Long-term antidepressant treatments result in a tonic activation of forebrain 5-HT1A receptors

N-(3-((3-(trifluoromethyl)phenyl)selanyl)prop-2-yn-1-yl) benzamide induces antidepressant-like effect in mice: involvement of the serotonergic system

RATIONALE

Major Depressive Disorder (MDD) significantly impairs the quality of life for those affected. While the exact causes of MDD are not fully understood, the deficit of monoamines, especially serotonin and noradrenaline, is widely accepted. Resistance to long-term treatments and adverse effects are often observed, highlighting the need for new pharmacological therapies. Synthetic organic compounds containing selenium have exhibited pharmacological properties, including potential antidepressant effects.

OBJECTIVE

To evaluate the antidepressant-like effect of N-(3-((3-(trifluoromethyl)phenyl)selenyl)prop-2-yn-1-yl) benzamide (CF3SePB) in mice and the involvement of the serotonergic and noradrenergic systems.

METHODS

Male Swiss mice were treated with CF3SePB (1-50 mg/kg, i.g.) and 30 min later the forced swimming test (FST) or tail suspension test (TST) was performed. To investigate the involvement of the serotonergic and noradrenergic systems in the antidepressant-like effect of CF3SePB, mice were pre-treated with p-CPA (a 5-HT depletor, 100 mg/kg, i.p.) or the receptor antagonists WAY100635 (0.1 mg/kg, s.c., a 5-HT1A receptor antagonist), ketanserin (1 mg/kg, i.p., a 5-HT2A/2C receptor antagonist), ondansetron (1 mg/kg, i.p., a 5-HT3 receptor antagonist), GR110838 (0.1 mg/kg, i.p., a 5-HT4 receptor antagonist), prazosin (1 mg/kg, i.p., an α1-adrenergic receptor antagonist), yohimbine (1 mg/kg, i.p., an α2-adrenergic receptor antagonist) and propranolol (2 mg/kg, i.p., a non-selective beta-adrenergic receptor antagonist) at specific times before CF3SePB (50 mg/kg, i.g.), and after 30 min of CF3SePB administration the FST was performed.

RESULTS

CF3SePB showed an antidepressant-like effect in both FST and TST and this effect was related to the modulation of the serotonergic system, specially the 5-HT1A and 5-HT3 receptors. None of the noradrenergic antagonists prevented the antidepressant-like effect of CF3SePB. The compound exhibited a low potential for inducing acute toxicity in adult female Swiss mice.

CONCLUSION

This study pointed a new compound with antidepressant-like effect, and it could be considered for the development of new antidepressants.

5-hydroxytryphophan mitigates ergot alkaloid-induced suppression of serotonin and feed intake in cattle

The impact of ergot toxicosis on livestock industries is detrimental and treatments are needed in many countries. The objective of this study was to evaluate the effects of acute exposure to ergot alkaloids and 5-hydroxytryptophan (5-HTP) supplementation on feed intake, serotonin metabolism, and blood metabolites in cattle. Eight Holstein steers (538 ± 18 kg) fitted with ruminal cannulas were used in a replicated 4 × 4 Latin Square design experiment with a 2 × 2 factorial treatment structure. The treatments were the combination of 0 (E-) or 15 µg ergovaline/kg BW (E+) and 0 (5HTP-) or 0.5 mg of 5-hydroxy-l-tryptophan/kg BW (5HTP+) administered daily for 6 d. Toxic endophyte-infected tall fescue seed was used to supply the daily dose of ergovaline. Endophyte-free seed was used to equalize seed intake between treatments. Ground seed was placed into the rumen immediately before feeding. The 5-HTP was dissolved in water and infused into the abomasum via the reticulo-omasal orifice. Blood was collected from a jugular vein catheter at 0, 1, 2, 4, 8, and 24 h after treatment administration. Ergovaline without 5-HTP (E+/5HTP-) decreased dry matter intake (DMI) in comparison to steers without ergovaline and 5-HTP (E-/5HTP-). However, 5-HTP infusion in association with ergovaline (E+/5HTP+) normalized the DMI. Although E + did not affect (P > 0.05) the area under the curve (AUC) of serum 5-HTP, 5-hydroxyindoleacetic acid, tryptophan, and kynurenine, serum and plasma serotonin concentrations were decreased (P < 0.05). The infusion of 5-HTP increased (P < 0.05) the AUC of serum 5-HTP, serum and plasma serotonin, and serum 5-hydroxyindoleacetic acid. In conclusion, acute exposure to ergot alkaloids reduced DMI and circulating serotonin in cattle but 5-HTP administration showed potential to normalize both circulating serotonin and feed intake.

Techniques for Measurement of Serotonin: Implications in Neuropsychiatric Disorders and Advances in Absolute Value Recording Methods

Serotonin (5-HT) is a monoamine neurotransmitter in the peripheral, enteric, and central nervous systems (CNS). Within the CNS, serotonin is principally involved in mood regulation and reward-seeking behaviors. It is a critical regulator in CNS pathologies such as major depressive disorder, addiction, and schizophrenia. Consequently, in vivo serotonin measurements within the CNS have emerged as one of many promising approaches to investigating the pathogenesis, progression, and treatment of these and other neuropsychiatric conditions. These techniques vary in methods, ranging from analyte sampling with microdialysis to voltammetry. Provided this diversity in approach, inherent differences between techniques are inevitable. These include biosensor size, temporal/spatial resolution, and absolute value measurement capabilities, all of which must be considered to fit the prospective researcher's needs. In this review, we summarize currently available methods for the measurement of serotonin, including novel voltammetric absolute value measurement techniques. We also detail serotonin's role in various neuropsychiatric conditions, highlighting the role of phasic and tonic serotonergic neuronal firing within each where relevant. Lastly, we briefly review the present clinical application of these techniques and discuss the potential of a closed-loop monitoring and neuromodulation system utilizing deep brain stimulation (DBS).

Wistar Kyoto rats exhibit decreased serotonin neuronal firing and increased norepinephrine burst activity but dampened hippocampal α2-adrenoceptor sensitivity

BACKGROUND

Wistar Kyoto (WKY) rats manifest abnormalities in the function of monoamine receptors and transporters, as well as levels of these neurotransmitters in the brain. The present study assessed alterations in the firing activity of serotonin (5-hydroxytryptamine [5-HT]), norepinephrine (NE), and dopamine (DA) neurons, as well as the activity of 5-HT and NE receptors and transporters in the hippocampus.

METHODS

In vivo electrophysiological recordings were conducted in male WKY and Wistar rats. Extracellular single-unit recordings of 5-HT, NE, and DA neurons were performed. Recordings of pyramidal neurons were conducted in the medial prefrontal cortex (mPFC) and the hippocampus, where direct application of 5-HT and NE by iontophoresis was also carried out.

RESULTS

The mean firing rate of 5-HT neurons was significantly decreased in WKY compared to Wistar rats. The burst activity of NE neurons was significantly increased in WKY, while their mean firing activity was not changed. There was no alteration in the firing, burst, and population activity of DA neurons in WKY animals. In the hippocampus, a decrease in sensitivity of α2-adrenoceptors, but not 5-HT receptors, was observed. There was, however, no change in the activity of 5-HT and NE transporters. The firing activity of mPFC pyramidal neurons was similar in WKY versus Wistar rats.

CONCLUSION

In WKY rats, there was a decrease in the firing activity of 5-HT neurons. There was also an enhanced burst activity of NE neurons, accompanied by a reduction in sensitivity of the α2-adrenoceptor in the hippocampus, inferring a decrease in NE transmission.

