Optic Tectum Research Articles

Blind But Alive - Congenital Loss of atoh7 Disrupts the Visual System of Adult Zebrafish.

Vision is the predominant sense in most animal species. Loss of vision can be caused by a multitude of factors resulting in anatomic as well as behavioral changes. In mice and zebrafish, atoh7 mutants are completely blind as they fail to generate retinal ganglion cells (RGCs) during development. In contrast to mice, raising blind zebrafish to adulthood is challenging and this important model is currently missing in the field. Here, we report the phenotype of homozygous mutant adult zebrafish atoh7 mutants that have been raised using adjusted feeding and holding conditions. The phenotype of adult mutants was characterized using classical histology and immunohistochemistry as well as optical coherence tomography. In addition, the optokinetic response was characterized. Adult atoh7 mutants display dark body pigmentation and significantly reduced body length. They fail to form RGCs, the resulting nerve fiber layer as well as the optic nerve, and consequently behave completely blindly. In contrast, increased amounts of other retinal neurons and Müller glia are formed. In addition, the optic tectum is anatomically reduced in size, presumably due to the missing retinal input. Taken together, we provide a comprehensive characterization of a completely blind adult zebrafish mutant with focus on retinal and tectal morphology, as a useful model for glaucoma and optic nerve aplasia.

Genetically defined neuron types underlying visuomotor transformation in the superior colliculus.

The superior colliculus (SC) is a conserved midbrain structure that is important for transforming visual and other sensory information into motor actions. Decades of investigations in numerous species have made the SC and its nonmammalian homologue, the optic tectum, one of the best studied structures in the brain, with rich information now available regarding its anatomical organization, its extensive inputs and outputs and its important functions in many reflexive and cognitive behaviours. Excitingly, recent studies using modern genomic and physiological approaches have begun to reveal the diverse neuronal subtypes in the SC, as well as their unique functions in visuomotor transformation. Studies have also started to uncover how subtypes of SC neurons form intricate circuits to mediate visual processing and visually guided behaviours. Here, we review these recent discoveries on the cell types and neuronal circuits underlying visuomotor transformations mediated by the SC. We also highlight the important future directions made possible by these new developments.

Honey Enriched with Additives Alleviates Behavioral, Oxidative Stress, and Brain Alterations Induced by Heavy Metals and Imidacloprid in Zebrafish.

Environmental concerns have consistently been a focal point for the scientific community. Pollution is a critical ecological issue that poses significant threats to human health and agricultural production. Contamination with heavy metals and pesticides is a considerable concern, a threat to the environment, and warrants special attention. In this study, we investigated the significant issues arising from sub-chronic exposure to imidacloprid (IMI), mercury (Hg), and cadmium (Cd), either alone or in combination, using zebrafish (Danio rerio) as an animal model. Additionally, we assessed the potential protective effects of polyfloral honey enriched with natural ingredients, also called honey formulation (HF), against the combined sub-chronic toxic effects of the three contaminants. The effects of IMI (0.5 mg·L-1), Hg (15 μg·L-1), and Cd (5 μg·L-1), both individually and in combination with HF (500 mg·L-1), on zebrafish were evaluated by quantifying acetylcholinesterase (AChE) activity, lipid peroxidation (MDA), various antioxidant enzyme activities like superoxide dismutase and glutathione peroxidase (SOD and GPx), 2D locomotor activity, social behavior, histological and immunohistochemical factors, and changes in body element concentrations. Our findings revealed that all concentrations of pollutants may disrupt social behavior, diminish swimming performances (measured by total distance traveled, inactivity, and swimming speed), and elevate oxidative stress (OS) biomarkers of SOD, GPx, and MDA in zebrafish over the 21-day administration period. Fish exposed to IMI and Hg + Cd + IMI displayed severe lesions and increased GFAP (Glial fibrillary acidic protein) and S100B (S100 calcium-binding protein B) protein expression in the optic tectum and cerebellum, conclusively indicating astrocyte activation and neurotoxic effects. Furthermore, PCNA (Proliferating cell nuclear antigen) staining revealed reduced cell proliferation in the IMI-exposed group, contrasting with intensified proliferation in the Hg + Cd group. The nervous system exhibited significant damage across all studied concentrations, confirming the observed behavioral changes. Moreover, HF supplementation significantly mitigated the toxicity induced by contaminants and reduced OS. Therefore, the exposure to chemical mixtures offers a more complete picture of adverse impacts on aquatic ecosystems and the supplementation with bioactive compounds can help to reduce the toxicity induced by exposure to environmental pollutants.

