Chronic antidepressant potentiates spontaneous activity of dorsal raphe serotonergic neurons by decreasing GABAB receptor-mediated inhibition of L-type calcium channels

Integrative Research of Neuropharmacology and Informatics Pharmacology for Mental Disorder

Mental illness poses a huge social burden, accounting for approximately 14% of all deaths. Depression, a major component of mental illness, affects approximately 300 million people worldwide, mainly in developed countries, and is not only a major social burden but also a cause of suicide. The social burden of depression is estimated to increase further in developing countries, and overcoming it is a pressing issue for all countries, including Japan. Although clinical evidence has demonstrated the efficacy of serotonergic neurotransmission enhancers in the treatment of depression, the full picture of their therapeutic effects has not yet been fully elucidated. In this review, we show that the hyperactivity of serotonin neurons, especially those in the dorsal raphe nucleus, is commonly induced by various antidepressants within a period corresponding to the onset of their clinical efficacy. We established quantitative prediction methods for pharmacological activity using only chemical structures to translate the biological understanding of mental disorders, including major depressive disorders, into clinically effective therapeutics. Our method exhibited better performance than the previously reported methods of quantitative prediction, while targeting a larger number of proteins. Our article suggests the importance of integrative neuropharmacology and informatics-based pharmacology studies to understand the biological basis of mental disorders and facilitate drug development for these disorders.

The Role of GABA in the Dorsal Striatum-Raphe Nucleus Circuit Regulating Stress Vulnerability in Male Mice with High Levels of Shati/Nat8l

Depression is a frequent and serious illness, and stress is considered the main risk factor for its onset. First-line antidepressants increase serotonin (5-hydroxytryptamine; 5-HT) levels in the brain. We previously reported that an N-acetyltransferase, Shati/Nat8l, is upregulated in the dorsal striatum (dSTR) of stress-susceptible mice exposed to repeated social defeat stress (RSDS) and that dSTR Shati/Nat8l overexpression in mice (dSTR-Shati OE) induces stress vulnerability and local reduction in 5-HT content. Male mice were used in this study, and we found that dSTR 5-HT content decreased in stress-susceptible but not in resilient mice. Moreover, vulnerability to stress in dSTR-Shati OE mice was suppressed by the activation of serotonergic neurons projecting from the dorsal raphe nucleus (dRN) to the dSTR, followed by upregulation of 5-HT content in the dSTR using designer receptors exclusively activated by designer drugs (DREADD). We evaluated the role of GABA in modulating the serotonergic system in the dRN. Stress-susceptible after RSDS and dSTR-Shati OE mice exhibited an increase in dRN GABA content. Furthermore, dRN GABA content was correlated with stress sensitivity. We found that the blockade of GABA signaling in the dRN suppressed stress susceptibility in dSTR-Shati OE mice. In conclusion, we propose that dSTR 5-HT and dRN GABA, controlled by striatal Shati/Nat8l via the dSTR-dRN neuronal circuitry, critically regulate stress sensitivity. Our study provides insights into the neural processes that underlie stress and suggests that dSTR Shati/Nat8l could be a novel therapeutic target for drugs against depression, allowing direct control of the dRN serotonergic system.

Shared genetic loci and causal relations between schizophrenia and obsessive-compulsive disorder

Based on the clinical overlap between schizophrenia (SCZ) and obsessive-compulsive disorder (OCD), both disorders may share neurobiological substrates. In this study, we first analyzed recent large genome-wide associations studies (GWAS) on SCZ (n = 53,386, Psychiatric Genomics Consortium Wave 3) and OCD (n = 2688, the International Obsessive-Compulsive Disorder Foundation Genetics Collaborative (IOCDF-GC) and the OCD Collaborative Genetics Association Study (OCGAS)) using a conjunctional false discovery rate (FDR) approach to evaluate overlap in common genetic variants of European descent. Using a variety of biological resources, we functionally characterized the identified genomic loci. Then we used two-sample Mendelian randomization (MR) to estimate the bidirectional causal association between SCZ and OCD. Results showed that there is a positive genetic correlation between SCZ and OCD (r_g = 0.36, P = 0.02). We identified that one genetic locus (lead SNP rs5757717 in an intergenic region at CACNA1I) was jointly associated with SCZ and OCD (conjFDR = 2.12 × 10^-2). Mendelian randomization results showed that variants associated with increased risk for SCZ also increased the risk of OCD. This study broadens our understanding of the genetic architectures underpinning SCZ and OCD and suggests that the same molecular genetic processes may be responsible for shared pathophysiological and clinical characteristics between the two disorders.

