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The quantitative distribution of corticotropin-releasing hormone (CRH) in the brain and pituitary of the fish Oreochromis mossambicus (tilapia) was studied following the validation of a radioimmunoassay. Compared to the pituitary content, the brain contained 20 times more CRH. Eighty percent of the total brain content was located outside the hypothalamus, particularly in the telencephalon. Substantial amounts of CRH were also present in other regions devoid of hypophysiotropic neurons, such as the vagal lobe and optic tectum. Telencephalic and pituitary CRH co-eluted with the tilapia CRH(1-41)standard on reverse phase HPLC. In vitro CRH release by the telencephalon amounted to 5% of its content per hour, whereas release from the pituitary was negligible. We conclude that CRH in the brain of tilapia regulates pituitary and non-pituitary related functions, probably as a neurotransmitter or neuromodulator.

HPLC characterization of tilapia pituitary endorphins using an antibody specific for N-terminally acetylated endorphins yielded three major peaks in the neurointermediate lobe, but none in the pars distalis. The melanotropes secreted two of the immunoreactive products in vitro, one of which coeluted with Xenopus laevis N-ac-beta-END(1-8). This immunoreactive fraction also coeluted with diacetyl-alpha-MSH. Evidence is presented that the noteworthy corticotropic potency of this HPLC fraction, previously attributed to diacetyl-alpha-MSH, results from END and MSH acting in a coordinated fashion. Confinement stress had no effect on plasma N-ac-beta-END immunoreactivity, but led to a decrease in plasma alpha-MSH levels. Therefore, it seems unlikely that the corticotropic action of the peptides regulates the elevation of cortisol production that takes place during confinement, but it may play a role during other forms of stress that are known to activate the melanotropes.

The effect of melanin-concentrating hormone (MCH) on the release of alpha-melanocyte stimulating hormone (alpha-MSH) from the tilapia pituitary gland was studied in vitro. In a superfusion set up, 10 nM to 1 microM synthetic salmon MCH caused a concentration-dependent inhibition of alpha-MSH release from tilapia neurointermediate lobes (NILs). Immunoneutralization of MCH in tilapia NILs further indicated that endogenous MCH has an inhibitory effect on the melanotropes. The release of monoacetylated alpha-MSH release was more strongly inhibited by MCH than that of des-, and diacetylated alpha-MSH, indicating that MCH modulates the secretory signal of the melanotropes in a quantitative and qualitative manner. A high concentration of MCH (10 microM) substantially increased the release of alpha-MSH. Further evidence in support of a stimulatory action of high concentrations of MCH was provided by the observation that the MCH analogue MCH(2-17) at 10 and 35 microM enhanced alpha-MSH release as well. Therefore, we conclude that the response of pituitary melanotropes to MCH is biphasic, as was reported previously for the effects of MCH on other targets in fish and mammals. Under physiological conditions the inhibitory action of MCH on fish melanotropes most likely dominates.