Loss of Stim2 in zebrafish induces glaucoma-like phenotype.

Calcium is involved in vision processes in the retina and implicated in various pathologies, including glaucoma. Rod cells rely on store-operated calcium entry (SOCE) to safeguard against the prolonged lowering of intracellular calcium ion concentrations. Zebrafish that lacked the endoplasmic reticulum Ca2+ sensor Stim2 (stim2 knockout [KO]) exhibited impaired vision and lower light perception-related gene expression. We sought to understand mechanisms that are responsible for vision impairment in stim2 KO zebrafish. The single-cell RNA (scRNA) sequencing of neuronal cells from brains of 5days postfertilization larvae distinguished 27 cell clusters, 10 of which exhibited distinct gene expression patterns, including amacrine and γ-aminobutyric acid (GABA)ergic retinal interneurons and GABAergic optic tectum cells. Five clusters exhibited significant changes in cell proportions between stim2 KO and controls, including GABAergic diencephalon and optic tectum cells. Transmission electron microscopy of stim2 KO zebrafish revealed decreases in width of the inner plexiform layer, ganglion cells, and their dendrites numbers (a hallmark of glaucoma). GABAergic neuron densities in the inner nuclear layer, including amacrine cells, as well as photoreceptors significantly decreased in stim2 KO zebrafish. Our study suggests a novel role for Stim2 in the regulation of neuronal insulin expression and GABAergic-dependent vision causing glaucoma-like retinal pathology.

The influence of food web structure and foraging behaviour on visual system traits in a predatory freshwater fish

Lake trout (Salvelinus namaycush Walbaum, 1792) forages across macrohabitats (offshore vs nearshore) that likely pose different challenges to the visual system. We examined how differential foraging between macrohabitats challenges the visual system by sampling lake trout from four lakes with differing food web structures (e.g., presence/absence of cisco/smallmouth bass) and evaluating the association between nearshore energy use and visual traits. We found that lake trout foraging more nearshore had relatively larger eyes (corrected for body size) and larger optic tectum size only in lakes with the large pelagic cisco prey absent, while there was no association with these visual traits when cisco was present. Eye shape was not influenced by nearshore foraging. Our results suggest that nearshore foraging challenges the visual system more than offshore foraging but that this may be dependent on important food web structural attributes.

The telencephalon is a neuronal substrate for systemic inflammatory responses in teleosts via polyamine metabolism

Systemic inflammation elicits sickness behaviors and fever by engaging a complex neuronal circuitry that begins in the preoptic area of the hypothalamus. Ectotherms such as teleost fish display sickness behaviors in response to infection or inflammation, seeking warmer temperatures to enhance survival via behavioral fever responses. To date, the hypothalamus is the only brain region implicated in sickness behaviors and behavioral fever in teleosts. Yet, the complexity of neurobehavioral manifestations underlying sickness responses in teleosts suggests engagement of higher processing areas of the brain. Using in vivo models of systemic inflammation in rainbow trout, we find canonical pyrogenic cytokine responses in the hypothalamus whereas in the telencephalon and the optic tectum il-1b and tnfa expression is decoupled from il-6 expression. Polyamine metabolism changes, characterized by accumulation of putrescine and decreases in spermine and spermidine, are recorded in the telencephalon but not hypothalamus upon systemic injection of bacteria. While systemic inflammation causes canonical behavioral fever in trout, blockade of bacterial polyamine metabolism prior to injection abrogates behavioral fever, polyamine responses, and telencephalic but not hypothalamic cytokine responses. Combined, our work identifies the telencephalon as a neuronal substrate for brain responses to systemic inflammation in teleosts and uncovers the role of polyamines as critical chemical mediators in sickness behaviors.

Decreased GABA levels during development result in increased connectivity in the larval zebrafish tectum.