Dorsal raphe serotonergic neurons preferentially reactivate dorsal dentate gyrus cell ensembles associated with positive experience

Major depressive disorder (MDD) is among the most common mental illnesses. Serotonergic (5-HT) neurons are central to the pathophysiology and treatment of MDD. Repeatedly recalling positive episodes is effective for MDD. Stimulating 5-HT neurons of the dorsal raphe nucleus (DRN) or neuronal ensembles in the dorsal dentate gyrus (dDG) associated with positive memories reverses the stress-induced behavioral abnormalities. Despite this phenotypic similarity, their causal relationship is unclear. This study revealed that the DRN 5-HT neurons activate dDG neurons; surprisingly, this activation was specifically observed in positive memory ensembles rather than neutral or negative ensembles. Furthermore, we revealed that dopaminergic signaling induced by activation of DRN 5-HT neurons projecting to the ventral tegmental area mediates an increase in active coping behavior and positive dDG ensemble reactivation. Our study identifies a role of DRN 5-HT neurons as specific reactivators of positive memories and provides insights into how serotonin elicits antidepressive effects.

A selective serotonin reuptake inhibitor ameliorates obsessive-compulsive disorder-like perseverative behavior by attenuating 5-HT2C receptor signaling in the orbitofrontal cortex

Perseveration is a characteristic of patients with obsessive-compulsive disorder (OCD). Clinically, neuronal activity in the lateral orbitofrontal cortex (OFC) is increased in OCD patients. Successful treatment with selective serotonin reuptake inhibitors (SSRIs) reduces activity in the lateral OFC of OCD patients, but the precise mechanisms underlying this effect are unclear. Previously, we reported that repeated injection of the dopamine D₂ receptor agonist quinpirole (QNP) resulted in OCD-like deficits, including perseveration in a reversal learning task. QNP-treated mice showed hyperactivity in lateral OFC pyramidal neurons. The present study demonstrated that 4-week administration of an SSRI increased the rate of correct choice in a reversal learning task. Using the electrophysiological approach, we revealed that an SSRI decreased the activity of lateral OFC pyramidal neurons in QNP-treated mice by potentiating inhibitory inputs. The 4-week administration of an SSRI inhibited the potentiation of neuronal activity induced by a 5-HT_2C receptor agonist. Additionally, both 4-week administration of SSRI and acute application of 5-HT_2C receptor antagonist prevented the QNP-induced potentiation of inhibitory inputs to fast-spiking interneurons in the lateral OFC. Administration of a 5-HT_2C receptor antagonist to mice for 4 days increased the rate of correct choice in a reversal learning task. Collectively, these results indicate that chronic SSRI ameliorated perseverative behavior in QNP-treated mice by modulating inhibitory inputs in the lateral OFC. Short-term 5-HT_2C receptor blockade also ameliorated QNP-induced behavioral and neurological abnormalities by, at least in part, a common mechanism with chronic SSRI.

GABAB receptor intracellular signaling: novel pathways for depressive disorder treatment?

Affecting over 320 million people around the world, depression has become a formidable challenge for modern medicine. In addition, an increasing number of studies cast doubt on the monoamine theory of depressive disorder and, worryingly, antidepressant medications only significantly benefit patients with severe depression. Thus, it is not surprising that researchers have shown an increased interest in new theories attempting to explain the pathogenesis of this disease. One example is the excitatory/inhibitory transmission imbalance theory. These abnormalities involve glutamate and γ-aminobutyric acid (GABA) signaling. Studies on GABA_B receptors and their antagonists are particularly promising for the treatment of depressive disorders. In this paper, intracellular pathways controlled by GABA_B receptors and their links to depression are described, including the impact of ketamine on GABAergic synaptic transmission.

