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Defeating cancer as leukemia has been an up and down challenge. However, leukemia must be treated from the roots. Nowadays, the CRISPR-Cas9 system provided scientists the ability to manipulate the genetic information to correct mutations, rewrite genetic code, or edit immune cells for immunotherapy purposes. Additionally, such system is used for basic and clinical approaches in leukemia therapy. Lymphoid cancers including acute lymphoblastic leukemia (ALL) can be treated by performing gene editing or enhancing immune system through CART cells. Here, we present and detail therapeutic applications of the CRISPR/Cas9 system for immune cell therapy, and knock-out or knock-in of main genes promoting leukemogenesis or ALL progression. We also described current and future challenges, and optimization for the application of CRISPR/Cas9 system to treat lymphoid malignancies.

Acute lymphoblastic leukemia (ALL) has been established as the most common acute leukemia in children, accounting for 80–85% of cases. ALL occurs mostly in children and it is considered as a high-risk disease in the elderlies. ALL is characterized by a clonal disorder where the normal hematopoiesis is replaced by a malignant clonal expansion of lymphoid progenitors. Although many therapeutic strategies have been established to treat ALL leading to improved survival rates, the short-term and long-term complications derived from treatment toxicity represent a critical risk for patients. The treatment-related toxicity suggests a need for the development of new therapy strategies to effectively treat high-risk and low-risk disease. Nowadays, an important approach is focused on the identification of molecules involved in the mechanisms that lead to leukemia generation and progression to determine potential targets at the transcriptional level. MicroRNAs (miRNAs) are a group of key molecules that regulate signaling pathways related to lymphopoiesis. miRNAs participate in the regulation of hematopoietic differentiation and proliferation, as well as their activity. The present review details the recompilation of evidences about the relation between miRNAs and lymphopoiesis, ALL development and progression in order to propose and explore novel strategies to modulate ALL-related miRNA levels as a therapeutic approach.

Multidrug resistance protein-4 (MRP4) belongs to the ABC transporter superfamily and promotes the transport of xenobiotics including drugs. A non-synonymous single nucleotide polymorphisms (nsSNPs) in the ABCC4 gene can promote changes in the structure and function of MRP4. In this work, the interaction of certain endogen substrates, drug substrates, and inhibitors with wild type-MRP4 (WT-MRP4) and its variants G187W and Y556C were studied to determine differences in the intermolecular interactions and affinity related to SNPs using protein threading modeling, molecular docking, all-atom, coarse grained, and umbrella sampling molecular dynamics simulations (AA-MDS and CG-MDS, respectively). The results showed that the three MRP4 structures had significantly different conformations at given sites, leading to differences in the docking scores (DS) and binding sites of three different groups of molecules. Folic acid (FA) had the highest variation in DS on G187W concerning WT-MRP4. WT-MRP4, G187W, Y556C, and FA had different conformations through 25 ns AA-MD. Umbrella sampling simulations indicated that the Y556C-FA complex was the most stable one with or without ATP. In Y556C, the cyclic adenosine monophosphate (cAMP) and ceefourin-1 binding sites are located out of the entrance of the inner cavity, which suggests that both cAMP and ceefourin-1 may not be transported. The binding site for cAMP and ceefourin-1 is quite similar and the affinity (binding energy) of ceefourin-1 to WT-MRP4, G187W, and Y556C is greater than the affinity of cAMP, which may suggest that ceefourin-1 works as a competitive inhibitor. In conclusion, the nsSNPs G187W and Y556C lead to changes in protein conformation, which modifies the ligand binding site, DS, and binding energy.

The neuroendocrine transdifferentiation has been found in many cancer cell types, such as prostate, lung and gastrointestinal cells and is accompanied by a lower patient life expectancy. The transdifferentiation process has been induced in vitro by the exposure to different stimuli in human lung adenocarcinoma. The aim of this work was to identify the morphological characteristics of the neuroendocrine phenotype in a human lung cancer cell line, induced by two cAMP elevating agents (IBMX and FSK). Our results showed two phenotypes, one produced by IBMX with higher volume, cell size and increased number of secondary projections, and the other produced by FSK with higher area, roughness of the membrane, cell neurite percentage, number of outgrowths per cell and increased number of primary projections. In conclusion, we describe some morphological and ultrastructural characteristics of the neuroendocrine phenotype in A549 human lung cancer cell line promoted by IBMX and FSK to contribute to the understanding of the autocrine or paracrine signaling within the tumor microenvironment.

