Control of dorsal raphe serotonergic neurons by the medial prefrontal cortex: Involvement of serotonin-1A, GABA(A), and glutamate receptors

Simulating combined monoaminergic depletions in a PD animal model through a bio-constrained differential equations system

Introduction

Historically, Parkinson's Disease (PD) research has focused on the dysfunction of dopamine-producing cells in the substantia nigra pars compacta, which is linked to motor regulation in the basal ganglia. Therapies have mainly aimed at restoring dopamine (DA) levels, showing effectiveness but variable outcomes and side effects. Recent evidence indicates that PD complexity implicates disruptions in DA, noradrenaline (NA), and serotonin (5-HT) systems, which may underlie the variations in therapy effects.

Methods

We present a system-level bio-constrained computational model that comprehensively investigates the dynamic interactions between these neurotransmitter systems. The model was designed to replicate experimental data demonstrating the impact of NA and 5-HT depletion in a PD animal model, providing insights into the causal relationships between basal ganglia regions and neuromodulator release areas.

Results

The model successfully replicates experimental data and generates predictions regarding changes in unexplored brain regions, suggesting avenues for further investigation. It highlights the potential efficacy of alternative treatments targeting the locus coeruleus and dorsal raphe nucleus, though these preliminary findings require further validation. Sensitivity analysis identifies critical model parameters, offering insights into key factors influencing brain area activity. A stability analysis underscores the robustness of our mathematical formulation, bolstering the model validity.

Discussion

Our holistic approach emphasizes that PD is a multifactorial disorder and opens promising avenues for early diagnostic tools that harness the intricate interactions among monoaminergic systems. Investigating NA and 5-HT systems alongside the DA system may yield more effective, subtype-specific therapies. The exploration of multisystem dysregulation in PD is poised to revolutionize our understanding and management of this complex neurodegenerative disorder.

CaMKIV-Mediated Phosphorylation Inactivates Freud-1/CC2D1A Repression for Calcium-Dependent 5-HT1A Receptor Gene Induction

Calcium calmodulin-dependent protein kinase (CaMK) mediates calcium-induced neural gene activation. CaMK also inhibits the non-syndromic intellectual disability gene, Freud-1/CC2D1A, a transcriptional repressor of human serotonin-1A (5-HT1A) and dopamine-D2 receptor genes. The altered expression of these Freud-1-regulated genes is implicated in mental illnesses such as major depression and schizophrenia. We hypothesized that Freud-1 is blocked by CaMK-induced phosphorylation. The incubation of purified Freud-1 with either CaMKIIα or CaMKIV increased Freud-1 phosphorylation that was partly prevented in Freud-1-Ser644Ala and Freud-1-Thr780Ala CaMK site mutants. In human SK-N-SH neuroblastoma cells, active CaMKIV induced the serine and threonine phosphorylation of Freud-1, and specifically increased Freud-1-Thr780 phosphorylation in transfected HEK-293 cells. The activation of purified CaMKIIα or CaMKIV reduced Freud-1 binding to its DNA element on the 5-HT1A and dopamine-D2 receptor genes. In SK-N-SH cells, active CaMKIV but not CaMKIIα blocked the Freud-1 repressor activity, while Freud-1 Ser644Ala, Thr780Ala or dual mutants were resistant to inhibition by activated CaMKIV or calcium mobilization. These results indicate that the Freud-1 repressor activity is blocked by CaMKIV-induced phosphorylation at Thr780, resulting in the up-regulation of the target genes, such as the 5-HT1A receptor gene. The CaMKIV-mediated inhibition of Freud-1 provides a novel de-repression mechanism to induce 5-HT1A receptor expression for the regulation of cognitive development, behavior and antidepressant response.

The role of serotonin in depression-A historical roundup and future directions

Depression is one of the most common psychiatric disorders worldwide, affecting approximately 280 million people, with probably much higher unrecorded cases. Depression is associated with symptoms such as anhedonia, feelings of hopelessness, sleep disturbances, and even suicidal thoughts. Tragically, more than 700 000 people commit suicide each year. Although depression has been studied for many decades, the exact mechanisms that lead to depression are still unknown, and available treatments only help a fraction of patients. In the late 1960s, the serotonin hypothesis was published, suggesting that serotonin is the key player in depressive disorders. However, this hypothesis is being increasingly doubted as there is evidence for the influence of other neurotransmitters, such as noradrenaline, glutamate, and dopamine, as well as larger systemic causes such as altered activity in the limbic network or inflammatory processes. In this narrative review, we aim to contribute to the ongoing debate on the involvement of serotonin in depression. We will review the evolution of antidepressant treatments, systemic research on depression over the years, and future research applications that will help to bridge the gap between systemic research and neurotransmitter dynamics using biosensors. These new tools in combination with systemic applications, will in the future provide a deeper understanding of the serotonergic dynamics in depression.

Maturation of cortical input to dorsal raphe nucleus increases behavioral persistence in mice

The ability to persist toward a desired objective is a fundamental aspect of behavioral control whose impairment is implicated in several behavioral disorders. One of the prominent features of behavioral persistence is that its maturation occurs relatively late in development. This is presumed to echo the developmental time course of a corresponding circuit within late-maturing parts of the brain, such as the prefrontal cortex, but the specific identity of the responsible circuits is unknown. Here, we used a genetic approach to describe the maturation of the projection from layer 5 neurons of the neocortex to the dorsal raphe nucleus in mice. Using optogenetic-assisted circuit mapping, we show that this projection undergoes a dramatic increase in synaptic potency between postnatal weeks 3 and 8, corresponding to the transition from juvenile to adult. We then show that this period corresponds to an increase in the behavioral persistence that mice exhibit in a foraging task. Finally, we used a genetic targeting strategy that primarily affected neurons in the medial prefrontal cortex, to selectively ablate this pathway in adulthood and show that mice revert to a behavioral phenotype similar to juveniles. These results suggest that frontal cortical to dorsal raphe input is a critical anatomical and functional substrate of the development and manifestation of behavioral persistence.

Decision-making dynamics are predicted by arousal and uninstructed movements

During sensory-guided behavior, an animal's decision-making dynamics unfold through sequences of distinct performance states, even while stimulus-reward contingencies remain static. Little is known about the factors that underlie these changes in task performance. We hypothesize that these decision-making dynamics can be predicted by externally observable measures, such as uninstructed movements and changes in arousal. Here, using computational modeling of visual and auditory task performance data from mice, we uncovered lawful relationships between transitions in strategic task performance states and an animal's arousal and uninstructed movements. Using hidden Markov models applied to behavioral choices during sensory discrimination tasks, we find that animals fluctuate between minutes-long optimal, sub-optimal, and disengaged performance states. Optimal state epochs are predicted by intermediate levels, and reduced variability, of pupil diameter and movement. Our results demonstrate that externally observable uninstructed behaviors can predict optimal performance states and suggest that mice regulate their arousal during optimal performance.

Changes in serotonin neurotransmission as assayed by microdialysis after acute, intermittent or chronic ethanol administration and withdrawal

BACKGROUND

The serotonergic neurotransmitter system is involved in many ethanol-induced changes, including many behavioural alterations, as well as contributing to alcohol dependence and its withdrawal.

AIMS

This review has evaluated microdialysis studies where alterations in the serotonin system, that is, serotonin, 5-HT, or its metabolite 5-hydroxyindoleacetic acid, 5-HIAA, have been reported during different ethanol intoxication states, as well as in animals showing alcohol preference or not. Changes in 5-HT receptors and the 5-HT transporter are briefly reviewed to comprehend the significance of changes in microdialysate 5-HT concentrations.