Effects of bilateral sequential theta-burst stimulation on 5-HT1A receptors in the dorsolateral prefrontal cortex in treatment-resistant depression: a proof-of-concept trial

Theta-burst stimulation (TBS) represents a brain stimulation technique effective for treatment-resistant depression (TRD) as underlined by meta-analyses. While the methodology undergoes constant refinement, bilateral stimulation of the dorsolateral prefrontal cortex (DLPFC) appears promising to restore left DLPFC hypoactivity and right hyperactivity found in depression. The post-synaptic inhibitory serotonin-1A (5-HT1A) receptor, also occurring in the DLPFC, might be involved in this mechanism of action. To test this hypothesis, we performed PET-imaging using the tracer [carbonyl-11C]WAY-100635 including arterial blood sampling before and after a three-week treatment with TBS in 11 TRD patients compared to sham stimulation (n = 8 and n = 3, respectively). Treatment groups were randomly assigned, and TBS protocol consisted of excitatory intermittent TBS to the left and inhibitory continuous TBS to the right DLPFC. A linear mixed model including group, hemisphere, time, and Hamilton Rating Scale for Depression (HAMD) score revealed a 3-way interaction effect of group, time, and HAMD on specific distribution volume (VS) of 5-HT1A receptor. While post-hoc comparisons showed no significant changes of 5-HT1A receptor VS in either group, higher 5-HT1A receptor VS after treatment correlated with greater difference in HAMD (r = -0.62). The results of this proof-of-concept trial hint towards potential effects of TBS on the distribution of the 5-HT1A receptor. Due to the small sample size, all results must, however, be regarded with caution.

Low doses of lysergic acid diethylamide (LSD) increase reward-related brain activity

Renewed interest in classic psychedelics as treatments for psychiatric disorders warrants a deeper understanding of their neural mechanisms. Single, high doses of psychedelic drugs have shown promise in treating depressive disorders, perhaps by reversing deficits in reward processing in the brain. In addition, there are anecdotal reports that repeated ingestion of low doses of LSD, or "microdosing", improve mood, cognition, and feelings of wellbeing. However, the effects of low doses of classic psychedelics on reward processing have not been studied. The current study examined the effects of two single, low doses of LSD compared to placebo on measures of reward processing. Eighteen healthy adults completed three sessions in which they received placebo (LSD-0), 13 μg LSD (LSD-13) and 26 μg LSD (LSD-26) in a within-subject, double-blind design. Neural activity was recorded while participants completed the electrophysiological monetary incentive delay task. Event-related potentials were measured during feedback processing (Reward-Positivity: RewP, Feedback-P3: FB-P3, and Late-Positive Potential: LPP). Compared to placebo, LSD-13 increased RewP and LPP amplitudes for reward (vs. neutral) feedback, and LSD-13 and LSD-26 increased FB-P3 amplitudes for positive (vs. negative) feedback. These effects were unassociated with most subjective measures of drug effects. Thus, single, low doses of LSD (vs. placebo) increased three reward-related ERP components reflecting increased hedonic (RewP), motivational (FB-P3), and affective processing of feedback (LPP). These results constitute the first evidence that low doses of LSD increase reward-related brain activity in humans. These findings may have important implications for the treatment of depressive disorders.

The NLRP3 Inflammasome in Stress Response: Another Target for the Promiscuous Cannabidiol

Many psychiatric patients do not respond to conventional therapy. There is a vast effort to investigate possible mechanisms involved in treatment resistance, trying to provide better treatment options, and several data points toward a possible involvement of inflammatory mechanisms. Microglia, glial, and resident immune cells are involved in complex responses in the brain, orchestrating homeostatic functions, such as synaptic pruning and maintaining neuronal activity. In contrast, microglia play a major role in neuroinflammation, neurodegeneration, and cell death. Increasing evidence implicate microglia dysfunction in neuropsychiatric disorders. The mechanisms are still unclear, but one pathway in microglia has received increased attention in the last 8 years, i.e., the NLRP3 inflammasome pathway. Stress response and inflammation, including microglia activation, can be attenuated by Cannabidiol (CBD). CBD has antidepressant, anti-stress, antipsychotic, anti-inflammatory, and other properties. CBD effects are mediated by direct or indirect modulation of many receptors, enzymes, and other targets. This review will highlight some findings for neuroinflammation and microglia involvement in stress-related psychiatric disorders, particularly addressing the NLRP3 inflammasome pathway. Moreover, we will discuss evidence and mechanisms for CBD effects in psychiatric disorders and animal models and address its potential effects on stress response via neuroinflammation and NLRP3 inflammasome modulation.

Precision Medicine in Antidepressants Treatment

Precision medicine uses innovative approaches to improve disease prevention and treatment outcomes by taking into account people's genetic backgrounds, environments, and lifestyles. Treatment of depression is particularly challenging, given that 30-50% of patients do not respond adequately to antidepressants, while those who respond may experience unpleasant adverse drug reactions (ADRs) that decrease their quality of life and compliance. This chapter aims to present the available scientific data that focus on the impact of genetic variants on the efficacy and toxicity of antidepressants. We compiled data from candidate gene and genome-wide association studies that investigated associations between pharmacodynamic and pharmacokinetic genes and response to antidepressants regarding symptom improvement and ADRs. We also summarized the existing pharmacogenetic-based treatment guidelines for antidepressants, used to guide the selection of the right antidepressant and its dose based on the patient's genetic profile, aiming to achieve maximum efficacy and minimum toxicity. Finally, we reviewed the clinical implementation of pharmacogenomics studies focusing on patients on antidepressants. The available data demonstrate that precision medicine can increase the efficacy of antidepressants and reduce the occurrence of ADRs and ultimately improve patients' quality of life.

Linalool as a Therapeutic and Medicinal Tool in Depression Treatment: A Review

Depression is a prevalent disease worldwide, limiting psychosocial functioning and thequality of life. Linalool is the main constituent of some essential oils from aromatic plants, representing about 70% of these volatile concentrates. Evidence of the linalool activity on the central nervous system, mainly acting as an antidepressant agent, is increasingly abundant. This review aimed to extend the knowledge of linalool's antidepressant action mechanisms, which is fundamental for future research, intending to highlight this natural compound as a new antidepressant phytomedication. A critical analysis is proposed here with probable hypotheses of the synergic mechanisms that support the evidence of antidepressant effects of the linalool. The literature search has been conducted in databases for published scientific articles before December 2020, using relevant keywords. Several pieces of evidence point to the anticonvulsant, sedative, and anxiolytic actions. In addition to these activities, other studies have revealed that linalool acts on the monoaminergic and neuroendocrine systems, inflammatory process, oxidative stress, and neurotrophic factors, such as BDNF, resulting in considerable advances in the knowledge of the etiology of depression. In this context, linalool emerges as a promising bioactive compound in the therapeutic arsenal, capable of interacting with numerous pathophysiological factors and acting on several targets. This review claims to contribute to future studies, highlighting the gaps in the linalool knowledge, such as its kinetics, doses, routes of administration, and multiple targets of interaction, to clarify its antidepressant activity.

Molecular Signaling Mechanisms for the Antidepressant Effects of NLX-101, a Selective Cortical 5-HT1A Receptor Biased Agonist

Depression is the most prevalent of the mental illnesses and serotonin (5-hydroxytryptamine, 5-HT) is considered to be the major neurotransmitter involved in its etiology and treatment. In this context, 5-HT_1A receptors have attracted interest as targets for therapeutic intervention. Notably the activation of presynaptic 5-HT_1A autoreceptors delays antidepressant effects whereas the stimulation of postsynaptic 5-HT_1A heteroreceptors is needed for an antidepressant action. NLX-101 (also known as F15599) is a selective biased agonist which exhibits preferred activation of cortical over brain stem 5-HT_1A receptors. Here, we used behavioral, neurochemical and molecular methods to examine the antidepressant-like effects in rats of a single dose of NLX-101 (0.16 mg/kg, i.p.). NLX-101 reduced immobility in the forced swim test when measured 30 min but not 24 h after drug administration. NLX-101 increased extracellular concentrations of glutamate and dopamine in the medial prefrontal cortex, but no changes were detected in the efflux of noradrenaline or 5-HT. NLX-101 also produced an increase in the activation of pmTOR, pERK1/2 and pAkt, and the expression of PSD95 and GluA1, which may contribute to its rapid antidepressant action.

Brain 5-HT1A Receptor PET Binding, Cortisol Responses to Stress, and the Familial Transmission of Suicidal Behavior

BACKGROUND

The serotonin 1A (5-HT1A) receptor has been implicated in depression and suicidal behavior. Lower resting cortisol levels are associated with higher 5-HT1A receptor binding, and both differentiate suicide attempters with depression. However, it is not clear whether 5-HT1A receptor binding and cortisol responses to stress are related to familial risk and resilience for suicidal behavior.