The excess of many chemical substances in humans, such as copper has been reported to induce DNA damage and trigger cancer development...

It is known that growth hormone (GH) is expressed in immune cells, where it exerts immunomodulatory effects. However, the mechanisms of expression and release of GH in the immune system remain unclear. We analyzed the effect of growth hormone-releasing hormone (GHRH), thyrotropin-releasing hormone (TRH), ghrelin (GHRL), and somatostatin (SST) upon GH mRNA expression, intracellular and released GH, Ser133-phosphorylation of CREB (pCREBS133), intracellular Ca2+ levels, as well as B-cell activating factor (BAFF) mRNA expression in bursal B-lymphocytes (BBLs) cell cultures since several GH secretagogues, as well as their corresponding receptors (-R), are expressed in B-lymphocytes of several species. The expression of TRH/TRH-R, ghrelin/GHS-R1a, and SST/SST-Rs (Subtypes 1 to 5) was observed in BBLs by RT-PCR and immunocytochemistry (ICC), whereas GHRH/GHRH-R were absent in these cells. We found that TRH treatment significantly increased local GH mRNA expression and CREB phosphorylation. Conversely, SST decreased GH mRNA expression. Additionally, when added together, SST prevented TRH-induced GH mRNA expression, but no changes were observed in pCREBS133 levels. Furthermore, TRH stimulated GH release to the culture media, while SST increased the intracellular content of this hormone. Interestingly, SST inhibited TRH-induced GH release in a dose-dependent manner. The coaddition of TRH and SST decreased the intracellular content of GH. After 10 min. of incubation with either TRH or SST, the intracellular calcium levels significantly decreased, but they were increased at 60 min. However, the combined treatment with both peptides maintained the Ca2+ levels reduced up to 60-min. of incubation. On the other hand, BAFF cytokine mRNA expression was significantly increased by TRH administration. Altogether, our results suggest that TRH and SST are implicated in the regulation of GH expression and release in BBL cultures, which also involve changes in pCREBS133 and intracellular Ca2+ concentration. It is likely that TRH, SST, and GH exert autocrine/paracrine immunomodulatory actions and participate in the maturation of chicken BBLs.

Dementia is a syndrome characterized by the presence of a persistent cognitive impairment that interferes with the capabilities of an individual to carry out her/his personal, professional, and/or social activities. Purinergic receptors have been proposed as possible targets for treatment in different neurodegenerative disorders due to the fact that the A1 and A2 receptor functions were associated with neuromodulator effects within the brain. Adenosine A1 receptors (A1Rs) are widely distributed in the brain of most animals with a high density in the neocortex and the hippocampus; hence, this receptor has been linked to Alzheimer's disease (AD), because there was observed to be a decrease in patients, as well as, in different animal models. Several studies have explored the effects of A1R agonists as well as antagonist in AD, yet, there is a discrepancy between whether it is the agonists or the antagonists of adenosine receptors that produce beneficial effects in AD treatment.

Dementia caused by Alzheimer's disease (AD) is mainly characterized by accumulation in the brain of extra- and intraneuronal amyloid-β (Aβ) and tau proteins, respectively, which selectively affect specific regions, particularly the neocortex and the hippocampus. Sporadic AD is mainly caused by an increase in apolipoprotein E, a component of chylomicrons, which are cholesterol transporters in the brain. Recent studies have shown that high lipid levels, especially cholesterol, are linked to AD. Adenosine is an atypical neurotransmitter that regulates a wide range of physiological functions by activating four P1 receptors (A1, A2A, A2B, and A3) and P2 purinergic receptors that are G protein-coupled. A1 receptors are involved in the inhibition of neurotransmitter release, which could be related to AD. The aim of the present work was to study the effects of a lard-enriched diet (LED) on cognitive and memory processes in adult rats (6 months of age) as well as the effect of theobromine on these processes. The results indicated that the fat-enriched diet resulted in a long-term deterioration in cognitive and memory functions. Increased levels of Aβ protein and IL-1β were also observed in the rats fed with a high-cholesterol diet, which were used to validate the AD animal model. In addition, the results of qPCR and immunohistochemistry indicated a decrease in gene expression and distribution of A1 purinegic receptor, respectively, in the hippocampus of LED-fed rats. Interestingly, theobromine, at both concentrations tested, restored A1 receptor levels and improved cognitive functions and Aβ levels for a dose of 30 mg/L drinking water.

