5-HT1A autoreceptor levels determine vulnerability to stress and response to antidepressants

Density and function of central serotonin (5-HT) transporters, 5-HT1A and 5-HT2A receptors, and effects of their targeting on BTBR T+tf/J mouse social behavior

BTBR mice are potentially useful tools for autism research because their behavior parallels core social interaction impairments and restricted-repetitive behaviors. Altered regulation of central serotonin (5-HT) neurotransmission may underlie such behavioral deficits. To test this, we compared 5-HT transporter (SERT), 5-HT(1A) and 5-HT(2A) receptor densities among BTBR and C57 strains. Autoradiographic [(3) H] cyanoimipramine (1 nM) binding to SERT was 20-30% lower throughout the adult BTBR brain as compared to C57BL/10J mice. In hippocampal membrane homogenates, [(3) H] citalopram maximal binding (B(max) ) to SERT was 95 ± 13 fmol/mg protein in BTBR and 171 ± 20 fmol/mg protein in C57BL/6J mice, and the BTBR dissociation constant (K(D) ) was 2.0 ± 0.3 nM versus 1.1 ± 0.2 in C57BL/6J mice. Hippocampal 5-HT(1A) and 5-HT(2A) receptor binding was similar among strains. However, 8-OH-DPAT-stimulated [(35) S] GTPγS binding in the BTBR hippocampal CA(1) region was 28% higher, indicating elevated 5-HT(1A) capacity to activate G-proteins. In BTBR mice, the SERT blocker, fluoxetine (10 mg/kg) and the 5-HT(1A) receptor partial-agonist, buspirone (2 mg/kg) enhanced social interactions. The D(2) /5-HT(2) receptor antagonist, risperidone (0.1 mg/kg) reduced marble burying, but failed to improve sociability. Overall, altered SERT and/or 5-HT(1A) functionality in hippocampus could contribute to the relatively low sociability of BTBR mice.

Freud-2/CC2D1B mediates dual repression of the serotonin-1A receptor gene

The serotonin-1A (5-HT1A) receptor functions as a pre-synaptic autoreceptor in serotonin neurons that regulates their activity, and is also widely expressed on non-serotonergic neurons as a post-synaptic heteroreceptor to mediate serotonin action. The 5-HT1A receptor gene is strongly repressed by a dual repressor element (DRE), which is recognized by two proteins: Freud-1/CC2D1A and another unknown protein. Here we identify mouse Freud-2/CC2D1B as the second repressor of the 5-HT1A-DRE. Freud-2 shares 50% amino acid identity with Freud-1, and contains conserved structural domains. Mouse Freud-2 bound specifically to the rat 5-HT1A-DRE adjacent to, and partially overlapping, the Freud-1 binding site. By supershift assay using nuclear extracts from L6 myoblasts, Freud-2-DRE complexes were distinguished from Freud-1-DRE complexes. Freud-2 mRNA and protein were detected throughout mouse brain and peripheral tissues. Freud-2 repressed 5-HT1A promoter-reporter constructs in a DRE-dependent manner in non-neuronal (L6) or 5-HT1A-expressing neuronal (NG108-15, RN46A) cell models. In NG108-15 cells, knockdown of Freud-2 using a specific short-interfering RNA reduced endogenous Freud-2 protein levels and decreased Freud-2 bound to the 5-HT1A-DRE as detected by chromatin immunoprecipitation assay, but increased 5-HT1A promoter activity and 5-HT1A protein levels. Taken together, these data show that Freud-2 is the second component that, with Freud-1, mediates dual repression of the 5-HT1A receptor gene at the DRE.