Calcium, Bioenergetics, and Parkinson's Disease

Degeneration of substantia nigra (SN) dopaminergic (DAergic) neurons is responsible for the core motor deficits of Parkinson’s disease (PD). These neurons are autonomous pacemakers that have large cytosolic Ca²⁺ oscillations that have been linked to basal mitochondrial oxidant stress and turnover. This review explores the origin of Ca²⁺ oscillations and their role in the control of mitochondrial respiration, bioenergetics, and mitochondrial oxidant stress.

Antidepressant-resistant depression is characterized by reduced short- and long-interval cortical inhibition

BACKGROUND

Major depressive disorder (MDD) is highly heterogeneous and can be classified as treatment-resistant depression (TRD) or antidepressant-responsive depression (non-TRD) based on patients' responses to antidepressant treatment. Methods for distinguishing between TRD and non-TRD are critical clinical concerns. Deficits of cortical inhibition (CI) have been reported to play an influential role in the pathophysiology of MDD. Whether TRD patients' CI is more impaired than that of non-TRD patients remains unclear.

METHODS

Paired-pulse transcranial magnetic stimulation (ppTMS) was used to measure cortical inhibitory function including GABAA- and GABAB-receptor-related CI and cortical excitatory function including glutamate-receptor-related intracortical facilitation (ICF). We recruited 36 healthy controls (HC) and 36 patients with MDD (non-TRD, n = 16; TRD, n = 20). All participants received evaluations for depression severity and ppTMS examinations. Non-TRD patients received an additional ppTMS examination after 3 months of treatment with the SSRI escitalopram.

RESULTS

Patients with TRD exhibited reduced short-interval intracortical inhibition (SICI) and long-interval intracortical inhibition (LICI), as shown by abnormally higher estimates, than those with non-TRD or HC (F = 11.030, p < 0.001; F = 10.309, p < 0.001, respectively). After an adequate trial of escitalopram treatment, the LICI of non-TRD reduced significantly (t = - 3.628, p < 0.001), whereas the ICF remained lower than that of HC and showed no difference from pretreatment non-TRD.

CONCLUSIONS

TRD was characterized by relatively reduced CI, including both GABAA- and GABAB-receptor-mediated neurons while non-TRD preserved partial CI. In non-TRD, SSRIs may mainly modulate GABAB-receptor-related LICI. Our findings revealed distinguishable features of CI in antidepressant-resistant and responsive major depression.

Norepinephrine, neurodevelopment and behavior

Neurotransmitters play critical roles in the developing nervous system. Among the neurotransmitters, norepinephrine (NE) is in particular postulated to be an important regulator of brain development. NE is expressed during early stages of development and is known to regulate both the development of noradrenergic neurons and the development of target areas. NE participates in the shaping and the wiring of the nervous system during the critical periods of development, and perturbations in this process can alter the brain's developmental trajectory, which in turn can cause long-lasting and even permanent changes in the brain function and behavior later in life. Here we will briefly review evidence for the role of noradrenergic system in neurodevelopmental processes and will discuss about the potential disruptors of noradrenergic system during development and their behavioral consequences.

Ahnak scaffolds p11/Anxa2 complex and L-type voltage-gated calcium channel and modulates depressive behavior

Genetic polymorphisms of the L-type voltage-gated calcium channel (VGCC) are associated with psychiatric disorders including major depressive disorder. Alterations of S100A10 (p11) level are also implicated in the etiology of major depressive disorder. However, the existence of an endogenous regulator in the brain regulating p11, L-type VGCC, and depressive behavior has not been known. Here we report that Ahnak, whose function in the brain has been obscure, stabilizes p11 and Anxa2 proteins in the hippocampus and prefrontal cortex in the rodent brain. Protein levels of Ahnak, p11, and Anxa2 are highly and positively correlated in the brain. Together these data suggest the existence of an Ahnak/p11/Anxa2 protein complex. Ahnak is expressed in p11-positive as well as p11-negative neurons. Ahnak, through its N-terminal region, scaffolds the L-type pore-forming α1 subunit and, through its C-terminal region, scaffolds the β subunit of VGCC and the p11/Anxa2 complex. Cell surface expression of the α1 subunits and L-type calcium current are significantly reduced in primary cultures of Ahnak knockout (KO) neurons compared to wild-type controls. A decrease in the L-type calcium influx is observed in both glutamatergic neurons and parvalbumin (PV) GABAergic interneurons of Ahnak KO mice. Constitutive Ahnak KO mice or forebrain glutamatergic neuron-selective Ahnak KO mice display a depression-like behavioral phenotype similar to that of constitutive p11 KO mice. In contrast, PV interneuron-selective Ahnak KO mice display an antidepressant-like behavioral phenotype. Our results demonstrate L-type VGCC as an effector of the Ahnak/p11/Anxa2 complex, revealing a novel molecular connection involved in the control of depressive behavior.