T cell acute lymphoblastic leukaemia is a type of cancer that develops from lymphoid progenitors, and chemotherapy is corner stone of the treatment. Thiopurine drugs, consisting of 6-mercaptopurine, 6-thioguanine, and azathioprine (Aza), can effectively treat this disease. To be activated, Aza must first be biotransformed to 6-mercaptopurine by thiol groups in glutathione. However, glutathione is decreased in cancer cells due to high levels of reactive oxygen species (ROS). N-acetylcysteine could provide thiol groups for glutathione synthesis to biotransform Aza. Using flow cytometry, the ability of N-acetylcysteine to increase the antiproliferative and apoptotic effects of Aza without increasing ROS was tested in Jurkat cells. Individually, Aza 1.0 and 2.0 μM, as well as N- acetylcysteine 3.0 mM, induced apoptosis and cell cycle arrest. Together, Aza + N-acetylcysteine significantly reduced proliferation compared to that obtained with the individual drugs. Combination of N-acetylcysteine 3.0 mM with Aza 1.0 μM was as effective at inducing apoptosis as Aza 2.0 μM alone. The combination of N-acetylcysteine 3.0 mM + Aza 1.0 μM increased cell cycle arrest at the G2/M phase. We found that Aza 1.0 or 2.0 μM induced a significant increase in ROS compared to that in untreated cells, while N-acetylcysteine 3.0 mM and N-acetylcysteine 3.0 mM + Aza 1.0 μM kept ROS at control levels; the latter drugpairing represents a favourable combination to reduce oxidative stress in the presence of Aza. In conclusion, N-acetylcysteine augments antiproliferative and apoptotic effects of Aza without increasing ROS in vitro.

Background Major depressive disorders are characterized by their severity and long‐lasting symptoms, which make such disorders highly disabling illnesses. Unfortunately, 50% of major depressive patients experience relapses, perhaps partly because drug research has been performed only in animal models that screen for antidepressant drugs that appear to only ameliorate acute depression symptoms. The bilateral olfactory bulbectomy (OBX) animal model presents the advantage of mimicking the symptoms of chronic depression by means of brain surgery. Adenosine purinergic receptors A2A (A2AR) have been the target of interest in the field of psychiatric diseases. This study aimed to show which A2A receptor ligands exert antidepressive‐like effects in the OBX rat model. Methods Forty Sprague‐Dawley male rats were divided into four groups: control, OBX + vehicle, OBX + ZM 241385, and OBX + adenosine groups. Pharmacological treatment was administered for 14 days, and the rats were examined via the forced swim test (FST), open field test (OFT), and sucrose preference test (SPT). Results The OBX + ZM 241385 group exhibited decreased immobility time in the FST, decreased isolation time in the OFT, and reversed anhedonia behavior in the SPT compared to the vehicle group. However, no significant differences for adenosine treatment were found. Conclusions ZM 241385 administration (2 mg/kg i.p.) restored behavioral changes associated with OBX‐induced depression.

