[Research progress on the neural mechanism of the regulation of social isolation on innate behaviors].

Innate behavior is mainly controlled by genetics, but is also regulated by social experiences such as social isolation. Studies in animal models such as Drosophila and mice have found that social isolation can regulate innate behaviors through the changes at the molecular level, such as hormone, neurotransmitter, neuropeptide level, and at the level of neural circuits. In this review, we summarized the research progress on the regulation of social isolation on various animal innate behaviors, such as sleep, reproduction and aggression by altering the expression of conserved neuropeptides and neurotransmitters, hoping to deepen the understanding of the key and conserved signal pathways that regulate innate behavior by social isolation.

Adverse childhood experiences and mental health disorder in China: A nationwide study from CHARLS.

OBJECTIVES: Mental health disorder is highly prevalent worldwide, and factors attributable to its development have not been fully understood. We aimed to explore the association of adverse childhood experiences (ACEs) with mental health disorder using data from the China Health and Retirement Longitudinal Study (CHARLS). METHODS: CHARLS is an ongoing nationwide survey enrolling 17,708 residents aged ≥45 years in China. Total 20 ACE indicators were extracted. Unmeasured confounding and potential mediation were assessed, and effect-size estimates are expressed as odds ratio (OR) with 95 % confidence interval (CI). RESULTS: Of 10,961 assessable participants with complete information, 3652 were reported to have mental health disorder. The risk for mental health disorder was increased by 20.7 % for participants with one to less than four events of ACEs (OR: 1.207, 95 % CI: 0.711 to 2.049), and this number reached 102.7 % for participants with four or more events (OR: 2.027, 95 % CI: 1.196 to 3.436) relative to those with no ACE. In subgroup analyses, between-group risk estimates differed significantly for residence, marital status, personal asset, life satisfaction, and living standard. The E-value of 2.35 for ACEs of 4 or more events indicated the low likelihood of unmeasured confounders. In mediation analyses, the proportion eliminated was 5.1 % for continuous ACEs and 6.1 % for categorical ACEs (both P < 0.001). DISCUSSION: Our findings indicate that ACEs, especially with four or more events, can predict the significant risk of mental health disorder, underscoring the necessity of screening mental illness based on ACEs to reduce the resultant morbidity and mortality in China.

A neural pathway for social modulation of spontaneous locomotor activity (SoMo-SLA) in Drosophila.

Social enrichment or social isolation affects a range of innate behaviors, such as sex, aggression, and sleep, but whether there is a shared mechanism is not clear. Here, we report a neural mechanism underlying social modulation of spontaneous locomotor activity (SoMo-SLA), an internal-driven behavior indicative of internal states. We find that social enrichment specifically reduces spontaneous locomotor activity in male flies. We identify neuropeptides Diuretic hormone 44 (DH44) and Tachykinin (TK) to be up- and down-regulated by social enrichment and necessary for SoMo-SLA. We further demonstrate a sexually dimorphic neural circuit, in which the male-specific P1 neurons encoding internal states form positive feedback with interneurons coexpressing doublesex (dsx) and Tk to promote locomotion, while P1 neurons also form negative feedback with interneurons coexpressing dsx and DH44 to inhibit locomotion. These two opposing neuromodulatory recurrent circuits represent a potentially common mechanism that underlies the social regulation of multiple innate behaviors.

Asexuality in Drosophila juvenile males is organizational and independent of juvenile hormone.

Sexuality is generally prevented in newborns and arises with organizational rewiring of neural circuitry and optimization of fitness for reproduction competition. Recent studies reported that sex circuitry in Drosophila melanogaster is developed in juvenile males but functionally inhibited by juvenile hormone (JH). Here, we find that the fly sex circuitry, mainly expressing the male-specific fruitless (fruM ) and/or doublesex (dsx), is organizationally undeveloped and functionally inoperative in juvenile males. Artificially activating all fruM neurons induces substantial courtship in solitary adult males but not in juvenile males. Synaptic transmissions between major courtship regulators and all dsx neurons are strong in adult males but either weak or undetectable in juvenile males. We further find that JH does not inhibit male courtship in juvenile males but instead promotes courtship robustness in adult males. Our results indicate that the transition to sexuality from juvenile to adult flies requires organizational rewiring of neural circuitry.

