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Changes in expression of provasotocin and proisotocin genes during adaptation to hyper- and hypo-osmotic environments in rainbow trout

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The physiological roles of neurohypophysial hormones, vasotocin (VT) and isotocin (IT), are not yet clear in teleosts. Since information on responsiveness of hypothalamic neurosecretory neurons to environmental stimuli may contribute to an understanding of their physiological roles, effects of environmental hyper- and hypo-osmotic stimuli on expression of VT and IT precursor (proVT and proIT) genes in rainbow trout were investigated, using an in situ hybridization technique in which 46 mer synthetic oligonucleotides were used as hybridization probes. The probes corresponded to the mRNA loci encoding chum salmon proVT (-5 to 11) and proIT (-5 to 11), and were labeled at the 3′-end with 35S. Autoradiographic silver grains which represent the hybridization signals of proVT and proIT mRNAs were localized in both magnocellular and parvocellular neurons in the nucleus preopticus magnocellularis (NPOmg). Localizations of proVT and proIT hybridization signals coincided with those of VT- and IT-immunoreactive neurons in adjacent sections, and showed that proVT and proIT genes are expressed in separate neurons. The intensity of proVT hybridization signals as determined by grain counting in magnocellular neurons in the NPOmg was conspicuously decreased after transfer from fresh water (FW) to 80% seawater (SW). The proVT mRNA levels in SW trout were consistently lower than those of FW trout for up to 2 weeks. After return from 80% SW to FW, the proVT mRNA level increased, attaining the initial FW level. The proIT mRNA levels in SW trout were not statistically different from those in FW trout, except for the 1st day after transfer to SW. These results suggest that synthesis of proVT was elevated by transfer from higher to lower salinity, and that VT may have a physiological role in salmonid osmoregulation, especially in adaptation to a hypo-osmotic environment.

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avidin-biotin-peroxidase complex


arginine vasopressin


bovine serum albumin


ethylene diamine tetra-acetic acid

cpm :

counts per minute

FW :

fresh water


gromerular filtration rate


in situ hybridization

IT :


mRNA :

messenger ribonucleic acid

NPOmg :

nucleus preopticus magnocellularis




phosphate-buffered saline


saline sodium citrate

RT :

room temperature

SW :


VT :



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Correspondence to Susumu Hyodo.

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Hyodo, S., Urano, A. Changes in expression of provasotocin and proisotocin genes during adaptation to hyper- and hypo-osmotic environments in rainbow trout. J Comp Physiol B 161, 549–556 (1991).

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Key words

  • Vasotocin
  • Isotocin
  • Gene expression
  • Osmoregulation
  • Rainbow trout
  • Oncorhynchus mykiss