Preliminary investigation of the effect of oral supplementation of Lactobacillus plantarum strain SNK12 on mRNA levels of neurotrophic factors and GABA receptors in the hippocampus of mice under stress-free and sub-chronic mild social defeat-stressing conditions

The effect of Lactobacillus plantarum SNK12 (CPLP) supplementation on mRNA levels of hippocampal neurotrophic factors and gamma aminobutyric acid receptors (GABA_R) was tested. In Experiment 1, stress-free, unsupplemented and CPLP (4 × 10⁸ cells/head)-supplemented male C57BL/6J (B6) mice were the experimental animals. In Experiment 2, intruder (male, B6) mice [negative control; unsupplemented, sub-chronic mild social defeat stress (sCSDS)-induced; and CPLP-supplemented, sCSDS-induced] were exposed to aggressor mice (adult male Slc:ICR). mRNA levels of neurotrophic factors and GABA_R in hippocampal samples of these mice were analyzed. In CPLP-supplemented mice of both experiments, mRNA levels of bdnf, nt-3, and GABA_R were upregulated. Moreover, a tendency toward the improvement of habituation ability (Experiment 1) and behavior (Experiment 2) was observed in mice, which may be associated with upregulated neurotrophic factors and GABA_R. We demonstrated that oral supplementation of CPLP to stress-free and stress-induced mice upregulated mRNA levels of hippocampal neurotrophic factors and GABA_R.

GABAergic modulation of serotonergic neurons in the dorsal raphe nucleus

The dorsal raphe nucleus (DRN), located in the brainstem, is involved in several functions such as sleep, temperature regulation, stress responses, and anxiety behaviors. This nucleus contains the largest population of serotonin expressing neurons in the brain. Serotonergic DRN neurons receive tonic γ-aminobutyric acid (GABA)inhibitory inputs from several brain areas, as well as from interneurons within the same nucleus. Serotonergic and GABAergic neurons in the DRN can be distinguished by their size, location, pharmacological responses, and electrophysiological properties. GABAergic neurons regulate the excitability of DRN serotonergic neurons and the serotonin release in different brain areas. Also, it has been shown that GABAergic neurons can synchronize the activity of serotonergic neurons across functions such as sleep or alertness. Moreover, dysregulation of GABA signaling in the DRN has been linked to psychiatric disorders such as anxiety and depression. This review focuses on GABAergic transmission in the DRN. The interaction between GABAergic and serotonergic neurons is discussed considering some physiological implications. Also, the main electrophysiological and morphological characteristics of serotonergic and GABAergic neurons are described.

MT1 and MT2 melatonin receptors are expressed in nonoverlapping neuronal populations

Melatonin (MLT) exerts its physiological effects principally through two high-affinity membrane receptors MT1 and MT2. Understanding the exact mechanism of MLT action necessitates the use of highly selective agonists/antagonists to stimulate/inhibit a given MLT receptor. The respective distribution of MT1 and MT2 within the CNS and elsewhere is controversial, and here we used a "knock-in" strategy replacing MT1 or MT2 coding sequences with a LacZ reporter. The data show striking differences in the distribution of MT1 and MT2 receptors in the mouse brain: whereas the MT1 subtype was expressed in very few structures (notably including the suprachiasmatic nucleus and pars tuberalis), MT2 subtype receptors were identified within numerous brain regions including the olfactory bulb, forebrain, hippocampus, amygdala and superior colliculus. Co-expression of the two subtypes was observed in very few structures, and even within these areas they were rarely present in the same individual cell. In conclusion, the expression and distribution of MT2 receptors are much more widespread than previously thought, and there is virtually no correspondence between MT1 and MT2 cellular expression. The precise phenotyping of cells/neurons containing MT1 or MT2 receptor subtypes opens new perspectives for the characterization of links between MLT brain targets, MLT actions and specific MLT receptor subtypes.