MATERIALS AND METHODS

Changes in 5-HT content following acute, chronic and during ethanol withdrawal states are evaluated. In addition, the serotoninergic system was assessed in animals that have been genetically selected for alcohol preference to ascertain whether changes in this monoamine microdialysate content may contribute to alcohol preference.

RESULTS AND DISCUSSION

Changes occurred in 5-HT signalling in the limbic brain regions, increasing after acute ethanol administration in specific brain regions, particularly at higher doses, while chronic alcohol exposure essentially decreased serotonergic transmission. Such changes may play a pivotal role in emotion-driven craving and relapse. Depending on the dosage, mode of administration and consumption rate, ethanol affects specific brain regions in different ways, enhancing or reducing 5-HT microdialysate content, thereby inducing behavioural and cognitive functions and enhancing ethanol consumption.

CONCLUSION

Microdialysis studies demonstrated that ethanol induces several alterations in 5-HT content as well as its metabolites, 5-HIAA and 5-HTOL, not only in its release from a specific brain region but also in the modifications of its different receptor subtypes and its transporter.

Ketamine promptly normalizes excess norepinephrine and enhances dopamine neuronal activity in Wistar Kyoto rats

Ketamine acts primarily by blocking the N-methyl-D-aspartate (NMDA) receptor at the phencyclidine site. The rapid antidepressant properties of ketamine were demonstrated in the clinic and several behavioral models of depression in rodents. We hypothesized that the normalization of abnormal activity of monoamine neurons in Wistar Kyoto (WKY) rats contributes to the rapid antidepressant effects of ketamine. A single administration of ketamine (10 mg/kg, i. p) or saline was administered to anesthetized WKY rats before in vivo electrophysiological recordings of dorsal raphe nucleus (DRN) serotonin (5-HT), locus coeruleus (LC) norepinephrine (NE) and ventral tegmental area (VTA) dopamine (DA) neuronal activity. Pyramidal neurons from the medial prefrontal cortex (mPFC) were also recorded before and after a ketamine injection. In the VTA, ketamine elicited a significant increase in the population activity of DA neurons. This enhancement was consistent with findings in other depression-like models in which such a decreased population activity was observed. In the LC, ketamine normalized increased NE neuron burst activity found in WKY rats. In the DRN, ketamine did not significantly reverse 5-HT neuronal activity in WKY rats, which is dampened compared to Wistar rats. Ketamine did not significantly alter the neuronal activity of mPFC pyramidal neurons. These findings demonstrate that ketamine normalized NE neuronal activity and enhanced DA neuronal activity in WKY rats, which may contribute to its rapid antidepressant effect.

Lactobacillus reuteri ATG-F4 Alleviates Chronic Stress-induced Anhedonia by Modulating the Prefrontal Serotonergic System

Mental health is influenced by the gut-brain axis; for example, gut dysbiosis has been observed in patients with major depressive disorder (MDD). Gut microbial changes by fecal microbiota transplantation or probiotics treatment reportedly modulates depressive symptoms. However, it remains unclear how gut dysbiosis contributes to mental dysfunction, and how correction of the gut microbiota alleviates neuropsychiatric disorders. Our previous study showed that chronic consumption of Lactobacillus reuteri ATG-F4 (F4) induced neurometabolic alterations in healthy mice. Here, we investigated whether F4 exerted therapeutic effects on depressive-like behavior by influencing the central nervous system. Using chronic unpredictable stress (CUS) to induce anhedonia, a key symptom of MDD, we found that chronic F4 consumption alleviated CUS-induced anhedonic behaviors, accompanied by biochemical changes in the gut, serum, and brain. Serum and brain metabolite concentrations involved in tryptophan metabolism were regulated by CUS and F4. F4 consumption reduced the elevated levels of serotonin (5-HT) in the brain observed in the CUS group. Additionally, the increased expression of Htr1a, a subtype of the 5-HT receptor, in the medial prefrontal cortex (mPFC) of stressed mice was restored to levels observed in stress-naïve mice following F4 supplementation. We further demonstrated the role of Htr1a using AAV-shRNA to downregulate Htr1a in the mPFC of CUS mice, effectively reversing CUS-induced anhedonic behavior. Together, our findings suggest F4 as a potential therapeutic approach for relieving some depressive symptoms and highlight the involvement of the tryptophan metabolism in mitigating CUS-induced depressive-like behaviors through the action of this bacterium.

Stressor control and regional inflammatory responses in the brain: regulation by the basolateral amygdala

Increasing evidence has connected the development of certain neuropsychiatric disorders, as well as neurodegenerative diseases, to stress-induced dysregulation of the immune system. We have shown that escapable (ES) and inescapable (IS) footshock stress, and memories associated with ES or IS, can differentially alter inflammatory-related gene expression in brain in a region dependent manner. We have also demonstrated that the basolateral amygdala (BLA) regulates stress- and fear memory-induced alterations in sleep, and that differential sleep and immune responses in the brain to ES and IS appear to be integrated during fear conditioning and then reproduced by fear memory recall. In this study, we investigated the role of BLA in influencing regional inflammatory responses within the hippocampus (HPC) and medial prefrontal cortex (mPFC) by optogenetically stimulating or inhibiting BLA in male C57BL/6 mice during footshock stress in our yoked shuttlebox paradigm based on ES and IS. Then, mice were immediately euthanized and RNA extracted from brain regions of interest and loaded into NanoString® Mouse Neuroinflammation Panels for compilation of gene expression profiles. Results showed differential regional effects in gene expression and activated pathways involved in inflammatory-related signaling following ES and IS, and these differences were altered depending on amygdalar excitation or inhibition. These findings demonstrate that the stress-induced immune response, or "parainflammation", is affected by stressor controllability and that BLA influences regional parainflammation to ES or IS in HPC and mPFC. The study illustrates how stress-induced parainflammation can be regulated at the neurocircuit level and suggests that this approach can be useful for uncovering circuit and immune interactions in mediating differential stress outcomes.

Contribution of the opioid system to depression and to the therapeutic effects of classical antidepressants and ketamine

Major depressive disorder (MDD) afflicts approximately 5 % of the world population, and about 30-50 % of patients who receive classical antidepressant medications do not achieve complete remission (treatment resistant depressive patients). Emerging evidence suggests that targeting opioid receptors mu (MOP), kappa (KOP), delta (DOP), and the nociceptin/orphanin FQ receptor (NOP) may yield effective therapeutics for stress-related psychiatric disorders. As depression and pain exhibit significant overlap in their clinical manifestations and molecular mechanisms involved, it is not a surprise that opioids, historically used to alleviate pain, emerged as promising and effective therapeutic options in the treatment of depression. The opioid signaling is dysregulated in depression and numerous preclinical studies and clinical trials strongly suggest that opioid modulation can serve as either an adjuvant or even an alternative to classical monoaminergic antidepressants. Importantly, some classical antidepressants require the opioid receptor modulation to exert their antidepressant effects. Finally, ketamine, a well-known anesthetic whose extremely efficient antidepressant effects were recently discovered, was shown to mediate its antidepressant effects via the endogenous opioid system. Thus, although opioid system modulation is a promising therapeutical venue in the treatment of depression further research is warranted to fully understand the benefits and weaknesses of such approach.

Decision-making dynamics are predicted by arousal and uninstructed movements

During sensory-guided behavior, an animal's decision-making dynamics unfold through sequences of distinct performance states, even while stimulus-reward contingencies remain static. Little is known about the factors that underlie these changes in task performance. We hypothesize that these decision-making dynamics can be predicted by externally observable measures, such as uninstructed movements and changes in arousal. Here, combining behavioral experiments in mice with computational modeling, we uncovered lawful relationships between transitions in strategic task performance states and an animal's arousal and uninstructed movements. Using hidden Markov models applied to behavioral choices during sensory discrimination tasks, we found that animals fluctuate between minutes-long optimal, sub-optimal and disengaged performance states. Optimal state epochs were predicted by intermediate levels, and reduced variability, of pupil diameter, along with reduced variability in face movements and locomotion. Our results demonstrate that externally observable uninstructed behaviors can predict optimal performance states, and suggest mice regulate their arousal during optimal performance.