Introduction Lung cancer is one of the leading causes of deaths by cancer in both sexes worldwide. A primary concern of this disease is the presence of neuroendocrine (NE) phenotype which has been correlated to a decreased survival, an increased number of peripheral tumour cells and an increased percentage of metastasis. A key aspect of NE phenotype is the formation of secretory granules with the fundamental property of secreting a great variety of factors such as hormones and neurotransmitters. In recent years, there has been an increasing interest in the role of this factors as immunomodulators, a primary concern is their role in the bidirectional communication within the tumour microenvironment. It remains unclear the immunomodulator effect of factors secreted in NE phenotype of lung cancer. The objective of this work was to test the effect of factors secreted by the lung adenocarcinoma cell line (A549 NE ) with NE phenotype on the activity of cytotoxic T lymphocytes (CTL) in vitro . Material and methods Human lung adenocarcinoma A549 cells were treated with [cAMP] increasing agents up to 120 hours (isobutyl-1-methylxanthine (IBMX, 0.5 mM), forskolin (FSK, 0.5 mM) or both IBMX+FSK) to observe the changes in the phenotype acquisition: morphology and neurite number (LM), Chromogranin A (CgA by FACS). Co-cultures and fluorescence release were used to evaluate the interaction of cytotoxic activity of T lymphocytes (C: Jurkat) against target cells (T: A549/GFP) and to observe the effect of the neuroendocrine differentiation on the cytotoxic activity, using as target cells original A549 cells and transdifferentiated (A549 NE ). Results and discussions Changes in the A549 cell morphology were observed (size and presence of neurite-like projections), decreased proliferation rate and the overexpression of neuroendocrine marker chromogranin A were also observed in all treatments since 72 hours (except for CgA overexpression with IBMX treatment). The fluorescence release of A549/GFP after C: T co-cultures showed an increasing cytolytic time-dependent effect and C:T ratio. The NE phenotype acquisition diminishes the fluorescence release of target cells in 24 hour co-culture suggesting a decreased cytolysis. Conclusion The current data support the generation of a neuroendocrine phenotype from A549 cell line (A549 NE ), stable for 48 hours after cAMP stimuli withdrawal. In co-cultures with CTL, there is evidence of diminished cytolysis by means of the acquisition of the NE phenotype.

Background and Objectives: Acute lymphoblastic leukemia (ALL) is the most common type of cancer in childhood. The majority of patients respond to treatment, but those with resistant phenotypes suffer relapse or death. The antifolate methotrexate (MTX) is the most commonly used drug against ALL due to its efficacy. Once inside leukemic cells, MTX is metabolized into methotrexate polyglutamates (MTX-PG) by action of the enzyme folylpolyglutamate synthetase (FPGS), leading to a longer action compared to that of MTX alone. Materials and Methods: In this work, we demonstrated that the combination treatment of methotrexate and 5 and 10 mM glutamic acid could enhance methotrexate cytotoxicity in CCRF-SB (B-ALL) cells. In addition, MTX plus 20 mM glutamic acid was able to improve the synthesis of MTX-PG5. Results: All treatments induced an increase in FPGS expression compared to that of the control group. Furthermore, we detected different cellular expression patterns of FPGS in the different treatments. Conclusion: Based on these findings, we demonstrated that levels of methotrexate polyglutamates (MTX-PGs) could be a key determinant of methotrexate-induced cytotoxicity in CCRF-SB acute lymphoblastic leukemia cells.

The sodium-coupled neutral amino acid transporter 2 (SNAT2) is a protein that is expressed ubiquitously in mammalian tissues and that displays Na(+), voltage and pH dependent activity. This transporter mediates the passage of small zwitterionic amino acids across the cell membrane and regulates the cell homeostasis and its volume. We have examined the expression of SNAT2 mRNA and protein during the development of the rat cerebral cortex, from gestation through the postnatal stages to adulthood. Our data reveal that SNAT2 mRNA and protein expression is higher during embryogenesis, while it subsequently diminishes during postnatal development. Moreover, during embryonic period SNAT2 colocalizes with the radial glial cells marker GLAST, while in postnatal period it is mainly detected in neuronal dendrites. These findings suggest a relevant role for amino acid transport through SNAT2 in the developing embryonic brain.

TWIST is a transcription factor that belongs to the family of helix-loop-helix proteins involved in metastasis with essential role regulating cell movement during early development, as well as in the tumor progression and metastasis of many cancers including breast cancer. It will be interesting to study the relation among cancer chemotherapy, malnutrition and the transcription factors like TWIST in order to explore the risk to metastasis. We used breast cancer line MDA-MB-231. Cell cultures were treated with 5-Fluorouracil (5-Fu), as well as changes in serum and nonessentials amino acid (NEAA), to explore the cell viability and the cellular distribution of TWIST by immunocytochemistry. Our results indicate that cell viability decreased significantly with 5-Fu treatment whereas no changes were observed in malnutrition treatment. On the other hand, TWIST protein significantly increased its distribution in cytoplasm of treated groups with malnutrition as well as in those treated with 5-Fu compared with the control. These results suggest that TWIST translocation was modified by the treatments and further studies are necessary to suggest that TWIST could be a tag protein to avoid metastasis.