METHODS

[11C]CUMI-101 positron emission tomography imaging to quantify regional brain 5-HT1A receptor binding was conducted in individuals considered to be at high risk for mood disorder or suicidal behavior on the basis of having a first- or second-degree relative(s) with an early onset mood disorder and history of suicidal behavior. These high-risk individuals were subdivided into the following groups: high risk resilient having no mood disorder or suicidal behavior (n = 29); high risk with mood disorder and no suicidal behavior history (n = 31); and high risk with mood disorder and suicidal behavior (n = 25). Groups were compared with healthy volunteers without a family history of mood disorder or suicidal behavior (n = 34). Participants underwent the Trier Social Stress Task (TSST). All participants were free from psychotropic medications at the time of the TSST and PET scanning.

RESULTS

We observed no group differences in 5-HT1A receptor binding considering all regions simultaneously, nor did we observe heterogeneity of the effect of group across regions. These results were similar across outcome measures (BPND for all participants and BPp in a subset of the sample) and definitions of regions of interest (ROIs; standard or serotonin system-specific ROIs). We also found no group differences on TSST outcomes. Within the high risk with mood disorder and suicidal behavior group, lower BPp binding (β = -0.084, SE = 0.038, P = .048) and higher cortisol reactivity to stress (β = 9.25, 95% CI [3.27,15.23], P = .004) were associated with higher lethality attempts. There were no significant relationships between 5-HT1A binding and cortisol outcomes.

CONCLUSIONS

5-HT1A receptor binding in ROIs was not linked to familial risk or resilience protecting against suicidal behavior or mood disorder although it may be related to lethality of suicide attempt. Future studies are needed to better understand the biological mechanisms implicated in familial risk for suicidal behavior and how hypothalamic-pituitary-adrenal axis function influences such risk.

5-HT Receptors and the Development of New Antidepressants

Serotonin modulates several physiological and cognitive pathways throughout the human body that affect emotions, memory, sleep, and thermal regulation. The complex nature of the serotonergic system and interactions with other neurochemical systems indicate that the development of depression may be mediated by various pathomechanisms, the common denominator of which is undoubtedly the disturbed transmission in central 5-HT synapses. Therefore, the deliberate pharmacological modulation of serotonergic transmission in the brain seems to be one of the most appropriate strategies for the search for new antidepressants. As discussed in this review, the serotonergic system offers great potential for the development of new antidepressant therapies based on the combination of SERT inhibition with different pharmacological activity towards the 5-HT system. The aim of this article is to summarize the search for new antidepressants in recent years, focusing primarily on the possibility of benefiting from interactions with various 5-HT receptors in the pharmacotherapy of depression.

Chronic antidepressant treatment rescues abnormally reduced REM sleep theta power in socially defeated rats

The effects of chronic antidepressant (AD) treatment on sleep disturbances in rodent chronic stress models have not been thoroughly investigated. Here, we show that chronic social defeat stress (SDS) in rats induces prolonged social avoidance, alterations in sleep architecture (increased total rapid eye movement [REM] sleep duration, bout, and shortened REM latency), and contextual but not cued fear memory deficits, even 1 month after the last SDS. These abnormalities were associated with changes in electroencephalography (EEG) spectral powers, including reduced REM sleep theta power during the light phase. Chronic treatment with two different classes of antidepressants (ADs), imipramine and fluoxetine, significantly ameliorated these behavioral, sleep, and EEG abnormalities. Interestingly, REM theta power was normalized by chronic (1 month) but not 1 week AD administration and solely correlated with the ratio (an objective indicator) of social interaction 1 month after the last SDS. These data suggest that reductions in REM sleep theta power, an EEG parameter that has never been directly investigated in humans, is a core sleep symptom in socially defeated rats, and, potentially, also in patients with stress-related psychiatric disorders, including major depressive and posttraumatic stress disorders.

Antidepressant action of transcranial direct current stimulation in olfactory bulbectomised adolescent rats

BACKGROUND

Antidepressant drugs in adolescent depression are sometimes mired by efficacy issues and paradoxical effects. Transcranial direct current stimulation (tDCS) could represent an alternative.

AIMS/METHODS

We tested the antidepressant action of prefrontal tDCS and paroxetine (20 mg/kg, intraperitoneal) in olfactory bulbectomised (OBX) adolescent rats. Using enzyme-linked immunosorbent assays and in situ hybridisation, we examined treatment-induced changes in plasma brain-derived neurotrophic factor (BDNF) and brain serotonin transporter (SERT) and 5-HT-1A mRNA.

RESULTS

OBX-induced anhedonia-like reductions in sucrose preference (SP) correlated with open field (OF) hyperactivity. These were accompanied by decreased zif268 mRNA in the piriform/amygdalopiriform transition area, and increased zif268 mRNA in the hypothalamus. Acute paroxetine (2 days) led to a profound SP reduction, an effect blocked by combined tDCS-paroxetine administration. Chronic (14 days) tDCS attenuated hyperlocomotion and its combination with paroxetine blocked OBX-induced SP reduction. Correlations among BDNF, SP and hyperlocomotion scores were altered by OBX but were normalised by tDCS-paroxetine co-treatment. In the brain, paroxetine increased zif268 mRNA in the hippocampal CA1 subregion and decreased it in the claustrum. This effect was blocked by tDCS co-administration, which also increased zif268 in CA2. tDCS-paroxetine co-treatment had variable effects on 5-HT1A receptors and SERT mRNA. 5-HT1A receptor changes were found exclusively within depression-related parahippocampal/hippocampal subregions, and SERT changes within fear/defensive response-modulating brainstem circuits.

CONCLUSION

These findings point towards potential synergistic efficacies of tDCS and paroxetine in the OBX model of adolescent depression via mechanisms associated with altered expression of BDNF, 5-HT1A, SERT and zif268 in discrete corticolimbic areas.

Serotonin modulation of hippocampal functions: From anatomy to neurotherapeutics

The hippocampal region receives a dense serotoninergic innervation originating from both medial and dorsal raphe nuclei. This innervation regulates hippocampal activity through the activation of distinct receptor families that are expressed in excitatory and inhibitory neurons, terminals of several afferent neurotransmitter systems, and glial cells. Preclinical and clinical studies indicate that hippocampal dysfunctions are involved in learning and memory deficits, dementia, Alzheimer's disease, epilepsy and mood disorders such as anxiety, depression and post-traumatic syndrome disorder, whereas the hippocampus participates also in the therapeutic mechanisms of numerous medicines. Not surprisingly, several drugs acting via 5-HT mechanisms are efficacious to some extent in some diseases and the link between 5-HT and the hippocampus although clear remains difficult to untangle. For this reason, we review reported data concerning the distribution and the functional roles of the 5-HT receptors in the hippocampal region in health and disease. The impact of the 5-HT systems on the hippocampal function is such that the research of new 5-HT mechanisms and drugs is still very active. It concerns notably drugs acting at the 5-HT_1A,_2A,_2C,_4,6 receptor subtypes, in addition to the already existing drugs including the selective serotonin reuptake inhibitors.

Role of central serotonin and noradrenaline interactions in the antidepressants' action: Electrophysiological and neurochemical evidence

The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.