Fast antidepressant action of ketamine in mouse models requires normal VGLUT1 levels from prefrontal cortex neurons

The NMDA antagonist ketamine demonstrated a fast antidepressant activity in treatment-resistant depression. Pre-clinical studies suggest that de novo synthesis of the brain-derived neurotrophic factor (BDNF) in the PFC might be involved in the rapid antidepressant action of ketamine. Applying a genetic model of impaired glutamate release, this study aims to further identify the molecular mechanisms that could modulate antidepressant action and resistance to treatment. To that end, mice knocked-down for the vesicular glutamate transporter 1 (VGLUT1+/-) were used. We analyzed anhedonia and helpless behavior as well as the expression of the proteins linked to glutamate transmission in the PFC of mice treated with ketamine or the reference antidepressant reboxetine. Moreover, we analyzed the acute effects of ketamine in VGLUT1+/- mice pretreated with chronic reboxetine or those that received a PFC rescue expression of VGLUT1. Chronic reboxetine rescued the depressive-like phenotype of the VGLUT1+/- mice. In addition, it enhanced the expression of the proteins linked to the AMPA signaling pathway as well as the immature form of BDNF (pro-BDNF). Unlike WT mice, ketamine had no effect on anhedonia or pro-BDNF expression in VGLUT1+/- mice; it also failed to decrease phosphorylated eukaryote elongation factor 2 (p-eEF2). Nevertheless, we found that reboxetine administered as pretreatment or PFC overexpression of VGLUT1 did rescue the antidepressant-like activity of acute ketamine in the mice. Our results strongly suggest that not only do PFC VGLUT1 levels modulate the rapid-antidepressant action of ketamine, but also highlight a possible mechanism for antidepressant resistance in some patients.

Differential Modulation of Dorsal Raphe Serotonergic Activity in Rat Brain by the Infralimbic and Prelimbic Cortices

The reciprocal connectivity between the medial prefrontal cortex (mPFC) and the dorsal raphe nucleus (DR) is involved in mood control and resilience to stress. The infralimbic subdivision (IL) of the mPFC is the rodent equivalent of the ventral anterior cingulate cortex, which is intimately related to the pathophysiology/treatment of major depressive disorder (MDD). Boosting excitatory neurotransmission in the IL-but not in the prelimbic cortex, PrL-evokes depressive-like or antidepressant-like behaviors in rodents, which are associated with changes in serotonergic (5-HT) neurotransmission. We therefore examined the control of 5-HT activity by both of the mPFC subdivisions in anesthetized rats. The electrical stimulation of IL and PrL at 0.9 Hz comparably inhibited 5-HT neurons (53% vs. 48%, respectively). However, stimulation at higher frequencies (10-20 Hz) revealed a greater proportion of 5-HT neurons sensitive to IL than to PrL stimulation (86% vs. 59%, at 20 Hz, respectively), together with a differential involvement of GABA_A (but not 5-HT_1A) receptors. Likewise, electrical and optogenetic stimulation of IL and PrL enhanced 5-HT release in DR in a frequency-dependent manner, with greater elevations after IL stimulation at 20 Hz. Hence, IL and PrL differentially control serotonergic activity, with an apparent superior role of IL, an observation that may help to clarify the brain circuits involved in MDD.

The Role of α-Synuclein in the Regulation of Serotonin System: Physiological and Pathological Features

In patients affected by Parkinson's disease (PD), up to 50% of them experience cognitive changes, and psychiatric disturbances, such as anxiety and depression, often precede the onset of motor symptoms and have a negative impact on their quality of life. Pathologically, PD is characterized by the loss of dopamine (DA) neurons in the substantia nigra pars compacta (SNc) and the presence of intracellular inclusions, called Lewy bodies and Lewy neurites, composed mostly of α-synuclein (α-Syn). Much of PD research has focused on the role of α-Syn aggregates in the degeneration of SNc DA neurons due to the impact of striatal DA deficits on classical motor phenotypes. However, abundant Lewy pathology is also found in other brain regions including the midbrain raphe nuclei, which may contribute to non-motor symptoms. Indeed, dysfunction of the serotonergic (5-HT) system, which regulates mood and emotional pathways, occurs during the premotor phase of PD. However, little is known about the functional consequences of α-Syn inclusions in this neuronal population other than DA neurons. Here, we provide an overview of the current knowledge of α-Syn and its role in regulating the 5-HT function in health and disease. Understanding the relative contributions to α-Syn-linked alterations in the 5-HT system may provide a basis for identifying PD patients at risk for developing depression and could lead to a more targeted therapeutic approach.

Selective refinement of glutamate and GABA synapses on dorsal raphe 5-HT neurons during postnatal life

Serotonin (5-hydroxytryptamine, 5-HT) neurons are implicated in the etiology and therapeutics of anxiety and depression. Critical periods of vulnerability during brain development enable maladaptive mechanisms to produce detrimental consequences on adult mood and emotional responses. 5-HT plays a crucial role in these mechanisms; however, little is known about how synaptic inputs and modulatory systems that shape the activity of early 5-HT networks mature during postnatal development. We investigated in mice the postnatal trajectory of glutamate and GABA synaptic inputs to dorsal raphe nucleus (DRN) 5-HT neurons, the main source of forebrain 5-HT. High-resolution quantitative analyses with array tomography and ex vivo electrophysiology indicate that cortical glutamate and subcortical GABA synapses undergo a profound refinement process after the third postnatal week, whereas subcortical glutamate inputs do not. This refinement of DRN inputs is not accompanied by changes in 5-HT1A receptor-mediated inhibition over 5-HT neurons. Our study reveals a precise developmental pattern of synaptic refinement of DRN excitatory and inhibitory afferents, when 5-HT-related inhibitory mechanisms are in place. These findings contribute to the understanding of neurodevelopmental vulnerability to psychiatric disorders. This article has an associated 'The people behind the papers' interview.

Anxiolytic effect of Korean Red Ginseng through upregulation of serotonin and GABA transmission and BDNF expression in immobilized mice

Background

Anxiolytic properties of Korean Red Ginseng (KRG) have been previously reported. However, the exact mechanism(s) of action remains to be elucidated. The present study investigated the effect of KRG on immobilization-induced anxiety-like behaviors in mice and explored the involvement of the serotonin and GABA systems and BDNF in the anxiolytic action.

Methods

Mice were orally administered with KRG (200 mg/kg/day) for 4 weeks and immobilized once daily for 2 h. p-Chlorophenylalanine (p-CPA) was intraperitoneally injected on day 22-28, and flumazenil or bicuculline was injected on day 25-28. After behavioral evaluations, brains were dissected for biochemical analyses.

Results

KRG improved immobilization-induced anxiety-like behaviors in mice, as assessed by the elevated plus maze (EPM) and marble burying tests (MBT). The anxiolytic effect of KRG was comparable to that of fluoxetine, a reference drug clinically used for anxiety disorders. A serotonin synthesis inhibitor, p-CPA, blocked the effect of KRG in the EPM and MBT, indicating the requirement of serotonin synthesis for anxiolytic action. In addition, the anxiolytic effect of KRG was inhibited by bicuculline (a GABA_A antagonist) in MBT, implying the involvement of GABA transmission. Western blotting analyses revealed that KRG upregulated the expression of tryptophan hydroxylase and GABA_A receptor in the brain, which was blocked by p-CPA. Enhanced BDNF expression by KRG in the hippocampus was also indicated to mediate the anxiolytic action of KRG in immobilized mice.

