THE ONTOGENY OF PITUITARY-INTERRENAL AXIS IN THE ASIAN SEABASS (LATES CALCARIFER)

Abstract

Treatment with exogenous cortisol promoted the survival of two day-old larvae exposed to hypersalinity shock (60 %0). This is presumably through the stimulation of hypo-osmoregulat01y, and possibly other, mechanisms, since there was no effect on mortalities with hyposaline shock (3.5 %0). Radioimmunoassays indicated that fertilised eggs contained cortisol, presumably of maternal origin. The levels of the cortisol declined during embryogenesis. Levels started to rise from about 4 hours post-hatching (hph), that is about 20 hours post-spawning (hps), suggesting the onset of endogenous hormone production. Steroidogenesis by the larvae in viva was studied by chromatographic steroid analyses, using tritiated 17a-hydroxyprogesterone as the precursor. Eggs just before hatching (12-15 hps) metabolised the precursor into a variety of both free and glucuronide-conjugated metabolites, but the yield of the metabolites was ve1y low. Consistent with the natural post-hatching rise in the endogenous cortisol levels, larvae converted the steroid precursor into various free metabolites apparently including cortisol, at various ages from just after hatching; glucuronide-conjugated metabolites were detected from 1 day post-hatching (dph) onwards. Cortisol was established to be one of the metabolites at 4 clays post-hatching (dph) by recrystallisation to constant specific activity; it was impractical to do this at earlier ages. The development of the larval interrenals was monitored immunohistochemically using antisera to bovine adrenodoxin and cytochrome P-45021 , two proteins necessary for cortisol synthesis in mammals, as markers. Immune Reactivity to the two proteins were detectable in the developing interrenals from l and 1.5 dph respectively. This was 4 clays before this tissue could be distinguished by normal histology. A similar approach was used to investigate the development of adrenocorticotropin (ACTH) and other hormones in the pituitary. Although the pituitary could be identified histologically by 8 hph, hormones were first detectable xi immunohistochemically several hours later. ACTH and melanotropin, another potential interrenal trophic factor, first appeared at 19 hph, followed by somatolactin at 1 dph, and somatotrophin at 2 dph. Prolactin did not appear until 9 dph. Collectively, it is concluded that the the larval interrenals begin de nova cortisol production soon after hatching. However, the immunohistochemical evidence for steroidogenic capability comes later, as does the available evidence for pituitary control. The disparity between immunohistochemical and other data presumably reflect the relative insensitivity of the former approach.