Common beans ( Phaseolus vulgaris L.) contain a high proportion of undigested carbohydrates (NDC) that can be fermented in the large intestine to produce short-chain fatty acids (SCFA) such as acetate, propionate, and butyrate. The objective of the present study was to evaluate the composition and chemopreventive effect of a polysaccharide extract (PE) from cooked common beans ( P. vulgaris L) cv. Negro 8025 on azoxymethane (AOM) induced colon cancer in rats. The PE induced SCFA production with the highest butyrate concentrated in the cecum zone: 6.7 +/- 0.06 mmol/g of sample for PE treatment and 5.29 +/- 0.24 mmol/g of sample for PE + AOM treatment. The number of aberrant crypt foci (ACF) and the transcriptional expression of bax and caspase-3 were increased, and rb expression was decreased. The data suggest that PE decreased ACF and had an influence on the expression of genes involved in colon cancer for the action of butyrate concentration.

It is known that serotonin exerts its different nociceptive and motor functions by interacting with distinct receptors subtypes, which could be either G-protein coupled or ionotropic. Previous reports demonstrated the early activation of serotonin receptor transcripts during rat development, suggesting a potential role of the serotoninergic system during ontogeny. In this study we have compared the cellular distribution of three serotonin receptor subtypes: 5-HT 2C , 5-HT 5A , and 5-HT 7 . Immunocytochemical methods were used in slices of rat hippocampus obtained during the postnatal development. 5-HT 2C immunoreactivity was strong at all developmental stages in the CA1 region, whereas differences were observed between P0 and P5 in the CA3 region. The 5-HT 5A receptor immunosignal in CA1 and CA3 was strong at P0, decreased at P11, and then increased in the adult. The immunoreactivity to 5-HT 7 receptors was high in all regions at P0 and then decreased progressively during postnatal development; the signal was stronger for 5-HT 2C than for 5-HT 5A and 5-HT 7 receptors. Changes in the expression level of each receptor may result in differences in functional and pharmacological properties of the cells expressing them as well as in the hippocampal neuronal network. The distribution of the three serotonin receptor subtypes studied varied during the ontogeny, which supports their potential role during development and will help to understand their mechanisms.

Administering strychnine, a potent antagonist of glycine receptors, to pregnant rats caused marked toxic effects on the ensuing embryos. The embryotoxic effects of strychnine were compared with those induced by retinal palmitate during rat neurulation; and it was found that strychnine was stronger than retinal palmitate in a number of abnormalities such as anencephaly, general aplasy and abnormal cerebral vesicles. Although the glycine receptor beta1 subunit mRNA was found to be expressed in the embryos when strychnine was administered to the mother rats, its presence may not fully account for the toxic effects and it may be that strychnine is targeting also other molecules, such as the nicotinic receptor that has been found early in development.

The corpus callosum (CC) is the main white matter tract involved in interhemispheric brain communication. We establish that uptake of [3H]5-hydroxytryptamine (5-HT) in CC is partially inhibited by some antidepressants. Slices of the adult rat CC had a high-affinity uptake of 5-HT. About 80% of this uptake was Na+ dependent, with a Michaelis-Menten constant, Km, of 420 ± 80 nM and a rate of 5-HT uptake, Vmax, of 9.5 ± 0.8 pmol/mg protein/min. The 5-HT uptake was reduced ∼60% at pH 5 compared with that at pH 7. Fluoxetine (Prozac) inhibited only 43% of 5-HT uptake in a concentration-dependent manner, with an affinity constant, Ki, of 44.7 ± 10.0 nM. We also studied the effects of other monoamine uptake inhibitors, all at 10 μM, and found that zimelidine, imipramine, and clomipramine inhibited 5-HT uptake in the CC by ∼30–40%. The fluoxetine-insensitive 5-HT uptake was not altered by high concentrations of dopamine plus norepinephrine. The present data show that Na+-dependent 5-HT uptake occurs in the CC and optic nerve and that this uptake is partially sensitive to antidepressants and probably mediated by the serotonin transporter, which may be relevant during depression. © 2003 Wiley-Liss, Inc.

Serotonin is an ancient molecular signal and a recognized neurotransmitter brainwide distributed with particular presence in hippocampus. Almost all serotonin receptor subtypes are expressed in hippocampus, which implicates an intricate modulating system, considering that they can be localized as autosynaptic, presynaptic, and postsynaptic receptors, even colocalized within the same cell and being target of homo- and heterodimerization. Neurons and glia, including immune cells, integrate a functional network that uses several serotonin receptors to regulate their roles in this particular part of the limbic system.