Conclusion

KRG exhibited the anxiolytic effect in immobilized mice by multiple mechanisms of action, involving enhanced serotonin and GABA transmissions and BDNF expression.

Functional characterization of cannabidiol effect on the serotonergic neurons of the dorsal raphe nucleus in rat brain slices

Cannabidiol (CBD), the main non-psychoactive cannabinoid found in the cannabis plant, elicits several pharmacological effects via the 5-HT1A receptor. The dorsal raphe nucleus (DRN) is the main serotonergic cluster in the brain that expresses the 5-HT1A receptor. To date, the effect of CBD on the neuronal activity of DRN 5-HT cells and its interaction with somatodendritic 5-HT1A autoreceptors have not been characterized. Our aim was to study the effect of CBD on the firing activity of DRN 5-HT cells and the 5-HT1A autoreceptor activation by electrophysiological and calcium imaging techniques in male Sprague–Dawley rat brain slices. Perfusion with CBD (30 μM, 10 min) did not significantly change the firing rate of DRN 5-HT cells or the inhibitory effect of 5-HT (50–100 μM, 1 min). However, in the presence of CBD (30 μM, 10 min), the inhibitory effects of 8-OH-DPAT (10 nM) and ipsapirone (100 nM) were reduced by 66% and 53%, respectively. CBD failed to reverse ipsapirone-induced inhibition, whereas perfusion with the 5-HT1A receptor antagonist WAY100635 (30 nM) completely restored by 97.05 ± 14.63% the firing activity of 5-HT cells. Administration of AM251 (1 µM), MDL100907 (30 nM), or picrotoxin (20 μM) did not change the blockade produced by CBD (30 μM) on ipsapirone-induced inhibition. Our study also shows that CBD failed to modify the KCl (15 mM, 4 min)-evoked increase in [Ca2+]i or the inhibitory effect of ipsapirone (1 μM, 4 min) on KCl-evoked [Ca2+]i. In conclusion, CBD does not activate 5-HT1A autoreceptors, but it hindered the inhibitory effect produced by selective 5-HT1A receptor agonists on the firing activity of DRN 5-HT cells through a mechanism that does not involve CB1, 5-HT2A, or GABAA receptors. Our data support a negative allosteric modulation of DRN somatodendritic 5-HT1A receptor by CBD.

Preferential in vivo inhibitory action of serotonin in rat infralimbic versus prelimbic cortex: relevance for antidepressant treatments

The infralimbic (IL) cortex is the rodent equivalent of human ventral anterior cingulate cortex (vACC), which plays a key role in the pathophysiology and treatment of major depressive disorder (MDD). The modulation of glutamatergic neurotransmission in IL [but not in the adjacent prelimbic (PrL) cortex] evokes antidepressant-like or depressive-like behaviors, associated with changes in serotonin (5-HT) function, highlighting the relevance of glutamate/serotonin interactions in IL for emotional control. 5-HT modulates neuronal activity in PrL and cingulate (Cg) cortex but its effects in IL are largely unknown. We therefore compared the in vivo effects of 5-HT on pyramidal neuron activity in IL (n = 61) and PrL (n = 50) of anesthetized rats. IL pyramidal neurons were more responsive to physiological dorsal raphe stimulation (0.9 Hz) than PrL neurons (84% vs. 64%, respectively) and were inhibited to a greater extent (64% vs. 36%, respectively). Orthodromic activations (8% in PrL) were absent in IL, whereas biphasic responses were similar (20%) in both areas. Excitations were mediated by 5-HT2A-R activation, whereas inhibitions involved 3 different components: 5-HT1A-R, 5-HT3-R and GABAA-R, respectively. The remarkable inhibitory action of 5-HT in IL suggests that 5-HT-enhancing drugs may exert their antidepressant action by normalizing a glutamatergic hyperactivity in the vACC of MDD patients.

Effect of Psilocybin and Ketamine on Brain Neurotransmitters, Glutamate Receptors, DNA and Rat Behavior

Clinical studies provide evidence that ketamine and psilocybin could be used as fast-acting antidepressants, though their mechanisms and toxicity are still not fully understood. To address this issue, we have examined the effect of a single administration of ketamine and psilocybin on the extracellular levels of neurotransmitters in the rat frontal cortex and reticular nucleus of the thalamus using microdialysis. The genotoxic effect and density of glutamate receptor proteins was measured with comet assay and Western blot, respectively. An open field test, light–dark box test and forced swim test were conducted to examine rat behavior 24 h after drug administration. Ketamine (10 mg/kg) and psilocybin (2 and 10 mg/kg) increased dopamine, serotonin, glutamate and GABA extracellular levels in the frontal cortex, while psilocybin also increased GABA in the reticular nucleus of the thalamus. Oxidative DNA damage due to psilocybin was observed in the frontal cortex and from both drugs in the hippocampus. NR2A subunit levels were increased after psilocybin (10 mg/kg). Behavioral tests showed no antidepressant or anxiolytic effects, and only ketamine suppressed rat locomotor activity. The observed changes in neurotransmission might lead to genotoxicity and increased NR2A levels, while not markedly affecting animal behavior.

Differential expression of serotonin2B receptors in GABAergic and serotoninergic neurons of the rat and mouse dorsal raphe nucleus

The central serotonin_2B receptor (5-HT_2BR) modulates 5-HT and dopamine (DA) neuronal function in the mammalian brain and has been suggested as a potential target for the treatment of neuropsychiatric disorders involving derangements of these monoamine systems, such as schizophrenia, cocaine abuse and dependence and major depressive disorder. Studies in rats and mice yielded contrasting results on the control of 5-HT/DA networks by 5-HT_2BRs, thereby leading to opposite views on the therapeutic potential of 5-HT_2BR agents for treating the above disorders. These discrepancies may result from anatomo-functional differences related to a different cellular location of 5-HT_2BRs in rat and mouse brain. Using immunohistochemistry, we assessed this hypothesis by examining the expression of 5-HT_2BRs in 5-HT and GABAergic neurons of rats and mice within different subregions of the dorsal raphe nucleus (DRN), currently considered as the main site of action of 5-HT_2B agents. Likewise, using in vivo microdialysis, we examined their functional relevance in the control of DRN 5-HT outflow, a surrogate index of 5-HT neuronal activity. In the DRN of both species, 5-HT_2BRs are expressed in 5-HT cells expressing tryptophan hydroxylase 2 (TPH₂), in GABAergic cells expressing glutamic acid decarboxylase 67 (GAD67), and in cells expressing both markers (GAD67 & TPH₂; i.e., GABA-expressing 5-HT neurons). The proportion of 5-HT_2BR-positive cells expressing only TPH₂ was significantly larger in mouse than in rat DRN, whereas the opposite holds true for the expression in cells expressing GAD67 & TPH₂. No major species differences were found in the dorsal and ventral subregions. In contrast, the lateral subregion exhibited large differences, with a predominant expression of 5-HT_2BRs in TPH₂-positive cells in mice (67.2 vs 19.9 % in rats), associated with a lower expression in GAD67 & TPH₂ cells (7.9 % in mice vs 41.5 % in rats). Intra-DRN (0.1 μM) administration of the preferential 5-HT_2BR agonist BW 723C86 decreased and increased DRN 5-HT outflow in rats and mice respectively, both effects being prevented by the intra-DRN perfusion of the selective 5-HT_2BR antagonist RS 127445 (0.1 μM). Altogether, these results show the existence of anatomical differences in the cellular expression of 5-HT_2BRs in the rat and mouse DRN, which translate into an opposite control of 5-HT outflow. Also, they highlight the relevance of the subset of GAD67-positive 5-HT neurons as a key factor responsible for the functional differences between rats and mice in terms of 5-HT neuronal activity modulation.