3D-printed hydrogel particles containing PRP laden with TDSCs promote tendon repair in a rat model of tendinopathy.

Long-term chronic inflammation after Achilles tendon injury is critical for tendinopathy. Platelet-rich plasma (PRP) injection, which is a common method for treating tendinopathy, has positive effects on tendon repair. In addition, tendon-derived stem cells (TDSCs), which are stem cells located in tendons, play a major role in maintaining tissue homeostasis and postinjury repair. In this study, injectable gelatine methacryloyl (GelMA) microparticles containing PRP laden with TDSCs (PRP-TDSC-GM) were prepared by a projection-based 3D bioprinting technique. Our results showed that PRP-TDSC-GM could promote tendon differentiation in TDSCs and reduce the inflammatory response by downregulating the PI3K-AKT pathway, thus promoting the structural and functional repair of tendons in vivo.

GSTP1-mediated S-glutathionylation of Pik3r1 is a redox hub that inhibits osteoclastogenesis through regulating autophagic flux.

Glutathione S-transferase P1(GSTP1) is known for its transferase and detoxification activity. Based on disease-phenotype genetic associations, we found that GSTP1 might be associated with bone mineral density through Mendelian randomization analysis. Therefore, this study was performed both in vitro cellular and in vivo mouse model to determine how GSTP1 affects bone homeostasis. In our research, GSTP1 was revealed to upregulate the S-glutathionylation level of Pik3r1 through Cys498 and Cys670, thereby decreasing its phosphorylation, further controlling the alteration of autophagic flux via the Pik3r1-AKT-mTOR axis, and lastly altering osteoclast formation in vitro. In addition, knockdown and overexpression of GSTP1 in vivo also altered bone loss outcomes in the OVX mice model. In general, this study identified a new mechanism by which GSTP1 regulates osteoclastogenesis, and it is evident that the cell fate of osteoclasts is controlled by GSTP1-mediated S-glutathionylation via a redox-autophagy cascade.

Therapeutic potential and mechanisms of mesenchymal stem cell-derived exosomes as bioactive materials in tendon-bone healing.

Tendon-bone insertion (TBI) injuries, such as anterior cruciate ligament injury and rotator cuff injury, are the most common soft tissue injuries. In most situations, surgical tendon/ligament reconstruction is necessary for treating such injuries. However, a significant number of cases failed because healing of the enthesis occurs through scar tissue formation rather than the regeneration of transitional tissue. In recent years, the therapeutic potential of mesenchymal stem cells (MSCs) has been well documented in animal and clinical studies, such as chronic paraplegia, non-ischemic heart failure, and osteoarthritis of the knee. MSCs are multipotent stem cells, which have self-renewability and the ability to differentiate into a wide variety of cells such as chondrocytes, osteoblasts, and adipocytes. Numerous studies have suggested that MSCs could promote angiogenesis and cell proliferation, reduce inflammation, and produce a large number of bioactive molecules involved in the repair. These effects are likely mediated by the paracrine mechanisms of MSCs, particularly through the release of exosomes. Exosomes, nano-sized extracellular vesicles (EVs) with a lipid bilayer and a membrane structure, are naturally released by various cell types. They play an essential role in intercellular communication by transferring bioactive lipids, proteins, and nucleic acids, such as mRNAs and miRNAs, between cells to influence the physiological and pathological processes of recipient cells. Exosomes have been shown to facilitate tissue repair and regeneration. Herein, we discuss the prospective applications of MSC-derived exosomes in TBI injuries. We also review the roles of MSC-EVs and the underlying mechanisms of their effects on promoting tendon-bone healing. At last, we discuss the present challenges and future research directions.

