The Regulation of Prolactin Cells in the Rainbow Trout ( Oncorhynchus mykiss) : 1. Possible Roles for Thyrotropin-Releasing Hormone (TRH) and Oestradiol

Abstract

The regulation of rainbow trout prolactin (PRL) cells by hypothalamic and humoral factors has been investigated. PRL cell activity was assessed using acid gel electrophoresis to measure PRL release in vitro and image analysis to measure PRL cell nuclear area, an index of synthetic activity. 17β-oestradiol stimulate PRL synthesis and release during a 48-hr incubation. TRH reduced PRL synthesis and release and the use of bacitracin to prevent metabolism reduced the effect on release, though not that on synthesis, suggesting that a metabolite of TRH may have a role, at least in the former process. This was supported by the finding that the TRH metabolite, cyclo(His-Pro), was also potent in inhibiting PRL cell activity; hence, at least part of the action of TRH may be via metabolites. In some experiments, the PRL isoforms, PRLI and PRLII, could be separately measured and their release appeared to be differentially affected by TRH and cyclo(His-Pro). A 48-hr pretreatment with 17β-oestradiol to stimulate PRL cell activity did not prevent the inhibitory effects of TRH, rather the responses were enhanced. TRH and its metabolite(s) and oestradiol are concluded to regulate rainbow trout PRL secretion.