The role of serotonin neurotransmission in rapid antidepressant actions

RATIONALE

Ketamine has rapid antidepressant effects that represent a significant advance in treating depression, but its poor safety and tolerability limit its clinical utility. Accreting evidence suggests that serotonergic neurotransmission participates in the rapid antidepressant effects of ketamine and hallucinogens. Thus, understanding how serotonin contributes to these effects may allow identification of novel rapid antidepressant mechanisms with improved tolerability.

OBJECTIVE

The goal of this paper is to understand how serotonergic mechanisms participate in rapid antidepressant mechanisms.

METHODS

We review the relevance of serotonergic neurotransmission for rapid antidepressant effects and evaluate the role of 5-HT_1A, 5-HT_1B, 5-HT_2A, and 5-HT₄ receptors in synaptic plasticity, BDNF signaling, and GSK-3β activity. Subsequently, we develop hypotheses on the relationship of these receptor systems to rapid antidepressant effects.

RESULTS

We found that 5-HT_1A and 5-HT_1B receptors may participate in ketamine's rapid antidepressant mechanisms, while agonists at 5-HT_2A and 5-HT₄ receptors may independently behave as rapid antidepressants. 5-HT_1A, 5-HT_2A, and 5-HT₄ receptors increase synaptic plasticity in the cortex or hippocampus but do not consistently increase BDNF signaling. We found that 5-HT_1A and 5-HT_1B receptors may participate in rapid antidepressant mechanisms as a consequence of increased BDNF signaling, rather than a cause. 5-HT_2A and 5-HT₄ receptor agonists may increase BDNF signaling, but these relationships are tenuous and need more study. Finally, we found that ketamine and several serotonergic receptor systems may mechanistically converge on reduced GSK-3β activity.

CONCLUSIONS

We find it plausible that serotonergic neurotransmission participates in rapid antidepressant mechanisms by increasing synaptic plasticity, perhaps through GSK-3β inhibition.

GABAergic interneurons' feedback inhibition of dorsal raphe-projecting pyramidal neurons of the medial prefrontal cortex suppresses feeding of adolescent female mice undergoing activity-based anorexia

Anorexia Nervosa (AN) is characterized by voluntary food restriction, excessive exercise and extreme body weight loss. AN is particularly prevalent among adolescent females experiencing stress-induced anxiety. We used the animal model, activity-based anorexia (ABA), which captures these characteristics of AN, to reveal the neurobiology underlying individual differences in AN vulnerability. Dorsal raphe (DR) regulates feeding and is recruited when coping inescapable stress. Through chemogenetic activation, we investigated the role of mPFC pyramidal neurons projecting to DR (mPFC→DR) in adolescent female mice's decision to eat or exercise following ABA induction. Although the DREADD ligand C21 could activate 44% of the mPFC→DR neurons, this did not generate significant group mean difference in the amount of food intake, compared to control ABA mice without chemogenetic activation. However, analysis of individuals' responses to C21 revealed a significant, positive correlation between food intake and mPFC→DR neurons that co-express cFos, a marker for neuronal activity. cFos expression by GABAergic interneurons (GABA-IN) in mPFC was significantly greater than that for the control ABA mice, indicating recruitment of GABA-IN by mPFC→DR neurons. Electron microscopic immunohistochemistry revealed that GABAergic innervation is 60% greater for the PFC→DR neurons than adjacent Layer 5 pyramidal neurons without projections to DR. Moreover, individual differences in this innervation correlated negatively with food intake specifically on the day of C21 administration. We propose that C21 activates two antagonistic pathways: (1) PFC→DR pyramidal neurons that promote food intake; and (2) GABA-IN in the mPFC that dampen food intake through feedback inhibition of mPFC→DR neurons.

Serotonin Type 2a Receptor in the Prefrontal Cortex Controls Perirhinal Cortex Excitability During Object Recognition Memory Recall

Previous experiences can drive adaptive behavior based on different characteristics, including contextual ones. Indeed, contextual information can be used as a criterion to guide the recall of the most relevant memory trace and the inhibition of others. The medial Prefontal Cortex (mPFC) has been proposed as an area that plays a pivotal role in regulating the retrieval of memory traces in downstream regions. Also, we have shown that mPFC Serotonin 2a Receptors (5-HT2aR) modulates the retrieval of a contextually guided recognition memory task and modulates the retrieval and reconsolidation of memories in the Perirhinal Cortex (PRH). However, how the mPFC output mediated by the 5-HT2aR activity is modulating memory retrieval in the PRH is a question that remains unclear. To tackle this question, we analyzed neuronal activity in the PRH and mPFC, by measuring expression of the immediate early gene c-Fos. We combined behavioral, pharmacological and immunohistochemical techniques to examine how mPFC 5-HT2aR controls mPFC and the PRH activity. We found that blockade of mPFC 5-HT2aR increase the level of c-Fos expression in the PHR and that this increase correlates with animals' performance in the task. We also found an increase in c-Fos expression in the mPFC after mPFC 5-HT2aR blockade that does not correlate with the animals' behavioral response. However, these changes showed a significant correlation with those observed in the PRH. These results suggest that mPFC 5-HT2aR signaling may modulate the behavioral response during memory recall by controlling the neuronal activation in the PRH.

Fluoxetine exposure throughout neurodevelopment differentially influences basilar dendritic morphology in the motor and prefrontal cortices

The significance of serotonin (5HT) in mental health is underscored by the serotonergic action of many classes of psychiatric medication. 5HT is known to have a significant role in neurodevelopment, thus 5HT disruption during development may have a long term impact on brain structure and circuits. We previously generated a model of 5HT alteration throughout neurodevelopment by maternal administration of the selective serotonin reuptake inhibitor fluoxetine. We found resulting social behavior alterations in the offspring during both postnatal and adult ages. Previous work by others has indicated that early 5HT disruption influences neuronal morphology. Therefore, in the current study we sought to determine if dendritic morphological changes occur in areas involved in the social behavior deficits we previously observed, specifically the primary motor (M1) and medial prefrontal (mPFC) cortices. We quantified dendritic morphology of projection neurons in M1 and mPFC at postnatal day (P)10 and P79 in mice exposed to fluoxetine. Basilar dendritic complexity and spine density were persistently decreased in M1 fluoxetine-exposed neurons while in the mPFC, similar reductions were observed at P79 but were not present at P10. Our findings underscore that the developing brain, specifically the projection cortex, is vulnerable to 5HT system perturbation, which may be related to later behavioral disruptions.

Ketamine triggers rapid antidepressant effects by modulating synaptic plasticity in a new depressive-like mouse model based on astrocyte glutamate transporter GLT-1 knockdown in infralimbic cortex

OBJECTIVE

Recently, we reported on a new MDD-like mouse model based on a regionally selective knockdown of astroglial glutamate transporters, GLAST/GLT-1, in infralimbic cortex (IL) which evokes widespread changes in mouse brain associated with the typical alterations found in MDD patients. To further characterize this new MDD-like mouse model, here we examine some transcriptional elements of glutamatergic/GABAergic neurotransmission and neuroplasticity in forebrain regions in the GLT-1 knockdown mice. Furthermore, we assess the acute ketamine effects on these transcriptional processes.

MATERIAL AND METHODS

We used a small interfering RNA (siRNA) pool targeting GLT-1 mRNA to disrupt the GLT-1 transcription in mouse IL. Histological assays were performed to examine postsynaptic density protein-95 (PSD95), neuritin (NRN), glutamine acid descarboxilase-65 (GAD65), and GLT-1 mRNA expression in IL and hippocampus.