Escitalopram enhances the association of serotonin-1A autoreceptors to heteroreceptors in anxiety disorders

Selective serotonin reuptake inhibitors (SSRIs) represent one of the most common treatment options in major depression and anxiety disorders. By blocking the serotonin transporter, SSRIs modulate serotonergic neurotransmission as well as the function of autoreceptors and heteroreceptors. However, treatment-induced changes on a network level primarily remain unknown. Thus, we evaluated the association between serotonin-1A (5-HT1A) autoreceptors and heteroreceptors before and after SSRIs. Twenty-one patients with anxiety disorders underwent positron emission tomography using [carbonyl-11C]WAY-100635 before and after 12 weeks of escitalopram treatment; 15 of them completed the study protocol. Additionally, 36 drug-naive healthy controls were measured once. The 5-HT1A receptor binding potential (BPND) was quantified for the dorsal raphe nucleus (DRN) using a region-of-interest approach and for the entire brain by calculating parametric maps. Voxel-wise linear regression was applied between DRN autoreceptor and whole-brain heteroreceptor 5-HT1A BPND. Consistent with previous observations, healthy subjects showed widespread positive correlations of 5-HT1A BPND between autoreceptors and heteroreceptors. Comparing patients before versus after escitalopram treatment revealed enhanced associations of autoreceptor-to-heteroreceptor 5-HT1A BPND within the amygdala and hippocampus (R2=0.21-0.28 vs 0.49-0.81; p<0.05-0.001). In contrast, no significant SSRI-induced changes were found for correlations of heteroreceptor-to-heteroreceptor 5-HT1A BPND between several limbic regions. This interregional approach suggests a treatment-induced reinforcement of the association of 5-HT1A binding between autoreceptors and heteroreceptors specifically in areas involved in anxiety disorders. These findings provide complementary information about treatment effects on a network level and confirm the central role of the DRN as a prime regulatory area.

Measuring serotonin synthesis: from conventional methods to PET tracers and their (pre)clinical implications

The serotonergic system of the brain is complex, with an extensive innervation pattern covering all brain regions and endowed with at least 15 different receptors (each with their particular distribution patterns), specific reuptake mechanisms and synthetic processes. Many aspects of the functioning of the serotonergic system are still unclear, partially because of the difficulty of measuring physiological processes in the living brain. In this review we give an overview of the conventional methods of measuring serotonin synthesis and methods using positron emission tomography (PET) tracers, more specifically with respect to serotonergic function in affective disorders. Conventional methods are invasive and do not directly measure synthesis rates. Although they may give insight into turnover rates, a more direct measurement may be preferred. PET is a noninvasive technique which can trace metabolic processes, like serotonin synthesis. Tracers developed for this purpose are α-[¹¹C]methyltryptophan ([¹¹C]AMT) and 5-hydroxy-L-[β-¹¹C]tryptophan ([¹¹C]5-HTP). Both tracers have advantages and disadvantages. [¹¹C]AMT can enter the kynurenine pathway under inflammatory conditions (and thus provide a false signal), but this tracer has been used in many studies leading to novel insights regarding antidepressant action. [¹¹C]5-HTP is difficult to produce, but trapping of this compound may better represent serotonin synthesis. AMT and 5-HTP kinetics are differently affected by tryptophan depletion and changes of mood. This may indicate that both tracers are associated with different enzymatic processes. In conclusion, PET with radiolabelled substrates for the serotonergic pathway is the only direct way to detect changes of serotonin synthesis in the living brain.

5-HT1A receptor-regulated signal transduction pathways in brain

Serotonin is an influential monoamine neurotransmitter that signals through a number of receptors to modulate brain function. Among different serotonin receptors, the serotonin 1A (5-HT1A) receptors have been tied to a variety of physiological and pathological processes, notably in anxiety, mood, and cognition. 5-HT1A receptors couple not only to the classical inhibitory G protein-regulated signaling pathway, but also to signaling pathways traditionally regulated by growth factors. Despite the importance of 5-HT1A receptors in brain function, little is known about how these signaling mechanisms link 5-HT1A receptors to regulation of brain physiology and behavior. Following a brief summary of the known physiological and behavioral effects of 5-HT1A receptors, this article will review the signaling pathways regulated by 5-HT1A receptors, and discuss the potential implication of these signaling pathways in 5-HT1A receptor-regulated physiological processes and behaviors.

