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

Neural circuits underlying divergent visuomotor strategies of zebrafish and Danionella cerebrum

Many animals respond to sensory cues with species-specific coordinated movements.1,2 A universal visually guided behavior is the optomotor response (OMR),3,4,5,6 which stabilizes the body by following optic flow induced by displacements in currents.7 While the brain-wide OMR circuits in zebrafish (Danio rerio) have been characterized,8,9,10,11,12 the homologous neural functions across teleost species with different ecological niches, such as Danionella cerebrum,13,14,15 remain largely unexplored. Here, we directly compare larval zebrafish and D. cerebrum to uncover the neural mechanisms underlying the natural variation of visuomotor coordination. Closed-loop behavioral tracking during visual stimulation revealed that D. cerebrum follow optic flow by swimming continuously, punctuated with sharp directional turns, in contrast to the burst-and-glide locomotion of zebrafish.16 Although D. cerebrum swim at higher average speeds, they lack the direction-dependent velocity modulation observed in zebrafish. Two-photon calcium imaging and tail tracking showed that both species exhibit direction-selective encoding in putative homologous regions, with D. cerebrum containing more monocular neurons. D. cerebrum sustain significantly longer directed swims across all stimuli than zebrafish, with zebrafish reducing tail movement duration in response to oblique, turn-inducing stimuli. While locomotion-associated neurons in D. cerebrum display more prolonged activity than zebrafish, lateralized turn-associated neural activity in the hindbrain suggests a shared neural circuit architecture that independently controls movement vigor and direction. These findings highlight the diversity in visuomotor strategies among teleost species with shared circuit motifs, establishing a framework for unraveling the neural mechanisms driving continuous and discrete locomotion.

Divergent Visuomotor Strategies in Teleosts: Neural Circuit Mechanisms in Zebrafish and Danionella cerebrum

Many animals respond to sensory cues with species-specific coordinated movements to successfully navigate their environment. However, the neural mechanisms that support diverse sensorimotor transformations across species with distinct navigational strategies remain largely unexplored. By comparing related teleost species, zebrafish ( Danio rerio, ZF ) and Danionella cerebrum ( DC ), we investigated behavioral patterns and neural architectures during the visually guided optomotor response (OMR). Closed-loop behavioral tracking during visual stimulation revealed that larval ZF employ burst-and-glide locomotion, while larval DC display continuous, smooth swimming punctuated with sharp directional turns. Although DC achieve higher average speeds, they lack the direction-dependent velocity modulation observed in ZF . Whole-brain two-photon calcium imaging and tail tracking in head-fixed fish reveals that both species exhibit direction-selective motion encoding in homologous regions, including the retinorecipient pretectum, with DC exhibiting fewer binocular, direction-selective neurons overall. Kinematic analysis of head-fixed behavior reveals that DC sustain significantly longer directed swim events across all stimuli than ZF , highlighting the divergent visuomotor strategies, with ZF reducing tail movement duration in response to oblique, turn-inducing stimuli. Lateralized motor-associated neural activity in the medial and anterior hindbrain of both species suggests a shared circuit motif, with distinct neural circuits that independently control movement vigor and direction. These findings highlight the diversity in visuomotor strategies among teleost species, underscored by shared sensorimotor neural circuit motifs, and establish a robust framework for unraveling the neural mechanisms driving continuous and discrete visually guided locomotion, paving the way for deeper insights into vertebrate sensorimotor functions.

Research Highlights

Larval DC exhibit faster swimming than ZF , matching the direction of visual motion. DC execute OMR in smooth, curved swimming patterns, interspersed with sharp directional turns. ZF and DC share similar visuomotor neural architecture, recruiting pretectal and hindbrain regions. ZF and DC demonstrate lateralized encoding of turns, particularly in medial hindbrain neurons.

In Brief

Larval Danionella cerebrum respond to global visual motion cues in smooth, low-angle swimming patterns, interspersed with sharp directional turns, swimming consistently faster than zebrafish. Fouke et al. use behavioral tracking of freely moving and head fixed fish to reveal an evolutionarily conserved visuomotor neural architecture transforming visual motion cues into species-specific locomotor behaviors.