RESULTS

Knockdown of GLT-1 in mouse IL leads to decreased expression of PSD95 and NRN neuroplasticity mRNAs in IL and hippocampus, which was reversed by an acute dose of ketamine antidepressant. Likewise, a single dose of ketamine also increased the mRNA levels of GAD65 and GLT-1 in IL of GLT-1 knockdown mice, reaching the basal values of control mice.

CONCLUSIONS

The glutamatergic neuronal hyperactivity and deficits in the GABA system resulting from siRNA-induced astroglial glutamate transporter knockdown in IL can compromise the integrity/plasticity of neurocircuits affected in MDD. Suitable depressive-like animal models to address the neurobiological changes in MDD are an unmet need and the development of the GLAST/GLT-1 knockdown mouse model may represent a better option to understand the rapid-acting antidepressant effects of ketamine.

Smo-Shh Agonist Purmorphamine Prevents Neurobehavioral and Neurochemical Defects in 8-OH-DPAT-Induced Experimental Model of Obsessive-Compulsive Disorder

Obsessive-compulsive disorder is a mental disorder characterized by repetitive, unwanted thoughts and behavior due to abnormal neuronal corticostriatal-thalamocortical pathway and other neurochemical changes. Purmorphamine is a smoothened-sonic-hedgehog agonist that has a protective effect against many neurological diseases due to its role in maintaining functional connectivity during CNS development and its anti-inflammatory and antioxidant properties. As part of our current research, we investigated the neuroprotective effects of PUR against behavioral and neurochemical changes in 8-hydroxy-2-(di-n-propylamino)-tetralin-induced obsessive-compulsive disorder in rats. Additionally, the effect of PUR was compared with the standard drug for OCD, i.e., fluvoxamine. The intra-dorsal raphe-nucleus injection of 8-OH-DPAT in rats for seven days significantly showed OCD-like repetitive and compulsive behavior along with increased oxidative stress, inflammation, apoptosis, as well as neurotransmitter imbalance. These alterations were dose-dependently attenuated by long-term purmorphamine treatment at 5 mg/kg and 10 mg/kg i.p. In this study, we assessed the level of various neurochemical parameters in different biological samples, including brain homogenate, blood plasma, and CSF, to check the drug’s effect centrally and peripherally. These effects were comparable to the standard oral treatment withfluvoxamine at 10 mg/kg. However, when fluvoxamine was given in combination with purmorphamine, there was a more significant restoration of these alterations than the individualtreatmentswithfluvoxamine and purmorphamine. All the above findings demonstrate that the neuroprotective effect of purmorphamine in OCD can be strong evidence for developing a new therapeutic target for treating and managing OCD.

Development of prefrontal cortex

During evolution, the cerebral cortex advances by increasing in surface and the introduction of new cytoarchitectonic areas among which the prefrontal cortex (PFC) is considered to be the substrate of highest cognitive functions. Although neurons of the PFC are generated before birth, the differentiation of its neurons and development of synaptic connections in humans extend to the 3rd decade of life. During this period, synapses as well as neurotransmitter systems including their receptors and transporters, are initially overproduced followed by selective elimination. Advanced methods applied to human and animal models, enable investigation of the cellular mechanisms and role of specific genes, non-coding regulatory elements and signaling molecules in control of prefrontal neuronal production and phenotypic fate, as well as neuronal migration to establish layering of the PFC. Likewise, various genetic approaches in combination with functional assays and immunohistochemical and imaging methods reveal roles of neurotransmitter systems during maturation of the PFC. Disruption, or even a slight slowing of the rate of neuronal production, migration and synaptogenesis by genetic or environmental factors, can induce gross as well as subtle changes that eventually can lead to cognitive impairment. An understanding of the development and evolution of the PFC provide insight into the pathogenesis and treatment of congenital neuropsychiatric diseases as well as idiopathic developmental disorders that cause intellectual disabilities.

OUP accepted manuscript

The medial prefrontal cortex (mPFC) is thought to be central for flexible behavioral adaptation. However, the causal relationship between mPFC activity and this behavior is incompletely understood. We investigated whether transcranial direct current stimulation (tDCS) over the mPFC alters flexible behavioral adaptation during reward-based decision-making, targeting Montreal Neurological Institute (MNI) coordinates X = −8, Y = 62, Z = 12, which has previously been associated with impaired behavioral adaptation in alcohol-dependent patients. Healthy human participants (n = 61) received either anodal (n = 30) or cathodal (n = 31) tDCS versus sham tDCS while performing a reversal learning task. To assess the mechanisms of reinforcement learning (RL) underlying our behavioral observations, we applied computational models that varied with respect to the updating of the unchosen choice option. We observed that anodal stimulation over the mPFC induced increased choice switching after punishments compared with sham stimulation, whereas cathodal stimulation showed no effect on participants’ behavior compared with sham stimulation. RL revealed increased updating of the unchosen choice option under anodal as compared with sham stimulation, which accounted well for the increased tendency to switch after punishments. Our findings provide a potential model for tDCS interventions in conditions related to flexible behavioral adaptation, such as addiction.

Event-based control of autonomic and emotional states by the anterior cingulate cortex

Despite being an intensive area of research, the function of the anterior cingulate cortex (ACC) remains somewhat of a mystery. Human imaging studies implicate the ACC in various cognitive functions, yet surgical ACC lesions used to treat emotional disorders have minimal lasting effects on cognition. An alternative view is that ACC regulates autonomic states, consistent with its interconnectivity with autonomic control regions and that stimulation evokes changes in autonomic/emotional states. At the cellular level, ACC neurons are highly multi-modal and promiscuous, and can represent a staggering array of task events. These neurons nevertheless combine to produce highly event-specific ensemble patterns that likely alter activity in downstream regions controlling emotional and autonomic tone. Since neuromodulators regulate the strength of the ensemble activity patterns, they would regulate the impact these patterns have on downstream targets. Through these mechanisms, the ACC may determine how strongly to react to the very events its ensembles represent. Pathologies arise when specific event-related representations gain excessive control over autonomic/emotional states.

Post-Stroke Depressive Symptoms: Varying Responses to Escitalopram by Individual Symptoms and Lesion Location

OBJECTIVE

The efficacy of antidepressants in post-stroke depressive symptoms (PSD) varies. We aimed to examine whether the effect of escitalopram on PSD differs according to individual depressive symptoms and stroke lesion location.

METHODS

This is a post hoc analysis of EMOTION (ClinicalTrials.gov, NCT01278498), a randomized, placebo-controlled, double-blind trial that examined the efficacy of escitalopram on depression in acute stroke patients (237 with placebo, 241 with escitalopram). Depressive symptoms were evaluated with the 10-item Montgomery-Åsberg Depression Rating Scale (MADRS). Changes in MADRS and individual item scores at 12 weeks were compared between the treatment groups and among the stroke lesion location groups. Stroke lesion locations were grouped according to the anatomical distribution of serotonin fibers that originate from the midbrain/pons and spread to the forebrain via subcortical structures: "Midbrain-Pons," "Frontal-Subcortical," and "Others." Least-squares means were calculated to demonstrate the independent effect of lesion location.

RESULTS

Total MADRS scores decreased more significantly in the escitalopram than in the placebo group, while a significant effect of escitalopram was observed in only 3 items: apparent sadness, reported sadness, pessimistic thoughts. In the lesion location analyses, escitalopram users in the Frontal-Subcortical group showed significant improvement in total MADRS scores (placebo [n = 130] vs. escitalopram [n = 148], least-square mean [95% CI]: -2.3 [-3.5 to -0.2] vs. -4.5 [-5.5 to -3.4], p = .005), while those in the Midbrain-Pons and Others groups did not.

CONCLUSIONS

The effect of escitalopram on PSD may be more prominent in patients with particular depressive symptoms and stroke lesion locations, suggesting the need for tailored treatment strategies.