Triple reuptake inhibition of serotonin, norepinephrine, and dopamine increases the tonic activation of α2-adrenoceptors in the rat hippocampus and dopamine levels in the nucleus accumbens

Clinical studies have shown the therapeutic efficacy of an increase in dopamine (DA) transmission in treatment of major depressive disorder (MDD). In the present study, we investigated whether blockade of DA transporters in addition to serotonin (5-HT) and norepinephrine (NE) produced additional adaptations of monoaminergic systems. In vivo electrophysiological recordings were carried out in male anesthetized rats. Vehicle, the 5-HT reuptake inhibitor escitalopram, the NE/DA reuptake blocker nomifensine and their combination (triple reuptake inhibition; TRI) were delivered for 2 or 14 days. Firing activity of NE, 5-HT and DA neurons was assessed. Tonic activation of 5-HT_1A receptors and α₁- and α₂-adrenoceptors was determined in the hippocampus and extracellular DA levels in the nucleus accumbens (NAc). Unlike escitalopram, nomifensine and TRI administration increased the tonic activation of α₂-adrenoceptors in the hippocampus despite decreasing NE neuronal firing activity after 2 and 14 days of administration. The firing activity of 5-HT neurons was increased after prolonged nomifensine and TRI regimens, while addition of nomifensine to escitalopram prevented the early 2-day suppression of firing by 5-HT reuptake inhibition. The tonic activation of 5-HT_1A receptors was enhanced only with escitalopram. Whereas escitalopram and nomifensine decreased firing activity of DA neurons after a 2-day administration, their combination normalized it to baseline level after 14 days; this was accompanied by a robust increase in extracellular DA levels in the NAc. In summary, these results indicate that TRI increases NE and DA but not 5-HT transmission, suggesting a differential efficacy profile in MDD patients.

Psychedelics in Psychiatry: Neuroplastic, Immunomodulatory, and Neurotransmitter Mechanisms

Mounting evidence suggests safety and efficacy of psychedelic compounds as potential novel therapeutics in psychiatry. Ketamine has been approved by the Food and Drug Administration in a new class of antidepressants, and 3,4-methylenedioxymethamphetamine (MDMA) is undergoing phase III clinical trials for post-traumatic stress disorder. Psilocybin and lysergic acid diethylamide (LSD) are being investigated in several phase II and phase I clinical trials. Hence, the concept of psychedelics as therapeutics may be incorporated into modern society. Here, we discuss the main known neurobiological therapeutic mechanisms of psychedelics, which are thought to be mediated by the effects of these compounds on the serotonergic (via 5-HT_2A and 5-HT_1A receptors) and glutamatergic [via N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors] systems. We focus on 1) neuroplasticity mediated by the modulation of mammalian target of rapamycin-, brain-derived neurotrophic factor-, and early growth response-related pathways; 2) immunomodulation via effects on the hypothalamic-pituitary-adrenal axis, nuclear factor ĸB, and cytokines such as tumor necrosis factor-α and interleukin 1, 6, and 10 production and release; and 3) modulation of serotonergic, dopaminergic, glutamatergic, GABAergic, and norepinephrinergic receptors, transporters, and turnover systems. We discuss arising concerns and ways to assess potential neurobiological changes, dependence, and immunosuppression. Although larger cohorts are required to corroborate preliminary findings, the results obtained so far are promising and represent a critical opportunity for improvement of pharmacotherapies in psychiatry, an area that has seen limited therapeutic advancement in the last 20 years. Studies are underway that are trying to decouple the psychedelic effects from the therapeutic effects of these compounds. SIGNIFICANCE STATEMENT: Psychedelic compounds are emerging as potential novel therapeutics in psychiatry. However, understanding of molecular mechanisms mediating improvement remains limited. This paper reviews the available evidence concerning the effects of psychedelic compounds on pathways that modulate neuroplasticity, immunity, and neurotransmitter systems. This work aims to be a reference for psychiatrists who may soon be faced with the possibility of prescribing psychedelic compounds as medications, helping them assess which compound(s) and regimen could be most useful for decreasing specific psychiatric symptoms.

Long-term administration of cariprazine increases locus coeruleus noradrenergic neurons activity and serotonin1A receptor neurotransmission in the hippocampus

BACKGROUND

Cariprazine, the novel dopamine (DA) D₃-preferring D₃/D₂ and serotonin (5-HT)_1A receptor partial agonist, has activity as an adjunctive therapy in major depressive disorder (MDD).

AIMS

This study aims to investigate the effects of chronic cariprazine administration in combination with the selective serotonin reuptake inhibitor escitalopram on the activity of monoaminergic systems.

METHODS

Rats received cariprazine alone and in adjunct to escitalopram for 2 and 14 days and the firing activity of dorsal raphe nucleus 5-HT, locus coeruleus norepinephrine (NE) and ventral tegmental area DA neurons was assessed. 5-HT and NE neurotransmission in hippocampus pyramidal neurons was evaluated by assessing tonic activation of their 5-HT_1A, and α₁- and α₂-adrenergic receptors, using their selective antagonists.

RESULTS

Two and 14-day cariprazine regimens increased the firing rate of NE, but not 5-HT and DA neurons. Addition of cariprazine to escitalopram reversed the inhibitory effect of escitalopram on NE but not 5-HT and DA neurons. In the hippocampus, there was an increase in neurotransmission at 5-HT_1A receptors in cariprazine-treated rats, but no change in overall NE transmission by either regimen.

CONCLUSION

Cariprazine increased NE neuronal firing and reversed the escitalopram-induced inhibition of these neurons. Despite a lack of effect on 5-HT neuronal firing activity, there was an increase in tonic activation of hippocampus 5-HT_1A receptors by cariprazine alone but not with the combination. These effects provide a possible rationale for the clinical efficacy of cariprazine as an adjunctive strategy in patients with MDD.

Dentate nNOS accounts for stress-induced 5-HT1A receptor deficiency: Implication in anxiety behaviors

Background

Anxiety is a common disorder with high social burden worldwide. Dysfunction of serotonin‐_1A receptor (5‐HT_1A receptor) in the dentate gyrus (DG) of the hippocampus has been predominantly implicated in the anxiety behavior. However, the molecular mechanism underlying the deficiency of postsynaptic 5‐HT_1A receptor in regulating anxiety behavior remains unclear.

Methods

Using pharmacological and genetic methods, we investigated the role of detate nNOS in 5‐HT_1A receptor decline and anxiety behavior induced by chronic mild stress (CMS) in mice.

Results

Here we showed that local elevation of glucocorticoids in the DG accounted for chronic stress‐induced anxiety behavior. Neuronal nitric oxide synthase (nNOS) mediated chronic stress‐induced downregulation of 5‐HT_1A receptor in the DG through peroxynitrite anion (ONOO•) pathway but not cyclic guanosine monophosphate (cGMP) pathway. By using pharmacological tool drugs and nNOS knockout mice, we found that nNOS in the DG played a key role in chronic stress‐induced anxiety behavior.

Conclusions

These findings uncovered an important role of nNOS‐5‐HT_1A receptor pathway in the DG of the hippocampus in chronic stress‐induced anxiety. Accordingly, we developed a “dentate nNOS‐5‐HT_1A receptor closed‐loop” theory (stress‐glucocorticoids‐nNOS‐Nitric oxide‐ONOO•‐5‐HT_1A receptor ‐nNOS) of stress‐related anxiety.

Computational Analysis of Therapeutic Neuroadaptation to Chronic Antidepressant in a Model of the Monoaminergic Neurotransmitter and Stress Hormone Systems

The clinical practice of selective serotonin reuptake inhibitor (SSRI) augmentation relies heavily on trial-and-error. Unfortunately, the drug combinations prescribed today fail to provide relief for many depressed patients. In order to identify potentially more effective treatments, we developed a computational model of the monoaminergic neurotransmitter and stress-steroid systems that neuroadapts to chronic administration of combinations of antidepressant drugs and hormones by adjusting the strengths of its transmitter-system components (TSCs). We used the model to screen 60 chronically administered drug/hormone pairs and triples, and identified as potentially therapeutic those combinations that raised the monoamines (serotonin, norepinephrine, and dopamine) but lowered cortisol following neuroadaptation in the model. We also evaluated the contributions of individual and pairs of TSCs to therapeutic neuroadaptation with chronic SSRI using sensitivity, correlation, and linear temporal-logic analyses. All three approaches revealed that therapeutic neuroadaptation to chronic SSRI is an overdetermined process that depends on multiple TSCs, providing a potential explanation for the clinical finding that no single antidepressant regimen alleviates depressive symptoms in all patients.

