornithine dihydrochloride, reversed the protection afforded by sildenafil, suggesting that Akt/eNOS/sGC cascade does not participate in the protective mechanisms. Our data also show that the protective effect of sildenafil can be extended to vardenafil, another PDE5 inhibitor. In conclusion, sildenafil protects against MDMA-induced long-term reduction of indoles by a mechanism involving increased production of cGMP and subsequent activation of PKG and mitochondrial ATP-sensitive K(+) channel opening.

Oral phosphodiesterase type 5 inhibitors: nonerectogenic beneficial uses

INTRODUCTION

Phosphodiesterase type 5 (PDE5) hydrolyses cyclic guanylate monophosphate (cGMP) specifically to 5' GMP. PDE5 inhibitors were a breakthrough medication that addressed a previously unfulfilled medical need. They promoted vascular relaxation in the corpora cavernosa and penile erection during sexual stimulation. Sildenafil, vardenafil, and tadalafil were approved then introduced as effective treatments for male erectile dysfunction. This impact has stimulated academic, clinical, and industrial research.

AIM

To highlight the nonerectogenic beneficial uses of oral PDE5 inhibitors.

METHOD

A systematic review of published studies in this affair based on a Pubmed and medical subject heading databases search of all concerned articles.

MAIN OUTCOME MEASURES

Demonstrated beneficial as well as applicable uses of oral PDE5 inhibitors.

RESULTS

As chemical molecules, these drugs were shown to exert potential nonerectogenic beneficial effects. They showed efficacy as a useful adjunct in the management of pulmonary hypertension. Additional uses were extended to different utilities: essential hypertension, benign prostatic hyperplasia, gastrointestinal disorders, endothelial dysfunction, female sexual dysfunction, genital blood flow, exercise capacity, Raynaud's phenomenon, sperm motility, etc.

CONCLUSION

Exploring PDE5 inhibitors for their possible medical applications in diverse specialties seems to be beneficial in making use of these molecules for the welfare of humanity.

Going with the flow: trafficking-dependent and -independent regulation of serotonin transport

Antidepressant-, cocaine- and 3,4-methylenedioxymethamphetamine-sensitive serotonin (5-hydroxytryptamine, 5-HT) transporters (SERTs) are expressed on presynaptic membranes of 5-HT-secreting neurons to provide efficient uptake of the biogenic amine after release. SERTs also support 5-HT transport across platelet, placental, gastrointestinal and pulmonary membranes and thus play a critical role in central nervous system and peripheral nervous system 5-HT signaling. SERTs are subject to multiple levels of posttranslational regulation that can rapidly alter 5-HT uptake and clearance rates. Specific cell surface receptors are now known to regulate SERT trafficking and/or catalytic function, with pathways activating protein kinase C, protein kinase G and p38 mitogen-activated protein kinase receiving the greatest attention. Remarkably, disease-associated mutations in SERT not only impact basal SERT activity but also selectively impact one or more SERT regulatory pathway(s). In this review, we describe both trafficking-dependent and trafficking-independent modes of SERT regulation and also the suspected roles played in regulation by SERT-associated proteins. Elucidation of the SERT 'regulome' provides important depth to our understanding of the likely origins of 5-HT-associated disorders and may help orient research to develop novel therapeutics.

Serotonin transporter phosphorylation by cGMP-dependent protein kinase is altered by a mutation associated with obsessive compulsive disorder

Human serotonin transporter (hSERT) activity expressed in HeLa cells was stimulated by agents that release nitric oxide, stimulate soluble guanylyl cyclase, or activate cGMP-dependent protein kinase (PKG). This stimulation was blocked by a PKG inhibitor. A naturally occurring mutation, I425V, associated with obsessive-compulsive disorder and other neuropsychiatric disorders, activated hSERT and eliminated stimulation via the PKG pathway. Inhibitors of soluble guanylyl cyclase or PKG decreased activity of the I425V mutant, but not wild type, indicating that both wild-type and mutant transporters could exist in both high and low activity forms. Mutation of Thr-276 in the fifth transmembrane domain (TM5) to alanine or aspartate prevented activation of wild-type hSERT through the PKG pathway and also blocked the inhibition of I425V activity by inhibitors of the pathway. The accessibility of positions in TM5 near Thr-276 was modified in T276D, but not in I425V. These results are consistent with the hypothesis that PKG phosphorylates hSERT at Thr-276 and increases its activity by modifying the substrate permeation pathway formed, in part, by TM5. The effect of the I425V mutation may shift the balance of hSERT toward the phosphorylated form, possibly by interfering with the action of a phosphatase. However, association of hSERT with protein phosphatase 2A was not decreased in the I425V mutant.

