SSRI antidepressants differentially modulate serotonin reuptake and release in Drosophila

Measurement of Real-Time Serotonin Dynamics from Human-Derived Gut Organoids

The importance of the gut in regulating the brain-body immune axis is becoming increasingly evident. Interestingly, the brain and gut share many common signaling molecules, with serotonin being one of the most notable. In fact, the gut is the primary source of serotonin in the body. However, studying serotonin dynamics in a human-specific context remains a challenge. Human stem cell-derived models provide a promising avenue for studying signal transmission in well-controlled, in vitro environments. In this study, we report the first fast-scan cyclic voltammetry (FSCV) measurements of serotonin signaling in a newly developed enterochromaffin cell (ECC)-enriched gut organoid model. First, we characterize the stem cell-derived gut organoids and confirm they are enriched with ECCs, the key cell type responsible for producing and releasing serotonin in the gut. We then optimize an in vitro buffer that maintains cell viability while supporting FSCV measurements. Using this system, we detect spontaneous release events, which increase in frequency and amplitude following stimulation with forskolin (FSK) and 3-isobutyl-1-methylxanthine (IBMX). Finally, we confirm the identity of the signal as serotonin using a selective serotonin reuptake inhibitor (SSRI), which significantly delayed the reuptake profile. Our study introduces the first real-time measurement of serotonin signaling in a human-derived gut model. We believe this system will be essential for future research on serotonin's role in the gut and for potential novel drug target identification.

Coding principles and mechanisms of serotonergic transmission modes

Serotonin-mediated intercellular communication has been implicated in myriad human behaviors and diseases, yet how serotonin communicates and how the communication is regulated remain unclear due to limitations of available monitoring tools. Here, we report a method multiplexing genetically encoded sensor-based imaging and fast-scan cyclic voltammetry, enabling simultaneous recordings of synaptic, perisynaptic, proximate and distal extrasynaptic serotonergic transmission. Employing this method alongside a genetically encoded sensor-based image analysis program (GESIAP), we discovered that heterogeneous firing patterns of serotonergic neurons create various transmission modes in the mouse raphe nucleus and amygdala, encoding information of firing pulse frequency, number, and synchrony using neurotransmitter quantity, releasing synapse count, and synaptic and/or volume transmission. During tonic and low-frequency phasic activities, serotonin is confined within synaptic clefts due to efficient retrieval by perisynaptic transporters, mediating synaptic transmission modes. Conversely, during high-frequency, especially synchronized phasic activities, or when transporter inhibition, serotonin may surpass transporter capacity, and escape synaptic clefts through 1‒3 outlet channels, leading to volume transmission modes. Our results elucidate a mechanism of how channeled synaptic enclosures, synaptic properties, and transporters collaborate to define the coding principles of activity pattern-dependent serotonergic transmission modes.

Effects of serotonergic manipulation in the brainstem and hypothalamus of overnourished rats during lactation

Previous studies suggest that being overweight and obese is capable of altering brain function during critical periods due to the higher plasticity of the brain during development. However, more literature still needs to be on the immediate effect of overnutrition and serotonin modulation in brain areas. In this study, we aimed to evaluate the effects of serotoninergic manipulation on oxidative stress and pro-inflammatory markers in the brainstem and hypothalamus of overnourished rats. The fluoxetine treatment was performed from postnatal day 3 (PND3) to postnatal day 21 (PND21) to evaluate mitochondrial function, oxidative balance, and the mRNA levels of monoaminergic molecules such as serotonin and dopamine, pro-inflammatory cytokines, and BDNF. Neonatal overnutrition induces molecular and biochemical changes in the brainstem and hypothalamus. These nutritional disturbances during lactation dysregulate energy balance and cellular redox, inducing oxidative stress. Conversely, modulation of the serotoninergic system through pharmacological use of fluoxetine was able to reverse the deleterious effects of overnutrition during lactation, enabling better brain development and delaying the development of pathologies and oxidative stress.

Serotonin acts through multiple cellular targets during an olfactory critical period

Serotonin (5-HT) modulates early development during critical periods when experience drives heightened levels of plasticity in neurons. Here, we investigate the cellular mechanisms by which 5-HT modulates critical period plasticity (CPP) in the olfactory system of Drosophila. We first demonstrate that 5-HT is necessary for experience-dependent structural plasticity in response to chronic CO2 exposure and can re-open the critical period long after it normally closes. Knocking down 5-HT7 receptors in a subset of GABAergic local interneurons was sufficient to block CPP, as was knocking down GABA receptors expressed by CO2-sensing olfactory sensory neurons (OSNs). Furthermore, direct modulation of OSNs via 5-HT2B receptors in CO2-sensing OSNs and autoreceptor expression by serotonergic neurons was also required for CPP. Thus, 5-HT targets individual neuron types in the olfactory system via distinct receptors to enable sensory driven plasticity.