Preclinical neuroimaging of gene-environment interactions in psychiatric disease

Psychiatric disease is one of the leading causes of disability worldwide. Despite the global burden and need for accurate diagnosis and treatment of mental illness, psychiatric diagnosis remains largely based on patient-reported symptoms, allowing for immense symptomatic heterogeneity within a single disease. In renewed efforts towards improved diagnostic specificity and subsequent evaluation of treatment response, a greater understanding of the underlying of the neuropathology and neurobiology of neuropsychiatric disease is needed. However, dissecting these mechanisms of neuropsychiatric illness in clinical populations are problematic with numerous experimental hurdles limiting hypothesis-driven studies including genetic confounds, variable life experiences, different environmental exposures, therapeutic histories, as well as the inability to investigate deeper molecular changes in vivo . Preclinical models, where many of these confounding factors can be controlled, can serve as a crucial experimental bridge for studying the neurobiological origins of mental illness. Furthermore, although behavioral studies and molecular studies are relatively common in these model systems, focused neuroimaging studies are very rare and represent an opportunity to link the molecular changes in psychiatric illness with advanced quantitative neuroimaging studies. In this review, we present an overview of well-validated genetic and environmental models of psychiatric illness, discuss gene-environment interactions, and examine the potential role of neuroimaging towards understanding genetic, environmental, and gene-environmental contributions to psychiatric illness.

Cognitive Control as a 5-HT1A-Based Domain That Is Disrupted in Major Depressive Disorder

Heterogeneity within Major Depressive Disorder (MDD) has hampered identification of biological markers (e.g., intermediate phenotypes, IPs) that might increase risk for the disorder or reflect closer links to the genes underlying the disease process. The newer characterizations of dimensions of MDD within Research Domain Criteria (RDoC) domains may align well with the goal of defining IPs. We compare a sample of 25 individuals with MDD compared to 29 age and education matched controls in multimodal assessment. The multimodal RDoC assessment included the primary IP biomarker, positron emission tomography (PET) with a selective radiotracer for 5-HT1A [(11C)WAY-100635], as well as event-related functional MRI with a Go/No-go task targeting the Cognitive Control network, neuropsychological assessment of affective perception, negative memory bias and Cognitive Control domains. There was also an exploratory genetic analysis with the serotonin transporter (5-HTTLPR) and monamine oxidase A (MAO-A) genes. In regression analyses, lower 5-HT1A binding potential (BP) in the MDD group was related to diminished engagement of the Cognitive Control network, slowed resolution of interfering cognitive stimuli, one element of Cognitive Control. In contrast, higher/normative levels of 5-HT1A BP in MDD (only) was related to a substantial memory bias toward negative information, but intact resolution of interfering cognitive stimuli and greater engagement of Cognitive Control circuitry. The serotonin transporter risk allele was associated with lower 1a BP and the corresponding imaging and cognitive IPs in MDD. Lowered 5HT 1a BP was present in half of the MDD group relative to the control group. Lowered 5HT 1a BP may represent a subtype including decreased engagement of Cognitive Control network and impaired resolution of interfering cognitive stimuli. Future investigations might link lowered 1a BP to neurobiological pathways and markers, as well as probing subtype-specific treatment targets.

Comparison of Therapeutic Effects of TREK1 Blockers and Fluoxetine on Chronic Unpredicted Mild Stress Sensitive Rats

The animal model for depressive behavior due to chronic unpredicted mild stress (CUMS) is commonly used to evaluate antidepressant treatments. The CUMS model has faced some criticism because of the heterogeneity of behavioral effects. Spadin and SID1900 are TREK1 blockers with a quick antidepressant effect. However, to date, their effectiveness and the long-term therapeutic mechanisms are not known. We hypothesize that early intervention with TREK1 blockers can fully reverse depressive-like behaviors, that the chronic administration of TREK1 blockers has a more pronounced effect than the SSRI fluoxetine, and that its long-term therapeutic effects may be mediated by improvement of impaired neurogenesis. Furthermore, we optimized the use of the CUMS model for increased homogeneity by screening the rats after the CUMS induction procedure. Depressive-like behavior was assessed by a forced swimming test, sucrose preference, and open field tests. To evaluate neurogenesis, cell proliferation and newly generated cell apoptosis were measured in the hippocampal dentate gyrus. Of 32 rats that underwent the CUMS procedure, 26 rats that exhibited depressive-like behaviors were grouped as CUMS sensitive rats (CUMSS), while six that did not were grouped as CUMS resistant ones (CUMSR). The CUMSR rats exhibited minor neurogenesis impairments, while the CUMSS rats had a more pronounced effect. Treatment with TREK1 blockers could reverse depressive-like behaviors at least 1 week earlier than that of fluoxetine. Chronic administration of both the TREK1 blockers and fluoxetine could restore neurogenesis impairments. This study underlines the importance of model validation by determination of CUMS sensitivity. The TREK1 blockers were found to have an effect that was more rapid and more pronounced than that of fluoxetine. Therapeutic benefits after chronic administration were associated with a restoration of impaired neurogenesis.

Serotonin Signaling Trough Prelimbic 5-HT1A Receptors Modulates CSDS-Induced Behavioral Changes in Adult Female Voles

BACKGROUND

Most previous studies have focused on the effects of social defeat in male juvenile individuals. Whether chronic social defeat stress in adulthood affects female emotion and the underlying mechanisms remains unclear.

METHODS

Using highly aggressive adult female mandarin voles (Microtus mandarinus), the present study aimed to determine the effects of chronic social defeat stress on anxiety- and depression-like behaviors in adult female rodents and investigate the neurobiological mechanisms underlying these effects.

RESULTS

Exposure of adult female voles to social defeat stress for 14 days reduced the time spent in the central area of the open field test and in the open arms of the elevated plus maze and lengthened the immobility time in the tail suspension and forced swimming tests, indicating increased anxiety- and depression-like behaviors. Meanwhile, defeated voles exhibited increased neural activity in the prelimbic cortex of the medial prefrontal cortex. Furthermore, chronic social defeat stress reduced serotonin projections and levels of serotonin 1A receptors in the medial prefrontal cortex-prelimbic cortex. Intra-prelimbic cortex microinjections of the serotonin 1A receptor agonist 8-OH-DPAT reversed the alterations in emotional behaviors, whereas injections of the serotonin 1A receptor antagonist WAY-100635 into the prelimbic cortex of control voles increased the levels of anxiety- and depression-like behaviors.

CONCLUSIONS

Taken together, our results demonstrated that chronic social defeat stress increased anxiety- and depression-like behaviors in adult female voles, and these effects were mediated by the action of serotonin on the serotonin 1A receptors in the prelimbic cortex. The serotonin system may be a promising target to treat emotional disorders induced by chronic social defeat stress.

d-Lysergic acid diethylamide, psilocybin, and other classic hallucinogens: Mechanism of action and potential therapeutic applications in mood disorders

Depression and anxiety are psychiatric diagnoses commonly associated with low quality of life and low percentage of responsiveness by patients treated with currently available drugs. Thus, research into alternative compounds to treat these disorders is essential to guarantee a patient's remission. The last decade has witnessed a revamped interest for the application of psychedelic medicine for the treatment of mental disorders due to anecdotal reports and clinical studies which show that low doses of d-lysergic acid diethylamide (LSD) and psilocybin may have antidepressant effects. LSD and psilocybin have demonstrated mood-modulating properties likely due to their capacity to modulate serotonergic (5-HT), dopaminergic (DA) and glutamatergic systems. LSD, belonging to the category of "classic halluginogens," interacts with the 5-HT system through 5HT_1A, and 5HT_2A receptors, with the DA system through D₂ receptors, and indirectly also the glutamatergic neurotransmission thought the recruitment of N-methyl-d-aspartate (NMDA) receptors. Randomized clinical studies have confirmed its antidepressant and anxiolytic effects in humans. Thus, in this chapter, we will review the pharmacology of psychedelic drugs, report the most striking clinical evidence which substantiate the therapeutic potentials of these fascinating compounds in mood disorders, and look into the horizon of where psychedelic medicine is heading.