Rapid stimulation of presynaptic serotonin transport by A(3) adenosine receptors

The inactivation of synaptic serotonin (5-hydroxytryptamine, 5-HT) is largely established through the actions of the presynaptic, antidepressant-sensitive 5-HT transporter (SERT, SLC6A4). Recent studies have demonstrated post-translational regulation of SERT mediated by multiple Ser/Thr kinases, including protein kinases C and G (PKC and PKG) and p38 mitogen-activated protein kinase (MAPK), as well as the Ser/Thr phosphatase PP2A. Less well studied are specific surface receptors that target these signaling pathways to control SERT surface expression and/or catalytic rates. Using rat basophilic leukemia 2H3 cell line (RBL-2H3), we previously established that activation of A(3) adenosine receptors (A(3)AR) stimulates SERT activity via both PKG and p38 MAPK (Zhu et al., 2004a). Whether A(3)ARs regulate SERT in the central nervous system (CNS) is unknown. Here we report that the A(3)AR agonist N(6)-(3-iodobenzyl)-N-methyl-5'carbamoyladenosine (IB-MECA) rapidly (10 min) and selectively stimulates 5-HT transport in mouse midbrain, hippocampal, and cortical synaptosomes. IB-MECA-induced stimulation of 5-HT uptake is blocked by the selective A(3)AR antagonist 3-ethyl-5-benzyl-2-methyl-phenylethynyl-6-phenyl-1,4(+/-)dihydropyridine-3,5-dicarboxylate (MRS1191) and is absent from synaptosomes prepared from A(3)AR knockout mice. Kinetic analyses demonstrate that IB-MECA induces an increase of 5-HT transport V(max) with no significant change in K(m). As in RBL-2H3 cells, IB-MECA stimulation of synaptosomal 5-HT uptake can be blocked by preincubation with PKG antagonists N-[2-(methylamino)ethy]-5-isoquinoline-sulfonamide (H8) and DT-2 (YGRKKRRQRRRPPLRK(5)H), as well as by the p38 MAPK inhibitor SB203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole]. Chronoamperometry studies in the anesthetized rat hippocampus support a role for A(3)ARs in SERT regulation in vivo. Together, these results identify a novel, region-specific action of CNS A(3)ARs in the modulation of SERT-mediated 5-HT transport that may be relevant for the etiology and/or therapy of 5-HT-linked brain disorders.

Phosphorylation of threonine residue 276 is required for acute regulation of serotonin transporter by cyclic GMP

Cellular protein kinases, phosphatases, and other serotonin transporter (SERT) interacting proteins participate in several signaling mechanisms regulating SERT activity. The molecular mechanisms of protein kinase G (PKG)-mediated SERT regulation and the site of transporter phosphorylation were investigated. Treatment of rat midbrain synaptosomes with 8-bromo-cGMP increased SERT activity, and the increase was selectively blocked by PKG inhibitors. The V(max) value for serotonin (5-HT) transport increased following cGMP treatment. However, surface biotinylation studies showed no change in SERT surface abundance following PKG activation. (32)P metabolic labeling experiments showed increased SERT phosphorylation in the presence of cGMP that was abolished by selectively inhibiting PKG. Phosphoamino acid analysis revealed that cGMP-stimulated native SERT phosphorylation occurred only on threonine residues. When added to CHO-1 cells expressing SERT, 8-bromo-cGMP stimulated 5-HT transport and SERT phosphorylation. Mutation of SERT threonine 276 to alanine completely abolished cGMP-mediated stimulation of 5-HT transport and SERT phosphorylation. Although the T276A mutation had no significant effect on 5-HT transport or SERT protein expression, mutation to aspartate (T276D) increased the level of 5-HT uptake to that of cGMP-stimulated 5-HT uptake in wild-type SERT-expressing cells and was no longer sensitive to cGMP. These findings provide the first identification of a phosphorylation site in SERT and demonstrate that phosphorylation of Thr-276 is required for cGMP-mediated SERT regulation. They also constitute the first evidence that in the central nervous system PKG activation stimulates endogenous SERT activity by a trafficking-independent mechanism.

