Regulator of G-protein signaling 6 (RGS6) promotes anxiety and depression by attenuating serotonin-mediated activation of the 5-HT(1A) receptor-adenylyl cyclase axis.
ABSTRACT: Targeting serotonin (5-HT) bioavailability with selective 5-HT reuptake inhibitors (SSRIs) remains the most widely used treatment for mood disorders. However, their limited efficacy, delayed onset of action, and side effects restrict their clinical utility. Endogenous regulator of G-protein signaling (RGS) proteins have been implicated as key inhibitors of 5-HT(1A)Rs, whose activation is believed to underlie the beneficial effects of SSRIs, but the identity of the specific RGS proteins involved remains unknown. We identify RGS6 as the critical negative regulator of 5-HT(1A)R-dependent antidepressant actions. RGS6 is enriched in hippocampal and cortical neurons, 5-HT(1A)R-expressing cells implicated in mood disorders. RGS6(-/-) mice exhibit spontaneous anxiolytic and antidepressant behavior rapidly and completely reversibly by 5-HT(1A)R blockade. Effects of the SSRI fluvoxamine and 5-HT(1A)R agonist 8-OH-DPAT were also potentiated in RGS6(+/-) mice. The phenotype of RGS6(-/-) mice was associated with decreased CREB phosphorylation in the hippocampus and cortex, implicating enhanced G?(i)-dependent adenylyl cyclase inhibition as a possible causative factor in the behavior observed in RGS6(-/-) animals. Our results demonstrate that by inhibiting serotonergic innervation of the cortical-limbic neuronal circuit, RGS6 exerts powerful anxiogenic and prodepressant actions. These findings indicate that RGS6 inhibition may represent a viable means to treat mood disorders or enhance the efficacy of serotonergic agents.
Project description:Elevating serotonin (5-HT) levels with selective serotonin reuptake inhibitors (SSRIs) is the most widely used treatment for depression. However, current therapies are ineffective, have delayed benefit, or cause side effects in many patients. Here, we define a mechanism downstream of 5-HT1A receptors that mediates antidepressant-like behavior and is profoundly and selectively enhanced by genetic disruption of regulators of G protein signaling (RGS) activity at G(alpha)i2. Animals rendered insensitive to RGS protein regulation through a mutation in G(alpha)i2 (G184S) exhibited spontaneous antidepressant- and anxiolytic-like behaviors. Mice expressing RGS-insensitive G(alpha)i2 also exhibited increased cortical and hippocampal phosphorylation of glycogen synthase kinase-3beta, a constitutively active proapoptotic kinase that is inhibited through phosphorylation in response to serotonin, SSRIs, and 5-HT1 receptor agonists. Both behavioral and biochemical phenotypes were blocked by treatment with WAY 100635, a 5-HT1A-selective antagonist. RGS-insensitive mice were also 5-10 times more responsive to the antidepressant-like effects of the SSRI fluvoxamine and 5-HT1A-selective agonist 8-hydroxy-2-dipropylaminotetralin. In contrast, the antidepressant potency of agents acting through nonserotonergic mechanisms was unchanged as was 5-HT1A action on body temperature. The findings point to a critical role for endogenous RGS proteins to suppress the antidepressant-like effects of 5-HT1A receptor activation. By selectively enhancing the beneficial effects of serotonin, inhibition of RGS proteins represents a therapeutic approach for the treatment of mood disorders.