Altered Trek-1 Function in Sortilin Deficient Mice Results in Decreased Depressive-Like Behavior

The background potassium channel TREK-1 has been shown to be a potent target for depression treatment. Indeed, deletion of this channel in mice resulted in a depression resistant phenotype. The association of TREK-1 with the sorting protein sortilin prompted us to investigate the behavior of mice deleted from the gene encoding sortilin (Sort1^−/−). To characterize the consequences of sortilin deletion on TREK-1 activity, we combined behavioral, electrophysiological and biochemical approaches performed in vivo and in vitro. Analyses of Sort1^−/− mice revealed that they display: (1) a corticosterone-independent anxiety-like behavior, (2) a resistance to depression as demonstrated by several behavioral tests, and (3) an increased activity of dorsal raphe nucleus neurons. All these properties were associated with TREK-1 action deficiency consequently to a decrease of its cell surface expression and to the modification of its electrophysiological activity. An increase of BDNF expression through activation of the furin-dependent constitutive pathway as well as an increase of the activated BDNF receptor TrkB were in agreement with the decrease of depressive-like behavior of Sort1^−/− mice. Our results demonstrate that the TREK-1 expression and function are altered in the absence of sortilin confirming the importance of this channel in the regulation on the mood as a crucial target to treat depression.

Can we increase speed and efficacy of antidepressant treatments? Part I: General aspects and monoamine-based strategies

Major depressive disorder (MDD) is a severe psychiatric syndrome with high prevalence and socioeconomic impact. Current antidepressant treatments are based on the blockade of serotonin (5-hydroxytryptamine, 5-HT) and/or noradrenaline transporters. These drugs show slow onset of clinical action and limited efficacy, partly due to the activation of physiological negative feed-back mechanisms operating through autoreceptors (5-HT_1A, 5-HT_1B, α₂-adrenoceptors) and postsynaptic receptors (e.g., 5-HT₃). As a result, clinically-relevant doses of reuptake inhibitors increase extracellular (active) 5-HT concentrations in the midbrain raphe nuclei but not in forebrain, as indicated by rodent microdialysis studies and by PET-scan studies in primate/human brain. The prevention of these self-inhibitory mechanisms by antagonists of the above receptors augments preclinical and clinical antidepressant effects. Hence, the mixed ß-adrenoceptor/5-HT_1A antagonist pindolol accelerated, and in some cases enhanced, the clinical action of selective serotonin reuptake inhibitors (SSRI). This strategy has been incorporated into two new multi-target antidepressant drugs, vilazodone and vortioxetine, which combine 5-HT reuptake inhibition and partial agonism at 5-HT_1A receptors. Vortioxetine shows also high affinity for other 5-HT receptors, including excitatory 5-HT₃ receptors located in cortical and hippocampal GABA interneurons. 5-HT₃ receptor blockade by vortioxetine enhances pyramidal neuron activity in prefrontal cortex as well as cortical and hippocampal 5-HT release. It is still too soon to know whether these new antidepressants will represent a real advance over existing drugs in the real world. However, their development opened the way to future antidepressant drugs based on the prevention of local and distal self-inhibitory mechanisms attenuating monoamine activity.

Deep brain stimulation and fluoxetine exert different long-term changes in the serotonergic system

Both selective serotonin reuptake inhibitors (SSRIs) and ventromedial prefrontal cortex (vmPFC) deep brain stimulation (DBS) modulate serotonergic activity. We compared the acute (1 day) and long-term (12 days) effects of vmPFC stimulation and fluoxetine on serotonin (5-HT) release and receptor expression in rats. Samples to measure serotonin levels were collected from the hippocampus using microdialysis. Serotonin transporter (SERT), 5-HT_1A and 5-HT_1B mRNA were measured using in situ hybridization. [³H]8-OH-DPAT and [¹²⁵I]cyanopindolol autoradiography were used to measure 5-HT_1A and 5-HT_1B binding. Our results show that after fluoxetine injections serotonin levels were approximately 150% higher than at baseline. Twelve days later, pre-injection 5-HT extracellular concentration was substantially higher than on day 1. In contrast, serotonin levels following DBS were only 50% higher than at baseline. While pre-stimulation 5-HT on day 12 was significantly higher than on treatment day 1, no stimulation-induced 5-HT peak was recorded. SERT expression in the dorsal raphe was increased after acute fluoxetine and decreased following a single day of DBS. Neither fluoxetine nor DBS administered acutely substantially changed 5-HT_1A or 5-HT_1B binding. Chronic fluoxetine treatment, however, was associated with a decrease in [³H]8-OH-DPAT prefrontal cortex and hippocampus expression. In contrast, chronic DBS induced a significant increase in [¹²⁵I]cyanopindolol binding in the prefrontal cortex, globus pallidus, substantia nigra and raphe nuclei. mRNA expression of 5-HT_1A and 5-HT_1B in raphe nuclei was not altered by either treatment. These results suggest that fluoxetine and DBS modulate activity of the serotonergic system but likely exert their effects through different mechanisms.

Computational Model of Antidepressant Response Heterogeneity as Multi-pathway Neuroadaptation

Current hypotheses cannot fully explain the clinically observed heterogeneity in antidepressant response. The therapeutic latency of antidepressants suggests that therapeutic outcomes are achieved not by the acute effects of the drugs, but rather by the homeostatic changes that occur as the brain adapts to their chronic administration. We present a computational model that represents the known interactions between the monoaminergic neurotransmitter-producing brain regions and associated non-monoaminergic neurotransmitter systems, and use the model to explore the possible ways in which the brain can homeostatically adjust to chronic antidepressant administration. The model also represents the neuron-specific neurotransmitter receptors that are known to adjust their strengths (expressions or sensitivities) in response to chronic antidepressant administration, and neuroadaptation in the model occurs through sequential adjustments in these receptor strengths. The main result is that the model can reach similar levels of adaptation to chronic administration of the same antidepressant drug or combination along many different pathways, arriving correspondingly at many different receptor strength configurations, but not all of those adapted configurations are also associated with therapeutic elevations in monoamine levels. When expressed as the percentage of adapted configurations that are also associated with elevations in one or more of the monoamines, our modeling results largely agree with the percentage efficacy rates of antidepressants and antidepressant combinations observed in clinical trials. Our neuroadaptation model provides an explanation for the clinical reports of heterogeneous outcomes among patients chronically administered the same antidepressant drug regimen.

Role of 5-HT1A Receptor Stimulation in the Medial Prefrontal Cortex in the Sustained Antidepressant Effects of Ketamine

BACKGROUND

We previously reported that serotonergic transmission plays an important role in antidepressant effects of ketamine. However, detailed mechanisms have not been elucidated. Among the serotonin receptor subtypes, the serotonin1A receptor in the medial prefrontal cortex has an important role in depression. Here, we investigated the role of the medial prefrontal cortex serotonin1A receptor and its signaling mechanism in the antidepressant effects of ketamine.

METHODS

The role of serotonin1A receptor-mediated signaling mechanism (phosphoinositide-3 kinase/Akt) in the medial prefrontal cortex was examined in the mouse forced swimming test and western blotting.

RESULTS

Ketamine exerted antidepressant effects that lasted for 24 hours, and the sustained antidepressant effects were attenuated by intra-medial prefrontal cortex injection of a serotonin1A receptor antagonist, WAY100635. The sustained antidepressant effects were mimicked by intra- medial prefrontal cortex, but not systemic, administration of a serotonin1A receptor agonist, (±)-8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT). The sustained antidepressant effects of ketamine and 8-OH-DPAT were abrogated by intra- medial prefrontal cortex injection of a phosphoinositide-3 kinase inhibitor. Ketamine increased the phosphorylation of Akt in the medial prefrontal cortex at 60 minutes after administration, which was blocked by a serotonin1A receptor antagonist and a phosphoinositide-3 kinase inhibitor. Furthermore, the sustained antidepressant effects of ketamine and 8-OH-DPAT were attenuated by pretreatment of intra-medial prefrontal cortex injection of a mechanistic target of rapamycin complex-1 inhibitor.

CONCLUSIONS

These results indicate that selective stimulation of the medial prefrontal cortex serotonin1A receptor and subsequent activation of the phosphoinositide-3 kinase/Akt/mechanistic target of rapamycin complex-1 pathway may be necessary for ketamine to exert the sustained antidepressant effects, and that this mechanism could be targeted to develop a novel and effective approach for treating depression.