The purinergic receptors P2X(4) and P2X(6) are ion channels activated by ATP. These receptors are present in the gastrointestinal tract, and they are involved in synaptic transmission, taste sensation, and pain, among other functions. In this work, we studied the distribution of P2X(4) and P2X(6) receptors in proximal and distal regions of the gut newborn and adult rats. Using immunohistochemistry, purinergic receptors were found in gut epithelial cells and capillary vessels. In both proximal and distal regions of newborn rats, we observed P2X(4) signal in epithelial cells, whereas P2X(6) was present in capillary vessels in the proximal region and to a lesser extent in the distal region. In both regions of adult gut, we observed P2X(4) and P2X(6) immunostain in the capillary vessels. Semi-quantification indicated a significant difference in the amount of P2X(4) between proximal regions, whereas the P2X(6) content of both newborn regions differed from that in adult proximal gut. We conclude that P2X(4) and P2X(6) purinoreceptors are present in the gut from birth and that they are differentially distributed among regions.

Hydroxytryptamine (5-HT) is involved in a variety of different physiological processes and behaviors through the activation of equally diverse receptors subtypes. In this work we studied the changes on the expression of 5-HT5A receptors in rat hippocampus induced by leptin, an adipocyte-derived hor- mone that has been reported to participate in the modulation of food intake and in adult hippocampal neurogenesis. To study the effect of leptin on the 5-HT5A receptor gene expression a qRT-PCR was used and the distribution of those receptors in the hippocampus was visualized by immunohistochemistry. Rats were separated in four groups: control (untreated rats), leptin-treated, serotonin-treated and lep- tin + serotonin treated. The results showed that even though the 5-HT5A gene expression did not change in the hippocampus of any of the treated groups, in the rats treated with leptin and serotonin, the specific immunostaining for the 5-HT5A serotonin receptor decreased significantly in the dentate gyrus. © 2010 Elsevier Ireland Ltd. All rights reserved.

The distribution of the glycine receptor subunits alpha1-3 and beta in the developing rat cerebellum was studied from postnatal day 1 to adulthood by means of quantitative RT-PCR and immunohistochemistry. qRT-PCR of postnatal cerebella indicated the presence of mRNA for each subunit, with a relative expression of alpha2alpha3alpha1beta. The immunohistochemistry indicated a strong alpha2 signal in the Purkinje cells, internal and external granular layers. The alpha1-3 subunits had weak signals in the Purkinje cells and molecular layer. The alpha1 subunit was expressed at a low level and was also found in the white matter. The function of these receptors in neuronal and glial plasma membranes in early postnatal development remains to be determined.

In the central nervous system, serotonin (5-HT) may function as a mitogen as well as a neurotransmitter; and its early appearance suggests a potential role in development. The present experiments were done to determine the localization of the mRNA coding for the 5-HT 5A receptor during development of the rat brain. 5-HT 5A gene transcription was assessed by in situ hybridization, from E18 and during postnatal (PN) development. An intense signal of 5-HT 5A mRNA was found in the cerebral cortex and olfactory nucleus at E18, PN0 and PN5. A sharp decrease at PN11 was followed by an increase until reaching the adult level in the cerebral cortex; whereas in the olfactory nucleus, transcription remained weak. In contrast, in the hippocampal formation the signal was weak in the CA1, CA2 and CA3 regions at E18 and PO; increased at P5 and then decreased at P11 before attaining the adult level. We conclude that the gene coding for the 5-HT 5A receptor is already active in the embryonic rat brain and is differentially expressed during development.

There are several factors that suggest serotonin [5-hydroxytryptamine (5-HT)] plays a role as a neurotransmitter/neuromodulator within the retina. The presence of mRNAs encoding 5-HT receptors (5-HTR) of the types 5-HT2CR and 5-HT5AR within the rat retina was investigated using in situ hybridization of digoxigenin-labeled probes. The 5HT5AR probe produced no labeling, whereas the 5HT2CR probe hybridized in cells scattered in the inner nuclear and ganglion cell layers. Thus, the 5HT2CR gene is expressed by retinal neurons, some of which represent third-order neurons, either amacrine or ganglion cells. This suggests that 5-HT may modulate the outgoing signal from the retina.