Serotonin in depression and Alzheimer's disease: Focus on SSRI's beneficial effects

Depression is a complex and pervasive mental health disorder affecting millions globally. Serotonin, a critical neurotransmitter, plays a central role in the pathophysiology of depression. This review explores serotonin's multifaceted role in depression, focusing on its synthesis, bioavailability, receptor interactions, and the impact of various factors, including diet, stress, and gender differences. This review aims to provide a comprehensive understanding of serotonin's role in depression by examining its synthesis and structure, its bioavailability and dietary influences, and its interactions with stress and immune responses. Additionally, it investigates the influence of age, socioeconomic status, and gender on depression, and integrates findings from animal research to elucidate serotonin's impact on mood disorders and cognitive decline. A literature review was conducted using PubMed, Google Scholar, and Embase databases. Key focus areas included serotonin synthesis and receptor interactions, dietary effects on serotonin bioavailability, and the relationship between serotonin, immune responses, and stress. Gender differences, age-related factors, and socioeconomic influences on depression were also examined. Studies were thematically categorized and analyzed to provide a cohesive overview. Our review highlights that serotonin synthesis involves a complex enzymatic process, with recent structural studies revealing intricate receptor interactions. Dietary factors significantly impact serotonin levels, with interventions potentially modulating mood disorders. Stress and immune responses are linked to serotonin dynamics, with chronic stress exacerbating mood disorders and influencing cognitive decline. Animal studies underscore serotonin's role in mood regulation and cognitive function, while human research reveals how age, gender, and socioeconomic factors affect depression. The findings emphasize the need for a multidimensional approach to understanding and treating depression. Various factors, including diet, stress, and immune responses, influence serotonin's role in mood disorders. The review suggests potential therapeutic pathways involving dietary interventions and stress management. Furthermore, gender-specific considerations and the impact of age and socioeconomic status on depression outcomes highlight the need for tailored treatment strategies.

SSRI use during acute COVID-19 and risk of long COVID among patients with depression

BACKGROUND

Long COVID, also known as post-acute sequelae of COVID-19 (PASC), is a poorly understood condition with symptoms across a range of biological domains that often have debilitating consequences. Some have recently suggested that lingering SARS-CoV-2 virus particles in the gut may impede serotonin production and that low serotonin may drive many Long COVID symptoms across a range of biological systems. Therefore, selective serotonin reuptake inhibitors (SSRIs), which increase synaptic serotonin availability, may be used to prevent or treat Long COVID. SSRIs are commonly prescribed for depression, therefore restricting a study sample to only include patients with depression can reduce the concern of confounding by indication.

METHODS

In an observational sample of electronic health records from patients in the National COVID Cohort Collaborative (N3C) with a COVID-19 diagnosis between September 1, 2021, and December 1, 2022, and a comorbid depressive disorder, the leading indication for SSRI use, we evaluated the relationship between SSRI use during acute COVID-19 and subsequent 12-month risk of Long COVID (defined by ICD-10 code U09.9). We defined SSRI use as a prescription for SSRI medication beginning at least 30 days before acute COVID-19 and not ending before SARS-CoV-2 infection. To minimize bias, we estimated relationships using nonparametric targeted maximum likelihood estimation to aggressively adjust for high-dimensional covariates.

RESULTS

We analyzed a sample (n = 302,626) of patients with a diagnosis of a depressive condition before COVID-19 diagnosis, where 100,803 (33%) were using an SSRI. We found that SSRI users had a significantly lower risk of Long COVID compared to nonusers (adjusted causal relative risk 0.92, 95% CI (0.86, 0.99)) and we found a similar relationship comparing new SSRI users (first SSRI prescription 1 to 4 months before acute COVID-19 with no prior history of SSRI use) to nonusers (adjusted causal relative risk 0.89, 95% CI (0.80, 0.98)).

CONCLUSIONS

These findings suggest that SSRI use during acute COVID-19 may be protective against Long COVID, supporting the hypothesis that serotonin may be a key mechanistic biomarker of Long COVID.

Effect of depression and the antidepressant fluoxetine on osseointegration-A pre-clinical in vivo experimental study

OBJECTIVE

The aim of this study was to explore the effect of depression and selective serotonin reuptake inhibitors on implant osseointegration and bone healing.

METHODS

Forty-eight 6- to 8-week-old SPF Sprague-Dawley male rats were randomly divided into four groups: the Control group, the Fluoxetine group, the Depression group and the De&Flu group. The rats in the Depression group and the De&Flu group were subjected to a depression modelling process, and the rats in the Control group and the Fluoxetine group were raised normally. Then, a titanium implant was placed in the right tibia of each rat. In the Fluoxetine group and De&Flu group, fluoxetine was injected subcutaneously daily, while subcutaneously injecting physiological saline in the Control group and Depression group. Collecting serum from the rats used for ELISA. The surgical area was cut for microcomputed tomography and histology observation.

RESULTS

After 12 weeks, bone mineral density was lower in the De&Flu group than in the Control group, Depression group and Fluoxetine group. Bone mineral density was also lower in the Depression group and the Fluoxetine group than in the Control group. The percentage of bone-implant contact (BIC%) in De&Flu rats was lower than in the Control, Depression and Fluoxetine groups. The BIC% in the Depression group and the Fluoxetine group was lower than in the Control group.