Synergistic Effect of Reverse Drilling and Bone Dust on Femoral Tendon-Bone Healing After Anterior Cruciate Ligament Reconstruction in a Rabbit Model.

BACKGROUND: Anterior cruciate ligament (ACL) injuries and bone tunnel enlargement (BTE) after ACL reconstruction (ACLR) remain frequent issues. Bone dust (BD) produced by tunnel preparation with osteogenic ability and reverse drilling (RD), an easy compaction technique, make it accessible to enhance tendon-bone healing in the clinic. HYPOTHESIS: RD and BD synergistically promote tendon-bone healing by improving peritunnel bone and preventing BTE in femurs. STUDY DESIGN: Controlled laboratory study. METHODS: In total, 96 New Zealand White rabbits underwent ACLR. The semitendinosus tendon was freed before medial parapatellar arthrotomy. After the native ACL was transected, bone tunnels were prepared through the footprint of the native ACL. All animals were randomly assigned to 1 of 4 groups according to different tunnel preparation methods: group 1 (irrigation after extraction drilling [ED]; control group), group 2 (irrigation after RD), group 3 (no irrigation after ED), and group 4 (no irrigation after RD). BD was harvested by irrigating tunnels and was characterized by morphology and size. The specimens underwent microarchitectural, histological, and biomechanical evaluations at 4, 8, and 12 weeks postoperatively. RESULTS: Micro-computed tomography demonstrated more peritunnel bone and less BTE in the femurs of group 4 compared with the other groups. Histologically, BD possessed osteogenic activity in bone tunnels postoperatively. Meanwhile, group 4 regenerated a higher amount of the tendon-bone interface and more peritunnel bone than group 1. Biomechanically, group 4 showed higher failure loads and stiffness than group 1. However, peritunnel bone loss, active osteoclasts, and significant BTE were found in the femurs of group 1 and group 3 at 12 weeks postoperatively, while no strong correlation was found between BTE and inflammatory cytokines. Scanning electron microscopy and particle size analysis suggested that BD produced by ED and RD had no difference in size. CONCLUSION: Tendon-bone healing was facilitated by the synergistic effect of RD and BD in femurs. CLINICAL RELEVANCE: This study provides a more accessible and effective surgical strategy to promote tendon-bone healing after ACLR by increasing peritunnel bone and preventing BTE in femurs.

The doublesex gene regulates dimorphic sexual and aggressive behaviors in Drosophila.

Most animal species display dimorphic sexual behaviors and male-biased aggressiveness. Current models have focused on the male-specific product from the fruitless (fruM) gene, which controls male courtship and male-specific aggression patterns in fruit flies, and describe a male-specific mechanism underlying sexually dimorphic behaviors. Here we show that the doublesex (dsx) gene, which expresses male-specific DsxM and female-specific DsxF transcription factors, functions in the nervous system to control both male and female sexual and aggressive behaviors. We find that Dsx is not only required in central brain neurons for male and female sexual behaviors, but also functions in approximately eight pairs of male-specific neurons to promote male aggressiveness and approximately two pairs of female-specific neurons to inhibit female aggressiveness. DsxF knockdown females fight more frequently, even with males. Our findings reveal crucial roles of dsx, which is broadly conserved from worms to humans, in a small number of neurons in both sexes to establish dimorphic sexual and aggressive behaviors.

HemoglobinA1c Is a Risk Factor for Changes of Bone Mineral Density: A Mendelian Randomization Study.