Voltage and ionic regulation of human serotonin transporter in Xenopus oocytes

1. The serotoninergic system is known to be involved in the control of multiple behavioural and physiological functions. The serotonin (5-hydroxtryptamine; 5-HT) transporter (SERT), which controls the synaptic 5-HT concentration through re-uptake of this neurotransmitter into presynaptic terminals, has been a primary therapeutic target for various psychiatric and peripheral disorders. The aim of the present study was to identify the regulatory mechanism(s) of the human SERT (hSERT) in heterologously expressed oocytes. 2. The hSERT cRNA was transcribed in vitro and injected into Xenopus oocytes. The 5-HT-induced transporter currents were measured by voltage clamp. The effects of extracellular sodium or chloride were studied by replacement perfusion with tetramethylammonium-chloride (96 mmol/L) or sodium acetate (96 mmol/L). In addition, to alter the internal calcium concentration, CaCl2 (50 micromol/L) and inositol triphosphate (IP3; 50 micromol/L), with or without EGTA (2.5 mmol/L), were injected into oocytes. The specificity of 5-HT-sensitive currents was determined by the use of the SERT antagonist desipramine and niflumic acid to block background chloride currents. 3. The hSERT-expressing oocytes displayed voltage-dependent, 5-HT-induced currents that increased at negative potentials. Replacing extracellular sodium or chloride significantly decreased the hSERT currents by 89 and 45%, respectively (P < 0.05, n = 7 each). Injection of IP3 or CaCl2 increased the hSERT currents by approximately 65% (P < 0.05; n = 10 each) and the effect of IP3 was abolished by preinjection of EGTA. 4. These results demonstrate that hSERT activity is not only voltage dependent, but is also affected by intracellular calcium and extracellular sodium and chloride.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Dissection of an allosteric mechanism on the serotonin transporter: a cross-species study

The serotonin transporter (SERT), which belongs to a family of sodium/chloride-dependent transporters, is the major pharmacological target in the treatment of several clinical disorders, including depression and anxiety. Interaction with a low-affinity allosteric site on SERT modulates the ligand affinity at the high-affinity binding site. Serotonin (5-hydroxytryptamine) and certain SERT inhibitors possess affinity for both sites. In the present study, we report the characterization of a severely attenuated allosteric mechanism at the recently cloned chicken serotonin transporter (gSERT). A cross-species chimera study was performed, followed by species scanning mutagenesis. Residues important for the allosteric mechanism were mapped to the C-terminal part of SERT containing the transmembrane domains 10 to 12. We identified nine residues located in four distinct amino acid segments. The contribution of each segment and individual residues was investigated. Consequently, a gSERT mutant with a restored allosteric mechanism, as well as a human SERT (hSERT) mutant with a severely attenuated allosteric mechanism, was generated. The nine residues confer a functional allosteric mechanism for different combinations of ligands, suggesting that they contribute to a general allosteric mechanism at SERT. The finding of an allosteric mechanism at SERT is likely to be of physiological importance, in that serotonin was also found to act as an allosteric effector at duloxetine, RTI-55 and (S)-citalopram. Furthermore, the allosteric potency of 5-HT was found to be conserved for both hSERT and gSERT.

Human serotonin transporter variants display altered sensitivity to protein kinase G and p38 mitogen-activated protein kinase

Human serotonin [5-hydroxytryptamine (5-HT)] transporters (hSERT, 5HTT, and SLC6A4) inactivate 5-HT after release and are prominent targets for therapeutic intervention in mood, anxiety, and obsessive-compulsive disorders. Multiple hSERT coding variants have been identified, although to date no comprehensive functional analysis of these variants has been reported. We transfected hSERT or 10 hSERT coding variants and examined total and surface protein expression, antagonist recognition, and transporter modulation by posttranslational, regulatory pathways. Two variants, Pro339Leu and Ile425Val, demonstrated significant changes in surface expression supporting alterations in 5-HT transport capacity (V(max)). Regardless of basal transport activity, all SERT variants displayed a capacity for rapid, phorbol ester-triggered down-regulation. Remarkably, five variants (Thr4Ala, Gly56Ala, Glu215Lys, Lys605Asn, and Pro612Ser) demonstrated no capacity for 5-HT uptake stimulation after acute protein kinase G (PKG)/p38 mitogen-activated protein kinase (MAPK) activation. Epstein-Barr virus (EBV)-transformed lymphocytes natively expressing the most common of these variants (Gly56Ala) exhibited a similar loss of 5-HT uptake stimulation by PKG/p38 MAPK activators. HeLa cells transfected with the Gly56Ala variant demonstrated elevated basal phosphorylation and, unlike hSERT, could not be further phosphorylated after 8-bromo cGMP (8BrcGMP) treatments. These studies reveal cellular phenotypes associated with naturally occurring human SERT coding variants and suggest that altered transporter regulation by means of PKG/p38 MAPK-linked pathways may influence risk for disorders attributed to compromised 5-HT signaling.

