Serotonergic neuroepithelial cells of the skin in developing zebrafish: morphology, innervation and oxygen-sensitive properties

Abstract

SUMMARYIn teleost fish, O 2 chemoreceptors of the gills (neuroepithelial cells or NECs) initiate cardiorespiratory reflexes during hypoxia. In developing zebrafish, hyperventilatory and behavioural responses to hypoxia are observed before development of gill NECs, indicating that extrabranchial chemoreceptors mediate these responses in embryos. We have characterised a population of cells of the skin in developing zebrafish that resemble O 2 -chemoreceptive gill NECs. Skin NECs were identified by serotonin immunolabelling and were distributed over the entire skin surface. These cells contained synaptic vesicles and were associated with nerve fibres. Skin NECs were first evident in embryos 24-26hpost-fertilisation (h.p.f.), and embryos developed a behavioural response to hypoxia between 24 and 48h.p.f. The total number of NECs declined with age from approximately 300cells per larva at 3days post-fertilisation (d.p.f.) to ~120cells at 7d.p.f., and were rarely observed in adults. Acclimation to hypoxia (30mmHg) or hyperoxia (300mmHg) resulted in delayed or accelerated development, respectively, of peak resting ventilatory frequency and produced changes in the ventilatory response to hypoxia. In hypoxia-acclimated larvae, the temporal pattern of skin NECs was altered such that the number of cells did not decrease with age. By contrast, hyperoxia produced a more rapid decline in NEC number. The neurotoxin 6-hydroxydopamine degraded catecholaminergic nerve terminals that made contact with skin NECs and eliminated the hyperventilatory response to hypoxia. These results indicate that skin NECs are sensitive to changes in O 2 and suggest that they may play a role in initiating responses to hypoxia in developing zebrafish.

Serotonergic and cholinergic elements of the hypoxic ventilatory response in developing zebrafish

“…for three reasons: (1) only these cells have been shown to possess chemoreceptor properties in the gill (Jonz et al, 2004;Qin et al, 2010); (2) NECs appear to be functional as chemoreceptors in vivo beginning at 7d.p.f. (Jonz and Nurse, 2005); and (3) this time frame represents an important developmental period when the systems of gas exchange (Rombough, 2002) and O 2 sensing appear to be transferred to the gills from the skin Coccimiglio and Jonz, 2012). Our results indicate that the addition of new NECs in the gill filaments during development is inhibited by hypoxia up to 7d.p.f.…”

“…Coincident with these changes is the development of gill primordia at 3d.p.f. (Kimmel et al, 1995), a transition in O 2 chemosensitivity from an extrabranchial site to the gills Coccimiglio and Jonz, 2012), and innervation of serotonergic NECs of the gill filaments at 7d.p.f. (Jonz and Nurse, 2005).…”

Aquatic surface respiration and swimming behaviour in adult and developing zebrafish exposed to hypoxia

“…Unlike in adults, at this early developmental stage O 2 -sensitive gill chemoreceptors are not yet fully innervated (Jonz and Nurse, 2005) and so are not likely to participate in reflex responses. However, cutaneous NECs that receive innervation, retain 5-HT and exhibit O 2 -sensitive properties appear in zebrafish as early as 1 dpf (Coccimiglio and Jonz, 2012). Although these cells may be candidate chemoreceptors for initiation of ASR in early larvae, we were not able to accordingly modify ASR responses in larvae by immersion in 5-HT or ACh before 18 dpf.…”

“…But the progression in ASR throughout early life in fish has not previously been correlated with events of respiratory development within the same species. Zebrafish embryos respond to hypoxia by first increasing the frequency of pectoral fin and whole body movements as early as 2 dpf in order to increase flow across respiratory surfaces and improve diffusion; they develop a maximal ventilatory response to hypoxia between 5 and 7 dpf that coincides with innervation of gill NECs (Jonz and Nurse, 2005;Coccimiglio and Jonz, 2012), and this response develops an increasing reliance on central N-methyl-Daspartate (NMDA) receptors at 8 dpf (Turesson et al, 2006). In the present study, developing zebrafish initiated ASR behaviour at lower O 2 tensions than adults.…”

“…These cells first appear in the gill filaments of zebrafish larvae between 5 and 7 days postfertilization (d.p.f.) and are preceded by a cutaneous population of NEC-like cells in embryos (Jonz and Nurse, 2005;Coccimiglio and Jonz, 2012). In addition, cells expressing an acetylcholine (ACh) transporter were reported in the gills of zebrafish (Shakarchi et al, 2013) and the amphibious fish Kryptolebias marmoratus (Regan et al, 2011).…”