γ-aminobutyric acid (GABA) is an abundant neurotransmitter that plays multiple roles in the vertebrate central nervous system (CNS). In the early developing CNS, GABAergic signaling acts to depolarize cells. It mediates several aspects of neural development, including cell proliferation, neuronal migration, neurite growth, and synapse formation, as well as the development of critical periods. Later in CNS development, GABAergic signaling acts in an inhibitory manner when it becomes the predominant inhibitory neurotransmitter in the brain. This behavior switch occurs due to changes in chloride/cation transporter expression. Abnormalities of GABAergic signaling appear to underlie several human neurological conditions, including seizure disorders. However, the impact of reduced GABAergic signaling on brain development has been challenging to study in mammals. Here we take advantage of zebrafish and light sheet imaging to assess the impact of reduced GABAergic signaling on the functional circuitry in the larval zebrafish optic tectum. Zebrafish have three gad genes: two gad1 paralogs known as gad1a and gad1b, and gad2. The gad1b and gad2 genes are expressed in the developing optic tectum. Null mutations in gad1b significantly reduce GABA levels in the brain and increase electrophysiological activity in the optic tectum. Fast light sheet imaging of genetically encoded calcium indicator (GCaMP)-expressing gab1b null larval zebrafish revealed patterns of neural activity that were different than either gad1b-normal larvae or gad1b-normal larvae acutely exposed to pentylenetetrazole (PTZ). These results demonstrate that reduced GABAergic signaling during development increases functional connectivity and concomitantly hyper-synchronization of neuronal networks.

Neuro-protective effects of increased O-GlcNAcylation by glucosamine in an optic tectum traumatic brain injury model of adult zebrafish.

This study investigated the behavioral and molecular changes in the telencephalon following needle stab-induced injury in the optic tectum of adult zebrafish. At 3 days post-injury (dpi), there was noticeable structural damage to brain tissue and reduced neuronal proliferation in the telencephalon that persisted until 30 dpi. Neurobehavioral deficits observed at 3 dpi included decreased exploratory and social activities and impaired learning and memory (L/M) functions; all of these resolved by 7 dpi. The injury led to a reduction in telencephalic phosphorylated cAMP response element-binding protein and O-GlcNAcylation, both of which were restored by 30 dpi. There was an increase in GFAP expression and nuclear translocation of NF-κB p65 at 3 dpi, which were not restored by 30 dpi. The injury caused decreased O-GlcNAc transferase and increased O-GlcNAcase levels at 3 dpi, normalizing by 30 dpi. Glucosamine (GlcN) treatment at 3 dpi significantly restored O-GlcNAcylation levels and L/M function, also reducing GFAP activation. Glucose treatment recovered L/M function by 7 dpi, but inhibition of the hexosamine biosynthetic pathway by 6-diazo-5-oxo-L-norleucine blocked this recovery. These findings suggest that the O-GlcNAc pathway is a potential therapeutic target for addressing L/M impairment following traumatic brain injury in zebrafish.

The Visual Systems of Zebrafish.

The zebrafish visual system has become a paradigmatic preparation for behavioral and systems neuroscience. Around 40 types of retinal ganglion cells (RGCs) serve as matched filters for stimulus features, including light, optic flow, prey, and objects on a collision course. RGCs distribute their signals via axon collaterals to 12 retinorecipient areas in forebrain and midbrain. The major visuomotor hub, the optic tectum, harbors nine RGC input layers that combine information on multiple features. The retinotopic map in the tectum is locally adapted to visual scene statistics and visual subfield-specific behavioral demands. Tectal projections to premotor centers are topographically organized according to behavioral commands. The known connectivity in more than 20 processing streams allows us to dissect the cellular basis of elementary perceptual and cognitive functions. Visually evoked responses, such as prey capture or loom avoidance, are controlled by dedicated multistation pathways that-at least in the larva-resemble labeled lines. This architecture serves the neuronal code's purpose of driving adaptive behavior.

Antiparkinson potential of khellin on rotenone-induced Parkinson's disease in a zebrafish model: targeting MAO, inflammatory, and oxidative stress markers with molecular docking, MD simulations, and histopathology evidence