Exploring a post-traumatic stress disorder paradigm in Flinders sensitive line rats to model treatment-resistant depression II: response to antidepressant augmentation strategies

OBJECTIVE

Post-traumatic stress disorder (PTSD) displays high co-morbidity with major depression and treatment-resistant depression (TRD). Earlier work demonstrated exaggerated depressive-like symptoms in a gene×environment model of TRD and an abrogated response to imipramine. We extended the investigation by studying the behavioural and monoaminergic response to multiple antidepressants, viz. venlafaxine and ketamine with/without imipramine.

METHODS

Male Flinders sensitive line (FSL) rats, a genetic model of depression, were exposed to a time-dependent sensitisation (TDS) model of PTSD and compared with stress naive controls. 7 days after the TDS procedures, immobility and coping (swimming and climbing), behaviours in the forced swim test (FST) as well as hippocampal and cortical 5-hydroxyindoleacetic acid (5HIAA) and noradrenaline (NA) levels were analysed. Response to imipramine, venlafaxine and ketamine treatment (all 10 mg/kg×7 days) alone and in combination were subsequently studied.

RESULTS

TDS exacerbated depressive-like behaviour of FSL rats in the FST. Imipramine, venlafaxine and ketamine were ineffective as monotherapy in TDS-exposed FSL rats. However, combining imipramine with either venlafaxine or ketamine resulted in significant anti-immobility effects and enhanced coping behaviours. Only ketamine+imipramine (frontal-cortical 5HIAA and NA), ketamine alone (frontal-cortical and hippocampal NA) and venlafaxine+imipramine (frontal-cortical NA) altered monoamine responses versus untreated TDS-exposed FSL rats.

CONCLUSION

Exposure of FSL rats to TDS inhibits antidepressant response at behavioural and neurochemical levels. Congruent with TRD, imipramine plus venlafaxine or ketamine overcame treatment resistance in these animals. These data further support the hypothesis that exposure of FSL rats to a PTSD-like paradigm produces a valid animal model of TRD and warrants further investigation.

Roles of 5-HT1A receptor in the expression of AMPA receptor and BDNF in developing mouse cortical neurons

The possible interactions between serotonergic and glutamatergic systems during neural development and under the pathogenesis of depression remain unclear. We now investigated roles of 5-HT_1A receptor in the mRNA expression of AMPA receptor subunits (GluR1 and GluR2) and brain-derived neurotrophic factor (BDNF) using primary culture of cerebral cortex of mouse embryos. Neurons at embryonic day 18 were cultured for 3days or 14days and then treated with 5-HT_1A receptor agonist (8-OH-DPAT) for 3h or 24h. In neurons cultured for 3 days, 8-OH-DPAT treatment for both 3h and 24h increased the mRNA levels of BDNF and GluR1, but not GluR2. In neurons cultured for 14 days, however, 8-OH-DPAT had no effects on these mRNA levels. Next, we examined in vivo roles of 5-HT_1A receptor by administration of 8-OH-DPAT to newborn mice. Twenty-four hours after the oral administration of 8-OH-DPAT, the mRNA expression of BDNF was decreased in the frontal cortex, but had no effects on the mRNA expression of GluR1 and GluR2. Taken together, the present study suggests that 5-HT_1A receptor activation modulates mRNA expression of AMPA receptor subunit and BDNF in cortical neurons, and the effects are different between in vitro and in vivo.

Serotonin-1A receptor alterations in depression: a meta-analysis of molecular imaging studies

BACKGROUND

Postmortem studies of people who have successfully committed suicide and people with depression have implicated the serotonin-1A (5-HT1A) receptor system in the pathophysiology of depression. Several molecular imaging studies have investigated alterations in 5-HT1A receptors in patients with depression using positron emission tomography and have reported conflicting results.

METHODS

We performed a meta-analysis of studies investigating the relationship between depression and 5-HT1A binding. We conducted a comprehensive search of Medline, Embase, ScienceDirect, Scopus and Springer databases for relevant studies published between January 1999 and October 2015. The meta-analysis was conducted in accordance with the Meta-analysis of Observational Studies in Epidemiology guidelines.

RESULTS

Ten studies were included, comprising 218 patients with depression and 261 healthy controls. The results of these studies indicated a reduction in 5-HT1A receptors in mesiotemporal cortex, yielding a summary effect estimate of -0.8 (95 % CI -1.36, -0.24). Smaller reductions were reported in 5-HT1A receptor binding in the hippocampus, raphe nuclei, insular, anterior cingulate cortex and occipital cortex of people with depression. No clear effect of depression on 5-HT1A receptors was detected in the amygdala.

CONCLUSIONS

Reduced 5-HT1A receptor binding was associated with the pathology of depression and predicted altered serotonergic neurotransmission in various brain regions. These findings increase our understanding of the neurophysiological processes underlying depression.

Selective serotonin 5-HT1A receptor biased agonists elicitdistinct brain activation patterns: a pharmacoMRI study

Serotonin 1A (5-HT1A) receptors are involved in several physiological and pathological processes and constitute therefore an important therapeutic target. The recent pharmacological concept of biased agonism asserts that highly selective agonists can preferentially direct receptor signaling to specific intracellular responses, opening the possibility of drugs targeting a receptor subtype in specific brain regions. The present study brings additional support to this concept thanks to functional magnetic resonance imaging (7 Tesla-fMRI) in anaesthetized rats. Three 5-HT1A receptor agonists (8-OH-DPAT, F13714 and F15599) and one 5-HT1A receptor antagonist (MPPF) were compared in terms of influence on the brain blood oxygen level-dependent (BOLD) signal. Our study revealed for the first time contrasting BOLD signal patterns of biased agonists in comparison to a classical agonist and a silent antagonist. By providing functional information on the influence of pharmacological activation of 5-HT1A receptors in specific brain regions, this neuroimaging approach, translatable to the clinic, promises to be useful in exploring the new concept of biased agonism in neuropsychopharmacology.

Animal models of depression: molecular perspectives

Much of the current understanding about the pathogenesis of altered mood, impaired concentration and neurovegetative symptoms in major depression has come from animal models. However, because of the unique and complex features of human depression, the generation of valid and insightful depression models has been less straightforward than modeling other disabling diseases like cancer or autoimmune conditions. Today's popular depression models creatively merge ethologically valid behavioral assays with the latest technological advances in molecular biology and automated video-tracking. This chapter reviews depression assays involving acute stress (e.g., forced swim test), models consisting of prolonged physical or social stress (e.g., social defeat), models of secondary depression, genetic models, and experiments designed to elucidate the mechanisms of antidepressant action. These paradigms are critically evaluated in relation to their ease, validity and replicability, the molecular insights that they have provided, and their capacity to offer the next generation of therapeutics for depression.

Altered monoamine system activities after prenatal and adult stress: A role for stress resilience?

INTRODUCTION

Prenatal stress (PNS) and stress in adulthood are risk factors for development of major depressive disorder. The present study aimed to 1) confirm previous neuroendocrine and behavioral changes induced by PNS, and 2) to characterize the effect of early- and late life stress on the in vivo activity of monoamine systems.

METHODS

Gestational dams were restrained thrice daily under bright illumination from gestational day (GD)11-20. Behavior and neuroendocrine responses to the forced swim test (FST) were determined in adult (50-80 days) offspring, and electrophysiological single unit recordings of dorsal raphe nucleus serotonin (5-HT), ventral tegmental area dopamine (DA) and locus coeruleus norepinephrine (NE) neurons were obtained at baseline and 24h after the FST.

RESULTS

Gestational dams did not habituate to chronic restraint stress, and PNS reduced the birth weight of offspring. In adulthood, swim stress elevated CORT levels longer in PNS animals, while it had no effect on swim behaviors. Baseline firing activity of 5-HT neurons was decreased in PNS animals, while the firing activity of NE and DA neurons was increased. Swim stress had no effect on the firing on 5-HT neurons, but normalized the firing activity of catecholamine neurons in PNS animals.

