EXPRESSION OF MCH AND POMC GENES IN RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) DURING ONTOGENY AND IN RESPONSE TO EARLY PHYSIOLOGICAL CHALLENGES

The expression of the neuropeptide melanin-concentrating hormone (MCH) in two groups of hypothalamic neurones (NLT- and LVR-MCH neurones), a nd POMC in the pituitary corticotropes and melanotropes, has been exam ined in rainbow trout larvae using immunocytochemistry and quantitativ e in situ hybridization. The aim was to establish at what stage in ont ogeny these cells first respond to two physiological challenges-backgr ound color and stress. Trout reared in black or white trays showed ada ptive skin pigmentary changes at 10 days posthatching, when fish in a pale environment abruptly exhibited melanin aggregation from a prior d ispersed state, although the pigment cells were already competent to r espond to adrenalin and MCH in vitro at 3 days. Immunoreactive MCH was detectable in the neurohypophysis at hatching and MCH mRNA in the NLT -MCH neurones (which project to the pituitary) was enhanced at 7 days in the white-reared trout. Immunostainable POMC was also present in th e pars intermedia at hatching but their POMC mRNA was unaffected by ta nk color until 28 days, when it was enhanced in the black-reared trout . It is suggested that early pigment concentration depends on neural s ignals from the sympathetic nervous system in conjunction with MCH fro m the NLT rather than on a reduction in alpha MSH secretion from the p ars intermedia. MCH mRNA in the LVR-MCH neurones was increased on a pa le environment only 28 days after hatching, suggesting that these cell s play little role in the early adaptive pigment response. Previous st udies on the ontogeny of cortisol secretion indicate the hypothalamopi tuitary-interrenal axis can respond to stress by about 14 days. Howeve r, the pituitary ACTH cells showed no stress-induced changes in POMC m RNA until 28 days. ACTH release may therefore be dissociated from POMC transcription in the early stages of development. The LVR- and NLT-MC H neurones were both stimulated by stress, LVR-MCH mRNA responding by 14 days and NLT-MCH mRNA by 21 days. Melanotrope POMC mRNA was reduced by stress but the physiological significance ol this is not known. (C ) 1997 Academic Press.