Anticonvulsant and anxiolytic-like potential of the essential oil from the Ocimum basilicum Linn leaves and its major constituent estragole on adult zebrafish (Danio rerio)

The Ocimum species present active compounds with the potential to develop drugs for treating chronic disease conditions, such as anxiety and seizures. The present study aims to investigate the anticonvulsant and anxiolytic-like effect of the essential oil from O. basilicum Linn (OEFOb) leaves and its major constituent estragole (ES) in vivo on adult zebrafish (aZF) and in silico. The aZF were treated with OEFOb or ES or vehicle and submitted to the tests of toxicity, open-field, anxiety, and convulsion and validated the interactions of the estragole on the involvement of GABAergic and serotonergic receptors by molecular docking assay. The results showed that the oral administration of OEFOb and ES did not have a toxic effect on the aZF and showed anxiolytic-like effects with the involvement of GABAA, 5-HT1, 5-HT2A/2C and 5-HT3A/3B as well on anxiety induced by alcohol withdrawal. The OEFOb and ES showed anticonvulsant potential attenuating the seizures induced by pentylenetetrazole (PTZ) by modulation of the GABAA system. Both anxiolytic and anticonvulsant effects were corroborated by the potential of the interaction of ES by in silico assay. These study samples demonstrate the pharmacological evidence and potential for using these compounds to develop new anxiolytic and anticonvulsant drugs.

Neuroprotective effects of Shenghui decoction via inhibition of the JNK/p38 MAPK signaling pathway in an AlCl3-induced zebrafish (Danio rerio) model of Alzheimer's disease

ETHNOPHARMACOLOGICAL RELEVANCE

Alzheimer's disease (AD) is a multi-factorial degenerative disease, and multi-targeted therapies targeting multiple pathogenic mechanisms should be explored. Shenghui decoction (SHD) is an ancient traditional Chinese medicine (TCM) formula used clinically to alleviate AD. However, the precise mechanism of action of SHD as a therapeutic agent for AD remains unclear.

AIM OF THE STUDY

This study investigated the neuroprotective properties and potential mechanisms of action of SHD in mitigating AD-like symptoms induced by AlCl3 in a zebrafish model.

MATERIALS AND METHODS

Active components of SHD were detected using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Zebrafish were exposed to AlCl3 (200 μg/L) for 30 days to establish an AD zebrafish model. AlCl3-exposed zebrafish were treated with SHD or donepezil. Behavioral tests were used to assess learning and memory, locomotor activity, and AD-related anxiety and aggression in AlCl3-exposed zebrafish. Nissl staining and transmission electron microscopy were used to evaluate histological alterations in brain neurons. The concentrations of pro-inflammatory cytokines (tumor necrosis factor-α, TNF-α; interleukin-1β, IL-1β) were quantified using Enzyme-linked immunosorbent assay (ELISA). Markers of oxidative stress and cholinergic activity (acetylcholinesterase, AChE) were detected using biochemical assays. Western blotting and immunofluorescence were used to detect the protein expression levels of Aβ, p-tau, PSD-95, synaptophysin, TLR4, phosphorylation of NF-κB p65, p38, and JNK.

RESULTS

Fifteen SHD compounds were identified by UPLC-MS/MS analysis. SHD improved AlCl3-induced dyskinesia, learning and memory impairment, anxiety-like behavior, and aggressive behavior in zebrafish. AlCl3-exposed zebrafish showed AD-like pathology, overexpression of Aβ, hyperphosphorylated tau protein, marked neuronal damage, decreased expression of synaptic proteins, synaptophysin, and PSD-95, and impairment of synaptic structural plasticity. These effects were reversed by the SHD treatment. We also observed that SHD ameliorated oxidative stress and decreased AChE activity and inflammatory cytokine levels. These effects are similar to those observed for donepezil. Meanwhile, SHD could decrease the protein expression of TLR4 and inhibit phosphorylation of NF-κB, JNK, and p38 MAPK. These results demonstrate that SHD has the potential to exert neuroprotective effects, which may be partly mediated via inhibition of the JNK/p38 MAPK signaling pathway.