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.

Oligonucleotides as therapeutic tools for brain disorders: Focus on major depressive disorder and Parkinson's disease

Remarkable advances in understanding the role of RNA in health and disease have expanded considerably in the last decade. RNA is becoming an increasingly important target for therapeutic intervention; therefore, it is critical to develop strategies for therapeutic modulation of RNA function. Oligonucleotides, including antisense oligonucleotide (ASO), small interfering RNA (siRNA), microRNA mimic (miRNA), and anti-microRNA (antagomir) are perhaps the most direct therapeutic strategies for addressing RNA. Among other mechanisms, most oligonucleotide designs involve the formation of a hybrid with RNA that promotes its degradation by activation of endogenous enzymes such as RNase-H (e.g., ASO) or the RISC complex (e.g. RNA interference - RNAi for siRNA and miRNA). However, the use of oligonucleotides for the treatment of brain disorders is seriously compromised by two main limitations: i) how to deliver oligonucleotides to the brain compartment, avoiding the action of peripheral RNAses? and once there, ii) how to target specific neuronal populations? We review the main molecular pathways in major depressive disorder (MDD) and Parkinson's disease (PD), and discuss the challenges associated with the development of novel oligonucleotide therapeutics. We pay special attention to the use of conjugated ligand-oligonucleotide approach in which the oligonucleotide sequence is covalently bound to monoamine transporter inhibitors (e.g. sertraline, reboxetine, indatraline). This strategy allows their selective accumulation in the monoamine neurons of mice and monkeys after their intranasal or intracerebroventricular administration, evoking preclinical changes predictive of a clinical therapeutic action after knocking-down disease-related genes. In addition, recent advances in oligonucleotide therapeutic clinical trials are also reviewed.

Medial prefrontal cortical control of reward- and aversion-based behavioral output: Bottom-up modulation

How does the brain guide our actions? This is a complex issue, where the medial prefrontal cortex (mPFC) plays a crucial role. The mPFC is essential for cognitive flexibility and decision making. These functions are related to reward- and aversion-based learning, which ultimately drive behavior. Though, cortical projections and modulatory systems that may regulate those processes in the mPFC are less understood. How does the mPFC regulate approach-avoidance behavior in the case of conflicting aversive and appetitive stimuli? This is likely dependent on the bottom-up neuromodulation of the mPFC projection neurons. In this review, we integrate behavioral-, pharmacological-, and viral-based circuit manipulation data showing the involvement of mPFC dopaminergic, noradrenergic, cholinergic, and serotoninergic inputs in reward and aversion processing. Given that an incorrect balance of reward and aversion value could be a key problem in mental diseases such as substance use disorders, we discuss outstanding questions for future research on the role of mPFC modulation in reward and aversion.

Effect of chronic unpredictable mild stress on the expression profile of serotonin receptors in rats and mice: a meta-analysis

Chronic-stress-induced depression is recognized as a widespread public health concern. Selective serotonin reuptake inhibitors (SSRIs) have been the most common treatment for this illness. However, the role of 5-hydroxytryptamine (5-HT) receptor subtypes in stress-induced depression remains unclear. Evidence from Animal studies has reported a variety of results regarding the effects of chronic unpredictable mild stress (CUMS) on serotonin signaling pathways and 5-HT receptor subtypes. This divergence may rely on differences in protocols, methods, and studied pathways. Thus, the aim of this systematic review was to weigh the currently available findings regarding serotonin receptor changes in animal models of CUMS. Overall, our meta-analysis results showed the association of altered expression of 5-HT_1A receptors in the frontal cortex and 5-HT_2A receptors both in the whole cortex and the hypothalamus of rats following CUMS. Moreover, by using a qualitative-structured analysis and the application of risk-of-bias tools, we identified possible sources of data variation between the studied literature, which should be taken into account in future animal studies of chronic-stress induced depression.

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.

Serotonergic neurons in the treatment of mood disorders: The dialogue with astrocytes

Astrocytes were traditionally regarded as cells important to neuronal activity, providing both metabolic and structural supports. Recent evidence suggests that they may also play a crucial role in the control of higher brain functions. In keeping with this hypothesis, it is now well accepted that astrocytes contribute to stress but also react to antidepressant drugs as they express serotonergic transporters and receptors. However, the downstream mechanisms leading to the fine-tuned regulation of mood are still unknown. This chapter pays attention to the role of astrocytes in the regulation of emotional behavior and related serotonergic neurotransmission. In particular, it gives a current state of the clinical and preclinical evidence showing that astrocytes respond to environmental conditions and antidepressant drugs through the release of gliotransmitters and neurotrophic factors which in turn, influence serotonergic tone in discrete brain areas. This state-of-the-art review aims at demonstrating the remarkable potential for novel therapeutic antidepressant strategies targeting these glial cells.

The anterior cingulate cortex and event-based modulation of autonomic states

In spite of being an intensive area of research focus, the anterior cingulate cortex (ACC) remains somewhat of an enigma. Many theories have focused on its role in various aspects of cognition yet surgically precise lesions of the ACC, used to treat severe emotional disorders in human patients, typically have no lasting effects on cognition. An alternative view is that the ACC has a prominent role in regulating autonomic states. This view is consistent with anatomical data showing that a main target of the ACC are regions involved in autonomic control and with the observation that stimulation of the ACC evokes changes in autonomic states in both animals and humans. From an electrophysiological perspective, ACC neurons appear able to represent virtually any event or internal state, even though there is not always a strong link between these representations and behavior. Ensembles of neurons form robust contextual representations that strongly influence how specific events are encoded. The activity patterns associated with these contextually-based event representations presumably impact activity in downstream regions that control autonomic state. As a result, the ACC may regulate the autonomic and perhaps emotional reactions to events it is representing. This event-based control of autonomic tone by the ACC would likely arise during all types of cognitive and affective processes, without necessarily being critical for any of them.

Implication of 5-HT7 receptor in prefrontal circuit assembly and detrimental emotional effects of SSRIs during development

Altered development of prefrontal cortex (PFC) circuits can have long-term consequences on adult emotional behavior. Changes in serotonin homeostasis during critical periods produced by genetic or pharmacological inactivation of the serotonin transporter (SERT, or Slc6a4), have been involved in such developmental effects. In mice, selective serotonin reuptake inhibitors (SSRIs), administered during postnatal development cause exuberant synaptic connectivity of the PFC to brainstem dorsal raphe nucleus (DRN) circuits, and increase adult risk for developing anxiety and depressive symptoms. SERT is transiently expressed in the glutamate neurons of the mouse PFC, that project to the DRN. Here, we find that 5-HTR7 is transiently co-expressed with SERT by PFC neurons, and it plays a key role in the maturation of PFC-to-DRN synaptic circuits during early postnatal life. 5-HTR7-KO mice show reduced PFC-to-DRN synaptic density (as measured by array-tomography and VGLUT1/synapsin immunocytochemistry). Conversely, 5-HTR7 over-expression in the developing PFC increased PFC-to-DRN synaptic density. Long-term consequences on depressive-like and anxiogenic behaviors were observed in adults. 5-HTR7 over-expression in the developing PFC, results in depressive-like symptoms in adulthood. Importantly, the long-term depressive-like and anxiogenic effects of SSRIs (postnatal administration of fluoxetine from P2 to P14) were not observed in 5-HTR7-KO mice, and were prevented by co-administration of the selective inhibitor of 5-HTR7, SB269970. This study identifies a new role 5-HTR7 in the postnatal maturation of prefrontal descending circuits. Furthermore, it shows that 5-HTR7 in the PFC is crucially required for the detrimental emotional effects caused by SSRI exposure during early postnatal life.