In this study, the antiparkinson effect of khellin (KL) on rotenone-induced Parkinson's disease (PD) was examined in zebrafish. Initially, In silico evaluations, such as drug likeness and ADME/T analysis, confirmed the pharmacological viability of KL. Molecular docking and molecular dynamics (MD) analysis revealed stable binding interactions between KL and monamine oxidase B (MAO-B). Molecular docking results for KL and pioglitazone (CCl) revealed binding energies of −6.5 and −10.4 kcal/mol, respectively. Later, molecular dynamics (MD) studies were performed to assess the stability of these complexes, which yielded binding energies of −36.04 ± 55.21 and −56.2 ± 80.63 kJ/mol for KL and CCl, respectively. These results suggest that KL exhibits considerable binding affinity for MAO-B. In In vitro studies, according to the DPPH free radical scavenging assay, KL exhibited significant antioxidant effects, indicating that it can promote redox balance with an IC50 value of 22.68 ± 0.5 μg/ml. In vivo studies and evaluation of locomotor activity, social interaction, histopathology and biochemical parameters were conducted in KL-treated zebrafish to measure SOD and GSH antioxidant activity, the oxidative stress marker malondialdehyde (MDA), the inflammatory marker myeloperoxidase (MPO) and MAO-B. However, while the locomotor and social interaction abilities of the rotenone-treated zebrafish were significantly reduced, KL treatment significantly improved locomotor activity (p < 0.001) and social interaction (p < 0.001). KL alleviated PD symptoms, as indicated by significant increases in SOD (p < 0.01), GSH (p < 0.001), MDA (p < 0.001), MAO-B (p < 0.001) and MPO (p < 0.001) in rotenone-induced PD fish (p<0.001) significantly reduced activities. Histopathological studies revealed that rotenone-induced brain hyperintensity and abnormal cellularity of the periventricular gray matter in the optic tectum were significantly reduced by KL treatment. This study provides a strong basis for developing KL as a new candidate for the treatment of Parkinson's disease, with the prospect of improved safety profiles and efficacy.

Do Microplastics Have Neurological Implications in Relation to Schizophrenia Zebrafish Models? A Brain Immunohistochemistry, Neurotoxicity Assessment, and Oxidative Stress Analysis.

The effects of exposure to environmental pollutants on neurological processes are of increasing concern due to their potential to induce oxidative stress and neurotoxicity. Considering that many industries are currently using different types of plastics as raw materials, packaging, or distribution pipes, microplastics (MPs) have become one of the biggest threats to the environment and human health. These consequences have led to the need to raise the awareness regarding MPs negative neurological effects and implication in neuropsychiatric pathologies, such as schizophrenia. The study aims to use three zebrafish models of schizophrenia obtained by exposure to ketamine (Ket), methionine (Met), and their combination to investigate the effects of MP exposure on various nervous system structures and the possible interactions with oxidative stress. The results showed that MPs can interact with ketamine and methionine, increasing the severity and frequency of optic tectum lesions, while co-exposure (MP+Met+Ket) resulted in attenuated effects. Regarding oxidative status, we found that all exposure formulations led to oxidative stress, changes in antioxidant defense mechanisms, or compensatory responses to oxidative damage. Met exposure induced structural changes such as necrosis and edema, while paradoxically activating periventricular cell proliferation. Taken together, these findings highlight the complex interplay between environmental pollutants and neurotoxicants in modulating neurotoxicity.

Eye Size Does Not Change with Artificial Selection on Relative Telencephalon Size in Guppies (Poecilia reticulata)

Introduction: Variation in eye size is sometimes closely associated with brain morphology. Visual information, detected by the retina, is transferred to the optic tectum to coordinate eye and body movements towards stimuli and thereafter distributed into other brain regions for further processing. The telencephalon is an important visual processing region in many vertebrate species and a highly developed region in visually dependent species. Yet, the existence of a coevolutionary relationship between telencephalon size and eye size remains relatively unknown. Methods: Here, we use male and female guppies artificially selected for small- and large-relative-telencephalon-size to test if artificial selection on telencephalon size results in changes in eye size. In addition, we performed an optomotor test as a proxy for visual acuity. Results: We found no evidence that eye size changes with artificial selection on telencephalon size. Eye size was similar in both absolute and relative terms between the two selection regimes but was larger in females. This is most likely because of the larger body size in females, but it could also reflect their greater need for visual capacity due to sex-specific differences in foraging and mating behaviour. Although the optomotor response was stronger in guppies with a larger telencephalon, we found no evidence for differences in visual acuity between the selection regimes. Conclusion: Our study suggests that eye size and visual perception in guppies do not change rapidly with strong artificial selection on telencephalon size.

Label-free multiphoton imaging reveals volumetric shifts across development in sensory-related brain regions of a miniature transparent vertebrate.

