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Olfactory sensitivity to changes in environmental Ca2 in the freshwater teleost Carassius auratus: an olfactory role for the Ca2+ -sensing receptor?
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Abstract(s)
Olfactory sensitivity to changes in environmental Ca2+ has been demonstrated in two teleost species; a salmonid (Oncorhynchus nerka) and a marine/estuarine perciform (Sparus aurata). To assess whether this phenomenon is restricted to species that normally experience large fluctuations in external ion concentrations (e.g. moving
from sea water to fresh water) or is present in a much wider range of species, we investigated olfactory Ca2+ sensitivity in the goldfish (Carassius auratus), which is a
stenohaline, non-migratory freshwater cyprinid.
Extracellular recording from the olfactory bulb in vivo by electroencephalogram (EEG) demonstrated that the olfactory system is acutely sensitive to changes in external
Ca2+ within the range that this species is likely to encounter in the wild (0.05–3 mmol l–1). The olfactory system responded to increases in external calcium with
increasing bulbar activity in a manner that fitted a conventional Hill plot with an apparent EC50 of 0.9±0.3 mmol l–1 (close to both ambient and plasma free [Ca2+]) and an apparent Hill coefficient of 1.1±0.3 (means
± S.E.M., N=6). Thresholds of detection were below 50 mmol l–1. Some olfactory sensitivity to changes in external [Na+] was also recorded, but with a much higher
threshold of detection (3.7 mmol l–1). The olfactory system of goldfish was much less sensitive to changes in [Mg2+] and [K+]. Preliminary data suggest that Ca2+ and Mg2+
are detected by the same mechanism, although with a much higher affinity for Ca2+. Olfactory sensitivity to Na+ may warn freshwater fish that they are reaching the limit of their osmotic tolerance when in an estuarine environment. Olfaction of serine, a potent odorant in fish, was not dependent on the presence of external Ca2+ or Na+. Finally, the teleost Ca2+-sensing receptor (Ca-SR) was shown to be highly expressed in a subpopulation of olfactory receptor neurones by both immunocytochemistry and in situ hybridisation. The olfactory sensitivity to Ca2+ (and Mg2+) is therefore likely to be mediated by the Ca-SR. We suggest that olfactory Ca2+ sensitivity is a widespread phenomenon in teleosts and may have an input into the physiological mechanisms regulating internal calcium homeostasis.
Description
Keywords
Olfaction, Freshwater Fish Ca2+-sensing receptor Electroencephalogram Immunocytochemistry In situ hybridisation Carassius auratus.
Citation
Hubbard, P. C.; Ingleton, P. M.; Bendell, L. A.; Barata, E. N.; Canario, A. V. M.Olfactory sensitivity to changes in environmental Ca2 in the freshwater teleost Carassius auratus: an olfactory role for the Ca2+ -sensing receptor?, Journal of Experimental Biology, 205, 18, 2755-2764, 2002.