An Adenosine A2A Receptor Antagonist Improves Multiple Symptoms of Repeated Quinpirole-Induced Psychosis

Obsessive-compulsive disorder (OCD) is a neuropsychiatric disorder characterized by the repeated rise of concerns (obsessions) and repetitive unwanted behavior (compulsions). Although selective serotonin reuptake inhibitors (SSRIs) is the first-choice drug, response rates to SSRI treatment vary between symptom dimensions. In this study, to find a therapeutic target for SSRI-resilient OCD symptoms, we evaluated treatment responses of quinpirole (QNP) sensitization-induced OCD-related behaviors in mice. SSRI administration rescued the cognitive inflexibility, as well as hyperactivity in the lateral orbitofrontal cortex (lOFC), while no improvement was observed for the repetitive behavior. D₂ receptor signaling in the central striatum (CS) was involved in SSRI-resistant repetitive behavior. An adenosine A_2A antagonist, istradefylline, which rescued abnormal excitatory synaptic function in the CS indirect pathway medium spiny neurons (MSNs) of sensitized mice, alleviated both of the QNP-induced abnormal behaviors with only short-term administration. These results provide a new insight into therapeutic strategies for SSRI-resistant OCD symptoms and indicate the potential of A_2A antagonists as a rapid-acting anti-OCD drug.

Two eARCHT3.0 Lines for Optogenetic Silencing of Dopaminergic and Serotonergic Neurons

Dopaminergic and serotonergic neurons modulate and control processes ranging from reward signaling to regulation of motor outputs. Further, dysfunction of these neurons is involved in both degenerative and psychiatric disorders. Elucidating the roles of these neurons has been greatly facilitated by bacterial artificial chromosome (BAC) transgenic mouse lines expressing channelrhodopsin to readily enable cell-type specific activation. However, corresponding lines to silence these monoaminergic neurons have been lacking. We have generated two BAC transgenic mouse lines expressing the outward proton pump, enhanced ArchT3.0 (eArchT3.0), and GFP under control of the regulatory elements of either the dopamine transporter (DAT; Jax# 031663) or the tryptophan hydroxylase 2 (TPH2; Jax# 031662) gene locus. We demonstrate highly faithful and specific expression of these lines in dopaminergic and serotonergic neurons respectively. Additionally we validate effective and sensitive eArchT3.0-mediated silencing of these neurons using slice electrophysiology as well as with a well-established behavioral assay. These new transgenic tools will help expedite the study of dopaminergic and serotonergic system function in normal behavior and disease.

Ketamine-Induced Prefrontal Serotonin Release Is Mediated by Cholinergic Neurons in the Pedunculopontine Tegmental Nucleus

BACKGROUND

Ketamine rapidly elicits antidepressive effects in humans and mice in which serotonergic activity is involved. Although α4β2 nicotinic acetylcholine receptor (α4β2 nAChR) in the dorsal raphe nucleus plays a key role in the ketamine-induced prefrontal serotonin release, the source of cholinergic afferents, and its role is unclear.

METHODS

Prefrontal serotonin levels after ketamine injection were measured by microdialysis in rats. Electrolytic lesion of pedunculopontine tegmental nucleus and laterodorsal tegmental nucleus was made with constant direct current.

RESULTS

Bilateral lesion of the pedunculopontine tegmental nucleus, but not laterodorsal tegmental nucleus, attenuated prefrontal serotonin release induced by systemic ketamine. Intra-pedunculopontine tegmental nucleus, but not intra-laterodorsal tegmental nucleus ketamine perfusion, increased prefrontal serotonin release. This increase was attenuated by intra-dorsal raphe nucleus injection of dihydro-β-erythroidine, an α4β2 nAChR antagonist, or NBQX, an AMPA receptor antagonist.

CONCLUSIONS

These results suggest the ketamine-induced serotonin release in medial prefrontal cortex is mediated by cholinergic neurons projecting from pedunculopontine tegmental nucleus to dorsal raphe nucleus via α4β2 nAChRs.