The effects of acute and chronic stresses on vasotocin gene transcripts inthe brain of the rainbow trout (Oncorhynchus mykiss)

Secretion of adrenocorticotropic hormone (ACTH) from the fish pituitary, wh ich occurs in times of stress, is stimulated by several hypothalamic neurop eptides, one of which is arginine vasotocin (AVT). This study investigates whether gene expression for AVT is up-regulated during acute or chronic str ess. Rainbow trout (Oncorhynchus mykiss) were subjected to one of two forms of acute stress-either 2 h confinement followed by 2 h recovery, or captur e and transfer to low water for 2 min followed by 4 h recovery in their hom e tank before autopsy. In other experiments, these stresses were repeated d aily for 5 or 6 days (chronic stress). Quantification of AVT transcript pre valence in the parvocellular and magnocellular neurones of the preoptic nuc leus after in situ hybridization was used as a monitor of the AVT gene resp onse to stress. The results showed that acute confinement, but apparently n ot brief low-water stress, significantly increased AVT transcript prevalenc e in a group of parvocellular perikarya. When applied repeatedly, both form s of stress caused habituation, such that the AVT hybridization signal rema ined at control or even lower levels despite elevated pro-opiomelanocortin transcripts in the corticotropes and raised plasma cortisol concentrations. The AVT hybridization signal in the magnocellular perikarya showed no sign ificant response to either acute or chronic stress. The results support the idea that these parvocellular AVT neurones are involved in ACTH stimulatio n during acute stress, and that the system habituates to chronic stresses.