Asymmetries in motor behavior, such as human hand preference, are observed throughout bilateria. However, neural substrates and developmental signaling pathways that impose underlying functional lateralization on a broadly symmetric nervous system are unknown. Here we report that in the absence of overriding visual information, zebrafish larvae show intrinsic lateralized motor behavior that is mediated by a cluster of 60 posterior tuberculum (PT) neurons in the forebrain. PT neurons impose motor bias via a projection through the habenular commissure. Acquisition of left/right identity is disrupted by heterozygous mutations in mosaic eyes and mindbomb, genes that regulate Notch signaling. These results define the neuronal substrate for motor asymmetry in a vertebrate and support the idea that haploinsufficiency for genes in a core developmental pathway destabilizes left/right identity.

BackgroundAnimals use sensory cues to efficiently locate resources, but when sensory information is insufficient, they may rely on internally coded search strategies. Despite the importance of search behavior, there is limited understanding of the underlying neural mechanisms in vertebrates.ResultsHere, we report that loss of illumination initiates sophisticated light-search behavior in larval zebrafish. Using three-dimensional tracking, we show that at the onset of darkness larvae swim in a helical trajectory that is spatially restricted in the horizontal plane, before gradually transitioning to an outward movement profile. Local and outward swim patterns display characteristic features of area-restricted and roaming search strategies, differentially enhancing phototaxis to nearby and remote sources of light. Retinal signaling is only required to initiate area-restricted search, implying that photoreceptors within the brain drive the transition to the roaming search state. Supporting this, orthopediaA mutant larvae manifest impaired transition to roaming search, a phenotype which is recapitulated by loss of the non-visual opsin opn4a and somatostatin signaling.ConclusionThese findings define distinct neuronal pathways for area-restricted and roaming search behaviors and clarify how internal drives promote goal-directed activity.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-016-0346-2) contains supplementary material, which is available to authorized users.

The master circadian clock in fish has been considered to reside in the pineal gland. This dogma is challenged, however, by the finding that most zebrafish tissues contain molecular clocks that are directly reset by light. To further examine the role of the pineal gland oscillator in the zebrafish circadian system, we generated a transgenic line in which the molecular clock is selectively blocked in the melatonin-producing cells of the pineal gland by a dominant-negative strategy. As a result, clock-controlled rhythms of melatonin production in the adult pineal gland were disrupted. Moreover, transcriptome analysis revealed that the circadian expression pattern of the majority of clock-controlled genes in the adult pineal gland is abolished. Importantly, circadian rhythms of behavior in zebrafish larvae were affected: rhythms of place preference under constant darkness were eliminated, and rhythms of locomotor activity under constant dark and constant dim light conditions were markedly attenuated. On the other hand, global peripheral molecular oscillators, as measured in whole larvae, were unaffected in this model. In conclusion, characterization of this novel transgenic model provides evidence that the molecular clock in the melatonin-producing cells of the pineal gland plays a key role, possibly as part of a multiple pacemaker system, in modulating circadian rhythms of behavior.

SCN2A, encoding the neuronal voltage-gated Na + channel NaV1.2, is one of the most commonly affected loci linked to autism spectrum disorders (ASDs). Most ASD-associated mutations in SCN2A are loss-of-function, but studies examining how such mutations affect neuronal function and whether Scn2a mutant mice display ASD endophenotypes have been inconsistent. We generated a protein truncation variant Scn2a mouse model (Scn2a Δ1898/+ ) by CRISPR that eliminates the NaV1.2 channel's distal intracellular C-terminal domain and analyzed the molecular and cellular consequences of this variant in a heterologous expression system, in neuronal culture, in brain slices, and in vivo. We also analyzed multiple behaviors in wild type and Scn2a Δ1898/+ mice and correlated behaviors with clinical data obtained in human subjects with SCN2A variants. Expression of the NaV1.2 mutant in a heterologous expression system revealed decreased NaV1.2 channel function and cultured pyramidal neurons isolated from Scn2a Δ1898/+ forebrain showed correspondingly reduced voltage-gated Na + channel currents without compensation from other central nervous system voltage-gated Na + channels. Na + currents in inhibitory neurons were unaffected. Consistent with loss of voltage-gated Na + channel currents, Scn2a Δ1898/+ pyramidal neurons displayed reduced excitability in forebrain neuronal culture and reduced excitatory synaptic input onto the pyramidal neurons in brain slices. Scn2a Δ1898/+ mice displayed several behavioral abnormalities, including abnormal social interactions that reflect behavior observed in humans with ASD and with harboring loss-of-function SCN2A variants. This model and its cellular electrophysiological characterizations provide a framework for tracing how a SCN2A loss-of-function variant leads to cellular defects that result in ASD-associated behaviors.

Asymmetries in motor behavior, such as human hand preference, are observed throughout bilateria.However, neural substrates and developmental signaling pathways that impose underlying functional lateralization on a broadly symmetric nervous system are unknown. Here we report that in the absence of over-riding visual information, zebrafish larvae show intrinsic lateralized motor behavior that is mediated by a cluster of 60 posterior tuberculum (PT) neurons in the forebrain. PT neurons impose motor bias via a projection through the epithalamic commissure to the habenula. Acquisition of left/right identity is disrupted by heterozygous mutations in mosaic eyes and mindbomb, genes that regulate Notch signaling. These results define the neuronal substrate for motor asymmetry in a vertebrate and support the idea that developmental pathways that establish visceral asymmetries also govern acquisition of left/right identity. Results Larval zebrafish show persistent individual lateralized behavior in locomotor trajectoriesAfter loss of illumination, 6 days post-fertilization (dpf) larval zebrafish initiate a circular swimming behavior in which they repeatedly perform same-direction turn maneuvers in a restricted spatial area ( Figure 1A) (Horstick et al., 2017). Same-direction turn movements were sustained in individual larvae for two minutes (Figure 1B), however across the population there was no net tendency for larvae to

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