Effects of water restriction on social behavior and 5-HT neurons density in the dorsal and median raphe nuclei in mice

We explored here the hypothesis that temporary chronic water restriction in mice affects social behavior, via its action on the density of 5-HT neurons in dorsal and median raphe nuclei (DRN and MRN). For that, we submitted adult C57BL/6 J mice to mild and controlled temporary dehydration, i.e., 6 h of water access every 48 h for 15 days. We investigated their social behavior in a social interaction task known to allow free and reciprocal social contact. Results showed that temporary dehydration increases significantly time spent in social contact and social dominance. It also expands 5-HT neuron density within both DRN and MRN and the behavioral and neuronal plasticity were positively correlated. Our findings suggest that disturbance in 5-HT neurotransmission caused by temporary dehydration stress unbalances choice processes of animals in social context.

A Decade of Progress in Deep Brain Stimulation of the Subcallosal Cingulate for the Treatment of Depression

Major depression contributes significantly to the global disability burden. Since the first clinical study of deep brain stimulation (DBS), over 446 patients with depression have now undergone this neuromodulation therapy, and 29 animal studies have investigated the efficacy of subgenual cingulate DBS for depression. In this review, we aim to provide a comprehensive overview of the progress of DBS of the subcallosal cingulate in humans and the medial prefrontal cortex, its rodent homolog. For preclinical animal studies, we discuss the various antidepressant-like behaviors induced by medial prefrontal cortex DBS and examine the possible mechanisms including neuroplasticity-dependent/independent cellular and molecular changes. Interestingly, the response rate of subcallosal cingulate Deep brain stimulation marks a milestone in the treatment of depression. DBS achieved response and remission rates of 64–76% and 37–63%, respectively, from clinical studies monitoring patients from 6–24 months. Although some studies showed its stimulation efficacy was limited, it still holds great promise as a therapy for patients with treatment-resistant depression. Overall, further research is still needed, including more credible clinical research, preclinical mechanistic studies, precise selection of patients, and customized electrical stimulation paradigms.

Psilocybin and LSD have no long-lasting effects in an animal model of alcohol relapse

For most psychiatric disorders, including alcohol use disorder (AUD), approved pharmacological treatments are limited in their effectiveness, and new drugs that can easily be translated into the clinic are needed. Currently, great hope lies in the potential of psychedelics to effectively treat AUD. The primary hypothesis is that a single session of psychedelic-guided psychotherapy can restore normal brain function in AUD individuals and thereby reduce the risk of relapse in the long run. Here we applied three different treatment schedules with psilocybin/LSD in order to investigate relapse-like drinking in the alcohol deprivation effect (ADE) model. In contrast to the primary hypothesis, psychedelics had no long-lasting effects on the ADE in male and female rats, neither when administered in a high dosage regime that is comparable to the one used in clinical studies, nor in a chronic microdosing scheme. Only sub-chronic treatment with psilocybin produced a short-lasting anti-relapse effect. However, it is not a translatable treatment option to give psychedelics sub-chronically for relapse prevention. In conclusion, our results in the ADE model do not support the hypothesis that microdosing or high doses of psychedelic reduce relapse behavior. This conclusion has to be confirmed by applying other animal models of AUD. It could also well be that animal models of AUD might be unable to fully capture the therapeutic potential of psychedelic drugs and that only future large-scale clinical trials will be able to demonstrate the efficacy of psychedelics as a new treatment option for AUD.

A single-cell transcriptomic and anatomic atlas of mouse dorsal raphe Pet1 neurons

Among the brainstem raphe nuclei, the dorsal raphe nucleus (DR) contains the greatest number of Pet1-lineage neurons, a predominantly serotonergic group distributed throughout DR subdomains. These neurons collectively regulate diverse physiology and behavior and are often therapeutically targeted to treat affective disorders. Characterizing Pet1 neuron molecular heterogeneity and relating it to anatomy is vital for understanding DR functional organization, with potential to inform therapeutic separability. Here we use high-throughput and DR subdomain-targeted single-cell transcriptomics and intersectional genetic tools to map molecular and anatomical diversity of DR-Pet1 neurons. We describe up to fourteen neuron subtypes, many showing biased cell body distributions across the DR. We further show that P2ry1-Pet1 DR neurons - the most molecularly distinct subtype - possess unique efferent projections and electrophysiological properties. These data complement and extend previous DR characterizations, combining intersectional genetics with multiple transcriptomic modalities to achieve fine-scale molecular and anatomic identification of Pet1 neuron subtypes.

Depressive-like state sensitizes 5-HT1A and 5-HT1B auto-receptors in the dorsal raphe nucleus sub-system

Dorsal raphe (DR) and median raphe (MR) 5-HT neurons are two distinct sub-systems known to be regulated by 5-HT_1A and 5-HT_1B auto-receptors. Whether the auto-receptors in each sub-system are functionally altered in depressive-like state remains unknown. The present study is aimed to study a specific circuit (DR-ventral hippocampus and MR-dorsal hippocampus) within each sub-system to investigate changes in receptor sensitivity in the pathogenesis of depression. A mouse model of depression was developed through the social defeat paradigm, and was then treated with fluoxetine (FLX). 5-HT_1A auto-receptor in the neuronal cell body (DR or MR) and 5-HT_1B auto-receptor in the axonal terminal (ventral or dorsal hippocampus) were directly targeted by local perfusion of antagonists (5-HT_1A: WAY100635; 5-HT_1B: GR127935) through reverse microdialysis. Time courses of dialysate 5-HT measured at the axonal terminal were subsequently determined for each circuit. At baseline, 5-HT_1A and 5-HT_1B antagonists dose-dependently increased dialysate 5-HT, with sub-circuit specificity. In the depressive-like state, greater increases in dialysate 5-HT were observed only in the DR-ventral hippocampus circuit following local delivery of both antagonists, which were then fully restored following the FLX treatment. In contrast, no changes were observed in the MR-dorsal hippocampus circuit. Our results demonstrate differential changes in sensitivities of 5-HT_1A and 5-HT_1B auto-receptors in the DR-ventral hippocampus and MR-dorsal hippocampus circuits. 5-HT_1A and 5-HT_1B auto-receptors in the DR-ventral hippocampus circuit are sensitized in the depressive-like state. Taken together, these results suggest that the DR sub-system maybe the neural substrate mediating depressive phenotypes.

Functional differentiation in the human ventromedial frontal lobe: A data-driven parcellation

Ventromedial regions of the frontal lobe (vmFL) are thought to play a key role in decision-making and emotional regulation. However, aspects of this area's functional organization, including the presence of a multiple subregions, their functional and anatomical connectivity, and the cross-species homologies of these subregions with those of other species, remain poorly understood. To address this uncertainty, we employed a two-stage parcellation of the region to identify six distinct structures within the region on the basis of data-driven classification of functional connectivity patterns obtained using the meta-analytic connectivity modeling (MACM) approach. From anterior to posterior, the derived subregions included two lateralized posterior regions, an intermediate posterior region, a dorsal and ventral central region, and a single anterior region. The regions were characterized further by functional connectivity derived using resting-state fMRI and functional decoding using the Brain Map database. In general, the regions could be differentiated on the basis of different patterns of functional connectivity with canonical "default mode network" regions and/or subcortical regions such as the striatum. Together, the findings suggest the presence of functionally distinct neural structures within vmFL, consistent with data from experimental animals as well prior demonstrations of anatomical differences within the region. Detailed correspondence with the anterior cingulate, medial orbitofrontal cortex, and rostroventral prefrontal cortex, as well as specific animal homologs are discussed. The findings may suggest future directions for resolving potential functional and structural correspondence of subregions within the frontal lobe across behavioral contexts, and across mammalian species.