CONCLUSION

The present data confirm previously established effects on neuroendocrine and physiological measures, and demonstrate an altering effect of PNS and stress on monoamine system activities in adulthood. Since PNS did not result in a depressive-like phenotype, these central changes following PNS might play reflect adaptive changes contributing to stress resilience in adulthood.

Effects of serotonin in the hippocampus: how SSRIs and multimodal antidepressants might regulate pyramidal cell function

The hippocampus plays an important role in emotional and cognitive processing, and both of these domains are affected in patients with major depressive disorder (MDD). Extensive preclinical research and the notion that modulation of serotonin (5-HT) neurotransmission plays a key role in the therapeutic efficacy of selective serotonin reuptake inhibitors (SSRIs) support the view that 5-HT is important for hippocampal function in normal and disease-like conditions. The hippocampus is densely innervated by serotonergic fibers, and the majority of 5-HT receptor subtypes are expressed there. Furthermore, hippocampal cells often co-express multiple 5-HT receptor subtypes that can have either complementary or opposing effects on cell function, adding to the complexity of 5-HT neurotransmission. Here we review the current knowledge of how 5-HT, through its various receptor subtypes, modulates hippocampal output and the activity of hippocampal pyramidal cells in rodents. In addition, we discuss the relevance of 5-HT modulation for cognitive processing in rodents and possible clinical implications of these results in patients with MDD. Finally, we review the data on how SSRIs and vortioxetine, an antidepressant with multimodal activity, affect hippocampal function, including cognitive processing, from both a preclinical and clinical perspective.

Long-Term Citalopram Treatment Alters the Stress Responses of the Cortical Dopamine and Noradrenaline Systems: the Role of Cortical 5-HT1A Receptors

BACKGROUND

Cortical dopamine and noradrenaline are involved in the stress response. Citalopram, a selective serotonin reuptake inhibitor, has direct and indirect effects on the serotonergic system. Furthermore, long-term treatment with citalopram affects the dopamine and noradrenaline systems, which could contribute to the therapeutic action of antidepressants.

METHODS

The effects of long-term treatment with citalopram on the responses of the dopamine and noradrenaline systems in the rat prefrontal cortex to acute handling stress were evaluated using in vivo microdialysis.

RESULTS

Acute handling stress increased dopamine and noradrenaline levels in the prefrontal cortex. The dopamine and noradrenaline responses were suppressed by local infusion of a 5-HT1A receptor agonist, 7-(Dipropylamino)-5,6,7,8-tetrahydronaphthalen-1-ol;hydrobromide, into the prefrontal cortex. The dopamine response was abolished by long-term treatment with citalopram, and the abolished dopamine response was reversed by local infusion of a 5-HT1A receptor antagonist, (Z)-but-2-enedioic acid;N-[2-[4-(2-methoxyphenyl)piperazin-1-yl]ethyl]-N-pyridin-2-ylcyclohexanecarboxamide into the prefrontal cortex. On the other hand, long-term treatment with citalopram reduced the basal noradrenaline levels (approximately 40% of the controls), but not the basal dopamine levels. The noradrenaline response was maintained despite the low basal noradrenaline levels. Signaling from the 5-HT1A receptors and α2-adrenoceptors was not involved in the decrease in the basal noradrenaline levels but partially affected the noradrenaline response.

CONCLUSIONS

Chronic citalopram treatment differentially suppresses the dopamine and noradrenaline systems in the prefrontal cortex, and the dopamine stress response was preferentially controlled by upregulating 5-HT1A receptor signaling. Our findings provide insight into how antidepressants modulate the dopamine and noradrenaline systems to overcome acute stress.

The fatty acid amide hydrolase inhibitor URB597 modulates serotonin-dependent emotional behaviour, and serotonin1A and serotonin2A/C activity in the hippocampus

The fatty acid amide hydrolase (FAAH) inhibitor URB597 increases anandamide, resulting in antidepressant/anxiolytic-like activity, likely via CB1 receptor-mediated modulation of serotonin (5-HT) and norepinephrine (NE) neurotransmission. However, the relative importance of the 5-HT and NE systems in these effects and on effects of URB597 on postsynaptic 5-HT receptors remain to be determined. Using behavioural and electrophysiological approaches, we assessed the effects of acute-single and repeated URB597 treatment on responses predicting antidepressant/anxiolytic activity, and on hippocampal 5-HT1A and 5-HT2A/C receptor sensitivity. Acute-single or serial URB597 treatment, compared to vehicle, reduced immobility in the forced swim test (FST), increased open arm visits in the elevated plus maze and shortened feeding latency in the novelty-suppressed feeding test (NSFT). Repeated URB597 treatment yielded more profound behavioural effects, which were associated with an increase in hippocampal brain-derived neurotrophic factor (BDNF). The 5-HT synthesis inhibitor para-chlorophenylalanine (pCPA), but not the NE neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) prevented URB597-mediated antidepressant/anxiolytic-like response in the FST and NSFT, while DSP4 did not further affect URB597-mediated increase in raphe 5-HT neuron firing. Repeated URB597 administration decreased hippocampal pyramidal firing in response to 5-HT2A/C and 5-HT1A stimulation with 1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI) and 8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT), respectively, suggesting plastic adaptation of these receptors. The effects of acute-single and repeated URB597 administration on hippocampal cell firing in response to DOI or 8-OH-DPAT were similar in magnitude and intensity to the positive control citalopram. These data indicate that URB597 acts, either directly or indirectly, on the 5-HT system, increases hippocampal BDNF expression, and modifies 5-HT1A and 5-HT2A/C function.

Galanin (1-15) enhances the antidepressant effects of the 5-HT1A receptor agonist 8-OH-DPAT: involvement of the raphe-hippocampal 5-HT neuron system

Galanin N-terminal fragment (1-15) [GAL(1-15)] is associated with depression-related and anxiogenic-like effects in rats. In this study, we analyzed the ability of GAL(1-15) to modulate 5-HT1A receptors (5-HT1AR), a key receptor in depression. GAL(1-15) enhanced the antidepressant effects induced by the 5-HT1AR agonist 8-OH-DPAT in the forced swimming test. These effects were stronger than the ones induced by Galanin (GAL). This action involved interactions at receptor level since GAL(1-15) affected the binding characteristics and the mRNA levels of 5-HT1AR in the dorsal hippocampus and dorsal raphe. The involvement of the GALR2 was demonstrated with the GALR2 antagonist M871. Proximity ligation assay experiments indicated that 5-HT1AR are in close proximity with GALR1 and GALR2 in both regions and in raphe RN33B cells. The current results indicate that GAL(1-15) enhances the antidepressant effects induced by 8-OH-DPAT acting on 5-HT1AR operating as postjunctional or as autoreceptors. These results may give the basis for the development of drugs targeting potential GALR1-GALR2-5-HT1AR heteroreceptor complexes linked to the raphe-hippocampal 5-HT neurons for the treatment of depression.

Molecular Mechanisms of Cannabis Signaling in the Brain

Cannabis has been cultivated and used by humans for thousands of years. Research for decades was focused on understanding the mechanisms of an illegal/addictive drug. This led to the discovery of the vast endocannabinoid system. Research has now shifted to understanding fundamental biological questions related to one of the most widespread signaling systems in both the brain and the body. Our understanding of cannabinoid signaling has advanced significantly in the last two decades. In this review, we discuss the state of knowledge on mechanisms of Cannabis signaling in the brain and the modulation of key brain neurotransmitter systems involved in both brain reward/addiction and psychiatric disorders. It is highly probable that various cannabinoids will be found to be efficacious in the treatment of a number of psychiatric disorders. However, while there is clearly much potential, marijuana has not been properly vetted by the medical-scientific evaluation process and there are clearly a range of potentially adverse side-effects-including addiction. We are at crossroads for research on endocannabinoid function and therapeutics (including the use of exogenous treatments such as Cannabis). With over 100 cannabinoid constituents, the majority of which have not been studied, there is much Cannabis research yet to be done. With more states legalizing both the medicinal and recreational use of marijuana the rigorous scientific investigation into cannabinoid signaling is imperative.