Inhibition by L-prolyl-L-leucyl-glycinamide (PLG) of alpha-melanocyte stimulating hormone release from hypothalamic slices.

Release of alpha-MSH from male rat hypothalamic slices was studied using a sensitive bioassay (1-2 pg). Addition of 60 mM KCl to superfusion medium resulted in a twofold increase in alpha-MSH release compared to spontaneous release. Both spontaneous and potassium-induced release were inhibited in a dose-response manner by the tripeptide Pro-Leu-Gly-NH2 (PLG, or MIF-1); 0.04 microgram to 1 microgram PLG inhibited the alpha-MSH release but the lowest dose demonstrated a greater inhibitory effect; high concentrations of PLG, on the other hand, did not modify either spontaneous or potassium-evoked alpha-MSH release from the slices. Contrarily, DA did not modify either spontaneous or potassium- induced alpha-MSH release at any of the doses tested. These findings demonstrate that the inhibitory behavior of PLG and DA in the central nervous system (CNS) differs from their behavior towards alpha-MSH release in the pituitary. This suggests differences in the regulation of alpha-MSH release from the pituitary and the CNS.