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Dopamine (DA)-producing neurons are critically involved in the production of motor behaviors in multiple circuits that are conserved from basal vertebrates to mammals. Although there is increasing evidence that DA neurons in the hypothalamus play a locomotor role, their precise contributions to behavior and the circuit mechanisms by which they are achieved remain unclear. Here, we demonstrate that tyrosine-hydroxylase-2-expressing (th2+) DA neurons in the zebrafish hypothalamus fire phasic bursts of activity to acutely promote swimming and modulate audiomotor behaviors on fast timescales. Their anatomy and physiology reveal two distinct functional DA modules within the hypothalamus. The first comprises an interconnected set of cerebrospinal-fluid-contacting DA nuclei surrounding the 3 rd ventricle, which lack distal projections outside of the hypothalamus and influence locomotion through unknown means. The second includes neurons in the preoptic nucleus, which send long-range projections to targets throughout the brain, including the mid- and hindbrain, where they activate premotor circuits involved in swimming and sensorimotor integration. These data suggest a broad regulation of motor behavior by DA neurons within multiple hypothalamic nuclei and elucidate a novel functional mechanism for the preoptic DA neurons in the initiation of movement., Competing Interests: Declaration of Interests The authors declare no competing interests., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Postembryonic neurogenesis has been observed in several regions of the vertebrate brain, including the dentate gyrus and rostral migratory stream in mammals, and is required for normal behavior [1-3]. Recently, the hypothalamus has also been shown to undergo continuous neurogenesis as a way to mediate energy balance [4-10]. As the hypothalamus regulates multiple functional outputs, it is likely that additional behaviors may be affected by postembryonic neurogenesis in this brain structure. Here, we have identified a progenitor population in the zebrafish hypothalamus that continuously generates neurons that express tyrosine hydroxylase 2 (th2). We develop and use novel transgenic tools to characterize the lineage of th2(+) cells and demonstrate that they are dopaminergic. Through genetic ablation and optogenetic activation, we then show that th2(+) neurons modulate the initiation of swimming behavior in zebrafish larvae. Finally, we find that the generation of new th2(+) neurons following ablation correlates with restoration of normal behavior. This work thus identifies for the first time a population of dopaminergic neurons that regulates motor behavior capable of functional recovery., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Somatosensory neurons in teleosts and amphibians are sensitive to thermal, mechanical, or nociceptive stimuli [1, 2]. The two main types of such cells in zebrafish--Rohon-Beard and trigeminal neurons--have served as models for neural development [3-6], but little is known about how they encode tactile stimuli. The hindbrain networks that transduce somatosensory stimuli into a motor output encode information by using very few spikes in a small number of cells [7], but it is unclear whether activity in the primary receptor neurons is similarly efficient. To address this question, we manipulated the activity of zebrafish neurons with the light-activated cation channel, Channelrhodopsin-2 (ChR2) [8, 9]. We found that photoactivation of ChR2 in genetically defined populations of somatosensory neurons triggered escape behaviors in 24-hr-old zebrafish. Electrophysiological recordings from ChR2-positive trigeminal neurons in intact fish revealed that these cells have extremely low rates of spontaneous activity and can be induced to fire by brief pulses of blue light. Using this technique, we find that even a single action potential in a single sensory neuron was at times sufficient to evoke an escape behavior. These results establish ChR2 as a powerful tool for the manipulation of neural activity in zebrafish and reveal a degree of efficiency in coding that has not been found in primary sensory neurons.

Centrosomes are considered to be the major sites of microtubule nucleation in mitotic cells (reviewed in ), yet mitotic spindles can still form after laser ablation or disruption of centrosome function . Although kinetochores have been shown to nucleate microtubules, mechanisms for acentrosomal spindle formation remain unclear. Here, we performed live-cell microscopy of GFP-tubulin to examine spindle formation in Drosophila S2 cells after RNAi depletion of either gamma-tubulin, a microtubule nucleating protein, or centrosomin, a protein that recruits gamma-tubulin to the centrosome. In these RNAi-treated cells, we show that poorly focused bipolar spindles form through the self-organization of microtubules nucleated from chromosomes (a process involving gamma-tubulin), as well as from other potential sites, and through the incorporation of microtubules from the preceding interphase network. By tracking EB1-GFP (a microtubule-plus-end binding protein) in acentrosomal spindles, we also demonstrate that the spindle itself represents a source of new microtubule formation, as suggested by observations of numerous microtubule plus ends growing from acentrosomal poles toward the metaphase plate. We propose that the bipolar spindle propagates its own architecture by stimulating microtubule growth, thereby augmenting the well-described microtubule nucleation pathways that take place at centrosomes and chromosomes.