Background: As a valuable blood glucose measurement, HemoglobinA1c (HbA1c) is of great clinical value for diabetes. However, in previous observational studies, studies on its effect on bone mineral density (BMD) have different results. This study aimed to use Mendelian randomization (MR) to assess the effect of HbA1c on bone mineral density and fracture risk, and try to further explore whether this association was achieved through glycemic or non-glycemic factors. Methods: Take HbA1c measurement as exposure, and BMD estimated from quantitative heel ultrasounds (eBMD) and bone fractures as outcomes. Two-Sample MR Analysis was conducted to assess the causal effect of HbA1C on heel BMD and risk fracture. Then, we performed the analysis using two subsets of these variants, one related to glycemic measurement and the other to erythrocyte indices. Results: Genetically increased HbA1C was associated with the lower heel eBMD [odds ratio (OR) 0.91 (95% CI 0.87, 0.96) per %-unit, P = 3 × 10-4(IVW)]. Higher HbA1C was associated with lower heel eBMD when using only erythrocytic variants [OR 0.87 (0.82, 0.93), P=2× 10-5(IVW)]; However, when using only glycemic variants, this casual association does not hold. In further MR analysis, we test the association of erythrocytic traits with heel eBMD. Conclusion: Our study revealed the significant causal effect of HbA1c on eBMD, and this causal link might achieve through non-glycemic pathways (erythrocytic indices).

Cepharanthine Ameliorates Chondrocytic Inflammation and Osteoarthritis via Regulating the MAPK/NF-κB-Autophagy Pathway.

Osteoarthritis is a worldwide joint disease caused by abnormal chondrocytic metabolism. However, traditional therapeutic methods aimed at anti-inflammation for early-stage disease are palliative. In the present study, we demonstrated that cepharanthine (CEP), extracted from the plant Stephania cepharantha, exerted protective medicinal efficacy on osteoarthritis for the first time. In our in vitro study, CEP suppressed the elevated expression of matrix metalloproteinases (MMPs), a disintegrin and metalloproteinase with thrombospondin motifs 5 (ADAMTS5) and inducible nitric oxide synthase (iNOS) stimulated by IL-1ß or TNF-α by inhibiting the activation of MAPK and NF-κB signaling pathways, and upregulated the protein expression of aggrecan, collagen II, and Sox9. Also, CEP could reverse the reduced level of cellular autophagy in IL-1ß or TNF-α-induced chondrocytes, indicating that the protective effect of CEP on osteoarthritis was achieved by restoring MAPK/NF-κB-mediated autophagy. Furthermore, in a murine OA model, CEP mitigated cartilage degradation and prevented osteoarthritis in the CEP-treated groups versus the OA group. Hence, our results revealed the therapeutic prospect of CEP for anti-osteoarthritic treatment.

A male-specific doublesex isoform reveals an evolutionary pathway of sexual development via distinct alternative splicing mechanisms.

The doublesex/mab-3 related transcription factor (Dmrt) genes regulate sexual development in metazoans. Studies of the doublesex (dsx) gene in insects, in particular Drosophila melanogaster, reveal that alternative splicing of dsx generates sex-specific Dsx isoforms underlying sexual differentiation. Such a splicing-based mechanism underlying sex-specific Dmrt function is thought to be evolved from a transcription-based mechanism used in non-insect species, but how such transition occurs during evolution is not known. Here we identified a male-specific dsx transcript (dsxM2) through intron retention (IR), in addition to previously identified dsxM and dsxF transcripts through alternative polyadenylation (APA) with mutually exclusive exons. We found that DsxM2 had similarly masculinizing function as DsxM. We also found that the IR-based mechanism generating sex-specific dsx transcripts was conserved from flies to cockroaches. Further analysis of these dsx transcripts suggested an evolutionary pathway from sexually monomorphic to sex-specific dsx via the sequential use of IR-based and APA-based alternative splicing.

Dihydromyricetin ameliorates oxygen­glucose deprivation and re­oxygenation­induced injury in HT22 cells by activating the Wnt/ß­catenin signaling pathway.