Challenges and opportunities for drug discovery in psychiatric disorders: the drug hunters' perspective

Innovation is essential for the identification of novel pharmacological therapies to meet the treatment needs of patients with psychiatric disorders. However, over the last 20 yr, in spite of major investments targets falling outside the classical aminergic mechanisms have shown diminished returns. The disappointments are traced to failures in the target identification and target validation effort, as reflected by the poor ability of current bioassays and animal models to predict efficacy and side-effects. Mismatch between disease biology and how psychiatric diseases are categorized has resulted in clinical trials of highly specific agents in heterogeneous patients, leading to variable treatment effects and failed studies. As drug hunters, one sees the opportunity to overhaul the pharmaceutical research and development (R&D) process. Improvements in both preclinical and clinical translational research need to be considered. Linking pharmacodynamic markers with disease biology should provide more predictive and innovative early clinical trials which in turn will increase the success rate of discovering new medicines. However, to exploit these exciting scientific discoveries, pharmaceutical companies need to question the conventional drug research and development model which is silo-driven, non-integrative across the confines of a company, non-disclosing across the pharmaceutical industry, and often independent from academia. This leads to huge redundancy in effort and lack of contextual learning in real time. Nevertheless, there are signs that drug discovery in the 21st century will see more intentional government, academic and industrial collaborations to overcome the above challenges that could eventually link mechanistic disease biology to segments of patients, affording them the benefits of rational and targeted therapy.

Strain-specific outcomes of repeated social defeat and chronic fluoxetine treatment in the mouse

Social stress is a risk factor for affective disorders in vulnerable individuals. Although the biological nature of stress susceptibility/resilience remains to be elucidated, genetic variation is considered amongst the principal contributors to brain disorders. Furthermore, genetic predisposition may be determinant for the therapeutic outcome, as proposed for antidepressant treatments. In the present studies we compared the inherently diverse genetic backgrounds of 2 mouse strains by assessing the efficacy of a chronic antidepressant treatment in a repeated social stress procedure. C57BL/6J and BalbC mice underwent 10-day social defeats followed by 28-day fluoxetine treatment (10 mg/kg/mL, p.o.). In C57BL/6J, most of the social defeat-induced changes were of metabolic nature including persistently altered feed efficiency and decreased abdominal fat stores that were ameliorated by fluoxetine. BalbC mouse behavior was persistently affected by social defeat both in the social avoidance and the forced swim tests, and in either procedure it was restored by chronic fluoxetine, whereas their endocrine parameters were mostly unaffected. The highlighted strain-specific responsivity to the metabolic and behavioral consequences of social defeat and to the chronic antidepressant treatment offers a promising research tool to further explore the underlying neural mechanisms and genetic basis of stress susceptibility and treatment response.

Pet-1 is required across different stages of life to regulate serotonergic function

Transcriptional cascades are required for specification of 5-HT neurons and 5-HT modulated behaviors. Expression of several cascade factors extends across lifespan suggesting their control of behavior may not be temporally restricted to programming normal numbers of 5-HT neurons. We applied new mouse conditional targeting approaches to investigate ongoing requirements for Pet-1, a cascade factor required for the initiation of 5-HT synthesis but whose expression persists into adulthood. We found that Pet-1 was required after 5-HT neuron generation, for multiple steps in 5-HT neuron maturation including axonal innervation to the somatosensory cortex, firing properties, and 5-HT_1A and 5-HT_1B autoreceptor expression. Targeting Pet-1 in adult 5-HT neurons showed that it was still needed to preserve normal anxiety-related behaviors through direct autoregulated control of serotonergic gene expression. These findings show that Pet-1 is required across lifespan and therefore behavioral pathogenesis can result from both developmental and adult-onset alterations in serotonergic transcription.

Different susceptibility to social defeat stress of BalbC and C57BL6/J mice

Social stress may precipitate psychopathological disorders in susceptible individuals. The present experiments were focused on the biology beyond the differential susceptibility to social stress. Social defeat, an ethologically relevant stressor known to elicit different coping strategies, was used in two mouse strains differing for baseline emotionality, such as C57BL6/J and BalbC. In separate experiments, in both strains a single social defeat decreased home-cage activity without altering social aversion; it diminished body weight only in defeated BalbC mice. In longitudinal experiments, mice experienced repeated social defeats that induced multiple long-term consequences. Defeated C57BL6/J increased their body weight and food intake; defeated BalbC mice diminished their metabolic efficiency. Only defeated BalbC subjects exhibited increased social avoidance levels; no differences from controls were seen on forced swim test response in defeated mice of either strain. No long-term effects of social defeat were detected in peripheral biomarkers of stress, metabolic, and immune responses, although the analysis of selected internal organs revealed decreases in abdominal fat and gonadal organs in all defeated subjects. These results demonstrated a strain-distinctive profile in the susceptibility to social defeat stress, either acutely or chronically, with metabolic consequences more consistently found in C57BL6/J while social aversion induced predominantly in BalbC subjects.