Animals integrate information from different sensory modalities as they mature and perform increasingly complex behaviors. This may parallel differential investment in specific brain regions depending on the demands of changing sensory inputs. To investigate developmental changes in the volume of canonical sensory integration brain regions, we used third harmonic generation imaging for morphometric analysis of forebrain and midbrain regions from 5 to 90 days post fertilization (dpf) in Danionella dracula , a transparent, miniature teleost fish whose brain is optically accessible throughout its lifespan. Relative to whole brain volume, increased volume or investment in telencephalon, a higher order sensory integration center, and torus longitudinalis (TL), a midbrain visuomotor integration center, is relatively consistent from 5 to 30 dpf, until it increases at 60 dpf, followed by another increase at 90 dpf, as animals reach adulthood. In contrast, investment in midbrain optic tectum (TeO), a retinal-recipient target, progressively decreases from 30-90 dpf, whereas investment is relatively consistent across all stages for the midbrain torus semicircularis (TS), a secondary auditory and mechanosensory lateral line center, and the olfactory bulb (OB), a direct target of the olfactory epithelium. In sum, increased investment in higher order integration centers (telencephalon, TL) occurs as juveniles reach adulthood and exhibit more complex cognitive tasks, whereas investment in modality-dominant regions occurs in earlier stages (TeO) or is relatively consistent across development (TS, OB). Complete optical access throughout Danionella 's lifespan provides a unique opportunity to investigate how changing brain structure over development correlates with changes in connectivity, microcircuitry, or behavior.

Transcriptomic changes in the posterior pallium of male zebra finches associated with social niche conformance

Animals plastically adjust their physiological and behavioural phenotypes to conform to their social environment—social niche conformance. The degree of sexual competition is a critical part of the social environment to which animals adjust their phenotypes, but the underlying genetic mechanisms are poorly understood. We conducted a study to investigate how differences in sperm competition risk affect the gene expression profiles of the testes and two brain areas (posterior pallium and optic tectum) in breeding male zebra finches (Taeniopygia castanotis). In this pre-registered study, we investigated a large sample of 59 individual transcriptomes. We compared two experimental groups: males held in single breeding pairs (low sexual competition) versus those held in two pairs (elevated sexual competition) per breeding cage. Using weighted gene co-expression network analysis (WGCNA), we observed significant effects of the social treatment in all three tissues. However, only the treatment effects found in the pallium were confirmed by an additional randomisation test for statistical robustness. Likewise, the differential gene expression analysis revealed treatment effects only in the posterior pallium (ten genes) and optic tectum (six genes). No treatment effects were found in the testis at the single gene level. Thus, our experiments do not provide strong evidence for transcriptomic adjustment specific to manipulated sperm competition risk. However, we did observe transcriptomic adjustments to the manipulated social environment in the posterior pallium. These effects were polygenic rather than based on few individual genes with strong effects. Our findings are discussed in relation to an accompanying paper using the same animals, which reports behavioural results consistent with the results presented here.

Valine administration in the hypothalamus alters the brain and plasma metabolome in rainbow trout.

Central administration of valine has been shown to cause hyperphagia in fish. Although mechanistic target of rapamycin (mTOR) is involved in this response, the contributions to feed intake of central and peripheral metabolite changes due to excess valine are unknown. Here, we investigated whether intracerebroventricular injection of valine modulates central and peripheral metabolite profiles and may provide insights into feeding response in fish. Juvenile rainbow trout (Oncorhynchus mykiss) were administered an intracerebroventricular injection of valine (10 µg·µL-1 at 1 μL·100·g-1 body wt), and the metabolite profile in plasma, hypothalamus, and rest of the brain (composing of telencephalon, optic tectum, cerebellum, and medulla oblongata) was carried out by liquid chromatography-mass spectrometry (LC/MS)-based metabolomics. Valine administration led to a spatially distinct metabolite profile at 1 h postinjection in the brain: enrichment of amino acid metabolism and energy production pathways in the rest of the brain but not in hypothalamus. This suggests a role for extrahypothalamic input in the regulation of feed intake. Also, there was enrichment of several amino acids, including tyrosine, proline, valine, phenylalanine, and methionine, in plasma in response to valine. Changes in liver transcript abundance and protein expression reflect an increased metabolic capacity, including energy production from glucose and fatty acids, and a lower protein kinase B (Akt) phosphorylation in the valine group. Altogether, valine intracerebroventricular administration affects central and peripheral metabolism in rainbow trout, and we propose a role for the altered metabolite profile in modulating the feeding response to this branched-chain amino acid.NEW & NOTEWORTHY Valine causes hyperphagia in fish when it is centrally administered; however, the exact mechanisms are far from clear. We tested how intracerebroventricular injection of valine in rainbow trout affected the brain and plasma metabolome. The metabolite changes in response to valine were more evident in the rest of the brain compared with the hypothalamus. Furthermore, we demonstrated for the first time that central valine administration affects peripheral metabolism in rainbow trout.