Dihydromyricetin (DMY) is a natural flavonoid that possesses a wide range of pharmacological properties. The aim of the present study was to determine whether DMY could protect against nerve cell injury following ischemic stroke through antioxidant and neuroprotective effects. The effects of DMY on the viability, oxidative stress and apoptosis of HT22 cells following oxygen­glucose deprivation and re­oxygenation (OGD/R) were examined using MTT, lactate dehydrogenase (LDH), superoxide (SOD), malondialdehyde (MDA), western blot and TUNEL assays. Furthermore, Wnt/ß­catenin signaling proteins in OGD/R­stimulated HT22 cells were detected in the presence or absence of DMY. In a separate experiment, the effect of DMY on OGD/R­induced HT22 cell injury was also observed in the presence of the Wnt/ß­catenin inhibitor, XAV939. The results demonstrated that DMY had no impact on the survival of untreated HT22 cells, although DMY treatment significantly increased cell viability and inhibited cytotoxicity, oxidative stress and apoptosis following OGD/R. In addition, DMY upregulated the expression of Wnt/ß­catenin in OGD/R­stimulated HT22 cells. In conclusion, DMY protected HT22 cells from OGD/R­induced oxidative stress and apoptosis, and its effects may be mediated by the activation of the Wnt/ß­catenin signaling pathway.

The sex determination gene doublesex regulates expression and secretion of the basement membrane protein Collagen IV.

The highly conserved doublesex (dsx) and doublesex/mab-3 related (Dmrt) genes control sexually dimorphic traits across animals. The dsx gene encodes sex-specific transcription factors, DsxM in males and DsxF in females, which function differentially and often oppositely to establish sexual dimorphism. Here, we report that mutations in dsx, or overexpression of dsx, result in abnormal distribution of the basement membrane (BM) protein Collagen IV in the fat body. We find that Dsx isoforms regulate the expression of Collagen IV in the fat body and its secretion into the BM of other tissues. We identify the procollagen lysyl hydroxylase (dPlod) gene, which is involved in the biosynthesis of Collagen IV, as a direct target of Dsx. We further show that Dsx regulates Collagen IV through dPlod-dependent and independent pathways. These findings reveal how Dsx isoforms function in the secretory fat body to regulate Collagen IV and remotely establish sexual dimorphism.

The role of the cholinergic anti-inflammatory pathway in autoimmune rheumatic diseases.

The cholinergic anti-inflammatory pathway (CAP) is a classic neuroimmune pathway, consisting of the vagus nerve, acetylcholine (ACh)-the pivotal neurotransmitter of the vagus nerve-and its receptors. This pathway can activate and regulate the activities of immune cells, inhibit cell proliferation and differentiation, as well as suppress cytokine release, thereby playing an anti-inflammatory role, and widely involved in the occurrence and development of various diseases; recent studies have demonstrated that the CAP may be a new target for the treatment of autoimmune rheumatic diseases. In this review, we will summarize the latest progress with the view of figuring out the role of the cholinergic pathway and how it interacts with inflammatory reactions in several autoimmune rheumatic diseases, and many advances are results from a wide range of experiments performed in vitro and in vivo.

Genetic Associations Between IL-6 and the Development of Autoimmune Arthritis Are Gender-Specific.

Objectives: To find out the genetic association between IL6 and autoimmune arthritis. Methods: We performed a two-sample Mendelian randomization (MR) study using multiple genome-wide association studies (GWAS) datasets. Furthermore, a sex-stratified MR study was performed to identify sexual dimorphism in the association between IL6 and autoimmune arthritis. Then, LocusZoom plots were displayed based on the IL6R gene region to present evidence of genetic colocalization between diseases. Results: The MR result denoted a genetic association between the increased level of IL-6 signaling and risk of RA (ß=0.325, 95%CI 0.088, 0.561, p=7.08E-03) and AS (ß=1.240, 95%CI 0.495, 1.980, p=1.1E-03). Accordingly, sIL6R was found to have negatively correlation with the onset of RA (ß=-0.020, 95%CI -0.0320, -0.008, p=1.18E-03) and AS (ß=-0.125, 95%CI -0.177, -0.073, p=2.29E-06). However, no genetic association between IL6/sIL6R and PsA was detected. The gender-stratified MR analysis showed that IL6 was associated with AS in the male population, with RA in the female population, and with PsA in the male population. Additionally, ADAR, a gene identified by a sensitive test, could be the reason for the nonsignificant association between IL6 and PsA in a pooled population. Conclusion: Our findings showed that the overactive IL6 signal pathway led to autoimmune arthritis, especially in RA and AS. Sexual difference was also observed in IL6-intermediate susceptibility to autoimmune arthritis.