Project description:Estradiol regulates serotonin 1A (5-HT(1A)) receptor signaling. Since desensitization of 5-HT(1A) receptors may be an underlying mechanism by which selective serotonin reuptake inhibitors (SSRIs) mediate their therapeutic effects and combining estradiol with SSRIs enhances the efficacy of the SSRIs, it is important to determine which estrogen receptors are capable of desensitizating 5-HT(1A) receptor function. We previously demonstrated that selective activation of the estrogen receptor, GPR30, desensitizes 5-HT(1A) receptor signaling in rat hypothalamic paraventricular nucleus (PVN). However, since estrogen receptor-beta (ERbeta), is highly expressed in the PVN, we investigated the role of ERbeta in estradiol-induced desensitization of 5-HT(1A) receptor signaling. We first showed that a selective ERbeta agonist, diarylpropionitrile (DPN) has a 100-fold lower binding affinity than estradiol for GPR30. Administration of DPN did not desensitize 5-HT(1A) receptor signaling in rat PVN as demonstrated by agonist-stimulated hormone release. Second, we used a recombinant adenovirus containing ERbeta siRNAs to decrease ERbeta expression in the PVN. Reductions in ERbeta did not alter the estradiol-induced desensitization of 5-HT(1A) receptor signaling in oxytocin cells. In contrast, in animals with reduced ERbeta, estradiol administration, instead of producing desensitization, augmented the ACTH response to a 5-HT(1A) agonist. Combined with the results from the DPN treatment experiments, desensitization of 5-HT(1A) receptor signaling does not appear to be mediated by ERbeta in oxytocin cells, but that ERbeta, together with GPR30, may play a complex role in central regulation of 5-HT(1A)-mediated ACTH release. Determining the mechanisms by which estrogens induce desensitization may aid in the development of better treatments for mood disorders.
Project description:Several lines of evidence implicate 5-hydroxytryptamine (5-HT, serotonin) in the pathophysiology of mood disorders and suicide. However, it is unclear whether these conditions include morphological involvement of the dorsal raphe nucleus (DRN), the origin of most forebrain 5-HT innervation.We used morphometric, immunohistochemical, and molecular methods to compare the DRN in post-mortem tissue of 50 subjects (13 controls, 14 major depressive disorder [MDD], 13 bipolar disorder, 10 schizophrenia; 17 of the cases died by suicide). NeuN and PH8 antibodies were used to assess all neurons and serotonergic neurons respectively; 5-HT(1A) autoreceptor expression was investigated by regional and cellular in situ hybridization. Measurements were made at three rostrocaudal levels of the DRN.In MDD, the area of the DRN was decreased. In bipolar disorder, serotonergic neuronal size was decreased. Suicide was associated with an increased DRN area, and with a higher density but decreased size of serotonergic neurons. Total neuronal density and 5-HT(1A) receptor mRNA abundance were unaffected by diagnosis or suicide. No changes were seen in schizophrenia.The results show that mood disorders and suicide are associated with differential, limited morphological alterations of the DRN. The contrasting influences of MDD and suicide may explain some of the discrepancies between previous studies, since their design precluded detection of the effect.
Project description:The complexities of the involvement of the serotonin transmitter system in numerous biological processes and psychiatric disorders is, to a substantial degree, attributable to the large number of serotonin receptor families and subtypes that have been identified and characterized for over four decades. Of these, the 5-HT(1A) receptor subtype, which was the first to be cloned and characterized, has received considerable attention based on its purported role in the etiology and treatment of mood and anxiety disorders. 5-HT(1A) receptors function both at presynaptic (autoreceptor) and postsynaptic (heteroreceptor) sites. Recent research has implicated distinct roles for these two populations of receptors in mediating emotion-related behavior. New concepts as to how 5-HT(1A) receptors function to control serotonergic tone throughout life were highlights of the proceedings of the 2012 Serotonin Club Meeting in Montpellier, France. Here, we review recent findings and current perspectives on functional aspects of 5-HT(1A) auto- and heteroreceptors with particular regard to their involvement in altered anxiety and mood states.