CONCLUSIONS

Depression and fluoxetine negatively affect bone density and implant osseointegration independently, and this damaging effect is exacerbated when both factors are present. The mechanism may be related to the dysregulation of the hypothalamic-pituitary-adrenal axis and inflammation in the body.

Microdosing ketamine in Drosophila does not block serotonin reuptake, but causes complex behavioral changes mediated by glutamate and serotonin receptors

Microdosing ketamine is a novel antidepressant for treatment-resistant depression. Traditional antidepressants, like selective serotonin reuptake inhibitors (SSRIs), inhibit serotonin reuptake, but it is not clear if ketamine shows a similar mechanism. Here, we tested the effects of feeding ketamine and SSRIs to Drosophila melanogaster larvae, which has a similar serotonin system to mammals and is a good model to track depressive behaviors, such as locomotion and feeding. Fast-scan cyclic voltammetry (FSCV) was used to measure optogenetically stimulated serotonin changes, and locomotion tracking software and blue dye feeding to monitor behavior. We fed larvae various doses (1-100 mM) of antidepressants for 24 h and found that 1 mM ketamine did not affect serotonin, but increased locomotion and feeding. Low doses (≤10 mM) of escitalopram and fluoxetine inhibited dSERT and also increased feeding and locomotion behaviors. At 100 mM, ketamine inhibited dSERT and increased serotonin concentrations, but decreased locomotion and feeding because of its anesthetic properties. Since microdosing ketamine causes behavioral effects, we further investigated behavioral changes with a SERT16 mutant and low doses of other NMDA receptor antagonists and 5-HT1A and 2 agonists. Feeding and locomotion changes were similar to ketamine in the mutant, and we found NMDA receptor antagonism increased feeding, while serotonin receptor agonism increased locomotion, which could explain these effects with ketamine. Ultimately, this work shows that Drosophila is a good model to discern antidepressant mechanisms, and that ketamine does not work on dSERT like SSRIs, but effects behavior with other mechanisms that should be investigated further.

Electrochemical and biosensor techniques to monitor neurotransmitter changes with depression

Depression is a common mental illness. However, its current treatments, like selective serotonin reuptake inhibitors (SSRIs) and micro-dosing ketamine, are extremely variable between patients and not well understood. Three neurotransmitters: serotonin, histamine, and glutamate, have been proposed to be key mediators of depression. This review focuses on analytical methods to quantify these neurotransmitters to better understand neurological mechanisms of depression and how they are altered during treatment. To quantitatively measure serotonin and histamine, electrochemical techniques such as chronoamperometry and fast-scan cyclic voltammetry (FSCV) have been improved to study how specific molecular targets, like transporters and receptors, change with antidepressants and inflammation. Specifically, these studies show that different SSRIs have unique effects on serotonin reuptake and release. Histamine is normally elevated during stress, and a new inflammation hypothesis of depression links histamine and cytokine release. Electrochemical measurements revealed that stress increases histamine, decreases serotonin, and leads to changes in cytokines, like interleukin-6. Biosensors can also measure non-electroactive neurotransmitters, including glutamate and cytokines. In particular, new genetic sensors have shown how glutamate changes with chronic stress, as well as with ketamine treatment. These techniques have been used to characterize how ketamine changes glutamate and serotonin, and to understand how it is different from SSRIs. This review briefly outlines how these electrochemical techniques work, but primarily highlights how they have been used to understand the mechanisms of depression. Future studies should explore multiplexing techniques and personalized medicine using biomarkers in order to investigate multi-analyte changes to antidepressants.

Microdosing ketamine in Drosophila does not inhibit SERT like SSRIs, but causes behavioral changes mediated by glutamate and serotonin receptors

Recently, the FDA approved microdosing ketamine for treatment resistant depression. Traditional antidepressants, like serotonin selective reuptake inhibitors (SSRIs), block serotonin reuptake, but it is not clear if ketamine blocks serotonin reuptake. Here, we tested the effects of feeding ketamine and SSRIs to Drosophila melanogaster larvae, which has a similar serotonin system to mammals, and is a good model to track depression behaviors, such as locomotion and feeding. Fast-scan cyclic voltammetry (FSCV) was used to measure optogenetically-stimulated serotonin changes, and locomotion tracking software and blue dye feeding to monitor behavior. We fed larvae various doses (1-100 mM) of antidepressants for 24 hours and found that 1 mM ketamine did not affect serotonin, but increased locomotion and feeding. Low doses (≤ 10 mM) of escitalopram and fluoxetine inhibited dSERT and also increased feeding and locomotion behaviors. At 100 mM, ketamine inhibited dSERT and increased serotonin concentrations, but decreased locomotion and feeding due to its anesthetic properties. Since microdosing ketamine causes behavioral effects, we also investigated behavior changes with low doses of other NMDA receptor antagonists and 5-HT1A and 2 agonists, which are other possible sites for ketamine action. NMDA receptor antagonism increased feeding, while serotonin receptor agonism increased locomotion, which could explain these effects with ketamine. Ultimately, this work shows that Drosophila is a good model to discern antidepressant mechanisms, and that ketamine does not work on dSERT like SSRIs at microdoses, but affects behavior with other mechanisms.