Serotonin: a regulator of neuronal morphology and circuitry

Serotonin is an important neuromodulator associated with a wide range of physiological effects in the central nervous system. The exact mechanisms whereby serotonin influences brain development are not well understood, although studies in invertebrate and vertebrate model organisms are beginning to unravel a regulatory role for serotonin in neuronal morphology and circuit formation. Recent data suggest a developmental window during which altered serotonin levels permanently influence neuronal circuitry, however, the temporal constraints and molecular mechanisms responsible are still under investigation. Growing evidence suggests that alterations in early serotonin signaling contribute to a number of neurodevelopmental and neuropsychiatric disorders. Thus, understanding how altered serotonin signaling affects neuronal morphology and plasticity, and ultimately animal physiology and pathophysiology, will be of great significance.

Modifying 5-HT1A Receptor Gene Expression as a New Target for Antidepressant Therapy

Major depression is the most common form of mental illness, and is treated with antidepressant compounds that increase serotonin (5-HT) neurotransmission. Increased 5-HT1A autoreceptor levels in the raphe nuclei act as a “brake” to inhibit the 5-HT system, leading to depression and resistance to antidepressants. Several 5-HT1A receptor agonists (buspirone, flesinoxan, ipsapirone) that preferentially desensitize 5-HT1A autoreceptors have been tested for augmentation of antidepressant drugs with mixed results. One explanation could be the presence of the C(−1019)G 5-HT1A promoter polymorphism that prevents gene repression of the 5-HT1A autoreceptor. Furthermore, down-regulation of 5-HT1A autoreceptor expression, not simply desensitization of receptor signaling, appears to be required to enhance and accelerate antidepressant action. The current review focuses on the transcriptional regulators of 5-HT1A autoreceptor expression, their roles in permitting response to 5-HT1A-targeted treatments and their potential as targets for new antidepressant compounds for treatment-resistant depression.

RGS inhibition at G(alpha)i2 selectively potentiates 5-HT1A-mediated antidepressant effects

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.

Flexible Accelerated STOP Tetracycline Operator-knockin (FAST): a versatile and efficient new gene modulating system

We created the Flexible Accelerated STOP Tetracycline Operator (tetO)-knockin (FAST) system, an efficient method for manipulating gene expression in vivo to rapidly screen animal models of disease. A single gene targeting event yields two distinct knockin mice-STOP-tetO and tetO knockin-that permit generation of multiple strains with variable expression patterns: 1) knockout, 2) Cre-mediated rescue, 3) tetracycline-controlled transcriptional activator (tTA)-mediated misexpression, 4) tetracycline-controlled transcriptional activator (tTA)-mediated overexpression, and 5) tetracycline-controlled transcriptional silencer (tTS)-mediated conditional knockout/knockdown. Using the FAST system, multiple gain-of-function and loss-of-function strains can therefore be generated on a time scale not previously achievable. These strains can then be screened for clinically relevant abnormalities. We demonstrate the flexibility and broad applicability of the FAST system by targeting several genes encoding proteins implicated in neuropsychiatric disorders: Mlc1, neuroligin 3, the serotonin 1A receptor, and the serotonin 1B receptor.

Altered function of the serotonin 1A autoreceptor and the antidepressant response

In this issue of Neuron, Richardson-Jones et al. report on a novel genetic approach to conditionally and specifically dampen the function of the 5-HT(1A) autoreceptor in mice to study its role on the 5-HT system and on the antidepressant-like effect of the selective serotonin reuptake inhibitor (SSRI) fluoxetine.