Learning and visual discrimination in newly hatched zebrafish

Toxicity of different-sized cobalt ferrite (CoFe2O4) nanoparticles to Oncorhynchus mykiss at early development stages

As innovative and versatile agents with potential applications in a wide range of fields including medicine, electronics, wastewater treatment, cosmetics, and energy storage devices, magnetic nanoparticles (NPs) are significant attention. However, our knowledge of the harmful effects of different-sized NPs, particularly of their effects on aquatic animals, is limited. In this study, we evaluated the impact of different-sized (sub-2, 5, and 15 nm) cobalt ferrite (CoFe2O4) NPs on the biological parameters of rainbow trout (Oncorhynchus mykiss) embryos and larvae. The NPs were characterized using techniques such as high-resolution transmission electron microscopy (HRTEM) for imaging, X-ray diffraction (XRD) for crystallographic analysis, and energy-dispersive X-ray spectroscopy (EDX) for elemental analysis, and were tested for impact through a series of toxicity, genotoxicity, and biochemical assays at a concentration of 100 mg/L. The obtained results showed that toxicity of CoFe2O4 NPs depended on the size of NPs and the developmental stage of the fish. Our results, which revealed significant changes in biological parameters of O. mykiss under exposure to CoFe2O4 NPs, imply that these NPs may be not environmentally safe. The hierarchical cluster analysis showed that embryos of the control group were clearly separated from those exposed to NPs of various sizes. However, in the exposed larvae, the effects of control and the smallest-sized NPs (sub-2 nm) differed from those induced by larger NPs (5 nm and 15 nm). Additional research is necessary to comprehend the mechanisms underlying the observed variations, which would be advantageous for both managing the risk of NPs to humans and advancing the field of aquatic nanotoxicology.

Exploring the Importance of Environmental Complexity for Newly Hatched Zebrafish

The effects of an early impoverished social or physical environment on vertebrate neural development and cognition has been known for decades. While existing studies have focused on the long-term effects, measuring adult cognitive phenotypes, studies on the effects of environmental complexity on the early stages of development are lacking. Zebrafish (Danio rerio) hatchlings are assumed to have minimal interaction with their environment and are routinely reared in small, bare containers. To investigate the effects of being raised under such conditions on development of behaviour and cognition, hatchlings housed for 10 days in either an enriched or a standard environment underwent two cognitive tasks. The results were mixed. Subjects of the two treatments did not differ in performance when required to discriminate two areas. Conversely, we found a significant effect in a number discrimination task, with subjects from impoverished condition performing significantly worse. In both experiments, larvae reared in impoverished environment showed a reduced locomotor activity. Given the effects that enrichment appears to exert on larvae, a third experiment explored whether hatchlings exhibit a spontaneous preference for more complex environments. When offered a choice between a bare setting and one with objects of different shapes and colors, larvae spent over 70% of time in the enriched sector. Deepening these effects of an early impoverished environment on cognitive development is crucial for the welfare of captive zebrafish populations and for enhancing the quality and reliability of studies on larval zebrafish.

Learning and memory formation in zebrafish: Protein dynamics and molecular tools

Research on learning and memory formation at the level of neural networks, as well as at the molecular level, is challenging due to the immense complexity of the brain. The zebrafish as a genetically tractable model organism can overcome many of the current challenges of studying molecular mechanisms of learning and memory formation. Zebrafish have a translucent, smaller and more accessible brain than that of mammals, allowing imaging of the entire brain during behavioral manipulations. Recent years have seen an extensive increase in published brain research describing the use of zebrafish for the study of learning and memory. Nevertheless, due to the complexity of the brain comprising many neural cell types that are difficult to isolate, it has been difficult to elucidate neural networks and molecular mechanisms involved in memory formation in an unbiased manner, even in zebrafish larvae. Therefore, data regarding the identity, location, and intensity of nascent proteins during memory formation is still sparse and our understanding of the molecular networks remains limited, indicating a need for new techniques. Here, we review recent progress in establishing learning paradigms for zebrafish and the development of methods to elucidate neural and molecular networks of learning. We describe various types of learning and highlight directions for future studies, focusing on molecular mechanisms of long-term memory formation and promising state-of-the-art techniques such as cell-type-specific metabolic labeling.

A comparative analysis of Danionella cerebrum and zebrafish (Danio rerio) larval locomotor activity in a light-dark test

The genus Danionella comprises some of the smallest known vertebrate species and is evolutionary closely related to the zebrafish, Danio rerio. With its optical translucency, rich behavioral repertoire, and a brain volume of just 0.6 mm³, Danionella cerebrum (Dc) holds great promise for whole-brain in vivo imaging analyses with single cell resolution of higher cognitive functions in an adult vertebrate. Little is currently known, however, about the basic locomotor activity of adult and larval Danionella cerebrum and how it compares to the well-established zebrafish model system. Here, we provide a comparative developmental analysis of the larval locomotor activity of Dc and AB wildtype as well as crystal zebrafish in a light-dark test. We find similarities but also differences in both species, most notably a striking startle response of Dc following a sudden dark to light switch, whereas zebrafish respond most strongly to a sudden light to dark switch. We hypothesize that the different startle responses in both species may stem from their different natural habitats and could represent an opportunity to investigate how neural circuits evolve to evoke different behaviors in response to environmental stimuli.