Virtual screening identified natural Keap1-Nrf2 PPI inhibitor alleviates inflammatory osteoporosis through Nrf2-mir214-Traf3 axis.

Overactive osteoclastogenesis is involved in the inflammatory bone loss and could be target for therapy. Here, we applied transcription factor enrichment analysis using public inflammatory osteolysis datasets and identified Nrf2 as the potential therapeutic target. Additionally, in-silico screening was performed to dig out Nrf2-Keap1 PPI inhibitor and Forsythoside-ß was found to be the best-performing PHG compound. We firstly tested the effect of Forsythoside-ß in inflammatory osteoporosis models and found it was able to attenuate the bone loss by inhibiting osteoclastogenesis and activating Nrf2-signaling in vivo. Forsythoside-ß was capable to suppress the differentiation of osteoclast in time and dose-dependent manners in vitro. Further, Forsythoside-ß could inhibit the production of reactive oxygen species and induce Nrf2 nuclear-translocation by interrupting Nrf2-Keap1 PPI. Recently, Nrf2 was identified as the epigenetic regulator modulating levels of miRNA in various diseases. We discovered that Forsythoside-ß could suppress the expression of mir-214-3p, one of most variable miRNAs during osteoclastogenesis. To clarify the undermining mechanism, by utilizing chip-seq dataset, we found that Nrf2 could bind to promoter of mir-214-3p and further regulate this miRNA. Collectively, Forsythoside-ß was able to prevent bone loss through Nrf2-mir-214-3p-Traf3 axis, which could be a promising candidate for treating inflammatory bone loss in the future.

Correlation between lung infection severity and clinical laboratory indicators in patients with COVID-19: a cross-sectional study based on machine learning.

BACKGROUND: Coronavirus disease 2019 (COVID-19) has caused a global pandemic that has raised worldwide concern. This study aims to investigate the correlation between the extent of lung infection and relevant clinical laboratory testing indicators in COVID-19 and to analyse its underlying mechanism. METHODS: Chest high-resolution computer tomography (CT) images and laboratory examination data of 31 patients with COVID-19 were extracted, and the lesion areas in CT images were quantitatively segmented and calculated using a deep learning (DL) system. A cross-sectional study method was carried out to explore the differences among the proportions of lung lobe infection and to correlate the percentage of infection (POI) of the whole lung in all patients with clinical laboratory examination values. RESULTS: No significant difference in the proportion of infection was noted among various lung lobes (P > 0.05). The POI of total lung was negatively correlated with the peripheral blood lymphocyte percentage (L%) (r = - 0.633, P < 0.001) and lymphocyte (LY) count (r = - 0.555, P = 0.001) but positively correlated with the neutrophil percentage (N%) (r = 0.565, P = 0.001). Otherwise, the POI was not significantly correlated with the peripheral blood white blood cell (WBC) count, monocyte percentage (M%) or haemoglobin (HGB) content. In some patients, as the infection progressed, the L% and LY count decreased progressively accompanied by a continuous increase in the N%. CONCLUSIONS: Lung lesions in COVID-19 patients are significantly correlated with the peripheral blood lymphocyte and neutrophil levels, both of which could serve as prognostic indicators that provide warning implications, and contribute to clinical interventions in patients.

fruitless tunes functional flexibility of courtship circuitry during development.

Drosophila male courtship is controlled by the male-specific products of the fruitless (fruM) gene and its expressing neuronal circuitry. fruM is considered a master gene that controls all aspects of male courtship. By temporally and spatially manipulating fruM expression, we found that fruM is required during a critical developmental period for innate courtship toward females, while its function during adulthood is involved in inhibiting male-male courtship. By altering or eliminating fruM expression, we generated males that are innately heterosexual, homosexual, bisexual, or without innate courtship but could acquire such behavior in an experience-dependent manner. These findings show that fruM is not absolutely necessary for courtship but is critical during development to build a sex circuitry with reduced flexibility and enhanced efficiency, and provide a new view about how fruM tunes functional flexibility of a sex circuitry instead of switching on its function as conventionally viewed.

