Effect of acute restraint on hypothalamic pro-vasotocin mRNA expression in flounder, Platichthys flesus

Arginine vasotocin (AVT) stimulates release of adenocorticotrophin hormone (ACTH) in trout. However, AVT's role in fish hypothalamic-pituitary-interrenal-axis (HPIA) is not fully understood. Here, we examined distribution of AVT and glucocorticoid receptor (GR) in the magnocellular preoptic nucleus (PM) and the AVT/cortisol response to acute restraint in flounder. The GR/AVT distribution in the PM was determined using double immunohistochemistry (IHC). Flounder were confined in nets, immersed in water for 30 m. with plasma and tissue samples taken prior to, 3, 24 and 48 h post-confinement. Plasma osmolality, Na+, Cl- and cortisol were taken as indicators of HPIA activation. Plasma AVT was measured proVT mRNA expression in the PM was detected using in situ hybridisation (ISH) with a S35 labelled oligoprobe for homologous flounder proVT. Double IHC showed the presence of GR in AVT synthesising neurones of the PM. plasma Na+, Cl-, osmolality and cortisol (1.0 +/- 0.9 to 183.6 +/- 3.1 mM; p < 0.001) increased significantly 3 h post-restraint: recovering to control levels after 48 h. Plasma AVT levels did not change. However, a concomitant increase in proVT mRNA expression in the magnocellular (PMm) and gigantocellular (PMg) neurones of the PM was observed (11.1 +/- 1.8 to 55.2 +/- 9.1% 24 h post-restraint; p < 0.001) and levels still remained significantly elevated at 48 h (p < 0.01). This suggests that PMm and PMg AVT neurones are associated with HPIA activation following acute restraint, including potential cortisol negative feedback. The extended elevation of hypothalamus proAVT mRNA expression following a single acute stressor affords a possible mechanism to moderate sensitivity of the HPIA to subsequent challenges. (c) 2007 Elsevier Inc. All rights reserved.