Project description:The antidepressant efficacy of selective 5-HT reuptake inhibitors (SSRI) and other 5-HT-enhancing drugs is compromised by a negative feedback mechanism involving 5-HT(1A) autoreceptor activation by the excess 5-HT produced by these drugs in the somatodendritic region of 5-HT neurones. 5-HT(1A) receptor antagonists augment antidepressant-like effects in rodents by preventing this negative feedback, and the mixed ?-adrenoceptor/5-HT(1A) receptor antagonist pindolol improves clinical antidepressant effects by preferentially interacting with 5-HT(1A) autoreceptors. However, it is unclear whether 5-HT(1A) receptor antagonists not discriminating between pre- and post-synaptic 5-HT(1A) receptors would be clinically effective.We characterized the pharmacological properties of the 5-HT(1A) receptor antagonist DU-125530 using receptor autoradiography, intracerebral microdialysis and electrophysiological recordings. Its capacity to accelerate/enhance the clinical effects of fluoxetine was assessed in a double-blind, randomized, 6 week placebo-controlled trial in 50 patients with major depression (clinicaltrials.gov identifier NCT01119430).DU-125530 showed equal (low nM) potency to displace agonist and antagonist binding to pre- and post-synaptic 5-HT(1A) receptors in rat and human brain. It antagonized suppression of 5-hydroxytryptaminergic activity evoked by 8-OH-DPAT and SSRIs in vivo. DU-125530 augmented SSRI-induced increases in extracellular 5-HT as effectively as in mice lacking 5-HT(1A) receptors, indicating a silent, maximal occupancy of pre-synaptic 5-HT(1A) receptors at the dose used. However, DU-125530 addition to fluoxetine did not accelerate nor augment its antidepressant effects.DU-125530 is an excellent pre- and post-synaptic 5-HT(1A) receptor antagonist. However, blockade of post-synaptic 5- HT(1A) receptors by DU-125530 cancels benefits obtained by enhancing pre-synaptic 5-hydroxytryptaminergic function.
Project description:Multiple lines of evidence suggest that serotonin type 1A (5-HT(1A)) receptor dysfunction is involved in the pathophysiology of mood disorders, and that alterations in 5-HT(1A) receptor function play a role in the mechanisms of antidepressant and mood stabilizer treatment. The literature is in disagreement, however, as to whether 5-HT(1A) receptor binding abnormalities exist in bipolar disorder (BD). We acquired PET images of 5-HT(1A) receptor binding in 26 unmedicated BD subjects and 37 healthy controls using [¹?F]FCWAY, a highly selective 5-HT(1A) receptor radio-ligand. The mean 5-HT(1A) receptor binding potential (BP(P)) was significantly lower in BD subjects compared to controls in cortical regions where 5-HT(1A) receptors are expressed post-synaptically, most prominently in the mesiotemporal cortex. Post-hoc assessments involving other receptor specific binding parameters suggested that this difference particularly affected the females with BD. The mean BPP did not differ between groups in the raphe nucleus, however, where 5-HT(1A) receptors are predominantly expressed pre-synaptically. Across subjects the BPP in the mesiotemporal cortex was inversely correlated with trough plasma cortisol levels, consistent with preclinical literature indicating that hippocampal 5-HT(1A) receptor expression is inhibited by glucocorticoid receptor stimulation. These findings suggest that 5-HT(1A) receptor binding is abnormally reduced in BD, and this abnormality may particularly involve the postsynaptic 5-HT(1A) receptor system of individuals with a tendency toward cortisol hypersecretion.
Project description:Mood disorders cause much suffering and are the single greatest cause of lost productivity worldwide. Although multiple medications, along with behavioral therapies, have proven effective for some individuals, millions of people lack an effective therapeutic option. A common serotonin (5-HT) transporter (5-HTT/SERT, SLC6A4) polymorphism is believed to confer lower 5-HTT expression in vivo and elevates risk for multiple mood disorders including anxiety, alcoholism, and major depression. Importantly, this variant is also associated with reduced responsiveness to selective 5-HT reuptake inhibitor antidepressants. We hypothesized that a reduced antidepressant response in individuals with a constitutive reduction in 5-HTT expression could arise because of the compensatory expression of other genes that inactivate 5-HT in the brain. A functionally upregulated alternate transporter for 5-HT may prevent extracellular 5-HT from rising to levels sufficiently high enough to trigger the adaptive neurochemical events necessary for therapeutic benefit. Here we demonstrate that expression of the organic cation transporter type 3 (OCT3, SLC22A3), which also transports 5-HT, is upregulated in the brains of mice with constitutively reduced 5-HTT expression. Moreover, the OCT blocker decynium-22 diminishes 5-HT clearance and exerts antidepressant-like effects in these mice but not in WT animals. OCT3 may be an important transporter mediating serotonergic signaling when 5-HTT expression or function is compromised.