Diversity of Intraspecific Patterns of Brain Region Size Covariation in Fish.

Traits often do not evolve in isolation or vary independently of other traits. Instead, they can be affected by covariation, both within and across species. However, the importance of within-species trait covariation and, critically, the degree to which it varies between species has yet to be thoroughly studied. Brain morphology is a trait of great ecological and behavioral importance, with regions that are hypothesized to vary in size based on behavioral and cognitive demands. Sizes of brain regions have also been shown to covary with each other across various taxa. Here, we test the degree to which covariation in brain region sizes within species has been conserved across 10 teleost fish species. These 10 species span five orders, allowing us to examine how phylogenetic proximity influences similarities in intraspecific trait covariation. Our results showed a trend that similar patterns of brain region size covariation occur in more closely related species. Interestingly, there were certain brain region pairs that showed similar levels of covariation across all species regardless of phylogenetic distance, such as the telencephalon and optic tectum, while others, such as the olfactory bulb and the hypothalamus, varied more independently. Ultimately, the patterns of brain region covariation shown here suggest that evolutionary mechanisms or constraints can act on specific brain regions independently, and that these constraints can change over evolutionary time.

Environmental pollution of paraben needs attention: A study of methylparaben and butylparaben co-exposure trigger neurobehavioral toxicity in zebrafish

Parabens (PBs) are commonly utilized as preservatives in various commodities. Of all the PBs, methylparaben (MeP) and butylparaben (BuP) are usually found together at similar levels in the aqueous environment. Although a few studies have demonstrated that PBs are neurotoxic when present alone, the neurobehavioral toxic effects and mechanisms of coexisting MeP and BuP at environmental levels has not been determined. Neurobehavior is a sensitive indicator for identifying neurotoxicity of environmental pollutants. Therefore, adult female zebrafish (Danio rerio) were chronic co-exposure of MeP and BuP at environmental levels (5, 50, and 500 ng/L) for 60 d to investigate the effects on neurobehavior, histopathology, oxidative stress, mitochondrial function, neurotransmitters and gene expression. The results demonstrated that chronic co-exposure of MeP and BuP interfered with several behaviors (learning-memory, anxiety, fear, aggressive and shoaling behavior) in addition to known mechanisms of producing oxidative stress and disrupting energy. More intriguingly, chronic co-exposure of MeP and BuP caused retinal vacuolization and apoptosis in the optic tectum zone. It even has further effects on the phototransduction pathway, impairing optesthesia and leading to neurotransmitters dysregulation. These are critical underlying mechanisms resulting in neurobehavioral abnormalities. This study confirms that the pollution of multiple PBs by chronic co-exposure in aquatic environments can result neurobehavioral toxicity. It also suggests that the prolonged effects of PBs on aquatic ecosystems and health require close attention.

Amlodipine Attenuates Carrageenan-induced Oxidative Stress Targeting Transsynaptic Neuronal Damage by Promoting Survival of Retinal Ganglion Cells in Adult Zebrafish (Danio rerio).