Innate behaviors are behaviors that do not need to be learned. They include activities such as nest building in birds and web spinning in spiders. Another behavior that has been extensively studied, and which is generally considered to be innate, is courtship in fruit flies. Male fruit flies serenade potential mates by vibrating their wings to create a complex melody. This behavior is under the control of a gene called 'fruitless', which gives rise to several distinct proteins, including one that is unique to males. For many years, this protein ­ called Fru^M ­ was thought to be the master switch that activates courtship behavior. But recent findings have challenged this idea. They show that although male flies that lack Fru^M fail to show courtship behaviors if raised in isolation, they can still learn them if raised in groups. This suggests that the role of Fru^M is more complex than previously thought. To determine how Fru^M controls courtship behavior, Chen et al. have used genetic tools to manipulate Fru^M activity in male flies at different stages of the life cycle and distinct cells of the nervous system. The results revealed that Fru^M must be present during a critical period of development ­ but not adulthood ­ for male flies to court females. However, Fru^M strongly influences the type of courtship behavior the male flies display. The amount and location of Fru^M determines whether males show heterosexual, homosexual or bisexual courtship behaviors. Adult flies with lower levels of Fru^M show an increase in homosexual courtship and a decrease in heterosexual courtship. These findings provide a fresh view on how a master gene can generate complex and flexible behaviors. They show that fruitless, and the Fru^M protein it encodes, work distinctly at different life cycles to modify the type of courtship behavior shown by male flies, rather than simply switching courtship behavior on and off. Exactly how Fru^M acts within the fruit fly brain to achieve these complex effects requires further investigation.

Parallel Synaptic Acetylcholine Signals Facilitate Large Monopolar Cell Repolarization and Modulate Visual Behavior in Drosophila.

Appropriate termination of the photoresponse in image-forming photoreceptors and downstream neurons is critical for an animal to achieve high temporal resolution. Although the cellular and molecular mechanisms of termination in image-forming photoreceptors have been extensively studied in Drosophila, the underlying mechanism of termination in their downstream large monopolar cells remains less explored. Here, we show that synaptic ACh signaling, from both amacrine cells (ACs) and L4 neurons, facilitates the rapid repolarization of L1 and L2 neurons. Intracellular recordings in female flies show that blocking synaptic ACh output from either ACs or L4 neurons leads to slow repolarization of L1 and L2 neurons. Genetic and electrophysiological studies in both male and female flies determine that L2 neurons express ACh receptors and directly receive ACh signaling. Moreover, our results demonstrate that synaptic ACh signaling from both ACs and L4 neurons simultaneously facilitates ERG termination. Finally, visual behavior studies in both male and female flies show that synaptic ACh signaling, from either ACs or L4 neurons to L2 neurons, is essential for the optomotor response of the flies in high-frequency light stimulation. Our study identifies parallel synaptic ACh signaling for repolarization of L1 and L2 neurons and demonstrates that synaptic ACh signaling facilitates L1 and L2 neuron repolarization to maintain the optomotor response of the fly on high-frequency light stimulation.SIGNIFICANCE STATEMENT The image-forming photoreceptor downstream neurons receive multiple synaptic inputs from image-forming photoreceptors and various types of interneurons. It remains largely unknown how these synaptic inputs modulate the neural activity and function of image-forming photoreceptor downstream neurons. We show that parallel synaptic ACh signaling from both amacrine cells and L4 neurons facilitates rapid repolarization of large monopolar cells in Drosophila and maintains the optomotor response of the fly on high-frequency light stimulation. This work is one of the first reports showing how parallel synaptic signaling modulates the activity of large monopolar cells and motion vision simultaneously.