Biogenic Amine Neurotransmitter Transporters: Just When You Thought You Knew Them

Plasma membrane transporters have long been known to support the reuptake of biogenic amine neurotransmitters following release in the central and peripheral nervous systems. Using high-resolution imaging, patch-clamp and amperometric approaches, as well as molecular manipulations of transporter-regulatory pathways, surprising new details have been uncovered as to how transporters work and are influenced by signaling pathways and psychostimulants.

Involvement of the NMDA receptor, NO-cyclic GMP and nuclear factor K-beta in an animal model of repeated trauma

Post-traumatic stress disorder (PTSD) may be associated with shrinkage of the hippocampus, with glutamate release causally related to these events. Recent animal studies strongly implicate activation of the nitric oxide (NO)-cascade in anxiety and stress. Using an animal model of repeated trauma, the effect of stress was investigated on the hippocampal NO-cGMP signalling pathway, specifically the release of nitrogen oxides (NOx) and its modulation by NMDA receptor-, NO-, cGMP- and nuclear factor K-beta (NFK-beta)-selective drugs. Immediately after stress, rats received the glutamate NMDA receptor antagonist, memantine (MEM; 5 mg/kg i.p./d), the NO synthase inhibitor, 7-nitroindazole sodium salt (7-NINA; 20 mg/kg i.p./d), the cGMP-specific PDE inhibitor, sildenafil (SIL; 10 mg/kg i.p./d) or the NFkappa-beta antagonist, pyrollidine dithiocarbamate (PDTC; 70 mg/kg i.p./d), for 7 days. Stress significantly increased hippocampal NOx on day 7 post-stress, which was blocked by either 7-NINA or PDTC, while MEM was without effect. SIL, however, significantly augmented stress-induced NOx accumulation. Increased cGMP therefore acts as a protagonist in driving stress-related events, while both nNOS (neuronal NOS) and iNOS (inducible/immunological NOS) may represent a therapeutic target in preventing the effects of severe stress. The value of NMDA receptor antagonism, however, appears limited in this model.

p38 MAPK Activation Elevates Serotonin Transport Activity via a Trafficking-independent, Protein Phosphatase 2A-dependent Process

Presynaptic, plasma membrane serotonin (5-hydroxytryptamine; 5-HT) transporters (SERTs) clear 5-HT following vesicular release and are regulated through trafficking-dependent pathways. Recently, we provided evidence for a trafficking-independent mode of SERT regulation downstream of adenosine receptor (AR) activation that is sensitive to p38 MAPK inhibitors. Here, we probe this pathway in greater detail, demonstrating elevation of 5-HT transport by multiple p38 MAPK activators (anisomycin, H(2)O(2), and UV radiation), in parallel with p38 MAPK phosphorylation, as well as suppression of anisomycin stimulation by p38 MAPK siRNA treatments. Studies with transporter-transfected Chinese hamster ovary cells reveal that SERT stimulation is shared with the human norepinephrine transporter but not the human dopamine transporter. Saturation kinetic analyses of anisomycin-SERT activity reveal a selective reduction in 5-HT K(m) supported by a commensurate increase in 5-HT potency (K(i)) for displacing surface antagonist binding. Anisomycin treatments that stimulate SERT activity do not elevate surface SERT surface density whereas stimulation is lost with preexposure of cells to the surface-SERT inactivating reagent, 2-(trimethylammonium)ethyl methane thiosulfonate. Guanylyl cyclase (1H-(1,2,4)-oxadiazolo[4,3-a]-quinoxalin-1-one) and protein kinase G inhibitors (H8, DT-2) block AR stimulation of SERT yet fail to antagonize SERT stimulation by anisomycin. We thus place p38 MAPK activation downstream of protein kinase G in a SERT-catalytic regulatory pathway, distinct from events controlling SERT surface density. In contrast, the activity of protein phosphatase 2A inhibitors (fostriecin and calyculin A) to attenuate anisomycin stimulation of 5-HT transport suggests that protein phosphatase 2A is a critical component of the pathway responsible for p38 MAPK up-regulation of SERT catalytic activity.