CONCLUSIONS

Our findings revealed the therapeutic mechanism of SHD in mitigating AD progression and suggested that SHD is a potent neuroprotectant that contributes to the future development of TCM modernization and broader clinical applications.

A tale of two males: Behavioral and neural mechanisms of alternative reproductive tactics in midshipman fish

An amalgam of investigations at the interface of neuroethology and behavioral neuroendocrinology first established the most basic behavioral, neuroanatomical, and neurophysiological characters of vocal-acoustic communication morphs in the plainfin midshipman fish, Porichthys notatus Girard. This foundation has led, in turn, to the repeated demonstration that neuro-behavioral mechanisms driving reproductive-related, vocal-acoustic behaviors can be uncoupled from gonadal state for two adult male phenotypes that follow alternative reproductive tactics (ARTs).

Label-free, whole-brain in vivo mapping in an adult vertebrate with third harmonic generation microscopy

Comprehensive understanding of interconnected networks within the brain requires access to high resolution information within large field of views and over time. Currently, methods that enable mapping structural changes of the entire brain in vivo are extremely limited. Third harmonic generation (THG) can resolve myelinated structures, blood vessels, and cell bodies throughout the brain without the need for any exogenous labeling. Together with deep penetration of long wavelengths, this enables in vivo brain-mapping of large fractions of the brain in small animals and over time. Here, we demonstrate that THG microscopy allows non-invasive label-free mapping of the entire brain of an adult vertebrate, Danionella dracula, which is a miniature species of cyprinid fish. We show this capability in multiple brain regions and in particular the identification of major commissural fiber bundles in the midbrain and the hindbrain. These features provide readily discernable landmarks for navigation and identification of regional-specific neuronal groups and even single neurons during in vivo experiments. We further show how this label-free technique can easily be coupled with fluorescence microscopy and used as a comparative tool for studies of other species with similar body features to Danionella, such as zebrafish (Danio rerio) and tetras (Trochilocharax ornatus). This new evidence, building on previous studies, demonstrates how small size and relative transparency, combined with the unique capabilities of THG microscopy, can enable label-free access to the entire adult vertebrate brain.

Thalamic neurons drive distinct forms of motor asymmetry that are conserved in teleost and dependent on visual evolution

Brain laterality is a prominent feature in Bilateria, where neural functions are favored in a single brain hemisphere. These hemispheric specializations are thought to improve behavioral performance and are commonly observed as sensory or motor asymmetries, such as handedness in humans. Despite its prevalence, our understanding of the neural and molecular substrates instructing functional lateralization is limited. Moreover, how functional lateralization is selected for or modulated throughout evolution is poorly understood. While comparative approaches offer a powerful tool for addressing this question, a major obstacle has been the lack of a conserved asymmetric behavior in genetically tractable organisms. Previously, we described a robust motor asymmetry in larval zebrafish. Following the loss of illumination, individuals show a persistent turning bias that is associated with search pattern behavior with underlying functional lateralization in the thalamus. This behavior permits a simple yet robust assay that can be used to address fundamental principles underlying lateralization in the brain across taxa. Here, we take a comparative approach and show that motor asymmetry is conserved across diverse larval teleost species, which have diverged over the past 200 million years. Using a combination of transgenic tools, ablation, and enucleation, we show that teleosts exhibit two distinct forms of motor asymmetry, vision-dependent and - independent. These asymmetries are directionally uncorrelated, yet dependent on the same subset of thalamic neurons. Lastly, we leverage Astyanax sighted and blind morphs, which show that fish with evolutionarily derived blindness lack both retinal-dependent and -independent motor asymmetries, while their sighted surface conspecifics retained both forms. Our data implicate that overlapping sensory systems and neuronal substrates drive functional lateralization in a vertebrate brain that are likely targets for selective modulation during evolution.