Project description:Despite the widespread use of antidepressant medications that block serotonin (5-hydroxytryptamine; 5-HT) and/or norepinephrine (NE) transporters, such as SSRIs (selective serotonin reuptake inhibitors) or SNRIs (serotonin and norepinephrine reuptake inhibitors), the underlying neurobiological basis of action of these agents is poorly understood. Increases in serotonergic function are hypothesized to have beneficial effects on depressive symptoms. However, which of the 14 different neuronal receptors sensitive to 5-HT accounts for the therapeutic effects of SSRIs and SNRIs remains undetermined. The development of drugs that activate or block specific 5-HT receptors may help to circumvent the two main limitations of current antidepressants: low efficacy and delayed onset of therapeutic action. What follows is a short summary of the author's views on this matter.
Project description:Serotonergic antidepressant drugs have been commonly used to treat mood and anxiety disorders, and increasing evidence suggests potential use of these drugs beyond current antidepressant therapeutics. Facilitation of adult neurogenesis in the hippocampal dentate gyrus has been suggested to be a candidate mechanism of action of antidepressant drugs, but this mechanism may be only one of the broad effects of antidepressants. Here we show a distinct unique action of the serotonergic antidepressant fluoxetine in transforming the phenotype of mature dentate granule cells. Chronic treatments of adult mice with fluoxetine strongly reduced expression of the mature granule cell marker calbindin. The fluoxetine treatment induced active somatic membrane properties resembling immature granule cells and markedly reduced synaptic facilitation that characterizes the mature dentate-to-CA3 signal transmission. These changes cannot be explained simply by an increase in newly generated immature neurons, but best characterized as "dematuration" of mature granule cells. This granule cell dematuration developed along with increases in the efficacy of serotonin in 5-HT(4) receptor-dependent neuromodulation and was attenuated in mice lacking the 5-HT(4) receptor. Our results suggest that serotonergic antidepressants can reverse the established state of neuronal maturation in the adult hippocampus, and up-regulation of 5-HT(4) receptor-mediated signaling may play a critical role in this distinct action of antidepressants. Such reversal of neuronal maturation could affect proper functioning of the mature hippocampal circuit, but may also cause some beneficial effects by reinstating neuronal functions that are lost during development.
Project description:Nicotinic acetylcholine receptor (nAChR) blockers potentiate the effects of selective serotonin reuptake inhibitors (SSRIs) in some treatment-resistant patients; however, it is not known whether these effects are independent, or whether the two neurotransmitter systems act synergistically. We first determined that the SSRI fluoxetine and the nicotinic partial agonist cytisine have synergistic effects in a mouse model of antidepressant efficacy, whereas serotonin depletion blocked the effects of cytisine. Using a pharmacological approach, we found that the 5-HT1A agonist 8-OH-DPAT also potentiated the antidepressant-like effects of cytisine, suggesting that this subtype might mediate the interaction between the serotonergic and cholinergic systems. The 5-HT1A receptors are located both presynaptically and postsynaptically. We therefore knocked down 5-HT1A receptors in either the dorsal raphe (presynaptic autoreceptors) or the hippocampus (a brain area with high expression of 5-HT1A heteroreceptors sensitive to cholinergic effects on affective behaviors). Knockdown of 5-HT1A receptors in hippocampus, but not dorsal raphe, significantly decreased the antidepressant-like effect of cytisine. This study suggests that serotonin signaling through postsynaptic 5-HT1A receptors in the hippocampus is critical for the antidepressant-like effects of a cholinergic drug and begins to elucidate the molecular mechanisms underlying interactions between the serotonergic and cholinergic systems related to mood disorders.