Abstract
Dopaminergic neurons of the descending diencephalospinal system are located in the posterior tuberculum (PT) in zebrafish (Danio rerio), and correspond in mammals to the A11 group in hypothalamus and thalamus. In the larval zebrafish, they are likely the only source of central dopaminergic projections to the periphery. Here, we characterized posterior tubercular dopaminergic fibers projecting to peripheral sense organs, with a focus on the lateral line neuromasts. We labeled and identified catecholaminergic neurons and their projections by combining two immunofluorescence techniques, (i) using an antibody against Tyrosine hydroxylase, and (ii) using an antibody against GFP in transgenic zebrafish expressing in catecholaminergic neurons either membrane-anchored GFP to track fibers, or a Synaptophysin-GFP fusion to visualize putative synapses. We applied the CLARITY method to 6 days old whole zebrafish larvae to stain and analyze dopaminergic projections by confocal microscopy. We found that all lateral line neuromasts receive direct innervation by posterior tubercular dopaminergic neurons, and tracked these projections in detail. In addition, we found dopaminergic fibers projecting to the anterior and posterior lateral line ganglia, and extensive central dopaminergic arborizations around the terminal projection field of the lateral line afferent neurons in the hindbrain medial octavolateralis nucleus (MON). Therefore, dopaminergic innervation may affect lateral line sense information at different processing stages. Additional dopaminergic fibers innervate the trigeminal ganglion, and we observed fine catecholaminergic fibers in the skin with arborization patterns similar to free sensory nerve endings. We also detected potentially dopaminergic fibers innervating inner ear sensory epithelia. Therefore, the diencephalospinal A11-type dopaminergic system may broadly modulate peripheral senses. We also briefly report peripheral sympathetic catecholaminergic projections labeled in our experiments, and their innervation of the developing intestine, swim bladder and abdominal organs.
Highlights
Recent studies have revealed that dopaminergic (DA) neurons in the posterior tuberculum (PT) and ventral diencephalon of larval zebrafish correspond to the mammalian dopaminergic A11 system, which all depend on the transcription factor Orthopedia (Otp) (Ryu et al, 2007; Fernandes et al, 2013)
Based on observations that modulatory systems are effective at this stage (Burgess and Granato, 2007; Reinig et al, 2017), we focused our studies on the peripheral projection patterns of the far projecting A11-type DA neurons located in the PT at 6 dpf
We provide a comprehensive description of the peripheral projections of the diencephalic dopaminergic neurons with origin in the PT, which descend into the spinal cord and are homologous to the mammalian A11 group
Summary
Recent studies have revealed that dopaminergic (DA) neurons in the posterior tuberculum (PT) and ventral diencephalon of larval zebrafish correspond to the mammalian dopaminergic A11 system, which all depend on the transcription factor Orthopedia (Otp) (Ryu et al, 2007; Fernandes et al, 2013). The diencephalic DA systems in zebrafish comprise the A11 homologous groups (PTar, PTac, PTp, PTN; previously named DC2, DC4-6), as well as groups in the preoptic region, the pretectum, the ventral thalamus, and several hypothalamic groups (Rink and Wullimann, 2001). The catecholaminergic projections in the larval zebrafish central nervous system have been previously characterized (Rink and Wullimann, 2002; McLean and Fetcho, 2004; Tay et al, 2011), and the A11 homolog groups (PTar, PTac, PTp, PTN) have been shown to extend fibers to the subpallium, endohypothalamic tract (eht), rhombencephalon (RhE) and spinal cord (Table 1; Kastenhuber et al, 2010; Tay et al, 2011). There is to date no detailed anatomical analysis of DA fibers projecting into the periphery
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