BIPHASIC EFFECT OF MCH ON ALPHA-MSH RELEASE FROM THE TILAPIA (OREOCHROMIS-MOSSAMBICUS) PITUITARY

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 mu M 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 mu M) 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(217) at 10 and 35 mu M 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.