Transsynaptic degeneration of retinal ganglion cells (RGCs) likely plays a role in progressive loss of vision in glaucoma. Carrageenan (Carr), obtained from seaweed (Rhodophyceae), a sulphated polysaccharide extracted from a species of red algae (Chondrus crispus), is a reversible gel mostly used in eye-drop preparation due to its hydrocolloid nature. But Carr-induced inflammation causing oxidative stress in animals has not yet been evaluated. It was reported that amlodipine (AML) can inhibit inflammatory cytokines by lowering intracellular calcium concentration and increasing antioxidant defences. In this study, we examined the impact of AML administration on Carr-induced oxidative stress, causing free radial-mediated neurodegeneration and behavioural alteration in zebrafish. We aimed to evaluate the behavioural alteration in zebrafish using the novel tank diving test (NTDT) and the light-dark preference test (LDPT) as invaluable tools for analysing visual functions. To understand the role of the antioxidant defence system, oxidative stress biomarkers such as lipid peroxidation and CAT activity were analysed. RGCs of the retina of the eye and neurons of the optical tectum (TeO) in the brain were investigated by histopathological studies. Carr-treated groups swam in random patterns and showed an increased number of transitions and spent more time in alter zones, whereas controls prefer perceived motion. AML significantly attenuates the increased lipid peroxidation level and decreased CAT activity with Carr administration. Histopathological analysis showed an increase in the percentage of pyknotic cell counts after Carr treatment, whereas AML supplementation significantly reduced the pyknotic RGC cell count in the retina and TeO of the brain. Findings showed the effectiveness of AML as an antioxidant and neuroprotective agent by reducing pyckonsis in developmental RGCs and possible repair mechanisms against Carr-induced oxidative stress. This study may be useful to know the underlying mechanisms of weakening and eliminating central neurons for synaptic connections in the visual system of zebrafish.

Digest: How environmental light conditions shape the evolution of visual systems in birds.

How do varying environmental light conditions influence the evolution of avian visual systems? Fröhlich et al. (2024) demonstrate that nocturnal birds evolved broader corneas and slightly longer axial lengths than their diurnal counterparts, increasing light capture efficiency. Nocturnal species also tended to maintain or reduce the size of brain regions responsible for vision, i.e., the optic tectum and the visual wulst. These results highlight adaptive trends in nocturnal species, where evolutionary improvement in low-light performance of eyes may be accompanied by compromised brain function.

Neuron mapping in the Molly fish optic tectum: An emphasis on the adult neurogenesis process.

Teleost fish exhibit the most pronounced and widespread adult neurogenesis. Recently, functional development and the fate of newborn neurons have been reported in the optic tectum (OT) of fish. To determine the role of neurogenesis in the OT, this study used histological, immunohistochemical, and electron microscopic investigations on 18 adult Molly fish specimens (Poecilia sphenops). The OT of the Molly fish was a bilateral lobed structure located in the dorsal part of the mesencephalon. It exhibited a laminated structure made up of alternating fiber and cellular layers, which were organized into six main layers. The stratum opticum (SO) was supplied by optic nerve fibers, in which the neuropil was the main component. Radial bipolar neurons that possessed bundles of microtubules were observed in the stratum fibrosum et griseum superficiale (SFGS). Furthermore, oligodendrocytes with their processes wrapped around the nerve fibers could be observed. The stratum album centrale (SAC) consisted mainly of the axons of the stratum griseum centrale (SGC) and the large tectal, pyriform, and horizontal neurons. The neuronal cells of the SO and large tectal cells of the SAC expressed autophagy-related protein-5 (APG5). Interleukin-1β (IL-1β) was expressed in both neurons and glia cells of SGC. Additionally, inducible nitric oxide synthase (iNOS) was expressed in the neuropil of the SAC synaptic layer and granule cells of the stratum periventriculare (SPV). Also, transforming growth factor beta (TGF-β), SRY-box transcription factor 9 (SOX9), and myostatin were clearly expressed in the proliferative neurons. In all strata, S100 protein and Oligodendrocyte Lineage Transcription Factor 2 (Olig2) were expressed by microglia, oligodendrocytes, and astrocytes. In conclusion, it was possible to identify different varieties of neurons in the optic tectum, each with a distinct role. The existence of astrocytes, proliferative neurons, and stem cells highlights the regenerative capacity of OT. RESEARCH HIGHLIGHTS: The OT of the Molly fish exhibited a laminated structure made up of alternating fiber and cellular layers, which were organized into six main layers. Radial bipolar neurons that possessed bundles of microtubules were observed in the stratum fibrosum et griseum superficiale (SFGS). The stratum album central (SAC) consisted mainly of the axons of the stratum griseum centrale (SGC) and the large tectal, pyriform, and horizontal neurons. Inducible nitric oxide synthase (iNOS) was expressed in the neuropil of the SAC synaptic layer and granule cells of the stratum periventricular (SPV). Also, transforming growth factor beta (TGF-β), SRY-box transcription factor 9 (SOX9), and myostatin were clearly expressed in the proliferative neurons. The existence of astrocytes, proliferative neurons, and stem cells highlights the regenerative capacity of OT.